2.5 Esterification experiments using the synthetic broth
All batch esterification reactions were carried out in 200 mL round-bottomed flask. The two necked flask was equipped with a sampling port, and a spiral coil reflux condenser in the middle to recycle back liquid after vapors condensed during reaction runs. Oil bath fitted with temperature control was used to maintain the targeted reaction temperature. The whole reaction system was placed on a magnetic stirring plate for controlling the constant stirring of a magnetic stirring bar in the flask.
In a typical experiment, a mixture of methanol, PDO, glycerol and catalyst (Amberlyst-15) were fed into the flask, and the heating was started with condenser water on and with a constant stirring speed of 600 rpm. After the targeted temperature was reached, a mixture of acetic acid and butyric acid was injected into the flask via syringe to initiate the esterification reaction. 0.5 mL sample was withdrawn every 30 min for analyzing the reaction conversion of acetic acid and butyric acid. The initial mass ratio of methanol, PDO, glycerol, acetic acid and butyric acid for all experiments was kept constant at 27.6: 6.2: 3.6: 1.6: 1 to mimic the real composition in crude glycerol fermentation broth after ultrafiltration, concentration and methanol precipitation treatments described in Section 2.6. The total quantity of all reactants without catalyst was fixed at 100 g.
The esterification of acetic acid and butyric acid with methanol catalyzed by Amberlyst-15 in the synthetic broth was examined under the following different reaction conditions: temperature (25, 40, 50 and 70°C), catalyst loading (0, 5, 10, and 20% (w/w)), and water content (0, 2,5, 5 and 10% (w/w)). For different alcohols, same molar ratio of ethanol and 1-propanol were also tested to react with the acids. After the reaction was finished, the reaction mixture was filtered to separate the sample solution from the used solid catalyst. All results shown are averages with standard deviations of experiments conducted in triplicates.