The introduction of lignocellulosic biomass (LB) resources as a replacement source of green renewable energy has drawn significant recognition in regard to the rising demand for biofuels due to their abundance, absence of competition with food, and ability to produce sustainable value-added compounds, including biofuels
[7]. LB feedstocks provide a plentiful source of organic carbon that may be extensively used for their transformation into bio-based chemicals and biofuels with added value. LB is made up of primarily both polar and non-polar polymeric materials such as cellulose, hemicellulose, and lignin
[8]. Cellulose (40–60%), hemicellulose (20–40%), and lignin (10–25%) make up the majority of the components in LB products
[9]. Due to its abundance and low cost as a feedstock, LB can provide around 40% of the world’s energy demands
[10]. As of now, a variety of lignocellulose biomass including rice straw
[11], elephant grass
[12], switch grass
[13], palm wood
[14], agricultural waste
[15], algal biomass residues
[16], and textile mill waste including cotton spinning waste
[17] are utilized as bioethanol feedstocks. The cell wall’s composition, level of lignification, and cellulose’s crystallinity, are the major causes of the structural recalcitrance, enabling lignocellulose to resist chemical and biological deconstruction
[18]. It is challenging to convert LB into simple sugars because cellulose, a key component of LB, is bound to lignin and hemicellulose through different bonds. A known robust component of LB called lignin prevents the use of biocatalysts and enzymes to hydrolyze polymeric constituents such as cellulose and hemicellulose. Due to this, LB pre-treatment is the only method that can completely remove lignin and allow polysaccharide components to be fully utilized. After pre-treatment, the separated cellulose/hemicellulose-rich elements can be used for enzymatic hydrolysis and subsequent fermentation, while the segregated fractions can be further used to synthesize biochemicals. At the commercial level, for the pretreatment process employed by LB biorefineries, principal cost investment, energy consumption, and whole process efficiency are often taken into account. There are now a number of pretreatment techniques broadly classified as physical, chemical, and physicochemical procedures. The chemical pretreatment uses a variety of chemicals, including acid, alkaline, organosolv, and/or ionic liquids
[19]. Pretreatment frequently involves the use of acids in particular sulphuric acid and alkalies such as NaOH and CaO
[20]. These techniques can effectively extract lignin or digest hemicellulose to break down lignocellulose’s structural stubbornness
[21]. Their use is constrained, however, mainly due to the high-energy need and production of toxic harmful chemicals (hydroxymethyl furfural and furfural) that can impede the activity of biocatalysts employed in fermentation. Nonetheless, the acid and alkali treatment are the most widely used method for the pretreatment of biomass due to its low cost and ambient operating conditions. Additionally, the used acid/alkali in the pretreatment process must be recycled from the hydrolysate after completion of the process to minimize its hazardous impact to the environment, and reusing recycled acid/alkali can improve the process economy
[22]. Furthermore, scientists are looking for the best solution that can efficiently saccharize biomass by enzymatic approaches using hydrolases (cellulases and xylanases), oxidoreductases (laccases), recombinant feruloyl esterase, etc.
[23][24], and in some cases genetic lignocellulose modification
[21][24]. Contrarily, using biological techniques for LB pretreatment has a number of drawbacks, including the need for a catalyst that meets the stringent constraints, a lack of stability, and expensive manufacturing and purifying procedures.
2. Lignocellulosic Biofuel
The initial step in the institution of sustainable biofuel production is the capability to use suitable lignocellulosic biomass as feedstock for the selected product, which is otherwise considered trash and is often simply burned, leading to environmental pollution. Lignocellulosic biomass is a plentiful, affordable, renewable, and carbon-neutral resource that can be exploited to make second-generation biofuels without affecting the food security of the world. Its production is enormous on a global scale and accounts for 181.5 billion tons per year
[27][28]. Depending on the type, to varying degrees and proportions, these polymers are structured in an intricate, non-uniform, three-dimensional spatial configuration. The hydrophobic property of lignin, the crystalline structure of cellulose, and the encasement of cellulose by the lignin-hemicellulose matrix, which is firmly bound by hydrogen and covalent bonds, all have an impact on the resilience of lignocellulose
[29].
