Flexible and durable cellulose aerogels for highly effective oil/water separation

Abstract

In this paper, a facile and green method was presented to prepare flexible, ultralight, and hydrophobic cellulose aerogels (CA) on the chemical cross-linking of cellulose solution, lyophilization and subsequent hydrophobic modification with methyltrichlorosilane (MTCS) by a thermal chemical vapor deposition (CVD) process. The resultant cellulose aerogels had an interconnected and highly porous structure with a varied pore size owing to different starting solution concentrations. And also the cellulose aerogels were ultralight (as low as 0.027 g cm⁻³), but they exhibited improved mechanical properties with compress stress of 1.10–3.85 MPa. After silanization, the MTCS-modified cellulose aerogels showed durable hydrophobicity with an average water contact angle of 141°, even maintaining 131° after 5 days. Furthermore, the flexible and hydrophobic cellulose aerogels could rapidly collect oils and organic solvents both on the surface and bottom from water, and exhibited excellent adsorption performances (e.g. 59.32 g g⁻¹ of pump oil) as well as good recyclability. Thus, such a flexible and hydrophobic cellulose aerogel is a very promising material for oil spill cleanup and industrial oily wastewater purification.

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1. Introduction

With the rapid development of industry and the social economy, water pollution resulting from oil spillage, industrial discharge of organic solvents/dyes, and heavy metal ions have become more and more serious, which has caused severe damage to the environment, ecology, and human health.^1–5 To address this issue, various approaches including oil/water separation, photocatalytic degradation, and adsorption, have been proposed for water purification.^6–9 Among these techniques, the approach of using three-dimensional porous absorbents with a hydrophobic surface has been proven to be very promising for the fast removal of oil from the surface of water owing to their superior oil/water selectivity. Recently, inorganic sorbents including carbon sponges/aerogels,^10–14 graphene sponges/aerogels,^15–20 and synthetic polymers such as polyurethane sponges,^21,22 poly(alkoxysilane) organogels,^23,24 poly(m-phenylenediamine) aerogel²⁵ and polypyrrole foam²⁶ have been demonstrated to be highly efficient on oil/water separation. However, the critical problems for such functional materials are their expensive precursors, complicated fabrication procedures and complex equipment, as well as non-biodegradability, which are great adverse for their industry applications. Thus, the development of facile, economic, safe and eco-friendly strategy for massive production of alternative materials has become extremely urgent.

During recent years, a large amount of cellulose-based advanced materials have been developed as highly effective oil absorbents, because its sustainability, biodegradability, excellent mechanical properties, as well as abundant hydroxyl groups providing chemical active sites for hydrophobic modification. For example, Sai et al.²⁷ functionalized the web-like skeleton of bacterial cellulose aerogels using a trimethylsilylation reaction with trimethylchlorosilane in liquid phase for oil/water separation. The obtained hydrophobic bacterial cellulose aerogels were able to collect a wide range of organic solvents and oils. Yang et al.²⁸ prepared chemically cross-linked cellulose nanocrystal (CNC) aerogels based on hydrazone cross-linking hydrazide and aldehyde-functionalized cellulose nanocrystals, which can absorb dodecane of 72 g g⁻¹. Cellulose nanofibril (CNF) aerogels were demonstrated to have high oil capacities and good reusability.²⁹ These modified cellulose-based materials presented enhancement in adsorption capacity and selectivity to oil or organic solvents from water. However, separating long cellulose nanofibers or rod-like nanowhiskers from native cellulose generally consume more energy and produce more wastes. Therefore, it remains a challenge to produce large-scale cellulose absorbents from renewable biomass by facile and economic process.

Herein, a facile and effective method is presented to prepare flexible, ultralight, and hydrophobic cellulose aerogels based on chemical cross-linking of cellulose solution, lyophilization and subsequent hydrophobic modification with methyltrichlorosilane (MTCS) by thermal chemical vapor deposition (CVD) process. As a result, the MTCS-modified cellulose aerogels not only had highly porous structure, but also exhibited durable hydrophobicity. Especially, the hydrophobic cellulose aerogels could rapidly and efficiently collect oils and organic solvents both on the surface and bottom from water, and exhibited excellent adsorption performances as well as good recyclability.

