7863
J . Phys. Chem. 1989, 93. 7863-7868 spectroscopic observations and with the present results. It should be emphasized that the various spectroscopic measurements provide information that can be utilized only for the estimation of the value of the ratio K2u(dimer)/u(02). As in the current study, estimates of u(dimer) totally depend on the value assigned to K,. For example, recent studies of the pressure dependence of the 0, absorption spectrum in the Herzberg conTorr-' for t i n ~ u m ' -give ~ an average result of (2.0 f 0.3) X K2u(dimer)/g(oxygen) = K2A at 298 K. Using this value and Torr-] as estimated by Johnston et al., taking K , = 2.62 X we find that o(dimer)/u(O,) turns out to be close to 800. On the other hand, the value of the ratio of these cross sections is reduced to about 20, when the K2 value of Johnston is substituted by the value derived from the early estimate of HermanI7 according to which the concentration of the dimer in air at 760 Torr is I O i 4 molecules Evidence obtained in this study and in various other investigations in which different methods were used to assess the strength of the bonding forces in the oxygen dimer in the gas phase indicates that, except for very low temperatures of ca. 90 K, the oxygen molecules are very loosely bound in this species. Following the terminology used by Hirschfelder,,O the strongly bound and the weakly bound (O,), are described as bound dimer and metastable dimer, respectively. To some extent, the use of the term dimer for both the strongly and weakly interacting oxygen molecules is slightly misleading, especially in view of the fact that existence of the weakly bound species is not the only explanation for the observed effect of pressure on 0, absorbance in the Herzberg continuum. Thus, according to Blake and McCoy, this effect can (17) Herman, L. Ann. Phys. (Paris) 1939, I I , 548. (18) Pernot, C.; Durup, J.; Ozenne, J. B.; Beswick, J. A,; Cosby, P. C.; Moseley, J . T.J . Chem. Phys. 1979, 71, 2387. (19) Slanger, T. G.; Jusinski, L. E.; Black, G.; Gadd, G. E.Science 1988, 241, 945. (20) (a) Hirschfelder, J . 0.;Curtis, C. F.; Bird, R. B. Molecular Theory of Gases and Liquids; Wiley: New York, 1954. (b) Stogryn, D. E.; Hirschfelder, J . 0. J . Chem. Phys. 1959, 31, 1531.
be attributed to the Occurrence of enforced dipole transitions during the collision of free molecules.13 Whatever the case may be, the seemingly weak interaction between oxygen molecules brings about a sizeable change in the absorption cross sections in the Herzberg continuum which, in turn, is reflected in the primary photodissociation process. Basically, the nature of the metastable dimer is still obscure and depends on its lifetime. One could speculate that this dimer might undergo photodissociation along different channels than free oxygen does. For example, a ring-shaped complex could conceivably dissociate in a process that leads directly to ozone (O,),
+ hu
-
O3 + 0
(23)
Since in our system the 0 atoms thus formed will readily react with the excess of oxygen and form ozone, occurrence of this kind of photodissociation process would be kinetically indistinguishable from a single-step dissociation into 0, and two O(3P). The threshold for 0, dissociation lies at 242.4 nm,'* and therefore photodissociation of an oxygen dimer into two O(3P) could possibly be accompanied by formation of vibrationally excited ground-state oxygen molecules that carry some of the excess energy available when photons a t shorter wavelengths are absorbed. Again such a reaction would not alter the kinetics of ozone formation in our system. However, in the real atmosphere at low pressures, the vibrationally excited ground-state molecule could possibly have a sufficiently long lifetime to absorb solar radiation and dissociate. Evidence for the Occurrence of this kind of reaction of vibrationally excited ground-state oxygen molecules has been obtained in a recent study by Slanger et aI.l9 of the excimer-laser-induced 248-nm photolysis of oxygen. Based on the findings of our study and on spectroscopic evid e n ~ eas~ well , ~ ~as ~theoretical arguments brought forward by Blake and McCoy,13 the occurrence of these reactions of oxygen metastable dimers cannot be considered more than an exciting speculation. Registry No. 02,7782-44-7; (O&, 12596-83-7; 03, 10028-1 5-6.
Structure and Reactivity of Oxovanadate Anions in Layered Lithium Aluminate Materials Jen Twu and Prabir K . Dutta* Department of Chemistry, The Ohio State University, 120 West 18th Avenue, Columbus, Ohio 43210 (Received: February 17, 1989; In Final Form: May 8 , 1989)
Lithium aluminate, [LiAl,(oH),]+, examined in this study is characterized by positively charged layers of edge-sharing M(OH), octahedra. The neutrality of the material is maintained by anions, which exist between the layers. In this study, we have examined the ion exchange of vanadate ions from aqueous solution into the lithium aluminates. Powder X-ray diffraction patterns, diffuse reflectance, and Raman spectroscopy have provided information about the specific oxovanadate ions in the interlayers and their orientation. Thermal treatment of LiAI,(OH)6-V,07 was also examined. At temperatures below 100 'C, dimerization of the v,o7,-is found to occur, which is reversed upon rehydration. At higher temperatures, vanadate species primarily containing polymeric units of tetrahedral VO, are formed. These materials exhibit selective oxidation characteristics, as evidenced by the oxidation of o-xylene to o-tolualdehyde.
