Ultra-fine tuning of microporous opening size in zeolite by CVD

The porous structure of a CVD (chemical vapor deposition) layer on silicalite zeolite was studied in detailed by TEM (transmission electron microscopy). The pore mouth of silicalite was suggested to be narrowed by CVD on the basis of the adsorption results, where the rate of 1-methylpentane adsorption decreased as the amount of CVD is increased, while that of the isomer, n-hexane, was not affected. The CVD layer was oberved as amorphous in TEM images and distinguished from silicalite when the focus was adjusted so that zeolite pores were clearly observable. The CVD layer was found to uniformly cover all faces of crystals. The thickness of the CVD layer increased as more silicon alkoxide was deposited, and the observed thickness in TEM images was in agreement with that estimated from the amount of deposition. The lattice fringe pattern was observed continuing from silicalite to the edge of a particle through the CVD layer, which seemed amorphous at the just-focus point for the observation of zeolite pores. Therefore, one-dimensional channels were considered to exist in the CVD layer, and we suggest that the opening size of the micropore in the CVD layer can be tuned in the region of 0.1 nm, which is quite important on the shape-selectivity.