Dr. Valdés-Morales, Héctor

The influence of surface physical-chemical characteristics of Chilean natural zeolite on the catalytic ozonation of toluene is presented in this article. Surface characteristics of natural zeolite were modified by acid treatment with hydrochloric acid and ion-exchange with ammonium sulphate. Prior to catalytic ozonation assays, natural and chemically modified zeolite samples were thermally treated at 623 and 823 K in order to enhance Brønsted and Lewis acid sites formation, respectively. NaturalandmodifiedzeolitesampleswerecharacterisedbyN2 adsorptionat77K,elementalanalysis, X-ray fluorescence, and Fourier transform infrared (FTIR) spectroscopy, using pyridine as a probe molecule. The highest values of the reaction rate of toluene oxidation were observed when NH4Z1 and 2NH4Z1 zeolite samples were used. Those samples registered the highest density values of Lewis acid sites compared to other samples used here. Results indicate that the presence of strong Lewis acid sites at the 2NH4Z1 zeolite surface causes an increase in the reaction rate of toluene oxidation, confirming the role of Lewis acid sites during the catalytic ozonation of toluene at room temperature. Lewis acid sites decompose gaseous ozone into atomic oxygen, which reacts with the adsorbed toluene at Brønsted acid sites. On the other hand, no significant contribution of Brønsted acid sites on the reaction rate was registered when NH4Z1 and 2NH4Z1 zeolite samples were used.

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.