Note: Descriptions are shown in the official language in which they were submitted.
COOPERATIVE CHEMICAL ADSORPTION OF ACID GASES IN
FUNCTIONALIZED METAL-ORGANIC FRAMEWORKS
BACKGROUND
[0001] 1. Technical Field
[0002] The present technology pertains generally to fluid stream
separation
schemes and methods for producing metal-organic frameworks, and more
particularly to the production and use of metal-organic frameworks with
metal atoms that are coordinatively bound to polytopic linkers and ligands
lo that expose basic nitrogen atoms to the pore volumes and the flow of
gases.
[0003] 2. Background
[0004] Carbon dioxide generated from the combustion of fossil fuels
for
heat and electricity production is a major contributor to climate change and
ocean acidification. The predicted growth of the global economy and world
population in the near future will lead to an increased demand for energy,
resulting in even further increases in the concentration of CO2 in the
atmosphere. In 2012, coal and natural gas fired power plants released
more than 11.1 gigatons of carbon dioxide in to the atmosphere, which
accounts for nearly 30% of total global emissions.
[0005] To mitigate the effects of rising atmospheric CO2 levels
related to the
burning of fossil fuels, various strategies are used to control and capture
CO2 emissions. However, there few financial incentives to reduce CO2
emissions in many countries and existing carbon capture technologies are
simply too expensive to be practical at the scales required for large power
plants that can release several tons of CO2 per minute. The most
expensive component of any carbon capture and sequestration process is
usually the separation of CO2 from the other gases that are present in the
flue gas of a power plant. There is a need for the development of new
materials and processes to remove CO2 from flue gas using as little energy
and cost as possible.
[0006] While the exact composition of a flue gas depends on the design
of
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Date Recue/Date Received 2023-08-11
the power plant and the source of natural gas or coal, a mixture of mostly
N2, CO2, and H20 is present along with potentially more reactive gases that
are in lower concentrations, such as 02, S0x, NOx, and CO. Typical flue
gas is also released at ambient pressure and at temperatures ranging from
about 40 C to 80 C.
[0007] The separation of CO2 from H2 is also important in the context
of two
distinct applications: (i) the capture of pre and post combustion CO2
emissions like those produced from coal gasification power plants, and (ii)
the purification of hydrogen gas, which is synthesized on large scales
annually. Separation of CO2 from CH4 is another separation relevant to the
purification of natural gas, which can have up to 92% CO2 impurity at its
source. Carbon dioxide removal is required for approximately 25% of the
natural gas reserves in the United States. Removal of CO2, is typically
conducted at pressures between 20 bar and 70 bar with existing processes.
[0008] The removal of CO2 from low-pressure flue gas mixtures and other
CO2 gas separations is generally performed with aqueous amine solutions
that are selective for acid gases. Amines are known to be very selective
toward CO2 capture from flue gases or feedstock gases because of the
strong chemical bonds formed in the chemisorption process. However, the
use of these liquid materials has a number of drawbacks. Regeneration of
such absorbents is only possible at high temperatures and the system
therefore requires a high input of energy. In addition, corrosion inhibitors
need to be used with aqueous amine materials increasing cost, and amine
vapors can contaminate the gas streams that are being treated.
[0009] As a result of the large energy penalty for desorbing CO2 from such
liquid absorbents, solid adsorbents with significantly lower heat capacities
are frequently proposed as promising alternatives. Advanced solid
adsorbents also have the potential to decrease significantly the cost of CO2
removal from the effluent streams of fossil fuel-burning power plants.
[0010] Solid adsorbents, including zeolites, activated carbons, silicas,
and
metal-organic frameworks, have received significant attention as
alternatives to amine solutions, demonstrating high CO2 capacities and high
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selectivities for CO2 over N2, together with reduced regeneration energy
penalties. For example, zeolites have attracted attention as solid
adsorbents for carbon dioxide capture. Compared to aqueous
alkanolamine absorbents, zeolites require significantly less energy input for
adsorbent regeneration. However, zeolites have hydrophilic properties that
limit their application to separations that do not include water.
[0011] Activated carbon is another solid adsorbent for carbon dioxide
separations that requires less energy for regeneration and its hydrophobic
properties lead to better performance under moisture conditions compared
to zeolites. While the high surface area of activated carbon contributes to
much higher carbon dioxide capture capacities at high pressures, it does
not perform very well at low pressure ranges.
[0012] Metal organic frameworks, (M0Fs), an emerging class of
nanoporous crystalline solids built of metal coordination sites linked by
organic molecules, show promising properties for gas capture applications.
Due to their high surface areas and tunable pore chemistry, the separation
capabilities of certain metal-organic frameworks have been shown to meet
or exceed those achievable by zeolite or carbon adsorbents.
[0013] Although metal organic framework materials offer well-defined
porosity, high surface area, and tunable chemical functionalities, many
materials have hydrophilic properties that limit their application since it is
observed that the CO2 uptake capacity dramatically decreases in humid
conditions.
[0014] Accordingly, there is a need for efficient methods and
materials for
selectively separating constituent gases from a stream of gases that can be
performed at lower temperatures and pressures and regeneration energies
than existing techniques. There is also a need for materials and methods
that provide effective separations at low cost. The present invention
satisfies these needs as well as others and is generally an improvement
over the art.
BRIEF SUMMARY
[0015] The technology pertains to cooperative chemical adsorption of
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carbon dioxide in metal-organic frameworks and to metal-organic
frameworks as tunable phase-change adsorbents for the efficient capture
and separation of acid gases, as illustrated by carbon dioxide separations.
[0016] From the description herein it will be appreciated that
materials and
methods are provided that allow manipulation of a general mechanism
utilizing two adjacent amines on a metal-organic framework or other porous
structure to form an ammonium from one amine and a carbamate from the
other amine. The formation of one ammonium carbamate pair influences
the formation of ammonium carbamate on adjacent adsorption sites.
[0017] An acid gas is defined as any gas that can form a covalent bond with
an amine or other basic nitrogen group on the ligand or any gas that results
in the formation of ammonium with an amine upon adsorption. For
example, the methods can work for any gas that is capable of a chemical
reaction with an amine including CO2, S02, CS2, H2S, 503, SR2, RSH, NO2,
NO3, NO, BR3, and NR3 etc.
[0018] In the case of carbon dioxide separations, there are two
adsorption
sites that adsorb one CO2. Cooperativity occurs with more than just two
adsorption sites. A large number of amines adsorb CO2 at the same time
forming chains of ammonium carbamate. These chains spatially extend
along the pore surface in at least one direction. Aggregates can also form.
Adsorption sites adapt a regular, and repeating orientation. The new
orientation allows each site to contribute to the adsorption of two or more
CO2 molecules. Cooperative chemical adsorption may involve different
elements. The strength, nature, and number of covalent, coordinate,
hydrogen, and ionic bonds in the adsorbent or acid gas may increase or
decrease. New bonds form between the adsorbent and CO2 and existing
bonds between different components of the adsorbent may weaken or
break.
[0019] This cooperativity results in a large increase in the amount of
gas
adsorbed with only a small change in adsorption conditions. This is best
manifested as a discontinuity (step) in the adsorption isotherm. The metal-
organic frameworks for CO2 adsorption produce an unusually shaped
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Date Recue/Date Received 2023-08-11
isotherm (the relationship between CO2 adsorption amount and CO2
pressure at constant temperature). For traditional adsorbents, the first
derivative of the isotherm (in its functional function form of gas uptake
versus pressure) is always positive and its value decreases monotonically
as pressure is increased from low pressure to high pressure. For
cooperative adsorbents, the first derivative of the isotherm is also positive.
Before the step, the first derivative of the isotherm also decreases;
however, at the step point the value of the positive first derivative suddenly
increases over a pressure regime. After the step concludes, the first
derivative resumes the expected decrease with increasing pressure. It is
possible for more than one step to exist in each isotherm.
[0020] It was found that the reason for the isotherm shape is the
mechanism by which CO2 is adsorbed. An amine which was previously
bonded to a metal-organic framework is reorganized. The reorganization is
dependent on the identity of the metal atoms in the framework. The
mechanism is general to metal sites with closely spaced amines (or other
atoms) coordinated to them.
[0021] For measurements at two different temperatures, the isotherm
step
moves to higher pressures at higher temperatures. Unlike other adsorbents,
the shape of the isotherm allows the material to adsorb CO2 more efficiently
at higher temperatures. Most adsorbents adsorb CO2 less efficiently with
higher temperatures. Advantages of CO2 adsorption at higher temperatures
include reducing the amount of water adsorbed, reducing the size of the
adsorption bed, and reducing the temperature swing of the material
between adsorption and regeneration.
