J. Phys. Chem. B 1998, 102, 2207-2213
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Chemical Behavior of Lithium Ions in Reexpanded Li-Montmorillonites Marı´a D. Alba, Rafael Alvero, Ana I. Becerro, Miguel A. Castro,* and Jose´ M. Trillo Departamento de Quı´mica Inorga´ nica, Instituto de Ciencia de Materiales, UniVersidad de SeVillasConsejo Superior de InVestigaciones Cientı´ficas, P.O. Box 874, 41080 SeVilla, Spain ReceiVed: May 9, 1997; In Final Form: January 12, 1998
Chemical behavior of lithium ions in reexpanded lithium-montmorillonites has been investigated, their hydration capacity, interlamellar availability, and thermal reactivity being examined. Two experiments have been carried out: a thermal treatment to evaluate the possible recollapse of the sample, and an ionic exchange with sodium ions followed by a further thermal treatment to analyze its exchange capacity. Samples obtained have been characterized by means of XRD, FTIR, DTA/TG, and 29Si, 27Al, and 7Li MAS NMR. A complete recollapse of the sample after the thermal treatment and an extensive ionic exchange in NaCl solution were attained. The total reversibility of the process for lithium ions has been concluded, although irreversible damage in the silicate lattice is observed.
Introduction The design of new materials based on layered aluminosilicates requires a precise knowledge of the interaction mechanisms between the interlamellar cations and the lattice of the solid upon thermal and hydrothermal treatments, since these activation processes are commonly used when those materials are employed, for instance, as solid-acid catalysts or radioactive waste repository components.1,2 Within this general approach, the interaction between interlayer lithium cations and dioctahedral montmorillonite lattice under thermal treatment has become a classical subject. Early studies3 demonstrated that Li-saturated montmorillonite irreversibly loses its expandable character and hydration capacity and exhibits a greatly reduced exchange capacity, following heat treatment at 200-300 °C. The importance of this phenomenon, known as the Hofmann-Klemen effect, is illustrated by its use as a mineralogical test4 to distinguish between montmorillonites and beidellites. Several different theories, based on indirect experimental measurements, have emerged to explain the reaction mechanism of the Hofmann-Klemen effect, these being summarized into two main models: (a) migration of interlayer lithium cations into vacant octahedral sites of the montmorillonite lattice;5-7 (b) location of the ions in the bottom of the pseudohexagonal cavities of the basal surfaces in the tetrahedral layer of the clay.8,9 In very recent articles,10,11 we have provided direct experimental evidence that supports the second reaction mechanism. This was possible after attaining the reexpansion of the collapsed Li-montmorillonite by means of hydrothermal treatment with water vapor at 8.5 MPa and 300 °C for 24 h. In these papers, special attention was paid to the influence exerted by the reversible migration of lithium on the main constituent elements of the lattice, Si and Al, monitored basically through FTIR and 27Al and 29Si MAS NMR spectroscopies. As regards the chemical behavior of the lithium ions in the reexpanded sample, several questions arose from that study: Are the lithium ions now exchangeable? What about their hydration capacity? Would this reexpanded sample recollapse again upon * Please address all comunications to Dr. Miguel A. Castro.
