Elucidating silanol removal in MFI nanosized zeolites through controlled post-synthesis hydrothermal treatments

Abstract

The rational control of silanol groups is still an elusive dream in zeolite synthesis. To enhance our understanding of this phenomenon, we have performed various hydrothermal post-synthesis treatments on nanosized Silicalite-1 using sodium molybdate, sodium chloride, and ultrapure water to evaluate the alteration of silanol content. All treated samples exhibited a phase transition from orthorhombic to monoclinic confirmed by X-ray diffraction (XRD). Fourier-Transform Infrared (FTIR) and solid-state Nuclear Magnetic Resonance (NMR) spectroscopy showed a decrease in the concentration of silanol groups in the post treated samples. The treatment with sodium molybdate leads to the removal of approximately 98% of the silanol groups, while the samples treated with sodium chloride and ultrapure water showed a decrease of 90% and 20% respectively, compared to the parent silicalite-1. Additionally, the results from pyridine adsorption followed by FTIR show that the Parent-Silicalite-1 and the H₂O-Silicalite-1 samples contain a significant amount of defects, requiring 13.55 and 11.18 μmol, respectively, to saturate all the unsymmetrical bridging sites, while for the sample treated with sodium chloride only 1.07 µmol is needed to saturate all siloxane sites. In contrast, the treatment with sodium molybdate produced a free-defects zeolite. Molybdenum was found to contribute to the removal of both weak and strong hydrogen-bonded silanol groups through its insertion into the zeolite framework, while sodium played a significant role in condensing open bridges within the zeolite framework and removing less weak and strong hydrogen bonded silanol groups. This condensation of silanol groups seems to be facilitated by the polarization of silanol groups in presence of Na⁺ promoting the formation of siloxy groups that eventually condense easily with adjacent SiOH groups, liberating water molecules during the activation process of the samples.