Nanocatalyst-Based Biofuel Generation: An Update, Challenges and Future Possibilities

Abstract

Aggrandize industrialization and urbanization have resulted in many issues, such as increased energy demand, a plethora of waste output, and negative environmental consequences. As a result, there is excessive exploitation and over-usage of fuels and finite resources, which is paving the path for the exhaustion of fuels. Extensive use of these fossil-derived fuels has caused serious threats to the environment in terms of greenhouse gases emission leading to breathing troubles and other associated health hazards. In order to mitigate the harmful effects of fossil-derived fuels, researchers are more focused towards the production and application of bio-based fuels like bioethanol, biodiesel, biohydrogen etc. These biofuels are produced from crops and edible/non-edible materials and emit much lower pollution compared to fossil-derived fuels. Even though biofuels are effective alternatives, high operational costs with low production volume are the major limitations of this process, which the available technologies cannot handle. With increasing application of nanoparticles as catalysts in several sectors due to its unique properties such as high catalytic activity, surface to volume ratio, mechanical properties, etc., its application in biofuels production has been explored recently. The present review focuses on the application of nanocatalysts in various stages of biofuel production, different types of nanocatalyst used in the innovative era and for biofuels production and their merits and demerits. The supply of biofuels, such as feedstock is large, and with improved processing, we may be able to significantly lower our reliance on fossil fuels. The present review discusses the current updates, future possibilities, and challenges of biofuels production to help make the country self-reliant in the field of green energy.

1. Introduction

Renewable energy (RE) is generated by natural sources and is abundantly accessible from sources like sun, wind, water, waste, and heat from the Earth [1]. Humans have always relied heavily on coal, oil, and other fossil fuels to run everything from incandescent bulbs to vehicles to factories. The frequent use and burning of fossil fuels has generated historically high levels of greenhouse gases [2]. According to the World Health Organization (WHO), around 99% of the population globally breathes air that is polluted and causes a health risk, and more than 13 million humans die every year from avoidable environmental causes, including air pollution [3]. The combustion of fossil fuels is the main contributor to the hazardous levels of nitrogen dioxide and fine particulate matter. In 2018, air pollution from fossil fuels created $2.9 trillion in cost to the economy, healthcare systems, and society; this amounts to $8 billion a day [4]. Utilizing renewable energy sources like wind and solar help tackle air pollution and health problems in addition to climate change [5]. Since they are finite, fossil fuels could not be categorized as a renewable energy source. Burning fossil fuels releases CO₂ into the atmosphere, leading to climate change and global warming. Wood is a resource that can be replenished if it comes from sustainably managed forests; the process of turning wood processing industry waste into compressed briquettes and wood pellets can also be deemed waste recycling. Compressed biomass fuels provide more energy than burning logs [6]. However, burning both raw wood and rubbish releases particles into the sky [7]. India’s present economic growth attempts are leading to a rise in the country’s energy requirements. The availability of increasing volumes of energy is a prerequisite for a country’s economy to expand. The Ministry of Power (MoP), National Electricity Plan [NEP] has produced a comprehensive 10-year action plan with the goal of supplying electricity to the entire country, as well as a strategy that will ensure that power is distributed to the public efficiently and inexpensively [8]. The World Energy Council predicts that global electricity consumption will peak around 2030. India is one of the world’s top consumers of coal and imports expensive fossil fuel. Around six billion people live in countries that are net producers of fossil fuels; as they are in developing countries, they are vulnerable to geopolitical shocks and crises. The population of these countries comprises around 80% of the whole planet [9]. Renewable energy sources, however, are available all over the world, and their potential has not yet been realized. The International Renewable Energy Agency (IRENA) is an inter-governmental body with the responsibility of promoting acceptance and sustainable use of renewable energy sources, improving knowledge, and enabling cooperation. It has been predicted that 90% of the world energy might be produced from renewable sources by 2050. Nations may diversify their economies, protect themselves from the unpredictable price swings of fossil fuels, foster equitable economic growth, job creation, and eradicate poverty with the aid of renewable energy sources [9]. The significance of switching from conventional to renewable energy sources is becoming more and more obvious as we became more conscious of our environmental effects. The Environmental Protection Agency (EPA) claims that green energy, which includes electricity generated by solar, wind, geothermal, biogas, low-impact hydropower, and some qualified biomass sources, offers the most environmental benefit.

Nanotechnology has been shown to be helpful for the creation of biofuels when combined with other procedures, such gasification, pyrolysis, hydrogenation, and anaerobic digestion [10]. Thermal processing of biomass using catalytic techniques offers the opportunity to selectively obtain a limited range of products while lowering the energy requirements of the transformations at lower reaction temperatures. Heterogeneous catalysts, as opposed to homogeneous catalysts, enable conditions for continuous production and offer more effective methods for separating products and catalysts. They also do away with the quenching process [11]. The development of biofuels using nanotechnology and nanomaterials has emerged as a promising tool that offers low-cost methods to raise production quality. Because of their small size and distinctive qualities, such as their high surface area to volume ratio, high catalytic activity, high adsorption capacity, and stability, nanoparticles (NPs) have many benefits over other sources for the synthesis of biofuels [10]. The current review concentrated on the usage of nanocatalysts throughout several stages of the production of biofuels, such as the pre-treatment process, fermentation process, etc. There are many types of biofuel feedstock available, and with better processing, their use may be able to reduce our dependency on fossil fuels. The current review covers the most recent developments, future directions, and challenges in producing biofuels.

