A novel, non-metallic graphitic carbon nitride catalyst for acetylene hydrochlorination

Current status and perspective of metal-free materials as catalysts in acetylene hydrochlorination: active site, dopant, and mechanism

Acetylene hydrochlorination is one of the main catalytic routes for vinyl chloride monomer (VCM) production, in particular for coal-rich regions, with mercury or noble metal chlorides as conventional catalysts. In recent years, there is a fast-growing transition from metal-based catalysts to metal-free materials for acetylene hydrochlorination because of strict environmental regulation and strong motivation for sustainable development, and this trend is clearly exemplified in a series of studies on carbon, boron nitride, graphitic carbon nitride, and ionic liquids, which garnered significant interest as catalysts in acetylene hydrochlorination. In this review, the current status and development of these metal-free catalysts are summarized with a focus on the nature of active sites, doping effects, reaction mechanism, and optimization methods, in order to provide a timely account. For carbon materials, the essential role of nitrogen dopants in catalytic performance is elaborated and possible active nitrogen species are explored, and the dual dopant strategy is discussed in the frame of synergetic effects which could further boost the activity and stability. For the other metal-free materials, they exhibited a different pattern regarding reactant adsorption and reaction mechanism from the carbon catalysts, in particular a dual-site mechanism is found due to the balanced adsorption between HCl and C2H2. For each metal-free material, a short discussion is provided at the end of the section to shed light on the unique property which uncovered the difference from not only conventional metal catalysts but also the other metal-free materials. In the end, the key issues preventing the further improvements of metal-free catalysts and the practical way to replace metal catalysts are discussed. Overall, the current work paves the way for the future development of metal-free catalysts for acetylene hydrochlorination.

A low-temperature active and selective bimetallic Cu-In catalysts for hydrogenation of methyl 3-hydroxypropionate to 1,3-propanediol

The pathway for producing 1,3-propanediol (1,3-PDO) from methyl 3-hydroxypropionate (3-HPM) has great application potential. However, the reaction is sensitive to temperature and results in reduced product selectivity at high temperatures. This study explores the use of low-temperature active Cu-In bimetallic catalysts for the 3-HPM reaction. The Cu-1In/SiO2 catalyst exhibits superior catalytic performance with a 91.5 % yield of 1,3-PDO, surpassing that of the Cu/SiO2 catalyst by 264 % under identical conditions. Multiple characterization methods reveal the textural and physiochemical properties of the catalysts. The excellent catalytic performance of Cu-1In/SiO2 can be attributed to the introduction of CuIn alloy and highly dispersed In2O3. The interaction between copper and Indium species on the catalyst surface facilitates the dispersion of Cu species, while simultaneously highly dispersed In2O3 introducing new adsorption sites for reactants, thereby greatly improving its catalytic performance.

Atomic Cu-N-P-C Active Complex with Integrated Oxidation and Chlorination for Improved Ethylene Oxychlorination

Fine constructing the chemical environment of the central metal is vital in developing efficient single-atom catalysts (SACs). Herein, the atomically dispersed Cu on the N-doped carbon is modulated by introducing CuP moiety to CuNC SAC. Through fine-tuning with another heteroatom P, the Cu SAC shows the superior performance of ethylene oxychlorination. The Cu site activity of Cu-NPC is four times higher than the P-free Cu-NC catalyst and 25 times higher than the Ce-promoted CuCl₂ /Al₂ O₃ catalyst in the long-term test (>200 h). The selectivity of ethylene dichloride can be splendidly kept at ≈99%. Combined experimental and simulation studies provide a theoretical framework for the coordination of Cu, N, and P in the complex active center and its role in effectively catalyzing ethylene oxychlorination. It integrates the oxidation and chlorination reactions with superior catalytic performance and unrivaled ability of corrosive-HCl resistance. The concept of fine constructing with another heteroatom is anticipated to provide with inspiration for rational catalyst design and expand the applications of carbon-based SACs in heterogeneous catalysis.