Introduction Supported metal oxide catalysts have found wide applications in industrial processes.' For instance, vanadium oxides are used for selective oxidation of hydrocarbons to manufacture butadiene, maleic anhydride, acrolein, acetaldehyde, acetic acid, methanol, formaldehyde, and phthalic anhydride.2 Considerable interest ( 1 ) Kung, H. H . Ind. Eng. Chem. Prod. Res. Deu. 1988, 25, 171. (2) Dadyburjor, D. B.; Jewur, S . S . ; Ruckenstein, E. Catal. Reu.-Sci. Eng. 1919, 19, 293.
0022-3654/89/2093-7863!$01.50/0
exists in understanding the correlation between the structure of the metal oxide and Rs reactivity, as well as the role of the supp ~ r t . ~ Novel -~ methods for generating metal oxide layers on supports is an area of active research.&* The aim of these studies (3) Grasselli, R. K.; Burrington, J. D. Adu. Catal. 1980, 30, 133. (4) Ziolkowski, J. J . Catal. 1983, 80, 263. (5) Mori, K.; Miyamato, A,; Murakami, Y. J . Phys. Chem. 1984,88,2735. ( 6 ) Kijenski, J.; Baiker, A,; Greinski, M.; Dollenmeier, P.; Wokaun, A. J . Catal. 1986, 101, 1 .
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The Journal of Physical Chemistry, Vol. 93, No. 23, I989
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is to improve the performance of the catalyst by influencing its distribution and interaction with reactants and products. The role of specific vanadate species in well-defined coordination geometries is proposed to be responsible for the catalytic performance on different supports."' These materials are typically synthesized by impregnation of aqueous vanadate solution onto supports, followed by calcination. We have recently published a paper on a layered lithium aluminate hydroxide material that exhibits selective uptake of specific anions by the process of ion exchange from a complex equilibrium of ions in solution.I2 In this study, we focus on the ion exchange of vanadate ions. These ions exhibit a complex equilibria in solution, depending on both the pH and the concentration of vanadium. For a 0.1 M aqueous vanadate solution, these equilibria can be roughly summarized asI3
-
(V 1002dOH V 1~027(OH)5-,V IOOZS~-) decavanadate (pH 1-3) (VO(OH),, VO,(OH)-, V,0g3-, V4OI;-) metavanadate (pH 4-6) (V03(OH)2-, HV2073-,V20T6-) pyrovanadate (pH 8-1 1)
-
I
I 'L 400
800
600
Id00
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v0d3-
vanadate (pH
> 12)
The layered lithium aluminate material represented by the formula [LiA12(0H)6]+.X-can be pictured as sheets of lithium and aluminum atom octahedrally surrounded by hydroxyl group^.^^^'^ These sheets, which define the framework, are positively charged and need to be neutralized by the anions, X-. The anions position themselves between the sheets and can be readily ion-exchanged. These materials resemble the more commonly studied hydrotalcites, whose anion-exchange behavior has been well investigated.'b22 A recent communication by Pinnavaia and co-workers reported on the exchange of VI0O22- into a transition-metal hydrotalcite and the photoxidation of isopropyl alcohol, using this material as a catalyst.2k Dredzon has also reported the ion exchange of V,,02,6- into hydrotalcite via an organic acid exchanged material.22 We report here the ion exchange of vanadate ions into LiA12(OH)6+over a wide pH range. A combination of spectroscopy and diffraction measurements provides information about the ion-exchange process and the orientation of ions in the interlayers. Thermal treatment promotes the reaction between the vanadate ions inside the lithium aluminate layers and provides materials that exhibit selective catalytic properties toward the oxidation of o-xylene. This study indicates the feasibility of generating novel materials by ion exchange of polyoxometallic anions into layered compounds.
