Based on first-principles simulations,
we present that carbamate
formation can be kinetically more favorable than bicarbonate formation
at high stripping temperatures (>400 K) from the reaction between
CO2 and 2-amino-2-methyl-1-propanol (AMP) in aqueous solution,
while the latter tends to be predominant during CO2 capture
at low absorber temperatures (<330 K). This finding offers explanation
for the intriguing observation of oxazolidinone formation as the major
product of AMP degradation, which is known to occur via carbamate,
as also seen from thermal degradation of aqueous monoethanolamine
(MEA) in CO2 capture processes. From ab initio molecular dynamics simulations coupled with metadynamics sampling,
the free-energy barrier for carbamate formation is predicted to substantially
decrease from 11.7 to 5.5 kcal/mol with increasing temperature from
313 to 413 K in 25 wt % AMP solution whereas that for bicarbonate
formation increases from 9.6 to 12.4 kcal/mol. Likewise, the predicted
free-energy barrier for carbamate formation in aqueous MEA also decreases
with temperature but is significantly less compared to the AMP case.
Further analysis demonstrates that the increase of temperature results
in enhancing the disruption of the hydrogen bond network around the
basic nitrogen atom of AMP (or MEA), allowing more facile CO2 access to form carbamate. Our work provides new insight on the strong
temperature dependence of the CO2 capture mechanism and
kinetics in aqueous solutions of amines, arising from changes in the
hydrogen bond structure and dynamics around amines.
This study attempts to explain the well-known experimental observation that 1,3-Bis(2-aminoethyl)urea (urea) is preferentially formed over the other major product, 2-imidazolidone (IZD), from thermal degradation of aqueous ethylenediamine (EDA) during...
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