CHEMICAL ENGINEERING TRANSACTIONS

VOL. 78, 2020

A publication of

The Italian Association
of Chemical Engineering
Online at www.cetjournal.it

Guest Editors: Jeng Shiun Lim, Nor Alafiza Yunus, Jiří Jaromír Klemeš
Copyright © 2020, AIDIC Servizi S.r.l.

ISBN 978-88-95608-76-1; ISSN 2283-9216

Transesterification of Palm Oils Catalyzed by Ionic Liquid in

Microwave Heated Reactors

Sanphawat Phromphithaka, Nakorn Tippayawongb,*

aGraduate Degree Program in Energy Engineering, Faculty of Engineering, Chiang Mai University
bDepartment of Mechanical Engineering, Chiang Mai University, Chiang Mai, 50200, Thailand

n.tippayawong@yahoo.com

Biodiesel is a renewable, carbon neutral liquid fuel produced from vegetable oils or animal fats with alcohol via

catalytic transesterification reaction. Conventionally, alkaline-catalysed transesterification using convective

heating takes long time to complete. Alkaline catalysts used are usually non-recyclable, environmentally

unfriendly and could react with free fatty acids to form soap. Recently, ionic liquids have been shown to be

effective in catalyzing transesterification. Furthermore, microwave irradiation has been shown to boost heat

transfer to the reactants, making the reaction time shorter than the conventional heating. In this work,

biodiesel production from microwave-irradiated transesterification of palm oils with methanol using a green

ionic liquid was investigated. The ionic liquid (ChOH) was synthesized from choline chloride (ChCl) with

potassium hydroxide (KOH) and characterized by Fourier transform infrared spectroscopy. The batch

experiments were setup and carried out in an 800 W microwave oven with a flask reactor connected to a

condenser. The continuous experiments were setup and carried out with a polytetrafluoroethylene (Teflon)

tubing connected to a Teflon valve and a pump. Operating condition was oil to methanol ratio (1:15), reaction

time (8 min), power of microwave (800 W), and catalyst loading (4 wt%). Methyl ester content was determined

by gas chromatography and mass spectrometer.From the findings, it was shown that the ionic liquid was

effective in catalysing transesterification of palm oils. Microwave heating proved to accelerate the reaction in

short time. The methyl ester content was found to be 87 and 82 % for the batch and continuous experiments.

The ester content was quite low, compared to the conventional heating. Further investigation was needed to

find high biodiesel yield. The ChOH could also be reused several times but its effectiveness to produce

biodiesel may be degraded, in comparison with the fresh catalyst.

1. Introduction

Energy consumption has surged continually after the industrial revolution. Fossil fuel is the main energy

source for advancing the world, though pollutions such as sulfur oxides (SOx), nitrogen oxides (NOx), and

especially carbon oxides (COx) are emitted (Simasatitkul and Arpornwichanop, 2019). Many countries issued
a policy to reduce carbon emissions (Lee et al., 2018). Adoption of bio-renewable energy is one way to reduce

the climate change effect from fossil fuels such as pyrolysis (Prasertpong and Tippayawong, 2019),

gasification (Sittisun et al., 2019), and simple chemical conversion (Tippayawong and Chanhom, 2011).

Biodiesel is one of the bio-renewable energy sources, which is easy to produce. Feedstock is simple and

abundant. Palm oil is the main dominant biodiesel feedstock in South East Asian countries including

Indonesia, Malaysia, and Thailand (Silitonga et al., 2013). Palm oil mill effluent is posed an environmental

issue. It can solve with microalgal biotechnology (Kamyab et al., 2019). Biodiesel production from palm oil is

produced via transesterification by triglycerides, which the main chemical in palm oil, react with alcohol such

as methanol or ethanol and catalysts (Ullah et al., 2018), as shown in Figure 1.

In recent years, ionic liquids (ILs) have been accepted as new green chemicals that are capable of

revolutionizing chemical processes. ILs are basically organic salts which exist in liquid state at temperature

below 100 °C (Soares et al., 2019). ILs consist of cations and anion. Normally, the cations are composed of

organic, while anions can be organic or inorganic. They had the chemical interactions between components.