a further treatment at 300 °C? These three points, investigated thoroughly for years in the collapsed Li-montmorillonite, as stated before, have now been examined in the reexpanded sample and reported in the present paper. With this aim, we have performed two kinds of experiments, as outlined in the Figure 1: the submission of the reexpanded sample to a further thermal treatment at 300 °C for 24 h as well as to an ionic exchange process with Na+ ions. The changes experienced by the studied sample upon these experiments have been examined through X-ray diffraction (XRD), differential thermal analysis (DTA), Fourier-transform infrared spectroscopy (FTIR), and 29Si, 27Al, and 7Li magic angle spinning nuclear magnetic resonance (MAS NMR). The results obtained give a clear answer on the state of the Li-montmorillonite after the reexpansion experiment, completing the investigation of this classical topic of silicate chemistry, as well as providing useful information on the particular properties of the active part of a solid before and after a recovering treatment, this point being very important to understand general physicochemical processes of practical interest such as the deactivation of catalysts. Experimental Section Materials. From the two montmorillonites we employed in the former work on this investigationsWyoming and Trancos montmorillonitessthe latter has been selected for these experiments. Chemical data and structural characterization for this montmorillonite were given elsewhere.12 The structural formula for this sample when saturated with lithium (Li-MT) is the following:
Li0.87[Si7.64Al0.36]IV[Al3.09Fe0.28Mg0.69]VIO20(OH)4 The main criterion for the selection consisted of the higher level of tetrahedral substitutions, which allows the use of 27Al MAS NMR to study the structural changes occurring in the tetrahedral sheet and was crucial in the elucidation of the operative mechanism in the Hofmann-Klemen effect. The following abbreviations for the samples obtained from the different experiments sketched in Figure 1, along with a
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Figure 1. Diagramatic sketch displaying the two experiments carried out in this paper. In experiment I the initial sample of the study, the reexpanded Li-MT (named sample A), is heated at 300 °C for 24 h in order to examine its ability to be recollapsed, the heated sample being named sample B. In experiment II the initial sample (sample A) is exchanged with sodium ions to determine the exchangeability of the lithium ions contained in sample A. The sample thus obtained is called sample C. Finally, as a second step of this experiment II, sample C is submitted, as sample A in experiment I, to a further thermal treatment at 300 °C for 24 h. The heated sample is called sample D.
detailed description of the treatments carried out, are presented for their identification through the whole text: the starting sample of our experiments was the reexpanded lithiummontmorillonite attained in our former work11 from a Lisaturated sample initially collapsed by heating at 300 °C for 24 h in air. The reexpansion treatment was carried out by heating at 300 °C for 24 h under an atmosphere of water vapor at 8.5 MPa. This sample, the starting point of the experiments reported in this paper, will be named sample A. This sample A was, on one handsexperiment I in the Figure 1ssubmitted to a thermal treatment at 300 °C for 24 h in air, similar to that employed to collapse a fresh Li-montmorillonite, to investigate if this sample A was able to recollapse as a nontreated lithium sample, as well as to analyze the variation in the hydration degree of the lithium ions. The product obtained after this treatment will be named sample B. On the other hand, the ion exchangeability of sample A was tested by an ionic exchange process in a NaCl solution, in an attempt to obtain a sodium-saturated montmorillonite (experiment II in Figure 1). This process was performed by immersing aliquots of sample A in 50 mL centrifuge tubes containing molar solutions of high-purity sodium chloride in deionized water. The exchange reaction was left to continue for 6 h in shaking devices, the samples being recovered after these periods by centrifuging and decanting the liquid phases. This process was repeated five times to ensure a complete exchange. Finally, samples were washed with deionized water until washing liquids were chloride-free and dried at 50 °C. This sample was labeled as C. The resulting sample, presumably Na-montmorillonite if the exchange capacity of the reexpanded sample was not reduced, was submitted in turn to a further thermal treatment at 300 °C for 24 h in air, to examine whether its behavior is similar to a lithium-montmorillonite (Hofmann-Klemen effect) or to a sodium-montmorillonite (no appreciable effect). As shown in Figure 1, this sample was labeled as sample D. Throughout the present paper, the first two samples of our former work, namely, the initial fresh lithium-montmorillonite and that obtained after heating this initial sample (it will be
named as the standard collapsed lithium montmorillonite), will be employed for comparative purposes. Methods The set of samples obtained in the course of this investigation (Figure 1) has been studied through several techniques, properly selected to provide answers to the questions formulated in the Introduction. Thus, the basal reflections of samples B and D were determined by XRD to establish if sample A was able to recollapse after the thermal treatment and if sample C had either the typical properties of a Li-montmorillonite or a behavior associated with a sodium sample. Additionally, this experimental technique yields information on the crystallinity and the long order periodicity of the samples examined. X-ray patterns were obtained from random powdered samples with a Siemens Kristalloflex D-500 instrument, using Ni-filtered Cu KR, a 36 kV operating voltage, and 26 mA operating current. Diagrams were obtained from 70° to 3° 2θ at a scanning speed of 1° 2θ/min. The local environment of the main elements and group constituents of the samples has been monitored by means of MAS NMR of 7Li, 29Si, and 27Al, and FTIR. Additionally, 7Li MAS NMR was employed as a tool to detect the presence of lithium ions in samples C and D after the ionic exchange process. Since the value for the relative receptivity of the 7Li isotope is high, receptivity/13C ) 1.54 × 103,13 the absence of 7Li signals in the spectra were interpreted as lithium-free samples. MAS NMR measurements were recorded on a Bruker AMX 300 spectrometer equipped with a multinuclear probe. Powdered samples were packed in zirconia rotors. From samples spun at 4.0 kHz, 29Si spectra were obtained at 59.60 MHz, employing a pulse width of 4 µs (π/2 pulse length ) 6 µs), spectral width of 20 kHz, recycle delay of 1 s, 12000 scans, and 20 Hz Gaussian line broadening, chemical shifts being reported in ppm from tetramethylsilane. From samples spun at 3.5 kHz, 27Al spectra were recorded at 78.23 MHz using a pulse width of 4 µs (π/2 pulse length )
Chemical Behavior of Lithium Ions
Figure 2. X-ray diffraction patterns, obtained with Cu KR radiation, for samples A, B, C, and D as described in the text.
