Cellulose Nanocrystal Aerogels Research Articles

Effects of Freeze-Drying Processes on the Acoustic Absorption Performance of Sustainable Cellulose Nanocrystal Aerogels.

Cellulose aerogels have great prospects for noise reduction applications due to their sustainable value and superior 3D interconnected porous structures. The drying principle is a crucial factor in the preparation process for developing high-performance aerogels, particularly with respect to achieving high acoustic absorption properties. In this study, multifunctional cellulose nanocrystal (CNC) aerogels were conveniently prepared using two distinct freeze-drying principles: refrigerator conventional freezing (RCF) and liquid nitrogen unidirectional freezing (LnUF). The results indicate that the rapid RCF process resulted in a denser CNC aerogel structure with disordered larger pores, causing a stronger compressive performance (Young's modulus of 40 kPa). On the contrary, the LnUF process constructed ordered structures of CNC aerogels with a lower bulk density (0.03 g/cm3) and smaller apertures, resulting in better thermal stability, higher diffuse reflection across visible light, and especially increased acoustic absorption performance at low-mid frequencies (600-3000 Hz). Moreover, the dissipation mechanism of sound energy in the fabricated CNC aerogels is predicted by a designed porous media model. This work not only paves the way for optimizing the performance of aerogels through structure control, but also provides a new perspective for developing sustainable and efficient acoustic absorptive materials for a wide range of applications.

A pushed biosynthesis of 2,6-dihydroxybenzoic acid by the recombinant 2,3-dihydroxybenzoic acid decarboxylase immobilized on novel amino-modified lignin-containing cellulose nanocrystal aerogel

Production of 2,6-dihydroxybenzoic acid (2,6-DHBA) via enzymatic carboxylation of resorcinol by decarboxylases is of great promising but shows depressed equilibrium conversion. In this study, 2,3-dihydroxybenzoic acid decarboxylase from Aspergillus oryzae (2,3-DHBD_Ao) pushing the conversion towards carboxylation for efficient 2,6-DHBA biosynthesis was achieved. Meanwhile, a novel amino-modified and lignin-doped cellulose nanocrystal aerogel (A-LCNCA) with high specific surface area and prominent CO2 capture was prepared for 2,3-DHBD_Ao immobilization. 2,3-DHBD_Ao@A-LCNC contributed a further enhanced conversion of carboxylation with the maximal conversion of 76.2 %, which was correlated to both the activity of 2,3-DHBD_Ao and the high CO2 loading capacity of A-LCNCA. Moreover, 2,3-DHBD_Ao@A-LCNC exhibited superior performances in a wider range of temperature and higher concentrations of substrate, with a prolonged storage period longer than 30 days. After seven cycles reuse, 2,3-DHBD_Ao@A-LCNCA could retain 85.3 % of its original activity. These results suggest a considerable potential of 2,3-DHBD_Ao@A-LCNCA in the selective biosynthesis of 2,6-DHBA.

Bioinspired “aerogel grating” with metasurfaces for durable daytime radiative cooling for year-round energy savings

Passive radiative cooling strategy shows a great potential in mitigating global warming and reducing energy consumption. However, existing radiative coolers generally show poor solar-reflection and be easily dampened by environmental aging, making the cooling ineffective. Mimicking the biological design for thermoregulation has given promise for realizing high performance and long time cooling. Here, inspired by structural whiteness in butterflies, we developed a sustainable cellulose nanocrystal aerogel grating (CAG) with designable metasurfaces for efficient radiative cooling, which can be achieved by using copper meshes as well as manipulating the 2D confined freeze casting process. The simulation indicates that the regular tubes and irregular gully on the aerogel surface act as gratings to achieve diffraction and broadband reflection of sunlight. Such CAG exhibits ultrahigh solar reflectance (97.4 %), high infrared emittance (94 %), and anti environmental aging function (anti-ultraviolet light/ contamination). It achieves a sub-ambient temperature drop of 10.5 °C under direct sunlight and still approximately 9.4 °C even after 6 months of exposure to hot weather. Building simulations demonstrate that CAG can save 47 % of the worldwide energy consumption for cooling each year. This work provides insight into the design of bioinspired thermal-regulating materials to save energy consumption.

