Homogeneous, heterogeneous and enzymatic ...

Biotechnology Advances 28 (2010) 500–518

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Biotechnology Advances j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / b i o t e c h a d v

Research review paper

Homogeneous, heterogeneous and enzymatic catalysis for transesterification of high free fatty acid oil (waste cooking oil) to biodiesel: A review Man Kee Lam, Keat Teong Lee ⁎, Abdul Rahman Mohamed School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 Nibong Tebal, Pulau Pinang, Malaysia

a r t i c l e

i n f o

Article history: Received 10 December 2009 Received in revised form 16 March 2010 Accepted 20 March 2010 Available online 31 March 2010 Keywords: Biodiesel Waste cooking oil Homogeneous catalyst Heterogeneous catalyst Enzymatic Transesterification

a b s t r a c t In the last few years, biodiesel has emerged as one of the most potential renewable energy to replace current petrol-derived diesel. It is a renewable, biodegradable and non-toxic fuel which can be easily produced through transesterification reaction. However, current commercial usage of refined vegetable oils for biodiesel production is impractical and uneconomical due to high feedstock cost and priority as food resources. Low-grade oil, typically waste cooking oil can be a better alternative; however, the high free fatty acids (FFA) content in waste cooking oil has become the main drawback for this potential feedstock. Therefore, this review paper is aimed to give an overview on the current status of biodiesel production and the potential of waste cooking oil as an alternative feedstock. Advantages and limitations of using homogeneous, heterogeneous and enzymatic transesterification on oil with high FFA (mostly waste cooking oil) are discussed in detail. It was found that using heterogeneous acid catalyst and enzyme are the best option to produce biodiesel from oil with high FFA as compared to the current commercial homogeneous base-catalyzed process. However, these heterogeneous acid and enzyme catalyze system still suffers from serious mass transfer limitation problems and therefore are not favorable for industrial application. Nevertheless, towards the end of this review paper, a few latest technological developments that have the potential to overcome the mass transfer limitation problem such as oscillatory flow reactor (OFR), ultrasonication, microwave reactor and co-solvent are reviewed. With proper research focus and development, waste cooking oil can indeed become the next ideal feedstock for biodiesel. © 2010 Elsevier Inc. All rights reserved.

Contents 1. 2.

3. 4. 5.

6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . Biodiesel . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Composition of vegetable oils and fats . . . . . . . . . . 2.2. Composition of biodiesel . . . . . . . . . . . . . . . . Current status of biodiesel production . . . . . . . . . . . . . Biodiesel costing and potential of waste cooking oil as feedstock Waste cooking oil . . . . . . . . . . . . . . . . . . . . . . 5.1. Thermolytic reaction . . . . . . . . . . . . . . . . . . 5.2. Oxidative reaction . . . . . . . . . . . . . . . . . . . 5.3. Hydrolytic reaction . . . . . . . . . . . . . . . . . . . Catalysis in transesterification . . . . . . . . . . . . . . . . . 6.1. Homogeneous base-catalyzed transesterification . . . . . 6.2. Homogeneous acid-catalyzed transesterification . . . . . 6.3. Homogeneous acid and base-catalyzed transesterification: 6.4. Heterogeneous base-catalyzed transesterification . . . . 6.5. Heterogeneous acid-catalyzed transesterification . . . . . 6.5.1. Zirconium oxide (ZrO2) . . . . . . . . . . . . 6.5.2. Titanium oxide (TiO2) . . . . . . . . . . . . . 6.5.3. Tin oxide (SnO2) . . . . . . . . . . . . . . .

⁎ Corresponding author. Tel.: +60 4 5996467; fax: +60 4 5941013. E-mail address: [email protected] (K.T. Lee). 0734-9750/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.biotechadv.2010.03.002

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6.5.4. Zeolites . . . . . . . . . . . . . . . . . . . . . 6.5.5. Sulfonic ion-exchange resin . . . . . . . . . . . . 6.5.6. Sulfonic modified mesostructure silica . . . . . . . 6.5.7. Sulfonated carbon-based catalyst . . . . . . . . . 6.5.8. Heteropolyacids (HPAs) . . . . . . . . . . . . . . 6.6. Enzyme (biocatalyst) catalyzed transesterification . . . . . . 6.6.1. Mucor miehei (Lipozym IM 60) . . . . . . . . . . 6.6.2. Pseudomonas cepacia (PS 30) . . . . . . . . . . . 6.6.3. C. antarctica (Novozym 435). . . . . . . . . . . . 6.6.4. Bacillus subtilis . . . . . . . . . . . . . . . . . . 6.6.5. Rhizopus oryzae . . . . . . . . . . . . . . . . . . 6.6.6. Penicillium expansum . . . . . . . . . . . . . . . 7. Other methods or technologies for biodiesel production . . . . . . 7.1. Oscillatory flow reactor (OFR) for transesterification reaction 7.2. Microwave technology in transesterification reaction . . . . 7.3. Ultrasonic technology in transesterification reaction . . . . . 7.4. Co-solvent . . . . . . . . . . . . . . . . . . . . . . . . 8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction To date, fossil fuels account over 80.3% of the primary energy consumed in the world, and 57.7% of that amount is used in transportation sector (International Energy Agency I, 2006). On the other hand, the global consumption of diesel fuel is estimated to be 934 million tonnes per year (Kulkarni and Dalai, 2006). Thus, The World Energy Forum predicted that fossil oil will be exhausted in less than 10 decades, if new oil wells are not found (Sharma and Singh, 2009). The main reason that caused the fast diminishing of energy resources is due to rapid population and industrialization growth globally (Pimentel and Pimentel, 2006). Due to this phenomenon, the era of cheap crude oil no longer exists leading to high sky rocketing price of petroleum, bellicose conflicts and increasing the number of undernourished people especially from undeveloped countries. Fig. 1 presents the projection of world energy demand in the near future indicating that there is an urgent need to find more new renewable energies to assure energy security worldwide (Exxon Mobile, 2004). Renewable energy has been highlighted in the last ten years due to its potential to replace fossil fuel especially for transportation. Renewable energy sources such as solar energy, wind energy, hydro energy, and energy from biomass and waste have been successfully developed and used by different nations to limit the use of fossil fuels. Nevertheless, based on recent study from International Energy Agency (IEA), only energy produced from renewable sources and waste has the highest potential

Fig. 1. Projection of energy demand for the near future.

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511 511 511 512 512 513 513 513 513 514 514 514 514 515 515 515 516 516 516 516

among other renewable resources as shown in Fig. 2 (International Energy Agency I, 2008). Combustible renewable and waste accounted for 10.1%, compared to hydro energy 2.2% and other 0.6% (included geothermal, solar wind and heat). Hence, it is predicted that renewable energy from combustible energies such as biodiesel will enter the energy market intensively in the near future to diversify the global energy sources. 2. Biodiesel Biodiesel is an alternative diesel fuel derived from vegetable oils or animal fats (Vasudevan and Briggs, 2008). The main components of vegetable oils and animal fats are triglycerides or also known as esters of fatty acids attached to a glycerol. Normally, triglycerides of vegetable oils and animals fats consist of several different fatty acids. Different fatty acids have different physical and chemical properties and the composition of these fatty acids will be the most important parameters influencing the corresponding properties of a vegetable oils and animal fats (Gerhard Knothe and Krahl, 2004). Direct use of vegetable oils and animal fats as combustible fuel is not suitable due to their high kinematic viscosity and low volatility. Furthermore, its long term use posed serious problems such as deposition, ring sticking and injector chocking in engine (Muniyappa et al., 1996). Therefore, vegetable oils and animal fats must be subjected

Fig. 2. World total energy supply by fuel (Mtoe) in year 2006 (excluding electricity and heat tread). Total: 11,741 million tonnes of oil equivalent (Mtoe).

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to chemical reaction such as transesterification to reduce the viscosity of oils. In that reaction, triglycerides are converted into fatty acid methyl ester (FAME), in the presence of short chain alcohol, such as methanol or ethanol, and a catalyst, such as alkali or acid, with glycerol as a byproduct (Vasudevan and Briggs, 2008). Eq. (1) depicts the transesterification reaction. Another alternative way to produce biodiesel is through thermal cracking or pyrolysis. However, this process is rather complicated to operate and produce side products that have no commercial value (Sharma and Singh, 2009).

Table 2 Chemical structures of common FAME. Methyl ester

Formula

Common acronym

Molecular weight

Methy Methy Methy Methy Methy

C17H34O2 C19H38O2 C19H36O2 C19H34O2 C19H24O2

C16:0 C18:0 C18:1 C18:2 C18:3

270.46 298.51 296.50 294.48 292.46

palmitic stearate oleate linoleate linolenate

3. Current status of biodiesel production

ð1Þ 2.1. Composition of vegetable oils and fats Vegetable oils and animal fats usually have hydrophobic properties, which mean they are insoluble in water. As mention earlier, triglycerides are made up of 1 mol glycerol and 3 mol fatty acids. Fatty acids vary in terms of carbon chain length and number of unsaturated bonds (double bonds). Typical fatty acids compositions found in several vegetable oils are summarized in Table 1 (Ma and Hanna, 1999). Fatty acids that have no double bonds are termed “saturated” such as stearic acid. These chains contain maximum number of possible hydrogen atoms per atom carbon. Fatty acids that have double bonds are termed “unsaturated” such as Linoleic acid. These chains do not contain maximum number of hydrogen atoms due to the presence of double bond(s) on some carbon atoms. Normally, natural vegetable oils and animal fats are obtained in the crude form through solvent extracting or mechanically pressing, containing a lot of impurities such as free fatty acids, sterols and water. In fact, these free fatty acids and water content will have significant effect on the transesterification reaction, especially if a base catalyst is used. They could also interfere with the separation of FAME and glycerol during water washing (purification step) because of soap formation. 2.2. Composition of biodiesel

With the crude fossil fuel prices near all-time high, biodiesel has emerged as the fastest growing industries worldwide. Several countries especially United State and members of European Union are actively supporting the production of biodiesel from the agriculture sector. In year 2006, nearly 6.5 billion liters of biodiesel was produced globally as shown in Fig. 3 (TBW, 2008). It is interesting to note that 75% of the total biodiesel production comes from European countries. This is mainly due to substantial support from the European government such as consumption incentives (fuel tax reduction) and production incentive (tax incentives and loan guarantees) which will further catalyzed the growth of the biodiesel market in the next ten years. Besides that, the United States spent around US$ 5.5 billion to 7.3 billion a year (TBW, 2008) to accelerate biofuels production. As a result, around 450 million gallons of biodiesel was produced in the United States in the year 2007 compared to only 25 million gallons in year 2004 (Thurmond, 2008). This 1700% increment was indeed a shocking increase in the entire history of biodiesel production. However, by the year 2020, it is predicted that biodiesel production from Brazil, China, India and some South East Asia countries such as Malaysia and Indonesia could contribute as much as 20% (Thurmond, 2008). The driving forces for development of biodiesel in these countries are economic, energy and environmental security, improving trade balances and expansion of agriculture sector (Zhou and Thomson, 2009). In addition, Brazil, China and India each have set targets to replace 5% to 20% of total diesel with biodiesel by the year 2010 with emphasis on second generation non-edible feedstock (Thurmond, 2008). If governments from these countries continue to aggressively promote biodiesel generation and continue to invest in research and development for non-edible feedstocks such as jatropha, castor, algae and grease, the prospects to achieve biodiesel targets will be realized faster than anticipated. Fig. 4 depicts

Biodiesel is a mixture of fatty acid alkyl esters. In the case when methanol is used as reactant, it will be a mixture of fatty acid methyl esters (FAME) whereas if ethanol is used as reactant, the mixture will be fatty acid ethyl esters (FAEE). However, methanol is commonly and widely used in biodiesel production due to their low cost and availability. Based on different feedstock, the biodiesel produced will have different composition of FAME. Table 2 shows the common composition of FAME in biodiesel (Ma and Hanna, 1999).

