Nitrogen-modified activated carbon supported bimetallic gold–cesium(I) as highly active and stable catalyst for the hydrochlorination of acetylene

Abstract

In the challenging acetylene hydrochlorination to vinyl chloride over Au-based catalysts, Au–Cs^I catalysts are substantially more active and stable than their monometallic counterparts. Here we describe a novel nitrogen-modified activated carbon supported Au–Cs^I catalyst (1Au4Cs^I/NAC) that delivers stable performance for acetylene conversion reaching 90.1% and there was only 1.5% C₂H₂ conversion loss after 50 h under the reaction conditions of C₂H₂ hourly space velocity 1480 h⁻¹. After a careful characterization of all the catalysts, we concluded that the nitrogen atoms’ influence on the stability of the Au–Cs^I catalysts correlates with the strengthening of the adsorption of hydrogen chloride to the catalyst and consequently inhibits Au³⁺ reduction under the reaction conditions.

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Introduction

Acetylene hydrochlorination is an important coal-based technology for the industrial production of vinyl chloride.¹ Supported mercury chloride (HgCl₂) catalysts are currently the most convenient and the most practical catalysts for hydrochlorination of acetylene, but HgCl₂ is susceptible to reduction by acetylene and sublimation, which severely limits the production capacity and causes severe environmental problems.² Catalysts free from toxic mercury are thus desired.

Since Hutchings et al. carried out pioneering work to investigate the use of non-mercury catalysts for acetylene hydrochlorination,^3–5 a significant number of supported-metal chloride catalysts, including Bi³⁺,⁶ Cu²⁺,⁷ Pt²⁺,⁸ and Pd²⁺ chlorides,⁹ have been shown to display considerable activity towards catalyzing acetylene hydrochlorination, and the most widely used are based on supported AuCl₃ often in combination with other metals chlorides owing to its high intrinsic activity toward the formation of the desired vinyl chloride.^10–12 However, the deactivation rate of the AuCl₃ catalyst is still very high when the reaction used is under real commercial conditions with lifetimes in the range of a few hours.⁵ The practical application of Au catalysts for the hydrochlorination reaction is attractive but, at present, it is still desired that the stability should be improved.

Recently, nitrogen-doped carbon materials have attracted considerable attention as new promising materials.¹³ The presence of nitrogen atoms in the carbonaceous material tailors their surface properties accordingly, supporting a new idea to employ them as an active catalyst for mercury-free hydrochlorination.^14–17 In addition, recently Dai et al. reported that a AuCl₃/PPy–MWCNT catalyst showed the benefits of the nitrogen promotion of Au³⁺ catalysts.¹⁸ The enhanced catalytic performance was attributed to electron transfer from PPy to the Au³⁺ center, which increased the adsorption of hydrogen chloride. We have recently contributed to the field of Au-catalyzed acetylene hydrochlorination by discovering that the use of Cs^I as a promoter can facilitate more efficient Au³⁺ active species stabilization under much harsher conditions.¹⁹ These findings are useful for the design of highly active and stable acetylene hydrochlorination catalysts and have prompted us to consider whether the nitrogen-promoting effect can be applied to bimetallic catalysts, to give further improved performance in the acetylene hydrochlorination reaction. In this work the effect of modification of nitrogen to activated carbon supported Au–Cs^I catalysts has been investigated by synthesizing a 1Au4Cs^I/NAC catalyst and evaluating the material’s properties by several characterization techniques (XPS, STEM, TPD, TPR) comparing it to 1Au4Cs^I/AC catalysts as reference.

Experimental

Materials

Activated carbon (marked as AC, NORIT ROX 0.8, pellets of 0.8 mm diameter and 5 mm length); HAuCl₄·4H₂O (the content of Au assay 49.7%); urea (99 wt%); CsCl (99.9 wt%); H₂O₂ (10 wt%); glacial acetic acid (10 wt%); C₂H₂ (gas, 98%); HCl (gas, 99.999%).

