High-Adsorption, Self-Extinguishing, Thermal and Acoustic

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High-Adsorption, Self-Extinguishing, Thermal and AcousticResistance Aerogels Based on Organic and Inorganic Waste Valorization from Cellulose Nanocrystals and Red Mud Ge Zhu, Hui Xu, Alain Dufresne, and Ning Lin ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01244 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 18, 2018

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ACS Sustainable Chemistry & Engineering

High-Adsorption, Self-Extinguishing, Thermal and Acoustic-Resistance Aerogels Based on Organic and Inorganic

Waste

Valorization

from

Cellulose

Nanocrystals and Red Mud Ge Zhu, † Hui Xu, ‡ Alain Dufresne, § and Ning Lin †∗, † School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of Technology, #122 Luoshi Road, Wuhan 430070, P. R. China ‡ School of Chemistry and Materials Science, Ludong University, #186 Hongqi Zhong Road, Yantai 264025, P. R. China § Université Grenoble Alpes, CNRS, Grenoble INP, LGP2, F-38000, Grenoble, France

KEYWORDS. Cellulose nanocrystal; Red mud; Aerogel; Waste valorization; Organic-inorganic hybrid.

Corresponding Author. * Ning Lin, Email: [email protected].

ACS Paragon Plus Environment

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ABSTRACT. Waste transformation as the source to valuable materials is an effective strategy for the high-valued valorization in view of socio-economic and environmental issues. Inspired from the concept of organic-inorganic hybrid, we proposed the utilization of cellulose nanocrystals (CNC, extracted from biomass waste) and red mud (RM, from industrial waste) to fabricate composite aerogels as potential construction materials with the multifunction of adsorption, thermal insulation, acoustic resistance and flame retardancy. For equal loading level of modified CNC (Si-CNC) and RM (1:1, w/w) components, the obtained aerogel was chemically crosslinked with diisocyanate to enhance its structural stability and mechanical properties. It exhibited an improved compression modulus of 3.7 MPa together with high porosity (98.8%) and specific surface area (73.23 m2/g). As a result of the intrinsic features of organic Si-CNC (skeleton) and inorganic RM (particles aggregate), the Si-CNC/RM-1/c composite aerogel displayed significant functional performances such as magnetic conductivity, rapid oil adsorption of 30 times its mass, 20.8% reduction in thermal conductivity, 24.5% increase in sound-absorption coefficient, and highly efficient self-extinguishing behavior within 2 s.

INTRODUCTION The socio-economical concerns on the negative environmental impact and economic loss have driven wastes reutilization for the production of valuable energy and materials (i.e. valorization) which has gained more interests during last twenty years.[1] As one of the key objectives, the EU’s Roadmap to a Resource Efficient Europe has announced the highlights of wastes valorization on the high-quality recycling, energy and materials recovery by 2020. Turning waste into high-valued material affords the opportunity to develop recycled and low-cost materials and


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also scales down the demand for extraction of new resources.[2,3] The utilization of traditional wastes (including industrial waste, food and life wastes) has been explored in the development of materials for obtaining alternative building materials, reinforced additives, transition metal based catalysts, recycled packaging and even biomaterials.[4] General classification of the wastes can be divided as organic waste such as diverse plants and animals’ feedstock and processing byproducts, and inorganic waste including various large-scale industrial wastes. Cellulose nanocrystal (CNC) is a rigid and rod-like nanomaterial isolated from natural cellulose, a residual biomass widely existing in organic waste, such as straw, cotton, wood and tunicin.[5] As a highly-crystalline nanoparticle, the researchers are dedicated to the application of CNC in the fields of composites and functional materials, appealed by its remarkable mechanical properties,

special

surface

chemistry

and

biological

properties

(biocompatibility,

biodegradability and low toxicity).[6,7] When extracted from tunicin (the cellulose found in tunicate, a sea animal), the CNC is typically in the dimensions of several micrometers in length and 10-20 nanometers in diameter since the cellulose microfibrils in tunicin have a helical organization[8] and are highly crystalline. It exhibits therefore a ultrahigh aspect ratio (> 100) in comparison with CNC isolated from agro-resources such as wood or cotton (aspect ratio around 10-15).[9] Because of its superior modulus and high aspect ratio, the tunicate CNC has gained importance as potential reinforcement in composites (low percolating threshold) and for the development of structural nanomaterials (three-dimensional network).[10] Among various industrial by-products, red mud (RM) is a large scale inorganic waste derived from the Bayer process for the alumina production.[11] With aluminum, iron, silicon, titanium oxides and hydroxides as the main components, the global yearly production of RM is reported to be around 90 million tons and most RM is treated by land or sea dumping particularly in the


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developing countries. However, the random disposal of RM is expected to cause serious environmental issues, because of its hazardous features such as its alkaline nature and the chemical and mineralogical species present in RM.[12] Despite several attempts for the development of RM in the catalytic or adsorbed materials[13-15], its practical application is still restricted because of its low utilization and high treatment cost. Aerogels are porous materials fabricated from a gel in which the liquid molecules have been replaced with air by freeze-drying or critical point drying. Based on their high porosity and light weight properties, aerogel materials commonly exhibit remarkable advantages for the development of sorbents, thermal-insulating materials, functional templates and biomimetic substrates.[16,17] Cellulose nanocrystal (CNC) is a promising candidate as organic network for the construction of aerogel materials[18-21], typically with a porosity higher than 90% and a density lower than 0.1 g⋅cm-3. Heath and Thielemans firstly reported the preparation of aerogels from cotton CNC by a solvent exchange process from water to ethanol and supercritical CO2 drying, presenting a high specific surface area and low shrinkage.[22] Recently, a cellulosic scaffold aerogel was developed from acetylated CNC by freeze-drying, exhibiting multifunctions of hydrophobicity, oleophilicity and lipophilicity.[23] Nowadays, the emphasis on the development of cellulose-based aerogels focuses on the improvement of the porous microstructure stability, enhancement of the physical performances and exploiting additional functions or multifunctions for potential advanced applications. Based on the organic-inorganic hybrid strategy and purpose of wastes valorization, we propose in this study the combined utilization of rod-like cellulose nanocrystals and iron-containing red mud for constructing multifunctional composite aerogels. The tunicate cellulose nanocrystals extracted from the shell of sea pineapples (Halocynthia roretzi) provided the skeleton of the