12 µs), a spectral width of 50 kHz, a recycle delay of 0.3 s, 6000 scans, and 10 Hz Gaussian line broadening. Chemical shifts are reported in ppm from 0.1 M [Al(H2O)6]3+. The observation frequency for 7Li spectra was 116.64 MHz. A pulse width of 5 µs (π/2 pulse length ) 10 µs), a spectral width of 42 kHz, and a recycle delay of 0.5 s were used. Six thousand scans were collected for each measurement, and 10 Hz Gaussian line broadening was employed in the processing. Rotors were spun at 4.0 kHz, and chemical shifts were reported with respect to a saturated aqueous solution of LiCl. FTIR spectra were recorded using a Nicolet 510P spectrometer. The samples were suspended in water and allowed to sediment onto a holding sampler of cadmium telluride, the suspensions being evaporated to dryness overnight in order to form oriented aggregate films for examination. Cadmiun telluride windows, supplied by Perkin-Elmer, were selected as appropriate holders for this study because clay films were easily adhered on them. In addition, they presented a useful transmission range, 5000-360 cm-1, and can be heated to 300 °C. These oriented samples were studied in a specially designed evacuable cell with facilities for heating in situ the sample film. These techniques reveal the changes produced in the samples after the two experiments and, consequently, give knowledge on the location and availability of the lithium ions in the reexpanded Li-montmorillonites, the central topic of this paper. Finally, the last question formulated in the Introduction, that about the hydration capacity of the samples, was investigated by using differential thermal analysis and thermogravimetry. DTA and TG were carried out using a Setaram equipment, model TG-DTA 92, equipped with a graphite furnace and programmed to provide simultaneous plots of the weight change (TG) and the endo/exothermal changes (DTA) of a process as a function of the temperature, using calcined alumina as reference. The sample was maintained in an inert atmosphere of nitrogen throughout the heating period, and the temperature was increased at a linear rate of 8 °C min-1. Results and Discussion X-ray Diffraction Study. Figure 2 shows the variation of the XRD patterns of the Li-MT samples after the experiments described above. All the profiles contain both the basal reflections and the general hk diffractions characteristic of the
J. Phys. Chem. B, Vol. 102, No. 12, 1998 2209 smectites. Since the shape and the positions of the hk bands are not modified in any sample and taking into account that there are no new peaks that could be associated with new crystalline structures, the description of the results will be limited to the change of the basal reflections in both intensity and position. The basal spacing of sample A corresponds to an expanded structure, the separation of the structural layers being a function of the nature and the amount of water present in the interlayer space. The initial pattern obtained for the sample exhibited an unusually high basal spacing (d001 ) 17 Å) for a Limontmorillonite, probably due to the elevated water pressure employed in the preparation. Sequential dehydration experiments at 50 and 75 °C showed basal spacings compatible with two-water-layer and one-water-layer hydration states. After the thermal treatment at 300 °C for 24 h, the sample collapsed, showing a [001] reflection corresponding to 9.6 Å, the behavior of sample A being thus similar to that described for an initial lithium-saturated montmorillonite. As already pointed out,14 the [003] reflection intensity (2θ )28.2°) is increased in relation to that of the [001] one when the c parameter is close to the collapsed spacing. Assuming basal spacing values of 9.6 and 14.8 Å for the dehydrated and the hydrated phases (6 H2O molecules surrounding the Li+ ions), we have performed a calculation15 of the first- and third-order basal reflection intensities for these structures, giving the F003/ F001 ratio a value for the dehydrated phase 14 times higher than that obtained for the hydrated one. This calculation agrees with the experimental data in Figure 2 for samples A and B. We have completed the XRD study of this solid by trying to expand this sample with different solvents (water, ethylene glycol, and glycerol). There was no reexpansion in any case, as for a standard thermal-treated Li-montmorillonite. Regarding experiment II, samples C and Dsobtained after Na+-exchange of the sample A and further thermal treatment at 300 °C, respectivelysshowed a set of basal reflections characteristic of an expanded structure in both cases, d001) 12.4 Å. The