Nanocellulose Aerogels Research Articles

Cellulose Aerogels: Synthesis, Applications, and Prospects.

Due to its excellent performance, aerogel is considered to be an especially promising new material. Cellulose is a renewable and biodegradable natural polymer. Aerogel prepared using cellulose has the renewability, biocompatibility, and biodegradability of cellulose, while also having other advantages, such as low density, high porosity, and a large specific surface area. Thus, it can be applied for many purposes in the areas of adsorption and oil/water separation, thermal insulation, and biomedical applications, as well as many other fields. There are three types of cellulose aerogels: natural cellulose aerogels (nanocellulose aerogels and bacterial cellulose aerogels), regenerated cellulose aerogels, and aerogels made from cellulose derivatives. In this paper, more than 200 articles were reviewed to summarize the properties of these three types of cellulose aerogels, as well as the technologies used in their preparation, such as the sol–gel process and gel drying. In addition, the applications of different types of cellulose aerogels were also introduced.

High Aspect Ratio Carboxylated Cellulose Nanofibers Cross-linked to Robust Aerogels for Superabsorption-Flocculants: Paving Way from Nanoscale to Macroscale.

Charged nanocellulose (NC) with a high aspect ratio (larger than 100) extracted from animal or bacterial cellulose and chemical cross-linked NC aerogels have great promising applicability in material science, but facile fabrication of such NC aerogels from plant cellulose by physical cross-linking still remains a major challenge. In this work, carboxylated cellulose nanofiber (CNF) with the highest aspect ratio of 144 was extracted from wasted ginger fibers by a simple one-step acid hydrolysis. Our approach could easily make the carboxylated CNF assemble into robust bulk aerogels with tunable densities and desirable shapes on a large scale (3D macropores to mesopores) by hydrogen bonds. Excitingly, these CNF aerogels had better compression mechanical properties (99.5 kPa at 80% strain) and high shape recovery. Moreover, the CNF aerogels had strong coagulation-flocculation ability (87.1%), removal efficiency of MB dye uptake (127.73 mg/g), and moderate Cu2+ absorption capacity (45.053 mg/g), which were due to assistance mechanisms of charge neutralization, network capture effect, and chain bridging of high aspect ratio carboxylated CNF. This provided a novel physical cross-linking method to design robust aerogels with modulated networked structures to be a general substrate material for industrial applications such as superabsorbent, flocculation, oil-water separation, and potential electrical energy storage materials.

Controlling porosity and density of nanocellulose aerogels for superhydrophobic light materials

A very low-density oil-absorbing hydrophobic material was fabricated from cellulose nanofiber aerogels–coated silane substances. Nanocellulose aerogels (NCA) superabsorbents were prepared by freeze drying cellulose nanofibril dispersions at 0.2%, 0.5%, 0.8%, 1.0%, and 1.5% w/w. The NCA were hydrophobically modified with methyltrimethoxysilane. The surface morphology and wettability were characterized by scanning electron microscopy and static contact angle. The aerogels displayed an ultralow density (2.0–16.7 mg·cm-3), high porosity (99.9%–98.9%), and superhydrophobicity as evidenced by the contact angle of ~150° that enabled the aerogels to effectively absorb oil from an oil/water mixture. The absorption capacities of hydrophobic nanocellulose aerogels for waste engine oil and olive oil could be up to 140 g·g-1 and 179.1 g·g-1, respectively.

Conformal Ultrathin Coating by scCO2-Mediated PMMA Deposition: A Facile Approach To Add Moisture Resistance to Lightweight Ordered Nanocellulose Aerogels

A facile approach for adding moisture resistance to transparent nematic aerogels composed of individualized cellulose nanofibers (i-CNF) at full preservation of the anisotropic aerogel structure is presented. Sequential nitroxide-mediated oxidation and mechanical cellulose fiber delamination were applied to obtain i-CNF dispersions in water. Nematic ordering caused by repulsive forces between i-CNF surface carboxylate groups was set by acid-induced hydrogen bonding and gelation, respectively. Solvent exchange to acetone, impregnation with the PMMA, scCO2-mediated antisolvent precipitation of the secondary polymer, and scCO2 extraction of interstitial acetone afforded highly hydrophobic nanocomposite aerogels. Birefringence studies utilizing polarized light and the Michel–Levy Chart to evaluate interference patterns revealed that nematic i-CNF ordering is virtually not affected by the respective surface modification and scCO2 drying steps. Morphological studies provide evidence that the large internal surf...

