‘‘thesis’’ --- 2007/1/31 22:24 --- page i (#1)

A CATALYSIS-ENGINEERINGAPPROACH

TO

SELECTIVEHYDROGENATION ‘‘thesis’’ --- 2007/1/31 22:24 --- page ii (#2)

ii ‘‘thesis’’ --- 2007/1/31 22:24 --- page iii (#3)

A CATALYSIS-ENGINEERINGAPPROACH

TO

SELECTIVEHYDROGENATION

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft op gezag van de Rector Magnificus prof. dr. ir. J.T. Fokkema, voorzitter van het College voor Promoties, in het openbaar te verdedigen

op dinsdag 13 maart 2007 te 12:30 uur

door

Martinus Mathilda Pieter Zieverink

scheikundig ingenieur geboren te Sittard ‘‘thesis’’ --- 2007/1/31 22:24 --- page iv (#4)

Dit proefschrift is goedgekeurd door de promotoren: Prof. dr. J.A. Moulijn Prof. dr. F. Kapteijn

Samenstelling promotiecommissie: Rector Magnificus voorzitter Prof. dr. J.A. Moulijn Technische Universiteit Delft, promotor Prof. dr. F. Kapteijn Technische Universiteit Delft, promotor Prof. dr. W. Buijs Technische Universiteit Delft Dr. T. Boger Corning, Inc. Dr. E. Schwab BASF A.G. Prof. dr. J.M. Winterbottom University of Birmingham Dr. ir. M.T. Kreutzer Technische Universiteit Delft

Dr. ir. M.T. Kreutzer heeft als begeleider in belangrijke mate aan de totstandkoming van het proefschrift bijgedragen.

This research was financially supported by BASF A.G. and Corning, Inc.

c 2007 by M.M.P. Zieverink

All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilised in any form or by any means, electronic or mechanical, including protocopying, recording or any information storage and retrieval system, without written permission of the author.

ISBN: 978-90-5335-113-0 Keywords: Heterogeneous Catalysis / Hydrogenation / Isomerization / Edible oil ‘‘thesis’’ --- 2007/1/31 22:24 --- page v (#5)

Ter nagedachtenis aan Frans Otten en Tinus Zieverink ‘‘thesis’’ --- 2007/1/31 22:24 --- page vi (#6)

vi ‘‘thesis’’ --- 2007/1/31 22:24 --- page vii (#7)

CONTENTS

1 Introduction 1 1.1 Catalytic hydrogenation and diffusion effects ...... 1 1.2 Vegetable oil hydrogenation with monolithic catalysts ...... 3 1.3 Engineering monolithic catalysts ...... 3 1.4 Thesis outline and scope ...... 4

2 Isomerization Reactions in the Hydrogenation of 1-Dodecene 7 2.1 Introduction and motivation ...... 7 2.2 Experimental ...... 10 2.3 Results and discussion ...... 13 2.4 Conclusions ...... 24 2.5 Nomenclature ...... 24

3 Diffusional Effects on Isomerization Reactions: Hydrogenation of Methyl Oleate and Derivatives 27 3.1 Introduction ...... 27 3.2 Previous work and literature overview ...... 30 3.3 Experimental ...... 35 3.4 Results ...... 39 3.5 Modelling the effects of diffusion limitations ...... 49 3.6 Discussion ...... 51 3.7 Conclusions ...... 56 3.8 Nomenclature ...... 56

4 Kinetics of the Hydrogenation and Isomerization of 1,5,9-cis,trans,trans- Cyclododecatriene 61 4.1 Introduction ...... 61 4.2 Previous work ...... 65 4.3 Experimental ...... 66 4.4 Results ...... 70 4.5 Discussion ...... 72 4.6 Conclusions ...... 87

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CONTENTS

4.7 Nomenclature ...... 88

5 Introduction to Edible Oils and Hydrogenation Processes 91 5.1 The history of edible oil use ...... 91 5.2 Occurrence ...... 92 5.3 The chemistry of fats and oils ...... 92 5.4 Analysis of fats ...... 93 5.5 The invention of edible oil hydrogenation ...... 95 5.6 Hydrogenation processes ...... 96 5.7 Hydrogenated oil products ...... 97 5.8 Health issues associated with hydrogenated oil ...... 98 5.9 Conclusions ...... 100

6 Monolith Catalysts as Alternative to Slurry Systems: Hydrogenation of Ed- ible Oil 103 6.1 Introduction ...... 103 6.2 Experimental ...... 105 6.3 Results and discussion ...... 111 6.4 Economic evaluation of add-on unit ...... 119 6.5 Conclusions ...... 124 6.6 Nomenclature ...... 125

7 High(er) Performance Monoliths: Proof of Principle 131 7.1 Improving monolithic catalysts ...... 131 7.2 Experimental ...... 133 7.3 Results and discussion ...... 136 7.4 Conclusions ...... 138 7.5 Nomenclature ...... 138

8 Summary and Evaluation 141 8.1 Diffusion effects on hydrogenation and isomerization ...... 141 8.2 Concerning the use of monoliths ...... 145 8.3 Catalysis engineering ...... 146 8.4 Outlook on fat harding ...... 148

Samenvatting 149

Dankwoord 152

Publications and Presentations 154

Curriculum Vitae 156

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Chapter 1

Introduction

’Les Chimistes forment encore un peuple distinct, tres-peu` nombreux, ayant sa langue, ses lois, ses mysteres,` & vivant presque isole´ au milieu d’un grand peuple peu curieux de son commerce n’attendant presque rien de son industrie’ Paul Henry Thiry (1723-1789) in ‘Encyclopedie,´ ou dictionnaire raisonne´ des sciences, des arts et des metiers’´

1.1 Catalytic hydrogenation and diffusion effects

The father of catalytic hydrogenation is without doubt Paul Sabatier. He was the first to recognize the formation of unstable reaction intermediaries and discovered the use of finely divided metal catalysts, in addition to the introduction of catalytic supports. For his work he was awarded the Nobel Prize in Chemistry for 1912, sharing the prize with Victor Grignard who won it for discovering the reagent later named after him. Sabatier’s work laid the foundation for industrial processes such as methanol synthesis and vegetable oil hydrogenation. At first, it was thought that hydrogenation could only take place in the vapour phase, but Wilhelm Normann’s 1903 patent made clear that fatty acids could also be hydrogenated in the liquid phase (Normann, 1903). Working in the liquid phase of course has huge advantages; reaction volumes can be much smaller and no energy needs to be expended for vaporization. It does however add a layer of complexity: instead of a gaseous hydrogen/reactant mix flowing over a catalyst, hydrogen needs to be dissolved into the liquid in such a way that hydrogen and reactant can come into contact with the catalyst surface. However, since diffusivity in a liquid is at most one hundredth of that in a gas, the existence of gradients inside a porous catalyst particle cannot be ignored anymore. At some point, the reaction rate inside the catalyst will outstrip the rate at which the products can diffuse away. The first to treat this quantitatively was Ernest Thiele in a landmark paper (Thiele, 1939). He introduced a dimensionless number, later termed the Thiele modulus, that describes the effectiveness of a catalyst particle. This Thiele modulus is the ratio of the reaction rate that would be obtained without gradients, and the diffusion rate:

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CHAPTER 1

Figure 1.1: Catalyst effectiveness as a function of a dimensionless number. Figure taken from Thiele (1939). This number was later named Thiele modulus, symbolized by φ.

r r φ L (1.1) = Def f C

where L is the diffusion length, Def f the effective diffusivity inside the catalyst and C the reactant concentration. For a first order reaction r kC and = s k φ L (1.2) = Def f The effectiveness of a catalyst particle can be determined for a given geometry. Thiele showed that for a slab-like catalyst, the effectiveness η can be calculated with tanh φ η (1.3) = φ Results of these calculations for different situations are illustrated in Figure 1.1 which is taken from the original paper. An extension of Thiele’s theory can also be applied to consecutive reactions A B C, where the maximum amount of intermediate B that is formed will decrease by→ an→ increase of the ratio of reaction rate/diffusion rate. The first one to recognize this was Wheeler (1951), and experimental proof of Wheeler’s the- ory was given by Weisz and Swegler (1955) for the dehydrogenation of cyclohexane to cyclohexene and benzene. Hydrogenation of carbon-carbon double bonds present in for example olefins or fatty acids can also result in isomerization of these double bonds. They can shift position

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INTRODUCTION

and change conformation from cis to trans and vice versa. In this thesis, the effects of diffusion limitations are considered, not only for consecutive hydrogenations, but also on the isomerization reactions that accompany them.

1.2 Vegetable oil hydrogenation with monolithic catalysts

The explosive growth of the world population in the late nineteenth century posed a serious problem; the demand for butter and associated products could not be met solely from animal sources. Although large amounts of vegetable oils were available, these did not lend themselves towards applications where solid fats where wanted. The invention of fat-hardening in the beginning of the twentieth century changed all that; it was now possible to take a liquid vegetable oil and reduce the amount of unsaturation of the fats in a way such that a solid product remained. Vegetable oil hydrogenation is a consecutive reaction; tri-unsaturated fats are hydro- genated into di-unsaturates and so on. Fats are large molecules ( 2 nm) and as a result diffusion will often dominate the hydrogenation process, resulting≈ in increased formation of the final saturated product. However, it is often desired to remove the poly-unsaturates selectively; depending on the desired properties of the product, a certain distribution of poly- and mono-unsaturates is aimed at. To some extent, hydrogenation is always accom- panied by isomerization; the double bonds that have the cis conformation in vegetable oils can be isomerized into trans double bonds. Both saturated fats and trans fats are becoming increasingly suspect because of their adverse effects on human health (Mann, 2002). The fat-’hardening’ process, consisting of mixing oil, hydrogen and slurry catalyst in large stirred tanks operated in fed-batch mode has remained largely unchanged. Continu- ous processes were developed quite early (Lush, 1923; Bolton, 1927) but have not taken root. In this thesis, the use of monolithic catalysts for fat-hardening is investigated. Mono- liths are ceramic structures of parallel straight channels, with diameter of the channel on the order of one millimeter. Since the macroporous ceramic structure provides little or no anchoring sites for catalytic material, the monolith channels need to be washcoated first with a support material such as porous silica or alumina. With these structured catalysts there is no need for reactant/catalyst separation and continuous operation is in principle possible.

1.3 Engineering monolithic catalysts

The most common monoliths are made by extrusion of a ceramic paste and are used as exhaust gas catalyst carriers in the automotive industry. These ceramic (often cordier- ite) structures have the distinct advantage of being cheap, easy to manufacture on a large scale and are very resistant to heat shock. However, they are also very macroporous with

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CHAPTER 1

cordierite

high surface area ‘active layer’

low surface area ‘filler’

Figure 1.2: High Performance Monolith: channels pre-rounded with filler material, thin catalytic washcoat.

2 1 a total BET surface area of less than 4 m g− (Nijhuis et al., 2001). Any coating that is applied tends to enter into the macroporous structure of the cordierite. For gas-phase only reactions this does not present a problem; because of the much slower diffusion rates it does present a serious problem in liquid-phase reactions. At best the catalyst is inac- cessible to the reactants but more likely the diffusional path of the reactants is increased. This has for example been demonstrated by Crezee et al. (2003) for ruthenium deposited on carbon coated monoliths. Equally important, the coating material tends to accumulate in the channel corners, increasing diffusion length. In this thesis, a preparation method is outlined that overcomes these problems. First, the monolith channels are rounded with a low surface area filler material, followed by a thin layer of high surface area material onto which the active catalytic metal can be deposited (Figure 1.2). The reduced diffu- sional distance in these so-called High Performance Monoliths is thought to result in both increased activity and intermediate yields.

1.4 Thesis outline and scope

Chapter 2 is focussed on the hydrogenation and isomerization/double bond shift of the α-olefin 1-dodecene. Most of the work reported in literature is on the hydrogenation of relatively short olefins in the gas phase. Using 1-dodecene increases the complexity, but also allows a better distinction between the different forms of isomerization. Chapter 3 is concerned with the influence of controlled diffusion limitations on the hydrogenation and isomerization of methyl oleate, a fatty acid methyl ester. In contrast to Chapter 2, where the double bond to be hydrogenated was located at the end of the molecule, in methyl oleate the double bond is in the middle of the molecule, posing inter-

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INTRODUCTION

esting problems regarding the analysis of the reaction products. Conditions were chosen such that hydrogen was in abundant supply so that only reactant gradients existed inside the catalyst. The influence of diffusion limitations on the isomerization rates relative to the hydrogenation rates was determined and modelled. Chapter 4 makes a study of the kinetics of the hydrogenation of 1,5,9-cis,trans,trans- cyclododecatriene, a cyclic olefin, using a palladium on alumina slurry catalyst. This reaction is at first glance an ordinary consecutive reaction, but the existence of conform- ational isomers of these cyclic molecules has some interesting consequences on the ob- served kinetics. Once again, differences in hydrogenation and isomerization behaviour are observed. Chapter 5 gives an introduction into the use of vegetable oil, and the different ways in which oils are treated before becoming an edible product. Edible oil hydrogenation or fat-hardening technology is introduced, and the adverse health effects associated with hydrogenated fat are discussed. In Chapter 6, the use of monoliths for hydrogenation of vegetable oil is investig- ated. Comparisons are made with slurry catalysts and the economics of a process using monolith catalysts is detailed. One of the observations in Chapter 6 was that ordinary monolith catalysts suffer heav- ily from internal diffusion limitations as a result of the accumulation of catalytic wash- coat in the monolith corners. As a result, in Chapter 7 an improved method of monolith washcoating is discussed which gives so-called High Performance Monoliths. The hydro- genation of cyclododecatriene was used as a model reaction to demonstrate the effect a better washcoating method can have on reducing the amount of diffusion limitations. Finally, Chapter 8 gives an overview of the results and discusses the implications in broader terms.

Bibliography

Bolton, E., 1927. Recent advances in the hydrogenation of oils. J.S.C.I., 444T–446T. Crezee, E., Kooyman, P., Kiersch, J., Sloof, W., Mul, G., Kapteijn, F., Moulijn, J., 2003. Dispersion and distribution of ruthenium on carbon-coated ceramic monolithic cata- lysts prepared by impregnation. Catal. Lett. 90, 181–186. Lush, E., 1923. Hydrogenation. J.S.C.I. 62, 219T–225T. Mann, J., 2002. Diet and risk of coronary heart disease and type 2 diabetes. The Lancet 360, 783–789. Nijhuis, T., Beers, A., Vergunst, T., Hoek, I., Kapteijn, F., Moulijn, J., 2001. Preparation of monolithic catalysts. Catal. Rev. Sci. Eng. 43(4), 345–380. Normann, W., 1903. Process for converting unsaturated fatty acids or their glycerides into saturated compounds. British Patent 1515.

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CHAPTER 1

Thiele, E., 1939. Relation between catalytic activity and size of particle. Ind. Eng. Chem. 31(7), 916–920. Weisz, P., Swegler, E., 1955. Effect of intraparticle diffusion on the kinetics of catalytic dehydrogenation of cyclohexane. J. Phys. Chem. 59(9), 823–826. Wheeler, A., 1951. Reaction rate and selectivity in catalyst pores. Adv. Catal. 3, 249–327.

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Chapter 2

Isomerization Reactions in the Hydrogenation of 1-Dodecene

’One must suppose that it is an advance of some kind if experiments based only on a simple measurement of rate can be shown to be consistent with only nine reaction mechanisms’ Geoffrey C. Bond in Bond (2005)

2.1 Introduction and motivation

Interest in this chapter is focussed on the interplay between hydrogenation and iso- merization of carbon-carbon double bonds, in the hydrogenation of olefins over supported palladium catalysts. The suppression of isomerization reactions is of great interest in the hydrogenation or hardening of fats and fatty esters. The naturally occurring cis double bonds are partially hydrogenated while at the same time unwanted isomerization towards trans double bonds takes place. Although strictly speaking, they are not olefins, the alkyl side-chains of the double bonds in fats are expected to behave like hydrocarbons. A typ- ical fatty acid such as oleic acid has an alkyl chain of 18 carbons with the double bond between the ninth and tenth carbon which leaves ample room for double bond migration. Although a large volume of literature exists on the hydrogenation of olefins, it is mostly limited to gas-phase reaction systems. Little is published on the effect of hydro- gen on the cis-trans isomerization and double bond shift and whether these two processes share a surface reaction step. Cis-trans isomerization of a double bond at some point in- volves a spatial rotation of alkyl side chains, something not necessarily needed for double bond migration. As a result, it is entirely possible that these processes happen on a differ- ent timescale. Although 1-butene isomerization has been studied extensively, it only has a single internal double bond and as such double bond shift is limited from 1- to 2-butene. Pentene obviously has the same drawback. 1-Dodecene was chosen as a model compound because of the relative ease with which reaction products can be analyzed and its similarity to long chain fatty acids. It allows us to distinguish between the double bond shift from 1- to 2-dodecene and the subsequent cis-trans isomerization followed by migration of double bonds deeper into the molecule.

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CHAPTER 2

100

80

60

40

butene composition (%) 20

0 0 20406080100 conversion (%)

Figure 2.1: Change in the composition of the butene fraction during the hydrogenation of 1-butene over Pd-alumina at 313 K. 1-butene ( ); trans-2-butene (3); cis-2-butene (2); thermodynamic equilibrium (+). Data taken from Bond4 and Wells (1964); lines drawn to guide the eye.

Olefin hydrogenation on supported palladium catalysts: Overview

Most literature on olefin hydrogenation/isomerization on supported palladium cata- lysts concerns the hydrogenation of gaseous 1-butene. Rapid isomerization takes places at low temperatures (Figure 2.1) but decreases relative to the hydrogenation rate with in- creasing temperature (Bond and Wells, 1964; Bond et al., 1968; Winterbottom, 1962), in- dicating a lower activation energy for isomerization than for hydrogenation. Liquid phase hydrogenation of 1-hexene and 1-octene has been reported (Smits et al., 1996; Tamai et al., 1999; Ramesh et al., 2000) but without quantification of the different isomerization and hydrogenation reaction rates. Bond and Wells (1964) found the rate of isomerization of 1-butene on transition metals to be independent of the hydrogen pressure, while Hol- brook and Wise (1972) report a square root dependency on the hydrogen pressure. Reinig (1992) hydrogenated 1-butene on an alumina supported palladium catalyst: at hydrogen pressures up to 0.5 kPa the rate of isomerization increased strongly with pressure, fol- lowed by a slight decrease with further pressure increase. It should be noted that these were all gas-phase reaction systems, with butene and hydrogen partial pressures of less than 0.1 MPa.

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1-DODECENE

100

80

60

40 2-pentene yield (%) 20

0 0 20406080100 % hydrogenation

Figure 2.2: 2-pentene yields during the hydrogenation of 1-pentene over Pd-charcoal. trans-2- pentene (3); cis-2-pentene (2). Data taken from Bond and Rank (1965); lines drawn to guide the eye.

Double bond migration of α-olefins initially yields less 2-trans-alkene relative to the cis isomer than corresponds to thermodynamic equilibrium (Bond, 2005). This behaviour is operative in gaseous 1-butene (Holbrook and Wise, 1972; Carra´ and Ragaini, 1968) and has also been observed in liquid 1-pentene (see Figure 2.2). It has been argued by Carley (2002) that this is caused by molecular congestion at the active site which causes the 2-pentyl group to adopt the conformation of lowest molar volume (e.g. the cis form). Bond and Wells (1964) wrote an excellent review on the hydrogenation of (short) olefins on transition metals which despite its age is still to be recommended. A recent overview of the field can be found in Bond (2005).

In conclusion, although a large body of literature exists on the (gas phase) hydrogen- ation of short olefins, there is little in the way of quantitative data on the hydrogenation and isomerization of long olefins in the liquid phase.

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CHAPTER 2

Table 2.1: Catalyst characteristics. 2 1 SBET 210 3 m g ± − dpore 15 nm dp 17 µm Pd loading 1.06 wt% dispersion 23.4 %

9 2

10

7

6

1 5 8 3 4

4 8 12 16

Figure 2.3: Chromatogram of the reaction mixture. See Table 2.2 for identification of the peaks.

2.2 Experimental

Materials and Analysis

A commercial supported palladium on γ -alumina catalyst was used (Aldrich Chem- ical Company, Inc.). Catalyst properties are given in Table 2.1. 1-Dodecene (95% pure, Aldrich) was used as reactant. GC-MS showed that the remaining 5% was made up par- tially by methyl-undecenes. Analytical grade n-decane (Merck) was used as a solvent 3 resulting in a solution containing 10 wt% 1-dodecene (400 mol m− ). Analysis of the reaction products was carried out on a Chrompack 9001 Gas Chro- matograph equipped with a 50 m 0.25 mm 0.2 mm CP-Sil 88 fused silica column and a flame ionization detector. A× split ratio of× approximately 1:100 was used. Helium was used as a carrier gas. The column pressure was maintained at 150 kPa, and the flow 1 rate was 0.45 ml min− . The column was operated isothermally at 333 K and both the injector and detector were kept at 523 K. Seven clearly separated reactant peaks were de-

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1-DODECENE

peak 10: trans -2-DE )

-3 100

peak 9: cis -2-DE peak 6: trans -3-DE

50 peak 5: cis -3-DE and trans -4-DE concentration (mol m

0 0306090 peak 3: cis -4-DE time (min)

Figure 2.4: Concentration profiles of the internal isomers vs time during hydrogenation of 1-DE over a Pd/alumina catalyst at T 353 K and P 0.6 MPa. = =

tected, two of which could be attributed to 1-dodecene and dodecane; see Figure 2.3 and Table 2.2. The other peaks can be attributed to the formation of internal isomers. GC-MS showed that the reactant was contaminated with traces of n-methyl-undecenes, the precise position of the methyl groups remaining unknown. For convenience, 1-dodecene will be abbreviated to 1-DE and the sum of internal isomers to n-DE and dodecane to DA.

The concentration profiles in Figure 2.4 show the typical evolution of a consecutive reaction. Hydrogenation of 1-butene and 1-pentene is known to yield an initial trans-cis ratio of the 2-olefin of 2-2.5 (Carra´ and Ragaini, 1968; Ragaini and Somenzi, 1969; Bond and Wells, 1964; Bond and Rank, 1965). This allows peak 9 and 10 to be positively identified as respectively cis-2- and trans-2-dodecene. The next pair of reactants to be formed is cis-3- and trans-3-dodecene and indeed peak 5 and 6 might be attributed to them. However, after approximately 20 minutes, the area of peak 5 exceeds that of peak 6, making it unlikely that peak 5 consists only of cis-3-dodecene, as that would result in a trans-cis equilibrium below unity. Combined with the intuition that trans species elute before cis species on this type of GC-column, one arrives at the conclusion that peak 6 indeed belongs to trans-3-dodecene and that peak 5 is a combination of cis-3- and trans-4-dodecene. This leaves peak 3 to be most likely cis-4-dodecene.

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CHAPTER 2

Table 2.2: Chromatogram peak identification. peak reactant peak reactant 1 solvent 6 trans-3-dodecene 2 dodecane (DA) 7 n-methyl-undecene 3 cis-4-dodecene 8 1-dodecene (1-DE) 4 n-methyl-undecene 9 cis-2-dodecene 5 cis-3-dodecene and 10 trans-2-dodecene trans-4-dodecene

Table 2.3: Overview of experiments and reaction conditions. In all experiments C1-DE,0 400 mol 3 = m− .

TPCH2 DH2,eff DDE,eff rv,obs 8DE ∗ 8H2 3 9 2 1 9 2 1 3 1 K MPa mol m− 10− m s− 10− m s− kmol mcat− s− -- 353 0.4 18 1.4 0.20 19.0 70 214 353 0.6 28 1.4 0.20 24.6 90 184 353 † 0.8 36 1.4 0.20 29.5 108 165 353 1.0 45 1.4 0.20 33.9 124 152 373 0.8 39 1.8 0.23 48.0 152 194 393 0.8 42 2.3 0.27 74.1 208 222 ∗see Equation 2.9 †in duplo

Equipment and Procedure

The constant pressure hydrogenation experiments were carried out in a 700 ml batch autoclave, equipped with baffles and a self-inducing stirrer. Temperature was controlled within 1 K and the pressure within 8 kPa. Samples were taken through a sample line and stored for later analysis. A total reaction volume of 450 ml liquid was used. The 5 3 3 3 catalyst loading cat was kept constant at 3 10− mcatmreactor− (7.3 10− wt%). Prior to the experiments, the autoclave was flushed three· times with nitrogen· at high stirrer speeds to remove all air from the system. After reaching the reaction temperature, the desired hydrogen pressure was applied to the autoclave. No induction period was observed in any of the experiments.

Physical properties

The equilibrium hydrogen concentration was calculated using the expression given by Chaudhari et al. (2002). The diffusivity of hydrogen in n-decane was estimated with the Wilke-Chang equation. The diffusivity of the reactants in n-decane was estimated from a value reported for n-dodecane in n-octane by Rutten (1992) (D 1.1 10 9 m2 s 1 = · − − 12 ‘‘thesis’’ --- 2007/1/31 22:24 --- page 13 (#23)

1-DODECENE

@ 298 K). By correcting for temperature and viscosity according to the Stokes-Einstein correlation, an estimate of the reactant diffusivity at elevated temperatures was made. Correcting for catalyst porosity and assuming a tortuosity factor of approximately five, the effective diffusivity of the reactants inside the catalyst was estimated at Deff 0.1D. The values for the diffusivities and concentrations that were calculated for the= various experiments are summarized in Table 2.3.

Parameter estimation

A number of kinetic models were investigated in order to describe the hydrogenation and isomerization of 1-dodecene. To this end, the non-linear parameter estimation cap- abilities of Athena Visual Workbench 8.3 (Stewart and Associates, Madison WI) were used. The weighted sum of squared residuals (based on reactant concentrations) SSres was minimized

n
2 X Ci Ci SSres − (2.1) = σ 2 i 1 i = where

2 2 2 σ σ ( frel Ci ) (2.2) i = f ix + 3 The absolute error σ f ix in the measured concentration was estimated at 1.5 mol m− and the relative error frel at 0.05.

2.3 Results and discussion

Figure 2.5 shows a typical result. At first, 1-DE decreases rapidly, forming both 1- DA and n-DE whith the latter always going through a maximum. Next, the rate of 1-DE disappearance decreases. In Figure 2.6 the same data is shown on a logarithmic scale; the linear time dependency on the logarithmic scale clearly indicates first order processes. The parallel lines suggest that the activity towards hydrogenation of both n-DE and 1-DE are equal. This does however not take into account the contribution of the continuous isomerization taking place from 1-DE to n-DE and vice versa. As the equilibrium lies heavily to the side of n-DE, any difference in activity will be effectively masked by the fast isomerization reaction. In the next section it will be shown that 1-DE is in fact almost 25 times more reactive towards hydrogenation than n-DE.

Double bond shift of 1- to 2-dodecene and subsequent cis-trans isomerization

Figure 2.7 shows the influence of the hydrogen pressure on the trans-cis ratio of 2- dodecene as a function of the overall conversion of dodecene (i.e. the amount of dodecane

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CHAPTER 2

1.0

0.8

0.6 (-) i x 0.4

0.2

0.0 03060 time (min)

Figure 2.5: Formation of internal isomers during the hydrogenation of 1-dodecene. Molar frac- tion reactant 1-DE (3), sum of internal isomers n-DE (2) and hydrogenated product DA ( ) as a function of time. T 353 K and P 0.6 MPa. 4 = =

that is formed). As has been oberved for 1-pentene by Bond and Rank (1965), the double bond shift of 1-dodecene yields less trans-2 than the equilibrium value. The initial trans- cis ratio of 2-dodecene formed from 1-dodecene is approximately 1.8. The equilibration towards Ktc 3.5 is caused by subsequent cis-trans isomerization. The rate at which this equilibrium≈ is established relative to the hydrogenation is decreased with increasing pressure, so the hydrogenation has a higher apparent order in hydrogen than the cis-trans isomerization.

Carley (2002) explained the initial selectivity with which 2-cis-pentene is formed from 1-pentene in terms of molecular congestion at the active site, where the adsorbed 2-olefin complex adopts the conformation of lowest molar volume. It is interesting however to note that the difference in liquid molar volume at room temperature between trans-2- and cis-2-pentene is 2%, whereas for 2-undecene this is only 0.7%. Despite this small difference there is still a strong initial preference towards formation of cis-2-dodecene. The trans-cis equilibrium of 2-dodecene of 3.5-4 at 353 K is somewhat higher than what is calculated for gaseous hexene and heptene; see Table 2.4.

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1-DODECENE

1 20

16

0.1 12 lines in parallel (-) i x

8 -DE / 1-DE 0.01 n

4

0.001 0 0 204060 time (min)

Figure 2.6: Logarithm of the molar fraction of reactant 1-DE (3) and n-DE ( ); ratio of n-DE and 1-DE (2) as a function of time. T 353 K and P 0.6 MPa. 4 = =

0.4 4.0 increasing P 3.5 0.3 trans -2-DE increasing P 3.0 -dodecene

(-) 0.2 i cis

x cis -2-DE 2.5 / 2-

0.1 trans

2- 2.0

0.0 1.5 0 20406080100 0 20406080100 dodecene hydrogenated (%) dodecene hydrogenated (%)

Figure 2.7: Effect of hydrogen pressure on the cis-trans isomerization of 2-dodecene. P 0.4 MPa ( ); P 0.6 MPa (2); P 0.8 MPa ( ); P 1.0 MPa ( ), T 353 K. =  = = 4 = × =

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CHAPTER 2

0.4 increasing P 1.2

0.3 trans -2-DE

0.8 -dodecene increasing P

(-) 0.2 i x trans

trans -3-DE / 2- 0.4 0.1 trans 3-

0.0 0.0 0 20406080100 0 20406080100 dodecene hydrogenated (%) dodecene hydrogenated (%)

Figure 2.8: Effect of hydrogen pressure on the double bond shift of trans-2- to trans-3-dodecene. P 0.4 MPa ( ); P 0.6 MPa (2); P 0.8 MPa ( ); P 1.0 MPa ( ), T 353 K. =  = = 4 = × =

Table 2.4: trans-cis equilibrium of 2- and 3-hexene/heptene (Kabo et al., 1967; Pedley et al., 1986). 1Hi 1Si Ktc 1 1 1 kJ mol− J mol− K− 353 K 373 K 393 K trans / cis-2-hexene -3.4 -0.41 3.4 3.2 3.0 trans / cis-3-hexene -3.6 2.93 2.4 2.3 2.1 trans / cis-2-heptene -2.4 0.88 2.0 1.9 1.9 trans / cis-3-heptene -3.3 1.50 1.9 1.7 1.6

Double bond shift of trans-2- to trans-3-dodecene

Figure 2.8 shows the influence of hydrogen pressure on the ratio of trans-3- and trans- 2-dodecene as a function of the overall conversion of dodecene. The rate at which the double bond moves is decreased relative to the hydrogenation rate with increasing pres- sure. As with cis-trans isomerization, the hydrogenation therefore has a higher order in hydrogen than the double bond shift.

