Elsevier

Catalysis Today

Volume 352, 1 August 2020, Pages 337-344
Catalysis Today

Propane dehydrogenation over vanadium-doped zirconium oxide catalysts

https://doi.org/10.1016/j.cattod.2019.12.012Get rights and content

Highlights

  • Effects of V content on properties of V-doped ZrO2 (VZrO-x) were investigated.

  • Incorporation of V dopant in Zr lattice generated more Zrcus4+ cations.

  • VZrO-x showed superior PDH activity and recyclability than pure ZrO2.

  • Deactivation of VZrO-x was suppressed by co-feeding hydrogen.

Abstract

Bulk ZrO2 is a highly active and selective catalyst for dehydrogenation of propane (PDH), in which coordinatively unsaturated Zr cations (Zrcus4+) serve as active sites. Substitution of dopant ions into Zr lattice can improve its catalytic activity by generating more Zrcus4+ sites. In this work, a series of vanadium-doped ZrO2 metal oxides (VZrO-x) have been prepared and the influences of vanadium content on their properties have been systematically investigated. Various characterization techniques showed that an appropriate amount of vanadium dopant helps more Zrcus4+ sites to be created by a structural transformation and H2 pretreatment. However, excess vanadium dopant led to a negative effect on the catalytic activity owing to the formation of bulk-like V2O5 crystallites. The catalytic activity of VZrO-x is well correlated with the amount of Lewis acid sites because Zrcus4+ cations correspond to Lewis acid sites. The VZrO-8 catalyst exhibited two times higher activity than pure ZrO2. Moreover, for repeated cycles the activity was totally recovered by oxidative regeneration followed by reductive pretreatment. Finally, the performance test results showed that H2 co-feeding can further enhance the activity by suppressing coke deposition during PDH.

Introduction

In the chemical industry, propene is a widely used starting material for the production of polymers, oxygenates, and valuable chemical intermediates [1]. Large volumes of propene have been produced by steam cracking and fluid catalytic cracking. A steady increase in demand for propene over the past decade has made propene-selective processes necessary because conventional processes yield a mixture of olefins [[1], [2], [3], [4]]. In terms of the exclusive selectivity towards propene, the direct propane dehydrogenation (PDH) process has attracted attention in recent years. Moreover, the PDH process has the advantage of using a more economical feedstock, propane. The profitability of the PDH process is based on the price gap between propane and propene and high availability of cheap propane from shale resources. These facts further motivate research on PDH.

The non-oxidative dehydrogenation of propane (C3H8 ↔ C3H6 + H2) is a highly endothermic and thermodynamically limited reaction. Thus, the PDH process requires high reaction temperatures above 550 ℃ to provide a sufficient equilibrium conversion at ambient pressure [3,5]. The high operating temperature in the PDH process causes several issues, including severe coke deposition by the deep dehydrogenation of propane and agglomeration or sintering of active phases [6]. As a result, PDH catalysts rapidly lose their activity and they must be periodically regenerated after the short-time reaction [3,7,8]. There are two types of catalysts that have been used in commercial PDH processes: Pt-Sn/Al2O3 and CrOx/Al2O3 [3,[7], [8], [9], [10]]. Despite the high activity and selectivity of these catalysts, the high cost of Pt and the severe toxicity of the Cr6+ species require the development of alternative catalysts that have economic and environmental benefits.

The catalytic performances for the dehydrogenation of light alkanes have been reported for many metal oxides [3,[11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]]. It is known that coordinatively unsaturated metal cations (Mcusx+) that are next to oxygen vacancies serve as adsorption sites for light alkanes. Surface oxygen sites act as hydrogen acceptors, abstracting hydrogen from the adsorbed alkane molecules. For easily reducible metal oxides, the number of Mcusx+ sites and thus their catalytic performances can be tuned by pretreatment using a reducing gas, such as H2 and CO. The superior performances of pretreated bulk WOx and FeOx/Al2O3 catalysts compared to fully oxidized catalysts have been reported in previous studies [11,20]. In addition to the partial reduction of metal oxides, Mcusx+ sites can be created by structural transformation when metal oxides are mixed together. Ga2O3-Al2O3 solid solutions, which have more coordinatively unsaturated Ga3+ cations, exhibited higher catalytic activity than γ-Ga2O3 in propane dehydrogenation [12].

