1232
J. Phys. Chem. 1994,98, 1232-1237
Clinoptilolite and Heulandite Structural Differences As Revealed by Multinuclear Nuclear Magnetic Resonance Spectroscopy Raymond L. Ward' and H. Lawrence McKaguet Lawrence Livermore National Laboratory, University of California, Livermore, California 94550 Received: August 1 1 , 1993; In Final Form: November 16, 1993"
Wide-line proton N M R has revealed differences in the water environment in two zeolitic isomorphs: clinoptilolite and heulandite. The proton spectrum of clinoptilolite is Gaussian in shape, while that of heulandite is a (reduced splitting) Pake doublet. Studies of clinoptilolite from five different locations and heulandite from nine different sites agree with this observation. These results suggest a simple nondestructive N M R method for distinguishing heulandite from clinoptilolite. Dehydration experiments are particularly informative. Proton line widths of clinoptilolite as a function of the dehydration temperature reveal three types of water binding, none being absorbed water. Similar studies with heulandite reveal a change from the Pake doublet to a Gaussian, which is probably associated with the heulandite to heulandite B transformation. 27Al and 29Si N M R studies of clinoptilolite indicate a change in framework structure and/or cation binding with dehydration. 27AlN M R of heulandite exhibits an increase in line width with dehydration temperature to 175 "C. At this temperature the increase stops and the line width remains constant to 215 "C, the maximum temperature studied. This agrees with the proton studies and is attributed to the heulandite to heulandite B transformation. 29SiN M R of heulandite reveals a framework structural change and/or cation binding with dehydration. All of these observations are reversible upon rehydration.
Introduction Our interest in the amount of water and the nature of its binding in clinoptilolite bearing tuffs at the Nevada test site led to a multinuclear NMR study of clinoptilolite and its isomorph heulandite.' Clinoptilolite and heulandite are both naturally occurring aluminosilicates (zeolites) belonging to the same class and are considered to have the same framework structure (isomorphic) but different chemical compositions of the idealized unit cell.24 The unit cell constants for the monoclinic structures are, for clinoptilolite, a = 17.62 A, b = 17.91 A, c = 7.39 A, and 0 = 116'26' and, for heulandite, a = 17.70 A, b = 17.94 A, c = 7.42 A, and 0 = 116'24'. The chemical formula for clinoptilolite is Na~[Al&i30072].24H20 and for heulandite is Ca4[AlsSi28072je24H20. Actually each zeolite has a variety of cations (Ca, Na, K, Mg, etc.) with Na and K the most prevalent cations in clinoptilolite and Ca the most prevalent in heulandite. Clinoptilolite and heulandite are usually distinguished by their difference in thermal stability, their Si/Al ratio, and their cation content. Clinoptiloliteis stable to temperatures approaching 700 O C , has a Si/Al ratio >4, and has Na and K for cations. Heulandite undergoes a phase transition to a B form above 200 "C, is unstable at high temperatures, has a Si/Al ratio < 4, and has Ca for its major cation. We initially developed a proton NMR method for detecting and quantitating the amount of water in rocks from the Nevada test site.' Analysis has shown that clinoptilolite is the dominant water-containing substance, and there is no organic matter to consider. A simple integrationof the proton signal and comparison with a benzoic acid reference sample was sufficient for water determination to better than 5%. Benzoic acid was chosen as the reference standard because it can be obtained in high purity, does not contain absorbed moisture, and has a similar line shape and line width compared to those for clinoptilolite. High purity benzoic acid does have a much longer proton TIthan clinoptilolite (clinoptilolite usually contains small amounts of iron). A recycle time of 10 min was necessary for the quantitation of the benzoic acid signal, whereas 20 s was sufficient for clinoptilolite.