Table 1 Typical fatty acid composition (%) for different common oil source. Fatty acid

Soybean

Cottonseed

Palm

Lard

Tallow

Coconut

Lauric (C12:0) Myristic (C14:0) Palmitic (C16:0) Stearic (C18:0) Oleic (C18:1) Linoleic (C18:2) Linolenic (C18:3)

0.1 0.1 0.2 3.7 22.8 53.7 8.6

0.1 0.7 20.1 2.6 19.2 55.2 0.6

0.1 1.0 42.8 4.5 40.5 10.1 0.2

0.1 1.4 23.6 14.2 44.2 10.7 0.4

0.1 0.8 23.3 19.4 42.4 10.7 0.4

46.5 19.2 9.8 3.0 6.9 2.2 0.0

Fig. 3. Biodiesel production in year 2006. Total production: 6.5 billion liters.

M.K. Lam et al. / Biotechnology Advances 28 (2010) 500–518

Fig. 4. World biodiesel production and capacity.

a more recent world biodiesel production and capacity in the recent years (Thurmond, 2008). 4. Biodiesel costing and potential of waste cooking oil as feedstock Currently, the major concern for biodiesel production is economic feasibility. In a reality scenario, biodiesel production will not be favored without tax exemption and subsidy from government; as the production cost is higher than fossil derived diesel (Demirbas and Balat, 2006). The overall biodiesel cost consists of raw material (production and processing), catalyst, biodiesel processing (energy, consumables and labour), transportation (raw materials and final products) and local and national taxes (Haas et al., 2006). To date, most biodiesel plants are using refined vegetable oils as their main feedstock. Therefore, the cost of refined vegetable oils contributed nearly 80% of the overall biodiesel production cost (Lam et al., 2009b). Thus, it is undeniable that feedstock will be the most crucial variable affecting the price of biodiesel in the global market. A generic process model to estimate biodiesel capital and operating cost had been developed by Haas et al. and the result is shown in Fig. 5 (Haas et al., 2006). The model was designed on the basis of continuous transesterification process using crude, degummed soybean oil as the main feedstock and is dependent on the price of feedstock. In addition, the model was based on a process plant with an annual production capacity of 378,541,181 L (10 × 106 gal). However, some economic factors were excluded, such as internal rate of return, economic life span, corporate tax rate, salvage value, debt fracture, construction interest rate and long term interest rate, working capital, environment control equipment, marketing and distribution expenses, the cost of capital, and the

Fig. 5. Impact of feedstock prices on the predicted unit cost of biodiesel production from crude degummed soybean oil with crude glycerol co-product assigned a value of $0.15/ lb ($0.33/kg).

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existence of governmental credits or subsidies. Based on Fig. 5, when the feedstock cost is at US$ 0.52/kg (US$ 0.236/lb), the model estimated a final biodiesel production cost of US$ 0.53/L ($ 2.00/gal). It is clear that the cost of feedstock contributed the most, which accounted 88% of the total production cost. In addition, the model also estimated the process economic from the recovery of co-product, glycerol as illustrated in Fig. 6. It was assumed that glycerol was sold as low commercial grade glycerol with purity 80% w/w aqueous solution. It can be noted that returns from selling glycerol was not significant, only accounted to 6% reduction in the overall biodiesel production cost. In order to overcome this limitation, biodiesel manufacturer are focusing their attention on using low-cost feedstock such as waste cooking oil in order to ensure economic viability in biodiesel production. Waste cooking oil is far less expensive than refined vegetable oils and therefore has become a promising alternative feedstock to produce biodiesel. In fact, generation of waste cooking oil in any country in the world is huge, and may result to environmental contamination if no proper disposal method is implemented. Table 3 shows the estimated waste cooking oil produced in selected countries (Gui et al., 2008). Based on the table, waste cooking oil generated is more than 15 million tonnes. However, it should be noted that the actual amount of waste oil produced is much higher based on global production. If such amount of waste oil is successfully collected and converted to biodiesel, it will be sufficient to meet the European biodiesel production target at 10 million tonnes in year 2010 (Lam et al., 2009b). Apart from that, a recent study on the production cost of biodiesel using waste cooking oil as feedstock shows that the overall production costs of biodiesel can be reduced by more than half compared to virgin vegetable oil (Escobar et al., 2009). Furthermore, the production costs are even lower than fossil derived diesel as illustrated in Fig. 7 (Escobar et al., 2009). Hence, the high cost of feedstock can be overcome if waste cooking oil is used for biodiesel production. 5. Waste cooking oil Currently, world oil and fats production is about 154 million tonnes (MPOC, 2008). This figure refer to the production of 17 major oils and fats, comprising from vegetable oils (i.e. soybean, cottonseed, groundnut, sunflower, rapeseed, sesame, corn, olive, palm, palm kernel, coconut, linseed, and castor) and animal fats/oils (i.e. butter, lard, tallow, grease and fish oil). Most of this oil is used for deep-frying processes, after which could cause disposal problem. Serious water contamination may occur if no proper disposal method is implemented. Such scenario does not only contribute to pollution problems but is also harmful to human beings. In fact, EU has enforced a ban on the utilization of all waste oils as domestic animal feed because during frying process, many harmful compounds are formed. Eventually,

Fig. 6. Impact of market value of 80% (w/w) glycerol on the unit cost of biodiesel production with the soy oil feedstock assigned a value of $0.236/lb ($0.520/kg).

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Table 3 Quantity of waste cooking oil produced in selected countries. Country

Quantity (million tonnes/year)

China European United States Japan Malaysia Canada Taiwan

4.5 0.7–1.0 10.0 0.45–0.57 0.5 0.12 0.07

these harmful compounds will enter the human food chain during meat consumption (Kulkarni and Dalai, 2006). Since frying improves the taste of food, it has become a common method in food preparation. During frying, oil is heated under atmospheric condition at temperature of 160–190 °C (Gazmuri and Bouchon, 2009) for relative long period of time. In addition, the same oil or fat is used several times, mainly because of economical reasons. However, continuously using the same oil or fat for frying will causes various physical and chemical changes in the oil, depending on the type of oil and oil composition. Some physical changes observed in vegetable oil after frying are (i) an increase in viscosity, (ii) an increase in specific heat, (iii) a change in surface tension, and (iv) a change in color (Cvengros and Cvengrosova, 2004). Apart from that, oils are also subjected to three types of reactions during frying, mainly thermolytic, oxidative and hydrolytic (Mittelbach and Enzelsberger, 1999; Nawar, 1984). These three reactions will continuously cause the formation of many undesired and harmful compounds if the oil is used repeatedly. The toxicological effects of these compounds upon human consumption are still not completely known. However, if waste cooking oil is to be made feedstock for biodiesel production, the amount of polar compound in the waste cooking oil, especially free fatty acid (FFA) must be taken into consideration as it will greatly affect the transesterification reaction. Refined oil usually contains less than 0.5 wt.% FFA whereas for waste cooking oil, FFA contents range between 0.5 and 15 wt.% (Gerhard Knothe and Krahl, 2004).

5.1. Thermolytic reaction

5.2. Oxidative reaction Oxidative reaction occurs when oxygen in air dissolved in the oil or fat and reacts mainly with unsaturated acyglycerols (AG) resulting in the formation of various oxidation products. The main reactions involved in the oxidation reactions are summarized in Fig. 8 (Velasco and Dobarganes, 2002). RH represents triacylglycerol undergoing oxidation in one of its unsaturated fatty acyl groups. Initially, radicals — alkyl radicals (R˙) are formed. By the addition of oxygen, eventually alkylperoxyl radicals (ROO˙) are produced. Finally, alkoxyl radicals (RO˙) are formed due to the decomposition of hydroperoxides (ROOH) which produce various saturated and unsaturated aldehydes, ketones, hydrocarbons, lactones, alcohols, acids and esters. Most of these compounds will remain within the oil or fat, e.g. dimeric and polymeric acid, dimeric AG and polyglycerols as products of the radical reactions and increase the viscosity of the cooking oil. Others might be further decomposed through alkoxyradicals to volatile polar compounds, e.g. hydroxyl- and epoxyacids that escape from the oil (Cvengros and Cvengrosova, 2004). 5.3. Hydrolytic reaction The hydrolysis of triglycerides occurs when steam produced during the preparation of food. Part of the water quickly evaporates, but a certain part dissolved in the oil or fat and induces its cleavage to give higher fatty acids, glecerol, monoglycerides and diglycerides concentration (Kulkarni and Dalai, 2006). 6. Catalysis in transesterification The following section describes various catalysis methods for transesterification reaction of waste cooking oil and the potential of heterogeneous acid catalysts and enzymes towards a more sustainable biodiesel industry. However, due to limited availability of information regarding the transesterification of waste cooking oil for certain type of catalysts; other oils with high FFA oils are taken to re-present waste cooking oil so that the potential of these catalysts for transesterification can be easier understood. Nonetheless, study that uses waste cooking oil as feedstock will be given priority when discussing various technologies. Table 4 summarizes the reaction conditions for various types of catalysts

A thermolytic reaction occurs in the absence of oxygen at high temperatures. A series of alkanes, alkenes, lower fatty acids, symmetric ketones, oxopropyl esters, CO, and CO2 are produced from the saturated fatty acids in the oil. For unsaturated fatty acids, basically diametric compounds including dehydrodimers, saturated dimers and polycyclic compounds are formed. In addition, dimers and trimers may be formed when unsaturated fatty acids react with other unsaturated fatty acids through Diels–Alder reaction (Kulkarni and Dalai, 2006).

Fig. 7. Range of production costs for biodiesel and diesel in year of 2006.

Fig. 8. Simplified mechanism for oil oxidation reaction during frying.