[0022] It can be seen that this mechanism is different from how other
amine-based adsorbents capture CO2 or other acid gases and that by
understanding the mechanism it has been possible to tune the CO2
adsorption isotherm to match the step position with the partial pressure of
CO2 in the gas mixture. Initially, in the present mechanism for CO2 capture,
the CO2 binding involves breaking a nitrogen-element bond, where the
element is not hydrogen. All other amine-based adsorbents are understood
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Date Recue/Date Received 2023-08-11
to bind CO2 by breaking a nitrogen-hydrogen bond.
[0023] Secondly, it has been shown that the CO2 adsorption isotherm
step
position is related to the metal-amine bond strength. This is not how other
amine-based CO2 adsorbents work. It is possible to match the step position
to the concentration of gas for removal of an acid gas, particularly CO2, by
changing the strength of metal-amine bond in the framework. Adsorbent
stability is also increased by changing metal-amine bond strength.
[0024] The location of the isotherm step is also dependent upon many
things that can be controlled including the composition of the porous
adsorbent (preferably a metal-organic framework), the temperature of
adsorption, the pressure of the adsorptive, the composition of the gas
mixture, the entropy of the gas mixture, and the manner in which the
material was previously treated.
[0025] For example, costs can be reduced by adsorbing at higher
temperatures rather than lower temperatures in some settings. The heat
transfer from adsorbent to cooling fluid can be increased by adsorbing at
higher temperatures rather than at lower temperatures. Stepped isotherms
can also be used to raise or lower the temperature of regeneration thereby
reducing the cost of regeneration.
[0026] In other settings, the amount of non-target gases (H20, S02, N2,
etc.) that are adsorbed can be reduced by changing the substituent's on the
diamine. Adsorbent stability can be increased by using amines with boiling
points above regeneration temperature of the adsorbent (i.e. using less
volatile amines).
[0027] According to one aspect of the technology, a metal-organic
framework family is provided that has a functionalized surface having two
adjacent amines wherein an ammonium is formed from one amine and a
carbamate is formed from the other amine. The metal-organic frameworks
have a functionalized pore surface having adjacent amine adsorption sites
that adsorb at least one CO2 molecule. Adsorption sites adapt a regular,
and repeating orientation. The new orientation allows each site to contribute
to the adsorption of two or more CO2 molecules. Adsorption also occurs
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Date Recue/Date Received 2023-08-11
without a significant change in the volume of the adsorbent.
[0028] According to another aspect of the technology, a cooperative
chemical adsorption method is provided using a metal-organic framework
that has functionalized surface locations with two adjacent amines. An
ammonium is formed from one of the amines and a carbamate is formed
from the other amine and a CO2 molecule is adsorbed with the adjacent
amines.
[0029] In another aspect of the technology, the two adjacent
adsorption
sites are a subset of a plurality of adsorption sites where cooperativity
occurs and the amines adsorb CO2 at the same time and form chains or
aggregates of ammonium carbamate. These chains of ammonium
carbamate extend along the surface of the metal-organic framework in at
least one direction.
[0030] Carbon dioxide adsorption applications include: removing CO2
from
outside air; removing CO2 from air people breath; removing CO2 as a
greenhouse gas from the emissions of power plants; removing CO2 from
natural gas; removing CO2 from oxygen; sensor for the presence of CO2;
using the heat of adsorption for making heat; and the use of the adsorbent
as a heat pump.
[0031] Further objects and aspects of the technology will be brought out in
the following portions of the specification, wherein the detailed description
is for the purpose of fully disclosing preferred embodiments of the
technology without placing limitations thereon.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS
OF THE DRAWINGS
[0032] The technology described herein will be more fully understood
by
reference to the following drawing which is for illustrative purposes only:
[0033] FIG. 1 is a schematic flow diagram of a method of gas
separations
according to one embodiment of the technology.
[0034] FIG. 2A is an idealized adsorption isotherm with a typical
Langmuir-
type isotherm shape.
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Date Recue/Date Received 2023-08-11
[0035] FIG. 2B is a step shaped isotherm of one embodiment of the
present
technology.
[0036] FIG. 3A through FIG. 3C depicts the mechanism for CO2
adsorption
at three neighboring M¨mmen sites within an infinite one-dimensional chain
of such sites running along the crystallographic c axis of a mmen-
M2(dobpdc) compound. Simultaneous proton transfer and nucleophilic
attack of N on a CO2 molecule forms an ammonium carbamate species that
destabilizes the amine coordinated at the next metal site, initiating the
cooperative adsorption of CO2 by a chain reaction.
[0037] FIG. 4 depicts CO2 adsorption isotherms at 40 C shown on a linear
scale for N,N'-dimethylethylenediamine derivatives of Mg2(dobpdc),
Mn2(dobpdc), Fe2(dobpdc), Co2(dobpdc), and Zn2(dobpdc).
[0038] FIG. 5 depicts CO2 adsorption isotherms at 40 C shown on a
logarithmic scale for N,N'-dimethylethylenediamine derivatives of
Mg2(dobpdc), Mn2(dobpdc), Fe2(dobpdc), Co2(dobpdc), and Zn2(dobpdc).
[0039] FIG. 6 depicts CO2 adsorption isotherms at 40 C shown on a
logarithmic scale for N,N'-dimethylethylenediamine derivatives of
Mg2(dobpdc) and Co2(dobpdc). The difference in the isotherm step position
is attributable to changes in the metal-amine bond strength differences
between Mg and Co.
[0040] FIG. 7 depicts CO2 adsorption isotherms at 40 C shown on a
logarithmic scale for N,N'-dimethylethylenediamine and N,N-
dimethylethylenediamine derivatives of Mg2(dobpdc). The difference in the
isotherm step position is attributable to changes in the metal-amine bond
due to changes in the steric and electronic properties of the amine that is
bonded to the metal site.
[0041] FIG. 8 depicts CO2 adsorption isotherms at 100 C shown on a
logarithmic scale for N-methylethylenediamine, N-ethylethylenediamine,
and N-isopropylethylenediamine derivatives of Mg2(dobpdc). The difference
in the isotherm step position is attributable to changes in bond strengths
that result from the steric and electronic property differences that occur as
alkyl group of different sizes are included in the framework.
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Date Recue/Date Received 2023-08-11
[0042] FIG. 9 depicts CO2 adsorption isotherms at 25 C shown on a
logarithmic scale for N,N-dimethylethylenediamine, N,N-
diethylethylenediamine, and N,N-diisopropylethylenediamine derivatives of
Mg2(dobpdc). The difference in the isotherm step position is attributable to
differences in the strength of the tertiary amine with the accepted proton
(the ammonium species) and the strength of the ammonium group ¨
carbamate group ammonium interaction due to the presence of different
amounts of steric bulk.
[0043] FIG. 10 depicts CO2 adsorption isotherms at 120 C shown on a
logarithmic scale for ethylenediamine, N-methylethylenediamine, and N,N'-
dimethylethylenediamine derivatives of Mg2(dobpdc). The difference in the
isotherm step position is attributable to increasing the strength of
interaction
between the amine and CO2 that form the carbamate or increasing the
strength of the ammonium carbamate interaction can overcome the
increased metal-amine bond strength associated with primary amines.
[0044] FIG. 11 depicts CO2 adsorption isotherms at 50 C shown on a
logarithmic scale for N,N'-dimethylethylenediamine and N,N'-
dimethylpropylenediamine derivatives of Mg2(dobpdc). The difference in the
isotherm step position and number of steps is attributable to changing the
length of the bridge between the two diamines.
[0045] FIG. 12 depicts CO2 adsorption isotherms at 120 C shown on a
logarithmic scale for ethylenediamine, 1,2-diaminopropane, and 1,2-
diaminocyclohexane derivatives of Mg2(dobpdc). The difference in the
isotherm step position and number of steps is attributable to changing the
nature of alkyl groups on the alkyl bridge that connects the two diamines.
[0046] FIG. 13A depicts CO2 adsorption isotherms at 40 C shown on a
logarithmic scale for N,N'-dimethylethylenediamine derivatives of
Mg2(dobpdc) and Mn2(dobpdc). The difference in the isotherm step position
is attributable to changes in the entropy of the metal-organic frameworks
associated with translational, vibrational, and rotational motions of the
diamines prior to CO2 adsorption.
