Lewis–Brønsted Acid Pairs in Ga/H-ZSM-5 To Catalyze Dehydrogenation of Light Alkanes

Abstract

The active sites for propane dehydrogenation in Ga/H-ZSM-5 with moderate concentrations of tetrahedral aluminum in the lattice were identified to be Lewis-Brønsted acid pairs. With increasing availability, Ga and Brønsted acid site concentrations changed inversely, as protons of Brønsted acid sites were exchanged with Ga. At a Ga/Al ratio of 1/2, the rate of propane dehydrogenation was 2 orders of magnitude higher than with the parent H-ZSM-5, highlighting the extraordinary activity of the Lewis-Brønsted acid pairs. Density functional theory calculations relate the high activity to a bifunctional mechanism that proceeds via heterolytic activation of the propane C-H bond followed by a monomolecular elimination of H and desorption of propene.

The Nature and Catalytic Function of Cation Sites in Zeolites: a Computational Perspective

“…These experimental and computational findings highlight the bifunctional nature of the Cu/SSZ‐13 . Very recently, the extraordinary activity of Lewis‐Brønsted acid pairs of Ga/H‐ZSM‐5 for propane dehydrogenation was reported by Schreiber et al . The promotion effect of Lewis‐Brønsted acid pairs for methanol to aromatics reaction was also identified for the same system by solid‐state NMR spectroscopy .…”

Capturing spatially resolved kinetic data and coking of Ga–Pt supported catalytically active liquid metal solutions during propane dehydrogenationin situ

“…the exact mechanisms at play are still under investigation. Finally, a contribution of Pt-GaOx species, a known catalyst for PDH, 39,40 may also explain the higher initial activity. The reduction of said species by in situ formed H2 to the metallic state upon exposure to propane has been observed by means of in situ HRTGA-MS measurements (Figure 2).…”

Ga/HZSM-5 Catalysed Acetic Acid Ketonisation for Upgrading of Biomass Pyrolysis Vapours

“…Powder XRD revealed negligible change zeolite structure following the reaction ( Figure S7), however elemental analysis confirmed the presence of surface carbon postreaction for all xGa/HZSM-5 catalysts (falling from 12 wt% for the parent HZSM-5 and xGa/HZSM-5 samples to only 1 wt% for Ga2O3, Table S2). Acetone selectivity at iso-conversion increased with Ga loading at both 350 °C and 400 °C ( Figure 6), concomitant with the rise in weak:strong acid site ratio and Lewis acidity [73]. Vervecken also reported an increase in acetone selectivity >350 °C for acetic acid ketonisation over HZSM-5(100) [42], attributed to a higher activation energy for ketonisation that competing aromatisation (which forms xylenols, phenolics and other aromatics).…”

“…Although all xGa/HZSM-5 catalysts were stable for 5 h on-stream at 400 °C, future extended ageing and recycling tests are required to optimise formulation and performance. Acetone selectivity at iso-conversion increased with Ga loading at both 350 • C and 400 • C (Figure 6), concomitant with the rise in weak:strong acid site ratio and Lewis acidity [73]. Vervecken also reported an increase in acetone selectivity >350 • C for acetic acid ketonisation over HZSM-5(100) [42], attributed to a higher activation energy for ketonisation that competing aromatisation (which forms xylenols, phenolics and other aromatics).…”