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

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 CN groups of adsorbed d3-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 C2 and C3 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 C6+ 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.

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► Effect of Si- and Al-related defects on selectivity and life-time in MTH is examined.
► Defects in micro- and micro-mesoporous ZSM-5 are analysed by FTIR and 27Al MAS NMR.
► Low-defective framework of H-ZSM-5 enhances substantially catalyst life-time.
► Low-defective highly ordered H-ZSM-5 favours formation of C2 and C3 olefins.
► Leaching of HZSM-5 by NaOH and oxalic acid provides pores free of defects.