image
Home Journals Progress in Chemistry
Progress in Chemistry

Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

About  /  Aim & scope  /  Editorial board  /  Indexed  /  Contact  / 
Online first  /  Current issue  /  All issues  /  Top access  /  RSS

Progress in Chemistry >

2024 , Vol. 36 >Issue 4: 586 - 600

DOI: https://doi.org/10.7536/PC230813

Review

Preparation of Lignocellulosic Aerogel and Its Flame Retardant Modification

  • Wang Shuang 1 ,
  • Zhang Xin 1 ,
  • Sun Miao 1 ,
  • Duan Hongjuan 1 ,
  • Zhang Haijun , 1, * ,
  • Li Shaoping 2
Expand
  • 1 State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081,China
  • 2 Hubei Three Gorges Laboratory, Yichang 443007, China
*e-mail:

Received date: 2023-08-15

  Revised date: 2023-12-05

  Online published: 2024-03-15

Supported by

National Natural Science Foundation of China(52072274)

National Natural Science Foundation of China(52272021)

National Natural Science Foundation of China(52232002)

National Natural Science Foundation of China(U23A2055)

Open/Innovation Foundation of Hubei Three Gorges Laboratory(SK232006)

Fold

Abstract

lignocellulose aerogels possess excellent properties of low density,high porosity,low thermal conductivity and so on,making them widely utilized in thermal insulation,adsorption,catalysis,electromagnetic shielding,biomedical and other fields.Moreover,as a bio-based material,lignocellulose is a green,pollution-free,renewable,and sustainable material.in this paper,the latest research progress of wood-based cellulose and agricultural waste-based cellulose aerogels are reviewed.Then the current research status of lignocellulose aerogel preparation methods including freeze-drying,supercritical drying,and atmospheric drying,is summarized.in addition,for the flammability issues commonly found in lignocellulose aerogels,commonly used methods to improve the flame retardancy of lignocellulose aerogels are discussed in detail.Finally,this paper concludes the main problems in lignocellulose aerogel preparation methods and properties,and the future development direction in this field is proposed。

Contents

1 Introduction

2 Lignocellulose aerogels

2.1 Wood-based cellulose aerogels

2.2 Agricultural waste-based cellulose aerogels

3 Preparation of lignocellulose aerogel material

3.1 Freeze-drying preparation of lignocellulose aerogels

3.2 Supercritical drying preparation of lignocellulose aerogels

3.3 Atmospheric pressure drying preparation of lignocellulose aerogels

4 Flame retardants of lignocellulose aerogel

4.1 Inorganic flame retardants

4.2 Organic flame retardants

4.3 Organic/inorganic flame retardants

5 Conclusion and outlook

Key words： aerogel; lignocellulose; drying technology; flame retardant properties

Cite this article

Wang Shuang , Zhang Xin , Sun Miao , Duan Hongjuan , Zhang Haijun , Li Shaoping . Preparation of Lignocellulosic Aerogel and Its Flame Retardant Modification[J]. Progress in Chemistry, 2024 , 36(4) : 586 -600 . DOI: 10.7536/PC230813

1 Introduction

With the rapid development of modern industry,energy consumption has increased dramatically,and the development of environmentally friendly and energy-saving thermal insulation materials has attracted wide attention[1,2]。 Aerogel is an ideal thermal insulation material because of its low density,high porosity and low thermal conductivity[3,4]。 At present,there are mainly two kinds of aerogel materials:Inorganic and polymer.inorganic aerogels,such as silica,have been commercialized,but they still have some disadvantages,such as poor toughness,long preparation cycle,low service temperature and high cost[5,6]。 Organic polymer aerogels have excellent flexibility,easy processing and low cost,but they are usually prepared by non-renewable and non-degradable materials,which have the problems of heavy pollution and difficult degradation,limiting their wide application。
lignocellulose is one of the most abundant polysaccharides in nature,which is green,pollution-free,renewable and sustainable.At the same time,there are abundant active hydroxyl groups in Lignocellulose,which can crosslink with water or other crosslinking agents to form hydrogels with three-dimensional network structure,and it is an excellent material for aerogels[7,8]。 Compared with inorganic and polymer aerogels,lignocellulose aerogels not only have excellent properties such as low density,high porosity and low thermal conductivity,but also have the characteristics of renewable and eco-friendly,which is a new generation of environmentally friendly porous materials and has attracted wide attention of researchers[9]。 However,the low decomposition temperature and flammability of lignocellulose greatly limit the practical application of lignocellulose aerogels.Therefore,it is of great significance to improve the flame retardancy of lignocellulose aerogels。
based on this,the latest research progress of wood-based and crop waste-based cellulose aerogels was reviewed in this paper.According to the different drying methods,the research status of lignocellulose aerogels prepared by freeze-drying,supercritical drying and atmospheric drying were reviewed.the methods and processes to improve the flame retardancy of lignocellulose aerogels were discussed and reviewed.Finally,the existing problems and challenges in the field of lignocellulosic aerogel materials were summarized,and the future development direction of this field was prospected。

2 Lignocellulose aerogel

2.1 Wood based cellulose aerogel

Lignocellulose mainly comes from economic crops such as wood,cotton,bamboo and grass[10]。 Among them,wood is a low-cost and green cellulose composite with a natural layered and anisotropic structure[11]。 the wood-based cellulose aerogel is prepared by treating lightweight wood to remove lignin and hemicellulose while maintaining the structure of the wood,and then freeze-drying to obtain the lignocellulose aerogel with high porosity,high strength and anisotropy。
Balsa wood is often used as a raw material for lignocellulosic aerogels because of its low density.Garemark et al.First soaked basamu in sodium chlorite solution and dimethyl sulfoxide solution in turn,completely removed The water,and then soaked it in ionic liquid such as guanidine phosphate at 65℃for 12 H.the wet gel was freeze-dried to obtain cellulose aerogel[12]。 The results show that the unique structure makes the average pore size of cellulose aerogel about 20 nm,the specific surface area up to 280 m2/g,and the yield strength up to 1.2 MPa.Due to the anisotropic structure of wood,the thermal conductivity of the sample in the radial and axial directions is 37 mW/(m·K)and 57 mW/(m·K),respectively.The ionic liquid effectively dissolves part of the cellulose in the wood and connects the pipes inside the wood with each other,and the ultrapure water soaking makes the dissolved cellulose crystallize and form an interconnected cellulose network inside the pipes of the wood 。
the synthesis process of ionic liquids is complex,polluting and expensive.Therefore,Sun et al.Soaked balsa wood(Fig.1A)in sodium chlorite solution and NaOH solution to remove lignin and hemicellulose in The wood,and then prepared cellulose aerogel by freeze-drying after washing(Fig.1D)[13]。 The results show that the cellulose aerogel(Fig.1e,1f)maintains the porous structure of the raw material itself compared with the basamu(Fig.1b,1C),and the density of the cellulose aerogel is about 31.68 mg/cm3,the pore size distribution is between 2 and 40 nm,the specific surface area is up to 32.18 m2/g,the pore volume is 0.146 cm3/g,and the thermal conductivity is 33 mW/(m·K);At the same time,the strength of the cellulose aerogel is 33.04 kPa when the deformation is 60%,and the cellulose aerogel can recover completely after the load is removed,and has excellent compressibility and resilience 。
图1 (a)天然木材的光学图片;(b)天然木材的横截面扫描电子显微镜图片;(c)天然木材的切向截面扫描电子显微镜图;(d)木材基纤维素气凝胶的光学图片;(e)木材基纤维素气凝胶的横截面扫描电子显微镜图片;(f)木材基纤维素气凝胶的切向截面扫描电子显微镜图片[13]

Fig. 1 (a) Optical picture of the natural wood; (b) Agricultural section scanning electron microscopy images of natural wood; (c) Tangential Section scanning electron microscopy images of natural wood; (d) Optical picture of lignocellulosic aerogel; (e) Cross section scanning electron microscopy images of lignocellulosic aerogel; (f) Tangential Section scanning electron microscopy images of lignocellulosic aerogel[13]

