The reactivity of cellulose. III. Acid hydrolysis of eucalypt α‐cellulose in the intermediate molecular weight range

Abstract

AbstractAn α‐cellulose pulp from Eucalyptus wood was hydrolyzed under homogeneous conditions (mainly 67.5% sulfuric acid at 0 and 20°C.) and under heterogeneous conditions with small weight losses (sulfuric acid solutions of up to 50% concentration for 2 hr. at 50°C. and for periods of up to a week in 21.3% acid at 50°C.). Intrinsic viscosities, [η], of the regenerated samples and the residues, respectively, were determined from measurements in cupriethylenediamine solution. Between weight‐average degrees of polymerization of ca. 310‐70 the apparent rate constants decreased with time of hydrolysis. Comparison of data for homogeneous and heterogeneous conditions confirm the influence of supermolecular factors on the course of hydrolysis of the fiber. Finely ground cellulose is hydrolyzed more rapidly than intact fibers. The concept of a welldefined noncrystalline fiber fraction in which the β‐glucosidie linkages are the only labile bonds and are almost immediately accessible to the hydrolyzing agent is inconsistent with these data. It seems rather that in this material a sharp distinction cannot be drawn between the amorphous and crystalline regions. The observed activation energy for homogeneous hydrolysis, ca. 28 kcal./mole, is in agreement with that determined by other workers under different conditions. The Huggins‐Kraemer equation was followed for heterogeneously hydrolyzed samples, with k′ values of 0.47 for [η] > 2, rising to 0.60‐7 for [ηl] < 1. Regenerated samples gave an average k′ of 0.44 for [η] > 1. Infrared spectra of the hydrolyzed fibers revealed no modification of the cellulose I structure over the whole time series and up to 50% acid in the concentration series. Material regenerated from strong acid solution shows the cellulose II spectrum.