Design strategies for the development of a Pd-based acetylene hydrochlorination catalyst: improvement of catalyst stability by nitrogen-containing ligands

Acetylene hydrochlorination is an attractive chemical reaction for the manufacture of polyvinyl chloride (PVC), and the development efforts are focused on the search for non-mercury catalyst systems. Supported Pd-based catalysts have relatively high activity in the catalytic hydrochlorination of acetylene but are still deactivated rather quickly. Herein, we demonstrated that the atomically dispersed (NH4)2PdCl4 complex, distributed on activated carbon, enabled the highly active and stable production of the vinyl chloride monomer (VCM) through acetylene hydrochlorination under low temperature conditions. We found that the presence of nitrogen-containing ligands in the structure of the active center could remarkably improve the stability of the Pd-based catalysts when compared with the case of the conventional PdCl2 catalyst. Further analyses via X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR) show that the variations in the Pd dispersion, chemical state and reduction property are caused by the nitrogen-containing ligands. Temperature-programmed desorption (TPD) characterizations illustrated that the N-containing ligands over the (NH4)2PdCl4/AC catalyst might enhance the adsorption of HCl. These findings suggest that in addition to strategies that target the doping modification of support materials, optimization of the structure of the active center complexes provides a new path for the design of highly active and stable Pd-based catalysts.


Introduction
Polyvinyl chloride (PVC) is commonly used in various manufacturing processes due to its advanced performance in mechanical enhancement, chemical inertness and stability. PVC is produced by the polymerization of the vinyl chloride monomer (VCM), synthesized either via the oxychlorination reaction of ethylene or via hydrochlorination of acetylene, and the latter approach usually employs supported mercuric chloride as a catalyst. [1][2][3] However, mercuric chloride vapor may release into the environment easily under the reaction conditions and poison human beings and the environment. 4,5 The Minamata Convention on Mercury was approved by delegates from over 140 countries in 2013 for limiting the emission and use of hazardous mercury, and now, the treaty has entered into force. Considering the current severe mercury limitations, it is urgent to develop a new mercury-free catalyst for this process.
As the selection of non-mercury catalysts poses a signicant challenge, a number of catalytic formulations have been proposed including noble metal chlorides,  non-noble metal chlorides [30][31][32][33][34][35] and even non-metallic materials. [36][37][38][39][40][41][42][43][44] Among them, Pd catalysts have been considered to be very effective for the hydrochlorination of acetylene. For example, Hutchings et al. explored the catalytic performance of carbon supported with a series of metal chlorides and found that the conversion of acetylene to VCM decreased in the following order: Pd(II) > Hg(II) > Cu(II) $ Cu(I) > Ag(I) > Cd(II) > Zn(II). 45 Although the Pd-based catalysts exhibited excellent hydrochlorination activity, similar to most other precious metal catalysts, they could not maintain a high level of stability. It appears that the inherent volatilization loss of the Pd active species restrains the application of Pdbased catalysts for the hydrochlorination of acetylene. Therefore, the stabilization of volatile Pd active species under the reaction conditions has become an urgent technical issue to make acetylene hydrochlorination applicable.
To date, the main strategies used for the stabilization of Pd species are still focused on the doping modication of support materials to improve the stability of the metal catalysts by enhancing the interactions of the Pd species with the supports. For example, Wang et al. showed that upon adjusting the surface acidity of the support, an NH 4 F-modied Pd/HY catalyst delivered enhanced stability; 46 Bao et al. showed that upon doping of the carbon structure with certain nitrogen species, the stability of the PdCl 2 /AC-N catalyst was signicantly improved. 47 These experimental results suggest that one of the effective solutions to overcome the volatilization issue of Pd species is to develop a suitable support that can exhibit enhanced interaction with the Pd active centers. However, only few studies have been reported on the optimization of the structure of the active center to enhance the catalytic performance of Pd-based catalysts. Recently, it has been found that the coordination of an ionic liquid to the Pd active center can stabilize the Pd species against volatilization and then improve the catalytic stability. 48 In this study, the well-dened (NH 4 ) 2 -PdCl 4 complex was prepared, and an attempt was made to explore the catalytic performance of the carbon-supported (NH 4 ) 2 PdCl 4 complex catalysts. It was found that the (NH 4 ) 2 -PdCl 4 active center could signicantly stabilize the Pd species. Furthermore, the stabilization mechanism of Pd species in the (NH 4 ) 2 PdCl 4 /AC catalyst was proposed on the basis of TEM, XPS, ICP, H 2 -TPR and TPD characterization. To the best of our knowledge, this is the rst study that reports the use of nitrogen-containing Pd complexes in the preparation of supported Pd-based catalysts for acetylene hydrochlorination.