thermal treatment does not cause the irreversible collapse of the structure. This means that sample D has rehydrated after the thermal treatment and taken up water from the atmosphere. Sample C behaves, consequently, in a different manner to that observed for sample A, an extensive replacing of the interlamellar lithium ions being deduced. The unique change observed in the patterns of the two samples is related to the relative intensity of the [004] reflection (d ) 3.13 Å; 2θ ) 28.5°), which is increased for both samples. To interpret this variation, we have calculated, as before, the change in the intensities of the [001] and [004] reflections with varying composition of the interlamellar space for this montmorillonite, from fully lithium-homoionized to one containing sodium ions as exchangeable cations. Once more, the calculation coincides with the trend observed experimentally. Substitution in the interlamellar space of lithium ions by sodium ions causes an increase in the F004 and a reduction in the value of F001. Differential Thermal and Thermogravimetric Analyses. Figure 3 includes the DTA and TG curves for the four samples examined, together with those corresponding to the samples studied in our former work, for the sake of comparison. The y-axis values of each curve have been corrected in Figure 3 to be presented for equal amounts of sample. All the samples were conditioned at 55% relative humidity before each experiment, analyses being performed in the range from room temperature to 250 °C to explore the dehydration process. In this temperature range smectites lose their molecular water, adsorbed as a
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Figure 3. DTA and TG curves in the dehydration region for the initial (curve a) and heated at 300 °C for 24 h (curve b) lithiummontmorillonites, and for samples A (curve c), B (curve d), C (curve e), and D (curve f) as described in the text.
multilayer on the external surface, condensed in mesopores, or incorporated in the interlayer space. The different contributions of each type of water cause particular DTA and TG profiles, which reveal both the nature of the intercalated cation and its availability after each treatment. The DTA diagram obtained for the fresh lithium-saturated montmorillonite (curve a) shows a usual pattern for this kind of sample,16 which consists of a double endothermic peak with maximum values centered at 109 and 178 °C. This peak system has been interpreted17 as due to the result of a double contribution: a first set of different weakly adsorbed water molecules (on the external surface, in mesopores, and physically adsorbed in the interlayer space) is responsible for the first broad signal, and at higher temperatures, a second type of water molecules more strongly bonded accounts for the other band. This latter peak has been identified for those smectites saturated with divalent ions and Li+ and has been associated with the dehydration of the coordination sphere around the exchangeable cations, the existence of covalent bonds between the cations and the water molecules being postulated. The thermogram shows a continuous loss of weight for the temperature interval studied, with a slight slope variation in the boundary between the two steps. Once heated at 300 °C, the collapsed sample (curve b) exhibits a DTA profile with only a single peak, drastically reduced in intensity and slightly shifted at lower temperatures (90 °C). The dehydration of the fresh sample causes the disappearance of the higher temperature peak as well as the contributions of the water molecules weakly bonded between the structural sheets of the montmorillonite, the only desorbed water being that adsorbed on the external surface or condensed in mesopores. After the reexpansion treatmentssample A of this work (curve c)sboth the DTA and the TG lines resemble the results obtained for the initial Li sample, a qualitative recovering of all the different kinds of water molecules in the sample being deduced. A more detailed observation of the patterns reveals two small differences: the lower temperature DTA peak has been slightly sharpened, and the water content has been reduced to 94% of the initial value. The effect of the rehydration treatment on the textural properties of the sample (the SBET value is halved) can justify these observations. Regarding the two experiments carried out in this investigation, the DTA and TG analyses are especially relevant. Thermal
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Figure 4. FTIR absorption spectra (1225-475 cm-1 region) of samples A, B, and C as described in the text.
