Catalytic Processes for Intensified Vinyl Chloride Production

Abstract

Polyvinyl chloride (PVC) is the third most widely used plastic in today’s society. The industrial production of its monomer, vinyl chloride (commonly denoted as VCM), relies on hydrochlorination of acetylene or thermal dehydrochlorination of 1,2-dichloroethane (known as ethylene dichloride, EDC) forming equimolar amounts of HCl. EDC itself is produced by ethene chlorination with Cl2 or oxychlorination with HCl, enabling its recycling. However, while the state-of-the-art CuCl2-based oxychlorination catalyst is very selective, it suffers from stability issues. In this work, three major areas of research have been targeted, in particular i) the development of a more robust ethene oxychlorination catalyst, ii) the process intensification via combining oxychlorination and dehydrochlorination functions in one material or a dual catalyst system, and iii) the evaluation of a feedstock change from ethene to the much cheaper ethane. To this end, catalytic systems and corresponding mechanistic understanding is developed by assessment of performance and kinetics, combined with in-depth molecular level modeling of the complex reaction network. In a first step, recently discovered HCl oxidation catalysts were tested in ethene oxychlorination and compared to a CuCl2-based benchmark. Among these catalysts, RuO2 exhibited a low yield of EDC and VCM due to strong combustion, while CeO2 exhibited excellent stability and enabled up to 25% VCM and 25% EDC yield in a single pass. The selectivity was attributed to the bifunctional character of ceria, where redox centers oxychlorinate ethene to EDC, which is subsequently dehydrochlorinated to VCM over acid sites generated in situ. However, combustion or the formation of 1,2-dichlorethene (DCE) could not be suppressed, and deeper mechanistic understanding was yet missing. Therefore, first computational studies of the complete ethene oxychlorination and its related combustion network were conducted on the RuO2(110) surface, a relatively simple material for density functional theory (DFT) calculations, revealing that chlorination steps are kinetically controlled, whereas oxidation steps are under thermodynamic control. This enabled a reduction of the overall reaction network for investigation on a CeO2(111) surface. Catalytic evaluation and corresponding characterization revealed that the catalyst surface contains CeOCl, while the bulk phase remains CeO2, regardless of the starting phase. A higher degree of chlorination led to increased selectivity to EDC and VCM, while decreasing the selectivity to DCE and COx (CO+CO2). Combination of DFT with steady-state experiments and temporal analysis of products (TAP) then revealed that the most likely pathway of VCM formation proceeds via a cascade reaction on a chlorinated CeO2(111) surface. In contrast, oxygen vacancies facilitate the formation of DCE, while combustion can originate from reactants and products. To overcome these mechanistic limitations, other lanthanide oxides were investigated in ethene oxychlorination, revealing that the oxide forms of all compounds, except CeO2, transformed into their respective chlorides or mainly oxychlorides. Among those, europium oxychloride (EuOCl) led to a VCM selectivity of up to 96% at 20% conversion, while CeO2 still exhibited about double the activity with 30% VCM selectivity. The outstanding selectivity of EuOCl was attributed to a unique balance of mild redox and enhanced acid properties, which suppresses oxidation and boosts EDC dehydrochlorination. However, the high price and moderate activity of EuOCl motivated further efforts to design a stable and VCM selective CeO2-based ethene oxychlorination catalyst by nanostructuring the active phase onto a suitable carrier. Among the developed materials, CeO2 on monoclinic ZrO2 emerged as an outstanding catalyst, exhibiting 100% EDC selectivity at a 10-fold increased metal efficiency with respect to bulk CeO2. Use of acidic supports, such as Al2O3 or ZSM-5, resulted in high VCM selectivities, yet combustion and coking were pronounced. The observed performances were rationalized by suitable probe reactions, evidencing that a high HCl oxidation ability and low acidity are key for active and selective EDC production catalysts. Accordingly, we implemented a dual-catalyst system, featuring CeO2/ZrO2 for EDC formation and Ca-doped Al2O3 for EDC dehydrochlorination, leading to up to 100% VCM selectivity at 25% ethene conversion. In the quest to utilize ethane instead of ethene as feedstock for VCM production, recent studies reported a wide range of ethane oxychlorination catalysts, which mainly yield ethene, while VCM remains a minor by-product. Strikingly, the same catalysts selectively form VCM under equivalent reaction conditions in ethene oxychlorination. Combining quantitative catalytic tests, TAP, and DFT on iron phosphate, the origin of these diverging selectivity patterns was revealed. In particular, co-feeding ethane in ethene oxychlorination gave evidence that the alkane suppresses VCM formation in ethene oxychlorination, which originates from the hydrocarbon competition for active sites. These observations were extended by ethane co- feeding tests in ethene oxychlorination over a wide range of known oxychlorination catalysts, and corresponding DFT calculations indicated that the described phenomenon is material independent. Overall, the fundamental insight developed in this work exemplifies how an integrated approach of catalytic synthesis, characterization, and evaluation in combination with molecular- level rationalization are able to uncover complex reaction networks, providing guidelines for future developments within industrial VCM production.