[0047] FIG. 13B depicts isosteric heats of adsorption calculations
indicating
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Date Recue/Date Received 2023-08-11
nearly identical heats of CO2 adsorption onto the N,N'-
dimethylethylenediamine derivatives of Mg2(dobpdc) and Mn2(dobpdc).
Differences in CO2 step position are thus associated with entropic effects
associated with translational, vibrational, and rotational motions of the
diamines prior to CO2 adsorption.
[0048] FIG. 14 depicts H20 adsorption isotherms at 40 C shown on a
linear
scale for N,N-dimethylethylenediamine, N,N-diethylethylenediamine, and
N,N-diisopropylethylenediamine derivatives of Mg2(dobpdc). The difference
in the amount of water adsorbed at a particular pressure is attributable to
lo using alkyl groups of various sizes to reduce the amount of pore space
available for non-acid gas molecules to adsorb.
DETAILED DESCRIPTION
[0049] Referring more specifically to the drawings, for illustrative
purposes,
embodiments of the apparatus and methods for gas separations are
generally shown. One embodiment of the technology is described
generally in FIG. 1 to illustrate the methods. It will be appreciated that the
methods may vary as to the specific steps and sequence and the apparatus
may vary as to structural details without departing from the basic concepts
as disclosed herein. The method steps are merely exemplary of the order
that these steps may occur. The steps may occur in any order that is
desired, such that it still performs the goals of the claimed technology.
[0050] Turning now to FIG. 1, one method 10 for separating acid gases
from a stream of gases using functionalized porous frameworks with
controlled step-shaped isotherms is generally shown. The apparatus
configuration and separation conditions can be optimized for gas separation
capacity, temperature and pressure swings and regeneration energy.
[0051] At block 20 of the method of FIG. 1, an initial evaluation of
the
separation parameters is made. Typical separations include pre-
combustion feedstock gas separations such as the removal of carbon
dioxide from natural gas, digester gas, or syngas as well as post
combustion separations such as flue gas streams.
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Date Recue/Date Received 2023-08-11
[0052] The selection of the type of framework or particular framework
can
account for the composition of the gases to be treated and the temperature
and pressure at the time of presentation to the separator. The regeneration
energy and temperature swing requirements and framework cost, stability
and reactivity can also be considered in the selection of the framework
configuration and the functionalizing ligands.
[0053] The framework that is selected at block 20 is prepared at block
30 of
FIG. 1. The preferred frameworks for acid gas separations are porous
metal-organic framework compositions of metal atoms coordinatively bound
to polytopic organic linkers that have pores whose dimensions that permit
the flow of gases and have interior surfaces that expose coordinatively
unsaturated metal ions. The framework is further functionalized with
ligands that are bound to the coordinatively unsaturated metal ions that
expose basic nitrogen atoms to the pore volumes. Ligands are prepared
and the framework is functionalized at block 40.
[0054] The preferred frameworks that are prepared at block 30 are
metal
organic frameworks that have metal atoms in an oxidation state appropriate
to binding with both a polytopic linker and basic nitrogen ligand elements.
In one embodiment, the metal is one or more atoms selected from the
group Al, Be, Ca, Cd, Co, Cr, Cu, Fe, Mg, Mn, Ni, Sc, Ti, V, and Zn.
[0055] CO2 adsorption isotherms at 40 C shown on a linear scale and
logarithmic scale for N,N'-dimethylethylenediamine derivatives of
Mg2(dobpdc), Mn2(dobpdc), Fe2(dobpdc), Co2(dobpdc), and Zn2(dobpdc)
are shown in FIG. 4 and FIG. 5 respectively.
[0056] The structure of the metal organic framework is determined in part
by the rigid or semi-rigid polytopic organic linkers that are used in its
formation. Preferred polytopic linker molecules include aromatic
compounds with two or more functional groups such as pyrazolate (-
C3H2N2-), triazolate (¨C2HN3-), tetrazolate (¨CN4-) or carboxylate (-0O2-)
groups.
[0057] In one embodiment, the polytopic linker is composed of one or
more
linkers selected from the group: 1,3,5-benzenetripyrazolate; 1,3,5-
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Date Recue/Date Received 2023-08-11
benzenetristriazolate; 1,3,5-benzenetristetrazolate, 1,3,5-
benzenetricarboxylate; 1,4-benzenedicarboxylate; and 2,5-dioxido-1,4-
benzenedicarboxylate.
[0058] In another embodiment, the linkers containing at least two
cyclic
rings, two carboxylate groups, and two oxido groups such as 4,4'-dioxido-
3,3'-biphenyldicarboxylate and 4'-4"-dioxido-3',3"-terphenyldicarboxylate.
[0059] There are many combinations of metals and polytopic linkers
that
can be fashioned to provide metal organic frameworks with desirable pore
sizes and open metal sites. For example, in one embodiment, the
framework has a metal is selected from the group Ca, Fe, Mn, Cu, Co, Ni,
Cr, or Cd and the polytopic liker is 1,3,5-benzenetripyrazolate. In another
embodiment, the metal is selected from the group Ca, Fe, Mn, Cu, Co, Ni,
Cr, or Cd and the polytopic linker is 1,3,5-benzenetristetrazolate.
[0060] In another embodiment, the metal is selected from the group Cr,
Mn,
Fe, Co, Ni, or Cu and the polytopic linker is 1,3,5-benzenetristriazolate.
[0061] Another embodiment has a framework where the metal is selected
from the group Cd, Fe, Al, Cr, Ti, Sc or V and the polytopic linker is 1,3,5-
benzenetriscarboxylate or 1,4-benzenedicarboxylate.
[0062] Yet another framework has metal selected from the group Mg, Ca,
Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu or Zn and the polytopic linker is 2,5-
dioxido-1,4-benzenedicarboxylate.
[0063] Another preferred framework has a metal selected from the group
Mg, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu or Zn and the polytopic ligand is
4,4'-dioxidobipheny1-3,3'-dicarboxylate.
[0064] The functionalizing ligand that is selected and prepared at block 40
of FIG. 1, preferably has an amine that will expose basic nitrogen atoms
within the pore volume when bound to the metal organic framework.
However, other basic nitrogen groups can be used. For example, in one
embodiment, the basic nitrogen ligand is a primary, secondary, or tertiary
alkylamine. In another embodiment, the basic nitrogen ligand is a primary
or secondary imine. The preferred functionalizing ligand prepared at block
is a diamine. Suitable ligand diamines include: ethylenediamine,
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Date Recue/Date Received 2023-08-11
propylenediamine, butylenediamine, pentylenediamine, hexylenediamine,
1,2-propanediamine, 2,3-butanediamine, 1,2-diamino-2-methylpropane, N-
boc-ethylenediamine, N-ethylethylenediamine, N,N'-
diethylpropylenediamine, N,N-diethylethylenediamine, N-
isopropylethylenediamine, N,N'-diisopropylethylenediamine, N-
isopropylpropylenediamine, N,N'-diisopropylpropylenediamine, N,N1-
diisopropylethylenediamineõ N-methylethylenediamine, N,N'-
dimethylethylenediamine, N-methylpropylenediamine, N,N'-
dimethylpropylenediamine, 1,3-diaminocyclohexaneõ N,N-
dimethylethylenediamine, N,N,N'-trimethylethylenediamine, N,N,N',N'-
tetramethylethylenediamine, N-trimethylsilylethylenediamine, N,N-
bis(trimethylsilyl)ethyleneidmaine, N,N'-bis(trimethylsilyl)ethyleneidmaine,
N,N-dimethylpropylenediamine, N,N,N'-trimethylpropylenediamine,
N,N,N',N'-tetramethylpropylenediamine, diethylenetriamine, 2-(2-
aminoethyoxy)ethylamine, dipropylenetriamine, 1,2-diaminocyclohexane,
piperazine, and tris(2-aminoethyl)amine. Other ligands include 2-
(Diisopropylphosphino)ethylamine N-methylethanolamine, and
monoethanolamine.
[0065] Accordingly, the mechanism can work for separating any acid gas
that can chemically react with an amine including CO2, SO2, CS2, H2S, 503,
SR2, RSH, NO2, NO3, NO, BR3 and NR3.
