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