Solid-liquid two-phase absorbent and CO2 trapping process

Abstract

The invention relates to the field of gas trapping, and particularly discloses a solid-liquid two-phase absorbent and a CO2 trapping process, the solid-liquid two-phase absorbent comprises 2, 6-dimethyl piperazine and water, and the concentration of the 2, 6-dimethyl piperazine is 15 wt%-40 wt%. According to the invention, water is used as a medium of the two-phase absorbent, the use of an expensive and polluting organic solvent is avoided, the environmental protection performance of the absorbent is remarkably improved, and meanwhile, the absorbent can effectively improve the CO2 absorption capacity and desorption efficiency, has the advantages of low cost, low energy consumption and high efficiency, and is suitable for large-scale industrial application.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of gas capture, in particular to a solid-liquid two-phase absorbent and a CO2 capture process. BACKGROUND

[0002] With the increasing global greenhouse gas emissions, CO2 capture technology has become one of the key technologies to combat climate change. The homogeneous absorbent mainly using monoethanolamine (MEA) is the mainstream technology currently used, which has low CO2 absorption capacity and desorption efficiency, which limits its application on an industrial scale. The solid-liquid two-phase absorbent can solve these problems to some extent, however, the current solid-liquid two-phase absorbent mostly uses organic solvents as solvents, which has high pollution and solvent cost. If a solid-liquid two-phase absorbent based on water as solvent can be developed, it is expected to solve the problems of absorption, desorption performance, energy consumption and environmental protection. SUMMARY

[0003] In order to improve the environmental performance of the absorbent, and at the same time improve its CO2 absorption capacity and desorption efficiency, the present application provides a solid-liquid two-phase absorbent and a CO2 capture process.

[0004] In a first aspect, the present application provides a solid-liquid two-phase absorbent.

[0005] A solid-liquid two-phase absorbent, the solid-liquid two-phase absorbent comprising 2,6-dimethylpiperazine and water, the concentration of the 2,6-dimethylpiperazine being 15wt%-40wt%.

[0006] As an optional implementation, in the embodiments of the present application, the concentration of the 2,6-dimethylpiperazine is 20wt%-30wt%.

[0007] As an optional implementation, in the embodiments of the present application, the ESP_H of the 2,6-dimethylpiperazine is -1.0475a.u.-1.0485a.u.

[0008] As an optional implementation, in the embodiments of the present application, in the 2,6-dimethylpiperazine, the local electrostatic potential minimum value near the N atom near the substituted methyl is -40kcal / mol, and the local electrostatic potential minimum value far from the N atom near the substituted methyl is -41.1kcal / mol.

[0009] In a second aspect, the embodiments of the present application provide a CO2 capture process.

[0010] A CO2 capture process, comprising the following steps:

[0011] Absorbing CO2, reacting with the CO2 using the solid-liquid two-phase absorbent as described in the first aspect to generate a precipitate.

[0012] As an optional implementation, in the embodiments of the present application, the precipitate comprises a double-substituted carbamate.

[0013] As an optional implementation, in the embodiments of the present application, the reaction temperature of the solid-liquid two-phase absorbent with the CO2 is 35℃-50℃.

[0014] As an optional implementation, in the embodiments of the present application, the absorption capacity of the solid-liquid two-phase absorbent is 0.9 mol / mol-amine-1 mol / mol-amine.

[0015] As an optional implementation, in the embodiments of the present application, after the step of absorbing the CO2, the CO2 capture process further comprises:

[0016] Desorption, desorbing the precipitate to regenerate the 2,6-dimethylpiperazine; in the desorption step, the energy consumption of the regeneration of the 2,6-dimethylpiperazine is 2.6 GJ / t-CO2-2.9 GJ / t-CO2.

[0017] As an optional implementation, in the embodiments of the present application, in the desorption step, the desorption temperature is 90℃-110℃, the desorption pressure is standard atmospheric pressure, and the desorption time is 20 min-35 min.

