CARBOHYDRATES TO CHEMICALS:MONOSACCHARIDES

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Chapter -15

CARBOHYDRATES TO CHEMICALS

Vamsi Krishna Nunna,

INTRODUCTION

Carbohydrates are the most abundant class of organic compounds found in living organisms.

They originate as products of photosynthesis, an endothermic reductive condensation of

carbon dioxide requiring light energy and the pigment chlorophyll. Carbohydrates can be

written as carbon hydrates, Cn(H2O)n, hence their name.

n CO2 + n H2O + energy

CnH2nOn + n O2

The carbohydrates are a major source energy required for metabolism. Aside from the sugars

and starches that meet this vital nutritional role, carbohydrates also serve as a structural

material (cellulose), a component of the energy transport compound ATP, recognition sites

on cell surfaces, and one of three essential components of DNA and RNA.

MONOSACCHARIDES

Monosaccharides can be in turn can be classified as ketoses and aldoses basing on the functional

group present, i.e. ketone or aldehyde.

Glucose is a monosaccharide, an aldohexose and a reducing sugar. The general structure of

glucose and many other aldohexoses was established by simple chemical reactions.Hot

hydriodic acid (HI) was often used to reductively remove oxygen functional groups from a

molecule, and in the case of glucose this treatment gave hexane (in low yield). From this it

was concluded that the six carbons are in an unbranched chain. The presence of an aldehyde

carbonyl group was deduced from cyanohydrin formation, its reduction to the hexa-alcohol

sorbitol, also called glucitol, and mild oxidation to the mono-carboxylic acid, glucuronic

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Carbohydrates to Chemicals

acid. Somewhat stronger oxidation by dilute nitric acid gave the diacid, glucaric acid,

supporting the proposal of a six-carbon chain.

Fig. 15.1. Reactions of glucose

The five oxygens remaining in glucose after the aldehyde was accounted for were thought to

be in hydroxyl groups, since a penta-acetate derivative could be made. These hydroxyl

groups were assigned, one each, to the last five carbon atoms, because geminal hydroxyl

groups are normally unstable relative to the carbonyl compound formed by loss of water.

Glucose and other saccharides are extensively cleaved by periodic acid, thanks to the

abundance of vicinal diol moieties in their structure. This oxidative cleavage, known as the

Malaprade reaction is particularly useful for the analysis of selective O-substituted

derivatives of saccharides, since ether functions do not react. The stoichiometry of

aldohexose cleavage is shown in the following equation.

HOCH2(CHOH)4CHO + 5 HIO4 ----> H2C=O + 5 HCO2H + 5 HIO3

Configuration of Glucose

The four chiral centers in glucose indicate there may be as many as sixteen (24) stereoisomers

having this constitution. These would exist as eight diasteromeric pairs of enantiomers, and

the initial challenge was to determine which of the eight corresponded to glucose. This

challenge was accepted and met in 1891 by the German chemist Emil Fischer.

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15.3

The last chiral center in an aldose chain (farthest from the aldehyde group) was chosen by

Fischer as the D / L designator site. If the hydroxyl group in the projection formula pointed to

the right, it was defined as a member of the D-family. A left directed hydroxyl group (the

mirror image) then represented the L-family. It is important to recognize that the sign of a

compound's specific rotation (an experimental number) does not correlate with its

configuration (D or L). It is a simple matter to measure an optical rotation with a polarimeter.

Determining an absolute configuration usually requires chemical interconversion with known

compounds by stereospecific reaction paths.

Fischer projection formulas and names for the D-aldose family (three to six-carbon atoms)

are shown below.

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Carbohydrates to Chemicals

Emil Fischer made use of several key reactions in the course of his carbohydrate studies.

These are described here

Oxidation

Sugars may be classified as reducing or non-reducing based on their reactivity with Tollens',

Benedict's or Fehling's reagents. If a sugar is oxidized by these reagents it is called reducing,

since the oxidant (Ag(+) or Cu(+2)) is reduced in the reaction, as evidenced by formation of a

silver mirror or precipitation of cuprous oxide. The Tollens' test is commonly used to detect

aldehyde functions; and because of the facile interconversion of ketoses and aldoses under

the basic conditions of this test, ketoses such as fructose also react and are classified as

reducing sugars.

