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

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

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Chemical Engineering Journal xxx (xxxx) xxxx

Contents lists available at ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

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



Noor Azeerah Abasb, , Rozita Yusoffa, , Mohamed Kheireddine Arouac,d, Haliza Abdul Azizb, Zainab Idrisb a

Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia Advanced Oleochemical Technology Division (AOTD), Malaysian Palm Oil Board, No 6, Persiaran Institusi, Bandar Baru Bangi, 43000 Kajang Selangor, Malaysia c Centre for Carbon Dioxide Capture and Utilisation, School of Science and Technology, Sunway University, No. 5, Jalan Universiti, Bandar Sunway, 47500 Selangor, Malaysia d Department of Engineering, Lancaster University, Lancaster, United Kingdom b

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

stable and active proprie• Thermally tary heterogeneous catalyst was successfully developed.

optimized operating conditions • The via microwave heating were established.

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

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

A R T I C LE I N FO

A B S T R A C T

Keywords: Heterogeneous catalyst Palm-based glycol ester Mesoporous material Microwave heating

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.



Corresponding authors. E-mail addresses: [email protected] (N.A. Abas), ryusoff@um.edu.my (R. Yusoff).

https://doi.org/10.1016/j.cej.2019.122975 Received 3 May 2019; Received in revised form 13 September 2019; Accepted 25 September 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Noor Azeerah Abas, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.122975

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Fig. 1. Catalytic esterification set up via; a) microwave heating b) conventional heating.

1. Introduction

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

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

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phase separation, the catalyst can be reused and higher ester yield can be produced [10–13]. 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 [16–19]. 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 [20–25]. 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

Table 1 Summary of the variable levels investigated in the reactions. Type of catalysta

calcined Zn-Mg-Al, tin (II) oxalate, Amberlyst15

Reaction time (min) Temperature (oC) Catalyst dosage (wt. %) Reactant mole ratio (LA: DEG)

20, 40, 60, 90, 120, 150, 180 120, 150, 170, 190, 210 1, 3, 5, 7, 9 2: 1.0, 2:1.3, 2:1.5, 2:2.5

LA: lauric acid; DEG: diethylene glycol. a For preliminary catalytic screening only. The best catalyst on the activity will be chosen for further optimization of other parameters.

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 pTSA, 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

Fig. 2. X-ray diffraction (XRD) pattern of calcined Zn-Mg-Al catalyst. 3

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Fig. 3. N2 sorption curves and pore size distribution of both fresh and calcined Zn-Mg-Al catalysts. Table 2 Textural properties of Zn-Mg-Al catalysts.

Table 3 Surface area, acidity and basicity active sites of catalysts.

Catalyst

SBET (m2g−1)

Pore Volume (cm3g−1)

Pore Diameter (nm)

Atomic Ratio (M (II) : M(III))

Catalyst

Surface area (m2g−1)

Acidity (mmol.g−1)

Basicity (mmol.g−1)

Fresh Calcined

81 117

0.19 0.59

3.81 7.74

3.04: 1.00 3.09: 1.10

calcined Zn-Mg-Al tin (II) oxalate Amberlyst-15

117 52 43

0.65 0.30 1.7

0.35 NA NA

Fig. 4. Temperature-programmed desorption (TPD) spectra of calcined Zn-Mg-Al catalyst a) NH3-TPD (acidity active sites); b) CO2-TPD (basicity active sites); Conditions: Pre-treatment at 500 °C for 2 h. Temperature programme was set from 50 °C to 800 °C at ramping rate 5 °C min−1. Probe: NH3 (acidic active site) and CO2 (basic active site).

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knowledge on the use of double-layered hydroxide (LDH) hydrotalcitelike compound as catalyst in the production of glycol ester under microwave heating.