[0018] Compared with the prior art, the present application has the beneficial effects that:

[0019] The solid-liquid two-phase absorbent provided by the embodiments of the present application uses 2,6-dimethylpiperazine as the absorbent, and the specific concentration of 2,6-dimethylpiperazine reacts with CO2 to generate a double-substituted carbamate, and the solubility of the double-substituted carbamate in the aqueous solution is relatively low. With the absorption of CO2, the double-substituted carbamate gradually accumulates in the solution and precipitates to form a solid phase, thereby promoting the occurrence of solid-liquid phase separation and enhancing the absorption capacity of CO2. Moreover, the concentration of 2,6-dimethylpiperazine in the solid-liquid two-phase absorbent is controlled in the range of 15wt%-40wt%, which can produce a precipitate and maintain good mass transfer conditions in the solution, thereby continuously driving the improvement of the absorption capacity of CO2. When the concentration of 2,6-dimethylpiperazine is less than 15wt%, the precipitate produced by the reaction of 2,6-dimethylpiperazine with CO2 is not obvious, and the absorption capacity of CO2 is not improved. When the concentration of 2,6-dimethylpiperazine is greater than 40wt%, the absorption capacity of CO2 decreases, indicating that the high concentration of 2,6-dimethylpiperazine will inhibit the reaction and is not conducive to the generation of the precipitate.

[0020] The disubstituted carbamate precipitate generated by absorbing CO2 can promote the desorption of CO2 by heating or reducing the pressure. During the desorption process, the disubstituted carbamate precipitate releases CO2, and the reaction system is then regenerated to the initial liquid phase. Moreover, thanks to the higher thermal conductivity of the solid precipitate, the generation of the disubstituted carbamate precipitate in the present application promotes the improvement of the overall thermal conductivity of the system, and the desorption energy consumption is reduced accordingly. In addition, the 2,6-dimethylpiperazine that is regenerated after the disubstituted carbamate precipitate desorbs and releases CO2 is exothermic in the process of dissolving in water. This dissolution heat effect further accelerates the desorption process and reduces energy consumption. The solid-liquid two-phase adsorbent of the present application can maintain good adsorption stability without obvious performance degradation after multiple cyclic adsorption tests. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0022] Figure 1 Schematic diagram of the reaction of 2,6-dimethylpiperazine with CO2 to form a precipitate disclosed in the examples of this application;

[0023] Figure 2 This is a graph showing the cyclic adsorption test results of 2,6-dimethylpiperazine in Example 3 disclosed in this application;

[0024] Figure 3 The molecular electrostatic potential diagram of piperazine or its derivatives disclosed in this application;

[0025] Figure 4 The hydrogen atom electrostatic potential (ESP_H) diagram of piperazine or its derivatives disclosed in this application;

[0026] Figure 5 The precipitation analysis test results of Example 3 disclosed in this application are shown in FIG. 1 , wherein (a) is the carbon-13 nuclear magnetic resonance ( 13 C NMR) diagram; (b) is the mass spectrum, and (c) is the Fourier transform infrared spectroscopy (FT-IR) diagram. DETAILED DESCRIPTION

[0027] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0028] In the present application, the terms "upper", "lower", "left", "right", "front", "back", "top", "bottom", "inner", "outer", "middle", "vertical", "horizontal", "lateral", "longitudinal" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings. These terms are mainly used to better describe the present application and its embodiments, and are not intended to limit the indicated devices, elements or components to have a specific orientation, or to be constructed and operated in a specific orientation.

[0029] In addition, in addition to indicating the orientation or positional relationship, the above-mentioned partial terms can also be used to indicate other meanings, for example, the term "upper" can also be used to indicate a certain dependent relationship or connection relationship in some cases. For those skilled in the art, the specific meaning of these terms in the present application can be understood according to the specific situation.

[0030] In addition, the terms "mounting", "setting", "provided with", "connection", "connected" should be broadly understood. For example, it can be fixedly connected, detachably connected, or integrally constructed; it can be mechanically connected, or electrically connected; it can be directly connected, or indirectly connected through an intermediate medium, or internal communication between two devices, elements or components. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific situation.

[0031] In addition, the terms "first", "second" and the like are mainly used to distinguish different devices, elements or components (the specific types and structures may be the same or different), and are not intended to indicate or imply the relative importance and quantity of the indicated devices, elements or components. Unless otherwise stated, the meaning of "multiple" is two or more.

[0032] Compared with the absorbent using organic solvent, the solid-liquid two-phase absorbent using water as solvent has the advantages of more green and lower cost. However, the inventors found in the research process that the reaction product generated by the reaction of amine compounds such as piperazine (PZ) and many piperazine derivatives (such as N-methyl piperazine, N-ethyl piperazine) with CO2 usually shows the characteristics of not easy to dissolve in organic solvents but easy to dissolve in water, which leads to poor solid-liquid phase separation effect of piperazine and many piperazine derivatives based on water as solvent after use, low CO2 absorption capacity and desorption efficiency, which is the key problem limiting its wide application. Therefore, how to develop an environmentally friendly solid-liquid two-phase absorbent with excellent CO2 absorption capacity and desorption efficiency is of great significance.

