INTRODUCTION:
Majority of the world’s energy needs are supplied through petrochemical sources, coal and natural gases, with the exception of hydroelectricity and nuclear energy. Of all, these sources are finite and at current usage rates will be consumed shortly1. Diesel fuels have an essential function in the industrial economy of a developing country and used for transport of industrial and agricultural goods and operation of diesel tractor and pump sets in agricultural sector. Economic growth is always accompanied by commensurate increase in the transport. The high energy demand in the industrialized world as well as in the domestic sector and pollution problems caused due to the widespread use of fossil fuels make it increasingly necessary to develop the renewable energy sources of limitless duration and smaller environmental impact than the traditional one. This has stimulated recent interest in alternative sources for petroleum-based fuels. An alternative fuel must be technically feasible, economically competitive, environmentally acceptable, and readily available. One possible alternative to fossil fuel is the use of oils of plant origin like vegetable oils and tree borne oil seeds. This alternative diesel fuel can be termed as biodiesel.
It is defined by the World Customs Organization (WCO) as “a mixture of mono-alkyl esters of long-chain (C16-18) fatty acids derived from vegetable oil or animal fat, which is a domestic renewable fuel for diesel engines and which meets the specifications of ASTM D6751. Table 1 compares the ASTM standards for Diesel and biodiesel2.
Table 1: Comparison of the standards for diesel and biodiesel based on American Society for Testing and Materials (ASTM)
|
Property |
Diesel |
Biodiesel |
|
Standard Number Composition
Specific gravity (g/mL) Flash point (K) Cloud point (K) Pour point (K) Water (vol%) Carbon (wt %) Hydrogen (wt %) Oxygen (wt %) Sulphur (wt %) Cetane number |
ASTM D975 Hydrocarbon (C10-C21) 0.85
333-353 258-278 243-258 0.05 87 13 0 0.05 40-55 |
ASTM D6751 Fatty acid methyl ester(C12-C22) 0.88
373-443 270-285 258-289 0.05 77 12 11 0.05 48-60 |
Biodiesel though possesses all the favorable characteristics of diesel, scores over diesel in emission characteristics. In Table 2, the emission characteristics of pure biodiesel (B100) and that of 20% blend with 80% diesel (B20) are given relative to that of diesel.
Table 2: The average emissions of B100 and B20 20% blend with diesel as compared to normal diesel (in percentages)3
|
Emission |
B100 |
B20 |
|
Carbon monoxide Total unburnt hydrocarbons Particulate matter Nitrogen oxides Sulphates Air toxics |
- 48 - 67 - 47 + 10 - 100 - 60 to – 90 |
- 12 - 20 - 12 + 2 - 20 - 12 to - 20 |
It is often claimed that biodiesel has many environmentally beneficial properties. The main aspect of this claim is that it is a ‘carbon neutral’ since no net amount of carbon dioxide is released to the atmosphere4. However, this carbon neutrality is with respect to the amount of carbon dioxide produced by combustion of biodiesel and the amount of carbon dioxide absorbed by the plants for producing the biomass. However, this carbon neutrality is not maintained in other concurrent operations like fertilizer production, esterification, solvent extraction, refining, drying and transportation. To be able to assess the net balance in carbon neutrality one has to carry out a detailed life cycle analysis. In spite of these factors, biodiesel still has some advantages over the conventional fossil fuel based diesel. These considerations are presented in table 3.
Table 3: Advantages of Biodiesel over conventional petroleum based diesel
|
Parameters |
Characteristics exhibited by biodiesel in comparison to petroleum based diesel |
|
Engine modific ation
|
Blends of 20% biodiesel with 80% petroleum diesel (B20) can be used in unmodified diesel engines. The use of neat biodiesel (B100) may require certain engine modifications to avoid performance and maintenance problems. |
|
Source
|
One half of biodiesel can be manufactured from recycled oil or fat and the other half can be harnessed from sources like soya bean, rape seed oil, etc |
|
Toxicity |
Biodiesel is non toxic and biodegradable. Nearly 80% less carbon dioxide emission is seen, but emission of nitrogen oxides (precursor of ozone) is higher. |
|
Cetane number |
Biodiesel has a Cetane number of 100 as compared to the value of 40 for diesel. This parameter is related to ignition quality and hence biodiesel will allow cold starts and less idle noise |
|
Life of engine |
Biodiesel being a better lubricant can extend the life of the engine |
|
Odor |
Biodiesel provides a pleasant smell of popcorn or French fries |
Four primary production methodologies for producing biodiesel have been studied extensively. These include direct use and blending of oils, microemulsion, pyrolsis and transesterification5. Tranesterification process has been widely used to reduce the high viscosity of triglycerides and is a topic of great interest. Transesterification is one of the reversible reactions and proceeds essentially by mixing the reactants. However, the presence of a catalyst (a strong acid or base) accelerates the conversion.
Transesterification (Alcoholysis):
Transesterification is the general term used to describe the important class of organic reactions where an ester is transformed into another by interchange of the alkoxymoiety6-8.
The purpose of the transesterification process is to lower the viscosity of the oil. The first account of what we know as biodiesel today can be traced back to a patent of the University of Brussels (Belgium) describing the alcoholysis (often referred to as transesterification) of vegetable oils using ethanol (EtOH) in order to separate the fatty acids from the glycerol by replacing the glycerol with short linear alcohols9. Since then biodiesel production, feedstock & fuel properties, standard have been extensively studied, documented and critically reviewed10-14. The transesterification reaction can be catalyzed by both homogeneous and heterogeneous catalysts. In turn, the homogeneous catalysts include alkalis and acids. The most commonly used alkali catalysts are sodium hydroxide, sodium methoxide and potassium hydroxide. In this sense, numerous references can be found in the background literature. Sulfuric acid, hydrochloric acid and sulfonic acid are usually preferred as acid catalysts. The catalyst is dissolved into the methanol by vigorous stirring in a small reactor. The oil is transferred into the biodiesel reactor, and then the catalyst and alcohol mixture is pumped into the oil. The final mixture is stirred vigorously for 2 hr at 340K in ambient pressure. A successful transesterification reaction produces two liquid phases: ester and crude glycerin. Crude glycerin, the heavier liquid, will collect at the bottom after several hours of settling. Phase separation can be observed within 10 min and can be completed within 2 hr of settling. Complete settling can take as long as 20 hr. After settling is complete, water is added at the rate of 5.5% by volume of the methyl ester oil and then stirred for 5 min, and the glycerin is allowed to settle again. Washing the ester is a two-step process, which is carried out with extreme care. A water wash solution at the rate of 28% by volume of oil and 1 g of tannic acid per liter of water is added to the ester and gently agitated. Air is carefully introduced into the aqueous layer while simultaneously stirring very gently. This process is continued until the ester layer becomes clear. After settling, the aqueous solution is drained, and water alone is added at 28% by volume of oil for the final washing11–13.
