Fourier-transform infrared study of the role of zeolite Lewis sites in methanol reactions over HZSM-5 surfaces

Abstract

Methanol reactions over untreated and highly dealuminated HZSM-5, deactivated by chemisorbed pyridine or ammonia, have been studied by Fourier-transform infrared spectroscopy. Dimethylether is the end-product on partly deactivated surfaces. This study, consistent with previous investigations, reveals evidence for the involvement of zeolite Lewis acidity in methanol dehydration into dimethylether. The ability of pyridine to react with Lewis sites appears to be limited by the zeolite narrow channel structure and this results in the presence of some bare sites, which could be responsible for methanol dehydration into dimethylether. This is confirmed by using the less bulky and less interacting (but more basic) ammonia in a comparative study. Zeolite dealumination appears to modify ZSM-5 accessibility, particularly to bulky adsorbates, e.g. pyridine. The proportion of Lewis sites left unmasked over the partly deactivated surface corresponds with the amount of dimethylether formed in the gas phase. Methanol reaction over deuterated or dehydroxylated ZSM-5 rules out the possibility of involvement of zeolite Brønsted acidity in the formation of surface methoxy species, the key intermediate leading to dimethylether in methanol reactions.