Chem. Mater. 1993,5, 770-777
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Art ic 1es Preparation and Characterization of Pillared Magadiite She-Tin Wong and Soofin Cheng* Department of Chemistry, National Taiwan University, Taipei, Taiwan, Republic of China Received November 4, 1992. Revised Manuscript Received March 3, 1993 Aluminum Keggin ion-pillared magadiite was synthesized from Na-magadiite, NazSil402y1lH20,by ion-exchange with Keggin ion solution. The precursor for this pillaring reaction is n-hexylamine-intercalated magadiite, whereby the free interlayer spacing of H-magadiite was greatly expanded by n-hexylammonium ions and free amines. The presence of Keggin ion pillars in the interlayers of the product was confirmed by 27Aland 29Si MAS-NMR studies. XRD analysis showed that the resultant pillared product is highly disordered, but NZadsorptiondesorption analysis showed that the pillared magadiite contains both micro- and mesopores. The presence of pillars in the interlayers of magadiite was further confirmed by the higher thermal stability and surface area over Na-magadiite, as well as the possession of microporous structures. 2-Propanol dehydration studies on the acid-base properties of Keggin ion-pillared magadiite revealed that it consists mainly of acidic sites. These sites are the Si-OH groups of the silicate layers and the A1-OH groups of the Keggin ions. The latter was found to be stronger acid sites.
Introduction
In recent years, a large body of work has been published in the literature on pillared layer compounds. These compounds are considered the precursors for preparing porous materials for adsorption and catalysis. However, few reports fccused on pillared layered silicates. Layered silicates have very similar properties to the well-known clay materials. We therefore chose one of the more common layered silicates, Na-magadiite. The discovery of natural magadiite in lake beds at Lake Magadi, Kenya, was reported by Eugster in 1967.l However, it was believed that Na-magadiite was actually synthesized successfully several years earlier by McCulloch in 1952.2 Natural Na-magadiite was suggested to be precipitated from the alkaline brines which were rich in silica and low in alumina. Absence of alumina is undoubtedly of critical importance. Na-magadite is a sodium silicate with an ideal unit cell compositionof NazSi140z~llH20.Eugster' has speculated that the structure is built up of sheets of Si04 tetrahedra, and this layered structure was supported by McAtee et al.3 and Brindley.4 Their XRD experiments have shown that the basal spacing varied reversibly with the degree of hydrationldehydration. Recently, a hypothetical model structure of magadiite was proposed by Garces et al.5 In this model, the magadiite structure consists of layers of six-member rings of tetrahedra and blocks containing fivemember rings attached to both sides of the layers. The actual structure of Na-magadiite however remains unknown. (1) Eugster, H. P. Science 1967, 157, 1177. (2) McCulloch, L. J. Am. Chem. SOC.,1952, 74, 2453. (3)McAtee, Jr., J. L.; House, R.; Eugster, H. P. Am. Mineral. 1968, 53,2061. (4) Brindley, G. W. Am. Mineral. 1969, 54, 1583. (5) Garces, J. M.; Rocke, S. C.; Crowder, C. E.; Hasha, D. L. Clays Clay Minerals 1988, 36, 409.
The layers of Na-magadiite contain terminal oxygen ions that are neutralized by Na+ ions. The Na+ ions in Na-magadiite can be ion-exchanged with proton^,^^^ Ca2+ ions,4and organic cations.6 The cation exchange capacity (CEC) was found to be 60 mequivl100 g of dry sample.3 Alkylammonium ions have been shown to expand the interlayer spacing of magadiite to as large as 64 In the case of H-magadiite, the surface silanol groups can react with a large number of organic compounds to form organic intercalated c~mplexes.~ These organicswellingcomplexes of magadiite can then be used as precursors for pillaring reactions. Various types of pillars have been introduced in this way, e.g., organosiliconcompoundP1° and silica.11J2 Direct pillaring of H-magadiite by reaction with organosilicon compounds has also been achieved.l3 The objective of this work is to prepare alumine-silicate molecular sieves by pillaring layered silicate (magadiite) with polyoxo cations of aluminium. The pillars that were introduced between the magadiite interlayers are the polyoxo cations of aluminium or commonly termed Keggin The diameter of Keggin ions, [A~~~O~(OH)~~(HZO)~Z]~+. ions is about 8.6 A.14J5 Thus, the final product will be an acid catalyst with free interlayer spacing in the microporous range. Preliminary investigations of the synthesis and characterization of this Keggin ion-pillared magadiite are presented. (6) Lagaly, G.; Beneke, K.; Weiss, A. Am. Mineral. 1975, 60,642. (7) Lagaly, G.; Beneke, K.; Weiss, A. Am. Mineral. 1976, 60,650. (8) Hitzky, E. R.;Rojo, J. M.;Lagaly, G. ColloidPolym. Sei. 1985,263, 1025. (9) Hitzky, E. R.; Rojo, J. M. Nature 1980,287, 28. (10) Yanagisawa, T.; Kuroda, K.;Kato, C.Bull. Chem. SOC.Jpn. 1988, 61, 3743. (11)Landis,M.E.;Aufdembrink,B.A.;Chu,P.;Johnson,I.D.;Kirker, G. W.; Rubin, M. K. J . Am. C h e n . SOC. 1991,113, 3189. (12) Dailey, J. S.;Pinnavaia, T. J. Chem. Mater. 1992, 4 , 855. (13) Sprung, R.;Davis,M. E.;Kauffman, J. S.;Dybowski, C. Ind.Eng. Chem. Res. 1990,29, 213. (14) Johansson, G. Acta Chem. Scand. 1960,14, 769,771. (15) Rausch, W.V.;Bale, H. D. J. Chem. Phys. 1964,40, 3391.