Catalyst preparation

A commercially activated carbon NORIT ROX 0.8 was selected for the preparation of the support. The activated carbon was first pretreated with HNO₃ (65 wt%) at room temperature for 1 h. Subsequently, it was filtered, washed and then dried at 110 °C for 12 h (sample AC).

N-doped AC was prepared based on the process described below.²⁰ A mixture of AC (5 g), glacial acetic acid (3 mL) and distilled water (50 mL) was stirred for 30 min to obtain a carbon slurry. Urea (3 g), H₂O₂ (10 mL) and deionized water (30 mL) were added, and the solution was stirred for 24 h at room temperature in the dark. Finally, the solution was filtered and calcined at 500 °C under a nitrogen atmosphere for 1 h. The obtained sample was labeled as NAC.

Supported bimetallic Au–Cs^I catalysts were prepared using an incipient wetness impregnation technique. A HAuCl₄·4H₂O and CsCl solution in aqua regia was added dropwise to the NAC or AC support with agitated stirring. After the solution was homogeneously mixed with the support, the system was aged at 40 °C for 4 h, followed by drying at 110 °C for 12 h before use. The obtained catalysts were labeled as 1Au4Cs^I/NAC and 1Au4Cs^I/AC, respectively. Au loading in all the catalysts was fixed at 1.0 wt%.

Catalyst characterization

The sizes of the particles on the samples were also observed by a transmission electron microscope (TEM, Tecnai G2 F30 S-Twin, 300 kV). The solid samples were finely ground. The resultant fine powders were dispersed ultrasonically in ethanol and were then loaded on a copper grid (Beijing Zhongjingkeyi Technology Co., Ltd.).

The elemental surface composition of the catalysts was controlled by XPS acquired with a Kratos AXIS Ultra DLD spectrometer. XPS analysis was performed with a monochromatized aluminum X-ray source with a pass energy of the electron analyzer of 40 eV. The pressure in the sample analysis chamber was lower than 6 × 10⁻⁹ Torr during data acquisition. Binding energies are referenced to the C1s line at 284.8 eV.

Temperature-programmed desorption (TPD) experiments were performed in a tubular quartz reactor. The samples (about 75 mg) were first treated in situ at 180 °C for 0.5 h using pure HCl and then cooled to room temperature in the same atmosphere. The sample was swept with pure Ar at a flow rate of 30 mL min⁻¹ for 1 h to remove physisorbed and/or weakly bound species. TPD was performed by heating the sample from room temperature to 500 °C at a ramp rate of 10 °C min⁻¹ in pure Ar, and the TPD spectra were recorded by a quadrupole mass spectrometer (QMS 200 Omnistar).

Temperature-programmed reduction (TPR) experiments were carried out on a micro-flow reactor fed with a flow of hydrogen (5% in Ar) at a rate of 40 mL min⁻¹. The weight of the tested samples was (about) 75 mg. The temperature was increased from 30 to 850 °C at a rate of 10 °C min⁻¹. The hydrogen consumption was measured using a thermal conductivity detector (TCD).

Catalytic test

Catalysts were tested for acetylene hydrochlorination in a fixed-bed glass microreactor (i.d. 10 mm). Acetylene (10 mL min⁻¹, 1 bar) and hydrogen chloride (12 mL min⁻¹, 1 bar) were fed through a mixing vessel via calibrated mass flow controllers to a heated glass reactor containing catalyst (200 mg), with a total GHSV (C₂H₂) of 1480 h⁻¹. A reaction temperature of 180 °C was chosen, blank tests using an empty reactor filled with quartz wool did not reveal any catalytic activity, and quartz sand was used to extend the bed length, above and below the catalyst itself, separated by quartz wool. The gas phase products were passed through an absorption bottle containing NaOH solution to remove excess HCl first and then analysed on-line by GC equipped with a flame ionisation detector (FID). Chromatographic separation and identification of the products was carried out using a Porapak N packed column.

