Lanthanide compounds as catalysts for the one-step synthesis of vinyl chloride from ethylene

Abstract

The industrial manufacture of vinyl chloride relies on a two-step process involving CuCl2-catalyzed ethylene oxychlorination to ethylene dichloride followed by thermal cracking of the latter to vinyl chloride. This work evaluates a wide range of commercial and self-prepared lanthanide (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er) compounds for the one-step production of vinyl chloride from ethylene in a fixed-bed reactor at 623–773K and 1bar using feed ratios of C2H4:HCl:O2:Ar:He=3–6:1–9.6:1–7:3:80–92 and space times of 6–252ghmol−1 (based on ethylene). Ex situ characterization by X-ray diffraction, electron microscopy, and X-ray photoelectron spectroscopy reveals that the oxide forms of all compounds, except CeO2, transform into their respective (oxy)chloride. Among all studied systems, CeO2 shows the highest activity but suffers from combustion forming COx, while europium oxychloride (EuOCl) leads to the best vinyl chloride selectivity of 96% at 20% C2H4 conversion for over 100h on stream. Temperature-programmed reduction with H2, temperature-programmed desorption of NH3, and oxidation tests (C2H4, CO, and HCl oxidation) unravel the unique balance of mild redox and enhanced acid properties of EuOCl compared to CeO2, which suppress over-oxidation and boost ethylene dichloride dehydrochlorination. Strategies to couple the excellent selectivity of EuOCl with the high activity of CeO2 are demonstrated through the synthesis of homogeneous europium-cerium mixed oxides, combining two functions on a single surface. In addition, the engineering of a dual-bed reactor, integrating a CeO2 bed first to produce ethylene dichloride in high yield which is subsequently transformed to vinyl chloride over EuOCl leads to vinyl chloride yields of up to 30% per pass. These very promising findings constitute a crucial step for process intensification of polyvinyl chloride production and exploring the potential of rare-earth compounds in industrially-relevant reactions.