Synthesis of Structurally Stable Colloidal Composites as Magnetically

Mar 30, 2010 - ACS Catalysis 2011 1 (7), 674-681 ... Subramanian Tamil Selvan , Karthikeyan Narayanan. 2016 .... Chemical Communications 2013 49, 6093...
2 downloads 0 Views 2MB Size
Download PDF
Chem. Mater. 2010, 22, 2955–2961 2955 DOI:10.1021/cm100277k

Synthesis of Structurally Stable Colloidal Composites as Magnetically Recyclable Acid Catalysts Mathias Feyen,† Claudia Weidenthaler,† Ferdi Sch€ uth,† and An-Hui Lu*,‡ †

Max-Planck-Institut f€ ur Kohlenforschung, D-45470 M€ ulheim an der Ruhr, Germany, and ‡ State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116012, China Received January 27, 2010. Revised Manuscript Received March 18, 2010

In this study, we provide a simple and reproducible method for the preparation of highly active and recyclable colloidal acid catalysts. First, 16-heptadecenoic acid-functionalized magnetite nanoparticles were encapsulated in monodisperse cross-linked polymer spheres. This was achieved by emulsion copolymerization technique in an aqueous phase of styrene and divinylbenzene (DVB). Different ratios of styrene and DVB were used to tune the structural stability and surface morphology of the composites. With increase in DVB content, the surfaces of the colloidal composites become increasingly rougher. The obtained colloids were functionalized with sulfonic acid groups to obtain magnetically recyclable catalysts with Hþ contents in the range of 2.2-2.5 mmol g-1 and surface areas of 45-120 m2 g-1. For the condensation reaction of benzaldehyde and ethylene glycol, magnetic acid catalyst prepared only from DVB precursor was found to be active and with high selectivity and long-term stability. 1. Introduction Nanometer-sized magnetically separable catalysts are very attractive because of their high surface area and easy separation by a magnetic field, which can be superior to conventional separation techniques. Moreover, this kind of catalyst can bridge the gap between homogeneous and heterogeneous catalysis, that is, retain the advantages of homogeneous catalysis (fast reaction rate) and heterogeneous catalysis (easy removal of catalyst after reaction). For the synthesis of magnetically separable catalysts, magnetic nanoparticles (MNPs) are a crucial element. Although it can be possible to directly use MNPs as the catalyst, such MNPs usually need to be protected by a protective shell, consisting of surfactant, polymer, or inorganic materials, in order to avoid aggregation and deterioration of the magnetic cores.1-3 Especially for catalytic purposes, the long time stability of the MNPs is a crucial issue. Polymeric shells appear interesting in this field because they can be synthesized with a broad range of incorporated functionalities, which facilitates a defined adjustment of the surface polarity. Because magnetic recovery is only meaningful in liquid phase catalysis, the limited thermal stability is not a big problem. In the aqueous phase, polymers may actually be more stable than oxidic catalysts, depending on pH. After preparation, polymeric shells further allow an easy modification to support immobilized catalytically active species or other functional moieties on the magnetic core for applications in *Corresponding author. E-mail: [email protected].

(1) Lu, A.-H.; Salabas, E. L.; Sch€ uth, F. Angew. Chem., Int. Ed. 2007, 46, 1222–1244. (2) Jeong, U.; Teng, X.; Wang, Y.; Yang, H.; Xia, Y. Adv. Mater. 2007, 19, 33–60. (3) Shokouhimehr, M.; Piao, Y.; Kim, J.; Jang, Y.; Hyeon, T. Angew. Chem., Int. Ed. 2007, 46, 7039–7046. r 2010 American Chemical Society

