Breathing catalyst-supports: CO2 adjustable and magnetic recyclable “smart” hybrid nanoparticles

Anchao Fenga, Yun Wanga, Liao Penga, Xiaosong Wangb and Jinying Yuan*ac
aKey Lab of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China. E-mail: yuanjy@mail.tsinghua.edu.cn; Fax: +86-10-62771149; Tel: +86-10-62783668
bDepartment of Chemistry, Waterloo Institute of Nanotechnology, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada
cState Key Laboratory of Polymer Materials Engineering, Sichuan University, P. R. China

Received 12th September 2016 , Accepted 29th September 2016

First published on 29th September 2016


A facile route for fabricating CO2 gas stimulated, magnetically recyclable hybrid nanoparticle is designed and illustrated through introducing a CO2 sensitive polymeric matrix and gold nanoparticles (AuNPs) onto the surface of magnetic Fe3O4@SiO2 nanospheres. The accessibility of the AuNPs to the reaction substrates can be tuned following swollen or collapsed of CO2 sensitive shell, which may leads to an increase or decrease of catalytic activity. Therefore, the obtained hybrid system offers a novel strategy to adjust and control the catalytic activity through mild stimulation of CO2/N2, where the magnetic core assures a magnetically recyclable catalysis.


Introduction

Gold nanoparticles (AuNPs) are highly active in catalysis for versatile reactions.1–3 Immobilization of the particles within stimuli-responsive polymers allows the catalytic reactions to be stimulated by the external environment,4–8 including temperature, pH, light and redox reagents added.9–17 Carbon dioxide (CO2)-stimulated catalytic reactions are rarely explored,18 but highly desirable because that they are abundant, non-toxic, non-accumulating after the catalytic reactions.19–24 We and others have established the concept that CO2 can stimulate the solution behaviour of amidino and amino polymers, such as poly(N-amidino)dodecyl acrylamide (PAD), poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA), and poly(N,N-diethylaminoethyl methacrylate) (PDEAEMA).25–33 These polymers can be converted to be hydrophilic via reactions with CO2. The reverse reactions to recover the original hydrophobic polymers are readily achievable via bubbling the solution with an inert gas (such as argon) or nitrogen. These CO2 switch-on/-off cycles leave no contamination during the stimulate process, so it is highly desirable as a “green” trigger for smart systems. Using CO2-responsive block copolymers, we have created breathing vesicles that can reversibly swelling and shrinking via cycles of CO2 and N2 bubbling. This breathing property results in CO2-turned permeability of the membrane that has been used for size-selective release, separation and reaction.25,27 We have also produced a membrane with CO2-triggered switchable oil/water wettability.32 The membrane is produced from CO2-responsive polymer fibers. Zhao and his co-workers have demonstrated that the dispersibility of AuNPs in aqueous solution can be adjusted using thiol-terminated CO2-sensitive polymer as ligands for possible enhanced and stimulated catalytic behavior.18 Stability, processability and recyclability of the catalytic systems, however, remain to be the challenges for possible practical applications, which require rational and creative design of the systems.

By embedding AuNPs into the crosslinked matrix of CO2-responsive polymers, a long-term stability of the catalysts will be possible. The accessibility of the catalysts to the reaction substrates (Scheme 1a) can also be better tuned by the gating effect of the CO2 responsive matrix. The polymer matrix will be covered on magnetic NPs to achieve a processible and recyclable system for potential industrial applications and environment protection, especially for precious heavy metals.34–36 This novel type of nanocomposites can be achieved by the convergence of the newly emerged polymerization and NPs synthesis techniques.


image file: c6ra22762g-s1.tif
Scheme 1 Schematic illustration of (a) adjustable catalytic activity with gas, and (b) representative process for catalytic reaction using gas adjustable and magnetic recyclable catalyst-supports.

