8108
J. Phys. Chem. B 2000, 104, 8108-8115
States and Dynamic Behavior of Protons and Water Molecules in H3PW12O40 Pseudoliquid Phase Analyzed by Solid-State MAS NMR Sayaka Uchida, Kei Inumaru,† and Makoto Misono*,‡ Department of Applied Chemistry, Graduate School of Engineering, The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ReceiVed: February 9, 2000; In Final Form: June 6, 2000
The states and dynamic behavior of acidic protons and water molecules in solid H3PW12O40‚nH2O (0 < n < 6), which would be closely related to its pseudoliquid phase catalysis, were quantitatively elucidated by the comprehensive application of 31P, 1H, and 17O magic-angle spinning (MAS) NMR. 1H and 17O MAS NMR were sensitive to the local environment in the pseudoliquid phase (e.g., hydrogen bonding), and 31P MAS NMR was effective especially to quantify the states of the protons. At 173 K, several peaks appeared in the 31P MAS NMR spectra that were reasonably assigned to phosphorus atoms in polyanions (PW O 3-) having 12 40 different numbers of acidic proton(s) directly attached to them. Hence, the acidic protons reside either directly on the polyanions as “isolated acidic protons” or as H3O+ or H5O2+, and the amounts of these species were determined as a function of water content. The relative intensity of the 31P MAS NMR peaks obeyed binomial distribution for all the range of 0 < n < 6, which shows that the isolated acidic protons and protonated water molecules (H3O+ and H5O2+) are distributed uniformly (i.e., randomly) in the solid. The distribution of these species was well explained by the random removal of water from the solid bulk. At 298 K, the 31P MAS NMR peaks coalesced, revealing that the isolated acidic protons, which are the origin of the strong acidity in the pseudoliquid phase, migrate between the neighboring polyanions much faster than catalytic reactions. This is the first quantitatiVe observation of the protons in hydrated heteropolyacids by spectroscopic methods.
Introduction The catalytic properties of heteropoly compounds have drawn much attention owing to their versatility and greenness as catalysts, which have been demonstrated by both successful large-scale applications and promising laboratory results.1 Heteropolyacids are also good cluster models of mixed oxide catalysts, and therefore the catalytic processes can be described at the atomic/molecular level.1 Thus, spectroscopic study and chemical stoichiometry become very realistic as compared with the cases of the conventional mixed oxide catalysts. A remarkable property of the heteropolyacids when they are used as solid catalysts is the formation of “pseudoliquid phase”.2 Higher catalytic activities of solid heteropolyacids at low temperatures for several reactions such as dehydration and esterification are closely related to this property.1d,3 Polar molecules such as H2O, alcohols, and ethers readily move into or out of the three-dimensional bulk phase, sometimes expanding or shrinking the distance between the Keggin anions (e.g., PW12O403-), and catalytic reactions occur there.4 This is not the adsorption in the micropores of the heteropolyacids since there are no intrinsic micropores in the crystal structure.5 A large number of alcohols were readily absorbed during the dehydration as revealed by transient-response analysis using isotopically labeled alcohols.6 This pseudoliquid-phase behavior brought about unique selectivities7 as well as high catalytic activity. * To whom correspondence should be addressed. Telephone: +81-3-3342-1211, ext. 2523. Fax: +81-3-3340-0147. E-mail: misono@ cc.kogakuin.ac.jp. † Present address: Department of Applied Chemistry, Faculty of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan. ‡ Present address: Department of Environmental Chemical Engineering, Kogakuin University, 1-24-2 Nishi-shinjuku, Shinjuku-ku, Tokyo 163-8677, Japan.
To understand the fundamental nature of the pseudoliquidphase catalysis, detailed information of the acidic proton, such as its location, mobility, and interactions with reacting molecules, is indispensable. As for zeolites, it has been suggested that the mobility of the acidic protons plays significant roles in catalysis, in addition to the static properties such as deprotonation energy or acid strength.8 The interaction of acidic protons with small basic molecules such as water is an interesting subject, and the study of it will provide useful information about the acid catalysis of heteropolyacids, since water is often contained in the working state and plays important roles in their catalysis. As for zeolites, the interaction of H2O with Brønsted acidic protons has been studied by IR,9 MAS and static 1H NMR,10 neutron diffraction,11 and quantum chemical ab initio calculations.12 It has been revealed, for example, that water molecules exist as neutral hydrogen-bonded species or cationic species in zeolite channels, depending on the local environment and the acid strength.13,14 Furthermore, it may be pointed out that the states and dynamics of protons in solids have been a focus of many studies from the viewpoints of fundamental solidstate chemistry as well as from practical applications such as proton conductors.15 In the pseudoliquid phase of heteropolyacids, absorbed water molecules are surrounded by an electrostatic field of large Keggin anions. To understand the local interactions in solid acid catalysts, solid-state NMR has been very effective.16 In our early studies, we have directly detected by solid-state NMR the intermediates of the dehydration of ethanol17 and elucidated the dynamic behavior of CH3OH molecules18 in the pseudoliquid phase of H3PW12O40. Recently, we observed the delocalization of the acidic protons in H3PW12O40‚nH2O (n < 6) by solid-
10.1021/jp000508o CCC: $19.00 © 2000 American Chemical Society Published on Web 08/08/2000