(7) Bond. G.C.: Zurita. J . P.: Filamerz. S. ADD/.Catal. 1986. 22. 361. (8) Van Hengstum, A. J.; Van Ommen, J. G.;'Bosch, H.; Grellings, P. J . Appl. Caral. 1983, 5, 207. (9) Kozlowski, R.; Pettifer, R. F.; Thomas, J. M. J. Pfiys.Cfiem. 1983,87, 5 172. ( I O ) Naryana, M.; Narasimhan, C. S.; Kevan, L. J . Carol. 1983,79,237. ( I I ) Haber, J . ; Kozlowska, A.; Kozlowski, R. J . Carol. 1986, 102, 52. (12) Puri, M.; Dutta, P. J . Pfiys. Cfiem. 1989, 93, 376. ( I 3) Baes, C.H.; Mesner, R . E. The Hydrolysis of Cations; Wiley: New
York, 1976. (14) Serna, C. J.; Rendon, J . L.; Iglesias, J. E. Clays Clay Miner. 1982, 30, 180. (15) Reichle, W. T. CHEMTECH 1986, 86. (16) Bish, D. L. Bull. Mineral. 1980, 103, 107. ( I 7) Miyata, S. Clays Clay Miner. 1983, 31, 305. (18) Brindley, G. W.; Kikkawa, S.CIuys Clay Miner. 1980, 28, 87. (19) Itaya, K.; Chang, C. H.; Uchida, 1. Inorg. Cfiem. 1987, 26, 624. (20) (a) Giannelis, E. P.; Nwera, D. G.;Pinnavaia, T. J . Inorg. Cfiem. 1987, 26, 203. (b) Martin, K. J.; Pinnavaia, T. J. J . Am. Cfiem.SOC.1986, 108, 541. (c) Kwon, T.; Tsigdinos, G. A.; Pinnavaia, T. J . J. Am. Cfiem.Soc. 1988, 110, 3653. (21) Carrado, K. A.; Suib, S. L.; Skoularikis, N . D.; Coughlin, R. W. Inorg. Cfiem. 1986, 25, 4217. (22) Dredzon, M. Inorg. Cfiem. 1988, 27, 4628. (23) Dutta, P. K.; Shieh, D. C. Appl. Sperrrosc. 1985, 39, 343.
1
:
li .2.00
.
I/ 10.0-2
i
. ..
d U L ~
20.vI0
30.00
40.80
28 (degree) Figure 1. Powder X-ray diffraction pattern of (a) [LiAI,(OH),]Cl and (b) [LiAI,(OH)6]C1 exchanged with 0.1 M vanadate solution at pH 10. (c) Raman spectrum of the vanadate-exchanged material. Excitation: 457.9 nm.
Experimental Section Analytical reagent grade chemicals were used for all preparations described in this work. The preparation of the clay was adapted from the method by Serna et aI.I4 Aluminum (0.05 mol) was dissolved in 100 mL of 2 M N a O H solution. LiCl (0.25 M) was dissolved in this solution, and the mixture was heated for 48 h a t 90 OC. The products were washed with water followed by 0.1 M NaCI. Care was taken to exclude C 0 2 during the synthesis. Vanadate solutions were prepared by dissolving NaVO, in water, and the pH value was adjusted by mixing with either N a O H or HCI solution. Ion exchange of the clay was carried out with 0.1 M vanadate solution for 3-24 h at room temperature. During the ion exchange, the pH values of the solutions were monitored and adjusted, if necessary. The samples were washed with deionized water and air dried prior to the spectroscopic experiment. Heat activations of these materials were done under ambient conditions with an oven manufactured by Technical Products Corp. Powder X-ray diffraction was obtained with a Rigaku Geigerflex D/Max 2B diffractometer using Cu Ka radiation. The vanadium content of the clay was determined by X-ray fluorescence with a Kevex Model 0700 spectrometer. Intensity of the V K a radiation was measured following excitation with an Fe secondary target. The Raman spectra were recorded with either 10-50 mW of 457.9-nm radiation from a Spectra Physics argon-ion laser or 647.1-nm radiation from a Coherent krypton-ion laser. The scattered light was collected and dispersed through a Spex Model 1403 double monochromator and detected with a GaAs PMT with photon counting. A Shimadzu Model UV-265 spectrophotometer with a diffuse-reflectance attachment was used to obtain the electronic spectrum. BET/surface area measurements were conducted on a Micromeritics Pulse Chemisorb 2900 instrument. The catalytic performance of the samples for the oxidation of o-xylene was studied in a glass reactor containing 1 g of clay and 5 g of glass beads. The temperature of the reactor was maintained a t 350 O C . Air a t a velocity of 25 mL/min was passed through an o-xylene reservoir immersed in a temperature-controlled water
Structure and Reactivity of Oxovanadate in [LiAI2(oH),]+
L 400
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The Journal of Physical Chemistry, Vol. 93, No. 23, 1989 7865
I , -v
400
600
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1000
WAMNUMBER
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Figure 2. Raman spectrum of material obtained after exchange of [LiAI,(OH),]CI with 0.1 M vanadate solution at pH 5. Excitation: 457.9 nm. (Unwashed sample.)
bath. The products were trapped by condensation with cold water and were analyzed by a Varian Model 920 gas chromatograph, using known standards to estimate the retention times.
