Selective Oxidation of Propane to Propylene in the Presence of HCl over CeO2 and NiO-Modified CeO2 Nanocrystals

Selective oxidation of propane to propylene in the presence of HCl over CeO₂ and NiO-modified CeO₂ nanocrystals

Jincan Kang, Quanhua Xie, Huamin Zhang, Jun Cheng, Qinghong Zhang, Ye Wang*

State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, P. R. China

*Corresponding author: wangye@xmu.edu.cn

Abstract: Here, we report a new strategy for the oxidative dehydrogenation of propane by O₂ with outstanding propylene yield in the presence of HCl. CeO₂ is an efficient catalyst for the conversion of C₃H₈ to C₃H₆ by (O₂ + HCl). The reaction was structure-sensitive, and the catalytic behavior depended on the exposed facet of CeO₂ nanocrystals.  A propylene selectivity of 80% was achieved over an 8 wt% NiO−CeO₂ catalyst, offering a propylene yield of 55%. The structure−property correlation indicates that the surface oxygen vacancy and the surface chloride coverage are two crucial factors determining the activity and selectivity. The peroxide species formed by adsorption of O₂ on surface oxygen vacancies may activate chloride, generating a radical-like active chlorine species for the selective conversion of propane.

Keywords: Oxidative dehydrogenation, Propane, Propylene, CeO₂ nanocrystals, Hydrogen chloride

1. Scope

Oxidative dehydrogenation of C₃H₈ is an attractive reaction for C₃H₆ production, but the over-oxidation results in low C₃H₆ selectivity at considerable C₃H₈ conversions and the formation of undesirable CO₂. To increase the selectivity of the target product, which is more reactive than the substrate, is a challenging goal in selective oxidation catalysis. Despite some recent encouraging progress, high propylene selectivity (>70%) is still difficult to achieve at a high propane conversion (>30%).

Some studies have been devoted to the conversion of CH₄ and other lower alkanes by (O₂ + HX), but only a few studies have reported the conversion of propane by this strategy.^1,2 Besides lower olefins, RX (R = alkyl group) was also formed in the conversion of lower alkanes.² Here, we report the direct conversion of C₃H₈ to C₃H₆ by (O₂ + HCl) with a high single-pass yield using CeO₂-based catalyst.³ We demonstrate that the reaction is structure-sensitive, and the modification of CeO₂ by NiO can further enhance C₃H₆ yield. The roles of HCl and the reaction mechanism will be discussed.

2. Results and discussion

We first investigated the catalytic behaviors of various metal-oxide catalysts for the conversion of C3H8 by (O2 + HCl). The result shows that CeO2 is a promising catalyst, not only because CeO2 demonstrates the highest single-pass C3H6 yield among all the catalysts examined but also because it shows high stability. The catalytic behavior of CeO2 depended on its morphology or the exposed facets  The rates of C3H8 conversion and C3H6 formation decreased in the order of nanorods (exposing {110} + {100}) > nanocubes (exposing {100}) > nano-octahedra (exposing {111}) ≈ nanoparticle (exposing {111}). Thus, the {110} facet shows higher activity than the {100} facet, which was significantly higher than the {111} facet. We further compared the C3H6 selectivity at similar C3H8 conversion levels and found that the C3H6 selectivity decreased in the following sequence: nanocube > nanorod > nanooctahedron ≈ nanoparticle. Thus, the {100} facet is the most selective for C3H6 formation, followed by the {110} and {111} facets. In short, the CeO2-catalyzed conversion of C3H8 is a structure-sensitive reaction. The {110} facet is the most active for C3H8 conversion, whereas the {100} facet is the most selective for C3H6 formation.

We investigated the effect of various modifiers on the catalytic behavior of CeO2 nanorods. Among all the modifiers examined, NiO was the most efficient for promoting C3H6 formation. Both O2 and C3H8 conversions increased after the doping of NiO with a low content. The C3H6 selectivity increased gradually from 55% to 72% with an increase in NiO content to 8 wt%.  At the same time, the selectivities of CO and CO₂ decreased, and the selectivity of organic chlorides kept low (< 4%).

Our studies revealed that HCl played a crucial role in the selective formation of C3H6 over CeO2 based catalysts. CO2 was the major product in the absence of HCl (Figure 1A), indicating that CeO2 and NiO−CeO2 were complete oxidation catalysts for the oxidation of C3H8 by O2. Both C3H8 conversion and C3H6 selectivity increased with the partial pressure of HCl. C3H8 conversion of ~70% and C3H6 selectivity of ~80% were attained over the 8 wt% NiO−CeO2 catalyst at a P(HCl) of 25 kPa (Figure 1B). The single-pass C3H6 yield reached ~55%. We further confirmed that ~98% HCl could be recovered.

Figure 1. Effect of HCl partial pressure on the conversion of C₃H₈ by (O₂ + HCl). (A) CeO₂. (B) 8 wt% NiO−CeO₂.

We performed mechanistic studies for CeO2 and 8 wt% NiO−CeO2 catalysts (Figure 2). We uncovered that the oxidation of HCl by O2 to Cl2 (the Deacon reaction) occurred on our catalysts, but the formation of Cl2 was inhibited by the presence of C3H8. We characterized the CeO2 nanocrystals with different morphologies and the NiO−CeO2 catalysts with different NiO contents by UV-Raman and HCl chemisorption studies. The correlation of the characterization results with the catalytic behaviors suggests that C3H8 conversion activity depends on the concentration of oxygen vacancies, while the chemisorption amount of HCl determines C3H6 selectivity. In combination with DFT calculations, we propose that oxygen is adsorbed on the O-vacancy site on CeO₂ surfaces, forming O₂²⁻ species. And the HCl is activated by the O₂²⁻ species into Cl•-like species, which may be responsible for the activation of C3H8 and the selective formation of C3H6.

Figure 2. Proposed reaction mechanism for the oxidative dehydrogenation of C₃H₈ to C₃H₆ by (O₂ + HCl) over CeO₂-based catalysts.

3. Conclusions

CeO2 is an efficient and stable catalyst for the conversion of C3H8 to C3H6 by O2 in the presence of HCl. The reaction is structure sensitive and the catalytic behavior depends on the exposed facet of CeO2. CeO2 nanorods and nanocubes show the highest activity and the highest C3H6 selectivity, respectively. The modification of CeO2 nanorods with NiO increases catalytic performances, offering a C3H6 yield of ~55%. The structure−property correlation suggests that the concentration of surface O vacancies and the surface coverage of Cl⁻ are two crucial factors that determine the activity and selectivity. We propose that oxygen is adsorbed on the O-vacancy site on CeO₂ surfaces, forming O₂²⁻ species and the O₂²⁻ species oxidizes Cl⁻ into Cl•-like species, which accounts for the activation and selective conversion of C₃H₈ to C₃H₆.

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