2. Different Types of Renewable and Green Energy

Renewable energy is produced by using resources that are found naturally and slowly replenish themselves. Therefore, even if there are finite supplies of these resources at a given time, there is generally not a concern that they will run out completely. In addition, compared to conventional fossil fuels, RE generates fewer greenhouse gases and contaminants. Some examples of renewable energy are solar, wind, hydroelectric, geothermal, biomass, and marine energy [12]. Green energy, meanwhile, is a form of energy that has a positive impact that helps to save the environment from pollution and global warming. Unlike RE, it does not release any harmful carbon emissions and also reduces carbon footprints. Green energy is a subset of RE and includes only those resources that have the greatest environmental benefits [13]. For instance, wind power is an example of both green and renewable energy as trees can be planted and grown repeatedly, but burning wood is RE, not green energy, as it produces environmental pollution and is responsible for global warming after it is burnt. Different types of renewable energy source have been presented in Figure 1.

3. Different Types of Biofuels

As opposed to the drawn-out natural processes that leads to the formation of fossil fuels like oil, biofuels are swiftly harvested from biomass. Biofuels burn similarly to natural gas, and they are gradually but steadily replacing natural gas. Despite being created by the anaerobic decomposition of biomass, biogas primarily consists of methane gas. The majority of agricultural businesses use biogas, which is now packed in gas cylinders for consumption in homes. There is an urgent need for a novel sustainable strategy for producing bioenergy because of the concerning rise in our reliance on fossil fuels like oil, coal, and natural gases. One such method for achieving a sustainable development of bioenergy is the effective execution of a zero-waste discharge policy, which can only be achieved by turning organic wastes into bioenergy. The conversion of organic waste is strongly advised because waste management is essential for reducing the problem and threat of wastes. The main renewable energy sources, include biogas, bioethanol, coal, hydrogen, and biodiesel, are discussed below, along with their advantages and disadvantages [14,15]. The biomass utilized in the production of the various biofuels is categorized as follows (Figure 2).

Solid biofuels are made from solid, organic, non-fossil biomass that is derived from living things. These biomasses are widely used in the generation of electricity, energy, and heat. These biofuels are created from renewable industrial waste such as charcoal, fuelwood, wood pellets, wood scraps, and animal waste. Among the notable examples is biochar [16,17].

Liquid biofuels that are made from natural biomass or biodegradable fractions are included in these biofuels. Due to their high energy density, liquid biofuels are superior to solid and gaseous biofuels in many ways, making them the best choice for transportation, storage, and retrofitting. Bioethanol, biodiesel, and bio-oil are some of the most important examples of liquid biofuels; types of liquid biofuels include (a) those based on triglycerides, such as vegetable oil, pyrolytic oil, biodiesel, hydrogenated oil, and biogasoline, which are made from biomass; (b) biofuel feed stocks such as bio-oils, BTL diesel, and drop-in biofuels. These are all examples of lignocellulosic-based biofuels.

Gaseous biofuels, including low-density gaseous biofuels, are gaseous by nature. Those such as biogas, biohydrogen, and bio syngas are a few notable examples. During gasification or pyrolysis, the biowastes are transformed into gaseous biofuels. Subsequently, to generate power or heat, these biofuels are used in Otto engines coupled to an electricity generator [16].

3.1. Biodiesel

The trans-esterification method is used for biodiesel production. In this method, the heavier molecules in oil are broken down to simpler or lighter elements by alcohol in the presence of a catalyst. Biodiesel is produced from a range of edible plant oils, such as soybean, rapeseed, sunflower, palm, and coconut oils; non-edible oils used to make biodiesel include oil crops such as Jatropha curcas, Calophyllum inophyllum, Nicotiana tabacum, Ceiba pentandra, and Hevea brasiliensis [16]. Pure biodiesel is generally produced using concentrates from plants with high-energy content. It is manufactured by mixing fats and oils from animals and plants, respectively. Alcohol is also used in the production of biodiesel is alcohol, and grease from animals and plants is employed as a supplement. After innovations, production of biodiesel is completed in three stages: pre-treatment, reaction, and washing. Silva et al. created biodiesel (2019) using waste oil, wastewater scum, and home fat trap wastes [18]. By using microscale open column chromatography and triacylglycerol, they determined the contest of biodiesel esters [19]. One advantage of using biodiesel is its environmental friendliness; it largely reduces emissions of carbon dioxide, sulfur, carbon monoxide, and hydrocarbons. The ability of biodiesel to be used in unmodified vehicle engines is another advantage of it as a renewable energy. Through a simple production process, it is easy for waste cooking oils to be salvaged, and they are more biodegradable than fossil fuels. Although biodiesel has these advantages, it has disadvantages as well. It can lead to food competition since palm and soya bean oils give a higher yield of biodiesel, and the cost of producing biodiesel is high [20,21].

