Pressure-driven methanol intrusion in MFI-zeolites and its effects on the structural deformation in silicalite-1

MFI-zeolites are nowadays used in methanol-to-olefins (MTO) production processes as catalysts, representing an alternative to the high-energy demanding Steam Cracking process, which accounts for 95% of the worldwide olefins production. At ambient conditions, only the grain surfaces of the zeolite are supposed to be active in the MTO process. Applying hydrostatic pressure, the methanol molecules may be injected into the structural channels of the zeolites. The structure of MFI-zeolites is characterized by SiO4 interconnected tetrahedra, which define two major structural channels systems, confined by 10-members rings (10mR) of tetrahedra running along [010] and sinusoidal cavities along the [100] direction; minor rings, constituted by 6mR and 5mR units are also present. Comboni et al. (2020 Catalysis Today, 345, 88-96) studied, through in situ synchrotron X-ray powder diffraction, the capability of pressure to improve the methanol absorption process. In order to evaluate the magnitude of the methanol HP-driven injection process, also the intrinsic compressional behavior of silicalite-1 has been studied, using a non-penetrating pressuretransmitting fluid (i.e. silicone oil). A different compressional behavior characterizes the methanol and the siliconeoil ramps (hereafter, Sil-meth and Sil-soil), as a consequence of the intrusion of methanol within the silicalite-1 structural channels. Since no structural refinement was possible based on XRD data, we aim to define the framework deformation and methanol intrusion of silicalite-1 at varying pressure, through template-based geometric modeling, conducted on both Sil-meth and Sil-soil. This modeling identifies the flexibility intrinsic to the geometry and topology of the crystal structure, considered as a mechanical framework. Concerning the compression along the three principal crystallographic axes in Sil-meth, it has been observed that along the a- and the b-axis the structure behaves similarly, whereas along the c-axis, which does not correspond to any of the channel development directions, the structure shows a higher compression. Conversely, the Sil-soil ramp presents a more isotropic compression. As expected, the 10mR’s are clearly more compressible in Sil-soil, whereas in the Sil-meth ramp the intrusion of methanol leads to the phenomenon known as “pillar effect”. As pointed out by Comboni et al. (2020) for both the Pramps, a phase transition from monoclinic to orthorhombic (MOPT) occurs at about 0.5 GPa. Although the phase transition does not largely affect the unit-cell volume of silicalite-1, in both the Sil-soil and the Sil-meth, it strongly influences the behavior of the channels under HP, changing their compressional trend at 0.5 GPa. Lastly, the behavior of the Sil-soil is characterized by a “distributed” compression, which results in a rather equal magnitude of the compression of the 10mR, 6mR, and 5mR units. Conversely, in the Sil-meth, the compression of the 10mR is significantly modest, whereas takes place a clear distortion of the 5mR units, which small diameter does not allow any methanol intrusion process.