**4. Catalysts used for biodiesel production**

The catalysts used in the transesterification reaction, are extremely important to the group. The presence of a catalyst speeds up the reaction, increasing the yield of the final product. These catalysts are classified into two major categories: homogeneous catalysts and heterogeneous catalysts, each of which can further be divided into subgroups. The classification is shown in **Figure 6**.

### **4.1 Homogeneous transesterification**

### *4.1.1 Homogeneous base-catalyzed transesterification*

The base catalysts used for the process of Transesterification include KOH, NaOH, carbonates and corresponding potassium and sodium alkoxides such as sodium ethoxide, sodium methoxide, sodium butoxide and sodium propoxide. The alkaline catalyzed Transesterification reactions are 4000 times faster than acid catalyzed Transesterification reactions. As compared to acidic catalyst, the base catalyst are less corrosive to industrial equipments, hence alkaline catalysts are mostly employed in commercial. However, the base catalysedTransesterification reaction is affected significantly by the presence of free fatty acid (FFA) and moisture content in the feedstock. Therefore, the glycerides and alcohol used for Transesterification must be substantially anhydrous. It has been recommended that the FFA contents should be less than 2%, whereas the moisture content below 0.5 wt%. As the value of FFA is inversely proportional to the conversion efficacy, therefore small amount of water and high FFA contents present in animal fats and vegetable oils results in the deactivation of the catalyst and cause saponification (soap formation), which consequently decrease

**Figure 6.** *Catalysts used for biodiesel production.*

*Feasibility of Biodiesel Production in Pakistan DOI: http://dx.doi.org/10.5772/intechopen.101967*

**Figure 7.** *Mechanism for base-catalyzed transesterification [22].*

the biodiesel yield and renders the separation of glycerol and ester [21]. So, low free fatty acid content in triglycerides is required for base catalyzed Transesterification. Homogeneous acid catalyst is then referred for Transesterification.

#### *4.1.2 Mechanism*

Generally, the mechanism of base-catalyzed Transesterification of animal fats or vegetable oils involves four steps [13, 21]. In the first step, the base react with the alcohol gives an alkoxide and protonated catalyst. In the second step, nucleophilic attack of the alkoxide at the carbonyl group of the triglycerides and generates a tetrahedral intermediate. In the third step, alkyl ester and corresponding anion of diglyceride is produced. The final step involves the deprotonation of the catalyst to regenerate the active species that is able to start another catalytic cycle by reacting with the second molecule of the alcohol. Same mechanism is followed by the diglycerides and monoglycerides to convert to a mixture of alkyl esters and glycerol. The mechanism is summarized in the **Figure 7**.

#### **4.2 Homogeneous acid-catalyzed transesterification**

Mineral acids such as H2SO4, HCl and H3PO4are widely used for the acid catalyzed transesterification reaction. Acid catalysts are recommended for the oils that have higher free fatty acid contents such as waste oil or palm oil [23]. Such types of oils are first treated with acid catalyst (esterification) before the basic transesterification in order to convert the free fatty acids to esters. In this case, the FFA is esterified until the free fatty acid content becomes lower than 0.5% [24] In acid catalysis the oil is treated with acid catalyst and gives biodiesel and water but the water must be

**Figure 8.** *Mechanism for acid-catalyzed transesterification [25].*

removed immediately because it will results in the soap] formation in base catalyzed transesterification.

## *4.2.1 Mechanism*

In the acid catalyzed transesterification, the protonation of carbonyl group of the ester results in the formation of carbocation, which after a nucleophilic attack of the alcohol produces a tetrahedral intermediate. This intermediate then eliminates the glycerol to form a new ester and to regenerate the catalyst. This mechanism is related to a monoglyceride. However, this reaction can be extended to di- and triglycerides (**Figure 8**).

