Sn(II)/PN@AC catalysts: synthesis, physical-chemical characterization, and applications

In this study, the novel tin-based catalysts (Sn(II)/PN@AC) were prepared using the phosphorus and nitrogen dual-modified activated carbon as support and SnCl2 as active compounds, as well as then evaluated in acetylene hydrochlorination. Under the reaction temperature of 180 °C and an acetylene gas hourly space velocity (GHSV-C2H2) of 30 h–1, the 15%Sn(II)/PN@AC-550 showed the initial acetylene conversion of 100% and vinyl chloride selectivity over 98.5%. Additionally, the deactivation rate of 15%Sn(II)/PN@AC-550 reached 0.47% h–1, which was lower than that of 15%Sn(II)/AC-550 (1.02% h–1), suggesting that PN@AC-550 as novel support can retarded the deactivation of Sn(II)/AC-550 catalysts during acetylene hydrochlorination. Based on the catalytic tests and characterization results (XRD, Raman, BET surface area, TEM, C2H2-TPD, H2-TPR, XPS, FT-IR, TGA, and ICP), it demonstrated that PN@AC-550 as support could effectively improve the dispersion of tin species, retard the formation of coke deposition, lessen the oxidation of SnCl2 during the preparation process, as well as relatively inhibit the leach of tin species during the reaction. By combing the FTIR results and Rideal–Eley mechanism, we proposed that that HSnCl3 was transition state of SnCl2 in catalysis acetylene hydrochlorination and then adsorbed the acetylene to produce the vinyl chloride.


Catalyst preparation
H 3 PO 4 (0.54 g), C 2 H 4 N 4 (0.46 g), and AC (9.0 g) were firstly premixed in distilled water at 55 °C for 90 min, after which the sample was dried at 80 °C overnight. The obtained samples were calcinated at 550 °C for 4 h and then denoted as PN@AC. N@AC was prepared by the above-mentioned procedure.
Catalysts were prepared by the following processes. Firstly, SnCl 2 ·2H 2 O (0.47 g) was completely dissolved in ethanol. Subsequently, SnCl 2 solution was gradually added to carbon supports, followed by air drying at 80 °C for 12 h. The abovementioned described procedures were repeated to prepare 10%Sn(II)/PN@AC, 10%Sn(II)/N@AC, and 10%Sn(II)/AC, respectively.

Catalytic performance
The catalytic performance was tested in a fixed-bed quartz reactor (i.d.=10 mm). To remove water vapor, the reaction system was washed by hydrogen chloride for 30 min before the initial reaction. Then the gas mixture of HCl and C 2 H 2 (V C2H2 /V HCl = 1.0:1.1) was introduced into reactor containing 4.0 mL of catalysts with C 2 H 2 -GHSV = 30 h -1 or 60 h -1 at 180 °C [19]. The final products contained the unreacted hydrogen chloride, which was adsorbed by the medical soda-lime. Having experienced adsorption, the cleaned gas mixture was analyzed online by GC900 (GDX-301 column).

Catalyst characterization
The BET surface area and pore textural properties data of the catalysts was acquired by a nitrogen adsorption method using Quantachrome Nova2000e instruments. The X-ray diffraction (XRD) patterns of the catalysts was collected from a Shimadzu XRD-6000 instrument using Cu Ka radiation over the range of 10-80 °. Raman spectra of sample were performed on a Renishaw (514 nm laser source). Transmission electron microscopy (TEM) images were obtained using a JEM-2100F instrument with 300 keV acceleration voltages. X-ray photo-electron spectroscopy (XPS) analysis was conducted on an Esca Lab 250Xi spectrometer. Thermogravimetric analysis (TGA) experiments (NETZSCH STA 449F3 Jupiter instrument) carried out to study the coke deposition of catalysts. Inductively coupled plasma optical emission spectrometer (Agilent 720 ICP-OES) was used to determine the absolute content of tin elements in samples.
The Fourier transform infrared spectra (FT-IR) of the samples was measured by Biorad Excalibur FTS 3000). Acetylene temperature-programmed desorption (C 2 H 2 -TPD) and hydrogen temperature-programmed reduction experiments (H 2 -TPR) were performed on FINESORB-3010 instruments, respectively [3]. Adsorption capacity of catalysts for HCl were calculated by the titration method [4].

