Synthesis of cellulosic and nano-cellulosic aerogel from lignocellulosic materials for diverse sustainable applications: a review

Abstract Cellulosic aerogels are sustainable, biodegradable, and ultra-light porous materials with three-dimensional networks having high specific surface area. Depending on the source of precursor materials, they are categorized into plant-based aerogel, bacterial cellulosic aerogel. Different types of aerogels are also produced from microcrystalline cellulose (MCC), nanocrystalline cellulose (NCC), cellulose microfibril (CMF) and cellulose nanofibril (CNF). Furthermore, inorganic and organic substances are embedded to produce hybrid aerogel or composite aerogel for the enhancement of its performance in various fields. Mixing, gelation, solvent exchange, and drying (e.g., super critical carbon dioxide or freeze drying) are the basic steps involved in cellulosic aerogel synthesis. Based on the composition of precursors during aerogel synthesis, cellulosic aerogels have broad applications in various fields such as adsorbents, electrodes, sensors, captive deionization materials, catalysts, drug delivery, thermal and sound insulating materials. This review provided consolidated information on: (i) classification of cellulosic aerogels based on the sources of raw materials, (ii) processes involved to produce the cellulosic aerogel, (iii) cellulosic aerogel synthesized from MCC, NCC, CMF and CNF, (iv) nano particle doped cellulosic aerogel, and (v) its application in various field with future perspectives. Graphical Abstract


Introduction
One of the vital factors responsible for the environmental upset is the growing use of nonrenewable, non-biodegradable and non-sustainable products. [1]To address the raising concerns of environmental upsets (e.g., air, water and soil pollution), research has been directed toward producing bio-based, renewable, and sustainable products that improve the living standard of human civilization. [2]Primary source of bio-based renewable eco-friendly materials are plant biomasses that are obtained mainly as forest wastes and agricultural wastes.A considerable part of agriculture waste is burnt below the ideal condition of combustion causing a decrease of nutrients in the plant and an increase in air pollution due to formation of harmful gases along with particulate matter.[5] Several conventional methods like incineration, composting, surface mulching, and burning, are used for managing biowaste. [6]A considerable part of bio wastes can be valorized in producing different macromolecules.
Plant biomass basically consists of three macromolecules such as cellulose, hemicellulose and lignin and cellulose is in large proportions ($40%-60%, w/w). [7]Cellulose is produced naturally in large amounts in the biosphere which is an ecofriendly, biocompatible polysaccharide. [8]Furthermore, this polymer is a nontoxic, biodegradable biopolymer with molecular mass ranging from 1.44 Â 10 6 to 1.8 Â 10 6 g and density ranging from 1.52 to 1.54 g/cm 3 (20 C), having good tensile and compressive strength.Cellulose has a branched helical structure linked by hydrogen bonding between chains and has b-1, 4 glycosidic bonds, a-1, 4 glycosidic bonds, a-1, 6 glycosidic bonds of glucose units resulting its high strength.[11][12][13][14][15][16] Extracted cellulose can be converted into four types of cellulose (e.g., microcrystalline cellulose-MCC, nanocrystalline cellulose-NCC, cellulose microfibril-CMF, and cellulose nanofibril-CNF) depending on the structure and size of the cellulosic polymer.19] Aerogels are solid materials with a large number of micro/nano level pores resulting in a high internal surface area ($5-500 m 2 /g), high porosity ($81-99.9%),and low density (0.02 g/cm 3 -0.2g/cm 3 ).Furthermore, it has high rigidity, and low thermal conductivity, and can bear weight much more times than its weight. [20,21]Historically, in 1930, first time, SS Kistler obtained an aerogel from silica by evaporating the liquid using supercritical drying from wet gel. [22]ilica was the most common raw material for aerogel preparation. [23][26][27] Limitations of this type of aerogel are high production cost, high delicacy, and a tendency to break while a small load is applied. [21]These limitations have confined their uses and there was a need to develop a sustainable and biodegradable aerogel with high delicacy, more brittle and high mechanical strength.
Currently, attempts are being carried out to produce organic aerogels from natural substances containing cellulose as their main component. [3]Cellulose is being extracted from these biowastes by hydrolysis and bleaching methods. [28,29]Thereafter, this extracted cellulose is used as the raw material for the synthesis of cellulosic aerogels.The advantages of cellulose as a raw material for the synthesis of cellulosic aerogels are: (i) the abundance of renewable cellulose source in bio ecosystem; (ii) cellulose has helical structure stabilized by a large number of hydrogen bonds (intermolecular and intramolecular cross-linking of bonds form a stable three-dimensional structure making the synthesis process of aerogel simple and easy); (iii) modification of cellulose chains by treating them chemically and physically to enhance the mechanical and physical properties of cellulosic aerogel.Different types of modified celluloses (MCC, NCC, CMF and CNF) are used to produce MCCaerogel, NCC-aerogel, CMF-aerogel and CNF-aerogel.These cellulosic aerogels have a high surface area, low pore radius and are mechanically more stable than the cellulosic aerogel produced from the cellulose of larger dimensions.32][33][34][35] Apart from natural cellulose, bacterial cellulose has been used by researchers for the synthesis of cellulosic aerogel because of its high purity, crystallinity and distinct chemical and physical properties compared to cellulose produced from plant wastes. [36][39] Agitated fermentation and static fermentation are the two commonly used methods for bacterial cellulose production. [40]In case of bacterial cellulose, the resultant cellulose does not contain impurities like hemicellulose and lignin that may be present in the cellulose extracted from plant-based sources. [41][44][45] These hybrid aerogels have strong cross-linking structures attributing the enhanced mechanical strength and prevention of the permanent shrinkage of aerogels.Hydrophobic modification of cellulosic aerogel was done by adding methyltrimethyloxysilane (MTMS), trimethylchlorosilane (TMCS) and propylene glycol. [30,46][49] Different nano-particles (range within 1-100 nm) like copper, iron, gold, silver phosphorus, nitrogen, and nickel, are doped to produce nano-doped cellulosic aerogel.Nano-particle doped cellulosic aerogels have better physical and chemical properties than normal cellulosic aerogel.
][52] Nowadays, researchers are developing more sustainable cellulosic aerogels which are biocompatible and have no adverse response to these biomaterials in contacting with tissue.[55] Recent demands are on the production of nano-particles doped cellulosic aerogel that can meet the need applications in various fields like an advanced catalyst, and materials for electronic gadgets.The market value of cellulosic aerogels is estimated to be USD 638 million in the year 2020 and is expected to reach a value of up to USD 1045 million in the year 2025. [56]North America has the sole market of aerogel followed by Europe and Asia.However, limited consolidated reports are available on the information on NCC/CNF extracted from biowastes and NCC/CNF derived aerogel and nano particle doped cellulosic aerogel with their applications.This review focused on different types of cellulosic aerogel based on raw materials (plat based cellulose and bacterial cellulose).Also, emphasis has been given on the types of cellulosic aerogels derived from microcrystalline cellulose (MCC), nanocrystalline cellulose (NCC), cellulose microfibril (CMF), cellulose nanofibril (CNF) and modified composite aerogel by doping nano particles.It may be noted that MCC, NCC, CMF, CNF and modified composite cellulosic materials were produced from waste biomass.Furthermore, an overview of synthesis of cellulosic aerogel and its diverse industrial applications including biomedical sectors for each type of cellulosic aerogel have been documented.