Aerogels composed of lignocellulose and metal-organic frameworks have shown great potential for the removal and separation of pollutants in water.Tian et al.used balsa wood as raw material,first used sodium chlorite and glacial acetic acid mixed solution to remove lignin,then used NaOH solution to further remove hemicellulose,after repeated washing,prepared metal-organic framework material/wood-derived cellulose aerogel by freeze-drying method[14]。 The results show that the porosity of the prepared cellulose aerogel is as high as 91.54%and the specific surface area is 110 m2/g when the loading of the amino-functionalized metal-organic framework material is 36%;The sample has excellent adsorption capacity for iodine vapor and iodine solution,and the adsorption capacity is 704 mg/G and 248 mg/G,respectively.This is because the acid and alkali treatment makes the cellulose inside the balsa wood more fully exposed,forming a more loose layered structure 。
Agate rattan is rich in cellulose resources and can also be Used as a raw material for lignocellulose aerogel.Han et al.used manaoteng as raw material,delignified with 2 wt%sodium chlorite solution buffered with acetic acid,then further treated with NaOH solution for 7 H to remove hemicellulose,and finally freeze-dried to obtain cellulose aerogel[15]。 Silane-modified cellulose aerogels were successfully obtained by spraying 2%1 H,1 H,2 H,2 H-perfluorodecyltriethoxysilane solution onto the aerogel surface with a spray gun and drying at 105℃for 24 H.The density of the prepared cellulose aerogel is only 15 mg/cm3,and the porosity is 90.12%;The aerogel exhibits excellent hydrophobic and oleophobic properties,with an oil contact angle of 154°and a water contact angle of 164.1°;The prepared cellulose aerogel has a radial thermal conductivity of 30 mW/(m·K)and an axial thermal conductivity of 56 mW/(m·K),showing anisotropic heat transfer performance 。
Based on the anisotropic structure of wood itself,lignin and hemicellulose can be removed by special methods to prepare lignocellulosic aerogels with high porosity,low density and anisotropy.However,this process needs to destroy the internal structure of wood to achieve the purpose of extracting lignocellulose,and only freeze-drying process can be used to prevent the structural collapse of wet gel during drying.In addition,there are special requirements for the initial wood,such as low density,which makes the source of raw materials narrow.Moreover,the wood is not easy to process,which greatly limits the practical application of the process。
Compared with wood-based cellulose aerogel,wood pulp,which has been industrially treated to remove lignin,hemicellulose and other impurities from wood itself,has the advantages of easy processing and unlimited drying methods,and is also Used by researchers as a raw material for aerogel preparation.Dong Hao et al.used wood pulp bleached by sulfate as raw material and multi-walled carbon nanotubes as additives to prepare nanocellulose filaments by tetramethylpiperidine free radical oxidation,and then prepared cellulose/multi-walled carbon nanotubes carbon aerogel by freeze-drying and high temperature carbonization[16]。 The results show that the volume density of the prepared cellulose/multi-walled carbon nanotube carbon aerogel is 0.10 g/cm3,and the introduction of multi-walled carbon nanotubes significantly improves the electrochemical performance of the aerogel,the working voltage increases from 0.4 V to 0.6 V,the internal resistance decreases from 1.07Ωto 0.98Ω,and the specific capacitance increases from 172.45 F/G to 238.89 F/G(current density of 1 A/G),which can be used in the field of supercapacitors 。
in order to further reduce the density of lignocellulose aerogel,Song et al.First oxidized wood pulp fibers with tetramethylpiperidine,then added sodium borohydride,and then reacted In HCl for a period of time,followed by washing with deionized water,solvent exchange with ethanol and tert-butanol,and finally freeze-drying to obtain cellulose aerogel[17]。 The prepared cellulose aerogel has a porosity of up to 98.9%,a density of 16.3 mg/cm3,a specific surface area of 131.5 m2/g,and a tensile strength of up to 12.18 MPa.However,this method requires two solvent exchanges and freeze-drying of the alcogel,which is difficult to prepare and requires high equipment.Li et al.Obtained cellulose aerogel by atmospheric pressure drying using wood pulp fiber as raw material and sodium lauryl sulfate as additive[18]。 The results show that the prepared cellulose aerogel has a Young's modulus of 45.6 kPa,a density of 21.2 mg/cm3,and a porosity of 97.2%,which is due to the network structure formed by hydrogen bonding between fibers 。
aerogel with different shapes can be prepared by using industrial wood pulp as a raw material,and the preparation raw material of the wood pulp is not limited,so that most wood can be used as the raw material to prepare the Aerogel.However,the preparation process of wood pulp will produce a lot of pollution,and its treatment process is more complex。

2.2 Crop waste based cellulose aerogel

Agricultural wastes such as corn straw,rice straw,wheat straw,bagasse and corncob are also rich in lignocellulose,which are high-quality raw materials for the preparation of lignocellulose aerogels,but there are few related studies at present[19⇓⇓~22]。 At present,only a small part of these agricultural wastes are used for feed or making paper and ethanol,and most of them are used as fuel or discarded,causing a certain degree of harm to the environment。
Chen Kouqin et al.First soaked peanut shell powder in benzene-ethanol solution to remove the oil layer,then used acid-base treatment to remove the lignin and hemicellulose of peanut shell powder,and finally put it into an ultrasonic cell pulverizer for 1 H to obtain peanut shell cellulose[23]。 Then,they successfully prepared cellulose aerogel by freeze-drying method with methyltrimethoxysilane as modifier.The results show that the density of the prepared cellulose aerogel is 26 mg/cm3and the porosity is 98.4%.The hydrophobic angle can reach 152°,and it has super-hydrophobic properties,indicating that methyltrimethylsilane is successfully coated on the surface of cellulose aerogel.Although this method achieves the purpose of preparing aerogel from peanut shell,the treatment method is too complicated 。
in order to further shorten the experimental steps,Shi Tianlin et al.used poplar catkins as raw materials,first used chloroform to remove the waxy layer on the surface of poplar catkins,then used sodium chlorite solution and NaOH solution In turn to obtain cellulose suspension,and then used ultrasonic treatment for 5 min,and finally used freeze-drying method to prepare porous cellulose aerogel with honeycomb three-dimensional structure[24]。 The results show that the density of the prepared cellulose aerogel is 3.4 mg/cm3,and its porosity is as high as 99.77%,which shows the great advantages of poplar catkin fiber in the field of high value and low energy consumption 。
Chhajed et al.First used toluene/ethanol mixed solution to remove the wax on the surface of rice straw,then used sodium chlorite and sodium hydroxide solution to remove lignin and hemicellulose,and then prepared light(12 mg/cm3)and porous(99.5%)cellulose aerogel after non-directional freeze-drying at-40℃[25]。 They further sprayed a chloroform solution containing polylactic acid on the surface of the sample and air-dried it to obtain modified cellulose aerogel.The results show that the modified aerogel has a density of 22 mg/cm3,a specific surface area of 150 m2/g,an adsorption capacity of 28 G/G for n-hexane and 70 G/G for chloroform,and a compressive strength increased by 7.2 times(326 kPa,80%deformation)compared with the unmodified aerogel.At the same time,the hydrophobic angle of the sample is about 114°,which shows excellent hydrophobicity 。
Similarly,Wang et al.Mixed wheat straw powder with konjac glucomannan,gelatin and potato starch,and prepared composite cellulose aerogel by freeze-drying,thus avoiding the pretreatment of wheat straw[26]。 The results show that the density of the prepared cellulose aerogel is 43.3 mg/cm3,the porosity is 94.5%,and the thermal conductivity is 41.4 mW/(m·K).At the same time,the addition of wheat straw powder increased the pore size in the range of 10~50μm from 48.86%to 66.98%,which significantly reduced the pore size of composite cellulose aerogel 。
cellulose in agricultural wastes is usually wrapped by lignin and hemicellulose,which needs to be pretreated to remove hemicellulose and lignin to obtain relatively pure lignocellulose,and then Cellulose aerogel is prepared by appropriate process to achieve the purpose of high value-added utilization of agricultural wastes.However,the treatment process often causes a lot of pollution,hemicellulose and lignin can not be effectively utilized,and the preparation process of aerogel is too complex.in addition,the sources of most agricultural wastes are scattered,and it takes a lot of manpower and material resources to concentrate them,so the preparation of lignocellulose aerogel from agricultural wastes is still in the research stage。

3 Method for drying lignocellulose aerogel

the preparation of lignocellulose aerogel includes three processes:the dispersion of cellulose,the formation of gel and the drying of wet gel.After gelation,a stable three-dimensional network structure is formed,and the liquid solvent filled in the structure needs to be replaced by gas,while maintaining the original structure,so as to obtain aerogel.Therefore,the most critical step in the preparation of lignocellulosic aerogels is the selection of an appropriate drying technique[27]。 in the process of solvent removal,there are three common drying methods:freeze drying,supercritical drying and atmospheric drying.At present,the physical properties of lignocellulose aerogels prepared by these three methods,such as raw materials,density,porosity,compressive strength and thermal conductivity,are shown In Table 1。
表1 Comparison of physical properties of cellulose aerogels prepared by different methods

Table 1 Comparison of physical properties of cellulose aerogels prepared by different methods

Preparation method Raw materials Density
(mg/cm3)		 Porosity
(%)		 Compressive
strength(kPa)		 Specific surface area(m2/g) Thermal conductivity
(mW/(m·K))		 ref
Freeze-drying method MC, cyanuric acid 86.0 98.2 - - - 30
Cellulose, SH, BTA 68.0 99.8 11.9(50%) 73.9 - 31
CNF 38 97.6 305(85%) - 32
Cellulose 30.0 - - 156.7 - 34
Cellulose, polyethylene glycol - - - 3.9002 - 35
Cellulose, citric acid 85.0 85.0 - - - 36
Cellulose, PD, PL 25.0 98.5 - - - 37
BWP 7.0 99.5 15.2(50%) - - 40
BWP 47.6 99.4 24.5(70%) - - 42
Wood fiber, carbon nanotubes - - - 377.9 - 43
Cellulose, recumbent 11.0 99.0 - - - 46
Cellulose, chitosan 8.4 98.0 99.3(60%) - 28.0 47
Cellulose, chitosan 9.4 99.3 321.0(50%) - 28.0 49
CN, CMC 10.0 99.0 - - - 50
Cellulose, polyimide 46.0 97.7 - - 23.0 55
BP, polyvinyl alcohol, GO 10.2 99.4 40(80%) 23.4 - 56
Supercritical drying Cellulose 33.0 - - 400.0 28.0 64
Cellulose 15.0 99.0 37.5(50%) 500 65
Cellulose 39.4 96.4 200.0(50%) 385.0 - 66
Cellulose, ML 90.0 93.6 660.0 23.0 67
Atmospheric pressure drying method Cellulose, silane 54.0 - 210.12(20%) - 37.0 69
Cellulose 58.8 98.0 70.0(50%) 22.4 - 70
Cellulose, polylactic acid, starch 88.0 93.1 81.2(50%) - - 71
WP, borax 16.4 98.0 74.1(50%) - 45.0 72
WP, borax, chitosan 52.7 94.5 485.0(50%) - 68.0 73

(MC：microcrystalline cellulose;SH：sodium hypophosphite;BTA：butane tetracarboxylic acid;CNF：cellulose nanofibers;PD：polydopamine;PL：polyethyleneimine;BWP：bleached wood pulp;CN：carbon nanotubes;CMC：carboxymethyl cellulose;BP：bamboo powder;GO：graphene oxide;ML：methyltrimethylsilane;WP：wood pulp.)

