DOI: 10.3303/CET2188157 
 

 
 

 
 
 
 

 

 

 
 
 

 
 
 

 
 
 

 
 
 

 
 

 

 
 

 
 
 

 
 
 

 
 

 
 
 

 
 
 

 
 
 

Paper Received: 13 May 2021; Revised: 28 July 2021; Accepted: 13 October 2021 
Please cite this article as: Chuah L.F., Klemeš J.J., Bokhari A., Asif S., 2021, A Review of Biodiesel Production from Renewable Resources: 
Chemical Reactions, Chemical Engineering Transactions, 88, 943-948  DOI:10.3303/CET2188157 

CHEMICAL ENGINEERING TRANSACTIONS 

VOL. 88, 2021 

A publication of 

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

Guest Editors: Petar S. Varbanov, Yee Van Fan, Jiří J. Klemeš

Copyright © 2021, AIDIC Servizi S.r.l. 

ISBN 978-88-95608-86-0; ISSN 2283-9216 

A Review of Biodiesel Production from Renewable 
Resources: Chemical Reactions 

Lai Fatt Chuaha,b, Jiří Jaromír Klemešc, Awais Bokharic, Saira Asifc,* 
aMalaysia Marine Department, 42007 Port Klang, Selangor, Malaysia.  
bFaculty of Maritime Studies, Universiti Malaysia Terengganu, 21030 Kuala Terengganu, Terengganu, Malaysia. 
cSustainable Process Integration Laboratory, SPIL, NETME Centre, Faculty of Mechanical Engineering, Brno University of 

Technology, VUT Brno, Technická 2896/2, 616 00, Brno, Czech Republic 
 asif@fme.vutbr.cz 

Biodiesel has become an attractive renewable fuel due to carbon footprint reduction. Greenhouse gas footprint 
is minimised when the locally available renewable feedstock is utilised for cleaner biodiesel production. This 
biodiesel has similar characteristics of diesel oil which allows its use in compression motors without extensive 
engine modifications. One of the greenest choices to address the ever growing demand for cleaner energy is 
renewable biomass power. Contemporary development and work have been aimed at transforming these bio-
based resources into practical gas and liquid fuels that comply with the current petro-based chemical production 
infrastructure. Biodiesel is a prospective alternative to petroleum fuels due to its clean, renewable and 
ecologically beneficial characteristics. The transesterification cycle is applied to the processing of cleaner 
biodiesel manufacturing. Comprehensive research is underway to develop more effective biodiesel processing 
processes. The different new technologies are both solid catalysts and supercritical processes that are not 
catalytic. A review of the various methods used for the successful synthesis of biodiesel includes the 
homogeneous, heterogeneous and enzyme catalyst, microwave and ultrasonic synthesis followed by the 
supercritical synthesis of biodiesel. The chemical reactions of biodiesel, i.e. transesterification, esterification and 
interesterification have been presented by this paper and the merits and limitations of these chemical reactions 
for biodiesel discussed. 

1. Introduction

The use of fossil fuel has significantly increased through sustained developments in industrialisation, motoring 
and shipping (Chuah et al., 2021). Biodiesel is an alternative fuel for the replacement of petroleum diesel 
solution. Due to its similarity to diesel fuel, such as heated value and viscosity, Biodiesel can be used in a diesel 
engine without major alterations (Bokhari et al., 2016). There are three chemical reactions viz. 
transesterification, esterification and interesterification reactions to produce biodiesel (Yusup et al., 2015). 
Transesterification of oil with alcohol is the most popular process for biodiesel production. One mole of 
triglyceride required 3 moles of alcohol during transesterification (Chuah et al., 2016). The stepwise 
transesterification reaction is presented in Figure 1. Excess alcohol is needed to shift the reaction forward to the 
desired product, i.e. fatty acid methyl ester, due to the reversible nature of the transesterification reaction. Waste 
oil to alcohol molar ratio 1:6 is the most commonly used for transesterification, which is able to obtain 98 wt.% 
ester conversion (Issariyakul and Dalai, 2014). However, the optimum molar ratio of oil to alcohol is depending 
on the oil quality and type of oil used (Asif et al., 2017). An increase of oil to alcohol molar ratio to a certain level 
does not further increase the conversion during transesterification because the reaction mixture polarity 
increased and increased the solubility of glycerol back to the ester phase and promoting the reverse reaction 
between glycerol and ester or glycerides, consequently reducing the ester yield (Issariyakul and Dalai, 2014). 
In terms of ester conversion percentage in transesterification, potassium hydroxide as alkali catalyst is higher 
compared to sodium hydroxide, which is 96 (Issariyakul et al., 2007) and 76.9 wt.% (Meng et al., 2008) under 
operating parameters of waste oil to methanol molar ratio of 1:3, 1 wt.% alkali catalyst, 50 °C and 60 min reaction 
time. There are various alcohols that can be used in transesterification reaction, such as methanol, ethanol, 