Synthesis of cellulosic aerogel
Cellulosic aerogel is synthesized using cellulose (both plantbased cellulose and bacterial cellulose) as raw material.The quality of cellulosic aerogel largely depends on the quality of cellulose and synthesis techniques.Cellulosic aerogel can also be prepared using MCC/NCC/CMF/CNF as the raw material and these raw materials are obtained by various methods (eg; acid hydrolysis, grinding, 2,2,6,6-Tetramethylpiperidin-1-yloxyl (TEMPO) mediated modification of the cellulose).Different naturally occurring cellulosic materials (MCC, NCC, CMF, CNF) contain tiny microstructures and better mechanical strength, thus enabling them to produce aerogel with larger specific areas. [57]he methods of synthesis and arrangement properties of cellulosic aerogels depend on the purity, concentration, and shape of the cellulose.Figure 1 depicts the different steps involved to synthesize cellulosic aerogel from cellulose. [58]ellulose is the raw material for synthesis of cellulosic aerogel and this raw material is derived from either plant sources or bacterial sources.Furthermore, other types of celluloses (MCC, NCC, CMF and CNF) can be synthesized from raw cellulose.All types of celluloses can be used to produce cellulosic aerogel with diverse properties for different applications.The first step for aerogel preparation is the dispersion/dissolution of cellulose in the solvent to prepare a cellulose-solvent solution where gelation occurs (i.e., crosslinking between the cellulose and solvent molecules).In the second step, shaping and gelation are done depending on the required shape and size of the aerogel.Thereafter, the regeneration and solvent exchange is executed to remove the solvent from the wet gel.In this step, the resultant material is called cellulosic hydrogel.Hydrogels have different applications in biomedical fields like wound dressing and sensor for monitoring human motions with anti-bacterial properties. [59,60]Lastly, the wet gel is dried using freeze or supercritical drying.Further treatment is required to modify the aerogel into hydrophobic cellulose.
Hydrophobic modification of cellulosic aerogel requires adding methyltrimethyloxysilane (MTMS) to the aerogel in a container and putting it into a preheated oven for 12 hr. [30]rimethyl-chlorosilane (TMCS) is used as plasma for hydrophobic modification of the surface of the cellulosic aerogel by employing cold plasma theory. [61]Propylene glycol adipate was also added to the cellulose solution for modifying the cellulose hydrophobically. [46]The nano-doped cellulosic aerogels are also produced by doping nano-particles during synthesis.The detailed steps of aerogel synthesis and its characterizations are provided in the supplementary material.

Classification of cellulosic aerogel based on raw materials
Types of cellulosic aerogel can be classified into three broad groups based on the raw materials used: (i) plant-based cellulosic aerogel, (ii) bacterial cellulosic aerogel, and (iii) hybrid aerogel.The plant-based raw materials were treated with acids/alkali for removal of hemicellulose and lignin followed by bleaching to remove color materials resulting in white extracted cellulose.The extracted cellulose can further be treated with chemicals for the modification of its physical structure.Bacterial cellulose is produced through the fermentation process using microorganisms.These extracted/bacterial celluloses are used for the synthesis of cellulosic aerogel.Hybrid aerogels are prepared using two or more types of raw materials (organic or inorganic) where at least one raw material used was cellulose.

