J. Phys. Chem. C 2007, 111, 2185-2190
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Ionic Liquid-Assisted Immobilization of Rh on Attapulgite and Its Application in Cyclohexene Hydrogenation Shiding Miao, Zhimin Liu,* Zhaofu Zhang, Buxing Han, Zhenjiang Miao, Kunlun Ding, and Guimin An Beijing National Laboratory for Molecular Sciences, Center for Molecular Sciences, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, China ReceiVed: September 29, 2006; In Final Form: NoVember 30, 2006
Rhodium-attapulgite (Rh-Atta) catalyst was prepared by immobilizing Rh3+ on Atta via an ionic liquid (IL), 1,1,3,3-tetramethylguanidine lactate (TMG+L-), followed by reduction with hydrogen at 300 °C. The resulting composite was characterized by different techniques. X-ray photoelectron spectroscopy analysis showed that the loaded rhodium on Atta existed mainly in the form of Rh0 with a small amount of its oxides and was distributed uniformly on Atta with a particle size of less than 5 nm, as confirmed by transmission electron microscopy examination. X-ray diffraction analysis indicated that the layered channel structure of Atta was destroyed to some extent due to the impregnation of the IL and Rh. IR spectra indicated that the Lewis and Bronsted acid sites in Atta absorbed the TMG+ ions chemically. The specific surface areas determined by the BET method from N2 sorption isotherms decreased with the entrance of the IL and loaded Rh. The activity of the composite for cyclohexene hydrogenation was investigated, which exhibited much higher efficiency compared to those of other catalysts, and the turnover frequencies reached 2700 (mol of cyclohexene/mol of Rh)/h.
Introduction Ionic liquids (ILs), due to their unusual properties such as an extremely low vapor pressure, a wide liquid range, good chemical stability, high thermal stability, and excellent solvent characteristics, have been intensively investigated in chemical reactions and exhibited bright future perspectives.1 Generally, ILs are used as reaction media, which require a large amount of ionic liquids.2 On the basis of economic criteria and possible toxicological concerns, it is desirable to minimize the amount of the utilized ILs in a potential process. Recently, the immobilization of ILs onto solid supports and preparation of catalytic materials assisted by ILs have attracted significant attention. For example, an IL, [bmim][PF6] or [bmim][BF4], dissolving a Rh complex was successfully supported on silica gel and showed a high catalytic activity and outstanding stability for hydrogenation reactions.3 Rhodium complexes, such as Rh(PPh)3+, Rh(COD)(PPh3)2+, and [Rh(COD)(PPh3)2]BF4 (COD stands for 1,5-cyclooctadiene), were introduced into the interlamella surfaces of hectorite by direct ion exchange, which also exhibited good activities for olefin hydrogenation.4 Ru and Pd nanoparticles were immobilized on montmorillonite and SBA15 supports, respectively, with the help of ILs, which displayed high efficiency for hydrogenation of benzene and olefin.5 In comparison to pure ILs, supported ILs offer some additional features. For instance, they can be used as heterogeneous catalysts to be separated and recovered easily, and they may exhibit a synergetic effect. Moreover, they can assist effective catalyst dissolution in ILs to be immobilized on solid supports. Although a few examples of immobilized ionic liquids on solid supports have been reported in the literature, they are mainly used as support materials impregnated with ILs as catalysts.6 * To whom correspondence should be addressed. Fax: 861062562821. E-mail:
[email protected].
More recently, the IL-assisted immobilization of Rh complexes and Rh nanoparticles on a Al2O3 support has been reported via the one-step loading method.7 Several kinds of supports, including silica gel,8 SBA-15,9 polymer,10 montmorillonite etc.,5a have been used to immobilize ILs via different interactions. Natural clays are environmentally friendly materials with crystalline structures and relatively large surface areas which can be used as catalyst supports. In our previous work, we supported Ru on montmorillonite by an IL, which showed high efficiency for benzene hydrogenation.5a Attapulgite (Atta) is another natural mineral, generally denoted as (H2O)4(Mg,Al,Fe)5(OH)2Si8O20‚4H2O. Atta is made up of two bands of silica tetrahedra linked by magnesium ions in octahedral coordination which are connected by a continuous plane of tetrahedral basal oxygen atoms, and there exist H2O and exchangeable cations, such as H+, Na+, and K+, in the repeated units.11 Atta owns dented surfaces and thus a relatively higher surface area, a moderate cation exchange capacity (CEC), and high acidity, which give it a certain coordination capacity when it interacts with transition-metal ions.12 So far, Atta clays have been used as drilling fluids, paints, agricultural carriers, industrial floor absorbents, and catalyst supports.13 On the basis of the structure of Atta, it can absorb ILs or exchange with IL ions, and the combination of Atta with an IL may widen the application of Atta. However, there is no report about the immobilization of IL on Atta in a literature survey. In this paper, we report the IL-assisted immobilization of rhodium nanoparticles on Atta. The resulting composites were characterized in detail using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), etc. The catalytic activity of the composite for cyclohexene hydrogenation was also investigated.