3.2. Wood Fuel

In underdeveloped nations, about 80% of total wood use is for fuel. With the development of electricity, kerosene, and propane, dependency on fuelwood is predicted to progressively decline, but the rural poor’s strong reliance on it is predicted to last long into the twenty-first century. Fuelwood collecting is a significant contributor to deforestation, especially in rural and urban regions where continual collection causes the slow deterioration and eventual deforestation of accessible areas [22]. About half of the wood harvested worldwide is used to fuel appliances for cooking, heating, and electricity [23]. The biggest disadvantage of using wood fuels is emission. Socio-economic and environment factors create concerns about wood fuels. Wood fuels have positive impacts, such as development and rural employment in infrastructure, poverty reduction, income generation and a source of sustainable energy with climate change mitigation and carbon sequestration, it also has negative impacts [24]. The effects of conventional wood fuels on climate change depends on a number of different factors. One is combustion, which emits CO₂ and short-lived climatic forcing (SLCFs), such methane and black carbon aerosols (BC and OC) (CH₄). The carbonization of charcoal also results in the emission of CH₄ and many OC chemicals. Emissions are also produced during the long-distance transportation of some commercial wood fuels [25]. In addition to this, migrant labor, child labor, poor health, imbalance in economic effect, increased biomass competition, inflated prices of food and forest products, reduction of soil organic matter, changes in physical and chemical properties of water, loss of ecosystem, decrease in genetic diversity, and carbon emission from land use are also negative effects of wood fuels [26].

3.3. Biomethane

It is possible to convert biogas into biomethane, which is a recognized mature technology. Especially in North American nations, biomethane, also known as renewable natural gas (RNG), is created by bringing anaerobic-digestion-produced biogas up to the standards required for feed-in into the natural gas infrastructure. Thermochemical conversion can also yield biomethane, which is known as biosynthetic natural gas (bio-SNG) [27]. Biomethane is produced either by “upgrading” biogas (a process that removes any CO₂ and other impurities contained in the biogas) or by gasifying solid biomass and then methanating it, making it a nearly pure source of methane.

By removing elements like water and hydrogen, biogas is upgraded to biomethane. Halogenated hydrocarbons, siloxanes, oxygen, nitrogen, carbon monoxide, hydrogen sulphide, and initial stage particles are also removed. Carbon dioxide (CO₂) is removed in the second stage to increase the CH₄ (methane) content [27]. About 90% of the biomethane currently produced worldwide is created through upgrading biogas. The diverse properties of the gases included in biogas are used by modern technology to differentiate them, with barrier and water scrubbing accounting for over 60% of the world’s biomethane output today [28]. Compared to biogas, biomethane provides more potential uses, including (1) use as a car fuel for grid injection; (2) improved RE storage capacities through increased external heat usage, (3) increased biomethane energy density, and higher efficiency as a result of (4) a prolonged decoupling of production and usage in time and space. However, the method of upgrading biogas to biomethane is not always favorable because it increases costs, energy demand, and material use, which might have an impact on the environment [27].

Following the thermal gasification of solid biomass, woody biomass is initially decomposed in an environment with low oxygen content and high pressure (between 700 °C and 800 °C). In these conditions, the biomass produces a mixture of gases, principally CO, H₂O, and CH₄ (sometimes collectively called syngas). To produce a pure stream of biomethane, this syngas is purified to remove of any corrosive and acidic components. The subsequent step in the methanation process involves the employment of a catalyst, which promotes the reaction between the H₂O and CO or CO₂ to create CH₄. Any remaining CO₂ or H₂O are removed at the end of this process [29].

3.4. Bioethanol

Ethyl alcohol, often known as bioethanol, has the chemical formula of C₂H₅OH. It can be utilized either as pure ethanol or in a gasoline blend to make “gasohol.” Octane enhancers and bioethanol-diesel mixes are used to improve gasoline or lower its emissions. In comparison to octane gasoline, bioethanol has a variety of advantages, including a wider range of flammability limitations, quicker flames, and higher vaporisation temperatures. In comparison to petroleum fuel, bioethanol produces fewer airborne pollutants, is safer, and degrades more quickly. Due to its considerable contribution to lowering the use of crude oil and environmental pollution, bioethanol is the biofuel that is selected most frequently around the world. It can be created using a variety of feedstocks, such as sugar, starch, lignocellulosic, and algal biomass by microbial fermentation. Yeasts, particularly Saccharomyces cerevisiae, are the most common form of microorganism. Common microorganisms are utilized in the manufacture of ethanol due to their high ethanol productivity, strong tolerance for ethanol and the capacity to ferment a variety of substrates [29]. High octane number (108), low boiling point, increased heat from vaporisation, and equivalent energy content are all benefits of using bioethanol as a biofuel. Without altering the existing engine, automobiles can run on gasoline blended with up to 85% (v/v) bioethanol. Both the consumption of petroleum and greenhouse gas emissions can be considerably decreased through blending. Even though several bioethanol pilot plants have been put into operation around the globe, a great deal of work must be done to reduce production costs. Feedstock, enzyme, detoxification, and ethanol recovery costs are the four main cost factors [30].