First generation homogenous catalyst: Traditional or first generation homogeneous catalysts enjoy certain advantages over other catalysts including cost effectiveness, high activity, and easily attained reaction conditions (25–1300C, atmospheric pressure). However these same homogeneous catalysts, by virtue of the associated production process, face a variety of technical hurdles that limit their use for biodiesel production and eventually may cause the demise of the early biodiesel producers. Homogeneous catalysts are normally limited to batch-mode processing15. In addition, other steps in the biodiesel production process are time consuming and costly processing. These steps include oil pretreatment, catalytic transesterification, separation of Fatty Acid Methyl Ester (FAME) from crude glycerin, neutralization of waste homogeneous catalyst, distillation of accessory methanol, water washing of the FAME phase, and vacuum drying of the desired products16. Each of these steps introduce additional processing time and cost. As an example, separation of the products from the spent waste catalyst requires a post treatment with large volumes of water to neutralize the used catalyst in the product mixture. This creates an additional process burden by generating waste water that must be treated before release into the environment16. Other difficulties with using homogeneous catalysts center on their sensitivity to free fatty acid (FFA) and water in the source oil. FFAs react with basic catalysts (NaOH, KOH) to form soaps when the FFA and water content are above 0.50% and 0.06%, respectively17,18. This soap formation complicates the glycerol separation, and reduces the FAME yield. Water in the feedstock results in the hydrolysis of FAME in the presence of strong basic or acidic catalyst. Thus, some inexpensive oils, such as crude vegetable oils, waste cooking oil, and rendered animal fats, which generally contain a high content of FFA and water, cannot be directly utilized in existing biodiesel facilities with homogeneous catalysts. Likewise, the cost of oil feedstock in 2006 accounted for up to 80% of biodiesel production cost19, 20. So when petroleum diesel prices fell in 2008, the relatively expensive soybean derived biodiesel could not compete, forcing many biodiesel facilities to close. Therefore, part of the current solution is to develop a second generation technology based on heterogeneous catalysts that are capable of effectively processing less costly feed stocks high in FFAs and water content with a simpler and less costly processing method. However, production costs are still rather high compared to petroleum-based diesel fuel21. There are two main factors that affect the cost of biodiesel: the cost of raw material and the cost of processing. Processing costs could be reduced through simplified operations and eliminating waste streams. A solution to this problem could be transesterification in supercritical methanol without using any catalyst.22,23 As a matter of fact, in this case, the reaction is very fast (less than 5 min) and the absence of catalyst decreases downstream purification costs.22,23. Even if some production plants use this technology, the reaction requires very high temperatures (350–400 °C) and pressures (100–250 bar) and thus high capital costs.
Second Generation Heterogeneous Catalyst Advancements:
Recent developments in heterogeneous catalysis for biodiesel production has the potential to offer some relief to the biodiesel producers by improving their ability to process alternative cheaper feed stocks, and to use a shortened and less expensive manufacturing process. While homogeneous based process required batch mode operation, heterogeneous processes can be run in either batch or continuous mode giving the producers the option to continue with their current batch reactors or retrofit their operations with a packed bed continuous flow reactor operation. Heterogeneous catalysts in either mode are in a separate phase from the reaction products, thereby removing costly and time consuming water washing and neutralization steps to separate and recover the spent catalyst. Additionally, contaminated water from that process is greatly reduced and the need for waste water treatment minimized. The greatest advantage of the heterogeneous approach over the homogeneous method is the prolonged lifetime of the heterogeneous catalysts for FAME production. This attribute is generally related to the stability of the microcrystal structure of the catalyst surface. Poisoning and leaching of catalyst components can change the bulk and surface structure of the catalyst and cause catalyst deactivation quickly, if the catalyst is not formulated properly. The above three factors, catalytic activity, catalyst life and oil flexibility have a tremendous impact on the cost of biodiesel. The reaction conditions could be less drastic than the methanol supercritical process.
The reported heterogeneous catalysts are separated into two categories based on their operation temperature. For reaction temperatures lower than the flash point of biodiesel (1300C), this type of catalyst was referred to as a low temperature catalyst. For reaction temperatures greater than (1300C), were classified as high temperature catalysts. These catalysts are characterized by the need for additional safety considerations and require more energy intensive operations.
Low temperature catalysts:
As previously stated, studies of solid base catalysts began burgeoning in the 1970 s. Most dealt with common single Solid base catalysts metal oxides such as alkaline oxides and rare earth metal oxides. Subsequently, studies were expanded to include alkali metal exchanged zeolites, alkali metal ion-supported catalysts, and clay minerals such as hydrotalcites.
Solid Base Catalysts:
Alkaline Metal Salts on Porous Supports:
Alkali metals are the most common source of super basicity and are frequently selected as the active species for biodiesel production. The loading of many kinds of alkaline salts on supports have been reported as a way to prepare basic catalysts, such as NaOH24,25–28, KOH29,30, K2CO331, KI32, 33, KNO334, 35, KF36–39, and LiNO340,41. The supports for these catalysts include Al2O334, 38, Zeolite42, ZnO40 and SiO233.
An example of a commercialized super base catalyst is Na/NaOH/Al2O3. Kim et al.28 also tested its activity for soybean oil transesterification with methanol and found almost the same activity as homogeneous NaOH catalyst under optimized reaction conditions (FAME yield was 94% with a reaction temperature of 600C, reaction time of 2 hr, stirring speed 300 rpm, co-solvent n-hexane 10ml, amount of catalyst 1 g). The basicity is believed to be associated with the Lewis base concept according to the O 1s XPS results presented. Table 4 shows the effect of preparation on catalyst basicity. The oxygen 1s binding energy shifts downward as the Na and NaOH impregnation onto the γ -Al2O3 support progresses, indicating that the basicity increases together with the degree of impregnation. Consistent with O 1s binding energy, the catalyst’s FAME activity also proportionally increases. Work was also reported using a catalyst consisting of a mixed oxide of zinc and aluminum which promotes the transesterification reaction without catalyst loss. The reaction is performed at a higher temperature than homogeneous catalysis processes, with an excess of methanol.
