Preparation and application of biomass-based alkyl glycoside surfactant

Surfactants, often referred to as "industrial MSG", have a long history of development. With the development and progress of society, people's awareness of environmental protection continues to increase, and the research of surfactants is developing in the direction of greening. Alkyl glycoside is a kind of green mild nonionic surfactant synthesized from sugars and fatty alcohols. It is internationally recognized as the first choice of "green" functional surfactant [1]. The alkyl glycosides prepared based on renewable biomass resources have excellent physical and chemical properties and high ecological safety, and have the advantage that other types of surfactants are difficult to match. Various alkyl glycoside derivatives synthesized from alkyl glycosides can retain the advantages of alkyl glycosides and have more functions. At present, alkyl glycosides and their derivatives are widely used in personal care, plastic building materials, agricultural medicine, petrochemicals and other fields.

01 Preparation of alkyl glycosides

1.1 Raw materials

Alkyl glycosides are mainly synthesized from two kinds of raw materials, sugars and fatty alcohols, and have a wide range of raw material sources.

Sugar raw materials include glucose, starch, cellulose and straw. Glucose and starch are used as synthetic raw materials, and the reaction conditions are relatively mild, but they are themselves raw materials for food, and the production cost is relatively high. Cellulose as a raw material for synthesis, the reaction conditions are more severe, usually high temperature and high pressure, and high requirements for equipment [2]. Straw as a synthetic raw material, unlike glucose and starch, does not have any negative impact on the food supply [3]. The reaction can be carried out under mild conditions, but it has the disadvantages of complex products and darker colors. As a large agricultural country, the annual output of crop straw in China is very large. If straw is used as a raw material for sugar, it has a very large raw material advantage.

Fatty alcohol raw materials are usually long carbon chain alcohols, such as C8 ~ C12 higher alcohols. Low-carbon glycosides are not suitable for daily chemicals and other industries, only for some special industries [4]. Current research focuses on the production of long-chain alkyl glycosides using high-carbon alcohols as raw materials.

1.2 Synthesis process

At present, a large amount of research on alkyl glycoside synthesis has been carried out at home and abroad. There are many synthetic methods, mainly Fischer synthesis (direct glycosylation and transglycosylation) and enzyme catalysis.

The direct glycosylation method (one-step method) is currently the most studied synthetic method, and is one of the synthetic methods with more industrial applications. Under acid catalyst conditions, sugars and higher alcohols directly react to form alkyl glycosides and water. The synthesis method strictly controls the reaction process parameters, and can produce tasteless, light-colored high-quality alkyl glycosides. The direct glycosylation method also has shortcomings, and requires higher management and equipment during the synthesis process. At present, there are many domestic companies that use direct glycosylation to synthesize alkyl glycosides, such as Henan Kaipu Chemical, Shanghai Fakai Chemical, China National Chemical Institute, etc.

Transglycosylation (two-step method) is currently the most widely used industrial synthesis method. Under acidic catalyst conditions, short-chain alcohols and sugars are first subjected to a low-glycosylation reaction to form short-chain alkyl glycosides; then short-chain alkyl glycosides and long-chain alcohols undergo transglycosylation reactions to finally obtain long-chain alkyl glycosides . The cost of raw materials for transglycosylation is lower, and the reaction temperature is lower, which can reduce the generation of caramel, but the reaction is complicated, which will increase the equipment and operating costs, and there will be short-chain alkyl glycoside residues, increasing the cost of purification. Domestic production of alkyl glycosides by transglycosylation methods include Hubei Huahua Chemical, Changchun Kangbo Chemical, Jinling Petrochemical Research Institute, Jilin Chemical Research Institute, etc. [5].

Enzyme catalysis is the use of special enzymes (glycosidase, glycoside synthase and glycosyltransferase, etc.) to catalyze the direct production of fatty alcohols and sugars to alkyl glycosides. Glycosidases, also known as glycoside hydrolases, are stable in nature and can accept substrates with different structures. Directly using unprotected non-activated sugars as glycosyl donors, they are widely used in enzymatic glycosylation reactions [7]. Enzyme catalysis is also widely used in protein engineering, DNA recombination technology, the development and utilization of natural product gene cluster biosynthesis and computer modeling [8]. Enzyme catalysis has the advantages of specificity and high efficiency, but it has strict requirements on the environment and high cost. At present, it is mostly used in laboratory research, and there is little promotion of industrialization.

