Elsevier

Applied Catalysis A: General

Volume 566, 25 September 2018, Pages 15-24
Applied Catalysis A: General

Hydrochlorination of acetylene catalyzed by activated carbon supported highly dispersed gold nanoparticles

https://doi.org/10.1016/j.apcata.2018.08.012Get rights and content

Highlights

  • Au catalysts were prepared using several solvents for acetylene hydrochlorination.

  • The activity of the catalysts is linked with the intrinsic properties of solvents.

  • The optimal solvent can highly disperse and anchor the active species.

  • The optimal solvent greatly improves the catalytic performance of the catalysts.

  • Au-isopropanol/AC catalyst exhibits the outstanding activity.

Abstract

A series of Au catalysts were prepared with several representative solvents and evaluated for acetylene hydrochlorination. The results revealed that the activity of the catalyst is closely linked with the intrinsic properties of solvents. The catalytic performance of the catalysts increased with decreasing polarity of the solvents, and superior performance was achieved over the Au-isopropanol/AC catalyst, with a 84% stable conversion under reaction conditions of 180 °C and a gas hourly space velocity (GHSV) of 1200 h−1; relative increases of 700.0% and 483.3%, respectively, in acetylene conversion were achieved compared with that achieved with traditional catalysts prepared with water and aqua regia. The substitution of the highly polar water and aqua regia with weakly polar and volatile alcohols altered the crystallization process of Au nanoparticles (NPs) during their formation. The various edges or defects in the formed multiple-twinned or polycrystalline particles provided new active sites for reactants. The altered solvents may enhance the interaction between the support and the Au species, highly dispersing and anchoring the active species and inhibiting their agglomeration and loss during the reaction. Moreover, the interaction also strengthens the adsorption capacity for reactants of the catalysts, enhancing their catalytic performance. This approach may provide an effective reference for exploring environmentally benign mercury-free catalysts for acetylene hydrochlorination.

Graphical abstract

The catalytic activity of the Au catalyst is closely linked with the intrinsic properties of the solvents used in the preparation process.

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Introduction

Polyvinyl chloride (PVC) has a wide and important application as the second-largest general-purpose resin. Two synthetic methods are used to produce PVC: (i) ethylene oxychlorination and (ii) acetylene hydrochlorination. Acetylene hydrochlorination is mainly used in regions rich in coal resources. For example, more than 70% of the VCM produced in China is synthesized by acetylene hydrochlorination. Currently, the most widely used catalyst in this synthesis route is activated-carbon-supported mercury (II) chloride (HgCl2), in which the active components are easily sublimated. An estimated 1.02–1.41 kg of HgCl2 catalyst (HgCl2 content: 10–12 wt%) is consumed per tonne of PVC produced; however, approximately 25% of the HgCl2 is lost during the cycle. The loss of Hg not only causes catalyst deactivation but also causes severe environmental pollution and human health risks [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]]. In addition, more than 140 countries signed the ‘Minamata Convention on Mercury’ treaty in Japan, which aims to control the worldwide consumption of Hg. Therefore, a solution to the pollution problem caused by the Hg catalyst is needed, and the research and development of alternative non-Hg-based catalysts is underway.