Table 4 Preliminary catalytic screening of various types of catalysts in esterification of lauric acid (LA) and diethylene glycol (DEG) via microwave heating. Catalyst Type

Lauric Acid Conversion (%)

2. Experimental details

Blank (autocatalytic) Amberlyst-15 tin (II) oxalate calcined Zn-Mg-Al

15.8 31.6 65.4 95.4

2.1. 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 inhouse 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 without any treatment.

Operating conditions: T = 210 °C, t = 180 min, mole ratio LA to DEG = 2:1, 5% by wt. of catalyst dosage.

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

2.2. Catalyst preparation The Zn-Mg-Al catalyst system was synthesized by co-precipitation technique under pH controlled [41]. The process preparation began with the dilution of metal salt solution and precipitating agent in a single container. The metal salt solution comprised of divalent cation salt (212.85 g) and trivalent cation salt (93.75 g) were dissolved in 1 L of deionized water. The precipitating agent solution was prepared by dissolving the carbonate anion (42.4 g) and hydroxide anion (16 g) in 0.5 L of deionized water. Both solution were then mixed for 18 h at 60 °C under vigorous stirring. Later, the obtained precipitate was filtered, washed with hot deionized water until neutral pH. The precipitate was then dried at 120 °C, overnight. The dried catalyst underwent calcination process for 5 h at 600 °C before characterization analysis. 2.3. Catalyst characterization The synthesized of Zn-Mg-Al catalyst was analysed by X-ray Diffraction (XRD) using a diffractometer equipped with secondary monochrometer (Bruker D8 ADVANCE, Germany), CuKα1 radiation and interface to a DACO-MP data acquisition microprocessor provided

Fig. 5. Effect of reaction temperature on conversion of lauric acid. Reaction conditions: 5% calcined Zn-Mg-Al catalyst dosage, 300 rpm, 2:1 mol ratio (lauric acid: diethylene glycol) and 180 min. 5

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Fig. 6. Effect of reaction time on conversion of lauric acid. Reaction conditions: 190 °C, 5% of calcined Zn-Mg-Al catalyst dosage, 300 rpm and 2:1 mol ratio (lauric acid: diethylene glycol).

Fig. 7. Effect of reactant mole ratio (MR) lauric acid to diethylene glycol at final acid value (AV) obtained on lauric acid conversion, mono- and di-laurate compositions. Reaction conditions: 190 °C, 5% calcined Zn-Mg-Al catalyst dosage, 300 rpm and 90 min reaction time.

2.4. Esterification of palm-based lauric acid and diethylene glycol

with Diffract/AT software. A 2θ range from 5° to 80° was scanned at 0.02° s−1. A specific surface area was measured by nitrogen adsorption data using Brunauer- Emmett- Teller (BET) technique (BELCAT; MicrotracBEL Corporation, Japan) under relative pressure range from 0.05 to 0.98. The average distribution of pore sizes was calculated using the BJH method. The adsorption and desorption isotherms were obtained at −196 °C. Prior to each measurement, all samples were degassed at 120 °C for 5 h under vacuum. The acidity and alkalinity of the catalysts were determined using a Temperature-Programmed Desorption (TPD) method (BELCAT; MicrotracBEL Corporation, Japan) under the following conditions: Pretreatment at 500 °C for 2 h. Probe: NH3 (acidic active site) and CO2 (basic active site). The temperature programmed was set from 50 °C to 800 °C at ramping rate 5 °C min−1 [42].

The preliminary catalytic screening for the three types of heterogeneous catalysts (calcined Zn-Mg-Al, tin (II) oxalate and Amberlyst15) used in esterification of lauric acid (0.5 mol, 100 g) with diethylene glycol (0.25 mol, 26.5 g) were carried out using microwave system (MARS SYNTHESIS, CEM Corporation USA). The microwave system is equipped with two magnetrons, a 250-ml three-neck round bottom glass reactor, magnetic stirrer, condenser and fibre optic temperature sensor (Fig. 1). The operating conditions was set at temperature of 210 °C with reactants mole ratio was 2:1 (lauric acid: diethylene glycol), 5% catalyst dosage at 180 min. The reason to use high temperature was due to maximum boiling point one of the reactant; which is for lauric acid (299 °C) and diethylene glycol (245 °C). The best catalyst performed on the catalytic activity then will be used for further optimization of other parameters in order to maximize the di-ester 6

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Fig. 8. Effect of calcined Zn-Mg-Al catalyst dosage on lauric acid conversion. Reaction conditions: 190 °C, 300 rpm, 2:1.3 mol ratio (lauric acid: diethylene glycol) and 90 min reaction time.

