A comparative study on catalytic properties of solid acid catalysts for glycerol acetylation at low temperatures

Abstract

The catalytic properties of various types of solid acids, including silica–alumina, HUSY, dodecamolybdophosphoric acids supported on Nb2O5 (HPMo/Nb2O5) and mesoporous SBA-15 (HPMo/SBA-15), Amberlyst-15, sulfated ceria–zirconia (SCZ), propylsulfonic acid functionalized SBA-15 (PrSO3H-SBA-15), and sulfonic acid functionalized SBA-15 (SO3H-SBA-15) and microcrystalline cellulose (SO3H-Cell) catalysts, are studied for glycerol acetylation with acetic acid at low temperatures. Compared at the same acidity loading and similar glycerol conversion level (∼30%) below the equilibrium, the glycerol conversion turnover rate toward di- and triacetin is considerably higher on PrSO3H-SBA-15 and Amberlyst-15, with the rates shown in order as follows: PrSO3H-SBA-15>Amberlyst-15>HPMo/Nb2O5≥HPMo/SBA-15>HUSY>SCZ>SiO2–Al2O3. The catalytic properties of these solid acids are relatively stable under consecutive batch runs at 353K, whereas SO3H-SBA-15 and SO3H-Cell deactivate with a significant acidity loss due to hydrolysis of the grafted sulfonic acid groups. When similar type of the solid acids is compared, the acid strength affect the rate and selectivity, favoring a higher acid strength for the facilitation of the reactions (PrSO3H-SBA-15>Amberlyst-15; HUSY>SiO2-Al2O3; HPMo/Nb2O5≥HPMo/SBA-15). However, the proportional correlation between the acid strength and the glycerol conversion turnover rate does not hold when different types of solid acids are compared. The orders of magnitude higher glycerol conversion turnover rates with moderate Brönsted acid strength on the sulfonic acid functionalized catalysts suggest that the configuration of surface acid moieties attribute substantially to their catalytic activity for the reactions. The acid strength on SCZ is the highest, reflecting the super acidic nature of the sites formed on the sulfated metal oxide surface, but its catalytic activity for the reactions is inferior. The glycerol conversion turnover rates on the catalysts follow the Arrhenius type temperature dependence, and the characterization results indicate that the internal mass transfer resistance does not limit the reactions.