Synthesis and Characterization of a Cu2(pzdc)2(bix) [pzdc: 2,3-pyrazinedicarboxylate; Bix: 1,3-bis(imidazol-1-yl)benzene] Porous Coordination Pillared-Layer Network

Article abstract of DOI:10.1021/acs.cgd.7b01616

A porous coordination polymer (PCP) was prepared using pyrazine-2,3-dicarboxylate (pzdc) and the pillar ligand 1,3-bis(imidazol-1-yl)benzene) (bix) in an attempt to improve on the thermal stability and adsorption properties of Cu₂(pzdc)₂(L) (L = pillar ligand; typically dipyridyl-based). As-synthesized Cu₂(pzdc)₂(bix) showcases a highly crystalline structure (Monoclinic, P2₁/c, a = 11.836 ?, b = 13.005 ?, c = 11.389 ?, β = 107.64°) that exhibits characteristic diffraction peaks also found in the dipyridyl-based PCP counterparts. Thermal analysis based on results from thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC) and in situ high temperature X-ray diffraction (XRD) suggests both framework contraction and expansion during the elimination of physisorbed and coordinated water, phase transition, and prior to the eventual destruction of the structure, which takes place at ca. 556 K. The thermal stability limit is an improvement over those displayed by other Cu₂(pzdc)₂(L) materials by an average margin of ca. 55 K. Comparison of the adsorption of CO₂ vs N₂ both at cryogenic temperatures suggests a concomitant structural expansion in the presence of the former. This process appears to take place even at ambient temperatures and CO₂ pressures up to 50 atm; the isotherm profile showcases a hysteretic pathway and full adsorption saturation is not apparent. Furthermore, evaluation of CO₂ adsorption at different equilibration time intervals indicate that the structure relaxation time scale is larger than that of the adsorption kinetics; similar observations have been made before for Cu₂(pzdc)₂(bpy) (bpy: 4,4′-bipyridine). The presence of the imidazole groups, however, results in greater interactions with CO₂ (maximum ΔH_iso ～ 32 kJ mol^-1). Selectivity for CO₂ over CH₄ (10/90) based on the Ideal Adsorbed Solution Theory (IAST) reaches 8.4, which is a 10% and 75% increase when compared against Cu₂(pzdc)₂(bpy) and Cu₂(pzdc)₂(bpe) (bpe: 1,2-di(4-pyridyl)-ethylene).

Full text of DOI:10.1021/acs.cgd.7b01616

Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE
Article
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2
Synthesis and Characterization of a Cu(pzdc)(bix) [pzdc:
,3-pyrazinedicarboxylate; bix: 1,3-bis(imidazol-1-
yl)benzene] Porous Coordination Pillared-Layer Network
2
Rodinson R. Arrieta-Perez, Jose N Primera-Pedrozo, Jason Exley, and Arturo J. Hernandez-Maldonado
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01616 • Publication Date (Web): 02 Feb 2018
Downloaded from http://pubs.acs.org on February 3, 2018
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Synthesis and Characterization of a Cu (pzdc) (bix) [pzdc: 2,3ꢀpyrazinedicarboxylate; bix:
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,3ꢀbis(imidazolꢀ1ꢀyl)benzene] Porous Coordination PillaredꢀLayer Network
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a
a
b
Rodinson R. ArrietaꢀPérez, José N. PrimeraꢀPedrozo, Jason Exley
a
and Arturo J. HernándezꢀMaldonado *
a
Department of Chemical Engineering, University of Puerto RicoꢀMayagüez Campus
Mayagüez, PR 00681ꢀ9000
b
Micromeritics Instrument Corporation, Norcross, GA 30093
*
To whom correspondence should be addressed:
Phone: 787ꢀ832ꢀ4040 x3748; Fax: 787ꢀ834ꢀ3655; Eꢀmail: arturoj.hernandez@upr.edu
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Abstract
A porous coordination polymer (PCP) was prepared using pyrazineꢀ2,3ꢀdicarboxylate (pzdc) and
the pillar ligand 1,3ꢀbis(imidazolꢀ1ꢀyl)benzene) (bix) in an attempt to improve on the thermal
stability and adsorption properties of Cu (pzdc) (L) (L = pillar ligand; typically diꢀpyridylꢀ
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based). Asꢀsynthesized Cu (pzdc) (bix) showcases a highly crystalline structure (Monoclinic,
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P2 /c, a = 11.836 Å, b = 13.005 Å, c = 11.389 Å, β = 107.64˚) that exhibits characteristic
1
diffraction peaks also found in the diꢀpyridylꢀbased PCP counterparts. Thermal analysis based on
results from thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC) and in
situ high temperature Xꢀray diffraction (XRD) suggests both framework contraction and
expansion during the elimination of physisorbed and coordinated water, phase transition, and
prior to the eventual destruction of the structure, which takes place at ca. 556 K. The thermal
stability limit is an improvement over those displayed by other Cu (pzdc) (L) materials by an
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2
average margin of ca. 55 K. Comparison of the adsorption of CO vs. N both at cryogenic
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temperatures suggests a concomitant structural expansion in the presence of the former. This
process appears to take place even at ambient temperatures and CO pressures up to 50 atm; the
2
isotherm profile showcases a hysteretic pathway and full adsorption saturation is not apparent.
Furthermore, evaluation of CO adsorption at different equilibration time intervals indicate that
2
the structure relaxation time scale is larger than that of the adsorption kinetics; similar
observations have been made before for Cu (pzdc) (bpy) (bpy: 4,4’ꢀbipyridine). The presence of
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2
the imidazole groups, however, results in greater interactions with CO (maximum ꢁH ~ 32 kJ
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iso
ꢀ1
mol ). Selectivity for CO over CH (10/90) based on the Ideal Adsorbed Solution Theory
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4
(
IAST) reaches 8.4, which is a 10 and 75% increase when compared against Cu (pzdc) (bpy) and
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Cu (pzdc) (bpe) (bpe: 1,2ꢀdi(4ꢀpyridyl)ꢀethylene).
