Surface reactivity of zeolites type H-Y and Na-Y with methanol

Abstract

The surface reactions of methanol with zeolites type H-Y and Na-Y, between 20 and 350°C, were studied by infrared spectroscopy, gas liquid chromatography, adsorption isotherms and thermo-gravimetric techniques. At room temperature methanol is physically adsorbed on both zeolites. In the case of zeolite H-Y the adsorption occurs via hydrogen bonds between the OH-groups of the methanol molecules and the surface hydroxyl groups and the structural oxygen atoms of the zeolite. At 20°C, a part of the surface hydroxyl groups reacts with the methanol molecules, initiating the methoxylation of the surface, according to the scheme Si—OH + CH₃OH →SiOCH₃+ H₂O. This esterification of the silanol groups is enhanced as the temperature increases, and reaches a maximum at ∼130°C. Above 120°C a new reaction of the surface methoxyl groups with methanol occurs, with formation of dimethylether. The production of dimethylether reaches its maximum at 210°C. Around 250°C, secondary reactions forming gaseous cracking products, predominantly butane and propene, were detected. In the temperature range studied no chemical reaction of methanol with the surface of the dehydrated Na-Y was observed. Therefore, it is concluded that the surface activity of the zeolite H-Y in dehydration of methanol is mainly due to the high Brönsted acidic character of its surface. The analysis of the methoxylation reaction in terms of surface diffusion of the methanol molecules, and of the proton exchange frequency between adsorbed methanol molecules and the zeolite surface, shows that the lifetime of the protonated CH₃OH⁺ ₂ molecule is the rate controlling factor of the methoxylation process.

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