Counting Sites in Lewis Acid Zeolite Sn-Beta: Connecting Site Quantification Experiments and Spectroscopy To Investigate the Catalytic Activity for the Alcohol Ring Opening of Epoxides

Abstract

Sn-Beta is a promising catalyst for numerous reactions involved in biomass upgrading and fine chemical production, but it is complex with multiple types of active sites. The activity for Sn-Beta can be calculated on a per-site basis using site quantification experiments that involve adding a Lewis basic probe molecule, but it is not clear which types of sites are being titrated. Our work connects site quantification experiments with spectroscopic measurements to explain differences in the catalytic activity of materials crystallized for different amounts of time. For alcohol ring opening of epoxides, experiments reveal that Sn-Beta crystallized for 10 days (Sn-Beta-200-10d) is more active than Sn-Beta crystallized for 40 days (Sn-Beta-200-40d). These materials are investigated using site quantification experiments with three probestriethylamine, pyridine, or 2,6-lutidineto reveal the different fractions and types of sites. As the probe:Sn ratio is increased, these experiments result in two distinct slopes, indicating two distinct activities: high and low activity. The difference in activity between Sn-Beta-200-10d and Sn-Beta-200-40d can be attributed to the reduced fraction of high-activity sites. Although the two slopes have typically been assigned to open defect Sn sites for high activity and closed Sn sites for low activity, 15N NMR measurements of materials dosed with 15N-labeled pyridine contradict this assignment. Indeed, at low concentrations, pyridine adsorbs on both open defect and closed Sn sites whereas the low activity corresponds to pyridine binding to SnOH groups in addition to closed Sn sites. Overall, the identification of appropriate site quantification experiment parameters and the combination of these titrations with NMR techniques allows for the establishment of a synthesis–structure–activity relationship that has the potential to improve the performance of Sn-Beta.

Confined dual Lewis acid centers for selective cascade C–C coupling and deoxygenation

“…It has been reported that closed (Sn–(OSi) 4 ), hydrolyzed-open ((HO)–Sn–(OSi) 3 )⋯HO–Si), and defect-open ((HO)–Sn–(OSi) 3 ) Sn sites exist on SnBeta materials which could be quantified by varied approaches. 24,26,27 Based on the CD 3 CN-DRIFTS results (Fig. S4†), we postulate that both defect open and closed Sn sites exist on the SnBeta synthesized by liquid-phase grafting method.…”

“…To unravel the site requirements of this reaction on the obtained materials, the chemical and physical properties of conned dual Lewis acidic Ce and Sn centers were examined using XRD, 1 H-SSNMR, diffuse reectance (DR) UV-Vis (Ultraviolet-Visible Spectroscopy), and Diffuse Reectance Infrared Fourier Transform Spectroscopy (DRIFTS)-pyridine. By employing a well-established dealumination procedure, 14 framework Al species were extensively removed to yield Beta-deAl with a Si/Al = 600, as conrmed by 27 Al-solid state nuclear magnetic resonance (SSNMR) and inductively coupled plasma (ICP) analysis (Fig. S2 and Table S2 †, respectively).…”

Consequences of Pore Polarity and Solvent Structure on Epoxide Ring-Opening in Lewis and Brønsted Acid Zeolites

“…They attributed the rate increases to a decrease in the density of (SiOH) x groups that leads to entropic gains for ring-opening. More recent work used site quantification, rate measurements, and 15 N NMR of adsorbed pyridine to show that Sn sites adjacent to (SiOH) x nests provide greater turnover rates for epoxide ring-opening than Sn sites adjacent to a point SiOH defect . These findings indicate that the (SiOH) x structure near Sn atoms influences ring-opening kinetics, which may stem from differences in local solvent structure.…”

Heteroatom Lewis acid zeolites: synthesis, characterization and application in the conversion of biomass-derived oxygenates

“…, closed or open sites), and display different acidities, which all together will influence the catalytic performance. 34,35 Moreover, different synthesis methods may lead to different pore structures, crystallite sizes, and concentrations of defect sites (which determines the hydrophobicity or hydrophobicity), which will also affect the catalytic performance. 36–39 Significant advances in the qualification and quantification of the location, fine structure and acidity of heteroatom centers confined in zeolites have also been made in recent years.…”