'
Present address: Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, San Antonio, TX 78228-05 10. Abstract published in Aduance ACS Abstracts, January 1 , 1994. @
0022-365419412098-1232%04.50/0
Water molecules in zeolites can vary from tightly bound structural water to a loosely held isolated liquid type. Isolated mobile liquid and/or physically absorbed water is revealed by relatively narrow 1-2-kHz NMR line widths. This narrow line width type of water has not been observed in these studies. All water molecules, if constrained in their motion, will exhibit a magnetic dipoledipole interaction, which in an environment such as gypsum can produce line widths as large as 38 kHz. Fixed, randomly located water will result in a Gaussian NMR line shape, whereas an ordered environment will lead to a Pake doublet. Although NMR interactions are short range, the ordered environment must be long range to yield a sharp Pake doublet, otherwise a Gaussian shape will be observed.
Materials and Methods All NMR measurements were performed on a Bruker MSL300, except for one proton measurement on Poona heulandite that was performed at 400.13 MHz on a Bruker MSL-400 spectrometer. This measurement was made in order to confirm that the Pake5doublet splitting was indeed field independent and thus dipolar in nature. The zeolite dehydrations were carried out in an oven at the stated temperature and the samples cooled to room temperature in a desiccator over anhydrous calcium sulfate. The sample was put into an NMR tube and sealed with Teflon tape or in a magic angle spinning rotor with a tight-sealing cap for the NMR measurements. Rehydration experiments were carried out over water at room temperature in a closed vessel. Static proton NMR measurements were performed at 300.13 MHz in a dry nitrogen atmosphere. 27Aland 29Simagic angle spinning experiments were performed with either a 4- or 7-mm rotor. The 4-mm rotor was routinely spun at 7-10 kHz and the 7-mm rotor at 5 kHz. The 27Aland 29Si measurements were made at 78.205 and 59.62 MHz, respectively. Values of 7r/2 were 'H = 3.5 ps, 29Si = 3.25 ps, and 27A1= 6.5 ps. Clinoptilolite samples were from Hector, CA; Death Valley Junction, CA; Castle Creek, ID; Fish Creek Mountains, NV; and Creede, CO. Heulandite samples were from Poona, India; 0 1994 American Chemical Society
Clinoptilolite and Heulandite Differences
The Journal of Physical Chemistry, Vol. 98, No. 4, 1994 1233
)AL
224'Cand
HYDRATION
2 1
1
I
Q
I
I
4
I
1 0 0 200 300 400 5 0 0 600
0
Temp.
O C
Figure 1. Plot of the water content of Hector clinoptilolite, wt % vs dehydration temperature, determined by NMR measurements at room temperature: (inset) proton NMR spectrum for nondehydrated clinoptilolite; each division on the frequency axis represents 50 ppm. Reproducibilityof the water content measurement is better than 5% for high water content samples and -40% for samples dehydrated at 500 O C .
-1
I " " " '
-
g
23
22
c
o,"
211,'
P P \
I
I ~ . . . , . . . . I . . . . , .
200
0
200
400
600
800
TEMP. "C
Figure 2. Plot of the proton line width of Hector clinoptilolitemeasured at room temperature, FWHM vs dehydration temperature. Reproducibility of measuied line widths is better than 5% in all cases.
Aurangbad, India; Yacolt, Washington; Paterson, Passaic County, New Jersey; Goaz, Minas Gerais Brazil; Guanajuato, Mexico; Cascadia, Linn County, Oregon; Upper Clackmus River, Oregon; and Coonabaran, NSW, Australia. The commercial suppliers were Ward's Natural Science Establishment, Inc., Rochester, New York; Mineral Research, Clarkson, New York; and Minerals Unlimited, Ridgecrest, California. Results Proton NMR Studies. All clinoptilolite samples listed in the Materials and Methods section exhibited a Gaussian line shape, (Figure 1, inset), with a full width at half maximum (FWHM) of 15-25 kHz. The width depends on the water content and temperature history; at room temperature, the more water and narrower the line. A Gaussian line shape is indicative of randomly dispersed water molecules. For a given water concentration, the larger the line width the more rigidly fixed the water molecule. The dehydration of Hector clinoptilolite was followed in detail by proton NMR. A plot of the water content vs dehydration temperature (Figure 1) is a continuously decreasing curve with some scatter but no indication of breaks in the plot. Line width measurements, however, suggest three types of water (Figure 2). The line width, measured as the full width at half maximum (FWHM), increases from 18.25 kHz for untreated material at room temperature to a maximum of 23 kHz for material dehydrated a t 175 OC, then decreases to 17.5 kHz for material dehydrated a t 350 OC, and continues to increase to 21 kHz a t 700 O C . This behavior with Hector clinoptilolite has been observed in threeseparateNMRexperiments. Thedata presented in Figure 2 are the average of two with the indicated error ranges. Samples were heated overnight or longer at the given temperature. Since, generally, the greater the line width the more rigidly bound the water is, the data in Figure 2 lead to the conclusion that the water environment in clinoptilolite varies with water content through
100
.I
0
PPH
,
,
,
, , . ,
-100
I
.,I
.,
-200
Figure 3. Proton NMR spectra of Poona heulandite heated overnight at the following temperatures: room temperature and 110, 130, 150, 175, 200, and 224 ' C ; and rehydrated heulanditeafter dehydration at 224 OC.
