Research Article Cite This: ACS Catal. 2017, 7, 8572-8577
pubs.acs.org/acscatalysis
Catalytically Active Boron Nitride in Acetylene Hydrochlorination Pan Li,†,‡ Haobo Li,‡ Xiulian Pan,*,‡ Kai Tie,‡ Tingting Cui,‡ Minzheng Ding,‡ and Xinhe Bao*,†,‡ †
Department of Chemical Physics, University of Science and Technology of China, 96 Jinzhai Road, 230026 Hefei, P.R. China State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, 116023 Dalian, P.R. China
‡
S Supporting Information *
ABSTRACT: This study presents the discovery that porous boron nitride (p-BN) is active in acetylene hydrochlorination, although boron nitride (BN) is generally considered chemically inert. An acetylene conversion of 99% is achieved with a vinyl chloride selectivity over 99% at 280 °C at a gas hourly space velocity (GHSV) of 1.32 mL min−1 g−1. By contrast, the commercially available crystallized hexagonal BN (h-BN) exhibits no catalytic activity. Furthermore, this p-BN is rather durable as demonstrated by a 1000 h lifetime test. Catalytic tests, spectroscopic characterization, and theoretical calculations indicate that the activity likely originates from the defects and edge sites. Particularly, the armchair edges of BN can polarize and activate acetylene, which then reacts with gaseous HCl giving vinyl chloride as the product. KEYWORDS: porous boron nitride, metal-free catalyst, acetylene hydrochlorination, mercury-free catalyst, stability
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frequently investigated for applications such as CO2 capture,22 hydrogen storage,23 and oil removal,24 it is less studied in catalysis because it is generally considered to be chemically inert. However, modifications introducing defects and vacancies could disturb its electronic structure and therefore may render BN catalytically active. BN with boron vacancies was theoretically predicted to have the potential of activating CO2;25 carbon-doped h-BN was shown to be active as a metalfree photocatalyst for the reaction of CO2 and H2O to produce H2.26 Very recently, h-BN was demonstrated to be highly active and selective in oxidative-dehydrogenation reactions;27,28 the reported oxidative dehydrogenation of propane exhibited a propene selectivity of 79% and an ethene selectivity of 12% at a propane conversion of 14%.27 However, it is not known if boron nitride is catalytically active for other reactions. Herein, we report that porous BN (denoted as p-BN) can directly catalyze the acetylene-hydrochlorination reaction. Furthermore, it exhibits better performance than SiC@C−N and most nitrogen-doped carbon materials and thus could open a new avenue for further development of metal-free catalysts for vinyl chloride production from acetylene.
INTRODUCTION Poly vinyl chloride (PVC) is globally one of the most frequently used engineering plastics and is used in various aspects of human life because of its unique properties, such as its good resistance to photo and chemical degradation. However, in coal-rich countries such as China and India, the production of its monomer, vinyl chloride monomer (VCM), still relies on the electrophilic addition of HCl to acetylene catalyzed by mercuric chloride.1 The toxicity of mercuric chloride has prompted worldwide efforts to explore mercuryfree catalysts and more environmentally friendly process. Hutchings and co-workers investigated a range of metal chlorides and predicted AuCl3 to be the most active catalyst.2 Since then, extensive studies have been carried out on gold,1,3−13 and significant progresses have been made. While continuing efforts are made to bring gold catalysts toward commercialization, there are also wide interests in exploring non-noble-metal catalysts. Carbon-based materials, particularly nitrogen-doped carbon, have attracted wide attention as metalfree catalysts for a number of chemical reactions. The potential of nitrogen-doped carbon as a catalyst for acetylene hydrochlorination was also demonstrated recently.14−21 For example, SiC-supported N-doped carbon (SiC@C−N) catalyzed the reaction with a conversion of acetylene at around 80%, a vinyl chloride selectivity over 98% at 200 °C, and total gas hourly space velocity (GHSV) of 0.8 mL min−1 g−1.14 However, this activity slowly degraded with time on the stream, and acetylene conversion dropped by 4% over 150 h. Therefore, we set out to explore alternative metal-free catalysts. Hexagonal boron nitride (h-BN) is a 2D material analogous to graphite with a layered structure. Although h-BN is © XXXX American Chemical Society
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RESULTS AND DISCUSSION p-BN was prepared by thermal treatment of a mixture of boric acid and melamine in NH3 at 800 °C by adapting a reported method (see the Methods section for more details).23 Figure 1 shows that the catalyst exhibits a high activity in acetylene Received: June 9, 2017 Revised: October 23, 2017
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Figure 1. Catalytic performance of p-BN as functions of different parameters: (A) reaction temperature; (B) HCl/C2H2 ratio in the feed; (C) linear velocity, performed by increasing the catalyst loading from 1.0 to 3.0 g without changing the reactor or GHSV (filled symbols stand for the 3.0 g reaction, and open symbols stand for the 1.0 g reaction); (D) HCl partial pressures, with the total pressure fixed at 1.0 bar. (E) Conversion versus GHSV under reaction conditions of 280 °C, HCl/C2H2 = 1/1. (F) Stability test of ∼1000 h at optimized reaction conditions: 1.0 g catalyst, GHSV = 1.32 mL min−1 g−1, HCl/C2H2 = 1.2, and 280 °C. The red symbols (columns, circles, and squares) stand for the VCM selectivity, and the black symbols stand for the acetylene conversion.
