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Synthesis and Catalytic Activity of Cryptomelane-Type Manganese Dioxide Nanomaterials Produced by a Novel Solvent-Free Method Yun-shuang Ding,† Xiong-fei Shen,† Shanthakumar Sithambaram,‡ Sinue Gomez,† Ranjit Kumar,‡ Vincent Mark B. Crisostomo,‡ Steven L. Suib,*,‡ and Mark Aindow†,§ Institute of Materials Science and Department of Materials Science and Engineering, UniVersity of Connecticut, Unit 3136, Storrs, Connecticut 06269-3136, and Department of Chemistry, UniVersity of Connecticut, Unit 3060, Storrs, Connecticut 06269-3060 ReceiVed June 15, 2005. ReVised Manuscript ReceiVed July 18, 2005
Cryptomelane-type K-OMS-2 nanomaterials with high surface area (156 m2/g) have been synthesized via a low-temperature solvent-free method in a very short time (1 h). Field emission scanning electron microscopy and high-resolution transmission electron microscopy images reveal that these materials have nanorod morphologies with average diameters of about 10 nm and lengths of about 50 nm. These are different from the long fiberous morphologies of OMS-2 materials made by conventional reflux or hydrothermal methods. X-ray diffraction and Brunauer-Emmett-Teller studies indicate that these materials have small crystallite sizes (∼9.8 nm) and that they are mesoporous with a uniform pore size distribution centered at 12 nm. These K-OMS-2 materials show improved catalytic activity for the oxidation of alcohols compared with the conventional K-OMS-2 materials, which may be due to their higher surface areas and novel surface properties. This fast, inexpensive, and environmentally friendly solvent-free method has the potential of being used in scaled-up syntheses of K-OMS-2 and other transition-metal-ion-substituted manganese oxide nanomaterials.
1. Introduction One-dimensional (1D) transition-metal oxide nanomaterials (such as TiO2, VOx, MnOx, and ZnO2) have attracted much attention in recent years both for fundamental research and for practical applications in the electronic, optoelectronic, magnetic, and catalytic fields owing to their novel physicochemical properties, such as multiple d orbital electrons, quantum size effects, and space-confined transport phenomena, which depend on their particle sizes and shapes.1-4 Manganese dioxide forms one of the largest groups of transition-metal oxides due to the variety of different polymorphic crystallographic structures. Tunnel structure manganese oxide octahedral molecular sieve (OMS) materials, such as 3 × 3 tunnel (referred to as OMS-1), 2 × 2 tunnel (OMS-2), 2 × 4 tunnel (OMS-5), and 2 × 3 tunnel (OMS-6) structures, are porous materials which have been investigated extensively as low-cost and environmentally benign ionic and molecular sieves and efficient redox catalysts.5-9 These OMS materials have 1D * To whom correspondence should be addressed. E-mail: steven.suib@ uconn.edu. † Institute of Materials Science. ‡ Department of Chemistry. § Department of Materials Science and Engineering.
(1) (2) (3) (4) (5)
Martin, C. R. Science 1994, 266, 1961. Wang, X.; Li, Y. J. Am. Chem. Soc. 2002, 124, 2880. Ma, R., Bando, Y.; Sasaki, T. J. Phys. Chem. B 2004, 108, 2115. Sun Y.; Xia, Y. AdV. Mater. 2004, 16, 264. Shen, Y. F.; Zerger, R. P.; DeGuzman, R. N.; Suib, S. L.; McCurdy, L.;Potter, D. I.; O’Young, C. L. Science 1993, 260, 511. (6) DeGuzman, R. N.; Shen, Y. F.; Neth, E. J.; Suib, S. L.; O’Young, C. L.; Levine, S.; Newman, J. M. Chem. Mater. 1994, 6, 815. (7) Chen, X.; Shen, Y. F.; Suib, S. L.; O’Young, C. L. J. Catal. 2001, 197, 292.
Figure 1. Representation of the crystal structure for K-OMS-2.
open framework structures with different tunnel sizes, which are built up by edge-shared and corner-shared [MnO6] octahedral units leading to different pore sizes.10-12 Cryptomelane (OMS-2) is a form of manganese dioxide having a tunnel size of 0.46 nm × 0.46 nm constructed from edge-shared double [MnO6] octahedral chains, which are corner-connected to form 1D tunnels as illustrated in Figure 1 (viewed along the c axis). Cryptomelane has a mixed-valent manganese framework due to the coexistence of Mn(IV) and a minor amount of Mn(III).13,14 In the naturally occurring (8) Xia, G.-G.; Tong, W.; Tolentino, E. N.; Duan, N.-G.; Brock, S. L.; Wang, J.-Y.; Suib, S. L.; Ressler, T. Chem. Mater. 2001, 13, 1585. (9) Shen, X. F.; Ding Y. S.; Liu, J.; Laubernds, K.; Zerger, R. P.; Polverejan, M.; Son, Y. C.; Aindow, M.; Suib, S. L. Chem. Mater. 2004, 16, 5327. (10) Golden, D. C.; Chen, C. C.; Dixon, J. B. Science 1986, 231, 717. (11) Rziha, T.; Gies, H.; Rius, J. Eur. J. Mineral. 1996, 8, 675. (12) Shen, X. F.; Ding Y. S.; Liu, J.; Cai, J.; Laubernds, K.; Zerger, R. P.; Vasiliev, A.; Aindow, M.; Suib, S. L. AdV. Mater. 2005, 17, 805. (13) Post, J. E.; Burnham, C. W. Am. Mineral. 1986, 71, 1178. (14) Feng, Q.; Kanoh, H.; Miyai, Y.; Ooi, K. Chem. Mater. 1995, 7, 148.