Fig. 9. Recyclability of the solid catalysts (calcined Zn-Mg-Al, tin (II) oxalate and Amberlyst-15) on lauric acid conversion. Reaction conditions: 190 °C, 300 rpm, 5% catalyst dosage, 2:1.3 mol ratio of lauric acid to diethylene glycol and 90 min reaction time.

The evaluation of the catalyst recyclability was performed for eight times in esterification of lauric acid using optimized reaction parameters. For the purpose of comparison, the reusability of tin (II) oxalate and Amberlyst-15 were also performed. After the first cycle, the three catalysts (calcined Zn-Mg-Al, tin (II) oxalate and Amberlyst-15) are recovered from product mixture by vacuum filtration and centrifugation. The next step was overnight drying to remove moisture from catalyst cake and resumed to subsequent cycle without any pre-treatment.

content. Optimization of other parameters was performed with continuous magnetic stirring that fixed at 300 rpm, the reaction temperature was varied from 120 °C up to 210 °C. Other investigated variables were catalyst dosing, reactant mole ratio, time course and reusability of catalyst. Table 1 displays a summary of the variable parameters investigated in the reaction. Vessel was removed after allowing the reaction mixture to cool to 50 °C. The heterogeneous catalyst was separated by vacuum filtration after the reaction completed. Product mixture was then analysed using gas chromatography. The percentage conversion of lauric acid was represented as equivalent to the glycol esters formed. Data reported expressing an average value ± standard errors of the measurement conversion that performed in triplicate.

2.5. Gas chromatography analysis The reaction products were separated by gas chromatography 7

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Fig. 10. Profile of lauric acid conversion over time course using different heating system for production of diethylene glycol di-laurate. Reaction conditions: 190 °C, 300 rpm, 5% calcined Zn-Mg-Al catalyst dosage and 2: 1.3 mol ratio (lauric acid: diethylene glycol).

and selectively react due to ample active sites provided by calcined ZnMg-Al. This catalyst is potentially able to promote the esterification of lauric acid with diethylene glycol at ambient pressure, attributed to its mesoporous pore volume range from 2 to 50 nm. The acidic-basic properties of the calcined Zn-Mg-Al catalyst were determined by TPD of NH3 and CO2. For the purpose of comparison, the acidic-basic properties were also obtained for the two other commercial catalysts (tin (II) oxalate and Amberlyst-15). For example, Fig. 4 presented the NH3-TPD and CO2-TPD profiles of the calcined Zn-Mg-Al catalyst. The ammonia desorption curves (NH3-TPD) shows that desorption of ammonia reached a first maximum at 130 °C due to the existence of hydroxyl group in materials, attributed to the presence of Bronsted acid sites on catalyst surface [47,48]. High concentration of acidic active sites was measured in calcined Zn-Mg-Al catalyst with 0.65 mmol·g−1. The difference in the electronegativity of magnesium (Mg) and zinc (Zn) leads to the production of more acidic hydrotalcite-like compound structure [49–51]. The peaks obtained in the CO2-TPD profiles (Fig. 4b) can be consigned based on the analysis at the temperature range of 100 °C–500 °C. At this condition, literature reported that the Mg-Al mixed oxides are characterized by the presence of CO2 desorption peaks [52,53]. The behaviour of this CO2 desorption peaks as shown in Fig. 4b can be defined as a shoulder at 100 °C–500 °C, attributed to CO2 desorption from weak Bronsted hydroxyl group (weak-strength basic sites [54]. It is mentioning that calcined Zn-Mg-Al exhibits both acidic-basic active sites due to its bifunctional catalyst type. However, in the case of esterification, only acidic active site is being used. A summary of surface area, acidity and basicity active sites of the catalysts used in the esterification of diethylene glycol and lauric acid were listed in Table 3. The characteristics were analysed using Brunaeur-Emmett-Teller (BET) method and temperature program desorption (TPD), respectively. Amberlyst-15 exhibits the highest acidity with 1.7 mmol.g−1 although smaller surface area was measured (43 m2·g−1). As for tin (II) oxalate, the surface area and acidic active sites were determined to be 52 m2·g−1 and 0.30 mmol.g−1, respectively. In theory, the reactant is adsorbed on the surface of the catalyst and the effective concentration of the reactant is generally increased on the surface, the reactants are brought closer and the reaction speed is increased. The synergistic effect of textural properties could lead to the improvement of the catalytic performance [55,56].

