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Novel Coassembly Route to Cu-SiO2 MCM-41-like Mesoporous Materials Xuefeng Guo, Min Lai, Yan Kong, Weiping Ding,* and Qijie Yan* Lab of Mesoscopic Chemistry, Department of Chemistry, Nanjing University, Nanjing, 210093, China
C. T. Peter Au Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China Received June 23, 2003. In Final Form: December 14, 2003 A series of mesostructured Cu-SiO2 composites have been synthesized with sodium metasilicate (Na2SiO3) and cuprammonia nitrate (Cu(NH3)4(NO3)2) respectively used as Si and Cu sources. The synthetic procedures were conducted at room temperature, and cetyltrimethylammonia bromide was used as a template. Under our experimental conditions, ordered mesoporous Cu-SiO2 composites could be obtained with a copper content up to 16.8 wt %. Average pore diameters (2.80-3.15 nm), wall thickness (1.30-2.20 nm), and specific surface area (1020-690 m2/g) are found to vary linearly with copper content (0-16.8 wt %). Results of thermal gravimetry-differential thermal analysis reveal the collapse temperature of the order structure starts at ∼1250 K for mesoporous Cu-SiO2 with 16.8 wt % copper content. As indicated by the outcomes of inductively coupled plasma and X-ray photoelectron spectroscopy studies, copper is mainly incorporated inside the pore wall rather than embedded on the wall surface. Copper species strongly interact with silica, and calcination at high temperatures cannot cause phase separation between silica and copper oxide. Cu status in mesoporous Cu-SiO2 composites is similar to that in copper silicate in neighboring structures. Based on the results, a S+I-I+I- mechanism is proposed in which copper entities are surrounded by silicon species during synthesis of the mesostructured composite.
Introduction Since the discovery of the M41S family of silica-based mesoporous materials in 1992,1,2 increasing attention has been paid to the novel materials,3-6 mainly due to their great applications as catalysts, absorbents, and host materials based on their large internal surface area and narrow pore size distributions. Among the family, MCM41 has been extensively studied for its uniform hexagonal mesopores. The large pore size, varying from 20 to 100 Å, of the materials facilitates the flow of reactant and product molecules; in this case, the MCM-41 materials have been investigated as shape-selective catalysts for the production of bulky organic molecules. According to the results of previous studies,4 pure silica MCM-41 showed very limited catalytic activities. Hence, it is necessary to incorporate metal in the silicate framework to create active sites for catalytic interaction; metals such as Al, Ti, Cr, V, Sn, Co, Pd, Fe, Ga, Zr, and Mn have been respectively incorporated successfully in the mesoporous structures.1,7-15 It has been demonstrated (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710-712. (2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.; Olsen, D. H.; Sheppard, E. W.; Mccullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834-10843. (3) Huo, Q.; Margolese, D.; Ciesla, U.; Feng, P.; Gier, T. E.; Sierger, P.; Leao, R.; Petroff, P. M.; Schuth, F.; Stucky, G. D. Nature 1994, 368, 317-321. (4) Corma, A. Chem. Rev. 1997, 97, 2373-2419. (5) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56-77. (6) Soler-lllia, G. J. A. A.; Sanchez, C.; Lebeau, B.; Patarin, J. Chem. Rev. 2002, 102, 4093-4138. (7) Reddy, K. M.; Moudrakovski, I.; Sayari, A. J. Chem. Soc., Chem. Commun. 1994, 9, 1059-1060. (8) Abdel-Fattah, T. M.; Pinnavaia, T. J. Chem. Commun. 1996, 5, 665-666.
