U.S. patent application number 15/391321 was filed with the patent office on 2017-04-20 for hybrid zeolitic imidazolate frameworks controlling framework porosity and functionality by a mixed-ligand synthetic approach.
This patent application is currently assigned to GEORGIA TECH RESEARCH CORPORATION. The applicant listed for this patent is GEORGIA TECH RESEARCH CORPORATION. Invention is credited to Catherine Rose Blad, Christopher W. Jones, Sankar Nair, Joshua Allen Thompson.
Application Number | 20170107237 15/391321 |
Document ID | / |
Family ID | 48870781 |
Filed Date | 2017-04-20 |
United States Patent
Application |
20170107237 |
Kind Code |
A1 |
Thompson; Joshua Allen ; et
al. |
April 20, 2017 |
Hybrid Zeolitic Imidazolate Frameworks Controlling Framework
Porosity and Functionality by a Mixed-Ligand Synthetic Approach
Abstract
Metal-organic frameworks, in particular hybrid zeolitic
imidazolate frameworks (ZIFs), devices having hybrid ZIFs, and
methods for preparing hybrid ZIFs are disclosed herein. In some
embodiments, the method includes preparing a first solution
comprising a first imidazolate and a second imidazolate, preparing
a second solution comprising a metal ion, and combining the first
solution and the second solution to form the hybrid ZIF.
Inventors: |
Thompson; Joshua Allen;
(Atlanta, GA) ; Blad; Catherine Rose; (Atlanta,
GA) ; Jones; Christopher W.; (Mableton, GA) ;
Nair; Sankar; (Atlanta, GA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GEORGIA TECH RESEARCH CORPORATION |
Atlanta |
GA |
US |
|
|
Assignee: |
GEORGIA TECH RESEARCH
CORPORATION
Atlanta
GA
|
Family ID: |
48870781 |
Appl. No.: |
15/391321 |
Filed: |
December 27, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13754430 |
Jan 30, 2013 |
9527872 |
|
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15391321 |
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61592197 |
Jan 30, 2012 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01D 2257/504 20130101;
Y02P 20/151 20151101; B01D 2257/7025 20130101; C07F 3/06 20130101;
C01B 37/00 20130101; C01B 39/00 20130101; B01D 53/02 20130101; B01D
2253/204 20130101; Y02C 20/20 20130101; C07F 3/003 20130101; B01J
20/226 20130101; Y02C 20/40 20200801 |
International
Class: |
C07F 3/06 20060101
C07F003/06; B01D 53/02 20060101 B01D053/02 |
Claims
1. A metal-organic framework (MOF) comprising a hybrid zeolitic
imidazolate framework (ZIF) having: i. at least two, non-identical
linkers selected from the group consisting of: 2-methylimidazolate,
carboxaldehyde-2-imidazolate, benzimidazolate,
2-aminobenzimidazolate, and imidazolate, or a functionalized
derivative thereof, and iii. a metal ion, wherein the at least two
linkers are included in a solution having a total concentration of
y.
2. The MOF of claim 1, wherein the first linker is at a
concentration of y minus x.
3. The MOF of claim 1, wherein a second linker is at a
concentration x.
4. The MOF of claim 1, wherein the total concentration of the at
least two linkers in a solution is between about 10 mmol and about
50 mmol.
5. The MOF of claim 1, further comprising a functionalized hybrid
ZIF.
6. The MOF of claim 5, wherein the functionalized hybrid ZIF
comprises an aldehyde.
7. The MOF of claim 5, wherein the functionalized hybrid ZIF
comprises an amine.
Description
TECHNICAL FIELD
[0001] The various embodiments of the disclosure relate generally
to zeolitic imidazolate frameworks (ZIF) and their associated
fabrication processes, and more particularly to tunable, hybrid
ZIFs and to methods for making such structures.
BACKGROUND
[0002] Framework modification of porous materials has considerable
potential to enable tuning of material properties to increase
performance in a variety of applications such as separations,
catalysis, and chemical sensors. In particular, improving material
performance for separations is highly desirable to reduce the
overall process energy requirements. Typically, the two material
properties in porous materials that determine separation
performance are pore size (kinetic separation) and adsorption
selectivity (thermodynamic effect). From this perspective, a
nanoporous class of metal-organic frameworks (MOFs) called zeolitic
imidazolate frameworks (ZIFs) are an attractive option.
[0003] MOFs are a promising class of nanoporous materials for use
in separations and catalysis, among many other applications. ZIFs,
a subclass of these materials, have many advantageous properties
including good thermal and chemical stability, high microporosity,
and high surface area. The pore sizes of these materials (0.2-2 nm)
allow selective sieving and recognition of molecules. Recent
studies have also shown that ZIFs exhibit a "gate-opening"
phenomenon: as they interact with adsorbing molecules, they undergo
structural changes during adsorption, thereby allowing more
adsorbate molecules into the framework. Because the organic linker
components in the framework rotate to allow the above phenomena,
the nature of the organic linker has significant implications on
the selection and behavior of appropriate ZIF materials for
specific applications. For instance, ZIF-8 has a crystallographic
pore aperture of 0.34 nm as determined by X-ray diffraction;
however, there is increasing evidence that the as-made material
separates gases considerably larger than its pore aperture (e.g.,
C.sub.3H.sub.6/C.sub.3H.sub.8) more efficiently than gases closer
to its crystallographically determined pore size
(CO.sub.2/CH.sub.4).
[0004] In general, it is possible to tune the properties of MOFs
for specific applications using methods such as chemical or
structural modifications. One approach for chemically modifying a
MOF is to use a linker that has a pendant functional group for
post-synthesis modification. For example, ZIF-90, an
aldehyde-containing ZIF, can be modified using NaBH.sub.4 as a
reducing agent to generate alcohol groups. Another approach to
modification is to use organic ligands that can change the
structural characteristics of the material. MIL-53 exhibits a
flexible framework, but modification of the terephthalic acid
linker to include an amino functional group improves the separation
performance for CO.sub.2. Another recent approach to modification
is the use of a triazolate linker in which a C--H moiety of the
imidazole is replaced by a nitrogen atom, thereby allowing
crystallization of a hybrid material that does not disturb the
crystal structure of the original material. However, in the case of
using mixed linkers, determining appropriate ligand combinations a
priori is not always straightforward. It has been shown that the
use of ligands with bulky substituents produces new ZIF frameworks
with enhanced CO.sub.2 adsorption properties by preventing
crystallization of ZIF topologies with smaller unit cells and
network cages; however, this discovery came from using
high-throughput synthesis techniques. Similarly, the pore size of a
MOF can be tuned by increasing the length of bridging organic
linkers. A series of mixed-ligand Zn-based MOFs were transformed
from a nonporous material to one with relatively high surface area
and porosity by increasing the length of bridging dicarboxylic or
bipyridyl linkers.
[0005] Another way to modify surface properties is by postsynthetic
exchange (PSE) of the organic linkers or metal centers by heating
the MOF material in a solvent containing a different linker or
metal ion that exchanges into the material while maintaining the
crystal structure. Recently, the linker of ZIF-71
(4,5-dichloroimidazole) was successfully subjected to PSE with a
linker that is not otherwise found in ZIF structures
(4-bromoimidazole). ZIF-8 has also been subjected to PSE, replacing
the framework linkers (2-methylimidazole) with imidazole. This
produced a material with 85% substituted linkers while maintaining
the ZIF-8 crystal structure.
[0006] The different ZIF topologies can possess a variety of pore
sizes and surface properties. ZIFs have been studied for CO.sub.2
adsorption and membrane-based separations by both experiments and
computations. Although these materials normally have high CO.sub.2
capacity, the adsorption selectivity for typical gas pairs of
interest (e.g., CO.sub.2/CH.sub.4) tends to be low and comparable
with commercially available adsorbent materials such as BPL carbon.
Practically, increasing the adsorption selectivity would greatly
increase the potential for commercialization. Currently, very few
ZIF materials (e.g., ZIF-78) have shown significant
CO.sub.2/CH.sub.4 and CO.sub.2/N.sub.2 adsorption selectivities of
10 and 50, respectively, or more. Some large pore MOF structures
exhibit higher CO.sub.2 affinity and selectivity for these gas
pairs; however, these materials typically have open metal centers
that are susceptible to performance degradation from steam exposure
and poison from trace contaminants, which adversely affect CO.sub.2
capacity. Conversely, ZIFs have relatively high thermal and
chemical stability that permits modification of the surface
properties and have the added benefit of small pore apertures that
are promising for kinetic gas separations, which further improves
separation performance.
BRIEF SUMMARY
[0007] The various embodiments of the disclosure relate generally
to zeolitic imidazolate frameworks (ZIF) and their associated
fabrication processes, and more particularly to tunable, hybrid
ZIFs and to methods for making such structures.
[0008] An embodiment of the disclosure can be a method for forming
a hybrid zeolitic imidazolate framework (ZIT) having the steps of
preparing a first solution comprising a first imidazolate and a
second imidazolate, preparing a second solution comprising a metal
ion, and combining the first solution and the second solution to
form the hybrid ZIF. In one embodiment, the process can further
comprise activating the hybrid ZIF to remove impurities. For
example, the activating can comprise vacuum degassing between about
100.degree. C. and about 300.degree. C. In some embodiments, the
first imidazolate is different from the second imidazolate. In one
embodiment, the first imidazolate can comprise 2-methylimidazolate
and the second imidazolate can comprise
carboxaldehyde-2-imidazolate. In an alternative embodiment, the
first imidazolate can comprise 2-methylimidazole and the second
imidazolate can comprise benzimidazolate. In another embodiment,
the first imidazolate can comprise benzimidazolate and the second
imidazolate can comprise 2-aminobenzimidazolate. In yet another
embodiment, the first imidazolate can comprise 2-methylimidazolate
and the second imidazolate can comprise imidazolate.
[0009] The metal ion can comprise a transition metal. In one
embodiment, the metal ion can comprise zinc. Alternatively, the
metal ion can comprise cobalt.
[0010] In an embodiment, the method for forming a hybrid ZIF can
further comprise functionalizing the hybrid ZIF. The
functionalizing can comprise exposing the hybrid ZIF to a reactive
agent. For example, the reactive agent can comprise an aldehyde.
Alternatively, the reactive agent can comprise an amine.
