213
I n d . Eng. C h e m . Res. 1990,29, 213-220
Pillaring of Magadiite with Silicate Species Renato S p r u n g ? and Mark E. Davis* Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061
Jon S.K a u f f m a n a n d Cecil Dybowski Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716
Contact of organosilicon compounds with protonated magadiite (H-magadiite) produces partially pillared magadiite. The pillaring silicate species react with the magadiite layers to form new materials which show surface areas of 100-200 m2/g and microporosity after air calcination a t temperatures of 650 "C and below. Silicas with controlled pore size distributions are useful as separating agents, chromatographic column packings, and catalyst supports. Amorphous silicas or glasses with narrow pore size distributions are available with mean pore diameters of approximately 70 A and above (Smith and Gallegos, 1988). Also, there exist silica polymorphs that are molecular sieves. These materials have well-defined crystal structures and pores in the range of approximately 3-8 A, e.g., pure silica forms of sodalite (Bibby and Dale, 1985),ZSM-5 (Flanigen et al., 1978),and SSZ-24 (Zones, 1987). To date, there are no silicates that show well-defined pore structures in the range 10 to =70 A. Specifically, we are interested in the range from 10 to 20 A. Several classes of materials show well-defined pore structures in the range 10-20 A. VPI-5 (Davis et al., 1988) is a family of aluminophosphate-based molecular sieves with pores around 12.5 A. This structure has not been synthesized as a pure silicate as yet. Also, the pillaring of clays, e.g., bentonite and montmorillonite, can form pores in the 10-20-A range. Various compounds, Le., pillars, react with the clay layers (usually aluminosilicates) to prop open the sheets and form pores. In most cases, the pillaring agents are cationic and ion exchange for alkali ions which normally reside between the anionic layers. Vaughan (1988) presents a good historical perspective of this field. Of interest in connection with this work is the formation of silicate-clay complexes by Lewis et al. (1985). Polyhedral, three-dimensional silicate structures were obtained by cyclic polymerization of monoorganyltrichlorosilanes. These structures were then intercalated into bentonite. Although pore sizes were not reported, high surface area products, i.e., 150-400 m2/g, were obtained. The importance of this study is that polyhedral structures were intercalated, ensuring that the pillars contain polynuclear species. In order to have pores in the 10-20-A range, it is necessary to have more than one silicon atom in the pillar. The discovery of natural magadiite was reported by Eugster (1967), and its synthesis was given by Lagaly et al. (1975). Hydrated sodium magadiite (Na-magadiite) is a layered silicate with ideal unit cell composition of Na20.Si,,028.11H20(the structure of Na-magadiite remains unknown). The layers of magadiite contain terminal oxygen ions that are neutralized by sodium and/or protons. The interlayer distance is 15.6 A, and when the sodium is exchanged by hydrogen ions to give H-magadiite, the spacing decreases to 13.2 A. Upon dehydration of H-magadiite by heating to 300 "C, the interlayer distance decreases to 11.2 A, and further heating to 400 "C induces
* To whom correspondence should be addressed.
Current address: Department of Chemical Engineering, Maringa State University, P.O. Box 331, Maringa, Parana 87100,
Brazil.
0888-5885/90/2629-0213$02.50/0
condensation of the silanol groups to form siloxane bonds (Si-0-Si) between the layers. Intercalation of magadiite has been reported. Alkyl ammonium ions have been shown to produce interlayer spaces as large as 64 A (Lagaly et al., 1975). The quaternary ions exchange for the sodium ions residing within the layer spacings and expand the silicate layers. Recently, the organic derivatization of magadiite by chlorotrimethylsilane was reported (Yanagisawa et al., 1988). In that case, the chlorotrimethylsilane reacted with silanol groups to form the intercalated organosiloxane species ((CH,),SiCl + SiOH SiOSi(CH3)3+ HCI). It is important to note the distinction between ion exchange and reaction. With exchange, the intercalated compound forms an ionic association with terminal oxygens. However, with reaction, a covalent bond is formed. Thus, the reaction product is strongly held between the silicate layers. The objective of this work is to synthesize pure silicate materials through the reaction of magadiite with polyhedral, three-dimensional silica structures in order to produce novel microporous solids. Our final goal is to synthesize microporous silicas with pores of approximately 10-20-A diameter. We report here the results of our initial investigations which involve several new microporous materials. Synthetic procedures and detailed product characterizations are presented.