943



propanol, and butanol. Long-chain alcohol such as ethanol and propanol has higher miscibility with oil and lower 
reaction time compared to methanol. Methanol (short carbon chain) is the most commonly used in 
transesterification reaction due to its lower cost and higher nucleophilicity compared to ethanol (long carbon 
chain) (Issariyakul and Dalai, 2014). Issariyakul et al. (2007) reported that the use of methanol in 
transesterification reaction of waste fryer grease resulted in 82 wt.% biodiesel yield recovery, whereas only 62 
wt.% biodiesel yield recovery is observed when using ethanol. 
Esterification reaction occurs in a relatively straightforward way, in which one mole of free fatty acid reacts with 
one mole of alcohol to produce one mole of biodiesel and one mole of water. Both transesterification and 
esterification reactions (endothermic processes) can be significantly influenced by some operating parameters, 
such as reaction temperature, loading catalyst and a molar ratio of oil to alcohol. The immiscibility behaviour of 
glycerides and alcohol is often referred to as mass transfer resistance, which can be overcome by several 
methods such as rigorous mechanical stirring, supercritical condition, microwave, ultrasonic and hydrodynamic 
cavitation. In general, the activation energy for the transesterification process is higher compared to the 
esterification process. It could be attributed to the stability of the triglycerides molecules (esters), which required 
more energy to break prior to the reaction with alcohol in the transesterification process. Free fatty acid is ready 
to react in the esterification process due to its less stable molecular structure. Similarly to other carboxylic acids, 
a free fatty acid is capable of donating the proton (H+), which makes them susceptible to the reaction. 
The interesterification of triglycerides with methyl acetate instead of methanol provides a promising alternative 
to transesterification due to the formation of triacetin instead of glycerol (Maddikeri et al., 2014). One mole of 
triglyceride required 3 moles of methyl acetate during interesterification to produce one mole of triacetin and 3 
moles of alkyl esters (Figure 2). Interesterification has been mostly studied in the presence of enzymes or under 
supercritical conditions. However, both approaches have their own disadvantages. The drawbacks of the 
enzymatic approach require longer reaction time and higher production costs as well as difficulty at industrial 
scale due to the need for careful control of the reaction parameters and inherent slowness of the reaction, 
whereas the supercritical approach requires significantly high pressure (20 - 40 MPa) and temperature (350 – 
400 °C) as well as significant excess of methyl acetate with 1:42 molar ratio of oil to methyl acetate (Maddikeri  
et al., 2014). The high oil to methyl acetate molar ratio under supercritical resulted in a higher cost for the 
evaporation of the unreacted methyl acetate (boiling point: 56.9 °C). Although hydrodynamic cavitation (one of 
the intensification processes) has been used for biodiesel production from soybean, vegetable oil, Thumba oil, 
frying oil and Nagchampa oil based on the esterification/transesterification reactions, however for 
interesterification reaction, Maddikeri et al. (2014) are the first to attempt to apply hydrodynamic cavitation 
technically for biodiesel production. Throughout this paper, the chemical reactions of biodiesel are presented. 
The merits and limitations of these chemical reactions for biodiesel have been discussed in this paper. 

Figure 1:  Scheme for stepwise transesterification reaction 

Figure 2: Scheme for stepwise interesterification reaction 

+

OH2C C

O

R1

HC O C R2

O

H2C O C R3

O

C

H

H

H

O H +

Triglyceride Methanol Glycerol

Methyl esters 

OHH2C

HC OH

H2C OH

CO R1

O

C

H

H

H

CO R2

O

C

H

H

H

CO R3

O

C

H

H

H

3

OH2C

HC

H2C

O

O

C

C

C R3

O

R1

O

R2

O

+ C

O

H3C
OCH3

3

OH2C

HC

H2C

O

O

C

C

C CH3

CH3

O

CH3

O

C

O

R3 OCH3

O

+ 3

Triglyceride Methyl acetate Triacetin Methyl ester

944



2. Chemical reactions

Chemical reactions for biodiesel production from sustainable feedstock is attaining concerns and attention. A 
detailed discussion of different chemical reactions for biodiesel has been presented and discussed in the 
following sections. 