Surface-Tailored Nanocellulose Aerogels with Thiol-Functional Moieties for Highly Efficient and Selective Removal of Hg(II) Ions from Water

Developing an easily recyclable and reusable biosorbent for highly efficient removal of very toxic Hg(II) ions from bodies of water is of special significance. Herein, a thiol-functionalized nanocellulose aerogel-type adsorbent for the highly efficient capture of Hg(II) ions was fabricated through a facile freeze-drying of bamboo-derived 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidized nanofibrillated cellulose (TO-NFC) suspension in the presence of hydrolyzed 3-mercaptopropyl-trimethoxysilane (MPTs) sols. Notably, the modified aerogel was able to effectively and selectively remove more than 92% Hg(II) ions even in a wide range of Hg(II) concentrations (0.01–85 mg/L) or coexistence with other heavy metals. Besides, the adsorption capacity of the aerogel was not compromised much by the variation in pH values of Hg(II) solutions over a wide pH range. The fitting results of adsorption models suggested the monolayer adsorption and chemisorptive characteristics with the maximal uptake capacity as high as ...

Nanocellulose Aerogels Inspired by Frozen Tofu

Cellulose-based aerogels are reported that were generated using cellulose nanofibril (CNF) gels in an ambient drying process. These nancellulose aerogels were inspired by the preparation of a traditional Chinese food, frozen tofu, which undergoes significant toughening after freezing and thawing. It is this toughening mechanism that allows for the formation of nanocellulose aerogels which ordinarily would collapse upon ambient drying. By freezing hydrated CNF gel dispersions, solvent exchanging, and drying at ambient pressure, monolithic nanocellulose aerogels with high porosity (>98%), low density (as low as 0.018 g cm–3), and high surface area (>30 m2 g–1) were readily generated. Moreover, since no special reactors are required, these structures can be easily created over large areas. For example, a hydrated CNF gel was frozen at −72 °C, and then, the frozen gel was immersed directly in a 2-propanol bath to allow solvent exchange. The resulting alcohol-infused gel was dried at ambient pressure to fabric...

Tert-butyl alcohol used to fabricate nano-cellulose aerogels via freeze-drying technology

Aerogel, a highly porous material, is attracting increasing attention owing to low thermal conductivity and high specific surface area. Freeze-drying technology has been employed to produce nano-cellulose aerogels; however, the resultant product has low specific surface areas. Here, a modified approach to prepare nano-cellulose aerogels was reported, which involves tert-butyl alcohol as a solvent. Nano-cellulose aerogels were prepared via a spontaneous gelation fashion using calcium chloride solution, followed by tert-butyl alcohol solvent displacement and freeze drying. Addition of calcium chloride (0.25%) accelerated the physical gelation process. The application of tert-butyl alcohol as a solvent contributed to preservation of gel network. The obtained spherical nano-cellulose aerogels had a shrinkage rate of 5.89%. The specific surface area and average pore size was 164. 9666 m2 g−1 and 10.01 nm, respectively. Additionally, nano-cellulose aerogels had a comparable thermal degradation property when compared to microcrystalline cellulose. These biophysical properties make nano-cellulose aerogels as a promising absorption material.

Retention of lysozyme activity by physical immobilization in nanocellulose aerogels and antibacterial effects