M.K. Lam et al. / Biotechnology Advances 28 (2010) 500–518

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Table 4 Comparison of reaction conditions and performance for various types of catalysts used in transesterification of waste cooking oil. Catalyst

Reaction conditions

Performance

Comments

Reference

0.33

Yield = 88.8%

6

2

Yield = 87%a

Excess catalyst used led to soap formation –

Leung and Guo (2006) Demirbas (2009)

Methanol (20:1) Methanol (245:1) Methanol (30:1)

4 41.8a 1

20 4 69

Conversion = N 90% Yield = 99% Conversion = 99%

– – –

Wang et al. (2006) Zheng et al. (2006) Freedman et al. (1984)

Acid: Methanol (10:1) Base: Methanol (6:1) Acid: Methanol (9:1) Base: Methanol (7.5:1)a Acid: Methanol (7:1)

Acid: 2 Base: 1 Acid: 2 Base: 0.5a Acid: 0.4

Acid: 2 Base: 1 Acid: 2 Base: 1 Acid: 3

Conversion = 97%

–

Wang et al. (2006)

Yield = 96%

–

Patil et al. (2010)

Yield = 81.3%

–

Wan Omar et al. (2009)

Base: Methanol (7:1)

Base: Not clearly specified

Base: 3

0.85

1

Yield = 66%

Kouzu et al. (2008a)

4 5.35

2 0.5

Yield = 97.3% Yield = 71.7%

Calcium soap was detected – –

20

Packed-bed reactor was used –

Park et al. (2008)

0.37

FFA Conversion = 85% Yield = 26.6%

8

Yield = 92%

–

Lou et al. (2008)

4 Å zeolite was used simultaneously to adsorb water Nanostructured catalyst contain double acid sites –

Cao et al. (2008)

Type of alcohol (alcohol to oil molar ratio)

Catalyst loading, wt.%

Reaction time, h

Homogeneous base catalyst NaOH 60

Methanol (7:1)

1.1

KOH

Methanol (9:1)a

Temperature, °C

87

Homogeneous acid catalyst H2SO4 95 H2SO4 70 H2SO4 65

Two-step: Acid catalyst follow by base catalyst Ferric sulfate follow Acid: 95 by KOH Base: 65 Ferric sulfate follow Acid: 100 by KOH Base: 100 Ferric sulfate follow Acid: 60 by CaO Base: 60

Heterogeneous base catalyst CaO Reflux methanol Methanol (12:1) temperature: 60–65 K3PO4 60 Methanol (6:1) Oil palm ash 60 Methanol (18:1) Heterogeneous acid catalyst WO3/ZrO2 75 Zeolite Y (Y756)

460

Methanol to Oleic acid (19.4:1) Methanol (6:1)

Carbon-based catalyst derived from starch H3PW12O40·6H2O (PW12)

80

Methanol (30:1)

Not clearly specified Not clearly specified 10

65

Methanol (70:1)

3.7a

14

Yield = 87%

Zr0.7H0.2PW12O40 (ZrHPW)

65

Methanol (20:1)

2.1

8

Yield = 98.9%

ZS/Si

200

Methanol (18:1)

3

5

Yield = 98% a

SO2− 4 /TiO2–SiO2 SO2− 4 /SnO2–SiO2

200 150

Methanol (9:1) Methanol (15:1)

3 3

4 3

Enzymatic catalyst Pseudomonas cepacia (PS 30)

38.4

Ethanol (6.6:1)

13.7

2.47

Candida antarctica (Novozym 435) Novozym 435

30

Methanol (3:1)

4

50

30

Methanol (3:1)

4

50

Novozym 435

40

Methanol (4:1)

4

12

Bacillus subtilis encapsulated in magnetic particles (Magnetic cell biocatalyst) Rhizopus oryzae

40

Methanol (1:1)

3

72

40

Methanol (4:1)

30

Immobilized Penicillium 35 expansum on resin D4020

Methanol (1:1)

Not clearly specified

a

Self-estimation.

a

Yield = 90% Yield = 92.3%

– –

Guan et al., 2009 Chin et al. (2009)

Brito et al. (2007)

Zhang et al. (2009)

Jacobson et al. (2008) Peng et al. (2008) Lam et al. (2009a)

Second portion of lipase SP 435 (5 wt %) was added into the reaction media after 1 h Conversion = 90.4% Three-step batch methanolysis Conversion = 90.9% Three-step continuous methanolysis at different flow rate Yield = 88 Tert-butanol was used as co-solvent Yield = 90 Methanol was added in two stepwise

Wu et al. (1999)

30

Yield = 88–90%

Chen et al. (2006)

7

Yield = 92.8%

Yield = 96%

Methanol was added in three stepwise. Methanol was added in three stepwise. Blue silica gel was used to adsorb excess water.

Watanabe et al. (2001) Watanabe et al. (2001) Halim and Harun Kamaruddin (2008) Ying and Chen (2007)

Li et al. (2009)

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Table 5 Advantages and disadvantages of different types of catalysts used in transesterification of waste cooking oil. Type of catalyst

Advantages

Disadvantages

Homogeneous base catalyst

•Very fast reaction rate — 4000 times faster than acid-catalyzed transesterification •Reaction can occur at mild reaction condition and less energy intensive •Catalysts such as NaOH and KOH are relatively cheap and widely available

•Sensitive to FFA content in the oil

Heterogeneous base catalyst

•Relatively faster reaction rate than acid-catalyzed transesterification •Reaction can occur at mild reaction condition and less energy intensive •Easy separation of catalyst from product

•Sensitive to FFA content in the oil due to its basicity property

•High possibility to reuse and regenerate the catalyst

Homogeneous acid catalyst

Heterogeneous acid catalyst

•Insensitive to FFA and water content in the oil •Preferred-method if low-grade oil is used •Esterification and transesterification occur simultaneously Reaction can occur at mild reaction condition and less energy intensive •Insensitive to FFA and water content in the oil •Preferred-method if low-grade oil is used •Esterification and transesterification occur simultaneously •Easy separation of catalyst from product

Enzyme

•High possibility to reuse and regenerate the catalyst •Insensitive to FFA and water content in the oil •Preferred-method if low-grade oil is used •Transesterification can be carried out at low reaction temperature, even lower than homogeneous base catalyst •Only simple purification step is required

in transesterification of waste cooking oil whereas Table 5 lists out their advantages and disadvantages. 6.1. Homogeneous base-catalyzed transesterification Currently, biodiesel is commonly produced using homogeneous base catalyst, such as sodium hydroxide (NaOH) or potassium hydroxide (KOH) (Felizardo et al., 2006, Kulkarni and Dalai, 2006). These catalysts are commonly used in the industries due to several reasons: (i) able to catalyze reaction at low reaction temperature and atmospheric pressure; (ii) high conversion can be achieved in a minimal time, (iii) widely available and economical (Lotero et al., 2005). In fact, it was reported that the rate for base-catalyzed reaction would be 4000 times faster compared to acidic catalyst (Fukuda et al., 2001, Kulkarni and Dalai, 2006). However, the use of this catalyst is limited only for refined vegetable oil with less than 0.5 wt.% FFA (Wang et al., 2006) or acid value less than 1 mg KOH/g (Felizardo et al., 2006). Some researchers reported that base catalyst can tolerate higher content of FFA as shown in Table 6. Nevertheless, it is clear that the FFA content in oil feedstock should be as low as possible (ranging from less than 0.5 wt.% to less than 2 wt.%) for base-catalyzed transesterification reaction. Thus, if waste cooking oil with an average FFA content more than 6 wt.%, base catalyst is definitely not suitable to be used (Lotero et al., 2005). FFA consists of long carbon chain that is disconnected from glycerol backbone. They are sometimes called carboxylic acids. If an oil or fat containing high FFA such as oleic acid is used to produce biodiesel, alkali catalyst will typically react with FFA to form soap, which is highly undesirable (Nag, 2008, Yan et al., 2009, Kulkarni and Dalai, 2006). Eq. (2) shows a typical reaction between FFA (oleic acid)

•Soap will formed if the FFA content in the oil is more than 2 wt.% •Too much soap formation will decrease the biodiesel yield and cause problem during product purification especially generating huge amount of wastewater •Poisoning of the catalyst when exposed to ambient air

•Soap will be formed if the FFA content in the oil is more than 2 wt.% •Too much soap formation will decrease the biodiesel yield and cause problem during product purification •Leaching of catalyst active sites may result to product contamination •Very slow reaction rate •Corrosive catalyst such as H2SO4 used can lead to corrosion on reactor and pipelines •Separation of catalyst from product is problematic

•Complicated catalyst synthesis procedures lead to higher cost •Normally, high reaction temperature, high alcohol to oil molar ratio and long reaction time are required. •Energy intensive •Leaching of catalyst active sites may result to product contamination •Very slow reaction rate, even slower than acid-catalyzed transesterification •High cost •Sensitive to alcohol, typically methanol that can deactivate the enzyme

and base catalyst (KOH). This reaction is highly undesirable because it will deactivate the catalyst from accelerating the transesterification reaction. Furthermore, excessive soap in the products can drastically reduce the fatty acid methyl ester (FAME) yield and inhibit the subsequent purification process of biodiesel, including glycerol separation and water washing (Nag, 2008, Kulkarni and Dalai, 2006).

ð2Þ Apart from that, high water content in waste cooking oil also affects the methyl ester yield. When water is present, particularly at high temperatures, it can hydrolyze triglycerides to diglycerides and form free fatty acid. Eq. (3) shows the hydrolysis reaction. With the presence of base catalyst, the FFA will subsequently react to form soap as shown in Eq. (2). Thus, when water is present in the reactant, it

Table 6 Level of FFA recommended for homogeneous base catalyst transesterification. Author/reference

Recommended FFA (wt.%)

Ma and Hanna (1999) Ramadhas et al. (2005) Zhang et al. (2003a) Freedman et al. (1984) Kumar Tiwari et al. (2007) Sahoo et al. (2007)

b1 ≤2 b 0.5 b1 b1 ≤2

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generally manifests itself through excessive soap production. Apart from that, the soaps of saturated fatty acids tend to solidify at ambient temperatures and thus a reaction mixture with excessive soap may gel-up and form a semi-solid mass which is very difficult to recover (Felizardo et al., 2006).

ð3Þ 6.2. Homogeneous acid-catalyzed transesterification Since liquid base-catalyzed transesterification process poses a lot of problems especially for oil or fat with high FFAs concentration, liquid acid catalysts are proposed in order to overcome the limitations. To date, the most investigated catalysts for acid-catalyzed system are sulfuric acid (H2SO4) and hydrochloric acid (HCl). Acidcatalyzed transesterification holds an important advantage with respect to base-catalyzed process: acid catalyst is insensitive to the presence of FFAs in the feedstock (Kulkarni and Dalai, 2006) and can catalyzes esterification and transesterification simultaneously (Jacobson et al., 2008). Esterification is a chemical reaction in which two reactants, typically an alcohol (e.g. methanol) and an acid (e.g. FFA) react to form an ester as the reaction product. It was reported that acid catalysis is more efficient when the amount of FFA in the oil exceeds 1 wt.% (Zhang et al., 2003a, Canakci and Van Gerpen, 1999, Freedman et al., 1984). In addition, economic analysis has proven that acidcatalyzed procedure, being a one-step process, is more economical than the base-catalyzed process which requires an extra step to convert FFA to methyl esters (Zhang et al., 2003a, b). However, acid-catalyzed system is not a popular choice for commercial applications due to slower reaction rate, requirement of high reaction temperature, high molar ratio of alcohol to oil, separation of the catalyst, serious environmental and corrosion related problem (Jacobson et al., 2008, Wang et al., 2006). In a study of acid-catalyzed transesterification of waste cooking oil using H2SO4, Wang et al. reported that the yield of FAME increased with longer reaction time, higher methanol to oil ratio and higher catalyst loading. The conversion of waste cooking oil was more than 90% at a reaction time of 10 h with ratio of methanol to oil at 20:1 and 4 wt.% H2SO4 (with reference to weight of oil) (Wang et al., 2006). In another study, Freedman et al. reported 99% oil conversion by using 1 mol% of H2SO4 and methanol to oil ratio 30:1 for 69 h reaction time (Freedman et al., 1984). These data indicates that acid-catalyzed transesterification process requires more severe reaction conditions (such as longer reaction time) than base-catalyzed reaction. Lotero et al. investigated the reasons for the low activity of acidcatalyzed compared to base-catalyzed transesterification reaction. Fig. 9 shows the mechanism of acid-catalyzed reaction (Lotero et al., 2005). From the figure, it can be seen that the protonation of carbonyl group is the key step in the catalyst–reactant interaction. However, this initial chemical pathway has in turn increases the electrophilicity of the adjoining carbon atom, resulting in the intermediate molecules susceptible to nucleophilic attack. In contrast, the base catalysis takes on a more direct route, at which alkoxide ion is created initially and directly acts as a strong nucleophile. Fig. 10 shows the mechanism of base-catalyzed reaction (Lotero et al., 2005). This crucial different

Fig. 9. Homogeneous acid-catalyzed reaction mechanism for the transesterification of triglycerides: (1) protonation of the carbonyl group by the acid catalyst; (2) nucleophilic attraction of the alcohol, forming a tetrahedral intermediate; (3) proton migration and breakdown of the intermediate. The sequence is repeated twice for R2 and R3.

pathway (formation of electrophilic species by acid catalysis versus formation of stronger nucleophile by base catalysis) is ultimately responsible to the difference in catalytic activity in the transesterification reaction.