2.1 Homogeneous-catalysed 

Presently, homogeneous alkali catalysed, e.g. sodium methoxide, sodium hydroxide and potassium hydroxide 
transesterification is the commonly adopted process worldwide at an industrial scale for biodiesel production 
(Kulkarni and Dalai, 2006). There are several advantages of homogeneous alkali catalysed compared to the 
acid catalyst in transesterification: (i) widely available and cheap, (ii) high conversion of methyl ester in a shorter 
time, (iii) reaction can occur at mild reaction condition. Kulkarni and Dalai (2006) reported that homogeneous 
alkali catalysed transesterification could be 4,000 fold faster compared to the acidic catalyst. Lam et al. (2010) 
reported that some researchers claimed alkali catalysed could tolerate the free fatty acid value less than 0.5 
wt.% or less than 2 wt.% in order to prevent saponification occurrence. The soap formation during reaction 
resulted in a difficult separation of glycerol from the product, which resulted in ester yield reduction. Shahbazi 
et al. (2012) investigated biodiesel production from RBD (Refined, Bleached and Deodorised) palm oil with 1:6 
molar ratio of oil to methanol, 1 wt.% alkali catalyst (potassium hydroxide and sodium hydroxide) and agitation 
speed of 600 rpm for 60 min under 60 °C reaction temperature using the mechanical stirring method. They 
stated that KOH was considered the best catalyst for better conversion of methyl ester during transesterification 
reaction. Wan Ab Rashid et al. (2014) obtained a biodiesel yield of 91 wt. % through homogeneous base 
catalysed transesterification of cooking palm oil at 60 °C, 1:23 oil to methanol molar ratio, 5 wt.% catalyst loading 
and 3 min in milichannel reactor. Kafuku and Mbarawa (2010) studied transesterification of Moringa oleifera oil 
under conventional heating in the presence of potassium hydroxide. It was concluded that 82 wt.% conversion 
of biodiesel at 1:6 molar ratio of oil to methanol and 1 wt.% catalyst concentration was achieved after 60 min of 
reaction time. Kumar et al. (2013) reported that 90 wt.% conversion was obtained after only 40 min for 
conventional heating with 0.5 wt.% sodium hydroxide. With respect to other sources of oil, work on conventional 
heating for transesterification with sodium hydroxide or potassium hydroxide were reported, achieving 81.0 – 
99.4 wt.% conversions in 40 - 90 min and 73.9 – 99.5 wt.% yields in 30 – 120 min. Against conventional heating, 
it was also confirmed that the ultrasonic and milichannel reactors could enhance transesterification of oil with 
alcohol, allowing high yields and conversions into biodiesel at a shorter time. 

Table 1: Homogeneous catalysed used in biodiesel production 

Source of oil Reaction condition 
Maximum 

yield/conversion 
(wt. %) 