Herein, we report the synthesis of a CO2 responsive catalytic system, Fe3O4@SiO2@PDEAEMA–Au, which has a magnetic core covered by a crosslinked CO2-responsive polymer shell with AuNPs embedded. Gas stimulated swelling and shrinking of the polymer shells allows the stimulated accessibility of the AuNPs to substrates. The magnetic core renders the nanocomposites recyclable using magnetic bars and this recycle process does not affect the activity of the catalysts (Scheme 1b).

Experimental section

Materials

Iron(III) acetylacetonate (Fe(acac)3), 3-(trimethoxysilyl)propyl methacrylate (MPS, 98%), bis-acrylamide (MBA), 4-nitrophenol (4-NP), 1-octadecene and oleylamine (OA) were purchased from Acros (USA) and used as received. N,N-Diethylaminoethyl methacrylate (DEAEMA) was obtained from Tokyo Chemical Industry Co. Ltd (Japan) and was passed through a column of basic aluminum oxide prior to use. HAuCl4·4H2O and NaBH4 were purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China) and used as received. Azodiisobutyronitrile (AIBN, Beijing Chemical Technology Co., 98%) was recrystallized in n-hexane twice before use. Ethanol, ammonia, n-hexane, cyclohexane, tetrahydrofuran (THF) and tetraethylorthosilicate (TEOS) were obtained from Beijing Chemical Reagent Co. Ltd (China) and purified before use. Acetonitrile was refluxed with CaH2 and then distilled. Other chemical reagents were used as received.

Preparation of Fe3O4@OA nanoparticles

The Fe3O4@OA nanoparticles used in this research were prepared using a high thermal decomposition method. 1.06 g of Fe(acac)3 was dissolved in a mixture of 30 mL 1-octadecene and 30 mL OA. The solution was dehydrated at 90 °C for 1.5 h. Then, the solution was quickly heated to 300 °C and then was kept at this temperature for 1 h under an argon atmosphere. Ethanol (300 mL) was poured into the solution after it was cooled down to room temperature. The precipitate was collected using an external magnetic field and washed with ethanol 3 times. The product was dried under vacuum overnight.

Preparation of Fe3O4@SiO2 core–shell nanospheres

0.2 g Fe3O4@OA was added to a mixture containing 200 mL cyclohexane, 40 mL of 1-hexanol, 50 mL of triton X-100, and 8 mL of water. 4 mL of TEOS was added and then the solution was stirred for 6 h at room temperature. Then 2.5 mL of ammonia was added and the solution was stirred for another 12 h. The precipitation was washed with ethanol 3 times and collected using an external magnetic field. The product was dried under vacuum overnight.

Preparation of vinyl-ended Fe3O4@SiO2–MPS

A mixture of 0.2 g Fe3O4@SiO2, 100 mL ethanol and 2.5 mL MPS was placed in a dried 250 mL flask. The flask was stirred for 24 h before the product was washed with ethanol 3 times and collected using a magnet. Finally, the MPS modified Fe3O4@SiO2 was dried under vacuum overnight.

Preparation of Fe3O4@SiO2–PDEAEMA hybrid nanoparticles

0.1 g Fe3O4@SiO2–MPS was suspended in 50 mL of acetonitrile. A mixture of DEAEMA (0.6 g), MBA (0.2 g) and AIBN (8 mg) was then added to the above suspension in a 100 mL flask. The flask attached to a fractionating condenser and receiver was submerged in a heating mantle. The reaction mixture was heated from ambient temperature until the boiling state within 10 min, and the reaction system was kept under refluxing state for further 20 min. The polymerization was further carried out with distilling the solvent out of the reaction system within 3 h. Next, the particles were washed three times with THF and ethanol in turn, collected using a magnet, and finally dried under vacuum overnight.

Preparation of Fe3O4@SiO2–PDEAEMA–Au hybrid nanoparticles

0.05 g of Fe3O4@SiO2–PDEAEMA were dispersed in 10 mL of deionized water and treated with CO2 gas for 20 min with a flow rate of 20–30 mL min−1. Then 2 mL of 4 mM HAuCl4 aqueous solution was added and the mixture was stirred for 1 h to absorb the Au3+ on the surface of polymer matrix. 5 mL of 0.05 M NaBH4 aqueous solution was added dropwise under the ice-bath and the reaction lasted for 1 h. The final product was purified through washing with water three times and dried under vacuum oven until constant weight.