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aerogels, together with the complexion and fixation of various contents of red mud. The mechanical properties of the composite aerogels were synergistically enhanced by the presence of inorganic particles (red mud) and chemical crosslinking with isocyanate (HDI). Meanwhile, the composite aerogel containing moderate red mud contents displayed a homogeneously porous microstructure, as well as high porosity and specific surface area. Provided by the features of organic CNC and inorganic RM components, the developed aerogels possessed a combination of functions such as magnetic conductivity, oil/organic molecule absorbability, thermal and sound insulation as well as flame retardancy. The attempt of this work fulfilled the organic and inorganic wastes valorization of tunicate cellulose nanocrystals and red mud, and the development of multifunctional aerogels that could be potentially used as high-valued materials in the fields of construction, adsorption/separation and energy storage. EXPERIMENTAL SECTION Materials. Raw tunicate (sea pineapple) was collected as the organic source from the marineland at Weihai city, China. Red mud was kindly donated by the mining corporation in Anhui province, China. Methyltrimethoxysilane (MTMS) was used as silylating reagent for surface modification and was purchased from Macklin Corporation (Shanghai, China). Hexamethylene diisocyanate (HDI) was used as crosslinking agent for aerogels, and purchased from Aladdin Corporation (Shanghai, China). Potassium hydroxide (KOH), sodium hydroxide (NaOH), sodium hypochlorite (NaClO), glacial acetic acid and sulfuric acid (H2SO4) were supplied by Aladdin Corporation and used without any treatment. Extraction of cellulose nanocrystals (CNC) from tunicate. The extraction of CNC from raw tunicate (sea pineapple) involved both purification and hydrolysis processes.[24] Briefly, the dried


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mantle of the raw sea pineapple was crushed into powder, followed by the removal of proteins with an alkaline solution (5 wt% KOH). Repeated bleaching treatments using NaClO/acetic acid solution were then performed to remove pigments and impurities from the mixture. Thereafter, the suspension containing purified tunicate cellulose (tunicin) was mechanically treated with a Warring blender (8010S, USA). Regarding the hydrolysis process, concentrated sulfuric acid (98 wt%) was added dropwise to the purified tunicate cellulose suspension under vigorous mechanical stirring in an ice bath. The temperature was then raised to 50 °C, and kept for 3 h with a H2SO4 concentration of 55 wt% to release the cellulose nanocrystals. Finally, after successive washing/centrifugation and dialysis treatments, the aqueous suspension with homogeneously dispersed tunicate CNC was obtained. Surface silylation of CNC. The surface modification of CNC was based on the chemical reaction between the surface hydroxyl groups of CNC and MTMS (silylated agent) in water for 4 h at room temperature.[25] The modified cellulose nanocrystals (Si-CNC) were purified to remove free silane and unreacted polysiloxanes by dialysis in water for five days. Ball-milling treatment of red mud. The raw red mud (RM) was ball-milled with 6 mm-diameter zirconia balls (used for the ball milling of inorganic particles) in Planetary Mill at a rotation speed of 250 rpm for 24 h. The ball-milled RM powders were further sieved using nylon filter (0.106 mm) to remove the larger particles. Preparation of composite aerogels. The composite aerogels containing various contents of inorganic RM powders and organic Si-CNC components were fabricated by mixing 1 wt% SiCNC aqueous suspension (with regulated pH to 7.0 with 0.2 M NaOH solution) and as-prepared red mud (RM) aqueous suspension. The mixture was homogeneous dispersed by applying strong


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mechanical stirring (2 h) and ultrasonic treatment (15 min). The mixture suspension was poured into plastic molds and carefully moved into liquid nitrogen for freezing, followed by the freezedrying treatment to produce the composite aerogels. The content of organic (Si-CNC) and inorganic components (RM) in the prepared aerogels was regulated as 1 : 0, 0.25, 0.5, 1, 2, 4, 6 (w/w) with a fixed Si-CNC weight of 35 mg. Aerogel crosslinking with diisocyanate (HDI). In order to enhance the mechanical properties, the prepared composite aerogels were crosslinked using HDI in acetone (10 wt%) with triethylamine as the catalyst at 30 °C for 48 h.[26] After the reaction, the crosslinked aerogels were washed thoroughly with acetone to remove the unreacted reagents. For subsequent analysis and characterization, the nomenclatures Si-CNC/RM-x/c and Si-CNC/RM-x were used to refer to composite aerogels with or without HDI crosslinking (c), according to the RM content ranging from 0.25 to 6 (x). Characterization. Surface modification of CNC and composition analysis of RM. The surface silylation of cellulose nanocrystals was investigated by Fourier transform infrared spectroscopy (FTIR) on a FTIR iS5 spectrometer (Nicolet, Madison, USA) in the range 4000‒400 cm-1 and X-ray photoelectron spectroscopy (XPS) using a ESCALAB 250Xi equipment (Thermo Fisher Scientific, USA). The morphological observation of cellulose nanocrystals before and after modification was performed using transmission electron microscopy (TEM). A drop of diluted suspension containing 0.01 wt% CNC or Si-CNC was negatively-stained with 2% (w/v) uranyl acetate, and observed on a Tecnai G2 F30 instrument (FEI, USA) at 300 kV. X-ray diffraction (XRD) analysis was performed to investigate the crystallinity of the materials. The freeze-dried


