WO2024216296A1

WO2024216296A1 - Energy-efficient and stable modified amines and methods for co2 separation

Info

Publication number: WO2024216296A1
Application number: PCT/US2024/024692
Authority: WO
Inventors: Joo-Youp LEE; Soumitra PAYRA
Original assignee: University of Cincinnati
Current assignee: University of Cincinnati
Priority date: 2023-04-14
Filing date: 2024-04-15
Publication date: 2024-10-17
Anticipated expiration: 2025-10-14

Abstract

A sorbent chemical for CO₂ separation is provided. The sorbent chemical includes an amine functionalized with an epoxide chemical impregnated onto a mesoporous silica support. The amine is selected from the group consisting of polyethylenimine, tetraethylenepentamine, and pentaethylenehexamine. Also, the epoxide chemical is selected from the group consisting of isobutylene oxide (TBO), dimethyl- 1,2-epoxybutane (DMEB), epoxy octane (EO), styrene oxide (SO), phenyl glycidyl ether (PGE), glycidyl 4-methoxyphenyl ether (GMPE), butyl glycidyl ether (BGE), glycidyl isopropyl ether (GIPE), 2- (tert-butoxymethyl oxirane) (TBGE), and neopentyl glycol diglycidyl (NGD).

Description

ENERGY-EFFICIENT AND STABLE MODIFIED AMINES AND METHODS FOR CO₂ SEPARATION

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of the filing date of United States Provisional Application No. 63/459,465 filed April 14, 2023, United States Provisional Application No. 63/610,508 filed December 15, 2023, and United States Provisional Application No. 63/544,144 filed October 13, 2023, the disclosures of which are incorporated by reference herein in their entireties.

TECHNICAL FIELD

[0002] The present invention relates to methods for separating carbon dioxide (CO2).

STATEMENT REGARDING FEDERALLY FUNDED RESEARCH OR DEVELOPMENT

[0003] This invention was made with government support under DE-FE0032128 awarded by the U.S. Department of Energy. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

[0004] There is a broadly recognized need for systems and methods for reducing the amount of carbon dioxide in the atmosphere created by burning of fossil fuels and for providing a low cost, non-polluting renewable energy source as a substitute for fossil fuels. One method of carbon dioxide (CO2) separation generally involves the use of sorbent-coated structures. The sorbent separates CO2 from ambient air, but then the sorbent needs go through a desorption step to remain effective. The desorption step is crucial in achieving the energy efficiency and CO2 separation productivity of Direct Air Capture (DAC) systems. The temperature swing adsorption (TSA) process is a well-known method for CO2 desorption. However, its desorption time is longer due to the slow heating and cooling of the sorbent, thereby increasing the desorption cost. Therefore, a need still exists for a method of improving desorption time for CO2 separation systems. SUMMARY OF THE INVENTION

[0005] Certain exemplary aspects of the invention are set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of certain forms the invention might take and that these aspects are not intended to limit the scope of the invention. Indeed, the invention may encompass a variety of aspects that may not be explicitly set forth below. [0006] Embodiments of the disclosed invention are directed to a sorbent chemical for CO2 separation including an amine functionalized with an epoxide chemical impregnated onto a mesoporous silica support. The amine is selected from the group consisting of polyethylenimine, tetraethylenepentamine, and pentaethylenehexamine. Also, the epoxide chemical is selected from the group consisting of isobutylene oxide (IBO), dimethyl- 1,2-epoxybutane (DMEB), epoxy octane (EO), styrene oxide (SO), phenyl glycidyl ether (PGE), glycidyl 4-methoxyphenyl ether (GMPE), butyl glycidyl ether (BGE), glycidyl isopropyl ether (GIPE), 2- (tert-butoxymethyl oxirane) (TBGE), and neopentyl glycol diglycidyl (NGD).

[0007] In one embodiment, the epoxide chemical is selected from the group consisting of isobutylene oxide (IBO) and glycidyl isopropyl ether (GIPE). In another embodiment, the amine is polyethylenimine. In one embodiment, the amine is tetraethylenepentamine. In another embodiment, the amine is pentaethylenehexamine.

[0008] One embodiment of the present invention is a monolith block coated with the sorbent chemical described above. In another embodiment, the amine comprises one or more nitrogens (N) and the molar ratio of epoxide chemical to N in the amine is in the range from 0.1 to 0.4. In an alternative embodiment, the sorbent support also is selected from the group consisting of porous silica, alumina, and zeolite.

[0009] Another embodiment of the disclosed invention is directed to method of removing carbon dioxide from carbon dioxide-laden ambient air. In some embodiments, the sorbent support is the one described above. The method involves directing a flow of the carbon dioxide-laden ambient air through a monolith carbon dioxide capture structure that is supporting a sorbent, wherein the sorbent is capable of binding carbon dioxide to the sorbent, so as to remove carbon dioxide from the ambient air. Then, exposing said monolith carbon dioxide capture structure to microwave energy from a mobile microwave radiator. Next, regenerating the sorbent by directing microwave energy at the monolith carbon dioxide capture structure, thereby causing separation of carbon dioxide from the sorbent. Then, moving the mobile microwave radiator to another monolith carbon dioxide capture structure for consecutive carbon dioxide desorption. The movements of the microwave radiator from one monolith carbon dioxide capture structure to another monolith carbon dioxide capture structure are cyclically repeated.

[0010] In one embodiment, the monolith carbon dioxide capture structure is irradiated with microwave energy at a DC voltage greater than 5 V. In another embodiment, the monolith carbon dioxide capture structure is irradiated with microwave energy at a DC voltage in the range from 5- 8 V. In one embodiment, the monolith carbon dioxide capture structure is irradiated with microwave energy at a DC voltage of about 6.5 V.