[0011] An embodiment can be a metal-organic framework (MOF) that
can comprise a hybrid ZIF comprising a first imidazolate, a second
imidazolate, and a metal ion. In some embodiments, the first
imidazolate is different from the second imidazolate. In one
embodiment, the first imidazolate can comprise 2-methylimidazolate
and the second imidazolate can comprise
carboxaldehyde-2-imidazolate. In an alternative embodiment, the
first imidazolate can comprise 2-methylimidazole and the second
imidazolate can comprise benzimidazolate. In another embodiment,
the first imidazolate can comprise benzimidazolate and the second
imidazolate can comprise 2-aminobenzimidazolate. In yet another
embodiment, the first imidazolate can comprise 2-methylimidazolate
and the second imidazolate can comprise imidazolate.
[0012] The metal ion can comprise a transition metal. In one
embodiment, the metal ion can comprise zinc. Alternatively, the
metal ion can comprise cobalt.
[0013] In one embodiment, the ZIF can have a CO.sub.2/CH.sub.4
selectivity of at least 1.2 times greater than a non-hybrid ZIF. In
another embodiment, the ZIF can have a CO.sub.2/CH.sub.4
selectivity of at least 1.5 times greater than a non-hybrid ZIF. In
yet another embodiment, the ZIF can have a CO.sub.2/CH.sub.4
selectivity of at least 1.8 times greater than a non-hybrid ZIF.
Alternatively, the ZIF can have a CO.sub.2/CH.sub.4 selectivity of
at least 2 times greater than a non-hybrid ZIF.
[0014] In some embodiments, the hybrid ZIF can have
CO.sub.2/CH.sub.4 adsorption selectivity from about 2.5 to about
13.1. In other embodiments, the hybrid ZIF can have
CO.sub.2/CH.sub.4 adsorption selectivity of at least 10. In
alternative embodiments, the hybrid ZIF can have CO.sub.2/CH.sub.4
adsorption selectivity of at least 12. In another embodiment, the
hybrid ZIF can have CO.sub.2/CH.sub.4 adsorption selectivity of at
least 14. In yet another embodiment, the hybrid ZIF can have
CO.sub.2/CH.sub.4 adsorption selectivity of at least 15.
[0015] In an embodiment, the hybrid ZIF can have a pore size from
about 0.25 to about 0.40 nm. In another embodiment, the hybrid ZIF
can have a pore size less than about 0.40. In an alternative
embodiment, the hybrid ZIF can have a pore size of at least 0.40.
In an yet another embodiment, the hybrid ZIF can have a continuous
crystal structure.
[0016] In another embodiment, the hybrid ZIF can further comprise a
functionalized hybrid ZIF. The functionalized hybrid ZIF can
comprise an aldehyde. Alternatively, the functionalized hybrid ZIF
can comprise an amine.
[0017] Another embodiment of the disclosure can be a molecular
sieve device that can comprise a metal-organic framework (MOF)
comprising a hybrid ZIF that can comprise a first imidazolate, a
second imidazolate, and a metal ion. In some embodiments, the first
imidazolate is different from the second imidazolate. In one
embodiment, the first imidazolate can comprise 2-methylimidazolate
and the second imidazolate can comprise
carboxaldehyde-2-imidazolate. In an alternative embodiment, the
first imidazolate can comprise 2-methylimidazole and the second
imidazolate can comprise benzimidazolate. In another embodiment,
the first imidazolate can comprise benzimidazolate and the second
imidazolate can comprise 2-aminobenzimidazolate. In yet another
embodiment, the first imidazolate can comprise 2-methylimidazolate
and the second imidazolate can comprise imidazolate.
[0018] The metal ion can comprise a transition metal. In one
embodiment, the metal ion can comprise zinc. Alternatively, the
metal ion can comprise cobalt.
[0019] In one embodiment, the ZIF can have a CO.sub.2/CH.sub.4
selectivity of at least 1.2 times greater than a non-hybrid ZIF. In
another embodiment, the ZIF can have a CO.sub.2/CH.sub.4
selectivity of at least 1.5 times greater than a non-hybrid ZIF. In
yet another embodiment, the ZIF can have a CO.sub.2/CH.sub.4
selectivity of at least 1.8 times greater than a non-hybrid ZIF.
Alternatively, the ZIF can have a CO.sub.2/CH.sub.4 selectivity of
at least 2 times greater than a non-hybrid ZIF.
[0020] In some embodiments, the hybrid ZIF can have
CO.sub.2/CH.sub.4 adsorption selectivity from about 2.5 to about
13.1. In other embodiments, the hybrid ZIF can have
CO.sub.2/CH.sub.4 adsorption selectivity of at least 10. In
alternative embodiments, the hybrid ZIF can have CO.sub.2/CH.sub.4
adsorption selectivity of at least 12. In another embodiment, the
hybrid ZIF can have CO.sub.2/CH.sub.4 adsorption selectivity of at
least 14. In yet another embodiment, the hybrid ZIF can have
CO.sub.2/CH.sub.4 adsorption selectivity of at least 15.
[0021] In an embodiment, the hybrid ZIF can have a pore size from
about 0.25 to about 0.40 nm. In another embodiment, the hybrid ZIF
can have a pore size less than about 0.40. In an alternative
embodiment, the hybrid ZIF can have a pore size of at least 0.40.
In an yet another embodiment, the hybrid ZIF can have a continuous
crystal structure.
[0022] In an alternative embodiment, the hybrid ZIF can further
comprise a functionalized hybrid ZIF. The functionalized hybrid ZIF
can comprise an aldehyde. Alternatively, the functionalized hybrid
ZIF can comprise an amine.
BRIEF DESCRIPTION OF THE FIGURES
[0023] FIG. 1 illustrates compositional analysis of ZIF hybrid
frameworks by .sup.1H NMR, in accordance with an embodiment of the
disclosure.
[0024] FIG. 2a illustrates powder XRD patterns of ZIF-8-90 hybrids,
in accordance with an embodiment of the disclosure.
[0025] FIG. 2b illustrates powder XRD patterns of ZIF-7-8 hybrids,
in accordance with an embodiment of the disclosure.
[0026] FIG. 3 illustrates TEM-images of ZIF-8-90 hybrids at (a) 0%;
(b) 24%; (c) 43%; and (d) 100% OHC-IM loading, in accordance with
an embodiment of the disclosure.
[0027] FIG. 4 illustrates TEM images of ZIF-7-8 hybrids at (a) 33%;
(b) 91%; (c) 94%; and (d) 100% Bz-IM loading, in accordance with an
embodiment of the disclosure.
[0028] FIG. 5a illustrates differential mass loss curves of
ZIF-8-90 as determined from thermogravimetry in a 40% air-60%
nitrogen gas stream, in accordance with an embodiment of the
disclosure.
[0029] FIG. 5b illustrates differential mass loss curves of ZIF-7-8
hybrids as determined from thermogravimetry in a 40% air-60%
nitrogen gas stream, in accordance with an embodiment of the
disclosure.
[0030] FIG. 6a illustrates nitrogen physisorption isotherms of
ZIF-8-90, with isotherms stacked 150 cm.sup.3/g apart, in
accordance with an embodiment of the disclosure.
[0031] FIG. 6b illustrates nitrogen physisorption isotherms of
ZIF-7-8 hybrids at different loadings of OHC-IM and Bz-IM,
respectively, in accordance with an embodiment of the
disclosure.
[0032] FIG. 7a illustrates pore size distributions determined by
the HK method of ZIF-8-90 hybrids, in accordance with an embodiment
of the disclosure.
[0033] FIG. 7b illustrates pore size distributions determined by
the HK method of ZIF-7-8 hybrids, in accordance with an embodiment
of the disclosure.
[0034] FIG. 8a illustrates .sup.1H NMR-based composition analysis
of mixed-linker ZIFs prepared with 2-MeIM and 2-amBzIM linkers, in
accordance with an embodiment of the disclosure.
[0035] FIG. 8b illustrates powder XRD patterns of mixed-linker ZIFs
prepared with 2-MeIM and 2-amBzIM linkers, in accordance with an
embodiment of disclosure.
[0036] FIG. 9 illustrates N.sub.2 physisorption isotherms of
mixed-linker ZIFs with 2-amBzIM, in accordance with an embodiment
of the disclosure.
[0037] FIG. 10a illustrates CO.sub.2 and CH.sub.4 adsorption
isotherms ZIF-8-ambz-(14), in accordance with an embodiment of the
disclosure.
[0038] FIG. 10b illustrates CO.sub.2 and CH.sub.4 adsorption
isotherms for ZIF-8-ambz-(24), in accordance with an embodiment of
the disclosure.
[0039] FIG. 10c illustrates CO.sub.2 and CH.sub.4 adsorption
isotherms (c) ZIF-8-ambz-47, in accordance with an embodiment of
the disclosure.
[0040] FIG. 11 illustrates the strategy for producing a ZIF
material with a primary amine far enough removed from the aromatic
ring of the organic linker to have useful basicity, in accordance
with an embodiment of the disclosure.
[0041] FIG. 12a illustrates FTIR spectra of ZIF-8-90-(50) (squares)
and ZIF-en (circles), in accordance with an embodiment of the
disclosure.
[0042] FIG. 12b illustrates FI-Raman spectra of ZIF-8-90-(50)
(squares) and ZIF-en (circles), in accordance with an embodiment of
the disclosure.
[0043] FIG. 13a illustrates solution .sup.1H NMR of ZIF-8-90-(50)
(squares) and ZIF-en (circles), in accordance with an embodiment of
the disclosure.
[0044] FIG. 13b illustrates SS .sup.13C CP-MAS NMR spectra of
ZIF-8-90-(50) (squares) and ZIF-en (circles), in accordance with an
embodiment of the disclosure.
[0045] FIG. 14a illustrates powder XRD patterns of ZIF-8-90-(50)
and ZIF-en showing maintenance of crystal structure after
functionalization, in accordance with an embodiment of the
disclosure.
[0046] FIG. 14b illustrates N.sub.2 physisorption isotherms of
ZIF-8-90450) and ZIF-en, in accordance with an embodiment of the
disclosure.
[0047] FIG. 15a illustrates CO.sub.2 and CH.sub.4 adsorption
isotherms of ZIF-8-90-(50), in accordance with an embodiment of the
disclosure.