[0066] For carbon dioxide separations, a diamine or polyamine ligand
is
particularly preferred. In this case the metal-organic framework
composition has adjacent amine groups where exposure to CO2 results in
reversible formation of an ammonium carbamate complex from pairs of
adjacent amines. Here adjacent amine groups have basic nitrogen atoms
separated by less than 1 nm. This proximity allows a proton transfer to
occur. For example CO2 binding of this type is not achieved without proton
transfer from one amine to the next to form an ammonium carbamate ion
pair. The formation of a first ammonium carbamate complex lowers
energetic barriers that enable subsequent complexes to be formed under
the same conditions of temperature and pressure.
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Date Recue/Date Received 2023-08-11
[0067] One particularly preferred configuration for carbon dioxide
separations is a framework formed from a 4,4'-dioxidobipheny1-3,3'-
dicarboxylate polytopic ligand and a basic nitrogen ligand of N,N'-
dimethylethylenediamine and the metal is selected from the group Mg, Ca,
Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu or Zn.
[0068] The frameworks that are prepared at blocks 30 and 40, are
tunable
phase-change adsorbents that permit the cooperative chemical adsorption
of carbon dioxide and other acid gases in metal-organic frameworks for
efficient gas separations and capture.
[0069] It has been shown that the metal-organic frameworks for CO2
adsorption, for example, produce an unusually shaped isotherm (the
relationship between CO2 adsorption amount and CO2 pressure at constant
temperature) that gives these materials excellent properties. It was found
that the reason for the isotherm shape is the mechanism by which the acid
gas is adsorbed. The amine ligand which was previously bonded to a
metal-organic framework is reorganized. The reorganization is dependent
on the metal of the framework. The mechanism is general to metal sites
with closely spaced amines (or other atoms) coordinated to them. Unlike
other adsorbents, the shape of the isotherm allows the material to adsorb
CO2 more efficiently at higher temperatures as most adsorbents adsorb
CO2 less efficiently with higher temperatures.
[0070] FIG. 2A depicts variations in idealized CO2 adsorption behavior
with
temperature for a classical microporous adsorbent showing the usual
Langmuir-type isotherm shape. This can be compared with the isotherm
shape of FIG. 2B of the phase-change adsorbent showing a step-shaped
(sometimes referred to as 'S-shaped') isotherm. The double-headed black
arrow in FIG. 2A and FIG. 2B indicates the working capacity (that is, the
amount of gas removed) for a separation performed using a temperature
swing adsorption process in which selective adsorption occurs at
Pads and
Tiow and desorption is performed at P . des and Thigh for a classical
adsorbent
or Tmedium for the phase change adsorbent described herein.
[0071] FIG. 2A and FIG. 2B illustrates the extraordinary advantages
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Date Recue/Date Received 2023-08-11
associated with utilizing an adsorbent exhibiting step-shaped isotherms in a
temperature swing adsorption process versus the Langmuir-type isotherms
observed for most microporous adsorbents. For carbon capture
applications, a gas mixture containing CO2 at low pressure (Pads) and low
temperature (Tiow) is contacted with the adsorbent, which selectively
adsorbs a large amount of CO2. The adsorbent is heated to liberate pure
CO2 with a partial pressure of P . des, and is then reused for subsequent
adsorption/desorption cycles. For a classical adsorbent isotherm as shown
in FIG. 2A, the steepness of the isotherm gradually diminishes as the
temperature increases, necessitating a high desorption temperature to
achieve a significant working capacity for a separation. In contrast, for a
phase-change adsorbent of the type described here (FIG. 2B), the position
of the isotherm step shifts dramatically to higher pressures as the
temperature increases, such that a large working capacity can be achieved
with only a small increase in temperature. The CO2 adsorption isotherm can
be tuned to match the step position with the partial pressure of CO2 in the
gas mixture. For an efficient carbon capture process, one would ideally
create a phase-change adsorbent with a large vertical step positioned just
below the partial pressure of CO2 in the flue gas.
[0072] It has been shown that the CO2 adsorption isotherm step position is
primarily related to the metal-amine bond strength. Manipulation of the
bond strength will allow control over the isotherm step positions that are
shown in FIG. 2B. For example, the metal-heteroatom (particularly metal-
amine) bond strength, and therefore CO2 step position, can be adjusted by
changing the identity of the metal in the framework. Likewise, the metal-
heteroatom bond strength, and therefore the CO2 step position, can be
adjusted by changing the identity of the heteroatom.
[0073] For example, functionalization of Mg2(dobpdc) and Co2(dobpdc)
frameworks with N,N'-dimethylethylenediamine results in two frameworks
that vary by only the identity of the metal. As shown in FIG. 6, the
difference
in the isotherm step position is attributable to changes in the metal-amine
bond strength differences between Mg and Co. The strength of the amine-
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Date Recue/Date Received 2023-08-11
Co bond is expected to be stronger than the amine-Mg bond. For
cooperative adsorption to occur, the amine-metal bond must first break.
The stronger amine-Co bond results in a less exothermic adsorption
process. Thus, the step in the Co adsorbent occurs at higher pressures
than the Mg adsorbent because the bond strength was tuned to change the
overall enthalpy of the reaction as illustrated in FIG. 6.
[0074] Another way of tuning the metal-amine bond strength to change
the
step position is by changing the amine ligand. For example, Mg2(dobpdc)
can be functionalized with two isomers of dimethylethylenediamine. N,N-
dimethylethylenediamine (one primary amine and one tertiary amine
(abbreviated as 1 -ethyl-3 ) and N,N'-dimethylethylenediamine (two
secondary amines abbreviated for this discussion 2 -ethyl-2 ). For 1 -ethyl-
3 the primary amine is the better ligand for the metal and will coordinate
stronger than the all secondary amine 2 -ethyl-2 . Because the primary
amine-Mg bond is stronger than the secondary amine-Mg bond, the
adsorption reaction containing 1 -ethyl-3 will be less exothermic than the
reaction with the adsorbent containing only 2 -ethyl-2 . Thus, by changing
the nature of the amine preferentially coordinated to the metal cation the
position of the step can change.
[0075] Similarly, Mg2(dobpdc) can be functionalized with a series of
diamines containing one primary amine and one secondary amine. The
alkyl group of the secondary amine was varied to include N-
methylethylenediamine, N-ethylethylenediamine, and N-
isopropylethylenediamine. At any temperature the step position of the
methyl group containing material occurs before the step position of the ethyl
group-containing compound. Similarly, the step position of the ethyl group
occurs between the isopropyl containing material. These changes are
attributable to changes in bond strengths owing to the steric and electronic
property differences that occur as alkyl group of different sizes are included
in the framework as shown in FIG. 8.
[0076] Metal-heteroatom bond strength, and therefore CO2 step
position,
can also be adjusted by changing the identity of the substituents on the
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Date Recue/Date Received 2023-08-11
heteroatom. Specifically, metal-amine bond strength, and therefore CO2
step position, can be adjusted by changing identity of the amine steric
and/or electronic properties. As shown in FIG. 7, the difference in the
isotherm step position is attributable to changes in the metal-amine bond
owing to changes in the steric and electronic properties of the amine that is
bonded to the metal site.
[0077] For example, Mg2(dobpdc) can be functionalized N,N'-
dimethylethylenediamine and N,N'-dimethylpropylenediamine. By changing
the length of the bridge that separates the two diamines, the position and
number of steps in the isotherm can be changed since the orientation and
energetics of the ammonium carbamate changes. FIG. 11 depicts CO2
adsorption isotherms at 50 C shown on a logarithmic scale for N,N'-
dimethylethylenediamine and N,N'-dimethylpropylenediamine derivatives of
Mg2(dobpdc). The difference in the isotherm step position and number of
steps is attributable to changing the length of the bridge between the two
diamines.
[0078] Mg2(dobpdc) can be functionalized 1,2-diaminopropane and 1,2-
diaminocyclohexane. By changing the identity of the substituents on the
carbon bridge that separates the two amines, the position of the steps is
shifted versus ethylenediamine due to energy differences created by the
steric bulk. For example, FIG. 12 depicts CO2 adsorption isotherms at 120
C shown on a logarithmic scale for ethylenediamine, 1,2-diaminopropane,
and 1,2-diaminocyclohexane derivatives of Mg2(dobpdc). The difference in
the isotherm step position and number of steps is attributable to changing
the nature of alkyl groups on the alkyl bridge that connects the two
diamines.
