Article pubs.acs.org/Langmuir
A Mesoporous Silica Nanocomposite Shuttle: pH-Triggered Phase Transfer between Oil and Water Haixia Wang,† Hengquan Yang,*,† Huanrong Liu,† Yuhong Yu,‡ and Hongchuan Xin§ †
School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, P. R. China Yabao Pharmaceutical Group Co., Ltd., Yuncheng 044602, P. R. China § Key Laboratory of Biofuels, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, P. R. China ‡
S Supporting Information *
ABSTRACT: With a simple protocol, we synthesize a novel mesoporous silica nanocomposite shuttle that can reversibly transfer between an organic phase and water in response to the pH, due to the switchable surface hydrophobicity/hydrophilicity. Our synthesis protocol allows the phase transfer ability to be tuned in a controllable fashion. This nanocomposite shuttle exhibits a good ability to load various cargoes such as Pd(OAc)2, Pd nanoparticles, and organic molecules. The built-in properties of the nanocomposite shuttle lay the foundations for many innovative applications. As a proof of concept, we successfully demonstrate its application in separating and recycling Pd nanoparticle catalysts. The composite shuttle can take Pd nanoparticles to an organic phase for catalyzing hydrogenation of olefins and come back to an aqueous phase at the end of reaction, making in situ separation and recycling of nanocatalysts possible. This pH-driven round trip for catalysis can be repeated several times. Our investigations not only supply a novel nanocomposite shuttle with controllable properties but also open an innovative avenue to in situ separation and recycling of nanocatalysts, which can address the obstacles of the conventional methods such as centrifugation and filtration.
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aqueous phase after catalysis mission was accomplished.28 Lodge’s group presented polymersome and micelle shuttles that could transport hydrophobic organic dyes between ionic liquid and water.30,31 Zhao and Yang’s groups recently reported various thermo- and light-sensitive polymer brush-grafted particles that could undergo spontaneous transfer between aqueous and organic phases.32,33 Murray et al. demonstrated a ligand-exchange strategy to transfer colloidal nanocrystals between hydrophobic and hydrophilic media.34,35 Although significant progress has been made within the field, the shuttles that possess a ability to carry various cargoes are mainly limited to dendrimer, polymer micelle, and polymersomes.28−36 The situation does not meet the increasing demands of delivery of various cargoes, limiting the applications of the shuttle concept. Among various materials, mesoporous materials may be a good shuttle candidate due to its unique architecture.37−42 Its large pore size and high pore volume are helpful to accommodate various cargoes including molecules and metal nanoparticles. Its flexible synthesis protocol and ease of functionalization enable its architecture and surface properties to be facilely tailored according to the mission requirements. These intriguing properties make mesoporous materials accessible to many innovative applications in catalysis, adsorption/separation, and biomedicine. More specifically, a
INTRODUCTION The oil/water interface is prevailing in natural and artificial systems. The ability to reversibly transfer substances including molecules, supramolecular assemblies, and colloidal particles across the oil/water interfaces (sometimes called shuttle) is not only necessary for the biologically relevant process1−6 but also highly desirable for many innovative applications such as biomedical diagnose, drug delivery, separation/purification of nanoparticles, and catalyst recycling.7−10 For example, in biological systems, a biological molecule or colloidal particle can smartly regulate its affinity to a hydrophobic phase or water for crossing biological membrane barriers, which is currently inspiring researchers to design smart drug nanocarriers for diagnostic and therapeutic applications.11−14 In catalysis field,15−20 if a catalyst can reversibly transfer between two immiscible phases in response to environment changes, the successive catalyst separation and recycling can be in situ conducted without need for conventional separation methods such as filtration and centrifugation. Because of the importance in understanding the biological process and technologically applications, reversible transfer of various substances has been the subject of fundamental research and practical applications.21−27 Regarding supramolecular assembly and colloidal particle shuttles, a series of important contributions have recently cast bright light in the area.28−36 For example, Crooks and coworkers described a smart dendrimer shuttle that could carry Pd nanoparticles to transfer from an organic phase into an © 2013 American Chemical Society
Received: January 24, 2013 Revised: March 18, 2013 Published: March 19, 2013 6687
