Mesopore Formation in USY and Beta Zeolites by Base Leaching: Selection Criteria and Optimization of Pore-Directing Agents

Molecular criteria for the selection of organic pore-directing agents (PDAs) in NaOH leaching, that is, desilication, were investigated on USY and beta zeolites of distinct aluminum contents (Si/Al = 15-385). PDAs prove particularly useful for FAU and BEA topologies since they serve a dual purpose: tailoring the mesopore structure while preventing realumination and amorphization of the crystals. An efficient PDA is positively charged and contains ca. 10-20 carbon atoms, for example, TPA(+) or CTA(+). Compositional, textural, morphological, structural, and acidity studies performed on selected hierarchical zeolites confirmed the presence of extensive secondary porosity coupled to well-preserved zeolitic properties. Inclusion of TPA(+) in the alkaline solution led to the largest preservation of the intrinsic zeolite properties, whereas CTA(+) facilitates the reassembly of dissolved species during alkaline treatment. Finally, we report the preparation of mesoporous zeolites in a continuous-mode using a tubular reactor and a high-shear reactor, attaining productivities up to 100 times higher than in conventional batch preparation.