Theoretical framework for kinetics

“There are at least two competing ways of interpreting kinetics data. Some authors employ the framework of the zwitterion mechanism, as proposed by Caplow [121] and then championed by Danckwerts and Versteeg et al. [122] among others. This framework is wildly popular, and most of the data discussed in this section has been treated following its prerogatives. Conversely, one could employ the termolecular mechanism proposed by Crooks and Donnellan [123], which also has its fair share of support [124]. Though we shall not delve into a debate regarding both frameworks, as their implications when applied to water-lean solvents are practically the same, both of them warrant a short introduction.

According to the zwitterion mechanism, the reaction between CO2 and amines follows in two steps. The first is the direct reaction between one molecule of amine and one molecule of CO2 forming a zwitterion. The second is the deprotonation of this zwitterion by a base (which can be the amine itself or any other basic species in solution) forming the amine carbamate species.AmH+CO2k2⇌k-1AmH+COO-AmH+COO-+Bkb→AmCOO-+BH+

The rate of conversion of CO2 following the zwitterion mechanism is thus given by the equation below.-RCO2=CO2·Am1k2+k-1k2·∑kb·B

Since this mechanism assumes two consecutive reactions, the final rate will depend on which step is the rate-determining one (i.e. the slowest). Therefore, the final rate will follow two asymptotic behaviors. If 1/k2 ≫ k−1/(k2∙Σkb∙[B]), then the equation below holds. This is equivalent to a reaction where the zwitterion formation is the slowest step, typically due to the instability of the zwitterionic species.-RCO2≈k2·CO2·Am

Conversely, if 1/k2 ≪ k−1/k2∙Σkb∙[B], then it follows that the equation below holds.-RCO2≈k2k-1·CO2·Am·∑kb·B

This means that the conversion rate of CO2 will be between 1st and 2nd order with respect to the amine. And particularly in the case of water-lean solvents, where the possibility of the diluent itself acting as a base is usually quite remote, Versteeg et al. [122] suggest the approximation given by the equation below.-RCO2≈kbk-1·k2·CO2·Am2

The termolecular mechanism, on the other hand, proposes that the reaction between amine and CO2 happens in one singular step, and involves three molecules at once. These three are the amine, molecular CO2, and a third that can either be the amine itself or the diluent. A loosely bonded species is formed and quickly dissociated, so that the rate determining step of this mechanism is the three-molecular complexation. In other words, the equation below holds. The products of this reaction are the amine carbamate and a protonated species, which could either be the amine or the diluent.-RCO2=kAm·CO2·Am2+kD·CO2·Am·D

Crooks and Donnellan [123] have argued that the zwitterion mechanism rate equation is overparametrized, and Da Silva and Svendsen [124] have shown that both expressions satisfactorily cover the same range of experimental data. Therefore, results expressed in the language of the zwitterion framework can be readily converted to that of the termolecular mechanism and vice-versa.

The discussion above applies for primary and secondary amines. Ternary amines cannot form carbamates, and it has been often assumed that they absorb CO2 simply be enabling the formation of bicarbonate in aqueous solutions [125].Am+H2O→AmH++OH-CO2+OH-→HCO3-

A corollary of this mechanism is that tertiary amines should not be able to chemically absorb CO2 in nonaqueous solutions. This has been supported not only by Versteeg and van Swaaij [40] but by numerous other researchers. It is a fact that reactions in nonaqueous tertiary amine solvents are severely depressed. Li et al. [96] have identified that PEG200 mixed with 30 %wt. MDEA has properties similar to those of a physical solvent. Presumably the absence of water renders the amine almost, if not wholly, unreactive. S. Chen et al. [46] saw the same for ethanol–MDEA, while Pohorecki and Mozeński [112] saw the same for PC–TEA. All three authors, however, report an increase in the solubility of CO2 with the addition of the tertiary amine. Conversely, many others were able not only to identify these reactions, but also to measure them [37][126][127].

The dismissal of tertiary amine reactions in nonaqueous solvents comes together with the assumption that alkylcarbonates cannot be formed at relevant rates during CO2 absorption, a thesis also sustained by Versteeg and van Swaaij [40]. And yet, in recent years, several studies based on NMR spectroscopy have come out supporting the existence of alkylcarbonates in water-lean solvents [42][128][129][130], with one study by Behrens et al. [54] even proving that alkylcarbonates are present in CO2-loaded aqueous MEA. A good summary on recent findings is present in Cieslarova et al. [131]. Alkylcarbonate formation seems to be a promising thesis to explain the reactivity of tertiary amines in water-lean solvents.”

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