[0048] FIG. 15b illustrates CO.sub.2 and CH.sub.4 adsorption
isotherms of ZIF-en, in accordance with an embodiment of the
disclosure.
[0049] FIG. 16a-b illustrate IAST calculations of the selective
CO.sub.2 adsorption performance of ZIF materials assuming 25%
CO.sub.2/75% CH.sub.4 gas phase mixture, in accordance with an
embodiment of the disclosure.
DETAILED DESCRIPTION
[0050] Although many embodiments of the disclosure are explained in
detail, it is to be understood that other embodiments are
contemplated. Accordingly, it is not intended that the disclosure
is limited in its scope to the details of construction and
arrangement of components set forth in the following description or
illustrated in the drawings. The disclosure is capable of other
embodiments and of being practiced or carried out in various ways.
Also, in describing the preferred embodiments, specific terminology
will be resorted to for the sake of clarity.
[0051] It must also be noted that, as used in the specification and
the appended claims, the singular forms "a," "an" and "the" include
plural referents unless the context clearly dictates otherwise.
[0052] Also, in describing the embodiments, terminology will be
resorted to for the sake of clarity. It is intended that each term
contemplates its broadest meaning as understood by those skilled in
the art and includes all technical equivalents which operate in a
similar manner to accomplish a similar purpose.
[0053] Ranges may be expressed herein as from "about" or
"approximately" one particular value and/or to "about" or
"approximately" another particular value. When such a range is
expressed, another embodiment includes from the one particular
value and/or to the other particular value.
[0054] By "comprising" or "comprising" or "including" is meant that
at least the named compound, element, particle, or method step is
present in the composition or article or method, but does not
exclude the presence of other compounds, materials, particles,
method steps, even if the other such compounds, material,
particles, method steps have the same function as what is
named.
[0055] It is also to be understood that the mention of one or more
method steps does not preclude the presence of additional method
steps or intervening method steps between those steps expressly
identified. Similarly, it is also to be understood that the mention
of one or more components in a device or system does not preclude
the presence of additional components or intervening components
between those components expressly identified.
[0056] Various embodiments of the disclosure are directed to
zeolitic imidazolate frameworks (ZIF) and their associated
fabrication processes, and more particularly to tunable, hybrid
ZIFs and to methods for making such structures. A novel structural
modification approach towards tuning the properties of ZIF
materials is disclosed. In particular, hybrid ZIFs containing a
combination of different ligands in differing relative quantities
can be synthesized according to the present disclosure.
[0057] One particular advantage of this disclosure is the ability
to manufacture continuously tunable framework functionality or
microporosity. A further advantage is the ability to produce
homogenous crystal structure frameworks that would normally form
varying crystal structures. The present disclosure can improve upon
the non-hybrid ZIFs. For example, characterization by X-ray
diffraction and nitrogen physisorption demonstrates the formation
of a set of crystalline ZIF structures that can exhibit adsorption
properties different from their parent frameworks. Additionally,
continuous control over composition can be possible, as shown by
.sup.1H NMR spectroscopy. Furthermore, the present disclosure
relates to a method that can be a facile route whereby chemically
and thermally robust ZIFs can be subjected to continuous and
tunable alterations in chemical functionality or microporosity by
in-situ incorporation of various linkers, including various
imidazoles, including imidazole and benzimidazole derivatives.
[0058] By the method disclosed herein, the surface functionalities
in ZIF materials can be better controlled and improvement in gas
separations can be realized without severely altering the pore
volume of the material. Additionally, by the methods of the present
disclosure, in situ linker substitution can be performed in various
MOFs, including ZIFs, with two different linkers to introduce two
different functionalities in the material without changing the
crystal structure. For example, 2-aminobenzimidazole, a linker not
found in other ZIF materials, can be incorporated into the ZIF-8
structure, with good control over the linker substitution
stoichiometry. Although this linker contains a primary amine
functional group, it can have little effect on the CO.sub.2
adsorption affinity even at high substitution loadings. Without
wishing to be bound by theory, it is thought that the reduced
effect on the CO.sub.2 adsorption affinity can be due to the
aromaticity of the linker which reduces the basicity of the primary
amine.
[0059] Various MOFs can be manufactured using the methods of this
disclosure including, but not limited to mixed-linker ZIFs that can
undergo post-synthetic modification (PSM) without detrimental loss
of pore volume. Furthermore, ZIF materials can be produced that
have high gas adsorption selectivities including, but not limited
to, CO.sub.2/CH.sub.4 adsorption selectivity.
[0060] An embodiment of the disclosure can be a method for forming
a hybrid zeolitic imidazolate framework (ZIF) comprising the steps
of preparing a first solution comprising a first imidazolate and a
second imidazolate, preparing a second solution comprising a metal
ion, and combining the first solution and the second solution to
form the hybrid ZIF. In one embodiment, the process can further
comprise activating the hybrid ZIF to remove impurities. In some
embodiments, the first imidazolate is different from the second
imidazolate. In one embodiment, the first imidazolate can comprise
2-methylimidazolate and the second imidazolate can comprise
carboxaldehyde-2-imidazolate. In an alternative embodiment, the
first imidazolate can comprise 2-methylimidazole and the second
imidazolate can comprise benzimidazolate. In another embodiment,
the first imidazolate can comprise benzimidazolate and the second
imidazolate can comprise 2-aminobenzimidazolate. In yet another
embodiment, the first imidazolate can comprise 2-methylimidazolate
and the second imidazolate can comprise imidazolate. In some
embodiments, the first imidazolate can comprise 2-methylimidazolate
or benzimidazolate. In other embodiments, the second imidazolate
can comprise benzimidazolate, 2-aminoimidazolate, imidazolate, or
carboxaldehyde-2-imidazolate. Alternatively, one of ordinary skill
in the art would recognize that a variety of imidazolate linkers
with different functionalities can be used to form the hybrid ZIFs,
including imidazolate derivatives and benzimidazolate derivatives.
The functionalities can be the first imidazolate or the second
imidazolate. The various functionalities can include, but are not
limited to alkyl, amino, chloro, bromo, carbonyl, nitro, sulfonate,
hydroxy, hydroxo, aldehyde, organometallic functionalities, and the
like.
[0061] The metal ion can comprise a transition metal. For example,
the meal ion can comprise a first row transition metal such as
nickel, iron, zinc, or cobalt. In some embodiments, the metal ion
can comprise zinc or cobalt. In one embodiment, the metal ion can
comprise zinc. Alternatively, the metal ion can comprise
cobalt.
[0062] The activating can remove species such as solvent, unreacted
metal ions or unreacted imidazolate remaining in the pore of the
ZIF. The activating can comprise any ZIF activating process
commonly known in the art, including but not limited to, heat
treating, vacuum degassing. For example, the activating can
comprise vacuum degassing between about 100.degree. C. and about
300.degree. C.
[0063] In an embodiment, the method for forming a hybrid ZIF can
further comprise functionalizing the hybrid ZIF. The
functionalizing can comprise exposing the hybrid ZIF to a reactive
agent. A reactive agent can be any reagent known in the art that
can undergo a chemical reaction with the hybrid ZIF. For example,
the reactive agent can comprise an aldehyde.
[0064] Alternatively, the reactive agent can comprise an amine. In
one embodiment, the reactive agent cam comprise an aldehyde or an
amine.
[0065] An embodiment can be a metal-organic framework (MOP) that
can comprise a hybrid ZIF comprising a first imidazolate, a second
imidazolate, and a metal ion. In some embodiments, the first
imidazolate is different from the second imidazolate. In one
embodiment, the first imidazolate can comprise 2-methylimidazolate
and the second imidazolate can comprise
carboxaldehyde-2-imidazolate. In an alternative embodiment, the
first imidazolate can comprise 2-methylimidazole and the second
imidazolate can comprise benzimidazolate. In another embodiment,
the first imidazolate can comprise benzimidazolate and the second
imidazolate can comprise 2-aminobenzimidazolate. In yet another
embodiment, the first imidazolate can comprise 2-methylimidazolate
and the second imidazolate can comprise imidazolate.
[0066] The metal ion can comprise a transition metal. In one
embodiment, the metal ion can comprise zinc. Alternatively, the
metal ion can comprise cobalt.
[0067] The hybrid ZIF of the present disclosure can have an
increased selectivity for many gas pairs as compared to a
non-hybrid ZIF. Without wishing to be bound by theory, it is
thought that better selectivity can be derived from either a change
in pore size of the hybrid ZIF materials, or a change in surface
properties by introducing organic functional groups into the
framework. Having a smaller pore size can result in better
diffusion selectivity for small gas pairs while changing the
organic functional groups can increase the adsorption selectivity.
By way of non-limiting examples, the hybrid ZIF can have a greater
selectivity for the following gas pairs: CO.sub.2/CH.sub.4,
CO.sub.2/N.sub.2, O.sub.2/N.sub.2, C.sub.2H.sub.4/C.sub.2H.sub.6,
C.sub.3H.sub.6/C.sub.3H.sub.8, and the like. In one embodiment, the
hybrid ZIF can have a CO.sub.2/CH.sub.4 selectivity of at least 1.2
times greater than a non-hybrid ZIF. In another embodiment, the ZIF
can have a CO.sub.2/CH.sub.4 selectivity of at least 1.5 times
greater than a non-hybrid ZIF. In yet another embodiment, the ZIF
can have a CO.sub.2/CH.sub.4 selectivity of at least 1.8 times
greater than a non-hybrid ZIF. Alternatively, the ZIF can have a
CO.sub.2/CH.sub.4 selectivity of at least 2 times greater than a
non-hybrid ZIF.
[0068] In some embodiments, the hybrid ZIF can have
CO.sub.2/CH.sub.4 adsorption selectivity from about 2.5 to about
13.1. In other embodiments, the hybrid ZIF can have
CO.sub.2/CH.sub.4 adsorption selectivity of at least 10. In
alternative embodiments, the hybrid ZIF can have CO.sub.2/CH.sub.4
adsorption selectivity of at least 12. In another embodiment, the
hybrid ZIF can have CO.sub.2/CH.sub.4 adsorption selectivity of at
least 14. In yet another embodiment, the hybrid ZIF can have
CO.sub.2/CH.sub.4 adsorption selectivity of at least 15. In still
another embodiment, the hybrid ZIF can have a CO.sub.2/CH.sub.3
adsorption selectivity of at least 20. The present disclosure
refers to adsorption selectivity; however, one of ordinary skill in
the art to which this disclosure pertains would understand that
there can also be a difference or increase in diffusion selectivity
also.
