Introduction and Literature Review
Scheme 1.3: Interesterification reaction using methyl acetate
1.4 Catalysts for biodiesel production
As discussed in earlier section 1.3, biodiesel can be synthesized either in absence of catalyst or in presence of homogeneous, heterogeneous (alkali/ acid) or enzymatic catalysts [18,19]. Catalysts usually improve the kinetics of trans–
esterification reaction and the product yield as they are able to lower the surface tension
between the two immiscible phases, i.e. triglycerides and alcohol [15]. The classification of different catalysts used for biodiesel production is shown in Fig. 1.3.
Homogeneous catalyst, especially homogeneous alkali catalyst (NaOH/ KOH) are more commonly used for transesterification of feedstock with low FFA content. The presence of FFA and water always produce negative effects on transesterification reaction, since the presence of FFA and water causes soap formation, consumes catalyst and reduces catalyst effectiveness, all of which results in a low conversion of triglyceride to biodiesel [15]. Another major drawback of using homogeneous catalyst is contamination of by–product glycerol, expensive separation of the catalyst from the reaction mixture and generation of large amounts of waste–water during purification of biodiesel.
Figure 1.3: Classification of catalysts for biodiesel production.
Biodiesel synthesis by using solid catalysts is therefore a viable environmentally friendly alternative which minimize the consumption of large amounts of water during the purification of biodiesel [19]. Unlike homogeneous catalyst, the heterogeneous catalysts act in a different phase from the reaction mixture. Use of heterogeneous catalyst has the advantage of easy separation from the reaction mixture and recovery as well as reusability of the catalyst, potentially leading to higher efficiencies and lower
production costs [27,28]. Some process parameters for homogeneous and hetero–
geneous catalyst are compared in Table 1.2.
Table 1.2: Comparison between homogeneous and heterogeneous catalyst for biodiesel production [27]
Variable Homogeneous catalyst Heterogeneous catalyst Reaction rate and yield Fast and high conversion Slow and moderate
conversion Purification of product Difficult Easy
Methodology Batch operation Continuous operation
possible Presence of FFA and
water
Highly sensitive Low sensitive Catalyst reusability Not possible Possible
Cost Cheap (but overall process
becomes costly)
Potentially cheaper
Extensive research has been carried out in last few decades to develop different catalysts for biodiesel production. A brief discussion on different types of catalysts for biodiesel production is given in following sub–sections.
1.4.1 Alkali or base catalyst for biodiesel production
Due to mild operating conditions and higher reaction yield, homogenous base catalyst (NaOH/ KOH) is most widely used for biodiesel production for feedstocks with very low or negligible content of free fatty acids (typically FFA < 1% w/w). As low as 1 wt% of NaOH or KOH is sufficient to yield around 98% biodiesel from low fatty acid feedstock [29]. As discussed above, the homogeneous base catalyst in presence of FFA and water leads to soap formation. Moreover, fraction of catalyst retained in by–product glycerol act as contaminant and lowers the economy of the overall process [19].
To overcome these hurdles and improve the economics of the large scale transesterification process, the use of heterogeneous catalyst is a feasible solution, which can be easily separated from the reaction mixture for reuse, without contamination of the by–product glycerol. Since the ability of the base to abstract a
proton from the alcohol is directly connected to the base strength, stronger bases are in general more effective to initiate the transesterification of triglycerides in heterogeneous catalyst form [27]. It was reported by Malero et al. [30] that the metal oxide provides sufficient adsorptive sites for alcohol in transesterification reaction and concluded that the high transesterification activity of catalyst might be due to the manifestation of the dissociation of alcohol to RO– and H+ on basic sites of metal oxide catalyst surface. Several studies have reported the use of heterogeneous base catalysts synthesized with different methods using silica, zinc oxide, zirconia, zeolites, alumina, aluminosilicates, clays, activated carbon, etc. as supports, and alkali and alkaline earth oxides or their salts (like KOH, KF, KI, KNO3, K2CO3, NaOH, CaO, Ba(OH)2) as functional part [31–41].