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15.5

When the aldehyde function of an aldose is oxidized to a carboxylic acid the product is called

an aldonic acid. Because of the 2º hydroxyl functions that are also present in these

compounds, a mild oxidizing agent such as hypobromite must be used for this conversion

(equation 1). If both ends of an aldose chain are oxidized to carboxylic acids the product is

called an aldaric acid. By converting aldoses to its corresponding aldaric acid derivative, the

ends of the chain become identical .

Thus, ribose, xylose, allose and galactose yield achiral aldaric acids which are, of course, not

optically active. The ribose oxidation is shown in equation 2 below. Other aldose sugars may

give identical chiral aldaric acid products, implying a unique configurational relationship.

The examples of arabinose and lyxose shown in equation 3 above illustrate this result. A

Fischer projection formula may be rotated by 180º in the plane of projection without

changing its configuration.

Reduction

Sodium borohydride reduction of an aldose makes the ends of the resulting alditol chain

identical, HOCH2(CHOH)nCH2OH, there by accomplishing the same configurational change

produced by oxidation to an aldaric acid. Thus, allitol and galactitol from reduction of allose

and galactose are achiral, and altrose and talose are reduced to the same chiral alditol.

Derivatives of HOCH2(CHOH)nCHO

HOCH2(CHOH)nCO2H (an Aldonic Acid)

HOBr Oxidation

---->

HNO3 Oxidation

---->

H2OC(CHOH)nCO2H (an Aldaric Acid)

NaBH4 Reduction

---->

HOCH2(CHOH)nCH2OH (an Alditol)

Osazone Formation

The Osazone reaction was developed and used by Emil Fischer to identify aldose sugars

differing in configuration only at the alpha-carbon. The equation shows the general form of

the osazone reaction, which effects an alpha-carbon oxidation with formation of a bis-

phenylhydrazone, known as an osazone. Application of the Osazone reaction to D-glucose

and D-mannose demonstrates that these compounds differ in configuration only at C-2.

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Carbohydrates to Chemicals

Chain Shortening and Lengthening

These two procedures permit an aldose of a given size to be related to homologous smaller

and larger aldoses. Thus Ruff degradation of the pentose arabinose gives the tetrose

erythrose. Working in the opposite direction, a Kiliani-Fischer synthesis applied to arabinose

gives a mixture of glucose and mannose. Using these reactions, we can understand the

Fischer's train of logic in assigning the configuration of D-glucose.

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15.7

Anomeric Forms of Glucose

Two different crystalline forms of glucose were reported in 1895. Each of these gave all the

characteristic reactions of glucose, and when dissolved in water equilibrated to the same

mixture. This equilibration takes place over a period of many minutes, and the change in

optical activity that occurs is called mutarotation. These facts are summarized in the

diagram,

A simple solution to this dilemma is achieved by converting the open aldehyde structure for

glucose into a cyclic hemiacetal, called a glucopyranose, as shown in the following diagram.

The linear aldehyde is tipped on its side, and rotation about the C4-C5 bond brings the C5-

hydroxyl function close to the aldehyde carbon. For ease of viewing, the six-membered

hemiacetal structure is drawn as a flat hexagon, but it actually assumes a chair conformation.

The hemiacetal carbon atom (C-1) becomes a new stereogenic center, commonly referred to

as the anomeric carbon, and the α and β-isomers are called anomers.

Fig. 15.2. Anomers of glucose

Cyclic Forms of Monosaccharides

The preferred structural form of many monosaccharides may be that of a cyclic hemiacetal.

Five and six-membered rings are favored over other ring sizes because of their low angle and

eclipsing strain. Cyclic structures of this kind are termed furanose (five-membered) or

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Carbohydrates to Chemicals

pyranose (six-membered), reflecting the ring size relationship to the common heterocyclic

compounds furan and pyran shown on the right.

Ribose, an important aldopentose, commonly adopts a furanose structure, as shown in the

following illustration. The upper bond to this carbon is defined as beta, the lower bond then

is alpha.

Fig. 15.3. Formation of ribofuranose

The cyclic pyranose forms of various monosaccharides are often drawn in a flat projection

known as a Haworth formula, after the British chemist, Norman Haworth. These Haworth

formulas are convenient for displaying stereochemical relationships, but do not represent the

true shape of the molecules. These molecules are actually puckered in a chair conformation.

Examples of four typical pyranose structures are shown below, both as Haworth projections

and as the more representative chair conformers.

The size of the cyclic hemiacetal ring adopted by a given sugar is not constant, but may vary

with substituents and other structural features. Aldolhexoses usually form pyranose rings and

their pentose homologs tend to prefer the furanose form, but there are many counter

examples.

