Infrared Study of CO₂ Sorption over “Molecular Basket” Sorbent Consisting of Polyethylenimine-Modified Mesoporous Molecular Sieve

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Abstract

An infrared study has been conducted on CO2 sorption into nanoporous CO2 “molecular basket” sorbents prepared by loading polyethylenimine (PEI) into mesoporous molecular sieve SBA-15. IR results from DRIFTS showed that a part of loaded PEI is anchored on the surface of SBA-15 through the interaction between amine groups and isolated surface silanol groups. Raising the temperature from 25 to 75 °C increased the molecular flexibility of PEI loaded in the mesopore channels, which may partly contribute to the increase of CO2 sorption capacity at higher temperatures. CO2 sorption/desorption behavior studied by in situ transmission FTIR showed that CO2 is sorbed on amine sites through the formation of alkylammonium carbamates and absorbed into the multiple layers of PEI located in mesopores of SBA-15. A new observation by in situ IR is that two broad IR bands emerged at 2450 and 2160 cm−1 with CO2 flowing over PEI(50)/SBA-15, which could be attributed to chemically sorbed CO2 species on PEI molecules inside the mesopores of SBA-15. The intensities of these two bands also increased with increasing CO2 exposure time and with raising CO2 sorption temperature. By comparison of the CO2 sorption rate at 25 and 75 °C in terms of differential IR intensities, it was found that CO2 sorption over molecular basket sorbent includes two rate regimes which suggest two distinct steps: rapid sorption on exposed outer surface layers of PEI (controlled by sorption affinity or thermodynamics) and the diffusion and sorption inside the bulk of multiple layers of PEI (controlled by diffusion). The sorption of CO2 is reversible at 75 °C. Comparative IR examination of the CO2 sorption/desorption spectra on dry and prewetted PEI/SBA-15 sorbent revealed that presorbed water does not significantly affect the CO2−amine interaction patterns.

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