Influence of the Reaction Temperature on the Nature of the Active and Deactivating Species During Methanol-to-Olefins Conversion over H-SAPO-34

Abstract

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 between 673 and 773 K. The operando spectroscopy results suggest that the nature of the active species also influences the olefin selectivity. In fact, monoenylic and highly methylated benzene carbocations are more selective to the formation of propylene, whereas the formation of the group of low methylated benzene carbocations and methylated naphthalene carbocations at higher reaction temperatures (i.e., 673 and 773 K) favors the formation of ethylene. At reaction temperatures between 573 and 623 K, catalyst deactivation is caused by the gradual filling of the micropores with methylated naphthalene carbocations, while between 623 and 773 K the formation of neutral poly aromatics and phenanthrene/anthracene carbocations are mainly responsible for catalyst deactivation, their respective contribution increasing with increasing reaction temperature. Methanol pulse experiments at different temperatures demonstrate the dynamics between methylated benzene and methylated naphthalene carbocations. It was found that methylated naphthalene carbocations species are deactivating and block the micropores at low reaction temperatures, while acting as the active species at higher reaction temperatures, although they give rise to the formation of extended hydrocarbon deposits.

Kinetic and Deactivation Differences Among Methanol, Dimethyl Ether and Chloromethane as Stock for Hydrocarbons

“…At the same time, the catalyst has a significant amount of dimethyl and trimethyl naphthalenes (DBE=6 and carbon number=12 and 13, respectively). Those species are known to have a lower intrinsic methylation rate than polymethyl benzenes and to be precursors for more deactivating species . For CTH reaction the distribution of HCP species is also centered at tetramethyl benzenes, but the proportion of bulkier methylated benzenes and naphthalenes is neglected by the increase of the proportion of trimethyl benzenes and olefins (DBE=1).…”

“…Those species are known to have a lower intrinsic methylation rate than polymethyl benzenes [52] and to be precursors for more deactivating species. [53] For CTH reaction the distribution of HCP species is also centered at tetramethyl benzenes, but the proportion of bulkier methylated benzenes and naphthalenes is neglected by the increase of the proportion of trimethyl benzenes and olefins (DBE = 1). These results indicate that the HCP species are more evolved in the case of MTH and DTH than for CTH, and this evolution is linked with species with lower methylation activity, methylated naphthalenes in particular.…”

Recent trends and fundamental insights in the methanol-to-hydrocarbons process

“…3d,e). 25,26,42,43,44,45,46,47 The formation and characteristics of these bands are quite unique depending on the zeolite framework topology and acidity.…”

“…H-SAPO-34 and H-SSZ-13) employing a combination of operando UVvisible DRS and online gas chromatography/mass spectrometry. 13,42,47 Using a similar strategy involving operando UV-visible DRS, the same group very recently monitored the formation of active HCP species and the accumulation of coke molecules during both H-ZSM-5 and Mg-ZSM-5 catalysed MTH. 48 Such spatiotemporal UV-visible spectroscopic approach reveals the formation of a coke front at the beginning of reactor bed, which travels towards the end until full deactivation.…”

Mesoporogen‐Free Synthesis of Hierarchical SAPO‐34 with Low Template Consumption and Excellent Methanol‐to‐Olefin Conversion

“…In previous work, the methylsubstituted benzene speciesw ere considered as the active intermediates for the generation of ethylene and propylene. [13] According to the coke analyses, the main reason for the deactivation of conventionalm icroporous SAPO-34 catalysts is blocking of the outer surface rather than the absence of active intermediates.S evere coke deposition on the outer surface of the SAPO-34 catalysts led to inaccessibility of methanola nd active sites or active intermediates inside the catalyst, even though the active intermediates still existed within the deactivated catalysts;t his is consistent with previous works. [14] Moreover,o ur experimental results clearly revealed that the bulky multimethyl-substituted pyrene was located inside the intracrystalline hierarchical spaces, which demonstrated that the fabrication of ah ierarchical structure is ap owerful approach for improvingt he interaction between methanol and the active intermediates, accommodating more bulky coke species, and avoidingf ast deactivation.…”