New insights into the monoclinic-to-orthorhombic phase transition in MFI-zeolites

The so-called ZSM-5 zeolites have a MFI-type structure, and are characterized by an Al-doped siliceous framework (Fig. 1), with the Si/Al ratio ranging from 10 to infinity; when this ratio exceeds 1000, the zeolite is typically known as “Silicalite-1”. The ZSM-5 zeolites exhibit a complex polymorphism, as they may crystallize either in the orthorhombic P212121 space group, or in the more common orthorhombic Pnma and even in the monoclinic P21/n11 space groups (usually referred to as ORTHO and MONO, respectively). A monoclinic (P21/n11)-to-orthorhombic (Pnma) phase transition (hereafter MOPT) has been reported in ZSM-5 zeolites,1,2 in response to different variables. Several variables, such as temperature, pressure, framework chemistry, concentration of defects, as well as the nature and the concentration of the sorbate, may stabilize the Pnma or the P21/n polymorphs.3-6 In particular, the temperature and pressure at which the MOPT occurs largely depends on the framework composition. In order to obtain new insights into the mechanism of the MOPT at varying chemical and pressure conditions, we synthesized six chemically distinct MFI-zeolites, with framework-Si partially replaced by Al or B (counterbalanced by Na or H as extra-framework cations, see Tab. 1). Then, by in situ synchrotron X-ray powder diffraction experiments with a diamond-anvil cell, the high-pressure behavior of the synthesized MFI-zeolites has been studied using methanol and silicone oil as pressure-transmitting fluids. All the investigated zeolites crystallized in the monoclinic P21/n11 space group and experienced the MOPT during compression. The Na-Silicalite-1 (Table 1), for example, underwent the monoclinic-to-orthorhombic phase transition at ~ 0.5 GPa in silicone oil and at ~ 0.4 GPa in methanol. A preliminary analysis suggests that, when compressed in the non-penetrating silicone oil, the doped MFI zeolites experience the MOPT at lower pressures than the Na-SiO2 silicalite.