Highly compressible and hydrophobic anisotropic aerogels for

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Highly compressible and hydrophobic anisotropic aerogels for selective oil/organic solvent absorption Xuexia Zhang, Hankun Wang, Zhiyong Cai, Ning Yan, Minghui Liu, and Yan Yu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03554 • Publication Date (Web): 31 Oct 2018 Downloaded from http://pubs.acs.org on November 1, 2018

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ACS Sustainable Chemistry & Engineering

Highly compressible and hydrophobic anisotropic aerogels for selective oil/organic solvent absorption

Xuexia Zhang†,§, Hankun Wang*,†, Zhiyong Cai‡, Ning Yan§,#, Minghui Liu† and Yan Yu*,// †Institute

of New Bamboo and Rattan Based Biomaterials, International Center for Bamboo and

Rattan, Beijing, 100102, P.R. China. ‡Forest

Product Laboratory, USDA, Madison, WI 53726, USA.

§Faculty

of Forestry, University of Toronto, 33 Willcocks street, Toronto, On M5S3B3, Canada

#Department

of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College

Street, Toronto, On M5S3E5, Canada //College

of Material Engineering, Fujian Agriculture and Forestry University, Fuzhou, P.R. China.

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ABSTRACT Cellulose-based aerogels show great potential as absorbents for oil and chemical spill cleanup due to their low density and excellent absorption capacity. However, the hydrophility and inferior mechanical properties have often limited their practical applications. In this study, high-performance biomass-based aerogels were prepared by freeze-casting aqueous suspensions of polyvinyl alcohol (PVA) and cellulose nanofibrils (CNF) in the presence of hydrolyzed methyltrimethoxysilane (MTMS) sol. Successful silylation on the substrate surface was confirmed by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), thermal stability, and water contact angle measurements. Freeze casting successfully assembled highly aligned interconnected porous structure, resulting in the prepared aerogels with high modulus and strength in the aligned direction (along the freezing direction) and outstanding compression flexibility in the perpendicular direction (transverse to the freezing direction). The ultralow density (10.2 kg/m3), high hydrophobicity (water contact angel of 140º), and good compressive recovery (84% recovery of its original thickness after 100th compression tests), allowing the aerogel to absorb oils and organic solvents 45 to 99 times higher than their own weight. Meanwhile, good reusability was also observed with an absorption capacity greater than 84 % after 35 absorption-squeezing cycles. The novel aerogels prepared in this study are expected to have great potential for application in treating oil and chemical spills. KEYWORDS: Aerogel, Anisotropy, Silylation process, Freeze-casting, Oil absorption


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INTRODUCTION Water pollution from oil spills and chemical leaks are serious environmental problems that also lead to severe economic consequences. Therefore, there is an urgent need to develop more advanced technologies for the cleanup of oil pollutants1. Using porous absorbent materials for oil recovery is more attractive than other conventional methods (such as in-situ burning, chemical treatments, mechanical methods, and biodegradation), because of their high efficiency, low cost, and minimal effects adverse to the environment2,3. Aerogels, a class of highly porous and lightweight solid materials, display promising potential as ideal absorbents due to their large specific surface area and high porosity4. Although many advanced and efficient carbon-based aerogels based on carbon nanotubes (CNTs)5 or graphene6 have been reported to show extremely high absorption capacity and stable recyclability, high cost, complicated and energy-consuming processes involved in their fabrication restrict their practical applications7. Hence, the development of affordable aerogel materials with excellent properties for oil spill clean-up is a especially desirable. Very recently, aerogel based on cellulose nanofibrils (CNFs) has become a very promising candidate for oil absorption applications, due to their large specific surface area, high porosity, low density, and natural biodegradability8. However, a challenge for cellulose aerogel absorbent materials is their hydrophilicity (i.e. poor oil/water selective absorption) and inferior mechanical properties (i.e. poor shape recovery from deformation). Different strategies have been used to improve the hydrophobicity and oleophilicitily of cellulose aerogels, such as chemical vapor deposition process (CVD)9, atom layer deposition (ALD)10, esterification11, and sol-gel12. Among these methods, the gas phase reactions (e.g. CVD and ALD) were widely used. However, the strict preparation conditions and inhomogeneous grafting distribution within the aerogel hamper their large-scale production for industry applications13. To address these problems, Zhang et al.,14 proposed a synthetic process by directly freeze-drying CNF suspensions in the presence of acid-hydrolyzed methyltrimethoxysilane (MTMS) sols, yielding hydrophobic, flexible and ultralight weight aerogels. In the process, the silylation altered the microstructure of CNF aerogels remarkably, switching the regular porous cellular structure to random three-dimensional (3D) structure assembly of thin sheets after modification.