Koenigs-Knorr method, ketal alcoholysis method, tin tetrachloride method, these synthesis methods also have a lot of research, but they all have certain defects, and few industrial applications. The Koenigs-Knorr method has a high yield and is easy to separate and purify, but the heavy metal catalyst is more expensive, the production cost is high, and the waste liquid is easy to pollute the environment; the product of the ketal alcoholysis method of sugar has high selectivity, and the reaction process is easier to control, but The synthesis process is relatively complicated, and there will be a large number of by-products; the tin tetrachloride method has high selectivity, and there are also problems in the synthesis process, and the amount of organic solvent is large, and the cost is high [9].

In recent years, some auxiliary synthesis methods have appeared, such as ultrasonic and microwave-assisted synthesis. Hricovíniov et al. [10] catalyzed the glycosylation of D-xylose and D-lyxose with phosphomolybdic acid under microwave-assisted conditions. A series of alkyl chain glycosides with different chain lengths from C8 to C14 can be obtained in a short time , The yield is up to 73%. Zhou Dapeng et al [11] used NaHSO3 · H2O as a catalyst to synthesize dodecyl glycoside under microwave / ultrasonic radiation, and the conversion rate of glucose could reach 98.9%. These emerging auxiliary synthesis methods can greatly improve the reaction rate and yield, increase the controllability of the reaction, and have far-reaching research significance and application value.

1.3 Catalyst

There are different catalysts for different synthesis methods of alkyl glycosides. Here we mainly introduce the catalysts used in Fischer synthesis. Catalysts include 2 types, one-way catalytic systems and two-way catalytic systems [6].

The one-way catalytic system is usually a strong acid such as sulfuric acid, hydrochloric acid, phosphoric acid and p-toluenesulfonic acid, of which the organic acid is usually used in the direct glycosylation method, and the inorganic acid is usually used in the transglycosylation method; the two-way catalytic system includes inorganic and organic Inorganic catalysts are catalyzed by the main and co-catalysts, and organic catalysts usually have both catalysis and emulsification. In industry, the one-way catalytic system is more widely used.

In order to facilitate the recovery and separation of the catalyst, some studies have used solid acid catalysts. Fan Leming [12] used a self-made magnetic solid superacid SZT catalyst to selectively catalyze the conversion of glucose and starch to alkyl glucoside in the alcohol phase. In the reaction of synthesizing alkyl glycosides, the rapid recovery and recycling of solid acid are achieved by applying an external magnetic field. The resulting glucoside product has a light color and high selectivity, and the product is basically an alkyl monoglycoside. The use of solid acid catalysts has many advantages, but there are also problems such as high reaction temperature and long reaction time, which require more in-depth research. Selecting a catalyst that is easy to recover and separate is an important research direction of the alkyl glycoside industry.

In addition, the direct preparation of alkyl glycosides without catalysts has also been reported. Ludot et al. [13] successfully prepared decyl glycoside without catalyst. Using sulfolane as a solvent, sugar, decyl alcohol and sulfolane directly react to form decyl glycoside at a certain temperature, the yield can reach 83%, and the color of the glycoside product is lighter. This catalyst-free reaction provides new ideas for the green synthesis of alkyl glycosides.

1.4 dealcohol treatment

In the synthesis of alkyl glycosides, in order to improve the sugar conversion rate, usually excess alcohol is used. Therefore, the alcohol content in the reaction product is high, and dealcoholization treatment is required. At present, the methods for dealcoholization of alkyl glycosides include vacuum distillation separation, solvent extraction separation, supercritical fluid separation, combined separation of falling film evaporator and short path evaporator, combined separation of falling film evaporator and scraper evaporator, etc.

Vacuum distillation is simple in operation and requires low cost. The laboratory uses this method to purify alkyl glycosides. Industrial applications usually use combined separation methods, such as combined separation of falling film evaporator and short path evaporator, combined separation of falling film evaporator and scraper evaporator.According to different quality requirements for alkyl glycoside products, some companies use multi-level combination Separation device. The combined separation and dealcoholization effect is better, the amount of alkyl glycoside residual alcohol produced is lower, and the color is lighter. For example, Shanghai Fakai Chemical [14] uses a combination of falling film evaporator and scraper-type rotary film-forming evaporator for dealcoholization, which can obtain high-quality alkyl glycosides.

1.5 Decolorization

Alkyl glycoside products after dealcoholization treatment often still have a darker color and need to be decolorized. Common decolorization methods are physical decolorization and chemical decolorization.

