The potential of cellulose as a renewable chemical feedstock can be harnessed by its hydrolysis to smaller molecules. While complete hydrolysis of cellulose produces glucose, its partial hydrolysis yields cello-oligosaccharides (COS). The chemical structure of COS is analogous to cellulose albeit with a low degree of polymerization of 3-13 monomer units. As a result, COS plays a biological function in plants to indicate degradation of cell wall by pests and diseases. Consequently, COS can be used as an elicitor to preemptively enhance immune defense in plants and counter future threats. In addition, COS is useful as a non-digestible oligosaccharide for humans and has a market in functional food industry. Because of its potential biological application, it is necessary to synthesize COS free from toxic and corrosive impurities. We have been using acid functionalized carbon catalyst, as a completely heterogeneous catalytic system, to produce cello-oligosaccharides from cellulose. The carbon catalyst is made by air-oxidation of activated carbon to introduce acidic functional groups (mainly carboxyl). The catalyst has a porous polyaromatic structure, which adsorbs cellulose by CH-π and hydrophobic interaction. Because both cellulose and carbon are insoluble in water, a mechanochemical treatment is used to enhance contact between the catalyst and substrate before reaction. Upon hydrolysis in water the adsorbed cellulose reacts to form soluble COS, which are removed from the system to obtain a purified COS solution. The carbon catalyst shows higher hydrolysis activity for molecules with larger degree of polymerization and this might explain rapid hydrolysis of adsorbed cellulose to yield cello-oligosaccharides.

Synthesis of cello-oligosaccharides from cellulose by using carbon catalysts

ACS Spring 2022

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