Formaldehyde-Induced Deactivation of ZSM5 Catalysts during the Methanol-to-Hydrocarbons Conversion

High Resolution Image Download MS PowerPoint Slide Formaldehyde productivity and reactivity over zeolite catalysts are suggested to influence their selectivity and stability in the methanol-to-hydrocarbons (MTH) conversion. However, details of these relationships have yet to be established. In this study, we applied photoelectron photoion coincidence spectroscopy to examine the productivity of this intermediate over unpromoted and zinc-loaded ZSM-5 catalysts. Formaldehyde is the primary MTH product. Its production over unpromoted catalysts proceeds mainly via hydrogen transfer between methanol molecules, the rate of which is proportional to the concentration of strong Brønsted acid sites. In zinc-promoted ZSM-5, formaldehyde is mainly derived via methanol dehydrogenation, which provides up to a 12× higher productivity of this intermediate. The cumulative turnover capacity of the catalyst displays a strong negative correlation with its formaldehyde evolution activity. Formaldehyde evolution is enhanced at higher methanol concentrations and reaction temperatures, which are conditions also leading to enhanced coking. Complementary formaldehyde cofeeding experiments demonstrate that this intermediate enhances the formation of ethene and methylbenzenes over C 3+ alkenes, which is in agreement with high selectivities to the products of the arene cycle over the catalysts with high formaldehyde productivity. The addition of formaldehyde promotes most substantially the deactivation of the most stable ZSM-5 catalysts displaying inherently low productivity of this intermediate, rendering their lifetimes comparable to those of their less stable counterparts. Operando diffuse-reflectance UV–vis spectra show that small amounts of formaldehyde greatly promote the conversion of cyclopentadienyl and methylated benzenyl intermediates into bicyclic and polycyclic aromatics. Consistent with this, formaldehyde cofeeding results in increased coke accumulation in more stable catalysts, especially in their micropores, as supported by X-ray diffraction and photoelectron spectroscopy analysis. These findings represent direct evidence that the stability and product distribution of ZSM-5 catalysts are strongly governed by their formaldehyde evolution activity.