A Theoretical and Experimental Study on Esterification of Citric Acid with the Primary Alcohols and the Hydroxyl Groups of Cellulose Chain (n = 1-2) in Parched Condition

Dang T Nguyen,Quan T Pham,Pedro M Mancini

doi:10.1155/2020/8825456

Abstract

Esterification of citric acid (CA) with the primary alcohols and the hydroxyl groups of cellulose chain (n = 1-2) in parched condition were investigated by using density functional theory (DFT) method and a two-layer ONIOM approach. Geometry and energy of reactants, products, and transition state (TS) structures were optimized at B3LYP/6-311g (d, p) level and ONIOM (B3LYP/6-311g (d, p):PM3MM) level. The computational results show that the esterification occurs in the two main steps: the first step is the dehydration reaction of CA to form anhydrides of 5-membered ring and 6-membered ring and the second step is the ring opening reaction with the hydroxyl –OH groups to form the ester products. The energy barrier of dehydration reaction step is much higher than that of ring opening reaction step. Effect of substituent R in primary alcohol R-CH2OH (R: CH=CH2, CH2NHCH3, CH2OCH3, CH2Cl) and cellulose chain (1G, 2G) on the reactivity, which has negative inductive effect –I, is significant. The combination of calculation data and experiment data were applied to make findings more rigorous. The activation energy of CA was determined by using differential scanning calorimetry (DSC) and thermal gravimetric (TG) analysis to beEaexp = 47.8 kcal/mol; the experimental data favoured the dehydration reaction step of CA.

Highlights

Esterification is an important reaction in the chemical industries because the ester products are used in the many various applications such as solvents, flavors, plasticizers, pesticides, and emulsifiers [1]
The sulfated zirconia catalyst was used in the reaction of acetic acid and n-pentanol in batch reactor and the estimated enthalpy of the reaction was −2.03 kcal/mol and acetic acid adsorbed on the catalyst as the limiting step of the reaction [4], or a structured catalyst was developed from SiC foam materials with a ZSM-5 zeolite coating in the synthesis of n-amyl acetate [5]
Potential energy surface (PES) was established by setting the energy level of citric acid (CA) as the basepoint energy level; other energy levels of the reactant, products, and transition state (TS) structures are determined comparatively towards the zero energy level. e computational results are summarized and shown in Figure 5. e energy level diagram has the two TS structures, its shape like a double-hill. e estimated enthalpy of the reaction is in a range from −2.1 kcal/mol to −2.5 kcal/mol; the esterification is a slightly exothermic and domino reaction

Summary

Introduction

Esterification is an important reaction in the chemical industries because the ester products are used in the many various applications such as solvents, flavors, plasticizers, pesticides, and emulsifiers [1]. Condensation reaction between a carboxylic acid and a primary alcohol in homogeneous acid catalysts such as sulfuric acid at 110°C–130°C or heterogeneous acid catalysts such as ion exchange resin at 50°C–80°C is often used to obtain organic ester because of simplicity, economy, and environmental benefit. The esterification reaction between ethyl alcohol and acetic acid in sulfuric acid catalyst has been investigated in a batch reactor; the order of forward reaction was found to be. 1 [2], or a novel two-step process for the coproduction of ethyl and butyl acetates obtained higher purity using ion exchange resin (Dowex 50WX8) [3]. The sulfated zirconia catalyst was used in the reaction of acetic acid and n-pentanol in batch reactor and the estimated enthalpy of the reaction was −2.03 kcal/mol and acetic acid adsorbed on the catalyst as the limiting step of the reaction [4], or a structured catalyst was developed from SiC foam materials with a ZSM-5 zeolite coating in the synthesis of n-amyl acetate [5].

Methods

Results

Conclusion