Investigation of reaction parameters and kinetics for the synthesis of sorbitol-branched azelaic acid ester

The renewable polyol ester produced from the esterification of isosorbide (ISB) and azelaic acid (AA) is substitute to petro-based polyol ester. The reaction of producing isosorbide azelaic acid ester (IAAE) has not been researched intensively and reported in the open literature. Sorbitol or its anhydrides fatty acid ester (SFAE) is typically produced by a homogeneously catalysed process which suffers with the undesired product colouration, corrosive process environment and complex downstream separation process. The unidentified reaction kinetics and correlation of the composition distribution of the sorbitol and its anhydrides with the operating parameters could render to the off specification SFAE. The present work investigated the effect of reaction parameters and kinetics for the synthesis of IAAE through sequential reactions constituted of heterogeneously catalysed sorbitol (SL) dehydration and ISB esterification with AA. Owing to the unavailability of the IAAE standards, the chromatography techniques to quantify the reactants during the esterification process was developed. The gas chromatography analysis of samples derivatised using silylation II with steady heating and reduced carrier gas flow rate outperformed others, producing identical and sharp peak for AA, SL and its anhydrides. Prior to the esterification reaction, the present study dehydrated SL to its anhydrides using the best heterogeneous catalyst, Amberlyst 36 at different important operating parameters. The increase of catalyst loading from 5 to 7 wt% did not significantly affect the ISB yield. A higher temperature increased the reaction rate, whereas a prolonged reaction time increased the conversion of SL and yield of ISB to the maximum. In terms of giving a higher ISB yield during SL dehydration, Amberlyst 36 was found to outperform the other resin catalysts reported in the literature. Both SL conversion and ISB yield of >99% were recorded after a 4 h reaction at 423 K with a catalyst loading of 5 wt% and stirring speed of 300 RPM. The reaction kinetics was evaluated under a mass transfer resistances free condition at the reaction temperature ranged from 373 K to 423 K. The kinetic data well fitted to the Langmuir-Hinshelwood (LH2) model that took side reaction into account. The activation energy for dehydration SL to sorbitan (ST), dehydration ST to ISB and dehydration of SL to other side products such as humins were 109.22, 109.46 and 104.17 kJ/mol respectively. ISB produced from SL dehydration was reacted with AA catalysed by graphite to synthesis the monomer for renewable polyol ester. The critical parameters that influence the product distributions were investigated. It encompassed the stirring speed (0-500 RPM), catalyst particle size (18-120 MESH), catalyst loading (0-2 wt%), the molar ratio of AA to ISB (1:1 to 1:5 and 1:1 to 3:1) and reaction temperature (373-473 K). The best mass-transfer resistance-free condition that maximising the amount of isosorbide monoazelate (ISMA) was found in the reaction catalysed by 1 wt% of graphite catalyst with the particle size ranged 25-35 MESH and adopted an equimolar of AA and ISB with stirring speed of 300 RPM. Meanwhile, the best reaction temperature was identified as 433 K, considering the tradeoff between reasonable reaction rate and product quality. The Langmuir Hinshelwood Hougen Watson (LHHW) model well predicted the concentration profile of the esterification of ISB with AA, estimating activation energy of 26.12 kJ/mol. The current research has proven that the heterogeneously catalysed process, with sequential reactions of the SL dehydration followed by ISB esterification, is promising method to produced IAAE at milder condition