Zeolite Synthesis - American Chemical Society

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Chapter 10

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Silicate Species with Cagelike Structure in Solutions and Rapid Solidification with Organic Quaternary Ammonium Ions 1

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Isao Hasegawa and Sumio Sakka Institute for Chemical Research, Kyoto University, Gokasho, Uji-City, Kyoto-Fu 611, Japan Division of Molecular Engineering, Graduate School of Engineering, Kyoto University, Sakyo-Ku, Kyoto-City, Kyoto-Fu 606, Japan 1

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Organic quaternary ammonium ions are effective in forming silicate anions with cage-like structures. Formation of these silicate anions in solutions with the aid of the ammonium ions and the abrupt formation of silicate solids with the cubic octameric silicate structure (Si8O208-) from the solutions are described. Synthesis of silica-based materials with controlled skeleton structures, such as z e o l i t e s , requires c o n t r o l l i n g the structure of oligomeric s i l i c a t e species at the f i r s t reaction step. Organic quaternary ammonium ions, which are known as organic templates i n z e o l i t e synthesis (1_), have a role i n making up the s p e c i f i c structures of s i l i c a t e anions, whereas s i l i c a t e anions randomly polymerize i n aqueous solutions containing a l k a l i metal ions, resulting i n the presence of s i l i c a t e anions with d i f f e r e n t structures. A number of studies on the structures of s i l i c a t e species i n organic quaternary ammonium s i l i c a t e solutions and s o l i d s (2-27) indicated that the s i l i c a t e anions with cage-like structures, such as S i ^ O i ^ ~ (double three-membered ring), Sig02Q (double four-membered ring) and S i - ^ r / ^ (double five-membered ring) were formed. Such s i l i c a t e s with cage-like structures w i l l be u t i l i z e d as s t a r t i n g materials both i n the dissolved state and i n the s o l i d state. It i s possible to obtain such s i l i c a t e s as s o l i d s by evaporation to dryness of the organic quaternary ammonium s i l i c a t e aqueous solutions. In t h i s case, however, the process i s very slow and accordingly a problem of impurity incorporation a r i s e s . I t was reported that s i l i c a t e s o l i d s consisting of the double four-membered ring structure were abruptly separated out after an exothermal reaction on s t i r r i n g the mixture of (2-hydroxyethyl)trimethylammonium hydroxide aqueous solution and tetraethoxysilane (26). In the f i r s t section of t h i s paper, formation of s i l i c a t e species with cage-like structures i n organic quaternary ammonium s i l i c a t e solutions are reviewed. In the l a s t section, the process of the rapid s e l e c t i v e formation of the s i l i c a t e s o l i d s having the 0097-6156/89Λ)398-Ο140$06.00/0 ο 1989 American Chemical Society In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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double four-membered r i n g structure from the solutions i s described, which has been investigated i n the current study. Formation of S i l i c a t e Anions with Cage-like Structures by Organic Quaternary Ammonium Ions In t h i s section, some factors a f f e c t i n g the formation of s i l i c a t e species with cage-like structures are described. E f f e c t of the Structure of Organic Quaternary Ammonium Ions. The tetramethylammonium ion ( N ( C H o ) ^ ) , f i r s t introduced i n z e o l i t e synthesis by Barrer and Denny (30), and Kerr and Kokotailo (31)* i s e f f e c t i v e i n forming the cubic octameric s i l i c a t e anion ( S i g 0 2 Q ~ , cubic octamer) ( 2 - 1 6 ) . In the tetramethylammonium s i l i c a t e aqueous solutions at higher S 1 O 2 concentrations or c a t i o n - t o - s i l i c a molar r a t i o s (abbreviated to the N/Si r a t i o s ) , the cubic octamer i s singularly formed. The organic quaternary ammonium ions with three methyl groups and another functional group (the ammonium ions i n which another functional group for one methyl group of tetramethylammonium ions i s substituted), such as (2-hydroxyethyl)trimethylammonium (N (CH ) C H OH), phenyltrimethylammonium ( N ( C H ) C H ) and benzyltrimethylammonium (N^CH^gCn^C^H^) ions, also form the cubic octamer s e l e c t i v e l y ( 1 2 , 1 3 , 2 2 ) , although they do not form the same z e o l i t e s (32). This suggests that the i n t e r a c t i o n of s i l i c a t e anions and the organic quaternary ammonium ions during the s e l e c t i v e formation of the cubic octamer i s not influenced by a bulky group on the ammonium ion, and only three methyl groups on an organic quaternary ammonium ion are e s s e n t i a l , regardless of the other functional group being hydrophilic or hydrophobic. By using the ammonium ions with four a l k y l groups larger than the methyl group, the other types of s i l i c a t e anions with cage-like structures are formed. For example, i n tetraethylammonium ( N ( C H ) ) s i l i c a t e aqueous solutions, the prismatic hexameric s i l i c a t e anion ( S i ^ O ^ , prismatic hexamer) i s formed (13,14,17,18). The s i l i c a t e anion with a double five-membered ring structure i s mainly formed as a c r y s t a l l i n e s o l i d from the tetra-n-butylammonium (N (n-C^Ho) ) s i l i c a t e solutions whose N/Si r a t i o s range from 0.78 to 1 . 0 ( 2 0 , 2 1 ) . Pyridinium ions are also e f f e c t i v e i n forming s i l i c a t e anions with cage-like structures (27).

