J. Phys. Chem. 1993,97, 10389-10393
10389
Structural Studies of Pillared Saponite Liansheng Li, Xinsheng Liu,*J Ying Ge, and Ruren Xu Department of Chemistry, Jilin University, Changchun 130023, People's Republic of China
JoHo Rocbat and Jacek Klinowski' Department of Chemistry, University of Cambridge, Lensfeld Road, Cambridge CB2 IEW, U.K. Received: July 7, 1993' The layered aluminosilicate saponite has been pillared with [A11304(OH)24(H20)1~]~+ (referred to as Al13) and with the organosilicon compound NHz(CH2)3Si(OCzH5)3 (referred to as Si-R) and studied using XRD, FT'IR, 29Siand 27Al MAS NMR, and gas adsorption. In the case of Si-R the average length of the S i - O s i chain is seven Si atoms. Cross-linking between the tetrahedral layers and the pillars takes place upon calcination of the intercalated samples. The cross-linking sites can be either the Si or the A1 atoms in the Si-OH-A1 linkage in the tetrahedral layer, depending on the nature of the pillars. With All3, the bonding occurs on the Si site, while with SiO2-like species generated during calcination bonding occurs on the A1 site. Pillared saponite samples are microporous with a surface area of 240-280 m2/g and an average pore size of 8-9 A.
Introduction Three experiments demonstrate that the product is indeed saponite: (1) XRDdatamatch thosein the literature; (2) chemical Intercalation of clays such as montmorillonite with various composition (Na determined by atomic absorption, Mg by wet organic and inorganic compounds, simple molecules, and comchemicalanalysis,S a n d A1 by XRF) isNa,,47(Mg)6[Sia,ssAll,47]plexes has been investigated for many years.' Clays pillared with (3) solid-state NMR indicates that almost all inorganic complexes such as [ A ~ I ~ O ~ ( O H ) ~ ~ ( H(A113) ~ O ) I ~ ] ~020(OH)4-mH20; + A1 in the sample are tetrahedrally coordinated,which is consistent exhibit considerable thermal stability and large pore size, thus with the saponite structure. offering technological catalytic applications.2.3 Saponite, a trioctahedral member of the smectite family, is Intercalation with A 4 3 . Dried synthetic saponitewas dispersed built from two Si04 tetrahedral layers and one MgO6 octahedral in water (the weight ratio of solid to water = 1:9). The suspension layer arranged in a TOT sandwich (T = SiO4tetrahedral layer was then mixed with an AlI3solution (OH/Al = 2.5, [ A P ] = and 0 = MgO6 octahedral layer). Substitution of Si by A1 in 0.10 M) under stirring. The mixture was stirred at 60 "C for 2 the tetrahedral layer creates a negative charge which is comh. The solid was separated from the liquid phase, and the whole pensated by cations such as Na+ located in the interlayer space. process was repeated. The product was filtered, washed with Koizumi et ala4 first synthesized saponite via a solid-state distilled water, and dried in air. It was found to contain 1.8 hydrothermal reaction. Blumenthal et al.5also mention synthesis Al/uc (uc = unit cell). of saponite. Matsuda et a1.6 studied the chemical and catalytic Intercalation with Si-R. Saponite was intercalated with propertiesof saponite pillared with A1203. Jiang et aL7examined NH2(CH2)3Si(OC2H~)3 by dispersingsaponiteinto a 30%aqueous the intercalation of A113 into saponite and described the properties solution of N H ~ ( C H ~ ) ~ S ~ ( O C Zfollowed H S ) ~ , by refluxing the of the products. Nevertheless, structural information on pillared suspension for 24 h. The product was filtered, washed with saponitesis still lacking. We have earlier successfully intercalated distilled water, and dried in air. It was found to contain 5.1 an organosiliconcompound, NH~(CH~)~S~(OC~HS)~(S~-R), into Si/uc. the a-zirconium phosphatewith a layered structure and examined Techniques. Powder X-ray diffraction measurements were the structure of the product.