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15.9

Glycosides

Fig. 15.4. Glycoside formation

Acetal derivatives formed when a monosaccharide reacts with an alcohol in the presence of

an acid catalyst are called glycosides. This reaction is illustrated for glucose and methanol in

the diagram below. In naming of glycosides, the "ose" suffix of the sugar name is replaced by

"oside", and the alcohol group name is placed first. As is generally true for most aldols,

glycoside formation involves the loss of an equivalent of water. Since acid-catalyzed

aldolization is reversible, glycosides may be hydrolyzed back to their alcohol and sugar

components by aqueous acid.

Glycosides abound in biological systems. By attaching a sugar moiety to a lipid or

benzenoid structure, the solubility and other properties of the compound may be changed

substantially. Because of the important modifying influence of such derivatization, numerous

enzyme systems, known as glycosidases, have evolved for the attachment and removal of

sugars from alcohols, phenols and amines.

Chemists refer to the sugar component of natural glycosides as the glycon and the alcohol

component as the aglycon.Salicin, one of the oldest herbal remedies known, was the model

for the synthetic analgesic aspirin. Large classes of hydroxylated, aromatic oxonium cations

called anthocyanins provide the red, purple and blue colors of many flowers, fruits and some

vegetables. Peonin is one example of this class of natural pigments, which exhibit

pronounced pH color dependence. The oxonium moiety is only stable in acidic environments

and the color changes or disappears when base is added. The complex changes that occur

when wine is fermented and stored are in part associated with glycosides of anthocyanins.

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Carbohydrates to Chemicals

Finally, amino derivatives of ribose, such as cytidine play important roles in biological

phosphorylating agents, coenzymes and information transport and storage materials

Disaccharides

Two joined monosaccharides are called disaccharides and represent the simplest

polysaccharides. They are composed of two monosaccharide units bound together by a

covalent bond known as a glycosidic linkage formed via a dehydration reaction, resulting in

the loss of a hydrogen atom from one monosaccharide and a hydroxyl group from the other.

Some examples of disaccharides are;

Cellobiose: 4-O-β-D-Glucopyranosyl-D-glucose

Maltose : 4-O-α-D-Glucopyranosyl-D-glucose

Gentiobiose : 6-O-β-D-Glucopyranosyl-D-glucose Trehalose : α-D-Glucopyranosyl-α-D-glucopyranoside

Cellobiose, maltose and gentiobiose are hemiacetals they are all reducing sugars (oxidized

by Tollen's reagent). Trehalose, a disaccharide found in certain mushrooms, is a bis-acetal,

and is therefore a non-reducing sugar. Acid-catalyzed hydrolysis of these disaccharides

yields glucose as the only product. Enzyme-catalyzed hydrolysis is selective for a specific

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15.11

glycoside bond, so an alpha-glycosidase cleaves maltose and trehalose to glucose, but does

not cleave cellobiose or gentiobiose. A beta-glycosidase has the opposite activity.

Although all the disaccharides shown here are made up of two glucopyranose rings, their

properties differ in interesting ways. Maltose, sometimes called malt sugar, comes from the

hydrolysis of starch. It is about one third as sweet as cane sugar (sucrose), is easily digested

by humans, and is fermented by yeast. Cellobiose is obtained by the hydrolysis of cellulose.

It has virtually no taste, is indigestible by humans, and is not fermented by yeast. Some

bacteria have beta-glucosidase enzymes that hydrolyze the glycosidic bonds in cellobiose and

cellulose. The presence of such bacteria in the digestive tracts of cows and termites permits

these animals to use cellulose as a food. Finally, it may be noted that trehalose has a

distinctly sweet taste, but gentiobiose is bitter. Sucrose, or cane sugar, is our most commonly

used sweetening agent. It is a non-reducing disaccharide composed of glucose and fructose

joined at the anomeric carbon of each by glycoside bonds (one alpha and one beta).

Polysaccharides

polysaccharides are large high-molecular weight molecules constructed by joining

monosaccharide units together by glycosidic bonds. They are sometimes called glycans. The

most important compounds in this class, cellulose, starch and glycogen are all polymers of

glucose. Cotton fibres are essentially pure cellulose, and the wood of bushes and trees is

about 50% cellulose.

