Propene Acetoxylation in the Liquid Phase-influence of Technological Parameters and Pretreatment of the Catalyst Support on the Course of the Process
Marcin Bartkowiak*, 1 and Stanisław Lenart2
1West-Pomeranian University of Technology, Szczecin, Institute of Organic Chemical Technology, Pulaskiego 10, PL 70-322 Szczecin, Poland 2West-Pomeranian University of Technology, Szczecin, Institute of Materials Science and Engineering , Piastów 19, PL 70-310 Szczecin, Poland
Abstract: The novel and more safe method for propene acetoxylation has been developed. The reaction is conducted in the liquid environment of acetic acid, which is also the one of the substrates for this process, in the presence of palladium catalyst supported onto activated carbon. The influence of acetic acid concentration, temperature and pretreatment of the carbon catalyst support on the selectivity of transformation to allyl acetate and propene conversion has been investigated. The influence of the parameters has been described using the following factors: the selectivity of transformation to allyl acetate in relation to propene consumed and degree of propene conversion. Preparation of the catalyst and its characteristics has been described. Catalysts were prepared by wet impregnation method and pelletized activated carbon was used as a support. Prepared catalysts were analysed using XRD, SEM-EDX and UV-VIS DRS methods. Carbon support was pretreated before impregnation due to the literature reports. Thus the final catalyst samples were different from one another in terms of quantity and degree of dispersion of active phase on the support surface. Activity of catalyst samples has been tested in propene acetoxylation as the one of reaction parameters.
Introduction addition of copper(II)chloride as the reoxidizing agent In the recent years acetyl derivatives of small has increased efficiency of the palladium catalyst. alkenes increased its significance in the production of This discovery stimulated many further scientific valuable intermediates in chemical industry. Aceto- works in the field of palladium catalyst chemistry and xylation is one of the industrial oxidative esterification became the significant contribution to the modern processes, where the oxygen indirectly supports the organic synthesis (1). Subsequent research on reaction between acetic acid and alkene. Acetoxylation palladium and the PGMs catalyzed organic reactions processes found the application as the competitive led to novel methods of the carbon-carbon bonds methods of production of many chemicals and they formation. The importance of these discoveries has replaced many older methods, which were more been appreciated in 2005 and 2010 in the Nobel Prize undesirable for the environment. in Chemistry. Acetoxylation processes require appropriate During the second half of the 20th century catalysts, homogeneous or heterogeneous depending palladium catalysts were significantly and continuously on the phase of substrates and the method implemented. improved and applied to the large number of chemical Platinum group metals (PGMs) are most often used as processes. One of them was the acetoxylation process. the catalysts for acetoxylation reactions. Especially In the half of 60's first experiments of ethylene and palladium is efficient and selective catalyst for propylene oxidation in acetic acid were described by numerous syntheses of alkene acetates. It was found in Belov et al (2). They used palladium chloride as a the middle of the 1950s, that palladium can be used as catalyst and glacial acetic acid as reaction environment very effective catalyst in the production of and substrate. acetaldehyde in the novel way called Wacker process, In the early 1970s acetoxylation of alkenes in where PdCl /CuCl was used as the catalytic system in 2 2 gaseous phase has been developed and progressively the homogeneous reaction environment. Moreover the improved by many scientific teams (3-5) during the
next two decades (e.g. Hoechst AG, Bayer AG, *Corresponding authors; E-mail address: [email protected] Philips Petroleum Co. and more). Complete study of
ISSN 1203-8407 © 2015 Science & Technology Network, Inc. J. Adv. Oxid. Technol. Vol. 18, No. 2, 2015 303 M. Bartkowiak and S. Lenart acetoxylation of small olefins in a gaseous phase was remember – the safety of the process. Chen and Lee presented in literature by Kunugi et al in 1970 (6). described the safety aspects of propene acetoxylation Tubular flow reactor with stationary catalyst bed was (13). Acetoxylation is a kind of the oxidation reaction, used in those experiments. Active carbon and alumina using highly flammable mixture of hydrocarbon and were used as catalyst supports and both metallic oxygen in the presence of catalyst and acetic acid. It's palladium and its salts were used as catalysts. important question to compose the feed mixture out In the early 