Conversion of methanol to hydrocarbons over H-MCM-22 zeolite: deactivation behaviours related to acid density and distribution
A series of H-MCM-22 zeolites with different Si/Al ratios (H-M22-x, x = 15–90) were hydrothermally synthesized by boron incorporation to investigate the influences of acid density on the methanol-to-hydrocarbon (MTH) performances and deactivation behaviors. The acid density of H-M22-x zeolites is nearly proportional to their aluminum contents; the residual boron barely affects the overall acidity of H-M22-x zeolites. 27Al and 29Si MAS NMR experiments support the existence of competitive occupancy of various framework T sites among boron, aluminum and silicon during zeolite synthesis, which leads to different distribution of Brønsted acid sites/framework aluminum among three types of pores in H-MCM-22 zeolites. Increasing the Si/Al ratio of H-M22-x zeolites leads to a gradual increase in the proportion of Brønsted acid sites in supercages while those in sinusoidal channels decrease. However, the acid density has a significantly larger impact than acid distribution on the MTH performances of H-M22-x zeolites. Increasing the acid density can accelerate the hydrogen transfer reactions in the MTH conversion and promote the formation of aromatics, ethene, and alkanes. The deactivation of H-M22-x zeolites with low acid density (x = 50–90) is mainly ascribed to coke covering the acid sites inside the micropores and partly blocking the pore mouths. Nevertheless, the deactivation of H-M22-x zeolites with high acid density (x = 15–30) is mainly caused by massive coke deposition on the external surface which blocks the pore mouths of inner micropores. The coke on the external surface exhibits a lower decomposition temperature than that inside the micropores. The deactivation behaviors of H-M22-x zeolites for the MTH reaction can be divided into three stages. Firstly, rapid deactivation of supercages occurred for all the H-M22-x zeolites in spite of their different acid densities, suggesting a much stronger influence of pore structure than acidity on the initial deactivation behaviors of H-M22-x zeolites. Secondly, the acid sites in sinusoidal channels of H-M22-x zeolites are highly resistant to coke formation, which dominates the MTH reaction in the second stage. Finally, with the continuous formation of coke in the surface pockets, which gradually covers the acid sites and pore mouths of sinusoidal channels, complete deactivation of H-M22-x zeolites can be observed. GC-MS and UV-vis analysis of partly or completely deactivated H-M22-x zeolites reveals that the retained species in H-M22-x zeolites gradually condense into bulky hydrocarbon species through successive hydrogen transfer reactions and form methylbenzenes, naphthalene, polymethylnaphthalenes, phenalenone, pyrenes and eventually graphite-like carbonaceous species.
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