Plant-based cellulosic aerogel
Cellulose is a bio-degradable polymer produced by linking the monomer units of glucose and is present in the cell wall in plants in large proportions ($40%-60% w/w). [62]ellulose extracted from different plants and agricultural wastes is termed plant-based cellulose.Thus, the waste generated from agricultural and forest residues can be used for the extraction of cellulose using acid hydrolysis and/or other conventional methods. [63]Scientific reports revealed that different lignocellulosic materials (e.g., sugarcane bagasse, wheat straw, rice straw, tea stem waste coir fibers, and waste paper) are used for the synthesis of cellulosic aerogel as a source of raw materials (Table 1).Researchers used different solvents (e.g., NaOH, CH 4 N 2 S, NH 2 CONH 2 , polyethylene glycol-PEG, polyvinyl alcohol-PVA, urea) to synthesize the cellulosic aerogel.The use of ionic liquids (costly solvent) as solvent has been reported.Most of the studies has used freeze-drying techniques for the drying of aerogel.Limited supercritical drying has been reported for the drying of aerogel.Produced cellulosic aerogels were light weight (density: 0.012-0.15g/cm 3 ), highly porous (porosity: 90%-99%), and a high specific surface area (22-525 m 2 /g).
de Oliveira et al., extracted cellulose from rice husks, oats husks, and eucalyptus wastes with an average diameter ranging between 16 and 28.8 nm.These extracted celluloses were used to synthesize cellulosic aerogel.Comparative studies were done among the produced aerogels using three different cellulose nanocrystals of rice husks, oats husks, and eucalyptus wastes.The water absorption capacities of the produced aerogels were within 264%-403%.The relative crystallinity of these celluloses was measured and the eucalyptus nanocrystal cellulose had the highest relative crystallinity of 95% and rice husk nanocrystal cellulose had the lowest relative crystallinity of 60%. [64]ble 1.Properties of bio-waste generated cellulosic aerogels along with solvent and drying technique.Cotton linter Ionic solvent, N,Ndimethylformamide Supercritical drying -99% 358 [69]   Picea abies and Pinus sp.H 2 O, sodium chlorite Supercritical drying --400 [68]   Note: PEG: polyethylene glycol; PVA: polyvinyl alcohol; IL: ionic liquid; DMSO: dimethyl sulfoxide; HAc: hydrogen acetate.
Chine et al., had analyzed the loading and releasing capacity of methylene blue on cellulosic aerogel where cellulose was extracted from sugarcane bagasse.The specific surface areas of cellulosic aerogel for different concentrations of cellulose solutions (1-5% w/v) were between 22 and 525 m 2 /g.The cellulosic aerogel synthesized using 1%-5% (w/v) cellulose concentration had the highest specific surface area of 525 m 2 /g with a loading capacity of 6.4 mg/mg and releasing rate of 23 hr at a pH of 7.4. [65]tudies has been reported that aerogel prepared using two different drying techniques has different values of physical properties like pore size, and specific surface area.The cellulosic aerogel prepared using two techniques: (i) cyclic freezing at À20 C and then thawing at 20 C and (ii) cyclic freezing at À96 C with liquid nitrogen and thawing at 20 C has been compared.The specific surface area of cellulosic aerogel preparation using the second process is much higher than that of the first process. [66]Wijaya et al., extracted cellulose nanocrystals from passion fruit used for the synthesis of cellulosic aerogel, and its application as a drug carrier was evaluated.The loading of the tetracycline drug was calculated as 129.46 mg/g as a maximum at pH 3. In a phosphate buffer solution at pH 7.2, the release of tetracycline was maximum which was about 82.21% of the loaded drug. [67]ivaraman et al., used the CNF extracted from Picea abies and Pinus sp., as the raw material for producing super-insulating nanocellulose aerogel.Supercritical drying was used for cellulose hydrogel.Almost 10% shrinkage in cellulose aerogel (<1 wt% biomass) was observed after cellulose dying.Furthermore, aerogel shrinkage increases with a decrease in the weight percentage of the cellulose. [68]In a similar study, supercritical drying was used for drying the cellulose hydrogel.Ionic solvent and N,N-dimethyl formamide (DMF) were used as the solvent for cellulose dispersion.Cellulose dispersion time dropped to 3 min from 12 hr with the addition of DMF in an ionic solvent.Furthermore, the addition of DMF decreased the moisture sensitivity of the ionic solvent. [69]ellulosic aerogels synthesized from different biowastes (like pineapple wastes, waste newspaper, coir fiber, rice straw) have different adsorption capacities.3,70] Cellulosic aerogel prepared from wastepaper showed an adsorption capacity of 16 g/g while another study reported the adsorption capacity of waste paper derived cellulosic aerogel within range of 33-70 g/g [71,72] .Cellulosic aerogel prepared using wheat straw has comparative adsorptive capacities of 17.4, 17.3, 16.8 g/g in removing dyes like indigo blue, rhodamine B and methyl orange respectively. [70]nother cellulosic (waste newspaper) aerogel has an adsorption capacity of 22 g/g in adsorbing chloroform. [71]urthermore, cellulosic aerogel prepared from coir fiber has an adsorption capacity of 62 g/g for adsorbing methylene blue dye. [73]hermal conductivity of a cellulosic (rice straw) aerogel was in the range of 0.034-0.036W/m.K which has the potential to replace commercial products like porous fly ash-based foam and propylene foam. [74]Another similar product prepared using tea stem waste had a thermal conductivity of 0.030 W/m.K and was considered as a promising future as flame retardant as fire got extinguished after 263 s while 80 mm cellulosic aerogel was rekindled. [75]t could be noted from the above information that the densities of the aerogel produced using ionic liquids as solvents are almost similar to that of the aerogels produced using other conventional solvents.The porosity of the aerogels was also almost in the same range.Thus, the conventional solvents and freeze-drying method can be used for the production of sustainable and eco-friendly aerogels for commercial purposes.Also, optimized process conditions are required to establish the production of cellulosic aerogel on a commercial scale based on the raw material used.Raw materials could be used based on their abundance in a specific region or country.It also may be noted that cellulosic aerogels from plant-based cellulose are tested in various applications like absorption of oils, dyes, and chemicals, thermal and electrical insulating materials, flame retardant, and drug carrier.The market value of this aerogel is likely to expand since it is a sustainable product and has promise in different applications.

Bacterial cellulose aerogel
Cellulose produced by microbes such as Acetobacter sp. and Gluconacetobacter sp. was used as the raw material to produce aerogel.Bacterial cellulose had better physiochemical characteristics than plant cellulose, although they had similar molecular formulas.This might be due to the absence of lignin and hemicellulose in bacterial cellulose which is attributed to a more purified form, higher crystallinity, higher water-holding capacity, and higher porosity. [76]Synthesis of bacterial cellulose is costly and investigations were done of different growth media such as kitchen wastes, [29] industrial wastes, [77] and other synthetic media.Bacterial cellulose was produced as pellicles which were then washed with distilled water for further processes.Table 2 represents the scientific reports regarding bacterial cellulose using different bacterial strains.The density of aerogels, produced from different bacterial cellulose was compared when the hydrogel dried using freeze-drying or supercritical carbon dioxide drying.It was observed that the difference was negligible.Revin et al., cultivated Gluconacetobacter sucrofermentans for extracting the extracellular cellulose.This extracted cellulose was used to produce cellulosic aerogel with low thermal conductivity (0.0257 W/m/ C) and high acoustic absorbing coefficients in the frequency range of 250-5000 Hz. [38] Similarly, Gluconacetobacter hansenii was used for the production of cellulose-based aerogel with good insulating properties.This material had shown a low thermal conductivity of 0.013 W/m/ C which could be used as an insulating material for building envelop. [39]Furthermore, cellulosic aerogel from Gluconacetobacter xylinum extracted cellulose was synthesized by supercritical drying which had a very low density of 8.25 mg/cm 3 with a high specific surface area of 200 m 2 /g. [78]Bacterial nano-cellulosic aerogel was used as a distillation membrane with membrane thickness, salt rejection percentage, temperature polarization coefficient flux 218 ± 30 lm, 8.42 ± 0.21 kgm 2 /h, 99.87 ± 0.05%, 0.711 ± 0.012 respectively. [79]In another study, surface modification using trimethylchlorosilane was done on bacterial cellulosic aerogel.This surface modification resulted in a web-like structure with good adsorption capacity.This hydrophobically modified cellulosic aerogel (HBCA) has an adsorbing capacity of up to 185 g/g for adsorbing oils and organic solvents. [80]Wang et al. used hydrolysate extract from kitchen waste for cellulose production by growing Acetobacter xylinum using static fermentation.The cellulosic aerogel from this extracted cellulose has an adsorption capacity of 48.2 (w/w) toward adsorbing cooking oil.This material has good recyclability with an adsorption capacity of almost 89% of the first cycle when recycled up to 10 times. [81]acterial cellulose aerogel produced using cellulose extracted from Gluconacetobacter sp. has many applications, such as a thermally insulating material, acoustic insulating material, and a membrane.These aerogels have prospects as a thermal insulating material for protecting buildings from high atmospheric temperatures.These aerogels have their application as a sound absorber and have future potential in manufacturing soundproof rooms.They also could be used as a sophisticated water-purifying membrane.Bacterial cellulose aerogel produced using cellulose extracted from Acetobacter sp. has good adsorbing capacity along with good recyclability.Furthermore, as bacterial cellulose is a purified form of cellulose, it has prospering applications in biomedical fields, which were reviewed by Picheth et al. and Gorgieva et al. [82,83]