-
Experimental Section a. Samples. Na-magadiite was synthesized by the following procedure. Sodium hydroxide was dissolved in water after which Ludox HS-40 colloidal silica was added, and the solution was stirred for 1 h. The molar concentration of the reaction mixture was Na20.5SiO2-122H20. The reaction mixture was heated at autogenous pressure in Teflon-lined autoclaves to either 175 "C for 21.5 h or 100 "C for 49 days. Titration of Na-magadiite was performed according to the procedure of Lagaly et al. (1975). A suspension of 1.5 g of Na-magadiite per 20 mL water was stirred for 1h. The suspension was then titrated with 0.1 N HC1 to a final pH of 2.0 and allowed to remain stirring at this pH for an additional 24 h. The H-magadiite was recovered by filtering, washing with small amounts of water, and vacuum drying for at least 8 h at ambient temperature. Phenyltrichlorosilane (PhSiCl,) and cyclohexyltrichlorosilane (C,HI1SiC1,) were obtained from Petrarch Systems Inc. and used to form polyhedral silica structures for the pillaring of H-magadiite. The complete pillaring procedure was performed in two steps: (i) polymerization of the monoorganyltrichlorosilane and (ii) reaction with H-magadiite. To begin step i, 0.4 mL of water was added to a solution consisting of 0.7 mL of monoorganyltrichlorosilane in 6.3 mL of methanol at 0 "C. This mixture was stirred for 1 h at 0-3 "C. Separately, a suspension of 0 1990 American Chemical Society
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Ind. Eng. Chem. Res., Vol. 29, No. 2, 1990
Table I. Pillaring Conditions sample A B
c
D E
temp, "C 0-3 24 24 0-3 (3-3
final pH 6.6 6.7 6.7 6.3 below 1
silane ligand phenyl cyclohexyl phenyl phenyl phenyl
synthesis temp of parent Na-magadiite, "C 100 100 100
175 I75
1.2 g of H-magadiite in 15 mL of water was placed into an ice bath with stirring. Step ii was accomplished using a variety of conditions which are listed in Table I. In a typical procedure (sample A), the organosilane-containing mixture was rapidly poured into the H-magadiite suspension and the pH adjusted to 6.6 by addition of ammonium hydroxide. The temperature was maintained at 0-3 "C. After 2 h of contact, the total reaction mixture was filtered. The recovered solid was washed with water and vacuum dried at ambient temperature. Samples B and C were prepared at room temperature rather than 0-3 "C. No pH adjustment by ammonium hydroxide was performed in the preparation of sample E. Samples were calcined in air at either 470 or 650 "C. For calcination at 470 "C, the samples were heated to 470 "C over a period of 1.5 h and maintained at 470 "C for 1 h. For calcination a t 650 "C, the temperature was reached in 2 h and maintained for 1 h. b. Analysis. Magic angle spinning 13Cand ?Si NMR spectra were recorded on a Bruker MSL-300 spectrometer. The 13C spectra were obtained at a frequency of 75.468 MHz and a rotation rate of 3-4 kHz. The %Sispectra were taken at a frequency of 59.637 MHz and a spinning rate of 3-4 kHz. All chemical shifts are reported relative to TMS. Cross-polarization (CP) was employed for several spectra. Thermogravimetric analyses (TGA) and differential thermal analyses (DTA) were performed in air on a Du Pont 950 thermogravimetric analyzer and 900 differential analyzer, respectively. Infrared measurements were performed on an IBM IR/32 FTIR. The X-ray powder diffraction patterns were recorded on a Nicolet I2 automated diffraction system using Cu K a radiation and a diffracted beam graphite monochromator. Adsorption capacities of vapor-phase adsorbates and surface areas were measured by using a McBain-Bakr balance. Scanning electron micrographs (SEMs) were obtained from a Cambridge Instruments Steroscan 200 scanning electron microscope. Bulk chemical analyses for carbon, silicon, and sodium were performed by Galbraith Laboratories, Knoxville, TN.
