Study of the conversion of methanol to dimethyl ether on zeolite HZSM-5 using in situ flow MAS NMR

The conversion of methanol to gasoline (MTG) range hydrocarbons on zeolite catalyst HZSM-5 has been studied extensively using solid-state NMR. We have studied the reaction under batch and flow conditions using an isolated flow variable-temperature (VT) MAS NMR probe. This probe was developed to study heterogeneous catalysis reactions in situ at temperatures greater than 300 degrees C with reactant flow. In the batch studies, when 13C-labeled methanol was adsorbed on zeolite HZSM-5, sealed, and heated to 250 degrees C, dimethyl ether was formed. Two-dimensional exchange NMR shows that dimethyl ether was in equilibrium with methanol at 250 degrees C. When 13C-methanol was flowed over HZSM-5 at temperatures > or = 200 degrees C, only dimethyl ether was observed. Between 160 degrees C and 200 degrees C, both methanol and dimethyl ether were observed. The flow results are significant in that they suggest that there is no equilibrium between methanol and dimethyl ether in the catalyst at high temperatures, and that surface methoxy groups do not exist on the catalyst at high temperatures.