Hybrid aerogel
Aerogel can be organic or inorganic depending on the raw materials used for aerogel preparation.To incur the properties of two types of raw materials in a single aerogel, two raw materials can be used to prepare a single aerogel.Depending on the raw materials used, it can be organicorganic aerogel, organic-inorganic aerogel.
Carbon nanotubes with different mass ratios were added to the carbon nanofibril dispersion extracted from rice straw to prepare an organic-organic hybrid aerogel showing good adsorption efficiency for removing methylene blue and congo red from wastewater. [84]Graphene nanoplates/nanofibril/polyaniline (PANI) were mixed at a ratio of 2:2:1 to prepare organic-inorganic aerogel.Hybrid aerogel was synthesized using banana waste and paper waste.The resultant aerogel exhibited a stable hierarchical skeleton with good strain-tolerating capacity, a hydrophobic property, and a good adsorption capacity toward absorbing oils. [10]he produced hybrid aerogels had better structural and physical properties than cellulosic aerogels and the latter was more friable than the hybrid aerogels.Improved compressible strength, hydrophobicity and a network structure, are the most common characteristics of hybrid aerogels leading to a large field of applications.

Types of cellulosic aerogel based on the different particle sizes of celluloses
Cellulosic aerogel can also be classified into different categories based on the size of cellulose particles used for aerogel synthesis.These are: (i) microcrystalline cellulosic aerogel (MCC-aerogel), (ii) nanocrystalline cellulosic aerogel (NCCaerogel), (iii) cellulose microfibril aerogel (CMF-aerogel), and (iv) cellulose nanofibril-aerogel (CNF-aerogel).These aerogels are derived from various types of cellulose such as microcrystalline cellulose (MCC), nanocrystalline cellulose (NCC), cellulose microfibril (CMF), and, cellulose microfibril (CNF).][87][88] Concise information on different types of cellulose (MCC/NCC/CMF/CNF) is provided in Table 3 (readers are referred to the references in Table 3 for further information about MCC, NCC, CMF, and CNF).In this section, the focus has been made to discuss aerogels synthesized from MCC, NCC, CMF, and CNF as raw materials.

Microcrystalline cellulose aerogel (MCC-aerogel)
Microcrystalline cellulosic aerogel (MCC-aerogel) can be produced from microcrystalline cellulose (MCC) as the precursor material.MCC has a crystal size of around 10-60 mm and can be extracted from different naturally occurring cellulosic polymer (Table 3).MCCs are synthesized by partial de-polymerization of cellulose to remove the amorphous part of the cellulose polymer resulting in higher crystallinity (up to 80%). [89,90]Different mechanical and chemical processes like ultrasonication, enzyme mediated process, reactive extrusion, mechanical grinding and acid hydrolysis (HCl, HBr, H 2 SO 4 ) are employed to produce MCC (Table 3).Recently, Wang et al., extracted MCC from corncob with a pore volume 0.78 Â 10 À2 cm 3 /g and particle size was 55.46 lm.The hemicellulose and lignin part was removed using p-toluenesulfonic acid (p-TsOH) treatment. [91]It may be noted that the properties of MCC largely depend on the process of synthesis and the type of acid used. [92]This synthesized MCC has been used to produce MCC-aerogel.Zhao et al., synthesized MCC-aerogel using two different processes namely water bath heating technique and nano selfassembly method. [30]In nano self-assembly method, the authors thawed the frozen MCC solution and stirred it vigorously at room temperature.Afterwards, the mold (used for shaping) containing the solution was immersed in a methanol bath for regeneration.The resultant MCC hydrogel was then supercritically dried using carbon dioxide.The specific surface areas of the aerogel produced from the water bath heating technique and nano self-assembly method were 180.28 m 2 /g and 154.37 m 2 /g with average pore diameters at 28.58 and 25.71 nm respectively.These MCC-aerogels had shown a maximum oil removal capacity of 12 g/g.In this direction, Wei et al. produced MCC-aerogel with modification in dispersion step.They added dopamine to the MCC solution which triggered self-polymerization.Then, the resulting solution was kept on water bath for 3 min and it turned into gel.The gel was washed with distilled water and freeze dried to produce the MCC-aerogel. [31]The specific surface area of produced MCC-aerogel was higher by increasing dopamine content.This aerogel adsorbed methylene blue up to 110 mg/g after 48 hr of adsorption.In another study hydrophilic/oligophobic MCC-aerogel was prepared using a quaternarized N-halamine siloxane monomer.The materials with under water have been endowed with good oleophobic and anti-bacterial properties.The aerogels have very high separation efficiencies (over 99.9%) for separating water and oil. [93]nocrystalline cellulose aerogel (NCC-aerogel) Nanocrystalline cellulose (NCC) is a rod-shaped nanocrystal with a diameter of 10-80 nm.[94,95] The synthesis chemistry of both MCC and NCC is similar.MCC and NCC's synthesis procedures generally depend on the type of acid used for acid hydrolysis and subsequent chemical/mechanical steps as it defines the surface chemistry of the nanocrystals (Table 3).Oriana et al., reviewed the detailed effect of various acids used for nanocrystal production, structural chemistry and physical properties.[63] Shamskar et al., produced NCC by sulfuric acid hydrolysis of the cotton stalk and the cotton bleached pulp.Thereafter, NCC-aerogel was synthesized from produced NCC. NCC-aergel derived from cotton named as C-NCC-aerogel and CS-NCC-aerogel from cotton stalk.The length, width and surface area of C-NCC-aerogel and CS-NCC-aerogel were about 450 nm, 25 nm, 91 m 2 /g and 100-850 nm, 25 nm, 94 m 2 /g, respectively.The authors concluded that there was minimal difference in their properties (C-NCC and CS-NCC).[32] Zheng et al., prepared NCC from cotton linter using sulfuric acid hydrolysis and this NCC was used to a produce cellulose/NCC-aerogel with reinforced material properties.This synthesized cellulose/NCC-aerogel showed high deformability, ranging between 80% to 90% strain without having any break when Abbreviations: MCC: microcrystalline cellulosic; NCC: nanocrystalline cellulosic; CMF: cellulose microfibril; CNF: cellulose nanofibril.

