Advances in Biodiesel Production from Microalgae

: Biofuels, as a renewable, eco-friendly, and cost-effective energy source, can reduce the dependence on fossil fuels. The researchers considered different approaches for obtaining high biodiesel yields from microalgae biomass. This work aims to present an overview of the feasibility of microalgae use in biodiesel production. Therefore, biodiesel production from microalgae oil via the transesterification process was explained in detail. The application of non-catalytic transesteri-fication and catalytic transesterification was reviewed. The achievements in the application of homogenous catalysts, heterogeneous catalysts, and enzymatic catalysts for microalgae oil trans-esterification were discussed. The present technologies for biodiesel production from microalgae need more improvements to increase their efficiencies and reduce costs. Therefore, future research should focus on the development of effective catalysts for biodiesel production from microalgae biomass.


Introduction
The excessive consumption of fossil fuels has led to global warming and environmental pollution [1,2].Thus, the development of new, clean, and sustainable alternative energy sources is necessary [1][2][3].Biofuels are a promising alternative to fossil fuels as they are a renewable energy source [4,5].Over the last few decades, various feedstocks have been investigated for biofuel production [4].Until now, four biofuel generations have been identified [6,7].Biofuels of the first generation are produced from food crops, oil seeds (soybeans, sunflower seeds, rapeseeds), and animal fats using fermentation and esterification techniques [8][9][10][11].One of the main issues concerning this biofuel is the competition with food and fiber production for land and water usage [5,9].Biofuels of the second generation are produced from lignocellulosic biomass through cellulose hydrolysis, followed by sugar fermentation [9,12,13].One of the main disadvantages of this method is the need for large areas of land.This issue was overcome by the third-generation biofuels that use algal biomass [9,13].Microalgae have several advantages, such as: (1) they use atmospheric carbon dioxide as a carbon source for their growth; (2) they can be used for wastewater bioremediation; (3) they can accumulate high amounts of lipids; (4) they can grow under extreme environmental conditions; (5) they need less water than terrestrial crops; (6) they have no need of herbicides or pesticides; (7) they can utilize different types of water sources for their growth; (8) they can perform oxygenic photosynthesis using water; (9) they produce valuable compounds that can be extracted from their biomass [5,8,[14][15][16].The production of fourth-generation biofuel implies the direct conversion of solar energy into fuel from raw materials [6].
The BD production process from microalgae involves the following main steps: cultivation, harvesting, oil extraction, and transesterification [23,24].Microalgal BD is biodegradable, non-toxic, has no sulfur, and can reduce carbon emissions.Additionally, it has a similar chemical composition to petroleum diesel [4,5,13,14].However, the obtained BD is unstable, the implementation of the technology at industrial scale is difficult and expensive and the process requires large quantities of organic solvents for oil extraction from dry biomass.The harvesting, drying, and extraction of oil from biomass is difficult [4,13,25].The microalgal BD production cost is also higher than that of fossil diesel [4,26].According to Chung et al. [27], to cost-effectively produce BD from microalgae, the availability of microalgal biomass, synthetic extraction procedure, and production of high-quality microalgal BD must be considered.The fatty acid composition of biomass affects the yield and quality of BD [28].Microalgae biomass contains various components that can be extracted concomitantly or after BD extraction.Consequently, it is a valuable feedstock for the biorefinery approach [29].The biorefinery concept implies the complete valorization of microalgae biomass into various products.Thus, the costs involved in BD production from microalgae can be reduced [30][31][32].Gong and You [33] used Chlorella vulgaris microalgae to produce BD and four bioproducts (hydrogen, propylene glycol, glycerol tert-butyl ether, and poly-3-hydroxybutyrate).The process involving cultivation, harvesting, lipid extraction, remnant treatment, biogas utilization, biofuel production, and bioproduct manufacturing reduced the BD production cost to $2.79 per gasoline gallon equivalent.Additionally, Dong et al. [34] developed an integrated algal biorefinery process that can reduce the cost of microalgal BD by $0.95 per gasoline gallon equivalent (9% reduction).
This review aims to highlight the potential of microalgae biomass for BD production.A brief overview of BD production from microalgae via the transesterification process is presented.The advantages and disadvantages of non-catalytic transesterification and homogenous, heterogeneous, and enzymatic catalysis for the transesterification of microalgal oil to produce BD are discussed.This review also provides up-to-date information on the performances of the alkaline (homogeneous and heterogeneous), acidic (homogeneous and heterogeneous), and enzymatic catalysts for the production of BD from microalgae.In addition, the reaction conditions of the transesterification process and the obtained conversions and yields of BD are summarized.