Protons and H2O in H3PW12O40 Pseudoliquid Phase state 31P MAS NMR.19 Here, the combination of 31P MAS NMR with 1H MAS NMR was demonstrated to be very powerful. Therefore, in this study, we attempted to characterize in detail the states and dynamic behavior of acidic protons and water molecules in hydrated pseudoliquid phase of H3PW12O40‚nH2O (0 < n < 6) by combining 31P, 1H, and 17O solid-state magicangle spinning (MAS) NMR. The quantities of several protonic species (“isolated acidic protons” and protonated water molecules) at different hydration levels have been determined, and the rate of the migration of “isolated acidic protons” among Keggin-type heteropolyanions was estimated. The fundamental information obtained in this study will make possible the better understanding of the catalysis in the psuedoliquid phase at the atomic/molecular level. It is further hoped that the present study provides useful information about the states and dynamics of protons and water in not only solid acid catalyst but also solid inorganic compounds in general. Experimental Section Materials. H3PW12O40‚nH2O was supplied by Nippon Inorganic Color and Chemical Co. and was purified by extraction with diethyl ether followed by recrystallization from water. This process was repeated several times until 31P MAS NMR gave a single peak. The hydration level, n, was determined to be ca. 22 for the recrystallized materials by thermogravimetry (TG). Sample Preparation for Solid-State MAS NMR Measurements. Purified H3PW12O40‚22H2O (ca. 0.25 g) was placed in a Pyrex cylindrical glass cell (25 mm in diameter), which was connected to a glass-made high-vacuum system. The thickness of the sample bed was ca. 1 mm. Samples with n ) 0 and 6 were prepared as described previously,19 by evacuation at 493 K for 2 h and at 298 K for 4 h, respectively. The water contents after the above treatments were confirmed by TG. The hydration levels of the samples with 0 < n < 6 were controlled in two different ways, i.e., absorption and desorption methods. In the absoprtion method, a calculated amount of H2O vapor was contacted with H3PW12O40‚0H2O at room temperature. Under the present conditions, the pressure of water in the vacuum system became zero, indicating that all H2O molecules introduced in the vaccum system were absorbed by the sample. Blank experiments (in the absence of heteropolyacid) confirmed that error in the value of n due to the absorption of H2O onto the glass wall was less than 3%. In the desorption method, H3PW12O40‚6H2O was dehydrated at 373-423 K to form samples with 0 < n < 6. To determine the value of n, the desorbed water was all collected with liquid N2 into a cell of which the volume is known and the amount was calculated from the vapor pressure at 298 K. The errors in the values of n were less than 3% as estimated by blank tests. For 17O MAS NMR measurements, oxygen exchange with H217O (20% 17O) was carried out as follows. After dehydration of H3PW12O40, an excess amount of H217O (liquid) was introduced to the sample and allowed to exchange at 333 K for 24 h in a closed glass cell. The degree of oxygen exchange was followed by the relative intensity of the17O solution NMR signals for the three types of oxygen in PW12O403- (i.e., terminal and two types of bridging oxygen). The 17O signal appeared first for the terminal oxygen and then the two types of bridging oxygen. The 17O signal for the inner oxygen (PO4) did not appear, which indicates that the oxygen exchange occurs only between outer oxygen of PW12O403- and H217O. Oxygen exchange was carried out until the relative intensity of the 17O signals achieved the theoretical value calculated from the formula (i.e., terminal:bridging:bridging ) 2:1:1). After this treatment, the excess water was removed by evacuation to
J. Phys. Chem. B, Vol. 104, No. 34, 2000 8109
Figure 1. 31P MAS NMR spectra of H3PW12O40‚nH2O (MAS rate ) 3 kHz): (a) n ) 0 at 298 K; (b) n ) 0 at 173 K; (c) n ) 6 at 298 K; (d) n ) 6 at 173 K. 32 scans were taken for (a). 4 scans were taken for (b)-(d).
dryness and then the hydration level was controlled to be n ) 0 or 6 by the method described above. After the above procedures, the sample was transferred directly into a small glass cell (ca. 0.1 cm3), which was connected to the vacuum system. The small cell was sealed by firing while keeping the sample at 77 K. Solid-State MAS NMR Measurements. MAS NMR spectra were recorded with a Chemagnetics CMX-300 Infinity spectrometer operating at 7.05 T, equipped with a variabletemperature CPMAS probe. The sealed sample was set into a zirconia rotor (7.5 mm in diameter). 1H (300 MHz), 17O (41 MHz), and 31P (121 MHz) MAS NMR spectra were recorded using single-pulse excitation. When proton decoupling was applied for 31P, the peaks were slightly sharpened, while neither the positions nor the relative intensities of the peaks changed. Therefore, nondecoupled spectra were adopted in the discussion below to avoid any effects from incomplete decoupling. The π/2 pulse width and pulse delay were 4.5 µs and 20-200 s for 1H and 4.0 µs and 2000 s for 31P, respectively. The MAS rate was 3-5 kHz, and the measurements were carried out in the range of 173-298 K. The chemical shifts were expressed with reference to tetramethylsilane (1H) and 85% H3PO4 (31P). Adamantane (1H: 1.91 ppm) and NH4H2PO4 (31P: 1.00 ppm) were used as external standards for the calibration of chemical shifts. 17O MAS NMR was measured with a pulse width of 2 µs (π/6 pulse) and pulse delay of 1 s, using H217O (0 ppm) as a standard of chemical shifts. Results and 31P MAS NMR of H3PW12O40‚nH2O (n ) 0, 6). Figure 1 shows 31P MAS NMR spectra of H3PW12O40‚0H2O and H3PW12O40‚6H2O measured at 298 and 173 K. For H3PW12O40‚0H2O, a sharp peak was observed at -11.0 ppm (Figure 1a,b). H3PW12O40‚6H2O gave a peak at -15.6 ppm (Figure 1c,d). The chemical shifts and the line widths of these peaks were little affected by temperature in the range of 173298 K. The single peak for H3PW12O40‚6H2O is consistent with the crystal structure20 in which all polyanions are equally hydrogen bonded by H5O2+ cations. Figure 2 shows 1H MAS NMR spectra of the same samples as in Figure 1. Spectra were recorded at 298 and 173 K. H3PW12O40‚0H2O (Figure 2a) gave a single peak at 9.0 ppm, accompanied by spinning sidebands (SSB’s). The peak at 9.0 ppm is assignable to the acidic protons of H3PW12O40 since 1H
8110 J. Phys. Chem. B, Vol. 104, No. 34, 2000
Uchida et al.