Physical decolorization mainly uses adsorbents to adsorb the coloring substances in alkyl glycosides. The adsorbents used include activated carbon, bentonite, diatomaceous earth, zeolite and macroporous resin. Activated carbon is the most commonly used adsorbent decolorizing agent, not only has decolorizing function, but also has the function of removing odor. Physical decolorization can achieve the decolorization function, but the decolorization effect is limited, and there is the problem of target product adsorption loss during the decolorization process.

Chemical bleaching mainly uses bleaching agents to destroy the chromogenic groups in alkyl glycosides, including oxidative decolorization, reductive decolorization and photodecolorization. Commonly used decolorizing agents include hypochlorite, chlorine dioxide hydrogen peroxide, ozone, peroxyacid, sodium dithionite, sodium boron hydrogenate and sodium bisulfite. In industrial production, hydrogen peroxide is usually used for oxidative bleaching, and some bleaching aids are added. Yang Chunguang et al. [15] used hydrogen peroxide to decolorize alkyl glycosides, which can produce very light-colored alkyl glycosides with klette chroma less than 20. There are also some studies using light decolorization, such as ultraviolet irradiation or mercury lamp irradiation, which have obvious decolorization effect on alkyl glycosides [16].

02 alkyl glycoside derivatives

As the research on the synthesis of alkyl glycosides becomes more mature, the research on alkyl glycoside derivatives becomes more and more. In the early 20th century, Dow chemistry in the United States synthesized alkyl glycoside derivatives, such as dimethyl and trimethyl β-glucoside and 6-alkoxyethyl glucoside. In 1999, the China National Institute of Chemical Industry first synthesized the alkyl polyglycoside sulfosuccinate disodium salt. Since then, China has synthesized derivatives such as alkyl polyglycoside sulfates, phosphate salts, and carboxylic acid esters [17]. Domestic research on alkyl glycoside derivatives has begun to flourish.

Alkyl glycoside derivatives mainly include alkyl glycoside quaternary ammonium salt, alkyl glycoside inorganic acid ester, alkyl glycoside organic acid ester, alkyl glycoside sulfonate, alkyl glycoside betaine, branched chain alkyl glycoside, etc. [18] . Compared with alkyl glycosides, these alkyl glycoside derivatives have better performance and more functions.

The reaction of alkyl glycoside with chlorosulfonic acid and sodium sulfite can synthesize alkyl glycoside sulfonate, which has stronger temperature resistance, water solubility and foaming performance. Wang Fengshou et al. [19] introduced a preparation method of alkyl glycoside hydroxypropyl sulfonate in the patent. The process is simple and easy to operate. The obtained alkyl glycoside sulfonate not only has nonionic surfactant alkyl glycoside Due to the introduction of sulfonate, the water solubility of the product has also been improved. In this way, Suga Nate 160NC products can be made and used directly in shampoo shower gels for infants.

Alkyl glycoside can be synthesized with concentrated sulfuric acid, sulfur trioxide, etc. to synthesize alkyl glycoside sulfate. Polysaccharide sulfate has anti-HIV and HSV effects, can be used in medicine, is a potential antiviral drug, and has attracted the attention of the medical community [20].

The reaction between alkyl glycoside and quaternary ammonium salt, the product alkyl glucoside quaternary ammonium salt has the advantages of two surfactants, foaming, mildness, biodegradability, etc. have been significantly improved. Cationic alkyl glycosides synthesized on the basis of alkyl glycosides have good temperature resistance and inhibition, and have been successfully used in oilfield drilling fluids [21].

The reaction between alkyl glycoside and phosphorus pentoxide can synthesize alkyl glycoside phosphate, and the wettability, emulsification and dispersibility and solubilization are all enhanced. Song Bo et al. [22] used dodecyl and tetradecyl glycosides as raw materials to synthesize alkyl glycoside phosphate salts of different chain lengths, which have better surface properties than alkyl glycosides.

Alkyl glycoside reacts with citric acid and citric anhydride to synthesize alkyl glycoside citrate. It is non-irritating and has good detergency and stability. It has excellent low-temperature solubility and easy rinsing. Compounding various additives. Zhang Xiaohan et al. [23] used alkyl glycosides and anhydrous citric acid as raw materials to synthesize alkyl glycoside citrate using a self-made composite catalyst. After adding various additives, the human body has good affinity, no irritation and no irritation. Residue, with a strong sterilization effect, very easy to degrade, a new type of laundry detergent suitable for washing infant clothes.

