The recovery process and esterification method can be applied to producing chemical feedstock molecules from free carboxylic acids derived from a fermentation broth. An advantage of the present invention is that one can use free carboxylic acids directly from a fermentation broth and generate corresponding esters therefrom without the need to isolate or purify the acids from the fermentation broth, as is necessary in conventional extractions from broth. In com› parison to certain fermentation processes that neutralize or convert the carboxylic acids to their salts, the present process provides an easier way to isolate and extract car› boxylic acids from a fermentation broth. The present process eliminates a need for titration and neutralization of the fermentation broth that can precipitate metal salts, and certain purification steps to produce a stock platform chemi› cal. The free carboxylic acids are converted into esters, which are simpler to process and extract by distillation or other purification techniques without the use of expensive and complicated chromatographic separation colunms or resins. For instance in a conventional process, one would need to use ion exchange chromatography to isolate the acids. A small amount of salts may unavoidably carry-over after the ion exchange. Hence, one may require multiple units of operation to purify the acid to an acceptable quality level. With each added operational unit the costs of the overall process increases. Moreover, in synthesizing the ester of the acid, one can recover the salt as a carbonate or hydroxide, which can be used to regenerate the fermentation broth, and minimize waste. By converting the organic acids to their corresponding esters, we can avoid such issues. 

Conventionally, esters are produced when carboxylic acids are heated with alcohols in the presence of an acid catalyst. The mechanism for the formation of an ester from an acid and an alcohol are the reverse of the steps for the acid-catalyzed hydrolysis of an ester. The reaction can go in either direction depending on the conditions used. In a typical esterification process, a carboxylic acid does not react with an alcohol unless a strong acid is used as a catalyst. The catalyst is usually concentrated sulfuric acid or hydrogen chloride. Protonation makes the carbonyl group 

more electrophilic and enables it to react with the alcohol, 45 which is a weak nucleophile. 

In general terms, the present esterification method involves a reaction of fermentation-derived, free organic carboxylic acid with an alcohol in a carbon dioxide (CO$_{2}$ )- predominant atmosphere in substantial absence of any other acid catalyst to produce esters. The esterification reaction is performed in solution under conditions that are either at supercritical, critical or near critical temperatures and/or pressures for at least one of the alcohol or CO$_{2}$ . Under such conditions, we believe that CO$_{2 }$ self-generates or functions in situ as an acid catalyst, and regenerates back after the esterification reaction is completed. It is believed that a reactive intermediate (monoalkylcarbonic acid) is being made in situ in large enough quantities to drive esterification and affect ester production. This intermediate, having a similar pKa ( e.g., -4-5) as the free carboxylic acid, functions as a carbonic acid, which is much weaker than the usual strong acids. The observed trend of greater ester conversion at higher temperatures adduces a relatively large energy of activation for this process. As used herein, the term "sub- 

stantial absence" refers to a condition in which an acid catalyst is either largely or completely absent, or is present in de mininmis or trace amount ofless than catalytic efficacy. In other words, no other acid catalyst is present, or is present at a level less than 10%, 5%, 3%, or 1 % weight/weight relative to the carboxylic acid in the reaction. 

An advantageous feature of the inventive process is that activation of the free carboxylic acid as an acyl halide (e.g., fluoride, chloride, bromide) or by using strong mineral acids is Uilllecessary. Acy! halides are inconvenient to use because these species are inherently reactive, have issues with sta› bility, waste treatment, and can be cumbersome and costly to make. 

In the present process, carbon dioxide functioning as a catalyst instead of the usual strong acids removes the need to introduce a strong acid into the esterification reaction. This feature can circumvent the usual need to adjust pH values in order to remove the catalyzing acid, enabling a simpler and cleaner synthesis. One can simply proceed to filter the resultant product to remove alkali or alkaline earth metal carbonate or other salts. A cleaner product will save costs in purification and downstream processing for conver› sion to other chemical feedstock. 

The process described herein is a more environmentally benign way of producing esters. As it is believed that the carbon dioxide can self-generate an acid catalyst in situ in the presence of the alcohol during the esterification reaction, the present method does not require the use or addition of another acid catalyst species. In other words, the reaction kinetics with CO$_{2 }$ alone can drive the esterification in the substantial absence of any other acid catalyst. To reiterate, the present process does not require activation of free acids as, for example, an acyl chloride or by strong acids (i.e., Fischer esterification). 

In general, the esterification is conducted at an operational or reaction temperature between about 150° C. to about 250° C., at a reaction pressure of between about 400 psi and 2,500 psi or 3,000 psi (gauge), for an extended period, such as about 4 hours, up to about 12 hours. Typically, the tempera› ture can be in a range between about 170° C. or 190° C. to about 230° C. or 245° C. (e.g., 175° C., 187° C., 195° C. or 215° C.), and the operational pressure is between about 900 psi or 950 psi and about 2,200 psi or 2,400 psi ( e.g., 960 psi, 980 psi, 1020 psi or 1050 psi). Alternatively, the temperature can be in a range between about 180° C. to about 245° C. (e.g., about 185° C. or 200° C. or 210° C. to about 220° C. or 235° C. or 240° C.), and the operational pressure is between about 1000 psi and 2.350 psi (e.g., 1,100 psi, 1,200 psi, 1,550 psi, 1,750 psi, 1,810 psi, or 1,900 psi). Other temperatures may be within a range, for example, from about 160° C. or 185° C. to about 210° C. or 225° C., and other operational pressures may be within a range, for example, from about 1,150 psi or 1,500 psi to about 1,800 psi or 2,000 psi. 

These reaction temperatures and pressures correspond to supercritical, critical or near critical conditions for the alcohol( s) or CO$_{2}$ . Table 1 lists, for purpose of illustration, critical parameters for some common solvents (i.e., metha› nol, ethanol, 1-propanol, 1-butanol, water, and CO$_{2}$ ). 

