Production of biodiesel via catalytic upgrading and refining of sustainable oleagineous feedstocks

Production of biodiesel via catalytic upgrading and refining of sustainable oleagineous feedstocks

6

N.A. Tajuddin, A.F. Lee, K. Wilson Aston University, Birmingham, United Kingdom

6.1

Introduction

Diminishing world oil sources are of great concern, leading to a drive for alternative sources of clean and sustainable energy supplies. It has become increasingly obvious that continued reliance on fossil fuel energy resources is unsustainable, due to both depleted world reserves and associated greenhouse gas emissions (Su and Guo, 2014; Hook, 2009; Sharma and Singh, 2009; Ma and Hanna, 1999; Basha et al., 2009). Supplementing petroleum consumption with renewable biomass source is the first step to reduce global dependency on fossil fuels (Pasqualino et al., 2006) and improve energy security. There are many research initiatives aimed at developing alternative renewable biodiesels and energy resources. However, alternate resources of first- and second-generation of biodiesels derived from terrestrial crops and nonedible crops place an enormous strain on world food markets, which contributes to water shortages and destruction of the world’s forests. Therefore, biodiesels specifically derived via transesterification reaction using heterogeneous solid base and solid acid catalysts are considered to be a technically-viable alternative energy resource that is devoid of the major drawbacks of the first- and second- generation biodiesels. Commercially, there are four different techniques to produce biofuel from triglycerides, namely direct-use and blending, microemulsions, thermal cracking (pyrolysis), or transesterification (Afify et al., 2010). Out of these methods, transesterification is the most energy efficient due to the mild conditions that can be employed for conversion of the triglyceride to the fatty acid methyl ester (FAME) and is the fuel widely referred to as biodiesel (Knothe, 2010). Transesterification is a process of converting triglycerides (TAG) from vegetables oil or fats to fatty acid methyl ester (FAME) in the addition of alcohol such as methanol in the presence of acid, base, or enzyme catalyst (Fig. 6.1).

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R

O

Catalyst R1

+ R2–OH

R

O R2

O

+ R1–OH

O

Ester

Alcohol

Ester

Alcohol

Figure 6.1 General equations of transesterification reaction.

6.1.1

Major issues in biodiesel production

In this section, two major research problems have been identified. The first issue is related to the fossil oil depletion and the second relates to how homogeneous catalysts are not suitable to use in biodiesel transesterification reaction. These will be explained in the following subsection.

6.1.1.1

Oil depletion issues

The World Energy Forum has predicted that fossil-based oil, coal, and gas reserves will be exhausted in less than another 10 decades (Sharma and Singh, 2009). Exxon Mobil has developed an annually outlook view on future trends in energy supply, demand, and technology. In a report on population growth versus energy consumption, future energy demand has revealed which types of fuel will meet the demand. According to this report, the world’s population will rise 25% from 2010 to 2040, equating to a growth of nearly nine billion (Mobile, 2011). Along with this urban growth, energy consumption and fuel usage also will be increased, thus as fossil fuels are limited their availability may be prolonged by decreasing the overall consumption (De La Torre Ugarte et al., 2007). Fig. 6.2 illustrates the global peak oil discoveries and production over the past centuries. Billions of oil-equivalent barrels 80

80

60

60 Discovered oil volumes

40

Oil demand

20

0 1900

40

20

1920

1940

1960

1980

2000

0 2020

Figure 6.2 The peak oil discovery and the peak oil production for the past 100 years. Adapted from Longwell, H.J., 2002. The future of the oil and gas industry: past approaches, new challenges. World Energy 5(3), 100e104. Available at: http://www.aspo-australia.org.au/ References/Exxon-WE-Longwell-dec-02.pdf; Tsoskounoglou, M., Ayerides, G., Tritopoulou, E., 2008. The end of cheap oil: current status and prospects. Energy Policy 36(10), 3797e3806. Available at: http://www.sciencedirect.com/science/article/pii/S0301421508002322 (accessed 26.08.15.).

Production of biodiesel via catalytic upgrading and refining of sustainable feedstocks

123

While the demand keeps increasing across the years, consumption of oil is faster than its discovery, thus petroleum production is decreasing and will be depleted sooner as shown. These predictions are described by Hubbert peak theory, which says that when the peak production is passed, production rates enter an exponential decline (Hubbert, 1956; Management et al., 2005). The depletion or decline process is a natural phenomenon that accompanies the development of all nonrenewable resources. The depletion of the world’s crude oil reserve, increasing crude oil prices, and issues related to conservation have brought about renewed interest in the use of biobased materials. The desire to replace petroleum-based material with environmental-friendly and sustainable alternatives has stimulated the development of oil-based materials such as biodiesel. Emphasis on the development of renewable, biodegradable, and environmentally friendly industrial fluids, biodiesel has resulted in the widespread use of natural oils and fats from nonedible purposes.

6.1.1.2

Problems of homogeneously catalyzed biodiesel production

Conventionally, the most frequent method used to produce biodiesel is using homogenous acid-base catalyst. Homogeneous base catalysts such as sodium and potassium hydroxides have been reported to yield high conversion of vegetable oil to methyl ester, for instance (Atadashi et al., 2011). However, there are many drawbacks; in particular, catalyst recovery is almost impossible and saponification issues that in the end lead to separation and purification problem (Knothe, 2010; Wilson and Lee, 2012; Atadashi et al., 2013). Thus, biodiesel and glycerol by-product have to undergo extensive washing, which is energy intensive, generating vast quantities of aqueous waste, as well as being a time-consuming for the process. Homogeneous catalysts are also often very sensitive toward free fatty acid (FFA) and water content in triglycerides (Sani et al., 2014). If the feedstock contains more than three wt% FFA, the typical homogeneous catalysts such as sodium hydroxide (NaOH), potassium hydroxide (KOH), or methoxide will have tendencies to form soap and water (or methanol if using methoxide ions) through the saponification process (Wilson and Lee, 2012; Moser, 2011) (Fig. 6.3). This leads to quenching steps that render separation of catalyst as almost impossible (Yan et al., 2010). Complete removal of the catalyst is essential, as several researcher reported residual homogeneous catalyst can corrode engine components (Demirbas, 2007, 2002; Lotero et al., 2005). In view of the limitations associated with homogeneous catalysts, suitable heterogeneous catalysts are needed as a matter of urgency for green chemistry, as O

O R

OH FFA

+ NaOH (or NaOCH3)

R

O–Na+

+ H2O (or CH3OH)

Soap

Figure 6.3 Formation of unwanted side-products during saponification reaction.

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emphasized by Dhakshinamoorthy et al. (2011), with the advantages of heterogeneous catalysts widely reported (Wilson and Lee, 2012; Atadashi et al., 2011; Borges and Díaz, 2012; Lee and Wilson, 2014). The use of solid catalysts in industrial transesterification is generally preferable because they are more environmentally benign. To exploit the simplified downstream operation of a heterogeneously catalyzed process, along with improved process economics and potential for producing higher purity biodiesel (Atadashi et al., 2011; Helwani et al., 2009; Chew and Bhatia, 2008), it is important that the catalyst is stable and does not exhibit any leaching of the active phase during reaction.

6.2 6.2.1

General background to biodiesel Biodiesel as an alternative fuel

The urge to find sustainable replacements for transport fuel has led to a drive to find renewable, nontoxic, and carbon-neutral biofuels. Biodiesel is a fuel composed of mono-alkyl ester derived from vegetables oil or fats oil, which is proven to contribute to reductions in the world’s dependence on fossil oils (Murugesan et al., 2009). The most significant advantages of biodiesel usage over fossil fuel are: 1. Biodiesel can be produced from plant, algae, or waste oil, including sunflower, soybean, palm, and canola oil, among others. This factor contributes to the reduction of greenhouse gas (GHG) emission simultaneously (Lin et al., 2011). Care must be taken, however, to ensure that land-use issues do not arise when using such feedstocks. 2. Biodiesel is biodegradable, and hence is effective in reducing environmental pollution, degrading about four times faster than petro-diesel. The biodegradability is faster due to the presence of oxygen content. Pasqualino et al. (2006) stated that more than 98% biodiesel is degraded after 28 days compared to 50 and 56% of diesel fuel and gasoline, correspondingly. 3. Biodiesel has remarkable potential to lessen the world’s dependency on petroleum-based oil. The transportation sector is the most dependent on petroleum-based fuel such as diesel, gasoline, and liquid petroleum gas (LPG) and compressed natural gas (CNG) (Demirbas, 2002). Sharma and Singh (2009) reported that biodiesel produces nontoxic emissions, because it does not contain carcinogenic components and has a low sulfur content. In addition, biodiesel significantly reduces exhaust emissions such as unburnt hydrogen, carbon monoxides, and particulate matters (Sharma and Singh, 2009). Makareviciene and Janulis (2003) studied the environmental effect of using rapeseed oil and reported hydrocarbon emissions to be decreased by 53%, carbon monoxide (CO) by 7.2%, and smoke density is reduced by 72.6% when rapeseed oil was compared to fossil fuels. This contributes to a reduction in greenhouse effect of 782.87 g/kWh. 4. Biodiesel is also a safer fuel that gives better engine performance due to the higher cetane number (CN) and flash point compared to petrol diesel (Table 6.1). The essential property of diesel performance is the ability to auto ignite, which is determined by the cetane number (Nakkash and Al-Karkhi, 2012). The higher the cetane number, the better performance of the engine.

