Spectroscopic Signatures of VOC Physisorption on Microporous Solids. Application for Trichloroethylene and Tetrachloroethylene Adsorption on MFI Zeolites O. Bertrand,* G. Weber, S. Maure, V. Bernardet, J. P. Bellat, and C. Paulin Laboratoire de Recherches sur la Re ´ actiVite ´ des Solides, UMR 5613, 9, aVenue Alain SaVary - B. P. 47870-21078 DIJON Cedex, France ReceiVed: April 8, 2004; In Final Form: May 2, 2005 This paper presents an experimental infrared spectroscopic study of the physisorption of trichloroethylene (TCE) and tetrachloroethylene (PCE) on a self-supported high silica ZSM5 zeolite. The evolution of the shape, area, and location of vibration bands of both the adsorbent and the adsorbate is analyzed with respect to the number of sorbed molecules. The state of the adsorbed phase is characterized upon adsorption by comparing the location of the investigated vibration bands with the location of the corresponding vibration bands of the chloroalkenes in gaseous, liquid, and solid phases. The singular behavior of PCE with respect to TCE is seen from the modification of vibration bands of both the adsorbed phase and the adsorbent upon loading. The adsorption process proceeds by stages for PCE, whereas it appears continuous for TCE. Particular micropore loadings are evidenced at 4 and 6.5 molec.uc -1 for PCE and at 6 molec.uc -1 for TCE, in agreement with previous macroscopic and microscopic data. In addition, the presence of admolecules induces at least one emerging vibration band located at around 1715 cm -1 , mainly due to a contribution of the microporous surface of the adsorbent. 1. Introduction Volatile organic compounds (VOCs) are the byproducts of many industrial processes. These compounds are causing concerns due to the dangers they pose to human health and to the destruction of the ozone layer in the stratosphere. 1,2 Catalytic oxidation, thermal incineration, biological treatments, or regen- erative thermal treatments are destruction methods currently used to control VOC emission. 3-6 Another interesting alternative consists of recovering the organic substances by adsorption processes. For recovery methods, organic compounds are separated from the polluted effluent, then concentrated and recycled in the industrial process. Activated carbons are often used as industrial adsorbents because of their high adsorption capacity and their interesting cost in use. In counterpart, they have disadvantages in that they are hydrophilic, inflammable, and incompletely regenerable, which hydrophobic zeolites do not show. The choice of the adsorbent mainly lies in two considerations. At first, it must have pores larger than the largest molecules to be adsorbed. This criterion is stricter for zeolites than for activated carbons or polymers because zeolites show uniform and narrower pore size distribution. Moreover, the adsorbent must exhibit a high enough adsorption affinity of the compounds but up to a certain extent to be regenerated at low temperature. One way to optimize the choice of an adsorbent for an industrial application is to characterize pure gas adsorption equilibria, then to determine physicochemical mechanisms during the adsorption process. Physisorption of organic compounds on MFI zeolites usually gives rise to classical type I isotherms. However, stepped isotherms may be sometimes observed, as for the adsorption of some aromatic compounds (benzene, toluene, ethylbenzene, p-xylene, bromobenzene...) 7-16 or some linear paraffinic hy- drocarbons (n-hexane, n-heptane). 17,18 The origin of stepped isotherms was largely discussed in the literature. 19-23 For most authors, it is related to a phase transition of the adsorbate and/ or the adsorbent. Concerning chloroalkene derivatives, we have more particu- larly studied the interaction of tetrachloroethylene (PCE) and trichloroethylene (TCE) on high silica MFI zeolites using thermogravimetry, microcalorimetry, in situ X-ray diffraction, and in situ neutron diffraction. These chlorinated solvents of molecular size close to the pore opening of the zeolite of around 0.6 nm 24,25 do not interact in the same way with MFI zeolites (Figure 1). Indeed, sorption isotherms for TCE are of type I at 298 K, whereas those for PCE display a step at half loading (4 molec.uc -1 ) and at very low relative pressure (p/p 0 ) 0.02). 26-29 Moreover, the adsorption capacity of MFI zeolites for these VOCs is higher for TCE than for PCE. The singular character of PCE was also underlined by the discontinuous shape of * Corresponding author. E-mail: odile.bertrand@u-bourgogne.fr. Figure 1. Shematic representation of the microporous network of MFI zeolites, which consists of interconnected straight and sinusoidal channels (sites I, sinusoidal channels; sites II, straight channels; sites III, intersections of straight and sinusoidal channels). 13312 J. Phys. Chem. B 2005, 109, 13312-13321 10.1021/jp048442a CCC: $30.25 © 2005 American Chemical Society Published on Web 06/18/2005