* We now present the results of a carried out on a Rigaku D/MAX-IIIA X-ray diffractometer, structural study of saponite pillared with the All3 cation and with operating at 30 kV and 30 mA with Cu Ka radiation (A = 1.5418 the organosiliconcompound. We used 27Aland 29Simagic-angleA). 27AlMAS NMR spectra were recorded at 104.2MHz using spinning (MAS) NMR, powder X-ray diffraction, infrared very short and powerful (0.6 ps, equivalent to 9" pulse angle) spectroscopy, and gas adsorption. radio-frequency pulses and a recycle delay of 0.5s. Rotors were spun in air at 10 kHz. Chemical shifts are quoted in ppm from Experimental Section external aqueous Al(NOp),. 29Si MAS NMR spectra were Synthesis of Saponite. Solution A was prepared by dissolving recorded at 79.5 MHz with rotors spinning in air at 3-5 kHz. 2.95 g of NaOH in 15 mL of H20, followed by adding 4.84 mL z9Si-1H cross-polarization (CP/MAS) spectra were measured of water glass (Si02,413 g/L, Na2O/SiO2 = 3.09) with stirring. with a single contact, a contact time of 5 ms, a proton 90" pulse Solution B was made by dissolving 2.27 g of Al~(S04)3.18H20 of 5 ps,and a recycle delay of 2 s. High-power proton decoupling in 10 mL of H20, and solution C was made by dissolving 7.39 spectra were recorded with radio-frequency pulses equivalent to g of MgS0407H20 in 20 mL of H20. Solution B was added 50" pulse angle and a recycle delay of 20 s. Chemical shifts are dropwise into solution A with stirring, and then solution C was quoted in ppm from external tetramethylsilane (TMS). added in the same manner. The mixture was stirred until a Infrared spectra were recorded on a Nicolet 5DX FT-IR homogeneousgel was obtained. This was transferred into a sealed instrument using the KBr wafer technique. High-temperature autoclave, kept at 290 "C for 12 h, and then cooled to room IR spectra were recorded after keeping the sample for 5 min at temperature. The solid product was separated from the mother the desired temperature. liquor by filtration, washed with distilled water, and dried in air. Measurement of Surface Area and Pore Size. The specific t Present address: Department of Chemistry and Biochemistry,University surface area and the most probable pore size of pillared saponite of Notre Dame, Notre Dame, IN 46566. were determined by adsorption of N2 at 77 K. Saponite pillared t Present address: Dept. Quimica, Universidade de Aveiro, 3800 Aveiro, with AllJ was heated to 250 "C for 2 h at lo" Torr prior to the Portugal. adsorption measurements. Samples intercalated with NHz(CH2)sAbstract published in Advance ACS Absrracrs, September 1, 1993. 0022-3654/93/2097-lO389$04.00/0
0 1993 American Chemical Society
Li et al.
10390 The Journal of Physical Chemistry, Vol. 97, No. 40, I993
TABLE I: Change of d Spacings ( 6 1 ) (in A) in Intercalated Saponite upon Calcination sample
22OC
2OOOC
4OOOC
600OC
All3-saponite Si-R-saponite
19.6 23.2
19.6 22.6
18.8 17.7
18.8 17.7
Si(OC2HS)3were calcined at 450 "C for 2 h and then heated at 300 "C for 2 h at 10-4 Torr. The surface area and the most probable pore size were calculated using the MMBB3 computer program9 involving a planar-pore model.
