Catalysts in Production of Biodiesel: A Review

Copyright © 2007 American Scientific Publishers All rights reserved Printed in the United States of America

Journal of Biobased Materials and Bioenergy Vol. 1, 1–12, 2007

REVIEW

Catalysts in Production of Biodiesel: A Review K. Narasimharao∗ , Adam Lee, and Karen Wilson∗ Department of Chemistry, University of York, York YO10 5DD, UK Biodiesel is a renewable substitute fuel for petroleum diesel fuel which is made from nontoxic, biodegradable, renewable sources such as refined and used vegetable oils and animal fats. Biodiesel is produced by transesterification in which oil or fat is reacted with a monohydric alcohol in the presence of a catalyst. The process of transesterification is affected by the mode of reaction, molar ratio of alcohol to oil, type of alcohol, nature and amount of catalysts, reaction time, and temperature. Various studies have been carried out using different oils as the raw material and different alcohols (methanol, ethanol, butanol), as well as different catalysts, notably homogeneous ones such as sodium hydroxide, potassium hydroxide, sulfuric acid, and supercritical fluids or enzymes such as lipases. Recent research has focused on the application of heterogeneous catalysts to produce biodiesel, because of their environmental and economic advantages. This paper reviews the literature regarding both catalytic and noncatalytic production of biodiesel. Advantages and disadvantages of different methods and catalysts used are discussed. We also discuss the importance of developing a single catalyst for both esterification and transesterification reactions.

Keywords:

CONTENTS 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Biodiesel Usage Facts . . . . . . . . . . . . . . . . . . 1.2. Biodiesel Feed Stocks . . . . . . . . . . . . . . . . . . 2. Biodiesel Production . . . . . . . . . . . . . . . . . . . . . . 2.1. Base Catalyzed Transesterification . . . . . . . . . . 2.2. Acid-Catalyzed Transesterification . . . . . . . . . . 2.3. Enzyme Catalysts . . . . . . . . . . . . . . . . . . . . . 2.4. Supercritical Transesterification . . . . . . . . . . . . 2.5. Esterification of Free Fatty Acids . . . . . . . . . . . 2.6. Simultaneous Transesterification and Esterification 2.7. Biodiesel from Vegetable Oil via Pyrolysis . . . . . 3. Future Scope of Research . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

1 2 3 4 4 6 7 7 8 9 9 10 10 11

1. INTRODUCTION The depletion of world petroleum reserves and increased environmental concerns have stimulated the search for alternative renewable fuels that are capable of fulfilling an increasing energy demand.1 In recent decades, research concerning and knowledge about the external benefits of renewable raw materials have intensified the efforts for sustainable energy sources. Biodiesel plays a major role in this field because of the world-wide research, development, and deployment activities of this sustainable energy source.2 ∗

Authors to whom correspondence should be addressed.

J. Biobased Materials and Bioenergy 2007, Vol. 1, No. 1

Biodiesel fuel (fatty acid methyl ester (FAME)) from vegetable oil, which primarily contains triglycerides (TGs) and free fatty acids (FFAs), is considered the best candidate for diesel fuel substitute in diesel engines and is used neat (100% biodiesel) or can be blended with petroleum diesel.3 It is an attractive alternative (or extender) to petroleum diesel fuel due to well-known advantages, including the following: (i) it provides the potential for lower dependence on petroleum crude oil, (ii) it is a renewable resource, (iii) it provides the potential for reduced greenhouse gas emissions because of the closed CO2 cycle (Scheme 1), (iv) it has a lower combustion emission profile (especially SOx ), (v) it provides the potential for enhancement of rural economies, (vi) it is biodegradable, (vii) it can be used without engine modifications, (viii) it provides good engine performance, (ix) improved combustion is exhibited because of its oxygen content, (x) it exhibits low toxicity, and finally (xi) it has the ability to be blended in any proportion with regular petroleum-based diesel fuel.4 It can also be used in boilers or furnaces designed to use heating oils or in oil-fueled lighting equipment. 1556-6560/2007/1/001/012

doi:10.1166/jbmb.2007.002

1

Catalysts in Production of Biodiesel: A Review

Waste Oil

Fatty acid esters

fica

teri

ses

n Tra

tion

Narasimharao et al.