Lumped reaction model: 1-DE vs n-DE

Figure 2.6 indicates that the internal isomers can be treated as a single lump, as far as the rate of hydrogenation is concerned. Therefore, a simple reaction model based on the scheme in Figure 2.9 is used. The only isomerization reaction considered is the double bond shift from 1- to 2-dodecene, further internal isomerization reactions are not taken into account because of the lumping. The following rate expressions are employed, using apparent reaction orders for hydrogen and first order in the alkene:

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1-DODECENE

1-DE

r1 DA

riso,1 riso,2 r [] 2

n-DE

Figure 2.9: Simplified reaction scheme for the hydrogenation and isomerization of 1-dodecene.

m1 r1 kh1C1-DE C (2.3) = H2

m2 r2 kh2Cn-DE C (2.4) = H2

n riso,1 kisoC1-DE C (2.5) = H2

n riso,2 kiso Keq Cn-DE C (2.6) = H2 where the reaction rate constants are exponentially dependent on temperature    Eai 1 1 ki ki,ref exp (2.7) = − R T − Tref

The equilibrium constant Keq describes the relation between the isomerization reac- tions of 1-DE and n-DE and is dependent on temperature according to

 1H  1 1  Keq Keq,ref exp (2.8) = − R T − Tref The results of the parameter estimations are given in Table 2.5 and Figure 2.10. There is little significant difference in activation energy for both hydrogenation reactions as fit 1 shows. Therefore, in fit 2 these have been made equal. Since the activation energy for the isomerization was also almost equal to that of the hydrogenation reactions, fit 3 was carried out using only one activation energy for all three reactions. The order in hydrogen for the hydrogenation reactions approaches 0.6 and the double bond shift of 1-DE to n-DE shows little or no dependency on hydrogen. Since fit 3 gave little significance to the order in hydrogen for the isomerization, it was set to zero in fit 4. In this way, with almost no increase in the SSres, the amount of parameters could be reduced from 11 to 7. This simple model, despite its limitations, describes the experimental data very well, with the exception of low n-DE fractions ( 0.005). At high conversions towards the ≤ 17 ‘‘thesis’’ --- 2007/1/31 22:24 --- page 18 (#28)

CHAPTER 2

1 2 1 1.0 1 1.0 DA

0.8 0.8

0.1 0.1 (-) (-) 0.6 0.6 -DE -DE n -DE n n (-) (-) x x DA DA or x x or 0.4 0.4 1-DE 0.01 1-DE 0.01 x x 1-DE 0.2 0.2

0.001 0.0 0.001 0.0 0 60 120 180 240 0 60 120 180 time (min) time (min)

3 4 1 1.0 1 1.0

0.8 0.8

0.1 0.1 (-) (-) 0.6 0.6 -DE -DE n n (-) (-) x x DA DA x x or 0.4 or 0.4

1-DE 0.01 1-DE 0.01 x x

0.2 0.2

0.001 0.0 0.001 0.0 0 60 120 0 60 120 time (min) time (min)

5 6 1 1.0 1 1.0

0.8 0.8

0.1 0.1 (-) (-) 0.6 0.6 -DE -DE n n (-) (-) x x DA DA x x or 0.4 or 0.4

1-DE 0.01 1-DE 0.01 x x

0.2 0.2

0.001 0.0 0.001 0.0 060 03060 time (min) time (min)

Figure 2.10: Model fit 4 to experimental results. Drawn lines for modelling results. 1-DE ( ); n-DE (2); DA ( ). T 353 K, P 0.4 MPa (1); T 353 K, P 0.6 MPa (2); T 353 K,P 0.8 MPa (3, in4 duplo);= T 353= K, P 1.0 MPa= (4); T 373= K, P 0.8 MPa= (5); T 393= K, P 0.8 MPa (6). ======

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1-DODECENE

Table 2.5: Results of the parameter estimation and the approximate 95% confidence intervals. Ap- parent rate constants at Tref 373 K. = parameter fit 1 fit 2 fit 3 fit 4 units m 3m 1 kh1,ref 22.8 12.6 12.4 1.5 12.5 1.5 12.7 1.5 mol− m− s− ± ± ± ± m 3m 1 kh2,ref 2.0 0.6 2.6 0.3 2.6 0.3 2.6 0.3 mol− m− s− ± ± ± ± n 3n 1 kiso,ref 16.6 8.3 19.8 9.5 15.7 6.8 22.6 1.2 mol m s ± ± ± ± − − − m1 0.45 0.15 0.62 0.03 0.62 0.03 0.62 0.03 - ± ± ± ± m2 0.70 0.08 m1 m1 m1 - n 0.10±0.14 0.05≡ 0.13 0.10≡ 0.12 ≡ 0 - ± ± ± ≡ 1 Eah,1 22.2 5.3 24.8 0.8 25.1 0.7 25.2 0.7 kJ mol− ± ± ± ± 1 Eah,2 26.4 2.9 Eah,1 Eah,1 Eah,1 kJ mol− ± ≡ ≡ ≡ 1 Eaiso 29.3 4.2 30.9 3.8 Eah,1 Eah,1 kJ mol ± ± ≡ ≡ − Keq,ref 12.6 0.8 12.3 0.7 12.6 0.7 12.7 0.7 - ± ± ± ± 1 1Heq 5.5 4.0 7.8 2.2 5.5 1.7 5.1 1.6 kJ mol − ± ± ± ± − SSres 106.3 107.3 109.1 109.7 # of pars 11 9 8 7

saturated product dodecane, the mixture of n-DE isomers is reduced to a single compon- ent, most likely 4-cis-dodecene. This isomer appears less active than the other internal isomers, as Figure 2.10 demonstrates, and as a result the model deviates from the ex- perimental results at high conversions. 4-cis-DE is approximately six to eight times less active than the other internal isomers and 30 to 40 times less reactive than the terminal ol- efin. It should however be considered that the concentrations approach the absolute error in the measurements and so the deviation from the model becomes less relevant at high conversion levels. 1 The apparent activation energy for hydrogenation Eah 25 kJ mol− is quite low: 1 = Ramesh et al. (2000) reported Eah 43 55 kJ mol− for octene hydrogenation us- ing anchored palladium complexes. External= − mass-transfer limitations are unlikely, since 1 in that case the observed activation energy should have been 10-15 kJ mol− as has in- deed been reported by Smits et al. (1996), who hydrogenated octene with a highly active monolithic catalyst. The observed initial rate rv,obs was used to calculate the Weisz-Prater number 8:

2  dp  rv,obs 6 8i (2.9) = Di,effCi Two numbers were calculated, one for the reactant hydrogenation and one for hy- drogen. The calculations indicate that the reactions are heavily diffusion limited in both reactant and hydrogen (Table 2.3), confirming the existence of internal diffusion limita- tions. A first order in the alkene will not be changed by these limitations.

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CHAPTER 2

The modelling results show that the terminal double bond is approximately five times more active than the internal double bonds. Since the difference in reactivity between the α-olefin and the internal olefins is masked by internal diffusion limitations, the difference will be more pronounced under kinetic conditions. For an internal diffusion limited reac- tion it can be shown that rv,obs √k and as a result the real difference in activity will be of the close order of 25. ∼ The data presented by Tamai et al. (1999) on 1-hexene hydrogenation with supported palladium showed a similar behaviour although the difference in rates was not quantified by the authors. Terasawa et al. (1978) found that 1-hexene was 7 times as reactive as 2-hexene when hydrogenated with polymer-bound palladium. Modelli et al. (1984) found that 1-hexene was four times more reactive than cyclohexene, and also that 1-octene was more reactive than cyclooctene. In homogeneous hydroformylation with Co-based com- plexes, terminal olefins are three times more reactive than internal olefins. With Rh-based complexes the difference can be as high as a factor of forty (Howe-Grant, 1996). The order in hydrogen of the isomerization of 1- to 2-dodecene is lower than of the hydrogenation, a clear indication that different rate-determining steps are operative. The large uncertainty in the isomerization order m could lead to the conclusion that m 0 (fit 4). However, some hydrogen must be present for isomerization to take place. This= was demonstrated by carrying out an experiment where the supply of hydrogen was stopped after having reached a certain conversion: both the hydrogenation and isomerization re- actions stopped completely. The equilibrium value Keq that describes the equilibrium between the internal double bonds and the terminal double bond decreases from 240 for butene to 140 for heptene. As Figure 2.11 shows, the value found for 1-dodecene is at the least in line with this decreasing trend.

Differentiation between the isomerization reactions

A model that not only describes the difference between terminal and internal double bonds, but also takes into account the internal isomerization reactions will be much more complex. A complete description of dodecene at equilibrium requires 10 equilibrium constants which are, unfortunately, not available from literature; each equilibrium con- stant will also be dependent on temperature. To avoid being caught in a quagmire of variables the investigation will therefore be limited to the influence of pressure for the data collected at T 353 K. The composition of dodecene at equilibrium was reduced = to three equilibrium constants: K1t for the 2-trans-DE / 1-DE equilibrium, Ktc for the cis-trans equilibrium and Kdb for the distribution of the double bonds. The latter was set at unity implying the assumption that the internal double bonds are distributed equally at equilibrium (Figure 2.12). The modelling approach taken was the same as in the previous section with different rates of hydrogenation of terminal dodecene and internal dodecenes but with 1-DE, 2-cis- and 2-trans-DE, 3-trans-DE, and DA fitted independently. The lumped peaks in the chro-

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1-DODECENE

250

200

150

100 -olefin / 1-olefin (-) n 50

0 456789101112 # of carbon atoms

Figure 2.11: Equilibrium ratio of internal double bonds and terminal double bond at T 353 K (Kabo et al., 1967; Pedley et al., 1986). =

Kdb Kdb Kdb Kdb K1t 2-tr 3-tr 4-tr 5-tr 6-tr

1-DE Ktc Ktc Ktc Ktc Ktc

K / K 1t tc 2-cis 3-cis 4-cis 5-cis 6-cis

Kdb Kdb Kdb Kdb

Figure 2.12: Simplified description of 1-dodecene at equilibrium. Kdb 1. =

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CHAPTER 2

Table 2.6: Modelling of the different isomerization reactions. Results of the parameter estimation and the approximate 95% confidence intervals. Apparent rate constants at Tref 353 K. = parameter value units m 3m 1 kh1 1-DE DA 10.0 0.7 mol− m− s− → ± m 3m 1 kh2 n-DE DA 1.9 0.1 mol− m− s− → ± n 3n 1 kiso,1 1-DE 2-trans-DE 1.8 kiso2∗ mol− m− s− → ≡ · n 3n 1 kiso,2 1-DE 2-cis-DE 8.9 1.5 mol− m− s− → ± n 3n 1 kiso,3 cis-trans isomerization 15.7 4.5 mol− m− s− ± n 3n 1 kiso,4 double bond shift 22.5 2.9 mol m s ± − − − K1t 7.9 0.8 - ± Ktc 3.2 0.2 - ± m 0.58 0.02 - n 0.11±0.04 - ± ∗initial selectivity

DA DA kCCm hi2H2

n kCCn kKCC iso,4 i H2 iso,1 1 t i H 2 2-trans 3-trans ... kCCn kCCn iso,1 i H 2 iso,4 i H2 kCCm hi1H2 kKCCn kCCn DA 1-DE iso,3 tc i H2 iso,3 i H2

kCCn kCCn iso,2 i H2 iso,4 i H2 2-cis 3-cis ... n kCCn kKKCC(/) iso,4 i H2 iso,2 1 t tc i H2

kCCm hi2H2 DA DA

Figure 2.13: Reaction model taking into consideration hydrogenation, cis-trans isomerization and double bond shift.

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1-DODECENE

1 2 1.0 1.0

0.8 0.8

0.6 0.6 (-) (-) i i x x 0.4 0.4

0.2 0.2

0.0 0.0 0 60 120 0 60 120 time (min) time (min)

3 4 1.0 1.0

0.8 0.8

0.6 0.6 (-) (-) i i x x 0.4 0.4

0.2 0.2

0.0 0.0 0 60 120 0 60 120 time (min) time (min)

Figure 2.14: Effect of hydrogen pressure on the isomer formation. Drawn lines for the combined modelling results. 1-DE ( ); 2-trans-DE (2); 2-cis-DE ( ); 3-trans-DE ( ); DA ( ). P 0.4 MPa (1); P 0.6 MPa (2); P 0.8 MPa (3); P 1.0 MPa4 (4), T 353 K.× ∗ = = = = =

matogram were not taken into consideration. The complete reaction model is summarized in Figure 2.13. Results of the parameter estimation are shown in Table 2.6. The model describes the experimental data very well as is illustrated in Figure 2.14. The orders in hydrogen for hydrogenation and isomerization are essentially the same as in the simpli- fied model, as are the apparent hydrogenation rate constants. The rates of isomerization of the three isomerization reactions are of the same order of magnitude. Apparently there is little difference in rate between the shift of a double bond along the carbon chain and cis-trans isomerization.

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CHAPTER 2

2.4 Conclusions

In the hydrogenation of 1-dodecene three different types of isomerization can be re- cognized: double bond shift from 1- to 2-dodecene, cis-trans isomerization and double bond shift further into the molecule. The three isomerization reactions can all be de- scribed by an apparent first order behaviour in the alkene and a zero order behaviour in hydrogen, compared to a higher order for the hydrogenation. This indicates that these three reactions share a common surface reaction step, different from the hydrogenation reaction. There was no significant difference in the activation energy for hydrogenation and isomerization, in contrast to what has been observed in gas-phase hydrogenation of short olefins on palladium catalysts (Bond et al., 1968). The rate of cis-trans isomeriza- tion and double bond shift are almost equal and as such these reactions probably share the same rate limiting step of the adsorbed activated complex. Double bond shift of 1- to 2-dodecene initially yields more cis than predicted by the thermodynamic equilibrium. This steric effect which is operative in short olefins appears to also be in effect in a long olefin. As expected the terminal double bond in dodecene is up to 25 times as reactive towards hydrogenation as the internal double bonds which can all be treated as equally reactive. The combined hydrogenation and isomerization reactions can be described with a limited number of parameters; for modelling purposes it is sufficient to suppose that the double bonds in dodecene are evenly distributed with the exception of the terminal double bond.

2.5 Nomenclature

DA dodecane 1-DE 1-dodecene n-DE lump of internal dodecenes 3 C concentration, mol m− 2 1 D diffusivity, m s− 1 Ea activation energy of hydrogenation, J mol− 1 Eaiso activation energy of isomerization, J mol− dp catalyst particle diameter, m 1 1H enthalpy of isomerization, J mol− m 3m 1 kh hydrogenation rate constant, mol− m− s− n 3n 1 kiso isomerization rate constant, mol− m− s− K equilibrium constant, dimensionless P pressure, MPa 3 1 r reaction rate, mol mcat− s− 3 1 rv,obs observed initial rate, mol mcat− s− 1 1 R gas constant, 8.314 J mol− K− 1 1 1S entropy of isomerization, J mol− K−

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1-DODECENE

SSres weighted sum of squared residuals, dimensionless T temperature, K x molar fraction, dimensionless 3 3 cat volumetric catalyst loading, mcat mreactor− 8 Weisz-Prater number, dimensionless

Bibliography

Bond, G., 2005. Metal-catalysed reactions of hydrocarbons. Springer, New York, Ch. Hydrogenation of alkenes and related processes, pp. 291–355.

Bond, G., Rank, J., 1965. Proc. 3rd Int. Congr. Catal. Vol. 2. Elsevier, Amsterdam, p. 1225.

Bond, G., Webb, G., Wells, P., 1968. Hydrogenation of olefins. Part 4.-Reaction of n- butenes with hydrogen catalyzed by alumina supported ruthenium and osmium. Trans. Far. Soc. 64, 3077.

Bond, G., Wells, P., 1964. The mechanism of the hydrogenation of unsaturated hydrocar- bons on transition metal catalysts. Adv. Catal. 15, 91–226.

Carley, A., 2002. In: Surface chemistry and catalysis. Kluwer academic, New York, pp. 319–323.

Carra,´ S., Ragaini, V., 1968. On the mechanism of 1-butene isomerization on supported palladium. J. Catal. 10, 230–237.

Chaudhari, R., Jaganathan, R., Mathew, S., 2002. Hydrogenation of 1,5,9- cyclododecatriene in fixed-bed reactors: down- vs. upflow modes. AIChE Journal 48(1), 110–125.

Holbrook, L., Wise, H., 1972. Role of hydrogen in butene isomerization catalyzed by supported and unsupported palladium. J. Catal. 24, 315–319.

Howe-Grant, M. (Ed.), 1996. Kirk-Othmer Encyclopedia of Chemical Technology. Vol. 17. Wiley-Interscience, NY, pp. 902–919.

Kabo, G., Andreevskii, D., Savinetskaya, G., 1967. Isomerization equilibrium of second- ary n-monochloroheptanes and n-heptenes. Neftekhimiya 7(3), 364–368.

Modelli, A., Scagnolari, F., Innorta, G., Foffani, A., Torroni, S., 1984. Kinetic features of olefin hydrogenations catalyzed by polymer-anchored palladium acetate. J. Mol. Catal., 361–373.

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Pedley, J., Naylor, R., Kirby, S., 1986. Thermochemical data of organic compounds, 2nd Edition. Cambridge University Press. Ragaini, V., Somenzi, G., 1969. Kinetic study of 1-butene hydrogenation on supported palladium. J. Catal. 13, 20–23. Ramesh, B., Sadanand, D., Reddy, S., 2000. Hydrogenation of 1-alkenes catalysed by anchored montmorillonite palladium(II) complexes: a kinetic study. Transition Met. Chem. 25(6), 639–643. Reinig, D., 1992. Ph.D. thesis, TH Darmstadt. Rutten, P., 1992. Diffusion in liquids. Ph.D. thesis, TU Delft. Smits, H., Stanckiewicz, A., Glasz, W. C., Fogl, T., Moulijn, J., 1996. Selective three- phase hydrogenation of unsaturated hydrocarbons in a monolithic reactor. Chem. Eng. Sci. 51(11), 3019–3025. Tamai, H., Ikeya, T., Yasuda, H., 1999. Hydrogenation of 1-hexene and hexadienes by ultrafine Pd particles supported on the surface of PrPo4 hollow particles. J. Colloid Interface Sci. 218, 217–224. Terasawa, M., Kaneda, K., Imanaka, T., Teranishi, S., 1978. Study of hydrogenation of olefins catalyzed by polymerbound palladium (II) complexes. J. Catal. 51, 406–421. Winterbottom, J., 1962. Ph.D. thesis, University of Hull.

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Chapter 3

Diffusional Effects on Isomerization Reactions: Hydrogenation of Methyl Oleate and Derivatives

’The unfortunate aspect of its [capillary gas chromatagraphy] alliance with new compu- terized peak identification technology is that a deep background knowledge of fatty acids or other lipids will no longer be required of operators and responsible parties receiving more and more information will be too harassed by the sheer volume of information to detect the inevitable misidentifications. One hopes that none of these will be critical to health and safety’ (Ackmann, 2002)

3.1 Introduction

Natural fats and oils are materials of great complexity: the composition of fatty acid triglycerides is notoriously difficult to analyse. The fatty acids are usually separated from the glycerides for easier analysis, for example by gas chromatography. Sunflower oil for example will contain palmitic, stearic, oleic, linoleic and traces of various other acids. Ve- getable unsaturated fatty acids have double bonds on fixed places on the carbon chain and these double bonds are always in the cis configuration. However, when a fat is hydrogen- ated in order to decrease the amount of saturation, a number of unwanted side reactions can take place. The position or nature of the double bonds can change (by positional and geometrical isomerization). As a consequence, this will increase the difficulty of analyz- ing the reaction products. The following chapter will focus on the hydrogenation of a mono-unsaturated fatty ester, in an effort to reduce the reaction system to the most simple form. Insights obtained from this simple system could then be translated towards more complicated and realistic mixtures of fatty acid esters or triglycerides.

Oleic acid and its derivatives

Oleic acid (cis-9-octadecenoic acid) is a deceivingly simple molecule: a chain of eighteen carbon atoms, which ends in a carboxylic acid group. All carbon-carbon bonds

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CHAPTER 3

A MeO

O

MeO B

O

MeO C

O

Figure 3.1: Methyl oleate (A), methyl elaidate (B) and methyl stearate (C).

Table 3.1: Melting points in ◦C of assorted fatty acids, fatty acid methyl esters and triglycerides. acid methyl ester triglyceride stearic 69.6 39.1 73.0 elaidic 45.0 42.0 oleic 16.3 -19.9 5.5

are saturated with the exception of the ninth and tenth carbon counting from the acid group, the precise middle of the chain. Here two hydrogen atoms have been removed and a double bond has been formed. As a consequence there is a minimal difference in mass 1 between oleic acid and its saturated product stearic acid: 282 vs 284 g mol− . At the same time the difference in the melting point is large: 16.3 vs 69.6 ◦C. The unsaturated double bond in octadecenoic acid can be either in the cis or the trans configuration. The melting point of elaidic acid (trans-9-octadecenoic acid) is 45 ◦C, between the melting points of oleic and stearic acid. Apparently elaidic acid molecules can pack tighter in the solid phase than oleic acid molecules. This can be understood by the hindered configuration of the carbon chains emanating from the cis double bond (Figure 3.1). The trans form is thermodynamically more stable: at room temperature the equilibrium ratio of elaidic acid to oleic acid is approximately five to one. An overview of the melting points of the various acids, methyl esters and triglycerides is given in Table 3.1. In 1887, Van’t Hoff, Le Bel and Wislicenus had shown that the difference between maleic acid (cis) and fumaric acid (trans) which have the same molecular mass and are not optically active, is caused by the double bond that defines a plane and cannot rotate

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METHYL OLEATE

freely as a single bond (Pledge, 1959). The existence of elaidic acid (or iso-oleic as it was then called) in hydrogenated oils was first proven by Moore (1919). After laborious experimentation Moore showed that these iso-oleic acids consisted of a mixture of elaidic acid and acids for which the double bond had migrated to a certain extent. At 180 ◦C, the ratio between iso-oleic and oleic acid was found to be 2.3 to 1, possibly the first mention of a trans-cis equilibrium constant for fatty acids! Iso-oleic acid had the same mass and degree of unsaturation as oleic acid but with a higher melting point. It was formed when a vegetable oil was hydrogenated using a heterogeneous catalyst. Research reported as early as 1923 showed that the rate at which elaidic acid was formed was dependent on the operating conditions at which an oil was hydrogenated (Lush, 1923). However, elaidic acid is not the only isomer that can be formed from oleic acid: a bewildering array of isomers is possible. If one does not limit the location of the double bond to the ninth en tenth carbon atom, then no less than 31 isomers of oleic acid are possible (including itself): 2-16 cis-octadecenoic acid, 2-16 trans-octadecenoic acid and 17-octadecenoic acid. Since in the latter molecule the double bond is located at the end, only one ’isomer’ exists. The formation of positional isomers was conclusively shown by Scheffers (1930) who showed that the products formed after hydrogenation of oleic acid also contained 8 and 10 cis-octadecenoic acid together with the corresponding trans isomers. Starting from elaidic acid similar products were found. The author inferred (correctly) that this migration of the double bond would probably not be limited to one position but could occur along the length of the carbon chain. The work of Scheffers was limited by the analytical methods used. Unsaturated fatty acids had to be ’broken’ into an acid and an alkane. By finding the chain length of the acids formed the position of the double bond in the original molecule could be determ- ined. Only with the advent of gas chromatography was the effort involved in this greatly reduced.

Saturation, trans-cis and positional isomerization

In short, three types of reactions take place during the hydrogenation of oleic acid or an ester thereof. First, the addition of two hydrogen atoms resulting in the saturated stear- ate. Under the operating hydrogenation conditions that one is likely to find, this reaction will be irreversible. The other two reactions are isomerization reactions, where the double bond either changes position or ’flips’ from the cis to the trans form or vice versa. No hydrogen is consumed in these latter reactions although hydrogen does play an important role. A reaction scheme incorporating all 31 isomers of methyl octadecenoate and their isomerization reactions is given in Figure 3.2. For reasons of clarity the hydrogenation reactions are not shown. As Figure 3.2 shows, an exact description of the reaction system requires a plethora of equilibrium and reaction rate constants. A short overview of the literature on the trans-cis

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CHAPTER 3

kp,cisKp,2 kp,cisKp,7 kp,cisKp,8 kp,cis kp,cis 2-cis ... 8-cis 9-cis 10-cis ... 16-cis

kp,cis kp,cis kp,cis kp,cisKp,9 kp,cisKp,10 ktcKtc,2 ktc ktcKtc,16 ktc 17 k K kp,transKp,7 k K k k 2-trans p,trans p,2... 8-trans p,trans p,8 9-trans p,trans 10-trans p,trans ... 16-trans

kp,trans kp,trans kp,trans kp,transKp,9 kp,transKp,10

Figure 3.2: Simplified reaction scheme. Hydrogenation reactions have been omitted.

equilibrium in n-octadecenoic acids and the equilibrium distribution of the double bonds is therefore given in the next section.

3.2 Previous work and literature overview

Equilibrium data

Litchfield et al. (1963) used catalysts over which the isomerization is fast compared to the hydrogenation. Using both selenium and HNO2 as catalyst, they determined that an equilibrium mixture contains 75-80 % elaidic acid and 20-25 % oleic acid. They ob- served that an equilibrium mixture of di-unsaturated fatty acids also contained 75-80 % trans fatty acids, randomly distributed among the double bonds present. Gut et al. (1979) and Grau et al. (1986) determined the cis-trans ratio from a parameter fit of kinetic rate expressions. Both found the equilibrium constant to be independent of pressure and to 1 decrease slightly with temperature (1Hiso -2.5 kJ mol− ). The isomerization rate in- creased relative to the hydrogenation rate with≈ decreasing pressure (=hydrogen concentra- tion) and increasing temperature. The latter indicates that the isomerization reaction has a higher activation energy than the hydrogenation. Jonker et al. (1997) reported Ea,iso 1 1 ≈ 45 kJ mol− and Ea,hyd 30 kJ mol− . Other authors used thiyl radicals prepared by γ -radiolysis to reversibly≈ attack the double bonds of methyl oleate and Di-Oleoyl Phos- phatidyl Choline (DOPC) allowing trans-cis equilibration (Chatgilialoglu et al., 2000, 2002; Adhikari et al., 2001). Munzing¨ (1986) calculated the equilibrium constant from group contributions. The available literature data is summarized in Figure 3.3. All exper- imental literature data were correlated with the following expression using a least-squares method

1Htc 1Stc ln Ktc − (3.1) = RT + R 1 1 1 with 1Stc = 4.22 1.48 J mol− K− and 1Htc = -2.55 0.55 kJ mol− which is rep- resented by the solid± line in Figure 3.3. The accompanying± lines indicate the 95% confid- ence interval. The difference in the formation enthalpy was also measured by calorimetry for both the acids and the methyl esters by Rogers and Siddiqui (1975) and Rogers et al.

30 ‘‘thesis’’ --- 2007/1/31 22:24 --- page 31 (#41)

METHYL OLEATE

6

5

4 (-) 3 tc K

2

1

0 273 323 373 423 473 T (K)

Figure 3.3: trans-cis equilibrium constants for methyl octadecenoate and octadecenoic acid as a function of temperature. Litchfield et al. (1963) (+); Chatgilialoglu et al. (2000) (2); Adhikari et al. (2001) ( ); Chatgilialoglu et al. (2002) ( ); Gut et al. (1979) ( ); Grau et al. (1986) ( ); Gunstone et al. (1967)4 ( ); dotted line as calculated× by Munzing¨ (1986). ∗ • ◦

1 (1977) and found to be around -4 kJ mol− , which is slightly higher. Although there is ample data on the value of the equilibrium between oleate and elaidate, little is known when the double bond is in a different position. Gunstone et al. (1967) used heating with selenium and irradiation by ultra violet light in the presence of diphenyl sulphide to cata- lyze isomerization. Apart from the 12 isomers all the equilibrium mixtures were found to contain 74-80% of trans ester, independent of the position of the double bond. As the 12 isomers are far removed from the 19 isomers that were used for experimentation it is felt that this difference in the isomerization constant can be ignored. There is precious little data on the mutual equilibrium of the different n-octadecenoic species (with n 2 17). Isbell et al. (1992) used an acidic clay to produce estolides from oleic acid.= As a− side-reaction, protonated double bonds migrated over the carbon chain. From the reported trans-cis ratio of 1.3 at 523 K it is clear that equilibrium has not been reached in their reaction product (see Figure 3.3). However, a more or less equal distribution of double bonds around the original n 9 can be observed, with a slight preference towards the end of the carbon chain away= from the acid group (Figure 3.4). Brown and Swidler (1962) reported an almost even distribution of double bonds on octadecenoic acid after prolonged exposure to an acidic catalyst at high temperature.

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CHAPTER 3

30

25

20

15

10 % of unsaturated

5

0 2345678910111213141516 double bond position

Figure 3.4: Position of the double bond along the carbon chain after treatment of oleic acid with an acidic clay at 523 K. Data taken from Isbell et al. (1992).

This is in contradiction with thermodynamic data published on hexene and heptene (Kabo et al., 1979, 1967): at equilibrium there is a clear preference of the double bond for the centre of the molecule. Since the precise equilibrium distribution of the double bonds is unknown, a flex- ible approach is to assume a Gaussian distribution symmetrically around the ninth and tenth carbon. This allows an almost equal equilibrium distribution (large σ) or a more pronounced distribution (small σ). An example of this is shown in Figure 3.5, where the equilibrium distribution along the carbon chain is given for increasing values of the standard deviation σ. Based on the equilibrium distribution, the individual equilibrium constants K p,i can be calculated. For large values of σ, the double bonds will be evenly distributed at equilibrium and so K p,i 1 for i 2 to 16. = = A Gaussian distribution is the result of assuming that the double bond in the centre of the molecule has the lowest free energy, and that this free energy increases with the square of the distance from the centre. I.e.

2 1E f ree,i (i µ) (3.2) ∼ − According to Helmholtz the chance of any position occurring depends on the available free energy

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METHYL OLEATE

40

30

20 % of unsaturated 10

0 2345678910111213141516 double bond position

Figure 3.5: Example of a Gaussian distribution of double bonds at equilibrium. σ 1 (white), σ 2 (black), σ 4 (grey) and σ 10 (dashed). = = = =

  1E f ree,i  2 Pi exp − exp (i µ) (3.3) ∼ RT ∼ − which can be rewritten as a Gaussian distribution ! (i µ)2 Pi exp − (3.4) ∼ 2σ 2 − where σ is the standard deviation of the distribution. The results of equation 3.4 must always be normalized in such a way that the sum of Pi equals one. The assumption is made that the centre of the distribution is the double bond between the ninth and the tenth carbon (µ 9). Two simplifications are made in this approach: the (stabilizing) elec- tronic effects= of the ester group on the second double bond are ignored and the existence of the seventeenth double bond is ignored (P17 0). ≈ Hydrogenation and isomerization kinetics

Dutton et al. (1968) hydrogenated methyl oleate at atmospheric pressure over suppor- ted palladium and platinum catalysts. For both catalysts it was observed that the trans

33 ‘‘thesis’’ --- 2007/1/31 22:24 --- page 34 (#44)

CHAPTER 3

50

40

30

20 % of unsaturated 10

0 678910111213 double bond position

Figure 3.6: Position of the double bond for cis (dark) and trans (light) at 20% conversion in the hydrogenation of methyl oleate towards methyl stearate on a supported platinum catalyst. Data taken from Dutton et al. (1968).

double bonds migrated faster than the cis double bonds. An example of this is given in Figure 3.6. Note how the trans double bonds appear to be more ’spread out’ than the cis double bonds. The overall rate of positional isomerization was found to be faster on a palladium catalyst. Heertje et al. (1974) investigated isomerization and double bond migration on a silica supported nickel catalyst using tritium. In the double bond migra- tion products, tritium was incorporated with half the specific activity per molecule in the gas phase, pointing towards hydrogen abstraction prior to addition of tritium to a double bond. From this they postulated the existence of different sites for trans-cis isomerization and positional isomerization; respectively NiH2 and NiH. Van der Plank (1972a,b) and Van der Plank and Van Oosten (1975) hydrogenated at ambient pressure using silica sup- ported nickel catalysts and found that starting from either oleate or elaidate trans double bonds migrated faster than cis double bonds. Several simultaneous processes take place during hydrogenation of Fatty Acid Methyl Esters (FAMEs): double bonds can saturate, migrate or isomerize from cis to trans or vice versa. Assuming dissociative adsorption of hydrogen the surface intermediates of the FAMEs are of most interest. Chemisorption of an unsaturated carbon-carbon bond can be achieved by either abstraction of a hydrogen atom or by the addition of a hydrogen

34 ‘‘thesis’’ --- 2007/1/31 22:24 --- page 35 (#45)

METHYL OLEATE

atom. The former leads to the formation of the so-called allylic π-complex, the latter to the Half Hydrogenated State (HHS) or σ -complex. In the σ -complex, free rotation of the double bond becomes possible resulting in geo- metrical isomerization. By subsequent addition / abstraction of hydrogen from adjacent carbon atoms positional isomerization is also possible. The addition of a second hydro- gen is assumed to be the rate limiting step for hydrogenation. In the HHS, there is no longer a difference between trans and cis, i.e. the adsorbed molecule no longer contains information concerning its previous state. As a consequence both the hydrogenation and the double bond migration rates should be equal for both species unless steric factors play a role. When a π-complex is formed, geometrical isomerization will always coincide with positional isomerization. Two cis and two trans π-complexes can be formed (see Figure 3.7). Based on the spatial arrangement of these intermediate complexes, Van der Plank and Van Oosten (1975) argue that the steric stability of these π-complexes decreases in the order of IV > III II > I. This difference in steric stability must then be reflected in different surface concentrations.≥ If the rate of hydrogen addition is the same for all surface isomers, the rate of positional isomerization of trans double bonds should be larger than that of cis double bonds. The scant literature on hydrogenation of mono-unsaturated FAME has in common that experiments were carried out at low (atmospheric) pressures, where kinetic differences in the positional isomerization of trans and cis double bonds as postulated in Figure 3.7 will be the largest. Since allylic π-complexation requires a hydrogen atom to be abstracted, one expects this kind of adsorption to become less likely at higher hydrogen concentra- tions, resulting in smaller kinetic differences between cis and trans double bonds.

3.3 Experimental

Catalyst preparation

The catalyst was prepared by wet impregnation of a γ -alumina substrate using palladium-acetate in toluene. 50 ml of toluene containing a known amount of palla- dium acetate was slowly added to a vigorously stirred suspension of alumina particles (Puralox SBa-200, Alfa Aesar, Table 3.2) in toluene. After overnight stirring, the excess toluene was decanted. The remaining slurry was yellowish in color and was dried in air at room temperature. After drying for one hour at 333 K, the powder was calcined in air 1 for one hour at 623 K with a heating rate of 2 K min− . Finally, the powder was reduced 1 in a gaseous stream of 100 ml min− containing equal parts of nitrogen and hydrogen 1 during two hours at 453 K, with a heating rate of 2 K min− . The catalyst was passiv- ated overnight at room temperature in nitrogen containing a trace of oxygen (less than 1%). Four catalysts were prepared containing approximately 0.9, 0.3, 0.1 and 0.05 wt% palladium, assuming that all palladium precursor ended up on the alumina carrier.

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‘cisoid’ (I) cis-10 (cis-8) H R H R cis-9 RC CH +H RCH CH -H H C9 C10 H C9 C10 H H -H H H H H +H RR C C H C9 C10 H ‘transoid’ (II) trans-10 H H +H (trans-8) H H H H -H RC CR +H RCH CR

H C9 C10 H C9 C10 H H -H H H

‘cisoid’ (III) cis-10 H (cis-8) R H R trans-9 H C +H RCH CH H C C -H 9 10 H C C C H 9 10 H H -H H H R +H C H R

H C9 C10 H C H R H ‘transoid’ (IV) trans-10 H (trans-8) +H R H H H C +H RCH CR H C C -H 9 10 H C C C R 9 10 H -H H H H

Figure 3.7: Adsorbed allylic π-complexes; top half from cis, bottom half from trans monoenes (Van der Plank and Van Oosten, 1975).

The palladium surface area was determined with CO chemisorption using a Quanta- chrome Autosorb 1-C. The samples were first dried in vacuum, followed by reduction in a hydrogen atmosphere for two hours and three hours of evacuation, all at 373 K. The adsorption measurements were performed at 303 K, dispersion was calculated assuming a CO:Pd=1:1 stoichiometry. The dispersion of the catalysts with the high loadings is less than of the low loadings. It is possible that the actual palladium content is lower than that calculated from the amount of palladium salt added to the solution containing the alumina particles. As a result, the dispersions reported in Table 3.3 are minimum values.