10.1021/cm051294w CCC: $30.25 © 2005 American Chemical Society Published on Web 09/03/2005
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mineral cryptomelane, potassium or barium ions and small amounts of water are present in the tunnels.13 The potassium ions and water molecules can be ion-exchanged with other inorganic cations with appropriate sizes, such as Cu2+, Co2+, and Ni2+. The metal ions in the tunnel not only balance the charge of the mixed Mn2+, Mn3+, and Mn4+, but can also be active sites for selective catalysis. The special open tunnel structure and the mixed valences of manganese in porous OMS-2 lead to its unique physicochemical properties, such as the ability to undergo ion exchange, and reversible multielectron redox transformations. These properties make OMS-2 a promising material for heterogeneous redox catalysts, chemical sensors, and battery materials.15 Therefore, the syntheses of OMS-2 nanomaterials with high purities, small particle sizes, high surface areas, and high yields have been attractive goals for materials scientists in recent years. Giovanili first synthesized OMS-2 via a layered structure manganese oxide precursor (K-birnessite) followed by hydrothermal treatments to transform the layered structure to the tunnel structure at 120-250 °C for 2 days.16,17 Another route for preparing OMS-2 is to reflux Mn2+ with KMnO4 or H2O2 solutions in nitric acid medium at 100-120 °C for 1-2 days.18-21 OMS-2 has also been synthesized by hydrothermal treatment of Mn2+ with different kinds of oxidants, such as (NH4)2S2O8 and K2S2O8.22-28 Recently, sol-gel routes have been developed by Ching et al. to prepare OMS-2 materials via reactions between KMnO4 and organic reducing agents such as fumaric acid ((E)-butenedioic acid).29 The crystallographic forms and purity of the final products mainly depend on the concentration ratios of the reagents. Some other routes also have been explored to synthesize OMS-2, including microwave heating30 and high-temperature solid-state reactions.31 Both the reported hydrothermal and reflux routes need solvents in the reaction system and usually require long reaction times (>24 h). The crystallographic forms of the final OMS-2 products depend strongly on the pH, temperature, and stoichiometric
ratios of the reactants. Not only are the structure and purity of the products sensitive to the reaction conditions, but also the quantities of products prepared using hydrothermal methods are limited because of reactor size limitations. For example, an about 0.5 g yield is obtained from a 23 mL lined autoclave. The high reaction temperatures required (600-1000 °C) and complicated process control are limiting factors for conventional solid-state synthetic routes. This usually results in low surface areas (as low as 10 m2/g) or mixed phases.32,33 In summary, the synthesis methods for OMS-2 nanomaterials reported to date usually require relatively long reaction times (1-2 days) and complicated control of reaction conditions. Materials prepared by these methods also have relatively low surface areas (∼10-90 m2/g). Therefore, the development of rapid and simple synthetic approaches for large-scale production of OMS-2 nanomaterials with high surface area, high purity, and uniform shape and size is still the main challenge for commercialization of these functional materials. Recently, nanosized sulfated zirconia with Brønsted acidic sites has been synthesized by a solvent-free method followed by calcination.34 Here we report a fast one-step method to synthesize nanorods of the pure K-OMS-2 phase with very high surface areas (∼160 m2/g) from solvent-free reaction between potassium permanganate and manganese acetate (Mn(Ac)2‚4H2O) at a temperature as low as 80 °C within 1 h. This synthetic method not only eliminates the use of the solvent but also significantly reduces the reaction time (from 24 h). The solvent-free process, short synthesis time, low cost, and high yield make this approach a highly attractive route for production of OMS-2 porous nanomaterials on an industrial scale. The catalytic properties of the synthesized OMS-2 materials have also been evaluated via oxidation of alcohols. The K-OMS-2 nanorods synthesized by this new solvent-free method show higher catalytic activities than K-OMS-2 materials prepared by the conventional reflux method.
