The equilibrium CO2 uptake of each silylamine under 1 bar of CO2 at 25 °C was measured by gravimetry (Table 1). The CO2 uptake is reported both in moles of CO2 per mole of amine as well as moles of CO2 per kilogram of solvent. The former is useful to discern stoichiometric effects, whereas the latter is more useful to judge industrial viability compared to other solvent systems.
On average, the silylamines summarized in Table 1 exhibit an equilibrium CO2 capacity of 0.59 moles of CO2 per mole of amine. This combines CO2 absorption from the chemical reaction of CO2 with the silylamine and physical absorption of CO2 into the newly formed ionic liquid. Previous silylamine studies have shown that under 1 bar of CO2 at 25 °C, physical absorption contributes approximately 0.01–0.02 moles of CO2 per mole of amine to the overall CO2 capacity [0.59 moles of CO2 per mole of 3-(aminopropyl)tripropylsilane) (TPSA)].13 The majority of the CO2 uptake is the result of the chemical reaction of CO2 with the silylamine. The conventional stoichiometric CO2 capacity of an amine that reacts with CO2 to form ammonium-carbamate ion pairs is 0.5 moles of CO2 per mole of amine. However, our silylamines exhibit an enhanced CO2 capacity. An in-depth investigation of the chemical reaction of CO2 with the silylamines has shown that a stabilized carbamic acid species is formed amidst a network of ammonium-carbamate ion pairs (Figure 1).13 The results established that the theoretical CO2 capacity for the chemical reaction was in fact 0.67 moles of CO2 per mole of amine. Several of the silylamines (TEtSA, αMe-TEtSA, STEtSA, and SDMESA) discussed herein approach this limit (Table 1).
Variation of the distance between the amine and the Si atom does not adversely affect the CO2 capacity until the tether is increased to a butyl chain (Table 1). TEtSBA forms a low-melting solid immediately upon reaction with CO2; the comparatively lower equilibrium CO2 uptake (i.e., incomplete reaction) observed is likely the result of occlusion of the silylamine within the solid carbamate salt. Otherwise, the variation of the linker chain from methylene to ethylene and propylene did not alter the CO2 uptake significantly; a deviation of only 0.02 moles of CO2 per mole of amine is observed between TEtSMA, TEtSEtA, and TEtSA.
Compared to TEtSA, the inclusion of a single methyl group in the α position to the amine (αMe-TEtSA) does not affect the CO2 capacity; both TEtSA and αMe-TEtSA show a CO2 uptake of 0.63 moles of CO2 per mole of amine. However, the introduction of a second methyl group at the α position (α,αDMe-TEtSA) decreases the CO2 capacity on a molar basis by 27 %. The decreased CO2 uptake is a result of the increased steric hindrance at the reactive amine site, which lowers the stability of the carbamate species formed upon reaction with CO2. Interestingly, the inclusion of a methyl group in the β position relative to the amine (βMe-TEtSA) causes a slight decrease in the CO2 capacity; βMe-TEtSA has a capacity of 0.59 moles of CO2 per mole of amine compared to 0.63 moles of CO2 per mole of amine for TEtSA.
Unsaturation along the alkyl chain backbone between the Si atom and amine has little effect on the CO2 uptake compared to its saturated analogue. Unsaturated trans-TEtSA exhibits a CO2 capacity of (0.61±0.03) moles of CO2 per mole of amine, whereas saturated TEtSA exhibits a similar CO2 capacity of (0.63±0.01) moles of CO2 per mole of amine. Unsaturated trans-α,αDMe-TEtSA exhibits a CO2 capacity of (0.47±0.03) moles of CO2 per mole of amine, whereas saturated α,αDMe-TEtSA shows a CO2 capacity of (0.46±0.01) moles of CO2 per mole of amine. An additional unsaturated silylamine with a longer alkyl chain around the Si atom, trans-α,αDMe-TPSA, was also investigated and its CO2 uptake was slightly lower than that of the other unsaturated silylamines at 0.40 moles of CO2 per mole of amine.
