An investigation of the conformational behavior of a chlorinated hydrocarbon, 1,1,2-trichloroethane, adsorbed in zeolites by Raman spectroscopy

Abstract

The conformational behavior of 1,1,2-trichloroethane (TCE) adsorbed in two representative zeolites (silicalite-1 and zeolite L) was investigated by Fourier transform (FT)-Raman spectroscopy in combination with thermogravimetric analysis (TGA) and powder X-ray diffraction (XRD). The results indicate that the conformational properties of the TCE molecules inside a zeolitic host are strongly influenced by both the framework topology and the existence of charge balancing cations. When TCE is adsorbed inside completely siliceous silicalite-1, the population of the trans conformer increases significantly relative to pure liquid. Adsorption of TCE also induces a phase transition from the monoclinic to orthorhombic crystal system in the framework of silicalite-1 at high loading levels. At low loading, the TCE molecules are adsorbed inside the channels of silicalite-1. When at high loading levels, TCE molecules reside at both the channel intersection as well as the midsection of the channels. For zeolite L, the presence of an extra-framework cation shows a strong influence on TCE conformation. The strong interaction of the cation with the large dipole moment of the gauche isomer leads to a large increase in the gauche population. Raman data provide direct evidence that the cations in the framework interact much more strongly with the gauche isomer than the trans isomer.