Full length articleScreening of active center and reactivity descriptor in acetylene hydrochlorination on metal-free doped carbon catalysts from first principle calculations
Introduction
The Polyvinyl chloride (PVC) is the raw chemical materials for a wide range of engineering plastics [[1], [2], [3], [4], [5]]. Generally, PVC is obtained from the polymerization of vinyl chloride monomer (VCM). There are two major routes to VCM production which is the coal-based hydrochlorination of acetylene and the oil-based oxychlorination of ethane respectively [4,6,7]. In China which has a large reservoir of coal, VCM is mainly obtained from hydrochlorination of acetylene. The conventional catalyst for hydrochlorination of acetylene is the supported mercury chloride. However, the mercury based catalysts is toxic and prone to sublimation which is hazardous to human health and detrimental to the environmental protection [[1], [2], [3]]. It has been pointed out that the loss of mercury from the commercial production of VCM is over 600 tons per year, which is more than 50% of the world expenditure of mercury [3]. The latest Minamata accord signed in Japan, where the substitution for mercuric chloride in the acetylene hydrochlorination reaction was mandated [[8], [9], [10]]. Hence, it is indispensable to develop more efficient and environmental-friend catalysts for the sustainable development of PVC industry. Various alternative noble metal chloride catalysts to mercury have been proposed such as RuCl3, K2PdCl4, and AuCl3 [[11], [12], [13], [14]]. Among them, gold based catalysts arouse the intensive research interests due to its extraordinary catalytic performance [3,10]. On the other hand, the gold-based non-mercury catalysts are still facing several challenges such as stability, rare reservoir, and high price which hinder its commercialization [15]. Therefore, searching for high efficiency and low cost catalysts is crucial to the sustainable VCM production process.
The nanostructured carbon materials such as carbon nanotube, graphene, and nanodiamond have been demonstrated to be effective metal-free catalysts in various catalytic reactions such as oxidative dehydrogenation [[16], [17], [18], [19], [20], [21]], oxygen reduction reaction [[22], [23], [24], [25]], direct dehydrogenation [26], and CO2 electoreduction reaction etc. [[27], [28], [29], [30]]. Furthermore, the properties of the nanostructured carbon materials can be further adjusted by heteroatom doping. Boron and nitrogen are two most often used dopants because they have similar atomic radius with carbon which can potentially enable them to enter carbon matrix. On the other hand, boron and nitrogen has one less or more valence electron than carbon which resembles p or n doping respectively. It is expected that boron and nitrogen doping will bring the distinct effects to carbon host. Furthermore, doping pyridinic nitrogen atoms around the vacancy site in graphene enhance the catalytic performance of Pd catalyst in CO oxidation as documented in recent literature report [31]. In fact, the boron and nitrogen doping exactly exhibit the opposite effects in CO oxidation reaction on the supported single Au catalyst on carbon nanotube as revealed from our previous work [32]. The boron and nitrogen doping create negative and positive charged Au atom which consequently influence the reactants adsorption and reaction mechanism.
Recently, the nitrogen doped carbon materials reveal high conversion, high selectivity, and excellent stability in acetylene hydrochlorination reaction which is summarized in Table S1. The outcome from these studies indicates the promising future of metal-free doped carbon catalysts to replace the conventional metal catalysts. For example, Zhou et al. applied nitrogen doped carbon nanotube as catalyst in acetylene hydrochlorination reaction. The catalyst show decent TOF and excellent stability. Moreover, they found there is a linear relation between quaternary nitrogen with the conversion. Therefore, the quaternary nitrogen is identified as active center in reaction which is supported by DFT calculations [33]. Li et al. grow a thin carbon film above SiC substrate which has graphene flakes at the outer layers and nitrogen is doped at same time. The synthesized carbon/SiC composite has a remarkable performance in acetylene hydrochlorination which delivers a 80% conversion and 98% selectivity at 200 °C and keeps stable performance up to 150 h. Interestingly, the active center is proposed to be pyrrolic nitrogen, more precisely the carbons atom bonding with pyrrolic nitrogen, from the analysis of model compound and DFT calculations in this work [8]. In another work, Chao et al. used nitrogen doped ZIF-8 as the catalyst for acetylene hydrochlorination. The catalysts show an acetylene conversion of 92% at 220 °C and a 200 h long-term test indicates the super stability of the catalyst. Moreover, they identifies that the carbons bonding with pyridine nitrogen are the active sites in the reaction [9]. From these studies and others shown in Table S1, it can be concluded that the nitrogen doped carbon materials are indeed highly efficient and stable catalysts for acetylene hydrochlorination reaction. On the other hand, it is completely lacking an agreement on the possible active sites on the nitrogen doped carbon catalysts in acetylene hydrochlorination. The pyridinic, quaternary and pyrrolic nitrogens or carbon atoms around nitrogen are proposed to be the active center. This uncertainty seriously hinders the understandings on the reaction mechanism and further optimization of catalytic performance. Furthermore, there is still missing an accountable description on the reaction pathway and mechanism of acetylene hydrochlorination on the doped carbon catalysts.
