Dechlorination of pentachlorophenol in supercritical carbon dioxide with a zero-valent silver–iron bimetallic mixture

Abstract

A continuous stream of pentachlorophenol (PCP, 10–20 mg min⁻¹) in supercritical carbon dioxide (scCO₂) was dechlorinated efficiently by a heated column (25 × 1 cm diameter) of a zero-valent silver–iron (Ag⁰/Fe⁰) bimetallic mixture. Dechlorination efficiencies in successive 10 min fractions of reactor eluate were influenced appreciably by the temperature and pressure within the reactor column(s), the flow rate of the mobile phase and especially by the composition of the feedstock solvent. During 1 h of operation at 450 °C, organically-bound chlorine was liberated, virtually quantitatively, from a 20% (w/v) feedstock stream (0.1 ml min⁻¹ merged with 4 ml min⁻¹ scCO₂), and deposited as chloride ion on the surfaces of the metal particles. Sea sand, maintained under identical conditions, was capable of dechlorinating the substrate only partially (50% loss of the GC peak area for substrate; only partially dechlorinated products). By contrast, the dechlorination was virtually quantitative with the Ag⁰/Fe⁰ support. There was no evidence of PCP substrate or chlorinated aromatics among the products. In addition to approximately equal quantities of phenol and methylated phenols [o-and m-cresol; dimethyl- (2,4-, 2,6-, 3,4-, 3,5-, 3,5-dimethyl)phenol and trimethyl-(2,3,6-, 2,4,6-trimethyl)phenol] the remaining 5% of the product mixture consisted of methylated benzenes (m- and p-xylene, 1,2,4-trimethyl-, pentamethyl- and hexamethyl-benzene). If the feedstock solvent was changed from methanol to propan-2-ol, the product distribution was changed to ca. 50% phenol, 36% 1,2,4-trimethylbenzene and the remainder methylated phenols. In extended operation the reactor was run continuously for 14 h without apparent loss of activity provided that the chloride was washed from the Ag⁰/Fe⁰ metal surfaces at ca. 3 h intervals.