Catalytic reduction of 4-NP for magnetically recyclable catalysis

0.36 mL of 4-NP aqueous solution (5 mM, 5 × 10−6 mol), 3.6 mL of NaBH4 (0.2 M, 2 × 10−3 mol) aqueous solution and 25.5 mL of water were mixed in a colorimetric tube. The UV-vis absorption at 400 nm of this mixture was recorded as A0. We introduced 0.5 mL of catalyst aqueous dispersion (5.0 mg mL−1, CO2 treated) into the mixture with gentle shaking. The bright-yellow solution faded rapidly as the catalytic reaction proceeded. In a given time of 5 min, The UV-vis absorption at 400 nm was recorded as At and the conversion yield of the 4-NP was calculated as A0At/A0. Simultaneously, the catalyst-supports were separated by a magnetic bar and subsequently redispersed in 0.45 mL of water under CO2 treatment. The recovery of the catalyst-supports was further tested by recycling it for two more times and the conversion yields were calculated as above.

The kinetic process of gas stimulated catalytic reduction

The kinetic process can be studied by calculate a kinetic rate constant K as the specific parameter for different systems. Because excess NaBH4 was added to the reaction as compared to 4-NP, the reduction rate was assumed to be independent of the NaBH4 concentration, and a pseudo first-order kinetic equation could be applied to evaluate the catalytic rate. The correlation of ln(Ct/C0) versus the reduction time t was estimated to be linear, and the slope was estimated as the kinetic rate constant K (C0 and Ct are the initial and monitored concentration of 4-NP and t is the reaction time). The ln(Ct/C0) can be measured from the relative intensity of absorbance At/A0 (At and A0 are the peak absorbance at time t and 0). In a typical catalytic reaction, 0.036 mL of 4-NP aqueous solution (5 mM, 5 × 10−7 mol), 0.36 mL of NaBH4 (0.2 M, 2 × 10−4 mol) aqueous solution and 2.55 mL of water were mixed in a colorimetric tube. We introduced 50 μL of gas treated catalyst aqueous dispersion (1.0 mg mL−1) into the mixture with gentle shaking. Specifically, 50 μL of water treated with CO2 for 15 min was added as a blank catalyst to exclude the possibility of CO2 gas influencing the catalytic substrates other than influencing the catalyst. The bright-yellow solution faded gradually as the catalytic reaction proceeded. To study the catalytic kinetics, UV-vis spectra between 250 nm to 500 nm were recorded in a certain time interval during the process of catalytic reaction.

Characterization

Fourier transform infrared (FT-IR) spectra were recorded on an AVATAR 360 ESP FT-IR spectrometer. Thermogravimetric analysis (TGA) was carried out on a TGA 2050 thermogravimetric analyzer at a heating rate of 10 °C min−1 from 30 °C to 900 °C under a nitrogen atmosphere. The magnetic properties were tested by vibrating sample magnetometry (VSM) performed on a LakeShore 7307 vibrating sample magnetometer with an applied field between –11 kOe and 11 kOe at room temperature. The morphologies and sizes of the samples were characterized using a JEM-2010 transmission electron microscope (TEM) and the elemental composition was evaluated using energy-dispersive X-ray spectroscopy (EDXS) attached to the TEM apparatus. The concentration of samples for TEM characterization was kept around 1–2 mg mL−1, and a small drop of nanoparticle dispersion was dropcast onto a carbon-coated copper grid without staining. The average size and zeta potential were analyzed by a Malvern 3000HS Zetasizer (DLS) using a monochromatic coherent He–Ne laser (633 nm) as the light source and a detector that detected the scattered light at an angle of 90°.

