An overview of solid base heterogeneous catalysts for

Catalysis Reviews Science and Engineering

ISSN: 0161-4940 (Print) 1520-5703 (Online) Journal homepage: http://www.tandfonline.com/loi/lctr20

An overview of solid base heterogeneous catalysts for biodiesel production Akshey Marwaha, Amit Dhir, Sunil Kumar Mahla & Saroj Kumar Mohapatra To cite this article: Akshey Marwaha, Amit Dhir, Sunil Kumar Mahla & Saroj Kumar Mohapatra (2018): An overview of solid base heterogeneous catalysts for biodiesel production, Catalysis Reviews To link to this article: https://doi.org/10.1080/01614940.2018.1494782

Published online: 11 Jul 2018.

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CATALYSIS REVIEWS https://doi.org/10.1080/01614940.2018.1494782

An overview of solid base heterogeneous catalysts for biodiesel production Akshey Marwahaa, Amit Dhirb, Sunil Kumar Mahlac, and Saroj Kumar Mohapatraa a

Department of Mechanical Engineering, Thapar Institute of Engineering & Technology, Patiala, India; School of Energy and Environment, Thapar Institute of Engineering & Technology, Patiala, India; c Department of Mechanical Engineering, I.K. Gujral Punjab Technical University, Hoshiarpur, India b

ABSTRACT

ARTICLE HISTORY

The alcoholysis process requires high activity catalysts for biodiesel production. Heterogeneous catalysts have been proven to possess highly active nature and are environment-friendly. The present article emphasizes on various types of solid base catalysts that have been used in the recent past for the production of biodiesel by transesterification of oils. The parameters and conditions affecting the transesterification reaction and biodiesel yield have also been mentioned in the article. Heterogeneous catalysts have the capability to be recycled for many runs in the process without greatly abating the biodiesel yield. Also, such catalysts possess noncorrosive nature, thus making the biodiesel safe to be used in engine without any damage. The exploitation of waste materials as catalysts would reduce the overall production cost of biodiesel. Calcium-based catalysts in the reviewed literature have shown promising outcomes for the future use and would make the process economical for large-scale industrial applications.

Received 19 December 2017 Accepted 26 June 2018 KEYWORDS

Biodiesel; cost effective; heterogeneous catalyst; transesterification; yield

1. Introduction Modern world scenario has changed drastically over the past few decades in terms of energy demand and availability. Humungous use of resources by the mankind will certainly lead to a shortage of resources in the near future. The need to search for alternative sources of energy has massively increased due to the changing lifestyle of humans and necessity of comfort. Power generation industries, agricultural and transportation sector have a huge dependence on fossil fuels for energy requirement. The major drawback of using fossil fuels is the generation of CO2, which is responsible for the growth of greenhouse gases emission, thus resulting in global warming. CO2 is the major constituent of greenhouse gases. This has led to anthopogenic climate change which is likely to continue further for several centuries. Climate change has proved to be the greatest future challenge for the mankind.[1–3] CONTACT Akshey Marwaha [email protected] Department of Mechanical Engineering, Thapar Institute of Engineering & Technology, Patiala, India Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lctr. © 2018 Taylor & Francis

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The scarcity of petroleum reserves has led to use of renewable energy sources produced from biomass feedstocks.[4] Major complication with using biodiesel is the high viscosity when raw oil is used as motor fuel directly. Unlike gasoline and diesel, biodiesel cannot be used directly as fuel. This is due to the presence of water, free fatty acids, odorants, sterols, phospholipids, and other impurities in the crop-derived oils. Otherwise, slight modifications may be required in the engine of the vehicle to incorporate the use of biodiesel in it.[5] Biodiesel, also known as FAME (fatty acid methyl ester), is a type of alternative fuel derived from edible or non-edible crops. In terms of chemical definition, biodiesel comprises monoalkyl esters of long chain fatty acids.[5,6] Direct use and blending, microemulsions, thermal cracking (pyrolysis), and transesterification are the four main methods to produce biodiesel. Of these, transesterification reaction is the most widely used process.[7] The reaction involves the conversion of triglycerides (fats) to methyl esters assisted by the presence of a catalyst.[8] Transesterification reaction is a three-step process and involves reversible reactions.[9] Di- and monoglycerides are formed as intermediates in the reaction. As per the stoichiometry, 3 mol of methyl ester and 1 mol of glycerol are formed when 3 mol of triglyceride reacts with 1 mol of short chain alcohol.[10–12] The obtained yield is dependent upon the variations of molar ratio of methanol to oil, reaction temperature, catalyst amount, and reaction time. The fatty acids produced can differ by length of carbon chains, the number, orientation, and position of double bonds in the chain.[5] Biodiesel is one of those sources which have higher heating values (39–41 MJ/ kg) greater than that of coal (32–37 MJ/kg) but lower than gasoline (46 MJ/ kg).[13] It acts as a lubricity improver and thus helps in reducing wear of fuel pumps. Biodiesel is a nontoxic, renewable, and biodegradable fuel as compared to petro-diesel fuels. Also, the usage of biodiesel will help in maintaining a balance among environment, agriculture and economic development.[5] The quality of biodiesel is surrendered by the poor cold flow nature and autooxidation of biodiesel.[14] The cold flow behavior is observed at decreasing temperatures. Biodiesel tends to become a gel at low temperatures which can result in clogging of lines or filters. This leads to a problem when using biodiesel blends in vehicles in the winter.[15] Biodiesel may be subjected to conditions resulting in oxidation and subsequent degradation when exposed to air during use or storage which affects the quality and performance of the fuel. This phenomenon is measured by a property called oxidative stability. The presence of double bonds in the chains of fatty acid molecules is responsible for autoxidation of biodiesel.[15,16] This is affected by the presence of by-products formed during decomposition. Various biodiesels may tend to undergo oxidation in different conditions, thus resulting in a difference of stability of the fuel.[17] Treatment with antioxidant additives such as hydrogen to reduce the double bonds is a cost-effective way to improve the oxidative stability of biodiesel.[18] NOx emissions are also