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stirring. After further stirring for 12 h and then aging overnight, the precipitation was isolated and washed with water. Afterward, the solid powder was extracted with hot ethanol (12 h, three times). After being dry under vacuum, C or N nanocomposite was obtained . Octyltriamine-Bifunctionalized Silica Composites 6C-1N and 2C-1N. A mixture of 505 mL of methanol, 400 mL of water, 3.0 g of CTAC (cetyltrimethylammonium chloride), and 2.2 mL of aqueous NaOH (1 mol/L) solution was stirred for 30 min at room temperature. 8.61 mmol of TMOS (tetramethyl orthosilicate), 1.515 × 6/7 mmol of octyltrimethoxysilane, and 1.52 × 1/7 mmol of (MeO)3SiCH2CH2CH2(NHCH2CH2)2NH2 were added into this mixture under stirring. After further stirring for 12 h and further aging overnight, the precipitation was isolated and washed with water. Afterward, the resultant solid powder was extracted with hot ethanol (12 h, 3 times). After being dry under vacuum, 6C-1N was obtained. For 2C-1N, the gel composition is 8.61 mmol of TMOS (tetramethyl orthosilicate), 1.52 × 2/3 mmol of octyltrimethoxysilane, and 1.52 × 1/3 mmol of (MeO)3SiCH2CH2CH2(NHCH2CH2)2NH2. Other conditions and procedures are the same as those of 6C-1N. Phase Transfer. In a typical experiment, 4 mL of deionized water was first added into a vial containing 0.02 g of composite. Then, 4 mL of oil (ether, ethyl acetate, or other organic solvents) was then added into the vial. 1 mol/L HCl and NaOH solutions were used to adjust the pH. Adsorption of Pd and Loading Pd. 1.0 g of 2C@1N was added into 12 mL of toluene containing 0.017 g of Pd(OAc)2. After stirring for 4 h at room temperature, the Pd-adsorbed solid was reduced with NaBH4 in a mixture of toluene and ethanol (V/V = 20/1). Adsorption of Organic Compounds. For toluene, 8 mg of the solid materials was added into 36 mL of an aqueous toluene solution with a concentration of 196.7 ppm. The adsorption tests were conducted under the stirring conditions. The concentration of the residual 4-heptylphenol in water was monitored with high performance liquid chromatography (HLPC, Vmethanol/VH2O = 20:80, at 218 nm). For 4-heptylphenol, 8 mg of the solid materials was added into 36 mL of an aqueous 4-heptylphenol solution with a concentration of 11.5 ppm. The concentration of the residual 4-heptylphenol in water was also monitored with high performance liquid chromatography (HLPC, Vmethanol/VH2O = 90:10, at 278 nm). Catalytic Reaction. 1 mmol of olefin, the shuttle catalyst (containing 0.5 mol % Pd with respect to substrate), 20 mL of water, and 15 mL of ether were mixed in a flask. H2 was introduced through a balloon at ambient pressure. (To clearly demonstrate the catalyst dispersion and phase transfer, we used an excessive water and solvent.) The hydrogenations were conducted at 30 °C for a given time. At the end of reaction, a few drops of diluted HCl solution (1 mol/L) were added into the reaction system, and the pH of water was adjusted to 3−4. After stirring, the solid catalyst transferred into the water, and the upper organic layer was isolated for GC determination. Characterization. The small-angle X-ray powder diffraction analysis was performed on Rigaku D/max rA X-ray diffractometer (at 40 kV and 30 mA with Cu Kα radiation). N2 physical adsorption was measured by using an ASAP2020 volumetric adsorption analyzer. Before measuring, all samples were outgassed at 120 °C under vacuum for 6 h. The surface area was calculated from the adsorption branch in the relative pressure range of 0.05−0.15 using Brunauer−Emmett− Teller (BET) equation. Pore diameters were determined from the adsorption branch of the isotherm using the BJH method. The total pore volume was calculated at a relative pressure of P/P0 > 0.99. FT-IR spectra were performed on Thermo-Nicolet-Nexus 470 infrared spectrometer. Transmission electron microscope (TEM) images were obtained on a JEM-2000EX (operated at 200 kV). 300 MHz spectrometer: for 13C CP-MAS NMR experiments, 75.4 MHz resonant frequency, 4 kHz spin rate, 4 s pulse delay. 1.0 ms contact time, hexamethylbenzene as a reference compound; for 13Si MAS NMR experiments, 79.6 MHz resonant frequency, 4 kHz spin rate, 4.0 s pulse delay, TMS as a reference compound. Thermal gravimetric analysis (TG) was performed with a NETZSCH TG analyzer (Germany) under a nitrogen atmosphere from room temperature to
series of recent investigations indicate that template-containing mesoporous silica is an interesting composite for various applications.43−47 Silica part provides a rigid inorganic framework, and the confined micelles inside nanopores create a unique organic microenvironment to trap hydrophobic molecules and stabilize metal nanoparticles. For example, Stucky and Denoyel found that the template-containing mesoporous silica materials could be used as confined micelles to efficiently adsorb hydrophobic compounds from water.43 Aida and co-workers also demonstrated that templatecontaining mesopores were extremely active in catalyzing acetalization of cyclohexanone owing to the amphiphilic nanospace.44 Chen’s group and our group reported on a facile preparation of metal-supported catalysts with the aid of template inside the mesoporous channels and demonstrated that the metal nanoparticles confined in the templatecontaining mesopores were highly active.45,46 Teramae and co-workers determined the diffusion coefficient of metal complexes inside the template-containing mesopores and discovered that the diffusion rate of a neutral compound inside the template-containing mesopores was comparable to that the template-free pores.47 On the basis of these findings and our experimental experience,46 we believe that template-containing mesoporous silica composite is an interesting colloidal particle shuttle because it combines inorganic silica and the confined micelles where surfactant molecules are organized in a spatially particular fashion. This unique structure and composite may bring about new functions. However, the mesoporous silicarelevant shuttle has not been explored so far. In this contribution, for the first time, we synthesize a mesoporous silica nanocomposite shuttle according to a delayed condensation48−50 and examine its phase transfer ability and capacity to load molecular cargoes. As a proof of concept, we further explore its innovative application in separation and recycling of nanocatalysts that is highly difficult for the conventional methods such as centrifugation and filtration.