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METHYL OLEATE

Table 3.2: Characteristics of the catalyst support Puralox SBa-200. 2 1 SBET 173 3 m g ± − dpore 4-5 nm 3 3 porosity 0.57 cm cm− D10 6 µm D50 36 µm D90 95 µm

Table 3.3: Catalyst properties. 2 1 catalyst theoretical loading (wt%) S (m g− ) dispersion (%) 1 0.045 0.076 55 2 0.091 0.15 56 3 0.31 0.77 38 4 0.86 2.1 38

Equipment

The hydrogenation experiments were carried out in a 500 ml batch slurry autoclave, equipped with baffles and a self-inducing stirrer. Temperature was controlled within 1 K and pressure within 8 kPa. Samples were taken through a sample line and stored for later analysis. A total liquid reaction volume of 270 ml was used. The reactant mixture 3 consisted of approximately 13 mol m− methyl oleate or elaidate (99+% pure, Sigma- 4 3 3 Aldrich) in n-decane. The catalyst loading cat was 1.25 10− mcat mreac− . Prior to the experiment, the autoclave was flushed three times with nitrogen· at high stirrer speeds to remove all air from the system. After reaching the reaction temperature, hydrogen pressure was applied to the autoclave to a total pressure of 2.0 MPa. The hydrogen con- 3 centration under these conditions was estimated at 93 mol m− , using a correlation given by Chaudhari et al. (2002).

Analysis

The analysis of the reaction products was carried out on a Chrompack 9001 equipped with a 50 m 0.25 mm 0.2 mm CP-Sil 88 fused silica column and a flame ionization detector. A split× ratio of× approximately 1:200 was used. Depending on the conversion between 1 and 4 µl of sample was injected. Helium was used as a carrier gas. The 1 column pressure was maintained at 150 kPa, and the flow rate was 0.45 ml min− . The column was operated isothermally at 443 K and both the injector and detector were kept at 523 K. AOCS official method Ce 1f-96 (2002) recommends an operating temperature of 448 K but slightly better separation was obtained at a lower temperature.

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The methyl esters of 6, 9 and 11 trans-c18:1 and 6, 7, 9, 11 and 12 cis-c18:1 were purchased from Sigma-Aldrich and Alltech. It was found that the order of elution was 6, 9 and 11 trans-c18:1, followed by 7, 6, 9, 11 and 12 cis-c18:1. The elution of 7 cis-c18:1 before 6 cis-c18:1 on a CP-Sil 88 or similar column has also been observed by Christie (1989). It is well established that the retention time increases as the double bond position is increased from the 8th to the 16th position for both the cis species (Christie, 1989; Scholfield, 1981; Ratnayaka and Pelletier, 1992) and the trans species (Kramer et al., 2001, 2002; Ratnayaka and Pelletier, 1992). 6,7 and 8 cis-c18:1 are reported to be in- separable as are 6,7 and 8 trans-c18:1 (Christie, 1989; Scholfield, 1981; Ratnayaka and Pelletier, 1992; Kramer et al., 2002; Ackmann, 2002). 17-c18:1 and 16 cis-c18:1 elute at the same time (Christie, 1989). 16 trans-c18:1 and 14 cis-c18:1 elute at the same time (Kramer et al., 2002; Ackmann, 2002) as do 3 cis-c18:1 and 12 cis-c18:1 (Christie, 1989; Scholfield, 1981). 6, 7 and 8 cis-c18:1 and 12 and 13 trans-c18:1 show almost complete overlap (Kramer et al., 2002; Ackmann, 2002; AOCS official method Ce 1f-96, 2002; Chrompack Application note 899 - GC, 2004). Based on the available literature, the chromatograms were analysed by fitting them to 15 Gaussian curves using a least squares method, each curve with a peak width of approximately 0.034 minutes. The specific retention time for each peak was determined either from the available model compounds or deduced from literature. A list of the peaks is given in Table 3.4 together with the relevant literature data. The numbers in bold face refer to internal standards that were used. Peaks for 2 and 3 trans c18:1 were never observed as they probably disappear under the peak for methyl stearate. Peaks for 2,4 and 5 cis-c18:1 are thought to be somewhere under the lower trans peaks. An example of a chromatogram and the results of the peak-fit are shown in Figure 3.8. Using the areas of the various peaks it becomes possible to calculate the distribution of the double bonds amongst the cis and trans species. Figure 3.9 is an abstracted version of Figure 3.8. Peak areas are represented by the height of the bars. The top row of numbers on the x-axis represents the position of the cis double bonds, the bottom row the position of the trans double bonds.

Reactant diffusivity

The diffusivities of methyl oleate and methyl elaidate in n-decane were unknown and had to be estimated. Shieh and Lyons (1969) measured the diffusivity of hexadecane in n- 10 2 1 10 octane and n-dodecane at 298 K and found D 5.7 10− m s− and D 6.8 10− 2 1 = · = · m s− , respectively. Assuming a Stokes-Einstein dependency on temperature (D T/η) and a square root dependency on molecular mass (D √M), one finds that the∼ ∼ 9 2 1 diffusivity of methyl oleate in n-decane at 373 K should lie between 1.7 10− m s− 9 2 1 · and 1.9 10− m s− . According to Yaws (1995) the diffusivities of several cis and trans alkenes· in water have the same value within one percent, so it was assumed that the diffusivity of both methyl oleate and methyl elaidate in n-decane at 373 K is 1.8 10 9 m2 · − 38 ‘‘thesis’’ --- 2007/1/31 22:24 --- page 39 (#49)

METHYL OLEATE

3

saturated 4

5 8 6

7 9

FID response (A.U.) 2 10 1 14 11 12 13 15

12 13 14 15 time (min)

Figure 3.8: Example of a gas-chromatogram of the reaction products. The numbers refer to the peaks in Table 3.4.

1 s− . The diffusivity of hydrogen in n-decane at 373 K was estimated with the Wilke and 8 2 1 Chang equation to be 1.8 10− m s− . Correcting for catalyst porosity and assuming a tortuosity factor of five,· the effective diffusivity of the reactants inside the catalyst was estimated at Deff 0.1D. =

3.4 Results

Catalyst activity

The reaction was found to be first order in the concentration of the double bonds as shown in Figure 3.10. The conversion X is defined as the total fraction of unsaturates that have been hydrogenated towards methyl stearate, or

39 ‘‘thesis’’ --- 2007/1/31 22:24 --- page 40 (#50)

CHAPTER 3

30 3

20 4 5 6 7 8

10 % of unsaturated 9 10 2 11 12 13 1 14 15

0 6,7,8 9 10 11 3,12 13 14 15

4 5 6,7,8 9 10 11 12 13,14 15 16

Figure 3.9: Abstracted chromatogram based on data presented in Figure 3.8. Numbers refer to the peaks in Table 3.4. Top row numbers on x-axis for cis isomers, bottom for trans isomers.

Ct 0 C X unsat= sat t −0 (3.5) = Cunsat=

The first order rate constant kobs was not linearly dependent on the active palladium surface area: the observed rate increased more than proportional with the surface area (Figure 3.11). This could be caused by some poison in the feed. If each experiment is carried out with the same amount of poison present, this effect will be largest at low palladium loadings since in that case a relatively high percentage of active sites will be poisoned. The amount of external mass transfer was estimated by calculating the external CaLS number for both reactant and hydrogen

rv,obs CaLS,i (3.6) = kLS,i a0 Ci where

Di 6 kLS,i a0 Sh (3.7) = dp dp

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METHYL OLEATE

Table 3.4: Assignment of cis and trans species to peaks in the chromatogram. peak trans cis reference 1 4 2 5 3 6,7,8 Christie (1989); Scholfield (1981) Ratnayaka and Pelletier (1992); Bertucco and Vetter (2001) Kramer et al. (2002) 4 9 5 10 6 11 7 12 8 13,14 6,7,8 Kramer et al. (2002); Ackmann (2002) AOCS official method Ce 1f-96 (2002) Chrompack Application note 899 - GC (2004) 9 9 10 15 10 Kramer et al. (2002) 11 11 12 3,12 Christie (1989); Scholfield (1981) 13 13 14 16 14 Kramer et al. (2002); Ackmann (2002) 15 15

The Sherwood number Sh was estimated using the correlation given by Sano et al. (1974) for two-phase stirred tank reactors

!0.25 d4ρ3  η 0.33 Sh p 2 0.4 3 (3.8) = + η ρ Di

1 where  is the power dissipation per kilogram of liquid, estimated at 10 W kg− . This results in Sh 11. Using equation 3.9 the Weisz-Prater number 8 is calculated to determine the extent= to which internal diffusion limitations play a role

2  dp  rv,obs 6 8 (3.9) = Def f,i Ci The results of the calculations are summarized in Table 3.5. The estimated liquid to solid mass transfer is too low: for catalyst 4 the value of Ca exceeds one. There is however a large uncertainty in these calculations: the correlation of Sano et al. (1974) is for two-phase reactors without a gas-phase. It can be concluded that catalyst 4 suffers to a certain degree from external mass transfer limitations. The other catalyst are free of external transport limitations.

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1.E+05

8.E+04

cat 6.E+04 e )/ X

4.E+04 -ln(1-

2.E+04

0.E+00 0 60 120 time (min)

Figure 3.10: Catalyst hydrogenation activity. Catalyst 1 (3); catalyst 2 (2); catalyst 3 ( ); catalyst 4 (X). 4

Table 3.5: Estimation of parameters for verifying the presence of external mass transfer and internal diffusion limitations.

cat rv,obs CMeO CH2 CaLS,MeO CaLS,H2 8MeO 8H2 1 34 15.5 92.9 0.02 0.01 0.43 0.01 2 56 14.6 92.9 0.04 ≤0.01 0.77 0.01 3 370 13.1 92.9 0.31 ≤0.01 5.7 0.08 4 2781 13.1 92.9 2.3 ≤0.03 42 0.60

Methyl oleate vs methyl elaidate

The distribution of the double bonds during the hydrogenation of methyl oleate and elaidate is shown in Figure 3.12. For the sake of clarity the distributions have been nor- malized (the total amount of unsaturated molecules equals 100%). As the conversion increases, the double bonds spread out over the carbon chain showing a slight preference for the end of the chain away from the ester group. Note that the third, eighth, tenth, twelth and fourteenth bars in the graphs give cumulative amounts. An interesting phenomenon is that when starting from oleate (cis), the trans double bonds spread out faster over the carbon chain. Conversely, starting from elaidate (trans)

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METHYL OLEATE

250

200 ) -1 s

cat 150 -3 m liq 3 100 (m obs k 50

0 0 0.5 1 1.5 2 2.5 2 -1 surface area (m Pd gcat )

Figure 3.11: Catalyst activity vs palladium surface area.

Table 3.6: Ratio of 10/9 cis and 10/9 trans at 30% conversion. 10 trans 10 cis 9 trans 9 cis starting from 9 cis (oleate) 0.34 0.08 starting from 9 trans (elaidate) 0.12 0.40

the cis double bonds spread out faster. In Table 3.6 the ratio of 10/9 cis and 10/9 trans is given at low conversion: starting from oleate, 10/9 trans exceeds 10/9 cis and vice versa starting from elaidate. The rate of the double bond migration thus appears to be related to the starting material. In Figure 3.13, the trans-cis ratios of various positions of the double bond are plotted versus conversion. The rate at which equilibrium is reached is dependent on the location of the double bond: the further a double bond is removed from the 19 double bond, the faster trans-cis equilibrium is approached.

Trans-cis equilibrium

The influence of internal diffusion limitations on the overall trans-cis ratio was invest- igated by using catalysts with varying palladium loading to hydrogenate methyl oleate. Figure 3.15 shows that the trans-cis ratio at a given conversion is dependent on the Weisz- Prater number 8 (equation 3.9), which is a measure for the severity of diffusion limita-

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40 A B 50

40 30

30 20

20 % of unsaturated % of unsaturated 10 10

0 0 6,7,8 9 10 11 3,12 13 14 15 6,7,8 9 10 11 3,12 13 14 15

4 5 6,7,8 9 10 11 12 13,14 15 16 4 5 6,7,8 9 10 11 12 13,14 15 16

C D 50 50 82%

40 40

30 30

20 20 % of unsaturated % of unsaturated

10 10

0 0 6,7,8 9 10 11 3,12 13 14 15 6,7,8 9 10 11 3,12 13 14 15

4 5 6,7,8 9 10 11 12 13,14 15 16 4 5 6,7,8 9 10 11 12 13,14 15 16

Figure 3.12: Distribution of double bonds during the hydrogenation of methyl oleate at 30% (A) and 80% (B) conversion, and of methyl elaidate at 29% (C) and 82% (D) conversion. Top row numbers on x-axis for cis isomers, bottom for trans isomers.

tions. The overall composition of the reactant mixture is given in Table 3.7. The amount of trans formed at a given amount of saturation decreases as the diffusion limitations of the reactant methyl oleate increase.

The results were fitted to a simple first order model in which the cis and trans isomers are lumped. This model describes the results with two observable parameters, the ob- served trans-cis isomerization rate constant ktc,obs, and the observed hydrogenation rate constant kh,obs (Figure 3.14). The results of these fits are given in Table 3.7. The observed isomerization rate decreases relative to the hydrogenation rate with increasing diffusion limitations.

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METHYL OLEATE

6 30

5 25

4 20 (-) (-)

3 15

trans / cis 2 trans / cis 10

1 5

0 0 0 20406080100 0 20406080100 saturated (%) saturated (%)

Figure 3.13: trans-cis ratio of 19 ( ), 110 ( ), 111 (2) and 112 (3), during the hydrogenation of methyl oleate (left) and methyl elaidate× (right).4 Dashed line for thermodynamic equilibrium.

cis kh,obs

ktc,obsKtc ktc,obs saturated

trans kh,obs

Figure 3.14: Lumped reaction scheme.

Positional equilibrium

The distribution of the double bonds can be quantified by calculating the first and second moments of the distributions with

16 P iCi,iso i 2 µ1 iso = (3.10) , 16 = P Ci,iso i 2 =

16 P 2 (i µ1,iso) Ci,iso i 2 − µ2 iso = (3.11) , 16 = P Ci,iso i 2 = 45 ‘‘thesis’’ --- 2007/1/31 22:24 --- page 46 (#56)

CHAPTER 3

Table 3.7: Composition of reactant mixture at 80% conversion with reaction at various degrees of internal limitations and observed trans-cis isomerization rate. trans ktc,obs catalyst 8MeO trans cis cis kh,obs 1 0.4 15.6 4.4 3.5 0.68 2 0.8 15.6 4.4 3.5 0.44 3 6 14.3 5.6 2.5 0.32 4 42 11.7 8.3 1.4 0.18

4

3 (-) cis

/ 2 trans 1

0 0 20406080100 saturated (%)

Figure 3.15: Effect of diffusion limitations on trans-cis ratio during methyl oleate hydrogenation 8MeO 0.4 (3); 8MeO 0.8 (2); 8MeO 5.7 ( ) and 8MeO 42 (X). Lines for apparent first order= fits, dashed line for= thermodynamic= equilibrium.4 =

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METHYL OLEATE

2.5

2.0

1.5 (-) 2,cis

µ 1.0

0.5

0.0 0 20406080100 saturated (%)

Figure 3.16: Effect of diffusion limitations on the cis double bond distribution as expressed in the second moment µ . 8 0.4 (3); 8 0.8 (2); 8 5.7 ( ) and 8 42 (X). 2 MeO = MeO = MeO = 4 MeO =

in which iso is either cis or trans. The first moment µ1 equals the averaged location of the double bond. The second moment µ2 is a measure for the spread of the double bonds around the averaged location. The contribution of overlapping isomers in the GC peaks were given equal weight, e.g. the area of GC peak 3 was divided equally amongst 6,7, and 8 trans-c18:1. Since several isomers overlap and some isomers are missing from the chromatogram results, have to be interpreted with care. In fact, this method only produces useful results for the cis isomers. The first moment of the trans isomers µ1,trans always shows a bias towards the end of the molecule, wereas µ1,cis has a value of 9 0.5. ±

A plot of µ2,cis vs conversion in Figure 3.16 shows that the spread of the cis double bonds over the carbon chain at a given conversion is decreased if the reaction is carried out at increased diffusion limitations of methyl oleate. The distribution of double bonds at 80% conversion becomes narrower with increasing diffusion limitations as Figure 3.17 shows. The distribution of the cis isomers in Figure 3.17 at 80% conversion can be de- scribed by µ2,cis 2.4, 2, 1.3 and 0.9 respectively. The rate of positional isomerization decreases with increased= diffusion limitations, which is in line with that observed for trans-cis isomerization.

47 ‘‘thesis’’ --- 2007/1/31 22:24 --- page 48 (#58)

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40 40 A B

30 30

20 20 % of unsaturated % of unsaturated 10 10

0 0 6,7,8 9 10 11 3,12 13 14 15 6,7,8 9 10 11 3,12 13 14 15

4 5 6,7,8 9 10 11 12 13,14 15 16 4 5 6,7,8 9 10 11 12 13,14 15 16

40 40 C D

30 30

20 20 % of unsaturated % of unsaturated 10 10

0 0 6,7,8 9 10 11 3,12 13 14 15 6,7,8 9 10 11 3,12 13 14 15

4 5 6,7,8 9 10 11 12 13,14 15 16 4 5 6,7,8 9 10 11 12 13,14 15 16

Figure 3.17: Effect of diffusion limitations on the double bond distribution at 80% conversion of methyl oleate. 8MeO 0.4 (A); 8MeO 0.8 (B); 8MeO 6 (C) and 8MeO 42 (D). Top row numbers for cis isomers,= bottom for trans=isomers. = =

48 ‘‘thesis’’ --- 2007/1/31 22:24 --- page 49 (#59)

METHYL OLEATE

30 84%

cis 12 dikkere lijnen

20

10 % of unsaturated %

0 6,7,8 9 10 11 3,12 13 14 15 6,7,8 9 10 11 3,12 13 14 15 6,7,8 9 10 11 3,12 13 14 15 4 5 6,7,8 9 10 11 12 13,14 15 16 4 5 6,7,8 9 10 11 12 13,14 15 16 4 5 6,7,8 9 10 11 12 13,14 15 16

Figure 3.18: Double bond distribution during hydrogenation of 12 cis-octadecenoate at 9%, 35% and 90% conversion. Top row numbers for cis isomers, bottom for trans isomers.

Symmetrical distribution?

The distribution of the double bonds along the carbon chain appears to be more or less symmetrical around the middle 19 position. However, this could be the result of a random redistribution around the starting 19 position and not because of a preference for 19. In this respect, an experiment starting with the double bond at position 12 gave interesting results. Figure 3.18 shows the distribution of the double bonds at three dif- ferent conversions. A shift of 112 to the middle of the molecule can clearly be seen. In Figure 3.19 the first moment of the cis isomers has been calculated. At high conversions the mean of the distribution is close to 110 which confirms the direct observation of the double bond shift in Figure 3.18.

3.5 Modelling the effects of diffusion limitations

To investigate the effect of diffusion limitations on the hydrogenation and isomeriz- ation of octadecenoates a model was developed. The model was based on the reaction scheme given in Figure 3.2. A number of simplifying assumptions were made:

The trans-cis equilibrium is equal for all positions of the double bond (12 through • 116) (Ktc 3.8 at 373 K). = At equilibrium a Gaussian distribution of the double bonds is assumed. Based on a • given standard deviation σ the positional equilibrium constants K p,i are calculated.

Hydrogen is not limiting i.e. there are no hydrogen gradients inside the catalyst. • All reactions are first order in the concentration of double bonds. • The trans and cis double bond migration rates are assumed to be equal. • 49 ‘‘thesis’’ --- 2007/1/31 22:24 --- page 50 (#60)

CHAPTER 3

12

11 (-) 1,cis µ 10

9 0 20406080100 saturated (%)

Figure 3.19: First moment of the cis isomers, during hydrogenation of 12 cis-octadecenoate.

No external diffusion limitations. • For the cis and trans species the following rate equations were defined

ri,cis ( k p(1 K p,i ) ktc Ktc kh)Ci cis (3.12) = − + − − k p K p,i Ci 1 cis k pCi 1 cis ktcCi trans + − + + +

ri,trans ( k p(1 K p,i ) ktc kh)Ci trans (3.13) = − + − − k p K p,i Ci 1 trans k pCi 1 trans ktc KtcCi cis + − + + + where i=2 to 16. For 2 trans and 2 cis the equations are reduced since there is neither 1 cis nor 1 trans. Likewise for 17-c18:1 where the double bond is at the end of the carbon chain. This results in a total of 31 rate equations. For modelling of a batch slurry reactor, the following reaction / diffusion equation inside a spherical particle was solved for each component

2 ∂ Ci Def f r 0 (3.14) ∂z2 − =

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METHYL OLEATE

∂C dp with boundary conditions z 0, ∂z 0 and z 2 , C Cbulk. The change in bulk concentration of each component= is given= by = =

∂Cbulk,i 6 ∂Ci cat Def f (3.15) d ∂t = −dp ∂z z p = 2 The extent of diffusion limitations of the reactants can be expressed with the Thiele modulus defined by s dp kh φ (3.16) = 6 Def f

3.6 Discussion

Equilibrium data

The overall trans-cis equilibrium constant Ktc at 373 K was found to be on the order of 3.5 (Figure 3.15), which is within range of the value calculated from the correlation derived from literature data (Equation 3.1). The latter yields Ktc 3.8 (3.2-4.3). With the hydrogenation of methyl oleate (or 9 cis-octadecenoate)= the distribution of the cis double bonds as expressed in the second moment appears to stabilize between 2 < µ2,cis < 2.5 (Figure 3.16) at high conversions. Whether there is real equilibrium is at first glance unclear. The first moment µ1,cis of the cis double bonds does not deviate much from the original central ninth carbon-carbon double bond. I.e. the central weight of the distribution does not shift significantly. However, when 12 cis-octadecenoate is hydrogenated, the double bond tends to migrate back to the middle of the carbon chain. A clear shift from the twelfth to the tenth position is observed. If the double bonds were distributed evenly at equilibrium, one would expect the first moment to have remained at the value of the initial (twelfth) position. Since the first moment clearly shifts towards the middle of the carbon chain, it can be concluded that at equilibrium a non-equal distribution of double bonds must exist. When both trans-cis equilibrium and positional equilibrium have set in as is the case in Figures 3.15 and 3.16, µ2,cis µ2,trans. = Within 1 < σ < 3 the second moment µ2 of a Gaussian distribution is almost equal to the standard deviation σ. It is therefore concluded that the equilibrium distribution of the double bonds in methyl octadecenoate at 373 K can be approximated by a Gaussian distribution with 2 < σ < 2.5.

Influence of diffusion limitations on isomerization reactions

The rate at which trans-cis equilibrium is reached is influenced by the severity of dif- fusion limitations. In catalyst 1, which suffers from mild limitations only, the observed

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CHAPTER 3

4

3 (-)

2 trans / cis

1

0 0 20406080100 saturated (%)

Figure 3.20: Modelling the effect of diffusion limitations on trans-cis isomerization. Catalyst 1 (3); 2 (2); 3 ( ) and 4 (X). 4

Table 3.8: Parameters used in modelling of the effects of diffusion limitations. 1 catalyst kh (s− ) 1 2.5 2 5.0 3 161 4 8960 10 2 1 Def f 1.8 10 m s · − − dp 36 µm Ktc 3.8 (-) σ 2.2 (-)

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METHYL OLEATE

0.8

kinetic: k tc / k h = 0.77

0.6 h,obs 0.4 / k tc,obs k 0.2

0 0 1020304050

Φ MeO

Figure 3.21: Modelling the effect of diffusion limitations on trans-cis isomerization. Observed isomerization rate over hydrogenation rate vs the amount of diffusion limitations.

trans-cis isomerization rate is 0.68 times the hydrogenation rate. In contrast, for catalyst 4 this is only 0.18, meaning that the isomerization rate is relatively slower (Table 3.7). This behaviour was fitted using the model described in Section 3.5. The set of constants used is given in Table 3.8. The only difference between the various catalysts is the amount of reactant diffusion limitations caused by the increasing reaction rates. First order hydro- genation rate constants kh were calculated from the reactant Weisz-Prater numbers 8MeO in Table 3.5. An optimum was found for ktc 0.77kh. This means that under kinetic conditions, the trans-cis isomerization rate is 77%= of the hydrogenation rate. As conditions become more limiting, the ratio of the observed trans-cis isomerization rate constant ktc,obs and hydrogenation rate constant kh,obs decreases and the rate of isomerization appears to be slower. The results of the fit are shown in Figures 3.20 and 3.21 and are in good agreement with the experimental data. In contrast to Figure 3.15, differences in isomerization rate relative to the hydrogenation rate are modelled by the effects of diffusion on a single set of kinetic rate constants, instead of using four sets of apparent rate constants. If one assumes that the mechanism underlying trans-cis isomerization and positional isomerization is the same, it seems likely that k p ktc. Based on this assumption, the = second moment µ2,cis was calculated from the modelled data, the results of which are given in Figure 3.22. It should be noted that not the complete calculated data-set was

53 ‘‘thesis’’ --- 2007/1/31 22:24 --- page 54 (#64)

CHAPTER 3

2.5

2.0

1.5 (-) 2,cis µ 1.0

0.5

0.0 0 20406080100 saturated (%)

Figure 3.22: Modelling the effect of diffusion limitations on the cis double bond distribution as expressed in the second moment µ cis. Catalyst 1 (3); 2 (2); 3 ( ) and 4 (X). 2, 4

used (containing all 31 isomers) but rather a condensed set, which has been modified so as to give abstracted chromatograms. From these the second moment was calculated using the same method that was used for the experimental data. This approach ensures that the same bias is made as results from the assumptions made in calculating µ2,cis from the experimental data. The model calculations show for catalysts 1-3 a clear underestimation of the rate of positional isomerization. Even under mildly limiting conditions, the experimental second moment increases faster than the calculated one. As a consequence, the assumption of equal isomerization rates seems disproven: the rate of positional isomerization is faster than the trans-cis isomerization rate. This points towards different rate determining steps for both types of reactions, and gives some credence to the existence of π-complexes described in Section 3.2.

Positional isomerization: difference between cis and trans?

Reports in literature on higher rates of positional isomerization of trans double bonds compared to cis double bonds could not be confirmed. However, these reports were based on experiments carried out at atmospheric pressure, where differences in the mode of adsorption are expected to be the strongest, resulting in large differences. The results

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METHYL OLEATE

4 20 ∆9

3 kinetic 15

(-) (-) φ =0.71 ∆10 2 10 φ =0.71 ∆11 ∆12 trans / cis trans / cis ∆12 1 ∆11 5 ∆10 ∆9 kinetic

0 0 0 20406080100 0 20406080100 saturated (%) saturated (%)

Figure 3.23: calculated trans-cis equilibrium starting from oleate (left) and elaidate (right). Dashed lines give trans-cis ratio of 19, 110, 111 and 112 at φ 0.71. Solid lines give overall trans-cis ratio for kinetic and diffusion limiting conditions. =

presented in Figure 3.13 seem to indicate that depending on the starting material, either cis or trans double bonds tend to migrate faster. This kind of symmetry is baffling, since how would molecules know from which starting material they originate? At first glance, the trans-cis ratio of 19 and 110 appears to be moving away from equilibrium at high conversion. One explanation could be that the equilibrium ratio at 373 K is lower than 3.8 but this is clearly contradicted by the available literature. Van der Plank (1972b) suggested that a change in the reaction orders of hydrogenation and isomerization at high conversions could be responsible for this behaviour. Since the reaction system was very diluted, the surface coverage of the unsaturates could not significantly have changed during the experiments. Most likely it is an artifact due to unresolved peaks resulting in skewed values of the trans-cis ratio. 10 cis and 15 trans overlap in the chromatogram so 10 trans 10 trans at high conversions the ratio 10 cis becomes 10 cis 15 trans , lowering the actual value. Moreover, starting with methyl elaidate this phenomenon+ is absent. The high trans-cis value of 112 ( 3.8) could be due to contribution of underlying 4 and 5 cis peaks to 12 trans peaks, the≥ location of which is unknown. The effect of diffusion limitations on the product distribution was calculated using the model described in Section 3.5. Calculations were made for both methyl oleate and elaidate, assuming ktc k p 0.77kh. Overall trans-cis equilibrium was set at 3.8 and an equal distribution of= double= bonds at equilibrium was assumed (σ ). In fact, any distribution can be chosen since only the trans-cis equilibria at individual→ ∞ positions are considered. Modelling results are shown in Figure 3.23, starting respectively from oleate and elaidate. When diffusion limitations are absent, equilibrium is reached at the same rate for all positions of the double bond. This is the line marked ’kinetic’ in the graphs. However, even at a low value for φ a significant effect can be seen: starting from methyl oleate the

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CHAPTER 3

trans double bonds appear to move faster resulting in lower trans-cis ratio for 19 and increasingly higher for 110, 111 and 112. Starting from methyl elaidate the reversed is seen and the cis double bonds appear to move faster! This is in agreement with what is observed experimentally in Figures 3.12 and 3.13. The apparent symmetry is an artifact caused by diffusion effects and not caused by kinetic differences. Based on the experimental results it can therefore not be concluded that an intrinsic difference between trans and cis double bonds exists. The modelling results suggest that even weak diffusion limitations will mask any intrinsic difference between trans and cis double bonds.

3.7 Conclusions

The hydrogenation of mono unsaturated fatty methyl esters provided a number of in- sights in the effects of diffusion limitations on isomerization reactions. Both the rate of trans-cis and positional isomerization were diminished with increasing diffusion limita- tions. At equilibrium, the trans-cis equilibrium was found to be in line with literature and the distribution of double bonds at equilibrium could be described by a Gaussian distribution with σ 2.2. Depending on the starting material, either cis or trans bonds appear to move faster≈ along the carbon chain. It is shown that this is an effect of diffusion limitations, and can effectively mask any real (kinetic) difference between cis and trans double bonds. Finally, the experimental results suggest that positional isomerization is somewhat faster than trans-cis isomerization. This points towards different underlying rate determining steps for both reactions.

3.8 Nomenclature

FAME - Fatty Acid Methyl Ester methyl oleate - methyl 9 cis-octadecenoate methyl elaidate - methyl 9 trans-octadecenoate methyl stearate - methyl octadecanoate c18:1 - methyl octadecenoate 19 - double bond (either cis or trans) on position 9, counting from the acid carbon

a0 specific geometric particle area, m2 3 C concentration, mol m− Ca Carberry number, dimensionless 2 1 D diffusivity, m s− 2 1 Def f effective diffusivity, m s− D50 median particle size, µm dp catalyst particle diameter, m

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METHYL OLEATE

dpore catalyst pore diameter, nm 1E f ree free energy, J 1 1Htc enthalpy of isomerization, J mol− 1 kh hydrogenation rate constant, s− 1 kLSa0 liquid to solid mass transfer constant, s− 1 ktc trans-cis isomerization rate constant, s− 1 k p positional isomerization rate constant, s− K p positional equilibium constant, dimensionless Ktc trans-cis equilibrium constant, dimensionless 1 M molecular mass, g mol− P chance, dimensionless 3 1 r rate, mol mcat− s− 1 1 R gas constant, 8.314 J mol− K− 3 1 rv,obs observed rate, mol mcat− s− 2 1 S surface area, m g− Sh Sherwood number, dimensionless 2 1 SBET BET surface area, m g− 1 1 1Stc entropy of isomerization, J mol− K− T temperature, K X conversion, dimensionless z axis of integration, m 1  power input per unit mass, W kg− 3 3 cat catalyst loading, mcat mreactor− 8 Weisz-Prater number, dimensionless φ Thiele modulus, dimensionless η viscosity, Pa s µ1 first moment, dimensionless µ2 second moment, dimensionless 3 ρ liquid density, kg m− σ standard deviation of the equilibrium double bond distribution, dimensionless

Bibliography

Ackmann, R., 2002. The gas chromatograph in practical analyses of common and uncom- mon fatty acids for the 21st century. Anal. Chim. Acta 465, 175–192.

Adhikari, S., Sprinz, H., Brede, O., 2001. Thiyl radical induced isomerization of unsat- urated fatty acids: determination of equilibrium constants. Res. Chem. Intermed. 4-5, 549–559.

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AOCS official method Ce 1f-96, 2002. Determination of cis- and trans fatty acids in hydrogenated and refined oils and fats by capillary GLC. Bertucco, A., Vetter, G. (Eds.), 2001. High Pressure Process Technology: Fundamentals and Applications. Elsevier, Ch. Hydrogenation under supercritical single-phase condi- tions, pp. 496–508. Brown, L., Swidler, R., 1962. Process for isomerization of oleic acid and its derivatives. US 3,065,248. Chatgilialoglu, C., Altieri, A., Fischer, H., 2002. The kinetics of thiyl radical-induced reactions of monounsaturated fatty acid esters. J. Am. Chem. Soc. 124, 12816–12823. Chatgilialoglu, C., Ferreri, C., Ballestri, M., Mulazzani, Q., Landi, L., 2000. cis-trans isomerization of monounsaturated fatty acid residues in phospholipids by thyil radicals. J. Am. Chem. Soc. 122, 4593–4601. Chaudhari, R., Jaganathan, R., Mathew, S., 2002. Hydrogenation of 1,5,9- cyclododecatriene in fixed-bed reactors: down- vs. upflow modes. AIChE Journal 48(1), 110–125. Christie, W., 1989. Equivalent chain-lengths of methyl ester derivatives of fatty acids on gas chromatography. J. Chromatogr. 447, 305–314. Chrompack Application note 899 - GC, 2004. Fatty acid methyl esters (cis/trans c18:1 isomers) of milk. Dutton, H., Scholfield, C., Selke, E., Rohwedder, W., 1968. Double bond migration, geo- metric isomerization, and deuterium distribution during heterogeneous catalytic deu- teration of methyl oleate. J. Catalysis 10, 316–327. Grau, R., Cassano, A., Baltanas, M., 1986. Kinetics of methyl oleate catalytic hydro- genation with quantitative evaluation of cis-trans isomerization equilibrium. Ind. Eng. Chem. Process Des. Dev. 25, 722–728. Gunstone, F., Ismail, I., Lie Ken Jie, M., 1967. Fatty acids part 14. The conversion of the cis octadecenoic acids to their trans isomers. Chem. Phys. Lipids 1(3), 264–269. Gut, G., Kosinka, J., Prabucki, A., Schuerch, A., 1979. Kinetics of the liquid-phase hy- drogenation and isomerization of sunflower seed oil with nickel catalysts. Chem. Eng. Sci. 34, 1051–1056. Heertje, I., Koch, G., Wosten,¨ W., 1974. Mechanism of heterogeneous catalytic cis-trans isomerization and double-bond migration of octadecenoates. J. Catalysis 32, 337–342. Isbell, T., Kleiman, R., Erhan, S., 1992. Characterization of monomers produced from thermal high-pressure conversion of meadowfoam and oleic acids into estolides. J. Am. Oil Chem. Soc. 69(12), 1177–1183.

