Dr. Valdés-Morales, Héctor

In this study, the effect of zeolite chemical surface characteristics on the oxidative regeneration of toluene saturated-zeolite samples is investigated. A Chilean natural zeolite (53% clinoptilolite, 40% mordenite and 7% quartz) was chemically modified by acid treatment with hydrochloric acid and by ion-exchange with ammonium sulphate. Thermal pre-treatments at 623 and 823 K were applied and six zeolite samples with different chemical surface characteristics were generated. Chemical modification of natural zeolite followed by thermal out-gassing allows distinguishing the role of acidic surface sites on the regeneration of exhausted zeolites. An increase in Brønsted acid sites on zeolite surface is observed as a result of ammonium-exchange treatment followed by thermal treatment at 623 K, thus increasing the adsorption capacity toward toluene. High ozone consumption could be associated to a high content of Lewis acid sites, since these could decompose ozone into atomic active oxygen species. Then, surface oxidation reactions could take part among adsorbed toluene at Brønsted acid sites and surface atomic oxygen species, reducing the amount of adsorbed toluene after the regenerative oxidation with ozone. Experimental results show that the presence of adsorbed oxidation by-products has a negative impact on the recovery of zeolite adsorption capacity.

In this study, the influence of zeolite surface sites on the adsorption of volatile organic compounds (VOCs) is evaluated. Chilean natural zeolite of mordenite kind (86% mordenite, and 14% quartz) is used as a parent material. It is chemically modified using hydrochloric acid (2.4 mol dm−3) and thermally out-gassed at 550 °C. Textural characteristics are determined by nitrogen adsorption at 77 K. Acid–base properties of mordenite surface sites are assessed by temperature-programmed desorption, using ammonia and carbon dioxide as the base and acid probe molecules, respectively. Diffuse reflectance infrared Fourier transform spectroscopy studies applied here provide key information to understand surface interactions among adsorbed VOC molecules and active sites of mordenite samples. The presence of moisture reduces mordenite adsorption capacity toward VOCs. Results indicate that Brønsted and Lewis acid sites of mordenite surface could be mainly responsible of the abatement of VOCs. Weak base aromatic VOC molecules such as benzene, toluene and p-xylene seem to be adsorbed by a surface mechanism that includes interaction with Brønsted acid sites in the form of proton-donating hydroxyl groups of mordenite surface, forming hydrogen bonds; and with Lewis acid sites, generating a Lewis acid–base adduct.