Production of biodiesel via catalytic upgrading and refining of sustainable feedstocks

Table 6.1

125

Characteristic of petrol diesel versus biodiesel

Fuel property

Diesel

Biodiesel

Fuel standard

ASTM D975

ASTM PS 121

C10eC21 HC

C12eC22 FAME

36.6
103

32.6
103

1.3e4.1

1.9e6.0

0.85

0.88

848

878

Water (ppm by wt.)

161

0.05% max

Carbon (wt.%)

87

77

Hydrogen (wt.%)

13

12

Oxygen (by diff.) (wt.%)

0

11

Sulfur (wt.%)

0.05 max

0.0e0.0024

Fuel composition 3

Lower heating value (MJ/m ) Kinematic viscosity at Specific gravity at Density at

15 C

Boiling point

40 C

2

(mm /s)

15.5 C 3

(kg/m )

( C)

188e343

182e338

Flash point

( C)

60e80

100e170

Cloud point

( C)

15 to 5

3 to 12

35 to 15

15 to 10

Pour point

( C)

Cetane number

40e55

48e65

Stoichiometric air/fuel ratio (wt./wt.)

15

13.8

Adapted from Joshi, R.M., Pegg, M.J., 2007. Flow properties of biodiesel fuel blends at low temperatures. Fuel 86 (1e2), 143e151. Available at: http://www.sciencedirect.com/science/article/pii/S0016236106002122 (accessed 26.08.15.) with permission from Elsevier.

5. Biodiesel improves the energy security of nations, particularly developing nations without fossil fuel reserves, with potential to play an important role in world primary energy. Producing biodiesel can help a country lessen their reliance on imported fuel or reduce the impact of volatile foreign markets. Biodiesel can also help support local economies where it creates a new job infrastructure for people (Lin et al., 2011).

Even though biodiesel comprises advantages as mentioned above, it also has limitation. Among the disadvantages of biodiesel or direct use of oils as a fuel are: 1. The production of biofuel from edible oils (first generation of biodiesel) has increased many environmental issues such as competition over land usage for crops agricultural and biofuel production. In conjunction with this, food shortage, raised food prices, and destruction of biodiversity as deforestation occurred (Lin et al., 2011) are major causes for concern. The production of biodiesel from second-generation feedstocks should be encouraged to make these more viable/profitable and will be discussed in depth in Section 6.2.3.1.

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2. Oil deterioration and incomplete combustion are frequent problems associated with the direct use of vegetable oil as a fuels (Peterson et al., 1983). While polyunsaturated fatty acids are very vulnerable to polymerization and gum formation at higher temperature, this can be improved by blending vegetable oil with diesel. A blend of 70/30 rapeseed oil with diesel has been reportedly used to power a single-cylinder diesel engine for long run-hours (850 h) without having any adverse effects (Peterson et al., 1983). 3. Other problems include carbon deposits, coking, and trumpet formation in injectors, oil ring sticking, and thickening and lubricating oil gelling. Ma and Hanna (1999) addressed the above problems and also suggested few potential solutions such as preheating engines prior to starting, adjusting injection timing, and use of higher compressor engines.

6.2.2

The biodiesel production process

Biodiesel refers to lower alkyl esters of long chain fatty acids, which are synthesized either by transesterification with lower alcohols or esterification of fatty acids (Lotero et al., 2005). As mentioned in Section 6.1, transesterification of vegetables oil is the most conventional method to produce biodiesel, as the physical properties of the resulting methyl ester (biodiesel) are similar to diesel fuel and the process is relatively simple. Methanol is the most preferable alcohol to be used due to its low cost and industrial availability. The major components in vegetable oils or fats are triglycerides.

6.2.2.1

The transesterification reaction

Transesterification is a reversible reaction in which there is an exchange of the alkyl group of an ester with alkyl group of an alcohol as shown in Fig. 6.1 (Section 6.1), and can be catalyzed by an acid or base catalyst. This reaction also can be accomplished by addition of enzymes (biocatalysts) particularly lipases (Fukuda et al., 2001; Meher et al., 2006; Garcia et al., 2008). Biodiesel is commonly composed of fatty acid methyl ester (FAME) and can be produced by triglycerides from vegetable oils by transesterification of methanol, as illustrated in Fig. 6.4. The reaction of triglycerides with methanol proceeds in a stepwise manner to produce the intermediates of diglyceride and monoglyceride, with subsequent reactions with methanol producing glycerol and biodiesel. The mechanisms for base and acid catalyzed transesterification are shown in Fig. 6.5 and Fig. 6.6 (Meher et al., 2006). In the basic conditions, the nucleophilic of alkoxide ion (RO) attacks the carbonyl group of triglycerides ions and forming a tetrahedral intermediate. This process is followed by the rearrangement of the intermediate producing one molecule of methyl ester and diglyceride ion, with a further nucleophilic attack on the electrophile producing glycerol and FAME (biodiesel). For the acid-catalyzed reaction, carbonyl group is activated toward nucleophilic attacks by protonation of Hþ. Principally, carbonyl is protonated first before OH due to carbonyl group is most nucleophilic compare to OH. The net effect of protonation leads to weaken the carbon oxygen p bond, thus makes carbonyl carbon a stronger electrophile. As alcohol is introduced at this stage, proton transfer will react more quickly and produce intermediary. Acid will greatly facilitates elimination of leaving groups where water will be eliminated and in the end ester will be produced.

Production of biodiesel via catalytic upgrading and refining of sustainable feedstocks

HOCH2

R1COOCH2 Catalyst +

R2COOCH

CH 3OH

R2COOCH

R3COOCH2

R3COOCH2

Triglyceride

Diglyceride

+

R1COOCH3

+

R2COOCH3

+

R3COOCH3

HOCH2

HOCH2 Catalyst R2COOCH

127

+

CH3OH

HOCH R3COOCH2

R3COOCH2

Monoglyceride

Diglyceride HOCH2 Catalyst

HOCH

+

CH3OH

HOCH2 HOCH

R3COOCH2

HOCH2

Monoglyceride

Glycerol

Overall reaction HOCH2

R1COOCH2 Catalyst R2COOCH

+

3CH3OH

HOCH

R3COOCH2

HOCH2

Triglyceride

Glycerol

R1COOCH3 +

R2COOCH3 R3COOCH3 Fatty acid methyl ester (biodiesel)

Figure 6.4 The overall transesterification reaction of triglyceride.

6.2.3 6.2.3.1

Oil feedstocks for biodiesel production First and second generation biodiesel fuels

Conventionally, biodiesel is produced from a chemical reaction called transesterification, as explained in Section 6.2.2.1. Biodiesel can be classified into two types: first generation and second generation of biodiesel. The first generation of biodiesel is derives from edible feedstock such as soybean oil, rapeseed oil, palm oil, sunflower oil, and linseed (Moser, 2009; Santacesaria et al., 2012); however, these sources of biodiesel provoked much concern over competition of land use for agriculture versus energy crop cultivation (Naik et al., 2010; Wilson and Lee, 2012; Santacesaria et al., 2012). These oils also have different fuel properties when compared to diesel fuel, most notably higher density, viscosity, and flash point, as well as lower cetane number and heating values, which raised environmental issues relating to incomplete combustion and production of higher levels of emissions (Knothe, 2010; Meher et al., 2006).