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1
. Introduction
Metal Organic Frameworks (MOFs) have become increasingly important due to exceptional
chemical and textural properties, making them great candidates in some fields of science such as
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gas storage and delivery, separation technologies and catalysis, luminescence, biomimetic, drug
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delivery, chemical sensors.
These materials include porous coordination polymers (PCPs),
formed by combination of a thermodynamically stable layer [i.e., Cu(pzdc)] (pzdc: pyrazineꢀ2,3ꢀ
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dicarboxylate) and a choice of a wide range of pyridylꢀtype pillar ligands.
Such assembly is
simple yet successful in the tailoring of both the size and chemical environment of the
framework pores and channels.
PCPs that contain copper nodes (Cu (pzdc) (L); i.e., L = diꢀpyridylꢀbased ligand), sometimes
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known as CPLꢀn, were discovered by Kitagawa and coꢀworkers. These manifoldꢀlike
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frameworks consist of 3ꢀD structures that exhibit a 1ꢀD pore channel system.
CPLꢀn
materials have also been extensively studied by HernandezꢀMaldonado and coꢀworkers to
2
7ꢀ32
establish relationships between chemical, structural, textural, and adsorption properties.
An
interesting aspect of Cu (pzdc) (L) materials is the capability for tailoring the accessible surface
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2
for the uptake of light gases such as CO . Functionalities arising from the pillar ligands provide a
2
range of interactions with adsorbates; for instance, the presence of the carbonyl (C=O) group in
the ligand Nꢀ(4ꢀpyridylꢀisonicotinamide (pia) appears to enhance interactions between CO and
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Cu (pzdc) (pia) at low loadings in comparison to the ligands 4,4ꢀazopyridine (apy) or 1,2ꢀdiꢀ(4ꢀ
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2
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pyridil)ꢀethylene (bpe).
Cu (pzdc) (L) materials also display concomitant structural changes that arise in upon the
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inclusion of guest molecules or simply by changes in pressure and temperature. For instance,
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Cu (pzdc) (bpy) [bpy: 4,4’ꢀbipyridine] undergoes pore contraction upon the adsorption of guest
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benzene and water. Cu (pzdc) (bpy) also undergoes substantial structural changes upon an
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increase in temperature as evidenced by in situ highꢀtemperature Xꢀray powder diffraction
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(
XRD) and C crossꢀpolarization magic angle spinning nuclear magnetic resonance.
The
results suggest long and localꢀrange changes in the channels of the structure due to an apparent
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ꢀ1
local framework distortion giving as consequence a surface area reduction from 633 m g at
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ꢀ1
3
73 K to 123 m g at 423 K. Similar results have been reported for other Cu (pzdc) (L)
2 2
materials (i.e., L = 4,4’ꢀazopyridine (apy); 1,2ꢀdi(4ꢀpyridyl)ꢀethylene (bpe); Nꢀ(4ꢀpyridylꢀ
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isonicotinamide (pia); or 1,2ꢀdi(4ꢀpyridyl)ꢀglycol (dpyg)). On the other hand, Cu (pzdc) (bpy)
2
2
also evidenced a longꢀrange structural changes upon CO uptake at moderate pressure and
2
31
ambient temperature. According to a set of in situ XRD adsorption tests, the structural changes
are apparently linked to a combination of local changes induced by adsorbateꢀadsorbent
interactions and external forces being exerted onto the crystals.
As seen above, the pillar ligands usually employed for the syntheses of Cu (pzdc) (L) are usually
2
2
based on a family of 2ꢀconnecting Nꢀdonor diꢀpyridylꢀbased neutral organic molecules.
Differences between these ligands lie in the characteristic functional group that resides between
the pyridine rings, also known as a spacer group and that gives unique chemical properties to the
moiety and, as a consequence, to the framework. In general, these Nꢀdonor pyridylꢀtype ligands
are also considered rigid, imposing this limits on both flexibility and stability in Cu (pzdc) (L)
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2
frameworks. Here we present the synthesis and characterization of a Cu (pzdc) (L) based on the
2
2
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3ꢀ35
diꢀimidazoleꢀbased ligand L = 1,3ꢀbis(imidazolꢀ1ꢀyl)benzene or bix
both flexibility and stability, particularly in light of potential adsorption based applications for
these materials. The Cu (pzdc) (bix) crystal structure was elucidated using a combination of high
in an attempt to increase
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resolution Xꢀray diffraction (XRD) and Rietveld refinement techniques. The material was also
characterized via thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC),
in situ highꢀtemperature XRD, and N and CO adsorption at cryogenic temperatures. In
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addition, we present results of CO adsorption isotherms at ambient temperature, moderate/high
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gas pressures, and at different equilibration time intervals (t ), which allows probing the
e
structural dynamics of the framework material upon interactions with the adsorbate. The
adsorption selectivity capabilities of Cu (pzdc) (bix) were estimated via isosteric heats of
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adsorption and adsorption of single component CH and N also at ambient temperature.
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. Experimental Section
2.1 Materials
Reagents used for the synthesis of 1,3ꢀbis(imidazoleꢀ1ꢀyl)benzene (bix) and Cu (pzdc) (bix)
2
2
were potassium carbonate (K CO , 99% purity), 1,3ꢀdiazaꢀ2,4ꢀcyclopentadiene (imidazole, 99%
2
3
purity), α,α’ꢀdichloroꢀpꢀxylene (C H (CH Cl) , 98% purity), 2,3ꢀpyrazinedicarboxylic acid
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2
2
(H pzdc, 97% purity) and copper(II) perchlorate hexahydrate (Cu(ClO )·6H O, 98% purity).
