Cellulose, acetylation hydrolysis

By-product acetic acid is obtained chiefly from partial hydrolysis of cellulose acetate [9004-35-7]. Lesser amounts are obtained through the reaction of acetic anhydride and cellulose. Acetylation of saHcyHc acid [69-72-7] produces one mole of acetic acid per mole of product and the oxidation of allyl alcohol using peracetic acid to yield glycerol furnishes by-product acid, but the net yield is low. 

The solution process consists of four steps preparation of cellulose for acetylation, acetylation, hydrolysis, and recovery of cellulose acetate polymer and solvents. A schematic of the total acetate process is shown in Figure 9. 

Processes for Triacetate. There are both batch and continuous process for triacetate. Many of the considerations and support faciUties for producing acetate apply to triacetate however, no acetyl hydrolysis is required. In the batch triacetate sulfuric acid process, however, a sulfate hydrolysis step (or desulfonation) is necessary. This is carried out by slow addition of a dilute aqueous acetic acid solution containing sodium or magnesium acetate (44,45) or triethanolamine (46) to neutrali2e the Hberated sulfuric acid. The cellulose triacetate product has a combined acetic acid content of 61.5%. 

Fig. 7. Combined sulfur during preparation of cellulose acetate hydrolysis of sulfate and esters (6). Acetylation schedule A, mixer charged with linters and acetic acid B, minor portion of catalyst added C, began cooling to 18°C D, acetic anhydride added and continued cooling to 16°C E, significant portion...

Recent Developments. A considerable amount of cellulose acetate is manufactured by the batch process, as described previously. In order to reduce production costs, efforts have been made to develop a continuous process that includes continuous activation, acetylation, hydrolysis, and precipitation. In this process, the reaction mixture, ie, cellulose, anhydride, catalyst, and solvent, pass continuously through a number of successive reaction zones, each of which is agitated (92,93). In a similar process, the reaction mass is passed through tubular zones in which the mixture is forced through screens of successively small openings to homogenize the mixture effectively (94). Other similar methods for continuous acetylation of cellulose have been described (95,96). 

Sulfuric acid is a powerful esterification catalyst. It has been widely applied with mixtures of acetic acid and acetic anhydride to promote acetylations of numerous substances. Use of this catalyzed reaction for starch acetylation, however, has not risen to pre-eminence among starch acetylation methods as it has done among cellulose acetylations, although both reactions were discovered at the same time. The underdevelopment of this reaction in the starch field may be due to the following causes (1) sulfuric acid, a powerful acetylation catalyst, strongly catalyzes the hydrolysis of starch molecules and cannot be used for starch acetylations in the concentrations found most effective for cellulose reactions (2) most investigations of this reaction have been made on whole granules... 

Cellulose triacetate is often known as primary cellulose acetate, and partially hydrolyzed material is called secondary cellulose acetate. Many physical and chemical properties of cellulose acetylation products are strongly dependent on the degree of esterification, which is measured by the acetyl content (i.e., the weight of acetyl radical (CH3CO-) in the material) or acetic acid yield (i.e., the weight of acetic acid produced by complete hydrolysis of the ester). 

Cellulose triacetate is obtained by the esterification of cellulose (qv) with acetic anhydride (see Cellulose esters). Commercial triacetate is not quite the precise chemical entity depicted as (1) because acetylation does not quite reach the maximum 3.0 acetyl groups per glucose unit. Secondary cellulose acetate is obtained by hydrolysis of the triacetate to an average degree of substitution (DS) of 2.4 acetyl groups per glucose unit. There is no satisfactory commercial means to acetylate direcdy to the 2.4 acetyl level and obtain a secondary acetate that has the desired solubiUty needed for fiber preparation. 

When the acetylation is completed, microscopic examination of the solution should reveal no undissolved residues. The reaction is terrninated by adding water to destroy the excess anhydride and provide a water concentration of 5—10% for hydrolysis. A 10—25% cellulose acetate concentration is typical. 

Hydrolysis. The primary functions of hydrolysis are to remove some of the acetyl groups from the cellulose triester and to reduce or remove the combined acid sulfate ester to improve the thermal stabiUty of the acetate. 

As mentioned in Section 22.1 the probability of acetylation of any one cellulosic group is strongly dependent on its position in the fibre. Since they cannot be dissolved before acetylation it will be realised that some molecules will be completely acetylated whilst others may be untouched. It is thus necessary first to acetylate completely the cellulose and the resultant triacetate material, which is soluble in certain solvents, may then be back-hydrolysed in solution. Under these conditions the probabilities of hydrolysis of any acetyl groups in one molecule will be similar to the reaction probabilities of these groups in another molecule and products with a reasonably even degree of substitution less than three may be obtained. 

Barsha and Hibbert16 also demonstrated by meins of methylation, acetylation, acetolysis and hydrolysis experiments that the membranes synthesized by the action of A. xylinum on D-fructose and on glycerol were chemically identical with cotton cellulose. 

Other abundant carbohydrates, such as hemicelluloses and pectin, are usually highly branched and thus not very suitable for fiber and film production. Hemicelluloses and some pectins are also acetylated in the native state, which makes them more resistant to enzymatic hydrolysis (20,21) and changes their solubility properties (9-77,75). Branching does not, however, preclude their utilization in such potentially large markets as thickeners and adhesives. Xylans, for example, show such a strong adhesion to cellulose fibers that they are very difficult to remove completely by both acidic and alkaline pulping processes (22). 

Model of Deterioration Mechanisum, As acetyl content decreases due to hydrolysis or oxidation of ester bonding, solute permeability increases. Then concentration of solute( in this case sodium hypochlorite) in the membrane increases and the hydrolysis or oxidation rate increases and so on. It will be more reasonable to assume that hydrolysis or oxidation rate of cellulose acetate in the active surface layer may be accelerated by the action of the nascent oxygen generated from sodium hypochlorite. 

Partially acetylated cellulose (i.e., cellulose with less than three ester groups per repeat unit) is produced by an indirect route. Direct synthesis yields an inhomogeneous product due to insolubility of cellulose in the reaction mixture. Some chains are completely acetylated while others may be completely unreacted. A partially acetylated product is usually produced by controlled hydrolysis of the triacetate. The triacetate is soluble in the reaction mixture and complete solubility ensures that the final product will be more homogeneous. Hydrolysis of the triacetate is carried out by controlled reversal of the esterification reaction by the addition of water or dilute acetic acid. 

The basic cellulose unit contains three hydroxyl groups. The triester cellulose triacetate forms when cellulose is reacted with glacial acetic acid. Hydrolysis removes some of the acetate groups to form a secondary ester, which averages about 2.4 acetyl groups per unit rather than three. The secondary ester is then dissolved in acetone and the solution ejected through a spinneret to form fibers. Cellulose acetate processed in this manner is referred to as acetate rayon, but it may be more commonly known by its trade name Celanese. 

The synthesis started with levoglucosenone 4, available by the pyrolysis of cellulose, e.g. old newspapers. Bromination-dehydrobromination gave the enantiomerically-pure Diels-Alder dienophile 5, which was combined with isoprene to give predominantly the crystalline adduct 1. Hydrolysis and acetylation led to 6, which was carried on to the geometrically-defined allylic alcohol 7 via reduction with Zn-Cu couple. Overman rearrangement of 7 proceeded with high facial control, to give 8. 

Another approach to achieve a fast and total breakdown of cellulose could be the preparation of a soluble derivative and its homogeneous hydrolysis. An example of an acetylation-acetolysis-hydrolysis sequence is given by Hestrin (4). Decomposition and darkening is considerable. 

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