Chapter 14
Depolymerization of Cellulose and Cellulose Triacetate in Conventional Acetylation System
Downloaded by COLUMBIA UNIV on March 18, 2013 | http://pubs.acs.org Publication Date: April 17, 1998 | doi: 10.1021/bk-1998-0688.ch014
Shu Shimamoto, Takayuki Kohmoto, and Tohru Shibata Research Center, Daicel Chemical Industries, Ltd., 1239 Shinzaike, Aboshi-ku, Himeji, Hyogo, 671-12 Japan
In a conventional system for the acetylation of cellulose, acetic acid, acetic anhydride and sulfuric acid act as a diluent, an acetylation reagent and a catalyst, respectively; the sulfuric acid acts as a catalyst not only for acetylation but also for depolymerization. The depolymerization behaviors of cellulose and cellulose triacetate were studied so that the degree of polymerization of the final product could be predicted. Model experiments in which the acetylation reagent was absent revealed that the depolymerization of cellulose in earlier stages of the reaction proceeds considerably faster than that of cellulose triacetate, and depolymerization of cellulose triacetate is random whereas that of cellulose is not. Simulations of the final degree of polymerization of the product were carried out using the activation energies obtained by the model experiments. The estimated degree of polymerizations showed good agreements with the experimentally obtained ones when the depolymerization rate of cellulose was assumed to be proportional to the amount of sorbed sulfuric acid.
Among known systems for the acetylation of cellulose, one comprised of acetic acid, acetic anhydride and sulfuric acid has been widely employed in commercial productions of cellulose triacetate (CTA) and cellulose diacetate because of its efficiency (7). Sulfuric acid catalyzes depolymerization as well as acetylation in the system. Although there are some studies on the depolymerization during the pre treatment (2) and acetylation (J), a quantitative prediction of degree of polymerization (DP) of the final product based on depolymerization kinetics of cellulose and the intermediate product has not been reported as far as we know. In an attempt to accomplish this, depolymerization behaviors of cellulose and C T A were studied in the system in the absence of the acetylation reagent in this paper. Result and Discussion Evaluation of DP of cellulose. Although there are several known methods to evaluate DP of cellulose, we established an novel method in which cellulose was converted to C T A without remarkable degradation in order to apply the same MarkHouwink-Sakurada equation to the evaluation of depolymerization of cellulose and 194
©1998 American Chemical Society In Cellulose Derivatives; Heinze, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
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C T A . That was to avoid a systematic error caused by applying different MarkHouwink-Sakurada equations to cellulose and CTA. In order to examine the degradation during the acetylation using a lithium chloride / dimethyl acetamide mixture, cellulose regenerated from C T A with hydrazine was converted to CTA again by the method described in the experimental section. The original and the resulting CTA were analyzed by a GPC low angle laser light scattering ( G P C - L A L L S ) technic. The results are shown in figure 1 as molecular weight distribution curves. The distribution curves were almost identical although there was a slight difference at the higher molecular weight region, and the difference in weight average molecular weight was quite small, only 8 %. From these results, it was concluded that there is no remarkable degradation of cellulose during the acetylation using a lithium chloride / dimethyl acetamide mixture. Depolymerization of cellulose and C T A in acetic acid / sulfuric acid system. Depolymerization of cellulose and C T A in an acetic acid / sulfuric acid system in the absence of an acetylation reagent were studied separately. Cellulose degraded much faster than CTA in earlier stages of the reaction as shown in figure 2. It is well known that number average DP at reaction time t (DP ) in a random depolymerization is given by the following kinetic equation (5,4). mt
1
DPn,t
1
-+ k t
DPn,0
(1)
D P , o is the initial D P n and k is the rate constant of depolymerization. Weight average D P ( D P W ) at reaction time t can be expressed by the similar manner by assuming a Schulz - Zimm type distribution of D P and the polydispersity factor of 2 (5). n
1
DPw,t
1
- I-k.«
DPw,o
+
(2)
2
Equation 2 means that a plot of D P W versus time t gives a linear relationship in a random depolymerization. C T A gave an almost straight line in the plots whereas cellulose did not, as shown in figure 3. These results suggest that depolymerization of C T A in the system is random but that of cellulose is not. In figure 3a, temperature dependence of CTA depolymerization is clearly seen ; the higher the temperature is the faster the depolymerization rate is, as expected. The activation energy of the C T A depolymerization obtained by a plot of the logarithmic rate constant versus the reciprocal temperature was 13.7 kcal/mol. On the other hand, cellulose depolymerization showed much smaller dependence on temperature (figure 3b). It is known that sulfuric acid is sorbed on cellulose in an acetic acid / sulfuric acid system (
