J. Phys. Chem. 1996, 100, 17351-17360
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Solid-State 27Al and 29Si NMR and 1H CRAMPS Studies of the Thermal Transformations of the 2:1 Phyllosilicate Pyrophyllite John J. Fitzgerald* and Abdullatef I. Hamza Department of Chemistry, South Dakota State UniVersity, Brookings, South Dakota 57007-0896
Steven F. Dec and Charles E. Bronnimann† Department of Chemistry, Colorado State UniVersity, Fort Collins, Colorado 80523-1872 ReceiVed: May 23, 1996; In Final Form: August 8, 1996X
Solid-state multinuclear magic-angle spinning (MAS) NMR studies of the thermal transformations of the 2:1 phyllosilicate mineral, pyrophyllite, over the temperature range 150-1350 °C are reported. 27Al and 29Si NMR and 1H CRAMPS techniques have been used to follow the progress of dehydroxylation between 150 and 550 °C. At 550 °C, pyrophyllite is completely dehydroxylated in 7 days to pyrophyllite dehydroxylate, an aluminosilicate intermediate containing 5-coordinate aluminum, on the basis of the MAS 27Al NMR measurements at 14 T. MAS 27Al and CP/MAS (cross-polarization) and SP/MAS (single-pulse) 29Si NMR results indicate that the dehydroxylate is formed prior to the separation of the silica-alumina layer. At 950 °C, the thermally induced transformation of pyrophyllite anhydride results in separation of the silica-alumina layer. A transition-alumina-type phase, containing 4- and 6-coordinate aluminum, is formed between 950 and 1050 °C. In addition, a high content of amorphous silica glass and a small amount of a poorly ordered Si/Al-containing mullite phase forms between 950 and 1050 °C. At 1250-1350 °C, the 29Si NMR shows that this glassy silica is converted to cristobalite, while the 27Al NMR indicates that this process is accompanied by conversion of octahedral aluminums to tetrahedral aluminums, possibly by incorporation of aluminums into an amorphous Si/Al-containing phase. The mechanism of dehydroxylation and of the higher temperature transformations of pyrophyllite dehydroxylate are discussed in light of these multinuclear solid-state MAS NMR results.
Introduction Pyrophyllite, a phyllosilicate mineral, is a 2:1 aluminosilicate structural clay consisting of an octahedral gibbsite-like aluminum hydroxide sheet layered between two tetrahedral silicate sheets.1-4 Each octahedral aluminum is bonded to three other octahedral aluminums with one linkage to an adjacent aluminum via two cis-hydroxide groups and the remaining via an apical oxygen of the SiO4 units of the tetrahedral sheets (Figure 1, top). The dehydroxylation of pyrophyllite to pyrophyllite dehydroxylate following heating at 550 °C for 7 days has been described as a homogeneous process involving reaction of two cis-OH groups (adjacent to each other on an Al octahedron) with the liberation of water. Wardle and Brindley3,4 determined the crystal structures of both pyrophyllite and pyrophyllite dehydroxylate (Figure 1, top and bottom, respectively). The anhydride, pyrophyllite dehydroxylate, was shown to consist of 5-coordinate, distorted, trigonal bipyramidal AlO5 structural units in the aluminum oxide layer sandwiched between two distorted but intact tetrahedral silica layers. The formation of 5-coordinate aluminum sites in pyrophyllite dehydroxylate is consistent with the homogeneous reaction of the adjacent OH groups to liberate water and the formation of a bridging oxide midway between adjacent aluminum atoms.5 Higher temperature thermally induced transformations of pyrophyllite dehydroxylate have been less thoroughly examined, although such investigations have significant bearing on our understanding of the high-temperature formation of ceramic/glass materials. * Author to whom correspondence should be addressed. † Present address: Chemagnetics, Inc., 2555 Midpoint Dr., Fort Collins, CO 80525. X Abstract published in AdVance ACS Abstracts, October 1, 1996.
S0022-3654(96)01499-2 CCC: $12.00
Figure 1. Structural diagrams3,4 of pyrophyllite (top) and pyrophyllite dehydroxylate (bottom).