catalysis and biotechnology.4 Polymer coatings on magnetic nanoparticles are one of the most used approaches to realize the protection and functionalization of the magnetic cores, because of the wider selection of available polymers.5 Various polymers such as PMMA,6 polystyrene,7-10 PGMA,11 poly(N-isoropyl acrylamide),12 poly(MMA-DVB-GMA) spheres, 13 etc. have been used to prepare polymer-coated spherical magnetic composites, targeted at biomedical and optical applications.14-16 However, only a few publications demonstrate a perfect and defined encapsulation of MNPs inside polymer spheres, leading to both long-term stable dispersions in polar phases and immobilization of catalytically active species.17-19 (4) Huber, D. L. Small 2005, 1, 482–501. (5) Gupta, A. K.; Gupta, M. Biomaterials 2005, 26, 3995–4021. (6) Hong, R. Y.; Feng, B.; Cai, X.; Liu, G.; Li, H. Z.; Ding, J.; Zheng, Y.; Wei, D. G. J. Appl. Polym. Sci. 2009, 112, 89–98. (7) Xu, X. L.; Friedman, G.; Humfeld, K. D.; Majetich, S. A.; Asher, S. A. Adv. Mater. 2001, 13, 1681–1685. (8) Xu, X. L.; Friedman, G.; Humfeld, K. D.; Majetich, S. A.; Asher, S. A. Chem. Mater. 2002, 14, 1249–1254. (9) Faridi-Majidi, R.; Sharifi-Sanjani, N.; Agend, F. Thin Solid Films 2006, 515, 368–374. (10) Yang, C.; Guan, Y.; Xing, J.; Liu, J.; Shan, G.; An, Z.; Liu, H. AIChE J. 2005, 51, 2011–2015. (11) Ma, Z.; Guan, Y.; Liu, H. J. Appl. Polym. Sci., Part A 2005, 43, 3433–3439. (12) Aqil, A.; Vasseur, S.; Duguet, E.; Passirani, C.; Benoit, J. P.; Jerome, R.; Jerome, C. J. Mater. Chem. 2008, 18, 3352–3360. (13) Liu, H.; Guo, J.; Jin, L.; Yang, W.; Wang, C. J. Phys. Chem. B 2008, 112, 3315–3321. (14) Majetich, S. A.; Jin, Y. Science 1999, 284, 470–473. (15) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335–1338. (16) Zarur, A. J.; Ying, J. Y. Nature 2000, 403, 65–67. (17) Xia, A.; Hu, J.; Wang, C.; Jiang, D. Small 2007, 3, 1811–1817. (18) Nishio, K.; Masaike, Y.; Ikeda, M.; Narimatsu, H.; Gokon, N.; Tsubouchi, S.; Hatakeyama, M.; Sakamoto, S.; Hanyu, N.; Sandhu, A.; Kawaguchi, H.; Abe, M.; Handa, H. Colloids Surf., B 2008, 64, 162–169. (19) Xu, H.; Cui, L.; Tong, N.; Gu, H. J. Am. Chem. Soc. 2006, 128, 15582–15583.

Published on Web 03/30/2010

pubs.acs.org/cm

2956

Chem. Mater., Vol. 22, No. 9, 2010

For applications in catalysis, good magnetic recyclability and mechanical stability of these polymer spheres is a crucial point, which has been not highlighted yet. As known, polymer networks like PMMA and polystyrene always exhibit swelling effects in a solvent. As a consequence, this allows solvent molecules to directly contact the magnetic cores, which can lead to corrosion of them if there are any oxidative or acidic reagents present. To avoid or minimize that effect, one strategy is to prepare polymer shells with a high degree of cross-linking. For instance, by adding divinylbenzene (DVB) to styrene, the stability of the resulting polymer network will be enhanced. This kind of polymer can be converted to a solid acid catalyst through a sulfonation treatment. It has been widely accepted that environmentally benign and recoverable catalysts are highly desirable. Solid acid catalysts as substitute would be highly preferable over liquid acids (e.g., H2SO4, HF, and H3PO4), which cause difficulties in product separation, equipment corrosion, and environmental pollution. In this study, we demonstrate a synthetic method for the preparation of structurally stable, below 100 nm sized magnetically separable spheres that consist of magnetite cores and highly cross-linked poly (styrene-co-divinylbenzene) (PSD) protecting shells. These colloidal composites can be modified with sulfonic acid groups, thus serving as magnetically recyclable acid catalysts, which were tested in the condensation reaction between ethylene glycol and benzaldehyde. To the best of our knowledge, no similar approaches have been published so far.

