Acetylation processes

A theoretical explanation of the pretreatment action is that the acetic acid soaking greatly reduces the hydrogen bonding of cellulose chains to each other thus, it permits much better diffusion of anhydride into the inner structure of the cellulose, and the rate of diffusion is more uniform than would otherwise be possible. 

Theoretical studies have explained the behavior of these acids as catalytic agents. The effective strength of acidity is the controlling factor in the esterification reaction. This esterification is in a non-aqueous system, usually in the presence of acetic acid as a solvent. The explanation must therefore be approached on the basis of a non-aqueous system. When acetic acid is the solvent medium, this material is considered to combine with hydrogen ions in a manner corresponding to water in aqueous systems, giving the following parallel equations  

Acetic acid in this system may be considered as slightly basic since it combines with hydrogen ions. Salts such as sodium acetate become strong bases. Acids may vary from their corresponding strengths in aqueous solutions in a manner dependent upon the relative tendencies of water and acetic acid to take up hydrogen ions. Measurements have 

Conant and HalP termed perchloric and sulfuric acids super-acids in acetic acid because of their exceptional strength as compared with other acids. Hydrochloric and nitric acids are only slightly stronger than acetic acid itself. 

Various details of procedure are known for the esterification of cellulose with acetic anhydride in acetic acid. solvent. The factors of pretreatment, catalyst concentration, acetylation temperature and time of reaction are kept in balance in order that products of satisfactory appearance and the desired range of viscosity will be obtained. Modifications of this general formula usually involve the use of different solvents to replace part or all of the acetic acid. 

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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). 

The importance of strictly adhering to the conditions above set out in the acetylation process has been emphasised by several analysts. According to Durrans the anhydrous sodium acetate acts rather as a... 

To 10 c.c. of the oil (otto of rose or rose-geranium oil) 10 c.c. of formic acid 100 per cent, (specific gravity 1 22) is added, and the mixture gently boiled under a reflux condenser for one hour. The mixture is cooled, 100 c.c. of water added, and the whole transferred to a separator. The aqueous layer is rejected, and the oil washed with successive quantities of water as in the acetylation process. The formylated oil is dried with anhydrous sodium sulphate, and about 2 grams neutralised and saponified with alcoholic potash in the usual manner. The percentage of citronellol is then calculated from the following formula —... 

Acetylation.—Gitronellal may be quantitatively estimated by the ordinary acetylation process i when the aldehyde is quantitatively converted into isopulegyl acetate, which is then determined by saponification with potash in the ordinary way. Dupont and Labaume have attempted to base a method for the separation of geraniol from citronellal in citron-ella oils on the fact that the citronellal oxime formed by shaking with hydroxylamine solution at the ordinary temperature is not converted into an ester by subsequent acetylation, but into the nitrile of citronellic acid which is stable towards" alkali during the saponification process. 

Asymmetric reductive acetylation process was also applicable to acetox-yaryl ketones. For example, m-acetoxyacetophenone 10 was transformed to... 

Figure 8.4 A schematic of the fibre acetylation process developed by BP Chemicals (Sheen, 1992).

Centre (Sheen, 1992) (Figure 8.4). In this trial, 11 tonnes of acetylated fibre was produced for evaluation, and this led to the development of a continuous fibre acetylation process. The process used a solvent and catalyst-free acetylation system, because the use of either would complicate a commercial process and introduce unnecessary costs. A range of reaction conditions were studied and the following conclusions were drawn ... 

The capital cost of the batch acetylation plant was calculated at 12 million and that of the continuous process plant at 5.5 million. It was assumed that the acetylation facility would be integrated with a traditional fibreboard plant, so that fibre production and drying costs were not included in the analysis. The analysis showed that wood fibre could be acetylated to a WPG of 20 % for a cost of 562 per tonne in a batch process and 384 per tonne in a continuous process. BP subsequently lost interest in the wood acetylation process and sold the pilot plant. 

Table 8.3 Process costings for a batch and continuous fibre acetylation process...

Figure 8.5 A schematic of the A-Cell wood fibre acetylation process.

