Synergistic effect between nitrogen-doped sites and metal chloride for carbon supported extra-low mercury catalysts in acetylene hydrochlorination

Abstract

Carbon-supported mercury catalysts are extensively employed in calcium carbide-based polyvinyl chloride (PVC) industries, but the usage of mercury-based catalysts can pose an environmental threat due to the release of mercury into the surrounding area during the operation period. In this study, a highly active and stable mercury-based catalyst was developed, utilizing the nitrogen atom of the support as the anchor site to enhance the interaction between active sites (HgCl₂) and the carbon support (N-AC). Thermal loss rate testing and thermogravimetric analysis results demonstrate that, compared to commercial activated carbon, N-doped carbon can effectively increase the heat stability of HgCl₂. The obtained mercury-based catalysts (HgCl₂/N-AC) exhibit significant catalytic performance, achieving 2.5 times the C₂H₂ conversion of conventional HgCl₂/AC catalysts. Experimental analysis combined with theoretical calculations reveals that, contrary to the Eley-Rideal (ER) mechanism of HgCl₂/AC, the HgCl₂/N-AC catalyst follows the Langmuir-Hinshelwood (LH) adsorption mechanism. The nitrogen sites and HgCl₂ on the catalyst enhance the adsorption capabilities of the HCl and C₂H₂, thereby improving the catalytic performance. Based on the modification of the active center by these solid ligands, the loading amount of HgCl₂ on the catalyst can be further reduced from the current 6.5% to 3%. Considering the absence of successful industrial applications for mercury-free catalysts, and based on the current annual consumption of commercial mercury chloride catalysts in the PVC industry, the widespread adoption of this technology could annually reduce the usage of chlorine mercury by 500 tons, making a notable contribution to mercury compliance, reduction, and emissions control in China. It also serves as a bridge between mercury-free and low-mercury catalysts. Moreover, this solid ligand technology can assist in the application research of mercury-free catalysts.