Thin-Layered Sheets of VOHPO4Â·0.5H2O Prepared from VOPO4

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Thin-Layered Sheets of VOHPO4‚0.5H2O Prepared from VOPO4‚2H2O by Intercalation-Exfoliation-Reduction in Alcohol Naoki Yamamoto, Norihito Hiyoshi, and Toshio Okuhara* Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan Received March 26, 2002. Revised Manuscript Received July 2, 2002

Intercalation of alcohol into layers of VOPO4‚2H2O crystallites, exfoliation using alcohol (containing three to five carbon atoms) into delaminated sheets, and subsequent reduction by reflux in alcohol into thin-layered VOHPO4‚0.5H2O were investigated using XRD, IR, SEM, determination of the absorption of alcohol, elemental analysis, and TG/DTA. Stepwise thermal treatment of an alcohol suspension of VOPO4‚2H2O crystallites, initially having shapes of large square platelets (length of about 20 µm and thickness of about 1 µm), resulted in the corresponding homogeneous solutions. XRD, IR, and SEM data were used to explain the intercalation of alcohol and exfoliation in forming the solutions containing the delaminated sheets. The exfoliated sheets were reconstructed as thin-layered VOPO4‚2H2O with small flakelike shapes by the removal of the alcohol. Refluxing the VOPO4‚2H2O solution of 1-propanol or 1-butanol yielded flower-petal-like crystallites (length of about 5 µm and thickness of about 0.1 µm) of the corresponding vanadyl alkyl phosphates VO[(OH)1-x(OR)x]PO3‚ (H2O)y(ROH)z (R ) alkyl group). In contrast, refluxing a solution of secondary alcohol such as 2-butanol yielded fragment-like crystallites (length of about 1 µm and thickness of about 0.1 µm) of the pure VOHPO4‚0.5H2O phase. These results demonstrated that the processes of intercalation, exfoliation, and reduction in alcohol are critical in the final microstructure of the vanadium(IV) phosphate compounds.

Introduction Intercalation has been widely used as a method for creating new reaction fields in interlayered spaces of inorganic layered materials and for providing novel electronic properties.1 Generally, the extended distances between the layered space depend greatly on the size of the guest molecules. An oxovanadium phosphate, VOPO4‚2H2O, has a layered structure, in which the VO6 octahedra and the PO4 tetrahedra form V-O-P sheets by edge-sharing.2 These sheets are combined by means of weak hydrogen bondings between the water molecules and the PO4 groups of the sheet. It is wellknown that VOPO4‚2H2O possesses a high ability to intercalate,3 and to date, intercalation compounds with amine,4-11 amide,12 alcohol,13-17 carboxylic acid,18 amino * To whom correspondence should be addressed. Phone/Fax: +8111-706-4513. E-mail: [email protected]. (1) Intercalation Chemistry; Whittingham, M. S., Jacobson, A. J., Eds.; Academic Press: New York, 1982. (2) Tietze, H. R. Aust. J. Chem. 1981, 34, 2035. (3) Ladwig, G. Z. Anorg. Allg. Chem. 1965, 338, 266. (4) Beneke, K.; Lagaly, G. Inorg. Chem. 1983, 22, 1503. (5) Benesˇ, L.; Hyklova´, R.; Kalousova´, J.; Votinsky´, J. Inorg. Chim. Acta 1990, 177, 71. (6) Nakajima, H.; Matsubayashi, G. Chem. Lett. 1993, 423. (7) De Stefanis, A.; Tomlinson, A. A. G. J. Mater. Chem. 1995, 5, 319. (8) Nakajima, H.; Matsubayashi, G. J. Mater. Chem. 1995, 5, 105. (9) De Stefanis, A.; Foglia, S.; Tomlinson, A. A. G. J. Mater. Chem. 1995, 5, 475. (10) Kinomura, N.; Toyama, T.; Kumada, N. Solid State Ionics 1995, 78, 281. (11) Yatabe, T.; Nakano, M.; Matsubayashi, G. J. Mater. Chem. 1998, 8, 699.