[0011] In another embodiment, the epoxide chemical is selected from the group consisting of isobutylene oxide (TBO) and glycidyl isopropyl ether (GIPE). In one embodiment, the amine of the sorbent is polyethylenimine. In another embodiment, the amine of the sorbent is tetraethylenepentamine. In one embodiment, the amine of the sorbent is pentaethylenehexamine.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The objects and advantages of the disclosed invention will be further appreciated in light of the following detailed descriptions and drawings in which:

[0013] FIG. 1A is a schematic representation of PEI conjugation with the epoxide chemical Isobutylene oxide (IBO).

[0014] FIG. IB is a schematic representation of PEI conjugation with the epoxide chemical 3,3- Dimethyl-l,2-epoxybutane (DMEB).

[0015] FIG. 1C is a schematic representation of PEI conjugation with the epoxide chemical Epoxy octane (EO).

[0016] FIG. ID is a schematic representation of PEI conjugation with the epoxide chemical Styrene oxide (SO).

[0017] FIG. IE is a schematic representation of PEI conjugation with the epoxide chemical Phenyl glycidyl ether (PGE).

[0018] FIG. IF is a schematic representation of PEI conjugation with the epoxide chemical Glycidyl 4-methoxyphenyl ether (GMPE).

[0019] FIG. 1G is a schematic representation of PEI conjugation with the epoxide chemical Butyl glycidyl ether (BGE). [0020] FIG. 1H is a schematic representation of PEI conjugation with the epoxide chemical Glycidyl isopropyl ether (GIPE).

[0021] FIG. II is a schematic representation of PEI conjugation with the epoxide chemical 2- (tert- butoxymethyl oxirane) (TBGE).

[0022] FIG. 1J is a schematic representation of PEI conjugation with the epoxide chemical Neopentyl glycol diglycidyl (NGD).

[0023] FIG. 2 is a graph showing CO2 adsorption-desorption profiles for 2 TSA cycles (adsorption: 400 ppm CO2 in air at 30 °C for 90 min, desorption: N2 at 100 °C for 20 min).

[0024] FIG. 3 is a graph showing calculated CO2 swing capacities after 1st cycle.

[0025] FIG. 4 is a graph showing CO2 adsorption-desorption profiles for 2 TSA cycles (adsorption: 400 ppm CO2 in air at 30 °C for 90 min, desorption: N2 at 100 °C for 20 min).

[0026] FIG. 5 is a graph showing calculated CO2 swing capacities after 1st cycle.

[0027] FIG. 6A is a graph showing 100 cycles of CO2 adsorption-desorption plotted for 0.2 EB- PEI/silica sorbent relative to PEI/silica sorbent with respect to weight (%) changes vs. time at the CO2 desorption temperature of 100° C. FIG. 6B is a graph showing CO2 adsorption capacities of 0.2 EB-PEVsilica sorbent relative to PEI/silica sorbent after 1st and 100th cycles of run.

[0028] FIG. 7A is a graph showing 100 cycles of CO2 adsorption-desorption plotted for 0.2 IBO- PEI/silica sorbent relative to PEI/silica sorbent with respect to weight (%) changes vs. time at the CO2 desorption temperature of 100° C. FIG. 7B is a graph showing CO2 adsorption capacities of 0.2 IBO-PEI/silica sorbent relative to PEI/silica sorbent after 1st and 100th cycles of run.

[0029] FIG. 8A is a graph showing 100 cycles of CO2 adsorption-desorption plotted for 0.2 EO- PEI/silica sorbent relative to PEI/silica sorbent with respect to weight (%) changes vs. time at the CO2 desorption temperature of 100° C. FIG. 8B is a graph showing CO2 adsorption capacities of for 0.2 EO-PEI/silica sorbent relative to PEVsilica sorbent after 1st and 100th cycles of run.

[0030] FIG. 9A is a graph showing 100 cycles of CO2 adsorption-desorption plotted for 0.2 DMEB-PEI/silica sorbent relative to PEI/silica sorbent with respect to weight (%) changes vs. Time at the CO2 desorption temperature of 100° C. FIG. 9B is a graph showing CO2 adsorption capacities of 0.2 DMEB-PEI/silica sorbent relative to PEVsilica sorbent after 1st and 100th cycles of run.

[0031] FIG. 10 is a graph showing DSC data for CO2 saturated samples of unmodified PEI/silica and modified EC -PEI/silica sorbents. [0032] FIG. 11 is a graph showing DSC data for CO2 saturated samples of modified EC-PEI/silica sorbents.

[0033] FIG. 12 is an equation for calculating the adsorption and desorption capacities of monolith blocks.

[0034] FIG. 13 is a schematic of the experimental set-up for performance evaluations of sorbent- washcoated monolith blocks.

[0035] FIG. 14 is an image showing the experimental set-up for performance evaluations of monolith blocks.

[0036] FIG. 15 is a graph showing microwave calibration with 1 liter of water at different DC voltages.

[0037] FIG. 16 is a graph showing temperature profile with respect to MW DC Voltage and Energy.

[0038] FIG. 17 is an image showing IR images of 0.1 IBO-PEI-doped silica monolith at different MW power.

[0039] FIG. 18A is a schematic diagram of the MW cavity and three different monolith positions where temperatures were measured. FIG. 18B is a schematic diagram of thermocouple positions inside the monolith block and FIGs 18C, 18D and 18E are their corresponding temperature histograms.

[0040] FIG. 19 is a series of photographs of MW set-up and recording the temperature using thermocouples.

[0041] FIG. 20A is an image showing IR images of 0.1 IBO-PEI monolith after MW irradiation at 6.5 V (a) before and (b) after flipping its position. 1 and 2 signify the positions of the monolith. [0042] FIG. 20B is an image showing (a) IR images of (a) 0.15 g, (b) 0.3 g, and (c) 0.45 g of 0.1 IBO-PEI solution after MW irradiation at 6.5 V for 60 s

[0043] FIG. 20C is an image showing IR images of 0.1 IBO-PEI monolith after MW irradiation (a) before, (b) after vacuum drying, and (c) undoped silica monolith after vacuum drying.