3.1 Preparation of Lignocellulose Aerogel by Freeze-drying

Freeze-drying is the most widely used drying process in the preparation of lignocellulosic aerogels,by freezing the lignocellulosic wet gel to a solid state,and then sublimating the solvent(mainly water)in the inner gel directly from the solid state to a gaseous state at a certain low pressure and temperature,and maintaining the porous structure of the aerogel[28]。 Freeze-drying is the most commonly used method to remove the solvent from lignocellulose wet gel,which can reduce the capillary force by eliminating the difference between gas and liquid phases,and can ensure that the prepared sample has the characteristics of high porosity and low bulk density[29]。
Zhang et al.Used nanocrystalline cellulose as raw material,cyanuric chloride as modifier,and chloropropyltriethoxysilane as crosslinking agent to prepare cellulose aerogel by freezing in a refrigerator for 12 H,and then freeze-drying[30]。 The results show that the density of the prepared sample is only 86 mg/cm3,the porosity is as high as 98.2%,and the hydrophobicity is excellent due to the crosslinking effect of chloropropyltriethoxysilane.Although the presence of cyanuric acid endows aerogels with excellent antibacterial properties,the modification of cyanuric acid and the preparation of nanocrystalline cellulose are still too complex 。
In order to solve the problem of complex preparation process of nanocrystalline cellulose,Liu Yanbo et al.First prepared cellulose suspension,and then added butane tetracarboxylic acid and sodium hypophosphite as binder and catalyst respectively.After stirring for 6 H,the mixture was frozen in the refrigerator,thawed and replaced with tert-butyl alcohol,and then frozen in the refrigerator.The freeze-dried sample was cross-linked in the oven to obtain cellulose aerogel with a density of 68 mg/cm3and a porosity of 99.8%[31]。 The prepared cellulose aerogel has an average pore size of 2.2 nm and a specific surface area of 73.9 m2/g.The sample can fully recover after 50%compression,and its compressive stress is 11.9 kPa.The adsorption capacity of the aerogel for paraffin oil is as high as 115.4 G/G.However,the preparation process of the cellulose aerogel needs to be replaced by tert-butyl alcohol,the preparation process is complex,and the requirements for freeze-drying equipment are also high 。
In order to solve the problem of complex tert-butyl alcohol replacement process,Chen Xiaoxing et al.Mixed cellulose nanofibers,wet strength agent and deionized water,frozen them in a refrigerator,and then dried them in a freeze dryer to successfully prepare cellulose aerogel with a density of 38 mg/cm3and a porosity of 97.6%[32]。 When the compression of aerogel is 85%,its strength is 305 kPa;in addition,the results also show that compared with conventional freezing,the cellulose aerogel prepared by rapid freezing In liquid nitrogen has lower specific surface area,but its structure is more uniform and its pore size distribution is relatively narrow[33]。
Liu Xiaoting et al.Used liquid nitrogen freezing to improve the freezing rate of cellulose wet gel[34]。 They first dissolved cellulose in 1-allyl-3-methylimidazolium chloride ionic liquid,then put it into acetic acid solution,and repeatedly obtained cellulose wet gel,then frozen the wet gel in liquid nitrogen and dried it for 48 H,and finally successfully prepared cellulose aerogel.The results show that the prepared cellulose aerogel has a rich pore structure,its density is lower than 30 mg/cm3,and its specific surface area is as high as 156.7m2/g 。
Similarly,Wang Zepeng et al.prepared cellulose/polyethyleneimine aerogel by compounding nanocellulose with polyethylene glycol,sonicating in an ice bath,freezing with liquid nitrogen,and then freeze-drying[35]。 With the addition of polyethylene glycol,the specific surface area of cellulose/polyethyleneimine aerogel decreased from 6.17 m2/g to 3.9 m2/g,the average pore size decreased from 16 nm to 3.9 nm,and the tensile strength increased to 1.5 MPa.It improves the mechanical properties of aerogel to a certain extent,and also provides a new idea for the filtration of PM2.5 。
In order to further improve the mechanical properties of lignocellulose aerogel,Wang et al.First prepared oxidized cellulose nanofibers using tetramethylpiperidine as an oxidant,and then usedε-polylysine as an antibacterial agent.ε-polylysine was grafted onto nanocellulose by carbodiimide coupling reaction at room temperature using 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and N-hydroxysuccinimide as activators[36]。 The modified cellulose was mixed with binder citric acid at 80℃,and the wet cellulose aerogel was prepared by esterification and crosslinking reaction between citric acid and the hydroxyl groups of cellulose.The wet cellulose aerogel was frozen in liquid nitrogen to form pores,and then dried in a freeze dryer for 48 H to obtain light,（85 mg/cm3）and porous(85%)cellulose aerogels.The results show that the mechanical properties and antibacterial properties of aerogel can be significantly improved by usingε-polylysine as antibacterial agent and citric acid as crosslinking agent.Compared with the blank sample,the tensile strength of the aerogel increased by 2 times(26.44 MPa)when the addition ofε-polylysine was 0.8 wt%,and the sample also had more than 99.9%antibacterial efficiency.Although this method can make the aerogel have excellent mechanical properties and antibacterial properties,the treatment process of raw materials is too complicated 。
The polydopamine coating not only provides abundant binding active sites,but also contains functional groups for further reaction of polyethyleneimine.Based on this,Tang et al.First grafted polydopamine onto the surface of cellulose,then crosslinked it with polyethyleneimine to form cellulose wet gel,and finally freeze-dried it to obtain cellulose aerogel with a density of only 25 mg/cm3,a porosity of up to 98.5%and a compression resilience[37]。 The maximum adsorption capacity of the prepared cellulose aerogel for methyl orange and Cu2＋ions was 265.9 mg/G and 103.6 mg/G,respectively,indicating excellent performance 。
Although freezing in liquid nitrogen can guarantee the prepared samples with sufficiently high porosity and small pore size,the resulting disordered structure can hinder the directional heat transfer,electrical conduction,and other functional applications of aerogels[38,39]。 In recent years,researchers have adjusted the directional growth of ice crystals by selecting the appropriate cold source(liquid nitrogen or cold ethanol),and then changed the pore structure of lignocellulose aerogel to make it have better performance[40]。 Zhou et al.used cellulose as the raw material,polyvinyl alcohol as the binder,and graphene oxide as the reinforcing agent to adjust the pore structure of cellulose aerogel by liquid nitrogen directional freezing(Fig.2a),liquid nitrogen non-directional freezing(Fig.2b),and refrigerator freezing(Figs.2C)[41]。 the results show that the aerogel prepared by liquid nitrogen directional freezing has an anisotropic porous structure,and its compressive strength at 80%strain is 0.22 MPa,which is 2 times that of refrigerator freezing and 1.5 times that of liquid nitrogen non-directional freezing,respectively.They further modified the three kinds of cellulose to prepare hydrophobic aerogels,and the existence of oriented structure made the aerogels show faster adsorption rate and larger adsorption capacity for a variety of oils and organic compounds。
图2 (a)定向冷冻干燥示意图;(b)非定向冷冻干燥示意图;(c)冰箱冷冻干燥示意图[41] (纤维素纳米纤维(CNF);聚乙烯醇(PVA);氧化石墨烯(GO))

Fig. 2 Schematic representation of (a) the directional freeze-drying method;(b) the non-directional freeze-drying method and (c) the refrigerator freeze-drying method[41] (Cellulose nanofiber (CNF); polyvinyl alcohol (PVA); graphene oxide (GO))