[0033] To this end, the application provides a solid-liquid two-phase absorbent and a CO2 capture process, so as to ensure environmental protection of the solid-liquid two-phase absorbent and effectively improve the CO2 absorption capacity and desorption efficiency of the solid-liquid two-phase absorbent.

[0034] The technical solutions of the application will be further described below with reference to the embodiments and the accompanying drawings.

[0035] In a first aspect, the application provides a solid-liquid two-phase absorbent.

[0036] The solid-liquid two-phase absorbent comprises 2,6-dimethylpiperazine and water, and the concentration of the 2,6-dimethylpiperazine is 15wt%-40wt%.

[0037] The inventors have unexpectedly found that, unlike conventional piperazine and many piperazine derivatives, by using 2,6-dimethylpiperazine as an absorbent, the 2,6-dimethylpiperazine of a specific concentration reacts with CO2 to generate a double-substituted carbamate salt, and the solubility of the double-substituted carbamate salt in an aqueous solution is relatively low. With the absorption of CO2, the double-substituted carbamate salt gradually accumulates in the solution and precipitates to form a solid phase, thereby promoting the occurrence of solid-liquid phase separation and enhancing the CO2 absorption capacity. Moreover, when the concentration of the 2,6-dimethylpiperazine in the solid-liquid two-phase absorbent is greater than or equal to 15wt% and less than 40wt%, a solid phase can be continuously generated in the solution while maintaining good mass transfer conditions, thereby continuously driving the improvement of the CO2 absorption capacity and desorption efficiency. When the concentration of the 2,6-dimethylpiperazine is less than 15wt%, the precipitation generated by the reaction of the 2,6-dimethylpiperazine with CO2 is not obvious, and the improvement effect of the CO2 absorption capacity is not good. When the concentration of the 2,6-dimethylpiperazine is greater than 40wt%, the mass transfer condition efficiency decreases, thereby affecting the reaction and being not conducive to the generation of the precipitation.

[0038] The double-substituted carbamate salt precipitate generated by absorbing CO2 can promote the desorption of CO2 by heating or reducing the pressure. In the desorption process, the double-substituted carbamate salt precipitate releases CO2, and the reaction system is regenerated into an initial liquid phase state. Moreover, thanks to the higher thermal conductivity of the solid precipitate, the generation of the double-substituted carbamate salt precipitate in the application promotes the improvement of the overall thermal conductivity of the system, and the desorption energy consumption is reduced. In addition, after the double-substituted carbamate salt precipitate releases CO2, the newly generated 2,6-dimethylpiperazine is dissolved in water, which is an exothermic process. This dissolution heat effect further accelerates the desorption process and reduces the energy consumption. The application can maintain good adsorption stability without significant attenuation after multiple cycle adsorption tests.

[0039] The mechanism of the above reaction to generate the double-substituted carbamate salt precipitate is as follows: Figure 1As shown, first, two molecules of 2,6-dimethylpiperazine (26DMPZ) react with CO2 to form a mono-substituted carbamate anion (26DMPZmc - ) and protonated 2,6-dimethylpiperazine (26DMPZH + ); the mono-substituted carbamate anion continues to participate in the reaction of 2,6-dimethylpiperazine with CO2 to form a di-substituted carbamate dianion (26DMPZdc 2- ) and protonated 2,6-dimethylpiperazine. Subsequently, the di-substituted carbamate dianion binds with two molecules of protonated 2,6-dimethylpiperazine through ionic interaction to form an insoluble di-substituted carbamate salt (26DMPZdcs), triggering a phase transition of the system from a homogeneous liquid to a solid-liquid biphasic system, and finally resulting in precipitation.

[0040] It should be noted that the "concentration" mentioned in the present application is the mass concentration, i.e., the mass percentage of 2,6-dimethylpiperazine in the solid-liquid two-phase absorbent. Exemplary low, the concentration of 2,6-dimethylpiperazine can be 15wt%, 20wt%, 25wt%, 30wt%, 35wt% or 40wt%, etc.

[0041] Preferably, the concentration of 2,6-dimethylpiperazine is 20wt% to 30wt%.

[0042] By further controlling the concentration of 2,6-dimethylpiperazine within the above-mentioned preferred range, the precipitation particles produced by the reaction of 2,6-dimethylpiperazine with CO2 are more uniform and settle rapidly, thereby further improving the absorption capacity and desorption efficiency of the solid-liquid two-phase absorbent for CO2.

