Ultra-low Ru-promoted CuCl₂ as highly active catalyst for the hydrochlorination of acetylene

Abstract

Here we report a novel catalyst consisting of 400 ppm Ru and 4.24 wt% Cu supported on carbon nanotubes for the hydrochlorination of acetylene. We observed a synergy between Ru and Cu and obtained a highly active catalyst. The TOF of Cu400Ru/MWCNTs was higher than that of the HgCl₂ catalysts.

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Vinyl chloride is a monomer for industrially important polymeric material polyvinyl chloride, and catalytic hydrochlorination of acetylene is one of the methods for the manufacture of vinyl chloride on an industrial scale.¹ Commercial processes use carbon-supported mercuric chloride (HgCl₂) as a catalyst, but this catalyst is highly toxic and short-lived due to the loss of mercuric chloride, which has encouraged the search for non-mercuric catalytic systems.² To develop a novel and green catalyst, significant efforts have been made in the past few decades.^3–11 Among a wide variety of available non-mercuric catalysts, AuCl₃ supported on carbon shows unbeatable hydrochlorination properties with extremely high activity and selectivity.^12–18 Although promising performances can be achieved, AuCl₃ catalysts suffer from active Au³⁺ species being reduced to Au⁰ under the reaction conditions and rapid loss of their activity.^19,20 The addition of a second element, such as La,²¹ Co,²² Bi,²² Cs²³ and Cu,²⁴ to the monometallic AuCl₃ will definitely improve the catalytic performance, particularly the stability. Despite these impressive results, most studies have observed that this type of Au-based bimetallic catalyst is not as stable as is needed for practical applications and deactivates with time-on-stream. Another alternative is to employ Ru-based catalysts for acetylene hydrochlorination.^25–28 However, the rising cost of noble metals is also the main hindrance to their large-scale practical applications. As reducing costs of catalysts is desirable for the industrial production of VCM, the development of a catalyst featuring low toxicity, high catalytic activity, easiness to obtain, low-loading and good thermostability is of great interest.

As a substitute, non-precious metal-based catalysts have been studied significantly for acetylene hydrochlorination. For instance, Taiyo Kaken Co Ltd²⁹ revealed that supporting SnCl₂ on AC after its pretreatment with NH₃ performed well with 98% conversion of C₂H₂ and 99% selectivity for VCM under a GHSV of 50 h⁻¹. Deng et al.³⁰ reported that the ternary metal catalyst of SnCl₂–BiCl₃–CuCl/AC had 97% conversion and 95% selectivity initially. Wei et al.³¹ synthesized a bicomponent Bi–Cu catalyst with 30% of the activity of HgCl₂ and demonstrated the catalyst in a 20 ton per year continuous fluidized bed reactor over 700 h. Despite tremendous efforts in this direction, all efficient catalysts heretofore are still based on precious metals, and it remains difficult for non-precious metal-based catalysts to achieve both high activity and stability (SnCl₂ and BiCl₃ are both highly volatile for their low boiling points), which match those of commercial HgCl₂. Thus, a robust catalytic system with high activity and low cost has not been found yet, largely narrowing the selection range of competitive candidates as HgCl₂ catalyst's replacements.

Among a wide variety of available non-noble catalysts, Cu has a low cost and good thermostability and has potential as an alternative to the toxic HgCl₂ catalyst. However, it is plagued by the low activity of Cu-based catalysts for the hydrochlorination of acetylene when compared with HgCl₂. Very recent advances in using nitrogen-doped carbon nanotube (N-CNT) materials, as the substrates of Cu-based catalysts, have shown their promising future in acetylene hydrochlorination.³² Here, we present a new family of very active and selective catalyst materials for the hydrochlorination of acetylene based on Cu–Ru/MWCNTs. A clear synergistic effect was observed between Cu and Ru components which form a very active sites, showing superior catalytic activity comparable to that of the existing non-precious metals catalysts, although not as high as that of the state-of-the-art Au catalyst. The findings provide clear evidence that, similar to precious metals, the well-designed precious metals-free counterparts also have great potential for highly efficient catalytic acetylene hydrochlorination.