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EXPERIMENTAL SECTION
Chemicals. Tetramethyl orthosilicate (TMOS), (MeO)3SiCH2CH2CH2(NHCH2CH2)2NH2, (MeO)3Si(CH2)7CH3, cetyltrimethylammonium chloride (CTAC), and most of olefins were purchased from the Aladdin Company. Pd(OAc)2 was purchased from Shanghai Boka Company (China). Nanocomposite (xC@yN) Synthesis. A mixture of 505 mL of methanol, 400 mL of water, 3.0 g of CTAC (cetyltriethylammonium chloride), and 2.2 mL of aqueous NaOH (1 mol/L) solution was stirred for 30 min at 25 °C. x/(x + y) × 8.61 mmol of TMOS (tetramethyl orthosilicate) in combination with 1.01 mmol of octyltrimethoxysilane was added into this mixture under stirring, where x (= 6 or 3, 2, 1) and y (= 1) represent the molar fractions of the used siliceous precursors for core formation and shell formation, respectively. After stirring for 1.5 h, y/(x + y) × 8.61 mmol of TMOS together with 0.505 mmol of (MeO)3SiCH2CH2CH2(NHCH2CH2)2NH2 was added into above mixture. After further stirring for 10 h and aging overnight, the precipitation was isolated and washed with water. After being dry, the powder was extracted with hot ethanol (12 h, repeated three times). Octyl-Monofunctionalized Mesoporous Silica Nanocomposite (C) and Triamine-Monofunctionalized Mesoporous Silica Nanocomposite (N). A mixture of 505 mL of methanol, 400 mL of water, 3.0 g of CTAC (cetyltrimethylammonium chloride), and 2.2 mL of aqueous NaOH (1 mol/L) solution was stirred for 30 min at room temperature. 8.61 mmol of TMOS (tetramethyl orthosilicate) admixed with 1.515 mmol of octyltrimethoxysilane or (MeO)3SiCH2CH2CH2(NHCH2CH2)2NH2 was added into this mixture under 6688
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900 °C with a heating rate of 20 °C/min. Pd content was analyzed with an inductively coupled plasma-atomic emission spectrometry (ICP-AES, AtomScan16, TJA Co.). C and N content analysis was conducted on Vario EL (Elementar). X-ray photoelectron spectra (XPS) were recorded on a Kratos Axis Ultra DLD, and the C1S line at 284.8 eV was used as a reference. Water contact angles were measured on a KRÜ SS DSA100. Before measurement, all powder samples was dried and then compressed into a pellet, similar to preparing FT-IR sample pellet (ca. 4 MPa). The conversion and yield were determined by Agilent 7890A GC (using bromobenzene as an internal reference).
struct a mesoporous core (the molar ratio of octylsilane to TMOS was 15/85). After the core formation, a mixture of triamine silane [(MeO)3SiCH2CH2CH2(NHCH2CH2)2NH2] and TMOS (the molar ratio of triamine silane to TMOS was also 15/85) was added to grow a mesoporous shell surrounding the core. The shell thickness and the core diameter were adjusted by varying the molar fraction of the added siliceous precursors but keeping the total siliceous precursors constant. The synthesized composites were denoted as xC@yN, where x (= 6 or 3, 2, 1) and y (= 1) represent the molar fractions of the used siliceous precursors for core formation and shell formation, respectively. Take 2C@1N for example, the molar fraction of siliceous precursors added in the first step for the core formation is 2/3 of the total siliceous precursors, and the molar fraction of siliceous precursors added in the second step for the shell formation is 1/3 of the total siliceous precursors. The obtained materials were subjected to a hot alcohol extraction to remove a portion of template that is unstably adsorbed on the composites (12 h, repeating three times). For the sake of comparison, we also synthesized an octylmonofunctionalized silica nanocomposite (named as C), triamine-monofunctionalized silica nanocomposite (N), reversed triamine-functionalized core/octyl-functionalized shell silica nanocomposite (1N@2C, as opposed to 2C@1N), and octyltriamine-bifunctionalized silica nanocomposite (2C-1N and 6C-1N, via one step of condensation instead of the delayed condensation). Their detailed synthesis is included in the Supporting Information. The TEM images for representative samples 2C@1N and 1C@1N are displayed in Figure 1. 2C@1N consists of uniform discrete microspheres with a diameter of 200−300 nm (Figure 1a). Its surface is relatively smooth. The core and shell show different pore architectures (Figure 1b). The pores (ca. 2 nm) in the core are observed to be radially aligned, whereas the pore ordering on the shell is relatively irregular. Its analogous samples such as 6C@1N and 3C@1N show the same morphology and pore structure as 2C@1N. However, when the core dimension further decreases and the shell dimension increases to the case of 1C@1N, the morphology and pore structure undergo an apparent change. In spite of a spherical morphology (200−300 nm in diameter, Figure 1c), there are many ravines present on the surface of the microspheres, and a boundary between the core and shell can be observed in the more magnified TEM image (Figure 1d). These considerable differences between core and shell structures may result from the different compositions in the sol−gel process. The smallangle X-ray diffraction patterns of all these composites show a diffraction peak at 2θ = 2.5°, indicating a mesoporous structure (Figure S1, Supporting Information). N2 sorption analysis reveals that these synthesized nanocomposites have a relatively low specific surface area (20−30 m2/g), which is due to the presence of a portion of templates inside the nanopores (Figure S2 and Table S1). The low specific surface area determined with N2 sorption does not mean that the composite cannot load molecular cargoes at room temperature because the N2 sorption measurement is conducted under −196 °C, and under these conditions the organic ligands in nanopore are so rigid that N2 does not sufficiently enter the interior the before measuring equilibrium. More explanations are supplied in notes of Table S1). To further confirm the mesoporous structure of the nanocomposite framework, the composites were treated with a CH3OH/H2O solution containing NaCl for complete removal of templates.51 The template-free composites show an
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RESULTS AND DISCUSSION Synthesis and Characterization. The key for achieving a colloidal particle shuttle between an oil and water phase is to construct a smart surface that can respond to environment changes. We envision a template-containing mesoporous silica nanosphere that is composed of an octyl-functionalized mesoporous core and a triamine-rich mesoporous shell (as shown in Scheme 1). The octyl-functionalized mesoporous Scheme 1. Structure of a Core/Shell-Structured Mesoporous Silica Composite Shuttle and the Proposed Mechanism for Reversible Phase Transfer between an Organic Phase and Water Triggered by pHa
a
The upper layer is an organic phase, and the bottom layer is water.