Conversion Techniques for BD Production from Microalgae
The transesterification reaction occurs in three steps: triglycerides are converted into diglycerides, then into monoglycerides, and finally into esters (BD) and glycerol (by-product) [5,8,19,35].A successful transesterification reaction is achieved when two phases are separated, an oil phase containing the esters and a glycerol layer [13].The triglyceride transesterification reaction for BD production is presented in Figure 1 [36].Transesterification can be classified into non-catalytic and catalytic transesterification [37].

Figure 1.
Overall triglyceride transesterification reaction for BD production [36] "Used with permission of [Royal Society of Chemistry], from [36]; permission conveyed through Copyright Clearance Center, Inc.".

Non-Catalytic Transesterification
In non-catalytic transesterification, triglycerides are converted into BD using alcohol under supercritical conditions [37].The process requires a short reaction time to achieve high yields, needs a simple product purification process, and has high feedstock tolerance compared with catalyzed transesterification [8,[37][38][39][40].However, the process has high operating costs, is energy-intensive, and uses higher oil to alcohol molar ratios, higher temperatures, and pressures than catalytic transesterification [37,38,41].
The conversion of microalgae oil intoto fatty acid methyl esters (FAME) by non-catalytic transesterification has been investigated using various supercritical fluids, such as methanol (MeOH), ethanol (EtOH), dimethyl carbonate (DMC) and methyl acetate (MeOAc) [17,37,40,42].Felix et al. [42], investigated the non-catalytic in situ transesterification of Chlorella vulgaris oil with a moisture content of 80 wt% under subcritical conditions using MeOH.The optimum conditions were found to be 220 °C, 2 h, and 8 mL MeOH per gram of biomass, revealing a FAME yield of 74.6% with respect to the maximum theoretical FAME obtainable at a process power consumption rate of 0.47 kWh [42].Recently, the thermal-assisted Fenton reaction, combined with non-catalytic transesterification by the ultrasonication method, was studied to convert Chlorella sorokiniana CY-1 lipid into FAME.A conversion yield in the 85.7-92.8%range was obtained at 25 °C, a MeOH: lipid ratio of 6:1, 24 kHz, and continuous cycle conditions for 2-10 min [43].
A summary of studies conducted for non-catalytic transesterification of microalgae oil into BD is given in Table 1.A high BD yield of 99.3% in a short time (30 min) was achieved after the direct transesterification of Spirulina platensis [44].Liu et al. [45] obtained a high reaction conversion after investigating the production of BD from Chlorella protothecoides oil by non-catalytic transesterification using MeOH.Their results demonstrated that at 200 bar, 9:1 MeOH:oil molar ratio, and 4 min reaction time for temperatures in 300 to 400 °C range, the reaction conversion increased from 19.3 to 95.5% [45].Using response surface methodology, Nan et al. [40] studied the BD production from Chlorella protothecoides microalgae oil in supercritical MeOH and EtOH.Results indicated an optimal BD yield of 90.8% and 87.8%, for MeOH and EtOH, respectively.The optimal conditions for FAME were found to be: 320 °C, 152 bar, 19:1 alcohol-to-oil molar ratio, 31 min residence time, and 7.5 wt% water content.Additionally, the optimal conditions for fatty acid ethyl ester (FAEE) were: 340 °C, 170 bar, 33:1 molar ratio, 35 min, and 7.5 wt% [40].Further, the conversion of Schizochitrium limacinum microalgae to BD by non-catalytic transesterification using various supercritical fluids, namely MeOH, DMC, and MeOAc was considered.The conversion was >90% for MeOH after 40 min at 543 K, followed by 50% for DMC after 30 min at 643 K, and 40% for MeOAc after 40 min at 643 K [17].
Several studies were conducted to investigate the production of BD from wet microalgae biomass (Table 1).In this context, Tsigie et al. [47] carried out the in situ BD production from wet Chlorella vulgaris (80% moisture content) under subcritical conditions and obtained a maximum BD yield of 0.29 g/g dry biomass.Additionally, the same biomass (80% moisture content) was studied for BD production through in situ lipid hydrolysis and supercritical transesterification with EtOH.Thus, lipids were extracted from the hydrolysis solids and then transesterified.The extraction efficiency of the crude BD yield ranged from 56 to 100% [48].Jazzar et al. [49] performed in situ supercritical MeOH transesterification of isolated native microalgae, identified as Chlorella sp. and Nannochloris sp.(75 wt% of moisture) based on 18S rRNA gene sequencing followed by a DNA similarity search.Maximum BD yields of 45.6 wt% and 21.8 wt% were obtained for Chlorella sp. and Nannochloris sp., respectively [49].