The catalysts were prepared using co-impregnation of the metals in aqua regia solution using an incipient wetness technique with CuCl₂·2H₂O as the precursor for Cu and RuCl₃ as the precursor for Ru (the detailed procedure is described in the ESI†). The BET specific surface areas of all the MWCNTs supports are very similar to each other and range between 112 and 115 m² g⁻¹. It can be concluded that the impregnation of the support active components did not significantly alter the surface area or the pore volume of the catalyst material. To check for a distinct amount of Cu in the bulk region, ICP analysis was further conducted and the results are summarized in Table 1. Quantitative analysis revealed a Cu concentration of approximately 4.36 and 4.24 wt% for the Cu/MWCNTs and Cu400Ru/MWCNTs catalysts, respectively, which was a little lower than the target loading. Fig. 1 shows the XRD spectra of the Cu/MWCNTs and Cu400Ru/MWCNTs samples. The peaks at 2θ = 26.28° and 2θ = 43.79° correspond to the (002) plane of the graphite structure and the (100) plane of the disordered amorphous carbon. For the Cu/MWCNTs sample, diffraction lines were identified at 15.97°, 21.92° and 33.95°, which were assigned to the (100), (200) and (311) planes of the fcc structure of CuCl₂, respectively (JCPDS file, 01-071-2288). In addition, typical XRD of the CuCl sample showed 2θ values at 28.73°, 47.89° and 56.29°, which are indexed as (111), (220) and (310) planes, respectively (JCPDS file, 01-081-1841). Other phases such as Cu⁰ were not detected. In the Cu400Ru/MWCNTs sample, broad diffraction lines corresponding to CuCl₂ were identified, whereas diffraction lines of the CuCl phase became hardly visible due to peak broadening or most of Cu species presenting the CuCl₂ form. Typical transmission electron microscopy (TEM) images of Cu/MWCNTs and Cu400Ru/MWCNTs are shown in Fig. 2. As can be seen, after the deposition of CuCl₂ on MWCNTs, the CuCl₂ nanoparticles (NPs) were uniformly distributed over the entire surface (Fig. 2a). The average size of CuCl₂ NPs is about 3.12 nm (Fig. 2c). The TEM image of Cu400Ru/MWCNTs sample is shown in Fig. 2b. It shows that the CuCl₂ NPs are also uniformly dispersed on the MWCNTs support. The mean particle size of the CuCl₂ NPs calculated from the TEM images is 2.95 nm. Thus, when Ru was added to the Cu/MWCNTs, the mean particle size of CuCl₂ NPs decreased, suggesting that Ru is efficient in further dispersing the CuCl₂ NPs. From these characterization data, it is concluded that all the compounds are homogeneously and highly dispersed on the catalyst materials.

Table 1 Surface area related to Cu loading as measured by N₂-physisorption and atomic composition of Cu-based catalysts; derived from ICP data

Catalysts	BET surface area (m^2 g^−1)	Content of Cu (wt%)
Cu400Ru/MWCNTs	112.3	4.24
Cu/MWCNTs	112.7	4.36
Ru/MWCNTs	114.5	–

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Fig. 1 XRD patterns of the fresh Cu/MWCNTs, Ru/MWCNTs and Cu400Ru/MWCNTs.

Fig. 2 TEM images of the fresh Cu/MWCNTs (a) and Cu400Ru/MWCNTs (b), and corresponding particle size distributions of Cu/MWCNTs (c) and Cu400Ru/MWCNTs (d).

The catalytic performance of the prepared materials for acetylene hydrochlorination was evaluated in a fixed-bed glass reactor under conditions of 180 °C, C₂H₂ gas hourly space velocity (GHSV) = 180 h⁻¹ and a feed HCl/C₂H₂ volume ratio = 1.2 (the detailed procedure is described in the ESI†). First, we examined the hydrochlorination of acetylene in gas phase without using a catalyst. No evident activity was observed by GC during 24 h on stream. Cu nominal loading in all the catalysts was fixed at 6.0 wt% at various Ru loadings, namely, 100, 200, 400, and 600 ppm, and were denoted as Cu100Ru/MWCNTs, Cu200Ru/MWCNTs, Cu400Ru/MWCNTs, and Cu600Ru/MWCNTs, respectively. A complete list of the catalysts prepared, including bare MWCNTs support, Cu/MWCNTs, Ru/MWCNTs, Cu200Ru/MWCNTs, Cu400Ru/MWCNTs and Cu600Ru/MWCNTs, and their respective activity and selectivity data for the hydrochlorination of acetylene are summarized in Table 2. In addition, Fig. 3 shows C₂H₂ conversion as a function of time-on-stream of Cu/MWCNTs, Ru/MWCNTs, Cu400Ru/MWCNTs and Cu/MWCNTs + Ru/MWCNTs catalyst materials. Cu/MWCNTs + Ru/MWCNTs is a mixture of Cu/MWCNTs and Ru/MWCNTs with the ratio of Cu400Ru/MWCNTs.