1990s Sano et al. developed and the range of flammability. Unfortunately the presence implemented into industry (Showa Denko K.K.) novel of palladium catalyst makes the additional complica- method of propene acetoxylation. The catalyst for this tions, because in the presence of catalyst the safe process was Pd and Cu (or other reoxidant) onto alkene/oxygen ratio can vary in unknown way. Thus alumina support (7). Next three decades brought the acetoxylation in gaseous phase requires appropriate considerable increase of the number of new catalysts and precise control of the process parameters. for alkene acetoxylation, new oxidation agents and However the risk of ignition or explosion is still new mechanistic studies of the interactions between present in this method. Moreover the gas phase substrates and catalyst (1, 8-11). method requires increased pressure and temperature, The main and most wanted product of the propene thus the economical aspects of the process should be acetoxylation process is allyl acetate. It is very take over consideration. valuable compound for modern chemical industry. Acetoxylation of liquid hydrocarbons has been Allyl acetate can be used i.e. for the low-waste described in literature (1, 14-17). In most cases there production of glycerol epichlorohydrin (12) and it also were used typical homogeneous catalysis systems finds numerous applications in the production of the with palladium salts or complexes as active agents. auxiliary compounds for many industrial applications. From literature reveals that homogeneous palladium Propene catalytic acetoxylation can be presented catalysts used in acetoxylation processes are generally in the following reaction equations: selective and efficient but they have some dis- advantages too. Major disadvantage is difficult separation of post- reaction mixture and solution of catalyst, especially it's difficult to implement in the industrial scale. Thus the economic costs of tech-
Acetoxylation of propene using palladium nology implementation catalysts also leads to the formation of other by- and production increase. products: isopropenyl acetate (eq. 2) and n-propenyl The major hazards in propene acetoxylation can acetate (eq.3). Moreover some diacetates are formed be decreased using liquid phase method, but it's in the trace amounts. Isopropenyl acetate undergoes necessary to improve the separation of post-reaction hydrolysis reaction to form acetone. Carbon dioxide is mixture. The simple way is to use the catalyst the predominant by-product of this reaction and has to supported not homogeneous. The novel and more safe be removed from the post-reaction stream in the method for propene acetoxylation has been developed. absorber. The reaction is conducted in the liquid environment of The most widespread method of allyl acetate acetic acid, which is also the one of the substrates for production is gas-phase method using heterogeneous this process. palladium catalyst. In fact it is Showa Denko This method is the combination of the standard technology (7) modified and improved for the better method of acetoxylation of propene in gaseous phase efficiency. This method has been investigated by with the liquid phase methods of acetoxylation, where many companies and research teams over the last two the homogeneous catalysts were used usually. decades, as it can be found in the patent offices' Described process of propene acetoxylation makes use databases. There is a small number of journal of palladium catalyst supported onto activated carbon publications concerning acetoxylation of alkenes in and suspended in reaction mixture. gaseous phase, but there is an important aspect to Activated carbon used as support has a lot of
304 J. Adv. Oxid. Technol. Vol. 18, No. 2, 2015 M. Bartkowiak and S. Lenart advantages (18). It is inert both in acetic or basic environment, moreover it has very extended specific surface thus fulfil a condition to maintain the catalyti- cally active phase (metals or metal compounds) in a highly dispersed state. Carbon as a catalyst support has sufficient stability to be used in many catalytic processes. PGMs (platinum group metals) can be easily deposited onto its surface in many known ways described in literature. Moreover activated carbon is a convenient kind of support due to the easy metal recovery from waste catalysts – by simple incineration. The main disadvantage of activated carbon is its low mechanical strength, which results in progressive crushing of catalyst pellets in some processes, especially when the catalyst bed is not stable (fluidized etc.). In this work has been presented the experimental results of propene acetoxylation in liquid acetic acid medium using Pd/C catalysts prepared on modified supports. Influence of acetic acid concentration, temperature and pretreatment of the catalyst support on the course of the process has been examined.
