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

Abstract

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 structuredirecting 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 H 2 O vapor adsorption isotherms show that M-MFI materials synthesized with an HF/SDA ratio of 1.5 adsorb 4−10-fold less H 2 O than M-MFI synthesized with equal amounts of HF and SDA and 100 times less H 2 O 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 H 2 O 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.

Quantification of Intraporous Hydrophilic Binding Sites in Lewis Acid Zeolites and Consequences for Sugar Isomerization Catalysis

“…In contrast, post-synthetically prepared Sn-Beta with low initial defect density (parent Si/Al > 100, <0.6 Al (unit cell) −1 ) contain micropores that allow methanol to pack in structures similar to those of liquid methanol (ρ M,eff > 0.90; see Table ) because of its dilute intraporous silanol density (<1.4 silanol (unit cell) −1 ; see Table ). These results highlight the ability of silanol groups to facilitate adsorption of polar molecules within zeolitic micropores, as previously observed for water adsorption (293 K) on Ti-Beta and Ti-MFI zeolites of varied silanol density. Similar to the case of methanol studied here (Figure ), IR spectra collected in flowing water (298 K, P / P 0 = 0.10–0.75) indicate that water forms extended hydrogen-bonded networks within the micropores of high-defect Ti-Beta samples, but not in low-defect Ti-Beta samples under the conditions studied.…”

Ordered Hydrogen-Bonded Alcohol Networks Confined in Lewis Acid Zeolites Accelerate Transfer Hydrogenation Turnover Rates

“…These data are consistent with the preferential adsorption and nucleation of small water or alcohol clusters at hydrophilic binding sites (e.g., metal heteroatoms, SiOH defects, or H + sites) confined within zeolites prior to the formation of extended solvent structures. ,,, Increasing the equilibrium coverage of 2-butanol results in stronger 2-butanol monomer peak intensities and is accompanied by a blue-shift (toward 3620 cm –1 ) and sharpening of this feature. This phenomenon likely reflects the adsorption of 2-butanol monomers within a less polar intraporous solvating environment due to the absence of coadsorbed 2-butanol molecules that modify the local electric field near the OH dipole. − The vibrational peak characteristic of 2-butanol monomers undergoes a red-shift with increasing 2-butanol coverage in hydrophobic Si-Beta-F (3635 to 3609 cm –1 , section S6 of the Supporting Information), suggesting that the intraporous solvating environment becomes more polar as increasing amounts of 2-butanol are adsorbed within the purely siliceous pores of Si-Beta-F.…”

“…These data are consistent with the preferential adsorption and nucleation of small water or alcohol clusters at hydrophilic binding sites (e.g., metal heteroatoms, SiOH defects, or H + sites) confined within zeolites prior to the formation of extended solvent structures. 28,49,92,93 Increasing the equilibrium coverage of 2-butanol results in stronger 2-butanol monomer peak intensities and is accompanied by a blue-shift (toward 3620 cm −1 ) and sharpening of this feature. This phenomenon likely reflects the adsorption of 2-butanol monomers within a less polar intraporous solvating environment due to the absence of coadsorbed 2-butanol molecules that modify the local electric field near the OH dipole.…”

Aldol Condensation and Esterification over Ti-Substituted *BEA Zeolite: Mechanisms and Effects of Pore Hydrophobicity

“…Rates and selectivities for catalytic reactions in metalsubstituted zeolites (e.g., Ti-and Sn-incorporated BEA) are influenced by the polarity of the framework, which can be manipulated through synthesis procedures that control the presence of hydrophilic moieties such as silanol (i.e., SiOH) groups, 21,22 but these effects have not been examined for aldol condensation or esterification reactions. Suitable methods to control the densities of SiOH include postsynthetic modification, 23−25 silanol nest healing, 26,27 and hydrothermal synthesis mediated by fluoride.…”