Supported ionic liquid-palladium catalyst for the highly effective hydrochlorination of acetylene
Graphical abstract
Introduction
Vinyl chloride is a monomer (VCM) for industrial manufacture of polyvinyl chloride, and the mass production of vinyl chloride is widely known as catalytic hydrochlorination of acetylene (C2H2) [1]. Nearly 40% of the global VCM capacity is produced via calcium carbide method using mercuric chloride (HgCl2) supported on activated carbon as catalyst [2]. However, these processes are not green because mercury is a volatile and toxic material, which does damages to human health and environment [3]. In addition, inevitably HgCl2 is lost from the catalyst, which significantly limits the catalyst lifetime. Recently, the annual loss of mercury from commercial VCM production is 600–1000 tons, which is roughly 50% world consumption [4]. It is unsustainable since mercury deposit is rapidly being depleted. Therefore, there is an urgent need to develop a novel non-mercury catalyst for acetylene hydrochlorination.
To obtain highly active and stable acetylene hydrochlorination catalysts, various robust catalysts include transition metals [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], non-precious metals [18], [19], [20], [21] and even nonmetallic materials [22], [23], [24], [25] have been investigated. Among them many studies have focused on gold and palladium chlorides as better environmentally benign alternatives. Thereafter Au-based catalysts were widely studied and wide efforts are being made. The activity and stability of Au-based catalysts have been constantly improved to bring this catalyst to commercialization recently [4]. In comparison, Pd-based catalysts received much less attention and there were only a few reports although it also exhibited a high activity. This is likely due to the low stability of Pd-based catalyst under reaction conditions. For example, Nkosi et al. [26] reported that the PdCl2/C catalysts showed better acetylene conversion rate than that HgCl2/C catalysts. Unfortunately, although PdCl2/C acting as an acetylene hydrochlorination catalyst has a high initial activity, it still has some intrinsic defects, for example, the leaching of PdCl2 active component will decrease the catalytic activity dramaticlly, which limits the commercial application of Pd-based catalysts [27]. A lot of efforts have been devoted to promote the stability of Pd-based catalysts but the weakness remains. For example, Wang et al. [28] has demonstrated a supported Pd catalyst over HY zeolites for acetylene hydrochlorination. The Pd-based catalysts with NH4F modification preformed the higher stability owning to changes of the surface acidity, less carbon deposition and Pd loss, but the lifetimes were shorter and there were still 44.7% Pd species leaving. In addition, Wang et al. [29] also investigated the deactivation of Pd/NFY catalysts and suggested that by decreasing the Pd loss, the Pd-K/NFY catalyst could improve its stability. However, inductive coupled plasma (ICP) analysis showed that there still 37.8% Pd species leaving after reacting for 10 h compared with the fresh Pd-based catalyst. Despite the significant improvements for this Pd-based catalyst, it remains a great challenge to develop a active and robust catalytic system for acetylene hydrochlorination.
Herein, we report a Pd-supported-ionic-liquid-phase catalyst aiming to enhance the stability of non-mercury Pd-based catalysts for acetylene hydrochlorination reaction. This Pd-supported-ionic-liquid-phase catalyst was first introduced to ensure the catalytic components dissolved in continuously operated reactions via the combination of high solvent polarity and nonvolatility by Mehnert and co-workers [30], [31], [32], [33], [34], [35], [36]. Previously, we utilized supported-ionic-liquid-phase to preserve the promotional effect of Au-based catalysts in the acetylene hydrochlorination [37], [38]. The specific combination of 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) ionic liquid and activated carbon support was chose to immobilize volatile catalytically active Pd complexes in this work. Owning to the possible coordinating ability of ionic liquid to Pd atoms and the strong confinement effect, the as-grown Pd complexes are confined within porous supports. The significantly enhanced activity and stability suggest the likelihood of non-Hg-based catalysts towards commercial VCM production.
Section snippets
Chemicals
Activated carbon (AC, ROX 0.8, pellets of 0.8 mm diameter and 5 mm length) and PdCl2 (the content of Pd assay ≥ 59%) were purchased from NORIT and sigma-aldrich, respectively. The acetylene (gas, 98.0%) was purified by potassium dichromate solution to remove hydrogen sulfide and phosphine, and then dehydrated with zeolite-5A. Hydrogen chloride (HCl, greater than 99.9%) and nitrogen (N2, greater than 99.9%) were used without further purification. 1-butyl-3-methyl-imidazolium chloride ([Bmim]Cl)
Catalytic performance of Pd-IL/AC catalysts
The prepared 0.5Pd-10IL/AC and 0.5Pd/AC catalysts were investigated for acetylene hydrochlorination in a lab-scale fixed-bed reactor at 160 °C with C2H2 gas hourly space velocity (GHSV) of 740 h−1. Reactions without PdCl2 showed negligible catalytic activity (<10% conversion) for both IL/AC and bare AC catalysts, which eliminates the possible contribution of the ionic liquid to the reaction activity, as shown in Fig. 1a. 0.5Pd/AC catalyst is active to the acetylene hydrochlorination reaction.
Conclusions
In conclusion, we have successfully developed for the first time an environmentally friendly and highly efficient acetylene hydrochlorination process in a Pd-supported-ionic-liquid-phase catalysis system. Superior performance with 98.6% conversion of C2H2 and above 99.8% selectivity to VCM was obtained over the 0.5Pd-10IL/AC catalyst. Furthermore, the 0.5Pd-10IL/AC catalyst demonstrated a steady catalytic performance with a negligible loss of C2H2 conversion after 10 h reaction, which exceeds
Acknowledgements
Financial support from the National Natural Science Foundation of China (NSFC; grant No. 21606199, 21476207) are gratefully acknowledged.
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