6.3. Homogeneous acid and base-catalyzed transesterification: two steps Since homogeneous acid and base catalysts have their own advantages and limitations, some studies have attempted to use a combination of both catalysts to synthesis biodiesel from oil containing high FFAs. Initially, acid catalyst was employed to convert FFAs to ester through esterification. When the FFAs content in the oil drops to lower than 0.5–1 wt.%, transesterification of the oil can then be performed by using a base catalyst. Canakci and Van Cerpen have developed a pilot plant to produce biodiesel from feedstock with high FFAs content via two steps method; esterification and transesterification (Canakci and Van Gerpen, 2003). The feedstock was first treated with H2SO4 to reduce the level of FFAs to below 1 wt.%, followed by transesterification process catalyzed by homogeneous base KOH. Although high FAME yield can be obtained, but the rate of FFA esterification reaction was relatively very slow. Thus, higher amount of acid catalyst are required to accelerate the rate of reaction. The drawback of this two-step process is even more pronounced due to the requirement of extra separation steps to remove the catalyst in both stages. Although problem of catalyst removal from the first stage can be avoided by using base catalyst from the second stage through neutralization

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M.K. Lam et al. / Biotechnology Advances 28 (2010) 500–518

Fig. 10. Homogeneous base-catalyzed mechanism for the transesterification of triglycerides: (1) production of the active species, RO−; (2) nucleophilic attack of RO− to carbonyl group on triglycerides, forming a tetrahedral intermediate; (3) intermediate breakdown; (4) regeneration of the RO− active species. The sequence is repeated twice for R2 and R3.

process, the use of extra base catalyst will add to the cost of biodiesel production (Kulkarni and Dalai, 2006). 6.4. Heterogeneous base-catalyzed transesterification To date, many solid base catalysts have been developed for biodiesel production, such as basic zeolites, alkaline earth metal oxides and hydrotalcites. On top of that, alkaline earth metal oxides especially calcium oxide, CaO have attracted much attention due to their relatively high basic strength, low solubility in methanol and can be synthesized from cheap sources like limestone and calcium hydroxide (Zabeti et al., 2009). Kouzu et al. (2008a) reported that CaO obtained from calcinations of pulverized limestone, CaCO3 at 900 °C for 1.5 h in the flow of helium gas exhibited substantially good result in transesterification of refined soybean oil. The yield of FAME was 93% after 1 h reaction time at methanol reflux temperature and methanol to oil ratio 12:1. Fig. 11 shows the mechanism of CaO as heterogeneous base catalyst in transesterification. However, the yield of FAME dropped to 66% when waste cooking oil with FFA content 2.6 wt.% was used under the same reaction condition. It is obvious that the basic sites of CaO were poisoned by strong adsorption of FFAs on the surface of the catalyst (Kouzu et al., 2008a). Consequently, a portion of the catalyst changed into calcium soap by reacting with the FFAs adsorbed, resulting in low recovery of catalyst. Kouzu et al. further reported that the concentration of Ca in reaction product was

Fig. 11. CaO as heterogeneous base catalyst mechanism in transesterification of triglycerides: (1) abstraction of proton from methanol by the basic sites to form methoxide anion; (2) methoxide anion attacks carbonyl carbon in a molecule of the triglyceride leading to the formation of alkoxycarbonyl intermediate; (3) alkoxycarbonyl intermediate further transformed into a more stable form: FAME and anion of diglyceride; (4) methoxide cation attracts the anion of diglyceride leading to the formation of diglyceride. The sequence is repeated twice for R2 and R3.

3065 ppm which exceeded the basic standard of biodiesel, the concentration of mineral matter should be below 200 ppm (Kouzu et al., 2008a). In addition, some researchers also pointed out that soluble substance from CaO can leached out during transesterification. Gryglewicz (1999) stated in his paper that calcium oxide slightly dissolves in methanol. Lopez et al. conducted transesterification of sunflower oil with methanol in which a slight amount of calcium oxide was found to dissolved in the reaction product (Granados et al., 2007). Kouzu et al. (2008b) further identified the soluble substance as calcium diglyceroxide in which CaO reacted with glycerol during transesterification of soybean oil with methanol. Thus, an extra purification step is needed such as ion-exchange resin to remove the soluble content in the biodiesel (Kouzu et al., 2009). Apart from that, Granados et al. (2007) used activated CaO as a solid base catalyst in the transesterification of sunflower oil to investigate the role of water and carbon dioxide on the deterioration of the catalytic performance upon contact with air for different period of time. The study showed that CaO was rapidly hydrated and carbonated in the air. No calcium oxide peak was detected in the samples after exposed to air for more than 20 days. It was further reported that the active sites of CaO were poisoned due to chemisorption of carbon dioxide and water on the surface sites to form carbonates and hydroxyl groups, respectively. However, the catalytic activity of CaO can be regenerated if CaO is subjected to an

M.K. Lam et al. / Biotechnology Advances 28 (2010) 500–518

activation treatment at 700 °C in order to remove the main poisoning species (the carbonate groups) from the surface. However, leaching of the catalyst was still observed in the transesterification reaction although prior thermal treatment was employed. Besides, magnesium oxide (MgO) which is produced by direct heating of magnesium carbonate or magnesium hydroxide was also investigated on its catalytic activity in transesterification. Di Serio et al. (2006) reported that MgO was efficient in transesterification of soybean oil, however high reaction temperature (180 °C) is required. At low reaction temperature (100 °C), MgO catalyst exhibited very low catalytic performance since the FAME yield observed was less than 20%. This result was in agreement with Cantrell et al. (2005) and Gryglewicz (1999), who reported low or even no activity when using MgO in transesterification reactions performed at 60 °C. In fact, MgO has the weakest basic strength among group II oxides, e.g. CaO and strontium oxide (SrO) (Kouzu et al., 2008a). Nevertheless, mixed magnesium–alumina (Mg–Al) oxide prepared by using hydrotalcites (Mg6Al2(OH)16CO34H2O) as precursor and calcined at high temperature has manifested the basic sites of the catalyst (Xie et al., 2006). More than 90% of FAME yield was observed when using Mg–Al oxide as catalyst but high reaction temperature is still required (Di Serio et al., 2006). Besides that, high concentration of Mg and Al ions were found to leach out from the catalyst, resulting in the requirement of extra purification step (Oku et al., 2005). However, a more in-depth study on using Mg–Al in transesterification of high FFAs oil is required since the effect of FFAs content towards the catalyst performance is still unknown. 6.5. Heterogeneous acid-catalyzed transesterification Currently, biodiesel research is focused on exploring new and sustainable solid acid catalysts for transesterification reaction. In addition, it is believed that solid acid catalysts have the strong potential to replace liquid acid catalyst (Jacobson et al., 2008). The advantages of using solid acid catalyst are (1) they are insensitive to FFA content, (2) esterification and transesterification occurs simultaneously (Kulkarni and Dalai, 2006), (3) eliminate the washing step of biodiesel (Jitputti et al., 2006), (4) easy separation of the catalyst from the reaction medium, resulting in lower product contamination level, (5) easy regeneration and recycling of catalyst and (6) reduce corrosion problem, even with the presence of acid species (Suarez et al., 2007). In fact, the development of heterogeneous catalyst system holds an important factor to be incorporated into a continuous flow reactor (Lotero et al., 2005). Such continuous process can minimize product separation and purification costs, making it economically viable and able to compete with commercial petroleum-based diesel fuel (de Almeida et al., 2008). The ideal solid acid catalyst for transesterification reaction should have characteristics such as an interconnected system of large pores, a moderate to high concentration of strong acid sites, and a hydrophobic surface (Kulkarni and Dalai, 2006). However, research on direct use of solid acid catalyst for biodiesel production has not been widely explored due to its limitation of slow reaction rate and possible undesirable side reactions. Furthermore, there is a knowledge gap on the fundamental studies dealing with reaction pathway of triglycerides on solid acids. The following section will give an overview of various solid acid catalysts reported so far for biodiesel production. 6.5.1. Zirconium oxide (ZrO2) There have been several studies on the usage of zirconium oxide (ZrO2) as a solid acid catalyst for transesterification of different feedstock due to its strong surface acidity. The acidity property can even be enhanced by coating the surface of this metal oxide with anions like sulfate and tungstate. This can be done by impregnating ZrO2 with acidic solution such as sulfuric acid (H2SO4) to become sulfated zirconia, SO2− 4 /ZrO2 (Miao and Gao, 1997). Jitputti et al. (2006) reported that SO2− 4 /ZrO2 can gives promising results in

509

transesterification of palm kernel oil and crude coconut oil with methyl ester yield reaching as high as 90.3% and 86.3%, respectively. However, when unsulfated ZrO2 was used as catalyst instead of SO2− 4 / ZrO2, only 64.5% (palm kernel oil) and 49.3% (crude coconut oil) of methyl ester yield were attained, respectively. This eventually indicates that modification of metal oxide surface acidity is the key factor in obtaining high conversion of triglycerides. Apart from that, combination of alumina, Al2O3 with ZrO2 and modification of ZrO2–Al2O3 with tungsten oxide (WO3) not only provides high mechanical strength but also enhances the acidity of the catalyst (Jacobson et al., 2008). The addition of Al2O3 further stabilizes the tetragonal phase of ZrO2 support and also prevents the growth of WO3 particles. Furuta et al. (2004a) evaluated the performance of tungstated zirconia–alumina (WZA) and sulfated zirconia–alumina (SZA) in the transesterification of soybean oil with methanol at 200– 300 °C using a fixed bed reactor under atmospheric pressure. WZA was found to have a higher activity in transesterification as compared to SZA. However, the authors did not elaborate much on the causes of the improved activity of WZA catalyst. Nevertheless, high reaction temperature (250 °C) and long reaction time (20 h) were needed in order to achieve 90% conversion. Similar results was also reported by Jacobson et al. where by catalyst was prepared by impregnating 10 wt. % of WO3 on to ZrO2–Al2O3. Relatively low ester yield was obtained (65%) when the transesterification reaction was carried out at lower reaction temperature of 200 °C and shorter reaction time of 10 h (Jacobson et al., 2008). On the other hand, Faria et al. (2009) proposed a reaction mechanism for ZrO2 supported on SiO2 in transesterification of soybean oil, as shown in Fig. 12. Thus, a more detail understanding on reaction pathway by ZrO2 catalyst in transesterification can be obtained. Application of ZrO2 in esterification of waste cooking oil was also reported by Park et al. (2008). In their study, WO3 was incorporated into ZrO2 rather than impregnating with H2SO4. It was found that WO3/ZrO2 has higher stability than SO2− 4 /ZrO2, and therefore avoiding the leaching of acid sites into the reaction media. Even if WO3 leached into the reaction media, it does not contaminate the product (Park et al., 2008, Park et al., 2010). From the study, it was found that 85% of FFA conversion was attained in a packed-bed reactor after 20 h of reaction time at 75 °C, but decrease to 65% and remained stable thereafter. The reason given was due to the oxidation of WO3 after long term exposure to FFA (a reducing agent) that resulted to a decrease in catalytic activity. Therefore, leaching of WO3 was rule out as the main reason for catalyst deactivation. In addition, WO3/ZrO2 could be simply regenerated by air re-calcination. However, further study on catalyst and process optimization and also the oxidation state of WO3 are still required. Despite the high acidity of SO2− 4 /ZrO2, however it is known to suffer significant deactivation during liquid-phase transesterification, possibly due to sulfate leaching (Omota et al., 2003). Catalyst leaching was tested by dissolving fresh SO2− 4 /ZrO2 catalyst with water. The pH of the suspension was found to decrease quickly as a result of sulfate groups hydrolyzed to become H2SO4 and HSO− 4 . This will then cause the transesterification to occur via homogeneous acid catalysis and therefore interfere with the measurements of heterogeneous catalytic activity. Recently, a new preparation method for SO2− 4 /ZrO2 was proposed by using chlorosulfonic acid, HSO3Cl instead of the conventional impregnation of H2SO4 (Yadav and Murkute, 2004). The prepared SO2− 4 /ZrO2 exhibited higher catalytic activity in transesterification and no sulfate leaching was observed. However, HSO3Cl is a very hazardous chemicals, even an exposure for a very short time can cause death or major fatal injury (Kapias and Griffiths, 2001). 6.5.2. Titanium oxide (TiO2) Titanium dioxide (TiO2) is among transition metal oxides that have attracted attention for biodiesel production due to their acidic