Catalytic Transesterification
Catalytic transesterification is classified based on the type of catalyst used in homogeneous and heterogeneous catalysis [37,41,50].Alkaline (homogeneous and heterogeneous), acidic (homogeneous and heterogeneous), and enzymatic catalysts can be used in the transesterification of lipids extracted from microalgae [37,39,41].
The alkali-catalyzed transesterification occurs in four steps.First, the reaction of the base with alcohol takes place, producing an alkoxide and a protonated catalyst.Further, a tetrahedral intermediate is generated.Finally, alkyl ester and anionic diglyceride are formed, breaking down the tetrahedral intermediate, and the catalyst is regenerated [8,41].The alkali-catalyzed transesterification can be 4000 times faster than acid-catalyzed transesterification [38,51,52].
An alkaline catalyst can be used at low temperatures and pressures but is not recommended when the biomass percentage of unsaturated free fatty acids (FFAs) is high [37].When the percentage of FFAs of oil weight is higher than 0.5 wt%, the alkaline catalysts will react with FFAs to produce soaps [8,53,54].The saponification reaction lowers the FAME yield, makes the separation of FAMEs and glycerol difficult (soap acts as an emulsifier), and induces catalyst loss [38,55].Moreover, impurities such as water or inorganic/organic species can also affect the reaction rate and BD yield [39].
Acid-catalyzed transesterification involves the protonation of the carbonyl group of the ester.Thus, a carbocation is generated.The carbocation attacks the alcohol to produce a tetrahedral intermediate.The tetrahedral intermediate produces FAMEs, and finally, the catalyst is reprotonated and regenerated [8].Acid-catalyzed transesterification can be used to process raw materials with a high content of FFA (>1%).However, acid-catalyzed transesterification needs a long reaction time, higher amounts of reagent, and higher temperatures and pressures than alkali-catalyzed processes [8,38,41].Moreover, the acid catalyst is corrosive, and the process is not economically viable at a commercial scale [37,41].
The transesterification of microalgal lipids using enzymes (e.g., lipase) has gained attention for BD production due to the elimination of the adverse effects caused by the chemical catalysts [39,56,57].The lipase transesterification process requires less energy, a low molar ratio of alcohol to oil, moderate reaction conditions, low operation costs for BD production and the esters to be easy to recover [37,52,56,57].Additionally, the enzymes can be separated from BD and glycerol and reused several times [37,39].However, the recovery of glycerol from the immobilized lipase surface, the high cost of lipase enzyme, the loss of enzyme activity after some time, the lower reaction rates, and the usually incomplete reaction rates make this method not financially viable and challenging to apply at a commercial scale [37,52].

Homogeneous Catalysts Alkaline Catalysts
Homogeneous catalysts are widely used in the biofuel industry due to their high reaction rate and short reaction time [58].Among the homogeneous catalysts, sodium hydroxide (NaOH), sodium methoxide (CH3ONa), and potassium hydroxide (KOH) are the most commonly used [59].
A summary of homogeneous catalysts application for BD production from various microalgae species is given in Table 2. Karthikeyan et al. [64] obtained a methyl ester yield of 83.8% from Neochloris oleoabundans using a 3.70 g/L of NaOH, 1:6 molar ratio of oil to MeOH at 60 °C after 65 min.
A two-step in situ process has been applied to obtain a high FAME yield from Chlorella sorokiniana UTEX 1602.Thus, an in situ pre-esterification step using a heterogeneous catalyst (Amberlyst-15) before base-catalyzed transesterification was applied to reduce the FFA content from the biomass.The used pre-esterification conditions were 30 wt% Amberlyst-15, MeOH to biomass ratio of 2 mL/g, and 90 °C for 1 h.A FAME yield of 95.5 ± 1.5% was obtained after transesterification using 0.3% KOH, 4:1 MeOH: biomass at 90 °C after 15 min [60].
Sivaramakrishnan and Incharoensakdi [61] evaluated the methyl ester yields obtained by two-step transesterification and direct transesterification using Scenedesmus sp. and Chlorella sp.The BD yields obtained from two-step transesterification were 95.0% and 89.0% using Scenedesmus sp. and Chlorella sp., respectively, while for direct transesterification they were 96.0 and 92.0%[61].Additionally, Martinez-Guerra et al. [62] evaluated the BD yield of two single-step extractive-transesterification methods under microwave irradiation using Chlorella sp.biomass.High conversion yields were achieved using NaOH after transesterification under microwave irradiation with EtOH as solvent/reactant (96.2%) and with EtOH as reactant and hexane as solvent (94.3%) [62].Moreover, Martinez-Guerra et al. [65] compared the microwave and ultrasound effects on Chlorella, sp.BD production using EtOH as a solvent.The highest FAEE conversions for microwave and ultrasound methods were 96.2% and 95.0%, respectively.The findings showed that the ultrasound method provided a higher FAEE conversion at low solvent ratios, while the microwave method showed the best performance at lower power levels [65].