3.5. Biocoal

Effective biomass usage has recently attracted a lot of attention. Pyrolysis is a procedure for converting biomass which is of major interest. The pyrolysis method enables the creation of biochar, which can take the place of fossil fuels as an alternative energy source. Additionally, biochar can be transformed into liquid fuel for use in automobiles and as a substitute for petroleum-based products (gasoline, aviation kerosene, and diesel fuel). In recent years, the agriculture sector has employed biocoal more frequently as a premium complex fertiliser with special qualities [31]. It has been suggested that torrefied biomass may one day replace coal. Making biomass more similar to coal in terms of its characteristics as a solid fuel is one of the primary goals of torrefaction. Biocoal must follow the rules for solid fuels set forth by various regulatory authorities, although it is a unique fuel created by the thermal treatment of raw biomass. In contrast to thermally treated fuel that is already well-established on the market, this type of charcoal has a different productoin process [32]. Cheng and Huang et al. demonstrated that it is possible to make biocoal using the extensively produced and easily accessible biomass, such as agricultural and forestry wastes. A 3% share of agricultural wastes make up the estimated 146 billion tonnes of biomass produced annually globally. Therefore, it would be a tremendous opportunity for developing nations to switch from traditional fossil fuel-based electricity to renewable bioenergy based on biocoal [33].

3.6. Biohydrogen

Biohydrogen is a clean and regenerative energy source which can be manufactured through numerous methods, including electrolysis of water, thermos-catalytic reformation of organic molecules rich in hydrogen, and biological methods. Ongoing hydrogen (H₂) production technniques, such as steam reforming of natural gas and thermal cracking of coal gasification, are not environmentally friendly. However, biological H₂ production is a potential alternative, and mechanisms like direct biophotolysis, indirect biophotolysis, photofermentations, and dark fermentations are used for the synthesis (Figure 3) [34]. The synthesis of biohydrogen from industrial wastewater has attracted attention lately. To apply the biohydrogen generation process on a large scale, it is necessary to explore in-depth knowledge of lab scale parameters and new strategies. The operational settings have a significant impact on the production of biohydrogen [35].

3.6.1. Dark Ferementation

The biological approach (dark fermentation) is regarded as the most environmentally benign and sustainable of these technologies, and it is one of the most widely used procedures for producing biohydrogen. Dark fermentation has been observed to be an favorable procedure that involves a two-phase anaerobic treatment system, even if only 15–20% of the theoretical hydrogen potential of carbohydrates is recovered. Using anaerobic sludge as the inoculum, microbial species analysis of hydrogen-producing cultures reveals the presence of Streptococcus bovis, Clostridium cellulosi, Clostridium acetobutylicum, and Clostridium tyrobutyricum. The anaerobic glycolytic breakdown of carbohydrates typically leads to the formation of fermentative hydrogen. Theoretically, 12 moles of H₂ can result from the complete oxidation of 1 mole of hexose to CO₂ [36]. The dark fermentation technique coverts complex products to simpler ones through hydrolysis [37]. The advantage of creating H2 through fermentation is that it may do so under comfortable conditions and also profit from the remaining biomass valorization. Dark fermentation is more appealing since it has better production rates than photofermentative methods as well as the ability to utilize organic and wastewater wastes [36]. By further combining biohydrogen generation technology with additional products like bioalcohol, volatile fatty acids (VFAs), and CH4, the effectiveness and economics of the overall process may be improved [38].

3.6.2. Photo Fermentation

Purple non-sulfur (PNS) photosynthetic bacteria, which can develop into photoheterotrophs, photoautotrophs, or chemoheterotrophs, are responsible for photo fermentation. Under photoheterotrophic circumstances (light, anaerobiosis, organic electron donor), these bacteria create H₂. In order to make biohydrogen, green algae, and photosynthetic bacteria, respectively, the hydrogenase and nitrogenase enzymes are synthesized during photosynthesis. Rhodospirillum rubrum, Rhodo pseudomonas palustris, Rhodobacter sphaeroides O.U. 001, Rhodobacter sphaeroides RV, Rhodobacter sulfidophilus, and Rhodobacter capsulatus are the principal PNS bacteria involved in H₂ generation. The benefits of this method over using green algae and cyanobacteria for water photolysis include the fact that oxygen does not interfere with the process and that these bacteria can be employed in a range of environments (i.e., batch processes, continuous cultures, and immobilised systems) [36,37].

3.6.3. H₂ Production from Algae

Biodiesel, bioethanol, and biogas have all been produced using microalgae as a feedstock. Different microbial species have been employed as feedstock for the synthesis of biohydrogen, and substantial research has been conducted on the species Chlorella, Scenedesmus, and Saccharina [37].

