Heterogeneous catalysis of adipic acid esterification by titanium phosphate activated with SOCL₂

Current trends in the development of environmentally friendly polymer materials, in particular biodegradable and biocompatible polymers, naturally stimulate active research for plasticizers with a similar set of properties. Dialkyl esters of aliphatic dicarboxylic acids, such as derivatives of adipic acid and sebacic acid, act as a promising alternative to toxic phthalate plasticizers that dominate the market. Their key advantages are biodegradability, low migration capacity, and minimal ecotoxicity. However, their synthesis remains a significant technological barrier, as it is still mainly based on the use of homogeneous mineral acids in the role of esterification catalysts. This is associated with fundamental disadvantages: high corrosive activity, the need for neutralization and flushing, as well as formation of large volumes of toxic salts and wastewater, which directly contradicts the principles of "green" chemistry and sustainable development. Heterogeneous catalysts are considered as a strategically important solution that minimizes the negative impact on the environment due to possibilities of regeneration and organization of flow processes. Unlike homogeneous systems, they contain stable Bronsted or Lewis acid centers fixed on the developed surface of a heat-resistant inorganic substrate. Despite significant progress in increasing their activity and selectivity, the key technological limitation for widespread adoption remains the relatively low kinetics of esterification of long-chain substrates. In this paper, a method of acid-free activation of titanium phosphate using thionyl chloride is proposed. This approach makes it possible to transform inert surface hydroxyl groups into highly active Lewis chloride centers. It is shown that catalytic activity in the esterification reaction correlates with the residual content of the -OH groups on the surface, which are converted into chlorides during activation, significantly enhancing acidic properties of the material. Interestingly, amorphous TiP samples, devoid of pronounced crystalline structure and morphological features, are the most active both before and after treatment, which is probably due to the higher specific surface area.