58 ‘‘thesis’’ --- 2007/1/31 22:24 --- page 59 (#69)

METHYL OLEATE

Jonker, G., Veldsink, J., Beenackers, A., 1997. Intrinsic kinetics of 9-monoenic fatty acid methyl ester hydrogenation over nickel-based catalysts. Ind. Eng. Chem. Res. 36, 1567–1579. Kabo, G., Andreevskii, D., Radyuk, Z., 1979. Thermodynamics of isomerization of n- hexenes. Neftekhimiya 10(3), 330–334. Kabo, G., Andreevskii, D., Savinetskaya, G., 1967. Isomerization equilibrium of second- ary n-monochloroheptanes and n-heptenes. Neftekhimiya 7(3), 364–368. Kramer, J., Blackader, C., Zhou, J., 2002. Evaluation of two gc-columns for analysis of milkfat with emphasis on CLA, 18:1, 18:2 and 18:3 isomers, and short- and longchain FA. Lipids 37(8), 823–835. Kramer, J., Cruz-Hernandez, C., Zhou, J., 2001. Conjugated linoleic acids and octadecen- oic acids: Analysis by GC. Eur. J. Lipid Sci. Technol. 103, 594–632. Litchfield, C., Lord, J., Isbell, A., Reiser, R., 1963. cis-trans isomerization of oleic, li- noleic and linolenic acids. J. Am. Oil Chem. Soc. 40, 553–557. Lush, E., 1923. Hydrogenation. J.S.C.I. 62, 219T–225T. Moore, C., 1919. The formation of solid iso-oleic acids by the hydrogenation of ordinary liquid oleic acid. J. Soc. Chem. Ind. 38, 320T. Munzing,¨ M., 1986. Kinetische Untersuchungen zur selektieven Hydrierung und Isomer- isierung pflanzlicher Ole¨ an Kopferchromit. Ph.D. thesis, ETH Zurich.¨ Pledge, H., 1959. Science since 1500. Harper and Brothers, NY, p. 196. Ratnayaka, W., Pelletier, G., 1992. Positional and geometrical isomers of linoleic acid in partially hydrogenated oils. J. Am. Oil Chem. Soc. 69(2), 95–102. Rogers, D., Hoyte, O., Ho, R., 1977. Heats of hydrogenation of large molecules. Part 2.-six unsaturated and polyunsaturated fatty acids. J. Chem. Soc. Faraday Trans. I 74, 46–52. Rogers, D., Siddiqui, N., 1975. Heats of hydrogenation of large molecules. i. Esters of unsaturated fatty acids. J. Phys. Chem. 79(6), 574–577. Sano, Y., Yamaguchi, N., Adachi, T., 1974. Mass transfer coefficients for suspended particles in agitated vessels and bubble columns. J. Chem. Eng. Jpn. 7(4), 255–261. Scheffers, H., 1930. Intramoleculaire omleggingen bij de hydreering van esters van en- kelvoudig onverzadigde vetzuren. Ph.D. thesis, TU Delft. Scholfield, C., 1981. Gas chromatographic equivalent chain lengths of fatty acid methyl esters on a Silar 10c glass capillary column. J. Am. Oil Chem. Soc. 58, 662–663.

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Shieh, J., Lyons, P., 1969. Transport properties of liquid n-alkanes. J. Phys. Chem. 73(10), 3258. Van der Plank, P., 1972a. A criterion for detecting mass transport effects on double bond migration during the hydrogenation of methyl oleate using nickel-on-silica catalysts. J. Catalysis 26, 42–50. Van der Plank, P., 1972b. Isomerization phenomena during hydrogenation of methyl oleate during hydrogenation of methyl oleate and methyl elaidate over nickel-silica catalysts. J. Am. Oil Chem. Soc. 49, 327–332. Van der Plank, P., Van Oosten, H., 1975. Study of the mechanism of double-bond isomer- ization in methyl 9-octadecenoates. J. Catalysis 48, 223–230. Yaws, C. (Ed.), 1995. Handbook of transport property data. Gulf Publishing Company, Houston.

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Chapter 4

Kinetics of the Hydrogenation and Isomerization of 1,5,9-cis,trans,trans-Cyclododecatriene

‘This ring!’ [Frodo] stammered. ’How, how on earth did it come to me?’ Tolkien (1954)

4.1 Introduction

In this chapter, the consecutive hydrogenation of 1,5,9-cis,trans,trans- cyclododec- atriene (ctt-CDT) towards cyclododecadiene (CDD) followed by cyclododecene (CDE) and finally cyclododecane (CDA) is investigated. It is used as a model reaction for the reaction of poly-unsaturated alkenes towards the intermediate dienes and monoenes. In addition to being an interesting model reaction, the mono-unsaturated intermediate cyc- lododecene is a key intermediate for 12-laurolactam and 1,10-decanedicarboxylic acid

+H2 r ttt-CDT 1 +H2 r r iso,1 iso,2 tt-CDD r6 +H2 r2 r r ctt-CDT iso,5 iso,6 r7 t-CDE r10 r 3 r r riso,3 riso,4 ct-CDD iso,9 iso,10 CDA r4 r8 r cct-CDT riso,7 riso,8 c-CDE 11 r5 cc-CDD r9 ccc-CDT

Figure 4.1: Reaction scheme including all isomers.

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25

20

15 mol% 10

5

0 0 60 120 time (min)

Figure 4.2: Isomer formation during the hydrogenation of ctt-CDT at T 433 K and P 2.5 MPa. ctt-CDT (+); ttt-CDT (*); cct-CDT (-); tt-CDD ( ); ct-CDD (X); cc=-CDD ( ); trans=-CDE (3); cis-CDE (2). 4

production, which are used in the pharmaceutical industry. Other applications of cyc- lododecene are in the production of dyes and chemicals for crop protection (Streck and Hartig, 1990; Benaissa et al., 1996). The final product cyclododecane has some use in the preservation of paper and archeological artifacts (Bruckle¨ et al., 1999) and in the produc- tion of polyamid-12 (Streck and Hartig, 1990). In Figure 4.1 the complete reaction scheme is given, including all the isomers that can theoretically be formed. The reactant ctt-CDT can isomerize into ttt-, cct- and ccc-CDT although the latter was never detected in the reaction mixture. Each isomer can in turn hydrogenate into a diene, which can in turn hydrogenate into monoenes. The monoenes yield the final product cyclododecane. The hydrogenation reactions (horizontal) and iso- merization reactions (vertical) run in parallel so that complex mixtures of products are formed. It should be noted that it is possible for double bond shift to take place in the trienes and dienes, even forming conjugates, further increasing the number of isomers. The analysis of the reaction products indeed showed a number of unidentified species, more of which in the next section. An example of the isomer formation is shown in Figure 4.2 where the complete composition is given during a hydrogenation reaction. The isomers of equal saturation, although they are equal in a chemical sense, have very different spatial configurations. This is illustrated in Figure 4.3 for the trienes. The ’flat’ structure-formula is given together with a spatial view of a lowest energy config-

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1,5,9-cis,trans,trans-CYCLODODECATRIENE

1,5,9 cis,trans,trans-CDT

1,5,9 cis,cis,trans-CDT

1,5,9 trans,trans,trans-CDT

1,5,9 cis,cis,cis-CDT

Figure 4.3: Examples of spatial conformations of the isomers of 1,5,9-cyclododecatriene. 63 ‘‘thesis’’ --- 2007/1/31 22:24 --- page 64 (#74)

CHAPTER 4

30 2.10

25 2.05

20 2.00

15 1.95 t/c ttt/ctt K K

10 1.90

5 1.85

0 1.80 233 283 333 383 433 T (K)

Figure 4.4: Configuration equilibrium constant of two CDT isomers and both CDE isomers (Thorn- Csanyi´ and Ruhland, 1999b; Cope et al., 1960). Dotted lines are extrapolations from literature data.

uration. Furthermore, it is even possible that for a single isomer more than one stable configuration exists (Pawar et al., 1999; Anet and Rawdah, 1979; Rawdah and El-Faer, 1996, 1995). This difference in spatial conformation could give rise to different reactivit- ies of the isomers, especially if one or more double bonds in the structure is ’shielded’. Literature on the equilibrium composition of the various isomers is scarce. Thorn- Csanyi´ and Ruhland (1999a) reported on the ttt/ctt-CDT equilibrium ratio during poly- merization/depolymerization of 1,4-polybutadiene and found Kttt/ctt 9 at 298 K in toluene. In another paper, the enthalpy and entropy of isomerization= in methyl- 1 cyclohexane were reported at 1Httt/ctt 10.2 kJ mol− and 1Sttt/ctt 15.3 J 1 1 = − = − mol− K− (Thorn-Csanyi´ and Ruhland, 1999b). Within the temperature range measured only traces of cct-CDT were detected. It is a long established peculiarity of cycloalkenes that the cis isomers are more stable than the trans isomers up to a ring size of 10 carbons (Cope et al., 1960, 1959). From cyclo-undecene onwards, the trans isomers are preferred. Advances in molecular modelling even allow for this behaviour to be calculated (Barrows and Eberlein, 2005). Cope et al. (1960) determined the monoene equilibrium in acetic acid 1 1 at different temperatures and found 1Ht/c 1.7 kJ mol− and 1St/c 10.0 J mol− 1 = = K− . The available equilibrium data is summarized in Figure 4.4, where the dotted lines are extrapolations of the available literature data to the highest temperature achieved in this work (433 K).

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1,5,9-cis,trans,trans-CYCLODODECATRIENE

+H2 +H2 +H2 CDT CDD CDE CDA

Figure 4.5: Lumped reaction scheme for CDT hydrogenation.

4.2 Previous work

Barinov et al. (1974) reported an apparent activation energy for the hydrogenation of CDT on Pd/alumina of approximately 40 kJ/mol, but for rather large catalyst particles (dp 0.25 mm). In the absence of relevant information it was not possible to determine the regime≤ in which the measurements were made. Stuber¨ et al. (1995) carried out batch hydrogenations of undiluted CDT with Pd on alumina in a stirred autoclave at different pressures (0.15-1.2 MPa), stirrer speeds (13- 45 rps), catalyst loadings and temperatures (413-453 K) using Pd on alumina particles. Measurements were reportedly made in the absence of both external mass transfer limit- ations and internal diffusion limitations. A lumped reaction scheme was used to describe reaction kinetics (Figure 4.5) where the various CDT, CDD and CDE isomers are lumped together. Based on this scheme, they proposed a reaction model based on Langmuir- Hinshelwood kinetics. The product of the rate constants and the adsorption constants 1 kK had an activation energy of 49-54 kJ mol− . In the pressure-range studied, hydrogen adsorption was found to have negligible effects on site coverage distribution and was dis- regarded in the kinetic equation. However, the effect of pressure on the selectivity could only be accounted for by increasing the order for hydrogen of the final hydrogenation step to the saturated product to 1.36. As they note themselves, there is no clear chem- ical reason why the hydrogenation of CDE towards CDA should have a higher order in hydrogen compared to the first two hydrogenation steps. In an extension of the work of Stuber¨ et al. (1995), Benaissa et al. (1996) modelled the kinetic behaviour at varying pressure based on all the isomers that were formed. The influence of temperature was not investigated. A reaction scheme involving hydrogena- tion and isomerization as in Figure 4.1 was proposed. Since ccc-CDT was only present in traces it was excluded from the model. Good fit results were reported with a total of 11 hydrogenation rate constants, 10 isomerization rate constants and 8 hydrocarbon ad- sorption constants, excluding dodecane adsorption. It was furthermore observed that the adsorption constants increased with the number of cis double bonds present. However, their results also indicate that trans-CDE should adsorb more strongly than ttt-CDT and the difference in the adsorption constants varied within five orders of magnitude, which seems rather large.

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CHAPTER 4

Scope

In this chapter an attempt will be made to model the behaviour of this complex reactive system in a simple manner. Previous attempts at describing the reaction kinetics were de- pendent on large numbers of parameters, whose physico-chemical relevance are doubtful. The purpose of this investigation is to find the middle road between finding a meaning- ful kinetic model and using a plethora of parameters that only describe the observations. Special attention is given to the isomerization reactions that take place concurrently with the hydrogenation reactions.

4.3 Experimental

Catalyst preparation and characterization

The method used was essentially the same as described in chapter 3.3. Palladium was deposited on a γ -alumina carrier by means of ion-exchange. For this purpose,50 ml of toluene containing a known amount of palladium acetate was slowly added to a vigorously stirred suspension of alumina particles (Puralox SBa-200, Alfa Aesar, Table 4.2) in toluene. After overnight stirring, the excess toluene was decanted. The remaining slurry was yellowish in color and was dried in air at room temperature. After drying for one hour at 333 K, the powder was calcined for one hour at 623 K with a heating 1 1 rate of 2 K min− . Finally, the powder was reduced in a gaseous stream of 100 ml min− containing equal parts of nitrogen and hydrogen during two hours at 453 K, with a heating 1 rate of 2 K min− . The catalyst was passivated overnight at room temperature in air diluted with nitrogen in such a way that only traces of oxygen were present. The palladium surface area was determined with CO chemisorption using a Quanta- chrome Autosorb 1-C. Reduction and evacuation were performed both for two hours at 373 K, the adsorption isotherms were subsequently measured at 308 K. The measured 2 1 palladium surface area was 0.155 m g− which based on the theoretical palladium load- ing and spherical particles, corresponds with a dispersion of 38%.

Materials and Analysis

99 % pure 1,5,9-cis,trans,trans-cyclododecatriene (ctt-CDT) was kindly provided by BASF Aktiengesellschaft. Traces of 1,5,9-trans,trans,trans cyclododecatriene were found to be present. Analytical grade n-decane (Merck) was used as a dilutent resulting in a solution containing 10 wt% CDT. The adiabatic temperature rise after full hydrogenation of pure CDT is approximately 1400 K and to prevent intra-particle temperature gradients the reactant must be diluted. Also, the final product CDA has a melting point of 334 K and thus towards the end of a reaction tends to solidify in the sample line. Analysis of the reaction products was carried out on a Chrompack 9001 Gas Chromatograph equipped

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1,5,9-cis,trans,trans-CYCLODODECATRIENE

8 2 7 10 6

3 5

4 12 13 11

1 9 12

0 5 10 15 20 25 time (min)

Figure 4.6: Example of a chromatogram. Peaks are identified in Table 4.1.

Table 4.1: Chromatogram peak identification. peak number component peak number component 1 solvent 7 ttt-CDT 2 CDA 8 ct-CDD 3 t-CDE 9 isomers of CDD 4 c-CDE 10 ctt-CDT 5 tt-CDD 11 cct-CDT 6 cc-CDD 12 isomers of CDT

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CHAPTER 4

Table 4.2: Catalyst support characteristics (γ -alumina). 2 1 SBET 173 3 m g ± − dpore 4-5 nm 3 1 porosity 0.37 cm g− 3 ρ 1800 kg m− D10 6 µm D50 36 µm D90 95 µm

with a 50 m 0.25 mm 0.2 mm CP-Sil 88 fused silica column and a flame ionization detector. A split× ratio of× approximately 1:100 was used. Helium was used as a carrier gas. The column pressure was maintained at 150 kPa, and the flowrate was 0.45 ml 1 min− . The column was operated isothermally for 20 minutes at 363 K after which all reactants had eluted. After that, the temperature was ramped to 463 K during five minutes. Both the injector and detector were kept at 523 K. The various isomers of the triene, diene and monoene were identified by comparison with isomers obtained from Aldrich. Also a single hydrogenation of 1,5,9-trans,trans,trans-cyclododecatriene was carried out to positively identify 1,5-trans,trans-cyclododecadiene. An example of a chromatogram is given in Figure 4.6. The identification of the various peaks is given in Table 4.1. The unidentified isomers of CDD and CDT never exceeded 5% of the total amount and were not included in the kinetic modelling.

Equipment and Procedure

Hydrogenation experiments were carried out in a 700 ml batch autoclave, equipped with baffles and a self-inducing stirrer. Temperature was controlled within 1 K and pres- sure within 8 kPa. Samples were taken through a sample line and stored for later analysis. Typically, a total reaction volume of 450 ml was used. The catalyst loading was kept constant at 0.036 wt%. Prior to the experiments, the autoclave was flushed three times with nitrogen at high stirrer speeds to remove all air from the system. After reaching the reaction temperature, the desired hydrogen pressure was applied to the autoclave. Unless mentioned otherwise, pressure will refer to the partial hydrogen pressure.

Conversion

The conversion Xdb is defined as the fraction of double bonds in the original mixture that have been hydrogenated or

Cdb,0 Cdb Xdb − (4.1) = Cdb,0 where the total double bond concentration equals

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1,5,9-cis,trans,trans-CYCLODODECATRIENE

Table 4.3: Overview of experiments and reaction conditions.

TPCH2 CCDT DH2,eff DCDT,eff 3 3 9 2 1 9 2 1 K MPa mol m− mol m− 10− m s− 10− m s− 433 0.33 26 348 3.43 0.34 433 0.81 55 348 3.43 0.34 433 1.58 101 348 3.43 0.34 433 2.50 155 345 3.43 0.34 413 2.51 147 356 2.83 0.31 393 2.48 136 366 2.30 0.27 373 2.50 127 375 1.83 0.23 348 2.51 116 390 1.33 0.19

Cdb 3CCDT 2CCDD CCDE (4.2) = + + Physical Properties

The hydrogen solubility was calculated using the expression given by Chaudhari et al. (2002) for mixtures of CDT and n-decane. The diffusivity of hydrogen in n-decane was estimated with the Wilke-Chang equation. The diffusivity of the reactants (CDT, CDD and CDE) in n-decane had to be estimated from a value reported for n-dodecane in n- 9 2 1 octane by Rutten (1992) (D 1.1 10− m s− at 298 K). By correcting for temperature and viscosity according to the= Stokes-Einstein· correlation, an estimate of the reactant dif- fusivity at elevated temperatures was made. The effective diffusivities inside the catalyst particles were taken as one tenth of the bulk diffusivities as a reasonable assumption. The values for the diffusivities and concentrations that were calculated for the various experiments are summarized in Table 4.3.

Parameter estimation

The weighted sum of residuals SSres was minimized using the least squared method of Athena Visual Workbench 8.3 (Stewart and Associates, Madison WI)

n
2 X Ci Ci SSres − (4.3) = σ 2 i 1 i = where the total error σ is defined as

2 2 2 σ σ ( frel Ci ) (4.4) i = f ix + 3 The absolute error σ f ix was estimated at 10 mol m− and the relative error frel at 0.05. The absolute error was based on the lowest concentration that could be detected, and

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CHAPTER 4

1.0

0.8

0.81 MPa 0.33 MPa 1.59 MPa 0.6

0.4 2.5 MPa mol fraction (-)

0.2

0.0 3 -3 1 time*ε (s m cat m liq)

Figure 4.7: Influence of pressure on CDT hydrogenation. Lumped reactant profiles versus normal- ized time at T 433 K. CDT (x); CDD ( ); CDE (2); CDA (3). = 4

the relative error was based on the standard deviation of repeated sample measurements. Errors in sampling time were not taken into account. The total dataset consisted of a total of 117 measurements, each containing nine concentrations, resulting in a total of 1053 datapoints. Of these, 10 were given zero weight as for these points no concentration could be determined.

4.4 Results

In Figures 4.7 and 4.8 the reactant profiles are shown as a function of the product of time and catalyst loading. Note that all trienic, dienic and monoenic isomers are lumped into CDT, CDD and CDE respectively. The concentration profiles remain largely un- changed by temperature: only at 348 K significantly more intermediates are formed. The pressure on the other hand has a larger effect: a decrease in pressure shows a large in- crease in intermediates CDD and CDE formation. Also, the hydrogenation reaction as a whole can be described as first order in the concentration of the double bonds. This is illustrated in Figure 4.9 where plots ln(1 Xdb) versus time result in a straight line. The same behaviour was observed with− changes− in temperature (not shown). Figures 4.10 and 4.12 show the reactants and intermediates divided into their respect- ive isomers according to the scheme in Figure 4.1. There are three CDT isomers, three CDD isomers and two CDE isomers respectively. Figures 4.11 and 4.13 use the same

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1,5,9-cis,trans,trans-CYCLODODECATRIENE

1.0

0.8

0.6

348 K 433 K 413 K 373 K 0.4 393 K mol fraction (-)

0.2

0.0 1 3 -3 time*ε (s m cat m liq)

Figure 4.8: Influence of temperature on CDT hydrogenation. Lumped reactant profiles versus nor- malized time at P 2.5 MPa. CDT (x); CDD ( ); CDE (2); CDA (3). = 4

4

3 ) (-) db

X 2 -ln(1-

1

0 01234 3 -3 -1 time*ε (m cat m liq s )

Figure 4.9: First order behaviour in the concentration of double bonds at T 433 K. P 2.5 MPa ( ); P 1.6 MPa (2); P 0.8 MPa ( ); P 0.33 MPa (X). = =  = = 4 =

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CHAPTER 4

scheme to show the ratio of the isomers. For easy comparison between experiments, the data is shown in the conversion domain. Figure 4.10 shows the effect of pressure on the various isomers. As pressure is de- creased, the amounts of ttt- and cct-CDT that are formed by isomerization increase, while at the same time the yield of the dienic and monoenic isomers increases. By decreasing the pressure from 2.5 to 0.33 MPa, the maximum yield of ttt- and cct-CDT increases from 2.5 to 6 mol% and from 2 to 3.5 mol% while the initial reactant ctt-CDT decreases faster in the conversion domain. In contrast to these differences in the amounts of inter- mediates/isomers that are formed as a result of changes in hydrogen concentration, Figure 4.9 showed that the hydrogenation reaction as a whole can be approximated as first order in the concentration of the double bonds.

4.5 Discussion

Initial behaviour

All experiments have the same initial concentration of ctt-CDT. This makes the ana- lysis of the initial isomerization and hydrogenation reactions of the trienes relatively straightforward. Because initially the consecutive hydrogenations do not yet occur at an appreciable rate, the reaction rates of the trienes can be directly obtained from the slopes of the various CDT isomers at t 0. Figure 4.14 is an Arrhenius plot of the inital rates of formation of cct- and ttt-CDT.= Subtracting these rates from the rate of disappearance of ctt-CDT yields the hydrogenation rate of the latter. As Figure 4.14 shows the initial rates of isomerization of ctt-CDT towards cct-CDT and ttt-CDT have activation energies 1 1 comparable to the hydrogenation reaction; 48 and 44 kJ mol− versus 49 kJ mol− . Figure 4.15 shows the same initial rates as a function of the hydrogen concentration. The initial isomerization rates of ctt-CDT towards cct-CDT and ttt-CDT are lower and have a weaker dependency on the hydrogen concentration than the hydrogenation rate which is first order in hydrogen. This is in line with the behaviour observed in Figures 4.10 and 4.12, where the maxima of the ctt-CDT isomers cct- and ttt-CDT increase with decreasing pressure, and vary little with changes in temperature, with the exception of T 348 K. = Diffusional masking

The Weisz-Prater number 8 for the hydrogenation was calculated based on the initial observed rates rv,obs using Equation 4.5 to determine the extent of internal diffusion lim- itations during the reaction. Two numbers were calculated, one for the reactant and one for hydrogen. The results of these calculations are summarized in Table 4.4. The rates are expressed in moles of hydrogen (or double bonds) consumed per cubic meter of catalyst per second.

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1,5,9-cis,trans,trans-CYCLODODECATRIENE

Table 4.4: Experimental results and calculated Weisz-Prater numbers.

TP initial rate 8CDT 8H2 CDDmax CDEmax 3 1 K MPa mol mcat− s− --%% 433 0.33 270 0.08 0.11 46 65 433 0.81 1080 0.32 0.21 44≥ 60 433 1.58 2000 0.60 0.21 41 55 433 2.50 3700 1.12 0.25 30 30 413 2.51 1850 0.62 0.16 32 30 393 2.48 1170 0.43 0.13 29 33 373 2.50 500 0.21 0.08 33 30 348 2.51 115 0.06 0.03 41 43

2  dp  rv,obs 6 8i (4.5) = Di,effCi The calculations indicate that the reactions are only moderately diffusion limited in both reactant and hydrogen. However, a large margin of error is introduced by uncertain- ties in Di,eff and dp, making interpretation of 8 1 ambiguous. Stuber¨ et al. (1995) performed experiments with decreasing particle sizes≈ and found a true activation energy 1 1 of approximately 50 kJ mol− which is in line with the 49 kJ mol− found here, from which it is concluded that the results presented here are true kinetics. Since the isomeriz- ation reactions are almost an order of magnitude slower than the hydrogenation reactions, these also do not suffer from internal diffusion limitations.

(Apparent) equilibria

Figures 4.11 and 4.13 show the ratios of ttt- and cct-CDT with the reactant ctt-CDT. Both are clearly favoured over ctt-CDT as the value of the ratios increases to more than ten. Extrapolation of the data of Thorn-Csanyi´ and Ruhland (1999b) presented in Figure 4.4 gives Kctt/ttt 0.4 at 433 K. The lowest value found in Figure 4.13 for Kctt/ttt is approximately 0.5;= however the ratio appears to trend down to an even lower value. If ttt-CDT is less active towards hydrogenation than ctt-CDT an overshoot is possible; if the rate of hydrogenation of ttt-CDT cannot keep up with the rate at which it is formed by isomerization from ctt-CDT, the ratio of these two will eventually exceed the equilibrium value. A further illustration of the triene distribution is given in Figure 4.16, where the triene composition is given at varying conversion levels for all experiments. The shaded area represents the ttt-CDT and ctt-CDT equilibrium as estimated from the data of Thorn- Csanyi´ and Ruhland (1999b). At high conversion levels this area is reached and even exceeded, again pointing towards an overshoot. It should be noted however, that at high

73 ‘‘thesis’’ --- 2007/1/31 22:24 --- page 74 (#84)

CHAPTER 4 (-) (-) db db X X 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1

5 0 8 4 0

30 25 20 15 10 16 12

CDE (mol%) CDE t- (mol%) CDE c- (-) (-) (-) db db db X X X 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1

8 4 0 5 0

16 12 25 20 15 10 4 3 2 1 0

-CDD (mol%) -CDD tt (mol%) -CDD ct (mol%) -CDD cc (-) (-) (-) db db db X X X 0 0.2 0.4 0.6 0.8 1 0 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1

80 60 40 20

100 4 3 2 1 0 4 3 2 1 0

-CDT (mol%) -CDT ttt (mol%) -CDT ctt (mol%) -CDT cct

Figure 4.10: Influence of the temperature on the isomer formation at P 2.5 MPa. T 348 K (*); T 373 K (X); T 393 K ( ); T 413 K (2); T 433 K ( ). = = = = 4 = =  74 ‘‘thesis’’ --- 2007/1/31 22:24 --- page 75 (#85)

1,5,9-cis,trans,trans-CYCLODODECATRIENE (-) db X equilibrium with of decrease temperature 0 0.2 0.4 0.6 0.8 1

2.5 2.0 1.5 1.0

CDE cis / trans (-) (-) (-) db db db X X X 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1

8 4 0 8 6 4 2 0

16 12 10 6 5 4 3 2 1 0

CDD tt / ct CDD cc / ct CDD cc / tt (-) (-) (-) db db db X X X 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 1 10 1 0 0.2 0.4 0.6 0.8 1 0.1

100 10

0.1

1000 100 3 2 1 0

CDT ttt / ctt CDT cct / ctt CDT cct / ttt

Figure 4.11: Influence of the temperature on the isomer ratio at P 2.5 MPa. T 348 K (*); T 373 K (X); T 393 K ( ); T 413 K (2); T 433 K ( ). = = = = 4 = =  75 ‘‘thesis’’ --- 2007/1/31 22:24 --- page 76 (#86)

CHAPTER 4 (-) (-) db db X X 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1

0 0

50 40 30 20 10 20 10

-CDE (mol%) -CDE t (mol%) -CDE c (-) (-) (-) db db db X X X 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1

8 4 0 5 0

20 16 12 25 20 15 10 6 5 4 3 2 1 0

-CDD (mol%) -CDD tt (mol%) -CDD ct (mol%) -CDD cc (-) (-) (-) db db db X X X 0 0.2 0.4 0.6 0.8 1 0 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1

80 60 40 20

100 6 5 4 3 2 1 0 4 3 2 1 0

-CDT (mol%) -CDT ttt (mol%) -CDT ctt (mol%) -CDT cct

Figure 4.12: Influence of the hydrogen pressure on the isomer formation at T 433 K. P 2.5 MPa ( ); P 1.6 MPa (2); P 0.8 MPa ( ); P 0.33 MPa (X). = =  = = 4 = 76 ‘‘thesis’’ --- 2007/1/31 22:24 --- page 77 (#87)

1,5,9-cis,trans,trans-CYCLODODECATRIENE (-) db X 0 0.2 0.4 0.6 0.8 1

2.5 2.0 1.5

CDE cis / trans (-) (-) (-) db db db X X X 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1

8 6 4 2 0

12 10 5 4 3 2 1 0 5 4 3 2 1 0

CDD tt / ct CDD cc / ct CDD cc / tt (-) (-) (-) db db db X X X 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 1 10 1 0 0.2 0.4 0.6 0.8 1 0.1

100 10

0.1

1000 100 2 1 0

CDT ttt / ctt CDT cct / ctt CDT cct / ttt

Figure 4.13: Influence of the hydrogen pressure on the isomer ratio at T 433 K. P 2.5 MPa ( ); P 1.6 MPa (2); P 0.8 MPa ( ); P 0.33 MPa (X). = =  = = 4 = 77 ‘‘thesis’’ --- 2007/1/31 22:24 --- page 78 (#88)

CHAPTER 4

10000 ) -1

s 1000 cat -3

100

10 initial rate (mol m

1 0.27 0.29 0.31 0.33 0.35 1000 / RT

Figure 4.14: Arrhenius-plot of the initial hydrogenation rate of ctt-CDT (3); isomerization towards cct-CDT ( ) and isomerization towards ttt-CDT (2), P=2.5 MPa. 4

4000 1000

3000 750 ) ) -1 -1 s s cat cat cat -3 2000 500 -3 (mol m (mol m 1000 250 initial isomerization rate initial hydrogenation rate initial hydrogenation

0 0 0 50 100 150 -3 c H2 (mol m )

Figure 4.15: Influence of hydrogen concentration on the initial hydrogenation rate of ctt-CDT (3); isomerization towards cct-CDT ( ) and isomerization towards ttt-CDT (2), T =433 K. 4

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1,5,9-cis,trans,trans-CYCLODODECATRIENE

ctt-CDT

0 100

10 90

20 80

30 70

40 60

50 50

60 40

70 30

80 20

90 10

100 0 cct-CDT 0 102030405060708090100ttt-CDT

Figure 4.16: Ternary composition of the CDT isomers for all experiments. Xdb 0 0.25 ( ); = − ◦ Xdb 0.25 0.50 ( ); Xdb 0.50 0.75 (2); Xdb 0.75 1.0 (3). Shaded area for equilibrium as estimated= − from the5 data of= Thorn-Cs− anyi´ and Ruhland= (1999b).−

conversions the concentrations of the trienes become low and as a result the exact position of the points in the ternary triangle becomes less accurate. In the dienes tt- and ct-CDD are favoured over cc-CDD, the former slightly more so but it is unclear if true equilibrium has set in. Tentative values for the equilibrium composition of the dienes at 433 K are ct:tt:cc=1:1.7:0.4 within a large margin of error. The monoenes equibrilate at 433 K to trans/cis 2.1, which is in line with the results ≈ of Cope et al. (1960) who found Kt/c=2.08 at T 433 K (see also Figure 4.4). = Kinetic modelling

All the hydrogenation reactions are first order within the experimental range. The fact that all reactions are first order in the organic compounds means that the surface of the catalyst is always far from saturation with the organics. As a result, saturation of the active site will not be considered and also the typical Langmuir-Hinshelwood-Hougen-Watson denominators that are needed to take such saturation into account. The hydrogenation

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CHAPTER 4

) 30 0.33 MPa 300 -3 0.81 MPa ) -3

20 1.59 MPa 200 -CDT] (mol m 2.5 MPa cct -CDT] (mol m

10 100 ctt [ -CDT, ttt [

0 0 3 -3 time*ε (s m cat m liq)

Figure 4.17: Influence of hydrogen concentration on the formation by isomerization of cct-CDT and ttt-CDT at T =433 K. ttt-CDT (3); cct-CDT (2); ctt-( ), lines for modelling results. 4

reactions are all first order in hydrogen and the isomerization reactions have an order in hydrogen lower than one (Figure 4.15). However, even a simple model would consist of 25 parameters (two reaction orders, two activation energies and 21 rate constants). The approach taken here is therefore to first model the behaviour of the CDT isomers only which reduces to a model containing 11 parameters, and use these results to model the behaviour of all isomers. This is possible since the behaviour of the CDT isomers is not influenced by the reactions of the CDD isomers formed from them (see Figure 4.1) but only by their own interactions. With this, the implicit assumption is made that the kinetic behaviour of the CDD and CDE isomers is essentially the same as that of the CDT isomers.