Table 4 XPS analysis of O 1s orbital of four catalysts
|
Catalyst
|
Binding energy of O 1s (eV) |
Biodiesel yield (%) |
|
γ-Al2O3 |
538.8 |
5 |
|
NaOH/γ-Al2O3 |
538.5 |
60 |
|
Na/ γ-Al2O |
537.5 |
70 |
|
Na/NaOH/ |
535.5 |
78 |
|
γ-Al2O3 |
|
|
Xie et al.34 and Vyas et al.35 investigated the activity of KNO3/Al2O3. Xie et al.34 pointed out that the active phase was K2O derived from KNO3 at high temperature, and the surface Al–O–K groups were the main active sites. Cui et al.38 prepared KF/γ-Al2O3 and found there were two types of basic sites on the catalyst. The strong basic sites (super basic) promote the transesterification reaction at low temperature (650C), while the basic sites with medium strength require a higher temperature to process the reaction. Later, Boz et al.39 prepared KF catalysts loaded on nano-c-Al2O3 and Wang et al.43 loaded KF on malodorous CaO–MgO. They both found that the catalyst’s FAME activity is closely related to the basic nature of the catalyst and also to the high surface to volume ratio and porosity of the catalyst.
Reaction conditions: Methanol/oil molar ratio is 6:1, reaction temperature is 60°C, and stirring speed is 300 rpm. All data are taken from literature28.However, in spite of the high activity of the supported alkaline catalysts, they have important limitations. First, these catalysts like their homogeneous alkaline hydroxide counterparts have a low tolerance to FFA and water in raw materials. At this time, there is no report of using this kind of catalyst for directly processing crude oils which have a high Total Acid Number (TAN). Only refined oils can be used with these catalyst systems. Secondly, some researchers have observed lixiviation of catalyst components into reaction mixtures. Arzamendi et al.44 found that 55% of K2CO3, 20% of Na2CO3 and 15% of Na3PO4 dissolved into the reaction mixtures and catalyzed the transesterification reaction. Also, Xie et al.34 found KNO3/Al2O3 catalysts have a high solubility in water and were therefore unstable in the transesterification system.
However, Noiroj et al.45 found that the type of support strongly affected the activity and leaching of the active species of the catalyst. In this case, the amount of leached potassium of the KOH/Al2O3 was higher than that of the KOH/NaY catalyst. And they found that the interaction between active phase and support affected the leaching results. Additionally, Ramos et al.46 prepared sodium hydroxide on a zeolite support and hypothesized the presence of a homogeneous-like mechanism where the alkali methoxide species were leached out.
Alkaline Earth Metal Oxide Catalysts:
Much attention has been paid to alkaline earth metal oxides since they have shown less solubility in reaction mixtures and less corrosion in comparison to supported alkaline catalysts. As solid super base can be synthesized from alkaline metal oxides, researchers started from pure alkaline metal oxides. In fact, alkaline metal oxides have already been used as base catalysts in many organic reactions. For example CaO is widely used for the isomerization of 5-vinylbicyclo hept-2-ene (VBH) to 5-ethylidenebicyclo hept-2-ene (ENB)47,48, synthesis of 1, 3-dialkylurea from ethylene carbonate and amine51 and the synthesis of monoglyceride49. With respect to biodiesel production, the basicity of this type of metal oxide catalyst has been shown to have an influence on its activity for FAME generation. The basic strength of the Group II metal oxides follows the order: MgO > CaO > SrO > BaO .Corresponding research has demonstrated the catalyst’s activity for transesterification of oil with methanol follows the same order50–52. But, compared to a homogeneous NaOH catalyst, the above alkaline earth metal oxides show a relatively low transesterification activity. In particular, MgO exhibits almost no activity in transesterification of vegetable oils into biodiesel. Pure CaO reacts at a slow rate and requires about 6–24 hr. to reach a state of reaction equilibrium53–55. BaO is not suitable for biodiesel production because it dissolves in methanol and forms some noxious species56,57. Conversely, SrO, has a high activity and is insoluble in methanol, but will react strongly with CO2 and water in the air to form unreactive SrCO3 and Sr(OH)2. Furthermore, these strontium compounds are difficult to regenerate by calcining, requiring temperatures above 12000C58. As a partial solution to these limitations, recent work has focused on using mixed metal oxides to enhance the basicity of CaO or MgO-based catalysts and elevate their respective selectivity for FAME.