Although the obtained aerogels exhibited good recyclability using the rinsing-drying recovery method, it is time-consuming and energy intensive. Alternatively, mechanical squeezing appears to be a simpler, faster, and more cost-effective method to recover the absorbed oil in terms of practical applications15. To date, the majority of the reported oil absorbents based on CNF still suffers from poor compression robustness, resulting in dramatically reduced absorption capacity after mechanical squeeze16. Thus, the preparation of mechanical robust and hydrophobic aerogel materials that can withstand multiple squeezing-absorption cycles remains to be a challenge. Freeze-casting is a simple and cost effective technique for creating highly anisotropic porous structure with superior mechanical performance17,18. Typically, this procedure begins with the unidirectional freezing of aqueous suspension, followed by the unidirectional growth of ice crystals, and the aligned porous structure replicates the shapes of the unidirectional grown ice crystals19. In this study, we prepared anisotropic aerogels with lamellar structure through a facile silylation process and freeze-casting technique, using biodegradable poly(vinyl alcohol) (PVA) and CNF as starting materials. To the best of our knowledge, this is the first time that the mechanical properties and hydrophobicity of CNF aerogels have been simultaneously optimized by freeze-casting PVA/CNF suspensions, together with hydrolyzed methyltrimethoxysilane (MTMS) sols. The obtained aerogels absorbed a wide range of organic solvents and oils with absorption capacities of 45 - 99 times their own weights, and exhibited outstanding reusability with oil sorption retention of over 84% after 35 cycles of absorption − squeezing tests. MATERIALS AND METHODS Materials. Moso bamboo (Phyllostachys pubescens Mazel)) culms of four years were obtained from a plantation located in Zhejiang province, China. Polyvinyl alcohol (PVA, MW~ 95 000 g/mol), and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO, 98%) were purchased from Sigma-Aldrich. Sodium chlorite, acetic acid, potassium hydroxide, methyltrimethoxysilane (MTMS, 98%), and other chemicals were obtained from Aladdin Chemistry Co. Ltd and used as received without any purification. Deionized (DI) water was used in all experiments. Preparation of CNFs. The bamboo powders successively went through a series of chemical