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Handbook of Biofuels Production

ROH

RʹCOO

CH2

RʺCOO

CH

+

B

RO– + BH+

–OR

RʺCOO

OCRʹʺ

H 2C

CH2

RʹCOO +

(1)

CH

O

RʹCOO

CH2

RʺCOO

CH

H 2C

RʺCOO

OR O

C

Rʹʺ

CH2 +

CH H 2C

O–

C

(2) Rʹʺ

O–

O–

RʹCOO

OR O

H 2C

BH+

RʹCOO

CH2

RʺCOO

CH

H2 C

+

RʹCOO

CH2

RʺCOO

CH H2C

ROOCRʹʺ

(3)

B

(4)

O–

+ OH

Figure 6.5 Mechanism reaction of transesterification by base-catalyst. Reprinted from Meher, L., Vidyasagar, D., Naik, S., 2006. Technical aspects of biodiesel production by transesterificationda review. Renewable and Sustainable Energy Reviews 10(3), 248e268. Available at: http://www.sciencedirect.com/science/article/pii/S1364032104001236 (accessed 10.07.14.) with permission from Elsevier.

The so-called second-generation biodiesel feedstocks are derived from nonfood oil sources, and have been developed to reduce the dependency on edible oil (Lee and Lavoie, 2013). Jatropha curcas, mahua, jojoba oil, tobacco seed, salmon oil, and seamango represent some of these energy crops (Naik et al., 2010), in addition to increased use of waste cooking oils, restaurant grease and animal fats such as beef tallow and pork lard, which are also considered to be second-generation feedstocks. Table 6.2 shows the comparison of petroleum fuel, the first and second generation of biodiesel as well as other biofuels derived from sugars and lignocellulose that form part of the renewable fuels landscape.

6.2.3.2

Nonedible vegetable oils and their lipid composition

Due to ecological and economic issues from the first generation of biodiesel, nonedible vegetable oils have become a robust in biodiesel technology. Numerous studies have reported on the potential of nonedible oil used in the production of biodiesel from sources such as Jatropha (Mofijur et al., 2012; Diana da Silva Ara
ujo et al., 2014; Berchmans and Hirata, 2008; Li et al., 2014), Pongamia (Dwivedi and Sharma, 2014, 2015), rapeseed (Pullen and Saeed, 2015), mahua (Shadangi and Mohanty, 2014;

Production of biodiesel via catalytic upgrading and refining of sustainable feedstocks

H+

O R

O

+

H

O

R

H O

H

O H

+

–H+

R

O

O

H

R

H O

H

+

H

O

H

O

R

O

R

H+

O

129

O R R

O +

H

H O H

H

R

H R

O+

O R

H H O +

H

O

R H O R

–O +

–H+

R O

H

O R

Figure 6.6 Mechanism reaction of esterification by acid-catalyst. Reproduced from Meher, L., Vidyasagar, D., Naik, S., 2006. Technical aspects of biodiesel production by transesterificationda review. Renewable and Sustainable Energy Reviews 10(3), 248e268. Available at: http://www.sciencedirect.com/science/article/pii/S1364032104001236 (accessed 10.07.14.) with permission from Elsevier.

Nayak and Pattanaik, 2014), olive (Lama-Mu~ noz et al., 2014; L
opez et al., 2014), rice bran (Hasan et al., 2014; Rani et al., 2015), linseed (Shah et al., 2014), palm oil (Rashid et al., 2014; Abu-Hamdeh and Alnefaie, 2015; Shajaratun Nur et al., 2014), tallow (Kwon et al., 2014; Chakraborty and Sahu, 2014), lard (Sarantopoulos et al., 2014), and also waste cooking oil (Farooq et al., 2015; Hamze et al., 2015). Fatty acids may be defined as organic acids that occur in natural triglycerides and are monocarboxylic acids ranging from C4 to C28 atoms in straight chains, which will usually have either a saturated hydrocarbon chain or may contain from one to six double bonds. Typically, fats are produced by animals and oils by plants, with both mainly in the form of triglyceride molecules. Triglycerides are composed of one to three fatty acids attached to a glycerol backbone by ester linkages (Fig. 6.7). Other glyceride species, such as diglycerides and monoglycerides, are obtained from triglycerides (TGs) by the substitution of one or two acid moieties, respectively, with hydroxyl groups. Table 6.3 compiled the most common fatty acid that is normally found in vegetable oil. Biodiesel production from algae is another prospective avenue that is receiving increased attention from biodiesel researchers. There is a strong view among industry professionals that algae represents the most optimal feedstock for biofuel production in the long run (Adenle et al., 2013; Fasahati and Liu, 2015, 2016; Simionato et al., 2013; Dahiya, 2015; Noraini et al., 2014). There are concerns

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Handbook of Biofuels Production

Comparison of petroleum fuel, the first and second generation of biodiesel

Table 6.2

Criteria/ type

Petroleum refinery

First generation

Second generation

Feedstock

Crude petroleum

Edible vegetable oils, corn sugar, etc.

Nonedible crops, waste biomass, ie, cellulose, lignin, etc. and aquatic biomass (algae)

Products

CNG, LPG, diesel, petrol, kerosene, jet fuel

FAME or biodiesel, corn ethanol, sugar alcohol

Bio oil, hydrotreating oil, lignocellulosic ethanol, butanol, mixed alcohols

Problem

Depletion of petroleum reserve, environmental pollution, economic, and ecological problems

Limited feedstock (food vs. fuel), blended partly with conventional fuel

Still under development to reduce the cost (advance technology)

Advantages

e

Environmentally friendly, economic, and social security

No food competition Environmentally friendly

Adapted from Naik, S.N., et al., 2010. Production of first and second generation biofuels: a comprehensive review. Renewable and Sustainable Energy Reviews 14 (2), 578e597. Available at: http://linkinghub.elsevier.com/retrieve/pii/ S1364032109002342 (accessed 23.05.14.) with permission from Elsevier.

O CH2 O C R 1 CH

OH

CH2 OH Monoglycerides

O CH2 O C R 1 O CH O C R 2 CH2 OH Diglycerides

O CH2 O C R 1 O CH O C R 2 O CH2 O C R 3 Triglycerides

Figure 6.7 Glycerides compound in oil and fats.

over the cost of production and nutrient requirement that need to be carefully managed. These second-generation biofuels will help to reduce the demand on land as algal cultivation can be conducted in large scale pond facilities that do not place a demand on agricultural land (Mata et al., 2010). Algae is claimed to be the only feedstock that can be produced on a scale large enough to meet targets for biofuel production in a short time (Chisti, 2007). Algae was reported as one of the best oil sources for biodiesel, producing up to 250 times the amount of oil per acre as soybeans (Chisti, 2007), having the potential to

Production of biodiesel via catalytic upgrading and refining of sustainable feedstocks

Table 6.3

131

Common fatty acid found in vegetables oil

Name of fatty acids

Chemical name of fatty acids

Structure

Molecular formula

Lauric

Dodecanoic

12:0

C12H24O2

Myristic

Tetradecanoic

14:0

C14H28O2

Palmitic

Hexadecanoic

16:0

C16H32O2

Stearic

Octadecanoic

18:0

C18H36O2

Oleic

Octadecenoic

18:1

C18H34O2

Linoleic

Octadecadienoic

18:2

C18H32O2

Linolenic

Octadecatrienoic

18:3

C18H30O2

Arachidic

Eicosanoic

20:0

C20H40O2

Gadoleic

Eicosenoic

20:1

C20H38O2

Behenic

Docosanoic

22:0

C22H44O2

Erucic

Docosenoic

22:1

C22H42O2

Lignoceric

Tetracosanoic

24:0

C24H48O2

Nervonic

Tetracosenoic

24:1

C24H46O2

produce enough automotive fuel to replace current petroleum and diesel usage. Typically, algae oil contains linolenic (18:3), linoleic (18:2), oleic (18:1), stearic (18:0), palmitic (16:0), and myristic acids (14:0) (Demirbas¸, 2008; Hoekman et al., 2012). Algae are often classified into the following major groupings, which are Cyanobacteria (Cyanophyceae), green algae (Chlorophyceae), yellow-green algae (Xanthophyceae), golden algae (Chrysophyceae), red algae (Rhodophyceae), brown algae (Phaeophyceae), diatoms (Bacilleriophyceae), and pico-plankton (Eustigmatophyceae). Many species of microalgae have a high content of oil in the range of 80% by weight dry biomass (Krohn et al., 2011; Spolaore et al., 2006; Metting, 1996; Schenk et al., 2008), with green algae (Chlorophyceae) recognized as having the most potential for biodiesel production (Tran et al., 2013). Fig. 6.8 shows the lipid contents in various algae species, with Botryococcus braunii showing the highest lipid content (75%), followed by Schizochytrium sp. (72%) and Nannochloropsis sp. (68%). However, it should be noted that lipid contents vary depending on the algae growth condition. Schenk et al. (2008) mentioned that microalgae are among the most convenient sources of biodiesel due to their all-year-round availability. Microalgae cultured in an open pond was found to have a productivity that exceeds the yield of the best oilseed crops, producing 12,000 l ha1 biodiesel compared with 1190 l ha1 for rapeseed oil.