Figure 1. Scheme of apparatus for acetoxylation of propene in Experimental liquid phase. Reagents Acetoxylation of propene was conducted in liquid contains a portion of granulated catalyst (Pd+Cu onto phase in acetic acid environment. Molecular oxygen activated carbon) and ran under atmospheric pressure. was used as an oxidant together with an excess of Propene, nitrogen and oxygen were introduced to the acetic acid. Propene is the main substrate and it was reactor through the flow control valves, and the introduced to the reactor in gaseous phase. Gaseous porous sparger mounted in the bottom of the reactor. nitrogen was used as inert gas to blow through the Gaseous reagents were passed through the column of reaction line in post-process conditions and optionally liquid and the catalyst. The last one was present in the as a diluent for substrate mixture. Propene 2.8 was quasi-fluidized state during the gas flow. Reactor was supplied by Linde Gas Poland, oxygen 4.0, hydrogen heated by built-in electric heater with temperature 4.0 and nitrogen 2.8 by Messer Poland, acetic acid controller. 99,5% by POCh Gliwice Poland. Precursors of the Important section of apparatus is absorber of catalysts were delivered from Aldrich: PdCl2 99% and organic products with the system of gas circulation. It CuCl2 99%, as well as standards for GC : allyl acetate is simple method for separation of allyl acetate and 99% and isopropenyl acetate 99%. Toluene 99,8% by-products from aqueous medium. The peristaltic from POCh Gliwice Poland was used as solvent in the pump draws in the mixture of reaction gases and absorber. Active carbon AG used as support for the vapours from the top of reactor to the absorber filled catalyst was supplied by Gryfskand, Poland. with toluene (or other appropriate organic solvent), where organic products dissolves. Purified gaseous Apparatus mixture of substrates returns to the reactor through the The novel type of apparatus was proposed for this pump, the feed line and sparger. There is additional process - a liquid phase reactor equipped with a advantage using recirculation system – this operation circulation pump to reflux reaction gases back to the improves mixing intensity in the reaction tube. liquid reaction mixture. Construction of the apparatus Unreacted substrates are evacuated from reactor is shown in Figure 1. through the condenser to the last section of apparatus - the cold trap, which is used to condense unreacted Procedure of Process Operation propene. Mass balance of propene was calculated Acetoxylation has been conducted in the tubular from the amount introduced to the apparatus and glass reactor. Reactor was filled with acetic acid, concentration of propene in the stream of unreacted
J. Adv. Oxid. Technol. Vol. 18, No. 2, 2015 305 M. Bartkowiak and S. Lenart gaseous reagents (at the end of line), measured with ignition was very low. Moreover the method used for GC method. acetoxylation – in the excess of acetic acid aqueous Additionally these calculations were checked by solution is relatively safe. From preliminary studies mass control of unreacted propene captured in the revealed that dilution of substrate mixture with inert cold trap. The cold trap was a battery of three glass gas decreases the degree of alkene conversion, thus it cylinders filled with glass Raschig rings to increase was decided to use the mixture of pure reagents the heat-exchange surface. These cylinders were without diluent (N2). Oxygen concentration in the cooled to -55 ºC in the cryostat. reaction stream should not be lower than 33 vol.% As the main factors describing the acetoxylation (stoichiometric limit), while from economical point of of propene were used degree of conversion of propene view it is favourable to decrease alkene concentration and selectivity of transformation to allyl acetate in down to the safe limit. Thus the acceptable range of relation to propene consumed. Degree of conversion propene concentration is 53-67 vol.%. The lower limit of propene has been presented as amount of reacted of this range was selected for described studies, so the propene divided by total amount of propene passed process was conducted with the slight excess of through the reactor. Conversion has been calculated oxygen. using equations below (the first one has been used with GC analyses of reaction stream input and output, Preparation of Catalysts. Modification of the second one has been used with mass calculations Support Surface of cold trap and gas source). Catalysts for propene acetoxylation were prepared using wet impregnation method (7, 20-23). First , activated carbon AG used as support was sieved to
