Biomass and industrial wastes as resource materials for Aerogel

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Biomass and industrial wastes as resource materials for Aerogel preparation: opportunities, challenges and research directions Nilofar Asim, Marzieh Badiei, Mohammad A. Alghoul, Masita Mohammad, Ahmad Fudholi, Md Akhtaruzzaman, Nowshad Amin, and Kamaruzzaman Sopian Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02661 • Publication Date (Web): 23 Aug 2019 Downloaded from pubs.acs.org on August 24, 2019

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Industrial & Engineering Chemistry Research

Biomass and industrial wastes as resource materials for Aerogel preparation: opportunities, challenges and research directions Nilofar Asim1*, Marzieh Badiei2, Mohammad A. Alghoul3, Masita Mohammad1, Ahmad Fudholi1, Md Akhtaruzzaman1, Nowshad Amin4, Kamaruzzaman Sopian1 1

Solar Energy Research Institute, Universiti Kebangsaan Malaysia, Malaysia 2 Independent Researcher: Razavi 16, 91777-35843 Mashhad, Iran 3 Center of Research Excellence in Renewable Energy Research Institute, King Fahd University of Petroleum & Minerals, Saudi Arabia 4 Institute of Sustainable Energy, Universiti Tenaga Nasional, Malaysia * Corresponding author: [email protected]; [email protected].

1 ACS Paragon Plus Environment

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Abstract: Aerogels with extraordinary properties have attracted much attention due to their suitability for different technologies. However, the drawbacks of aerogels, such as expensive raw material and costly preparation methods, have motivated researchers to seek alternative materials. Meanwhile, rapid economic development has driven the generation of massive agricultural, biomass, and industrial wastes. Despite the huge potential of recycling wastes for preparation of various aerogels, related investigations are limited. This study reviews the variety of waste biomasses used for preparation of different types of aerogels and their applications, as well as the technologies used in their preparation. This study will draw the attention of researchers due to the potential of biomass as sustainable resource for the production of aerogels. Future research challenges and directions for the commercial application of aerogels are presented.

Keywords: biomass; industrial wastes; aerogel; sustainable preparation

1. Introduction: The challenge of conventional aerogel preparation has been overcome by extracting pure precursor, mainly polysaccharide-based precursors, and recycling silica precursor from different types of wastes. Bio-based aerogels, a class of porous biodegradable materials, are renowned because of their low density and large specific surface area. The first results of aerogel synthesis were reported by S. S. Kistler 1 and followed by some trials in 1988

2, 3, 4,

which led to the preparation of porous cellulose compounds from cellulose xanthate 5. The utilization of natural renewable polysaccharides in the preparation of bio-based aerogel materials from multiple sources, such as alginate, cellulose, chitosan (CS), lignin, pectin, proteins, and starch, represents an interesting innovation in this field


6, 7, 8, 9, 10.

The search

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results from Scopus search engine showed the increasing trend for using different waste and biomass as resource materials for aerogel preparation (Figure 1.)

Figure 1. The trend for aerogels utilizing various biomass and waste as resource materials. In fact, natural biopolymers are remarkable precursors for the preparation of bio-based aerogels. The use of these biopolymers potentially adds value to low-cost waste products. Notably, using agricultural and industrial wastes decreases wastes and reduces air and water pollutions by high percentage. The majority of biopolymers are obtained from agricultural wastes like lignocellulosic biomass, including forestry wastes, straws, and stovers of field crops

11,

such as corn and soybean; however, marine biomass

12

and solid urban residues

13

have similar attracted interest. Bio-based aerogels have typical features, such as sustainability, biodegradability, strong hydrophilicity, and large surface area. As-prepared aerogels are potential candidates for thermal insulation adsorbents

16.

14,

drug delivery systems

15,

and

Moreover, environment-friendly production of silica aerogel from wastes,

specifically agricultural residues, like rice and wheat husk 17, has been widely reported. Conversion of biomass wastes as sustainable natural resource into biodegradable aerogels is a significant sustainable process. Biodegradability factor is a substantial characteristic of the aerogel that makes it an eco-friendlier material compared with conventional aerogels. Utilizing biomass and wastes, reduces their negative environmental impacts


and provides


sustainability of nature. It also results in resource efficiency as well as lowering waste disposal cost. A series of excellent reviews have summarized the fabrication process, structures, and applications of aerogels, but considerably less attention has been placed specifically on those from wastes. In this review, we provide basic data on various aerogels with emphasis on the most abundant inedible biopolymers, mainly cellulose, lignin, chitin, and chitosan, as well as silica inorganic aerogels obtained from waste resources. The type of waste, type of extracted monomer, monomer structure, modifications and processing method such as drying conditions definitely influence on the physiochemical properties and structure of each individual aerogels. Here, a detailed study of the aerogels prepared from wastes (agricultural, industrial, municipal) based on more than 200 articles and regarding to their preparation, applications, and the technologies used in the preparations of different types of aerogels is surveyed in this article. 2. Aerogel preparation methods: The sol–gel method, as a chemical synthesis route, is a common approach for the preparation of aerogels. Freeze drying is commonly performed to prepare carbon-based aerogels, while ambient pressure drying is mostly conducted to synthesize silica aerogel from waste materials. The preparation of hydrophobic carbon-based aerogel is mostly realized by introducing a hydrophobic function on the surface of the aerogel via chemical modification 18 or pyrolysis

19, 20.

Drying of alcogels is regarded as one of most important step in the

preparation of aerogels 21. Given that freeze drying is not preferred for industrial use, research on the preparation of aerogel by air- and oven-drying methods have been ongoing 22. Loading nanomaterials on aerogel for various applications has been done via different methods, such as atomic layer deposition, hydrothermal, in situ synthesis, and ion exchange 23, 24, 25. Doshi 19


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reviewed bio-based aerogel preparation methods for oil removal application. Cellulose, as one of the most important and abundant sustainable materials in nature, is extensively used to prepare nanocellulose due to its promising properties and potential use in many applications. Groups of nanocellulose, nanofibril cellulose, and nanocrystal nanocelluloses have been used in various applications as renewable and sustainable materials. The huge potential of using agricultural and industrial wastes for preparing nanocellulose render the latter more interesting because its composition is directly dependent on its sources (agriculture biomass, marine animals, algae, fungi, and industrial waste)

26, 27, 28.

Figure 2 shows the different

techniques that could be utilized to prepare cellulose nanofibers, nanocellulose and aerogel from agricultural biomass. Chemical pretreatment in this case include acid hydrolysis, alkaline hydrolysis, oxidation, and organic solvents/ionic liquids. In enzymatic preparation, the enzyme catalyzes the hydrolysis of cellulose fibers

28.

Rajinipriya et al.

28

reviewed the

importance of waste materials in the preparation of nanocellulose. Considering of increasing demand for nanocellulose and environmental concern, they proposed more research concern on sustainable raw materials such as agricultural and industrial wastes as resource materials. Regarding to lignin, Figure 3 shows precipitation of lignin from black liquor obtained from pulp and paper industry through the Sequential Liquid-Lignin Recovery and Purification (SLRP) process. Black liquor that is composed mainly of 10–50% lignin by weight

44.

Furthermore, Figure 4 depicts the schematic of marine waste processing for preparation of proteins, pigments, chitin, CS, and their oligomers. Bernd et al.

29

used pyrolysis to

synthesize carbon nanostructures, such as carbon fiber, nanotubes, and graphite, from wood sawdust. Ravi et al.

30

extensively reviewed the preparation methods of different carbon

nanomaterials using biomass and plastic wastes as potential resource materials. The possibility of syntheses of other carbon allotropes, such as graphitic structure and nanotubes, using cellulose from waste biomass must be considered for the preparation of



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other carbon-based aerogels. Sevilla et al. 31 synthesized graphitic carbon nanostructures with cellulose as its source material. They used hydrothermal treatment, impregnation, and thermal treatment for this preparation. However, Zhang et al. 32 used a one-step simple method using KCl and various ferric salts for preparation of porous graphitic carbon from willow catkins biomass. In this method, graphitization and activation were performed simultaneously. Recently, Yang et al.

