Flexible superhydrophobic polysiloxane aerogels for oil–water separation via one-pot synthesis in supercritical CO₂

Abstract

Porous aerogels are an inspiring absorbent for oil–water separation and water purification. In particular, hydrophobic silica aerogels from abundant and cheap sources offer both economical and environmental benefits. Conventional silica aerogels from alkoxy siloxane precursors involve time-consuming and laboursome multistep processes. Herein, we demonstrate the formation of flexible superhydrophobic polysiloxane aerogels through a facile hydrosilylation from functionalized polydimethylsiloxane in supercritical carbon dioxide. This robust aerogel exhibits a high oil absorption capacity, superior recyclability and extraordinary mechanical properties even under harsh heating and cooling cycles. These characters favor this polysiloxane aerogel as being more competitive than common alkoxy silica ones for oily water treatment, oil spill clean-up and oil recovery.

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Introduction

Fast oil–water separation is becoming a global problem for solving the water pollution issues and conserving water resources.^1,2 Frequent oil spill accidents and oil-contaminated industrial wastewater have had severely bad effects on the environment and ecological system.^3–5 It is extremely urgent to develop effective and inexpensive oil–water separating materials for the oily pollution in water systems. Hydrophobic aerogels, as a class of highly porous material filled with air, demonstrated a higher oil absorption capacity, longer retention time, and better oleophilicity and reusability than normal absorbents (e.g. inorganic mineral clays,⁶ synthetic polymeric materials,^7,8 natural cellulose-based materials^9,10). Recently, a variety of hydrophobic aerogels with different chemical compositions have been reported as good candidates for oil–water separation, including ultralight carbon-based aerogels,^11–14 nanostructured metal oxide aerogels^15,16 and cellulose fibers from natural resources and waste.^17–19

Among the diverse kinds of hydrophobic aerogels, silica aerogels, as the first kind of aerogel from abundant and cheap sources, could be the ideal alternative for oil absorption in practical and industrial applications. Compared with inorganic silica ones, the organic silicon gels possess superhydrophobic surfaces and a certain degree of compression properties, due to the rich methyl groups directly bonded onto the silicon atoms and the flexible network backbones from Si–O/Si–C bonds. Several kinds of hydrophobic silica aerogels have been prepared, such as the silica aerogel microspheres,²⁰ magnetic silica sponges,²¹ marshmallow-like porous silica gels^22–25 and the polymer reinforced silica composite aerogels.^26–28 All of these silica aerogels were prepared from alkoxy siloxane precursors and had to go through multistep processes, including hydrolysis, pre-condensation, aging, solvent exchange, drying and/or further surface modification.^25,29 Developing novel kinds of hydrophobic silica aerogels through a facile route is interesting, but still remains challenging.

In this study, we present a facile method to prepare flexible superhydrophobic polysiloxane aerogels directly from functionalized polydimethylsiloxanes (PDMS) using a one-pot hydrosilylation reaction in supercritical CO₂. Compared to the previous sol–gel process, an efficient and controllable hydrosilylation reaction was applied to prepare the aerogels, which can allow modulation of the crosslinking degree of the polysiloxane aerogels. Besides, the aerogels directly from PDMS were inherently superhydrophobic without tedious surface post-modifications. Furthermore, the hydrosilylation in supercritical CO₂ integrated the gelation and drying techniques, avoiding time-consuming multi-steps, and provided a robust and flexible porous skeleton with characterized microstructures. The resultant polysiloxane aerogels were used as oil absorbents for oil–water separation, and exhibited a high oil absorption capacity, superior recyclability and extraordinary mechanical properties even under harsh heating and cooling cycles.

Experimental

Materials

Tetramethyldisiloxane (M^HM^H), octamethylcyclotetrasiloxane (D₄), 1,3,5,7-tetramethylcyclotetrasiloxane (D^H₄), 1,1,3,3-tetramethyl-1,3-divinyldisiloxane (M^ViM^Vi) were purchased from Beijing HWRK Chem Co. Ltd. Toluene, H₂SO₄ (98%) and CuCl₂ 2H₂O were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. CO₂ (>99.95%) was provided by Beijing Analysis Instrument Factory. The Karstedt catalyst solution (platinum-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex solution in xylene, Pt 2%) was purchased from Aladdin Industrial Corporation. All chemicals were used without further purification. Oil Red O was purchased from Sigma-Aldrich Shanghai Trading Co. Ltd.