acid,19 pyridine,20 and poly(ethylene oxide)21 have been reported. Furthermore, intercalation compounds with some cationic species, such as Na+, have been obtained through a redox mechanism in the presence of a reductant.22-24 As an extension of the intercalation studies, exfoliation, which is the formation of delaminated sheets by unlimited swelling of the layered compound in solvent, of layered materials such as clays,25 zirconium phos(12) Lara, M. M.; Real, L. M.; Lopez, A. J.; Gamez, S. B.; Garcia, A. R. Mater. Res. Bull. 1986, 21, 13. (13) Benesˇ, L.; Votinsky´, J.; Kalousova´, J.; Klikorka, J. Inorg. Chim. Acta 1986, 114, 47. (14) Benesˇ, L.; Melańova´, K.; Zima, V.; Kalousova´, J.; Votinsky´, J. Inorg. Chem. 1997, 36, 2850. (15) Benesˇ, L.; Melańova´, K.; Trchova´, M.; E Å apkova´, P.; Kalousova´, J.; Votinsky´, J.; Zima, V. Eur. J. Inorg. Chem. 1999, 2289. (16) Benesˇ, L.; Zima, V.; Melańova´, K. J. Inclusion Phenom. 2001, 40, 131. (17) Benesˇ, L.; Zima, V.; Melańova´, K. Eur. J. Inorg. Chem. 2001, 1883. (18) Benesˇ, L.; Votinsky´, J.; Kalousova´, J.; Handlı´, K. Inorg. Chim. Acta 1990, 176, 255. (19) Zima, V.; Benesˇ, L.; Melańova´, K. Solid State Ionics 1998, 106, 285. (20) Johnson, J. W.; Jacobson, A. J.; Brody, J. F.; Rich, S. M. Inorg. Chem. 1982, 21, 3820. (21) Melańova´, K.; Benesˇ, L.; Zima, V.; Vahalova´, R. Chem. Mater. 1999, 11, 2173. (22) Jacobson, A. J.; Johnson, J. W.; Brody, J. F.; Scanlon, J. C.; Lewandowski, J. T. Inorg. Chem. 1985, 24, 1782. (23) Lara, M. M.; Lopez, A. J.; Real, L. M.; Bruque, S.; Casal, B.; Ruiz-Hitzky, E. Mater. Res. Bull. 1985, 20, 549. (24) Matsubayashi, G.; Ohta, S.; Okuno, S. Inorg. Chim. Acta 1991, 184, 47. (25) Kleinfeld, E. R.; Ferguson, G. S. Science 1994, 265, 370.

10.1021/cm020292y CCC: $22.00 © 2002 American Chemical Society Published on Web 08/20/2002

Thin-Layered Sheets of VOHPO4‚0.5H2O

phate,26 niobic acid,27 and titanic acid28 has been recently reported. Exfoliations of VOPO4‚2H2O through the intercalation of either 4-butylaniline in THF or acrylamide in alcohol were reported.29-32 However, following the removal of the solvents, it was shown that the recovered intercalation compounds possessed morphologies of the crystallites that were different from those of their starting precursers.30-32 Furthermore, attempts to intercalate amine directly into VOHPO4‚ 0.5H2O33-35 resulted in the destruction and collapse of the structure.31 Vanadyl pyrophosphate, (VO)2P2O7, is the active component of a commercial catalyst used for the selective oxidation of n-butane into maleic anhydride.36 However, the yield of maleic anhydride using this process is currently low, and therefore, development of new catalysts with improved performances is highly desirable. It is generally accepted that the selective plane of the crystallites of (VO)2P2O7 is (100), where the pair sites V4+(dO)sOsV4+ are well-arranged, but the side planes are nonselective.37,38 Because the defects of the plane also greatly influence the catalytic performance,37,39 it is reasonable to assume that the microstructures (shape, size, and thickness) of (VO)2P2O7 crystallites are key factors in controlling the selectivity. (VO)2P2O7 is usually prepared by the pyrolitic transformation of VOHPO4‚0.5H2O, in which the morphology of the precursor is maintained. There are many reports that describe the preparation methods of VOHPO4‚ 0.5H2O,37,40-45 including the use of VOPO4‚2H2O as a starting material. Johnson et al.42 demonstrated that VOHPO4‚0.5H2O can be obtained by the direct reduction of VOPO4‚2H2O using alcohol. Hutchings et al.43-45 reported that the morphology of the resulting VOHPO4‚ (26) Kelle, S. W.; Kim, H.-N.; Mallouk, T. E. J. Am. Chem. Soc. 1994, 116, 8817. (27) Abe, R.; Shinohara, K.; Tanaka, A.; Hara, M.; Kondo, J. N.; Domen, K. Chem. Mater. 1997, 9, 2179. (28) Sasaki, T.; Nakano, S.; Yamauchi, S.; Watanabe, M. Chem. Mater. 1997, 9, 602. (29) Hiyoshi, N.; Yamamoto, N.; Terao, N.; Nakato, T.; Okuhara, T. Stud. Surf. Sci. Catal. 2000, 130, 1715. (30) Nakato, T.; Furumi, Y.; Okuhara, T. Chem. Lett. 1998, 611. (31) Nakato, T.; Furumi, Y.; Terao, N.; Okuhara, T. J. Mater. Chem. 2000, 10, 737. (32) Yamamoto, N.; Okuhara, T.; Nakato, T. J. Mater. Chem. 2001, 11, 1858. (33) Guliants, V. V.; Benziger, J. B.; Sundaresan, S. Chem. Mater. 1994, 6, 353. (34) Alagna, L.; Prosperi, T.; Tomlinson, A. A. G. Mater. Res. Bull. 1987, 22, 691. (35) Guliants, V. V.; Benziger, J. B.; Sundaresan, S.; Wachs, I. E.; Jehng, J.-M. Chem. Mater. 1995, 7, 1493. (36) Centi, G.; Trifiro, F.; Ebner, J. R.; Franchetti, V. M. Chem. Rev. 1988, 88, 55. (37) Busca, G.; Cavani, F.; Centi, G.; Trifiro, F. J. Catal. 1986, 99, 400. (38) Okuhara, T.; Inumaru, K.; Misono, M. ACS Symp. Ser. 523 1993, Chapter 12, 156. (39) Satsuma, A.; Tanaka, Y.; Hattori, T.; Murakami, Y. Appl. Surf. Sci. 1997, 121/122, 496. (40) Johnson, J. W.; Johnson, D. C.; Jacobson, A. J.; Brody, J. F. J. Am. Chem. Soc. 1984, 106, 8123. (41) Shimoda, T.; Okuhara, T.; Misono, M. Bull. Chem. Soc. Jpn. 1985, 58, 2163. (42) Bordes, E.; Courtine, P.; Johnson, J. W. J. Solid State Chem. 1984, 55, 270. (43) Ellison, I. J.; Hutchings, G. J.; Sananes, M. T.; Volta, J. C. J. Chem. Soc., Chem. Commun. 1994, 1093. (44) Sananes, M. T.; Ellison, I. J.; Sajip, S.; Burrows, A.; Kiely, C. J.; Volta, J. C.; Hutchings, G. J. J. Chem. Soc., Faraday Trans. 1996, 92, 137. (45) Hutchings, G. J.; Sananes, M. T.; Sajip, S.; Kiely, C. J.; Burrows, A.; Ellison, I. J.; Volta, J. C. Catal. Today 1997, 33, 161.