Aerogels prepared from aqueous dispersions of anionic and cationic cellulose nanofibrils (CNFs) were investigated as solid supports for enzymes and silver nanoparticles and to elicit a sustained antibacterial effect. The imparted stabilization in dry conditions was studied with aerogels that were cast after mixing the enzymes with CNFs followed by dehydration (freeze-drying). The activity of lysozyme immobilized in the given CNF system was analyzed upon storage in liquid and air media. In contrast with aqueous solutions of free, unbound enzyme, which lost activity after the first day, the enzyme immobilized physically in unmodified and cationic CNF presented better stability (activity for a longer time). However, the enzyme activity was reduced in the case of anionic CNF, which was prepared by TEMPO-mediated oxidation (TO-CNF). Both humidity and temperature reduced the stability of the enzyme immobilized in the respective CNF aerogel. The antibacterial activity of CNF aerogels carrying lysozyme was also tested against gram-negative and gram-positive bacteria. The results were compared with those obtained from CNF systems loaded with silver nanoparticles (AgNP) after in situ synthesis via UV reduction. Storage in cold or dry conditions preserved the activity and antibacterial performance of enzyme-loaded CNF aerogels. As expected, the lysozyme-containing aerogels showed lower inhibition than the AgNP-containing aerogel. In this latter case, the antibacterial activity depended on the concentration and size of the nanoparticles. Compared to unmodified CNF and TO-CNF, the aerogels prepared with cationic CNF, loaded with either lysozyme or AgNPs, showed remarkably better antibacterial activity. Similar experiments were conducted with horseradish peroxidase, which confirmed, to different degrees, the observations derived from the lysozyme systems. Overall, the results indicate that non-toxic and biodegradable CNF is a suitable support for bio-active materials and is effective in protecting and retaining enzymatic and antibacterial activities.

Nanocellulose Aerogels for Supporting Iron Catalysts and In Situ Formation of Polyethylene Nanocomposites

Aerogels of nanocellulose (NC) prepared by freeze‐drying of cellulose nanofibrils (CNF) hydrogels and followed by impregnation with methylaluminoxane serve as nanoporous organic supports for immobilizing single site iron catalysts such as bisiminopyridine iron(II) complexes. The resulting catalyst systems, exploiting renewable biomaterials as organic supports, are highly active in low pressure ethylene polymerization. They afford simultaneous control of high density polyethylene (HDPE) particle morphology and facile NC dispersion within the HDPE matrix. In the early stage of ethylene polymerization, mesoscopic shape replication and NC‐mediated templating yield platelets containing an NC core and a HDPE shell, as confirmed by scanning electron microscopy (SEM) of virgin polyethylene powders. Opposite to conventionally dried CNF hydrogels, forming large agglomerates, this facile NC aerogel‐mediated in situ NC/HDPE nanocomposite formation is vastly superior to melt compounding of HDPE with NC, failing to produce such fine NC dispersions. On increasing NC content to 3.0 wt%, both Young's modulus (+50%) and tensile strength (+40%) increase at the expense of elongation at break (−80%). According to the SEM analysis of NC/HDPE morphology, the dispersion of NC nanosheets together with the in situ formation of “shish‐kebab” polyethylene fiber‐like structures accounted for HDPE matrix reinforcement.

Cellulose Nanofibril Aerogels: Synergistic Improvement of Hydrophobicity, Strength, and Thermal Stability via Cross-Linking with Diisocyanate.

A facile gelation cross-linking approach was devised to fabricate meso- and macroporous cellulose nanofibril (CNF) aerogels with multiple improved properties. CNF hydrogels made using a freezing-thawing method with a 94 kPa modulus were solvent exchanged with acetone and then cross-linked with methylene diphenyl diisocyanate (MDI) to produce aerogels with significantly improved compressive properties that follow a power law increment against aerogel density with impressive 1.69, 2.49, and 1.43 scaling factors for Young's modulus, yield stress, and ultimate stress, respectively. The optimally cross-linked aerogels had nearly tripled specific surface area (228 m2/g) and doubled pore volume (1 m3/g) from numerous new 9-12 nm wide mesopores as well as significantly improved thermal stability (43% char residue at 500 C vs 9.1% for un-cross-linked aerogel). Cross-linking also made the amphiphilic CNF aerogel highly hydrophobic and capable of completely separating chloroform from water via simple filtration. These nanocellulose aerogels show great promise for efficient and continuous separation of oils and hydrophobic liquids from water.

Nanoscale engineering of nitrogen-doped carbon nanofiber aerogels for enhanced lithium ion storage

After carbonization of bamboo nanocellulose aerogels in ammonia gas, the obtained nitrogen-doped carbon nanofiber (N-ACNF) aerogels with a tunable nanostructure were found to be ideal candidates for high-performance anodes in LIBs.

