ORIGINAL PAPER

Advancements in Heterogeneous Catalysis for Biodiesel Synthesis

Shuli Yan • Craig DiMaggio • Siddharth Mohan •

Manhoe Kim • Steven O. Salley • K. Y. Simon Ng

Published online: 14 April 2010

� Springer Science+Business Media, LLC 2010

Abstract Heterogeneous catalysts are promising for the

transesterification reaction of vegetable oils to produce

biodiesel and have been studied intensively over the last

decade. Unlike the homogeneous catalysts, heterogeneous

catalysts can be easily separated from reaction mixture and

reused for many times. They are environmentally benign and

could be easily operated in continuous processes. This

review classifies the solid catalysts into two categories based

on their catalytic temperature, i.e. high temperature catalysts

and low temperature catalysts. The nature of the catalysts can

be specified into solid bases and solid acids. Three aspects,

catalyst activity, catalyst life and oil flexibility, will be

reviewed. Two kinds of heterogeneous catalysts, reported by

IFP Inc. and by WSU, respectively, show a high catalytic

activity, long catalyst life and low leaching of catalyst

components. These two catalysts also show ability to

simultaneously catalyze esterification and transesterifica-

tion, and can be used in half-refined or crude oil system

which provide a potential for greatly decrease the feedstock

cost.

Keywords Biodiesel � Heterogeneous catalyst �Transesterification � Esterification � Solid base � Solid acid

1 Introduction

Due to growing worldwide demand for energy and its

resulting impact on the environment, it is becoming

increasingly important to search for sustainable alternative

fuels. Among the many possible sources, biodiesel derived

from vegetable oil attracted early attention as a promising

fuel for substitution or blending with petroleum based

diesel fuel, because biodiesel and petroleum diesel share

similar physical and chemical properties [1]. Thus, pure

biodiesel or biodiesel blends can be used in conventional

compression-ignition engines without the need for engine

modifications [2]. Furthermore, certain properties of bio-

diesel, such as flash point, cetane number, ultralow sulfur

content, lubricity, biodegradability, and smaller carbon

footprint were all superior to petroleum diesel [3, 4].

1.1 Biodiesel Chemical Background

Biodiesel is an industry adopted term for a mixture of fatty

acid alkyl esters, that are a product of the transesterification

reaction of triglycerides with methanol. In that reaction,

three alkyl esters are produced from one triglyceride mol-

ecule. A second product, glycerol, is also produced in a

molar ratio of 1:1 glycerol:triglyceride, further adding to

the value of processing oils. These reactions are repre-

sented by:

This well established process, introduced in the nine-

teenth century, has been used to exploit the fatty acid and

triglyceride content of a variety of natural oils for biodiesel

S. Yan � C. DiMaggio � S. Mohan � M. Kim �S. O. Salley � K. Y. S. Ng (&)

Department of Chemical Engineering and Material Science,

Wayne State University, Detroit, MI 48202, USA

e-mail: sng@eng.wayne.edu

123

Top Catal (2010) 53:721–736

DOI 10.1007/s11244-010-9460-5

fuel over the years. Among these renewable oils and fats

are soybean oil [5], rapeseed oil [6], cotton seed oil [7],

sunflower seed oil [8], jojoba oil [9], waste cooking oil

[10], chicken fat [11], lard [12], and beef tallow [3].

Typical alcohols used as the reactant in this biodiesel

process include methanol [5], ethanol [13] and butanol

[14], with methanol being the most widely used because of

its lower cost.

Some of the first industrial processes to create biodiesel

relied on either strong base or strong acid homogeneous

catalysts for this transesterification reaction. Examples of

the base catalyst are potassium hydroxide [9, 13] and

sodium hydroxide [14, 15], while sulfuric acid has been

used as an acid catalyst [16]. The reaction mechanism for

strong base catalyzed transesterification is shown in Fig. 1

[17–19]. Lee et al. [19] described the first step in the

synthesis of alkyl esters as the formation of an alkoxide ion

(RO–) through proton transfer from the alcohol using the

base catalyst; the alkoxide ion then attacks a carbonyl

carbon on the triglyceride molecule and forms a tetrahedral

intermediate ion (step 2). This ion rearranges to generate a

diglyceride ion and alkyl ester molecule (step 3). The

diglyceride ion reacts with the protonated base catalyst,

which generates a diglyceride molecule and returns the

base catalyst to its initial state (step 4). The resulting

diglyceride is then ready to react with another alcohol

molecule, thereby starting the next catalytic cycle until all

the glyceride molecules have been converted to alkyl esters

(biodiesel).

Strong acid catalyzed transesterification is illustrated in

Fig. 2 [20]. This process can simply be described as the

protonated carbonyl group nucleophilicly attacks the alco-

hol, forming a tetrahedral intermediate; the proton then

migrates, and the intermediate decomposes forming a new

ester. This process can be extended to di- and mono-glyce-

rides as well. Additionally, research shows that heteroge-

neous catalysis, both base and acid, also follow the above

mentioned mechanism for alkyl ester production [19].

1.2 First Generation Homogeneous Catalysts and Their

Limitations

Traditional or first generation homogeneous catalysts enjoy

certain advantages over other catalysts including cost-

effectiveness, high activity, and easily attained reaction

conditions (25–130 �C, atmospheric pressure). However,

O

O

R2 O

R1

O

O

R3

O

ROH + B R O- + B H

+

+ R O-

O

O

R2 O R1

O

O

R3

O

OHR

O

O

R2 O R1

O

O

R3

O

OHR

O

O

R2 O-

O

R3

O

+ R1 O

O

R

O

O

R2 O-

O

R3

O

+ B H+

O

O

R2

O

R3

O

OH

+ B

(1)

(2)

(3)

(4)

Fig. 1 Reaction mechanism of

base catalyzed

transesterification [17, 19]

722 Top Catal (2010) 53:721–736

123

these same homogeneous catalysts, by virtue of the asso-

ciated production process, face a variety of technical hur-

dles that limit their use for biodiesel production and

eventually may cause the demise of the early biodiesel

producers. Homogeneous catalysts are normally limited to

batch-mode processing [21]. In addition, other steps in the

biodiesel production process also require time consuming

and costly processing. These steps include oil pretreatment,

catalytic transesterification, separation of fatty acid/methyl

ester (FAME) from crude glycerin, neutralization of waste

homogeneous catalyst, distillation of accessory methanol,

water washing of the FAME phase, and vacuum drying of

the desired products [22]. Each of these steps introduce

additional processing time and cost. As an example, sep-

aration of the products from the spent waste catalyst

require a post treatment with large volumes of water to

neutralize the used catalyst in the product mixture. This

creates an additional process burden by generating waste

water that must be treated before release into the envi-

ronment [22].

Other difficulties with using homogeneous catalysts

center on their sensitivity to free fatty acid (FFA) and water

in the source oil. FFAs react with basic catalysts (NaOH,

KOH) to form soaps when the FFA and water content are

above 0.50 and 0.06%, respectively [5, 23]. This soap

formation complicates the glycerol separation, and reduces

the FAME yield. Water in the feedstock results in the

hydrolysis of FAME in the presence of strong basic or

acidic catalyst. Thus, some inexpensive oils, such as crude

vegetable oils, waste cooking oil, and rendered animal fats,

which generally contain a high content of FFA and water,

cannot be directly utilized in existing biodiesel facilities

with homogeneous catalysts. Likewise, the cost of oil

feedstock in 2006 accounted for up to 80% of biodiesel

production cost [24, 25]. So when petroleum diesel prices

fell in 2008, the relatively expensive soybean derived

biodiesel could not ompete, forcing many biodiesel facili-

ties to close. Therefore, part of the current solution is to

develop a second generation technology based on hetero-

geneous catalysts that are capable of effectively processing

less costly feedstocks high in FFAs and water content with

a simpler less costly processing method.