Results and Discussion Figure 1 shows the X-ray powder diffraction patterns for Na-magadiite, H-magadiite, sample A, and sample A calcined to 470 "C in air. Na-magadiite synthesized at 100 and 175 "C showed essentially the same X-ray pattern, and the patterns compare well with those reported elsewhere (Lagaly et ai., 1975; Garces et al., 1988). The X-ray patterns of the H-magadiites obtained from Na-magadiites synthesized at 100 and 175 "C show essentially the same pattern except for the relative intensity of the highest d-spacing peak which is slightly greater for the H-magadiite prepared from the 100 "C synthesis of Na-magadiite. The interlayer distances for Na-magadiite and H-magadiite are 15.7 and 13.6 A, respectively, and agree with those given by Lagaly et al. (1975). Figure 1illustrates the X-ray pattern of sample A. Samples A, B, and C give similar X-ray patterns. Thus, no effect is observed when varying
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Figure 1. X-ray powder diffraction patterns of (I) Na-magadiite, (11) H-magadiite, (111) sample A, and (IV) sample A calcined to 470 "C in air.
the temperature and the organyl ligand from phenyl to cyclohexyl. Although the highest d spacing of sample A appears to be similar to that for Na-magadiite, it is much more diffuse, indicating a nonuniform lattice spacing. (From scanning electron micrographs, the particle size of sample A has not significantly changed from that of Namagadiite.) Also, since the peaks in the 25-30" 20 range are completely different, sample A is a unique phase from Na-magadiite. Figure 2 shows that the X-ray patterns of sample D and sample E are different than those of samples A, B, and C. Although the X-ray patterns of the Na-magadiite prepared at 100 and 175 "C are similar, apparently there is a difference in these samples that affects the pillaring process (compare A to D). The X-ray pattern of sample E shows a number of high d spacings. Thus, the pillaring process at low pH appears to yield a significantly different product from those obtained a t near neutral conditions. Figure 1 shows the X-ray pattern of sample A calcined to 470 "C in air. The X-ray pattern of sample A after calcination to 650 "C in air is essentially the same as illustrated for the 470 "C treatment. Notice the fairly diffuse nature of the intensity at 20 < 10". These X-ray data confirm that we have correctly synthesized Na-magadiite and H-magadiite. They show also that new compounds are formed by the pillaring and calcination processes. The condition of the samples depends greatly upon the pillaring procedure used. Except for sample E, the samples are somewhat tacky and difficult to dry even under vac-
l
Ind. Eng. Chem. Res., Vol. 29, No. 2, 1990 215 Table 11. Surface Areas of Samples SamDle calcination temD. "C surface area, m2/e 35 H-magadiite" 470 A
470 650 470 650 470 550 550
A B
B C H-magadiiteb
E
205 176 148 131 86 25 146
"From Na-magadiite synthesized at 100 "C. bFrom Na-magadiite synthesized a t 175 "C.
