Effects of cage shape and size of 8-membered ring molecular sieves on their deactivation in methanol-to-olefin (MTO) reactions

Four kinds of 8-membered ring (8-MR) small-port molecular sieves with CHA (SAPO-34), ERI (UZM-12), LTA (UZM-9), and UFI (UZM-5) topologies were prepared to investigate the effects of the cage shape and the cage size on their catalytic activities and on deactivation behaviors in methanol-to-olefin (MTO) reactions. UZM-5, -9, and -12 zeolites with Si/Al molar ratios of 5-6 showed high initial activities in the MTO reaction owing to their sufficient acidities, and they were highly selective to lower olefins such as SAPO-34 molecular sieve. However, they were rapidly deactivated in the order of CHA < LTA < ERI < UFI The UV-VIS and NMR spectroscopy examinations of the materials accumulated in the cages of the 8-MR catalysts indicated that the concentration of large fused polycyclic aromatics was high in the cages of an easily deactivated 8-MR catalyst. The selective and stable catalytic performance of SAPO-34 molecular sieve in the MTO reaction is explained by the suitable shape and size of its cages, which are thus capable of preserving stably the active intermediates, multialkyl benzenes. They are also capable of suppressing the formation of fused polycyclic aromatics that would cause its deactivation. (c) 2008 Elsevier B.V. All rights reserved.