Preparation of Centimeter-Sized Zeolite Crystals by Hydrothermal Seeded Growth
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ABSTRACT Centimeter-sized crystals of pyridine dodecasil 3C (all silica zeolite ZSM-39) have been prepared using hydrothermal seeded growth techniques and characterized by differential scanning calorimetry, nuclear magnetic resonance, optical microscopy and powder X-ray diffraction.
INTRODUCTION
Zeolites containing organic guest molecules are attractive candidates for high-technology materials, since their properties can be fine-tuned through systematic variation of the guest molecule within a given zeolite framework1 . Their application has been inhibited, however, by the unavailability of large-sized, high-purity crystals suitable for macroscopic manipulation and properties measurement. Indeed, even millimeter-sized single crystals of this type have been classified as "giant crystals," and the problems associated with growing larger samples are welldocumented 2 . We demonstrate here, however, that seeded hydrothermal growth of macroscopic, highly pure zeolite host-guest complexes can be achieved through judicious choice of silica sources and carefully optimized reaction conditions. Clathrasil dodecasil-3C 3 17SiO 2 .G, the all silica zeolite ZSM-39 4 ,was selected for study on several grounds. Dodecasil 3C (D3C) has the MTN framework structure 5 containing large hexadecahedral cages, each of which houses a single organic guest molecule, G, in the pure material. The largest pore in the structure is defined by a ring of six oxygen atoms, and diffusion of the guest species out of the host framework is therefore not possible. Since guest molecule concentration will influence the macroscopic properties of the material, the guarantee of fixed concentration through cage entrapment serves to maintain sample purity and homogeneity. Another favorable feature of the D3C system is its simple stoichiometry. Hydrothermal seeded growth already has the disadvantage of a large number of concentration, time, and temperature variables to contend with, and every effort was made to minimize the number of additional synthetic parameters. In the case of pyridine D3C, the specific material treated here, a relatively simple recipe could be employed consisting of water, pyridine, hydrofluoric acid, and a silica source 6 . A further advantage offered by the D3C family is the 819 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
availability of 2 - 3 mm sized single crystals 6 that are sufficiently large to use as seeds for the crystal growth experiments. Finally, it should be noted that D3C is one member of a large class of all-silica zeolites 7 , implying that the specific techniques developed for the growth of large D3C crystals should be more generally applicable.
CRYSTAL GROWTH
Crystals of pyridine dodecasil 3C up to 1 cm in size were grown by a hydrothermal seeded growth procedure from an aqueous pyridine-HF-Si02 reaction mixture at 190'C under autogenous pressure (Figure 1). The nutrient consisted of a mixture of two silica sources, fused silica in the form of rods and fumed silica. The reactor geometry show
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