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Nanomaterials: stimulants for biofuels and renewables, yield and energy optimization

Nanoparticles play a vital role in energy and yield optimization of biofuels for competitive cost and commerical production. The role of nanomaterials, functions, various characterization techniques, nucleation and growth kinetics have been discussed in this review.


Introduction
The world is actively exploring new avenues in the search for renewable alternatives to conventional energy resources, attributing to their cost-competitiveness and non-polluting properties. Fossils have a detrimental effect on the environment due to the greenhouse gases and toxic emissions 1 . The production of biofuels using waste organic matter could be a bridge between cost-effectiveness and sustainable development. The biofuels used to mitigate these environmental challenges are further classified into two groups (1) primary biofuels (2) secondary biofuels. 2,3 Primary biofuels are directly obtained from forests, animals, plants, and crop residues whereas secondary biofuels are obtained using biomass in combination with nanomaterials or microorganisms. The secondary biofuels are further categorized in three parts based on the type of biomass/feedstock and technology used. The first-generation biomass is derived from edible crops wheat, barley, sunflower, corn, sugarcane, and sorghum whereas the second generation biofuels are derived mostly from lignocellulosic biomass, cob, jatropha, straw, grass, cassava, and miscanthus. Meanwhile, microalgal species, algal biomass are used in third-generation fuels and they can produce biogas, biohydrogen, and biodiesel under diverse kinetics conditions. Alternatively, in the new definition, the utilization of carbon dioxide as feedstock to produce biofuels is linked to third-generation biofuels. Moreover, second, and third-generation biofuels are more effectively utilized at larger scales as these do not lead to deforestation, water shortage and food security issues. 4 Commercialization of second and third generation biofuels requires solid economics, and is dependent on conversion rates, high-value products, feedstock and pretreatment cost. Thus effectiveness of biofuel production process is indirectly linked to optimized carbon utilization and energy optimization. Nanomaterials could be significant in energy optimization and enhancing overall product yield. 5,6 Several researches in the recent decade suggest notable improvement in overall biofuel yield in nanomaterial-assisted systems. Despite numerous scientific achievements, second-generation biofuel production is hampered by a plethora of technical hurdles that must be overcome to improve commercial viability. High lignin content reduces the extraction of fermentable sugars from lignocellulosic biomass, formation of inhibitors reduces metabolic process and makes pretreatment steps cost intensive. 7 Efforts are being made to address challenges with the pretreatment process, hydrolysis, and biofuel yield using nanotechnology. The extremely small-sized nanomaterials (1-100 nm) 8 are accredited for their higher interfacial surface area, porosity, high selectivity, and more active sites. 9,10 In addition nanomaterials are mostly (except enzymes) associated with high chemical stability, crystallinity, higher absorption capacity, and improved catalytic activities. 11 Nanomaterials are effectively utilized in different domains of science and technology like cancer treatment 12 , mass transfer, water treatment, drug delivery, biological sensing and biofuels production, etc. Nanomaterials can be classified into 5 categories 13 (1) Metallic and metal oxides (2) Non-metal oxides (3) Biomolecules (4) Enzymes or biocatalyst (5) Carbon-based nanomaterials. In the context of biofuels, nanomaterials enhance chemical kinetics/catalytic activities and electron transfer, reduce formation of inhibitors, and improve anaerobic microbes activities. The control over size, morphology, and geometry at the synthesis level is eminent for catalytic activities and stability of nanomaterials. The use of nanomaterials for yield and energy optimization is in the nascent stage. A detailed description of nanoparticles nucleation and growth kinetics, stability criterion have been presented in this paper. Various nanomaterials, synthetic and characterization techniques along with the effect of crystal arrangement in context of biofuel production have been discussed in this review (Scheme 1). Miscellaneous process intensification approaches like microreactors, ultrasonication, microwave irradiation, and vibration in combination with nanomaterials have been discussed for improvement in product yield and energy optimization. [14][15][16] Several literatures suggest nanomaterials combined with microreactor technology may contribute to energy optimization. [17][18][19] In a continuous flow channel reactor, improved hydrodynamics, higher interfacial area, and flow mixing inside these capillaries in presence of nanoparticles is a major contributor in energy optimization and mass transfer enhancement in liquid-liquid and gas-liquid systems. Liquidliquid extraction, chemical reactions, and CO2 capture technologies are some applications of nanoparticle-assisted systems. Several catalysts have been synthesized and tested through experimentation and simulations to determine the best-suited precursors for the same, different processes requiring different organic or inorganic moieties to catalyze the reactions. The yield and energy optimization of biofuel production using nanomaterials has gained attention in recent years to make biofuel products more commercially viable and competing. Various organic, metallic, metal oxide and carbonbased nanomaterials are extensively tested for yield optimization of biofuels.

2.Potential Nanomaterial
Bioenergy is as effective and efficient as it is claimed but there are some issues to be addressed. Lower conversion, feedstock availability, primitive technology, and catalytic poisoning are major concerns with bioenergy tapping. Nanomaterials' assistance is expected to tackle these challenges. The nanomaterials are structures of size range 1 to 100 nm and prepared in various dimensions and geometries. 20 Nanotechnology is a frontier domain with a unique combination of science and technology which deals with synthesis, characterization, controlling the shape and size of nanomaterial within nano range (Figure 2). Nanomaterials find applications in optoelectronics, microfluidics, drug delivery, biofuels, fuel cells, and other energy storage and generation 21 although, the focus of this review is biofuels. Various metal, metal oxide, and carbon-based nanomaterials have been tested for mass transfer applications. The majority of research suggests nanoparticleassisted systems enhance overall heat and mass transfer rates due to diffusiophoresis and nanoparticles Brownian motion. Some of these studies regarding CO2 capture enhancement 22 using nanoparticle is shown in Figure 1.  The different nanomaterials used for biofuels applications are classified based on material and structural features in Table 1  Higher stability, Improved multifunctional properties [33][34][35] Hybrid FeMo, CuMo Synergetic catalytic activity 36 The aforementioned nanoparticles are prepared in different geometries (Nanocage, cubic, ring, spherical, nanotube, nanowire) which depend on precursor and reaction conditions. Hybrid and core-shell nanoparticle systems have ample potential to accelerate future developments in biofuels. The optochemical characteristics of nanoparticles depend on shape, size, and precursor properties. It has been observed that facecentered cubic(FCC), hexagonal closed packing (HCP) and body-centered cubic(BCC) collectively contributed to 68% of stable lattice structures and hexagonal, rhombohedral (RHO), orthorhombic (ORTH), simple cubic(SC), monoclinic (MON), double hexagonal closed packed (DHCP) and tetragonal(TET) accounted for the rest. 37 The critical size plays a decisive role in the stability of crystal structure. The stability of FCC and BCC nanocrystals is decided if the size is smaller and larger than the critical size respectively. The cohesive energies of a stable nanoparticle system should be higher. The BCC nanoparticle systems are stable if nanoparticles size is larger than critical size and vice versa for FCC crystals. 38 Most of the s and p block oxides are used as basic and acidic heterogeneous catalysts respectively for biodiesel production but p-block oxides offer relatively higher stability at elevated temperatures. Elements of d-block have variable oxidation states and valency which facilitates intermediate complexes thereby new routes for chemical reactions with low oxidation state. The magnetic and catalytic properties of transition metals are remarkable hence used as hybrid and complex nanoparticles (Figure 3). Listed below are the nanomaterials used for catalyzing various processes. The catalytic activity of nanomaterials is structure sensitive and depends on the existence of site and low energy electronic fluctuation. The transition metals have large coordination Please do not adjust margins Please do not adjust margins numbers as they are surrounded by metallic nonmagnetic or weakly magnetic atoms which create electronic fluctuations.
These electronic fluctuations lead to catalytic activities. 39 Zheng et al observed enhanced catalytic activity of transition metalnonmetal catalyst (MX) for hydrogen evolution reaction(HER) that depends on the principal quantum number and electronegativity of nonmetallic atoms. Larger electronegativity and principal quantum number significantly improve HER activity of Ni-nonmetal complex in the second period. 40 Surface defects have been extensively analyzed and found that reactivity of surface defects (Steps) are more than terraces attributing to their larger contact area. 41 Metal oxides and non-metal oxides are thermally stable at elevated temperatures (>1000 K) whereas metallic nanoparticles are sensitive to oxidation at elevated temperatures. Most of the metallic nanoparticles are stable up to 400 K but stability will increase beyond 800 K if metal nanoparticles supported over SiO2 or Al2O3. 42,43 The Carbon nanotubes (CNT) are stable temperatures above 3400 K. 44