Reduced smog & CO2

REVIEW

Triglycerides

Rapeseed or soybean oil Carbon neutral CO2

Scheme 1. CO2 life cycle for biodiesel.

Due to the recent increased awareness and development in this area, the objective of this review is to give fundamental insight into the production of biodiesel by different catalytic materials. In an effort to identify the ideal catalyst characteristics for biodiesel synthesis, this review also strives to compare the efficiency and applicability

of a number of liquid and solid catalysts in the production of biodiesel. 1.1. Biodiesel Usage Facts As early as the beginning of the 20th century Rudolf Diesel proposed vegetable oil as a fuel for his engine.5 During World War Two, vegetable oil was examined in “up-to-date” diesel engines, while in 1940, vegetable oil methyl and ethyl esters were used in France and Belgium as a fuel for buses. High consumption of conventional energy resources and increasing emission regulations has motivated an intense search for alternative fuels over the decades. Many proposals have been made regarding the availability and applicability of an environmentally friendly fuel that could be domestically available.6 Methanol, ethanol, compressed natural gas (CNG), liquefied natural gas (LNG), liquefied petroleum gas (LPG), and vegetable oils have all been considered as alternative fuels. In the 1930s and 1940s vegetable oils were used as diesel fuels from time to time, but usually only in emergency situations.7 Researchers have also concluded that vegetable

Dr. Narasimharao completed his Master of Science in 1996 from Andhra University, Vizag, India and a Ph.D. in 2002 from the Indian Institute of Chemical Technology, Hyderabad, India. After post doctoral positions in the University of Missouri-Kansas City, USA and The Technical University of Kaiserslautern, Germany he was awarded a Royal Society Indian visiting fellowship to work at the University of York (2005–06).

Dr. Lee has a B.A. (Natural Sciences) and Ph.D. from the University of Cambridge and is currently a Senior Lecturer in Physical Chemistry at the University of York. Prior to this he was a Lecturer in Physical Chemistry at the University of Hull, and conducted post-doctoral research at Cambridge and Johnson Matthey Technology Centre. Adam was awarded the 2000 C R Burch prize of the British Vacuum Council and 2004 Fonda-Fasella Elettra prize for outstanding contributions to vacuum and synchrotron science, respectively.

Dr. Wilson has a B.A. (Natural Sciences) and Ph.D. from the University of Cambridge and an M.Sc. in catalysis from Liverpool University. Following postdoctoral research both at Cambridge and within the Green Chemistry Group at the University of York she was appointed in 1999 to a Lecturership in Physical Chemistry in the Department of Chemistry at University of York.

2

J. Biobased Materials and Bioenergy 1, 1–12, 2007

Narasimharao et al.


vehicles and has also been reported to deteriorate polymers, such as polyurethane foam materials. 1.2. Biodiesel Feed Stocks Biodiesel is defined as the alkyl monoesters of fatty acids from renewable resources, such as vegetable oils, animal fats, and waste restaurant oils and greases. Biodiesel development can be found in 28 countries, Germany and France being the largest producers of biodiesel fuel in the world.12 There are several choices for vegetable oil sources. In the United States and Brazil soybean oil is a source that is already scaled up for biodiesel production. Nevertheless, other sources, such as rapeseed (in Europe), sunflower, peanut, cotton, palm oil, coconut, babassu, and especially castor oil, may also used in different parts of the world once their cultivation can achieve an economic up-scaling.4 The alcohol source is generally methanol, though ethanol and butanol have also been used. In general the physical and chemical properties and the performance of ethyl esters are comparable to those of the methyl esters. Methyl and ethyl esters have almost the same heat content, but the viscosities of the ethyl esters are slightly higher.13 Engine tests demonstrate that methyl esters produce slightly higher power and torque than ethyl esters,14 the latter also tending to cause more injector coking than methyl esters. The other alcohols are less reactive than methanol, and there are some technological problems in its industrial use. Different oils and fats contain a range of fatty acid concentrations (Table I), and as a result, biodiesel production costs are highly dependent on the feedstock type. The cost of the fat or oil used to produce biodiesel clearly affects the cost of the finished product, constituting up to 60– 75% of the overall financial burden; therefore, less expensive raw materials are preferred.15 To produce biodiesel economically, a production facility must have access to low-value feedstock, develop quality, high-value coproducts, and enjoy a cost-effective and high-yielding process. Vegetable oils, such as soybean or rapeseed oil, are significantly more expensive than beef tallow or yellow grease, despite the higher free fatty acid content of the latter. In the UK recycled cooking oil is currently the main feedstock of choice for biodiesel production due to its low purchase price. Once the limited quantities of used

