Dr. Valdes-Morales, Hector

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.

A great number of pollution problems come as a result of the emission of Volatile Organic Compounds (VOCs) into the environment and their control becomes a serious challenge for the global chemical industry. Adsorption is a widely used technique for the removal of VOCs due to its high efficiency, low cost, and convenient operation. In this study, the feasibility to use a locally available clay, as adsorbent material to control VOCs emissions is evaluated. Natural clay is characterised by different physical-chemical methods and adsorptive interaction features between VOCs and natural clay are identified. Toluene (T), methanol (M) and benzaldehyde (B) are used here as representatives of three different kinds of VOCs. Adsorption isotherms onto natural clay and faujasite-Y type zeolite (Fau Y) are obtained at room temperature. According to Langmuir model data, maximum adsorption capacities (qm) of Fez natural clay and zeolite toward methanol (M), toluene (T) and benzaldehyde (B) at 300 K are 8, 0.89 and 3.1 mmol g–1, and 15, 1.91 and 13.9 mmol g–1 respectively. In addition, the effect of temperature on the adsorption of toluene onto natural clay is evaluated in the range from 300 to 323K. An increase on temperature reduces the adsorption capacity of natural clay toward toluene, indicating that an exothermic physical adsorption process takes place. The enthalpy of adsorption of toluene onto Fez natural clay was found to be –54 kJ mol–1. A preliminary cost analysis shows that natural clay could be used as an alternative low cost adsorbent in the control of VOCs from contaminated gas streams with a cost of US$ 0.02 kg–1 compared to Fau Y zeolite with US$ 10 kg–1.

Tomatoes (Solanum lycopersicum L.), one of the most consumed vegetables worldwide, are climacteric fruit in which ripening is accompanied by quickly increased respiration and ethylene production. Ethylene stimulates ripening and senescence that finally may result in detrimental effects by promoting unwanted softening, grainy structure, accelerated pigment synthesis and chlorophyll loss in tomatoes. Therefore, most postharvest technology strategies are focused on the minimization of ethylene production, inhibition of its action and removal of ethylene from storage facilities. The aim of the present work was to study the ethylene adsorption capacity of a novel copper-zinc-based ethylene scavenger supported on natural zeolite and the effects of ethylene scavenging on quality attributes of tomatoes during their postharvest shelf life. Tomatoes (control, natural zeolite, zeolite doped with copper and zinc) were stored in hermetically sealed glass desiccators, in darkness at 20 °C and a relative humidity of 88 %. Production rates of ethylene and carbon dioxide were determined during 15 days of storage of tomato fruit by monitoring their headspace concentrations as a function of time using gas chromatography. Physical parameters, such as size, weight, colour and texture, and chemical attributes, including moisture, soluble solids, titratable acidity, reducing sugars and lycopene, were determined at the start of the experiment and after 8 and 15 days of tomato storage. Ethylene production diminished in 50 % for modified zeolite and in 7 % for natural zeolite during the first week, while major concentration peaks appeared for both zeolite treatments at 9.5 days. Moreover, modified zeolite delayed tomato respiration during the first six days. This adsorbent was able to shift the respiration peak compared to control treatment in time due to the incorporation of copper and zinc. Increased respiration and ethylene production rates in presence of both zeolites after 1 week of tomato storage trigger the decay of organic acids and part of the soluble solids. In addition, natural zeolite significantly reduced Young’s modulus at the end of storage, which can be attributed to the increased ethylene accumulation of about 40 % compared to control tomatoes. Furthermore, red colour evolution was promoted by natural zeolite, while modified zeolite induced the greatest delay of colour development in tomatoes. Additionally, the use of natural zeolite results to significantly higher increase of lycopene synthesis compared to tomatoes stored in presence of modified zeolite. Natural zeolite doped with copper and zinc cations favours ethylene removal and delays tomato fruit ripening. However, the single use of natural zeolite should be reconsidered due to its ripening promoting effects in tomatoes. Finally, the incorporation of copper and zinc cations to a zeolite support is a new, emergent postharvest technology to slow down fruit ripening that may create new commercial opportunities for fresh-market tomatoes

Ethylene stimulates ripening and senescence by promoting chlorophyll loss, red pigment synthesis, and softening of tomatoes and diminishes their shelf-life. The aim of this work was to study the performance of a novel copper- and zinc-based ethylene scavenger supported by ion-exchange on a naturally occurring zeolite by analyzing its ethylene adsorption capacity and the influence of ethylene scavenging on quality attributes during the postharvest life of tomatoes. The influence of copper- and zinc-modified zeolites on ethylene and carbon dioxide concentrations and postharvest quality of tomatoes was compared with unmodified zeolite. Interactions among ethylene molecules and zeolite surface were studied by diffuse reflectance infrared Fourier transform spectroscopy in operando mode. The percentage of ethylene removal after eight days of storage was 57% and 37% for the modified zeolite and pristine zeolite, respectively. The major ethylene increase appeared at 9.5 days for the modified zeolite treatment. Additionally, modified zeolite delayed carbon dioxide formation by six days. Zeolite modified with copper and zinc cations favors ethylene removal and delays tomato fruit ripening. However, the single use of unmodified zeolite should be reconsidered due to its ripening promoting effects in tomatoes at high moisture storage conditions, as water molecules block active sites for ethylene adsorption.