P P remove dust, then washed with distilled hot water. where: C0 and C means concentrations of propene in The support was boiled for 6-8 hours in hydrochloric reaction stream analysed in the inlet and outlet of the P P acid solution to remove ash constituents. Next it was reactor; n0 and n are the molar amounts of propene boiled in distilled water for 6 hours. This operation conducted to the reactor and collected from reactor was repeated 4 times with fresh portion of water. outlet respectively. Carbon was then dried and outgassed in vacuum Selectivity of transformation to allyl acetate in evaporator. relation to consumed propene has been calculated Impregnating agent was prepared by dissolving using the following equation. PdCl2 and CuCl2 in hydrochloric acid solution to 2+ 2+ prepare precursor solution of Pd and Cu with , known concentration. This solution was analysed in where: nAA is molar amount of allyl acetate produced UV-VIS spectrophotometer to prepare calibration P P per unit time and (n0 - n ) is the molar amount of curve for next analyses of residual impregnating propene consumed per unit time. agent. Acetoxylation of propene is conducted under From literature reveals, that activated carbon atmospheric pressure, insignificantly increased by the surface can be modified in different ways (16, 20). hydrostatic pressure of acetic acid in reactor and Modification of carbon surface can improve its resistance of flow in feed line and sparger. properties as a catalyst support. However there are no Temperature range of reaction is below the clear-cut conclusions about the influence of carbon boiling point of water – acetic acid solutions (100,1- modifications. It was considered, that it would be 112 ºC) and is from 70 to 95 ºC. useful to check how big is the impact of AG carbon The safety aspects of the process were took under surface modification on the course of acetoxylation consideration in relation to process parameters (volu- process. metric ratio of main substrates - propene and oxygen) Three methods of carbon modification have been due to explosion risk. Range of flammability for selected to implement. Modification of the carbon propene is 0,9-2,0% in the air and 2,1 - 52,8 in pure surface was conducted before the impregnation. oxygen (19). All the experiments were carried out Carbon has been divided into three portions. First with constant volumetric ratio of gaseous reagents portion of carbon has been oxidized in the stream of equal to 1:1. Thus in the reaction mixture and in the molecular oxygen, to increase the amount of oxide post-reaction gas stream the concentration of propene active sites onto carbon surface. The second one has was slightly below the upper flammability limit been initially reduced in the stream of molecular (calculated in relation to pure oxygen), so the risk of hydrogen, to clean the surface from oxide groups and
306 J. Adv. Oxid. Technol. Vol. 18, No. 2, 2015 M. Bartkowiak and S. Lenart other useless impurities. The last portion of carbon impregnation process were carried out using UV-VIS was then heated in the stream of inert nitrogen, thus spectrometer Spectroquant Pharo300 (Merck). UV- the surface has been cleaned and the amount of active VIS analysis is a simple, fast and accurate method for sites has been mildly modified. These three portions determining precursor concentration in the post- of carbon supports were named as Kx, where x is the impregnation solution. appropriate gas symbol (hydrogen, oxygen and nitrogen). Spectrophotometry UV-VIS DRS Temperatures, gas flows and the time of the Diffuse reflectance UV-VIS spectra of catalyst carbon modifications were as follows: samples were recorded using Jasco V-650 spectrometer
Table 1. Technological parameters of carbon modifications equipped with horizontal integrating sphere attachment (PIV-756) for powdery samples. Temperature Time Gas flow Sample Reagent 3 (ºC) (min) (cm /min) Gas Chromatography and GC-MS System KH 300 190 H2 20 In order to determine the composition of products KO 170 200 O2 20 of propene acetoxylation, the method of gas KN 300 190 N2 20 chromatography was used. Analyses were carried out on a Trace GC 2000 chromatograph (Thermo- Portion of support was weighted, and placed into Finnigan). A RTX-5 capillary column Restek 30 m x Erlenmeyer flask. Distilled water was then added to 0,53 mm x 1,5 μm was used. The detector temperature the flask in amount sufficient to cover support pellets was 280 ºC , whereas of sample chamber was 250 ºC . with layer of 2-3 milimeters height. Then the The column temperature was programmed as follows: precursor solution was added dropwise to the flask. isothermally at 40 ºC for 5 min, then increase to 80 ºC Total amount of precursor solution introduced to the at the rate 15 ºC/min and isothermally at 80 ºC for 10 flask was controlled by weight. Amount of precursor min. The flow gas was as follows: air 200 cm3/min, adsorbed onto carbon was determined by UV-VIS hydrogen 35 cm3/min, pressure of carrier gas (helium) analysis of the solution after impregnation. Impregnated 45 kPa with constant pressure mode. The identification support was then dried and outgassed in vacuum of components of post-reaction mixtures was evaporator. performed by a comparison of the retention time of From literature reveals that alkalization of peaks on the chromatograms with the retention time of impregnated support can improve catalytic