Microwave-Assisted Continuous Flow for the Selective Oligomerization of Glycerol

Considering the programmed decrease in oil resources, the concept of biorefinery is in the spotlight of the chemical industry [1-5]. In particular, the use of vegetable oils is a promising alternative to meet future challenges [6-8]. Among the oilseed chemicals, glycerol is an outstanding example with great potential for conversion into valuable products. Glycerol, propan-1,2,3-triol, is a symmetrical polyol with three hydroxy (OH) groups: two identical primary OHs and a secondary OH with a similar pKa and therefore close reactivity. Several chemical modifications obtained by oxidation, hydrogenolysis, etherification, esterification, dehydration and oligomerisation of glycerol have led to the production of a large number of value-added chemicals with specific applications in the polymer, agrochemical and pharmaceutical industries. The continuous oligomerisation of glycerol for the production of polyglycerol was carried out for the first time under microwave activation. In the presence of potassium carbonate, a homogeneous commercial catalyst, different parameters: temperature, flow rate and residence time were studied. The main linear and branched-chain diglycerol and triglycerol regioisomers were characterised and the distribution of the different oligomers was quantified. Further investigations in cyclic mode, combined with short distance distillation, allowed the mixture to be enriched with glycerol digesters and thus to obtain the following mixture: glycerol dimer (50.2 % by weight), glycerol trimer (22.1 wt%), glycerol tetramer (9.5 wt%) and glycerol pentamer (4.3 wt%).