J. Phys. Chem. 1992, 96, 415-421
415
Electron Spin Resonance and Electron Spin Echo Modulation Study of Activated Pd*+-Doped, Ai,,-Pillared Laponite Clay: Evidence for the Migration of Palladium Cations from an Ai,,-Piliar to the Laponite Layer with Increasing Activation Temperature Ravi K. Kukkadapu, Vittorio Luca, and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: May 23, 1991)
Electron spin resonance (ESR), electron spin echo modulation (ESEM) and X-ray photoelectron spectroscopy (XPS) studies have been carried out on [Pd(NH3)4I2+-doped,Al13-pillaredLaponite (Pd-Al13-Lap)clay activated in flowing oxygen at 250 to 500 OC for 14 to 16 h. ESR and XPS studies indicate that some of the Pd2+cations are converted into Pd3+. Activation at 250 “C results in a broad ESR resonance from Pd3+in an axial environment (species A). ESEM indicates that species A is Pd3+bound to an A113-pillar. At activation temperatures greater than 300 OC an ostensibly isotropic signal develops (species B). ESEM data recorded for species B shows no 27A1or 7Li modulation and therefore species B is assigned to Pd3+ bound to the Laponite layers at sites remote from Li substitution sites in the octahedral sheet. At activation temperatures from 360 to 500 OC,two ESR signals form (species A1 and A2) that are also indicative of Pd3+in an axially symmetric environment. These signals appear near the resonant field of species A. ESEM indicates that species A1 is due to Pd3+ cations bound to the Laponite layers in the vicinity of a Li substitution site which could be either within the pseudohexagonal cavities in the basal oxygen surface or at crystallite edges. ESEM analysis of species A2 gives no 27A1or 7Li modulation as with species B, although the ESR suggests that species A2 occupies a site of lower symmetry than species B. Reduction of Pd-Al13-Lapwith hydrogen gives ESR signals C, D, and E characteristicof a paramagnetic Pd species with orthorhombic symmetry, XPS analysis of this reduced sample indicates that these species are Pd+ cations.
Introduction Flocculation of clays with polyhydroxymetal cations (e.g. Al13-polymer)leads to the formation of either “laminated” or “delaminated” pillared clays depending upon the particle size of the clay.’ Large particle size clays (0.05-2.0-pm diameter), like natural hectorite and montmorillonite, result in the formation of laminated pillared clays, whereas delaminated pillared clays are characteristic of clays of small particle size (10.05-pm diameter) like the synthetic hectorite which is trademarked as Laponite. Figure 1 shows (a) a proposed “house-of-cards” structure of Al,,-delaminated clay (Al13-Laponiteclay) and (b) a schematic model of Al,3-laminated, pillared clay (Al13-montmorilloniteor hectorite). Al13-Laponiteis X-ray amorphous and consists of micropore and macropore volumes. In contrast, AlI3-montmorillonite clay shows good c axis order by X-rays and is predominantly microporous as are zeolites. Laponite is a trioctahedral clay with each layer consisting of two outer tetrahedral and one center octahedral sheets. The layer charge arises from isomorphous substitution of some Mg2+in the octahedral sheet by Li+ cations. In contrast, montmorillonite is dioctahedral and the layer charge arises from the substitution of some octahedral A13+ by Mg2+cations. In this study, we report characterization of an activated [Pd(NH3)4]2+-dopedAll ,-Laponite (Pd-Al13-Lap)by X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopy. This study extends work on [Pd(NH3)4]2+-dopedAll,-montmorillonite clay (Pd-Al,,-m~nt).~The Pd3+ and Pd+ species, formed in “activated” (heating in flowing oxygen) Pd-AlI3-Lapat 500 OC, have analogous ESR parameters to those in Pd-Al13-montactivated under similar conditions.z It has been suggested that the Pd3+ and Pd+ species in Al13-montmorilloniteclay interact with the surface of the three-sheet layer of the clay rather than interacting with the pillar. However, there is no direct evidence for such a hypothesis. ESEM can provide information on the location of paramagnetic cations doped into porous solids,provided the sample gives spin echoes under the experimentalconditions. Pd-AlI3-mont does not give spin echoes under the experimental conditions. This (1) Rnnavaia, T. J. In HeferogeneousCa~alysis; Shapiro, B. L.,Ed.;Texas A&M University Press: College Station, TX, 1984; p 142. (2) Luca,V.; Kukkadapu, R. K.; Kevan, L. J. Chem. Soc.,Faraday Trans.