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powders were analyzed on a D8 Advance X-ray diffractometer (Bruker, Germany) using Cu Kα radiation (λ = 0.154 nm) at 40 kV and 60 mA, with diffraction angles (2θ) ranging from 5° to 40°.The components and composition analysis of RM was performed by X ray fluorescence (XRF, AxiosmAX Advanced, The Netherlands). The morphology of the ball-milled RM powder was observed by scanning electron microscopy (SEM, Hitachi S-4800 instrument) at an accelerating voltage of 20 kV. Crosslinking, microstructure and macroscopic properties of composite aerogels. The crosslinking reaction between the aerogel and diisocyanate was investigated by FTIR with KBr pellets using a spectral width ranging from 4000 to 400 cm-1. The composite aerogels were cut in the transverse direction, coated with gold using a sputter coater to observe their cross-sectional morphology by SEM at an accelerating voltage of 10 kV. The values of specific surface area and pore characteristics for composite aerogels were determined by a Micrometrics Tri Star II 3020 nitrogen adsorption-desorption apparatus. The specific surface area was determined by the Brunauer-Emmett-Teller (BET) method from linear region of the isotherms in the 0.06–0.20 relative P/P0 pressure range. The pore size distribution was derived from the desorption branch of the isotherms by the Barrett–Joyner–Halenda (BJH) method. The apparent density (ρaerogel) and skeletal density (ρs) of the prepared aerogels were calculated from their dimensions and weight using the following equations:


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where

and

are the weight and volume of the aerogel.

represent the weight

fractions of the Si-CNC component and the RM component, respectively, in the aerogel. and

are the densities of silylated cellulose nanocrystals and RM, measured by a

densitometer (Micromeritic AccuPyc II 1330, USA) operating at room temperature. The porosity of the aerogel was calculated as the volume fraction of voids within the aerogel using the following equation:[27]

Contact angle measurements, mechanical and thermal properties of composite aerogels. Surface contact angle measurements were carried out for the cross-section of the composite aerogels on an instrument (POWEREACH, China) with absorbed water (100 µL) by a micro-injection needle. The value of the contact angle was determined at three different positions on each sample with a stable droplet on the cross-section of aerogels. Regarding raw RM, the powder was compacted as plate-like sample with smooth surface under a pressure of 20 MPa for contact angle measurements. Compression tests were performed on cylindrical aerogels with dimensions 15 mm (diameter) × 17 mm (height) using CMT6503 universal testing machine (SANS, Shenzhen, China) equipped with a 500 N load cell with two flat-surface compression stages. The compression properties for the aerogels were characterized both in air (dry state) and in water (wet state). The compression strength (σb) and compression modulus (E) were determined from the stress-strain curves recorded with a cross-head speed of 5 mm/min.


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The thermal stability of composite aerogels was determined by thermogravimetric analysis (TGA, STA449F3, TA, USA) at a heating rate of 10 °C/min from 25 °C to 900 °C under nitrogen atmosphere. Oil absorption test. Cashew oil was selected as the model for the study of oil absorption capability of the aerogels. The cylindroid aerogel was weighed before and immersed in 20 mL cashew oil. Superfluous free oil on the surface of the aerogel was carefully removed using filter paper, and weighed. The oil absorption capacity

where

and

(g/g) of the aerogel was calculated as follows:

are the weights of the aerogels before and after oil absorption.

The reusability of aerogels was evaluated by rinsing the aerogels at saturation adsorption of toluene for 20 min three times to complete the oil desorption, and then dried under vacuum for 4 h at 50 °C for the subsequent re-adsorption experiments. Organic molecule adsorption. 2,4-dichlorophenol was selected as organic and harmful molecule for the adsorption experiments of the aerogels. The analysis of 2,4-dichlorophenol concentration from the solutions was performed by UV spectroscopy (UV-2600, Shimadzu) at the characteristic length of 284 nm. The 2,4-dichlorophenol aqueous solutions with gradiently changing concentrations from 5 × 10-5 to 5 × 10-3 M (10 g/L adsorbent dose) were prepared at pH = 4 (regulation with 1 M HCl solution). All the experiments were carried out in a thermostatic shaker incubator (TS-100C, Shanghai), and the adsorption duration was controlled for 3 h at 30 °C to achieve the saturation adsorption of the aerogel. After adsorption, the aerogel was carefully taken out and the solution was centrifuged to remove the solid residue. The