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0.5H2O was dependent on the nature of the alcohol used in the reduction of VOPO4‚2H2O. Herein we describe the new method of synthesis of the precursors involving the intercalation of alcohol and exfoliation in alcohol, followed by reduction of VOPO4‚ 2H2O with alcohol. These chemical processes were systematically characterized using XRD, IR spectroscopy, SEM, and determination of the absorption of alcohol. Changes in the shape of the crystallites during the above processes were monitored by SEM. The size and length of the crystallites, the oxidative states of the V, and the surface areas of the precursors were measured. We have previously reported that this novel method is promising in the preparation of highly selective catalysts.46 Experimental Section Materials. V2O5 and H3PO4 (85%) were obtained from Wako Pure Chemical Industries Ltd. All alcohols were purchased from Wako, with the exception of 2-pentanol, which was purchased from Tokyo Kasei Co., and were used without further purification. Preparation of VOPO4‚2H2O. VOPO4‚2H2O was prepared according to the literature.2 A mixture of V2O5 (24 g), H3PO4 (85%, 133 cm3), and H2O (577 cm3) was refluxed at 388 K for 16 h. The resulting precipitate was collected by filtration, washed with acetone, then dried at room temperature, and confirmed as VOPO4‚2H2O2,47 by XRD and IR. SEM revealed that the obtained VOPO4‚2H2O consisted of square plate crystallites with a length of about 20 µm and thickness of about 1 µm. Preparation of VOHPO4‚0.5H2O. For reference purposes, a sample of the precursor VOHPO4‚0.5H2O was prepared from the direct reduction of VOPO4‚2H2O as follows.42 A suspension of powdered VOPO4‚2H2O (5 g) in 2-butanol (50 cm3) was stirred under reflux for 18 h. The resulting light-blue solid, which was identified as VOHPO4‚0.5H2O, was collected by filtration and washed with acetone. This standard precursor, VOHPO4‚0.5H2O, was denoted as P-4.48 As previously reported,48 P-4 possessed a platelet shape with a length of 3 µm and a thickness of 0.2 µm. Intercalation with Alcohol. A suspension of powdered VOPO4‚2H2O (1 g) in each alcohol (20 cm3), containing three to five carbon atoms, was stirred at room temperature for 24 h, and allowed to stand until segmentation of the solid was complete. The solid was collected by filtration under dry N2 conditions to avoid exposure to an ambient atmosphere. Exfoliation of VOPO4‚2H2O. A suspension of powdered VOPO4‚2H2O (0.5 g) in each alcohol (50 cm3) was placed in a flask (100 cm3) equipped with a condenser, and was heated stepwise with vigorous stirring. As summarized in Table 1, the temperature and time required to obtain a homogeneous alcoholic solution of exfoliated VOPO4 varied with the type of alcohol. Reduction of Exfoliated VOPO4 Sheets. The obtained homogeneous alcoholic solutions of exfoliated VOPO4 were further heated for 24 h to form light-blue precipitates under conditions that are summarized in Table 1. The resulting lightblue solid was collected by suction filtration, washed with acetone (100 cm3), and then dried overnight under ambient conditions. Characterization. Adsorption of gaseous alcohol into the layers of anhydrous VOPO4 was measured at 288 K (methanol) or at 298 K (ethanol, 1-propanol, and 2-propanol) using a static gas adsorption system (BELSORP SA18, BEL, Japan). The (46) Hiyoshi, N.; Yamamoto, N.; Okuhara, T. Chem. Lett. 2001, 484. (47) R’Kha, C.; Vandenbarre, M. T.; Livage, J. J. Solid State Chem. 1986, 63, 202. (48) Igarashi, H.; Tsuji, K.; Okuhara, T.; Misono, M. J. Phys. Chem. 1993, 97, 7065.

