New insights in catalyst deactivation and product distribution of zeolites in the Methanol-To-Hydrocarbons (MTH) reaction with methanol and dimethyl ether feeds
Methanol (MeOH) and dimethyl ether (DME) have been compared as feedstock for the Methanol-To-Hydrocarbons (MTH) reaction over H-ZSM-5 (MFI), H-SSZ-24 (AFI) and H-SAPO-5 (AFI) catalysts at 350 and 450 °C. Several clear observations were made. First, the MeOH-DME equilibrium is not always established in the MTH reaction, because the rate of MeOH dehydration to DME is similar to the rates of the methylation reactions over strong Brønsted acid sites. In the presence of weak acid sites (i.e. the AlPO framework of SAPO-5), which are nearly inactive to hydrocarbons formation, the MeOH-DME equilibrium can be reached. Second, the MTH activity is ostensibly higher for DME compared to MeOH. Third, the carbon conversion capacity of the catalysts is generally higher (up to 16 times higher under the conditions used in this work) with a DME feed compared to a MeOH feed. Incorporation of AlPO-5 as dehydration catalyst before or mixed with a H-SSZ-24 catalyst for MTH, leads to lower MeOH concentrations in the reaction mixture, and a significant increase of the conversion capacity. Finally, a MeOH feed results in a higher selectivity for aromatic products and ethylene, pointing to a larger contribution of the arene cycle, compared to a DME feed. We hypothesize, that MeOH causes formation of formaldehyde, while DME does not. Formaldehyde is a known coke precursor, which provides an explanation for the faster deactivation of zeolites in a MeOH feed.