0897-4756/93/2805-0770$04.00/0 0 1993 American Chemical Society
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Experimental Procedures Synthesis of Na-Magadiite. Na-magadiite was prepared hydrothermally according to the method of Lagaly et al.6 This method involved heating a mixture of 9 mol of SiOz, 2 mol of NaOH, and 75 mol of H20 at 100 "C for 4 weeks. The white precipitate formed was carefully washed with water, avoiding a pH below 9. The product was thenair driedat room temperature. Preparation of n-hexylamine-intercalatedmagadiite. A twostep method was used. Na-magadiite was initially converted to the proton form by titrating with dilute HC1 (0.12 N) to pH is: 2, and the H-magadiite was then suspended in pure liquid amine a t room temperature. Preparation of Keggin Ion-Pillared Magadiite. This compound was prepared from n-hexylamine-intercalated magadiite by ion-exchange with Keggin ion solution. Keggin ion solutions were prepared by using n-hexylamine to hydrolyze aluminum chloride in aqueous solution a t 50 OC.l8 The exchange process was normally carried out for 1day at 50 OC and a solution concentration of 0.6 N (in terms of Al), unless otherwise stated. The pH of the Keggin ion solution was adjusted to about 4.2. A control experiment was carried out to exchange the interlayer n-hexylamine species (cations and free molecules) with 1N NaCl solution instead of Keggin ion. CharacterizationTechniques. BET surface area measurements with NO were done volumetrically. The adsorptiondesorption isotherms were obtained with a Cahn TG-121 system with sensitivity in the order of 0.1 kg. The powder XRD patterns were recorded on nonoriented samples with a Philips PW 1840 automated powder diffractometer, using Ni-filtered Cu Ka radiation. Transmission IR spectra in the lattice vibrational region were recorded with a Perkin-Elmer 983 spectrometer. Spectra were taken with samples dispersed in KBr pellets. Diffuse reflectance infrared Fourier transform (DRIFT) spectra in the v(OH) region were taken with a Bomem MB 100 FTIR spectrometer on powdered samples. 27Aland %Simagic angle spinning (MAS) NMR spectra were taken with a Bruker MSL 500 spectrometer. The spinning frequencies were 5 and 3 kHz, and the numbers of scans were 1000and 500,for 27Al and 'Wi samples, respectively. The observed frequencies, external references, and pulse intervals were as follows: Wi: 99.361 MHz, TMS, 10 8. 27Al: 130.319 MHz, AlC13 solution, 500 ms. TGA and DTA were performed on a du Pont 951 thermogravimetric analyzer and 1600differential analyzer, respectively. The heating rate used was 10 OC/min under flowing nitrogen. The amount of NHa desorbed in the temperature-programmed desorption (TPD) experiments was determined by using a du Pont 951 thermogravimetric analyzer. The samples were dehydrated at 450-500 "C prior to NH3 adsorption. The N2 carrier gas and NH3 were dried throughmolecular sieveand NaOH traps. Physisorbed NH3 molecules were removed by heating the sample (10 OC/min) to 200 "C and then isothermally for 30 min. Then, the sample was analyzed by heating to 450 or 500 OC, and then of isothermally for 30 minutes. The effective acid strength (Ho) the catalyst was determined colorimetrically. This was done by adding indicators of different pK. values to the dehydrated catalyst powder suspended in benzene or heptane.17 The indicators were benzalacetophenone (pK, = -5.6), dicinnamalacetone (pK, = -3.0), and 4-(phenylazo)diphenylamine(pK, = +1.5). Chemical analysis was done on an inductively coupled plasma atomic emission spectrometer (ICP-AES, Model ICAP 9Ooo). Dehydration of 2-Propanol. About 0.27 g of H-magadiite or Keggin ion-pillared magadiite was pretreated in a N2 flow at 400 OC for 2-3 h. After cooling the sample to the desired reaction temperature, the N2 flow was directed through a reservoir of 2-propanol maintained at 13 "C. The flow rate through the catalyst was maintained at 21-23 mL/min, and the partial pressure of 2-propanol in the Nz flow is 20 Torr. The reaction products were analysed on a Shimadzu GC-8Agaschromatograph, with a Poropak S column and a FID detector. GC analysis on the reactant showed that 2-propanol did not contain any significant quantity of impurities. (16) Wong, S. T.;Cheng, S. Inorg. Chem. 1992, 31, 1165. (17) Goldstein, M.S.Experimental Methods in Catalytic Research; Anderson, R. B., Ed.; Academic Press: New York, 1968; Vol. 1, p 361.