Solid-state NMR investigations of the thermally induced formation of pyrophyllite dehydroxylate and its high temperature transformations have to date been limited.5,6 MacKenzie et al.5 used both 29Si and 27Al at 4.7 T to study these reactions up to 1350 °C. In the work by MacKenzie et al.5, 27Al NMR results showed drastic reductions in the total aluminum intensity upon dehydroxylation, with only 10% of the aluminums being observed by 27Al NMR at 550 °C. The corresponding 29Si NMR up to 550 °C showed that the -95 ppm signal due to pyrophyllite decreased with concomitant increases in a -101 © 1996 American Chemical Society
17352 J. Phys. Chem., Vol. 100, No. 43, 1996 ppm peak due to the dehydroxylate. At 900 °C, the 27Al NMR showed peaks at 41.7-66 and -7 ppm assigned to 4-coordinate and 6-coordinate aluminums, respectively, while the 29Si NMR showed the -101 ppm peak. At higher temperatures (11001350 °C), a range of 4-coordinate (40.6-48 ppm) and 6-coordinate peaks (at -8 ppm) appeared in the 27Al NMR consistent with mullite. At 1150-1350 °C, the 29Si NMR showed that pyrophyllite dehydroxylate is converted to amorphous silica, cristobalite (-110 ppm region), and poorly ordered mullite (-87 ppm). Frost and Barron6 reported the use of 27Al and 29Si NMR to examine the dehydroxylation of pyrophyllite at 550 °C for up to 7 days, with MAS 27Al NMR measurements at a 7.0 T magnetic field strength and 3-5 kHz sample spinning. MAS 29Si NMR obtained with 1H decoupling was used by Frost and Barron6 to follow the time course of dehydroxylation. These measurements revealed that the silica layers were maintained throughout the formation of the dehydroxylate. The original 29Si NMR signal at -95 ppm due to 3Q-type Si(OSi) (OAl ) 3 2 silicon atoms was observed to decrease in intensity concomitant with conversion to pyrophyllite anhydride [more than 90% dehydrated after 168 h (7 days) at 550 °C]. A new 29Si NMR signal at -101 ppm was observed to simultaneously increase in intensity upon partial or complete formation of pyrophyllite dehydroxylate. The change in the 29Si NMR chemical shift of the 3Q-type Si resonance signal of pyrophyllite to that of pyrophyllite dehydroxylate was hypothesized as due to significant changes in the Si-O-Si bond angles (132.2° to 137.3°), the Si-Si interatomic distances (2.98 to 3.03 Å), the average Si-O-Al bond angles (123.8° to 128.6°), and the Si-Al interatomic distances (3.15 to 3.10 Å) as also reported by MacKenzie et al.5 By contrast, the MAS 27Al NMR spectra (taken at 7.0 T) of partially and completely dehydroxylated pyrophyllite-derived solids obtained during the formation of pyrophyllite dehydroxylate reveal the gradual loss of the 5 ppm signal due to 6-coordinate aluminum in pyrophyllite, until virtually no signal is observed.6 The loss of the octahedral Al signal was attributed to the formation of 5-coordinate aluminum in the dehydroxylate. The loss of signal intensity was postulated to be due to a very large increase in the linewidth of the 27Al NMR resonance as a result of either chemical shift anisotropy or quadrupolar line broadening mechanisms, due to decreased symmetry around the aluminum. Recent MAS 27Al NMR studies at a magnetic field strength of 14 T and 16.0 kHz spinning speeds by Fitzgerald et al.7 have confirmed that the alterations from 6-coordinate Al in pyrophyllite to 5-coordinate Al in pyrophyllite dehydroxylate do indeed produce major changes in both the quadrupole coupling constant (qcc) and the asymmetry parameter (η) of the aluminum atoms. MAS 27Al NMR at 14.0, 11.7, and 8.4 T have demonstrated the advantages of both high-field and high-speed sample spinning conditions to observe the Al signals of the highly distorted 5-coordinate aluminums. In particular, the 27Al NMR spectrum of pyrophyllite dehydroxylate exhibits a second-order quadrupolar powder pattern for the 5-coordinate Al site. The simulated spectrum at 14.0 T has a qcc ) 10.5 MHz and η ) 0.6. Lower field spectra at 11.7 T show only a partially observable 5-coordinate signal due to increases in the second-order quadrupolar line-broadening effects at this lower field, whereas at 8.4 T the 5-coordinate 27Al NMR signal was unobservable due to severe second-order line-broadening effects at this even lower field. These measurements thus provided the first observation of the highly distorted 5-coordinate aluminum site in pyrophyllite dehydroxylate, with the large electric field gradient of aluminum nuclei in this material
Fitzgerald et al. requiring studies at 14.0 T to obtain a well-defined MAS 27Al NMR line shape for theoretical analysis. In the work reported herein, detailed solid-state MAS 27Al NMR studies of the dehydroxylation of pyrophyllite to pyrophyllite dehydroxylate up to 550 °C and the thermally induced transformations of pyrophyllite dehydroxylate up to 1350 °C are described. In addition to the high-field (14.0 T) high-speed MAS 27Al NMR described, solid-state single-pulse (SP)/MAS and cross-polarization (CP)/MAS 29Si NMR and 1H CRAMPS (combined rotation and multiple-pulse spectroscopy) NMR results are also reported. These results provide a detailed understanding of the changes in the chemical environments of the aluminum, silicon, and proton sites that occur during the thermal transformations of pyrophyllite over