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high when biodiesel blends are used.[19] Also, the cost of biodiesel production is a limitation. Biodiesel can be competitive in the market only if excise tax is not applied to it. So, the most crucial factor is the economics of biodiesel production.[20] 2. Potential feedstocks and their properties for biodiesel production Different regions have availability of different crops; therefore, the biodiesel feedstock depends on climate, local soil conditions, and availability,[21] as the feedstock constitutes roughly 75% of overall cost of biodiesel production. Therefore, for a commercial production, feedstock must be economical to use and should be easily available.[17] The biodiesel feedstock may be of thee different types: edible vegetable oils, nonedible vegetable oils and animal fats, and microalgae and fungi oil. Soybean, peanut, canola, sunflower, palm, coconut oil come in the first category while jatropha, karanja, neem oil, tung oil belong to the second category.[17,21–24] Edible oils such as rapeseed, sunflower, soybean, and palm oil have been mainly used for the commercial production. But cost of these oils is more than the diesel fuel. Due to its high cost, commercialization of biodiesel from edible sources is not suitable. Overuse of edible food crops will eventually lead to a shortfall of food in developing countries.[25] So, the main focus has now been diverted toward waste cooking oils (vegetable) or frying oil, soapstocks, and animal fats as they possess potential for making of low-cost biodiesel.[26–28] Utilization of animal fats as feedstocks can avoid the exploitation of edible crops.[29] The use of nonedible oils and waste cooking oils is very relevant especially in developing countries like India where usage of edible oils as fuel is not economical and practical.[30,31] Almost 80% of crude oil is imported in India to meet the demands.[32] Therefore, nonedible crop such as castor oil has good potential due to its high yearly production and ability to grow on dry land.[33] In addition, nonedible crops such as Jatropha curcas most likely reclaim wasteland with no special conditions required for their growth whereas edible crops tend to grow in limited areas with suitable conditions required for the respective crop.[34,35] More focus on optimization, kinetics, and improvement of biodiesel yield from nonedible sources is needed to avoid dependency on food crops.[36–39] Research in biotechnology has been done to develop biocrops as an alternative to edible crops. This has led to the progress of genetically engineered plants possessing economic, energetic, and environmental advantages over edible crops. It accounts for a sustainable way of obtaining biodiesel with bio-energy crops without destroying edible crops. Switchgrass, miscanthus, and big bluestem are such bio-crops having superior characteristics over food crops.[40,41] The physicochemical properties of various feed stocks have been shown in Table 1.[26,41,44,45]

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Table 1. Physicochemical properties of various oils. Feedstock (oil) Corn Cottonseed Linseed Peanut Rapeseed Safflower Sesame Soya bean Sunflower Palm Babassu Jatropha Karanja Neem Castor Mahua Tallow

High heating value (MJ/kg) 39.5 – 39.3 – 39.7 39.5 39.3 39.6 39.6 – – 39–40 – – 37.4 36.0 –

Density (kg/m3) 909:5 915 923:6 903 – 914:4 – 913:8 916:1 918:0 946:0 912 936.5 918.5 955.0 960 903

Kinematic viscosity at 38°C (or 40°C) mm2/s 34:9 33.7 (at 40°C) 27.2 40 (at 40°C) 37:0 31:3 35:5 32:6 33:9 39:6 30:3 55 (at 30°C) 43.61 (at 40°C) 50.30 (at 40°C) 251.20 (at 40°C) 24.50 (at 40°C) 51.2 (at 40°C)

Flash point 277 234 241 271 246 260 260 254 274 267 150 240 – – – 232 201

Pour point (° C) −40.0 −15.0 −15.0 −6.7 −31.7 −6.7 −9.4 −12.2 −15.0 – – – – – – – –

Cetane number Reference 37:6 [42] 33.7 [26] 34.6 [43] 34.6 [26] 37.6 [41] 41.3 [42] 40.2 [41] 37.9 [43] 37.1 [41] 42:0 [42] 38:0 [43] 40–45 [41] – [32] – [32] 42.3 [32] – [32] 40.2 [26]

Chemical composition of various edible and nonedible oils has been discussed in Table 2.[26,27,29,45–48,51,52] Table 3 discusses the oil yield obtained per year from different sources. The properties of biodiesel from different feed stocks depend upon structural features of alcohol and fatty acid species. These structural features include a degree of unsaturation, chain length, and branching of chain. Also, cetane number, viscosity, heat of combustion, and oxidative stability are some of the properties which are influenced by the structural features.[64] Ethanol-blended methyl ester has shown improved performance in properties of biodiesel. Although most of the properties of biodiesel are comparable to diesel fuel, the low-temperature properties of biodiesel are not suitable for use. Cold flow behavior of biodiesel can be improved by blending it with ethanol and kerosene. Cold Filter Plugging Point tends to increase with the increase of storing time of biodiesel. Also, the smoke and NOx emission level tend to decrease with blending of ethanol. The biodiesel follows Newtonian behavior down to the pour point value.[19,65,66]

3. Homogenous catalysis Homogeneous base catalysts are the most common catalysts used in the industrial applications. These catalysts react in the same phase as the reactants. KOH, NaOH, CH3NaO, and CH3OK are some of the widely used base catalysts that are highly reactive, require mild temperatures and molar ratios, and give high yields. Reaction rates by homogeneous catalysts are faster than

Tr: Traces.

Oil Corn Cottonseed Canola Linseed Peanut Rapeseed Safflower Sesame Soya bean Sunflower Jatropha Karanja Neem Mahua Tallow

C 14:0 0.05 – 0.06 0 Tr 0 – 0 0 0 – 0 0.2–0.26 – 2–8

C 16:0 10.99 28.3 4.20 5 6–9 3.49 6.50 13 10.58 6.08 12.8 10.2 13.6–16.2 24.5 24–37

C 18:0 2.02 0.9 1.70 2 3–6 0.85 2.45 4 4.76 3.26 7.3 7 14.4–24.10 22.7 14–29

C 20:0 0.42 0 0.59 0 2–4 – – 0 – – – – 0.8–3.4 – 1.2

C 22:0 0.13 0 0.32 0 1–3 – – 0 – – – – – – 0

C 24:0 0.16 0 0 0 – – – 0 – – – – 0 – –

Fatty acid composition wt.%

Table 2. Chemical composition of various edible and non-edible oils. C 18:1 27.03 13.3 58.51 20 53–71 64.40 15.1 53 22.52 16.93 44.8 51.8 49.1–61.9 37 40–50

C 18:2 57.39 57.5 21.19 18 13–27 22.30 73.6 30 52.34 73.73 34 17.7 2.3–15.8 14.3 1.5

C 18:3 0.96 0 10.12 55 0 8.23 0.11 0 7.19 0 1.1 3.6 – 0 0

C 22:1 0 0 0.50 0 0 – 0 0 – – – – – – 0

References [46] [47,48] [46] [43] [27] [26] [43] [43] [29] [26] [49] [50] [43] [50] [27]