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E f f e c t of the Structure of S i l i c o n Sources. Hoebbel et a l . used s i l i c i c acid sols, s i l i c i c acid gels, or A e r o s i l as a s i l i c a source of tetramethylammonium s i l i c a t e aqueous solutions (9). In the solutions at the conditions that a N/Si r a t i o i s 1 . 0 and S 1 O 2 concentration i s ca. 1.4 mol dm" , the d i s t r i b u t i o n s of s i l i c a t e anions are almost the same, and the cubic octamer i s a dominant species, although the degradation rates of the s i l i c a sources are d i f f e r e n t . This suggests that the cubic octamer i s formed i n the tetramethylammonium s i l i c a t e aqueous solution, regardless of the type of s i l i c a source with t e t r a - f u n c t i o n a l i t y . Tetraalkoxysilanes ( S ï ( O R ) a , R denotes an a l k y l group) can be used as a s i l i c a source as well ( 4 , 1 2 , 1 4 ) . When methyltriethoxysilane ( 0 1 3 5 1 ( 0 0 2 ^ ) 3 ) , a t r i - f u n c t i o n a l s i l a n e , i s used as a s i l i c a source and i s added to (2-hydroxyethyl)trimethylammonium hydroxide aqueous solutions, however, the species

In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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with the cubic octameric structure i s not formed (Hasegawa, I.; Sakka, S., unpublished data.)* In the solutions, a number of methylsilsesquioxane species, formed by the hydrolysis of methyltriethoxysilane, with different structures are present even under the conditions where the cubic octamer i s dominant i n the aqueous s i l i c a t e solutions. This indicates that the use of a s i l i c a source with t e t r a - f u n c t i o n a l i t y i s required for the selective structure formation with the aid of organic quaternary ammonium ions. Effect of Temperature. The temperature of a s i l i c a t e solution also a f f e c t s the polymerization of s i l i c a t e anions i n the solution. The d i s t r i b u t i o n of s i l i c a t e anions i n an organic quaternary ammonium s i l i c a t e solution at a fixed N/Si r a t i o and S1O2 concentration varies with the temperature of the solution (7,8,13,14,16). Ray and Plaisted (8) reported the temperature dependence of the d i s t r i b u t i o n of s i l i c a t e anions i n the tetramethylammonium s i l i c a t e aqueous solution at a N/Si r a t i o of 2/3 and a S1O2 concentration of 1.0 mol dm" . The amount of the cubic octamer i n the solution decreases with increasing temperature, and the cubic octamer p r a c t i c a l l y disappears above 50 C, indicating that the cubic octamer i s unstable at higher temperatures. However, Groenen et a l . (14) found that the cubic octamer remained i n a s i g n i f i c a n t concentration even at 85 °C, which was close to the temperature of actual z e o l i t e formation, i n the tetramethylammonium s i l i c a t e aqueous solution at a N/Si r a t i o of 1.0 and a S1O2 concentration of 1.3 mol dm . Effect of Water. The tetramethylammonium ion also contributes to the selective formation of the cubic octamer i n methanolic s i l i c a t e solutions, prepared by mixing a solution of tetramethylammonium hydroxide i n methanol, a small amount of water and tetraalkoxysilane (15). In t h i s solution, the amount of water present i n the solution has an important r o l e . The cubic octamer i s formed when the h^O/Si r a t i o i s above 4.0. As tetraethoxysilane i s used as a s i l i c a source, water i s consumed for the hydrolysis of tetraethoxysilane, and the r a t i o of 4.0 corresponds to the amount of water for the complete hydrolysis of tetraethoxysilane. It should be noted that, above the r a t i o of 4.0, the recovery of the cubic octamer increases with increasing h^O concentration although the N/Si r a t i o and S1O2 concentration are kept constant, indicating that the presence of water i s necessary for the selective formation. Tetraalkylammonium ions are known as the structure forming agents for water; the structure of water around the ion has a c e r t a i n degree of order (33). B r i e f l y , i t appears that the selective formation of s i l i c a t e species results from the cooperation of organic quaternary ammonium ions and water. Effect of Water-Miscible Organic Solvent. The d i s t r i b u t i o n of s i l i c a t e species i s d i f f e r e n t i n the aqueous and methanolic system of tetramethylammonium s i l i c a t e solutions, although the N/Si r a t i o and S1O2 concentration are the same. In the aqueous solutions at lower S1O2 concentrations, low-molecular weight species, such as the monomer and dimer, are present together with the cubic octamer (5,12). In contrast, i n the methanolic solutions at lower S1O2 concentrations, highly cross-linked polymeric species are present along with the cubic octamer (Hasegawa, I.; Sakka, S.; Sugahara, Y.;

