On the role of acidity in bulk and nanosheet [T]MFI (T = Al3+, Ga3+, Fe3+, B3+) zeolites in the methanol-to-hydrocarbons reaction

The influence of framework substituent (Al3+, Ga3+, Fe3+ and B3+) and morphology (bulk vs. nanometer-sized sheets) of MFI zeolite on acidity and catalytic performance in the methanol-to-hydrocarbons (MTH) reaction was investigated. The Brønsted acid density and strength decreased in the order Al(OH)Si > Ga(OH)Si > Fe(OH)Si >> B(OH)Si. Pyridine-15N NMR spectra confirmed differences in the Brønsted and Lewis acid strengths but also evidenced site heterogeneity in the Brønsted acid sites. Due to the lower efficiency with which tervalent ions can be inserted into the zeolite framework, sheet-like zeolites exhibited lower acidity than bulk zeolites. The sheet-like Al-containing MFI zeolite exhibited the greatest longevity as MTH catalyst, outperforming its bulk [Al]MFI counterpart. While the lower acidity of bulk [Ga]MFI led to better catalytic performance than bulk [Al]MFI, the sheet-like [Ga]MFI sample was found to be nearly inactive due to lower and heterogeneous Brønsted acidity. All Fe- and B-substituted zeolite samples displayed very low catalytic performance due to their weak acidity. Based on product distribution, the MTH reaction was found to be dominated by the olefins-based catalytic cycle. The small contribution of the aromatics-based catalytic cycle was larger for bulk zeolite than for sheet-like zeolite, indicating that shorter residence time of aromatics can explain the lower tendency to coking and enhanced catalyst longevity.

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