Cellulose As a polymer of glucose, cellulose has the formula (C6H10O5)n where n ranges

from 500 to 5,000, depending on the source of the polymer. The glucose units in cellulose are

linked in a linear fashion, as shown below. The beta-glycoside bonds permit these chains to

stretch out, and this conformation is stabilized by intramolecular hydrogen bonds. A parallel

orientation of adjacent chains is also favored by intermolecular hydrogen bonds. Although an

individual hydrogen bond is relatively weak, many such bonds acting together can impart

great stability to certain conformations of large molecules. Most animals cannot digest

cellulose as a food, and in the diets of humans this part of our vegetable intake functions as

roughage and is eliminated largely unchanged.

Some animals (the cow and termites, for example) harbour intestinal microorganisms that

breakdown cellulose into monosaccharide nutrients by the use of beta-glycosidase enzymes.

Cellulose is commonly accompanied by a lower molecular weight, branched, amorphous

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Carbohydrates to Chemicals

polymer called hemicellulose. In contrast to cellulose, hemicellulose is structurally weak and

is easily hydrolyzed by dilute acid or base. Also, many enzymes catalyze its hydrolysis.

Hemicelluloses are composed of many D-pentose sugars, with xylose being the major

component. Mannose and mannuronic acid are often present, as well as galactose and

galacturonic acid.

Fig. 15.5. Cellulose structure

Fig. 15.6. Representative partial structure of amylose

Starch is a polymer of glucose, found in roots, rhizomes, seeds, stems, tubers and corms of

plants, as microscopic granules having characteristic shapes and sizes. Most animals,

including humans, depend on these plant starches for nourishment. The structure of starch is

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15.13

more complex than that of cellulose. The intact granules are insoluble in cold water, but

grinding or swelling them in warm water causes them to burst. The released starch consists of

two fractions.

About 20% is a water soluble material called amylose. Molecules of amylose are linear

chains of several thousand glucose units joined by alpha C-1 to C-4 glycoside bonds.

Amylose solutions are actually dispersions of hydrated helical micelles. The majority of the

starch is a much higher molecular weight substance, consisting of nearly a million glucose

units, and called amylopectin. Molecules of amylopectin are branched networks built from

C-1 to C-4 and C-1 to C-6 glycoside links, and are essentially water insoluble.

Fig. 15.7. Representative partial structure of amylopectin

Glycogen is a polysaccharide of glucose (Glc) which functions as the primary short term

energy storage in animal cells. Glycogen is the analogue of starch, a less branched glucose

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Carbohydrates to Chemicals

polymer in plants, and is commonly referred to as animal starch, having a similar structure

to amylopectin. Glycogen forms an energy reserve that can be quickly mobilized to meet a

sudden need for glucose, but one that is less compact than the energy reserves of triglycerides

(fat). It is made primarily by the liver and the muscles, but can also be made by the brain.

The uterus also stores glycogen during pregnancy to nourish the embryo.

Glycogen is a highly branched polymer that is better described as a dendrimer of about

60,000 glucose residues and has a molecular weight between 106 and 107 daltons (~4.8

million). Most of Glc units are linked by α-1,4 glycosidic bonds, approximately 1 in 12 Glc

residues also makes -1,6 glycosidic bond with a second Glc, which results in the creation of

a branch. Glycogen does not possess a reducing end.

Fig. 15.8. Glycogen

Synthetic Modification of Cellulose

Cotton, probably the most useful natural fiber, is nearly pure cellulose. Crude cellulose is

also available from wood pulp by dissolving the lignan matrix surrounding it. These less

desirable cellulose sources are widely used for making paper. In order to expand the ways in

which cellulose can be put to practical use, chemists have devised techniques for preparing

solutions of cellulose derivatives that can be spun into fibers, spread into a film or cast in

various solid forms. A key factor in these transformations are the three free hydroxyl groups

on each glucose unit in the cellulose chain, --[C6H7O(OH)3]n--. Esterification of these

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15.15

functions leads to polymeric products having very different properties compared with

cellulose itself.

Cellulose Nitrate, first prepared over 150 years ago by treating cellulose with nitric acid, is

the arliest synthetic polymer to see general use. The fully nitrated compound, --

[C6H7O(ONO2)3]n--, called guncotton, is explosively flammable and is a component of

smokeless powder. Partially nitrated cellulose is called pyroxylin. Pyroxylin is soluble in

ether and at one time was used for photographic film and lacquers. The high flammability of

pyroxylin caused many tragic cinema fires during its period of use. Furthermore, slow

hydrolysis of pyroxylin yields nitric acid, a process that contributes to the deterioration of

early motion picture films in storage.