510

M.K. Lam et al. / Biotechnology Advances 28 (2010) 500–518

Fig. 12. Proposed catalytic cycle for SiO2/ZrO2 in transesterification.

properties. In addition, introduction of sulfuric group on the surface of TiO2 will enhance the acid strength of the catalyst. However, there are still very limited study reported in the literature describing the application of such catalyst for transesterification of vegetable oils (de Almeida et al., 2008). Recently, Chen et al. (2007) had evaluated the 2− catalytic activity of SO2− 4 /TiO2 and SO4 /ZrO2 for transesterification of cotton seed oil high in FFAs content to FAME. It was interesting to find that the activity of this catalyst is proportional to its specific surface 2 area. SO2− 4 /TiO2 with a specific surface area of 99.5 m /g can achieved a higher yield of 90% as compared to SO2− /ZrO with a specific surface 4 2 area of 91.5 m2/g, which can only achieved a yield of 85%. However, this catalyst requires high reaction temperature (230 °C), a negative factor for industrial application. When lower reaction temperature (120 °C) and reaction time (1 h) were used, the FAME yield obtained was only 40% (de Almeida et al., 2008). It was proposed that the reactivity of SO2− 4 /TiO2 can be increased by introducing a secondary metal, SiO2 to produce SO2− 4 /TiO2–SiO2 (Peng et al., 2008). By adding SiO2 to SO2− 4 /TiO2, the specific surface area of the catalyst increased to 258 m2/g with an average pore diameter of 10.8 nm. The synthesized SO2− 4 /TiO2–SiO2 was then subjected to transesterification of refined cotton seed oil blended with 50% oleic acid. The optimum FAME yield obtained was more than 90% at reaction temperature 200 °C, methanol to oil ratio 9:1, catalyst loading 3 wt.% and a shorter reaction time of 3 h. However, the reaction temperature is still considered very high if compared to homogeneous catalyst which lies between 60 and 100 °C. Apart from that, more in-depth research especially on the leaching characteristic of this catalyst must be carried out to justify its potential for industrial biodiesel production. 6.5.3. Tin oxide (SnO2) Tin oxide, SnO2 is n-type semiconductor material with a wide band gap (∼ 3.6 eV); it is transparent to visible light and reflects infrared light (Gutierrez-Baez et al., 2004). This optical and electrical

properties make SnO2 an excellent material for several applications in catalysis, conductivity, gas sensing, ceramics, plastics and biomedicine (Toledo-Antonio et al., 2003). Normally, SnO2 with mesostructure form can be obtained by hydrolyzing inorganic precursor such as meta-stannic acid (SnO·H2O) (Furuta et al., 2004a) in the presence of cationic, anionic and neutral surfactants as structure director (Gutierrez-Baez et al., 2004). However, most of SnO2 mesostructure is not stable and collapsed during calcination. Thus, in order to increase the mesostructure stability, anionic surfactant with phosphate or sulfate has been proposed such as the synthesis of mesoporous sulfated tin oxide, SO2− 4 /SnO2 (Gutierrez-Baez et al., 2004). The phosphate or sulfate groups will then remain bounded to the surface of SnO2, which will further stabilized its mesostructure walls and increasing its thermal stability. 2− Apart from SO2− 4 /ZrO2, SO4 /SnO2 is another solid super acid catalyst that has potential in transesterification reaction. Results from temperature programmed desorption (TPD) of ammonia and adsorption heat of argon (Ar) indicates that the acid strength of SO2− 4 /SnO2 2− was higher than that of SO2− 4 /ZrO2. Consequently, SO4 /SnO2 have shown superior catalytic activity than SO2− 4 /ZrO2 in esterification of noctanoic acid with methanol at temperature below 150 °C (Furuta et al., 2004b). However, report concerning the application of SO2− 4 /SnO2 in transesterification reaction is still very limited, typically for oil or fat containing high FFA. In addition, among the few studies reported in the literature, the findings reported on its catalytic performance in transesterification reaction have not been conclusive. The limited study carried out on SO2− 4 /SnO2 may be due to its complicated preparation procedure particularly owing to the difficulty in obtaining oxide gels from its salt as compared to SO2− 4 /ZrO2 that can be easily prepared (Khder et al., 2008, Matsuhashi et al., 2001). Therefore, a more widespread testing of SO2− 4 /SnO2 should be carried out to obtain more data and information for this promising material as potential solid catalyst for transesterification reaction.

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6.5.4. Zeolites Zeolites are microporous crystalline solids which contain silicon (Si), aluminium (Al) and oxygen in their framework. One of the common applications of zeolite (inorganic solid catalyst) is for the production of organic compounds such as ester (Balaji and Chanda, 1998). This is because the characteristic of zeolite can be tailored made to suit its function. For instance, zeolite can be synthesized with different crystal structures, pore sizes, Si/Al ratios and protonexchange levels (Lotero et al., 2005). Therefore, the acid strength of the catalyst can be controlled by changing the aluminosilicate framework such that it fits specific reaction requirements (Corma and Garcia, 1997). However, it should be noted that low surface acidity may caused slow reaction rate whereas extremely high acidity may cause deactivation due to coking or possible formation of undesirable by-products. Apart from that, zeolite also has another key feature. Since zeolite with specific pore structure and surface hydrophobicity can be tailored made according to substrate's size and polarity (Lotero et al., 2005), therefore only molecules with appropriate dimension are allowed to enter the zeolite cavity and diffusing through the pores (Xavier et al., 2009). Although zeolite has numerous advantages over other heterogeneous catalysts, however its catalytic activity in transesterification reactions is relatively low. This is mainly due to diffusion limitation of bulky reactants (triglycerides) into the microporous structure of zeolite (Kiss et al., 2006). Triglycerides with an average molecular size of 2 nm suffered mass transfer resistance in the zeolite's micropore (1–2 nm). Therefore, it is believed that transesterification reaction only occurs on the external surface of zeolite crystal. In order to overcome this limitation, the pore size and structure of zeolite must be adjusted by varying the Si/Al ratio. Generally, higher Si/Al ratio resulted in zeolite with larger-pore size (Okuhara, 2002) but with weaker acidic strength (Chung et al., 2008). Thus, although zeolite with larger-pore size may overcome the diffusion limitation problem but the reaction rate is still rather slow due to low acidity strength. Brito et al. (2007) reported the application of different zeolite Y catalysts in transesterification of waste cooking oil. Several types of zeolite Y with different concentration of Al2O3 and Na2O were utilized in the reaction, however, the catalysts were found to give poor performance. The highest biodiesel yield attained was only 26.6% although the reaction was carried out at high reaction temperature of 460 °C, methanol to oil molar ratio of 6 and reaction time of 22 min. Similar reports on the low activity of zeolite catalyst when used in transesterification were also reported in recent publications (Shu et al., 2007, Kiss et al., 2006, Okuhara, 2002). 6.5.5. Sulfonic ion-exchange resin Ion-exchange resins are insoluble macroporous polymer that is capable to exchange specific ions within the polymer itself with other ions in a solution or reaction media. Normally, sulfonic ion-exchange resins are co-polymers of divinylbenzene (DVB), styrene and sulfonic acid groups (as the active sites-Brønsted acidity) (Özbay et al., 2008). The polymer structure of the resin is mainly characterized by the composition of the cross-linking component (normally DVB), which will then determine its surface area and pore size distribution (Pääkkönen and Krause, 2003). Besides that, their catalytic activity is also strongly dependent on their swelling properties because the swelling capacity limits reactant accessibility to the acid sites and thus affects their overall activity (Feng et al., 2010). Common types of acidic ion-exchange resin are such as Amberlyst-15, Amberlyst-35 and Nafion SAC-13. These catalysts were reported to give good performance in FFA esterification, but, weak in transesterification (Chen et al., 1999; Vicente et al., 1998). Application of Amberlyst-15 with acidic functional groups has exhibited excellent catalytic activity in esterification reaction. Kiss et al. tested the activity of several solid acid catalysts in the esterification of dodecanoic acid with 2-ethylhexanol at 150 °C. Amberlyst-15 was

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found to require the least reaction time to achieve 90% conversion compared to sulfated zirconia and Nafion-NR50. Similar good results in esterification by using acidic Amberlyst-15 were also reported by Heidekum et al. (1999) and Chen et al. (1999). However, Amberlyst15 was found to give low performance in transesterification reaction. At a relative low reaction temperature (60 °C), the conversion of sunflower oil to FAME was reported to be only 0.7%, using the following reaction conditions: atmospheric pressure for 8 h reaction time and 6:1 methanol to oil molar ratio (Vicente et al., 1998). In another study, Dos Reis et al. reported the transesterification of Babassu coconut oil using Amberlyst-15 (Dos Reis et al., 2005). It was reported that a rather good triglycerides conversion of 80% can be achieved only if the methanol to oil ratio used is increased to 100:1. The reaction temperature and time are at 60 °C and 8 h respectively. This finding was rather expected as the activity of solid acid catalysts in transesterification is normally low at low reaction temperature. Consequently, if Amberlyst-15 is to be used, it is necessary to increase the reaction temperature to 150–200 °C to obtain sufficiently fast reaction rate. However, most ion-exchange resins such as Amberlyst15 have low thermal stability and become unstable at temperature above 140 °C (Lotero et al., 2005). Thus, this problem certainly limits their application to reactions that require high temperatures. Study regarding the deactivation of polystyrene sulfonic acid resins in esterification of high FFAs oils at a higher reaction temperature was reported and discussed extensively by Tesser et al. (2005). 6.5.6. Sulfonic modified mesostructure silica Mesostructure materials such as silica have an exceptional potential to be utilized as heterogeneous acid catalyst in biodiesel production. This mesoporous materials (silica) consist of large mesopores in which can significantly minimize the diffusion problem for reactants to access into the active sites of the catalyst (Mbaraka and Shanks, 2006). Apart from that, the physical and chemical properties of these mesoporous materials can be manipulated by incorporating suitable organic or inorganic functional groups into the mesoporous silica matrix (Mbaraka et al., 2006). For instance, in order to obtain a solid acid catalyst, organosulfonic groups can be incorporated onto the mesoporous silica material as shown in Fig. 13 (Melero et al.). The organosulfonic acid anchored on mesoporous silica acts as Brønsted acid (active sites) that is suitable to catalyze esterification and transesterification reactions. Mesostructure silica materials functionalized with propylsulfonic acid (SO3H) groups have been reported to have good catalytic activity in esterification of refined and unrefined oil (Melero et al.; Melero et al., 2009). To date, application of sulfonated silica in transesterification of waste cooking oil is still limited. Most of the recent studies reported focused on transesterification of refined oil rather than oil with high FFA and waste cooking oil. Mbaraka et al. (2006) reported the utilization of acidic mesoporous silica in esterification of beef tallow. From the study, it was found that propylsulfonic acid-functionalized mesoporous silica (SBA-15-SO3H) has an excellent catalytic activity in esterification. Nearly 95% conversion of FFA from beef tallow was attained at relatively short reaction time of 30 min, at reaction temperature of 120 °C and methanol to FFA ratio of 20. Unfortunately, the catalytic activity of SBA-15-SO3H reduced drastically for the subsequent reaction cycles due to accumulation of organic or carbonaceous matter on the catalyst surface that blocks the acidic sites. However, leaching of the active sites was rule out as the reason for catalyst deactivation since the authors overcome this problem by incorporating hydrophobic agent (propyltrimethoxysilane) onto SBA15-SO3H in order to increase its hydrophobicity. This bi-functionized (acidic and hydrophobic) mesostructure silica would have lower pores polarity, thereby reducing the interaction between polar impurities (contained in the beef tallow) with the sulfonic acid groups. Results showed that high FFA conversion (84%) was

