1048
Langmuir 1991, 7, 1048-1050
Synthesis of Zeolite A from Reactants Enclosed in Reverse Micelles Prabir K. Dutta' and Daniel Robins Department of Chemistry, The Ohio State University, 120 West 18th Avenue, Columbus, Ohio 43210-1173 Received December 13, 1990. In Final Form: March 11, 1991 This study involves synthesisof a zeolite from aqueous reactants dispersed in a reverse micellar medium. Zeolite A crystals are grown from silicates and aluminates dissolved in the water pools of an AOTisooctane system. The interactions of the solubilized reactants in the micelles lead to the formation of insoluble aluminosilicateparticles trapped within the AOT matrix. Zeolite growth appears to occur from these AOT encapsulated particles and leadsto uniform sized crystals of 1-2 pm. The presence of gravitational forces makes it impossible to keep the aluminosilicate particlesdispersedin the hydrocarbon medium after they reach a certain size. However, the potentially interesting aspect of this procedure is the initial interaction between micelle-solubilized aluminate and silicate ions that may provide alternate routes for zeolite nucleation. The formation of aluminosilicate zeolites is a complex process governed by the nature of reactants, presence of structure directing inorganic and organic cations, and effects such as aging, temperature, and stirring.'p2 Various depolymerization-polymerization equilibria, competing nucleation reactions, and crystal growth processesall occur during zeolite f ~ r m a t i o n . This ~ diverse chemistry is responsible for the discovery of more than 50 different framework types. However, this is only a small percentage of the possible frameworks that can be synthesized. Introduction of novel synthetic schemes, such as the use of organic solvents4or new structure directing ions6have been instrumental in making new framework structures. In this communication, we present the synthesis of zeolite A by using reactants dispersed in reverse micelles, which are aggregates of amphiphilic molecules in hydrocarbon solvents.6 The polar head groups of the surfactant molecules point toward the center of the aggregate and form a core that can solubilize water. These water pools can vary in size and, in this study, form the medium that contains the dissolved aluminate and silicate reactants. Specifically,reactant compositions that lead to the formation of zeolite A crystals are examined. Syntheses of small metal and semiconductor clusters have been reported in reverse micelle systems.'
Experimental Section All materials were obtained from commercialsourcesand used as received. Sodium 1,2-bis(2-ethylhexyloxycarbonyl)-l-ethanesulfonate, abbreviated as AOT (99%), silica gel (Davisil,200-425 mesh), and 2,2,4-trimethylpentane (isooctane, 99%+) were obtained from Aldrich. A1 powder (-40 mesh) and NaOH were obtained from Alfa and J. T. Baker, respectively. AOT (11.53 g) was dissolved in 450 mL of isooctane and 2 mL each of silicate and aluminate (1.0 M metal in 1.5 M NaOH) solution was sequentially added to the hydrocarbon solution, with vigorous stirring. After 30 min, the aqueous layer formed at the bottom of the reaction vessel was removed. The volume of this layer was typically 2 mL,indicating that only 50% of the added aqueous
* Author to whom all correspondence should be addressed.
(1) Breck, D. W. Zeolite Molecules Sieves; Wiley: New York, 1974. (2) Szostak, R. Molecular Sieues;Van Nostrand-Reinhold: New York, 1989. (3) Barrer, R. M. Hydrothermal Chemietryofzeolites;Academic Preas: New York, 1982. (4) Bibby, D. M.; Dale, M. P. Noture 1985,317, 157. (5) Lawton, S. L.; Rohrbaugh, W. J. Science 1990,247, 1319. (6) Luisi, P. L.; Straub, B. Reuerse Micelles-Technological and Biological Releuance; Plenum: New York, 1986. (7) Fendler, J. H. Chem. Reu. 1987,87, 877.
solution is being dispersed in ieooctane. The resulting solution waa clear and was heated to 80 O C in an oven. The unheated samplea remained opticallyclear for period of hours. After various periods of heating, the residue was recovered by filtration and extensively washed with ethanol to remove the AOT. Typical yields of solid products were -50 mg. Powder X-ray diffraction patterns were obtained with a Rigaku-GeigerflexD-Max2B diffractometer. Scanningelectron micrographs were taken with the Hitachi 5-510 microscope.