properties standard substances. A quantitative composition of of prepared catalyst, but some authors described their post-reaction mixtures was established by the method catalyst preparation without alkalization. of external standard. It was necessary to check out the influence of alkalization on the yield of prepared catalysts. An additional confirmation of peaks arrangement Impregnated and dried supports were divided into was achieved by the method of standard addition. GC- two parts. One part of the each catalysts was alkalized MS system was used as final identification tool for all using solution of NaOH, second part was left without constituents in post-reaction mixture and toluene from alkalization. Thus six samples of catalysts were the absorber. Analyses were carried out on a Trace obtained, described as Kx and KxA (alkalized). GC 2000 chromatograph (Thermo-Finnigan) equipped The last step in catalyst preparation was the with quadrupol MS detector with electron ionisation reduction of supported precursors. Reduction was worked in the condition: the source temperature 200 carried out in the steel tubular reactor, heated by ºC, the detector voltage 50V, the emission current 150 electronically controlled heater. Reactor was filled μA and ionisation energy 70 eV. Capillary column with impregnated support and connected to the source J&W DB-5MS was used. Analytical parameters were of molecular hydrogen. Temperature of the reduction the same as during simple GC analyses. was 110-130 ºC, under atmospheric pressure. Nitrogen 4.0 was used as inert gas used before and after SEM and X-Ray Microanalysis reduction (during warming and cooling of the reaction Prepared catalysts were analyzed for their micro- bed). After the stage of reduction the catalyst was structure and elemental analysis using scanning dried and stored under argon. electron microscope JEOL JSM 6100 operating at 20 kV and equipped with EDX detector (Energy Analytical Methods and Procedures Dispersive X-Ray Spectroscopy) ISIS 300 Oxford Spectrophotometry UV-VIS Instruments. EDX is a simple and accurate analytical Analyses of impregnating agent solution during technique used in qualitative and quantitative
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Figure 2. Example mass spectra of reaction products acquired during GC-MS analyses. elemental analysis of the solid surfaces, esp. metals calculated for the assumed process time, constant for with high Z (atomic number). each experiment and equal to 2 hours. Conversion of Elemental analysis of catalyst surface was propene was calculated from the results of quantitative conducted and the surface map of palladium and GC analyses of samples taken from the outlet gas copper on the support surface was obtained. Results of stream. Additionally these calculations were confirmed SEM-EDX allow to determine the degree of dispersion by weight of cold trap and the amount of propene of metals on the support and the composition of the introduced to the reactor during the process. Selectivity active phase of the catalyst. Catalyst samples were to allyl acetate and propene conversion were assumed outgassed in vacuum evaporator and stored under as two main factors which characterize the propene argon. Then analysed directly as small pellets, without acetoxylation process. Other organic by-products were any breaking up or special forming. Surface analyses determined qualitatively by GC/GC-MS analysis, but were conducted with a given number of repetitions were not balanced. (10-15) in different points of each sample. Elemental Qualitative analysis of post-reaction mixture compositions of each sample were took as arithmetic proved the presence of expected products as reveals mean value of all measurements made for this sample. from the literature. Allyl acetate as main product was produced together with typical by-products for this XRD Analysis reaction: isopropenyl acetate, n-propenyl acetate, Powder X-ray diffraction analyses of obtained diacetates and acetone as hydrolysis reaction product. catalysts were carried out using Philips X'Pert PRO Allyl alcohol was not presented in the post-reaction diffractometer. The radiation was filtered CuKα. The stream, because hydrolysis of allyl acetate requires diffraction patterns were recorded in the range of 10⁰ different reaction conditions and proper catalyst. to 90⁰. Example mass spectra of reaction products were presented below in Figure 2. Results and Discussion Resulting mass spectra are not very clear, but all The influence of acetic acid concentrations, the most important signals were found in the results. temperature and catalyst support pretreatment on the Thus it was enough to determine the presence of the course of the acetoxylation process were investigated. products in post-reaction mixture. The main product of propene acetoxylation is allyl Preliminary studies of the influence of acetic acid acetate. Isopropenyl acetate, acetone and carbon concentration and temperature on the course of the dioxide are the by-products of the reaction. The ranges process were conducted and the results are as follows. of changes of studied parameters as well as methods of catalyst surface modifications were assumed on the Influence of Acetic Acid Concentration basis of the literature data. Selectivity of transformation A series of syntheses was performed in order to to allyl acetate in relation to reacted propene was determine the influence of the acetic acid concentration
