Methanol to hydrocarbons conversion: Why dienes and monoenes contribute differently to catalyst deactivation?

Dienes, formed at much lower concentrations than monoenes via alkylation of monoenes with formaldehyde (HCHO), have much higher propensity than monoenes to foment catalyst deactivation in methanol-to hydrocarbons (MTH) conversion over zeolites via HCHO-mediated deactivation pathways. The mechanistic basis for why monoenes and dienes contribute differently to cause catalyst deactivation during MTH is investigated using independent kinetic studies of HCHO reactions with representative monoenes and dienes, i.e., propylene and 1,3-butadiene, over zeolites. Despite 3,6-dihydro-2H-pyran being the predominant product for both propylene/HCHO and 1,3-butadiene/HCHO reactions, a combination of transient and steady-state rate measurements and stoichiometric experiments reveals distinct persistent intermediates being involved in the addition of HCHO to different surface-bound alkyl species derived from propylene and butadiene. These persistent intermediates desorb only via HCHO and water-mediated proton transfer steps and as such can be sequestered on the catalyst. A nonlinear mixed-effects kinetic model reveals that the intermediate formed during HCHO-diene reactions has a higher steady-state surface coverage and its desorption via HCHO and water mediated routes exhibit > 8x and > 3x, respectively, higher rate constants than those during HCHO monoene reactions. These results evidence the distinct identity, surface coverage, and reactivities of surface bound alkyl intermediates as critical factors in the much more pernicious effects of dienes compared to monoenes on catalyst lifetime during MTH.