three stages of different water mobility. The maximum line width of 23 kHz is detected at 175-200 O C , where the water content has dropped from 14+% a t room temperature to 5% at 200 OC. The line width changes rather drastically from 23 to 17 kHzat about 400 OC, whereas the water content has only changed from 5 to 4%. This line width decrease indicates a change from a rigid environment to a more mobile one. Dehydration from 400 to 600 O C again removes little water but produces a fairly linear increase in the line width with dehydration temperature and a more rigid water environment. The proton spectrum of heulandite, unlike that of clinoptilolite, is a Pake doublet (Figure 3). These spectra were obtained from Poona heulandite heated overnight at the temperatures noted. The Pake splitting, 16.4 kHz, is substantially less than that observed for powdered CaS04.2Hz0.38 kHz. The best resolved proton spectrum is that obtained from Poona heulandite heated at 110 O C . This spectrum indicates additional smaller splittings. Water loss for heulandite, as measured by NMR, like with clinoptilolite, is a smoothly decreasing curve when water content is plotted against temperature (Figure4). Therearenoindications of breaks in the curve. The general shape of the proton resonance does change appreciably, however, with water loss. Increased water absorption, to maximum saturation, does not change the Pake splitting. Water loss by heating, however, does reduce the splitting and gradually changes the general appearance of the Pake doublet to that of a Gaussian curve for samples dehydrated above 200 O C (Figure 3). The FWHM measured from the top of one of the doublet peaks is plotted as a function of the dehydration temperature (Figure 5). This curve differs substantially from the clinoptilolite curve shown in Figure 2. The line width increases linearly from 20.2 kHz for nonheated heulandite to 22 kHz at approximately 175 OC. At this temperature there is a larger change in the line width, with the width reaching 25.75 kHz a t 225 OC. The Pake doublet splitting decreases over this temperature range (Figure 3), the spectrum changing to a Gaussian above the transition temperatureof about 175 OC. Since heulandite is known to undergo a framework
Ward and McKague
1234 The Journal of Physical Chemistry, Vol. 98, No. 4, 1994
"c 0
100 150 Temp. "C
50
200
250
Figure 4. Plot of Poona heulandite water content determined by proton NMR measurements at room temperature, wt % ' vs dehydration temperature. Reproducibility of water content is the same as for Figure 1. 1
9
I
I
BO
24 3
22 20 1
0
I
L
I
I
I
60
L
40
I
PPH
t
20
1
0
.
I
.
I
-20
Figure 6. 27AlMAS NMR spectra of Poona heulandite as received (bottom), heated to 204 OC overnight (middle), and heated to 204 OC and then stored over water for a few days (top). All spectrawere obtained with a ~ / 1 2pulse, 600 scans, a recycle time of 1 s, and a spinning rate of 6 kHz. UlNOPllLOIiTE
0
0
50
100 150 Temp. "C
200
250
Figure 5. Plot of the proton line width for Poona heulandite measured at room temperature, FWHM vs dehydration temperature. Reproducibility of measured line widths is the same as for Figure 2.