core is expected to provide a hydrophobic environment to trap organic cargoes. Because of protonation and deprotonation of triamine on the shell at different pHs, the hydrophilicity/ hydrophobicity of the nanocomposite surface can be reversibly switched. Such a switchable surface affinity to oil and water drives reversible phase transfer of the composite nanosphere between an organic phase and water. Our synthesis of the functional group-segregated, core/shellstructured silica composite microsphere followed up the delayed condensation strategy,48−50 as shown in Scheme 2. This strategy allows the molecular functionalities to be selectively positioned in the inner core or on the outer shell through a stepwise addition of siliceous precursors, making the independent control over the internal and external surface property possible. In the first step, a mixture of octyltrimethoxysilane and tetramethyl orthosilicate (TMOS) was added to a cetyltrimethylammonium chloride (CTAC) solution to con6689
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Scheme 2. Schematic Illustration of the Synthesis of Octyl-Functionalized Core/Triamine-Rich Shell-Structured Mesoporous Silica Nanocomposites through a Delayed Condensation Strategy
Figure 1. TEM images of 2C@1N and 1C@1N: (a) 2C@1N, bar = 100 nm. (b) 2C@1N, more magnified, bar = 20 nm. (c) 1C@1N, bar = 100 nm. (d) 1C@1N, more magnified, bar = 20 nm.
IV type of N2 sorption isotherms (Figure S3), which belongs to the type of a mesoporous material. Their surface area, pore volume, and pore size dramatically increase to the level of a typical mesoporous material (the textural parameters are also summarized in Table S1). In the 13C CP-MAS NMR spectrum of 2C@1N (Figure 2a), the signals at 54 and 61 ppm are assigned to the N-neighboring C atoms of template and triamine, respectively. The signals of other C atoms of octyl group, triamine, and template are also found between 12 and 42 ppm. The 29Si CP-MAS NMR spectrum of 2C@1N (Figure 2b) clearly exhibits both Q-band and T band, which corresponds to Si(−O−Si)x(OH)4−x and Si(C)(−O−Si)x(OH)3−x. The presence of T band indicates that functional groups are incorporated in the composite through a covalent linkage. In the FT-IR spectra of these composites (Figure S4), a strong absorbance at 2800−3000 cm−1 is attributed to the C−H stretching variation and the peak at 1080 cm−1 is ascribed to the Si−O−Si linkages. The solidstate NMR and FT-IR results both confirm the coexistence of organic moieties and inorganic ingredients on the nanocomposites. To quantitatively determine the loadings of octyl, triamine, and template (CTAC), we further carried out elemental analysis and TG analysis. The elemental analysis results of the composites and template-free composites are summarized in Table 1. From the comparison of the C contents of the composites and their template-free counterparts, the
Figure 2. Solid-state NMR spectra of 2C@1N: (a) 13C CP-MAS NMR and (b) 29Si MAS NMR.
loadings of template on these composites are estimated to be 10−13 wt %. From the N and C contents of the template-free composites, we can conclude that triamine and octyl groups are nearly quantitatively introduced onto the nanocomposites. The TG results support the elemental analysis findings (Figure S5). To clarify the distribution of octyl and triamine groups on the composites, we employed X-ray photoelectron spectroscopy (XPS) to investigate the elemental compositions on the surface layer (Figure 3; the detecting depth is ca. 10 nm for nonporous materials).52 C, N, Si, and O elements are all found on these nanocomposites. Comparing the molar ratios of N/C obtained from elemental analysis and XPS determination, one can infer the functionality distribution on the nanocomposites 6690
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Table 1. Comparison of Elemental Analysis Results and XPS Results of the Nanocomposites and the Template-Free Nanocomposites nanocomposites 6C-1N 6C@1N 3C@1N 2C@1N 1C@1N a
Na (wt %)
Ca (wt %)
1.51 1.47 1.90 2.23 2.92
31.50 31.45 31.39 30.30 26.26
template-free nanocomposites Na (wt %)
Ca (wt %)
N/Ca (EA)
N/Cb (XPS)
N/C (theor)
20.76 18.73 18.75 17.71
0.036 0.035 0.052 0.072 0.14
0.007 0.06 0.07 0.12 0.15
d d d 0.25 0.25
0.86 1.14 1.58 2.75
Determined by elemental analysis. bDetermined by XPS. cAssuming the template molecules are homogeneously distributed in the whole materials. Detecting depth > estimated shell thickness. The shell thickness was estimated on the basis of the molar fraction of added siliceous precursors.