(15) Suib, S. L. Annu. ReV. Mater. Sci. 1996, 26, 135. (16) Giovanoli, R.; Balmer, B. Chimia 1981, 35, 53. (17) Golden, D. C.; Dixon, J. B.; Chen, C. C. Clays Clay Miner. 1986, 34, 511. (18) Strobel, P.; Charenton, J. C. ReV. Chim. Miner. 1986, 23, 125. (19) DeGuzman, R. N.; Shen, Y. F.; Neth, E. J.; Suib, S. L.; O’Young, C. L.; Levine, S.; Newsam, J. M. Chem. Mater. 1994, 6, 815. (20) Chen, X.; Shen, Y. F.; Suib. S. L.; O’Young, C. L. Chem. Mater. 2002, 14, 940. (21) Villegas, J. C.; Garces, L. J.; Gomez, S.; Durand, J. P.; Suib, S. L. Chem. Mater. 2005, 7, 1910. (22) Hypolito, R.; Valarelli, J. V.; Giovanoli, R.; Netto, S. M. Chimia 1984, 38, 427. (23) Golden, D. C.; Chen, C. C.; Dixon, J. B. Clays Clay Miner. 1987, 35, 271. (24) Cai, L.; Liu, J.; Willis, W. S.; Suib, S. L. Chem. Mater. 2001, 13, 2413. (25) Wang, X.; Li, Y. Chem. Commun. 2002, 764. (26) Wang, X.; Li, Y. Chem.sEur. J. 2003, 9, 300. (27) Prieto, O.; Del Arco, M.; Rives, V. J. Mater. Sci. 2003, 38, 2815. (28) Liu, J.; Makwana, V.; Cai, J.; Suib, S. L.; Aindow, M. J. Phys. Chem. B 2003, 107, 9185. (29) Ching, S.; Roark, J. L. Chem. Mater. 1997, 9, 750. (30) Zhang, Q.; Luo, J.; Vileno, E.; Suib, S. L. Chem. Mater. 1997, 9, 2090. (31) Kim, S. H.; Kim, S. J.; Oh, S. M. Chem. Mater. 1999, 11, 557.
2.1. Synthesis. Preparation of K-OMS-2. In a typical experiment, 9.48 g (0.06 mol) of KMnO4 and 22.05 g (0.09 mol) of Mn(Ac)2‚ 4H2O (stoichiometric ratio KMnO4:Mn(Ac)2‚4H2O ) 2:3) powders were mixed and ground homogeneously in a mortar. The mixed powders were then placed in a capped glass bottle and maintained at 80 °C for 4 h. The resulting black product was thoroughly washed with deionized water several times to remove any ions which may remain in the product, and finally dried at 80 °C in air overnight. To investigate the effects of the reaction temperature on the crystal phase of the final products, the temperature was varied from 40 to 150 °C. Phase evolution with reaction time was also studied by fixing the reaction temperature at 80 °C and increasing the synthesis time from 30 min to 3 h. All other procedures followed the method described above.
2. Experimental Section
(32) Liu, J.; Son, Y. C.; Cai, J.; Shen, X. F.; Suib, S. L.; Aindow M. Chem. Mater. 2004, 16, 276. (33) Longo, J. M.; Horowitz, H. Z.; Clavenna, L. R. AdV. Chem. Ser. 1980, 186, 139. (34) Sun, Y.; Ma, S.; Du, Y.; Yuan, L.; Wang, S.; Yang, J.; Deng, F.; Xiao, F.-S. J. Phys. Chem. B 2005, 109, 2567.
5384 Chem. Mater., Vol. 17, No. 21, 2005 For comparison, K-OMS-2 was also prepared by the conventional reflux method in HNO3 medium at 120 °C for 24 h (stoichiometric ratio KMnO4:Mn2+ ) 2:3), which was denoted as K-OMS-2R. Detailed information about sample preparation can be found in the literature.19 2.2. Characterization. Structure and Morphology. The structure and phase purity of the prepared materials were analyzed by X-ray diffraction (XRD) using a Scintag PDS 2000 diffractometer with a Cu KR X-ray source (λ ) 1.54 Å) in a step scan mode with a scanning rate of 0.02 deg s-1 in the 2θ range from 5° to 85°. The crystal structure of the materials was further investigated by highresolution transmission electron microscopy (HRTEM) using a JEOL 2010 FasTEM instrument with an accelerating voltage of 200 kV. TEM sample preparation was performed by dispersing the sample in 1-butanol, dropping the dilute solution onto a holeycarbon-film-coated 300-mesh copper grid, and evaporating the butanol. The structure evolution of the potassium cryptomelane with reaction temperatures was investigated. XRD data were collected using the same step scan mode. The morphologies of the materials were characterized using a Zeiss DSM 982 Gemini field emission scanning electron microscopy (FESEM) instrument with a Schottky emitter. Powder samples were dispersed in acetone and dropped onto a gold-coated silicon wafer, and then the wafer was mounted onto a stainless steel sample holder using silver conductive paint. Chemical Composition. The chemical composition of the materials was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) after the samples were dissolved in a solution of 37 wt % HCl. The average oxidation state (AOS) of the manganese was obtained using a potentiometric titration method that has been described in detail elsewhere.35 Thermal Stability. The thermal stability of the materials was studied by performing thermogravimetric analyses (TGA) and temperature-programmed desorption with mass spectroscopic detection (TPD-MS). TGA was carried out on a Hi-Res TGA 2950 thermogravimetric analyzer with a 60 mL/min N2 flow from 25 to 700 °C at a heating rate of 20 °C/min. TPD analysis data were collected in an apparatus constructed specifically for such measurements and equipped with an MSS-RGA