[0044] FIG. 21 is a photograph of an experimental set-up used for CO2 adsorption.

[0045] FIG. 22 is a schematic diagram of experimental set-up for microwave-assisted CO2 desorption.

[0046] FIG. 23 is a photograph of an experimental set-up used for the MW-assisted CO2 desorption. [0047] FIG. 24 is graph showing temperature vs voltage for undoped and PEI-doped sorbent- wash coated monolith block.

[0048] FIGs 25A and 25B are graphs showing purity profiles of CO2 desorbed from PEI-600 blocks. FIGs 25C and 25D are graphs showing purity profiles of CO2 desorbed from EB-PEI blocks. FIGs 25E and 25F are graphs showing purity profiles of CO2 desorbed from IBO-PEI blocks.

[0049] FIG. 26A is a graph showing mass spectra for air in the lab. FIG. 26B is a graph showing mass spectra for CO2 with >99.8% purity. FIG. 26C is a graph showing mass spectra for the gas desorbed from IBO-PEI block before radiation (BR). FIG. 26D is a graph showing mass spectra for IBO-PEI block after radiation (AR). FIG. 26E is a graph showing mass spectra for CO2 before and after MW radiation for IBO-PEI block. FIG. 26F is a graph showing mass spectra for H2O before and after MW radiation for IBO-PEI block. FIG. 26G is a graph showing mass spectra for NH3 before and after MW radiation for IBO-PEI block.

[0050] FIG. 27A is a graph showing temperature profiles of Joule heating during heating and cooling at four different locations of PEI-doped monolith block. FIG. 27B is a graph showing cooling time comparison between Joule heating and MW heating of PEI block.

DEFINITIONS

[0051] As used herein, the term “about,” when referring to a value or to an amount of mass, weight, time, volume, voltage, pH, size, concentration or percentage is meant to encompass variations of ±20% in some embodiments, ±10% in some embodiments, ±5% in some embodiments, ±1% in some embodiments, ±0.5% in some embodiments, and ±0.1% in some embodiments from the specified amount, as such variations are appropriate to perform the disclosed method.

DETAILED DESCRIPTION

[0052] One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, numerous implementation-specific decisions must be made to achieve the developers’ specific goals, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking for those of ordinary skill having the benefit of this disclosure.

[0053] In one embodiment, the present invention involves the use of novel epoxide chemicals that have good electron withdrawing capabilities to lower the bonding strengths between modified amine sites and CO2 molecules and thus the desorption energy requirements. The modifications can increase steric hindrance between amine sites and thus suppress amine degradations. In one embodiment, the present invention is a sorbent chemical for CO2 separation comprising polyethylenimine functionalized with an epoxide chemical (EC-PEI) impregnated onto a mesoporous silica support, wherein the epoxide chemical is selected from the group consisting of isobutylene oxide (IBO), dimethyl- 1,2-epoxybutane (DMEB), epoxy octane (EO), styrene oxide (SO), phenyl glycidyl ether (PGE), glycidyl 4-methoxyphenyl ether (GMPE), butyl glycidyl ether (BGE), glycidyl isopropyl ether (GIPE), 2- (tert-butoxymethyl oxirane) (TBGE), and neopentyl glycol diglycidyl (NGD).

[0054] In another embodiment, the present invention is a sorbent chemical for CO2 separation comprising tetraethylenepentamine functionalized with an epoxide chemical (EC-TEPA) impregnated onto a mesoporous silica support, wherein the epoxide chemical is selected from the group consisting of isobutylene oxide (IBO), dimethyl- 1,2-epoxybutane (DMEB), epoxy octane (EO), styrene oxide (SO), phenyl glycidyl ether (PGE), glycidyl 4-methoxyphenyl ether (GMPE), butyl glycidyl ether (BGE), glycidyl isopropyl ether (GIPE), 2- (tert-butoxymethyl oxirane) (TBGE), and neopentyl glycol diglycidyl (NGD).

[0055] In yet another embodiment, the present invention is a sorbent chemical for CO2 separation comprising pentaethylenehexamine functionalized with an epoxide chemical (EC-PEHA) impregnated onto a mesoporous silica support, wherein the epoxide chemical is selected from the group consisting of isobutylene oxide (IBO), dimethyl- 1,2-epoxybutane (DMEB), epoxy octane (EO), styrene oxide (SO), phenyl glycidyl ether (PGE), glycidyl 4-methoxyphenyl ether (GMPE), butyl glycidyl ether (BGE), glycidyl isopropyl ether (GIPE), 2- (tert-butoxymethyl oxirane) (TBGE), and neopentyl glycol diglycidyl (NGD).

Microwave heating

[0056] Another embodiment of the present invention involves the use of microwave heating to improve desorption time. The desorption step is crucial in achieving the energy efficiency and CCh separation productivity of a Direct Air Capture (DAC) system. The temperature swing adsorption (TSA) process is a well-known method for CO2 desorption. However, its desorption time is longer due to the slow heating and cooling of the sorbent, thereby increasing the desorption cost. However, Microwave (MW) heating is fast, contactless, and direct. The polar molecules will align themselves in the presence of an electromagnetic field. In the presence of different MW powers, these molecules continuously rotate by aligning themselves in the field. This is called dipole rotation or dipolar polarization. Rotating molecules push, pull, and collide with other molecules (through electrical forces), distributing the energy to adjacent molecules and atoms in the material. The energy transfer process from the source to the sample is a form of radiative heating. In one embodiment of the present invention, MW energy is applied to a sorbent- washcoated monolith to improve CO2 desorption. The sorbent may comprise the EC-conjugated amines described above.

[0057] In another embodiment, MW radiation is applied using mobile microwave units. MW radiation is applied to one monolith block after another on the same array. In other words, a MW radiator keeps moving from left to right (or vice versa) and then from right to left. Those blocks that received the radiation can switch to adsorption once the temperature cools down. This system allows the monolith blocks to remain stationary through an adsorption-desorption cycle.