a wide temperature range. These NMR results corroborate the advantages of the simultaneous use of multinuclear solid-state NMR approaches to study thermally induced solid-state reaction processes for clay minerals of significance in materials science. Experimental Section Analysis and Thermal Treatment of Pyrophyllite. Elemental analysis of %Al2O3 and %SiO2 (theoretical 28.02% and 65.93%) was 28.58% and 61.59% by atomic absorption (AA) analysis. AA analysis was obtained by fusion of 0.1-0.2 g mineral samples with 1.2 g of lithium metaborate in graphite crucibles. Following heating for 30 min at 1000 °C, the melt was poured into 50 mL of 5% HNO3, dissolved, and diluted to 100 mL, and the resulting solution was diluted into the standard curve range with addition reagents as used for the AA standards. Aluminum AA standards ranged from 10 to 100 ppm of Al and contained 50 000 ppm of ammonium molybdate [(NH4)6Mo7O24‚4H2O], 5000 ppm of KCl, 0.1 M HCl, and 24 000 ppm of LiBO2 in 5% HNO3. AA measurements were obtained using a Varian AA-6 AA spectrophotometer at 309.3 nm with a N2O/ C2H2 burner. Silicon AA analysis used 10-80 ppm standards containing 1000 ppm of Al, 5000 ppm of KCl/0.1 M HCl, and 24 000 ppm of LiBO2 solution in 5% HNO3 at 251.6 nm using a N2O/C2H2 burner. Addition reagents were necessary to overcome interelement interferences that occur with Si/Alcontaining solutions. Total weight loss at 1050 °C due to water was 5.17% compared to 5.00% theoretical. Water loss determinations and calcination of mineral samples was carried out in a Blue Model M 10A electric furnace over the temperature range of 1101350 °C. In addition, the reference mullite was obtained from Coors Ceramics, Golden, CO. X-ray Powder Diffraction. X-ray powder diffraction patterns were obtained using a Philips-Norelco diffractometer equipped with a graphite monochrometer, a Cu KR X-ray source at 1.5418 Å, and a Dynamaster Model 64A recorder over the 2θ range 1-45°. Reference wavelength calibration was obtained using standard corundum or quartz standards. XRD patterns for pyrophyllite reveal numerous lines including three intense lines at 2θ (and d spacings) of 9.51° (9.26 Å), 19.15° (4.40 Å), and 29.0° (3.08 Å) that are comparable to those reported by Brindley and Wardle.4 These XRD lines of calcined pyrophyllite samples were used to monitor both its conversion to pyrophyllite dehydroxylate and other higher temperature calcined solids. The conversion to pyrophyllite dehydroxylate at 550 °C over 14 days produced a decrease in the intensity of the pyrophyllite lines. Three intense lines were observed at 2θ (and d spacings) of 9.35° (9.26 Å), 18.3° (4.43Å), and 28.4° (3.14 Å), identical to those of pyrophyllite dehydroxylate as reported by Wardle.3 At 7 days of calcination at 550 °C, the XRD lines for pyrophyllite dehydroxylate are the most intense.
Thermal Transformations of Phyllosilicate Pyrophyllite Calcination of pyrophyllite at 150 and 350 °C produced XRD patterns identical to those of pyrophyllite, whereas solids calcined at 950, 1050, and 1150 °C were nearly X-ray amorphous. The XRD of the 1150 °C sample does show several intense, very broad peaks: one at 2θ value of 21.84° (4.07 Å) due to amorphous cristobalite and several intense, broad peaks at 26.05°, 33.30°, and 42.68° (3.42, 2.69, and 2.12 Å) due to poorly ordered mullite. At 1250 and 1350 °C, the XRD patterns show a very intense, sharp peak at 21.84° (4.07 Å) assigned to crystalline cristobalite and three intense, sharp peaks at 26.05, 33.30, and 42.68° due to crystalline mullite. The percent content of mullite is between 10 and 15%. The peaks in the XRD pattern for the 1350 °C sample, in particular, are very narrow, indicative for both mullite and cristobalite of increased longrange ordering in the sample. Solid-State 27Al NMR Measurements. 27Al NMR spectra were recorded at 156.4 MHz on a Bruker AM-600 NMR spectrometer equipped with an Oxford medium-bore 14.1 T magnet using a “home-built” probe.8 The chemical shift reference, by sample substitution, was an aqueous 1 M AlCl3‚ 6H2O solution. A 90° pulse for this solution was 6.5 µs. MAS speeds of about 14-15 kHz were obtained using 4.5 mm (outer diameter) Vespel spinners. The solid-state 27Al NMR spectra were recorded using 0.5 µs excitation pulses, 50 ms relaxation delays, and 6 ms acquisition times. Typical spectra consisted of 5000 scans, acquired at a sweep width of 166.67 kHz using 2048 data points zero-filled to 8092, with 50 Hz line broadening due to exponential multiplication. Solid-State 29Si NMR Measurements. 