In one or more embodiments, the improved solvent systems disclosed herein are, comprise, include, or consist of one or more amine-based systems or one or more liquid amine-based solutions (collectively referred to hereinafter as “the amine system”, “the amine systems”, “the amine solution”, or “the amine solutions”) comprising one or more absorption solvents and/or one or more amine-based solvents (collectively referred to hereinafter as “the amine solvent” or “the amine solvents”). In at least one embodiment, the amine solvents of the improved solvent systems disclosed herein may be, comprise, include, or consist of one or more cyclic amines, one or more heterocyclic amines, one or more tertiary amines, one or more triols, one or more stereo hindered cyclic amines, or a combination or blend thereof. In some embodiments, the amine solvents may be, comprise, include, or consist of one or more organic compounds having at least one heterocycle feature, one or more alcohol groups, one or more amines groups, one or more ether groups, one or more piperazine groups, or a combination thereof. In an embodiment, the amine solvents disclosed herein may be, comprise, include, or consist of one or more heterorganic compounds, such as, for example, heterocyclic amines, one or more piperazines, one or more piperazine derivatives, or a combination or blend thereof. In other embodiments, the amine solvents may be, comprise, include, or consist of at least one morpholine, at least one piperazine, at least one piperidine, at least one triethanolamine, or a combination or a blend thereof. In one or more embodiments, the amine solvents may be, comprise, include, or consist of at least one of 2,5-dimethylpiperazine, 2-methylpiperazine, homopiperazine, N-n-butylaminoethanol, N-(2-aminoethyl)piperazine, 1-methylpiperazine, 2-(dimethylamino)-2-methylpropan-1-ol, trans-2,5-dimethylpiperazine, 1-ethylpiperazine, N-(2-hydroxyethyl)piperazine, thiomorpholine, 1,4-dimethylpiperazine, 2-(4-methylpiperazin-1-yl)ethanamine, 1-amino-2-butanol, 2,5-diethylpiperazine, 2-ethylpiperazine, 1-(2-hydroxyethyl)-4-methylpiperazine, 2-butylaminoethanol, diisopropanolamine, 2-(methylamino)ethanol, 1-amino-4-methylpiperazine, or a combination or blend thereof.

In some embodiments, the CO2 of at least one gaseous stream may be absorbed by the amine solvents disclosed herein to form at least one absorption system that comprises one or more resulting products (hereinafter “the resulting product” or “the resulting products”) and at least one absorption and/or at least one capture solution (collectively referred to hereinafter as “the capture solution”). The resulting products may comprise or consist of a single or sole resulting product or a complex mixture of at least two resulting products. The single or sole resulting product may be, comprise, include, or consist of one or more stable carbamate products and/or the complex mixture of at least two resulting products may be, comprise, include, or consist of at least one stable carbamate product or one or more stable carbamate products, at least one carbonate product or one or more carbonate products, at least one bicarbonate product or one or more bicarbonate products, or a mixture or blend thereof.

In one or more embodiments, at least one governing factor for determining the formation of the resulting product of CO2 absorption by amine solvent at maximum conversion may be a novel concept of inter-site stability of carbamate and protonated sites as shown in FIG. 1. It is shown, theoretically through computational work, that as CO2 is absorbed by the amine solvents, the lone pair electrons of nitrogen atom of amine attack the electrophilic carbon atom of CO2 to form at least one zwitterion (i.e., equation 1). The at least one zwitterion then transfers the proton to at least one different site to form at least one carbamate ion (i.e., equation 2). The carbamate sites and protonated sites are intermolecularly in contact making this state to be critical or at least substantially critical to the overall stability of the carbamate product. If one or more protonated sites are relatively stable in the presence of one or more carbamate sites and vice versa, the resulting product of absorption will remain as a stable carbamate even in the presence of water or other forms of one or more OH— ions. On the other hand, if one or more protonated sites, in the presence of one or more carbamate sites are unstable or not relatively or substantially stable, or vice versa, one or more carbamate products will decompose into one or more carbonate/bicarbonate products (i.e., equation 4). It may be notably important to emphasize that the overall stability of one or more carbamate products in the capture solution may be the result of the collaboration between the relative stability of one or more protonated sites in the presence of one or more carbamate sites, and vice versa, or in other word, the “inter-site stability” of these protonated and carbamate sites. Practically, the inter-site stability may be achieved by structurally modulating site structure to designate a fixed role for specific molecular site type, one of which is more prone to protonation, and the other is more prone to carbamate ion formation as shown in FIG. 2. This novel concept of inter-site stability of one or more carbamate sites and one or more protonated sites provides or brings forth an opportunity to modify the amine systems, which form unstable carbamate, into amine systems that only form stable carbamate. Through structural modulation, one or more geometrical properties and/or one or more chemical properties of one or more protonated sites and one or more carbamate sites may be simultaneously tuned in such a manner that the one or more geometrical properties and/or the one or more chemical properties may collaboratively stabilize the carbamate products. In some embodiments, the one or more geometrical properties and/or the one or more chemical properties may be, comprise, include, or consist of basicity, steric hindrance, nucleophilicity, or a combination thereof. It may be worth noting that even though one or more amine systems follow at least one zwitterions mechanism, other amine solvents, especially when amine molecules are small enough, may also participate in at least one intermolecular reaction to form carbonate/bicarbonate.

