Generation of hierarchical porosity in beta zeolite by post-synthesis treatment with the cetyltrimethylammonium cationic surfactant under alkaline conditions.

Abstract Commercial Beta zeolite (Si/Al = 19) was treated with aqueous solutions of hexadecyltrimethylammonium (CTA) in the presence of NaOH or NH 4 OH to generate hierarchical porosity in the material. The combination of a basic media and surfactant (CTA) allows generation of mesoporosity while preserving the crystallinity and acidity of the parent Beta zeolite. In addition, characterization by XRD and TEM gave no evidence for the formation of segregated mesoporous phases. TEM and N 2 adsorption isotherms suggest the presence of substantial intracrystalline mesoporosity (average pore size of 3 nm) in the Beta zeolite crystals, which was more pronounced for samples treated with CTA/NaOH solutions. The concentration of total Brønsted acid sites was determined by pyridine adsorption, and it increased in the treated samples as compared with the parent Beta zeolite. The strongest Brønsted acid sites slightly increased for the CTA/NH 4 OH sample, while they decreased when ammonium hydroxide was replaced by NaOH. All these results indicate that the basic character (i.e. strength) of the aqueous solution affects the degree of the zeolite mesostructuring process. The activity of the treated samples in the isomerization-disproportionation (I-D) of m-xylene was related to the concentration of the strongest Brønsted acid sites. The treatment with sodium hydroxide or ammonia aqueous solutions led to an I/D ratio lower than that of the parent Beta zeolite, evidencing the benefits of the additional mesoporosity by enhancing the formation of the bulkiest bimolecular intermediate required to form disproportionation products. Graphical abstract Image 1 Highlights • Zeolite Beta containing intracrystalline mesopores has been obtained by a post-synthesis methodology. • Time and nature of alkaline solution affects the surfactant incorporation to generate mesoporosity. • Crystallinity and Si/Al ratio was preserved after hierarchical material was obtained by post-synthesis treatment. [ABSTRACT FROM AUTHOR]