Cellulose Nanocrystal Gels with Tunable Mechanical Properties from Hybrid Thermal Strategies

Gels are useful materials for drug delivery, wound dressings, tissue engineering, and 3D printing. These various applications require gels with different mechanical properties that can be easily tuned, also preferably excluding the use of chemical additives, which can be toxic or harmful to the body or environment. Here, we report a novel strategy to synthesize cellulose nanocrystal (CNC) gels with tunable mechanical properties. Sequential freeze-thaw cycling and hydrothermal treatments were applied to CNC suspensions in different orders to give a series of pristine CNC hydrogels. Freeze-drying of the hydrogels also afforded a series of lightweight CNC aerogels. The mechanical properties of the hydrogels and aerogels were studied by rheological measurements and compression strength tests, respectively. Specifically, the complex modulus of CNC hydrogels ranged from 160 to 32,000 Pa among eight different hydrogels, while Young's modulus of CNC aerogels was tuned from 0.114 to 3.98 MPa across five different aerogels. The microstructures of aerogels were also investigated by scanning electron microscopy and X-ray microtomography, which revealed remarkable differences between the materials. Solvent sorption-desorption tests showed that the reinforced networks have excellent stability over the basic CNC aerogels in ethanol, demonstrating a material enhancement from the preparation strategies we developed. Thermal conductivity and thermal stability for these materials were also investigated, and it was found that the lowest thermal conductivity was 0.030 W/m K, and all of the aerogels are generally stable below 280 °C. These characteristics also expand the potential applications of this family of CNC gels to lightweight supporting materials and thermal insulators.

Metal oxide cluster-assisted assembly of anisotropic cellulose nanocrystal aerogels for balanced mechanical and thermal insulation properties.

Cellulose nanocrystal (CNC) materials grant abundant possibilities for insulation, however, their extensive application is hindered by the intrinsic tradeoff between their thermal insulating performance and mechanical properties. Here, we show that CNC aerogels with balanced thermal and mechanical performance can be fabricated via a 1 nm metal oxide cluster (phosphotungstic acid, PTA)-assisted unidirectional freeze-drying processing. The as-prepared hybrid aerogels with hierarchical porous structures consisting of layer-by-layer CNC nanosheets enable the decoupling of the strengthening of mechanical properties and the enhancement of thermal insulating capabilities. Within layered structures, the surface-doped nanosized PTA clusters with negative charges behave as dynamic physical cross-linking points, and continuous networks of PTA-doped CNC can be formed via multiple supramolecular interactions (e.g., electrostatic attractions and hydrogen bonds). The afforded stable three-dimensional network structures are able to withstand externally applied forces and large deformations, endowing the aerogels with excellent mechanical performance. Moreover, the inter-layer gap is dominated by nanopores, endowing much lower thermal conductivities along the radial direction in comparison to the axial direction. The addition of PTA clusters also contributes to the obvious enhancements of the fire-retardant properties. Our discoveries provide a facile approach for the design and scalable production of CNC-based insulation materials with optimized mechanical properties and additional fire-retardant properties.

Effect of Particle Size and Concentration of Nanocellulose on Thermal Properties of Nanocellulose Aerogel

Traditional building thermal insulation materials are mostly from petrochemical materials or high energy consumption processing process, which seriously restricts the sustainable development of the economy. Therefore, it is urgent to develop high-performance green insulation materials with high efficiency, energy saving, light weight and environmental protection. In this paper, nanocellulose aerogels made from cellulose nanocrystal (CNC) and cellulose nanofibril (CNF) with different concentrations were prepared by freeze-drying process. The effect of the concentration of nanocellulose on the thermal properties of aerogel was fully studied by infrared thermal imager, thermogravimetric analyser, and thermal conductivity meter. The study found that the overall thermal insulation performance of CNF aerogel was better than that of CNC aerogel. The concentration had a significant effect on the thermal insulation performance and thermal conductivity of the two aerogels. Among all the prepared nanocellulose aerogels, CNF aerogel with a concentration of 1% showed the best thermal insulation performance, which had a temperature difference of 24.1°C and a thermal conductivity of 0.030 W/mK.

