Synergistic effect of functional groups in carbonaceous spheres on the formation of fuel enhancers from glycerol

Abstract Carbonaceous microspheres with tunable surface chemistry were prepared via sustainable hydrothermal carbonization combined with subsequent thermal treatment and/or SO3H-functionalization (with sulfuric acid or 4-benzenediazonium sulfonate). Various techniques (including SEM and TEM, elemental and textural analysis, potentiometric titration, TGA method, and XPS measurement) were used for determination of physico-chemical properties of the obtained materials. The results revealed that the effectiveness of applied acidic modifications was related to the structural features of the samples. Catalytic properties of the sulfonated carbons were further tested in glycerol acetylation with acetic acid performed in a batch mode at 80 °C. The experiments showed that the carbonaceous spheres treated with sulfuric acid were very active in the reaction, giving high glycerol conversion and high combined selectivity to diacetin (DA) and triacetin (TA), important fuel enhancers, after a relatively short time. Especially good results were achieved using SO3H-functionalized spheres richly decorated with oxygen groups such as carboxylic and hydroxyl/phenol ones. Finally, reusability measurements demonstrated good catalyst stability in subsequent reaction cycles. Additional investigations of the re-used system confirmed only small leaching of active sites from the tested material.