2
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Methanol (99.8% purity) was used during the syntheses or for washing samples upon recovery;
denatured ethanol (95% purity) was used during synthesis of Cu (pzdc) (bix). All reagents were
2
2
obtained from SigmaꢀAldrich and used as received. Ultraꢀhigh purity grade CO , N and CH
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2
gases (Praxair, Inc.) were used during the acquisition of adsorption data. High purity grade He
gas (Praxair, Inc.) was used as a carrier gas during thermogravimetric analysis and in situ highꢀ
temperature XRD tests. All gases were preꢀtreated with zeolitic and hydrocarbon traps beds to
remove any traces of impurities and humidity.
2
.2 Synthesis of 1,3ꢀbis(imidazoleꢀ1ꢀyl)benzene
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,3ꢀbis(imidazoleꢀ1ꢀyl)benzene (bix) was synthesized as prescribed in a recipe presented
3
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elsewhere. A solution containing 46.4 mmol of imidazole (3.16 g) and 4.46 mmol of α, α’ꢀ
dichloroꢀpꢀxylene (0.78 g) in 50 mL of methanol was heated under reflux at 338 K. After 18 h,
methanol was removed via evaporation; leaving a yellow remainder which was later dissolved in
a solution containing 6.13 g of K CO in 100 mL of water. The resulting mixture was held
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overnight at ambient pressure and temperature. Upon filtration, the resulting product was a white
dehydrated crystalline solid (i.e., bix).
2
.3 Synthesis of Cu (pzdc) (bix)
2 2
Cu (pzdc) (bix) was synthesized at ambient pressure and temperature adapting procedures
2
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7
previously reported for the preparation of CPLꢀn type materials. One mmol of H pzdc (0.1681
2
g) and 0.5 mmol of bix (0.1006 g) were dissolved in 100 mL of a solution containing 1:1 NaOH
0
.04M and ethanol. The resulting solution mixture was dropwise added to 100 mL of a aqueous
solution containing 1 mmol of Cu(ClO )·6H O (0.37 g). The final solution was stirred for 24 h
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and the precipitate was then vacuum filtered and washed with equal amounts of methanol and
distilled/deionized water. The resulting blue solid crystals were dried overnight at 363 K to
remove excess solvents.
2
.4 Thermal Gravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), and
Xꢀray Diffraction (XRD)
All TGA tests were carried out using a TAꢀQ500 unit. A sample of Cu (pzdc) (bix) (~10 mg)
2
2
was placed onto a platinum holder suspended from the instrument weight balance and heated
ꢀ
1
ꢀ1
from ambient temperature to 1,173 K at 1 K min under a constant flow helium (60 mL min ).
TGA was realized to verify its thermal decomposition temperature profile, and to identify the
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optimal degassing temperature for activation of the material prior to adsorption tests. DSC tests
were performed in the Galbraith Laboratories, Inc. facilities in Knoxville, TN, USA. A Mettlerꢀ
Toledo DSC instrument was employed for those tests; a heating rate of 1 K minꢀ1 was employed
during the tests. Further details about the standard DSC procedures can be found elsewhere.
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XRD patterns were gathered using a Rigaku ULTIMA III Xꢀray diffraction unit. The
diffractometer is equipped with cross beam optics and a CuK target operating at 40 kV and 44
α
mA. XRD was employed for the following purposes: (1) verify the structure of the asꢀ
synthesized pillar ligand bix and (2) collect a highꢀresolution diffraction data for asꢀsynthesized
Cu (pzdc) (bix) for structure refinement. All XRD tests were performed with a scanning speed of
2
2
ꢀ
1
0
.04° min and a step size of 0.005° in the 2 ꢀ 60° 2θ range. The unit cell parameters of
Cu (pzdc) (bix) were determined using the CRYSFIRE program and the DICVOL91 indexing
2
2
37
routine. Afterwards, a set of refined unit cell parameters were obtained using a structureꢀless Le
3
8
Bail fitting method. A combination of two isostructural models based on Cu (pzdc) (dpyg) (or
2
2
CPLꢀ7) and Cu (pzdc) (bpp) were employed as a starting point for the elucidation of the fine
2
2
19, 39
structure of Cu (pzdc) (bix).
The resulting model was then refined via the Rietveld technique
2
2
using GSAS and the EXPGUI graphical interface, with a pseudoꢀVoigt peak profile and a
4
0, 41
background function of 22ꢀterm shifted Chebyshev polynomials.
The unit cell parameters
were refined during the last stages of the overall refinement procedure; hydrogen atoms were
directly added to the refined, final structure.
In situ highꢀtemperature XRD measurements were obtained using a Rigaku ReactorX attachment
fitted with a continuous beryllium window. This allows for complete enclosure and heating of
the sample under an inert atmosphere and still permits the passages of Xꢀray. Upon calibration of
the goniometer in a BraggꢀBrentano focusing mode, the ReactorX module was attached to the
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ꢀ1
system. Diffraction analyses were performed using a helium flow of 60 mL min for a fixed 2θ
diffraction angle range, voltage, current, and step size. Powders of asꢀsynthesized
Cu (pzdc) (bix) were loaded onto a thin black quartz sample plate and heated from 303 K to 903
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0
1
2
3
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7
8
9
0
1
2
3
4
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7
8
9
0
K by means of infrared heating and using a temperature ramp of 1 K min . The diffraction
ꢀ1
scanning speed was raised to 3.5° min to avoid large temperature differences between the
starting and ending points of each diffraction pattern. Temperature differences were no greater of
1
5 K. In situ ramp and hold XRD measurements were performed to further verify the structural
stability of Cu (pzdc) (bix) at a degassing or activation temperature prescribed by the TGA tests.
