Double decarbonylation of terephthaloyl chloride: a dual versatile route to para-dichlorobenzene in liquid and gas phases

Abstract

Recent advances in innovative recycling technologies enable the efficient recovery of high-quality monomers like terephthalic acid (TPA) from polyethylene terephthalate (PET) waste. Building on these advancements, Syensqo aims to produce sustainable high-performance polymers, such as Ryton® polyphenylene sulfide (PPS), using TPA as a starting material. To address this alternative, the double decarbonylation of terephthaloyl chloride (TdCl), derived from TPA, to produce para-dichlorobenzene (p-DCB), a key monomer of PPS, was evaluated. Two types of processes were assessed: liquid and gas phase decarbonylation. Both approaches achieved a similar turnover frequency of ∼30 h⁻¹ in p-DCB. In the liquid phase, the Wilkinson catalyst RhCl(PPh₃)₃ allows the transformation of TdCl into p-DCB with a maximum yield of 90% obtained under inert and solvent-free conditions in 4 hours at 220 °C. However, this route was hindered by the formation of regulated by-products, raising environmental concerns. In the gas-phase reaction, Pd-based catalysts were used, achieving up to 95% yield to p-DCB in 1 hour at 360 °C, with a total weight hourly space velocity (WHSV) of 25 h⁻¹. Switching the catalyst support from alumina to carbon significantly enhanced the catalytic activity by promoting the formation of an active carbide phase and reducing catalyst deactivation. Finally, a full manufacturing cost (FMC) assessment from TPA to p-DCB, covering variable costs, fixed costs, and its environmental footprint, is reported.