The most common alkaline catalysts employed in biodiesel development are sodium hydroxide (NaOH) and potassium hydroxide (KOH) [15] [16] [17] . The use of other alkaline catalysts such as Carbonates, Methoxide, Sodium ethoxide, Sodium propoxide and Sodium butoxide have also been investigated [18] . The high yield production of fatty acid alkyl esters under conditions of low temperature and pressure within a short duration make this method economically viable [19] [20] .

The main drawback of this approach is the feedstock purity requirement. The presence of free fatty acids, water and some other impurities in the feedstock has a pronounced effect on the process of transesterification [16] [21] . Besides, the complicated multi-step downstream operations of end products is another hindrance. Moreover, the treatment of the generated wastewater from this process incurs additional cost which further impede the economical sustainability of the method. The amount of wastewater produced is approximately 0.2 ton for every ton of biodiesel produced. The need for extensive downstream processing makes alkaline transesterification expensive and a threat to environmental sustainability [22] .

The requirement for reaction homogeneity has informed the use of homogeneous base catalyst in biodiesel synthesis. Homogeneous catalysts are very effective and cheap reagents, nevertheless, the limitation of product purification required subsequently for their methyl esters washing escalates the cost. Besides, catalyst reuse is a problem after enormous energy, water, and time have been expended [23] .

In practice, NaOH is preferred over potassium hydroxide because it causes less emulsions and is less expensive. There are various reports of use of NaOH as a catalyst for biodiesel production [23] . The use of KOH as base catalyst in the transesterification process has been widely reported also, however, KOH is reported to offer better fuel performance compared to NaOH and the separation of glycerol from biodiesel is easier with KOH when utilized as a catalyst; therefore it is preferred over NaOH [23] [24] .

Sodium methoxide (NaOCH₃) is more effective than sodium hydroxide as a catalyst because it disintegrates into CH₃O₂ and Na⁺ and does not produce water in contrast to NaOH/KOH. Moreover only 50% of it is required compared to NaOH. But the catalyst is less common due to its higher cost. It was found that 0.5% sodium methoxide and 1% sodium hydroxide exhibited similar results with methanol-to-oil molar ratios of 6.44. The use of NaOCH₃ is reported more in literature as observed by [25] [26] . Similarly, Potassium methoxide (CH₃OK) is a base catalyst which can also be used for transesterification. Although it has been tested by many researchers, very few have recommended using it on regular basis [27] [28] [29] .

The heterogeneous catalysis approach in biodiesel production reduces the difficulties of separation of products and catalyst, resulting in the generation of lower volume of effluent and reusability of the catalyst [30] [31] . Currently, reports in literature suggests the use of various acid and basic heterogeneous catalysts, however, heterogeneous chemical catalysis generally generate low yields compared to homogeneous alkaline catalysts [30] [32] .

In contrast, heterogeneous (solid) base catalysts are easily separated simply by filtration and can be reused several times. Commonly employed solid base catalysts include the alkaline earth metal oxides, zeolites, KNO₃ loaded on Al₂O₃, BaO, SrO, CaO and MgO [23] . Among the alkaline earth metal oxides CaO is the most widely used as a catalyst for transesterification producing up to 98% FAME yield in the first reaction cycle [33] . The reactivity of CaO can be improved further by subjecting it to calcination. However, the reusability of the catalyst for further reaction cycle remains in question. Boron and carbon group elements have been extensively investigated. Boron group elements particularly aluminum and Al₂O₃ serving as support for various other metal oxides, halides, nitrates and alloys are used for biodiesel production [34] [35] [36] .

Besides, transition metals have also been reported with great catalytic effectiveness in biodiesel production. Common among the transition metals and their oxides employed in biodiesel synthesis are ZnO, TiO, TiO₂/SO₂₂ and ZrO₂/SO₄ [23] . Zeolites as catalysts possess the characteristics of acidic sites and shape selectivity. Zeolites vary in pore structure and the inner electric fields from their crystal as well as their surface properties contribute to their varying catalytic properties. Zeolites can accommodate a wide variety of cations such as Na⁺, K⁺, Ca²⁺, Mg²⁺ and many more, which contribute to their basic nature. Zeolites as potential heterogeneous catalysts for the preparation of biodiesel have been investigated in myriad of studies [37] .

Lipases (E.C.3.1.1.3) are a class of hydrolase that are known to be responsible for hydrolysis of acyl glycerides. They are produced from microbial plant and animal species. Industrial production levels of these enzymes are from microbial sources. Organisms “Generally Regarded As Safe (GRAS) such as Aspergillus species, Penicillia species and Candida species etc. are the sources from which these enzymes have been produced. Lipase possesses further importance in that they exhibit stability at diverse pH range, temperature and organic solvent and are known to be capable of existing in intracellular and extracellular nature [7] .