In another study,Chen et al.First bleached wood pulp with tetramethylpiperidine as an oxidant to obtain oxidized fiber nanocellulose,then directionally and non-directionally frozen it in ethanol at-30℃,and finally freeze-dried it for 48 H to obtain cellulose aerogel with high porosity(99.5%)and low density（7 mg/cm3）[40]。 the results show that the strength of cellulose aerogel prepared by Directional freezing is 15.2 kPa when the deformation is 50%,which is 1.5 times that of non-directional freezing drying.the aerogel has a faster longitudinal liquid transport velocity(4.95 mm/s,3 times that of non-directional drying).directional freeze-drying creates a bottom-up temperature gradient during freezing,allowing solvent ice crystals to grow in this direction.in this process,the cellulose in the solution is squeezed out between the ice crystals and the solid-liquid separation is realized,and finally the cellulose aerogel with a layered structure is prepared by using a freeze-drying machine,which presents a honeycomb structure in the transverse direction and an oriented tunnel structure in the longitudinal direction,and the unique structure endows the oriented frozen sample with more excellent mechanical properties。
Lignocellulose aerogel has also been used in the field of energy storage.Chen et al.First used tetramethylpiperidine to oxidize and bleach wood pulp to prepare oxidized cellulose nanofibers,and added silver nanowire suspension,then directionally frozen in liquid nitrogen,and then freeze-dried to prepare cellulose aerogel[42]。 The density of the prepared cellulose aerogel is 47.6 mg/cm3,the porosity is 99.39%,and the average pore size is 23.9μm;When the deformation is 70%,the compressive strength of the aerogel is 24.5 kPa,and the aerogel also has excellent conductivity 。
Similarly,Xueqi Chen et al.Used eucalyptus fibers and multi-walled carbon nanotubes as raw materials to prepare composite conductive carbon aerogel with specific surface area up to 377.9 m2/g by liquid nitrogen directional freezing,freeze-drying and high temperature heat treatment in turn[43]。 the aerogel has excellent conductivity and high specific capacity,and the capacitance retention rate is as high as 131.7%after 5000 charge-discharge cycles(current density 2 A/G).the composite conductive carbon aerogel material has a three-dimensional crosslinked and interlinked structure,and a large number of holes exist in the interior of the composite conductive carbon aerogel material,so that the morphology and the structure of the composite conductive carbon aerogel material are similar to those of a wood tracheid.At the same time,a large number of nitrogen-doped carbon microvilli are grown in situ on the aerogel wall under the induction of the multi-walled nanotubes,and the carbon microvilli rich in N and O elements greatly improve the specific surface area and the defect content of the carbon aerogel,which is more beneficial to the application of the carbon aerogel in the field of electrochemical energy storage。
Lignocellulose aerogel has also been used in the field of adsorption.Li et al.Successfully prepared cellulose aerogel with highly oriented microchannel structure by mixing cellulose suspension with chitosan solution,directional freezing in liquid nitrogen,and drying in a freeze dryer for 24 H[44]。 the maximum adsorption capacity of the prepared cellulose aerogel for Pb(Ⅱ)was 252.6 mg/G,and the removal rate of Pb(Ⅱ)was 85%even after five adsorption cycles,indicating that the prepared cellulose aerogel had excellent adsorption performance for Pb(Ⅱ]。
Rectorite has large interlayer spacing,large specific surface area,good hydrophilicity and cation exchange characteristics,and shows great potential in the field of wastewater treatment[45]。 Chen et al.Used cellulose and rectorite as raw materials to make the low-temperature cold source(ethanol at-50℃)only contact with the bottom of the flat-bottomed glass mold,so that ice crystals grow along the bottom-up temperature gradient.Finally,the anisotropic cellulose nanofiber composite aerogel was prepared by freeze-drying,and its saturated adsorption capacity and removal rate for methylene blue were 120.0 mg/G and 96.1%,respectively,which were significantly better than those of pure cellulose nanofiber aerogel and rectorite powder[46]。 the introduction of the rectorite powder enhances the adsorption of the aerogel to the dye,and the cellulose nanofiber can make the dispersion of the rectorite powder more uniform,which is beneficial to the full contact between the rector and the dye molecules,thereby improving the adsorption capacity of the composite aerogel to the methylene blue。
In order to prepare lignocellulose-based aerogels with low density,good mechanical elasticity and excellent adsorption properties,Zhang et al.Prepared cellulose aerogels by directional freeze-drying process using cellulose and chitosan as raw materials and glutaraldehyde as crosslinking agent[47]。 They first used liquid nitrogen directional freezing,then freeze-dried for 48 H,and finally dried in vacuum at 110℃/1 H to obtain hydrophobic cellulose aerogel.The prepared cellulose aerogel has a density of 8.4 mg/cm3and a porosity of 98%;Even after 100 cycles at 60%strain(99.3 kPa),the aerogel does not undergo significant permanent deformation;At the same time,the transverse and axial thermal conductivities of aerogel are 28 mW/(m·K)and 36 mW/(m·K),respectively,showing anisotropic low thermal conductivity.The aerogel has a hydrophobic angle of 148°and an oil absorption capacity of 82~253 G/G,and shows excellent selectivity and high absorption capacity for various oils and organic solvents 。
Electromagnetic shielding is also one of the application areas of lignocellulosic aerogels.Chen et al.Successfully prepared an anisotropic composite aerogel composed of cellulose nanofibers and silver nanowires by coating Fe3O4nanoparticles on the surface of silver nanowires using a chemical co-precipitation method with cellulose nanofibers as raw materials[48]。 The composite aerogel has low density(16.76 mg/cm3),good saturation magnetization(4.21 emu/G)and electrical conductivity(0.02 S/cm);In addition,the aerogel has anisotropic electromagnetic shielding ability.This is due to the synergistic effect of cellulose nanofibers,silver nanowires and Fe3O4.However,the mechanical properties of the prepared aerogels need to be further improved 。
Polymer coating can significantly improve the stability of lignocellulosic aerogel structure and change its surface wettability.Zhang et al.Prepared cellulose aerogel by directional freeze-drying method with 3-mercaptopropyltrimethoxysilane instead of methyltrimethylsilane[49]。 They first soaked the dried cellulose aerogel in cyclohexane solution containing polydimethylsiloxane,trimethylolpropane triacrylate and photoinitiator for 30 min,then took it out and irradiated it with ultraviolet for 30 s,and successfully prepared cellulose aerogel with light（9.42 mg/cm3）,high porosity(99.29%)and superhydrophobicity(168.0°).The results show that the axial strength of the modified sample is 3 times higher than that of the unmodified sample(321 kPa).When the deformation is 50%,the permanent deformation of the aerogel is only 12.2%even after 1000 cycles of compression.The sample has anisotropic thermal conductivity of about 28 mW/(m·K)in the horizontal direction and about 38 mW/(m·K)in the axial direction.Under the action of ultraviolet radiation,the—SH functional group in the cellulose aerogel and the—C=C functional group in the solution undergo a thiol-ene click reaction(Fig.3),which significantly improves the mechanical properties of the aerogel 。
图3 紫外线诱导的硫醇-烯点击反应过程[49]

Fig. 3 Reaction process of the UV-induced thiolene click reaction[49]

Hajian et al.Further improved the mechanical properties of cellulose aerogels by adding carbon nanotubes[50]。 They first added 50 wt%of carbon nanotubes to the prepared 10 G/L carboxymethyl cellulose solution,mixed them evenly and then directionally frozen them in liquid nitrogen,and finally successfully prepared cellulose aerogel by freeze-drying for 48 H.The prepared sample has high porosity(99%)and low density(10 mg/cm3)by the physical crosslinking of carbon nanotubes and carboxymethyl cellulose solution;When the deformation of aerogel is 90%,there is no obvious permanent deformation after 200 compression cycles,showing excellent compressibility and resilience 。
In order To further adjust the microstructure of lignocellulose aerogel,researchers placed a low thermal conductivity wedge between the suspension and the cold source,such as polydimethylsiloxane polymer.to achieve the horizontal and vertical temperature gradients generated simultaneously during the freezing process of the wet gel,so that the aerogel has a unique layered structure(Figure 4A),and its performance is more excellent[51⇓⇓~54]。 Zhang et al.Successfully prepared cellulose aerogels by non-directional freeze-drying,single-directional freeze-drying and double-directional freeze-drying,respectively,using cellulose as raw material and polyimide as binder[55]。 The results show that the bidirectionally frozen sample has a density of 46 mg/cm3and a compressive modulus of 3 MPa,and the radial thermal conductivity of the aerogel is only 23 mW/(m·K)due to the special layered structure formed by bidirectionally freezing and drying,which is significantly lower than that of the non-directionally frozen sample[42 mW/(m·K)]and the unidirectionally cooled sample[66 mW/(M·K)],showing more excellent thermal insulation performance 。
图4 (a)双向冷冻技术示意图[55];(b)纤维素气凝胶合成示意图[56]

Fig. 4 (a) Schematic illustration of the bidirectional freeze- drying fabrication process[55]; (b) Process flow chart of synthesizing cellulose aerogel[56]

Similarly,Zhang et al.Used bamboo powder as raw material and treated it with sodium hydroxide and sodium chlorite in turn to completely remove lignin and hemicellulose[56]。 Cellulose aerogel with ultra-low density,（10.2 mg/cm3）,high porosity(99.4%)and hydrophobic(hydrophobic angle 140°)was successfully prepared by bi-oriented freeze-drying for 48 H,using tetramethylpiperidine as oxidant,polyvinyl alcohol as binder,and methyltrimethylsilane as modifier,and then drying the obtained sample in an oven at 90℃for 3 H.The specific surface area of the prepared cellulose aerogel is as high as 23.4 m2/g,and the prepared cellulose aerogel has excellent adsorption capacity for various oils and organic matters;It shows high strength in the axial direction(40.3 kPa when the deformation is 80%),excellent compression resilience in the transverse direction,and no obvious permanent deformation is observed after 100 times of repeated compression resilience under the condition of 70%deformation,which is due to the layered structure of the prepared cellulose aerogel 。
Although freeze-drying can control the pore structure of cellulose aerogels by adjusting the temperature gradient,the number of mesopores in the prepared samples is small,which makes the thermal insulation performance of aerogels relatively poor[57]。 At the same time,the freeze-drying method has the disadvantages of long preparation cycle,complex process and difficult to prepare large-size samples,which limits the practical application of this method[58]。

