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A facile method to fabricate superoleophilic and hydrophobic polyurethane foam for oil–water separation
Materials Letters 159 (2015) 345–348
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
A facile method to fabricate superoleophilic and hydrophobic polyurethane foam for oil–water separation Ying Pan a, Jing Zhan a,b, Haifeng Pan a,c, Bihe Yuan a,c, Wei Wang a, Lei Song a,n, Yuan Hu a,c,nn a
State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, PR China School of Civil Engineering and Environmental Engineering, Anhui Xinhua University, Hefei, Anhui 230088, PR China c Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute for Advanced Study, University of Science and Technology of China, Suzhou, Jiangsu 215123, PR China b
art ic l e i nf o
a b s t r a c t
Article history: Received 11 January 2015 Received in revised form 29 June 2015 Accepted 5 July 2015 Available online 6 July 2015
A facile and novel method to fabricate superoleophilic and hydrophobic flexible polyurethane foam (FPUF) is presented in this work. The modified FPUF was prepared by utilizing positively charged chitosan and negatively charged titanate nanotubes together with subsequent modification of dodecyl mercaptan. The roughness of the foam surface improved significantly by increasing the bilayers number of the coating. The modified FPUF could rapidly and selectively absorb various kinds of oils up to 29 times of its weight, and the absorbed oils could be collected by a simple squeezing process. Furthermore, after 50 absorption cycles, the modified FPUF could still maintain its high absorption capacity. & 2015 Published by Elsevier B.V.
Keywords: Multilayer structure Superoleophilic Porous materials Oil–water separation
1. Introduction Oil spill accidents and oily wastewater in industrial production can cause ecological and environmental problem [1]. Lots of the sea birds and mammals have been killed by contacting with the spilt oil. The underwater plants are also seriously damaged [2]. Therefore, separating oil from water is an urgent problem to solve. In recent years, many materials have been developed as oil absorbents, including inorganic, organic and composite materials, such as sawdust bed [3], vermiculite [4], rubber power [5], hydrophobic aerogels [6], exfoliated graphite [7], graphene [8] and polyurethane foams [9]. Polyurethane foam has shown noticeable oil adsorption capability due to its special features such as enough space for adsorption, low density, open-cell structure, high porosity and industrial production. Hamid et al., prepared polyurethane nanocomposite foam modified by integrating cloisite 20A nanoclay into the matrix. Compared with the pure polyurethane foam, both oil removal efficiency and adsorption capacity of the modified foam increased [10]. In the present work, we report a facile, novel and feasible approach to fabricate superoleophilic and hydrophobic flexible polyurethane foam (FPUF). Typically, the foam surface was n
Corresponding author. Corresponding author at: State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, PR China. E-mail addresses: [email protected] (L. Song), [email protected] (Y. Hu). nn
http://dx.doi.org/10.1016/j.matlet.2015.07.013 0167-577X/& 2015 Published by Elsevier B.V.
alternately deposited by the positively charged chitosan and the negatively charged titanate nanotubes (TNTs) using layer by layer (LbL) self-assembly technique, followed with the modification of dodecyl mercaptan. The modified FPUF has good potential application in oil–water separation.
2. Experimental Materials: Flexible polyurethane foam (DW30, density: 0.03 g/cm3) was obtained from Jiangsu Lvyuan New Material Co., Ltd. Chitosan (viscosity: 50–800 mPa s, degree of deacetylation 80–95%), titanium dioxide, sodium hydroxide, tetramethylammonium hydroxide (TMAOH, 25% aqueous solution) and hydrochloric acid (HCl 36–38%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Polyacrylic acid (PAA, Mw 3000) was received from Aladdin Industry Co., Ltd. 2.1. Preparation of solution TNTs were prepared by the hydrothermal method as described in reference [11]. TNTs were dispersed in 0.2 M TMAOH aqueous solution and were stirred at room temperature for 24 h to yield a colloidal TNT dispersion (0.8 wt%). Chitosan solution was dissolved in pH ¼ 5 deionized water with mechanical stirring for 24 h to prepare 0.5 wt% homogeneous solution.