Results and discussion

The synthesis for the hybrid nanoparticles is outlined in Fig. 1. The monodisperse Fe3O4 nanoparticles37–42 were prepared by high temperature (300 °C) reductive decomposition of Fe(acac)3 in oleylamine (OA). Such MNPs were coated with a layer of OA and are only dispersible in hexane and other nonpolar or weakly polar organic solvents. As revealed by transmission electron microscopy (TEM) (Fig. 2a), the resulting Fe3O4@OA has a diameter of about 9 ± 1 nm. This particle was coated with a uniform silica layer (18 ± 2 nm in thickness) resulting in Fe3O4@SiO2 microspheres (Fig. 2b) via the hydrolysis and condensation of tetraethylorthosilicate (TEOS) in an ethanol–ammonia mixture. The sharp contrast between the core and the shell as shown in Fig. 2b is attributed to the difference in the electron densities for the magnetic NPs and silica. The FT-IR spectrum of the Fe3O4@SiO2 displays characteristic peaks at 3335 cm−1 and 951 cm−1 (Fig. S1a), which are attributed respectively to the stretching vibrations of the hydroxyl group on the surface of silica and Si–OH vibration.37 The surface of this core–shell magnetic NPs was further functionalized using 3-(trimethoxysilyl)propyl methacrylate (MPS). The hydrolysis and condensation reactions of MPS on the surface of Fe3O4@SiO2 nanoparticles endow the particles with reactive methacrylate groups. FT-IR spectrum reveals the peak at 1635 cm−1 corresponding to the stretching vibrations of the vinyl groups from the methacrylates (Fig. S1a). This surface methacrylate is crucial to the subsequent polymerization step to adhere a layer of polymer matrix on the surface the magnetic NPs.
image file: c6ra22762g-f1.tif
Fig. 1 Schematic illustration of the synthesis of Fe3O4@SiO2@PDEAEMA–Au core–shell–corona hybrid systems.

image file: c6ra22762g-f2.tif
Fig. 2 TEM images of (a) Fe3O4@OA in ethanol; (b) Fe3O4@SiO2 in ethanol; (c) Fe3O4@SiO2@PDEAEMA in water with no gas stimulus; (d) Fe3O4@SiO2@PDEAEMA–Au in water with no gas stimulus and (e) EDX spectroscopy of the Fe3O4@SiO2@PDEAEMA–Au.

In situ distillation precipitation polymerization (DPP), an effective technique for the preparation of microspheres with uniform size and shape,43,44 was employed to coat the Fe3O4@SiO2 core–shell NPs with a crosslinked CO2 sensitive polymer shell. The polymerization of N,N-diethylaminoethyl methacrylate (DEAEMA) is performed in the presence of bis-acrylamide (MBA), the crosslinking agent, in acetonitrile. The resultant nanocomposite was examined using TEM and SEM (Fig. 2c and S2). The TEM image exhibits the core–shell–corona structure with dark Fe3O4 core, grey silica shell and light grey polymer corona (Fig. 2c). The thickness of the outmost polymer layer is about 10 nm. The SEM image exhibits dispersed Fe3O4@SiO2@PDEAEMA microspheres with a narrow size distribution (Fig. S2). A strong absorption peak at 1728 cm−1 due to stretching vibration of the carbonyl unit of ester group in the IR spectrum of the nanocomposites (Fig. S1a) further demonstrated that the shell of PDEAEMA polymers has been successfully coated onto the magnetic nanoparticles through DPP. Thermogravimetric analysis (TGA) indicates that the mass loss of Fe3O4@SiO2 and Fe3O4@SiO2@PDEAEMA at 800 °C under a nitrogen atmosphere is 19.02 and 40.91% (Fig. S1b), respectively, from which it is deduced that the nanocomposites contains 27.04 wt% of polymer (details in ESI). After treated with CO2 for 30 min, the zeta potential as measured for the particle reached +29.0 mV, as shown in Fig. S3a, supporting the formation of positive ammonium ions of the surface coated polymers.28,45

