Abstract
A polystyrene sulfonic acid-functionalized mesoporous silica (SBA-15-PSSA) catalyst was synthesized via an established multistep route, employing 2-bromo-2-methylpropionyl bromide as initiator of atom transfer radical polymerization. Fourier-transform infrared spectroscopy, thermogravimetric/differential thermal, Brunauer–Emmett–Teller, and transmission electron microscopy analyses revealed outstanding structural characteristics of the catalyst, including highly ordered mesopores, high surface area (726 m2/g), and adequate estimated concentrations of active sites (0.70 mmol H+/g). SBA-15-PSSA’s catalytic performance was evaluated in the esterification of acetic acid and n-heptanol as a model system at various temperatures (50–110 °C), catalyst loads (0.1–0.3 g), and reaction times (0–160 min). The conversion percentage of acetic acid was found to increase with the temperature, catalyst load, and reaction time. Furthermore, results indicated a fast conversion in the first 20 min of the reaction, with remarkable conversion values at 110 °C, reaching 86%, 94%, and 97% when the catalyst load was 0.1, 0.2, and 0.3 g, respectively; notably, at this temperature, 100% conversation was achieved after 60 min. At 110 °C, the reaction conducted in the presence of 0.3 g of catalyst displayed more than 6.4 times the efficiency of the uncatalyzed reaction. Such activity is explained by the concomitant presence in the polymer of strong sulfonic acid moieties and a relatively high hydrophobic surface, with adequate numbers of active sites for ester production.
Highlights
Solid acid catalysts have emerged as green alternatives to their liquid acid counterparts, such as sulfuric, hydrofluoric, hydrochloric, and nitric acids, which constitute a major source of harmful industrial waste and cannot be recycled
Pore-expanded mesoporous silica (SBA-15) was prepared implementing a modification of the method described in several reports [10,16,19]
Catalyst Santa BarbaraAmorphous-15 (SBA-15)-PSSA was successfully prepared implementing a four-step protocol: production of mesoporous and high surface area SBA-15, grafting of APTS as a source of the amine functionality used for the subsequent immobilization of bromo-2-methylpropionyl bromide (BMPB) acting as the atom transfer radical polymerization (ATRP) initiator
Summary
Solid acid catalysts have emerged as green alternatives to their liquid acid counterparts, such as sulfuric, hydrofluoric, hydrochloric, and nitric acids, which constitute a major source of harmful industrial waste and cannot be recycled. A substantial class of these catalysts consists of silica-supported sulfonic acids. These species have demonstrated an outstanding catalytic performance in a wide range of chemical transformations, including substrate alkylation, etherification, esterification, retro-esterification, and gas-phase glycerol dehydration [1,2,3,4,5,6,7]. Among silica-supported sulfonic hybrid solid catalysts, (poly)styrene sulfonic acid-modified Santa Barbara. Amorphous-15 (SBA-15) materials are very attractive, in the catalysis of esterification reactions, with activities comparable to the commercially available sulfonic-based catalysts Nafion.
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