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Fig. 13. Organosulfonic acid-functionalized mesoporous silica.

maintained for the second cycle of reaction. However, in-depth research and analysis work using sulfonic modified silica in transesterification reaction (typically from high free fatty acid oil) should be carried out to justify the potential of this solid acid catalyst. 6.5.7. Sulfonated carbon-based catalyst Carbon-based catalyst is classified as a type of sugar catalyst, in which sugar, starch or cellulose is carbonized incompletely at temperature below 500 °C (Hara, 2009). The incomplete carbonized carbon is then immersed in concentrated sulfuric acid, H2SO4 (purity N96%) and heated to 150 °C for 15 h under the flow of N2 (Lou et al., 2008, Takagaki et al., 2006, Nakajima et al., 2007). The resulting catalyst was claimed to be the first of its kind and is called as sulfonated carbon-based catalyst which comprise high density functional group such as sulfonic group (SO3H) and carboxyl group (COOH) (Hara, 2009). The structure of a typical carbon-based catalyst derived from D-glucose is schematically shown in Fig. 14 (Takagaki et al., 2006). The minimum unit in this material is a nanographene sheet (ca. 1 nm) comprised of 10–20 carbon sixmembered rings (Hara, 2009). In addition, the sulfonated carbon material has a high Hammett acid strength (H0) of −8 to −11, which is almost comparable to concentrated H2SO4 (Okamura et al., 2006). Furthermore, leaching of SO3H group was not observed in the esterification of high free fatty acid oil (Takagaki et al., 2006). Such finding not only open up a new route to minimize extensively the usage of H2SO4 in the industries, but also lead to a greener approach for

Fig. 14. Proposed schematic structure of the carbon materials prepared from D-glucose. The materials are amorphous carbons consisting of polycyclic aromatic carbon sheets with phenolic OH and COOH groups in addition to SO3H groups.

producing acid-based catalyst. However, it should be noted that the carbon-based catalyst could not be prepared by sulfonation of an incomplete carbonized resin, amorphous glassy carbon, activated carbon or natural graphite (Hara, 2009). Heating these carbon materials with H2SO4 will only result to low density SO3H groups and do not function as a solid acid catalyst. Up to date, there are only minimum study that reported on the use of carbon-based material for biodiesel production, typically from waste cooking oil. In a recent study reported by Lou et al., various sulfonated carbon-based catalysts were derived, mainly from starch, cellulose, sucrose and D-glucose (Lou et al., 2008). It was found that carbon-based catalyst derived from starch has a relatively larger-pore volume (0.81 cm3/g) and pore size (8.2 nm) allowing better reactants access to the SO3H sites. Apart from that, starch derived carbon-based catalyst also have a relatively higher total acid sites (1.97 mmol/g) and higher sulfur content (5.9 wt.%). In addition, high FAME yield (92%) was attained from transesterification of waste cooking oil at reaction temperature of 80 °C, methanol to oil molar ratio of 30, catalyst loading of 10 wt.% (referred to weight of oil) and reaction time of 8 h. Furthermore, the catalyst was found to be stable even after fifty cycles of repeated reaction. Nevertheless, this catalyst must still be further improved such as proper optimization work on the catalyst preparation as well as transesterification reaction conditions. 6.5.8. Heteropolyacids (HPAs) Heteropolyacids (HPAs) catalyst has attracted researcher's attention recently due to their excellent water tolerant ability, poses strong Brønsted acidity (stronger than conventional homogeneous acid, H2SO4) and high catalytic activity and stability (Sivasamy et al., 2009, Narasimharao et al., 2007). Typical HPAs which are easily available are H3PW12O40, H4SiW12O40, H3PMo12O40 and H4SiMo12O40 (Zhang et al., 2010). In addition, adding appropriate ratio of salt (Cs+, NH+ 4 and Ag+) to HPAs will dramatically increase its surface area and allow easier accessibility of reactant to its active sites (Narasimharao et al., 2007). However, it should be noted that HPAs can be slightly soluble in the reaction media and resulting to homogeneous reaction, in which contributed to the overall reaction rate (Sivasamy et al., 2009). Therefore, leaching of active sites may also easily occur and cause serious catalyst deactivation. Application of HPAs in biodiesel synthesis from waste cooking oil was reported by Cao et al. (2008). H3PW12O40·6H2O (PW12) was used as the HPAs catalyst and the waste cooking oil contained high amount of FFA (15.65%) and water content (0.1%). Optimum yield of 87%% biodiesel was attained at reaction temperature of 65 °C, methanol to oil molar ratio of 70:1 and 14 h of reaction time. At the same time, 4 Å zeolite was introduced into the reaction media as an adsorbent to adsorb water. Although PW12 has a high tolerance towards FFA content and is stable even after 5 reaction cycles, however, relatively high methanol to oil molar ratio and long reaction time may restrict the application of this catalyst in industrial scale. Apart from that, the catalyst was found not stable when reaction temperature was

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increased more than 60 °C. The authors claimed that this observation is due to the nature of waste cooking oil that contains many undesirable compounds that will cause side reactions when carrying out transesterification reaction at higher temperature. On the other hand, attempt was made to synthesize HPA with higher acidity by introducing Lewis acid into HPA, making it contains both Brønsted and Lewis acid sites. This was done by loading Brønsted acidic HPAs on Lewis acidic supporters, such as ZrO2, TiO2 or Ta2O5 (Zhang et al., 2009). Proper coordination of a Lewis acid to a Brønsted acid could enhance its original acidity and therefore forming a bifunctional acid site catalysts. Zhang et al. (2009) studied the potential of this double acidic sites of HPAs in transesterification of waste cooking oil. Zr0.7H0.2PW12O40 (ZrHPW) with nanotube structure was successfully synthesized using natural cellulose fiber as a template. ZrHPW exhibited high acidity capacity with 1350 μmol/g acid sites was detected, much higher than its original HPW (892 μmol/g). This high density acid sites is comparable to recent studies reported in the literature, such as carbonized glucose (1550 μmol/g) and mesoporous sulfated silica zirconia (1260 μmol/g). Apart from that, high biodiesel yield of 98.9% was attained at reaction temperature of 65 °C, methanol to oil molar ratio of 20, catalyst loading of 2.1% and reaction time of 8 h. Moreover, after 5 reaction cycles, biodiesel yield still remain at 95%. 6.6. Enzyme (biocatalyst) catalyzed transesterification Enzymatic transesterification especially those using lipase has drawn researcher's attention in last ten years due to the downstream processing problem posed by chemical transesterification. Huge amount of wastewater generation and difficulty in glycerol recovery are among problems that eventually increase the overall biodiesel production cost and being not environmental benign. In contrast, enzyme catalysis proceeds without the generation of by-products, easy recovery of product, mild reaction condition, insensitive to high FFA oil and catalyst can be reuse (Kulkarni and Dalai, 2006). These advantages prove that enzyme catalyzed biodiesel production has high potential to be an eco-friendly process and a promising alternative to the chemical process. However, it still has its fair share of constraints especially when implemented in industrial scale such as high cost of enzyme, slow reaction rate and enzyme deactivation (Bajaj et al., 2010). The following section reviews some of the potential enzymes which have been studied in transesterification of waste cooking oil to biodiesel with emphasis given on the reaction and optimum condition used. 6.6.1. Mucor miehei (Lipozym IM 60) In a study by Nelson et al., Lipozym IM 60 was found to show a promising result in transesterification of tallow with high FFA content. Based on the study, two different types of alcohols were investigated: (1) primary alcohols, such as methanol, ethanol, propanol, butanol and isobutanol and (2) secondary alcohol, such as isopropanol and 2-butanol. The effect of adding solvent (hexane) into the reaction mixture was also studied. The purpose of introducing solvent into enzymatic transesterification process is to increase the solubility between methanol and glycerol and therefore minimizes the possibility of enzyme deactivation caused by methanol and glycerol (Halim and Harun Kamaruddin, 2008). It was found that Lipozym IM 60 is a promising enzyme in transesterification of tallow with primary alcohol resulting in biodiesel yield as high as 93–99%. The optimum reaction conditions were: reaction temperature at 45 °C, stirring speed of 200 rpm, 5 h reaction time, 0.34 molar of triglyceride in hexane, methanol to oil molar ratio of 3 and 12.5–25% enzyme (by weight of tallow). However, for secondary alcohol, the yield of biodiesel obtained was relatively low. Only 19–24% of biodiesel yield was obtained using the same reaction condition as primary alcohols. No explanation was