The tertiary amino groups in the crosslinked PDEAEMA shell can serve as chelators for metallic nanoparticles through the gold–amine complexation. HAuCl4 was first bound to the tertiary amino groups and then reduced using NaBH4 generating AuNPs embedded Fe3O4@SiO2@PDEAEMA microspheres. The relatively dense polymer crosslinked network structure can further stabilize the nanoparticles and limit the particle size. As shown in Fig. 2d, the small, dark spheres of the Au nanocolloids are well-dispersed in the outmost PDEAEMA matrix without aggregation. The energy dispersive X-ray (EDX) spectroscopy (Fig. 2e) shows the existence of Au elements along with C, O, Si, Fe, Cu from the nanocomposites and the copper grid. The zeta potential for the gold embedded particle (+27.4 mV) is similar to that for the precursor (+29.0 mV), which proved sufficient electrostatic sites for the stability of AuNPs.

The accessibility of the AuNPs can be adjusted via swelling or shrinking the PDEAEMA shells by alternatively bubbling the solution with CO2 and N2. Consequently, gas-stimulated catalytic reactivity can be achieved.18,37,44,45 The gas-sensitive of the nanocomposites was examined using a dynamic light scattering (DLS) technique and zeta potential measurement. After CO2 bubbling, the tertiary amines on PDEAEMA shell sensitively converted to positive ammonium ions15,30 and the nanocomposites was therefore positively charged with zeta potential of +27.4 mV. DLS analysis revealed that the hydrodynamic diameter (Dh) was 73.8 nm (Fig. S4). After the CO2 treated nanocomposites were purged with N2 for 15 min, the zeta potential reduced to +1.66 mV (Fig. S3c) due to the deportation of the CO2. Consequently, the particles agglomerated resulting in Dh of 210.3 nm (Fig. S4) due to the drop in the zeta potential and hydrophobic collapse. These agglomerates can be segregated by bubbling the solution with CO2. This gas stimulated cycle of agglomeration and segregation can be performed repeatedly by alternating treatment of CO2 and N2.

The magnetic properties of Fe3O4@SiO2, Fe3O4@SiO2@PDEAEMA and Fe3O4@SiO2@PDEAEMA–Au are illustrated in Fig. 3a. As shown in Fig. 3a, there are no obvious hysteresis and no magnetic remanence for all the samples, indicating that the particles are superparamagnetic. The saturation magnetization (Ms) values decreased when the Fe3O4@SiO2 is coated with PDEAEMA and functionalized with AuNPs. This is attributed to the increase in the total mass of the nanocomposites.41 Nevertheless, the Ms intensity for the nanocomposites, Fe3O4@SiO2@PDEAEMA–Au, remains to be around 9.7 emu g−1, which is strong enough to be magnetically adsorbed by an external magnetic field. As shown in Fig. 3b, the solution of the nanocomposites is wine red in colour. After placing a magnetic bar near the vial for 5 min, the solution turns to colourless, suggesting that all particles are separated from the solution and aggregated toward the magnetic bar. Such accumulated nanoparticles can be re-dispersed after the removal of the magnet bar (Fig. 3b). The catalytic activity of the nanocomposites was examined using a Au-catalyzed model reaction to convert 4-nitrophenol (4-NP), one of the most common organic pollutants in industrial and agricultural wastewaters, to 4-aminophenol (4-AP).46–48 The catalytic reaction can be followed by UV-Vis measurement. 4-NP has an UV-Vis absorption at 400 nm (A-400), whereas 4-AP absorbs UV-Vis at 300 nm (A-300). The variation in A-400 during the reactions indicates the consumption of 4-NP and is used to evaluate the catalytic activity. When excess amount of catalysts were added, the reaction completed within 5 min (ref. 49–52) and the nanocomposites were recycled using a magnetic bar. The separated nanocomposites were re-dispersed in water and treated with CO2 for the next cycle of the reaction. The yield for each cycle of the reaction is compared in Fig. 3c. As shown in Fig. 3c, the conversion yield for the reaction remained ca. 90% after three cycles of the catalytic reactions.