Nanomaterial synthesis routes
Several methodologies have been used to synthesize nanomaterial which can be categorized as top-down (Physical methods) and bottom-up (Chemical methods) approaches ( Figure 4 a & b). [45][46][47] The physical methods are advantageous as large quantities can be produced via these. Bottom-up strategies have effective control over size and geometry but only produce materials in small quantities. Top-down approaches are best suited for materials with macroscopic connection and long-range order whereas bottom-up approaches are effectively utilized in short-range order at nanoscales. Thermodynamic stability of nanoparticles is dependent on aggregation state, crystal geometry, shape, size, and surface chemistry. 48 The nanoscale size of particles increases surface area per unit volume resulting in an increase in surface energy and the number of low coordination surface atoms. 48,49 Aggregation occurs when nanoparticles undergo clustering due to short diffusion distances and can be used as gas and solution phase nanoparticle instability indicator. The collision theory is used to model nucleation and growth kinetics. Nanoparticles must collide to form clusters. 50

Top-down Approach
A destructive approach in which larger molecules are fragmented into smaller units, which are further transformed into Nanoparticles of desired size and shape. The decomposition of larger fragments into smaller ones. These approaches are mostly physical or mechanical like cutting, grinding, attrition, spraying, electrolysis, milling, physical vapor deposition, laser ablation, chemical etching, and sputtering.

Please do not adjust margins
Please do not adjust margins Bello et al. 51 prepared coconut shell nanoparticles by this approach using ceramic balls for the milling process. They reported the size of NPs decreased with time and the brown color of NPs faded away hence proving a change in the characteristic of the material at the nano level.

Bottom-Up Approach
This approach involves nanoparticles being formed from relatively simpler molecules.  54 There are also numerous ways. Among these, the development of various types of NPs, ranging from single metal NPs to bimetallic and trimetallic NPs, has played a significant role. Metal NPs are commonly employed in catalysis, and metal alloys and core-shell NPs are two types among the most important. However, core-shell NPs offer benefits over alloy NPs in catalytic processes. Core diameters, shell thicknesses, sizes, and elemental compositions of cores and shells may all be customized for the former. 55,56 The epitaxial growth mode produces a monometallic surface; the island growth mode produces a bimetallic surface, and the trimetallic core-shell cluster structure produces a trimetallic surface. The core-shell cluster structure is more complex than the other two kinds of NPs, and it is expected to include more electrocatalytic active sites. [56][57][58] Hybrid nanoparticles were also prepared using bottom-up synthesis approaches. Hybrid nanoparticles consist of more than one nanoparticle and are used for synergetic effect over chemical kinetics. Hybrid nanoparticles are prepared via sol-gel, photochemical, seeding growth approach, coprecipitation, sonochemical, and green synthesis routes. 59,60 Core-shell nanoparticles are heterostructures that consist of metallic or metal oxide core with semiconductor metal/nonmetal oxide layer that facilitates stability. Moreover, core-shell nanoparticles also improve multifunctional characteristics and physiochemical properties of catalyst core. 61 Core-shell nanoparticles prepared via precipitation, co-precipitation, hydrolysis, solvothermal, hydrothermal, wet chemical, and direct reduction methods. In a broad sense core-shell nanoparticles are categorized into four types (1) organic/organic (2) inorganic/inorganic (3) inorganic/organic (4) organic/inorganic. Silica is a more common inert coating material as it reduces bulk density and increases suspension stability of core particles. 62 In bottom-up synthesis initially small and highly unstable atomic/molecular fragments are formed and clustering initiated. The stability of these nucleates increases as the fragment size approaches critical radius of nucleation. whereas nuclei smaller than critical radius are dissolved in solution to reduce overall free energy (Figure 4 c). When the size of these clusters is greater than the critical nucleate size (rnucleate >> rcritical) layer by layer or diffusion led growth of nanoparticles starts. 63 here ∆ is volume free-energy and γ is surface energy/unit area. No nucleation occurs when ∆ is zero and nucleation spontaneously occurs when ∆ negative. The reduction of Gibbs free energy is driving force for both nucleation and growth phases. At a critical size of nuclei rcritical= r*, ∆ = 0 , the Gibbs free energy barrier is ∆ * = 16 3 (∆ ) 2 and critical radius of nucleation * = ( − 2 ∆ ). Here ∆ * is free energy potential for stable nuclei formation that has critical radius of nucleation (r*). The nucleation would only occur above the supersaturation level, which corresponds to the energy barrier for nuclei formation, with no nucleation occurring above the equilibrium solubility. The concentration or supersaturation decreases after a certain nucleation period and Gibbs free energy reduce. No new nuclei would form once the concentration or supersaturation starts to decrease and growth phase of these formed nuclei begins till the concentration of solution reaches equilibrium concentration or solubility level. In other words, no nucleation occurs when the concentration of embryo/growth species rises over the equilibrium concentration (Figure 4

d).
Nucleation, on the other hand, begins when the concentration exceeds the minimum saturation level required for critical free energy. A rapid increase in nucleation rate has been observed at that point although growth phase cannot proceed at this point. A simultaneous increase in growth and nucleation rate has been observed once nuclei are formed just above Cmin. It is recommended for uniform size distribution of nanoparticles that all nuclei are formed with same size at same time. Moreover, with same subsequent growth rate, mono-sized nanoparticles are obtained. 64 When nuclei are established, they begin to grow at the same time. Nucleation and growth are inseparable processes above the minimum concentration; nonetheless, they proceed at distinct speeds. Nucleation for a shorter period is desired to achieve monosized nanoparticles. To achieve rapid nucleation, the concentration of growing species suddenly increases to very high supersaturation and drops rapidly below Cmin for nucleation. Existing nuclei continue to proliferate until the concentration of growing species is decreased to Cequilibrium (Csolubility). The formation of monosized nanoparticles is achieved by controlling growth process through a change in pH, temperature and other kinetics conditions (Figure 4e). Subsequent growth kinetics of nuclei plays an important role in uniform size distribution nanoparticles. The growth kinetics of nuclei is a multistep process (1) generation of embryo/ growth species (2) diffusion of embryo from bulk to growth surface (3) adsorption of embryo over growth surface (4) surface growth through irreversible incorporation. The growth process over growth species is either diffusion controlled or surface force controlled. The growth controlled by surface process is either monolayer or polynuclear ( Figure 5 a and b). Each nucleus that forms on the crystal face gives rise to a monoatomic layer, according to monolayer growth theory, which means growth rate is proportional to crystal face area and nucleation rate. In solution, many nuclei formed simultaneously. In polynuclear growth, it was assumed every nucleus is taken part in the formation of a separate monolayer. 65 The rate of transferring embryo to growth surface is known as diffusion whereas the irreversible incorporation of these embryones over growth surface is surface force controlled. Sol-gel and direct reduction method is used to prepare most of core-shell, hybrid and metallic nanoparticles ( Figure 5 c and d). In sol-gel methodology, fast and instantaneous nucleation leads to small and monosized nanoparticle size; and slow nucleation results in large and uneven nanoparticle size distribution. Once nucleation is achieved, the rate of nucleation and growth decreases as the degree of supersaturation decreases (Figure 5  e). The size of clusters increased over time with decrease in degree of supersaturation. Metal-support interaction is important in heterogeneous catalysis because it can affect the heterostructural features of metallic nanoparticles (NPs), such as degree of dispersion, shape, and nucleation. As a result, a key topic in catalytic research is how to make use of metal-support interactions to optimize NP shape for high catalytic activity, selectivity, and sintering resistance. The metal-support interaction, which is important in the deposition of size-selected metal clusters onto a well-defined support surface, is another important component in Pt-based catalysis. 66 y-Al2O3 (001) deformations, metal-support interactions, and nucleation energies display an odd-even oscillation pattern on the y-Al2O3(001) surface as the Ptn cluster size increases. The metal-support interaction and nucleation energies of an even Ptn cluster size are lower than those of its neighboring odd ones, but the y-Al2O3 (001) deformation energy is greater. The nanoparticle morphological shift from 2D to 3D is controlled by both metal-surface and metal-metal interactions, which are competing. When the metal-support interaction prevails on the y-Al2O3 (001) surface, smaller clusters such as Pt1, Pt2, Pt3, and Pt4 have a planar 2D character. Larger clusters, such as Pt5, Pt6, Pt7, Pt8, and Pt13, prefer a two-layer structure with one or more Pt atoms on the top layer not directly engaging with the support when the metal-metal interaction is dominant. The Pt4 cluster is the most stable 2D structure in this study, whereas the Pt5 and Pt6 clusters represent the transition from 2D to 3D, and the Pt7 cluster is the smallest 3D structure. 66 Recently, Duan et al. 54 observed enhanced electrocatalytic activities over Au/Pd core-shell nanoparticles with mushroomlike Pt clusters. Pt clusters placed on a Pd shell over Au core nanoparticles (Au@Pd@Pt NPs) have been discovered to have extraordinarily high electrocatalytic activity for the electrooxidation of formic acid. They proposed the mushroom growth mode as a new model for metal atoms with high cohesive energy, such as Pt, growing on other metal surfaces. Compared to typical island (pyramid) form Pt clusters, mushroom-shaped clusters can provide unique active sites with a higher coordinate number, which is better for certain reactions. This could explain why Au@Pd@Pt and Pd@Pt NPs have such strong electrocatalytic activity. The use of more adjustable parameters while synthesizing NPs are necessary to improve further electrocatalytic and chemical activities. Duan et al. suggest that high index surfaces of Au core NPs results in a high index surface on the Pt shell. The mushroom-shaped Pt clusters that formed on such a surface could be able to boost electrocatalytic activity even more. Another method is to use alloying, such as alloyed metal surfaces and mushroom clusters, to bring several active site components (such as Au, Pd, Pt, and Ru) together. Due to the existence of electrochemical Ostwald ripening, Song et al. 67 discovered that the growth process of Au particles on virgin MoS2 is somewhere between diffusion-limited and reaction-limited. The Au particles at the MoS2 edge had a greater size distribution and more orientation variation than those in the interior. Sulfur vacancies on MoS2 cause Au particle diffusion and coalescence throughout the development phase, unlike pristine MoS2. The size difference is attributed to exposed molybdenum atoms at the edge with dangling bonds that can strongly interact with Au atoms, but sulfur atoms on the MoS2 interior have no dangling bonds and only weakly interact with gold atoms, according to DFT calculations. Furthermore, S vacancies on MoS2 form strong nucleation centers, which can facilitate Au nanoparticle diffusion and coalescence. 67 DFT simulations demonstrate, the size difference is attributable to the fact that exposed molybdenum atoms with dangling bonds at the edge may strongly contact with Au atoms, but sulfur atoms on the MoS2 interior have no dangling bonds and only weakly interact with gold atoms. Furthermore, S vacancies on MoS2 act as strong nucleation centers, allowing Au nanoparticle diffusion and coalescence to be facilitated.
➢ Morphological Characterization of Nanoparticles-➢ Structural Characterization of NanoParticles-➢ Particle Size/Surface area characterization ➢ Optical characterization Apart from these, Density functional theory (DFT) calculations are widely used for magnetic, oxides, biomolecules and metallic catalysts to determine electronic configuration, magnetism, stability, and reaction mechanism. Furthermore, nanoparticle growth kinetics and aggregation state are analyzed using Dynamic Light Scattering (DLS). Catalyst pore volume and fouling characteristics are analyzed using BET absorption isotherms. STEM and XRD are useful tools to analyze metal crystallite composition and catalyst surface segregation. Some of these techniques and their characterizing properties are listed below in Table 2.