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Figure 1. XRD patterns of (a) Na-magadiite, (b) H-magadiite, and (c) HIM. The general notation used for the Keggin ion-pillared magadiite is KPM-a, where denotes the temperature a t which KPM was calcined in air for 2 h. Accordingly,KPM-25 denotes uncalcined KPM sample.
Results
(I) XRD Study. The XRD patterns of as-synthesized Na- and H-magadiite are shown separately in parts a and b of Figure 1,respectively. The interlayer spacings of the air-dried H- and Na-magadiite were observed a t 13.6and 15.0 A, respectively. The peaks in H-magadiite are broadened, and several diffraction peaks which were observed in Na-magadiite are missing. However, the XRD pattern of Na-magadiite can be reproduced by ionexchanging H-magadiite with 1M NaZC03 solution. The XRD patterns of an n-hexylamine complex of H-magadiite is shown in Figure IC. This n-hexylamine-intercalated magadiite (HIM) gives a large interlayer spacing of 30.3
A.
The thermal stabilities of Na-magadiite and KPM were studied by calcining the samples in air for 2 h at various temperatures. Figure 2A shows the selected XRD patterns of Na-magadiite at 25 "C, and after calcination at 300, 500, and 700 "C. The peaks become broad at high temperatures, and at 500 "C, a new broad peak around d = 4.1 A was apparent beside the main peak. A t 600 "C, Na-magadiite transformed to cristobalite and a small amount of quartz. Rearrangement of cristobalite to quartz was almost completed at 700 "C. Tridymite, which was reported to develope at 700 "C, was not observed in our study? The corresponding XRD patterns of KPM calcined at various temperatures are shown in Figure 2B. For KPM-25, the peak at an interlayer spacing of 30.3 A in the precursor (HIM) disappeared. At the same time, a broad peak centered at about 14.2 A appeared. After the interlayer n-hexylamine species was removed by calcining KPM to 300 "C for 2 h, strong scattering in the low-angle region was observed in the XRD pattern. Only a shoulder
Wong and Cheng
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Figure 2. (A) XRD patterns of (a) Na-magadiite, and after calcination in air at (b) 300 "C, ( c ) 500 "C, and (d) 700 "C. (B) XRD patterns of (a) KPM-25, (b) KPM-300, ( c ) KPM-500, and (d) KPM-700.
appeared in about the same region. An accurate determination of this interlayer spacing is prevented by an intense initial background. When KPM was calcined at 500 and 700 "C, the shoulder diminished. It was also noted
that the main peaks around d = 3.34 8, became better resolved than Na-magadiite at increasing calcination temperatures. From the interlayer spacings of the dehydrated Namagadiite samples, the basal thickness of the layer can be determined. In the present case, the broadening effect prevents accurate determination of these values. However, from the results of Lagaly et a1.: the interlayer spacing remained at 11.5 8, between 200 and 500 "C. At these temperatures, the sample was almost dehydrated, and only Na+ ions remained in the interlayers. The basal thickness (interlayer spacing - Na+ diameter) of the Na-magadiite layers can thus be determined to be 9.6 8,from the above calculation. (11) Adsorption-Desorption Study. The BET surface area of Na-magadiite is determined to be 39 m2/g. This value is close to that reported for H-magadiite of 35 m2/g.'3 Reconstituted Na-magadiite by exchanging HIM with NaCl solution in the control experiment showed only a slight increase in surface area (56 m2/g)compared to the as-synthesized Na-magadiite. The measured surface areas of KPM-300, 500, and 700 are 263, 248, and 261 m2/g, respectively. The corresponding microporous volumes determined by extrapolation of their N2 adsorption versus PIP0 plots (to PIP0 = 0) are 0.088,0.073, and 0.085 cm3/g of dry sample. I t was noted that a KPM product prepared by pillaring at room temperature for 1 day gave a