Cellulose Acetate, --[C6H7O(OAc)3]n--, is less flammable than pyroxylin, and has replaced it

in most applications. It is prepared by reaction of cellulose with acetic anhydride and an acid

catalyst. The properties of the product vary with the degree of acetylation. Some chain

shortening occurs unavoidably in the preparations. An acetone solution of cellulose acetate

may be forced through a spinnerette to generate filaments, called acetate rayon that can be

woven into fabrics.

Viscose Rayon , is prepared by formation of an alkali soluble xanthate derivative that can be

spun into a fiber that reforms the cellulose polymer by acid quenching. The following general

equation illustrates these transformations. The product fiber is called viscose rayon.

H3O(+)

NaOH

RO-CS2(-) Na(+)

RO(-) Na(+) + S=C=S

ROH

cellulose

viscose solution

rayon

Catalytic conversion of carbohydrates

Carbohydrates are the main source of renewables used for the production of bio-based

products. Sucrose and starch are the major sources. Polysaccharides such as inulin are

gaining importance as a source of fructose. Two carbohydrates of animal origin, lactose and

chitin are also used commercially.

Multistep reactions carried out by cascade catalysis without intermediate product

recovery decrease operating time and may reduce considerably the amount of waste

produced. For example, sorbitol can be obtained in one-pot reaction from starch-derived

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Carbohydrates to Chemicals

polysaccharides using ruthenium supported on acidic Y-zeolite. The acidic sites of the zeolite

catalyze the polysaccharide hydrolysis yielding transiently glucose, which is hydrogenated to

sorbitol on ruthenium. Similarly, 2,5-Furanedicarboxylic acid, a potential substitute for

terephtalic acid is obtained in one-pot reaction over a bifunctional acidic and redox catalyst

consisting of cobalt acetylacetonate encapsulated in solgel silica. In the three former

examples, the reactions steps took place on heterogeneous catalysts. However, cascade

catalysis without recovery of intermediate products may involve enzymatic catalysis,

homogeneous catalysis and heterogeneous catalysis. Combination of enzymatic and chemical

steps can give a better yield.

Hydrogenation of glucose and derivatives

Glucose issued from starch or sucrose hydrolysis is hydrogenated to sorbitol , a commodity

product used in food, pharmaceutical and chemical industries as well as an additive in many

end-products.Mannitol and gluconic acid are the main by-products of this reaction.

Fig.

15.9. Hydrogenation of glucose

Catalysts allowing a 100% conversion and 99% selectivity are required. Also, they should be

stable after many recycling operations or for extended period of time on stream in continuous

reactor. Most of the industrial production is still conducted batch-wise on Raney nickel

catalysts promoted with electropositive metal atoms such as molybdenum and chromium ,but

because of the risk of nickel or metallic promoter leaching, they tend to be replaced by

carbon supported ruthenium catalysts which are also more active. However, active carbon

Synthetic Strategies in Chemistry

15.17

powders are difficult to handle and recycle in batch operation, therefore a continuous process

with formed carbon support are desirable.

Hydrogenation of glucose to sorbitol was achieved on ruthenium catalysts supported on

activated carbon cloths (ACC) obtained by carbonization and CO2 activation of woven rayon

.Catalyst 0.9 wt.% Ru/ACC was loaded with ruthenium by cationic exchange or anionic

adsorption both giving an homogeneous distribution of 2 nm ruthenium particles in carbon

fibers. The ACC was clamped on a support fitting along the autoclave walls thus allowing an

easy recycling of the catalyst since, unlike catalysts in powder form, no filtration are required

and there is no attrition or leaching.

There is a great interest to convert C6 carbohydrates available in large supply from starch

or sucrose into C5 and C4 polyols that are little present in biomass and find many

applications in food and non-food products. Glucose can be converted to arabitol by an

oxidative decarboxylation of glucose to arabinonic acid followed by hydrogenation to

arabitol.The main drawback of this reaction is the formation of deoxy products. Aqueous

solutions (20 wt. %) of arabinonic acid were hydrogenated on Rucatalysts in batch reactor.

The selectivity was enhanced by adding small amounts of anthraquinone-2-sulfonate (A2S),

which decreased the formation of deoxy by-products.

Fig. 15.10. Oxidative decarboxylation of glucose

Dehydroxylation of carbohydrates

Deoxyhexitols consisting of C6 diols, triols, and tetrols are well suited to replace polyols

derived from petrochemistry for applications in polyester and polyurethane manufacture.

