Monolithic TS-1 Nanocrystals Prepared via Ammonia-Assisted Dry-Gel Conversion for Propylene Epoxidation

Monolithic TS-1 nanocrystals synthesized by dry-gel conversion show the advantage of high yield. In order to explore the crystallization mechanism during the process, experiments were designed to compare in detail the differences between the dry-gel conversion and the hydrothermal method as well as their effects on crystallization. The powder TS-1 nanocrystals obtained through the hydrothermal method exhibited a smooth and nonstacked morphology. Carbon nanotubes on the surface of nickel foam, as the carrier of a monolithic catalyst, provided abundant nucleation sites for the crystallization of TS-1, reducing the volume of crystal nucleation and the required Gibbs free energy. The nanocrystals obtained in the vapor phase were formed by the accumulation of small particles with obvious mesopores. Nanocrystals could not be obtained under anhydrous conditions. Increasing the concentration of ammonia–water in the vapor phase increased the rate of gel dissociation and the concentration of the gel in the liquid phase, thereby enhancing the crystallization rate. The mechanisms by which ammonia promoted titanium entering the framework and reduced the defect sites were proposed by molecular simulation.