[0043] In some embodiments, the ESP_H of 2,6-dimethylpiperazine is -1.0475a.u. to -1.0485a.u.

[0044] ESP_H is the electrostatic potential at the hydrogen atom in the amine group of the molecule, and the more negative the value, the stronger the affinity for protons. ESP_H is calculated by quantum chemistry software such as Gaussian or ORCA.

[0045] The ESP_H of unsubstituted piperazine is -1.0472a.u., and the ESP_H of 2,6-dimethylpiperazine is more negative than that of unsubstituted piperazine, meaning that 2,6-dimethylpiperazine can more effectively bind with protons during the desorption stage, prompting the decomposition of the di-substituted carbamate salt and the release of CO2. As a result, the desorption efficiency of the precipitate is improved, and the energy consumption is reduced.

[0046] In some embodiments, the local electrostatic potential minimum near the N atom close to the substituted methyl group in 2,6-dimethylpiperazine is -40 kcal / mol, and the local electrostatic potential minimum near the N atom far from the substituted methyl group is -41.1 kcal / mol.

[0047] The local electrostatic potential minimum near the N atom close to the substituted methyl group and the local electrostatic potential minimum near the N atom far from the substituted methyl group are obtained by measuring the molecular electrostatic potential of 2,6-dimethylpiperazine using quantum chemistry software such as Gaussian or ORCA.

[0048] In unsubstituted piperazine, the local ESP minimum near both nitrogen atoms is -41.2 kcal / mol. In 2,6-dimethylpiperazine, the presence of a methyl group adjacent to the nitrogen atom causes the ESP minimum near the nitrogen atom to increase, indicating a decrease in nucleophilicity, and theoretically, the reactivity of 2,6-dimethylpiperazine with CO2 is weakened. However, experiments have found that the reaction of 2,6-dimethylpiperazine with CO2 produces a precipitate, causing the reaction to continuously move in the direction of generating a precipitate, so the CO2 absorption capacity of 2,6-dimethylpiperazine is actually improved compared to unsubstituted piperazine.

[0049] In a second aspect, the embodiments of the present application provide a CO2 capture process.

[0050] A CO2 capture process, comprising:

[0051] The CO2 is adsorbed by reacting with the solid-liquid two-phase absorbent as mentioned in the first aspect to generate a precipitate.

[0052] Once the precipitate appears, solid-liquid separation is formed, which can cause the reaction to move in the direction of generating a precipitate, thereby continuously producing a precipitate and improving the CO2 absorption capacity. Moreover, the precipitate serves as a high-concentration CO2 carrier, and CO2 can be recovered by desorption of the precipitate, which is beneficial to shorten the energy consumption and time required for CO2 recovery and improve the CO2 separation efficiency.

[0053] In some embodiments, the precipitate comprises a double-substituted carbamate salt.

[0054] Unlike the highly water-soluble mono-substituted product or hydroxylated product formed with piperazine and its derivatives, the double-substituted salt is almost insoluble in water, forming a stable solid phase. The formation of the precipitate solid phase not only allows high-capacity carbon sequestration, but also completely decomposes back to 2,6-dimethylpiperazine and CO2 under desorption conditions, so that the absorbent maintains good cycle performance without significant degradation.

[0055] In some embodiments, the reaction temperature of the solid-liquid two-phase absorbent and CO2 is 35℃-50℃. Preferably, the reaction temperature of the solid-liquid two-phase absorbent and CO2 is 40℃.

[0056] The reaction temperature is within the above range, taking into account the reaction kinetics and energy consumption. When the reaction temperature is lower than 20℃, the reaction rate decreases significantly. It is found that, when the temperature is around 40℃, the absorption rate of 2,6-dimethylpiperazine is close to the maximum value. At the same time, 2,6-dimethylpiperazine does not need additional refrigeration or heating when performing CO2 adsorption, and can be operated at room temperature, thereby greatly reducing the operating cost.

[0057] In some embodiments, the absorption capacity of 2,6-dimethylpiperazine is 0.9 mol / mol-amine-1 mol / mol-amine.

[0058] Since the reaction of 2,6-dimethylpiperazine with CO2 can continuously produce precipitates, the reaction is continuously driven to the side of generating precipitates, which promotes the increase of the CO2 adsorption capacity to the range of 0.9 mol / mol-amine-1 mol / mol-amine, which is significantly higher than that of piperazine and its derivatives. This also indicates that the generation of precipitates plays an important role in driving the increase of the CO2 adsorption capacity.

