THE METHODS FOR MANUFACTURING ALKYL GLUCOSIDES

Fischer glycosidation is the only method of chemical synthesis that has enabled the development of today’s economic and technically perfected solutions for large-scale production of alkyl polyglucosides. Production plants with capacities of over 20,000 t/year have already been realized and enlarge the product range of the surfactants industry with surface-active agents based on renewable raw materials. D-Glucose and linear C8-C16 fatty alcohols have proved to be the preferred feedstocks. These educts can be converted to surface-active alkyl polyglucosides by means of direct Fischer glycosidation or two-step transglycosidation via butyl polyglucoside in the presence of acid catalysts, with water as a by-product. The water has to be distilled from the reaction mixture in order to shift the reaction equilibrium toward the desired products. During the glycosidation process, inhomogeneities in the reaction mixture should be avoided, as they lead to excessive formation of so-called polyglucosides, which are highly undesirable. Many technical stratagems therefore concentrate on homogenizing the educts n-glucose and alcohols, which are poorly miscible due to their difference in polarity. During reaction, glycosidic bonds are formed both between fatty alcohol and n-glucose and between the n-glucose units themselves. Alkyl polyglucosides consequently form as mixtures of fractions with different numbers of glucose units at the long-chain alkyl residue. Each of these fractions, in turn, is made up of several isomeric constituents，since the n-glucose units assume different anomeric forms and ring forms in chemical equilibrium during Fischer glycosidation and the glycosidic linkages between D-glucose units occur in several possible bonding positions. The anomer ratio of the D-glucose units is approximately α/β＝ 2: 1 and appears difficult to influence under the described conditions of Fischer synthesis. Under thermodynamically controlled conditions, the n-glucose units contained in the product mixture exist predominantly in the form of pyranosides. The mean number of n-glucose units per alkyl residue, the so-called degree of polymerization, is essentially a function of the molar ratio of the educts during manufacture. Owing to their pronounced surfactant proper[1]ties, special preference is given to alkyl polyglucosides with degrees of polymerization between 1 and 3, for which approximately 3-10 mol fatty alcohol must be used per mole of n-glucose in the process.

The degree of polymerization decreases as the excess fatty alcohol increases. Excess fatty alcohols are separated and recovered by a multi-step vacuum distillation process with falling film evaporators, so that thermal stress can be kept to a minimum. The evaporation temperature should be just high enough and the contact time in the hot zone just long enough to ensure sufficient distillation of the excess fatty alcohol and flow of the alkyl polyglucoside melt without any significant decomposition reaction. A series of evaporation steps can be advantageously used to separate first the low-boiling fraction, then the main amount of fatty alcohol, and finally the remaining fatty alcohol, until the alkyl polyglycoside melts as a water-soluble residue.

Even under the mildest conditions for the synthesis and evaporation of fatty alcohols, undesirable brown discoloration will occur, and bleaching processes are required to refine the product. One method of bleaching that has proven suitable is to add an oxidizing agent, such as hydrogen peroxide, to an aqueous formulation of alkyl polyglycoside in an alkaline medium in the presence of magnesium ions.

The multiple studies and variants used in the synthesis, post-processing and refining process guarantee that even today, there is still no widely applicable “turnkey” solution to obtain a specific product grade. On the contrary, all the process steps need to be formulated. Dongfu provides some suggestions for the solution design and technical solutions, and explains the chemical and physical conditions for the reaction, separation and refining process.

All three main processes – homogeneous transglycosidation, slurry process, and glucose feed technique-can be used under industrial conditions. During transglycosidation, the concentration of the intermediate butyl polyglucoside, which acts as a solubilizer for the educts D-glucose and butanol, must be kept over about 15% in the reaction mixture so as to avoid inhomogeneities. For the same purpose, the water concentration in the reaction mixture employed for direct Fischer synthesis of alkyl polyglucosides must be kept at less than about 1 % . At higher water contents there is a risk of turning the suspended crystalline D-glucose into a tacky mass, which would subsequently result in bad processing and excessive polymerization. Effective stirring and homogenization promote the fine distribution and reactivity of the crystalline D-glucose in the reaction mixture.

Both technical and economic factors have to be considered when selecting the method of synthesis and its more sophisticated variants. Homogeneous transglycosidation processes based on D-glucose syrups appear especially favorable for continuous production on a large scale. They allow permanent savings on crystallization of the raw material D-glucose in the value-added chain, which more than compensate for the higher one-time investments in the transglycosidation step and the recovery of butanol. The use of n-butanol presents no other disadvantages, since it can be recycled almost completely so that the residual concentrations in the recovered end products are only a few parts per million, which can be considered noncritical. Direct Fischer glycosidation according to the slurry process or glucose feed technique dispenses with the transglycosidation step and the recovery of butanol. It can also be performed continuously and calls for slightly lower capital expenditure.

In the future, the supply and price of fossil and renewable raw materials, as well as the further technological progress in the production of alkyl polysaccharides, will have a decisive impact on the market capacity and production capacity of development and application. Base polysaccharide already has its own technical solutions that can provide important competitive advantages in the surface treatment market for companies that develop or have adopted such processes. This is especially true when prices are high and low. The manufacturing cost of the manufacturing agent has risen to the usual level, even if the price of local raw materials drops slightly, it may fix the substitutes for surfactants and may encourage the installation of new alkyl polysaccharide production plants.