4. Different Generation of Biofuels

4.1. First Generation

Starch- and sugar-based feedstocks are the two categories of edible feedstocks used to create first generation biofuels. Wheat, barley, corn, and potatoes are a few examples of feedstocks that include starch. Sugarcane and sugar beet are among the feedstocks used for sugar-based products. Corn, sugarcane, and wheat are the most often used edible feedstocks for the manufacture of first generation biofuels. Via a fermentation process, the feedstock is transformed into biofuels. To purify the biofuels, the final product int eh process passes through a distillation chamber. Before being fed to the fermentation chamber, starch-based feedstocks must also undergo a pre-treatment procedure because the long-chain polymer structure of glucose found in starch-based feedstocks must be broken down to a shorter chain in order for the feedstock to be successfully converted into biofuels [39]. Feedstocks like sunflower, canola, and corn produces biodiesel, sorghum, sugarcane, and oil palm produces bioethanol [40].

4.2. Second Generation

The primary biomass used in the production of second-generation fuels is lignocellulose. These biofuels have advantages over first-generation biofuels since they use a variety of fuel routes. Second-generation biofuels are distinguished by the fact that they utilize a non-food source (lignocellulose biomass, field crop residues, forest product residues, or fast-growing dedicated energy crops). The gasoline produced is a “drop-in” replacement for conventional petroleum-based fuels, which means that it can be blended indefinitely or used unblended in existing cars. Cellulosic ethanol, which is created by fermenting sugars obtained from the cellulose and hemicellulose portions of lignocellulose biomass, is the primary kind of second-generation biofuel now in use or being developed. Nonetheless, biobutanol can be utilized as a drop-in replacement for gasoline without blending restrictions, even if the fuel output is currently lower than that of ethanol [41,42].

4.3. Third Generation

Environmental deterioration, industrialization, and the quickening rate of fossil fuel depletion have all fueled the need for the development of sustainable fuel alternatives. As a result, interest in biofuels has increased, particularly third-generation biofuels made from microalgae because they do not threaten food or land supply [43]. Microalgae-based third-generation biofuel is an effective way to combat climate change and global energy insecurity. The most commonly used definition of third-generation biofuels is fuels made from algal biomass, which have a substantially different growth yield than traditional lignocellulosic biomass. The lipid content of the microorganisms is typically a limiting factor in the production of biofuels from algae [44].

5. Role of Nanocatalyst for Biofuel Production

A substance or material that exhibits catalytic properties and at least one nanoscale dimension, physically or in terms of internal structures, is referred to as a nanocatalyst (NC). Because there is more surface area available for reacting with the reactants thanks to NC’s high surface-to-volume ratio, the catalyst performs better overall. They can be divided into two major categories based on whether a catalyst is present in the same phase as the substrate: heterogeneous catalyst and homogeneous catalyst [11]. Depending on the types of feedstock materials used, several types of biofuels like bioethanol, bioethanol, biogas, and biodiesel are produced. One of these is the manufacturing of biodiesel, which accelerates the development of clean and sustainable fuel. Biodiesel is produced utilizing a variety of techniques, including pyrolysis, direct blending, micro-emulsion, and trans-esterification, with a critical discussion focusing on boosting biodiesel output with nanocatalysts [45]. Biodiesel is a liquid fuel made of long-chain fatty acid esters generated from micro- and macro algal oil, animal fats, and vegetable and nut oils. One of the innovative and promising methods for producing biodiesel on a large scale is the use of nanocatalysts (Figure 4). Using a nanocatalyst, which generates various fatty acid methyl esters compositions, makes it simple to change its physico-chemical properties (FAME) [45,46].

The price of producing biodiesel is influenced by nanocatalyst characteristics, including catalytic activity, specificity, and overall stability. Degradation due to physical, mechanical, and thermal factors has an impact on catalytic activity. Due to their small particle size, which has a major impact on catalytic activity and methyl ester yield, powdered acidic and basic catalysts is unsuitable for industrial production. There are numerous nanocatalyst changes being made [45,47]. The different types of nanocatalysts and the reasons for their use as biofuel are described in

Table 1. (a) Different types of NC. (b) Different nanomaterials used and the reason for their uses in biofuel production.