Acid Catalysts
Homogeneous acid transesterification is performed in the presence of concentrated HCl or H2SO4 [41].Many researchers have studied the homogeneous acid transesterification of microalgae oil, as shown in Table 3. Velasquez-Orta et al. [70] evaluated the FAME production from marine microalgae Nannochloropsis oculata and freshwater microalgae Chlorella sp.via in situ transesterification using H2SO4 as catalyst.A FAME yield of 73.0 ± 5.0% and 92.0 ± 2.0% was obtained from Nannochloropsis oculata and Chlorella sp., respectively [70].Additionally, a high FAME yield of 96.9 ± 6.3 wt% was achieved from Chlorella vulgaris [67].Kim et al. [71] obtained FAME yields between 72.6 and 100% using different HCl dosages from wet Nannochloropsis gaditana via in situ transesterification.
Response surface methodology was employed to investigate various parameters required to obtain high FAMEs from dry hydrodictyon microalgae through in situ transesterification.The maximum BD yield of 89.9% was achieved in 60.4 min at 50 °C [72].
A one-step process to convert the Chlorella pyrenoidosa oil containing about 90% of water into BD was applied [73].Cao et al. [73] reported that water had a negative effect on BD production at a lower temperature of 90 °C.A BD yield of 92.5% was achieved after 180 min reaction time [73].Im et al. [74], achieved a conversion yield of 90.6% after in situ transesterification of wet Nannochloropsis oceania microalgae containing 65 wt% moisture.A conversion yield of 90.0 ± 0.6 wt% was obtained via the direct transesterification of dry Chlorella spp.microalgae after a reaction time of 120 min [75].
It was observed that shorter reaction times were sufficient for high BD yields [50].For instance, Li et al. [76] investigated in situ BD production from Chlorella pyrenoidosa, cultivated in rice straw hydrolysate, and reported a 95.0%BD yield obtained in 120 min reaction time.
Nautiyal et al. [77] reported BD yields of 79.5% and 74.6% by simultaneous extraction and transesterification using hexane from Spirulina platensis and pond water algae.A BD yield of 68.0% was obtained using heterotrophic microalgal oil of Chlorella protothecoides, using 100% H2SO4 based on oil weight [78].
Taguchi analysis was considered to optimize the in situ transesterification using H2SO4 for BD production from Scenedesmus sp.The results revealed a maximum yield of 48.4 ± 0.2% [69].Carvalho Júnior et al. [79] reported a BD yield of 23.1 ± 2.8% (m/m) via in situ methanolysis from Nannochloropsis oculata biomass using HCl as catalyst.Ehimen et al. [80] investigated the in situ transesterification process using diethyl ether as a co-solvent under mechanical stirring and ultrasonic agitation at 24 kHz.A BD yield of 0.297 ± 0.002 g/g was obtained from the in situ transesterification process performed under ultrasonic agitation, compared to a yield of 0.283 ± 0.001 obtained using mechanical stirring [80].
Several researchers also studied microwave-assisted transesterification using homogeneous acid catalysts.For instance, Binnal and Babu [81] obtained a BD yield of 97.1% from microalga Nannochloropsis oculata with a moisture content of 80 wt% using a low-cost microwave reactor.First, the ethanolic KOH was used to convert the algal lipids into soap.Further, the soap was separated and subjected to simultaneous acidulation and esterification to produce BD [81].Sharma et al. [82] obtained a BD yield of 84.0% from Chlorella vulgaris via acid-based catalyzed transesterification in a microwave-assisted reactor.The heterogeneous catalysis process can be used when the lower-quality feedstocks contain high free fatty acid and moisture contents.The heterogeneous catalysis process is slower and gives a lower ester yield than the homogeneous catalysis process [50].Heterogeneous catalysts generally appear in a solid form and act at different phases in the liquid reaction mixture [84].The heterogeneous catalysts can be recovered, separated, and used again.Thus, the production costs are reduced [50,[84][85][86][87].Even so, the price of heterogeneous catalysts is a few times higher than homogeneous ones due to the complicated synthesis procedures of supported heterogeneous catalysts [50].The reaction time required for heterogeneous reactions could be 5 times longer than those of homogeneous transesterification.Thus, catalyst stability, an essential criterion for industrial application, can be affected [39,88].Long reaction times can also lead to the deactivation of metal catalysts [39].Thus, catalyst regeneration is required.The catalyst deactivation can also occur when its pore structure is destroyed.As a result, the diffusion of lipids through the pores of solid catalysts is inhibited [39,89].The main advantages of acid catalysts over alkaline heterogeneous catalysts for BD production are reduced catalyst deactivation and product contamination [89].

