Role of formaldehyde in promoting aromatic selectivity during methanol conversion over gallium-modified zeolites

Abstract

Gallium-modified HZSM-5 zeolites are known to increase aromatic selectivity in methanol conversion. However, there are still disputes about the exact active sites and the aromatic formation mechanisms over Ga-modified zeolites. In this work, in situ synchrotron radiation photoionization mass spectrometry (SR-PIMS) experiments were carried out to study the behaviors of intermediates and products during methanol conversion over Ga-modified HZSM-5. The increased formaldehyde (HCHO) yield over Ga-modified HZSM-5 was found to play a key role in the increase in aromatic yields. More HCHO was deemed to be generated from the direct dehydrogenation of methanol, and Ga2O3 in Ga-modified HZSM-5 was found to be the active phase. The larger increase in aromatic production over Ga-modified HZSM-5 after reduction‒oxidation treatment was found to be the result of redispersed Ga2O3 with smaller size generating a larger amount of HCHO. This study provides some new insights into the internal driving force for promoting the production of aromatics over Ga-modified HZSM-5.

Maximizing the catalytic performance of FER zeolite in the methanol-to-hydrocarbon process by manipulating the crystal size and constructing a bifunctional system

“…However, this observation is opposite to the reported effect of Y 2 O 3 with ZSM-5 zeolite: the hydrogen transfer product, including dienes, was reported to be decreased after intra-pellet physical mixing with Y 2 O 3 . 43,45 Based on this observation, two independent hydrogen transfer routes ( i.e. , methanol- and olefin-induced) were proposed.…”

“…However, this observation is opposite to the reported effect of Y 2 O 3 with ZSM-5 zeolite: the hydrogen transfer product, including dienes, was reported to be decreased after intra-pellet physical mixing with Y 2 O 3 . 43,45 Based on this observation, two independent hydrogen transfer routes (i.e., methanol-and olefin-induced) were proposed. 45,49 However, in our case, we hypothesize that Y 2 O 3 inter-mixed with zeolites significantly affects methanol-induced hydrogen transfer only, while the olefin-induced hydrogen transfer route still produces dienes.…”

“…39 Several strategies are usually employed in zeolite catalysis to extend the catalyst lifetime, including adjusting the acid site density, 21 modifying the morphology, 40,41 introducing mesoporosity, 42 and incorporating metals or constructing bifunctional systems. 43 Nevertheless, the primary aim is to decelerate the arene cycle of the dual cycle mechanism, which could be achieved in different ways, as indicated above. Herein, the role of formaldehyde (HCHO), a disproportionation product of methanol formed during the early phases of MTH catalysis (2CH 3 OH → CH 4 + HCHO), needs to be carefully evaluated.…”

Revealing Formaldehyde-Mediated Methanol-to-Aromatics Reactions over Zn-Modified Zeolites by Observing the Oxygenated and Polyunsaturated Intermediates

“…Generally, HCHO in MTH reactions was suggested to be mainly formed via three pathways, i.e., (1) disproportionation of methanol, 12,17 (2) HT from methanol to alkenes, 13,14 and (3) catalytic dehydrogenation of methanol. 25,36 To learn the origin of HCHO over Zn-modified HZSM-5, the time-resolved tendencies of methane, C2−C4 alkanes, H 2 , CO, and some other hydrocarbons were studied in a continuous-flow fixed-bed reactor (1 bar) and are presented in Figures 2f−i and S8. Methane is considered a key product that generated with HCHO in a 1:1 stoichiometry through the disproportionation reaction of methanol on BASs.…”

“…Besides, there are two possible pathways for the formation of naphthalenes in a cyclopentadiene-based system. One depends on the alkylation of two molecules of cyclopentadiene (36) and the second starts with Diels−Alder cyclopentadiene addition (37), both followed by isomerization and the dehydrogenation reaction. 59…”