The secondary silylamine STEtSA exhibits a CO2-uptake capacity of (0.65±0.01) moles of CO2 per mole of amine, which is on a par with that of its primary amine analogue TEtSA ((0.63±0.01) moles of CO2 per mole of amine). The secondary silylamine SDMESA exhibits a significantly higher CO2-uptake capacity (0.62 moles of CO2 per mole of amine) than its primary amine counterpart, which reached a CO2 uptake of only 0.42 moles of CO2 per mole of amine. DMESA immediately forms a solid upon reaction with CO2, whereas SDMESA does not.
The CO2 uptake at 40 °C was investigated for a select group of silylamines to evaluate the effect of temperature on the CO2 absorption (Table 2). An absorption temperature of 40 °C is representative of the flue-gas temperature of a post-combustion stream in a coal-fired power plant. Of the three silylamines investigated at 40 °C, only TEtSA showed a (modest) drop of 5 % in CO2 capacity at this elevated temperature.
Table 2. CO2 uptake capacity at 40 °C for a select group of silylamines.
|Silylamine||CO2 uptake at 40 °C|
| ||[mol molamine−1]||[mol kgamine−1]|
The reversal temperature is the point at which the reversible ionic liquid releases CO2 and reverts back to the silylamine. We determined the reversal temperature of the reversible ionic liquids experimentally by using differential scanning calorimetry (DSC). The temperatures of the reversal events were determined by the intersection of the baseline of the event and the tangent to the peak of the event (see Supporting Information for an example thermogram).
The lower the temperature of reversal, the smaller the amount of energy required to heat the reversible ionic liquid from the capture temperature to the point at which the CO2 is released. Of the structural modifications investigated, the effect of branching along the alkyl chain backbone, unsaturation of the propyl backbone, and the order of the amine (1° or 2°) showed the strongest influence on reversal temperature (Table 3). The reversal temperature of TEtSA is included as a baseline. These experimental results are in agreement with the trends predicted previously for the reversal temperatures of these ionic liquids by the quantum-chemical approach COSMO-RS.12
Table 3. Reversal temperature of reversible ionic liquids influenced by structural modifications.
|Silylamine||Reversal temperature [°C]||Silylamine||Reversal temperature [°C]|
|DMESA||80±8[e]|| || |
We examined the effect of the alkyl tether between the Si atom and the amine and found that decreasing the alkyl chain length from three C atoms (TEtSA) to two C atoms (TEtSEtA) results in the formation of a solid that melts at 49 °C. The reversal temperature increases by almost 40 °C. Unlike TEtSEtA, the additional reversible ionic liquids presented in Table 3 show a decrease in the reversal temperature from that of the TEtSA base comparison.
The introduction of a single methyl group, α (αMe-TEtSA) or β (βMe-TEtSA), to the amine results in a reduction of the reversal temperature of 19 and 14 °C, respectively. The introduction of two methyl groups α to the amine results in a further reduction in the reversal temperature as shown for α,αDMe-TEtSA (43 °C). As capture conditions dictate a flue-gas temperature of 40 °C, a reversal temperature at or below 40 °C would result in incomplete conversion to the reversible ionic liquid. Although the reversal temperature of α,αDMe-TEtSA is low, it evidences our ability to affect the reversible ionic liquid properties significantly by altering the silylamine structure.
The reversal temperatures of reversible ionic liquids with an unsaturated propyl chain (trans-TEtSA, trans-α,αDMe-TEtSA, trans-α,αDMe-TPSA) between the amine and Si atom were also investigated. A mixture of cis and trans unsaturated silylamines were initially proposed, however, only the trans isomers were synthesized. These unsaturated reversible ionic liquids show reversal temperatures that are over 20 °C less than those of their saturated counterparts. We postulate that this must be related to the entropic effect that results from the inflexible, locked conformation of the unsaturated reversible ionic liquids.
The reversal temperature of the secondary amines decreases significantly (30, 37 °C) compared to that of TEtSA (71 °C). Unlike the branched amines, both STEtSA and SDEMSA exhibit capacities equal to the corresponding primary amine at 25 °C. However, the capture conditions must be carefully balanced because these silylamines may not reach full conversion to the reversible ionic liquid at 40 °C.
