Catalysis – Innovative Applications in Petrochemistry and Refining DGMK Conference October 4-6, 2011, Dresden, Germany

Reaction Pathways for Catalytic Gas-Phase Oxidation of Glycerol over Mixed Metal Oxides W. Suprun, R. Gläser, H. Papp Institute of Chemical Technology, Universität Leipzig, Germany

Abstract Glycerol as a main by-product from bio-diesel manufacture is a cheap raw material with large potential for chemical or biochemical transformations to value-added C3-chemicals. One possible way of glycerol utilization involves its catalytic oxidation to as an alternative to petrochemical routes. However, this catalytic conversion exhibits various problems such as harsh reaction conditions, severe catalyst coking and large amounts of undesired by-products. In this study, the reaction pathways for gas-phase conversion of glycerol over transition metal oxides (Mo, V und W) supported on TiO2 and SiO2 were investigated by two methods: (i) steady state experiments of glycerol oxidation and possible reactions intermediates, i.e., , 3-hydroxy propionaldehyde and , and (ii) temperature-programmed surface reaction (TPSR) studies of glycerol conversion in the presence and in the absence of gas-phase . It is shown that the supported W- , V and Mo-oxides possess an ability to catalyze the oxidation of glycerol to acrylic acid. These investigations allowed us to gain a deeper insight into the reaction mechanism. Thus, based on the obtained results, three possible reactions pathways for the selective oxidation of glycerol to acrylic acid on the transition metal-containing catalysts are proposed. The major pathways in presence of molecular oxygen are a fast successive destructive oxidation of glycerol to COx and the dehydration of glycerol to acrolein which is a rate-limiting step

Introduction Glycerol is the main by-product of transesterification of vegetable oils for the production of bio-diesel. Currently, about 1,2 Mio. t/a of raw glycerol is produced in the EU-countries. The massive increase in the bio-diesel production goes hand in hand with the availability of large amounts of crude glycerol, which must be valorised. An economical study has shown that a competitive production of acrylic acid from glycerol may be possible if the price of glycerol would be less than 300 US$/t [1]. In 2010 refined glycerol still had a price of 450 to 500 US$/t. But crude glycerol was only around 80 to 100 US $/t. This makes crude glycerol a potentially very competitive raw material for the production of basic and fine chemicals. The catalytic dehydration of glycerol to acrolein, acetol and/or hydrogenolysis to propanediols was intensively investigated at the last decade [2], but literature about a conversion of glycerol to acrylic acid without intermediates is very limited. The synthesis of acrylic acid is currently based on the selective oxidation of acrolein or over complex multicomponent Mo-based catalyst. Titania and silica are suitable catalyst supports for selective gas-phase conversion of hydrocarbons to carboxylic acids. As part of our ongoing research for the catalytic conversion of glycerol [3, 4], we have studied the catalytic behaviour of catalysts containing Mo, V and W oxides supported on titania and silica in absence and presence of oxygen. The presented work is aimed to clarify the role of transition metal oxide components and some reaction intermediates for dehydration and/or selective oxydehydration of glycerol.

DGMK-Tagungsbericht 2011-2, ISBN 978-3-941721-17-3 253 Catalysis – Innovative Applications in Petrochemistry and Refining