2. Experimental section

2.1. Materials

Cotton linter with α-cellulose content of more than 95% is provided by Heze Sanmu Health Materials Co. Ltd. (Shandong, China). NaOH, urea, N,N′-methylenebisacrylamide (MBA) and methyltrichlorosilane (MTCS) are purchased from Sinopharm Chemical Co. Ltd.(Shanghai, China). All the chemical reagents are analytical grade, and used without further purification.

2.2. Fabrication of cellulose aerogels

The cellulose aerogels (CA) were fabricated based on chemical cross-linking of N,N′-methylenebisacrylamide (MBA). Briefly, desired cellulose was dispersed into 100 g of 7 wt% NaOH/12 wt% urea aqueous solution to form transparent solution with cellulose concentration of 2 wt%, 2.5 wt%, 3 wt% and 4 wt% according to our previous reports.^30,31 Subsequently, 0.6 g of MBA cross-linkers was directly added into the cellulose solutions at room temperature and stirred for 40 min to obtain homogeneous solution. Then, the mixture was kept for 4 h to transform into cellulose hydrogels. The cellulose hydrogels were washed with deionized water to remove any residues, and finally freeze-dried to obtain cellulose aerogels.

2.3. Hydrophobic modification of cellulose aerogels

Hydrophobic cellulose aerogels were prepared by chemical vapor deposition technique using methyltrichlorosilane (MTCS).^29,32 A small glass bottle containing 1 mL of MTCS was placed in a desiccator together with cellulose aerogels. The desiccator with the cover closed was kept at ambient temperature for 24 h for the silanation reaction. Subsequently, the treated aerogels were kept in a vacuum oven at ambient temperature for 24 h to eliminate the excess silane. The hydrophobic cellulose aerogels (i.e. MTCS-modified cellulose aerogels) were finally obtained.

2.4. Characterization

2.4.1. Analysis of structure and morphology. Rheological measurement of the mixed solution composed of cellulose and MBA was performed at 25 °C on RM-200C rheometer (Hapro electric technology Co. Ltd, China) at a constant shear frequency of 1 Hz. The elastic storage modulus (G′) and the viscous loss modulus (G′′) were recorded to monitor the gelation behavior and mechanical properties. The surface morphologies of the obtained aerogels were observed by S-4800 field emission scanning electron microscopy (FESEM, Hitachi, Japan) after sputtering with gold. The chemical structures of the aerogels were characterized using Nicolet 5700 Fourier transform infrared spectroscopy (FTIR, Thermo Electron Corp., USA) ranging from 4000 to 400 cm⁻¹ at a resolution of 4 cm⁻¹ using KBr disk method. The surface chemical compositions of the aerogels were determined by K-Alpha X-ray photoelectron spectroscopy (XPS, Thermo Electron Corp., USA). The C 1s, O 1s, and Si 2p peaks were deconvoluted into the components consisting of a Gaussian/Lorentzian line shape function on a linear background using XPS software. The water contact angle (WCA) of the aerogels was measured using a contact analyzer (DCA-322, Thermo Cahn Corp., USA). Five contact angle measurements at different places were carried out for each sample. Compression testing was performed on a universal testing machine (HS-3000A, ShangHai HeSon Instrument Co. Ltd, China) with compressing speed of 10 mm min⁻¹ to 80% of its original height, and the samples were cylinder with a diameter of 20 mm and a height of 13 mm. Five replicates were tested for each sample.

2.4.2. Estimation of density, pore volume and porosity. Density of the aerogels was calculated based on the ratio of its mass to volume. The porosity of the aerogels was calculated based on the bulk density (ρ_b) and skeletal density (ρ_s = 1.528 g cm⁻³) of cellulose aerogels using eqn (1).³³

The pore volume (V_p, milliliters of pores in 1 g of aerogels) of the aerogels was calculated through the uptake of water in the aerogels. Water is a non-solvent for cellulose, which only penetrates into the pores of the samples. Thus, V_p was calculated according to the following eqn (2).³⁴

where, M_wet is the weight of the samples immersed in water until it reached swelling-equilibrium. M_dry is the weight of dry samples, and ρ is the density of water (0.995 g mL⁻¹, 30 °C). All samples were carried out in triplicate and then the average values are considered.