[0079] The step is associated with a large change in entropy that
occurs
when disordered amines and CO2 become organized into chains. The initial
entropy of the framework is related to the strength of the metal-amine bond
and to what extent the diamine can move with translation, vibrational, and
rotational degrees of freedom. Thus, two adsorbents can have steps in
different positions despite very similar heats of adsorption owing to entropy
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Date Recue/Date Received 2023-08-11
differences between the materials. This is exemplified by Mg2(dobpdc) and
Mn2(dobpdc) which possess nearly identical heats of adsorption but
different step positions related to the extent that amines are dynamic on the
pore surfaces. See FIG. 13A and FIG. 13B. The differences in CO2 step
position are thus associated with entropic effects associated with the
translational, vibrational, and rotational motions of the diamines prior to
CO2
adsorption.
[0080] The strength of the C-N bond in a carbamate and the strength of
the
N-H bond of the ammonium can be changed by altering the steric and/or
electronics of the amine. The strength of the ionic interactions between the
ammonium and carbamate can also be changed by altering the stenos
and/or electronics of both amines. The strength of the carbamate-metal
bond can also be changed by changing the identity of the metal.
[0081] For example, Mg2(dobpdc) can be functionalized ethylenediamine,
N-methylethylenediamine, and N,N'-dimethylethylenediamine. FIG. 10
depicts CO2 adsorption isotherms at 120 C shown on a logarithmic scale
for ethylenediamine, N-methylethylenediamine, and N,N'-
dimethylethylenediamine derivatives of Mg2(dobpdc). The difference in the
isotherm step position is attributable to increasing the strength of
interaction
between the amine and CO2 that form the carbamate or increasing the
strength of the ammonium carbamate interaction can overcome the
increased metal-amine bond strength associated with primary amines.
[0082] The step of the adsorbent containing all primary amines occurs
prior
to the step of the adsorbent containing one primary and one secondary
amine. Similarly, the step of the adsorbent containing one secondary amine
occurs before the step of the adsorbent containing two secondary amines.
Thus, increasing the strength of interaction between the amine and CO2
that form the carbamate or increasing the strength of the ammonium
carbamate interaction can overcome the increased metal-amine bond
strength associated with primary amines.
[0083] Similarly, Mg2(dobpdc) can be functionalized with a series of
diamines containing one primary amine and one tertiary amine. The identity
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Date Recue/Date Received 2023-08-11
of the alkyl groups on the tertiary amines can be varied to include N,N-
dimethylethylenediamine, N,N-diethylethylenediamine, and N,N-
diisopropylethylenediamine. Due to the reduced steric bulk of the primary
amine, the primary amine is expected to coordinate to the metal cation in all
three cases. Because of the presence of hydrogen atoms on only the
primary amine, the carbamate must form on the primary amine end of the
diamine while ammonium must form of the carbamate end of the diamine.
At any temperature, the step position of the dimethyl containing adsorbent
occurs before the step of the diethyl containing adsorbent. Similarly, the
step of the diethyl containing adsorbent occurs before the step of the
diisopropyl containing compound. These changes are attributable to
differences in the strength of the tertiary amine with the accepted proton
(the ammonium species) and the strength of the ammonium group ¨
carbamate group ammonium interaction.
[0084] FIG. 9 depicts CO2 adsorption isotherms at 25 C shown on a
logarithmic scale for N,N-dimethylethylenediamine, N,N-
diethylethylenediamine, and N,N-diisopropylethylenediamine derivatives of
Mg2(dobpdc). The difference in the isotherm step position is attributable to
differences in the strength of the tertiary amine with the accepted proton
(the ammonium species) and the strength of the ammonium group ¨
carbamate group ammonium interaction owing to the presence of different
amounts of steric bulk.
[0085] The adsorption properties can also be changed by altering the
character of the connections between the two amines, including (adding
extra alkyl groups to the connection, using cyclic hydrocarbons (such as
cyclohexane), and changing the chirality of the amine positions.
[0086] The adsorption of acid gases is related to the interaction of
multiple
amines with acid gases to cooperatively adsorb molecules. Other non-acid
gases do not adsorb via cooperative mechanisms including H20, N2, and
hydrocarbons (including but not limited to CH4). Through variation of the
amine sterics, the surface area available for other gases to adsorb onto can
be changed without altering the volumetric capacity of the adsorbent for
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Date Recue/Date Received 2023-08-11
acid gases such as CO2. Thus, variations of the amine stenos can be used
to increase or decrease the amount of other gases adsorbed onto other
accessible pore spaces.
[0087] The adsorption of acid gases, especially CO2, occurs via
insertion of
CO2 into the metal-amine bonds to form carbamates. During desorption, the
potential for amine loss exists due to the disconnection between the amine
and the framework. Thus, the rate of adsorbent degradation can be
controlled by changing the strength of the metal-amine bond, such that
stronger bonds will reduce amine volatility. Furthermore, heavier amines
will generally exhibit increased boiling points. Thus, inclusion of steric
groups can be used to decrease amine volatility such that the boiling point
of the pure amine will be a higher temperature than the optimum adsorbent
regeneration temperature. In addition, the difference in the amount of water
adsorbed at a particular pressure is attributable to using alkyl groups of
various sizes to reduce the amount of pore space available for non-acid gas
molecules to adsorb as shown in FIG. 14.
[0088] Accordingly, exercise of control over the various bond
strengths can
allow control over: (i) optimum adsorption temperature/pressure, (ii)
optimum desorption temperature/pressure; and (iii) the heat of
adsorption/desorption.
[0089] Once the functionalized framework has been prepared and the
operational parameters have been identified, a mixture stream of gases can
be exposed to the solid-phase material for separation at block 50 of FIG. 1
to produce a gaseous stream depleted in CO2 and a solid-phase
composition enriched in CO2 or other acid gas. The separation conditions
as well as the composition of the solid-phase material are also controlled to
optimize the separations at block 50.
[0090] At block 60, the solid-phase composition enriched in acid gas
is
regenerated so that it can be reused. This is usually accomplished by
exposure of the solid-phase composition to elevated temperatures to
release the separated gas that is subsequently removed from the system.
[0091] To increase the fraction of CO2 removed from the gas stream the
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Date Recue/Date Received 2023-08-11
location of the isotherm step should be shifted to lower pressures. This can
be accomplished by varying the relative strength of the amine-0O2 and
amine-MOF interactions.
[0092] To reduce the temperature of regeneration a higher pressure
step is
more advantageous than a lower pressure step. This can be accomplished
by varying the relative strength of the amine-0O2 and amine-MOF
interactions as well.
[0093] To reduce the regeneration energy, it may be favorable to
choose an
amine that is optimally adsorbed at pressure higher or lower than
atmospheric pressure as demonstrated here. Thus, the amine can be
chosen to allow for regeneration to occur under vacuum or pressurized
conditions.
[0094] To reduce the enthalpy of adsorption/desorption, an adsorbent
with a
step at a higher pressure over a lower pressure is desirable. This can be
accomplished by varying the relative strength of the amine-0O2 and amine-
MOF interactions.
[0095] To change the step position, the entropy the amines have on the
surface of the framework can be changed. This can be accomplished by
changing the rate of diamine exchange, which is dependent on the strength
of the metal¨amine bond and varies for each metal.
[0096] To increase heat removal from the bed during adsorption it is
advisable to increase temperature differential between the adsorption bed
temperature and the temperature of the heat sink. By shifting the isotherm
through variation of the bond strengths, it is possible to design an
adsorbent that can effectively remove CO2 at high temperatures.
[0097] Adsorption is favorable when the free energy of the phase
containing
ordered chains of ammonium carbamate is lower in energy than the
configuration that allows for adsorption to occur via non-cooperative
processes. This is related to the enthalpy of adsorption, the entropy of the
solid phase and the entropy of the gas phase. Thus, optimum adsorption
and desorption conditions can be controlled by altering the entropy of the
gas phase surrounding the adsorbent. The entropy of the gas mixture can
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Date Recue/Date Received 2023-08-11
be changed by varying the temperature of the gas phase, the pressure of
the gas phase, or the composition of the gas phase. For example, during a
process that results in the co-adsorption of multiple adsorptives
simultaneously (for example CO2 and H20), the presence of multiple gases
during desorption can be used to decrease the temperature of desorption
owing to the increased entropy of the mixed gas phase during the
desorption process.
[0098] The technology described herein may be better understood with
reference to the accompanying examples, which are intended for purposes
of illustration only and should not be construed as in any sense limiting the
scope of the technology described herein as defined in the claims
appended hereto.
[0099] Example 1
[00100] In order to demonstrate the technology, several diamine-
appended
metal-organic frameworks (mmen-M2(dobpdc) (mmen = N,N'-
dimethylethylenediamine; M = Mg, Mn, Fe, Co, Zn; d0bpdc4- = 4,4'-
dioxidobipheny1-3,3'-dicarboxylate) were produced and tested.