2
2
ꢀ1
The asꢀsynthesized sample was heated to that activation temperature at 1 K min and
held/soaked at that condition for 10 h. Xꢀray diffraction patterns were collected every hour at a
ꢀ1
scanning speed of 2° min in the 2 ꢀ 60° 2θ range.
2
.5 Equilibrium Gas Adsorption Measurements, and Estimation of Textural Properties and
Isosteric Heat of Adsorption
Lowꢀpressure equilibrium adsorption data were obtained using a Micromeritics ASAP 2020
volumetric adsorption unit fitted with turbo molecular drag pumps. Activation of the sample was
carried out prior to each adsorption test using the ASAP 2020 thermal vacuumꢀdegassing stage
or module. A sample of asꢀsynthesized Cu (pzdc) (bix) was loaded (~100 mg) into a quartzꢀ
2
2
ꢀ1
made tube fitted with an isolation valve. The sample was then evacuated at a rate of 50 mmHg s
ꢀ1
to reach a degassing pressure of ca. 10 ꢂmHg, heated at a rate of 10 K min in situ to 373 K
(
activation temperature), and held at that condition for 10 hours. Upon completion of the
degassing stage, the sample was allowed to reach ambient temperature under vacuum and then
backfilled with helium (inert gas) to allow its transfer to the analysis port under isolation. Prior to
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each adsorption test, the samples were evacuated using vacuum provided by a turbo molecular
drag pump to effectively remove all the inert gas.
Cu (pzdc) (bix) textural properties were estimated by means of nitrogen and carbon dioxide
2
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
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0
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1
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9
0
equilibrium adsorption data gathered at 77 and 194.5 K, respectively. Surface area was estimated
using the BET isotherm model. Pore volume was estimated with the Modified DubininꢀAstakhov
42, 43
(MDA) isotherm model
and assuming a liquidꢀlike adsorbed phase at pore saturation (i.e.,
molecules tightly packed). The MDA model is given by the following expression:
(1)
where q is the equilibrium adsorption loading amount, q is the adsorption loading amount at
o
saturation (complete pore filling), R is the universal gas constant, T is temperature (K), β is the
affinity coefficient of the adsorbate, E is the characteristic energy of adsorption, P/P is the
o
relative pressure, n is the heterogeneity coefficient, α is a fitting parameter and K is Henry’s law
constant.
Carbon dioxide isosteric heat of adsorption ∆Hads was estimated using single component
equilibrium adsorption data at different temperatures (273 ꢀ 323 K) and evaluated at constant
surface loading. The calculations were made using the classic ClausiusꢀClapeyron equation:
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1
1
1
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1
1
1
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2
2
2
2
2
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3
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d ln P
−
ꢀHads = R
(2)
d 1 T
(
)
at constant
loading
Moderate high pressure CO single component gas equilibrium adsorption and desorption
2
0
1
2
3
4
5
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7
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9
0
1
2
3
4
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0
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0
1
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3
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9
0
isotherms were obtained using a Micromeritics ASAP 2050 extended pressure volumetric
adsorption instrument (up to 7 atm). In an attempt to assess the dynamics behind the
concomitant CO adsorption/ Cu (pzdc) (bix) framework relaxation/expansion process, each set
2
2
2
of isotherms was gathered using equilibration time intervals (t ) of 15, 60 and 120 seconds. All
e
the adsorbent samples were activated using the procedure that was employed for ASAP 2020
adsorption tests. As will be discussed later, the desorption isotherms legs at each t were fitted
e
with the MDA model (Eqn. 1).
Highꢀpressure (up to 50 atm) CO single component adsorption isotherms were measured using
2
the Particulate Systems HPVAꢀII 100 at Micromeritics Instrument Corporation (Georgia, US), a
static volumetric system, connected to a high vacuum source. Between 250ꢀ450 mg of each
material was used for these tests. Samples were heated to 373 K under vacuum for a minimum
of four hours preceding analyses. Prior to each analysis on the HPVAꢀII 100 system a blank run
was carried out with the same analysis gas, temperature, and pressure steps as the sample
experiment. Blank run data were extracted from the sample analysis data to correct for small
errors resulting from the increasing density of CO at elevated pressures. A wait time of at least
2
10 minutes was implemented after equilibrium at each adsorption/desorption pressure step before
continuing the experiment. This equilibration time was the maximum allowable due to
instrument usage logistics. Sample temperature was maintained at 298 K using a recirculating
water bath.
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3
. Results and Discussion
.1 Crystal Structure Determination and Thermal Behavior
3
Figure 1 shows that the asꢀsynthesized Cu (pzdc) (bix) possesses a highly crystalline structure
2
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
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3
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0
1
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9
0
that exhibits characteristic peaks found also in CPLꢀlike solid frameworks, namely those that
appear at 7.7, 11.3 and 13.32˚ in 2θ. These correspond to crystalline planes (100), (110) and
(011) and are directly related to the formation of 2D Cu(pzdc) layers. In other words, these
reflections are closely associated to geometry of the material pore gallery. Figure 1 also shows
what appears to be slight peak broadening in some reflections and this could be related to local
distortions that result in the coordination between Nꢀdonor imidazoleꢀbased ligands and first row
4
4
transition metals, similar to what is observed in some 2D coordination polymers.
In general, the Cu (pzdc) (bix) structure consists of a 3D framework with 1D channels along c
2
2
direction, as shown in Figure 2, with pillar ligand molecules interconnecting pzdc layers along a.
