(544gr) Catalytic Thiophene Oxidation By Groups 4 and 5 Zeolite BEA with H2O2: Mechanistic and Spectroscopic Evidence for the Effects of Metal Lewis Acidity and Solvent Lewis Basicity

Sulfoxidation of thiophenes with hydrogen peroxide (H₂O₂) is a promising alternative to hydrodesulfurization (HDS) for the desulfurization of crude-oil streams to produce low- and ultra-low-sulfur content fuels and gasolines, as HDS requires long residence times and harsh reaction conditions to remove highly-substituted polyaromatic thiophenes. Highly disperse group 4 and 5 transition metals grafted onto mesoporous silica and incorporated into zeolites selectively activate H₂O₂ for sulfide oxidation, which produces sulfoxide moieties that enable facile methods for removing sulfur from product streams (e.g., liquid-liquid extraction).

Here, we present a combination of kinetic data and in situ UV-visible spectroscopy to identify and implicate specific surface species for sulfoxidation and the mechanism by which 2,5-dimethylthiophene (C₆H₈S; i.e., a model substituted thiophene) oxidation occurs [1]. Product selectivities as a function of C₆H₈S conversion show that C₆H₈O is resistant to further oxidation, as this is process breaks aromaticity. Notably, these product selectivities differ from non-aromatic sulfide oxidation (e.g., thioanisole) [2], which are readily oxidized to form the corresponding sulfone (i.e., the product resulting from two sequential oxidation reactions). Sulfoxidation rates vary by five thousand among this series of materials, while H₂O₂ selectivities do not vary significantly. Mechanistic interpretation of kinetics measured in the absence of mass-transfer restrictions show that group 4 and 5 metal atoms (Ti, Zr, Nb, and Ta) incorporated into zeolite *BEA (M-BEA) reversibly adsorb then irreversibly activate H₂O₂ to form pools of hydroperoxide and peroxide intermediates (collectively referred to as M-(O₂)). These reactive intermediates then combine with C₆H₈S to form the corresponding sulfoxide (C₆H₈SO). UV-vis spectra collected in situ corroborate kinetic analysis that implies surfaces are saturated with specific surface species (e.g., M-(O₂) and sulfoxide saturated surfaces at low and high [C₆H₈S]:[H₂O₂], respectively) under different well-defined kinetic regimes. This mechanistic insight allowed us to measure activation enthalpies (ΔH^‡) for C₆H₈S oxidation (ΔH^‡_Ox) and H₂O₂ decomposition (ΔH^‡_Dec). ΔH^‡_Ox and ΔH^‡_Dec are similar in value and decrease with a similar functional dependence on the adsorption enthalpy for CD₃CN bound to Lewis acid sites, likely because both reactions involve the interaction of an electron lone pair (on S and O atoms of C₆H₈S and H₂O₂, respectively) with the reactive M-(O₂) intermediates. These differences in ΔH^‡_Ox between the four M-BEA and lack of differences between ΔH^‡_Ox and ΔH^‡_Dec for a given M-BEA explain why there are such large differences in the rates of sulfoxidation without a corresponding change in H₂O₂ selectivities.

Measured turnover rates for C₆H₈S sulfoxidation depend strongly on the solvent for that catalytic reaction and change by more than ten thousand. For example, the rates of reaction are immeasurable (<10^-7 (mol C₆H₈SO)(mol Ti • s)^-1; 0.01 M C₆H₈S, 0.01 M H₂O₂) when dimethylsulfoxide is used as a solvent, whereas, rates are several orders of magnitude higher in acetonitrile (~0.007 (mol C₆H₈SO)(mol Ti • s)^-1). UV-vis spectra collected in situ on Ti-BEA show that differences in turnover rates are attributed to competition between these Lewis basic solvents and H₂O₂ for adsorption onto the Lewis acidic active sites, which result in decreased rates (by decreasing the relative abundance of M-(O₂) intermediates). More generally, turnover rates depend exponentially on the solvent nucleophilicity (N₁), which is shown for a variety of academic- and industrially-relevant solvents (acetonitrile, methanol, ethanol, acetone, and p-dioxane) as well as solvent mixtures (methanol and acetonitrile).

Together, the data and interpretations presented in this work provide a self-consistent mechanism for the sulfoxidation of C₆H₈S with H₂O₂ over M-BEA, and demonstrates that the rates of reaction depend highly on the electron affinity of the active site, while H₂O₂ selectivities are unaffected. Moreover, this work identifies the dependence of turnover rates on solvent nucleophilicity (a proxy for “Lewis base strength”) and shows that these Lewis basic solvents competitively adsorb to the active sites to form Lewis acid-base adducts, which decrease turnover rates. Consequently, this work shows that turnover rates for sulfoxidation are highest when highly electrophilic active sites (i.e., stronger Lewis acids) are paired with non-coordinating solvents (i.e., weaker Lewis bases) and will guide the design of increasingly productive catalytic systems for sulfide oxidation.

References:

[1] Bregante, D.T.; Patel, A.Y.; Johnson, A.M.; Flaherty, D.W.; In Review

[2] Thornburg, N.E.; Notestein, J.M.; ChemCatChem 2017, 9, 3714-3724.