3.2 Preparation of Lignocellulose Aerogel by Supercritical Drying

supercritical drying is a mature technology for the preparation of aerogels.By controlling the pressure and temperature,the solvent in the lignocellulose gel is transformed into a Supercritical state.in this state,the solvent in the aerogel is transformed into a fluid without liquid-gas phase distinction and discharged from the gel,and finally the aerogel with complete structure is obtained[59]。 Supercritical drying of wet gel is the most important step in aerogel production because it is able to maintain the three-dimensional pore structure of the gel,giving it properties such as high porosity,low density,and large surface area[60]。 Compared with freeze-drying,supercritical drying can better maintain the initial mesoporous structure of lignocellulose wet gel,further increase its specific surface area and reduce its thermal conductivity[61]。
Chang Huanjun et al.First prepared cellulose aerogel from carbon nanotubes and cellulose by freeze-drying and supercritical drying,respectively[62]。 The results show that the specific surface area of the aerogel prepared by supercritical drying is 5.6 times higher(266 m2/g)and the pore volume is 5.13 times higher(0.77 cm3/g)than those of the freeze-dried sample.Compared with the layered structure formed by freeze-drying(caused by the directional growth of ice crystals),the cellulose aerogel prepared by supercritical drying shows a filamentous porous structure,and its pore structure is mainly mesoporous,while the freeze-dried sample is mainly macroporous 。
Because there are abundant hydroxyl groups on the surface of lignocellulose,the modified functional groups can be combined on the surface of cellulose through chemical processes,and then the cellulose aerogel can be directly modified.Wang et al.Prepared cellulose aerogels by freeze-drying and supercritical drying,respectively,using microcrystalline cellulose as raw material,3-(2-aminoethylamine)propylmethyldimethoxysilane as modifier,and calcium chloride as binder[63]。 The results show that compared with freeze-drying,the specific surface area of cellulose aerogel prepared by supercritical drying increases from 120 to 262 m2/g,the pore volume increases from 0.47 to 1.08 cm3/g,and the average pore size decreases from 17.7 to 10.4 nm.In addition,they found that the cellulose aerogel obtained by supercritical drying showed an interconnected 3D network nanostructure,the size of the pores was more uniform,and the shrinkage of the specimen was smaller 。
Similarly,Sivaraman et al.First used tetramethylpiperidine to oxidize cellulose and subsequently employed supercritical drying to prepare cellulose aerogel with a density of only 33 mg/cm3,a thermal conductivity as low as 28 mW/(m·K),and a specific surface area of 400 m2/g[64]。 Plappert et al.Prepared cellulose aerogel by supercritical drying[65]。 The results show that the specific surface area of the prepared cellulose aerogel is up to 500 m2/g,the porosity is 99%,and the prepared cellulose aerogel is hydrophobic(hydrophobic angle is 119.4°).The results of Lin et al.Showed that when the solid content was 4 wt%,the cellulose aerogel prepared by supercritical drying process had a porosity of 96.4%,a density of 39.4 mg/cm3,a specific surface area of 385 m2/g,a deformation of 50%,and a compressive strength of 0.2 MPa,showing excellent strength[66]。
The performance of cellulose aerogel can be further improved by constructing a secondary network inside the lignocellulose aerogel through polymerization.Zhang et al.Constructed a SiO2network in cellulose aerogel by adding methyltrimethylsilane[67]。 They boiled the prepared fiber hydrogel in NaOH solution for 1 H,washed it with distilled water and ethanol,and then dried it by supercritical drying to obtain cellulose aerogel.The composite cellulose aerogel was soaked in a mixed solution of acetic acid,urea,cetyltrimethylammonium bromide and methyltrimethylsilane at 80℃for 2 d,washed with ethanol,and then dried by supercritical drying.The results show that the density of the prepared cellulose aerogel is 90 mg/cm3,the porosity is 93.6%,and the pore volume is 10.4 cm3/g;At the same time,the sample also has excellent flexibility(when the radius of curvature is 1 mm,it does not break),good mechanical strength(it does not break under a load of 2.7×104times its own weight)and high thermal insulation performance(the thermal conductivity is 23 mW/(m·K)).Compared with the unmodified cellulose aerogel,the specific surface area of the modified composite aerogel increases from 135 to 660 m2/g;The average pore size decreased from 2942 nm to 22 nm.The composite aerogel also exhibits excellent hydrophobicity with a water wetting angle of 139°.This is due to the polymerization of methyltrimethylsilane inside the cellulose aerogel to form a"hard"secondary network,which synergizes with the initial network of cellulose aerogel to improve the performance of cellulose aerogels 。
Although supercritical drying can maintain the mesoporous structure of lignocellulose aerogel to the greatest extent and prepare lignocellulose aerogel with excellent performance,there are still many problems in the actual production:(1)the equipment used in the supercritical drying process is expensive,the equipment used is an autoclave,and the airtightness requirement is high;(2)the experimental cycle of supercritical drying process is long,the output is low,and the cost is high;(3)the process of supercritical drying is complicated,and the solvent needs to be replaced before the drying of wet gel.These shortcomings have severely limited the development of lignocellulosic aerogels prepared by supercritical drying,making it only suitable for the preparation of a small amount of high-performance products。

3.3 Preparing lignocellulose aerogel by atmospheric pressure drying

The solvent in the lignocellulose aerogel network gel structure is mainly water,which has a large surface tension.As shown in Fig.5,during solvent evaporation,large capillary pressures(F1,F2,F3,and F4)are generated,The resultant forces(F1',F2',F3',and F4')act on the pore wall between the gas-liquid interface in the horizontal direction,which leads to the collapse and destruction of the pore structure[68]。 in order to achieve atmospheric pressure drying,it is necessary to replace water with a solvent with low surface tension,or add some oil-soluble surfactant to reduce the surface tension,thereby reducing the pressure of the internal capillary.Hasan et al.Used cellulose as raw material and silane as modifier to soak the obtained cellulose aerogel in ethanol solution by impregnation method,and replaced the water in the gel with acetone to reduce the surface tension of the solvent,and finally prepared cellulose aerogel[69]。 Moreover,they also modified aerogel with polysiloxane particles to prepare hydrophobic cellulose aerogel with water contact angle greater than 130 degrees,which provides a new idea for large-scale preparation of cellulose aerogel in an environmentally friendly,economical and efficient manner。
图5 干燥过程中气凝胶孔壁表面毛细作用力示意图[70]

Fig. 5 Schematic diagram of capillary force on the pore wall of aerogel material during drying[70]

in addition,the collapse of aerogel during atmospheric pressure drying can also be reduced or eliminated by enhancing the connection of pore networks.Li et al.Used glycyloxypropyltrimethoxysilane and branched polyethyleneimine to improve the connection strength between cellulose,and replaced water In the gel with acetone to reduce the surface tension of the solvent[70]。 The silicon hydroxyl groups generated by the hydrolysis of glycyloxypropyltrimethoxysilane are crosslinked with the hydroxyl groups on the surface of cellulose,and at the same time,the epoxide groups of glycyloxypropyltrimethoxysilane and the amine groups of branched polyethyleneimine have strong interaction,thereby increasing the crosslinking density of the gel network.This improved cross-linked 3D network helps the gel pore walls resist the stress generated by capillary pressure during drying.Compared with the freeze-dried sample,the cellulose aerogel prepared by atmospheric drying has similar density(58.82 g/cm3),large specific surface area（22.4 m2/g,which is increased by 3.3 times)and similar compression resilience(when the deformation is 50%,there is no obvious permanent deformation after five times of compression resilience).However,this method still requires the use of low surface energy substances to replace water in order to reduce the structural collapse of aerogel due to capillary pressure during atmospheric drying 。
The researchers also Prepared lignocellulosic aerogels by atmospheric pressure drying through measures such as building stronger connections and increasing pore size.Zhang et al.prepared cellulose aerogel by foaming process to produce large pore size,using polylactic acid as binder to achieve firm connection,and finally drying under normal pressure[71]。 They first heated and gelatinized the prepared starch solution,then added paper fiber and poly yoghurt powder in turn,stirred them evenly,and then added sodium lauryl sulfate to foam for 10 minutes.The obtained wet foam was then dried at 60℃for 6 H and then at 150℃for 20 min.The density of the prepared cellulose aerogel is 88 mg/cm3,the porosity is 93.18%,and the compressive strength is 81.24 kPa;However,the degradation rate of the aerogel was as high as 95%after 56 days at room temperature,which could not meet the daily use 。
in order to further solve the problems of poor strength and easy degradation of lignocellulose aerogel,Zhao et al.Used the phenomenon that higher plants In nature improve their strength by crosslinking with trace borate and polysaccharide.Cellulose aerogel was successfully prepared by ambient pressure drying method using bleached softwood pulp as raw material,borax as crosslinking agent and sodium lauryl sulfate as foaming agent[72]。 They first dried and crushed the pulp fibers and prepared a pulp suspension with a concentration of 1.5 wt%,then added borax and sodium dodecyl sulfate in turn,filtered through a Buchner funnel to remove excess water,and then dried overnight at ambient temperature and placed at 90℃for 12 H.The results show that the cellulose aerogel prepared by ambient drying has no obvious volume shrinkage due to the crosslinking effect of borax and fiber,and its porosity is as high as 98%,density is only 16.4 mg/cm3,and thermal conductivity is 45 mW/(m·K).Compared with the sample without borax,the compressive strength of the sample without borax is increased by 28 times(74.1 kPa,when the deformation is 50%),and the sample has better flame retardancy and excellent self-extinguishing property.However,the sample still has some shortcomings,such as poor antibacterial performance,low strength,and can not meet the daily use 。
On the basis of boron ion crosslinked cellulose,Wu et al.Added chitosan and polyacrylamide as reinforcing agents to further improve the performance of cellulose aerogel[73]。 The results show that cellulose aerogel has high strength(compressive stress of 485 kPa at 50%strain)and low density(（52.65 mg/cm3）)and high porosity(96.49%)due to the crosslinking effect of chitosan and polyacrylamide with cellulose.The thermal conductivity of the prepared cellulose aerogel is 68 mW/(m·K),and the prepared cellulose aerogel has the properties of flame retardancy,sound insulation,antibiosis and the like 。
aerogel was prepared By solvent exchange method with low surface energy substance to replace the water in lignocellulose hydrogel,and then dried at atmospheric pressure,which greatly saved the preparation cost.However,this method is expensive and harmful to human body.by adopting a firmer crosslinking method and increasing the pore size of the lignocellulose wet gel,the connection between lignocellulose networks can be enhanced,the effect of capillary force during drying is reduced,the purpose of normal-pressure drying without solvent exchange is achieved,the preparation process is further simplified,and the preparation cost is reduced;However,the large pore size of lignocellulose Aerogel will affect its thermal conductivity,which is not conducive to the expansion of its application range。

4 Lignocellulose aerogel flame retardant

lignocellulose has low decomposition temperature and is easy to burn,so the application range of Lignocellulose aerogel is limited[74]。 By improving the flame retardancy of lignocellulose aerogel,its application field can be expanded[75]。 At present,the way to improve the flame retardancy is to add different kinds of flame retardants to lignocellulosic aerogels.flame retardants can be divided into inorganic flame retardants,organic flame retardants and organic/inorganic composite flame retardants according to their types。

4.1 Inorganic flame retardant

The mechanical properties and flame retardancy of lignocellulose aerogels can be effectively improved by mixing lignocellulose with inorganic substances,such as aluminum hydroxide,graphene oxide and montmorillonite[76⇓⇓⇓~80]。
Yuan et al.Introduced Al(OH)3nanoparticles into cellulose aerogel.During combustion,the Al(OH)3in the aerogel nanopores decomposes into alumina and releases water vapor.The high endothermic property of water(1300 J/G)can reduce the temperature and dilute the combustible gas,which ultimately inhibits the combustion of the sample[81]。 In addition,as shown in Figure 6A,the Al(OH)3nanoparticles form a protective layer around the cellulose,which further protects the cellulose from being burned.The results show that the total heat release rate of the cellulose/Al(OH)3composite aerogel decreases from 11.3 kJ/G to 1.7 kJ/G,and the carbon residue increases from 14%to 60%,compared with the single-phase cellulose aerogel.The results show that the flame retardancy of cellulose aerogel is greatly improved by the introduction of Al(OH)3 。
图6 (a)纤维素/Al(OH)3复合气凝胶的透射电子显微镜(TEM)图片[81];气凝胶燃烧前(b)和燃烧10 s后(c)的静态图片[82]

