Fine structure investigations on seeded dry gel and FER zeolite in a novel VPT process

An unusual crystallization kinetic curve composed of four different stages was determined for FER zeolite, which was transformed from a seeded Na2O–SiO2–Al2O3 dry gel (SDG) in tetrahydrofuran (THF)/H2O vapor with a method of vapor phase transport (VPT) at 433 K for 72 h. The evolution of morphology and fine structures of the intermediate samples were investigated with FT–IR, SEM, TEM, HRTEM, SAED and N2 adsorption to reveal the crystallization mechanism of SDG by VPT. The initial large globoids of SDG, which are composed of nano-scale amorphous globules, are found to diminish and disperse into smaller particles. Some mesoporous voids with the diameter of 10 nm are observed in SDG, which provide the channels for THF/H2O vapor to enter and then react with the gel. The mesopores in the SDG gradually decrease to form dense flake-like dry gel, which is thought to be the primary part of gel crystallization in the VPT process. FER zeolite crystallites appear at 10.5 h, which are found to grow on the interface of dense dry gel. Most of the untransformed dry gel further becomes to the flake-like crystallites for another 25 h. The dry gel disappears at 36 h, and a re-crystallization stage occurs from 36 h to 72 h.