29Si NMR spectra were recorded on a home-built Nicolet NT-200 NMR spectrometer optimized for CP/MAS 29Si NMR experiments at 39.74 MHz, with both 29Si and 1H radio frequency field strengths of approximately 41.5 kHz, and a wide-bore 4.7 T magnet. Samples were contained in either cut and sealed 8 mm diameter NMR quartz tubes (for CP/MAS) or 8 mm diameter drilled Delrin rods (capacity 400 mg) supported in a modified Gaytype spinner10 and spun at speeds from 1.8 to 2.2 kHz in a homebuilt probe.9 Typical CP/MAS spectra were obtained under the following NMR conditions: 8 µs 29Si and 1H 90° pulse, 2 s delay times, a 1H-29Si contact time of 7.5 ms for most samples, and a 25.65 ms signal acquisition time. NMR spectra consisting of 10002000 scans were acquired with a 10 kHz sweep width and 1024 data points, zero-filled to 2048, with 25 Hz line broadening. In the SP/MAS 29Si NMR experiments, using high-power proton decoupling, 29Si pulse widths of 2.50-5.00 µs (20-60° tip angles), delay times from 250 to 600 s, and acquisition times of 25.65 ms were used to obtain NMR spectra (usually overnight) using 144 or 256 scans, 10 kHz sweep width, 1024 data points zero-filled to 2048, and 40 Hz line broadening. In addition, solid-state SP/MAS 29Si NMR spectra were also obtained at 119.27 MHz on a Bruker AM-600, using a homebuilt probe at 2.2 kHz sample spinning. All 29Si NMR spectra were externally referenced to the signal for tetrakis(trimethylsilyl)methane (TTMSM), which was assigned a chemical shift of 0.00 ppm. 29Si chemical shifts relative to this reference are ca. -2.0 ppm lower than chemical shift values obtained using TMS as an external reference. 1H CRAMPS NMR Measurements. 1H CRAMPS NMR spectra were obtained at 187 MHz on a modified Nicolet NT200 NMR spectrometer, using the BR-24 pulse sequence.11 The cycle time for the BR-24 pulse sequence varied between 108 and 144 s, corresponding to a pulse spacing of 3.0-4.0 s. Samples were sealed off under vacuum in a thick-walled 5 mm o.d. (2 mm i.d.) glass tube. Repetition delays of 3-6 s and
J. Phys. Chem., Vol. 100, No. 43, 1996 17353 between 64 and 256 repetitions were used to obtain signal-tonoise ratios of typically 100:1. Chemical shifts were determined by external referencing via substitution of samples containing TTMSM and are reported here relative to tetramethylsilane (TMS). Results The thermal conversion of pyrophyllite over the temperature range up to 1350 °C involves three primary stages: (1) conversion of pyrophyllite to pyrophyllite dehydroxylate at 550 °C; (2) disruption of the silica-alumina structural interface in pyrophyllite dehydroxylate at 950 °C with the formation of an alumina-rich (transition alumina) and silica-rich (amorphous silica) phase, in addition to a minor poorly ordered mullite component; and (3) crystallization of significant amounts of cristobalite between 1250 and 1350 °C and additional alterations in the 4-coordinate/6-coordinate aluminum ratio of the transition alumina phase. The structural changes in the aluminum oxide layer from solid-state 27Al NMR are the most significant in terms of understanding these thermally induced processes because major alterations occur in the aluminum coordination number and site symmetry.13-18 SP/MAS and CP/MAS 29Si NMR have been most useful in the temperature range 150-550 °C to monitor the dehydroxylation process leading to the formation of pyrophyllite dehydroxylate and to obtain information regarding the disruption of the alumina-silica layers near 950 °C as well as additional crystallization and/or phase transformations that are associated with the silicon atom sites at higher temperatures (g1150 °C). 1H CRAMPS NMR has also been useful to monitor the dehydroxylation process, by examining the signals due to the Al-OH-Al proton populations and the residual protons in pyrophyllite anhydride. First StagesDehydroxylation of Pyrophyllite. MAS 29Si NMR spectra of various samples of thermally treated pyrophyllite were obtained at 39.7 and 119.2 MHz under high-power 1H decoupling using single-pulse (SP) measurements and 1H29Si cross-polarization (CP) conditions (for 39.7 MHz only). The 29Si NMR results at 119.2 MHz are shown in Figure 2 for samples heated at 350, 500, and 550 °C for 24 h. The corresponding MAS 27Al NMR spectra of samples heated at 150, 350, and 550 °C for 24 h are given in Figure 3. The degree of dehydroxylation (R), where
R ) actual weight loss/maximum weight loss is also given for each sample based on weight loss measurements. Up to 500 °C, the SP/MAS 29Si NMR of pyrophyllite shows primarily a single resonance in the -95.0 to -96.7 ppm range due to 3Q-type Si(OSi)3(OAl2) silicon sites in the silicate layer of pyrophyllite, while the sample heated at both 500 and 550 °C for 24 h shows a 29Si NMR signal at -101.3 ppm due to pyrophyllite dehydroxylate. The CP/MAS 29Si NMR spectra, measured at 39.7 MHz with a 1H-29Si contact time of 25.65 ms and a delay time of 2 s, for samples of pyrophyllite calcined at 550 °C for 4 h and at 550 °C for 24 h, are also shown in Figure 4. The CP/MAS 29Si NMR spectrum of pyrophyllite heated at 550 °C for 24 h, the temperature of initial conversion to pyrophyllite dehydroxylate, shows a broadened resonance with a chemical shift of ca. -92 ppm, assigned to silicon atoms of the remaining undehydroxylated pyrophyllite contained in these samples.5,6,24 The signal/noise ratio of the 29Si NMR peak observed for the pyrophyllite sample calcined at 550 °C for 24 h is drastically reduced in comparison with the sample calcined at 550 °C for 4 h. The signal intensity decrease is attributed to nearly complete dehydroxylation of the 24 h calcined sample. The nearly complete elimination of hydroxyl protons following dehydroxylation drastically reduces the proton population needed
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Figure 2. Solid-state MAS 29Si NMR spectra at 119.27 MHz and 2.2 kHz sample spinning speeds for pyrophyllite-derived solids following calcination from 350 to 1050 °C for 24 h of heating.