[0069] By the present disclosure, hybrid ZIFs can be manufactured
that have reduced pore volumes compared with their counterpart
non-hybrid ZIFs. Reduced poor sizes can lead to better kinetic
separation performance using pure hybrid ZIF membranes and/or
hybrid ZIF composite membranes. Furthermore, the reduction in the
average pore size of the hybrid ZIF can enable the control of the
gate-opening phenomena observed in the interaction of ZIF materials
with molecules such as N.sub.2 and O.sub.2 In an embodiment, the
hybrid ZIF can have a pore size from about 0.25 to about 0.40 nm.
In another embodiment, the hybrid ZIF can have a pore size less
than about 0.40. In an alternative embodiment, the hybrid ZIF can
have a pore size of at least 0.40.
[0070] In an alternative embodiment, the hybrid ZIF can have a
continuous crystal structure. It has been unexpectedly found that
in the present disclosure, the crystal structure can be
near-identical to a non-hybrid ZIF in terms of the arrangement of
metal ions and the linkers. A non-hybrid ZIF is herein defined as a
ZIF comprising only one imidazolate linker. Typically, only one
structure can be obtained when synthesizing a ZIF with one
particular organic linker. For example, the only structure observed
with 2-methylimidazole as the linker is ZIF-8. It can be possible
to manufacture a ZIF with a mixture of linkers, but this can
require specific amounts of each linker. A hybrid ZIF, in this
case, can be a material that can be synthesized with a different
linker composition from the original crystal structure and the
linker composition can be tunable by changing the ratios of the
linkers in the composition. Maintenance of the overall crystal
structure can be advantageous because it can allow for the creation
of a continuously tunable met of materials starting from a
`baseline` ZIF that can be thermally and chemically stable, but
each member of the set can exhibit different in applicability to a
range of separations. It would be clear to one of ordinary skill in
the art that the identity of the linkers can change depending on
the extent of hybridization.
[0071] In one embodiment, the hybrid ZIF can further comprise a
functionalized hybrid ZIF. The functionalized hybrid ZIF can
comprise an aldehyde. Alternatively, the functionalized hybrid ZIF
can comprise an amine.
[0072] Another embodiment of the disclosure can be a molecular
sieve device that can comprise a metal-organic framework (MOF)
comprising a hybrid ZIF that can comprise a first imidazolate, a
second imidazolate, and a metal ion. In some embodiments, the first
imidazolate is different from the second imidazolate. In one
embodiment, the first imidazolate can comprise 2-methylimidazolate
and the second imidazolate can comprise
carboxaldehyde-2-imidazolate. In an alternative embodiment, the
first imidazolate can comprise 2-methylimidazole and the second
imidazolate can comprise benzimidazolate. In another embodiment,
the first imidazolate can comprise benzimidazolate and the second
imidazolate can comprise 2-aminobenzimidazolate. In yet another
embodiment, the first imidazolate can comprise 2-methylimidazolate
and the second imidazolate can comprise imidazolate.
[0073] The metal ion can comprise a transition metal. In one
embodiment, the metal ion can comprise zinc. Alternatively, the
metal ion can comprise cobalt.
[0074] In one embodiment, the ZIF can have a CO.sub.2/CH.sub.4
selectivity of at least 1.2 times greater than a non-hybrid ZIF. In
another embodiment, the ZIF can have a CO.sub.2/CH.sub.4
selectivity of at least 1.5 times greater than a non-hybrid ZIF. In
yet another embodiment, the ZIF can have a CO.sub.2/CH.sub.4
selectivity of at least 1.8 times greater than a non-hybrid ZIF.
Alternatively, the ZIF can have a CO.sub.2/CH.sub.4 selectivity of
at least 2 times greater than a non-hybrid ZIF.
[0075] In some embodiments, the hybrid ZIF can have
CO.sub.2/CH.sub.4 adsorption selectivity from about 2.5 to about
13.1. In other embodiments, the hybrid ZIF can have
CO.sub.2/CH.sub.4 adsorption selectivity of at least 10. In
alternative embodiments, the hybrid ZIF can have CO.sub.2/CH.sub.4
adsorption selectivity of at least 12. In another embodiment, the
hybrid ZIT can have CO.sub.2/CH.sub.4 adsorption selectivity of at
least 14. In yet another embodiment, the hybrid ZIF can have
CO.sub.2/CH.sub.4 adsorption selectivity of at least 15.
[0076] In an embodiment, the hybrid ZIF can have a pore size from
about 0.25 to about 0.40 nm. In another embodiment, the hybrid ZIF
can have a pore size less than about 0.40. In an alternative
embodiment, the hybrid ZIF can have a pore size of at least 0.40.
In an yet another embodiment, the hybrid ZIF can have a continuous
crystal structure.
[0077] In an embodiment, the hybrid ZIF can further comprise a
functionalized hybrid ZIF. The functionalized hybrid ZIF can
comprise an aldehyde. Alternatively, the functionalized hybrid ZIT
can comprise an amine.
[0078] It is to be understood that the embodiments and claims
disclosed herein are not limited in their application to the
details of construction and arrangement of the components set forth
in the description and illustrated in the drawings. Rather, the
description and the drawings provide examples of the embodiments
envisioned. The embodiments and claims disclosed herein are further
capable of other embodiments and of being practiced and carried out
in various ways. Also, it is to be understood that the phraseology
and terminology employed herein are for the purposes of description
and should not be regarded as limiting the claims.
[0079] Accordingly, those skilled in the art will appreciate that
the conception upon which the application and claims are based may
be readily utilized as a basis for the design of other structures,
methods, and systems for carrying out the several purposes of the
embodiments and claims presented in this application. It is
important, therefore, that the claims be regarded as including such
equivalent constructions.
Example 1
[0080] Materials.
[0081] Sodium formate (99%, NaCO.sub.2H), 2-methylimidazole (99%,
2-MeIM), Zn(NO.sub.3).sub.2.6H.sub.2O (99%) and benzimidazole (99%,
Bz-IM) were obtained from Sigma-Aldrich. Methanol (MeOH),
dimethylformamide (DMF), and caboxaldehyde-2-imidazole (99%,
OHC-IM) were obtained from Alfa Aesar. All materials were used
without any further purification.
[0082] Synthesis of ZIF-8-90 Hybrids.
[0083] A solution of 20 mmol NaCO.sub.2H, (20-x) mmol 2-MeIM (ZIF-8
linker) and x mmol OHC-IM (ZIF-90 linker) in 50 mL MeOH was
prepared. The value x varied between 0-20 to alter the ratio of
OHC-IM:2-MeIM in solution. In order to fully dissolve the OHC-IM
ligand, the solution was heated to 50.degree. C. until it became
clear. A separate solution was prepared with 5 mmol
Zn(NO.sub.3).sub.2.6H.sub.2O and 50 mL deionized H.sub.2O. After
the MeOH solution cooled to room temperature, the Zn salt solution
was poured into the IM solution and allowed to stir at room
temperature for 1 hr. The resulting milky solution was centrifuged
at 10000 rpm for 5 min, and the precipitate was redispersed in 45
mL MeOH and washed three times. The powder was dried in an oven at
85.degree. C.
[0084] Synthesis of ZIF-7-8 Hybrids.
[0085] A solution of 20 mmol NaCO.sub.2H, (20-x) mmol 2-MeIM and x
mmol of Bz-IM (ZIF-7 linker) in 50 mL MeOH was prepared. Like the
ZIF-8-90 hybrids, the value x was changed to alter the Bz-IM:2-MeIM
ratio. A separate solution was prepared with 5 mmol
Zn(NO.sub.3).sub.2.6H.sub.2O and 50 mL DMF. The Zn salt solution
was poured into the IM solution and allowed to stir at room
temperature. Times for crystal formation varied between 1-48 hrs.
The resulting suspension was centrifuged at 10000 rpm for 10 min,
and the precipitate was redispersed in 45 mL MeOH. The product was
washed three times and then recovered by vacuum filtration and
dried in an oven at 85.degree. C.
[0086] Characterization Methods.
[0087] Powder X-ray diffraction (XRD) was performed at room
temperature on an X'Pert Pro PANalytical X-Ray Diffractometer using
Cu K.alpha. radiation of wavelength .lamda.=1.5406 .ANG..
Measurements were carried out from 4-50.degree. 2.theta., using an
X'celerator detector with low background sample holders. For unit
cell volume calculations in the ZIF-8-90 system, an internal
standard (.alpha.-Al.sub.2O.sub.3) was added to the powder samples,
and the diffraction pattern was shifted appropriately (typically by
about 0.1.degree. 2.theta.) such that the peak positions of the
internal standard were correctly reproduced. Structureless (Le
Bail) refinement of the full XRD patterns (excluding the internal
standard peaks) was carried out with the Expo2009 package, to
obtain a highly accurate cubic lattice constant (a) and hence the
unit cell volume (a.sup.3). Thermogravimetric and decomposition
analyses were performed on a Netzsch STA-409-PG thermogravimetric
analyzer (TGA) and differential scanning calorimeter (DSC). Powder
samples were heated from room temperature to 900.degree. C. with a
ramp rate of 10 K/min in a diluted air stream (40% air-60%
nitrogen). Smoothed differential mass loss curves were analyzed to
determine decomposition temperature of hybrid materials. Solution
.sup.1H NMR measurements were performed using a Mercury Vx 400 MHz
spectrometer by digesting crystals using d4-acetic acid
(CD.sub.3CO.sub.2D) as the solvent. To determine the fraction of
imidazole linkers in the framework of each sample, the areas of
each peak were normalized to either the aldehyde proton of OHC-IM
(9.84 ppm) or the 2 position proton of Bz-IM (9.05 ppm). Particle
size and morphology were examined using a JEOL 100CX transmission
electron microscope (TEM) operating at 100 keV. Samples were
dispersed in isopropanol and a drop of the dispersion was added to
the TEM grid. Nitrogen physisorption measurements were carried out
at 77 K on a Micromeritics ASAP 2020 surface area analyzer. Samples
were first degassed for 18 hrs at 200.degree. C. (ZIF-8-90 hybrids)
or 250.degree. C. (ZIF-7-8 hybrids) to remove occluded solvent
molecules (H.sub.2O or DMF). Horvath-Kawazoe (HK) equations were
used to analyze nitrogen physisorption isotherms of hybrid
materials. The BET, Langmuir and t-plot micropore volume methods
were used to analyze the relative surface properties of the
hybrids.
