Elsevier

Chemical Engineering Journal

Volume 382, 15 February 2020, 122975
Chemical Engineering Journal

Production of palm-based glycol ester over solid acid catalysed esterification of lauric acid via microwave heating

https://doi.org/10.1016/j.cej.2019.122975Get rights and content

Highlights

  • Thermally stable and active proprietary heterogeneous catalyst was successfully developed.

  • The optimized operating conditions via microwave heating were established.

  • Microwave heating proceeded faster than conventional heating in catalytic esterification of lauric acid.

  • The proprietary heterogeneous catalyst can be reused up to six cycles.

Abstract

This study involved in maximizing the conversion of lauric acid to glycol ester via esterification with diethylene glycol, aided by calcined Zn-Mg-Al catalyst in a 250-ml reactor using microwave heating. Preliminary catalytic screening involving three types of catalysts (tin (II) oxalate, Amberlyst-15 and calcined Zn-Mg-Al), resulted in the conversion of lauric acid obtained were 65.4%, 31.6% and 95.4% using tin (II) oxalate, Amberlyst-15 and calcined Zn-Mg-Al, respectively. In addition, conversions obtained from the solid acid catalysts appeared to be higher than autocatalytic esterification of only 15.8%. The optimum operating condition for esterification via microwave heating was established at 190 °C, 2:1.3 mol ratio of lauric acid to diethylene glycol with 5% of catalyst dosage at 90 min. Calcined Zn-Mg-Al catalyst under optimised condition gives 98.2% of lauric acid conversion. The recyclability of the catalysts in the esterification of lauric acid with diethylene glycol were also carried out. It shows that calcined Zn-Mg-Al and tin (II) oxalate both can be used for six cycles as compared to Amberlyst-15 catalyst that has lost part of its activity after the third cycle. The microwave heating remains attractive for heating catalytic esterification as it accelerates the reaction speed at shorten period of time from 8 h to 1.5 h as compared to conventional heating.

Introduction

There is an increasing the trend of chemical industries toward new processes that should meet stringent environmental or energy requirement such as generation of nearly zero waste chemicals, less energy, and sufficient uses of product chemicals in various application. Glycol esters are the new chemical compound derived from fatty acid esterified with diol under optimum condition that have widely potential functions. Glycol esters as emulsifier can be applied in household and personal care products [1], as coalescent aid in paint formulations [2], additives for engine oil [3] and biolubricants [4], [5], not to forget as phase change material in energy storage [6]. In 2017, Technavio’s analyst forecast the global emulsifier market to grow at a CAGR of 7.16% during the period 2017–2021 [7]. The esters can be produced either by direct esterification of diol reacted with fatty acids in the presence of catalyst or even conducted in autocatalytic mode. Self-esterification process can be proceeded successfully by considering significant factors such as reactants properties, operating conditions and materials stability [8]. For example, the autocatalytic esterification between lauric acid and glycerol was well performed at 130 °C with smaller dosing of methyl lactate in the reaction for miscibility purposes [9].

Typically in chemical industry, esterification was conducted in the presence of homogeneous acid catalyst such as sulphuric acid (H2SO4) or para-toluenesulphonic acid (p-TSA). The optimized process could produce maximum content of esters yield, however this homogeneous catalysed reaction may generate enviromental and corrosion problem. At higher temperature of homogeneous catalytic esterification using p-TSA, may lead to the formation of catalyst ester in the product mixture with high toxicity and harmful to others. Furthermore, since homogeneous catalyst is in the same phase with the product mixture, the purification process later requires additional steps such as neutralization and tedious separation. This will increase the production cost.

Hence, in order to overcome the problems, an attractive route which is the use of heterogeneous solid acid catalysts for the esterification was proposed for more environmentally friendly and economical process. The drawbacks could be avoided by simple filtration of solid–liquid phase separation, the catalyst can be reused and higher ester yield can be produced [10], [11], [12], [13]. A study reported on the use of ion exchange resin in the production of ethylene glycol mono- and di-acetate. The reaction was conducted by varying reaction temperature from 60 °C to 90 °C at different reactant ratio between 0.5 and 1.0 [14]. Another process reported was reaction between propylene glycol with acrylic acid aided by Amberlyst-15 was performed. The influence of the reaction conversion over reaction temperature from 60 °C to 80 °C and reactants mole ratio were investigated [15]. Most studies were reported in the literature concerning the catalysts used in the esterification like sulfonic exchange acid resins [16], [17], [18], [19]. The results obtained showed that ion exchange resin managed to catalyse the esterification into ester but the temperature is limited up to only 120 °C due to its poor thermal stability. An improvement is required in getting the maximum content of ester yield using an efficient and selective catalyst with good thermal stability.