that transition metals can be incorporated in MCM-41 while retaining the uniform pore size of the mesoporous material. Copper is a metal known to show redox properties, and ions of copper have been introduced to mesoporous silica by means of ion exchange, impregnation, and grafting.16-19 It has been reported that Cu-containing mesoporous silica can be synthesized directly by mixing copper complexes with a suitable organofunctional silicon alkoxide.20,21 But it is difficult to introduce the divalent metal into the framework of tetravalent silicon without destroying the mesoporous structures, and indeed, the amount of copper that could be retained in a mesoporous silica framework is usually small. Wang et al. reported that a copper amount of above 3.02 wt % would result in the collapse of the ordered mesoporous framework of MCM-41.22 Karakassides et al.21 have reported a synthesis of mesoporous Cu-SiO2 composite with Si(OCH3)4 and (9) Tuel, A.; Gontier, S.; Teissier, R. Chem. Commun. 1996, 5, 651652. (10) Reddy, M. K.; Wei, B.; Song, C. Catal. Today 1998, 43, 261-272. (11) Rhee, C. H.; Lee J. S. Catal. Today 1997, 38, 213-219. (12) Kosslick, H.; Lischke, G.; Landmesser, H.; Parlitz, B.; Storek, W.; Fricke, R. J. Catal. 1998, 176, 102-114. (13) Yonemitsu, M.; Tanaka, Y.; Iwamoto, M. J. Catal. 1998, 178, 207-213. (14) Zhang, W.; Pinnavaia, T. J. Catal. Lett. 1996, 38, 261-265. (15) Jentys, A.; Pham, N. H.; Vinek, H.; Englisch, M.; Lercher, J. A. Microporous Mater. 1996, 6, 13-17. (16) Poppl, A.; Hartmann, M.; Kevan, L. J. Phys. Chem. 1995, 99, 17251-17258. (17) Xu, J.; Yu, J.; Lee, S. J.; Kim, B. Y.; Kevan, L. J. Phys. Chem. B 2000, 104, 1307-1314. (18) Luca, V.; Maclachlan, D. J.; Bramley, R. R.; Morgan, K. J. Phys. Chem. 1996, 100, 1793-1800. (19) Paul, P. P.; Heimrich, M. J.; Martin, J.; Michael, A. Catal. Today 1998, 42, 61-71. (20) Karakassides, M. A.; Fournaris, K. G.; Travlos, A.; Petridis, D. Adv. Mater. 1998, 10, 483-486. (21) Karakassides, M. A.; Bourlinos, A.; Petridis, D.; Coche-Guere`nte, L.; Labbe`, P. J. Mater. Chem. 2000, 10, 403-408.
10.1021/la0351055 CCC: $27.50 © 2004 American Chemical Society Published on Web 02/26/2004
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{[(CH3O)3Si(CH2)3NHCH2CH2NH2]2}Cu respectively used as Si and Cu sources, and they found that a Cu content higher than 5 wt % results in the collapse of the ordered mesoporous structure of MCM-41. In this paper, we report that ordered mesoporous MCM-41 materials with a copper amount up to 16.8 wt % can be synthesized via a novel coassembly route involving sodium metasilicate and cuprammonia at room temperature with cetyltrimethylammonium as a template. Experimental Section Synthesis. In a typical synthesis procedure, 1.85 g of cetyltrimethylammonium bromide (C16TMABr) and 5.7 g of Na2SiO3‚9H2O were dissolved in 50 cm3 of distilled water followed by cooling to 273 K in an ice-water bath. Then 10 cm3 of cuprammonia solution composed of a suitable amount of Cu(NO3)2 and 25% aqueous ammonia was added at 273 K, resulting in a dark blue solution. With constant stirring, diluted sulfuric acid (1 mol L-1) was added to regulate the pH to a value of 9.0. After further stirring for 3 h, a blue solid was separated by centrifugation. The products, designated as 5Cu-MCM, 10Cu-MCM, and 20Cu-MCM (the figures 5, 10, and 20 indicate the nominal content of wt % copper in the product), were washed repeatedly with distilled water and ethanol for a number of times and were then dried at room temperature. For characterization of copper status in the Cu-SiO2 mesoporous composites, one sample composed of aerogel silica impregnated with 10 wt % Cu was prepared for comparison and designated as 10Cu-IP gel in the following text. The as-synthesized samples were heated to 823 K at a rate of 1 K min-1 in a dry air stream and were calcined at that temperature for 5 h. Thus obtained, the products were pale blue Cu-SiO2 MCM-41 