C-doped boron nitride nanotubes for the catalysis of acetylene hydrochlorination: A density functional theory study

doi:10.1016/j.mcat.2020.110853

Molecular Catalysis

Volume 488, June 2020, 110853

https://doi.org/10.1016/j.mcat.2020.110853 Get rights and content

Abstract

The mechanism of a novel mercury-free catalyst, carbon-doped boron nitride nanotubes (BNNTs), for acetylene hydrochlorination reaction was investigated by density function theory (DFT) calculations. Two types of carbon-doped BNNTs with different diameters, boron substituted and nitride substituted with carbon, were studied in detail as the catalyst of the acetylene hydrochlorination reaction. Results show the adsorption of C₂H₂ on carbon-doped BNNTs is dominant in the adsorption process due to the stronger interaction of C₂H₂ with carbon-doped BNNTs. HCl can be dissociated on carbon-doped BNNTs with small diameter during the adsorption process. The C₂H₂ is chemically adsorbed on the doped impurity C atom where it is activated to continue the addition reaction with the gaseous HCl molecule. The rate-limiting step is the splitting of HCl molecule and the attack of H and Cl atoms on C double bond C bond of the activated C₂H₂ to form the transition state. The reaction can take place easily on carbon-doped BNNTs and a low energy barrier of 28.47 kcal/mol is found. The doping site and curvature have a slight impact on the catalytic performance in the reaction based on the comparison of energy barrier. The study reveals that the doped impurity C atom can largely improve the activity of BNNTs, and carbon-doped BNNTs can be an effective non-metal catalyst in the acetylene hydrochlorination reaction.

Graphical abstract

The doped C atom influences the electronic properties, adsorption properties and catalytic performance of boron nitride nanotubes (BNNTs). C-doped BNNTs (B or N substituted by C atom) exhibit high activity in acetylene hydrochlorination.

Introduction

In current days, the concept of green chemistry comes to be valued by many countries due to the issues of growing environmental pollutions [1]. The essence of green chemistry is to design the production method which can reduce or eliminate harmful substances in chemical reaction [2,3]. Polyvinyl chloride (PVC) is widely used in a variety of industrial fields, including pipes, electric cables, healthcare etc. Millions of tons of PVC are consumed each year according to previous statistics [4]. In coal-rich countries such as China, the acetylene hydrochlorination reaction is commonly used to produce the vinyl chloride monomer (VCM), which is then polymerized to produce PVC [5,6]. In this conventional VCM production route, the mercuric chloride (HgCl₂) are largely employed as the catalysts of the acetylene hydrochlorination [6]. The HgCl₂ catalysts supported on activated carbon (AC) possess high catalytic performance in the acetylene hydrochlorination. However, most of HgCl₂ will volatilize into gas at reaction temperature higher than 200 °C and the loss of HgCl₂ in to the air is considerable each year [7]. The lost toxic mercury can pollute water resource and damage human health, which is not consistent with the concept of green chemistry. Therefore, it is necessary to develop novel effective mercury-free catalysts to substitute for conventional HgCl₂ catalysts in the VCM production route [8]. There are many kinds of alternative catalysts for mercury chloride, such as the noble metal and metal-free catalysts. The Au-based catalysts have played important roles in noble metal catalysts [6]. Hutching et.al first proposed that the gold catalyst was very effective for acetylene hydrochlorination and synthesized the Au catalyst supported on AC (Au/AC) [9,10]. Furthermore, a series of methods were employed to enhance the activity and stability of gold catalysts, such as the addition of other metal elements, and the substitution of the supporter (eg. Au/CeO₂) [[11], [12], [13], [14], [15]]. The Au-based and other noble metal catalysts have high catalytic performance in experiments, but the large-scale applications of noble metal catalysts in VCM production are also limited by the scarcity and high price of these materials.

Metal-free catalysts can be used as excellent alternatives of mercury chloride catalyst due to abundant resources, better selectivity, low-cost and environment-friendly characteristic [16]. Metal-free catalysts can be doped to obtain the character of Lewis acid, which improves their catalytic activity in acetylene hydrochlorination [17,18]. Up to now, there are various metal-free catalysts that were prepared from carbon related materials, such as tubular nitrogen-doped carbon nanotubes (N-CNTs) [17], thin graphitic carbon nitride (g-C₃N₄) [19], porous 13X zeolite and layered nanocomposite by growing N-C layer out of the silicon carbide granules (SiC@N-C) [20,21]. Above-mentioned catalysts showed high selectivity, good activity and thermal conductivity on the basis of theoretical computation and experiment [17,[19], [20], [21]]. Besides, other metal-free catalysts such as the porous boron nitride (p-BN) synthesized by Li et al. exhibit better performance than most N-CNTs [22]. This found indicated that the use of BN materials to catalyze the acetylation of acetylene is promising.

