Browsing by Author "Rezai, Alireza"

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Catalytic activation of alkanes on noble metal-loaded zeolites : experimental studies and simulation of the dehydroalkylation of toluene with ethane
(2012) Rezai, Alireza; Weitkamp, Jens (Prof. Dr.-Ing.)
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 selectivity to the title reaction. At a high space velocity, the formation of methane can be entirely avoided. Furthermore, the disproportionation of toluene is also absent, resulting in 100 % selectivity to ethyltoluenes and hydrogen. Decreasing the space velocity results in an increase in conversions as well as the yield of ethyltoluenes. However, at space velocities lower than about 3.0 h-1, the selectivity to ethyltoluenes decreases since the toluene disproportionation reaction can compete with the dehydroalkylation reaction. With increasing conversion, C1 to C4 alkanes are also formed to a larger extent, as well as other aromatics including benzene, xylenes, and ethylbenzene. A maximum yield of ethyltoluenes is observed at a conversion of 13 % at 90 h on stream and medium space velocities. It is suggested that this maximum is a result of secondary reactions of ethyltoluenes. Hydrocracking experiments confirm that methane forms mainly from secondary reactions of products or reactants. A noble metal is required for a dehydroalkylation catalyst with good activity. However, the noble metal can also promote undesired side reactions including hydrocracking or hydrogenolysis and, mainly, the hydrodealkylation of the reactants and products. Although on Pt/H-ZSM-5 catalysts the highest toluene conversion is observed, on Pd/H-ZSM-5 the highest yields to ethyltoluenes can be achieved. This is as a result of Pd being less active for hydrodealkylation. Hence, further experiments in the membrane reactor were carried out using a Pd/H-ZSM-5 catalyst. Since high selectivity to the desired products can be achieved, the ultimate goal of increased yields can be achieved by improving conversion. This was attempted by application of a membrane reactor, which may improve yields by selectively removing a product and hence shifting equilibrium. Prior to catalytic experiments in a membrane reactor, a model was developed applying fundamental thermodynamics in order to gauge the maximum attainable shift in equilibrium by the selective removal of hydrogen in a membrane reactor. The model considers chemical reaction equilibrium and simultaneously the hydrogen partial pressure equilibrium across the membrane. From the viewpoint of hydrogen recovery, the molar flow of the sweep gas must be about 10 times higher than the molar flow of the alkane to ensure a high hydrogen recovery from the reactor. Under these conditions, an optimum shift in equilibrium can be achieved with a minimum dilution of the hydrogen on the sweep side. Dehydroalkylation reactions show interesting advantages in comparison to, more widely studied, dehydrogenation reactions. Since dehydrogenation reactions typically result in a net increase in the number of moles, low pressure is favored. However, for the membrane reactor, removal of hydrogen means that high conversions can still be maintained at high pressure. However, in the case of dehydroalkylation of toluene with ethane, the number of moles does not change. Thus, in a membrane reactor an increase in conversion can be achieved at higher pressures since more hydrogen can permeate at higher pressure. However, there is a limit to which the equilibrium can be shifted, depending on the thermodynamics of the given reaction. The model suggested here may be applied to simply estimate optimum conditions for specific reactions in order to achieve the highest attainable conversions when applying membrane reactors. The modeling results are confirmed by experimental results. Experiments show that the dehydroalkylation of toluene with ethane can be successfully performed in a membrane reactor. Since the reaction is taking place under mild conditions, the catalytic performance of the bifunctional zeolite in the membrane reactor is stable with time on stream. Experiments demonstrate large improvements with tripled conversion and almost tripled yield to the desired ethyltoluenes by increasing the sweep gas flow rate and the pressure on the reaction side. Selectivity to methane and ethylbenzene, formed from hydrodealkylation reactions that consume hydrogen, is reduced. Increasing pressure has a positive effect on conversion since the net number of moles does not change during this reaction. However, at high pressure, the toluene disproportionation reaction is dominant. Unfortunately, choosing a low contact time to successfully eliminate the toluene disproportionation is not possible here, since under such conditions the produced hydrogen is swept out of the reaction zone before it can permeate. Hence, a careful optimization of the catalyst and the reaction conditions in the membrane reactor is needed in order to achieve industrially relevant yields.