Dispersions of transition-metal-based phases in mesostructured silica matrixes: Preparation of high-performance catalytic materials

The finding that ordered mesostructured silica (OMS) matrixes can stabilize guest nanocrystals has transformed the preparation of advanced catalytic materials. Today, it is possible to tailor the materials in such a way that high loading of guest catalytic phases (CPs) may be achieved at different locations in the host mesoporous matrix and, in some cases, to combine several different chemical functionalities in a single nanocomposite. The conditions for the preparation of host-guest catalytic composites have been elaborated, as have the preparation and testing of representative nanocrystal ensembles, i.e. WS₂, MoS₂, WO₃, ZrO₂ (tetragonal), TiO₂ (anatase), Cs_2.5H_0.5PW₁₂ O₄₀ clusters of alumina phase and molecular species H₃PW₁₂O₄₀, inserted in an OMS (SBA-15 and MCM-41) matrix at 35-80 wt.% loading. Special fixation methods provide uniform distribution of nanoparticles in the silica nanotubes and at the outer surface of the OMS microcrystals at high loadings. CPs with high thermostability as a result of their stabilization in the OMS matrix demonstrated superior performance by a factor of 1.5-5 in hydrotreating, acidic and oxidation reactions of hydrocarbons compared with conventional reference catalysts. Diffusion limitations in catalytic reactions performed with CP/OMS composites were estimated to be negligible. The only factor limiting the accessibility of CP to reacting molecules was partial plugging of the OMS mesopores.