Results and discussion

The XPS results shown in Fig. 1a confirm the elemental compositions of 1Au4Cs^I/AC and 1Au4Cs^I/NAC samples. The peaks at about 86.1, 200.6, 284.8, 399.5, 531.7 and 725.6 eV can be assigned to the binding energy of Au4f, Cl2p, C1s, N1s, O1s and Cs3d, respectively. The XPS spectrum of AC indicates that as the annealing temperature increases to 500 °C on doping the carbon with nitrogen atoms, the nitrogen signal can be clearly detected. Fig. 1b demonstrates the high-resolution XPS spectra based on the deconvoluted N1s spectra. The peaks at 398.6, 400.4, 402.7 eV correspond to pyridinic N, pyrrolic N and quaternary N, respectively.¹⁵ XPS analysis (Fig. 1b) reveals that three types of nitrogen species coexist in the 1Au4Cs^I/NAC catalyst. Recent observations suggest that carbon-based catalyst support materials can be systematically doped with nitrogen to create strong, beneficial catalyst-support interactions by increased support/catalyst chemical binding (or anchoring) which substantially enhances catalyst activity and stability.²¹

Fig. 1 XPS spectra of (a) fresh 1Au4Cs^I/AC and 1Au4Cs^I/NAC samples normalized at 284.8 eV of C1s, (b) N1s spectrum for the fresh 1Au4Cs^I/NAC catalyst.

The gold content by mass of the catalyst as determined by ICP-MS was 0.94 ± 0.07 wt% for 1Au4Cs^I/AC and 0.95 ± 0.05 wt% for 1Au4Cs^I/NAC. Fig. 2 shows two typical TEM micrographs. The small bright dots represent the metal particles; particles below 5 nm are dominant in 1Au4Cs^I/AC, though some are found up to 6 nm in diameter (Fig. 2a). In contrast, particles below 3 nm are dominant in 1Au4Cs^I/NAC (Fig. 2b). 1Au4Cs^I/AC in particular showed a large particle size distribution. Most of the particles in 1Au4Cs^I/AC had particle sizes below 4 nm, with a small fraction between 4 and 6 nm. For 1Au4Cs^I/NAC, hardly any particles larger than 4 nm were observed. The average particle size for 1Au4Cs^I/AC was 3.7 nm and for 1Au4Cs^I/NAC 2.3 nm. This indicates that adding nitrogen atoms had caused a decrease in Au NP particle size.

Fig. 2 Representative STEM figures of fresh 1Au4Cs^I/AC (a) and 1Au4Cs^I/NAC (b) and corresponding particle size distributions.

The catalytic performance of Au-based catalysts for acetylene hydrochlorination is shown in Fig. 3. To accelerate the deactivation process of Au-based catalysts, higher GHSV (1480 h⁻¹) was used than that reported in the literature. NAC catalysts display very poor catalytic activity (5% conversion) for the acetylene hydrochlorination reaction (not shown), while no conversion was detected in AC. In fact, experimental studies and theoretical simulations revealed that the carbon atoms bonded with pyrrolic nitrogen atoms as active sites and can deliver limited activity as has been reported by Bao’s group before.¹⁵

Fig. 3 Conversion of acetylene to VCM in acetylene hydrochlorination over (a) 1Au4Cs^I/NAC and (b) 1Au4Cs^I/AC catalysts, respectively.

The catalyst evaluation data in Fig. 3b shows that 1Au4Cs^I/AC had good performance with the acetylene conversion of 1Au4Cs^I/AC catalyst decreasing from 89.2% down to 80.1% after running for 50 h, indicating that 1Au4Cs^I/AC is slightly deactivated under these reaction conditions. As a comparison, the 1Au4Cs^I/NAC catalyst exhibits a similar initial performance as 1Au4Cs^I/AC, and its C₂H₂ conversion reaches 90.1% at 4 h (Fig. 3a). At the same time, 1Au4Cs^I/NAC had stable conversion without significant decline in activity over the reaction time (89.6% at 50 h). Thus, the results suggest that the nitrogen-doped catalysts (1Au4Cs^I/NAC) exhibited improved catalytic stability as well as a slightly increased catalytic activity. This indicates the effectiveness of nitrogen doping in tuning the reactivity. Dai and coworkers¹⁸ have indicated an improved activity and stability upon the addition of nitrogen atoms to Au-based acetylene hydrochlorination catalysts (AuCl₃/PPy–MWCNT). On the one hand, it has been reported that the incorporation of nitrogen atoms increased the electron density of the Au³⁺ center via the transfer of lone pair electrons on the nitrogen atom to the Au³⁺ center. On the other hand, the Au³⁺ species is the electron donor in the adsorption process of hydrogen chloride. Thus, it is most likely that this promoting effect of 1Au4Cs^I/NAC bimetallic catalysts during acetylene hydrochlorination is related to the increased electron-donating capability of Au³⁺ for hydrogen chloride adsorption, which is due to the increased electron density of Au³⁺ as a result of electron transfer from nitrogen atoms. This will be discussed in the HCl-TPD analysis. It should be highlighted that for all the tests carried out and reported in the current study, the selectivity for vinyl chloride was virtually 100% with trace amounts (<0.1%) of 1,2-dichlorethane and chlorinated oligomers only produced throughout the entire run period according to the TCD spectrum.

