Methanol loading dependent methoxylation in zeolite H-ZSM-5

Area of Science:

• Catalysis Science
• Surface Chemistry
• Materials Science

Background:

• H-ZSM-5 zeolites are crucial catalysts in chemical transformations.
• Understanding methanol interaction with acidic sites is key for optimizing catalytic processes.
• Methoxylation is a fundamental reaction pathway in hydrocarbon conversion.

Purpose of the Study:

• To investigate the effect of methanol loading on H-ZSM-5 on the methoxylation reaction rate.
• To elucidate the structural changes of adsorbed methanol at different loadings.
• To correlate surface species and reaction products using advanced spectroscopic techniques.

Main Methods:

• Operando diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS) to probe surface species.
• Mass spectrometry (MS) to monitor reaction products.
• Systematic variation of methanol loading on H-ZSM-5 (Si/Al ≈ 25).
• Density Functional Theory (DFT) calculations for vibrational frequency analysis.

Main Results:

• Methoxylation rate increases with methanol loading per Brønsted acidic site.
• No methoxylation occurs at loadings of 1 molecule per acidic site or less.
• Hydrogen-bonded methanol transitions from neutral to protonated geometry with increasing loading.
• Experimental results are supported by DFT calculations of vibrational frequencies.

Conclusions:

• Methanol loading is a critical factor controlling the rate of methoxylation on H-ZSM-5.
• The geometry of adsorbed methanol dictates its reactivity.
• Operando DRIFTS/MS combined with DFT provides comprehensive insights into catalytic mechanisms.