[00101] All reagents and solvents were obtained from commercial sources
at
reagent grade purity or higher. A 10% v/v stock solution of N,N--
dimethylethylenediamine (mmen) in hexanes was used for amine
functionalization reactions. The mmen solution was stored under N2 and
was kept free of H20 contamination by the inclusion of freshly ground CaH2
in the 200 m L Schlenk flask. The compound H4(dobpdc) was synthesized
using conventional methods.
[00102] For the synthesis of Mg2(dobpdc), H4dobpdc (27.4 mg, 0.10 mmol)
was added to a 20-ml glass scintillation vial and Mg(NO3)2.6 H20 (64.0 mg,
0.25 mmol), and 10 ml of mixed solvent (55:45 MeOH:DMF) were
subsequently added. The vial was sealed with a PTFE-lined cap and placed
in a 2 cm deep well plate on a 393 K hot plate. After 12 h a white powder
formed on the bottom and walls of the vial. The reaction mixture was then
decanted and the remaining powder was soaked in DMF at 343 K for 12
hours, after which the solvent was decanted and replaced with fresh DMF.
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Date Recue/Date Received 2023-08-11
This process was repeated 6 times over the course of 3 days. The solvent
was switched to Me0H and the process repeated until by infrared
spectroscopy the amide stretch of DMF was no longer apparent. The solid
was then collected by filtration and fully desolvated by heating under
dynamic vacuum (<10 pbar) at 523 K for 24 h to afford 23.3 mg (0.073
mmol), 73% of Mg2(dobpdc).
[00103] A similar synthesis scheme was utilized to produce: 33.8 mg
(0.0889
mmol), 89% of Mn2(dobpdc); 2.395 g (6.28 mmol), 93% of Fe2(dobpdc);
54.1 mg (0.139 mmol), 93% of Co2(dobpdc); 21.4 mg (0.0534 mmol), 53%
of Zn2(dobpdc) and 39.3 mg (0.101 mmol), 68% of Ni2(dobpdc) for
analysis.
[00104] Laboratory powder X-ray diffraction patterns were collected on
a
Bruker AXS D8 Advance diffractometer equipped with Cu-Ka radiation (X =
1.5418 A), a Gdbel mirror, a Lynxeye linear position-sensitive detector, and
mounting the following optics: fixed divergence slit (0.6 mm), receiving slit
(3 mm), and secondary beam SoIler slits (2.5 ) . The generator was set at
40 kV and 40 mA, due to the oxygen sensitivity of Fe2(dobpdc) and mmen-
Fe2(dobpdc), X-ray diffraction patterns were collected in sealed glass
capillaries placed on the powder stage. Infrared spectra were collected on a
Perkin-Elmer Spectrum 400 equipped with an attenuated total reflectance
(ATR) accessory. Thermogravimetric analysis (TGA) was carried out at a
ramp rate of 2 C/min in a nitrogen flow with a TA Instruments Q5000.
Elemental analyses for C, H, and N were performed at the Microanalytical
Laboratory of the University of California, Berkeley.
[00105] Example 2
[00106] To further demonstrate the operational principles of the
methods,
spectroscopic and diffraction measurements were undertaken to determine
the mechanism of CO2 uptake leading to a steep adsorption step for
adsorbents such as mmen-Mg2(dobpdc). In particular, powder X-ray
diffraction studies, which were performed on the isostructural compound
mmen-Mn2(dobpdc) due to the greater crystallinity of its base framework,
provided detailed structural information on how CO2 binds within the
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Date Recue/Date Received 2023-08-11
channels of the material. Diffraction data collected at 100K before and after
exposure of a sample to 5 mbar of CO2 showed the unit cell volume
contracting by just 1.112(8)%, but revealed large changes in the relative
intensity of selected diffraction peaks.
[00107] Complete structural models were developed for both data sets using
the simulated annealing method, as implemented in TOPAS-Academic,
followed by Rietveld refinement against the data. Before exposure to CO2,
the mmen molecules were bound through one amine group to the Mn+2
sites with a Mn¨N distance of 2.29(6)A , whereas the other amine lay
exposed on the surface of the framework. Counter to the initial assumption
that the uncoordinated amine groups would serve to bind CO2, CO2
adsorption instead occurred by means of full insertion into the Mn¨N bond,
resulting in a carbamate with one 0 atom bound to Mn at a distance of
2.10(2)A . The second 0 atom of the carbamate had a close interaction of
2.61(9)A with the N atom of a neighboring mmen, resulting in chains of
ammonium carbamate running along the crystallographic c axis of the
structure. The observed ammonium carbamate N...0 distance was similar
to the distance of 2.66-2.72A in a single crystal of puremmen-0O2 (methyl
(2-(methylammonio) ethyl) carbamate). This well-ordered chain structure
was maintained at 295 K, as determined from a full Rietveld refinement
against data collected at this temperature. Thus, the adsorption of CO2 at
ambient temperatures is associated with a structural transition to form an
extended chain structure held together by ion pairing between the metal-
bound carbamate units and the outstretched ammonium group of a
neighboring mmen molecule.
[00108] The foregoing structural information enabled the formulation of
a
detailed mechanism for the adsorption of CO2 in phase-change adsorbents
of the type mmen-M2(dobpdc). As shown in Fig. 3A through FIG. 3C, the
uncoordinated amine of a mmen molecule acts as a strong base to remove
the acidic proton from the metal-bound amine of a neighboring mmen
molecule. Deprotonation occurs only in the presence of CO2, such that
simultaneous nucleophilic addition of CO2 results in the formation of a
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Date Recue/Date Received 2023-08-11
carbamate with an associated ammonium countercation. At suitable
temperatures and pressures, rearrangement of the carbamate is possible
such that the M¨N bond is broken and a M-0 bond is formed. The ion-
pairing interaction causes the mmen molecule to stretch, destabilizing the
M¨N bond and facilitating insertion at the next metal site. This cooperative
effect will propagate until a complete one-dimensional ammonium
carbamate chain has formed. Indeed, it is this cooperativity that leads to the
sudden uptake of a large amount of CO2 and a steep vertical step in the
adsorption isotherm.
[00109] Despite being labile, the amines were stable to evacuation under
vacuum at high temperatures. This unexpected lability seems to allow
substitution, but not elimination, reactions to occur rapidly under conditions
relevant to carbon capture. Furthermore, the sudden adsorption of CO2 in
this compound is thus associated with a transition from a dynamic surface
state to a well-ordered extended surface structure. Accordingly, the reaction
with CO2 can be considered to be thermodynamically non-spontaneous at
low pressures because of the large decrease in entropy associated with this
transition. Indeed, the molar entropy of gas-phase CO2 was found to be the
primary determinant of the step pressure for phase-change adsorbents.
Step pressures for all five phase-change metal organic frameworks were
shown to be linearly correlated with the gas-phase entropy of CO2 as a
function of temperature.
[00110] Example 3
[00111] The mechanism of CO2 adsorption suggests that variation of the
metal amine bond strength should provide a method of manipulating the
isotherm step position. The CO2 adsorption isotherms series of the mmen-
M2(dobpdc)(M=Mg, Mn, Fe, Co, Ni, Zn) compounds were measured at 25,
40, 50 and 75 C. With the exception of the Ni compound, which showed
normal Langmuir-type adsorption behavior, all of the materials showed
sharp isotherm steps that shifted to higher pressure with increasing
temperature. Analysis of the isotherm steps at 25 C yielded Hill
coefficients of 10.6, 5.6, 7.5, 11.5 and 6.0 for M=Mg, Mn, Fe, Co and Zn,
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Date Recue/Date Received 2023-08-11
respectively, reflecting the cooperative nature of the CO2 adsorption
mechanism.
[00112] For a given temperature, the step position varies in the order
Mg <
Mn < Fe < Zn < Co, in good agreement with the published series for
octahedral metal complex stabilities. The lack of a step for the Ni
compound, even at very high pressures is attributable to the exceptional
stability of the Ni¨mmen bond, which prevents carbamate insertion from
taking place under the conditions surveyed.
[00113] The trend in calculated adsorption energies was directly
correlated
with the calculated metal-amine bond length. Thus, similar variations in
tuning step position will be possible for the M2(dobpdc) series by altering
the sterics of the amine bound to the metal, as well as the spacer between
the two amine groups. Hence, depending on the concentration of CO2
present in a gas mixture, an adsorbent can be rationally designed to match
the optimum process conditions depicted in FIG. 1.
