Development of Intracrystalline Mesoporosity in Zeolites through Surfactant-Templating

Abstract

Novel insights into the surfactant-templating process leading to the formation of tailored intracrystalline mesoporosity in USY zeolite are presented in the light of the changes in the textural, morphological, and chemical properties of this zeolite produced during its treatment in a basic solution of cetyltrimethylammonium bromide (CTAB). The inability of analogous surfactants with bulkier heads to produce mesoporosity suggests that individual CTAB molecules can actually enter the zeolite through its microporosity. Once inside, the surfactant molecules self-assemble to produce the micelles responsible for the formation of mesoporosity causing the expansion of the zeolite crystals, as evidenced by He pycnometry measurements. The analysis of ultramicrotomed samples by transmission electron microscopy evidenced the formation of uniform intracrystalline mesoporosity throughout the entire crystals. In order to investigate an alternative method, namely, the dissolution and reassembly of zeolites, this was performed in USY leading to the formation of composite materials, which are distinctly different from the zeolite with intracrystalline mesoporosity obtained by surfactant-templating. Finally, it was proved that the presence of mesoporosity in the initial zeolite is not needed for the surfactant-templating to occur. This was verified by surfactant-templating of a NaY zeolite, which does not present the large mesopores found in USY.