#### **4.3 Bio-catalyst (enzyme) catalyzed Transesterification**

In enzyme catalyzed Transesterification, the reaction is catalyzed by various lipases such as candida rugasa, candida Antarctica, immobilized lipase (lipozyme RMIM) pseudomonas cepacia, pseudomonas spp. Or rhizomucarmiehei. The yield of biodiesel greatly depends on the type of enzyme used [23]. 60% biodiesel yield was achieved from transesterification of soyabean oil using commercially avalaibleimobalized lipase (Lipozyme RMIM) [26, 27]. More importantly sufficient time is required for the enzyme catalyzed Transesterification as compared to base catalyzed Transesterification. However, the various parameters such as pH, temperature, solvent, type of micro-organism that generate enzyme etcmust be optimized to achieve the industrial goals. This process is highly selective, more efficient, produces less side products or waste i.e., environmentally favorable and involves less consumption of energy because reaction can be carried out in mild conditions [28].

*Feasibility of Biodiesel Production in Pakistan DOI: http://dx.doi.org/10.5772/intechopen.101967*

Arumugam et al. [29] used the sardine oil (byproduct of fish industry) as a low cost feedstock for the production of biodiesel. The FFA content of the oil was high (32mgKOH/G of oil) and the lipase enzyme immobilized on activated carbon was used for the Transesterification. Various reaction conditions were optimized such as methano/oil ratio 9:1, water content 10 v/v% and temperature 30°C. Reusability of the catalyst was studies for 5 cycles and 13% drop in FAME yield occurred.

#### **4.4 Heterogeneous Transesterification**

In heterogeneous catalysis, the phase of the catalyst is different from the phase of the reactants. Heterogeneous catalysts are very important in various fields such as industrial bulk chemical production, synthesis of selective chiral molecueles and energy [30]. Various process problems associated with homogeneous Transesterification, such as regeneration or separation of the catalyst, soap formation, disposal of byproducts, treatment of waste effluents and corrosion in case of acid catalyst have been solved by the use of heterogeneous Transesterification. Heterogeneous catalysts they are easily recovered at the end of the reaction by decantation or filteration, reusablility, show potential activity, selectivity, longer catalyst lifetimes and cost effective green process [31]. Interestingly heterogeneous catalysts could be used in certain harsh conditions such as high temperature and pressure. Heterogeneous catalysts may be solid base catalyst or solid acid catalyst.

Heterogeneous catalysts can be designed to bring out entrapment and grafting of the active molecules on the surface or inside the pores of the solid support such as alumina, silica or ceria. Mixed metal oxides [32], transition metal oxides [33], ion exchange resin [34], Alkali earth metal oxides [35] and alkali metal compounds supported on zeolite or alumina [36] have been used in different chemical reactions such as aldol condensation, isomerization, oxidation, Michael condensation, Knoevenagel condensation, and transesterification [37].

#### *4.4.1 Heterogeneous base catalyst*

Heterogeneous base catalysts are used to overcome the constraints such as saponification that hinders the glycerol separation from the layer of methyl ester associated with the homogeneous base catalysts. These catalysts show superior catalytic activities under mild conditions and are non-corrosive, environmentally friendly, have less disposal problems and easily separated from the reaction mixture [38, 39]. Moreover, the properties of these catalysts can be tuned accordingly to enhance activity, selectivity and longer catalyst lifetime. Various metal-based oxides such as alkali metal, alkaline earth metals and transition metal oxides can be used as a base catalyst for the biodiesel production from oils by trans-esterification process. The structure of metal oxides consists of cations (positive metal ions) that possess Lewis acid characteristics and anions (negative oxygen ions) that possess Brønstedbase characteristics. The combination of Lewis acid and Bronsted base characteristics make them potential catalyst for transesterification reaction.

#### *4.4.1.1 Alkaline earth and alkali metal-based catalyst*

Alkaline earth metal oxides such as CaO, MgO, BaO, BeO and SrOhave successfully been used as a catalysts for biodiesel production by many researchers.

Calcium oxide is favored ecofriendly material that haslonger life time because it is cheap catalyst, moderate reaction conditions and high activity. Generally,

calcium hydroxide and calcium nitrate are used as precursors for the CaO production. Recently, several calcium-rich waste materials such as mollusk shell and bones, chicken eggshells have been used for CaO synthesis to minimize the biodiesel production cost, problem of waste disposal.

Demirbas [40] described the supercritical conditions effect on the sunflower oil catalytic Transesterification in the presence of 3 wt% of CaO with 60–120 mesh size, 40: 1 of alcohol/oil molar ratio, at pressure of 24 MPa and 252°C The author reported 98.9% yield of methyl ester in reaction time of 26 min.