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Table 3. Oil content of various feedstocks used for biodiesel production. Oil type Edible

Nonedible

Other sources

Feedstock used Soybean Sunflower Canola Corn Rapeseed Palm Castor Linseed Jatropha Karanja Rubber Tetraselmis suecica (Africa) Algae Algae (50 g/m2/day at 50% TAG [triacyglycerols]) Microalgae (with 30 wt.% oil from biomass) Microalgae (with 50 wt.% oil from biomass)

Oil yield (L/ha) 446 952 974 172 1190 2308.85 1026 (kg/ha) 478 741 7800 (kg/ha) 150 (kg/ha) 66,000 (kg/ha) 100,000 98,500 58,700 136,90

References [53] [54] [55] [54] [54] [56] [57] [58] [55] [55] [59] [60] [61] [62] [63] [63]

homogenous acid-catalyzed reactions; however, the former are susceptible to water thus forming soaps in the solution, eventually consuming the catalyst. Thus, emulsification of catalyst in solution leads to no possibility of catalyst regeneration. This leads to reduced methyl ester yields, as the glycerol formed may also mix with methyl esters. The removal of catalyst from the solution after reaction is not practical as excess water is required for biodiesel washing and neutralization by an alkali or acid.[67–70] Homogeneous acid catalysts are suitable for the alcoholysis of oils containing high levels of free fatty acid content. Sulfuric acid and methanesulfonic acid are good acid catalysts. Esterification of fatty acids is a faster reaction than the transesterification of triglycerides, as the former is a one-step process, whereas the latter requires the steps for complete reaction.[71] However, acid catalysts require higher temperature (≥100°C) and longer reaction times (≥4 h).[72] The corrosive nature of acid catalyst may damage the equipment, and the yield produced is relatively lower yield with base catalysts. Using RSM (Response Surface Methodology) along with CCD (Central Composite Design) has been utilized vastly to obtain suitable parameters for an economical biodiesel production.[29]

4. Transesterifiction by heterogeneous catalyst In heterogeneous catalysis, transesterification reaction proceeds with a catalyst having a different phase than that of the reactants. With easy separation of glycerol from biodiesel and reusability of catalyst, heterogeneous catalysis qualifies as a green process,[73] where high-quality glycerol with high purity levels (at least 98%) is obtained alongside biodiesel.[74] Figure 1[12,68] depicts the mechanism of conversion of triglyceride and fatty acids to biodiesel.

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Figure 1. Mechanism of transesterification of triglyceride (i) and esterification of free fatty acid (ii).[12,68]

While using waste cooking oil, high FFA content in oil hinders the biodiesel conversion. The two-step process consisting of esterification of FFA and then transesterification of triglyceride is reduced to single-step process. Urea can be produced by using CO2 along with ammonia formed in the synthesis of glycerol carbonate and dimethyl carbonate, thus reducing the carbon foot print along with complete utilization of by-product glycerol.[75] Transesterification can be of catalytic or non-catalytic type as shown in Figure 2.[25,27,40,68,76,77] Noncatalytic methods include Biox process and supercritical technique where biox cosolvent process converts lipids (feedstock) to methyl esters by using inert cosolvents to generate an oil-rich system and supercritical method incorporates the use of very high pressures and temperatures to achieve high yield within very short times. From the viewpoint of heterogeneous catalysts, the most crucial factors for successful application in industries are low cost, availability, activity, and reusability.[78] Advancements in the field of nanoparticles as catalysts have rose due to high selectivity and stability. High activity from nano-sized catalyst has been achieved due to a large surface area because of pores on their surface. Sodium titanate nanotubes have gained popularity as nano-catalyst for a more active conversion of feedstocks.[79] Solid acid and base

Figure 2. Classification of transesterification reaction.[40,68,76]

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heterogeneous catalysts come under the category of inorganic type catalysts along with oxides, sulfides, and metal salts. Carbon-based catalysts as supports possess high surface area and thermal stability, thus having qualities of good heterogeneous catalyst.[28] Organic type catalysts include ion exchange resins and enzymes. Biocatalysts in form of immobilized enzymes have also been used in transesterification reactions. Heterogeneous catalysts are preferred for continuous catalytic transesterification process, unlike homogeneous catalysts which are suitable for batch production only.[37] Catalytic activity, catalyst life, and oil flexibility greatly affect the cost of biodiesel. In this way, heterogeneous catalyst is eco-friendly and cost-effective. These catalysts may also be classified on the basis of their operating temperatures below and above 130°C as low-temperature and high-temperature catalysts respectively.[80,81] Heterogeneous catalysts can be regenerated when their activity begins to reduce after several runs of cycle. The cause of deactivation of a catalyst may be of thee types: mechanical, chemical, and thermal. Design studies in the recent past have concluded that powdered catalyst may be unsuitable for production at industrial scale due to the small particle size of catalyst as it may affect catalytic activity and methyl ester yield. The morphology and shape of catalyst play a huge role in heterogeneous catalyst. Spherical catalysts have good stability and are easy to separate from reaction mixture.[40] Heterogeneous catalysts are capable of superseding the homogeneous catalysts for the commercial biodiesel production, as it will spare uneconomical generation of wastewater in the future.[81,82] Calcium oxide, for example, has potential as a solid catalyst due to its low cost and high activity; however, soap solution poses problem when CaO is used for high FFA oils.[83] Solid acid heterogeneous catalysts prove to be more effective in conversion of low-grade feedstock without minimal catalyst deactivation.[84–86] Therefore, there is a need to discover the ideal heterogeneous catalyst having low cost, high activity, high stability, and high recyclability.[14,87,88]

5. Solid base catalysts High specific surface area along with a high concentration of basic sites available on the surface ensures high catalytic activity of basic oxide catalysts. Solid base catalysts manage to give the same yield result with lower temperatures and less time.[86] The only limitation of these catalysts is their tendency to form soaps when FFA content of the oil is quite high (>2%).[44] CaO-based catalysts are widely popular because of their low cost and high effectiveness.[89]