Heterogeneous Alkaline Catalysis
The heterogeneous alkaline catalysis features easy final product separation and a fast reaction rate [90].The most widely used heterogeneous alkaline catalysts are CaO and MgO due to their low solubility in MeOH and high catalytic activity [41,90].A summary of the used heterogeneous alkaline catalysts in the transesterification process of microalgae oil is given in Table 4. Ma et al. [91] used a KOH/Al2O3 catalyst for the in situ transesterification of microalgae Chlorella vulgaris for BD production.The BD yield was 89.5 ± 1.6 wt% after 5 h of reaction at the optimum working conditions (10 wt% of KOH/Al2O3 and 60 °C) [91].
The BD production from the same Chlorella vulgaris biomass increased to 67.3 ± 2.2% and 71.0 ± 3.3% when the in situ transesterification using microwave irradiation and ultrasound irradiation, respectively, was carried out for a reaction time of 60 min using a KF/CaO catalyst.Moreover, the highest BD yield of 93.1 ± 2.4% was achieved after in situ transesterification of microalgae using combined ultrasound and microwave irradiation in 45 min [92].
Kazemifard et al. [88] used a mixed microalgae biomass for in situ transesterification using a magnetic KOH/Fe2O3-Al2O3 nanocatalyst.A conversion of 95.6% of microalgae lipids into esters was achieved after a reaction time of 6 h at 65 °C [88].
Table 5 presents the data on the application of heterogeneous acid catalysts for the transesterification of microalgae oil.Guldhe et al. [86] obtained a maximum BD conversion of 98.3% from Scenedesmus obliquus using 15 wt% chromium-aluminum catalyst at 80 °C for 4 h.A slightly lower BD conversion of 94.6% was obtained from the same microalgal biomass using 15 wt% tungstated zirconia (WO3/ZrO2) at 100 °C in 3 h [98].Additionally, a phosphotungstic acid-modified zeolite imidazolate framework (HPW/ZIF-67) was studied as an acid-base bifunctional heterogeneous catalyst for BD production from Chlorella vulgaris.The achieved conversion efficiency of the catalyst was 98.5% at 200 °C after 90 min [51].Lower BD conversion of 51.9 and 71.4% were obtained using 4 wt% of WO3/ZrO2 from Scenedesmus sp. after microwave and ultrasound-assisted in situ transesterification, respectively [99].Loures et al. [83] achieved a BD yield of 98% using Nb2O5/SO4 as a heterogeneous acid catalyst at 250 °C for 4 h in a pressurized stainless-steel reactor.
Recently, a carbon-based heterogeneous catalyst (DMB), synthesized by carbonization of de-oiled Tetradesmus obliquus KMC24 microalgae biomass followed by sulfonation, was studied for BD production [100].The optimum conditions for a maximum FAME yield of 94.2% were MeOH/oil molar ratio of 11:1, a catalyst concentration of 4 wt%, a temperature of 70 °C, and a reaction time of 8 h [100].