Feyen et al.

2.1. Chemicals. Ammonium peroxodisulfate (g98.0%), ammonium hydroxide solution (28 wt %) in water, and iron(II)chloride tetrahydrate (g99.0%) were purchased from Fluka. 16-Heptadecenoic acid was prepared following the synthesis of Mirviss et al.20 The final product was received with a purity of 96% analyzed by ESI-MS (ESI mass spectra were recorded by using a Micromass ZMD quadrupole mass spectrometer with z-spray alignment of the ESI source (capillary voltage 3.5 kV, desolvation temperature 150 °C). The flow rate into the mass spectrometer was set to 400 mL h-1). Iron(III) chloride hexahydrate (g99%), divinylbenzene (tech. 80% mixture of isomers) and fuming sulfuric acid were bought from Sigma Aldrich, whereas glycidyl methacrylate (g95%) was received from TCI. All chemicals were used as received except divinylbenzene and styrene (>99%, Fluka), which were freshly destilled at 50 °C under a vacuum to remove the inhibitors. 2.2. Synthesis and Stabilization of Fe3O4 Nanoparticles in Aqueous Phase. Magnetite particles were prepared using a modified procedure originally described by Massart et al. (see the Supporting Information).21 In a typical synthesis, 5.0 mmol FeCl3 3 6H2O and 2.5 mmol FeCl2 3 4H2O were dissolved in 10 mL of Millipore water (18.2 MΩ cm). This solution was injected dropwise into an aqueous solution of ammonium hydroxide (2 wt %) at 90 °C under vigorous mechanical stirring. After 30 min the formed black material was collected with a magnet to remove the supernatant. All steps were performed under argon.

The stabilization of the iron oxide particles in aqueous media was then achieved by adding a mixture of 0.7 mmol 16-heptadecenoic acid dissolved in 5.0 mL of an aqueous solution of ammonia (2 wt %). After 1 h of stirring at 50 °C, a stable dispersion was received and divided into 6 equal parts by volume. 2.3. Encapsulation of the Fe3O4 Nanoparticles in Polymer Spheres. In a typical synthesis, the amount of GMA is kept constant at 2.32 mmol, whereas the total amount of styrene and DVB is fixed as 29.45 mmol. To achieve polymer with different cross-linking degree, polymerization were carried out with 2, 20, 40, 60, 80, and 100% DVB with respect to the total amount of monomers (styrene and DVB). The mixed monomers were added to the above-mentioned six dispersions, respectively. After 1 h of moderate mechanical stirring at 50 °C, each mixture was diluted with 142 mL of warm ammonia solution (1.3 wt %) and then quickly heated to 70 °C and held at this temperature for 20 min. The polymerizations were initiated by adding 0.17 mmol of (NH4)2S2O8 dissolved in 2 mL of H2O (mQ). The reactions were continued for 17 h at constant stirring rate, resulting in gray dispersions that were characterized by TEM without further purification. The obtained samples were denoted as Fe3O4@DVB-x, with x=2, 20, 40, etc., representing 2%, 20%, 40%, etc., DVB in the mixed monomers. 2.4. Sulfonation of Magnetic Spheres. In a typical reaction, 400 mg dried magnetic polymer spheres was dispersed in 10 mL oleum and shaken for 10 min. Afterward, the dispersion was dropwise added into 100 mL Millipore water. The particles were collected by decantation and retaining the spheres with a magnet from the acidic solvent. The particles were washed five times, each time with 10 mL Millipore water and separated by magnetically assisted decantation. The particles were dried at 50 °C and then ready for catalytic test. 2.5. Catalytic Tests. The condensation reaction of ethylene glycol and benzaldehyde was performed according to literature.22 The catalyst recycled by a magnet was washed three times with ethanol to remove the entire residue, dried at 50 °C, and then tested under the same catalytic reaction conditions repeatedly. The GC measurements were recorded on a HP-6890-504 gas-chromatograph equipped with a 15 m FFAP column and flame ionization (FID) detector working at 350 °C. The injected samples were heated under a 0.5 bar H2 atmosphere with 6 K min-1 from 60 to 230 °C. The individual response factors were experimentally determined from the pure chemicals (g99%) as follows: benzaldehyde=1.115, ethylenglycol=3.200, 2-phenyl1,3-dioxolane = 1.400, and benzoic acid = 1.500. 2.6. Characterization. The X-ray powder patterns were recorded on a Stoe STADI P transmission diffractometer in Debye-Scherrer geometry (Mo K(alpha)1, 0.70930 A˚) with a primary monochromator and a position sensitive detector. TEM and SEM analyses were carried out with Hitachi HF 750 and Hitachi S-5500 microscope, respectively. All samples were prepared on lacey carbon film supported by a copper grid. TGA was performed on a NETZSCH STA 449C thermobalance. The measurement was carried out under air with a heating rate of 10 °C/min. The IR spectra of samples were collected on a Magna-IR 750 Nicolet FTIR spectrometer using an ATR cell. Materials used for IR, XPD and TG characterization were thoroughly washed with water and ethanol and dried. N2 sorption measurements were performed with a Micrometrics ASAP 2010 instrument. Magnetic properties of the sample have been measured using a superconducting quantum interference