d
Figure 3. XPS spectra of the nanocomposites. Figure 4. Distributions of the composites in water, ether, and ether/ water biphasic system (0.02 g of nanocomposite, 4 mL of water, and 4 mL of ether). (A) The liquid is water. (B) Adding ether into water to form a biphasic system. (C) Adding drops of aqueous HCl (1 mol/L) to (B) and adjusting the water pH to 3−4. (D) Adding drops of NaOH (1 mol/L) solution to (C) and readjusting the pH to 9−10.
because the former reflects the bulky elemental compositions and the latter gives surface elemental nanocomposition information. Table 1 also summarizes the molar ratios of N/ C obtained from these two methods. Elemental analysis reveals that the molar ratios of N/C on 6C-1N and 6C@1N are nearly equal (0.036 vs 0.035). However, XPS results disclose that the molar ratio of N/C for 6C@1N (0.06) is much higher than that for 6C-1N (0.007). Furthermore, it is noteworthy that for all the investigated composites the N/C ratios on the surface layer are all higher than those of the whole materials, suggesting that the outer shell layer is indeed rich in triamine. The significant differences confirm that the delayed condensation strategy is an effective approach to control the functionality distribution. For 2C@1N and 1C@1N, the surface N/C ratios determined by XPS are lower than the theoretical values. These results indicate that the triamine-functionalized shell is contaminated with a little amount of octyl groups due to the surface inhomogeneity caused by less reactivity of octyltrimethoxysilane and the delayed condensation method itself.48−50 Phase Transfer Ability. We examined the phase transfer ability of these nanocomposites (Figure 4). The octylmonofunctionalized composite C floats on water (fail to disperse in water, Figure 4A), while triamine-monofunctionalized composite N is well dispersed in water (Figure 4A). These differences originate from the different surface properties. The former is hydrophobic due to the presence of octyl groups, and the latter surface is hydrophilic since triamine is a hydrophilic group. More differences were observed in a biphasic ether/ water system (Figure 4B). C is distributed in the upper layer (ether), whereas N is distributed in the bottom layer (water). Regardless of the pH in an acidic or a basic range, the
distributions of C in ether and N in water are not changed (Figure 4C,D). Distinctly different from these monofunctionalized composites, the core/shell-structured nanocomposites 6C@1N, 3C@1N, 2C@1N, and 1C@1N initially float on water (Figure 4A). For the biphasic ether/water system, these four composites are all distributed in the ether layer. When a few drops of HCl solution are added and the pH of water is adjusted to 3−4, these four composites cross the oil/water interface and migrate to the water layer after stirring or shaking. More interestingly, when the pH of water is readjusted to 9−10 using NaOH, they move back to the upper ether layer. These phenomena clearly demonstrate reversible pH-triggered phase transfer behavior of these nanocomposites. However, 1C@3N (with an increased shell thickness) is observed to always reside in water regardless of the pH changes. To further confirm the role of the functionality distribution, we examined a reversed core/shell-structured composite 1N@2C (as compared to 2C@ 1N) that was composed of a triamine-functionalized core and an octyl-functionalized shell. Different from 2C@1N, 1N@2C resides in the ether layer and cannot transfer between water and ether in response to the pH (Figure 4). Additionally, 2C-1N synthesized through one step of condensation is not able to transfer between ether and water (Figure 4). These remarkable contrasts point to a fact that the phase transfer ability of these 6691
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Figure 5. Water contact angles of various composites with different core and shell dimensions.
composites is strongly dependent on the thus-designed distributions of the functional group. Moreover, it was found that the phase transfer ability of the nanocomposites could be also tuned by varying the molar ratio of octylsilane to triamine silane (Figure S6). The wettability (water contact angle measurements) may account for their differences in the dispersibility in water and the partitioning behavior in the biphasic system. The water contact angles for various nanocomposites are reflected in a form of histogram (Figure 5). The contact angle of C and the contact angle of N are quite different. The former was determined as 120.9°, whereas the latter was measured as 59.3° (Figure 5). Interestingly, the water contact angles of the core/ shell-structured composites 6C@1N, 3C@1N, 2C@1N, and 1C@1N fall in between the values of C and N. They gradually decrease in the range of 97.2° and 84.3° as the core dimension increases and shell dimension decreases. The relative hydrophobicity enables these four composites to reside in the organic phase. 1C@3N with the increased shell thickness is hydrophilic since its water contact angle is as low as 63.6°. The water contact angle measurements also provide some insights into the phase transfer reversibility (Figure 6; their images in Figure S7). Water contact angles of the fresh composites 6C@1N, 3C@1N, 2C@1N, and 1C@1N were determined as 97.2°, 93.5°, 86.6°, and 84.3°, respectively. After treatment with a diluted HCl solution, their contact angles considerably decreased down to 72.5°, 71.4°, 68.3°, and 63.1°. After the HCl-treated samples were further treated with NaOH, their water contact angles restored to 94.8°, 94.1°, 86.8°, and 83.7°. These values are very close to those of the fresh samples. The reversibility in wettability well accounts for the reversible phase transfer. These reversible changes were attributable to the acid−base reactions of triamines on the shell with H+ or OH− (as shown in Scheme 1).53 Triamine on the shell was protonated upon meeting with HCl. The protonated triamines made the surface of composites hydrophilic due to bearing charges. The resultant hydrophilicity drove the composites to move into water for lowering the interface energy. Addition of
Figure 6. Water contact angles of the nanocomposites and the nanocomposites after treatment with HCl and NaOH. Note: “treatment with H+”: the fresh nanocomposite was mixed with a HCl (1 mol/L) solution (pH 3−4) and further stirred for 0.5 h. Afterward, the sample was washed with water and ethanol and dried at 80 °C. The “treatment with NaOH”: the HCl-treated nanocomposite was mixed with a NaOH (1 mol/L) solution and further stirred for 0.5 h. Afterward, the sample was washed with water and ethanol and dried at 80 °C.