In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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Kuroda, K. ; Kato, C. J . Chem. S o c , Chem. Commun, i n press.). Moreover, the amount of the cubic octamer formed i n the methanolic system at higher S 1 O 2 concentrations i s larger than that i n the aqueous system (15), suggesting that methanol increases the degree of polymerization of s i l i c a t e species and s t a b i l i z e s the higher molecular weight species. I t i s reported that dimethyl sulfoxide ((CH ) S0) also has such an effect (14,34).

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Effect of Addition of Sodium Ions to Tetramethylammonium S i l i c a t e Aqueous Solution. In z e o l i t e synthesis, a l k a l i metal cations are combined with organic quaternary ammonium ions to produce z e o l i t e s with d i f f e r e n t structures from the one produced with only the organic quaternary ammonium ion (V). I t i s then expected that other types of s i l i c a t e species are formed i n the s i l i c a t e solutions when organic quaternary ammonium ions and a l k a l i metal cations coexist. In such s i l i c a t e aqueous solutions, however, a l k a l i metal cations only act to suppress the a b i l i t y of the organic quaternary ammonium ions to form s e l e c t i v e l y s i l i c a t e species with cage-like structures (13,14,28,29). Sodium ions added to the tetramethylammonium s i l i c a t e aqueous solutions as an aqueous sodium hydroxide solution i n h i b i t the formation of the cubic octamer (13,28,29). The amount of the cubic octamer i n the solutions decreases with increasing amount of sodium ions added. When the concentration of the sodium ion i s higher than twice that of the tetramethylammonium ion, no cubic octamer i s formed i n the system, and the d i s t r i b u t i o n of s i l i c a t e anions i n the solutions becomes almost the same as that i n the s i l i c a t e solutions containing only sodium ions (29). Engelhardt and Rademacher reported that the amount of the cubic octamer formed i n sodium tetramethylammonium s i l i c a t e aqueous solutions decreased with increasing Na/(Na+N) molar r a t i o , i n other words, as tetramethylammonium ions were displaced by sodium ions i n keeping the amount of 0H~ ions fixed (28). This means that 0H~ ions do not cause the decrease i n the amount of the cubic octamer but only sodium ions cause. Considering that water, with the aid of tetramethylammonium ions, i s necessary for the selective formation of the cubic octamer, the decrease i n the amount of the cubic octamer with increasing amount of sodium ions may be explained by the destruction of the interaction between tetramethylammonium ions and water molecules by sodium ions which consume water for the hydration. Rapid S o l i d i f i c a t i o n of Organic Quaternary Ammonium S i l i c a t e s In t h i s section, the r e s u l t s of our study on the rapid s o l i d i f i c a t i o n of organic quaternary ammonium s i l i c a t e s are presented. Experimental Preparation of Solutions. Tetraethoxysilane i s used as a s i l i c a source. By adding tetraethoxysilane to a tetramethylammonium or a (2-hydroxyethyl)trimethylammonium hydroxide aqueous solution, two kinds of mixtures i n which a N/Si r a t i o was 1 . 0 and a S 1 O 2 concentration was 2.22 mol kg were prepared. For comparison, another mixture consisting of tetraethoxysilane and a sodium hydroxide aqueous solution with a Na/Si molar r a t i o of 1 . 0 and a S 1 O 2 concentration of 2.22 mol kg was prepared.