PREPARATIVE BIOCHEMISTRY & BIOTECHNOLOGY
folded to 180 .This aerogel has an adsorption capacity reaching up to 17 g/g in adsorbing pump oil and could be recycled up to 10 times.The addition of NCC in cellulosic aerogel increased the compressive modulus of aerogel to almost 489.1 kPa, which was two to four times compared to that of pure cellulosic aerogel. [96]In another study, ammonium persulphate (APS) was used as a strong oxidant for removing the amorphous region of cellulose during the synthesis of NCC.APS were cost-effective strong oxidants, resulting in carboxylated NCC from the biomass without pretreatment by removing the non-cellulosic components. [97]ohd et al., extracted NCC from an oil palm empty fruit bunch and, this extracted NCC was used to produce NCCaerogel and modified (amino silane) NCC-aerogel.Structural and internal modification was noted with increased roughness, pore volume, average pore diameter and surface area.The modified NCC-aerogel's carbon-dioxide absorption capacity has reached 8.8 mg/g.The authors concluded that this aerogel was a promising adsorbent in the CO 2 capture field. [33]llulose microfibril aerogel (CMF-aerogel) Cellulose microfibrils (CMF) contains both crystalline and amorphous part of cellulose.These are long and thin interconnected fibrils with three-dimensional structures with a uniform internal dimensional distribution.Aerogel synthesized from CMF is termed as CMF-aerogel.A number of research reports related to CMF-aerogel is very limited as the research focus has been shifted toward the CNF-aerogel since it has better promise than CMF-aerogel.Rostamitabar et al. produced cellulose-chitosan aerogel microfiber, which had high porosity (85%), low density (0.18 g/cm 3 ) and high surface area (300 m 2 /g).It was nontoxic and had shown anti-bacterial properties toward E. coli and S. aureus and could be a promising candidate in the wound dressing industry. [34]llulose nanofibril aerogel (CNF-aerogel) Cellulose nanofibril aerogel (CNF-aerogel) are produced from cellulose nanofibrils as the raw material.CNFs are produced by pelleting cellulose microfibril (CMF) using physical treatments like ultrasonication and also produced from MCC using a high-pressure homogenizer (20,000 psi). [98,99]NF (1-4 mm in length) are long and thin fibers interconnected to each other by hydrogen bonds resulting in a three-dimensional agglomerate structure. [100]To avoid the agglomeration, different types of pretreatments were carried out, (i) acid hydrolysis of cellulose to prepare smaller cellulose particles; [98] (ii) chemical modification of cellulose by replacing the hydroxyl groups using coupling agents like maleic acid; (iii) chemical modification of cellulose fibers using TEMPO mediated oxidation. [35,101]opakumar et al., extracted CNF from eucalyptus pulp and this CNF was modified using silane to produce modified CNF-aerogel.This modified aerogel showed good adsorption capacity toward adsorbing crystal violet at a rate of 0.150 g/g. [102]Bhandari et al., investigated synthesized CNF-aerogel in biomedical application where the produced aerogel was used for delivering water soluble drugs in a drug delivery system.Investigations revealed that the physical and mechanical properties of CNF-aerogel (e.g., mucoadhesive capacity, swelling index, floating time) were changed after loading the drug.Therefore, the hygroscopic and water swelling capacity of CNF-aerogel has made it a prospering material for drug delivery. [103]In another study, authors extracted CNF from bleached eucalyptus pulp using a grinder, in which the blades were moving at 1500 rpm for up to 40 cycles.To improve the physical properties of the CNF-aerogel, maleic acid and sodium hypophosphite has been added to the CNF suspension to produce cross-linked CNF-aerogel.The cross-linked CNF-aerogel was hydrophobic and had water absorbency up to 70 g/g. [104]Zhuang et al., extracted CNF from bamboo powder by grinding the pretreated cellulose suspension in an ultra-fine grain motor grater.Then the solution was homogenized to prepare 37.1% (w/v) nanofiber slurry.This CNF slurry was used to produce modified CNF-aerogel using EPTMAC (2, 3epoxypropyl trimethyl ammonium chloride) as a modifier and CNF/PVA as a cross-linking agent.This modified aerogel has shown good adsorption capacity of 146 mg/g for adsorbing small microplastics. [105]In a similar study, CNFaerogel spheres prepared was in the range of 1-3 mm using silane modified technique.This modified CNF-aerogel had shown maximum absorption capacities of 1080 mg/g, 177 mg/g, and 342 mg/g toward removing different pollutants like Cu 2þ , phenol and aniline, respectively. [106]mparative analysis of MCC-aerogel, NCC-aerogel, CMF-aerogel and CNF-aerogel The shape and structure of these types of aerogels (MCCaerogel, NCC-aerogel, CMF-aerogel and CNF-aerogel) depend on (i) the source of the cellulose, from which the cellulose has been extracted (MCC, NCC, CMF, CNF) (ii) the process of extraction of MCC/NCC/CMF/CNF from the raw cellulose (iii) the solvent used for dispersion, and (iv) the drying method for conversion of hydrogel to aerogel.Structural characteristics of MCC-aerogel varies between supercritical carbon dioxide (ScCO 2 ) drying and freeze-drying.The aerogel produced using ScCO 2 drying are like sponges with small pore sizes, physically more compact and are less negatively affected by surface tension.Whereas, the aerogel produced using freeze-drying method forms sheet like structures in the internal region due to the penetration of ice crystals through the inner pore of the structure. [30]CC-aerogel and CNF-aerogel has improved mechanical strength, and specific area.The structural difference between them is on a microscopic level.In NCC-aerogel the cellulose particles are shorter in length while in CNF-aerogel, the cellulose fibers are of greater length.Due to the increased flexibility of CNFs and their propensity for entanglement, CNFs lend themselves more readily to hydrogel formation than NCCs. [107]CC-aerogel has a non-intersecting cylindrical mesoporous structure. [32,33]CNF-aerogel is both mesoporous and microporous with pore size up to several hundred micrometers forming a three-dimensional interconnected structure. [108]Smaller configuration (nanoscale) of NCC-aerogel has enhanced their colloidal property and form more stable colloidal solution when dispersed in the solvent.They also have high specific surface area, specific strength, modulus and unique optical properties. [109]Therefore, researchers have been more driven toward NCC-aerogel and CNFaerogel.