Supported CaO Catalysts:
Although earlier work showed weak FAME activity for pure CaO catalysts, more recent research has shown that the smaller particle size of CaO catalysts can increase the total amount of base sites and base strength, which leads to an improved activity in the oil transesterification reaction. Reddy et al.59 tested the activity of nano crystalline CaO and found it active even at room temperature. However, as Gryglewics60, 61 and Martyanov and Sayari62 pointed out, pure CaO converted to form a suspensoid due to its poor mechanical strength, which would lead to difficulties in separating the waste catalyst from biodiesel and glycerol products after transesterification. Since these findings could have a potential impact on industrial applications, many researchers have tried to solve this problem by applying CaO on different metal oxide supports. In particular, CaO has been combined with ZnO63, MgO44, 64, Al2O364, 65, Zeolite46, 64, SiO264, 66, and La2O367 with improved base characteristics, activity and catalytic life. The calcined calcium zincates was used as a solid catalyst for the methanolysis of sunflower oil to FAME resulting in yields higher than 90% after 45 min of reaction63. The reaction conditions of the heterogeneous process (600C, methanol: sunflower oil molar ratio of 12, 3 wt% catalyst) were very similar to those observed under homogeneous conditions (KOH dissolved in methanol). Yan et al.64 investigated the effects of a second metal oxide by impregnating CaO on basic oxides such as MgO, neutral oxides such as SiO2, and acidic oxides such as Al2O3 and zeolites HY. In this work, the best results were obtained for a catalyst with 16.5% of CaO loading on MgO. The same catalyst also possessed the strongest base strength and largest number of base sites. The conversion of rapeseed oil using this catalyst reached 92% at 64.50C. Further work by Yan et al67. revealed the active centers of the CaO/MgO catalyst. CO2-TPD profiles of CaO/MgO showed that there were two types of basic sites each with a different strength. The desorption peaks of CO2 at 6000C were attributed to the strong basic sites corresponding to unbounded O2- anions, while CO2 desorption peaks at low temperature (3500C) were attributed to the weak basic sites related to oxygen in both Ca2+–O2- and Mg2+–O2- pairs. Yan et al.58,64,67 also quantified the effects of base property on oil transesterification. They found that the activity of CaO/MgO linearly increased with base amount, and base amount linearly increased with CaO loading when the CaO loading was lower than 16.5%. Later, Yan et al.68 combined CaO with the basic oxide La2O3, and used the Hammett method to determine the base properties of catalysts. They found that the binary metal oxides had a higher base strength and a wider base site distribution than pure CaO and La2O3 catalysts individually, and they also had a higher reactivity than CaO and La2O3 individually. Using this binary metal oxide catalyst with a 3:1 of molar ratio of Ca to La, they found the FAME yield reached 94.3% within 60 min at just 580C. This suggested a reaction rate much closer to that of a homogeneous NaOH catalyzed processes and that heterogeneous catalysts were capable of attaining the same activities as homogeneous catalysts. Equally important here, this work indicated that CaO type heterogeneous catalysts also showed a high tolerance to water and FFA which is present in unrefined raw oil feedstocks. This implies these heterogeneous catalysts have a potential for biodiesel production. Until now, many CaO based catalysts were reported to be more tolerant than the supported alkaline catalysts. Yan et al.64 reported that conversion of rapeseed oil by CaO/MgO reached as high as 98% when the water content of the raw oil was in the range of 0–2 wt% and the total acid number was below 7.4 mg KOH/g (FFA content around 3.7%). Later, Yan et al.68 reported that CaO–La2O3 was active when the oil contained 10% of water and when the FFA content was lower than 3.5%. They then tested the basicities of the catalysts which had adsorbed small amounts of FFA and water, respectively, naming the catalysts Ca3La1–FFA andCa3La1–water. Yan and co-workers68 found that the basic properties of the Ca3La1–water catalyst are much closed to the fresh CaO–La2O3 catalyst; therefore, it can be assumed that CaO–La2O3 shows a high tolerance to small amounts of water, while the basic property of the Ca3La1–FFA catalyst notably decreased both the base strength and sites, indicating CaO–La2O3 has a low tolerance for FFA in oils. Further characterization results indicated that there are Lewis base sites and Bronsted base sites on the surface of fresh CaO–La2O3 catalysts and both of these base sites are active centers for oil transesterification with methanol. Infact, the addition of small quantities of water can change the Lewis base sites into Bronsted base sites suggesting Ca3La1–water is still active in transesterification. Conversely, FFA will react with and bind to both base sites resulting in poisoning of the catalyst. Therefore, Ca3La1–water FFA shows a low activity for FAME production. Further work by Yan et al.68 using CaO–La2O3 for processing crude soybean oil, crude palm oil and waste cooking oil, which satisfy the limitation of water content lower than 10% and FFA lower than 3.5%, showed a FAME yield in excess of 95% within 3 hr. For the oils with a high FFA content, dilution with refined oil will lower the FFA content of the mixture again allowing the use of a CaO–La2O3 catalyst to produce a high FAME yield. Equally important to the usefulness of a catalyst is its lifetime. Reddy et al.59 found that nano crystalline calcium oxide particles deactivated after eight cycles with soybean oil and after only three cycles with higher FFA content poultry fat. Similarly, Kawashima et al.54,55 found evidence of decreased FAME activity with CaTiO3 and CaZrO3 catalysts after as few as three cycles. To overcome these limitations, therefore, it is important to understand the mechanism of catalyst deactivation. In general, there are three pathways by which catalyst deactivation can occur: poisoning, blocking of reactant fragments and lixiviation. Many studies have already paid close attention to poisoning due to high levels of either FFA or water present in raw materials64,68, so they will not be discussed here. But little research has been performed that focuses on the effects of exposing a stored catalyst to CO2, moisture, and O2 present in the air. As stated by Busca69, base catalysts can easily react with these components of ambient air to form very stable surface species like carbonate, hydroxide, and epoxide which cover the basic sites and deactivate the base catalysts. Yan et al.70 found that when comparing the activity of a fresh CaO–La2O3 catalyst to one exposed to air for 12 hr, the yield of FAME sharply decreased from 96.8 to 34.5%, and the total base amount decreased from 14.0 to 1.5 mmol CO2/g. They later attributed this decrease in activity to moisture and CO2 in air restructuring the surface of CaO–La2O3 catalyst from metal oxides to hydroxide and carbonate. Other research groups have found that the reused base catalysts have a lower basic strength and a lower activity than the fresh catalysts71,72. This was explained by the blockage of active surface sites on the catalyst by strongly adsorbed intermediates or product species. In particular, Martyanov and Sayari62 studied reused catalysts (CaO, Ca(OCH3)2) and found that surface adsorbed butyric acids were the most likely species responsible for the catalyst deactivation. Catalyst lixiviation or leaching is another frequently encountered pathway for catalyst deactivation. Many researchers have studied the leaching of catalyst components into reaction mixtures. Kouzu et al. using a pure CaO catalyst71 found that the calcium concentration would be as high as 3065 ppm in the FAME when waste cooking oil was used as the source oil. Later Kouzu et al.72 showed calcium in the products of transesterified refined soybean oil. In that case, he found that the calcium content in glycerol was about 2000 ppm and calcium content in FAME was around 10–100 ppm. Similarly, Granados et al.73,74 studied the leaching of species from solid CaO and the role of these species in the catalytic reaction. He pointed out that CaO can react with glycerol to form Calcium glyceroxide which is more soluble than CaO and active in oil transesterification. He found that the solubility of CaO in alcohol is around 0.6 mg/mL and when CaO was less than 1 wt% the major reaction mechanism is homogeneous. When the catalyst loading is greater than 1 wt% CaO, the total homogeneous contribution is much smaller than that arising from the heterogeneous sites74. Thus, it is very apparent that determining the homogeneous contribution to the FAME yield of a CaO-based catalyst system, and other like catalysts, is as important as quantifying the heterogeneous component.