In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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Analytical Procedure. The structures of s i l i c a t e anions i n the solutions and s o l i d s have been examined with the t r i m e t h y l s i l y l a t i o n technique combined with gas-liquid chromatography and S i NMR. The molecular weight d i s t r i b u t i o n was measured by applying gel permeation chromatography to the t r i m e t h y l s i l y l a t e d derivatives. The method of Lentz (35,36) was adopted for t r i m e t h y l s i l y l a t i o n of the aqueous s i l i c a t e solutions. The mixture of cone, hydrochloric acid, water, 2-propanol and hexamethyldisiloxane was used as the t r i m e t h y l s i l y l a t i n g reagent. Trimethylsilylated derivatives obtained were adaptable to gas-phase analysis. The d i s t r i b u t i o n of s i l i c a t e species i n solutions, which was analyzed quantitatively by the t r i m e t h y l s i l y l a t i o n technique combined with gas-liquid chromatography, was expressed as the SiC^ recovery, that i s , the percentage of s i l i c a as a s i l i c a t e species i n t o t a l s i l i c a component in the solutions. For t r i m e t h y l s i l y l a t i o n of the s i l i c a t e s o l i d s , the method of Gotz and Masson (37) was used. The mixture of chlorotrimethylsilane, hexamethyldisiloxane and 2-propanol was used as the t r i m e t h y l s i l y l a t i n g reagent. Conditions for gas-liquid and gel permeation chromatographies were described elsewhere (12). S i NMR spectra were recorded with JEOL GSX-400 and FX-200 spectrometers. The spectra of the s i l i c a t e solutions were recorded at 79.42 MHz using a pulse angle of 45°, a delay time of 5 s and an acquisition time of 0.084 s with a GSX-400. A teflon-made tube was used as sample holder. The spectra of the s i l i c a t e s o l i d s were acquired at 39.64 MHz under conditions of magic angle spinning using a delay time of 6 s and an a c q u i s i t i o n time of 0.2048 s. Chemical s h i f t s are given with reference to an external sample of tetramethylsilane.

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Results and Discussion Organic Quaternary Ammonium Hydroxide-Tetraethoxysilane Mixture. The variation i n the temperature of the mixture of (2-hydroxyethyl)trimethylammonium hydroxide aqueous solution and tetraethoxysilane with the s t i r r i n g time i s shown i n F i g . 1. The temperature of the mixture gradually r i s e s . I t i s noticed that the exothermal reaction s t a r t s at ca. 29 °C. Before the exothermal reaction, the mixture i s heterogeneous, consisting of two phases, an organic phase of tetraethoxysilane and an aqueous phase of (2-hydroxyethyl)trimethylammonium hydroxide solution. The mixture becomes a single phase solution just when the temperature of the solution reaches a maximum of ca. 61 °C. When the temperature i s lowered, s o l i d s begin to precipitate. The whole solution completely s o l i d i f i e s into a mass of wet powder when the temperature drops to 44 °C. In the mixture of tetramethylammonium hydroxide aqueous solution and tetraethoxysilane, almost the same tendency i s seen i n the variation i n the temperature with the s t i r r i n g time as seen i n the mixture containing (2-hydroxyethyl)trimethylammonium ions described above. With decreasing temperature, the s i l i c a t e s o l i d s are also deposited from the solution which has become a single phase after the exothermal reaction. Sodium Hydroxide-Tetraethoxysilane Mixture. The exothermal reaction in the mixture of sodium hydroxide aqueous solution and

