At present, there are three main production methods of polyglycerol:
①Polyglycerol was extracted from crude glycerol distillation residue.
②It is directly synthesized from 3-chloro-1,2-propylene oxide or glycidyl and glycerol.
③The refined glycerol was heated and dehydrated under alkaline conditions to form polyglycerol.
The first method is to extract crude glycerol distillation residue with alcohols and other organic solvents, and then to obtain polyglycerol by desolvation. In order to obtain light-colored polyglycerol, activated carbon decolorization under acidic conditions can be used. In order to obtain high purity and low degree of polymerization glycerol, acetylated distillation and short-range distillation under high vacuum (0.1-0.3 mmHg) can be used.
The method is to react glycerol with 3-chloro-1,2-propylene oxide and then decompose hydrochloric acid by adding water. It is difficult to control and refine the reaction. Polyglycerol with high degree of polymerization can be prepared by adding glycidyl into glycerol or polyglycerol and reacting in the presence of KOH or stannic tetrachloride, but the product has a deeper color and is difficult to refine. When carboxylic acid catalysts were used, glycidyl was dripped into glycerol or polyglycerol at 100 ~ 120 (?) C, the easily refined polyglycerol could be obtained. Light-colored polyglycerol can be obtained by dropping glycidyl into glycerol or polyglycerol at 115-125 C with phosphoric acid of 0.01%-10% polyglycerol as catalyst. Polyglycerol can be refined by noble metal hydrogenation catalyst, degassing and hydrogenation with nitrogen or steam to refine crude polyglycerol.
The third method is the most common way to synthesize polyglycerol. The general operation is to dehydrate and condensate at high temperature under alkaline conditions. The dehydration reaction between hydroxyl groups can occur between glycerol molecules or within molecules. Therefore, dehydrated polymers of glycerol can form various structures because of the different positions of dehydration between hydroxyl groups. Such as straight chain, branched chain, ring chain, etc. With the increase of degree of polymerization, its structure is more complex. Because primary hydroxyl groups have the same reactivity, high degree of polymerization glycerol exists in low degree of polymerization glycerol. Therefore, polyglycerol is actually a mixture of different degrees of polymerization. In the study of glycerol polymerization, degree of polymerization, color and odor have become the central issues.
In order to obtain high-quality polyglycerol (less glycerol content, less cyclic compounds, relatively concentrated degree of polymerization, light color, less odor), the degree of polymerization can be controlled by measuring the hydroxyl value (or refractive index, viscosity, etc.) of the polymer (the relationship between hydroxyl value and refractive index can be referred to in the appendix); by controlling the evaporation of generated water, the glycerol content can be gradually reduced without evaporating glycerol. Segmented reaction to reduce or reduce the content of glycerol and cyclic compounds. Under the protection of vacuum and nitrogen, it can prevent coke gas from being produced by oxidation at high temperature and deepen the color of oxidation. The final product can be treated by activated carbon or ion exchange resin to obtain colorless and tasteless products.