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2.2. LbL deposition process
3. Results and discussions
FPUF was firstly immersed in 0.1 wt% poly (acrylic acid) solution for 5 min to obtain negatively charged sample. Then the treated FPUF was successively immersed in the solutions of chitosan and TNTs for 2 min. Each time before soaking in the solutions, the foam was washed in the deionized water for 2 min. One bilayer film was built up by a chitosan single-layer and a TNTs single-layer. When the bilayers reached to the desired number, the samples were dried into a 70 °C convection oven overnight and then stored in a desiccator for 24 h to eliminate the residual water. Then, the treated FPUFs were immersed into 0.01 M dodecyl mercaptan ethanol solution for 24 h. Finally, the samples were washed with ethanol and dried in a 70 °C oven for 1 h. The FPUFs coated with 3, 6 and 9 bilayers and followed with the modification of dodecyl mercaptan were named as FPUF-1, FPUF-2 and FPUF-3, respectively.
The surface morphological evolution of the FPUFs is investigated by SEM, as shown in Fig. 1a. The pure FPUF sample shows a smooth and clean surface. After covered with the coating, the surfaces of the treated FPUFs become rough. On the surface of FPUF-1, the TNTs are not easily identified. Instead, some agglomerates appear on the skeleton and the 3 bilayers coating does not distribute uniformly on the foam surface. With 6 bilayers coating, the outline of TNTs can be obviously observed on the foam surface. For the FPUF-3, it is observed that numerous TNTs uniformly anchored on the skeleton of the foam, indicating that the modified FPUF foam with rough surface has been successfully prepared by LbL self-assembly method. EDX spectra of pure and modified FPUFs are shown in Fig. 1b as a representation to monitor the element composition of samples. The Ti element is only detected in the modified FPUFs, indicating the presence of TNTs on the surfaces of modified FPUFs. Thermal stability of the pure FPUF and modified FPUFs is investigated by TGA. The degradation of the FPUFs under nitrogen atmosphere is divided into two stages. As shown in Fig. 1c, the surface modification delays the thermal degradation of the foams at the second stage. Moreover, the char residue of the modified samples at 750 °C improves with increasing the bilayers, due to the more adsorbed TNTs on the FPUFs. Fig. 2a and b shows the photographs of water droplet and soybean oil droplet on the pure FPUF and FPUF-3. After modified with dodecyl mercaptan, the FPUF-3 becomes superoleophilic. When the pure FPUF and FPUF-3 are placed on soybean oil simultaneously (Fig. 2c and d), the pure FPUF floats on the oil while the FPUF-3 sinks into the oil within 3 s. Furthermore, the FPUF-3 exhibits excellent selective absorption of oil from water. Once a piece of FPUF-3 touches the soybean oil layer on the water surface, it absorbs the oil slick rapidly, resulting in a transparent water
2.3. Characterization The morphologies of the pure and modified FPUFs, which were pre-coated with a gold layer, were observed using scanning electron microscope. (SEM, FEI SIRION 200, USA). The Thermogravimetric analysis (TGA) of sample under nitrogen atmosphere was examined on a TGA-Q5000 apparatus (TA Instruments Inc., USA) from 50 to 750 °C at a heating rate of 20 °C/min. The water contact angle (WCA) of the samples was measured with a drop-shape analysis system (Krüss DSA100, Germany) at three different points for each.
Fig. 1. SEM images (a) and EDX spectra (b) of the pure and modified FPUFs; TG and DTG (c) curves of the pure and modified FPUF.
Y. Pan et al. / Materials Letters 159 (2015) 345–348
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Fig. 2. Digital photographs of water droplet and soybean oil droplet on the pure FPUF (a) and FPUF-3 (b). The photos of the pure FPUF (c) and FPUF-3 (d) in soybean oil. A series of photos for process of absorption soybean oil from the surface of water (e–g).