In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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tetraethoxysilane also s t a r t s at ca. 29 °C, the same temperature as that for the mixture containing the organic quaternary ammonium ions. The solution becomes turbid just when the exothermal reaction s t a r t s . At the maximum temperature (61 °C), viscous s o l s t a r t s to precipitate, indicating that the hydrolysis of tetraethoxysilane and the random polymerization of the hydrolyzed product take place very rapidly when the temperature of the solution reaches a maximum. Reaction Process. In order to elucidate the reaction process before the exothermal reaction, gas-liquid chromatography was performed on both the top and bottom phases of the sample containing (2hydroxyethyl)trimethylammonium ions. The top phase was analyzed by direct i n j e c t i o n into a gas chromatograph. The bottom phase was analyzed after t r i m e t h y l s i l y l a t i o n by the method of Lentz. Throughout the reaction, only tetraethoxysilane i s detected from the top phase. The gas chromatograms of the t r i m e t h y l s i l y l a t e d derivatives obtained from the bottom phase at various time i n t e r v a l s are shown i n F i g . 2. After 1 h of s t i r r i n g , only the monomer (SiO^ ) i s detected, which would have been formed by the hydrolysis of tetraethoxysilane. After 2 h of s t i r r i n g , the small peak corresponding to the dimer (SioOy ) appears. I t i s assumed that the amount of hydrolyzed product or tetraethoxysilane increases with time, and the polymerization proceeds i n the bottom aqueous phase. As the exothermal reaction occurs after 200 min of s t i r r i n g , the cubic octamer and the species with molecular weights higher than the cubic octamer are recovered from the bottom phase. As indicated with the mixture containing sodium ions, i t appears that the polymerization of s i l i c a t e species following the hydrolysis of tetraethoxysilane proceeds abruptly when the exothermal reaction occurs. Actually, the amount of tetraethoxysilane i n the top phase decreases pronouncedly as the temperature r i s e s , and the mixture becomes a single phase within a few minutes of the exothermal reaction. The quantitative analysis performed on the d i s t r i b u t i o n of s i l i c a t e anions obtained at the maximum temperature i n the single phase solution with the t r i m e t h y l s i l y l a t i o n technique combined with gas-liquid chromatography indicates that the recovery of monomer i s 0.2% and that of the cubic octamer i s 10.3%. The other low-molecular weight species are not recovered from the solution. I t i s assumed that the rest of s i l i c a component i s present as the species with molecular weights higher than the cubic octamer. The presence of higher molecular weight species at the maximum temperature i s c l e a r l y seen from Fig. 3 showing the gel permeation chromatogram of the t r i m e t h y l s i l y l a t e d derivatives obtained from the single phase solution at the maximum temperature. In the investigation of the d i s t r i b u t i o n of s i l i c a t e anions i n the (2-hydroxyethyl)trimethylammonium s i l i c a t e aqueous solutions at a N/Si r a t i o of 1.0 and d i f f e r e n t S1O2 concentrations, i t was reported that the recovery of the cubic octamer increased with increasing S1O2 concentration, and the recovery was ca. 93 % i n the solution at a S i 0 concentration of 1.75 mol dm (12). The recovery of the cubic octamer i n the present single phase solution i s too low compared to the S1O2 concentration of the solution high at 2.22 mol kg" . The low recovery of the cubic octamer from the solution at the maximum temperature may be attributed to the high temperature of the -3

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In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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Time / min F i g u r e 1. V a r i a t i o n i n the temperature o f the m i x t u r e o f ( 2 h y d r o x y e t h y l ) t r i m e t h y l a m m o n i u m h y d r o x i d e aqueous s o l u t i o n and t e t r a e t h o x y s i l a n e w i t h the s t i r r i n g t i m e . (Reproduced w i t h p e r m i s s i o n from Ref. 26. C o p y r i g h t 1988 The C h e m i c a l S o c i e t y o f Japan.)

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Washington, D.C. 20036 In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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solution. I t i s known that the amount of cubic octamer i n tetramethylammonium s i l i c a t e aqueous solutions diminishes with the r i s e of temperature of the solutions (7,8,13,14,16). S i l i c a t e Skeleton Structure of the (2-Hydroxyethyl)trimethylammonium S i l i c a t e S o l i d . Figure 4 shows the S i magic angle spinning NMR spectrum of the s o l i d formed from the (2-hydroxyethyl)trimethylammonium hydroxide-tetraethoxysilane mixture after standing i n a desiccator for dryness. Only one sharp peak appears at -98.74 ppm which i s assigned to the s i l i c a t e building unit of the tri-branching group, Si(0Si) (0"). The value of the chemical s h i f t i s i n intimate agreement with that of tetramethylammonium s i l i c a t e , SigOoQÎNiCHg^g, which has the double four-membered ring structure i n the s i l i c a t e skeleton (-97.7 or -99.3 ppm) (38,39). One large peak assigned to Sig0oQ(Si(CHo)3)g appears on the gas chromatogram of the s o l i d t r i m e t h y l s i l y l a t e d by the method of Gotz and Masson. These indicate that the s o l i d consists only of the cubic octameric s i l i c a t e structure. The mixture of tetramethylammonium hydroxide and tetraethoxysilane also gives the s o l i d consisting only of the Sig02Q s i l i c a t e skeleton structure. Although the s i l i c a t e skeleton structure of the (2hydroxyethyl)trimethylammonium s i l i c a t e s o l i d , deposited from the solution, consists of the SigO^Q s i l i c a t e structure solely, the recovery of the cubic octamer from the solution at the maximum temperature i s only 10.3%, indicating that the selective formation of s i l i c a t e structure i n the s o l i d i s promoted very rapidly with lowering temperature.

