Alkyl polyglycosides synthesis

The glycosidation of the reducing sugars obtained from hemicelluloses leads to the alkyl polypentosides surfactants. Glycosylation needs fatty alcohols as raw materials. Natural fatty alcohols derived, for example, from coconut or palm kernel oil are used in the alkyl polyglycoside synthesis to buUd up the hydrophobic part of the surfactants. These alcohols are used as blends obtained after transesterification and fractionation of oils leading to corresponding fatty acid methyl esters, and... 

FIG. 3 Carbohydrate sources for industrial-scale alkyl polyglycoside synthesis. DE dextrose equivalent. 

Rhode, O., Weuthen, M. and Nickel, D. (1997) New nonionic derivatives of alkyl polyglycosides - synthesis and properties, in Alkyl Polyglycosides - Technology, Properties and Applications, Chapter 8, Wiley-VCH Verlag GmbH, Weinheim, pp. 139-149. 

Fig. 8.40 Synthesis of alkyl polyglycosides and their fatty acid esters.

A present-day boom of interest to alkyl polyglycosides APG, known however for hundred years, is caused by several reasons finding of technologically acceptable and cost-effective ways of their synthesis, availability of renewable resources (starch, treacle and alcohols of natural origin), environmental compatibility, high surface activity, excellent usage and dermatological properties [4-6 24]. 

One successful solution to overcome the problem of nonselective derivatization of carbohydrates was the glucosidation found by Fischer [12], The technical process is an adaptation of Fischer s synthesis using long-chain fatty alcohols and a controlled degree of polymerization. So alkyl polyglycosides (APG) with different application properties can be produced. 

Figure 4A.45 Synthesis and manufacturing processes of alkyl polyglycosides. (Figures 4A.44 and 4A.45 adapted from Refs 80,91,130,131.)...

Emil Fischer discovered the synthesis of alkyl glycosides by reaction of glucose and alcohol in the presence of an acidic catalyst. During the course of the further development of alkyl polyglycosides, several laboratory methods have been developed for synthesis of a variety of substances and to facilitate study of their physicochemical properties [3,12-15]. The various syntheses... 

Basically, all processes for the reaction of carbohydrates to alkyl polyglycosides by the Fischer synthesis can be attributed to two process variants, namely, direct synthesis and the transacetalization process. In either case, the reaction can be carried out in batches or continuously. 

Direct synthesis is simpler from the equipment point of view [18-20]. In this case, the carbohydrate reacts directly with the fatty alcohol to form the required long-chain alkyl polyglycoside. The carbohydrate used is often dried before the actual reaction (e.g., to remove the crystal-water in case of glucose monohydrate = dextrose). This drying step minimizes side reactions that take place in the presence of water. 

Highly degraded glucose syrup [dextrose equivalents (DE) > 96] can react in a modified direct synthesis. The use of a second solvent and/or emulsifiers (e.q., alkyl polyglycoside) provides for a stable fine-droplet dispersion between alcohol and glucose syrup [21,22]. 

In the second stage, the short-chain alkyl glycoside is transacetalized with a relatively long-chain alcohol (C12/14-OH) to form the required alkyl polyglycoside. If the molar ratios of carbohydrate to alcohol are identical, the oligomer distribution obtained in the transacetalization process is basically the same as in the direct synthesis. 

If fatty alcohols containing 16 or more carbon atoms per molecule are used in the synthesis of alkyl polyglycosides, the products obtained are soluble in water in only very low concentrations. They are referred to in the following text as water-insoluble alkyl polyglycosides. In these alkyl polyglycoside types, the nonpolar character predominates due to the long-chain alkyl group. These cannot be used as surfactants but instead are mainly used as emulsifiers in cosmetic formulations [30-32]. 

The observations of the reaction of glucose with dodecanol/tetradecanol largely apply to the synthesis of water-insoluble alkyl polyglycosides, such as hexadecyl/octadecyl polyglycosides. The acid-catalyzed reaction is car-... 

FIG. 3 Flow diagram for the synthesis of long-chain alkyl polyglycosides. 

So far as the indnstrial prodnction of alkyl polyglycosides is concerned, processes based on the Fischer synthesis have been successfully adopted. Development work over 20 years has enabled the efficiency of this synthesis ronte to be increased to a level where it has finally become attractive for indnstrial application. Optimization work, particnlarly in the nse of long-chain alcohols, snch as dodecanoFtetradecanol, has resulted in distinct improvements in product quality and process economy. 

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