Fluorescent Probe of Aminopolymer Mobility in Bulk and in Nanoconfined Direct Air CO₂ Capture Supports

Abstract

Poly(ethylenimine) (PEI) is widely recognized as an efficient carbon capture medium. When loaded onto mesoporous oxide supports, the polymer becomes particularly attractive for direct air capture (DAC) applications given the high surface area of the composites, the low volatility of the polymer, and the excellent cyclability of the system. As polymer segmental mobility is coupled with CO 2 uptake and diffusion, understanding how that mobility is influenced by nanoconfinement will ultimately be critical to the development of more efficient DAC systems. Here, we discuss our development of a fluorescent probe molecule based on tetrakis(4hydroxyphenyl)ethylene. As the fluorescence intensity of this molecule and the shape of the emission spectra are strongly dependent on the viscosity of the supporting medium, doping PEI-composites with this fluorescent probe can provide sensitive indication of polymer glass transition and/or melting temperatures across a wide range of temperatures (−100 to +100 °C). Herein, we demonstrate how this molecule can be used as a ratiometric probe to study bulk PEI dynamics and confinement effects in mesoporous silica as influenced by pore functionality, polymer fill fraction, and polymer architecture.

Fundamental Insight into Humid CO2 Uptake in Direct Air Capture Nanocomposites Using Fluorescence and Portable NMR Relaxometry

“…We previously detailed the merits of employing fluorescent probes based on tetraphenylethylene (TPE) in a proof-of-concept study for qualitatively comparing aminopolymer mobility across confined mesoporous DAC composites. 35 Here, we develop these probes further to study branched PEI mobility in model DAC systems as a function of real-world operating conditions, beginning with moisture uptake as a function of RH exposure. PEI composites with γ-Al 2 O 3 are quite hygroscopic; 24 a composite that is 40 wt % PEI can uptake as much 1.7 equivalents in weight of water relative to PEI when allowed to equilibrate in a stream of humid N 2 (vide inf ra).…”

“…Correll et al recently demonstrated that THPE emits near 460 nm and appears blue in a frozen/glassy/ aggregated state, while it appears green and emits closer to 530 nm in a viscous PEI solution. 35 While TPE derivatives are much more commonly employed as AIE rather than ratiometric probes, that initial work demonstrated how a ratiometric analysis of the emission of THPE could be used to accurately assess aminopolymer and small-molecule melting points and glass transitions. The efficacy of the probe was benchmarked in 12 systems with melting points spanning ∼200 °C, both in the bulk and in confined mesoporous oxides.…”

“…Various fluorescent probes have been employed to study polymer mobility and have proven particularly useful for studying mobility in thin films and in confinement where other techniques encounter limitations. − We recently detailed the development of a powerful fluorescent probe designed specifically to study aminopolymer mobility, and we demonstrated its usefulness in studying DAC sorbents . Here, we build on that foundational study by examining polyethylenimine (PEI) mobility as a function of CO 2 uptake across a wide range of temperatures and relative humidity (RH) in a mesoporous γ-Al 2 O 3 composite.…”

Probing the Morphology and Mobility of Amines in Porous Silica CO2 Sorbents by 1H T1–T2 Relaxation Correlation NMR

“…), the dynamic motions of the confined amines and their relationship with CO 2 uptake kinetics and capacity remain relatively poorly understood. [23][24][25]35 A primary challenge with quantifying the mobilities and structures of the amine compounds is the difficulty in characterizing their structures and morphologies due to their confinement in the porous support. Examples from the literature include a report characterizing PEI mobility under confinement in SBA-15 silica by quasielastic neutron scattering (QENS) 24 and a study probing CO 2 diffusivity within PEI/ amorphous silica sorbents via FTIR studies.…”

“…For instance, amines buried toward the pore walls are not as effective in capturing CO 2 as amines farther from the pore walls due to diffusive barriers. ,, Although the impact of the chemical structure of supported amines has been well-correlated with CO 2 uptake performance on the molecular level (e.g., the role of primary, secondary, tertiary amines; − the role of accessible vs sterically hindered amines, − etc. ), the dynamic motions of the confined amines and their relationship with CO 2 uptake kinetics and capacity remain relatively poorly understood. − , …”