As shown in Table 4, the activation barriers are small for the direct
reaction, even in the most polar solvent (smaller than 13 kcal
mol-1). Therefore, all reactions must show a fast
kinetics. From the viewpoint of CO2 recovering after
capture, it is desirable that the reverse reactions also have a small
activation barrier. In this way, the thermodynamics of the reaction is
the determinant parameter for the feasibility of the capture process. By
selecting from Table 3 the anions with negative value for the Gibbs free
energy change (in any solvent) and activation barrier for either the
direct or the reverse reaction around 10 kcal mol-1 or
below (Table 4), which would then combine kinetics and thermodynamics
features favoring the reaction and its reverse, the following species
show up: CH3S-,
C2H5S-,
F- and CN-.
Additionally, important cases are those where changing the solvent
dielectric constant changes the spontaneity of the reaction. This was
found for F-, CN-,
CH3S- and
C2H5S-. In these
cases, we could think of a capture process in a less polar solvent and
thus CO2 recovering in a more polar one. Therefore, some
solvent gradient could be designed to combine the two processes, capture
and regeneration.
As an example of such a behavior, Figure 5 shows the profile for the
reaction between CN- and CO2. Among
all systems studied, CN- is the one which shows a
clear dependency of the transition state energy with the solvent
polarity. Curiously, as observed before, the effects of THF on the
energy values are closer to the effects of water than to those of
toluene. In this case, we additionally computed the effect of
chloroform, a solvent with a dielectric constant (ɛ = 4.81) intermediary
between that of THF and toluene. The results are also shown in Figure 6.
The values for chloroform are between those of THF and toluene, but
closer to THF.