lower surface area (122 m2/g, after calcination at 300 "C). However, the surface area was even lower if the exchange time was increased from 1 to 3 days at 50 "C (61 m2/g, after calcination at 300 "C).Therefore, to obtain a pillared product of high surface area, the optimal condition is 50 "C for reaction temperature and 1 day for the exchange period. Figure 3 shows the N2 adsorption-desorption isotherms for Na-magadiite and KPM-300. For Na-magadiite, the initial uptake of N2 corresponding to micropore filling is very small. The hysteresis loop characteristic of mesoporous material is not significant. In the case of KPM, the initial uptake of N2 is much higher than that of Namagadiite. A small hysteresis loop is observed on the isotherm at PIP0 between 0.4 and 1.0. POis the saturated vapor pressure of N2 at 25 "C. These P/Po values correspond to pore radii between 1.7 and 20 nm. (111) IR Studies. Transmission IR spectra of Namagadiite and KPM on calcination at different temperatures are shown in parts A and B of Figure 4, respectively. These spectra are shown only in the lattice vibrational region from 1500 to 200 cm-'. The main absorption peaks of Na-magadiite appear between 1000 and 1250cm-' with
Chem. Mater., Vol. 5, No. 6, 1993 773
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a maximum at about 1085 cm-'. These peaks become broader as the calcination temperature increases, and at 700 "C, a different spectrum is observed. In KPM, the main absorption region resolved into two main peaks at 1232 and 1077 cm-I. In other words, the peak at about 1181cm-' observed in Na-magadiite vanished. Progres-
sively calcining KPM to higher temperatures does not affect its IR spectrasignificantly. At 700 "C, the spectrum is quite similar to that of Na-magadiite at room temperature. The growth of the peak around 1181 cm-l is apparent. DRIFT spectra in the v(0H) region from 4000 to 2500 cm-l for H-magadiite, KPM-BOO,and KPM-700 are shown in Figure 5. The samples were dehydrated under flowing N2. The spectra of all samples show a broad envelope of peaks. At 200 "C,three peaks can be identified at 3622, 3450, and 3242 cm-I for H-magadiite. This envelope becomes narrower at increasing temperatures due to the decreased contribution of lower frequency peaks. At 500 "C, a prominent peak is observed at 3667 cm-l. For KPM500 and KPM-700, however, no such peak narrowing was observed. (IV) TGA Studies. The TGA profiles and derivatives (DTG) of Na-magadiite, H-magadiite, and HIM in Nz environments are shown in Figure 6. With the exception of the HIM profile, the DTG peaks (and those peaks in Figures 7 and 8) before 200 "C are due to dehydration reactions, and they were omitted in the subsequent discussion. The DTG profile of Na-magadiite in Nz did not reveal any weight change in the temperature region between 200 and 800 "C. Similar results were found for other reaction gases such as 0 2 and H2 (H2:N2 = 1:9).In H-magadiite, two weight loss peaks are observed at 382 and 572 "C. These peaks are absent in the DTG profile of Na-magadiite where the protons are replaced by Na+ ions. It can be seen that only one prominent weight loss at 88 "C is observed in HIM. This peak is due to the
Wong and Cheng
774 Chem. Mater., Vol. 5, No. 6,1993 242.c
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desorption of interlayer n-hexylamine species. Weight losses due to adsorbed water are not observed, and this was confirmed by TGA-mass spectrometry analysis. From the weight loss observed in the HIM profile, the number of moles of interlayer n-hexylamine species/mol of H-magadiite (Mo/Mm) can be determined. The value of Mol Mm for HIM is determined to be about 3, which is comparable to the value obtained by chemical analysis.
Figure 9. (A) *IA1MAS NMR spectrum of KPM-300. (B) BSi MAS NMR spectra of (a) H-magadiite, (b) H-magadiite, after calcination in air at 300 "C,and (c) KPM-300.