In some embodiments, the present methods disclose herein comprise at least two scenarios or chains of events to exemplify applications of the novel concept of inter-site stability of one or more carbamate sites and one or more protonated sites in improving chemistry of capture amine solvents starting from those, producing carbonate/bicarbonate at equilibrium conversion as shown in FIG. 3. Based on the principle of inter-site stability or role designation, the improved solvent systems disclosed herein may induce the sole formation of stable carbamate as the amine solution is fully converted. In a first scenario or chain of events, the induction of the stable carbamate product may be provided, achieved, and/or completed through at least one intra-molecular structure modulation. In the second scenario or chain of events, at least one inter-molecular structure modulation may provide and/or induce the stability to achieve or produce the stable carbamate product.

In the first scenario or chain of events, at least one amine solution containing 2,5-dimethyl-piperazine in water as a diluent is provided as shown in FIG. 4. At the low amine conversion, the dominant product is carbamate, while at higher conversion, the formed carbamate decomposes into bicarbonate. In this amine solution, both of the protonated site and the carbamate site are structurally equivalent, in the form of NH₂⁺(CHCH₃)CH₂and NCOO⁻(CHCH₃)CH₂, respectively. Both of the protonated site and the carbamate site may be exposed to some steric effect by a methyl group at the alpha position. The equivalency of the protonated and carbamate sites makes the carbamate product less stable. In order to prohibit the decomposition of carbamate, inter-site stability of both of the protonated and carbamate sites may be improved. There are at least two methods or processes to intra-molecularly modulate the structure disclosed herein. A first method or process may be to impose a greater steric hindrance on the protonated site at the same time removing the steric hindrance on the carbamate site. The modulation may be achieved by adding a methyl group to the alpha position for the protonated site, while removing the methyl group at the alpha position for the carbamate site. This modification may enhance or substantially enhance the preference of NH(CHCH₃)₂site to be protonated as compared to NH(CH₂)₂site, which may be preferable to be a carbamate site. As a result, at maximum CO2 loadings, the formed carbamate is stabilized even in the presence of water. A second method or process to improve carbamate stability through structural modification starting with solution of 2,5-dimethyl-piperazine may be to change, convert, or turn one nitrogen site (hereinafter “N site”) into a tertiary amine site, which may be designated as a protonated site by moving a methyl group from the alpha position to a 1-position, while removing the steric hindrance on the other N site, which may serve as a carbamate site. Consequently, at equilibrium conversion, the resulting carbamate product may be stabilized with NH⁺(CH₂)₂CH₃site to be the protonated site while NH(CH₂)₂holds CO2 as the carbamate site. In both modifications, one site becomes preferable to be protonated as compared to the other and the other site becomes for prone toward holding CO2 as carbamate. Such role designation stabilizes the inter-site interaction in the capture solution, thus promoting the overall stability of the carbamate product. In the second method or process, both of the protonated site and the carbamate site are intra-molecularly connected in the same amine molecule.

In one or more embodiments, a different method or process may be utilized with 2,5-dimethyl-piperazine to illustrate the inter-molecular structure modulation as shown in FIG. 5. The two N sites of 2,5-dimethyl-piperazine are equivalent and exposed to weak steric effect. Instead of changing, converting, or tuning one of these N sites, a new molecular site may be added into the improved solvent system. In some embodiments, the new molecular site to be added may be NH(CH₂)₂of morpholine, with considerably lower basicity or lower pKa of conjugated acid. With lower basicity and less steric exposure, the NH(CH₂)₂site of morpholine may be more prone to hold CO2 and form the carbamate ion while the NH(CH₂)₂CH₃sites of 2,5-dimethyl-piperazine may be more preferable for protonation. As a result, the aqueous mixture of morpholine and 2,5-dimethyl-piperazine with a molar ratio of about 2:1 may form at least one stable carbamate product. In one or more embodiments, the molar ratio of morpholine to 2,5-dimethyl-piperazine may be from about 0.5:1 to about 3:1, from about 1:1 to about 3:1, from about 1:1 to about 2:1, about 0.5:1, about 1:1, about 2:1, or about 3:1.