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5.1. Metal oxides 5.1.1. Direct use of metal oxides Gryglewicz et al.[90] discussed the scope of using low-cost CaO catalyst for methanolysis of rapeseed oil where ultrasonication along with 10 wt.% THF (tetrahydrofuran) improved the reaction rate to achieve conversion in 2 h at 4.5:1 molar ratio of methanol to oil. Kouzu et al.[91] employed CaO catalyst to achieve a yield of 93% in reaction time of 1 h, which elevated to above 99% yield in 2 h time. Also, comparison of catalytic activity of MgO, CaO, and SrO was studied as depicted in Figure 3. Liu et al.[92] used CaO for transesterification of soya bean oil and concluded that a certain amount of water in reaction mixture enhanced the biodiesel yield. Ninety-five percent yield was obtained in 3 h using a molar ratio of 12:1, catalyst loading of 8 wt.%, reaction temperature of 65°C with water addition in methanol at 2.03% where the activity of CaO was stable even after 20 cycles of use. Kawashima et al.[6] studied the acceleration of CaO activity achieved by stirring it with methanol for 1 h at 25°C and successive drying. Activated CaO showed higher catalytic activity than nonactivated CaO. CaO–glycerin complex formed during reaction accelerated the reaction. An amount of 0.1 g CaO, 3.9 methanol, 15 g rapeseed oil with 1.5 h activation time were optimal parameters to obtain 90% FAME yield in 3 h at 60°C. Boey et al.[93] reviewed the potential of CaO as catalyst and observed that CaO with its diverse nature can be used in as individual, loaded, or in mixed form and also as support for different catalysts. Conversion of CaCO3 to CaO has proved to be the most cost-effective way to use the catalyst. A study by Di Serio et al.[94] revealed that MgO prepared by calcination of Mg(CO3)4·Mg(OH)2 at 400°C for 18 h exhibited high activity with high surface area (229 m2/g), pore volume (0.61 cm3/g), and specific basicity

Figure 3. Effect of MgO, CaO, and SrO on FAME yield at 0.5 and 1 h reaction time.[91]

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Figure 4. Pore volume distribution of various catalysts.[94]

(893 CO2 µmol/g). The large mean sized pores on catalyst surface helped in achieving over 90% yield from soybean oil at 200°C temperature in 1 h time. Figure 4 shows the pore volume values of different catalysts. CHT and MgO (c) and MgO(I) possess mesopores whereas MgO(II) and MgO(III) possess micropores. Figure 5 displays FAME yields achieved with various catalysts used.

Figure 5. Biodiesel yields with various catalysts at T = 180°C and 1 h reaction time.[94.

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Liu et al.[95] achieved maximum yield of 95% from soybean oil in 30 min using 3 wt.% SrO catalyst with 12:1 molar ratio below 70°C reaction temperature. SrO was used for 10 cycles of run with minimal activity reduction. Similar results were obtained when a study on ultrasonic-assisted transesterification with CaO, SrO, and BaO conducted by Mootabadi et al.[96] revealed that 20 kHz ultrasonication at 50% amplitude improved the yield in lesser time and lower molar ratio when compared to mechanical stirring (800 rpm). BaO and SrO showed that better yield results over CaO and catalyst amount of 3 wt.%, molar ratio of 9:1, and reaction time of 1 h were considered optimum parameters. The high tendency of BaO for leaching in biodiesel resulted in a loss of catalyst, whereas SrO possessed lesser leaching tendency and high activity. Choudhury et al.[97] utilized ultrasound technology (35 kHz, 35 W) for transesterification of JCO with CaO catalyst. The reaction proceeded in two steps: esterification with H2SO4 followed by transesterification with Ca(OMe)2 where a molar ratio of 11:1 with 64°C temperature and 5.5 wt.% catalyst resulted in over 90% yield in 2.2 h time. CaO calcined at 900°C (for 3 h) reacted with methanol, thus giving Ca(OMe)2, an active catalyst validated with XRD results. Ultrasonification enhanced the diffusion of methanol–oil mixture, thus the methoxy formation resulting in better yields in shorter times. 5.1.2. Loaded metal oxides In loaded metal oxides, the precursors used for the preparation of the loaded element are supported on base catalyst. Watkins et al.[98] used Li-loaded CaO catalyst, where high-Li loading resulted in a drop of surface area and catalytic activity. Li loading of 1.23 wt.% was found optimal for the formation of methyl butanoate. X-ray photoelectron spectroscopy and diffuse reflectance infrared fourier transform spectroscopy measurements showed that optimal loading agreed with the formation of electron deficient surface Li+ species and –OH species at defect sites on support. Doping of Li elevates base strength of CaO. Meher et al.[99] compared catalytic performance of Li/CaO catalyst with Na/CaO and K/CaO for transesterification of Karanja oil where 94.9% FAME yield was obtained with optimal parameters: 12:1 molar ratio, 65°C reaction temperature, and 2 wt.% Li/CaO in 8 h. Li/CaO possessed higher basicity than Na/CaO and K/CaO, thus Li/CaO proved to be reliable alternative to homogeneous catalysts especially when FFA of oil is >1%. Kinetics and mechanism study of Zr/CaO carried out by Kaur et al.[51] for conversion of JCO revealed that with 15 wt.% Zr loading on CaO, the catalyst calcined at 700°C performed the best. Catalyst characterization of 15-Zr/CaO-700 was executed with SEM and TEM techniques where SEM indicated irregular shape of Zr/CaO formed in 0.5–2 µm size range, whereas TEM revealed the formation of nano-clusters of Zr/CaO in quasi-spherical shape with

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Figure 6. FE-SEM and TEM image of 15-Zr/CaO-700 catalyst.[51]

average size of about 30 mm. Figure 6 displays the SEM and TEM images of 15-Zr/CaO-700 catalyst where nano-clusters of Zr/CaO are in quasi-spherical shape. Ninety-nine percent yield was achievable with 5 wt.% catalyst and molar ratio of 15:1 (methanol:oil) at 65°C and 21:1 (ethanol:oil) at 75°C. Koras–Nowak criterion test confirmed that kinetics of reaction were independent of transport phenomena. A study on Li-loaded NiO used for the ethanolysis of waste cottonseed oil by Kaur et al.[100] found that basic strength and stability of catalyst increased with 5 wt.% Li impregnation. Calcination temperature of 600°C was optimal for activation of catalyst and its activity showed increment with enhanced Li doping. 5-Li/NiO-600 catalyst with 5 wt.% catalyst, 600°C calcination temperature, and ethanol/oil ratio of 12:1 with 65°C temperature gave over 98% FAEE yield. Basic metal ions caused saponification on interaction with high FFA content in oil. Figure 7 indicates consistent loading of Li ions over NiO. Cu-doped zinc oxide nano-catalyst tested by Gurunathan et al.[87] gave high yield of 97.18% with molar ratio of 10:1, 55°C reaction temperature, and 10 wt.%

