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Synthesis and Characterization of CoSBA-1 Cubic Mesoporous Molecular Sieves A. Vinu,†,‡ J. Deˇdecˇek,§ V. Murugesan,‡ and M. Hartmann*,† Department of Chemistry, Chemical Technology, University of Kaiserslautern, P.O. Box 3049, D-67653, Kaiserslautern, Germany, Department of Chemistry, Anna University, Chennai 600 025, India, and Heyrovsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, CZ-182 23 Prague 8, Czech Republic Received February 25, 2002 Revised Manuscript Received April 26, 2002 Introduction The discovery of ordered materials with pore sizes in the mesopore range by researchers of the Mobil Oil company1,2 has initiated an intensive research effort resulting in more than 2000 publications. Among these materials, cubic phases, such as MCM-48 and SBA-1, have received only little attention compared to hexagonal structures, viz. MCM-41 and SBA-15. Huo et al. reported the synthesis of SBA-1 under strongly acidic conditions employing a so-called S+X-I+ mechanism.3-5 SBA-1 is a cubic phase, which possesses a threedimensional cage-type structure with pore openings of ≈1.8-2 nm. Pure silica mesoporous materials possess a neutral framework, which limits their application in catalysis and adsorption. To obtain materials with potential for catalytic applications, it is necessary to modify the nature of the amorphous walls by incorporation of heteroelements. However, because of the strongly acidic synthesis conditions, the preparation of SBA-1 containing heteroatoms in the amorphous walls is difficult. Recently, the incorporation of V, Ti, and Mo into SBA-1 mesoporous materials has been reported.6-9 Cobalt-substituted molecular sieves are of tremendous interest in catalysis because these materials contain redox-active sites. Recently, Thomas et al.10 reported the * To whom correspondence should be addressed. Phone: +49-631205-3559. Fax: +49-631-205-4193. E-mail:
[email protected]. † University of Kaiserslautern. ‡ Anna University. § Academy of Sciences of the Czech Republic. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. T.; Chen, C. T.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (3) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schu¨th, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176. (4) Huo, Q.; Leon, R.; Petroff, P. M.; Stucky, G. D. Science 1995, 268, 1324. (5) Huo, Q.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 1147. (6) Dai, L.-X.; Tabata, K.; Suzuki, E.; Tatsumi, T. Chem. Mater. 2001, 13, 208. (7) Dai, L.-X.; Teng, Y.-H.; Tabata, K.; Suzuki, E.; Tatsumi, T. Chem. Lett. 2000, 794. (8) Dai, L.-X.; Tabata, K.; Suzuki, E.; Tatsumi, T. Microporous Mesoporous Mater. 2001, 44-45, 573. (9) Che, S.; Sakamoto, Y.; Yoshotake, H.; Terasaki, O.; Tatsumi, T. J. Phys. Chem. B 2001, 105, 10565.