(20) Mirviss, S. B. J. Org. Chem. 1989, 54, 1948–1949. (21) Massart, R.; Cabuil, V. J. Chem. Phys. 1987, 84, 1247–1250.

(22) Xing, R.; Liu, N.; Liu, Y.; Wu, H.; Jiang, Y.; Chen, L.; He, M.; Wu, P. Adv. Funct. Mater. 2007, 17, 2455–2461.

2. Experimental Section

Article

Chem. Mater., Vol. 22, No. 9, 2010

2957

Figure 1. (Left) Powder XRD patterns of (a) Fe3O4 nanoparticles, (b) HDA-functionalized Fe3O4 nanoparticles, (c) Fe3O4@DVB-100, (d, e) Fe3O4@DVB-100-H before and after catalysis; (right) TEM image of Fe3O4 nanoparticles.

Scheme 1. Schematic Illustration of Synthetic Pathway Towards Polymer Stabilized Magnetic Separable Catalysts; HDA, 16-Heptadecenoic acid; GMA, Glycidyl Methacrylate

device (SQUID) magnetometer. Acid-base titrations were done with the system 848 Titrino plus (Methrohm Ion Analysis), using 0.1 M aqueous solution of NaOH as the titration base. XPS measurements were performed with a Kratos HSi spectrometer with a hemispherical analyzer. The monochromatized Al KR X-ray source (E = 1486.6 eV) was operated at 15 kV and 15 mA. For the narrow scans, an analyzer pass energy of 40 eV was applied. The hybrid mode was used as lens mode. The base pressure during the experiment in the analysis chamber was 4  10-7 Pa. To account charging effects, all spectra have been referred to C 1s at 284.5 eV.

3. Results and Discussions The overall synthetic procedure is schematically illustrated in Scheme 1. First, the inorganic Fe3O4 cores were prepared by the well-known alkaline hydrolysis of ferric and ferrous chloride in aqueous phase.21 The synthesized Fe3O4 MNPs were stabilized and functionalized using 16heptadecenoic acid (HDA),20 also in aqueous phase. Subsequently, the functionalized MNPs were encapsulated within the copolymer PSD, in which the amount of the DVB was varied in order to achieve different crosslinking degrees. Finally, the obtained composite was treated with oleum to create the SO3H groups in the polymer shells. As shown in Figure 1, the crystal phases are indentified as Fe3O4 with a small proportion of γ-Fe2O3. The particle sizes calculated based on the Scherrer equation23 with the assumption of spherical particles are around (23) Scherrer, P. Nachr. Ges. Wiss. G€ ottingen, Math.-Phys. Kl. 1918, 2, 98.