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given for this observation. On the other hand, in a solvent-free system (without the addition of hexane), biodiesel yield dropped significantly to merely 20–65% when methanol or ethanol were used as primary alcohol. Nevertheless, if branched primary alcohol (isopropanol) was used in a solvent-free system, high yield of biodiesel (97%) could be maintained (Nelson et al., 1996). The main advantage of using secondary alcohol in biodiesel production is to reduce the solidification point and subsequently improve its cloud point and pour point characteristic (Salis et al., 2005). 6.6.2. Pseudomonas cepacia (PS 30) Other than Lipozym IM 60, Nelson et al. (1996) also screened the potential of PS 30 in transesterification of tallow to biodiesel. However, the efficiency of the enzyme in catalyzing the transesterification reaction was low, even when solvent was introduced into the reaction mixture. Biodiesel yield obtained was merely 13.9–28.8% for primary alcohols (methanol, ethanol and isobutanol) and 44.1% for secondary alcohol (isopropanol). The reaction conditions reported are; reaction temperature at 45 °C, stirring speed of 200 rpm, 5 h reaction time, 0.34 molar of triglyceride in hexane, methanol to oil molar ratio of 3 and 12.5–25% enzyme by weight of tallow. Although Nelson et al. reported that PS 30 was not a good enzyme, nevertheless, Wu et al. still attempted to study the same enzyme but by optimizing the transesterification reaction using response surface methodology (RSM) (Wu et al., 1999). In his study, restaurant grease was used as feedstock and ethanol with 95% purity was used as the source of primary alcohol. Using the regression equation developed, it was predicted that an optimum biodiesel yield of 85.4% can be obtained at the following optimum conditions: 38.4 °C, 2.47 h, 13.7% lipase (PS-30), and grease to ethanol molar ratio of 1:6.6. However, the apparent yield of biodiesel obtained was way below 85% when using the predicted optimum conditions. Consequently, the authors improved the biodiesel yield by introducing a second portion of lipase PS-30 into the reaction mixture after 1 h of reaction. Unfortunately, this method did not improve the biodiesel yield significantly. On the other hand, if the second portion was changed to lipase SP 435 (Candida antarctica) with 5% (wt) of loading, 96% of ethyl ester yield can be obtained. Nevertheless, neither lipase PS-30 nor SP435 when used individually could give high yield as predicted using RSM. Recently, enzymatic transesterification reached another important milestone with the use of immobilization technology. The purpose of immobilization is to provide a more rigid external backbone for lipase molecule which will result in a faster reaction rate (Knezevic et al., 1998). Lipase can be immobilized into ion-exchange resin, photocross linkable resin, silica beads and alumina through adsorption, cross-linking, entrapment and covalent bonding method. The advantages of immobilized enzymes over free enzymes are easier lipase recovery, high stability, insensitive to solvent, and reusable. Hsu et al. immobilized PS-30 within a phyllosilicate sol–gel matrix (IM PS-30) and was subsequently subjected to transesterification using waste grease oil (Hsu et al., 2002). It was found that IM PS-30 was able to convert 84–94% of grease oil to biodiesel for both primary and secondary alcohols. The reaction was conducted at 50 °C, alcohol to grease molar ratio of 1:4, 100 mg IM PS-30, and reaction time of 18 h. In addition, the enzymatic activity of IM PS-30 did not deactivate even after 48 h of reaction as the lipase was strongly constrained within the matrix. Furthermore, by adding molecular sieve (to remove water) in the reaction mixture, the biodiesel yield increased by as much as 20% with shorter reaction time. 6.6.3. C. antarctica (Novozym 435) C. antarctica was first used by Nelson et al. (1996) in transesterification of high FFA tallow to produce biodiesel. It was observed that Novozym 435 has high enzymatic activity when secondary alcohol (2-butanol) was used in a solvent-free system. Biodiesel yield of 96.4% was obtained at the following reaction conditions: 0.34 molar

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of tallow, reaction temperature at 45 °C, alcohol to tallow molar ratio of 3:1, stirring speed of 200 rpm and reaction time of 16 h. Besides that, adding suitable amount of water (6 mol% based on tallow) into the system was found to promote ester formation when secondary alcohol was used with Novozym 435 (Nelson et al., 1996). This is because water is an essential element to maintain the catalytic activity of enzymes. However, the presence of too much water may cause hydrolysis of oil in which is undesirable and eventually will lead to lower conversion of oil to biodiesel. In another study reported by Watanabe et al. (2001), stepwise methanol addition was introduced during transesterification in order to avoid deactivation of enzyme by methanol and to prolong its durability. Waste cooking oil was used as the main feedstock whereas immobilized Novozym 435 was used as the enzyme. The reaction was conducted with three different ways: (1) three-step batch (fixed flow rate) methanolysis, (2) three-step flow methanolysis and (3) onestep flow methanolysis. For the three-step batch reaction, the amount of methanol used was divided equally in each step. 90.4% conversion of waste oil to biodiesel was obtained after the third step. In addition, for the three-step continuous feed, methanol at a flow rate of 6.1, 6.3 and 4.2 mL/h was introduced into the first, second and third reactor, respectively and 90.9% of biodiesel conversion was achieved. On the other hand, for one-step methanolysis, waste oil was diluted initially with 90% methyl ester (biodiesel) with weight ratio of 3:1 and equimolar amount of methanol, relative to total fatty acids in the waste oil. The mixture was then fed into a bioreactor containing 3 g of Novozym 135 at a flow rate of 4 mL/h and 90% of biodiesel conversion was achieved. Apart from that, in order to reduce the overall production cost of enzyme, Novozym sp. 99–135 was immobilized on low-cost textile cloth (Chen et al., 2009). The immobilization carrier of textile cloth was initially activated by mixing with co-fixing agents consisted of polyethylene glycol, tween and span (surfactants), gelatin and lecithin. The lipase solution was then blended with activated immobilization carrier and dried in air prior to use. Three reactors are connected in series to form a three-step reaction system in which glycerol was separated at each step. Waste cooking oil was used as the main feedstock with high acid value of 143 mg KOH/g. The optimum reaction condition of lipase/hexane/water/waste cooking oil weight ratio of 25:15:10:100, reaction temperature of 45 °C and reactant flow of 1.2 ml/min gave 91% yield of biodiesel. However, for this continuous process, biodiesel yield was found to decrease to 76% after running the reaction at optimum condition continuously for 100 h. The expected reasons are: (1) the glycerol absorbed on the surface of immobilized lipase restricts mass transfer between substrate and enzyme, and (2) enzyme was poisoned by methanol. Most researchers prefer to use hexane as co-solvent in enzymatic transesterification as it can enhance higher activity and posed good stability. However, hexane is a hydrophobic solvent in which hydrophilic compounds used as subtract (alcohol) or obtained byproduct (glycerol) are easily immiscible in hydrophobic reaction medium. This has resulted to low solubility of both mediums and high possibility to deactivate the enzyme (Halim and Harun Kamaruddin, 2008). In a study done by Halim and Harun Kamaruddin, tert-butanol was used as an ideal solvent in a reaction mixture containing waste cooking palm oil, methanol and Novozym 435. High solubility of methanol and glycerol in tert-butanol can eventually eliminate the negative effect of methanol and glycerol on enzyme activity. From the study, it was found that 88% of biodiesel yield can be obtained at reaction temperature 40 °C, methanol to oil molar ratio of 4:1, 4% Novozym and 12 h reaction time. 6.6.4. Bacillus subtilis Ying et al. was the first group of researchers that used B. subtilis for transesterification of waste cooking oil to biodiesel (Ying and Chen,

2007). B. subtilis was initially encapsulated within the net of hydrophobic carrier with magnetic particles (Fe3O4), and then the secreted lipase can be conjugated with carboxyl at the magnetic polymicrosphere surface. This magnetic cell biocatalyst (MCB) was claimed to have better dispersion during transesterification and can easily be separated from the reaction mixture by subjecting to an external magnetic field. From the study, it was found that biodiesel yield can reached up to 90% at reaction temperature 40 °C, pH 6.5, loading of 3.0% MCB, adding methanol in two stepwise and 72 h reaction time. Furthermore, MCB can be easily regenerated without losing its enzymatic activity.

6.6.5. Rhizopus oryzae Chen et al. investigated the enzymatic conversion of waste cooking oil using immobilized R. oryzae lipase (Chen et al., 2006). A three-step batch transesterification reactor was used and stepwise process was introduced in the reactor to reduce the poisoning of enzyme by methanol. The optimum reaction condition was reported at 40 °C, methanol to oil molar ratio of 4, immobilized lipase to oils weight ratio of 30%, pressure of 1 atm and reaction time of 30 h. Biodiesel yield in the range of 88–90% can be obtained under these conditions.

6.6.6. Penicillium expansum Immobilized P. expansum on resin D4020 was reported by Li et al. (2009) to have high enzymatic activity in transesterification of waste oil to biodiesel. The process was further enhanced with the addition of adsorbents such as molecular sieve and blue silica gel into the reaction mixture with the purpose to absorb excess water produced during esterification of FFA with methanol. Excess water will not only result to aggregation of the enzyme in hydrophobic media, hence reducing its enzymatic activity, but also have a negative effect on the enzyme's stability. Optimum biodiesel yield of 92.8%% was obtained when the reaction was carried out at 35 °C, stirring speed of 200 rpm, 2 g of waste oil, 0.4 g t-amyl alcohol, 0.96 g blue silica gel, 168 U immobilized penicillium expansum and 7 h of reaction time. One molar equivalent of methanol was added at reaction time of 0, 1 and 3 h. Apart from that, the synthesized enzyme displayed higher stability in waste oil with 68.4% of the original enzymatic activity retained even after recycle and reuse for 10 batches.

7. Other methods or technologies for biodiesel production Currently, most commercial scale biodiesel plants use batch or continuous-type reactor to produce biodiesel from refined vegetable oils. In addition, homogeneous base catalysts such as potassium hydroxide and sodium hydroxide are the most widely used catalysts due to their fast reaction rate and mild reaction conditions. However, if waste cooking oil is to be used as the main feedstock to produce biodiesel, a two- step process may be required in which the first step is esterification process to reduce FFA content in the oil and the second step is transesterification process. However, this two-step process generates a lot of waste water and is not environmental benign. Alternatively, as presented earlier, heterogeneous and enzymatic catalysts have the potential to overcome the problems posed by homogeneous catalysts. However, most of the studies reported are carried out at laboratory scale. Therefore, if heterogeneous or enzymatic based transesterification process need to be scaled up to industrial scale, mass and heat transfer limitation must be carefully addressed. The following section reviews some of the potential technologies that can facilitate heterogeneous and enzymatic transesterification system to overcome the mass and heat transfer limitation and thus obtaining higher yield of biodiesel in a shorter reaction time.

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7.1. Oscillatory flow reactor (OFR) for transesterification reaction Oscillatory flow reactor (OFR) was first introduced by Harvey et al. (2003) to produce biodiesel through some improvement in mixing intensity between reactants. OFR is a novel type of continues flow reactor, consisting of tubes containing equally spaced orifice plate baffles. Therefore, an oscillatory motion is superimposed upon the net flow of the process fluid, creating flow patterns conducive for efficient heat and mass transfer, whilst maintaining plug flow regime (Harvey et al., 2003). In addition, each baffle essentially behaves as a stirred tank that lead to excellent mixing and suspension by creating vortices between orifice baffles and oscillating fluid (Zheng et al., 2007). This is an essential element in designing a biodiesel reactor especially when heterogeneous catalysts are used due to the presence of three immiscible phases (oil–alcohol–catalyst) at the initial stage of reaction. Thus, improvement in mixing and suspension of catalysts tend to produce higher yield of biodiesel in a shorter reaction time compared to conventional batch-type stirred tank reactor. Apart from that, OFR allows longer residence time as the mixing is independent of the net flow and hence the reactor length-to-diameter ratio can be reduced. This is an important plus point if the process is scaled up for commercial application in order to reduce the overall capital and pumping cost. Harvey et al. applied OFR in the production of biodiesel from waste cooking oil and pure rapeseed oil (Harvey et al., 2003). The reaction was performed at temperature of 20–70 °C, residence time of 10– 30 min and molar ratio of methanol to oil was maintained at 1.5. Pure sodium hydroxide (32.4 g) was dissolved in pure methanol initially at 40 °C for 1 h. It was found that at 50 °C and 30 min of reaction time, nearly 99% of biodiesel was produced. Moreover, the product contains negligible amount of triglyceride and diglyceride. However, some traces of monoglyceride were detected (Noureddini and Zhu, 1997). Nevertheless, Harvey et al. (2003) concluded that in-depth study on OFR in transesterification with heterogeneous catalyst is promptly required as OFR is ideal for suspending solid catalysts or polymer supported catalysts. 7.2. Microwave technology in transesterification reaction In recent years, electromagnetic energy utilization and development has gained much interest by many research groups (Corsaro et al., 2004). Microwave irradiation is one of the examples as this process posed several advantages such as higher yields of cleaner product, minimum energy consumption and environmentally benign compared to conventional heating in various chemical reactions (Groisman and Gedanken, 2008). In fact, conventional heating process suffers significant drawback due to its limitation such as it is dependent on the thermal conductivity of materials, specific heat and density (Groisman and Gedanken, 2008). Apart from that, conventional heating is rather slow and heat is not distributed uniformly in a reaction vessel, resulting to more energy (than the theoretical value) is required for a particular chemical reaction (Mutyala et al., 2010). Moreover, direct contact between the hot reaction vessel surface with reaction media (reactants) may results to product decomposition especially when heated for long period of time. In contrast, microwaves transfer energy in a form of electromagnetic and not thermal heat reflux. The oscillating microwave field tends to oscillate polar ends of molecules or ions continuously (Marra et al., 2010; Azcan and Danisman, 2007). Consequently, collisions and friction between the moving molecules is created and generate heat (Marra et al., 2010). Heat is therefore directly deposited into the reaction media and resulted to rapid temperature increase throughout the sample (Liu and Cheng, 2009, Azcan and Danisman, 2007). Thus, higher yield of products can be obtained in a short reaction time. However, the major drawbacks of using microwave irradiation for biodiesel synthesis are the scaling-up of the process from laboratory