Results and Discussion
'+
X-ray Diffraction. Intercalation of [All304(OH)u(H~O) 121 cations and N H ~ ( C H ~ ) ~ S ~ ( O Cmolecules ~ H S ) ~ into synthetic saponite is reflected in the increase of the dwl spacing of the intercalatedsamples: from 13.2 to 19.6 A for saponiteintercalated with All3 and to 23.2 A for saponite intercalated with Si-R. Calcination of the intercalated saponites causes a decrease of the interlayer spacings, reflecting changes of the pillars in the interlayer space, and the onset of cross-linking between the pillars and the tetrahedral aluminosilicate layers. Table I gives the do01 spacing for saponite intercalated with A113 and Si-R upon calcination at different temperatures. The spacing for sa intercalated with A113 decreases slightly from 19.6 to 18.8 with increasing temperature and remains constant as the temperature is increased further to 600 "C. The decrease of the do01 spacing by 0.8 A implies that processes such as dehydroxylation of All3 and cross-linking of the pillar with the tetrahedral layers of saponite may occur. The height of the All3 pillar, estimated to be 9.2 A on the basis of the thickness of TOT layer of saponite, 9.6 A, compares well with the dimensions of All3 cation.1° However, at temperatures between 200 and 400 "C, the do01 spacing of Si-R intercalated saponite undergoes a significant decreasefrom 23.2 to 17.7 A, correspondingto the decomposition of the organosilicon species in the interlayer space. The large 17.7-Afinal dspacing impliesthat the pillars are still present but have been transformed. Above 400 "C, do01 no longer changes (see Table I). The height of the new pillar formed in saponite is estimated to be 7.7 A, approximately equal to the thickness of two layers of Si04 tetrahedra. X-ray diffraction shows that pillared saponite samples are thermally stable above 600 "C. Infrared Spectroscopy. Figure 1 shows the infrared spectra in the region 2000-400 cm-l of the parent syntheticsaponite,samples intercalated withAll3and Si-R, and intercalated samplescalcined at different temperatures. The absorption bands in the spectra are listed in Table I1 and assigned by reference to the literature.8J1J2 A comparison of the spectra in Figure la-c reveals several features which reflect interactions of the pillars with the tetrahedral layers upon intercalation. In saponite intercalated with Al13, shifts toward higher wavenumbers are observed of the 1054-and 988-cm-1 bands correspondingto Si(Al)-O stretching vibrations, and shifts toward lower wavenumbersof the bands at 738 and 652 cm-l are assigned to Si-0-A1 (see Table 11). In addition, relative intensities of the bands at 820 (Al-OH-Mg) and 6 9 1 cm-1 (Si-O-A1 vibration) are reduced (see Figure 1). The bands at 457 and 443 cm-1, due to Si-0-Mg bending vibrations, overlap at 455 cm-1. In samples intercalated with Si-R, apart from band shifts (see Table 11), new bands at 1224, 1138, 772, 603, and 585 cm-1 are found. Studies of the intercalation of a-zirconium phosphate with Si-R* show that the pillars undergohydrolysisand polymerization during intercalation to form NH3+(CH2)3Si(OH)2OSi(OH)2(CH2)3NH3+ dimers and NH3+(CH2)3Si(0H)z [OSi(OHMCH2)pNH3+]2 trimems Also in the present case, intercalation of the organosiliconcompound is accompanied by hydrolysis and polymerization (see below). The new absorption bands in the IR spectrum are accordingly assigned to the vibrations of the organic groups and Si-O-Si linkages (see Table 11). The most significant changes of the IR spectrum observed upon calcination of saponite intercalated with All3 is thedecrease
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Figure 1. (a) Infrared spectra of synthetic saponite. (b) As-prepared andcalcinedsaponiteintercalatedwith A113. (c) As-prepared andcalcined saponite intercalated with Si-R.
in intensity of the Si-0-AI bending at 646 cm-l (see Figure 1b). This indicates that calcination brings about chemical reactions between the All3 pillars and the tetrahedral layers and that the site of attack of the tetrahedral layer by All3 is probably related to Si-O-A1 linkages. Upon calcination of saponite intercalated with Si-R, vibrations of organic groups of the polymerized
The Journal of Physical Chemistry, Vo1. 97, No. 40, 1993 10391
Structural Studies of Pillared Saponite
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d=
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Figure 2. 29Si and IHL9Si CP/MAS NMR spectra of (a) synthetic saponiteand (b) saponitecalcined at 200 OC under different instrumental
conditions.