33

prepared a graphitic mesopore carbon by catalytic carbonization of

sucrose. Agricultural wastes are not only excellent sources of cellulose but also contain silica and other metal oxides as main constituents. Hence, these wastes can possibly be used for silica and silica aerogel preparations

34, 35.

Vaibhav et al.

34

used alkali/acid treatment to

prepare pure nanosilica from rice husk, bamboo leaves, sugarcane bagasse, and groundnut shell. Thermal treatment and hydrothermal activation were used to prepare nanosilica from rice husk

36.

Adebisi et al.

37

reviewed the preparation methods for the synthesis of high-

purity silicon nanoparticle using biomass waste materials. Many other industrial wastes consist of a high percentage of silica, alumina, and other metal oxides in their composition, and they can be used to prepare silica and silica aerogel. Waste silicon sludge, fly ash, coal gangue, red mud, copper slug, and mine wastes are some potential industrial wastes that can be used to prepare silica, alumina, and their aerogels

13, 38, 39, 40.

Figure 5 shows some

preparation methods for extracting silica and alumina from silicon sludge and fly ash.


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Figure 2. Different methods of nanocellulose and aerogel production from agricultural biomass (dp is the particle diameter). Adapted in part with permission from 41, 42, 43. Copyright 2015 IntechOpen, 2014 Elsevier, 2017 The Royal Society of Chemistry. 7 ACS Paragon Plus Environment


Figure 3. Sequential liquid-lignin recovery and purification (SLPR) process. Reproduced with permission from 44 . Copyright 2018 AIMS Press.

Figure 4. Schematic diagram for preparation of chitin and chitosan from marine wastes. Reprinted with permission from 45. Copyright 2014 Springer.


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Figure 5. extracting SiO2 and alumina from (a) sludge, (b) rice husk and (c) fly ash. Adapted in part with permission from 38, 46, 47. Copyright 2015 Elsevier, 2014 Elsevier, 2013 Longdom. Considering almost same sol-gel and freeze-drying methods for preparation of various aerogels, the emphasis will focus on finding more simple and economic methods for extracting the base componenst from different agricultural /industrial wastes for aerogel’s preparation. Then, finding more green and economic pretreatment methods such as liquid hot water

48, 49

to enhance the extraction of main component from waste biomass is very

important. The potential of combining different approaches such as microwave

50

and

ultrasonic to commonly used pretreatments methods to enhance their performance must consider as well

51, 52.

However, the optimum pretreatment method solely depends on the

nature of waste materials and final required components. 3. Aerogels from Cellulose:

The first report on cellulose aerogels was published in 2001 by Tan, et al.

53.

Cellulose

aerogels are novel green biodegradable nanoporous materials that have attracted extensive 9 ACS Paragon Plus Environment


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interest among scientists. Cellulose, as the precursor for the preparation of aerogels, is the most abundant, sustainable, and environment-friendly biopolymer in nature that is available in many biomass wastes and could be regarded as one of the best precursors for the preparation of carbon-based aerogel due to its low cost, sustainability, and environmentfriendly properties 54, 55. Figure 6 compares the chemical structure of cellulose with chitin and chitosan that differ by the group at position C2. These are the main polysaccharides used for the preparation of aerogels. The prepared cellulose aerogels have unique features and excellent properties, such as low density, high porosity, and high specific surface area with cross-linked 3D network 56, 57.

These features makes these aerogels suitable in the preparation of absorbents

10, 14,

membranes 58, filters 59, templates 60, 61, energy storage devices, biomedicine 62, and insulators 63.

Generally, the cellulous gels are formed through dissolution of cellulose fibers in an

aqueous or organic solvent, followed by its regeneration

64.

Several different crystalline

structures of cellulose are known resulting from the biomass source of cellulose, extraction methods and post-treatment processes. Nanocellulose has a higher degree of crystallinity in comparison with other types of cellulose. According to Takeshi and Yoda 65, nanocellulose can be obtained in two forms of cellulose nanocrystals (CNC) and cellulose nanofibrils (CNF) based on separation method. Nanocellulose, with nanoscale dimensions and unique morphology, has distinctive characteristics, mainly high surface area 66, ultra-lightweight 67, and outstanding mechanical strength

68.

The different morphologies, mechanical and surface properties as well as

hydrophilicity of aerogel compounds are due to the presence of nanocellulose and its derivatives derived from biomass resources

69, 70, 71

which directly relate to the pretreatment

and extraction methods 51, 52, 72 .


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However, the economic barrier of the current nanocellulose extraction methods is the cost of chemical reagents and energy required during the separation processes 30. This issue could be considered determining factor that hinders the development of cellulose aerogels. Hence, overcoming all these technical drawbacks are further emphasized to allow Lignocellulosic biomass to be used as sustainable feedstock for various applications. The performance characteristics of cellulose aerogels are greatly influenced by the biomass source of cellulose 73, 74.

The reason is that physical and chemical properties of cellulose fibers like degree of

polymerization (DP) and degree of crystallinity and extraction process conditions

77.

75, 76

are determined by the types of biomass

Additionally, the drying technology that is used to

produce a particular type of cellulose aerogel greatly influences its morphology i.e. specific surface area and pore size distribution 78, 79. Cellulose has been introduced as a promising raw material for the preparation of aerogel via various methods. The different sources of nanocellulose cotton fibers

12, 89

wood chips

84, 85,

and wheat straw

90.

bagasse

86,

80, 81, 82

mainly include rice straw 83,

paper waste and recycled paper

For example, Lin et al.

89

87,

bamboo leaf

88,

fabricated hydrophobic and flexible

cellulose aerogel from cotton linter and was an efficient, renewable, and reusable oil sorbent. Nguyen et al.

87

prepared aerogels from recycled cellulose fibers obtained from paper waste

with a low thermal conductivity of 0.029–0.032Wm−1K−1. These aerogels, which showed satisfactory heat insulation performance, can be used in medium- and low-temperature heat insulation materials. Furthermore, the homogeneous nanoporous cellulose aerogels were synthesized from waste cotton fabrics 91. The prepared composite aerogel exhibited moderate thermal conductivity of 0.081 W m-1 k-1, indicating the excellent heat-insulating performance 92, 93.

In addition, Shi and coworkers 94 utilized cotton linter in the freeze-drying process. The

fabricated cellulose aerogel exhibited low density of around 0.2-0.4 g cm-3 and a low thermal conductivity of up to 0.029 W m-1 k-1 suitable for insulating materials and heat insulation



applications. Fan et al.

95

prepared cellulose aerogels from newspaper waste, which, as

municipal solid waste, contains cellulose fibers

96.

The mesoporous cellulose aerogels

showed low density of approximately17.4–28.7 mg cm-3 with oil and organic solvent adsorption capacity. Kenawy et al.

97

fabricated highly porous aerogel from nano-fibrillated

cellulose extracted from rice straw. Jian et al.

98

synthesized cellulose aerogel with 3D

network from wheat straw featured with very low density (approximately 40 mg.cm–3) and large specific surface area (approximately 101 m2.g–1). Moreover, cellulose aerogel was modified using trimethylchlorosilane to improve its absorption capability for oil and dye contaminants. In 2013, Nguyen et al.

99

synthesized hydrophobic cellulose aerogel from

waste paper and modified using methyltrimethoxysilane. Seantier et al. 70 used bleached cellulose fibers and cellulose nanofibers (NFC or CNC) from date palm tree (Phoenix dactylifera L). Aerogels were prepared from cellulose nanofibers and exhibited better-quality properties. NFC aerogels are commonly introduced as thermal superinsulating materials

100.

According to results, cellulose nanoparticles are added to the

cellulose fiber to decrease thermal conductivity as a result of the decrease in pore sizes and pore wall thickness in the resulting aerogels. Cellulose nanofibers are flexible and can create 3D networks even at very low concentrations. Numerous studies have focused on the fabrication of nanofibrous aerogels from cellulosic resources. The sustainable amphiphilic cellulose-based aerogels have shown tunable mechanical strength due to the presence of both hydrophilic hydroxyls and hydrophobic pyranose rings. Table 1 summarizes the latest studies on cellulose aerogels from waste materials.


