Propane dehydrogenation over vanadium-doped zirconium oxide catalysts
Graphical abstract
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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2023, Journal of Industrial and Engineering ChemistryCitation Excerpt :However, the optimal active species is still controversial, the investigation and adjustment of these active species need further research in the future. The coordinatively unsaturated Zr sites on the ZrO2 surface are very critical for the dehydrogenation of light alkanes [162–164]. Otroshchenko et al. used seven preparation methods to prepare ZrO2 catalysts for controlling the activity and selectivity in PDH [165].