Results and Discussion ton Exchange of Oxovanadate Anions. The ion-exchange process involving the replacement of interlayer chloride ions in [LiA12(0H)6]+CI- by vanadate ions from a 0.1 M aqueous solution of sodium vanadate was investigated over the range of pH 2-14. At pH's exceeding 13, it was not possible to ion-exchange vanadates into the lithium aluminate, since the hydroxide anions selectively replace the interlayer chloride ions. Ion exchange between pH 8 and 1 1 resulted in the formation of a vanadate-exchanged material, whose powder X-ray diffraction pattern and Raman spectrum are shown in parts b and c of Figure 1, respectively. Figure l a is the diffraction pattern for the lithium aluminate chloride. Comparison of parts a and b of Figure 1 clearly indicates that the gallery height increases from 3 ( d m l= 7.8 A) to 6 A (doe, = 10.8 A) as the chloride ions are replaced by the vanadate ions. No residual reflections corresponding to the chloride ions in the interlayer are evident after ion exchange, indicating complete exchange. This is also supported by the X-ray fluorescence measurements, which indicate a vanadium loading of 19 wt %. The Raman spectrum in Figure I C is characterized by bands at 350,483,605, 837, and 878 cm-'. The band at 605 cm-I has been assigned to the AI-0 stretching motion of the framework,I2 whereas the rest of the bands arise from the vanadate species. Ion exchange between pH 5 and 6 also leads to complete exchange of vanadate species for chloride ions. Figure 2 shows the Raman spectrum of the ion-exchanged material a t pH 5. The X-ray diffraction of this material is identical with that shown in Figure 1 b, with a gallery height of 6 A, but the Raman spectrum is quite distinct. The major Raman bands are observed at 358, 520, 606, 877, and 944 cm-I. The band a t 877 cm-l is similar to that observed upon ion exchange at pH 8-1 1, whereas the other bands are indicative of the presence of new vanadate species. Ion exchange at this pH range also results in the appearance of weak Raman bands at 330 and 977 cm-'. These bands disappear upon extensive washing, indicating that the species responsible for these bands are adsorbed on the surface. When the acidity of the ion-exchanging solution is lowered to pH 3-4, the exchange of vanadates still occurs, but as shown in the X-ray diffraction pattern in Figure 3, reflections due to interlayer chloride are present, indicating incomplete exchange. In order to obtain the exchanging solutions at these lower pH's, acidification of these solutions was carried out with HCI. Thus, under these conditions, vanadate ions compete with chloride ions for ion exchange. In these samples, the gallery height is 6 A, and the Raman bands are observed at 326, 353, 521, 605, 835,946, and 982 cm-I. The bands at 326 and 982 cm-I are not removed by extensive washing, as was observed in the range between pH 5 and 6. Exchange at values lower than pH 3 led to progressive destruction of the lithium aluminate material, as evidenced by the appearance of a broad band in the diffraction pattern at 20-25
28 (degree) Figure 3. (a) Powder X-ray diffraction pattern and (b) Raman spectrum of [LiAI,(OH),]CI exchanged with 0.1 M vanadate solution at pH 3.
Excitation: 457.9 nm. prominent Raman band of these species occurs in the 8001000-cm-l region due to the stretching motion of the V-0 functionality. Several trends are manifested by this band. Its frequency increases as the number of terminal oxygen atoms decrease. For example, the frequencies of the V-0 stretch for tetrahedrally coordinated vanadium with four, three, two, and one terminal oxygen atoms occur at 827, 877, 945, and 1030 cm-I, respectively. Increasing polymerization along with branching at the vanadium atom to form oxovanadate species should therefore lead to an increase in the V - 0 stretching frequency in the Raman spectrum. Such effects have been reported for molybdates and silicates and can be understood as arising from increased bond order of the terminal-metal-oxygen atoms as polymerization at the metal center proceed^.^^,^^ Another general feature in the vibrational spectra of metal oxoanions is the increase in frequency of the terminal-metal-oxygen stretch as the other oxygen atoms around the metal center are p r ~ t o n a t e d . ~ ~ . ~ ~ The Raman spectrum of the vanadate-exchanged material at pH 10 (Figure IC) is best assigned to V2074 ions, which in solution exhibits bands at 351, 503, 810, 850, and 877 cm-l due to 6(V03), v(V0V) (s), v(V0V) (as), v ( V 0 3 )(as), and v(V03) (s) motions, HV20,)-, and (V03),x- ( n respectively. The presence of vo43-, > 3) can be definitely excluded due to the lack of the corresponding prominent Raman bands at 340,820,915, and 945 cm-I. However, HV042-, which is characterized by Raman bands at 351, 545, and 877 cm-I cannot be considered to be absent in the interlayer, based on the Raman data alone. The weakness of the Raman band at 545 cm-' makes it difficult to distinguish between HV042- from V2074-. The major components in the ion-exchanging solution at pH 10 and 0.1 M concentration of vanadium are V20,4-, HV20,3-, and HV042- and are present in equilibrium molar ratios of 6:3:1.13 W e have noted in an earlier study, that ions with higher charge densities are preferentially ion-exchanged into [LiAl2(oH)6]+; e.g., for the phosphate and sulfate series,12 the exchange selectivity followed the order Po43-> HP042- > H2PO4I- and > HS041-. The present observation that V20,4- is preferentially exchanged over HV20,3- is in agreement with the previous studies. The much lower concentration of HV042- in the exchanging solution as compared to v2074-ion would also favor the ion exchange of the latter species. On the basis of these factors, we conclude that V20,4- is the predominant species in the interlayers of [LiAI,(OH),]+. Information about the orientation of the V207"- anion can be derived from the gallery height of 6 A. This ion is formed by the sharing of two VO4 tetrahedra as in V03-0-VO,. The two distinct arrangements are
A.