Nanomaterials application in Biofuels
The nanoparticle application in biofuel production is attracting attention nowadays with regard to its higher interfacial surface area promoting chemical kinetics of biochemical reactions. Nanomaterials such as core-shell, nanocages nanotubes, hybrid, nanofibers, nanorings, and magnetic nanoparticles systems have been tested for this purpose. The functions of these nanomaterials in enhancing biofuel production are different in each biofuel (Table.3)   This section reviews several nanomaterials-assisted attempts to improve biofuel production and energy optimization. Various biofuels like biodiesel, bioethanol, HVO, biogas, and biohydrogen-related studies have been discussed to draw a clearer picture.

Biodiesel
Biodiesel is methyl/ethyl esters of Fatty acid (FAME/FAEE) derived from oils/wax or natural fatty acids via transesterification. The transesterification process is overall driven by alkaline/acid catalysts. Several batch and continuous reactor systems are applied to increase overall FAME yield including microreactor, oscillatory baffled reactor and microstructured reactors. Higher FAME yield and improved product quality may be obtained by using microreactors and process intensification techniques like vibration, pulsation, and microwave. 18,68 Several acid and base-catalyzed heterogeneous catalysts are studied for biodiesel production. Biodiesel has reduced emissions, lubricity, high flash point, biodegradability, lower sulfur content, and higher miscibility with conventional diesel. 69,70 Biodiesel is a promising contender for conventional diesel fuel due to low cost, reduced NOx emission, and high cetane number though, high viscosity and poor cold-flow properties hinder its utilization in a conventional diesel engine. The limitations with theoretical yield and purity of FAME are resolved by using miniaturized reactors and heterogenous acid/base catalysts. The homogeneous acid/alkaline catalysts have been scrutinized in the past decade but the lower product quality, higher separation cost, corrosion, saponification probability, high water usage limits their applications at a larger scale.

ARTICLE
Please do not adjust margins Please do not adjust margins ATP-dependent direct uptake of zero-valent iron follows metalloenzyme, electron transfer, and reduction pathways. 73

