High-pressure cation-exchange treatment of a ZSM-5 zeolite

ZSM-5 zeolite is an aluminosilicate that usually has a high silica-to-alumina ratio. It contains two sets of nanopores. One set of nanopores is straight, with the cross-sectional size of 5.3 5.6 Å; the other set is Sshaped, with the cross-sectional size of 5.2 5.5 Å. An A1 atom at the center of an AlO4 tetrahedron connects to adjacent SiO4 tetrahedron by sharing an O atom and thus generates a negative framework charge counterbalanced by extra-framework cations, such as alkaline or alkalineearth cations. If the alumina content is relatively high, the nanopore surface can be acidic. Such ZSM-5 zeolites are chemically reactive and/or catalytically active, having been widely applied for hydrocarbon interconversion, catalysis, etc. Often, the as-synthesized ZSM-5 zeolites need to be further modified to control the properties of inner surfaces of nanopores, such as the degree of hydrophobicity. For instance, for active liquid spring or programmable catalysis, the liquid infiltration needs to be controlled by an external pressure, and the working pressure must be adjusted in an appropriate range. One commonly used technique is cation-exchange. As H-form zeolite crystals are immersed in an aqueous solution of electrolyte, cations can diffuse into the network. The framework defect sites can be effectively deactivated. Consequently, the nanopore walls become more nonpolar and less wettable to water; i.e., the effective degree of hydrophobicity increases. For a highly hydrophobic zeolite, however, the ordinary cation-exchange treatment can be relatively inefficient because the electrolyte solution does not enter the nanopores spontaneously, and thus the cation-exchange only takes place at positions close to the pore opening and the process can be relatively slow. A promising alternative for introducing transition metal cations into extra-framework positions of zeolites is the solid-state ion-exchange technique. However, the process usually demands high vacuum or with inert gas protection.