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Table 1. Reaction Conditions, Oxidation Numbers of Vanadium, Surface Areas, and Particle Sizes of the Precursors reaction conditions

properties

reduction of exfoliated VOPO4 alcohol

exfoliation temp (K)

stirring temp (K)

drying temp (K)

1-PrOH 2-PrOH 1-BuOH 2-BuOH i-BuOH 1-PeOH 2-PeOH

343 343 343 363 343 363 363

363 363 383 383 383 403 403

298 298 298 298 298 333 333

VPa P-4b a

exfoliated VOPO4

reduced compounds

particle size (µm)

SA n in Vn+ (m2‚g-1) length

particle size (µm) SA thickness n in Vn+ (m2‚g-1) length thickness

4.87 4.95 4.80 4.73 4.78 4.38 4.48

12 9 16 6 13 10 6

5 2 5 6 5 1 5

0.075 0.107 0.056 0.152 0.069 0.105 0.154

4.99

1

10

1.053

4.07 4.03 3.95 4.04 4.03 4.07 4.09

7 9 6 6 11 7 6

3.97

3

5 0.5 6 2 2 4 1

0.106 0.116 0.124 0.135 0.069 0.107 0.156

4

0.270

Starting material, VOPO4‚2H2O. VOHPO4‚0.5H2O obtained from the direct reduction with 2-butanol (see the text). b

adsorption amounts at each equilibrium pressure were estimated from the decrease in the pressure. Before the introduction of alcohol vapor, VOPO4‚2H2O was dehydrated by evacuation at 433 K to anhydrous VOPO4. Powder XRD patterns were recorded on a Rigaku MiniFlex diffractometer (Ni-filtered Cu KR radiation). In some cases, the samples were covered with Mylar films (Rigaku Co.) to avoid exposure to ambient moisture. IR spectra of the solids were measured as KBr disks using a Bio-rad FTS-7 spectrometer. SEM images were obtained using a Hitachi S-2100A scanning electron microscope. All microanalyses for the weight percent of carbon and hydrogen atoms were carried out at the Center for Instrumental Analysis, Hokkaido University. The contents of V and P were measured by an inductively coupled plasma atomic emission spectrometer (Shimadzu ICPS-8000), in which the sample powder was dissolved into hot H2SO4, and the solution was diluted with water to about 30 ppm V and P. The content of oxygen in the sample was calculated by subtracting the sum of the weights of V, P, C, and H. The surface area of the solid, which was pretreated at 433 K for 3 h under vacuum, was measured using a BET method of N2 adsorption at 77 K using an automatic adsorption system (BELSORP28SA, BEL, Japan). The average oxidation number of vanadium in the solid was determined using a redox titration method (KMnO4, FeSO4(NH4)2SO4‚6H2O, and (NH4)2S2O8).49

Results Formation of Intercalation Compounds of VOPO4‚nH2O with Alcohol. Figure 1 shows the XRD patterns of the intercalation compounds of VOPO4‚2H2O with alcohol as well as VOPO4‚2H2O itself. The intercalation compounds were obtained by mixing a drop of the primary alcohol and powdered VOPO4‚2H2O, and placed on the glass sample holder at room temperature. As shown in Figure 1c, the wet sample with 1-butanol exhibited a peak at 2θ ) 5.2°, which corresponds to 1.70 nm of the interlayer distance. In contrast, as shown in Figure 1d, isobutanol (2-methyl-1-propanol) afforded a compound showing a peak at 2θ ) 6.23°, which corresponds to 1.40 nm. Since VOPO4‚2H2O itself shows a peak at 2θ ) 11.88° (0.74 nm) as shown in Figure 1a, the compounds obtained from these alcohols possess interlayer distances that are greater than that of VOPO4‚2H2O. Similarly layered compounds (intercalation compounds) were obtained using methanol, ethanol, 1-pentanol, 1-hexanol, and 1-octanol (data not shown), (49) Hodnett, B. K.; Permanne, P.; Dolmen, B. Appl. Catal. 1983, 6, 231.