mass spectroscopy detector (MKS instrument). About 50 mg of sample was loaded into a quartz tube, degassed in a 30 mL/min He flow overnight, and then heated under a 30 mL/min He flow at a heating rate of 3 °C/min from 25 to 725 °C. Surface Area and Porosity. A Micromeritics ASAP 2010 instrument was used to measure the surface areas and pore size distributions of the materials. Samples were pre-degassed at 150 °C for about 10 h to remove water and other physically adsorbed species. The N2 isothermal adsorption and desorption experiments were performed at relative pressures (P/P0) from 10-3 to 0.995 and from 0.995 to 0.01, respectively. Lewis Acidity and Basicity. NH3 and CO2 chemisorptions were also conducted using a Micromeritics ASAP 2010 instrument (chemisorption attachment) to quantitatively measure the amount of strong Lewis acidic and basic sites, respectively, in the materials. About 1.0 g of the fresh material was first evacuated at 150 °C for 2 h to remove the adsorbed species from the surface to reveal acidic and basic sites followed by adsorption measurements at 35 °C at gas pressures from 100 to 700 mmHg. After the first adsorption measurement, the material was evacuated at the analysis temperature (35 °C) for 1 h, and then a second adsorption was measured. The first measurement analyzes the total amount of gas adsorption resulting from physisorption, weak chemisorption, and strong (35) Brock, S. L.; Sanabria, M.; Suib, S. L.; Urban, V.; Thiyagarajan, P.; Potter, D. I. J. Phys. Chem. B 1999, 103, 7416.
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Figure 2. X-ray diffraction patterns of (a) K-OMS-2 materials prepared by the solvent-free method and (b) K-OMS-2 materials prepared by the reflux method. (c) Cryptomelane standard pattern.
chemisorption. The second measurement analyzes the amount of adsorption resulting from physisorption and weak chemisorption. Therefore, the difference in the adsorption between the two measurements is the amount of strong chemisorption. 2.3. Catalytic Application. OMS-2 materials have already shown good catalytic activity in the case of alcohol oxidations.36 In this work, the synthetic K-OMS-2S nanorods were tested as catalysts for oxidation of three kinds of alcohols. The activities of K-OMS2S nanorods were compared with those of K-OMS-2 materials prepared using the reflux method. Chemicals used in this reaction are 2-thiophenemethanol, furfuryl alcohol, and cyclohexanol, which were purchased from Sigma-Aldrich and were used without any further purification. Oxidation of alcohols was carried out using toluene as the solvent. In a typical experiment, the reaction mixture consisted of 5.0 mmol of substrate (alcohol), 10 mL of toluene, and 50 mg of catalyst. The mixture was refluxed using a water condenser in a 50 mL round-bottom flask at 110 °C in air for 4 h. The reaction mixture was filtered to separate the catalyst and products. The organic substrates were then analyzed with gas chromatography/ mass spectrometry (GC/MS). Two injections were done for each analysis, and the average is reported. A blank reaction was performed by using the above-mentioned conditions except that the catalyst was omitted. In the catalytic oxidation reaction of the cyclohexanol, the reaction mixture was refluxed in air for 20 h instead of 4 h. Other conditions were kept the same.
3. Results 3.1. Characterization. Structure and Morphology. All reflections in the XRD patterns of the prepared materials, as shown in Figure 2a, are in good agreement with the standard pattern of the pure tetragonal cryptomelane-Q phase [space group I4/m (No. 87)] with lattice parameters a ) b ) 0.981 nm and c ) 0.285 nm (Figure 2c, JCPDS 29-1020). No impurity phases were observed. Comparing the XRD pattern, Figure 2b, of K-OMS-2R made by the conventional reflux with that of K-OMS-2S made by the solvent-free method (Figure 2a), diffraction peak broadening effects in (36) Son, Y. C.; Makwana, V. D.; Howell, A. R.; Suib, S. L. Angew. Chem., Int. Ed. 2001, 40, 4280.
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Table 1. Crystallite Sizes and BET Surface Areas of the OMS-2 Materials sample
crystallite size (nm)
BET surface area (m2/g)
micropore vola (cm3/g)
mesopore vola (cm3/g)
macropore vola (cm3/g)
total pore volb (cm3/g)
average pore diametera (nm)
K-OMS-2S K-OMS-2R K-OMS-2c
9.8 18 -
156 91 11
0.01 0.001 -
0.48 0.29 -
0.03 0.17 -
0.52 0.46 -
12.4 20.3 -
a Calculated by the BJH method. b Single point of the total pore volume at P/P ) 0.995. c Liu, J.; Son, Y. C.; Cai, J.; Shen, X. F.; Suib, S. L.; Aindow 0 M. Chem. Mater. 2004, 16, 276. A dash indicates data were not available.
novel solvent-free method are much smaller than those of the K-OMS-2 materials prepared by aqueous routes.
