The flexibility window in zeolites

Abstract

Today synthetic zeolites are the most important catalysts in petrochemical refineries because of their high internal surface areas and molecular-sieving properties. There have been considerable efforts to synthesize new zeolites with specific pore geometries, to add to the 167 available at present. Millions of hypothetical structures have been generated on the basis of energy minimization, and there is an ongoing search for criteria capable of predicting new zeolite structures. Here we show, by geometric simulation, that all realizable zeolite framework structures show a flexibility window over a range of densities. We conjecture that this flexibility window is a necessary structural feature that enables zeolite synthesis, and therefore provides a valuable selection criterion when evaluating hypothetical zeolite framework structures as potential synthetic targets. We show that it is a general feature that experimental densities of silica zeolites lie at the low-density edge of this window--as the pores are driven to their maximum volume by Coulomb inflation. This is in contrast to most solids, which have the highest density consistent with the local chemical and geometrical constraints.

Flexibility windows and phase transitions of ordered and disordered ANA framework zeolites

“…Both analcime and pollucite thus lie towards the less dense edge of the flexibility window, consistent with the behaviour of other zeolite frameworks [11].…”

“…It has recently been observed that known zeolite frameworks are typically perfectible over a wide range of densities [11]. This range is termed the "flexibility window".…”

In situ solid-state NMR and XRD studies of the ADOR process and the unusual structure of zeolite IPC-6

“…To explain this we take note of the beautiful work of Sartbaeva, Dawson and coworkers who showed that having some 'flexibility' is an important criterion in determining whether a zeolite can be successfully prepared. 16,17 Figure 5 shows how this concept can be applied to explain why the IPC-6 structure forms. The distance between these two layers is then fixed at that found for IPC-2P as the layers are connected by the rearrangement.…”

Flexibility in MOFs: do scalar and group-theoretical counting rules work?

“…Depending on the way the bodies are assembled, the structure can be rigid or flexible. If it is flexible, it can deform and the modes of 2 | Dalton Trans., 2015, 00, [1][2][3][4][5][6][7][8][9] This journal is © The Royal Society of Chemistry 2015 deformation are called mechanisms. A mechanism also corresponds to a degree of freedom of the structure.…”

“…A first class of approaches consists in exploring the phonon spectrum of zeolites and identifying vibration modes that conserve the shape and size of tetrahedra and have low frequency; these modes are variously referred to as rigid unit modes 14 (RUM) or floppy modes 15 depending on the implementation. More recently 9 , the concept of a flexibility window has emerged, where many zeolites remain flexible in a range of density; it is postulated that zeolites with a large flexibility window are more realisable 16,17 . In a few rare studies, rigid-units 18,19 and flexibility windows 20 type approaches have been applied to MOF-like structures.…”