Adsorption of Oil by 3-(Triethoxysilyl) Propyl Isocyanate-Modified Cellulose Nanocrystals

Oil leaks into water bodies and increased organic pollutants harm the environment and ecosystem in several ways, and cleaning up oil spills from water bodies is a global challenge. This research aimed to construct modified cellulose nanocrystal (CNC) based aerogels with 3-triethoxysilyl propyl isocyanate (TEPIC) to evaluate their potential application in oil adsorption. Here, a freeze-drying method was employed to make CNC aerogels. The aerogels were characterized using scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) analysis, porosity and density measurements, Fourier transform infrared spectroscopy (FTIR), water contact angle (WCA) measurement, compressive strength, and oil adsorption capacity. SEM results confirmed that the aerogels have a largely porous structure, including a community of uniformly interconnected cellulose fibers. Moreover, the studied aerogels had a low density due to the high porosity. Also, the small pore diameter and high specific surface area were confirmed by the BET evaluation. FTIR confirmed the existence of functional groups and strong hydrogen bonding between CNC/TEPCI/Urea molecules. All TEPIC-modified CNC aerogels had water contact angle values greater than 130° indicating their hydrophobicity. The highest oil and glycerol adsorption was obtained with the use of modified CNC aerogels. Thus, the sample modified with 3 wt% TEPIC showed the highest adsorption capacities of 130 ± 7.22, 120 ± 4.75, and 95.28 ± 4.82 gg−1 for motor oil, vegetable oil and glycerol, respectively. The results of this study showed that ultra-light, hydrophobic and oil adsorbent materials based on chemically modified CNC aerogels can successfully be fabricated.

Hydrophobic and lipophilic cellulose nanocrystal aerogel prepared by methyltrichlorosilane via vapor‐phase reaction

AbstractHydrophobic and lipophilic cellulose nanocrystal (CNC) aerogel was prepared by vapor‐phase reaction using methyltrichlorosilane (MTS). The water contact angle of the MTS‐CNC aerogel was up to 148.5°. The residual Cl element in MTS was negatable in the modified CNC aerogel. The MTS‐CNC aerogel only adsorbed the paraffin, but no water was adsorbed. The pseudo‐first‐order kinetic model could well describe the adsorption of the liquid paraffin oil on the CNC aerogel and the maximum adsorption capacity was about 60 g g−1 at 40°C. Multiple steps controlled the whole adsorption of oil on the MTS‐CNC aerogel, but 90% of the maximum adsorption capacity of paraffin was taken up in the first 1 min. The vacuum desorption could effectively remove the adsorbed oil at 80°C, and the good performance of the MTS‐CNC aerogel in adsorption and desorption was retained during 5 cycles. Therefore, the MTS‐CNC aerogel could be a potential candidate for removing oil from the oil‐spilled water system.

Comparison of characteristics of the cellulose nanocrystal aerogels aminosilane-functionalized through gas-phase reaction

Comparison of characteristics of the cellulose nanocrystal aerogels aminosilane-functionalized through gas-phase reaction

One-step in-situ green synthesis of cellulose nanocrystal aerogel based shape stable phase change material

