Search

Amine functional groups in polymeric facilitated transport membranes (FTMs) selectively facilitate CO2 transport across the membrane over other gases to give rapid and selective CO2 separation from mixed gas streams. However, the mechanisms by which the amine functional group facilitates CO2 transport are not well understood. Additionally, water vapor is known to enhance CO2 permeation across amine-based FTMs, but how exactly water influences the amine-facilitated CO2 transport mechanism is unknown. To address these fundamental aspects of CO2 facilitated transport, we have developed a novel operando transmission FTIR spectroscopy tool to directly observe the formation of CO2 transport intermediates (e.g. carbamate, bicarbonate) formed in amine-based FTMs under realistic operating conditions, including in humid CO2 gas mixtures, and correlate formation of specific CO2 transport intermediates to CO2 permeation rates measured simultaneously for the same sample. This work describes the design of this operando transmission FTIR spectroscopy tool and demonstrates its utility by investigating the mechanism of water-enhanced CO2 transport across polyvinylamine (PVAm), a prototypical FTM for CO2 separations. We show that CO2 moves across PVAm most likely as carbamate (R-NHCOO−) species, based on infrared band positions and literature assignments, and formation of carbamate species is facilitated by water in the CO2/N2 feed gas mixture. During exposure of PVAm to dry CO2/N2 feeds, carbamate species or other intermediates were not observed spectroscopically, and the CO2 flux across the membrane was below the detection limit. We speculate that water may facilitate the formation of carbamate, and thus enhance CO2 transport across the membrane, by catalyzing deprotonation of a zwitterion (R-NH2+COO−) intermediate to give carbamate species. This work clarifies the unknown mechanism of CO2 facilitated transport across PVAm, and the role of water in the transport mechanism, and establishes a new operando spectroscopy tool that can be used with a wide range of membrane systems to probe transport mechanisms, fouling mechanisms, and changes in membrane structure/performance over time. [ABSTRACT FROM AUTHOR]

Atomistic-level understanding of the interaction of α,β-unsaturated aldehydes and their derivatives with late transition metals is of fundamental importance for the rational design of new catalytic materials with the desired selectivity towards C[double bond, length as m-dash]C vs. C[double bond, length as m-dash]O bond partial hydrogenation. In this study, we investigate the interaction of acrolein, and its partial hydrogenation products propanal and allyl alcohol, with Pd(111) as a prototypical system. A combination of infrared reflection-absorption spectroscopy (IRAS) and temperature programmed desorption (TPD) experiments was applied under well-defined ultrahigh vacuum (UHV) conditions to obtain detailed information on the adsorption geometries of acrolein, propanal, and allyl alcohol as a function of coverage. We compare the IR spectra obtained for multilayer coverages, reflecting the molecular structure of unperturbed molecules, with the spectra acquired for sub-monolayer coverages, at which the chemical bonds of the molecules are strongly distorted. Coverage-dependent IR spectra of acrolein on Pd(111) point to the strong changes in the adsorption geometry with increasing acrolein coverage. Acrolein adsorbs with the C[double bond, length as m-dash]C and C[double bond, length as m-dash]O bonds lying parallel to the surface in the low coverage regime and changes its geometry to a more upright orientation with increasing coverage. TPD studies indicate decomposition of the species adsorbed in the sub-monolayer regime upon heating. Similar strong coverage dependence of the IR spectra were found for propanal and allyl alcohol. For all investigated molecules a detailed assignment of vibrational bands is reported.