Supported MgO Catalysts:
One of the drawbacks of using a CaO-based catalyst is the low BET surface area associated with the catalyst. Because activity is closely related to surface area for many catalyst systems, loss of active surface area through deactivation can have a proportionally larger effect on the product yield. Therefore, one solution would be simply to use more catalyst in industrial application design. However, this introduces additional cost into the plant design and material usage. To avoid these concerns some research groups have turned to metal oxide catalysts from hydrotalcites which are well dispersed, have a high surface area, and are characterized by a strong base property. One such example is a MgO based catalyst. Cantrell et al.52 reported on a supported MgO–Al2O3 catalyst which was active in transesterification of glyceryl tributyrate using methanol. He found that the BET surface area was as high as 166 m2/g.Using the same catalyst, Xie et al.75 found that the Mg/Al molar ratio also had an effect on FAME activity. Using a Mg/Al ratio of 3, 773 K calcination temperature, 15:1 M ratio of soybean oil: methanol, and a catalyst dosage of 7.5 wt%, oil conversion was found to be 67% after 9 hr. Other work by Li et al.76, using mixed oxides from Mg–Co–Al–La hydroxide, found those catalysts maintained its activity for 7 recycles in a batch reactor. Additional work on the MgO–Al2O3 catalyst, by Fraile et al.77, found that the reaction mechanism relied on the residual alkaline ions as the main source of strong basicity and catalytic activity in the transesterification of sunflower oil with methanol.
Base Resin Catalysts:
Not only inorganic bases, but also some organic bases were also tested for biodiesel production78,79, especially for base resin catalysts. Shibasaki-Kitakawa et al.80 investigated the transesterification of triolein with ethanol using various commercial resin catalysts. They found that the anion-exchange resins, such as Diaion PA308, PA306s, HPA25 (Mitsubishi Chemical C., Ltd, Tokyo, Japan), exhibited much higher catalytic activity than the cation exchange resins like PK208 (Mitsubishi Chemical C., Ltd, Tokyo, Japan). The anion-exchange resins characterized by a lower cross-linking density and a smaller particle size produced both a high reaction rate and conversion. The best catalytic performance was obtained on Diaion PA306s resin, which yielded over 80% conversion of soybean oil to ethyloleate after 3 hr reaction at 323 K. However, other groups have found less satisfactory results using base resin catalysts. In particular, Aracil and coworkers81 used an anion-exchange resin in the transesterification of sunflower oil to biodiesel and found the conversion was less than 1% after 8 h at a typical reaction temperature of 333 K. Kimet al.82 found that trace amounts of CH3ONa, functioning as a homogeneous catalyst, exhibited a synergetic effect with the resin catalyst for conversion.
Biont Shell Based Catalysts:
Recently, catalysts derived from renewable materials, such as shrimp shell83,84, turtle shell83, crab shell83,oyster shell85 and egg shell86 have been employed for conversion of oils to FAME. Previously, these catalysts were generally considered as waste. The major components of these biont shells are chitin, protein and CaCO3. Normally, disposal of these waste materials from seafood processing are an economic or environmental problem for entrepreneurs and local governments. However, biodiesel production catalysts prepared from these ‘‘wastes’’ are a promising ‘‘green’’ technology. Xie et al.83 first reported preparing biont shell supported KF catalysts for biodiesel production and found methyl ester yields from rapeseed oil as high as 97.5% within 3 h under optimal reaction conditions. Yang et al.84 reported, a three step preparation procedure for a shrimp shell catalyst which included incomplete carbonization of the shrimp shells, loading KF onto the altered shells, and activation. In a different approach, Nakatani et al.85 and Wei et al.86 prepared CaO catalysts by simple calcination of oyster shells and egg shells, high in CaCO3(95%), at 700–10000C. Although these catalysts proved active for biodiesel synthesis, further work is required to limit the amount of Ca leaching to improve the catalyst life and tolerance to water and FFA in oil feedstock.
Solid Acid Catalysts:
Even though many heterogeneous base catalysts have been reported as highly active for biodiesel synthesis, they still cannot tolerate acidic oils with FFA content [3.5%] such as yellow and brown grease. However, sulfur based acidic Top Catal and homogeneous catalysts such as H2SO4 show a much higher tolerance to FFA and water than the basic homogeneous NaOH and KOH catalysts, suggesting these catalysts may be more suited for processing acid oils. Using this line of reasoning, some researchers turned to investigating sulfur based heterogeneous acid catalysts for converting acidic oils into biodiesel.
Sulfated Zirconia Based Catalysts:
Sulfated metal oxides show super acid properties because of the interaction between the sulfate group and the metal oxide centers. These kinds of catalysts, including sulfated zirconia87–89 and sulfated tin oxides90 have been widely used in esterification and transesterification reactions under mild conditions. Kiss et al.87 studied several solid acid catalysts (zeolites, ion-exchange resins, and mixed metal oxides) as catalysts for the esterification of dodecanoic acid with 2-ethylhexanol, 1-propanol, and methanol. That work revealed that sulfated zirconia was the most active for esterification. Later, Garcı´a et al.91 investigated the activity of sulfated zirconia for soybean oil transesterification. That group found that the catalyst preparation method had a significant effect on the resulting catalyst activity. Under optimized conditions (120 0C , 1 hr and 5 wt% of catalyst) and using sulfated zirconia prepared by a solvent-free method, the methanolysis of soybean oil was 98.6% and ethanolysis was 92.0%. The sulfated zirconia prepared by standard methods88 was poor catalyst for soybean oil methanolysis (conversion of 8.5%) and conventional zirconia even less. Similarly, Suwannakarn et al.89 studied the activity and stability of a commercial sulfated zirconia catalyst for transesterification of tricaprylin with a series of aliphatic alcohols at 1200C. He found that the catalytic activity decreased as the number of carbons in the alkyl chain of the alcohol increased. In addition, the sulfated zirconia catalyst exhibited significant activity loss with subsequent reaction cycles. Characterization of the recycled catalysts showed that the concentration of the SO42- moieties in the sulfated zirconia had permanently decreased. Essentially, the SO42- species were leached out. As explained by Yadav and coworkers92,93, the sulfate groups leached out as H2SO4 and HSO4-, which in turn gave rise to a homogeneous acid catalysis which interfered with activity measurements of the intended heterogeneous catalyst.