In some embodiments, an aqueous piperidine system and aqueous triethanolamine is provided or utilized as shown in FIG. 5. While a solution of piperidine may absorb CO2 aqueous to form unstable carbamate, which may decompose into bicarbonate at maximum loadings, triethanolamine may or may only react with CO2 to form bicarbonate. Like in the first method or process, the resulting carbamate product may be stabilized by applying the concept of inter-site stability, but through the intermolecular modulation in this embodiment. The NH(CH₂)₂site of piperidine is highly basic, thus strongly prefers to be protonated. Similarly, triethanolamine is a tertiary amine, which cannot accommodate CO2 binding. The reasonable inter-site stability induction may be addition of a new molecular site that is preferable to hold CO2 as a carbamate ion. Each starting amine solvent may be mixed with piperazine, whose NH(CH₂)₂sites are medium basic and less exposed to steric effect, with the molar ratio of about 2:1. In both resulting capture solutions, carbamate NCOO⁻(CH₂)₂sites of piperazine are stabilized in the presence of a NH⁺(CH₂)₂site of the triethanolamine and in the presence of the NH₂⁺(CH₂)₂site of the piperidine. This enhanced inter-site stability promotes, substantially promotes, or only promotes selectivity toward formation of carbamate at maximum conversion. Through examples disclosed herein, it is demonstrated that, the principle of inter-site stability of the protonated sites and the carbamate sites may play at least one role in determining the selectivity of the final absorption or resulting products and/or may be flexibly applied in an amine structure designing phase to target resulting products that are, include, comprise, or consist of stable carbamate products.

The improved solvent systems and methods disclosed herein may comprise an engineering-scale development of absorption technology that may be based on one or more multiple aspects including, for example, but not limited to predictive process control, rigorous process monitoring, or a combination thereof. However, the one or more multiple aspects may be hindrance because the underlying challenges behind this hindrance may be a lack or misunderstanding of mechanistic insights to identify potential amine systems, whose product distribution, with fixed physical/chemical properties, is stable under process operation conditions. Although it has been accepted that most neat capture solutions might form carbamate when capturing carbon dioxide, the inevitably significant amount of water in the flue gas is commonly attributed to be the main reason for the formation of carbonate/bicarbonate or even complex mixtures of these products. In this disclosure, a new concept of inter-site stability is introduced to rationalize the capture product formation as well as to effectively guide amine solvent design based on chemical structure of the molecule with the amino sites. The fundamental mechanism of this inter-site stability is experimentally and computationally substantiated by an observed “hopping” phenomenon of proton and carbamate between different amino sites in the amine solution. To demonstrate the application of this mechanistic understanding, some amine systems, forming unstable carbamate as product in the presence of water, are also modified based on the inter-site stability rationale to induce higher product stability and uniformity.

EXAMPLES

In a first example, an aqueous solution of morpholine contacts CO2 and only carbamate is formed. This is evidenced, as shown in FIGS. 6 and 7, by the presence of C═O asymmetric, and symmetric stretching bands at 1530 cm⁻¹, 1422 cm⁻¹, respectively, as well as N—COO— stretching band at 1274 cm⁻¹. It is worth noting that, as the CO2 loading increases, the C—N stretching band at 1097 cm⁻¹shifts to 1104 cm⁻¹and increases strongly in intensity. Such behavior may be assigned to the protonation of NH(CH₂)₂amino site of morpholine. At theoretically maximal CO2 loadings, 2 moles of morpholine are required to capture 1 mole of CO2 as carbamate product. When neat 2,5-dimethyl-piperazine is added to the fully converted solution of morpholine, most carbamate product featuring bands experience negligible change except for the C—N band of protonated amino site at 1104 cm⁻¹, whose intensity is notably reduced. The intensity reduction of C—N band at 1104 cm⁻¹suggests the consumption of protonated NH(CH₂)₂amino sites of morpholine. As the new blend of amino sites solution absorbs more new molecules of CO2, signature bands of carbamate, including C═O asymmetric, and symmetric stretching band, as well as N—COO— stretching band from morpholine are observed to increase in intensity. The increase in the carbamate bands by morpholine corresponding to CO2 absorption in the second reaction stage indicates the availability of NH(CH₂)₂amino sites on morpholine. This supports the hypothesis that proton from NH₂⁺(CH₂)₂of morpholine “hops” to 2,5-dimethyl-piperazine, as demonstrated in FIG. 8, which then opens more amino sites for more carbamate formation. Besides, the asymmetrical stretching band of carbamate formed from 2,5-dimethyl-piperazine is not observed. This absence of carbamate formed by added amine implies that all newly formed carbamate ions are held by amino site of morpholine. The preference in protonation of specific amino site in this case of study may be assigned to the difference in the basicity of nitrogen lone pair. Comparing the basicity of amino sites, the pKa of amino site of morpholine is less than that of 2,5-dimethyl-piperazine. Thus, it is more preferable for 2,5-dimethyl-piperazine to be protonated in the mixed solution with morpholine.