1400
0
1200
1000
contraction to a B form, it is reasonable to equate our N M R observations to this transformation,6 although our data show a lower temperature (25-30 "C) than the range normally observed: 202 f 3 O C , by X-ray diffraction techniques. One factor that we were unable to control was the effect of cooling the sample to room temperature after the dehydration. Ideally the spectra should be obtained at the dehydration temperature with a heated N M R probe. Furthermore, placing the zeolite in a closed vessel over water for a few days completely restored the Pake doublet or non-B type structure (top spectrum Figure 3). *'A1 NMR. The effect of dehydration on the framework structure of clinoptilolite and heulandite is reflected in the line width of the single 27Al resonance located a t 54.3 ppm for clinoptilolite and 55.8 ppm for heulandite (Figure 6) with respect to aqueous Al(II1) ion. Figure 6 illustrates the changes in line width observed for heulandite upon dehydration and the reversibility of the process. Loss of water accentuates a high-field asymmetry in the 27Al line width for both cinoptilolite and heulandite. We have repeated our dehydration experiments on clinoptilolite and heulandite and have used 27AlMAS to determine the effects on the zeolite framework. The *'A1 line width (FWHM) for clinoptilolite plotted as a function of the dehydration temperature appears in Figure 7. The line width increases smoothly with no indication of abrupt changes that would correlate with the proton line widths of Figure 2. The line broadening is completely reversible. Samples heated to 300 O C , and then stored in a desiccator over water for 1-2 days, exhibit 27Alline widths in agreement with that of the original hydrated material. 27Al line width measurements are repeatable to at least 5%. The 27Alline width for dehydrated heulandite as a function of the dehydration temperature appears in Figure 8. In contrast to the 27AlN M R results for clinoptilolite and in agreement with proton studies of heulandite, there is a break in the line width plot at approximately 175 OC. At this temperature, the 27Alline width stops increasing and is constant to 215 O C . Since other studies have revealed that heating heulandite beyond 215 OC
o
o
800
t
0
50
100
200
150
250
300
Temperature, 'C
Figure 7. Plot of the 27AlNMR line width for Hector clinoptilolite, measured as in Figure 6, FWHM vs dehydration temperature. HEULANDITE
B
1200 0
5
a
50
150 200 250 Temperature, "C Figure8. Plot of the2'A1 NMR line width for Poona heulandite, measured as in Figure 6, FWHM vs dehydration temperature. 100
tends to destroy the zeolitic framework, the dehydration was not carried beyond this temperature. This break in the plot of the 27Al line width vs the dehydration temperature is in the same temperaturerange as that revealed by protonNMRand isascribed to the transformation to the B phase. Again, the 27AlNMR data, like the proton results, yield a lower temperature for the onset of this transition than do X-ray diffraction techniques. Again, as with clinoptilolite, the effect on the 27Alline width of heulandite is reversible upon rehydration, with the line width returning to 750 Hz. %i NMR. We have studied the effect of dehydration on the
The Journal of Physical Chemistry, Vol. 98, No. 4, 1994
Clinoptilolite and Heulandite Differences
h , J
I
.
1
.
-60
~
-70
*
1
-80
*
1
-BO
.
1
-100 PPI4
.
1
-110
.
-120
1
1
-130
1
.
1
.
-140
Figure 9. 29SiNMR spectrum for hydrated Poona heulandite (lower curve) and heulandite heated at 228 “Covernight (upper curve).
29Si NMR spectra of heulandite and clinoptilolite. Since 29Siis a “dilute” spin Z = ‘ 1 2 nucleus, the same techniques used in I3C NMR spectroscopy of solids can be brought to play to study the effect of dehydration on the zeolitic framework. One important difference in the case of zeolites, in contrast to carbon NMR, is the small amount of covalently bound hydrogen and the fact that oxygen intervenes between silicon and hydrogen in these systems. The small concentration of SiOH is readily apparent by the large number of cross-polarization magic angle spinning (CPMAS) acquisitions, >50 000, that are required to yield the same signalto-noise ratio in the 29Sispectra as 100 accumulationsusing simple Bloch MAS acquisitions. Similarily, proton decoupling has little effect on the resolution of 29Sispectra. We have observed similar moderately resolved 29SiNMR spectra for both heulandite and clinoptilolite as reported by Dehydration reduces the resolution for both zeolites, producing a Gaussian curve for each when more than two-thirds of the water has been removed. Figure 9 shows the 29Si spectra of heulandite obtained under simple Bloch acquisition with proton decoupling for hydrated (lower spectrum) and partially dehydrated heulandite (upper spectrum). The relatively poor resolution of the hydrated material precludes a detailed study of the dehydration process. Rehydration does, however, restore the four-peak spectrum from the Gaussian for both heulandite and clinoptilolite.