Figure 2. 1H MAS NMR spectra of H3PW12O40‚nH2O (MAS rate ) 3 kHz): (a) n ) 0 at 298 K; (b) n ) 0 at 173 K; (c) n ) 6 at 298 K; (d) n ) 6 at 173 K. Asterisks denote spinning sidebands. 64 scans were taken for (a) and (b). 256 scans were taken for (c) and (d). Figure 4. 31P MAS NMR spectra of H3PW12O40‚nH2O (0 < n < 6) prepared by desorption method. The spectra were taken at 298 (a - e) and 173 K (f - j) (MAS rate ) 3 kHz). (a) and (f) n ) 6; (b) and (g) n ) 4.0; (c) and (h) n ) 2.1; (d) and (i) n ) 0.5; (e) and (j) n ) 0.1 scan was taken for (g)-(i). 4 scans were taken for (a), (c), (d), (f), and (j). 32 scans were taken for (b) and (e).
Figure 3. 17O MAS NMR spectra of H3PW12O40‚nH2O (MAS rate ) 5 kHz): (a) n ) 6 at 298 K, 12 000 scans; (b) n ) 6 at 223 K, 4450 scans; (c) n ) 0 at 298 K, 30 000 scans; (d) n ) 0 at 223 K, 13 000 scans.
there is no other proton. Upon cooling to 173 K (Figure 2b), the chemical shift, the line width, and intensities of SSB’s remained almost unchanged. The results observed at 298 K are consistent with those of the earlier studies.21 For hexahydrate H3PW12O40‚6H2O (Figure 2c), a very broad signal was observed. Considering that all protons and water form H5O2+ in hexahydrate,20 the signal is attributable to H5O2+. The broadness may be due to strong dipole-dipole interaction as will be described later. Upon cooling to 173 K (Figure 2d), a small sharper peak accompanied by SSB’s appeared. The total peak area (including SSB’s) of the spectrum was much smaller (one-tenth) than that of H3PW12O40‚0H2O (Figure 2b), indicating that the major part of this signal was not detectable due to its broadness. 17O MAS NMR of H PW O ‚nH O (n ) 0, 6). As for 3 12 40 2 17O NMR of an aqueous solution of ca. 0.5 M PW O 312 40 (spectrum not shown), well-resolved signals for terminal W ) O oxygens (766 ppm) and bridging W-O-W oxygens (424 and 406 ppm for edge-sharing and corner-sharing groups) were obtained in agreement with the previous study.22 17O MAS NMR spectra of hexahydrate (n ) 6) and anhydrous sample (n ) 0) are given in Figure 3. Hexahydrate gave spectra with spinning sidebands (Figure 3a,b). The positions of the
isotropic peaks were confirmed by varying the MAS speed from 4 to 5 kHz. Terminal W ) O oxygen (denoted by Ot) appeared at 707 ppm, and two kinds of bridging W-O-W oxygen (Ob) showed splittings (438, 428 ppm and 397, 378 ppm). This splitting is probably attributed to quadrupolar effects.23 The positions of the peaks of two kinds of Ob remained almost unchanged from the solution spectrum (424 to 438 and 428 ppm and 406 to 397 and 378 ppm) while that of Ot shifted ca. 60 ppm upfield. This fact indicates that the peak of Ot is very sensitive to its environment. Cooling the hexahydrate to 223 K caused no detectable change in the spectrum. As for the anhydrous sample (Figure 3c), the signals of Ob broadened and the signal of Ot disappeared. This broadening may be due to degeneration of the crystallization state24 as will be discussed in the later section. The intensity of the Ob signals decreased sharply upon cooling to 223 K (Figure 3d). Variable-Temperature MAS NMR of H3PW12O40‚nH2O (0 < n < 6) Prepared by the Desorption Method. The results obtained by the desorption method are described since they gave more uniform composition. Figure 4 shows the 31P MAS NMR spectra of H3PW12O40‚nH2O (0 < n < 6) measured at 173 and 298 K. At 298 K (Figure 4a-e), peaks were observed at -11.0 ppm (the same position as n ) 0), -15.6 ppm (the same position as n ) 6), -14.8 ppm (sharp), and at around -12 to -14 ppm (broad). The broad peak tended to shift to a lower field with a decrease in the water content. As will be described below, the broad peak is formed by the coalescence of several peaks. Upon cooling to 173 K (Figure 4f-j), the broad peak split into several sharp peaks. In these spectra, essentially five peaks were observed at ca. -11.0, -11.9, -13.5, -14.8, and -15.6 ppm. The intensities (i.e., area) of these peaks systematically changed with the hydration levels. As the water content decreased (from Figure 4f to Figure 4j) the peak at the highest field (-15.6 ppm) monotonically decreased, while peaks at intermediate fields (-14.8, -13.5, and -11.9 ppm) increased at first and then decreased. The peak at the lowest field (-11.0 ppm) monotonically increased as the water content decreased. The variations of these peak intensities are summarized in Table 1. The assignment of 31P MAS NMR peaks and the behavior
