日本ゼオライト学会　刊行物 Publication of Japan Zeolite Association

Zeolites offer finely tunable environment for the preparation of nanoclusters with well defined structure, that is, the nuclearity, structure, and catalytic properties of the intrazeolite nanoclusters can be controlled simply by modifying the pore structure, composition, and cation of the host zeolite. Thus the nanoclusters encaged in zeolites are expected to provide structureactivity correlations, which are of great importance in catalyst design, as well as novel catalytic systems including bifunctional catalysis. In the present review, we focus on metal sulfide nanoclusters encapsulated in zeolite pores from the view points of the preparation, structure, and catalytic properties. We stress here the importance of the host-guest interactions of precursors, intermediates, and product nanoclusters with the host zeolite. Hexacarbonyl molybdenum Mo(CO)₆ was accommodated in the supercage of faujasite zeolite, followed by sulfidation. The thermal stability and reactivity of intermediate Mo subcarbonyl species, the precursors to Mo sulfide nanoclusters, are elucidated by the interaction strength with zeolite framework oxygen, the electron density of which is strongly modified by the Al/Si ratio and counter cation. The resultant structure of Mo sulfide nanoclusters thus depends on the composition of the host zeolite; Mo dinuclear sulfide clusters with a composition of Mo₂S₄ are formed when the Al/Si ratio is larger than 0.25, while MoS₂ nanoclusters are produced when the ratio is lower than 0.25. The Mo-Mo atomic distance of Mo₂S₄ is increased as the Al content increases. It has been found that the intrinsic activity of Mo₂S₄ is increased with decreasing Mo-Mo atomic distance. The dynamic structural transformations between Mo₂S₄ and Mo₄S₆ nanoclusters in H₂-H₂S/H₂ treatments are described. Co₂Mo₂S₆ nanoclusters with a thiocubane structure are successfully prepared by introducing Co(CO)₃NO to Mo₂S₄/NaY, followed by second sulfidation. It is noteworthy that the significantly high thermal stability of Mo₂S₄ and Co₂Mo₂S₆ nanoclusters is due to moderate interactions with the framework oxygen as sold ligands.