Effect of acidity on the properties of silica-aluminas synthesized by sol-gel method

In the present work amorphous silica-aluminas were synthesized by the coprecipitation method during the hydrolysis of an alcohol solution of tetraethoxysilane (with a tetraethoxysilane: alcohol mass ratio of 1: 1) and 6% aqueous solution of aluminum nitrate at pH values of 1, 3, and 10. The Si/Al molar ratio for all synthesized samples were 4.72 (± 0.29). The amorphous character of the investigated materials was confirmed by X-ray phase analysis. According to the results of scanning electron microscopy, it was found that the resulting powders have particles with a size of 1-20 μm. It was shown that the conditions of synthesis affected the specific surface area and porosity of the materials under study. By the method of low-temperature adsorption-thermodesorption of nitrogen it was established that silica-aluminas obtained under acidic conditions were microporous materials. For the sample obtained under alkaline conditions (pH = 10), the contribution of macropores is very significant. A decrease in surface area is observed as the pH of the synthesis increases. The Hammett indicator method was used to identify and quantify surface centers of different acidity. All studied silica-aluminas are characterized by the presence of both Brønsted basic (pKax from 7 to 12.8) and acidic (pKax from 0 to 7) centers, and Lewis basic (pKax from -4.4 to 0) with a pronounced maximum at pKax = 1.02. It was found that the synthesis conditions had a significant effect on the concentration of active centers. The values of the Hammett function are practically the same for the 3 studied silica-aluminas and describe the studied samples as materials of medium acidity. The variety of Lewis and Brønsted centers on the surface indicates the amphoteric properties of the materials under study. This gives the samples the properties of polyfunctional sorbents and catalysts.

ZnO/Ionic Liquid Catalyzed Biodiesel Production from Renewable and Waste Lipids as Feedstocks

A new protocol for biodiesel production is proposed, based on a binary ZnO/TBAI (TBAI = tetrabutylammonium iodide) catalytic system. Zinc oxide acts as a heterogeneous, bifunctional Lewis acid/base catalyst, while TBAI plays the role of phase transfer agent. Being composed by the bulk form powders, the whole catalyst system proved to be easy to use, without requiring nano-structuration or tedious and costly preparation or pre-activation procedures. In addition, due to the amphoteric properties of ZnO, the catalyst can simultaneously promote transesterification and esterification processes, thus becoming applicable to common vegetable oils (e.g., soybean, jatropha, linseed, etc.) and animal fats (lard and fish oil), but also to waste lipids such as cooking oils (WCOs), highly acidic lipids from oil industry processing, and lipid fractions of municipal sewage sludge. Reusability of the catalyst system together with kinetic (Ea) and thermodynamic parameters of activation (ΔG‡ and ΔH‡) are also studied for transesterification reaction.

Charge-tunable graphene dispersions in water made with amphoteric pyrene derivatives

Pyrene derivatives with biomolecular functional groups (lysine and taurine) have been used to produce stable, concentrated and biocompatible graphene dispersions with amphoteric properties.

Novel Amphoteric Cryogels for Cd2+ Ions Removal from Aqueous Solutions

In this work, amphoteric cryogels based on N,N-dimethyl acrylamide, methacrylic acid and allylamine, crosslinked by N,N-methylenebisacrylamide were synthesized by free-radical polymerization in cryo-conditions. The synthesized cryogels were used for the removal of cadmium ions from aqueous solutions under different pH values. The chemical structure was studied by FTIR, porosity by nitrogen adsorption and morphology by scanning electron microscopy and texture analyzer. The amphoteric properties of cryogels were studied by zeta potential measurements. Adsorption tests revealed that cryogels exhibit 3 times higher adsorption capacity at pH 6.0 than at pH 4.0. The maximum adsorption capacity of the amphoteric cryogels for Cd2+ was 113 mg/g, at pH 6.0 and initial Cd2+ concentration 100 ppm. The results suggest that the predominant removal mechanism is ion exchange between sodium, which initially presents in the structure of the cryogel, and cadmium from the aqueous phase. Recovery studies suggested that the cryogels used can be regenerated and efficiently reused.

The amphoteric properties of g-C3N4 nanosheets and fabrication of their relevant heterostructure photocatalysts by an electrostatic re-assembly route

A stable colloid of g-C3N4 nanosheets was prepared on a large scale via a H2SO4 exfoliation route.