[0088] Crystallization.
[0089] In preparing ZIF hybrids, some observations can be made
about the behavior of the induction period and crystallization
process. In all syntheses described here, the non-solvent induced
crystallization (NSIC) technique was used. By introducing a
non-solvent, rapid crystallization can occur due to a drastic
solubility change in the reaction solution, and there can be a
significant reduction (or elimination) of the induction period that
involves precursor formation and crystallite nucleation. Because
ZIF-8 has been synthesized in both non-solvents used in this study,
we hypothesized that addition of NaCO.sub.2H to the reactants would
increase the nucleation time due to competitive coordination with
the Zn.sup.2+ metal center. Furthermore, by using NaCO.sub.2H and
the linkers in equimolar amounts, both linkers included in the
framework can likely be largely deprotonated before addition of the
Zn salt solution, thereby allowing for a more random distribution
of linkers. Otherwise, deprotonation of the linker is driven by the
energy of formation of the framework, and thus the ZIF with the
more favorable lattice energy is more likely to crystallize in pure
form rather than a phase containing a random mixture of
linkers.
[0090] Upon adding the Zn salt solution in the ZIF-8-90-hybrid
case, the solution remained clear for 30-60 s. A crystal suspension
then formed rapidly. Crystal yields were 20-25% based on the
fraction of added Zn incorporated into the crystals. ZIF-7-8
hybrids behaved quite differently during crystallization. Several
solvent systems were employed, but only the DMF-MeOH solvent system
yielded crystalline materials over a wide range of Bz-IM:2-MeIM
ratios reported here Upon adding the Zn-DMF solution, a long
induction period of 1-4 hrs was observed in the case of low Bz-IM
percentages (0-10%). Thus, these reactions were all carried out for
48 hrs at room temperature. However, at 25% Bz-IM, there was rapid
crystal formation, and hence reactions with higher Bz-IM
percentages were only carried out for 1 hr. Yields varied
considerably with the percentage of Bz-IM used, with 10% Bz-IM
having the lowest (.about.1% based on Zn) and 100% Bz-IM having the
highest crystallization yield (67% yield based on Zn).
[0091] Composition Analysis.
[0092] .sup.1H NMR was used to quantify the fraction of
substituting linker (Bz-IM or OHC-IM relative to 2-MeIM) in the
crystals. Because the NSIC technique was used to form the hybrid
materials, it can be expected that the linker with the lower
solubility in the non-solvent can be incorporated in precursors to
a larger extent than the other linker. FIG. 1 shows the fraction of
substituting linker used in the reactant solution versus the
corresponding fraction that resulted in the framework, as
determined by .sup.1H NMR. Both OHC-IM and Bz-IM are incorporated
into the framework at higher fractions than they are present in
solution. It has been shown that ZIF-8 crystals grow in methanol
solutions at room temperature by a nucleation-limited mechanism
without addition of another coordinating linker. Considering that
nucleation is further slowed by additives in the solution
(NaCO.sub.2H), it is likely that the OHC-IM and Bz-IM linkers are
incorporated into nuclei precursors more favorably than the 2-MeIM
linker due to the rapid solubility change by addition of the
non-solvent during synthesis. The hybrid composition can clearly be
controlled; however in the case of ZIF-7-8 hybrids, a precise
control of the synthesis can be necessary to reach any arbitrary
linker composition.
[0093] Crystal Structure.
[0094] FIGS. 2a 1and 2b show XRD patterns of the ZIF-8-90 and
ZIF-7-8 hybrids, respectively. ZIF-8 and ZIF-90 have nearly
identical I-43m cubic unit cells; structure refinement has shown
that the unit cell dimension differs by 2-3%. This makes the XRD
identification of separate phases during hybrid crystallization
difficult. We obtained the unit cell volumes of the ZIF-8-90
hybrids using the Le Bail refinement technique. To account for peak
position errors when measuring powder XRD patterns, the patterns
were corrected using an .alpha.-Al.sub.2O.sub.3 internal standard.
There is an overall systematic increase in the unit cell volume
with increasing OHC-IM fraction. This volume changes by 2.7% as the
hybrid composition goes from pure ZIF-8 to pure ZIF-90.
[0095] On the other hand, ZIF-7 and ZIF-8 have considerably
different crystal structures. Based on refinement, ZIF-7 has a
rhombohedral R-3 space group while ZIF-8 has the cubic I-43m space
group. Differences in XRD patterns are therefore easily discernible
(FIG. 2b). Up to 35% Bz-IM loading, the framework still maintains a
cubic structure characteristic of ZIF-8, and thereafter transitions
to the R-3 space group. Between 79-92% Bz-IM, significantly
disordered materials are obtained. The diffraction pattern of 79%
Bz-IM, in particular, has an apparent superposition of both ZIF-8
and ZIF-7 phases; however, our subsequent TEM images show clearly
that a physical two-phase mixture of ZIF-7 and ZIF-8 crystals does
not exist. Because these materials have considerably different
crystal structures, there can be compositional ranges wherein a
hybrid with a completely random distribution of linkers is unable
to crystallize. Therefore, this XRD pattern can represent an
intergrowth of ZIF-7 and ZIF-8 phases. However, distinguishing an
intergrown hybrid material from a hybrid that has a truly random
linker distribution would require a detailed microcrystallographic
study (e.g., with electron diffraction) and the development of
suitable intergrowth models. For example, we attempted to perform
an indexing of the three XRD patterns mentioned above; however, the
number of distinct usable peaks is only about 10 and therefore a
reliable and unambiguous indexing was not possible to address the
question of whether or not an intergrown material was formed.
[0096] No XRD patterns are shown for hybrids between loadings of
35-79% Bz-IM because of the narrow synthesis range (7-8% Bz-IM in
solution) needed to obtain these materials. It is presently unclear
whether hybrids in this composition range can be successfully
obtained by a highly accurate adjustment of the reactant
composition, or whether the materials in this range are unstable
and hence transform to one of the phases obtained at lower or
higher Bz-IM loadings. Further modification of the procedure could
yield a material with a better range of tunability by producing
more favorable crystallization conditions for the ZIF-7-8 hybrids.
The XRD patterns shown, and the following TEM and nitrogen
physisorption data, do not indicate the formation of a simple
physical mixture of ZIF-7 and ZIF-8; instead, a single crystalline
phase forms in every case.
[0097] Particle Size and Morphology. TEM was used to examine
changes in particle size or morphology with the linker composition.
FIG. 3 shows TEM images of ZIF-8-90 hybrids at different OHC-IM
loadings. At 0% OHC-IM (pure ZIF-8), particles show a monodisperse
distribution centered at approximately 250 nm, exhibiting the
typical rhombic dodecahedral morphology of ZIF-8 particles when
synthesized with NaCO.sub.2H. As the OHC-IM loading increases, the
particle size increases, growing to more than 1 .mu.m at 24%
OHC-IM. Significantly, all the TEM images showed only one particle
population and thereby suggest formation of a single-phase hybrid
material. If ZIF-8 and ZIF-90 had precipitated separate phases, the
particle sizes would likely be considerably different, since the
pure ZIF-8 phase (FIG. 3a) exhibits sub-micron particles while the
pure ZIF-90 phase (FIG. 3d) exhibits micron-sized particles. TEM
images of ZIF-7-8 hybrids are shown in FIG. 4. The particle
morphology of these materials is less well-defined than in the
ZIF-8-90 hybrids, being closer to spherical in most cases than the
rhombic dodecahedral morphology. This observation can support the
formation of a single crystal phase different from both ZIF-7 and
ZIF-8 at the intermediate loadings of Bz-IM, considering that ZIF-7
has been shown to form spherical or rod-like morphologies and ZIF-8
forms the rhombic dodecahedral morphology in the presence of an
additive. The particle size of the disordered hybrid material shown
in FIG. 4b is considerably smaller than that of the other materials
in FIG. 4. As 2-MeIM is still the major organic linker in the
solution, it can be preventing crystallization of the R-3 phase and
resulting in low crystallization yields and the disordered
rhombohedral-like structure shown by XRD. Like the ZIF-8-90
hybrids, there are not different particle size populations or
morphologies that indicate a physical mixture of the two
phases.
[0098] Thermal Decomposition. Previous studies on thermal
decomposition of ZIFs showed the existence of a wide range of
decomposition temperatures that also depend on synthesis
conditions. Thermogravimetric analysis (TGA) was used here to
elucidate stability changes in the hybrid materials. To minimize
the influence of solvent mass loss, samples were first degassed
under vacuum for 12 hours at either 200.degree. C. (ZIF-8-90) or
250.degree. C. (ZIF-7-8). FIGS. 5a and 5b show differential mass
loss curves for ZIF-8-90 and ZIF-7-8, respectively. ZIF-8 (0%
OHC-IM) shows the highest thermal stability for the ZIF-8-90
hybrids, with nearly no decomposition observed until 450.degree. C.
At 24% OHC-IM loading, decomposition begins closer to 400.degree.
C., and above 50% OHC-IM loading, the hybrid frameworks are
thermally stable up to 300.degree. C. Aldehydes typically oxidize
easily in air, and this is the likely cause of the lower thermal
stability under exposure to diluted air at elevated temperatures.
On the other hand, the onset of decomposition in ZIF-7-8 hybrids
remains unchanged at 400-450.degree. C. Materials at high loadings
of Bz-IM (79-100%) are stable above 500.degree. C. This increase in
decomposition temperature of the framework can be attributed to the
greater stability provided by the aryl groups of Bz-IM.