10.1021/jp066398o CCC: $37.00 © 2007 American Chemical Society Published on Web 01/18/2007
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TABLE 1: Chemical Composition (%) of the Purified Atta Used in This Work SiO2
Al2O3
Fe2O3
FeO
MgO
CaO
Na2O
K2O
TiO2
P2O5
MnO
loss on ignition
61.23
11.42
7.2
0.17
9.07
0.34
0.29
1.32
1.07
0.023
0.11
7.54
Experimental Section Starting Materials. The Atta clay used in this work was provided by Hongyan Mining Co., which was leached with 2 M HCl aqueous solution at room temperature for 5 h, followed by chloride elimination via rinsing the clay with deionized water and calcination under a flow of air at 250 °C to remove the organic compounds, and then the clay was pulverized and sieved down to 120 mesh. The sieved Atta was dispersed in doubly distilled water with a mass ratio of Atta:H2O ) 1:99, and the dispersion was vigorously stirred for 6 h to obtain a homogeneous suspension. The clay suspension was sedimentated naturally for 72 h, and the upper suspension was collected and dried at 200 °C for 24 h. The obtained Atta was used for the synthesis of Atta-IL-Rh composites in this work, and its chemical composition is shown in Table 1. Other reagents including RhCl3 and cyclohexene (AR) were provided by Beijing Reagents Co., and the IL (1,1,3,3tetramethylguanidine lactate, TMG+L-) was synthesized on the basis of procedures reported previously.14 Preparation of Atta-IL-Rh Composites. A 6.0 g sample of the purified Atta was dispersed in a 50 mL aqueous solution containing 0.6 g of TMG+L- with stirring for about 4 h. Then the Atta was separated via centrifugation and treated with fresh IL aqueous solution again. These procedures were repeated three times. One part of the Atta treated with IL aqueous solution was dried at 200 °C for 24 h and named Atta-IL. The other was redispersed in 10.0 mL of RhCl3 aqueous solution with a concentration of 2.0 mg/mL. Then the clay was dried at 60 °C
under vacuum after the water was removed via evaporation and named Atta-IL-Rh3+. Part of the Atta-IL-Rh3+ was hydrogenated with H2 at 300 °C for 2 h, resulting in a composite named Atta-IL-Rh. The mass content of Rh in the composite was 1.0 wt %, which was calculated on the basis of the amounts of the clay and the RhCl3 added. For comparison, we prepared the catalyst via direct dispersion of Atta into RhCl3 aqueous solution and hydrogenation at 300 °C and named it Atta-Rh. Characterization. The XPS spectra of the as-prepared samples were collected on an ESCALab220i-XL spectrometer at a pressure of about 3 × 10-9 mbar using Mg KR as the excitation source (hν ) 1253.6 eV) and operating at 15 kV and 20 mA, and the C1s binding energy was set to 284.8 eV as the energy calibration. The XRD patterns were collected on an X-ray diffractometer (SW, X’PERT) operated at 40 kV and 10 mA with nickel-filtered Cu KR radiation (λ ) 1.54060 Å). The nitrogen sorption analysis was performed on an ASAP-2405N instrument at liquid nitrogen temperature. Prior to the adsorption, the sample was degassed at 300 °C for 12 h at 10-4 Torr on a high-vacuum line. FTIR spectra were recorded by a Bruker IFS25 spectrometer using the KBr pellet technique. TEM was performed on a transmission electron microscope (Philips, Tecnai F30) equipped with an ultra-high-resolution pole piece that can result in a point resolution of 0.17 nm at a operating voltage of 300 kV, and the images were electronically captured using a CCD camera. SEM examination was carried out on a scanning electron microscope (JEOL JSM-4300) operated in
Figure 1. IR spectra (a) and magnified spectrum (b) of the prepared samples Atta, Atta-IL, and Atta-IL-Rh.
Figure 2. X-ray photoelectron spectra: (a) survey spectrum and (b) Rh 3d spectrum of the Atta-IL-Rh catalyst.
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Figure 3. XRD patterns of the purified Atta (a, JCPDS number 290855), Atta-IL (b), Atta-IL-Rh (c), and Atta-Rh (d).
Figure 5. SEM images of the Atta-IL-Rh nanocomposite.