Heteropolyacid Catalysts:
A series of heteropolyacid (HPAs) catalysts
have also attracted much attention due to their high activity in biodiesel
formation reactions, both transesterification and esterification. Alsalme et
al.94 studied some HPA catalysts and compared them with some
homogeneous and heterogeneous catalysts such as H2SO4,
Amberlyst-15, and zeolites HY and H-Beta. The intrinsic catalytic activity,
expressed as turn over frequency (TOF), of the HPA catalyst is significantly
higher than that of the conventional acid catalysts in these reactions. They
also tested the catalytic activity and acid strength of several kinds of HPA
catalysts. The TOF values decreased with decreasing catalyst acid strength in
the order: H3PW12O40
Cs2.5H0.5PW12O40
> H4SiW12O40 > 15% H3PW12O40/Nb2O5,
15%H3PW12O40/ZrO2, 15% H3PW12O40/TiO2
[H2SO4[HY, H-Beta[Amberlyst-15. They found that Cs2.5H0.5PW12O40
exhibits high catalytic activity as well as high resistance to leaching. The
other types of supported HPA catalysts suffered from leaching and exhibited a
significant homogeneous component to the catalyst’s activity caused by the
leached HPA. Pesaresi et al.95 studied the catalytic mechanisms of
CsxH4-x SiW12O40 (x = 0.8–4) in the
transesterification of C4 and C8 triacyglycerides and
esterification of a C16 FFA. The catalyst material, loading C1.3
Cs per Keggin, provided an insoluble, heterogeneous catalyst active for
both transesterification and esterification, with reactivity correlating with
the number of accessible H+ sites residing within the mesopore
structure.For loadings B0.8 Cs per Keggin, transesterification activity arises
from the homogeneous contribution. Narasimharao et al.96
investigated structure related activity for CsxH3-xPW12O40
(x = 0.9–3). Materials with the Cs content in the range x = 2.0–2.7 were well
dispersed, having a high surface areas *100 m2/g-1 and
high Bronsted acid strength. CsxH3-xPW12O40
was active in both esterification of palmitic acid and transesterification of
tributyrin. Further work showed an optimum performance occurs for Cs loadings
of x = 2.0–2.3, correlating with the accessible surface acid site density. These
catalysts were recovered for three times and leaching of soluble
heteropolytungstate wasn’t observed. Other HPAs were also reported. Katada et
al.97 found that H4PNbW11O40, H3PW12O40
and the heteropolyacid derived solid acid catalyst, H4PNbW11O40/WO3–Nb2O5,
were highly active for the transesterification of triolein with ethanol. But,
H4PNbW11O40 and H3PW12O40 dissolved into the
reaction mixture; H4PNbW11O40/WO3–Nb2O5
was insoluble to the reaction mixture. Further study showed that the activity
of H4PNbW11O40/WO3–Nb2O5
was sensitive to calcination temperature, and calcination around 773 K provided
a highly active catalyst. The activity was observed in the co-presence of water
(3.9 wt%) and oleic acid (5 wt%). In a fixed-bed continuous-flow reaction, it
maintained the yield of FAME around 25–40% for 4 days.
Organically-Functionalized Acid Catalysts:
The purpose of preparing organically-functionalized acid catalysts is to overcome the shortcomings of other acid catalysts, such as leaching and low surface area. Some attempts have been made with the sulfonic acid ion exchange resins, such as Poly (DVB) resin sulfonated with H2SO498, Amberlyst-35(Rohm & Haas)98, Amberlyst-15 (Rohm & Haas)94,99, and Nafion SAC-13100. Rezende et al.98 prepared different polymer supports based on styrene and divinylbenzene which were conveniently functionalized with sulfonic acid. In order to obtain an appropriate triglyceride conversion at low temperature (650C), it was necessary to use a high ratio of methanol to oil (50:1–300:1) and high catalyst dosage (25–50%). Under the optimal conditions FAME yields reached a maximum value over 90% using a sulfonated poly (DVB) ion exchange resin which had 442 m2/g of specific surface area and 3.4 meqH+ g-1 of acid capacity. Some polymer based catalysts were claimed to be active for both oil transesterification and fatty acid esterification reaction in unrefined oil systems. As an example, Soldi et al.101 prepared sulfonated polystyrene compounds where sulfonation was between 5.0 and 6.2 mmol SO3H/g of dry polymer. That work showed conversion of beef tallow, with a 53 mg KOH/g acid number, reached 70% within 18 hr.
Natural Based Catalysts:
A novel type of renewable catalyst has been prepared from various carbohydrates such as D-glucose, sucrose, cellulose and starch102–104. These catalysts were made by incomplete carbonization of carbohydrates followed by sulfonation. The incomplete carbonization of D-glucose leads to a rigid carbon material consisting of small polycyclic aromatic carbon sheets in a three dimensional sp3-bonded structure103. Sulfonation has been demonstrated to provide a highly stable solid with a high density of active SO3H sites. This type of catalyst was found to be physically robust without leaching of SO3H groups during use. This resulted in remarkable catalytic performance for FAME formation reactions for both transesterification and esterification103,105,106. In studies by Lou et al. [106], carbohydrate-derived acid catalysts had been successfully applied to biodiesel production with higher fatty acid oils, such as waste oils with high acid values. Variables such as starting material, carbonization temperature and time, and sulfonation temperature and time for catalyst preparation all had a significant impact on the catalytic and textural properties of the prepared solid acids. Under optimal reaction conditions (800C, 20: 1 of molar ratio of methanol to oil, 10 wt% of catalyst loading, over a starch-derived sulfonic acid catalyst), the FAME yield was measured at about 92% after 8 hr’s reaction. Furthermore, the starch derived solid acid catalyst proved exceptionally stable under reaction conditions.
High Temperature Catalysts:
Even though solid acid catalysts exhibit improved activity for converting acid oils into FAME, most of them show a relative low reaction rate and deactivate quickly in comparison to solid base catalysts. Intensifying reaction conditions, by increasing reaction temperature and pressure, has been shown to effectively accelerate the reaction rate and prolong the catalyst lifetime. Some of the more successful examples of these catalysts that function at higher temperature and pressure, including both solid base and acid catalysts, were tested in subcritical or supercritical methanol flow conditions (240 0C, 8 MPa) and reported below.