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pretreatments including acidified sodium chlorite (75 ℃ for 1h, 5 times), 2 wt% potassium hydroxide (90 ℃, 2h), 5 wt% potassium hydroxide (90 ℃, 2h), and acidified sodium chlorite (75 ℃, 1h), to remove lignin and hemicellulose. To facilitate nanofibrillation, the purified samples were then oxidized with sodium hypochlorite using TEMPO as a catalyst under neutral conditions (60 ℃, 48 h)20. The samples were thoroughly washed and filtrated. The oxidized samples were dispersed in water with a concentration of 0.6 wt% and then nanofibrillated using a high-pressure microfluidizer (Microfluidizer, M-110EH-30, Microfluidics Corp., USA) five times through interaction chambers of 200 and 87 μm at 10,000 psi. The resulting bamboo-extracted CNF suspension was stored at 4 ℃ before future utilization. Preparation of PVA/CNF solutions. PVA solution was prepared by dissolving PVA (5 g) in 100 ml of water at 85 ℃ for 12 h. The PVA solution (6g, 0.05 g/ml), CNF suspension (50g, 0.6 wt%), and a desired amount of DI water with a total concentration in water of 0.6 wt% were mixed together in a flask and stirred for 1 h. The weight ratio between PVA and CNF was 1:1. Preparation of hydrophobic and anisotropic aerogels by freeze-casting. First, the obtained PVA/CNF suspension was adjusted to a pH of 4 with 0.5 M hydrochloric acid (HCl) solution. The polysiloxanes sol was prepared separately by dropwise adding MTMS (2.45 g, 40 mmol/g(CNF)) in water (80 ml, pH = 4), and then stirred for 10 min. The freshly prepared solution was added dropwise into PVA/CNF suspension and stirred at room temperature for 2h. Silylated PVA/CNF suspensions were frozen using a custom-built freeze-caster as shown in Figure S2. The directional freezing of suspensions was achieved by pouring them into rectangular Teflon molds placed on a copper block, which was immersed in an ethanol/liquid-nitrogen bath to create a uniaxial thermal gradient from bottom upward, and then covered with a Teflon lid. The suspensions were frozen from the bottom to top, while the ethanol/liquid-nitrogen level was kept up to the height of the copper block. Once completely frozen, the samples were immediately transferred to a freeze-dryer (FreeZone plus 2.5 L, Labconco Corp., Kansas, MO, U.S.A) and freeze-dried at -80 ℃ under vacuum for 48 h. Thereafter, the silylated samples were taken out and placed in an oven for 3 h at 90 ℃ to ensure complete silylation. The aerogels were sealed in plastic bags for further



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characterizations. Density and porosity. Apparent densities were calculated by measuring their masses and dimensions. The porosity was calculated using following equations:

Porosity  (1 

 ) 100% s

(1)

where ρ and ρs are the densities of the aerogel and solid materials, respectively. Considering the aerogel as a composite of cellulose, PVA and polysiloxane, the density of the solid materials (ρs) was calculated according to Equation (2) based on the solid density of each component and their weight ratios used in the formulation:

s 

1 wCNF

 CNF



wPVA

 PVA



w poly ( MTMS )

(2)

 poly ( MTMS )

where w and ρ are the weight fraction and solid densities of different components, and ρCNF, ρPVA, and ρpoly(MTMS) were fixed at 1500, 1267, and 1900 kg/m3, based on data in the literature 9,14. Characterizations. The morphology of obtained CNF was investigated through AFM (Bruker, Dimension ICON). For SEM observation, the aerogel samples were first frozen by liquid nitrogen to prevent deformation, then cut with a razor blade either parallel or perpendicular to the direction of freeze-casting, sputter-coated with platinum, and imaged using a scanning electron microscope (FE-SEM, XL30, FEI, USA). The BET specific surface area was determined by nitrogen physisorption using a Micrometric ASAP 2460 automated system. FT-IR spectra were recorded by a Nicolet IS10 FT-IR spectrometer (Thermo Fisher Scientific, USA) in the range of 400-4000cm-1 with a resolution of 4cm-1. The surface compositions of aerogel samples were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Escalab 250Xi, US) employing Al Kα (1486.6 eV) as the radiation source. The surface spectra was gathered in the range of 0-1100 eV at a takeoff angle of 90°. Thermal stability measurements were carried out using a Theromogravimetric analyzer (TGA, Q 50 TA Instruments, USA) from 30 to 700 ℃ at a 10 ℃/min heating rate under N2 protection. The surface wettability of aerogels was measured by static contact angle analysis using a contact angle goniometer (OCA20, Dataphysics Instrument, Germany). The volume of the water droplet was fixed


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at 4.0 μL, and five different positions were tested. Compressive properties and shape recovery ability. The compression tests were performed using Instron 5848 testing system (Instron Co. USA) equipped with a 50 N load. Cubic samples of ~20 mm side length were compressed either parallel or perpendicular to the freeze-casting direction, after they had been conditioned at 50% relative humidity and 23 ℃ for at least 24 h. The stress-strain curves were measured with a strain rate of 5 mm/min to 80% strain. Compression modulus was determined from the slope of the initial linear region of the stress-strain curves. A 100-cycle loading-unloading cyclic test was performed by measuring stress-strain curves at a strain rate of 200 mm/min. Three replicates were measured for each group. After the sample had been unloaded, the shape recovery ability was assessed according to the following formula:

S(%)  100   final

(3)

Where εfinal is the strain at the final position when load detected reached 0 N. Oil/solvent absorption capacity and reusability of aerogel. To explore the oil or solvent absorption of the aerogels, the samples were weighted and immersed in various oils (gasoline, diesel, pump oil, corn oil, mineral and motor oil) and solvents (acetone, ethanol, toluene, hexane, chloroform, and DMSO) for 2 minutes to reach equilibrium. Then, the soaked aerogels were taken out and weighted after the excess surface oil/solvent of aerogels was removed by filter paper. Absorption capacity (Q, g/g) was calculated according to following equation:

Q( g / g ) 

W1  W0 W0

(4)

where W0 and W1 are the weights of the aerogels before and after absorption, respectively. All the measurements were carried out in triplicate and the average values were presented as the final results. Aerogel reusability was evaluated by simply manual squeezing due to its convenient operation and environmental friendliness. Absorption and squeezing of the aerogels were tested for 35 cycles using gasoline as the model oil. RESULTS AND DISCUSSION



Structural characterization. The original unmodified aerogels were fabricated by a freeze-casting technique, taking PVA and CNF as the starting materials. PVA is an inexpensive polymer that can be water-soluble, biocompatible and biodegradable, exerting significant function in manufacturing the sustainable aerogel21. CNF was isolated from bamboo powder using the TEMPO-oxidized pretreatment followed by microfluidization. The morphology of the as-prepared CNFs was characterized by AFM imaging (Figure S1), revealing nanofibrils with widths ranging from 10 to 30 nm. To synthesize modified aerogels, PVA/CNF suspension was directly freeze-dried in the presence of acid-hydrolyzed MTMS sols using freeze-casting technique. The mechanism for the hydrophobic modification of aerogels is illustrated in Figure 1. First, MTMS was acid-hydrolyzed in water, yielding reactive silanol groups (Si-OH). Then the silanol sol was added dropwise into PVA/CNF suspension, and two reactions simultaneously occurred during the mixture stage: (i) the silanol groups may directly react with the hydroxyl groups of substrate thereby forming Si-O-C bonds, (ii) the silanol groups may also condense themselves on the substrate surface, forming rigid polysioxanes with stable Si-O-Si bonds. Subsequently, the silylated suspension was unidirectionally frozen using a custom-built freeze-caster (Figure S2). Finally, polysioxanes formed rigid layers on the substrate surface through dehydration reactions when the water was removed during the freeze-drying stage. Due to the simplicity of the freeze-casting process, aerogels may be easily shaped into cylinders, cubes, and cuboids (Figure S3).


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Freeze-casting

Freeze-drying

Ice crystal

MTMS

CNF

PVA

Figure 1. Mechanism scheme for the synthesis of modified PVA/CNF aerogels

The hydrophobic modification was characterized by FTIR, XPS, and TGA techniques. Figure 2a shows the FTIR spectra of aerogels before and after silylation. After MTMS modification, stretching vibrations of Si-OH at ca. 920 cm-1 increased significantly, and new bands at the stretching vibrations of Si-C and/or Si-O-Si at ca.770 cm-1 and -CH3 deformation vibrations at ca. 1272 cm-1 of the polysiloxanes appeared, which indicated the successful formation of polysiloxanes on the substrate surface as expected22.

