Modeling and Experimental Study on the Thermal Decomposition of Perfluorooctanesulfonic Acid (PFOS) in an α-Alumina Reactor

Pyrolysis of perfluorooctanesulfonic acid (PFOS) (200–450 PPMV) was studied in an α-alumina flow reactor under plug flow conditions at temperatures from 450 to 1000 °C and helium flow velocities of 100, 200, and 300 mL min–1. Major products at the lowest temperatures were HF, SO2, and perfluorooctanyl fluoride (C8F16O). A new low-temperature product, C2F4, was detected in significant quantities from the decomposition of PFOS. The pyrolysis mechanism was studied by quantum chemical calculation at the b3lyp/GTLarge//b3lyp/6-31G(d,p) and G4MP2 levels of theory to develop a thermochemical analysis and preliminary chemical kinetic model for the decomposition. Alongside the previously postulated initiation of pyrolysis via an α-sultone intermediate to C8F16O, we have discovered a competitive direct fission route for PFOS into C8F17 radicals and HOSO2. C8F17 rapidly fissions CF2 radicals, which are the source of the observed C2F4. At the lowest temperatures, an acceleration in the rates of production of C2F4, HF, and SO2 was observed in the α-alumina reactor when compared with predictions of the kinetic model. An alumina nanocluster was subjected to quantum chemical analysis to show that PFOS can be both chemisorbed and physisorbed on an alumina surface, a process that might explain the experimental observation of an acceleration of decomposition in an α-alumina reactor at low temperatures.