Griffith and co-workers have reported on the Raman spectra of vanadate ions in solution as a function of pH.24 The most
(24) (a) Griffith, W. P.; Wilkins, T.D.J . Chem. Sor. A 1966, 1087. (b) Griffith, W. P.; Lesniak, P. J. B. J . Chem. SOC.A 1969, 1066.
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The Journal of Physical Chemistry, Vol. 93, No. 23, I989 o?,
.
.
.
Twu and Dutta
.
0
v
0A
V\? 0
L
0
0’;’o
....* A
O’O + + + + + +
B
From the single-crystal data on Mg2V207,25 we estimate that the dimension of the V20T4- ion corresponding to arrangements A and B are 7.8 and 5.0 A, respectively. The van der Waals radii of the oxygen atoms have been taken into account. On the basis of the observed gallery height of 6 A, arrangement B appears to be more likely. Also, it is to be noted that different orientations of the anion are possible in arrangement B, involving rotation around the V-0-V plane. For an ordered arrangement of the lithium and aluminum atoms in the octahedral sheet,I4 the surface area around each unit positive charge is 25 A2. I n the arrangements A and B, the ion (4 units of negative charge) is distributed over areas approximating 25 and 40 A2, respectively. The energetics of the ion-exchange process favor the more charge-delocalized distribution as in B, indicating that the energy required to spread the layers further apart (arrangement A ) is not compensated for in the vertical orientation. In order to account for the gallery height of 6 8, for arrangement B, a layer of water molecules sandwiching the v2074-is required. The spaces between the vanadate ions is also filled with water molecules. This system bears a resemblance to hydrated salt forms of tu-zirconium phosphates.26 Upon exchange between pH 5-6, the Raman spectral features ~the band at 877 cm-’ appear weakly in due to v ~ 0 7 including the spectrum (Figure 2). The set of prominent Raman bands at 360, 520, and 944 cm-l is assigned to V40124-,which in solution exhibits prominent bands at 360 and 945 cm-1.23924The Raman bands at 330 and 980 cm-I that are removed upon washing are ~ . ~ ~ within this characteristic of a V,,02,6- ~ p e c i e s . ~Therefore, pH range, V4012eis the predominant species exchanging into the [LiAi,(OH)6]+, with a minor amount of V2074-. The similarity - - V40,24--exchanged in gallery hei ht between the v ~ 0 7 ~and material of 6 results from the fact that the smallest dimension of these two units is similar. In this geometrical arrangement for V4012-, unit negative charge is distributed over a larger area as compared to the V20,“- anion. At the lowest pH (3-4), the major species in the interlayers is still V401;- ions, along with a minor amount of V,,OZsb. No Raman bands due to V207@are observed. The presence of V,0028b is indicated by the Raman bands at 320 and 980 cm-I, which cannot be removed upon washing. The gallery height in this case is also of the order of 6 A, as observed in the previous samples. In contrast to this study, Pinnavaia and co-workers have found that Vlo0286-can be completely ion-exchanged into hydrotalcites containing zinc aluminum, zinc chromium and nickel aluminum in the framework, with a gallery height of 7.1 Dredzon has also noted complete exchange of V,,OZs6- into a magnesium-aluminum hydrotalcite via preexchange of organic ions.22 Since v],O2s6- is only a minor component in the lithium aluminate at pH 3-4, reflections corresponding to the major species V40,;- are only observed in the diffraction patterns. Attempts to increase the loading of V,o0,86versus V401?- by adjusting the concentration and p H were unsuccessful. The major difference between the lithium aluminate and the transition-metal hydrotalcite is in the layer surface area per unit charge: 25 and 16.5 A2, respectively. The more concentrated charge density in the transition-metal hydrotalcite may
w
(25) Gopal, R.; Calvo, C. Acta Crystallogr. 1974, 830, 2491. (26) Alberti, G . Arc. Chem. Res. 1978, 1 1 , 163. (27) So, H.; Pope, M. T. Inorg. Chem. 1972, 11, 1441. (28) Selbin, J.: Morpugo, L. J . Inorg. Nucl. Chem. 1965, 27. 363. ( 2 9 ) Onodera, S.; Ikegami, Y. Inorg. Chem. 1980, 19, 615. (30) Lippens, B. C.; de Boer, J . H. Arra Crystallogr. 1964, 17, 1312. (3 I ) West, F. P.; Glasser, F . P. J . Solid State Chem. 1972, 4, 20. (32) Bubnova, R.; Filatov, K . Powder Diffraction File; International Center for Diffraction Data: Swarthmore, U. K., 1988. 32-606.