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Handbook of Biofuels Production

Algae species

Schizochytrium sp. Spirulina maxima Nitzschia sp. Crypthecodinium cohnii Isochrysis sp. Nannochloropsis sp. Scenedesmus dimorphus Scenedesmus obliquus Dunaliella salina Dunaliella bioculata Chlorella minutissima Chlorella sorokiniana Chlorella protothecoides Chlorella emersonii Chlorella vulgaris Nannochloris sp. Tetraselmis suecica Dunaliella primolecta Botryococcus braunii Neochloris oleoabundans Chlorella sp. Choricystis minor Phaeodactylum tricornutum Cylindrotheca sp.

0

10

20

30

40 % wt

50

60

70

80

Figure 6.8 Lipid contents in various algae growth (%wt). Reproduced from reference Hoekman, S.K., et al., 2012. Review of biodiesel composition, properties, and specifications. Renewable and Sustainable Energy Reviews 16(1), 143e169. Available at: http://www.sciencedirect.com/science/article/pii/S136403211100390X (accessed 26.05.14.) with permission from Elsevier.

6.3 6.3.1

Recent robust technology in biodiesel catalysis Homogeneous vs. heterogeneous catalysis

In Section 6.1.1.2, the undesirable homogeneous catalysts used in biodiesel have been discussed in detail. Although transesterification using a conventional alkalicatalyzed process gives high conversion levels of triglycerides to their corresponding methyl esters in short times, the reaction has several drawbacks. Table 6.4 summarizes the drawbacks of homogeneous catalysts and the advantages of heterogeneous catalysts. Due to these disadvantages, research on the transesterification reaction using heterogeneous catalysts for biodiesel production has increased over the past decade (Lee and Wilson, 2014). Fig. 6.9 summarizes the classification of catalysts. Zhang et al. (2003) argued there is a considerable incentive for the substitution of liquid bases by solid bases for the following reasons: (1) energy intensive product/catalyst separation, (2) corrosiveness, and (3) the costs associated with the disposal of spent or neutralized caustics. In other words, the use of heterogeneous catalysts allows a more environmentally friendly process to be used for biodiesel production. Furthermore, McNeff et al. (2008) agreed that the use of heterogeneous catalysts could enable the design of an efficient,

Production of biodiesel via catalytic upgrading and refining of sustainable feedstocks

133

Drawbacks of homogeneous catalysts and the advantages of heterogeneous

Table 6.4

Drawbacks of homogeneous catalysts

Advantage of heterogeneous catalysts

Difficult to separate the catalysts from product

Easily separated for recycling

Technically difficult to recycle the catalyst

Possible for reusability

Catalyst need to be removed or washed by a large amount of hot water

Minimize the product separation and purification costs

Produce large amount of industrial wastewater

Economically viable to compete with commercial petro-base diesel fuel

Formation of soap in base catalysts

No formation of soap by the end of the reaction

Homogeneous acid catalysts are corrosive to equipment

Noncorrosive and nontoxic catalysts

Waste material– based catalyst Acid catalyst Boron group–based catalyst

Homogeneous catalyst Base catalyst Catalyst

Transition metal oxides and derivatives

Biocatalyst

Enzyme-based catalyst

Base heterogeneous

Alkali metal oxides and derivatives

Heterogeneous catalyst

Acid heterogeneous

Ion – exchange resins

Mixed metal oxides and derivatives

Carbon group–based catalyst

Figure 6.9 Classification of catalysts. Adapted from Chouhan, A.P.S., Sarma, A.K., 2011. Modern heterogeneous catalysts for biodiesel production: a comprehensive review. Renewable and Sustainable Energy Reviews 15(9), 4378e4399. Available at: http://www.sciencedirect.com/science/article/pii/ S1364032111003595 (accessed 26.05.14.). Copyright 2011 Elsevier.

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continuous process, and improve the economics of biodiesel production. Furthermore, the use of heterogeneous catalysts will avoid the production of soaps through free fatty acid neutralization as reported by Guo and Fang (2011), which simplifies the posttreatment (ie, separation and purification) processes. According to Chouhan and Sarma (2011) porosity and surface basicity are among important factors to be considered prior to selection of a good heterogeneous catalyst. Additionally, basic strength will determine the catalytic activity of a catalyst. According to Lee and Wilson (2014), solid base catalysts are preferable for the transesterification of high purity oil with low FFA as they are more active compared to solid acids. Numerous studies have been undertaken on heterogeneous biodiesel catalysts ranging from alkali metal oxides (Ebiura et al., 2005; Arzamendi et al., 2007), alkaline metal oxides (Tittabut and Trakarnpruk, 2008; Mootabadi et al., 2010; Di Serio et al., 2006; Vujicic et al., 2010; Su et al., 2013), mixed metal oxides (Xie and Zhao, 2014; Teng et al., 2010; Gao et al., 2010; Umdu et al., 2009), transition metal oxides (Jitputtii et al., 2006; Yang and Xie, 2007; Madhuvilakku and Piraman, 2013; Wen et al., 2010), ion-exchange resins (Alsalme et al., 2008; Li et al., 2012; ShibasakiKitakawa et al., 2013), sulphated oxides (Rattanaphra et al., 2012; Boffito et al., 2013), boron group (Tormin et al., 2011; Wang et al., 2011; Shajaratun Nur et al., 2014), carbon groups (Fang et al., 2012; Fu et al., 2013; Shu et al., 2010), enzymes (Yun et al., 2013; Taher et al., 2014; Zhao et al., 2014), and waste catalytic materials (eg, egg shells or sources of calcite) (Patle et al., 2014; García-Moreno et al., 2014; Wen et al., 2010). Waste material is another promising avenue in solid heterogeneous catalyst. It has been claimed to be effective in transesterification reaction, especially involving egg shells (Joshi et al., 2015; Boro et al., 2014), clams (Girish et al., 2013), and cockle (Boey et al., 2011), while natural dolomite also has been adapted by in biodiesel reaction (Wilson et al., 2008). This will be discussed in detail in Section 6.3.2.

6.3.2

Solid base catalysts

Solid base catalysts exhibit excellent activity for transesterification of triglycerides and would be the favored choice for conversion of oils with low free-fatty-acid content. Numerous solid base catalysts have been explored for the transesterification of triglycerides, spanning oxides of group IIA elements; CaO (Boro et al., 2014; Liu and Zhang, 2012; Hu et al., 2011; Istadi et al., 2015b), Mg (Mahdavi and Monajemi, 2014; Jeon et al., 2013; Manríquez-Ramírez et al., 2013; Teixeira et al., 2013), SrO (Dias et al., 2012), BaO (Zhang et al., 2012), carbonates of group IA and IIA elements; and CaCO3 (Hsieh et al., 2010), MgCO3 (Zeng et al., 2014), SrCO3 (Sirisomboonchai et al., 2015; Rashtizadeh et al., 2014), BaCO3 (Sirisomboonchai et al., 2015), transition metal oxides (Wang et al., 2012; Lee and Taufiq-Yap, 2015), basic zeolites (Shu et al., 2007; Wu et al., 2013; Carrero et al., 2011), and hydrotalcites (Sun et al., 2014a; Cantrell et al., 2005; Trakarnpruk and Porntangjitlikit, 2008; Teng et al., 2010; Woodford et al., 2012). Table 6.5 tabulates the most recent heterogeneous solid base catalysts used in biodiesel synthesis.