1800

1600

1400

100

MTMS-modified 102.6 eV

O 1s unmodified MTMS-modified

Intensity (a.u.) unmodified MTMS-modified

Si 2p

b

96

98

100

C 1s

102

104

106

108

Binding energy (eV)

Si 2p

110

1000 -1

Wavenumber(cm )

800

600

60 40

unmodified MTMS-modified

20

v(Si-C)MTMS v(Si-O-Si)MTMS 1200

c

80

Weight (%)

v(Si-OH)MTMS δ(C-H)MTMS

Intensity (a.u.)

a Transmittance(%)

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


1000

800

600

400

200

Binding energy (eV)

0

0

0

100

200

300

400

500

600

Temperature (℃ )

Figure 2. Characterization of the unmodified and modified PVA/CNF aerogels: (a) FITR spectra (b) XPS wide scan spectra, with inserted images of Si 2p narrow scan spectrum of MTMS-modified aerogel (c) TGA curves


700


The interfacial bonds between polysioxanes and PVA/CNF substrate were further confirmed by XPS analysis, as shown in Figure 2b. Compared to unmodified aerogels, two peaks at 154.08 and 103.08 eV, attributing to Si 2s and Si 2p, respectively, were intensified remarkably in the spectrum of the MTMS-modified PVA/CNF aerogels, which suggested the presence of silicon on the substrate surface23. The peak of Si 2p at 102.6 eV was assigned to covalent bonds of Si-O-C, which confirmed that polysiloxane was chemically bonded onto the PVA/CNF surface through dehydration reaction. The TGA curves of the unmodified and silylated PVA/CNF aerogels are shown in Figure 2c, and the degradation data are summarized in Table S1. Silylation modification did not change the thermal stability of the aerogels until around 200 ℃. However, beyond 200 ℃, the MTMS-modified aerogels showed improved thermal stability compared to their unmodified counterparts. After treatment with MTMS, higher values in both T30% ( 397.7 ℃ vs 254.1 ) and Tmax (268.0 vs 248.64 ℃ ) were observed for the modified aerogels compared to the unmodified samples (Table S1). The improved thermal stability should be due to the rigid polysioxane coating that could retard the thermal decomposition of the modified aerogels24. a

b

c

d

Figure 3. SEM images of the unmodified (a - b) and MTMS-modified aerogels (c - d)


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at both cross and longitudinal sections

The morphologies of the aerogels before and after silylation were studied by examining their cross and longitudinal sections. As shown in Figure 3, the macropores made up with interconnected CNF sheets were clearly observed in the cross sections (Figure 3a and c), and the alignment of these pores was confirmed as the layered structure could be clearly observed in the longitudinal section (Figure 3b and d) regardless of the modification. The distinctive morphologies at two sections indicated the anisotropic structure of the aerogels and it was well preserved after modification, which overcome the limitation that aqueous silylation process might significantly change the microstructure of MTMS modified aerogels reported by the previous study14. This is mainly due to the employed freeze casting technique, in which laminar ice crystals grow along the temperature gradient from bottom upward during freezing, producing aerogels with aligned lamellar structure after sublimation17. The aligned porous structure offers numerous micro-channels for liquid transformation and storage, which are beneficial for oil treatments25. The freeze-cast aerogels have densities of 6.6 and 10.2 kg/m3 for original unmodified and MTMS-modified PVA/CNF hybrid aerogels, respectively (Table 1). Both aerogels exhibited ultralight and highly porous (~ 99.5%) characteristics. The specific surface area was measured by the BET method, and a slight decrease of BET surface area was noted (28.40 m2/g for original aerogels and 23.41 m2/g for modified aerogels), as shown in Table 1. Table 1. Structural characteristic of the unmodified and MTMS-modified PVA/CNF aerogels

density samples

ρa (kg/m3)a

ρs (kg/m3)b

porosity (%)

BET surface area (m2/g)

unmodified

6.6

1375

99.5

28.4

MTMS-modified

10.2

1782

99.4

23.4

Note: aρa is the apparent density of the aerogels; bρs is the density of the solid scaffold.