In a second example, the same amine blending containing 2,5-dimethyl-piperazine and morpholine is utilized, but in reversal experimental order was studied to shed more light on the site transfer as shown in FIGS. 9 and 10. When the aqueous solution of 2,5-dimethyl-piperazine absorbs carbon dioxide at low loading, only carbamate product is detected by IR spectroscopy with the fingerprint of asymmetric C═O stretching appears at 1518 cm⁻¹as shown in FIG. 9. Surprisingly, as morpholine is slowly added to the reacted solution of 2,5-methyl-piperazine, the C═O stretching band initially drops in intensity and shifts to higher wavenumber at 1530 cm⁻¹afterward. The emergence of the peak at 1530 cm⁻¹coincides with the signature band of carbamate formed by morpholine. This observation strongly illustrates that in the capture solution, the carbamate site spontaneously “hops” from 2,5-dimethyl-piperazine to morpholine amino site. From the structural viewpoint, carbamate formed by 2,5-dimethyl-piperazine are under stronger steric hindrance, caused more by nearby methyl groups, than morpholine. Consequently, the NH(CH₂)₂amino site of morpholine is relatively preferable to hold the carbamate moiety in the mixed solution with 2,5-dimethyl-piperazine as shown in FIG. 10. In some embodiments, the preference of some amino structures to be protonated or to hold carbamate moiety may be rationalized; however, the spontaneous transfer or “hop” of those moieties within an amino site blend has not been previously reported.

In one or more embodiments, such spontaneous “hopping” behavior as described herein may imply that the appearance of a more preferable type of site, as compared to the present site population in the amine solution, for protonation or carbamate moiety holding may cause at least one rearrangement of the carbamate and proton moieties. In other words, the solution, after new combination type of sites is added, is more stable. Following this line of rationale, the concept of “inter-site stability” of carbamate and protonated sites may be a governing factor for the stability of carbamate products. The inter-site stability states that within a population of amino sites, the stability of carbamate may be relative to the protonated nitrogen site and an increase in the preference distinguishment for protonation or carbamate holding between different types of sites may enhance the stability of carbamate product. The spirit of the inter-site stability may be summarized in the FIG. 11. Besides being justified from the “hopping” phenomenon described herein, the inter-site stability may be rationalized from chemistry studies. It has been shown, theoretically through computational work, that as CO2 is absorbed by the amine solvents, the lone pair electrons of nitrogen atom of amine attack the electrophilic carbon atom of CO2 to form a zwitterion. The zwitterion then transfers the proton to a different site to form carbamate ion. The carbamate and protonated sites are intermolecularly in contact making this state to be critical to the overall stability of the carbamate product. If protonated sites are relatively stable in the presence of carbamate sites and vice versa, the final product of absorption will remain as stable carbamate even in the presence of water or other forms of OH ions. On the other hand, if protonated sites, in the presence of carbamate sites are not relatively stable, or vice versa, carbamate products will decompose into carbonate/bicarbonate products. It is notably important to emphasize that the overall stability of carbamate products in the solution is the result of the collaboration between the relative stability of protonated sites in the presence of carbamate sites, and vice versa, or in other word, the “inter-site stability” of these sites. Practically, the inter-site stability may be achieved by structurally modulating site structure to designate a fixed role for specific site type, one of which is more prone to protonation, and the other is more prone to carbamate ion formation. This novel concept of inter-site stability of carbamate and protonated sites brings forth the opportunity to modify amine systems, which form unstable carbamate, into amine systems that only form stable carbamate. Through structural modulation, geometrical and chemical properties of protonated and carbamate sites such as basicity, steric hindrance, nucleophilicity, and so on may be simultaneously tuned in such a manner that they collaboratively stabilize the carbamate products. It is worth noting that even though most amine systems follow the zwitterions mechanism as mentioned above, a few amine solvents, especially when amine molecules are small enough like with MEA, also participate in termolecular reaction to form carbonate/bicarbonate.

In a further example, two scenarios are provided and examined to exemplify applications of the concept of inter-site stability of carbamate and protonated sites for improving the chemistry of capture amine solvents starting from those, producing carbonate/bicarbonate at equilibrium conversion. The improved solvent systems and methods disclosed herein will or may induce the sole formation of stable carbamate as the solution is fully converted based on the principle of inter-site stability or role designation. In a first scenario, the induction of stable carbamate product is done through the intra-molecular structure modulation, whereby, inter-molecular structure modulation induces the stability that is presented in the second scenario.

First Scenario: Inter-Sites Stability Induction by Intramolecular Site Modulation

The first scenario utilizes an amine solution containing 2,5-dimethyl-piperazine in water as a diluent. At the low amine conversion, the dominant product is carbamate. The FTIR spectra of this amine solution at low CO2 loading exhibits main evolving bands around 1520 cm