Synthesis of the methyl hydrogen polysiloxanes (A1 and A2)

Different molar ratios of D^H₄, D₄ and M^HM^H were placed in a glass flask followed by addition of H₂SO₄ (98%) (5 wt% of total silane amount) under stirring. The molar ratios of D₄/D^H₄/M^HM^H were 6.25/1.25/1 for A1 and 5.75/1.75/1 for A2. Then the reaction was continued for 5 hours at room temperature. After that, toluene and water were added. The toluene layer was separated and washed with water three times. The resulting organic layer was dried over anhydrous magnesium sulfate and filtered. The solvent was removed using a rotary evaporator and a colorless liquid product was obtained. The characterization of the chemical structures for the precursors A is shown in the ESI and Fig. S1–S4.†

Synthesis of the vinyl-terminated polydimethylsiloxane (B)

D₄ (36.91 g, 0.124 mol) and M^ViM^Vi (3.09 g, 0.0166 mol) were placed in a glass flask, and the initiating amount of H₂SO₄ (98%) (2 g, 0.02 mol) was added under stirring. Then the reaction was continued for 5 hours at room temperature. After that, toluene and water were added. The toluene layer was separated and washed with water three times. The resulting organic layer was dried over anhydrous magnesium sulfate and filtered. The solvent was removed using a rotary evaporator and a colorless liquid product, B (36.2 g), was obtained with 90.5% yield. The characterization of the chemical structures for the precursors B is shown in the ESI and Fig. S1–S4.†

Preparation of the aerogel

Equal molar amounts of Si–H and vinyl from a methyl hydrogen polysiloxane (A) and vinyl-terminated PDMS (B), respectively, were added into a 50 mL high-pressure reactor equipped with a magnetic stirrer. Then, CO₂ was charged into the reactor at 40 °C until the whole system became homogeneous. Then 100 ppm of total weight of the reactants Karstedt’s catalyst was injected into the cell and quickly mixed well. After gelation for 4 hours without agitation, the CO₂ was released (Fig. 1).

Fig. 1 (a) Synthesis route of the polysiloxane aerogel in scCO₂. (b) Schematic description of the one-pot preparation for the target polysiloxane aerogel in scCO₂.

Characterization

¹H NMR and ²⁹Si NMR measurements were carried out using Bruker AV-400 and AV-300 NMR instruments, respectively, using the residual protonated solvent as an internal standard. Mass spectra (MALDI-TOF-MS) were determined using a Bruker BIFLEX III Mass spectrometer. Transmission electron microscopy (TEM) samples were examined with a JEM2200FS (200 keV). Before measurement, the samples were ground into pieces and then dispersed in dry ethanol and sonicated for 1–2 min. After that, the sample solution was cast on a carbon-coated copper grid. After one minute, the excess sample solution was carefully removed using a piece of filter paper. The surface microstructure was examined using a Scanning Electron Microscopy (SEM) (6700F) instrument. Contact angle data were obtained using a Krüss Drop Shape Analysis System-100 (DSA 100) by a sessile water drop method with 5 μL liquid drops. Thermogravimetric analysis (TGA) was performed using a 7 Series thermal analysis system (Perkin-Elmer). The sample was heated from 30 to 800 °C at a rate of 5 °C min⁻¹ in a dynamic nitrogen atmosphere with a flow rate of 70 mL min⁻¹. The bulk density was obtained from the weight/volume ratio of the specimens. The skeletal density was measured using an UltraPYC 1200e automatic density analyzer from Quantachrome Instruments. The porosity (%) of each sample was calculated as (1 − ρ_b/ρ_s) × 100%, where ρ_b and ρ_s refer to the bulk and skeletal densities, respectively. The specific surface area and pore size were determined using a PoreMaster-60-17 from Quantachrome Instruments.

Results and discussion

In this work, a series of aerogels were prepared with a methyl hydrogen polysiloxane (A) and vinyl-terminated PDMS (B) through a hydrosilylation reaction in supercritical CO₂ (Table S1†). This method is simple, time-saving, and environmentally friendly, avoiding troublesome solvent exchange, complicated drying processes, surface modification and the use of organic solvents.

Structure, hydrophobicities and mechanical properties of the obtained aerogels

Fig. 2a and d show photographs of the two polysiloxane aerogels, A1–B and A2–B, obtained from the reaction of B with precursors A1 and A2, respectively. They display a greatly soft and elastic appearance. The bulk densities of the A1–B and A2–B aerogels were 227 and 258 mg cm⁻³, respectively (Table S1†). Their microstructures were further characterized using SEM and TEM. The SEM images (Fig. 2b and e) show that both the A1–B and A2–B aerogels have a three-dimensional interconnected network, a highly porous structure, Pt particles that were uniformly distributed in the aerogel (Fig. S5†) and characteristic crosslinked bulb structures with diameters of about 8 μm. After pounding in a mortar to break the bulb structures, both the A1–B and A2–B aerogels also demonstrate nanoporous structures with a pore size in the range of 30–180 nm (Fig. 2c and f) and, because a highly porous structure is conducive to the rapid transport of gas and liquid in the aerogel, they can be used for oil absorption. Notably, A1–B shows more inter-connective nanopores and less regional aggregations than A2–B, which is beneficial to enhance the porosity and specific surface area (Table S1 and Fig. S6†). The difference in microstructure of the two aerogels can be attributed to their specific chemical compositions. As shown in Fig. 1, A2 has a high mole percentage of Si–H, so even at the same reaction concentration, the A2–B aerogel will have more crosslinking. During the gelation, the more crosslinks in A2–B will lead to faster precipitation and more aggregates in scCO₂ than that of A1–B, resulting in the reduced porosity and specific surface area of the A2–B aerogel compared to that of the A1–B aerogel.