Table I. Fatty acid distribution of some vegetable oils and animal fat. Fatty acid distribution (% by weight) Feed stock Rapeseed oil Sunflower oil Safflower oil Soybean oil Beef tallow Yellow grease

C14:0 36 243

C16:0 349 608 860 1058 2432 2324

C16:1 3.79

J. Biobased Materials and Bioenergy 1, 1–12, 2007

C18:0

C18:1

C18:2

C18:3

Saturation level (%)

085 326 193 476 2025 1296

64.40 16.93 11.58 22.52 37.43 44.36

2230 7373 7789 5234 2–3 697

8.23

434 934 1053 1534 4763 3863

8.19 0.67

3

REVIEW

oils hold promise as replacement fuels for modern diesel engines since their calorific value is comparable to that of diesel. However, their use in direct injection diesel engines is restricted by some unfavorable physical properties, particularly their viscosity, which is about 11–17 times higher than that of diesel fuel. Consequently, vegetable oil causes poor fuel atomization, incomplete combustion, and carbon deposition on the injector and valve seats, resulting in serious engine fouling.4 Different approaches have been considered to reduce the high viscosity of vegetable oils: (a) dilution of 25 parts vegetable oil with 75 parts diesel fuel, (b) microemulsions with short chain alcohols, (c) pyrolysis, (d) catalytic cracking, and (e) transesterification with ethanol or methanol to produce fatty acid esters commonly known as biodiesel fuel.8 Biodiesel is renewable, nontoxic, and biodegradable. However, while biodiesel is definitely renewable, the fact that it cannot displace a significant fraction of current petroleum-based fuel consumption means that it does not really allow us to make much progress toward using it as a sustainable energy supply. Indeed if all of the available vegetable oil and animal fat were used in biodiesel synthesis we could only replace about 15% of the current demand for on-highway diesel fuel.9 Nontoxicity and biodegradability are useful characteristics, but they are only significant when the fuel is used in its pure form, as is common in Germany and Austria. For the 20% and lower blends that are common in the United States, the diesel fuel portion of the blend determines the toxicity and biodegradability.10 Biodiesel does provide a reduction in harmful emissions, such as SOx , CO, hydrocarbons, particle matter, and soot, as well as NOx in optimized diesel engines, together with lower net CO2 emissions. The solvent properties of biodiesel can also cause some problems when used initially in diesel engines. Sediments and sludge formed in old diesel storage tanks can become dispersed upon initial use of biodiesel, resulting in blocked filters, but once the contaminants are removed no further maintenance issues arise. Indeed the superior lubricity of biodiesel can even help reduce engine wear.11 However, pure biodiesel is not compatible with natural rubber, so it can degrade some fuel system components made of natural rubber such as hoses, gaskets, and seals found in pre-1993


REVIEW

industrial frying oils have been fully given over to biodiesel, additional raw feedstocks may come from the cultivation of rapeseed crops.16

2. BIODIESEL PRODUCTION Vegetable oils and animal fats are comprised of a complex mixture of triglycerides and other minor components, such as free fatty acids, gums, waxes, etc. Triglycerides are esters of glycerol with three chains of aliphatic or olefinic FFAs of variable length (12–24 carbons). Direct use of oil diluted with solvents to form microemulsions lowers viscosity and improves ignition characteristics but can result in some engine performance problems.17 The pyrolysis of vegetable oil has been investigated for over 100 years as a route to synthesize petroleum. While this tends to produce more biogasoline than biodiesel fuel,18 it is viewed by some companies as an economic method to produce valuable alternative biofuels. Among all the proposed methods to convert oils to biodiesel, transesterification of the triglycerides seems to be the best choice, as the physical characteristics of fatty acid esters (biodiesel) are very close to those of diesel fuel.19 Furthermore, the methyl or ethyl esters of fatty acids can be burned directly in unmodified diesel engines, with very low deposit formation and a by-product (glycerol) that has commercial value.20 In the biodiesel production process, transesterification is the chemical reaction between triglycerides and alcohol in the presence of a catalyst to produce monoesters. The long and branched chain triglyceride molecules are transformed to monoesters and glycerol. The transesterification process consists of a sequence of three consecutive reversible reactions, which include conversion of triglycerides to diglycerides, followed by the conversion of diglycerides to monoglycerides. The glycerides are converted into glycerol and yield one ester molecule in each step. The overall transesterification reaction can be represented by the reaction Scheme 2. Several aspects, including the type of catalyst (alkaline or acid), alcohol/vegetable oil molar ratio, temperature, purity of the reactants (mainly water content), and free fatty acid content, have an influence on the course of the transesterification.21 The alcohol/vegetable oil molar ratio is one of the main factors that influences the transesterification. An excess of the alcohol favors the formation of the products. On the other hand, an excessive amount of alcohol makes the recovery of the glycerol difficult. The high

