FTIR and 27Al MAS NMR analysis of the effect of framework Al- and Si-defects in micro- and micro-mesoporous H-ZSM-5 on conversion of methanol to hydrocarbons

Abstract

A series of micro- and micro-mesoporous H-ZSM-5 zeolites with highly regular and, on the other side, a defective framework were prepared by synthesis and post-synthesis treatment, and their activity was investigated in transformation of methanol to low olefins, aromatics and paraffins. 27Al MAS NMR and FTIR spectroscopy of OH groups and C N groups of adsorbed d 3-acetonitrile were used for analysis of the population of framework Al- and Si-related defective sites. Beside framework Al atoms in regular Td coordination, perturbed Td coordinated Al atoms and partly extra-framework Al species, and the internal Si–OH groups as Si-related defects were identified. The increase in the concentration of framework Al- and Si-defective sites in microporous H-ZSM-5 was manifested in methanol transformation in the increased yields of aromatics and paraffins, while highly regular framework preferred formation of low C 2 and C 3 olefins. The non-defective micro-mesoporous H-ZSM-5 prepared by alkaline and subsequent oxalic acid leaching of the zeolite with silanols typical for external crystal surface gave substantially longer catalyst life-time. The micro-mesoporous H-ZSM-5 (synthesized by using carbon particles) exhibited high content of defective sites, i.e., inner silanols, interacting OH groups, perturbed framework Al atoms and Al-Lewis sites, provided a slight increase in the yield of paraffins and C 6 + aromatics, and particularly much lower catalyst life-time compared to the sample of similar composition and synthesis procedure carried out without carbon presence. These findings point out on the importance of highly regular framework of H-ZSM-5 for methanol to olefin transformation, and on the way of preparation of micro-mesoporous H-ZSM-5 by desilication with alkaline solution followed by acid leaching of electron acceptor sites, resulting in the high life-time of the catalyst.