Figure 4. X-ray diffraction pattern obtained after thermal treatment of a [LiAI,(OH)6]CI-V,0, sample: (a) room temperature; (b) 80 OC, 24 h; (c) 200 OC, 5 h; (d) 300 ‘C, 1 h; (e) 300 OC, 3 h; (f) 450 ‘C, 24 h.
favor the more compact V100286-ion. This is supported by the observation of Pinnavaia and co-workers2k that ion exchange of V4012-ion into the transition-metal hydrotalcites leads to a gallery height of 4.7 8, as compared to 6 8, observed in this study. The smaller spacing can be attributed to the stronger electrostatic interaction due to the higher charge density in the transition-metal hydrotalci te. In summary, this study indicates that the vanadate ions exchanged into the lithium aluminate is a function of the pH of the exchanging solution. V2074- is selectively ion exchanged between pH 8-1 1. At pH 5-8, V4012-4is the major species along with minor quantities of v ~ 0 7 ~For - . the most acidic solutions (pH 3-5), v40,24is still the major species along with VloOZ8”ions. Thermal Treatment of LiA12(OH)6-V207.It is well recognized that in order to generate catalytically active materials from hydrotalcites, thermal decomposition of the support is n e c e ~ s a r y . ~ ~ - ~ ~ The fate of the vanadate ion, v 2 0 7 4 - exchanged a t p H 10, and its interaction with the support were investigated as a function of temperature. Figure 4 shows the powder diffraction pattern a t different temperatures as the sample is heated from ambient to 450 “C. There is a gradual decrease in the gallery height as the sample is heated. The layer structure begins to disappear at temperatures beyond 300 O C . At temperatures over 450 “C, a new phase is observed in the diffraction patterns. The gallery heights upon heating the sample at 80, 200, and 300 O C are 3.4, 2.3, and 1.8 A, respectively. The starting material has a gallery height of 6 8,. Rehydration of these heated samples resulted in a change of the gallery heights to 6, 2.4, and 1.8 8, for the 80, 200, and 300 O C samples, respectively. The sample heated to 80 “C upon hydration regenerates the diffraction pattern of the starting material. In the thermal decomposition of hydrotalcites with anions such as carbonates, reversibility has been reported (33) Busetto, C.; Del Piero, G.; Manara, G. J . Catal. 1984, 85, 260. (34) Reichle, W. T. J . Catal. 1985, 94, 547. (35) Reichle, W. T.; Kang, S. Y.; Everhardt, D. S . J . Catal. 1986, 101, 352.
Structure and Reactivity of Oxovanadate in [LiA12(OH)6]+
A
ko
600
Bi
to the exhaustion of the interlayer water molecule necessary for the reaction. The dimerization leads to vanadium atoms with two terminal oxygen atoms. Upon heating to temperatures of 200 OC and above, further polymerization involving the formation of more condensed vanadate species can be excluded, since the Raman spectrum shows no evidence of bands in the range of 1000 cm-I due to vanadium atoms bonded to one terminal oxygen atom.23s24 Instead, the V02(s) stretch decreases in frequency from 945 to 906 cm-I, whereas the VO,(s) stretch increases from 877 to 906 cm-I. The layer structure is evident until heat treatments of 300 "C, with progressively decreasing gallery heights. This also supports the conclusion from the Raman data that random condensation around the vanadium atom is not occurring, for it would necessitate increased spacings. W e propose that at these temperatures, hydroxide-mediated polymerization and depolymerizatior, reactions occur, leading to oligomeric chains of VO, tetrahedra linked by oxygen atoms. The vanadium atom, on the average, contains two terminal oxygen atoms, leading to a V 0 2 stretch at 906 cm-l. Such species resemble the metavanadates: (VO,),"-, whose most prominent Raman band occurs between 925-940 cm-1.29 The hydroxide ions necessary for creating the metavanadate species in the lithium aluminate are produced in the above-discussed dimerization reaction and progressive dehydroxylation of the lithium aluminate framework. The bands at 270 and 320 nm in the diffuse reflectance spectra are also supportive of VO, units. At temperatures in excess of 450 "C, both the Raman spectrum and X-ray diffraction indicate the creation of new vanadate species. In the Raman spectrum (Figure 50, sharp peaks at 791, 818, and 832 cm-', along with a broad band at 945 cm-I is observed. The anions, set of sharp peaks are characteristic of the discrete vo43whereas the broad peak is in the range for m e t a ~ a n a d a t e s .The ~~ diffraction data (Figure 4f) indicate sharp peaks at 20 = 37.5", 45.8", and 66.4', which are readily assigned to y-A1203.30The broader peaks at 20 = 18.7", 20.8', 28.2", 32.4', and 58.8" are indicative of Li,V04 and LiV0,,31*32 in agreement with the Raman spectrum. These results suggest that at temperatures of 450 "C, the lithium aluminate framework is being destroyed, with formation of lithium oxide and alumina, analogus to the thermal ~ - ~basic ~ L i 2 0 reacts with decomposition of h y d r o t a l ~ i t e s . ~The the vanadates to form Li3V04 and LiVO,. The above observations can be summarized in the following scheme:
-
lDMl
WAVENUMBER
Figure 5. Raman spectra of samples obtained after thermal treatment of [LiAIz(OH)6]-Vz07: (a) room temperature; (b) 80 O C , 24 h; (c) 200 O C . 3 h; (d) 300 O C , 6 h: (e) 400 O C , 6 h; (f) 450 OC, 24 h.