Scheme 2. Transesterification of TGs with alcohols.

4

Narasimharao et al.

molar ratio of alcohol to vegetable oil interferes with the separation of glycerine, because there is an increase in solubility. When glycerin remains in solution, it helps drive the equilibrium back to the left, lowering the yield of esters. So, the ideal alcohol/oil ratio has to be established empirically, considering each individual process. There are different transesterification processes that can be applied to synthesize biodiesel: (a) base-catalyzed transesterification, (b) acid-catalyzed transesterification, (c) enzyme-catalyzed transesterification, and (d) supercritical alcohol transesterification. 2.1. Base Catalyzed Transesterification 2.1.1. Homogeneous Catalysts Biodiesel is currently synthesized using homogeneous alkaline catalysts because the transesterification reaction by an acid catalyst is much slower than the base-catalyzed reaction. There are several comprehensive studies of basecatalyzed transesterification.21–23 The most common basic catalysts are potassium hydroxide (KOH), sodium hydroxide (NaOH), sodium methoxide (NaOCH3 ), and sodium ethoxide (NaOCH2 CH3 ). Daranoko et al.24 studied the two-step transesterification reaction of waste oils by using stoichiometric amounts of methanol and the necessary amounts of KOH, supplemented with the exact amount of KOH to neutralize acidity. Both reactions were completed in 30 min in the temperature range 40–60 C. Parameters such as density, viscosity, water content, and energy content were investigated. It was concluded that a twostep, alkaline-catalyzed transesterification reaction is an economic method for biodiesel production from used vegetable oil. The activity and efficiency of nonionic bases as catalysts for the transesterification of vegetable oils in homogeneous media has also been studied.25 In a first study, the catalytic activity of some guanidines was compared. It was observed that 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) produces more than 90% of methyl esters after 1 h with only 1 mol% in the reaction mixture. The other bases tested under the same experimental conditions gave yields below 66%. The catalytic activity of these materials is not directly related to the relative basicity of compounds. The catalytic activity of TBD was also compared with that of typical industrial catalysts and the yields obtained with TBD were close to those observed with NaOH with no by-products such as soaps, which are easily formed when alkaline metal hydroxides are used. When compared to potassium carbonate, TBD was always more active, even at low molar concentrations. Although TBD is less active than sodium methoxide at only 0.5 mol%, it does not require any special reaction conditions. Due to the excellent performance of TBD in the transesterification of vegetable oils, catalytic activity of several alkylguanidines was investigated under the same reaction conditions (27.2 mmol of rapeseed oil, 62.5 mmol J. Biobased Materials and Bioenergy 1, 1–12, 2007

Narasimharao et al.

2.1.2. Heterogeneous Catalysts Even though homogeneous catalyzed biodiesel production processes are relatively fast and show high conversions with minimal side reactions, they are still not very costcompetitive with petrodiesel. For instance, the basic catalysts used in the process, generally KOH and NaOH, are neutralized with phosphoric acid after the reaction, and the resultant salts constitute vast quantities of undesired waste chemicals. The other disadvantages include that (1) the catalyst cannot be recovered and must be neutralized at the end of the reaction, (2) there is limited use of continuous processing methodologies,30 and (3) the processes are very sensitive to the presence of water and free fatty acids; consequently they need a high quality feedstock (refined vegetable oils) to avoid undesired side reactions (hydrolysis and saponification) or additional reaction steps to first convert/eliminate the free fatty acids. Biodiesel synthesis using solid catalysts instead of homogeneous catalysts could potentially lead to cheaper production costs by enabling reuse of the catalyst and opportunities to operate in a fixed bed continuous process.31 J. Biobased Materials and Bioenergy 1, 1–12, 2007