Protons and H2O in H3PW12O40 Pseudoliquid Phase
J. Phys. Chem. B, Vol. 104, No. 34, 2000 8111
TABLE 1: Observed and Calculated (in Parentheses) Relative Peak Intensities of the 31P MAS NMR Spectra (173 K) of H3PW12O40‚nH2O by the Desorption Method relative intensity/% na 0 0.5 1.6 2.1 3.0 4.2 4.8 6
-10.6 - -11.0 ppm, mb ) 3
-11.9 - -12.1 ppm, m)2
-13.2 - -13.8 ppm, m)1
-14.8 - -15.0 ppm, m)0
-15.6 ppm, m)0
100 (100) 57 (60) 13 (18) 8 (9) 0 (1) 0 (0) 0 (0) 0 (0)
0 (0) 39 (34) 49 (42) 36 (33) 11 (6) 4 (5) 3 (3) 0 (0)
0 (0) 4 (6) 32 (32) 39 (41) 36 (35) 31 (30) 30 (28) 0 (0)
0
0
0
0
6
0
17
0
53
0
24
41
14
53
0
100
sum of m ) 0c 0 (0) 0 (0) 6 (8) 17 (17) 53 (58) 65 (65) 67 (69) 100 (100)
mavd 3 2.53 1.70 1.35 0.58 0.39 0.35 0
a Number of water molecules per polyanion. b m isolated acidic protons per polyanion. See text. c (-14.8 to -15.0 ppm) + (-15.6 ppm). See text. d Averaged m per polyanion.
Figure 6. 1H MAS NMR spectra of H3PW12O40‚nH2O (0 < n < 6) prepared by the desorption method. The spectra were taken at 298 (a - e) and 173 K (f - j) (MAS rate ) 3 kHz). (a) and (f) n ) 6; (b) and (g) n ) 4.0; (c) and (h) n ) 2.1; (d) and (j) n ) 0.5; (e) and (j) n ) 0. 64 scans were taken for all samples except for n ) 6 (256 scans).
Figure 5. Variable-temperature 31P MAS NMR spectra of H3PW12O40‚ 2.1H2O prepared by the desorption method (MAS rate ) 3 kHz): (a) 298 K; (b) 273 K; (c) 248 K; (d) 173 K. 4 scans were taken for (a)(c). 1 scan was taken for (d).
of their intensities will be quantitatively discussed later in terms of the distribution of “isolated acidic protons” in the pseudoliquid phase. As shown in Figure 4, H3PW12O40‚2.1H2O gave four distinct peaks at 173 K (Figure 4h) while it gave a broad signal at 298 K (Figure 4c). The temperature dependency of 31P MAS NMR was measured in detail for H3PW12O40‚2.1H2O (Figure 5). Upon raising the temperature, the four peaks moved toward the center and merged into a broad peak at -13.3 ppm. This change was reversible with temperature. The “center of gravity” of four
peaks at 173 K (Figure 5d) was calculated to be -12.9 ppm, which is close to -13.3 ppm. These facts show that the broad peak at 298 K (Figure 5a) is a coalesced peak of the four signals observed at 173 K (Figure 5d). The distances between the neighboring peaks are 1.3-1.6 ppm, showing that the interconversion of these species at 298 K is faster than ca. 200 Hz. Figure 6 gives the 1H MAS NMR spectra of the same samples as shown in Figure 4. At 298 K, a peak appeared at around 9 ppm accompanied by SSB’s (except for the case of H3PW12O40‚ 6H2O; Figure 6a,f). The position of the peak shifted to low field from 9.0 to 9.6 ppm with increasing water content (Figure 6e to Figure 6b). All water molecules should be protonated to form (H2O)xH+ (x ) 1, 2, ...), though it is very unlikely that oligomers higher than H5O2+ (x ) 2) exist for 0 < n < 6, as these samples are prepared by dehydration of hexahydrate, where only H5O2+ exists. As for zeolites loaded with small amounts of water, the theoretical 1H NMR chemical shifts (ab initio calculation by MP2 method) for hydroxonium ion H3O+ and H5O2+ were reported to be 13.4 and 10.6 ppm, respectively.12b Considering
8112 J. Phys. Chem. B, Vol. 104, No. 34, 2000
Figure 7. 31P (a - e) and 1H (f - j) MAS NMR spectra of H3PW12O40‚ nH2O (0 < n < 6) at 298 K prepared by the absorption method (MAS rate ) 3 kHz): (a) and (f) n ) 6; (b) and (g) n ) 3.0; (c) and (h) n ) 2.0; (d) and (i) n ) 0.8 (e) and (j) n ) 0. 4 scans were taken for (a). 32 scans were taken for (b)-(e). 64 scans were taken for (j). 256 scans were taken for (f)-(i).