[0058] In one embodiment, monolith blocks are enclosed and the blocks stay in the same enclosure for both CO2 adsorption and desorption. For example, 4 blocks stacked in an 2x2 array can be located in an enclosure. If we call these stacked blocks a module, a microwave radiator will move from one module to the other.

EC-conjugated amine material synthesis

[0059] To functionalize amine with various epoxide chemicals, the molar ratio of epoxide chemicals to the nitrogen content in amine was varied between 0-0.4. Then, the resultant reaction mixture was stirred at room temperature for a selected period of time to ensure the conjugation of epoxide chemicals (EC) to PEI. Subsequently, the alcoholic or aqueous solution containing epoxide chemical conjugated amine was used for impregnation onto a mesoporous silica support. Following impregnation, the synthesized sorbent materials were dried at -50-70 °C in vacuum oven overnight to completely remove the solvent. Preparation of mesoporous silica support

[0060] Various different supports such as porous silica, alumina, and zeolites can be used to impregnate EC-conjugated amines. In one embodiment, porous silica support is used to synthesize EC-conjugated modified amine sorbents. Mesoporous silica was synthesized using a spray-drying technique. To obtain a mesoporous silica network structure, the spray-dried silica particles were calcined at -400-600 °C. Multiple different amine forms such as TEPA, PEHA, polyethyleneimine (PEI), etc. can be conjugated to ECs including isobutylene oxide (IBO), 3,3-Dimethyl-l,2- epoxybutane (DMEB), epoxy octane (EO), styrene oxide (SO), phenyl glycidyl ether (PGE), Glycidyl 4-methoxyphenyl ether (GMPE), butyl glycidyl ether (BGE), glycidyl isopropyl ether (GIPE), 2- (tert-butoxymethyl oxirane) (TBGE), and neopentyl glycol diglycidyl (NGD). Among such amines, PEI is selected to represent the results below. EC-conjugated PEI (EC-PEI hereafter) was impregnated onto the mesoporous silica support to synthesize the EC-PEI/silica sorbent.

EXAMPLES

Example 1

[0061] The CO2 adsorption-desorption performances for the aforementioned EC-PEI/silica sorbents were evaluated in thermogravimetric analyzer (TGA). Prior to the analysis, all sorbents were degassed at 100 °C for 10 min under N2 flow. Then, CO2 adsorption was performed using 400 ppm CO2 in air for 90 min and the adsorption temperature was set to 30 °C. After the adsorption, the gas was switched to -99.99% N2 for desorption, and the temperature raised to 100 °C. Then, the desorption temperature was held for 20 min to achieve a complete desorption process. The CO2 adsorption performance results are summarized in FIG. 2 and FIG. 3.

Example 2

[0062] Additional EC-PEEsilica sorbents were evaluated for CO2 adsorption as shown in FIG. 4 and FIG. 5. The slopes of the CO2 adsorption for most EC-PEI/silica sorbents are steeper than that for the unmodified PEI/silica sorbent, indicating that the modified EC-PEEsilica sorbents show faster CO2 adsorption kinetics as shown in FIG. 4.

Example 3 [0063] The adsorption-desorption cyclic performance test was run by repeating the adsorption and desorption process for 100 cycles. The TGA performance results for the selected different modified PEI sorbents of 0.2 EB-PEI/sorbent, 0.2 IBO-PEI/silica sorbent, 0.2 EO-PEI/silica sorbent, and 0.2 DMEB-PEI sorbent were recorded. The CO2 working capacities of the 1st and 100th cycles are shown in FIGs 6, 7, 8, and 9. As shown in Figures 6-9, all the modified PEI sorbents exhibited higher stability and comparable performances to unmodified PEI after 100 cycles. As expected, the unmodified PEI/silica sorbent showed the highest CO2 adsorption capacity (1.81 mmol/g) from the 1st cycle, but it gradually decreased to 1.23 mmol/g after the 100th cycle. The reduction in the CO2 adsorption capacity for the unmodified PEI/silica sorbent after the 100th cycle run was -33%. In contrast, all the modified PEI sorbent samples showed much lower loss of CO2 adsorption capacities during the 100 cycles. After repeated sorption cycles, this lower cyclic stability of unmodified PEI/silica sorbents could be correlated with the degradation of amine or amine leaching. Meanwhile, the reduced loss in CO2 adsorption capacity by modified PEI sorbents could also be associated with enhanced steric hindrance between amine sites and reduced basicity between CO2 and the amine moieties during the cyclic operation. All the modified PEI sorbents showed enhanced thermal stability as well as reduced loss in CO2 sorption capacities relative to the unmodified PEVsilica sorbent.

[0064] Referring to FIG. 6A, 100 cycles of CO2 adsorption-desorption were plotted for 0.2 EB- PEI/silica sorbent relative to PEI/silica sorbent with respect to weight (%) changes vs. time at the CO2 desorption temperature of 100° C. FIG. 6B shows the CO2 adsorption capacities of 0.2 EB- PEI/silica sorbent relative to PEVsilica sorbent after 1st and 100th cycles of run.

[0065] Referring to FIG. 7A, 100 cycles of CO2 adsorption-desorption were plotted for 0.2 IBO- PEI/silica sorbent relative to PEI/silica sorbent with respect to weight (%) changes vs. time at the CO2 desorption temperature of 100° C. FIG. 7B shows the CO2 adsorption capacities of 0.2 IBO- PEI/silica sorbent relative to PEVsilica sorbent after 1st and 100th cycles of run.

[0066] Referring to FIG. 8 A, 100 cycles of CO2 adsorption-desorption were plotted for 0.2 EO- PEI/silica sorbent relative to PEI/silica sorbent with respect to weight (%) changes vs. time at the CO2 desorption temperature of 100° C. FIG. 8B shows the CO2 adsorption capacities of for 0.2 EO-PEI/silica sorbent relative to PEI/silica sorbent after 1st and 100th cycles of run.