(a)
Nanocatalysts	Description	Examples and (Mode of Synthesis)	Feedstock Source	Reference
Metal-oxide-based	They are conventionally used for production of biodiesel.	Nano-MgO, MgO/MgFe2O4, KOH/Fe2O3/Al2O3, Na2SiO3	Goat fat, Sunflower oil, Palm oil, corn oil, Canola oil, Soybean oil, animal fat, cooking oil	[45,48,49]
Carbon-based	The physical and chemical properties of nanocatalysts made from carbon materials including graphene, carbon nanotubes, and reduced graphene oxides have been characterized and correspond to various morphologies and sizes of the resulting no composite materials. It has been discovered that nanocatalysts with acidic properties, high porosity, and large surface areas have better catalytic activity.	KOH loaded MWCNTs (Impregnation method) silicon carbide/sodium hydroxide-graphene oxide (In-situ Impregnation method) Sulfonated biochar and activated carbon (Pyrolysis method)	Canola oil, Rapeseed oil oleic acid, used cooking oil, oleic acid, vegetable oil	[45,50,51,52]
Zeolite-based	Zeolite materials are now being used more frequently than before. The existence of active acidic and basic sites, high catalytic activity, easily modifiable structures using various functions, and metal exchange are only a few of the distinctive qualities they notably possess.	Zeolite/chitosan/KOH Lanthanum-natural zeolite(La/NZA) (impregnation method), ZSM-5 (nanosheets) (Hydrothermal method), Li/NaY zeolite (Hydrothermal and microemulsion)	Waste cooking oil, crude palm oil Linoleic acid, shea butter, castor oil	[45,53,54,55,56]
(b)
Nanoparticle	Examples	Remarks		References
Carbon-based	CNT *, graphene, MWCNT **, fullerene	greater thermal conductivity, inertness, and stability		[57,58]
Core–shell	Ag/SiO2, Au/SiO2, Ni/SiO2, Fe3O4/SiO2, Au/TiO2, Fe/C, FeNi/SiO2, ZnO/SiO2	better multi-functionality and greater stability		[57,59,60]
Hybrid	FeMo, CuMo	active catalysis in combination		[57,61]
Metal/non-metal oxide	TiO2, Fe3O4, ZnO, CaO, SiO2, Al2O3	increased melting point and thermal stability		[57]
Metallic	Ni, Ag, Au, Rh, Pd, Pt	moderate temperature stability and increased catalytic activity		[57,62]

* Carbon nanotubes; ** Multi-walled CNT.

a,b, respectively.

Trans-esterifying oils from plants, animals, or oleaginous microorganisms, often produce fatty acid methyl esters, or biodiesel, after they have been reacted with an alcohol such as methanol. The main issues with this method, though, are saponification, catalyst deactivation, and slow reaction rates [63]. Therefore, experts thought that nanotechnology might be employed efficiently to overcome these problems. Nanotechnology has become the most promising technology in recent years, with groundbreaking uses in a wide range of industries. Classically, it is described as the creation, manufacture, and use of materials with atomic or molecular precision at sizes between 1 and 100 nanometers (nm) [64].

5.1. Metal-Oxide-Based NCs

This group of NC has been considered most hopeful and therefore widely used in biodiesel production from a variety of feedstock. SiO₂/ZrO₂ catalyst synthesized by Faria et.al using the sol-gel method with high surface area has a good efficiency as the catalyst can be reused for more than six cycles of transesterification [65]. Potassium bi-tartrate (C₄H₄O₆HK) loaded ZrO₂ NC having size range if 10–40 nm was used in biodiesel production as reported by Qiu et al. in 2011 [66]. The TiO₂-ZnO NC showed an improved activity than compared to other sulfated metal oxides. This is primarily due to the acidic strength of TiO₂ particles. These catalysts showed an impressive efficacy in biodiesel production from cooking oils [67,68].

5.2. Magnetic NCs

Similar to solid supported NCs, magnetic NC have recently gained a lot of interest from scientists. These NCs can be used for more than one cycle of transesterification due to their exceptional magnetic characteristics. There have been numerous attempts in this area, and numerous magnetic NCs have been created and used in the transesterification of various feedstock for the creation of biodiesel [64]. NC KF/CaO-Fe₃O₄, developed by Hu et.al [69], is another example of magnetic NC which is novel and is used for biodiesel synthesis; it is made from Stillingia oil. This reported NC has a diameter of 50 nm and is highly efficacious as it can be reused to up to multiple times with hardly any loss in activity. A total of 90% of the catalyst can be recovered when used under the condition of 65 °C, a methanol/oil molar ratio of 12:1, and a catalyst concentration of 4% after 3 h of reaction [64]. A quick, modest, and economically feasible Nano technological approach using magnetic NC for biodiesel production from soybean oil has been developed by Alves et.al. The authors used a co-precipitation to create a mixture of magnetic iron/cadmium and iron/tin oxide nanoparticles, and the resulting NCs were tested for their effectiveness in the hydrolysis, transesterification, and esterification of soybean oil and its fatty acids. At 200 °C and 1 h of reaction time, esterification aided by iron/tin oxide nanoparticles produced high yields of about 84%. Additionally, these NCs were magnetically recovered and employed an additional four times without losing any of their activity, but an activity loss was seen for the iron/cadmium oxide nanoparticles catalyst [70]. In a different work, Feyzi and Norouzi reported using a combination of two different procedures, namely the sol-gel and incipient wetness impregnation methods, to synthesize a Ca/Fe₃O₄@SiO₂ NC. In addition, this magnetic NC was used to produce biodiesel. Thus, produced NCs were shown to have high magnetic characteristics. The findings obtained showed that the utilized magnetic NCs had highly effective catalytic activity under ideal conditions, as evidenced by the greatest biodiesel yield (about 97%). The NC’s magnetic properties allowed for repeated use without noticeably losing any of its catalytic properties [71]. Erdem et al. proposed the innovative method for surface modification of magnetic iron oxide NPs by silica layer via the Stöber process, followed by functionalization of chloro-sulfonic acid. As a result, magnetic nanoparticles with acid functionalization served as powerful NCs for the generation of biodiesel. Additionally, in the palmitic acid-methanol esterification process, the catalytic activity of coated and non-coated solid (acids-functionalized), which is one of the typical industrial processes for the manufacture of biodiesel, NC was assessed. According to the authors, adding a silica layer to the surface of magnetic nanoparticles only slightly hinders their ability to magnetize. However, because of their relatively porous structure and the steadiness of their magnetic core in an acidic reaction medium thanks to a covering process, the authors were able to demonstrate accelerated mass transportation. The produced acid-functionalized NC demonstrated promising acid catalyzed esterification as a result of its characteristics [72].