Recognizing the best catalyst for a reaction

Heterogeneous catalysis is immensely important, providing access to materials essential for the well-being of society, and improved catalysts are continuously required. New catalysts are frequently tested under different conditions making it difficult to determine the best catalyst. Here we describe a general approach to identify the best catalyst using a data set based on all reactions under kinetic control to calculate a set of key performance indicators (KPIs). These KPIs are normalized to take into account the variation in reaction conditions. Plots of the normalized KPIs are then used to demonstrate the best catalyst using two case studies: (i) acetylene hydrochlorination, a reaction of current interest for vinyl chloride manufacture, and (ii) the selective oxidation of methane to methanol using O₂ in water, a reaction that has attracted very recent attention in the academic literature.

Cationic Covalent Triazine Network: A Metal-Free Catalyst for Effective Acetylene Hydrochlorination

Vinyl chloride, the monomer of polyvinyl chloride, is produced primarily via acetylene hydrochlorination catalyzed by environmentally toxic carbon-supported HgCl2. Recently, nitrogen-doped carbon materials have been explored as metal-free catalysts to substitute toxic HgCl2. Herein, we describe the development of a cationic covalent triazine network (cCTN, cCTN-700) that selectively catalyzes acetylene hydrochlorination. cCTN-700 exhibited excellent catalytic activity with initial acetylene conversion, reaching ~99% and a vinyl chloride selectivity of >98% at 200 °C during a 45 h test. X-ray photoelectron spectroscopy, temperature programmed desorption, and charge calculation results revealed that the active sites for the catalytic reaction were the carbon atoms bonded to the pyridinic N and positively charged nitrogen atoms (viologenic N+) of the viologen moieties in cCTN-700, similar to the active sites in Au-based catalysts but different from the those in previously reported nitrogen-doped carbon materials. This research focuses on using cationic covalent triazine polymers for selective acetylene hydrochlorination.

Deactivation and Regeneration of Nitrogen Doped Carbon Catalyst for Acetylene Hydrochlorination

The poor stability of carbon materials doped with nitrogen limited their development in acetylene hydrochlorination. Therefore, investigating the deactivation reasons of carbon catalysts and researching regeneration methods became the research focus. Herein, carbon-nitrogen materials were synthesized by one-step pyrolysis, which using biomass materials with high nitrogen content, the synthesized material was used in an acetylene hydrochlorination reaction. The acetylene conversion rate of D-GH-800 catalyst was up to 99%, but the catalytic activity decreased by 30% after 60 h reaction. Thermogravimetric analysis results showed that the coke content was 5.87%, resulting in catalyst deactivation. Temperature-programmed desorption verified that the deactivation was due to the strong adsorption and difficult desorption of acetylene by the D-GH-800 catalyst, resulting in the accumulation of acetylene on the catalyst surface to form carbon polymers and leading to the pore blockage phenomenon. Furthermore, based on the catalyst deactivation by carbon accumulation, we proposed a new idea of regeneration by ZnCl₂ activation to eliminate carbon deposition in the pores of the deactivated catalyst. As a result, the activity of D-GH-800 was recovered, and lifetime was also extended. Our strategy illustrated the mechanism of carbon deposition, and the recoverability of the catalyst has promising applications.

The Effect of sp2 Content in Carbon on Its Catalytic Activity for Acetylene Hydrochlorination

We report the influence of sp² content in carbon catalyst on the catalytic activity for acetylene hydrochlorination. Nanodiamonds (NDs) were used as the precursor and calcinated under different temperatures. The resulting ND500, ND700, ND900, and ND1100 catalysts were characterized, and the sp² content increased with increasing calcination temperature. The specific activities of the catalysts first increased and then decreased with increasing sp² content. The highest catalytic activity could be obtained in the ND-900 catalyst with a sp² value of 43.9%. The density functional theory results showed that the adsorption sites for acetylene and hydrogen chloride were located at the interface between sp² and sp³ configuration.

Construction of highly dispersed Au active sites by ice photochemical polishing for efficient acetylene hydrochlorination

Compared to traditional Au/AC, ice-photochemical polishing results in atomically dispersed AuClx-like multi-sites, yielding a significantly improved performance of Au/AC-F1I1 catalysts.