Fig. 6 (a) Transmission electron microscopy (TEM) image of Cellulose/Al(OH)3 composite aerogel[81]; Static images of aerogel before combustion (b) and after combustion for 10 s (c)[82]

In addition to aluminum hydroxide,sodium bicarbonate can also be used to improve the flame retardancy of lignocellulosic aerogels.Farooq et al.Prepared cellulose/sodium bicarbonate composite aerogel by freeze-drying method using cellulose aerogel as matrix and sodium bicarbonate as flame retardant[82]。 The results show that compared with the single-phase cellulose aerogel,the combustion rate of the aerogel can be reduced from 5.84 cm/s to 0.33 cm/s by adding 10 wt%sodium bicarbonate without affecting the thermal conductivity[about 0.028 W/(m·K)].Due to the thermal decomposition of sodium bicarbonate to release CO2and water during the combustion process,the aerogel shows self-extinguishing phenomenon after the fire source is removed.As shown in Figures 6 B and 6 C,when the addition of sodium bicarbonate reaches 20 wt%,the cellulose/sodium bicarbonate composite aerogel shows the best flame retardant performance in terms of self-extinguishing and pyrolysis,and does not produce harmful gases in flameless pyrolysis 。
Montmorillonite is a silicate mineral with a special layered structure,which has been widely used as a flame retardant for polymer aerogels[83]。 Chen Yanguo et al.Prepared cellulose/graphene/montmorillonite composite aerogel with graphene oxide and montmorillonite as flame retardants[84]。 As the content of graphene and montmorillonite increases,the porous structure of cellulose aerogel becomes more compact and ordered.the uniformly dispersed graphene nanosheets can give full play to their barrier effect,prevent oxygen from entering the interior,and inhibit the transfer of combustible gas and heat.the results of thermogravimetric analysis show that graphene and montmorillonite can effectively improve the high temperature stability of cellulose aerogel,and the carbon residue after combustion increases from 2.8%of single-phase cellulose aerogel to 14.7%of composite aerogel.Compared with the single phase aerogel,the peak heat release rate and the total heat release of the composite aerogel with 20 wt%montmorillonite and 5 wt%graphene decreased by 67.8%and 57.6%,respectively,indicating that the graphene and montmorillonite have a good synergistic flame retardant effect。
In addition to these traditional inorganic flame retardants,metal-organic frameworks,as an emerging class of porous materials,are also used as flame retardants to reduce the fire risk of polymer materials[85⇓~87]。 In the family of metal-organic frameworks,zeolitic imidazolate framework-8 is a porous crystalline substance composed of zinc ions linked by 2-methylimidazolium bridging ligands.Nabipour et al.Prepared zeolitic imidazolate framework-8 composite cellulose aerogel by freeze-drying layer-by-layer assembly method[88]。 The results show that the peak heat release rate and total heat release value of pure cellulose aerogel are 140.4 kW/m2and 13.0 MJ/m2,respectively.The peak heat release rate and total heat release value of the composites decreased with the increase of the mass fraction of zeolitic imidazolate ester framework-8.Among them,the peak heat release rate and total heat release value of the composite cellulose aerogel assembled with three-layer zeolitic imidazolate framework-8 were reduced to 47.8 kW/m2and 9.9 MJ/m2,respectively,which were 66%and 24%less than those of pure cellulose aerogel.The reason is that ZnO formed by the decomposition of zeolite imidazolate framework-8 promotes the formation of carbon layer,which not only delays the release of heat,but also prevents the release of combustibles into the combustion zone.In addition,the formation of metal nanoparticles also forms a highly dense protective layer on the surface of the material,thus blocking the combustion 。
the introduction of inorganic flame retardants can effectively improve the flame retardancy of lignocellulose aerogels,but only in the case of a larger amount can achieve a better flame retardant effect.At the same time,the addition of inorganic flame retardants may also destroy the three-dimensional porous network structure of lignocellulose aerogels,which affects the mechanical structure and performance of aerogels to some extent。

4.2 Organic flame retardant

Compared with inorganic flame retardants,organic flame retardants have higher flame retardant efficiency and smaller dosage。
the use of phosphorus-containing organic flame retardant compounds can solve The problem of large addition of inorganic flame retardants.Guo et al.Prepared lightweight and flame-retardant cellulose composite aerogels by freeze-drying combined with crosslinking using N-hydroxymethyldimethylphosphonopropionamide and 1,2,3,4-butanetetracarboxylic acid as additives[89]。 The results show that the total heat release value of flame retardant cellulose aerogel is significantly lower than that of pure cellulose aerogel.The total heat release of the sample without flame retardant is 5.4 MJ/m2,while the total heat release of the aerogel with flame retardant is only 2.0 MJ/m2,which is reduced by 62.96%.The carbon residue of aerogel after combustion also increases from 0.15 wt%to 40.37 wt%,which is about 268%higher.This is due to the fact that during the combustion process,the flame retardant N-hydroxymethyldimethylphosphonopropionamide decomposes into some intermediates,dehydrates and oxidizes cellulose,resulting in the formation of coke and the termination of combustion 。
phosphorus-containing flame retardants mainly produce phosphorus-containing intermediates through their decomposition reactions at high temperatures,which reduce the oxygen supply in the combustion process,while promoting the formation of carbide,and ultimately block the heat transfer.Huang et al.Added a new phosphorus-containing flame retardant modifier synthesized from 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide(DOPO)as the main raw material to cellulose aerogel,and prepared cellulose composite aerogel by esterification and freeze-drying[90]。 The results show that the flame retardancy of cellulose composite aerogel is gradually improved with the increase of flame retardant content,and the peak heat release rate and total heat release rate of the composite aerogel with 25%flame retardant are reduced to 34.7 kW/m2and 0.87 MJ/m2,respectively,which are 67%and 60%lower than those of pure cellulose aerogel.The phosphorus-containing flame retardant first decomposes to form phosphoroxyl radicals during the initial degradation process,which can inhibit the chain reaction of gas-phase combustion,resulting in a decrease in the total heat release value.In addition,the flame retardant can also produce phosphoric acid or polyphosphoric acid during the combustion process,which can induce the pyrolysis of cellulose matrix to form a stable coke layer and inhibit the heat transfer,thus reducing the fire risk of cellulose composite aerogel 。
Nitrogen-containing organic flame retardants are also used to improve the flame retardancy of lignocellulosic aerogels because they are similar to phosphorus-containing organic flame retardants.Zhang et al.Improved the flame retardancy of cellulose aerogels by adding 3-(2-aminoethylamino)propylmethyldimethoxysilane[91]。 The results show that the adsorption capacity of the aerogel for CO2can reach 1.68 mmol/G at room temperature and pressure,and the CO2can prevent the combustion of combustibles,thus improving the flame retardant effect of cellulose aerogel 。
Although organic compound flame retardants have high flame retardant efficiency and good compatibility with lignocellulose suspension,they also produce toxic gases in the process of heating,which is harmful to human body and the environment。

4.3 Organic/inorganic composite flame retardant

organic/inorganic composite flame retardants have also been used to prepare lignocellulose aerogels because of the advantages of both inorganic and Organic flame retardants。
the addition of flame retardants containing nitrogen/phosphorus or nitrogen/boron can play a synergistic role in flame retardancy,but the effect of the synergistic effect of nitrogen/boron on the flame retardancy of lignocellulose aerogel is not clear.Therefore,Wang et al.Prepared cellulose composite aerogels using boric acid and melamine formaldehyde resin as crosslinking agents[92]。 They first prepared cellulose suspension,then added boric acid and melamine formaldehyde resin to the cellulose suspension by mechanical stirring,and finally prepared cellulose composite aerogel by freeze-drying process.The results show that when the mass ratio of cellulose,melamine formaldehyde resin and boric acid is 1.5∶1.5∶1.05,the limiting oxygen index of the cellulose aerogel without crosslinking agent increases from 17.8%to about 85%,and the UL-94 fire rating reaches V0,indicating that the material has good self-extinguishing property.This is due to the dehydration of boric acid to form a carbonized layer,which limits the spread of oxygen and heat in the material,while the thermal decomposition of melamine formaldehyde resin releases NH3,which prevents the spread of flame,indicating that boric acid and melamine formaldehyde resin have a synergistic effect on the flame retardancy of aerogel 。
Polydopamine has been used to modify the surface of various materials because of its good adhesion and oxidation resistance.Han et al.Used polydopamine to modify cellulose aerogel.They first polymerized polydopamine to the surface of cellulose in situ,and then introduced Fe3+into the mixture through the strong coordination bond between the catechol group in polydopamine and metal ions to prepare polydopamine-modified cellulose aerogel[93]。 The results show that the limiting oxygen index of the composite aerogel with a mass ratio of 1∶0.5(polydopamine accounts for 0.5)increases from 18.7%to 33.1%,which is due to the scavenging of free radicals by polydopamine,thus inhibiting the fuel supply in the combustion process.In addition,Fe3+can effectively promote the formation of char during combustion,and the latter can act as a physical barrier to delay combustion,which also contributes to the improvement of flame retardancy of polydopamine/cellulose composite aerogel 。
in conclusion,the introduction of inorganic,organic and organic/inorganic flame retardants into lignocellulosic aerogels can improve the flame retardancy of the latter,but there are also some problems In the application process:
(1)Inorganic flame retardants have the advantages of low toxicity,good thermal stability and low price,but the addition amount is large,and the compatibility with lignocellulose is not good。
(2)Organic flame retardants have the problem of producing a large amount of smoke and harmful gases during combustion。
(3)Although organic/inorganic flame retardants have the advantages of both inorganic and organic flame retardants and have more efficient flame retardant effect,the preparation process is more complex,and their compatibility with lignocellulose suspension is poor,so they are difficult to be uniform and dispersed[94]。

5 Conclusion and prospect

lignocellulose aerogel is a hot topic in the current research field because of its low cost,wide source of raw materials,low density,high porosity and low thermal conductivity.in this paper,the latest research status of lignocellulose aerogel was reviewed,and the latest research progress of three kinds of lignocellulose aerogel preparation and drying technology was introduced.Although lignocellulose has a wide range of sources,lignocellulose aerogel can be prepared by atmospheric drying,freeze-drying and supercritical drying,and the introduction of flame retardants into lignocellulose aerogel can improve its flame retardancy,there are still the following problems in practical application(Fig.7):
图7 木质纤维素气凝胶干燥技术及其阻燃剂方面目前存在的问题以及未来研究方向[82]

Fig. 7 Current problems and future research tends of lignocellulosic aerogel drying technologies and flame retardants.