CDT isomers only As seen in the previous section, the initial rates of hydrogenation and isomerization have a different dependency on the hydrogen concentration. Since the cct and ttt isomers can only be formed by isomerization from ctt-CDT, their rate of formation relative to the rate of hydrogenation allows a way to determine the effect of the hydrogen concentration on both reactions, using all available data instead of the initial points only. Based on the scheme in Figure 4.1, the hydrogenation and isomerization reaction rates are defined respectively as

m ri ki C C j (4.6) = H2 80 ‘‘thesis’’ --- 2007/1/31 22:24 --- page 81 (#91)

1,5,9-cis,trans,trans-CYCLODODECATRIENE

400

) 15 -3 348 K 373 K 393 K 300 ) -3 413 K 433 K 10 200 -CDT] (mol m cct -CDT] (mol m

5 ctt 100 [ -CDT, ttt [

0 0 3 -3 time*ε (s m cat m liq)

Figure 4.18: Influence of temperature on the formation by isomerization of cct-CDT and ttt-CDT at P=2.5 MPa. ttt-CDT (3); cct-CDT (2); ctt-( ), lines for modelling results. 4

Table 4.5: Isomerization and hydrogenation of the three CDT isomers. Results of the parameter estimation and the approximate 95% confidence intervals. rate constant at 393 K estimated value units 2 1 3m m k1 (ttt diene) (1.98 0.22) 10− s− m mol− → ± · 2 1 3m m k2+k3 (ctt diene) (2.85 0.25) 10− s− m mol− → ± · 2 1 3m m k4+k5 (cct diene) (1.76 0.24) 10− s− m mol− → ± · 2 1 3n n kiso,1 (ttt ctt) (1.41 0.35) 10− s− m mol− → ± · 1 3n n kiso,2 (ctt ttt) 0 s− m mol− → 2 1 3n n kiso,3 (ctt cct) (1.22 0.30) 10− s− m mol− → ± · 1 3n n kiso,4 (cct ctt) 0 s m mol → − − parameter m 0.94 0.02 - n 0.50± 0.05 - ± 1 Ea 42.5 0.4 kJ mol− ± 1 Eaiso 42.1 1.3 kJ mol ± −

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CHAPTER 4

+2 H2 tt-CDD

r12

ct-CDD CDA r13

cc-CDD r14

Figure 4.19: Direct route of the CDD isomers towards cyclododecane based on non-equilibrium adsorption of the CDE isomers.

n riso,i kiso,i C C j (4.7) = H2 where the apparent rate constants are exponentially dependent on temperature accord- ing to    Eai 1 1 ki ki,ref exp (4.8) = − R T − Tref

with Tref 393 K. The results of the fit are shown in Table 4.5 and Figures 4.17 and 4.18. With the= exception of T 348 K, P 2.5 MPa the model conforms well to the data. In line with what was observed= in Figure= 4.14 for the initial rates only, the hydro- genation and isomerization reactions almost have the same activation energy. ’Back- isomerization’ of ttt- and cct- towards ctt-CDT apparently is not significant. Close inspection of Figures 4.11 and 4.13 showed that for the trienes no (apparent) equilib- rium sets in within the measured domain, and as a result ttt ctt cct simplifies to ttt ctt cct. As expected from the initial rate data shown↔ in Figure↔ 4.15, the hydro- genation← is→ first order in the concentration of hydrogen, and the isomerization has order half in hydrogen. A similar result was reported by Smith (1966) on the hydrogenation of cyclododecene on a colloidal Pt catalyst: a decrease in pressure affected the hydro- genation reaction more than the isomerization, hence a lower order in hydrogen for the isomerization.

All isomers Using the parameters determined from the trienes only, an attempt was made to fit the behaviour of all the isomers once again based on the scheme in Figure 4.1. That is, the whole data set was fitted using the parameters determined from the CDT-isomers. The variation with temperature of the ratio of riso,9 over riso,10 (isomerization of the

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Table 4.6: Isomerization and hydrogenation of all isomers. Results of the parameter estimation and the approximate 95% confidence intervals. Apparant rate constants at 393 K. Model 1 Model 2 Model 3 2 1 3m m k1 (ttt tt) 2.0 2.0 2.0 10 s m mol → ≡ ≡ ≡ · − − − k2 (ctt tt) 0.4 0.1 0.4 0.1 0.2 0.2 ” → ± ± ± k3 (ctt ct) (2.9-k2) (2.9-k2) (2.9-k2)” → ≡ ≡ ≡ k4 (cct ct) 1.7 0.4 1.7 ” → k5 (cct cc) (1.8-k4) (1.8-k4) (1.8-k4)” → ≡ ≡ ≡ k6 (tt t) 1.0 0.5 0 0 ” → ± k7 (ct t) 0 1.3 0.2 1.4 0.3 ” → ± ± k8 (ct c) 0.7 1.0 0.2 1.0 0.2 ” → ± ± k9 (cc c) 9.8 1.8 0 0 ” → ± k10 (t sat) 0 0.5 0.1 0.4 0.4 ” → ± ± k11 (c sat) 2.6 0.2 0.0 0.2 0.7 ” → ± ± 5 1 3o o k12 (tt sat) 5.8 1.8 0 10 s m mol → ± ≡ · − − − k13 (ct sat) 7.8 1.1 10.8 0.8 ” → ± ± k14 (cc sat) 0 0 ” → ≡ 2 1 3n n kiso,1 (ctt ttt) 1.4 1.4 1.4 10 s m mol → ≡ ≡ ≡ · − − − kiso,2 (ttt ctt) 0 0 0 ” → ≡ ≡ ≡ kiso,3 (cct ctt) 1.2 1.2 1.2 ” → ≡ ≡ ≡ kiso,4 (ctt cct) 0 0 0 ” → ≡ ≡ ≡ kiso,5 (ct tt) 3.9 3.9 4.2 3.7 16.8 8.3 ” → ± ± ± kiso,6 (tt ct) 4.9 5.1 8.9 5.0 30.8 11.3 ” → ± ± ± 3 1 3n n kiso,7 (cc ct) 11.0 13.0 1.6 8.3 1.0 10 s m mol → ± ± · − − kiso,8 (ct cc) 54.1 6.7 64.7 39.9 ” → ± 2 1 3n n kiso,9 (c t) 60.5 21.1 29.5 15.8 50.4 10− s− m mol− → ± 1 1 ± 1 · kiso,10 (t c) kiso,9 K − kiso,9 K − kiso,9 K − ” → ≡ t/c ≡ t/c ≡ t/c m 0.94 0.94 0.94 - n ≡ 0.50 ≡ 0.50 ≡ 0.50 - o ≡ ≡ 2.00 ≡ 2.00 - ≡ ≡ 1 Ea 42.5 42.5 42.5 kJ mol− ≡ ≡ ≡ 1 Eaiso 42.1 42.1 42.1 kJ mol− SSres ≡765≡ 411≡ 422 - # of pars. 13 16 14 -

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monoenes) was fixed using the equilibrium data of Cope et al. (1959), leaving only kiso,9 to be determined and further reducing the number of parameters. Lack of equilibrium data made it impossible to use this approach with the dienic species. Results of this fit are given in Table 4.6 as Model 1 and are shown in Figure 4.20 as the dashed lines. The behaviour of the dienes is captured reasonably well at equal pressure but deviates significantly at decreasing temperature. This is to be expected since the model does not account for shifting of the diene equilibria with temperature. Also, the amounts of cis and trans-cyclododecene formed do not change with changing pressure as observed experimentally and reach the same maximum regardless of operating conditions. The approach taken here is based on the implicit assumption of fast reactant adsorption-desorption. Reactants in bulk are thought to always be at equilibrium with the catalytic surface. However, the results appear to indicate that for the monoenes this is not the case. Lowering the pressure increases the intermediate monoene yield which can- not be accounted for if all consecutive hydrogenations have the same dependency on the hydrogen concentration. Some of the monoenes that are formed from hydrogenation of the dienes are thus not transported back to the bulk but stay on the catalyst surface. This opens up a direct route from the dienes to the saturated cyclododecane, which then must be second order in hydrogen, as compared to the first order of the stepwise hydrogenation. This is depicted in the scheme in Figure 4.19 and adds three rate equations:

o ri ki C C j (4.9) = H2 where o equals two. Non-equilibrium adsorption of intermediates has been observed in the hydrogenation of aromatic amines, see for example Jian et al. (1997) and in the hydrogenation of α-methyl styrene on a Pd/alumina catalyst (Ahn et al., 1986). Ahn et al. (1986) concluded that both adsorption and surface reaction determine the hydro- genation reaction, the former slightly more so. As a result, there is some evidence that non-equilibrium adsorption may be needed to describe hydrogenation reactions. A second parameter estimation with a shunt reaction from CDD to CDA, labelled model 2, was therefore carried out. The addition of three parameters to Model 2 reduces the sum of squared residuals by more than 40% and gives a qualitative description of both the influ- ence of temperature and pressure on the behaviour of the isomers. In fact, it is possible to simplify Model 2 by only considering the direct route from ct-CDD towards cyclodo- decane (r13) which only slightly increases the sum of squared residuals (Model 3). The apparent lack of fit for cis,cis-CDD could be caused by the exclusion of unidentified iso- mers which contribute at most 5% to the total amount. The rate of isomerization increases on the order of 1:5:10 for CDT, CDD and CDE respectively regardless of the model used to fit the data (Table 4.6). This could well mean that as the molecule becomes less strained, ’flipping’ of a double bond gets easier. The rate constants concerning the isomerization of the monoenes have to be considered with caution however: as Figures 4.11 and 4.13 show, trans- and cis-CDE are formed almost at equilibrium. This makes determination of the isomerization rate difficult, if not

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Concentration (mol m-3) 10 15 20 10 20 100 200 300 400 0 5 0 0 1 2 3 4 67 56 8 5 s m 3 cat cct ctt ttt m -3 liq 20 40 60 80 100 10 15 20 25 0 20 40 60 80 0 5 0 cc tt ct 120 160 20 40 60 80 40 80 0 0 t c

Figure 4.20: Fit of kinetic model to experimental data. Dashed lines for Model 1, solid lines for Model 3. T 433 K, P 2.5 MPa (1); 433 K, 15.8 MPa (2); 433 K, 0.81 MPa (3); 433 K, 0.33 MPa (4); 413= K, 2.5 MPa= (5); 393 K, 2.5 MPa (6); 373 K, 2.5 MPa (7); 348 K, 2.5 MPa (8). 85 ‘‘thesis’’ --- 2007/1/31 22:24 --- page 86 (#96)

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1. H2 + 2* 2H*

2. A + * A* B + * B*

3. A* + H* AH* + * 2 rkKK33= HA[A] [H 2* ]θ B* + H* BH* + *

4. AH* BH* rkKKK44= HAHA[A] [H 2* ]θ

2 5. AH* + H* P + 2* rkKKK55= HAAH[A][H 2* ]θ BH* + H* P + 2*

1 θ* = 1+++KKiKiHiiH [H]22∑∑ [] [] [H] iAB==,, iAB

Figure 4.21: Surface reaction steps for hydrogenation and isomerization. A and B represent cis and trans double bonds, P the saturated C-C bond.

impossible. On the other hand, fast isomerization would mask any difference in the rate of hydrogenation of both isomers.

Isomerization vs hydrogenation

The different orders in hydrogen for the hydrogenation (1st order) and isomerization (0.5 order) of CDT indicate that the reaction rates are governed by different rate determin- ing steps. If one considers the hydrogenation / isomerization of a double bond in terms of the classic Horiuti-Polanyi approach it becomes possible to define the following possible rate determining steps (Figure 4.21):

1. dissociative adsorption of hydrogen

2. adsorption / desorption of the reactants (cis and trans or A and B)

3. addition / abstraction of a single hydrogen, creating a surface intermediate

4. isomerization of the surface intermediate

5. hydrogenation: addition of a second hydrogen to the surface intermediate followed by rapid product desorption

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1,5,9-cis,trans,trans-CYCLODODECATRIENE

R2 CH CH R1 ‘cis’ ‘trans’ R2 CH CH R1 3 H R1 43H R1 2 fast C C fast 2 slow +H* +H* H R R2 H R2 CH CH R1 2 R2 CH CH R1 C C -H* -H* * * * H H *

+H* 5 slow 5

R1CH2 CH2R2

Figure 4.22: Proposed mechanism for the hydrogenation and isomerization of CDT. Numbers refer to the reaction steps in Figure 4.21.

Making either step 3 or 4 rate determining would result in a 0.5 order in hydrogen for the isomerization. However, if formation of the surface intermediate (step 3) were rate-limiting for the isomerization, it would at the same time determine the rate of the hydrogenation step 5 and give the latter a 0.5 order in hydrogen as well. This is clearly not the case; the insertion of the second surface hydrogen into the surface intermediate (step 5) determines the rate hydrogenation, resulting in a first order.

4.6 Conclusions

The hydrogenation and isomerization of cyclododecatriene and its isomers is an ex- cellent example of a very complex reaction system, ruled by both reaction kinetics and thermodynamic equilibria. The approach taken by Benaissa et al. (1996) towards this sys- tem was one of brute force, describing the system by a multitude of parameters yielding a good fit of their data. However, as a result of this process the actual values of these parameters have little, if any, physical meaning. In contrast, in this Chapter a bottom-up approach was taken, starting from initial rate data, followed by a description of the first reaction steps and finally a model describing the whole system, all the while making use of available thermodynamic data.

Hydrogen and intermediate formation

A paradox in the results presented here is in the effect of the hydrogen pressure / concentration on the formation of intermediates. The results show that an increase in intermediates formation is linked to the formation of certain isomers. At the same time, the overall reaction appears perfectly first order in the double bond concentration. On

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the one hand certain double bonds appear more or less active, while taken as a whole all double bonds appear to exhibit the same reactivity! This property of cyclododecatriene makes it an excellent molecule for catalyst performance testing. It can be used for a simple determination of catalyst activity or as a way to determine catalyst selectivity. The yield of intermediates can be used as an easy way to compare the effectiveness of different catalysts.

Isomerization vs hydrogenation

It has been shown that the rate of isomerization and hydrogenation of the double bonds in CDT is governed by different rate determining steps. This is illustrated in Figure 4.22. The rate at which the extremity ’R2’ rotates along the free C-C axis in fact determines the rate of isomerization. This could also explain why the rate of isomerization increases as the number of double bonds in the molecule decreases. As the strain in the molecules de- creases, rotation of ’R2’ will become easier and hence faster. Although the hydrogenation is a bimolecular reaction between an adsorbed complex and a surface hydrogen, it is in general much faster than the mono molecular isomerization. Only at hydrogen concentra- 3 tions below approximately 0.5 mol m− is the rate of isomerization of ctt-CDT towards ttt- and cct-CDT higher than the hydrogenation rate (cf. Table 4.6).

Reactant non-equilibrium adsorption

The reactant ctt-CDT is some 30% more reactive towards hydrogenation than its iso- mers ttt- and cct-CDT, whereas these isomers appear thermodynamically more stable. The kinetic behaviour of the three CDT isomers was used to develop a kinetic model comprising all eight isomers. It was found that the measurements could only be accur- ately described by assuming non-equilibrium adsorption of the CDE isomers. I.e. the molecules tend to ’stick’ more to the catalyst surface if only a single double bond is present. This could be caused by the absence of strain which is so clearly present in the dienes and trienes.

4.7 Nomenclature

3 C concentration, mol m− 2 1 D diffusivity, m s− 1 Ea activation energy of hydrogenation, J mol− 1 Eaiso activation energy of isomerization, J mol− dp catalyst particle diameter, m 1 1Hi/j enthalpy of isomerization, J mol− 1 k reaction rate constant, s− P pressure, MPa

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3 1 r reaction rate, mol mcat− s− 3 1 rv,obs initial reaction rate, mol mcat− s− 1 1 R gas constant, 8.314 J mol− K− 1 1 1Si/j entropy of isomerization, J mol− K− SSres weighted sum of squared results, dimensionless T temperature, K Xdb double bond conversion, dimensionless 3 3 cat volumetric catalyst loading, mcatm reactor− 8 Weisz-Prater number, dimensionless

Bibliography

Ahn, B., Smith, J., McCoy, B., 1986. Dynamic hydrogenation studies in a catalytic slurry reactor. AICHE Journal 32(4), 566–574. Anet, F., Rawdah, T., 1979. The conformation of cis-cyclododecene, evidence from dy- namic NMR spectroscopy and iterative force-field calculations. Tetrahedon Letters 22, 1943–1946. Barinov, N., Makarovskii, I., Mushenko, D., Blandin, Y. V., 1974. Hydrogenation of cyc- lododecatriene on a palladium catalyst. Zh. Prikl. Khim. 47(12), 2711–2714. Barrows, S., Eberlein, T., 2005. cis and trans isomers of cycloalkenes. J. Chem. Edu. 82(9), 1334–1339. Benaissa, M., Le Roux, G., Joulia, X., Chaudhari, R., Delmas, H., 1996. Kinetic mod- eling of the hydrogenation of 1,5,9-cyclododecatriene on Pd/Al2O3 catalyst including isomerization. Ind. Eng. Chem. Res. 35, 2091–2095. Bruckle,¨ I., Thornton, J., Nichols, K., Strickler, G., 1999. Cyclododecane: technical note on some uses in paper and objects conservation. JAICDE 38(2), 162–175. Chaudhari, R., Jaganathan, R., Mathew, S., 2002. Hydrogenation of 1,5,9- cyclododecatriene in fixed-bed reactors: down- vs. upflow modes. AIChE Journal 48(1), 110–125. Cope, A., Moore, P., Moore, W., 1959. Relative stabilities of cis and trans cyclononene, cyclodecene, cycloundecene and cyclododecene. J. Chem. Soc. 81(12), 3153. Cope, A., Moore, P., Moore, W., 1960. Equilibration of cis- and trans-cycloalkenes. J. Chem. Soc. 82(7), 1744–1749. Jian, M., Kapteijn, F., Prins, R., 1997. Kinetics of the hydrodenitrogenation of ortho- propylaniline over NiMo(P)/Al2O3 catalysts. J. Catal. 168, 491–500.

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Pawar, D., Davis, K., Brown, B., Smith, S., Noe, E., 1999. Conformational study of trans-cyclododecene by dynamic NMR spectroscopy and computational methods. J. Org. Chem. 64(13), 4580–4585. Rawdah, T., El-Faer, M., 1995. Conformational analysis of cis,trans,trans-1,5,9- cyclododecatriene. Tetrahedon Letters 36(19), 3381–3384. Rawdah, T., El-Faer, M., 1996. The static and dynamic conformational properties of cis,cis,trans-1,5,9-cyclododecatriene. Tetrahedon Letters 37(24), 4267–4270. Rutten, P., 1992. Diffusion in liquids. Ph.D. thesis, TU Delft. Smith, G., 1966. Mode of hydrogen attack during hydrogenation of olefins - hydrogena- tion and isomerization of cis- and trans-cyclodecene. J. Catal. 5(1), 152–157. Streck, R., Hartig, H., 1990. Herstellung und Anwendungsmoglichkeiten¨ von cyclodo- decatrien und cyclooctadien. Chem. Ing. Tech. 62(11), 888–891. Stuber,¨ F., Benaissa, M., Delmas, H., 1995. Partial hydrogenation of 1,5,9- cyclododecatriene in three phase catalytic reactors. Catal. Today 24(1-2), 95–101. Thorn-Csanyi,´ E., Ruhland, K., 1999a. Quantitative description of the methathesis poly- merization/depolymerization equilibrium in the 1,4-polybutadiene system, 1. Influence of feed concentration and temperature. Macromol. Chem. Phys. 200(7), 1662–1671. Thorn-Csanyi,´ E., Ruhland, K., 1999b. Quantitative description of the methathesis poly- merization/depolymerization equilibrium in the 1,4-polybutadiene system, 3. Influence of the solvent. Macromol. Chem. Phys. 200(12), 2606–2611. Tolkien, J., 1954. In: The Fellowship of the Ring. Allen & Unwin, Ch. The Shadow of the Past.

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Chapter 5

Introduction to Edible Oils and Hydrogenation Processes

’In animals which do not posses a rumen (pigs, geese, rodents) the fatty acids derived from plants are taken up by the intestines and used unaltered in the deposition of storage fat. These storage fats are therefore of a soft consistency. In ruminants, however, the plant fatty acids in the rumen are extensively hydrated (saturated) by the bacterial rumen flora and the saturated fats are then absorbed from the intestine and built into the storage fats. Beef fat is therefore of a firm consistency. Since these alterations are brought about by bacteria, and considering that 60-90% of beef protein is also of bacterial origin, con- sumption of a steak is, in the final analysis, at the expense of bacteria, and consumption of a pork chop at the expense of fodder plants.’ (Schlegel, 1993)

5.1 The history of edible oil use

The use of animal fats (other than the eating of Mammoth-steak) goes back to the dawn of human civilization. Wooden sticks dipped in burning fat provided light to the Paleolithic artists working in dark caves creating images that still inspire awe to contem- porary viewers. The ancient Egyptians created candles by burning a reed in animal fat and sesame oil had religious significance in ancient Egypt. Poppyseed has been found in the remains of Bronze-age bread; linseed and cottonseed have been found in Bronze-age settlements together with grindstones. Soybeans are mentioned as a source of oil in a 2838 B.C. Chinese document. Up to the middle of the 19th century, tallow and butter (i.e. animal fats) were the most important edible fats in Europe, lard and vegetable oils playing only a minor role. Towards the end of the 19th century, the production of vegetable oil by hydraulic pressing and solvent extraction was introduced. This process gave relatively high oil yields but necessitated posttreatment of the oils by neutralisation, bleaching, and deodorization. Margarine was developed between 1866 and 1869 by a Frenchman with the delightful name of Hippolyte Mege´ Mouries in response to a contest initiated by Emperor Louis Napoleon II for a reasonably priced fat that could be used as a replacement for butter. He

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processed beef tallow (predominantly stearate) until he obtained a liquid fat, which after mixing with skimmed milk gave a solid product. A patent was issued on 20 October 1869 in Paris (# 86480): ’Demande d’un brevet d’invention de quinze ans pour la production de certains corps gras d’origine animale’. The invention of margarine gave further impetus to the oil-processing industry. With the discovery of oil hydrogenation (hardening) at the beginning of the 20th century, liquid oils could be converted into spreadable, consistent fats. In the 1930s, interesterification and fractionation were developed as further methods to modify the consistency of oils and fats.

5.2 Occurrence

Fats and oils are constituents of all forms of plant and animal life. However, few plants and animals produce oil in sufficient quantities to be commercially significant. Nowadays, most vegetable oils are derived from the seeds of annual plants such as soybean, cotton- seed, peanut, sunflower, corn and rapeseed. Another major source are the oil-bearing fruits and nuts of trees such as coconut, palm, palm kernel and olive. Oil content of the materials may vary between 3 and 70 % of the total weight of the seed, nut, kernel or fruit. Meat fats are derived almost exclusively from three kinds of domestic animals: hogs, cattle and sheep. The bulk of the world’s milk-fat production consists of butterfat from cow’s milk. Fats and oils are a unique class of agricultural product in that a high degree of interchangeability among them is possible for many products and uses. However, for a satisfactory substitution, additional processing and/or blending may be necessary. Where we speak of oils and fats, we only distinguish between their state at ambient temperature: liquid or solid. This means that a product which is called a fat in moderate climates can be called an oil in a relatively warm climate. Until the development of the hydrogenation process, animal fats in the form of lard, tallow and butter were the major source of edible fats. Hydrogenation made it possible for vegetable oils to be converted into fat forms that people were accustomed to, with greater flavor stability at lower cost. It also allowed for the conversion of highly unstable (oxidative) fish oils into products that could be stored for extended periods. The total annual world production of oils and fats in 2005 is expected to be in excess of 120 million tons, of which 80% is intended for human consumption and approximately 5% is fed to livestock (Gunstone and Hamilton, 2001). The remainder is used by the oleochemical industry.

5.3 The chemistry of fats and oils

Fats are mixtures of fatty acid triglycerides. The most common fatty acids in vegetable oil are the octadecanoic acids, but also shorter and longer chainlengths of up to 24 carbon atoms are possible. A list of common fatty acids and their melting points is given in Table

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Table 5.1: Names and designations of some common fatty acids. See also Figure 5.1. fatty acid trivial name short acid mp tri-glyceride mp (◦C) (◦C) tetradecanoic acid myristic c14:0 54.4 57.0 hexadecanoic acid palmitic c16:0 62.9 65.5 octadecanoic acid stearic c18:0 69.6 73.0 cis-9-octadecenoic acid oleic cis c18:1 16.3 5.5 trans-9-octadecenoic acid elaidic trans c18:1 45.0 42.0 cis,cis-9,12-octadecadionic linoleic c18:2 -5.0 -13.1 cis,cis,cis-9,12,15- α-linolenic c18:3 -11.0 -24.2 octadecatrienoic acid

Table 5.2: Fatty acid composition in wt% of some important oil feedstocks. fatty acid sunflower oil soybean oil palm oil myristic 0 0 0-15 palmitic 3-7 7-11 22-46 stearic 1-5 2-6 1-5 oleic 36-72 15-33 36-68 linoleic 13-45 43-56 2-20 α-linolenic 0-1 5-11 0

5.1. Figure 5.1 shows the most important octadecanoic acids. The degree of unsaturation of the fatty acids in the triglyceride determines the melting properties of the fat or oil as can be seen from the different melting points. The fatty acid composition of some important oil feedstocks is given in Table 5.2. All natural oils and fats are unsaturated up to a certain degree, and often an increase in saturation is needed in the desired product. In fats of vegetable origin virtually all double bonds are in the cis configuration. In fats derived from ruminants (e.g. milk fat) up to 5% of the fatty acids will be in the trans configuration. The names of the fatty acids can be confusing; α-linolenic acid is a member of the so- called ω-3 fatty acid group, which is currently widely being used in food products health claims. ω-3 fatty acids are a family of polyenoic acids with three or more unsaturated centres separated from each other by one carbon and having the first unsaturated centre three carbons from the last carbon.

5.4 Analysis of fats

The elucidation of the chemical nature of fats was initiated by Scheele, who produced glycerol from olive oil around 1780. Chevreul subsequently (ca. 1815) recognised that

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O

OH stearic acid O

OH

oleic acid O

OH

linoleic acid O

OH

α-linolenic acid

Figure 5.1: Some common fatty acids. See also Table 5.1.

fats were predominantly esters of fatty acids and glycerol. One of the earliest chem- ical characterization methods is the so-called Iodine Value (IV) which is defined as the amount of iodine (in grams) that has to be added to 100 g of fat for complete saturation. Although it gives no information on the nature of the fats (mono, di, tri-unsaturated or cis and trans), it is still in use as a simple way to define the (un)saturation of a fat. Sunflower oil for example has an IV of 125-136 and soybean oil has an IV of 120-141. Chemical determination of the Iodine Value requires a titration which is not practicable in the on- line monitoring of industrial processes. Other ways have been developed that allow easy determination of the Iodine Value, such as the refractive index of the oil or the use of spectrophotometry (Thomaidis and Georgiou, 2000). Attempts to determine the compos- ition of fats directly with HPLC have been reported (Macher and Holmqvist, 2001) but have not yet caught on. Before the invention of Gas Chromatography (GC), the composition of the various fatty acids in vegetable oils was determined by the so-called lead salt-ether separation. In this method, the difference in solubility of lead salts of fatty acids in ether was used. A disadvantage of this method was that trans fatty acids (’iso’oleic) that are formed during hydrogenation partly appeared as saturated fatty acids. GC was in fact developed to solve an analytical problem with short chain fatty acids, and is now the most commonly used tool in the analysis of fats (Ackmann, 2002).

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Determination of the total amount of trans fats can be carried out with Fourier- Transform Infrared Spectroscopy. However, the presence of conjugated dienes or trienes can skew the results. This method also does not give information on the fatty acid dis- tribution (Mossoba et al., 2003). Presently in industrial practice the composition of a fat is determined by a single GC-run, using a 50 or 100 m highly polar capillary column. Sometimes a pre-fractionating step is carried out by silver-ion chromatography where the trans and cis isomers are separated. The major disadvantage of GC analysis is that the fats must be saponified first, followed by esterification of the fatty acids. Direct GC analysis of fats is impossible because of the high boiling points of tri-glycerides. Thus all inform- ation regarding the position of the fatty acids on the glycerol ’backbone’ is lost with GC analysis. A comprehensive review of the available methods and their limitations can be found in Mossoba et al. (2003).

5.5 The invention of edible oil hydrogenation

The chemistry involved in hydrogenation is simple: hydrogen molecules catalytically react with unsaturated double carbon bonds, forming saturated carbon bonds. At the same time, the natural occurring cis double bonds can isomerize into the thermodynamically more stable trans double bonds. Liquid phase hydrogenation, which resembles the tech- nology of today, was invented on 27 February 1901 by Wilhelm Normann while working for Leprince & Sieveke in Herford, Germany. It was first revealed in a German patent (Normann, 1902) and a few months later in a British patent (Normann, 1903). These pat- ents described a process whereby a finely divided metal catalyst was mixed with oil, then hydrogen was introduced into the mixture. Normann demonstrated not only that hydro- genation in the liquid phase was possible, but that an inexpensive metal such as nickel could be used instead of the previously used precious metals. The first commercial application of the Normann invention occurred in 1905-1906 and involved the processing and hydrogenation of whale oil. The British firm of Joseph Crossfield and Sons obtained the rights to the Normann patent in 1908 and subsequently sold them to Procter and Gamble in 1909. In 1911, Procter and Gamble began selling of hydrogenated cotton oil that was produced using liquid-phase hydrogenation. In 1907, the Crossfield and Sons factory produced 2000 kg of hydrogenated oil per day. As early as 1914 some 24 factories worldwide hydrogenated some 200-230 million kg per year and by 1918 the number of factories had increased to 65 (Normann, 1922). There are two main reasons for hydrogenating oil. One is to change the naturally occurring oils and fats into physical forms with consistency and handling characteristics required for functionality. Oil characteristics such as creaming capability, frying stability, (sharp) melting properties can be tailored in this way. Another reason for hydrogenation is to increase oxidative stability. Flavour stability is necessary to maintain product ac- ceptability for prolonged periods after processing, packaging, shipping and the final use of the product. Poly-unsaturated fatty acids oxidize rapidly, giving oil a rancid taste.

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Figure 5.2: First fat-hardening autoclave as constructed by Wilhelm Normann. Note the impeller which is placed too low according to modern insights.

5.6 Hydrogenation processes

After having succesfully hardened oil with laboratory equipment, Normann was faced with the difficulty of making the process work on a larger scale. In March 1903 he wrote to a friend: ’In der Raffinationssache habe ich die ’Freude’ erlebt, dass die Reaktion im Reagenzglase sehr schon¨ ging und im Grossen gar nicht. Jetzt kann ich wieder von vorn anfangen und herausknobeln, woran es liegt’. It was soon found that hydrogen dissolved poorly into the oil, and the beneficial effect of operating under enhanced pressure was discovered. A photograph and diagram of the first autoclave constructed by Normann while at Leprince & Sieveke shows a striking resemblance to modern equipment (Fig. 5.2). Vigorous stirring also increased reaction rates, but a patent application was denied by the Imperial Patent-office, on the grounds that the matter was obvious. If only more bureaucrats had such in-depth knowledge of transport phenomena! As Normann himself noted in 1922, ’fat hardening consists of the intimate mixing of oil and catalyst with hydrogen’ (Normann, 1922). Historically, two types of processes can be distinguished: the ’Normann process’ and the ’Wilbuschewitsch process’. In the former, hydrogen is dispersed into a mixture of oil and catalyst. Current batch processes are of the Normann type. The latter process disperses oil and catalyst into a hydrogen atmosphere. An example of a modern Wil- buschewitsch type process is the Buss loop reactor which is usually operated batchwise. A comprehensive overview of various industrial and laboratory reactors can be found in

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Edvardsson and Irandoust (1994). The conventional reactor technology applied today is based on slurry systems, often operated in a batch or semi-batch mode. In these slurry systems, the solid catalyst is provided in a powder form (comprising 0.05-0.1 % of the weight of the oil) and mixed with the liquid reactants in a stirred tank with a volume of 5- 20 m3. The solid particles as well as the gas bubbles are kept suspended within the liquid phase by means of mechanical agitation and chaotic mixing, e.g. via a stirrer. The reactor is heated by heating coils to 493-563 K at 1-6 bar pressure. However, reaction does not take place at constant temperature and pressure but rather the reactor is slowly pressurized during heat-up. As a result, conventional fat hardening is carried out according to certain ’recipes’ with little in the way of actual rationalization. Two modes of operation exist: recirculation and dead-end. In the recirculation sys- tem, hydrogen is sparged at the bottom of the reactor, withdrawn from the headspace and returned. In the dead-end system, hydrogen is dispersed by means of a vortex creating agitator (O’Brien, 1998). An example of a dead-end process is given in Figure 5.3. Some- times the liquids are transported by way of pressure differences instead of by pumps. After hydrogenation is completed to the desired degree, the oil is filtered to remove the catalyst (which may be reused). Finally, post-bleaching is carried out to remove any traces of nickel, undesired colors and oxidation products. Bleaching conditions are usu- ally addition of 0.1 to 0.2 wt% bleaching earth together with phosphoric acid or citric acid as chelating agent. Health legislation demands levels of nickel below 0.1 ppm. After bleaching, the bleaching earth is filtered and the oil then pumped to a storage tank; it may later be blended with other harder or softer fats or oils to make margarine or shortening. Industrial slurry hydrogenation thus results in two separate waste streams: a catalyst-rich oil slurry which may be reused several times and bleaching earth contaminated with nickel salts that has to be disposed of as chemical waste, the latter on the order of 1-2 kg per ton of product. An example of a slurry hydrogenation is given in Figure 5.4 where the changing oil composition, the iodine value and the total amount of trans are given versus time. Note how the poly-enes are quickly converted into monoenes, while at the same time isomer- ization into trans isomers takes place.

5.7 Hydrogenated oil products

Since literally hundreds of consumer products exist, trying to manufacture each one by tailored hydrogenation would be a logistical nightmare. As a result, most manufactur- ers use a basestock system with a limited number of hydrogenated stock products from which different products can be made by blending (O’Brien, 1998). The three elementary basestocks are

brush hydrogenated. To increase the shelf-life of a liquid oil, only linolenic fats are • hydrogenated. These are sensitive to oxidation resulting in taste degradation.

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hydrogen catalyst filter press

spent catalyst

bleaching earth

citric acid

reactor

post-bleach tank

product

Figure 5.3: A commercial slurry oil hydrogenation process.

partially hydrogenated. These are used in blends for margarine, frying-shortenings, • fillings and other products requiring a sharp melting point and flavor stability, while having the right firmness at room temperature. An example of a partially hydrogen- ated soybean oil is given in Table 5.3.

low IV hard fats. Almost all double bonds are saturated. Can be called stearins if • the iodine value approaches zero.

