(465d) Catalytic Consequences of Framework Polarity for Ethanol Dehydration on Sn-Beta Zeolites

Jason S. Bates, Brandon C. Bukowski, Jeffrey P. Greeley, Rajamani Gounder*

Charles D. Davidson School of Chemical Engineering, Purdue University, West Lafayette, IN 47907

*rgounder@purdue.edu

Lewis acidic Sn-Beta zeolites containing different silanol defect density and framework metal site coordination (open: (HO)-Sn-(OSi)₃, closed: Sn-(OSi)₄) have been recognized to behave catalytically different in reactions of oxygenated compounds. In aqueous-phase glucose isomerization, titration and quantification of sites in Sn-Beta zeolites using pyridine and CD₃CN [1], together with theoretical estimates of 1,2-intramolecular hydride shift activation barriers [2], indicate that open sites are the dominant active sites. Furthermore, glucose isomerization initial rate constants (373 K, per open Sn) are >10× higher for open sites confined within Sn-Beta-F (fluoride-mediated, low-defect) than Sn-Beta-OH (hydroxide-mediated, high-defect) [1], which has been ascribed to the presence of extended hydrogen-bonded water structures that increase activation free energies in high-defect pores. Here, we employ gas-phase ethanol dehydration as a probe reaction to clarify the kinetic consequences of defect density in the absence of bulk solvent effects, and assess the influence of reaction conditions on active Sn site coordination.

Bimolecular ethanol dehydration rates to form diethyl ether (DEE) were measured over low-defect Sn-Beta-F and high-defect Sn-Beta-OH zeolites at 404 K, ethanol pressures between 0.5–35 kPa, and water pressures between 0.1–50 kPa. Rate data were interpreted using a mechanism-derived rate expression that describes measured reaction orders, supported by density functional theory calculations and microkinetic modeling to discriminate among candidate reaction pathways and surface intermediates. Infrared spectroscopic quantification of open and closed Sn sites using CD₃CN titrants before and after reaction indicated that Sn sites change structure dynamically during catalysis, leading to turnover rates (per total Lewis acidic Sn) that are invariant (within ~2–3× at 404 K) among six Sn-Beta-OH samples with varying Sn content (Si/Sn=30–95). Polar silanol groups located within the pores of Sn-Beta-OH preferentially stabilize, relative to Sn-Beta-F, dimeric intermediates that involve adsorption of a second polar water or ethanol molecule with an ethanol monomer, reflected in more negative apparent water orders at high water pressure, in lower apparent ethanol orders at high ethanol pressure, and in ~4× higher values of the regressed thermodynamic equilibrium constants (at 404 K) associated with ethanol-water dimer formation. These kinetic observations are consistent with single-component adsorption isotherms and two-component adsorption measurements that indicate higher uptakes of water relative to ethanol in high-defect micropores. Hence, differences in DEE formation rates (404 K) between the two materials arise solely from differences in prevalent surface coverages of inhibiting and reactive dimers rather than the intrinsic rate constants for DEE formation, which are insensitive to framework polarity.

[1] Harris et al., J. Catal. 335 (2016) 141–154.

[2] Bermejo-Deval et al., Proc. Natl. Acad. Sci. 109 (2012) 9727–9732.