Single-site zinc on silica catalysts for propylene hydrogenation and propane dehydrogenation: Synthesis and reactivity evaluation using an integrated atomic layer deposition-catalysis instrument

•Synthesis of isolated Zn(II) alkyl sites on SiO2 via atomic layer deposition.•Single-site nature of Zn(II) alkyl sites was confirmed by spectroscopic measurements (e.g., XAS).•Open-shell, tricoordinate ethyl-Zn(II) sites exhibit higher activity.•High Zn(II) dispersion produces Zn/SiO2 catalysts with improved activity and stability.

Alkyl-zinc and zinc oxide-type sites were synthesized via atomic layer deposition on high-surface-area silica using an integrated atomic layer deposition-catalysis instrument (I-ALD-CAT). One-cycle ALD experiments using diethylzinc (DEZ) afforded Zn/SiO2 systems that provided key insights into the reactivity and stability of Zn sites as a function of dispersion at the submonolayer level. The I-ALD-CAT tool design allowed for systematic comparison of the reactivity of different grafted zinc sites. Open-shell 16-electron, tricoordinate ethyl zinc-silica sites exhibit higher activity in propane hydrogenation-dehydrogenation compared to 18-electron, tetracoordinate zinc oxide-type centers. Silica surface saturation with Zn(II) sites (∼75% of a monolayer) results in facile zinc agglomeration and catalyst deactivation under reaction conditions. Reduced DEZ dosing coupled with thermal substrate pretreatment techniques (e.g., dehydration under vacuum) resulted in increased Zn dispersion and produced Zn/SiO2 catalysts with improved activity and stability under propylene hydrogenation (200 °C) and propane dehydrogenation (550 °C) conditions.

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