Fig. 4 shows the TPR profiles for fresh and used Au-based catalysts. For all the catalysts, a distinguishable hydrogen consumption peak in the range of 400–800 °C can be observed, which can be attributed to the reduction of the surface groups of the activated carbon at the carbon surface, which releases CO and CO₂ with concomitant reduction of oxygenated groups by H₂ in the TPR stream. More specifically, the profile can be fitted with two contributions. The reduction bands in the range of 400–650 °C can be assigned to the decarboxylation reactions of carboxylic acid and carboxylic anhydride groups,²² while above 650 °C, these are likely due to the reduction of lactones and phenols.²³ As mentioned before, this phenomenon on the carbon matrix should be considered a consequence of using HNO₃ in the washing step, which induced modifications on the carbon surface inducing oxygenated functional groups, as well as changes in the textural properties of the carbon support. In addition, the analysis of the 1Au4Cs^I/AC sample led to assigning the reduction band around 322 °C to Au³⁺ to Au⁰ (Fig. 4a). Besides, a straightforward decrease of temperature in the reduction band of Au³⁺ to 309 °C was observed (Fig. 4b) for 1Au4Cs^I/NAC, this shift indicates that strong interactions between Au species and nitrogen atoms exists in 1Au4Cs^I/NAC catalysts. Through comparing the TCD signals with a standard, the fractions of different Au species in these fresh catalysts can be estimated. This allows an estimation of ca. 46.8, and 38.8% for the 1Au4Cs^I/AC and 1Au4Cs^I/NAC catalysts, respectively. For used 1Au4Cs^I/AC, the reduction peak of Au³⁺ to Au⁰ is evidently decreased compared with the fresh catalyst, which shows a degree of Au³⁺ reduction to Au⁰ during acetylene hydrochlorination. In contrast, the reduction peak of Au³⁺ to Au⁰ is barely decreased for used 1Au4Cs^I/NAC compared with the fresh catalyst. Also comparing the TCD signals with a standard, allows an estimation of ca. 27.8 and 36.2% for the used 1Au4Cs^I/AC and 1Au4Cs^I/NAC catalysts, respectively. This result showed that the presence of nitrogen atoms in the 1Au4Cs^I/NAC catalyst partly inhibited the reduction of Au³⁺, which can further stabilize the catalytic active Au³⁺ species in the reaction process of AuCl₃ catalysts. This result is consistent with the excellent catalytic activity and stability of the 1Au4Cs^I/NAC catalyst. It is worth noting that although the results in this study revealed that the 1Au4Cs^I/AC presents a higher amount of Au³⁺ compared with the 1Au4Cs^I/NAC, the activity of 1Au4Cs^I/AC is no more than 1Au4Cs^I/NAC. In fact, according to Hutchings et al., although Au³⁺ is clearly needed for the reaction, exceeding a given threshold will not further increase the activity.²⁴ By using TPR determinations, the optimal Au³⁺ amount would be about ca. 30%, with excess amounts not affecting the reactivity of the catalyst, which is consistent with the catalytic performance of 1Au4Cs^I/AC and 1Au4Cs^I/NAC catalysts (see Fig. 3).