1.3 Second Generation Heterogeneous Catalyst

Advancements

Recent developments in heterogeneous catalysis for bio-

diesel production has the potential to offer some relief to the

biodiesel producers by improving their ability to process

alternative cheaper feedstocks, and to use a shortened and

less expensive manufacturing process. Whereas homoge-

neous based process required batch mode operation,

O

O

R2 O

R1

O

O

R3

O

H+

O

O

R2 O

R1

O+

O

R3

O

H

O

O

R2 O

R1

O+

O

R3

O

H

+ R4 OH

O

O

R2 O R1

OH

O

R3

O

O+

H

R4

O

O

R2 O R1

OH

O

R3

O

O+

H

R4

O

O

R2 OH

O

R3

O

+ R1 O

O

R4 + H+

(1)

(2)

(3)

Fig. 2 Reaction mechanism of

acid catalyzed

transesterification [17, 20]

Top Catal (2010) 53:721–736 723

123

heterogeneous processes can be run in either batch or con-

tinuous mode giving the producers the option to continue

with their current batch reactors or retrofit their operations

with a packed bed continuous flow reactor operation. Het-

erogeneous catalysts in either mode are in a separate phase

from the reaction products, thereby removing costly and

time consuming water washing and neutralization steps to

separate and recover the spent catalyst. Additionally, con-

taminated water from that process is greatly reduced and the

need for waste water treatment minimized.

The greatest advantage of the heterogeneous approach

over the homogeneous method is the prolonged lifetime of

the heterogeneous catalysts for FAME production. This

attribute is generally related to the stability of the micro-

crystal structure of the catalyst surface. Poisoning and

leaching of catalyst components can change the bulk and

surface structure of the catalyst and cause catalyst deacti-

vation quickly if the catalyst is not formulated properly.

The above three factors—catalytic activity, catalyst life

and oil flexibility—have a tremendous impact on the cost

of biodiesel. Because of this, we have undertaken a review

of the past and current heterogeneous catalyst technology

with these aspects as the focus. In this paper, the reported

heterogeneous catalysts are separated into two categories

based on their operation temperature. For reaction tem-

peratures lower than the flash point of biodiesel (130 �C),

we refer to this type of catalyst as a low temperature cat-

alyst. For reaction temperatures greater than 130 �C, we

classified these catalysts as high temperature catalysts.

These catalysts are characterized by the need for additional

safety considerations and require more energy intensive

operations. To further reduce the field of catalysts for this

review, only heterogeneous catalyst technologies which

provide the potential for decreasing the biodiesel process

cost and feedstock cost will be discussed.

2 Low Temperature Catalysts

As previously stated, studies of solid base catalysts began

burgeoning in the 1970 s. Most dealt with common single

metal oxides such as alkaline oxides and rare earth metal

oxides. Subsequently, studies were expanded to include

alkali metal exchanged zeolites, alkali metal ion-supported

catalysts, and clay minerals such as hydrotalcites.

2.1 Solid Base Catalysts

2.1.1 Alkaline Metal Salts on Porous Supports

Alkali metals are the most common source of super basi-

city and are frequently selected as the active species for

biodiesel production. The loading of many kinds of

alkaline salts on supports have been reported as a way to

prepare basic catalysts, such as NaOH [2, 26–29], KOH

[30, 31], K2CO3 [32], KI [33, 34], KNO3 [35, 36], KF [37–

40], and LiNO3 [41, 42]. The supports for these catalysts

include Al2O3 [33, 39], Zeolite [43], ZnO [41] and SiO2

[34].

An example of a commercialized super base catalyst is

Na/NaOH/Al2O3. It is used for the alkylation of cumene,

and the isomerization of safrole, dimethyl butene and vi-

nylbicyclo heptene [44]. Kim et al. [29] also tested its

activity for soybean oil transesterification with methanol

and found almost the same activity as homogeneous NaOH

catalyst under optimized reaction conditions (FAME yield

was 94% with a reaction temperature of 60 �C, reaction

time of 2 h, stirring speed 300 rpm, co-solvent n-hexane

10 mL, amount of catalyst 1 g). The basicity is believed to

be associated with the Lewis base concept according to the

O 1s XPS results presented. Table 1 shows the effect of

preparation on catalyst basicity. The oxygen 1s binding

energy shifts downward as the Na and NaOH impregnation

onto the c-Al2O3 support progresses, indicating that the

basicity increases together with the degree of impregnation.

Consistent with O 1s binding energy, the catalyst’s FAME

activity also proportionally increases.

Xie et al. [35] and Vyas et al. [36] investigated the

activity of KNO3/Al2O3. Xie et al. [35] pointed out that the

active phase was K2O derived from KNO3 at high tem-

perature, and the surface Al–O–K groups were the main

active sites. Cui et al. [39] prepared KF/c-Al2O3 and found

there were two types of basic sites on the catalyst. The

strong basic sites (super basic) promote the transesterifi-

cation reaction at low temperature (65 �C), while the basic

sites with medium strength require a higher temperature to

process the reaction. Later, Boz et al. [40] prepared KF

catalysts loaded on nano-c-Al2O3 and Wang et al. [45]

loaded KF on malodorous CaO–MgO. They both found

that the catalyst’s FAME activity is closely related to the

basic nature of the catalyst and also to the high surface to

volume ratio and porosity of the catalyst.

Table 1 XPS analysis of O 1s orbital of four catalysts

Catalyst Binding energy

of O 1s (eV)

Biodiesel yield

(%)

c-Al2O3 538.8 5

NaOH/ c-Al2O3 538.5 60

Na/ c-Al2O3 537.5 70

Na/NaOH/ c-Al2O3 535.5 78

Reaction conditions: Methanol/oil molar ratio is 6:1, reaction tem-

perature is 60 �C, and stirring speed is 300 rpm. All data are taken

from literature [29]

724 Top Catal (2010) 53:721–736

123

However, in spite of the high activity of the supported

alkaline catalysts, they have important limitations. First,

these catalysts like their homogeneous alkaline hydroxide

counterparts have a low tolerance to FFA and water in raw

materials. At this time, there is no report of using this kind

of catalyst for directly processing crude oils which have a

high total acid number (TAN). Only refined oils can be

used with these catalyst systems. Secondly, some

researchers have observed lixiviation of catalyst compo-

nents into reaction mixtures. Arzamendi et al. [46] found

that 55% of K2CO3, 20% of Na2CO3 and 15% of Na3PO4

dissolved into the reaction mixtures and catalyzed the

transesterification reaction. Also, Xie et al. [35] found

KNO3/Al2O3 catalysts have a high solubility in water and

were therefore unstable in the transesterification system.

However, Noiroj et al. [47] found that the type of support

strongly affected the activity and leaching of the active

species of the catalyst. In this case, the amount of leached

potassium of the KOH/Al2O3 was higher than that of the

KOH/NaY catalyst. And they found that the interaction

between active phase and support affected the leaching

results. Additionally, Ramos et al. [48] prepared sodium

hydroxide on a zeolite support and hypothesized the pres-

ence of a homogeneous-like mechanism where the alkali

methoxide species were leached out.

