Reactivity Of Surface Methoxy Species On Acidic Zeolites

Mechanistic study of heterogeneously catalyzed reactions is not only of fundamental importance for refinement of theories in modern chemistry,but also of practical importance for optimization of existing industrial processes and development of new ones.Surface methoxy species is an important intermediate in methanol conversion on acidic zeolites,and it also exists as a form of the simplest carbocation CH3+ on acidic zeolites.Knowledge of their reactivity is directly related to the mechanism of the first C-C bond formation in methanol conversion,and also benefit to the establishment of carbocation chemistry in heterogeneous catalytic systems.In this work,the reactivity of surface methoxy species was investigated by in situ 13C MAS NMR and GC-MS analysis.The main results obtained are summarized in the following:1.Decomposition of surface methoxy speciesOn various acidic zeolites(the strength of acidity follows the order of H-ZSM-5>H-SAPO-34>H-Y>H-[B]-ZSM-5),surface methoxy species could always be transformed into complex hydrocarbons upon thermal treatments.There is an obvious induction period during which light alkanes and olefins were formed before aromatics and cyclic tertiary carbenium caions.With the decrease of acidity of zeolites,the decomposition temperature increases and the main primary products are changed from alkanes(on H-ZSM-5)to alkenes(on H-[B-]ZSM-5).C2-species(ethane or aldehyde)were always observed as primary products on H-SAPO-34 or Na-Y with low acidic concentration.These results provide the experimental evidences to support the direct mechanism for methanol conversion.2.Reaction of surface methoxy species and alkanesSmall inert alkanes(such as methane,ethane,propane or isobutane)were first used as probe molecules to react with surface methoxy species at temperature lower than the decomposition temperature of surface methoxy species.The reaction temperature decreased with increasing the number of substituent groups of alkanes.Notably,during the reaction between 13C-labeled surface methoxy species and non-labeled Cn alkanes,the single-labeled Cn+1 alkanes were the mainly primary products on zeolite H-Y,but on zeolite H-ZSM-5,the single-labeled Cn+1 alkanes were always produced with accompanied by the formation of almost 100%13C-labeled methane.This research sheds light on the low-temperature transformation of alkanes by co-conversion with methanol.3.Carbenium-ion-like property of surface methoxy speciesBased on the decomposition of surface methoxy species and its reaction with alkanes,we first demonstrated that surface methoxy species on zeolite H-ZSM-5 could take part in the hydride transfer,direct methylation,and carbene insertion reactions.At temperatures T<471 K,surface methoxy species reacts with propane trough hydride transfer reaction,generating methane as a characteristic product.On the other hand,surface methoxy species could react with methane via direct methylation and carbene insertion to produce ethane at T=473 K.In analogy to the activity of CH3+ species in superacid solutions,the reactivity of surface methoxy species on acidic zeolites involves C-H bond activation and C-C bond formation.It not only provides new insight into carbocation chemistry of solid acids,but also helps to understanding of the first C-C bond formation in methanol conversion.