(606e) The Catalytic Consequences of Silanol Densities within Titanium BEA on Alkene Epoxidation with Hydrogen Peroxide

The rational design of catalytic systems to economically utilize “green” oxidants for epoxidations requires clear understanding of structure-function relationships that govern rates and selectivities. Epoxidation rates and selectivities depend exponentially on the extent of electron exchange between the Lewis acidic metal atom and the reactive oxygen moiety (i.e., the LMCT band of O→M for M-OOH species) of the epoxidation-active intermediate when groups 4 and 5 transition metals are substituted into the framework of zeolite *BEA (M-BEA) [1]. Relative to the same metal atoms grafted into mesoporous SiO₂, M-BEA selectively stabilizes the epoxidation transition state, because attractive van der Waals interactions are greater within the voids of *BEA than mesoporous SiO₂ [2]. These two independent material properties (i.e., active site electron affinity and dimensions of the confining void) are not, however, the only chemical or physical descriptors that relate to liquid-phase catalysis over M-BEA. For example, the “hydrophilic” or “hydrophobic” character of the confining voids in Ti- and Sn-BEA (i.e., the density of silanol groups) also greatly affect rates of glucose isomerization [3] and Baeyer-Villiger oxidation [4]. This work seeks to unravel the catalytic consequences of changing the silanol nest ((SiOH)₄) density of Ti-BEA on alkene epoxidation.

Here, we compare structural and catalytic properties of a series of seven Ti-BEA catalysts that contain a range of (SiOH)₄ densities (0 – 5 per unit cell), which are synthesized either by post-synthetic modification of commercial Al-BEA or direct hydrothermal synthesis (to produce Ti-BEA with extremely low (SiOH)₄ densities). The densities of (SiOH)₄ are controlled by changing the Si/Al ratio in the starting Al-BEA (initial Si/Al = 12.5 – 250); lower Si/Al ratios result in higher densities of (SiOH)₄. Three independent techniques (i.e., ²⁹Si MAS-NMR, H₂O physisorption, and infrared spectroscopy) show that the number of SiOH groups depends directly on the initial Si/Al ratio of the parent BEA. Turnover rates and selectivities (i.e., the productive use of H₂O₂) for 1-octene epoxidation increase by factors of 20 and 10, respectively, at standard conditions (1 mM 1-octene, 10 mM H₂O₂, in CH₃CN, 313 K), when the mean number of (SiOH)₄ per unit cell increases from ~0 to 5. Isothermal 1-octene adsorption experiments (0.1 mM 1-octene, 40 mM H₂O, in CH₃CN, 313 K) show that the uptake of 1-octene does not depend on the density of (SiOH)₄. Activation enthalpies decrease (25 to 13 kJ mol^-1) with decreasing (SiOH)₄ density(5 to 0 (SiOH)₄ per unit cell), which is attributed to the stabilizing van der Waal interactions between the hydrophobic pore walls and -CH₂ units of 1-octene. These observed differences in activation enthalpies among Ti-BEA with varying (SiOH)₄ densities are counter to what is expected based upon measured differences in epoxidation rates.

These data, in conjunction with transition state theory formalisms, suggest that transition states for 1-octene epoxidation do not form from distinct precursor states that differ between the zeolite samples with different (SiOH)₄ densities. Thus, the differences in turnover rates are attributed to changes in the ratio of thermodynamic activities of 1-octene within BEA to that of the transition state for epoxidation at the active site. The intra-zeolite 1-octene concentration remains constant for all (SiOH)₄ densities, which shows that 1-octene prefers to adsorb to hydrophobic regions (e.g., pristine areas of pores) within all Ti-BEA. The activities of the transition states may not change significantly with (SiOH)₄ density, because Ti atoms prefer to bind species to maintain octahedral coordination. The remaining coordination sites of tetrahedrally coordinated Ti-atoms are occupied by solvent molecules (e.g., CH₃CN, CH₃OH) whose proximity cause these species to dictate the activity of the transition state to a greater degree than distal SiOH groups of the framework. Epoxidation, however, depends on 1-octene molecules that are near Ti-OOH moieties, rather than hydrophobic regions, and are highly influenced by nearly hydrophilic regions (e.g., (SiOH)₄ nests that may permanently reside near active sites) and result in increased thermodynamic activities. Turnover rates for highly defective Ti-BEA (~5 (SiOH)₄ per unit cell) decrease by a factor of three when CH₃OH replaces CH₃CN as the solvent (1 mM 1-octene, 10 mM H₂O₂, 313 K). This comparison suggests that interactions between the transition state and solvent molecules co-coordinated to the active site changes the thermodynamic activity of the transition state to a greater extent than that of 1-octene within the pore. Collectively, these structure-function relationships show that both the rates and selectivities for alkene epoxidations can be increased by increasing the electrophilicity of the active site, maximizing attractive van der Waal interactions between the transition state and confining void, and introducing high densities of silanol nests.

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