Catalyst preparation
The (NH 4 ) 2 PdCl 4 complex was synthesized according to the literature. 49,50 The representative procedures were conducted as follow: 35.3 mg H 2 PdCl 4 (Sigma-Aldrich) and 23 mg NH 4 Cl (Sigma-Aldrich) were dissolved in 8 ml water at 80 C via ultrasonication for 60 min to obtain yellow-brown (NH 4 ) 2 PdCl 4 crystals, and then, the catalyst preparation was conducted using an incipient wetness impregnation technique; aer this, 3 g of active carbon was added to the abovementioned mixture under agitation. Aer drying the mixture for 12 h at 110 C in an oven under vacuum, the synthesized catalyst was obtained, named (NH 4 ) 2 PdCl 4 /AC. A carbon-supported PdCl 2 catalyst (PdCl 2 /AC) was synthesized as a reference catalyst following the abovementioned methods except for the addition of the reagent NH 4 Cl. Unless otherwise specied, the loading amount of Pd in all Pd-based catalysts was 0.5 wt%.

Catalyst characterization
XPS spectra were obtained by the Kratos AXIS Ultra DLD apparatus, with a monochromatized aluminum X-ray source, and the passing energy was 40 eV. The C 1s line (284.8 eV) was adopted as the corrected benchmark for all the measured spectra. The specic surface areas were measured using N 2 adsorptiondesorption at 77 K via the Micromeritics ASAP 2000 instrument. TEM was used to characterize the catalyst morphology and microstructures via the Tecnai G2 F30 S-Twin electron microscope. The TPD was conducted in a tubular quartz reactor.
Herein, 75 mg of each catalyst sample was initially treated with pure C 2 H 2 or HCl at 180 C for 30 min; aer adsorption and sweeping with pure Ar for 60 min at the gas ow rate of 30 ml min À1 , a temperature-programmed route was carried out from 25 C to 550 C at the heating rate of 10 C min À1 .

Catalytic test
Activity tests for the hydrochlorination of acetylene were conducted in a heterogeneous xed-bed reactor. Prior to the reaction, nitrogen (N 2 ) was passed through the reactor for 30 min to remove any water and air remaining. Then, C 2 H 2 (5 ml min À1 , 1 bar) and HCl (6 ml min À1 , 1 bar) were added to the clean and dry reactor using a mass ow controller. The output gas products were passed through a vessel with the NaOH solution to remove excess HCl. The composition of the output was analyzed by gas chromatography (GC-9790A).