TABLE Ik Infrared Frequencies (in cm-l) and Assignments of the Bands for the Parent and the Intercalated Samples of Saponite A11 1-sawnite sawnite Si-R-sawnite assignment 1649 free water 1640 1640 NH3+ 1520 1406 structural OH 1403 1224 -CH2CHzCHr Si(Al)-O stretching 1138 Si(Al)-Ostretching 1054 1046 1058 Si(Al)-Ostretching 99 1 988 978 Si-O-Al, Al-OH-Mg 824 820 822 172 NH3+ in-plane wiggle Si-O-A1 738 126 Si-0-Al 690 683 69 1 664 Si-0-A1 652 646 Si-oSi 603 Si-O-Al? 585 Mg-OH or Si-O-A1 527 523 529 Si-0-Mg bending 457 455 455 Si-0-Mg bending 443 ~
~
~~
organosilicon species disappear, and the bands at 683,664,603, and 585 cm-1 decreasein intensity. This reflects the decomposition of the organic parts of the intercalated species and the onset of cross-linking between the pillars and the tetrahedral layers of saponite. 29Si and Z7AI MAS NMR. Figure 2 shows 29SiMAS spectra with high-power decoupling and29SiJH CP/MASNMR spectra of the parent saponite and the sample calcined at 200 OC. The spectrum of the parent sample contains three peaks with chemical shifts of -84.5, -89.4, and -98.8 ppm. Following Weiss et al.,13 we assign them to Si(ZAl), Si(lA1) and Si(OA1) sites in the tetrahedral layer. A comparison of 29Si MAS and 29Si-1H CP/ MAS spectra reveals that there are few OH groups around the Si sites in the structure. This indicates that the structure of synthetic saponite is not compatible with the model proposed for 2:l clay minerals by Edelman and Favejee.'4 According to this model, someapical oxygensat the tetrahedral silicate layers would be replaced by hydroxyl groups. Comparing the spectrum of synthetic sample with the very similar spectrum of the sample calcined at 200 O C further confirms this conclusion. 2 7 A l MAS NMR spectra of syntheticsaponiteand of the sample calcined at 200 OC show one strong peak at 63.5 ppm and one weak peak at ca. 8 to -3 ppm (Figure 3). The former is due to 4-coordinated A1 in the tetrahedral layer and the latter to 6-coordinated Al. The 6-coordinated A1 may be created by the substitution of Mg by A1 in the octahedral layer or from ion exchange of hydrated AP3 cations. However, the broad and powder pattern-like appearance of the peak from 6-coordinated A1 suggests that substitution of a small amount of octahedral Mg by A1 has taken place in the octahedral layers. From the 27Al MAS NMR studies it is clear that A1 atoms incorporated in the
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Figure 3. Z7Al MAS NMR spectra of synthetic saponite (bottom) and
saponite calcined at
200
OC (top). 03
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Figure 4. 29SiMAS NMR spectra of as-prepared and calcined saponite intercalated with A113.
synthetic saponite are mainly substituted for Si atoms in the tetrahedral layers. Figure 4 shows 29Si MAS NMR spectra of the saponite intercalated with All3 and of samples calcined at different temperatures. TheZ9SiMAS NMR spectrum of the intercalated sample dried at room temperature (22 "C) is exactly the same as that of synthetic saponite in which three peaks with chemical shifts of -84.5, -89.4, and -93.8 ppm are found. This implies that All3 introduced during intercalation does not significantly interact with the tetrahedral layers of saponite, the process proceeding by ion exchange. However, calcination of the sample at 400 OC changes the relative intensity of the Z9Si NMR peaks. Thepeakat-89.4ppm,corresponding toSi(1Al) in the tetrahedral layers, slightly decreases. This agrees with IR results, which showed a decrease in intensity of the band around 650 cm-1 (Si0-Al) (Figure lb). More profound changes of the 29SiNMR spectrum are seen for the sample calcined at 600 O C . The Si(1Al) and Si(OA1) peaks at -89.4 and -93.8 ppm are shifted to -90 and -95 ppm, respectively. The intensity of the Si(lA1) peak at -90 ppm further decreases and the Si(OA1) peak at -95 ppm becomes dominant. The intensity of the Si(2Al) peak at
10392 The Journal of Physical Chemistry, Vol. 97, No. 40, 1993
Li et al. r
(D 0
?
600°C
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150
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-50
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PPm
Figure 5. 27AlMAS NMR spectra of as-prepared and calcined saponite
intercalated with All3.
