Insight into the factors affecting the reactivity of sulfonic acid species anchored on hyper-cross-linked polymers in esterification

Abstract

The global market for organic esters was estimated at around $89.4 billion in 2022, and is expected to continue to grow, reaching $127.4 billion in 2029. The wide application of mineral acids as homogeneous catalysts for the synthesis of organic esters causes several economic and environmental problems. For this reason, a lot of effort is undertaken to make esterification a more sustainable process. Nowadays, particular attention is paid to the development of new solid acid catalysts containing sulfonic acid groups (–SO3H) supported on organic polymers. This study fits into this scientific trend and aims at providing insight into the factors affecting the reactivity of –SO3H species anchored on hyper-cross-linked polymers (HCPs). For this purpose, polymers characterized by different cross-linking densities but with a comparable surface area of ca. 540–620 m2/g and a similar pore size of ca. 3.7 nm were applied as supports for anchoring of –SO3H through post-synthetic sulfonation. The loading of sulfonic acid species in all prepared catalysts varied from 1.19 to 2.22 mmol/g. Catalytic activity of the sulfonated HCPs (sHCPs) was evaluated in esterification of acetic acid with different alcohols. It was found that the key factors affecting the reactivity of –SO3H supported on HCPs are hydrophilicity of the polymer surface and the localization of the sulfonic acid species on the external surface of the polymer matrix. The latter was the most favoured during the post-synthetic sulfonation of the polymer characterized by the highest cross-linking density. In the case of this material, the –SO3H species were approximately 7 times more reactive than those of Amberlyst-15 (TOF = 11.36 vs. 1.57 h−1, respectively). The most efficient sHCP used in this study reached approximately two times higher conversion of n-butanol than Amberylst-15 (53.6 vs. 27.5 %) despite of the significantly lower loading of sulfonic acid species in the former polymer (2.22 vs. 4.66 mmol/g, respectively). It was also established that the catalytic esterification on the surface of sHCPs proceeded according to the Langmuir-Hinshelwood mechanism, in which chemisorption of the alcohol is the rate-determining step. Moreover, sHCP catalysts could be reused four times without significant deactivation.