Methanol-To-Aromatics conversion (MTA) has important practical significance in the chemical industry field. ZSM-5 is commonly used as a catalyst for MTA. Using different loadings of phosphorus, zinc modification and hydrothermal treatment, the catalytic performance of modified catalysts (Zn/P/ZSM-5) in MTA conversion was varied significantly. With the unique advantages of solid-state NMR characterization, the structure of solid catalyst can be studied at atomic level. From the 1H, 27Al, 29Si, 31P NMR analysis, combined with the results of catalytic evaluation and NH3-TPD, pyridine IR experiments, the key information we obtained is that, firstly, the reduce in catalytic performance was associated with the degree of dealumination caused by hydrothermal treatment. After phosphorus modification, further dealumination occurred, the more phosphorus loaded, the more severely dealunination happened. Extra-framework aluminum showed up-field shift in the 27Al spectra, indicating affected by phosphorus incorporation. In the 31P spectra, Several P species appeared, and dealumination occurred simultaneously, the catalyst showed better reactivity than that with only hydrothermal treatment, indicating some P species were responsible for the promotion of this reaction. In this case, phosphorus incorporation will increase the hydrothermal stability and modulate acidity of the catalysts, but too much P loading will generate polyphosphate and pyrophosphate species, which will reduce the reactivity. Secondly, zinc was incorporated into the P-modified catalyst, forming some new species, such as zinc phosphate. With the decrease in polyphosphate and pyrophosphate species, some of the Bronsted acid sites which originally covered by P species were released, and aromatics yields and selectivity of BTX over the catalyst were raised obviously. To a Zn/P/ZSM-5 catalyst, proper zinc-loading will decrease the reaction-negative phosphorus species and promote aromatization, but too much zinc will accelerate the formation of coke. In our work, catalyst with Si/Al molar ratio of 30, modified by 2%(w) of original phosphorus source, pared with 1.0%(w) zinc loading, exhibited highest aromatics yields and BTX selectivity. These findings may be the reliable guidance to the industrial application of the modified ZSM-5.

Solid-state NMR study on the effect of modified H-ZSM-5 on methanol-to-sromatics conversion

248th National Meeting (2014)

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