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scale to industrial production scale and process safety (Vyas et al., 2010). A few reports concerning the usage of microwave irradiation in transesterification have been reported recently (Lertsathapornsuk et al., 2008, Barnard et al., 2007, Leadbeater and Stencel, 2006). One of the interesting studies is the development of a continuous flow microwave reactor to produce biodiesel from waste cooking oil as reported by Barnard et al. (2007). In that study, transesterification reaction was performed using a commercially available multimode microwave apparatus (CEM MARS). Initially, a 10 L mixture of waste cooking oil, methanol and catalyst was prepared (1:6 ratio of oil to methanol and 1 wt.% KOH as catalyst) and placed in a holding tank. The mixture was then pumped into the microwave reactor vessel at a flow rate of 2 L/min and heated to 50 °C using microwave power of 1600 W. After 10 min, the products were pumped out from the reactor. It was found that the overall biodiesel conversion achieved 97.9%. Furthermore, when the flow rate of the reaction mixture was increased to 7.2 L/min, 98.9% of biodiesel conversion can still be obtained. Apart from that, the study also reported the overall energy consumption by microwave irradiation and conventional heating as shown in Table 7. The results clearly show that microwave irradiation process would be significantly more energy-efficient than conventional heating in a continuous biodiesel production process. 7.3. Ultrasonic technology in transesterification reaction Ultrasonic technology has been recognized as an effective method to enhance mass transfer rate between immiscible liquid–liquid phases within a heterogeneous system (Ji et al., 2006). Therefore, it has been widely used in various biological and chemical reactions to improve the yield within a shorter reaction time. Ultrasound is defined as sound with frequency beyond human ear can respond. The normal sound frequency that can be detected by human lies between 16 and 18 kHz, but frequency for ultrasound generally lies between 20 kHz and 100 MHz (Vyas et al., 2010). This high frequency sound wave will compresses and stretches the molecular spacing of a medium in which it passes through. Thus, molecules will be continuously vibrated and cavities will be created. As a result, micro fine bubbles are formed through sudden expansion and collapse violently, generating energy for chemical and mechanical effects (Colucci et al., 2005). Furthermore, the collapsed bubbles will disrupt the phase boundary and impinging of the liquids to create micro jets, leading to intensively emulsification of the system (Ji et al., 2006). Ultrasonic technology in transesterification has proven to be an efficient mixing tool and provides sufficient activation energy to initiate the reaction (Singh et al., 2007). Ultrasonic-assisted transesterification does not only shorten reaction time, but also minimize the molar ratio of alcohol to oil and reduce energy consumption

Table 7 Energy estimation for the preparation of biodiesel using conventional and microwave heating. Entry

Reaction conditions

Energy consumption (kJ/L)a

1 2

Conventional heatingb Microwave continuous flow (7.2 L/min feedstock flow) Microwave continuous flow (2 L/min feedstock flow)c Microwave heating (4.6 L batch reaction)

94.3 26.0

3 4e a

60.3 (92.3)d 90.1

Normalized for energy consumed per liter of biodiesel prepared. b On the basis of values from the joint U.S. Department of Agriculture and U.S. Department of Energy 1998 study into the life cycle inventory of biodiesel and petroleum diesel for use in an urban bus. c Assuming a power consumption of 1700 W and a microwave input of 1045 W. d Assuming a power consumption of 2600 W and a microwave input of 1600 W. e Assuming a power consumption of 1300 W, a microwave input of 800 W, a time to reach 50 °C of 3.5 min, and a hold time at 50 °C of 1 min.

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compared to conventional mechanical stirring method (Vyas et al., 2010). However, up to now, there are only few studies applying ultrasonic technology in transesterification of waste cooking oil (Refaat and El Sheltawy, 2008, Wang et al., 2007). Wang et al. explored the potential of ultrasonic technology in enzymatic transesterification of high acid value waste oil (Wang et al., 2007). Commercial immobilized Novozym 435 from C. antarctica was utilized as a biocatalyst in the system. It was found that the enzymatic activity was enhanced with the assistance of low frequency and mild energy ultrasonic sound wave. Under the optimal ultrasonic assistant reaction conditions, such as 8% oil quantity of Novozym 435, molar ratio of propanol to oil 3:1, frequency of ultrasonic assistant 28 kHz occupied at power 100 W, reaction temperature at 40–45 °C, an overall biodiesel (propyl oleate) conversion of 94.86% was achieved in only 50 min. Furthermore, ultrasonic sound wave tends to reduce the adsorption of biodiesel and glycerol on the surface of immobilized Novozym 435. As a result, Novozym 435 can be recycle to use with clean appearances, well decentralizations, no agglomeration, easy washing and well operational stability. 7.4. Co-solvent Transesterification reaction is a slow reaction process which typical requires 30 min to few hours to drive the reaction towards completion, depending on the type of catalyst used. This is because the reactants used in transesterification (triglyceride and alcohol) are not miscible and therefore causing mass transfer limitation especially at the initial time of the reaction. In order to accelerate the rate of transesterification reaction, researchers have recently introduced cosolvent such as tetrahydrofuran (THF), hexane and diethyl ether (DME) into the reaction mixture with the aim to increase the solubility and subsequently improve the mass transfer rate between oil and methanol phase (Guan et al., 2009). Experimental results have shown that THF is indeed a good co-solvent that can accelerate biodiesel production within a shorter reaction time, either for homogeneous or heterogeneous system (Chai et al., 2007, Yang and Xie, 2007, Peña et al., 2009). However, one of the disadvantages of using THF is the co-solvent must be separated out from the final product upon completion of reaction. Although it can be easily distilled out together with methanol, however separation between methanol and THF becomes rather problematic since both chemicals have close boiling point. Note that distilled methanol should be further purified before it is subjected to the next cycle of reaction. In another study, biodiesel itself was used as co-solvent since it is miscible in oil and methanol phase (Park et al., 2009). Furthermore, biodiesel does not need to be separated out since it itself is the final product in the transesterification reaction. In the study by Park et al., it was reported that when biodiesel was added as co-solvent, transesterification reaction can be completed within a relatively shorter reaction time (20 min) as compared to system without co-solvent (60 min). Nevertheless, when co-solvent was used in heterogeneous transesterification of waste cooking oil, a contra observation was reported (Guan et al., 2009). Result showed that the addition of cosolvent THF or DME into the reaction media, caused a drop in biodiesel yield. Based on this finding, the authors suggested that mass transfer between the methanol and oil phase in a heterogeneous catalytic system was not affected significantly even with the addition of cosolvent. Furthermore, the authors observed that when THF was added into the reaction mixture, the solid catalyst easily stick with glycerol and agglomerate on the reactor wall. Thus, catalyst was deactivated as a result of agglomeration. A similar observation was also reported in a homogeneous system when waste cooking oil was utilized as the feedstock (Sabudak and Yildiz, 2010). The authors concluded that the positive effect of THF addition was found to be insignificant because the reactor used for the transesterification reaction was equipped

with a mechanical agitator and circulation pump that is capable to homogenize the mixture sufficiently even without the use of cosolvent. Furthermore, it is more economical to improve the mass transfer limitation through mechanical stirring rather than using cosolvent as additional energy is required to distillate the co-solvent. Nevertheless, more studies are required to verify the effect of cosolvent in heterogeneous transesterification system particularly for enzyme catalyst. 8. Conclusion Biodiesel is a renewable and alternative fuel to petroleum-based diesel that is non-toxic, biodegradable and does not contribute net carbon emission to the atmosphere. Currently, biodiesel is produced through transesterification reaction from vegetable oils such as rapeseed, soybean and palm oil. However, the high prices of these oils in the global market have sharply increased the overall biodiesel production cost and making it not economically viable as compared to petrol based diesel. Furthermore, the oils are important commodities in the human food supply chain and therefore its conversion to biodiesel in a long run may not be sustainable. As an alternative, this paper has addressed the potential of using waste cooking oil, as a cheap and economical feedstock for biodiesel production. Nevertheless, current commercial technology (homogeneous base catalyst) was found not suitable for the transesterification of waste cooking oil due to its high FFA content. On the other hand, using homogeneous acid catalyst requires longer reaction time and could potentially cause corrosion on equipment. Therefore, it is clear that research on heterogeneous catalyst, either base or acid type, should be carried out extensively to develop a suitable catalyst to convert waste cooking oil to biodiesel with special emphasis on catalyst deactivation and regeneration. Apart from that, enzymatic transesterification is another possible way to produce biodiesel from waste cooking oil due to its high stability towards FFA content in oils. However, the enzyme should be synthesized in a cheaper way and available for commercial use. Apart from the development of suitable heterogeneous and enzymatic catalysts for biodiesel production from waste cooking oil, one major drawback still exist. Up-scaling of laboratory scale process involving immiscible phases to commercial scale is not an easy task due to mass and heat transfer limitation. Nevertheless, recent advances in technologies such as oscillatory flow reactor (OFR), microwave irradiation, ultrasonic technology and co-solvent have shown high potential in overcoming the limitation. These technologies not only facilitate transesterification reaction in terms of increasing mixing intensity, heat and mass transfer rate, but also proved to be more energy-efficient as compared to conventional heating process. Thus, in order to materialize the technology for converting waste cooking oil to biodiesel using heterogeneous and enzyme catalysts, extensive research work on process scaling-up and more advance research on developing sustainable catalysts should be conducted simultaneously. Acknowledgements The authors would like to acknowledge the funding given by Universiti Sains Malaysia (Research University Grant No. 1001/ PJKIMIA/814062, Short-term Grant No. 304/PJKIMIA/6039015, Research University Postgraduate Research Grant Scheme No. 1001/ PJKIMIA/8031018 and USM Fellowship) for this project. References Azcan N, Danisman A. Alkali catalyzed transesterification of cottonseed oil by microwave irradiation. Fuel 2007;86:2639–44. Bajaj A, Lohan P, Jha PN, Mehrotra R. Biodiesel production through lipase catalyzed transesterification: an overview. J Mol Catal B Enzym 2010;62:9-14.

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