As early as 1994, researchers proposed the existence of boron nitride nanotubes (BNNTs) which can be synthesized from hexagonal boron nitride (h-BN) in theory [23]. Later on, Chopra et al. synthesized BNNTs by arc-discharge method in experiment [24]. Subsequently, the structures and properties of BNNTs are extensively investigated [[25], [26], [27]]. BNNTs have constant band gap which results in great electrical insulation [28]. Moreover, BNNTs possess good resistance to oxidation and high temperature, which increase stability of BNNTs in severe environment [29,30]. The large ratio surface, high chemical stability and outstanding biocompatibility of BNNTs lead to their excellent performance in sensor [31] and drug delivery applications [32,33]. As a superb metal-free materials, BNNTs have also been explored extensively for their catalytic application in a number of reactions by theoretical computation, such as methanol dehydrogenation [34], CO oxidation by N₂O [35], and decomposition of nitrous oxide [36]. The results indicate BNNTs can be used as fine metal-free catalysts and doped BNNTs can have better catalytic performance than pure BNNTs [[34], [35], [36], [37]]. Among various dopant atoms, carbon doping is a common strategy because C and BN systems have similar lattice parameters, which indicates B single bond CN nanostructures could be easily constructed [38]. Besides, it is revealed that the introduction of carbon atom decreases the band gap of BNNTs, which indicates carbon atom can tune the electronic properties of BNNTs [39,40]. The formed donor-type or accepter-type defects are active sites when the boron or nitride atom is substituted by carbon. Many theoretical studies also proved carbon doping for BN materials can enhance the adsorption properties of hydrogen, increase the electrocatalytic reactivity in oxygen reduction reaction, and improve the catalytic activity in N₂O reduction reaction and acetylene hydrochlorination [[41], [42], [43], [44]]. In the experimental study of the p-BN for acetylene hydrochlorination, the commercially available crystallized hexagonal BN (h-BN) exhibited no catalytic activity while the p-BN prepared by thermal treatment of a mixture of boric acid and melamine in NH₃ showed high performance [22]. The X-ray photoelectron spectroscopy also indicated the existence of C in the sample while the catalytic effect of these carbon atoms is unclear. In another experimental study on the catalytic mechanism of g-C₃N₄ for acetylene hydrochlorination, results indicated that the C and N atoms had important roles in the acetylene hydrochlorination [19]. It is thus crucial to study the role of C in the BN materials and the catalytic mechanism of the C atoms in BN materials needs to be analyzed for their high catalytic performance in the acetylene hydrochlorination reaction.

In this study, C-doped BNNTs as the catalyst of the acetylene hydrochlorination were investigated by using density functional theory (DFT) calculations. The adsorption of C₂H₂ and HCl on C-doped BNNTs were first analyzed. The adsorption energy and band gap were calculated to investigate the catalytic performance of C doped BNNTs with different doping sites. Meanwhile, the effect of curvature of BNNTs was also analyzed. The results indicate the C-doped BNNTs hold high catalytic activity, which was independent of doping site and curvature.

Section snippets

Computational method

The dispersion incorporated density functional theory (DFT-D) calculations were performed by using DMol³ module in material studio package [45,46]. All geometry optimizations were performed by DFT-D method with the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) as the exchange-correlation potential. The dispersion correction was described with Tkatchenko-Scheffler (TS) scheme [47,48]. Double numerical plus d-functions (DNDs) were used as the basis set. The global

Optimized structures of C-doped BNNTs

Single-walled BNNTs of armchair (n, n), with index of n = 3, 4, 6, and 8, were chosen in the study for investigating the curvature effect of the nanotubes. The BNNTs with a carbon atom substituted on the boron (C_B-BNNTs) or nitrogen (C_N-BNNTs) monovacancy, were investigated for comparison. To observe the structural difference before and after doping, the optimized geometry structures and bond lengths of pristine, C_B- and C_N- (3, 3) BNNTs are shown in Fig. 1 and Table 1, respectively. For

Conclusion

C-doped BNNTs (C_B- and C_N- BNNTs) as an effective catalyst for acetylene hydrochlorination were studied comprehensively by DFT studies. Compared with the pristine BNNTs, C-doped BNNTs were activated by the introduced C dopant. The band gap of BNNTs are narrowed by the doped C atom and electrons are accumulated around C atom which makes the C site an active catalytic center. The interactions between C-doped BNNTs and C₂H₂ are much stronger than HCl in acetylene hydrochlorination. The

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Nos. 21403003, 21403004 and 61671019).

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C-doped boron nitride nanotubes for the catalysis of acetylene hydrochlorination: A density functional theory study

Abstract

Graphical abstract

Introduction

Section snippets

Computational method

Optimized structures of C-doped BNNTs

Conclusion

Acknowledgements

Appl. Catal.

Fuel Process. Technol.

Appl. Catal.

J. Catal.

J. Catal.

Mat. Sci. Eng. R.

Mater. Chem. Phys.

Comput. Theor. Chem.

Theor. Chem. Acc.

Comput. Theor. Chem.

Appl. Surf. Sci.

Comput. Theor. Chem.

Appl. Surf. Sci.

Appl. Surf. Sci.

Sens. Actuators B

Appl. Surf. Sci.

Meeting the challenges to sustainability through green chemistry

Green Chem.

Green chemistry: principles and practice

Chem. Soc. Rev.

Green chemistry puts down roots

Nature

China goes green: cleaner production of chemicals

Green. Process. Synth.

Production of acetylene and acetylene-based chemicals from coal

Chem. Rev.

Discovery, development, and commercialization of gold catalysts for acetylene hydrochlorination

J. Am. Chem. Soc.

Reactivation of a supported gold catalyst for acetylene hydrochlorination

J. Chem. Soc. Chem. Commun.

Heterogeneous non-mercury catalysts for acetylene hydrochlorination: progress, challenges, and opportunities

Green Chem.

Self-regeneration of Au/CeO2 based catalysts with enhanced activity and ultra-stability for acetylene hydrochlorination

Nat. Commun.

Non-mercury catalytic acetylene hydrochlorination over bimetallic Au–Ba(ii)/AC catalysts

Catal. Sci. Technol.

Self-regeneration of Au/CeO₂ based catalysts with enhanced activity and ultra-stability for acetylene hydrochlorination