Abstract
Rosin esters are widely applied as masticatory substances and beverage stabilizers, while classical acid-catalysed processes will lead to metal residue or environmental issues. Super/subcritical CO2–enriched high temperature liquid water (HTLW) as a green acid catalyst in the esterification reaction of rosin with glycerol was investigated. The pH of CO2–H2O binary system, as calculated based on gas–liquid equilibrium, charge balance and chemical equilibrium equations, ranged from 3.49 to 3.70 depending on the reaction conditions, indicating effective acid catalysis. Response surface methodology experiments showed the optimum conditions were 3.5 h, 3.9 MPa CO2 pressure, a rosin-to-glycerol molar ratio of 1.32 and 269°C, and an enhanced esterification yield of 94.74% was achieved, which was superior to that obtained using a ZnO catalyst. It was found that the esterification kinetics was a pseudo first-order reaction, and the enthalpy and entropy of activation were calculated using the Arrhenius–Polanyi equation. The presence of super/subcritical CO2-enriched HTLW catalyst can decrease the activation energy and significantly accelerate the reaction rate.
Highlights
Rosin esters are widely applied as masticatory substances and beverage stabilizers, while classical acid-catalysed processes will lead to metal residue or environmental issues
Response surface methodology experiments showed the optimum conditions were 3.5 h, 3.9 MPa CO2 pressure, a rosin-to-glycerol molar ratio of 1.32 and 269°C, and an enhanced esterification yield of 94.74% was achieved, which was superior to that obtained using a ZnO catalyst
The esterification of rosin has to be carried out at elevated temperatures and using a highly active catalyst [3]. Alkaline catalysts such as CaO, Ca(OH)2 and LiOH were used for the esterification of rosin, these have been largely abandoned because they affect the colour of the rosin ester [5]
Summary
Rosin esters are widely applied as masticatory substances and beverage stabilizers, while classical acid-catalysed processes will lead to metal residue or environmental issues. Super/subcritical CO2–enriched high temperature liquid water (HTLW) as a green acid catalyst in the esterification reaction of rosin with glycerol was investigated. Compared with the original rosin, rosin esters exhibit good oxidation resistance and reduced brittleness, low acid values, high thermal stability, high softening points and light colour, and can be widely applied as coating and adhesives, solder fluxes, neutralizers, masticatory substances, base beverage stabilizers, slow-release coating materials and film forming agents [1,5,6]. Resin acids have a tricyclic phenanthrene structure that results in high steric hindrance, such that there is a high activation energy for esterification reactions. For this reason, the esterification of rosin has to be carried out at elevated temperatures and using a highly active catalyst [3]. There is a requirement for research regarding the green catalytic conversion of rosin to esters
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