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initiate the esterification reaction. 0.5 mL sample was withdrawn every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.