Figure 4. Nitrogen sorption isotherms (b, Atta; 9, Atta-IL; 2, AttaIL-Rh).
high-vacuum mode at 15 kV, which provided general textural information on the samples. Catalytic Activity Test. All hydrogenation reactions were carried out in a 30 mL stainless steel autoclave with a magnetic stirrer. In a typical experiment, 0.102 g of catalyst and 12.3 g of cyclohexene were loaded into the autoclave, and the air in the reactor was replaced by H2 within 10 min. Then the reactor was moved to a water bath with the desired temperature, and H2 stored in a cylinder was connected to the reactor. The temperature of the water bath was controlled with a temperature controller (Haake D3) with an accuracy of (0.1 °C. The pressure of the reaction system was monitored using a pressure transducer (Foxboro/ICT model 93). The reaction system was stirred by a magnetic stirrer at 300 rpm. The products were collected via centrifugation (12000 rpm) and were analyzed by a gas chromatograph (Agilent 4890 D) equipped with an Innovax capillary column and a Varian FID-GC flame ionization detector. The turnover frequencies (TOFs) were determined for ∼100% conversion of cyclohexene. Results and Discussion IR Analysis. The FTIR analysis was performed to obtain information about the as-prepared samples. In Figure 1a, the band appearing at 3614 cm-1 in the three samples represents the stretching mode of the surface hydroxyl groups on the layer as well as the adsorbed water (zeolitic water) bonded to the different cations in the octahedral coordination of Atta.15 The band near 1207 cm-1 was attributed to the in-layer Si-O-Si stretching, and that at 801 cm-1 may correspond to the AlO-Si bond in Atta.16 An enlarged spectrum (Figure 1b) shows
the difference between Atta and Atta-IL or Atta-IL-Rh. The band at 1618 cm-1 is attributed to the stretching of CdN, and bands at 1587, 1650, and 1577 cm-1 can be assigned to -NH and -NH2, indicating the presence of IL in the Atta-IL and Atta-IL-Rh composites. According to a previous report,17 the stretching of CdN occurred at 1614 cm-1, while the vibration shifted toward higher frequency by about 4 cm-1 in our samples (1618 cm-1), which probably resulted from the direct coordination between the nitrogen of TMG+ and the Mg2+ ions in MgO6 polyhedra.18 The Lewis and Bronsted acid sites were situated at 1630 and 1650 cm-1, respectively, as identified by the adsorption of n-butylamine probe molecules.19 In this situation, we assume that the Lewis and Bronsted acid sites absorbed the TMG+ ions via chemical bonding, confirmed by the presence of bands at 1650 and 1652 cm-1 for the Atta-IL and AttaIL-Rh samples, respectively. In addition, -CH3 (1413 and 2945 cm-1) was also found in Atta-IL and Atta-IL-Rh, further confirming the existence of TMG+ in this composite. Generally, Atta is formed by an alternation of blocks and cavities (tunnels) that grow up in the c axis direction, and the tunnels are filled with coordinated water molecules bonded to Mg2+ ions located at the edges of octahedral sheets. Silanol groups (SiOH) are present on the “external surface” of the silicate particles, which may act as Bronsted acid sites and Lewis sites to give cling sites for the basic species.20 Additionally, the considerable substitution of aluminum by magnesium and iron in the octahedral layer provides Atta a moderately high layer charge. In this work, cations of the IL can access the electronegative substrate via electrostatic interaction; meanwhile the oxygen ions associated with tetrahedra on ribbon edges may attract cations or molecules via hydrogen bonding to the -Nd in TMG+, as proposed by Brzezinski et al.21 Moreover, the roughness of the surface of Atta also facilitates the absorption of some ions and/or organic molecules.22 Therefore, the ionic liquid was easily immobilized onto the surface of Atta. As reported,23 TMG+ has a strong ability to coordinate transition
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Figure 6. TEM images of the purified Atta (a) and Atta-IL-Rh (b), HRTEM images of Atta-IL-Rh (c, d), SAED pattern of Atta-IL-Rh (e), and TEM image of Atta-Rh (f).