[0067] Referring to FIG. 9A, 100 cycles of CO2 adsorption-desorption were plotted for 0.2 DMEB-PEI/silica sorbent relative to PEI/silica sorbent with respect to weight (%) changes vs. Time at the CO2 desorption temperature of 100° C FIG. 9B shows the CO2 adsorption capacities of 0.2 DMEB-PEI/silica sorbent relative to PEI/silica sorbent after 1st and 100th cycles of run.

Example 4

[0068] The desorption energy requirements were investigated using differential scanning calorimetry (DSC) for unmodified and EC-conjugated modified PEI sorbents. The sorbents were saturated with 400 ppm CO2 in air in TGA. Then, the heat of desorption was measured in a N2 flow at a heating rate of 1 °C/min and temperatures ranging from -20 to 150 °C. The heat of desorption values for the 0.1 IBO-PEVsilica, 0.1 DMEB-PEI/silica, and 0.1 GIPE-PEI/silica sorbents were 51, 25, and 46% less than that for the unmodified PEI/silica sorbent, respectively, in FIG.10. The data are summarized in Table 1.

Table 1 - Heat of desorption data of unmodified PEI/silica and modified EC-PEI/silica sorbents

Note: PEI/silica sorbent was used as a baseline sorbent

Example 5

[0069] The heat of desorption was reduced by increasing the molar ratio of epoxide chemicals to N in amine from 0.1 to 0.2. FIG. 11 shows that as the degree of EC modification increases, the heat of desorption decreases and the onset of endothermic peak shifts to lower temperature. This result indicates that EC modification can not only reduce the heat of desorption but also CO2 desorption temperatures. Example 6

[0070] The EC-conjugated modified PEI sorbents in a powdered form were washcoated onto monolith blocks for performance evaluations. Sorbent support was first washcoated onto monolith blocks and then EC-conjugated modified PEI was impregnated onto the coating layer. The impregnation was performed by first, mixing a desired amount of unmodified or EC-conjugated modified PEI with water at 50 °C and stirring for 48 h to ensure the conjugation of EC to PEI. It should be noted that other solvents such as methanol, isopropanol, and ethanol can also be used for the conjugation of EC to PEI under the same reaction conditions. Next, the as-prepared PEI solution is impregnated onto the sorbent support washcoated monolith block and dried overnight in a pre-heated vacuum oven set to 70 - 80 °C. The type of PEI and the amount of PEI impregnated onto each monolith block are summarized in Table 2.

[0071] Then, oven-dried monolith blocks were placed in the tubular reactor by wrapping it with ceramic blanket to ensure no gas leakage around the monolith block. A schematic and a picture of the experimental set-up for the performance evaluations of unmodified and modified PEI sorbent-washcoated monolith blocks are shown in FIGs 13 and 14. Since the block was exposed to air during impregnation and assembling, some CO2 was pre-adsorbed onto it. The adsorbed CO2 was degassed by flowing N2 gas into the monoliths at 8 L/min and maintaining the exit gas temperature at ~90 °C using heating tape. Once the CO2 concentration at the outlet reached ~40 ppm, heating and N2 were stopped. After the monolith cooled down to room temperature (24 °C), an adsorption experiment was carried out by introducing 10 L/min air at room temperature until the monolith was saturated. This adsorption operation usually took -18-22 h. After that, a desorption step was performed until an outlet CO2 concentration reached -40 ppm by flowing N2 gas at -90 °C. Inlet and outlet CO2 concentrations along with the temperatures of gas and monolith were recorded to obtain breakthrough curves. The adsorption and desorption capacities of the monolith blocks were calculated from the equation shown in FIG. 12. The performances of representative sorbent-washcoated monolith blocks are summarized in Table 2. Table 2, Adsorption and desorption capacities of monolith blocks impregnated with unmodified and modified PEI prepared in water.

Example 7

[0072] A 50% (wt) mixture of 0.1 isobutylene oxide (IBO)-conjugated poly(ethyleneimine) (MW = 600) (PEI) (9.04 g of 0.1 IBO-PEI) was impregnated onto silica support washcoated onto monolith blocks and dried. Here, 0.1 is the molar ratio of IBO to the nitrogen in PEI. IBO was used to oxidize the primary amine to the secondary amine in PEI. Such a preferential alkylation of 1° amines to 2° amines is highly desirable not to sacrifice the CO2 adsorption capacities. A bare silica (i.e., without 0.1 IBO-PEI)-washcoated monolith block was also used as a control.

Example 8

[0073] MW calibration: Before applying the MW energy to the monolith blocks, the MW energy radiation was calibrated using 1 liter of water at different DC voltages (V), controlling the power going into the MW cavity (model: BP-120, Microwave Research and Applications, Inc., frequency: 2.45 GHz and rectangular cavity). It can be seen from FIG. 15 that up to 5 V, no power is absorbed by water. The water started absorbing the power after 5 V and linearly absorbed up to 8 V. The power absorption started decreasing after 8 V and then saturated from 9 V. Only a linear profile (5-8 V) was considered for calibration. Example 9

[0074] 0.1 IBO-PEI-impregnated and bare silica monolith blocks were kept inside the MW cavity and irradiated at different DC voltages for 60 seconds to record the temperature using a thermocouple. The thermocouple was placed at the center of the monolith block. The temperature profile is also plotted against the energy. It can be seen from FIG. 16 that up to 5 V, there is no significant increase in the temperature for both blocks, similar to the MW calibration graph. Then, with increasing the voltage, both blocks started absorbing MW power and started to be heated. However, 0.1 IBO-PEI-doped silica absorbed more MW energy than the bare silica monolith. MW heating is also known as dielectric heating. MW electromagnetic radiation always heats a dielectric material. At higher frequencies, this heating is caused by molecular dipole rotation of the molecule. Molecular rotation occurs in materials containing polar molecules having an electrical dipole moment. The ceramic monolith made of magnesium-alumino-silicate cordierite is a MW inactive material. Silica is a non-polar compound because of its linear and symmetrical shape. However, PEI is a polar compound with a dielectric constant ~9. Therefore, PEI absorbs the MW electromagnetic radiation, whereas the ceramic monolith does not. As a result, the IBO- PEI-doped silica monolith block could reach higher temperatures than the silica-coated monolith block. At 6.5 V, the 0.1 IBO-PEI impregnated and bare monolith temperatures were 96 and 44 °C, respectively. This temperature rise was confirmed with IR images captured using the FLIR C5 camera. It can be seen in FIG. 17 that the temperatures recorded using both approaches are almost the same.

