Key-Note Invitada: ATR-FTIR for Spectrokinetic Analysis at Liquid/Solid Interfaces

COLLINS, SEBASTIAN

Congreso; VI International Congress on Operando Spectroscopy; 2018

Attenuated total reflection (ATR) infrared (IR) spectroscopy is a powerful tool to investigate reaction pathways in liquid(reactive)/solid(catalyst) systems, because it provides the detection of adsorbed species on a catalyst under reaction conditions. Catalysts are commonly deposited on an internal reflection elements (IRE) as layers of powders or as films (e.g. metal film), and they are exposed to the liquid phase reactants. In order to study the evolution of adsorbed intermediates, transient techniques are usually applied, such as stop-flow and concentration-modulation excitation spectroscopy (c-MES) [1,2]. However, to obtain quantitative information, that is, determining intrinsic reaction rates, the chemical engineering aspects of an ATR flow-through cell must be developed. Particularly, mass transport into the ATR cell has to be characterized. We have developed an optimized flow-through ATR microfluidic cell to assess intrinsic kinetic parameters of reactions under chemical control. This ATR cell possesses no dead-volume and has a uniform fluid velocity profile across the cell. The mass transport dynamic from the bulk solution to the surface of the ATR crystal -where the catalyst is deposited- is described by a convection-diffusion transport model [2]. The mass-transfer limitation in the ATR cell was validated by studying the adsorption/desorption and oxidation of CO on both a non-porous (Pt film) and a porous catalyst layer (Pt/Al2O3). Then, complex surface reactions where investigated. The mechanism of acetonitrile hydrogenation to amines using toluene as solvent was investigated on Pt and Ni as metal films and supported catalysts, Pt/Al2O3 and Ni/SiO2. Reaction intermediates were identified and track in time. Acetonitrile is adsorbed linearly (on top) on Pt and bridged on Ni, which determines the selectivity of the catalyst. Imine (CH3CH=NH), as key intermediate, is hydrogenated to ethylamine, and condensed to diethylamine and triethylamine and ammonia as a byproduct. The time-evolution of the IR signals were modeled using a proposed microkinetic mechanism and intrinsic kinetic constants were obtained under chemical control.Finally, examples of in situ/operando studies of enzymes by FTIR-ATR spectroscopy will be presented. Particularly, a new technique based on experiments of modulation excitation spectroscopy (MES) will be presented. Pure and supported lipases have been investigated regarding its conformational changes from the band distribution of the Amide I signal. To achieve this, protocols of isotopic exchange with D2O have been developed in order to remove spectral interferences. Using these methods, the synthesis, characterization and reactivity of Rhizomucor meihei lipase supported on pure chitosan and functionalized with glutaraldehyde was investigated. Operando ATR showed the activity of these biocatalysts in the hydrolysis reaction of ethyl stearate. Moreover, we were able to identify the acyl intermediate adsorbed in the active site of Candida antárctica during in the interaction of R/S ketoprofen [3].