Although several metal chlorides, including Pt4+ [9,10,[20], [21], [22]], Pd2+ [[21], [22], [23], [24]], Ru2+ [14,[25], [26], [27]], Sn2+ [28], have been studied as possible Hg-free catalysts for acetylene hydrochlorination, Au-based catalysts have been demonstrated to be the most active catalysts to replace the Hg-based catalysts [[29], [30], [31], [32], [33]]. For example, Hutchings proposed that the standard reduction potential of metal ions is a more appropriate parameter for correlating metal active components and their catalytic activity; as such, Au(III) should exhibit higher catalytic activity than other metal catalysts because of its higher standard reduction potential [3,6]. However, although the catalytic activity of the Au-based catalyst is excellent, it is easily deactivated by (i) the reduction of Au3+ species to Au° [5], (ii) the formation of coke deposits and (iii) the aggregation of Au particles [34,35]; which limits its industrial application. Consequently, many scholars have investigated Au-based catalysts. Conte et al. [36] adopted aqua regia as the impregnation solvent for HAuCl4 and studied the effect of Pd, Pt, Ir, Rh and Ru additives on the catalytic performance of Au catalysts; their results showed that the additives did not improve the catalytic performance and that the undoped Au was the most active catalyst for long-term use in acetylene hydrochlorination. Shen et al. [31] used aqua regia as the solvent and investigated the effect of CuCl2 as a promoter on the catalytic performance of Au–Cu/C catalysts for acetylene hydrochlorination; their results showed that the dispersion of active species and the surrounding electron cloud density of Au3+ were improved upon the addition of Cu species, resulting in stable activity with no discernible decline at 200 h under the reaction conditions of 160 °C, a total gas hourly space velocity (GHSV) of 50 h−1 and a gas volume ratio VHCl/VC2H2 of 1.15. In our previous works [11,33,37], we prepared bimetallic and ternary Au-based catalysts to improve the catalytic performance of the single Au catalyst; the results indicated that the additives could inhibit the aggregation of active Au species and that La(III) and Co(III) additives, which possess higher standard reduction potentials, could also inhibit the reduction of Au3+ to Au°; the Co(III) and Cu(II) components could augment the relative content of Au+ and Au3+, respectively, in Au-based catalysts. Consequently, we found that the ternary Au1Co(III)3Cu(II)1/SAC (SAC = spherical activated carbon) catalyst exhibited the best performance. Wei et al. [30] synthesized an Au-based catalyst by complexing Au with KSCN and found that the -SCN complex substantially decreased the electrode potential of Au3+ and thus reduced the likelihood of its reduction, resulting in higher reactivity and selectivity. They also introduced Bi into the Au-aqua regia solution and prepared Au-Bi/C catalysts; X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure analyses showed that the addition of Bi made Au species exist in the form of Au+, which greatly enhanced the catalytic activity of the catalyst [38]. Dai et al. [13] used polypyrrole to modify the carrier and found that electron transfer between N atoms and Au3+ centres could increase the adsorption of hydrogen chloride onto the catalyst and improve the activity and stability of the catalyst. Hu et al. [39] investigated the effect of boron-modification of AC on the catalytic performance of Au catalysts. Their results showed that the modification of boron species is favourable to stabilize the active Au3+ species and inhibit catalyst sintering during the reaction, thereby improving the catalytic stability. Li et al. [40] synthesized an Au–IL/AC (IL = ionic liquid) catalyst and found that the active components can be well developed in an IL homogeneous medium; the Au(III) complexes were stabilized in the Au(III) form by forming an Au(III)–IL complex, resulting in a catalyst with an excellent specific activity and superior long-term stability, with 99.4% acetylene conversion and only 3.7% loss after running for 300 h under reaction conditions of 180 °C and a GHSV(C2H2) of 30 h−1. They also demonstrated that Au(III) catalysts could be stabilized through modifying a supported-IL-phase Au(III) catalyst with CuCl2 and found that the reduced Au° could be re-oxidized in situ to Au3+ species by CuCl2 during the reaction and further stabilized by the electron transfer from Cu to these active species, their catalyst delivered stable catalytic performance with no obvious loss of conversion after 500 h under typical industrial reaction conditions [41].