5.3. Zeolites/Nanozeolites

Another type of catalyst utilized in the commercial manufacture of biodiesel is zeolites (Zes). Due to their strong acidic nature, high surface area, shape selectivity, and distinctive molecular sieving capabilities, zeolites are renowned for their promising catalytic performance and have been used for a wide range of catalytic applications for many years (reference) [73]. Given the numerous catalytic uses for Zes, including the manufacture of biodiesel, the scientific community is currently concentrating on improving the efficacy of the development and implementation of nanozeolites (NZe). Because of their huge exterior surface areas and great dispersibility in both aqueous solutions and organic media, NZe—hydrophilic supports—enable improved access of the enzymes to the substrate [64]. de Vasconcellos et al. investigated the possibility of using NZe with different crystallographic structures functionalized with (3-aminopropyl) trimethoxysilane (APTMS) and cross-linked with glutaraldehyde as solid supports for immobilization of lipase recovered from Thermomyces lanuginosus. The generated enzyme-NZe complexes were also shown to play a part in the ethanolysis transesterification of microalgae oil to fatty acid ethyl esters (FAEEs) by the authors. The findings of this study demonstrated that the utilized NZe functionalized with APTMS and cross-linked with glutaraldehyde demonstrated ability to immobilize greater amounts of enzymes, and these enzyme-NZe complexes also reported to exert relatively higher enzymatic activities than free enzyme (non-functionalized) [74]. In their study, Al-Ani et al. created the basic cation-rich hierarchical ZesX and Y by combining post-synthesis alterations and ion exchange, and then assessed the effectiveness of producing biodiesel from vegetable oils. Their results demonstrated promising catalytic performance of the generated cation-rich hierarchical Zes, as evidenced by the enhanced conversion of triglycerides through the transesterification of vegetable oils [63].

5.4. Nanohydrotalcites

Another substance that is common in nature is hydrotalcite; it has enormous potential for usage, causing it to attract increasing amounts of attention. Given the wide-ranging applications of hydrotalcite, recent research has focused on the manufacture of nano-hydrotalcites, also known as anionic clays or aluminum-magnesium layered double hydroxides. The hydrotalcite compounds belong to the category of positively charged, two-dimensional, nanostructured anionic clays that contain two different types of metallic cations which are accommodated by tightly packed OH groups. To produce FAMEs, Dias et al. synthesized cerium modified Mg-Al hydrotalcites, which were then utilized as catalysts in the methanolysis of soybean oil. The results indicated that this type of catalyst can produce FAMEs with yields of more than 90%. Another study showed how to make Mg-Al nano-hydrotalcite, employing it as a catalyst for the transesterification of pongamia oil for the first time. At a temperature of 65 °C and a 6:1 molar ratio of methanol and catalyst, a maximum biodiesel conversion of roughly 90.8% was attained [75].

6. Efficiency of Biofuels

Soybean biodiesel has significant benefits over maize grain ethanol among the food-based biofuels. In addition to reducing greenhouse gases (GHGs) by 41% when compared to diesel, biodiesel also eliminates several main air pollutants and has a negligible effect on the environment and human’s health through the emission of N, P, and pesticides [76]. The energy efficiency of corn, wheat, sugarcane, rapeseed, soybean, sunflower, and palm oil, i.e., first generation of biofuels, are from roughly 26% to 68%, 16% to 85%, 78% to 100%, 43% to 80%, 27% to 100%, 10% to 79%, 67% to 139%, and 7%, respectively. Compared to wheat, corn, and molasses, cassava ethanol has a lower energy efficiency. The location of the feedstock’s production (such as the nation, the kind of soil, and the biofuel conversion process) has a significant impact on its energy efficiency. When compared to rapeseed and soybean oils, the energy efficiency of palm oil is almost nine times higher [39]. Rapeseed and canola biodiesel is particularly effective at powering heavy equipment and other vehicles. Biodiesel-powered engines are typically more effective than gasoline-powered ones.

