Capture and release of acid gasses using tunable organic solvents with aminopyridine

Abstract

A class of water lean, organic solvents that can bind with various acid gasses to form acid gas bound molecules having a high degree of intramolecular hydrogen bonding which enables their use as regenerable solvents for acid gas capture. Unlike the other devices described in the prior art, the present invention takes advantage of shortened distances between the portions of the molecule that form hydrogen bonds within the structures when loaded with an acid gas so as to create a molecule with a higher internal bonding affinity and a reduced proclivity for agglomeration with other molecules.

Claims

1. A method for capturing and releasing an acid gas from a stream containing the acid gas, the method comprising the steps of; contacting the stream with a gas selective capture sorbent comprising a liquid amino pyridine that reversibly binds an acid gas under a first set of conditions and releases said acid gas under a second set of conditions, whereby the acid gas binds to the gas selective capture sorbent to form a charge neutral bound sorbent under the first set of conditions, and releasing the acid gas from the sorbent by shifting the polarity of the sorbent away from the charge neutral state.

2. The method of claim 1 wherein the acid gas is released and the gas selective capture sorbent is regenerated without the use of a co-solvent.

3. The method of claim 1 wherein the acid gas is selected from the group consisting of CO.sub.2, SO.sub.2, COS, CS.sub.2, H.sub.2S and combinations thereof.

4. The method of claim 1 wherein the amino pyridine contains at least one R-group that is an alkyl amine or a dialkyl amine.

5. The method of claim 1 wherein the amino pyridine is selected from 2-MAMP; 3-MAMP; 4-MAMP; 2-EAMP; 4-EAMP; 2-MAEP; and derivatives and combinations thereof.

6. The method of claim 1 wherein the amino pyridine is a secondary amino pyridine that includes at least one R-group with a carbon number selected from C1 to C18.

7. The method of claim 6 wherein the at least one R-group is selected from the group consisting of methyl, ethyl, propyl, iso-propyl, butyl, iso-butyl, aryl, and combinations thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows an example of one exemplary embodiment of the invention.

(2) FIG. 2 shows an example of another embodiment of the invention

(3) FIGS. 3A and 3B show the structures of various other embodiments of the invention.

(4) FIGS. 4 and 5 show the rates of CO.sub.2 release and sorbent regeneration in various examples of the invention

(5) FIG. 6 shows the recyclability of one example of the present invention over several load and unload cycles.

DETAILED DESCRIPTION OF THE INVENTION

(6) The following description includes various embodiments of the present invention. It will be clear from this description of the invention that the invention is not limited to these illustrated embodiments but that the invention also includes a variety of modifications and embodiments thereto. Therefore the present description should be seen as illustrative and not limiting. While the invention is susceptible of various modifications and alternative constructions, it should be understood that there is no intention to limit the invention to the specific form disclosed, but, on the contrary, the invention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention as defined in the claims.

(7) The present detailed description includes various modified CO.sub.2 binding organic liquids (CO.sub.2BOLs) and various designs for tuning such materials to effectuate acid gas capture while imparting desired characteristics such as lowered levels of viscosity to the resulting bound materials. While these embodiments are shown and described in the context of CO.sub.2, it is to be understood that the invention is not limited thereto but includes other acid gasses as well. In one set of embodiments, water-lean non-aqueous amine-based solvents that form zwitterionic carbamates for acid gas capture are described. In another set of embodiments examples structures of various materials including various types of amino pyridines are shown as are acid gas capture methods and processes that utilize these materials.

(8) In one typical application and usage, the present invention includes a series of water lean or non-aqueous sorbents that are able to bind acid gasses such as CO.sub.2, SO.sub.2, COS, CS.sub.2, etc. at standard temperature and pressures (STP) to form liquid carbamate salts with a generally high gravimetric capacity (20 wt %) but retain a desired level of viscosity. As such the bound sorbents may be pumped or transferred from one location to another which enables the acid gas in one location to be captured under a first set of process conditions and then moved to another location where the acid gas can be removed by subjecting the bound material to a second set of conditions and ideally the sorbent regenerated for later use. Stripping of these acid gasses and regeneration of the underlying sorbent can take place when conditions, such as temperature, pressure or other stimuli are altered. This enables a cyclical process to take place wherein acid gasses are captured, bound, and then subsequently released with these binding materials recycled for reuse.