Figure 3. FESEM images of K-OMS-2 materials prepared by (a) the solvent-free method and (b) the reflux reaction.
Figure 4 is a series of typical HRTEM images obtained from the K-OMS-2S materials. These nanorods exhibited well-defined lattice fringes. Examples of the three main types of lattice fringes observed in such images are shown in Figure 4b-d. The spacings of these lattice fringes are 0.68, 0.30, and 0.48 nm, corresponding to those of the {110}, {310}, and {200} planes of the cryptomelane structure, respectively. These are three of the most widely spaced {hk0} planes with finite structure factors in the cryptomelane structure. As such, the HRTEM observations are consistent with the nanorods having a major axis of [001] and a circular cross-section such that rods lying on the planar carbon support film will be oriented with a random azimuthal 〈uV0〉 direction parallel to the electron beam direction. The chemical composition of the K-OMS-2S materials was determined using ICP-OES. The molar ratio of potassium to manganese is about 0.14. The AOS of Mn in the manganese oxide is 3.73. The composition is K0.147Mn1.08O2‚0.06 H2O as determined by ICP-OES combined with AOS results.
Figure 4. HRTEM images of the K-OMS-2S nanorods prepared by the solvent-free method at 80 °C for 4 h.
the latter one are more extensive, implying that K-OMS-2S has smaller crystallite sizes than K-OMS-2R. Thus, the crystallite sizes of K-OMS-2R and K-OMS-2S calculated from the (211) diffraction lines using Scherrer’s equation are 18 and 9.8 nm, respectively. The K-OMS-2S materials prepared by the low-temperature solvent-free method are all nanoparticles as shown in FESEM images (Figure 3a). HRTEM studies revealed the details of the morphologies for these nanoparticles, which adopt a very fine, uniform nanorod shape. The range of diameters of these nanorods is about 7-10 nm, and the range of lengths is about 20-80 nm (Figure 4). No impurity phases were observed in the high-magnification TEM pictures. The morphologies are different from the conventional fibrous morphology of K-OMS-2R made by the reflux method reported in the literature.19-21,28 Nanofibers synthesized via the reflux method are usually a few tens of nanometers in diameter, and the lengths of these nanofibers vary from a few hundred nanometers to several micrometers as shown in Figure 3b. The dimensions of the K-OMS-2 nanorods prepared by this
Surface Area and Porosity. The analysis results of the surface area, average pore size, and pore volume of K-OMS2R and K-OMS-2S are summarized in Table 1. As shown in Table 1, the Brunauer-Emmett-Teller (BET) surface area of K-OMS-2S is 156 m2/g, which is much higher than that of K-OMS-2R (91 m2/g). The crystallite sizes and average pore diameters of K-OMS-2S are 9.8 and 12.4 nm, respectively, which are much smaller than those of K-OMS-2R (18 and 20.3 nm, respectively). The smaller average pore size may explain why the surface area of K-OMS-2S is much larger than that of K-OMS-2R. K-OMS-2 materials prepared by the solvent-free method show isotherms of N2 adsorption/desorption similar to those for K-OMS-2R. A typical type II adsorption isotherm (hysteresis loop) for K-OMS-2S is shown in Figure 5. The inset of Figure 5 shows a mesopore size distribution calculated by the Barrett-Joyner-Halenda (BJH) method. The BJH plot shows a narrow pore size distribution in the range of 2-30 nm for the K-OMS-2S materials. Only a major pore size distribution peak centered at 12 nm is observed. A large total pore volume of 0.52 m3/g is also observed. The mesopore volume is 0.48 m3/g, which constitutes about 92% of the total pore volume of the samples. The macropore volume is about 0.03 m3/g. An average pore size of about 12.4 nm was calculated by the BJH method. A small amount of micropores, about 0.01 m3/g, was also observed in the samples. As determined by the t-plot method, 2% of the surface area is contributed by micropores. Therefore, the
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Figure 5. (A) N2 absorption/desorption isothermal plot of K-OMS-2S and (B) the pore size distribution plot by the BJH method.
