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

Abstract

Bifunctional catalysis in zeolites possessing both Brønsted and Lewis acid sites offers unique opportunities to tailor shape selectivity and enhance catalyst performance. Here,w ee xamine the impact of framework and extra-framework gallium species on enriched aromatics production in zeolite ZSM-5. We compare three distinct methods of preparing Ga-ZSM-5 and reveal direct (single step) synthesis leads to optimal catalysts compared to post-synthesis methods.U sing ac ombination of state-of-the-art characterization, catalyst testing,and density functional theory calculations,weshow that Ga Lewis acid sites strongly favor aromatization. Our findings also suggest Ga(framework)-Ga(extra-framework) pairings, which can only be achieved in materials prepared by direct synthesis,are the most energetically favorable sites for reaction pathwaysl eading to aromatics.C alculated acid site exchange energies between extra-framework Ga at framework sites comprised of either Al or Ga reveal as ite-specific preference for stabilizing Lewis acids,w hich is qualitatively consistent with experimental measurements.T hese findings indicate the possibility of tailoring Lewis acid siting by the placement of Ga heteroatoms at distinct tetrahedral sites in the zeolite framework, whichc an have am arked impact on catalyst performance relative to conventional H-ZSM-5.

Aromatics Production via Methanol-Mediated Transformation Routes

“…It was found that such a high dispersion of Ga facilitated the formation of active gallium species, which have a good synergism with Brønsted acid sites. 250,251 Lin et al prepared a series of nanosized [Al]-, [Ga, Al]-, and [Ga]-HZSM-5 catalysts with a fixed Si-to-M 3+ ratio (M = Al or Ga) via the direct hydrothermal synthesis method. 43 Among all tested catalysts, [Ga, Al]HZSM-5 with a total Ga/Al ratio of 0.5 but a low extra-framework Ga-to-Brønsted acid ratio of 0.06, was found to be the most effective in the methanol aromatization, exhibiting an initial aromatic selectivity of 79.5% at 500 °C.…”

Study of the Hydrogen Pretreatment of Gallium and Platinum Promoted ZSM-5 for the Ethane Dehydroaromatization Reaction

“…The XPS spectra for the reduced catalyst in the Ga 2p 3/2 region show a loss of peak intensity, indicating the high dispersion of the metal species on the zeolite surface. For the red-Ga and red-GaPt catalysts, the intensity loss indicates the penetration of the Ga species into the zeolite pores and ion exchange of the metal promotors with the Brønsted acid sites. ,,− XPS of the red-Ga and red-GaPt catalysts exhibited a shift in speciation to the higher binding energy of 1120.0 eV. The shift to the higher binding energy when compared to those of the fresh catalysts indicates the increased presence of Ga + and/or GaO + on the external surface following the reduction of the catalyst.…”

“…17,18,35,37 Specifically, it has been shown for Gamodified ZSM-5 that the intrazeolite Ga species of the catalyst can dehydrogenate ethane to form ethylene, which then can be further oligomerized and aromatized by the Ga Lewis acid sites species and the Brønsted acid sites. 35,37,38 In this work, further investigation of the synergistic effects of the bimetallic GaPt promoted ZSM-5 catalyst was performed. The effectiveness of the hydrogen pretreatment of gallium and platinum promoted ZSM-5 was tested for the ethane dehydroaromatization reaction.…”

“…This step has been shown to be promoted by the metal sites on the zeolite catalysts. ,,, Following the dehydrogenation of ethane, the second step is considered to be the oligomerization of olefins, cyclization, and final dehydrogenation to aromatics. The oligomerization of the olefins has been shown to occur on the Brønsted acid sites of the metal promoted zeolite catalyst. ,,, Specifically, it has been shown for Ga-modified ZSM-5 that the intrazeolite Ga species of the catalyst can dehydrogenate ethane to form ethylene, which then can be further oligomerized and aromatized by the Ga Lewis acid sites species and the Brønsted acid sites. ,, …”

Two-Step Process for Feasible Conversion of Ethane to Aromatics: Concept and Demonstration

“…Regarding the roles of Ga in the reaction system, it is one of the most efficient catalyst components for olefin aromatization, − and it also catalyzes conversions of light alkanes such as propane and butanes to aromatics. The Cyclar process is an example of this application.…”