With the continuous deepening of the research and application of alkyl glycosides, new alkyl glycoside derivatives continue to appear. These derivatives with better performance and new functions have great application potential, and they will play a role in various fields together with alkyl glycosides in the future.
03 Application of alkyl glycosides

3.1 Detergent

Alkyl glycosides can be used in detergents with little irritation, high safety, and easy degradation. Conventional detergents mainly use sulfate, alcohol ether carboxylate or sulfonate surfactants as active substances, alkanolamides as thickeners, and C8 ~ C16 quaternary ammonium salt cationic surfactants as fungicides, which are more irritating. , Easy to remain, and difficult to degrade [23]. In addition, alkyl glycosides have a good synergistic effect with commonly used anionic and nonionic surfactants. When compounded, they can reduce the amount of surfactants and have better resistance to low temperatures and hard water. Due to these excellent properties, alkyl glycosides have been used in various high-end detergent products.

3.2 Cosmetics

Alkyl glycosides can be used in cosmetics, are rich in foam, delicate, mild and non-irritating, and have emulsification, moisturizing and good compound synergistic effect [24]. Alkyl glycoside products are highly safe and can be used in infants and young children. For example, alkyl glycoside sulfonate derivatives have been widely used in infant and child care products. Alkyl glycosides are increasingly used in bath products, shampoos, skin care products and other products, and are favored by consumers.

3.3 Food processing

Alkyl glycosides can be used in food additives to promote food emulsification, with foaming and thickening effects. The addition of alkyl glycosides can disperse the combination of fat and water in the food, and the various ingredients in the food can be evenly mixed to improve the taste of the food, increase the stability of the food, and extend the storage time of the food. As a green and safe food additive, alkyl glycosides have broad application prospects in the food processing industry.

3.4 Agricultural Medicine

Alkyl glycosides can be used in pesticides and pharmaceutical products. Alkyl glycosides have good wetting and penetration properties, which can promote the absorption of pesticides by crops; alkyl glycosides are easy to biodegrade and have good hygroscopicity. They can be used as emulsifiers and have synergistic effects on pesticides such as herbicides and insecticides [ 25]. Research abroad has used alkyl glycosides as surfactants to prepare microemulsions with a colloidal structure and used them as carriers in medical drugs [26]. C8 ~ C12 alkyl glycosides have broad-spectrum antibacterial properties against bacteria and fungi, and can be used for medical disinfection and cleaning. Alkyl glycosides have excellent compatibility. After compatibility with Chinese herbal medicine, they have a stable appearance and excellent medicinal properties [27].

3.5 Petrochemical

Alkyl glycosides can be used in drilling fluids to suppress collapse and lubrication, and to improve the solid phase capacity and temperature resistance of drilling fluids. It can also be used as a viscosity reducer and fluid loss agent for drilling fluids, as a dispersant and retarder for cement slurries, as an emulsifier for oil-in-water drilling fluids, and as a foaming agent for microfoam systems [21]. The high-performance drilling fluids formulated with alkyl glycosides and their derivatives have now achieved large-scale application and played an important role in the petrochemical industry.

In addition, alkyl glycosides are also widely used in textile, paper, leather, and waste treatment. In the treatment of waste, the use of alkyl glycoside surfactants can accelerate the dissolution, hydrolysis and acidification of organic waste during anaerobic decomposition, reducing the time of waste treatment [28]. In the textile industry, alkyl glycosides can be used in all aspects of textile production, as detergents, scouring agents, defoamers, dispersants, etc. In the tanning industry, alkyl glycosides can be used in the synthesis of leather chemicals and the tanning process [29].

04 Conclusion

As a biomass-based surfactant, alkyl glycosides have many advantages. They have rich raw material sources, green synthetic processes, excellent product performance, and broad development prospects. In this paper, the preparation process of alkyl glycosides is introduced in detail, including the source of raw materials, synthesis process, catalyst selection and dealcoholization and decolorization methods, and the specific application of alkyl glycosides and their derivatives in various fields is introduced by category. It can be found that the related research of alkyl glycosides is developing rapidly, and the application fields are also expanding, which has important research significance and application prospects.

With the continuous progress of the alkyl glycoside industry, it should also be noted that there are still deficiencies. The synthesis process of alkyl glycosides needs to be further optimized, seeking a milder reaction condition; the decolorization method needs to be further improved to prepare products with lighter color; the industrial application of derivatives needs to be further explored and applied to various industries as soon as possible. As more and more attention is paid to the concept of green development, biomass-based alkyl glycosides and their derivatives will surely have great development and wider application, serving the national economy and people's livelihood.


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