Desilication mechanism revisited: Highly mesoporous all-silica zeolites enabled through pore-directing agents

摘要

The role of pore-directing agents (PDAs) in the introduction of hierarchical porosity in silicalite-1 in alkaline medium was investigated. By incorporation of various PDAs in aqueous NaOH, homogenously distributed mesopores were introduced in 2.5 Iμm silicalite-1 crystals. It was proven for the first time that framework aluminum is not a prerequisite for the introduction of intracrystalline mesoporosity by desilication. The pore-directing role is not directly exerted by framework trivalent cations metals, but by species on the external surface of the zeolite. The inclusion of metal complexes (Al(OH)4-, Ga(OH)4-) and tetraalkyl ammonium cations (tetramethyl ammonium (TMA+), tetrapropyl ammonium (TPA+)) in the alkaline solution led to distinct mesopore surface areas (up to 286 m2 g-1) and pore sizes centered in the range of 5-20 nm. In the case alkaline treatment was performed in the presence of Al(OH)4-, all aluminum partially integrated in the zeolite giving rise to both Lewis and Brànsted acidity. Apart from the concentration and location, the affinity of the PDA to the zeolite surface plays a crucial role in the pore formation process. If the PDA is attracted too strongly (e.g., TMA+), the dissolution is reduced dramatically. When the pore-directing agent is not attracted to the zeolite's external surface, excessive dissolution occurs (standard alkaline treatment). TPA+ proved to be the most effective PDA as its presence led to high mesopore surface areas (>200 m2 g-1) over a broad range of PDA concentrations (0.003-0.1 M). Importantly, our results enable to extend the suitability of desilication for controlled mesopore formation to all-silica zeolites.