Biochemical or thermochemical processes are typically used for transforming lignocellulosic biomass into bioenergy. The biochemical approach uses microorganisms and/or a variety of enzymes to reduce the feedstock into fermentable sugars, which are then fermented to make biofuels, such as bioethanol, biogas, biobutanol, biohydrogen, biodiesel, and so on. Biochemical procedures typically include very mild reaction conditions. The inclusion of a pretreatment step (chemical, physical, or biological), prior to hydrolysis improves the overall procedure and makes it commercially feasible. The thermochemical route, on the other hand, includes pyrolysis, liquefaction, and gasification to produce a variety of fuels, such as, ethanol, renewable diesel, and aviation fuel. The presence of moisture has a detrimental impact on product yields and emissions, but the thermochemical conversion can employ a wider variety of feedstock and is unaffected by lignin in the biomass.
2.1. Bioethanol
Separate hydrolysis and fermentation (SHF), simultaneous saccharification and fermentation (SSF), Pre-saccharification and simultaneous saccharification and fermentation (PSSF) and consolidated bioprocessing (CBP) are the primary adopted techniques for bioethanol production
[30]. Considering bioethanol, the saccharification-fermentation process is the main biological mechanism for converting lignocellulosic biomass into bioenergy. According to this method
[31], biomass is hydrolyzed to create monosaccharides, which are subsequently fermented to generate ethanol. An alternative to this is gasification-fermentation, which eliminates the intricate saccharification stage, addressing a key downside of saccharification-fermentation. LB is thermally gasified to create synthetic gas (syngas), which is made of CO, H
2, CO
2, and N
2, and is subsequently fermented to create biomaterials, such as bioethanol
[32]. Despite decades of development and research targeted at raising the market value of biomass, commercialized bioenergy production from lignocellulose biomass still needs technological and financial advancement
[9].
2.2. Biobutanol
Biobutanol (C
4H
10O), also known as butyl alcohol, is a renewable biofuel that has an advantage over bioethanol due to its higher energy density, immiscibility in water, lower Reid vapor pressure, low toxicity, and compatibility with existing infrastructure
[33]. It is primarily generated by the acetone-butanol-ethanol (ABE) fermentation process, which entails the microbial fermentation of sugars from biomass feedstocks into butanol using particular bacterial strains, such as
Clostridium species. Butanol’s toxicity to microorganisms, problems with butanol recovery later on, the choice of biomass, and pretreatment, which have an impact on large-scale synthesis, are the main obstacles of the ABE process
[34]. The biobutanol industries create a variety of high-value byproducts, including plastics, fibers, solvents, and coatings. They also serve as an important precursor for many chemicals with added value including butyl acetate, acrylic acid, adhesives, and glycol ethers in addition to producing the primary transportation fuel, all of which have the potential to boost economic growth through a variety of product alternates
[35].
2.3. Biodiesel
Biodiesel is a clean energy source that reduces greenhouse gas emissions, maintains ecological balance, and is compatible with existing infrastructure. It is derived from biological sources such as edible and non-edible oils, animal fats, and waste cooking oils
[36]. Traditional physicochemical processes include transesterification, esterification, pyrolysis, and micro-emulsion, among which transesterification, wherein triglycerides and alcohol react in the presence of a catalyst to yield fatty acids alkyl ester and glycerol at low temperature and pressure, is cost-effective and yields high-quality products. However, conventional production methods have reached their maximum efficiency, making biodiesel less competitive than petroleum-based diesel
[37]. In recent years, there has been a growing demand for innovative, clean, and enhanced technology to accelerate the reaction times, use less energy and catalysts, and maintain excellent biodiesel quality. To that end, a variety of techniques, including microwave, ultrasonic, supercritical, hydrodynamic cavitation, reactive distillation, membrane, plasma, cosolvent, rotatory, and plug flow reactors, have been investigated for biodiesel production
[38].
2.4. Biohydrogen
Hydrogen, with its high energy density, is used in industries as a fuel and renewable energy source. However, traditional techniques such as water electrolysis and auto-thermal processes are economically unviable due to their high-power requirements. Biohydrogen, a carbon-neutral process, offers potential benefits over thermochemical and electrochemical methods
[39]. It can be produced using pure sugars or waste substrates such as lignocellulosic biomass and microalgae. It is produced by dark fermentation, photofermentation, or a combination of these methods. However, the generation and output of biohydrogen are influenced by substrate availability, inoculum origin, and operational factors. A study by Patel et al. established a low-cost biohydrogen manufacturing technique using agricultural waste. H
2 production peaked at 37 °C and pH 8.5. The highest H
2 yield was measured in wheat straw pre-hydrolysate (WSPH) at 2.54 ± 0.2 mol-H
2/mol-reducing sugar and in pre-treated wheat straw enzymatic-hydrolysate (WSEH) at 2.61 ± 0.1 mol-H
2/mol-reducing sugar
[40]. Advancements in technology and renewable energy sources make biohydrogen a promising option for a cleaner and more sustainable future
[41].