Fig. 2 Photographs, and SEM and TEM images of the A1–B (a–c) and A2–B (d–f) aerogels synthesized at 22 MPa and a concentration of 136 mg mL⁻¹.

One of the most important features of a polysiloxane aerogel is its superhydrophobicity (Fig. 3a and c). The surface hydrophobicities of the A1–B and A2–B aerogels were investigated by using water-contact angle measurements. Both of them are superhydrophobic with a contact angle over 150° (Fig. 3b and d) with neither chemical nor physical surface treatment. The hydrophobic features of the aerogels are due to the microscopic rough surface from the intrinsic porous structures and the presence of plenty of methyl groups.

Fig. 3 Optical images of one water droplet deposited on the A1–B (a) and A2–B surface (c). The water was colored using CuCl₂. The water contact angles for A1–B (b) and A2–B (d) are 154° and 150°, respectively.

For many silica aerogels, low mechanical strength limited their applications. But in this work, this problem can be solved using the one-pot hydrosilylation reaction of methyl hydrogen polysiloxane (A) and vinyl-terminated PDMS (B) in supercritical CO₂. Stress–strain curves of uniaxial compression tests on the A1–B and A2–B aerogels demonstrate their high compressibility and remarkable flexibility (Fig. 4a). The A1–B aerogel could recover to its original shape after 55% deformation over 100 cycles of repeated compression at room temperature, 150 °C and −50 °C, respectively (Fig. 4b–d and S7a†). The shape recovery and persistence indicate that the polysiloxane aerogels possess integrated structures and good mechanical stability. A silica aerogel without further chemical or physical surface treatment will break and collapse with small stress, because it has many silanol groups on the surface and the silanol groups that are close together will further react to form siloxane bonds during a compression test. But for the obtained polysiloxane aerogels without silanol groups, the elastic PDMS-like backbones, highly porous gel networks and repulsive interactions between the methyl groups endow the aerogel with high compressibility, remarkable flexibility and no tendency to collapse during compression tests. Compared with traditional crumbly silica aerogels (Table S2†),^20,30–33 the excellent toughness of the A1–B aerogel is especially significant for reusable oil absorbents in industrial applications. It is noteworthy that A1–B has better flexibility than A2–B under the same uniaxial compression (Fig. 4a), which can be attributed to the fact that A1–B has less chemical crosslinks than A2–B.

Fig. 4 (a) Stress–strain curves of uniaxial compression tests on the A1–B and A2–B aerogels. 100 cycles of repeated compression of the A1–B aerogel at room temperature (b), 150 °C (c) and −50 °C (d).

As a result of the excellent mechanical stability of the polysiloxane aerogels, the superhydrophobic aerogels also exhibit considerable robustness against organic solvents (Fig. S4b†). Meanwhile, the polysiloxane aerogels are stable up to ca. 400 °C from thermogravimetric analysis (Fig. S8 and Table S1†). The thermal stability of the as-synthesized polysiloxane aerogels is much better than that of common oil–water separation materials from carbon-based polymers, for which the initial decomposition temperature is ca. 200 °C (Table S3†).^17,34

Absorption capacities of the obtained aerogels

With high porosity, superhydrophobicity, high compressibility and remarkable flexibility, the polysiloxane aerogels can be considered as ideal candidates for oil–water separation. To further verify the feasibility for oil–water separation applications, n-hexane was chosen as a model absorbate to investigate the separation performance of the A1–B aerogel. As shown in Fig. 5a, n-hexane is completely separated from water quickly using the A1–B aerogel and also can be all squeezed out from the aerogel by hand. Even after 100 cycles of absorption and squeezing-out, not only does the A1–B aerogel’s absorption performance still remain stable with no significantly decrease (Fig. 5b), but also its morphology has no damage, indicating its excellent recyclability (Fig. S7b†). The reusability of the polysiloxane aerogel as an oil absorbent can be attributed to its robust skeleton. Furthermore, the superhydrophobic A1–B aerogel displays excellent absorption capacities towards a wide range of organic solvents, up to 4.7–14.5 times of its dry weight depending on the density of the organic liquid (Fig. 5c and S9†). The aerogel could be recycled and recovered by a facile squeezing-out, even for the high-density organic solvents (e.g., chloroform) and viscous oils (e.g., mineral oil with a comparable kinetic viscosity of ca. 44.6 mm s⁻¹ at 40 °C to medium crude oil³⁵). Therefore, this kind of polysiloxane aerogel can be used for regeneration of the oil. Particularly, A1–B demonstrates a better absorption ability than A2–B, probably due to its lower crosslinking density and larger specific surface area than A2–B (Table S1†). It is worth noting that the oil absorbency of the A1–B polysiloxane aerogel is much more superior to that of many other reported silica ones (Table S4†).^31,32 So the excellent absorption capacity of the polysiloxane aerogel is not only due to the highly porous framework, oleophilic nature, capillary action and robust skeleton, but is also attributed to sufficient swelling of the cross-linked flexible polysiloxane network in organic solvents.