uum. In addition, a considerable fraction of solid is literally "glued" to the magnetic stirring bar and the walls of the reaction vessel at the end of the preparation of samples B and C. Tacky products are characteristic of syntheses performed at pH = 6.6. When an acidic solution is utilized, as in sample E, a free-flowing, nontacky powder is obtained. According to Voronkov and Lavrent'yev (1982), acidcatalyzed reactions of organosilanes favor cyclization while base-catalyzed reactions tend to yield two- and three-dimensional structures. Thus, sample E may be the only sample that is pillared by cyclic silicon-containing structures. SEMs of samples A-E reveal that there are no phases present other than the reacted magadiite (see Figure 3). Apparently, the tacky nature of samples A-D is from a surface coating of organosilanes species rather than from the addition of a second amorphous phase. The morphology of samples A-E is that bf magadiite. Inspection of all samples heated to 650 "C in air shows that the morphology does not change upon calcination to 650 "C. However, substantial changes are observed in magadiite and the pillared products heated to 850 "C. Table I1 shows the nitrogen BET surface areas obtained from the various materials. All samples were heated to 350 "C under vacuum for approximately 8 h prior to analysis. H-magadiite cannot adsorb N2 within the interlayer spacing. Thus, the surface area reported is from the external surface area of the particles. All samples (A-E) show increased surface area over H-magadiite, indicating that some pillaring has occurred. Sample A gives the highest surface area. Also, notice that the surface area of the sample decreases with increasing calcination temperature. Sample A was pillared at 0-3 "C. Pillaring at room temperature resulted in a silicate (sample C) that gave a lower surface area. However, the use of cyclohexyltrichlorosilane produced a material (sample B) with higher surface area than that prepared with phenyltrichlorosilane (sample C). Sample E has a surface area of 146 m2/g after calcination in air to 550 "C. From SEM and X-ray analyses, we do not detect the presence of extraneous phases. Also, if an extraneous phase were present
IO
20
30
40
50
28, degreee Figure 2. X-ray powder patterns of (I)sample A, (11)sample D,and (111) sample E.
in an amount below the detection limits by SEM and X-ray analyses, its surface area would have to be beyond the known limits of surface area for silicates in order to create the values shown in Table 11. Thus, the surface areas reported for samples A-E must be the result of some of the magadiite layers being propped open by silicate species (serve as the pillaring agents). In order to test for the size of the micropores formed in the pillard magadiites, vapor-phase probe molecules with various kinetic diameters were contacted with the solids in a McBain-Bakr balance. All samples were activated by heating to 350 "C under vacuum overnight. Table I11 lists the adsorption capacity of samples A-C for various adsorbates with kinetic diameters less than 10 A. It appears that the adsorption of organics is the same to within experimental error for a given sample. For example, sample A shows an adsorption capacity between
Table 111. Adsomtion Caoacitv - - of Silicates kinetic diameter, A 3.64 3.64 4.30 6.00 6.20 8.5OC
sample A adsorbate
Nz N2 n-hexane cyclohexane neopentane triisopropylbenzene
PIPOU 0.05 0.3 0.3 0.3 0.4 1.0
(470 "C) 0.082 0.090 0.066 0.056 0.073 0.070
capacity? cm3/g sample A sample B (650 "C) (470 "C)
sample C (470 "C)
0.068 0.083 0.063 0.056
0.060 0.066 0.043 0.030
0.032 0.040 0.027 0.023
0.049
0.050
0.028
cm3 of liquidlg. Density of 'All adsorptions performed at room temperature except for N2which was a t liquid N, temperature. adsorbate assumed to be the liquid density at adsorption temperature. Although the kinetic diameter is 8.5 A, the molecular diameter is 9.4 A, while the thickness is 5.3 A.
216 Ind. Eng. Chem. Res., Vol. 29, No. 2, 1990
Figure 3. Scanning electron micrograph of Na-magadiite (left top and bottom) and sample E (right top and bottom).
