MECHANISM AND KINETICS OF ENZYMATIC HYDROLYSIS OF NATIVE AND SOLVENT TREATED CELLULOSE

TERRY YU-CHIA CHOU, Purdue University

Abstract

This work deals with the effect of crystalline structures on the hydrolysis of cellulose by Trichoderma viride enzymes. Attempts were also made to derive kinetic expressions for the heterogeneous reaction systems. When native cellulose in cotton linters is hydrolyzed by a dilute acid, the degree of polymerization of the solid substrate drops quickly from 2000 (TURN) 2200 to about 100, then levels off at this value even after an extensive degree of hydrolysis of the solid residue as judged by its weight loss. This is known as the "Level Off Degree of Polymerization" or LODP of the native cellulose. When cellulose is treated with such swelling agents as phosphoric acid (85%), its crystalline structure changes from its native form known as Cellulose I into a regenerated form known as Cellulose II. When regenerated cellulose, Cellulose II, is hydrolyzed, DP of the solid residue will also reduce to a new LODP of about 30. The molecular weight distribution of the solid cellulosic residues after different degrees of enzymatic hydrolysis was determined by gel permeation chromatography (GPC). In this study, the appearances of maxima on the molecular weight distribution curves at DP values of about 130 to 150 and of about 28 to 36 are considered as evidences of the presence, in the solid substrate, of Cellulose I and of Cellulose II, respectively. A series of the GPC chromatograms from native cellulose after different time periods of enzymatic hydrolysis are shown in Fig. 15. A peak at 150 DP appeared soon after the hydrolysis was started and its presence persisted until the hydrolysis is nearly completed (89% weight loss after 48 hours). When a native cellulose was treated with 85% phosphoric acid for a long period of time (P18H), all cellulose is expected to be changed into Cellulose II. The corresponding GPC chromatograms are shown in Figure 16. A peak at 36 DP appeared and persisted to the very end of the hydrolysis (98% weight loss). When a cellulose sample was treated with 85% phosphoric acid to an extent less than a complete dispersion (or swelling), the treated cellulose, afterwards, is expected to be a mixture of untreated native cellulose (Cellulose I) and regenerated cellulose (Cellulose II). When such a sample was subjected to the enzymatic hydrolysis, the corresponding GPC chromatograms indeed showed both peaks at 130 DP (for example, see Fig. 17 for the P2H sample). From such observations, a model of enzymatic hydrolysis of native and regenerated cellulose has been formulated. Attempts were then made to derive rate expressions. It was assumed that one part of the cellulose molecules was hydrolyzed to yield soluble sugars directly and another part was broken down to yield the intermediates, type I and/or type II chain segments. The intermediates could be further hydrolyzed to soluble sugars. The reaction scheme can be depicted as follows: cC ('k(,1)) I or II ('k(,2)) S (1-c)C ('k(,3)) S Apparent first order kinetics was assumed for each of the above steps. The kinetic constants were obtained by least squares fitting of the resulting kinetic equations with the experimental data. This model is consistent with the trend of the disappearance of the intact cellulose molecules and of the generation of type I or type II chain segments and of the soluble sugars.

Degree

Ph.D.

Subject Area

Chemical engineering

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