Method Reference 

RBD palm oil 

1 wt.% KOH, 60 °C, 
60 min, 600 rpm, 1:6 
oil to methanol molar 

ratio 

90 (conversion) Mechanical stirring Shahbazi et al., 2012 

Waste cooking 
oil 

1 wt.% NaOH, 50 °C, 
90 min, 1:6 oil to 

methanol molar ratio 

89.8 
(conversion) Mechanical stirring Meng et al., 2008 

Waste cooking 
oil 

1 wt.% KOH, 70 °C, 
60 min 

98.2 
(conversion) Mechanical stirring Agarwal et al., 2012 

Jatropha oil 
1 wt.% NaOH, 60 °C, 

90 min, 1:5.4 oil to 
methanol molar ratio 

98 ( yield) Mechanical stirring Chitra et al., 2005 

Hibiscus 
sabdariffa 

1.5 wt.% KOH, 60 °C, 
60 min, 600 rpm, 1:8 
oil to methanol molar 

ratio 

99.4 
(conversion) Mechanical stirring 

Nakpong and 
Wootthikanokkhan, 

2010  

Moringa 
oleifera 

1 wt.% KOH, 60 °C, 
60 min, 400 rpm 82 (conversion) Mechanical stirring 

Kafuku and Mbarawa, 
2010 

945



2.2 Heterogeneous-catalysed 

The most pressing disadvantages of heterogeneous acid catalyst transesterification are high cost for catalyst 
synthesis, required high reaction temperature, high chemical consumption and long reaction time due to mass 
transfer limitation between liquid reactants and solid catalyst. In general, the reaction temperature should be 
kept below the boiling point of the reactant, e.g. methanol (65 °C) and ethanol (78 °C). Heterogeneous acid 
catalyst required a high temperature up to 200 °C, and pressure is needed to be applied to the reaction mixture 
in order to maintain the reacting alcohol in a liquid state (Issariyakul and Dalai, 2014). However, the advantages 
of heterogeneous acid catalyst transesterification are less sensitivity to free fatty acid, and water content in the 
oil, esterification and transesterification can be performed simultaneously, no washing is required resulted in a 
reduction of the acidic wastewater, easy separation of catalyst from the reaction mixture, easy recovery, 
regeneration and recycling of catalyst and less corrosion to the equipment (Chuah et al., 2017). Lotero et al. 
(2005) stated that the ideal solid acid catalyst for the transesterification of waste cooking oil should have 
characteristics such as an interconnected system of large pores, a moderate to a high concentration of strong 
acid sites and a hydrophobic surface. A more in-depth study using a solid acid catalyst in the transesterification 
of high free fatty acid oil is required to overcome its slow reaction rate and undesired side reaction. Miao and 
Gao (1997) reported that the acid property could be enhanced by impregnating ultrafine crystalline zirconium 
oxide (ZrO2 - metal oxide) with sulphuric acid. Jitputti et al. (2006) claimed that without sulphate of ZrO2 as a 
catalyst used in transesterification of crude palm kernel oil and crude coconut oil, only 69.0 wt.% and 54.3 wt.% 
of methyl ester content were achieved. It could be concluded that modification of metal oxide surface acidity is 
the key factor in obtaining high methyl ester conversion. Application of ZrO2 supported La2O3 in the 
transesterification of sunflower oil with methanol to produce biodiesel was also reported by Sun et al. (2010). 
Transesterification reaction was performed with 1:30 oil to methanol molar ratio with the presence of catalysts 
(ZrO2, La2O3 and 7 - 28 wt.% La2O3/ZrO2) at a reaction temperature of 200 °C for 5 h reaction time under 
magnetic agitation. They found that the highest methyl ester conversion of 96 wt.% was attained with a catalyst 
of 21 wt.% La2O3/ZrO2 calcinated at 600 °C. However, they also claimed that with a single catalyst (unsupported 
catalyst), i.e. ZrO2 and La2O3, in transesterification reaction of sunflower oil, only 31.0 wt.% and 80.8 wt.% of 
methyl ester conversion were obtained. Table 2 compares the reaction condition and performance of 
homogeneous and heterogeneous catalysed used in biodiesel production. The insights into the structure‐
function interaction of heterogeneous catalysts lead increasingly to the conclusion that catalysts are by far not 
materials with static surface and bulk structures. Instead, they are dynamic entities that can modify their structure 
(for example surface morphology or composition) depending on the local reaction conditions in the reactor. 
The process conditions are not affected by the mass flow rate into the process. It is affected due to the presence 
of free fatty acid content, acid value, triglycerides, saturation or unsaturation in the feedstock oils. 

Table 2: Heterogeneous catalysed used in biodiesel production 

Source of oil Catalyst Reaction condition 
Maximum 

yield/conversion 
(wt. %) 

Method Reference 

Crude palm 
kernel 

SO42-/SnO2 
200 °C, 60 min, 350 

rpm, 50 bar under N2, 
1:60 oil to methanol 
molar ratio, 3 wt.% 

catalyst 

95.4 
(conversion) Mechanical 

stirring 
Jitputti et 
al., 2006 

SO42-/ZrO2 
88.3 

(conversion) 

Crude 
coconut oil 

SO42-/SnO2 
95.8 

(conversion) Mechanical 
stirring SO42-/ZrO2 

93.0 
(conversion) 

Sunflower oil 

ZrO2 

200 °C, 5 h, 1:30 oil 
to methanol molar 

ratio, 5 wt.% catalyst 

31 (conversion) 

Mechanical 
stirring 

Sun et al., 
2010 

La2O3 
80.8 

(conversion) 

21% 
La2O3/ZrO2 

96 (conversion) 

Scenedesmus 
sp. 