treatment of sample A (curve d) reverts to the patterns associated with the collapsed sample, profiles similar to those of the heattreated sample being obtained. On the other hand, ion-exchange of sample A produces a considerable change in the diagrams obtained (curve e). The double-peak system is now reduced to a single band around 125 °C, compatible with an extensive substitution of Li+ (responsible for the higher temperature peak) by Na+ ions, and the TG profile does not show the twocomponent system, the dehydration being completed at 150 °C. After thermal treatment at 300 °C (curve f), the results show a similar behavior, but a remarkable reduction in the intensity is clearly observed, probably due to the rehydration conditions (sample D was rehydrated at room temperature, not being placed in water at any time). FTIR Spectroscopy. Figure 4 shows the FTIR spectra in the region 1225-475 cm-1 for samples A, B, and C. The sample D spectrum is not included in Figure 4 due to its analogy with the sample C one. This region includes the vibrational bands of the silicate anion and the OH bending (librational) ones. Concerning the vibrations of the tetrahedral sheet (Si-O anion), the low effective symmetry of the dioctahedral layered silicates imposes distortions on the structure, which makes the general treatment of the spectra rather complicated. It is possible to divide the band system into two groups: the Si-O stretching vibrations, appearing in the 950-1200 cm-1 region (the complex Si-O absorption pattern being composed at least of four different contributions18), and the Si-O bending vibrations, observable in the two strong absorptions at 485 and 540 cm-1. Although detailed interpretations of the different bands are not possible in these materials because of its complexity, it is known that the distortion of the tetrahedral silicon-oxygen framework causes changes in frequency values of each mode of vibration (the position of each band being a function of the angles Rswhich indicates the rotation of the tetrahedra about their vertical axessand τ, the Oapical-Si-Obasal angle) and that the increase in the water content produces a narrowing and an increase in intensity of the Si-O stretching band system.16 On the basis of this information, the Si-O bands in the sample A spectrum were described11 as due to the complete restoration of the initial situation in the tetrahedral sheet after the rehydration experiment, the abstraction of the Li+ ions to the interlamellar space being deduced. After our experiment I the Si-O bands of sample B suffer the same changes observed for a standard collapsed Li-montmorillonite: broadening, highfrequency shift, and a decrease in intensity. These modifications
Chemical Behavior of Lithium Ions
Figure 5. FTIR absorption spectra (950-650 cm-1 region) of Trancos Li-montmorillonite initial (fresh in the figure), heated at 300 °C for 24 h (collapsed in the figure), sample A, and sample C (as described in the text).
can be interpreted by the distortion of the tetrahedral sheet when Li+ ions are incorporated and by the decrease in the water content of the sample as established by the DTA/TG study. There was no appreciable change in the spectra recorded after experiment II for the samples C and D regarding the Si-O bands, which are rather insensitive to the nature of the interlayer cations in hydrated montmorillonites.19 However, the lack of variation of the Si-O signals in sample Dsnot shown in Figure 4swith respect to sample C shows the differential behavior of the montmorillonite after the ionic exchange process, a considerable amount of hydrated Na+ ions in the interlayer space being inferred. Since in our previous work there was an unclear aspect in relation with the partial recovery of the OH bending modes after the rehydration experiment, the OH band region has been separated in a different plot, Figure 5, the spectra obtained for the fresh and the standard collapsed Li-montmorillonite being incorporated. The librational frequencies of the hydroxyl species are indicated in the figure, a reasonable agreement between them and the structural formula being observed for the initial sample. Upon heating at 300 °C the intensity of the OH bending modes decreases and their positions shift to high frequencies. In sample A, after the rehydration experiment, only a partial recovery of the OH modes was observable, the generation of protons being postulated to interpret the observed result. This interpretation can be reexamined with these experiments: if the ionic exchange process carried out in experiment II was effective and, consequently, the repulsive force on the proton of the OH groups exerted by the electric field of the sourrounding ionsswas eliminated, the OH bands should now be considerably recovered after performing an extensive exchange of the cationic species affecting these groups.20 Because of the clear enhancement of the OH signals observed in the sample C spectrum, the explanation postulated in our former work is reinforced. Solid-State NMR. 29Si and 27Al MAS NMR has been applied to the whole set of samples, following the same procedure as in the other techniques. 7Li MAS NMR signals could be obtained only after experiment II. Samples C and D failed to give Li signals even for the maximum receiver gain allowed for the instrument, an extensive ionic exchange with Na+ ions being once more deduced.