(9) The solvents described in the present application have low thermal regeneration temperatures (70-100 C.) and low CO.sub.2-rich solvent viscosity compared to other water-lean materials. The regeneration temperature can be lowered further to 60 C. by applying polarity swing assisted regeneration (PSAR) making it feasible to utilize lower grade heat from the power plant for acid gas stripping resulting in energy savings. While others have utilized aqueous acid gas capture technologies made up of primary and secondary alkanol amines such as monoethanolamine (MEA) or diethanolamine (DEA) in water for chemical absorption of acid gas capture materials like CO.sub.2, high regeneration temperatures (>120 C.), high steel corrosion due to the water load, and thermal degradation render these types of materials undesirable. In addition, most of these other solvents when amine-based require a co-solvent such as water or an added organic to dissolve the carbamate salt. The present embodiments which are amine-based operate in much less harsh (milder) conditions and do not require a co-solvent to dissolve the CO.sub.2 carrier to enable processing.

(10) In one set of embodiments principles and methods for the tunability of acid gas capture binding organic liquids for acid gas capture such as CO.sub.2 binding in CO.sub.2BOLs are described. A series of discoveries made at Pacific Northwest National Laboratory in Richland, Wash. USA by Vanda Glezakou, Roger Rousseau, and others have shown that single component CO.sub.2-rich CO.sub.2BOLs do not exist entirely as zwitterionic species but rather in a dynamic equilibrium between alkyl carbonic acid (the acid form [A]) and the zwitterion form [Z]. Tests performed on that zwitterion form revealed that various aspects of acid gas capture (and CO.sub.2 capture specifically) by water-lean solvent systems can be controlled by deliberate molecular modifications. Specifically, it was found that close proximity of amine and alcohol moieties and tunable acid/base equilibria play important roles in determining CO.sub.2 adsorption kinetics and bulk liquid viscosity.

(11) The close proximity of the quanidine and alcohol moieties facilitates the concerted mechanism of CO.sub.2 binding by the nucleophilic alcohol and concurrent proton transfer to the quanidine. The overall effect is fast CO.sub.2 binding kinetics associated with low entropic contribution to the free energy barrier. This proximity also enables stronger internal H-bonding that favorably reduces viscosity. A high acidity at the alcohol site allows for a more efficient CO.sub.2 activation at the transition state and an efficient proton transfer to the amine. In some embodiments non-charged CO.sub.2 capture solvent systems obtained by adjusting the acid/base properties of the solvent molecules, so that a significant fraction of the CO.sub.2-loaded molecules can exist in a partial or non-charged (acid) form may be preferable. This adjustment can be achieved by either increasing the acidity of the alcohol or by decreasing the basicity of the amine. Experiments have shown that non-charged CO.sub.2 capture systems exhibit appreciably lower viscosities than the analogous zwitterionic form due to decreased ionic interactions. The guidelines outlined here for controlling CO.sub.2 uptake kinetics and viscosity reduction can be ubiquitously applied to both carbonate and carbamate solvent systems.

(12) This proximity also enables stronger internal H-bonding that favorably affects viscosity and assists in enabling acid gas bound moieties to be pumped from one location to another. In one set of experiments the presence of a high acidity at the alcohol site allows for a more efficient CO.sub.2 activation at the transition state and an efficient proton transfer to the amine. This then leads to the concept that by adjusting the acid/base properties of the solvent molecules so as to obtain a charge neutral molecule rather than an ionic molecule as the art currently describes, that CO.sub.2 capture systems that exhibit appreciably lower viscosities than the analogous zwitterionic forms could be designed.

(13) Referring first to FIG. 1, an example of one exemplary embodiment of the invention is shown. FIG. 1 shows the methodology for the capture of an acid gas, in this case CO.sub.2 by a binding organic liquid containing liquid. From experiments it has been shown that the acid gas binding free energy is one of the deciding criteria in the design of gas separation solvents. In this particular instance, the binding organic liquid has been designed with angles that allow for a closer hydrogen bonding proximity between the positive portion of the capture molecule and the negative portion of the bound CO.sub.2 which results in reducing the charge on the zwitterion moving the molecule toward a charge neutral arrangement and a resulting desired lower viscosity because the individual molecules are structured in such a way so as to reduce agglomeration of CO.sub.2-bound molecules. These lower viscosities in turn enable higher loadings of the bound CO2, and flow and pumpability of the gas capture liquid to be maintained.