2.1.2 Alkaline Earth Metal Oxide Catalysts

Much attention has been paid to alkaline earth metal oxides

since they have shown less solubility in reaction mixtures

and less corrosion in comparison to supported alkaline

catalysts. As solid super base can be synthesized from

alkaline metal oxides, researchers started from pure alka-

line metal oxides. In fact, alkaline metal oxides have

already been used as base catalysts in many organic reac-

tions. For example CaO is widely used for as the isomer-

ization of 5-vinylbicyclo [2.2.1] hept-2-ene (VBH) to

5-ethylidenebicyclo [2.2.1] hept-2-ene (ENB) [49, 50],

synthesis of 1, 3-dialkylurea from ethylene carbonate and

amine [51] and the synthesis of monoglyceride [52]. With

respect to biodiesel production, the basicity of this type of

metal oxide catalyst has been shown to have an influence

on its activity for FAME generation. The basic strength of

the Group II metal oxides follows the order: MgO \CaO \ SrO \ BaO. Corresponding research has demon-

strated the catalyst’s activity for transesterification of oil

with methanol follows the same order [53–55]. But, com-

pared to a homogeneous NaOH catalyst, the above alkaline

earth metal oxides show a relatively low transesterification

activity. In particular, MgO exhibits almost no activity in

transesterification of vegetable oils into biodiesel. Pure

CaO reacts at a slow rate and requires about 6–24 h to

reach a state of reaction equilibrium [56–58]. BaO is not

suitable for biodiesel production because it dissolves in

methanol and forms some noxious species [59, 60]. Con-

versely, SrO, has a high activity and is insoluble in meth-

anol, but will react strongly with CO2 and water in the air

to form unreactive SrCO3 and Sr(OH)2. Furthermore, these

strontium compounds are difficult to regenerate by cal-

cining, requiring temperatures above 1200 �C [17]. As a

partial solution to these limitations, recent work has

focused on using mixed metal oxides to enhance the

basicity of CaO or MgO-based catalysts and elevate their

respective selectivity for FAME.

2.1.2.1 Supported CaO Catalysts Although earlier work

showed weak FAME activity for pure CaO catalysts, more

recent research has shown that the smaller particle size of

CaO catalysts can increase the total amount of base sites

and base strength, which leads to an improved activity in

the oil transesterification reaction. Reddy et al. [61] tested

the activity of nanocrystalline CaO and found it active even

at room temperature. However, as Gryglewics [6, 62] and

Martyanov and Sayari [63] pointed out, pure CaO con-

verted to a form of suspensoid due to its poor mechanical

strength, which would lead to difficulties in separating the

waste catalyst from biodiesel and glycerol products after

transesterification. Since these findings could have a

potential impact on industrial applications, many

researchers have tried to solve this problem by applying

CaO on different metal oxide supports.

In particular, CaO has been combined with ZnO [64],

MgO [46, 65], Al2O3 [65, 66], zeolite [48, 65], SiO2 [65,

67], and La2O3 [68] with improved base characteristics,

activity and catalytic life. Rubio-Caballero et al. [64] used

the calcined calcium zincate as a solid catalyst for the

methanolysis of sunflower oil to FAME resulting in yields

higher than 90% after 45 min of reaction. The reaction

conditions of the heterogeneous process (60 �C, methanol:

sunflower oil molar ratio of 12, 3 wt% catalyst) were very

similar to those observed under homogeneous conditions

(KOH dissolved in methanol). Yan et al. [65] investigated

the effects of a second metal oxide by impregnating CaO

on basic oxides such as MgO, neutral oxides such as SiO2,

and acidic oxides such as Al2O3 and zeolite HY. In this

work, the best results were obtained for a catalyst with

16.5% of CaO loading on MgO. The same catalyst also

possessed the strongest base strength and largest number of

base sites. The conversion of rapeseed oil using this cata-

lyst reached 92% at 64.5 �C. Further work by Yan et al.

revealed the active centers of the CaO/MgO catalyst. CO2-

TPD profiles of CaO/MgO showed that there were two

types of basic sites each with a different strength. The

desorption peaks of CO2 at *600 �C were attributed to the

strong basic sites corresponding to unbounded O2- anions,

while CO2 desorption peaks at low temperature (*350 �C)

Top Catal (2010) 53:721–736 725

123

were attributed to the weak basic sites related to oxygen in

both Ca2?–O2- and Mg2?–O2- pairs. Yan et al. [17, 65, 68]

also quantified the effects of base property on oil transeste-

rification. They found that the activity of CaO/MgO linearly

increased with base amount, and base amount linearly

increased with CaO loading when the CaO loading was lower

than 16.5%. Later, Yan et al. [70] combined CaO with the

basic oxide La2O3, and used the Hammett method to deter-

mine the base properties of catalysts. They found that the

binary metal oxides had a higher base strength and a wider

base site distribution than pure CaO and La2O3 catalysts

individually, and they also had a higher reactivity than CaO

and La2O3 individually. Using this binary metal oxide cat-

alyst with a 3:1 of molar ratio of Ca to La, they found the

FAME yield reached 94.3% within 60 min at just 58 �C.

This suggested a reaction rate much closer to that of a

homogeneous NaOH catalyzed processes and that hetero-

geneous catalysts were capable of attaining the same activ-

ities as homogeneous catalysts.

Equally important here, this work indicated that CaO

type heterogeneous catalysts also showed a high tolerance

to water and FFA which is present in unrefined raw oil

feedstocks. This implies these heterogeneous catalysts have

a potential for biodiesel production. Until now, many CaO-

based catalysts were reported to be more tolerant than the

supported alkaline catalysts. Yan et al. [65] reported that

conversion of rapeseed oil by CaO/MgO reached as high as

98% when the water content of the raw oil was in the range

of 0–2 wt% and the total acid number was below 7.4 mg

KOH/g (FFA content around 3.7%). Later, Yan et al. [70]

reported that CaO–La2O3 was active when the oil con-

tained 10% of water and when the FFA content was lower

than 3.5%. They then tested the basicities of the catalysts

which had adsorbed small amounts of FFA and water,

respectively, naming the catalysts Ca3La1–FFA and

Ca3La1–water. Yan and co-workers found that the basic

properties of the Ca3La1–water catalyst are much closed to

the fresh CaO–La2O3 catalyst; therefore, it can be assumed

that CaO–La2O3 shows a high tolerance to small amounts

of water, while the basic property of the Ca3La1–FFA

catalyst notably decreased both the base strength and sites,

indicating CaO–La2O3 has a low tolerance for FFA in oils.

Further characterization results indicated that there are

Lewis base sites and Bronsted base sites on the surface of

fresh CaO–La2O3 catalysts and both of these base sites are

active centers for oil transesterification with methanol. In

fact, the addition of small quantities of water can change

the Lewis base sites into Bronsted base sites suggesting

Ca3La1–water is still active in transesterification. Con-

versely, FFA will react with and bind to both base sites

resulting in poisoning of the catalyst. Therefore, Ca3La1–

FFA shows a low activity for FAME production. Further

work by Yan et al. [70] using CaO–La2O3 for processing

crude soybean oil, crude palm oil and waste cooking oil,

which satisfy the limitation of water content lower than

10% and FFA lower than 3.5%, showed a FAME yield in

excess of 95% within 3 h. For the oils with a high FFA

content, dilution with refined oil will lower the FFA con-

tent of the mixture again allowing the use of a CaO–La2O3

catalyst to produce a high FAME yield.

