Carbohydrates

Carbohydrates are by far the most abundant class of renewables. The big three among the carbohydrates (Figure 2.6) are the glucose polymers (glucans), cellulose and starch, and the disaccharide sucrose. Chitin is also widespread, but its actual production — from waste material of the seafood industry — is small. Its monomer, glucosamine, is receiving much attention as a health supplement.

Some natural polysaccharides find use [31] as thickening and stabilizing agents (hydrocolloids) [32] in food and beverages. Sources can be seaweed (agar, alginate, carrageenan), seeds (guar gum, locust bean gun), fruits (pectin), or bacteria (xanthan gum). A newcomer in the polysaccharide field is the fructan inulin.

2.9 CELLULOSE

Wood harvested annually for energy, construction, and paper, cardboard, and hygiene products amounts to over 3 billion m3 [33]. In many countries, forestry is in a

HO Cellulose

HO Cellulose

Starch

HO Sucrose

Starch

HOHX O

HO Sucrose

FIGURE 2.6 Carbohydrates, the big three.

sustainable balance, i.e., harvesting is fully compensated by replanting; however, this is not the case in all countries. Certification is an instrument here.

The cellulose demand for paper amounts to some 200 x 106 t/yr. Recycle streams contribute substantially to this demand. Regenerated cellulose includes fibers (rayon, mainly used in tires) and films (cellophane was once the leading clear packaging film). Classic cellulose solvents used in regeneration are carbon disulfide/sodium hydroxide and an ammoniacal copper solution. More recent solvents are N-methyl-morpholine-N-oxide and phosphoric acid. The most recently developed cellulose solvents are ionic liquids. For example, 1-butyl-3-methyl-imidazolium chloride dissolves 100 g/l of cellulose at 100°C [34]. Improved solubility of carbohydrates in ionic liquids is observed when dicyanamide is applied as the anion [35].

Cellulose derivatives [36] can be divided into nonionic and anionic materials. The degree of substitution (up to three) is always an important variable. The nonionic derivatives comprise ethers (hydroxyethyl and hydroxypropyl cellulose, methyl cellulose) and esters (cellulose acetate, cellulose nitrate). The anionic carboxymethyl cellulose is produced in the greatest volume. The cellulose derivatives serve a broad spectrum of applications. For instance, cellulose acetate is applied in cigarette filters, as membranes, as fibers, etc. The anionic class can be extended by 6-carboxycellu-lose [37] and by 2,3-dicarboxycellulose; both materials show promise but are still in the research and development stage. A challenge is to arrive at cellulose-based superabsorbing materials.

2.10 STARCH

Starch is a mixture of a linear a-1,4-glucan (amylose, see Figure 2.6) and a branched glucan (amylopectin), containing also 1,4,6-bonded glucose units. Generally, the weight ratio for amylopectin:amylose is about 75:25, but high-amylopectin starches can be obtained by genetic modification of corn or potato.

The big starch-containing grains, wheat, rice, and corn, are each annually produced at amounts of over 600 x 106 t. Some 40 x 106 t of starch is industrially isolated. The dominant raw material (almost 80%) is corn.

The starch serves food and nonfood applications. In Europe the ratio is about 1:1. The largest nonfood application in Europe as well as in the United States is in the paper and board area. Both native and modified starches are applied here. Of the starch derivatives used in papermaking, cationized starch is of particular importance. Here, starch is equipped with C3-chains carrying a quaternary ammonium group. The reagent is made by reacting epichlorohydrine with trimethylamine.

Figure 2.7 lists the major starch-derived chemicals and materials. The conversion pathways are marked with either a circle or a square, indicating whether the conversion step is industrially biocatalyzed or chemocatalyzed, respectively. For instance, the starch-to-vitamin C route involves consecutively enzymatic steps (hydrolysis), a metal-catalyzed hydrogenation, a biocatalytic bacterial regioselective oxidation to L-sorbose, and chemocatalytic protection/oxidation/deprotection/ring-closure steps. North China Pharmaceutical Corp. possesses fermentation technology for the direct conversion of glucose to vitamin C. With an annual production of over 100,000 t, vitamin C is becoming a bulk chemical.

Sorbose Ring
FIGURE 2.7 Starch network.