Experimental Titania and silica catalysts loaded with Mg, Mo, W and V oxides were prepared by impregnation of precursors with aqueous solutions of magnesium nitrate, methavanadate, ammonium tungstate and ammonium molibdate. The following commercial 2 precursors were used: TiO2 (Millenium; BET 77 m /g) and SiO2 (Köstropur 050612: CWK 2 Bad Köstritz; BET: 305 m /g). The preparation of Ti-PO4 and Ti-Mo-W-V-PO4 catalysts was carried out by simultaneous impregnation of TiO2 with aqueous solutions of the corresponding metal salts and phosphoric acid. The impregnated samples were dried at 110 °C and calcined in air for 4 h at 450 °C. The atomic relation between metal, titania or silica was kept constant, i.e.: M:Ti or M:Si = 1:10 and Ti / PO4 = 12:1. The catalyst composition and notation of the prepared samples are listed in Tab.1. Textural properties were determined by adsorption–desorption isotherms of N2 using an ASAP 2010 apparatus (Micromeritics). Powder XRD patterns were recorded on a Bruker D8- Advance X–ray diffractometer using a nickel-filtered Cu Ka (0.15418 nm) source at 40 kV and 50 mA. Temperature programmed reduction of catalyst samples was carried out with a H2/Ar- mixture on an AMI 100 (Altamira) instrument equipped with a TCD detector. The catalysts were pretreated in a flow at 300 °C for 30 min and after that cooled down to 30°C. The H2-TPR measurements were recorded from 30 to 700 °C in a H2/Ar flow (5 vol% H2; 50 ml min−1, heating rate: 10 K min−1). Catalytic tests for dehydration and oxidation of glycerol were carried out in a continuous flow fixed–bed reactor at atmospheric pressure. Aqueous solution of glycerol (5 wt%) was injected using a liquid flow controller and then evaporated at 200 °C (Bronkhorst) prior to entering the reactor. The concentrations of glycerol and in the gas-feed were 0.18 vol% and 12 vol%, respectively. The product samples were analyzed by GC (Chrompack 9001) equipped with a capillary column (OPTIMA–WAX, 30 m MN) and −1 an FID detector: TP: 100 °C (2 min), 12 K min , 245 °C (5 min). Additionally CO and CO2 were analysed on-line using the FTIR Gas–Analyser 1301 (INNOVA).

Results and discussion Textural properties and total acidity of the investigated catalysts are presented in Table 1. The specific surface area decreased after impregnation, especially for catalysts loaded with

Tab. 1 Physico-chemical properties of catalysts loaded with transition metal oxides . Specific BET area Average pore Total acidity 2 Catalyst (m /g) diameter (Å) mmolNH3/g Support TiO2 SiO2 TiO2 SiO2 TiO2 SiO2 Mo 74 275 83 114 415 355 W 73 - 79 - 468 - Mg 71 - 86 - 65 - Mo-W 68 245 87 132 522 434 Mo-V 64 215 85 125 502 448 Mo-W-V 52 204 86 150 565 460

Mo-W-V-PO4 48 195 105 135 675 524

two or three metal oxides. The average pore diameter decreased after impregnation of TiO2 from 127 to ~75 Å and SiO2 from 150 to ~115 Å. All XRD patterns (Fig. 1) show intense peaks at a 2θ region of 20-35 corresponding to the metal oxides i.e: MoO3, V2O5, and WO3 and prove that the structure of the amorphous silica and crystalline anatase titania was not changed or destroyed during calcination at 450°C. Reflexes typical for molybdenum, vanadium and tungsten oxides indicate that the metal oxides were successfully introduced. No differences were observed between crystalline phase of mixed metal oxide Mo-W-V and samples additionally loaded with phosphoric acid. This indicated that no metal phosphates,

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e.g., heteropolyacids (i.e. HPWO, HPMoO or HPVO) were formed during the impregnation in presence of phosphoric acid or calcination. It is generally accepted that the catalytic activity in the dehydration of glycerol is related to the catalyst acidity [2]. Therefore, TPD of were performed for all catalysts to characterize their total acidic properties. Tab. 1 shows that the incorporation of W-, V- and Mo-oxide led to an increased acidity. In contrast, the total acidity of titania loaded with Mg- oxide decreased which indicated basic properties of the TiO2-MgO systems.

o V b a x Mo * W

* * * * Ti-W Si-Mo-W x Ti-Mo-W Si-Mo

o Ti-Mo

Intensity (a.u.) Intensity Si-Mo-W-V Intensity (a.u.) Ti-Mo-V o o o Si-Mo-V Ti-Mo-W-V Si-Mo-W-V-PO Ti-Mo-W-V-PO 4 4 10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 2 Theta (Grad) 2 Theta (Grad) Fig.1. X–ray diffraction patterns of titania (a) and silica (b) loaded with Mo, W and V- oxides.

Comparison of dehydration activity. Glycerol possesses primary and secondary OH-groups and offers two main reaction pathways for dehydration with formation of hydroxyacetone and acrolein [2]. Additionally, different O-containing compounds such as , acetaldehyde, propionaldehyde, allylalcohol, , , phenol and COx were detected. The effect of V-, W-, Mg-oxides and PO4 on the performance of the supported titania catalysts in the formation of main dehydration products of glycerol in absence of oxygen for the temperatureregion 180-330°C is presented in Fig. 2.

