Formation of HCO+ ion by protonation of carbon monoxide in zeolites: the origin of catalytic activity in methanol carbonylation

Abstract

We show that CO can be protonated by Brønsted acid sites to form the superelectrophilic HCO + cation in H-zeolites such as mordenite. This reactivity stems from the confining environment of small 8-ring side pockets of H-MOR. HCO + is a catalytically active super-electrophile: it carbonylates dimethyl ether and hydrocarbons to methyl acetate and aldehydes.Zeolites are industrially important crystalline and microporous materials composed of tetrahedral SiO4 and AlO4 units [1]. They are used industrially as adsorbents and catalysts for cracking, reforming, methanol to olefin (MTO) and other reactions [1][2][3][4][5]. Despite intense research, various catalytic processes that occur on zeolites took decades to understand mechanistically. Methanol to olefin process, for example, is thought to occur through hydrocarbon pool mechanism with alkylated aromatics being the catalytically important intermediates [6]. The origin of cracking activity of steamed/dealuminated zeolites has been discussed for over six decades and attributed to enhanced Bronsted acidity of steamed zeolites [2]. Very recently, we revealed that increased cracking catalytic activity stems from the presence of Al2O3 clusters in the steamed/dealuminated zeolite with penta Al1O5 sites that break the first alkane C-H bonds very effectively with the following steps occurring on Bronsted acid sites [7]. Among recent developments of zeolites catalytic chemistry, methanol carbonylation activity to form acetic acid from (CH3OH + CO) or methyl acetate from (dimethyl ether + CO) that was first described on H-Zeolite catalysts by Fujimoto [8] has recently gained a lot of attention.Experimental efforts [8][9][10][11] identified that zeolites containing 8-membered rings, such as H-MOR, FER, SSZ-13 and SSZ-39 possess good activity for this catalytic process whereas large-pore