Figure 7 shows the TGA and DTG profiles for KPM-25 and KPM-300 in Nz environments. In KPM-25, two peaks are observed at 242 and 446 "C. In addition, the peaks at 382 and 572 "C observed in H-magadiite are absent. In KPM-300, two peaks are observed at 468 and 575 "C.The peak at 382 "C observed in H-magadiite is absent. The main difference between the profiles of KPM-25 and KPM300 is the absence of the peak at 242 "C in the latter. However, both samples gave a peak around 446-468 "C, which is not found in H- or Na-magadiite. (V) DTA Studies. The DTA profiles of Na-magadiite, H-magadiite, and KPM-300 are shown in Figure 8. The DTA profiles of Na- and H-magadiite are similar. Groups of small peaks are observed at about 344,524, and 702 "C in Na-magadiite. They are less prominent in H-magadiite. Two other exothermic peaks were found at about 939 and 1071 "C for all samples. The DTA profiles of KPM-300 shows an additional broad exothermic peak centered at 508 "C. (VI) MAS NMR Studies. The 27A1MAS NMR spectrum of KPM-300 is shown in Figure 9A. Two main
Chem. Mater., Vol. 5, No. 6, 1993 775
Pillared Magadiite
magadiite was confirmed by comparing their XRD patterns
O 015
with those reported in the l i t e r a t ~ r e . ~However, J~ the I interlayer spacings were found to differ slightly from the 292 F
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resonances are observed at 6.453 and 65.590 ppm. The two small resonances (marked with asterisks) are the spinning side bands. The29Si MAS NMR spectra of KPM300, H-magadiite, and H-magadiite after calcined a t 300 "C, are compared in Figure 9B. In H-magadiite, resonances appeared a t -101.65, -112.47, and -114.67 ppm. The first resonance diminished and a broad resonance appeared at -112.03 ppm, in the spectrum of calcined H-magadiite sample. The spectrum of KPM-300 is similar to that of H-magadiite, except that the peak intensity is weaker and the peaks are broader so that the latter two resonances observed in H-magadiite are now less well resolved. (VII) Chemical Analysis. The Na/Si mole ratio of the as-synthesized Na-magadiite was found to be 3.6/14. No A1 was detected in the sample. In KPM-600, the amount of Na was negligible, and the Al/Si mole ratio was found to be 9.1/14. (VIII)Acidity. The acidity of H-magadiite and KPM500 was studied by carrying out NH3 TPD experiments and surface titrations with Hammett indicators. Figure 10 compares the NH3 TPD profiles of H-magadiite and KPM-500. The latter sample was gray in color, indicating the presence of residual coke on the surface. The NH3 TPD profile of H-magadiite does not show any peak due to desorption of chemisorbed NH3. In the NH3 TPD profile of KPM-500 however, a broad peak was observed at about 292 "C. The colorimetric experiment with Hammett indicators also showed that Keggin ion hydroxyls have an effective strength or HOvalue 5-5.6. In this case, a lower HOvalue implies stronger acid strength. For H-magadiite, -1.5 IHO5 +3.0. The acid-base properties of H-magadiite and KPM500 samples were further studied by 2-propanol dehydration reactions. In the 2-propanol dehydration study on H-magadiite, an initial conversionof 2.9% was achieved at 250 "C. Here, conversion is defined as the percentage of 2-propanol converted to propene and acetone. The product contains 13.47 % propene and 86.53 % acetone. The conversion increase to 18.0% when the reaction temperature was raised to 350 "C. Then, the propene selectivity increase to 79.97 % ,with only 20.03 % acetone. With KPM-500, the conversion is 100% even at 250 "C. The product contains mainly propene, Le., 99.89%.
Discussion (I) Ion-Exchange and Intercalation Properties of Na- and H-Magadiite. The identity of H- and Na-
literature values, due to the different degree of hydration/ dehydration. For example, interlayer spacings of 13.2 and 15.6 A for H- and Na-magadiite respectively have been reported by Lagaly et aLsJ The IR spectra for magadiite are also similar to those reported by Rooney et al.l8 H-magadiite showed greater structural disorder than Namagadiite, and it can be attributed to the increase in convoluted nature of the silicate layers as the Na+ ions are replaced by the smallerprotons.l9 Lagalyet aL20attributed the broadening of XRD peaks in their intercalation compounds to the change in layer folding which destroyed the parallel orientation of layers. The ease of interconversion between H- and Na-magadiite structures indicates that they are structurally related. The chemical compositions of Na-magadiite and its reconstituted form were shown to be identical by Eugster.1 H-magadiite has been known to form intercalation complexes with different kinds of neutral compounds. In the attempt to prepare alkylamine complexes by direct reaction of H-magadiite with pure alkylamines, Lagaly et ale7found that the interlayer spacing is only slightly enhanced to about 14A. However, when the H-magadiite is first treated with dimethyl sulfoxide (DMSO) and then with alkylamines, large interlayer spacings were observed. Surprisingly, we found that the preintercalation step with DMSO is not needed in our studies. The interlayer spacing of HIM is in close agreement with the 29.8 A of Lagaly et al? Since alkyl chains grow by 1.27 A per added carbon, the free spacing of about 21 A (interlayer spacing - basal thickness) suggests that the interlayer hexylamine species formed double layers, with the ammonium heads pointing toward the basal layers. For a monolayer of dimethyldidodecylammoniumcations [ ( C I ~ H ~ & ( C H ~ ) ~arranged N + I perpendicular to the magadiite layers, the free interlayer spacing has been calculated to be 18.2 A by Lagaly et alq6Accordingly, the observed free interlayer spacing in this case may indicate that the n-hexylamine species in the bilayer is arranged perpendicular to the basal surface and that the hydrocarbon tails from opposite layers do not overlap each other. Since one mole of H-magadiite consists of 2 mol of protonic acid sites, the calculated Mo/Mm value showed that we have an excess of about 1mol of free n-hexylamine present in HIM. If the distribution of Si-OH groups is uniform throughout the structure, the mean distances between them will be about 7 we7 To accommodate the free n-hexylamine molecules between the n-hexylammonium ions, the bilayer must be closely packed, and this may play a part in stabilizing the bilayer. The suggested close packing of the interlayer is further supported by the absence of interlayer adsorbed water. The original pattern of Na-magadiite can be restored by replacing the interlayer organic species in the complex, such as HIM by Na+ ions. Therefore, the structure of magadiite is not much altered during the intercalation process. (11) Evidence of Pillaring of Interlayers in KPM. The diameter of the Keggin ion is about 8.6 A. If the (18)Rooney, T. p.;Jones, B. F.; Neal, J. T. Am. Mineral. 1969,54, 1034. (19) Liebau, F. Structural Chemistry of Silicates-Structure, Bonding,and Classification; Springer-Verlag: Berlin, 1986; p 200. (20) Lagaly, G.; Beneke, K.; Dietz, P.; Weiss, A. Angew. Chem., Znt. Ed. Engl. 1974, 13, 819.