Biocatalysts are gaining more attention nowadays and have the potential to outperform chemical catalysts for biodiesel production. The development of enzymes in biodiesel synthesis is aimed to offshoot the various concerns associated with chemical catalysis. New biochemical routes to biodiesel production, based on the use of enzymes, are becoming very interesting [18] [38] [39] [40] . Several lipases from microbial strains, including Pseudomonas fluorescens, Pseudomonas cepacia, Rhizomucor miehei, Rhizopus oryzae, Candida rugosa, Thermomyces lanuginosus, and Candida antarctica, have been reported to have record success in transesterification activity. The recent studies using methyl acetate as the acyl acceptor and soybean oil illustrate that the use of this acyl acceptor does not lead to inhibition of the enzyme [41] - [47] .

Commercially, the prospect of biological catalyst is becoming promising in biodiesel synthesis due in part to enhanced production processes and feasibility of use in new operational environment, thus organisms that are capable of enzyme secretion have been modified genetically for optimal secretion and for improved catalytic activities such that no less than 75% of industrial market have been fortified by hydrolytic enzymes like protease, lipase and amylase. Research enquiry into these lipases are becoming increasing, though low production have been reported in current enzyme market, a constraint attributable to the feedstock selection, operational time and purification requirement [48] .

In addition to propensity of hydrolyzing triacylglycerol (TAG) to glycerol and free fatty acids, esterification, transesterification and aminolysis in non-aqueous media are some other processes that have utilized the activities of lipase. Besides, they are among the hydrolases with enormous industrial propensities and have find application in other production processes such as confectioneries, detergent, chemical and pharmaceutical industries [48] .

Lipases possess tremendous potential for application in biotechnology because they are capable of acting at the aqueous and non-aqueous interface which differentiates them from other enzymes. Purification to homogeneity may not be necessary be required but a partial purification to a certain degree is recommended for use in industries [48] . Despite the tremendous advantage of enzymatic alcoholysis, this approach however has some shortcomings which include loss of enzyme activity due to deleterious alcohol effects, water deactivation and glycerol inhibition [18] [49] . Moreover, the high cost of enzyme production still remain the major impediment to commercialization of enzyme-catalyzed processes.

Supercritical alcohol transesterification is process of biodiesel production designed to offshoot the many issues such as huge volume of wastewater effluent and feedstock selectivity [50] associated with conventional transesterification with chemical catalyst. Nevertheless, supercritical alcohol method is faced with high temperatures requirement (350˚C - 400˚C) and pressures (10 - 25 MPa) and therefore the high capital cost.

In an effort to lessen the effect of these drawbacks, a catalyst-free technique for the transesterification of vegetable oils using an alcohol under supercritical conditions has been suggested, subject to the advantages of fuel quality and environmental considerations [51] [52] [53] . Based on the available reports in literature, this method of catalyst-free alcoholysis reactions at high temperature and pressure conditions enable robust phase solubility and decreased mass-transfer restrictions. The reaction rate is incredibly increased in the supercritical state thereby ensuring reaction completion in shorter period while a less difficult separation and purification steps is ensured [51] .

Moreover, the absence of a catalyst is augmented by the presence of the alcohol which serves a dual purpose of reactant as well functions as a catalyst [54] . Besides, supercritical method (SCM) has been reported to be more tolerant with the associated water and free fatty acids in the feedstock compared to the usual alkali-catalyzed procedure. Therefore, the technique is much more effective to various types of vegetable oils, this has raised unprecedentedly the interest in research involving the use of this approach.

The economic analysis of biodiesel synthesis by homogeneous alkaline catalysis and SCM routes had been investigated previously [55] [56] . The energy utilization was observed to be similar in the two approaches. Higher energy requirement in the SCM due to the heating involve, is the cause for the higher costs, however, this cost is compensated for by the less downstream operation requirement for the product, which alternatively is the reason for higher cost in the usual homogenous catalytic production approach.

Marchetti and Errazu [57] weighted different methods for biodiesel production with low-valued oil resource that are predominantly rich in free fatty acids. They submitted that the SCM is an attractive alternative from technological perspective. Besides, economic consideration informs low wastewater generation and a food grade glycerin production as a byproduct. Thus, SCM approach is an important and promising procedure in biodiesel production.

Available research works in literature highlight some measures that can apparently lower the exorbitant operating costs and product degradation. These suggested measures include the use of co-solvent, hydro-esterification approach, utilization of heterogeneous catalyst especially from renewable sources, a two- step method of triglycerides hydrolysis in subcritical water followed by esterification of the hydrolysates and employing different reactor configurations with increased mass transfer operating in continuous mode (micro-reactor or packed bed reactor).