(1)the pore size of lignocellulose aerogel prepared by freeze-drying is usually large,which requires high equipment,and the in-situ growth of ice crystals will affect the uniformity of the pore size of aerogel.the internal structure of aerogel can be adjusted by directional freeze-drying technology,but this method has some shortcomings,such as complex process,difficult to prepare large-size samples and long cycle.Supercritical drying can preserve the mesoporous structure of aerogel to the greatest extent,but the process is complex and requires high equipment,so it is only suitable for a few high-quality products.the atmospheric pressure drying method has the advantages of simple operation and low equipment requirement,but usually requires solvent replacement;Solvents are not only expensive,but also harmful to human health.Secondly,the lignocellulose aerogel prepared by atmospheric pressure drying technology has large pore size and low strength。
(2)the inorganic flame retardant has the disadvantages of large addition amount and poor compatibility with lignocellulose;organic flame retardants release a lot of smoke and toxic gases when they burn,which are harmful to the environment and human health.the Organic/inorganic composite flame retardant has the problems of poor dispersion with lignocellulose and complex operation。
(3)lignocellulose resources have not been used with high added value.At present,Lignocellulose is mainly obtained from cash crops such as wood and cotton,and agricultural wastes such as rice straw,straw and bagasse are relatively less used[8]。 Moreover,the process of extracting lignocellulose is complex,lignin and hemicellulose are not effectively utilized,and the pollution is serious。
Based on the above problems,we believe that the main research directions in the field of lignocellulosic aerogel materials in the future are as follows(Fig.7):
(1)that dry technology of the lignocellulose aerogel is improved,a simpl drying technology is further developed,the preparation process of the lignocellulose aerogel is simplify,and the large-scale production and the wide application of the lignonlose aerogel are realized。
(2)Explore the appropriate natural and cheap raw materials as flame retardants,effectively improve the flame retardancy of lignocellulose aerogel,and solve the flammable shortcomings of lignocellulose aerogel in daily use。
(3)Improve the technology of extracting lignocellulose from agricultural wastes,realize the effective utilization of lignin and hemicellulose,and further realize the purpose of utilizing agricultural wastes with high added value。

References

Publishing order | Descend order by publishing year | Descend order by cited within
[1]
Han L, Dong L H, Zhang H J, Li F L, Tian L, Li G Q, Jia Q L, Zhang S W. J. Eur. Ceram. Soc., 2021, 41(10): 5127.

[2]
Tian L, Liang F, Dong L H, Li J Y, Jia Q L, Zhang H J, Yan S, Zhang S W. J. Am. Ceram. Soc., 2021, 104(2): 1110.

[3]
Huang Z, Zheng Y, Zhang H, Li F, Zeng Y, Jia Q, Zhang J, Li J, Zhang S. J Mater Sci Technol, 2021, 94.

[4]
Li K Z, Yuan G Q, Dong L, Deng G, Duan H J, Jia Q L, Zhang H J, Zhang S W. Sep. Purif. Technol., 2022, 302: 122065.

[5]
Han L, Dong L H, Li F L, Duan H J, Zhang H J, Li G Q, Jia Q L, Zhang S W. J. Eur. Ceram. Soc., 2022, 42(6): 2699.

[6]
Han L, Khalil A M E, Wang J K, Chen Y, Li F L, Chang H, Zhang H J, Liu X Y, Li G Q, Jia Q L, Zhang S W. Sep. Purif. Technol., 2021, 278: 119605.

[7]
Klemm D, Kramer F, Moritz S, Lindström T, Ankerfors M, Gray D, Dorris A. Angew. Chem. Int. Ed., 2011, 50(24): 5438.

[8]
Jayan S S, Jayan J S, Saritha A. Int. J. Biol. Macromol., 2023, 248: 125928.

[9]
Guo W W, Chen S, Liang F W, Jin L P, Ji C P, Zhang P, Fei B. Int. J. Biol. Macromol., 2023, 246: 125343.

[10]
Abbasi Moud A. Int. J. Biol. Macromol., 2022, 222: 1.

[11]
Shahid A M, Sangeetha U K, Sahoo S K. Ind. Crops Prod., 2023, 202: 117076.

[12]
Garemark J, Perea-Buceta J E, Rico del Cerro D, Hall S, Berke B, Kilpeläinen I, Berglund L A, Li Y Y. ACS Appl. Mater. Interfaces, 2022, 14(21): 24697.

[13]
Sun H, Bi H J, Lin X, Cai L P, Xu M. Polymers, 2020, 12(1): 165.

[14]
Tian S L, Yi Z D, Chen J Q, Fu S Y. J. Hazard. Mater., 2023, 443: 130236.

[15]
Han X S, Wu W J, Tian Z W, Wang J W, Zhang L, Han J Q, Li S S, Jiang S H. Compos. Commun., 2022, 33: 101199.

[16]
Dong H, Li X, Niu L, Liu Z M. Journal of Forestry Engineering, 2023, 8(05): 121.

( 董浩, 李旭, 牛力, 刘志明. 林业工程学报, 2023, 8(05): 121.)

[17]
Song Y H, Wu T, Bao J K, Xu M H, Yang Q L, Zhu L P, Shi Z Q, Hu G H, Xiong C X. Carbohydr. Polym., 2022, 288: 119407.

[18]
Li J B, Yang X, Xiu H J, Dong H L, Song T, Ma F Y, Feng P, Zhang X F, Kozliak E, Ji Y. Ind. Crops Prod., 2019, 128: 186.

[19]
Debnath B, Haldar D, Purkait M K. Carbohydr. Polym., 2021, 273: 118537.

[20]
Li S M, Chen G. J. Clean. Prod., 2020, 251: 119669.

[21]
Kamel R, El-Wakil N A, Dufresne A, Elkasabgy N A. Int. J. Biol. Macromol., 2020, 163: 1579.

[22]
Asim N, Badiei M, Alghoul M, Mohammad M, Fudholi A, Akhtaruzzaman M, Amin N, Sopian K J I, Research E C. Ind. Eng. Chem. Res., 2019.

[23]
Chen K Q, Wang L M, He H M, Ling H L, Xu W. Functional Material, 2022, 53(02): 2107.

( 陈扣琴, 王黎明, 郝慧敏, 凌杭丽, 徐伟. 功能材料, 2022, 53(02): 2107.)

[24]
Shi T L, Wang H K, Wu Y, Xu L, Zhan X X. Furniture, 2018, 39(04): 14.

( 史甜霖, 王汉坤, 吴燕, 胥莉, 詹先旭. 家具. 2018, 39(04): 14.)

[25]
Chhajed M, Verma C, Sathawane M, Singh S, Maji P K. Mar. Pollut. Bull., 2022, 180: 113790.

[26]
Hou J, Wang Y, Xue H, Dou Y. Polymers (Basel), 2018, 10(9).

[27]
Chen Y M, Zhang L, Yang Y, Pang B, Xu W H, Duan G G, Jiang S H, Zhang K. Adv. Mater., 2021, 33(11): 2005569.

[28]
De France K J, Hoare T, Cranston E D. Chem. Mater., 2017, 29(11): 4609.

[29]
Wei G, Zhang J M, Usuelli M, Zhang X F, Liu B, Mezzenga R. Prog. Mater. Sci., 2022, 125: 100915.

[30]
Zhang Y, Yin M L, Lin X H, Ren X H, Huang T S, Kim I S. Chem. Eng. J., 2019, 371: 306.

[31]
Liu Y B, Chen Y X, Chen Q, Zhou C, Hu K, Dong C, Yang B. Cotton Textile Technology, 2022, 50(11): 44.

( 刘延波, 陈云霞, 陈倩, 周聪, 胡楷, 董超, 杨波. 棉纺织技术, 2022, 50(11): 44.)

[32]
Chen X X, Zeng Z W, Chen W. Journal of Wuhan University of Technology, 2020, 42(07): 8.

( 陈晓星, 曾志文, 陈伟. 武汉理工大学, 2020, 42(07): 8.)

[33]
Gong C, Shi Y, Ni J P, Yang X B, Liu Y Z, Tian C. J. Korea TAPPI, 2016, 48(6): 17.

[34]
Liu X T, Yang H, Shen Q H, Sheng J S, Guo X Z. Journal of Ceramics, 2020, 41(03): 415.

( 刘晓婷, 杨辉, 申乾宏, 盛建松, 郭兴忠. 陶瓷学报, 2020, 41(03): 415.)

[35]
Wang Z P, Zhao K Y, Cai Y X, Huang J D, Sun Y, Zhang W B. Journal of Forestry Engineering, 2023, 8(04): 87.

( 王泽鹏, 赵柯豫, 蔡雨欣, 黄景达, 孙毅, 张文标. 林业工程学报, 2023, 8(04): 87.)

[36]
Wang C, Cao H, Jia L Y, Liu W L, Liu P T. Carbohydr. Polym., 2022, 291: 119568.