Figure 1. XRD patterns of (a) VOPO4‚2H2O, the intercalation compounds of VOPO4‚2H2O with (b) 1-propanol, (c) 1-butanol, (d) isobutanol, and (e) 2-propanol, and the samples obtained from VOPO4‚2H2O with (f) 2-butanol and (g) 2-pentanol. Asterisks indicate diffraction peaks due to the Mylar film.

in which the interlayer distances were observed to increase in relation to the use of alcohols with longer chain lengths of the alkyl group. Note that exposure of the intercalation compound with 1-butanol to air at room temperature for 24 h resulted in its reversion to VOPO4‚2H2O, as indicated by the XRD pattern and IR spectrum. For intercalation compounds with secondary alcohols, an XRD peak at 2θ ) 6.94°, corresponding to 1.27 nm, was observed for 2-propanol, as shown in Figure 1e, indicating the extension of the interlayer distance of VOPO4‚2H2O by intercalation. However, addition of 2-butanol or 2-pentanol to VOPO4‚2H2O did not result in any changes to the XRD pattern of the original VOPO4‚2H2O, signifying that intercalation of these alcohols does not occur, at least under these conditions. The sorption isotherms of gaseous alcohols by anhydrous VOPO4 are shown in Figure 2. Since the surface area of anhydrous VOPO4 is only 1 m2‚g-1, the monolayer adsorption corresponds to less than a 1.3 × 10-3 alcohol/VOPO4 molar ratio. As can be seen in Figure 2, the amounts of sorption for methanol, ethanol, and 1-propanol were above 4 × 10-1 alcohol/VOPO4. This indicates the presence of absorption, not only adsorption. It was shown that the initial relative pressure at which the absorption begins increases as the molecular


Figure 2. Isotherms of the uptake of alcohol to anhydrous VOPO4 at 288, 298, 298, and 298 K for (O) methanol, (b) ethanol, (4) 1-propanol, and (2) 2-propanol, respectively.

size of the alcohol increases; absorptions of methanol, ethanol, and 1-propanol were observed to start at about 0.1, 0.3, and 0.5 fractions of the relative pressure, p/p0, respectively. It was shown that the relative pressure, at which the adsorption of alcohol occurred, became higher as the molecular size became larger. In contrast, only slight absorption of 2-propanol into anhydrous VOPO4 was observed. Exfoliation of VOPO4‚2H2O. Changes in the color of the liquid and in the state of the solid were not detected for a suspension of VOPO4‚2H2O in THF, which was heated stepwise at 303 K for 1 h and then at 323 K for 1 h, even with additional heating at 340 K for 36 h; within 24 h at room temperature without stirring, the solids of this suspension were observed to settle completely. On the other hand, stirring VOPO4‚2H2O in 1-butanol at 303, 323, and 343 K for 1 h at each temperature, or in 2-butanol at 303, 323, 343, and 363 K for 1 h at each temperature, resulted in the disappearance of all solid VOPO4‚2H2O, and in the formation of a yellow-green homogeneous solution. The homogeneity of the solution remained unchanged for at least 3 days of standing at room temperature. 1-Propanol, isobutanol, and 2-propanol also gave the corresponding

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homogeneous solutions after similar thermal treatment; however, in the case of the 1-propanol solution, after standing for 3 days at room temperature, a small amount of precipitate was detected, which was shown using XRD and IR spectroscopy to be the intercalated VOPO4 with 1-propanol. Homogeneous solutions were not formed for the suspensions of VOPO4‚2H2O in ethanol or methanol with heating at 343 or 333 K for 12 h. For the five cases in which homogeneous solutions were formed, following the removal of each alcohol by treatment at 333 K, XRD patterns of the recovered solid in all cases exhibited a peak (2θ ) 12.36°) corresponding to that of VOPO4‚2H2O. Accordingly, the IR spectra of these recovered solids were identical to that of VOPO4‚ 2H2O. However, as shown in the SEM images in Figure 3, the crystallites of the recovered solids consisted of aggregates of twisted thin layers with similar sizes, which were significantly different from the starting VOPO4‚2H2O crystallites (square platelets, Figure 3a). Following treatment at 433 K in a vacuum, the surface areas of the solids recovered from the 1-butanol, isobutanol, and 2-butanol solutions were 16, 13, and 6 m2‚g-1, respectively, whereas that of the starting VOPO4‚2H2O was only 1 m2‚g-1. The thickness of the recovered solids was estimated using eq 1,

2 4 + ) SF t L

(1)

where, t, S, L, and F are thickness (µm), surface area (m2‚g-1), length (µm), and density (g‚cm-3), respectively. The length of the crystallite was determined using SEM measurements. The density of VOPO4‚2H2O (2.298 g‚cm-3) was estimated using the lattice constant and molecular weight.50 As summarized in Table 1, the thicknesses of the resulting thin layers of VOPO4‚2H2O were estimated to be 0.056-0.154 µm. Although significantly lower than that of the starting VOPO4‚2H2O (1.14 µm), the thicknesses of these solids are too large to be estimated from the line widths of their XRD peaks.

Figure 3. SEM images of (a) VOPO4‚2H2O, and the powdered samples recovered from VOPO4‚2H2O exfoliated in (b) 1-propanol, (c) 1-butanol, (d) isobutanol, (e) 2-propanol, and (f) 2-butanol.