Fig. 5 (a) Separation of n-hexane from water using the A1–B aerogel, and the hexane is colored with Oil Red O. (b) Weight gain of the A1–B aerogel during 100 cycles of n-hexane absorption. (c) Absorption capacities of the A1–B and A2–B aerogels for various organic solvents, as indicated by weight gain.

Mechanism for the formation of the polysiloxane aerogels in scCO₂

To investigate the hydrosilylation in scCO₂, the effects of different experimental conditions on the formation of the polysiloxane aerogels were investigated. Firstly, upon the decrease of the reaction concentration, the resulting A2–B aerogels have more unconsolidated structures. Their microscopic structures reveal some regional small bulb aggregates and unevenly distributed bigger pores over hundreds of micrometers (Fig. S10†). The porosities of the A2–B aerogels synthesized at concentrations of 68, 95 and 136 mg mL⁻¹ are 86.8%, 86.0% and 76.5%, respectively, which suggests that the porosity decreases with the increasing of sample concentration. Under a high reaction concentration, the skeleton density is constant, but an increased bulk density could result in a decrease of pore size, which is consistent with the SEM results (Fig. 2e and S10†). Moreover, with the decrease of the concentration, the A2–B’s contact angle increases (Table S1†), indicating enhanced hydrophobicity. This probably is due to the more porous surface with lower concentration. It is worth noting that the A1–B aerogel could not be formed at a concentration lower than 136 mg mL⁻¹ (Table S1†) because of the reduced crosslinking for A1–B. So it could not form stable three-dimensional interconnected networks, but precipitation occurred. Secondly, the polysiloxane aerogels could not be formed under 16 MPa. Although the precursors, A1 and B, can be well solubilized in scCO₂ at 16 MPa, during the gelation, the crosslinked A1–B slowly precipitated and finally hard and fragile monoliths were formed, which display as an amorphous closely packed bulk aggregate (Fig. S11†). This suggests that scCO₂ can not only work as a solvent to dissolve the precursors and a reaction medium for gelation, but can also favor the gel skeleton formation at a high enough pressure. For comparison, we prepared the A1–B gel in dioxane by freeze-drying. The gel appears as a shrunk plastic and the network collapses (Fig. S12†), which is similar to the reported silicon aerogels formed in organic solvents.^36,37 This further proves that scCO₂ is critical for the aerogel formation.

Based on the above experimental results, a mechanism for the formation of the polysiloxane aerogels is proposed in Fig. 6. The two precursors (A and B) are first dissolved in scCO₂, forming a homogeneous solution. During the hydrosilylation process, the crosslinked polysiloxanes have worse solubility than the monomers in scCO₂ and could partially precipitate as micron-sized aggregates as shown in Fig. 2. Meanwhile, the scCO₂ could suffuse, swell and support the microstructures in the aggregates and the whole skeleton of the final aerogel. Therefore, after the degassing and drying procedure polysiloxane aerogels which preserve the skeleton structure are obtained.

Fig. 6 Schematic illustration for the formation of polysiloxane aerogels in scCO₂.

Conclusions

In conclusion, an excellent superhydrophobic polysiloxane aerogel has been prepared through a simple and green one-pot hydrosilylation cross-linking in scCO₂. The polysiloxane aerogel exhibits a high oil absorption capacity, superior recyclability, extraordinary mechanical properties and robust stability regardless of the harsh external conditions, such as various organic solvents, and heating and cooling tests. It is believed that the distinct chemical composition from the polydimethylsiloxane and the highly porous microstructures afford the resulting aerogel with superhydrophobicity, outstanding elasticity and reusability. The robust polysiloxane aerogel has great potential for the industrial applications of oily water treatment, oil spill clean-up and oil recovery.

Acknowledgements

We are grateful for the support from the National Basic Research Program of Ministry of Science and Technology of China (2012CB933201) and the National Natural Science Foundation of China (51303187 and 51225306).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra13023a

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