(14) A series of simulations on various capture molecules (in this case CO.sub.2BOLs) including IPADM-2-BOL, IPADM-3-BOL, IPATBM-2-BOL, and PADM-2-BOL showed that solvated CO.sub.2 in the vicinity of the alcohol where the radial rC-O distances were less than 2.00 binds CO.sub.2 in the form of an alkylcarbonate, while the H atom that originally belonged to the OH group remains on the guanidine N. For radial distances greater than 2.20 , 1-IPADM-2-BOL remains in its alcohol form, and CO.sub.2 is mostly linear with the OCO angle averaging 175. The angle decreases to 165 for rC-O distances between 2.0 and 2.2 . CO.sub.2 binding happens in an effectively concerted mechanism: at rC-O 2.00 , the LOCO angle decreases to 165 for rC-O distances between 2.0 and 2.2 . CO.sub.2 binding happens in an effectively concerted mechanism: at rC-O distances of 2.00 , the LOCO angle becomes 150 with a simultaneous H transfer to the nitrogen of the guanidine base. The CO.sub.2 structure is consistent with a partial charge transfer to form a CO.sub.2 (-) moiety and subsequent formation of a CO.sub.3-moiety in the IPADM-2-BOL. This stronger hydrogen bonding, coupled with charge neutrality, contributes to a reduction in agglomeration in the acid gas rich environment. This reduction in agglomeration in turn results in overall reduced viscosity in the system. In other embodiments, viscosity is adjusted based upon monitoring the free energy profile (i.e., G ([A][Z]) kJ/mol) and tuning the acid-base equilibria (Keq=[A]/[Z]) between the solvated and bound CO.sub.2 states.

(15) The free energy profile for binding CO.sub.2 to IPADM-2-BOL proceeds with a barrier of 16.51.2 kJ/mol and a binding free energy of 5.81.6 kJ/mol. The binding free energy is consistent with the experimentally obtained values for diazabicyclo[5.4.0]-undec-7-ene (DBU) containing dual-component CO.sub.2BOLs that range between 5.7 to 9.7 kJ/mol. This implies that at 40 C., there is an equilibrium between solvated and bound CO.sub.2. The free energy barrier of 16.5 kJ/mol and the activation energy of 9.8 kJ/mol are compatible with the experimental observation that this process readily occurs at 40 C.

(16) For aqueous monoethanolamine (MEA) capture liquids, the energy barrier is more than twice that of CO.sub.2BOLs: density functional methods give a barrier of 35.5 kJ/mol for dry MEA, and from 16 kJ/mol up to 63 kJ/mol for wet MEA; activation free energy estimates with the Arrhenius relation from experimental data are 46.7 kJ/mol. An estimate of activation free energy is only 7 kJ/mol higher than the activation energy, which is indicative of a small entropic contribution at the transition state, owing to the proximity of the acid/base moieties in the single component systems: unlike dual component systems, solvent re-organization at the transition state is not required. The relatively low barrier then suggests that capture in CO.sub.2BOLs is likely to be diffusion limited. Because the solvent viscosity increases exponentially with CO.sub.2 loading, the capture rate will decrease as more CO.sub.2 is added. These phenomena were observed when the CO.sub.2 absorption rates of single and dual-component CO.sub.2BOL solvents where measured with wetted-wall experiments. These findings and understandings enable the design of liquid carbamates that will have increased CO.sub.2 uptake capability, and enable the design of carbamate salts with decreased viscosities. By tuning the proximity of the alcohol and the amine so as to create structures that when bound to an acid gas have a preference for their own internal hydrogen bonding rather than agglomeration with other structures and creating molecules that are preferentially more charge neutral than ionic, the problems of high viscosity in an acid gas rich state as exists in other embodiments is reduced.

(17) Inspired by these findings, single component amines (e.g., amino pyridines) with various pendant coordinating bases were designed to create structures having a high degree of internal hydrogen bonding and/or acid-base equilibria favoring the non-charged acid state (e.g., [1:1] acid [A]:zwitterion [Z] ratio) upon capture and binding of the acid gas in the bound material.

(18) In one set of embodiments, modified CO.sub.2 binding organic liquids with structures favoring internal hydrogen bonded species in the bound material were utilized that gave resulting viscosities that were reduced compared to those forming primarily zwitterionic species. For example a modified 1-IPADM-2-BOL was created that formed 34% internal (neutral charged) hydrogen bonded species and 66% external hydrogen bonded species that gave a resulting viscosity of 110 (cP) at a CO.sub.2 loading of 25 wt %.

