Intrinsic flexibility of porous materials; theory, modelling and the flexibility window of the EMT zeolite framework

Abstract

Framework materials have structures containing strongly bonded polyhedral groups of atoms connected through their vertices. Typically the energy cost for variations of the inter-polyhedral geometry is much less than the cost of distortions of the polyhedra themselves -as in the case of silicates, where the geometry of the SiO 4 tetrahedral group is much more strongly constrained than the Si-O-Si bridging angle. As a result, framework materials frequently display intrinsic flexibility, and their dynamic and static properties are strongly influenced by low-energy collective motions of the polyhedra. Insight into these motions can be obtained in reciprocal space through the 'rigid unit mode' (RUM) model, and in real-space through template-based geometric simulations. We briefly review the framework flexibility phenomena in energy-relevant materials, including ionic conductors, perovskites and zeolites. In particular we examine the 'flexibility window' phenomenon in zeolites and present novel results on the flexibility window of the EMT framework, which shed light on the role of structure-directing agents. Our key finding is that the crown ether, despite its steric bulk, does not limit the geometric flexibility of the framework.

Pressure-induced symmetry changes in body-centred cubic zeolites

“…This mirrors the predictions of Fletcher et al . [ 33 ] from geometric simulations and our previous high-pressure study of the EMT framework containing the 18C6 molecule [ 34 ]. However, there is a discrepancy as the flexibility of the filled and empty EMT frameworks is identical, unlike that seen herein for the FAU framework.…”

“…Consequently, this leads to a decrease in compressibility as a function of pressure. However, other work has shown how the intrinsic flexibility window and compression of the EMT framework is unperturbed by the occlusion of 18-crown-6 ether (18C6) [ 33 , 34 ]. This is of particular interest, as 18C6 is used as an organic additive in the preparation of EMT-type zeolites like EMC-2 [ 35 , 36 ], where synthesis without 18C6 has not been achieved [ 37 ].…”

Defining the flexibility window in ordered aluminosilicate zeolites

“…In a study on a siliceous faujasite structure, we have distinguished the intrinsic window, governed by the framework alone, from the extrinsic window controlled by interactions with extra-framework content inside small sodalite (SOD) cages [ 10 ]. Interestingly, we found that this is not always the case, as we have shown with crown ether and without in larger t-wof and t-wou cages in EMC-2 zeolite [ 11 ]. The geometric simulation approach is not limited to tetrahedral systems, and has, for example, been applied successfully to networks of regular and Jahn–Teller distorted octahedra in manganite perovskites [ 12 – 14 ].…”

Intrinsic Flexibility of the EMT Zeolite Framework Under Pressure

“…Figure 2 shows the variation in lattice parameters of the filled and empty (i.e., with and without 18C6) zeolite EMC-2 as a function of pressure. The flexibility window of the EMT framework obtained from geometric simulations, which suggest the window to be identical for both the empty and 18C6 filled zeolite EMC-2, is overlaid for comparison [25]. This comparison indicates that our measurements were performed well within the window boundaries and, consequently, no distortions of the TO4 tetrahedra are anticipated over the range of pressures studied.…”

“…This strongly suggests a competition between the FAU and EMT phases during crystallisation. Simulation results further suggest that the macrocation influences the free-energy landscape during the crystallisation process rather than acting as a geometric template [25]. [28] are highlighted.…”