hydrochlorination. At 200 °C and GHSV = 1.2 mL min−1 g−1, equivalent to 40 h−1 (HCl/acetylene = 1/1), the conversion of acetylene reaches 72% with the vinyl chloride selectivity above 96%. p-BN is rather robust and can be used at a higher temperature. For example, acetylene conversion rises to 90% at 280 °C, and vinyl chloride selectivity reaches over 99% (Figure 1A). However, increasing the temperature beyond 300 °C does not further improve the conversion. In contrast, no acetylene conversion is detected over the commercially available h-BN under the same conditions. Figure 1B shows that the conversion increases to 95% with a HCl/C2H2 ratio in the feed up to 1.2. Figure 1C demonstrates that the reaction is further facilitated at a higher linear velocity. For example, when the catalyst loading is increased from 1.0 to 3.0 g in the same reactor, which raises the linear velocity from 0.53 to 1.59 cm/min, acetylene conversion increases from 92 to 94% at the same temperature and GHSV while the selectivity to vinyl chloride remains the same. It is anticipated that the conversion can be further enhanced at a higher linear velocity, which is often the case in practical applications when catalysts are loaded into a reactor with a much larger length/diameter
ratio . For comparison with gold-based catalysts, we tested the p-BN catalyst under a higher GHSV (120 h−1), and the catalyst still exhibits 81% conversion with a selectivity over 99% (Figure 1E). Moreover, the catalysts are reproducible, as indicated in Figure S1. In addition, the p-BN catalyst exhibits rather good durability, as displayed in Figure 1F. At the optimized reaction conditions, the catalyst gives a conversion of over 99% with a selectivity to vinyl chloride of 99%. After ∼1000 h on the stream, the conversion decreased slightly to 95%. Note that the GHSV is similar to that reported for gold-based catalysts in ref 10. In order to examine its durability, the catalyst was tested at a much higher GHSV of 9.24 mL min−1 g−1 (equivalent to 300 h−1). The results in Figure S2 show that a conversion close to 90% and a selectivity close to 99% are obtained. Although the conversion decreases to 80% after 200 h, the catalyst can be regenerated, as shown in Figure S2. The regenerated catalyst exhibits an initial conversion of 87% under the same conditions. Among reported metal-free catalysts under similar conditions, this p-BN exhibits rather high activity and durability for acetylene hydrochlorination.21,29−31 It is surprising that p-BN 8573
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enlarged layer distance is frequently reported for 2D layered materials because of restacking, the incorporation of defects, and a decrease in layer number.33 The X-ray-photoelectron-spectroscopy (XPS) N1s feature at 398.0 eV and B1s peak at 190.4 eV (Figure 3B,C) demonstrate the presence of the B−N bonds. The much weaker C1s feature at a binding energy of 288.3 eV34 and the N1s feature at 399.2 eV can be associated with the sp2-bonded carbon in Ncontaining aromatic rings.26,35 This implies the presence of some C−N groups, which should arise from pyrolysis of the melamine precursor. In addition, the 192.0 eV peak in the B1s spectrum corresponds to B−O,36,37 likely resulting from surface impurities or from the unreacted boric acid. The 400.7 eV N1s peak indicates the presence of N−H groups,26,38 reflecting the presence of edges saturated with H, likely originating from the NH3 treatment. However, this feature is not observed for h-BN, indicating a concentration of edge sites too low to be detectable. The FTIR spectra in Figure 3E confirm again that p-BN possesses the intrinsic structure of boron nitride, as