region without any contaminations (Fig. 2e–g). The water and oil contact angle (OCA) of the pure and modified FPUF are shown in Fig. 3a and b, respectively. Compared with the pure one, the WCA of FPUF-1 decreases. This may be due to the presence of hydrophilic chitosan and TNTs in the coating. However, the WCA of FPUF-3 increases to 128°, ascribing to the rougher surface as shown in Fig. 1a. Compared with the pure FPUF, the OCA of the modified FPUFs decrease from 85° to 0°, indicating that the superoleophilic surface has been created by dodecyl mercaptan. The absorption capacity (k) is calculated by the weight-gain ratio according to the following equation: k ¼(M2 M1)/M1, where M1 and M2 represent the weight of the sponge before and after absorption of oil or organic solvent, respectively. Fig. 4a shows the absorption capacity of the FPUFs for n-hexane, diesel oil and soybean oil. The modified FPUFs exhibit high absorption capacities at a range of 20–29, depending on the viscosity and density of the oil or organic solvent. With the increasing of the bilayers covered on the FPUF, the absorption capacity decreases slightly. This is due
to the weight gain of the FPUFs after covering with more bilayers. The absorption capacity of the FPUF-3 as a function of the absorption cycle for soybean oil is presented in Fig. 4b. After 6 absorption cycles, the absorption capacity of FPUF-3 decreases. The reduction in absorption capacity may be explained by the fact that the volume of the foam shrank by the squeeze process during the oil removal. During 6–50 absorption cycles, the absorption capacity maintains at a stable value. The morphology of the FPUF-3 after 50 absorption cycles is shown in Fig. 4c. It can be found that the surface is still covered with the coating, which means that the coating can interact well with substrate.
4. Conclusions Superoleophilic and hydrophobic FPUF was prepared by LbL self-assembly method. The roughness of the foam surface could be controlled by changing the bilayer number. The as-prepared
Fig. 3. WCA (a) and OCA (b) of the pure and modified FPUF.
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Fig. 4. Absorption capacity of the modified FPUF for different oils and organic solvent (a). Effect of absorption cycle on the FPUF-3's absorption capacity of soybean oil (b). SEM images of the FPUF-3 after 50 absorption cycles (c).
modified FPUF separated the oil from the water successfully and exhibited high oil absorption capacity. Furthermore, the absorbed oils could be collected by a simple squeezing process, and after 50 absorption cycles, the modified foam still showed high absorption capacity and maintained the rough surface. Due to the cheap material, simple preparation technique and excellent performance, the treated FPUF will be a promising candidate for oil–water separation.
Acknowledgments The work was financially supported by the National Basic Research Program of China (973 Program) (No. 2014CB931804) and the National Natural Science Foundation of China (No. 51473154 and No. 51303165).
References [1] D. Wang, T. Silbaugh, R. Pfeffer, Y.S. Lin, Removal of emulsified oil from water by inverse fluidization of hydrophobic aerogels, Powder Technol. 203 (2010)
298–309. [2] Y. He, Y. Liu, T. Wu, J. Ma, X. Wang, Q. Gong, et al., An environmentally friendly method for the fabrication of reduced graphene oxide foam with a super oil absorption capacity, J. Hazard. Mater. 260 (2013) 796–805. [3] A. Cambiella, E. Ortea, G. Rios, J. Benito, C. Pazos, J. Coca, Treatment of oil-inwater emulsions: performance of a sawdust bed filter, J. Hazard. Mater. 131 (2006) 195–199. [4] D. Mysore, T. Viraraghavan, Y.C. Jin, Treatment of oily waters using vermiculite, Water Res. 39 (2005) 2643–2653. [5] A.L. Ahmad, S. Bhatia, Sumathi S. Ibrahim, Adsorption of residual oil from palm oil mill effluent using rubber power, Braz. J. Chem. Eng. 22 (2005) 371–375. [6] L. Han, J. Zhu, J. Kang, Y. Liang, Y. Sun, Catalytic wet air oxidation of highstrength organic coking wastewater, Asia-Pac. J. Chem. Eng. 4 (2009) 624–627. [7] S. Keav, A. Martin, J. Barbier, D. Duprez, Deactivation and reactivation of noble metal catalysts tested in the catalytic wet air oxidation of phenol, Catal. Today 151 (2010) 143–147. [8] C. Wu, X. Huang, X. Wu, R. Qian, P. Jiang, Mechanically flexible and multifunctional polymer-based graphene foams for elastic conductors and oil–water separators, Ad. Mater. 25 (2013) 5658–5662. [9] H. Li, L. Liu, F. Yang, Oleophilic polyurethane foams for oil spill cleanup, Proc. Environ. Sci. 18 (2013) 528–533. [10] A.A. Nikkhah, H. Zilouei, A. Asadinezhad, A. Keshavarz, Removal of oil from water using polyurethane foam modified with nanoclay, Chem. Eng. J. 262 (2015) 278–285. [11] R. Ma, Y. Bando, T. Sasaki, Directly rolling nanosheets into nanotubes, J. Phys. Chem. B 108 (2004) 2115–2119.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
