Methanol adsorption in zeolites - A first-principles study

The methanol to gasoline (MTG) conversion process, using a zeolite catalyst, is of major commercial importance. However, the first step of the reaction, involving methanol adsorption on the zeolite catalyst, is still not well understood. This paper describes first-principles calculations performed on periodic zeolite models to investigate the nature of methanol adsorption. We have examined a number of possible geometries for this adsorption and found that the nature of the adsorbed species can depend on the particular zeolite structure. In more open ring structures, as found in chabazite, the stable form of methanol is found to be protonated, in contrast to results of previous calculations on cluster models. However, in the sodalite structure methanol is found To be simply physisorbed. The vibrational spectra of the adsorbed species have been studied and compared to experimental results. It is found that both chemisorbed methanol and physisorbed methanol give strongly red-shifted O-H stretcking frequencies, but the former can be distinguished by the H-O-H bending mode.