Reactivity of Surface Methoxy Species

Formation of Surface Methoxy Species during the Conversion of Methanol to DME on Acidic Zeolites 

Among the early investigations of methanol adsorption and conversion on acidic zeolites, most of the H and C MAS NMR experiments were performed under batch reaction conditions with glass inserts in which the catalyst samples were fused. Zeolites HZSM-5 76a,204,206,264-272), HY 71,72), H-EMT 273), HZSM-12 274), HZSM-23 275), H-erionite 275), H-mordenite 271,272), and H-offretite 275,276), silicoaluminophosphates H-SAPO-5 271,274), H-SAPO-11 274), and H-SAPO-34 76,277,278), as well as montemorillonite 279) and saponite 279) were investigated as catalysts. 

At reaction temperatures of r 523 K, the conversion of methanol on acidic zeolites is dominated by a dehydration of methanol to DME 210). Two mechanisms have been proposed for the formation of DME. In the indirect pathway (Eqs. (27a, b)), methanol molecules adsorbed on bridging OH groups react first to give methoxy species (ZOCH3), which subsequently react with another methanol molecule to give DME 280,281)  

Z stands for the zeolite framework. In the direct pathway (Eq. (28)), two methanol molecules react with each other on one Bronsted acid site 282). This pathway involves the simultaneous adsorption and reaction of two methanol molecules, with the formation of one DME and one water molecule in a single step  

A number of theoretical studies have been performed to improve our understanding of the adsorption and conversion of methanol on acidic zeolites 245,283-288). Applying non-local periodic density functional calculations. Gale and co-workers 284) suggested that both pathways, described in Eqs. (27a, b) and (28), are energetically reasonable routes. By contrast, Blaszkowski and van Santen 286) found 

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To unambiguously elucidate the reactivity of surface methoxy species, the preparation of pure methoxy species on the catalyst surface is an important prerequisite. This preparation can be achieved by a SF protocol, which starts with a flow of C-enriched methanol into acidic zeolites at room temperature, followed by a purging of the catalyst with dry nitrogen at room temperature and subsequently at higher temperatures (74,262. The latter step progressively removes the surplus of methanol and DME, together with water produced by the conversion of methanol. 

The reactivity of surface methoxy species was further investigated with various probe molecules that were thought to possibly be involved in the MTO process, including water, toluene (representing aromatics), and cyclohexane (representing saturated hydrocarbons) (263). It was found that surface methoxy species react at room temperature with water to form methanol, which indicates the occurrence of a chemical equilibrium between these species at low reaction temperatures (Scheme 15) (263). 

Until now, the detailed mechanism involved in the MTG/MTO process has been a matter of debate. Two key aspects considered in mechanistic investigations are the following the first is the mechanism of the dehydration of methanol to DME. It has been a matter of discussion whether surface methoxy species formed from methanol at acidic bridging OH groups act as reactive intermediates in this conversion. The second is the initial C—C bond formation from the Ci reactants. More than 20 possible mechanistic proposals have been reported for the first C-C bond formation in the MTO process. Some of these are based on roles of surface-bound alkoxy species, oxonium ylides, carbenes, carbocations, or free radicals as intermediates (210). 

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