The common cations are nitrogen or phosphorous containing compounds (Zhao and Baker, 2013) such as

DOI: 10.3303/CET2078008

Paper Received: 09/04/2019; Revised: 19/07/2019; Accepted: 24/10/2019
Please cite this article as: Phromphithak S., Tippayawong N., 2020, Transesterification of Palm Oils Catalyzed by Ionic Liquid in Microwave
Heated Reactors, Chemical Engineering Transactions, 78, 43-48 DOI:10.3303/CET2078008

alkylammonium, 1,3-dimethylimidazolium and tetramethyl phosphonium. The common choices of anions

include BF4
−, 𝐶𝑙−,and 𝐶𝐻3CO2

−. Many ILs are proven to be poorly biodegradable and relatively toxic. A new

interesting IL is called deep eutectic solvents (DES). It is formed from the mixture of organic halide salts with

hydrogen bond donor to a complex organic agent. They can form eutectic mixtures, which are in liquid state at

temperatures below 100 °C. DESs have some advantages over ILs, such as easy preparation, high purity, low

cost, no reactivity with water, non-toxicity and biodegradability (Chen and Mu, 2019). The deep eutectic ILs

became a target of studying biodiesel production. Choline chloride (ChCl) is one of the interesting deep

eutectic ILs, which can be developed to catalyze for biodiesel synthesis. The choline is not expensive and

biodegradable. Fan el at. (2013) studied biodiesel production via transesterification using choline ionic liquid

compound as a catalyst. The result showed that choline hydroxide (ChOH) was best for biodiesel production

with conventional heating. It needs long reaction time to be synthesized.

Figure 1: Biodiesel production via transesterification

Microwave is an alternative heating method for reducing the reaction time because microwave irradiation

delivers the energy directly to the target. The transfer of heat is more effective than the conventional heating

(Motasemi and Ani, 2012). Lin et al. (2013) investigated the use of the 4-allyl-4-methylmorpholin-4-ium

bromine ([MorMeA][Br]) ionic liquid as a catalyst in a batch process. It was shown that maximum biodiesel

yield of 89 % could be achieved with a reaction time of 6 min. The catalyst can be reused for seven times

repeatedly. Tippayawong et al. (2012) improved microwave irradiation technique by adopting a flow reactor.

The improved microwave was used in biodiesel production using jatropha oil, methanol and sodium methoxide

(NaOCH3) as a catalyst. Results showed that jatropha oil can be converted to biodiesel with a yield of 97 %

within 0.5 min.

It was notable that reported works on the combined use of ionic liquids and microwave irradiation were

relatively rare for transesterification of vegetable oils. From the existing literature, no report was found for ionic

liquid catalyzed and microwave heated reaction in a continuous flow reactor.

The objective of this study was to investigate ChOH ionic liquid catalyst for transesterification in batch and

continuous microwave heating processes. Palm oil was used as a representative vegetable oil. To the authors’

knowledge, this was the first time that ChOH ionic liquid was investigated in a microwave heated, continuous

flow reactor.

2. Material and methods

2.1 Materials

Palm oil was purchased by Morakot Industries plc.in Bangkok. Methanol (AR, 99.9 %), and n-Hexane (AR, 95

%) were bought from RCl Labscan. Choline chloride ( 98 %), potassium hydroxide ( 85 %) and 1-butanol

(99.9 %) were purchased from Sigma-Aldrich. The standard heptadecanoic acid-methyl ester was obtained

from Dr. Ehrenstorfer GmbH (99.3 %).

2.2 Ionic liquid preparation and analysis

Choline hydroxide (ChOH) was synthesized from equimolar choline chloride and potassium hydroxide in

methanol, shown in Figure 2.

Figure 2: Synthesis of choline hydroxide using choline chloride and potassium hydroxide

The solution was subjected to stirring at 60 °C for 24 h (Fan el at., 2013). Thereafter, it was cooled to room

temperature and potassium chloride was removed by filtration. The excess methanol was evaporated away

from the mixture to obtain the ChOH ionic liquid catalyst. Basicity of the ionic liquid in methanol solution was
determined by a pH meter and Fourier transform infrared spectroscopy (FT-IR). The liquid film was used for

recording FT-IR of ChOH on a FT-IR spectrophotometer in the 4,000 – 400 cm−1 scanning ranges.