[37]
Tang J T, Song Y, Zhao F P, Spinney S, da Silva Bernardes J, Tam K C. Carbohydr. Polym., 2019, 208: 404.

[38]
Yan G H, Chen B L, Zeng X H, Sun Y, Tang X, Lin L. Carbohydr. Polym., 2020, 244: 116492.

[39]
Lee J, Deng Y L. Soft Matter, 2011, 7(13): 6034.

[40]
Chen Y M, Zhou L J, Chen L, Duan G G, Mei C T, Huang C B, Han J Q, Jiang S H. Cellulose, 2019, 26(11): 6653.

[41]
Zhou L J, Zhai S C, Chen Y M, Xu Z Y. Polymers, 2019, 11(4): 712.

[42]
Chen Y, Zhang L, Mei C, Li Y, Duan G, Agarwal S, Greiner A, Ma C, Jiang S. ACS Appl. Mater. Interfaces, 2020, 12(31): 35513.

[43]
Chen X Q, Tang Q H, Chang L, Guo M H. Journal of Forestry Engineering, 2023, 8(04): 110.

( 陈雪琦, 唐启恒, 常亮, 郭明辉. 林业工程学报, 2023, 8(04): 110.)

[44]
Li Y Q, Guo C F, Shi R H, Zhang H, Gong L Z, Dai L B. Carbohydr. Polym., 2019, 223: 115048.

[45]
Wu Y, Qiu X D, Cao S Y, Chen J J, Shi X W, Du Y M, Deng H B. J. Colloid Interface Sci., 2019, 539: 533.

[46]
Chen Y M, Hanshe M, Sun Z X, Zhou Y, Mei C T, Duan G G, Zheng J J, Shiju E, Jiang S H. Int. J. Biol. Macromol., 2022, 207: 130.

[47]
Zhang M L, Jiang S, Han F Y, Li M M, Wang N, Liu L F. Carbohydr. Polym., 2021, 264: 118033.

[48]
Chen Y M, Luo H, Guo H T, Liu K M, Mei C T, Li Y, Duan G G, He S J, Han J Q, Zheng J J, Shiju E, Jiang S H. Carbohydr. Polym., 2022, 276: 118799.

[49]
Zhang M L, Jiang S, Li M M, Wang N, Liu L F, Liu L Y, Ge A X. Carbohydr. Polym., 2022, 288: 119370.

[50]
Hajian A, Fu Q L, Berglund L A. Compos. Sci. Technol., 2018, 159: 1.

[51]
Bai H, Walsh F, Gludovatz B, Delattre B, Huang C L, Chen Y, Tomsia A P, Ritchie R O. Adv. Mater., 2016, 28(1): 50.

[52]
Zhao N F, Yang M, Zhao Q, Gao W W, Xie T, Bai H. ACS Nano, 2017, 11(5): 4777.

[53]
Yang M, Zhao N F, Cui Y, Gao W W, Zhao Q, Gao C, Bai H, Xie T. ACS Nano, 2017, 11(7): 6817.

[54]
Gao H L, Zhu Y B, Mao L B, Wang F C, Luo X S, Liu Y Y, Lu Y, Pan Z, Ge J, Shen W, Zheng Y R, Xu L, Wang L J, Xu W H, Wu H A, Yu S H. Nat. Commun., 2016, 7: 12920.

[55]
Zhang X, Zhao X Y, Xue T T, Yang F, Fan W, Liu T X. Chem. Eng. J., 2020, 385: 123963.

[56]
Zhang X X, Wang H K, Cai Z Y, Yan N, Liu M H, Yu Y. ACS Sustainable Chem. Eng., 2019, 7(1): 332.

[57]
Zou F X, Budtova T. Carbohydr. Polym., 2021, 266: 118130.

[58]
Berglund L, Nissilä T, Sivaraman D, Komulainen S, Telkki V V, Oksman K. ACS Appl. Mater. Interfaces, 2021, 13(29): 34899.

[59]
Zaman A, Huang F, Jiang M, Wei W, Zhou Z W. Energy Built Environ., 2020, 1(1): 60.

[60]
Şahin İ, Özbakır Y, İnönü Z, Ulker Z, Erkey C. Gels, 2018, 4(1): 3.

[61]
Buchtová N, Pradille C, Bouvard J L, Budtova T. Soft Matter, 2019, 15(39): 7901.

[62]
Chang H J, Zhang L F, Lv S Y, Wang S Q. Wood Science and Technology, 2022, 36(05): 43.

( 常焕君, 张龙飞, 吕少一, 王思群. 木材科学与技术, 2022, 36(05): 43.)

[63]
Wang X Y, Zhang Y, Wang S Q, Jiang H, Liu S, Yao Y, Zhang T M, Li Q. J. Mater. Sci., 2018, 53(18): 13304.

[64]
Sivaraman D, Siqueira G, Maurya A K, Zhao S Y, Koebel M M, Nyström G, Lattuada M, Malfait W J. Carbohydr. Polym., 2022, 292: 119675.

[65]
Plappert S F, Quraishi S, Nedelec J M, Konnerth J, Rennhofer H, Lichtenegger H C, Liebner F W. Chem. Mater., 2018, 30(7): 2322.

[66]
Lin W H, Jana S C. Microporous Mesoporous Mater., 2021, 310: 110625.

[67]
Zhang Y J, Qu J, Ji Q, Zhang T T, Chang W J, Hao S M, Yu Z J C. Carbon, 2019, 155: 353.

[68]
Wang B H, Zhang W B, Zhang W, Mujumdar A S, Huang L X. Dry. Technol., 2005, 23(1/2): 7.

[69]
Hasan M, Gopakumar D A, Arumughan V, Pottathara Y B, Sisanth K S, Pasquini D, Bracic M, Seantier B, Nzihou A, Thomas S, Rizal S, Khalil A H P S. Polymers (Basel), 2019, 11(3).

[70]
Li Y Z, Grishkewich N, Liu L L, Wang C, Tam K C, Liu S Q, Mao Z P, Sui X F. Chem. Eng. J., 2019, 366: 531.

[71]
Zhang Y X, Liao J M, Li J, Guo S S, Mo L H, Liu Z, Xiong Q G. Environ. Pollut., 2022, 306: 119450.

[72]
Abdul Razzaq A, Yao Y Z, Shah R, Qi P W, Miao L X, Chen M Z, Zhao X H, Peng Y, Deng Z. Energy Storage Mater., 2019, 16: 194.

[73]
Wu M Y, Yu G, Chen W, Dong S, Wang Y R, Liu C, Li B. Carbohydr. Polym., 2022, 278: 118963.

[74]
Huang Y J, Zhou T, He S, Xiao H, Dai H M, Yuan B H, Chen X F, Yang X B. Appl. Surf. Sci., 2019, 497: 143775.

[75]
Le Thanh N T. Mater. Today Proc., 2022, 66: 2726.

[76]
Golitz P. Angew. Chem., 2001, 40(1): 3.

[77]
Zhu J D, Xiong R J, Zhao F X, Peng T P, Hu J, Xie L, Xie H S, Wang K, Jiang C W. ACS Sustainable Chem. Eng., 2020, 8(1): 71.

[78]
Li Y Z, Wang B J, Sui X F, Xu H, Zhang L P, Zhong Y, Mao Z P. Cellulose, 2017, 24(9): 3815.

[79]
Chen Z J, Zhang Y, Zheng L, Zhang H, Shi H H, Zhang X C, Liu B. Biomater. Adv., 2022, 134: 112549.

[80]
Horrocks A R. Polym. Degrad. Stab., 2011, 96(3): 377.

[81]
Yuan B, Zhang J, Yu J, Song R, Mi Q, He J, Zhang J. Sci China Chem, 2016, 59(10): 1335.

[82]
Farooq M, Sipponen M H, Seppala A, Osterberg M. ACS Appl. Mater. Interfaces, 2018, 10(32): 27407.

[83]
Chen H B, Wang Y Z, Sanchez-Soto M, Schiraldi D A. Polymer, 2012, 53(25): 5825.

[84]
Chen Y G, Li Z Y, Li X H, Wu Z S. China Plastics, 2019, 33(01): 33.

( 陈艳果, 李志伟, 李小红, 吴志申. 中国塑料, 2019, 33(01): 33.)

[85]
Wang X G, Qi P, Zhang S J, Jiang S L, Li Y C, Sun J, Fei B, Gu X Y, Zhang S. Mater. Today Chem., 2023, 30: 101550.

[86]
Song K P, Pan Y T, Zhang J, Song P G, He J Y, Wang D Y, Yang R J. Chem. Eng. J., 2023, 468: 143653.

[87]
Lyu P, Hou Y, Hu J, Liu Y, Zhao L, Feng C, Ma Y, Wang Q, Zhang R, Huang W, Ma M. Polymers (Basel), 2022, 14(23).

[88]
Nabipour H, Nie S B, Wang X, Song L, Hu Y. Cellulose, 2020, 27(4): 2237.

[89]
Guo L M, Chen Z L, Lyu S Y, Fu F, Wang S Q. Carbohydr. Polym., 2018, 179: 333.

[90]
Huang J L, Wang X, Guo W W, Niu H X, Song L, Hu Y. J. Taiwan Inst. Chem. Eng., 2022, 131: 104159.

[91]
Zhang T M, Zhang Y, Jiang H, Wang X Y. Environ. Sci. Pollut. Res., 2019, 26(16): 16716.

[92]
Wang L, Sánchez-Soto M, Fan J, Xia Z P, Liu Y. Polym. Adv. Technol., 2019, 30(7): 1807.

[93]
Han F Y, Huang H, Wang Y, Liu L F. Cellulose, 2021, 28(17): 10987.

[94]
Fang Y, Sun W. Textile Journal, 2022, 43(01): 43.

( 方寅春, 孙卫昊. 纺织学报. 2022, 43(01): 43.)

Options
Outlines

/

〈 〉