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terminal oxidation of n-pentane, n-hexane, and n-octane by air yielding alcohols, aldehydes, and the corresponding acids using a cobalt-containing molecular sieve with AEI structure. In the present study, the synthesis of the mesoporous molecular sieve CoSBA-1 with different Co content is reported. UV-vis-NIR spectroscopy shows that the cobalt species are present in a highly dispersed state and have a trigonal pyramidal coordination. Cobalt-containing SBA-1 was synthesized under acidic conditions using cetyltriethylammonium bromide (CTEABr) as the surfactant, tetraethyl orthosilicate (TEOS) as the silica source, and Co(II)acetate tetrahydrate as the cobalt source. A typical synthesis procedure for CoSBA-1 was as follows: Solution A was prepared by adding 0.81 g of CTEABr to an aqueous solution of HCl. The obtained solution was cooled at 0 °C and homogenized for 30 min. TEOS and Co(CH3COO)2 were precooled to 0 °C and then added to solution A under vigorous stirring. Stirring was continued for another 5 h at 0 °C. Thereafter, the reaction mixture was heated to 100 °C for 1 h. The solid product was recovered by filtration and dried in an oven at 100 °C overnight. The molar composition of the gel was 1:0.1-0.5:0.2:27.556:350-700 TEOS:CoO:CTEABr:HCl:H2O. The assynthesized material was then calcined in air by raising the temperature from 20 to 550 °C with a heating rate of 1.8 K/min and keeping the sample at the final temperature for 10 h. The powder X-ray diffraction patterns of the calcined materials were collected on a SIEMENS D5005 diffractometer using Cu KR (λ ) 0.154 nm) radiation. Nitrogen adsorption isotherms of the calcined samples were recorded at 77 K on a Quantochrome Autosorb 1 sorption analyzer. The samples were outgassed for 3 h at 250 °C under vacuum (p < 10-5 mbar) prior to the adsorption experiments. Diffuse reflectance (DR) UV-vis-NIR spectra were collected using a Perkin-Elmer Lambda 19 UV-vis-NIR spectrometer equipped with a diffuse reflectance integrating sphere coated with BaSO4, which also served as a standard. NO was adsorbed on the evacuated samples at ambient temperature for 15 min at the pressure of 132 hPa. Thereafter, samples were evacuated at ambient temperature for 1 min and transferred into the UVvis spectrometer. Figure 1 exhibits the XRD patterns of SBA-1 and various CoSBA-1 materials prepared under different synthesis conditions. Table 1 gives the preparation conditions and textural properties of the resulting cobalt containing cubic mesoporous molecular sieves in comparison to the data of the parent material SBA-1. As displayed in Figure 1, the calcined samples exhibit XRD patterns characteristic of the cubic phase SBA-1, which can be indexed in the space group pm3 h n. The data of CoSBA-1 match well with the data reported in the literature,3,5 showing that the structural integrity is retained after Co insertion. It is noteworthy that CoSBA-1 materials can be prepared from starting gels (10) Thomas, J. M.; Raja, R.; Sankar, G.; Bell, R. G. Nature 1999, 398, 22
10.1021/cm025544+ CCC: $22.00 © 2002 American Chemical Society Published on Web 05/15/2002
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Figure 3. UV-vis spectra of rehydrated calcined samples: (s) CoSBA-1(A), (- - -) CoSBA-1(B), (‚‚‚) CoSBA-1(C), (-‚-‚-) CoSBA-1(D), and (-‚‚-‚‚-) CoSBA-1(E).
Figure 1. Powder XRD patterns for the various calcined samples: (a) Pure siliceous SBA-1, (b) CoSBA-1(A), (c) CoSBA1(B), (d) CoSBA-1(C), (e) CoSBA-1(D), and (f) CoSBA-1(E). Table 1. Synthesis Conditions and Textural Properties of SBA-1 and CoSBA-1 with Different Co Concentrations as Determined by Atomic Absorption Spectroscopy
sample CoSBA-1(A)a CoSBA-1(B)b CoSBA-1(C)c CoSBA-1(D)d CoSBA-1(E)e SBA-1f
Co content (wt %) 0.15 0.30 0.80 0.22 0.42
a0 (nm)
ABET (m2/g)
pore volume (cm3/g)
dp,HK (nm)
dp,BJH (nm)
7.56 7.74 7.55 7.52 7.41 7.77
1290 1280 940 1120 1020 1430
0.64 0.62 0.42 0.49 0.44 0.68
1.8 1.8 1.8 1.8 1.8 1.8
1.9 1.9 1.8 1.8 1.8 2.0
a 1/0.2/28/350/0.1 TEOS/CTEABr/HCl/H O/Co(CH COO) . b 1/ 2 3 2 0.2/28/350/0.3 TEOS/ CTEABr/HCl/H2O/Co(CH3COO)2. c 1/0.2/28/ d 350/0.5 TEOS/CTEABr/HCl/H2O/ Co(CH3COO)2. 1/0.2/56/700/0.2 TEOS/CTEABr/HCl/H2O/Co(CH3COO)2. e 1/0.2/56/700/0.5 TEOS/ CTEABr/HCl/H2O/Co(CH3COO)2. f 1/0.2/56/700 TEOS/CTEABr/ HCl/H2O.
