A novel titanosilicate with MWW structure.I.Hydrothermal synthesis,elimination of extraframework titaniu,and characterizations

摘要

A novel titanosilicate with MWW topology, Ti-MWW, has been prepared by an acid treatment on a corresponding lamellar precursor which is hydrothermally synthesized with the coexistence of boron and titanium using piperidine (P1) or hexamethyleneimine (HM) as a structure-directing agent. The MWW precursor can be synthesized to have a Si/Ti ratio as low as 10 when the Si/B ratio of the gel is maintained at 0.75. Both the materials synthesized using PT and HM exhibit the crystal form of thin platelets, while the latter shows a larger crystal size. Besides the tetrahedral Ti species, the precursor always contains the octahedral Ti species showing a UV—vis band at 260 nm, regardless of the Ti content. Calcination of the precursor results in a partial condensation of the octahedral Ti species to form the anatase phase, which is hardly removed by the acid treatment. MWW-type titanosilicate nearly free of both anatase and boron, however, is successively prepared by a cyclic treatment on the lamellar precursor, that is, an acid treatment, subsequent calcination, and a further acid treatment. After evacuation at 773 K, the titanosilicate thus prepared shows a characteristic IR band at 960 cm’ not observed for the Ti-free sample. The intensity of the 960 cm~(-1) band increases linearly with the Ti content up to a level corresponding to Si/Ti = 40, indicating the limitation of incorporating Ti into the framework of MWW by the present method.