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

A novel titanosilicate with MWW topology, Ti-MWW, has been prepared by an a cid treatment on a corresponding lamellar precursor which is hydrothermally synthesized with the coexistence of boron and titanium using piperidine (P I) or hexamethyleneimine (HM) as a structure-directing agent. The MWW precu rsor can be synthesized to have a Si/Ti ratio as low as 10 when the Si/B ra tio of the gel is maintained at 0.75. Both the materials synthesized using PI and HM exhibit the crystal form of thin platelets, while the latter show s 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 par tial condensation of the octahedral Ti species to form the anatase phase, w hich is hardly removed by the cid treatment. MWW-type titanosilicate nearly free of both anatase and boron, however, is successively prepared by a cyc lic treatment on the lamellar precursor, that is, an acid treatment, subseq uent calcination, and a further acid treatment. After evacuation at 773 K, the titanosilicate thus prepared shows a characteristic W band at 960 cm(-1 ) 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/T i = 40, indicating the limitation of incorporating Ti into the framework of MWW by the present method.