contribution of the micropores to the total pore volume is small. Lewis Acidity and Basicity. Chemisorption studies of NH3 and CO2 were used to quantitively measure the strong acidic sites and basic sites of the materials. The isothermal adsorption plots are shown in Figure 6. The amounts of strong chemisorption for NH3 and CO2 per gram of the sample are 10.8 cm3 and 1.35 cm3, respectively, as obtained by extrapolating the irreversible adsorption (strong chemisorption) to zero pressure. Here, it was assumed that each NH3 and CO2 molecule only adsorbs on one strong active site. Therefore, the calculated concentrations of strong acidic sites and basic sites in the materials are 0.48 and 0.06 mmol/g of sample, respectively. Thermal Stability. The results of TGA analyses of K-OMS2S and K-OMS-2R are shown in Figure 7. From TGA profiles, K-OMS-2S shows thermal behavior similar to that of K-OMS-2R except for an about 2% difference in weight loss. K-OMS-2S showed three major weight losses between 25 and 750 °C. The first weight loss (about 4%) is before 250 °C owing to desorption of physisorbed H2O (Figure 7). The release of chemically adsorbed water may cause the slow weight loss (∼2%) between 250 and 500 °C. The second significant weight loss (about 6%) occurs between 500 and
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640 °C, which could be due to the elution of lattice oxygen species of the materials as confirmed by the first oxygen peak in TPD-MS data (Figure 8B). This indicates that the tunnel structure of the K-OMS-2 is not stable any more. When the temperature is over 640 °C, the product starts to lose lattice oxygen from the framework structure and decomposes to the Mn3O4 phase (XRD data are not shown here). TPD profiles of H2O and O2 elution from the K-OMS2S samples at elevated temperatures are shown in Figure 8. One major H2O desorption peak is observed from 50 to 200 °C. Therefore, the TPD measurements indicate that the weight loss at 120 °C in the TGA profile is mainly due to the loss of H2O. Then two major O2 elution peaks were detected at about 550-720 °C. K-OMS-2S starts to lose structural oxygen from the framework of the species at about 640 °C and transforms into the Mn3O4 spinel structure at about 720 °C. The TPD results are in good agreement with TGA results. The thermal stability of K-OMS-2S is about 550 °C in an inert atmosphere. Temperature Effects. Since the reaction temperature is an important factor that can affect the growth of the crystal and phases of the final products, in this work the reaction temperatures were varied from 40 to 150 °C to investigate temperature effects on the preparation and phase transitions of cryptomelane when the reaction times were fixed at 4 h. The XRD patterns (Figure 9) show that a poorly ordered manganese oxide phase was obtained at 40 °C. Two main peaks at 2θ of about 37.7° and 65.9° are due to diffraction from the MnO6 octahedral units, but the arrangements of these MnO6 building blocks are random. At 60 °C, a cryptomelane phase is present. When the temperature was increased to 150 °C, the cryptomelane pattern disappeared and another phase was formed which was identified as hausmannite (Mn3O4, JCPDS 24-734). Time Effects. XRD was also used to study the formation processes of cryptomelane by characterizing the samples prepared at different reaction times. Figure 10 shows XRD patterns of the samples synthesized at different reaction times by the solvent-free method. The cryptomelane phase was formed even within 30 min. Since the particles formed in such a short reaction time may be very small, the peak
Figure 6. Measurements of Lewis acidic and basic sites in the K-OMS-2S materials by (A) NH3 chemisorption and (B) CO2 chemisorption, respectively.
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Chem. Mater., Vol. 17, No. 21, 2005 5387 Table 2. Oxidation of Alcohols Catalyzed by K-OMS-2S and K-OMS-2R substrate 2-thiophenemethanol
product
catalysta
2-formylthiophene
K-OMS-2R K-OMS-2S blank furfuryl alcohol furfuraldehyde K-OMS-2R K-OMS-2S blank cyclohexanol cyclohexanone K-OMS-2R K-OMS-2S K-OMS-2Sc
conversion selectivity (%) (%) 40b 72 0 10 31 0 5 15 20
100b 100 0 100 100 0 100 100 100
a The amount of the catalyst used is 50 mg. b Data from Son, Y. C.; Makwana, V. D.; Howell, A. R.; Suib, S. L. Angew. Chem.,Int. Ed. 2001, 40, 4280. c The amount of the catalyst used is 100 mg.
Figure 7. TGA measurement for K-OMS-2S in a N2 atmosphere from 25 to 750 °C.