Fig. 4 TPR intensities for the reduction of Au³⁺ species for fresh and used 1Au4Cs^I/AC (a) and 1Au4Cs^I/NAC (b) catalysts.

Previous literature studies ascribed the activity of the Au-based catalyst to the presence of Au³⁺ species postulating them as active sites. In order to obtain a correlation between the activity of VCM production and the amount of Au³⁺ clusters made on the catalyst surface and the valence state changes of the catalyst structure before and after the reaction, samples tested for the acetylene hydrochlorination at different compositions were then carefully analysed by XPS. It should be noted that where more than one Au species was evident, curve fitting was employed to determine the ratio of each species (Fig. 5). The XPS spectra of the Au4f level for all samples were deconvoluted into three pairs of peaks for Au⁰, Au³⁺ and Au⁰-s species, respectively.^24,25 Table 1 lists the binding energy and the relative content of each Au species in the fresh and used catalysts. The full-width-half-maximum (FWHM) values of Au peaks are also summarized in Table 1. It is worth mentioning that the FWHM of Au³⁺ peaks obtained for both fresh and used bimetallic catalysts are very close. Due to the reduction property of carbon towards Au³⁺, there is a large number of Au⁰ in the fresh Au-based catalysts (Table 1).⁴ It should also be stressed that some small metallic gold clusters (Au⁰-s) are also formed in the fresh 1Au4Cs^I/AC and 1Au4Cs^I/NAC catalysts, which coincided well with the previous publications.^24,25 However, the Au⁰-s species are inactive and not involved in the reaction, since the reduced catalysts also contain these Au⁰-s clusters and they displayed negligible catalytic activity (<5% conversion) for acetylene hydrochlorination under the same conditions (not shown), illustrating that the presence of Au³⁺ was indispensable for high catalytic activity and the Au⁰-s species are inactive and not involved in the reaction. It has also been confirmed by other researchers^24,25 that reduced Au-based catalysts containing only Au⁰-s and Au⁰ clusters cannot activate acetylene directly. In fresh 1Au4Cs^I/AC and 1Au4Cs^I/NAC samples, the relative content of Au³⁺ is 32.6% and 31.4%, respectively (Table 1). A higher content of Au³⁺ in 1Au4Cs^I/AC can be observed. Under the reaction conditions, Au³⁺ is reduced into Au⁰, contributing to the deactivation of the Au catalyst. In the case of 1Au4Cs^I/AC catalysts after reaction, only 20.5% Au³⁺ species are present (Table 1), demonstrating that a reduction and/or agglomeration of Au³⁺ species occurs in the reaction process. In contrast, in the used 1Au4Cs^I/NAC catalyst, there are still large amounts of Au³⁺ (30.6%), indicating that the incorporation of the nitrogen atoms helps to keep the active Au³⁺ species stable at a high temperature under a reducing atmosphere. The fact that the amounts of Au³⁺ obtained by XPS are lower than those obtained by H₂-TPR can be ascribed to the fact that XPS is known to cause chemical and structural changes within several investigated molecules. For example, Karadas published an overview of the HAuCl₄ photoreduction process from Au³⁺ to Au⁰ when exposed to 1253.6 eV Mg Kα (nonmonochromatized) radiation.²⁶ Together with further contributions from Ozkaraoglu et al., they showed that the disappearance of Au³⁺ peaks in the XPS spectra occurs via first-order kinetics but at differing rates.^27,28 In view of this, the fact that Au³⁺ content obtained by XPS is always lower than that obtained by TPR is reasonable. In combination with the catalytic performances of these catalysts (Fig. 3), it is reasonable to conclude that a high content of Au³⁺ leads to the best catalytic performance, as displayed by the 1Au4Cs^I/NAC.

Fig. 5 XPS spectrum and simulation for the samples: (a) fresh 1Au4Cs^I/AC catalyst, (b) used 1Au4Cs^I/AC catalyst, (c) fresh 1Au4Cs^I/NAC catalyst, (d) used 1Au4Cs^I/NAC catalyst.