Discussion Knowlton9 et al. (1981) studied the dehydration of Hector clinoptilolite by thermogravimetric methods. They interpreted their results in terms of three types of water: adsorbed water, loosely bound water, and tightly bound zeolitic water. They proposed that water molecules near the middle of a channel, where the electrostatic potential due to the framework charge is low, are loosely bound, whereas water molecules at sites near the surface of a channel, where the electrostatic potential is high and where the water molecules may experience multiple hydrogen bonds, are tightly bound. Adsorbed water is not apparent in the clinoptilolite proton NMR experiments. Adsorbed water from an NMR standpoint should exhibit a much narrower line width than observed here. We have discussed this point previously’ and pointed out that the water absorbed by powdered Si02 exhibits a line width of only 1-1.5 kHz. Our results suggest that any absorbed water is taken
1235
up by the zeolite and distributed among the various types of water within the zeolite. We observed a Gaussian line shape for all samples of clinoptilolite that we examined. The line width and area and not the shape are the only parameters that vary. The line width decreases coiitinuously with increasing water content, but only to about 6 kHz, where the zeolite is saturated with water. The narrow line width peak of 1.5 kHz, normally observed for adsorbed water on crystalline solids, is not observed. Hey and Bannister4 first reported, from X-ray studies, that heulanditeand clinoptilolite are identical in structure. Heulandite from the various sites listed in the Materials and Methods section, however, exhibits an entirely different proton NMR spectrum than that of clinoptilolite. Instead of a Gaussian-shaped absorption curve, heulandite exhibits a classical Pake doublet, with a reduced splitting, 16.4 kHz, from that of CaS04.2H20, 38 kHz. The Pakedoublet is indicative of a very ordered arrangement of water molecules. One experiment at 400 MHz, on Poona heulandite, produced the same doublet splitting as that measured at 300 MHz. This result is indicative of a magnetic field independent dipoldipole interaction. Pakeswas thefirst touseprotonNMR toexaminesinglecrystals of gypsum, CaS04.2H20. Each water molecule gave rise to two resonances whose separation in frequency units depends on the product of the proton magnet moments, the inverse cube of the distance separating the protons, and the orientation of the protonproton vector with respect to the external magnetic field, BO, direction (as described by the spherical harmonic (3 cos2 0 - 1)) and is further independent of the magnetic field strength. The broadening of the resonance observed for CaS04.2H20 is primarily due to the magnetic dipoledipole interaction of neighboring protons superimposed on the Pake splitting. In powdered C a S 0 ~ 2 H 2 0the total width is 65 kHz. Gutowsky and PakeIO studied the effect of motion on the shape and splitting of the doublet. They determined that if there is rotational motion about the internuclear axis, and if this rotation is rapid on an NMR time scale, the shape remains the same while the doublet separation, 38 kHz, is reduced by half. The Poona heulandite splitting of 16.4 kHz is 43% of that of CaS04-2H20. All samples of heulandite listed in the Materials and Methods section produce a doublet splitting of between 14.8 and 16.6 kHz with an average value of 15.8 & 0.7. Ducrosl’ was one of the first to examine the water environment in zeolites by proton NMR. His primary interest was with chabazite, but he also examined heulandite, natrolite, analcime, and edingtonite. A single crystal of chabazite exhibited a proton doublet structure, with the separation of the doublet given by the expression
AH = k(3 cos2 a! - 1) where a is the angle between the symmetry axis of the crystal and the external magnetic field direction and k is a parameter specific for chabazite. At room temperature k = 3 kHz. This corresponds to a maximum separation of 6 kHz. For a sample of powder chabazite, a doublet was observed with a value of A?I = k/2. Ducrosll studied the effect of water content on the parameter k in chabazite and observed an increase in the separation of the doublet with a decrease in the water content for a defined orientation of the crystal. He attributed this increase in the doublet splitting to an increase in the anisotropy of the crystalline environment with water loss. He proposed that water molecules diffuse between at least two different binding sites on a time scale that is rapid compared to the NMR frequency separation of the two sites. One binding site, presumably a wall site, is more anisotropic in nature than the other. The number of sites and/or distortion at the wall site increases with dehydration. The other site is less distorted and is perhaps represented by water within the cage of the zeolitic structure and is not involved with the cage