-84.5 ppm increases slightly, and a small peak with a chemical shift of -79 ppm appears (see Figure 4). This new resonance could be attributed to the Si(2Al,,1Alp) environment, where s denotes saponite and p the pillar. It follows from the 29SiMAS NMR spectra of the calcined samples that there is bonding between the A113 and the Si(nA1) (n = 1,2) linkages. The slight increase in relative intensity of the peaks of Si(2Al) and appearance of the new Si(2A1,,1Alp) peak must be at expense of the Si( 1Al) and Si(2Al) peaks. The shifts of the peaks from Si(lA1) and Si(OA1) linkages reflect the structural adjustment in response to the reactions between Si-OH-A1 bridges and Al13pillars. The bridge O H groups in the Si-0-A1 linkages are formed through charge balancing of Si-O-A1 by H+liberated from the All3 species upon calcination.15 Reactions between the linkages and pillars could produce either Si-0-AI, or A1-O-AI, linkages, depending on to which part of the Si-OH-A1 linkage the A113 attacks. From the 29Si MAS NMR studies, it is obvious that the former is the case. Reactions between the Si-OH-A1 bridges and the All3 pillars take place on the Si site rather than on the A1 site and cause a decrease of the intensity of the Si( 1Al) peak in the 29Si MAS N M R spectrum (see Figure 4). The 27Al MAS NMR spectrum of the saponite intercalated with Al13, shown in Figure 5, contains two peaks with chemical shifts of 64 and 6 ppm. The peak from tetrahedral aluminum at 64 ppm is a superimposition of the peak from saponite itself (63.5ppm, see Figure 3) and Al13. The octahedral peakat 6 ppm is mainly from the All3 pillars. Calcination of the samples does not change the spectrum significantly by comparison with the uncalcined sample (see Figure 5). The slight increase in intensity of the octahedral peak may be due simply to the change of symmetry of the A113 pillars which makes the peak more visible, a phenomenon observed for the All3 polycation.16J7 The slight increase of the line width of the tetrahedral A1 peak, as seen from Figure 5, reflects effects of the All, pillars on the A1 in the tetrahedral layers. Figure 6 shows 29SiMAS NMR spectra of Si-R intercalated saponite and samples calcined at different temperatures. The spectrum of the sample intercalated with S i 4contains two groups of peaks, corresponding respectively to saponite and to intercalated polymerized organosilicon species. The peaks at -84.5,-89.6, and -94.0 ppm belong to saponite (compare the spectrum of synthetic saponite), while the peaks a t -59 and -68 ppm are due to the intercalated polymerized organosilicon species. Polymerization of the organosilicon compound during intercalation is
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Figure 6. 29SiMAS NMR spectra of as-prepared and calcined saponite
intercalated with Si-R. Saponite NH3+
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F i p e 7. Schematic representation of intercalated polymerized organosilicon species in the interlayer space of saponite.
known to occur from a comparison of the chemical shifts (-59 and -68 ppm) of the intercalated sample with that of the starting organosilicon compound (-42.2 ppm).8 From thechemical shifts of the peaks,18J9 the peaks a t -59 and -68 ppm are attributed respectively to the Q2 and Q3 environments, as shown in Figure 7. The relative intensity of the peaks (Z@/Ze = 2.5,measured by spectral deconvolution) suggests that the polymerized organosilicon species in the interlayer space have an average Si-oSi length of seven Si atoms. A two-dimensional schematic representation of the intercalated polymerized organosilicon species is given in Figure 7. With increasing temperature, the intercalated polymerized organosilicon species gradually decompose and the organic parts are removed (see Figure 6). At 400 O C , 29Si MAS N M R peaks of the organosilicon species disappear and new peaks a t -101 and ca.-110ppmemerge. At 600 OC,saponitepeaksat-84.5,-89.6, and -94.0 ppm shift to -85,-90.6,and -95.1 ppm, respectively,
The Journal of Physical Chemistry, Vol. 97, No. 40, 1993 10393
Structural Studies of Pillared Saponite
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Figure 8. 2'Al MAS NMR spectra of as-prepared and calcined saponite intercalated with Si-R.