[0059] In some embodiments, after the step of adsorbing CO2, the CO2 capture process further comprises:

[0060] Desorption, desorbing the precipitate to regenerate 2,6-dimethylpiperazine; in the desorption step, the energy consumption for regenerating 2,6-dimethylpiperazine is 2.6 GJ / t-CO2-2.9 GJ / t-CO2.

[0061] The low solubility of the precipitate reduces the sensible heat of the solution, and CO2 exists in the form of chemical bonds in the solid phase, which can be released by only destroying the ion pair during desorption, thereby simultaneously reducing the demand for sensible heat and latent heat. Compared with the traditional monoethanolamine (MEA) with an energy saving rate of more than 30%, the energy saving rate of the present application is more than 30%, thereby directly achieving the core purpose of the present application of "low energy consumption regeneration".

[0062] In some embodiments, in the desorption step, the desorption temperature is 90℃-110℃, the desorption pressure is standard atmospheric pressure, and the desorption time is 20 min-35 min.

[0063] Under the conditions of 90℃-110℃ and one standard atmospheric pressure (i.e. normal pressure), the precipitate can release more than 90% of CO2 within 20 min, and the performance retention rate is greater than 90% after ten continuous cycles, indicating that the present application can effectively shorten the desorption time and improve the desorption efficiency.

[0064] The technical solutions of the present application will be further described below in combination with more specific embodiments.

[0065] Example 1

[0066] The embodiment of the present application provides a solid-liquid two-phase absorbent which is mixed by 2,6-dimethylpiperazine and water, wherein the concentration of 2,6-dimethylpiperazine is 15wt%.

[0067] Example 2

[0068] The embodiment of the present application provides a solid-liquid two-phase absorbent, which is different from the embodiment 1 in that the concentration of 2,6-dimethylpiperazine is 20wt%, and the rest is consistent with the embodiment 1.

[0069] Example 3

[0070] The embodiment of the present application provides a solid-liquid two-phase absorbent, which is different from the embodiment 1 in that the concentration of 2,6-dimethylpiperazine is 30wt%, and the rest is consistent with the embodiment 1.

[0071] Example 4

[0072] The embodiment of the present application provides a solid-liquid two-phase absorbent, which is different from the embodiment 1 in that the concentration of 2,6-dimethylpiperazine is 40wt%, and the rest is consistent with the embodiment 1.

[0073] Comparative Example 1

[0074] The comparative example of the present application provides an absorbent, which is different from the embodiment 3 in that the unsubstituted piperazine (PZ) is used to replace 2,6-dimethylpiperazine, and the rest is consistent with the embodiment 3.

[0075] Comparative Example 2

[0076] The comparative example of the present application provides an absorbent, which is different from the embodiment 3 in that the N-methylpiperazine (NMPZ) is used to replace 2,6-dimethylpiperazine, and the rest is consistent with the embodiment 3.

[0077] Comparative Example 3

[0078] The comparative example of the present application provides an absorbent, which is different from the embodiment 3 in that the N-ethylpiperazine (NEPZ) is used to replace 2,6-dimethylpiperazine, and the rest is consistent with the embodiment 3.

[0079] Comparative Example 4

[0080] The comparative example of the present application provides an absorbent, which is different from the embodiment 3 in that the N-isopropylpiperazine (NIPZ) is used to replace 2,6-dimethylpiperazine, and the rest is consistent with the embodiment 3.

[0081] Comparative Example 5

[0082] The present application provides an absorbent, which is different from Example 3 in that 2-methylpiperazine (2MPZ) is used instead of 2,6-dimethylpiperazine, and the rest is consistent with Example 3.

[0083] Example 6

[0084] The present application provides an absorbent, which is different from Example 3 in that N-aminoethylpiperazine (N2AEPZ) is used instead of 2,6-dimethylpiperazine, and the rest is consistent with Example 3.

[0085] Example 7

[0086] The present application provides an absorbent, which is different from Example 3 in that N-hydroxyethylpiperazine (N2HEPZ) is used instead of 2,6-dimethylpiperazine, and the rest is consistent with Example 3.

