“Coking” of zeolites during methanol conversion: Basic reactions of the MTO-, MTP- and MTG processes

Deactivation of acidic zeolite catalysts during methanol conversion is investigated for elucidating how spatial constraints interfere mechanistically. Detailed product composition – including retained organic matter – is determined in a time resolved mode. At 270–300 °C with H-ZSM-5, first unsaturated hydrocarbons are formed—methane being the indicative co-product. Then the reaction rate increases auto-catalytically, but soon declines because of exhaustive pore filling. The retained organic matter consists mainly of ethyl-trimethyl-benzene- and isopropyl-dimethyl-benzene molecules. Alkylation of benzene rings with ethene and propene produces the deactivating molecules. At 475 °C, alkylation of benzene rings with olefins has shifted to the reverse, reactivating the H-ZSM-5 catalyst. Coke forms slowly on the surface of H-ZSM-5 crystallites. Spatial constraints suppress the formation of 2-ring aromatics. With the wide pore zeolite H-Y, fast deactivation is noticed—bigger aromatic molecules can be formed and are retained. Methanol reactions on the protonic catalyst sites are visualized as CH3+ attack for methylation and dehydrogenation, methane being the hydrogen-rich co-product. Methanol conversion on zeolites H-ZSM-58, H-EU-1 and H-Beta is comparatively investigated. Zone ageing is discussed for favorable reactor design. It is shown, how a multi-compound product composition is the source of information for elucidating complex reaction mechanisms.