Maximizing the selectivity to triacetin in glycerol acetylation through a plastic waste-derived carbon catalyst development and selection of a reaction unit

Abstract

Carbon catalysts derived from poly(ethylene terephthalate), PET, enriched with surface acidic sites were prepared and used in glycerol acetylation to increase the glycerol conversion and formation of higher acetins, i.e., diacetins and triacetin (DA and TA, respectively). Two approaches of carbon functionalization were applied – treatment with concentrated sulfuric acid or modification with benzenediazonium sulfonate (BDS) generated in-situ. The as-prepared carbons were characterized applying different techniques (e.g., elemental analysis, potentiometric titration, TG method, or XPS studies), and used as catalysts in glycerol esterification with acetic acid under different conditions. Using the synthesized carbons in the process carried out in a classical reaction mode (i.e., batch and reflux conditions, no water removal) enabled us to significantly increase the reaction rate and yields of the produced di- and triacetins compared to the reaction without a catalyst. The most active sample showed a high content of sulfonic groups and gave about 95 % conversion of glycerol and 55 and 20 % selectivity to DA and TA, respectively, just within 1 h of the process at 110 °C. Moreover, the used catalyst caused a significant increase in the TA yield when the reaction was performed with the addition of toluene compared to the toluene-free process. This was related to the continuous removal of water from the reactor and shifting the reaction equilibrium toward the higher esters as well as due to the perfectly-tailored properties of the obtained carbon sample.