A hierarchical zeolite microsphere prepared by an eco-friendly and practical route for efficient reaction of bulky molecules

Abstract

The hierarchical zeolite ZSM-5 was synthesized in a CH3CH2OH–H2O hydrothermal system by using hexadecyltrimethyl ammonium bromide (CTAB) as sole template at a low crystallization temperature (130 °C). The effects of ethanol and CTAB on crystallization process and morphologies of ZSM-5 products were investigated. Ethanol molecules in synthesis system were beneficial to speed up crystallization, though crystal growth in NaOH-CTAB-H2O system was more difficult. The existence of CTAB resulted in highly crystalline and pure phase hierarchical ZSM-5 microspheres assembled by nanocrystals. The traces of crystallization evolution by XRD, FT-IR, N2 adsorption/desorption, SEM/TEM, 29Si MAS NMR, 27Al MAS NMR spectra and TGA revealed that the hierarchical ZSM-5 microspheres were formed through the transformation of mesostructured amorphous aluminosilicates, in which the initial surfactant micelles were gradually excluded and thus acted as a molecule structure-directing agent to direct the crystallization of MFI nanozeolite. At the same time, the CTAB may disrupt nanocrystals further growth and aggregate them into hierarchical microsphere through CTA+. Pyridine (Py) and pivalonitrile (Pn) adsorbed FT-IR spectra showed that the hierarchical ZSM-5 microspheres possessed large number of accessible acid sites due to the introduction of mesopores into zeolite. Therefore, it displayed much higher catalytic activities for both large molecules 1,3,5-trimethylbenzene (TMB) transformation and 1,3,5-triisopropylbenzene (TIPB) cracking reactions than conventional MFI zeolite, and the high selectivity to isomerization products (1,2,3- and 1,2,4-TMB) and benzene product from over-cracking.