Figure 7. FESEM and HTEM image of 5-Li/NiO-600 catalyst.[100]

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catalyst in 1 h time. Results from experimental data revealed first-order kinetic model suitable for reaction. GC–MS analysis confirmed the presence of FAME in reaction mixture. Similar results were obtained by Baskar et al.[35] who used ferromagnetic zinc oxide catalyst. Structural properties of ZnO were enhanced with iron loading. The reaction kinetics was dependent on reaction time and temperature and first-order model fitted into kinetic model obtained gave the activation energy. A molar ratio of 12:1, reaction temperature of 55°C, and 14 wt. % catalyst loading resulted in 91% conversion within 50 min time. Mahesh et al.[101] used KBr-loaded CaO catalyst to produce biodiesel from WCO. Characterization of catalyst was performed with SEM, XRD, and FTIR. ANOVA analysis predicted optimum parameters as 12:1 molar ratio, 3 wt.% catalyst amount, and 1.83 h of reaction time giving a yield of 83.6%. 5.2. Metal hydroxides

Sarve et al.[102] tested the capability of Ba(OH)2 catalyst for biodiesel production from sesame oil under sonication (1.2 kW power and 20 kHz frequency). RSM predicted the optimal parameters of the reaction as 1.79 wt. % catalyst concentration, 6.69:1 molar ratio, 31.92°C reaction temperature, and 40.30 min reaction time. A percentage of 98.6 of yield was achieved with 1.13% difference between predicted and actual FAME values where sensitive analysis revealed catalyst amount as most crucial factor for better FAME yield. Later, Sarve et al.[103] conducted a two-step conversion of kusum oil with esterification (with H2SO4) followed by tranesterification by Ba(OH)2. A percentage of 96.8 of conversion was recorded with conditions: molar ratio (methanol/oil) of 9:1, catalyst amount of 3 wt.%, reaction temperature of 50° C, and 80 min reaction time. The reaction kinetics boosted with increased reaction temperature as indicated by second-order kinetics reaction which fitted well experimentally. 5.3. Mixed metal oxides

Taufiq-Yap et al.[104] compared catalytic activity of CaMgO and CaZnO with CaO, MgO, and ZnO for conversion of JCO. Catalyst amount of 4 wt.%, molar ratio of 15:1, reaction time of 6 h, and reaction temperature of 65°C were optimal parameters. Although CaO gave slightly higher conversion, CaMgO and CaZnO maintained their activity even after six runs of use, whereas the activity of CaO reduced considerably after the fourth run. CaMgO was found to be more active than CaZnO. Figures 8 and 9 display SEM images of CaMgO and CaZnO, respectively, where the former is densely agglomerated and the latter showed homogenous distribution of tiny spherical particles. Calcium oxide has also been used as loading on catalyst due to its basicity. Martínez et al.[105] used prepared CaO nanoparticles supported on faujasite

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Figure 8. SEM image of CaMgO.[104]

Figure 9. SEM image of CaZnO.[104]

zeolite (NaX) and varied the loading amount of CaO to verify its effect on the yield of FAME. Results showed that larger yield is achieved with higher basicity of catalyst. Faujasite zeolites have relatively higher basicity which further increases with induction of alkali earth oxides such as CaO or BaO. Molar ratio of methanol to oil at 6:1, catalyst with a loading of 16%, and reaction time of 6 h gave the highest yield of 93.5%. Results from X-ray diffraction showed that addition of oxides does not notably change the structure of faujasite. Figure 10 reveals linear relationship between CaO concentration and FAME yield up to 16 wt.% concentration. Figure 11 showing SEM image of catalyst indicates the quasi-spherical shape of NaX zeolite. CaO can also be supported on high surface area supports having high porosity such as silica or alumina. CaO supported on meso-macroporous silica successfully helped in conversion of palm oil in presence of methanol, as claimed by Witoon et al.[106] Highly porous bimodal silica support was compared with uni-modal mesoporous silica. Uni-modal and bimodal silica were represented by 50 CaO/U and 50 CaO/B, respectively, with only

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Figure 10. Effect of CaO concentration on FAME yield.[105]

Figure 11. SEM image of CaO nano-16%/NaX.[105]

difference being in addition in the former for preparation. CaO-loaded silica supports were rendered active with calcination of 800°C for 4 h. High loading of CaO was necessary to obtain high yields of FAME due to enhanced basicity of CaO. A molar ratio of 12:1, reaction temperature of 60°C, and catalyst amount of 5 wt.% resulted in high yield (94.15%) of bimodal silica material 50 CaO/B-325 (pellet size ~335µm) in the first run. Activity of the catalyst was endurable with sustainable yield of 88.87% after fifth run of reuse without any treatment. Bimodal silica material performed better against the uni-modal catalyst but involves more expensive procedure than the latter. Mixed oxides of calcium, CaO–NiO and CaO–Nd2O3, were made use of by Teo et al.[107] When compared with CaO, both catalysts exhibited high activity, basicity, and stability. Unlike CaO in which leaching phenomena are common, the two catalysts were easy to separate from solution. No saponification resulted from the use of CaO–NiO and CaO–Nd2O3. Both the catalysts underwent calcination of 900°C for 6 h resulting in active crystalline

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phases. Molar ratio of 15:1, catalyst amount of 5 wt.%, and reaction temperature of 65°C resulted in a yield of over 80% for both the catalysts. However, CaO–NiO possessed superior activity than CaO–Nd2O3. The activity of both the catalysts was sustained at over 80% even after the sixth reuse cycle. CaO–MgO-loaded Al2O3 was used as catalyst with ethanol to produce biodiesel from cottonseed oil by Mahdavi et al.[108] CaO/MgO catalysts in mass ratio 8:2 were dried at 70°C for 1 day followed by calcination at 700°C for 5 h. Figure 12 displays the SEM image of 10 wt.% CaO–MgO/Al2O3 revealing fine stack of Ca and Mg oxide particles settled on the Al2O3 surface. Thus, high basicity of CaO–MgO catalyst can be achieved by increasing the amount of CaO. The desorption curves of CaO-MgO/Al2O3 catalyst prepared with varied amounts (wt. %) of CaO were observed at different temperatures as shown in Figure 13. Box–Behnken design was utilized to speculate the optimal parameters of molar ratio as 12.24, reaction temperature of 95.63°C, and14.4 wt.% loading of CaO–MgO on alumina to achieve 97.62% FAEE yield. Experimental results chimed well with 92.45% conversion by the use of molar ratio of 8.5:1, 95°C reaction temperature, and 12.5 wt.% loading. The catalyst saw almost negligible loss of activity even after four cycles of run.