Solid basic materials such as MgO, Al–Mg hydrotalcites, Cs-exchanged sepiolite, and mesoporous MCM-41 have been used as catalysts for glycerol transesterification with triglycerides.32 MgO and low Al content hydrotalcites behave as active catalysts for triolein glycerolysis. Other esters have been used for glycerol transesterification. Metal oxides like MgO, CeO2 , La2 O3 , and ZnO have been used as solid base catalysts for the transesterification of glycerol with stoichiometric amounts of methyl stearate in the absence of solvent.33 These catalysts are active, but the selectivity to mono-, di-, and triesters is similar to that obtained by using homogeneous basic catalysts (40% monoester at 80% conversion). Schuchardt et al.34 heterogenized guanidines on organic polymers such as cellulose and poly(styrene/ divinylbenzene) for the transesterification of vegetable oils. The guanidine-containing cellulose shows a slightly reduced activity compared to guanidine in the homogeneous phase, giving a conversion of 30% after 1 h, when used at 5 mol%. This guanidine-containing cellulose was used in a continuous reactor containing 100 g of the catalyst with a 2:1 alcohol/oil mixture pumped at 60 C at a rate of 0.48 liters/h. However, they observed an incomplete reaction attributed either to leaching of the catalyst or to its irreversible protonation. In order to circumvent the leaching of the guanidines from the polymers, Sercheli et al.35 reported a method to encapsulate N ,N ,N -tricyclohexylguanidine (TCG) in the supercages of a hydrophobic Y zeolite by the reaction of dicyclohexylcarbodiimide and cyclohexylamine. These encapsulated guanidines showed low activity in the transesterification of vegetable oils (14% conversion after 5 h), as diffusion of the triglycerides through the channels of the Y zeolite is slowed due to steric hindrance. The guanidines heterogenized on gel-type poly(styrene/divinylbenzene) showed a slightly lower activity than their homogeneous analogues but allowed the same high conversions after prolonged reaction times. However, they also slowly leached from the polymers, allowing only nine catalytic cycles.36 Insoluble salts of amino acids have also found usage as catalysts for the methanolysis of triglycerides.37 Some metal salts of amino acids such as those of copper, zinc, cadmium, nickel, lanthanum, cobalt, calcium, magnesium, and iron were tested. Zinc arginate catalysts are also applied for the methanolysis of palm oil with a methanol:oil molar ratio of 6:1 and deliver 67 wt% of methyl ester. However, reasonable reaction rates could only be achieved at temperatures above 130 C, and it is unclear whether these amino acid catalysts are reusable or not. The transesterification of soybean oil using calcium carbonate as a catalyst has been investigated.38 Conversions above 95% are achieved at 260 C for ethyl esters, using flow reactors with residence times of approximately 18 min. No decrease in the activity of calcium carbonate 5

REVIEW

of methanol, and 1.0 mol% of catalyst at reaction temperature of 70 C) for the sake of comparison.26 The TBD was always the most active (90% yield in 1 h reaction time); however, 1,3-dicyclohexyl-2-n-octylguanidine (DCOG), 1,1,2,3,3-pentamethylguanidine (PMG), 7-methyl-1,5,7triazabicyclo[4.4.0]dec-5-ene (MTBD), and 1,2,3-tricyclohexylguanidine (TCG) also showed yields of 64%, 47%, 49%, and 74% in 1 h reaction time. The activity order of the catalysts is TBD > TCG > DCOG > MTBD > PMG, in line with their relative base strengths. Metal complexes27 28 of the type M(3-hydroxy-2methyl-4-pyrone)2 (H2 O)2 , where M = Sn, Zn, Pb, and Hg, have been used for soybean oil methanolysis under homogeneous conditions. Sn and Zn complexes showed great activities for this reaction, achieving yields of up to 90% and 40%, respectively, in 3 h, using a molar ratio of 400:100:1 (methanol:oil:catalyst), without emulsion formation. Recently,29 liquid amine-based catalysts were also successfully applied for transesterification of refined vegetable and frying oil. Four amines (diethylamine (DEA), dimethylethanol amine (DMAE), tetramethyldiaminoethane (TEMED), and tertramethylammonium hydroxide (TMAH) (as 25% in methanol)) were used. The highest conversion of 98% was achieved with TMAH as a catalyst at 65 C in 90 min. In these cases, a large amount (13%) of liquid amine catalysts is required for the transesterification. Amine catalysts used in the reaction also act as a solvent for both reactants and products, helping to shift the reaction equilibrium toward the products.