for cross-polarization. A more complete perspective on the changes in the silicon atom environments during the dehydroxylation process can be seen by comparing the CP/MAS 29Si NMR spectra of the two pyrophyllite samples calcined at 550 °C with the corresponding SP/MAS 29Si NMR spectra as given in Figure 4. At 550 °C for 4 h, the SP/MAS 29Si NMR spectrum shows two resonances at -92.6 and -98.5 ppm, assigned to the silicon atoms of pyrophyllite (with adjacent protons) and the silicon atoms of pyrophyllite dehydroxylate (with no adjacent protons), respectively. These results are consistent with those observed by Frost and Barron.6 By
contrast, the CP/MAS 29Si NMR spectrum shows only the pyrophyllite silicon signal since this solid contains nearby protons in the Al-OH-Al layer of the mineral that facilitate 1H-29Si cross-polarization transfer. The relative signal intensity of the NMR peak at -92.4 ppm indicates that ca. 20% of pyrophyllite remains, thereby providing a means to quantify the conversion of pyrophyllite to pyrophyllite dehydroxylate as reported initially by Frost and Barron.6 At 550 °C calcination for 24 h, the SP/MAS 29Si NMR spectrum shows only the signal due to the pyrophyllite dehydroxylate. By contrast, the CP/ MAS 29Si NMR spectrum of this sample shows only the silicon
Thermal Transformations of Phyllosilicate Pyrophyllite
J. Phys. Chem., Vol. 100, No. 43, 1996 17355
Figure 3. Solid-state MAS 27Al NMR spectra at 156.4 MHZ and 14-15 kHz sample spinning speeds for pyrophyllite-derived solids following calcination from 150 to 1050 °C for 24 h of heating. The value R refers to the degree of dehydroxylation.
resonance at -92.4 ppm of pyrophyllite, the signal intensity being drastically reduced due to limited proton populations in the samples as also noted in the 1H CRAMPS NMR spectrum of this sample (Vide infra). The CP/MAS 29Si NMR signal of the remaining pyrophyllite accounts for 7 days. 1H CRAMPS NMR of Various Pyrophyllite Solids. 1H CRAMPS NMR spectra measured at 187 MHz for pyrophyllite and various pyrophyllite samples calcined at 150 °C for 4 h, at 550 °C for 4 h, and at 550 °C for 24 h are given in Figure 7. The 1H CRAMPS spectrum of pyrophyllite gives a single resonance at 2.4 ppm due to protons of the bridging hydroxide ligands occupying cis-positions on the aluminum octahedron of the gibbsite-like aluminum hydroxide layer of pyrophyllite. The large line width for the proton signal assigned to the AlOH-Al moiety is probably the result of 1H-27Al dipolar coupling.11,31 The 1H CRAMPS spectra of pyrophyllite calcined at 150 °C (and 350 °C, not shown) are nearly identical to the corresponding spectrum of pyrophyllite, showing a single peak at 2.4 ppm. At these temperatures, dehydroxylation of these samples is very limited, with dehydration values (R) of 0.00 and 0.18, respectively. The MAS 27Al NMR and SP/MAS 29Si NMR spectra of these two samples are also similar to the corresponding spectra of pyrophyllite. At 550 °C calcination for 4 h (R ) 0.62), the 1H CRAMPS signal corresponding to the Al-OH-Al group in pyrophyllite decreases in intensity and is substantially broadened as a result of dehydroxylation due to removal of 62% of the structural water. The increased peak broadening is probably a consequence of the presence of a range
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Figure 4. CP/MAS and SP/MAS 29Si NMR spectra at 39.7 MHz and 1.8-2.2 kHz sample spinning speeds for pyrophyllite-derived solids following calcination at 550 °C for 4 and 24 h.
of different Al-OH-Al sites remaining during the intermediate stages of dehydroxylation leading to the formation of pyrophyllite anhydride. The SP/MAS 29Si NMR of this sample indicates 65% conversion to pyrophyllite dehydroxylate. The 1H CRAMPS signal broadening probably also reflects a significant degree of disorder in sample regions that are not completely dehydroxylated. At 550 °C calcination for 24 h, the dehydroxylation is ca. 95% complete (R ) 0.94). The dramatic reduction in the 1H CRAMPS signal, and the low signal/noise of the CP/MAS 29Si NMR signal (Figure 4), support these observations. The 1H CRAMPS spectrum show a very broad resonance centered at 2.5 ppm over a 10 ppm range of the chemical shift scale and substantial tailing of the broad signal to the higher chemical shift region. The sharp signals in the spectrum at ca. 0 and 5 ppm are due to rotor lines, a particular problem for lossy samples.32 The broad 1H NMR resonance at 2.5 ppm is probably due to any remaining undehydroxylated pyrophyllite. In addition, the asymmetrical nature of the broad resonance suggests that another broad resonance is centered in a region from 5 to 6 ppm. This latter signal intensity may be due to residual protons in the dehydroxylated pyrophyllite which may be associated with Al-OH-Si groups due to migration of protons during the latter stages of dehydroxylation. 1H CRAMPS resonances in this region have been observed previously for silica-alumina solids11 as well as dehydroxylated and dealuminated kaolinite materials.30 For calcination at time periods greater than 24 h at 550 °C, and at calcination temperatures higher than 550 °C, the other calcined pyrophyllite samples do
Fitzgerald et al.
