Understanding the Impact of Pore-Polymer Interactions on the Mobility of Poly(ethyleneimine) Confined in Mesoporous SBA-15: Quasi-Elastic Neutron Scattering Studies

Supported amines are considered a promising class of CO₂ sorbents based on their capability of capturing CO₂ from various sources such as flue gas stream from power plants (5-15% CO₂) as well as ambient air (~400 ppm). Among supported amines, poly(amines) physically impregnated within the mesopores of solid oxide supports have been extensively used for achieving high amine contents (i.e., high capacity) and physical/chemical stability (i.e., limited loss via evaporation/oxidative degradation of amine compounds).¹ Within supported amine systems, the physical properties of the poly(amines) play key roles in determining CO₂ uptake performance. In particular, the chain dynamics of poly(amines) govern diffusion rates of CO₂, which ultimately determine CO₂ sorption/desorption dynamics.² Therefore, understanding the dynamics of poly(amine) species confined in solid supports will promote designing optimal supported amines for CO₂ capture.

Herein, we study the impact of the interaction modes between the pore surface and poly(amines) with a model system comprised of branched-poly(ethyleneimine) (BPEI) supported in mesoporous SBA-15 silica. Chain mobilities of BPEI are quantitatively analyzed using quasi-elastic neutron scattering (QENS) in tandem with coarse-grained molecular dynamics (MD) simulations. The pore-polymer interactions are tuned by grafting organosilanes having different end groups to the silica surface, leading to different levels of attraction between BPEI and the pore walls of silica. Chain mobilities of BPEI undergoing four different interaction modes with pore walls are investigated. The first mode brings covalent tethering of amine sites of BPEI to pore surface via halide-amine reaction, where alkyl halides are grafted onto pore surfaces. The second interaction is acid-base and hydrogen bonding interactions based on silanols decorating the native SBA-15 silica, which strongly interact with amine groups of BPEI. The third interaction considers relatively weak interaction with pore surface and BPEI by substituting acidic silanols into basic aminoalkyl groups on the pore walls, so that pore walls interact with BPEI mainly by inter-amino hydrogen bonding. Lastly, repulsive, van der Waals interaction (via grafting aliphatic chains on pore walls) yields the weakest interaction between pore walls and BPEI. QENS (High-Flux Backscattering Spectrometer; HFBS at NIST) and MD simulation revealed that the chain mobilities of BPEI are significantly influenced by different surface groups appearing on the pore walls. The strong/transient interaction (acid-base, hydrogen bonding) due to silanol-BPEI interaction yielded the slowest motions, covalent bonding and repulsive interactions resulted in slightly increased mobility, and hydrogen bonding (aminoalkyl-BPEI; without acid-base interaction) brought significantly recovered mobility as compared to bulk PEI. We hypothesize that PEI’s mobility depends not only on the attraction level with the pore walls but also on its chain conformation in the presence of surface capping groups on the pore walls. To understand further, these results are currently being coupled with additional QENS experiments, alongside insights developed via other techniques.

References:

(1) Choi, S.; Drese, J. H.; Jones, C. W. Adsorbent Materials for Carbon Dioxide Capture from Large Anthropogenic Point Sources. ChemSusChem 2009, 2 (9), 796–854.

(2) Holewinski, A.; Sakwa-Novak, M. A.; Carrillo, J.-M. Y.; Potter, M. E.; Ellebracht, N.; Rother, G.; Sumpter, B. G.; Jones, C. W. Aminopolymer Mobility and Support Interactions in Silica-PEI Composites for CO2 Capture Applications: A Quasielastic Neutron Scattering Study. J. Phys. Chem. B 2017, 121 (27), 6721–6731.