Biohydrogen
Improve electron transfer rate, decrease the dissolved oxygen (DO) levels in the culture, improve efficacy of the oxygen-sensitive hydrogenase, provide nutrition to microbial growth, accelerate electron transfer between ferredoxin and hydrogenase. [74][75][76] Act as enzyme carrier, enhance separability of enzyme during hydrolysis and pretreatment, cofactor for enzymatic activities, aid to structural stability of enzymes, ability to modulate oxidation-reduction potential. 77-79 Fischer Tropsch synthesis Improve FTS activity, olefin selectivity, CO conversion rates. 80 Alternatively, heterogeneous solid and enzymatic (nanomaterial) catalysts have gained momentum due to their environmental and economical benefits. Degirmenbasi et al. 81 observed nearly ~ 97% FAME yield at relatively low temperatures using CaO nanoparticles impregnated with K2CO3. The higher stability of impregnated catalyst is reported with functionalized CaO with 3-10% K2CO3 and catalyst maintained yield and product quality for up to five cycles in a batch reactor at 9:1 alcohol/oil ratio. Sahani et al. 82 observed 98.4% conversion using BaCeO3 (Ba/Ce = 1:1) with 1.2 wt% catalyst loading at 1:19 oil/methanol in agitated batch reactor. The BaCeO3 demonstrated excellent 6 th cycle reusability with 81% conversion in Karanja oil and methanol system. Kawashima et al. 83 have worked over Calcium based multimetallic oxide nanoparticles and reported enhancement up to 92% with Ca2Fe2O5. The heterogeneous catalysts include alkaline oxides or alkaline earth metal oxides supported with zeolite or hydrotalcite. Several heterogeneous catalysts like zinc ethanoate/SiO2, zinc stearate/SiO2, MoO3/ZrO2, WO3/ZrO2, WO3/ZrO2-Al2O3, MoO/SiO2 and TPA/ZrO2, have been analyzed at laboratory scale. 84 The advantages of these catalysts are low cost, higher ester yield, larger surface area, higher purity glycerol, low maintenance, cheaper separation, lower operation costs. 85 It requires a low effluent water load which makes it environmentally benign. 70 However, cost and energy consumption are considerably higher than the homogenous catalysts. The commercialization of heterogeneous catalysts is promising to attribute to insensitivity for free fatty acids and elimination of purification unit 86 but it is under development as only CaO loaded microcapsules are available. CaO is most often used as a heterogeneous catalyst due to high activity, long catalytic life, availability, and moderate reaction conditions. The co-extrusion process is used to encapsulate CaO in amphiphilic alginate. The conversion occurs in pores of microcapsule when fatty acid and methanol are passed through the reactor. 87 It has been observed that a heterogeneous catalyst requires a higher transesterification temperature than its counterpart which might elevate the overall cost of operation. Heterogeneous catalysts derived from natural resources (lemon/orange peel and eggshell) are cost-effective alternatives of CaO catalyst 88,89 with a capacity to effectively downsize plants, reduce basic waste-water discharge, and reduce environmental impact hence, transesterification could be scale-up to industrial levels with more sustainability. The use of enzymes for transesterification has several environmental and operational advantages like mild reaction conditions, higher selectivity, high oil/alcohol ratio, no side reactions, enzyme biodegradability, low enzyme recovery cost, high-quality glycerol, improved stability, and recyclability of enzymes. Despite several advantages, enzymatic transesterification faces issues like lower reaction rates, higher production cost, and catalyst inhibition. 90 Innovations like recombinant DNA technology open paths for novel, inexpensive, and highly efficient enzymatic catalysts that may reduce the overall cost of operation. 90,91 Various enzymes like Rhizopus oryzae lipase (ROL), triacylglycerol lipase (TGL), pseudomonas cepacia lipase (PCL), esterases (carboxyl ester hydrolase-CEH) are used for transesterification. TGL converts long-chain fatty acids to glycerol and fatty acid methyl/ethyl esters. 91 whereas, esterases convert short-chain fatty acids to transesterification products. 91 Komintarachat et al. demonstrate incredible 97.5 % FAME yield using waste cooking oil and MOx catalyst supported on a porous support. They reported that the activity MOx catalyst on these porous supports based on biodiesel yield is in order Al2O3 > SiO2 > SnO2 > ZnO. 92 Most used enzymatic catalysts include Pseudomonas, Candida, and Rhizopus are derived from natural sources like fungi, yeast, and bacteria. Alcoholic reactant, oil/alcohol ratio, reaction temperature, and immobilized/free enzyme ratio are factors that affect biodiesel production other than origin and type of enzyme . 90 Enzyme immobilization is a Please do not adjust margins Please do not adjust margins technique where enzymes are immobilized or attached over solid surface through physical and chemical methods. Various techniques like cross-linked enzyme aggregates (CLEAs) and Protein-coated microcrystals (PCMCs) are used for enzyme immobilization. Covalent attachment, entrapment, adsorption, and cross liking are the primary approaches used for stable immobilization of enzymes over solid surfaces. 93,94 Higher biodiesel yield was reported with immobilized lipase (Thermomyces lanuginosus, Pseudomonas cepacia, and Candida Antarctica). The immobilized lipase system is highly effective in enzyme recovery and purification units and produces high purity glycerol. Batch reactors and continuous plug flow reactors are used at an industrial scale with free or immobilized enzyme systems however, continuous plug flow reactors for biodiesel production have upper-hand over batch reactors in different aspects like high product quality, lower labor cost, stable operational condition, easy control, and automation 90,95,96 though, low conversion yield and higher residence time are important factors that need to be addressed. Various Enzyme, acid, and base-catalyzed systems are used in the last decade. The commercial viability, water usage, and product yield are major emphasis while selecting heterogeneous and bionanomaterials for transesterification. Although, each nanomaterials system has its advantages and disadvantages listed below in Table 4. Transesterification of ester hydrolysis is a reversible reaction just like Fischer esterification. Therefore excess amount of alcohol is required that acts as a nucleophile. Protonation, nucleophilic addition, proton transfer, and elimination of alkyl alcohol are subsequent steps in the acid-catalyzed reaction. The nucleophile eventually replaces alkoxide in the ester. The series of steps like nucleophilic addition, elimination of OR', and deprotonation of carboxylic acid take place in base-catalyzed reactions (Figure 6). The heterogeneous base catalysts are advantageous due to higher catalyst lifetime, low cost, higher activity than the solid acid catalyst. 97,98 Although, solid base catalysts are sensitive to Free Fatty Acid (FFAs) content. The solid acid catalysts have lower activity but it is insensitive towards FFAs, easy separation and simultaneously conduct transesterification and esterification process. 99,100 Encapsulated, acid catalysts are proton donors while base catalysts are proton accepters that can drive overall reaction kinetics in their respective mechanisms. The comparative analysis of solid acid/base and biocatalyst is given below in Table 5.

Hydrotreated Vegetable Oils (HVO)
The low energy density of biodiesels in low-duty diesel engines limits its application in both steady-state and transient conditions. Biodiesel being highly viscous faces issues of cold flow property which may affect cold start performance and fuel injection in the engine. 101,102 Hydrotreated vegetable oils have improved combustion and flow properties than biodiesel/ fatty acids. The calorific value, oxidation stability and cold flow properties of fatty acids/biodiesel improve with hydrotreatment. Catalytic hydroprocessing of fatty acids is an alternative substitute for biodiesel with improved properties. HVO is paraffinic hydrocarbons with a straight-chain configuration that is free from sulfur and aromatics. HVO has improved cold flow properties, higher calorific value, oxidation stability, and high cetane number. Catalytic hydrotreatment consists of series of steps, the first step consists of conversion of triglycerides into several intermediates (carboxylic acid, monoglycerides, and diglycerides) which are further converted into alkanes via pathways namely 103 (i) Hydrodeoxygenation (HDO) (ii) Hydrogenation (iii) Hydrodecarboxylation (HDC). HVO has a higher combustion value attributing to low density due to paraffinic nature, improved hydrogen content, and low lubricity due to absence of oxygen and sulfur. 104 Plethora of works has been reported on hydrotreatment of vegetable oils but the catalyst was of macro range hence not fully utilized and a lot of novel properties were left unexplored. Unsupported CoMoS nanocatalyst was employed to investigate the hydroprocessing of WCO in a batch autoclave reactor by Zhang et al. 105 It was observed that the reaction pathways for oxygen elimination were majorly decarboxylation/hydrodecarbonylation routes with a rate constant thrice that of hydrodeoxygenation. The hydrodeoxygenation process was immune to gradual sulfur deficiency, unlike hydrodecarbonylation activity which declined. The HDO route via direct reduction to aldehyde was significantly slower than alcohol route hence less significant. The juxtaposition of the C18/C17 ratio of unsupported versus lewis acid-supported catalysts played a pivotal role in hydrodeoxygenation selectivity and enhanced product quality. Supported catalysts showed a significant lead in the ratio over the unsupported counterpart due to lack of acidity which is abundant in Al2O3 support. Side reactions observed include hydrocracking, polymerization, hydrogenation, and cyclization. The reduction of fatty acids is noted to be the rate-determining step and the hierarchy of the rate constants are Please do not adjust margins Please do not adjust margins decarbonylation > hydrodeoxygenation > decarboxylation. A study by Abdulkareem-Alsultan et al. 106 reported carbon-based nanocatalysts for catalyzing deoxygenation reaction with multiple benefits like catalytic stability, resistance to coke formation, and increased product selectivity, and reduced production costs due to abundant carbon source. The carbonbased catalysts observe better stability due to their mild acidity. The dominating reaction path in deoxygenation (DO) by carbonbased catalysts is N4-metal moiety bonded with carbon that plays important role in oxygenation reduction in the DO process. There is a drawback to these catalysts as they cannot be synthesized without nitrogen. N-doped carbon catalysts show increased adsorption of reducing oxygen along with decomposition of peroxides. A catalyst synthesized by Shu et al 107 with sulfonation of vegetable oil asphalts treated by carbon observed high catalytic activity due to dense acid sites and conversion of 94.8 wt. % with catalyst/oil value of 0.2 wt. %. Oh et al. 108 concluded that carbon moiety is where oxygen reduction takes place after employing three types of carbon. Alsultan et al. 109 incorporated the acidity of carbon support and basicity of metal oxide in CaO-La2O3/AC nanocatalyst, using phosphoric acid to activate the carbon. The yield obtained over the catalyst was 72% of hydrocarbons through decarbonylation and decarboxylation and retained catalytic stability and conversion rates up to six cycles. In another study with Ag2O3-La2O3/AC nanocatalysts 110 , they obtained higher conversion yields of 89% and product selectivity of 93%. The latter was found to be more stable with an 80% constant yield without vigorous reaction conditions. Coke formation with these catalysts was under 1.5-1.8 wt % at pH 7. Multiwalled carbon nanotube-based catalyst was synthesized by Asikin-Mijan 111 for deoxygenation reaction of Jatropha curcas oil and observed high yield and selectivity with 5 wt % catalyst loading in a semibatch reactor. A high yield of the product was observed with Ni/MWCNT but higher selectivity was offered by Ni-Co/MWCNT with negligible coke formation. Reddy et al. 112 produced highly active CaO nanocatalyst with 90.61 m 2 /g surface area and 66 nm size from seashells to produce biodiesel from crude jatropha curcas oil through bi-step transesterification. The yield at 5.15:1 (mol/mol) ethanol to pretreated oil and 0.02:1 (w/w) catalyst ratio was found to be 98.5%. After six catalytic cycles, an average of 95.8% yield was retained and the fuel property was uncompromised with the standards. An exponential decline was observed after the sixth cycle in the production of biodiesel. Studies in this domain are not just limited to experiments but researchers are trying to simulate mathematical modeling. One such study was done by Muharam et al. 113 where they developed a mathematical model for hydrotreating vegetable oil in a slurry bubble column reactor for biodiesel production using NiMo-P/Al2O3 as a catalyst. The column developed was 7.14 m tall and 2.68 m in diameter. The simulation was run at 34.5 bar and 598 K. The simulation results showed 97.73% conversion of triglyceride and the yield of product was 83.34% with a purity of 77.23%. Vachova et al. 114 studied the hydrotreatment of rapeseed oil with sulfide CoMo/γ-Al2O3 and NiMo/γ-Al2O3 catalysts. A trickle bed reactor was employed for carrying out experiments under pressures 8 and 4 MPa and 320−380 °C temperature. Marginal amounts of aromatic hydrocarbons and, at elevated temperatures, higher contents of isoalkanes and cycloalkanes were obtained with yield directly proportion to reaction temperature. NiMo catalyst generated more aromatics since it has more dehydrogenation activity than CoMo catalyst (Figure 7). The liquid n-alkanes obtained have very high cetane value which declined with increasing temperature as aromatics and isoalkanes were formed. 114 The following graph depicts an abstract of the products obtained through these catalysts.