(HP5890, Agilent Technologies, United States of America) equipped with SGE-HT5 capillary column (30 m × 250 µm × 0.1 µm) and detected by flame ionization detector (FID) at 360 °C. The oven was set initially at 100 °C, held for 5 min and increased at ramping rate of 6 °C min−1 to 350 °C. The flowrate of helium gas (as a carrier) was set at 20 ml min−1 and the injector temperature with split ratio of 10:1 was set at 350 °C. 3. Results and discussion 3.1. 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 (1 0 0), (0 0 2), (1 0 1), (1 1 0), (1 0 3) and (1 1 2) planes matched with zinc magnesium aluminium hydroxide carbonate hydrate pattern (Zn0.33 Mg0.67 Al0.33 (OH)2 (CO3)0.167 (H2O)0.5) [44]. Physisorption studies were carried for both fresh and calcined ZnMg-Al catalyst to determine the textural properties, the isotherm adsorption/desorption and pore size distribution. Fresh Zn-Mg-Al catalyst as precursor was prepared up to drying step and to compare the properties changes with calcined Zn-Mg-Al catalyst. Based on Fig. 3a, both fresh and calcined Zn-Mg-Al catalysts have the characteristic of mesoporous materials as they show type IV isotherm with pronounced N2 hysteresis loop [45]. These catalysts attributed to the capillary condensation in the mesoporous medium at relative pressure of 0.65–0.98. As can be observed in Fig. 3b, both catalysts exhibit the narrow pore size distribution at the range of 12–120 Å (1.2–12 nm) as demonstrated in the BJH pore-size distribution. Table 2 summarized the textural properties of Zn-Mg-Al catalysts. The specific surface area measured for fresh Zn-Mg-Al catalyst was 81 m2g−1. The surface area was increased to 117 m2g−1 when the catalyst sample was calcined at 600 °C. The result agreed with the principle of calcination that larger surface area can provide larger active site [46]. Hence, lauric acid and diethylene glycol molecules could effectively 8

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with water formation. Thus, prolonged reaction time was not feasible to the reaction and the reaction duration for esterification aided by calcined Zn-Mg-Al catalyst can be established for 90 mins and would be used for further experiments.