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Figure 3. Influence of acetic acid concentration on the course of Figure 4. Influence of temperature on the course of propene propene acetoxylation. acetoxylation. on the course of acetoxylation of propene in liquid active centres for further reaction steps (24-27). phase. Reactions were carried out in the following As shown in Figure 3, increase of AcOH concen- conditions: propene flow = 10 cm3/min, oxygen flow tration is directly proportional to increase of conversion = 10 cm3/min, recirculation flow – 360 cm3/min, and selectivity. First publications describing aceto- temperature 90 ºC, catalyst sample KHA. Results of xylation reaction (op.cit. 1,2) shows that many the experiments are presented in Figure 3. experiments conducted in homogeneous environment Propene conversion and selectivity of transforma- were carried out using glacial acetic acid. However in tion to allyl acetate increase along with the increase of reaction conducted with heterogeneous catalyst is acetic acid concentration. This increase of conversion preferable to use solution of AcOH due to more and yield of AA were more significant in the range of efficient separation of reaction products, because concentration 10-50 wt.% and less significant above solubility of allyl acetate (limited in water) increases the concentration of 50 wt.%. Thus the optimal with increase of acetic acid concentration and more of concentration of acetic acid in the reaction mixture is allyl acetate remains in post-reaction mixture instead in the range 40-50 wt.%. Using the higher concen- of be transferred to the absorber of organics. tration of acetic acid is economically and environ- mentally unfavourable. From the results reveals that Influence of Temperature acetic acid concentration equal to 40% is the optimal In order to determine the influence of temperature value for the next studies. on the course of propene acetoxylation its value was Influence of acetic acid concentration on the varied in the range from 70-95 ºC. A series of syntheses reaction can be explained as follows. was performed under the following conditions: A detailed mechanism of heterogeneous propene concentration of AA = 40 wt%, catalyst sample KHA, acetoxylation has not been verified yet. Few proposed reagent flows = 10 cm3/min, recirculation flow – 360 mechanisms were described in literature for similar cm3/min. The experimental results are presented in process of vinyl acetate formation, with debate Figure 4. regarding whether Pd is active as Pd(0) or Pd(2+). Degree of conversion of propene and selectivity to Moiseev et al (14) presented theory of Pd(0) active allyl acetate increase significantly along with the sites. Other researchers argued that active site is increase of temperature. Optimal temperature for probably surface acetate intermediate formed by propene acetoxylation in liquid phase is 90 ºC and adsorption of acetic acid onto surface of catalyst. This must not exceed 95-100 ºC (temperature close to the mechanism is consistent with Langmuir - Hinshelwood boiling point of reaction mixture for optimal range of type of surface reaction mechanism. acetic acid concentration). Increase of temperature of In summary - from literature reveals that most propene acetoxylation promotes higher degree of convincing theory of reaction mechanism of propene propene conversion and selectivity of transformation acetoxylation is similar to theory of VAM (vinyl to allyl acetate. It can be explained with fundamentals acetate monomer) production. The rate determining of heterogeneous catalytic fluid-solid reaction. step of reaction is C-O coupling of vinyl (allyl) and Individual steps of simple reaction in such acetic species. More active sites on the surface of conditions are well known (diffusion from bulk phase catalyst can be formed proportionally to amount of to external surface and then into the pores, adsorption acetic acid and alkene adsorbed. Thus increase of - reaction - desorption and diffusion of the products acetic acid concentration promotes the number of from pores to the bulk phase.
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Table 2. Elemental composition of the surface of analysed catalysts.
Sample name Pretreatment Pd [%] Cu [%] Pd/Cu ratio Pd conc. [wt %]
KH H2/Δ 93,7 6,3 15 3,5
KHA H2/Δ 95,0 5,0 19 3,4
KO O2/Δ 91,7 8,3 11 2,9
KOA O2/Δ 94,0 6,0 16 2,8
KN N2/Δ 95,5 4,5 21 2,6
KNA N2/Δ 95,7 4,3 22 2,5
Figure 5. SEM image and EDX mapping of KHA catalyst sample Figure 6. SEM image and EDX mapping of KH catalyst sample (mag. x1000). (mag. x1000).