It would not be possible to have such an infrastructure in place in the start-up phase of a wood acetylation industry. Such a process would rely upon there being sufficient quantities of acetylated wood available as a feedstock, and this would require many years of production. Furthermore, the costs involved in setting up such a plant would in all probability make the acetylation process uneconomic at present. Only when the industry matures would it be possible to bear the costs of building the necessary extra processing facilities. 

These modifications result in a furnish that can be converted into composites of any desired shape, density, and size and provide an opportunity for a manufacturer to distinguish a product line based on quality, uniformity, and performance. Of the various reaction systems studied to date, a simple noncatalyzed acetylation process appears to be closest to implementation on a commercial beisis. 

In 1963 J.H. Rubinstein and H. Taybi reported a rare human disease that induced mental retardation, an increased risk of neoplasia and physical abnormalities, including broad thumbs, big and broad toes, short stature and craniofacial anomalies. The molecular basis of the Rubinstein-Taybi syndrome (RTS) was later discovered to be a disruption of one copy of the human CREB binding protein (CBP or CREB-BP) gene that encodes the histone acetyltransferase CBP [3]. In addition to CBP, all mammals have a closely related acetyltransferase, p300. Retrospectively, RTS was the first disease found to be due to a defective acetylation process [1-3]. 

The trans acetylation process is commercially less attractive with higher plant costs and alcohol recycling but several major producers have patents on this process [54-57]. 

The fibrous acetylation process is performed in the presence of a suitable liquid, such as benzene, in which the reaction product is insoluble and which thereby retains the fiber form. For fibrous acetylation vapor-phase treatment with acetic anhydride can also be used. Besides sulfuric acid, perchloric acid and zinc chloride have been used as catalysts. 

The conversion of the amino group into amide (3289.77-3365.14 cm ) confirmed the acetylation process. The appearance of a characteristic peak at 1725.2 cm (C = 0) in the IR spectra of the graft copolymerized biomaterial compared with its unmodified biomaterial, and this confirmed the formation of the grafted biomaterial (Figure 3.7). 

In attaining a high alpha-cellulose with simultaneous removal of pentosan, there is a considerable loss of other material, presumably hexosan in nature. Although higher purity can be attained by treatment with cold, concentrated sodium hydroxide, this results in mercerization of the cellulose. Mercerized cellulose is of no use in the acetylation process unless it is specially treated in order to avoid inactivation by drying. ... 

This production expansion was made possible in no small degree by a sharp reduction in the cost of manufacture. From an original market price of 1.45 a pound in 1926, cellulose acetate dropped steadily to. 50 a pound in 1936, and further to. 30 in 1940. In addition to a lower cost as a result of the increasing quantity of production, these reductions in price were aided greatly by reductions in the cost of raw materials. Acetic acid became much cheaper during this period, and the cost of conversion of the acid to its anhydride was aided by improved process development. Recovery of acetic acid from aqueous solution also became cheaper with the adoption of extraction and azeotropic distillation processes, replacing the original recovery by evaporation of neutralized solutions. In addition, technical developments in the acetylation process increased the economy of plant unit operations. 

Cellulose acetate is used in films, cigarette filters, and moldings. It is made by reacting cellulose with acetic anhydride. U.S. 5,608,050 (to Eastman) describes an acetylation process that improves the thermal stability of the polymer. U.S. 5,962,677 (to Daicel) describes a process for improving the processing properties of the polymer. Estimate the cost of production of the polymer by each route. 

One important variation of this general procedure which is in commercial practice is the use of a chlorinated solvent, such as methylene chloride, as the solvent in the reaction. Since methylene chloride has a low boiling point, the temperature of the reaction is controlled by refluxing. Andther process, which has been used to a lesser degree, is a fibrous acetylation process. In this ipethod, the reaction is carried out in the presence of several parts of benzene or a similar nonsolvent. The cellulose does not dissolve, but is converted to cellulose triacetate while remaining in fibrous form. The catalyst in this process is getaerally perchloric acid. 

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