Enthalpy of Regeneration
The enthalpy of regeneration is the primary contributor to the overall energy required to release absorbed CO2 and regenerate the silylamine. The structural modifications presented here show both positive and negative deviations from the enthalpy of regeneration of the baseline silylamine TEtSA (Table 4).
Table 4. Enthalpy of regeneration of reversible ionic liquids influenced by structural modifications.[a]
|Silylamine||Heat of regeneration [kJ mol−1]||Silylamine||Heat of regeneration [kJ mol−1]|
|DMESA||130±14[f]|| || |
The enthalpies of regeneration for the reversible ionic liquids were calculated from the DSC thermograms. Heat supplied to the system [kJ mol−1] was calculated based on the gravimetric uptake of CO2.
An increase of the length of the alkyl tether between the amine and Si atom (TEtSBA) results in a significant increase in the heat of regeneration (152 kJ mol−1) with an enthalpy of regeneration almost double that of TEtSA (83 kJ mol−1). This may be a result of intramolecular interactions, which result in the stabilization of the ammonium-carbamate and carbamic acid species.13
As presented in the CO2 capacity section, the introduction of two methyl groups in the α position to the amine (α,αDMe-TEtSA) results in the incomplete conversion of the silylamine to the reversible ionic liquid. This is likely because of steric hindrance, which causes destabilization of the carbamate ion.14 As a result, the enthalpy of regeneration should be lower, although it appears to be higher than expected (114 kJ mol−1) compared to TEtSA. However, because of the decreased equilibrium CO2 uptake of α,αDMe-TEtSA, the enthalpy cannot be directly compared to that of its unbranched analogue (TEtSA); the enthalpies are calculated based on their respective equilibrium CO2 uptake. The unsaturated amines, with branching in the α position to the amine (trans- α,αDMe-TEtSA, trans- α,αDMe-TPSA) show a decrease in the enthalpy of regeneration of 18 and 62 °C, respectively.
Reversible ionic liquid viscosity
The viscosities of the reversible ionic liquid species formed after reaction with 1 bar of CO2 at 25 °C (at equilibrium conversions) are reported in Table 5. We recognize that some of the viscosities of the reversible ionic liquids reported at 25 °C are not industrially viable. Nevertheless, several important points must be emphasized: 1) viscosities at equilibrium conversion at 25 °C are important to characterize the structure–property relationships of the reversible ionic liquids, 2) industrial processes are usually conducted at temperatures higher than 25 °C (commonly 40 °C), which further reduces the viscosity of the reversible ionic liquid, and 3) in the real world it is not necessary for the CO2 uptake to go to the full equilibrium CO2 uptake, which allows an opportunity to control the viscosity of the system (see below).
Table 5. Reversible ionic liquid viscosity of the silylamines after reaction with 1 bar CO2 measured at 25 and 40 °C.
| ||25 °C||40 °C|
|variation of the alkyl chain length between the Si atom and amine|
|branching along the alkyl chain backbone|
|unsaturation in the alkyl chain backbone|
The results given in Table 5 demonstrate that the manipulation of the silylamine structure can affect the reversible ionic liquid viscosity. The reversible ionic liquid viscosities reported here vary from <100 to over 6000 cP at 25 °C.
The length of the alkyl tether between the amine and Si atom has a significant effect on the reversible ionic liquid viscosity. With only a methylene linker, the reversible ionic liquid viscosity is in the order of 2000 cP, whereas the reversible ionic liquid forms a solid with the longest linker (butylene). We anticipate that at intermediate lengths (ethylene and propylene) a trend of increasing reversible ionic liquid viscosity would be observed.
However, although TEtSA, which has a propylene linker, falls within the viscosity range of TEtSMA and TEtSBA, the reversible ionic liquid that has an ethylene linker, TEtSEtA, forms a solid after complete reaction with CO2. It is not clear why TEtSEtA does not follow the viscosity trend of the other reversible ionic liquids. The trend exhibited by TEtSMA, TEtSA, and TEtSBA may be explained by the proximity to the Lewis acidic Si atom.