The distance between them is c.a. 10.050 Å when measured along b. From a chemical ambient
perspective, the coordination complex around the copper metal nodes is exactly like the one
found in CPLꢀn porous coordination polymers. It consists of a distorted squareꢀpyramidal
2
+
configuration due to the presence of a JahnꢀTeller distortion around Cu . The five complexing
agents linked to the coordination center are delineated by three pzdc units and a half a bix pillar.
The Cu (pzdc) (bix) crystalline structure was refined by a combination of highꢀresolution
2
2
diffraction and Rietveld refinement techniques, as shown in Figure 1 and Table 1. A full list of
atomic parameters, interatomic distances, and angles are presented in the Supporting Information
file (Tables S1 and S2).
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2
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5
5
6
Attempts to explain the structural response of CPLꢀn variants to external stimuli such as pressure
2
6, 29, 31, 45
or temperature have been made in the past.
Motivated by those findings, we subjected
Cu (pzdc) (bix) to TGA, DSC, and in situ highꢀtemperature XRD tests (Figure 3).
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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9
0
1
2
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9
0
1
2
3
4
5
6
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8
9
0
Cu (pzdc) (bix) has a thermal stability of up to about 510 K and the profile decomposition shape
2
2
is very similar to those observed in other CPLꢀlike framework material. The first weight loss
observed corresponds to the release of water that is loosely bound or contained within the pore
gallery structures (8.46 wt. %). This amount corresponds to two guest water molecules located
within the asymmetric unit cell. The second weight loss corresponds to organic pyrazine groups
and a small fraction of bix (total of 49.24 wt.%) while the third weight loss corresponds to the
complete elimination of the rest of the pillar ligand bix (22.14 wt. %). The remnant corresponds
to copper (about 20 wt. %). These results commensurate with those based on the chemical
composition observed in the crystalꢀrefined structure: 8.20, 47.30, 28.13 and 16.13 wt.%,
respectively.
Figure 3 also shows DSC and stacks of in situ highꢀtemperature Xꢀray diffraction patterns. Nonꢀ
uniform shifting and broadening of diffraction peaks were observed upon an increase in
temperature. For instance, peak reflections corresponding to (100) and (110) (associated with the
pore galleries of the PCP) experienced changes associated with diffraction angle and peak
broadening. The specific diffraction pattern changes can be related to expansion in the pore
gallery and structural breathing due to release of the physisorbed water. DSC measurements
meanwhile showed two endothermic peaks centered at 368 and 428 K, suggesting that removal
of water is taking place stepwise. The former peak is related to bulk water (W1) release, as
readily observed in the TGA profile, while the latter is attributed to water molecules strongly
adsorbed or coordinated to the PCP structure (W2). From Figure 4, the W1 water molecules are
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plausibly located near the benzene ring of the bix ligand while W2 water molecules are
coordinated to imidazole rings. It is known that the dipole moment for benzene and for imidazole
rings are is 0 and 3.61 D, respectively, suggesting that there is a stronger interaction between W2
molecules and the PCP framework. This correlates well with (110) peak shifting to lower
diffraction angles being more pronounced at higher temperatures (> 400 K). These results also
correlate well with data gathered from TGA profile, which indicated that the material continues
to lose weight upon treatment at temperatures greater of 373 K (Figure 3).
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
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4
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0
1
2
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4
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9
0
1
2
3
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9
0
1
2
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9
0
1
2
3
4
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9
0
1
2
3
4
5
6
7
8
9
0
It can be seen from the in situ XRD data that peak broadening for both (100) and (110) planes
increases as temperature increases. This suggests that, in principle, a structural defect within the
framework might be taking place, which in turn could induce inhomogeneous strain within the
crystal. This could also lead to a pore that is both contracted and expanded. It should be noted
that a nonꢀidentifiable phase transition was observed in the DSC profile at temperatures below
4
50 K, indicating maybe that a microporousꢀtoꢀmesoporous material transition is primarily
associated to structural defects.
The DSC profile also shows a pronounced exothermic peak centered at 467 K and another one
centered at 465 K. Pyrazine ligands and bix pillars both decompose at temperatures near to the
temperature of Cu (pzdc) (bix) destruction, which is ca. 556 K. Therefore, the endothermic peak
2
2
at 465 K and the exothermic peak at 467 K are both associated to decomposition and phase
transition of the framework, respectively. The phase transition is linked to a crystal symmetry
change evidenced by the shifts to higher and lower diffraction angles for (100) and (110) planes,
respectively (Figure 3). It should be noted that in the process of decomposition, Cu (pzdc) (bix)
2
2
could plausibly reach a nonꢀreversible new phase before undergoing a complete decomposition.
This contrasts somewhat with studies that were made for Cu (pzdc) (bpy) (bpy: 4,4’ꢀbipyridine)
2
2
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1
1
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2
2
2
2
2
2
2
2
2
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46
or CPLꢀ2, which found that the thermally induced structural changes were mainly related to a
shift of (100) to higher diffraction angles. Furthermore, the decomposition temperature limit of
Cu (pzdc) (bix) (556 K) is higher than that of Cu (pzdc) (bpy) (523 K), Cu (pzdc) (apy) (510
2
2
2
2
2
2
0
1
2
3
4
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9
0
1
2
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9
0
1
2
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0
1
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0
1
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4
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9
0
K), Cu (pzdc) (bpe) (490 K), Cu (pzdc) (pia) (520 K) or Cu (pzdc) (dpyg) (453 K), probably
2
2
2
2
2
2
due to the imidazole groups. Bix ligands have been used in nonꢀpyrazine based 2D coordination
polymers because of variable coordination positions conformations, which also results in
3
6
improved thermal stability. The imidazole based ligand donates a lone pair of electrons to a
metal node while enhancing the basic nature of the ligand itself. This in turn results in a much
stronger metalꢀnitrogen coordination bond. Imidazole groups are more basic than pyridine
groups, so it is expected to see a thermal stability enhancement in PCP materials when
substituting the latter with the former.