1.4.2 Acid catalyst for biodiesel production
The direct application of base catalyst is not possible for non–edible oils feedstock due to their high FFA content, which leads to saponification. Thus, the single step transesterification process is converted in two step process. The first step of the process reduces the FFA content in oil by esterification with methanol and acid catalyst and the second step is conventional transesterification process, in which triglyceride portion of the oil reacts with methanol and base catalyst to form ester and glycerol [18].
Alternatively, the acid catalysts are capable of performing simultaneous esterification and transesterification reaction. The commonly used homogenous acid catalysts are H2SO4, HF, H3PO4, HCl, and p–toluene sulfonic acid etc. [20, 42, 43]. The major drawbacks of acid catalyst are: (i) their highly corrosive, (ii) hazardous in nature, and (iii) ~ 4000 times slower kinetics as compared to the homogenous base catalyst [44].
Because of the lower activity of the acid catalyst as compared to the base catalyst (both
in homogenous and heterogeneous form), the reactions are generally carried out at higher temperature and pressure [30].
In recent years, a substantial progress has been made on the development of heterogeneous acid catalyst for biodiesel production. Several heterogeneous acid catalysts have been tested in the FFA esterification as well as simultaneous esterification and transesterification reaction, such as sulphated metal oxides, mesoporous silica, modified zeolites, metal organic framework (MOF) structures, ion exchange resins, polymer supported sulphonic groups, carbon–based supports with functionalized acid groups, etc. [44–52]. Therefore, acid heterogeneous catalysts can be considered as an alternative to minimize the environmental damage and reduce the cost of biodiesel production.
1.4.3 Enzymes as a catalyst for biodiesel production
In biological systems, interesterification and transesterification reactions occur naturally. Enzyme catalysed transesterification is another alternate path to produce biodiesel. The most common enzyme used for transesterification reaction is lipase.
However, due to the high cost of the enzyme, proper recovery and recycle of the enzyme is essential for the economy of the process [53,54]. Over past few years, lipase from various sources immobilized on different supports has been studied by many researchers. Lipase from Candida Antarctica immobilised on mesoporous silica [55], Candida rugosa on nanofibrous poly–membrane [56], Thermomyces lanuginosus immobilized on microporous polymer [57], Thermomyces lanuginosus immobilised on styrene divinylbenzene copolymer [58], Burkholderia immobilised on hydrophobic magnetic particles [59], Candida Antarctica, Thermomyces lanuginosus and Rhizomucor miehei immobilised on epoxy–functionalised silica [60,61], Aspergillus
niger immobilised on micro–porous biosilica [62], have been used. Jegannathan et al.
[63] and Zhao et al. [64] reviewed many other feasible supports such as calcium alginate beads, anion exchange resin, silica–polymer, acrylic resin and biosilica. for immobilization of lipase. The common aspects of these studies involved optimizing the reaction conditions (solvent, temperature, pH, etc.) in order to establish suitable characteristics for an industrial application. However, the reaction yields as well as the reaction times are still unfavourable compared to the alkali catalysed transesterification systems.
1.4.4 Ionic–liquid as a catalyst for biodiesel production
In recent years, the ionic liquids (ILs) have been also investigated as an alternate to heterogeneous alkali and acid catalyst for biodiesel production [65]. Ionic liquids (ILs) with variety of structures have been considered as green reaction mediumdue to their negligible volatility, excellent thermal stability and high solubility [66]. Brønsted acid ionic liquid containing an alkane–sulfonic acid group was reported suitable for production of biodiesel from various feedstocks [67].
The ionic liquids exhibit good catalytic activities, especially, 1–(4–sulfonic acid) butylpyridinium hydrogen sulfate was reported to have similar catalytic activity as concentrated sulfuric acid [68]. The ionic liquid catalysts could be recovered and reused after distillation. However, the ionic liquid catalysed transesterification reaction required higher reaction temperature (> 423 K) and longer reaction time as compared to the alkali catalysts [66].