[0087] Experiment 1

[0088] CO2 absorption capacity test

[0089] A mixed gas of 15% CO2 by volume and 85% N2 by volume is used as the test gas, and the test gas is introduced into a three-necked flask containing the test solution at a flow rate of 400 mL·min-1. The temperature of the test solution is maintained at 40°C by heating, and the concentration change of the outlet CO2 is monitored by a smoke analyzer (GAS Tiger 6000-2L). The cumulative absorption capacity of 200 min is taken as the CO2 absorption capacity. The total volume of absorbed CO2 is obtained by integrating the curve of time and concentration change and multiplying by the smoke flow rate, and then converted to molar quantity and divided by the molar quantity of the amine to obtain the absorption capacity (unit: mol / mol amine). -1 The test gas is introduced into a three-necked flask containing the test solution at a flow rate of 400 mL·min-1. The temperature of the test solution is maintained at 40°C by heating, and the concentration change of the outlet CO2 is monitored by a smoke analyzer (GAS Tiger 6000-2L). The cumulative absorption capacity of 200 min is taken as the CO2 absorption capacity. The total volume of absorbed CO2 is obtained by integrating the curve of time and concentration change and multiplying by the smoke flow rate, and then converted to molar quantity and divided by the molar quantity of the amine to obtain the absorption capacity (unit: mol / mol amine).

[0090] The absorbents of the above examples and comparative examples are used as test solutions for CO2 absorption test, and the results of CO2 absorption capacity test are shown in Table 1.

[0091] Experiment 2

[0092] CO2 desorption test

[0093] The test solution loaded with CO2 is heated to 100°C and maintained for 30 min using a heating mantle, and the released CO2 is measured cumulatively using a mass flow controller (Sevenstar-CS200, China). The desorption energy consumption is recorded by a power meter (UT230A-II, China) connected to the heating mantle.

[0094] The absorbents of the above examples and comparative examples are used as test solutions for CO2 absorption test, and the results of CO2 absorption capacity test are shown in Table 1.

[0095] Table 1

[0096]

[0097]

[0098] As shown in Table 1, the piperazine and its derivatives used in Comparative Examples 1-7 generally have a phenomenon that when the CO2absorption capacity is high, the corresponding desorption amount is low, and the desorption energy consumption is high. In sharp contrast, Example 3 of the present application not only maintains a high CO2absorption capacity, but also achieves a high desorption amount and a low desorption energy consumption. This breakthrough advantage is due to the core difference: the product of the absorbent in Comparative Examples 1-7 after reacting with CO2has high solubility in aqueous solution and cannot precipitate as a solid; while the product of 2,6-dimethylpiperazine after reacting with CO2can be further converted and ultimately form a solid precipitate. The formation of the precipitate effectively drives the reaction equilibrium to move towards the product direction, thereby breaking the conventional performance limitation rule that "high absorption capacity must be accompanied by low desorption amount and high energy consumption".

[0099] Further analysis of the data of Examples 1 to 4 in Table 1 shows that the concentration of 2,6-dimethylpiperazine is a key parameter affecting the CO2capture performance. Within the preferred concentration range (15wt% to 40wt%), as the concentration increases, the CO2absorption capacity shows a trend of increase. However, when the concentration exceeds the upper limit of the range, the CO2absorption capacity no longer increases, and the desorption amount decreases and the desorption energy consumption significantly increases. This is because too high concentration of 2,6-dimethylpiperazine leads to a sharp increase in the viscosity of the solid-liquid mixture formed during the absorption / desorption process, which severely inhibits the mass transfer rate of CO2in the system, thereby hindering the efficiency of the absorption and desorption processes and increasing the energy consumption required for desorption.

[0100] Experiment Three

[0101] Cyclic Performance Test

[0102] The CO2absorption test was performed first using the CO2absorption test method of Experiment One, and then the CO2desorption test was performed using the CO2desorption test method of Experiment Two (wherein the desorption time was shortened to 20 minutes, and the other conditions remained unchanged). This was a one-cycle test, and the cyclic performance of Example 3 was tested for 10 cycles, and the results are shown in Table 3. Figure 2

[0103] In the rapid absorption-desorption cyclic performance test with a desorption time of 20 minutes, the absorption capacity of 2,6-dimethylpiperazine was stable at about 0.65 mol / mol-amine, and the desorption amount was about 0.60 mol / mol-amine, with excellent cyclic stability, indicating that even with a shortened desorption time, high-efficiency capture can still be maintained, further reducing energy consumption.

[0104] Experiment Four​

[0105] Molecular electrostatic potential test of piperazine or its derivatives

[0106] The molecular electrostatic potential of piperazine or its derivatives used in Comparative Examples 1-7 and Example 3 was tested, and the test results are shown in Figure 3 .

