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.