Catalytic performance of Pd-based catalysts
The catalytic performances of (NH 4 ) 2 PdCl 4 /AC and PdCl 2 /AC catalysts are shown in Fig. 1 at the reaction temperature of 100 C and C 2 H 2 GHSV of 100 h À1 . We could observe that the (NH 4 ) 2 PdCl 4 /AC catalyst showed superior catalytic performance than that of the PdCl 2 /AC catalyst, resulting in a 99.7% acetylene conversion (Fig. 1a) and 99.5% VCM selectivity (Fig. 1b). Compared with PdCl 2 /AC, the (NH 4 ) 2 PdCl 4 /AC catalyst also shows a remarkable robust catalytic behavior under these conditions. Note that the acetylene activity of the catalyst (NH 4 ) 2 PdCl 4 at this low temperature (100 C) was equivalent to that of the typical Pd-based catalyst reported at high temperatures (typically exceeding 140 C, as listed in Table S1 †). The effect of space velocity on the hydrochlorination of acetylene was further investigated. As shown in Fig. 1c, the acetylene conversion remained at 85% and 57% as the space velocity of C 2 H 2 was increased from 100 h À1 to 200 h À1 and then to 600 h À1 at 100 C, respectively. We have demonstrated that the designed (NH 4 ) 2 PdCl 4 /AC catalyst is suitable for the hydrochlorination of acetylene in a wide range of space velocities.
To further clarify the structural evolution of the active phase for PdCl 2 /AC and (NH 4 ) 2 PdCl 4 /AC, the kinetic behaviors were discussed by calculating the approximate activation energy (E a ) (Fig. 1d). Ea was calculated based on the Arrhenius plots (ln(R) versus 1/T), and all the kinetic data were obtained and calculated at the level of <15% yield of vinyl chloride, which excluded the effect of internal and external diffusion. Contrary to the E a calculated for the PdCl 2 /AC catalyst, the E a of (NH 4 ) 2 PdCl 4 /AC was markedly decreased from 40 to 25 kJ mol À1 . The improvement in the catalytic performance may be assigned to the modulation of the intrinsic structure of the catalysts as well as the synergetic effect of Pd and the nitrogen-containing ligands on the catalytic performance.