Equally important to the usefulness of a catalyst is its

lifetime. Reddy et al. [61] found that nanocrystalline cal-

cium oxide particles deactivated after eight cycles with

soybean oil and after only three cycles with higher FFA

content poultry fat. Similarly, Kawashima et al. [57, 58]

found evidence of decreased FAME activity with CaTiO3

and CaZrO3 catalysts after as few as three cycles.

To overcome these limitations, therefore, it is important

to understand the mechanism of catalyst deactivation. In

general, there are three pathways by which catalyst deac-

tivation can occur: poisoning, blocking of reactant frag-

ments and lixiviation. Many studies have already paid close

attention to poisoning due to high levels of either FFA or

water present in raw materials [65, 70], so they will not be

discussed here. But little research has been performed that

focuses on the effects of exposing a stored catalyst to CO2,

moisture, and O2 present in the air. As stated by Busca

[69], base catalysts can easily react with these components

of ambient air to form very stable surface species like

carbonate, hydroxide, and epoxide which cover the basic

sites and deactivate the base catalysts. Yan et al. [70] found

that when comparing the activity of a fresh CaO–La2O3

catalyst to one exposed to air for 12 h, the yield of FAME

sharply decreased from 96.8 to 34.5%, and the total base

amount decreased from 14.0 to 1.5 mmol CO2/g. They

later attributed this decrease in activity to moisture and

CO2 in air restructuring the surface of CaO–La2O3 catalyst

from metal oxides to hydroxide and carbonate. Other

research groups have found that the reused base catalysts

have a lower basic strength and a lower activity than fresh

the catalysts [71, 72]. This was explained by the blockage

of active surface sites on the catalyst by strongly adsorbed

intermediates or product species. In particular, Martyanov

and Sayari [63] studied reused catalysts (CaO, Ca(OCH3)2)

and found that surface adsorbed butyric acids were the

most likely species responsible for the catalyst deactivation

in their experiments.

Catalyst lixiviation or leaching is another frequently

encountered pathway for catalyst deactivation. Many

researchers have studied the leaching of catalyst compo-

nents into reaction mixtures. Kouzu et al. using a pure CaO

catalyst [71] found that the calcium concentration would be

as high as 3065 ppm in the FAME when waste cooking oil

was used as the source oil. Later Kouzu et al. [72] showed

calcium in the products of transesterified refined soybean

oil. In that case, he found that the calcium content in

726 Top Catal (2010) 53:721–736

123

glycerol was about 2000 ppm and calcium content in

FAME was around 10–100 ppm. Similarly, Granados et al.

[73, 74] studied the leaching of species from solid CaO and

the role of these species in the catalytic reaction. He

pointed out that CaO can react with glycerol to form Ca

diglyceroxide which is more soluble than CaO and active

in oil transesterification. He found that the solubility of

CaO in alcohol is around 0.6 mg/mL and when CaO was

less than 1 wt% the major reaction mechanism is homo-

geneous. When the catalyst loading is greater than 1 wt%

CaO, the total homogeneous contribution is much smaller

than that arising from the heterogeneous sites [74]. Thus, it

is very apparent that determining the homogeneous con-

tribution to the FAME yield of a CaO-based catalyst sys-

tem, and other like catalysts, is as important as quantifying

the heterogeneous component.

2.1.2.2 Supported MgO Catalysts One of the drawbacks

of using a CaO-based catalyst is the low BET surface area

associated with the catalyst. Because activity is closely

related to surface area for many catalyst systems, loss of

active surface area through deactivation can have a pro-

portionally larger effect on the product yield. Therefore,

one solution would be simply to use more catalyst in your

industrial application design. However, this introduces

additional cost into the plant design and material usage. To

avoid these concerns some research groups have turned to

metal oxide catalysts from hydrotalcites which are well-

dispersed, have a high surface area, and are characterized

by a strong base property. One such example is a MgO

based catalyst. Cantrell et al. [55] reported on a supported

MgO–Al2O3 catalyst which was active in transesterifica-

tion of glyceryl tributyrate using methanol. He found that

the BET surface area was as high as 166 m2/g. Using the

same catalyst, Xie et al. [75] found that the Mg/Al molar

ratio also had an effect on FAME activity. Using a Mg/Al

ratio of 3, 773 K calcination temperature, 15:1 M ratio of

soybean oil: methanol, and a catalyst dosage of 7.5 wt%,

oil conversion was found to be 67% after 9 h. Other work

by Li et al. [76], using mixed oxides from Mg–Co–Al–La

hydroxide, found those catalysts maintained its activity for

7 recycles in a batch reactor. Additional work on the MgO–

Al2O3 catalyst, by Fraile et al. [77], found that the reaction

mechanism relied on the residual alkaline ions as the main

source of strong basicity and catalytic activity in the

transesterification of sunflower oil with methanol.

2.1.3 Base Resin Catalysts

Not only inorganic bases, but also some organic bases were

also tested for biodiesel production [78, 79], especially for

base resin catalysts. Shibasaki-Kitakawa et al. [80] inves-

tigated the transesterification of triolein with ethanol using

various commercial resin catalysts. They found that the

anion-exchange resins, such as Diaion PA308, PA306s,

HPA25 (Mitsubishi Chemical C., Ltd, Tokyo, Japan),

exhibited much higher catalytic activity than the cation-

exchange resins like PK208 (Mitsubishi Chemical C., Ltd,

Tokyo, Japan). The anion-exchange resins characterized by

a lower cross-linking density and a smaller particle size

produced both a high reaction rate and conversion. The

best catalytic performance was obtained on Diaion PA306s

resin, which yielded over 80% conversion of soybean oil to

ethyloleate after 3 h reaction at 323 K. However, other

groups have found less satisfactory results using base resin

catalysts. In particular, Aracil and coworkers [81] used an

anion-exchange resin in the transesterification of sunflower

oil to biodiesel and found the conversion was less than 1%

after 8 h at a typical reaction temperature of 333 K. Kim

et al. [82] found that trace amounts of CH3ONa, func-

tioning as a homogeneous catalyst, exhibited a synergetic

effect with the resin catalyst for conversion.

2.1.4 Biont Shell Based Catalysts

Recently, catalysts derived from renewable materials, such

as shrimp shell [83, 84], turtle shell [83], crab shell [83],

oyster shell [85] and egg shell [86] have been employed for

conversion of oils to FAME. Previously, these catalysts were

generally considered as waste. The major components of

these biont shells are chitin, protein and CaCO3. Normally,

disposal of these waste materials from seafood processing

are an economic or environmental problem for entrepreneurs

and local governments. However, biodiesel production cat-

alysts prepared from these ‘‘wastes’’ are a promising

‘‘green’’ technology. Xie et al. [83] first reported preparing

biont shell supported KF catalysts for biodiesel production

and found methylester yields from rapeseed oil as high as

97.5% within 3 h under optimal reaction conditions. Yang

et al. [84] reported, a three step preparation procedure for a

shrimp shell catalyst which included incomplete carbon-

ization of the shrimp shells, loading KF onto the altered

shells, and activation. In a different approach, Nakatani et al.

[85] and Wei et al. [86] prepared CaO catalysts by simple

calcination of oyster shells and egg shells, high in CaCO3

(95%), at 700–1000 �C. Although these catalysts proved

active for biodiesel synthesis, further work is required to

limit the amount of Ca leaching to improve the catalyst life

and tolerance to water and FFA in oil feedstock.

