Cellulose acetylation catalyst

Most cellulose acetate is manufactured by a solution process, ie, the cellulose acetate dissolves as it is produced. The cellulose is acetylated with acetic anhydride acetic acid is the solvent and sulfuric acid the catalyst. The latter can be present at 10—15 wt % based on cellulose (high catalyst process) or at ca 7 wt % (low catalyst process). In the second most common process, the solvent process, methylene chloride replaces the acetic acid as solvent, and perchloric acid is frequentiy the catalyst. There is also a seldom used heterogeneous process that employs an organic solvent as the medium, and the cellulose acetate produced never dissolves. More detailed information on these processes can be found in Reference 28. 

Perchloric acid is a weU-known acetylation catalyst, especially in the fibrous method of preparing cellulose triacetate. Unlike sulfuric acid, perchloric acid does not combine with cellulose (78), ie, it does not form esters, and therefore virtually complete acetylation (DS 3.0, 44.8% acetyl) occurs. However, the extremely corrosive nature of perchloric acid and explosive nature of its salts have precluded its use industrially as an acetylation catalyst. 

Zinc chloride is a Lewis acid catalyst that promotes cellulose esterification. However, because of the large quantities required, this type of catalyst would be uneconomical for commercial use. Other compounds such as titanium alkoxides, eg, tetrabutoxytitanium (80), sulfate salts containing cadmium, aluminum, and ammonium ions (81), sulfamic acid, and ammonium sulfate (82) have been reported as catalysts for cellulose acetate production. In general, they require reaction temperatures above 50°C for complete esterification. Relatively small amounts (<0.5%) of sulfuric acid combined with phosphoric acid (83), sulfonic acids, eg, methanesulfonic, or alkyl phosphites (84) have been reported as good acetylation catalysts, especially at reaction temperatures above 90°C. 

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). 

Although there has been much discussion of the chemistry of cellulose acetylation, it is now generally agreed that the sulfuric acid is not a catalyst in the normal sense of the word, but rather that it reacts with the cellulose to form a sulfo ester. The acetic anhydride is the reactant that provides the acetate groups for esterification. The acetylation mixture consists of the output from the acetic anhydride recovery unit, being about 60 percent acetic acid and 40 percent acetic anhydride, in an amount 5-10 percent above the stoichiometric requirement, to which has been added 10-14 percent sulfuric acid based on the weight of cellulose used. The reaction is exothermic and requires that the heat be dissipated. 

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... 

A broader exploitation of cellulose solubility in ionic liquids lies in modification of cellulose into more useful forms. Cellulose acetate is currently the most useful derivative of cellulose, with uses in photographic film, as a synthetic fibre and as a component of adhesives. Acetylation of cellulose has been carried out effectively in l-allyl-3-methylimidazolium chloride [Amim][Cl], [C4mim CI], [C2mim][Cl], l-butyl-2,3-dimethylimidazolium chloride [C4dmim][Cl] and l-allyl-2,3-dimethyl-imidazolium bromide [Admim][Br] [136, 159-162], The first report of cellulose acetylation in an ionic liquid involved a one-step reaction in [Amim][Cl] without catalyst, yielding cellulose acetate with a controllable degree of substitution [136], Yields of cellulose acetate of up to 86% have been obtained via reaction in [C4mim] [Cl] at 80 °C for 2 h [160],... 

More recently, Eastman Chemical Company published three patents involving the use of ionic liquids containing carboxylate anions for cellulose acetylation [163-165], Ionic liquids with carboxylate anions resulted in faster acetylation of cellulose at lower temperatures relative to the rate in ionic liquids with the same cation and a chloride anion. Transition metals such as zinc were found to be good catalysts for the acetylation of cellulose in ionic liquids [164],... 

A comparison of cellulose acetate solution properties as a function of wood-pulp source and acetylation catalyst level is shown in Table 11.2. 

Cellulose dissolved in suitable solvents, however, can be acetylated in a totally homogeneous manner, and several such methods have been suggested. Treatment in dimethyl sulfoxide (DMSO) with paraformaldehyde gives a soluble methylol derivative that reacts with glacial acetic acid, acetic anhydride, or acetyl chloride to form the acetate (63). The maximum degree of substitution obtained by this method is 2.0 some oxidation also occurs. Similarly, cellulose can be acetylated in solution with dimethylacetamide—paraformaldehyde and dimethylformamide-paraformaldehyde with a potassium acetate catalyst (64) to provide an almost quantitative yield of hydroxymethylceUulose acetate. 

In the fibrous acetylation process, part or all of the acetic acid solvent is replaced with an inert dilutent, such as toluene, benzene, or hexane, to maintain the fibrous stmcture of cellulose throughout the reaction. Perchloric acid is often the catalyst of choice because of its high activity and because it does not react with cellulose to form acid esters. Fibrous acetylation also occurs upon treatment with acetic anhydride vapors after impregnation with a suitable catalyst such as zinc chloride (67). 