Results and Discussion The aqueous silicate or aluminate solutions are not dispersed within the isooctane and remain as a separate layer in the absence of AOT. Clearly, the detergent is helping solubilizethe reactant solutions in the hydrocarbon medium. From the extensive literature on H20-AOThydrocarbon systems! it is quite clear that the primary mechanism for dispersing the HzO in the hydrocarbon is via the reverse micelles formed by aggregation of AOT molecules. This would suggest that the silicate and aluminate species are indeed present in the aqueous pool of the reverse micelles, considering their polar character. In order to provide a more definitive proof of the distribution of silicate specieshomogeneously through the hydrocarbon medium, as would be expected in the case of dissolution of these species in the water pools of the reverse micelles, we synthesized silicomolybdicacid (H&i(MsO,)6), which is a well-known reagent used for analytical determination of silicates.8 This material is characterized by an electronic transition at -305 nm in aqueous s o l ~ t i o n This . ~ solution was readily solubilized in the hydrocarbon medium in the presence of AOT. The electronic spectrum of the homogeneous micellar mixture is shown in Figure 1and exhibits a transition at 313 nm, slightly red-shifted from the aqueous solution, presumably due to a different environment in the water pooh of the reverse micelle. This clearly indicates that polyoxosilicate species can indeed be dispersed in the water pools of the micelle. The following observations were made during the heating of the isooctane-reverse micelle-aluminatesilicate solution. The clear solution became cloudy throughout after 1h of heating. The cloudiness cleared with further heating along with the formation of a white precipitate at the bottom of the reaction vessel. X-ray powder diffraction patterns of the precipitated solid a t
-
(8)Iler, R. K. The Chemistry of Silica; John-Wiley: New York, 1979.
(9) Garnett, H. E.; Walker, J. Analyst 1964, 89, 642.
0743-7463/91/2407-1048$02.50/00 1991 American Chemical Society
Langmuir, Vol. 7, No. 6,1991 1049
Letters
0
z P
8
2 3
300
350 Nanometers
Figure 1. Absorption spectrum of an aqueous solution of silicomolybdic acid dispersed in AOT-isooctane system.
Zeolite Na-A
2
0 10
20
30
2 Thoto
Figure 2. X-ray powder diffraction pattern of zeolite A synthesized in the AOT-isooctane system.
this stage only showed a small angle reflection at 28 = 3.35' due to AOT, which disappeared after extensive washing of the sample with ethanol. The first evidence of zeolite A crystals was obtained after -8 h of heating, along with the reflection at 28 = 3.35" due to AOT. The AOT is removed after washing extensively with ethanol and the XRD pattern of this material is shown in Figure 2. In order to gain further insight into the crystallization process, SEM micrographs were obtained after 1 and -8 h of heating for a partially and completely washed solid. These are shown in Figures 3 and 4. For the sample heated for an hour, partial washing shows the presence of aggregates of AOT (Figure 3a). Upon extensive washing and complete removal of AOT, distinct particles of -1 pm dimension are observed (Figure 3b,c). The sample is amorphousat this stage. Figure 3 suggeststhat amorphous aluminosilicate particles are dispersed within the AOT matrix. Figure 4a shows the micrograph for a partially washed sample after 8 h of heating. Distinct cubic crystals of zeolite A surrounded by sheaths of AOT molecules are seen in Figure 4a (as confirmed by the XRD peak at 29 = 3.35'). More extensive washing leads to the removal of the AOT molecules and disappearance of the low-angle reflection in the diffraction pattern and appearance of distinct crystals as seen in the micrograph of Figure 4b. Most of these crystals are separated from each other,
Figure 3. SEM pictures of solid obtained after an hour of heating: (a) partial washing of AOT; (b) complete washing of AOT; (c) magnified image of part b.
though intergrowths of cubic crystals were also observed at times. The crystals are all within 1-2 pm in size. The ratio of H20 to AOT ( 4) used in these experiments should result in water pools