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Figure 6. 29Si (59.60 MHz) MAS NMR spectra of samples A, B, C, and D (described in Figure 1).
Figure 6 shows the 29Si MAS NMR spectra for the initial sample and those treated under experiments I and II. The 29Si signal for sample A shows a single band at -96.3 ppm, almost completely restored to the initial chemical shift found in the fresh Li-montmorillonite. This signal is attributable21 to the tetrahedral silicon linked through oxygen to three other silicon atoms, the slight asymmetry toward a low field being assigned to silicon atoms with a neighboring aluminum tetrahedron, Q3(1Al). After the thermal treament at 300 °C, the 29Si peak width is increased and the position is shifted to a higher field, the spectrum recorded being equal to that obtained for a standard collapsed Li-montmorillonite. The reversibility of the collapse is thus again shown. While the displacement in the chemical shift value is explained by means of a partial neutralization of the layer charge, the change in the width is interpreted on the basis of variations in the distribution of the Si-O-Si angles of the SiO4 tetrahedra after the treatment. Both facts are in accordance with the location of the dehydrated Li+ ions in the hexagonal holes of the tetrahedral layer, where they influence the position and the line width of the 29Si signal. Following experiment II, the 29Si signal does not vary in position after the ionic exchange, which is compatible with the reported data for initial montmorillonites homoionized in different inorganic exchangeable cations.22 However, this result is not compatible with the interpretation made in our former paper regarding the incomplete restoration of the 29Si signal for sample A, and irreversible changes in the tetrahedral framework are inferred from the 29Si signals, even after performing experiment II. This information can be of special interest in evaluating the chemical properties of solids, such as many heterogeneous catalysts, which have to undergo cyclic regeneration treatments. Upon heating treatment at 300 °C, sample D shows a similar MAS NMR signal without any change in its chemical shift position. Likewise, this result agrees with an extensive ionic exchange process of the exchangeable Li+ ions in the sample and corroborates the above-mentioned explanation on the irreversible damage caused by these treatments on the tetrahedral sheet of the silicates. 27Al MAS NMR spectra for the samples under the different treatments are shown in the Figure 7, which have been collected at a spinning rate of 3.5 kHz in order to remove as much as possible the spinning sidebands of the AlVI peak from the spectral region where the AlIV peak appears. The spectrum for
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Figure 7. 27Al (78.23 MHz) MAS NMR spectra of samples A, B, C, and D (described in Figure 1). The asterisk signifies spinning sidebands.
sample A consists of two principal components at around 0 and 67 ppm, which can be assigned to octahedral and tetrahedral Al3+ ions, respectively. As previously reported,10,11 the AlIV signal disappears upon heating at 300 °C the initial fresh Limontmorillonite, this result being explained by the distortion exerted on the aluminum tetrahedra after lithium migration. As this effect is not observable when lithium ions occupy octahedral vacancies,23,24 their location in the hexagonal holes of the tetrahedral layer was concluded. The tetrahedral aluminum signal was recovered after the reexpansion treatment, thus providing direct spectroscopic evidence and confirming the location of Li+ ions in the standard collapsed lithiummontmorillonites. 27Al spectra from experiments I and II give complementary evidence about the ability of the reexpanded sample to recollapse as well as to effectively exchange the interlamellar ions. Thus, sample B shows an unmodified AlVI signal, but does not exhibit any peak for the tetrahedral aluminum, as was the case of a standard collapsed Li-montmorillonite. On the other hand, there are no appreciable changes in the AlVI or AlIVsignals after the ionic exchange in sample C, and after the heat treatment of this sample, no distortion of the aluminum tetrahedra was deduced. Once more, extensive substitution of Li+ ions by Na+ ions should be inferred. As regards 7Li MAS NMR measurements, Figure 8 includes several spectra corresponding to the initial fresh lithiummontmorillonite, the standard collapsed one, the reexpanded sample A of this paper, and the recollapsed sample B. The initial 7Li (spectrum a) shows a single resonance whose central transition center of gravity is located around 0 ppm, and the line width at half-height is 13 ppm. A few small spinning sidebands complete the description of this spectrum, associated with a structure of hydrated Li+ ions similar to that found in aqueous solution. The collapse of this sample upon heating at 300 °C (spectrum b) causes a decrease in the line width as well as a marked enhancement of the spinning sidebands. These facts, as already reported by Luca et al.,25 can be explained on the basis of a lower quadrupole interaction of dehydrated Li+ ions, thus causing extensive spinning sidebands due to satellite transitions to appear.26 The reexpanded sample A (spectrum c) shows a set of signals very similar to those of the initial
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Figure 8. 7Li (116.64) MHz) MAS NMR spectra of the Limontmorillonite: (a) initial; (b) heated at 300 °C for 24 h; (c) sample A; and (d) sample B (both as described in the text and in Figure 1). The asterisk signifies spinning sidebands.