Catalytic characterization
As shown, (NH 4 ) 2 PdCl 4 /AC illustrated excellent catalytic performance for the hydrochlorination of acetylene. To elucidate the inuence of nitrogen-containing ligands on the catalyst microstructure, valence and adsorption properties, a series of structural characterizations were employed to analyze the Pdbased catalysts. XRD analysis was employed to probe the characteristics of the support and the Pd species. No discernible Pd 0 characteristic diffraction peaks were observed in the XRD patterns of the fresh (NH 4 ) 2 PdCl 4 /AC and PdCl 2 /AC catalysts; this suggested that most of the Pd species were present in small nanoclusters and/or non-crystalline isolated atoms (Fig. 2). Fig. 3 shows an HAADF-STEM image of the (NH 4 ) 2 PdCl 4 /AC and PdCl 2 /AC catalysts. As observed, the HAADF-STEM analysis clearly demonstrated the existence of isolated Pd atoms with high number density in (NH 4 ) 2 PdCl 4 /AC (Fig. 3a). They are observed as bright spots evenly distributed on the surface of carbon. The light dots cannot be attributed to nitrogen species as nitrogen species cannot be distinguished from carbon in the current analysis mode. Therefore, the light dots must be assigned to Pd species. Furthermore, since the light gray dot has the typical size of 2-3Å, these dots should be mainly assigned to single atoms/cations. For PdCl 2 /AC, it can be observed that except for the presence of isolated Pd atoms as big bright white spots, the image also illustrates the presence of Pd clusters with a few atoms (Fig. 3b). This observation provides a good reason to believe that the single isolated Pd atoms can be the active sites for the reaction, providing the observed promotion of the catalytic activity. More importantly, except for the amorphous diffraction peaks of carbon, no discernible diffraction peak was detected in the XRD pattern of the used (NH 4 ) 2 PdCl 4 /AC and PdCl 2 /AC catalysts (Fig. 2); this indicated that the used Pd-based catalysts did not show the aggregation of particles.
Further insights into the material structure were obtained by X-ray photoelectron spectroscopy, and the deconvolution results are shown in Table 1 and Fig. 4. It is apparent that the catalyst prepared with the (NH 4 ) 2 PdCl 4 precursor presents a relatively high amount of N species (3.12%), which is absent in the PdCl 2 /AC sample; this indicates that nitrogen-containing ligands can remain stable during the preparation process. More importantly, the proportion of surface N/C has no dramatic changes before and aer the reaction (Table 1 and Fig. 4b); this indicates that the (NH 4 ) 2 PdCl 4 phase does not obviously decompose during the reaction. The structure of the (NH 4 ) 2 PdCl 4 phase can remain relatively stable on the AC support. In addition, the surface content of Pd is 0.32% and 0.31% for the fresh (NH 4 ) 2 PdCl 4 /AC and PdCl 2 /AC catalysts according to the XPS analysis, respectively, suggesting that the Pd species have been well dispersed on the support. Aer the reaction, the content of the Pd species reduces obviously to only 0.18% for PdCl 2 /AC; this may be explained by the leaching of Pd    Fig. 4a displays the high-resolution spectra for the Pd 3d regions of Pd-based catalysts. The binding energies of the Pd 3d 5/2 signals for the fresh (NH 4 ) 2 PdCl 4 /AC catalyst at 337.4 eV and 335.1 eV correspond to the Pd 2+ and Pd 0 species (Fig. 4a). 53,54 It is well-accepted that Pd in the cationic form works as an active species for acetylene hydrochlorination. However, the Pd 3d 5/2 signals of the (NH 4 ) 2 PdCl 4 /AC catalyst, corresponding to Pd 2+ , which represent a negative shi of 0.3 eV as compared to that of the PdCl 2 /AC catalyst (337.7 eV), demonstrate that the electronic structure of Pd changes with the incorporation of nitrogen-containing ligands. Table 3 lists the relative ratio of Pd 2+ /Pd 0 for the fresh and used Pd-based catalysts. The results indicate that the Pd 2+ /Pd 0 ratio in the used catalysts observably decreases when compared with that for the fresh catalysts; this indicates that the active Pd 2+ species has been reduced under the reaction conditions. However, the (NH 4 ) 2 PdCl 4 /AC catalyst experiences a slower reduction during the reaction for the Pd 2+ species when compared with the PdCl 2 / AC catalyst. This is likely one of the reasons for the rapid deactivation observed for PdCl 2 /AC as compared to that of (NH 4 ) 2 PdCl 4 /AC. Thus, nitrogen-containing ligand modalities that can enhance the stability of the Pd 2+ species are very important for the practical applications of these catalysts. H 2 -TPR was employed to analyse the reduction ability of the active cationic Pd species. For both PdCl 2 /AC and (NH 4 ) 2 PdCl 4 / AC, a discernible characteristic reduction band in the range of 500-800 C can be observed (Fig. 5). This band was attributed to the reduction of the surface oxygenated groups on the AC support. 55 Apart from this, the clear bands at around 160-250 C were due to the reduction of the cationic Pd species. The TPR proles were analyzed by peak-differentiation-imitating analysis (Fig. S1 †). 56 Through calibration with a CuO standard, the Pd 2+ content for the fresh catalysts can be estimated (Table S2 †); the estimated Pd 2+ contents for the fresh PdCl 2 /AC and (NH 4 ) 2 PdCl 4 /AC catalysts are ca. 76.3% and 80.2%, respectively, which are consistent with the XPS analysis results. A more pronounced change occurred in the reduction temperature of the Pd 2+ species. It can be seen clearly from the graph that for the (NH 4 ) 2 PdCl 4 /AC catalyst, the peak for Pd 2+ reduction is evidently increased as compared to that of PdCl 2 /AC (Fig. 5). The increased reduction temperature of (NH 4 ) 2 PdCl 4 /AC can be attributed to the existence of nitrogen-containing ligands, which probably inhibit the reduction of the cationic Pd species.
The surface areas of the Pd-based catalysts were evaluated via the low-temperature N 2 adsorption/desorption experiments. Table 4 lists the catalyst texture parameters of the Pd-based catalysts. It is shown that the AC support has a microporous   structure, and the specic surface area is up to 1162.1 m 2 g À1 . Fresh catalysts, including PdCl 2 /AC and (NH 4 ) 2 PdCl 4 /AC, show specic surface areas lower than that of the AC support probably due to the blockage of pores by the active Pd species. In addition, the used catalysts exhibit lower specic surface areas when compared with those of the fresh catalysts. For example, about 43.1% of the specic surface area is lost aer reaction for 10 h. The loss of the active surface area may be caused by carbon deposition (acetylene may oligomerize over the catalyst) on the catalyst surface, which may result in clogged pores and decreased catalyst activity; moreover, this is likely the cause for catalyst deactivation. However, the surface area loss was only 20.8% for (NH 4 ) 2 PdCl 4 /AC. This result indicates that the amount of coke deposition is signicantly reduced when (NH 4 ) 2 PdCl 4 is used for Pd-based catalysts although the underlying mechanism for this reduction requires further investigation.