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Figure 6. The structure of cellulose, chitin and chitosan. Reprinted with permission from 101. Copyright 2017 MDPI AG.


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Table 1.: Overview of studies on cellulose-based aerogels Waste materials

Type of Aerogel



Cellulose aerogel

Cellulose from cotton linter







Potential application

Ref 102



Cellulose oxidation

adsorption and separation of oil as



crosslinking

well as organic solvent



freeze drying



oxidation

CNFs from eucalyptus

Silane-modified CNF



Cellulose oxidation

water treatment & thermal

bleached Kraft pulp

aerogels



crosslinking

insulation



freeze drying



oxidation

Composite hybrid CNF –



freeze drying

Electroactive & supercapacitor

MnOx aerogel



calcination

materials

cellulose / chitosan aerogel



gelation

Adsorption and separation of



freeze-drying

oil/seawater separation



gelation

Adsorption and separation of heavy



freeze drying

metal ions

CNFs from wood flour

cellulose nanofibrils

& chitosan 

Preparation process

Waste paper

Cellulose/chitosan


103

104

105

106

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













Wheat straw

Newspaper waste

Waste denim

Waste newspaper

Recycled waste fibers

SnO2–znO - cellulose



NaOH/PEG suspension

photocatalytic degradation of

aerogels



Cellulose hydrogel

rhodamine B



synthesis of SZ@CA



freeze drying

Cellulose composite with



gelation

Adsorption and separation of Congo

Fe3O4 powder



freeze drying

Red

Cellulose aerogel



utilizing ionic liquid solvents through dissolution

Adsorption and separation of oil

108



regeneration



freeze- drying



gelation

Adsorption and separation of organic

95



freeze drying

pollutants



Silylated CNF suspensions


109



freeze-drying 110

Cellulose aerogel

cellulose aerogel

96

Pruning waste of

non-crosslinked and



gelation

Adsorption and separation, heat

blueberry tree

crosslinked cellulose aerogel



Freeze-drying

insulator

Waste box board and

Cellulose aerogel



Gelation


111



freeze drying



silanization



Gelation


112



vacuum freeze drying

pollutants

milk-container Board



107

Paper waste

Cellulose monolite aerogels











Waste newspaper

Bamboo fibers

Pine needles

Waste paper &

Cellulose aerogel

Cellulose /AlOOH aerogel

cellulose aerogels

Cellulose aerogel

cardboard











Waste cotton fabrics

Wastepaper

Wastepaper

Wheat straw

Wastepaper

Cellulose aerogel

Cellulose aerogel

Cellulose aerogel

Cellulose aerogel

cellulose Aerogel



gelation



Freeze-drying



gelation



Freeze-drying



gelation



Freeze-drying



gelation



Freeze-drying



modification with clay and APP powders



gelation



Freeze-drying



gelation



Freeze-drying



cross-linking



silanization



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113

thermal insulator

24


114

thermal insulator

115

thermal insulator

91


116

gelation

Thermal insulator, Adsorption and

87



Freeze-drying

separation



gelation


117



freeze drying



gelation

crude oil spill cleaning

99



Freeze drying


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

Silanization gelation



Seaweeds,

Polysaccharide (cellulose,





wastes of

chitin, chitosan) aerogels



supercritical drying

Cellulose Aerogel



gelation



freeze drying

Catalysis; Adsorption and separation

118

Adsorption and separation

119

the seafood industry 

bleached softwood

kraft pulp



4.

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Aerogels from Lignin

Lignin is a natural polymer and a high carbon containing renewable resource with heterogeneous aromatic composition

120.

In plant cell walls, lignin provides regular and

proper connection with other components, especially cellulose 121 . The chemical structure of lignin is not exactly known due to two main reasons. First, the monomers are connected with each other through various inter-unit linkages. Second, lignin is obtained from variety of sources and species. Regardless of the source, all lignins are polymers of 4-hydroxycinnamyl alcohol i.e. pcoumaryl alcohol, its 3-methoxylated derivatives i.e. coniferyl as well as 3,5-methoxylated derivatives that is sinapyl alcohol (Figure 7). Lignin has many covalent bonds in the structure along with some strong noncovalent interactions such as hydrogen bonding that have caused stability and resistivity against degradation and makes the commercial use very difficult

122.

However, the aromatic

structure of lignin and its various chemical functional groups i.e. hydroxyl, methoxyl, carbonyl and carboxyl groups allow it to react with other substances.

Figure 7 : Chemical Structure of monomeric units of lignin. Adapted with permission from 123.

Copyright 2014 Elsevier.

In general, lignin is more normally the massive waste of the pulp and paper industry as well as cellulosic ethanol industry

44

.

It is the main fuel for the energy balance of pulping 18

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process. So one of the major goals in lignin material research is to find high-end uses and high value applications. Regarding their physical and chemical properties, generally, the major difference among types of lignin originates from the pulping process and the minor difference from the plant sources 124 . Kraft lignin is produced from the Kraft pulping process, which produces more than 50 tons per year equals to 85% of world lignin production 122, 125 . However, the utilization of Kraft lignin is limited. Therefore, the aim of researchers is to valorize Kraft lignin to prepare value-added chemicals for large-scale industrial applications 126 . Kraft pulping is characterized by increasing the amount of phenolic hydroxyl group in lignin structure and condensing the structures during the cooking process. Therefore, kraft lignin has high contents of ash and carboxyl group. On the other hand, the lignin obtained from organosolv pulping has high purity with low molecular weight and narrow range of molecular weight distribution. Until now the Kraft process is the leading pulping process all over the world 122 . Lignin-based aerogels are special class of nanostructure materials for the use of lignocellulosics. In the synthesis process, lignin biopolymer with three phenolic monomers is chemically crosslinked to form a 3D network structure gels

127

. Chen et al. (2011)

128

produced a lignin-resorcinol-formaldeyde (LRF) aerogel under supercritical ethanol drying conditions. Grishechko et al. (2013)

129

studied the lignin-phenol-formaldehyde (LPF)

aerogels. Resorcinol has been reported as expensive raw material that contains toxic material. Hence, the use of lignin is considered as an alternative to resorcinol as carbon raw material. In other study

130

synthetic phenol was replaced with tannin that is natural phenolic

compound and further supercritically dried to form aerogels. Tanin can be extracted from leaves of plants, and mainly from wood and bark of several trees


131

. The goal of the work


was to replace toxic compounds such as phenol and resorcinol with the natural phenolic compounds for the sustainable production of advanced materials. The problem was that the aforementioned aerogels have fragile structures. Thereby, researchers 132 used bacterial celulose (BC) to toughen the LRF aerogel. Wang et al. (2016) 133

developed lignin aerogels of nanocellulose which acts as an adhesion agent.

Moreover, Quraishi et al. (2015)

60

developed alginate-lignin aerogels that were bioactive

and featured good as scafolds for tissue engineering and regenerative medicine (TERM) 134 . Lignin has outstanding characteristics including ecofriendly nature, availability, low cost, biodegradability and biological effect as well as its reinforcement effect in composite’s structure that has made it an ideal candidate for variety of applications especially in polymer matrices for high performance composites as well as food and other industrial applications 135, 136

. Table 3 presents the studies conducted on preparation of aerogels from Lignin.


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Table 2: Overview of studies on Lignin-based aerogels Waste materials

Type of Aerogel



enzymatically isolated

Lignin-Resorcinol-

lignin

Formaldehyde copolymer aerogel



lignin-derived carbon

Graphene/Lignin- aerogel

(LDC)

Preparation process 

polycondensation of resorcinol with


Ref

Thermal insulator

128

formaldehyde and ignin 

gelation



supercritical ethanol drying



Graphene-oxide / lignin water suspension

Ultrahigh Electromagnetic



freeze drying to form foams

Interference Shielding

137

Performance





alginate–lignin hybrid



lignin, Na-alginate & CaCO3 suspension

scaffolds for

aerogels



CO2 induced gelation

tissue engineering & regenerative




medicine



lignin-tanin-formaldehyde solution

biomedical and

softwood black liquor &



hydrogel formation

environmental applications.