3.2. Screening of catalysts Preliminary screening of three types of catalysts (calcined Zn-Mg-Al, tin (II) oxalate, Amberlyst-15) were carried out to compare the performance of the catalyst in the esterification of lauric acid with diethylene glycol. The results are listed in Table 4. From this table, autocatalytic esterification (blank reaction) gives the lowest conversion of lauric acid (15.8%). With the incorporation of heterogeneous catalyst in this reaction, exhibit increasing in activity towards production of diethylene glycol di-laurate. The conversion follows the order of calcined Zn-Mg-Al > tin (II) oxalate > Amberlyst-15 > autocatalytic. The activity of Amberlyst-15 gives the lowest conversion as compared to tin (II) oxalate and calcined Zn-Mg-Al. Although the acidic active site concentration of Amberlyst-15 is high and theoretically may result in high yield, this catalyst have poor thermal stability with maximum temperature only up to 120 °C [58]. Thus, the low conversion rate of this catalyst is mainly due to the decomposition of the catalyst at the operating temperature of the reaction, 210 °C. Calcined Zn-Mg-Al catalyst gives the highest conversion with > 95% conversion of lauric acid. This catalyst is a hydrotalcite -like compound comprises of the formation of layered structure containing 3 metals in one phase. Based on physicochemical properties of this bifunctional catalyst, it is thermally stable at high temperature up to 500 °C and exhibit both acidic and basic active site [59]. For this study, it can be said that the most important factor for the reaction that influence the yield of ester are the temperature and the catalyst. High temperature and the presence of the catalyst that is thermally stable will result in high yield. Other factors that contribute to higher yield are high acidity, large surface area and pore space [60]. Based on the result of the preliminary screening, it is confirmed that the best catalyst was calcined Zn-Mg-Al. Thus, this catalyst would be used for further optimization of the esterification via microwave heating.

3.5. Effect of reactants mole ratio The catalytic activity of calcined Zn-Mg-Al was further inspected by varying reactants mole ratio with other parameters be fixed. Five mole ratios were chosen, from 2:1 to 2:2.5 to investigate the effect of reactants mole ratio of lauric acid to diethylene glycol on the formation of diethylene glycol (DEG) mono- and di-ester content as can be seen in Fig. 7. Excess of diethylene glycol in the reaction is an alternative way to increase conversion rate of lauric acid. This also to speed up the forward reaction in order to eliminate water completely as by-product. It can be observed from the plot that as the mole ratio of lauric acid to diethylene glycol increased from 2:1 to 2:1.3, the yield of DEG di-ester content is increased from 91.9% to 96.2% corresponding to 98.2% lauric acid conversion and acid value of 3.34 mg KOH·g−1 sample. The yield of DEG di-ester content decreased drastically to 56.8% with the increasing reactant mole ratio up to 2: 2.5. The results obtained might be due to the surplus of diethylene glycol employed in the reaction lead to lower lauric acid concentration as more diol diluted in the mixture. Hence, the optimum reactant mole ratio of lauric acid to diethylene glycol selected for this esterification reaction was 2:1.3. 3.6. Effect of catalyst dosage The effect of calcined Zn-Mg-Al catalyst dosage on esterification conversion was investigated by varying the catalyst at 1%, 3%, 5%, 7% and 9%. The results are plotted in Fig. 8. The conversion percentage increased from 90.8% to 98.2% with the increasing of catalyst dosage from 1% to 5%, respectively. However, as the calcined Zn-Mg-Al catalyst dosage was further increased to 7% and 9%, there is no significant difference on the lauric acid conversion was observed. Thus, the calcined Zn-Mg-Al catalyst dosage of 5% is sufficient to give maximum conversion of 98.2%.

3.3. Effect of reaction temperature In this work, the temperature was varied at 120, 150, 170, 190 and 210 °C with other parameters remained constant (5% calcined Zn-Mg-Al catalyst dosage, 2:1 mol ratio of lauric acid to diethylene glycol, 300 rpm and 180 min reaction time). The profile trend for lauric acid conversion over time was plotted and shown in Fig. 5. It can be seen from the figure, esterification conducted at 120 °C only managed to convert less than 45% of lauric acid. As the reaction temperature increased to 150 °C, 170 °C, 190 °C and 210 °C, the conversion are 83.5%, 94.6%, 96.7% and 95.4%, respectively. The latter condition (210 °C) shows approximately 1.3% decreased in lauric acid conversion as compared to the reaction conducted at 190 °C. This probably because of the diethylene glycol may evaporate at 210 °C which is close to the boiling point of diethylene glycol (220–240 °C). Similar conversion trend was plotted when the reaction was performed at temperature over 170 °C. In the first 90 mins, the conversion was increased proportionally to the temperature. Later, the conversion is slowing as the esterification reached a plateau and equilibrium. Therefore, 190 °C was chosen to be the best temperature to perform the esterification process.

