In situ imaging of the sorption-induced subcell topological flexibility of a rigid zeolite framework

Abstract

The crystallographic pore sizes of zeolites are substantially smaller than those inferred from catalytic transformation and molecular sieving capabilities, which reflects flexible variation in zeolite opening pores. Using in situ electron microscopy, we imaged the straight channels of ZSM-5 zeolite with benzene as a probe molecule and observed subcell flexibility of the framework. The opening pores stretched along the longest direction of confined benzene molecules with a maximum aspect change of 15%, and the Pnma space group symmetry of the MFI framework caused adjacent channels to deform. This compensation maintained the stability and rigidity of the overall unit cell within 0.5% deformation. The subcell flexibility originates mainly from the topologically soft silicon-oxygen-silicon hinges between rigid tetrahedral SiO 4 units, with inner angles varying from 135° to 153°, as confirmed by ab initio molecular dynamics simulations.

Real-time regeneration of a working zeolite monitored via operando X-ray diffraction and crystallographic imaging: how coke flees the MFI framework

“…We note that a very similar ellipsoidal distortion has been directly observed upon both benzene [48] and para-xylene [49] adsorption in H-ZSM-5 using (scanning) transmission electron microscopy ((S)TEM). It is therefore proposed that aromatic molecules, be it ordered adsorbed species [48,49] or coke as in the present study, induce this structural change of the MFI framework through steric strain. This transition from elliptical to circular can be quantified in terms of a 10 ring aspect ratio, as defined in [48,49], and this key structural parameter is shown in Figure 4.…”

“…S8 of the ESI. † We note that a very similar ellipsoidal distortion has been directly observed upon both benzene 48 and para-xylene 49 adsorption in H-ZSM-5 using (scanning) transmission electron microscopy ((S)TEM). It is therefore proposed that aromatic molecules, be it ordered adsorbed species 48,49 or coke as in the present study, induce this structural change of the MFI framework through steric strain.…”

“…It is therefore proposed that aromatic molecules, be it ordered adsorbed species [48,49] or coke as in the present study, induce this structural change of the MFI framework through steric strain. This transition from elliptical to circular can be quantified in terms of a 10 ring aspect ratio, as defined in [48,49], and this key structural parameter is shown in Figure 4. A large numerical value of the aspect ratio corresponds to an oval 10-ring, whereas a lower value describes a more circular ring shape (Figure 3).…”

“…This transition from elliptical to circular can be quantified in terms of a 10 ring aspect ratio, as defined in, 48,49 and this key structural parameter is shown in Fig. 4.…”

“…This analysis also sheds considerable light on why the a minus b parameter is such a powerful descriptor for coke content and catalyst deactivation. The near identical behaviour of (a − b) and the 10 ring aspect ratio known to be sensitive to rigid aromatic molecules, 48,49 means that (a − b) is insensitive to species that have little to do with catalyst deactivation, such as adsorbed volatile products, substituents on aromatic rings, and the presence of water. Water is not discussed here, but the insensitivity of (a − b) and the 10 ring aspect ratio can easily be demonstrated in hydration-dehydration experiments (not shown).…”