The capability of 1 H and 13 C flow magic-angle spinning NMR is demonstrated for monitoring the capture of CO 2 on the surface of mesoporous amine-functionalized silica and the progress of heterogeneously catalyzed CO 2 hydrogenation over the microporous titanosilicate Pt/ETS-10 under in situ conditions. The custom-built gas-handling system allows to maintain a controllable and stable delivery of reaction gases to the sample compartment containing porous material over a period of hours. Using calibration of NMR signals, we obtained the amounts of reactants and products. The presented approach demonstrates a complementary NMR-based in situ method for quantitative investigation of CO 2 capture and conversion over nanoporous solids.
In this contribution, the preparation of hierarchically structured ETS-10-based catalysts exhibiting notably higher activity in the conversion of triolein with methanol compared to microporous titanosilicate is presented. Triolein, together with its unsaturated analog trilinolein, represent the most prevalent triglycerides in oils. The introduction of mesopores by post-synthetic treatment with hydrogen peroxide and a subsequent calcination step results in the generation of an additional active surface with Brønsted basic sites becoming accessible for triolein and enhancing the rate of transesterification. The resulting catalyst exhibits a comparable triolein conversion (≈73%) after 4 h of reaction to CaO (≈76%), which is reportedly known to be highly active in the transesterification of triglycerides. In addition, while CaO showed a maximum conversion of 83% after 24 h, the ETS-10-based catalyst reached 100% after 8 h, revealing its higher stability compared to CaO. The following characteristics of the catalysts were experimentally addressed – crystal structure (X-ray diffraction, transmission electron microscopy), crystal shape and size (scanning electron microscopy, laser diffraction), textural properties (N2 sorption, Hg porosimetry), presence of hydroxyl groups and active sites (temperature-programmed desorption of NH3 and CO2, 29Si magic angle spinning nuclear magnetic resonance (NMR)), mesopore accessibility and diffusion coefficient of adsorbed triolein (pulsed field gradient NMR), pore interconnectivity (variable temperature and exchange spectroscopy experiments using hyperpolarized 129Xe NMR) and oxidation state of Ti atoms (electron paramagnetic resonance). The obtained results enabled the detailed understanding of the impact of the post-synthetic treatment applied to the ETS-10 titanosilicate with respect to the catalytic activity in the heterogeneously catalyzed transesterification of triglycerides.
Xe NMR spectroscopy is applied under in situ and in operando conditions to study the mixing process in a multicomponent liquid mixture with partially miscible components. The process of mixing of an oil-methanol mixture was triggered by an industrially relevant catalytic transesterification reaction to form fatty acid methyl esters and glycerol. Up to date, kinetic limitations in liquid-phase reactions originating from the poor miscibility of the reacting species have been addressed solely under ex situ conditions, typically by chromatography. In the approach presented here, xenon gas, solvated in the reacting species, acts as a sensor, providing information on the progress of mixing and on the composition during the course of the catalytic reaction. We believe that this study offers a new tool to the set of established techniques for addressing mixing and/or separation processes in liquids, including but not limited to the ones resulting from catalytic reactions.
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