Conte et al. [42] investigated the effect of Au-based catalysts on the hydrochlorination of acetylene using different solvents such as aqua regia, concentrated hydrochloric acid and concentrated nitric acid. They also investigated the effect of the different preparation conditions on the catalytic activity of Au-based catalysts. their results showed that the impregnation solvent (HCl, HNO3, aqua regia) and the drying temperature not only affect the functional groups on the surface of the carrier, but also affect the Au3+/Au° ratio; they also showed that the acetylene conversion rate is not related to the total Au3+ amount but to the amount of Au3+ on Au/C interfaces [5]. Among the catalysts they investigated, the one prepared using aqua regia as the solvent, inhibited the agglomeration of the Au components. Li et al. [43] pretreated AC at different temperatures and assessed the catalytic performance of the supported Au catalysts towards acetylene hydrochlorination; their results indicated that the AC provides active sites for the activation of acetylene and that Au3+ at the AuCl3/C interface is the active site of the catalyst. Hutchings’ group [44] recently performed an in situ X-ray absorption fine structure study of Au/C catalysts under reaction conditions and showed that highly active catalysts comprise single-site cationic Au entities whose activity correlates with the Au(I) : Au(III) ratio; They also showed that the Au/C catalysts were supported analogues of single-site homogeneous Au catalysts and proposed a mechanism based on a redox couple of Au(I)–Au(III) species. Zhao et al. [45] demonstrated that organic aqua regia (OAR) can be used as a green alternative solvent to traditional aqua regia to activate Au/AC catalysts for this reaction; the variations of Au particle size and chemical state caused by the OAR treatment promoted Au oxidation and high dispersion, leading to both improved activity and enhanced stability of the resultant Au(H2O)/AC (OAR) catalyst.

In view of these previously reported results, the solvents exert a non-trivial effect on the catalytic performance of the single undoped Au/AC catalyst. However, the solvents used in the previous studies mainly focused on aqua regia, water and acid solutions; the common relationship between the properties of solvents and the performance of the catalysts was not unmasked. Therefore, in the present work, we selected several representative solvents used in the preparation of Au/AC catalysts to correlate their properties with the catalytic behavior of the resultant catalysts. The results show that the catalytic activity of the Au catalyst is closely linked with the intrinsic properties of the solvents used in the preparation process; the substitution of the highly polar water and aqua regia with weakly polar and volatile alcohols alters the crystallization process of Au nanoparticles (NPs) during their formation and the various edges or defects in the formed multiple-twinned or polycrystalline particles provide new active sites for reactants. The altered solvents may enhance the interaction between the corresponding functional groups on the support and the Au species, highly dispersing and anchoring the active species during the preparation process and inhibiting their agglomeration and loss during the reaction. Moreover, the interaction also enhances the reactants adsorption capacity of the Au catalysts, thereby enhancing their catalytic performance. The obtained results reveal that the highly dispersed Au NPs also exhibit good catalytic activity for acetylene hydrochlorination, and these results provide further theoretical guidance for the improvement of Au-based catalysts.

Section snippets

Materials

The coconut AC was purchased from S. S. Activated Carbon Industry Science and Technology Co., Ltd. Chloroauric acid tetrahydrate (HAuCl4·4H2O; 47.8% Au content) was purchased from Tianjin Yingda Rare Chemical Reagents Factory. All of the solvents used in impregnation process were analytical-reagent grade; none of the solvents required further purification.

Catalyst preparation

AC was stirred in HCl solution (1 mol L−1) at 70 °C for 5 h and then filtered and washed to pH 7 with deionized water to remove Na, Fe, and

Catalytic performance of Au-based catalysts

Fig. 1 shows the catalytic performance of the Au-based catalysts for acetylene hydrochlorination. As shown in Fig. 1a, the solvent used in the preparation process strongly affects the catalytic activity. The conversion of acetylene with bare AC as catalyst rapidly decreases to approximately 2%. When water is used as the solvent, the prepared Au-water/AC catalyst exhibits an initial conversion of 35%, which decreases to 11% within 24 h. By comparison, the Au-aqua regia/AC catalyst shows a

Conclusions

Carbon-supported Au catalysts were synthesized for the direct synthesis of VCM, and the effects of the properties of several representative solvents used in the preparation on the catalytic behavior of the resultant catalysts were also studied in detail. The results of performance tests revealed that the catalytic performance of the resultant Au catalysts increased with decreasing polarity of the adopted dispersing solvents and that superior performance could be attained with the

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This project is supported by National Natural Science Foundation of China (Grant No. 21706167, 21776179), Fok Ying Tung Education Foundation (No. 161108), Program for Changjiang Scholars and Innovative Research Team in University (No. IRT_15R46), Yangtze River Scholar Research Project of Shihezi University (No. CJXZ201601), and the Start-Up Foundation for Young Scientists of Shihezi University (RCZX201507).

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