Nonoxidative methane conversion on molybdenum/H-ZSM5 catalysts for use in a hydrogen-transport membrane reactor

Natural gas conversion with high yields of C2+ hydrocarbons in a single-step process has proven to be an elusive industrial goal. This thesis covers proof-of-concept research towards the demonstration of enhanced CH4 conversion in a catalytic hydrogen-transport membrane reactor. Detailed reaction-transport simulations showed that nonoxidative conversion of CH4 to C2--C10 hydrocarbons and aromatics requires catalytic activation of both CH4 and C2 intermediate products in order to achieve high reaction rates at 1038 K. Deposition of solid carbon products can be prevented by using reaction temperatures below 1000 K or shape-selective catalysts. Simulations predict CH4 conversion to C2--C10 products with about 90% yields at 1038 K when removing hydrogen from the system at rates corresponding to 10 mum SrZr0.95Y0.05O3 perovskite films.; The synthesis, structure, and function of Mo/H-ZSM5 catalysts for non-oxidative CH4 aromatization were investigated in order to determine its suitability for use in a hydrogen-transport membrane reactor. Physical mixtures of MoO3 and H-ZSM5 interact via solid-state migration of MoOx oligomers on the zeolite surface between 623 and 973 K. MoOx exchanges at acid site pairs within ZSM5 channels as (Mo 2O5)+2 species, producing one molecule of H2O per Mo dimer above 743 K. Mo concentrations greater than 4 wt% lead to sublimation of (MoO3)n above 773 K and to extraction of framework Al from the zeolite lattice to form Al2(MoO 4)3 above 843 K.; (Mo2O5)2+ species reduce and carburize to form MoCNHY species bonded to ZSM5 lattice oxygens after exposure of MoOx/H-ZSM5 to CH4 above 933 K. CH 4 activation on MoCn sites leads to production of C2H 4, which is converted into C6--C10 aromatic products on ZSM5 acid sites. The rate-limiting step in CH4 aromatization is the removal of H atoms from adsorbed intermediates on acid sites; this step is catalyzed by recombinative desorption of H2 at MoC n sites. Catalyst deactivation occurs by saturation of MoCn sites with carbon atoms, which inhibit CH4 activation and H 2 desorption, and by growth of adsorbed intermediates on acid sites, which leads to unreactive carbon species. MoCn sites can be regenerated by H2 treatment at 950 K, while carbon deposits on acid sites are only removed by temperature-programmed treatment in air to 973 K.; Dense SrZr0.95Y0.05O3 perovskite films were selected for the membrane reactor because of their ability to selectively transport hydrogen and of their chemical stability at 773--973 K in carburizing conditions of CH4 pyrolysis. Thin (20 mum) SrZr0.95Y 0.05O3 films supported on porous SrZr0.95Y 0.05O3 substrates were prepared via spin-coating of SrZr 0.95Y0.05O3 powder (made from glycine-nitrate combustion synthesis) onto a porous substrate prepared from physical mixtures of SrZr0.95Y0.05O3 and carbon black. An experimental membrane reactor using glass o-rings to seal the ceramic membrane to the reactor walls was constructed in order to carry out CH4 conversion on Mo/H-ZSM5 catalysts placed on top of thin SrZr0.95Y0.05O 3 membranes at 873--973 K.