Ca.ta.lysts for Acetylation. Sulfuric acid is the preferred catalyst for esterifying cellulose and is the only known catalyst used commercially for this function. The role of sulfuric acid during acetylation has been discussed (77,78). In the presence of acetic anhydride, sulfuric acid rapidly and almost quantitatively forms the cellulose sulfate acid ester (77). Even in the absence of anhydride, the sulfuric acid is physically or mechanically retained (sorbed) on the cellulose. The degree of absorption is a measure of the reactivity or accessibiUty of different celluloses. 

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...

High temperature acetylation of cellulose above 50°C produces cellulose acetate from low purity wood pulp cellulose in shorter reaction times. In a high temperature method recently disclosed (102), cellulose reacts with 200—400% acetic anhydride in the presence of <5% acid catalyst at 68—85°C for 3—20 min. After the acid catalyst is neutralized with magnesium acetate, the cellulose acetate is hydrolyzed at 120°C for two hours (103). Several modified catalyst systems have been developed for acetylation of cellulose above 90°C (89,90). 

As previously discussed, solvents that dissolve cellulose by derivatization may be employed for further functionahzation, e.g., esterification. Thus, cellulose has been dissolved in paraformaldehyde/DMSO and esterified, e.g., by acetic, butyric, and phthalic anhydride, as well as by unsaturated methacrylic and maleic anhydride, in the presence of pyridine, or an acetate catalyst. DS values from 0.2 to 2.0 were obtained, being higher, 2.5 for cellulose acetate. H and NMR spectroscopy have indicated that the hydroxyl group of the methy-lol chains are preferably esterified with the anhydrides. Treatment of celliflose with this solvent system, at 90 °C, with methylene diacetate or ethylene diacetate, in the presence of potassium acetate, led to cellulose acetate with a DS of 1.5. Interestingly, the reaction with acetyl chloride or activated acid is less convenient DMAc or DMF can be substituted for DMSO [215-219]. In another set of experiments, polymer with high o -celliflose content was esterified with trimethylacetic anhydride, 1,2,4-benzenetricarboylic anhydride, trimellitic anhydride, phthalic anhydride, and a pyridine catalyst. The esters were isolated after 8h of reaction at 80-100°C, or Ih at room temperature (trimellitic anhydride). These are versatile compounds with interesting elastomeric and thermoplastic properties, and can be cast as films and membranes [220]. 

Acetate A general name for processes for making cellulose acetate fibers. Cellulose is acetylated, dissolved in acetone, and spun into fibers by injecting through orifices into heated chambers. Cellulose mono-acetate is made by acetylating with a mixture of acetic acid, acetic anhydride, and sulfuric acid as the catalyst. Cellulose tri-acetate is made in a similar fashion, but using perchloric acid as the catalyst, and dry-spinning from a solution in ethanol/ methylene chloride. Cellulose tri-acetate fibers were first made commercially by Courtaulds in London in 1950. 

Cellulose acetate is the most important ester derivative of cellulose. It is produced by acetylation of cellulose using acetic anhydride in acetic acid in the presence of a strong acid catalyst (usually sulfuric acid). In Eq. 9-29 the symbol is a general means of representing a polymer molecule minus the functional group of interest and —OH specifically... 

The first application of the ferrous ion-hydrogen peroxide initiation for polymerizing vinyl monomers on and into cellulose fibers has been reported by Landeias and Whewell (41) in three successive papers. They are apparently the first who applied the "anchored catalyst technique, which other people have termed "in situ polymerization to cellulose grafting. The authors internally deposited methyl methacrylate, acrylonitrile, styrene, methyl vinyl ketone and methacrylamide in amounts between 10 and 80%. No attempt had been made to determine if actual grafting had occurred. In 1961 Richards (42) studied this question in great detail. Products obtained by polymerization of acrylonitrile and of styrene in viscose rayon were acetylated. Fractionation of... 

Acetylation of cellulose to the triacetate has been carried out without breaking down of the structure with acetic anhydride containing pyridine to help open up the cell wall structure and to act as a catalyst (71). This led Stamm and Tarkow (72) to test the liquid phase reaction on wood. High dimensional stabilization without break down of the structure was obtained, but excessive amounts of chemical were used. They hence devised a vapor phase method at atmospheric pressure that proved suitable for treating veneer up to thicknesses of 1/8 inch. Acetic anhydride pyridine vapors generated by heating an 80-20% mixture of the liquids were circulated around sheets of veneer suspended in a box lined with sheet stainless steel. Hardwood veneer,... 

The industrial preparation of cellulose diacetate employs acetic anhydride with sulphuric acid as catalyst. The reaction is conducted at low temperature and cellulose starts to dissolve in the acetylation bath as the reaction progresses. The reaction is conducted until practically full acetylation. The homogeneous solution obtained is then hydrolysed to reduce the DS to 2.4. Precipitation in dilute acetic acid, then washing with water and finally drying produce cellulose acetate flakes. 

Catalysis, enzyme-substrate and intermediate compound theory in homo-and heterogeneous, V, 51 Catalysts, for acetonation, III, 51 for acetylation of starch, I, 284, 286 Bourguel s, II, 109, 110, 113 for esterification of cellulose, I, 312 in oxidation of carbohydrates by halogens, III, 177... 

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