materials. Characterization. The X-ray diffraction (XRD) patterns of the samples were recorded using a Shimadzu XD-3A diffractometer operating with Cu KR radiation and a Ni filter. The N2 sorption isotherms and Brunauer-Emmett-Teller (BET) surface area of the Cu-incorporated MCM-41 materials were measured at 77 K using a Micromeritics ASAP 2000 sorption analyzer. The pore size distribution was calculated using the Barrett-Joyner-Halenda (BJH) model. All the samples were degassed at 573 K under a vacuum before analysis. Transmission electron micrographs of the as-synthesized materials were recorded by a JEM-2010 UHRTEM (ultrahighresolution transmission electron microscope) with an acceleration voltage of 200 kV. Samples for transmission electron microscopy (TEM) were prepared by dipping a Cu TEM grid, coated with holey carbon film, into a colloidal suspension of particles dispersed in ethanol, which were then air-dried and stored in a vacuum chamber. The chemical compositions of the samples were measured by using a Jarrell-Ash 1100 inductively coupled plasma (ICP) spectrometer. The samples were completely dissolved with suitable acid before analysis. Thermal gravimetry (TG) and differential thermal analysis (DTA) curves were recorded on a TA Instrument 2100 at a heating rate of 5 K min-1 in N2 streams. The X-ray photoelectron spectroscopy (XPS) analyses were conducted on an ESCALAB MK-II spectrometer equipped with a Mg KR X-ray source. The carbon 1s peak at 284.6 eV was used as the reference for binding energies. The powdered samples were mounted onto the sample holder and were degassed overnight at room temperature and pressures of the order of 10-8 mbar before analysis.
Results and Discussion X-ray diffraction patterns of as-synthesized and calcined samples are shown in Figures 1 and 2, respectively. An intense diffraction (100) peak at about 2θ ) 2° and additional (110), (200), and (210) peaks are obviously observed for 5Cu-MCM, 10Cu-MCM, and 20Cu-MCM samples, indicating a MCM-41 structure. No ordered (22) Wang, L.; Velu, S.; Tomura, S.; Ohashi, F.; Suzuki, K.; Okazaki, M.; Osaki, T.; Maeda, M. J. Mater. Sci. 2002, 37, 801-806.
Figure 1. XRD patterns of the as-synthesized Cu-MCM materials: (a) MCM-41, (b) 5Cu-MCM, (c) 10Cu-MCM, and (d) 20Cu-MCM.
Figure 2. XRD patterns of the Cu-MCM materials (calcined at 823 K): (a) MCM-41 (b) 5Cu-MCM, (c) 10Cu-MCM, and (d) 20Cu-MCM. Table 1. Chemical Composition (ICP) and Structural Properties of Calcined MCM Materials
sample
Cu content (wt %)
surface area (BET, m2/g)
pore size (BJH, nm)
a0 (nm) (a0 ) 2d100/x3)
wall thickness (nm)
MCM-41 5Cu-MCM 10Cu-MCM 20Cu-MCM
0 4.81 7.93 16.8
1172 1020 850 691
2.70 2.78 2.86 3.15
3.92 4.08 4.64 5.37
1.22 1.30 1.78 2.22
mesoporous materials can be obtained with a nominal Cu content exceeding 20 wt % in our synthesis, for any lowangle peaks cannot be observed in XRD patterns. After calcination at 823 K, the (100), (110), and (200) peaks, and thus the ordered mesoporous structures, are still reserved for all three Cu-containing samples (Figure 2). For the pure silica sample, however, the (110), (200), and (210) peaks vanish upon heating at the same temperature, although the (100) diffraction still is intense, suggesting the calcined pure silica sample is not so ordered compared to the current Cu-containing silica mesoporous material. It appears, under our experimental conditions, that the samples with copper incorporation are more regular to some extent than the pure silica sample. The copper contents verified by ICP analysis are 4.81, 7.93, and 16.8 wt % for 5Cu-MCM, 10Cu-MCM, and 20Cu-MCM, respectively, although the nominal amounts of copper added were 5, 10, and 20 wt % (Table 1). Table 1 also includes data of specific surface areas (BET), average pore diameters, lattice parameters (a0 ) 2d100/x3), and
Coassembly Route to Mesoporous Materials
Figure 3. Variation of specific surface area, pore size, and wall thickness with Cu content.