sample, again with small spinning sidebands. When this solid is reheated at 300 °C, sample B, the 7Li MAS NMR signal obtained (spectrum d) resembles that of the standard collapsed lithium sample, thus showing again the reversibility of the collapse-reexpansion-recollapse cycle. As already noted, it was not possible to obtain any 7Li spectra for the samples prepared from experiment II, even when the instrumental receiver gain was set to its maximum value. Therefore, a successful ionic exchange replacing the initial lithium ions is attained during the second experiment. General Considerations. Following the results obtained through the different experimental techniques employed, a set of assessments can be made on the chemical behavior of the lithium ions in the reexpanded Li-montmorillonites. First, lithium ions show an availability and a reactivity similar to those encountered in a fresh Li-montmorillonite, although a partial hydrolysis is observed. Regarding their availability, these ions are extensively exchanged when the sample is immersed in a NaCl solution. The exchanged sample (sample C) shows a typical XRD basal spacing for a hydrated structure, which could not be irreversibly collapsed after thermal treatment at 300 °C and subsequent rehydration in air (sample D); it exhibits as well a single-endothermic-peak DTA profile between room temperature and 250 °C and presents 29Si and 27Al NMR spectra compatible with a sodium-homoionized clay; likewise, there is neither a shift in the 29Si band position nor disappearance of the AlIV peak after the thermal treatment. FTIR OH bending modes from sample C revealed a complete restoration of the initial bands for a fresh montmorillonite, which indicated a disappearance of the effects produced after the reexpansion by the proton generation. Since there is not any signal associated with the existence of either protons or exchangeable lithium ions in sample C and the lithium content in this sample is below the detection limit of the NMR technique, the experimental results can be explained in this point through one of these two ways: the degree of lithium hydrolysis in the reexpanded sample is low enough to produce a 7Li MAS NMR spectrum corresponding to hydrated ions (not hydrolyzed ones), but high enough to generate protons observable by FTIR, these protons
Chemical Behavior of Lithium Ions being exchanged during the intercalation process; or, in the second hypothesis, during the sample C preparation the hydrolysis reaction of lihium ions is reverted in the intercalated solution, lithium ions being completely exchanged during the process. As regards the reactivity, lithium ions still have the property of migration to the silicate lattice after thermal treatment to the same extent as in a fresh Li-montmorillonite. XRD, FTIR, NMR, and DTA/TG measurements agree with this assessment. Second, the hydration capacity of the sample as a whole, and of the lithium ions in particular, is reverted in the reexpanded sample to its initial capability. The water content observed in the TG curve and the double endothermic DTA peak clearly reflect this reversibility. From this study, due to the similarity between the DTA/TG profiles for the initial fresh Li-montmorillonite and sample A, we can deduce that the hydrolysis degree should not be in any case very high because, if it were the case, a new band for the dehydroxylation of the hydrolyzed species would be detected in the thermal analysis. These results provide additional information about the reversibility of the process. Although a reversible behavior has been identified for the lithium ions after the reexpansion treatment, the solid matrix has suffered irreversible damage, as observed in the 29Si MAS NMR signals for samples C and D. In summary, the reexpansion of the collapsed lithiummontmorillonites produces a complete reversibility in the location and availability of the exchangeable cations in both its hydration state and reactivity under different treatments, a low degree of hydrolysis being observed. Likewise, the silicate lattice suffers some textural and structural damage after the hydrothermal treatment, this aspect being of importance in connection with the deactivation of catalysts. Acknowledgment. Spanish DGICYT (Project PB94-1426) is acknowledged for financial support. References and Notes (1) Pinnavaia, T. J. Science 1983, 220, 365.
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