The branching along the alkyl chain backbone of the silylamine has a significant effect on viscosity. With an added methyl group in the α position to the amine (αMe-TEtSA), the reversible ionic liquid viscosity increases slightly (6915 vs. 6088 cP). However, when two methyl groups are in the α position (α,αDMe-TEtSA), the viscosity decreases to 1257 cP at 25 °C. The decrease in viscosity is likely because of the decreased CO2 uptake. Unreacted silylamine (viscosity<100 cP) could still be present. A 1000 cP decrease is seen in the reversible ionic liquid viscosity of βMe-TEtSA compared to that of TEtSA.
Unsaturation in the alkyl chain backbone causes a 36 % decrease in the reversible ionic liquid viscosity at 25 °C for trans-TEtSA compared to the saturated reversible ionic liquid counterpart. A significant reversible ionic liquid viscosity decrease is also seen in trans-α,αDMe-TEtSA (<100 cP) versus its saturated analogue α,αDMe-TEtSA (1257±338 cP).15 We found a similarly low reversible ionic liquid viscosity for trans-α,αDMe-TPSA. Notably, these two silylamines have a lower equilibrium CO2 capacity than the other silylamines as they do not reach their full theoretical CO2 capacity (0.67 moles of CO2 per mole of amine) at 25 °C.
Although the secondary silylamine STEtSA does not exhibit a decreased CO2 capacity relative to its TEtSA analogue, it has a markedly lower viscosity. At (135±14) cP, the reversible ionic liquid viscosity of STEtSA is nearly 98 % lower than that of TEtSA. Similarly, although DMESA forms a solid upon reaction with CO2, its secondary analogue, SDMESA has an ionic viscosity of only (117±16) cP.
At 40 °C, the viscosity of the reversible ionic liquid drops by over 80 % on average (excluding those for which measurements are <100 cP or form solids). For example, at 25 °C, the viscosity of TEtSMA is (2373±206) cP, whereas the viscosity is only (625±58) cP at 40 °C. As long as the reversal temperature is above 40 °C, a significant decrease in CO2 capacity is not expected.
Control of the reversible ionic liquid viscosity
A unique feature of the reversible ionic liquid systems discussed herein is that the viscosity varies with CO2 uptake following a hockey-stick-like relationship (Scheme 10). For 3-(aminopropyl)tripropylsilane (TPSA), the viscosity of the partially converted solvent (which contains both silylamine and reversible ionic liquid) is 620 cP at a CO2 uptake of 0.45 moles of CO2 per mole of amine at 25 °C.11 However, with only a slight increase in CO2 uptake to 0.50 moles of CO2 per mole of amine, the viscosity increases by over 50 % to 1528 cP.
This curve is even more valuable for the results obtained at 40 °C (Figure 2). Compared to the 25 °C curve, it is immediately apparent that at 40 °C higher CO2 uptakes can be achieved at lower viscosities. For example, at 25 °C with 0.45 moles of CO2 per mole of amine, the viscosity of TPSA is 620 cP. If measured at 40 °C, a viscosity of 620 cP is not reached until an uptake that nears 0.52 moles of CO2 per mole of amine. Notably, at a CO2 uptake of 0.39 moles of CO2 per mole of amine, the viscosity is only 144 cP—a viscosity that nears industrial viability.
A plot of CO2 uptake versus viscosity for the branched amine α,αDMe-TEtSA at 25 °C is shown in Figure 3. Although α,αDMe-TEtSA does not reach the same CO2 uptake as TPSA at 25 °C, the curves are similar in shape. The α branching affects only the end point; although the equilibrium CO2 uptake is lower, the viscosity exhibits behavior similar to that of TPSA prior to the end point. At higher conversions, α,αDMe-TEtSA deviates slightly with a sharper rise in viscosity. For example, at 0.45 moles of CO2 per mole of amine, the viscosity of α,αDMe-TEtSA is nearly 1500 cP, whereas TPSA does not reach a similar viscosity until 0.50 moles of CO2 per mole of amine.