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Figure 5. TEM image of calcined 20Cu-MCM.
Figure 6. TG-DTA curves of 20Cu-MCM material. Table 2. Results of ICP and XPS Characterization
Figure 4. Nitrogen sorption isotherms for calcined 20CuMCM. The inset shows the pore diameter distribution determined by the BJH method.
wall thickness of the samples. All the samples possess quite large surface areas. The 5Cu-MCM exhibits a surface area of 1020 m2/g, much larger than that reported in refs 20 and 21, in which the mesoporous Cu-SiO2 with 4.7 wt % Cu content gives 500-600 m2/g when heated at similar temperatures and the higher content of Cu leads to collapse of the ordered mesostructures. With an increase in copper content from 0 to 16.8 wt % in the current synthesis, the surface area of the Cu-SiO2 materials decreases from 1172 to 691 m2/g; basically a linear relationship with copper content can be addressed. The average pore size and wall thickness also linearly vary with the copper content, as shown in Figure 3. In other words, by adopting our synthetic method, one can adjust porous properties of these materials by regulating the copper content. The type-IV isotherms of N2 sorption of the samples show a characteristic step at around P/P0 ) 0.40 of capillary condensation of N2 molecules in uniform mesopores, as depicted in Figure 4. The narrow and sharp pore size distribution curve, indicative of uniform mesoporosity for 20Cu-SiO2, is also shown in Figure 4 (inset). Figure 5 shows the TEM image of 20Cu-MCM, which reveals the well-ordered mesoporous structure possessed by the sample. As calculated from the TEM picture, the pore diameter is around 3.2 nm, in good agreement with the value of 3.15 nm given by N2 adsorption measurement (Table 1). The typical TG-DTA results of as-synthesized 20CuMCM are shown in Figure 6. The TG curve showed three
sample 5Cu-MCM 10Cu-MCM 20Cu-MCM 10Cu-IP gel
ICP Cu/Si (atom)
XPS Cu/Si (atom)
1/20.9 1/13.1 1/5.0
1/46.8 1/30.7 1/4.2
binding energy (eV) Si2p Cu2p5/2 O1s 103.2 103.3 103.2 103.4
935.8 935.7 935.8 933.4
532.5 532.5 532.5 532.7
major weight loss steps before 1473 K. The first step at temperatures lower than 450 K is due to the loss of absorbed water. The second step appearing between 450 and 750 K can be assigned to decomposition and release of surfactant in N2 flow. The final small step higher than 750 K should be the further polymerization of a few remaining Si-OH clinging to the framework. The total weight loss is about 57%. The DTA curve exhibits a large endothermic process between room temperature and 1250 K, associated with desorption of water, decomposition of surfactant, and the further polymerization of silanol. The signal peak starting at ∼1250 K along with no weight loss might indicate the collapse of the ordered framework, which shows the high thermal stability possessed by the Cu-MCM samples. The results of surface (XPS) and bulk (ICP) compositions of the Cu-containing samples are listed in Table 2. For the samples 5Cu-MCM and 10Cu-MCM, the surface contents of copper are much lower than the bulk one. This may suggest the Cu species are buried in the framework of the channel walls. The further XPS analysis performed on sample 10Cu-MCM with etching by an Ar+ beam for varied periods of time shows that the Cu/Si ratio changed from 1/30.7 (0 min) to 1/27.1 (10 min) to 1/18.6 (25 min), which supports the speculation that the copper species was buried by silica in the pore wall. 20Cu-MCM is an
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Figure 7. High-angle XRD patterns of 5Cu-MCM, 10CuMCM, 20Cu-MCM, and 10Cu-IP gel calcined at 823 K for 5 h.