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concentration of the raffinate containing unabsorbed 2,4-dichlorophenol was analyzed by UV spectroscopy. Thermal insulation and acoustic resistance. The values of thermal conductivity for the aerogels were measured using a thermal constant analyzer (TPS-2500S, Hot Disk, Sweden) with transient plane source method (TPS) at 30 °C in accordance with the standard of ISO22007-2.2. Cylindrical aerogels with a diameter of 29 mm and a height of 10 mm were tested on an impedance tube device (B&K, Zhejiang) in the frequency range from 100 to 6400 Hz to measure the sound-absorption coefficient, according to the standard ISO10534-2 (1998).[28] Flammability test. The fire resistance performance of the aerogels was photographed with a lab spirit lamp. Further analysis of the combustion behavior of the aerogels was carried out with a cone calorimeter device (Fire Testing Technology, FTT, England) under a heat flux of 35 kW m−2.[29] RESULTS AND DISCUSSION Surface silylation of CNC and chemical crosslinking of composite aerogels. The preparation route of composite aerogels is shown in Figure 1, including (I) extraction of cellulose nanocrystals (CNC) from tunicin, (II) surface silylation of CNCs (Si-CNC), (III) ball-milling treatment of red mud (RM), (IV) freeze-drying and fabrication of composite aerogels, and (V) proposed chemical crosslinking reaction with HDI. Derived from its rod-like morphology and high aspect ratio, tunicate CNC exhibited the apparent birefringence behaviour in aqueous suspension and porous aerogel after freeze-drying. The surface silylation converted the hydrophilic hydroxyl groups of CNC to hydrophobic


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methyl siloxy for Si-CNC. The brown RM powder was observed by SEM to occur as aggregates of diverse inorganic oxides. The homogeneous dispersion of Si-CNC and RM particles in aqueous suspension can be achieved by mechanical stirring and ultrasonic treatment, and the prepared aerogels exhibited a uniform appearance. The chemical crosslinking of the aerogel with HDI was expected to occur between the surface hydroxyl groups of Si-CNC and RM, which promoted improved structural stability and mechanical properties for the composite aerogels.

Figure 1. Preparation route for composite aerogels in this study. (I) Isolation of tunicate CNC by the acid hydrolysis (panels: appearance of freeze-dried CNC and birefringence photo of CNC aqueous suspension); (II) surface silylation of CNC; (III) ball-milling of RM (panels: SEM


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images and aggregate model of RM particles); (IV) freeze-drying treatment and (V) chemical crosslinking reaction for the composite aerogels. The rod-like morphology of CNCs before and after modification as well as the spherical shape of red mud particles was observed by TEM and SEM, as shown in Figure 2. Tunicate CNC exhibited ultrahigh aspect ratio with several µm in length and 15-25 nm in width for both CNC and silylated Si-CNC, while RM was observed as aggregates of various inorganic particles.

Figure 2. TEM images for CNC (A, B) and Si-CNC (C, D); SEM images for RM at magnification of 10,000 (E) and 50,000 (F). The chemical modification was performed to regulate the surface properties of CNC with the replacement of hydroxyl groups by methyl siloxy groups. As shown in Figure 3 (A), the surface silylation on Si-CNC can be confirmed by the additional stretching bands at


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1373.1, 1274.7 and 780.6 cm-1 in the FTIR spectra.[25] Further proof for the surface modification was provided by XPS result, tracing the apparent signals of silicon element located at 152 eV for Si2s and 103 eV for Si2p in the pattern for Si-CNC (Figure 3 B). The comparison of XRD patterns for CNC and Si-CNC (Figure S1 A) indicated the preservation of the original crystalline structure after surface modification with typical features of cellulose I (native cellulose) at 2θ angles 14.7° and 34.4° (

, 16.4°

, 22.8°

). Figure S1 (B) showed the crystalline characteristics of the main

inorganic components in RM involving iron oxide, aluminum oxide and titanic oxide, which was consistent with the XRF results on the composition analysis of RM in Table S1 (Supporting Information).

Figure 3. (A) FTIR spectra of Si-CNC/CNC, (B) XPS spectra of Si-CNC/CNC. The possible crosslinking reaction between Si-CNC and RM via HDI was investigated. The FTIR spectra of the Si-CNC, RM, HDI, RM/c and Si-CNC/c were shown in Figure 4 (A). It can be confirmed by additional peaks located at 1621, 1573 and 1254 cm-1 corresponding to the –CO–NH, N–H stretching and C–N bending from the covalent


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bonding between –OH and –NCO groups.[30] Similar results can also be found in the spectra for crosslinked composite aerogels as shown in Figure 4 (B), which proved the reasonable crosslinking strategy based on the use of HDI to enhance the structural stability of nanocellulose-based composite aerogels.

Figure 4. FTIR spectra for (A) raw inorganic and organic components: (a) RM, (b) SiCNC, (c) HDI, (d) RM/c, and (e) Si-CNC/c; (B) crosslinked aerogels: (f) Si-CNC/RM0.25/c, (g) Si-CNC/RM-0.5/c, (h) Si-CNC/RM-1/c, (i) Si-CNC/RM-2/c, (j) Si-CNC/RM4/c, and (k) Si-CNC/RM-6/c. Microstructure and macroscopic physical properties of composite aerogels. The visual appearance of fabricated aerogels is shown in Figure 5 (A and B). Because of their ultralow density, the prepared aerogels can stand on a leaf, and the red colour of composite aerogels gradually deepened with the increase in RM content. It is worth noting that the major component in RM is iron oxide (34.06% from XRF results in Table S1), which endows the composite aerogels with magnetic conductibility. As shown in Figure 5 (C), the composite aerogel Si-CNC/RM-1/c can be easily attracted by a magnet


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(Video_1 in Supporting Information), and the applied magnetic field can also induce free motion of aerogels (Video_2 in Supporting Information). Aiming in developing multifunctional aerogels, the property of magnetic conductibility is indeed important to be synergistically associated with its other functions. Due to the hydrophobicity of the SiCNC backbone, the neat aerogel Si-CNC/c and composite aerogels Si-CNC/RM-0.25/c can float on water (Figure 5 D and E).

