Catalytic Consequences of Oxidant, Alkene, and Pore Structures on Alkene Epoxidations within Titanium Silicates

Daniel T. Bregante, Jun Zhi Tan, Rebecca L. Schultz, E. Zeynep Ayla, David S. Potts, Chris Torres, David W. Flaherty

Research output: Contribution to journalArticlepeer-review

Abstract

Ti atoms incorporated into the framework of zeolite *BEA (Ti-BEA) or grafted onto SBA-15 (Ti-SBA-15) catalyze alkene epoxidation with hydrogen peroxide (H2O2), t-butyl hydrogen peroxide (TBHP), or cumene hydroperoxide (CHP). The rates of epoxidation, however, differ by orders of magnitude depending on the combination of an oxidant, an alkene, and a support used. Within Ti-BEA, the rates of 1-octene epoxidation with H2O2 are 30 and 170 times greater than reactions with TBHP or CHP, respectively. In contrast, 1-octene epoxidation rates in Ti-SBA-15 with H2O2 are 7- A nd 40-fold higher than in reactions with TBHP or CHP, respectively. Moreover, comparisons of 1-alkene (C6-C10) epoxidations within Ti-BEA and Ti-SBA-15 show that the turnover rates depend differently on the length of alkene reactants depending on the oxidant identity, which is due to complex interactions among the alkene, the activated oxidant, and the solvent-filled pore of the Ti-silicate. Thermochemical analyses of apparent activation free energies within a Born-Haber cycle reveal three distinct contributions that affect catalysis, which include charge transfer between Ti-OOR and CâC functions within the transition state; the adsorption of the alkene into the silicate pores; and the structural rearrangement of the reactive Ti-OOR intermediates and solvent to accommodate the alkene. First, epoxidations with H2O2 give the highest rates among these oxidants because Ti-OOH intermediates are more electrophilic than Ti-OOtBu or Ti-OOcumyl species as a consequence of electron donation from alkyl groups that increase intrinsic barriers for O-atom transfer. Second, differences in the epoxidation rates between Ti-BEA and Ti-SBA-15 largely reflect the changes in the stability of the alkenes adsorbed within the pores of each silicate. Third, the distinct sensitivities of epoxidation rates on oxidant identity within Ti-BEA and Ti-SBA-15 are caused by differences between the inner-sphere interactions among Ti-OOR intermediates and adsorbed alkenes that depend on the surrounding environment. We present a thermodynamic model that quantitatively describes how inner-sphere interactions among epoxidation transition states depend on the steric bulk of the reacting species and how these interactions are conferred by the topology of the surrounding pore. The mesopores of Ti-SBA-15 allow transition states to access conformations that lower the free energy of the complex relative to analogous transition states in Ti-BEA, which explains why epoxidation rates in mesoporous solids are less sensitive to the identity of the oxidant than within microporous silicates.

Original languageEnglish (US)
Pages (from-to)10169-10184
Number of pages16
JournalACS Catalysis
Volume10
Issue number17
DOIs
StatePublished - Sep 4 2020

Keywords

  • cumene hydroperoxide
  • hydrogen peroxide
  • inner sphere
  • oxidations
  • solid-liquid interfaces
  • steric interactions
  • tert-butyl hydroperoxide
  • zeolites

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)

Fingerprint Dive into the research topics of 'Catalytic Consequences of Oxidant, Alkene, and Pore Structures on Alkene Epoxidations within Titanium Silicates'. Together they form a unique fingerprint.

Cite this