An Investigation of the Behavior of Completely Siliceous Zeolite ZSM-5 under High External Pressures

Zeolites are microporous aluminosilicates with well-defined channels and cavities. They have been widely used as ionexchangers, molecular sieves, sorbents, and catalysts. The stability of zeolites is crucial to their applications. Many experimental and theoretical studies have been concentrated on their thermal stability. On the contrary, their behavior under external high pressures has received less attention. Only a handful of the zeolite topologies have been studied under high pressures. 13 Much of the early work has been focused on (1) the pressureinduced structural transformations between crystalline phases; (2) pressure-induced hydration and the compressibility of zeolites affected by nonframework species; (3) the effect of pressure on ionic conductivity of zeolites; and (4) pressureinduced amorphization (PIA) and the reversibility of this process upon decompression. Initially discovered in hexagonal ice, PIA has been the subject of many studies 21 and found to occur in various materials. Pressure-induced amorphous state appears to be a universal property of condensed materials. The PIA in zeolitic systems has been investigated experimentally and theoretically in the past decade. 26 Pressure-induced amorphous phases of several zeolites can partially or totally revert back to the initial zeolite’s crystalline structure upon decompression. The recent work by Greaves and co-workers has shown that upon compression a zeolite may transform to two different amorphous phases: (1) a low-density amorphous (LDA) phase, which is a socalled “perfect glass” with relatively ordered structure and negligible configuration entropy, and (2) a high-density amorphous (HDA) phase with a normal density of aluminosilicate glass. This phenomenon of the occurrance of more than one amorphous phase with the same chemical composition but different densities and entropies is called polyamorphism. Usually, the topology of the material in the crystalline phase is preserved in the LDA phase, but lost in the HDA phase. 32 Zeolite ZSM-5 is a representative member of a class of highsilica zeolites with applications in adsorption, catalysis, 35 and thin films/membranes. It is often used as a model system for zeolite property study. ZSM-5 consisting of 4-, 5-, 6-, and 10membered rings has a framework with two interconnected channel systems: sinusoidal 10-memebred ring (10-MR) channels along the [100] direction, interconnected with straight 10MR channels that run parallel to the [010] direction (Figure 1). A tortuous pore path is present in the [001] direction. The threeletter code for this topology assigned by the International Zeolite Association is MFI. ZSM-5 is usually synthesized in the presence of tetrapropylammonium cations (TPA), and the template ions are found in the intersections of the two channel systems. Completely siliceous ZSM-5 (Si-ZSM-5) is also known as silicalite-1. Its framework is neutral and hydrophobic. The behavior of Si-ZSM-5 under pressures was first investigated by Liu et al. They suggested that the Si-ZSM-5 irreversibly transforms into zeolite ZSM-11 at 4 GPa. Since the study did not use any in situ technique, no information on the structure change during the compression/decompression is available.More recently,