2.4.3. Liquid adsorption capacity. To determine liquid adsorption capacity of the hydrophobic cellulose aerogels, various types of oils or organic solvents including pump oil, colza oil, ethyl acetate, chloroform, cyclohexane, n-hexane, methylbenzene, and petroleum ether were used. The MTCS-modified cellulose aerogels were weighted (W₀), and then immersed in a given solvent for 20 min. The swollen samples were taken out, and then weighted (W) after the sample surface was wiped with a filter paper to remove excess organics. The corresponding adsorption capacity (C_m) was calculated according to the following eqn (3).³⁵

To study the reusability, the oil/organic swollen samples was squeezed by hand to remove the absorbed solvent. The weight of the aerogels before organic adsorption, after adsorption, and after squeeze for removal of organic was measured during each cycle. Four samples were tested for each experiment.

3. Results and discussion

3.1. Gelation behavior and mechanical properties

To monitor the gelation time of cellulose solution containing MBA and the storage modulus of the formed cellulose hydrogels, the viscoelastic properties of the mixed systems were determined at 25 °C. Fig. 1 shows the time dependence of storage modulus (G′) and loss modulus (G′′) for different concentrations of cellulose solutions with MBA. All systems showed a similar rheological behavior, revealing a viscous liquid at initial time. G′ was smaller than G′′, then G′ increased more rapidly than G′′ with prolonging time, finally G′ exceeded G′′, thus the system behaved as gel state. This crossover (G′ = G′′) is a well-known phenomenon in cross-linking reaction, and it can be described by the gel point (t = t_gel).^36,37 The results revealed a transition of the cellulose liquid phase to a gel state resulting from the formation of cross-linked network structure (shown in Scheme 1). Moreover, it was found that all gel points appeared at different times when the hydrogel systems were prepared with different cellulose concentrations. The gel point of 2 wt% cellulose hydrogel appeared at 265 min, while decreased to 165 min when cellulose concentration increased to 4 wt%. The shortened gelation time could be explained by that the crosslinking density progressively increased with the increases of cellulose concentration in the presence of MBA, consequently more molecular chain entanglement and aggregation occurring, which contributed to the formation of cross-linked network. In addition, the maximum storage modulus of the hydrogels increased from 2.68 to 110 Pa within 300 min when the cellulose concentration increased from 2 to 4 wt%, indicating excellent flexibility of cellulose hydrogels resulting from their strong cross-linked network structure.

Fig. 1 Time dependence of storage modulus G′ and loss modulus G′′ for cellulose solutions with cellulose concentrations of 2 wt%, 2.5 wt%, 3 wt%, and 4 wt%, respectively.

Scheme 1 Schematic drawing for the preparation of cellulose aerogels and proposed reaction mechanism.

Further, the mechanical properties of the cellulose aerogels prepared from different cellulose concentrations were determined, and the compression stress–strain behavior was shown in Fig. 2. Two apparent regions were observed, i.e. a slowly increasing stress response below 60% strain and a dramatically increasing stress response above 60% strain. The compress stress of cellulose aerogels obtained from 2, 2.5, 3 and 4 wt% cellulose solution was 0.27, 0.35, 1.27 and 1.82 MPa at 60% strain, respectively. Moreover, at 80% strain, the compress stress of corresponding cellulose aerogels increased to 1.10, 1.77, 2.83 and 3.85 MPa, respectively, which much higher than previous reports, such as 54.5 kPa for CNF aerogels,³⁸ 3.7 kPa for CNC aerogles,²⁸ and 12.1 kPa for carbon aerogles³⁹ compressed 80%. Furthermore, it was found that the cellulose aerogels obtained from higher concentration of cellulose solution exhibited higher stress at the same compressive strain compared to the aerogels from low concentration, owing to their larger proportion of chemically cross-linked parts. And also, the stress–strain curves of the cellulose aerogels obtained from 2 and 2.5 wt% cellulose solution revealed that the cellulose aerogels could be compressed to large strains (0–50%) at relatively low stress (0 to 0.15 MPa), indicating their excellent elasticity.

Fig. 2 Typical compression stress–strain curves of the cellulose aerogels from 2, 2.5, 3, and 4 wt% cellulose concentrations. The inset picture shows the aerogel had excellent flexibility.