An average basestock system can be based on as little as seven hydrogenated oils, with the iodine value ranging from 109 to 8. This allows the manufactures to produce almost all required specifications by blending two or more basestocks.

5.8 Health issues associated with hydrogenated oil

A high intake of saturated fats has been associated as an important risk factor for coronary heart disease (CHD) as early as the 1950s (Keys, 1980). Declining rates in CHD are thought to be caused by improved composition of foodstuffs, reduced cigarette smoking and better medical treatments. However, increasing obesity levels and the asso- ciated type 2 diabetes are still to benefit from reduced saturated fatty acid intake (Mann, 2002).

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60 140

120 50 cis -C18:2

trans- C18:1 100 40 cis- C18:1 C18:0 80 30 60 Iodine Value

amount (wt%) 20 40

10 trans- C18:2 20

0 0 0 60 120 180 0 60 120 180 time (min) time (min)

Figure 5.4: Slurry hydrogenation of soybean oil at T 373 K and P 20 Mpa: composition, iodine value and amount of trans vs time. Note that palmitic= acid (c16:0, 11= wt%) must be included to complete the mass balance.

Table 5.3: Characteristic data of a partially hydrogenated soybean oil (Gunstone and Padley, 1997). soybean oil product mp, ◦C n.a. 35 IV 132.8 80 trans, wt% 0 28 c18:0 4 12 c18:1 23.1 63 c18:2 53.1 13 c18:3 7.4 0.1

When consumed in large enough amounts, trans fatty acids increase LDL-cholesterol and decrease HDL-cholesterol, which is expected to increase the risk of CHD. A recent study concluded that a high intake of trans fatty acids, regardless of their source, contrib- utes to the risk of CHD (Oomen et al., 2001). Combining the results of five large cohort studies, it was found that a 2% increase of trans fats in the total energy intake increases the relative risk of CHD to 1.25 0.14. ± Recent research has shown that saturated and trans fats trigger liver metabolism, res- ulting in an upsurge in the production of precursors to LDL cholesterol (’bad’ cholesterol) to those of manufactured ones. Typically, between 2 and 5 % of ruminant fat is in the trans form. Since the ratio of trans and saturated fats from animal sources are fairly constant, it is impossible to distinguish between the health effects of both types (Aro, 2001). Ac- cording to European guideliness, less than 2% of the daily energy intake should consist of trans fatty acids (Luzzi and James, 2000). The American Food and Drug Administration

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(FDA) will require food manufacturers to list the amount of trans fat on food labels from January 2006 (Ault, 2003).

Background to the effects of trans fats

There is as yet no clear understanding of the mechanisms and phenomena that cause the detrimental health effects of the consumption of large amounts of trans fats in or- ganisms. On a fundamental level, trans isomers could have consequences in a physical (changes in membrane structure) or biochemical manner (enzyme driven metabolisms). Membranes made of trans fatty acids can form a more rigid packing of phospholipids, changing the microviscosity or thermal phase behaviour (Fig. 5.5). Changes in the mor- phology of mitochondrial membranes were observed in yeast auxotrophs which were fed with trans unsaturated fatty acids. As a result, these yeasts had only 10-20 % of their normal respiratory activity (Tung et al., 1991).

Pseudomonas putida can respond to membrane-disrupting stress factors such as or- ganic solvents or heat shock by changing the composition of the membrane by isomeriz- ation. An enzyme called cis-trans isomerase can isomerize cis fatty acids into trans fatty acids without transient saturation or shifting of the double bond (von Wallbrunn et al., 2003). In response to 4-chlorophenol, the trans-cis ratio of unsaturated fatty acids in Pseudomonas putida was increased from 0.5 to 3 (Neumann et al., 2003). The isomerase can only reach the double bonds inside the membrane bilayer when the membrane fluid- ity is disturbed, an interesting example of biological self-regulation. However, there is no way back to the cis conformation; only by replacing the trans fatty acids can the mem- brane be restored to its previous condition. Kratsch et al. (2003) found that the enzymatic cyclooxygenation conversion of arachidonic acid (all-cis 5,8,11,14-eicosatetraneoic acid) was halted if only one double bond was isomerized into the trans form, proof of the en- zyme’s very selective behaviour.

5.9 Conclusions

Fat hardening has been practiced for more than a century, producing fats of the right consistency for consumer products. Depending on the product application, reaction con- ditions were sometimes even chosen to maximize the formation of trans fats. Insights into the negative health effects of both saturated and trans fats are triggering research into new processes that minimize the formation of either one or both. There are also several inherent drawbacks connected to the current process in the form of slurry hydrogenation. Therefore in the next chapter, a fresh look is taken at the venerable fat-hardening process.

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Figure 5.5: Schematical depiction of a membrane consisting of phospholipids with chains in the cis or in the trans configuration (adapted from Chatgilialoglu et al. (2000)).

Bibliography

Ackmann, R., 2002. The gas chromatograph in practical analyses of common and uncom- mon fatty acids for the 21st century. Anal. Chim. Acta 465, 175–192. Aro, A., 2001. Complexity of issue of dietary trans fatty acids. The Lancet 357, 732–733. Ault, A., 2003. FDA says it will require trans-fat amounts on food labels. The Lancet 363, 218. Chatgilialoglu, C., Ferreri, C., Ballestri, M., Mulazzani, Q., Landi, L., 2000. cis-trans isomerization of monounsaturated fatty acid residues in phospholipids by thyil radicals. J. Am. Chem. Soc. 122, 4593–4601. Edvardsson, J., Irandoust, S., 1994. Reactors for hydrogenation of edible oils. J. Am. Oil Chem. Soc. 66(7), 235–242. Gunstone, F., Hamilton, R. (Eds.), 2001. Oleochemical manufacture and applications. Sheffield Academic Press Ltd., p. 3. Gunstone, F., Padley, F. (Eds.), 1997. Lipid technologies and applications. Marcel Dekker, NY, Ch. Hydrogenation of edible oils, technology and applications, pp. 265–303. Keys, A., 1980. Seven countries: a multivariate analysis of death and coronary heart disease. Harvard University Press, Cambridge, MA. Kratsch, S., Drossler,¨ K., Sprinz, H., Brede, O., 2003. Thiyl radicals in biosystems: in- hibition of the prostaglandin metabolism by the cis-trans-isomerization of arachidonic acid double bonds. Arch. Biochem. Biophys. 416, 238–248.

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Luzzi, A., James, W., June 2000. European diet and public health: the continuing chal- lenge. Tech. rep., Eurodiet. Macher, M.-B., Holmqvist, A., 2001. Triacylglycerol analysis of partially hydrogenated vegetable oils by silver ion HPLC. J. Sep. Sci. 24, 179–185. Mann, J., 2002. Diet and risk of coronary heart disease and type 2 diabetes. The Lancet 360, 783–789. Mossoba, M., Kramer, J., Delmonte, P., Yurawecz, M., Rader, J., 2003. Official methods for the determination of trans fats. AOCS Press, Champaign, Il. Neumann, G., Kabelitz, N., Heipieper, H., 2003. The regulation of the cis-trans isomerase of unsaturated fatty acids in Pseudomonas Putida : correlation between CTI activity and K+-uptake systems. Eur. J. Lipid Sci. Technol. 105, 585–589. Normann, W., 1902. Verfahren zur Umwandlung ungesattigter¨ Fettsauren¨ oder deren Gly- ceride in gesattigte¨ Verbindungen. Deutsches Reichspatent Nr. 141029. Normann, W., 1903. Process for converting unsaturated fatty acids or their glycerides into saturated compounds. British Patent 1515. Normann, W., 1922. Uber den gegenwartiger¨ Stand der Fetthartung.¨ Angew. Chem. 63, 437–440. O’Brien, R. (Ed.), 1998. Fats and oils; formulating and processing for applications. Tech- nonomic publishing company, Inc., pp. 81–96. Oomen, C., Ocke, M., Feskens, E., van Erp-Baart, M.-A. J., Kok, F., Kromhout, D., 2001. Association between trans fatty intake and 10-year risk of coronary heart disease in the Zutphen elderly study: a prospective population-based study. The Lancet 357, 746–751. Schlegel, H., 1993. General Microbiology, 7th Edition. Cambridge University Press, p. 453. Thomaidis, N., Georgiou, C., 2000. Direct parallel flow injection multichannel spectro- photometric determination of olive oil iodine value. Anal. Chim. Acta 405, 239–245. Tung, B., Unger, E., Levin, B., Brasitus, T., Getz, G., 1991. Use of an unsaturated fatty acid auxotroph of Saccharomyces Cerevisiae to modify the lipid composition and func- tion of mitochondrial membranes. J. Lipid Res. 32, 1025–1038. von Wallbrunn, A., Richnow, H., Neumann, G., Meinhardt, F., Heipieper, H., 2003. Mech- anism of cis-trans isomerization of unsaturated fatty acids in Pseudomonas Putida. J. Bacteriol. 185(5), 1730–1733.

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Chapter 6

Monolith Catalysts as Alternative to Slurry Systems: Hydrogenation of Edible Oil

‘Die Ausfuhrung¨ der Hartungsverfahrens¨ ist im allgemeinen grundsatzlich¨ die gleiche geblieben wie zu Anfang. Sie besteht im innigen Durchmischen von Ol¨ und Katalysator mit Wasserstoff, was teils mit Hilfe von Ruhrwerken,¨ teils durch Zerstaubung¨ vorgenom- men wird. Hierzu sind die verschiedenartigsten Apparate ersonnen worden, zum Teil von so phantasticher Art, dass sie eigentlich nur ein Kopfschutteln¨ bewirken konnen’¨ (Nor- mann, 1922)

6.1 Introduction

Although practised industrially for many decades, batch slurry fat-hydrogenation technology has some distinct shortcomings. The most obvious is the difficult separation of the suspended slurry catalyst. The filtration of the catalyst is costly, time consuming, and - in essentially all cases - results not only in additional waste streams but also in loss of the solid catalyst and some product. This loss of catalyst has in many cases prevented the use of more effective and selective but also more expensive precious metal catalysts. Another disadvantage is the power consumption of the impellers that are used for mixing and dispersion of gas into the liquid. Commercial continuous reactors are either tubular or flow reactors with well stirred compartments. In these reactors, hydrogen is dispersed in a catalyst/oil slurry as it flows and so the problem of catalyst/oil separation remains (Edvardsson and Irandoust, 1994). Fixed bed technology was commercialized as early as the 1920s (Lush, 1923; Bolton, 1927) but does not appear to have taken root. Although there is no shortage of data published on laboratory scale fixed bed reactors (Boyes et al., 1992, 1995; Heldal et al., 1989; Moulton and Kwolek, 1982; Zhu et al., 1997; Snyder et al., 1978; Mukherjee et al., 1975), at current there appear to be no commercial fixed bed processes. Several alternative designs based on monolith catalysts have been proposed within the last few years as a potential solution to the problems associated with slurry operation

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Figure 6.1: Sunflower oil hydrogenated with a monolith catalyst (left) and with slurry catalyst (right).

(Broekhuis et al., 2001; Boger et al., 2003; Heiszwolf et al., 2001; Edvinsson Albers et al., 1998). All these designs have in common that they benefit not only from the unique mass transfer characteristics of monolith catalysts but also allow for a significant reduction in the complexity of the process. In the conventional process, significant workup is required to remove the catalyst from the hydrogenated oil in a filtration step, followed by post- bleaching to remove any traces of nickel that are left in the oil. The contribution of the post-hydrogenation workup to the process economics is significant. It represents about 20% of the total operating costs, and as much as 50% if the consumption of hydrogen at catalyst is excluded (Hamm and Hamilton, 2000). Figure 6.1 shows two batches of sun- flower oil, one hydrogenated with a monolith catalyst and the other with a slurry catalyst. The dark colour is caused by the catalyst particles still present in the oil, underlining the importance of the filtration step. In Figure 6.2 simplified flowsheets of a conventional batch slurry system and of a system outfitted with an add-on monolith reactor are given. After a batch of oil is hydro- genated in the conventional process it is transported either by pump or by gas pressure to a filter press where the catalyst is removed. After that, the oil is treated in a separate tank with bleaching earth and chelating agent which in turn need to be removed. The proposed retrofit with a monolith catalyst in effect removes the need for filtration and post treat- ment since no catalyst particles remain in the liquid. Addition of a pump to recirculate the liquid is needed. Gas can be introduced by gravity-driven natural circulation, making use of traditional liquid distribution devices such as showerheads (Heiszwolf et al., 2001). Alternatively, a liquid motive gas ejector can be used that entrains and compresses the hy- drogen (Machado et al., 2005). This produces a fine dispersion of gas bubbles in the liquid and serves as an effective gas-liquid contactor. The ejector serves as a gas compressor and so the gas-liquid dispersion delivered to the inlet of the reactor can be delivered at a higher pressure than the outlet pressure. In the conventional mode of operation, the stirrer speed

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hydrogen

hydrogen catalyst filter press

spent catalyst

bleaching earth

citric acid monolith reactor reactor

post-bleach tank pump

product

Figure 6.2: Conventional hydrogenation process (left) retrofit with add-on monolith catalyst (right). Note that also in the latter case it is necessary to do post-bleaching.

must be high enough to ensure both catalyst suspension and the transport of hydrogen into the oil. With the retrofit, the stirrer speed can be decreased as long as the heat transfer of the oil to the internal coils remains sufficient. Although monolith catalysts have already been proposed for use in edible oil hydro- genation years ago (Bird et al., 1979), this concept has not found broad attention. In this chapter, the performance of monolith catalysts in the hydrogenation of edible oil will be investigated and the question to what extent they could be an attractive alternative to cur- rent technology will be addressed. Emphasis lies on the possibility of catalyst reuse and the economical implications of the use of a noble metal catalyst. Parts of this chapter were also published in Boger et al. (2004).

6.2 Experimental

Catalysts

Experiments were carried out with a large variety of catalysts supplied by Corning Inc. All were based on palladium as the active component. Palladium has been pro- posed as a more active catalyst material and it has been shown that only low palladium concentrations are required (Savchenko and Makaryan, 1999). The supplied monolith catalysts were prepared using cordierite honeycomb supports, washcoated with high sur- face silica. Washcoat loadings were typically between 12-15 wt%. Preparation methods as described in Nijhuis et al. (2001) were used. To enable good comparison, monolith

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Table 6.1: Characteristics of catalysts used. catalyst 1 2 3 4∗ 5 coating silica silica silica silica silica/alumina cell density (cpsi) 400 400 400 - 200 wt% coating 10 11 15 - 12 dwc , d p (µm) 11 11 25 13 20 d pore (nm) 11.1 6-7 12.6 12.6 10.3 wt% Pd on coating 7.7 3.4 6.9 3.4 2.0 Figure 3. Schematic of stirrers used in the experiments. (A) Premex stirrer, (B) monolith stirrer8 ∗slurry catalyst and (C) screw impeller stirrer (SISR)19.

10 18

34 21

17

42 A B C

Figure 6.3: SchematicsFigure 4. of Comparison the stirrers of hydrogenation that were results used: with (A) slurry Premex (powder, open stirrer; symbols) (B) and monolith monolith stirrer; (C) screw impeller stirrercatalyst or (solid SISR. symbols) under identical process conditions.

100 100 sat trans cis Saturated Monene Diene Triene 80 80 catalyst and slurry catalyst were prepared in comparable fashion and with comparable properties. Characteristics60 of the catalysts that were60 tested are given in Table 6.1.

40 40 Equipment 20 20 Concentration in wt% Concentration in wt%

0 0 All experiments140 were carried100 out60 in a20 300 ml140 autoclave,100 which60 could20 be equipped with different stirringa arrangements,IV shown inb Figure 6.3. AllIV experiments with slurry catalyst were performed with the self gas-inducing Premex stirrer (Figure 6.3A). The

monolith experiments were mainly run with the gas inducing monolith stirrer (Figure 6.3B). A few experiments were operated with the screw impeller stirrer (SISR, Figure

6.3C) to demonstrate that the results obtained21 are independent of the monolith reactor configuration and are generally applicable to the replacement designs proposed in the

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Table 6.2: Composition of the soy-oil used in the experiments. Component Content (wt%) Palmitic Acid c16:0 11.3 Stearic Acid c18:0 3.9 Oleic Acid 9 cis-c18:1 23.1 Linoleic Acid 9,12 cis,cis-c18:2 54.4 α-Linolenic Acid 9,12,15 cis,cis,cis-c18:3 7.2 Others 0.4 ≤ IV 131.4

literature. The monolith stirrer used is described in Sandee et al. (2001), with the addition of a small pipe between the monolith inserts to make the stirrer gas-inducing. The stirrers were operated at high rotational speed (1800 rpm for the monolith stir- rer and 2000 rpm for the slurry) to achieve good mixing and mass transfer. Independent pressure-step experiments were performed to determine the gas to liquid mass transfer kL a (Dietrich et al., 1992) for the three stirrers. The results indicated that for all experi- ments the criterion for absence of gas to liquid mass transfer limitations, Ca < 0.05, was fulfilled. The experiments were run in a semi-batch mode (constant hydrogen pressure) and the amount of oil in the reactor was between 140 and 220 g. The catalyst concentration in the slurry experiments was varied between 0.03 and 0.1 wt%. In the monolith experiments higher loadings of up to 2 wt% were used since the monolith catalyst contained the inert cordierite backbone onto which the catalyst phase was applied. The amount of active catalyst mass in the reactor was of the same order of magnitude as for the slurry exper- iments. The slurry catalyst was used only once whereas the monolith catalyst was used over several cycles in most cases. This was done to get information about the catalyst deactivation and reusability.

Reactant

The oil used was pure bleached soybean oil provided by Unichema International. The composition of the oil used in all experiments is given in Table 6.2. The raw soybean oil did not contain any trans isomers. The overall saturation of an oil or fat is often expressed in the iodine value or IV. Per definition, the IV equals the amount of iodine (in grams) that has to be added to saturate all the double bonds present in 100 grams of fat. As such, decreasing the IV of a ton of oil by one requires 39.4 mol of hydrogen. The IV can be determined by titration, IR-spectroscopy or refractive index, but can also accurately be calculated when the exact composition of a fat sample is known e.g. by gas-chromatography. The IV of the soybean oil used was 131.4 which corresponds to a 3 total concentration of double bonds of approximately 4400 mol m− .

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140

120

100

80

60 Iodine Value 40 δIV/δt = -1.13 min-1 20 -1 -1 = -0.045 mmol H2 g oil min 0 0 60 120 180 time (min)

Figure 6.4: Determining catalyst activity from a plot of the Iodine value IV versus time.

Conversion and catalyst activity

The conversion is defined as the fraction of the double bonds, initially present in the oil, that have been saturated and can be calculated from the iodine value with

IV0 IV X − (6.1) = IV0 Catalyst activity is determined at t 0 by plotting the iodine value versus time. An example of this is given in Figure 6.4,= where the IV is plotted versus time for a slurry 1 1 experiment. The initial rate is 0.045 mmol goil− s− ; 200 grams of oil were hydrogenated 7 3 3 1 with 1.3 10− m of slurry catalyst. From this an (initial) rate of 1350 mol mcat− s− can be calculated.·

Experimental procedure

All experiments were carried out as follows. The autoclave was filled with a known amount of oil and catalyst (either slurry or monoliths). At high stirring speed and ambient temperature the autoclave was then flushed several times with hydrogen in order to remove the air. At low stirring speed and at ambient (hydrogen) pressure, the autoclave was heated. Having reached the desired temperature, the stirrer speed was increased (2000 rpm for slurry and 1800 rpm for monolith experiments) and hydrogen pressure applied.

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solvent linoleate

palmitate oleate

MUFA PUFA stearate FID response (A.U.) FID response (A.U.) linolenate

0 5 10 15 20 25 12 13 14 15 16 17 18 19 20 21 22 time (min) time (min)

Figure 6.5: Chromatogram of partially hydrogenated soy-oil (left), area of interest (right). MUFA: Mono Unsaturated Fatty Acid methyl esters, PUFA: Poly Unsaturated Fatty Acid methyl esters.

Samples were taken at regular intervals through a sample line and stored for later analysis. Depending on the number of samples taken, a total of between one and three grams of oil was withdrawn from the reactor. At the end of an experiment, the gas inside the autoclave was vented and the autoclave allowed to cool down. When sufficiently cool enough to handle the autoclave was removed and the used oil stored. When performing consecutive experiments, the monolith catalyst was stored in fresh soy oil in-between runs in order to minimize the exposure to air.

Analysis

The fat samples were converted into Fatty Acid Methyl Esters (FAMEs) using the boron trifluoride method (AOCS official method Ce 2-66, 2002). The analysis of the reaction products was carried out on a Chrompack 9001 equipped with a 50 m 0.25 mm 0.2 mm CP-Sil 88 (Cyanopropyl Polysiloxane) fused silica column and a× flame ionization× detector. A split ratio of approximately 1:200 was used. Depending on the concentration, between 1 and 4 µl of sample was injected. Helium was used as a carrier gas. The column pressure was maintained at 150 kPa, and the flowrate was 0.45 ml 1 min− . The column was operated isothermally at 443 K and both the injector and detector were kept at 523 K. AOCS official method Ce 1f-96 (2002) recommends an operating temperature of 448 K but slightly better separation was obtained at a lower temperature.

The chromatogram

Figure 6.5 shows an example of a chromatogram of partially hydrogenated soy-oil. The first peak is palmitate or methyl hexadecanoate: as it is a saturated compound that is not formed during reaction, the relative peak area will remain the same. The palmitate

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Table 6.3: Assignment of cis and trans PUFA to peaks in the chromatogram. peak component 16 trans isomer of linoleate 17 9,12 trans,trans-c18:2 18 9,12 cis,trans-c18:2 19 9,12 trans,cis-c18:2 20 all cis α-linoleate 21 trans isomers of linolenate 22 all cis linolenate

20 23

18

19 16 17 FID response (A.U.) 22 21

15 16 17 18 19 20 21 22 time (min)

Figure 6.6: Chromatogram of partially hydrogenated soy-oil focussing on the PUFAs.

peak was used as a check for the accuracy of the analysis and was found to be on the order of 1 wt%. Stearate is the final saturated product formed by complete saturation of oleate, linoleate and linolenate. Two important regions can be recognized in the chromatogram: the first one contains all the Mono Unsaturated Fatty Acid methyl esters or MUFAs and the second one the Poly Unsaturated Fatty Acid methyl esters or PUFAs. The identific- ation of the peaks belonging to the MUFAs is described in detail in Chapter 3, which focusses on the hydrogenation of methyl oleate.

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Table 6.4: Composition of oil hydrogenated by monolith catalyst 1 at IV 80 (wt%). ≈ Batch run# 1 3 7 IV 73.9 76.0 80.0 c16:0 12.1 12.5 11.8 c18:0 23.0 22.9 22.6 cis-c18:1 20.8 18.2 16.9 trans-c18:1 23.3 23.8 21.3 cis-c18:2 14.7 18.4 18.5 trans-c18:2 5.4 2.6 6.7 cis-c18:3 0.7 1.6 1.7

PUFA identification

Given the large number of isomers that can be formed theoretically from linoleate and linolenate, one would expect their analysis to be exceedingly difficult. However, a num- ber of simplifying rules hold: all isomers of linoleate elute before linoleate (peak #20), whereas all isomers of linolenate (peak #22) elute before between linoleate and linolen- ate (AOCS official method Ce 1f-96, 2002). In the latter area one also finds c20:0 and cis-c20:1 (AOCS official method Ce 1f-96, 2002). It is generally accepted that the elu- tion order of the main isomers of linoleate is 9,12 trans,trans-c18:2 (peak #17) followed by 9,12 cis,trans-c18:2 (peak #18) and 9,12 (peak #19) trans,cis-c18:2 (Scholfield, 1981; AOCS official method Ce 1f-96, 2002; Kramer et al., 2002; Naglic and Smidovnik, 1997). As one can see in Figure 6.6, there are several (minor) peaks in the same area, these are conjugated isomers of linoleate (CLA or Conjugated Linolenic Acid) (Mossoba et al., 1991). The correct analysis and identification of CLAs is a field in itself (Kramer et al., 2001): however, with the increased interest in the (positive) health effects (Whigham et al., 2000; Chardigny et al., 2001; Belury, 2002), this field is likely to expand quickly. An overview of the assignment of the various peaks in the chromatogram is given in Table 6.3.

6.3 Results and discussion

Monolith reuse

Common issues with the reuse of catalysts are the loss of activity and changes in se- lectivity. In Figure 6.7, the activity of catalyst 1 is given for seven subsequent batches. The activity is defined as the amount of double bonds hydrogenated per kilogram of palladium per second. After an initial decline in activity, stable operation is reached at approxim- ately 55% of the original activity. The cause of catalyst deactivation is not completely understood. A significant part might be due to coking of active sites, as the activity of

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2.0 ) -1 s

-1 1.5 Pd 2400 rpm

1.0

0.5 Initial activity (mol kg

0.0 1234567 Run #

Figure 6.7: Activity of monolith catalyst 1, tested in monolith stirrer in seven subsequent batches. T 393K, P 2.1 MPa. = =

similar catalysts could partially be recovered after treatment in hydrogen at elevated tem- peratures. Edvardsson et al. (2001) hydrogenated the FAME of sunflower oil with slurry catalysts that were filtered from the product and reused. Most extensive deactivation was found for a Pd/γ -alumina catalyst, while Pt based catalysts showed little or no deactiva- tion. Extensive coke formation was found to be responsible for the deactivation and the acidity of the support was an important factor herein as high acidity favours polymeriz- ation and cyclization. Another explanation could be fouling of the catalyst pores; this would also explain why catalyst 2, which has the smallest pore size, deactivates the most. As Figure 6.8 shows, the selectivity of the catalyst is effectively unaffected. In Table 6.4, the product composition is given at a double bond conversion of 0.4 or IV 80 for the first, third and seventh run. In batch six and seven, the stirrer speed was increased≈ from 1800 to 2400 rpm with no appreciable change. From this it is concluded that under these conditions, external mass transfer from gas to the bulk liquid does not play a role when using the monolith stirrer. To further validate the monolith stirrer, a series of con- secutive experiments with the SISR and monolith stirrer was carried out with catalyst 2 (Figure 6.9). Although a different catalyst was used, a similar deactivation behaviour was observed and there was also no difference in results between the monolith stirrer and the SISR.

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80 60 cis

50 diene monoene 60 40

40 30 trans saturated amount (%) amount (%) 20 20 saturated 10 triene

0 0 0 0.2 0.4 0.6 0 0.2 0.4 0.6 conversion (-) conversion (-)

Figure 6.8: Selectivity of monolith catalyst 1, tested in monolith stirrer in seven subsequent batches. T 393 K, P 2.1 MPa. First run (black symbols), third run (white symbols), seventh run (grey symbols).= =

12 SISR )

-1 10 s -1 Pd 8

6 SISR Monolith 4 Stirrer SISR 2 Initial activity (mol kg

0 1234 Run #

Figure 6.9: Initial activity of monolith catalyst 2, tested in both SISR and monolith stirrer in four subsequent batches. T 393K, P 2 MPa. = =

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60 80 monoene cis 50 diene 60 40 trans

30 saturated 40

amount (%) amount (%) 20 20 10 saturated triene 0 0 0 0.2 0.4 0.6 0 0.2 0.4 0.6 conversion (-) conversion (-)

Figure 6.10: Effect of pressure on the selectivity, monolith catalyst 3. T 393 K, P 0.9 Mpa (open symbols) and P 2.1 MPa (closed symbols). = = =

80 80

p =17 bar p =0.5 bar 60 1.2 2.2 60 5.1 p =52 bar 40 7.3 40 20 monoene concentration (%) monoene concentration (%)

0 20 0 20406080 0 5 10 15 20 25 time (min) time (min)

Figure 6.11: Influence of reaction pressure on intermediate monoene yield. (left) Cottonseed oil hydrogenated with Ni (Pihl and Scho¨on,¨ 1971) (right) canola oil hydrogenated with Pd (Hsu et al., 1986).

Influence of operating conditions

The most important operating conditions are temperature, pressure and catalyst-to- oil ratio. Experiments were carried out at sufficiently high stirrer speeds to ensure it had no influence. An experiment was performed in which one blade of the monolith stirrer was loaded with a monolith catalyst and the other blade with bare monolith support. This experiment was compared to one in which both blades were loaded with catalyst, thus doubling the catalyst-to-oil ratio. The reaction rates in both experiments scaled with the amount of catalyst and the same selectivity profiles were found, demonstrating the

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experimental reproducibility. These results also demonstrate that under the experimental conditions, the rate at which hydrogen is transferred into the oil was not limiting. The influence of temperature was determined with monolith catalyst 3. Prior to the ex- periments at 395 K, 375 K and 353 K, three runs were performed at 395 K to make sure the catalyst was properly deactivated. The apparent activation energy of the hydrogenation 1 was 20 kJ mol− while the activation energy of the trans formation was approximately 1 35 kJ mol− . At a given conversion, an increase in temperature will therefore result in a relatively higher amount of trans isomers. The influence of pressure was determined with monolith catalyst 3 at 0.9 and 2.1 MPa (Figure 6.10). Fresh catalyst was used for each experiment. Increase in pressure clearly decreases both the formation of trans iso- mers and of the monoenes. The activity increased by only 50%, indicating a square root dependence on the hydrogen concentration. Higher apparent activation energies for trans formation have been reported for both palladium (Zajcew, 1960; Hsu et al., 1988; Ahmad et al., 1979; Riesz and Weber, 1964) and nickel supported slurry catalysts (Veldsink et al., 1997; Jonker, 1999). A higher ap- parent activation energy for isomerization can be explained in terms of hydrogen supply. With an increase in temperature at a given pressure more hydrogen will be consumed, res- ulting in a lower concentration at the catalyst surface. If the isomerization reaction has a lower order dependence on hydrogen than the hydrogenation, this will result in relatively more trans formation at increased temperatures. The decrease of trans formation at in- creased pressure has been reported for Pd slurry supported catalysts by Gut et al. (1979); Hsu et al. (1988); Rylander (1970); Zajcew (1960) and can be explained in similar terms. Zajcew (1960) hydrogenated soy oil in a sparged batch slurry autoclave and found that the amount of trans formed decreased with increasing agitation rate. Increased agitation results in higher hydrogen concentration at the catalyst surface and thus relatively less trans formation. Many authors have found that a decrease in pressure increases the yield of the monoenic intermediates; Pihl and Scho¨on¨ (1971); Susu et al. (1978); Bern et al. (1975); Hashimoto et al. (1978); Susu and Ogunye (1981); Wisniak and Albright (1961); Eldib and Albright (1957) for nickel and Hsu et al. (1986, 1988); Ahmad et al. (1979) for pal- ladium based catalysts. This behaviour is often explained by the existence of different reaction orders in hydrogen. However, if an increase in hydrogen concentration (=pres- sure) increases the overall reaction rate, internal mass transport limitations of the reactants will also increase resulting in a lower yield of the intermediates, given that the first re- actant is the most active. Two examples of the influence of pressure on the formation of monoenes are given in Figure 6.11 for both a nickel and a palladium slurry catalyst.

Monolith and slurry catalyst comparison

A comparison was made between a slurry catalyst and monolith catalyst, both based on the same silica support. Both catalysts underwent the same procedure for metal de- position to make a comparison possible. In Figure 6.12, the composition of the product

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Table 6.5: Composition of oil hydrogenated by monolith catalyst 3 and slurry catalyst 4 at IV 80. ≈ feed monolith slurry IV 131.4 79.4 81.1 c16:0 11.3 12.4 11.0 c18:0 3.9 15.1 12.3 cis-c18:1 23.1 29.3 29.7 trans-c18:1 0 22.9 28.0 cis-c18:2 54.4 10.7 6.1 trans-c18:2 0 9.6 12.5 cis-c18:3 7.2 0 0

80 80

monoene cis 60 60 diene

40 40 trans amount (%) amount (%) saturated 20 20

saturated triene 0 0 0 0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8 conversion (-) conversion (-)

Figure 6.12: Comparison of monolith catalyst 3 (open symbols) and slurry catalyst 4 (closed sym- bols). T 393 K and P 2.1 MPa. = =

is given in terms of the overall hydrogenation of double bonds. Comparing the results, it can be observed that the product distribution is essentially the same up to a conversion of 0.3 or an iodine value of approximately 90. At this point, all the molecules with three double bonds (linolenic acid) have been hydrogenated into more saturated species. As the hydrogenation continues, distinct differences can be observed between the two catalysts. With the slurry system, lower amounts of dienes and saturates are observed at higher con- centrations of monoenes. The monolith catalyst produces significantly less trans isomers, at the expense of increased formation of saturates. The product composition at iodine value 80 is given in Table 6.5. The difference in selectivity behaviour can be explained in terms of hydrogen avail- ability. The high flow rate through the monolith channels ensures a constant renewal of hydrogen on the catalyst surface, resulting in a concentration at the catalyst surface that is close to the bulk concentration. The slurry particles on the other hand are moving with the liquid in the reactor and hydrogen gradients can develop over the liquid film surrounding

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Table 6.6: Activity comparison of monolith catalyst 3 and slurry catalyst 4. monolith slurry 3 1 rv,obs (mol mcat− s− ) 410 5000 dwc , d p (µm) 25 13 3 Pd loading (kgPd mcat− ) 103 51 Pd dispersion (%) 6 21

the particle. As a result, the hydrogen concentration at the catalyst surface will be lower than the bulk hydrogen concentration. A lower availability of hydrogen in turn yields more intermediates and more formation of trans isomers. In Table 6.6, the activity of both catalysts is given. Based on catalyst volume the slurry catalyst is twelve times as active as the monolith catalyst, despite the higher palladium loading on the monolith washcoat. However, when corrected for dispersion, the slurry catalyst has 1.7 times as much palladium surface area. Estimation of the Weisz-Prater number showed that the reaction was limited in fat for both catalysts. For an internally limited reaction the catalyst effectiveness is equal to the reciprocal Thiele modulus: s 1 k Cn 1 η with φ L v − (6.2) = φ = Def f

where L is the catalyst characteristic diffusion length (L dp/6 for a spherical cata- = lyst particle and L dwc for a catalyst layer), kv the rate constant, n the reaction order = and Def f the effective diffusivity of the reactant. Assuming that the reaction constant is proportional to the active palladium surface results in ksl 1.7kmono. The ratio between the observed rates of the slurry and the monolith catalysts= can be calculated with

r obs (slurry) ηk (slurry) v, v (6.3) rv,obs (monol) = ηkv(monol) Substituting the correct values results in a ratio of approximately fifteen which is in good agreement with the experimentally determined ratio of twelve. The difference in activity between the slurry catalyst and the monolith catalyst is thus explained by 1. a higher palladium dispersion on the slurry catalyst and 2. the longer diffusional path inside the monolith washcoat. A more general comparison between both types of catalysts is made in Figures 6.13 through 6.15, where the product composition is plotted versus the catalyst activity at dif- ferent conversion levels. A number of research catalysts prepared and supplied by Corn- ing Inc., both slurry and monolith catalysts, were tested. All catalysts consist of palladium on a silica support. The monoliths are cordierite washcoated with silica. Two different cell densities were used: 200 and 400 cpsi. The palladium loading on the washcoat was varied between 2 and 8 wt%, and the average washcoat thickness was varied between 10 and 55 µm.