In press.
may be due to the presence of too much paramagnetic Fe3+within the montmorillonite lattice. Laponite is devoid of significant paramagnetic impurities. Nuclei of 7Li (abundance = 92.6%) located in the octahedral sheet and nAl nuclei (abundance = 100%) in the Al13-pillarhave nuclear spins of I = 3 / 2 and I = 5/2, respectively. Paramagnetic metal species in Laponite clay give good spin echoes;3therefore, using ESEM it should be possible to determine whether the paramagnetic Pd species in the Pd-All,-Lap system are located in the vicinity of 7Li and/or 27Alnuclei. In this study, we examine (a) the paramagnetic Pd species formed by activation and (b) the migration of the Pd species from an Al13-pillarto the surface of the three-sheet Laponite layer as the temperature of activation is increased. These results are compared with those obtained in Pd zeolites& and Pd-Al13-mont clay.2
Electron Spin Echo Modulation ESEM is a pulsed ESR technique used to measure weak hyperfine interactionsof a paramagnetic species with the surrounding magnetic nuclei (lattice and/or adsorbate) which are not normally resolved by conventional ESR spectroscopy. These weak hyperfine interactions provide useful structural information. A detailed description of ESEM theory and analysis has been p~blished.~ in a two-pulse experiment, the time between the two pulses ( T ) is varied. The echo appears at time T after the second pulse. As T is increased, the echo amplitude traces out a decay envelope which may be modulated as a result of weak dipolar electronnuclear hyperfine interactions of neighboring magnetic nuclei within about 0.6 nm. The decay is modulated at about the Larmor frequency of the interacting nuclei. For example, in the present system modulation from both 27Alor ’Li nuclei can be observed if a paramagnetic Pd cation is close to 27Alnuclei of the A113-pillar or close to 7Li nuclei in the Laponite Layer. In a three-pulse spin echo experiment the time T between the first and second pulse is kept fixed, while the time between the (3) Kukkadapu, R. K.; Kevan, L.J . Phys. Chem. 1989, 93, 1654. (4) Naccache, C.; Priiet, M.; Mathieu, M. V. Adv. Chem. Ser. 1973,121, 266. (5) Ghosh, A. K. Kevan, L.J . Phys. Chem. 1988, 92, 4439. (6) Ghosh, A. K.; Kevan, L. J . Phys. Chem. 1989, 93, 3747. (7) Kevan, L. In Time Domain Electron Spin Resonance; Kevan, L., Schwartz, R. N., Eds.;Wiley-Interscience: New York, 1979; Chapter 8.
0022-365419212096-415%03.00/0 0 1992 American Chemical Society
416 The Journal of Physical Chemistry, Vol. 96, No. 1, 1992
Micropore regions
8,
Macropore region
b)
-\
I I I I I
k
Micropore region
I
I I
I
_i
Figure 1. Schematic structures of (a) delaminated (AII3-Laponite)and (b) laminated (AI,,-montmorillonite or hectorite) pillared clays. Open circles indicate AIl3 polymer ions.
second and third pulse, T, is varied. The echo appears a t time T after the third pulse. The echo amplitude is recorded as a function of T. It is possible to suppress the modulation of one nucleus by selecting suitable r values. In the present study 7Li modulation is recorded in an Al,,-Laponite clay sample by suppressing 27Almodulations and vice versa. The resulting modulated decay can be simulated as a function of the number N of approximately equivalent nearest nuclei coupled to the paramagnetic metal ion, the average distance R to the nuclei, and the isotropic hyperfine coupling constant Ais,,.