WO3/ZrO2 

80 °C, 20 min, 350 
rpm, 1:45 oil to 

methanol molar ratio, 
4 wt.% catalyst 

71.4 
(conversion) Ultrasonic 

Guldhe et 
al., 2014 

946



3. Conclusions

Because of its renewable nature and environmental benefits, biodiesel is a promising and more appealing fuel 
for diesel engines. The main issue to consider is that biofuels are more expensive than fossil fuels. Using low-
quality feedstock, such as non-edible oils and animal fats, which do not compete with food supply and land for 
food cultivation, is thought to be an effective way of lowering biodiesel production costs. The paper demonstrates 
that biodiesel feedstock can be non-edible oil or edible oil. Biodiesel is made from edible or inedible oil via a 
transesterification reaction in which the organic group (alkyl) of alcohol is substituted with the organic group of 
a triglyceride, resulting in fatty acid alkyl ester and glycerol. Biodiesel production is a simple process that does 
not require a licence or complicated technology. The alkaline catalysed transesterification method is the most 
commonly used because it is the quickest reaction, has the highest yield, has the mildest reaction conditions, is 
the least expensive, corrosive, and toxic. Catalysts are important in increasing yield and conversion, as well as 
saving energy required to run the reaction at higher temperatures without a catalyst. Acids and alkalis are 
commonly used as liquid catalysts in the production of biodiesel. Solid catalysts based on transition metals and 
noble metals are used in the production of renewable fuels. By applying homogeneous catalyst 
transesterification in biodiesel production, about 5-fold lower methanol consumption, 5-fold shorter time reaction 
and 3-fold lower temperature reaction compared to heterogeneous catalyst transesterification. The second 
major contributor to the environmental footprint after the raw material is the energy efficiency in biodiesel 
production.  It has been observed that about 4-fold reduction in the environmental footprint using homogeneous 
catalyst transesterification compared to heterogeneous catalyst transesterification. In terms of separation time 
(time-saving) and free fatty acid sensitivity, heterogeneous catalyst transesterification proved to be more 
advantageous over homogeneous catalyst transesterification. In addition, in-situ (trans) esterification is an 
alternative route in biodiesel production. It involves lesser steps as it eliminates the need for lipid or oil extraction 
prior to transesterification. Commercial units are in operation all over the world, producing massive amounts of 
biodiesel each year, demonstrating that biodiesel is a mature technology for industrial applications, particularly 
in countries where biomass is abundant. Eventually, the amount of diesel consumed in the world is massive, 
having a negative impact on the economy and the environment. As a result, accelerating the use of biodiesel 
will have a significant impact on the environment, vehicle engines, independence from crude oil, investment, 
and the economy by creating jobs. 

Acknowledgements 

The authors gratefully acknowledge the Ministry of Transportation Malaysia, Malaysia Marine Department, 
Public Service Department of Malaysia, Dato’ Hj. Baharin Dato’ Abdul Hamid, Dato’ Hazman Hussein, Haji Wan 

Endok Wan Salleh, Haji Roslee Mat Yusof, Captain Abdul Samad Shaik Osman, Mohd Hafiz Abdul Majid, Chief 
Engineer Kevin Chin Ket Vui, Chief Mate Lim Poh Keong, Prof. Dr. Ir. Suzana Yusup, Chong Zhi Kai, Captain 
Shamsir Mohamed, Haji Badrul Hisham Umar, Prof. Dr. Nor Hasni Osman (UUM), Jenna Tan Ying Min, Tan 
Lee Chwin, Chew Kuan Lian, Teh Bee Bee, Loh Chong Hooi, Timmy Chuah Tim Mie and Ong Shying Weei for 
their support. The authors have been also supported by the EU project "Sustainable Process Integration 
Laboratory – SPIL", project No. CZ.02.1.01/0.0/0.0/15_003/0000456 funded by EU "CZ Operational Programme 
Research, Development and Education", Priority 1: Strengthening capacity for quality research, which has been 
gratefully acknowledged. 

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