9.0 nm ( 1.0 nm. This result is in good agreement with our TEM observations (Figure 1). After a series of treatments including functionalization (Fe3O4@HDA) and encapsulation (Fe3O4@DVB-100), the magnetite nanoparticles are stable, as revealed by the XRD patterns in Figure 1. The sample codes are explained in the Experimental Section. It should be noted that the use of oleic acid instead of the HDA for stabilization of the colloidal solution did not result in the formation of water-dispersible magnetic nanoparticles. We thus use self-prepared HDA as the stabilizer to prepare water dispersible magnetite nanoparticles (see Experimental Section). The successful functionalization of Fe3O4 particles with HDA was confirmed by FT-IR spectroscopy (Figure 2) by comparing the spectra of Fe3O4 and Fe3O4@HDA. As seen in Figure 2, there are bands at 1525 and 1431 cm-1 that can be attributed to the bidentate interaction between the deprotonated carboxylate function of the HDA and the iron oxide surface.24,25 Further, the presence of terminal vinyl groups in Fe3O4@HDA is revealed by the absorption bands resulting from stretching and deformation vibrations at wave numbers 3088 and 910 cm-1. The XPS (Figure 5 b and d) of Fe3O4@HDA reveal the presence of carbon and two oxygen species on the surface which can be correlated to the carboxylate function of the surfactant. Based on TGA results of the carefully washed (24) Rochiccioli-Deltcheff, C.; Franck, R.; Cabuil, V.; Massart, R. J. Chem. Res. 1987, 1209–1232. (25) De Palma, R.; Peeters, S.; Van Bael, M. J.; Van den Rul, H.; Bonroy, K.; Laureyn, W.; Mullens, J.; Borghs, G.; Maes, G. Chem. Mater. 2007, 19, 1821–1831.

2958

Chem. Mater., Vol. 22, No. 9, 2010

Feyen et al.

Figure 2. FTIR spectra of (a) Fe3O4 nanoparticles, (b) HDA-functionalized Fe3O4 nanoparticles, (c) Fe3O4@DVB-100, (d) Fe3O4@DVB-100-H.

Fe3O4@HDA colloids (see Figure S1 in the Supporting Information), where a weight loss of 3.7% was observed, the surface concentration of HDA was estimated as 5.2 molecules per nm2 surface. The encapsulation of the functionalized Fe3O4 particles in PSD with controlled cross-linking degree was achieved in the presence of GMA. Our experiments show that without GMA, the magnetite nanoparticles cannot be uniformly coated with the polymer. Without GMA, there are always some big clusters of trapped MNPs in the polymer matrix, and mostly on the external surface of the polymer spheres. Thus, the presence of GMA in the synthesis is a crucial factor for obtaining uniform spheres with magnetite nanoparticles encapsulated inside. GMA can enhance the polarity of the monomer mixture and thus facilitate penetration of the Fe3O4 particles inside the monomer droplets. This is consistent with the results from the literature.11,13,18 Figure 3 a-f shows that all magnetic particles are well-encapsulated in polymer spheres with a size of about 90 nm. By considering at least 300 particles of each batch, we observed that only less than 4% of the polymer spheres were formed without magnetic nanoparticles embedded. Because the number of defective particles is quite small, the materials were used without further purification, although a magnetic separation would be possible. Such small particles, usually below 100 nm, are very useful for biomedical applications,26 but also for liquid phase catalytic reactions, where mass transfer can be an issue. With the increase in the amount of DVB, the surfaces of the composite spheres become rougher, leading to higher surface areas. Furthermore, N2 adsorption analyses reveal that the specific surface areas of Fe3O4@DVB-2 (2% DVB of total monomer) and Fe3O4@DVB-100 (pure DVB as monomer) are 45 and 120 m2 g-1, respectively. Again, the rough surface samples show a higher surface area, which is desirable for catalysis. The surface morphologies of the representative samples Fe3O4@DVB-2 and Fe3O4@DVB-100 were investigated with high-resolution SEM. As seen in Figure 4, sample Fe3O4@DVB-2 consists of perfect spherical particles (26) Berry, C. C. J. Mater. Chem. 2005, 15, 543–747.