[0099] Porosity. FIGS. 6a and 6b show nitrogen physisorption
isotherms of ZIF-8-90 and ZIF-7-8 hybrids. Inclusion of either
OHC-IM or Bz-IM reduces the maximum quantity adsorbed in comparison
to pure ZIF-8, thus decreasing the micropore volume of the
framework. It has been shown that ZIF-90 exhibits hysteresis at
P/P.sub.0.about.0.4, and this was attributed to a constriction in
the micropores. As the OHC-IM loading increases in the framework,
the single inflection point seen in ZIF-8 at
P/P.sub.0.about.5.times.10.sup.-3 (attributed to a gate-opening
mechanism that allows more nitrogen into the micropores) turns into
two inflection points as pure ZIF-90 is approached: one inflection
still characteristic of the gate-opening effect at lower nitrogen
activity (e.g., P/P.sub.0.about.10.sup.-4) and the other attributed
to constriction in the micropores at higher nitrogen activity
(e.g., P/P.sub.0.about.0.4). To confirm that these inflection
points and the isotherms are indeed a result of hybrid materials
(containing a single phase with an essentially random dispersion of
linkers in the framework), a 50-50 (molar) physical mixture of
ZIF-8 and ZIF-90 crystals was prepared. As shown in FIG. 6a, the
inflection points in the corresponding isotherm are characteristic
of the parent framework materials and not of the hybrids at similar
overall composition (e.g. 43% OHC-IM). This is a further indication
that the materials prepared with our synthetic method are single
phases with different microporosity from pure ZIF-8 or ZIF-90.
[0100] FIG. 6b shows physisorption isotherms of the ZIF-7-8
hybrids. As the Bz-IM loading increases, the micropore volume is
severely reduced; for example, there is a 40% micropore volume
reduction from pure ZIF-8 at only 20% Bz-IM loading. The inflection
typical of ZIF-8 decreases and disappears as the loading increases.
This indicates that the bulky Bz-IM linkers are much less amenable
to rotational displacement. Thus, we hypothesize that inclusion of
Bz-IM in the framework reduces and even eliminates gate-opening
phenomena in the hybrid ZIF materials when probed with cryogenic
nitrogen adsorption. In fact, the shape of the isotherm (Type I)
with 20% Bz-IM loading is similar to the isotherm predicted for
ZIF-8 by grand canonical Monte Carlo (GCMC) simulations that assume
a rigid framework. The progressive suppression of the gate-opening
phenomena in these hybrids provides a method for tuning the average
pore size and the flexibility so that the material can be adapted
to specific applications (separations, catalysis). Although ZIF-7
has been shown to admit condensable gases (e.g. CO.sub.2, ethane)
via a structural change, the pores are not accessible to nitrogen
at 77 K, as shown in FIG. 6b.
[0101] Horvath-Kawazoe analysis was used to analyze the relative
pore size distributions (PSDs) of the hybrid materials. It has been
shown that some assumptions of these equations are not physically
accurate and therefore do not provide correct absolute values of
the PSDs. For example, the main pore size for both ZIF-8 and ZIF-90
predicted by the HK method are larger than the pore sizes
determined crystallographically. However, the HK method can be
reliably used for relative comparison of a series of structurally
related materials. FIGS. 7a and 7b show the PSDs for both hybrid
material types. ZIF-8 (0% OHC-IM) shows two pore size
distributions, centered at .about.0.4 nm and .about.0.75 nm. The
former represents the limiting pore diameter of the ZIF-8 windows.
Considering the gate-opening mechanism, the latter PSD is
interpreted as resulting from rotational displacement of linkers to
allow further adsorption of nitrogen, and not due to an actual
secondary pore system (e.g. the SOD cages) in the framework. This
interpretation is supported by the fact that while the main pore
size becomes larger with increasing OHC-IM loading (the pure ZIF-90
material shows a primary pore size of .about.0.5 nm), the secondary
PSD is quite different from the original ZIF-8 material and hence
is unlikely to originate from a secondary pore system such as the
ZIF cages, which maintain essentially the same dimensions in all
the ZIF-8-90 hybrids. Interestingly, the gate-opening effect is
still apparent in ZIF-90, an observation not shown or explained
previously. This effect in ZIF-90 occurs at a significantly lower
relative pressure (P/P.sub.0.about.10.sup.-4) than typically
observed for ZIF-8 (P/P.sub.0.about.5.times.10.sup.-3). Considering
this difference, the gate-opening in ZIF-90 and in hybrids with
higher OHC-IM loading can be interpreted as being more easily
induced than in pure ZIF-8.
[0102] PSDs of the ZIF-7-8 hybrids are shown in FIG. 7b. As noted
before, the isotherm inflection is related to the second PSD. At 7%
Bz-IM loading, the intensity of the second PSD is reduced
considerably, and it disappears at 20% Bz-IM. This indicates that
Bz-IM groups block the rotational displacement of the imidazole
linkers. The primary PSD shifts to overall smaller values, and at
20% Bz-IM, a PSD centered at .about.0.32 nm appears. This shift to
smaller pore sizes indicates that Bz-IM is effectively reducing the
average pore size.
Example 2
[0103] Materials.
[0104] Sodium formate (99%, NaCO.sub.2H), 2-methylimidazole (98%,
2-MeIM), Zn(NO.sub.3).sub.2.6H.sub.2O (99%), 2-aminobenzimidazole
(97%, 2-amBzIM), and carboxaldehyde-2-imidazole (99%, OHC-IM) were
obtained from Alfa Aesar. Methanol (MeOH) and dimethylformamide
(DMF) were obtained from BDH. Ethylenediamine (99%, en) was
obtained from Sigma-Aldrich. All chemicals were used as received
without further purification.
[0105] Synthesis of ZIF-8-ambz-(x).
[0106] A solution was prepared with (20-x) mmol 2-MeIM, x mmol
2-amBzIM, and 5 mmol NaCO.sub.2H in 50 mL deionized H.sub.2O. The
value x was varied between 0-10 to change the ratio of
2-MeIM:2-amBzIM in solution. To fully dissolve the 2-amBzIM, the
solution was heated to 70.degree. C. for at least 2 hrs in a round
bottom flask until the solution turned clear. A separate solution
containing 5 mmol Zn(NO.sub.3).sub.2.6H.sub.2O in 50 mL DMF was
also prepared. After the imidazole solution cooled to room
temperature, the Zn-salt solution was added and allowed to stir for
1 hr. The solution was then centrifuged at 10,000 rpm for 5 min and
the precipitate was washed with MeOH. This washing was repeated
three times, and then the precipitate was recovered by vacuum
filtration and dried in an oven at 85.degree. C. The yield of
product was approximately 20-25% based on Zn.
[0107] Synthesis of ZIF-8-90-(50).
[0108] Synthesis of ZIF-8-90-(50) is a scaled-up reaction of a
previously reported procedure. A solution was prepared containing
12.6 mmol OHC-IM, 87.4 mmol 2-MeIM, and 100 mmol of NaCO.sub.2H in
250 mL MeOH. To fully dissolve all the OHC-IM, this solution was
heated to 50.degree. C. in a sealed polyethylene bottle for 2 hrs.
A separate solution was then prepared with 25 mmol
Zn(NO.sub.3).sub.2.6H.sub.2O in 250 mL deionized H.sub.2O. After
the imidazole solution had cooled to room temperature, the Zn-salt
solution was added and allowed to stir for 1 hr. The solution was
centrifuged at 10,000 rpm for 5 min, and the precipitate was washed
with MeOH after pouring off the supernatant. This was repeated
again, and then the precipitate was recovered by vacuum filtration.
The recovered product was dried in an oven at 85.degree. C. The
yield was approximately 20% based on Zn.
[0109] Functionalization of ZIF-8-90-(50).
[0110] The ZIF-8-90-(50) samples were functionalized with
ethylenediamine to form ZIF-en samples. First, a dispersion of 0.2
g ZIF-8-90-(50) was prepared in 25 mL MeOH. Then, at room
temperature, 1 mL ethylenediamine was added to the dispersion.
Sealed in a Teflon cup, the ZIF dispersion was heated to 80.degree.
C. and stirred for 24 hrs. After cooling to room temperature, the
powder was washed with MeOH and recovered by vacuum filtration. The
powder sample turned from white to yellow after reaction.
[0111] Materials Characterization.
[0112] The ZIF materials were analyzed by powder X-ray diffraction
(XRD) using an X'Pert Pro PANalytical X-ray Diffractometer.
Diffraction measurements were done from 3.5-50.degree. 2.theta.
using an X'celerator detector. N.sub.2 physisorption measurements
were done on a Micromeritics ASAP 2020 surface area analyzer at
-196.degree. C. ZIF-8-ambz-(x) samples were degassed at 250.degree.
C. under vacuum for 18 hrs before physisorption analysis, whereas
ZIF-8-90-(50) and ZIF-en samples were degassed at 150.degree. C.
under vacuum for 12 hrs. The BET surface area and t-plot micropore
volume methods were used to analyze the relative surface properties
of each sample. Fourier-transform infrared (FTIR) and
Fourier-transform Raman (FT-Raman) spectroscopy were performed on
ZIF-8-90-(50) and ZIF-en samples. The powder samples were prepared
in KBr pellets for FTIR and then analyzed on a Bruker Vertex 80v
FIR Analyzer from 4000-400 cm.sup.-1. For FT-Raman, powder samples
were packed tightly in NMR tubes and analyzed on a Bruker RAM II
FT-Raman Analyzer from 4000-400 cm.sup.-1. To determine the linker
composition in the framework, all samples were analyzed with
solution .sup.1H nuclear magnetic resonance (NMR) spectroscopy on a
Mercury Vx 400 MHz spectrometer after digesting samples using
d4-acetic acid (CD.sub.3CO.sub.2D). Solid state (SS).sup.BC cross
polarization-magic angle spinning (CP-MAS) NMR was performed on a
Bruker 300 MHz spectrometer, using a spinning rate of 10 kHz with a
4 mm rotor and collecting a minimum of 5,000 scans. ZIF
decomposition stability was tested on a Netzsch STA-409-PG
thermogravimetric analyzer (TGA). Powder samples were heated from
room temperature to 900.degree. C. with a ramp rate of 10.degree.
C./min in a diluted air stream (25% air/75% N.sub.2).
[0113] Adsorption Measurements.
[0114] Adsorption measurements for CO.sub.2 and CH.sub.4 were
carried out in a custom-built, constant-volume apparatus. Samples
were tested at temperatures of 35, 55, and 75.degree. C. to provide
data at multiple temperatures, facilitating calculation of heats of
adsorption for each gas and sample. Pressure ranges tested were
typically from 0-1000 kPa. Samples were degassed at 100.degree. C.
under vacuum for at least 12 hrs before testing. Ultra-high purity
CH.sub.4 (99.999%) and bone-dry CO.sub.2 (99.999%) were used in all
adsorption measurements.