(19) In another set of embodiments, five modified CO.sub.2BOLs (1-MEIPADM-2-BOL, 1-IPADM-2-BOL, 1-IPADM-3-BOL, 1-IPATBM-2-BOL, PADM-2-BOL) were utilized. Table 1 lists fractions of internal hydrogen bonded species formed in these sorbent liquids at a CO.sub.2 loading of 25 wt %.

(20) TABLE-US-00001 TABLE 1 Internal H-Bonded Viscosity Viscosity CO.sub.2 Binding Species (MD Simulation) (Experimental) Organic Liquid (P.sub.int) (cP) (cP) 1-MEIPADM-2-BOL 52% 114 75 1-IPADM-2-BOL 34% 149 171 1-IPADM-3-BOL 21% 190 270 1-IPATBM-2-BOL 2% 499 Very viscous 1-PADM-2-BOL <1% 950 Solid
As these data show, selecting or modifying structures of the sorbent liquid and/or tuning acid-base equilibria to obtain an increasing number of internal hydrogen-bonded species in the bound material reduces viscosity of these CO.sub.2 binding organic liquids.

(21) In another set of experiments, various CO.sub.2 binding organic liquids were modified and tuned. The acid-base equilibria between the (solvated) organic acid and the conjugate base (i.e., bound CO.sub.2 state) in the capture liquid were adjusted, decreasing the free energy profile, and increasing the number of charge neutral species that resulted in decreasing the viscosity in the resulting CO.sub.2-bound material.

(22) In one example, modified CO.sub.2 binding organic liquids were created with reduced acidity of the pendant R-group of the coordinating base. A IPADM-2-BOL was modified by attaching an oxime moiety to the alcohol group of the coordinating base, reducing the free energy from 21.6 kJ/mol to 3.1 kJ/mol and tuning the acid-base equilibrium yielding a ratio of neutral-to-charged species (acid [A]:zwitterion [Z]) in the capture liquid from 1/4000 to 3/1 in the bound material. This resulted in lower viscosity and improved capture capacity.

(23) In another set of embodiments, modified CO.sub.2 binding organic liquids were utilized with structures modified to reduce basicity of the pyridine core, for example, by attaching acidic or electronegative moieties to the core structure. A 1-IPADM-2-BOL was modified by attaching fluorine to the pyridine core reducing the free energy in the capture liquid from 21.6 kJ/mol to 5.4 kJ/mol and tuning the acid-base equilibrium yielding a ratio of neutral-to-charged species in the capture liquid from 1/4000 to 8/1 in the bound material, resulting in lower viscosity and improved capture capacity.

(24) In another set of embodiments, a class of novel amino pyridine solvents was created. These non-aqueous amines demonstrated a high CO.sub.2 capture capacity regeneration temperatures less than 100 C., durability for absorption and regeneration over multiple cycles without degradation, and a high water tolerance. The structures of various examples of these materials are shown in FIGS. 2-3.

(25) In one set of embodiments, these amino pyridine solvents were created that were liquids at both CO.sub.2 free and rich states, and were structurally predisposed to stabilize the incipient carbamic acid upon reaction with CO.sub.2 through formation of stable 7 and 8 membered rings, respectively, via hydrogen bonding by the acidic proton and the 2-pyridine nitrogen. These solvents utilize the amine chemistry to bind CO.sub.2 as carbamates, with internal hydrogen bonds that enable liquid products, thus allowing for water-free or -lean CO.sub.2 capture liquids that otherwise would be unachievable under standard amine compositions of matter. These aminopyridine materials have unprecedented high CO.sub.2 capture capacity 20 wt %, with low regeneration energy. These amino pyridines form a new class of materials with potential applications in acid gas capture from flue gas of coal fired power plants, separation of acid gasses from natural gas streams and biogas, co-capture of acid gasses such as CO.sub.2, SO.sub.2, CO.sub.2, CS.sub.2 and H.sub.2S, and other combinations of acid gasses from natural gas and biogas streams.

(26) In one set of experiments six of these non-aqueous amine examples (2-MAMP, 3-MAMP, 4-MAMP, 2-EAMP, 4-EAMP, 2-MAEP) were shown to capture CO.sub.2 with high capture capacity. All of these example compounds were liquids in the CO.sub.2-rich state at room temperature, with a viscosity less than 300 cP at 40 C. Table 2 shows the CO.sub.2 capture capacity at 25 C. of a set of six of these compounds. Table 3 shows the CO.sub.2 capture capacity at 40 C. These results demonstrate that the CO.sub.2 capture capabilities of these amine materials are approximately equivalent. The slight increase in capture capacity at 40 C. is due to a reduced viscosity due to the rise in temperature from the exothermic reaction of CO.sub.2 with amines.