Figure 2. Nitrogen adsorption isotherms at 77 K of various calcined samples.
with nSi/nCo ratios as low as 5 without significant loss in structural order. The amount of cobalt in the samples, however, is significantly lower (typically below 1 wt %, cf. Table 1). The nitrogen adsorption isotherms are of type IV of the IUPAC classification (Figure 2). The specific surface areas, specific pore volumes, and pore diameters calculated from the nitrogen adsorption experiments are summarized in Table 1. The specific pore volumes of the CoSBA-1 materials range from 0.45 to 0.68 cm3/g and the pore size is determined to be ≈1.8 nm using the Horvath-Kawazoe (HK) method with the Saito-Foley modification for cylindrical pores. The specific surface area decreases from 1430 m2/g for SBA-1
to 940 m2/g for the CoSBA-1(C), which is the sample with the highest cobalt content. The specific pore volume decrease with increasing metal content of the sample. A reduction of the HCl/TEOS and HCl/CTEABr molar ratios in the starting gel by one-half results in samples with a higher degree of structural ordering and a higher amount of incorporated cobalt. The lower amount of incorporated cobalt in the samples D and E (synthesized under more acidic conditions) compared to other CoSBA-1 samples is tentatively attributed to higher solubility of the Co species in the concentrated aqueous solution of HCl. Similar findings have also been observed by Dai et al. for vanadium-substituted SBA-1 molecular sieves.6 Figure 3 exhibits normalized UV-vis spectra of rehydrated and calcined CoSBA-1 materials. All samples exhibit absorption bands in three regions of the UVvis spectra. The complex band between 14500 and 22000 cm-1 reflects the well-known d-d transitions of Co(II) ions.11 Bands above 35 000 cm-1 are typically assigned to either charge transfer (CT) bands, reflecting the interaction of Co(II) ions with the molecular sieve matrix, or own absorption of the molecular sieve.12-14 The broad band around 27 000 cm-1 could correspond to CT bands of Co(III) ions. However, the intensity of this band does not change significantly with cobalt loading in the matrix, which indicates that this band is not connected with the presence of Co(III) ions in the SBA-1 matrix, but should be attributed to defect sites in the amorphous walls. Similar absorption bands were also observed for cobalt-free AlMCM-41 and the zeolites MCM-22 and beta.15 Moreover, the extinction coefficients of CT bands are significantly higher compared to those of d-d transitions (two or more magnitudes), and, hence, assuming that the band around 27 000 cm-1 corresponds to the Co(III) ions, these ions represent only a negligible part of the Co ions present in CoSBA-1 samples. We therefore conclude that exclusively Co(II) ions are present in calcined CoSBA-1 as evident from their d-d transitions. In this region, four bands with maxima at 15300, 17000, 19200, and 20000 cm-1 are (11) Lever, A. B. P. Inorganic Electronic Spectroscopy; Elsevier: Amsterdam, 1984. (12) Kaucky´, D.; Deˇdecˇek, J.; Wichterlova´, B. Microporous Mesoporous Mater. 1999, 31, 75. (13) Deˇdecˇek, J.; Kaucky´, D.; Wichterlova´, B. Microporous Mesoporous Mater. 2000, 35-36, 483 (14) Deˇdecˇek, J.; Wichterlova´, B. J. Phys. Chem. B 1999, 103, 1462. (15) Deˇdecˇek, J., unpublished results.
