Selective elimination of phenol from hydrocarbons by zeolites and silica-based adsorbents-Impact of the textural and acidic properties

Enhancing the Acidity Window of Zeolites by Low-Temperature Template Oxidation with Ozone

Revisiting the impact of the first and often deemed trivial postsynthetic step, i.e., a high-temperature oxidative calcination to remove organic templates, increases our understanding of thermal acid site evolution and Al distributions. An unprecedented degree of control over the acidity of high-silica zeolites (SSZ-13) was achieved by using a low-temperature ozonation approach. Fourier transform infrared spectroscopy of adsorbed probe molecules and solid-state NMR spectroscopy reveal the complexity of the thermal evolution of acid sites. Low-temperature activated (ozonated) zeolites maintain the original Brønsted acidity content and high defect content and have virtually no Lewis acidity. They also preserve the "as-made" Al distribution after crystallization and show a clear link between synthesis conditions and divalent cation capacity, as measured with aqueous cobalt ion uptake. The synthesis protocol is found to be the main contributor to Al proximity, yielding record high exchange capacity when ozonated. After conventional calcination at 500-600 °C, however, the presence of water leads to the gradual depletion of Brønsted acid sites, in particular, in small crystals. This work indicates that low-temperature ozonation followed by thermal activation at different temperatures can be used as a novel tool for tuning the amount and nature of acid sites, providing insights into the activity of zeolites in acid-catalyzed reactions, such as CO2 hydrogenation to dimethyl ether, and thereby expanding the possibilities of rational acidity tuning.

A Cooperative OSDA Blueprint for Highly Siliceous Faujasite Zeolite Catalysts with Enhanced Acidity Accessibility

A cooperative OSDA strategy is demonstrated, leading to novel high-silica FAU zeolites with a large potential for disruptive acid catalysis. In bottom-up synthesis, the symbiosis of choline ion (Ch⁺ ) and 15-crown-5 (CE) was evidenced, in a form of full occupation of the sodalite (sod) cages with the trans Ch⁺ conformer, induced by the CE presence. CE itself occupied the supercages along with additional gauche Ch⁺ , but in synthesis without CE, no trans was found. The cooperation, and thus the fraction of trans Ch⁺ , was closely related to the Si/Al ratio, a key measure for FAU stability and acidity. As such, a bottom-up handle for lowering the Al-content of FAU and tuning its acid site distribution is shown. A mechanistic study demonstrated that forming sod cages with trans Ch⁺ is key to the nucleation of high-silica FAU zeolites. The materials showed superior performances to commercial FAU zeolites and those synthesized without cooperation, in the catalytic degradation of polyethylene.

Adsorption of Toluene and Water over Cationic-Exchanged Y Zeolites: A DFT Exploration

In this study, density functional theory (DFT) calculations have been performed to investigate the adsorption mechanisms of toluene and water onto various cationic forms of Y zeolite (LiY, NaY, KY, CsY, CuY and AgY). Our computational investigation revealed that toluene is mainly adsorbed via π-interactions on alkalis exchanged Y zeolites, where the adsorbed toluene moiety interacts with a single cation for all cases with the exception of CsY, where two cations can simultaneously contribute to the adsorption of the toluene, hence leading to the highest interaction observed among the series. Furthermore, we find that the interaction energies of toluene increase while moving down in the alkaline series where interaction energies are 87.8, 105.5, 97.8, and 114.4 kJ/mol for LiY, NaY, KY and CsY, respectively. For zeolites based on transition metals (CuY and AgY), our calculations reveal a different adsorption mode where only one cation interacts with toluene through two carbon atoms of the aromatic ring with interaction energies of 147.0 and 131.5 kJ/mol for CuY and AgY, respectively. More importantly, we show that water presents no inhibitory effect on the adsorption of toluene, where interaction energies of this latter were 10 kJ/mol (LiY) to 47 kJ/mol (CsY) higher than those of water. Our results point out that LiY would be less efficient for the toluene/water separation while CuY, AgY and CsY would be the ideal candidates for this application.

Adsorption of Pb(II) ions from contaminated water by 1,2,3,4-butanetetracarboxylic acid-modified microcrystalline cellulose: Isotherms, kinetics, and thermodynamic studies

Microcrystalline cellulose (MCC) has been utilized as an adsorbent material for the removal of Pb(II) ions from aqueous solution after treatment with 1,2,3,4-butanetetracarboxylic acid (BTCA) at elevated temperature to obtain MMCC. The resulting adsorbent was characterized for point of zero point charge (pHZPC), estimation of carboxyl content, Fourier transform infrared spectroscopy (FT-IR), scan electron microscopy (SEM), and textural properties, including surface area, and subsequently utilized for the removal of Pb(II) ions from aqueous solution. The adsorption process was probed by investigating the effect of adsorbent dose, pH of solution, temperature, agitation time, and Pb(II) ion concentration. The results showed successful functionalization of MCC using BTCA, significantly improved the binding properties of the adsorbent towards Pb(II) ions. Isothermal adsorption data was analyzed using Langmuir, Freundlich and Temkin models, evaluated via nonlinear regression analysis. The maximum adsorption capacity was found to be 1155 mg/g (at pH 5 and 30 °C) from Langmuir theory, and appears independent of surface area. The Freundlich model was found to provide the best fit and the constant n was determined to be 2.69, indicating that adsorption of Pb(II) ions onto MMCC is favorable. Kinetic modelling showed good agreement for the pseudo-second order kinetic model, supporting the theory that chemisorption is involved in the adsorption process, which is promoted by a high density of active sites. Thermodynamic analysis showed that the adsorption of Pb(II) ions onto MMCC was endothermic and nonspontaneous; hence, MMCC offers an effective method of Pb(II) ion removal from aqueous solutions, with potential for water remediation processes.

Treatment of oil refinery effluent using bio-adsorbent developed from activated palm kernel shell and zeolite

This study investigated the potential of palm kernel shell (PKS) as a biomass feed for adsorbent production. This work aims at synthesizing green adsorbent from activated PKS by integrating iron oxide and zeolite. The newly developed adsorbents, zeolite-Fe/AC and Fe/AC, were analyzed for surface area, chemical composition, magnetic properties, crystallinity, and stability. The adsorbent efficiency in removing effluent from the palm oil mill was evaluated. The influence of operating parameters, including adsorbent dosage, H2O2, reaction time, and initial solution pH for adsorption performance was studied. The Fourier transform infrared analysis revealed that the adsorbents contain functional groups including OH, N-H, C[double bond, length as m-dash]O and C[double bond, length as m-dash]C, which are essential for removing pollutants. The SEM-EDX analysis shows holes in the adsorbent surface and that it is smooth. The adsorption study revealed that under optimized conditions, by using 4 g L-1 of adsorbent and 67.7 mM H2O2, zeolite-Fe/AC was able to remove 83.1% colour and 67.2% COD within 30 min. However, Fe/AC requires 5 g L-1 of adsorbent and 87.7 mM to remove 86.8 percent and 65.6 percent, respectively. This study also showed that zeolite-Fe/AC has higher reusability compared to Fe/AC. Among Freundlich and Temkin models, the experimental data were found to be best fitted with the Langmuir isotherm model. The kinetic analysis revealed that for both adsorbents, the adsorption process fitted the pseudo-second-order model (R 2 = 0.9724). The finding reflects monolayer adsorption of zeolite-Fe/AC and Fe/AC. This study thus demonstrates the applicability of low-cost green adsorbents produced from PKS to treat oil refinery effluent and other recalcitrant wastewaters.