2.2 Solid Acid Catalysts

Even though many heterogeneous base catalysts have been

reported as highly active for biodiesel synthesis, they still

cannot tolerate acidic oils with FFA content [3.5%, such

as yellow and brown grease. However, sulfur based acidic

Top Catal (2010) 53:721–736 727

123

homogeneous catalysts such as H2SO4 show a much higher

tolerance to FFA and water than the basic homogeneous

NaOH and KOH catalysts, suggesting these catalysts may

be more suited for processing acid oils. Using this line of

reasoning, some research turned to investigating sulfur

based heterogeneous acid catalysts for converting acidic

oils into biodiesel.

2.2.1 Sulfated Zirconia Based Catalysts

Sulfated metal oxides show superacid properties because of

the interaction between the sulfate group and the metal

oxide centers. These kinds of catalysts, including sulfated

zirconia [87–89] and sulfated tin oxides [90] have been

widely used in esterification and transesterification reac-

tions under mild conditions. Kiss et al. [87] studied several

solid acid catalysts (zeolites, ion-exchange resins, and

mixed metal oxides) as catalysts for the esterification of

dodecanoic acid with 2-ethylhexanol, 1-propanol, and

methanol. That work revealed that sulfated zirconia was

the most active for esterification. Later, Garcıa et al. [91]

investigated the activity of sulfated zirconia for soybean oil

transesterification. That group found that the catalyst

preparation method had a significant effect on the resulting

catalyst activity. Under optimized conditions (120 �C, 1 h

and 5 wt% of catalyst) and using sulfated zirconia prepared

by a solvent-free method, the methanolysis of soybean oil

was 98.6% and ethanolysis was 92.0%. The sulfated zir-

conia prepared by standard methods [88] was poor for

soybean oil methanolysis (conversion of 8.5%) and con-

ventional zirconia even less so. Similarly, Suwannakarn

et al. [89] studied the activity and stability of a commercial

sulfated zirconia catalyst for transesterification of trica-

prylin with a series of aliphatic alcohols at 120 �C. He

found that the catalytic activity decreased as the number of

carbons in the alkyl chain of the alcohol increased. In

addition, the sulfated zirconia catalyst exhibited significant

activity loss with subsequent reaction cycles. Character-

ization of the recycled catalysts showed that the concen-

tration of the SO42- moieties in the sulfated zirconia had

permanently decreased. Essentially, the SO42- species

were leached out. As explained by Yadav and coworkers

[92, 93], the sulfate groups leached out as H2SO4 and

HSO4-, which in turn gave rise to a homogeneous acid

catalysis which interfered with activity measurements of

the intended heterogeneous catalyst.

2.2.2 Heteropolyacid Catalysts

A series of heteropolyacid (HPAs) catalysts have also

attracted much attention due to their high activity in

biodiesel formation reactions, both transesterification and

esterification. Alsalme et al. [94] studied some HPA catalysts

and compared them with some homogeneous and hetero-

geneous catalysts such as H2SO4, Amberlyst-15, and zeolites

HY and H-Beta. The intrinsic catalytic activity, expressed as

turnover frequency (TOF), of the HPA catalyst is signifi-

cantly higher than that of the conventional acid catalysts in

these reactions. They also tested the catalytic activity and

acid strength of several kinds of HPA catalysts. The TOF

values decreased with decreasing catalyst acid strength

in the order: H3PW12O40 & Cs2.5H0.5PW12O40 [ H4SiW12

O40 [ 15%H3PW12O40/Nb2O5, 15%H3PW12O40/ZrO2, 15%

H3PW12O40/TiO2 [ H2SO4 [ HY, H-Beta [ Amberlyst-15.

They found that Cs2.5H0.5PW12O40 exhibits high catalytic

activity as well as high resistance to leaching. The other types

of supported HPA catalysts suffered from leaching and

exhibited a significant homogeneous component to the cat-

alyst’s activity caused by the leached HPA. Pesaresi et al.

[95] studied the catalytic mechanisms of CsxH4-x

SiW12O40 (x = 0.8–4) in the transesterification of C4 and C8

triacyglycerides and esterification of a C16 FFA. The catalyst

material, loading C1.3 Cs per Keggin, provided an insoluble,

heterogeneous catalyst active for both transesterification and

esterification, with reactivity correlating with the number of

accessible H? sites residing within the mesopore structure.

For loadings B0.8 Cs per Keggin, transesterification activity

arises from the homogeneous contribution. Narasimharao

et al. [96] investigated structure related activity for CsxH3-

xPW12O40 (x = 0.9–3). Materials with the Cs content in the

range x = 2.0–2.7 were well dispersed, having a high sur-

face areas *100 m2/g-1 and high Bronsted acid strength.

CsxH3-xPW12O40 was active in both esterification of pal-

mitic acid and transesterification of tributyrin. Further work

showed an optimum performance occurs for Cs loadings of

x = 2.0–2.3, correlating with the accessible surface acid site

density. These catalysts were recovered for three times and

leaching of soluble heteropolytungstate wasn’t observed.

Other HPAs were also reported. Katada et al. [97] found

that H4PNbW11O40, H3PW12O40 and the heteropolyacid-

derived solid acid catalyst, H4PNbW11O40/WO3–Nb2O5,

were highly active for the transesterification of triolein with

ethanol. But, H4PNbW11O40 and H3PW12O40 dissolved into

the reaction mixture; H4PNbW11O40/WO3–Nb2O5 was

insoluble to the reaction mixture. Further study showed

that the activity of H4PNbW11O40/WO3–Nb2O5 was sensi-

tive to calcination temperature, and calcination around

773 K provided a highly active catalyst. The activity

was observed in the co-presence of water (3.9 wt%) and

oleic acid (5 wt%). In a fixed-bed continuous-flow reaction,

it maintained the yield of FAME around 25–40% for

4 days.

728 Top Catal (2010) 53:721–736

123

2.2.3 Organically-Functionalized Acid Catalysts

The purpose of preparing organically-functionalized acid

catalysts is to overcome the shortcomings of other acid

catalysts, such as leaching and low surface area. Some

attempts have been made with the sulfonic acid ionic-

exchange resins, such as Poly (DVB) resin sulfonated with

H2SO4 [98], Amberlyst-35(Rohm & Haas) [98], Amber-

lyst-15 (Rohm & Haas) [94, 99], and Nafion SAC-13[100].

Rezende et al. [98] prepared different polymer supports

based on styrene and divinylbenzene which were conve-

niently functionalized with sulfonic acid. In order to obtain

an appropriated triglyceride conversion at low temperature

(65 �C), it was necessary to use a high ratio of methanol to

oil (50:1–300:1) and high catalyst dosage (25–50%). Under

the optimal conditions FAME yields reached a maximum

value over 90% using a sulfonated poly (DVB) ion-

exchange resin which had 442 m2/g of specific surface area

and 3.4 meqH? g-1 of acid capacity. Some polymer based

catalysts were claimed to be active for both oil transeste-

rification and fatty acid esterification reaction in unrefined

oil systems. As an example, Soldi et al. [101] prepared

sulfonated polystyrene compounds where sulfonation was

between 5.0 and 6.2 mmol SO3H/g of dry polymer. That

work showed conversion of beef tallow, with a

53 mg KOH/g acid number, reached 70% within 18 h.

2.2.4 Natural Based Catalysts

A novel type of renewable catalyst has been prepared from

various carbohydrates such as D-glucose, sucrose, cellulose

and starch [102–104]. These catalysts were made by

incomplete carbonization of carbohydrates followed by

sulfonation. The incomplete carbonization of D-glucose

leads to a rigid carbon material consisting of small poly-

cyclic aromatic carbon sheets in a three dimensional sp3-

bonded structure [103]. Sulfonation has been demonstrated

to provide a highly stable solid with a high density of active

SO3H sites. This type of catalyst was found to be physi-

cally robust without leaching of SO3H groups during use.

