Framework–OSDA interactions and charge compensation in pure silica MFI zeolites revealed by solid-state NMR

Host-guest interactions in pure silica MFI zeolites synthesized in fluoride media using a range of organic structure-directing agents (OSDAs) (TEA, TPA, TPMA, TBA and TBMA) were investigated with emphasis on the influence of OSDA symmetry. Solid-state NMR techniques, notably bidimensional 1 H– 19 F heteronuclear correlation ( 1 H– 19 F HMQC) and the Rotational Echo Double Resonance ( REDOR) were employed. Fluoride bonds silicon at the Si9 site and depending on the OSDA also the Si12, giving rise to characteristic 19 F NMR signals at –65 and –80 ppm, respectively. Bulky OSDAs reduces fluoride incorporation and promotes bonding at Si12 site along with the formation of siloxy-silanol defects. Triple Quantum-Single Quamtum (TQ-SQ) 1 H NMR spectra confirm the involvement of three neighboring silanol groups in the siloxy-silanol defects. 1 H– 13 C HMQC spectra revealed that these defects are located near the terminal methyl groups of the longer OSDA chains and are distributed along both straight and sinusoidal channels of the MFI structure. REDOR experiments demonstrated that in MFI zeolites synthesized with asymmetric trialkyl methyl ammonium OSDAs, fluoride at the Si9 site is closer to the N–CH 3 group, as confirmed by theoretical calculations. These findings highlight the critical role of OSDA structure in directing charge distribution and defect formation during zeolite crystallization. The study showcases the power of advanced solid-state NMR methods for probing framework–OSDA interactions, which are essential for tailoring zeolite properties for different applications.