Bioethanol
Bioethanol prepared via 2 nd generation using lignocellulosic biomass is a low-cost synthesis route and alternative to firstgeneration ethanol production from molasses. Lignocellulosic biomass consists of hemicellulose (20-35%), lignin (15-20%), cellulose (35-50%), ash, and others (15-20%) approximately. 115 The bioethanol production using lignocellulosic biomass is carried out in subsequent stages (1) Pretreatment [Conversion of indigestible polysaccharides to digestible polysaccharides (Glucan, Xylan)] (2) Hydrolysis [Conversion of glucan, xylan into hexose and pentose] (3) Fermentation (Conversion of hexose and pentose into ethanol). 116 The use of nanoparticles promotes overall ethanol yield and reduces acetic acid formation. The schematic representation of the nanoparticleassisted hydrolysis process is given below in Figure 8. Sanushi et al. 117 reported 1.6 fold increase in bioethanol production and a 2.10 fold decrease in acetic acid concentration using NiO nanoparticles at pretreatment stage. Bioethanol like other biofuels is considered a sustainable alternative since it is renewable and has the potential to replace most petroleumbased fuels. 118 The limiting step in bioethanol production is catalytic conversion (hydrolysis) of lignocellulosic biomass for carbohydrates degradation (hemicellulose and cellulose) into simple fermentable sugars. Current practices for the conversion of complex cellulosic biomass into sugars are cost-inefficient. The complexity and high cost of separation can be resolved with nanoparticle application. Several types of nanoparticles such as nickel cobaltite, iron oxide, zinc oxide, nanocomposites, etc. have been utilized in biofuel production. Commonly enzymatic and acid catalysts are used in catalytic conversion of various lignocellulosic biomass; however, immobilization of enzymes/biocatalysts is being considered as the most promising technique. Several biocatalysts such as β-glucosidase, cellulase, laccase, cellobiose, and xylanase are immobilized on nanomaterials support to form nanobiocatalyst which enhances catalytic potency and stability of enzymes. Furthermore, the use of magnetic nanoparticles (magnetic nanobiocatalysts) provides easy recovery and reusability of biocatalysts. Thus, the application of nanoparticles and nanobiocatalysts is promising in developing an economically viable and environmentally sustainable approach for bioethanol production. 118, 119 Cherian et.al 120 studied bioethanol production by cellulase immobilization onto MnO2 nanoparticles. They immobilized cellulase derived from Aspergillus fumigatus JCF on manganese dioxide nanoparticles which provided support as well as enhanced the catalytic activity of cellulase. The immobilization of cellulase onto MnO2 resulted in an increase in characteristic nanoparticle size from 76 to 101 nm, furthermore, the system had higher thermal stability than free enzyme up to 700 o C for two hours. Recyclability of immobilized catalase also increased significantly in five cycles with 60% activity retention. Cellulase immobilization on MnO2 nanoparticles, combined with yeast for SSF, gave a bioethanol concentration of approximately 21.96 g/L. 120 Sanusi et al. 117 studied the use of NiO NPs as biocatalysts for the production of ethanol. They reported improved bioethanol production with potato peel waste using NiO NPs, emphasizing the possibilities of using nanoparticles to increase biofuel production with starch-based agricultural biomass. Primarily, bioenergy production from lignocellulosic biomass is carried out via saccharification-fermentation process, but it has a drawback of complex saccharification, and to address this problem gasification-fermentation has emerged as a better substitute to this process. The process of gasificationfermentation, involves thermal gasification of lignocellulosic biomass to generate synthetic gas (syngas) comprised of CO2, CO, N2, and H2 which is further fermented to produce various bio substances like bioethanol. Kim et.al studied the use of nanoparticles for better bioethanol yield in syngas fermentation with Clostridium ljungdahlii. They examined six types of nanoparticles i.e. palladium on alumina, palladium on carbon, hydroxyl-functionalized SWCNT, silica, iron (III) oxide, and alumina, where silica nanoparticles performed best for enhancing mass transfer in the gas-liquid system, and their activity was further enhanced by modification of silica nanoparticles' hydrophilic surface by hydrophobic alkyl functional groups. Silica nanoparticles with methyl functionalization enhanced the dissolved concentrations of CO2, CO, and H2 by 200.2%, 272.9%, and 156.1%, respectively, and provided better activity than silica nanoparticles that were unmodified or isopropyl-functionalized for mass transfer enhancement. The use of nanoparticles enhanced ethanol production by 166.1% but suffered inefficient use of nanoparticles. 118,121 However, in another work, Kim et.al studied the application of magnetic nanoparticles for increasing bioethanol production through syngas fermentation. They examined two types of nanoparticles namely methylfunctionalized cobalt ferrite-silica (CoFe2O4@SiO2-CH3) and methyl-functionalized silica to enhance mass transfer of gaseous precursors to fermentation broth in syngas-water system which is the rate-determining step in syngas fermentation. CoFe2O4@SiO2-CH3 magnetic nanoparticles exhibited enhanced syngas mass transfer. The ethanol yield increased by 126.9% and 213.5%, respectively by addition of SiO2-CH3 and CoFe2O4@SiO2-CH3 nanoparticles compared to a control. A neodymium magnet was used to recover CoFe2O4@SiO2-CH3 nanoparticles and reused for five cycles to test recyclability, which confirmed their activity retained for mass transfer enhancement. 122 Although, lignocellulosic materials are majorly used for bioethanol production but are afflicted by relatively high cost compared to other feedstocks, 123 and lignin content with low biodegradability. 124 Thus, microalgal biomass which contains no lignin may prove to be an innovative alternative for production of bioethanol. Harun et al. 125 investigated bioethanol production from one such microalgal biomass. The relevance of microalgae (Chlorococum sp.) as a precursor for bioethanol production via yeast (Saccharomyces bayanus) fermentation was studied. They observed that 10 g L −1 of lipid-extracted microalgae debris produced a maximum of 3.83 g L −1 ethanol concentration.