Figure 12. SEM image of 10 wt.% CaO–MgO (8:2)/Al2O3.[108]

Figure 13. Desorption curves from CO2-TPD profiles of CaO–MgO (8:2), 20 wt.% CaO–MgO (8:2)/ Al2O3, 25 wt.% CaO–MgO (8:2)/Al2O3, 10 wt.% CaO–MgO (8:2)/Al2O3.[108]

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A solid base mixed oxide catalyst with CaO loading on CeO2 was prepared by Wong et al.[109] Loading of 50% CaO on cerium oxide was found optimum in terms of better basicity and stability (more basic sites) with low leaching effect. Analysis by BET test revealed high surface area of the catalyst. Reaction temperature of 65°C with molar ratio of 12:1 and 5 wt.% catalyst loading for 4 h resulted in 95% yield. The catalyst when compared with commercial CaO showed superior endurability when reused for seven runs. But its activity reduced substantially after the sixth run due to leaching of CaO. A study by Kesić et al.[110] challenged the facts of recent studies regarding calcium-based perovskites. Perovskites containing small amount of CaO showed activity even at lower temperatures (~60°C) conflicting with the claim that CaTiO3, CaMnO3, CaZrO3, and Ca2Fe2O5 are not active at 60°C temperature. But for the complete transesterification of sunflower oil, higher temperatures of about 165°C were necessary. At 165°C temperature, CaTiO3, CaMnO3, and CaZrO3 successfully achieved high yield of above 90% within 2 h. The low-temperature methanolysis was largely dependent on structural properties of CaO and perovskites. The study positively proved that even small amount of CaO can boost the activity of reaction.[110]

5.4. Alkaline earth metal alkoxides

Liu et al.[111] presented a method to utilize calcium ethoxide as a catalyst for the transesterification of soybean oil. It was observed that the catalyst performed better than calcium oxide in terms of activity. A large number of small-sized pores were observed on the catalyst surface as shown in Figure 14. The catalyst held a high surface area of 15.02 m2/g with pore volume of 0.100 cm3/g. It was observed that the catalyst performed better than calcium oxide in terms of activity. A high yield of 95% was observed by using 3 wt.% of catalyst, 12:1 molar ratio of methanol/oil, and 65°C reaction temperature in 1.5 h duration. Deshmane et al.[76] observed that the use of ultrasound technology enhanced the mass transfer process for the conversion of soybean oil using calcium methoxide as catalyst. The transesterification reaction with and without

Figure 14. SEM image of calcium ethoxide.[111]

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Figure 15. SEM image of calcium methoxide.[112]

Figure 16. SEM image of calcium methoxide.[113]

ultrasonication seemed to follow a pseudo first-order kinetics. A molar ratio of 9:1 methanol/oil, catalyst amount of 1 wt.%, and 65°C reaction temperature ensured yield over 90% in 90 min time. A higher frequency of 611 kHz clearly boosted the kinetics of the reaction. Calcium methoxide, Ca(OCH3)2, was used to produce biodiesel from microalgae oil (Nannochloropsis oculata) by Teo et al.[112] Maximum yield of 92% was achievable with catalyst in presence of methanol. BET analysis revealed high surface area of catalyst (30.5 m2/g) when compared to that of CaO (6.3 m2/g). Large number of pores was available on the catalyst surface. The thin plates formed a flower-like structure on catalyst surface as shown in Figure 15. A larger molar ratio accelerated the kinetics of reaction with larger yield produced. With reaction temperature of 60°C, 3 wt.%, and molar ratio of 30:1 for duration of 3 h gave a yield of 92% FAME. XRD patterns revealed calcium methoxide at lower degrees and CaO at higher degrees. TGA tests displayed decomposition of CaCO3 at 600°C. DTA curves confirmed conversion of CaCO3 to CaO. Up to five runs of reuse, catalyst showed great stability and maintained >90% yield but reduced significantly on the sixth run. Impurities present in the microalgae oil were primarily responsible for reduced activity after some runs.[112] Suwanthai et al.[113] achieved similar results by developing a statistical model with an experimental design to obtain the optimized conditions of the reaction. Calcium methoxide used as catalyst possessed high porosity and

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high surface area with large number of pores observed as shown in Figure 16 where a flower-shaped structure was observed. The RSM technique predicted a quadratic model giving optimum conditions as 11.5:1 molar ratio, 2.71 wt. % catalyst amount, and 175 min of reaction time to obtain FAME yield of 95.99%. 5.5. Aluminum oxide (alumina)

Alumina possesses high surface area, thus making it ideal for the role of catalyst support. Arzamendi et al.[114] compared the activity of aluminasupported NaOH heterogeneous catalyst to that of homogeneous NaOH. It was found out that the transesterification rate in the initial stage was governed by catalyst-to-methanol ratio and thus increases with the loading of NaOH in the alcohol (methanol). As for the case of NaOH/γ-Al2O3, its activity was largely dependent upon the methanol-to-oil molar ratio. This can be attributed to the fact that with the increase of molar ratio, the methanol adsorption enhances, thus resulting in higher performance of the catalyst. Oil conversion of 88% was achievable with molar ratio (methanol/ oil) of 12:1 which increased to 100% when the latter was changed to 24:1 ratio. Solid base catalysts like CaO may react with free fatty acids and tend to cause saponification in the solution.[2,115] The waste shell derived catalysts have not reported the presence of soaps which is an advantage when compared to individual base catalysts. Xie et al.[116] experimented with biont shell, obtained from turtle shell for the transterification of rapeseed oil. A tristep procedure consisting of incomplete carbonization followed by KF impregnation and activation was done to prepare the catalyst. The presence of incompletely carbonized chitin when reacted with KF enhanced the activity of a catalyst. When catalyst was carbonized (incompletely) at 500° C, impregnated in 25 wt.% KF solution, and finally activated at 300°C, a yield of 97.5% was obtained with 3 wt.% catalyst amount in 3 h. Larger surface area (63.43 m2/g) of catalyst BSC-2 is attributed to presence of macro pores responsible for generation of active centers and thus high activity of catalyst. SEM analysis revealed a layer covered with micro-nanometer crystals. Ultrasonication technology, as compared to mechanical stirring, ensures faster reaction rates, reduces the catalyst amount and reaction temperature required, and also gives better yield for a lower molar ratio of methanol to oil, thus adding to the economy of process.[117–121] Nano-base catalyst KF/γAl2O3 was used for synthesis of biodiesel from soybean oil by Shahaki et al.[122] The catalyst was obtained with calcination of KF and γ-alumina at 500°C for 3 h. The catalyst underwent characterization by SEM, TEM, XRD, and FT-IR techniques. XRD results displayed the activity with the formation of K3AlF6 (potassium aluminum fluoride) and K2O (potassium oxide). Reaction