Figure 5. MAS 27Al NMR spectra at 14 T for pyrophyllite-derived solids following calcination at 550 °C for up to 14 days. Conditions were similar to those in Figure 3.
Figure 6. Solid-state MAS 27Al NMR spectrum of pyrophyllite dehydroxylate at 14 T (156.4 MHz and 14-15 kHz sample spinning speeds): experimental (top) and computer-simulated (bottom) spectra.
not exhibit observable 1H CRAMPS signals using similar measurement conditions, presumably due to reduced proton populations. These samples have large R values (>0.96) consistent with their higher temperature treatment. Second StagesSeparation of Silica-Alumina Layers. The second stage of the thermally induced transformations of pyrophyllite occurs in the temperature range 750-1050 °C on the basis of the SP/MAS 29Si NMR given in Figure 2 and the 27Al NMR given in Figure 3. From 750 to 900 °C, the SP/ MAS 29Si NMR spectra at 119.2 MHz show that the principal
Thermal Transformations of Phyllosilicate Pyrophyllite
Figure 7. 1H CRAMPS NMR spectra (187 MHz and 2-3 kHz sample spinning speeds) for pyrophyllite-derived solids following calcination from 150 to 550 °C for 24 h of heating.
resonance due to pyrophyllite dehydroxylate occurs at -100.3 to -101.3 ppm. At 950 °C, a dramatic change is observed in the 29Si NMR spectrum as evidenced by the broad resonance at -108 ppm and an additional minor peak at -88 ppm. The former resonance is assigned to amorphous silica (4Q) and the latter resonance to poorly disordered mullite. Similar 29Si NMR spectra were observed for the samples heated at 1050 and 1150 °C. These results together with the corresponding changes in the 27Al NMR spectra of these samples (Vide infra) provide direct evidence that the disruption in the silica-alumina layers in pyrophyllite dehydroxylate occurs at 950 °C. The 27Al NMR spectra of the pyrophyllite-derived solid obtained following heating at 750 °C for 24 h shows a complex spectrum with a sharp signal intensity maximum at 12 ppm and at -9.3 ppm, due mainly to 5-coordinate aluminum in the dehydroxylate. In addition, a broader signal in the chemical shift range of 25-35 ppm is likely associated with other 5-coordinate aluminums that may have undergone major distortions prior to separation of the silica-alumina layers. At 950 and 1050 °C, the 27Al NMR spectra show the presence of two intense broad spectral features at 3.4 and 63.0 ppm and at 2.6 and 65.3 ppm, respectively. The 27Al NMR spectra of these solids suggest that the disruption of the silica-alumina layer between 950 and 1050 °C is accompanied by conversion of the 5-coordinate Al in the dehydroxylate to a mixture of 4-coordinate and 6-coordinate aluminums due to a disproportionation reaction. The appearance of the 4- and 6-coordinate signals is generally similar to that of the 27Al NMR spectrum of γ-alumina12 (Figure 8) with two notable exceptions. First, the tetrahedral peak is more intense than in the γ-alumina NMR spectrum, and second, the tetrahedral and octahedral peaks are
J. Phys. Chem., Vol. 100, No. 43, 1996 17357 both substantially broader than those seen in the 27Al NMR spectra of γ-alumina.12 The increased peak intensity and peak line width of the tetrahedral signal suggest that additional tetrahedral aluminums in the 50 ppm region, in addition to the 63-65 ppm region, are present following separation of the silica-alumina layer. The peak at 63-65 ppm is assigned to a poorly ordered alumina phase, while the 27Al NMR signal observed in the 50 ppm region is likely associated with a poorly ordered silica-alumina phase [Al(OSi)4 sites] or possibly mullite,15,17,18 which forms during the segregation of the silica and alumina phases at these higher temperatures. The formation of such phases has been reported for the high-temperature solidstate transformation of kaolinite on the basis of solid-state 27Al NMR measurements.20-23 The increased linewidth of the octahedral signal at 2-3 ppm is probably a result of chemical shift dispersion due to a wide range of a range of different local AlO6 sites, which may be adjacent to both octahedral and tetrahedral aluminums12 in transition-type alumina as well as due to 