Oligomerization of glycerol - a critical review

The oligomerization of glycerol to preferentially di- and triglycerol is reviewed, with primary focus on the use of heterogeneous acidic and basic catalysts. Low molecular-weight oligomers have found a wide field of applications in cosmetics, food industry and polymer production. The growing market intensified research work on the selective catalytic oligomerization of glycerol. Performing the reaction of glycerol in the presence of microporous and mesoporous solid catalysts aims at exerting shape-selective effects on the reaction, suppressing the abundant formation of cyclic isomers and cutting further polymerization of the target products. Enhanced selectivity to diglycerol is observed over some type of catalysts, but the solids suffer from leaching of active alkaline cations from the solid, severe deterioration of crystallinity of zeolites and even dissolution of the solids in the hot glycerol during batch reaction at temperatures in the range of 240-260 degrees C. In those cases it is difficult to separate homogeneous and heterogenous reaction routes, and the shape-selective effects are levelled off. The oligomerization is a consecutive reaction, and complete conversion of glycerol favours formation of high molecular-weight glycerol oligo-and polymers. To achieve maximum yield of diglycerol, the reaction has to be interrupted at glycerol conversions of ca. 50%. Alternative reaction engineering is required to overcome the inherent disadvantages of a batch reaction. Examples will be given for a selective glycerol oligomerization under reduced pressure in a so-called fall-film reactor using super-acidic polymers as catalysts.