Synthesis and Characterization of ZSM-5/MCM-41 Meso/Microporous Composite Molecular Sieve Incorporated by Lanthanum

A ZSM-5/MCM-41 meso/microporous composite molecular sieve incorporated by lanthanum, noted as La-ZSM-5/MCM-41, was first synthesized under hydrothermal condition using sodium silicate (Na₂SiO₃) and aluminium sulphate (Al₂(SO₄)₃) as Si and Al sources, sulfurate solution of lanthanum oxide (La₂O₃) as La supply and cetyltrimethyammonium bromide (CTMABr) and tetrepropylammonium bromide (TPABr) as mesoporous and microporous templates. There are several main procedures performed during the synthetic process, involving the introduction of rare earth La, the addition of ZSM-5 seed crystal and the two-step crystallization. The composite was characterized by means of XRD, N₂ adsorption, SEM, IR, TG-DTG and DTA. The results of XRD and N₂ adsorption measurement indicate that the synthetic sample possesses a composite phase containing mesoporous MCM-41 structure and microporous ZSM-5 structure, a dual pore structure whose pore size distribution lies respectively in mesoporous and microporous region, and rare earth La can replace Si existing in the framework of the composite sample. SEM micrograph of La-ZSM-5/MCM-41 sample shows the nonuniform agglomerates assembled by many spherical particles of different sizes closely combined with each other, which differ greatly in the morphology from pure ZSM-5 and MCM-41 molecular sieves. Moreover, the characteristic absorbing bands of La₂O₃ cannot be seen in La-ZSM-5/MCM-41 sample′s IR spectra. In comparison with that of ZSM-5/MCM-41 composite, La-ZSM-5/MCM-41 sample′s main IR vibrational peaks vary to low wavenumber and its weight loss peaks also shift to high temperature as well as a new and light exothermic peak appears in its DTA curve. All these phenomena demonstrate the replacement between Si in the framework of ZSM-5/MCM-41 composite and rare earth La, and prove the successful synthesis of ZSM-5/MCM-41 composite molecular sieve incorporated by rare earth La.