The confinement of phase change material (PCM) within 3D porous structure has been identified as an attractive and efficient strategy to fabricate high performance shape stable PCM (SSPCM). Nevertheless, the conventional fabrication techniques suffer limitations such as cumbersome preparation process, the usage of toxic reagents and high-cost additives. Herein, an ingenious and green one-step in-situ synthesis method was developed to prepare SSPCM, which was constructed by the combination of polyethylene glycol (PEG) within chemically cross-linked cellulose nanocrystal (CNC) aerogel. The as-prepared CNC/PEG phase change aerogel composite exhibited excellent shape stability, even when compressed upon the melting point of PEG, the composite still maintained its original shape without any leakage. Meanwhile, the composite possessed high phase change enthalpy of 145.8 J/g and extraordinary cyclic reversibility after 100 thermal cycles. Specifically, the thermal conductivity and mechanical properties of the novel CNC/PEG phase change aerogel composite were dramatically improved due to the higher thermal conductivity of CNCs and the chemically cross-linked CNC framework. This work established a promising sustainable approach for the exploitation of CNC/PEG phase change aerogel composite with outstanding comprehensive properties, which would offer great opportunities for applications in thermal energy storage systems.

Graphene oxide and polyethyleneimine cooperative construct ionic imprinted cellulose nanocrystal aerogel for selective adsorption of Dy(III)

Because of dysprosium’s unique physical and chemical properties and limited supply, the price of rare earth dysprosium has been high in recent years. Therefore, the study of the method of high efficiency selective separation of dysprosium has the double value of scientific research and practical economy. In this paper, we used periodic cellulose nanocrystals as the basic structure, polyethylenimine and graphene oxide were introduced, combined with imprinting technology, to construct a porous imprinted aerogel and use it for selective adsorption of Dy(III). The physical and chemical properties were characterized by SEM, FT-IR and TGA. It was proved that both polyethylenimine and graphene oxide were crosslinked effectively with cellulose nanocrystals. Adsorption experiments showed that the composite imprinted aerogel could selectively adsorb dysprosium effectively, and the maximum adsorption capacity for Dy(III) was 36.495 mg g−1. The reproducibility experiment showed that aerogel had good regeneration ability. In conclusion, cellulose nanocrystal aerogel, which is environmentally friendly, efficient and repeatable, is expected to provide a new direction for the recovery of rare earth elements.

Chiral Nematic Cellulose Nanocrystal/Germania and Carbon/Germania Composite Aerogels as Supercapacitor Materials

The development of novel aerogel materials with chiral nematic ordered structures offers exciting pathways for the fabrication of multifunctional hybrid materials with enhanced functionality. Aerogels prepared from cellulose nanocrystals are especially interesting due to their unique structural properties. To promote applicability in energy storage materials, it is often necessary to incorporate metals and metal oxides into three-dimensional porous nanostructures. In this study, germania was incorporated into a chiral nematic cellulose nanocrystal aerogel using a sol–gel method. Interestingly, our approach does not disturb the order of the original chiral nematic CNC aerogels, providing hybrid aerogels with a large concentration of randomly distributed GeO2 nanoparticles and specific surface areas up to 705 m2 g–1. Carbonization of the composite material afforded a highly ordered material with no collapse during compression and good shape recovery after release. The combination of the electrochemical double layer capacitance provided by the carbonaceous skeleton and the pseudocapacitive contribution from the GeO2 nanoparticles resulted in materials with a maximum Cp of 113 F g–1 that exhibited good capacitance retention. To push the boundaries of safer energy storage devices based on renewable resources, we demonstrate the preparation of an all-cellulose solid-state symmetric supercapacitor.

Ultrathin‐Walled 3D Inorganic Nanostructured Networks Templated from Cross‐Linked Cellulose Nanocrystal Aerogels

AbstractA key challenge in the development of materials for applications in the fields of opto‐ and nanoelectronics, catalysis, separation, and energy conversion is the ability to fabricate 3D inorganic semiconductive nanostructures in a precisely‐controlled and cost‐effective manner. This work describes the fabrication of 3D nanostructured TiO2 monoliths by coating ultraporous cross‐linked cellulose nanocrystal (CNC) aerogel templates with TiO2 layers of controlled thickness via atomic layer deposition (ALD). Following calcination, the resulting hollow inorganic ultraporous 3D networks form the thinnest self‐supporting semiconductive structure (7 nm) fabricated directly on a conductive substrate. The CNC‐templated ALD–TiO2 electrodes are applied toward photoelectrochemical water splitting. The results show that a TiO2 coating as thin as 15 nm produces a maximum water splitting efficiency, resulting in materials savings and reduced fabrication time.