Nano-particles doped cellulosic aerogel
49] Nano-particles doped cellulosic aerogel is a type of organicinorganic hybrid aerogel where nano-particles are the inorganic part.Nano-particles like silver, gold, phosphorus, nitrogen, copper and nickel are doped in cellulosic aerogels or composite cellulosic aerogels.
Table 4 demonstrates a few examples of nano-particle doped cellulosic aerogels along with their process of synthesis and application.Li et al. had shown that the modification of carbon nanofiber aerogel by doping with phosphorus had increased its charge transfer, specific capacitance and electro-sorption capacity due to increased surface area.Therefore, this aerogel can be used as a promising material for applications like capacitive deionization.The electro-sorption capacity of phosphorus doped aerogel and undoped aerogel was recorded as 16.20 mg/g and 12.81 mg/g, respectively in 1000 mg/L NaCl solution, signifying a great difference. [116]One study showed that by modifying the cellulosic aerogel with nitrogen nano-particles has significantly increased its charge transfer and specific capacitance of 17.29 mg/g -in 1000 mg/L NaCl solution whereas undoped aerogel had specific capacitance of 12.81 mg/g in 1000 mg/L NaCl solution.Authors reported that nitrogen doped aerogels find applications in the captive deionization fields. [117]ano doped cellulosic aerogel has found its utilities as a catalyst.Investigations has reported that MnO 2 /N doped cellulosic aerogel demonstrated as a very promising candidate as a catalyst in reducing bio-derivatives into aldehydes resulting in almost 90-100% conversion along with high dispersibility in the reactants and easy separability from the reaction media, whereas the undoped cellulosic aerogel has negligible catalytic activity. [56]In another study, Cu 2 O nanoparticle doped in cellulosic aerogel was utilized as photocatalyst where xenon lamp was used as the light source.This study showed the increased degradation rate of methylene blue to 95.79% compared to pure Cu 2 O having a degradation rate of 73.59%. [118]Furthermore, comparative analysis of catalytic activity of copper doped cellulosic aerogel and nickel doped cellulosic aerogel were reported a much better performance of copper cellulosic aerogel than that of nickel cellulosic aerogel.Cu-cellulosic aerogel reduced 4-nitrophenol to 4-aminophenol in 8 min reaction cycle and was reused five times with a reaction time of 60 min.On the other hand, Ni-cellulosic aerogel having large particle size and tends to agglomerate, requiring 160 min to complete the reaction cycle.The authors concluded that the copper doped cellulosic aerogel can be used as an effective catalyst. [47]ble 4. Nanoparticle doped in cellulosic aerogel with their process of synthesis and applications.

Nanoparticle
Process for synthesis Application References

Phosphorus doped in bacterial cellulose
Dipping bacterial cellulose hydrogel in phosphoric acid solution and then carbonized Promising material in capacitive deionization and has high specific capacitance [116]   Nitrogen doped in bacterial cellulosic aerogel Bacterial cellulose hydrogel thermally treated in ammonia atmosphere at higher temperature Increased captive deionization by the produced aerogel [117]   MnO 2 -N doped in cellulosic aerogel Manganese acetate added to cellulose-NaOH solution and the solution then heated to allow gelation and the aerogel obtained after drying followed by carbonization in nitrogen atmosphere Efficient catalyst for producing aldehydes from bio-derivatives [56]   Cu 2 O nano-particle doped in cellulose-based aerogel In-situ deposition by immersing cellulose-based aerogel (CBA) in CuSO 4 solution and then adding hydrazine hydrate to form Cu 2 Ofunctionalized CBA Used as visible light photocatalyst in degrading pollutants and water remediation [118]   Cu(ll) and Ni(ll) doped in bacterial cellulosic aerogel singularly.
Facile immersion of cellulose hydrogel in CuSO 4 /Ni(CH 3 COO) 2 solution followed by adding NaBH 4 solution then freeze drying the prepared hydrogel Used as a catalyst for reducing 4 nitrophenol to 4-aminophenol [47]   Fe 3 SO 4 doped in bacterial cellulosic aerogel Bacterial cellulose immersed in ferric chloride and ferrous chloride solution then freeze dried to form magnetic bacterial cellulose and the product used to prepare MBC aerogel Promising adsorbent in removing various organic solvents and oils [49]   Gold (Au) doped in PEI (poly ethylene imine)/CNF aerogel PEI/CNF aerogel immersed in AuSO 4 solution followed by its immersion in NaBH 4 Efficient catalyst for decolorizing cationic and anionic dyes [120]   Silver (Ag) doped in polyaniline/cellulosic aerogel Electrodeposition on polyaniline/cellulosic aerogel operated at 3.6 V Increased electroconductivity and is used as electrode [119]   Magnetite nano-particles doped in Graphene oxide/Cellulose fiber aerogel CNF/GO solution is prepared followed by dispersion of magnetite nanoparticle.The hydrogel is prepared after transferring the solution in glass container and adding ascorbic acid and heating it.
Effective Removal efficiency for removing gold from gold cyanide solution.[122]  Studies revealed that changing the amount of same nanoparticles doped in a cellulosic aerogels, changes different physical properties of the nano doped aerogel.For example, by increasing (by weight percentage from 0.46 wt% to 4.54 wt%) amount of silver nano-particles electrodeposited on the surface of the cellulose/polyaniline aerogel enhanced the conductivity to 4.59 Â 10 À2 S C/m from 3.45 Â 10 À2 S C/m.Also, conductivity and specific capacitance of 4.54 wt% silver doped cellulose/polyaniline aerogel increased to 0.94 S C/m and 217 F/g respectively. [119]n similar study, Chen et al. produced manganese oxide/nitrogen doped CNF aerogel using TEMPO-oxidized CNF.An asymmetric supercapacitor was fabricated together in which the activated carbon was the negative electrode and doped CNF-aerogel was used as the positive electrode.Maximum energy density and power density of 23.3 W h/kg and 600 W/kg, respectively at 0.5 A/g was delivered by the positive electrode.It retained 99.2 t of the initial capacitance after 3000 cycles a t 0.5 A/g. [101]n another study, the adsorption capacity of Fe core cellulosic aerogel with different Fe 3 O 4 concentrations was investigated and the resultant adsorption efficiency was in the range between 37 À 87 g/g for a wide variety of organic solvents and oils.The Fe core cellulose is a lightweight, durable for almost 100 stress strain cycles without any deformation in shape and has magnetic properties which varies with Fe 3 O 4 concentration and allows it to be separated easily from the waste water without any direct contact using magnets and can be recycled and reused. [49]Zhang et al., investigated the efficiency of Au doped poly ethylene imine/cellulose nanofiber (PEI/CNF) aerogel for removing different organic dyes like methylene blue and 4nitrophenol. [120]Arash et al. prepared nano-particle doped graphene/cellulosic aerogel using magnetite nano-particles.Magnetite nano-particles were prepared by dissolving FeCl 2 Á4H 2 O and FeCl 3 Á6H 2 O in HCl solution and water solution, respectively.Afterwards, the prepared solution was added to the aqueous solution of deoxidized ammonia, under continuous stirring.The precipitate was washed with deionized water, centrifuged and dried overnight at 50 C. [121] GCM (Graphene aerogel/cellulose fibers/magnetite nano-particles) has good adsorption capacity toward adsorbing gold (Au).The Au adsorption capacity of GCM increases (100 to 130 mg/g) with an increase in the concentration of GCM composite from 1 to 3 g/L. [122]pplications of nano-doped cellulosic aerogel ais vast and technology should be developed as per need.For example, nano-doped cellulose aerogel has the potential to be used on a large scale in deionization of water in a water purifying system for purified and ion free drinking water.Furthermore, nano-doped cellulose has the potential as an adsorbent in removing dyes, oils, heavy metals from waste water and shows magnetic properties when doped with iron.Moreover, it has potential in different industries like textile, leather, paint, and mining industries.Nano-doped cellulosic aerogel has been used as an electrode and supercapacitor.It has a good scope in developing electrical batteries using these.It may be noted that nano-particles doped cellulosic aerogels have the promise as advanced catalysts for executing time consuming chemical reactions in a small interval of time with high conversion.