Table 6.5

Recent heterogeneous solid base catalytic-transesterification Conversion or yield obtained

Year published

Mg-Zn mixed oxides

Co-ppt, impregnation, and urea hydrolysis method

Soybean oil in methanol ratio 12:1 to 24:1

Yield: Mg3Zn1:90%

2015

Pasupulety et al. (2015)

CaFeAl

Co-ppt, calcined at 750 C for 3 h

Soybean oil in methanol ratio 6:1, 9:1, 12:1 and 15:1

Yield: 90% at 12:1 ratio

2015

Lu et al. (2015)

MgAl mixed oxide (Mg1Zn2Al1, Mg1Co2Al1, Mg3Al0.6Fe0.4, and Mg3Al0.6La0.4)

Co-ppt, calcined at 773K for 8 h

Soybean oil in methanol

Yield: Mg1Zn2Al1:64.7%, Mg1Co2Al1:66.4%, Mg3Al0.6Fe0.4:95.7%, Mg3Al0.6La0.4:98.3%

2014

Sun et al. (2014b)

CaOeLa2O3

Co-ppt, calcined at 800 C for 6 h

Jatropha oil in methanol ratio 24:1

Yield: 86.51%

2014

Taufiq-Yap et al. (2014)

SrCO3

Modified solegel and coppt method. Calcined at 900 C for 1 h

Soybean oil in methanol (ratio 12:1)

Conversion: 98%

2012

Lima et al. (2012)

Ca(C3H7O3)2/CaCO3

Calcined CaCO3 in a He at 900 C for 1.5 h

Soybean oil, canola oil, and sunflower oil in methanol ratio 30:1

Yield: soybean oil:75% sunflower oil:70%. Canola oil: 63%

2010

Hsieh et al. (2010)

References

Mixed oxides

Carbonates

Continued

135

TE oil sources and solvent used

Catalysts

Production of biodiesel via catalytic upgrading and refining of sustainable feedstocks

a

Preparation method/ remarks

Continued a

136

Table 6.5

Preparation method/ remarks

TE oil sources and solvent used

Conversion or yield obtained

Year published

Calcined between 600 and 900 C for 2 h

Palm kernel oil (PKO) in methanol (ratio 30:1)

Methyl ester content: >98% with calcined dolomite at 800 C

2010

Ngamcharussrivichai et al. (2010)

Ferric-manganese doped tungstated/ molybdenum nanoparticle

Impregnation reaction followed by calcination at 600 C for 3 h

Waste cooking oil in methanol (ratio 25:1)

Yield: 92.3%  1.12

2015

Alhassan et al. (2015a,b)

Sr3Al2O6

Solegel method, calcined in air at 900 C

Soybean oil in methanol (ratio 25:1)

Yield: 95.7  0.5%

2014

Rashtizadeh et al. (2014)

SrO, CaO, ZnO, TiO2, and ZrO2

Calcination of zinc hydroxide at 800 C for 5h

Rapeseed oil in methanol

Yield: 95% at 250 C

2010

Yoo et al. (2010)

Tungsten zirconia (WZ) oxide

Calcined at 800 C for 3 h

Triacetin in methanol

2007

Lopez et al. (2007)

Catalysts Dolomites (CaMg(CO3)2)

References

Transition metal oxides

MgCaAlHT

Co-ppt, calcined at 450 C for 4 h

Jatropha oil in methanol ratio 6:1

Conversion: 90% for Mg/Ca ¼ 1

2015

Guzm
anVargas et al. (2015)

AleCa hydrotalcite

Co-ppt, calcined at 550 C for 5 h

Soybean oil in methanol

Yield: 87.4%

2014

Sun et al. (2014a,b)

Handbook of Biofuels Production

Hydrotalcites or also known as layered double hydroxide (LDH)

Co-ppt, calcined at 140 C and 200 C

Soybean oil in methanol

Yield: 91.71% at 140 C

2014

Liu et al. (2014a,b)

Mg/AleCO3 HT

Urea  method(urea NO3  molar ratio of 3.0) calcined at 500 C for 4h

Refined microalgae oil in methanol

Conversion: 90.3%

2014

Zeng et al. (2014)

Metallic (Fecralloy) monoliths based on MgeAl HT MM Mg:Al HT

Co-ppt, calcined at 500 C

Sunflower oil in methanol

62e77% Oil conversion after 10 h

2013

Reyero et al. (2013)

Free-alkali co-ppt, template with polystryne

Triglycerides (C4eC18)

C4 TAG is 2e3 times faster than that of the C18 TAG

2012

Woodford et al. (2012)

Zeolite X and A from flyash

Alkaline fusion method through ion exchange and calcined at 900 (10)  C for 2 h

Refined mustard oil in methanol (ratio 18:1).

Conversion: 84.6%

2015

Volli and Purkait (2015)

Hierarchical ZSM-5 (h-ZSM-5) and Beta (h-Beta) zeolites

Functionalise zeolitic with organosilanes, calcined in air at 550 C for 5 h

Algae oil in methanol

Yield/conversion value is not stated. A recovered production phase around 50 wt%

2011

Carrero et al. (2011)

Zeolites of mordenite, beta and X

Impregnation with sodium acetate then calcined at 550 C for 15 h

Refined sunflower oil in methanol (ratio 6:1)

Zeolite 3NazX FAME wt %: 95.1%

2008

Ramos et al. (2008)

Basic zeolites

TE, transesterification; co-ppt, co-precipitation method.

137

a

Production of biodiesel via catalytic upgrading and refining of sustainable feedstocks

Zr-Zn-Al hydrotalcite

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Handbook of Biofuels Production

The hydrotalcite (HT) family is one class of solid base catalyst that has attracted much attention in solid baseecatalyzed biodiesel production since it possesses good anion-exchange properties (Allada et al., 2002; Sels et al., 2001), and tunable base strength for the transesterification reaction (Sels et al., 2001; Xie et al., 2006; Debecker et al., 2009). Hydrotalcite (HT)-like compounds are a family of anionic clays that are also known as layered double hydroxides (LDH) (Wang and Jehng, 2011). They are represented by the general formula M2þ 1x M3þ x ðOHÞ2 An x=n  yH2 O where M2þ are divalent anions (eg, Mg2þ, Zn2þ, Mn2þ, Ni2þ, Co2þ, Fe2þ), M3þ are trivalent metal ions (eg, Al3þ, Cr3þ, Fe3þ, Co3þ, Ga3þ) and An is the interlayer anion (Nishimura et al., 2013; Tronto et al., 2013). HTs also possesses a unique characteristic known as the retrotopotactical effect or as it is more commonly called a “Memory Effect,” which means after calcination they can be reconstructed to their layered structure upon rehydration. Calcination at desired temperature will significantly enhance hydrotalcite surface area due to the Mg-Al HT converting to MgO or Mg(Al)O mixed oxide with a higher surface area and well-dispersed mixed oxides. Xie et al. (2006) reported that calcination at a high temperature (773K) produced the significant catalytic activities (66% conversion), while calcination beyond that resulted lower basicity due to formation of spinel phase. Through rehydration with suitable anions and liquid flow, the interlayer of OH will be reconstructed. Nishimura et al. (2013) observed that reconstructed HT produced a higher Br€ onsted basicity due to formation of OH anions compared to CO3 2 ions in as-synthesized HT (Fig. 6.10). MgeAl hydrotalcites have been applied for TAG transesterification of both poorand high-quality oil feeds. A key development has been in the utilization of alkali-free routes to prepare HT that employ NH3OH and NH3CO3 solutions for the precipitation, thereby overcoming leaching issues related to residual Na and K (Cantrell et al., 2005). Increasing Mg:Al ratio is found to increase the surface charge in the layers, which correlates with the base strength and the rate of C4 transesterification (Fig. 6.11). The bulky nature of oil triglycerides causes serious mass-transport limitations and poor accessibility of base sites in bulk hydrotalcites. Woodford et al. (2012) addressed this issue through the synthesis of macroporous Mg-Al hydrotalcites, which were prepared by adapting the method of Geraud et al. (2006) to an alkali-free method, in which size-controlled polystyrene nanospheres were used as a physical template to introduce

Reconstruction using memory effect

Calcination

Mg/AI/An-

Mg(AI)O

n-

Mg/AI/A

Figure 6.10 Illustration of as synthesized, calcined, and reconstructed hydrotalcites. Reprinted from reference Didier Tichit, B.C., 2003. Catalysis by hydrotalcites and related materials. CATTECH 7(6), 206e217. Available at: http://link.springer.com/article/10.1023/B: CATT.0000007166.65577.34 with permission from Springer.

