Environmental syntheses of nanosized zeolites with high yield and monomodal particle size distribution
Introduction
Molecular sieves such as zeolites are being used as ion exchangers, adsorbents, and catalysts and in a number of additional applications. They are commonly synthesized from dense aluminosilicate gel systems giving micron-sized crystals at fairly high yields based on the amount of aluminosilicate in the gel [1]. During the last decade, there has been a growing interest in the reduction of zeolite particle sizes going from micron- to nanometer-sized dimensions, i.e., here the goal is to prepare zeolite crystals in the range of 10–1000 nm [2], [3]. One would expect a significant change in the properties of these nanosized zeolites in comparison with the conventional molecular sieves, especially in the fields of sensing, membranes, microelectronics, and other applications [4], [5], [6], [7], [8], [9], [10]. In addition, the decrease of the particle size causes a strong increase of the surface to volume ratios [11], and this is expected to be of significant importance in catalytic reactions [12]. Numerous studies have been devoted to the synthesis of different zeolite structures in the nanosize range, such as FAU-, LTA-, MFI-, LTL- and BEA-type molecular sieves, and their further use for the preparation of two- and three-dimensional constructs [13], [14], [15], [16].
Clear precursor solutions with an excess of organic templates are generally used to prepare nanosized zeolites. If these zeolites are present as discrete particles in solution, they are often called colloidal molecular sieves. These systems require fast nucleation with minimal aggregation of the particles during the entire crystallization process [17]. Depending on the zeolite structures, the aggregation of the particles can be avoided by decreasing the content of alkali cations in the precursor systems, or by complete substitution of the inorganic base by organic templates such as tetraalkylammonium cations [2], [3], [18].
In order to prepare nanosized zeolite crystals, the precursor systems should have a high degree of supersaturation, since high supersaturation tends to result in high nucleation rates, a large number of nuclei, and thus in the smallest particle sizes [19], [20]. In aluminosilicate gels, the supersaturation is strongly influenced by the pH of the solution [21]. In addition, high alkalinity would permit a decrease of the synthesis temperature [22], [23], thus avoiding de-saturation in the reaction systems that are expected to produce additional zeolite material in subsequent synthesis steps [24]. Unfortunately, the use of organic templates in the OH− form, for instance, tetramethylammonium (TMAOH), tetraethylammonium (TEAOH), tetrabutylammonium (TBAOH) or tetrapropylammonium (TPAOH) hydroxides makes the synthesis of colloidal zeolites a very expensive process. Moreover, the crystalline yield of nanosized zeolites is typically low, and rarely exceeds a few percent. As a consequence, a high amount of the initial reactants remains unused in the solutions at the end of the synthesis, including a high fraction of the expensive organic templates, which are lost during the purification stage of the crystalline particles.
This paper reports the preparation of colloidal zeolite crystals with LTA and FAU type structures using a multi-step synthesis procedure, where the yield of the zeolites can be increased considerably. At the end of each stage of the hydrothermal treatment, the non-reacted mother liquor is separated from the zeolite phase, and re-used for subsequent syntheses. Thus the type of the crystalline phase and the mean size of the zeolite crystals are kept constant for each individual aluminosilicate system. Hence, the utilization of the organic template molecules has been increased significantly.
Section snippets
Multi-step syntheses of colloidal zeolites
The colloidal FAU and LTA type crystals were synthesized form clear precursor solutions having the following molar composition:
FAU: 0.57TMA2O:0.007Na2O:0.15Al2O3:1SiO2:50H2O,
LTA: 3.59TMA2O:0.1Na2O:0.5Al2O3:1SiO2:170H2O.
The above colloidal solutions were prepared by mixing tetramethylammonium hydroxide pentahydrate (Aldrich), aluminiumisopropoxide (Aldrich), colloidal silica (Ludox HS-30), sodium hydroxide (Aldrich, 98%), and doubly distilled water at ambient conditions under vigorous stirring
Results and discussion
The evolution of FAU and LTA crystals growing in clear aluminosilicate solutions by multi-step approach was followed with several physicochemical methods. The changes in the nature, particle size distribution and crystalline yield of the zeolite phases after each separate step were examined. Previously, it has been demonstrated that the nucleation and subsequent growth of zeolites occur when the necessary species are present in solution at the required concentrations [1], [21].
The XRD patterns
Conclusion
A multi-step synthesis approach for nanosized zeolites has been developed, based on the repeated use of supernatant aluminosilicate solutions after the separation of crystalline LTA and FAU colloidal particles. The nanosized FAU zeolites synthesized in three subsequent synthesis batches have about the same particle size and high colloidal stability in water suspensions, but slightly different chemical composition. The change in the chemical composition of the crystals is attributed to a gradual
Acknowledgment
The financial support of the DFG-CNRS bilateral program and PROCOPE is greatly acknowledged.
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