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Figure 4. Effect of various treatments and NO adsorption on the visible spectra of calcined CoSBA-1(C). (a) Rehydrated (s), (b) dried in flowing oxygen at 350 °C (- - -), evacuated at 470 °C (‚‚‚), and (d) NO adsorbed at room temperature on the evacuated sample (‚-‚-‚).
observed for the rehydrated sample (Figure 4). The effect of various treatments and subsequent NO adsorption on the visible spectra of sample CoSBA-1(C) is also shown in Figure 4. The band around 20000 cm-1 disappears after dehydration of the sample in flowing oxygen and subsequent evacuation at high temperature (470 °C). Therefore, the Co(II) ions in dehydrated samples are characterized by a triplet in the visible region. Bands at 14700, 17300, and 19500 cm-1 are observed for the sample dried in oxygen and bands at 14600, 17300, and 19900 cm-1 are found for the evacuated sample. Band positions were estimated by deconvolution of the spectra using lines of Gaussian shape and by analysis of the second derivative mode of the spectra. Thus, only one type of Co(II) ions is present in dehydrated CoSBA-1 and its coordination depends on the treatment conditions. The triplet ascribed to Co(II) ions is only slightly different in hydrated and dehydrated CoSBA-1. It was found that extraframework Co(II) ions located in ion-exchange sites in hydrated zeolites and mesoporous molecular sieves exhibit octahedral coordination. In contrast, Co(II) ions in SBA-1 are predominately in nonoctahedral coordination and presumably occupy framework (referring to the amorphous walls in mesoporous molecular sieves) positions. Triplets between 15000 and 19000 cm-1 are typically assigned to tetrahedrally coordinated Co(II) ions, viz. in the framework of cobalt-containing aluminophos-
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phates and zeolites.16 For these species, a similar triplet is also observed in the NIR region. However, this triplet is missing in the NIR spectra of our CoSBA-1 samples (not shown). Similar spectra (a triplet in the visible region and no triplet in the NIR region) were reported by Sˇ poner et al.17 for the aluminophosphates CoAPO-5 and CoAPO-11. They explained their findings by creation of oxygen vacancies in the coordination sphere of the cobalt sites, which results in the formation of trigonal pyramidal cobalt centers. By analogy, we tentatively assume that the cobalt ions in CoSBA-1 are not tetrahedrally coordinated but possess a trigonal pyramidal symmetry. The Co(II) ions characterized by a single band at 20000 cm-1 in hydrated CoSBA-1 are tentatively assigned to framework Co(II) ions, which resume octahedral symmetry by coordinating with additional water molecules. After dehydration, the water molecules are lost and the coordination of theses cobalt ions changes to the same coordination that is observed for the majority of Co(II) ions. Adsorption of NO (Figure 4) is followed by dramatic changes in the visible spectrum of CoSBA-1. The bands characteristic of dehydrated samples have disappeared and new broad bands at 14900, 17400, and 20600 cm-1 are found. This indicates that Co(II) ions in CoSBA-1 are located on the SBA-1 channel surface and, hence, are accessible for guest molecules. Therefore, the present study has demonstrated that the incorporation of cobalt(II) ions into the amorphous walls of SBA-1 can be controlled by adjusting the hydrochloric acid concentration. UV-vis DRS spectra revealed that the majority of the cobalt ions in the CoSBA-1 samples occupy the framework position in the surface layer of the CoSBA-1 channel walls. Further extension of this work, including adsorption and catalytic studies, is currently underway. Acknowledgment. Financial support of this work by Deutsche Forschungsgemeinschaft, Fonds der Chemischen Industrie, and Volkswagen-Stiftung is gratefully acknowledged. CM025544+ (16) Uytterhoeven, M. G.; Schoonheydt, R. A. Microporous Mater. 1994, 3, 265. (17) Sˇ poner, J.; C ˇ ejka, J.; Deˇdecˇek, J.; Wichterlova´, B. Microporous Mesoporous Mater. 2000, 37, 117.