Example 10

[0075] The temperature uniformity of the monolith blocks was tested by irradiating the blocks at 6.5 V for 60 seconds. After the irradiation, the monolith block was immediately removed from the MW cavity. The temperatures were recorded at four different places by keeping the thermocouples inside the monolith block. The monolith block was kept horizontally inside the cavity and measured the temperatures at three different positions: left (L), half (H), and right (R), as shown in FIG. 18A. A schematic of the thermocouple positions is given in FIG. 18B, and their corresponding temperature histograms are shown in FIGs 18C-18E. Some pictures of the MW set-up and temperature recording using the thermocouples are shown in FIG. 19. It can be seen from thermocouple temperatures and IR videos that the temperature distribution was not uniform throughout the monolith block. This can be attributed to either non-uniform impregnation of IBO- PEI or non-uniform MW radiation inside the cavity or a combination of both.

Example 11

[0076] The monolith block was flipped opposite its initial position and irradiated by keeping all other parameters the same to know the MW radiation uniformity (FIG 20A, images (a) and (b)). We observed from IR images that the right side of the monolith always gets heated by MW irradiation, which is attributed to the uneven distribution of MW radiation. The temperature difference in the monolith may come from the inhomogeneous impregnation of PEI on the silica support. To further confirm this statement, we have recorded the temperature of only 0.1 IBO-PEI solution at 6.5 V at different weight loading. It can be seen from the IR images that with an increase in the 0.1 IBO-PEI amount, the temperature of the solution increased at 6.5 V, as shown in FIG. 20B, images (a), (b) and (c). Therefore, where the PEI amount is higher in the monolith, the temperature is higher during MW heating, i.e., inhomogeneous impregnation of PEI on silica support.

[0077] To further confirm this statement, the temperature of only 0.1 IBO-PEI solution at 6.5 V was recorded at different weight loading. It can be seen from the IR images in 20B, images (a), (b) and (c), that with an increase in 0.1 IBO-PEI amount, the temperature of the solution increased at 6.5 V. Therefore, the non-uniform temperature distribution of the 0.1 IBO-PEI monolith block could also come from the non-uniform impregnation of 0.1 IBO-PEI onto the silica support washcoated onto the monolith block.

[0078] The IR camera cannot measure the temperature of the monolith when it is inside the glass reactor. In this example, the monolith was kept inside the tubular glass reactor during MW radiation. Therefore, the IR camera was used to monitor the temperature distribution of the monolith blocks after MW radiation. Water droplets formed on the inner wall of the glass tube during MW heating. This indicates that moisture was adsorbed inside the monolith during the adsorption of CO2 from the air, which can interfere with the temperature recording because water is MW active and has the highest dielectric constant at room temperature (78.4). To examine the effect of water adsorbed onto the monolith on the temperature distribution, both 0.1 IBO-PEI and bare monolith blocks were dried at 70 °C in a vacuum oven overnight. Then, the MW heating was performed at the same MW power (6.5 V). A temperature difference was observed before and after vacuum drying. After the vacuum drying, the temperature of the monolith block decreased from 75 to 65 °C (FIG. 20C, image (b)). To achieve our desired temperature range (70-90 °C), the MW power was increased to 7.1 V for the vacuum-dried 0.1 IBO-PEI-impregnated monolith block. The temperature range found at 7.1 V was 55-92 °C. However, the bare monolith block remained unaffected by vacuum drying, as shown in FIG. 20C, image (c). This indicates that the bare monolith block is not heated well regardless of the presence of water vapor adsorbed. However, the 0.1 IBO-PEI-impregnated monolith block benefits from the synergistic collaboration of PEI and water vapor.

Example 12

[0079] CO2 in ambient air was captured in monolith blocks inside a tubular glass reactor at a 15 LPM flow rate from a compressed air line. The saturation of the monolith block was confirmed when the CO2 concentrations at the reactor outlet were close to the inlet concentration. The experimental set-up used for the CO2 adsorption is shown in FIG. 21.

Example 13

[0080] The monolith block saturated with CO2 in ambient air was placed inside a glass reactor in the microwave (MW) chamber, as shown in FIG. 22. The air inside the glass reactor was evacuated with a vacuum pump while ball valve 1 (BV1) was opened and BV2 was closed. After removing the remaining air inside the reactor, the BV1 was closed. Then, MW was radiated for 2 min at 6.5 DC voltage (DCV). After radiation, the desorbed gas was collected in a sampling bag connected to the pump outlet by opening BV1. Then, the sampling bag was connected to the CO2 analyzer to record the purity of CO2 gas desorbed using Lab View. The vacuum pressure was relieved by opening the BV2. The experimental set-up used for the MW-assisted CO2 desorption is shown in FIG. 23.

Example 14

[0081] MW energy was calibrated using 1 L of water with respect to DC voltage to control the power supplied to the MW. The MW power supplied to the MW and absorbed by the 1 L water with respect to DC voltage is shown in FIG. 15. It shows that the power output and the power consumed by water increased with increasing voltage. Changes in temperatures are plotted for a sorbent support-washcoated monolith block (i.e., without any dopant, square symbol) and a PEI- impregnated sorbent-washcoated monolith block (circle symbol) as shown in FIG. 24. There was no significant temperature increase up to 5 DC V. The monolith block with bare silica sorbent support showed no significant temperature increase above -40 °C.