776 Chem. Mater., Vol. 5, No. 6, 1993 pillaring process with Keggin ions is successful, then a XRD peak is expected at an interlayer spacing of 18.2 A. To have a clear view of this peak, the interlayer n-hexylamine species in KPM-25 not exchanged by Keggin ions or protons should be removed. TGA results showed that they decomposed at about 242 "C. After the interlayer n-hexylamine species is removed by calcining KPM to 300 "C, no well-defined peak was observed in the abovementioned region. Therefore, the pillared structure is not well ordered. Although the XRD results showed that the KPM was not well ordered, the results from surface area and adsorption-desorption measurements further support the presence of pillars in the interlayers of magadiite. The free spacing of a dehydrated Na-magadiite is only 1.9 A. Therefore, a Nz molecule with kinetic diameter of 3.64 A cannot access the interlayem21 Hence, only the external surface area is accessible. In the case of KPM, the interlayer Na+ ions are replaced by the bulky Keggin ions. Thus, the surface area of KPM is expected to be much higher than Na-magadiite. Indeed, the surface area of KPM shows an increase of more than 6-fold over Namagadiite. Contribution to the surface area of KPM from the fragmentation of the layered structure during the exchange process is negligible. This can be seen by comparing the surface area of the reconstituted Namagadiite in the control experiment with that of assynthesized Na-magadiite. Therefore, the large increase in surface area of KPM is primarily due to the pillaring of the layers, The surface area of KPM is comparable to magadiite pillared with silicate species (205 m2/g)13but smaller than that with silica (520-680 m2/g).11J2 The N2 adsorption-desorption results showed that micropores and mesopores are created in the KPM structure. To have more direct evidences on the pillaring of the interlayers with Keggin ions, 29Si and 27AlMAS NMR studies were carried out. The 29SiMAS NMR spectrum of H-magadiite is similar to that reported by Dailey and Pinnavaia.12 This sample exhibits a Q3 HOSi(0Si)B resonance at -101.65 ppm and a doublet Q4Si(0Si)l lines at -112.47 and -114.67 ppm. After the H-magadiite sample was calcined at 300 "C, the Q3 resonance diminished, due to the cross-linking of interlayer Si-OH groups. In the 29Si MAS NMR spectrum of KPM-300, however, Q3resonance persists. This indicates that the interlayers of KPM-300 are prevented from cross-linking by some species. Furthermore, the similarity between the 29SiMAS NMR spectra of H-magadiite and KPM-300 shows that the layer structure remained intact in KPM-300. From the results of 27AlMAS NMR, the resonances which are observed at 6.453 and 65.590 ppm correspond to the octahedral and tetrahedral aluminium sites in the Keggin ion. Similar spectra and resonances were also observed on Keggin ion-pillared clays.22 Therefore, the results of 29Siand 27AlMAS NMR confirmed the presence of Keggin ion pillars in the interlayers of KPM. In the KPM sample, the Al/Si mole ratio was determined to be 9.1114. In a study on Keggin ion-pillared clay, the charge on the Keggin ion in the product has been shown to depend on the pH of the exchange mediuma23At a pH of about 4.2, which was the pH of our exchange medium, the expected charge is between 3+ and 4+. Accordingly, (21) Breck, D. W. Zeolite Molecular Sieves-Structure, Chemistry, and Use; Wiley-Interscience: New York; 1974; p 636. (22) Fripiat, J. J. Cataly. Today 1988, 2, 281. (23) Vaughan, D. E. W. Cataly. Today 1988,2, 187.