0.05 and 0.07 cm3/g for the hydrocarbons. The small differences in the uptakes could be due to packing and/or experimental error. Because the value of PIP, for the experiments with triisopropylbenzene is listed near unity, the possibility of capillary condensation in the voids between the particles must be considered. However, since the uptake of triisopropylbenzene follows that of the other organics for all samples illustrated, the effects of capillary condensation must be small. Notice that the adsorption of N2always exceeds that of the hydrocarbons. Again, this may be due to packing. Finally, the absorption capacities appear to correlate with the BET surface area. This is, the higher the BET surface area, the larger the adsorption capacity. The BET surface areas and the adsorption capacities for hydrocarbons show that the calcined, pillared magadiites do in fact have microporosity (N2isotherms follow type I behavior typical of microporous materials). At this point, we cannot deduce the pore size or the pore size distribution. However, we do know that there are pores with minimum dimensions of 6.2 A X 9.4 A. This is because neopentane and triisopropylbenzene are absorbed. Also, the shape of the pores is unknown. It is easy to rationalize the -6-A distance due to the fact that the organic ligand is a six-membered carbon ring. Since the pillaring species contains a t least one organic ligand (vide infra), then there must be space to accommodate it. The
9.4-A distance could be achieved by recognizing that the pores may be somewhat rectangular in shape and have an interpillar dimension of 9.4 A or above. Further adsorption experiments with larger probe molecules are necessary to determine more precisely the size and shape of the pores. Thus far we have shown that magadiite can be partially pillared and that after calcination the solids possess microporosity. Next we provide information concerning the composition of the materials, the structure of the silicate pillars, and the types of bonding that occur between the pillars and the magadiite layers. The chemical compositions of Na-magadiite, H-magadiite, and the pillared compounds were obtained by combining TGA/DTA results with bulk elemental analyses for sodium and carbon. The TGA data obtained from Namagadiite and H-magadiite are illustrated in Figure 4. The weight loss from Na-magadiite is coincident with an associated endotherm on a DTA scan from the desorption of water. The water loss and sodium content give an approximate unit cell composition of 0.85Na20-Si,,028* 7.5H20, which compares well with that of Lagaly et al. (1975). (The degree of hydration can be quite different due to sample treatments.) The slightly lower sodium content in our sample probably indicates that there are some hydroxyl groups present also between the layers. Upon titration of Na-magadiite to give H-magadiite, the sodium content decreases dramatically and the TGA
Ind. Eng. Chem. Res., Vol. 29, No. 2, 1990 217 I
Q)
0 C 0
c 4-
0
100
200
300
400
500
600
700
800
Temperoture, O C Figure 4. Thermogravimetric analysis of (I) H-magadiite, (11) Namagadiite, and (111) sample A. All analyses were performed in air.
changes. The weight losses below 300 "C are attributed to water desorption, while those above 300 "C are presumably from the water losses via condensation of the silanol groups to form siloxane bonds (Rojo et al., 1988). The composition of our H-magadiite deduced from TGA and bulk sodium analyses is xH.0.02Na20-Si,4028.yH20 (x denotes an unknown number of silanol groups, while y denotes that the level of hydration is variable). The pillared compounds gave TGA results similar to that illustrated in Figure 4 for sample A. The initial weight loss is from water desorption. The weight losses occurring after approximately 300 "C are from desorption and/or combustion of the organyl ligands in the pillaring species. Specifically, sample E contained 0.036 wt % sodium (Na-magadiite, 3.65 wt %; H-magadiite, 0.12 wt %) and 11.6 wt % carbon. It shows also a 13.8 wt % loss above 300 "C from TGA, which is within reasonable agreement with the bulk carbon analysis. Thus, the chemical compositions of Na-magadiite and H-magadiiteare comparable to those obtained elsewhere (Lagaly et al., 1975, Garces et al., 1988, Rojo et al., 1988),and the pillared compounds show a significant amount of organic group present. If we assume (i) that the organic group is the phenyl ligand of the starting species (vide infra), (ii) that the volume it occupies is at least as large as it would be if it were a liquid (density = 0.88 g/cm3),and (iii) that all the organic groups reside between the magadiite layers, then the organic group volume is 0.15 cm3/g of solid. This value gives an upper bound to the expected adsorption volume. Since the true adsorption volumes for sample E are below this amount, they are within the limits of feasibility. Figure 5 shows infrared spectra for sample A. When treated to 160 "C under vacuum, the infrared spectrum of sample A reveals a number of bands above 2000 cm-l. The absorption at 3630 cm-' is assigned to silanol groups (Rojo et al., 1988). The band at 3250 cm-' may be due to ammonium ions that are from the neutralization of the pillared compound with ammonium hydroxide (vide supra). Weaker bands at 3050, 2950, and 2850 cm-' are assigned to phenyl groups. Several stretching frequencies below 2000 cm-' are characteristic of phenyl groups also. Upon calcination of sample A in air to 470 "C and rehydration by exposure to ambient air during the transfer from the calcination furnace to the infrared spectrometer, the band around 3650 cm-' remains as well as less intense features slightly below 3000 cm-'. These results indicate that not all of the silanol groups are reacting with the pillaring agents. Also, there remains residual organic species after calcination in air at 470 "C (bands slightly below 3000 cm-'). The 29SiNMR spectra of Na-magadiite, H-magadiite, sample E, and sample E calcined to 470 "C are illustrated
v)
C
e k8
4000
3000
2000
1000
Wovenumber, cm-'
Figure 5. Infrared spectra of sample A (I) dehydrated under vacuum at 160 O C and (11) calcined at 470 "C and dehydrated under vacuum at 250 "C.