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Re-heating of the (2-Hydroxyethyl)trimethylammonium S i l i c a t e Solid. When the (2-hydroxyethyl)trimethylammonium s i l i c a t e s o l i d i s heated again, the s o l i d melts into a single phase solution i n several minutes. This solution s o l i d i f i e s again on cooling. The s o l i d obtained after re-heating and cooling also consists of the Sig02Q s i l i c a t e structure, showing that the t r a n s i t i o n between the s o l i d and the single phase solution of (2-hydroxyethyl)trimethylammonium s i l i c a t e with temperature i s r e v e r s i b l e . ^S± NMR spectrum of the solution, obtained by re-heating of the s o l i d at ca. 50 °C, i s shown i n F i g . 5. Gel permeation chromatogram of the t r i m e t h y l s i l y l a t e d derivatives of s i l i c a t e species i n the solution i s almost the same as that shown i n F i g . 3, and the presence of species with molecular weights higher than the cubic octamer i s confirmed. A very small peak observed at -90.1 ppm i s assigned to the prismatic hexamer, possibly formed by the depolymerization of the cubic octamer. Main peaks observed at the range between -99.5 and -100 ppm are attributed to tri-branching units i n the s i l i c a t e structures. The large sharp peak at -99.5 ppm i s assigned to the cubic octamer. The other peaks, seen at the range between -99.6 and -100 ppm, would be assigned to the higher molecular weight species, which are confirmed by gel permeation chromatography. This might indicate that the higher molecular weight species consist solely of tri-branching units and have cage-like structures larger than the double four-membered ring, such as the double f i v e - or six-membered ring structure. It i s conjectured that, on heating, s i l i c a t e species with larger Z

In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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10.

HASEGAWA AND SAKKA

Silicate Species with Cagelike Structures

J

I

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-100 6

-150

/ ppm

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Figure 4. S o l i d state S i magic angle spinning NMR spectrum of (2-hydroxyethyl)trimethylammonium s i l i c a t e . (Reproduced with permission from Ref. 26. Copyright 1988 The Chemical Society of Japan.)

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150

rings would be formed by cleavage of some of the siloxane bonds i n the cubic octamer and subsequent reformation, and on cooling, the cubic octamer would be e a s i l y formed by the combination of s i l i c a t e species with larger rings. This would explain the rapid and reversible t r a n s i t i o n between the s o l i d and solution of (2hydroxyethyl)trimethylammonium s i l i c a t e . In the mixture of sodium hydroxide and tetraethoxysilane, viscous s o l precipitates at the maximum temperature. The s o l formation shows that polymeric s i l i c a t e species with uncontrolled structures are formed. I t should be r e c a l l e d that the sodium ion i s not e f f e c t i v e i n c o n t r o l l i n g the structure of s i l i c a t e species. In the mixture of organic quaternary ammonium hydroxide and tetraethoxysilane, a single phase solution i s obtained at the maximum temperature. This suggests that the structure of higher molecular weight species i n the solutions containing organic cations at the maximum temperature i s d i f f e r e n t from that formed i n the mixture containing sodium ions. I t seems that the structure of higher molecular weight species i n the solution containing the organic quaternary ammonium ions at the maximum temperature i s also controlled by the organic cations, possibly r e s u l t i n g i n the rapid s o l i d i f i c a t i o n of organic quaternary ammonium s i l i c a t e s . Conclusions Tetramethylammonium or (2-hydroxyethyl)trimethylammonium s i l i c a t e s o l i d , consisting s o l e l y of the SigC^Q s i l i c a t e structure, was abruptly separated out by cooling the single phase solution which was obtained after an exothermal reaction by mixing the organic quaternary ammonium hydroxide aqueous solution and tetraethoxysilane. The change between the s o l i d and the solution was reversible and very rapid. Acknowledgments The authors wish to express their sincere thanks to Prof. Chuzo Kato and Dr. Kazuyuki Kuroda of Waseda University for valuable discussions and to Dr. Yoshiyuki Sugahara of Waseda University for recording S i NMR spectra. y

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RECEIVED

December 22,

1988

In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.