Transglycosidation processes using D-glucose as a raw materials.
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 into surface-active alkyl polyglycosides by direct Fischer glycosylation or by two-step transglycosides of butyl polyglycoside in the presence of an acid catalyst, with water as a by-product. Water must be distilled from the reaction mixture to shift the reaction equilibrium toward the desired product. In the glycosylation process, inhomogeneities in the reaction mixture should be avoided because they can lead to excessive formation of so-called polydextrose, which is highly undesirable. Therefore, many technical strategies focus on the homogenous educts n-glucose and alcohol, which are difficult to miscible due to their different polarities. 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 average number of normal glucose units per alkyl residue, the so-called degree of polymerization, is basically a function of the molar ratio of educts during the manufacturing process. Due to their remarkable surfactant properties, alkyl polyglycosides with a degree of polymerization between 1 and 3 are particularly preferred, for which reason about 3-10 moles of fatty alcohols must be used per mole of normal glucose in this method.
The degree of polymerization decreases at an increasing excess of fatty alcohol. The excess fatty alcohol is separated and recovered by means of multistep vacuum distillation processes with falling-film evaporators, which make it possible to keep the thermal stress at a minimum. The evaporation temperature should be just high enough and the contact time in the hot zone just long enough to ensure adequate distillation of the excess of fatty alcohol and flow of the alkyl polyglucoside melt, without the occurrence of any considerable decomposition reactions. A series of evaporation steps can be favorably employed to separate first low-boiling fractions, then the main quantity of fatty alcohol, and finally the remaining fatty alcohol until the alkyl polyglucoside melts are obtained as water-soluble residues.
Even when synthesis and evaporation of the fatty alcohol are performed under the most gentle conditions, undesired brown discoloration occurs, calling for bleaching processes to refine the products. One bleaching method that has proved suitable is the addition of oxidants such as hydrogen peroxide to aqueous preparations of alkyl polyglucosides in alkaline medium in the presence of magnesium ions.
The manifold investigations and variants employed during synthesis, workup, and refining show that even today there are still no generally applicable “turnkey” solutions for obtaining specific product grades. On the contrary, all process steps need to be worked out, mutually adjusted, and optimized. This chapter has provided suggestions and described some practicable ways to devise technical solutions, as well as stating standard chemical and physical conditions for conducting reactions, separation, and refining processes.
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.
The future availability and prices of fossil and renewable raw materials, as well as further technical advances in production and application of alkyl polyglucosides, may be expected to have a decisive influence on the development of the latter’s market volume and production capacities. The viable technical solutions that already exist for the production and use of alkyl polyglucosides may give a vital competitive edge in the surfactants market to companies that have developed or already employ such processes. This is particularly true in the event of high crude oil prices and low cereal prices. Since the fixed manufacturing costs are certainly on a customary level for bulk industrial surfactants, even slight reductions in the price of native raw materials may urge the substitution of surfactants commodities and may clearly encourage the installation of new production plants for alkyl polyglucosides.
Post time: Jul-11-2021