4-coordinate aluminums in poorly ordered mullite. The solids formed in this intermediate temperature region therefore contain principally a transition-type alumina phase [with both tetrahedral and octahedral aluminum atoms, the former of the structural unit Al(OAl)4], a small amount of a Si/Al phase [with tetrahedral Al(OSi)4 units and octahedral AlO6 sites], and a minor R-alumina component [with Al(OAl)6 sites]. Third StagesFormation of Mullite, Cristobalite, and an Amorphous Transition Alumina. The third stage of the thermal conversion of pyrophyllite occurs between 1150 and 1350 °C. For the sample heated at 1150 °C, the SP/MAS 29Si NMR shows similar spectra to the samples heated at 950 and 1050 °C, with a very broad resonance centered between -111.7 and -112.4 ppm, assigned to amorphous silica (and possibly some minor Si/Al glass phase), as well as a less intense resonance at -88.1 and -87.9 ppm that is assigned to poorly ordered mullite. [The SP/MAS 29Si NMR spectrum of a pure mullite sample (Figure 8) shows three resonances at -84, -87, and -90 ppm.] At 1250 and 1350 °C, the SP/MAS 29Si NMR spectra of thermally treated pyrophyllite samples show drastic changes with the appearance of a sharp resonance at -112 ppm assigned to crystalline cristobalite, in addition to less intense peaks at -73 and -87 ppm. The latter resonance is assigned to poorly crystallized mullite. The XRD patterns for these pyrophyllite samples heated at 1250 and 1350 °C also show considerable narrowing of the peaks assigned to cristobalite, suggesting that the long-range ordering in the separated silica phase increases, consistent with the 29Si NMR results. The 27Al NMR spectra of pyrophyllite sample heated between 1150 and 1350 °C, together with reference spectra of γ-alumina and mullite are shown in Figure 8. These spectra of thermally treated pyrophyllite solids heated in this temperature region indicate that the final stages of thermal transformation of the intermediate transition alumina phase derived from pyrophyllite dehydroxylate involves a shift in the relative intensities of the octahedral and tetrahedral peaks in the 27Al NMR spectra, with the tetrahedral aluminum signal increasing with increases in the heating temperature. The 27Al NMR spectra of pyrophyllite heated at 1150 and 1250 °C is assigned to a type of transition alumina that has a higher Td/Oh aluminum peak ratio than that of the γ-alumina sample. In addition, the tetrahedral peak is much broader than that observed for γ-alumina and is likely composed of at least two overlapping resonances. For the sample heated at 1350 °C, the 27Al NMR spectrum shows a further increase in the intensity of the tetrahedral signal in comparison with a γ-aluminum sample, particularily in the 55
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Fitzgerald et al.
Figure 8. Solid-state SP/MAS 29Si NMR spectra (left) and MAS 27Al NMR spectra (right) for pyrophyllite-derived solids calcinated from 1150 to 1350 °C for 24 h. Also included are the SP/MAS 29Si NMR and 27Al NMR spectra of mullite (Coors Ceramics) and the MAS 27Al NMR spectrum of γ-alumina (Alpha D2). NMR conditions were similar to those in Figures 2 and 3.
ppm region, that corresponds to the region for 4-coordinate aluminums contained in silica-aluminas. These changes in the aluminum coordination number accompany the formation of cristobalite from amorphous silica glass on the basis of 29Si NMR results, suggesting that some additional octahedral aluminums may be incorporated into 4-coordinate aluminum sites of the remaining silica glassy matrix at these temperatures. The 27Al NMR spectrum of mullite shown in Figure 8 has a similar line shape to the spectra of the various pyrophyllite-derived solids, with minor differences in the relative intensities of the Td/Oh peak ratios. However, the major differences observed in the 29Si NMR spectra for the pyrophyllite-derived samples (Figure 8) indicate that the similarities of the 27Al NMR do not accurately reflect the major phase differences between these pyrophyllite-derived samples containing high contents of cristobalite and a small amount of mullite (Vide infra).