exception, which shows a bit higher surface Cu/Si ratio to the bulk. Generally, it may be proposed that the mesostructured Cu-SiO2 composite precipitated in a sandwichlike form, that is, the copper species is in fact surrounded by silicon species. The binding energy data of Si2p, Cu2p5/2, and O1s are also listed in the table. The binding energies of Si2p and O1s are similar for all three Cu-MCM and Cu-IP gel samples and can be reasonably attributed to Si-O species. The Cu2p binding energies of the three Cu-MCM samples are much higher than that of Cu-IP gel, in which the Cu species can be considered as CuO.23 The much higher binding energies of Cu2p in Cu-MCM samples than that of CuO reveals the Cu atoms are surrounded by silicon in Cu-MCM samples, most likely in the structure of Cu-O-Si; the higher electron affinity of silicon causes movement of electrons from copper to silicon and then the increase in binding energy of Cu2p. This implies that the neighboring structure of Cu in Cu-MCM samples is similar to that in copper silicate. Figure 7 depicts the high-angle X-ray diffraction patterns of 5Cu-SiO2, 10Cu-SiO2, 20Cu-SiO2, and 10Cu-IP gel after calcination at 823 K for 5 h. No distinguishable peaks referring to copper oxide for Cu-MCM samples are visible, indicating the phase separation between CuO and SiO2 did not happen upon heat treatment. Comparably, sharp diffraction peaks corresponding to CuO for the 10Cu-IP gel are clearly observed. This reveals the strong interaction between copper and silica, in agreement with the result of XPS. The surrounding (23) Espinos, J. P.; Morales, J.; Barranco, A.; Caballero, A.; Holgado, J. P.; Gonzalez-Elipe, A. R. J. Phys. Chem. B 2002, 106, 6921-6929.
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structure of Cu in Cu-MCM is similar to that of copper silicate, although the phase of copper silicate is not detected by XRD. A formation mechanism, named the S+I-I+I- ion pair mechanism, where S+, I-, I+, and I- represent surfactant cations (CTA+), SiO32-, Cu(NH3)42+, and SiO32-, respectively, would be suggested for the current synthesis of mesoporous Cu-SiO2 composites. After the S+I-I+I- ion pairs formed in basic solution, adding sulfuric acid lowered the pH of the mixture and made the inorganic species polymerize and precipitate, followed by the formation of mesostructured materials. Cu2+ ions polymerize much more weakly than Si species; therefore, more Si species polymerization is needed to stabilize the resulting mesostructure. With the increase of Cu content within a certain range, the sandwich-type channel wall became thicker and thicker, just as the results listed in Table 1. And meanwhile, the increase of Cu species (Cu(NH3)42+, I+) weakened the electric interaction between surfactant and inorganic species (S+I-) in S+I-I+I- ion pairs and resulted in bigger pores (Table 1). Too much copper would destroy the ion pair assembly and then the mesostructures. In the process of calcination, part of the Cu ions might move to the surface from the interlayer of the sandwichtype channel wall, leading to a bit higher surface Cu content such as, for instance, 20Cu-MCM. We may describe the structure of current Cu-MCM samples as copper silicate like species buried in silica walls. The thicker walls lead to higher thermal stability than pure mesoporous silica. The mechanism for formation of the current Cu-MCM samples may be applicable for synthesis of other mesoporous composites of transition metals with silica, for instance, Zn-SiO2, Ni-SiO2, and so forth. Conclusions A novel synthetic way has been successfully developed for synthesis of mesoporous Cu-SiO2 composites with contents of copper up to 16.8 wt % at room temperature. The mesoporous Cu-SiO2 composites possess mesopores of 2.80-3.15 nm in diameter and high specific surface areas of 1000-691 m2/g corresponding to 5-16.8 wt % copper contents after calcination at 823 K. The mesostructured Cu-SiO2 materials would possess a sandwichlike channel wall with copper silicate like species buried between silicas and high thermal stability. Acknowledgment. The financial support of the National Natural Science Foundation of China (No. 20173026) and the Modern Analysis Center of Nanjing University is gratefully acknowledged. LA0351055
