Ga-Substituted Nanoscale HZSM-5 in Methanol Aromatization: The Cooperative Action of the Brønsted Acid and the Extra-Framework Ga Species

Abstract

A series of nanosized [Al]-, [Ga, Al]-, and [Ga]-HZSM-5 catalysts with a fixed Si-to-M3+ ratio (M = Al or Ga) was synthesized by a seed-induced crystallization method. In order to reveal the catalytic nature of the extra-framework Ga species, an acid treatment was applied to selectively extract Lewis acidic amorphous Ga cations of as-synthesized catalysts. A comparative evaluation of freshly prepared and acid-treated catalysts in methanol conversion to aromatics showed the dehydrogenative nature of the extra-framework Ga species, which is essential in the enhancement of aromatics. Among tested catalysts, [Ga, Al]-HZSM-5 with a low extra-framework Ga-to-Brønsted acid ratio (0.06) was the most effective. The efficacy is due to the contact synergy of the extra-framework Ga species and the Brønsted acid, by which aromatics originated from the dual-cycle mechanism of methanol conversion can be accelerated.

Recent Advances on Gallium-Modified ZSM-5 for Conversion of Light Hydrocarbons

“…Metal precursors and preparation procedures are vital for the successful fabrication of metal modified zeolites. Conventionally, Ga-containing salts [ 45 , 46 , 47 , 48 , 49 , 50 ] (gallium nitrate, etc. ), trimethyl gallium [ 51 ], gallium chloride [ 52 ], gallium oxide [ 53 , 54 ], etc.…”

Ethylene Dehydroaromatization over Ga‐ZSM‐5 Catalysts: Nature and Role of Gallium Speciation

“…It has been reported [10] that solution ion exchange can lead to the formation of crystalline Ga x O y that is not visible in the PXRD pattern of Ga-Z2;h owever,s amples prepared by ion exchange with higher Ga concentrations (Table S1) clearly show the presence of crystalline Ga x O y by PXRD (i.e.peaks at 2q = 21.68 8 and 37.18 8 in Figure S4). [22] It has also been reported [16] that PXRD patterns can provide evidence of Ga incorporation in framework sites based on shifts in two characteristic MFI peaks,( 501) and (303), when al arger Ga atom replaces as maller Al atom. Fora ll H-Ga-ZSM-5 samples in this study,these two peaks (at 2q = 23.08 8 and 23.88 8, respectively) exhibit no observable shifts compared to commercial zeolite,Z -ref ( Figure S3).…”

“…[11c,14] Jones and co-workers [7b] developed amethod using synthesis mixtures with 3-mercaptopropyl-trimethoxysilane (MPS) to generate gallosilicates with ah igh percentage of Ga Lewis acid sites occluded as extra-framework species into zeolite pores without directly forming covalent bonds with framework oxygens.T he resulting gallosilicate catalyst (i.e.G a-MFI in the absence of Al) enhanced rates of propane dehydrogenation compared to gallosilicates prepared by conventional methods.Shu and coworkers evaluated several Ga-ZSM-5 catalysts obtained via different methods (i.e.d irect synthesis,i on exchange,i mpregnation, and physical mixing) on n-heptane aromatization. [15] They reported the Ga-ZSM-5 catalyst prepared by direct synthesis exhibited the highest BTX selectivity,w hich they attribute to enhanced Lewis acidity and the presumed mesoporosity incurred by the incorporation of framework Ga. Moreover,t hey hypothesized that both liquid-phase ion exchange and direct synthesis lead to Ga substitution in framework sites.O nt he contrary,H sieh et al [16] argued that Ga is less prone to be incorporated into the zeolite framework (compared to Al) based on their analysis of [Ga, Al]-ZSM-5 catalysts obtained via seeded synthesis.…”

“…Moreover, they hypothesized that both liquid‐phase ion exchange and direct synthesis lead to Ga substitution in framework sites. On the contrary, Hsieh et al . argued that Ga is less prone to be incorporated into the zeolite framework (compared to Al) based on their analysis of [Ga, Al]‐ZSM‐5 catalysts obtained via seeded synthesis.…”

Insight into the Formation of COX By‐Products in Methanol‐to‐Aromatics Reaction over Zn/HZSM‐5: Significantly Affected by the Chemical State of Surface Zn Species

“…[1][2][3] HZSM-5 zeolite is regarded as the most appropriate catalytic-material for this process, because of its adjustable acidic properties, high hydrothermal stability and regular three-dimensional 10-ring pore matching with the light aromatic molecules. [4][5][6] Generally, the significantly enhanced selectivity of aromatics can be obtained over the metal modified HZSM-5 (in particular, the catalysts containing Zn, Ag or Ga), [1,4,5,[7][8][9][10][11][12][13][14][15][16][17][18][19] owing to the interaction-dehydrogenation [5,13,[20][21][22][23] of metal species with acid sites. For instance, Bi et al obtained an aromatic selectivity of up to 77.9 % at the time-on-stream of 10 min over a Zn-modified HZSM-5 (SiO 2 /Al 2 O 3 = 38) with the retention of more medium-strength Brønsted acid sites at 753 K and 2.0 h À 1 WHSV.…”