NaOH caused deprotonation, and the surface properties of the composites returned hydrophobic. The affinity to organics again drove the particles to transfer into the organic phase. It should be noted that both the introduction of octyl groups in the core and reservation of a potion of templates inside the pores were found to be absolutely necessary for the reversible phase transfer. It can be explained by the possibility that the thus-obtained strongly hydrophobic microenvironment is helpful to prevent water from entering into the pore interior (cause some irreversible changes). Besides the ether/water system, these nanocomposites can reversibly shuttle between water and ethyl acetate. However, for less polar solvents such as hexane, toluene, and benzene, it was found that the phase transfer did not occur, and the nanocomposites always remained in the organic phase despite 6692
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Figure 7. Photographs for the phase transfers of 2C@1N in the biphasic ether/water system (a) and 1C@1N in the biphasic ethyl acetate/water system (b).
recycling of metal nanocatalysts that is difficult for the conventional methods such as centrifugation and filtration but highly desired in catalysis community.58,59 The nanocomposite shows a high adsorption capacity for Pd(OAc)2. 1.7 wt % Pd(OAc)2 (with respect to nanocomposite) was found to be completely absorbed by 2C@ 1N. The high adsorption capacity confirms its good ability to load molecular cargoes. After reduction with NaBH4, a Pdsupported catalyst Pd/2C@1N was obtained (with a Pd loading of ca. 0.8 wt %). As TEM images showed (Figure 8),
the pH changes. The phase transfer ability is related to the solvent nature. It is well-known that colloidal particles with moderate hydrophobicity/hydrophilicity (the water contact angles are in vicinity of 90°) prefer to adsorb at an oil/water interface. Crossing the oil/water interface needs to overcome a high energy obstacle according to the modified eq 1:10,54 E = πR2γow(1 ± cos θ )2
(1)
where R refers to the particle radius, γow is the interfacial tensions of oil/water, and θ is the contact angle that particle makes at the oil/water interface (the cos within the bracket is negative for transfer of the particle into the water phase). For 100 nm-scaled particles, E is estimated to be as high as 104−106 KBT (KB is Boltzmann constant), leading to the thermal difficulty in detachment of particles from the interface. This is main reason that there are only few samples of phase transfer of above 100 nm-sized particles.16,23,32 According to this equation, the energy E is dependent on the interfacial tensions of oil/ water and reaches the maximum at θ = 90°. Obviously, the low interfacial tensions of oil/water lead to a low energy obstacle. The theoretical predications well agree with our experimental findings. Among above five solvents, ethyl acetate and ether have the least interface intension of oil/water (see Table S2). Just in these two biphasic systems, phase transfer of these nanocomposites occurs. Additionally, Pickering emulsion inversion probably causes “phase transfer”.55−57 Our optical microscopy observations reveal that there are not emulsion droplets present in both the oil and water phase, which excludes the possibility of Pickering emulsion inversion mechanism in our system. (The conductivity measurements and oil/water volume fraction measurements also support this conclusion). Impressively, the pH-triggered two-way phase transfer of the nanocomposite shuttles could be repeated several times. The appearance of the phase transfer of the nanocomposites in the ether/water and ethyl acetate/water systems is displayed in Figure 7. After eight round trips, this nanocomposite could still rapidly transfer between water and organic phases through varying the water pH. (The visual appearances for the two-way phase transfers of 3C@1N and 1C@1N are included in Figures S8 and S9.) Applications. Next, we employ this nanocomposite shuttle to carry metal nanoparticles, to address the separation and
Figure 8. TEM images of Pd nanoparticles loaded on 2C@1N; the bar is 20 nm. Note: the low contrast between pore wall and channel is probably attributable to the presence of templates inside the pores.
only little portion of Pd nanoparticles are observed on the external surface (Figure 8a). Most of Pd nanoparticles are located inside the nanopores of the nanocomposite (Figure 8b). In agreement with the recently reported results, the good dispersion of Pd is due to the interactions of the cationic head groups of templates with metal surface.45,46 The good ability of the nanocomposite to load cargoes is further confirmed by the adsorption of benzene and 4-heptylphenol. As displayed in Figure S10, the nanocomposite 2C@1N has a higher adsorption capacity and faster adsorption rate for these two 6693
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Pd/1C@1N also highlights the excellent recyclability (Figure S11). The shuttle catalyst Pd/2C@1N shows good activity toward various olefins (Table 2). Complete conversions of these olefins were afforded under mild conditions. The product−catalyst separation was facilely accomplished based on the phase transfer strategy.
