Alkoxy group surface-bound

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). 

Since the reaction of M0O3 with alcohols to form surface-bound alkoxides has been studied in detail [18,19], initial investigations of the Nb2W40i9 ion have focused on the binding of alkoxy groups. The reactions outlined in Scheme I summarize the pathways available in solution to esters Nb2W40j 9R, R = CH3, CH3CH2, (CH3)2CH, and (0113)30,... 

Additional chemical evidence for the assignment of the 58 ppm resonance to the methoxy species III was the observation that it also formed from methyl bromide and methyl chloride in relative yields consistent with the leaving group stability T > Br > Cl. Methyl iodide was adsorbed on several zeolites with different Si/Al ratios, and the intensity of the 58 ppm resonance correlated with the A1 content, as it must for a framework-bound alkoxy. The final example of chemical evidence for the assignment regards the expected chemistry of species such as III and VI upon exposure to moisture. The Si-O-C linkage is easily hydrolyzed on a silica gel surface to form alcohols and/or ether. As demonstrated in Fig. 16, the species assigned to III readily hydrolyzes to methanol and dimethyl ether, whereas the proposed ethoxy species formed from ethyl iodide- C hydrolyzed to ethanol upon exposure to atmospheric moisture. 

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