Promoting propane dehydrogenation with CO₂ over Ga₂O₃/SiO₂ by eliminating Ga-hydrides

Abstract

Due to the shortage supply of propylene and the development of shale gas, there is increased interest in on-purpose propane dehydrogenation (PDH) technology for propylene production. Ga-based catalysts have great potential in PDH, due to the high activity, low carbon deposit and deactivation. Ga-hydrides formed during PDH reduce the rate, selectivity and yield of propylene. In this contribution, CO₂ is introduced into PDH as a soft oxidant to eliminate the unfavorable intermediate species Ga^δ+-H_x re-generating Ga³⁺-O pairs, and also minimize coke deposition thereby improving the catalytic performance. In situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy experiments show that CO₂ can effectively eliminate Ga^δ+-H_x. At different temperatures, co-feeding CO₂ during PDH over Ga₂O₃/SiO₂ catalysts with different loadings significantly improves the stability of the conversion and selectivity, especially the latter, and provide a new dimension for improving the performance of PDH process.

Introduction

Propylene is the second most utilized building block of the petrochemical industry after ethylene and is used for the synthesis of polypropylene, propylene oxide, acrylic, etc. Propylene is mainly produced by naphtha steam cracking and fluid catalytic cracking [1]. However, crackers have been converted from naphtha to ethane units, with ethane feedstocks predominantly producing ethylene, thereby reducing propylene production. At the same time, the demand for propylene has gradually increased, resulting in a shortage of propylene supply. With the development of shale gas, there is increasing interest in on-purpose technologies for propylene production, such as catalytic propane dehydrogenation (PDH) [2, 3].

Currently, PtSn/Al₂O₃ and Cr₂O₃/Al₂O₃ catalysts are commercially used, but challenges still exist, including insufficient C₃H₆ selectivity and deactivation caused by carbon deposition [1, 4]. A number of metal oxides, such as Ga₂O₃ [5, 6, 7, 8, 9, 10], ZnO [11, 12, 13], VO_x [14, 15, 16, 17], MoO_x [18], are also active for the PDH. Ga-based catalysts have great potential in PDH, due to the low carbon deposition and high activity. Xu et al. [9] reported that the propylene selectivity can reach 74% at propane conversion of 39% over Ga₂O₃/ZrO₂. Szeto et al. [8] have found that the mononuclear alumina-supported Ga sites also show promising activity and high selectivity toward propylene at 550 °C.

In recent years, Ga-based catalysts have attracted more attention. SiO₂ supported well-defined Ga(III) single-site catalysts have been developed, which show higher activity and selectivity towards propylene compared with other single-site catalysts based on Fe, Co, and Zn [11] [19, 20, 21, 22]. There have also been many studies on the active sites of Ga-based catalysts. Getsoian et al. [23] suggested that features previously assigned to Ga⁺ in XANES may also be interpreted as low-coordinate Ga³⁺-H_x or Ga³⁺-R_x species on Ga-impregnated zeolites and SiO₂-supported single-site catalysts. Cybulskis et al. [24] investigated the nature of active sites for PDH on single-site Ga/SiO₂, and showed that Ga³⁺ alkyl and hydride intermediates have a great influence on PDH performance. Only four coordinate, single-site Ga³⁺-O species can cleave the C–H bonds of C₃H₈. The less stable Ga^δ+-H_x species is formed under high temperature H₂ treatment, and the rate of its transformation to Ga³⁺-O can affect the rate of propane dehydrogenation. Rane et al. [25] studied the PDH activity of Ga⁺ and GaO⁺ in zeolite ZSM-5, and showed that the monovalent Ga⁺ species are more active than GaH₂⁺ species, and compared with these cations, GaO⁺ cations are the most active. Phadke et al. [26] investigated mechanism of PDH over Ga/H-MFI, and reported that [GaH]²⁺ cations are the catalytically active centers for PDH and H₂ inhibits the dehydrogenation by reaction with [GaH]²⁺ cations to from [GaH₂]⁺-H⁺ cation pairs.

Due to its promotional effect, CO₂ has been extensively used as a mild oxidant in oxidative dehydrogenation reactions. CO₂ can alleviate the chemical equilibrium in oxidative dehydrogenation by the effective removal of H₂ through coupling with reverse water gas shift (RWGS) reaction. Compared with O₂, CO₂ suppresses the unwanted total oxidation products due to its lower oxidizing ability. CO₂ can also modify the surface and/or bulk composition of the catalyst, thereby affecting the structural, electronic, acid-base, adsorption, diffusion, and redox characteristics [27]. We speculate that the addition of CO₂ in PDH may accelerate the transformation of Ga^δ+-H_x species to Ga³⁺-O pairs to re-generate the active sites for C–H bond activation of propane, thus improving the catalytic performance. Meanwhile, the introduction of CO₂ induces the change of the Gibbs free energy (ΔG) of propane dehydrogenation. Without CO₂, ΔG is 8.3 kJ/mol (600 °C), while it is 15.3 kJ/mol (600 °C) after the introduction of one molar equivlent of CO₂. In this work, the role of Ga-hydrides in the PDH of supported gallium oxide is studied using in situ DRIFT, and the results show the effectiveness of CO₂ to eliminate Ga-hydrides. With the introduction of CO₂, the dehydrogenation performance was indeed improved, especially the propylene selectivity. This work provides an effective way to improve the dehydrogenation performance of Ga₂O₃/SiO₂ catalysts, and also provides insight into the catalytically active centers.