High-pressure cold methanol intrusion in MFI-zeolites

In the last decades, several efforts were devoted to explore the P-mediated intrusion of molecules in microporous compounds since this can lead to new routes of tailoring functional materials, bearing a potentially relevant technological impact. MFI-zeolites are also used as catalysts in some olefins-production processes, representing an appealing alternative to the high-energy demanding Steam Cracking process, which actually accounts for 95% of the total worldwide olefins production.1-3 The applicative importance of MFI-type zeolites is due to their unique structure formed by (Al,Si)O4 tetrahedra connected in such a way that a pore system, consisting of two intersecting channels, occurs within its zeolitic framework. The employment of zeolites as synthesis catalysts allows milder synthesis conditions, leading to a lower energy consumption and, therefore, lower greenhouse emissions. Furthermore, in recent years MFI-zeolites have been used in the promising methanol-to olefins synthesis process, which, being able to obtain olefins directly from methanol in place of oil bears a potential breakthrough industrial impact. It is worth to underline usually, at ambient conditions, only the surfaces of the zeolite crystallites are believed to be active in the methanol-to olefins process. However, induced by pressure, the methanol molecules may penetrate and diffuse through the zeolitic channels.4 This may bear a significant impact in the industrial applications of this zeolite as a catalyst, since a “cold” intrusion of methanol into the zeolite cavities might pave the way to increase the efficiency of the methanol-to-olefins conversion process. In this regard, we synthesized and then studied, by in situ synchrotron X-ray powder diffraction experiments, the high-pressure behavior of six MFI-zeolites with different chemical composition (reported in Tab. 1). Consistently with the previous studies,5 all the synthesized zeolites are monoclinic (space group P21/n11) at ambient pressure, although a monoclinic-to-orthorhombic phase transition (MOPT) is reported to occur at P > 1 GPa.5 Analyzing the pressure-volume data and the diffraction patterns, we were able to ascertain: i) all the MFI zeolites compressed in silicone oil have overall the same bulk compressibility (Fig. 1), ii) there are differences, among the different zeolites, in the magnitude of the methanol adsorption (e.g., Fig. 2), iii) the MOPT is influenced by both crystal chemistry and sorbate (methanol) loading. Overall, this study provides useful information about the optimal chemical composition of a potential MFI-catalyst in the methanol-to-olefins conversion process operating at high-pressure conditions.