Solid Base Catalysts:
Several strong base catalysts have been tested at high reaction temperatures. One of which, CaO investigated by Demirbas107, was operated under supercritical methanol conditions. When the temperature was 2520C, transesterification was completed within 6 min with 3 wt% CaO and 41:1 methanol/oil molar ratio. In other work on Ca based catalysts, Suppes et al.108 found that CaCO3 was active when the temperature was greater than 2000C and required about 18 min to essentially convert all the oil; FFA in the oil was esterified by CaCO3 and did not appear to inhibit the catalyst; Also, no decrease in activity of the calcium carbonate was observed after weeks of utilization suggesting little leaching or deactivation. Separately, Barakos et al.109 used both non-calcined and calcined Mg–Al–CO3 hydrotalcite catalysts in refined cottonseed oil, acidic cottonseed oil, and crude animal fat feedstock. Mg–Al–CO3 hydrotalcite catalysts were active in both transesterification and esterification. The activity of the calcined catalyst was lower than the non-calcined catalyst. But, the non-calcined catalyst showed evidence of deactivation when recycled. However, additional information regarding catalyst leaching and stability was not presented in this work.
Solid Acid Catalysts:
(a) Sulfate Salts:
As with the low temperature catalysts, the activity of the sulfated salt family of catalysts is based on the presence of sulfonic acid sites, which can be considered as the heterogeneous counterpart of sulfuric acid. Also, the acid strength of the catalyst still has an important role in the transesterification reaction. Some factors, such as the preparation technology and suitable selection of support, greatly influenced the acid site distribution on the catalyst.
For instance, Jothiramalingam and Wang15 reported that catalysts prepared from a stronger acid precursor containing benzene sulfonic acid groups had a higher acid strength and were more active than those containing only propylsulfonic acid groups. Chen et al.110 presented evidence that good catalytic performance of the sulfated silica-zirconia material was attributed to an improved preparation process which resulted in a higher dispersion of zirconia, thus creating a higher acid site density. The carriers for sulfonic acid include not only some inorganic metal oxides (zirconia oxide15,110, tin oxide111, stannia112), but also some meso structured silica113 and carbon materials such as multi-wall carbon nanotubes114, 115 and asphalt115. In their work, Jitputti et al.112 evaluated the activities of sulfated zirconia and stannia for crude palm kernel oil and coconut oil conversion to biodiesel. They showed minor activity at 200 0C, which subsequently decreased with additional recycling. They associated the decrease in activity to both sulfate leaching and active site poisoning. In other work, Melero et al.113 prepared propylsulfonic acid SBA-15 material and found it highly active for the conversion of refined and crude palm oil and soybean oil. The catalytic performance was attributed to the large surface area and pore diameter of the mesoporous support. However, they also found a slight decrease of activity in recycled catalyst testing. To remedy this, they pointed out that further work would be performed to enhance the strength of acid sites and control the surface properties of the silica support in order to enhance the durability of these sulfonated meso structure catalysts. Shu et al.115 prepared sulfonation of carbonized vegetable oil asphalt and sulfonated multi-walled carbon nanotubes (s-MWCNTs). They found that the asphalt-based catalyst showed higher activity than the s-MWCNTs for the production of biodiesel and that this behavior might be correlated to the high acid site density of asphalt catalysts resulting from its loose irregular network and large pores. Using the asphalt based catalyst, the conversion of cottonseed oil achieved 89.93% when the methanol/cottonseed oil molar ratio was 18.2, reaction temperature 2600C, reaction time 3 hr, and a catalyst/cottonseed oil mass ratio of 0.2%. Also, the asphalt based catalyst can be re-used. Shu et al.114,115 thought that the sulfonated polycyclic aromatic hydrocarbons provided an electron-withdrawing function to keep the acid sites stable.
Heteropolyacid Catalysts:
Sunita et al.116 compared the activities of zirconia supported isopoly and heteropoly tungstate catalysts. Zirconia-supported heteropoly tungstate possessed a high total acidity and showed superior catalytic performance compared to zirconia-supported heteropoly tungstate catalysts. Under their reaction conditions of 2000C, methanol/oil molar ratio 15, and 15% WO3/ZrO2 calcined at 7500C the ZrO2 catalyst achieved 97% conversion of oil. Kulkarni et al.117 impregnated tungstophosphoric acid on four different supports such as hydrous zirconia, silica, alumina and activated carbon, and used them for converting low quality canola oil containing about 20 wt% FFA to biodiesel. The hydrous zirconia supported tungstophosphoric acid was found to be the most active. At 2000C, 1:9 oil to alcohol molar ratio, and 3 wt% catalyst loading a maximum ester yield of 90 wt% was observed.
Other Catalysts:
Except for the above two types of catalysts, other weaker solid acid catalysts were also tested for activity in biodiesel formation reactions. These included many types of zeolites and phosphate based catalyst systems. For instance, Britoet al.118 studied the activity of several commercialized Y-type zeolites in a continuous tubular reactor at atmospheric pressure and within a temperature range of 200–476 0C. The results showed that higher temperature accelerated transesterification reaction rate. With used fry oil, they found that the optimal reaction conditions for transesterification with methanol could be achieved with zeolite Y530 at 4660C, 12.35 min residence time, and a methanol/oil molar ratio of only 6. In other work, some phosphate salts have also been reported active for biodiesel formation. One such example is by Li and Xie119 who prepared Fe3--vanadyl phosphate. When the transesterification reaction was performed at a molar ratio of methanol to oil of 30:1, reaction temperature of 473 K, reaction time of 3 hr, and a catalyst loading of 5 wt%, the maximum conversion of soybean oil was found to be 61.3%. The importance of this work is that it showed the activity of this catalyst was not significantly affected by the presence of free fatty acids and water in the reactant mixture. In addition, the catalyst also exhibited catalytic activity for the esterification of free fatty acids with methanol. Unfortunately, the same catalyst slowly deactivated after 5 recycles. Serio et al.120 suggested that the deactivation of vanadyl phosphate catalyst was strongly affected by reaction temperature. In effect, higher reaction temperatures accelerated the deactivation process. However, catalyst leaching at higher temperatures was not the root cause of deactivation. Instead, surface characterization work showed that deactivation was primarily due to the progressive reduction of surface vanadium from V5 toV3 by methanol where V5 and intermediate V4 species were active and V3 was inactive. Further work revealed that the deactivation was reversible and catalyst activity could be restored by simple oxidation. Yet another catalyst technology based on Fe–Zn double metal cyanide complexes was tested by Sreeprasanth et al.121. The catalysts were hydrophobic, and contained only Lewis acidic sites. Bronsted acid sites as well as basic sites were absent. These catalysts proved active for both the transesterification and esterification of unrefined and waste cooking oils. Thus Lewis acidic sites were found to be active centers for both the transesterification and esterification reactions and the surface hydrophobicity of these catalysts improved their tolerance to water.