3
.2 Estimation of Textural Properties from Gas Adsorption at Cryogenic Temperatures
Figure 5 shows the N and CO equilibrium adsorption isotherms gathered for Cu (pzdc) (bix) at
2
2
2
2
77 and 194.5 K, respectively, after activating the sample at 373 K under vacuum for several
hours. The adsorption/desorption profiles follow a Type I isotherm as suggested by International
of Pure and Applied Chemistry (IUPAC) classifications) and this is characteristic of microporous
2
ꢀ1
solids. Using the N adsorption data, we estimated a BET surface area of ca. 100 m g and a
2
3
ꢀ1
pore volume of 0.09 cm g . While the adsorption saturation loading for N can be readily
2
ꢀ1
obtained from in Figure 5 (i.e., 1.215 mmol g ), for CO the isotherm profile indicates that
2
saturation was not achieved within the prescribed relative pressure range. Still, a theoretical
value can be calculated under the assumption that both adsorbates would yield similar surface
areas (i.e., kinetic diameters are similar) and based on the following relationships:
1
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2
S
α
q
σ
S
α
q
CO ,sat CO2
2
σ
(3)
(4)
N2
N ,sat N2
CO2
2
2


q
σ
CO ,sat
N2
2






≈
q
σ
1
1
1
1
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1
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1
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1
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2
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2
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3
3
3
3
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4
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5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
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9
0
1
2
3
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9
0
N ,sat
2
CO2
where S is the surface estimated for adsorbate i, q the adsorption loading amount at saturation,
i
i,sat
and σ is the adsorbate kinetic diameter (σ = 3.65 Å and σCO2 = 3.2 Å). Assuming also that the
i
N2
adsorbent framework remains rigid during the adsorption, equation 4 yields a CO adsorption
2
ꢀ
1
loading at saturation of ca. 1.5 mmol g . This value contrasts considerably with what the
adsorption isotherm data suggest, even without extrapolation. A possible explanation to this is
that the crystal structure is perhaps undergoing a concomitant structural expansion during CO2
adsorption; this has been reported before for Cu (pzdc) (bpy) materials also at cryogenic
2
2
4
5
temperatures. The CO isotherm as shown in Figure 5B also suggests a dynamical adsorption
2
behavior that is represented by a inflections points at low relative pressure and changes in slope
that are absent in the case of N2.
3.3 CO Adsorption at Ambient Temperature and Equilibration Time Intervals (t )
2
e
High pressure CO adsorption and desorption data gathered for asꢀsynthesized Cu (pzdc) (bix) at
2
2
2
2
98 K (Figure 6) present a hysteretic pathway probably associated to a pore expansion and
contraction. These data were captured upon an equilibration time of 10 min for each point due to
instrument usage logistics. However, our group has already shown that equilibration time
intervals (t ) play an important role in the determination of actual structural relaxation upon
e
3
1, 32
changes induced by CO adsorption;
the rate of structural expansion is smaller than the rate
2
of adsorption and therefore the equilibrium adsorption amounts will increase simultaneous an
1
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increase t . In an attempt to verify such behavior Cu (pzdc) (bix), we gathered CO adsorption
e
2
2
2
isotherm data at ambient temperature and pressures up to 7 atm using different t values. The
e
apparent equilibrium state for each adsorption or desorption datum was corroborated by a nearꢀ
0
1
2
3
4
5
6
7
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9
0
1
2
3
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0
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0
zero slope condition present in at least 11 consequent pressure transient points (e.g., for t = 100
e
sec, a total of 1100 sec worth of data per section per pressure step).
Figure 7 shows that, once again, the hysteretic loop persists even at large t values. The loadings
e
observed at 7 atm increased considerably between a t of 15 and 60 sec, but appeared to level of
e
afterwards. Furthermore, hysteresis gap increased with t , which suggests dynamics between
e
framework relaxation time, adsorbateꢀadsorbent interactions and external force exerted onto the
Cu (pzdc) (bix) crystals.
2
2
In an attempt to quantify this expansion, we estimated textural properties by fitting the MDA
model to the desorption profiles shown in Figure 6. The results are presented in Table 2. The
results show that an increase in t results in an increase in micropore volume and this suggests
e
the Cu (pzdc) (bix) framework is expanding. The C parameter also increases with t , not
2
2
e
surprisingly since the former is directly proportional to the pore width of the framework (i.e.,
39
surface energy potential decreases as C increases.
3
.4 Isosteric Heat of Adsorption of Carbon Dioxide and Selectivity
The imidazoleꢀbased ligand bix, unlike the diꢀpyridylꢀbased ligand traditionally used in CPLꢀn
materials, will produce specific adsorbateꢀadsorbent interactions that are significant. Imidazole
rings have a dipole moment, or polarity, larger than that of pyridyl rings (3.6 vs. 2.2 D,
respectively). Such difference will result in better, stronger interactions with adsorbates
possessing large dipole or quadrupole moments; the location of the imidazole rings within the
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Cu (pzdc) (bix) pore galleries make them readily accessible to small molecules such as CO ,
2
2
2
which has a quadrupole moment that is three times as large as that of N and an order of
2
magnitude larger than that of O . Possible evidence of the interaction of CO with the imidazole
2
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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3
3
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3
3
3
4
4
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6
0
1
2
3
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9
0
1
2
3
4
5
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9
0
1
2
3
4
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6
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8
9
0
1
2
3
4
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9
0
1
2
3
4
5
6
7
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9
0
rings comes in the form of the MDA n parameter (heterogeneity factor) values presented in
Table 2.