Narasimharao et al.

was observed after weeks of utilization, but these catalytic systems need high energy requirements. The methanolysis of rapeseed oil was tested in the presence of caesiumexchanged NaX faujasites, mixed magnesium–aluminum oxides, magnesium oxide, and barium hydroxide as catalysts for different methanol:oil ratios.39 Barium hydroxide was particularly catalytically active for a methanol:oil molar ratio of 6:1 under methanol reflux after a reaction time of only 1 h; oil conversion was about 80% with a nearly quantitative ester molar fraction. On the other hand, caesium-exchanged NaX faujasites and mixed magnesium–aluminum oxides required a long reaction time (3 h) and a high methanol:oil molar ratio to achieve high yields in methyl esters. The transesterification of soybean oil with methanol in the presence of a series NaX faujasite zeolite, titanosilicalites with Na and K as cations (ETS-10), zeolite, and metal catalysts has also been studied.40 ETS-10 catalysts offered higher conversions (>80%) than zeolite-X type catalysts; however, this may reflect a homogeneous route as alkali methoxide species were leached out. Na/NaOH/-Al2 O3 heterogeneous alkaline catalysts were applied for the transesterification of soybean oil with methanol using hexane as cosolvent.41 The best result (>90%) was obtained after 2 h using a methanol:oil molar ratio of 9:1 at 60 C. We have reported a series of Li-promoted CaO catalysts42 with Li loadings in the range 0.26–4.0 wt% which are effective in the transesterification of glyceryl tributyrate and methanol to methyl butanoate. A Li content of 1.23 wt% was found to give the optimum activity toward methyl butanoate formation (Fig. 1). Li doping increases the base strength of CaO, and XPS and DRIFTS measurements reveal that the optimum loading correlates with the formation of an electron deficient surface Li+ species and associated –OH species at defect sites on the support. High Li loadings result in bulk LiNO3 formation 3 Li (Li/CaO)

Initial rate (mmol·min–1·g(cat)–1)

REVIEW


Mg (hydrotalcite)

2.2. Acid-Catalyzed Transesterification The transesterification reaction can also be catalyzed by Brønsted acids, preferably sulfonic and sulfuric acids, but the reactions rates are low and require relatively high temperatures to get high product yields.45 According to an acid-catalyzed mechanism for esterification,46 carboxylic acids can be readily formed by hydrolysis of the carbocation intermediate formed upon protonation of the ester. This suggests that acid-catalyzed transesterification should be carried out in the absence of water to avoid the competitive formation of carboxylic acids and concomitant reduction in the yields of alkyl esters.

2

2.2.1. Homogeneous Catalysts

1

0 0

2

4

6

8

10

12

Loading (mmolg–1) Fig. 1. Catalyst activity as a function of Li or Mg loading for Li/CaO and Mg:Al hydrotalcite catalysts in the transesterification of glyceryl tributyrate with methanol.

6

and a drop in surface area and corresponding catalytic activity. In another publication,43 we synthesised Mg–Al hydrotalcite materials with different Mg compositions by an alkali-free coprecipitation route. Trace alkali residues found in materials prepared using conventional methods using alkali carbonates and hydroxides can be problematic due to leaching and product contamination in liquid phase reactions. All materials showed very good performance for the liquid phase transesterification of glyceryl tributyrate with methanol for biodiesel production. The rate increases steadily with Mg content (Fig. 1), with the Mg rich catalyst an order of magnitude more active than conventional solid base MgO. The rate of reaction also correlates with intralayer electron density, which can be associated with increased basicity. Surface characterization reveals that in both classes of solid base the electronic state of the surface Li or Mg species can be correlated with catalyst activity. Xie et al.44 tested alumina loaded with different potassium precursor catalysts for the transesterification of soybean oil. The conversion to methyl esters over the catalysts is in the following order: KI/Al2 O3 > KF/Al2 O3 > KOH/Al2 O3 > KNO3 /Al2 O3 > K2 CO3 /Al2 O3 > KBr/ Al2 O3 . KI/Al2 O3 demonstrated superior catalytic activity compared to the other catalysts; however, to achieve the highest conversion correspondingly high catalyst loadings (35 wt%) and activation temperatures are needed, and none of these materials were tested for leaching or reusability.

