The Scott group conducts both fundamental and applied research in reactions, surface chemistry, and catalysis. Our goal is to understand the interactions and transformations of molecules at gas-solid and liquid-solid interfaces by creating highly uniform active sites. We use advanced techniques in organometallic and coordination chemistry, surface science, spectroscopy, kinetics, mechanistic analysis and modeling to investigate, design and reengineer heterogeneous catalysts. Our group includes both chemical engineering and chemistry students, working to solve important current problems at the interface of chemistry and reaction engineering.
Designing Self-Activating Catalysts for Olefin Metathesis and Polymerization
Olefin metathesis is the redistribution of hydrocarbon chain lengths by rearranging the substituents on C=C bonds, for example:
It is widely used in manufacturing commodity chemicals, surfactants, polymers, and many specialty chemicals.
Atomically-dispersed catalysts (typically, based on Mo, W, or Re) on oxide supports (typically, silica or alumina) show the remarkable ability to self-activate, offering the prospect of being able to design catalysts that can also reactivate themselves on demand. The active sites are metal carbenes. We are investigating mechanisms of self-activation and deactivation in collaboration with Prof. Al Stiegman (FSU), Dr. Mostafa Taoufik (CPE-Lyon) and Dr. Régis Gauvin (Chimie Paristech).
A significant primary kinetic isotope effect suggests that perrhenate activates by a mechanism involving C-H activation:
Read about our recent work in: J. Am. Chem. Soc. 2018, 129, 8912; ACS Catal. 2018, 8, 1728; ACS Catal. 2017, 7, 7442; J. Am. Chem. Soc. 2016, 129, 8912; J. Phys. Chem. C. 2011, 30, 133; Top. Catal. 2011, 30, 133; J. Am. Chem. Soc. 2007, 129, 8912.
Chlorination of alumina results in a dramatic increase in activity and stability of a grafted CH3ReO3 catalyst:
Isolated strong Lewis sites (i.e., Lewis acid sites remote from hydroxyl groups) are relatively more abundant on amorphous and chlorinated Al2O3. Their presence is linked to the generation of active sites:
Exploring the Spatial Distribution of Active Sites on Surfaces
Hydroxyl groups present on oxide surface terminations are the principal sites of attachment for molecular catalysts. For high surface area amorphous oxides, thermal pretreatment reduces their surface density and allows the formation of stable, isolated active sites. According to X-ray absorption spectroscopy (EXAFS), the absence of metal-metal scattering paths provides evidence of these isolated sites. Or does it? Grafted GaR3 has such paths, showing that site pairing that persists even at very low hydroxyl densities. In collaboration with Prof. Songi Han (UCSB) and Prof. Baron Peters (UIUC), we are investigating the distribution of paramagnetic VCl4 on silica and alumina using electron paramagnetic resonance (EPR) spectroscopy and computational modeling.
The absence of room temperature EPR signals for VCl4/SiO2 (in contrast to VCl4/Al2O3) suggests strong dipolar coupling of V(IV) centers. It implies that hydroxyl grafting sites are co-located. EPR signals emerge at low T, when the rotational motion of the V(IV) centers is frozen.
Understanding Alkane Dehydrogenation with Operando X-ray Absorption Spectroscopies
An operando spectroscopy involves observing the active sites in real-time, while reactions are taking place. In collaboration with Dr. Simon Bare (SSRL), Prof. Jean-Sabin McEwen (WSU), Dr. Mostafa Taoufik (CPE-Lyon), and Prof. Adam Hock (UIC), we are studying the evolution of gallium sites in the presence of propane, propene and H2 at temperatures up to 550 °C using operando X-ray absorption spectroscopies (XANES and EXAFS) at the Stanford Synchrotron Lightsource.
Exploring Catalytic Mechanisms in Upgrading Biomass to Renewable Fuels and Chemicals
Selectively 13C-labeled model compounds such as 2-phenoxy-1-phenyethanol are synthesized to study the mechanisms of catalytic lignin conversion to aromatic monomers. Operando solid-state NMR studies of the reaction progress at elevated temperatures and pressures reveal the evolution of individual species in the reaction network.
Read about our recent work in: Chem. Sci. 2020 (in press); Green. Chem., 2020, 22, 550; J. Am. Chem. Soc. 2019, 141, 17370; ACS Catal., 2019, 9, 7204; ChemCatChem, 2019, 11, 190; J. Phys. Chem. C, 2018, 122, 8209; ACS Catal. 2017, 7, 3489; ACS Catal. 2016, 6, 8286; Catal. Sci. Technol. 2015, 5, 1540; ACS Catal. 2014, 4, 2165; Angew. Chem. Int. Ed. 2013, 52, 10349.
Probing P-Zeolite Catalysts using Advanced Solid-State NMR Methods
P-modified zeolites show high selectivity in acid-catalyzed reactions for biomass upgrading to renewable chemicals, such as the conversion of furanics to butadiene or p-xylene.
With the Catalysis Center for Energy Innovation (CCEI) and Prof. Songi Han (UCSB), we are studying the nature of the P-sites and their dynamic behavior under reaction conditions, using solid-state NMR methods with advanced pulse sequences to distinguish sites with similar chemical shifts, and the enhanced sensitivity of Dynamic Nuclear Polarization (DNP) to reveal spatial correlations.