Characterization of supports
The structure of different supports was firstly studied, including the BET surface area and the pore volume. The value of AC (983.0 m 2 ·g -1 , 0.48 m 3 ·g -1 ) was higher than that of N@AC (786.3 m 2 ·g -1 , 0.38 m 3 ·g -1 ) and PN@AC (725.1 m 2 ·g -1 , 0.33 m 3 ·g -1 ). It is demonstrated that the nonmetal additives are filled into the partial pores of AC ( Figure 1a and Table 1). Figure  1b shows that the two diffraction peaks of AC were not affected by the introduction of nonmetal additives [20], inferring the well dispersion of promoter on AC surface [21]. As shown in Figure 1c, the I D /I G value gradually declines in the order of PN@AC(1.63)> N@AC(1.23)> AC (0.85), suggesting that PN-doping does lead to the more abundant defect of PN@AC and consequently improve the catalytic behavior of AC in acetylene hydrochlorination [22][23][24].

Characterization of Sn-based catalysts
As evident from Table 1 and Table 2, the load of SnCl 2 can reduce the specific surface area and the pore volume of supports. Furthermore, besides the characteristic peaks of carbon, the discernible peaks of SnCl 2 (PDF#72-0137) is not observed in tin-based catalysts (Figure 2a), inferring the dispersion of tin compounds on support surface [20,21]. As shown in Figures  2b and 2c, numerous black particles are dispersed on the support surface. Meanwhile, Figure 2d illustrates that no large particles are reunited on the PN@AC, indicating that Sn species are stably and homogeneously dispersed on the PN@AC.

Catalytic performance
The catalytic performances of different catalysts were tested in acetylene hydrochlorination, and the results are shown in Figures 3a-c. The acetylene conversion on the Sn(II)/PN@AC increased as the SnCl 2 content (5 wt%-15 wt%). When the SnCl 2 content is 15 wt%, Sn(II)/PN@AC achieves an initial acetylene conversion of 100%, with vinyl chloride selectivity of

Synergistic effect between Sn and supports
The status of tin, nitrogen and phosphorus species in catalysts was characterized by XPS analysis (Figure 4a and Table 3). As Figure 4b illustrates, Sn-based catalysts behaved the two types of Sn species including Sn 2+ (~486.7 eV) and Sn 4+ (~487.7 eV) [26][27][28]. This result is consistent with H 2 -TPR results ( Table 4). As shown in Figure 4e and Table 4, the H 2 reduction peaks of Sn 2+ and Sn 4+ in catalysts (15%Sn(II)/N@AC and 15%Sn(II)/PN@AC) was lower than the standard consumption peaks (467.4 and 542.9 °C) [26][27][28], inferring that the nonmetal element additives can retard the oxidation of Sn 2+ during the synthesis process and lessen the leach of Sn species during the reaction. The determination of Sn 2+ and Sn 4+ values in Tables (Table 4 and Table 5) was calculated on the normalization of peak areas. Figure 4c and Figure 4d confirm the existence of N-P and Pyridinic N, which can promote the reactivity of catalysts significantly [25,29,30]. The Pyridinic N content of 15%Sn(II)/N@AC was calculated to be 0.9 wt%, which is close to the 15%Sn(II)/PN@AC (0.88 wt%). It is revealed that phosphorus atoms bonded with nitrogen in the pyridine structure (ortho-position NP) is the main factor on the performance of 15%Sn(II)/AC according to the present work (Figure 1a, Figure 4d, and Table 6) and a recently published paper [25].  As shown in Figure 4c, the electron binding energies at 135.0 eV, 133.5 eV, and 132.6 eV can be assigned to P-O, P-N, and P-C [23,24,31]. Particularly, the addition of SnCl 2 into catalysts shift the P-N binding energy from 133.5 eV to 133.2 eV. Moreover, a positive shift (0.3 eV) can be observed in the binding energy of Sn 2+ for 15%Sn(II)/PN@AC versus 15%Sn(II)/ AC. Therefore, it is attributing this shift to the synergistic effect between ortho-position NP and Sn species [32,33].

