Seed-assisted synthesis of Ga-MFI slabs and nanocrystal agglomerates and comparative study of acidic properties

Abstract

A series of Ga-MFI zeolites were one-step synthesized following the seed-assisted strategy without using additional organic template. The effects of synthesis parameters on the framework fabrication, morphology evolution, and gallium incorporation were systematically investigated by XRD, N2-adsorption, UV-Raman, STEM, TEM, solid state NMR, NH3-TPD, and Py-IR analyses. The Si/Ga seed exhibited a highly efficient structure-directing capability in the assembly of MFI framework and the amount of the costly tetrapropylammonium hydroxide (TPA+) was thus decreased to less than one-tenth in comparison with the traditional synthesis. Crystallization temperature and pre-aging conditions played the key roles in the Ga-MFI zeolite synthesis. Both of the morphology and the intraframework gallium concentration can be facilely controlled by adding the mineralizer and surfactant. The addition of NH4F and CTAB was favorable for the formation of the slabs-form product but against to the gallium substitution. Only with the help of Si/Ga seed, the synthesized Ga-MFI nanocrystal agglomerates possess a highly accessible hierarchical mesopore structure that consisted solely of interparticle-type mesopores. Simultaneously, it also exhibited a remarkable gallium incorporation, which ensured the great acidic properties, including strong acidic strength and high acid amount. As a consequence, the Ga-MFI nanocrystal agglomerates performed the excellent catalytic activity in the LDPE degradation.