[0107] Depend on Figure 3 As can be seen, in unsubstituted piperazine (PZ), the local ESP minima around both nitrogen atoms are –41.2 kcal / mol, demonstrating a completely symmetrical electronic environment. For the monoalkyl-substituted series (NMPZ, NEPZ, and NIPZ), the ESP distribution exhibits a distinct differentiation: the introduction of the alkyl group reduces the electron density near the substituted nitrogen atom, resulting in a positive shift in the ESP minimum. Simultaneously, the unsubstituted nitrogen atom becomes slightly enriched due to electron cloud rearrangement, resulting in a slight negative shift in the ESP minimum. Importantly, this effect becomes more pronounced with increasing alkyl chain length.

[0108] It is worth noting that when the nitrogen center is converted into a tertiary amine due to substitution, its steric hindrance increases significantly, the channel directly involved in the formation of carbamate is blocked, and it mainly acts as a proton acceptor. Based on the above mechanism, it can be expected that the CO2 absorption capacity of the four can be calculated according to NMPZ. <NEPZ<NIPZ<PZ的顺序递增。

[0109] In 2MPZ and 26DMPZ substituted with ortho-methyl groups, the increase in the number of methyl groups causes the ESP minimum near the two nitrogen atoms to move in the positive direction as a whole, indicating that the nucleophilicity is simultaneously weakened; the corresponding CO2 reaction activity decreases accordingly, and the absorption capacity is PZ>2MPZ>26DMPZ from high to low.

[0110] Experiment 5

[0111] Hydrogen Atom Electrostatic Potential (ESP_H) Test

[0112] The hydrogen electrostatic potential (ESP_H) of piperazine or its derivatives used in Comparative Examples 1-7 and Example 3 was tested, and the test results are shown in FIG. Figure 4 .

[0113] It should be noted that to obtain physically meaningful ESP_H values, the contribution of the nucleus itself was deducted during the calculation to avoid infinite values. A more negative ESP_H value indicates a stronger attraction between the proton and the amine nitrogen, a greater tendency for the proton to recombine, and thus a more favorable desorption process.

[0114] like Figure 4 As shown, Figure 4 (a) is the electrostatic potential of hydrogen atoms of unsubstituted piperazine, Figure 4(b) the electrostatic potential of the hydrogen atom of N-methylpiperazine, Figure 4 (c) the electrostatic potential of the hydrogen atom of N-ethylpiperazine, Figure 4 (d) the electrostatic potential of the hydrogen atom of N-isopropylpiperazine, Figure 4 (e) the electrostatic potential of the hydrogen atom of 2-methylpiperazine, Figure 4 (f) the electrostatic potential of the hydrogen atom of 2,6-dimethylpiperazine.

[0115] From Figure 4 (a) it can be seen that in unsubstituted piperazine (PZ), both secondary amine groups show the same ESP_H value (–1.0472 a.u.) due to their molecular symmetry. In combination with Figure 4 (b), Figure 4 (c), Figure 4 (d) it can be seen that upon introduction of an alkyl substituent (such as methyl, ethyl or isopropyl) on one of the nitrogen atoms, the ESP_H value of the amine group on the opposite side is significantly negatively shifted. This indicates that the increased volume of the substituent enhances the electron-donating ability of the derivative, which can strengthen its electrostatic interaction with the proton and increase the CO2 desorption efficiency.

[0116] Further, as shown in Figure 4 (e), the two secondary amine sites of 2-methylpiperazine (2MPZ) show ESP_H values of –1.0477 a.u. and –1.0475 a.u., respectively, as shown in Figure 4 (f), the corresponding values for 2,6-dimethylpiperazine are further reduced to –1.0481 a.u. and –1.0479 a.u., which indicates that the methyl substitution on the adjacent nitrogen atom leads to a significant increase in the negative ESP_H value. Therefore, 2,6-dimethylpiperazine exhibits a stronger proton binding ability, which has a positive effect on the CO2 desorption efficiency.

[0117] Experiment six

[0118] Precipitate analysis test

[0119] The precipitate of Example 3 was subjected to an analysis test: during the absorption process, samples were collected at 0 min, 10 min, 20 min, 30 min, 50 min, respectively, and after the end of the absorption, samples of the liquid and solid phases were collected, for a total of 7 samples.

[0120] After freeze-drying to remove water, all samples were subjected to 13 CNMR test (relaxation delay 20 s, 180 scans);

[0121] FT-IR spectra of the dried solid phases were tested using a Thermo Fisher Nicolet iS50 Fourier Transform Infrared Spectrometer;

[0122] A Thermo Scientific QE Plus high-resolution mass spectrometer (HRMS) was used to analyze the positive and negative electrospray ionization (ESI) mass spectra of the solid phase.