A facile method to fabricate superoleophilic and hydrophobic polyurethane foam for oil–water separation Ying Pan a, Jing Zhan a,b, Haifeng Pan a,c, Bihe Yuan a,c, Wei Wang a, Lei Song a,n, Yuan Hu a,c,nn a
State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, PR China School of Civil Engineering and Environmental Engineering, Anhui Xinhua University, Hefei, Anhui 230088, PR China c Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute for Advanced Study, University of Science and Technology of China, Suzhou, Jiangsu 215123, PR China b
art ic l e i nf o
a b s t r a c t
Article history: Received 11 January 2015 Received in revised form 29 June 2015 Accepted 5 July 2015 Available online 6 July 2015
A facile and novel method to fabricate superoleophilic and hydrophobic flexible polyurethane foam (FPUF) is presented in this work. The modified FPUF was prepared by utilizing positively charged chitosan and negatively charged titanate nanotubes together with subsequent modification of dodecyl mercaptan. The roughness of the foam surface improved significantly by increasing the bilayers number of the coating. The modified FPUF could rapidly and selectively absorb various kinds of oils up to 29 times of its weight, and the absorbed oils could be collected by a simple squeezing process. Furthermore, after 50 absorption cycles, the modified FPUF could still maintain its high absorption capacity. & 2015 Published by Elsevier B.V.
Keywords: Multilayer structure Superoleophilic Porous materials Oil–water separation
1. Introduction Oil spill accidents and oily wastewater in industrial production can cause ecological and environmental problem [1]. Lots of the sea birds and mammals have been killed by contacting with the spilt oil. The underwater plants are also seriously damaged [2]. Therefore, separating oil from water is an urgent problem to solve. In recent years, many materials have been developed as oil absorbents, including inorganic, organic and composite materials, such as sawdust bed [3], vermiculite [4], rubber power [5], hydrophobic aerogels [6], exfoliated graphite [7], graphene [8] and polyurethane foams [9]. Polyurethane foam has shown noticeable oil adsorption capability due to its special features such as enough space for adsorption, low density, open-cell structure, high porosity and industrial production. Hamid et al., prepared polyurethane nanocomposite foam modified by integrating cloisite 20A nanoclay into the matrix. Compared with the pure polyurethane foam, both oil removal efficiency and adsorption capacity of the modified foam increased [10]. In the present work, we report a facile, novel and feasible approach to fabricate superoleophilic and hydrophobic flexible polyurethane foam (FPUF). Typically, the foam surface was n
Corresponding author. Corresponding author at: State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, PR China. E-mail addresses: [email protected] (L. Song), [email protected] (Y. Hu). nn
http://dx.doi.org/10.1016/j.matlet.2015.07.013 0167-577X/& 2015 Published by Elsevier B.V.
alternately deposited by the positively charged chitosan and the negatively charged titanate nanotubes (TNTs) using layer by layer (LbL) self-assembly technique, followed with the modification of dodecyl mercaptan. The modified FPUF has good potential application in oil–water separation.
2. Experimental Materials: Flexible polyurethane foam (DW30, density: 0.03 g/cm3) was obtained from Jiangsu Lvyuan New Material Co., Ltd. Chitosan (viscosity: 50–800 mPa s, degree of deacetylation 80–95%), titanium dioxide, sodium hydroxide, tetramethylammonium hydroxide (TMAOH, 25% aqueous solution) and hydrochloric acid (HCl 36–38%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Polyacrylic acid (PAA, Mw 3000) was received from Aladdin Industry Co., Ltd. 2.1. Preparation of solution TNTs were prepared by the hydrothermal method as described in reference [11]. TNTs were dispersed in 0.2 M TMAOH aqueous solution and were stirred at room temperature for 24 h to yield a colloidal TNT dispersion (0.8 wt%). Chitosan solution was dissolved in pH ¼ 5 deionized water with mechanical stirring for 24 h to prepare 0.5 wt% homogeneous solution.