metal ions; therefore, the supported IL on Atta may have the ability to stabilize some metal nanoparticles. XPS Analysis. The as-prepared composites were characterized by XPS, and Figure 2a shows the survey spectrum of AttaIL-Rh. It was indicated that no elemental chlorine was detectable, which suggests that RhCl3 was completely converted during the H2 reduction process. The Rh 3d XPS spectrum of Atta-IL-Rh is displayed in Figure 2b. Compared to the binding energy (BE) values of Rh 3d in pure rhodium metal foil,24 both the predominant peak at 307.0 eV and the deconvolved peak at 311.8 eV are attributed to Rh0 species, and the peaks at 308.5 and 313.3 eV are assigned to the partially oxidized rhodium species,25 which may result from the slight oxidation of Rh0
nanoparticles exposed to air. XPS analysis shows that the Rh content on the surface of Atta-IL-Rh was about 0.3%, much lower than the initial stoichiometric quantities of Rh (1.0 wt %), implying some Rh was intercalated into the interlayers of Atta. XRD Analysis. Figure 3 gives the XRD patterns of Atta, Atta-IL, Atta-IL-Rh, and Atta-Rh. Atta-Rh shows an XRD pattern similar to that of Atta, which suggests that the Atta in Atta-Rh retained its original structure without any damage. However, for Atta-IL and Atta-IL-Rh, the d110 peaks displayed a lower intensity and a wider peak shape and shifted slightly to the lower angle, compared to that for Atta. Also, the peaks in the 2θ range of 18-30° significantly broadened. These
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TABLE 2: Results of Catalyst Performance in Hydrogenation of Cyclohexene entry
catalyst
substrate/Rh (mol/mol)
T (°C)
P(H2) (MPa)
time (h)
conversion (%)
TOF (h-1)
1 2a 3b 4c 5d 6 7e 8 9f 10g
Atta-IL-Rh Atta-IL-Rh Atta-IL-Rh Atta-IL-Rh Atta-IL-Rh Atta-Rh Atta-IL-Rh3+ Atta-IL [Rh(cod)(suphos)]-Pd0/SiO2 Pd/[Bimim][BF4]
15000 15000 15000 15000 15000 15000 15000 15000 525 500
30 30 30 30 30 30 30 30 25 40
0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3
5.5 5.5 5.6 6.0 6.3 48 45.0 72 0.5 5
>99.0 98.7 >99.0 >99.0 >99.0 51.5 98.5 3.51 100 100
2700 2692 2652 2475 2250 328 1048 100
a -d Atta-IL-Rh catalyst was used the second, third, fourth, and fifth times, respectively. e Atta-IL-Rh3+ used without reduction by H2 at 300 °C. f Based on data from ref 30, using a combined Pd and grafted Rh complex as the catalyst. g Based on data from ref 31, using Pd in 1-n-butyl3-methylimidazolium hexafluorophosphate ([Bimim][BF4]) as the catalyst.
observations indicate that the structures of Atta in Atta-IL and Atta-IL-Rh became disordered to some extent (the peak observed at 2θ ) 8.6° corresponding to d110 ) 1.04 nm) and confirm that some substances, e.g., IL or Rh, were impregnated into the channels of Atta, leading to some expansion or collapse of the Atta framework. On the pattern of Atta-IL-Rh, no detectable diffraction peak is assigned to Rh species, which indicates that there are no large Rh particles in the Atta-ILRh composites. N2 Sorption Analysis. To verify the change of the surface and porous characteristics of Atta in Atta-IL and Atta-ILRh, the as-prepared samples were evaluated by N2 adsorption (Figure 4). All samples exhibited sorption isotherms of type IV according to the IUPAC classification,26 and the specific surface areas were determined by the BET method in the relative pressure range of 0.05-0.20 from nitrogen adsorption isotherms.27 Similar to reports in the literature,28 the specific surface area of the clay decreased from 181.0 m2/g for the purified Atta to 164.2 m2/g for Atta-IL and further to 142.5 m2/g for AttaIL-Rh, which might result from the absorption of the IL and deposition of nanoparticles on Atta. The small hysteresis of the composite indicates less mesoporosity, suggesting that the Rh nanoparticles might occupy the mesopores, leaving relatively little free volume in the clay.29 Morphology. Figure 5 shows SEM images of Atta-IL-Rh. It is clear that Atta exhibits a fibrous structure, and some fibers form straight parallel aggregates. These fibers stacked up, leading to pores with different sizes. In a magnified image (Figure 5b), the size of a single fiber can be clearly observed, which was about 40-60 nm in width and 1-10 µm in length. TEM examination gives more information about the microstructures of the prepared composites. Figure 6a shows a typical TEM image of the purified Atta, which indicates that Atta is composed of densely packed fibers with smooth surfaces. For the Atta-IL-Rh composite, there are numerous nanoparticles attached along the fibers, and the particle size is less than 5 nm, as shown in Figures 6b-d. A large magnified TEM image, shown in Figure 6c, displays a fairly narrow particle distribution, which is located on the rods and in the cavities of Atta (as denoted by arrows). From the image shown in Figure 6d, it can be clearly observed that the size of the particles is around 3 nm. The selected area electron diffraction (SAED) pattern shows few spots, but it is dominated by weakly developed rings in two directions (Figure 6e), indicating poor crystallinity. However, the size of Rh nanoparticles in Atta-Rh is around 25 nm (Figure 6f), much larger than that of Rh nanoparticles in AttaIL-Rh. Therefore, it can be deduced that the IL played an important role in stabilizing the nanoparticles in Atta-IL-Rh composites.