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65 monoliths

55

45 monoene (%) 35

slurry catalysts 25 10 100 1000 10000 -3 -1 r v,obs (mol m cat s )

Figure 6.13: Monoene amount versus catalyst activity of various monolith and slurry catalysts at IV=115/X 0.12 (3), IV=90/X 0.31 (2) and IV=70/X 0.47 ( ). Open symbols either slurry or 400 cpsi= monoliths, closed symbols= 200 cpsi monoliths. =T 3934 K and P 2.1 MPa. = =

There is a clear correlation between catalyst activity and the products that are formed. A more active monolith catalyst forms more monoene and less saturated product, and also more trans isomers. As the catalyst becomes more active, hydrogen becomes depleted in- side the monolith channels, which results in the change of product composition. A similar effect is noted for the 200 cpsi monoliths: the larger the monolith channel, the lower the mass transfer enhancement caused by developing flow inside the channels. The 200 cpsi monoliths thus suffer more from hydrogen depletion than their 400 cpsi counterpart with the resulting change in product composition. As a rule, the more active monolith catalysts are, the more similar they behave to slurry catalysts.

Effect of washcoat thickness and loading

In Figure 6.16, the activity of the monolith catalysts is plotted against the reciprocal washcoat thickness. Activity decreases steeply with increasing washcoat thickness, which is what would be expected for a diffusion limited reaction. Under these conditions the catalyst effectiveness scales with the reciprocal Thiele modulus:

1 1 n 1 n n p + rv,obs ηkvC kvC kv Def f C 2 (6.4) = ≈ φ = L

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50 monoliths

40

30 (%)

trans 20

10 slurry catalysts

0 10 100 1000 10000 -3 -1 r v,obs (mol m cat s )

Figure 6.14: trans amount versus catalyst activity of various monolith and slurry catalysts at IV=115/X 0.12 (3), IV=90/X 0.31 (2) and IV=70/X 0.47 ( ). Open symbols either slurry or 400= cpsi monoliths, closed= symbols 200 cpsi monoliths.= T 3934 K and P 2.1 MPa. = =

the rate constant kv is equal to the amount of available palladium surface area which is the product of the amount of palladium present and the palladium dispersion. Since all data can be correlated by a single line, it appears that this product is more or less a constant and that increased palladium loading is counterweighted by lower dispersion. The scatter in the results is caused by uncertainties in both L and kv. A washcoat inside a square monolith channel tends to accumulate in the channel corners and as such the average thickness calculated by assuming flat washcoat layers gives only an estimated guess of the real diffusion length, as is illustrated in Figure 6.17. The results indicate strong diffusion limitations of the fat molecules inside the catalytic layers and are in agreement with the low observed activation energies.

6.4 Economic evaluation of add-on unit

Monolith catalyst design

Having shown the feasibility of monolith use for hydrogenation of vegetable oil in the previous section, the question of practicability arises. The main challenge is the catalyst activity; can a batch of oil be hydrogenated in a reasonable amount of time with a real-

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30 monoliths

25

20

15

sarurated (%) 10

5 slurry catalysts

0 10 100 1000 10000 -3 -1 r v,obs (mol m cat s )

Figure 6.15: Saturated amount versus catalyst activity of various monolith and slurry catalysts at IV=115/X 0.12 (3), IV=90/X 0.31 (2) and IV=70/X 0.47 ( ). Open symbols either slurry or 400 cpsi= monoliths, closed symbols= 200 cpsi monoliths. =T 3934 K and P 2.1 MPa. = =

Table 6.7: Activity of catalysts at T 393 K and P 2.1 MPa. = = catalyst run# activity 1 3 1 3 1 mol kgPd s− mol mcat− s− mol mmonolith− s− 1 1 1.9 220 7 7 1.0 120 4 2 1 10.8 760 21 4 2.3 160 4 3 1 4.0 410 28 2 0.7 75 5 4∗ - 100 5000 - ∗slurry catalyst

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200 ) -1

s 150 cat -3

100 (mol m v,obs r 50

0 0 0.02 0.04 0.06 0.08 0.1 L -1 (µm-1)

Figure 6.16: Effect of washcoat thickness on monolith hydrogenation catalyst activity. Palladium loading on the silica washcoat: 2 wt% ( ), 5 wt% (2) and 7.5 wt% ( ). T 393 K and P 2.1 MPa.  ◦ = =

average thickness

Figure 6.17: Rounding of silica washcoat inside a monolith channel, resulting in thick layers in the corners and thin layers on the flat surfaces.

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Table 6.8: Cost estimation of a 400 cpsi 0.23 m3 monolith catalyst containing 0.6 kg of palladium, needed to brush hydrogenate 10 tons of oil in two hours. cost amount manufacturing cost ex. metal 20000 $/m3 $4650 metal cost 7500 $/kg $4430 metal recovery cost 10% 2000 $/m3 $465 metal recovery 90% -6750 $/kg $-4000 total cost $5545

istically sized monolith catalyst? Table 6.7 shows the activity of the monolith catalysts 3 1 that were tested. After deactivation, a rate of 5 mol mmonolith− s− seems feasible at 393 K and 2.1 MPa. At operating conditions of 433 K and 0.5 MPa, this translates to 4 mol 3 1 1 mmonolith− s− , assuming an activation energy of 20 kJ mol− and a square root dependency on pressure. As an example, consider the retrofitting of a ten ton medium batch reactor. Soy oil is brush hydrogenated and the iodine value has to be reduced from 132 to 115. A 400 cpsi monolith coated with a 25 µm washcoat loaded with 2.5 wt% palladium is used, which amounts to 2.6 kg palladium per cubic meter of monolith. Reducing the iodine value of one ton of oil by one IV requires 39.4 mol of hydrogen. To achieve a reaction time of two 3 1 3 hours at a rate of 4 mol mmonolith− s− a monolith volume of 0.23 m would be required. In Table 6.8, a cost estimation of the monolith catalyst is made. The palladium present in the monolith would cost almost $4500 as such. However, most of the metal can be reclaimed by the manufacturer for a certain fee. This reduces the actual cost of the metal significantly; it makes up only 15% of the total monolith catalyst cost. The major cost is in the preparation of the monolith catalyst; washcoating with silica and the deposition of palladium on the washcoat. Machado et al. (2005) estimates the cost of a monolith catalyst at $50000/m3; this however presupposes small scale use of monoliths in a niche market (fine chemicals). In a large scale application as is considered here catalyst demand will be much higher and as a result prices can be expected to drop. The cost of the monolith catalyst can be reduced by increasing the activity per volume. This can be done by an increase in active surface area or by increasing the palladium load- ing on the catalyst washcoat. In Table 6.9, the cost of a number of catalysts is estimated using the 400 cpsi and 2.5 wt% Pd as a reference.

Slurry operation

The catalyst cost of conventional slurry operation is dependent on the amount of cata- lyst that is needed, the catalyst cost and the costs associated with removal of the catalyst from the product stream. Commercial nickel based catalysts such as Johnson Matthey’s Pricat catalysts and Engelhard’s Nycosel catalysts can cost anywhere between $5 and $15 per kilogram, depending on the application. The amount used per ton of oil is also vari-

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Table 6.9: Cost estimation of monolith catalyst with varying activity. catalyst 1 2 3 4 cell density (cpsi) 400 400 600 600 Pd on washcoat (%) 2.5 5.0 2.5 5.0 Pd weight (kg) 0.59 0.79 0.50 0.67 3 1 rV,obs (mol mmonolith− s− ) 4 6 6 9 3 Vmonolith (m ) 0.23 0.16 0.16 0.10 catalyst cost ($) 5560 4000 3790 2780

Table 6.10: Catalyst cost estimation for standard slurry operation. Adapted from Hamm and Hamilton (2000). catalyst loading 0.6 kg/ton catalyst cost 10 $/kg 6 $/ton catalyst recovery 0.3 $/kg 0.18 $/ton citric acid 1.2 $/kg 0.5 kg/ton 0.6 $/ton bleaching earth 0.5 $/kg disposal 0.1 $/kg 1 kg/ton 0.6 $/ton oil loss 0.7 $/kg 1 kg/ton 0.7 $/ton capital write off∗ 2.0 $/ton total 10.1 $/ton ∗Post-treatment equipment

able, but a reasonable amount would be 0.5 kg per ton (Hamm and Hamilton, 2000). This brings the catalyst cost to around $6 per ton of oil. Added to this is another $2 for envir- onmental costs related to catalyst and bleaching earth disposal and the cost of chelating agent that is used to remove traces of nickel left in the product (Hamm and Hamilton, 2000). Typical edible oil specifications call for nickel concentrations lower than 0.1 ppm. Also there are the capital investment costs related to the filtering, pressing and bleaching installations which add another $2 per ton. This brings the total catalyst cost for slurry operation to a total of approximately $10 per ton (Table 6.10). With a monolith add-on unit there would be no need for capital investments in filtering equipment; however, extra investments will have to be made for the reactor housing and a recirculation pump. These will be considered negligible compared to the considerable costs of an entire train of post treatment units. The reduced electricity cost by stirring the holding tank at a lower rate will be more or less offset by the cost of running the pump.

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40

30

1 20 2

3

catalyst cost ($/ton) 10 4

0 0 20406080100 # of batches

Figure 6.18: Estimation of catalyst cost per ton of product. 10 ton batches, batch time 2 hour and 1IV=17. Horizontal line for conventional nickel slurry catalyst. For legend see Table 6.9.

Comparison

The significant investment cost of a monolith catalyst can only be recouped by using it a number of times, so that the catalyst cost per ton of product drops to an acceptable level. Figure 6.18 shows the catalyst cost per ton of product as a function of reuse for four different monolith catalysts. The first catalyst as detailed in Table 6.8 will be cost-effective after 55 runs. Either doubling the palladium loading or increasing the cell density from 400 to 600 cpsi both result in a 50% increase of activity with little difference in catalyst cost (catalyst 2 and 3 in Table 6.9) and are cost-effective after 40 runs. In the most advantageous case (catalyst 4 in Table 6.9), 35 batches are needed to compete with the slurry process. Increasing the operating pressure will of course increase the reaction rate, further reducing the monolith volume needed and costs associated. However, there may be limits to the pressure at which existing equipment can be operated.

6.5 Conclusions

Hydrogenation or hardening of vegetable oil with monolith catalysts is very well pos- sible. Monoliths can be reused multiple times, with little change to product composition. Also, the enhanced transfer of hydrogen to the catalyst surface in the monolith channels

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is apparent in the decreased formation of both intermediate monoenes and trans isomers when compared to slurry catalyst. The efficiency of the palladium deposited on the mo- nolith washcoat is much lower than the palladium in slurry catalysts. This is caused by the longer diffusional lengths in the monolith washcoats and the lower dispersion of pal- ladium. In order to make monoliths more active, or to reach equal activities with less palladium, thinner washcoats are needed. Lowering resistance to diffusion could also be achieved by using more macro porous materials. The data presented in Figures 6.13 and 6.15 suggest that at a given temperature and pressure a trade-off exists between the formation of undesired trans and saturated fats. Less trans results in more saturates and vice versa. The desired product composition will dictate the operating conditions. The existing equipment, however, limits the operating conditions to hydrogen pressures of five to six bars. Two major catalyst manufacturers have recently announced their interest in platinum based catalysts for production of low trans products (Mangnus and Beers, 2004; Furlong, 2004). Platinum catalysts have the advantage over palladium catalysts in that they are somewhat more selective towards the di-unsaturated fats, resulting in higher intermediate yields and lower trans formation at a given iodine value and are reportedly less prone to deactivation. A disadvantage of plat- inum is its high price; depending on market fluctuations it is three to four times more ex- pensive than palladium. Positive health effects of better products may offset these higher costs. The economic viability of an add-on unit replacing the conventional slurry technology is, somewhat surprisingly, not dictated by the cost of palladium. Since most of the metal can be reused the cost per kilogram is only 10-20% of the actual metal price. However, higher active metal surface areas could further lower the amount of precious metal ne- cessary to achieve the desired rates. The largest contribution to cost is the manufacturing cost of the monolith catalyst which is not expected to drop much below $20,000 per m3. Cost reduction must therefore be achieved by improving the quality of the catalytic wash- coat so that higher rates per monolith volume can be achieved. This would enable equal production rates at lower monolith volume and accompanying costs. A large scale reaction such as vegetable oil hydrogenation might however not be the most ideal case for retrofitting existing equipment. The low catalyst cost per ton of product makes economic competition difficult. Small scale production of fine chemicals has more potential for the use of monoliths (Machado et al., 2005). The higher catalyst costs and higher added value of the product make this a more likely candidate for the use of monoliths as the return on investment is much faster.

6.6 Nomenclature

3 C concentration, mol m− CaGL Carberry number for gas-liquid mass transfer, dimensionless dwc average washcoat thickness, µm

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d p average particle size, µm d pore average pore size, nm 2 1 Def f effective diffusivity, m s− IV iodine value, grams of I2 added to 100 g of oil 1 IV drop per kg oil equals approximately 0.04 mol H2 1 kL a gas to liquid mass transfer coefficient, s− 1 kv rate constant, s− L characteristic diffusion length, m n reaction order, dimensionless P hydrogen pressure, MPa T temperature, K 3 1 rv,obs reaction rate, mol mcat− s− 3 1 rV,obs reaction rate, mol mmonolith− s− V volume, m3 X conversion, dimensionless η catalyst effectiveness, dimensionless φ Thiele modulus, dimensionless

Bibliography

Ahmad, M., Priestley, T., Winterbottom, J., 1979. Palladium-catalyzed hydrogenation of soybean oil. J. Am. Oil Chem. Soc. 56, 571–577. AOCS official method Ce 1f-96, 2002. Determination of cis- and trans fatty acids in hydrogenated and refined oils and fats by capillary GLC. AOCS official method Ce 2-66, 2002. Preparation of methyl esters of fatty acids. Belury, M., 2002. Dietary conjugated linoleic acid in health: physiological effects and mechanisms of action. Annu. Rev. Nutr. 22, 505–531. Bern, L., Hell, M., Scho¨on,¨ N., 1975. Kinetics of the hydrogenation of rapeseed oil: II. rate equations of chemical reactions. J. Am. Oil Chem. Soc. 52(10), 391–394. Bird, A., Priestley, T., Winterbottom, J., 1979. Process for the hydrogenation of a veget- able oil. US patent 4,163,750. Boger, T., Roy, S., Heibel, A., Borchers, A., 2003. A monolith loop reactor as an attractive alternative to slurry reactors. Catal. Today 79(1-4), 441–451. Boger, T., Zieverink, M., Kreutzer, M., Kapteijn, F., Moulijn, J., Addiego, W., 2004. Monolithic catalysts as an alternative to slurry systems: Hydrogenation of edible oil. Ind. Eng. Chem. Res. 43, 2337–2344.

126 ‘‘thesis’’ --- 2007/1/31 22:24 --- page 127 (#137)

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Bolton, E., 1927. Recent advances in the hydrogenation of oils. J.S.C.I., 444T–446T. Boyes, A., Chughtai, A., Khan, Z., Raymahassay, S., Sulidis, A., Winterbottom, J., 1995. The cocurrent downflow contactor (CDC) as a fixed bed and slurry reactor for catalytic hydrogenation. J. Chem. Tech. Biotechnol. 64, 55–65. Boyes, A., Chughtai, A., Lu, X., Raymahassay, S., Sarmento, S., Tiltston, M., Winterbot- tom, J., 1992. The cocurrent downflow contactor (CDC) reactor - chemically enhanced mass transfer and reaction studies for slurry and fixed bed catalytic hydrogenation. Chem. Eng. Sci 47(13-14), 3729–3736. Broekhuis, R., Machado, R., Nordquist, A., 2001. The ejector-driven monolith loop re- actor - experiments and modeling. Catal. Today 69(1-4), 87–93. Chardigny, J., Bretillon, L., Sebedio, J.-L., 2001. New insights in health effects of trans α-linolenic acid isomers in humans. Eur. J. Lipid Sci. Technol. 103, 478–482. Dietrich, E., Mathieu, C., Delmas, H., Jenck, J., 1992. Raney-nickel catalyzed hydrogen- ations: gas-liquid mass transfer in gas-induced stirred slurry reactors. Chem. Eng. Sci. 47(13-14), 3597–3604. Edvardsson, J., Irandoust, S., 1994. Reactors for hydrogenation of edible oils. J. Am. Oil Chem. Soc. 66(7), 235–242. Edvardsson, J., Rautanen, P., Littorin, A., Larsson, M., 2001. Deactivation and coke form- ation on palladium and platinum catalysts in vegetable oil hydrogenation. J. Am. Oil Chem. Soc. 78(3), 319–327. Edvinsson Albers, R., Houterman, M., Vergunst, T., Grolman, E., Moulijn, J., 1998. Novel monolithic stirrer reactor. AIChE Journal 44, 2459. Eldib, I., Albright, L., 1957. Operating variables in hydrogenating cottonseed oil. Ind. Eng. Chem. 49(5), 825–831. Furlong, K., 2004. The low trans challenge. Oils and Fats International July issue, 30–31. Gut, G., Kosinka, J., Prabucki, A., Schuerch, A., 1979. Kinetics of the liquid-phase hy- drogenation and isomerization of sunflower seed oil with nickel catalysts. Chem. Eng. Sci. 34, 1051–1056. Hamm, W., Hamilton, R. (Eds.), 2000. Edible oil processing. Sheffield Academic Press, Sheffield. Hashimoto, K., Muroyama, K., Nagata, S., 1978. Kinetics of the hydrogenation of fatty oils. J. Am. Oil Chem. Soc. 48(6), 294–295. Heiszwolf, J., Engelvaart, L., Van den Eijnden, M., Kreutzer, M., Kapteijn, F., Moulijn, J., 2001. Hydrodynamic aspects of the monolith loop reactor. Catal. Today 56, 805.

127 ‘‘thesis’’ --- 2007/1/31 22:24 --- page 128 (#138)

CHAPTER 6

Heldal, J., Moulton, K., Frankel, E., 1989. Fixed-bed continuous hydrogenation of soy- bean oil with palladium-polymer supported catalysts. J. Am. Oil Chem. Soc. 66(7), 979–982.

Hsu, N., Diosady, L., Graydon, W., Rubin, L., 1986. Heterogeneous catalytic hydrogena- tion of canola oil using palladium. J. Am. Oil Chem. Soc. 63(8), 1036–1042.

Hsu, N., Diosady, L., Rubin, L., 1988. Catalytic behaviour of palladium in the hydrogen- ation of edible oils. J. Am. Oil Chem. Soc. 65(3), 349–356.

Jonker, G., 1999. Hydrogenation of edible oils and fats. Ph.D. thesis, Rijksuniversiteit Groningen.

Kramer, J., Blackader, C., Zhou, J., 2002. Evaluation of two gc-columns for analysis of milkfat with emphasis on CLA, 18:1, 18:2 and 18:3 isomers, and short- and longchain FA. Lipids 37(8), 823–835.

Kramer, J., Cruz-Hernandez, C., Zhou, J., 2001. Conjugated linoleic acids and octadecen- oic acids: Analysis by GC. Eur. J. Lipid Sci. Technol. 103, 594–632.

Lush, E., 1923. Hydrogenation. J.S.C.I. 62, 219T–225T.

Machado, R., Broekhuis, R., Nordquist, A., Roy, B., Carney, S., 2005. Applying mono- lith reactors for hydrogenation in the production of specialty chemicals-process and economic considerations. Catal. Today 105, 305–317.

Mangnus, G., Beers, A., 2004. Reduction of oils at reduced TFA content. Oils and Fats International July issue, 33–35.

Mossoba, M., McDonald, R., Armstrong, D., Page, S., 1991. Hydrogenation of soy- bean oil: a thin-layer chromatography and gas chromatography/matrix isolation/fourier transform infrared study. J. Agric. Food Chem. 39, 695–699.

Moulton, K., Kwolek, W., 1982. Continuous hydrogenation of soybean oil in a trickle-bed reactor with copper catalyst. J. Am. Oil Chem. Soc. 59(8), 333–337.

Mukherjee, K., Kiewitt, I., Kiewitt, M., 1975. Stationary catalysts for the continuous hydrogenation of fats. J. Am. Oil Chem. Soc. 52, 282–288.

Naglic, M., Smidovnik, A., 1997. Use of capillary gas chromatography for determining the hydrogenation level of edible oils. J. Chromatogr. A 767, 335–339.

Nijhuis, T., Beers, A., Vergunst, T., Hoek, I., Kapteijn, F., Moulijn, J., 2001. Preparation of monolithic catalysts. Catal. Rev. Sci. Eng. 43(4), 345–380.

Normann, W., 1922. Uber den gegenwartiger¨ Stand der Fetthartung.¨ Angew. Chem. 63, 437–440.

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HYDROGENATIONOFEDIBLEOIL

Pihl, M., Scho¨on,¨ N.-H., 1971. Kinetics of the hydrogenation of cottonseed oil in the presence of solid catalyst. Acta Polytech. Scand. Chem. Ind. Met. Ser. 100, 4–36. Riesz, C., Weber, H., 1964. Catalysts for selective hydrogenation of soybean oil. II. com- mercial catalysts. J. Am. Oil Chem. Soc. 41(6), 400–403. Rylander, P., 1970. Hydrogenation of natural oils with platinum metal group catalysts. J. Am. Oil Chem. Soc. 47, 482–486. Sandee, A., Ubale, R., Makkee, M., Reek, J., Kramer, P., Moulijn, J., Van Leeuwen, P., 2001. ROTACAT: A rotating device containing a designed catalyst for highly selective hydroformulation. Adv. Synth. Catal. 343(2), 201–206. Savchenko, V., Makaryan, I., 1999. Palladium catalysts for the production of pure mar- garine. Platinum Metals Rev. 43(2), 74–82. Scholfield, C., 1981. Gas chromatographic equivalent chain lengths of fatty acid methyl esters on a Silar 10c glass capillary column. J. Am. Oil Chem. Soc. 58, 662–663. Snyder, J., Dutton, H., Scholfield, C., 1978. Laboratory-scale continuous hydrogenation. J. Am. Oil Chem. Soc. 55(4), 383–386. Susu, A., Ogunye, A., 1981. Nickel catalyzed hydrogenation of soybean oil: I. kinetic, equilibrium and mass transfer determinations. J. Am. Oil Chem. Soc. 58(6), 657–661. Susu, A., Ogunye, A., Onyegbado, C., 1978. Kinetics and mechanism of nickel catalysed palm oil hydrogenation. J. Appl. Chem. Biotechnology 28, 823–833. Veldsink, J., Bouma, M., Scho¨on,¨ N., Beenackers, A., 1997. Heterogeneous hydrogena- tion of vegetable oils: a literature review. Catal. Rev.-Sci.Eng. 39, 253–318. Whigham, L., Cook, M., Atkinson, R., 2000. Conjugated linoleic acid: implications for human health. Pharm. Res. 42(6), 503–510. Wisniak, J., Albright, L., 1961. Hydrogenating cottonseed at relatively high pressure. Ind. Eng. Chem. 53(5), 375–380. Zajcew, M., 1960. The hydrogenation of fatty oils with palladium catalyst. III Hydrogen- ation of fatty oils for shortening stock. J. Am. Oil Chem. Soc. 37, 11–14. Zhu, G., Lei, Y., Wang, Q., Yang, X., 1997. A tubular reactor for continuous hydrogen- ation of oleic acid under moderate conditions using a thin hydride layer of hydrogen storage alloy LaNi4.8Cu0.2 as a catalyst. J. Alloys. Compd. 253-254, 689–691.

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CHAPTER 6

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Chapter 7

High(er) Performance Monoliths: Proof of Principle

’Please write music like Wagner, only louder’ Sam Goldwyn, instructing a composer for a movie

7.1 Improving monolithic catalysts

Cordierite monolithic structures have a number of distinct advantages: they are cheap and easy to manufacture, resistant to thermal shock and harsh chemical environments. However, since the cordierite itself has little or no surface area for anchoring a (metal) catalyst it needs to be coated with some sort of high surface area carrier. Silica, γ -alumina and carbon are most commonly used. An indepth review on the coating of cordierite monoliths can be found in Nijhuis et al. (2001) and Vergunst et al. (2001); the following will be limited to some general observations. Carbon washcoating generally involves dipping monoliths into liquid polymer which is subsequently carbonized and/or oxidized (Garcia-Bordeje et al., 2001). For silica and alumina two major coating procedures exist; slurry coating and sol-gel coating. With the first method, monoliths are dipped in a slurry consisting of approximately 5 µm particles which is consequently removed, leaving a (relatively) thin layer of silica or alumina after drying. As was seen in the previous chapter, this method still leaves something to be desired: coating material tends to accumulate in the corners of the monolith channels (Kolb and Cerro, 1993) as is illustrated in Figure 7.1. After impregnation this will result in increased diffusion length in the corners, unless a so-called egg-shell impregnation with the active material is possible. Another option is to choose a cordierite monolith with round channels. Although monoliths with round channels are commercially available, they do not yet extend into the range of high cell densities (100 cpsi or higher). A shared disadvantage of all coating methods is that the coating material tends to enter into the macroporous structure of the cordierite, where at best it is inaccessible to reactants but more likely increases the diffusional path of the reactants. This has for

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Figure 7.1: Results of conventional monolith slurry coating. Silica (left) and γ -alumina (right) coating. Note the accumulation in the channel corners.

example been demonstrated by Crezee et al. (2003) for ruthenium deposited on carbon coated monoliths. An attractive solution to overcome the problems associated with coating directly onto square monolith channels is to first round these channels with some sort of inert material of low surface area, onto which the high surface area carrier can be deposited. This then leads to the perhaps somewhat grandiosely named High Performance Monolith Catalysts (HPMC). It is thought that by avoiding accumulation of catalytic material in the channel corners and by having an overall thinner catalyst layer better selectivities can be achieved (Perez-Cadenas´ et al., 2005). An added advantage of round channels over square ones is that they offer higher gas to liquid mass transfer under conditions of Taylor flow (Kreutzer et al., 2005).

Using intermediate yield to determine φ

The selective hydrogenation of cyclododecatriene was used as a model reaction to de- termine the extent to which the catalysts suffered from reactant diffusion limitations. The kinetics of this reaction were exhaustively discussed in Chapter 4. In short it is a con- secutive reaction of the type CDT CDD CDE CDA, where CDD and CDE are the intermediates cyclododecadiene and→ cyclododecene,→ → and CDA the fully saturated product cyclododecane. Internal diffusion limitations of the reactants will tend to decrease the maximum amount of intermediates formed and must therefore be avoided. This is illus- trated in Figure 7.2 where the maximum yield of CDE for a known kinetics is calculated as a function of the amount of internal diffusion limitations, expressed in the Thiele modulus φ. Based on the maximum yield of CDE that is measured, the Thiele modulus describ- ing that particular catalyst can be estimated. In this example, catalyst 1 shows a higher maximum yield of CDE and as such suffers less from diffusion limitations than catalyst 2.

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70

measured under kinetic conditions 60

catalyst 1 50

40 catalyst 2

maximum CDE yield (%) maximum CDE 30

20 0 5 10 15

φ 1 (-)

Figure 7.2: Maximum yield of intermediate CDE as function of the Thiele modulus of the first reaction step. Calculated from kinetic data for CDT hydrogenation at P 1.5 MPa. =

7.2 Experimental

Two monolith catalysts where manufactured and tested; the first a conventional cata- lyst were the carrier material (γ -alumina) was deposited directly onto the cordierite, the second having the channels rounded with inert α-alumina prior to coating with the carrier.

Preparation of conventional monolithic catalysts (MC)

Cordierite monoliths were coated with a γ -alumina washcoat using a dipcoating 2 method. The monoliths used had square cells, a cell density of 62 cells cm− (400 cpsi), a wall thickness of 0.18 mm, a length of 5 cm and a diameter of 1 cm. 150 grams of γ -alumina (Puralox SBa-200, mean particle size 36 µm) was milled with 130 ml of de- mineralised water, 83 grams of 20wt % colloidal alumina for 23 hours in a ball mill. Nitric acid was added to the solution in order to lower the pH to 3.5. At the end of the milling, one part of the coating solution was diluted with water in order to obtain a vis- cosity of approximately 30 mPa s. Dried monolith pieces of approximately 5 cm length were dipped into the coating solution for 1 minute. The excess liquid was blown out from the monolith channels using pressurized air. Monoliths were then dried overnight at room temperature while horizontally rotated in order to obtain a uniform coating. Finally, the 1 monoliths were calcined for 4 hours at 723 K with a heating/cooling rate of 10 K min− .

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Figure 7.3: Monolith coated three times with α-alumina and a top layer of γ -alumina.

This method yielded a coating with an average thickness of approximately 30 µm, with more material in the channel corners and less on the channel walls.

Preparation of high performance monolithic catalysts (HPMC)

40 grams of α-alumina (AKP 50, mean particle size 0.3 µm, Sumitomo Chemicals ltd.) was slowly added to 100 ml of demineralised water agitated by air and ultrasound. Monolith pieces were then dipped into this suspension for 1 minute, after which the excess liquid was blown out using pressurized air. After being dried in air for five hours, the monoliths were dipped in an aluminium phosphate solution to increase the adherence of the next layer. This procedure was repeated three times, and resulted in monoliths with clearly rounded channels. As a final step, γ -alumina was applied using the methodology described in the previous section, resulting in a layer thickness of 15-20 µm. An example of a monolith prepared in this fashion is illustrated in Figure 7.3. BET analysis showed 2 1 that the α-alumina layers had a surface area of 2.5 m g− and the γ -alumina had one of 2 1 approximately 170 m g− , both in line with the surface areas of the respective starting materials.

Palladium deposition and activation

Monolith pieces were immersed in toluene in a holder inside a glass beaker which was vigorously stirred. To this, a toluene solution containing a known amount of palladium acetate was added during the course of three hours. The colour of the solution slowly changed from orange-red to colourless. The monoliths were then dried overnight at room 1 temperature and calcined for two hours at 673 K with a heating rate of 5 K min− . Finally, 1 the catalyst was reduced in a gaseous stream of 50 ml min− containing equal parts of 1 nitrogen and hydrogen during two hours at 453 K, with a heating rate of 2 K min− . The catalyst was passivated overnight at room temperature in nitrogen containing a trace of

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HIGHPERFORMANCEMONOLITHS

Table 7.1: Monolithic catalyst properties. MC HPMC dwc ∗(µm) 28 15 Pd loading† (wt%) 1 2 ∗Average thickness of the γ -alumina layer †Amount of palladium on the γ -alumina

Figure 7.4: Turbine autoclave used for monolith testing. Figure 10 : Alternate reactors for multiphase kinetics measurements oxygen (less than 1%). The propertiesa) Turbine of the catalysts Reactor that were b) used Screw are summarized Impeller Stirred Reactor in Table 7.1. The amount of palladium was determined by means of ICP on samples of comparable make.

Equipment and procedure

The catalysts were tested in a so-called turbine autoclave, dedicated for monolith test- ing (Figure 7.4). It consists of a number of turbine blades mounted on a hollow shaft which at sufficient stirrer speeds produces a gas-liquid froth that is recirculated at high speed through the monoliths which are placed inside the shaft. Hydrogen is fed continu- ously and samples can be withdrawn through a sample line for analysis. The speed at 1 which this froth moves through the monolith channels is very high (> 1 m s− ) which

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CHAPTER 7

4000 70

60 HPMC HPMC

) 3000

-1 50

s MC cat

-3 40 2000 30

MC yield (%) CDE

rate (mol m 20 1000 10

0 0 0.0 0.5 1.0 1.5 2.0 0.4 1.0 1.5 2.0 P (MPa) P (MPa)

Figure 7.5: Observed rate per γ -alumina washcoat volume (left) and maximum yield of the reaction intermediate CDE at T 398 K (right). =

precludes any interference from external film diffusion effects. The experimental proced- ure was essentially equal to the one described in Chapter 4, including the analysis of the reaction products.

7.3 Results and discussion

The measured observed rates per washcoat volume are summarized in Figure 7.5; the HPMC is more active than the MC which is not unsurprising, given the higher palladium loading based on which one would expect a 40% increase in activity. The maximum yield of the intermediate CDE is always higher for the HPMC, which is even the more active catalyst. This intermediate yield was used to determine the Thiele modulus φ of both catalysts: Figure 7.6 shows the yield of CDE at different hydrogen pressures as a function of φ. The lines in Figure 7.6 were calculated based on the true kinetics of CDT hydrogenation as determined from Pd/γ -alumina slurry catalysts. From the measured CDE yields, the appropriate Thiele modulus can be determined for both catalysts. It is clear that φ is lower for the HPMC at any pressure and as a result the HPMC is the more efficient catalyst. Figure 7.7 shows the Thiele moduli of both catalysts as determined from Figure 7.6; φ1,MC is approximately 1.7 times as large as φ1,HPMC. At 0.4 MPa the effectiveness η of the catalyst washcoat inside the HPMC is approximately 63%, compared to 26% for the MC. This also shows that in order to be able to compete with a slurry catalyst, the washcoat thickness on the HPMC needs to be decreased even further. For η 95% the Thiele modulus needs to be 0.4 which means a further reduction of the washcoat≥ thickness by a factor of four. ≤

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HIGHPERFORMANCEMONOLITHS

70

60

50

40 0.4 MPa

1.0 MPa

maximum CDE yield (%) maximum CDE 30 P =1.5 MPa

20 0 5 10 15

φ 1 (-)

Figure 7.6: Using the measured maximum CDE yield to determine the Thiele modulus. MC (dashed lines); HPMC (solid lines).