Recently, a bulk ZrO2 catalyst has been reported to exhibit a high activity and selectivity in PDH [14,22,23]. A substitution of dopants with lower oxidation states or with different radii into ZrO2 lattice leads to structural transformation, creating defect sites [24,25]. The accompanied increase in the amount of Zrcus4+ cations and oxygen vacancies further improved catalytic performance of ZrO2-based catalysts for dehydrogenation of light alkanes [[25], [26], [27], [28]]. Vanadium oxide is known as an active catalyst for oxidative dehydrogenation of light alkanes [[29], [30], [31], [32]]. In this study, we first demonstrated that vanadium can improve the catalytic performance of the ZrO2 catalyst by creating more active sites of Zrcus4+ cations. To identify the influence of the dopant content, a series of vanadium-doped ZrO2 metal oxides (VZrO-x) were prepared via the co-precipitation method. The physical and chemical properties of VZrO-x were systematically investigated by various techniques. The catalytic performances and recyclability of VZrO-x were compared to those of ZrO2 in the temperature range of 550–600 ℃. Moreover, the influence of H2 co-feeding on the deactivation rate of VZrO-x with the time-on-stream was investigated.

Section snippets

Catalyst preparation

VZrO-x catalysts with various vanadium contents were prepared by the co-precipitation method. In a typical synthesis, specified amounts of ZrO(NO3)2∙xH2O (Sigma Aldrich, 99%) and VCl3 (Sigma Aldrich, 97%) were dissolved in 200 mL of deionized water to obtain a total cation concentration of 0.1 M. While vigorously stirring the solution at room temperature, an aqueous ammonia solution (Samchun Pure Chemical, 28–30 wt%) was added dropwise until the pH of 9 was reached. The solution was aged at 100

Structural properties of the VZrO-x catalysts

Transition metal dopants can significantly affect the physico-chemical properties of ZrO2 [24]. The influence of the amount of vanadium dopant on the crystalline phase of VZrO-x, where x = [V]/([Zr]+[V]) × 100, was investigated using X-ray diffraction (XRD). Generally, pure ZrO2 has a monoclinic phase that is thermodynamically stable below 1175 ℃ [33]. As shown in Fig. 1, the synthesized ZrO2 clearly exhibited diffraction peaks at 2θ = 28.3° and 31.8°, corresponding to the monoclinic phase (●).

Conclusion

In this study, we showed that the addition of vanadium dopant in ZrO2 can lead to significant improvement in catalytic activity and recyclability in PDH. The superior catalytic performances of VZrO-x resulted from the creation of additional Zrcus4+ cations, which are Lewis acid sites. The activity of VZrO-x was well correlated with the amount of Lewis acid sites. An appropriate amount of vanadium led to the substitution of the vanadium dopant into Zr lattice. These VOx species enabled more Zrcus

CRediT authorship contribution statement

Namgi Jeon: Conceptualization, Investigation, Writing - original draft. Hyeongju Choe: Investigation. Beomgyun Jeong: Investigation. Yongju Yun: Conceptualization, Supervision, Writing - original draft, 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.

Acknowledgements

This research was supported by LG Chem and the National Research Foundation of Korea (NRF) grant (NRF-2019R1C1C1002846) funded by the Ministry of Science and ICT. The NAP-XPS analysis was supported by the Korea Basic Science Institute under the R&D program (Project No. D38700, T38607, and C39121) supervised by the Ministry of Science and ICT.

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