Fermentative processes for the polyester monomer 1,3-propanediol and for the diabetic-friendly sweetener mannitol have been developed by DuPont and zuChem [38], respectively. In chemocatalysis, sometimes steps can be combined; thus starch can be directly converted to sorbitol by applying a bifunctional Ru-HUSY zeolitic catalyst [39]. The outer surface of the zeolite provides the Bronsted acidity required for the starch hydrolysis. The Ru component catalyzes the hydrogenation of glucose. This process has recently come into industrial practice.

Note that there are also some commercial green surfactants shown in Figure 2.7: the sorbitan esters, known for a long time, and the more recently developed (Henkel) alkyl polyglucosides (APG surfactants) and the N-methyl glucamides (Procter and Gamble/Hoechst). Together with dicarboxylate-polysaccharides as Ca-complexing materials, peracetylated sugar polyols as peracetate precursors, and carboxymethyl cellulose as antiredeposition agent, it is not difficult to imagine production of fully green detergent formulations.

Lactic acid, made by fermentation of glucose or sucrose in an enantiomeric pure form, is used traditionally in meat preservation, but it is increasingly being developed as a key chemical. Major outlets are in esters (biodegradable solvents and bread improvers) and especially in the green polyester poly-L-lactate. The volume of lactic acid (racemate) produced using the chemical process, starting with acetaldehyde, is much less than that produced by fermentation.

Anionic starches can be obtained by sulfatation, by carboxymethylation, or by oxidation. Starch oxidation skills have improved substantially. In particular, the TEMPO-catalyzed oxidation [40] displays an amazing selectivity. Indeed, potato starch has been oxidized to 6-carboxy starch with a selectivity of >98% at 98% conversion. Salt-free enzymatic TEMPO oxidations (O2/laccase/TEMPO) have also been patented recently. Here, 6-carboxylate as well as 6-aldehyde groups are introduced. Gallezot et al. reported recently [41] on the use of iron tetrasulfonatophthalocyanine as catalyst and hydrogen peroxide as oxidant in the oxidation of several starches. Carboxyl as well as carbonyl functions are introduced, leading to hydrophilic materials.

For further starch derivatives, the reader is referred to a recently published book [42].

2.11 SUCROSE

With its present-day world market price of about 26 cents/kg, the disaccharide sucrose [43] is probably the cheapest chiral compound. The three largest producers are Brazil and India (sugar cane) and the European Union (EU) (sugar beet). The top four exporters are Brazil >> Thailand > EU > Australia.

Some industrial sucrose conversions are shown in Figure 2.8. Part of the conversions, e.g., alcohol manufacture, are executed with molasses, the mother liquor of the sugar crystallization. Potential side products of beet sugar manufacture are pectin (from the pulp) and betaine and raffinose (from the molasses).

Sucrose can be transesterified with fatty acid methyl esters toward mono- and di-esters, applied as emulsifiers or to the sucrosepolyesters (SPEs), which materials have been proposed as fat replacers. The fully esterified sucroseoctaacetate is known for its bitterness.

Invert sugar -j> Mannitol

Sucrose-octa-sulfate

Polyethers

FIGURE 2.8 Sucrose as a key chemical.

Full use of the hydroxyl groups of sucrose is also made in the reaction with ethene oxide or propene oxide, leading to polyether polyols that are used in polyurethane manufacture.

The high-intensity sweetener sucralose was recently admitted to the American market. In sucralose, three hydroxyl groups of sucrose have been replaced by chlorine. This pertains to the 1- and 6-positions in the fructose part and to the 4-position in the glucose part of sucrose. Sucralose has a similar taste profile to sucrose [44]; it is nontoxic, nonnutritive, and 60 times more stable to acid hydrolysis than sucrose. Sucralose is 650 times as sweet as sucrose.

Sucrose is biocatalytically isomerized (Südzucker), whereby the 1 ^ 2 bond between glucose and fructose changes to a 1 ^ 6 bond. The disaccharide obtained is named Palatinose, which upon hydrogenation gives a 1:1 mixture of two C12 systems under the name Palatinit. Both Palatinose and Palatinit are commercial sweetening compounds. Due to the low rate of hydrolysis compared with sucrose, both systems are suitable for diabetics and are mild to the teeth.

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