even after heating samples to temperatures of 500 "C. The new crystalline phase formed by heating to temperatures exceeding 450 "C is characterized by reflections at 20 = 18.7", 20.8", 28.2', 32.4", 37.5". 45.8', 58.8", and 66.4". In order to understand the structural changes that occur upon thermal treatment, diffuse reflectance and Raman spectroscopy were also carried out on the thermally treated samples. The electronic spectra of the material heated to 200 and 400 "C exhibit bands at 270 and 320 nm, respectively. These are representative d(e) of charge-transfer transitions a(t2) d(e) and a(t,) arising from tetrahedral v a n a d a t e ~ . ~ ~ % ~ * The Raman spectra of the thermally treated LiA12(OH),-V207 material as a function of temperature are shown in Figure 5. Upon heating to 80 "C, a decrease in intensity of the bands at 350, 483, and 877 cm-' due to v 2 0 7 , - are observed, along with the growth of bands at 338, 363, 514, and 942 cm-I. For samples heated to 200 "C, broadening of the prominent bands at 877 and 945 cm-I is observed. Upon further heating (300 "C), these bands approach each other, and finally merge at 906 cm-' for samples heated to 400 "C. The band at 350 cm-' progressively loses intensity as a function of heating. Beyond 450 "C, the intensity of the band at 906 cm-' decredses along with the growth of sharp Raman peaks at 79 I , 8 18, and 832 cm-' and a broad band at 945 cm-'. The new peaks that appear upon heating to 80 OC can readily be assigned to V40124-ion (Figure 5b).23324 Therefore, initial heating promotes the dimerization reaction 2V207+ + 2 H 2 0 F? V40124-+ 40H-. As shown in Figure 4b, this reaction brings about a decrease of gallery height by 2.4 A, possibly due to the loss of the interlayer water used up in the reaction. However, this process is reversible and rehydration generates the starting LiA12(OH)6-V207 material. This is a novel example of reversible interlayer dimerization. This reaction reaches a steady state after 4-6 h of heating at 80-1 20 "C, after which the relative intensities of the Raman bands due to V207" (877 cm-I) and V401z" (945 cm-I) do not change. The incomplete conversion could be due
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The Journal of Physical Chemistry, Vol. 93, No. 23, 1989 7867
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It is of considerable interest to contrast this scheme with the extensive studies done on the thermal decomposition of oxovanadates on other supports, including Al2O3, Si02, Ti02, MgO, and Ce02.3W' Raman spectroscopy has been used in these studies and allows for a direct comparison with the present work. The initial Raman spectrum obtained upon impregnation of vanadates onto supports depends on the p H of the solution.40 Samples prepared by impregnation of vanadate solutions a t p H 10 on A120, exhibit a broad band at 940 cm-I (estimated half-width, -75 cm-I). The width of this peak is a reflection of the different vanadate species (all with V 0 2 terminal groups) that are adsorbed on the surface. In comparison, the lithium aluminate at pH 10 selectively adsorbs the vzo74ion. Calcination of the aluminasupported vanadates in air at 450 "C leaves the 940-cm-' band
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(36) Roozeboom, F.; Medema, J.; Gellings, P. J. Z . Phys. Chem. (Munich) 1978. III. 215.