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Figure 4. XRD patterns of (a) P-4, and the precipitates obtained by refluxing the solution of VOPO4‚2H2O exfoliated in (b) 1-propanol, (c) 1-butanol, (d) isobutanol, (e) 2-propanol, and (f) 2-butanol.

Reduction of Delaminated Sheets of VOPO4‚ 2H2O with Primary Alcohols. In the cases where the homogeneous solutions of VOPO4‚2H2O with primary alcohols were refluxed for 24 h (at 363, 383, and 383 K for 1-propanol, 1-butanol, and isobutanol, respectively), light-blue precipitates were formed. The XRD patterns of these precipitates, which were dried at room temperature, are presented in Figure 4. Although the precipitate resulting from refluxing in isobutanol exhibited an XRD pattern (Figure 4d) similar to that of P-4 (Figure 4a), the relative intensities were greatly different from that of P-4. However, the precipitates obtained from refluxing in 1-propanol and in 1-butanol exhibited XRD patterns that also differed from that of VOHPO4‚ 0.5H2O, with the main peaks at 2θ ) 9.04° (0.98 nm, Figure 4b) and at 2θ ) 8.22° (1.07 nm, Figure 4c), respectively. The precipitates obtained from refluxing in 1-pentanol and in 1-hexanol provided XRD patterns with intense peaks at 2θ ) 6.70° (1.55 nm) and at 2θ ) 4.84° (1.82 nm), respectively (data not shown). It was confirmed that the interlayer distance correlated well with the number of carbon atoms of these primary alcohols, except for isobutanol. The IR spectra of these crystallites are presented in Figure 5. The IR spectrum of the product of 1-butanol (Figure 5b) exhibited various absorption bands of lattice vibrations (1100-400 cm-1) due to ν(PsO) (1179 and 977 cm-1) and ν(VdO) (977 cm-1), and due to ν(CsOH) (3554 cm-1), ν(CsH) (2967, 2940, and 2881 cm-1), ν(PsOsC) (1155 and 1029 cm-1), and δ(PsOsC) (784 cm-1),51 and is markedly different from that of P-4 (Figure 5a). The crystallites from refluxing in 1-propanol also provided an IR spectrum similar to that of 1-butanol (data not shown). On the other hand, the IR spectrum of the crystallites from isobutanol (Figure 5d) was consistent with that of VOHPO4‚0.5H2O. SEM micrographs of these light-blue crystallites are given in Figure 6. As shown in Figure 6a, P-4 consists of platelike crystallites, each with a length of about 4 µm. In contrast, SEM images of the crystallites from 1-propanol and 1-butanol (Figure 6b,c, respectively) (50) Bordes, E.; Courtine, P.; Pannetier, G. Ann. Chim. 1973, 8, 105. (51) Bellamy, L. J. The Infra-red Spectra of Complex Molecules; Chapman and Hall: London, 1975; p 353.

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Figure 5. IR spectra of (a) P-4, and the precipitates obtained by refluxing the solution of VOPO4‚2H2O exfoliated in (b) 1-butanol, (c) isobutanol, and (d) 2-butanol.

show rose-petal shapes, and furthermore, images of the crystallites from isobutanol (Figure 6d) show a unique shape, which can be described as “small buds of a flower”. The physical data of these crystallites are summarized in Table 1. Reduction of Exfoliated Sheets of VOPO4‚2H2O with Secondary Alcohols. Refluxing the homogeneous solutions of VOPO4‚2H2O in secondary alcohols for 24 h, at 363 K for 2-propanol and at 383 K for 2-butanol, also resulted in the formation of light-blue precipitates. The XRD patterns and IR spectra of these precipitates are presented in Figures 4 and 5, respectively. In contrast to the cases of primary alcohols, the XRD patterns and IR spectra of the precipitates formed in secondary alcohols were consistent with those of P-4 (Figures 4 and 5). The SEM images of the compounds obtained from 2-propanol and 2-butanol are shown in Figure 6. It is worthy to note that these precipitates consisted of small fragments of crystallites with a length of about 1 µm. The physical data of these crystallites are listed in Table 1. Discussion We have initially discovered that thermal treatment of a suspension of VOPO4‚2H2O in alcohol affords a homogeneous solution, and the following possibilities were considered for its formation: (1) alcohol was intercalated into VOPO4‚2H2O to form an intercalation compound, which then swelled infinitely, and finally exfoliated, or (2) VOPO4‚2H2O was simply dissolved in alcohol. We have observed that the temperatures at which the suspensions changed into homogeneous solutions were lower for primary alcohols as compared to secondary alcohols. This observation, and considering that primary alcohols intercalated more readily into the VOPO4‚2H2O layers than secondary alcohols (Figure 2), led to our assumption that the formation of a homogeneous solution is related to intercalation. As mentioned earlier, VOPO4‚2H2O has shown high capabilities of intercalation for a variety of guest molecules.4-24 Recently, we have reported that the VOPO4-4-butylaniline intercalation compound can be delaminated in THF to form thin sheets.29,30 In addition, with the use of SEM, we have directly detected the swelling of the VOPO4‚ 2H2O layers by the intercalation of 4-butylaniline.29-31


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Figure 6. SEM images of (a) P-4, and the precipitates obtained by refluxing the solution of VOPO4‚2H2O exfoliated in (b) 1-propanol, (c) 1-butanol, (d) isobutanol, (e) 2-propanol, and (f) 2-butanol.

