Hydrocellulose alkaline degradation

In the present study, the role of cellulose physical structure in alkaline reactions was investigated by comparing the alkaline degradation of highly crystalline (cellulose I) fibrous hydrocellulose with that of amorphous (noncrystalline) hydrocellulose. The amorphous substrate was taken as a cellulose model the reactivity of which would most closely approximate that of alkali-soluble cellulose. The availablity of such an approximation to the inherent reactivity of cellulose allowed evaluation of the effects of the more highly ordered structure of the fibrous hydrocellulose. 

Experimental Approach. The experimental study was a comparison of the alkaline degradations of fibrous and amorphous hydrocelluloses in oxygen-free 1.0 NaOH, at 60 and 80 C. The fibrous hydrocellulose was predominantly crystalline (cellulose I) and therefore served as a substrate which would undergo alkaline reactions with significant physical structure effects. In contrast, the amorphous hydrocellulose was noncrystalline (9,10). Thus, it was a substrate which would experience substantially less structural constraint during its alkaline reactions. 

During the course of the alkaline degradations, both physical and chemical structures of the hydrocelluloses were monitored. Hydroxyl accessibility (13) was determined as a practical measure of the fraction of molecules accessible to the alkaline medium. The crystalline structure was characterized by x-ray diffraction (14). 

Alkaline Degradations - Change in Physical Structure. The hydroxyl accessibility of the fibrous hydrocellulose was initially 51.4 0.8%. In contrast, the amorphous substrate had an accessibility of 99.2 1.0%. Exposure of the fibrous hydrocellulose to the alkaline media caused the accessibility to decrease slightly to 50.7 1.0% and 49.1 1.2% at 60 and 80°C, respectively, but accessibility did not change significantly during the reaction periods (0-168 hr). 

Degradation Procedure. Alkaline degradations were conducted in 316 stainless steel laboratory digesters (10). Hydrocellulose substrate... 

During the alkaline degradation of cellulose and cellobiose, a yellow chromo-phore is formed whose spectral properties are dependent on pH e.g. Am 290 nm at pH 12 was shifted to 256 nm at pH 6 for both hydrocellulose and cellobiose. It is suggested that the chromophore is a jS-hydroxy-a-enone which, at acid pH values, tautomerizes to an aliphatic jS-diketone. It was concluded that the chromophore is a minor product since the molar absorption coefficients of such compounds are very large (> 1 x 10 ) and the apparent molar absorption coefficient for hydrocellulose was only about 900. 

Hydrocellulose, as expected, is degraded more rapidly in alkaline solutions, and, likewise, prehydrolysis of wood pulps accelerates and increases their alkali absorption during pulping. Loss of weight from the boiling of hydrocelluloses with dilute alkaline solutions can be greatly diminished by prereduction with sodium borohydride. ... 

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