This resulted in remarkable catalytic performance for

FAME formation reactions for both transesterification and

esterification [103, 105, 106]. In studies by Lou et al. [106],

carbohydrate-derived acid catalysts had been successfully

applied to biodiesel production with higher fatty acid oils,

such as waste oils with high acid values. Variables such as

starting material, carbonization temperature and time, and

sulfonation temperature and time for catalyst preparation

all had a significant impact on the catalytic and textural

properties of the prepared solid acids. Under optimal

reaction conditions (80 �C, 20: 1 of molar ratio of metha-

nol to oil, 10 wt% of catalyst loading, over a starch-derived

sulfonic acid catalyst), the FAME yield was measured at

about 92% after 8 h’s reaction. Furthermore, the starch-

derived solid acid catalyst proved exceptionally stable

under reaction conditions.

3 High Temperature Catalysts

Even though solid acid catalysts exhibit improved activity

for converting acid oils into FAME, most of them show a

relative low reaction rate and deactivate quickly in com-

parison to solid base catalysts. Intensifying reaction con-

ditions, by increasing reaction temperature and pressure,

has been shown to effectively accelerate the reaction rate

and prolong the catalyst lifetime. Some of the more suc-

cessful examples of these catalysts that function at higher

temperature and pressure, including both solid base and

acid catalysts, were tested in subcritical or supercritical

methanol flow conditions (240 �C, 8 MPa) and reported

below.

3.1 Solid Base Catalysts

Several strong base catalysts have been tested at high

reaction temperatures. One of which, CaO investigated by

Demirbas [107], was operated under supercritical methanol

conditions. When the temperature was 252 �C, transeste-

rification was completed within 6 min with 3 wt% CaO

and 41:1 methanol/oil molar ratio. In other work on Ca

based catalysts, Suppes et al. [108] found that CaCO3 was

active when the temperature was greater than 200 �C and

required about 18 min to essentially convert all the oil;

FFA in the oil was esterified by CaCO3 and did not appear

to inhibit the catalyst; Also, no decrease in activity of the

calcium carbonate was observed after weeks of utilization

suggesting little leaching or deactivation.

Separately, Barakos et al. [109] used both non-calcined

and calcined Mg–Al–CO3 hydrotalcite catalysts in refined

cottonseed oil, acidic cottonseed oil, and crude animal fat

feedstock. Mg–Al–CO3 hydrotalcite catalysts were active

in both transesterification and esterification. The activity of

the calcined catalyst was lower than the non-calcined cat-

alyst. But, the non-calcined catalyst showed evidence of

deactivation when recycled. However, additional informa-

tion regarding catalyst leaching and stability was not pre-

sented in this work.

3.2 Solid Acid Catalysts

3.2.1 Sulfate Salts

As with the low temperature catalysts, the activity of the

sulfated salt family of catalysts is based on the presence

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123

of sulfonic acid sites, which can be considered as the

heterogeneous counterpart of sulfuric acid. Also, the acid

strength of the catalyst still has an important role in the

transesterification reaction. Some factors, such as the

preparation technology and suitable selection of support,

greatly influenced the acid site distribution on the cata-

lyst. For instance, Jothiramalingam and Wang [21]

reported that catalysts prepared from a stronger acid

precursor containing benzene sulfonic acid groups had a

higher acid strength and were more active than those

containing only propylsulfonic acid groups. Chen et al.

[110] presented evidence that good catalytic performance

of the sulfated silica-zirconia material was attributed to

an improved preparation process which resulted in a

higher dispersion of zirconia, thus creating a higher acid

site density.

The carriers for sulfonic acid include not only some

inorganic metal oxides (zirconia oxide [21, 110], tin oxide

[111], stannia [112]), but also some mesostructured silica

[113] and carbon materials such as multi-wall carbon

nanotubes [114, 115] and asphalt [115]. In their work,

Jitputti et al. [112] evaluated the activities of sulfated

zirconia and stannia for crude palm kernel oil and coconut

oil conversion to biodiesel. They showed minor activity at

200 �C, which subsequently decreased with additional

recycling. They associated the decrease in activity to both

sulfate leaching and active site poisoning. In other work,

Melero et al. [113] prepared propylsulfonic acid SBA-15

material and found it highly active for the conversion of

refined and crude palm oil and soybean oil. The catalytic

performance was attributed to the large surface area and

pore diameter of the mesoporous support. However, they

also found a slight decrease of activity in recycled cata-

lyst testing. To remedy this, they pointed out that further

work would be performed to enhance the strength of acid

sites and control the surface properties of the silica sup-

port in order to enhance the durability of these sulfonated

mesostructure catalysts. Shu et al. [115] prepared sulfo-

nation of carbonized vegetable oil asphalt and sulfonated

multi-walled carbon nanotubes (s-MWCNTs). They found

that the asphalt-based catalyst showed higher activity than

the s-MWCNTs for the production of biodiesel and that

this behavior might be correlated to the high acid site

density of asphalt catalysts resulting from its loose

irregular network and large pores. Using the asphalt based

catalyst, the conversion of cottonseed oil achieved

89.93% when the methanol/cottonseed oil molar ratio was

18.2, reaction temperature 260 �C, reaction time 3.0 h,

and a catalyst/cottonseed oil mass ratio of 0.2%. Also, the

asphalt based catalyst can be re-used. Shu et al. [114,

115] thought that the sulfonated polycyclic aromatic

hydrocarbons provided an electron-withdrawing function

to keep the acid sites stable.

3.2.2 Heteropolyacid Catalysts

Sunita et al. [116] compared the activities of zirconia

supported isopoly and heteropolytungstate catalysts. Zir-

conia-supported heteropolytungstate possessed a high total

acidity and showed superior catalytic performance com-

pared to zirconia-supported heteropolytungstate catalysts.

Under their reaction conditions of 200 �C, methanol/oil

molar ratio 15, and 15% WO3/ZrO2 calcined at 750 �C the

ZrO2 catalyst achieved 97% conversion of oil. Kulkarni

et al. [117] impregnated tungstophosphoric acid on four

different supports such as hydrous zirconia, silica, alumina

and activated carbon, and used them for converting low

quality canola oil containing about 20 wt% FFA to bio-

diesel. The hydrous zirconia supported tungstophosphoric

acid was found to be the most active. At 200 �C, 1:9 oil to

alcohol molar ratio, and 3 wt% catalyst loading a maxi-

mum ester yield of 90 wt% was observed.

3.2.3 Other Catalysts

Except for the above two types of catalysts, other weaker

solid acid catalysts were also tested for activity in biodiesel

formation reactions. These included many types of zeolite

and phosphate based catalyst systems. For instance, Brito

et al. [118] studied the activity of several commercialized

Y-type zeolites in a continuous tubular reactor at atmo-

spheric pressure and within a temperature range of 200–

476 �C. The results showed that higher temperature

accelerated transesterification reaction rate. With used fry

oil, they found that the optimal reaction conditions for

transesterification with methanol could be achieved with

zeolite Y530 at 466 �C, 12.35 min residence time, and a

methanol/oil molar ratio of only 6.

In other work, some phosphate salts have also been

reported active for biodiesel formation. One such example is

by Li and Xie [119] who prepared Fe3?-vanadyl phosphate.