Synergistic effect between Sn and reactants
To study the effect of various adsorption reactants on the structural conformation of SnCl 2 , we used FT-IR techniques to analyze three samples including Sn(II)/AC-C 2 H 2 , Sn(II)/AC-HCl, and Sn(II)/AC-N 2 ( Figure 5), which represent that SnCl 2 / AC was respectively pretreated at 200 °C for 1 h under acetylene, hydrogen chloride, and nitrogen atmosphere, respectively. Two characteristic adsorption bands at ~1121 cm -1 and ~3397 cm -1 are observed in Sn(II)/AC-HCl, suggesting that the gaseous HCl reacted firstly with Sn species. Figure 4a shows that Sn(II)/AC catalysts during acetylene hydrochlorination contains SnCl 4 and SnCl 2 , the latter of which is the main formation of Sn species. Based on the analysis of our previous study [3], owing to SnCl 4 and HCl are electron-receptor, the adsorption of HCl on the SnCl 2 sites forms the HSnCl 3 [34][35][36]. The results of FT-IR spectra are no obvious difference between Sn(II)/AC-N 2 and Sn(II)/AC-C 2 H 2 , implying that SnCl 2 cannot make bond with C 2 H 2 . Combining the Eley-Rideal mechanism and previous work [34][35][36][37][38], it is indicated that HSnCl 3 is as transition state of SnCl 2 in the catalysis of acetylene hydrochlorination.
HCl adsorption experiments was applied to study the HCl adsorption capacity of catalysts, and the results are displayed in Figure 6a. The adsorption capacity of HCl on 15%Sn(II)/PN@AC, 15%Sn(II)/N@AC, and 15%Sn(II)/AC are 0.64  mmol·g -1 , 0.49 mmol·g -1 , and 0.28 mmol·g -1 , respectively. The published papers finds that HCl adsorbs on Pyridinic N firstly [30], but the Pyridinic N content in 15%Sn(II)/PN@AC is close to 15%Sn(II)/N@AC (Table 6). It is implying that the synergy effect between Sn and NP may be responsible for the highly HCl adsorption capacity and thus improve the catalytic behaviors of tin-based catalysts in acetylene hydrochlorination, based on the above results.
As shown in Figure 6b, the adsorption area for C 2 H 2 of catalysts decreases in the order 15%Sn(II)/PN@AC>15%Sn(II)/N@ AC>15%Sn(II)/AC. When N and P-codoped 15%Sn(II)/AC can result in higher C 2 H 2 adsorption capacity as compare to 15%Sn(II)/N@AC and 15%Sn(II)/AC. This result indicates that NP is responsible for the C 2 H 2 adsorption capacity, which is well agree with the previous study [25].

Inactivation of tin-based catalysts
After 40 h reaction, the reduction of BET surface area in used tin-based catalysts is listed in Table 2, the loss of BET surface area in 15%Sn(II)/PN@AC, 15%Sn(II)/N@AC and 15%Sn(II)/AC reaches 138.6 m 2 ·g -1 , 241.1 m 2 ·g -1 , and 299.7 m 2 ·g -1 . Figure 7 shows the TGA curves of catalysts before and after reaction. Based on the same type fresh-and used-catalysts, the weight loss in the temperature range of 150-470 °C was mainly originated from the coke deposition on the catalysts surface in acetylene hydrochlorination [39,40], and the results is listed in Table 7. The amount of coke deposition for the 15%Sn(II)/PN@AC, 15%Sn(II)/N@AC, and 15%Sn(II)/AC catalysts are 1.70%, 3.61%, and 3.70% (Figures 7a and  b). Thus, the synergistic effect of Sn species and NP in 15%Sn(II)/PN@AC can accelerate the anti-coking ability, which, consequently, prolonging the lifetime of the Sn-based catalysts. The 75.9%, 72.3%, and 65.4% of the initial Sn content is lost from 15%Sn(II)/AC, 15%Sn(II)/N@AC, and 15%Sn(II)/PN@AC, respectively, after 40 h reaction ( Table 8), demonstrating that the leach of tin species is a deactivation reason of tin-based catalysts and manifesting that NP additives can retard the loss of tin compounds during acetylene hydrochlorination [41].

Conclusion
Although carbon supported-HgCl 2 as catalyst exhibits considerable catalytic performance, its toxicity and sublimation can be led to the serious environmental pollution. Additionally, the Minamata convention will be forbidden the utilization of mercury-based materials. But this study reported that Sn/PNAC as catalysts for acetylene hydrochlorination was prepared using nontoxic compounds. This work also finds that the stability of 15%Sn(II)/PN@AC catalysts correlate with N and P additives. After careful characterizations and additional catalytic tests, PN@AC supports can not only make the tin compounds dispersion well, but also strengthens the reactants adsorption of catalysts. According to the XPS, C 2 H 2 -TPD, H 2 -TPR, and HCl adsorption experiments results, it is found that the better catalytic behavior of Sn(II)/PN@AC is mainly attributed to the synergy between ortho-position NP and Sn species. Additionally, the reaction mechanism was proposed as follows, the adsorption of HCl on SnCl 2 forms the transition state (HSnCl 3 ) is the initial step and then react with C 2 H 2 to produce the vinyl chloride. Such a finding provides the guidance to develop the tin-based catalysts for acetylene hydrochlorination.

Acknowledgment
This work is supported by PhD research startup foundation of Pingding Shan University (PXY-BSQD-202110)