Catalytic oxidation of toluene by ozone over NaX, NaY and MCM-41 adsorbents

Oxidation of toluene over NaX, NaY and MCM-4 1 adsorbents by ozone was studied. The combined use of ozone and the various micro- or meso-porous adsorbents aimed to take advantage of the strong oxidizing capability of ozone. At the same time the residual ozone would be minimized due to the enhanced catalytic reaction in the porous structure. The Lewis acid sites in the adsorbents were believed to decompose ozone into atomic oxygen, and the subsequent reactions would then degrade the adsorbed toluene into CO₂ and H₂O. Powdered X-ray Diffraction (PXRD) and X-ray Fluorescence Spectroscopy (XRF) were used to identify the phase purity and the chemical composition of the adsorbents. The acid strength and the density of acid sites in the adsorbents were characterized by NH₃-TGA methods. The toluene oxidation performance was evaluated in both dry (0%RH) and humid (50%RH) environments with 6ppm ozone at and 1 .5ppm toluene at the inlet. A comparison was made of the amount of adsorbent for the removal of 90% of toluene during steady state conditions. The use of ozone in the system reduced the bed length substantially for the tested adsorbents. In the dry conditions, the MCM-4 1 required the smallest amount of material to achieve the 90 % reduction target, followed by NaY and NaX. In the more humid environment, extra amounts of MCM-4 1 and NaX adsorb ents were required to reach the target as compared with the dry conditions. The performance of NaY, however, was more stable in the humid environment as NaY is a hydrophobic zeolite which adsorbs only a small amount of water. The residual ozone could be minimized by extending the bed-length of the adsorbents. A material balance analysis of the major species in both the effluents and the adsorbents showed that about 20-25% of the removed toluene was carried out via ozone related catalytic oxidations while the rest was done by adsorption only.