When the transesterification reaction was performed at a

molar ratio of methanol to oil of 30:1, reaction temperature

of 473 K, reaction time of 3 h, and a catalyst loading of

5 wt%, the maximum conversion of soybean oil was found

to be 61.3%. The importance of this work is that it showed

the activity of this catalyst was not significantly affected by

the presence of free fatty acids and water in the reactant

mixture. In addition, the catalyst also exhibited catalytic

activity for the esterification of free fatty acids with meth-

anol. Unfortunately, the same catalyst slowly deactivated

after 5 recycles. Serio et al. [120] suggested that the deac-

tivation of vanadyl phosphate catalyst was strongly affected

by reaction temperature. In effect, higher reaction temper-

atures accelerated the deactivation process. However, cata-

lyst leaching at higher temperatures was not the root cause of

deactivation. Instead, surface characterization work showed

730 Top Catal (2010) 53:721–736

123

that deactivation was primarily due to the progressive

reduction of surface vanadium from V5? to V3? by methanol

where V5? and intermediate V4? species were active and

V3? was inactive. Further work revealed that the deactiva-

tion was reversible and catalyst activity could be restored by

simple oxidation.

Yet another catalyst technology based on Fe–Zn double-

metal cyanide complexes was tested by Sreeprasanth et al.

[121]. The catalysts were hydrophobic, and contained only

Lewis acidic sites. Bronsted acid sites as well as basic sites

were absent. These catalysts proved active for both the

transesterification and esterification of unrefined and waste

cooking oils. Thus Lewis acidic sites were found to be

active centers for both the transesterification and esterifi-

cation reactions and the surface hydrophobicity of these

catalysts improved their tolerance to water.

3.3 Amphoteric Metal Oxide

Some amphoteric metal oxides, such as PbO, PbO2, and

ZnO, have attracted attention from researchers because of

their adjustable basic and acid properties. Singh and Fer-

nando [122] found that the FAME yield reached 89% using

amphoteric PbO and PbO2. However, further testing

showed Pb content in the glycerol and biodiesel products

was as high as 2000 ppm which implied some dissolution

of the catalyst. With respect to ZnO based catalyst systems,

two candidates appear to offer the best next generation

catalyst solution for improved biodiesel synthesis. One is a

catalyst composed of zinc aluminum oxides from Institut

Francais Du Petrole (Vernaison, France); the other is a

catalyst of zinc lanthanum oxides from Wayne State Uni-

versity (MI, USA).

3.3.1 Zinc–Aluminum Catalyst for the Esterfif-HTM

Process

The Esterfif-HTM process was developed by the French

Institute of Petroleum (IFP) and commercialized by Axens.

It was first used in an industrial context in 2006, by Sofi-

proteol in Sete [123]. This process uses a heterogeneous

catalyst, a spinel mixed oxide of zinc and alumina metals.

The use of heterogeneous catalysts eliminates the need for

catalyst recovery and washing steps—and associated waste

streams—required by processes using homogeneous cata-

lysts such as sodium hydroxide or sodium methylate. The

process chart is shown in literature [22]. The catalyst

section includes two fixed bed reactors (CSTR), fed with

vegetable oil and methanol at a given ratio. Excess meth-

anol is removed after each reactor by partial evaporation.

Then, esters and glycerol are separated in a settler. The

remaining glycerol is collected and the residual methanol

removed by evaporation.

A patent [124] awarded to IFP described an acid catalyst

with a formula of ZnAl2O4, xZnO, yAl2O3 (with x and y

being in the range of 0–2) which originated from a hy-

drotalcite precursor. The BET surface area is between 50

and 200 m2/g and a pore volume is greater than 0.3 cm3/g.

In the continuous process, the reaction temperature is

required to be between 210 and 250 �C, pressure between

30 and 50 bar, and VVH (volume of injected oil/volume of

catalyst/hour) from 0.3 to 3. Using these conditions, they

showed a FAME yield of 91% based on a catalyst with a

surface area of 65 m2/g, pore volume of 0.63 cm3/g under

the conditions of 240 �C, 50 bar, 160 min of contact time.

This patent also addressed the catalyst lifetime. They found

that on the 14th day, the FAME yield decreases to 30.9% at

240 �C, 50 bar, 1 VVH. The structure of the deactivated

catalyst is not reported in this patent. A subsequent IFP

patent [125] states that this catalyst is quite sensitive to

water. In fact they maintained the water concentration

below 1000 ppm, which implies that the oil feedstocks

used in Esterfif-HTM process must be well refined.

3.3.2 Zinc Lanthanum Catalysts

A series of zinc and lanthanum containing catalysts were

developed by Yan et al. [68, 126, 127]. This type of cat-

alyst exhibits weak basic properties and is composed of

ZnO, La2CO3 and LaOOH. Chief among its attributes are

activity, longevity, FFA and water tolerance, and oil

flexibility.

The ZnLa catalysts demonstrate high catalytic activity.

When using refined soybean oil, FAME yields as high as

95% under reaction conditions of 60 min, 200 �C, 500 psi,

36:1 M ratio of methanol to oil, and 2.3 wt% catalyst

dosage in a stirred batch reactor.

Yan et al. [127] reported the Zn3La1 catalyst which had

a 3:1 M ratio of zinc to lanthanum was recycled 17 times in

a batch reactor without loss of activity and maintained a

high FAME yield (*92.3%) for 70 days in a continuous

tubular flow reactor (Fig. 3a, b). Additional testing showed

the catalyst still active after more than 100 days of use

[128]. At this point, there is no report of any other heter-

ogeneous catalyst with a longer catalyst life than this

Zn3La1 catalyst for biodiesel production.

As part of the above work, food-grade soybean oils

combined with 5.20%, 10.13%, 15.21% and 30.56% of

oleic acid and 1.03%, 3.12% and 5.07% water were tested.

The results (Tables 2 and 3) show that all the oils were

converted within 150 min to FAME (*96%) even with

FFA content as high as 30.56% or water content as high as

5.07%. This suggests that by properly controlling the

temperature of the reaction, hydrolysis reactions in the

presence of water can be minimized, which will allow for

higher FAME yields from higher FFA and water content

Top Catal (2010) 53:721–736 731

123

feedstocks. Furthermore, this catalyst was found to be

relatively insensitive to species in air such as CO2, mois-

ture and O2 in air, which poison base catalysts.

Zn3La1 was used to process multiple unrefined and

waste oils, i.e. crude corn oil from DDGs, crude algae oil,

crude coconut oil, crude palm oil, crude soybean oil, waste

cooking oil, food grade soybean oil with 3% water and 5%

FFA addition. Figure 4 shows that all the oils were com-

pletely converted to FAME within 3 h in a batch reactor.

This is an impressive result considering the fatty acid

composition and total acid number (TAN) of these oils

(Table 4). Here, it should be noted that crude corn oil from

DDGs contains 93% triglycerides and has a TAN as high as

25.19 mg KOH/g. Similarly, crude algae oil contains only

80% triglycerides, and has a TAN of 26.38 mg KOH/g,

while crude coconut oil has a TAN of 8.48 mg KOH/g.