reflected by the bands at around 800 and 1400 cm−1, corresponding to the B−N−B (bending vibration) and B−N (stretching vibration) bonds, respectively.11 In addition, the 3430 cm−1 band is attributed to O−H or N−H bonds. Elemental mapping using energy-dispersive X-ray spectroscopy indicates that B, N, C, and O are distributed homogeneously across the sample (Figure S3). The above results demonstrate that p-BN and h-BN share the same intrinsic boron nitride structure in the bulk. However, the unique morphology of small stacks of few-layer-flakes endows p-BN with a substantial amount of edges and defects. This is also reflected by its very high BET surface area (824 m2/ g) and by the presence of a large amount of micropores and mesopores (Figures S4 and S5). By contrast, the well crystallized h-BN has a surface area of only 29 m2/g. It is known that electronic structures may be perturbed on edge sites and defects, where unsaturated atoms are readily bonding with other heteroatoms.39,40 This could induce catalytic activities, analogous to those of graphene or carbon nanotubes.39 It appears that the oxygen groups in BN are
gives high activity, considering that boron nitride is generally considered to be chemically inert. Such inertness is reflected by the undetectable activity of the commercially available h-BN. The high-resolution transmission electron micrograph (HRTEM) images in Figure 2 show that p-BN exhibits a
Figure 2. HRTEM images of p-BN at different magnifications and measured layer distances.
typical layered structure of boron nitride, essentially composed of stacks of small flakes. The number of layers falls in the range of 3−10. The typical layer distance falls in the range of 0.35− 0.40 nm, which is similar to that reported previously for boron nitride prepared with a similar method.23,32 It is larger than the 0.33 nm reported for the commercial h-BN, likely as a result of the presence of defects. X-ray diffraction (XRD) in Figure 3A shows that p-BN is poorly crystallized with respect to h-BN. However, two characteristic peaks of BN crystal at 24.9 and 42.8° are still discernible, which correspond to the BN (002) and (100) planes, respectively.23,26 The slightly down-shifted diffraction angle of BN (002) suggests an enlarged layer distance,32,33 consistent with the TEM observation. An
Figure 3. Structure of p-BN in comparison with h-BN. (A) XRD, (B) XPS spectra of N1s, (C) B1s, (D) C1s, (E) FTIR spectra. 8574
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Figure 4. DFT calculations showing different sites for acetylene adsorption. (A) Possible sites on the zigzag edge. (B) Possible sites on the armchair edge and in the plane. (C) Adsorption energies on different sites. (D) Optimized structure for acetylene adsorption on site 4 on the armchair edge. (E) Charge distribution upon adsorption of acetylene on site 4. Blue balls represent nitrogen atoms, light pink balls represent boron atoms, gray balls represent carbon atoms, and white balls represent hydrogen atoms.
increases, reaches a maximum at 0.55 bar, and then levels off. This is typical behavior for an Eley−Rideal mechanism.41 The optimized structure in Figure 4D shows that the acetylene molecule adsorbs with one C atom bonding to B and the other C to the adjacent N. Interestingly, the molecule is polarized significantly by the B−N site due to the electronegativity differences between B and N (Figure 4E), resulting in one C carrying −0.97 e and the other carrying +0.62 e. Thus, acetylene is activated, which facilitates the adsorption of HCl and hence the reaction. This is consistent with TPD in Figure S6c,d. Consequently, the hydrochlorination reaction proceeds.