Tanin from barks of



solvent exchange

wattle trees




lignin from wheat straw

kraft lignin from

Tannin-lignin aerogels


60

130








kraft lignin from

Lignin–phenol–

softwood black liquor

formaldehyde aerogels

lignin from crop wastes

kraft and organosolv

Lignin-based aerogels

lignin-based aerogels



sol-gel polymerization of lignin and phenol with

Page 22 of 80

Thermal insulation

129

138

formaldehyde 

solvent exchange






sol-gel polymerization of lignin and resorcinol

carbon electrodes for

with formaldehyde

supercapacitors



solvent exchange



ambient drying process



pyrolysis up to 1050 C



sol-gel polycondensation of lignin with

lignins

formaldehyde 

solvent exchange



freeze drying


supercapacitors

139

Page 23 of 80 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46








alkali lignin (MW 10000)

wheat straw lignin

Wheat Straw lignin (AS

BC-LRF aerogel

Lignin aerogel

Lignin aerogel

lignin)



Hardwood lignin from beech chips

Lignin aerogel



lignin-resorcinol-formaldeyde (LRF) gel

oil/water separation,



impregnated BC gel with LRF gel by sonication

supercapacitors and sensors



CO2 supercritical drying



lignin crosslinking & gelation



solvent exchange



supercritical drying of lignin gels



Fenton‐type pre‐activation of lignin



crosslinking of oxidized lignin



solvent exchange



supercritical CO2 drying in N2









solvent exchange





132

Thermal insulator

127

Electrochemical applications

140


140




softwood lignin from









solvent exchange




Lignin – graphene oxide



catalytic graphitization of kraft lignin

(LGO) – CNF aerogel



oxidation of lignin graphene oxide



reduced lignin graphene oxide



assembly of LGO & CNF



freeze drying

Lignin aerogel

pine kraft lignin



kraft lignin


Page 24 of 80


140

Water resistance, thermal stability

141

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5. Aerogels from Chitin and Chitosan: 5.1.

Chitin:

Chitin -based aerogels fabricated from abundant natural macromolecules of chitin as raw material, have been widely studied. Next to cellulose, chitin is the most abundant (1011 tons produced annually) renewable biopolymers on earth

142.

In fact, the major material obtained

from marine biomass is the biopolymer chitin with molecular structure consisting of β-(1,4)linked linear chains of N-acetyl-d-glucosamine. Isolated chitin chains have different degrees of acetylation depending on their sources of origin and process of extraction

143.

Chitin is

converted to chitosan through different degrees of deacetylation. The utilization of chitin in aerogel preparation has attracted interest because of its low cost, abundancy of waste material resources, and significant characteristic properties, such as biocompatibility, non-toxicity, as well as thermal and chemical stability. Different size distributions of chitin ( nanocrystals ChNCs and nanofibers ChNFs ) can be generated from the same resource while they can been utilized as raw materials for aerogel preparation 4. Chitin nanofibers and nanocrystals, as renewable resources with unique structures and properties, can be isolated from the shells of crustaceans, i.e., crab, shrimp, and lobster, and in fish waste via different methods 6. It is also found in fungi, insects (ants), annelids, cephalopods (squid and octopus), and so on. However, the most common industrial renewable resource for chitin is mainly seafood-processing residues, i.e., crab and shrimp shells

5, 7, 8, 16, 53.

According to reports, an enormous quantity of crustacean shell wastes

(approximately more than 6 mega tons) are produced every year 33. Disposal of these wastes has been a real challenge for industry; hence, production of chitin, chitosan and their derivatives for utilization in various value-added industrial platforms has extreme importance. It should be mentioned that extraction process and treatment conditions would extremely influence on morphological appearance and physicochemical characteristic of resulting chitin. Development of profitable technology for extraction of chitin and its derivatives is of great interest among academic and industries 144. Several publications have reported the production of chitin aerogels and their potential applications in different areas. For example, Tsutsumi and coworkers

56

prepared highly

porous aerogels with high specific surface area (up to 289 m2 g–1) from squid pen chitin nanofibrils through freeze-drying process. The numbers of recent publications on the fabrication of chitin aerogels are presented in Table 3.



5.2.

Page 26 of 80

Chitosan:

Nevertheless, extensive removal of acetyl groups of chitin produces chitosan that is a natural polyamine. Chitosan is a biocompatible polycationic biopolymer obtained via alkaline deacetylation of chitin. It is a long-chain copolymer of β-(1→4)-linked 2-acetamido-2-deoxyβ-D-glucose units with acetamide groups in the C2 position. The source of origin, variables of deacetylation process and purification conditions can influence the characteristics of final product, resulting in chitosan molecules with different chemical and physical qualities 145. The physicochemical properties of chitosan are characterized by its deacetylation degree and molecular weight. Among many characteristics, the degree of deacetylation, which defines the content of free amino groups in the polysaccharides, is one of the more important chemical characteristics, which influences the performance of chitosan in many of its applications

146, 147.

During deacetylation process, acetyl groups of chitin are replaced with

amino groups. The chemical structures of chitin and chitosan were compared with that of the cellulose in Figure 6. Given its mechanical and structural properties, chitosan has gained interest as a polysaccharide for synthesis of different types of nanostructured porous materials in different fields. The main characteristic of chitosan -based aerogel is its highly porous structure with extended surface area. The presence of amino groups allows chitosan to react easily with electrophilic reagents like aldehydes and acids. chitosan has many distinctive physicochemical and biological properties, such as the formation of gels with polyanions 3. Fabricating chitosan aerogels could improve the availability of chitosan functional groups for chemical reactions. For example, Ricci et al.

148

successfully prepared chitosan aerogel

microspheres from marine crustaceans with high surface areas of up to 350 m2g-1 and were applied as catalysts in asymmetric aldol reactions. Similarly, Quignard and his coworkers

12

used squid pen and crab shell chitosan with high degrees of amine groups to prepare chitosan aerogel particles, which demonstrated highly porous and fibrous microstructures. This was followed by thermal decomposition of aerogel into N-doped carbonaceous /carbon aerogels /equivalent. However, chitosan has some weaknesses that limit its applications. These mainly include weak chemical stability, poor mechanical strength, low surface area, and dissolution only in acidic solution

87;

chitosan is insoluble in many solvents. Numerous chitosan modification

methods were found to be effective and harmless to the environment and which improved the 26 ACS Paragon Plus Environment

Page 27 of 80 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


inherent properties of chitosan. Two biopolymer functional groups are responsible for the different chemical modifications, resulting in materials with more desirable characteristics. Chemical modifications of chitosan, typically either at the amino group (the secondary C2 NH2 group) or the hydroxyl group (the primary C6 OH and secondary C3 OH groups), aim to produce derivatives with enhanced properties to widen its applications. These derivatives are considerably easier to fabricate into aerogels because they are water or acid soluble 149. One physical method for improving the mechanical strength of chitosan aerogels involves blending of cellulose nanofibers into a biopolymer matrix. This method resulted in greater mechanical strength and thermal insulation, along with higher adsorption capacity of composites. Cellulose and chitosan are abundant natural, renewable, biocompatible, and biodegradable polymers. Zhang et al.

105

prepared reinforced nanofibrilated cellulose

composite aerogels by integrating NFC into chitosan matrix via the freeze-drying method. chitosan –cellulose composite aerogels have also presented improved adsorption ability

88.

Moreover, organic–inorganic hybrid aerogels are another group of aerogels with improved thermal characteristics 88, 89, 150. Ebisike and his coworkers 86 prepared chitosan –silica hybrid aerogels from the biomass waste of crab shell and bamboo leaves to fabricate chitosan –silica hybrid aerogel [(chitosan) hA]. CS biopolymer was hybridized with varieties of inorganic compounds, such as silica 85 and chondroitin sulfate (ChS) 82. Various applications, such as adsorption 94, catalysts 77, 80, biosensors 81, tissue engineering 82, 151,

and controlled release of bioactive molecules 6, have been reported for chitosan aerogels.