Figure 8 makes comparison of low pressure CO equilibrium adsorption data and isosteric heat
2
3
9
profiles in Cu (pzdc) (bix), Cu (pzdc) (bpp) and Cu (pzdc) (dpyg). XRD patterns for these
2
2
2
2
2
2
PCPs are presented in the Supplementary Information file (Figure S1). In general, all the
ꢀ1
apparent interaction energies fall within the physical adsorption range (ꢁHiso <40 kJ mol ).
However, it is evident from the data that the interactions between CO and Cu (pzdc) (bix) are
2
2
2
significant in comparison. Furthermore, at 0.85 atm CO , the uptake in Cu (pzdc) (bix) is three
2
2
2
times that the one seen in Cu (pzdc) (bpp) and an order of magnitude larger than the seen in
2
2
Cu (pzdc) (dpyg). Given that these adsorbent materials are isostructural to one another, it is
2
2
evident that the imidazole based ligands play a significant role in the adsorption of CO . The
2
ligand along with the copper node is a significant contributor to the overall surface potential or
field.
To compare the CO adsorption capacity over N and CH , we also gathered single component
2
2
4
adsorption isotherms for those gases. The results are collected in Figure 9; selectivity toward
CO is quite evident, most certainly because of specific interactions between its strong
2
permanent quadrupole moment, and the imidazole based ligands and metal node (i.e., Cu). It
should be noted that the CH adsorbed amounts are larger compared to those obtained for N and
4
2
this plausibly attributed to the induced octopole moment that CH exhibits when in contact with
4
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47ꢀ50
the electric field generated by the adsorbent surface generated by metals
and its slightly large
5
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polarizability.
CO /CH selectivity profiles were generated using the adsorption data shown in Figure 9 and the
2
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0
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Ideal Adsorbed Solution Theory (IAST). This theory provides a framework to elucidate the
suitability of an adsorbent to achieve a particular separation under equilibrium conditions and
involves the assumption that the mixing of the adsorbed phases is ideal. The selectivity is
calculated using the following expression:
x
y
x
CH4
(
(
CO2
)
)
SIAST
=
(7)
y
CO2
CH4
where x and y are the molar fraction of CO or CH in the adsorbed and gas phase, respectively.
i
i
2
4
Details on the equations that govern IAST as well as the calculations methods is available
4
3
elsewhere. Figure 10 shows IAST profiles for Cu (pzdc) (bix) and different gas phase
2
2
CO /CH compositions (i.e., 10/90, 20/80, and 50/50), representing bulk level separation case
2
4
scenarios. IAST phase diagrams are shown in the Supplementary Information file (Figure S2).
For the case of 10/90 gas mixtures, Cu (pzdc) (bix) showcases a maximum IAST selectivity of
2
2
8
.4 at 298 K and 1 atm, which is a 10 and 75% increase in comparison with selectivity values
reported for Cu (pzdc) (bpy) and Cu (pzdc) (bpe), respectively, at similar temperature, pressure
2
2
2
2
3
2
and gas composition.
Conclusions
A porous coordination pillaredꢀlayer network based on bis(imidazole) Nꢀdonor pillar ligand was
synthesized and characterized. A Rietveld refinement confirmed a material structurally similar to
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those found in the Cu (pzdc) (L) family. Results based on thermogravimetric analysis and in situ
2
2
high temperature XRD reported that the material composition and structure is stable up to ca.
556 K, an increase in stability of an average of 55K compared to previous Cu (pzdc) (L)
2
2
1
1
1
1
1
1
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0
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materials. In situ tests also evidenced a nonꢀtypical behavior of the structure using temperature as
external stimuli in which the structure experimented a pore gallery expansion and distortion upon
the release of physisorbed and coordinated water. Adsorption of N at 77 K and CO at 194.5 K
2
2
confirmed the porous nature of the material while suggesting a concomitant pore expansion upon
the uptake of the latter. The expansion was further demonstrated with results of CO adsorption
2
at 298 K and at different equilibration time intervals, t . The presence of a hysteretic adsorptionꢀ
e
desorption pathway moderate pressure and still present a higher pressure (50 atm) was related to
a structural expansion previously reported for Cu (pzdc) (L) materials. Both isosteric heats of
2
2
adsorption as well as IAST based selectivity calculations suggest that the bix imidazole rings are
responsible for additional CO interactions within the Cu (pzdc) (bix) framework, in contrast
2
2
2
with diꢀpyridylꢀbased Cu (pzdc) (L) materials.
2
2
Acknowledgements
Support was provided by the National Aeronautics and Space Administration (NASA) Award
NNX13AD38A. We also wish to acknowledge support from the National Science Foundation
(NSF) sponsored Puerto Rico Institute for Functional Materials Postdoctoral Fellowships
Program under the Grant No. EPSꢀ1002410.
Supporting Information. Xꢀray crystallographic information file (CIF) for asꢀprepared
Cu (pzdc) (bix), tables with the corresponding refined atomic parameters, interatomic distances
2
2
and angles, and IAST adsorption isotherm profiles. Crystallographic information data are also
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available from the Cambridge Crystallographic Data Centre (CCDC) upon request
(http://www.ccdc.cam.ac.uk CCDC deposition number 1484122).
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10.
42) Kapoor, A.; Ritter, J. A.; Yang, R. T. On the DubininꢀRadushkevich Equation for
Adsorption in Microporous Solids in the Henry's Law Region. Langmuir 1989, 5, 1118.
43) Do, D. D. Adsorption Analysis: Equilibria and Kinetics; 1998.
(
(
44) Carballo, R.; Castiñeiras, A.; Covelo, B.; GarcıaꢀMartınez, E.; Niclos, J.; Vazquezꢀ
Lopez, E. M. Solid State Coordination Chemistry of Mononuclear MixedꢀLigand
Complexes of Ni(II), Cu(II) and Zn(II) with aꢀHydroxycarboxylic Acids and Imidazole.