[00114] Although stepped adsorption isotherms have been observed
previously in solid adsorbents, the origin of the step reported here is unique
and distinct from all previously reported mechanisms. In contrast to most
metal-organic frameworks showing such behavior, the isotherm steps
reported here are not attributable to pore-opening, gate opening or pore-
closing processes.
[00115] Several features unique to the mmen-M2(dobpdc) series permitted
phase transitions of this type to be observed. First, for solid ammonium
carbamate chains to form, the metal-amine coordinate bond must be
capable of rearrangement. Thus, only amines tethered to the solid surface
through coordinate bonds rather than covalent bonds can undergo the
rearrangement shown in FIG. 3. Second, a homogeneous surface with
appropriately positioned adsorption sites, which is dictated by the location
of open metal sites within the pores of the metal-organic framework, is
necessary. Thus, a very limited number of metal-organic framework
materials would be able to mimic the adsorption behavior and it is likely that
no amine-functionalized mesoporous silica sorbent could be engineered
-26-
Date Recue/Date Received 2023-08-11
precisely enough to meet these requirements. Notably, in contrast to the
pore expanded derivatives of M2(dobdc) reported here, amine
functionalization of the parent Mg2(dobdc) compound was not reported to
result in stepped adsorption isotherms.
[00116] Example 4
[00117] Effective adsorbents for carbon capture must possess large
working
capacities for processes occurring at temperatures above 40 C and at CO2
partial pressures near 0.15 bar for coal flue gas or near 0.05 bar fora
natural gas flue stream. On this basis, the location of the isotherm steps for
the Mg and Mn compounds makes them better suited for this application
than the Fe, Co or Zn compounds, which are better suited for separations
from gas mixtures with higher CO2 concentrations. To assess the utility of
these phase-change adsorbents for capturing CO2 in a pure temperature
swing adsorption process, adsorption isobars were collected under dynamic
gas flow. Samples of mmen-Mg2(dobpdc) and mmen-Mn2(dobpdc) were
activated, saturated with 100% CO2 and then cooled isobarically to room
temperature under three differentCO2-containing gas mixtures: 100%, 15%
and 5%. The resulting isobars reveal how small changes in temperature
induced large changes in the quantity of CO2 adsorbed.
[00118] Phase change adsorbents showed very large working capacities
when used in temperature swing adsorption processes. For mmen-
Mg2(dobpdc) to give a working capacity in excess of 13 wt%, the material
must simply swing between 100 C and 150 C. Similarly, the working
capacity of mmen-Mn2(dobpdc) was in excess of 10 wt% when cycled
between 70 and 120 C. In particular, to simulate a pure temperature swing
adsorption process accurately, 15% CO2 in N2 was flowed over the samples
during the cooling phase, whereas 100% CO2 was used during heating
phases.
[00119] In contrast to aqueous amine absorbents that use heat
exchangers
to save sensible energy costs, the greater working capacities and smaller
temperature swings of phase-change adsorbents allow more economical
processes to be developed for a high-enthalpy adsorbent without the use of
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Date Recue/Date Received 2023-08-11
a heat exchanger. Because phase-change adsorbents saturate with CO2 at
their transition point, it is not necessary for adsorption to occur at the
lowest
possible temperature. Whereas we previously showed thatmmen-
Mg2(dobpdc) can operate effectively under standard flue gas adsorption
conditions (40 C).
[00120] In addition, it was observed that the phase-change adsorbents
operated more efficiently at higher adsorption temperatures than at lower
temperatures. Because classical adsorbents must operate at the lowest
possible adsorption temperature to maximize working capacity, only phase-
change adsorbents can enable high-temperature adsorption processes to
be considered.
[00121] Adsorbing CO2 at elevated temperatures affords several
additional
process benefits besides directly decreasing sorbent regeneration energy.
In particular, overcoming the competitive adsorption of water vapor, which
is present in flue gas at high concentrations, presents a serious challenge
for solid adsorbents. Amine-based solid adsorbents fare better than those
using a purely physical adsorption mechanism, because they are known to
retain their affinity for CO2 under humid conditions. However, even for
systems where the amine reactivity with CO2 is unaffected by the presence
of water, the physical adsorption of water on non-amine binding sites
increases the overall regeneration energy of the material.
[00122] The mmen-Mg2(dobpdc) also adsorbed nearly 90% less water at
100 C than at 40 C. Thus, the energy penalty associated with desorbing
co-adsorbed water can be substantially decreased by performing CO2
adsorption at a high temperature, obviating the need for strict flue gas
dehumidification. No changes to the CO2 adsorption isotherm were
apparent after exposure to water at 40 C or 100 C, indicating the stability
of the mmen-Mg2(dobpdc) in the presence of water vapor even at high
temperatures.
[00123] The high effective operating temperatures of mmen-Mg2(dobpdc)
and mmen-Mn2(dobpdc) offer opportunities for cost savings beyond just
decreases in the regeneration energy. Because of the exothermic nature of
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Date Recue/Date Received 2023-08-11
all adsorption processes, the incorporation of labor and material intensive
coolant pipes into an adsorbent bed (a component of the considerable
infrastructure cost for carbon capture) is necessary to maintain isothermal
adsorption conditions. The rate of heat transfer from a sorbent bed to the
coolant pipes, which contain surface temperature water at 25 C, is
primarily dependent on the heat transfer coefficient of the sorbent, the total
contact area between the sorbent and the coolant pipes, and the
temperature differential between the sorbent and the coolant. The physical
size of adsorption units is dictated, to a great extent, by the need to
provide
sufficient contact area between the coolant and sorbent for effective heat
removal.
[00124] For processes that are limited by heat transfer rather than
mass
transfer, which is likely for many CO2 capture processes using solid
adsorbents, the use of high temperatures will maximize the temperature
differential between the coolant and the sorbent, substantially reducing the
overall bed size by reducing the size of the necessary contact area. By
increasing the coolant¨sorbent temperature differential from about 15 C to
nearly 75 C, adsorption bed size could potentially be reduced fivefold.
[00125] In turn, smaller adsorbent beds would reduce the pressure drop
across the adsorbent, reduce the size and cost of the required capital
equipment, and allow as little as one-fifth as much adsorbent to be used. By
decreasing these other system costs, new classes of adsorbents have the
ability to reduce the cost of carbon capture substantially beyond simply
decreasing the sorbent regeneration energy.
[00126] From the discussion above it will be appreciated that the
technology
described herein can be embodied in various ways, including the following:
[00127] 1. A method for acid gas separations, the method comprising:
(a)
determining concentration of an acid gas from a stream of a mixture of
gases; (b) preparing a porous metal organic framework of metal atoms
bound to polytopic organic linkers; (c) selecting basic nitrogen ligands
capable of binding with unsaturated metal ions of the organic framework
with a binding strength; (d) binding the basic nitrogen ligands to
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coordinatively unsaturated metal ions that expose nitrogen atoms to pore
volumes of the framework; and (e) contacting the framework with a stream
of a mixture of gases; (f) wherein acid gas is adsorbed to the basic nitrogen
ligands; and (g) wherein a step position of a produced isotherm is matched
to the concentration of an acid gas from the stream of a mixture of gases.
[00128] 2. The method of any preceding embodiment, wherein the gas
mixture contains at least one of the following gases CO2, SO2, CS2, H2S,
503, SR2, RSH, NO2, NO3, NO, BR3, NR3 where R is an organic moiety.
[00129] 3. The method of any preceding embodiment, wherein the metal
atoms of the framework are atoms selected from the group of atoms
consisting of Al, Be, Ca, Cd, Co, Cr, Cu, Fe, Mg, Mn, Ni, Sc, Ti, V, and Zn.
[00130] 4. The method of any preceding embodiment, wherein the
polytopic
linker is selected from the group 1,3,5-benzenetripyrazolate, 1,3,5-
benzenetristriazolate, 1,3,5-benzenetristetrazolate, 1,3,5-
benzenetricarboxylate, 1,4-benzenedicarboxylate; 2,5-dioxido-1,4-
benzenedicarboxylate, 4,4'-dioxidobipheny1-3,3'-dicarboxylate and 4'-4"-
dioxido-3',3"-terphenyldicarboxylate.
[00131] 5. The method of any preceding embodiment, wherein the basic
nitrogen ligand is an alkylamine selected from the group of a primary,
secondary, or tertiary alkylamine.
