Acetylation of cellulose in densified gas

1. A process for producing a cellulosic ester product, the process comprising the steps of:

esterifying a cellulose containing material in the presence of a densified gas with an esterifying agent under pressure to form an intermediate cellulosic ester having a first degree of substitution; and

de-esterifying the intermediate cellulosic ester with a de-esterifying agent, in the presence of the densified gas, to form the cellulosic ester product having a second degree of substitution, which is less than the first degree of substitution,

wherein the densified gas is a near supercritical fluid or a supercritical fluid; and

wherein the esterification and de-esterifying steps occur in the presence of a catalyst selected from a group consisting of trifluoroacetic acid, alkyl sulfonic acids, aryl sulfonic acid, fluorinated alkylsulfonic acid, acetic acid, citric acid, ascorbic acid, tetrabutyl ammonium hydrogen sulfate, 4-(dimethylamino) pyridine, sodium carbonate, pyridine, potassium carbonate, sodium bicarbonate, tetrabutyl ammonium hydroxide, choline hydroxide, sodium acetate, potassium acetate, zinc acetate and ammonium acetate.

2. The process of claim 1 , wherein the first degree of substitution is from 2.5 to about 3.

3. The process of claim 1 , wherein the densified gas is selected from a group consisting of carbon dioxide, nitrogen, hydroflorcarbons, methane, ethane, propane, ethylene, proplyene, 1,1-difluoroethane, 1,1,1,2-tetrafluorethane, and trifluoromethane.

4. The process of claim 1 , wherein the esterifying agent comprises an anhydride.

5. The process of claim 1 , wherein the esterifying agent is selected from a group consisting of acetic anhydride, ketene, C 1-18 carboxylic anhydride, triflouroacetic acid anhydride, C 1-18 carboxylic acid and mixtures thereof.

6. The process of claim 1 , wherein the esterifying step occurs at a pressure of at least 5 MPa.

7. The process of claim 1 , further comprising:

reducing the pressure of the reaction system; and

recovering and recycling at least a portion of the densified gas.

8. The process of claim 7 , further comprising adding additional densified gas to the reaction to extract soluble materials prior to reducing the pressure of the reaction system.

9. The process of claim 1 , wherein the catalyst is recovered and recycled to the de-esterifying step.

10. The process of claim 1 , wherein the esterifying step is conducted at a temperature from 30° C. to 200° C.

11. The process of claim 1 , wherein the second degree of substitution is from 0.5 to 2.9.

12. The process of claim 1 , wherein the second degree of substitution is from 2.35 to 2.55.

13. The process of claim 1 , wherein the de-esterifying agent is selected from a group consisting of water, methanol, ethanol, isopropanol, butanol, pentanol, isobutanol, R—OH, wherein R is from C 1 to C 20 alkyl group, and mixtures thereof.

14. The process of claim 1 , wherein the de-esterifying step occurs at a pressure of at least 5 MPa.

15. A process for making cellulose acetate, comprising:

acetylating a cellulose containing material in the presence of an acid catalyst, an esterifying agent, and near supercritical fluid carbon dioxide or supercritical fluid carbon dioxide to form an intermediate composition; and

de-esterifying the intermediate composition with a de-esterifying agent in the presence of the acid catalyst and the near supercritical fluid or the supercritical fluid carbon dioxide, to form the cellulose acetate with a degree of substitution less than 3,

wherein the acid catalyst is triflouroacetic acid.

16. The process of claim 15 , wherein the esterifying agent comprises an anhydride.

17. The process of claim 15 , further comprising recovering and recycling the carbon dioxide.