#### *4.4.1.2 Mixed metal-based catalyst*

Mixed metal oxides consist of two or more type of metal cations. Oxides may be binary, ternary and quaternary and so on with respect to the presence of the number of different metal cations [41]. Mixed metal-based oxides are mainly used as basic catalyst depending on the mixture of the catalyst. More importantly, the basicity of these catalysts can be tuned by changing their chemical composition and procedure for synthesis. Similarly, activation energy, type of synthesis method and structure of the catalyst have a strong impact on the final basicity of the mixed metal oxides.

It has been reported that, calcining MgO with ZrO2 gives a bimetallic oxide MgO-ZrO2having high basicity character and is almost unaffected by dissolution. Similarly, MnO, CuO and CuO supported on Al2O3 have been investigated in transesterification reaction at room temperature, yielded upto 97%. Al2O3-ZnO mixed oxide and rare earth oxides were studied but require high temperature for biodiesel production from vegetable oils. Calcium bimetallic oxides such as CaCeO3, CaZrO3, CaMnO3, CaTiO3 and Ca2Fe2O5 have also been investigated for the transesterification at 60°C, which displayed good activity and reusability [42, 43].

Xie et al. [44] used the Zinc aluminate catalyst (ZnAl2O4) in a batch processing for the biodiesel production from waste cooking oil. More than 95% ester yield was obtained at temperature greater than 150 C, alcohol to oil molar ratio 40:1, stirrer speed of 700 rpm, reaction time of 2 h and varying the catalyst amount in the range of 1–10 wt%. The catalyst was reused for 3 cycles and the yield reduced after the 3 run. The authors reported that the decrease may be due to the carbon deposition on the surface catalyst or loss of tiny particles of the catalyst during the process of recovery.

Basic catalyst may have several problems during the process of transesterification because they are sensitive to free fatty acid content. If the free fatty acid content is higher than 2 wt %, soap formation occurs resulting in decrease in the yield of biodiesel. The downstream purification process raises problems such as producing a large amount of wastewater [45].

#### *4.4.2 Heterogeneous acid catalysts*

#### *4.4.2.1 Metal oxides/mixed metal oxides*

Metal oxides such as FeTiO, ZrFeO, ZrFeTiO and Cesium-doped heteropolyacid have been used successfully as solid acid catalysts for the Transesterification of oil using ethanol and methanol as a solvent. Acid catalysts are insensitive to water content and free fatty acid (FFAs) present in the feedstock and is a are preferable method for cheaper feedstock [45].

Alhassan et al. [46] developed Ferric-manganese-based solid catalyst by impregnating the support material of sulfated zirconia with Fe2O3-MnO. The catalyst

#### *Feasibility of Biodiesel Production in Pakistan DOI: http://dx.doi.org/10.5772/intechopen.101967*

wascalcined for 3 h at 600°C. The synthesized catalyst was then used for the waste cooking oil Transesterification. The author found 96.5% yield of biodiesel under optimum reaction conditions of oil to alcohol molar ratio of 1:20, at 180°C temperature and catalyst loading of 3 wt%. The yield of the catalyst remained the same (96.5%) for 6 runs but decreased upto 87% upon the seven run. They reported that the decrease may be due to blockage of the energetic centers as a result of the accumulation of triglycerides in the pores of the catalyst.

### *4.4.2.2 Heteropoly acid derivatives*

Heteropolyacids and their salts are also used as solid acid catalysts for the biodiesel production. HPAs withKeggin structure can be prepared very easily as compared to other HPAs. They possess high thermal stability and are preferably used for production of biodiesel from different feedstocks. Keggin-type HPA has a low specific surface area, which can be overcome using appropriate supportive material. Similarly, HPAs supported on the carriers are used in biodiesel production because of their structural mobility and superacidity.

Sakthivel et al. [47] used the tungstophosphoric acid (HPW) and MCM-48 supported HPW catalysts for the esterification of long chain fatty acids and alcohol in supercritical CO2 (sc-CO2) medium. High yield was obtained in the supercritical CO2 medium due to the rapid diffusion of reactants and products in the MCM-48 channels and high contact of the reactants with the catalyst.

