Aminopolymer Mobility and Support Interactions in Silica-PEI Composites for CO₂ Capture Applications: A Quasielastic Neutron Scattering Study

Abstract

Composite gas sorbents, formed from an active polymer phase and a porous support, are promising materials for the separation of acid gases from a variety of gas streams. Significant changes in sorption performance (capacity, rate, stability etc.) can be achieved by tuning the properties of the polymer and the nature of interactions between polymer and support. Here we utilize quasielastic neutron scattering (QENS) and coarse-grained molecular dynamics (MD) simulations to characterize the dynamic behavior of the most commonly reported polymer in such materials, poly(ethylenimine) (PEI), both in bulk form and when supported in a mesoporous silica framework. The polymer chain dynamics (rotational and translational diffusion) are characterized using two neutron backscattering spectrometers that have overlapping time scales, ranging from picoseconds to a few nanoseconds. Two modes of motion are detected for the PEI molecule in QENS. At low energy transfers, a "slow process" on the time scale of ∼200 ps is found and attributed to jump-mediated, center-of-mass diffusion. A second, "fast process" at ∼20 ps scale is also found and is attributed to a locally confined, jump-diffusion. Characteristic data (time scale and spectral weight) of these processes are compared to those characterized by MD, and reasonable agreement is found. For the nanopore-confined PEI, we observe a significant reduction in the time scale of polymer motion as compared to the bulk. The impacts of silica surface functionalization and of polymer fill fraction in the silica pores (controlling the portion of polymer molecules in contact with the pore walls), are both studied in detail. Hydrophobic functionalization of the silica leads to an increase of the PEI mobility above that in native silanol-terminated silica, but the dynamics are still slower than those in bulk PEI. Sorbents with faster PEI dynamics are also found to be more efficient for CO capture, possibly because sorption sites are more accessible than those in systems with slower PEI dynamics. Thus, this work supports the existence of a link between the affinity of the support for PEI and the accessibility of active sorbent functional groups.

Performance of a silica-polyethyleneimine adsorbent for post-combustion CO2 capture on a 100 kg scale in a fluidized bed continuous unit

“…Between 107 to 122 h, the inlet H2O concentration to the sorption reactor was 8.2 vol.% where Table 1 lists the condition used (nos. [25][26][27][28] As the temperature was increased to 50, 55, 61, 65 and 70 o C, the sorption performance of sorbents did not change to a significant extent, unlike under dry conditions. Between 50 and 70 o C with wet gas, the CO2 removal efficiency and a dynamic sorption capacity were above 80% and 6.5 wt.%, respectively.…”

“…Compared with K-based solid sorbents, amine-based ones have been known to be promising as they can reduce the regeneration energy because of its lower heat of adsorption and high dynamic sorption capacity. Among amine-based sorbents, silica-polyethyleneimine (PEI) has been widely developed by Sayari and coworkers [12][13][14][15][16][17][18], Jones and coworkers [19][20][21][22][23][24][25], Choi and coworkers [26][27][28][29][30][31] and Snape and coworkers [32][33][34][35][36]. Sayari and coworkers [16][17][18] and Jones and coworkers [22][23][24][25] reported the largest single body of work on the application of amine-grafted silica materials for CO2 adsorption.…”

“…Among amine-based sorbents, silica-polyethyleneimine (PEI) has been widely developed by Sayari and coworkers [12][13][14][15][16][17][18], Jones and coworkers [19][20][21][22][23][24][25], Choi and coworkers [26][27][28][29][30][31] and Snape and coworkers [32][33][34][35][36]. Sayari and coworkers [16][17][18] and Jones and coworkers [22][23][24][25] reported the largest single body of work on the application of amine-grafted silica materials for CO2 adsorption. Choi and coworkers hypothesized that the high propensity of PEI to oxidative degradation may be associated with the occurrence of transition metal impurities within the polymer.…”

Exploring the Acid Gas Sorption Properties of Oxidatively Degraded Supported Amine Sorbents

“…showing an average pore diameter of 81 Å, all consistent with the previous literature. [15,19,20] SBA-15 was then impregnated with both branched 800 MW PEI (henceforth known as PEI-SBA), and fully methylated branched 800 MW PEI (henceforth known as MePEI-SBA). The latter was synthesized as stated above, and its phase purity was confirmed through 1 H NMR ( Figure S2), showing excellent agreement with previous work by Sayari et al [24] When discussing the physicochemical properties, we present our findings per gram of SBA (gSBA -1 ), to normalize the sorbent to equivalent quantities to allow more insightful structural comparisons.…”

“…The reduction in CO2 uptake with more dilute feedstocks is likely due to less frequent CO2-amine interactions. [19,20] Further, less sites can initiate the stronger CO2-amine interactions required to selectively capture CO2 at low concentrations (> 60 kJ mol -1 ). [15,40] The CO2 uptake for 400 ppm CO2 between PEI-SBA and PEI90-SBA decreased by 29 % (from 0.71 mmol g -1 to 0.52 mmol g -1 ); however, the organic content only decreased by 15 % (from 0.48 g gSBA -1 to 0.41 g gSBA -1 ).…”