Ti-Mo Ti-Mo-V Ti-Mo-W Ti-Mo-V-W Ti-PO4 Ti-Mg 60 60 Allylalcohol Hydroxyacetone

40 40

20 20 Selectivity (%) Selectivity

0 0 180 210 240 270 300 330 210 240 270 300 330 80 60 Acetic acid Acrolein 60 40

40

Selectivity (%) Selectivity 20 20

0 0 180 210 240 270 300 330 180 210 240 270 300 330 Temperature (°C)

Fig. 2. Catalytic performance of titania catalysts loaded with PO4, Mg-, Mo- ,W- and V-oxide during dehydration of glycerol at different temperature. Data analysed after 4h of reaction.

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The results of GC-analysis show that all investigated catalysts exhibit full conversion only at temperature higher than 270°C. At temperature lower than 240°C the glycerol conversion was between 10 and 40%.The C-balance for catalytic tests at 180-240°C was between 55 and 60%. This indicated that glycerol was oligomerised to heavy glycerol oligomers which are also responsible for the rapid catalyst deactivation after a TOS of 2-4 h. It was found that acetic acid is a main reaction product at 180°C. Obviously a part of adsorbed glycerol directly undergoes destructive cleavage with participation of transition metal oxide as active lattice oxygen. As shown in Fig.2 the formation of acetic acid and allyl alcohol also strongly depends on reaction temperature. The selectivity to acetic acid increased at temperatures higher than 270°C. Allyl alcohol was a main product detected with a selectivity of 25-55% in the low temperature region between 210 and 240°C. The acidic catalyst TiO2-PO4 showed the highest selectivity to acrolein and hydroxyacetone and exhibited a low selectivity of about 3-5% to allyl alcohol and acetic acid. This indicated that active lattice oxygen of transition metal oxide was involved in the formation of these two products. Basic TiO2-MgO favored the formation of glycidol and hydroxyacetone and caused rapid catalyst deactivation due to deposition of carbonaceous compounds.

Additionally, it was found that the high activity of TiO2-PO4 is limited only to times on stream between 2-5 h. After 5h TOS the conversion rapidly decreased and reached 25-40% at 20h TOS. It should be emphasised that TiO2 catalysts containing individual transition metals like Mo, W and V oxide or a mixture of Mo-V-W-oxides and/or loaded with PO4 acid component give excellent long-time stability beyond 24 h (Fig.3). At the same time, the catalysts loaded with Mo-V-W-oxides showed lower selectivity to the formation of acrolein and hydroxyacetone and higher selectivity to COx, acetic acid and propionic acid. 80 Acrolein 100 Ti-PO 4 Ti-Mo-W-V 60 80

60 40 Ti-PO 4 40 Conversion (%) Selectivity (%) Ti-PO 20 4 20 Mo-V-W-PO Ti-Mo-V-W-PO 4

0 0 0 5 10 15 20 25 0 5 10 15 20 25 TOS (h) TOS (h)

Fig. 3. Performance of titania supported catalysts loaded with PO4 and Mo-V -W-oxide in the dehydration of glycerol in absence of oxygen at 300°C.

In general, catalysts containing individual and mixed metal oxide showed improved stability against deactivation which can be attributed to the oxidative properties of the transition metal oxide components that enhance coke removal [3,4].

Catalytic conversion of glycerol in presence of oxygen. This part of the paper was motivated by a direct conversion of glycerol to acrylic acid. As an alternative to the current acrolein or proplylene route, acrylic acid can be produced in tandem-type reactors with glycerol dehydration and acrolein oxidation. However, it would be

