Influence of the Reaction Temperature on the Natureof the Active and Deactivating Species During Methanol-to-OlefinsConversion over H-SAPO-34

摘要

The selectivity toward lower olefins during the methanol-to-olefins conversion over H-SAPO-34 at reaction temperatures between 573 and 773 K has been studied with a combination of operando UV–vis diffuse reflectance spectroscopy and online gas chromatography. It was found that the selectivity toward propylene increases in the temperature range of 573–623 K, while it decreases in the temperature range of 623–773 K. The high degree of incorporation of olefins, mainly propylene, into the hydrocarbon pool affects the product selectivity at lower reaction temperatures. The nature and dynamics of the active and deactivating hydrocarbon species with increasing reaction temperature were revealed by a non-negative matrix factorization of the time-resolved operando UV–vis diffuse reflectance spectra. The active hydrocarbon pool species consist of mainly highly methylated benzene carbocations at temperatures between 573 and 598 K, of both highly methylated benzene carbocations and methylated naphthalene carbocations at 623 K, and of only methylated naphthalene carbocations at temperatures between673 and 773 K. The operando spectroscopy results suggest that thenature of the active species also influences the olefin selectivity.In fact, monoenylic and highly methylated benzene carbocations aremore selective to the formation of propylene, whereas the formationof the group of low methylated benzene carbocations and methylatednaphthalene carbocations at higher reaction temperatures (i.e., 673and 773 K) favors the formation of ethylene. At reaction temperaturesbetween 573 and 623 K, catalyst deactivation is caused by the gradualfilling of the micropores with methylated naphthalene carbocations,while between 623 and 773 K the formation of neutral poly aromaticsand phenanthrene/anthracene carbocations are mainly responsible forcatalyst deactivation, their respective contribution increasing withincreasing reaction temperature. Methanol pulse experiments at differenttemperatures demonstrate the dynamics between methylated benzene andmethylated naphthalene carbocations. It was found that methylatednaphthalene carbocations species are deactivating and block the microporesat low reaction temperatures, while acting as the active species athigher reaction temperatures, although they give rise to the formationof extended hydrocarbon deposits.