Acidic catalyst may have several problems such as very slow reaction rate, corrosive to reactors and pipelines. Normally, high reaction temperature, high oil to methanol molar ratio and long reaction time are required [45].

## *4.4.3 Heterogeneous bifunctional (acid: base) catalysts*

As the alkali catalyzed transesterification of the feedstock with higher FFA contents can produce low yield of biodiesel, because the FFA reacts with the alkali catalyst and produce the foam that results in separation and emulsification problems [48]. To solve this problem, a two steps catalytic process for the biodiesel production is recommended. In the first step, the free fatty acid contents of the feedstock are esterified using the acidic catalyst such as ferric sulfate or sulfuric acid. In the second step, biodiesel are produced by the transesterification using the basic catalyst such as CaO or ZnO. The problem of the catalyst removal in the first step can be avoided by neutralizing the acid catalyst by using the extra alkaline catalyst in the second step. But the use of extra catalyst can increase the overall cost of the biodiesel production. The residues of the acidic or alkaline catalyst in the products of biodiesel can cause the engine problems because the acidic catalyst can attack the metallic parts of the engine. On the other hand, basic catalyst can produce higher level of incombustible ash. Therefore, both the catalyst must be removed properly from the biodiesel to avoid the aforementioned problems [49, 50]. Further, it can be concluded that there is substantial room for the development of an efficient and effective catalyst for profitable biodiesel technology (**Figure 9**).

Recently, bifunctional heterogeneous catalysts has been introduced to solve the drawbacks adhere with the solid base/acid catalyst and develop more economical biodiesel technology. The bifunctional heterogeneous solid catalyst can be used as an alternative for the biodiesel production that can promote both esterification and Transesterification simultaneously [52].

#### **Figure 9.**

*Schematic representation of operating principle of bifunctional catalyst [51].*

In recent years, bifunctional heterogeneous catalysts have been used widely for the production of industrial fine chemicals. The bifunctionality concept has been designed to drive complex reactions through the advance approach of combining two hostile functions, such as acid and base, with cooperative interactions between their active sites precisely positioned functional groups [53]. Therefore, bifunctional heterogeneous catalyst can perform simultaneous esterification and transeseterification of free fatty acids and triglycerides respectively without being affected by the water content present or produced during the formation of biodiesel [54].

#### *4.4.3.1 Mechanism*

Generally, heterogeneous reactions involve three steps such as adsorption, surface reaction and desorption [55]. In the first step, carbonyl group of free fatty acids (FFA) adsorbs on acid sites while methanol adsorb on the basic site of the catalyst to produce carbocation and oxygen anion for esterification and transesterification respectively. In the second step, at the surface of the catalyst, nucleophilic attacked carbocation and oxygen anion at each methanol hydroxyl group and triglyceride carbonyl group for esterification and transesterification reactions, respectively. The nucleophilic attack would generate tetrahedral intermediate. Finally, the product (FAME) is formed from desorption of hydroxyl group and alkyl triglycerides from catalyst surface after breaking the -OH and -C-O- bond respectively, while the deprotonated catalyst regenerated the active species for starting another catalytic cycle. Glycerol, H2O, are produced as by-product during esterification and transesterification reactions (**Figure 10**).

#### *4.4.3.2 Transition metal-based catalysts*

Transition metals such as Ni, Fe and Co based compounds have been extensively investigated as bifunctional heterogeneous catalyst for biodiesel production. The TiO and MnO have shown good catalytic activity for biodiesel production. These catalysts have been used for the simultaneous esterification of FFAs and transesterification of triglycerides under continuous flow conditions by using low grade feedstock with high fatty acids contents ofupto 15%.

**Figure 10.**

*Mechanism for esterification and transesterification reactions on a bifunctional heterogeneous catalyst [56].*

Cannilla et al. [57] used a novel MnCeOx system for the transesterification of refined sunflower with the methanol. The performance of such catalyst was compared with that of common acid supported catalyst. The results showed that MnCeOx system have a superior activity especially by operating at low temperature i.e., ≤120°C. The catalytic performance was the result of synergic role played by the presence of both base/acid character and textural porosity.