The most common acid catalysts employed are H2 SO4 and HCl. Freedman et al.23 showed that the methanolysis of soybean oil, in the presence of 1 mol% of H2 SO4 with an alcohol/oil molar ratio of 30:1 at 65 C, takes 50 h to reach complete conversion of the vegetable oil (>99%), while butanolysis (at 117 C) and ethanolysis (at 78 C), using the same quantities of catalyst and alcohol, take 3 and 18 h, respectively. Al-Widyan et al.47 worked on the transesterification of used palm oil under various conditions using different concentrations of HCl, H2 SO4 , and excess ethanol. They concluded that at higher catalyst concentrations (1.5–2.25 M) J. Biobased Materials and Bioenergy 1, 1–12, 2007

Narasimharao et al.

2.2.2. Heterogeneous Catalysts The use of commercial sulfonic ion-exchange resin was also reported for production of biodiesel.48 The methanolysis of babassu and soybean oil was compared using Amberlyst-15 with sulfuric acid as a catalyst, with the cationic-exchange resin found to exhibit better activity than the homogeneous catalyst.49 The methanolysis of soybean oil was carried out using different solid super acids such as tungstated zirconia-alumina (WZA), sulfated tin oxide (STO), and sulfated zirconia-alumina (SZA),50 of which the WZA catalyst was the most effective, achieving conversions >90% at temperatures above 250 C after 20 h. Amberlyst-15, SZ, Nafion NR50, and WZ showed reasonably good activities at moderate temperatures of 60 C, indicating that these are suitable alternatives to the homogeneous systems which can overcome the drawbacks of corrosion and handling of liquid mineral acids. Transesterification has also been conducted on a simple TG (triolein) with ethanol using various ion-exchange resins as a heterogeneous catalyst.51 The anion-exchange resins with a lower cross-linking density and a smaller particle size gave a high reaction rate and high conversion. Another advantage of the resins is that they could be recycled in batch transesterification without any loss in the catalytic activity. The anion-exchange resins exhibited much higher catalytic activities than their cation-exchange counterparts. A continuous transesterification reaction was carried out using an expanded bed reactor packed with the most active resin. The reactor system permitted the continuous production of ethyl oleate with a high conversion. Kaita et al.52 synthesized aluminum phosphate catalysts with various Al/P molar ratios and used the resultant materials for the transesterification of kernel oil with methanol. According to the authors, these catalysts were thermally stable with good reactivity and selectivity to methyl esters; however, their applications still required high temperatures (200 C) and high methanol-to-oil molar ratios (60:1). 2.3. Enzyme Catalysts Although enzyme-catalyzed transesterification processes are not yet commercially developed, new results have been reported in recent articles and patents. The common aspects of these studies involve optimization of the J. Biobased Materials and Bioenergy 1, 1–12, 2007