1236 The Journal of Physical Chemistry, Vol. 98, No. 4, 1994
walls. In contrast to Ducros’s observations, we observed a decrease in the Pake splitting with a decrease in water content. Why are the proton spectra of heulandite and clinoptilolite so different? One anticipated phenomenon that could be the cause is motion. Stockton et al.,Iz in their study of vesicle motion by deuterium NMR, examined the collapse of quadrupole powder patterns by motion. Computer simulated line shapes (Figure 1 in their article), obtained using a slow-motional theory developed by Freed et a1.,13 are remarkedly similar to those exhibited by heulandite through stages of dehydration (Figure 3). Since the Hamiltonian describing the quadrupole coupling of a nucleus of spin Z = 1 is functionally identical to that of two dipolar coupled protons, one can consider the applicability of their results to heulandite and clinoptilolite. In the slow-rotational limit, calculations yield a Pake doublet; as the rotational rate increases, the doublet merges into a Gaussian line shape. Heat-induced polymorphic transformations of heulandite were first observed by Rinne14 and have been further studied by many others including Alietti,Is Boles,6 and Alietti16 et al. Three separate phases A, B, and I have been identified.I7 Heating heulandite above 200 OC produces a contracted phase, B, which is stable for an indefinite period. Phase B can rehydrate into phase I, which appears to be another stable form of heulandite. Since our studies do not cover the same extended temperature range, nor do we have crystallographic information, we will refer to the heulandite transformation that we observe simply as the formation of the B phase. Simonot-Grange and Cointot18reported that at the end of the heulandite to heulandite B transformation heulandite is about 70% hydrated. This agrees fairly well with our proton N M R data as well as with our 27Al measurements. Hambley and Taylor19 used neutron diffraction to study heulandite and partially dehydrated heulandite in order to establish the hydrogen-bonding network and to try and elucidate some of the processes which occur on dehydration. They used relatively mild dehydrating conditions to avoid the possibility of a collapse of the structure, which they report occurs when more than about 40% of the water is removed.18 They observed that heating to 70 O C under vacuum for 18 h leads to significant changes in the lattice constants. The largest changes are a reduction of the b parameter of the unit cell and an increase in the angle 8. These changes are consistent with a partial collapse of the zeolitic cavities toward heulandite B, the heat collapsed phase. Hambley and Taylori9state that cation location and water location play a critical role in controlling the thermal stability. Their analysis established that there is movement of cations and water molecules when heulandite is partially dehydrated. Since cation sites in heulandite are bonded to framework oxygens, depletion of these sites could lead to collapse of the framework as a result of new electrostatic forces. They suggest that the Si/Al ratio influences the thermal behavior primarily by determining the number of extra framework ions. Ogawazo and Bregerzl et al. report that removal of even loosely bound water results in a structural contraction of heulandite. Bregerzi et al. also suggest that Ca ions bind water that is classified as tightly bound in heulandite, whereas the more loosely bound water is associated with monovalent cations such as N a and K. We have performed preliminary ion-exchange experiments in an attempt to correlate the proton N M R spectra with cation types, with inconclusive results. Although we exchanged Ca for Na and K ions in Hector clinoptilolite, we observed no change in the Gaussian line shape exhibited by clinoptilolite water molecules. As of yet, we have not been successful in exchanging either N a or K for Ca ions in Poona heulandite. We were able to increase the content of K and Na in Poona heulandite but detected no change in the water Pake pattern of heulandite. 27Alhas a nuclear spin of Z = 5 / 2 and a quadrupole moment eQ, and as such interacts with electric field gradients eq. Quadrupole interactions depend on the orientation of the electric