Page 28 of 80

Table 3.: Overview of studies on chitin and chitosan-based aerogels Waste materials

Aerogel

Preparation process


Ref



crab shell

Chitosan / silica hybrid aerogel

sol-gel method by combining an inorganic network

Adsorption & separation of Cd2+ &

85



bamboo leaves

in the presence of an organic polymer

heavy metal



Chitosan



Prawn chitin nanofiber



wood cellulose nanofiber





Chitosan

Ultrapure chitosan

Chitosan /graphene oxide aerogel

Chitin nanofibril aerogel

Cu-chitosan aerogel

Chitosan/ chondroitin sulfate (chs)



Chitosan

chitosan aerogels



gelation



crosslinking



freeze drying



aq. suspension



solvent exchange



freeze drying



gelation



supercritical CO2 drying 

gelation



freeze drying



gelation


adsorption of CO2 molecules

150

Catalysis

57

Heterogeneous Catalyst

80

wound healing

82

nanomedicine applications

83

Page 29 of 80 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46


 

Chitosan and cellulose from cotton

chitosan/oxidized cellulose

linter

aerogel





wastepaper & chitosan

Chitosan & Nanofibrillated

Wastepaper/chitosan aerogel

chitosan/ cellulose aerogel

cellulose



Chitosan

chitosan aerogel modified with Au+2



Chitosan & medical adsorbent

Chitosan/cellulose aerogel

cotton



Chitosan from crab & shell

Chitosan/silica hybrid aerogel

supercritical CO2 drying



Cellulose hydrogel formation



cellulose hydrogel oxidation



Schiff base reaction of hydrogel and chitosan



freeze drying



Modification 

Wastepaper/chitosan hydrogel formation



freeze drying



gelation



freeze drying



Wet gel preparation



freeze drying



gelation



freeze drying



gelation


Adsorption & separation of oil

102

Adsorption & separation of cu+2 &

106

heavy metals


105

Catalysis

68

Adsorption & separation of Congo

88

Red (CR)

thermal insulators, catalysis and

86


silica from bamboo leaves 

Chitin from carapace powders

Nanofibrillated chitin/Ag2O aerogel



partially deacetylated α-chitin

α-chitin nanofiber/nanowhisker

nanofiber/nanowhisker (DEChN)

(DEChN) aerogels

Page 30 of 80



ambient pressure drying

biomedical materials



gelation

Adsorption & separation of



Freeze drying

58

radioiodine anions



Alkali deacetylation



solvent exchange



freeze drying



Hydrothermal gelation of chitin aqueous

Biomedicine, photocatalysis, gas

suspensions

sensing


59


from Crab shell from food waste (Nantong, China) 

chitin nanocrystals from dried

Chitin aerogel

crab shells



Chitin from shrimp shells

CNFs from corn husks and



CNCs from shrimp shells



Chitosan powder

freeze drying



chitin nanowhisker aerogel



surface modification of chitin nanowhiskers



chitin nanowhisker/ carbon



CNC3 & CNT hybrid aerogel



freeze drying

nanotubes hybrid aerogel





Chitin/cellulose hybrid aerogel

translucent and ultralight





gelation



Freeze drying

gelation through cross-linking with



60

152


anti-bacterial and anti-oxidant

61

thermal insulators

63

Page 31 of 80 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46


Chitosan aerogel

formaldehyde 



Chitosan powder

chitosan/carbon nanotubes aerogel



α-chitin powder



Chitosan powder

α-chitin aerogel

translucent and ultralight


CNTs & CS aq. Suspension



Crosslinking



Freeze-drying



gelation with ethanol






gelation through cross-linking with



Chitosan powder



fiber reinforced silica / chitosan nanocomposite aerogel



Chitosan powder

Chitosan–silica composite aerogels

81

adsorption & energy materials

catalyst supports, biomedical

2

materials

thermal insulators

65

formaldehyde

chitosan aerogel in the visible region

sensors, medical restorative,


gelation of sodium silicate & chitosan






gelation




thermal insulation, sorption, catalysis

Adsorption & separation of Congo


red

153

67




Chitosan



Bamboo powder

chitosan aerogel beads supported by microfibrilated cellulose



agarose from red seaweed &



chitosan powder



Chitin nanofibers from squid-pen



Gelation through ball dropping method



freeze drying

Chitosan/agarose composite aerogel

Chitin aerogel

(Sepioteuthis lessoniana)



Chitosan from shrimp

hybrid monolith aerogel of chitosan (CTS) - Graphene Oxide





Chitin powder

Chitosan

Chitin nanofiber aerogels

Chitosan / montmorillonite hybrid



cross-linked gel formation



Freeze drying



Gelation



freeze drying



Gelation



freeze drying



Page 32 of 80


89

formaldehyde in indoor air


154

catalysis of aqueous Knoevenagel

56

condensation reaction

Adsorption & separation of CO2

65

aqueous NaOH–urea suspension

biomaterials,

155



gelation from ethanol

heat or sound insulators, catalysis






gelation of an intercalated “chitosan–


montmorillonite” solution,

methylene blue

aerogel microspheres 

super-critical CO2 drying


156

Page 33 of 80 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46




chitosan (79% DD) from India

Chitosan Aerogel

Sea Food Products



Chitosan nanofibers from

Chitosan aerogel

crustaceans



α-chitin from crab shell



β-chitin from squid pen



Chitosan

Chitosan aerogel

chitosan aerogel



gelation






gelation






gelation






gelation



crosslinking

biomedical

66

Heterogeneous Catalyst

148

Catalysis

12

adsorption & separation of anionic

157

surfactant sodium dodecylbenzenesulfonate (SDBS)



Chitosan from squid pen

Chitosan aerogel



Supercritical CO2 fluid extraction.



Wet gel formation,



freeze drying


catalysis or adsorption & separation

158


Page 34 of 80

6. Aerogels from Pectin: Pectin is a linear polysaccharide found as a primary constituent of plant cell walls and is responsible for the rigidity and cohesion between cells. It is an abundant and inexpensive commercial product, and its chemical structure is composed of linear polymers of galacturonic acid linked by 1,4 glycosidic bonds. Carboxylic acid functional groups are found in galacturonic acid chains that are partially esterified on the C6 position using methanol. The percentage of methyl-esterified groups in the pectin chain is referred to as degree of methylation or degree of esterification (Figure 8).

Figure 8: Chemical structure of pectin molecule. Reprinted with permission from 159. Copyright 2010 MDPI AG.

Pectin is a major industrial waste biomass polysaccharide that is mainly obtained from fruit juice industries as abundant, inexpensive, and high-volume commercial waste product. The main sources are apple pomace and citrus peel because of their abundance and rich protopectin content. Pectin has the ability to form gels, and pectin aerogels are excellent thermal-insulating compounds that are commonly fabricated through dissolution–gelation– coagulation process, followed by supercritical drying. Thermal conductivity increases with increasing pectin concentrations. According to reports

160, 161,

the obtained thermal

conductivity values are in the range of 0.016–0.020 W m−1K−1. Moreover, pectin aerogels have been reported to have considerable mechanical strength. The first pectin aerogel or 34 ACS Paragon Plus Environment

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aeropectin preparation has been reported by White and his coworkers

162.

