Acetals formed from glycerol find applications in several areas, their use as fuel additives being the most promising, since it results in total integration in the biodiesel chain and eliminates one of the obstacles to its production. In order to convert glycerol into acetals, the use of p-sulfonic acid calix[4]arene supported on silica (CX4SO₃HSi(n)) and microwave-assisted heating was proposed, and quantitative analysis was performed by ¹H NMR. The effects of the main reaction parameters were evaluated, such as temperature, reaction time and the amount of heterogeneous catalyst, using benzaldehyde and glycerol as model substrates. From the optimized conditions (0.5 mmol of glycerol, 0.5 mmol of benzaldehyde, 5 wt% of the catalyst CX4SO₃HSi(n), at 110 °C in 20 min), the generality of the reaction was evaluated, obtaining twelve products with yields ranging from 6 to 99%. The important aspects of this protocol are the following: solvent-free reaction, water as the sole waste, being metal-free, and shorter reaction times.

Heteropolyacids (HPAs) are well known for their application as catalysts in organic synthesis owing to their oxidizing capabilities and strong Brönsted acidity. However, a notable drawback of HPAs is their solubility in various reaction media, which has prompted the development of strategies to convert them into heterogeneous catalysts through immobilization on diverse materials. This review aims to describe recent advancements in the utilization of alumina as a support for HPAs and their applications as heterogeneous catalysts in the synthesis of organic compounds. Various strategies, methodologies, loading techniques, and the nature of HPAs, along with their acid and redox properties, are analyzed and compared. Several organic reactions, including oxidation processes, esterification, and the condensation synthesis of heterocycles, such as chromones and pyrroles, are explored.

A new series of composite materials (PLMTPA) based on tungstophosphoric acid (TPA) included in a polymeric matrix of polyacrylamide (PLM), with a TPA:PLM ratio of 20/80, 40/60, and 60/40, were synthesized and well characterized by FT-IR, XRD, ³¹P MAS-NMR, TGA-DSC, and SEM-EDAX. Their acidic and textural properties were determined by potentiometric titration and nitrogen adsorption–desorption isotherms, respectively. Considering ³¹P MAS-NMR and FT-IR analyses, the main species present in the samples is the [PW₁₂O₄₀]³⁻ anion that, according to XRD results, is highly dispersed in the polymeric matrix or appears as a noncrystalline phase. The thermogravimetric analysis revealed that PLMTPA materials did not undergo any remarkable chemical changes up to 200 °C. Additionally, the PLMTPA materials showed strong acid sites whose number increased with the increment of their TPA content. Finally, PLMTPA materials were used as efficient and recyclable noncorrosive catalysts for the synthesis of 2-benzazepines and related compounds. Good yields (55–88%) and high purity were achieved by a Pictet-Spengler variant reaction between N-aralkylsulfonamides and s-trioxane in soft reaction conditions: low toluene volume, at 70 °C, for 3 h. The described protocol results in a useful and environmentally friendly alternative with operative simplicity. The best catalyst in the optimized reaction conditions, PLMTPA60/40₁₀₀, was reused six times without appreciable loss of activity.