Methanol to hydrocarbons over large cavity zeolites: Toward a unified description of catalyst deactivation and the reaction mechanism

The reaction mechanism for the conversion of methanol to hydrocarbons over three large cavity zeolites, H-beta, H-MCM-22, and H-mordenite, has been investigated. 13C methanol was co-reacted with 12C benzene to study the buildup and further reactions of the intermediates formed. Co-reaction was required, as these aromatic intermediates will not be formed from pure methanol at temperatures low enough to actually monitor these events. The reactions were followed by dissolving quenched catalysts in HF followed by extraction of the organic compounds and analysis by GC–MS. The same hydrocarbon compounds are formed inside the pores of three zeolites, and it is the most substituted methylbenzenes that function as reaction intermediates in the hydrocarbon pool mechanism for the conversion of methanol. The heptamethylbenzenium cation was for the first time detected and shown to serve as a key reaction intermediate in zeolite catalysts other than H-beta. The formation of bicyclic coke precursors was also investigated, and progress has been made toward a more complete description of the reactions leading to catalyst deactivation. Quantum chemical calculations have shed light on the processes leading to coke precursors. The profound similarities between H-beta, H-mordenite, and H-MCM-22 shown herein constitute a significant step toward a unified understanding of the MTH reaction over acidic zeolites.