Exfoliation of layered compounds, such as clays, chalcogenides, titanic acid, and niobic acid, in polar solvents, such as an aqueous solution of hydroxylamine chloride, have been reported.25-27 Kleinfeld et al.25 showed by ellipsometry a linear increase in the thickness of multiplayer structures for clays with the number of cycles of polyelectrolyte poly(diallyldimethylammonium chloride) and hectorite adsorption. Kelle et al.26 demonstrated that restacked solid prepared as a thick film of zirconium phosphate by drying the colloid gives diffraction lines typical of an intercalation compound. They further observed many sheets depending on the size and crystallinity of the precursor R-ZrP crystallites with TEM. Abe et al.26 showed the TEM micrograph of the exfoliated thin layers that consist of an even number of niobate sheets. Since the bonding between the layers of VOPO4‚2H2O is weak, it is reasonable to assume that the exfoliation of VOPO4‚2H2O can be carried out directly in alcohol. Sasaki et al.28 claimed that the exfoliated suspension of titanic acid in an aqueous solution of tetrabutylammonium hydroxide does not exhibit XRD peaks due to the crystallites. Similarly, in this study, the VOPO4‚ 2H2O solution of 1-butanol did not exhibit XRD peaks. As shown in Figure 3, the recovered solids (VOPO4‚ 2H2O) from the homogeneous alcohol solutions consisted of crystallites with flower-petal-like morphologies, which are significantly different from that of the original VOPO4‚2H2O. We thus reason that these changes in the morphologies are attributable to intercalation-exfoliation reactions. Alternatively, the possibility of the dissolution of VOPO4‚2H2O in alcohol was also considered. We reason that if VOPO4 dissolves in alcohols, homogeneous solutions should form whether from anhydrous VOPO4 or from VOPO4‚2H2O. However, when anhydrous VOPO4 was used instead of VOPO4‚2H2O, under conditions identical to those described above, homogeneous solutions using 1-butanol and 2-butanol were not formed. Therefore, the dissolution of VOPO4 into alcohol is insignificant in the scope of our present studies. Morphological Change by Exfoliation-Reduction. As summarized in Table 1, when the obtained

homogeneous solution was refluxed, the oxidation number of the vanadium ion decreased from about 5 to nearly 4. At the same time, it is important to note that the changes in the morphology, composition, and structure of the reduced compounds significantly depend on the type of alcohol present. As representative cases, the exfoliation-reduction processes involving 1-butanol and 2-butanol will be discussed. Whereas 2-butanol resulted in a pure VOHPO4‚0.5H2O phase, 1-butanol afforded a V4+ compound that was different from the well-known precursor VOHPO4‚0.5H2O, as presented in Figures 4-6 and Table 1. On the basis of microanalysis results (24.79 wt % vanadium, 15.83 wt % phosphorus, 3.76 wt % carbon, and 2.95 wt % hydrogen), the composition of the V4+ compound from 1-butanol was proposed as V1.00P1.05C0.64H6.01O6.77. The XRD pattern of the reduced compound (Figure 4c) shows a resemblance to that of vanadyl n-butyl phosphate, VOH0.5(n-C4H9)0.5PO4‚0.5(n-C4H9OH), as previously reported by Kamiya et al.52-54 and Du Pont’s researchers.55 This previously reported alkyl phosphate is identified as a layered compound having an ester bond (P(dO)sOsn-C4H9),51-55 and accordingly an ester bond was similarly identified for our compound using IR spectroscopy (Figure 5b, 1154 and 785 cm-1). There are two kinds of alcohols, ester and adsorbed alcohol, in vanadyl alkyl phosphates.52,53 By heating, the latter was desorbed readily, but the former was stable up to 500 K and decomposed at 570 K. The layered structure was supported by the fact that good correlation was observed between the interlayer distance of these compounds with primary alcohols and the length of the alkyl groups of the alcohol. Similar trends were previously reported for alkyl phosphates56 and alkyl phosphites.45,57-60 From the data of TG/DTA, the num(52) Kamiya, Y.; Nishikawa, E.; Satsuma, A.; Mizuno, N.; Okuhara, T. Sekiyu Gakkaishi 2001, 44, 265. (53) Kamiya, Y.; Ueki, S.; Hiyoshi, N.; Yamamoto, N.; Okuhara, T. Submitted for publication. (54) Hiyoshi, N.; Yamamoto, N.; Kamiya, Y.; Nishikawa, E.; Okuhara, T. Catal. Today 2001, 71, 129. (55) Horowitz, S. H.; Carron, M. C. Patent WO 98/15353, 1998. (56) Johnson, J. W.; Jacobson, A. J.; Butler, W. M.; Rosenthal, S. E.; Brody, J. F.; Lewandowski, J. T. J. Am. Chem. Soc. 1989, 111, 381.

