Enhancement in microporosity and catalytic activity on grafting silica and organosilica moieties in lamellar titanium phosphate framework

Abstract

New silica-grafted analogs of lamellar semicrystalline titanium phosphate (sc-TiOP), silicotitanium phosphate (STP) and organic–inorganic hybrid silicotitanium phosphate (HSTP) have been synthesized under hydrothermal condition without the assistance of any template or structure-directing agent under acidic pH conditions. These materials were characterized by elemental analysis, powder XRD, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), N2 sorption and spectroscopic studies. N2 adsorption studies revealed that the surface area increased significantly when the Si atoms were incorporated into the layered titanium phosphate framework. The surface area improves further with the grafting of organosilane moiety in HSTP material. Fourier-transformed infrared (FT IR), UV–vis and X-ray photoelectron spectroscopic (XPS) results suggested the presence of Si–O–P, Ti–O–Si bonds and octahedral coordination of Ti(IV) in these novel microporous materials. Si- and Si–CH2CH2–Si-grafted materials showed considerable enhancement in catalytic activity towards the liquid phase partial oxidation of styrene, α-methyl styrene, cyclohexene and acrylic acid using dilute H2O2 as an oxidant. These materials also showed excellent catalytic activity and selectivity in ammoximation of cyclohexanone to cyclohexanone oxime in the presence of aqueous ammonia and dilute aqueous H2O2 with reference to the silica-free sc-TiOP analogue.