Protease Biosensors Based on Peptide-Nanocellulose Conjugates: From Molecular Design to Dressing Interface

The development of point of care diagnostic protease sensors applied to wound healing has received increased interest in chronic wound treatment as an interface for chronic wound dressings. Biosensor technology and the use of nanomaterials have grown exponentially in recent years. A biosensor is fundamentally a biomolecule (functioning as a transducer) attached to a transducer surface, which is activated by a biochemical property that prompts a detection signal specific to a target biomarker. For instance, nanomaterials are often derivatized with a biomolecule that provides selectivity and sensitivity for diagnostic biomarkers. Nanocellulose can be prepared as a transducer surface from an assortment of sources like wood and cotton in a variety of physical forms such as nanocrystals, nanocellulose composites, and nanocellulose aerogels that possess ideal properties including biodegradability, biocompatibility, functionality, and a high specific surface area. Interfacing nanocellulosic biosensors with a wound dressing having protease-lowering properties allows in situ sensor detection selectivity and sensitivity to monitor the effectiveness of the dressing and titer of protease removed from the wound.

The interaction of wood nanocellulose dressings and the wound pathogen P. aeruginosa

Chronic wounds pose an increasingly significant worldwide economic burden (over £1 billion per annum in the UK alone). With the escalation in global obesity and diabetes, chronic wounds will increasingly be a significant cause of morbidity and mortality. Cellulose nanofibrils (CNF) are highly versatile and can be tailored with specific physical properties to produce an assortment of three-dimensional structures (hydrogels, aerogels or films), for subsequent utilization as wound dressing materials. Growth curves using CNF (diameter <20nm) in suspension demonstrated an interesting dose-dependent inhibition of bacterial growth. In addition, analysis of biofilm formation (Pseudomonas aeruginosa PAO1) on nanocellulose aerogels (20g/m2) revealed significantly less biofilm biomass with decreasing aerogel porosity and surface roughness. Importantly, virulence factor production by P. aeruginosa in the presence of nanocellulose materials, quantified for the first time, was unaffected (p>0.05) over 24h. These data demonstrate the potential of nanocellulose materials in the development of novel dressings that may afford significant clinical potential.

Fabrication and characterization of nano-cellulose aerogels via supercritical CO2 drying technology

In this study, we developed a novel technique to fabricate nano-cellulose aerogels. Cellulose solution was dropped to calcium chloride solution and nano-cellulose aerogels were formed through supercritical CO2 drying. The induction of calcium chloride solution maintained the shape of gel and facilitated physical gelation process. The supercritical CO2 drying aided in preservation of the original gel network and structure. The resultant spherical nano-cellulose aerogels were characterized with respect to shrinkage, specific surface area, thermal degradation, and micro-structure. They exhibited a nano-porous network structure composed of mesopores, with a high specific surface area of up to 353m2/g and an average pore size of 8.86nm. They showed a weak shrinkage of 4.03%. The nano-cellulose aerogels had similar thermal degradation behaviors to microcrystalline cellulose and may have wide engineering applications.

Flexible and Porous Nanocellulose Aerogels with High Loadings of Metal-Organic-Framework Particles for Separations Applications.

Flexible and porous aerogels are obtained by combining functional metal-organic frameworks (MOFs) and structural cellulose nanocrystals via a straightforward water-based sol-gel process, followed by freeze-drying. The aerogels have a hierarchical porous structure with controllable MOF loading up to 50 wt%. These hybrid materials can be used as absorbents for water purification.

Bacterial Nanocellulose Aerogel Membranes: Novel High-Porosity Materials for Membrane Distillation

We fabricated, characterized, and tested novel fibrous aerogel membranes in direct contact membrane distillation (MD) to elucidate the effects of a model high-porosity membrane material on MD performance. Unsupported bacterial nanocellulose aerogels exhibit higher porosity, thinner fibers, and lower bulk thermal conductivity than any previously reported MD materials. Modeling and experiments demonstrate that these material properties confer significantly higher intrinsic membrane permeability and thermal efficiency than symmetric PVDF phase inversion membranes with lower porosity. Development of macroporous fibrous membranes with aerogel-like porosity and thermal conductivity (>98% and <0.03 W m–1 K–1, respectively) in thinner-film formats may further improve MD flux.

