文摘
On the basis of density functional theory including dispersion correction [ωB97XD/6-311+G(2df,2p)//B3LYP/6-311G(d,p)], the thermodynamics and kinetics of the reactions of CH3OH and CH3OCH3 over H-ZSM-5 have been systematically computed. For the reaction of the methylated surface (CH3OZ) with CH3OH, CH3OCH3 formation is kinetically controlled and the competitive formation of CH2O + CH4 is thermodynamically controlled, in agreement with the observed desorption temperatures of CH3OH, CH3OCH3, and CH2O under experimental conditions. For the reaction between ZOCH3 and CH3OCH3, the formation of the framework stabilized (CH3)3O+ is kinetically controlled, consistent with the NMR observation at low temperature, and the competitive formation of surface CH3OCH2OZ + CH4 is thermodynamically controlled. On the basis of the thermodynamically more favored CH2O and CH3OCH2OZ, there are two parallel routes for the first C–C bond formation, from the coupling of CH3OCH2OZ with CH3OH and CH3OCH3 as well as from the coupling of CH2O with CH3OH and CH3OCH3. The most important species is the methylated surface (CH3OZ), which can react with CH3OH and CH3OCH3 to form the corresponding physisorbed CH2═O and chemisorbed CH3OCH2OZ, and they can further couple with additional CH3OH and CH3OCH3 to result in first C–C formation, verifying the proposed formaldehyde (CH2O) and methoxymethyl (CH3OCH2OZ) mechanisms.