Figure 5. (A) Appearances of the aerogels (from left to right: 0, 0.25, 0.5, 1, 2, 4, 6 RM content in the aerogel). (B) the composite aerogels Si-CNC/c and Si-CNC/RM-0.25/c on a leaf, (C) magnetic attraction of the composite aerogel Si-CNC/RM-1/c, (D and E) the neat aerogel Si-CNC/c and composite aerogel Si-CNC/RM-0.25/c floating on water; SEM images of the cross-sectional morphology for crosslinked aerogels at the same magnification: (F) Si-CNC/c, (G) Si-CNC/RM-0.5/c, (H) Si-CNC/RM-1/c, (I) SiCNC/RM-6/c. The microstructure of the aerogels was investigated by observing their cross-sectional morphology by SEM as shown in Figure 5 (F to I). The neat aerogel Si-CNC/c exhibited a


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network-like structure with interconnected pores (Figure 5 F), which can also be observed for the composite aerogels containing 0.5 and 1 parts of RM (Figure 5 G and H). The presence of moderate RM contents in Si-CNC/RM-0.5/c and Si-CNC/RM-1/c preserved the porous microstructure of the nanocellulose-based skeleton, which was induced by sublimation of ice crystals formed during the freeze-drying process.[31,32] However, the superfluous introduction of inorganic RM inevitably induced the occlusion of micro/nano pores and therefore resulted in the collapse of the porous structure, as shown for the composite aerogel Si-CNC/RM-6/c (Figure 5 I). The prepared aerogels exhibited typical porous and light-weight performances with high specific surface area and porosity, and low density, as summarized in Table 1. Due to the imbalance of the density for both components (1.540 g·cm-3 for Si-CNC[33] and 2.645 g·cm-3 for RM measured by densitometry), the value of the apparent density (ρaerogel) and skeletal density (ρs) for the composite aerogels gradually increased with the increase in the RM content. For moderate RM loading levels (< 2 parts), the obtained aerogel retained the high porosity (> 98%) from the Si-CNC skeleton, as previously reported for nanocellulose based porous materials.[34-36] Indeed, it has been reported that the randomly orientated nanocellulose can build a network in the microstructure and therefore contributes to the high porosity of the formed aerogel.[15] Moreover, the composite aerogel Si-CNC/RM-1/c had the highest specific surface area value (73.23 m2/g), indicating the lower agglomeration of the organic/inorganic components and potential adsorption or penetration of nitrogen molecules into the RM particles. In agreement with SEM observations, the RM loading level significantly affects the microstructure and morphology of the pores in the aerogel. A high pore volume is preserved for the


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composite aerogel Si-CNC/RM-1/c in contrast with the low pore volume observed for the composite aerogel Si-CNC/RM-6/c as shown in Figure S2. Table 1. Macroscopic physical parameters for the obtained aerogels: specific surface areas (BET), pore volume from N2 adsorption (Va, BJH) and desorption experiments (Vd, BJH), apparent density (ρaerogel), skeletal density (ρs) and calculated porosity.

Samples Si-CNC/c Si-CNC/RM-0.25/c Si-CNC/RM-0.5/c Si-CNC/RM-1/c Si-CNC/RM-2/c Si-CNC/RM-4/c Si-CNC/RM-6/c

BET

Va, BJH

Vd, BJH

ρaerogel

ρs

Porosity

m2/g

cm3·g-1

cm3·g-1

g·cm-3

g·cm-3

%

46.24 55.24 64.16 73.23 66.96 22.27 13.88

0.080 0.069 0.085 0.072 0.089 0.040 0.026

0.056 0.058 0.050 0.040 0.086 0.039 0.025

0.013 0.015 0.018 0.023 0.034 0.056 0.076

1.540 1.680 1.807 1.947 2.156 2.243 2.399

99.1% 99.1% 99.0% 98.8% 98.4% 97.5% 96.8%

The influence of the RM content on the surface properties of the composite aerogels was investigated by water contact angle measurements. As shown in Figure S3, RM being mainly composed of diverse inorganic particles possessing numerous hydrophilic hydroxyl groups, it exhibited a low water contact angle value (31.7°). Regarding the organic component (CNC), silylation was effective in converting the hydrophilic nature of unmodified CNC (water contact angle around 0°) to hydrophobic (134.5° for Si-CNC). The value of the water contact angle for both the cross-section and the surface of composite aerogels gradually decreased with the introduction of hydrophilic RM, but still preserved a high water contact angle (> 90°) attributed to the hydrophobic Si-CNC skeleton.


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The mechanical performances of the composite aerogels in dry (air) and wet (water) states and

comparison

of

compression

stress-strain

curves

between

crosslinked

and

uncrosslinked aerogels are shown in Figure 6. The expected improvement of the mechanical properties of the aerogel provided by HDI crosslinking was confirmed by the significant enhancement in both the compression modulus and strength from 0.11 MPa (E) and 0.02 MPa (σc) for the uncrosslinked aerogel Si-CNC to 1.29 MPa (E) and 0.11 MPa (σc) for the crosslinked aerogel Si-CNC/c in the dry state. Adding inorganic RM promoted the enhancement of the mechanical properties, exhibiting a gradual and simultaneous increase in both the compression modulus and strength of the composite aerogel (Figure 6 A and B). For composite aerogels Si-CNC/RM-1/c and Si-CNC/RM-6/c, the value of E and σc was 2.8-8.5 times and 1.3-1.6 times higher, respectively, than those for the neat aerogel Si-CNC/c (crosslinked). A notable increase of 33.2-99.6 times and 7.0-8.5 times, respectively, was observed compared to the neat aerogel Si-CNC (uncrosslinked). It is worth noting that the aerogels prepared in this study possess a remarkable compression modulus as high as 3.7-10 MPa in contrast with previous studies on aerogels with values in the 0.001-3 MPa (Figure 6 E). It can be attributed to the synergistic effect of chemical crosslinking with HDI and composite enhancement induced by the inorganic RM component. Considering the operational environment, the mechanical performance of the composite aerogels in water was investigated, indicating the preservation of high compression modulus and strength for the hydrophobic aerogels in the wet state. In comparison with the performance in air, the slight change observed in E and σc values for the composite aerogels in water may be ascribed to the impact of water molecules during the compressive tests.[37]


