Volatile organic compounds (VOC’s) are considered as one of the main air pollutants, and legislation has already been introduced to reduce their emission in many countries. In addition to main emission sources such as petroleum industries, there are many local sources such as painting, printing and laundering etc. which emit low concentration VOC’s.
Modified adsorption system (MAS) was suggested to control VOC’s stream with low concentration and high flow rate. The MAS was based on the procedure of adsorption followed by treatment of the concentrated VOC such as incineration (catalytic or thermal) or recovery. Moreover there have been attempts to develop a dual-functional adsorbent/catalyst medium which could integrate adsorption and catalytic incineration unit to simplify the control unit and maximize the energy efficiency.
In this study, toluene (target VOC) adsorption/desorption tests over several adsorbents (hydrophobic zeolites and activated carbon) were carried out to select a good VOC adsorbent for a dual-functional adsorbent/catalyst medium. Hydrophobic zeolite HY was found to show high adsorption capacity and good desorption features, comparable to activated carbon and other hydrophobic zeolites with different structure. Therefore transition metal loaded zeolite HY was proposed as a dual functional adsorbent/catalyst medium. The Ag/HY catalyst among M/HY catalysts (M=Fe, Ni, Mn, Cu, Cr, Co, Ag) showed the lowest temperature for the complete conversion of toluene as measured by the temperature programmed surface reaction (TPSR). Therefore Ag/HY could be selected as a dual functional adsorbent/catalyst medium.
Activity of Ag(6wt%)/HY catalyst for the deep oxidation of toluene increased with the Si/Al ratio of zeolite HY. Byproducts such as benzene, CO, and coke were produced during toluene oxidation over catalysts using zeolite HY with the low Si/Al ratio. Therefore zeolite HY(80) with the largest Si/Al ratio was favored as catalyst support for the deep oxidation of toluene.
Activity of Ag/HY(80) catalyst for the deep oxidation of toluene increased with the Ag loading. The characterizations of Ag/HY(80) catalysts such as XRD (X-ray diffraction), TPR (temperature programmed reduction), $O_2-TPD$ (temperature programmed desorption), and XPS (X-ray photoelectron spectroscopy) were carried out to explain the nature of active center of Ag catalyst for deep oxidation of toluene. From these results, it could be inferred that the presence of metallic Ag phase seemed to play an important role to create silver oxide species or partially oxidized Ag surface onto the surface of metallic Ag phase under reaction condition (i.e. oxygen environment), and these silver oxide species might act as an active redox site during the oxidation reaction.