Table 2 The conversion (X_A) and selectivity (S_VC) obtained for the different catalyst materials under investigation

Catalysts	Nominal composition of catalysts^a	XA^b (%)	SVC (%)
a The catalyst composition is represented by the mass percentage of the active species and the molar ratio of the components.b Reaction conditions: T = 180 °C, V(HCl)/V(C2H2) = 1.2, GHSV(C2H2) = 180 h^−1.
Cu100Ru/MWCNTs	100 ppm Ru, 6 wt% Cu	21.4	99.94
Cu200Ru/MWCNTs	200 ppm Ru, 6 wt% Cu	33.8	99.92
Cu400Ru/MWCNTs	400 ppm Ru, 6 wt% Cu	51.6	99.93
Cu600Ru/MWCNTs	600 ppm Ru, 6 wt% Cu	51.5	99.92
Cu/MWCNTs + Ru/MWCNTs	400 ppm Ru, 6 wt% Cu	11.5	99.89
Cu/MWCNTs	6 wt% Cu	7.9	99.86
Ru/MWCNTs	400 ppm Ru	6.1	99.91
MWCNTs	Bare MWCNTs	1.8	99.86

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Fig. 3 Conversion of acetylene to VCM in acetylene hydrochlorination over Cu/MWCNTs, Ru/MWCNTs, Cu400Ru/MWCNTs and Cu/MWCNTs + Ru/MWCNTs catalysts.

The bare MWCNTs support displays a very low activity (<2% C₂H₂ conversion) and is therefore regarded to be inactive for acetylene hydrochlorination reaction. Evidently, the undoped Cu/MWCNTs displayed low activity. For the fresh Cu/MWCNTs and Ru/MWCNTs catalysts, conversions of 7.9% and 6.1% were obtained after a 4 h reaction, respectively. Interestingly, when Cu/MWCNTs are doped with 400 ppm Ru, the activity was significantly enhanced, the conversion of C₂H₂ could reach ca. 51.6%, being 6.5 times that of the undoped Cu/MWCNTs, and the mixture sample shows activity (11.5%) similar to the sum of Cu/MWCNTs and Ru/MWCNTs but not Cu400Ru/MWCNTs, indicating an excellent synergistic effect of the two components that reached a highly active catalytic level. This observation is interesting, considering that such distinct changes in catalytic properties are induced by the addition of only 400 ppm of Ru.

In the following, Cu–Ru/MWCNTs catalysts containing different amounts of Ru were used to catalyze the reaction. Table 2 shows the dependence of the conversion of the C₂H₂ reactant on the amount of Ru species used in the Cu–Ru/MWCNTs catalysts. When the amount of Ru species increased from 100 to 600 ppm, the conversion increased at first and then reached a plateau. The maximum C₂H₂ conversion could be observed when the amount of Ru species was 400 ppm. Generally, increasing the amount of active species is favorable for the conversion of reactants. It is considered that the loading of Ru species does not correlate with the activity of the catalyst, meaning Ru is not the species mainly responsible for the acetylene hydrochlorination activity of the catalysts. In order to study the stability of the catalyst, we have performed a stability test for longer time durations. The result is shown in Fig. S1.† As shown in Fig. S1,† the conversion of C₂H₂ is reduced by only 5% after 65 h under the conditions of 180 °C and 30 h⁻¹ of gas hourly space velocity (GHSV, C₂H₂ based), indicating that the stability of Cu400Ru/MWCNTs is excellent.

The temperature-programmed reduction (TPR) technique was used to study the chemical reducibility of Cu species in the Cu-containing samples in hydrogen within the range of 100–700 °C. Fig. 4a displays the TPR profiles of Cu/MWCNTs, Cu–Ru/MWCNTs and Ru/MWCNTs. Notice that a similar total peak area is obtained in Cu/MWCNTs and Cu–Ru/MWCNTs, showing that they both have the same Cu^δ+ species populations (43%). From the figure, we observe the different onset reduction temperatures and the amounts of the reduction peaks for Cu/MWCNTs and Cu–Ru/MWCNTs. For the Cu/MWCNTs catalysts, only one reduction peak was observed and the onset reduction temperature is at 350 °C and reaches its maximum at 427 °C, which is attributed to Cu⁺. It is particularly noted that the reduction of Cu–Ru/MWCNTs occurs at a lower temperature (352 °C), before the rapid formation of large amounts of Cu phase at the expense of the Cu⁺ phase at a higher temperature centered at 406 °C. The newly formed H₂ consumption peak at 352 °C and the strong consumption peak attributed to Cu²⁺ are considerably lower than the standard consumption peaks attributed to Cu²⁺ and Cu⁺ (385 °C and 420 °C); thus, the incorporation of Ru³⁺ clearly promotes the reduction of active Cu species at a lower temperature, suggesting the strong interaction between Cu and Ru species. It should be noted that no H₂ consumption peak of Ru/MWCNTs can be observed, which may be due to the low Ru loading and the high distribution of Ru species on the surface of the support.