Discussion First StagesMechanism of Pyrophyllite Dehydroxylation and Pyrophylllite Dehydroxylate Structure. The dehydroxylation of pyrophyllite has been shown by water loss measurements and DTA analysis to involve two overlapping endothermic processes at 520 °C (30% water loss) and at 673 °C (60% water loss).3-6 Heating at 950 °C is required to remove the remaining 10% water to produce a completely anhydrous mineral under the rapid heating (10 °C/min) conditions of such analysis. Structural changes occurring in pyrophyllite during its conversion to the dehydroxylate have been sought using solidstate 27Al and 29Si NMR as reported by Barron and Frost6 and MacKenzie et al.,5 although only the 27Al NMR results reported herein have been sufficiently detailed to elucidate the coordination number changes of the aluminums during this process. The
Thermal Transformations of Phyllosilicate Pyrophyllite 27Al NMR results reported herein readily show that the conversion of 6-coordinate to 5-coordinate aluminum occurs in a nearly quantitative fashion as a result of peak narrowing of the 5-coordinate Al NMR second-order powder pattern signal at the large magnetic field (14 T) and fast spinning speeds employed. Dehydroxylation of the pyrophyllite samples used is essentially complete in 7 days at 550 °C. The 27Al NMR spectrum of the 5-coordinate AlO5 sites in pyrophyllite dehydroxylate consists of a second-order quadrupolar powder pattern with a large electric field gradient at the aluminum nucleus (qcc ) 10.5 MHz; η ) 0.6) and is consistent with a highly distorted, trigonal bipyramidal AlO5 structural unit.7 The isotropic chemical shift of 29 ppm is consistent with the range observed for other AlO5 sites (30.9-36 ppm) found in related aluminumoxygen systems.25,28 The qcc and η parameters obtained for pyrophyllite dehydroxylate are the largest observed to date for authentic 5-coordinate AlO5 sites, in contrast to those of andalusite25 (qcc ) 5.9 MHz; η ) 0.69-0.70), barium aluminum glycolate (qcc and η undetermined but much less),25-27 augelite (qcc ) 5.7 MHz, η ) 0.85), and senegalite (qcc ) 2.87 MHz with η ) 0 or qcc ) 2.48 MHz with η ) 1).28 The very large electric field gradient observed for the 5-coordinate, highly distorted trigonal bipyramidal AlO5 sites in pyrophyllite is likely a result of the structural constraints imposed on the aluminum oxide layer formed in the dehydroxylate as a result of the rigid silicate layers above and below it. The two adjacent silicate layers are intact following complete dehydroxylation on the basis of the X-ray crystal structure of Wardle3 and the solid-state 29Si NMR work of Frost and Barron.6 The crystal structure results of Wardle and Brindley4 clearly show that the geometry of the SiO4 units in pyrophyllite dehydroxylate, including the Si-O-Si bond angles and Si-Si bond distances, is significantly distorted following the dehydroxylation process. The SP/MAS and CP/MAS 29Si NMR results of this work support the previous 29Si NMR results of Barron and Frost6 since only two resonances are observed due to 3Q-type Si(OSi)3(OAl2) sites in either pyrophyllite (at -95 ppm) or pyrophyllite dehydroxylate (at -100 ppm). The chemical shift of the resonance due to the silicon sites is shifted by 5 ppm due to significant changes in the Si-O-Si bond angles (132.2° to 137.3°) as well as other structural alterations accompanying the dehydroxylation. Furthermore, the 3Q-type Si(OSi)3(OAl2) silicon sites have second-nearest-neighbor 6-coordinate aluminums in pyrophyllite and second-nearest-neighbor 5-coordinate aluminums in the dehydroxylate. The major structural alterations in the silicon sites are probably a consequence of the aluminum coordination number changes upon dehydroxylation. Most significantly, the 29Si NMR results reported here establish that the Al-O-Si linkages are intact throughout the complete dehydroxylation at 550 °C heating for 14 days. The 1H CRAMPS spectra for pyrophyllite and various dehydroxylated solids provide a means to monitor the removal of protons from Al-OH-Al moieties based on the intensity of the 2.4 ppm 1H NMR peak. For example, at 550 °C heating for 4 and 24 h, conversion to the dehydroxylate (65% and 95%, respectively) can be quantified on the basis of the decrease in the 1H NMR signal intensity, consistent with the interpretation of both the 27Al and 29Si NMR results. On the basis of these multinuclear NMR results reported herein and those of Barron and Frost,6 the dehydroxylation of pyrophyllite to pyrophyllite dehydroxylate involves a random condensation of cis-OH groups in the Al-OH-Al layer of pyrophyllite. Loss of 1 mol of water by condensation of two adjacent OH groups of the aluminum octahedron produces two major structural changes in the 2:1 dioctahedral layered clay
J. Phys. Chem., Vol. 100, No. 43, 1996 17359 mineral: (1) rearrangement of the octahedral 6-coordinate AlO6 sites to distorted, 5-coordinate trigonal bipyramidal AlO5 sites and (2) distortions in the tetrahedral SiO4 silicon sites adjacent to the dehydroxylated aluminum sites. A random mechanism of dehydroxylation by condensation of cis-OH groups of the Al-OH-Al layer is consistent with the observations from these studies and is essentially 95% complete following heating for 24 h at 550 °C. Second StagesHigh-Temperature Transformations of Pyrophyllite Dehydroxylate. In the temperature range of 750950 °C, intact pyrophyllite dehydroxylate undergoes phase separation accompanying the disruption of the Si-O-Al linkages. The 27Al NMR results reported herein indicate that phase separation into an alumina-rich and a silica-rich phase is preceeded by distortions of the 5-coordinate aluminum sites in pyrophyllite dedyroxylate at 750 °C (24 h heating), while the 29Si NMR show that silica-alumina phase separation does not actually occur until 950 °C. At this latter temperature, the 29Si NMR shows that the silica phase formed consists of primarily an amorphous glassy silica phase (with possible minor Si/Al glassy phase), as well as a minor poorly ordered mullite component. At 950 °C heating for 24 h, the 27Al NMR spectra show that conversion of the alumina layer containing AlO5 sites (δiso) 29) into a discrete alumina layer consisting of AlO4 (δiso) 63-65.3) and AlO6 (δiso) 2.6-3.4) structural moieties generally similar to that of γ-alumina occurs,12 although the relative Td/ Oh peak ratios and line widths are considerably different. This transition alumina phase is likely much less ordered or glassy as evidenced by the chemical shift dispersion observed for these two broad resonances. This transition-type alumina phase persists to 1050 °C, possibly with the initial formation of