Ward and McKague field gradient with respect to the direction of the external magnetic field, Bo. In powdered samples, because all orientations of crystallites are present, all but the central transition are spread over a wide range determined by the quadrupole coupling constant e2qQ/h. As a result, noncentral transitions are usually too broad to observe. The central, l / 2 to J/2, transition, however, is not affected by first-ordered quadrupole interactions and is often the only transition observed in powder spectra. The central transition is, however, affected by second-order quadrupolar interactions. This causes frequency shifts and line broadenings proportional to (e2qQ/h)2/vLwhere V L is the Larmor frequency of the 27Al nucleus. 27Al is a 100% abundant isotope and because of its quadrupole moment has a short spin lattice relaxation time T I , allowing fast NMR pulse repetition rates. In aluminosilicates (zeolites), the A104 tetrahedra are connected exclusively to Si04 tetrahedra. Lowenstein’s rule forbids A1-O-A1 linkages. This produces a single environment and a single resonance in the +SO to +80 ppm range from the usual reference, aqueous A l ( H ~ 0 ) 6 ~ + . Octahedral aluminum ions exhibit a resonance near 0 ppm. Although the quadrupole line width of 27Alis inversely proportional to the Larmor frequency, once this value is determined by the spectrometer, the line width can be further reduced by magic angle spinning (MAS) techniques. The actual reduction depends on the ratio of spinning speed to the static line width. Chemical shift anisotropy and dipolar couplings, if present, are effectively removed by MAS. These characteristics make 27A1an ideal probe of changes in zeolitic structures. The dehydration of clinoptilolite and heulandite results in an increase in the 27Alline width for both these zeolites. This line width change is the result of an increase in the electric field gradient experienced by the aluminum nuclei. This change in the electric field gradient could be the result of structuralchanges in the zeolitic framework, distortion of bond angles, etc., or they could be due to the water/cation displacement. The 27Al line width plotted against the dehydration temperature increases linearly for clinoptilolite (Figure 7). This implies a distortion in the electric field gradient at the aluminum site that continues to increase with the dehydration temperature and is “locked-in” to the water concentration. Whereas for heulandite, the linear increase in the 27Al line width stops where the heulandite B contraction19occurs and then remains constant for the temperature range examined (Figure 8). 29SiNMR is similar in many respects to I3C NMR. Both nuclei have a nuclear spin Z = l/2, low natural abundance (4.5% for 29Si),and can be considered a “dilute” spin nucleus. Silicon in zeolites, unlike most carbon environments, does not have directly bonded protons, with the nearest covalently bound proton environment resulting from small quantities of SiOH groups. Water molecules are plentiful, however. We have observed that in heulandite and clinoptilolite water does not have a direct NMR effect on the 29SiNMR spectra. By this we mean that the effect of proton decoupling has little effect on the 29SiMAS spectra of heulandite and clinoptilolite. Similarly, only small concentrations of SiOH are detected by CPMAS. Water does have an indirect effect on the spectra, as noted in Figure 9, where its removal has led to a change in the zeolitic framework and subsequent broadening in the 29Sispectra. As noted, rehydration removes this distortion and returns the better resolved spectra. Since the chemical shift of 29Si is a sensitive function of S i - M i bond angles, S i 4bond lengths, the kind and number of the next nearest neighbors, i.e., SiO(Al), and the presence of cations, 29Sispectra should be a sensitive monitor of framework distortions and displacements of cations. At present we feel, however, that better resolution, perhaps a t higher magnetic fields, is required for a more detailed study. Summary The proton NMR signal from powdered stationary samples of clinoptilolite is a Gaussian shaped curve with a line width of