broadening effect in the XRD pattern is significant. A clear XRD pattern of the pure cryptomelane phase was observed when the reaction time was increased to 1 h. The relative intensities of the diffraction peaks did not change significantly when the time was prolonged from 1 to 4 h, which indicates that cryptomelane can be synthesized within a very short time, 1 h, compared with the 1-2 day reaction time for conventional reflux or hydrothermal methods. 3.2. Catalytic Activity. The oxidation of alcohols to carbonyl compounds is of great interest to the fine chemical industry and academia. In this work, oxidation reactions were carried out to examine the catalytic activities of the K-OMS2S nanomaterials prepared by the solvent-free route. K-OMS2R synthesized by the reflux method was also used for these oxidation reactions. 2-Formylthiophene, furfuraldehyde, and cyclohexanone were produced from the catalytic oxidation of 2-thiophenemethanol, furfuryl alcohol, and cyclohexanol, respectively. The conversion and selectivity results are listed in Table 2. For the oxidation of 2-thiophenemethanone, a 40% conversion was obtained when K-OMS-2R was used as the catalyst. The selectivity toward 2-formylthiophene was 100%. Improved conversion was obtained when K-OMS-2S nanorods synthesized by the solvent-free method were used. The
conversion was increased to 72%, and the selectivity for 2-formylthiophene was retained (100%). In the blank reaction where the catalyst was omitted, no 2-formylthiophene was detected in the final product. The use of K-OMS-2S also enhanced the conversion from 10% to 31% in the oxidation of furfuryl alcohol. For the oxidation of cyclohexanol to cyclohexanone, K-OMS-2R shows very poor conversion (5%) although the selectivity is high (100%). K-OMS-2S nanomaterials caused the conversion to increase to 15% while still maintaining excellent selectivity (100%). When the amount of the catalyst was doubled, the conversion increased to 20%. 4. Discussion K-OMS-2 materials were prepared by a solvent-free method. Unlike soft-aqueous-phase routes that have fast atom diffusion rates, in this procedure, the starting materials, KMnO4 and Mn(Ac)2‚4H2O powders, must be processed by grinding thoroughly, so as to get well-mixed components. As the reaction proceeds, Mn2+ is oxidized by potassium permanganate to get higher valence Mn species. The mixed components changed color from purple to black within 30 min at 80 °C, which means that this redox reaction occurs in a very short time. The products collected at this time do not show the cryptomelane phase but the MnO6 octahedral building blocks with random arrangements (see Figure 10). Increasing the reaction time to 1 h, the poorly ordered MnO6 octahedral building blocks transform to the stable cryptomelane phase with an ordered tunnel structure (4.6 Å × 4.6 Å) as indicated by the XRD patterns in Figure 10.
Figure 8. TPD-MS profiles of (A) H2O and (B) O2 elution from K-OMS-2S from 25 to 750 °C.
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Figure 9. X-ray diffraction patterns of K-OMS-2S materials prepared at different temperatures for 4 h by the solvent-free reaction.
Figure 10. X-ray diffraction patterns of K-OMS-2S materials prepared in different reaction times at 80 °C.
Potassium cations in the reactant play an important role for the formation of the tunnel structure. When KMnO4 was replaced by other oxidizing agents, such as NaMnO4 and Mg(MnO4)2, the cryptomelane structure OMS-2 was not obtained but instead γ-MnO2 was formed, which has 1 × 1 and 1 × 2 intergrowth tunnel structures. The XRD patterns of manganese oxides synthesized with different oxidizing agents are not shown here. This means that the cryptomelane structure forms only in the presence of K+. A possible explanation for this is that hydrated K cations have a size similar to the tunnel structure size of cryptomelane (about 0.46 nm × 0.46 nm). They could therefore be acting as templates and stabilizing the structure. However, the sizes of Na+ (0.095 nm) and Mg2+ (0.065 nm) cations are too small to maintain the tunnel structure. The template effects of K+ for the cryptomelane structure have also been reported previously by Feng et al.37 Our studies indicate that the stoichiometric ratio between potassium permanganate and manganese chloride exerts a relatively small influence on the final phase of the synthesized products. When the KMnO4:Mn(Ac)2‚4H2O molar ratio was decreased from 2:3 to 1:3 or increased from 2:3 to 4:3, (37) Feng, Q.; Kanoh, H.; Ooi, K. J. Mater. Chem. 1999, 9, 319.
Ding et al.
K-OMS-2 was obtained in all cases but with a little variety in the intensities of the diffraction peaks. The XRD patterns are shown in Figure S1 in the Supporting Information. The results show that, unlike reflux, hydrothermal, or sol-gel methods, the stoichiometric ratio of the reactants is not a critical factor to obtain K-OMS-2 in this solventless synthesis method. 4.1. Morphology and Structure. The K-OMS-2 materials synthesized by this solvent-free method show a homogeneous nanorod morphology. These nanorods exhibit smaller dimensions (about 50 nm in average length and about 10 nm in diameter) than conventional K-OMS-2 materials, which usually exhibit a fibrous morphology with fibers a few micrometers long, diameters of tens of nanometers, and aspect ratios in the range of about 10:1 to 100:1. However, the nanorods have lower aspect ratios varying from 2:1 to 10:1. The diameters of these nanorods are about 7-10 nm, which are in good agreement with the particle size values (9.8 nm) calculated from the Scherrer equation, and are much smaller than those of OMS-2 materials produced by conventional reflux methods. This is consistent with BET measurements, since smaller particle sizes will usually result in higher surface areas. The adsorption/desorption hysteresis confirmed that the K-OMS-2S materials made by this novel synthetic method are porous. From the data in Table 1, the pores in the materials are of three kinds: micropores (∼2%), mesopores (∼92%), and macropores (∼6%). Most of the pores in the materials are mesopores (2-30 nm), which contribute to the majority of the total pore volume. In contrast, for conventional K-OMS-2R, mesopores only account for 63% of the total pore volume, and the rest are macropores. In addition, K-OMS-2S materials prepared by this method show a uniform mesopore size at about 12 nm. Materials synthesized by the conventional methods, such as the reflux method, usually contain mesopores with many different pore sizes, and a significant proportion (∼37%) of these are macropores. 4.2. Catalytic Reactivity. The physical and chemical properties of the catalysts, such as surface areas, pore sizes, cation vacancies, and average oxidation states of the manganese, are closely related to the catalytic activities of OMS-2 materials.38 The catalytic activities of OMS-2 nanomaterials were evaluated via the oxidation of three kinds of alcohols. The conversions in the reaction were found to be related to the surface areas of the materials. The higher the surface area, the better the catalytic performance. In the catalytic reaction of 2-thiophenemethanol oxidation, the conversion of the substrate (2-thiophenemethanol) was increased from 40% to 72% when K-OMS-2S was used as the catalyst instead of K-OMS-2R. In the case of oxidation of furfuryl alcohol, K-OMS-2S shows 20% higher conversion than K-OMS-2R. For the reaction of cyclohexanol oxidation, conventional K-OMS-2 materials are not very active. However, high surface area K-OMS-2S nanomaterials prepared by the solvent-free method make the conversion increase from 5% to 15%. When the amount of catalyst was increased (38) Makwana, V. D.; Son, Y.-C.; Howell, A. R.; Suib, S. L. J. Catal. 2002, 210, 46.
