Dr. Valdés-Morales, Héctor

Natural and synthetic zeolites with different properties (porous structure, SiO2/Al2O3 ratio, acidity and Fe-loading) were evaluated as adsorbents/catalysts for octamethylcyclotetrasiloxane (D4) removal in dynamic adsorption tests. BEA type zeolites, with high content of Lewis and Brønsted sites, promoted the catalytic D4 ring-opening leading on the formation of smaller a-x-silanediols, which are narrower molecules able to diffuse into the channel system. Wet oxidation processes were used for the regeneration of a spent BEA zeolite, including ozonation and Fenton-like treatment. Both treatments were optimized to recover almost completely the D4 uptake of the iron-exchanged Fe-BEA in the first use. Thus, its feasibility to be reused was evaluated in successive adsorption/oxidation cycles, recovering up to 80% in at least three subsequent steps. However, in further cycles the accumulation of D4 and/or by-products led to a successive decline in the catalytic activity of the zeolites, hampering not only the capacity to transform D4 into lineal silanediols, thus reducing the adsorption capacity, but also the catalytic activity towards promoting Fenton-like reactions during regeneration.

In this study, a hydrophobic synthetic zeolite, namely ZSM-5 is chosen as an adsorbent/ catalyst for toluene removal. Experimental results showed that toluene adsorption onto ZSM-5 was favourable, following a Langmuir adsorption isotherm model. ZSM-5 zeolite was regenerated using gaseous ozone at low temperature. Adsorbed toluene was oxidised, releasing mainly CO2 and H2O. Traces of oxidation by-products such as acetic acid and acetaldehyde were formed and remained adsorbed after the oxidativate regeneration with ozone. After four successive cycles of adsorption/ozonation, the adsorption efficiency was not affected (92%–99%). These results showed that volatile organic compound (VOC) removal by adsorption onto ZSM-5 zeolite followed by ozone regeneration could be used as a promising hybrid process for the control of VOC emissions in terms of efficiency.

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.