2.5. Biogas
Biogas is one of the most important renewable energy sources to solve the environmental and energy challenges and serves as a substitute to natural gas or transportation fuel. Biogas refers to a mixture of gases produced by anaerobic organisms via the fermentation of organic materials such as plant materials, agricultural waste, food waste, sewage, municipal waste, and compost without the presence of oxygen
[42]. This process is known as bio-methanation, and it primarily produces methane and carbon dioxide with minute amounts of hydrogen sulphides and siloxanes. The main variables affecting the effectiveness of biogas production procedures include organic loading rate, pH, carbon to nitrogen ratio, temperature, retention duration, and mixing rate. According to their sensitivity to temperature, the microorganisms utilized in the bioreactor are divided into three main groups: psychrophilic (15–25 °C), mesophilic (35–40 °C), and thermophilic (55–60 °C)
[43]. However, the actual use of lignocellulose-based material in the anaerobic digestion process is limited because of the biomass’s resistant nature, which results in poor digestion efficiency and biodegradation. Further developments, such as the use of several pretreatment techniques, microbial inoculum, and the application of chemical (NaOH and CaO) and biological (white-rot and brown-rot fungi) additives, are being prioritized in order to speed up microbial growth and the rate of biogas production
[44]. In an investigation, researchers have studied the biogas production from pineapple waste, in which both the biogas and methane production showed significant increases (mL/day) from longer to shorter HRT. The maximal values (HRT 5 days, OLR 5 g/COD/day with recirculation) were 55,130 and 30,322 mL/day, respectively
[45]. Its ability to harness methane from organic waste, its various applications in energy generation, cooking, and transportation, as well as its role in fertilizer production, make biogas a valuable asset in the transition toward a cleaner and more sustainable energy system.
3. Nanoparticle Application in Biofuel Generation
Lignocellulosic biofuels, obtained from plentiful organic sources provide a hopeful pathway for sustainable energy. Nevertheless, the conversion of these substances into biofuels encounters obstacles as a result of intricate compositions and ineffective decomposition mechanisms. Nanoparticle or nanomaterial application plays a transformative role by acting as a catalyst in the conversion of lignocellulosic materials into biofuels. NMs are defined as materials that have components or particles with at least one dimension in the nanometer scale, typically ranging from one nanometer to a few hundred nanometers. Furthermore, NMs where all dimensions are at the nanoscale are called NPs. Nanoparticles are essential for speeding up the decomposition of the resistant components in lignocellulose, which improves the overall effectiveness of biofuel manufacturing. Their catalytic characteristics greatly enhance the efficiency and standard of biofuels obtained from these renewable raw materials, representing a crucial breakthrough in sustainable energy generation. The application of nanomaterial at different process stages for improving the biofuel quality and yield is represented in Figure 1.
Figure 1. Possible avenues for the utilization of nanoparticles for various biofuel production.
NP application during the synthesis of biofuels plays a role in improving procedural effectiveness by boosting pretreatment, hydrolysis, and reaction rate throughout the fermentation process. The key determining factors for the development of the desired product include reaction time, size, surface area, shape, nature, and type of biomass
[46].
3.1. Bioethanol
Application of nanomaterials in bioethanol production is numerous and encompasses all stages of the lignocellulose to bioethanol process, ranging from pretreatment to fermentation. For example, Kim et al. (2023) explored the use of cerium-doped iron oxide nanoparticles for the simultaneous pretreatment and saccharification of raw corn cob biomass
[47]. These modified nanoparticles exhibit laccase and cellulase/hemicellulase mimicking properties, which further aids in the pretreatment. The synthesized NPs are of spherical shape with a size ranging from 40 to 70 nm. During simultaneous pretreatment and saccharification, a small amount of cellulose/hemicellulose enzymes are used. During the pretreatment process, approximately 44% of delignification was achieved due to the laccase mimicking properties of NPs. Further, the hydrolysate resulted in a maximum ethanol production of 21.7 g/L. In another study, Iron oxide NPs of size 70–100 nm synthesized with the
Spinacia oleracea leaves extract for bioethanol production from Corncob yielded 53.7% ethanol
[48].