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efficient to integrate two target reactions on one catalyst. Mixed Mo-, V-, W-oxides are known as effective catalysts for partial oxidation of acrolein [5, 6]. On the other hand, the catalyst acidity is an important property for dehydration of glycerol [1]. Therefore, we tested the catalytic activity of titania and silica samples loaded with Mo, V, W oxides and PO4 in the presence of molecular oxygen. The oxidation reaction was performed at 300°C with a constant molar relation of O2/glycerol 3:1. The major product in presence of Mo- W-V is acetic acid with a selectivity of about 30-45%. It was found that supported individual V and binary Mo-V oxides favored the total oxidation and showed the highest selectivity to acetic and propionic acid of about 50-62 and 15-20%, respectively. The selectivity to acrylic acid in glycerol oxydehydration at constant reaction conditions are compared in Fig.4. The catalysts supported on titania generally showed higher selectivity to acrylic acid in comparison to corresponding catalysts on silica support. This effect can be attributed by a higher total acidity and surface density of metal oxides on titania as well as to different interactions of the transition metal oxides with the support according to the Mars-van- Krevelen mechanism [7].

30 SiO2 4 TiO2 25 Mo-W-V-PO 20 Mo-W-V

15 Mo-V

Selectivity (%) 10 MoW Mo

5 4 PO

. 0

Fig. 4 Acrylic acid selectivity for titania and silica catalysts loaded with PO4 and Mo-,V-,W- oxides. Reaction conditions: T: 300°C, GHSV: 400 h-1 (glycerol)

Mo-V-W based catalysts doped with PO4 showed promising results in the one-step oxydehydration of glycerol, leading to the highest selectivity for acrylic acid of about 27% - the highest reported in the literature so far [8]. At full conversion, the selectivity to acetic acid was always high (32-55 %), however, total oxidation was also observed. In presence of oxygen acrolein was not fully oxidized to acrylic acid. The obtained results show that for the selective conversion of glycerol to acrylic acid two types of active centers e.g. acidic and redox are necessary. Obviously the best performance to a selective one step oxidation can be obtained by optimization of the total catalyst acidity, total content of mixed transition metal oxide and molar ratio Mo:W:V.

Reaction pathways. The complexity of the dehydration of glycerol in the presence of oxygen is largely due to the fact that not only the formed C2-C3- and hydroxyacetone can be oxidized but also highly active OH groups of glycerol leading to oxy-functionalized glycerol derivatives. To classify the reaction pathways, we employed several major products as reactants to investigate their behavior at the same reaction conditions, i.e., 300°C in the presence of oxygen. The multi component Mo-V-W-PO4 catalyst supported on titania was used.

Acetaldehyde: 68% conversion of acetaldehyde was obtained with a selectivity of 85 % and 12 % to acetic acid and CO2, respectively. It was proposed by Dimitru et al. [9] that acetaldehyde can further undergo over acid catalysts aldol-condensation. However, we did

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not observ the formation of acrolein and crotonaldehyde or corresponding acids in the presence of oxygen.

Hydroxypropionaldehyde: The conversion of hydroxypropionaldehyde at 300°C was complete. 30% acrolein, 28% acrylic acid, 12% acetaldehyde, 10% acetic and 5% CO2 were detected. The incomplete carbon balance may be due to formed carbonaceous deposits.

Hydroxyacetone: The conversion of hydroxyacetone was only 45%.This suggests that this carbonyl compound is less active to the oxidative conversion than acetaldehyde, acroleine and hydroxypropionaldehyde. The products were acetone (22 %), acetic acid (20%), acetaldehyde (21%) and CO2 (5%).

Acrolein: The conversion of acrolein was about 75 % with selectivities to acetaldehyde of 22%, acrylic acid of 18%, acetic acid of 30% and COx of 25% as main products, indicating that the oxidation capability of the Mo-V-W-PO4 catalyst is not high enough for a selective conversion to acrylic acid. The reason for that may be a too high content of active lattice oxygen in this particular catalyst which results in high activity for destructive C-C cleavage or total oxidation.