and their relative intensity is significantly altered. The peak at -95.1 ppm becomes dominant. This indicates that new bonds between the Si-OH-A1 linkages and new pillars are formed upon calcination. As in the case of saponite pillared with All3, OH groups are also supplied by the pillars, but via the decomposition of the -NH3+ groups. The peak around -85 ppm is still due to Si(2Al) sites and the peak at -90.6 ppm to Si(lA1) sites. The peakat-95.1 ppmispartiallyduetoSi(OA1) sitesinthetetrahedral layers of saponite and partially to Si,( 1A1,) sites of the newly formed pillars. The peak at -101 ppm is due to the Si(OA1) site of the new pillars but coordinated by one OH group, and the peak at -1 10 ppm is due to Si(OA1) sites of the new pillar with a nature of amorphous silica. The absence of the new peaks at -101 and -1 10 ppm in the case of saponite pillared with All3 confirms this assignment. Direct evidence comes from Z7Al MAS NMR spectra given in Figure 8. It is seen that the spectrum of the sample dried at 22 "C is similar to that of the parent saponite (compare Figure 3). As the temperature increases to 400 "C, the tetrahedral A1 peak narrows from ca. 12.5 to ca. 9 ppm. This is due to removal of nitrogen atoms (14Nis a quadrupolar nucleus) of the organosilicon species which created asymmetricenvironmentsfor the tetrahedral A1 in the tetrahedral layers (direct interactions between the negative charge of Si-0-A1 linkages and the positive charge of the NHs+ groups). Further increase of the calcination temperature has little effect on peak width (ca. 10 ppm for the sample calcined at 600 "C) but splits the tetrahedral A1 peak into a doublet. A peak at 63.5 ppm and a shoulder at ca. 53 ppm are observed. This change stronglyindicatesthat cross-linking occurs between the Si-OH-A1 linkages with the new Si02 pillars. The cross-linkinglifts parts of the A1 in the tetrahedral layers toward higher field (Figure 8). The presence of the shoulder in the 27Al MAS NMR spectrum clearlydemonstrates that bonding between the tetrahedral layers and the new pillars occurs at the A1 side of the Si-OH-A1 linkages and that new A I - M i , linkages are formed. There is no splitting in the 27Al MAS NMR spectrum (not shown) of the parent saponite measured under the same conditions. Mechanism of PiUring. Previous studies on All3 pillaring in clays15~16J0~21 have shown that whether or not the cross-linking between a tetrahedral layer and a pillar takes place depends on the location of the charge in the clay. If the charge is located in the octahedral layer, as in montmorilloniteand laponite, crosslinking does not occur.lS However, if the charge is located in the tetrahedral layer such as in the case of beidelite, cross-linking
does take place.20921 Since in saponite the location of the charge is similar to that in beidelite, it would be expected that crosslinking should take place for saponites intercalated with A113 and Si-R. The results given above clearly show that this is the case. Cross-linking between tetrahedral layers and pillars does take place and requires temperatures higher than 400 OC. Cross-linkingcan occur either on the Si site or on the A1 site of a Si-OH-A1 linkage. For the cross-linkingof A113 species in beidelite, both mechanisms were p r ~ p o s e d . l ~ However, . ~ ~ - ~ ~ the present study demonstrates that which site of the Si-OH-A1 linkage is involved in cross-linkingdepends also on the nature of the pillars. For saponite intercalated with All3, cross-linking occurs on the Si site, while for the silica-like pillar, the bonding occurs on the A1 site. The results obtained from Z7Al and 29Si MAS NMR of the All3 and SiOz-like pillared saponites cannot be interpreted on the basis of the same attack site (see Figures 6 and 8). Because, as revealed by NMR (see Figure 2), the starting saponite does not have the structure proposed by Edelman and Favejee,14 the inversion mechanism15JO must apply. The A104 and Si04 in the tetrahedral layers undergo an inversion of the apical oxygen in the tetrahedral layers from down to up as the calcination temperature increases (>400 "C). This inversion provides a suitable structure for the cross-linking to occur with the pillars. Surface Area and Pore Size. Measurements of NZadsorption isotherms on calcined saponites pillared with All3 and Si-R gave their surface areas and average pore sizes: 280 m2/g and 8.3 A for the former and 243 m2/g and 9.1 A for the latter.
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