19


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Figure 6. Compression modulus (A) and strength (B) for composite aerogels with various RM contents (under the 50% compression strain); Compression stress-strain curves for uncrosslinked aerogels (C) and crosslinked aerogels (D) in the dry state; and comparison of the compression modulus with values reported for CNC-based aerogels in previous reports[20,38-41] (E). Abbreviation in panel E: red mud (RM), poly(vinyl alcohol) (PVA), sodium alginate (SA), whey protein (WP), montmorillonite (MMT).


20

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Thermogravimetric analysis (TGA) was performed to evaluate the thermal degradation behavior for the composite aerogels with the addition of inorganic RM, as shown in Figure S4. The aerogels exhibited a two-step thermal degradation process attributed to cellulose, with a first-step degradation temperature (Td1) at about 320 °C and a secondstep degradation temperature (Td2) at about 446 °C (Figure S4 A). This two-step degradation process was attributed to highly sulfated, and less accessible and then less sulfated interior regions, respectively.[42] The inorganic RM component possessed the higher thermal stability, and therefore remained as the residue after the thermal degradation of cellulose. It was apparent that the composite aerogels containing higher RM loading levels can leave more residues during the TGA experiments (Figure S4 B). Advanced functional performances of composite aerogels. Because of ultrahigh porosity and specific surface area, small molecules or ions are expected to easily penetrate into the internal structure of the aerogel and therefore endows this material with high adsorption performance. The absorption ability of composite aerogels was investigated using cashew oil and 2,4-dichlorophenol model molecules. As shown in Figure 7 (A to D), the composite aerogel Si-CNC/RM-1/c presented effective oil/water separation. It can absorb cashew oil rapidly from the water within 10 seconds (Video_3 in Supporting Information). Figure 7 (E) showed the influence of RM content on the oil saturated adsorption capacity of the composite aerogels. It shows an increased tendency for low RM contents and then a declined tendency for higher RM contents. This result is consistent with the microstructure observation (SEM images) for aerogels. The introduction of moderate RM contents can preserve the original structure of the Si-CNC skeleton, while superfluous introduction of RM results in the collapse of the porous


21


Page 22 of 40

structure and therefore induces a decrease in the oil adsorption amount for the composite aerogels. The optimal composite aerogels, i.e. Si-CNC/RM-0.5/c and Si-CNC/RM-1/c displayed oil adsorption capacity as high as 33-36 g/g, which indicated that the prepared aerogels can adsorb oil more than 30 times their own weight. The comparison of oil adsorption capability between the reported porous materials and composite aerogels obtained in this study were summarized in Table S2, which indicated the superior oil adsorption performance of Si-CNC/RM aerogels. The reusability of the composite aerogels Si-CNC/RM-0.5/c and Si-CNC/RM-1/c was evaluated by repeated oil rinsingadsorption treatments, which reflected the remarkable structure integrity of these two aerogels that kept saturated adsorption capacity at high levels (as shown in Figure 7 F and G). It is worth noting that the developed aerogels exhibit promising potential in practical applications, particularly for oil/water separation performance in combination with their magnet conductivity.


22

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Figure 7. Process of oil-water separation for the prepared aerogel Si-CNC/RM-1/c (from A to D); oil absorption capacity of the aerogel (saturated adsorption) with various RM contents (E); reusability evaluation of aerogel Si-CNC/RM-0.5/c (F) and aerogel SiCNC/RM-1/c (G) for 10 cycles of oil rinsing-adsorption experiments. 2,4-dichlorophenol was chosen as the model molecule to evaluate the adsorption performance of small organic molecules by the aerogels. As shown in Figure 8 (A), the characteristic wavelength of 2,4-dichlorophenol in water was determined as 284 nm from the UV absorption spectrum. When adding RM, the saturated adsorption of 2,4dichlorophenol by the aerogels increased from 3.4 mg/g for the neat aerogel Si-CNC/c to 6.0 mg/g and 5.5 mg/g for the composite aerogels Si-CNC/RM-0.5/c and Si-CNC/RM-1/c. It can be attributed to the structural preservation of the porous Si-CNC skeleton structure


23


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and effective uptake of 2,4-dichlorophenol by the RM component. In fact, it has been reported that neat inorganic RM displays adsorption ability to phenols due to the charge attraction between its positive surface and negatively-charged phenolic species.[43] However, it also should be pointed out that neat RM material exhibited only 1.2 mg/g maximum adsorption to the 2,4-dichlorophenol from this previous study[43], extremely lower than the performances of the porous composite aerogels (5.5-6.0 mg/g) prepared in this study.