The pectin

extracted from citrus peel was fabricated through thermal gelation of pectin solution, dissolving in acidic solution, solvent exchange with ethanol, and finally drying with supercritical CO2. The recent reports on the preparation of pectin-based aerogels are summarized in Table 4



Page 36 of 80

Table 4.: Overview of studies on Pectin-based aerogels Waste materials

Aerogel

Preparation process


Ref



magnetic pectin aerogel



Gelation

magnetic targeting drug delivery vehicles

163






aq. suspension,

chromatography, heterogeneous catalysis

164



freeze drying

and large molecule adsorption



Polymerization,

thermal insulator



Coagulation,






gelation,



freeze-drying



modification with nanoclay

pectin from apple–citrus and citrus peel







Pectin (from citrus fruit peel)

Apple pomace Pectin

Pectin from citrus



Pectin from citrus peel



Pectin from apple pomace



High methoxyl pectin

Pectin aerogel

Polyaniline/Pectin Aerogels

Pectin/Clay Aerogels

Monolithic pectin aerogel

methoxyl pectin–xanthan aerogel

62

165



Dissolution

thermal



gelation

super insulator



coagulation






ethanol-induced gelation

coating on medical-grade



Supercritical CO2 drying

stainless steel


160

166

Page 37 of 80 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46




High-methoxyl (Pectin)



Dissolution in water,



gelation of pectin in ethanol






Dissolution in water



Solvent Exchange




pectin-TiO2 nanocomposite



aerogels



Pectin aerogel

and low-methoxyl





Citrus pectin

Amidated pectin

Pectin Aerogel

Thermal insulator

161

Thermal insulator

167

Gelation

potential

168


insulated food packaging porous carriers in oral drug delivery

169

packaging and insulation materials

170



low-methoxyl citrus

amidated low-methoxyl apple (AF)



cationic gelation



apple pectins

and citrus (CF) pectin aerogels






Pectin from apples

Pectin/ Boron nitride nanosheets



Gelation

(BNNSs) aerogel



freeze-drying



Page 38 of 80

7. Carbon Aerogels: After the initial synthesis of carbon aerogels

171,

the field largely became popular. This

exceptional class of materials, with its high surface area, electrical conductivity, chemical inertness, and compatibility with the environment makes it suitable for energy storage, absorbent, supercapacitors, and catalysts. Carbon-based aerogels are unique carbonaceous materials with unique high-surface-area. They are prepared through hydrothermal carbonization of carbonaceous precursor; this is a method wherein hydrothermal treatment process is followed by pyrolysis of biomass carbon in the presence of N2 or Ar at temperatures above 600 °C to obtain extremely pure carbon structure

25.

Sol–gel and

pyrolysis are common methods for preparing carbon aerogel. Chandrasekaran et al.

172

reviewed many carbon-based aerogel preparation methods, such as inkjet and 3D printing, which make them an attractively growing field for study and various applications. Despite the synthesis of carbon nanotube aerogel in the last decade using surfactants and via freeze drying and critical point drying 173, its preparation is regarded as very challenging. Although carbon is the main element in all agricultural, plastics and other biobased waste materials, mostly its proper extraction isn’t easy and economical, and more research must be conducted in this field. This material has become even more attractive due to its unique thermomechanical and electrical properties. Their unique characteristics, such as biodegradability, environmental compatibility, and high surface area, have made them special porous materials for applications such as adsorption

155,

environmental desalination

174,

and energy storage

62.

While common adsorbents, such as activated carbon, have sorption capacity less than 100 g. g-1, carbon aerogels, as important solid adsorbents, have shown unique adsorption capacity of up to 900 g. g-1 20, 175, 176. Their hydrophobic surfaces allow selective and effective adsorption of oils, organic pollutants, and heavy metals from wastewater

106.

Moreover, their unique

electrical conductivity is beneficial in electrical/electrochemical applications, such as super capacitors 177, electrocatalysis and electrosensing 178, as well as batteries 104. Since the sol-gel reaction chemistry control the morphology and properties of carbon aerogels, aerogels with specified applications and properties could develop considering sol-gel precursors, polymerization catalyst and reaction solvent (Figure 9)

179.

As shown, the Carbon aerogel

(CA) structure can be modified either during the sol–gel polymerization step, through the introduction of additives or templates to the reaction mixture, or through gas- or solutionphase reactions on the surfaces of the CA framework after the pyrolysis step. 38 ACS Paragon Plus Environment

Page 39 of 80 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 9. Synthetic scheme showing the versatility associated with carbon aerogel synthesis. (Reprinted with permission from ref. 179. Copyright 2011 Royal Society of Chemistry.) Using carbon allotropes, various carbon-based aerogel with different properties could be developed. Traditional carbon aerogel consists of 3D network of interconnected amorphous carbon

163, 172.

Chandrasekaran et al.

172

reviewed many carbon-based aerogel preparation

methods, such as inkjet and 3D printing, which make them an attractively growing field for study and various applications. The most common carbon-based aerogels that have been introduced as novel porous materials are carbon nanotube (CNT) 173, 180, graphene 181, 182, and carbon nanofiber aerogels

183.

Moreover, various types of carbonaceous wastes have been

used as raw materials to prepare carbon-based functional materials due to their carbon richness, low cost, and natural abundance. They mainly consist of abundant agricultural byproducts e.g., grass, leaves, rice husks and straws, coconut coirs, and shells, as well as municipal solid wastes, such as waste paper and newspaper. The second group is non-



Page 40 of 80

degradable municipal and industrial wastes, including different industrial polymeric products, such as plastic bottles, plastic bags, and industrial wastewaters. Graphene 184, 185, 186 and graphite aerogels are other carbon-based aerogels that are of interest to researchers due to their attractive properties. However, only limited studies have been published on the syntheses of CNT and grapheme aerogels materials. For example, Wang and his coworkers

188

187

from carbonaceous waste raw

prepared grapheme aerogels using

cigarette filters. The fabricated composite aerogels area ultralight (ρ=7.6 mg cm-3) and have high mechanical strength (0.07 MPa), and they are used as electromagnetic wave absorbers. Nevertheless, plenty of reports have utilized natural biofibers, mostly cellulose nanofibers, as starting materials for the preparation of emerging carbon nanofiber aerogels

189.

Cellulose

aerogels, with cross-linked 3D network, could be pyrolyzed into carbon aerogels to make composites

43, 190.

Cellulose-nanofiber aerogels have been used as the main source for novel

carbon aerogels produced via pyrolysis. Carbon nanofiber aerogels can be derived from several sources, mainly nanocelluloses and nanochitins, as the most abundant biopolymer nanofibril

97.

In fact, cellulose fiber has been introduced as precursor for the preparation of

porous cellulose aerogels, making it a special candidate for carbon aerogel preparation For example, low-cost and abundant bamboo pulp fibers

192

191.

were used as promising

precursors for the preparation of a novel lightweight carbon aerogel (5.65 mg cm-3) with excellent mechanical property, high hydrophobicity, and large specific surface area (379.39 m2g-1). Moreover, its excellent absorption behavior allows it to be used in future water treatment applications. In another case, waste paper or cotton, as raw material, was used to fabricate sponge-like hydrophobic and porous carbon microbelt aerogels and carbon fiber aerogels via a facile route

193.

Waste newspaper is extensively used as raw material for

fabricating carbon-based aerogel. Bi and coworkers (2013) 20 used twisted carbon fibers from cotton fibers and carbon microbelts from waste paper to produce porous and hydrophobic aerogels to adsorb a variety of organic solvents and oils in water purification systems. The latest publications on carbon aerogels from waste materials are listed in Table 5.