The aminosilane functionalization of cellulose nanocrystal aerogel via vapor‐phase reaction and its CO2 adsorption characteristics

AbstractThe cellulose nanocrystal (CNC) aerogel was functionalized using aminosilane via vapor‐phase reaction and then the modified CNC aerogel was characterized by several techniques such as Fourier transform infrared spectroscopy, FE‐SEM, and elemental analysis. Finally, the CO2 adsorption on the aminosilane‐grafted CNC aerogel was featured using a dual‐site Langmuir adsorption model. The result showed triethylamine significantly enhanced the amine loading, being 7.06 mmol g−1 at 120°C. The primary amine groups in the aminosilane survived the vapor‐phase reaction. The amine groups were homogeneously distributed inside the aerogel due to its porous structure. The dual‐site Langmuir model could well describe its CO2 sorption characteristics. The adsorption capacity was up to 2.57 mmolCO2 g−1 at 25°C and 101.33 kPa, of which the chemisorption entirely dominated, and it decreased only 3%–4% after six runs. Therefore, these features positively suggested the vapor‐phase reaction provided a new and feasible method to functionalize CNC aerogel for the capture of CO2.

Stabilization of Metastable Indomethacin α in Cellulose Nanocrystal Aerogel Scaffolds.

Indomethacin (IM) is a small molecule active pharmaceutical ingredient (API) that exhibits polymorphism with the γ-form being the most thermodynamically stable form of the drug. The α-form is metastable, but it exhibits higher solubility, making it a more attractive form for drug delivery. As with other metastable polymorphs, α-IM undergoes interconversion to the stable form when subjected to certain stimuli, such as solvent, heat, pH, or exposure to seed crystals of the stable form. In this study, IM was crystallized into cellulose nanocrystal aerogel scaffolds as a mixture of the two polymorphic forms, α-IM and γ-IM. Differential scanning calorimetry (DSC) and Raman spectroscopy were used to quantitatively determine the amount of each form. Our investigation found that the metastable α-IM could be stabilized within the aerogel without phase transformation, even in the presence of external stimuli, including heat and γ-IM seed crystals. Because interconversion is often a concern during production of metastable forms of APIs, this approach has important implications in being able to produce and stabilize metastable drug forms. While IM was used as a model drug in this study, this approach could be expanded to additional drugs and provide access to other metastable API forms.

Preparation of an Amine-Modified Cellulose Nanocrystal Aerogel by Chemical Vapor Deposition and Its Application in CO2 Capture

At present, cellulose nanocrystal (CNC) aerogels with high adsorption properties for CO2 are mostly prepared by the liquid phase method which leads to the problem of modifier loss. In this work, a ...

Chemically Cross-Linked Cellulose Nanocrystal Aerogels for Effective Removal of Cation Dye.

In this study, porous aerogels were prepared by directional freeze-drying via cross-linking cellulose nanocrystals (CNCs) with poly(methyl vinyl ether-co-maleic acid) (PMVEMA) and poly(ethylene glycol) (PEG). The thermal properties and physical adsorption performance toward cation methylene blue dye of the obtained CNC aerogels were investigated. The maximum degradation temperature was increased from 324°C of CNCs to 355°C of cross-linked CNC aerogels. The dye adsorption isotherm results showed that the maximum methylene blue adsorption capacity of CNC aerogels was 116.2 mg g−1, according to the Langmuir model, which was mainly due to the electrostatic attractions between negatively charged carboxyl groups or sulfonate groups on the CNC aerogles and cation MB molecules. The reusability test showed that the CNC aerogels contained the same dye adsorption performance in five adsorption/desorption cycles. Overall, this study described an ideal alternative for water purification with high dye adsorption capacity and enhanced physical performance.

