Dechlorination of pentachlorophenol in supercritical carbon dioxide with azero-valent silver-iron bimetallic mixture

A continuous stream of pentachlorophenol (PCP, 10-20 mg min(-1)) in supercr itical carbon dioxide (scCO(2)) was dechlorinated efficiently by a heated c olumn (25 x 1 cm diameter) of a zero-valent silver-iron (Ag-0/Fe-0) bimetal lic 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 especia lly by the composition of the feedstock solvent. During 1 h of operation at 450 degreesC, organically-bound chlorine was liberated, virtually quantita tively, from a 20% (w/v) feedstock stream (0.1 ml min(-1) merged with 4 ml min(-1) scCO(2)), and deposited as chloride ion on the surfaces of the meta l particles. Sea sand, maintained under identical conditions, was capable o f dechlorinating the substrate only partially (50% loss of the GC peak area for substrate; only partially dechlorinated products). By contrast, the de chlorination was virtually quantitative with the Ag-0/Fe-0 support. There w as no evidence of PCP substrate or chlorinated aromatics among the products . In addition to approximately equal quantities of phenol and methylated ph enols [o-and m-cresol; dimethyl- (2,4-, 2,6-, 3,4-, 3,5-, 3,5-dimethyl)phen ol and trimethyl-(2,3,6-, 2,4,6-trimethyl)phenol] the remaining 5% of the p roduct mixture consisted of methylated benzenes (m- and p-xylene, 1,2,4-tri methyl-, 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 w ithout apparent loss of activity provided that the chloride was washed from the Ag-0/Fe-0 metal surfaces at ca. 3 h intervals.