Figure 8. Absorption capacity of the aerogels (saturated adsorption) towards the organic molecule 2,4-dichlorophenol containing various RM contents at 30 °C (A); adsorption of 2,4-dichlorophenol as a function of its concentration in aqueous solution for neat and


24

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composite aerogels at 30 °C (B); Freundlich isotherm for 2,4-dichlorophenol adsorption by Si-CNC/c (C), Si-CNC/RM-0.5/c (D), Si-CNC/RM-1/c (E); and Langmuir isotherms for 2,4-dichlorophenol adsorptionby Si-CNC/c (F), Si-CNC/RM-0.5/c (G), Si-CNC/RM1/c (H). By changing Ce (equilibrium concentration of the adsorbate), the relative adsorbed amount (qe) was calculated according to the following equation:

where

and

are the amount of adsorbed 2,4-dichlorophenol in the aerogel and the weight of

the dry aerogel, respectively. The adsorption behavior of the neat and composite aerogels towards 2,4-dichlorophenol at various concentrations is illustrated in Figure 8 (B). The relatively low saturated adsorption observed for neat Si-CNC/c in comparison with the relatively higher adsorption for the composite aerogels indicates the effective adsorption of 2,4-dichlorophenol by RM. The adsorption isotherms of 2,4-dichlorophenol by aerogels were expressed by both Langmuir and Freundlich models to analyze the possible adsorption mechanism, as shown in Figure 8 (C to H). Freundlich isotherm equation:

where

is the saturation capacity (mg/g) and

is the calculated empirical parameter.


25


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Langmuir isotherm equation:

where the parameters

and

are the adsorption binding constant (L/mg) and the saturation

capacity (mg/g), respectively. It is observed that adsorption data for the prepared aerogels provided an excellent fit to Langmuir isotherm with correlation coefficients of 0.9961 to 0.9985, which indicates the monolayer type adsorption on the homogeneous surface and adsorbed sites for the aerogels.[44] Regarding the Freundlich isotherm model, the calculated parameters

ranged from 0.309 to 0.324 (< 0.5, as

shown in Table S2 in Supporting Information) showing the high adsorption ability towards 2,4dichlorophenol for the prepared aerogels.[45] The thermal conductivity of the prepared aerogels was measured to evaluate their thermal insulation performance, as shown in Figure 9(A). The neat aerogel Si-CNC/c exhibited a thermal conductivity of 23.1 mW⋅m-1⋅K-1 similar to that of air (25.4 mW⋅m-1⋅K-1), which can be attributed to its low density and high porosity[46] (full of air). When aiding inorganic RM, the value of the thermal conductivity for the composite aerogel gradually decreased because of its higher density and therefore higher fraction of solid phase for the heat hindrance in comparison with neat aerogel.[47] Furthermore, the presence of inorganic RM component at moderate loading levels preserved the high porosity for the aerogel, which may produce the tortuous pathway for heat diffusion and subsequently result in the further reduction of the thermal conductivity. The composite aerogels with RM contents higher than 1 parts exhibited the lowest thermal conductivity values ranging from 17.5 to


26

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18.5 mW⋅m-1⋅K-1, which are comparable or even lower than the values previously reported for nanocellulose-based aerogels[28,48-50] and commercially available insulators (e.g. polyurethane foam with 24 mW⋅m-1⋅K-1, polystyrene foam with 41 mW⋅m-1⋅K-1 and mineral wools with 35-44 mW⋅m-1⋅K-1).[51] As a consequence of porous structures, the composite aerogels are expected to have high sound absorption ability. The soundabsorption coefficient of the composite aerogels at various frequencies is shown in Figure 9 (B). The absorption coefficient was poor at low frequency, but improved quickly as the frequency increased. The gradual decrease with a reduction of absorption coefficient by 24.5% and 50.2% for aerogels Si-CNC/RM-1/c and Si-CNC/RM-6/c in comparison with that of the neat aerogel Si-CNC/c at the mid-frequency of 2000 Hz was observed. At 6000 Hz, the sound-absorption coefficient of the composite aerogels reached approximately 78.7% and 70.2% for aerogels Si-CNC/RM-0.5/c and Si-CNC/RM-1/c, which was larger than those of NCF aerogels

[52]

and wood-based panels such as fiberboard, particleboard,

plywood. [53] The sound absorption ability of the aerogels was positively associated with their densities, indicating the effective hindrance or absorption of the solid and inorganic RM component Figure 9 (C).


27


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Figure 9. Thermal conductivity (A), sound-absorption coefficient (B), proposed mechanism of heat and sound barrier effect (C) of the composite aerogels containing various RM contents. Adding inorganic components in polymeric materials is an effective strategy to enhance its fire retardancy.[54,55] In this study, the dispersed RM component provided high fire protection property to the composite aerogels despite the presence of the inflammable nanocellulose matrix. As shown in Figure 10, neat aerogel Si-CNC/c started to burn immediately to flame contact, and burnt out during 12.5 seconds. In comparison, the composite aerogel Si-CNC/RM-0.5/c extinguished the combustion after 9 seconds, and surprisingly this self-extinguishing behavior was observed to occur within 2 seconds for


28

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the composite aerogels Si-CNC/RM-1/c and Si-CNC/RM-6/c. The whole process of the aerogels combustion can be viewed in the Videos_4 to 7 in the Supporting Information. The preservation of the original brown color after flaming for these two composite aerogels demonstrated the rapid and highly-efficient suppression to the combustion induced by the RM component, even under repeated ignitions (Videos_8 in the Supporting Information). It is worth noting that the self-extinguishing property of the prepared aerogel is extremely important for construction materials since the potential fire threat can be prevented in practical application.