Predominantly, development of heterogeneous catalysts from renewable source received attention due to the basic nature of the carbonaceous constituents within these materials. Various studies available in literature (Table 2) sourced for materials such as seafood shells, animal bones and industrial wastes materials. These renewable heterogeneous catalyst are explored to overcome limitations of the SCM which majorly is the capital cost due to high temperature and pressure requirement. The materials are cheap and can be easily recovered and reused. Besides, no neutralization of the wastewater generated in the conventional process is required and it can be operated at similar reaction conditions as that of the conventional alkaline transesterification. Thus, the cheap nature and sustainable benefit of these materials make this method a promising and viable method. Researchers have explored these approach for sustainable biodiesel development.

Among the various works available in literature is the work of Viriya-empikul et al. [58] . They utilized resources such as waste egg shell, golden apple snail shell and Meretrix venus derived from industrial waste. These materials were

calcined in air at 800˚C for 2.4 h optimal conditions. They employed the synthesized catalyst in transesterification of palm olein oils. A comparative performance was observed as that obtainable with alkaline catalyst and recorded high catalytic activity in term of FAME conversion well above 90% in 2 h. Obadiah et al. [59] employed waste animal bones as cheap resource for biodiesel production from palm oil esterification. The catalyst was calcined to transform the calcium phosphate in the bone to hydroxyapatite at 800˚C temperature. 20 wt.% of the catalyst produced incredible result at mild reaction condition to achieve 96.78% conversion of methyl esters. The catalyst was observed to be comparable in performance with that observed with laboratory grade CaO.

Biodiesel production utilizing waste shell of Turbonilla striatula as solid oxide was investigated by Boro et al. [60] . The shell was calcined at ranges of temperature for 4 h and was used in transesterification of mustard oil. Optimum calcination temperature of 800˚C was determined and the calcined catalyst exhibit enhanced surface area and higher pore volume. 3 wt.% of the catalyst yielded 93.3% biodiesel at mild reaction conditions in about 6h. The catalyst could as well be reused only about two cycles as pronounced catalytic activity was lost after the second cycle. Catalytic efficiency of waste industrial eggshell as heterogeneous catalyst in microwave-assisted transesterification of palm oil was studied by khemthong et al. [61] . The waste eggshell was calcined in air at 800˚C for 4 h. The calcined catalyst exhibit high CaO content of about 99.2% and was observed to be efficient in transesterification of palm oil producing 96.7% FAME conversion with 15 wt.% of catalyst. The catalyst also showed high reusability potential.

Nair et al. [62] synthesized heterogeneous catalyst from clamshell (Mereterix mereterix) which was calcined at 900˚C for 3.5 h. This catalyst was employed in transesterification of waste frying oil at 60˚C for 3 h. A biodiesel yield and conversion of about 89% and 97% respectively was obtained optimally with 3.0 g of the catalyst. Besides, chicken bones derived heterogeneous catalyst was employed in the transesterification of waste cooking oil in biodiesel production by Farooq et al. [63] . This catalyst was calcined at 900˚C and was observed to exhibit good catalytic activity. Utilization of the catalyst generated 89.33% yield of the product with 5.0 g of the catalyst under mild reaction conditions within short reaction time of 4 h.

Nakatani et al. [64] produced biodiesel from soy bean oil using combusted oyster shell waste as heterogeneous catalyst. Calcination temperature was 700˚C and optimum biodiesel was achieved using 25 wt.% the catalyst at 5 h reaction time to produce high purity soybean methyl esters of yield above 70%. Wei et al. [65] applied calcined waste eggshell as cheap heterogeneous catalyst in transesterification of soybean oil. The calcination of the eggshell was done between 200˚C - 1000˚C temperature ranges for 2 h. the study resulted in more than 95% yield of soybean biodiesel in 3 h using 3 wt.% of the catalyst. Reusability test showed that the catalyst could be reused up to 13 times with no apparent diminished catalytic activity.

Application of waste fish scale calcined above 900˚C for 2 h was investigated by Chakraborty et al. [66] . The calcined fish scale was utilized as heterogeneous catalyst, the dose was 1.01 wt.% relative to oil and feasibility of the catalyst was possible for up to 6 times reaction cycles. Moreover, Niju et al. [67] utilized modified eggshell in transesterification of waste frying oil. To obtain high quality methyl ester, the modification was calcination-hydration-dehydration approach to transform the material to an oxide with high catalytic efficiency. The application of the catalyst in biodiesel synthesis from waste frying oil ensure 94.52% conversion of the oil with 5 wt.% of the modified catalyst in 1 h reaction time. Reusability of the catalyst inform probable use of up to 6 cycles with yield above 90%.