Amphoteric Metal Oxide:
Some amphoteric metal oxides, such as PbO, PbO2, and ZnO, have attracted attention from researchers because of their adjustable basic and acid properties. Singh and Fernando122 found that the FAME yield reached 89% using amphoteric PbO and PbO2. However, further testing showed Pb content in the glycerol and biodiesel products was as high as 2000 ppm which implied some dissolution of the catalyst. With respect to ZnO based catalyst systems, two candidates appear to offer the best next generation catalyst solution for improved biodiesel synthesis. One is a catalyst composed of zinc aluminum oxides from Institute Francais Du Petrole (Vernaison, France); the other is a catalyst of zinc lanthanum oxides from Wayne State University (MI, USA).
Zinc–Aluminum Catalyst for the Esterfif-HTM Process:
The Esterfif-HTM process was developed by the French Institute of Petroleum (IFP) and commercialized by Axens. It was first used in an industrial context in 2006, by Sofiproteol in Se`te123. This process uses a heterogeneous catalyst, a spinel mixed oxide of zinc and alumina metals. The use of heterogeneous catalysts eliminates the need for catalyst recovery and washing steps—and associated waste streams—required by processes using homogeneous catalysts such as sodium hydroxide or sodium methylate. The process chart is shown in literature16. The catalyst section includes two fixed bed reactors (CSTR), fed with vegetable oil and methanol at a given ratio. Excess methanol is removed after each reaction by partial evaporation. Then, esters and glycerol are separated in a settler. The remaining glycerol is collected and the residual methanol removed by evaporation. A patent124 awarded to IFP described an acid catalyst with a formula of ZnAl2O4, xZnO, yAl2O3 (with x and y being in the range of 0–2) which originated from a hydrotalcites precursor. The BET surface area is between 50 and 200 m2/g and a pore volume is greater than 0.3 cm3/g. In the continuous process, the reaction temperature is required to be between 210 and 250 Deg. C, pressure between 30 and 50 bar, and VVH (volume of injected oil/volume of catalyst/hour) from 0.3 to 3. Using these conditions, they showed a FAME yield of 91% based on a catalyst with a surface area of 65 m2/g, pore volume of 0.63 cm3/g under the conditions of 2400C, 50 bar, 160 min of contact time. This patent also addressed the catalyst lifetime. They found that on the 14th day, the FAME yield decreases to 30.9% at 240 °C, 50 bar, 1 VVH. The structure of the deactivated catalyst is not reported in this patent. A subsequent IFP patent125 states that this catalyst is quite sensitive to water. In fact they maintained the water concentration below 1000 ppm, which implies that the oil feedstocks used in Esterfif-HTM process must be well refined.
Zinc Lanthanum Catalysts:
A series of zinc and lanthanum containing catalysts were developed by Yan et al.67,126,127. This type of catalyst exhibits weak basic properties and is composed of ZnO, La2CO3 and LaOOH. Chief among its attributes are activity, longevity, FFA and water tolerance, and oil flexibility. The Zn La catalysts demonstrate high catalytic activity.
When using refined soybean oil, FAME yields as high as 95% under reaction conditions of 60 min, 200 °C, 500 psi, 36:1 M ratio of methanol to oil, and 2.3 wt% catalyst dosage in a stirred batch reactor. Yan et al.127 reported the Zn3La1 catalyst which had a 3:1 M ratio of zinc to lanthanum was recycled 17 times in a batch reactor without loss of activity and maintained at high FAME yield (*92.3%) for 70 days in a continuous tubular flow reactor (Fig. 1a, b).
Fig. 1 FAME yield (a) in the batch reactor using the recycled Zn3La1 catalyst. Note the catalyst was reused for 17 times. The average yield of soybean methyl esters is 93.7% (b) in the continuous reactor. Note that this catalyst has run for 70 days, and the average yield of FAME during stable stage is 92.3%127
Table 5: Yield of FAME in the presence
of different FFA addition
|
FFA addition (%) |
Yield of FAME at different reaction time (%) |
|||||
|
5 min |
10 min |
20 min |
40 min |
60 min |
90 min |
|
|
0 |
10 |
16 |
52 |
83 |
88 |
93 |
|
5.20 |
38 |
72 |
83 |
– |
95 |
92 |
|
10.13 |
73 |
83 |
88 |
92 |
– |
92 |
|
15.21 |
89 |
– |
95 |
– |
98 |
92 |
|
30.56 |
75 |
85 |
93 |
95 |
– |
01 |
Reaction conditions: catalyst amount of Zn3La1 is 2.3 wt%, molar ratio of methanol to oil is 36:1, reaction temperature is 200 C. All data are taken from literature67
Fig. 2 FAME yield of crude corn oil from DDGs, crude algae oil, crude coconut oil, crude palm oil, crude soybean oil, waste cooking oil, food grade soybean oil and food grade soybean oil with 3% water and 5% oleic acid addition. Reaction conditions: 126 g of oil, 180 g of methanol, 3 g of catalyst, 200 C, 500 psi, in the batch stir reactor. Note that all of these oils was converted into FAME within 3 h 127
Table 6 : Yield of FAME in the presence of different water addition
|
Water addition(%) |
Yield of FAME at different reaction time (%) |
|||
|
20 min |
60 min |
90 min |
150 min |
|
|
0 |
52 |
88 |
93 |
94 |
|
1.03 |
50 |
80 |
90 |
96 |
|
3.12 |
22 |
74 |
89 |
93 |
|
5.07 |
13 |
57 |
78 |
90 |
Reaction condition : catalyst amount of Zn3la1 is 2.3 wt% molar ratio of methanol to oil is 36.1,reaction temperature is 200.All data are taken from literature67.