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ber of 1-butanol molecules intercalated in the compound was determined to be 0.1, the same as that in previous reports.52,53 In conclusion, the compound obtained from 1-butanol can be expressed as VO{H0.9(n-C4H9)0.1}PO4‚ (H2O)1.6‚(n-C4H9OH)0.1. Among the primary alcohols, only isobutanol was shown to afford the pure VOHPO4‚0.5H2O phase (Figures 4 and 5). This exception of isobutanol among the primary alcohols is attributable to the configuration of ester bonding within the interlayer compound. It is reasonable to assume that the reaction between the POH group of VOHPO4 layers and the primary alcohol forms the ester bond, which results in the expansion of the interlayer distance. However, in the case of isobutanol, if an ester bond is formed, strong repulsive interactions are expected between the bulky isobutyl group of P(dO)sOsi-C4H9 and the oxygen atoms of the neighboring V2O4 group. By this reasoning, instead of the formation of an ester bond between POH and isobutanol, the direct reduction of VOPO4‚2H2O to VOHPO4‚0.5H2O by isobutanol through the dehydrogenation of isobutanol occurs. In the case of the reaction of the exfoliated VOPO4‚2H2O with secondary alcohols, direct reduction to VOHPO4‚0.5H2O would precede the formation of the corresponding alkyl phosphates, due to a similar steric effect of the secondary alkyl groups. In terms of morphological changes, the shapes of the crystallites remained nearly unchanged before and after the reductions using primary alcohols (Figures 3 and 6). On the other hand, the reduction using 2-butanol afforded smaller fragments of VOHPO4‚0.5H2O crystallites (Figures 3 and 6). It should be noted that, as described above, the structure as well as the composition of the reduced compound was different: VOHPO4‚ 0.5H2O for 2-butanol and VO{H0.9(n-C4H9)0.1}PO4‚ (H2O)1.6‚(n-C4H9OH)0.1 for 1-butanol. The lateral bonding of the VO{H0.9(n-C4H9)0.1}PO4‚(H2O)1.6‚(n-C4H9OH)0.1 sheets is possibly stabilized by the ester bonding to maintain the dimensions of the crystallites. In other words, the sheets of VOPO4 (or VOHPO4) would be broken into smaller fragments during the reduction step using secondary alcohols, such as 2-butanol. The exfoliation-reduction processes for VOPO4‚2H2O to the V4+ compounds having different microstructures are summarized in Scheme 1. It should be emphasized that the shape, length, and thickness of the crystallites varied greatly through these processes. The intercalationexfoliation process could be utilized as a novel method in the preparation of nanostructurally controlled catalyst precursors. As previously reported,46 smaller fragments of VOHPO4‚0.5H2O crystallites, which are de(57) Johnson, J. W.; Jacobson, A. J.; Brody, J. F.; Lewandowski, J. T. Inorg. Chem. 1984, 23, 3842. (58) Huan, G.; Jacobson, A. J.; Johnson, J. W.; Goshorn, D. P. Chem. Mater. 1992, 4, 661. (59) Sabbar, E. M.; de Roy, M. E.; Ennaqadi, A.; Gueho, C.; Besse, J. P. Chem. Mater. 1998, 10, 3856. (60) Johnson, J. W.; Brody, J. F.; Alexander, R. M. Chem. Mater. 1990, 2, 198.

Yamamoto et al. Scheme 1. Exfoliation and Reduction Processes to the V4+ Compound with Different Morphologies

rived from 2-butanol, provided a highly selective catalyst for the selective oxidation of n-butane. The obtained catalyst was about 3 times more active than the conventional catalyst. Furthermore, the selectivity to maleic anhydride reached about 75%, demonstrating the superiority of this novel process in the preparation of functional catalysts. Conclusions Our present study has demonstrated that the layers of VOPO4‚2H2O can be exfoliated in various alcohols to form the corresponding homogeneous solutions containing the delaminated sheets. The exfoliated layers were reconstructed to VOPO4‚2H2O by the removal of the alcohol solvents, where the recovered solid VOPO4‚2H2O consisted of flower-petal-like crystallites. Refluxing the homogeneous solution afforded the reduced layered compounds, the microstructure of which changed depending on the type of alcohol. Simple primary alcohols, such as 1-propanol and 1-butanol, yielded layered vanadyl alkyl phosphates, whereas secondary alcohols afforded pure VOHPO4‚0.5H2O with the morphology of smaller fragments. Acknowledgment. This work was supported in part by the New Energy and Industrial Technology Development Organization (NEDO), Japan. CM020292Y

Thin-Layered Sheets of VOHPO4Â·0.5H2O Prepared from VOPO4

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