3.2. Biohydrogen
Anaerobic bacteria used in dark fermentation break down carbohydrate-rich substrate and create hydrogen. With an energy density of 140 MJ/kg, which is higher than that of coal (24 MJ/kg) and petrol (44 MJ/kg), biohydrogen (H
2) has a lot of promise as a cheap, renewable, carbon-free, and ecologically friendly fuel
[49]. The kind of raw materials used, the nutrients that are accessible (such as C, N
2, PO
43−, and SO
42−), and other operational circumstances all have an impact on the synthesis of biohydrogen. To boost microbial development and enhance the activity of enzymes involved in producing H
2, researchers are proposing novel strategies, such as the use of mixed substrates, mixed microbial culture, and usage of nanomaterials and carbon-based biomaterials
[50]. An important way to produce bio-H
2 is nanotechnology-based pretreatment on lignocellulosic biomass structures. The cost of the procedure is decreased since the chemicals are readily recyclable and usable again
[51]. Titanium NPs produced 127% more biohydrogen when combined with sugarcane bagasse and anaerobic sludge
[52]. In a comparable manner, using palladium NP in a mixed culture also included with glucose produced 9% H
2 [53].
3.3. Biobutanol
A sustainable, environmentally friendly and perhaps practical alternative fuel to conventional petrol is biobutanol. Zinc oxide (ZnO) NPs were utilized as a catalyst in fermentation from coffee bean husk, a byproduct of farm waste chosen as raw material, and biobutanol was effectively produced by ABE fermentation. High butanol production and 0.36 g/L alcohol, 70.5% sugars were observed with 10 g of ZnO NPs utilized as catalyst
[54]. In another study by Gandarias et al.
[55], employing distinct bimetallic and trimetallic catalysts made by sol-immobilization under reaction conditions of 100 °C, 6 h, at 3 bar O
2, n-butanol solution yielded butyraldehyde, butyl butyrate, and butyric acid as a conversion product. In a case, Pt/C metal(s)/support NP have a 91.8% conversion rate of n-butanol with a yield of 23.2% butyraldehyde, 67.9% butyric acid, and 8.8% butyl-butyrate.
3.4. Biogas
Research is ongoing to determine the effect of nanoadditives on the anaerobic digestion (AD) process and, as a result, the output of biogas. The study examined the effects of titanium dioxide, zinc oxide, and silver nanoparticles (with an average size of at least 1 dimension <100 nm) on the process of methanogenesis in mesophilic batch anaerobic digestion of primary sludge. The findings indicated that none of the NPs used had a significant impact on methane generation. The methane production rates, measured in m
3 of CH
4 per kilogram of volatile solids, ranged from 0.08 to 0.13. There was no statistically significant difference observed between the control groups and experimental sets for the examined NPs
[56]. The majority of previous studies have suggested that methane production is decreased by increased NP concentrations. The results on several conductive materials, including graphene oxide (GO), carbon fibers, activated carbon, iron oxides, and biochar, support direct interspecies electron transfer, which is a state-of-the-art method to increase biomethane output. Mixed anaerobic culture with graphene oxide on the anaerobic fermentation process of assam lemon showed 219.64 mL/g VS fed improvement of biomethane yield
[57].
3.5. Biodiesel
The process of producing biodiesel can be done in a number of ways, including transesterification, pyrolysis or cracking, and micro-emulsion. The most widely used technique for producing biodiesel among them is triglyceride transesterification of methanol or ethanol in the presence of a chemical or biological catalyst. This may be done by immobilizing lipase on magnetic nanoparticles (MNPs), which will increase the triacylglycerol (TAG) conversion in the presence of a catalyst. Another way of biodiesel production is through whole cell immobilization of recombinant
Aspergillus oryzae, which was engineered for the generation of biodiesel and expressed the
Candida antarctica lipase B gene (r-CALB), according to a study by Adachi et al.
[58]. In another study, solid catalyst NPs derived from oil-palm empty fruit bunches were used as a renewable catalyst for biodiesel production. The study observed the highest palm-oil to biodiesel conversion of up to 97.90% when using 1% palm bunch ash nanocatalyst, which was produced by heating empty fruit bunches at 600 °C, and a 3 h reaction time.
[59].