image file: c6ra22762g-f3.tif
Fig. 3 (a) VSM curves of Fe3O4@SiO2, Fe3O4@SiO2@PDEAEMA and Fe3O4@SiO2@PDEAEMA–Au. (b) Photographs of the magnetic separation. The Fe3O4@SiO2@PDEAEMA–Au dispersed in water in the absence (left) and presence (right) of an external localized magnetic field. (c) Reusability of the Fe3O4@SiO2@PDEAEMA–Au as a catalyst for the reduction of 4-NP with NaBH4.

Gas-stimulated catalytic reactivity was evaluated via the catalytic kinetics using the catalysts treated with different number of gas stimulation cycles. The reaction was first performed after either CO2 aeration for 15 min or N2 aeration for 30 min. The reactions were monitored using UV-vis spectroscopy. For the system with CO2 treatment, the reaction was faster and the intensity of A-400 dropped to nearly zero within 14 min, while there were still ca. 30% unreacted substrates after one hour of the reactions if the system was bubbled with N2 (Fig. 4a and b). In other words, the activity of the catalysis can be adjusted using the gases. The kinetics for the reactions, after the second round of gas bubbling with CO2 (2nd CO2), N2 (2nd N2), and the third round of gas bubbling with CO2 (3rd CO2) are presented in Fig. 4c. The linear relations between ln (At/A0) and time are clear, and the slope was estimated as the kinetic rate constant K and plotted in Fig. 4d. As shown in Fig. 4d, the reaction rate constants K for the system treated with 1st, 2nd and 3rd CO2 stimulation are 0.226, 0.171, 0.154, respectively, whereas only 0.017, 0.023 for the 1st and 2nd N2 treated systems (detailed calculation in ESI). The rate constant K can be resumed to its original high level after twice of CO2 treatment, while kept at a low level by N2. It is therefore concluded that the change of catalytic activity is responsive to the gas stimulation, which can maintain a stable level under activation by CO2 and deactivation by N2.


image file: c6ra22762g-f4.tif
Fig. 4 Time-dependent UV-vis spectral changes of 4-NP catalyzed by Fe3O4@SiO2@PDEAEMA–Au nanocomposites with (a) CO2 treatment and (b) N2 treatment. (c) The plots of ln(At/A0) versus the time t at different gas stimuli. (d) The variation of the kinetic rate constant K as a function of gas stimuli cycles.

Conclusions

In summary, we report a novel catalyst-supports, CO2 gas adjustable and magnetic recyclable “smart” hybrid nanoparticles. By embedding AuNPs into the crosslinked matrix of CO2-responsive polymers, a polymer/inorganic nanoparticle hybrid nanocomposites as a CO2 adjustable and magnetic recyclable catalyst, Fe3O4@SiO2@PDEAEMA–Au, is prepared and studied. This hybrid system has good catalytic activity for the model reduction of 4-NP to 4-AP in aqueous solution. The catalytic activity can be easily adjusted by alternative gas bubbling of CO2 and N2. In addition, the Fe3O4@SiO2@PDEAEMA–Au can be magnetically recovered and reused with a stable catalytic activity for at least three times of recycling, which will be an advantage for industrial applications in terms of the cost and environmental protection. The combination of gas stimulation, easy separation and stable performance of the catalysts renders the designed materials valuable for industrial applications.

Acknowledgements

The authors would like to acknowledge the financial support for this work from the National Natural Science Foundation of China (21374053, and 51573086). This project is also supported by the Opening Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University) (Grant No. sklpme 2014-4-26), and special fund of State Key Joint Laboratory of Environment Simulation and Pollution Control (No. 15K07ESPCT).

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Footnote

Electronic supplementary information (ESI) available: Detailed synthesis and characterizations. See DOI: 10.1039/c6ra22762g

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