Catalytic activation of alkanes on noble metal-loaded zeolites : experimental studies and simulation of the dehydroalkylation of toluene with ethane

With the recent progress in chemical reaction engineering and highly active, multi-functional catalysts, the activation of relatively inert alkanes may be achieved. The direct use of short-chain alkanes, especially methane and ethane, as basic chemical feedstocks represents an attractive alternative to alkenes. Significant reserves of alkanes are found in nature in the form of wet natural gas, whereas alkenes have to be synthesized via other chemical reactions. The activation of alkanes can be achieved by oxidative and non-oxidative methods. Reactions of oxidative activation are often thermodynamically favored, e.g., due to formation of the stable but economically unattractive by-products water and/or carbon dioxide. In contrast, reactions of non-oxidative activation may produce hydrogen as a valuable by-product, but the equilibrium conversions of such reactions are generally low, and thus, measures have been taken to improve this situation, e.g., shifting equilibrium by removing a product. The alkylation of aromatics with alkanes, in particular, represents a potential new route for the synthesis of important feedstock chemicals such as n-propylbenzene and cumene, ethyltoluenes, and ethylbenzene. Estimates for current worldwide annual production capacities of ethylbenzene and cumene, the two most important chemicals produced by alkylation of aromatics, amount to 25·106 t·a-1 and 10·106 t·a-1, respectively. Conventional alkylating agents, including alkenes, alcohols and alkyl halides, have to be pre-synthesized via other processes, typically from an alkane feedstock. Alternatively, the direct alkylation of aromatics with alkanes may result in reduced costs and process intensification, since alkanes are much cheaper than other typical alkylating agents, abundantly available and pre-synthesis steps can be avoided. Therefore, the focus of the present work has been the heterogeneously catalyzed activation of ethane by direct application of ethane as an alkylating agent during toluene dehydroalkylation to ethyltoluenes on noble metal-loaded zeolite catalysts. Due to the thermodynamic stability of ethane, the desired reaction is strongly limited by equilibrium. Therefore, during the present work efforts were concentrated on experimental work in conjunction with theoretical modeling in order to better understand and hence improve the yield to the title reaction. The effect of pressure was investigated by a non-ideal gas model complemented by kinetic assumptions. At 300 °C, the thermodynamic model is in agreement with experimental results. However, at 350 °C conversions are well over the expected values calculated from the same model. The supra-equilibrium may be due to the formation of hydrogen-rich methane, acting as a hydrogen sink, hence shifting equilibrium towards the formation of ethyltoluenes. This was attributed to the consumption of hydrogen as a result of methane formation. Since methane is mainly formed by consumption of hydrogen, i.e., it is a secondary reaction, the space velocity was varied to improve…