Sulfur Promotion in Au/C Catalyzed Acetylene Hydrochlorination

The formation of highly active and stable acetylene hydrochlorination catalysts is of great industrial importance. The successful replacement of the highly toxic mercuric chloride catalyst with gold has led to a flurry of research in this area. One key aspect, which led to the commercialization of the gold catalyst is the use of thiosulphate as a stabilizing ligand. This study investigates the use of a range of sulfur containing compounds as promoters for production of highly active Au/C catalysts. Promotion is observed across a range of metal sulfates, non-metal sulfates, and sulfuric acid treatments. This observed enhancement can be optimized by careful consideration of either pre- or post-treatments, concentration of dopants used, and modification of washing steps. Pre-treatment of the carbon support with sulfuric acid (0.76 m) resulted in the most active Au/C in this series with an acetylene conversion of ≈70% at 200 °C.

Constructing green mercury-free catalysts with single pyridinic N species for acetylene hydrochlorination and mechanism investigation

A green bifunctional polymer for acetylene hydrochlorination is directly used as a catalyst and then used as a precursor to prepare an N-doped carbon catalyst.

Waste cigarette butt-derived nitrogen-doped porous carbon as a non-mercury catalyst for acetylene hydrochlorination

The development of advanced carbon materials as metal-free catalysts holds great importance for mercury catalyst replacement in acetylene hydrochlorination.

Nitrogen-Doped Carbon-Assisted One-pot Tandem Reaction for Vinyl Chloride Production via Ethylene Oxychlorination

A bifunctional catalyst comprising CuCl2 /Al2 O3 and nitrogen-doped carbon was developed for an efficient one-pot ethylene oxychlorination process to produce vinyl chloride monomer (VCM) up to 76 % yield at 250 °C and under ambient pressure, which is higher than the conventional industrial two-step process (≈50 %) in a single pass. In the second bed, active sites containing N-functional groups on the metal-free N-doped carbon catalyzed both ethylene oxychlorination and ethylene dichloride (EDC) dehydrochlorination under the mild conditions. Benefitting from the bifunctionality of the N-doped carbon, VCM formation was intensified by the surface Cl*-looping of EDC dehydrochlorination and ethylene oxychlorination. Both reactions were enhanced by in situ consumption of surface Cl* by oxychlorination, in which Cl* was generated by EDC dehydrochlorination. This work offers a promising alternative pathway to VCM production via ethylene oxychlorination at mild conditions through a single pass reactor.

Progress and Challenges of Mercury-Free Catalysis for Acetylene Hydrochlorination

Activated carbon-supported HgCl2 catalyst has been used widely in acetylene hydrochlorination in the chlor-alkali chemical industry. However, HgCl2 is an extremely toxic pollutant. It is not only harmful to human health but also pollutes the environment. Therefore, the design and synthesis of mercury-free and environmentally benign catalysts with high activity has become an urgent need for vinyl chloride monomer (VCM) production. This review summarizes research progress on the design and development of mercury-free catalysts for acetylene hydrochlorination. Three types of catalysts for acetylene hydrochlorination in the chlor-alkali chemical industry are discussed. These catalysts are a noble metal catalyst, non-noble metal catalyst, and non-metallic catalyst. This review serves as a guide in terms of the catalyst design, properties, and catalytic mechanism of mercury-free catalyst for the acetylene hydrochlorination of VCM. The key problems and issues are discussed, and future trends are envisioned.

Synergistic effect of two action sites on a nitrogen-doped carbon catalyst towards acetylene hydrochlorination

Whether the reaction pathway is steady or dynamic over the whole life cycle of a catalyst process can facilitate our understanding of its catalytic behavior. Herein, the dynamic reaction pathways of nitrogen-doped carbon catalysts are investigated in acetylene hydrochlorination. When triggered, the reaction follows the Langmuir-Hinshelwood mechanism with pyrrolic N and pyridinic N as dual active sites. However, pyridinic N is deactivated first, due to the strong adsorption of hydrogen chloride, causing the reaction to further run with pyrrolic N as the single active site and follow the Eley-Rideal mechanism. This work provides a new promising way to study the catalytic behavior of nitrogen-doped carbon catalysts.