detrimental in this reaction because upon treatment in oxygen, p-BN almost completely loses its activity for acetylene hydrochlorination. In order to understand the role of carbon, we have increased the amount of melamine by increasing the mass ratio of melamine to boric acid from 1/4 to 1/1 during synthesis. This results in incorporation of more carbon in the final product: surface C increases from 20 to 30%, as measured by XPS. However, the catalytic activity is not much influenced. Therefore, there might be other factors playing roles in the catalytic activity. We further looked into the catalytic role of the edge sites. Ball milling is a frequently employed method to disaggregate large graphene agglomerates into smaller flakes and to generate more edges.39 Our experiments reveal that ball milling does increase the surface area of h-BN to 39 m2/g, resulting in some catalytic activity in acetylene hydrochlorination (conversion of 4.4% under standard conditions). Because we do not have direct experimental evidence for the actual structures of the edge sites, we turned to density functional theory (DFT) when exploring the possible active sites on the edges in acetylene hydrochlorination (see Supporting Information for more details). Both zigzag and armchair edges with different sites were considered, as displayed in Figures 4, S7, and S8. The results show that acetylene hardly adsorbs on sites 1 and 2 of zigzag edges or on site 3 of the armchair edge (Figure 4A,B). Nor does it adsorb on site 5 within the plane of BN, as the adsorption energy is positive, exceeding +0.5 eV. Figure 4C reveals that only site 4 on the armchair edge is favorable for the adsorption of acetylene, although it is still relatively weak, having an adsorption energy of −0.17 eV. The temperatureprogrammed desorption (TPD) experiments in Figure S6a show that desorption of acetylene takes place at a relatively low temperature (in the range of 40−150 °C). In comparison, pBN exhibits almost no activity of HCl adsorption as demonstrated in HCl-TPD (Figure S6b), which is confirmed by the theoretical calculation. Therefore, the reaction likely proceeds with acetylene adsorption, followed by a reaction with gas phase HCl, that is, following the Eley−Rideal mechanism. This is further validated by the experiments displayed in Figure 1D, in which the HCl partial pressure is varied. Acetylene conversion increases when the partial pressure of HCl
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CONCLUSION It is demonstrated here that porous BN composed of few-layer stacks actively catalyzes the conversion of acetylene to vinyl chloride, although its commercial counterpart h-BN does not show any detectable activity. Acetylene conversion reaches as high as 99%, and the selectivity to vinyl chloride reaches 99% at 280 °C, HCl/C2H2 = 1.2 and GHSV = 1.32 mL min−1 g−1 (equivalent to 44 h−1). Detailed characterization of the compositions and structures of p-BN in comparison with those of h-BN suggests that both catalysts share the same intrinsic boron nitride structure in the bulk. Catalytic activity likely originates from the abundant defects and edge sites. DFT calculations show that the armchair edge is catalytically active, and acetylene molecules are polarized significantly over the B− N sites. The reaction proceeds with the adsorbed acetylene reacting with gaseous HCl, following the Eley−Rideal mechanism. However, the role of different defect sites, such as those on the bended layers, cannot be ruled out completely and will require further systematic investigation. Furthermore, the performance can be optimized by manipulating the reaction conditions. For example, acetylene conversion is facilitated at a higher HCl/C2H2 ratio and with a higher linear velocity of the feed, whereas the selectivity to vinyl chloride is not influenced much. This bodes well for the use of larger-scale shell-tube reactors in practical applications, where a much higher linear velocity is applied. In addition, this p-BN is rather durable, as demonstrated in a 1000 h test. It is more durable than the reported N-doped carbon catalysts under similar conditions. Therefore, the finding here provides an 8575
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packed into the quartz reactor, and the reaction was performed under the condition of HCl/C2H2 = 1.2/1 at 280 °C. Calculation Details. DFT calculations were performed using Vienna ab initio simulation packages (VASP)42 with the projector-augmented wave (PAW) method.43 All calculations were based on the same generalized gradient-approximation method with the Perdew-Burke- Ernzerhof (PBE)44 functional for the exchange-correlation term. All energy differences were evaluated using dispersion-corrected PBE-D3.45 The plane wave cutoff was set to 400 eV. The Brillouin zone was sampled by a 2 × 1 × 1 Monkhorst−Pack46 k-point sampling for structural optimizations and a 6 × 1 × 1 k-point grid for chargedensity calculations. The convergences of energy and forces were set to 1 × 10−5 and 0.05 eV/Å, respectively. The BN edge was simulated with a periodically repeated BN-nanoribbon model in rectangular supercells. The vacuum thicknesses perpendicular and parallel to the ribbon plane were set to about ∼12 and ∼20 Å, respectively.
alternative route for designing a new type of mercury-free, metal-free catalyst for vinyl chloride synthesis from acetylene. Furthermore, p-BN may also be applicable in the activation of other alkynes. This could broaden the already widely explored potential applications for BN.