3.7. Recyclability of catalyst Fig. 9 shows the results of the reusability study for the three heterogeneous catalysts in successive cycles of esterification batches. The result highlighted that both calcined Zn-Mg-Al and tin (II) oxalate both can be used for six cycles as compared to Amberlyst-15 that lost part of its activity in the third cycle. Low activity performance of Amberlyst-15 is mainly due to its low thermal stability, < 120 °C. The results show that strong evidence of the calcined Zn-Mg-Al stability with less than 2% decrease in conversion (six cycles). 3.8. Comparison between conventional heating vs microwave heating over reaction time Fig. 10 shows the profile of lauric acid conversion over time of catalytic esterification at optimum condition obtained from esterification reaction via microwave heating and conventional heating. From the plot, it can be observed after 90 mins of reaction, the catalytic reaction via conventional heating gives a conversion around 72%, while reaction performed via microwave heating gives lauric acid conversion of 98.2%. Nevertheless, under the same parameters it took about 8 h to achieve the similar conversion (98.05%) via conventional heating. Unlike conventional heating which only heats the material at the surface, microwave energy penetrates deep into the material and supplies energy. As a result, heat can be generated throughout the whole volume of the material. Therefore, rapid and uniform distribution of heat in the material can be achieved through microwave heating [61]. The effect of microwave irradiation in chemical reactions is a combination of the

3.4. Effect of reaction time Investigation of the effect of the reaction time on the lauric acid conversion was conducted for the catalytic esterification of lauric acid. From the result plotted in Fig. 6, it can be seen that the lauric acid conversion improved rapidly from 20 mins (65.4%) to 90 mins (96.7%). The increased in conversion might be due to the collisions between the solid catalyst (calcined Zn-Mg-Al) and reactant molecules as the reaction time proceed. However, the conversion profile remains stagnant as the time was prolonged from 90 to 180 mins. This might be due to the reversible reaction occurred where hydrolysis took place. In hydrolysis, water molecules may break the ester linkage as the surface saturated 9

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thermal effect and non-thermal effects, i.e., overheating, hot spots and selective heating, and non-thermal effects of the highly polarizing field, in addition to effects on the mobility and diffusion that may increase the probabilities of effective contacts. In addition, the catalytic esterification of lauric acid via microwave heating experienced a shorter time (1.5 h) as compared to conventional heating (8 h). The energy transfer from microwave radiation directly passes through the vessel wall into the molecules mixture during reaction and using dipole rotation mechanism so called dielectric heating for interaction between molecules and chargers in electric field [62].

[12]

[13]

[14] [15] [16]

4. Conclusions

[17]

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 temperature, time, catalyst dosage and reactants mole ratio were examined towards lauric acid conversion. At optimized operating conditions of 190 °C, 300 rpm, reactant mole ratio of 2:1.3 (lauric acid: diethylene glycol) and 5% by wt. of calcined Zn-Mg-Al catalyst dosage yields 96.2% of DEG di-laurate at 90 min reaction course. An alternative esterification method which is by using microwave heating was at least 5 times faster than the conventional method, affording a significant saving of time and energy. It is proven that calcined Zn-Mg-Al can be recycled up to six times as is. To the best of our knowledge, this study discovered the use of heterogeneous calcined Zn-Mg-Al catalyst in lauric acid conversion to DEG di-laurate. Further investigation is suggested for the use of DEG di-laurate that potential for the use in various industries applications by analysing the specific physicochemical properties required such as for plasticizer, cosmetic and personal care, fuel, or even in paint and coating.

[18] [19] [20]

[21]

[22]

[23]

[24]

[25]

[26] [27] [28] [29]

Acknowledgement

[30]

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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