Sulphur-doped activated carbon as a metal-free catalyst for acetylene hydrochlorination

A series of sulfur-doped spherical activated carbon (SAC) catalysts were prepared with phenyl disulfide (C12H10S2) as a sulfur source for acetylene hydrochlorination. The S-doped catalyst exhibits preferable catalytic performance compared to that of the blank carrier with the reaction conditions of GHSV of 90 h-1 and at 180 °C. The catalysts were characterized by N2 adsorption/desorption (BET), elemental analysis (EA), thermogravimetric analysis (TG), temperature-programmed desorption (TPD), Raman spectrum (Raman) and X-ray photoelectron spectroscopy (XPS). The results indicate that the presence of sulfur species is favorable to promote the ability of reactant adsorption and inhibit carbon deposition. In addition, the electronic and chemical properties of catalysts were investigated by density functional theory (DFT) simulation. It is illustrated that the introduction of sulfur species can not only change the spin density and charge density but also create more active sites on a carrier. The single sulfur doped carbon material catalysts were designed for the first time and the desirable results make it a green catalyst for the industrial application of acetylene hydrochlorination.

Understanding Surface Basic Sites of Catalysts: Kinetics and Mechanism of Dehydrochlorination of 1,2-Dichloroethane over N-Doped Carbon Catalysts

The production of vinyl chloride (VCM) by pyrolysis of 1,2-dichloroethane (DCE) is an important process in the ethylene-based poly(vinyl chloride) industry. The pyrolysis is performed at temperatures above 500 °C, gives low conversions, and has high energy consumption. We have shown that N-doped carbon catalysts give excellent performances in DCE dehydrochlorination at 280 °C. The current understanding of the active sites, mechanism, and kinetics of DCE dehydrochlorination over N-doped carbon catalysts is limited. Here, we showed that pyridinic-N on a N-doped carbon catalyst is the active site for catalytic production of vinyl chloride monomer from DCE. The results of CO2 and DCE temperature-programmed desorption experiments showed that the pyridinic-N catalytic sites are basic, and the mechanism of dehydrochlorination on a N-doped carbon catalyst involves a carbanion. A kinetic study of dehydrochlorination showed that the surface reaction rate on the N-doped carbon catalyst was the limiting step in the catalytic dehydrochlorination of DCE. This result enabled clarification of the dehydrochlorination mechanism and optimization of the reaction process. These findings will stimulate further studies to increase our understanding of the relationship between the base strength and catalytic performance. The results of this study provide a method for catalyst optimization, namely modification of the amount of pyridinic-N and the base strength of the catalyst, to increase the surface reaction rate of DCE dehydrochlorination on N-doped carbon catalysts.

Nitrogen-doped porous carbon from biomass with superior catalytic performance for acetylene hydrochlorination

Acetylene hydrochlorination is an important aspect of the industrial synthesis of polyvinyl chloride, but it requires a toxic mercury chloride catalyst. Here we report a green, highly efficient and low cost nitrogen-doped soybean meal carbon (SBMC) catalyst obtained from the simple carbonization of biomass soybean meal (SBM) in the presence of zinc chloride. This material exhibits excellent catalytic performance during acetylene hydrochlorination, with an initial acetylene conversion greater than 99% and 98% selectivity for vinyl chloride at 200 °C over 110 h. Analyses by X-ray photoelectron spectroscopy and temperature programmed desorption as well as catalytic activity evaluations show that pyridinic species are the active sites for hydrogen chloride, while pyrrolic N species are the main active sites for acetylene. An analysis of charge calculations based on model catalysts further indicates that the activity of pyrrolic N species essentially determines the performance of the SBMC catalyst. This investigation of the mechanism of acetylene hydrochlorination over SBMC confirms that such nitrogen-doped catalysts have two different active sites for the adsorption and activation of hydrogen chloride and acetylene molecules. This mechanism is different from that associated with metal chloride catalysts such as HgCl₂. This SBMC catalyst is a potential alternative to HgCl₂@AC catalysts for vinyl chloride synthesis and suggests a new means of designing carbon catalysts with basic surfaces for acetylene hydrochlorination.

A green, highly efficient and low-cost nitrogen-doped soybean metal carbon (SBMC) catalyst obtained from the simple carbonization of biomass soybean meal (SBM) in the presence of zinc chloride.

Boron-doped carbon nanodots dispersed on graphitic carbon as high-performance catalysts for acetylene hydrochlorination

Boron-doped carbon nanodot materials, comprising evenly distributed BC₃-nanodots in a layered carbon matrix, are prepared through a pre-assembly assisted carbonization synthetic strategy.