these values of chemical shift, the observed peaks in Figure 6 (9-9.6 ppm) can be regarded as a coalesced peak of the acidic protons (9 ppm) and the protonated water molecules. Upon cooling to 173 K, no significant change was observed in the position of the peaks except for the decrease in their line widths. Reproducibility of these data was confirmed for samples prepared separately. MAS NMR of H3PW12O40‚nH2O (0 < n < 6) Prepared by the Absorption Method. Hydrated samples were also prepared by absorption of water into anhydrous sample (absorption method). The patterns of 31P MAS NMR spectra shown in Figure 7a-e were different from those prepared by the desorption method (Figure 4). The peak at -14.8 ppm appeared even at n as low as 0.8 (Figure 7d). As n increased (Figure 7d to Figure 7a), the peak at intermediate chemical shifts (ca. 12-14 ppm) did not grow while a peak at -15.6 ppm appeared and grew much. The spectra little changed upon cooling to 173 K (not shown). Figure 7f-j shows the 1H MAS NMR spectra. A peak appeared at 9 ppm accompanied by SSB’s. In this case, the peak positions little changed with the change of n from 0 to 6. When n increased, the intensity of the 9 ppm peak decreased monotonically, and a broad peak appeared. For n ) 2 and 3, a sharp signal appeared at 7.6 ppm though the peak area was very small (less than one-tenth of the main peak). These results were very reproducible. The differences of 31P and 1H MAS NMR between the two preparation methods will be discussed further in the later section. Discussion 1. States of Acidic Protons and Water Molecules in H3PW12O40‚nH2O (n ) 0, 6). The crystal structure of H3PW12O40‚6H2O (hexahydrate) has been determined by neutron diffraction.20 Polyanions (PW12O403-) are packed in a cubic
Uchida et al. structure and all acidic protons are present in the form of H5O2+ cations (cf. H5O2+ (c) in Figure 9 below). The two H2O molecules in H5O2+ are bound together by an unusually short hydrogen bond (O‚‚‚O distance: 0.24 nm). Because of the short hydrogen bond distance, strong dipole-dipole interactions, which cause significant broadening, must exist as in the case of protons in ice, of which 1H NMR signal is very broad unless special techniques such as multipulse sequences are used.25 As for 31P MAS NMR of the hexahydrate (Figure 1c,d), a single sharp peak was observed, indicating the uniformity and high symmetry of the structure. The acidic protons in anhydrous sample (H3PW12O40‚0H2O) gave 1H MAS NMR spectra (Figure 2a,b) that are much narrower than that of the hexahydrate because of the smaller dipole-dipole interaction. These protons are called hereafter “isolated acidic protons”, as these protons have no hydrogen bond with water molecules. The 31P MAS NMR of the dehydrated sample showed a peak at -11.0 ppm (Figure 1a,b), much lower field than hexahydrate. A similar low-field shift has been observed for CsxH3-xPW12O40.26 Cs3PW12O40 is isostructural with the hexahydrate of H3PW12O40, where all H5O2+ cations of PW12O403- are replaced by Cs+. Cs3PW12O40 gave a 31P MAS NMR peak at -14.8 ppm,26 while anhydrous H3PW12O40 gave a lower field chemical shift (-11.0 ppm) indicating that the isolated acidic protons caused a significant low-field shift. 2. States of Proton and Water in H3PW12O40‚nH2O (0 < n < 6) at Low Temperature (173 K). a. Distribution of Isolated Acidic Protons Analyzed by 31P MAS NMR. Here the 31P MAS NMR spectra will be discussed quantitatively to provide detailed information about the states of protons. As reported previously, in the case of CsxH3-xPW12O40 (0 < x < 3), 31P MAS NMR exhibited four peaks, which were clearly resolved even at room temperature.26 These peaks at -10.9, -11.9, -13.5, and -14.9 ppm were reasonably assigned to polyanions having different numbers of protons (3, 2, 1, and 0 proton(s)) directly attached to the polyanions. The relative intensities of these peaks were in good agreement with those expected from the binominal distribution. This means that (3 - x) protons per polyanions were randomly distributed among the entire polyanions. Different interpretation has recently been proposed for Cs salts prepared in a different way,27 but we believe our interpretation is correct for our samples that are nearly uniform and have the predetermined compositions. The same interpretation may be applied to H3PW12O40‚nH2O (0 < n < 6) studied in this work. Here, the results of the desorption method are discussed, since they gave uniform samples. The31P MAS NMR of these samples at 173 K (Figure 4f-j) split into several peaks, while the spectra at 298 K (Figure 4a-e) were not well resolved. The positions of the split peaks observed at 173 K are very similar to those for CsxH3-xPW12O40 at 298 K. For example, H3PW12O40‚4.0H2O exhibited essentially four major peaks at -11.9, -13.7 (shoulder at -13.3), -14.8, and -15.6 ppm (Figure 4g). The former three peaks correspond to the peaks at -11.9 (m ) 2), -13.5 ppm (m ) 1), and -14.9 ppm (m ) 0) of CsxH3-xPW12O40, respectively. It should be noted that the extra peak at -15.6 ppm was at the same position as that of the hexahydrate. The peaks observed for H3PW12O40‚ 2.1H2O (-10.6, -11.9, -13.5, and -14.9 ppm, Figure 5d) were very close to those for CsxH3-xPW12O40 (-10.9, -11.9, -13.5, and -14.9 ppm). Thus, the peaks at -10.6, -11.9, -13.5, and -14.9 ppm correspond to polyanions having 3, 2, 1, and 0 isolated acidic proton(s) that directly attach to polyanions, and the peak at -15.6 ppm similar to hexahydrate (see below).
