CONTINUOUS-FLOW CVD METHOD TO CONTROL THE PORE-OPENING SIZE OF ZEOLITES

A continuous-flow CVD (chemical vapor deposition) method by which a large amount of zeolite can be modified uniformly to control the pore-opening size is studied. In this study, Si(OCH3)4 is deposited on Na-type and H-type mordenites, the pore-opening size narrowed by deposition is evaluated by competing cracking of octane isomers with different diameters, and the amount of coke residue formed by deposition is measured by thermal gravimetric measurement. On Na-type mordenite, silica can be deposited uniformly at higher temperatures such as 593K. On H-type mordenite, however, silica can be deposited uniformly only at a lower temperature such as 343 K owing to the secondary reaction of the produced CH3OH. During heating of the bed after deposition, coke and H2O are produced from CH3OH and influence the apparent pore-opening size depending on the position of the bed. When the sample is treated with steam, however, the coke is removed and the structure of the deposited silica layer may be rearranged to widen the pore-opening size. Owing to these roles of steam treatment, the pore-opening size of H-type mordenite also can be narrowed uniformly along the bed.