After reaction, the TAN of all the oils is significantly

0 2 4 6 8 10 12 14 16 180

10

20

30

40

50

60

70

80

90

100Y

ield

of F

AM

E

%

Recycle times

0 10 20 30 40 50 60 70 800

20

40

60

80

100

Yie

ld o

f FA

ME

(%

)

Time (day)

a

b

Fig. 3 FAME yield (a) in the batch reactor using the recycled

Zn3La1 catalyst. Note the catalyst was reused for 17 times. The

average yield of soybean methyl esters is 93.7% (b) in the continuous

reactor. Note that this catalyst has run for 70 days, and the average

yield of FAME during stable stage is 92.3% [127]

Table 2 Yield of FAME in the presence of different FFA addition

FFA

addition (%)

Yield of FAME at different reaction time (%)

5 min 10 min 20 min 40 min 60 min 90 min

0 10 16 52 83 88 93

5.20 38 72 83 – 95 92

10.13 73 83 88 92 – 92

15.21 89 – 95 – 98 92

30.56 75 85 93 95 – 01

Reaction conditions: catalyst amount of Zn3La1 is 2.3 wt%, molar

ratio of methanol to oil is 36:1, reaction temperature is 200 �C. All

data are taken from literature [68]

Table 3 Yield of FAME in the presence of different water addition

Water addition

(%)

Yield of FAME at different reaction time (%)

20 min 60 min 90 min 150 min

0 52 88 93 94

1.03 50 80 90 96

3.12 22 74 89 93

5.07 13 57 78 90

Reaction conditions: catalyst amount of Zn3La1 is 2.3 wt%, molar

ratio of methanol to oil is 36:1, reaction temperature is 200 �C. All

data are taken from literature [68]

0 20 40 60 80 100 120 140 160 180 2000

20

40

60

80

100

Crude coconut oil Waste cooking oil Crude soybean oil Crude palm oil Crude algae oil Crude corn oil from DDGs Food grade soybean oil NaOH H

2SO

4

Food grade soybean oil with 3 % water and 5 % FFA addition

FA

ME

con

tent

%

Reaction time min

Fig. 4 FAME yield of crude corn oil from DDGs, crude algae oil,

crude coconut oil, crude palm oil, crude soybean oil, waste cooking

oil, food grade soybean oil and food grade soybean oil with 3% water

and 5% oleic acid addition. Reaction conditions: 126 g of oil, 180 g

of methanol, 3 g of catalyst, 200 �C, 500 psi, in the batch stir reactor.

Note that all of these oils was converted into FAME within 3 h [127]

732 Top Catal (2010) 53:721–736

123

reduced. For instance, the TAN of algae oil after reaction is

0.94 mg KOH/g and that of corn oil from DDGs is

1.32 mg KOH/g. This implies that during the reaction

process, esterification of FFA with methanol is simulta-

neously performed with the transesterification of triglyc-

erides with methanol. Where traditional homogeneous

catalysts would have been deactivated using these high

TAN/FFA feedstocks, the Zn3La1 shows a remarkably

high activity for biodiesel formation reactions with a

variety of feedstocks.

Unlike previously reported solid catalysts, the Zn3La1

catalyst is very stable. Yan et al. pointed out that the Zn

and La contents in the FAME product are only 6 and

2 ppm. The Zn and La contents in the glycerine phase were

measured at only 8 and 4 ppm after a short induction

period when the yield of FAME stabilized (Fig. 5a, b). The

low level of Zn and La in FAME and glycerin products

suggests that Zn3La1 is a true heterogeneous catalyst with

a very stable crystal structure that does not deactivate under

reaction conditions.

Figure 6 illustrates the reaction pathway for biodiesel

formation. Where other catalysts have failed, this catalyst

can successful handle all the components of a crude feed-

stocks. In particular, there are three major components to

these inexpensive oils: triglyceride, FFA, and water. Thus,

there are four major reactions: transesterification of tri-

glyceride with methanol, which results in the formation of

FAME; esterification of FFA with methanol, which results

in FAME; hydrolysis of FAME, which consumes FAME;

and hydrolysis of triglycerides, which results in FFA. The

transesterification and esterification reactions will lead to

higher yields of FAME. However, hydrolysis reactions will

lead to lower FAME yields. With appropriate control of the

reaction temperature, Yan et al. [68] was able to maximize

the transesterification and esterification reactions while de

emphasizing the oil and biodiesel hydrolysis reactions.

4 Summary and Future Opportunity

The use of heterogeneous catalysts for biodiesel production

is an emerging research field which has been quickly

growing over the last 10 years. Most of the reported

Table 4 Fatty acid composition and TAN of food grade soybean oil, crude soybean oil, crude palm oil, waste cooking oil, crude corn oil from

DDGs, crude algae oil and crude coconut oil [127]

Fatty acid

components

Food grade

soybean oil (%)

Crude soybean

oil (%)

Crude palm

oil (%)

Waste cooking

oil (%)

Crude corn oil

from DDGs (%)

Crude algae

oil (%)

Crude coconut

oil (%)

C 12:0 0 0 0 0 0 0 49.13

C 14: 0 0 0.27 0.21 0 0 2.72 19.63

C 16: 0 11.07 13.05 41.92 11.58 11.50 20.91 10.12

C 16: 1 0.09 0.39 0.23 0.18 0 10.62 1.79

C 18: 0 3.62 4.17 3.85 4.26 4.77 6.95 2.83

C 18: 1 20.26 22.75 42.44 24.84 26.26 33.33 7.59

C 18: 2 57.60 52.78 11.30 53.55 56.20 18.45 2.75

C 18: 3 7.36 6.59 0.04 5.60 1.27 1.16 0.15

Others 0 0 0 0 0 6.86 6.01

TAN 0.03 6.62 0.48 7.56 25.19 26.38 8.48

0 10 20 30 40 50 60 70 80

0

100

200

300

400

500

600

0

20

40

60

80

100

La content in FAME phase

Time (day)

Zn

and

La c

onte

nt (

ppm

)

Yield of FAME

Yie

ld o

f FA

ME

%

Zn content in FAME phase

0 10 20 30 40 50 60 70 800

20

40

60

80

100

-200

0

200

400

600

800

1000

1200

1400

1600

1800

Zn

and

Laco

nten

t(pp

m)

Yield of FAME

Yie

ld o

f FA

ME

%

Time (day)

La content in glycerine phase Zn content in glycerine phase

a

b

Fig. 5 Zn and La contents in a FAME product and b glycerine

product. Note that after 3 days metal contents in FAME product

reached a low level; after 7 days metal contents in glycerine product

reached a low level [127]

Top Catal (2010) 53:721–736 733

123

catalysts can be divided into two types, solid base and solid

acid catalysts, according to their active center. Both Lewis

acid–base sites and Bronsted acid–base sites have the

ability to catalyze the oil transesterification reaction.

Therefore, catalyst activity is closely related to the acid/

base strength. Other texture properties of the catalyst also

impact the catalyst’s activity, such as specific surface area,

pore size, pore volume and active site concentration.

Modification of reaction conditions, such as increasing

reaction temperature (130–250 �C), pressure (100–

1000 psi), catalyst quantity (3–10 wt%), and methanol/oil

molar ratio (10:1–42:1) is effective for obtaining high

FAME yields. Reported catalysts, operating at high tem-

perature, exhibit a low base or acid strength which many

research groups have demonstrated is the basis for

improved activity, improved tolerance to FFA and water,

and extended catalyst lifetimes. Within these high tem-

perature catalysts, two catalysts stand out as leading tech-

nologies: one is the ZnO–Al2O3 catalyst from IFP and the

other is the ZnO–La2O3 catalyst developed at Wayne State

University. Additional work is still required to find ways to

rejuvenate or re-active catalysts that have failed because of

poisoning, leaching, or loss of surface area. Finally, the

most recognized drawbacks of heterogeneous catalysts are

their slow reaction rates in comparison to homogeneous

catalysts. Perhaps, by enhancing the number and type of

active sites and intensifying reaction conditions, to mini-

mize mass transfer limitations, these catalysts may be able

to overcome this limitation as well.

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