Solvent Mediated Interactions on Alkene Epoxidations in Ti-MFI: Effects of Solvent Identity and Silanol Density

Abstract

Selective alkene oxidation rates within zeolite pores reflect differences in contributions from thermodynamic nonidealities introduced by noncovalent interactions at solid−liquid interfaces and spatial constraints enforced by the zeolite topology. Epoxidation turnover rates for 1-hexene in Ti-MFI zeolite differ by 1000-fold across 10 combinations of solvents, including alcohols and acetonitrile, over Ti-MFI with distinct silanol defect densities. Solvent mediated noncovalent interactions persist across an extended binding environment that influences the stability of epoxidation transition states. Apparent activation enthalpies (ΔH App ⧧ ) and entropies (ΔS App ⧧) span 80 kJ mol −1 and 200 J mol −1 K −1 , respectively, with the most positive values for both appearing in methanol. Hydrophilic (i.e., (SiOH) x defect rich) Ti-MFI-OH catalysts in methanol give the greatest turnover rates because entropic gains associated with the disruption of hydrogen-bonded networks of intrapore methanol and water dominate activation free energies. In situ infrared spectra show that the adsorption of 1,2-epoxyhexane to Ti sites disturbs the equilibrium solvent structure within pores in ways that reflect the density of the hydrogen bond donor and acceptors near active sites. This interpretation agrees with measured enthalpies for adsorption of 1,2-epoxyhexane to Ti active sites within solvent filled pores that correlate with ΔH App ⧧ across solvent and catalyst combinations. The displacement of solvent molecules and the associated rupture and formation of hydrogen bonds among solvents, pore walls, and nearby reactive molecules during catalysis and adsorption events incur substantial excess contributions that govern the reaction rates and barriers. These realizations explain the particular success of alkene epoxidations in mixtures of H 2 O 2 and methanol over hydrophilic forms of Ti-MFI in industrial processes (e.g., the hydrogen peroxide−propylene oxide process). These findings demonstrate the gains achieved by appropriately pairing organic solvents with microporous catalysts to manipulate the intrinsic kinetics of reactions and the thermodynamics of adsorption at solid−liquid interfaces.

Understanding Rates and Regioselectivities for Epoxide Methanolysis within Zeolites: Mechanism and Roles of Covalent and Non-covalent Interactions

“…The cleaning procedure was repeated if the H 2 O–H 2 O injection did not give sufficiently low heat rates. Notably, this cleaning procedure is less harsh than treatments using HNO 3 and NaOH ,,,, we reported previously. This method reduces the risk of corrosion and metal dissolution at components of the ITC cell while still leading to measurements with a consistent baseline and high signal-to-noise ratio.…”

“…Previous studies demonstrate that changing the active metal ,, and intrapore solvent structure ,,,, strongly influence rates and selectivities for alkene epoxidation reactions through changes in the stability of reactive species. Intuitively, changing these variables must also influence the stability of the reactive species for epoxide ring-opening.…”

Hydrophilic Ti-MWW for catalyzing epoxidation of allyl alcohol

“…Bregante et al reported that the large number of Si-OH groups in Ti-Beta near the Ti active centers disrupted the transition state of epoxidation reaction by hydrogen bonding with H 2 O molecular clusters, generating an entropy increase for the reaction. The highly hydrophilic Ti-Beta gave an epoxidation turnover rate that was 100 times larger than that of hydrophobic Ti-Beta in the 1-hexene epoxidation reaction [22][23][24][25] .…”

Double Four Ring Units‐Containing Zeolites: Synthesis, Structural Modification and Catalytic Applications

“…[101] Besides, post-synthetic strategies allow the incorpora-tion of such heteroatoms into zeolite catalysts that cannot be obtained by direct synthesis. [17d,102] In terms of the epoxidation of alkenes and cycloalkenes, titanosilicate zeolites, such as Ti-MWW, [9b,103] Ti-MFI, [104] Ti-Beta, [105] and Ti-MOR, [106] are an excellent class of catalysts. Numerous studies have demonstrated that many factors, including the coordination states and location distributions of Ti active species, hydrophilicity/hydrophobicity of frameworks or active species, micropore sizes of zeolitic channels, etc., influence catalytic activities.…”