Wong and Cheng
the calculated AlISi mole ratio in a 100% ion-exchange KPM product will lie between 8.7114 and 6.5114, respectively. In addition, the calculated Al/Si mole ratio by assuming a 10076 exchange by AP+ species in the pillaring reaction is 0.7114. Therefore, the experimentally determined AlISi value of 9.1114 further supports the 27AlMAS NMR results that Keggin ions are present in KPM. However, small contribution from other aluminium species to the total A1 loading is also possible. The IR spectra of KPM are also different from that of Na-magadiite. These differences are not attributed to the replacement of hexylammonium ions by protons, which can happened since the pillaring solution is acidic. In fact, the IR spectra of magadiite showed little changes when the Na+ ions in Na-magadiite were exchanged with protons, except the small peak of Na-magadiite at 1235 cm-l was absent. In addition, the spectra of naturally occurring Na-magadiite and its acid derivative were reported similar as well.18 Therefore, the changes observed in the spectra of KPM with respect to that of Na-magadiite must be due to pillaring of the interlayers. In Namagadiite, strong absorption in the region 1250-950 cm-1 has been attributed to the v(Si-0) vibration of the magadiite framework, with Si in the tetrahedral configuration.8J8 The frequency of the main peak shifted from 1085 cm-l in Na-magadiite to 1077 cm-' in KPM, which may be due to the overlapping of A1-0 vibration. The reported Si-0 and A1-0 vibrations in Si and A1 gels calcined at 450 "C are very similar, i.e., 1082 and 1075 cm-l, respectively.24 Lagaly and Beneke' have suggested that condensation of Si-OH groups of neighboring layers of H-magadiite occurred at about 400 "C. Accordingly,their DTA profiles showed a small endothermic peak at about 380 "C for H-magadiite and not Na-magadiite. Therefore, the weight loss peak observed in the DTG profile of H-magadiite at 382 "Cis due to cross-linking of surface hydroxyl groups of opposite layers. This peak is absent in the DTG profile of Na-magadiite where the protons are replaced by Na+ ions. The DTG peak of H-magadiite at 382 "C is not observed in the KPM-25 and KPM-300 profiles either. This is because the Keggin ions prop up the interlayers and prevent the cross-linking of hydroxyl groups of neighboring layers. Therefore, band narrowing as a result of dehydroxylation and cross-linking was not observed in the 4OH) region of KPM. The broad v(OH) band in the KPM sample probably arises from the hydrogen-bonding interaction between the hydroxyls of Keggin ions and silicate layers. A new DTG peak was observed in the region between 400 and 500 "C in both DTG profiles of KPM. This peak is characteristic of the Keggin ion, and has been observed in Keggin ion-pillared buserite.16 The exothermic nature of this weight loss further suggests that it involved water loss, possibly from condensation reactions involving hydroxyl groups of Keggin ions and the Si-OHgroups of the magadiite surface. (111) Thermal Stability of Na-Magadiite and KPM. The thermal stability of Na-magadiite and the KPM structures were studied by calcining the samples at various temperatures. The changes that occurred in the XRD patterns of Na-magadiite below 500 "C,such as decreased interlayer spacing and broadening of peaks, can be ascribed simply to dehydration of Na-magadiite. In fact, when the sample calcined at 400 "Cwas rehydrated with water, (24) Zheng,L.; Hao, Y.; Tao, L.; Zhang,Y.; Xue, Z. Zeolites 1992,12, 374.
Pillared Magadiite the original XRD pattern was almost reproduced with interlayer spacing of 14.7 A. The XRD patterns of Namagadiite calcined at 300 and 500 "C appeared to be similar to that of H-magadiite, indicating some structural deformations during dehydration. Structural transformation of magadiite starts at 500 "C, as shown by a new broad peak at about d = 4.1 A. Rehydration of the sample at this stage can only partially reproduce the original pattern of Na-magadiite. Concomittantly, the IR peaks of Namagadiite calcined at 500 "C broadened, indicating disorder in the layered structure. The disorder in the structure at this stage leads to structural transformations at higher temperature, as observed by both IR and XRD. The thermal stability of KPM is higher than Namagadiite. XRD and IR analysis showed that the layered structure of KPM is stable to at least 700 OC, although it is highly disordered at this stage. The degree of disorder however is less than Na-magadiite, as shown by the greater resolution of the main XRD and IR peaks. The integrity of the pillared structure at this temperature is further supported by its high surface area and pore volume. These results further showed that the pillars are distributed into the internal surface and not just sitting at the edge of the interlayers. Deng et have reported low thermal stability of Keggin ion-pillared KHSi205 (without F- ions) using ethylammonium intercalate as intermediate. This suggests that the pillars might be sitting at the edge of the interlayers. Therefore, preswelling of H-magadiite with long-chainorganic molecules is important in order to create interlayer free spacing large enough for the diffusion of pillaring agent into the interlayer. The presence of pillars in the