Enzyme Catalysts
Several parameters affect enzymatic transesterification, such as temperature, alcohol-to-oil ratio, alcohol selection, organic solvents, water content, pH, and enzyme loading.Many lipases remain active below 70 °C, but the optimum reaction temperature depends on the alcohol-to-oil molar fraction, the type of organic solvent, and the immosystem used.Additionally, a slight excess of alcohol over alcohol/oil molar ratio of 3−5:1 is needed for the enzymatic transesterification process [56].Different alcohols, such as MeOH, EtOH, propanol, isopropanol, and isobutanol, are used in the enzymatic transesterification reaction, out of which MeOH and EtOH have a lower cost than other alcohols [41,56].However, these two alcohols can easily denature lipases.This issue can be overcome by using organic solvents, which can increase the alcohol solubility, thus protecting the enzyme from the degradation caused by the alcohol.It has been found that the optimum water content for most lipases is 10−20%, and the optimum pH of enzyme activity ranges between 7.5 and 8.5.Additionally, an enzyme concentration of 0.7 mg/mL was enough to achieve the maximum reaction rate [56].
Enzymatic transesterification (ET) of microalgal lipids has been studied for BD production, as shown in Table 6.Navarro López et al. [120] investigated the production of FAME from wet Nannchloropsis gaditana biomass via direct enzymatic transesterification using the lipase Novozyme 435.A FAME conversion of 99.5% was achieved after 56 h adding MeOH in 3 stages and t-butanol to decrease lipase deactivation [120].Tian et al. [121] developed a novel lipase-mediated process for BD conversion from Schizochytrium sp.oil.The authors stated that a FAME yield of 95.0% could be obtained with the combined use of lipase NS81006 and Novozym435 [121].
Tran et al. [101] achieved a high conversion of 97.3% from the oil of Chlorella vulgaris ESP-31 using immobilized Burkholderia sp.C20 lipase.Sánchez-Bayo et al. [110] obtained a FAEE conversion of 97.2 mol% from Isochrysis galbana using lipase B from Candida antarctica and Pseudomonas cepacia supported on SBA-15 mesoporous silica.Bayramoglu et al. [115] examined the conversion of microalgae Scenedesmus quadricauda oil to BD with free and immobilized lipase on the biosilica-polymer composite.Conversions of 85.7% and 96.4%, with the free and immobilized enzymes, respectively, were achieved.Tran et al. [113] carried out a one-step transesterification process to directly convert Chlorella vulgaris ESP-31 oil with MeOH into BD using immobilized Burkholderia lipase as the catalyst.A FAME conversion of 95.1% was obtained using hexane as solvent [113].Additionally, Guldhe et al. [109] achieved a conversion efficiency of 95.4% at a reaction temperature of 50 °C and a MeOH to oil ratio of 3:1 from Acutodesmus obliquus oil using immobilized Candida rugosa lipase.
Huang et al. [24] investigated the use of a recombinant lipase (GH2) for the catalytic conversion of microalgae Chlorella vulgaris oil mixed with MeOH or EtOH for BD production.Conversion rates higher than 90.0% were obtained [24].
As can be noted, the reaction time of the enzymatic transesterification process is 3 to 6 times longer than that of acid-/base-catalyzed transesterification.Moreover, the cost of enzymes, such as Novozym ® 435, is much higher than that of other enzymes and acid/base catalysts.Thus, enzymes which are novel, cheap, and tolerant to alcohols must be developed to improve the BD production yield and reduce the cost involved in the process [39].

Conclusions
Microalgae, as photosynthetic microorganisms with simple growing requirements, have been reported as promising feedstock for biofuel production, but several issues must be overcome.More research and technical development are necessary for microalgae biodiesel production to become competitive in terms of costs with fossil fuels.Special attention should be given to the transesterification process to make biodiesel production more cost-effective and environmentally friendly.The optimum conditions of the transesterification process (solvent/oil molar ratio, amount of catalyst, reaction temperature, and reaction time) should be optimized to maximize biodiesel production.More advanced research should be conducted on developing stable, reusable catalysts with high performance at a reduced cost.Extensive work on scaling up the technology for converting microalgal oil into biodiesel should be carried out.Additionally, the development of innovative techniques for the large-scale cultivation and harvesting of microalgae, the selection of microalgae species with high oil contents, and the utilization of wastewater for microalgae growth represent aspects that need more attention to reduce costs of the overall biofuel production process.

Table 2 .
Homogeneous alkaline catalysts studied for the transesterification of microalgae oil.

Table 3 .
Summary of homogeneous acid catalysts studied for the transesterification of microalgae oil.

Table 4 .
BD production using various heterogeneous alkali catalysts.

Table 5 .
Summary of heterogeneous acid catalysts studied for the transesterification of microalgae oil.

Table 6 .
Enzymatic BD production from different feedstocks.