Applications of cellulosic aerogel
Cellulosic aerogel has broad applications in various fields (e.g., as adsorption of oil and harmful heavy metals from waste water, as a drug delivery system, as a sensor, as a thermal and acoustic insulating material, electronic material, catalysis).Figure 2 depicts the schematic of wide ranges of applications in different broad sectors.It may be kindly noted that specified applications of different types of cellulosic aerogels investigated by researchers are already discussed in respective sections ("Classification of cellulosic aerogel based on raw materials" section and "Types of cellulosic aerogel based on the different particle sizes of celluloses" section).Furthermore, applications of nano-particles doped cellulose aerogel are already tabulated in Table 4 and are illustrated in "Nano-particles doped cellulosic aerogel" section.In addition, more applications of cellulosic aerogel in different sectors with its drying method are tabulated in Table 5.
Moreover, highly porous air-filled structures of cellulosic aerogel results in its low thermal conductivity.For example, cellulosic aerogel prepared from sugarcane bagasse had a thermal conductivity ranging between 0.031 and 0.042 W/mK.Also, thermos-gravimetric analysis at a temperature ranging from 25 C to 800 C showed a weight loss of 18% and 25% for the temperature ranges 25 C to 150 C and 150 C to 250 C, respectively, attributing its use as an insulating material. [13]Similarly, Gupta et al., analyzed the activity of 1 wt% NFC aerogel as a thermal insulator and its value of thermal conductivity was 0.0255 W/m K. Furthermore, Shi et al. prepared a cellulosic aerogel with a thermal conductivity of 0.029 W/m K, which was almost equal to air's thermal conductivity at normal atmospheric conditions. [123,124]Therefore, the above prepared cellulosic aerogels were suitable for their usage as an insulating material in low to medium insulation applications.
One of the most widely reported studies on cellulosic aerogel is their utilities and efficiencies as an adsorbent.In this direction, Thai et al., studied the adsorption efficiency of cellulosic aerogel for removing oils from wastewater.The cellulose was extracted from the sugarcane bagasse, and it could absorb 25 g of oil per gram of cellulosic aerogel. [13]nother study showed the sorption capacity of cellulosic aerogel made from waste newspaper to eliminate waste engine oil, vegetable oil and other liquids like methanol, ethyl acetate, ethanol, chloroform.This aerogel had good sorption capacity for all the liquids and chloroform had the highest sorption capacity of 22 g/g cellulosic aerogel attributing the promising material for the removal of various oils and organic liquids from the waste water.This aerogel could be recycled and reused 10 times. [71]number of reports are available heightened the adsorption capacity of cellulose-based aerogel in removing various heavy metals from wastewater.In this direction, Song et al. investigated the removal efficiency of carbon dots/NFC aerogel toward removing Cr(III) from wastewater where NFC was prepared from poplar wood fiber.Here, Cr(III) removal efficiency was observed around 95% after reaching equilibrium in 120 min at pH of 6 with 100 ppm initial metal concentration.Results demonstrated that carbon dots/NFC aerogel was a promising adsorbent in wastewater treatment. [125]In another study, the adsorption capacity of nanocellulose doped with Fe 3 O 4 nano-particles with magnetic properties was investigated for removing various heavy metals like lead, chromium and copper.The adsorption capacity of aerogel produced with a 1:1 mass ratio of cellulose and ferroferric oxide , was highest for Cr(VI) removal ($2.2 mg/g of adsorbent) while the adsorption capacity of Pb(II) and Cu(II) was 1.25 mg/g and 0.4 mg/g of adsorbent respectively.
Recently, cellulosic aerogel has found an increased potential usage in biomedical applications.In an investigation, Zhao et al., has shown the applicability of cellulose-based aerogel as a drug delivery system where cellulose prepared from banana pulp grafted with PEI.In this synthesized aerogel, sodium salicyalate drug was loaded using static adsorption and the loading efficiency was analyzed for varying initial drug concentration ranging between 5 to 3000 mg/L for 24 hr with maximum loading value of 287.39 mg/g.The drug release reached equilibrium in 10 hr resulting in effective and prolonged drug release. [126]In another study, CNF aerogel was investigated as the oral drug delivery system which was used for delivering the drug bendamustine hydrochloride with 19% loading of CNF aerogel.Here, it has been observed that 69% of the drug loaded in aerogel was released in 24 hr at 1.2 pH medium. [103]Rostamitabar et al., prepared MCC-chitosan aerogel which is a prospering anti-bacterial material and can be used for wound dressing and as a drug delivery system.The Drug loading (ibuprofen powder) capacity of the MCC-Chitosan aerogel increases with increasing aerogel fiber. [34,127]Wang et al., observed that cellulose triacetate (CTA) aerogel enabled uniform distribution of paracetamol particles which has high potential Table 5. Applications of cellulosic aerogel produced from natural wastes along with its drying method.