Sorbitol was taken as model molecule to study the hydrogenolysis to C4C6 products. To

improve the selectivity to deoxyhexitols, catalysts and reaction temperature were optimized

to favor the rupture of COH bonds (dehydroxylation reactions) rather than CC bond

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Carbohydrates to Chemicals

rupture. Copper-based catalysts, which have a low activity for hydrogenolysis of CC bonds,

were employed to treat 20 wt.% aqueous sorbitol solutions in the temperature range of 180

240 8C. In contrast, operating in the presence of palladium catalysts at 250 0C under 80 bar

of hydrogen pressure, cyclodehydration reactions of sorbitol and mannitol occurred with

formation of cyclic ethers: isosorbide, 2,5-anhydromannitol, 2,5-anhydroiditol, and 1,4-

anhydrosorbitol .

Catalytic oxidation of mono- and di-saccharides

Oxidation reactions are widely used for upgrading carbohydrates to varieties of high added

value chemicals used in detergents or pharmaceuticals (Vitamin C). To replace the non-green

hypochlorite agent by environmentaly friendly reagents, the catalytic system was improved.

Oxidation reactions with H2O2 mediated by metal phthalocyanine catalysts have also proved

very efficient to oxidize various carbohydrates including the oxidation of insoluble substrates

such as native starch.

Fig. 15.11.

Hydrogenolysis of sorbitol to C6 polyols

Glucose oxidation to gluconic acid, a biodegradable chelating agent and an intermediate in

food and pharmaceutical industry, was achieved with air oxidation in the presence of

palladium catalysts. Unpromoted palladium catalysts were active in glucose oxidation, but

the rate of reaction was low because of the over-oxidation of Pd-surface, and side oxidation

reactions decreased the selectivity. Using PdBi/C catalysts (5 wt.% Pd, Bi/Pd = 0.1)

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15.19

prepared by deposition of bismuth on the surface of 12 nm palladium particles, the rate of

glucose oxidation to gluconate was 20 times higher and the selectivity at near total

conversion was high on the fresh and recycled catalysts. Bismuth was assumed to act as a

cocatalyst protecting palladium from over-oxidation because of its stronger affinity for

oxygen. The metal-catalyzed oxidation gave comparable selectivity and higher productivity

than enzymatic glucose oxidation.

Catalytic Conversion of Polysaccharides

Polysaccharides are widely available renewable polymers but it is difficult to find cost

effective process to convert them to valuable end-products. Due to its large availability and

low cost, native starch has been used for a long time in the preparation of different end-

products.

To meet specific hydrophilic properties native starch has been either modified by oxidation

or by grafting hydrocarbon chains .Hydrophilic starch obtained by partial oxidation is widely

used in paper and textile industries and can be potentially applied in a variety of applications,

e.g., for the preparation of paints, cosmetics, and super absorbents. The oxidation occurs at

the C6 primary hydroxyl group or at the vicinal diols on C2 and C3 involving a cleavage of

the C2C3 bond to give carbonyl and carboxyl functions.

Fig. 15.12. Hydrophilic starch obtained by partial oxidation

Several transition metal catalysts based on Fe, Cu or W salts (0.010.1 mol%) have been

proposed to activate H2O2, which is a well-suited oxidant from an environmental and

economical point of view. However, the concentration of metal ions was quite high and

heavy metals were retained by the carboxyl functions of oxidized starch, which has good

complexing properties. Efficient catalytic methods for native starch oxidation withH2O2 in

the presence of iron tetrasulfophthalocyanine (FePcS) were proposed .Oxidation of starch

aqueous suspension in the presence of iron phthalocyanine gives both carboxylic and

carbonyl groups.

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Carbohydrates to Chemicals

To prepare more hydrophobic starches for specific applications, partial substitution of starch

with acetate, hydroxypropyl, alkylsiliconate or fatty-acid ester groups were described in the

literature. This can also be achieved by grafting octadienyl chains by butadiene

telomerization.

Fig. 15.13. Grafting of octadienyl chains on sucrose through butadiene telomerization

The reaction was first conducted with success on sucrose. The degree of substitution (DS)

obtained was controlled by the reaction time. Thus under standard conditions (0.05%

Pd(OAc)2/TPPTS, NaOH (1N)/iPrOH (5/1), 50 8C) the DS was 0.5 and 5 after 14 and 64 h

reaction time, respectively. The octadienyl chains were hydrogenated quantitatively in the

presence of 0.8 wt.% [RhCl(TPPTS)3] catalyst in H2O/EtOH (50/10) mixture yielding a very

good biodegradable surfactant.

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