Design and optimization of catalysts and membrane reactors for the non-oxidative conversion of methane

Kinetic-transport simulations were used in order to explore the relevant thermodynamic and kinetic barriers in the non-oxidative conversion of CH4 in a membrane reactor and the effects of continuous hydrogen removal and of catalytic sites on CH4 conversion and on the attainable yields of useful C-2-C-10 products. A sensitivity analysis of homogeneous CH4 pyrolysis pathways showed that sites that activate methane to form methyl radicals or ethene and the conversion of ethene to aromatics increased pyrolysis rates, but led to impractical reactor residence times with or without H-2 removal. Catalysts, such as Mo/H-ZSM5, which increase the rate of both CH4 conversion to ethene and of ethene aromatization are required in order to overcome the kinetic barriers in homogeneous pyrolysis pathways. Homogeneous models were modified by incorporating non-elementary catalytic steps with rate constants obtained from experimental residence time studies on Mo/H-ZSM5. Simulations using this homogeneous-heterogeneous kinetic model were used in order to determine the benefits of continuous H-2 removal on the yields of desired C-2-C-10 hydrocarbons and on the required residence times and to obtain rigorous criteria for the design of catalysts and membranes for direct methane conversion reactions. Bifunctional catalysts able to catalyze the required steps and the removal of H-2 across ceramic membranes can lead to almost complete CH4 conversion at similar to 1000 K at practical reactor residence times ( < 100 s). This performance requires catalytic reactors with intermediate values of the ratio of characteristic reaction and permeation times (delta = 1-10), which in turn require the use of thin dense ceramic films (10-100 mum) in order to achieve these 8 values for practical reactor diameters. (C) 2002 Elsevier Science Ltd. All rights reserved.