20

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mechanism and kinetics of tranesterification with ultrasonication (20 kHz frequency, 45.1 W power) were compared with mechanical stirring. A superior yield of 95% was achieved with molar ratio of 12:1, 50°C reaction temperature, catalyst amount of 2 wt.%, and 40 min of reaction time. Further, with mechanical stirring, yield of 76% was obtained with molar ratio of 15:1, 65°C temperature with 3 wt.% catalyst amount for duration of 6 h. Certainly, ultrasound-assisted transesterification helps in boosting the kinetics of reaction in lesser time without any additional treatment. SEM images of KF/γ-Al2O3 revealed the high distribution of KF all over the surface of alumina. The particle diameter of the catalyst reduced under sonication (after reaction) as displayed in Figures 17 and 18. As can be seen in SEM images, after ultrasonication reaction, KF particles are distributed all over the surface of alumina signifying uniform support achieved with effective ultrasound resulting in higher catalytic activity. Gao et al.[123] prepared highly stable monolithic KF/γ-Al2O3/HC catalyst capable of withstanding high-pressure drop prepared by coating of HC with γ-

Figure 17. SEM image of KF/γ-Al2O3 before reaction.[120]

Figure 18. SEM image after reaction under ultrasonic reaction.[120]

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21

Al2O3 followed by impregnation by KF solution. The catalyst prepared with 15% γ-Al2O3 and 50% KF solution impregnation confirmed to be most optimum loading amounts. An optimal molar ratio of 18:1, reaction temperature of 140°C, and reaction time of 33 min ensured over 96% conversion of palm oil. Similar alkaline catalytic centers identical to KF/γ-Al2O3 were loaded on HC (honeycomb ceramic) as indicated by SEM and XRD analysis. Xie et al.[124] prepared Al2O3-supported potassium iodide by impregnation of alumina with an aqueous solution of potassium. Loading of KI significantly enhanced catalytic activity and high conversions were obtained. Highest catalytic activity was obtained with 35 wt.% loading of KI on alumina and calcination temperature of 773 K for 3 h. Maximum conversion of 96% was obtained with an optimal molar ratio of 15:1, catalyst amount of 2.5 wt.%, and reaction time of about 8 h. Conversion of oil is dependent on the basicity of the catalyst. Basic sites increase with more decomposed amount of KI on support. K2O species and Al–O–K groups formed by decomposition of KI provide the most active basic sites for reaction.[124] 5.6. Waste-derived catalysts

Economical catalysts can be produced from biological and industrial wastes of no use which are commonly disposed of in landfills. Some of these wastes are rich in calcium content and thus have potential in the production of green heterogeneous catalyst. Waste egg shells are commonly found locally around us on a daily basis. As far as the composition of an eggshell is concerned, carbonate (CaCO3) content is the highest with 98.2%, followed by 0.9% magnesium carbonate and 0.9% phosphate.[125] Waste-derived catalysts have great potential in energy efficient and green synthesis of biodiesel.[126–128] Catalysts derived from waste shells of eggs, oyster, mollusk, shrimp are found in abundance and have a green impact on environment. Various wastes utilized for preparing solid base catalysts are enlisted in Table 4. Use of such catalysts eliminates the raw waste, which otherwise is going to be dumped in landfills, and subsequently provides the benefit of being costeffective catalyst.[131] Also, calcium-based catalysts prepared by commercial precursors (nitrates, carbonates, hydroxides) increase the overall cost of biodiesel production; however, use of waste shell derived catalysts can significantly reduce the production cost for various industrial applications. Using waste shells also adds value to the waste generated apart from being used as a catalyst. This helps in developing eco-friendly process and reducing the cost of catalyst used.[133] The merits and demerits of various homogeneous and heterogeneous catalysts have been shown in Table 5.[105,151–153] Heterogeneous catalysts possess an edge over homogeneous catalysts in terms of greater biodiesel yield and continuous recovery and reuse of catalyst.

CaO (Angel wing shell)

Maneerung et al. Roschat et al. Khemthong et al. Chen et al. Suryaputra et al.

16. 17. 18. 19. 20.

22. Syazwani et al.

Piker et al. Roschat et al. Nakatani et al. Boro et al. Boey et al. Sirisomboonchai et al. Buasri et al. Rezaei et al. Girish et al. Jaiyen et al.

6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

CaO (Waste obtuse horn)

Viriya-empikul et al.

5.

CaO (Waste mud crab shell) CaO (Waste chicken egg shells and mollusk shells: golden apple snail and Meretrix venus) CaO: Waste chicken egg shell Golden apple snail Meretrix venus CaO (Chicken eggshell) CaO (River snail shell) CaO (Oyster shell) CaO (Waste Turbonilla striatula shell) CaO (Crab and cockle shell) CaO (Scallop shell) CaO (Mussel, cockle and scallop shells) CaO (Mussel shell) CaO (Bivalve clam shell) CaO (Natural dolomite rock and waste mixed seashells) CaO (Chicken manure) CaO (Hydrated lime) CaO (Waste chicken eggshell) CaO (Waste ostrich eggshell) CaO (Waste capiz shell)

Catalyst CaO (Waste chicken eggshell) CaO (Waste chicken eggshell)

21. Lee et al.

Boey et al. Viriya-empikul et al.

3. 4.

S. No. Author 1. Wei et al. 2. Sharma et al.

Table 4. Various waste-derived catalysts.

