Structure-Function Relationships for Dispersed Early Transition Metal Atoms on Porous Oxides
Abstract
Nearly 10 billion kilograms of propylene oxide (PO) are produced every year, and PO produced by the chlorohydrin process required 5.2 billion kilograms of chlorine (Cl2) and 3.7 billion kilograms of alkali hydroxides (e.g., Ca(OH)2) in 2015 alone. Many other current methods for epoxidations also rely on the use of Cl2, which produces a slew of toxic and caustic by-products, or the use of stoichiometric amounts of organic peroxides or peracids (e.g., tertbutylhydroperoxide), which generates co-products that must be regenerated. Hydrogen peroxide (H2O2) can replace the use of Cl2-alkali hydroxides and organic oxidants entirely in epoxidations and oxidations. Titanium silicalite-1 (TS-1), which was first synthesized in the late 1970Õs,7 has since been adopted industrially for several H2O2-mediated oxidation reactions. Despite this success, non-productive H2O2 decomposition, limited substrate scope of the MFI framework (pore diameter ~0.5 nm), and the lack of clear design rules for similar heterogeneous epoxidation catalysts has prevented the use of this chemistry in many epoxidation chemistries where organic peroxides are still prevalent. To overcome these shortcomings, we need to understand how the particular combination of a reactive metal center and the solvating environment of the zeolitic framework made TS-1 successful for a few specific oxidation reactions. With such information, we can design atom-efficient catalysts to selectively activate H2O2 and form reactive intermediates with the appropriate coordinative and electronic attributes to epoxidize larger (e.g., C=6) alkenes with minimal H2O2 decomposition. The over-arching goals of this proposal are to develop a fundamental understanding and structure-function relationships for H2O2 activation and subsequent alkene epoxidation and H2O2 decomposition reactions on group 4-6 transition metals in the framework of zeolites or dispersed on high surface area metal oxide supports. Our aims include: Aim 1, identify how properties of the metal atoms (e.g., valence, electrophilicity) determine the identity of the reactive species (e.g., MOOH or M-(À2-O2)) and barriers for epoxidation or H2O2 decomposition; Aim 2, determine how the pore diameter of the catalyst influences rates and selectivities by solvation effects; Aim 3, investigate the effect of the silanol density within the zeolite pores (i.e., hydrophilicity) on epoxidation rates and selectivities; and Aim 4, establish how the electronic properties of the active site, those of the reactive species, and epoxidation rates and selectivities change across a series of metal-oxide supports with varying electrophilicities. Achieving these aims will answer fundamental questions regarding how the complex combinations of material physicochemical properties affect epoxidation rates and selectivities, and moreover, how we might design catalysts for (ep)oxidations of specific substrates. We will achieve Aims 1-4 using a combination of kinetic, thermodynamic, and spectroscopic (in situ Raman, infrared, and UV-vis) measurements to probe the elementary steps involved in the activation of H2O2 and epoxidation of alkenes and the nature of the reactive surface intermediates that are formed. The proposed work leverages the unique combination of skills within the PIÕs lab in determining reaction mechanisms, obtaining kinetic measurements, and applying state-of-the-art spectroscopic methods to directly observe surface intermediates at solid liquid interfaces.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Feb 14, 2019
- Source ID
- W911NF1810100
Entities
People
- David W Flaherty
Organizations
- Army Contracting Command
- United States Army
- University of Illinois Urbana–Champaign