Fischer Tropsch synthesis
Please do not adjust margins Please do not adjust margins Fischer Tropsch is a pivotal synthetic route for catalytic formation of hydrocarbons of different molecular weights from carbon monoxide and hydrogen in syngas. Researchers are mainly focused on developing catalysts for linear alkanes with high molecular weight and diesel production hence transition metal catalysts are being relied upon for this purpose and it is well established that at the nano level the catalytic efficiency grows exponentially for these metals. [126][127][128] Fuels derived from Fischer Tropsch synthesis is of high quality as it consists of low aromatic and almost zero sulfur content. Recent studies demonstrate iron-based nanoparticles are efficient due to high interfacial area and low microporosity. The microemulsion technique is considered ideal for preparing such nanomaterials for Fischer Tropsch synthesis. These iron-based nanocatalysts prepared via microemulsion techniques demonstrated higher selectivity and reactivity towards oxygenation. Chemical activity and selectivity are strongly correlated to geometric alignment and electronic configuration of nanoparticles, which relies on particle size, crystallographic phase, and morphology. 80 It has been reported that HCP Ni nanoparticles demonstrated higher H2 selectivity and low CH4 formation than FCC Ni nanoparticles. 80,129 In a study by Chin et al. 130 Fischer Tropsch synthesis was performed in a microchannel reactor with arrays of multiwalled carbon nanotubes as a catalyst. A dense Al2O3 film was deposited on FeCrAlY foam via metal-organic CVD. Carbon nanotubes were uniformly deposited on the substrate through catalytically decomposing ethylene. Thus prepared catalyst offered high thermal conductivity coherent with the property of CNT which vents out active catalytic sites of any residual heat from the exothermic operating at temperature as high as 265 o C. The unique microstructure enhanced mass transfer as the catalytic turn-over improved four-folds with selectivity towards CH4 being satisfactory at 240 o C above which methanation was favoured. 130 In a study by Coronel-Garcı´a et al. 131 , the catalytic activity of supported Co/SBA-15 promoted by ruthenium was tested for Fischer Tropsch synthesis. The catalyst was synthesis wetness impregnation method with 20%Co and 1-5wt%Ru. A significant reduction was observed in the specific surface area and increment in pore volume attributing to partial blockage of pores by Co3O4 aggregates and pore array destruction by increasing Ru content but offered higher selectivity for heavier alkanes and reduced selectivity for methane as metal phase reduced easily. Methane selectivity decreased with increasing Co 3+ and Co 2+ ions on the catalyst surface. ruthenium promoted catalyst enhanced selectivity for C5+ hydrocarbons from 66.9% to 90.3% and reduced methane selectivity from 20.4% to 2.4%. Park et al. 132 studied Fe2O3 nanoparticles supported on alumina as a catalyst for FTS. Strong interactions were observed between support and iron oxide particles which elevated the reduction temperature. Reduction of bigger iron oxide particles underwent easily but they suppressed the particle dispersion. It was also observed that CO adsorption and iron dispersion decreases with increasing particle size but reduction degree increased with the same. The optimum particle size suited for FTS was found to be 6.1 nm. A dependency on size was observed for CO conversion and a three-fold increase in TOF from 0.06 to 0.187 s −1 was reported on increasing particle size from 2.0 nm to 6.1 nm. 132 Yang et al. 133 investigated the catalytic activity of cobalt NPs supported by mesoporous carbon with nitrogen doping for FTS. It was observed that interfacial interaction between support and active site is of importance to this heterogeneous catalyst and affected the reduction behavior, dispersion of cobalt on support, and elevation in reduction temperature for smaller particles. They also observed that sp 2type nitrogen in the carbon framework acted as an active site for cobalt nucleation. The novel structure facilitated electron transfer at the interface between cobalt oxide and support. Small particle size was observed to favor lighter hydrocarbons and the TOF first increases from 7.2 × 10 −2 s −1 to 23.4 × 10 −2 s −1 and then levels off to 24.5 × 10 −2 s −1 with increment in size of particles. 133 Yang et al. 134 studied vacuum impregnation is used to deposit Ru nanoparticles inside pores of TiO2 nanotube (TNT) for FTS catalytic applications. CO conversion and activity retention were also tested for Ru deposition outside TNT which was found to be significantly lower than Ruin/TNT. The catalyst also depicted higher selectivity for the C19+ carbon chain. Confinement in TNT enhances the selectivity and activity by facilitating charge transfer, modulating Ru electronic properties, limiting metal sintering and leaching for higher performance in FTS, and promoting α-olefin re-adsorption which facilitates the formation of longer chains. 134 The role of crystal structures of metallic nanoparticles on FTS activity are listed below in Table 6.  Higher conversion of CO in HCP is due to the higher density of surface defects and stacking fault. 136,137 HCPCo consists of more active B5 sites and follows direct CO activation pathways rather than H-assisted pathways in CoFCC. 80