Production of biodiesel via catalytic upgrading and refining of sustainable feedstocks

Transesterification activity / mmol h–1 g–1(cat)

150

Mg 2+

120

–0.02

Al 3+

–0.04

OH

90

OH

60

–0.06

30 0

13

18

21

24

–0.08

( ) Δ Intra-layer e– density / e.Å–2

Increasing pKBH +

180

139

Mg content / wt%

Figure 6.11 Impact of Mg:Al hydrotalcite surface basicity on their activity toward tributyrin transesterification. Adapted from reference Cantrell, D.G., et al., 2005. Structure-reactivity correlations in MgAl hydrotalcite catalysts for biodiesel synthesis. Applied Catalysis A: General 287(2), 183e190. Available at: http://www.sciencedirect.com/science/article/pii/S0926860X05002140 (accessed 23.06.14.) with permission from Elsevier.

macropores. MacroHT was found to exhibit a 10-fold enhancement in the normalized activity for the transesterification of long-chain triglycerides (C12 and C18) when compared to conventional HT (Fig. 6.12), which was attributed to increased accessibility of the base sites in macropores. Shorter-chain C4 triglycerides showed less impact of macroporosity due to their smaller size and improved diffusion of this reactant. It was interesting to note that in both cases, spiking the reaction with glycerol has a significant detrimental impact on reaction rate, suggesting that strongly bound glycerol may be implicated in catalyst deactivation. Dolomite is a naturally abundant material, used widely for construction applications, which comprises Mg(CO3)-Ca(CO3) layers in an arrangement very similar to calcite (CaCO3) and is an interesting precursor to generate solid base catalysts. As the carbonate form, dolomite is relatively inactive, however upon calcination, dolomite forms an intermixed MgO-CaO composite that shows excellent activity for biodiesel production. A study on transesterification reactivity over dolomite catalyst has been established by Wilson et al. (2008). This research proven the uncalcined dolomite was inactive

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Handbook of Biofuels Production

300 nm

(b)

○, ∆ MacroHT , ▲ ConvHT 30

30

Trilaurin conversion / %

Tributyrin conversion / %

(a)

+Glycerol

20

10

0

○, ∆ MacroHT , ▲ ConvHT +Glycerol

20

10

+Glycerol

0 0

50

100

Reaction time / min

150

0

50

100

150

200

Reaction time / min

Figure 6.12 Nanoengineered macroporous Mg:Al hydrotalcite impact on surface basicity on their activity toward tributyrin transesterification. Adapted from Woodford, J.J., et al., 2012. Better by design: nanoengineered macroporous hydrotalcites for enhanced catalytic biodiesel production. Energy & Environmental Science 5(3), 6145. Available at: http://xlink.rsc.org/?DOI¼c2ee02837a (accessed 24.07.14.) with permission from The Royal Society of Chemistry.

for transesterification, due to lack of base sites. Calcination at 900 C somehow exhibited remarkable activity in transesterification of C4-C8 as well as in higher bulkier triglycerides (C16-C18). Exceptional consequences has also been obtained in transesterification of olive oil where promising outstanding conversion of more than 90% within a 3-h reaction (Fig. 6.13) (Wilson et al., 2008). The application of nano-crystalline MgO or CaO in transesterification as heterogeneous catalysts has also attracted much interest. Montero et al. (2010) successfully employed nano-crystalline MgO for the transesterification of tributyrin, producing conversions between 60 and 80% after 24 h. In this study, nano-crystalline MgO was synthesized through a sol-gel method using supercritical drying to form a precursor with w3-nm cubic MgO nanocrystals. Results have demonstrated the catalytic activity of calcined nano-crystalline MgO in transesterification is dependent on size evolution of surface electronic structure, where in this case (110) and (111) facets are much more dynamic in tributyrin reaction. TEM and XPS both have proven that

Production of biodiesel via catalytic upgrading and refining of sustainable feedstocks

141

900°C

Dolomite Mg

2+

MgO 20 nm

900ºC

CO32–

calcination

Ca2+

CaO 60 nm

C18

500

6 5

400

C8

C4

4

2

100

1

0

0

Hydrotalcite

200

Li/CaO

3

CsHPW

300

Activity/mmol.h–1.g(cat)–1

Activity/mmol.h–1.g(cat)–1

600

900°C dolomite (MgO/CaO)

Figure 6.13 (Top) SEM of fresh and 900 C calcined dolomite with scheme showing corresponding structures. (Bottom) Catalytic activity of calcined Dolomite for the transesterification of short- and long-chain TAGs with methanol benchmarked against literature solid acid and base catalysts. Reproduced from Wilson, K., et al., 2008. The application of calcined natural dolomitic rock as a solid base catalyst in triglyceride transesterification for biodiesel synthesis. Green Chemistry, 654e659. Available at: http://dx.doi.org/10.1039/b800455b with permission from The Royal Society of Chemistry.

MgO with low coordination surface attributes more activity in a mild transesterification reaction of tributyrin (Fig. 6.14) (Montero et al., 2009). Cs-doped MgO is another interesting avenue in heterogeneous solid base catalysts. Doping alkaline-earth oxides with alkali metals results in significant enhancement in catalytic transesterification rates, through the resulting increase in surface basic properties (Berger et al., 2007). Woodford et al. (2014) further explored Cs-promoted MgO nano-catalysts prepared via co-precipitation, for the transesterification of C8, C12, and C18 (olive oil) bulky triglycerides. XRD identified the formation of a Cs-Mg mixed hydroxycarbonate in the calcined catalyst with characteristic reflections

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Handbook of Biofuels Production

NanoMgO

2.2

21.4

Auger param. α /eV

1039.6

21.2

1039.5

21.0

1039.4 20.8

1039.3

20.6

TOF/mmol h–1 g–1 m–2

1039.7

21.6

Δ Ek/eV

NanoMgO at 500ºC

High basicity

2.0 1.8 1.6 1.4 1.2

1039.2

20.4

Low basicity

1.0 2

4

6

8

10 12 14 16 18

Crystallite size/nm

20.4

20.6

20.8

21.0

21.2

21.4

Δ Ek /eV

Figure 6.14 (Top) Fresh NanoMgO TEM images showing well-defined 3-nm cubic- (100) oriented MgO nanocrystallites in an amorphous matrix. Following 500 C annealing these are converted into w13 nm wide defective crystallites exposing (110) facets (Bottom) Relationship between MgO nanocrystal particle size and surface polarizability (DEk) and Auger parameter and XPS analysis, along with correlation of polarizabilty (DEk) with activity for transesterification. Adapted from Montero, J.M., et al., 2010. In situ studies of structure-reactivity relations in biodiesel synthesis over nanocrystalline MgO. Chemical Engineering Journal 161(3), 332e339 and Montero, J.M., et al., 2009. Structure-sensitive biodiesel synthesis over MgO nanocrystals. Green Chemistry. Available at: http://dx.doi.org/10.1039/b814357a with permission from Elsevier and The Royal Society of Chemistry accordingly.

at 2q ¼ 10 e30 . When compared to NanoMgO-500 as a benchmark, this Cs-MgO species has been adapted in transesterification of long-chain TAGs. Cs-MgO exhibited excellent catalytic activity compare to NanoMgO-500 where is found to be deactivated at first 300 min (Fig. 6.15). The reasons behind this still remain unclear as the author could not attributed to either mass-transport limitations or attainment of thermodynamic equilibrium (Woodford et al., 2014).

6.3.3

Solid acid catalysts

A wide range of inorganic and polymeric solid acids are commercially available; however, their applications for the transesterification of oils into biodiesel are less-

Production of biodiesel via catalytic upgrading and refining of sustainable feedstocks

DAG

FAME

–H

+

CH3

CH3

CH3OH

|

| + O– H | |

O– H

|

+

|

+ TAG

O

O O

70

H C(H C) H C

60

CH (CH ) CH

O O

50 40 30

Cs-MgO

20

Nano MgO

10

Cs-MgO

90

Trilaurin conversion/%

Tricaprylin conversion/%

H C(H C) H C

80

12

100

O

90

70 60 50 O

40 30

H C(H C) H C

300

600

900

Reaction time/min

1200

1500

O O

H C(H C) H C

CH (CH ) CH

O O

0 0

O

20 10

0

10

Nano MgO

80

0

300

Triolein conversion / %

100

600

8 6 4

Cs-MgO 2

Nano MgO

0 900

Reaction time/min

1200

1500

0

300

600

900

1200

1500

Reaction time/min

143

Figure 6.15 Formation of crystalline Cs2Mg(CO3)2(H2O)4 phase within co-precipitated Cs-doped MgO and resulting synergy in the transesterification of short- and long-chain TAGs with methanol compared with undoped nano-crystalline MgO. Adapted from Woodford, J.J., et al., 2014. Identifying the active phase in Cs-promoted MgO nanocatalysts for triglyceride transesterification. Journal of Chemical Technology and Biotechnology 89(1), 73e80 with permission from the John Wiley and Sons.

