Adsorption and catalytic destruction of trichloroethylene in hydrophobic zeolites

Abstract

Several chromium exchanged ZSM-5 zeolites of varyingSiO 2/Al 2O 3 ratio were prepared and investigated ambient (23°C) adsorption and subsequent oxidative destruction (250–400°C) of gaseous trichloroethylene (TCE, Cl 2C=CHCl) in a humid air stream. With an increase in theSiO 2/Al 2O 3 ratio from 30 to 120, the TCE saturation capacity of these dual-function sorbent/catalyst (S/C) media was found to increase from 6.0 to 10.1 wt% in a humid air stream. This phenomenon was attributed to an increase in hydrophobicity coupled with reduced steric hindrance and site competition for the adsorption of TCE molecules in the competitive adsorption of TCE and water. Ambient TCE adsorption experiments carried out in dry air showed the same trend, which was attributed to increasing organophilicity of the S/C media with an increase in the SiO 2/Al 2O 3 ratio. In order to gain knowledge of physisorption sites for TCE molecules in the ZSM-5 structure, temperature-programmed desorption over a temperature range of 30–300°C and in-situ FT-IR studies at ambient conditions were also carried out. These studies revealed that in all zeolites (except for Cr-ZSM-5 withSiO 2/Al 2O 3 ratio of 120) TCE interacted with terminal silanol (SiOH) and AlOH groups. At temperatures ≥ 300°C (with the exception of Cr-ZSM-5 withSiO 2/Al 2O 3 ratio of 120), allS/C media showed >95% TCE destruction efficiency. Based on its high adsorption capacity and high activity for oxidative destruction of TCE, it is concluded that the Cr-ZSM-5S/C medium with aSiO 2/Al 2O 3 ratio of 80 gives preferred performance both as a sorbent and a catalyst.