Study On Base-Catalyzed Oligomerization Of Glycerol

Glycerol, the main by-product of the interesterificationis, is becoming surplus with the blossoming of biodiesel. The utilization of the biomass glycerol in deep-processing is an immediate concern. The etherification of glycerol to polyglycerol is one of the efficient routes to utilize the simplest trihydric alcohol.In this paper, alkali catalytic method was applied. Inorganic alkalies, inorganic salts and layered sodium silicates were used as the catalysts for oligomerization of glycerol and the advanced HPLC analysis method was applied. Meanwhile, the effects of reaction condition on the polymerization and product distribution were investigated. The main contents and results are listed as follows:Firstly, oxidimetry and HPLC analysis methods for polyglycerol were compared by analyzing the oligomerization products of glycerol catalyzed by NaOH. HPLC thus appears to be a useful tool for monitoring the production of desired mixtures of polyglycerols and for quantitative analysis to control the reaction process. The experimental results showed that a considerable conversion could be maintained and the polyreaction can be controllable under 240-260℃. The catalyst dosage had little effect on glycerol conversion when it excessed 1.5 wt%. Secondly, the catalytic performance of inorganic alkalies and salts was investigated at 260℃and the same mole ratio (0.1 mol%) catalyst dosage. Both the alkali metal hydroxides and their carbonates showed high catalytic activity, the selectivity to the (di-+tri-)glycerol was between 63%-71%, when the conversion rate of polymerization was about 80%. The output of cyclic diglycerol added with the increasing of glycerol conversion. Among the alkaline metal hydroxides and carbonates, Mg(OH)2 and CaCO3 was respectively the weakest activity catalyst. Mg2(OH)2CO3 had a high selectivity for diglycerol. The catalytic activity of sodium and calcium salts was related to the alkalinity and solubility of their anions in the glycerol polymerizing system.Finally, the catalytic effect of layered sodium silicates, a new type of catalyst for the oligomarization of glycerol, was investigated in detail. The experiment results indicate the following reactivity order:a-Na2Si2O5≈(3-Na2Si2O5≈δ-Na2Si205> Kanemite> Magadiite, which is in accordance with their basicity. The catalytic performance ofδ-Na2Si205 was revealed:260℃seemed to be a suitable temperature with 1-2 wt%catalyst concentration when the products of interest were liner ones. The crystal transfer and a slow-release alkalinity controlled the generation of cyclic diglycerol and other by-products at a relatively low rate, the proper amount ofδ-Na2Si2O5 catalyst should be 1-2 wt%.The crystallinity of the layered sodium silicates had been losing and releasing its alkalinity, furthermore, the catalytic reaction started as a heterogeneously base-catalyzed reaction, then translated into homogeneous catalytic polymerization.