Single-Atom X/g-C3N4(X = Au1, Pd1, and Ru1) Catalysts for Acetylene Hydrochlorination: A Density Functional Theory Study

The mechanisms of the single-atom X/g-C3N4(X = Au1, Pd1, and Ru1) catalysts for the acetylene hydrochlorination reaction were systematically investigated using the density functional theory (DFT) B3LYP method. The density functional dispersion correction obtained by the DFT-D3 method was taken into account. During the reaction, C2H2 and HCl were well activated and the analysis of the adsorption energy demonstrated the adsorption performance of C2H2 is better than that of HCl. The catalytic mechanisms of the three catalysts consist of one intermediate and two transition states. Moreover, our results showed that the three single-atom catalysts improve the catalytic activity of the reaction to different degrees. The calculated energy barrier declines in the order of Pd1/g-C3N4 > Ru1/g-C3N4 > Au1/g-C3N4, and the energy barrier for the Au1/g-C3N4 catalyst was only 13.66 kcal/mol, proving that single-atom Au1/g-C3N4 may be a potential catalyst for hydrochlorination of acetylene to vinyl chloride.

Acetylene hydrochlorination over boron-doped Pd/HY zeolite catalysts

A novel boron-doped Pd/HY zeolite catalyst for acetylene hydrochlorination was prepared and exhibited an outstanding catalytic performance (the acetylene conversion was maintained at >95% for about 30 h). The boron species can stabilize catalytically active Pd²⁺ species and weaken carbon deposition and Pd²⁺ reduction during the reaction, thus improving the catalytic stability.

B doping partly weakens carbon deposition and Pd²⁺ reduction, thus enhancing catalytic stabilities of Pd/HY catalysts for acetylene hydrochlorination.

Highly effective carbon-supported gold-ionic liquid catalyst for acetylene hydrochlorination

The sulfur-containing ionic liquid (IL) trimethylsulfonium iodide (C3H9SI) was used to synthesize an efficient non-mercuric catalyst with HAuCl4·4H2O as a precursor and spherical active carbon (SAC) as a support. Various Au-IL/SAC catalysts were synthesized using the incipient wetness impregnation technique and applied to acetylene hydrochlorination. The 0.3% Au-IL/SAC catalyst showed the best catalytic performance, with an acetylene conversion of 90% at a temperature of 170 °C and gas hourly space velocity (GHSV) of 360 h-1 using water as the solvent. The catalyst also displayed excellent long-term stability: C2H2 conversion was maintained at 97% for up to 200 h (T = 170 °C, GHSV = 90 h-1). Brunauer-Emmett-Teller surface area, thermogravimetric analysis, temperature programmed desorption, X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy results together showed that the C3H9SI additive significantly improved the dispersion of Au species and inhibited coke deposition on the catalyst surface during the acetylene hydrochlorination reaction. The superior activity and stability of the Au-IL/SAC catalyst make it a green catalyst for the industrial application of acetylene hydrochlorination.

Efficient treatment of brine wastewater through a flow-through technology integrating desalination and photocatalysis

Many current treatments for brine wastewaters are energy-intensive, chemical-intensive, and involve independent process in the removal of salts and contaminants. We demonstrate that through the integration of capacitive deionization and photocatalysis reactions within carbon nanotubes (CNTs) based membrane system, we are able to realize the purification and desalination of wastewaters via single-step, energy-efficient, and environmentally friendly route. We firstly designed the membrane system consisting of graphitic carbon nitride (g-C₃N₄), CNTs membrane, and poly(vinyl alcohol)-formaldehyde (PVF) foam. Then, two identical membrane systems were used as permeable electrodes and photocatalytic microreactors to construct the flow-through setup. The tests of the setup with a variety of dye solution, antibiotics solution, and actual wastewaters prove that wastewaters passing through the setup promptly turn to clean water with significantly decreased salinity. This is because the setup can use C₃N₄ modified CNTs membrane to adsorb organic contaminants and inorganic ions and decompose contaminants via photocatalysis reactions. In addition, by discharging the setup, its adsorption capacity towards salts is easily recovered. Consequently, the flow-through setup is observed to exhibit stable performance for concurrent removal of organic contaminants and inorganic salts in multiple cycles.