Biohydrogen production
Various metabolic routes are used to produce molecular hydrogen using anaerobic bacteria. The kinetics of the metabolic process is dependent on pH, temperature, retention time, and substrate concentration. Nanoparticle introduction in the anaerobic biohydrogen process improves electron transfer and ability to react faster with electron donors. 141 Thus nanoparticle addition stimulate activity of microorganism in dark/photo fermentation biohydrogen process. Three welldeveloped processes have been utilized for biohydrogen production in recent decade namely (1.) Dark fermentation (2.) Photo fermentation (3.) Photocatalysis. 142 The initial pH effect, glucose concentration, and nickel nanoparticle concentration on biohydrogen yield was studied by Mullai et al. 143 at a mesophilic temperature of 30-35 ˚C using anaerobic microflora. The yield of biohydrogen using catalyst was found to be 2.54 mol of H2/mol of glucose with a production of 4400 ml. The process optimization was done by using a central composite design (CCD) with response surface methodology (RSM). The initial concentration of 14.01 g/L glucose was taken at a pH of 5.61 and concentration of nickel nanoparticles was 5.67 mg/L. The effect of initial concentration of substrate and concentration of nickel nanoparticles was significant in optimization process of biohydrogen production. As a result, it was concluded that Nickel nanoparticles increased production by 22.71%. Gadhe et al. 144 used dairy wastewater in batch to test the effects of nickel oxide (NiO) and hematite (Fe2O3) nanoparticles on biohydrogen production. It was found that using only NiO NP, a specific hydrogen production rate (SHPR) of 42.69 mmol/g VSS.d and maximum hydrogen yield (HY) of 16.75 mmol/g COD which was 43% and 24% enhanced respectively compared to control was obtained. While the combination of Fe2O3 and NiO NP on biohydrogen production implied that the maximum HY and SHPR of about 17.2 mmol/g COD, and 47.67 mmol/g VSS.d, respectively, can be obtained from dairy wastewater when the concentration of Fe2O3 was 50 mg/L, and that of NiO was 10 mg/L, respectively. The highest significant relative enhancement in the yield was recorded to be 27%, and SHPR to be 59%, suggesting that the combination was about 1.2-1.5 times more effective for enhanced recovery of hydrogen from dairy wastewater as compared to control, and sole nanoparticles addition. Similarly, the impact of Ni nanoparticles and Ni-graphene nanocomposite (Ni-Gr) on hydrogen production was studied by Elreedy et al. 145 using industrial wastewater containing mono-ethylene glycol (MEG) via anaerobic digestion. Batch reactors with different dosages of Ni nanoparticles and Ni-Gr nanocomposites with concentrations ranging from 0 to 100 mg/L were supplemented. Maximum achieved hydrogen yields were 24.73 ± 1.12 and 41.28 ± 1.69 mL/g COD at a dosage of 60 mg/L for Ni nanoparticles and Ni-Gr nanocomposites, respectively. A considerable enhancement of 23% and 105% in the production were registered at an optimum dosage of 60 mg/L for Ni nanoparticles and Ni-Gr nanocomposites, respectively, compared to the control without any nanomaterials. However, Please do not adjust margins Please do not adjust margins further increase in the dosage of Ni nanoparticles and Ni-Gr nanocomposites to 100 mg/L resulted in a significant decrease in the yield to 20.80 ± 1.12 and 24.24 ± 1.13 mL/g COD, respectively. By a non-linear regression model, 129% was recorded to be the highest enhancement at the dosage of 50 mg/L Ni-Gr nanocomposites at an initial pH of 5.0. Recalcitrant lignin in the lignocellulosic biomass is a major barrier in biofuel production as depicted by Shanmugam et al. 146 to convert it into biofuels, removal of recalcitrant lignin is important from the lignocellulosic biomass. An approach of enzyme-mediated degradation of lignin without generating any toxic intermediates was used. Enzyme immobilization onto polymercoated magnetic nanoparticles enhances support, anchorage, and easy reuse. Fe3O4@SiO2-chitosan nano-support was used to immobilize Trichoderma asperellum laccase. The optimum immobilization conditions determined by response surface methodology were 58.37 mg Fe3O4@SiO2-chitosan, 24.11 μg enzyme concentration at pH 5.95, and 1.14% N-ethyl-N′-(3dimethyl aminopropyl) carbodiimide (EDAC) which gave maximum immobilization efficiency of 92.41% and a yield of 91.23%. The immobilized laccase was found to have a higher delignification potential of 84.46% than the free enzyme. Biohydrogen yield was 2.8 mol H2/mol of sugar and hydrogen production rate was 25 L H2/L-d, respectively. An alternative to traditional hydrogen productions is provided by biological photoinduced fermentation as studied by Ji et al. 147 on biohydrogen production the investigation at near IR region coupled to a near-field enhancement by silica-core gold-shell nanoparticles (NPs) over a varied range of acetate concentrations (5-40 mM) and light intensities (11-160 W/m 2 ) was done. The kinetic data were modelled using modified Monod equations containing light intensity effects. The yields of CO2 and hydrogen produced were determined as 0.83 mol/mol of acetate and 2.31 mol/mol of acetate respectively. The maximum increase in these gas productions was found to be 113% and 115% respectively without affecting the bacterial growth rates (6.1-8.2 mg-DCM/L/hour) by adding nanoparticles. Whereas the highest H2 production rate was 0.81 mmol/L/hour. The energy conversion efficiency is directly related to NPs concentration but inversely with the intensity according to the simulation model. Complete hydrogenation of toxic 2-chlorobiphenyl was also demonstrated by using Pd catalysts and biohydrogen produced. Surface defect engineering is also a route to enhance biohydrogen production as shown by Nadeem et al. 148 from corn stover agricultural residue. SnO2 nanoparticles were annealed in both oxidative and reductive environments (Ar 95% + H2 5%) and their effects were compared. It was found that in reducing gas environment the charge carriers were created in the form of oxygen vacancies which increased the active sites in comparison to the oxidative environment. XRD and SEM techniques were used to study the nanoparticles produced. The reflectance spectra were also studied by using Diffuse reflectance spectroscopy. The shift in-band energy was seen as redshift after annealing (0.28 eV and 0.56 eV respectively for oxidative and reductive environments). An increase in hydrogen production by nanoparticle annealed in reducing and air environment was found to increase to 25.64% (147 ml/h) and 16.24% (136 mL/h), respectively. On the other hand, accumulated production of hydrogen was found to be 23.636% (345 mL) and 11.83% (312 mL) for both methods respectively. So, surface engineering was able to modify the charge flow rate and hence biohydrogen production by the fermentation process. Another catalyst supported by biopolymer chitosan was used by Sewwandi et al. 149 which was named nano zerovalent iron catalyst (n ZVI) to enhance biohydrogen production. Enterobacter aerogenes were used for fermentation of glucose under mesophilic conditions with a temperature of 37˚ C. The formation of a biopolymer-supported nanocatalyst was confirmed by TEM images. The biohydrogen yield was found to increase to 30% with a production of 243 mL H2/g glucose at a pH of 6.8 and a nanoparticle concentration of 150 mg/L. The enhancement was supposed to occur due to the availability of Fe 2+ by the designed catalyst in a continuous manner. But the presence of propionic acid and ethanol declined the yield. In a recent study by Sinharoy et al. 150 , extracts of green tea were used to synthesize iron nanoparticles and their effect was studied on the biohydrogen production by carbon monoxide (CO) from anaerobic biomass. Various spectroscopies revealed polyphenols, alkaloids and flavonoids from the extract acted as capping agents. CO solubility in water was observed to increase by 56% and biohydrogen production to be 1.58 ± 0.13 mmol/l. Continuous bioreactor enhanced the yield of hydrogen to 30.7 mmol/l in the presence of the nanoparticles. The continuous gas lift bioreactor with internal loops was made from Perspex material indigenously. Biohydrogen concentration grew exponentially with increase in concentration of iron nanoparticles. Requirement of optimum nanoparticle concentration was shown in a study by Zhang et al. 151 . It was observed that with moderate concentration of nickel, iron hybrid nanoparticles (~100 mg/L), gave better hydrogen yield whereas excess nanoparticle concentration inhibited the H2 production. The nickel and iron from the particles promoted the hydrogenase and ferredoxin activity and synthesis. Tawfik et al. 152 used black liquor from paper industry as biomass for hydrogen production via dark fermentation. The hydrogen yield enhanced up to four folds in the presence of Graphene/hydroxyapatite nanoparticles and three folds in the presence of graphene or hydroxyapatite. The activity of other enzymes was also observed to be improved upon immobilization. Zhong et al. 153 employed an expanded granular sludge bed reactor for biohydrogen production via glucose fermentation. The magnetite nanoparticles improved acetic acid and ethanol accumulation as well as formed electron conductive chain to promote electron transfer in the sludge reactor. The fermentation was confirmed to be changed to ethanol-type from butyrate-type through respective bacterial abundance. Khan et al. 154 demonstrated that green synthesized Ag nanoparticles through two ways namely henna with C beijerinckii KTCC and actinomycetes showed varied effects on biohydrogen production. The latter inhibited whereas former increased the hydrogen yield after 72 h incubation period.

Biogas
Biogas is a blend of gases primarily methane and CO2 derived through Anaerobic Digestion (AD) of organic matter. Biogas constitutes methane (40-60%) which is majorly used in domestic heating and cooking. The production of biogas through anaerobic is a cost-effective alternative as compared to other thermochemical waste-to-energy routes. Methane-rich biogas is produced when biomass is digested anaerobically by certain bacteria. These bacteria are natural inhabitants of swamps, water bodies, lakes, and even in the mammalian digestive system. New researches are mainly focused on increasing methane content with various alkalis, acids, steam explosion-based pretreatment routes adopted to enhance methane fraction and overall yield. Although, the risk of toxic release, corrosion, and inhibitors hinders pretreatment. Highpressure and multistage anaerobic digesters are extensively used to enhance overall methane yield and selectivity. 155,156 Two-stage digester systems in which acidogenesis/ hydrolysis and methanation/acetogenesis processes are performed separately. The liquid phase acidification process is a biphasic hydrolysis process where gas and liquid phase participate simultaneously to form acetate and methane is acetogenesis/methanogenesis. The high pressure (4-50 bar) anaerobic digester system recently gained attention as researchers observed >95% methane yield in a single-stage digester. A high-pressure biogas digester aims to increase methane fraction and reduce CO2 yield as CO2 absorbs in water at higher pressures. Hydrolysis, acidogenesis, acetogenesis, and methanogenesis are four subsequent steps for biogas production. Organic matter is decomposed into water-soluble molecules in the first hydrolysis step in presence of hydrolytic microbes. The hydrolytic microbe is generated from extracellular enzymes. The complex molecules (proteins, fats, and carbohydrates) are hydrolyzed into simple less complex molecules (fatty acids, amino acids, and sugars) during hydrolysis. In acidogenesis acidogenic bacteria (acidogens) transform hydrolysis products into low molecular weight carbon moieties (alcohols, volatile fatty acids (VFAs)). In the next acetogenesis step, acetogens CO2 and VFAs convert into acetic acid, hydrogen. In the last step, methanogens convert CO2 and acetic acids, and acetates into methane through methanogenesis. 155,157 The reaction kinetics of microbial enzymes are highly interactive and complex. Despite continuous mixing quantitative presence of microbial groups is irregular throughout the reactor that influences overall reaction dynamics. Moreover, methanogenesis is considered a ratelimiting step in biogas genesis as methanogens have retarded growth rate. Consequently, methanogenesis is most sensitive to redox temperature, pH variations, redox, and inhibitors. Microalgal based biogas generation process is the most commercially viable. Although, dilute nature microalgae culture leading to limited availability of biomass that leads to poor activity of anaerobic microbes thus reduce performance of anaerobic digestion. Nanomaterials including metals, nanofibers, CNT, and others have been successfully employed for efficient microalgal growth, and higher methane yield. 158 Zero valent iron, metallic and metal oxides, and carbon-based NPs are extensively tested in the last decade. 159 Inclusion of magnetite (Fe3O4) nanoparticles in two-stage anaerobic digester increase 15.1 hydrogen yield and 58.7% methane yield at solid loading 100 mg/L. The total 46.9% reduction in volatile suspended solid is reported with 100 mg/L Fe3O4 nanoparticle loading. 160 Although, lignocellulosic biomass is difficult to digest in two-stage anaerobic digesters due to presence of hemicellulose and lignin. The alkaline pretreatment before anaerobic digestion is necessary. Khalid et al. reported 57%, 60% improvement in biogas and methane yield using 2% NaOH pretreatment before anaerobic digestion. Combining both alkaline and Fe3O4 nanoparticles strategy in anaerobic digester have synergetic effect and maximum 100 % and 129% increase in biogas and methane yield reported. 161 The zerovalent iron form NPs DOM complex that binds with metal receptors on methanogen-cell. The ATP-dependent direct uptake of zerovalent iron follows metalloenzyme, electron transfer, and reduction pathways to form methane within methanogens (Figure 9). It has been reported that NPs demonstrate an appositive effect on the concentration of ammonia and greatly decrease H2S formation. Ni and Co NPs are promising metallic systems for higher methane yield. 162 Optimization study by Zaidi et al. demonstrated that maximum cumulative biogas production using green algae is with 1mg/L Ni Nps loading at 7 pH and 37 0 C. 163 In another study 70.46% enhancement of methane yields and 90.47% reduction in H2S production was reported using cattle manure and 2 mg/L Ni nanoparticle concentration. The addition of Ni nanoparticles increases 31.2% sulfide value and that eventually improves fertilizer value. 164 It is noted that α-Fe2O3, and zero-valent nano iron is less toxic to microalgae cells and contributes to algal growth as well. α-Fe2O3, in particular, demonstrated lower microalgal toxicity among other forms of iron. 158 Geng et al. 165 reported that addition of zerovalent iron in digester improves dewaterability of algal sludge and enhances overall methane production. The rapid hydrolysis rate improved from 2.23 to 5.98 in two substrate models and sludge dewatering saved 74.72% cost using zero-valent Iron than conventional systems. The focus is now devoted to the commercialization of nanoparticle-added systems by optimizing cost and environmental constraints. The preparation of nanoparticles from industrial waste or byproducts is a novel approach to reduce the overall operational cost of biogas

