| The effects of pulsed electric fields on alcohol-acid esterification under roomtemperature were systematically studied in this paper. Three analysis have been done thatanalyzed the effects of PEF on esterification in the watery system compared to the waterlesssystem, analyzed the effects of PEF on esterification with catalyst compared to esterificationwithout catalyst and analyzed the effects of PEF on esterification about short carbon chaincarboxylic acid compared to long carbon chain. The mechanism of esterification via pulsedelectric field application was explored from various angles. The main conclusions obtainedwere introduced as follows:1. The effects of pulsed electric fields on ethanol-acetic acid system esterification werestudied. The results indicate that PEF could promote the esterification under differentreaction conditions. The reaction rate significantly increased with increasing the PEFintensity. At the same time, it also could be found that the effect of PEF on esterification wasmore significant at lower than at higher temperatures. when the molar ratio of ethanol toacetic acid was lower than5:1, the reaction rate increased with the molar ratio of ethanol toacetic acid. But when the molar ratio was changed from3:1to5:1, the growth of the reactionrate became slower. If the PEF intensity used is0,6.6,13.3, and20.0kV/cm, the meanactivation energy (Ea) for esterification is76.64,71.50,67.50and59.10kJ/mol, respectively.The relationship between the applied PEF strength (E) and the activation energy decrease(ΔEa) can be that the reduced value of the activation energy was increased with the rising ofthe electric field strength.2. The effects of pulsed electric fields on ethanol-propionic acid system esterificationwere studied. The results indicate that PEF could promote the esterification under differentreaction conditions. when the PEF intensity was lower than13.3kV/cm, the reaction rateincreased with increasing the PEF intensity, and the reaction rate was an optimal value as thePEF intensity was13.3kV/cm. When PEF intensity continue to increase, the reaction ratealmost did not increased with increasing the PEF intensity. At the same time, it also could befound that the effect of PEF on esterification was more significant at lower than at highertemperatures. If the PEF intensity used is0,6.6,13.3, and20.0kV/cm, the mean activationenergy (Ea) for esterification is77.05,72.60,62.85and62.49kJ/mol, respectively. Thereduced value of the activation energy was not linear with the increase of the electric fieldstrength, and the reduced value was an optimal value as the PEF intensity was13.3kV/cm.when the PEF intensity was lower than13.3kV/cm, The relationship between the applied PEF strength (E) and the activation energy decrease (ΔEa) can be that the reduced value of theactivation energywas increased with the rising of the electric field strength. when the PEFintensity was higher than13.3kV/cm, The relationship between the applied PEF strength (E)and the activation energy decrease (ΔEa) should be expressed in a nearly horizontal straightline.3. The effects of pulsed electric fields-sodium bisulfate catalyst on ethanol-acetic acidsystem esterification were studied. The results indicated that the esterification was promotedby PEF-sodium bisulfate catalyst, and the synergy effect was greater than the sum of thesingle effect. If the PEF intensity used was20.0kV/cm, the mean activation energy (Ea) foresterification was decreased from48.50kJ/mol to47.80kJ/mol.4. The mechanism of PEF treatment on esterification in waterless system was exploredusing the energy analysis, FTIR and1HNMR technology. The results indicated that Themechanism of PEF on esterification in waterless system was breaking hydrogen bonds andpolarizing polar group. It lead to the fast formation of reactive intermediates and theactivation energy decreasing, and the reaction rate increased.The effects of PEF on the activation energy in three waterless systems were compared.The effect of PEF on esterification of ethanol-propionic acid was different from PEF onethanol-acetic acid. The reason was that the different carbon chain structure between the twoled to the different effect of PEF on the hydrogen bonds in the two systems. The effect of PEFon esterification with catalyst was strongest, but the effect of PEF on the activation energywas smallest. It showed that the mainly reason of the reaction rate increasing via PEF wasnot decrease of activity energy. It should be that in the system with lower activation energy,the effect of PEF on molecule structure led to the more collision probabilities. So, the reactionrate increased.5. The effects of pulsed electric fields on ethanol-acetic acid watery systemesterification were studied. The results indicated that the effects of PEF on the water systemwas greater than on the waterless system, and the effects of PEF on the water systemincreased with increasing PEF intensity. When the molar ratio of ethanol to acetic acid inwatery system was changed, the reaction rate increased with the higher acetic acidconcentration. If the PEF intensity used was20.0kV/cm, the mean activation energy (Ea) foresterification was decreased from52.20kJ/mol to42.00kJ/mol.6. The mechanism of PEF treatment on esterification in watery system was exploredusing the essence of acetic acid watery system via PEF application. There were mainly tworeasons: First, PEF treatment improve ionization levels of acetic acid to forms more H＋. The presence of H＋lead to the fast formation of reactive intermediates and the activation energydecreasing, and the reaction rate increased. Second, PEF treatment destroyed hydrated ions ormolecules to form more bare ion or molecule. Reaction molecules could meet very often, sothe reaction rate increased. The relationship between reaction rate and activation energy invarious system was compared via PEF application. The result showed that tthe reaction rateincreased more in the higher activation energy system under PEF treatment. |
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