IR and XRD Amorphous Synthesis Residue Showing Zeolitic Micropores
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1217-Y03-01
IR and XRD amorphous synthesis residue showing zeolitic micropores Anthony S.T. Chiang1, Sewn-Yi Fun1, Jan-Sen Wu1, Tseng-Chang Tsai2 1. Chem& Mater. Eng., National Central University, Chung-Li,Taiwan. 2. Applied Chemistry Dept., National University of Kaohsiung, Kaohsiung, Taiwan. ABSTRACT In the preparation of zeolite nanocrystals from the hydrothermal reaction of clear solution, the zeolite products are typically collected by high speed centrifugation. For Beta zeolite, the crystalline yield is often low, thus a good fraction of silicates reminds in the supernatant. These XRD amorphous materials turn out to be uniform nanoparticles that, after calcination, showed similar micropore structure as that of the collected beta zeolite nanocrystals. TPD measurements of hexane isomers further indicated that both the crystalline and the amorphous products were more selective toward the smaller hexane molecule. INTRODUCTION Beta zeolite, having a three-dimensional interconnected channel system with 12membered elliptical openings of 0.64 X 0.76 nm, is a good catalyst for the isomerization [1] or hydroisomerization (with metal loaded) of C6-C8 n-alkanes [2], as well as an effective adsorbent for the separation of hexane isomers [3]. The synthesis of beta zeolite is however not straightforward, and often suffers from low yield. This is even worse when one intends to reduce the diffusion resistance by producing smaller nanocrystals following the clear sol synthesis approach. The crystalline yield could be as low as 20% to 45% in different reports. For example, in the patent of Buskens et al [4], the calcined weight of beta zeolite separated from the mother liquor under 13,000 rpm centrifugation was only 42% of the total oxides used in the starting gel. Camblor et al [5] centrifuged at 16,000 rpm to separate their colloidal beta zeolite, with yield below 25%. Even with the addition of sodium and centrifuged at 50,000g for 1h, the yield of colloidal beta zeolite obtained by Schoeman et al [6] was still only ~20 wt%. The effect of Si/Al ratio on the synthesis of beta nano-zeolite was studied by Mintova et al [7] with a sodium containing recipe. The zeolite yield, after purified by centrifugation (20,000 rpm, 60 min) and water wash, was only ~40% when Si/Al=25 and ~8% for pure silica recipe. It is therefore interesting to learn exactly what was left in the supernatant when the colloidal beta zeolite was removed. The knowledge of its properties may then be used to judge the necessity of the centrifugation step for the target application. EXPERIMENTALS In this study, nano-size (~50 nm) BEA zeolite was synthesis by a two-step hydrothermal reaction of a clear synthesis solution, first at 90oC for an extended period of time then increased to 160oC. In a typical run, 2.58 g of AIP (Aluminum isopropoxide, Merck, ＞98%) was first dissolved in 47.85 g of aqueous TEAOH (Tetraethylammonium hydroxide, V.P. Chemicals 35 wt%), then diluted with 123 g of DI water. The pH value was 13.88 at this instance. Then, 65.9 g of TEOS was divid
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