Figure 10. (A) Photographs showing the behavior for neat and composite aerogels during flammability tests; plots of heat release rate (HRR, B) and total heat release (THR, C) for the aerogels. The cone calorimetry was performed to further investigate the flame retarding properties of the prepared aerogels, with the curves of heat release rate (HRR) and total heat release (THR) in Figure 10 (B and C) and combustion parameters in Table 2. Served as the


29


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thermally stable phase for the composite aerogels, the inorganic RM component embedded into the organic cellulose-based skeleton, which provided the thermal insulation and protection layer during the combustion.[56] As shown in Figure 10 (B and C), the intensities of the HRR and THR vs. time plots for the composite aerogels gradually reduced with the increase of the RM loading levels, indicating the effective suppression of RM to the heat transferring during the combustion and therefore the promising fire-retardant effect exerted by the RM component. Table 2 showed the combustion parameters of the aerogels containing various loading levels of RM from the cone calorimetry analysis. The tendencies of gradual reduction for the combustion parameters can be observed from the heat release rate (HRR), peak heat release rate (pKHRR), total heat release (THR) and total smoke release (TSR), which indicated the improved flame retardancy and protection to the organic cellulose skeleton depending on the inorganic RM component even at the same solid content of inflammable Si-CNC component in various aerogels. Table 2. The mean values of combustion parameters from the cone calorimetry analysis for the aerogels (TTI, time to ignition; HRR, heat release rate; pKHRR, peak heat release rate; TTPHRR, time to peak HRR; THR, total heat release; TSR, total smoke release).

TTI

HRR

THR

TSR

s

kW/m2

kW/m2

s

MJ/m2

m2

Si-CNC/c

1

34.34

175.36

5

3.62

0.0385

Si-CNC/RM-0.5/c

1

28.31

138.45

8

2.54

0.0285

Si-CNC/RM-1/c

2

24.73

113.37

10

2.24

0.0233

Si-CNC/RM-4/c

2

21.31

96.77

15

2.12

0.0143

Si-CNC/RM-6/c

2

17.73

94.00

15

1.73

0.0108

Samples

pKHRR TTPHRR


30

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CONCLUSIONS This study was an attempt to valorize waste for developing multifunctional aerogels from tunicate cellulose nanocrystal (CNC) and red mud (RM). Composite aerogels were prepared through freeze-drying of the skeleton of silylated CNCs and adding various RM contents. Crosslinking of ensuing aerogels was performed using hexamethylene diisocyanate (HDI). The presence of rigid RM enhanced the mechanical performance of the composite aerogels, which exhibited high compression modulus and strength values in both the dry and wet state compared with previously developed aerogels. Considering the preservation of the porous microstructure attributed to the nanocellulose-based skeleton, the content of introduced RM in the composite aerogels was recommended to be controlled at moderate loading levels (< 2 parts according to the weight of Si-CNC). Therefore, the optimal composite aerogel Si-CNC/RM-1/c was obtained, exhibiting a combination of multifunctional performances, e.g. (i) reasonable magnetic conductivity ascribed to iron-containing RM; (ii) high oil and organic molecules adsorption capacity attributed to hydrophobicity and high-porosity features; (iii) barrier properties to thermal and acoustic waves; and (iv) effective self-extinguishing performance resulting from the fire-resistance of inorganic components. The fabricated aerogels achieved the objective of high-valued waste valorization, and displayed promising potential for application as construction material. ASSOCIATED CONTENT Supporting Information. The Table S1 to S3, Figures S1 to S4 and Videos 1 to 8 are shown in the Supporting Information.


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AUTHOR INFORMATION Corresponding Author. * Ning Lin, Email: [email protected]. Address: 122 Luoshi Road, Wuhan University of Technology, Wuhan 430070, P. R. China. Tel: +86-27-87152611; fax: +86-27-87152611. ACKNOWLEDGMENT This study was supported by the National Natural Science Foundation of China (51603159) and Youth Chenguang Program of Science and Technology in Wuhan (2016070204010102). The authors also wish to acknowledge the financial support of Natural Science Foundation of Hubei Province (2017CFB490). REFERENCES (1) Lin, C. S. K.; Pfaltzgraff, L. A.; Herrero-Davila, L.; Mubofu, E. B.; Abderrahim, S.; Clark, J. H.; Koutinas, A. A.; Kopsahelis, N.; Stamatelatou, K; Dickson, F.; Thankappan, S.; Mohamed, Z.; Brocklesby, R.; Luque, R. Food waste as a valuable resource for the production of chemicals, materials and fuels. Current situation and global perspective. Energ. Environ. Sci. 2013, 6 (2), 426−464. DOI 10.1039/c2ee23440h. (2) Tonini, D.; Martinezsanchez, V.; Astrup, T. F. Material resources, energy, and nutrient recovery from waste: are waste refineries the solution for the future? Environ. Sci. Technol. 2013, 47 (15), 8962−8969. DOI 10.1021/es400998y. (3) Wang, J.; Qian, W.; He, Y.; Xiong, Y.; Song, P.; Wang, R. Reutilization of discarded biomass for preparing functional polymer materials. Waste Manage. 2017, 65, 11−21. DOI 10.1016/j.wasman.2017.04.025. (4) Balakrishnan, M.; Batra, V. S.; Hargreaves, J. S. J.; Pulford, I. D. Waste materials – catalytic opportunities: an overview of the application of large scale waste materials as resources for


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The multifunctional aerogels with high-adsorption, self-extinguishing, thermal and acoustic-resistance are obtained from cellulose nanocrystals and red mud by waste valorization. 49x31mm (600 x 600 DPI)


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High-Adsorption, Self-Extinguishing, Thermal and Acoustic

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