16

12

(-) 8 1 φ

4

0 0.0 0.5 1.0 1.5 P (MPa)

Figure 7.7: Thiele moduli determined from Figure 7.6 as a function of the reaction pressure. MC ( ); HPMC (2). 

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7.4 Conclusions

It has been shown that by pre-rounding of monolith channels with inert α-alumina it is possible to create thin, uniform washcoat layers. By knowing the true kinetics of a consecutive reaction, the amount of intermediates formed was used to estimate the amount of diffusion limitations. The so-called High Performance Monolith Catalyst combines increased catalyst activity with higher maximum intermediate yield and suffered less from internal diffusion limitations than a normal monolithic catalyst. These results clearly show the advantages of the rounding of monolith channels prior to applying the washcoat; a higher rate is combined with increased formation of wanted intermediates. In practical terms, this means that for the production of cyclododecene one could suffice with both a smaller reactor and spend less on separation of the reaction products. In practice, it might be easier to use monoliths with hexagonal channels for multiphase applications since these already approach a round geometry, and as a result less cross sectional area will be blocked by the rounding.

7.5 Nomenclature

CDA cyclododecane CDD cyclododecadiene CDE cyclododecene CDT cyclododecatriene HPMC High Performance Monolithic Catalyst MC Monolithic Catalyst dwc average washcoat thickness, µm P hydrogen pressure, MPa 3 1 rv,obs reaction rate, mol mcat− s− T temperature, K η catalyst effectiveness, dimensionless φ Thiele modulus, -

Bibliography

Crezee, E., Kooyman, P., Kiersch, J., Sloof, W., Mul, G., Kapteijn, F., Moulijn, J., 2003. Dispersion and distribution of ruthenium on carbon-coated ceramic monolithic cata- lysts prepared by impregnation. Catal. Lett. 90, 181–186.

Garcia-Bordeje, E., Kapteijn, F., Moulijn, J., 2001. Preparation and characterisation as- pects of carbon-coated monoliths. Catal. Today 69, 357–636.

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HIGHPERFORMANCEMONOLITHS

Kolb, W., Cerro, R., 1993. The motion of long bubbles in tubes of square cross section. Phys. Fluids A, 1549–1557. Kreutzer, M., Kapteijn, F., Moulijn, J., Heiszwolf, J., 2005. Multiphase monolith reactors: Chemical reaction engineering of segmented flow in microchannels. Chem. Eng. Sci. 60(22), 5895–5916. Nijhuis, T., Beers, A., Vergunst, T., Hoek, I., Kapteijn, F., Moulijn, J., 2001. Preparation of monolithic catalysts. Catal. Rev. Sci. Eng. 43(4), 345–380. Perez-Cadenas,´ A., Zieverink, M., Kapteijn, F., Moulijn, J., 2005. High performance monolithic catalysts for hydrogenation reactions. Catal. Today 105, 623–628. Vergunst, T., Linders, M., Kapteijn, F., Moulijn, J., 2001. Carbon based monolithic struc- tures. Catal. Rev. Sci. Eng. 43(3), 291–314.

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CHAPTER 7

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Chapter 8

Summary and Evaluation

’Denn was man schwarz auf weiß besitzt, kann man getrost nach Hause tragen.’ from Goethe’s Faust

8.1 Diffusion effects on hydrogenation and isomerization

The investigation of the mechanisms involved in olefin hydrogenation has a long and distuingished history. The work of Bond and Wells is still being cited and used as a frame of reference for research. Likewise, the Horiuti-Polanyi approach is still proving useful in providing simple yet meaningful mechanisms, resulting from the interaction of various surface reaction steps. In fact, as was seen in Chapter 4, the hydrogenation and isomer- ization of a single double bond in cyclododecatriene can be described quite satisfactorily with the Horiuti-Polanyi approach. The rates of isomerization and hydrogenation of the double bonds in cyclododecatriene (CDT) were found to have a reaction order in hy- drogen of 0.5 and 1.0 respectively. A reaction model was proposed based on different rate-determining steps for hydrogenation and isomerization (Figure 8.1). The insertion of the first hydrogen forming a half-hydrogenated surface intermediate is fast (step 3). The rate of isomerization is governed by the speed at which the alkylic extremities rotate (step 4), and the rate of hydrogenation by the second hydrogen insertion (step 5). So far, so good: an industrially relevant heterogeneously catalyzed gas-liquid hydro- genation / isomerization of a single double bond can be described much the same as a model reactant such as gaseous butene. However, in most practical (industrial) hydrogen- ations the occurrence of diffusion limitation inside the catalyst pores obfuscates the actual surface reaction steps. Therefore an in-depth study was made of three-phase catalytic hy- drogenation of four different reactants; 1-dodecene, methyl oleate, cyclododecatriene and vegetable oil. All these reactants have in common that they consist of a large number of carbon atoms, be it in a linear or cyclic form. Each contains one or more double bonds that can isomerize in conformation and in position. Focus was on a better understanding of both parallel isomerization reactions, for example in 1-dodecene and methyl oleate, and

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R2 CH CH R1 ‘cis’ ‘trans’ R2 CH CH R1 3 H R1 43H R1 2 fast C C fast 2 slow +H* +H* H R R2 H R2 CH CH R1 2 R2 CH CH R1 C C -H* -H* * * * H H *

+H* 5 slow 5

R1CH2 CH2R2

Figure 8.1: Proposed mechanism for the hydrogenation and isomerization of a single double bond in cyclododecatriene.

consecutive hydrogenations such as from cyclododecatriene towards cyclododecadiene and cyclododecene. During the hydrogenation of 1-dodecene rapid isomerization takes place, moving the double bond along the carbon chain, forming a number of different internal isomers. In Figure 8.2 the molar fractions of the reactant 1-dodecene, the internal dodecenes formed by isomerization and the hydrogenated product dodecane are shown as a function of time. The disappearance of 1-dodecene appears to take place in two distinct first order regimes; in the first, rapid isomerization towards the internal isomers takes place and in the second, both 1-dodecene and the internal isomers appear to have the same reactivity towards hy- drogenation. However, when isomerization between these two is taken into account, it is found that 1-dodecene is 25 times more reactive towards hydrogenation than the internal dodecenes. This is an excellent example of the interplay between hydrogenation and iso- merization, and how this can lead to the wrong conclusions. As a model this reaction proved to be intractable; the very high rate of reaction of the primary double bonds makes separation of the reaction mechanism from the diffusion effects very difficult. The approach taken was therefore reversed in Chapter 3; the severity of diffusion lim- itations was purposely changed in order to determine its effect. Methyl oleate (methyl 9-cis-octadecenoate) was hydrogenated with slurry catalysts with increasing palladium loadings. In this way the reaction rate was increased together with the associated internal diffusion limitations. It was demonstrated that the rate at which isomerization takes place relative to the hydrogenation rate is dependent on the severity of reactant diffusion lim- itations. This is illustrated in Figure 8.3: with increasing diffusion limitations, the rate at which trans-cis equilibrium is reached appears to decrease. The conversion levels at which the double bonds migrated along the carbon chain were diffusion controlled in a similar way (Figure 8.4). The most striking effect was that depending on the conforma- tion of the starting molecule, either the cis or trans double bonds appeared to be moving

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SUMMARY AND EVALUATION

1

dodecane

internal dodecenes

(-) 0.1 i x apparent equal activity

1-dodecene

0.01 03060 time (min)

Figure 8.2: Hydrogenation of 1-dodecene. Formation of less reactive internal isomers causes a decrease in the rate at which 1-dodecene disappears, resulting in two first order reaction regimes for 1-dodecene.

faster relative to each other along the carbon chain. It could be shown that this is an arti- fact, caused by diffusion limitations of the reactants. The effects of diffusion limitations on trans-cis isomerization could be described quantitatively, the effects on geometric iso- merization only qualitatively. The rate of cis-trans isomerization in the linear molecules 1-dodecene and methyl oleate could be described without the need for discriminating between the different pos- itions of the double bonds (i.e. in the middle or more towards the end of the molecule). Apparently, the mass of the extremities that have to rotate in order for isomerization to take place do not determine the rate. On the other hand in, the isomers of cyclododec- atriene and its products the rate of isomerization was found to increase with a decrease in the number of double bonds present in the molecule. As the strain in the molecules decreases, rotation of ’R2’ will become easier and hence faster. Although the hydrogen- ation is a bimolecular reaction between an adsorbed complex and a surface hydrogen, it is much faster than the monomolecular isomerization. Only at hydrogen concentrations 3 below approximately 0.5 mol m− is the rate of isomerization of ctt-CDT towards ttt- and cct-CDT higher than the hydrogenation rate. In summary, the rate of isomerization reactions relative to the hydrogenation rate can be very sensitive to the extent of internal diffusion limitations of the reactants. This can lead to surprising and sometimes counterintuitive results such as cis or trans double bonds

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4

3 increasing diffusion limitations (-)

2 trans / cis

1

0 0 20406080100 saturated (%)

Figure 8.3: Hydrogenation of methyl oleate with different Pd/γ -alumina catalysts. Influence of diffusion limitations on the cis-trans isomerization rate relative to the hydrogenation rate. Lines for modelling results.

40 40

30 30

20 20 % of unsaturated % of unsaturated 10 10

0 0 6,7,8 9 10 11 3,12 13 14 15 6,7,8 9 10 11 3,12 13 14 15

4 5 6,7,8 9 10 11 12 13,14 15 16 4 5 6,7,8 9 10 11 12 13,14 15 16

Figure 8.4: Distribution of the double bonds in methyl octadecenoate at 80% conversion. Left hydrogenated under kinetic control, right under diffusion limiting conditions. The top row numbers denote the position of the cis double bonds, the bottom row numbers the position of the trans double bonds.

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SUMMARY AND EVALUATION

70 'High Performance' Monolith Catalyst

Monolith Catalyst 60 measured CDE yield (%) measured CDE

50 012345 φ (-)

Figure 8.5: Decreasing the amount of diffusion limitations by way of monolith channel rounding prior to washcoating. Formation of the intermediate cyclododecene in the hydrogenation of cyc- lododecatriene.

that appear to migrate faster, or the apparent slowing down of cis-trans isomerization with increasing catalytic activity. Based on these results, a final layer of complexity is to be expected when hydrogen is not in abundant supply inside a catalyst. As a consequence, careful attention must always be paid to the Weisz-Prater or Thiele moduli of the reactants since they give a first indication towards the presence and extent of these phenomena.

8.2 Concerning the use of monoliths

The use of monoliths as carriers of catalytic material has been touted extensively in recent years for gas-liquid reaction systems. They offer low pressure drop when com- pared to other structured catalysts and the promise of enhanced gas to liquid and liquid to solid mass transfer when operated in the so-called Taylor flow regime. Easy separation of catalyst and reactant is an added benefit. However, the expense in manufacturing a monolithic catalyst cannot be taken for granted. One would be hard pressed to compete economically with an age-old process such as the (slurry) hydrogenation of vegetable oil. However, in case of fine chemicals manufacture where the added value of the product is high, monoliths can offer an attractive alternative to slurry technology. Monolith structures such as the cordierite monoliths used throughout this thesis are cheap and sturdy. For many years now, they have been the support of choice for auto- motive gas-exhaust catalysts. For use in gas-liquid reaction systems however, several problems arise. The decreased diffusivity of molecules in the liquid phase can cause dif- fusional limitations, with all the associated problems for selectivity. The characteristic

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CHAPTER 8

diffusional length inside a spherical particle equals one-sixth of the particle diameter, whereas for a slab-like coating on a monolith catalyst it equals the coating thickness, i.e. if all catalyst characteristics such as metal loading, porosity, tortuosity and so on can be kept equal one is still faced with the task of manufacturing a catalyst layer with a thickness of one-sixth of the diameter of the slurry catalyst one is trying to replace. The macropor- ous nature of cordierite makes this a difficult if not impossible task, in combination with the surface tension forces that tend to let a coating solution accumulate in the corners of a monolith channel. A method has been outlined that avoids these problems by first rounding the monolith channels with an inert material, on which a thin washcoat of even thickness could be deposited. These so-called High Performance Monolithic Catalysts (HPMC) indeed show better performance than monolith catalysts washcoated in a more conventional manner (MC). This is illustrated in Figure 8.5, were the observed Thiele modulus of a HPMC is compared with that of a MC for the hydrogenation of cyclododecatriene. Pre-rounding of monolith channels opens up an interesting new way of preparing monolithic catalysts, where attention can now be focussed on the preparation of even thinner washcoat layers. Monoliths with hexagonal channels could be a good alternative for square channels, as their shape will make rounding even easier.

8.3 Catalysis engineering

Heterogeneous catalysis, or more specifically heterogeneously catalyzed liquid phase hydrogenations, ranges from the mundane to the complex. The behaviour of a linear olefin such as 1-dodecene can be described satisfactorily with a limited number of parameters; it is enough to distinguish between the terminal double bond and the internal double bonds. This compared to the cyclic molecule cyclododecatriene whose behaviour can only be described by assuming subtle differences between the various isomers that are formed during reaction. Hydrogenation of methyl oleate can result in formation of no less than 31 isomers, posing quite a challenge in analyzing the reaction products. More importantly, this reaction demonstrates the sometimes subtle and confusing results of reactant diffusion limitations. For example, the rate at which cis-trans isomerization takes place relative to the rate of hydrogenation is controlled by the extent of diffusion limitations. Catalysis engineering is the art of designing a catalyst in such a way, that an optimal balance between activity and selectivity is found. The first decides the size of a reactor configuration, the second the amount of money and energy that has to be put into purific- ation of the products, cost of the raw materials and processing of the waste by-products. In Chapter 6, the possibilities for replacing a slurry catalyst with supported catalyst were looked at; a one-on-one translation of one into the other was not possible. The needs of a succesful monolithic catalyst require careful optimization which is not to be underestim- ated.

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SUMMARY AND EVALUATION

'ideal world' the hard way 1.0

Investigate what is 0.8 going on here...

0.6 by looking here...

0.4 catalyst effectiveness 0.2 ...to determine this

0.0 0.01 0.1 1 10 100 extent of diffusion limitations

Figure 8.6: Sometimes it will be easier to investigate the effects of diffusion limitations on a certain reaction system by considering ’worse’ catalysts, than to strive towards the perfect catalyst.

In Chapter 7, it was seen that one can come a long way in designing a monolithic catalyst such that it approaches a conventional slurry catalyst. In this case, the engineering part has almost delivered its full potential; for further increases in yield one must turn back to catalysis. A better active metal, or combinations of metal must then be found that will give intrinsically better performance.

In an ideal world, one would measure the true kinetics of a reaction and based on this predict what the effects would be of say, increasing catalyst size or activity. In other words, translate the behaviour of an ideal ’academic’ catalyst towards a ’real’ industrial catalyst. However, changing one catalyst property will often have unexpected effects on another; reducing catalyst particle size by grinding can cause changes in pore size, metal sintering and so on. Next to this, it can be very difficult to measure the true kinetics: sometimes the reaction rates are very high as is the case for 1-dodecene hydrogenation, or the reactants so bulky that their diffusivity will always remain a problem (e.g. fats). In these cases, the existence of diffusion limitations must be acknowledged and their effect studied. As illustrated in Figure 8.6, it will often be easier to ’make things worse’ than to prepare an ideal catalyst that still bears resemblance to the real catalyst under investigation.

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CHAPTER 8

8.4 Outlook on fat harding

It has been observed many times before that techology and technical feasibility are no panacea for society’s problems. It has been argued that the explosive growth of the world population in the twentieth century was to some extent made possible by the large scale application of the Haber-Bosch ammonia synthesis. The same ammonia that was used to manufacture fertilizers could just as easily serve as a nearly unlimited source of raw material for the manufacture of explosives. Only five years after the successful commercialization of ammonia synthesis, the unprecedented carnage of the first world war would prove in extremo that there is always a downside to technology and innovation. Fat hardening, although it was invented and reached early maturity in the same epoch has of course not had the same impact on our history. However, without it the diet of many people would have been severely nutritionally limited for many decades, even if one takes into account the harmful effects of the trans fats that have been consumed on such a large scale for so long. Now that a point has been reached where it may become possible to harden fats with significantly less formation of trans fats, it may no longer be economically viable to do so. Consumer perception in Western Europe now almost equates hydrogenation or hardening with unhealthy trans fats; manufacturers will find it easier to look for new sources of non-hydrogenated fat with the right properties than to change this perception. Because of this, any catalytic solution to the problem of trans formation in the hydrogenation of edible oils will most likely be too late in the offering.

Martijn Zieverink The Hague, 2007

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SAMENVATTING

Introductie. Heterogene katalyse houdt zich bezig met chemische reacties waarbij minstens twee en vaak ook drie fases betrokken zijn: een vaste, een vloeistof en een gas fase. De katalysator is meestal een vaste stof, vaak een metaal, dat in de vorm van kleine deeltjes (orde van grootte: nanometers) is aangebracht op een poreuze drager. De reactanten bestaan uit een vloeistof en een gas, die samen in contact moeten worden gebracht met het katalysatoroppervlak. In de praktijk betekent dit vaak dat de katalysator in poedervorm in een intensief geroerd vat met reactant wordt gedaan, waardoorheen vervolgens het gas wordt geborreld. Als proces heeft dit een aantal voordelen; het is een redelijk eenvoudig en bewezen concept. Belangrijk nadeel is dat het poeder na afloop verwijderd/afgefilterd moet worden. De chemische reactie die in dit proefschrift wordt onderzocht en beschreven is de hydrogenering van enkelvoudig onverzadigde bindingen tussen koolstofatomen. Deze dubbele binding kan niet alleen verzadigd worden, maar ook in een soort tussentoestand komen waarbij isomerisatie kan optreden. Dit heeft dan tot gevolg dat de dubbele binding gaat ’wandelen’ langs de koolstofketen (positionele isomerisatie) of van cis naar trans en vice versa isomeriseert. Dit laatste is van belang bij het hydrogeneren of harden van plantaardige vetten omdat in de natuur alleen cis dubbele bindingen voorkomen. Reactiekinetiek en diffusie-effecten. De snelheid en het gedrag van heterogeen ge- katalyseerde reacties worden zelden of nooit alleen bepaald door de soort katalysator. Een belangrijk aspect is de zogeheten stofoverdracht; de snelheid waarmee moleculen door diffusie een bepaalde afstand kunnen afleggen; bijvoorbeeld vanuit een gasbel naar de vloeistof, of vanuit de bulk van de vloeistof eerst door een dunne filmlaag aan de bui- tenkant van de drager en vervolgens door de poreuze drager naar de katalysatordeeltjes. Dit laatste heet diffusielimitering en kan soms onverwachte gevolgen hebben. In geval van een meervoudige reactie A B en vervolgens B C waarbij reactant A wordt om- gezet in B en B weer in C kan→ dit tot gevolg hebben→ dat er meer C wordt gevormd dan verwacht of gewenst. Doordat de intermediair B niet onmiddellijk de drager kan verlaten wordt de kans dat het doorreageert groter naarmate de tijd, die nodig is voor diffusie terug naar de vloeistof, langer wordt. In hoofdstuk 2 wordt gekeken naar de hydrogenering van 1-dodeceen; een reactie die zo snel verloopt dat deze onvermijdelijk diffusiegelimiteerd wordt. Dit heeft als gevolg dat de echte kinetiek, datgene wat er zich op katalysatorniveau afspeelt, ondergesneeuwd raakt. De waargenomen kinetiek daarentegen kon aan de hand van een aantal aannames

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met een simpel model beschreven worden. In hoofdstuk 3 wordt beschreven hoe de hy- drogenering van methyl oleaat en isomeren daarvan juist be¨ınvloed kan worden door te spelen met de mate waarin diffusielimiteringen een rol spelen. De resultaten in hoofdstuk 4 daarentegen zijn verkregen onder kinetisch bepaalde condities. Cyclododecatrieen be- vat drie dubbele bindingen die soms in de cis en soms in de trans toestand zijn, hetgeen resulteert in een complexe kinetiek tijdens de hydrogenering. Getracht is deze kinetiek te beschrijven met een zo simpel mogelijk model; een reactiemechanisme is voorgesteld dat de verschillende invloed van de waterstofconcentratie op de hydrogenerings- en isomeri- satiesnelheid verklaart. Een rode draad door hoofdstuk 2 tot en met 4 is het analyseren van complexe reactie mengsels met behulp van gas-chromatografie. Isomeren verschillen in chemische samen- stelling niet van elkaar en in fysische eigenschappen vaak heel weinig. Dit maakt het maken van een goed chromatogram en de juiste interpretatie daarvan een gecompliceerde zaak. Vetharding. Op 27 februari 1901 ontdekte de Duitser Wilhelm Normann dat onver- zadigde vetten in aanraking met waterstofgas in de aanwezigheid van een metaalpoeder reageerden tot verzadigde vetten. Meervoudig onverzadigde vetzuren worden zo terugge- bracht naar enkelvoudig of geheel verzadigde vetzuren, hetgeen de hardheid van het vet laat toenemen; vandaar de naam vetharding. Hoofdstuk 5 beschrijft dit proces in meer de- tail en geeft enige achtergrond bij het ontstaan en de gevolgen van de zogenaamde trans vetzuren. In hoofdstuk 6 wordt de traditionele manier van vetharding door middel van poederkayalysator in een geroerd vat vergeleken met het gebruik van een zogenaamde monolietkatalysator. Deze laatste bestaat uit een stuk keramiek opgebouwd uit honder- den of duizenden vierkante parallelle kanalen. Op de wand van deze kanalen wordt de poreuze drager aangebracht die het katalysatormetaal bevat. Groot voordeel hiervan is dat na afloop van de reactie er geen filtratie uitgevoerd hoeft te worden; de katalysator is immers ge¨ımmobiliseerd op de drager. Aangetoond is dat deze monolietkatalysator vele malen hergebruikt kan worden, hetgeen economisch interessant kan zijn. Daarnaast wor- den er bij gebruik van een monoliet minder trans vetzuren gevormd, hetgeen echter ten koste gaat van de selectiviteit naar enkelvoudig onverzadigde vetzuren. ’Catalysis engineering’. De term ’catalysis engineering’ omvat zowel de katalyse als het intrinsieke gedrag van reactanten die in contact komen met een katalyserend metaal oppervlak en alles dat vooraf gaat aan het daadwerkelijk in contact komen. Een belang- rijke conclusie van hoofdstuk 6 was dat het direct opbrengen van de katalysatordrager op de kanalen van een monoliet niet goed werkt: de drager neigt ernaar om in de hoeken van een kanaal op te hopen en dringt zelfs de keramische structuur binnen. Dit heeft als gevolg dat diffusieafstanden langer worden en selectiviteit naar gewenste producten la- ger. Hoofdstuk 7 richt zich daarom op de engineering van de monolietkatalysator. Een methode wordt beschreven om de kanalen eerst rond te maken met een inert materiaal om vervolgens een dunne, egale laag dragermateriaal te kunnen aanbrengen. Met behulp van deze techniek werden monoliet katalysatoren gemaakt die aanzienlijk beter presteerden dan de conventionele tegenhangers.

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Perspectief. Het voortschrijden van inzichten en technische mogelijkheden hoeft niet altijd tot nieuwe toepassingen te leiden. In het geval van vetharding is de huidige perceptie van de gemiddelde consument dat een product dat een gehard vet bevat altijd slecht is. De notie overbrengen dat vetharding niet noodzakelijkerwijs tot ongezonde trans vetten hoeft te leiden zou nog wel eens moeilijker kunnen blijken dan het overwinnen van de katalytische barrieres.`

Martijn Zieverink ’s Gravenhage, 2007

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DANKWOORD

Zoals zo vaak al is opgemerkt in dankwoorden van proefschriften, is promoveren allang geen soloaangelegenheid meer. Het is meer een (ontdekkings)reis die je alleen aflegt maar onderweg kom je wel de nodige mensen tegen die af en toe eens een stukje meelopen. Je maakt eens een praatje, steekt wat van elkaar op en vervolgens scheiden de wegen zich helaas vaak weer. Jacob en Freek, jullie stonden aan het begin klaar met een kaart en routebeschrijving. Toegegeven, er waren nogal wat witte plekken op die kaart en de aanwijzigingen waren ook niet altijd even duidelijk...desondanks ben ik heel tevreden over waar ik uiteindelijk ben beland. Michiel Kreutzer was mijn gids door de witte plekken, de dalen en de pieken. Michiel, ik kan je niet genoeg bedanken voor je geweldige begeleiding. Je stond altijd weer klaar met nieuwe ideeen,¨ dacht altijd mee en hebt eindeloos veel tijd en moeite in mij gestoken. Zonder jou had dit proefschrift niet zijn huidige vorm bereikt. Michiel Makkee wil ik bedanken voor het tonen van interesse in mijn werk en priveleven´ en omdat je altijd klaar stond als ik weer eens stoom moest afblazen. De discussies met Peter Verheijen over vakinhoudelijke zaken en (verre) afgeleides daarvan waren zeer waardevol en interessant. Ook ben ik Wim Buijs zeer erkentelijk voor het delen van zijn inzichten. Promoveren in de groep Moulijn (in de vijf jaar dat ik er deel van uitmaakte is de naam ervan drie keer veranderd; de aanduiding ’groep Moulijn’ blijft) betekent niet alleen maar onderzoek doen: zelf onderwijs geven, je eigen bescheiden kennis en vaardigheden weer overdragen is een belangrijk onderdeel van het werk. Ik beschouw het als een privilege dat ik meerdere studenten heb mogen begeleiden in de afsluiting van hun bachelor- of masteropleiding. Mark de la Vieter, Ivana Siriski, Daniel Costa, Carolina Faria, Bart Vermeer, Wouter Zwijnenburg; veel van jullie werk is door mij dankbaar opgenomen in dit proefschrift. Daarnaast heb ik veel van jullie geleerd over het omgaan met en begeleiden van mensen; ik hoop dat jullie niet teveel geleden hebben door de fouten die ik daarbij af en toe gemaakt heb. I immensely enjoyed the company of my roommates Archis Yawalkar, Tilman Schild- hauer and Manuel Baca, and of all the other individuals from the four corners of the world that roamed ’Het Lab’. Each of you taught me something of value from your own culture and background; I appreciate the wonderful diversity of this world a bit more because of you. Thanks also to Agust´ın Perez-Cadenas´ and Thorsten Boger for a fruitful cooperati- on. Dear Tracy Gardner, with much fondness I remember the many sessions in restaurants and pubs, discussing socialism, science and nearly everything else.

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Min of meer per toeval werd ik samen met Jasper Bakker aan het einde van mijn tijd in Delft betrokken bij het zogenaamde ’capillair’ project. Jasper, het was me een genoegen en ik kan niet wachten op de reeks publicaties die er aan zitten te komen! In mijn experimentele werk heb ik veel te danken aan de hulp van Harrie Jansma en Bart van der Linden; een woord van waardering ook voor Willem van den Bosch en Nico van der Knaap die op de meest ongelegen momenten toch altijd bereid waren om een gas- fles te wisselen. Els Arkenstein en Elly Hilkhuijsen wil ik bedanken voor de onvolprezen (administratieve) ondersteuning. Ik denk met plezier terug aan alle kletspraatjes die we hadden als ik weer eens moest wachten voor Jacob’s gesloten deur. Raffi Sharon wil ik bedanken voor het met frisse blik doorlezen van delen van het ma- nuscript en de vele waardevolle opmerkingen, inhoudelijk en anderzijds. Mijn paranimfen Joep Grooten en Jean-Pierre Gabriel¨ zijn mijn steun en toeverlaat geweest in moeilijke tij- den; het maakt de lol die we daarnaast vaak hebben alleen maar kostbaarder. Lieve Meike Holleman, jouw steun was doorslaggevend bij de laatste loodjes (Echt? Echt!). Tot slot wil ik dit proefschrift opdragen aan mijn grootvaders Frans Otten en Tinus Zieverink. Opa Frans heb ik vaak tot wanhoop gedreven met mijn eeuwige ’Waarom?’: ik hoop dat met dit boekje is aangetoond dat ik dat in ieder geval nog niet heb verleerd. Mijn voorliefde voor het vertellen van verhalen en anekdotes is rechtstreeks terug te voeren naar Opa Tinus. Een proefschrift over scheikundige onderwerpen is misschien niet de beste basis voor een goed en leesbaar verhaal. Als de lezer desondanks af en toe geboeid kon raken door het gebodene ben ik zeer tevreden.

Martijn Zieverink ’s Gravenhage, 2007

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PUBLICATIONS AND PRESENTATIONS

Publications

Analysis of transient permeation fluxes into and out of membranes for adsorption me- asurements. T.Q. Gardner, J.L. Falconer, R.D. Noble and M.M.P. Zieverink Chemical Engineering Science, 58(10), 2103-2112 (2003)

Monolithic catalysts as an alternative to slurry systems: hydrogenation of edible oil. T. Boger, M.M.P. Zieverink, M.T. Kreutzer, F. Kapteijn, J.A. Moulijn and W.P. Addiego Industrial and Engineering Chemistry Research, 43, 2337-2344 (2004)

High performance monolithic catalysts for hydrogenation reactions. A.F. Perez-Cadenas,´ M.M.P. Zieverink, F. Kapteijn and J.A. Moulijn Catalysis Today, 105, 623-628 (2005)

Combined hydrogenation and isomerization under diffusion limiting conditions. M.M.P. Zieverink, M.T. Kreutzer, F. Kapteijn and J.A. Moulijn, Industrial and Engineering Che- mistry Research, 44(25), 9668-9675 (2005)

Gas liquid mass transfer in lab-scale edible oil hydrogenation. M.M.P. Zieverink, M.T. Kreutzer, F. Kapteijn and J.A. Moulijn Industrial and Engineering Chemistry Research, 44(25), 9668-9675 (2005)

Selective hydrogenation of fatty acid methyl esters on palladium catalysts supported on carbon coated monoliths. A.F. Perez-Cadenas,´ M.M.P. Zieverink, F. Kapteijn and J.A. Moulijn Carbon, 44(1), 173-176 (2006)

Lectures

Reusable monolithic catalysts. An alternative for slurry vegetable oil hydrogenation. M.M.P. Zieverink, M.T. Kreutzer, F. Kapteijn and J.A. Moulijn NIOK symposium, 25 September 2003, Utrecht, The Netherlands

Effect of mass transfer on cis / trans isomerization of fatty acid methyl esters. M.M.P. Zieverink, M.T. Kreutzer, F. Kapteijn and J.A. Moulijn Netherlands’ Catalysis and Che- mistry Conference (NCCC) VI, 8 March 2005, Noordwijkerhout, The Netherlands

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High performance monolithic catalysts. A.F. Perez-Cadenas,´ M.M.P. Zieverink, F. Kap- teijn and J.A. Moulijn WOG workshop The Active Site: From Catalyst to Reactor, 19-20 May 2005, Brussels, Belgium

Combined hydrogenation and isomerization under diffusion limiting conditions. M.M.P. Zieverink, M.T. Kreutzer, F. Kapteijn and J.A. Moulijn 7th World Congress of Chemical Engineering (WCCE), 10-14 July 2005, Glasgow, Scotland

Poster presentations

Downscaling of an industrial hydrogenation: Fat Hardening on Lab Scale. M.M.P. Zieve- rink, M.T. Kreutzer, F. Kapteijn and J.A. Moulijn Netherlands Process technology Sym- posium (NPS3), 28-29 October 2003, Veldhoven, The Netherlands

Reusable monolithic catalysts: an alternative for slurry vegetable oil hydrogenation. M.M.P. Zieverink, M.T. Kreutzer, F. Kapteijn and J.A. Moulijn Netherlands Process tech- nology Symposium (NPS3), 28-29 October 2003, Veldhoven, The Netherlands

High performance monolithic catalysts. A.F. Perez-Cadenas,´ M.M.P. Zieverink, F. Kap- teijn and J.A. Moulijn Netherlands Catalysis and Chemistry Conference (NCCC) VI, 8 March 2005, Noordwijkerhout, The Netherlands

Hydrogenation of fatty acid methyl esters: evaluation of kinetic aspects. M.M.P. Zieve- rink, M.T. Kreutzer, F. Kapteijn and J.A. Moulijn 5th International Symposium on Ca- talysis in Multiphase Reactors and the 4th International Symposium on Multifunctional Reactors (CAMURE-5 and ISMR-4), 15-18 June 2005, Portoroz, Slovenia

Fast, safe, facile and economical determination of reaction characteristics by using fused silica capillaries. Jasper J.W. Bakker, M.M.P. Zieverink, M.T. Kreutzer, F. Kapteijn and J.A. Moulijn Netherlands Process technology Symposium (NPS6), 24-25 October 2006, Veldhoven, The Netherlands

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CURRICULUM VITAE

Martijn Zieverink werd geboren op 26 september 1975 te Sittard. Na het behalen van het Atheneum B diploma aan het Sintermeertencollege (voorheen Coriovallumcollege) te Heerlen studeerde hij drie maanden als scholarship student aan de Utah State University. In september 1993 begon hij met de studie scheikundige technologie aan de Technische Universiteit Delft. In maart 2001 studeerde hij af onder leiding van prof. dr. F. Kapteijn. In augustus 2001 begon hij als promovendus met zijn promotieonderzoek aan de Techni- sche Universiteit Delft, de resultaten waarvan staan beschreven in dit proefschrift. Op dit moment is hij werkzaam als Scientist Fats and Oils Physics bij Unimills (Member of the Golden Hope Group) te Zwijndrecht.

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