(174ce) Synergistic Enhancement of CO2 Adsorption Rate and Capacity in Polyamine-Based Protic Ionic Liquids Functionalized Highly Ordered Mesoporous Silica

Synergistic enhancement of CO₂ adsorption
rate and capacity in polyamine-based protic ionic liquids functionalized highly
ordered mesoporous silica

Wei
Zhang, Yi He*, Yao Shi*

Key Laboratory of Biomass Chemical Engineering of Ministry of
Education, Institute of Industrial Ecology and
Environment, College of
Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027,
China

Abstract:

It is incontestable that global warming
and anthropogenic climate change are associated with the increase in
atmospheric CO₂ concentration due to the large-scale burning of
fossil fuels starting from the industrial revolution. The development of
cost-effective and high-efficiency sorbents for CO₂ adsorption from
flue gas is a significant challenge in last decade. Polyamine-based protic
ionic liquids (PILs) are proposed as promising alternatives compared with
conventional ILs due to low-cost, simple-synthesis and high absorption
capacity, but the high viscosity and low adsorption/desorption rate limit its practical
applications. To overcome these challenges, a series of supported
polyamine-based protic ionic liquid sorbents based on tetraethylenepentammonium
nitrate ([TEPA][NO₃]) confined into SBA-15 have been synthesized,
characterized and evaluated towards CO₂ adsorption. The hybrid
sorbent with 66 wt% PIL exhibited excellent CO₂ adsorption rate and
enhanced uptake capacity. The CO₂ uptake capacity of the sorbent is
2.15 mmol/g at 333 K and 0.15 bar, a dramatic enhancement compared to bare
support (895%). In order to better understand the adsorption kinetics and the
rate-controlling step of the sorbent, the CO₂ uptake capacity was
plotted against t^0.5 using the intra-particle diffusion model
(IPDM). The adsorption rate of rate-controlling step in the sorbent was 147*10^-3
mmol g^-1 s^-0.5, at least two times higher than previously
reported ILs-functionalized and amine-modified support systems. In addition, the
thermostability of the sorbent was comparable to other ILs-functional molecular
sieve and was better than TEPA-modified porous materials. The FT-IR analysis
demonstrated that the PIL was successfully impregnated into the support and CO₂
was captured by amine groups in the PIL forming carbamate. According to adsorption isotherms and isosteric heat,
CO₂ adsorption of the sorbent involved both physisorption and
chemisorption. In order to understand the interactions between amine groups and
CO₂ at the molecular level, the DFT method was conducted to
calculate the binding energies for CO₂ adsorption over different
amine groups. The results suggested that CO₂ preferentially reacted
with secondary amine -N(2)H to form carbamate and the BE of the carbamate was
-64.0 kJ/mol consistent with
other theoretical and our experimental results. The long-term operating
performance of the sorbent evaluated by durable cyclic adsorption/regeneration
experiment and the results showed that after 10 adsorption/desorption cycles,
nearly 90% of original CO₂ adsorption capacity of the sorbent was
restored by N₂ steam desorption at 373 K. Overall, [TEPA][NO₃]
impregnated SBA-15 sorbent provide a low-cost, high adsorption capacity
alternative with fast adsorption kinetics, and desirable regenerability. The
high efficiency was proposed from the synergistic effect of highly affinity to
CO₂ of the PIL and fast intra-particle diffusion rate. Therefore, it
may serve as a promising candidate for CO₂ capture in industrial
applications.

Keywords: CO₂
adsorption; polyamine-based protic ionic liquids; SBA-15; Nanopore confinement;
DFT calculation