Cryptomelane-Type Manganese Dioxide Nanomaterials
to 100 mg from 50 mg, the conversion was 20%. The K-OMS-2 materials show very selective properties and give 100% selectivities in all three kinds of oxidation reactions no matter how the materials were prepared (reflux or solventfree methods). Since both K-OMS-2R and K-OMS-2S have the same 2 × 2 tunnel structures, they should have similar selectivity toward the substrates on the basis of shape selectivity of the tunnel structure.5,39 This hypothesis is supported by the results of all three catalytic reactions. The selectivities toward the final products are the same using K-OMS-2R and K-OMS-2S. The increase in the conversions by using K-OMS-2S may be due to the surface properties of the materials. Surface area is one of the most important factors to affect activities. The results are as expected in that the catalytic performances of K-OMS-2 materials are related to the surface areas of the materials. K-OMS-2 nanorods with higher surface areas have higher catalytic activity than the conventional K-OMS-2 materials because they have more active sites available for reaction. As indicated by the results of the pore size and particle size analyses, K-OMS-2S materials have smaller pores (∼12 nm) than K-OMS-2R (∼20 nm). In addition, K-OMS-2S nanorods are short and have low aspect ratios (2:1 to 10:1). In contrast, the fibrous K-OMS-2R materials have high aspect ratios (10:1 to 100:1). Since the tunnels of these nanorods are parallel to the [001] direction, which is the growth direction of the rods, there are more accessible tunnel sites for reaction in K-OMS-2S materials than in the long fibrous K-OMS-2R materials. This may explain why K-OMS-2S has better catalytic performance (conversion) than K-OMS-2R. Since the reaction was carried out at a relatively low temperature (80 °C), the fine particles formed during the reactions did not easily aggregate to form coarse crystallites as generally happens in high-temperature solid-state reactions. This is why the OMS-2 materials formed have uniform nanoparticle sizes. When the reaction temperature increases, (39) Thomas, J. M.; Raja, P.; Sankar, G.; Bell, R. G. Nature 1999, 398, 227.
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small crystallites aggregate to become coarse particles, and the BET surface areas of the materials decrease. With softaqueous-phase reactions, the crystalline seeds have free space to grow anisotropically along certain directions, so onedimensional long ordered crystallites can be formed. 5. Conclusion Pure-phase cryptomelane was synthesized successfully by a novel solvent-free reaction. Reactions were carried out at temperatures as low as 80 °C within a very short time. This reaction time (1 h) is significantly lower than the conventional 24 h reaction time. The as-prepared K-OMS-2 nanorods have higher surface area (∼160 m2/g) compared with those that were synthesized by conventional reflux, hydrothermal, or high-temperature solid-state reactions. The materials have uniform nanorod morphologies with average diameters of about 10 nm and lengths of about 50 nm. These K-OMS-2 materials are mesoporous and show uniform pore size distributions. The thermal stabilities of synthetic KOMS-2 nanomaterials are as high as 550 °C. These KOMS-2 materials show improved catalytic activity for the oxidation of alcohols compared with the conventional K-OMS-2 materials, which may be due to their higher surface areas and novel surface properties. In addition, no solvent is needed in this synthetic method. Therefore, the solventfree method is inexpensive, less polluting, and environmentally friendly. This rapid, low-temperature one-step solventfree method has the potential of being used in scaled-up syntheses of K-OMS-2 and other transition-metal-ionsubstituted manganese oxide nanomaterials. Acknowledgment. We thank the Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy, for financial support. We thank Dr. Francis S. Galasso and Dr. Yong-Chan Son for useful discussions. Supporting Information Available: XRD patterns of K-OMS-2 materials prepared with different reactant ratios (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM051294W