2.3 Experimental set-up and procedure

2.3.1 Batch reactor

The batch experiments were carried out in a 250 mL single neck reaction flask containing palm oil, methanol,

ChOH catalyst and boiling chips. The microwave reactor has a maximum power of 800 W. The flask reactor

was connected to a reflux condenser in order to condense vaporized methanol. The experimental setup is

shown in Figure 3. Initially the experimental mixture of about 100 mL was loaded into the flask. After the

reaction, it was placed at room temperature. The reaction products were separated using a funnel, then the

upper layer containing mainly methyl esters was collected to be purified by washing three times with water at

50 °C. Impurities, such as residue catalyst, glycerol and excess methanol were removed. Finally, water was

removed by heating at 150°C (de Almeida et al., 2015). The lower layer was glycerol and the ionic liquid.

2.3.2 Continuous flow reactor

The continuous flow reactor consists of a tank mounted with a magnetic stirrer, placed in an 800 W microwave

oven. The tube reactor was made from a poly-tetrafluoroethylene (Teflon) tubing of 3 mm ID, coiled into the

centre of the microwave and connected to the Teflon valve, the tank and a pump, also shown in Figure 3. The

mixture, around 100 mL of palm oil, methanol and ChOH, was loaded into the tank. The flow rate was

controlled by the valve and the pump. The biodiesel was purified in the same way as the batch experiment.

Figure 3: Overview of the batch and continuous biodiesel production experiments

2.4 Biodiesel analysis

For biodiesel analysis, the fatty acid methyl ester content was determined by GC/MS equipped with HP-5MS

capillary column (30 m×0.25 mm×0.25 μm). The heptadecanoic acid methyl ester (C17:0) was used an

internal standard. The operating condition for the gas chromatography measurements was inlet temperature

230 °C; oven temperature programmed from 45 °C hold for 2 min, 45 – 100 °C at 25 °C/min hold for 10 min,

100 – 190 °C at 5 °C/min hold for 10 min, 190 – 220 °C at 5 °C/min hold for 1 min, 220 – 250 °C at 10 °C/min

hold for 5 min,; injection volume 2 μL; 1.5 mL/min helium gas; The condition and the detector were similar to

Zhang et al.(2017).The methyl ester content was calculated from the following equation;

𝐶𝑒𝑠𝑡𝑒𝑟 =
∑ 𝐴 − 𝐴𝐸𝐼

𝐴𝐸𝐼
×

𝐶𝐸𝐼 × 𝑉𝐸𝐼
𝑚𝑜𝑖𝑙

× 100 % (1)

where C: total fatty acid methyl ester content (%); ΣA: sum of the peak area of fatty acid methyl ester from C14

to C24:1; AEI: the peak area of the internal standard, palmitic acid methyl ester; CEI: concentration of palmitic

acid methyl ester (mg/mL); VEI: volume of palmitic acid methyl ester (mL); m: mass of input biodiesel (mg).

3. Results and discussion

3.1 Characteristic of the ionic liquid

The ChOH obtained was a light orange liquid. The pH value in the methanol solution was more than 14. The

IR spectrum can be seen in Figure 4. The pattern observed was similar to those reported by Fan et al. (2013)

and De et al. (2011). From the IR spectrum, the featured bands for organic group were shown, in which the

O−H stretching was at 3,500 – 3,400 cm-1. Meanwhile, 3,032, 2,976, and 2,848 cm-1 represented the C–H

stretching of alkane. The C–H bending of CH2 and CH3 were at 1,484 and 1,364 cm-1. The C−N stretching was

shown at 1,299 cm-1 and the C−O stretching was at 1,088 cm-1.