Page 41 of 80 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46


Table 5.: Overview of studies on Carbon-based aerogels Waste materials 



Bagasse from sugarcanes

Type of Aerogel Carbon aerogel

lignin-derived carbon

Graphene/Lignin-

(LDC)

Derived Carbon aerogel



Preparation process

Potential Application

Ref

supercapacitor electrode

178



Gelation



Freeze-drying



Carbonization



chemical activation



Graphene-oxide / lignin water

Ultrahigh Electromagnetic

suspension

Interference Shielding



freeze drying to form foams



carbonization at 900 C at Ar

137

Performance

alkali lignin (MW

BC-LRF carbon



lignin-resorcinol-formaldeyde (LRF) gel

oil/water separation,

10000)

aerogel



impregnated BC gel with LRF gel by

supercapacitors and sensors

132

sonication 

CO2 supercritical drying 



kraft and organosolv

lignin-based carbon aerogels



Carbonization

sol-gel polycondensation of lignin with formaldehyde


supercapacitors

139


lignins



lignin from crop wastes

Lignin-based carbon



solvent exchange



freeze drying



carbonization at 1100 ̊C T N2



sol-gel polymerization of lignin and resorcinol

carbon electrodes for

with formaldehyde

supercapacitors

aerogels



Wheat Straw lignin (AS

Lignin carbogel

lignin)



Hardwood lignin from beech chips

Lignin carbogel

Page 42 of 80



solvent exchange



ambient drying process



pyrolysis up to 1050 C









solvent exchange






carbonization









solvent exchange






carbonization


138


140


140

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










from pine kraft




lignin



solvent exchange






carbonization

softwood lignin

Lignin carbogel

Waste newspaper

Waste pomelo peels

Cellulose from cotton linter

Carbon aerogel

Carbon aerogel

Cellulose/carbon aerogels



Gelation



Freeze drying



pyrolysis



hydrothermal carbonization



freeze-drying



pyrolysis



Gelation



Carbonization



activation



140


54


194

Adsorption & separation of CO2

195






eggplant

Animal skin waste

Carbon aerogel

Collagen-polypyrrole hybrid



Gelation



Freeze drying



Carbonization & post-pyrolysis



In situ oxidative polymerization



gelation



Freeze drying



air-limited calcination method



NaOH/urea aqueous solutions



freeze drying



carbonization



Activation



gelation



freeze-drying



pyrolysis

Page 44 of 80

Adsorption & separation of oils and

195

organic solvents

Biosensor, tissue engineering

196

Adsorption & separation of methylene

197

aerogels 





Cotton fibers

bamboo cellulose fibers

Waste durian shell

carbon aerogel

carbon aerogel

Carbon aerogel


blue (MB)

supercapacitor electrodes

Adsorption & separation of organic solvents /oil

148

163

Page 45 of 80 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46




Wood-derived nanofibers

Ultrathin Carbon nanofiber



gelation

aerogel






pyrolysis

supercapacitor electrodes

164

Adsorption & separation of dye

62

Adsorption & separation of oil and

165

under flowing inert

gas



Nitrogen-enriched

chitin powder

carbon

nanofiber aerogels



Nitrogen-doped

Waste cabbage leaves

carbon aerogels



typha

orientalis

cellulose

Carbon aerogel

fibers

porous



gelation






Carbonization



Hydrothermal gelation



freeze-drying



carbonization



gelation






Carbonization

organic solvents

Adsorption & separation of colored

198

diesel

under N2 atmosphere 

Plastic waste PET, HDPE, LDPE, waste

municipal

Polymeric aerogel



solid

mixed with Xylene and antioxidant



precipitation,



freeze-drying



166




Sisal leaves

Carbon

Aerogel/TiO2

Nanorods





gelation



freeze-drying



carbonization

Bamboo cellulose

cellulose nanofibers



CNF gels

nanofibers

(CNFs)/multi-walled



freeze-drying

carbon nanotubes



CNF aerogels dipped in

(MWCNTs) carbon aerogels 

carbonization

banana peel

banana peel/wastepaper



gelation



wastepaper

(BPWP) hybrid aerogel



freeze-drying



carbonization



In-situ oxidation

Cattail biomass

Carbon aerogel


199


200

energy storage, sensors and pressure-

169

MWCNT solution





Page 46 of 80

sensitive electronics.

Supercapacitor

170

Adsorption & separation of oil &

167

polymerization method 

leaves of Premna

carbon@SiO2@MnO2



gelation

microphylla (PM)

aerogel



freeze-drying



carbonization in N2 atmosphere

self-templated N-doped



gelation

carbon aerogel



freeze-drying,



carbonization in



Sisal leaves



Banana


organic solvents

supercapacitors

201

Page 47 of 80 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46


argon (Ar) atmosphere,  

watermelon

activation by CO2    

magnetite carbon aerogels (MCAs)

 

bleached softwood kraft

Carbon aerogel

pulp (BSKP)



puffed rice Popcorn





Natural apple pectin

premna microphylla leaves

Magnetic carbon aerogel

Carbon aerogel

Carbon aerogel

gelation freeze drying incorporating Fe3O4 nanoparticles into the networks calcination



NaOH/urea/H2O suspension



vacuum freeze-drying



carbonization



gelation



freeze-drying



carbonization



magnetization of popecorn



surface modification



Wet gel formation






carbonization



activation



gelation


Supercapacitors

Adsorption & separation of oil &

202

203

organic solvents

selective Adsorption & separation of

158

oil & organic solvents


204

pharmaceutical contaminants

Adsorption & separation

205






sisal fiber

biomass of apples (Malus

carbons aerogels

pumila Mill)







Waste newspaper

Bamboo pulp fibers

Waste cabbage

Carbon aerogel

Carbon aerogel

Nitrogen-doped

carbon

aerogel



cotton

Carbon fiber aerogel



freeze-drying



carbonization



gelation



freeze-drying



carbonization



gelation



freeze-drying



post-pyrolysis



gelation



freeze-drying



post-pyrolysis



Hydrothermal gelation



freeze-drying



carbonization



cotton calcination in air-limited

Page 48 of 80

electrochemical sensing

206

Adsorption & separation of organic

54

solvents

Adsorption & separation of Oil

192

Adsorption & separation of Oil;

207

supercapacitor

Adsorption & separation of dye

197

Adsorption & separation of Oils &

176

atmosphere 

Wastepaper

Carbon Microbelt Aerogel



gelation



freeze-drying



pyrolysis

Organic Solvents under


argon

Page 49 of 80 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46


atmosphere 



waste of shaddock (Citrus

carbon

maxima) peels

(CNANAs)

Cigarette filters

nanoballs

aerogels

graphene aerogels



gelation



freeze-drying



carbonization



carbonization

of

celluloses

electrocatalysis and electro sensing

108

electromagnetic wave absorption

188


162

Lithium Ion Storage

208

Supercapacitor electrode

206

fibers into CFs



Chitosan



reduction of GO into grapheme

graphene



carbonization of fibers

oxide/polydopamine



reduction of GO into graphene



gelation



freeze-drying



carbonization



gelation



freeze-drying



carbonization



activation

composite aerogel reinforced by chitosan 

Waste seaweed

Nano

Fe2O3

/

/N-doped

graphene aerogel



chitosan

Graphene-based

nitrogen

self-doped carbon aerogels




Page 50 of 80

Page 51 of 80 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8. Aerogel from Silica: Silica aerogels

209

with nanosized, open foam-like structures exhibit extremely low density

(containing up to 95% air in their volume), high specific area, high porosity, low thermal conductivity, and optical transparency, rendering them very attractive for a multitude of technological applications 210, 211. The capability to control the polarity of the surface between hydrophilic and hydrophobic properties, as well as excellent access option to its inner surface due to the open porous network, result in exceptional physico-chemical properties, rendering them suitable for thermal insulators, catalysts, adsorbents, and drug delivery 211. In nature, silica exists as sand, glass, quartz, and so on. Naturally occurring silica has crystalline structure, while synthetic silica, which is usually obtained from silicate solution, has amorphous structure 174. Silica aerogel is commonly produced using expensive and toxic precursors, such as tetramethoxysilane [Si(OCH3)4, TMOS] and tetraethoxysilane [Si(OC2H5)4, TEOS] 189. However, industrial and agricultural wastes containing silica in their respective compositions can be used as precursors for the preparation of silica-based aerogels. As an example, rice husk

212

has been abundantly reported to be one of the potential bioresources of silica.

According to literatures, rice husk ash, which is obtained from agricultural wastes and residues and is abundantly produced in the rice industry, consists of 92% amorphous pure silica. The composition of rice husk as an organic biomass consists of silica, lignin, cellulose, and hemicelluloses

175.

After burning the husk under controlled combustion, rice husk ash,

which contains approximately 90% to 98% silica, and some amount of metallic impurities are generated

176, 177

Utilization of these agricultural wastes could considerably reduce the high

cost of disposal and environmental problems because the extraction process of silica is cost effective. Rice husk is an abundant recyclable waste product, and its ash contains considerable levels of high-quality silica. Most importantly, pure silica obtained from rice



husk ash exists in an amorphous state with high surface area applications of silica aerogels include drug delivery systems

Page 52 of 80

178. 213,

The common practical adsorbent for pollution

treatment 214, and filler material for thermal management solution 212. Numerous reports have been made on silica aerogels prepared from rice husk ash

181.