External mass transfer can be slower or faster Temperature influences the sorption processes on depending on the state of catalyst-reactants system the catalyst surfaces and subsequently affects the (solid phase - liquid phase, solid phase - gas phase). reaction rate. Higher temperatures promote faster Moreover it depends on the flow rate conditions diffusion and mass transfer steps, thus decreases the (temperature, pressure, solvent etc.). Described impact of these steps on the overall reaction rate. catalytic system of propene acetoxylation is more Observed increase of reaction yield with increased complicated, because solid state catalyst contacts with temperature (Figure 4) can be interpreted as effect of reagents in liquid (AcOH) and gaseous phase (propene continuous decrease of mass transfer and diffusion and oxygen). From literature reveals (28 p.670) that in influence on the course of the process. binary reactant systems the solubility of gaseous reactants in the liquid phase affects increased reaction Influence of the Catalyst Support Pretreatment rate then improves the degree of conversion. A series of syntheses was performed to determine Moreover, depending on temperature conditions, the influence of the catalyst support pretreatment on the influence of internal mass transfer (into pores the course of the propene acetoxylation. Technological towards active sites) can be observed. Generally, for parameters of carbon support modifications were lower reaction temperatures higher influence of shown above in Table 1. Every catalyst sample were external mass transfer on the overall reaction rate analysed using SEM-EDX and XRD techniques. exists, but for higher temperatures internal mass transfer (forced flow in pores generated by pressure Elemental composition of the surface of catalyst difference across the pore) takes a part in the overall samples was presented in Table 2. reaction rate. (28 p. 672). SEM images of catalyst samples and EDX Described process utilizes slurry reactor type, mapping of metals on the surface of catalysts was where a number of steps of reaction exists such as shown in the Figures 5-10. gas-liquid and liquid-solid mass transfer, and chemical Saturation of the support with palladium and reaction. copper is high as well as degree of dispersion of active
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Figure 7. SEM image and EDX mapping of KOA catalyst sample Figure 9. SEM image and EDX mapping of KNA catalyst sample (mag. x1000). (mag. x1000).
Figure 8. SEM image and EDX mapping of KO catalyst sample Figure 10. SEM image and EDX mapping of KN catalyst sample (mag. x1000). (mag. x1000). phase on the surface of catalyst. Distribution of metals UV-VIS DRS analyses were recorded for each on the surface of carbon was generally uniform. catalyst sample. Unfortunately the spectra obtained The degree of dispersion of palladium onto carbon during analysis were not satisfying due to the high surface insignificantly differs for each sample and the absolute absorbance of the carbon samples and low best dispersion was achieved for KH and KHA concentration of the palladium (it not exceed 3.5 wt samples. XRD patterns of selected samples were %). Example of the KOA analysis using UV-VIS DRS shown in Figure 11 and 12. spectrometry was presented in Figure 13. Due to the low amount of active phase on the There are no characteristic bands on KOA carrier, only palladium peaks were detected in selected spectrum in comparison to the spectrum of pure samples (copper concentration was considerably lower support. The UV-VIS spectrum of KOA catalyst than Pd as shown in Table 2). No oxide form of sample hardly differs from the spectrum of pure AG palladium were detected, only Pd0 phase giving the carbon. characteristic diffraction peaks at 2Θ = 40⁰, 46.5⁰, Moreover the spectrum presented is very similar 67⁰, 82⁰ and 86.5⁰. The broad peaks of 2Θ = 25⁰ and to the spectra of the further catalyst samples (KHA, 44⁰ represents carbon structure of the support. KNA, KO, KH, KN).
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Figure 11. XRD patterns of K-samples of catalysts. Figure 13. UV-VIS DRS analysis of example catalyst (KOA) versus pure AG activated carbon. Measurement Information: Instrument name UV-VIS, Model name V-650, Accessory PIV- 756, Photometric mode Abs, Measurement range 900 - 190 nm, Data interval 1 nm, UV/Vis bandwidth 5.0 nm, Response Fast, Scan speed 400 nm/min, Change source at 340 nm, Light source D2/WI, Filter exchange Step, Correction Baseline.