Fig. 4 (a) TPR spectra of fresh Cu/MWCNTs, Ru/MWCNTs and Cu400Ru/MWCNTs catalysts; (b) XPS spectra of the Cu 2p_3/2 in Cu/MWCNTs and Cu400Ru/MWCNTs catalysts.

Furthermore, XPS spectra were used to investigate the valence state and relative amount of Cu species in Cu/MWCNTS and Cu400Ru/MWCNTS. The distinction between the Cu⁺ and Cu⁰ species was characterized through the examination of the Auger kinetic energy (KE) (Fig. S2†). According to the data, the Cu/MWCNTS and Cu400Ru/MWCNTS had a peak at about 916.0 eV with no visible peak at around 918.0 eV, suggesting the presence of Cu⁺ but not metallic copper species.^33,34 Curve fitting is employed to determine the ratio of each Cu species. In fresh samples of Cu/MWCNTS and Cu400Ru/MWCNTS catalysts, the relative content of Cu²⁺ is 66.9% and 90.3%, respectively, suggesting the existence of the strong interaction between Cu and Ru species.

There are two possible mechanisms by which the addition of a small amount of Ru species can enhance catalysis. One possible mechanism is that it directly creates a highly active reaction site on the surface of CuCl₂ NPs. For example, the supported Au alloyed Pd single-atom catalysts exhibited excellent performance for the Ullmann reaction of aryl chlorides, and the turnover number (TON) increased exponentially with a decrease of the amount of Pd in the catalysts.³⁵ The authors argued that the Au-alloyed Pd single-atom was proposed as the active site on the basis of HRTEM, XRD, EXAFS, and DRIFTS characterization and catalytic results. The other possible mechanism is that the Ru atom activates the Cu sites by modulating the electronic structure. Toshima³⁶ proposed that Au atoms located at the vertex or corner of Pd NPs act as highly reactive sites for aerobic glucose oxidation due to electron transfer from Pd. Thus, the mechanism of the synergistic effect between Ru and Cu is very interesting but needs to be studied further in more detail.

Finally, to prove the outstanding performance of this catalyst system, Cu400Ru/MWCNTs catalyst was compared to a list of acetylene hydrochlorination catalysts published in literature (note that the experimental conditions are not necessarily identical), as well as a commercial HgCl₂ catalyst, and this comparison is shown in Fig. 5. The TOF based on the total Cu metal was calculated to be 1.81 min⁻¹. To the best of our knowledge, these values are considerably higher than those reported previously for heterogeneous Cu catalysts (max TOF = 1.2 min⁻¹, based on the total metal), demonstrating that the interplay of Ru doping is essential for the observed catalyst modification effects. Compared to the precious-metal catalysts, Cu–Ru/MWCNTs have lower TOF than Au (6.96 min⁻¹) and Ru (2.71 min⁻¹), but the value is significantly higher than that of the Hg catalysts (1.56 min⁻¹), Ag (0.16 min⁻¹), Zn (0.15 min⁻¹) and Ba (0.03 min⁻¹), indicating that the practical application of Cu400Ru/MWCNTs catalysts for acetylene hydrochlorination is a promising alternative route.

Fig. 5 The calculated TOF values for the different catalyst materials reported in literature and the Cu400Ru/MWCNTs catalyst material.

Conclusions

In summary, different Cu–Ru-containing catalyst materials were examined for the hydrochlorination of acetylene into VCM and it was found that the synergistic effect between Cu and Ru led to a high activity (TOF = 1.81 min⁻¹). In spite of the slower conversion when compared with the AuCl₃ catalysts, this catalyst is superior to other transition-metal complexes known to catalyze this reaction (Fig. 5), and it was less toxic and more environmentally friendly. Thus, Cu400Ru/MWCNTs show great promise as green, mercury-free industrial hydrochlorination catalysts.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra03094c

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