For

example, silica was extracted from rice husk ash to prepare SiO2 aerogel via sol–gel process and supercritical drying of ethanol

215.

Likewise, Kumar and his coworkers

213

prepared

biocompatible silica aerogel microparticles from rice husk ash as promising drug delivery vehicles because of their large surface area and open pore structures. They used water-inmineral oil emulsion for sol–gel preparation, followed by supper critical drying with CO2. Other examples on the utilization of rice husk ash as silica resource for aerogel preparation are listed in Table 6. Other reports have proven that rice hull 216, wheat husk 17, and Bagasse 217

from agricultural wastes can be used for the production of silica aerogels. Considering of

using other resulted byproduct such as lignin, cellulose and hemicellulose during extraction and purification of silica from waste biomass as co product for different application could add more value for its commercialization. These silica aerogels have also been prepared from industrial wastes, mostly gold mine wastes and coal gangue. Details of these reports are explained in Table 6.


Page 53 of 80 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46


Table 6.: Overview of studies on Silica-based aerogels Waste precursor

Type of Aerogel

Preparation method

Application

Ref



Silica aerogel



gelation

Thermal insulator in cement-based

142



solvent exchange

mortar



drying at ambient pressure



gelation



solvent exchange









Water glass preparation



gelation



solvent exchange














Rice husks

Rice husk

Rice husk

Rice husk

Rice husk from Mazandaran

Silica aerogel

silica aerogel

γ-Fe2O3/ SiO2 aerogel

Silica microspheres

lands 

Rice Husk

Silica aerogel

thermal insulator

151

thermal insulator

218

Gelation

photoFenton- like degradation of

214



drying at 60oC

rhodamine B (RhB)



Silica gel formation

HPLC stationary phase

180



calcination at 550 oC



sol-gel method

thermal insulation,

212







Rice husk

Rice husk

silica aerogel beads

silica aerogel

Page 54 of 80



solvent exchange

chemical sensors, and energy storage




devices






ball dropping method

Adsorption and separation of oil; filler



surface modification (silylation)

material for thermal insulation






Silica aerogel nanoparticles incorporated

219, 220

Microwave and RF application

221

drug delivery

213

55

into poly(butylene succinate) 







Rice husk

Rice husk

Bagasse

Fly ash

silica aerogel



gelation

microparticles




TEOS-doped silica



gelation

thermal insulation, catalysis, acoustic

aerogel



atmospheric pressure drying

delay, drug delivery

Silica aerogel



gelation








Silica aq. gel









Silica aerogel


222

223

Page 55 of 80 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46
















Bagasse

Wheat husk

Gold mine waste

Coal gangue

Coal gangue

Coal gangue

Coal gangue

Silica aerogels

Silica aerogel

Silica aerogel

SiO2-Al2O3 aerogel

SiO2-Al2O3 aerogel

Silica aerogel

Silica aerogel



wet gel formation









gelation






gelation






Strong hydrophobic

224

Adsorption and separation

225

Thermal insulator for insulating plaster

226

gelation

Adsorption and separation; thermal

227




insulator



gelation





solvents



gelation

Thermal insulator

229



surface silylation






gelation

Thermal insulator

230



hydrophobization





228




silicon tetrachloride (SiCl4)

Silica aerogel

from polysilicon industry









Fly ash acid sludge

Rice hull

Rice hull

Bamboo leaf

silica aerogels

Silica aerogel

Silica aerogel

silica aerogel



gelation






gelation



surface silylation






gelation



drying at atmospheric pressure



gelation



solvent exchange



super critical drying



Preparation of water glass as a precursor

Page 56 of 80

thermal insulation

231

Strong hydrophobic material

232

Thermal insulator

222

thermal insulation, catalysis

216

Thermal insulator

233

for silica aerogel synthesis 

Rice husk ash

Silica aerogel



sol-gel



solvent exchange



vaccum oven drying

234


Page 57 of 80 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


9. Discussion and Future prospect of research: In the last decade, interest in developing aerogels for diverse applications, such as thermal insulator, catalysts, sensors, environmental, and biological applications, has increased. The unique properties of these aerogels and their preparations via flexible methods, such as sol– gel method, play important roles for their industrialization. The potentials of various materials for aerogel preparation show the importance of a fundamental understanding of materials science to design the optimized synthesis conditions and processes. One of the most important challenges that is still an active area of research is the complete study of the waste materials with variable composition and properties to determine the best design for the better utilization of natural resource and preparation of these aerogels while considering functionalization and making composite for their specified application. Since the sol-gel and freeze-drying methods are almost commonly used methods in the preparation of various aerogels, exploration of new, more green and economical methods such as liquid hot water for extraction and purification of waste materials for commercialization of aerogel production is very imperative. Meanwhile, the potential of combining different approaches such as microwave, ultrasonic and so on to commonly used pretreatments methods to modify their performance is very promising study concept. However, it is important to keep in mind that optimum pretreatment method strictly depend on waste materials characteristic as well as the components to be isolated. The separation of base materials from wastes is currently a major challenge in this field. For example, despite of presence of carbon as the base element in all agricultural, plastics and other biobased waste materials, proper and cheap extraction of carbon is still challenging subject that need more study. Techno-economic evaluation of monomer isolation from waste and biomass, which is a preliminary evaluation of mass and energy balances, has been performed by some authors. But there are not published data from economic standpoint on the cost of production of aerogels from varieties of industrial wastes and agricultural biomass in any of recorded articles in this review paper. This issue remains open and rich to researchers for the future development of aerogel production. Moreover, the potential of utilization of biowastes and industrial wastes for the synthesis of silica, alumina, and other metal oxides cannot be ignored as it provides inexpensive and sustainable resource materials for the preparation of metal oxide aerogels. Despite various research on the utilization of waste materials for aerogel preparation, extensive studies must 57 ACS Paragon Plus Environment


be conducted in this field to address them as industrial potential precursors for this application. Utilizing different byproducts such as lignin, hemicellulose and silica that made during extraction of specified materials such as cellulose from waste biomass must not be ignored. It could provide more value for their commercialization. Considering the promising development in the preparation of different carbon nanostructures, such as carbon fiber, nanocellulose, nanotube, graphite, and so on, from cellulose and lignocellulose biomass, which are the most abundant biomass on earth, their future use in the preparation of aerogel is extremely promising. 10. Conclusion: Despite the recent studies on aerogel preparation methods and precursor materials, the stateof-the-art aerogels cannot satisfy huge application demands. This contribution aimed to provide an overview of the utilization of waste biomass and industrial wastes by researchers and industries through categorizing the source of precursors, fabrication, synthesis process, and applications to emphasize the improvement of green and effective aerogel preparation methods. The limitations of these products and recommendations in future works are also detailed. The aforementioned natural resources are abundant and inexpensive, making it economical and sustainable for the preparation of aerogels while also addressing waste management problems. The conducted research and studies on this subject could definitely develop new reliable, cost-effective, and sustainable pathways for aerogel preparation. Future works must conduct in-depth study to provide the appropriate knowledge for understanding and establishing the relation between waste materials with variable composition and aerogels with specific requirements, considering different modifications, such as functionalization and making composite. Considering the direct dependence of characteristic such as morphology, surface and mechanical properties of prepared aerogels to its components derived from waste resources, developing optimum pretreatment and extraction methods is the most important future challenge. Waste materials are believed to have promising future as potential materials in aerogel preparation.

Acknowledgment: This study was partially supported by the Universiti Kebangsaan Malaysia’s grants (GUP2018-129 and GP-K019259). 58 ACS Paragon Plus Environment

Page 58 of 80

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Graphical abstract 337x111mm (96 x 96 DPI)



Figure 3 317x138mm (96 x 96 DPI)


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Figure 6 104x57mm (300 x 300 DPI)





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Figure 8



Figure 9


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Biomass and industrial wastes as resource materials for Aerogel

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