4,000 3,500 3,000 2,500 2,000 1,500 1,000 500

T
ra

n
s
m

it
ta

n
c
e
(

%
)

Wave number (cm
-1
)

3,500-3,400

3,032

2,976
2,848

1,484
1,364

1,299
1,088

Figure 4: IR spectrum of ChOH in methanol

3.2 Biodiesel yield

In this work, the ChOH catalyst was used in two different microwave heated processes, batch and continuous

flow reactors. Even though they were at the same condition, the methyl ester content obtained were different.

The microwave heated batch experiment gave 87 %, while the microwave heated continuous flow reactor

gave 82 % of biodiesel yield. The batch reactor appeared to give better yield than the continuous one because

it had the condenser enabling the methanol vapor phase to condense into liquid phase and return to the

mixture again. The batch process also had boiling chips for diffusing heat and harmonizing the oil and

methanol. Even through the continuous flow setup produced less ester content than the batch process by only

5 %, the batch reactor took longer time to set up and run for a single experiment. The continuous process was

more convenient.

The comparative study on the catalytic activities of different catalysts and different operating processes is

shown in Table 1. Abdullah et al. (2017) and Tippayawong and Sittisun (2012) reported biodiesel production

from the conventional heating and the continuous microwave heating. The result shows that the methyl ester

contents were similar, while the microwave heated continuous flow process was faster. The catalysts in both

reports could not be reused. The ChOH catalyst was used for biodiesel production with the conventional

heating by Fan et al. (2013). It was shown that high methyl ester content of 96 % could be obtained with no

soap formation and IL was reusable. In this work, the continuous process gave lower ester content than that

from Fan et al. (2013). But only a single condition was considered. It is interesting to further investigate the

continuous process by response surface methodology (Suwannapa and Tippayawong, 2017) combined with

Box-Behnken experimental design (Ding et al., 2018). The interesting variables may be the oil to methanol

ratio, flow rate, microwave power and catalyst loading, which affect the yields and properties of biodiesel

production. These variables were considered by Encinar et al. (2012) and Choedkiatsaku et al. (2015). The

optimum condition is expected to give high ester content.

Recycling of ChOH catalyst is also of great interest in this investigation. For the conventional heating, the

ChOH catalyst can be reused four times, while all the methyl ester contents obtained were above 80 %. From

this work, it was observed that the lower layer volume of the continuous process was less than that from the

batch process, affecting the amount of ChOH catalyst recovery. The quality and quantity of the desired ChOH

catalyst may be different from the conventional heating process.

Table 1: Comparison catalyst and process for biodiesel production

References Operation Reaction time

(min)

Ester content

(%)

Reusability

Abdullah et al. Conventional heating, palm oil/methanol

molar ratio of 1:20, KOH 1.5 wt%, 60°C

60 93 No

Tippayawong et al. Microwave heated continuous flow

reactor, jatropha oil/methanol molar ratio

of 1:6 , NaOCH3 1.0 wt%, microwave

power 800 W

0.5 96 No

Fan et al. Conventional heating, soybean oil/

methanol molar ratio of 1:9, ChOH 4

wt%, 60°C

150 97 Yes

This work Microwave heated batch, palm oil/

methanol molar microwave-assisted

ratio of 1:15, ChOH 4 wt%, microwave

power 800 W

8 87 Yes

This work Microwave heated continuous flow

reactor, palm oil/methanol molar ratio of

1:15 , ChOH 4 wt%, microwave power

800 W

8 82 Yes

4. Conclusions

The ChOH catalyst proved to be effective in biodiesel production via both batch and continuous reactors with

microwave heating. The methyl ester content of 87 % and 82 % were produced. Both processes spent only 8

min for production, faster than the conventional heating. The microwave heated continuous process gave a bit

less ester content than the microwave batch process. But, it was faster for all processes considered.

Nonetheless, the ChOH catalyst should be investigated further about optimization of methyl ester content and

recycling of the catalyst. Further analysis of the ChOH catalyst may include pH measurement and

characterization by nuclear magnetic resonance (NMR), since the pH meter may indicate the hydroxyl groups

in ChOH catalyst abd the NMR can give the structure of the ChOH catalyst.

Acknowledgments

We wish to acknowledge the support from Chiang Mai University and the Energy Policy and Planning Office.

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