N. oculata microalgae oil

Palm oil

WCO Palm oil Palm olein oil Palm oil Palm oil

WCO Palm oil Soybean oil Mustard oil Chicken fat WCO Palm oil WCO WFO Palm oil

Palm olein oil

Feedstock Soybean oil Pongamia pinnata oil (Karanja oil) Palm olein oil Palm olein oil 94.1 93.2 92.3

–

150:1, 9 wt.%, 60 min

15:1, 65°C, 7.5 wt.% 15:1, 65°C, 6 wt.%, 120 min 18:1, 15 wt.%, 4 min 9:1, 8 wt.%, 60 min 8:1, 60°C, 3 wt.%, 360 min, 700rpm 12:1, 5 wt.%, 360 min

86.75 (Conversion) 84.11

>90 97 96.7 92.7 93 ± 2.2

6:1, 5.8 wt.%, 660 min 97 12:1, 65°C, 5 wt.%, 90 min 98.5 ± 1.5 25 wt.%, 300 min 73.8 9:1, 65 ± 5°C, 3 wt.%, 360 min 93.3 13.8:1, 4.9 wt.%, 180 min >98 6:1, 65ͦ C, 5 wt.%, 120 min 86 9:1, 65ͦ C, 10 wt.%, 180 min 95 24:1, 12 wt.% 94.1 (Conversion) 18:1, 65°C, 8 wt.%, 180 min 95.84 30:1, 60°C, 10 wt.%, 180 min >98

>98 (purity) >90

FAME yield (or conversion/ purity)% 95 95

13.8:1, 65°C, 5 wt.%, 12:1, 10 wt.%

Conditions (molar ratio, temperature, catalyst amount, time) 9:1, 65°C, 3 wt.%, 180 min 8:1, 65°C, 2.5 wt.%, 150 min

(Continued )

[146]

[145]

[140] [141] [142] [143] [144]

[133] [134] [3] [135] [52] [115] [136] [137] [138] [139]

[132]

[130] [131]

Reference [36] [129]

22 A. MARWAHA ET AL.

26. Nisar et al.

24. Roschat et al. 25. Chen et al.

S. No. Author 23. Muciño et al.

Table 4. (Continued).

Catalyst

Sodium silicate (Rice husk) 30% RHA800-800 (Rice husk ash and chicken eggshell) CaO (Animal bones)

CaO (Sea sand)

Jatropha oil

Feedstock Safflower oil Soybean oil UCO Palm oil Palm oil

6:1, 70 ± 3°C, 6 wt.%, 180 min

12:1, 65°C, 2.5 wt.%, 30 min 9:1, 7 wt.%, 240 min

Conditions (molar ratio, temperature, catalyst amount, time) 12:1, 60°C, 7.5 wt.%.

96.1

FAME yield (or conversion/ purity)% 96.6% 97.5 95.4 97 91.5

[150]

[148] [149]

Reference [147]

CATALYSIS REVIEWS 23

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Table 5. Merits and demerits of various catalysts. Catalyst type Homogeneous base

Merits Cheap and widely available Faster reaction rate Require mild reaction conditions High biodiesel yield

Homogeneous acid

High biodiesel yield Converts high FFA feedstock (low-grade oil) easily Catalyzes esterification and transesterification simultaneously No soap formation Easy recovery of low-grade glycerol Presence of moisture and FFA content has no considerable effect Easy recovery and reuse of catalyst High selectivity and fast reaction rates Mild reaction conditions Absence of corrosive nature Longer catalyst life Minimal effect of high FFA and moisture Simultaneous esterification followed by transesterification Easy and eco-friendly recovery of catalyst

Heterogeneous base

Heterogeneous acid

Demerits High susceptibility to saponification causing reduced biodiesel yield Decrease in catalyst activity when exposed to ambient air Presence of moisture hinders reaction rate Wastewater treatment required for purification and recovery of catalyst Anhydrous conditions High temperature, molar ratio, catalyst concentration, and reaction times required for high yields Reduced catalytic activity upon presence of moisture in solution Corrosion by acid activity No catalyst recovery Lower reaction rate than homogeneous base catalysts

Anhydrous conditions Higher molar ratio, reaction temperature, and catalyst amount required as compared to homogeneous base catalyst Soap formation with use of high FFA oils

Complex procedure with use of high molar ratios, reaction temperature, and catalyst amount Lower acidic sites

6. Future perspective The endless demand to minimize the production cost of biodiesel has led to the search for more economical alternatives. Witnessing the current trends for lowcost catalysts, one can expect more challenges in the future to produce more sustainable and economical biodiesel. The “Food vs. Fuel” debate and the shortage of edible and nonedible oil worldwide for commercial production indicate the future trend toward more economical Ca-based catalysts procured from waste materials. Calcium has proven to be an effective and economical heterogeneous catalyst for biodiesel production. As the feedstock and catalyst are the main factors influencing the cost of biodiesel, waste oil and wastederived catalysts would be apt choices for green biodiesel production. Futuristic approach would be based on exploring more wastes of no use, thus adding more value and low cost toward heterogeneous catalysts. The future work must be targeted toward dismissing the leaching of Ca species in the solution. As Ca

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leaching tends to reduce the activity of catalyst after few runs of reuse, further research toward the mechanism and prevention of leaching is a crucial. Also, the use of nonedible oils and waste cooking oils contains high FFA content, thus increasing the leaching tendency of Ca-based catalysts. Further extensive research is required to devise a way for using waste oils along with calciumbased catalysts, necessary for producing an economical catalyst. Thus, future challenges will involve attempt to achieve 100% FAME yield with minimum cost of production for the commercial success of biodiesel.

7. Conclusion Both edible and nonedible sources have been utilized to meet energy demands by production of biodiesel. But due to “food vs. fuel” conflict, more focus has to be laid upon nonedible feed stocks for a more endurable and green process of production. Although the homogeneous catalysts have proven to give higher yields in a short time with low intensity conditions, their use is not tolerable for repeated use in a batch or continuous production. Thus, the development of heterogeneous catalysts is very crucial in production process. The results obtained from various studies mentioned in the present work indicate that heterogeneous base catalysts can easily replace the utilization of homogeneous catalysts in the near future for commercial use. These catalysts are porous and have high surface area due to which reactants adsorb on the surface of catalysts, unlike homogeneous catalysts which emulsify with reactants. Metal loadings doped on high surface area elements have been extensively used due to their versatile nature. Studies by different researchers have concluded that the employed heterogeneous catalysts can be reused for an average of six to eight cycles with only marginal decrease in their yields. But the leaching caused by the metal ions has destructive effect on the yields. With the presence of moisture and high FFA content in feedstocks, tendency of saponification also increases by the use of solid base catalysts. Better methods and technologies need to be introduced to control these effects to control the yields in an improved manner.

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