Figure 3. TEM images of Fe3O4 nanoparticles coated in polystyrene with different cross-linking degrees: (a) Fe3O4@DVB-2, (b) Fe3O4@DVB-20, (c) Fe3O4@DVB-40, (d) Fe3O4@DVB-60, (e) Fe3O4@DVB-80, and (f) Fe3O4@DVB-100 (insets are the high-magnification TEM images of corresponding samples.).

Figure 4. SEM images (a) Fe3O4 @DVB-2; b: Fe3O4 @DVB-100) of encapsulated Fe3O4 in cross-linked polystyrene spheres recorded at an acceleration voltage of 30 kV.

with a diameter of about 90 nm. The surfaces of these spheres are rather smooth. No visible humps corresponding to MNPs are present on the external surface of the spheres, indicating that magnetite nanoparticles are completely encapsulated within the polymer shells. The surfaces of Fe3O4@DVB-100 synthesized using only DVB (Figure 4) are rather rough. This sample consists of many spheres with an average diameter of about 90 nm, and a large number of small polymer particles (around 10 nm) on their external surface. Hence, SEM and TEM observations are in good agreement with each other. To check the amount of the encapsulated magnetite cores and obtain information on the thermal stability of the polymeric shells, TGA experiments were conducted by heating samples under air up to 800 °C. As seen in Figure S1 (Supporting Information), the decomposition temperature of the polymer increases with increasing amount of DVB, indicating a better heat resistance of such highly cross-linked polymer. When the amount of

Article

Chem. Mater., Vol. 22, No. 9, 2010

2959

Figure 5. TEM images of SO3H-functionalized samples (a) Fe3O4@DVB-2-H and (c) Fe3O4@DVB-100-H. XP spectra of (b) Fe3O4@DVB-2 and Fe3O4@DVB-2-H and (d) Fe3O4@DVB-100 and Fe3O4@DVB-100-H. The insets in b and d show the enlarged area between 172 and 162 eV for the samples Fe3O4@DVB-2-H and Fe3O4@DVB-100-H.

DVB exceeds 60%, the thermal decomposition behavior of the different magnetic composites is very similar at temperature below 450 °C. This is in good agreement with literature.27 The calculated weight percentage of the magnetite nanoparticles is, under the assumption that the residues consist of completely oxidized R-Fe2O3, about 8.6 ( 1.3 wt % for the whole sample series. The magnetic properties of the magnetic spherical composites were characterized by SQUID magnetization measurements at room temperature. The representative magnetization curves (M vs H) (normalized to the magnetite masses obtained from the TG results) of the as-made Fe3O4 nanoparticles Fe3O4@DVB-2 and Fe3O4@DVB-100 are shown in Figure S2 (Supporting Information). The absence of hysteresis indicates these composite spheres are superparamagnetic in nature at room temperature, i.e., no remanent magnetization and no coercivity was observed. The saturation magnetizations of the as-made Fe3O4 nanoparticles, Fe3O4@DVB-2 and Fe3O4@DVB-100 sample are about 77, 60, and 61 emu/g, respectively, at a maximum field of 1 T. This magnetization is sufficient to achieve complete separation of the composites with a commercial magnet within a few minutes. After the treatment with oleum, Fe3O4@DVB-2 and Fe3O4@DVB-100 bear SO3H functional groups on their surfaces (samples were named accordingly as Fe3O4@DVB2-H and Fe3O4@DVB-100-H). This has been proven by FT-IR, XPS and EDX analyses. The SdO vibration band at 1345 cm-1 (str.) (Figure 2) indicates the presence of the (27) Hattori, M.; Sudol, E. D.; El-Aasser, M. S. J. Appl. Polym. Sci. 1993, 50, 2027–2034. (28) Valix, M.; Cheung, W. H.; McKay, G. Langmuir 2006, 22, 4574– 4782.