3.2. Morphology and hydrophobicity

The chemically cross-linked cellulose aerogels were successfully prepared following a facile process consisting of mixing and freeze-drying (as shown in Scheme 1). No additional initiators or chemicals were required. The morphologies of the obtained aerogels from different cellulose concentrations were observed by FESEM, and shown in Fig. 3. Obviously, well-defined, interconnected, three-dimensional porous structure was formed when cellulose chains were regenerated from NaOH/urea aqueous solution and chemically cross-linked using MBA. Interestingly, due to the difference in cellulose concentration, the four aerogels exhibited different hierarchical porous structure. When the cellulose concentration was 2 wt%, the aerogel showed hierarchical porous structure with pore size in the range of 0.05–4 μm, and its three-dimensional network structure was constructed by filamentary cellulose matrix (Fig. 3a). With the increase of cellulose concentration, the size of pores in the cellulose aerogels gradually increased, correspondingly the pores had regular and very thin wall. When the cellulose concentration increased to 3–4 wt%, the average pore size of the cellulose aerogels exceeded 200 μm, even more than 500 μm (Fig. 3c and d). This indicated that the semi-rigid cellulose molecular chain played an important role in supporting the pore wall.⁴⁰

Fig. 3 FE-SEM images of the cellulose aerogels from different cellulose concentrations: 2 wt% (a), 2.5 wt% (b), 3 wt% (c), and 4 wt% (d). The inset picture in (a) shows the porous structure of the aerogel.

To further examine the effect of cellulose concentration on the microstructure of the aerogels, the density, porosity and the total volume of pores (V_p) of the cellulose aerogels were estimated, and listed in Table 1. It was found that the density of aerogels from 2 wt% cellulose solution could reach as low as 0.027 g cm⁻³, while the porosity and the total pore volume could be as high as 98.2% and 31.92 cm³ g⁻¹, respectively. Increasing cellulose concentration from 2 to 4 wt%, the density of aerogels gradually increased, while the porosity and the total pore volume decreased. But the cellulose aerogels also possessed an ultrahigh porosity of more than 96%, indicating light-weight and porous cellulose aerogels were successfully prepared. Remarkably, the prepared ultralight aerogel can stand on top of a leaf without a buckling (Scheme 1).

Table 1 Physical properties of cellulose aerogels from different starting cellulose solution concentrations

Sample	Density (g cm^−3)	Pore volume (cm^3 g^−1)	Porosity (%)
2 wt% CA	0.027	31.92	98.2
2.5 wt% CA	0.033	26.27	97.8
3 wt% CA	0.049	19.20	96.8
4 wt% CA	0.056	17.33	96.3
MTCS-modified CA	0.031	—	98.0

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To fabricate the hydrophobic aerogels, a facile thermal chemical vapor deposition of MTCS in a gaseous phase was performed to modify the hydrophilic cellulose aerogels from 2 wt% cellulose as a representative. The highly porous structure with pore size in the range of 0.05–4 μm and porosity of 98.0% was retained, confirming that the mild modified method using MTCS could not change the original microstructure and morphology of the cellulose aerogels (as shown in Fig. 4a). After modification, a similar ultralight cellulose aerogel with density of 0.031 g cm⁻³ was harvested (Table 1). While, the water contact angle experiments showed that the average water contact angle of the modified cellulose aerogels could reach ca. 141°, confirming the hydrophobic property of the modified aerogels (as shown in Fig. 4b). Moreover, the modified aerogel displayed durable hydrophobicity with water contact angle of 131° after 5 days. It was worth noting that the reaction of hydrophobic modification not only occurred on the surface of the aerogel, also in its interior. The modified aerogel cut off half also displayed high water contact angle of 128.4° (as shown in the inset in Fig. 4b). This result could be ascribed to its highly porous structure with an inherent 3D interconnected network, which could promote the rapid diffusion of MTCS, and permeating through the aerogel skeleton.

Fig. 4 FE-SEM image of the MTCS-modified cellulose aerogels from 2 wt% cellulose solution (a), and water contact angle of the MTCS-modified cellulose aerogels versus storage time (b). The inset picture shows the aerogel had excellent hydrophobicity not only on the surface of the aerogel, also in its interior.

