Search Results (4)

In this study, the homogeneous carboxylation of potassium, sodium, and lithium phenolates in DMSO solution at 100 °C by the Kolbe–Schmitt reaction was investigated. The impact of water, phenolate concentration, and cation nature on the yield of products and reaction selectivity was demonstrated. Based on the patterns observed, it was concluded that a complex cluster mechanism governs the carboxylation reaction in the solution. The use of a homogeneous reaction medium allowed for convenient testing of various additives to assess their impact on the reaction. Basic additives such as sodium salts of mesitol, tert-butylcalix[4]arene, sodium isopropyl, and tert-butyl cabonates were found to enhance the reaction, increasing the yield of hydroxybenzoic acids by 20% (to 61.6%). The main product in the DMSO solution was identified as 4-hydroxybenzoic acid, in contrast to the classical Kolbe–Schmitt method which typically yields 2-hydroxybenzoic (salicylic) acid. The use of ¹³C NMR spectroscopy enabled the observation of a “carbonate complex” in the solution for the first time, with the carbonate carbon displaying a chemical shift value of 142 ppm, an unusual finding for stable carbonates, and located between the signals of free dissolved CO₂ and carboxylate derivatives. Full article

Conversion of CO₂ into organic chemicals offers a promising route for advancing the circularity of carbon capture, utilisation, and storage in line with the international 2050 Net Zero agenda. The widely known commercialised chemical fixation of CO₂ into organic chemicals is the century-old Kolbe–Schmitt reaction, which carboxylates phenol (via sodium phenoxide) into salicylic acid. The carboxylation reaction is normally carried out between the gas–solid phases in a batch reactor. The mass and heat transfer limitations of such systems require rather long reaction times and a high pressure of CO₂ and are often characterised by the low formation of undesirable side products. To address these drawbacks, a novel suspension-based carboxylation method has been designed and carried out in this present study, where sodium phenoxide is dispersed in toluene to react with CO₂. Importantly, the addition of phenol played a critical role in promoting the stoichiometric conversion of phenoxide to salicylic acid. Under the optimal conditions of a phenol/phenoxide molar ratio of 2:1 in toluene, a reaction temperature of 225 °C, a CO₂ pressure of 30 bar, a reaction time of 2 h, and stirring at 1000 rpm, an impressive salicylic acid molar yield of 92.68% has been achieved. The reaction mechanism behind this has been discussed. This development provides us with the potential to achieve a carboxylation reaction of phenoxide with CO₂ more effectively in a continuous reactor. It can also facilitate the large-scale fixing of CO₂ into hydroxy aromatic carboxylic acids, which can be used as green organic chemical feedstocks for making various products, including long-lived polymeric materials. Full article

Carboxylation reactions using carbon dioxide (CO₂) as a reactant to produce new C-C bonds represent one of the most promising routes in carbon capture and utilization practices, which yield higher-atom and energy-efficient products. Kolbe–Schmitt-type reactions represent the carboxylation of aromatic compounds to their carboxylic acid derivatives. This study was the first and only to systematically investigate, thoroughly explain preparation procedures, and minutely describe the analytical methods of Kolbe–Schmitt and Marasse carboxylation of phenol. Most importantly, this study provides guidelines for the utilization of state-of-the-art technology in this century-old yet not sufficiently described reaction system. Kolbe–Schmitt carboxylation of phenol was found to be possible using sodium hydroxide (NaOH), potassium hydroxide (KOH), and sodium carbonate (Na₂CO₃), while the Marasse method was active only with potassium carbonate (K₂CO₃) as a reactant. The formation of metal phenoxide is the rate-determining step, which, however, could be more efficiently prepared under reflux. A new, simple, and repeatable HPLC method was described to identify and quantify all possible products of mono- and dicarboxylated phenols. It was found that all procedures result in the highest selectivity for salicylic acid (SA), followed by minor amounts of 4-hydroxybenzoic acid (4HBA) and 4-hydroxyisophthalic acid (4HiPh). Full article

Novel hybrid materials with integrated catalytic properties and hydrophobic response, C@Fe–Al₂O₃ hybrid samples, were presented and tested as catalysts for phenol reaction in aqueous solutions at atmospheric pressure and mild temperature conditions, using CO₂ as a feedstock. A series of carbon-coated γ-alumina pellets (C@Fe–Al₂O₃) were synthesized and characterized by TGA, Brunauer–Emmett–Teller (BET) method, Raman spectroscopy, SEM, TEM, and XPS in order to get comprehensive knowledge of their properties at the nanoscale and relate them with their catalytic behavior. The results obtained correlated their catalytic activities with their carbon surface compositions. The application of these materials as active catalysts in the Kolbe–Schmitt reaction for CO₂ conversion in aqueous media was proposed as an alternative reaction for the valorization of exhausts industrial effluents. In these early tests, the highest conversion of phenol was observed for the hybrid samples with the highest graphitic characteristic and the most hydrophobic behavior. Carboxylation products such as benzoic acid, p-hydroxybenzoic acid, and salicylic acid, have been identified under these experimental conditions. Full article