SO3H groups.28 Moreover, the XPS analyses (Figure 5b, d) show that the S/O atomic ratios of Fe3O4@DVB-2-H and Fe3O4@DVB-100-H are 1:3.3 and 1:4, respectively. These results confirm that SO3H groups have been grafted onto the polymer. The slight excess of oxygen may be due to the presence of residual GMA molecules. TEM characterization of these two sulfonated samples demonstrated that the morphology and the magnetic cores are retained. (Figure 5a, c). To check the dispersion characteristics of the acid catalysts in polar phases DLS-measurements were performed in aqueous phase (see Figure S3 in the Supporting Information). It reveals with detected hydrodynamic diameters (dh) of 91 nm for Fe3O4@DVB-2-H and 112 nm for Fe3O4@DVB2-H, that is in agreement with TEM observations, i.e., narrow size distributions and good dispersibility of both colloidal materials. XRD measurements confirmed that also the crystal phase of the inorganic cores is unchanged, indicating good protection of the magnetic core by the polymer matrix under the current experimental conditions. From SQUID magnetization measurements we could observe in both samples a slight decrease of the saturation magnetization (see Figure 7) during the functionalization step which was probably caused by slight oxidation effects occurring at surface near Fe3O4 nanoparticles in the polymer spheres. However, after this treatment the materials maintain fully attractable by a magnet. The amounts of immobilized protons on the polymer spheres were quantified by acid-base titration. It was shown that the Hþ contents of Fe3O4@DVB-2-H and Fe3O4@DVB100-H are 2.5 and 2.2 mmol g-1, respectively. These values are comparable with sulfonated ordered mesoporous polymers, though these mesoporous polymers have higher surface areas of 460 - 545 m2 g-1.22

2960

Chem. Mater., Vol. 22, No. 9, 2010

Feyen et al.

Table 1. Conversion of Benzaldehyde Catalyzed by the Acid Catalyst in Cyclohexane for a 1 h Reaction Period at 90 °C sample Fe3O4@DVB-2-H

Fe3O4@DVB-100-H

cycle no.

conversion of benzaldehyde (%)

2-phenyl-1,3,dioxane (conc %)

side products (conc %)

selectivity of 2-phenyl-1,3,dioxane (%)

1 2 3 4 5 1 2 3 4 5

53.3 61.9 61.1 72.2 67.9 49.6 51.1 52.1 54.0 55.5

49.3 60.0 58.5 59.9 61.3 44.1 46.1 46.4 50.4 49.3

2.3 1.1 2.1 3.4 6.6 5.2 4.7 5.3 3.5 6.1

95.4 98.1 96.5 94.7 91.0 89.5 90.7 89.8 93.6 89,0

We also calculated the expected SO3H-loading, both based on the calculated geometric surface area and the measured BET surface area, under the assumption of exclusive functionalization on the surface at a surface density of 1 SO3H nm-2 (for details, see the Supporting Information). Calculated loadings are on the order of 0.1 mmol g-1. This is by more than 1 order of magnitude lower than the actual loading determined by acid-base titration. One can thus conclude that SO3H-groups are also formed below the surface layer in the bulk of the polymer, due to deeper penetration of the sulfuric acid into the organic shells, resulting from the swelling effect of the polymer.29 In comparison, the Hþ content of polymer encapsulated magnetic spheres without sulfonation is below the detection limit of the instrument. The catalytic activity and the recyclability of these magnetic catalysts were demonstrated in the acid catalyzed condensation reaction of ethylene glycol and benzaldehyde to 2-phenyl-1,3-dioxolane (see Table 1). After each cycle, the magnetically recovered catalyst was washed three times with ethanol to remove residues from the condensation reaction. In the first five runs, Fe3O4@DVB-100-H showed nearly constant conversions of benzaldehyde (ca. 50%) with a selectivity toward 2-phenyl-1,3-dioxolane of ∼90% after 1 h. This catalyst is still magnetically separable after the five cycles. In the case of Fe3O4@DVB-2-H catalyst, an increasing conversion of benzaldehyde from 50% up to 72% and a selectivity toward 2-phenyl-1,3-dioxolane of ∼95% was observed under the same reaction conditions. However, during the repeated catalytic runs, a loss of the magnetic properties was observed for this catalyst. Thus catalyst Fe3O4@DVB-2-H has to be collected from the reaction mixture by centrifugation in the last three runs. As we mentioned above, the purpose of this study is the synthesis of structurally stable acid catalysts. Hence, after reaction, catalysts Fe3O4@DVB-2-H and Fe3O4@DVB100-H were collected and characterized again with TEM to check their structural stability. As seen in Figure 6a and b, the particles of Fe3O4@DVB-2-H lost their spherical shape by forming networks through “necks” between adjacent spheres. This is presumably because of the low mechanical stability (low cross-linking degree of the polymer network results in swelling and high dynamics within the polymer network). In contrast, Fe3O4@DVB(29) Bindschaedler, C.; Gurny, R.; Doelker, E.; Peppas, N. A. J. Colloid Interface Sci. 1985, 108, 83–89.