Protons and H2O in H3PW12O40 Pseudoliquid Phase
J. Phys. Chem. B, Vol. 104, No. 34, 2000 8113
Table 1 summarizes the relative intensities (i.e., area) of 31P MAS NMR observed for H3PW12O40‚nH2O at 173 K. According to the above discussion, the five peaks are assigned, to m ) 3, 2, 1, 0 (similar to CsxH3-xPW12O40) and 0 (similar to hexahydrate), as shown in Table 1. On the basis of the assignment, the average numbers of isolated acidic protons per polyanion (denoted by mav) can be calculated from the observed relative intensities of 31P MAS NMR (Table 1) by the following equation:
mav )
∑mI(m) ∑I(m)
Of the three acidic protons in H3PW12O40, mav is the number of isolated acidic protons and (3 - mav) protons are in the form of protonated water. If one assumes here that the isolated acidic protons are randomly distributed to polyanions, it is possible with the values of mav to calculate the relative intensities of 31P MAS NMR by the following equation:
I(m) )
( )(
mav 3! (3 - m)!(m)! 3
m
)
3 - mav 3
3-m
The calculated values are listed in Table 1 (in parentheses). The excellent agreement between the observed and calculated intensities supports again the above assignment of the 31P MAS NMR peaks and reveals that the isolated acidic protons are distributed randomly among polyanions in H3PW12O40‚nH2O. Thus, the 31P MAS NMR can differentiate experimentally the three acidic protons in H3PW12O40 to isolated acidic protons and protons bonded to water molecules. Here it is to be noted that, as far as 31P NMR is concerned, protons bonded to water molecules have only a small interaction with polyanion nearby. If interactions (i.e., hydrogen bonds) had large effects, numerous numbers of peaks would appear in 31P NMR, corresponding to the different number and strength of hydrogen bonds. We experimentally observed only five major peaks as discussed above. However, it is better to be noted that two distinct peaks were observed for m ) 0 (-14.9 and -15.6 ppm). The former has the same chemical shift as in Cs3PW12O40, which has no protons, and the latter has the same position as in H3PW12O40‚ 6H2O, which has no isolated acidic proton and all 12 terminal oxygens of the polyanion are equally hydrogen bonded to H5O2+. The intensity of the -15.6 ppm increased nearly proportionally to the amount of H5O2+ calculated in the next section. The split may be due to the change in the hydrogen bonding of protonated water from H3O+ to H5O2+. Another possibility is that the split is caused by different symmetry of hydrogen bonding with respect to polyanion (-15.6 ppm is for a symmetrical structure as in hexahydrate). Therefore, while the detailed mechanism is not clear yet, the two distinct states for m ) 0 are possibly related with the different degrees of hydrogen bonding of polyanions with H5O2+ or H3O+. b. The States of the Protonated Water Analyzed by 31P MAS NMR. The value of mav (isolated acidic proton) also provides information about the states of protonated water. The number of acidic protons that are bonded to water (i.e., not isolated acidic proton) is (3 - mav). As suggested by the results of 1H NMR, all water molecules are protonated and exist as H3O+ or H5O2+. If all protonated waters exist as H5O2+, the ratio between the water molecules and the acidic protons bonded to water, n/(3 - mav), should be equal to 2. The experimental values are distributed between 1 and 2, indicating the presence of 1:1 complex (H3O+). In fact, recent inelastic neutron scattering
Figure 8. Amount of H3O+ and H5O2+ per polyanion (n[monomer] and n[dimer]) in H3PW12O40‚nH2O (0 < n < 6) prepared by the desorption method. Symbols were calculated from the relative intensity of 31P NMR and were plotted as a function of n. Solid circles, H3O+; solid squares, H5O2+. Broken lines were calculated theoretically, assuming that H2O molecules were randomly removed from the hexahydrate structure.
measurement indicated the existence of H3O+ in H3PW12O40‚ nH2O for n ≈ 1.28 The numbers of H5O2+, n[dimer], and that of H3O+, n[monomer], per polyanion were obtained by solving the following equations:
2n[dimer] + n[monomer] ) n (for water) n[dimer] + n[monomer] ) 3 - mav (for acidic proton bonded to water) Then, n[dimer] becomes n - 3 + mav, and n[monomer] equals 6 - 2mav - n. These values are plotted as a function of n in Figure 8. n[monomer] exhibits a maximum at around n ) 3, while n[dimer] monotonically increased as n increased. Thus, the protonated water species in H3PW12O40‚nH2O are quantified for the first time. If one assumes that H2O molecules are randomly removed from the structure of the hexahydrate, n[dimer] and n[monomer] can be calculated numerically as a function of n. The results are shown by broken lines in Figure 8. The calculation well reproduced the experimental results. This demonstrates that water exists in the two states of H5O2+ and H3O+, the ratio of the two species following the random distribution. Figure 9 is the schematic illustration of the protonic species, i.e., isolated acidic proton, H3O+, and H5O2+. As for anhydrous sample (H3PW12O40‚0H2O), each polyanion interacts with three isolated acidic protons. When the water content increases, the water molecules are protonated in the pseudoliquid phase to form H3O+ and H5O2+, accompanied by the decrease in the amount of isolated acidic protons. c. Comparison between Desorption and Absorption Methods. Earlier study reported the exclusive formation of H5O2+ in H3PW12O40‚nH2O,29 while we observed both H5O2+ and H3O+. They adopted the “absorption method” to control the hydration levels, which may have given nonuniform samples. The n[monomer] and n[dimer] for the absorption method were also calculated from the observed31P MAS NMR intensities (Figure 10). Comparison of Figures 8 and 10 demonstrates that there is a significant difference between the two methods. For the absorption method, smaller amounts of H3O+ were formed at low values of n, while the formation of H5O2+ was dominant in the wide range of n (n > 1.5). The most influential difference between the two methods is probably the preparation temper-
8114 J. Phys. Chem. B, Vol. 104, No. 34, 2000
Figure 9. Schematic illustration of the protonic species in H3PW12O40‚ nH2O (0 < n < 6): (a) isolated acidic proton; (b) H3O+; (c) H5O2+.