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Handbook of Biofuels Production

frequently explored, in part reflecting their lower activity compared with basecatalyzed routes, in turn necessitating higher reaction temperatures to deliver suitable conversions. Solid acids have the advantage, however, over solid bases in that they are less sensitive to FFA contaminants then their solid base analogues, and hence can operate with unrefined or waste oil feedstocks containing FFA content (Lee and Wilson, 2014). Solid acids can either be used to remove fatty acid impurities by esterification as a pretreatment, or at higher temperatures, simultaneously esterify FFA and transesterify major TAG components, without soap formation, and thus reduce the number of processing steps to biodiesel (Girish et al., 2013; Lee et al., 2014; Wu et al., 2013). Heterogeneous solid acids have advantages over solid bases in term of them being less sensitive to FFA contaminants; thus they can be operated in unrefined or waste oil feedstocks (Lotero et al., 2005). Solid acids can also be used to remove impurities by pretreatment of esterification. In addition, they can also simultaneously esterify or transesterify TAG components without producing side reactions leading to soap formation. Moreover, it also reduces processing steps of biodiesel (Narasimharao et al., 2007; Kouzu et al., 2011; Lotero et al., 2005). Numerous studies have reported onsolid acidecatalyzed biodiesel production  using sulfated metal oxides (eg, SO4 2 ZnO (Istadi et al., 2015a,b), SO4 2 ZrO2   (Alhassan et al., 2015a,b; Yi et al., 2015), SO4 2 Nb2 O5 and SO4 2 TiO2 (Xie and Wang, 2013), H form zeolites (Wang et al., 2014), sulfonic ion-exchange resins (Fu et al., 2015), sulfonic modified mesostructured silica (Melero et al., 2010; Shao et al., 2013), sulfonated carbon-based catalyst (Poonjarernsilp et al., 2015), heteropolyacids (HPAs) (Alca~ niz-Monge et al., 2013), and acidic ionic liquids (ILs) (Ullah et al., 2015; Muhammad et al., 2015)). Recent publications on solid acid transesterification for biodiesel are summarized in Table 6.6. The ideal solid acid for esterification and transesterification should have characteristic such as strong Br€ onsted and/or Lewis properties to promote biodiesel reaction with significant rates, unique porosity or textural properties to minimize diffusional problem between long chain molecules, and a hydrophobic surfaces to promote absorption of oily hydrophobic species on catalyst surface (Su and Guo, 2014; Melero et al., 2009; Santacesaria et al., 2012).

6.3.3.1

Templated mesoporous materials: effect of pore networks and surface functionality

Tunable acidity and surface polarity as well as the ability to generate a well-defined pore network are crucial factors to be considered in controlling in-pore diffusion and absorption properties of heterogeneous acid catalysts. Hydrophobicity and hydrophilicity are the key properties to determine adsorption and desorption of reactants/ products at catalyst surfaces. Esterification of FFAs and transesterification of TAGs involve hydrophobic reactants (eg, TAGs or FFAs) and hydrophilic products (eg, water or glycerol). The existence of water in esterification prevents FFAs from approaching active sites properly hence deliberate the reaction rate. Meanwhile, in transesterification reactions, the hydrophilic glycerol product can bind strongly on polar surfaces inhibiting the absorption and diffusion of TAGs reactant. These facts lead

Recent heterogeneous solid acid catalysts for biodiesel production

Catalysts Mixed oxide group  Fe2O3-MnO-SO4 2 ZrO2

 S2 O8 2 ZrO2 eTiO2eFe3O4

WO3/ZrO2

 SO4 2 SnO  2, SO4 2SnO2 eSiO2, SO4 2 SnO2 -Al2O3

Oil sources and solvent used

Conversion or yield obtained

Year published

Impregnation reaction followed by calcination at 600 C for 3 h

Waste cooking oil containing 17.5% free fatty acids added to methanol and oil

Yield of 96.5  0.02%

2015

Alhassan et al. (2015a,b)

Co-ppt and impregnation methods

Cottonseed oil with various volumes of methyl acetate

98.5% in Zr/Ti molar ratio of 3:1 calcined at 550 C

2014

Wu et al. (2014)

Impregnation of Zr(OH)4 with an ammonium metatungstate, WO3 is calcined at 800 C

Soybean oil with 4 wt% oleic acid, oil, and methanol ratio 1:9

93% Conversion

2010

Park et al. (2010)

Impregnation method, calcined at 200, 300, 400, 500 C

Waste cooking oil in methanol

Yield of 92.3%

2009

Lam and Lee (2011)

Suspension polymerization method using styrene and diviniyl benzene

10 wt% catalyst loading, 40 wt% methanol (mol ratio of mefOH/ FFAs ¼ 10:1, acid value ¼ 64.9 mg KOH g1)

30% and 50% CLD resins exhibited 32.4% and 68.7% FFA conversion

2015

Fu et al. (2015)

References

Sulfonic acid group Macroporous cation exchange resin

Continued

145

Preparation method/ remarks

Production of biodiesel via catalytic upgrading and refining of sustainable feedstocks

Table 6.6

Continued

Catalysts  SO4 2 ZrO2

Carbon-mesoporous silica (CS) composite functionalized with sulfonic acid

146

Table 6.6

Preparation method/ remarks

Oil sources and solvent used

Conversion or yield obtained

Year published

Add NH3 aqueous solution in ZrOCl2$8H2O up to pH 8.5.

Oleic acid with methanol

90% Conversion

2013

Patel et al. (2013)

Carbonization of sucrose impregnated in SBA-15 mesoporous silica and its subsequent sulfonation

Esterification of palmitic acid and methanol, palmitic acid, and TE of soybean oil and methanol

Esterification conversion ¼ 98% TE yield: 99%

2012

Fang et al. (2012)

References

Heteropoly acids and polyoxometalates aqueous Immersion method: 50% PA and NaY were added in 20 ml of water

Free fatty oil oleic acid with ethanol

77.62% Conversion

2014

Liu et al. (2014a,b)

HPWO and CsHPWO supported on SiO2, MCM41, and ZrO2

Sol-gel hydro-thermal method and two-step impregnation method

Crude palm oil as called palm fatty acid distillate or PFAD)

Up to 92% FAME

2013

Trakarnpruk (2013)

Zr supported HPA

Suspension method

Sunflower oil with methanol

97% Conversion

2008

Sunita et al. (2008)

TE, transesterification, co-ppt, co-precipitation.

Handbook of Biofuels Production

PA/NaY (PA ¼ organic phosphonic acid)