Figure 6. TEM images in different magnifications of the samples Fe3O4@DVB-2-H (images a and b) and Fe3O4@DVB-100-H (images c and d) after five catalytic cycles.

100-H (Figure 6c and d) exhibits identical spherical morphology before and after five cycles of catalytic tests, which was supported by DLS experiments (see Figure S3 in the Supporting Information). Acid-base titration reveals that after the fifth cycle, the acidity of Fe3O4@DVB-2-H and Fe3O4@DVB-100-H are 1.6 mmol g-1 and 1.3 mmol, which are still sufficiently high concentrations to catalyze the condensation reaction. The results give an explanation for the comparable higher activity of Fe3O4@DVB-2-H, especially in the last three runs. Although the acidity of the recycled catalysts decreases, no regeneration step with sulfuric acid is required to reactivate the catalysts at least for the first five cycles. As described before, the magnetic properties of Fe3O4@DVB-2-H vanished after three cycles, whereas Fe3O4@DVB-100-H stayed magnetically separable after the reaction. These observations are stressed by the measured saturation magnetizations of the two catalysts before and after catalysis test. The corresponding M vs H curves at room temperature of both catalysts before and after five cycles are given in Figure 7. The two recorded curves of Fe3O4@DVB-100-H (Figure 7c, d) indicate the maintaining magnetic properties under reaction conditions with saturation magnetization values of 52 and 51 emu/g at a maximum field strength of 5 T (normalized to the magnetite masses based on the TG results). This data is stressed by XRD measurements, which prove the maintaining crystal structure of Fe3O4 in the used catalyst

Article

Chem. Mater., Vol. 22, No. 9, 2010

2961

degree with respect to the long-term stability of the catalysts. Thus, particularly the magnetically separable colloidal acid catalyst, sulfonic acid modified polydivinylbenzene, is a very interesting material in terms of achieving long-term stable solid acid catalyst. 4. Conclusion

Figure 7. M vs H curves measured at room temperature. a) (filled circles) Fe3O4@DVB-2-H, (b) (blank circles) Fe3O4@DVB-2-H after catalysis, (c) (filled triangles) Fe3O4@DVB-100-H and d) (blank triangles) Fe3O4@DVB100-H. The inset shows a photo of the catalyst Fe3O4@DVB-100-H after five catalytic cycles in the dispersed form (left side) and after magnet attraction (right side).

(see Figure 1). In contrary, the normalized values for Fe3O4@DVB-2-H (Figure 7a, b) dropped from 47 to 1 emu/g after five catalytic runs, revealing the inferior oxidation protection of the low cross-linked polymer shell. In combination with the TEM results, these findings clearly underline the importance of the cross-linking

We have demonstrated that colloidal magnetic composites (below 100 nm), namely poly(styrene-co-divinylbenzene) encapsulated Fe3O4 nanoparticles, can be prepared with different amounts of DVB. With increase in DVB content, the surfaces of the colloidal composites become increasingly rougher. The sulfonated magnetic composites can be used as long-term stable and recyclable acid catalysts, especially if only DVB is used as the polymer source. This catalyst exhibits unique properties by combining the advantages of high surface area, colloidal dispersibility in polar solvents, magnetic separability, and long-term stability. Acknowledgment. A.-H.L. thanks the NSFC (20873014) for the financial support. The authors thank the MaxPlanck-Institut f€ ur Kohlenforschung for providing basic funding. Supporting Information Available: Calculations and additional figures (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.