Exploration on the use of hydrotalcite-like compounds (HTLC) as heterogeneous catalysts in esterification of fatty acids have received considerable attention in different organic syntheses. HTLC comprises of metal divalent cation and trivalent cation such as magnesium (Mg2+) and aluminium (Al3+) formulated together with other transition metal (for example Zn, Ni, Cu etc.) layered double hydroxide (LDH). Their advantages revealed that this type of catalysts are thermally stable at high temperature, good mass transfer, high selectivity, environmental compatibility, can be reusable and widely studied for many organics syntheses under conventional heating [20], [21], [22], [23], [24], [25].

Microwave irradiation has been successfully applied in organic chemistry. Spectacular accelerations, higher yields under milder reaction conditions and higher product purities have been reported. The use of microwaves in chemical reaction is an alternative method to convective heating as the products are heated directly. It is an effective activation method that reduce the potential barrier and the thermodynamic product was produced easily. This was due to the interaction between the microwave energy and dipole moments of the starting materials [26], [27]. The operation is easily conducted, no overheating, saving the energy consumption, reduction of processing time, reducing the side reactions and increase the yield as well as improve the reproducibility are some advantages pointed out in the previous works [28], [29]. Several studies on organic syntheses have been performed via microwave heating in the presence of catalysts either with incorporation of enzyme [30], [31], ion exchange resin [32], homogeneous liquid catalyst [33] or heterogeneous solid catalysts [34], [35]. Some studies considered short chain fatty acid, such as 2, 4, 6-trimethylbenzoic acid [36], acetic acid [37] and propionic acid [38], or the used of other alcohols, like 2-ethylhexanol [39] and butanol [40]. Nevertheless, there is still lacks of information on the use of microwave heating on esterification of long chain fatty acids with diols in the presence of heterogeneous catalyst.

Therefore, this work reports on the use of calcined Zn-Mg-Al catalyst that is thermally stable at high temperature in the esterification of lauric acid with diethylene glycol without the use of solvent. The physicochemical properties of calcined Zn-Mg-Al was characterized by X-ray Diffraction (XRD), Brunauer-Emmett-Teller (BET) technique and Temperature Programmed Desorption (TPD) analysis. For the comparison purposes, two other types of catalyst (Amberlyst-15 and tin (II) oxalate) was also used on the esterification of lauric acid and diethylene glycol. Process optimization was carried out by investigating the effects of reaction temperature, reactants mole ratio, reaction time and catalyst dosage. Catalytic activity via microwave heating was compared with conventional heating conducted under the optimised conditions determined using microwave heating. The recyclability of the solid catalyst was also investigated. These results are a new contribution to the knowledge on the use of double-layered hydroxide (LDH) hydrotalcite-like compound as catalyst in the production of glycol ester under microwave heating.

Section snippets

Materials

Palm-based lauric acid was obtained from Emery Oleochemical (M) Sdn. Bhd, Malaysia. (Edenor C12, ≥98% purity). The commercial-grade of diethylene glycol (Sigma Aldrich, 99.9%) was purchased from Bumi Pharma Sdn Bhd, Malaysia. The chemicals were used as received. The hydrotalcite-like compound comprised of Zn-Mg-Al was developed in-house by the Malaysian Palm Oil Board (MPOB). The commercial catalysts such as tin (II) oxalate (Sigma Aldrich) and Amberlyst-15 (Dow Chemical Company) were used

Characterization of catalyst

In order to produce a selective and active solid catalyst, an X-ray diffraction (XRD) analysis was employed. XRD analysis identified the crystal structure, phase, crystal orientation and other structural parameters such as crystallinity and strain [43]. Diffractogram shows that calcined Zn–Mg–Al revealed the hydrotalcite-like compound pattern (Fig. 2). According to the ‘search and match’ technique and phase identification database, the pattern peaks of calcined Zn-Mg-Al corresponding to

Conclusions

High conversion of diethylene glycol (DEG) di-laurate was successfully obtained from catalytic esterification of lauric acid and diethylene glycol aided by calcined Zn-Mg-Al catalyst via microwave. This heterogeneous catalyst was synthesized, characterized and employed in the esterification under microwave heating. The catalyst managed to produce 117 m2g−1 for surface area with 0.65 mmol.g−1 acidic active sites and led to produce maximum lauric acid conversion (98.2%). The effect of reaction

Acknowledgement

The authors would like to thank the Director-General of MPOB for permission to publish the study in this journal. This study was funded by MPOB (M-RD08010000).

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