Experimental Section Laponite-RD is a synthetic hectorite clay provided by Laporte Industries, England. Na+ is the major interlayer cation. This clay was used as received. A1,3-Laponiteclay was prepared according to a procedure described el~ewhere.~ Elemental analysis of the samples indicated incorporation of 4.83 wt % aluminum (Galbraith Labs, Knoxville, TN). A [Pd(NH3),l2+-dopedZr4-Laponiteclay was also synthesized. Zr4-Laponiteclay was synthesized as described for AlI3-Laponite3 but using a Zr4-polymeric solution instead of an Al13-polymeric solution. The Zr,-polymeric solution was obtained by stirring an aqueous solution of ZrOClZ-8H20salt a t room temperature for 2 h.8 Pd was introduced by treating 1 g of Zr4- or AlI3-Laponiteclay with 0.275 g of [Pd(NH3),]ClZ.H2Osolution (Alpha) in 200 mL of water a t room temperature for 24 h. It was then thoroughly washed with distilled water, filtered, and air-dried at room temperature. Commercial atomic absorption analysis (Galbraith Labs) of this sample indicated incorporation of 2.8 wt % Pd. XRD patterns were recorded on a Phillips PW 1840 diffractometer, using a Cu K, line. Thermogravimetric analyses (TGA) were obtained on a Dupont 951 TG analyzer with a heating rate of 10 O/min. Oxygen (30 cm3/min) was used as the purge gas. The XPS spectra were measured with a Perkin-Elmer PHI Model 550 ESCA/SAM spectrometer using Mg K, X-rays at 1253.6 eV as the excitation source. The surface adsorbed carbon 1s line at 285.0 eV was used as the reference for the energy scale. A 25-eV analyzer pass energy was employed. The measuring error was f0.8 eV. The data were analyzed by computer programs (8) Kukkadapu, R. K.; Kevan, L. J . Chem. SOC.,Furuduy Trans. 1990, 4, 691.
Kukkadapu et al. provided in the ESCA spectrometer for smoothing, correction of inelsatic scattering, and curve resolution of overlapping peaks. A deconvolution algorithm for Gaussian line shapes was used. All the samples were checked for possible X-ray-induced reduction before and after recording XPS spectra by ESR. None of them showed any evidence of radiation reduction. The generation of paramagnetic Pd was also examined by ESR. A 0.05-g sample of Pd-All,-Lap was placed on a sintered glass disk inside a Pyrex tube reactor with stopcocks at both ends. The reactor was also connected via a side arm to a small quartz ESR tube (3 mm 0.d.) so that by reorienting the reactor an ESR spectrum could be recorded without exposure of the sample to air. The clay sample is heated in flowing oxygen (-30 cm3/min) while slowly increasing to a maximum temperature (250 to 500 "C) at which the heating is continued overnight (14 to 16 h). The average deviation of the temperature is f 2 0 OC. After heating, the qctivated sample is cooled to room temperature, and excess oxygen is pumped to a residual pressure of 0.6 nm from a Li site (site B). Figure 7a shows the ESEM spectrum of species A1 in PdAI,,Lap activated at 500 OC and recorded at T = 0.28 ps. The
-
modulation frequency of 5.14 MHz is characteristic of a weak hyperfine interaction between Pd3+and the 7Li nuclei present in the center octahedral sheet of a Laponite layer. Simulation of these data neglecting quadrupole interaction for a randomly oriented 7Li nucleus7 gives an interaction distance of 0.4 f 0.02 nm between Pd3+and one 7Linucleus in an octahedral sheet. No 27Al modulation is detected when T is set at 0.43 ps, suggesting that the Pd3+ is not in the vicinity of an AlI3-pillar. The tetrahedral sheet of a Laponite clay layer has hexagonal pockets with a diameter of -0.07 nm. It was previously hypothesized that small cations with ionic radii -0.07 nm (Li', Cuz+,Fe3+,etc.) migrate toward these pockets under high-temperature treatment in montmorillonite and hectorite clays.14 It has been shown by various techniques that exchangeable cations with ionic radii less than about 0.07 nm migrate toward the hexagonal pocket in a Fe3+-exchangedmontmorillonite and in Laponite clay under high-temperature treatment condition^.'^-'^ The presence of 7Li modulation with a Pd3+ to 7Li nuclear distance of 0.4 f 0.02 nm suggests that Pd3+species A1 is located near the hexagonal pockets of the tetrahedral sheet of the Laponite layer (site Al). This distance is consistent with the sum of the distance from the center of the octahedral sheet to the center of the oxygen a t o m in the peudohexagonal pockets (0.325 nm) and the Pd3+ cation radius (0.076 nm). Since the ESEM of species B shows no modulation from 7Li or 27Al,this suggests that species B is located on a Laponite layer not proximal to a 7Li site and not associated with an Al13-pi11ar (site B). That this is only a metastable site for Pd3+can be inferred from the significant reduction in the intensity of species B as the activation temperature increases from 460 to 500 OC. (14) Hofmann, V.; Klemen, R. Z. Anorg. Chem. 1950, 262, 95. (15) Luca, V.; Cardille, C. M. Clays Clay Miner. 1989, 4, 325. (16) Calvet, R.; Prost, R. Clays Clay Miner. 1971, 19, 325. (17) Ben, Hadj-Amana, A,; Besson, G.; Tchoubar, C. Clay Miner. 1987, 22, 325. (18) McBride, M. B.; Pinnavaia, T. J.; Mortland, M. M. Am. Miner. 1975, 60. .., 66. ~.