Elastic nanocellulose/graphene aerogel with excellent shape retention and oil absorption selectivity

In this work, a simple preparation method is developed to synthesize aerogels with great shape retention for oil absorption application. First, an elastic aerogel is prepared by ice-templating an aqueous suspension of cellulose nanocrystal (CNC) with polyethyleneimine (PEI) and epoxy cross-linker. After the lyophilization process, the prepared CNC aerogel shows highly porous structures and allows rapid absorption of solvents, including water, through capillary forces. In particular, the aerogel after compression recovers to its original shape without any deformation. To realize selective oil absorption, chemically modified graphene sheets are attached to the aerogel pore surface via a dip coating process. SEM images show that the graphene sheets form a uniform and dense monolayer on the pore surfaces of the CNC aerogel. This graphene coating improves hydrophobicity without compromising the overall porosity and elasticity of the aerogel. After surface modification, the CNC/graphene aerogel can selectively absorb organic solvents with a weight ratio ranging from 25 to 58 g/g. The CNC/graphene aerogel can be squeezed dried into a compact size, and retains its original shape rapidly in contact with liquids. The absorbed liquids can be collected by mechanical squeezing, and the absorption performance of this aerogel remains unchanged after 100 squeezing-swelling cycles. Several examples are also demonstrated to show the feasibility of applying this aerogel in oil spill control.

Ultralight Aerogels with Hierarchical Porous Structures Prepared from Cellulose Nanocrystal Stabilized Pickering High Internal Phase Emulsions.

Cellulose nanocrystal (CNC)-based aerogels with extremely low density and hierarchical porous structure were constructed via a facile Pickering-emulsion-templated strategy. In this method, aminated CNCs (CNC-NH2) were synthesized to stabilize o/w Pickering high internal phase emulsions (Pickering HIPEs). Amino groups were introduced to CNCs to decrease the net surface charges of CNCs, enhance their aggregation, and therefore achieve Pickering HIPEs stabilized by the particles of ultralow content (∼0.1 wt %). A series of CNC aerogels was then obtained by freeze drying these emulsions. The resulting aerogels were ultralight with a density that reached ca. 0.5 mg/cm3 (an order of magnitude lower than that previously reported for CNC aerogels) and an ultrahigh porosity (up to 99.969%). Contributed to the extremely low density, the thermal conductivity of the aerogels was around 0.021 W/(m·K) which is lower than that of air (0.024 W/(m·K)). This novel strategy could be applied to other materials, such as graphene and carbon nanotubes, to prepare ultralight aerogels with controllable porous structures and unique properties.

Flexible and Tough Cellulose Nanocrystal/Polycaprolactone Hybrid Aerogel Based on the Strategy of Macromolecule Cross-Linking via Click Chemistry

Cellulose nanocrystal (CNC) aerogel suffers from inherent brittleness, owing to the rigidity of crystals and the limited mobility between individual crystals. Conventional small molecule based cross-linking improves the toughness of CNC aerogel, but can hardly improve its flexibility. In this work, a macromolecule-based click cross-linking strategy with polycaprolactone diol as the cross-linker is employed to fabricate the CNC-PCL hybrid aerogel with enhanced toughness and flexibility. The molar ratio of CNC/PCL and the concentration of hydrogel are studied as the two key factors greatly affecting the mechanical performance of CNC-PCL aerogel. The resulted CNC-PCL hybrid aerogels show the highest compressive strength of 595 ± 100 kPa when the molar ratio of CNC/PCL is 1:0.1 (CP-0.1) and the highest shape recovery rate of 96.3 ± 2.5% when the molar ratio of CNC/PCL is 1:0.25 (CP-0.25). The surface polarity of CNC-PCL aerogel behaves as amphipathic due to the introduction of hydrophobic PCL. The thermal sta...