[00132] 6. The method of any preceding embodiment, wherein the basic
nitrogen ligand is an imine selected from the group of primary or secondary
imines.
[00133] 7. The method of any preceding embodiment, further comprising:
selecting a second type of basic nitrogen ligand capable of binding with
unsaturated metal ions of the organic framework with a binding strength;
and binding a combination of basic nitrogen ligands to coordinatively
unsaturated metal ions that expose nitrogen atoms to pore volumes of the
framework nitrogen ligands.
[00134] 8. The method of any preceding embodiment, further comprising:
selecting the basic nitrogen ligand based on steric and electronic properties
of the amines that form covalent bonds to the acid gas during adsorption.
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Date Recue/Date Received 2023-08-11
[00135] 9. The method of any preceding embodiment, further comprising:
selecting the basic nitrogen ligand based on steric and electronic properties
of the amines that accept protons during aggregate formation.
[00136] 10. The method of any preceding embodiment, further comprising:
selecting the basic nitrogen ligand based on potential ionic interactions
between partners in the aggregate.
[00137] 11. The method of any preceding embodiment, further comprising:
selecting the basic nitrogen ligand based on strength of interaction between
pairs of chains by selecting the number of carbons that separate two
amines of a diamine molecule.
[00138] 12. The method of any preceding embodiment, further comprising:
selecting the basic nitrogen ligand based on cyclic hydrocarbon molecules
bonded to the diamines to match the isotherm step position.
[00139] 13. A cooperative chemical adsorption method for acid gas
separations, the method comprising: (a) providing a porous metal-organic
framework; (b) functionalizing pore surfaces with a plurality of ligands
producing two adjacent amines that define adjacent adsorption sites; and
(c) adsorbing acid gas molecules with the adjacent amine adsorption sites;
(d) wherein a plurality of amines adsorb acid gas at the same time and
form covalently linked aggregates of more than one ammonium carbamate
ion pair; (e) wherein the aggregates spatially extend along the pore surface
in at least one dimension; and (f) wherein a gaseous stream depleted in
acid gas and a solid-phase composition enriched in acid gas is produced.
[00140] 14. The method of any preceding embodiment, further comprising:
matching a step position of a produced isotherm to a concentration of gas
for removal of an acid gas by changing the strength of the bond between
the metal and the basic nitrogen ligand.
[00141] 15. The method of any preceding embodiment, wherein the ligand
is
an alkylamine selected from the group of a primary, secondary, or tertiary
alkylamine.
[00142] 16. The method of any preceding embodiment, wherein the ligand
is
an imine selected from the group of primary or secondary imines.
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Date Recue/Date Received 2023-08-11
[00143] 17. The method of any preceding embodiment, wherein the
polytopic linker is 4,4'-dioxidobipheny1-3,3'- dicarboxylate and the basic
nitrogen ligand is N,N'-dimethylethylenediamine.
[00144] 18. A porous metal-organic framework composition for acid gas
separations, comprising:(a) a plurality of metal atoms bound to polytopic
organic linkers forming a porous metal-organic framework; and (b) a
plurality of ligands bound to coordinatively unsaturated metal ions that
expose nitrogen atoms to pore volumes of the framework; (c) wherein a
stepped isotherm is produced upon contact with a stream of mixed gases.
[00145] 19. The composition of any preceding embodiment, wherein the
metal atoms of the framework are atoms selected from the group of atoms
consisting of Al, Be, Ca, Cd, Co, Cr, Cu, Fe, Mg, Mn, Ni, Sc, Ti, V, and Zn.
[00146] 20. The composition of any preceding embodiment, wherein the
polytopic linker is an aromatic compound with two or more functional
azolate groups selected from the group of pyrazolate -C3H2N2-, triazolate ¨
C2HN3-, tetrazolate ¨CN4- and carboxylate (-0O2-) groups.
[00147] 21. The composition of any preceding embodiment, wherein the
polytopic linker is selected from the group 1,3,5-benzenetripyrazolate,
1,3,5-benzenetristriazolate, 1,3,5-benzenetristetrazolate, 1,3,5-
benzenetricarboxylate; 1,4-benzenedicarboxylate; 2,5-dioxido-1,4-
benzenedicarboxylate; and 4,4'-dioxidobipheny1-3,3'-dicarboxylate.
[00148] 22. The composition of any preceding embodiment, wherein the
basic nitrogen ligand is an alkylamine selected from the group of a primary,
secondary, or tertiary alkylamine.
[00149] 23. The composition of any preceding embodiment, wherein the
basic nitrogen ligand is an imine selected from the group of primary or
secondary imine.
[00150] 24. The composition of any preceding embodiment, wherein the
metal is selected from the group Ca, Fe, Mn, Cu, Co, Ni, Cr and Cd and the
polytopic ligand is 1,3,5-benzenetripyrazolate.
[00151] 25. The composition of any preceding embodiment, wherein the
metal is selected from the group Ca, Fe, Mn, Cu, Co, Ni, Cr and Cd and the
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Date Recue/Date Received 2023-08-11
polytopic ligand is 1,3,5-benzenetristetrazolate.
[00152] 26. The composition of any preceding embodiment, wherein the
metal is selected from the group of Cr, Mn, Fe, Co, Ni, and Cu and the
polytopic ligand is 1,3,5 benzenetristriazolate.
[00153] 27. The composition of any preceding embodiment, wherein the
metal is selected from the group of Fe, Al, Cr, Ti, Sc, and V and the
polytopic ligand is 1,3,5-benzenetriscarboxylate.
[00154] 28. The composition of any preceding embodiment, wherein the
metal is selected from the group of Fe, Al, Cr, Ti, Sc, and V and the
polytopic ligand is 1,4-benzenedicarboxylate.
[00155] 29. The composition of any preceding embodiment, wherein the
metal is selected from the group of Mg, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni,
Cu and Zn and the polytopic ligand is 2,5-dioxido-1,4-
benzenedicarboxylate.
[00156] 30. The composition of any preceding embodiment, wherein the
metal is selected from the group of Mg, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni,
Cu, and Zn and the polytopic linker is 4,4'-dioxidobipheny1-3,3'-
dicarboxylate.
[00157] 31. The composition of any preceding embodiment, wherein the
basic nitrogen ligand is an amine selected from the group of amines
consisting of ethylenediamine, propylenediamine, butylenediamine,
pentylenediamine, hexylenediamine, 1,2-propanediamine, 2,3-
butanediamine, 1,2-diamino-2-methylpropane, N-boc-ethylenediamine, N-
ethylethylenediamine, N,N'-diethylpropylenediamine, N,N-
diethylethylenediamine, N-isopropylethylenediamine, N,N'-
diisopropylethylenediamine, N-isopropylpropylenediamine, N,N'-
diisopropylpropylenediamine, N,N1-diisopropylethylenediamineõ N-
methylethylenediamine, N,N'-dimethylethylenediamine, N-
methylpropylenediamine, N,N'-dimethylpropylenediamine, 1,3-
diaminocyclohexane, N,N-dimethylethylenediamine, N,N,N'-
trimethylethylenediamine, N,N,N',N'-tetramethylethylenediamine, N-
trimethylsilylethylenediamine, N,N-bis(trimethylsilyl)ethyleneidmaine, N,N'-
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Date Recue/Date Received 2023-08-11
bis(trimethylsilyl)ethyleneidmaine, N,N-dimethylpropylenediamine, N,N,N'-
trimethylpropylenediamine, and N,N,N',N'-tetramethylpropylenediamine,
diethylenetriamine, 2-(2-aminoethyoxy)ethylamine, dipropylenetriamine,
1,2-diaminocyclohexane, piperazine, tris(2-aminoethyl)amine, 2-
(Diisopropylphosphino)ethylamine, N-methylethanolamine and
monoethanolamine.
[00158] Although the description herein contains many details, these
should
not be construed as limiting the scope of the disclosure but as merely
providing illustrations of some of the presently preferred embodiments.
Therefore, it will be appreciated that the scope of the disclosure fully
encompasses other embodiments which may become obvious to those
skilled in the art.
[00159] In the claims, reference to an element in the singular is not
intended
to mean "one and only one" unless explicitly so stated, but rather "one or
more." Furthermore, no element, component, or method step in the present
disclosure is intended to be dedicated to the public regardless of whether
the element, component, or method step is explicitly recited in the claims.
No claim element herein is to be construed as a "means plus function"
element unless the element is expressly recited using the phrase "means
for". No claim element herein is to be construed as a "step plus function"
element unless the element is expressly recited using the phrase "step for".
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