Compressive properties. Good mechanical properties and shape recovery are crucial for the collection of absorbed oils by simple squeezing method26. Since the aerogels had an anisotropic structure, compression was performed in both aligned (i.e. along the freezing direction) and perpendicular (i.e. perpendicular to the freezing direction) directions. The “a-” and “p-” letter represents the aerogels compressed in the “aligned direction” and the “perpendicular direction”,



respectively, as shown in Figure 4a. The compressive stress-strain curves of aerogels compressed in two directions are shown in Figure 4b, and the mechanical properties are summarized in Table 2. Under compression in the axial direction, aerogels exhibited the typical three-regime curves, including a linear elastic deformation stage at low strain, a plastic yielding plateau, and a final densification with a rapid rise of stress at high strain27, while no apparent linear regime prior to the plateau was observed when compressed in the perpendicular directions (see the inserted images in Figure 4b). The compressive modulus and maximum stress at 80% strain were both much lower in the perpendicular directions (Table 3). This mechanical anisotropy is mainly due to the anisotropic microstructure of the aerogel, as the uniform vertical alignment of sheets can bear high load in the aligned direction, which is consistent with other anisotropic aerogels and foams shown in the previous reports28,29. "a-"

direction

a

" p-"

d ir ec

t ion

p-unmodified a-unmodified p-MTMS-modified a-MTMS-modified

b

70

Compressive stress (kPa)

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

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60 10

50

8

40

6

30

4

20

2 0

10 0

0

2

0

4

20

6

8

40

10

60

80

Compressive strain (%)

Aerogel with aligned porous structure Figure 4. Compressive properties of PVA/CNF aerogels : (a) Schematic illustration of the PVA/CNF aerogels compressed in two different directions, in which “a-” and “p-” stand for “aligned to freezing direction” and “perpendicular to freezing direction”, respectively; (b) stress - strain curves for the unmodified and MTMS-modified PVA/CNF aerogels compressed in two directions.

Although the freeze-casted aerogels exhibited lower compressive modulus and strength in the perpendicular direction, their compression recovery properties were found to be much better in this direction (Table 2 and Figure S4). Interestingly, using simple freeze-casting technique, the unmodified PVA/CNF aerogels could recover up to 60 and 75% of its original thickness in the aligned and


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perpendicular direction at a large deformation of 80%, while the aerogels without freeze casting showed no recovery (Figure S4). After modification, the PVA/CNF aerogels exhibited good flexibility in the perpendicular direction with a recovery of 86% of its original height after 80% compression strain, whereas an obvious buckling effect and low recovery (56%) were observed in the aligned direction. Table 2. Mechanical properties of the unmodified and MTMS-modified PVA/CNF aerogels samples

Compression modulus

Maximum stress at 80%

Thickness recovery

(kPa)

strain (KPa)

at 80% strain (%)

p-unmodified

2.5±0.4

11.0±2.1

71.5±4.9

a-unmodified

120.7±8.5

56.0±4.2

60±2.6

p-MTMS-modified

6.4±0.2

13.0±1.4

83.5±2.1

a-MTMS-modified

80.0±10.7

40.3±6.8

55.8±5.4

a 1. Compression 2. Release 70% strain Compression in the perpendicular direction Cycle 1 Cycle 2 Cycle 10 Cycle 50 Cycle 100

3 p-MTMS-modified

2

Maximum stress (kPa)

4

5

Loading

1 Unloading

0 0

10

20

30

40

50

60

70

80

c

Maximum stress (kPa) Thickness recovery rate (%)

4

120 100 80

3

60

2

40

1

20

0

1

Compressive strain (%)

2

10

50

Thickness recovery (%)

b

5

Compressive stress (KPa)

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


0 100

Cycle time

Figure 5. (a) Schematic illustration of the changes in the aligned lamellar structure for modified PVA/CNF aerogels with cyclic compression in the perpenducular direction; (b) Cyclic compressive stress-strain curves and (c) maximum stress and thickness recovery of the aerogels under a cyclic compression test.



