Effectiveness of the reversible structural conversion of MWW zeolite for preparation of interlayer-expanded Ti-MWW with high catalytic performance in olefin epoxidation

Abstract

Interlayer-expanded MWW-type titanosilicate, Ti-IEZ-MWW, has been successfully prepared through the reversible structural conversion (RSC) of the MWW zeolite. First, the layered precursor of the MWW titanosilicates, Ti-MWW(P), was treated in an aqueous solution of HNO3 to remove octahedrally-coordinated Ti species in Ti-MWW(P) (Ti-MWW(P)-AT). This acid-treatment led to the partial removal of piperidine molecules, which served as pillaring species between the MWW layers as well as the structure-directing agent for the MWW-type zeolite, resulting in the condensation of silanol on the layer surface and formation of the three-dimensional (3D) MWW structure. Next, to promote the structural rearrangement from the 3D MWW structure to the original layered precursor, Ti-MWW(P)-AT was hydrothermally treated in the aqueous solution containing piperidine, forming a layered material (Ti-MWW(P)-RSC), which has almost the same structure as Ti-MWW(P). Finally, the interlayer silylation of Ti-MWW(P)-RSC was performed using diethoxydimethylsilane under acidic conditions. Thus prepared material, Ti-IEZ-MWW-RSC, contained unique framework Ti species and showed a higher catalytic activity in the epoxidation of cyclohexene with H2O2 compared to Ti-IEZ-MWW directly prepared from Ti-MWW(P) as well as the conventional 3D Ti-MWW.