Mechanism insight
The adsorption and activation of substrates are important steps in a catalytic reaction. Through TPD characterization, we studied the adsorption properties of the two substrates C 2 H 2 and HCl over the (NH 4 ) 2 PdCl 4 /AC and PdCl 2 /AC catalysts. In Fig. 6, the individual C 2 H 2 -TPD results exhibit that the desorption temperature of the surveyed catalysts follows the order of PdCl 2 /AC (387 C) < (NH 4 ) 2 PdCl 4 /AC (410 C), demonstrating that (NH 4 ) 2 PdCl 4 /AC displays strong adsorption capacity for C 2 H 2 , followed by PdCl 2 /AC. The reactivity of Pd 2+ towards C 2 H 2 is usually explained by p-coordination and scoordination between the Pd 2+ and triple bond of C 2 H 2 ; thereby, C 2 H 2 is activated. In addition, by comparing the desorption temperature of C 2 H 2 for the two catalysts, it was found that the adsorption capacity of (NH 4 ) 2 PdCl 4 /AC to C 2 H 2 was stronger than that of PdCl 2 /AC.  4 ] + , which indicates that the HCl molecule is activated during the adsorption process, there is another possibility for the mechanism of acetylene hydrochlorination over the (NH 4 ) 2 PdCl 4 /AC catalyst. The possible reaction mechanism is shown in Fig. 6c. The proposed reaction mechanism was further validated by kinetic experiments. As mentioned in Fig. 1d, compared with the E a calculated from PdCl 2 /AC, the E a of the (NH 4 ) 2 PdCl 4 /AC catalyst decreases signicantly from 40 to 25 kJ mol À1 . This is precisely due to the further activation of HCl molecules on [NH 4 ] + such that the energy required for the reaction becomes lower. Under the same conditions, (NH 4 ) 2 PdCl 4 /AC exhibits a higher catalytic performance than PdCl 2 /AC, as shown in Fig. 1a.  In summary, we adopted an innovative strategy to synthesize non-mercury catalysts using the compound (NH 4 ) 2 PdCl 4 instead of traditional PdCl 2 . The prepared catalyst (NH 4 ) 2 PdCl 4 / AC demonstrated excellent activity and stability in the hydrochlorination of acetylene. This indicated that the microelectronic environment of the active Pd 2+ sites was regulated and the reduction resistance of cationic palladium was improved via the addition of nitrogen-containing ligands [NH 4 ] + to the structure of the active center. In particular, nitrogen-containing ligand additive not only can enhance the dispersion of Pd species, but can also promote the activation ability of HCl and then reduce the activation energy of the reaction. Therefore, our study proves that the (NH 4 ) 2 PdCl 4 /AC catalyst can be a hopeful candidate for efficient, well-stabilized non-mercury catalysts in the manufacture of vinyl chloride.

Conflicts of interest
There are no conicts to declare.