Temperature programmed surface reaction (TPSR). It is known that glycerol in the presence of transition or noble metal catalysts can be partially oxidized to or/and glyceric [10-11]. In further consecutive oxidation steps glyceric acid, tartronic acid and hydroxylpurivic acid can be formed followed by oxidative C-C cleavage to lower carboxylic acid and COx. Supposedly, during the catalytic gas-phase conversion of glycerol over transition metal oxides in presence of oxygen partial oxidation to dihydroxyacetone, glyceric aldehyde, glyceric acid, tartronic acid and hydroxypurivic acid can also occur. However, none of these compounds could be detected by GC or HPLC analysis. In order to determine the oxidation states and thermal stability of these C3-compounds TPSR studies in a temperature region 100-800°C in presence of oxygen were carried out. Prior to TPSR measurement the catalyst samples was impregnated with 10% aqueous solution of corresponding C3-carboxylic compound and dried in He-Flow for 16 h at 65°C. Then TPSR analysis in a TPD apparatus coupled to a MS was performed. The obtained TPSR profiles in Fig. 5 for theTi-Mo-W-V-PO4 catalyst loaded with di-hydroxyacetone, glyceric aldehyde, 260°C

Pyrivic acid

Tartronic Acid

Glyceric aldehyde

Glyceric acid MS-Intensity (a.u)

Di-hydroxyacetone

0 200 400 600 800 1000 Temperature (°C)

Fig. 5. TPD profiles for the formation of CO2 over Ti-Mo-W-V-PO4 catalyst loaded with different C3-componds in O2/He flow . O2: 5 Vol%; HR: 10K/min.

258 DGMK-Tagungsbericht 2011-2 Catalysis – Innovative Applications in Petrochemistry and Refining

glyceric acid, tartronic acid and hydroxpyrivic acid indicate that C3-carboxyl compounds adsorbed at the catalyst surface posses very limited thermal stability and undergo cracking to CO2 already at temperatures of 180-270°C. In addition to CO2 formation, the presence of small quantities of , acetic acid and CO were detected during TPSR at temperatures between 200 and 250°C

Based on the above results, we propose a simple reaction network for glycerol conversion on bifunctional acid-redox catalysts in the presence of molecular oxygen (Scheme 1). Glycerol can be converted in acidic dehydration reactions to hydroxipropionaldehyde, acrolein and hydroxyacetone. Simultaneously to dehydration reactions all HO-groups of adsorbed glycerol can also be activated on the active redox centers and undergo selective oxidation to different C3-carboxylic acids. Due to their low thermal stability, the bicarboxylic C3 acids undergo rapid decarboxylation and/or successive destructive oxidation to acetic acid and COx. This alternative pathway prevents the selective conversion of glycerol in the dehydration reaction to acrolein. Obviously, the oxidation of HO-groups over redox centers proceeds with comparable activation energies and the dehydration of glycerol and the oxidation of acrolein to acrylic acid are the rate determining steps.

Scheme 1. Proposed reaction network for the competitive gas-phase dehydration and oxidation of glycerol: a) oxidation route to acrylic acid via primary dehydration; b) route of successive oxidation of prim. and sec. OH-groups or direct destructive cleavage.

Conclusions

The obtained results show that the selective transformation of glycerol to acrylic acid by direct oxidation greatly depends on the catalyst acidity as well as the selective dehydration of glycerol to acrolein. At the same time the redox activity for the selective conversion of acrolein plays an important role. The surface acidity of the support and the redox properties of the transition metal components need to be controlled in order to maintain two target reactions, i.e., dehydration of glycerol to acrolein and oxidation of acrolein to acrylic acid.

DGMK-Tagungsbericht 2011-2 259 Catalysis – Innovative Applications in Petrochemistry and Refining

Mo-W-V catalysts supported on titania and silica with equal molar ratio of metals of 1:1:1 prepared by impregnation were found to be very active for the destructive oxidation of glycerol to acetic acid with a selectivity of 45-55% at 100% conversion. The found low selectivity to acrylic acid (limited to ~ 25%) during catalytic gas-phase oxidation of glycerol over multi component Mo-V-W systems can be attributed to the simultaneous activation of OH-groups on acid and redox active centers for dehydration and oxidation reactions. The optimization of the oxidation ability for mixed metal oxide catalysts by changing the Mo:W:V ratios or by doping with other metal oxides should allow an efficient one step conversion of glycerol to acrylic acid.

Acknowledgments. This work has been financially supported by the Deutsche Forschungsgemeinschaft (PA 194/17). The help of Ms. B. Rulle for BET analysis of the solid samples and of B.Sc. S. Wiese and M.Sc. M. Nitzer for catalytic testing is gratefully acknowledged.

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