Figure 12. XRD patterns of KA-samples of catalysts. Figure 14. Influence of catalyst support modification on propene conversion. High absolute absorbance of powdered carbon supported catalysts may be the main reason of transformation to allyl acetate. Lower concentration of unsuitability of this analytical method in described copper (higher Pd:Cu ratio) results in the increase of case. However from literature reveals that PGM's can selectivity of transformation to acetate. be effectively determinated using UV-VIS DRS The degree of conversion of propene and method on the supports like silica, titania or alumina selectivity of transformation to allyl acetate in relation (29). to propene consumed increase along with an increase Reaction parameters of propene acetoxylation of Pd concentration on the surface of catalyst. were as follows: propene and O2 flow were set as 10 Alkalizing of impregnated support decreases the cm3/min, temperature 90 ºC, acetic acid concentration degree of conversion of alkene, but increases the in reaction mixture was 40 wt.%, recirculation flow – selectivity of transformation to allyl acetate in relation 360 cm3/min. Three syntheses were performed for to propene consumed. each catalyst sample. Results of the experiments are Modification of active carbon surface by heating presented in Figure 14 and 15. in an inert atmosphere (nitrogen treated samples KN, Modification of surface of catalyst support has a KNA) increases the amount of inert and weak acidic direct influence on the amount of adsorbed palladium functional groups. This impacts on the precursor precursor. The amount of palladium has changed for adsorption and reduces the possibility of sintering. subsequent samples in the range from 2,5 to 3,5 wt.%. Modification of active carbon surface by treating From EDX analysis and activity tests reveals that that with molecular oxygen and heat (KO and KOA Pd:Cu ratio has the influence on the selectivity of samples) increases the amount of acidic groups. This
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to 60 wt%, preferably 40-45 wt%, because the increase of reaction yield is not significant in given range, thus lower AcOH concentration is economically preferred. Influence of temperature on the reaction yield is significant and most favourable temperature of studied process is 90 ºC. Higher temperature (up to boiling point of acetic acid solution) is not preferred, because construction of apparatus is not adapted to work in such conditions. Moreover boiling of reaction mixture Figure 15. Influence of catalyst support modification on the would make it difficult to effectively separate allyl selectivity of transformation to allyl acetate in relation to propene acetate in the scrubber, and furthermore distillation of consumed. acetate from the solution is ineffective due to similar reduces significantly the hydrophobicity of the boiling temperatures of allyl acetate and acetic acid support and has an influence on precursor adsorption. solution (103 ºC and 101-102 ºC respectively). A Modification of the support surface by heating in number of such experiments have been carried out reducing atmosphere (KH and KHA samples) during preliminary studies and results were not decreases the amount of oxygen functional groups. satisfactory. These carbon support samples adsorbed more solution From literature reveals that active carbon supports of precursor, contrary to the assumptions revealed are very important in catalysis because of their from literature. The degree of propene conversion was inertness, thermal stability and large surface area. The higher using this series of catalysts than using N - and real disadvantage is natural origin of this support - it's 2 practically impossible to obtain the same subsequent O2-modified supports. batches of active carbon. Conclusions Described methods of support surface modification Studies of propene acetoxylation in varying were tested for one type of active carbon only, so the conditions have been conducted. Influence of selected results obtained in such experiments may vary for parameters on the course of the process has been different samples of active carbon. Experimental investigated. Novel and safer reaction system has been results show that optimal method for active carbon implemented for these studies. Slurry reactor type, AG is heating in hydrogen. Samples of carbon treated filled with solution of acetic acid as reaction environ- with H2 and used as supports, let to obtain the most ment and also the substrate, has been used. Hetero- efficient catalysts among tested. Degree of propene geneous catalyst (Pd+Cu supported onto powered conversion was 65-75 mol % and selectivity of activated carbon) suspended in acetic acid has been transformation to allyl acetate was 12-13 mol % for used. the assumed process conditions. The studies had one target: to develop more safe Conversion of propene during the single pass (gas and more energy-efficient method of allyl acetate flow through the reaction mixture) is rather low. In the production. Economical aspects - lower temperature industrial conditions it’s increased by the recirculation of the process and low pressure (lower energy of unreacted propene via compressor and the buffer consumption and more simple apparatus respectively) tank. were important in described studies. The safety aspect Under reaction conditions propene undergoes the - the risk reduction of ignition or explosion in the deep oxidation and CO2 is formed thus the relatively reactor was just as important. low selectivity to allyl acetate can be explained. Deep From the results reveals that favourable conditions oxidation is probably the result of low concentration for low-temperature and low-pressure propene of propene, approximately equal to upper flammability acetoxylation in aqueous medium are as follows. limit. Acetic acid concentration affects the degree of The preferable conditions for propene acetoxyla- propene conversion and selectivity of transformation tion in aqueous environment have been developed as a to allyl acetate in relation to consumed propene. result of preliminary optimization studies in novel Higher concentration of AcOH favours the increase of type of apparatus. Further studies are required to reaction yield. However it decreases the degree of reduce the amount of by-products and increase the extraction of allyl acetate from reaction environment. degree of conversion, but results described in this Optimal acetic acid concentration is in range from 40 paper are the good basis for the forthcoming
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