[0123] like Figure 5 As shown in (a), 13 In the C NMR spectrum, only the characteristic peak of 2,6-dimethylpiperazine was seen at 0 min, and new peaks at approximately 161.8 ppm appeared in the carboxyl region at 10 and 20 min, indicating the formation of monosubstituted carbamate; additional ring carbon peaks (C5, C6) appeared at 30 min, while the carboxyl region still showed a dominant single peak, indicating that monosubstituted carbamate was still the main product; a second carboxyl peak appeared at approximately 162.3 ppm at 50 min, and at the same time the solution began to become turbid, indicating that the formation of disubstituted carbamate occurred simultaneously with the precipitation process; two different carboxyl resonance peaks (approximately 161.8 ppm and approximately 160.4 ppm) were detected in the final solid product separated from the reaction, clearly confirming the presence of disubstituted carbamate species.

[0124] like Figure 5 As shown in (b), in the HRMS spectrum, a main ion peak was observed at m / z = 200.10, which is consistent with the calculated mass of the deprotonated disubstituted carbamate, confirming that it is the main product. Figure 5 As shown in (c), in the FT-IR spectrum, at 1680 cm -1 The strong absorption band at 1458 cm -1 and 1425cm -1 Additional OCO symmetric stretching vibration bands were observed, centered at approximately 3300 cm -1 (neutral NH) and about 3100-2800 cm -1 (Protonated NH + ) shows a broad NH stretching vibration band at 1100 cm -1 The characteristic CN stretching vibration band appearing nearby further confirmed that both nitrogen atoms on the piperazine ring had undergone substitution reactions.

[0125] Taken together, the above spectral test results jointly prove that the observed solid phase precipitate is mainly composed of disubstituted carbamate products formed by the reaction of 2,6-dimethylpiperazine and CO2.

[0126] The technical solutions disclosed in the embodiments of the present application are described in detail above, and the principles and implementation manners of the present application are described by applying specific examples. The above description of the embodiments is only used to help understand the technical solutions and core inventive points of the embodiments of the present application. Meanwhile, for those skilled in the art, according to the idea of the present application, the specific implementation manners and application ranges can be changed, and on the basis of the above, the content of the specification should not be understood as a limitation of the present application.

Claims

1. A solid-liquid two-phase absorbent, characterized in that: The solid-liquid two-phase absorbent includes 2,6-dimethylpiperazine and water, and the concentration of the 2,6-dimethylpiperazine is 15 wt% to 40 wt%.

2. The solid-liquid two-phase absorbent according to claim 1, characterized in that The concentration of the 2,6-dimethylpiperazine is 20 wt% to 30 wt%.

3. The solid-liquid two-phase absorbent according to claim 1 or 2, characterized in that: The ESP_H of the 2,6-dimethylpiperazine is -1.0475 au to -1.0485 au.

4. The solid-liquid two-phase absorbent according to claim 1 or 2, characterized in that: In the 2,6-dimethylpiperazine, the local electrostatic potential minimum value near the N atom close to the substituted methyl group is -40 kcal / mol, and the local electrostatic potential minimum value near the N atom far from the substituted methyl group is -41.1 kcal / mol.

5. A CO2 capture process, characterized in that: The CO2 capture process comprises the following steps: Adsorbing CO2, using the solid-liquid two-phase absorbent according to any one of claims 1 to 4 to react with the CO2 to generate a precipitate.

6. The CO2 capture process according to claim 5, characterized in that: The precipitate comprises the disubstituted carbamate.

7. The CO2 capture process according to claim 6, characterized in that: The reaction temperature of the solid-liquid two-phase absorbent and the CO2 is 35°C to 50°C.

8. The CO2 capture process according to claim 7, characterized in that: The absorption capacity of the solid-liquid two-phase absorbent is 0.9 mol / mol-amine to 1 mol / mol-amine.

9. The CO2 capture process according to any one of claims 5 to 8, characterized in that: After the step of adsorbing CO2, the CO2 capture process further comprises: Desorption: Desorbing the precipitate to regenerate the 2,6-dimethylpiperazine; in the desorption step, the regeneration energy consumption of the 2,6-dimethylpiperazine is 2.6 GJ / t-CO2 to 2.9 GJ / t-CO2.

10. The CO2 capture process according to claim 9, characterized in that: In the desorption step, the desorption temperature is 90° C. to 110° C., the desorption pressure is standard atmospheric pressure, and the desorption time is 20 min to 35 min.

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