interlayers of magadiite prevents the basal layers from cross-linking and therefore improves the thermal stability of the layered structure. (111) Acid-Base Properties of H-Magadiite and KPM. The presence of Si-OH groups on H-magadiite has already been shown by TGA and IR studies. They can be broadly divided into two types: a more free and a hydrogen-bonded one.26 The latter type is involved in cross-linking reaction at higher temperatures. The SiOH groups in H-magadiite are not strong enough to interact chemically with NH3 but are strong enough to involve in dehydration of 2-propanol. The hydrocarbon products in the reaction are propene and acetone. The former is an acid-catalyzed product whereas the latter is a base catalyzed one.21 The basic sites responsible for acetone formation are probably the surface oxides. One of the major disadvantages of H-magadiite as acid catalyst is the cross-linking of Si-OH groups at fairly low temperature (382 "C). An important question is whether Keggin ion contributes to the acidity of KPM. If the Si-OH groups are the only type of acid sites involved in the formation of propene, then the yield of propene (conversion x selectivity) over KPM-500 should be about 6.5 times higher than that of H-magadiite catalyst at similar reaction conditions. The reason is that the surface area of KPM is about 6.5 times that of H-magadiite. Since the propene yield over KPM500 at 250 "C is at least 255 times higher than that of H-magadiite catalyst, the surface area factor itself cannot explain entirely the increase in the number of acid sites. In addition, some of the original Si-OH groups in (25) Deng, Z. Q.;Lambert, J. F.; Fripiat, J. J. Chem. Mater. 1989,1, 640. (26) Rojo, J. M.; Hitzky, E. R.; Sanz, J. Inorg. Chem. 1988,27, 2785. (27) Wade, Jr., L. G. Organic Chemistry; Prentice-Hall: Englewood
Cliffs, NJ,1987.
Chem. Mater., Vol. 5, No. 6, 1993 777 H-magadiite have been involved in the exchange reaction for Keggin ions and blocked by coke from incomplete combustion of amine. As aresult, the acid sites on Keggin ions (Al-OH) must contribute to the conversion and propene yield. These are the acid sites on KPM-500 that interact chemically with NH3 as observed by NH3 TPD. The amount of chemisorbed NH3 desorbed from this sample corresponds to about 1.23 mmol of acid sites/g, or 3 sites/100 A2 of dry KPM sample. Similar concentration of acid sites was also reported for Keggin ion-pillared tetratitanate.28 The basic sites are also affected by the pillaring process, since the yield of acetone on KPM is much lower than that on H-magadiite. In KPM, A1-OH groups of the Keggin ions are stronger acid sites than the Si-OH groups of the magadiite layer. Such evidence is based on the TGA results that n-hexylamine species desorbed (or decomposed) at 242 "C in KPM-25 but desorbed at 88 "C in HIM. N-Hexylamine in the former material must interact chemically with the protons of the Keggin ions. In HIM however, the desorption temperature of 88 "C is the same as that for liquid amine alone. Therefore, the interlayer n-hexylamine species interacts only weakly with the Si-OH groups and complete protonation of intercalated amine may not have occurred. Further evidence came from the NH3 TPD experiment of H-magadiite, whereby the desorption peak due to chemisorbed NH3 is absent. Other evidence for the stronger acid strength of A1-OH over Si-OH groups came from the lower HOvalue in the colorimetric experiment. Thus, the most important outcome of these studies is the incorporation of rather strong acidic sites in KPM by the introduction of Keggin ion pillars in the interlayers of magadiite. The interlayer free spacing of KPM is large enough to allow dehydration of 2-propanol to occur. Conclusion The interlayer surfaces of H-magadiite were found to readily react with n-hexylamine molecules. The n-hexylamine-intercalated magadiite then acts as an intermediate in the synthesis of KPM. n-Hexylamine species expand the free interlayer spacing of H-magadiite and make the introduction of the bulky Keggin ion possible. The presence of Keggin ions in the interlayers of magadiite has been confirmed primarily by 27Aland 29SiMAS NMR studies. Other evidences came from thermal stability, surface area, pore structure, and 2-propanol dehydration studies. The latter showed that the free interlayer spacing of KPM was large enough for the diffusion of both reactant and products. Comparison of the IR, TGA, and TPD results between Na- or H-magadiite with KPM also demonstrate the presence of Keggin ions in the interlayers. Acid sites with higher strength than Si-OH sites of H-magadiite were detected in KPM with NH3 TPD and surface titration. These acid sites can be related to the presence of Keggin ion pillars. N2 adsorption-desorption isotherms revealed that the structure of KPM consists largely of micropores. DTA studies seem to show that the pillars are stable at least to about 800 O C . Acknowledgment. Financial support from the National ScienceCouncil of the Republic of China is gratefully acknowledged. The acknowledgment is further extended to Professor Shang-Bin Liu of the Academia Sinica, Institute of Atomic and Molecular Sciences, for using the NMR spectrometers. (28) Dhu, L. S. M.Sc. Thesis, National Taiwan University, 1990.
