High-Pressure In Situ NMR Methods for the Study of Reaction Kinetics in Homogeneous Catalysis

Abstract

High-pressure NMR methods for the study of the kinetics of gas−solution reactions are presented, and the importance/interplay of mass transport and chemical resistances are discussed. For reactions that are slow compared with mass transport, the true reaction kinetics can be obtained and used to confirm that the observed species are relevant to the catalytic reaction. Conversely, the ability to determine the catalyst speciation during the catalytic reaction aids interpretation of the kinetic data. When chemical reaction is fast compared with diffusion across the gas−liquid interface, reaction is shown to occur in the liquid layer; for such a transport-controlled reaction, chemical reaction can increase the concentration gradient across the boundary layer, enhancing diffusion both across the phase boundary and within the liquid layer, resulting in apparent positive orders in catalyst, substrate, or both. These dependencies do not reflect the chemical dependence of the reaction rate on these concentrations.

Development of a kinetic model of ethylene methoxycarbonylation with homogeneous Pd catalyst using a capillary microreactor

“…0.5 under the investigated conditions. This positive effect of CO has been observed in previous studies of alkene hydroformylation in a bubble column [16].…”

Reaction characterization by flow NMR: quantitation and monitoring of dissolved H2via flow NMR at high pressure

“…This method also allows for direct determination of k L a (the gasliquid volumetric mass transfer coefficient), which varies based on reactor configuration, temperature, and agitation conditions. Furthermore, this approach is superior to NMR analysis of reactions in sealed, high pressure sample tubes 5,6 because it allows for a constant head pressure to be maintained throughout the reaction in addition to maintaining agitation of the reaction. In short, reaction monitoring of pressurized reactions via a flow NMR approach affords the ability to obtain a holistic view of the reaction without introducing significant sampling biases.…”

Disulfonated xantphos for mass spectrometric mechanistic analysis

“…[1][2][3][4][5] Catalytically relevant species are typically much less abundant than the aforementioned and partitioned between precatalyst, resting state(s), decomposition products, and intermediates, and only a fraction may be on-cycle at any point in time. [6][7][8][9][10][11][12][13][14][15] Direct methods of investigation need to be sufficiently sensitive to delve down to very low concentration levels, and in electrospray ionization mass spectrometry (ESI-MS), chemists have a tool to do exactly that. However, the technique has an Achilles heel: it operates solely on ions, transferring them to the gas phase for analysis, and ignoring neutral compounds, which are unaffected by the electric fields used to manipulate charged species.…”