Effects of Hydrofluoric Acid Concentration on the Density of Silanol Groups and Water Adsorption in Hydrothermally Synthesized Transition-Metal-Substituted Silicalite‑1

• Published in: Chemistry of materials 2020-09, Vol.32 (17), p.7425-7437
• Main Authors: Bregante, Daniel T, Potts, David S, Kwon, Ohsung, Ayla, E. Zeynep, Tan, Jun Zhi, Flaherty, David W
• Format: Article
• Language: English
• Subjects: Chemistry; Materials Science
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/acs.chemmater.0c02405

The type and density of structural defects within zeolites and zeotypes affect the stabilities of adsorbed species, which, in turn, impact the performance of these materials as catalysts and adsorbents. Despite the recognized importance of silanol groups (SiOH) on the properties of a zeolite catalyst or adsorbent, the densities of SiOH have not been quantitatively linked to the concentration of hydroxide (OH–) and fluoride (F–) ions within the synthesis gel. Here, we present a method for the synthesis of siliceous or heteroatom-substituted MFI zeolites (M-MFI; M = Si, Ti, Nb, or Ta) with tunable densities of SiOH, which depend simply on the ratio of hydrofluoric acid (HF) to structure-directing agent (SDA; tetrapropylammonium hydroxide) used within the synthesis gel. The equilibrated ion exchange between OH– and F– ions forms tetrapropylammonium fluoride in situ, which does not lead to the formation of SiOH defects within M-MFI. Comparisons of infrared spectra from 15 distinct M-MFI materials show that the densities of SiOH groups within M-MFI decrease linearly with the ratio of HF/SDA, independent of the identity of the heteroatom within the framework. Materials synthesized within purely OH– media possess SiOH densities 3 and 100 times greater than analogous materials synthesized with HF/SDA ratios of 1 and 1.5, respectively. The use of HF forms metal fluoride complexes, detected by Raman spectroscopy, which are not readily incorporated into the zeolite framework during synthesis and lead to a decrease in the efficiency of transition-metal incorporation with increasing amounts of HF. The quantity of the heteroatom incorporated into the framework increases linearly with the concentration of metal precursor in the synthesis gel, which provides a method to mitigate the lower yields introduced by the use of HF. Comparisons between H2O vapor adsorption isotherms show that M-MFI materials synthesized with an HF/SDA ratio of 1.5 adsorb 4–10-fold less H2O than M-MFI synthesized with equal amounts of HF and SDA and 100 times less H2O than M-MFI synthesized in OH– media. Comparisons of water uptake within hydrophobic M-MFI materials show that framework Ti and Nb sites stabilize 5 and 7–8 H2O molecules, respectively, near saturation vapor pressures. These findings provide a flexible strategy to control the densities of silanol and hydroxyl groups (e.g., Nb-OH) within MFI and will likely extend to the synthesis of other zeolite frameworks.