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2.2. LbL deposition process
3. Results and discussions
FPUF was firstly immersed in 0.1 wt% poly (acrylic acid) solution for 5 min to obtain negatively charged sample. Then the treated FPUF was successively immersed in the solutions of chitosan and TNTs for 2 min. Each time before soaking in the solutions, the foam was washed in the deionized water for 2 min. One bilayer film was built up by a chitosan single-layer and a TNTs single-layer. When the bilayers reached to the desired number, the samples were dried into a 70 °C convection oven overnight and then stored in a desiccator for 24 h to eliminate the residual water. Then, the treated FPUFs were immersed into 0.01 M dodecyl mercaptan ethanol solution for 24 h. Finally, the samples were washed with ethanol and dried in a 70 °C oven for 1 h. The FPUFs coated with 3, 6 and 9 bilayers and followed with the modification of dodecyl mercaptan were named as FPUF-1, FPUF-2 and FPUF-3, respectively.
The surface morphological evolution of the FPUFs is investigated by SEM, as shown in Fig. 1a. The pure FPUF sample shows a smooth and clean surface. After covered with the coating, the surfaces of the treated FPUFs become rough. On the surface of FPUF-1, the TNTs are not easily identified. Instead, some agglomerates appear on the skeleton and the 3 bilayers coating does not distribute uniformly on the foam surface. With 6 bilayers coating, the outline of TNTs can be obviously observed on the foam surface. For the FPUF-3, it is observed that numerous TNTs uniformly anchored on the skeleton of the foam, indicating that the modified FPUF foam with rough surface has been successfully prepared by LbL self-assembly method. EDX spectra of pure and modified FPUFs are shown in Fig. 1b as a representation to monitor the element composition of samples. The Ti element is only detected in the modified FPUFs, indicating the presence of TNTs on the surfaces of modified FPUFs. Thermal stability of the pure FPUF and modified FPUFs is investigated by TGA. The degradation of the FPUFs under nitrogen atmosphere is divided into two stages. As shown in Fig. 1c, the surface modification delays the thermal degradation of the foams at the second stage. Moreover, the char residue of the modified samples at 750 °C improves with increasing the bilayers, due to the more adsorbed TNTs on the FPUFs. Fig. 2a and b shows the photographs of water droplet and soybean oil droplet on the pure FPUF and FPUF-3. After modified with dodecyl mercaptan, the FPUF-3 becomes superoleophilic. When the pure FPUF and FPUF-3 are placed on soybean oil simultaneously (Fig. 2c and d), the pure FPUF floats on the oil while the FPUF-3 sinks into the oil within 3 s. Furthermore, the FPUF-3 exhibits excellent selective absorption of oil from water. Once a piece of FPUF-3 touches the soybean oil layer on the water surface, it absorbs the oil slick rapidly, resulting in a transparent water
2.3. Characterization The morphologies of the pure and modified FPUFs, which were pre-coated with a gold layer, were observed using scanning electron microscope. (SEM, FEI SIRION 200, USA). The Thermogravimetric analysis (TGA) of sample under nitrogen atmosphere was examined on a TGA-Q5000 apparatus (TA Instruments Inc., USA) from 50 to 750 °C at a heating rate of 20 °C/min. The water contact angle (WCA) of the samples was measured with a drop-shape analysis system (Krüss DSA100, Germany) at three different points for each.
Fig. 1. SEM images (a) and EDX spectra (b) of the pure and modified FPUFs; TG and DTG (c) curves of the pure and modified FPUF.
Y. Pan et al. / Materials Letters 159 (2015) 345–348
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Fig. 2. Digital photographs of water droplet and soybean oil droplet on the pure FPUF (a) and FPUF-3 (b). The photos of the pure FPUF (c) and FPUF-3 (d) in soybean oil. A series of photos for process of absorption soybean oil from the surface of water (e–g).