3.3. Interface interaction between cellulose aerogel and MTCS

FTIR and XPS spectra were performed to investigate the silanization on the surface of cellulose aerogels. Fig. 5 shows the FTIR spectra of cellulose, cellulose aerogel from 2 wt% concentration before and after silanization. In the spectrum of raw cellulose (Fig. 5a), the characteristic peaks at 3354 cm⁻¹, 2899 cm⁻¹, 1425 cm⁻¹, 1162 cm⁻¹, 1063 cm⁻¹, and 893 cm⁻¹ are all the typical bands of cellulose molecules.^30,31,41 The peak at 1640 cm⁻¹ was assigned to the adsorbed water.^42,43 Compared with the spectrum of cellulose, a new peak at 1545 cm⁻¹ was found in the cellulose aerogels (Fig. 5b), which was attributed to the bending vibration of N–H. The characteristic peak at 1650 cm⁻¹ could be attributed to the stretching vibration of C=O, overlapping with that of adsorbed water. The shifting of band from 3354 cm⁻¹ in the spectra of cellulose to 3422 cm⁻¹ in cellulose aerogel cross-linked by MBA could be due to the overlapping of O–H and N–H stretching. Moreover, the peak at 1425 cm⁻¹ assigning to the stretching vibration of CH₂–OH disappeared in cellulose aerogels. These results confirmed that MBA was involved in the cross-linking reaction, and the active primary hydroxyl groups at the C₆ position preferentially participated in the reaction with MBA.⁴⁰ After silanization using MTCS (Fig. 5c), the vibrations of Si–C at 780 cm⁻¹ and –CH₃ deformation vibrations at 1271 cm⁻¹ of the siloxane compounds were found. The typical peaks of the Si–O–Si bonds of siloxane compounds in the 1000–1130 cm⁻¹ region overlapped with the C–O bonds present in cellulose.^29,44 This results confirmed the presence of strong interface interaction between the organosilane and the cellulose, accompanying with the covalent bonds between them, which would be beneficial for the maintenance of hydrophobicity in practical application.

Fig. 5 FTIR spectra of cellulose (a), 2 wt% cellulose aerogels (b) and MTCS-modified cellulose aerogels (c).

The XPS analysis was performed to monitor the surface element compositions of cellulose aerogels before and after modification, and shown in Fig. 6. Compared with unmodified cellulose aerogels, two new peaks at 154.3 eV and 102.8 eV, attributing to Si 2s and Si 2p, respectively, were found in the spectrum of the MTCS-modified cellulose aerogels, which indicated the presence of silicon on the surface of cellulose aerogels.^44,45 The high-resolution XPS spectra of C 1s, O 1s and Si 2p were further used to determine the change of chemical environment of surface elements. For the cellulose aerogels, the C 1s peak was deconvoluted into three distinct peaks at 284.6 eV, 286.3 eV, and 287.7 eV, ascribing to C–C, C–O/C–N, and O–C–O, respectively. In the O 1s spectrum, the binding energy at 532.1 eV could be assigned to C–O/C–OH.^31,46 After modification using MTCS, obvious differences could be found. The peak intensities of both C–O/C–N and O–C–O declined dramatically, meanwhile the binding energy of C–O/C–N and O–C–O shifted from 286.3 eV, and 287.7 eV to 286.7 eV and 288.3 eV, respectively. In the O 1s spectrum, the binding energy of C–O/C–OH shifted from 532.1 eV to 532.5 eV, and also the portion of C–O/C–OH was decreased, while a peak at 532.8 eV corresponding to Si–O was observed. These findings confirmed that during the silylation reaction, MTCS reacted with the surface hydroxyl groups of the cellulose aerogel. Methyl-silane radicals replaced the hydrogens of hydrogel groups and linked to the cellulose molecular chains by C–O–Si bonds, consequently cellulose aerogel transforming from hydrophilic to hydrophobic.^33,45,47 The existence of Si–O (102.8 eV) and Si–C (103.1 eV) reconfirmed the above hypothesis concerning hydrophobic modification mechanism.

Fig. 6 XPS survey spectra of 2 wt% cellulose aerogels before and after modification. High-resolution XPS spectra of C 1s, O 1s and Si 2p.