molecular cargoes, in comparison to a standard mesoporous material MCM-41 (template-free). These results may be contrary to most researchers’ thinking but are supported by some references and our studies. 43−47 The enhanced adsorption ability may be attributed to the unique microenvironment created by the inorganic framework and confined micelles. The good ability in loading metal nanoparticles and adsorption of molecular cargoes imply that the nanocomposite shuttle is applicable in catalysis. The hydrogenation of styrene in a biphasic oil/water system was used as a model reaction to evaluate the performances of the shuttle catalyst Pd/2C@1N. The reactions were conducted under ambient pressure of H2 in the presence of 0.5 mol % Pd (with respect to styrene). During the course of the reaction and at the end of reaction, Pd/2C@1N was observed to reside always in the organic layer (Figure 9, run 1, left image). To
Table 2. Results of Olefin Hydrogenations in the Ether/ Water Biphase in the Presence of the Shuttle Catalyst 2C@ 1Na
a
Reaction conditions: 20 mL of water, 15 mL of ether, 1 mmol of substrate, 66.5 mg of catalyst, ambient pressure of H2, 30 °C. b Reaction time. cDetermined by GC.
To further evaluate the nanocomposite shuttle (catalyst recycling) efficiency, we determined the residual Pd contents in the ether layer and water layer with ICP-AES (Table S3). In the first run, the residual Pd concentration in ether was determined as 0.2 ppm after the solid catalyst was transferred into water. The Pd concentration in water was as low as 0.027 ppm after the solid catalyst moved back to the ether phase. In the fifth cycles, the residual Pd in ether was 0.3 ppm, and the accumulative Pd residue in the water was 0.057 ppm. The low concentrations of the residual Pd in both water and the organic phase are due to the complete phase transfer. These results convincingly justify the high efficiency of the smart nanocomposite shuttle. Meanwhile, elemental analysis disclosed that N content of the solid catalyst used four times was very close to that of the fresh catalyst, indicating that the template could stably reside in the composite shuttle throughout the consecutive transfers (Table S4).
Figure 9. Photographs for the separation and recycling of the shuttle catalyst Pd/2C@1N through a pH-triggered phase transfer strategy (in the hydrogenation of styrene in a biphasic ether/water system). Note: the color change of the layer containing catalyst is due to the room light change with time.
separate the product from the shuttle catalyst, we added a few drops of a HCl solution and the pH of water was adjusted to 3−4 at the end of reaction. After stirring, the catalyst was observed to completely migrate to the lower water phase (Figure 9, run 1, middle image). The upper layer of organics was transferred through a simple decantation. Styrene was found to be fully converted to ethylbenzene within 60 min and a yield of 91% was achieved (Figure 9, run 1). The excellent activity may benefit from the distributions of the shuttle catalyst and substrate in the same phase. To recycle the catalyst, the pH of water was readjusted to 9−10 through addition of a little amount of a NaOH solution. Fresh styrene and solvent were directly added into the vessel. The shuttle catalyst Pd/2C@1N was observed to come back into the upper organic phase (Figure 9, run 1, right image). From the second to the eighth cycles, 99% conversions and above 90% yields were afforded within a slightly prolonged time. Throughout eight reaction cycles, the separation and recycling of the shuttle catalysts were facilely carried out on the basis of the pH-triggered phase transfer strategy (Figure 9, runs 2−8). Its sister shuttle catalyst
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CONCLUSIONS In summary, we demonstrate a simple, reliable, and controllable method for the synthesis of a smart mesoporous silica nanocomposite shuttle, which is essentially different from the existing systems such as dendrimer, polymer micelle, and polymersomes. This nanocomposite shuttle exhibits a good reversibility of phase transfer between an organic phase and water in response to the pH, and good ability to carry various cargoes such as Pd(OAc)2, Pd nanoparticles, and organic 6694
