Comparative study of MCM-22 and MCM-56 modified with molybdenum – Impact of the metal on acidic and oxidative properties of zeolites

Abstract

Hydrogen forms of layered MCM-22 and delaminated MCM-56 zeolites were modified with molybdenum species. The structure/texture and surface properties as well as catalytic activity in methanol transformation and dibutyl sulphide oxidation were compared for the two types of modified zeolites. The difference in the distribution of hydroxyls in HMCM-22 and HMCM-56 (more Brønsted acid sites (BAS) on HMCM-22, their high strength on HMCM-22, and domination of external silanols in HMCM-56) determines the location and properties of MoO3. Mo species interact with BAS on Mo/HMCM-22, which results in good dispersion of MoO3 and harder reducibility of molybdenum. Preferential interaction of Mo species with silanols on the external surface of HMCM-56 leads to larger crystallites of MoO3 which are easier to reduce. The strength of BAS is lower in HMCM-56 than in HMCM-22 and it is reduced in HMCM-22 after modification with Mo species. The lower strength of BAS in HMCM-56 results in a higher selectivity to 2-butene in the transformation of methanol, reaching 98.5% for Mo/HMCM-56. Mo species play a crucial role in the oxidation of sulphide to sulphoxide by H2O2. MoO3 is less stable in Mo/HMCM-56 because of the weaker interaction with the external zeolite surface. It is leached to the solution during oxidation of dibutyl sulphide. Therefore, Mo/HMCM-22, where Mo species are stronger bound to the surface, is more effective catalyst for this reaction.