Example 15

[0082] Three blocks named PEI, EB-PEI, and IBO-PEI were evaluated for the feasibility of temperature-vacuum swing desorption of CO2. PEI represents monolith block with PEI impregnated onto a mesoporous silica support. Similarly, EB-PEI and IBO-PEI represent modified PEI with 1,2 epoxy butane and isobutylene oxide, respectively.

[0083] PEI: The maximum vacuum pressure reached during the evacuation was -27.5” (inches) Hg. Therefore, the initial partial air pressure inside the reactor was 8%. The pressure increased to -26” Hg (i.e., AP = 1.5” Hg) after 1 min of MW radiation at 6.5 DCV. In contrast, after 2 minutes of MW radiation at 6.5 DC V, the pressure increased to -11” Hg (i.e., AP = 16” Hg). The gas collected in a sampling bag showed -91% CO2, and the rest of 8% is the mixture of other gases initially present in the reactor after evacuation during MW heating. The CO2 purity profile obtained from the gas collected in the sampling bag using the CO2 analyzer is shown in FIG. 25A, with a zoom-in version in FIG. 25B.

[0084] EB-PEI: The maximum vacuum pressure reached -27.9” Hg. Therefore, the initial partial pressure of air present inside the reactor was 6.7%. The pressure increased to -26.4” Hg (i.e., AP = 1.5” Hg) after 1 min of MW radiation at 6.5 DC V. However, after 2 minutes of MW radiation, the pressure increased to -7.9” Hg (i.e., AP = 20” Hg). The collected gas showed a CO2 purity of >93%, which is also consistent with the 6.7% initial air present in the reactor. The CO2 purity profile is shown in FIG. 25C and FIG. 25D.

[0085] IBO-PEI: The maximum vacuum pressure reached -28.3” Hg. Therefore, the initial partial pressure of air present inside the reactor was 5.4%. The pressure increased to 4.7” Hg (i.e., AP = 33” Hg) after 2 minutes of MW radiation. The collected gas showed a CO2 purity of >95%, which is also consistent with the 5.4% initial air present in the reactor.

[0086] The monolith blocks impregnated with the two modified PEIs (i.e., EB-PEI and IBO-PEI) can generate higher CO2 purities than that doped with unmodified PEI as a result of the MW radiation applied at 5.5 DC V for 2 min. A total of 6 min radiation was used to collect more gas in the sampling bag under continuous vacuum conditions during MW radiation. After every 2 min of radiation, the block was cooled down to room temperature and then radiated for the next 2 min. This result suggests that the modified PEIs can desorb CO2 with low desorption energy requirements due to low bonding strengths between the modified PEI sites and the CO2 molecules adsorbed.

Example 16

[0087] The purity of the gases desorbed from the monolith blocks doped with PEI and IBO-PEI was also determined using mass spectrometry (MS). The mass spectra were recorded using a table- top mass spectrometer (Pfeiffer Vacuum, ThermoStar) attached to a CH-Tron detector at a 900 SEM voltage. The capillary of the mass spectrometer was inserted inside the sampling bag through a rubber septum to analyze the desorbed gas. Every gas was analyzed five times to obtain consistent and reliable data. Before determining the gases desorbed from the blocks, indoor air collected in the lab and CO2 gas from technical-grade (>99.8%) CO2 gas cylinders were used as benchmark gases. Higher intensities for atomic nitrogen (N at 14 m/e), atomic oxygen (O at 16 m/e), N2 (at 28 m/e), and O2 (at 32 m/e) were shown by indoor air in the lab (indoor air hereafter). However, the CO2 peak (at 44 m/e), moisture (at 18 m/e), and several other peaks were also visible at low intensities, as the concentrations were much less than those of N2 and O2. The CO2 gas peak using the technical-grade CO2 gas cylinder showed the highest intensity for CO2 gas and very low intensities for nitrogen, moisture, and oxygen. Several small peaks, which were also detectable from the "indoor air" spectrum, were also shown in the spectrum.

[0088] Before MW radiation to the IBO-PEI block, the glass reactor was evacuated. Then, the gas was collected in a sample bag named IBO-PEI-BR where BR signifies before radiation. The gas collected after MW radiation was named IBO-PEI-AR, where AR signifies after radiation. On a linear scale, the mass spectrum of IBO-PEI-BR (FIG. 26C) is observed to be very close to that of indoor air (FIG. 26A). Additionally, the spectrum of IBO-PEI-AR (FIG. 26D) is observed to be very close to that of >99.8% CO2 gas (FIG. 26B). For clarity, a log scale was employed to plot the ion current intensities for CO2, water vapor (H2O), and ammonia (NH3) gases. In FIG. 26E, the proximity of the CO2 ion current intensity of IBO-PEI-BR to that of indoor air was observed. However, after MW radiation, the CO2 ion current intensity of IBO-PEI-AR was found to be close to that of >99.8% CO2. This data indicates that before MW radiation, the amount of CO2 gas desorbed was negligible. On the other hand, the CO2 gas desorbed from the IBO-PEI block after MW radiation was highly pure. The levels of water vapor and NH3 before and after the MW radiation, as shown in FIG. 26F and FIG. 26G, were not found to be significantly different from those in indoor air. This result suggests that high-purity CO2 recovery can be achieved without significantly expending energy to desorb water vapor and degrading IBO-PEI.