(13) Tokarz, M.; Shabtai, J. Clays Clay Miner 1985, 2, 89.
(19) Pinnavaia, T. J.; Tzou, M. S.; Landau, S . D.; Raythantha, R. H. J. 195.
M o l . Catal. 1984, 27,
J . Phys. Chem. 1992, 96, 421-425 Species A seems to be bound to Al,,-pillars (site A in Figure 9) as is demonstrated by the observation of distinct 27Almodulation in the ESE of this species recorded on samples activated at 250 OC. In proposing a site for the location of species A l , the observation of distinct 'Li modulation in the ESE signal recorded for this species must be accounted for. There are two possible sites at which Pd3+ could be located that would result in 7Li modulation. These are pseudohexagonal cavities (site A1 in Figure 9) near a Li site and at the edges of the Laponite crystallites (site A2) with proximal Li. Species A2, which like species B gives no modulation of the ESE signal, must be bound to the Laponite layers and cannot be
421
proximal to Li sites. Possible binding sites of species A2 and B are within pseudohexagonal cavities that are not proximal to Li sites, edge sites not proximal to Li sites, or the three basal oxygen atoms of S i 0 4tetrahedra which also are not proximal to Li sites.
Acknowledgment. We thank Dr. A. Ghosh for his initial contributions to this study. We thank Dr. D. Marton for recording XPS spectra and Laporte Industries for providing the Laponite. This research was supported by the Robert A. Welch Foundation, the National Science Foundation, and the Texas Advanced Research Program. Registry No. Pd2+,16065-88-6.
Orientational Disorder in Solid Cubane: A Thermodynamic and 13C NMR Study Mary Anne White,* Roderick E. Wasylishen, Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada B3H 453
Philip E. Eaton, Yusbeng Xiong, Kakumanu Pramod, and Nereo Nodari Department of Chemistry, University of Chicago, Chicago, Illinois 60637 (Received: May 31, 1991; In Final Form: July I I , 1991)
Through a combination of adiabatic calorimetry,differentialscanning calorimetry, and I3CNMR spectroscopy,cubane (CBH8, pentacyclo[4.2.0.~~503~8.0"~7]octane) has been found to exhibit rapid rmrientational motion in the solid state at room temperature. The molecules rotate more rapidly as the temperature is increased, and this motion culminates in a transition (Ttr= 394.02 f 0.04 K, A,,H = 5940 20 J mol-I, AJ = (1.849 f 0.006)R) to an orientationally disordered solid phase that exists for a short temperature range, 11 K, prior to melting at 405 K. The entropy change at the melting point (Af,,$ = (2.7 f 0.1)R) is typical for a solid composed of globular-shaped molecules with "plastic crystalline" characteristics.
