Tailoring Hollow and Plate-Like Silicalite-1 Zeolite via Mild Alkaline Etching: A Protection-Dissolution Mechanism Study

Hollow-structured MFI zeolite exhibits great potential as a high-performance catalyst, adsorbent, and low-k material. For the construction of interior voids in silicalite-1 (pure silica MFI-type) zeolite, alkaline etching by TPAOH aqueous solution is a typical top-down approach. However, tough conditions like high temperature (around 170 °C) and high pressure (generated in autoclave), which are unfavorable for the safety operation and large-scale application, are necessary for the creation of a hollow structure in the zeolite. In this work, alkaline etching at mild conditions of typically 80 °C and atmospheric pressure by using TMAOH, TEAOH, and TBAOH as etchants is comparatively investigated in detail on the basis of the preliminary study that concentrated on the TPAOH etchant. Hierarchical hollow silicalite-1 zeolites are successfully obtained at different processing times for TEAOH-/TBAOH-etching, while TMAOH-etching only leads to the formation of plate-like crystals even at relatively harsh treatment conditions. The thinning trend of the wall thickness of hollow silicalite-1 against etching time varies depending on the employed etchant. The adsorption affinity of organocations on the zeolite surface of silicalite-1 decreases in the order TMA+ > TEA+ ≥ TBA+ > TPA+. The protection-dissolution etching mechanism associated with different etchants is analyzed. Two more detailed routes of protection-dissolutioninternal (p-din) and protection-dissolutionexternal (p-dex) are, for the first time, identified for TEAOH/TPAOH/TBAOH and TMAOH, respectively. The zeolite film layers prepared from the hollow silicalite-1 seeds possess internal cavities, exhibiting good anticorrosion ability and low dielectric constant k values.