Furthermore, the MTMS-modified aerogels can even bear 100 cyclic loading-unloading compression tests in the perpendicular direction at a reduced strain of 70 % (Figure 5). Rapid shape recovery of the modified aerogel was observed during the unloading process, and no significant decrease in the maximum stress and thickness recovery was observed after 100 compression cycles, highlighting that the aligned porous structure were strong enough to withstand these cyclic compressions (Figure 5b and c, Movie S1). From these results, we speculated that the remarkable compressibility and flexibility of the aerogels were related to their anisotropic microstructure and hydrophobicity, and the illustration was shown in Figure 5a. During compression, stress was spread over through the interconnected pore walls and the strain was absorbed by the deformation of numerous pores. When the applied load was released, these squeezed pores could recover back to their original shape driven by the absorbed strain as well as the repulsive interactions between alkyl groups of polysiloxanes coating. Therefore, the aerogel could withstand large compressive strain without structural collapse. The high compressibility and flexibility make the MTMS-modified aerogels suitable for the oil cleaning applications requiring excellent shape recovery ability. Adsorption capacity and reusability. Since the modified PVA/CNF aerogels exhibited combined advantages of ultralight weight, hydrophobicity, and compression recovery properties, we further explored their potential use as selective oil absorbents. Effective oil/water selectivity is a crucial property for oil absorbents, particularly for oil spill cleaning in an aqueous environment30. After silylation, The MTMS-modified aerogels became hydrophobic as water formed droplets on the surface and maintained its round shape with a high contact angle of 140° for more than 10 min (Figure 6a). These tiny water droplets do not move when aerogels was upside down, indicating the formation of a Cassie impregnating wetting area31 . By dipping the modified aerogels into a mixture of oil/solvent and water, the aerogels could selectively remove the floating gasoline (Figure 6c and Movie S2), as well as the sunk chloroform (Figure 6d and Movie S3) completely within minutes. It is important to note that oil/solvent-absorbed aerogel continued to float on the water surface without oil/solvent release, further confirming the high hydrophobicity and selectivity of the modified aerogels. This phenomenon is beneficial for facile pickup of absorbents in practical applications.


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a

a a a a a30 s a ab aa aa aa aa aa a ca aa aa aa aa a da aaa aa baa aa a ea 100 a a 80 a a 60 a

Absorption capacity (g/g)

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


40

140°

142°

10 min

56 52 65 45 99 53 55 72 71 63 83 76

20 0

l e l ne anol ene ane orm SO olin ese oil oil l oi r oil o t i f n a p x u s l e th e ro D um or ner oto DM Ga C i M Ac E To H hlo P M C Diesel 72

Figure 6. (a) Water contact angel of the modified aerogels taken at 30s and 10 min (b) Hydrophobicity of the modified aerogel, and water has been dyed with blue color for easier visualization; (c-d) Images showing the selective removal of gasoline from the water surface and chloroform from under water ; (e)



absorption capacities of various oils and solvents.

Moreover, the modified PVA/CNF aerogels exhibited excellent absorption capacities, ranging from 45 to 99 g/g, toward a wide range of oils and solvents (Figure 5e). This should be attributed to the uniform alignment of numerous pores, not only allowing quick liquid absorption but also provide a large volume for the storage of the absorbed liquids. Significantly, the MTMS-modified aerogels have showed much higher absorption capacity than many recently reported aerogels and sponges, such as lignin/agarose/PVA aerogels (20 - 40 times)25, polymeric aerogels (25 – 53 times)32, chemical grafted NCF aerogels (20 - 46.6 times)33, cotton-based aerogels (30 - 59 times)23, winter melon carbon aerogels(16 - 50 times)34 and bacterial cellulose/silica aerogels (8 - 14 times)35. Nevertheless, the absorption capacity of the modified aerogels is still lower than that of carbon aerogels (50-190 times)36, graphene aerogel (100 - 250 times)26. However, these efficient carbon absorbents involve sophisticated preparation procedure and demanding production conditions, and thereby are expensive.


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a

b

t=2s

t=0s

t = -1 s

Absorption capacity (g/g)

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


t=5s

80

Absorbed mass Remnant mass

70 Gasoline

60 50 40 30 20 10 0

0

5

10

15

20

25

30

35

Cycle number Figure 7. (a) Rapid shape recovery of the MTMS-modified aerogel after absorbing gasoline (b) Reusability for absorption of gasoline with a compression-squeezing method.