Figure 10. Amount of H3O+ and H5O2+ per polyanion (n[monomer] and n[dimer]) in H3PW12O40‚nH2O (0 < n < 6) prepared by the absorption method. Symbols were calculated from the relative intensity of the 31P NMR (Figure 7) and were plotted as a function of n.
ature (373-423 K for desorption and 298 K for absorption). Slow diffusion of water molecules into the solid at low temperature may have caused the inhomogeneity for the absorption method. Therefore, the two methods would give the same NMR spectra if they were treated at a high temperature (further water desorption from the sample may occur), since the state of the water molecules would coincide after the treatment if one considers the high mobility of water molecules in the pseudoliquid phase above 423 K.2,4 d. Interpretation of 17O MAS NMR of H3PW12O40‚nH2O. The application of 17O NMR has been very effective toward the understanding of the bonding environments of oxygen in inorganic compounds.23a,30 We have obtained the 17O MAS NMR spectra of the hexahydrate with a good signal-to-noise ratio (Figure 3a), and the spectra remained unchanged upon cooling (Figure 3b). On the other hand, the anhydrous sample (n ) 0) (Figure 3c,d) gave significantly broad spectra in which Ot was not detectable. 17O (I ) 5/2) has a quadrupole, and hence the line shape is complex owing to the electric field gradient. Well-resolved spectra are obtained only when the quadrupole coupling constant (e2qQ/h) is negligible.23 In fact, X-ray powder diffraction revealed that dehydration of the hexahydrate reduces its crystallinity and crystal symmetry to a certain extent,24 which may induce significant broadening of 17O signals.
Uchida et al. The location of isolated acidic proton in dehydrated H3PW12O40 has been controversial (Ob or Ot). Kozhevnikov et al. have claimed by the comparison of 17O NMR spectra of dehydrated solid H3PW12O40 with its aqueous solution that the acidic proton is located on Ot.31 However, the spectrum of their “completely dehydrated sample” was quite similar to that of our hexahydrate and not to our anhydrous sample. While the reason for this discrepancy for anhydrous sample is not clear, it may be stated at least that the anhydrous sample absorbs water very rapidly. In contrast to Kozhevnikov et al., the studies of other groups including ours suggested that the isolated acidic proton is located on Ob (IR,32 LCAO-MO calculation,33 density functional quantum chemical calculation,34 and SEDOR NMR35). The present results at least demonstrated that the 17O NMR chemical shifts of H3PW12O40‚nH2O are highly sensitive not only to the position of the acidic proton but also to other factors such as the extent of hydration and crystallinity, although further experimental study is necessary to determine the location unambiguously. 3. Dynamic Behavior of the Acidic Protons and Its Relation with Catalysis. To begin with, it must be pointed out that the motions of proton in H3PW12O40‚nH2O can be classified into three types. First, there is diffusion of water molecules for a distance significantly longer than the size of the polyanion, which is slow compared with the NMR time scale. Second, there is migration of protons between neighboring anions, which was detected by our variable-temperature 31P MAS NMR measurements (ca. 200 Hz). Third, there are fast motions such as rotation of H3O+ and hopping of isolated acidic protons among the oxygens of a single polyanion, which were detected by neutron scattering (>Kilohertz).28,36 The second type of dynamics (interanion migration), which was presumed to be closely related to catalysis, are detected for the first time and will be discussed from the 31P MAS NMR of H3PW12O40‚nH2O. As the temperature increased from 173 to 298 K, the four peaks coalesced into a broad peak (Figures 4 and 5). The peaks at 173 K correspond to polyanions that have different numbers of acidic protons directly bonded to them, so that this is the first direct evidence that the isolated acidic proton, which is the origin of the strong acidity, is migrating among neighboring polyanions at 298 K (ca. 200 Hz). The peak at -11.0 ppm, which needed a higher temperature to coalesce (cf. Figure 4d), is assigned to H3PW12O40 molecules with no water molecule in their proximity (m ) 3) as discussed above. Therefore, it is suggested that the proton migration among polyanions is mediated by H3O+ and/or H5O2+. No coalescence at 298 K for the peaks at -14.8 and -15.6 ppm (cf. Figure 4b), which are assigned to PW12O403- completely surrounded by water molecules, is probably due to the slower motion of the water molecules (especially H5O2+) compared with isolated acidic protons. Let us compare the migration rate (ca. 200 Hz at 298 K) with the rate of catalytic reaction in the psuedoliquid phase. For example, the dehydration of 2-propanol proceeded at the rate of 2.5 × 10-4 molecule s-1 anion-1 at 363 K.6b The rate of ethanol dehydration at 403 K17 and that of pinacole rearrangement at 298 K37 were 5.1 × 10-2 and 6.0 × 10-2 molecule s-1 anion-1, respectively. In the case of 2-propanol,6b 4 molecule anion-1 of organic molecules were absorbed into the pseudoliquid phase during catalytic reaction. That is, the concentration of the organic molecules is comparable with that of the acidic protons. Therefore, the isolated acidic protons would migrate between neighboring polyanions at 106-103 times faster than the transformation of the organic molecule. In this sense, the