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147

to poor transesterification reactivity and deactivation of the catalysts at the same time (Su and Guo, 2014). Tuning of the hydrophobicity and hydrophilicity of mesoporous silica based catalysts can be readily achieved by incorporation of hydrophobic of alkyl group containing organosilica moieties into the catalyst. For this reason, investigation on sulfonic acid-modified mesostructured silica is gaining much attention. Templated mesoporous silica such as SBA-15, MCM-41 and periodic mesoporous organosilica (PMO) are reported to exhibit ample silinol groups. This intrinsic factor enables them to be easily functionalized with sulfonic acid group and a resulting large surface area, uniform pore-size distribution, high pore volume, and tunable pore channels and structure ordering (Su and Guo, 2014). Sulfonic acid functionalized silica normally prepared by using precursor of tetraethoxysilane through two popular methods: co-condensation and postsynthesis (Su and Guo, 2014). Melero et al. (2010) reported in their study, sulfonic acid-modified mesostructured silica were more active than ion-exchanged sulfonic acid resin (Amberlyst35 and SAC-13) in both esterification and transesterification reaction. In another study, Melero established the synthesis of sulfonic acid-modified mesostructured silica possessing a high thermal stability, high surface area, narrow pore-size distribution with well accessible acid sites (Melero et al., 2006). They also conveyed a possibility of tuning acidic strength by adapting suitable reagent with different electron withdrawing power (Melero et al., 2006). The application of pore-expended sulfonic acid SBA-15 has been demonstrated for the first time by Dacquin et al. (2012). In their study, the impact of pore-expended sulfonic functionalized SBA-15 toward palmitic acid in esterification and transesterification of tricaprylin and triolein has been explored. Large-pore SBA-15 was obtained by incorporation of trimethylbenzene (TMB) into Pluronic P123/tetraethyl orthosilicate (TEOS) and been aged for 1e3 days. Results showed pore diameters up to 14 nm were achieved through this method, with pore-expansion conferring >3-fold activity toward C16 FFAs esterification and C8/C18 transesterification reaction (Dacquin et al., 2012). In another study, the effect of pore network was evaluated by comparing 2D SBA-15 propylsulfonic acid (PrSO3H-SBA-15) and 3D KIT-6 propylsulfonic acid silica (PrSO3H-KIT-6) and applied in short and long chain esterification (Pirez et al., 2012; Dacquin et al., 2012). For both reactions, pore diameters are simultaneously increased with turnover frequency (TOF). Lee and Wilson (2014) emphasized SBA-15 with p6mm is notorious to transport bulk reaction media, whileKIT-6 with its interconnectedIa3d structure offers better in-pore accessibility of sulphonic acid sites. Results revealed PrSO3H-KIT-6 produced higher turnover frequency (40 and 70%) compared to SBA-15 toward propanoic and hexanoic acid (Fig. 6.16). However, pore accessibility remains challenging for esterification of C16-C18 long chains. The effect of hydrophobicity has been demonstrated using MCM-sulfonic acid catalysts co-functionalized with octyl groups (MCM-Oc-SO3H), which are found to exhibit enhanced turnover frequency compared to the parent MCM-sulfonic acid. Molecular dynamic simulations indicate that in addition to an increase in

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120 Propanoic acid

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CH3OH FFA

Biodiesel

t Fas

Esterification TOF/h–1

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Hexanoic acid Lauric acid

80

Palmitic acid

60 40 20 0 Amberlyst SBA-15 4.9 nm

Kit-6-80 Kit-6-100 Kit-6-120 5.2 nm 6.2 nm 7.0 nm

Solid acid

Figure 6.16 Comparison of mesoporous propylsulphonic acid KIT-6 and propylsulphonic SBA-15. Reprinted with permission from Pirez, C., et al., 2012. Tunable KIT-6 mesoporous sulfonic acid catalysts for fatty acid esterification. ACS Catalysis 2(8), 1607e1614. Copyright 2012 American Chemical Society.

hydrophobicity, the interaction of isolated sulphonic acid moieties with surface silanol groups is a primary cause of the lower acidity and activity of submonolayer samples within the MCM-SO3H series. Lateral interactions with octyl groups help to reorient sulphonic acid head-groups into the pore interior, thereby enhancing acid strength and associated esterification activity (Dacquin et al., 2010).

6.3.3.2

Hierarchical macroporousemesoporous solid acid and base materials

Formation of hierarchical macroporousemesoporous support materials have started to gain attention recently. Hierarchical macroporousemesoporous silica SBA-15 has been developed in order to promote bulky and viscous C16-C18 TAGs and to boost up their diffusion flows (Dhainaut et al., 2010). This catalyst has been synthesized by dual-templating hierarchical method using soft liquid crystalline surfactant and hard polystyrene nanosphere template (Fig. 6.17). The resulting sulfonic acid derivatized Bi-modal PrSO3H-MM-SBA-15 material is a macroporousemesoporous hierarchical catalyst offering high surface area and increased rate of reaction of both esterification and transesterification (Dhainaut et al., 2010). The increased activity of PrSO3H-MM-SBA-15 is attributed to a higher accessibility of sulfonic group toward mesopores as macropores act as rapid transport conduits to the active sites, and hence increase the mass transport in both transesterification of tricaprylin and esterification of palmitic acid (Dhainaut et al., 2010). This fascinating method also has successfully formed highly structure in

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Soft template Aluminum Isopropoxide/TEOS Pluronic P123 HNO3

149

Hard template (polystyrene)

Ageing (5 h)

Static conditions at 60°C Sedimentation of PS beads

Hierarchical catalyst supports for biodiesel synthesis

Highly organized macroporous–mesoporous Al2O3 Calcination under O2 at 600°C

Evaporation of supernatant

Precursor infiltration

Hybrid macro-mesophase formation

Figure 6.17 Dual templating route approaches toward hierarchical macroporousemesoporous silicas. Adapted from Lee, A.F., Wilson, K., 2014. Recent developments in heterogeneous catalysis for the sustainable production of biodiesel. Catalysis Today 242(Part A), 3e18. Available at: http:// www.sciencedirect.com/science/article/pii/S0920586114003034 (accessed 13.06.14.) with permission from Elsevier.

macroporousemesoporous alumina (Dacquin et al., 2009) with mesopores diameter of 200e500 nm and 5e20 nm, respectively. Using an identical hierarchical macroporousemesoporous SBA-15 support method has been employed to produce an alumina grafted Al-MM-SBA-15 support framework for alkali and nitrate-free synthesis of HT coatings from Mg(OMe)2 deposition. XRD revealed that HT/MM-SBA-15 exhibit smaller crystallite size compared to ConvHT with similar diffraction pattern while basicity is found to be similar (Fig. 6.18). Limiting conversions of 34 and 64% occurred after the first-hour reaction subjected to HT/MM-SBA-15 only composes a thin hydrotalcite coating and the majority of this catalyst is deposited in inert silica (Creasey et al., 2015).

6.4

Concluding remarks

This review has presented an overview of the impact of tuning both the surface properties and pore architectures of solid acid and base catalysts on their performance in biodiesel synthesis. Plant-oil viscosity and poor miscibility with light alcohols continue to hamper the use of new heterogeneous catalysts for continuous biodiesel production from both materials and engineering perspectives. Thus, the design of

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(a)

(b) Data Fit Al-SiO2 Al2O3

AI-MM-SBA-15

Si 2p XP signal

AI 2p XP signal

Data Fit Al-SiO2 SiO2

SBA-15

Al2O3 Micropores

AI-MM-SBA-15

MM-SBA-15

107

105 103 101 Binding energy/eV

99

77

75 73 Binding energy/eV

71

Figure 6.18 (Top) Highlight crystal structure of a-alumina with primitive cell with SEM/TEM micrographs of HT/MM-SBA-15 macropore network and hydrotalcite crystallites decorating macropores. (Below) (a) Si and (b) Al 2p XP spectra of parent MM-SBA-15 and Al-MM-SBA-15 following four alumina grafting cycles. Adapted from Creasey, J.J., et al., 2015. Facile route to conformal hydrotalcite coatings over complex architectures: a hierarchically ordered nanoporous base catalyst for FAME production. Green Chemistry 17(4), 2398e2405. Available at: http://dx.doi.org/10.1039/C4GC01689K with permission from Royal Society Chemistry.

pore networks with interconnecting macro and mesoporous channels has clear beneficial effects on reaction rates by improving in-pore diffusional properties. Likewise, control over surface hydrophobicity has been shown to be beneficial in esterification reactions where reactively formed water can be expelled from the active site, thereby hindering reverse hydrolysis processes.

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Widespread uptake and development of next-generation biodiesel fuels requires progressive government policies and incentive schemes to place biodiesel on a comparative footing with cheaper fossil-based fuels. The increasing use of waste or low-grade oil sources will improve the production costs of biodiesel, but presents a challenge for catalyst design due to the presence of impurities, which either require improved purification technology or design of catalysts that are robust to these components. Solid materials capable of simultaneous esterification and transesterification under mild conditions present a future challenge for catalyst scientists, although super acids may be one solution. Hierarchical solid acids may be employed to first hydrolyze TAGs, and then esterify the resulting FFAs to FAME. Process optimization needs collaboration between catalyst chemists, chemical engineers, and experts in molecular simulation to take advantage of innovative reactor designs. The future of biodiesel requires a concerted effort from chemists and engineers to develop catalysts and reactors in tandem. It is essential that technical advances in both materials chemistry and reactor engineering are pursued if biodiesel is to remain a key player in the renewable energy sector during the 21st century.

Acknowledgments We thank the EPSRC under EP/K000616/1, EP/F063423/1 and EP/G007594/3 for financial support and a Leadership Fellowship (AFL), and the Royal Society for the award of an Industry Fellowship to KW. NAT would also like to thank the Malaysian Ministry of Higher Education for Scholarship Funding throughout her research years.

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