In current work, the comprehensive first principle calculations on the role of different boron/nitrogen dopants and reaction mechanism in acetylene hydrochlorination on the doped carbon catalysts are performed. Although there is no reported boron doped carbon catalysts used in acetylene hydrochlorination, the inclusion of boron dopants will probably contribute to the optimization strategy of catalytic performance and the comparison between nitrogen doping will deepen the doping effect. Graphitic, pyridine, pyrrolic boron/nitrogen doped carbon nanotube and graphene are used to model the carbon catalysts. The interactions between acetylene and the doped carbon catalysts are carefully explored. The compassion of the binding energy of acetylene reveals the distinct role of various dopants. In particularly, the opposite effect of boron and nitrogen dopants is verified again for the acetylene adsorption. Furthermore, the reaction mechanism was explored for acetylene hydrochlorination and the important structures and reaction barrier along pathway are identified. The most active dopant is determined from the calculated barriers and a reactivity descriptor is proposed for screening the potential active doped carbon catalysts. Current work sheds light on the discovery of novel metal free catalyst based on doped carbon for acetylene hydrochlorination and expands the scope of the doping strategy for the carbon catalysts.
Section snippets
Computational methods
The calculations reported here were performed by using periodic, spin-polarized density functional theory (DFT) as implemented in the form of Vienna ab initio simulation package (VASP) [[34], [35], [36]]. For valence electrons, a plane-wave basis set was adopted with an energy cutoff of 400 eV and the ionic cores were described with the projector augmented-wave (PAW) method [37,38]. The Revised Perdew-Burke-Ernzerhof (RPBE) functional was used as the exchange-correlation functional
Results and discussion
The adsorptions of reactants, C2H2 and HCl, have apparent importance during the reaction. On the conventional gold catalysts, it is suggested that C2H2 has stronger adsorption energy than HCl and reaction begins with the adsorption of C2H2 via Eley-Rideal (ER) reaction mechanism [5,45,46].
To thoroughly investigate the effect of dopants on the adsorption of C2H2 and HCl, ten different sites on carbon nanotube are explored as the adsorption site as shown in Fig. 1 and Fig. S1 (HCl adsorption). In
Conclusions
In this work, the first principle calculations are performed to identify the active site and clarify the reaction pathway on the doped carbon catalyst in acetylene hydrochlorination reaction. Various nitrogen and boron configurations including pyridine, graphitic, and pyrrolic are under considerations. PDOS analysis reveals that the pyridine with three nitrogen/boron around a vacancy site has more populations around Fermi level than the others and has better reactivity. Acetylene molecule has
Abbreviations
- N
nitrogen
- B
boron
- C2H2
acetylene
- HCl
Hydrogen chloride
- d-d
dopant-dopant
- d-c
dopant-carbon
- c-c
carbon-carbon
- DFT
density functional theory
- PDOS
partial density of states
- HOMO
Highest molecular orbital
- LUMO
Lowest molecular orbital
- TS
transition state
Acknowledgment
This work is supported by NSFC (21573255, 51521091), “Strategic Priority Research Program” of the Chinese Academy of Sciences, Grant No. XDA09030103, Liaoning Provincial Natural Science Foundation (20180510014). B. L. thanks the financial grant from the Institute of Metal Research, Chinese Academy of Sciences (Y3NBA211A1). This work is also supported by State Key Laboratory of Catalytic Materials and Reaction Engineering (RIPP, SINOPEC). The computations are also supported by the Special
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