Synthesis and kinetics of tributyl citrate using aluminum dihydrogen tripolyphosphate supported on modified activated carbon as catalyst

Abstract

The mechanism of synthesis process of tributyl citrate using aluminum dihydrogen tripolyphosphate supported on modified activated carbon as a catalyst was investigated by gas chromatography-mass spectrometry, and the kinetic equation was also studied. The results showed that the influence of internal diffusions was eliminated with the particle size smaller than 180 µm, but the influence of external diffusions was eliminated with the stirring speed higher than 450 r/min. The yield of 97% for tributyl citrate was achieved under the the following optimized condition: 1:4.2 of acid to alcohol, 8% of dosage of catalyst accounting for total reaction amount, 5 h of reaction time for water segregating and refluxing, and 150 °C of reaction temperature. The apparent reaction activation energy was Ea = 61.4 KJ/mol. The mechanism of this catalytic reaction was considered to be attributable to the fact that the steps of the esterification of citrate to produce both monobutyl citrate and dibutyl citrate were very fast, but the rate of the formation of tributyl citrate was slow. The control step of total reaction was the reaction between dibutyl citrate and n-butanol to produce tributyl citrate.