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(9) Russell, T. P. Surface-Responsive Materials. Science 2002, 297, 964−967. (10) Edwards, E. W.; Chanana, M.; Wang, D. Y. Capping Gold Nanoparticles with Stimuli-Responsive Polymers to Cross Water−Oil Interfaces: In-Depth Insight to the Trans-Interfacial Activity of Nanoparticles. J. Phys. Chem. C 2008, 112, 15207−15219. (11) Bas, D.; Dorison-Duval, D.; Moreau, S.; Bruneau, P.; Chipot, C. Rational Determination of Transfer Free Energies of Small Drugs across the Water−Oil Interface. J. Med. Chem. 2002, 45, 151−159. (12) Cordle, R. A.; Lowe, M. E. The Hydrophobic Surface of Colipase Influences Lipase Activity at an Oil-Water Interface. J. Lipid Res. 1998, 39, 1759−1767. (13) Holowka, E. P.; Sun, V. Z.; Kamei, D. T.; Deming, T. J. Polyarginine Segments in Block Copolypeptides Drive Both Vesicular Assembly and Intracellular Delivery. Nat. Mater. 2007, 6, 52−57. (14) Zhang, J. L.; Srivastava, R. S.; Misra, R. D. K. Core-Shell Magnetite Nanoparticles Surface Encapsulated with Smart StimuliResponsive Polymer: Synthesis, Characterization, and LCST of Viable Drug-Targeting Delivery System. Langmuir 2007, 23, 6342−6351. (15) Cole-Hamilton, D. J. Homogeneous Catalysis-New Approaches to Catalyst Separation, Recovery, and Recycling. Science 2003, 299, 1702−1706. (16) Zeltner, M.; Schäatz, A.; Hefti, M. L.; Stark, W. J. Magnetothermally Responsive C/Co@PNIPAM-Nanoparticles Enable Preparation of Self-Separating Phase-Switching Palladium Catalysts. J. Mater. Chem. 2011, 21, 2991−2996. (17) Desset, S. L.; Cole-Hamilton, D. J. Carbon Dioxide Induced Phase Switching for Homogeneous-Catalyst Recycling. Angew. Chem., Int. Ed. 2009, 48, 1472−1474. (18) Buhling, A.; Kamer, P. C. J.; Van Leeuwen, P. W. N. M. Novel Amphiphilic Diphosphines: Synthesis, X-ray Structure, Rhodium Complexes, Use in Hydroformylation, and Rhodium Recycling. Organometallics 1997, 16, 3027−3037. (19) Jessop, P. G.; Mercer, S. M.; Heldebrant, D. J. CO2-Triggered Switchable Solvents, Surfactants, and Other Materials. Energy Environ. Sci. 2012, 5, 7240−7253. (20) Phan, L.; Andreatta, J. R.; Horvey, L. K.; Edie, C. F.; Luco, A.; Mirchandani, A.; Darensbourg, D. J.; Jessop, P. G. Switchable-Polarity Solvents Prepared with a Single Liquid Component. J. Org. Chem. 2008, 73, 127−132. (21) Jiang, H. B.; Jia, J. G. Complete Reversible Phase Transfer of Luminescent CdTe Nanocrystals Mediated by Hexadecylamine. J. Mater. Chem. 2008, 18, 344−349. (22) Chen, S.; Yao, H.; Kimura, K. Reversible Transference of Au Nanoparticles across the Water and Toluene Interface: A Langmuir Type Adsorption Mechanism. Langmuir 2001, 17, 733−739. (23) Horton, J. M.; Bao, C.; Bai, Z.; Lodge, T. P.; Zhao, B. Temperature- and pH-Triggered Reversible Transfer of Doubly Responsive Hairy Particles between Water and a Hydrophobic Ionic Liquid. Langmuir 2011, 27, 13324−13334. (24) Wei, Y. F.; Yang, J.; Ying, J. Y. Reversible Phase Transfer of Quantum Dots and Metal Nanoparticles. Chem. Commun. 2010, 46, 3179−3181. (25) Edwards, E. W.; Chanana, M.; Wang, D. Y.; Möhwald, H. J. Stimuli-Responsive Reversible Transport of Nanoparticles across Water/Oil Interfaces. Angew. Chem., Int. Ed. 2008, 47, 320−323. (26) Minami, H.; Mizuta, Y.; Kimura, A. Phase-Transfer Behavior of Cross-Linked Poly(acrylic acid) Particles Prepared by Dispersion Polymerization from Ionic Liquid to Water. Langmuir 2012, 28, 2523−2528. (27) Stocco, A.; Chanana, M.; Su, G.; Gernoch, P.; Binks, B. P.; Wang, D. Y. Bidirectional Nanoparticle Crossing of Oil-Water Interfaces Induced by Different Stimuli: Insight into Phase Transfer. Angew. Chem., Int. Ed. 2012, 51, 9647−9651. (28) Chechik, V.; Zhao, M. Q.; Crooks, R. M. Self-Assembled Inverted Micelles Prepared from a Dendrimer Template: Phase Transfer of Encapsulated Guests. J. Am. Chem. Soc. 1999, 121, 4910− 4911.
molecules. These built-in switchable properties lay the foundations for many innovative applications. As a proof of concept, we successfully demonstrate the feasibility of separation and recycling of Pd nanoparticle catalyst, using the nanocomposite shuttle as a carrier. The shuttle catalyst can be “in situ” isolated and recycled efficiently several times only through varying the pH. The high shuttle effectiveness is further justified by the retentive activity throughout the consecutive runs and the low level of Pd loss. Our investigations not only supply a novel nanocomposite shuttle with controllable properties but also open an innovative avenue to in situ separation and recycling of nanocatalysts that is highly difficult for the conventional methods such as centrifugation and filtration.
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ASSOCIATED CONTENT
S Supporting Information *
Textural parameters determined by N2 sorption; the interface tension of various oils; elemental analysis results of Pd and template loss; XRD patterns; N2 sorption isotherms; FT-IR spectra; TG curves; the distributions of 6Cx@1N15 and 1N@ 2C and 1N-2C in biphase; contact angles; the phase transfer of 3C@1N and 1C@1N; recycling of Pd/1C@1N. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Fax +86-351-7011688; Tel +86-351-7010588; e-mail
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the Natural Science Foundation of China (20903064 and 21173137), Program for the Top Young Academic Leaders, and the Top Young/Middle-aged Innovative Talents of Higher Learning Institutions of Shanxi (2011002 and 20120202).
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REFERENCES
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