reaction conditions (solvent, temperature, pH, type of microorganism which generates the enzyme, etc.) in order to establish suitable characteristics for an industrial application.53 However, the reaction yields as well as the reaction times are still unfavorable compared to those of the base-catalyzed systems. Lipase enzymes can also catalyze methanolysis of triglycerides. The most promising results were obtained by Fukuda et al. using immobilized Candida Antarctica lipase (Novozym 435).54 Shimada et al.55 found that Novozym435 was inactivated by shaking it in a mixture containing more than 1.5 M eq. of methanol to oil. Above this concentration, methanol is partially present as small droplets in the oil phase. These droplets are believed to cause enzyme deactivation. Therefore, methanol was added stepwise; after the addition of the third methanol equivalent, conversion to methyl esters was almost complete. The enzyme could be reused 50 times without loss of activity. The occurrence of free fatty acids did not affect the enzyme catalyst. Before the inlet of every reactor, 1 M eq. was added to the feed. Samukawa et al.56 reported an even more dramatic increase of the lipase efficiency when it was pretreated by a consecutive incubation in methyl ester and oil prior to reaction. The use of Novozym435 in methanolysis of triglycerides is also reported in supercritical carbon dioxide at 24.1 MPa and 50 C.57 High yields (90–95%) of fatty acid methyl esters could be obtained when the reaction was carried out at molar methanol/oil ratios of 25:1. Hsu et al.58 studied the optimization of alkyl ester production from grease using a phyllosilicate sol–gel immobilized lipase. According to the studies, it was concluded that the immobilized lipase was active from 40 to 70 C. Ester contents of 60–97% were highest when using a ratio of reactants of 2 mmol grease to 8 mmol alcohol and the biocatalyst was 10% (w/w) in the presence of a molecular sieve. Watanabe et al.59 investigated the enzymatic conversion of waste edible oil to biodiesel in a fixed-bed bioreactor. Three-step methanoloysis and onestep methanoloysis of waste oil were conducted using Candida antarctica lipase. As a result, 90% conversion from waste oil to biodiesel was obtained in both reactions. In general enzymatic reaction selectivity is high, and enzymes can be immobilized on support materials. However, enzymes are very expensive and exhibit unstable activities with reaction rates much lower than those possible with homogeneous base catalysts. 2.4. Supercritical Transesterification Saka and Kusdiana60 have developed a catalyst-free method for biodiesel fuel production by employing supercritical methanol. The supercritical treatment at 350 C, 43 MPa, and 240 s with a molar ratio of 42:1 in methanol is the optimum condition for transesterification of rapeseed oil to biodiesel fuel. The great advantage of this method 7

REVIEW

biodiesel with a lower specific gravity was produced in shorter reaction times. In their work, the variation of the specific gravity of the end product with time was used as an indicator for the effectiveness and completeness of the conversion process. Lower values of specific gravity were interpreted to indicate that more of the heavy glycerine was removed, which, in turn, meant more complete reaction. H2 SO4 offered better conversion levels than HCl at a catalyst concentration of 2.25 M.


REVIEW


was that free fatty acids present in the oil could be simultaneously esterified in the supercritical solvent. Variables such as the molar ratio of alcohol to vegetable oil and reaction temperature were investigated during the transesterification within this supercritical media.61 Increasing the reaction temperature within the supercritical regime resulted in increased ester conversion. Diosakou et al.62 reported an 85% conversion of soybean oil into methyl esters after 10 h reaction at 235 C and 6.2 MPa with a methanol:oil molar ratio of 21:1. The rate of the reaction can be dramatically improved when the reaction proceeds in supercritical methanol. Methanol reaches its supercritical stage at 239 C and 8.1 MPa and is believed to dissolve the oil completely. Under supercritical conditions of 350 C and 45 MPa, rapeseed oil was completely esterified in 240 s with a methanol:oil molar ratio of 42.63 Furthermore the presence of free fatty acids did not alter the yield or rate of the supercritical methanolysis reaction. This new supercritical methanol process requires a shorter reaction time and a simpler purification procedure because of the absence of dissolved catalyst. Of course this method does necessitate high temperature and pressure (and therefore energy costs) and also requires an expensive workup because of dissolution of the glycerol by-product in methanol under these reaction conditions. 2.5. Esterification of Free Fatty Acids The presence of free fatty acids in oils causes significant processing problems in standard biodiesel manufacturing, since the free fatty acid is readily saponified by the homogeneous alkali catalyst used to transesterify triglycerides, leading to a loss of catalyst as well as increased purification costs.64 The amount of free acids contained in different feedstocks is given in Table II. Turck et al.65 have investigated the negative influence of high FFA contents on the base-catalyzed transesterification of triglycerides. Free fatty acids react with the basic catalyst added for the reaction and give rise to soap, as a result of which some portion of the catalyst is neutralized and is therefore no longer available for transesterification. These high FFA content oils/fats are processed with an immiscible basic glycerol phase so as to neutralize the free fatty acids and cause them to pass over into the glycerol phase by means of monovalent alcohols. The main approach for improve the processing of free fatty acid oils is to first esterify the free fatty acids to Table II.

Amount of free fatty acids in different feed stocks.

Feed stock Refined vegetable oils Crude soybean oil Restaurant waste grease Animal fat Trap grease

8

Amount of free fatty acid (wt%)