Protons and H2O in H3PW12O40 Pseudoliquid Phase isolated acidic proton can be regarded to be “delocalized” in the time scale of catalytic reactions. This “delocalization” may be a unique characteristic of heteropolyacids different from other solid acids. For instance, acidic protons in zeolites such as H-ZSM-5 are bound in the proximity of Al-O-Si sites.38 We previously reported that CH3OH molecules are stoichiometrically protonated in the pseudoliquid phase of H3PW12O40 when the amount of absorbed CH3OH is less than the amount of acidic protons.18 On the other hand, only a part of CH3OH was protonated in H-ZSM-5 even when excess acidic sites are accessible for CH3OH molecules.39 The pseudoliquid phase of heteropolyacid serves the delocalized acidic protons of which the high mobility would contribute to the facile protonation of the reactant molecules in the three-dimensional reaction field. Conclusions (1) Three protonic species, i.e., isolated acidic protons, H3O+, and H5O2+ were quantitatively differentiated for the first time by solid-state MAS NMR in the pseudoliquid phase of H3PW12O40‚nH2O (0 < n < 6). Water molecules as well as acidic protons were nearly uniformly distributed in the solid. The variation of the amounts of H3O+ and H5O2+ were reasonably explained by a model assuming random desorption of water from the hexahydrate structure. (2) The isolated acidic protons, which are the origin of the strong acidity in the pseudoliquid phase, were found to migrate among the polyanions, much faster than the catalytic reactions. This delocalization would contribute to the facile protonation of the reactant molecule. Acknowledgment. The authors are grateful to Dr. S. Hayashi (National Institute of Materials and Chemical Research, Japan) and Prof. J. M. Dereppe (Universite´ Catholique de Louvain, Belgium) for helpful comments. References and Notes (1) As for the molecular chemistry of heteropoly compounds, see: (a) Hill, C. L., Ed. Polyoxometalates. Chem. ReV. 1998, 98, 1. (b) Pope, M. T., Mu¨ller A., Eds. Polyoxometalates: From Platonic Solids to AntiRetroViral ActiVity; Kluwer Academic Publishers: Dordrecht, Netherlands, 1994. (c) Pope, M. T. Heteropoly and Isopoly Oxometalates; Springer: Berlin, 1983. For catalysis, see: (d) Okuhara, T.; Mizuno N.; Misono, M. AdV. Catal. 1996, 41, 113. (e) Hill, C. L., Ed. Polyoxometalates in Catalysis. J. Mol. Catal. 1996, 114, 1. (2) (a) Misono, M.; et al. CSJ/ACS Chemical Congress, Honolulu, 1979: 1st Japan-France Seminar on Catalysis, Lyon, 1979. (b) Misono, M.; Sakata, K.; Yoneda, Y.; Lee, W. Y. Proceedings of the 7th International Congress on Catalysis, 1980, Kodansha (Tokyo); Elsevier: Amsterdam, 1981; p 1047. (3) (a) Okuhara, T.; Mizuno, N.; Lee, K. Y.; Misono, M. Acid-Base Catalysis; Tanabe, K., Hattori, H., Yamaguchi, T., Tanaka, T., Eds.; Kodansha: Tokyo, 1989; p 421. (b) Saito, Y.; Cook, P. N.; Niiyama, H.; Echigoya, E. J. Catal. 1985, 95, 49. (4) (a) Okuhara, T.; Tatematsu, S.; Lee, K. Y.; Misono, M. Bull. Chem. Soc. Jpn. 1989, 62, 717. (b) Misono, M.; Mizuno, N.; Katamura, K.; Kasai, A.; Konishi, Y.; Sakata, K.; Okuhara, T.; Yoneda, Y. Bull. Chem. Soc. Jpn. 1982, 55, 400. (5) (a) Ito, T.; Inumaru, K.; Misono, M. J. Phys. Chem. B 1997, 101, 9958. (b) Mizuno, N.; Misono, M. Chem. Lett. 1987, 967. (6) (a) Misono, M.; Okuhara, T.; Ichiki, T.; Arai, T.; Kanda, Y. J. Am. Chem. Soc. 1987, 109, 5535. (b) Okuhara, T.; Hashimoto, T.; Misono, M.; Yoneda, Y.; Niiyama, H.; Saito, Y.; Echigoya, E. Chem. Lett. 1983, 573. (7) (a) Okuhara, T.; Hibi, T.; Tatematsu, S.; Ichiki, T.; Misono, M. 9th Iberoamerican Symp. Catal. 1984, 623. (b) Saito, Y.; Niiyama, H. J. Catal. 1987, 106, 329. (c) Hibi, T.; Takahashi, K.; Okuhara, T.; Misono, M.; Yoneda, Y. Appl. Catal. 1986, 24, 69. (d) Okuhara, T.; Hibi, T.; Takahashi, K.; Tatematsu, S.; Misono, M. J. Chem. Soc., Chem. Commun. 1984, 697. (8) (a) Baba, T.; Komatsu, N.; Ono, Y.; Sugisawa, H. J. Phys. Chem. B 1998, 102, 804. (b) Baba, T.; Inoue, Y.; Shoji, H.; Uematsu, T.; Ono, Y. Microporous Mater. 1995, 3, 647.
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