Clinoptilolite and Heulandite Differences 15-25 kHz, depending on the water content. In order to better understand the nature of the water binding, we examined the effect of dehydration on water content as determined by N M R and the structureof the N M R signal. Dehydrationof clinoptilolite revealed no discontinuities in the water content as a function of the dehydration temperature, but the line width variation of the Gaussian-shaped curve did. Three regions of different water mobility were revealed by a plot of the N M R line width vs the dehydration temperature. Similar N M R line shapes were anticipated for heulandite, a structural isomorph of clinoptilolite. As noted, these two zeolites are distinguished from each other by their thermal stabilities, their Si/Al ratios, and their cation content, whereas their crystal structure parameters are quite similar. Proton N M R revealed, however, that heulandite exhibits a (reduced splitting) Pake doublet, indicative of long-range order for the water environment, rather than a Gaussian-shaped curve. Conversion of heulandite to its B form by dehydration converts the Pake doublet structure to a Gaussian curve, indicative of a change from an ordered to less ordered water environment. Five clinoptilolite samples from different sites were examined by proton N M R and in each case a Gaussian curve was observed. Heulandite, from nine different sites, was also examined, and in each case a Pake doublet with separations varying between 16.6 and 14.8 kHz was observed with an average of 15.8 f 0.7. All samples had been heated to 110 O C overnight. These results suggest a new nondestructive N M R method for distinguishing clinoptilolite from heulandite. Water molecules in the cavities of heulandite and clinoptilolite interact with their respective cations: Ca for heulandite and N a and K for clinoptilolite. The clinoptilolite N a and K cations can be readily exchanged for other cations including Ca, but the exchanged zeolite still exhibits a Gaussian proton N M R line shape. The Ca cations of heulandite are more difficult to exchange for other cations, and we have not been successful in doing so. 27Almagnetic angle spinning (MAS) N M R experiments of similarly treated samples revealed changes in the electric field gradientsat thealuminumsitesin heulandite that can be associated with the heulandite to heulandite B transformation. 27AlMAS N M R of dehydrated clinoptilolite revealed a continuous change in the electric field gradients at the aluminum sites with no indications of a break or change in slope up to the maximum
The Journal of Physical Chemistry, Vol. 98, No. 4, 1994 1237 temperature studied, 700 OC. 29Si MAS N M R of dehydrated heulandite and clinoptilolite also revealed structural changes in the zeolitic framework and/or cation binding, but the spectra are not as easy to interpret. Rehydration of heulandite and clinoptilolite did reveal that these processes are reversible, as determined by 'H,27Al,and 29Si NMR.
Acknowledgment. This work was performed under the auspices of the US. Department of Energy by the Lawrence Livermore National Laboratory under Contract W-7405-ENG-48. This work was supported in part by the Nuclear Test-Experimental Science Containment Program. We wish to thank Marty Phillippi of the Clorox Co., Pleasanton, CA, for the use of his 400-MHz spectrometer. References and Notes (1) McKague, H. L.; Hearst, J. R.; Ward, R. L.; Buckhard, N. R. Nucl. Geophys. 1992, 6, 359. (2) Alberti, A. Tschermaks Mineral. Petrogr. Mitt. 1975, 22, 25. (3) Koyama, K.; Takbuchi, Y . Z. Kristallogr. 1977, 145, 216. (4) Hey, M. H.; Bannister, F. A. Mineral. Mag. 1934, 23, 556. (5) Pake, G. E. J . Chem. Phys. 1948, 16, 327. (6) Boles, J. R. Am. Mineral. 1972, 57, 1463. (7) Nakata, S.: Asaoka. S.: Kondoh. T. In Studies in Surface Science andCatalysis; Murakami, Y . ,Iijima, A., Ward, J., Eds.; Elsevier:.Amsterdam, 1986: Vol. 28. D 71. (8) Lippmaa, E.; Mfigi, A.; Samonson, M.; Engelhardt, G. J.Am. Chem. Soc. 1981, 103, 4992. (9) Knowlton, G. D.; White, T. R.; McKague, H. L. Clays Clay Miner. 1981, 29, 401. (10) Gutowsky, H. S.; Pake, G. E. J. Chem. Phys. 1950, 18, 162. (11) Ducros, P. Bull. SOC.Fr. Mineral. Cristallogr. 1960, 83, 85. (12) Stockton, G. W.; Polnaszek, C. F.; Tulloch, A. P.;Hasan, F.; Smith, I. C. P. Biochemistry 1976, 15, 954. (13) Freed, J. H.; Bruno, G. V.; Polnaszek, C. F. J . Chem. Phys. 1971, .
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