(27) TABLE-US-00002 TABLE 2 Compound CO.sub.2 Wt % CO.sub.2 Mol % 2-MAMP 19.7 54.8 3-MAMP 19.5 54.1 4-MAMP 19.7 54.8 2-EAMP 18.2 56.5 4-EAMP 18.3 56.7 2-MAEP 17.8 55.2

(28) TABLE-US-00003 TABLE 3 Compound CO.sub.2 Wt % CO.sub.2 Mol % 2-MAMP 21.1 58.5 3-MAMP 20.0 54.9 4-MAMP 18.6 51.5 2-EAMP 14.0 43.3 4-EAMP 16.7 51.8 2-MAEP 19.3 59.8

(29) After the CO.sub.2 is captured and held by the compounds as zwitterionic carbamate salts under a first set of conditions, the carbamate salts can be subjected to a second set of conditions such as higher temperatures, a reduction in pressure, mixing with other materials, or other activities which will cause the bound CO.sub.2 or other acid gas to be released and the underlying sorbent to be regenerated. Table 4 shows the results of a set of experiments performed on these same six compounds, wherein temperatures were raised and the percentage of CO.sub.2 released was measured. These examples show that most of these amine based sorbent materials can be sufficiently regenerated at temperatures of 100 C., while 2-EAMP has a regeneration temperature of 80 C. The rates of regeneration of these various materials at various temperatures are shown in FIGS. 4 and 5. As these figures show, the rates of regeneration are relatively fast with most of these materials releasing the bulk of their captured CO.sub.2 within 5 minutes. Some materials such as 2-EAMP can be completely stripped at a lower temperature (80 C.) in less than 10 minutes. In other embodiments other materials may release at different temperatures including lower temperatures.

(30) TABLE-US-00004 TABLE 4 Compound 70 C. 80 C. 100 120 C. 2-MAMP 33.8 60.5 90.3 3-MAMP 31.1 51.0 77.5 4-MAMP 35.1 51.3 93.8 2-EAMP 85 95.8 4-EAMP 71.4 85.1 98.8 2-MAEP 34.6 76.4 90.3

(31) In addition to acid gas release and regeneration of the solvent using changes in temperature, regeneration can be effectuated or supported using swings in pressure or polarity. In another set of experiments embodiments of water-lean solvents were constructed that would switch polarity upon binding with an acid gas such as CO.sub.2. When subjected to an external induced polarity reduction these materials released the CO.sub.2 and were regenerated to their prior form and were capable of reuse for acid gas capture. This process, named polarity swing assisted regeneration (PSAR), can be coupled with other methodologies for regeneration and enables for example the regeneration of materials at a lower temperature than would ordinarily be required or expected. In one set of experiments CO.sub.2 rich 2-EAMP was able to be 81 percent stripped and regenerated at 60 C. using PSAR with decane as anti-solvent.

(32) Experiments run on these amino pyridine compounds also demonstrated that over repeated cycles of CO.sub.2 absorption at 25 C. and stripping and regeneration of the sorbent at 100 C. that the integrity of the underlying amino pyridine solvent remained intact with no signs of degradation. Furthermore, while viscosity increased with CO.sub.2 loading, the viscosity of these amino pyridine compounds at lower temperatures generally remained at less than or equal to the viscosity of other compounds at higher temperatures (viscosity of 2-MAEP at 40 C. is nearly the same as viscosity of IPADM-2 BOL at 75 C.). Furthermore, the addition of up to 10 percent water to these liquids did not demonstrate any negative effect upon the CO.sub.2 uptake and release. A mixture of CO.sub.2 rich 2-MAMP with ten percent water remained a liquid with no precipitate, and maintained a bicarbonate-to-carbamate ratio of 5:4 per .sup.13C NMR analysis.

(33) While various preferred embodiments of the invention are shown and described, it is to be distinctly understood that this invention is not limited thereto but may be variously embodied to practice within the scope of the following claims. From the foregoing description, it will be apparent that various changes may be made without departing from the spirit and scope of the invention as defined by the following claims.