Rezaei et al. [68] optimized biodiesel production from soybean oil utilizing calcined waste mussel shell as an eco-friendly, cheap catalyst. The calcination to convert calcium carbonate to CaO was due at temperature of 1050˚C optimum. The soybean methyl ester was produced with 12 wt.% of the calcined catalyst to generate 100% pure product and 94.1% yield. The reusability test of the catalyst ensure 5 times reaction cycles. Moreover, Buasri et al. [69] employed the activity of calcium oxide derived from waste shell of mussel, cockle and scallop as heterogeneous catalyst for transesterification of palm oil with methanol. The catalyst was prepared by calcination at 100˚C for 4 h. After 3 h reaction time, up to 95% conversion of the feedstock was achieved with 10 wt.% of the catalyst. The catalyst exhibit good activity and reusability up to 4 times reaction cycles.

Cyrtopleura costata (Angel wing shell) was utilized as heterogeneous catalyst in the conversion of microalgae oil for biodiesel synthesis by Osman et al. [70] . The catalyst (AWS) was modified through calcination at 800˚C and 900˚C over 2 h period to convert CaCO₃ to activate metal oxide phase. The study was discovered to offer higher basicity and surface area and was employed in the conversion of the feedstock. Optimized conditions of 1:150 and 9 wt.% for oil to methanol molar ratio and catalyst loading respectively produced a FAME yield of 84.11% in 1h reaction time. The study also recorded a 3 times reusability of the catalyst.

In another study, Bazargan et al. [71] employed palm kernel shell for transesterification of sunflower oil. The palm kernel shell haven been subjected to thermal treatment in a gasifier to produce a calcium oxide based biochar was calcined at 800˚C for 2 h, it was then utilized for catalytic conversion of feedstock to biodiesel utilizing 9:1 molar ratio of methanol to oil at 60˚C reaction temperature. The study recorded 99% conversion of the feedstock after 5 h reaction time. Moreover, Nisan et al. [72] applied calcined animal bone modified with potassium hydroxide as heterogeneous solid base catalyst for transesterification of high free fatty acid Jatropha. The study recorded high catalytic activity for transesterification of the feedstock. Optimum reaction condition of 9:1 methanol to oil molar ratio, catalyst dose of 6.0 wt.%, reaction temperature of about 70˚C at 3 h reaction time produced 96.1% FAME yield. The catalyst was reusable up to four cycle without pronounce deactivation.

Recently, Gohain et al. [73] utilized Musa balbisiana colla peel to produce an eco-friendly and highly effective heterogeneous base catalyst for biodiesel production through green practices. In the work, waste cooking oil was used as the feedstock to synthesize the biodiesel. An incredible 100% conversion was achieved by the catalyst which was modified after being washed and dried by burning in air and subsequent calcination of the formed ash at 700˚C for 4 h in a muffle furnace to obtain the calcined catalyst. 2 wt.% of the catalyst, 6:1 methanol to oil molar ratio, 60˚C reaction temperature and 3 h reaction time were found to be optimized conditions for conversion. Significant reduction in the biodiesel conversion was observed after each successive reuse of the catalyst which was attributed to leaching and loss of catalyst.

Waste ostrich and chicken eggshells as heterogeneous base catalysts were investigated by Tan et al. [74] for biodiesel production from waste cooking oil. The eggshell samples utilized in the study were manually crushed and sieved to obtain a particle size lesser than 0.5 mm. The fine powder was subsequently calcined in a muffle furnace at 1000˚C for 4 h. The prepared catalyst was then comparatively applied for conversion of the feedstock in a two-step transesterification reaction. The catalyst prepared from ostrich eggshell was discovered to produce higher FAME yield (96%) compared to the chicken eggshell (94%). This was attributed to larger surface area, improved basicity and smaller particle size of the ostrich-eggshell compared to chicken-eggshell. Optimum reaction condition recorded 12:1 methanol to oil molar ratio, 2h reaction time, 65˚C, reaction temperature and agitation of 250 rpm.

River snail shell was employed for transesterification of Palm oil in a study conducted by Roschat et al. [75] . The snail shell was calcined between 600˚C - 1000˚C for 3 h. The reaction was aided with a co-solvent and the catalytic performance of the prepared catalyst as well as the reaction kinetic was investigated. Optimum reaction conditions of 5 wt.% of the catalyst, 12:1 molar ratio of the methanol to oil, reaction temperature of 65˚C and reaction time of 1.5 h produced 98.5% yield of the fatty ester.