in Figure 6. Na-magadiite shows both Q3 (HOSi03 or Na+O-Si03)and Q @io4)type sites (notation of Lippmaa et al. (1980)). The Q4 region shows at least three resonances at -109.7, -111.1, and -113.7 ppm. The Q3 peak is at -99.1 ppm. These data are in agreement with those given elsewhere (Yanagisawa et al., 1988; Garces et al., 1988; Pinnavaia et al., 1986; Schwieger et al., 1985). The CP 29Sispectrum of Na-magadiite shows a clear increase in the Q3 signal, indicating a strong interaction between Si atoms and protons as expected. The '%i spectrum of H-magadiite reveals Q3- and Q4-typesites also. However, the Q3 resonance is at -101.4 ppm. Again, the CP '?3i spectrum of H-magadiite gives an increase in the Q3 signal. The 29Si NMR spectrum of sample E shows Q3- and Q4-type sites like the parent H-magadiite but gives a diffuse peak at -66.3 ppm as well. With CP, the -66.3 ppm peak is enhanced relative to the Q4-type resonances. This indicates that there is a proton source available for interaction with the silicon in the environments represented by the -66.3 ppm peak. The CP 13C NMR spectrum of sample E is illustrated in Figure 7. The two main resonances of 135 and 129 ppm enclose what appear to be two smaller peaks. A 13CNMR spectrum of trichlorophenylsilane in CDC1, reveals four resonances at 133.1, 132.8, 131.5, and 128.6 ppm in an intensity ratio of 2.1/1/1/2.1. Thus, there are phenyl ligands in the pillars of sample E and along with hydroxyl groups serve as the probable proton source for the CP of the -66.3 ppm resonance. The existence of Q3-typesites indicates that only a portion of the silanol groups in H-magadiite have reacted with the pillaring silicate species. We speculate that the silicon environments represented by the -66.3 ppm peak contain a phenyl ligand, at least one bridging oxygen atom to another silicon atom, and possibly one or more OH groups. We doubt that there is a unique environment but rather that several types exist due to the broadness of the resonance.
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Ind. Eng. Chem. Res., Vol. 29, No. 2, 1990
*'si
C P - * ' S ~ NMR
NMR
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1
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-60
-80
-100
-120
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-140
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PPM
- 40
-60
- 80
-100
-120
-140
-160
PPM Figure 6. %SiNMR spectra of (I) Na-magadiite, (11) H-magadiite, (111) sample E, (IV) sample E calcined in air at 550 "C.
The 29SiNMR spectrum of sample E calcined to 550 "C in air gives signal intensities over the range -100 to -115 ppm centered at near -110 ppm. The 93NMR spectrum of H-magadiite treated in the same manner (not shown) shows a peak that is from -110 to about -117 ppm with a maximum intensity at -112 ppm. Thus, calcination at 550 "C allows the internal silanol groups of H-magadiite to condense and form only Q,-type sites. Figure 6 shows
also the CP NMR spectrum of calcined sample E. The spectrum reveals peaks a t -91.1 and -101.7 ppm and a number bands around -110 ppm. The resonances at -91.1 and -101.7 ppm represent Q2- ((HO),SiO,) and/or Q & p e sites. Since calcination in air of sample E would combust all the phenyl ligands present in the as-synthesized material, the proton source for CP enhancement must be silanol groups. The Q3sites may be silanol groups that are
Ind. Eng. Chem. Res., Vol. CP
1
220
100
40
PPM F i g u r e 7. 19C NMR spectrum of sample E.
not able to condense upon calcination because of the pillaring distance between the magadiite layers and/or those formed from the removal of the phenyl ligands in the pillaring species. The Q2sites could be derived from
7
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