*
Introduction Polymorphism in polycyclic hydrocarbons has been known for many years.' It can arise due to the presence of an orientationally disordered phase that freezes out on cooling; one example of considerable current interest should be the fullerenes. Certainly, their known rotational disorder in the solid state2-, makes them candidates for polymorphism. For many years, there has been speculation of an orientationally disordered solid state in the cagelike hydrocarbon with the smallest number of atoms and highest symmetry: cubane (C8Hs,pentacyclo[4.2.0.02~5.03~8.04~7]octane). Indications that orientational disorder is possible are the nearly spherical molecular shape, the high vapor pressure (ca. 1 Torr at room temperature4), the low calculated barriers to single-particle re~rientation,~ and the apparent absence of welldefined shoulders in the 13CNMR spectrum of a static powder sample above T = 20 K.6 Although the room-temperature structure of cubane had_been determined by X-ray diffraction7 (trigonal, space group R3, 1 molecule per unit cell), the final disagreement factor (residual of 7.3%) was higher than one might expect for such a highly symmetric molecule if it were static. (1) For example: Parsonage, N. G.; Staveley, L. A. K. Disorder in Crystals; Clarendon Press: Oxford, U.K., 1978. (2) Yannoni, C. S.;Johnson, R. D.; Meijer, G.; Bethune, D. S.; Salem, J. R. J. Phys. Chem. 1991, 95, 9. (3) Tycko, R.; Haddon, R. C.; Dabbagh, G.; Glarum, S. H.; Douglas, D. C.; Mujsce, A. M. J. Phys. Chem. 1991, 95, 518. (4) Kybett, B. D.; Carroll, S.; Natalis, P.; Bonnell, D. W.; Margrave, J. L.; Franklin, J. L. J. Am. Chem. Soc. 1966, 88, 626. (5) Fyfe, C. A.; Harold-Smith, D. J. Chem. Soc.,Faraday Trans. 2 1975, 71, 967. (6) Facelli, J. C.; Orendt, A. M.; Solum, M. S.; Depke, G.; Grant, D. M.; Michl, J. J. Am. Chem. SOC.1986, 108, 4268. (7) Fleischer, E. B. J. Am. Chem. SOC.1964, 86, 3889.
A recent report8 of a Raman spectroscopic investigation of cubane provides evidence for a high-temperature solid-solid phase transformation. In this paper we address the question: Is there polymorphism in cubane? We present thermodynamic evidence for a transition to a high-temperature orientationally disordered phase of cubane and solid-state dynamical information based on variable-temperature I3C N M R relaxation studies.
Experimental Section The cubane sample used in these determinations was prepared at Chicago from cubane-1,4-dicarboxylicacid9using Barton methodology,1° viz. radical decarboxylation via photodecomposition/thermaldecomposition of the bis(thiohydr0xamic acid ester) in the presence of 2methyl-1-propanethiol as a hydrogen donor. The hydrocarbon was carefully purified by crystallizations from benzene and methanol, dried by transfer under vacuum through 4-A molecular sieves, and then finally sublimed at reduced pressure. Cubane is a colorless, crystalline solid at room temperature: the previously reported mp is 130 O C 9 It is stable at room temperature to oxygen, moisture, and light. The heat capacity of 0.949 g of cubane was measured by adiabatic heat pulse calorimetry for the temperature range 30 K 5 T I400 K. The calorimeter is described in detail elsewhere;" for these experiments, the upper temperature range of the calorimeter was extended 20 K beyond that of the earlier report. Due to the volatile nature of this sample, the calorimeter was filled and sealed at an ambient temperature of 5 O C , where, from thermodynamicdata: we estimated the vapor pressure to be lowered to 0.09 Torr. Differential scanning calorimetry (DSC) was carried out with a Perkin-Elmer DSC-7 calorimeter, on samples of mass about 5 mg (known (8) Dalterio, R. A.; Owens, F. J. Solid State Commun. 1988, 67, 673. (9) Eaton, P. E.; Cole, T. W., Jr. J. Am. Chem. SOC.1964, 86, 962. (10) Barton, D. H. R.; Crich, D.; Motherwell. W. B. J. Chem. SIX., Chem. Commun. 1983, 939. (1 1 ) Van Oort, M. J. M.; White, M. A. Rev. Sci. Imtrum. 1987,58, 1239.
0022-365419212096-42~%03.00/0 Q 1992 American Chemical Societv