Example 17

[0089] The heating and cooling performances of MW-assisted CO2 desorption were compared with those of Joule heating. For Joule heating, the PEI block was wrapped with carbon fiber, and both ends of the block were connected with positive and negative alligator clips. Subsequently, a 10 DC voltage was applied for heating, and the current was found to be 2.235 A. During the cooling period, the carbon fiber was removed from the block. A heating-cooling profile is displayed in FIG. 27A. The positions of the four different thermocouples placed inside the chamber of the monolith block are represented by the center, left corner, right corner, and bottom. It is observed in FIG. 27A that the temperature distribution inside the block was fairly uniform. [0090] The energy consumption required for MW heating to reach 80 °C was estimated and compared with Joule heating used for the PEI block. The energy required to reach 80 °C for MW heating was 48 kJ based on the electricity pulled out from the outlet, whereas for Joule heating, it was 67 kJ. However, it should be noted that energy requirement for the current MW would depend on the ageing and multiple parameters governing the electromagnetic field generation. In this context, if the energy requirement is determined based on calorimetry (i.e., energy absorbed by water from the MW calibration curve), it is 18 kJ. By contrast, the time required to reach 80 °C for Joule heating was ~45 min. Major energy savings were derived from 1 -minute MW heating. The cooling time is also crucial for a fast turnaround of the adsorption and desorption cycles. It was found that Joule heating required 33 min to cool down to 30 °C, while 13 min were required for MW heating to reach the same 30 °C, as shown in FIG. 27B. Therefore, MW heating is considered a more convenient, energy-efficient, and productive technology for CO2 desorption for Direct Air Capture.

[0091] All documents cited are incorporated herein by reference; the citation of any document is not to be construed as an admission that it is prior art with respect to the present invention. [0092] It is to be further understood that where descriptions of various embodiments use the term “comprising,” and / or “including” those skilled in the art would understand that in some specific instances, an embodiment can be alternatively described using language "consisting essentially of’ or "consisting of.”

[0093] While particular embodiments of the present invention have been illustrated and described, it would be obvious to one skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.

Claims

WHAT IS CLAIMED IS:

1. A sorbent chemical for CO2 separation comprising an amine functionalized with an epoxide chemical impregnated onto a mesoporous silica support, wherein: a. the amine is selected from the group consisting of polyethylenimine, tetraethylenepentamine, and pentaethylenehexamine; and b. the epoxide chemical is selected from the group consisting of isobutylene oxide (IBO), dimethyl-l,2-epoxybutane (DMEB), epoxy octane (EO), styrene oxide (SO), phenyl glycidyl ether (PGE), glycidyl 4-methoxyphenyl ether (GMPE), butyl glycidyl ether (BGE), glycidyl isopropyl ether (GIPE), 2- (tertbutoxymethyl oxirane) (TBGE), and neopentyl glycol diglycidyl (NGD).

2. The sorbent chemical of claim 1 wherein the epoxide chemical is selected from the group consisting of isobutylene oxide (IBO) and glycidyl isopropyl ether (GIPE).

3. The sorbent chemical of claim 1 wherein the amine is polyethylenimine.

4. The sorbent chemical of claim 1 wherein the amine is tetraethylenepentamine.

5. The sorbent chemical of claim 1 wherein the amine is pentaethylenehexamine.

6. A monolith block coated with the sorbent chemical of claim 1.

7. The sorbent chemical of claim 1 wherein the amine comprises one or more nitrogens (N) and the molar ratio of epoxide chemical to N in the amine is in the range from 0.1 to 0.4.

8. A method of removing carbon dioxide from carbon dioxide-laden ambient air, the method comprising: a. directing a flow of the carbon dioxide-laden ambient air through a monolith carbon dioxide capture structure that is supporting a sorbent, wherein the sorbent is capable of binding carbon dioxide to the sorbent, so as to remove carbon dioxide from the ambient air, b. exposing said monolith carbon dioxide capture structure to microwave energy from a mobile microwave radiator, c. regenerating the sorbent by directing microwave energy at the monolith carbon dioxide capture structure, thereby causing separation of carbon dioxide from the sorbent, d. moving the mobile microwave radiator to another monolith carbon dioxide capture structure for consecutive carbon dioxide desorption, and e. cyclically repeating said movements of the microwave radiator from one monolith carbon dioxide capture structure to another monolith carbon dioxide capture structure.

9. The method of claim 8 wherein the monolith carbon dioxide capture structure is irradiated with microwave energy at a DC voltage greater than 5 V.

10. The method of claim 8 wherein the monolith carbon dioxide capture structure is irradiated with microwave energy at a DC voltage in the range from 5-8 V.

11. The method of claim 8 wherein the monolith carbon dioxide capture structure is irradiated with microwave energy at a DC voltage of about 6.5 V

12. A method of removing carbon dioxide from carbon dioxide-laden ambient air, the method comprising: a. directing a flow of the carbon dioxide-laden ambient air through a monolith carbon dioxide capture structure that is supporting the sorbent of claim 1, wherein the sorbent is capable of binding carbon dioxide to the sorbent, so as to remove carbon dioxide from the ambient air, b. exposing said monolith carbon dioxide capture structure to microwave energy from a mobile microwave radiator, c. regenerating the sorbent by directing microwave energy at the monolith carbon dioxide capture structure, thereby causing separation of carbon dioxide from the sorbent, d. moving the mobile microwave radiator to another monolith carbon dioxide capture structure for consecutive carbon dioxide desorption, and e. cyclically repeating said movements of the microwave radiator from one monolith carbon dioxide capture structure to another monolith carbon dioxide capture structure.

13. The method of claim 12 wherein the epoxide chemical is selected from the group consisting of isobutylene oxide (IBO) and glycidyl isopropyl ether (GIPE).

14. The method of claim 12 wherein the amine of the sorbent is polyethylenimine.

15. The method of claim 12 wherein the amine of the sorbent is tetraethylenepentamine.

16. The method of claim 12 wherein the amine of the sorbent is pentaethylenehexamine.

17. The method of claim 12 wherein the monolith carbon dioxide capture structure is irradiated with microwave energy at a DC voltage greater than 5 V.

18. The method of claim 12 wherein the monolith carbon dioxide capture structure is irradiated with microwave energy at a DC voltage in the range from 5-8 V.

19. The method of claim 12 wherein the monolith carbon dioxide capture structure is irradiated with microwave energy at a DC voltage of about 6.5 V.