Precursor material for aerogel preparation Drying Application References
Cellulose from Sugarcane baggasse Freeze drying Adsorbent for oil and insulating material [13]   Waste newspaper Freeze drying Absorption of oil [71]   NFC and carbon dots from poplar wood powder Freeze drying Adsorption of heavy metal Chromium (III) from waste water [125]   Nanocellulose from Amorphafruticosa and Fe 3 O 4 Freeze drying Efficient adsorption efficiency in removing Pb (II), Cr (Vl) and Cu (II) from wastewater [137]   Cellulose nanofibril from banana pulp grafted with polyethyleneimine Freeze drying Promising drug delivery system for loading and releasing the drug NaSa (sodium salicylate) [126]   Cellulose nanofibril Freeze drying Oral gastroretentive drug delivery system [103]   Bacterial cellulose/multiwall Carbon nanotubules Supercritical drying Stain sensor with electrical conductivity [130]   Carbon nanotubules and carbon nanofibers Freeze drying Promising pressure and vapor sensor at room temperature [131]   Cellulose from pineapple waste Freeze Drying Insulating material of heat and sound [50]   Nano fibriliated cellulose from pinewood Freeze drying Thermal insulating material [123]   Cellulose from cotton linter Freeze drying Insulating material at low temperature [124]  in pharmaceutical applications. [128]Edwards et al., used peptide nanocellulose aerogels as a biosensor to detect the protease enzyme activity.The physical properties by mass spectral analysis of the aerogel signified that it was suitable for the application as a biosensor layer for an intelligent protease sequestrant wound dressing. [129]Hosseini et al., investigated the activity of BC/MWCNT aerogel, which can be used as a potential candidate as a strain sensor and a value of gauge factor of 21 and a response time of 390 min was obtained. [130]In another study, vapor and pressure sensing activity of CNF/CNT aerogel was analyzed.Chemoresistive response of CNF/CNT aerogel for various volatile organic compounds was analyzed, and its value was 0.10 for chloroform, which was the highest and by increasing the compressive strain and stress.However, the relative electrical resistance of the CNF/CNTs aerogel decreased gradually.The lowest relative electrical resistance was observed when the strain reached 70% and stress reached to around 177 kPa. [131]he recent trend is increasing on the nano-doped cellulosic materials for the developments in advanced catalysts, electronic materials and biomedical application like drug delivery, bio-sensors and these are already discussed in "Nano-particles doped cellulosic aerogel" section.

Conclusion and future perspectives
Aerogel is a solid material with numerous air pockets making it a very light material with a large surface area.Aerogels were first prepared using inorganic raw materials like silica and alumina.To make it biodegradable, biocompatible and cost-effective, cellulose has been used as the raw material for aerogel synthesis.Plant-based cellulose is a natural polymer present in plants and agricultural wastes.On the other hand, bacterial cellulose can be produced by certain bacterial strains such as Gluconacetobacter species and Acetobacter species.Production of bacterial cellulose is more costly and time consuming than that of cellulose extraction from plant sources.Cellulose can further be divided into four types based on their size and structure (e.g., MCC, NCC, CMF, and CNF).Different types of cellulose are used as raw materials for the synthesis of cellulosic aerogel.Various physical and chemical methods can be used to synthesize aerogels from MCC, NCC, CMF, and CNF.Applications of cellulosic aerogel are diversified in different fields by doping with different nano-particles like silver, gold, iron copper, nickel.Nano-particles doped cellulose aerogels have improved mechanical strength, higher specific surface area, and more adsorption capacity with a significant change in magnetic properties.Cellulosic aerogels find applications in different fields like thermal insulating material, sound insulating material, electrode, sensors, catalyst, adsorbing material and in drug delivery system.
Applications of cellulosic aerogels are ranging from small-scale biomedical products to larger-scale adsorbent products.However, challenges are still persistent before the commercialization of cellulose-based aerogels.For example, standardization of processing methods like synthesis, purification and establishment of eco-friendly methods and industrially feasible surface area are required.In this direction, life cycle analysis and techno-economic feasibility analysis are demanding to make the processes economic and environmentally friendly.Also, it is not well understood how the aerogel properties are affected by different sources of cellulose and their extraction, synthesis techniques.Moreover, for improving scalability, producing large units of aerogel and reducing the production costs of cellulose aerogel, other drying techniques like spray drying, spay freeze drying need to be explored.Overall, cellulosic aerogels have a wide area of applications and many more to explore along with commercializing these products for industrial, municipal, and pharmaceutical uses.More importantly, cellulosic aerogel is a sustainable and biodegradable product showing its high market demand in future.We expect that ongoing scientific research on cellulosic aerogels will further diversify the range of achievable applications and performance of cellulosic aerogels in both exquisite specialty materials and plentiful commodity materials.

Figure 1 .
Figure 1.Schematic diagram for synthesis of cellulose-aerogel/MCC-aerogel/NCC-aerogel/CMF-aerogel/CNF-aerogel from different types of celluloses.Cellulose is extracted from either plant sources via hydrolysis/bleaching or bacterial sources via fermentation.Modified cellulose (e.g., microcrystalline cellulose-MCC, nanocrystalline cellulose-NCC, CMF-cellulose microfiber and CNF-cellulose nanofiber) is synthesized from extracted cellulose by strong acid hydrolysis and ultrasonication.Cellulosic aerogel can be produced from both extracted cellulose and modified cellulose Key steps involved for the production of cellulosic aerogels from cellulose are dispersion, gelation and freezing/supercritical drying (nano doping is discussed in supplementary material).

Figure 2 .
Figure 2. Applications of cellulosic aerogels in diverse fields.

Table 3 .
Types of extracted cellulose based on cellulose particle size.