An insight into crystal morphology-dependent catalytic properties of MOR-type titanosilicate in liquid-phase selective oxidation

Mordenite zeolites, with controllable crystal morphologies grown orientedly along the c-axis, that is, the running direction of the 12-membered ring main channels, were hydrothermally synthesized. Corresponding Ti-containing mordenite (Ti-MOR) catalysts were then prepared by a secondary isomorphous substation technique, which combined together acid-assisted dealumination and sequential TiCl₄ vapor treatment. The diffusion properties of this obtained Ti-MOR catalysts with various morphologies were measured using n-butane as a probe molecule, and their catalytic properties were investigated in the liquid-phase oxidation reactions including ketone ammoximation and aromatics hydroxylation. The mordenite zeolites with smaller crystals, especially consisting of nanoparticles, were in favor of forming tetrahedrally coordinated Ti in TiCl₄ vapor treatment. The physicochemical properties and catalytic activity of Ti-MOR exhibited regular changes with an increasing crystal length along the c-axis. The aromatic hydroxylation was more sensitive than ketone ammoximation to the variety of crystal length. In particular, the adsorption capacity (Q_-), apparent diffusivity (D/L²), turnover number of ammoximation at 2.5 h and hydroxylation at 7 h decreased from 0.78 mmol g^-1, 22.7 × 10^-3 s^-1, 1783 mol (Ti-mol)^-1 and 70.5 mol (Ti-mol)^-1 to 0.59 mmol g^-1, 4.2 × 10^-3 s^-1, 500 mol (Ti-mol)^-1 and 3.4 mol (Ti-mol)^-1, respectively, when the crystal size of Ti-MOR was enlarged from 110 to 5160 nm. The adsorption properties and oxidation activity of Ti-MOR were found to exhibit a distinct change around a crystal length demarcation of ca. 1 μm. Further studies indicated that the oxidation activity of Ti-MOR was proportional to its apparent diffusivity.