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METHODS Materials. Boric acid and melamine were purchased from Sinopharm Chemical Reagent Company, and h-BN was purchased from Aladdin Reagent Company. Preparation of p-BN. Typically, 0.1 mol boric acid was dissolved in 300 mL deionized water and heated to 60 °C, followed by an addition of 0.025 mol melamine. The mixture was immediately transformed into a white, dense precipitate. Then the mixture was kept at 60 °C for 60 min under vigorous stirring, followed by heat treatment at 90 °C. A clear solution was obtained after heating to 90 °C. Subsequently, it was kept sealed at 90 °C for 6 h. Afterward, the solvent was evaporated at 90 °C. The resulting white powder was calcined in flowing Ar (100 mL/min) following a temperature program of heating by 2 °C/min to 550 °C, maintaining 550 °C for 2 h, heating by 5 °C/min to 800 °C, and maintaining 800 °C for 6 h. The resulting sample was further treated in NH3 at 800 °C for 6 h at a ramp of 2 °C/min, and the final product exhibited a white, powder form. The regeneration was carried out by transferring the sample to a different furnace, where it was treated in 50 mL/min NH3 at 800 °C for 2 h (a ramp of 2 °C/min), followed by Ar sweeping at room temperature to remove the remaining NH3. Characterization. FTIR was conducted on a Thermo Fisher Scientific Nicolet iS50 instrument. XRD was performed on a PANAlytical X’pert Pro-1 instrument. XPS was carried out on a Thermo ESCALAB 250Xi spectroscope, using Al Kα Xrays as the excitation source at a voltage of 15 kV. All binding energies were calibrated by contaminant C at 284.6 eV. TEM and HRTEM images were obtained on HT7700 and JEM-2100 electron microscopes, respectively, and EDX mapping was conducted in a JEOL JEM-2100 electron microscope. Before the TPD process, the catalyst was treated in He at 300 °C for 2 h to remove any impurities from the air. Then, it was exposed to 5% acetylene/He or 1% HCl/He for over 1 h to ensure a saturated adsorption, followed by sweeping of He at room temperature for over 1 h. Subsequently, the catalyst was heated in He with the effluents monitored by the online mass spectrometer. Catalytic Reaction. The acetylene hydrochlorination reaction was carried out in a vertically aligned, fixed-bed reactor with an inner diameter of 12 mm, under conditions: a catalyst loading of 1.0 g, a gas hourly space velocity 1.2 mL min−1 g−1, and a temperature of 200−280 °C. The effect of the HCl/C2H2 ratio on the catalytic performance was studied in the range of 1.0−1.2. The influence of C2H2 space velocity was tested in the range of 0.6−1.8 mL min−1 g−1. An online gas chromatograph (Tianmei 7900) equipped with a plot-Q capillary column and an FID detector was used to analyze the effluents. The 1000 h stability test was performed with HCl/C2H2 = 1.2/1 at 280 °C in powder (1.0 g) in a quartz reactor with a 7 mm inner diameter. The packing density of the p-BN powder is estimated to be 0.56 g/mL; thus, 1.2 mL min−1 g−1 is equivalent to 40 h−1. The effect of a higher GHSV of 9.24 mL min−1 g−1 (equivalent to 300 h−1) was also studied. The catalyst with 20−40 mesh was
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b01877. Sample reproducibility, the effect of a higher GHSV and corresponding regeneration result, element mapping results, BET surface area and pore size distribution, TPD results, and additional adsorption sites studied in DFT calculation (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (X.P.). *E-mail:
[email protected] (X.B.). ORCID
Xiulian Pan: 0000-0002-5906-6675 Xinhe Bao: 0000-0001-9404-6429 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the Ministry of Science and Technology of the People’s Republic of China (2016YFA0202803) and the National Natural Science Foundation of China (Nos. 21425312, 21373207, and 21621063).
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