7. Challenges for Biofuels Production

Becoming commercially competitive with fossil fuel remains a significant obstacle for biofuel. It has been suggested that biofuel production might be made more sustainable and cost-effective by combining it with the treatment of nitrogen-rich, municipal waste water and CO₂-rich fuel gas [77]. Several genetic engineering strategies should be combined to maximize biofuel output in order to create novel strains with commercial potential [78]. Incorporating biofuels into our current energy system requires extensive collaboration to solve the energy challenge. Fundamental mechanistic research, cell growth facility construction, genetic engineering of algae strains, and biofuel production condition optimization are just some of the examples of how these collaborative activities will help advance cultivation methodology development and technology innovation in biofuel production. It is crucial to conduct in-depth studies on photosynthetic mechanisms for biofuel production under varying conditions since this will provide knowledge on how best to maximize the growth of biomass of phototrophic organisms.

The availability of biomass, which may be easily collected from woods, plants, organic waste, agricultural waste, municipal solid waste, etc., is the major factor in the manufacture of biofuel. There are still a lot of obstacles to overcome and room for improvement when it comes to replacing commercially accessible petroleum-based lubricants. High operating costs are associated with lignocellulosic biomass pre-treatment techniques [79]. Algal biomass is also utilized to produce biodiesel since it can grow quickly, is oil-rich, and emits no carbon. It is thought that this could displace fossil fuels in the production of biodiesel. On the other hand, lipid extraction requires a lot of energy and algal biomass culture is expensive [80].

The technologies that are now available for producing sustainable biohydrogen still have poor yields and low efficiency, which presents hurdles and difficulties. Due to the release of molecular oxygen during photosynthesis, which might permanently inhibit hydrogenases, light-driven hydrogen generation by microalgae encounters certain difficulties. Aside from this, the cost of producing biohydrogen is not comparable with that of fossil fuels, which are used to produce hydrogen.

The high expense of the extraction process to recover the oil before converting it into biodiesel is a significant obstacle for the industrial usability of algal biodiesel. The recovery of lipids from algal biomass and their conversion to fatty acid methyl esters are the main difficulties with algal biodiesel [81].

8. Future Possibilities

The use of biofuel will replace petroleum-based products in the future since it is safer, cleaner, renewable, and more environmentally friendly. Researchers are now considering biofuels as a viable alternative to petroleum-derived fuels due to the limited supply and rising demand for these fuels. Biofuel generation although a cleaner and nontoxic process is still difficult. The accessibility of biomass, which may be easily collected from woods, plants, organic waste, agricultural waste, municipal solid waste, etc., is the major factor in the manufacture of biofuel. Although heterogeneous catalysis is thought to be one of the more promising methods for trans-esterifying fatty acids, there is still a lot of room for this idea to be developed. It can be accomplished by improving a number of related aspects, including the catalyst type and reactor layout. A redesigned catalyst and reactor symmetry that is simple to operate under a variety of dynamic operation situations is conceivable to develop. It will all work together to improve the strategy of catalytic processes so that they are in line with the shifting demands of this industry. In the near future, the likelihood of adopting biofuel as a green alternative energy source will increase dramatically. In this context, NCs are thought to be the most suitable catalysts for heterogeneous catalysis because they have a number of benefits, including increasing the effectiveness of the traditional heterogeneous catalysis approach. However, there is controversy around the toxicological problems of NCs, and serious study is required to address the toxicity impacts. There are differing views on the toxicity of the NCs used to produce biodiesel, but it is unquestionably essential to conduct further in-depth research to assess the toxicity of the various NCs. NCs have the potential to significantly increase overall productivity in the production of biofuels and biochemical. By consequence of their enormous surface area and robust catalytic activity, NCs have the ability to alleviate concerns with mass transfer resistance, rapid deactivation, and inefficiency. Overall, the use of NCs is crucial for the manufacture of sustainable biodiesel as well as cellulosic fuels, renewable product chemicals, and other bio-based products using the flexible bio refinery platform [64].

9. Conclusions

Nowadays, higher energy demand is a major problem for the entire world. However, dependable energy sources are required to satisfy the current energy loads. The majority of countries rely on nuclear and fossil fuels, which are non-renewable energy supplies that have a number of drawbacks and restrictions. The improvement of renewable energy offers sustainable supplies with several advantages in terms of socioeconomic considerations. One of the most promising alternatives to fossils fuels are biofuels. Contrary to traditional fossils fuels, which are non-renewable and emits damaging greenhouse gases, biofuels are clean and renewable fuels. The principal sources of biofuels include oils from plant seeds, animal fats, microorganisms, and algae. Essentially, they are produced through catalytic trans-esterification using homogeneous or heterogeneous nanocatalysts. Sodium hydroxide and potassium hydroxide are two alkaline nanocatalysts that are primarily employed in the trans-esterification of fat. However, these nanocatalysts make it challenging to trans-esterify inferior feedstock. To avoid the inherent problems with homogeneous catalysts, heterogeneous catalysts (such as lipase and solid catalysts) are in demand. So, the best course of action in reference to challenges and future possibilities is to behave as responsible risk reducers by lightening the load on the environment in terms of pollution and global warming by utilizing green biofuels to make the country self-sustainable in the field of green energy.