Reusability of the absorbents and the recoverability of pollutants are key criteria for oil/solvents cleanup, because most pollutants may be useful. Here, the absorption-squeezing process has been demonstrated for oil collection. Figure 7a and Movie S4 display the process of the absorption-squeezing cycles. Interestingly, the squeezed aerogels cannot recover its original shape. However, it can almost recover its original shape after re-absorbing oil within a few seconds. By compressing the aerogels in the perpendicular direction for oil collection, the absorption capacity of the aerogels slightly decreased from 58 to 49 times after 35 cycles, with less than 16% loss in the absorption capacity, indicating excellent reusability (Figure 7b). This is may be attributed to excellent flexibility and elasticity associated with the aligned porous structure of aerogels via freeze-casting



technique25. One main problem is that only ca. 57% of the absorbed oil can be collected by the squeezing process due to the oleophilicity of the modified aerogels. But compared to other methods, such as rinsing (solvent extraction), combustion (burning), and distillation (heating), squeezing is competitive because of its simplicity and sustainability30. CONCLUSIONS In summary, high-performance biomass-based aerogels were successfully prepared by combining facile aqueous silylation process with freeze-casting technique. The modification process was achieved by freeze-casting PVA/CNF suspension with the presence of MTMS-hydrolyzed sol. Owing to their low density (10.2 kg/m3), high porosity (99.4%), improved compressive recovery properties and hydrophobic characteristics, the modified aerogels possesse high absorption capacity of 45 - 99 times of their own weights, high oil/water selective sorption, and excellent reusability with absorption retention of more than 84% after 35 absorption-squeezing cycles. This is obviously time-efficient and environmentally friendly. Therefore, such aerogels are highly promising as absorbent materials to be used for oil spill control and environmental protection. ASSOCIATED CONTENT Supporting information Figure S1: (a) Optical images of as-prepared CNF suspension at 0.5 wt%; (b) AFM height image of diluted 0.1 wt% CNF suspension. Figure S2 Schematic illustration of a custom made freeze-caster. Figure S3: Optical images of aerogels with diverse shapes. Figure S4: Optical images of unmodified and MTMS-modified PVA/CNF aerogels recovering their original dimensions after compressed to 80% strain aligned or perpendicular to the freezing direction. “mom-directional” stands for unmodified PVA/CNF using non-directional freeze drying. Table S1: TGA data for unmodified and MTMS-modified aerogels Movie S1: Cyclic compression test for MTMS-modified PVA/CNF aerogel in the perpendicular direction Movie S2: Removal of gasoline from the water surface using MTMS-modified PVA/CNF aerogel Movie S3: Removal of chloroform from water using MTMS-modified PVA/CNF aerogel


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Movie S4: Reusability for absorption of gasoline using MTMS-modified PVA/CNF aerogel with a compression-squeezing method. AUTHOR INFORMATION Corresponding authors *E-mail:

[email protected];

[email protected];

Phone:

+86-10-84789812;

+86-10-84789909. ORCID Hankun Wang: 0000-0003-0338-1809; Yan Yu: 0000-0001-9060-8623 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We are grateful for the financial support from SFA 948 project (2015-4-48), National Key R&D Program of China (2017YFD0600804), Basic Scientific Research Funds of International Center for Bamboo and Rattan (1632018016) and the China Scholarship Council. REFERENCES (1) Meng, Y.; Young, T. M.; Liu, P.; Contescu, C. I.; Huang, B.; Wang, S. Ultralight carbon aerogel from nanocellulose as a highly selective oil absorption material. Cellulose 2015, 22(1), 435-447, DOI 10.1007/s10570-014-0519-5 (2) Ge, J.; Zhao, H.; Zhu, H.; Huang, J.; Shi, L.; Yu, S. Advanced sorbents for oil‐spill cleanup: recent advances

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Abstract Graphic

Freeze-casting

PVA

CNF

Freeze-drying

MTMS

Ice crystal

Synopsis Hydrophobic, flexible, and anisotropic aerogel were fabricated by freeze-casting suspensions of polyvinyl alcohol and cellulose nanofibrils, together with methyltrimethoxysilane sol.


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Highly compressible and hydrophobic anisotropic aerogels for

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