Preparation and characterization of zinc-doped gallophosphate molecular sieve
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The effect of Zn doping on crystallization of gallophosphate molecular sieve, cloverite, synthesized hydrothermally at 170 °C, using starting gels with ZnO, was investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), and molecular dynamics (MD) simulation, etc. The SEM observations and crystallinities estimated from XRD data revealed that Zn-doped cloverite with very good crystallinity was successfully prepared, and the addition of ZnO to the starting gel was effective in promoting the crystallization process in a short reaction time. The results of chemical analyses, estimated unit cell dimensions, and MD simulations suggested the substitution of Zn atoms for Ga atoms in the framework of Zn-doped cloverite.
I. INTRODUCTION
Gallophosphate molecular sieve, which is called cloverite (denoted as CLO below), was first synthesized from the Ga2O3–P2O5 system, using a fluoride medium.1 The CLO structure has three-dimensional 8- and 20membered ring channel systems, which are not connected to each other. The 8-membered ring channels with a diameter of 3.8 Å run through ␣-cage and rpa-cage (as observed in AlPO4-222) consisting of two types of double four-rings, which are basic structural units. The 20-membered ring channels with a diameter of about 13.2 Å, which are surrounded by the 8-membered ring channels, create a supercage with a body diagonal of about 30 Å at intersections. The supercage has windows with four-leafed clover shapes, because of the presence of terminal OH− ions, and cloverite was named after the shape of the windows. The 20-membered ring channels are, therefore, expected to be available to shape-selective sorption, catalytic processes, and so on. Several studies on CLO have been reported, i.e., syntheses, physicochemical properties, and applications.3–20 The incorporation of transition metals into the framework of CLO is of great interest in catalytic application utilizing shapeselectivity because of the existence of unusual and unique channel systems. We investigated the crystallization of transition metaldoped CLO, aiming at improving the physicochemical properties of CLO. Consequently, CLO containing Zn atoms has been successfully synthesized, and the addition of ZnO to the starting gel was found to be useful a)
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J. Mater. Res., Vol. 17, No. 10, Oct 2002 Downloaded: 20 Mar 2015
for the preparation of Zn-doped cloverite (denoted as Zn-CLO below) with good crystallinity. In this paper, we report the characterization of Zn-CLO prepared and the effect of Zn doping on the crystallization through the estimation of crystallinity. The locations of Zn atoms are, furthermore, examined by the molecular dynamics (MD) simulations, together with the configurations of quinuclidine templates in the cavities. II. EXPERIMENTAL
CLO and Zn-CLO were hydrothermally synthesized from starting gels with compositions of Ga 2 O 3 ⭈ xZnO ⭈ P2O5 ⭈ HF ⭈ 6 quinuclidine ⭈ 130 H2O (0 艋 x 艋 0.22),
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