Polyhedron 2004, 23, 1505.
(
45) Matsuda, R.; Kitaura, R.; Kubota, Y.; Kobayashi, T. C.; Takata, M.; Kitagawa, S.
Incommensurate guest adsorption in bellowsꢀshaped oneꢀdimensional channels of porous
coordination polymers. Microporous Mesoporous Mater. 2010, 129, 296.
(
46) GarcíaꢀRicard, O. J.; Fu, R.; HernandezꢀMaldonado, A. J. Thermally Induced Changes in
a Porous Coordination Polymer {Cu (pyrazineꢀ2,3ꢀdicarboxylate) (4,4’ꢀbipyridine)}
2
2
13
Studied via in Situ Xꢀray Diffraction and C CrossꢀPolarization Magic Angle Spinning
Nuclear Magnetic Resonance Spectroscopy. J. Phys. Chem. C 2011, 115, 3595.
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(
47) Pillai, R. S.; Peter, S. A.; Jasra, R. V. Adsorption of Carbon Dioxide, Methane, Nitrogen,
Oxygen and Argon in NaETSꢀ4. Microporous Mesoporous Mater. 2008, 113, 268.
(48) Snook, I. K.; Spurling, T. H. Intermolecular Interactions in Methane, Carbon
Tetrafluoride and Sulphur Hexafluoride. Journal of the Chemical Society, Faraday
Transactions 2 1972, 68, 1359.
1
1
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0
(
49) Sinai, J. J. Octopole Moment of Methane. J. Chem. Phys. 1964, 40, 3596.
50) Allen, M. P. A Molecular Dynamics Simulation Study of Octopoles in the Field of a
Planar Surface. Mol. Phys. 1984, 52, 717.
(
(51) RiveraꢀRamos, M. E.; HernandezꢀMaldonado, A. J. Adsorption of N and CH by Ionꢀ
2
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Exchanged Silicoaluminophosphate Nanoporous Sorbents: Interaction with Monovalent,
Divalent, and Trivalent Cations. Ind. Eng. Chem. 2007, 46, 4991.
(
52) Marathe, R. P.; Farooq, S.; Srinivasan, M. P. Modeling Gas Adsorption and Transport in
SmallꢀPore Titanium Silicates. Langmuir 2005, 21, 4532.
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Table 1. Data collection, crystallographic data and refinement parameters for Cu (pzdc) (bix)
2
2
Data collection
Phase
Xꢀray source
Wavelength (Å) 1.5418
CuK
ꢀ
Formula
C H CuN O
6
3
9
4
ꢀ1
Molecular Weight (g mol ) 380.78
ꢀ3
Step (º)
θ range (º)
0.005
ρ (g cm )
1.514
298
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
2 − 50
T (K)
Unit cell
Refinement
Crystal system
Space group
Monoclinic
Rp
0.0881
0.1150
0.0076
0.44208
P2 /c
wRp
Rexp
R(F )
1
a (Å)
b (Å)
c (Å)
β (˚)
11.836
13.005
11.389
107.64
1670.7
4
2
3
Volume (Å )
Z
Table 2: Modified DubininꢀAstakhov model parameters and pore volume data obtained from
CO desorption at 298 K.
2
Modified DubininꢀAstakhov (MDA) Parameters
Equilibration
V
mp
Time Interval
(s)
3
ꢀ1
qo
C
(ꢀ)
K
α
(ꢀ)
n
(ꢀ)
Std.
Deviation
(
cm g )
ꢀ1
ꢀ1
ꢀ1
a
(
mmol g )
(mmol g atm )
1
5
3.686
0.214
0.217
0.220
8.220
6498.976 1.723
4051.304 1.756
5374.623 1.183
± 0.0030
± 0.0095
± 0.0098
0.141
0.174
0.192
6
0
4.558
5.008
7.913
8.229
120
a
Standard deviation calculated based on residuals between the observed and calculated equilibrium loading amounts
for the complete pressure range.
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Figure Captions
Figure 1.
Observed and calculated powder Xꢀray diffraction patterns for Cu (pzdc) (bix).
2 2
The observed data are shown with markers points with the position of the Bragg
reflections and difference between calculated and observed intensities shown
below.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
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4
4
4
4
4
5
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 2.
Figure 3.
Structural views along [001] and [010], respectively. Hydrogen atoms and water
molecules are omitted for clarity.
Thermal gravimetric analysis and differential scanning calorimetry profiles, and
in situ high temperature diffraction patterns gathered for Cu (pzdc) (bix). All tests
2
2
ꢀ1
were performed using a dry helium atmosphere and a heating rate of 1 K min .
Figure 4.
Figure 5.
Approximate location of water molecules along the pore galleries of
Cu (pzdc) (bix).
2
2
(A) N and CO adsorption (filled markers) and desorption (empty marker)
2
2
isotherms gathered for Cu (pzdc) (bix) at 77 and 194.5 K, respectively; (B) Logꢀ
2
2
log scale plot for data shown in (A).
Figure 6.
Figure 7.
Figure 8.
High pressure CO adsorption (filled marker) and desorption (empty markers)
2
isotherms gathered for Cu (pzdc) (bix) at 298 K.
2
2
CO adsorption (filled markers) and desorption (empty markers) isotherms
2
gathered for Cu (pzdc) (bix) at 298 K and different equilibration time intervals, t .
2
2
e
(A) CO adsorption isotherms at 298 K and (B) CO isosteric heat of adsorption
2
2
(ꢁH ) profiles for Cu (pzdc) (bix), Cu (pzdc) (bpp) and Cu (pzdc) (dpyg). The
iso
2
2
2
2
2
2
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