Crystal-fluid interaction in MFI zeolites at high pressure

In the last decades, several studies have been dedicated to investigate the Pressure-induced intrusion of molecules in nanoporous compounds, as this type of phenomenon may lead to new routes for tailoring functional materials. The zeolites with the MFI-type structure topology are used as catalysts in some olefins-production processes, representing an appealing alternative to the high-energy demanding Steam Cracking process, which presently accounts for ~95% of the total worldwide olefins production [1]. At ambient conditions, only the surfaces of the zeolite crystallites are supposed to be active in the methanol-to-olefins process. Pressure could significantly improve the efficiency of the catalytic process, driving the injection and diffusion of the methanol molecules through the zeolitic channels. In the present work, already reported in [2] six MFI-type zeolites with different chemical composition, characterized Fe-, Al- and B-doped siliceous frameworks, balanced by Na or H as extra-framework cations, have been synthesized. Their compressional behavior has been investigated up to 2 GPa by in-situ powder synchrotron X-ray diffraction using two different hydrostatic pressure transmitting fluids: methanol (capable of entering the structural voids of the MFI zeolite) and silicone-oil (a polymeric fluid with a kinetic diameter of the molecules larger than the diameters of the structural channels). For each sample, the compressibility in silicone-oil has been found to be considerably higher than that in methanol. This difference in the bulk elasticity is due to the injection of methanol within the structural voids of the zeolites and the magnitude of the intrusion process seems to be influenced by the sample crystal chemistry. Due to the high number of experimental pressure points, a phase transition from the monoclinic polymorph (P21/n, stable at room pressure) to the orthorhombic polymorph (Pnma) (hereafter MOPT) has been identified at about 0.4 GPa for all the monoclinic zeolites compressed in silicone oil. Conversely, only two zeolites compressed in methanol (i.e., a penetrant fluid) show the MOPT and, in these cases, the phase transition shifted at greater pressure with respect to what observed with in the silicone oil experiments. A comparative analysis of the effect of pressure on the methanol adsorption by the MFI zeolites with different chemical composition may provide useful information on their application as catalysts in the methanol-to-olefins conversion processes.