Colloidal Structure-directing-agent-free Hydroxy-sodalite Nanocrystals Jianfeng Yao and Huanting Wang* Department of Chemical Engineering, Monash University, Clayton, VIC 3800, Australia, Tel: +61 3 9905 3449, Fax: +61 3 9905 5686, E-mail: [email protected]ABSTRACT Our previous work showed that colloidal structure- directing-agent-free hydroxy-sodalite nanocrystals could be synthesized by the direct transformation of silicalite nanocrystals. The structure directing agent TPAOH molecules trapped in the dried silicalite nanocrystals proved to play an important role in confining the structure transformation. In the presence of TPAOH, silicalite nanocrystals reacted with alkaline NaO-Al 2 O 3 aqueous solution and formed hydroxy-sodalite nanocrystals without substantial change in the sizes and morphologies. In this presentation, different colloidal silica sources were used to examine the effect of the crystalline structure of starting nanoparticles. The results indicated that sodalite crystals and a mixture of zeolite A and sodalite crystals were produced, and their sizes ranged from less than 100 nm to a couple of microns when amorphous colloidal silica nanoparticles were used. This again suggests that a combination of silicalite nanocrystals and TPAOH is advantageous for formation of hydroxy-sodalite nanocrystals. Keywords: silicalite, sodalite, nanocrystals, transformation, structure directing agent 1 INTRODUCTION There has been continuously growing interest in the synthesis and application of zeolite nanocrystals since colloidal zeolite suspension was first reported in the early 1970s. [1] Owing to the success of the synthesis of zeolite nanocrystals, it has become more convenient for studying the mechanisms of zeolite crystallization; the improved performance of zeolites has been seen in their traditional uses such as in catalysis, ion exchange and adsorption , and more importantly zeolite nanocrystals have brought about some new applications including low-k dielectrics in microelectronics, optoelectronics, chemical sensing and nanostructured membranes in fuel cells and separation processes. To produce colloidal zeolite nanocrystals, a large amount of organic structure directing agent (SDA) is required in most syntheses, and the use of SDA makes zeolite crystallization controllable. Recently, SDA-free syntheses of zeolite nanocrystals have been developed through the use of the space-confinement additives such as carbon black, starch, gelling polymer and even carbon nanotubes. However, a novel method for rational synthesis of the zeolite nanocrystals with narrow size distribution is still highly demanding. [1] Amongst various types of zeolites, sodalite is very attractive as functional material for a wide range of applications such as optical materials, waste management, hydrogen storage and hydrogen separation. Colloidal hydroxy-sodalite nanocrystal sols were synthesized by homogeneous nucleation in the presence of tetramethylammonium hydroxide (TPAOH). [2] By solid– solid transformation of pillared clay SDA-free, sodalite particles (500 nm) composed of small crystallites were synthesized. [3-5] Very recently, we have reported our findings that colloidal SDA-free hydroxy-sodalite nanocrsystals can be obtained by the transformation of silicalite-1 nanocrystals. [6] Silicalite nanocrystals were employed as the silica source for hydroxy-sodalite synthesis since synthesis of silicalite had been well established. After the incorporation of the alkaline solution and the transformation of crystal structure, the resulting hydroxy- sodalite nanocrystals possessed the similar sizes and morphologies as the original silicalite nanocrystals. In this presentation, we will further the use of different silica sources in the synthesis of sodalite and discuss the effect of silica source on sodalite growth. 2 EXPERIMENTAL 2.1 Sample preparation Two SiO 2 sols, i.e., Ludox HS-30 (Sigma Aldrich), and Snowtex 20L (20wt%, 40-50 nm, Nissan Chemicals) were used as received. TEOS (tetraethyl orthosilicate, Sigma- Aldrich) derived sol was prepared by mixing 10 g of TEOS, 7.1 g of deionized water and 1 g of 4 M HCl, followed by stirring at room temperature for 3h. TPAOH (tetrapropylammonium hydroxide) solution (1 M, Sigma Aldrich) was added under stirring into Ludox HS-30 sol, TEOS derived sol and Snowtex 20L sol, respectively. The silica sources with TPAOH were obtained by directly drying the three mixtures with a molar composition of 1 TPAOH: 4.8 SiO 2 : 44 H 2 O at 80-90 C. As a comparison, another batches of three silica sols were prepared and the dried under the same conditions without adding TPAOH. 439 NSTI-Nanotech 2006, www.nsti.org, ISBN 0-9767985-6-5 Vol. 1, 2006
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Colloidal Structure-directing-agent-free Hydroxy … · Colloidal Structure-directing-agent-free Hydroxy-sodalite Nanocrystals Jianfeng Yao and Huanting Wang* Department of Chemical
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