On the nature of active sites in alumina-supported zinc propane dehydrogenation catalysts

doi:10.1016/j.mcat.2020.111055

Molecular Catalysis

Volume 493, September 2020, 111055

https://doi.org/10.1016/j.mcat.2020.111055 Get rights and content

Highlights

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Interaction of Zn vapor with alumina results in the formation of Zn²⁺ surface cations.
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Zn²⁺ surface cations are active sites in hydrogen dissociation and propane conversion.
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Zn²⁺ surface sites in zinc-alumina catalysts are heterogeneous in their properties.

Abstract

Zinc-based materials are currently considered as an alternative for toxic chromium and costly platinum catalysts for propane dehydrogenation. So far however insufficient attention has been devoted to these materials in the literature. In this work we have revealed the nature of active sites in zinc-alumina (γ-alumina) catalysts. We have investigated zinc-alumina catalysts prepared by two different methods: traditional incipient wetness impregnation and zinc chemical vapor deposition (CVD) on alumina. The catalysts were characterized by infrared spectroscopy of adsorbed CO and H₂ molecules, TPR measurements and tested in propane dehydrogenation. It has been shown that high-temperature interaction of zinc vapor with alumina results in the formation of surface Zn²⁺ cations which are active in hydrogen dissociation and propane conversion. The amount of zinc that can be introduced by this procedure depends on the alumina dehydroxylation degree. It has been found that molecular hydrogen heterolytically dissociates on Zn²⁺ surface cations resulting in the formation of surface zinc hydride species. The Zn²⁺ sites on the surface of alumina are heterogeneous and are characterized by different band positions in the IR spectra of adsorbed CO and H₂ molecules. The Zn²⁺ sites are subjected to reduction under the reaction conditions.

Graphical abstract

Introduction

The selective catalytic conversion of light alkanes to alkenes is of growing interest because of rising demand for more efficient utilization of hydrocarbons feedstocks to produce useful petrochemicals and fuels. Significant attention has been garnered in recent years to propane dehydrogenation, because of the rising demand for propylene. Commercial dehydrogenation of propane processes (PDH) operate at temperatures above 500 °C (823 K). The catalysts used in this process generally are chromium oxide and platinum-based systems while many different materials have been explored. Platinum-based catalysts show excellent activity and good stability. However platinum is expensive and limitedly available. Chromium oxide catalysts are subjected to significant deactivation, because of carbon deposition requiring frequent catalyst regeneration. In addition chromium oxide catalysts because of their toxicity can cause environmental impact. All of these factors encourage searching for alternative systems for PDH.

Among the various materials that are actively investigated for PDH (vanadium oxide, gallium oxide, molybdenum oxide, indium oxide based materials) (for a review see [1]) zinc oxide (ZnO) based materials can be promising candidates, because of zinc low cost and low toxicity. The conversion of light alkanes to aromatics over zinc-exchanged ZSM-5 zeolites received considerable attention [2]. Zinc titanate was reported [3,4] exhibits high selectivity with high specific activity in dehydrogenation of propane and isobutane. Moderate activity and high olefin selectivity was also reported for Zn/Al₂O₃ [5] and Zn/SiO₂ [6,7] catalysts.

In this paper we report novel results on spectroscopic study of the nature of active sites on the surface of zinc-alumina catalysts and their catalytic behavior in propane dehydrogenation reaction. For the characterization of the surface sites we used diffuse reflectance spectroscopy of adsorbed carbon monoxide and hydrogen probe molecules. The effect of preparation method on the formation of catalytically active sites and catalytic performance of zinc-alumina catalysts was explored. Zinc-alumina catalysts were prepared by two different methods: conventional incipient wetness impregnation technique and high-temperature zinc vapor deposition (CVD) on alumina subjected to dehydroxylation at different temperatures. It was shown in the literature [8] that unlike conventional methods of zinc introduction (ion exchange, incipient wetness impregnation) high-temperature interaction of zinc vapor with the surface hydroxyls of support favors the formation of Zn²⁺ surface sites which are active in the catalytic transformations of alkanes [9].

Section snippets

Catalyst preparation

PURAL SB aluminum hydroxide (boehmite) from SASOL was used as a starting material for the preparation of the catalysts. The γ-alumina support was obtained by the calcination of this material at 800 K for 2 h. The BET specific surface area of the support measured by nitrogen adsorption was found to be 240 m²/g. Zinc-alumina catalysts were prepared by two different methods: (i) one sample was prepared by a conventional incipient wetness impregnation technique and (ii) two samples were prepared by

DRIFT spectroscopy characterization of freshly prepared catalysts

Fig. 2 shows the infrared spectra of carbon monoxide adsorbed on the zinc-alumina (Zn-Imp, Zn-CVD1 and Zn-CVD2) catalyst studied and on the parent alumina (preevacuated at 823 K). The spectra of the catalysts (traces 1–3) are characterized by a band in the region of 2202-2208 cm⁻¹. This band almost matches the position of CO adsorbed on Al³⁺ Lewis acid sites of the alumina surface (trace 4). However the intensities of the bands due to CO adsorption on the zinc-alumina catalysts are

Conclusion

In summary, in this work we have studied alumina supported zinc catalysts prepared by different methods: conventional wetness impregnation technique and high-temperature interaction of zinc vapor with alumina surface. The catalysts were characterized by DRIFT spectroscopy of adsorbed carbon monoxide and hydrogen probe molecules, TPR measurements and tested in propane dehydrogenation. The catalysts prepared by high-temperature interaction of zinc vapor with alumina are characterized by high

CRediT authorship contribution statement

Alexander I. Serykh: Conceptualization, Investigation, Resources, Writing - original draft, Writing - review & editing. Yury A. Agafonov: Investigation, Resources, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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On the nature of active sites in alumina-supported zinc propane dehydrogenation catalysts

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Catalyst preparation

DRIFT spectroscopy characterization of freshly prepared catalysts

Conclusion

CRediT authorship contribution statement

Declaration of Competing Interest

Appl. Catal.

J. Catal.

J. Catal.

J. Catal.

Microporous Mesoporous Mater.

J. Mol. Catal. A: Chemical

Chem. Eng. Res. Des.

Chin. Chem. Lett.

Chin. Chem. Lett.

Chem. Rev.

J. Appl. Chem. USSR

ACS Catal.

ACS Catal.

Phys. Chem. Chem. Phys.