Energy Optimization
The use of nanomaterials for biofuel yield and energy optimization is to improve economic viability. Since all biofuels are renewable and sustainable alternatives to conventional fossil fuels. So, the low cost of biofuel could induce people to be attracted to these renewable alternatives. Process intensification tools like microwave, mechanical agitation, ultrasonication, microchemical technologies along with heat transfer fluid could potentially be utilized at a larger scale. Although, some of these strategies have already been implemented at commercial scales. Microchemical technologies could be the best alternative for conventional batch reactor systems. Nandagopal et al. 168 compared various process intensification tools and reported that microsystems require nearly half the time to achieve maximum extraction percentage than conventional reactors. The order of mass transfer improvement in these strategies are, slug-based extraction > ultrasonic-assisted extraction > microwaveassisted extraction > batch extraction. 168

Heat transfer nanofluids
Nanofluids enhance augmented heating and cooling rates hence heat transfer properties have been utilized in various heat and mass transfer operations for biofuel production. Nanoparticles' efficacy in enhancing heat transfer coefficient is proven by researchers (Xuan et al. 169 , Maiga et al. 170 , Das et. al. 171 , Wang et al. 172 , Godson et al. 173 ) have unveiled its importance. Salman et al. 174 observed heat transfer enhancement in water is SiO2>Al2O3>ZnO>CuO. It was observed that heat transfer was regulated by surface properties, concentration, and size of nanoparticles. Brownian motion and thermophoresis are two fundamental mechanisms that enhance heat transfer. Since heat transfer and mass transfer processes are analogous in many aspects, the use of nanoparticles has received much attention in various mass transfer operations and kinetics studies. Nanoparticles enhance heat transfer coefficient by three molecular motions 173 (1.) Brownian motion (2.) Thermophoresis (3.) Diffusiophoresis.
(1.) Brownian motion is the randomized motion of colliding particles that increases convective currents in the flow which intensifies both heat and mass transfer. (2.) Thermophoresis works on soret effect where particles move as a result of the temperature gradient. Thermophoresis is advantageous for heat transfer especially natural convection where buoyancy and temperature play an important role. (3.) Diffusiophoresis is generally avoided as it involves the migration of particles from lower to higher concentration which agglomerates nanoparticles which is to be avoided especially in macro channels due to kinetic constraints. Thus, diffusiophoresis is highly undesirable in context of nanoparticle enhanced heat and mass transfer experiments. 173,175 Dhuriya et al. 175 have observed enhanced mass transfer with alumina nanoparticle as a result of diffusiophoretic mass transfer of fluorescein di-sodium dye in glass capillary (length 6 cm, I.D. 1 mm) tube. Mass transfer occurs due to the counter-current movement of dye molecules towards nanoparticles and vice versa. The counter-convective motion of dyes occurs in response to the diffusiophoretic movement of nanoparticles towards a higher concentration of dye as represented in Figure  10. Pang et al. 176 reported enhancement in the ammonia-water bubble absorption process in the presence of Ag-based nanofluids. They observed higher cooling rates and nearly 55% enhancement in absorption rates in presence of nanofluids. Nanofluids are expected to decrease energy consumption and potentially be used in biodiesel, HVO, and biohydrogen production processes. Apart from heat transfer, nanofluidassisted systems also reported enhancement in chemical kinetics and mass transfer which could play a pivotal role in energy optimization. The HTF could be beneficial to make biofuel synthesis route more energy-friendly as it gains/loses heat rapidly. Rapid and uniform cooling or heating improves overall energy efficacy of process in biofuel production.

Process Intensification
Biofuels could be a sustainable, renewable, and prospective source of energy to combat rising technological, environmental, and socioeconomic issues, especially for the transportation sector. The transportation sector heavily relies upon imported crude oil. Thus, bioethanol, HVO, and biodiesel could be best alternatives to conventional liquid fuels. The cost and economics of biofuel synthetic route are important to compete with conventional fuels. Process intensification strategies like microchemical technologies, supercritical reactors, hybrid systems are in development phase for higher product yield and rates. Emerging contact strategies like coiled flow reverser (CFR), coiled flow inverter (CFI), are continuous contacting

Emerging technologies and future prospective
Biofuels obtained using waste biomass via biochemical and chemical synthesis routes could be the best sustainable energy alternative for fossils in transportation sector. Although, challenges remain for biomass logistics, energy optimization, and higher product yield. These challenges could be addressed using various Process Intensification strategies individually or in combination with nanomaterials. These emerging technological advancements have an edge with higher conversion yield without substantial economic burden (Figure 11). The use of external fields (microwave, mechanical vibration, pulsation, ultrasonication) has been extensively explored. Most researchers have reported higher biofuel yields with these strategies. The use of ferrofluids has an added advantage over other nanoparticle systems as they are easy to separate and move in oscillating magnetic fields for improved mixing. The microchemical technologies 179,180 like conduit miniaturization, change of flow path geometry(helical, serpentine, 90 0 bends), and Coiled Flow Inverters (CFI) could potentially be used for continuous biofuel synthesis though, only liquid biofuels (HVO/Biodiesel) could be produced using microchemical technologies. For energy optimization of biofuel production process Heat transfer Fluids (HTF) and ionic liquids could be used in heating and cooling circuits. Moreover, these systems have already been tested in solar heating and industrial applications and could potentially be applied in biofuel production. The effective utilization of byproducts of biofuel synthesis for further energy utilization has future potential. In this line, use of glycerol (transesterification byproduct) has potential for hydrogen production via thermal decomposition which further improves overall process economy. Figure.11 Emerging technologies and future prospective for biofuel production and energy optimization

Conclusions
Fossil fuels have done irreparable damage to the environment and alternatives are of utmost requirement to stop further deterioration. Biofuels can cater to energy needs and at the same time not be extensively damaging to the planet by reusing the available biomass as feedstock. Current technologies are unable to optimally utilize the potential so the idea of utilizing nanomaterials to catalyze the process has been coined. The effect of electronic configuration and crystal structure on catalytic activity and selectivity of nanomaterials has been summarized. Nanomaterials are portraying great potential for biofuel production sustainably with commercial viability. Nanotechnology-based various conversion routes effectively convert waste biomass to value-added biofuel i.e. syngas, biodiesel, HVO, etc. The search for greener routes for biofuel production in combination with microchemical technologies for process intensification could further improve cost-effectiveness and make biofuel more competitive with fossils. Various studies reported enhanced yield, product quality, enhanced heat, and mass transfer while some shed light on further improvements with a special focus on energy alternatives for the transportation sector. On summarizing several literatures, it is observed that catalytic activity depends on the crystallographic arrangement, coordination number, ionization potential, oxidation state, ionic radii, and surface features. Moreover, Brownian motion, diffusiophoresis, and perturbation around nanoparticles could also vitally contributes to effective phase mixing. The focus of current researches is mainly on the commercial viability of biofuels with the help of microchemical technologies in combination with nanomaterials for synergetic process intensification.