Table 1 Quantification and identification from XPS of Au species over Au-based catalysts

Catalyst	Au species (%)			(Binding energy) ± (FWHM) (eV)
	Au^3+	Au^0	Au^0-s	Au^3+	Au^0	Au^0-s
1Au4Cs^I/AC fresh	32.6	59.2	8.2	(86.7) ± (1.4)	(84.3) ± (1.3)	(85.4) ± (1.7)
1Au4Cs^I/AC used	20.5	66.5	13.0	(86.7) ± (1.5)	(84.3) ± (1.4)	(85.5) ± (1.4)
1Au4Cs^I/NAC fresh	31.4	41.2	27.4	(87.0) ± (1.4)	(84.3) ± (1.2)	(85.1) ± (1.3)
1Au4Cs^I/NAC used	30.6	55.1	14.3	(87.3) ± (1.5)	(84.8) ± (1.1)	(85.8) ± (1.2)

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HCl-TPD is also used to characterize the active sites for the hydrogen chloride adsorption. As shown in Fig. 6, the 1Au4Cs^I/AC and 1Au4Cs^I/NAC catalysts presented a band between 50 and 400 °C, which was characteristic of hydrogen chloride desorption. However, the areas of the HCl-TPD peaks for the 1Au4Cs^I/NAC catalyst were much larger than for the 1Au4Cs^I/AC catalyst. This suggested an increase in chemisorbed hydrogen chloride on 1Au4Cs^I/NAC compared with 1Au4Cs^I/AC. The observed adsorption was caused by the interaction between nitrogen and Au³⁺, which changed the nature of the catalyst itself. The favorable adsorption of hydrogen chloride on 1Au4Cs^I/NAC can be explained by the transfer of nitrogen π electrons to the empty orbital of gold, which enhanced the electron-donating ability of gold and allowed the catalyst to combine with more hydrogen chloride.¹⁸ It is reported in the literature that when hydrogen chloride and acetylene coadsorbed on AuCl₃, the acetylene was co-catalyzed by hydrogen chloride and the AuCl₃ to produce vinyl chloride. In contrast, if the hydrogen chloride in the gas phase could not adsorb on the Au sites, the intermediate chlorovinyl was difficult to desorb from the AuCl₃ catalyst which resulted in the side reaction and the rapid deactivation of the AuCl₃ due to the loss of Cl atoms and consequently it loses its activity.²⁹ Based on the above understanding, the adsorption of hydrogen chloride is very important for the stability of AuCl₃ catalyst, for one thing, it is beneficial for improving the catalytic activity, for another, hydrogen chloride can cause Au to remain in its oxidative state and inhibit the active Au³⁺ species reduced by C₂H₂. Thus, it is clear that doping of carbon materials with nitrogen atoms can enhance the adsorption of hydrogen chloride and activate acetylene, and hence the hydrochlorination of acetylene.

Fig. 6 TPD profiles of hydrogen chloride on different samples: (a) 1Au4Cs^I/AC and (b) 1Au4Cs^I/NAC.

Conclusions

In summary, we have successfully demonstrated that doping with nitrogen can efficiently further enhance the catalytic performance of Au–Cs^I bimetallic catalysts. The doped 1Au4Cs^I/NAC catalysts with a total nitrogen content of 2.9 wt% are quite stable and more active for the hydrochlorination of acetylene. The enhanced catalytic performance may due to nitrogen atoms strongly interacting with Au species and strengthening the ability of the resultant catalysts to adsorb hydrogen chloride and inhibit the reduction of Au³⁺ to Au⁰ during acetylene hydrochlorination and hence improve the catalytic stability. The excellent catalytic performance of the 1Au4Cs^I/NAC catalyst demonstrated its potential as an alternative to mercury chloride catalysts for acetylene hydrochlorination.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (NSFC-21476207, 20976164, and 21303163) and National Basic Research Program of China (973 Program) (no. 2011CB710800), Qianjiang Talent Project in Zhejiang Province (QJD1302011) and Scientific Research Fund of Zhejiang Provincial Education Department (Y201328681).

Notes and references

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