[0115] Adsorption Analysis.
[0116] Isotherms obtained from adsorption measurements were fit to
a Toth isotherm to describe the heterogeneous surface resulting
from the mixed-linker structure:
q i = q sat b i p [ 1 + ( b i p ) t i ] 1 / t i ( 1 )
##EQU00001##
where q.sub.i is the capacity for adsorbing component i at
pressure, p, q.sub.sat is the saturation capacity, b.sub.i is the
affinity constant of component i, and t.sub.i is the heterogeneity
parameter. When t.sub.i is equal to 1, this equation becomes the
Langmuir isotherm model. The change in the affinity constant,
b.sub.i, the heterogeneity parameter, t.sub.i, and the saturation
capacity, q.sub.sat, with temperature can be described with the
following
b i = b i , 0 exp [ - .DELTA. H RT 0 ( T 0 T - 1 ) ] ( 2 ) t i = t
0 + .alpha. ( 1 - T 0 T ) ( 3 ) q sat , i = q sat , 0 exp [ .chi. (
1 - T T 0 ) ] ( 4 ) ##EQU00002##
where b.sub.i,0 is the pre-exponential affinity constant,
-.DELTA.H.sub.ads is the heat of adsorption at zero adsorbate
loading, T is the absolute temperature, T.sub.0 is the reference
temperature (35.degree. C.), .alpha. and t.sub.0 are parameters
used for thermal variation in the heterogeneity parameter, t.sub.i,
and q.sub.sat,0 and .chi. represent changes in saturation capacity
for each adsorbent. Isotherms at different temperatures for a
single gas were fit to the Toth isotherm by maximizing the
coefficient of determination (R.sup.2) for all data simultaneously.
The ZIF materials were analyzed with ideal adsorbed solution theory
(IAST) assuming a gas mixture of 25% CO.sub.2/75% CH.sub.4. To
perform the analysis, a thermodynamic criterion must be satisfied:
the spreading pressures of each component are equal to each other
(.pi..sub.i=.pi..sub.j), where .pi. is the spreading pressure
calculated as:
.pi. i = RT A .intg. 0 p i 0 q i p p ( 5 ) ##EQU00003##
Using Equation 1, the spreading pressure of component i can be
expressed in terms of isotherm parameters:
.pi. i A RT = .intg. 0 p i 0 q sat , i b i [ 1 + ( b i p ) t i ] 1
/ t i p ( 6 ) ##EQU00004##
Here, p.sub.i.sup.0 is obtained from the expression:
y.sub.ip=x.sub.ip.sub.i.sup.0 (7)
where y.sub.i is the gas phase mole fraction, p is the absolute
pressure, x.sub.i is the adsorbed phase mole fraction and
p.sub.i.sup.0 is the gas-phase pressure corresponding to
adsorbed-phase spreading pressure .pi., for the adsorption of pure
component i. The adsorption selectivity (.alpha..sub.1,2) for a
specific gas pair can be calculated as:
.alpha. 1 , 2 = ( x 1 / y 1 ) ( x 2 / y 2 ) ( 8 ) ##EQU00005##
and the capacity at p and T can be calculated by
1 q = x 1 q ( p 1 0 ) + x 2 q ( p 2 0 ) ( 9 ) ##EQU00006## and
q.sub.i=x.sub.iq (10)
To solve for x.sub.i and calculate selectivity and capacity,
Equations 6-7 were solved iteratively.
[0117] ZIF-8-ambz-(x).
[0118] As our previous study on mixed-linker ZIFs showed that the
inclusion of benzimidazole (Bz-IM) in a ZIF-8-like framework was
difficult to control, a set of ZIF materials were prepared
containing different proportions of 2-MeIM and 2-amBzIM linkers to
understand if this substitution could be better controlled while
maintaining a ZIF-8 crystal structure. FIG. 8a shows the fraction
of 2-amBzIM linkers included in the framework obtained from .sup.1H
NMR spectra and derived from syntheses using up to 50% 2-amBzIM in
the synthesis solution. In comparison to our previous study, we
find that inclusion of 2-amBzIM is more controllable than Bz-IM.
Because 2-amBzIM does not form any known single-linker ZIF
structure, it is likely that formation of the ZIF-7 structure with
Bz-IM is more thermodynamically favorable than any structure
containing 2-amBzIM; therefore, the ZIFs obtained with 2-amBzIM
represent a mixed-linker system with better control of linker
composition while maintaining the cubic I-43m structure of ZIF-8.
Powder XRD patterns (FIG. 8b) of the synthesized materials show
that the I-43m crystal structure is maintained up to high
substitution fractions of 2-amBzIM. At 47% 2-amBzIM in the
framework, a significant peak broadening is observed in the XRD
pattern. This is either due to the formation of a crystal with a
disordered distribution of the two linkers, or due to a high degree
of crystal strain arising from incorporation of the high fraction
of 2-amBzIM in the ZIF crystal structure.
[0119] Nitrogen physisorption was used to investigate the
gate-opening properties of these mixed-linker ZIFs. FIG. 9 shows
materials with modest loadings of 2-amBzIM (9.5% in the structure)
show no evidence of gate-opening effects compared to ZIF-8. There
is a reduction in micropore volume (see Table 1) with increasing
linker substitution.
TABLE-US-00001 TABLE 1 Linker substitution, framework density,
micropore volume and BET surface area of ZIF-8-ambz-(x),
ZIF-8-90-(50), and ZIF-en materials. t-plot BET Linker Framework
micropore Surface Substitution Density volume Area Sample (mol
%).sup.a (g/cm.sup.3).sup.b (cm.sup.3/cm.sup.3) (m.sup.2/cm.sup.3)
ZIF-8-ambz-(9) 9.5 0.934 0.586 1640 ZIF-8-ambz-(14) 14 0.983 0.406
1160 ZIF-8-ambz-(24) 24 1.024 0.322 880 ZIF-8-ambz-(47) 47 1.120
0.014 70 ZIF-8-90-(50) 48 0.983 0.630 1670 ZIF-en .sup. 27.sup.c
1.048 0.301 860 .sup.aCalculated from solution .sup.1H NMR, balance
is 2-MeIM .sup.bCalculated from assuming no change in unit cell
volume .sup.cTotal substitution = 22% en-IM + 5% OHC-IM
Significantly, at 47% 2-amBzIM loading, there is complete loss of
micropore volume and no uptake of N.sub.2 at -196.degree. C. To
activate this material properly, the powder sample needed to be
washed and soaked with MeOH at 50.degree. C. for 24 hrs to remove
occluded solvent molecules trapped in the pores during synthesis.
If degassing at 250.degree. C. was performed without this
activation step, there was complete loss of crystallinity. When
these samples were examined with TGA, the samples not soaked in
MeOH showed mass loss before the material decomposed, whereas the
samples soaked in MeOH showed no mass loss before decomposition.
Based on the above observations, it is very likely that the higher
2-amBzIM substitution has significantly reduced the effective pore
size and micropore volume of the ZIF material, blocking even small
N.sub.2 molecules from accessing the pores at -196.degree. C.
[0120] CO.sub.2 and CH.sub.4 adsorption measurements were used to
assess the changes in surface properties for the mixed-linker ZIFs
containing 2-amBzIM, compared to ZIF-8. For comparison, the heats
of adsorption at zero adsorbate loading for ZIF-8 are calculated to
be -15.6 kJ/mol and -12.3 kJ/mol for CO.sub.2 and CH.sub.4,
respectively (Table 2).
TABLE-US-00002 TABLE 2 Heats of adsorption for CO.sub.2 and
CH.sub.4 obtained from Langmuir and Toth isotherm fits, Henry's law
constants of ZIF materials and ideal selectivity of
CO.sub.2/CH.sub.4. K (cm.sup.3 (STP)/cm.sup.3 -.DELTA.H (kJ/mol)
adsorbent).sup.b Sample CO.sub.2 CH.sub.4 CO.sub.2 CH.sub.4 .alpha.
(ideal) ZIF-8.sup.a 15.6 12.3 12.5 5.3 2.4 ZIF-8-ambz-(14) 19.4
12.5 19.3 7.0 2.8 ZIF-8-ambz-(24) 20.4 12.4 24.7 7.9 3.1
ZIF-8-ambz-(47) 22.1 15.6 14.1 4.0 3.5 ZIF-8-90-(50) 25.6 20.0 41.3
7.4 5.6 ZIF-en 33.9 26.2 103 7.9 13.1 .sup.aValues for ZIF-8 are
taken from Refs. 7, 9, 31 .sup.bHenry's constants were predicted by
(q.sub.sat b)
Framework densities were calculated assuming no change in the unit
cell volume of ZIF-8, and these values are shown in Table 1 for
each sample presented. FIG. 10a shows the CO, and CH.sub.4
adsorption data and fits of the Toth isotherm for the
ZIF-8-ambz-(14) material. Open symbols denote CH.sub.4 adsorption,
closed symbols CO.sub.2. Squares: T=35.degree. C.; circles:
T=55.degree. C.; triangles: T=75.degree. C. The capacities reported
in this figure are presented in units of mmol/cm.sup.3, instead of
the more typical mmol/g. Because the mass adsorption capacity of
MOFs can often be misleading due to low bulk or framework
densities, the adsorption units used here reflect the material
capacity and performance more accurately. When comparing the
initial slope of the adsorption isotherms of the ZIF-8-ambz-(14)
material to those of ZIF-8, it is clear that there is not much
change in the affinity for CO.sub.2 or CH.sub.4 at this
substitution level of 2-amBzIM. Although the substituting linker
contains a primary amine functional group, the basicity of this
amine is greatly affected by the fused aromatic ring of the
imidazolate and benzyl groups. From the isotherm fit, the heats of
adsorption were found to be -19.4 kJ/mol and -12.5 kJ/mol for CO,
and CH.sub.4, respectively. The heat of adsorption for CH.sub.4 is
almost identical with that of ZIF-8. It has been shown by both
simulations and neutron scattering measurements that CH.sub.4
adsorbs in the 6-membered ring (MR) of ZIF-8 at lower pressures,
interacting directly with the C.dbd.C bond on the 2-MeIM linker.