Nanocellulose-derived highly porous carbon aerogels for supercapacitors

High-surface-area carbon aerogels are synthesized by pyrolysis of nanocellulose aerogels that are prepared by dissolving of cellulose microcrystalline in NaOH aqueous solutions followed by gelation, regeneration and freeze drying or supercritical CO2 drying. The obtained carbon aerogels are further treated via a CO2 activation process. The result shows that the nanocellulose aerogels dried by supercritical CO2 exhibit a high specific surface area (299 m2/g) and the obtained carbon aerogels preserve the interconnected 3D nanostructure and have a high specific surface area of 892 m2/g and a high pore volume of 1.80 cm3/g, which increase to 1873 m2/g and 2.65 cm3/g respectively after CO2 activation. The highly porous and interconnected nanostructure provides efficient migration of electrolyte ions and electrons, which makes the resultant activated carbon aerogels exhibit excellent electrochemical performance for supercapacitors. The specific capacitances reach 302 and 205 F/g at a current density of 0.5 and 20 A/g respectively within a potential window of −1.0 to 0 V in 6 M KOH solution. The capacitance retains 92% after 4000 cycles of charge/discharge, implying a good cycling stability.

Surface-restricted modification of nanocellulose aerogels in gas-phase esterification by di-functional fatty acid reagents

The gas-phase esterification of nanocellulose aerogels was implemented, using several fatty acid derivatives varying (1) the size of their aliphatic tails, (2) their reactive groups, namely acyl chloride or anhydride, (3) their mono or bi-functionality. The process parameters were adapted to the vaporization properties of each reagent in order to obtain efficient reagent transfer to the nanocellulose surface, as measured from the amount of deposited reagents. For each of these, the esterification extent was deduced from gravimetric measurements and spectroscopic analysis (FTIR, 13C CP/MAS NMR). The esterification with the mono-functional fatty acid derivatives progressed, as demonstrated earlier, from the surface of the substrates to their core. Strikingly, when bi-functional esterifying reagents such as cyclic anhydride and diacyl chloride were used, the derivatization was strictly limited to the nanocellulose surface, as evidenced from the correlation between the degree of substitution and the specific surface of the aerogel. The green gas-phase derivatization scheme reported here is thus a versatile process that can be easily adapted to many esterifying reagents, leading to interesting possibilities for selective surface topochemistry of the substrates.

Carbon aerogels from bacterial nanocellulose as anodes for lithium ion batteries

Carbon aerogels with large open pores and high surface area are fabricated via pyrolysis of a readily available natural resource, e.g., bacterial nanocellulose (BNC) aerogels. Freeze-drying of the BNC hydrogels is used to preserve the 3D open network structure upon calcination whereas using Fe(III) improves the yield and H/C ratio. These carbon aerogels are explored as anodes in lithium ion batteries where it is shown that they deliver superior capacity retention and rate performance compared to other carbon-based materials.

Super water absorbing and shape memory nanocellulose aerogels from TEMPO-oxidized cellulose nanofibrils via cyclic freezing–thawing

Mechanically robust cellulose nanofibril (CNF) aerogels with ultralow density (8 mg cm−3), superior porosity (99.5%), super water absorbency (104 g g−1 water/dried mass), high crystallinity (68.5%) as well as exceptional wet resilience and water activated shape recovery were facilely fabricated for the first time by ice-crystal templated self-assembly of TEMPO oxidized CNFs via cyclic freezing–thawing. With ultrathin widths (1–2 nm), high aspect ratios (several hundreds) and numerous surface polar hydroxyls and carboxyls, TEMPO oxidized CNFs behaved similar to aqueous soluble polymers to form strong freestanding hydrogels by repetitive freezing (−20 °C, 15 h) and thawing (room temperature, 9 h). The spaces occupied by the several hundred microns wide ice crystals were well preserved upon freeze-drying, deriving macroporous CNF aerogels with over 99% porosity of interconnected pores. The freezing induced self-assembling of CNFs was observed at a low concentration of 0.05%, whilst more ordered macroporous honeycomb structures were observed at and above 0.2%. Exchanging water in the CNF hydrogel with tert-butanol generated hierarchical CNF aerogels containing several hundred microns sized macroscopic as well as mesoscopic pores ranging from 2 to 90 nm with further improved specific surface area (117.8 m2 g−1), pore volume (1.19 cm3 g−1) and water absorption (116 g g−1). All CNF aerogels demonstrated super water absorbency, fast water-activated shape recovery in 4 s and reusability for at least 20 times.