Figure 7. TEM images of Atta-IL-Rh after it was used five times for hydrogenation of cyclohexene.
Catalytic Activity Test. To evaluate the catalytic characteristics of the as-prepared samples, we investigated the hydrogenation of cyclohexene using the prepared composites as catalysts. The reaction was performed at 30 °C and 0.3 MPa of H2 with a C6H12/Rh molar ratio of 15000, and the results are listed in Table 2. The TOFs, defined as moles of C6H10 consumed per mole of rhodium per hour, reached 2700 h-1 (Table 2, entry 1) when Atta-IL-Rh was used as the catalyst, which is much higher than that of the reported catalysts listed in Table 2 (entries 9 and 10). The loss of activity of the catalyst was not significant after it was used five times (entries 2-5), which indicates that the catalyst also possessed good stability.
2190 J. Phys. Chem. C, Vol. 111, No. 5, 2007 From the TEM images of Atta-IL-Rh used five times, shown in Figure 7, it can be observed that Rh nanoparticles were still firmly adhered on the surface of Atta and no remarkable aggregation of these nanoparticles occurred. To further explore the roles of different species in AttaIL-Rh composites, some control experiments over different samples were performed under the same hydrogenation conditions. As listed in Table 2, Atta-Rh showed less activity (entry 6), which might result from the large size of Rh particles decorated on Atta. This means that the Rh nanoparticles with a small size are favorable for this reaction. The catalytic activity of Atta-IL for cyclohexene hydrogenation was also investigated, which indicated that this sample had little effect on this reaction (entry 8). This implies that Atta only acted as a substrate and the IL as a stabilizing agent for Rh nanoparticles. Conclusion Rhodium nanoparticles can be supported on Atta with the aid of TMG+L-, and the resulting catalyst is active and stable for hydrogenation of cyclohexene. The catalyst not only combines the advantages of the natural clay and ILs, but also shows synergetic effects for the reaction. This green and effective method can also be used to prepare some other catalysts via immobilization of precious metal nanoparticles on natural clay by the IL. Acknowledgment. This work is financially supported by the National Natural Science Foundation of China (Grant Nos. 50472096 and 20533010). References and Notes (1) (a) Welton, T. Chem. ReV. 1999, 99, 2071-2084. (b) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772-3789. (c) Gelesky, M. A.; Umpierre, A. P.; Machado, G.; Correia, R. R. B.; Magno, W. C.; Morais, J.; Ebeling, G.; Dupont, J. J. Am. Chem. Soc. 2005, 127, 45884589. (2) Dupont, J.; Fonseca, G. S.; Umpierre, A. P.; Fichtner, P. F. P.; Teixeira, S. R. J. Am. Chem. Soc. 2002, 124, 4228-4229. (3) Mehnert, C. P.; Mozeleski, E. J.; Cook, R. A. Chem. Commun. 2002, 3010-3011. (4) (a) Valli, V. L. K.; Alper, H. Chem. Mater. 1995, 7, 359-362. (b) Pinnavaia, T. J.; Welty, Philip K. J. Am. Chem. Soc. 1975, 97, 38193820. (c) James, B. R. Catal. Today 1997, 37, 209-221. (d) Claver, C.; Ferna´ndez, E.; Margalef-Catala´, R.; Medina, F.; Salagre, P. J.; Sueiras, E. J. Catal. 2001, 201, 70-79. (5) (a) Miao, S. D.; Liu, Z. M.; Han, B. X.; Huang, J.; Sun, Z.; Zhang, J.; Jiang, T. Angew. Chem., Int. Ed. 2006, 45, 266-269. (b) Huang, J.; Jiang, T.; Gao, H.; Han, B.; Liu, Z.; Wu, W.; Chang, Y.; Zhao, G. Angew. Chem., Int. Ed. 2004, 43, 1397-1399. (6) (a) Benazzi, E.; Hirschauer, A.; Joly, J.-F.; Olivier, H.; Berhard, J.-Y. (Institute Francais du Petrole). Paraffins alkylation catalyst. Eur. Pat. EP 0 553 009, July 28, 1993. (b) Chauvin, Y.; Olivier, H.; Mussmann, L.
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