Because 2-amBzIM has similar adsorption sites, it is not surprising
that no change in the heat of adsorption of CH.sub.4 was seen in
ZIF-8-ambz-(x) materials compared with ZIF-8. An increase of about
4 kJ/mol in the heat of adsorption for CO.sub.2 in the
ZIF-8-ambz-(x) material relative to ZIF-8 is also consistent with
the introduction of a more polar functional group than the methyl
group on the Z linker; however, this does not greatly affect the
overall affinity for CO.sub.2 in this sample.
[0121] As FIGS. 10b and 10c show, with increasing substitution of
2-amBzIM in the ZIF framework, there is an overall increase in the
affinity for CO.sub.2 without much change in the affinity for
CH.sub.4. There is a concomitant increase in the heat of adsorption
for CO.sub.2. Table 2 summarizes the heats of adsorption obtained
for each material. With increasing fraction of 2-amBzIM, the heats
of CO.sub.2 adsorption show a monotonic increase, consistent with
the increasing binding energy due to the polar functional groups.
Although ZIF-8-ambz-(14) and ZIF-8-ambz-(24) showed excellent fits
to a Langmuir isotherm (t.sub.i=1), the Toth isotherm provided the
best overall fit for the ZIF-8-ambz-(47) material. It is likely
that the high substitution obtained in this sample causes
significant heterogeneity in the internal surface of the ZIF.
Interestingly, although N.sub.2 physisorption showed complete loss
of micropore volume at -196.degree. C., ZIF-8-ambz-(47) adsorbed
both CO.sub.2 and CH.sub.4 at ambient temperatures. This shows
these materials can have significant `breathing` and `flexibility`
effects that allow molecular adsorption and diffusion even when
containing bulky benzimidazolate-type linkers. Although no
gate-opening phenomena is observed from N.sub.2 physisorption,
there is clearly some freedom for the linkers to rotate or vibrate
to allow adsorption of larger molecules at ambient temperatures,
similar to behavior observed in ZIF-8. Table 2 shows the Henry's
constants obtained from isotherm fits and the ideal selectivity, as
well as data from ZIF-8 as a standard to compare the
CO.sub.2/CH.sub.4 selectivity. Because there is significant
reduction in the total adsorption capacity in the ZIF-8-ambz-(47)
sample, the Henry's constant decreases, but there is an overall
increase in selectivity as the 2-amBzIM fraction increases.
Considering that these materials also have a shrinking pore size
with increasing 2-amBzIM fraction, it is likely the overall
transport selectivity of these materials is higher than that of
ZIF-8 and hence the materials can be useful for membrane-based
CO.sub.2 separations.
[0122] Postsynthetic Modification of ZIF-8-90-(50). The previous
PSM attempts on ZIF materials showed a detrimental loss of
micropore volume after functionalization. We hypothesized that the
present mixed-linker synthetic strategy that allows control over
the amount of reactive sites can be able to prevent a total loss of
pore volume while still providing active sites for
functionalization. FIG. 11 shows the strategy for producing a ZIF
material with a primary amine far enough removed from the aromatic
ring of the organic linker to have useful basicity. FIGS. 12 a-b
show FTIR and FT-Raman spectra of ZIF-8-90-(50) and the modified
material ZIF-en. Several FTIR bands that show significant changes
are highlighted in FIG. 12a. Although the bands in the N--H
stretching region (3600-3200 cm.sup.-1) are broad, there is an
appearance of two symmetric bands and one asymmetric band after
functionalization, indicating the --NH.sub.2 moiety in ZIF-en. In
the carbonyl region (1700-1600 cm.sup.-1), there is a disappearance
of the band from the aldehyde and an appearance of a band in the
imine region, suggesting that the aldehyde is converted to an imine
by reacting with ethylenediamine; however, there can be an
appearance of --N--H bending in the same region, making it
necessary to also examine the FT-Raman spectra of ZIF-en. FT-Raman
spectra (FIG. 12b) confirm the disappearance of the .nu.(C.dbd.O)
at 1700 cm.sup.-1 and the appearance of the .nu.(C.dbd.N) band at
1650 cm.sup.-1; however, a weaker .nu.(C.dbd.O) band is still
present in ZIF-en, indicating that not all carbonyl groups were
reacted.
[0123] NMR spectroscopy was used to identify the functional groups
in the ZIF framework and also to determine if there is any
`cross-linking` caused by reaction of --NH.sub.2 groups of
ethylenediamine with nearby OHC-IM linkers. Solution .sup.1H NMR
spectra of the deuterated acid-digested ZIF materials are shown in
FIG. 13a. The chemical shift associated with the aldehyde
(.delta..about.9.9 ppm) nearly disappears after PSM, and there is
an appearance of a peak associated with an imine bonded to an
aromatic ring (.delta..about.8.5 ppm). Taken together, these
chemical shifts show a combined linker substitution of 27% (5%
aldehyde and 22% en) after functionalization compared with 48%
substitution prior to functionalization. This indicates 46% loss of
total OHC-IM linker during PSM, with 81% of the remaining OHC-IM
linkers in ZIF-en converted to the functionalized amine linker.
However, the TGA mass loss curves of these materials show no
significant changes in the total inorganic content (Zn). This
indicates that there is some overall loss of ZIF particles during
functionalization, but not the selective etching of a linker. It
should also be noted that the overall yield of ZIF following
functionalization was approximately 60% of the starting ZIF mass.
Therefore, the change in the relative fractions of the different
linkers in ZIF-en, compared to ZIF-8-90-(50), is best explained as
being due to some dissolution of the ZIF material during PSM. The
linkers dissolved in solution can then undergo PSE with the ZIF
material, changing the overall linker ratios in the final material.
It has already been shown that ZIF materials can easily dissolve in
certain solvents and even recrystallize upon cooling. When the
integrated area of the imine chemical shift (.delta..about.8.5 ppm)
was compared to the methylene group (.delta..about.4.2, 3.5 ppm),
we found a ratio of 1:2, suggesting no cross-linking of OHC-IM
linkers by ethylenediamine molecules. In the case of SS .sup.13C
CP-MAS NMR spectra (FIG. 13b), there was no appearance of an
aldehyde peak in the region of .delta..about.170-180 ppm following
functionalization likely due to low concentration in the framework.
Two methylene (--CH.sub.2) peaks appear at .delta..about.55-70 ppm
and 42 ppm and can be associated with the ethyl linker between the
imine and amine functional groups in Previously, the peak
associated with imine formation in ZIF-90 was attributed to a
chemical shift of 60 ppm, meaning the broad peak at
.delta..about.55-70 ppm is the methylene --CH.sub.2 bonded to imine
nitrogen. Therefore, the other peak at 42 ppm is a resonance of the
methylene --CH.sub.2 bonded to the primary amine nitrogen group.
The shoulder at 150 ppm is likely associated with C.dbd.N bond
formation from the imine. Overall, the NMR spectra suggest a high
conversion of the aldehyde groups of ZIF-8-90-(50) to imine groups
in ZIF-en, and there is no evidence of cross-linking of nearby
linkers by ethylenediamine.
[0124] Although there are changes in the relative fractions of the
two linkers following PSM, both powder XRD and N.sub.2
physisorption show excellent preservation of the ZIF crystal
structure and significant retention of micropore volume (FIGS. 14
a-b). From t-plot micropore volume calculations, a loss of 52% of
the pore volume was observed after functionalizing the ZIF
material. Considering that previous efforts to functionalize the
internal carbonyl groups of ZIF-90 led to near-complete loss of
micropore volume, our characterization results are encouraging.
They indicate that ZIF-en still allows gas diffusion through the
pores, and has good adsorption properties--both of which make it
potentially useful for gas separation applications.
[0125] FIGS. 15a and 15b show CO.sub.2 and CH.sub.4 adsorption
properties of ZIF-8-90-(50) and ZIF-en, respectively. Open symbols
denote CH.sub.4 adsorption, closed symbols CO.sub.2. Squares:
T=35.degree. C.; circles: T=55.degree. C.; triangles: T=75.degree.
C. As seen in Table 2, the heats of adsorption for both CO.sub.2
and CH.sub.4 increased after PSM of ZIF-8-90-(50). The heat of
adsorption values obtained for ZIF-en are close to typical values
for the commercial CO.sub.2 adsorbent zeolite 13X; however, the
affinity constants for ZIF-en are much lower than 13X (Table S1).
Unlike zeolite 13X, ZIF-en likely does not have high affinity for
H.sub.2O, which significantly affects separation performance in
13X. There is significant enhancement of CO.sub.2 affinity for
ZIF-en when compared to ZIF-8-90-(50) or previously published data
on ZIF-8. In addition, even though there is a 52% reduction in
micropore volume in ZIF-en, the CO.sub.2 capacity does not decrease
greatly. When comparing the Henry's law constants of ZIF-8-90-(50)
and ZIF-en, the ideal selectivity is found to increase from 6 to 13
after PSM, the latter being nearly 6 times higher than that of BPL
carbon. Overall, we conclude that the functionalization of
ZIF-8-90-(50) with ethylenediamine leads to a promising enhancement
of CO.sub.2/CH.sub.4 selectivity.
[0126] IAST Predictions. The IAST is useful for assessing an
adsorbent's performance in multicomponent systems. FIG. 16a shows
the adsorption selectivity from IAST calculations for the
adsorbents considered in this study. The single-component
adsorption parameters required for the IAST predictions are
obtained from our experimental data. Although our mixed-linker ZIFs
can have heterogeneous surfaces, thereby potentially decreasing the
accuracy of IAST predictions, it is still useful as a tool for
initial assessment of adsorption characteristics. Additionally, it
has been shown that if the chosen single-component adsorption model
fits the experimental data well, then IAST provides good
predictions of mixed gas adsorption. Overall, the materials studied
here show improvement over ZIF-8 and BPL carbon for
CO.sub.2/CH.sub.4 selectivity. Considering the difficulty and
expensive linkers required to make ZIFs with selectivities
comparable to ZIF-en (e.g., ZIF-78), it can be more prudent to
alter ZIF adsorption and gas separation properties by a
mixed-linker and/or PSM route rather than using more expensive
linkers if the materials are to be considered for practical
replacements of commercially available materials (BPL carbon).
Interestingly, ZIF-en shows an asymptotic behavior for selectivity
at low pressures. The mixed gas adsorption capacity for this sample
is shown in FIG. 16b. Like other amine-modified adsorbents, ZIF-en
can have high heats of adsorption (and thus selectivity) at low
partial pressures of CO.sub.2.
* * * * *