region without any contaminations (Fig. 2e–g). The water and oil contact angle (OCA) of the pure and modified FPUF are shown in Fig. 3a and b, respectively. Compared with the pure one, the WCA of FPUF-1 decreases. This may be due to the presence of hydrophilic chitosan and TNTs in the coating. However, the WCA of FPUF-3 increases to 128°, ascribing to the rougher surface as shown in Fig. 1a. Compared with the pure FPUF, the OCA of the modified FPUFs decrease from 85° to 0°, indicating that the superoleophilic surface has been created by dodecyl mercaptan. The absorption capacity (k) is calculated by the weight-gain ratio according to the following equation: k ¼(M2 M1)/M1, where M1 and M2 represent the weight of the sponge before and after absorption of oil or organic solvent, respectively. Fig. 4a shows the absorption capacity of the FPUFs for n-hexane, diesel oil and soybean oil. The modified FPUFs exhibit high absorption capacities at a range of 20–29, depending on the viscosity and density of the oil or organic solvent. With the increasing of the bilayers covered on the FPUF, the absorption capacity decreases slightly. This is due
to the weight gain of the FPUFs after covering with more bilayers. The absorption capacity of the FPUF-3 as a function of the absorption cycle for soybean oil is presented in Fig. 4b. After 6 absorption cycles, the absorption capacity of FPUF-3 decreases. The reduction in absorption capacity may be explained by the fact that the volume of the foam shrank by the squeeze process during the oil removal. During 6–50 absorption cycles, the absorption capacity maintains at a stable value. The morphology of the FPUF-3 after 50 absorption cycles is shown in Fig. 4c. It can be found that the surface is still covered with the coating, which means that the coating can interact well with substrate.
4. Conclusions Superoleophilic and hydrophobic FPUF was prepared by LbL self-assembly method. The roughness of the foam surface could be controlled by changing the bilayer number. The as-prepared
Fig. 3. WCA (a) and OCA (b) of the pure and modified FPUF.
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Fig. 4. Absorption capacity of the modified FPUF for different oils and organic solvent (a). Effect of absorption cycle on the FPUF-3's absorption capacity of soybean oil (b). SEM images of the FPUF-3 after 50 absorption cycles (c).
modified FPUF separated the oil from the water successfully and exhibited high oil absorption capacity. Furthermore, the absorbed oils could be collected by a simple squeezing process, and after 50 absorption cycles, the modified foam still showed high absorption capacity and maintained the rough surface. Due to the cheap material, simple preparation technique and excellent performance, the treated FPUF will be a promising candidate for oil–water separation.
Acknowledgments The work was financially supported by the National Basic Research Program of China (973 Program) (No. 2014CB931804) and the National Natural Science Foundation of China (No. 51473154 and No. 51303165).
References [1] D. Wang, T. Silbaugh, R. Pfeffer, Y.S. Lin, Removal of emulsified oil from water by inverse fluidization of hydrophobic aerogels, Powder Technol. 203 (2010)
298–309. [2] Y. He, Y. Liu, T. Wu, J. Ma, X. Wang, Q. Gong, et al., An environmentally friendly method for the fabrication of reduced graphene oxide foam with a super oil absorption capacity, J. Hazard. Mater. 260 (2013) 796–805. [3] A. Cambiella, E. Ortea, G. Rios, J. Benito, C. Pazos, J. Coca, Treatment of oil-inwater emulsions: performance of a sawdust bed filter, J. Hazard. Mater. 131 (2006) 195–199. [4] D. Mysore, T. Viraraghavan, Y.C. Jin, Treatment of oily waters using vermiculite, Water Res. 39 (2005) 2643–2653. [5] A.L. Ahmad, S. Bhatia, Sumathi S. Ibrahim, Adsorption of residual oil from palm oil mill effluent using rubber power, Braz. J. Chem. Eng. 22 (2005) 371–375. [6] L. Han, J. Zhu, J. Kang, Y. Liang, Y. Sun, Catalytic wet air oxidation of highstrength organic coking wastewater, Asia-Pac. J. Chem. Eng. 4 (2009) 624–627. [7] S. Keav, A. Martin, J. Barbier, D. Duprez, Deactivation and reactivation of noble metal catalysts tested in the catalytic wet air oxidation of phenol, Catal. Today 151 (2010) 143–147. [8] C. Wu, X. Huang, X. Wu, R. Qian, P. Jiang, Mechanically flexible and multifunctional polymer-based graphene foams for elastic conductors and oil–water separators, Ad. Mater. 25 (2013) 5658–5662. [9] H. Li, L. Liu, F. Yang, Oleophilic polyurethane foams for oil spill cleanup, Proc. Environ. Sci. 18 (2013) 528–533. [10] A.A. Nikkhah, H. Zilouei, A. Asadinezhad, A. Keshavarz, Removal of oil from water using polyurethane foam modified with nanoclay, Chem. Eng. J. 262 (2015) 278–285. [11] R. Ma, Y. Bando, T. Sasaki, Directly rolling nanosheets into nanotubes, J. Phys. Chem. B 108 (2004) 2115–2119.