(37) Chan, S. S.; Wachs, I. E.; Murrell, L. L.; Wang, L.: Hall, W. K. J . Phys. Chem. 1984, 88, 5831. ( 38) Honicke, D.; Xu, J. J . Phys. Chem. 1988, 92, 4699. (39) Le Coustumer, L. R.; Taouk, B: LeMeur, M.; Payen, M.; Guelton, M.; Grimblot, J. J . Phys. Chem. 1988, 92, 1230. (40) Payen, E.; Grimbolt, J.; Kasztelan, S. J . Phys. Chem. 1987, 91, 6642. (41) Saleh. R. Y.; Wachs, I. E.; Chan, S. S.; Cherish, C. C. J . Cufal.1986, 98, 102
7868 The Journal of Physical Chemistry, Vol. 93, No. 23, 1989 relatively unperturbed along with an increase in intensity around 800 cm-I. Since no well-defined Raman peaks are observed, it is difficult to postulate the presence of discrete species. The increased intensity at -800 cm-' has been assigned to polymeric tetrahedral vanadate species.36 If these samples are calcined in dry 02,a sharper peak at 1026 cm-' is observed,37which has been assigned to vanadium with single terminal oxygen atoms and/or neighboring vacancy sites.39 There is no change in the Raman spectrum for the LiA12(0H)6-V,07 sample after calcination in either dry or ambient conditions. Thus, the initial adsorption of vanadates onto other supports and subsequent polymerization is different from the lithium aluminate support. Another difference between the impregnated supports and this study is that even at 19 wt % V loadings on [LiAi,(OH),]+, no evidence for the formation of crystalline V 2 0 5 was noted, indicating a very welldispersed system. This is not surprising, since the lithium aluminate layers isolate the oxovanadate ions. Oxidation Reaction. Oxovanadate ions on supports have been extensively studied as catalysts for the oxidation reaction of hydrocarbons.'*2 As discussed above, the lithium aluminate material provides an unique support for incorporation of well-dispersed vanadate ions. The reactivity of the 0(-V0,-),O layer in the interlayers of the lithium aluminate generated by thermal decomposition of LiAI2(0H),-V,O7 at 350-450 "C was examined for the oxidation of o-xylene. In order to provide access to the vanadium sites in the interlayers, it is necessary to partially destroy the f r a m ~ w o r k . ~ ' -This, ~ ~ as has been shown above, is brought about by thermal decomposition. The surface area provides a sensitive measure of the accessibility to the internal sites of the system. Reichle and co-workers have shown that the surface area of hydrotalcites exhibits an increase as water molecules generated by dehydroxylation force their way out of the layers, thereby generating a porous s ~ p p o r t . ~The ~ , onset ~ ~ of higher surface area depends on the material and temperature, as shown recently for a series of h y d r o t a l c i t e ~ . ~The ~ surface are for LiA12(0H)6-V207 heated to 400 "C is 15 m2/g. The products from the reactor are collected by condensation of the vapor at room temperature
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(42) Kelkar, C. P.; Schutz, A,; Marrelin, G . In Perspective in Molecuiar Sieve Science; Flank, W. H., Whyte, T. E., Jr., Eds.; American Chemical Society: Washington, DC, 1988.
Twu and Dutta and found to be primarily o-tolualdehyde along with unreacted o-xylene. The ratio of o-tolualdehyde to o-xylene in the product is determined by gas chromatography to be 1 :6. Since analysis for the volatile gases (CO, C 0 2 ) was not carried out, an exact mass balance cannot be made. However, about 70%of the volume of o-xylene used in the reaction was recovered in the condensed product, indicating that the tetrahedral vanadates, under these reaction conditions, do to some degree selectively promote the oxidation of one of the methyl groups. The conversion to product over the catalyst is relatively low. A more detailed analysis of the products along with comparison to other supports is in progress. The LiA12(0H),-CI thermally decomposed under conditions described above for making the vanadate catalyst exhibit no reactivity in the oxidation of o-xylene, indicating the important role of the vanadium center.
Conclusions The major conclusions of the three areas examined in this study are as follows: (a) Ion exchange of vanadate ions into LiA12(0H),+ from an aqueous solution is influenced by pH. At pH 8-1 1, V2074- is selectively ion-exchanged, whereas as the pH is dropped, V4012is the major interlayer species, along with minor amounts of V2O.i' (pH 5-6) and V,00286-(pH 3-4). (b) Thermal treatment of LiA12(0H),-V@7 was carried out to temperatures of 500 "C. The layer structure is maintained until temperatures of 300 "C, as evidenced from the diffraction patterns. However, only samples heated until 100 "C will revert back to the original starting material upon rehydration. At these temperatures, VzO," dimerizes in the interlayers to form V,012-. Beyond 300 "C, polymeric O-(VO2),-0 species are formed. At 450 O C , the lithium aluminate structure is completely destroyed, leading to formation of Li3V04, LiV03, and AI2O3. (c) Material formed upon heat treatment of LiA12(0H)6+-V207 to 300-400 "C exhibits catalytic activity in the selective oxidation of o-xylene to o-tolualdehyde.
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Acknowledgment. We are grateful to the National Science Foundation (Grant CHE-8510614) for support of this work. Registry No. [LiA12(0H)6]CCI-, 68949-09-7; V2074-, 22466-30-4; o-xylene. 95-47-6.