3.4. Adsorption capacity and recyclability

The MTCS-modified cellulose aerogels may have great potential for the facile removal of various oils and organic solvents from water, owing to their 3D porous structure, low density, high porosity, hydrophobicity, oleophilicity, and robust stability. As shown in Fig. 7, the MTCS-modified cellulose aerogels were introduced to contact with the artificial organic pollutants (stained with Sudan red III), and could rapidly remove the floated colza oil and cyclohexane, as well as sank chloroform completely within 2 s.

Fig. 7 Absorption removal of red-colored colza oil (a), cyclohexane (b), and chloroform (c) from surface and bottom of water with the MTCS-modified cellulose aerogels.

To evaluate the adsorption capacity of the MTCS-modified cellulose aerogels quantitatively, a series of oils and organic liquids, such as pump oil, colza oil, ethyl acetate, chloroform, cyclohexane, n-hexane, methylbenzene and petroleum ether were investigated. It was found from Fig. 8 that MTCS-modified cellulose aerogels exhibited significantly improved adsorption capacity for all the organic liquids compared with cellulose aerogels. In the adsorption process, the liquids could rapidly penetrate into the interconnected porous structure of the aerogels, consequently, the aerogels could absorb the organic to equilibrium within a few minutes. For low viscosity ethyl acetate, chloroform, cyclohexane, n-hexane, methylbenzene and petroleum ether, the MTCS-modified cellulose aerogels could absorb to equilibrium within 5 min, while 20 min for high viscosity pump oil and colza oil. Moreover, the adsorption capacity for pump oil, colza oil, chloroform, and methylbenzene reached as high as 59.32, 55.85, 46.23, and 40.16 g g⁻¹, respectively, which were much higher than those of other polymer/carbon-based absorbents.^48–50 For ethyl acetate, cyclohexane, n-hexane, and petroleum ether, the adsorption capacity of the modified aerogels was more than 30 g g⁻¹. The difference in adsorption capacity of aerogel for various liquids is attributed to both the surface characteristics of aerogel and the features of liquids (density, surface tension, hydrophobicity).

Fig. 8 Adsorption capacities of cellulose aerogels and MTCS-modified cellulose aerogels for various organic liquids.

The reusability of absorbents and the recoverability of pollutants are significant properties for practical application. As illustrated in Fig. 9, direct squeezing was applied to recover the aerogels and gain the organic liquids (inset), unlike the combustion technique that will waste the precious raw materials. After ten cycles, the adsorption capacity of the MTCS-modified cellulose aerogels for representative chloroform slightly decreased from 1887.5 mg to 1591.9 mg, recovering ca. 84.3% of its initial adsorption capacity. For pump oil, the adsorption capacity decreased from 2106.7 mg to 1379.7 mg after ten cycles. The decrease in adsorption capacity may be due to the residual oil phase inside the aerogels, which cannot be removed by direct squeezing for its high viscosity. However, the adsorption capacity of the aerogels to chloroform and pump oil after two to ten cycles kept 1300–1600 mg, indicating a highly stable adsorption performance and excellent recyclability.

Fig. 9 Cyclic adsorption capacity of MTCS-modified cellulose aerogels for chloroform and pump oil. The inset picture shows the aerogels were recovered by squeezing.

4. Conclusions

Flexible, ultralight, and hydrophobic cellulose aerogels were prepared through a facile chemical cross-linking, lyophilization and subsequent hydrophobic modification with methyltrichlorosilane (MTCS). The silane was linked to the cellulose molecular chains by C–O–Si bonds, consequently leading to high hydrophobicity of the cellulose aerogel with both surface and inner structure. The MTCS-modified cellulose aerogels could rapidly and efficiently collect oils and organic solvents both on the surface and bottom from water, but also exhibited excellent adsorption performances as well as good recyclability. Thus, well-defined interconnected porous structure, high mechanical properties, and durable hydrophobic surface of the cellulose aerogels led to their highly effective removal of oil from water. Thus the hydrophobic cellulose aerogel is a very promising material for oily wastewater purification.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

The work is financially supported by the National Science Foundation of China (51303159, 51372226), National Science Foundation of Zhejiang Province (LY15E030003), and 521 Talent Project of Zhejiang Sci-Tech University.

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