Gas and liquid phase adsorption in isostructural Cu 3[biaryltricarboxylate]2 microporous coordination polymers

Article abstract of DOI:10.1039/c0cc03482g

N-Heteroarene substitution into biphenyl-based linkers enhances the uptake of electron-rich organosulfur molecules in a series of isostructural microporous coordination polymers.

Full text of DOI:10.1039/c0cc03482g

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COMMUNICATION
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Gas and liquid phase adsorption in isostructural
Cu3[biaryltricarboxylate]2 microporous coordination polymersw
Tae-Hong Park,a Katie A. Cychosz,a Antek G. Wong-Foy,a Anne Daillyb and
Adam J. Matzger*a
Received 25th August 2010, Accepted 16th November 2010
DOI: 10.1039/c0cc03482g
N-Heteroarene substitution into biphenyl-based linkers enhances
the uptake of electron-rich organosulfur molecules in a series of
isostructural microporous coordination polymers.
and trinuclear Cu(II) clusters,7 this highly porous material
has high H2 uptake and outstanding adsorption performance
for large organosulfur compounds of particular significance
in fuel desulfurization.4 N-Heteroarene containing biaryl-
tricarboxylate linkers (2–4) are related to 1 by the replacement
of C–H bonds with N. Although 4 led to a nonporous struc-
ture likely due to the presence of the pyridine-2,6-dicarboxylate
unit offering a competing coordination mode,8 three isostructural
MCPs were produced from the solvothermal reactions of 1–3
and Cu(NO3)2ꢀ2.5H2O. Their structures were confirmed by
X-ray crystallographyz and PXRD (Fig. 1), and these MCPs
exhibit BET surface areas of B3000 m2 gꢁ1 and very similar
pore diameters (ESIw). These isostructural MCPs constitute a
unique platform to explore, in isolation, the effect of linker
electronic nature upon guest adsorption.
Optimized interactions between host frameworks and guest
molecules by varying the structural and electronic environment
within pores can enhance adsorption capacity and selectivity of
porous solids in storage and separation applications. In
particular, the well-defined crystalline structure of microporous
coordination polymers (MCPs) allows evaluation of the collec-
tive effect of surface area, pore size/shape, and functionality
upon gas adsorption1,2 and separation from liquids.3,4 Besides
the significant contribution of the MCP structural features to
influencing adsorption properties, the fine tuning of the building
components provides a route to enhance affinity to adsorbates.
For example, H2 and CO2 gas adsorption capacity vary
dramatically with metal present in the isostructural M/DOBDC
(DOBDC = 2,5-dioxido-1,4-benzenedicarboxylate) series2e,f or
with linker functionality in the isoreticular metal–organic
frameworks (MOFs).2a,b However, assessment of pure linker
or metal substitution effects is often complicated by accompanying
changes in surface area, pore size,2 thermal stability,2d and
framework topology.5 This issue is solved here for a series of
three linkers that assemble into high surface area MCPs with
coordinately unsaturated metal centers.
The H2 and CO2 adsorption behaviors of UMCM-150(N)2
and UMCM-150(N)1 are very similar to that of UMCM-150
despite the surface decoration with the pyrimidine or pyridine
rings (Fig. 2). The excess gravimetric H2 uptakes of these
materials are B2.2 wt% at 77 K and 760 Torr; their
CO2 sorption isotherms monotonically reach the uptakes of
B120 mg gꢁ1 at ambient conditions (Table S1, ESIw) in
good agreement with Grand canonical Monte Carlo (GCMC)
simulations.2h Theoretical studies predict that N-heteroarenes
form a stable complex with H2 and CO2 in the molecular plane
in addition to facial coordination of the gas to the arene.6
However, the adsorption data, including very similar heats of
adsorption (ESIw), demonstrate that, in this case, replacement
with the N-heteroaryl ring does not substantially alter the
affinity to H2 and CO2 molecules for these isostructural frame-
works. This observation suggests that the coordinatively
unsaturated metal sites of the Cu(II) paddlewheel metal
Benzene rings comprise the inner surface of all the highest
performance MCPs; if experimentally feasible, substituting an
N-heteroaromatic ring for a benzene ring would be expected to
significantly change the pore wall electronic nature whilst only
minimally perturbing key structural parameters such as pore
size/shape.6 However, it is very rare that an N-heteroaryl-
based linker leads to the same framework as the analogous
benzene-based linker because different synthetic conditions
and ligand–ligand interactions lead to various metal clusters
and diverse networks.5
In order to probe the effect of linker electronic nature upon
adsorption in MCPs, UMCM-150 was chosen as a benchmark
structure. Constructed from the biphenyl-3,40,5-tricarboxylate
linker (1) and a combination of binuclear Cu(II) paddlewheel
a Department of Chemistry and Macromolecular Science and
Engineering Program, University of Michigan,
930 North University Ave, Ann Arbor, Michigan 48109, USA.
E-mail: matzger@umich.edu; Fax: +1 734-615-8853
b Chemical and Environmental Sciences Laboratory,
General Motors Corporation, Warren, Michigan 48090, USA
w Electronic supplementary information (ESI) available: Synthesis
and isotherms. CCDC 775769 and 775770. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c0cc03482g
Fig. 1 PXRD patterns of UMCM-150 and analogs. Simulated from
the UMCM-150 crystal structure (bottom); the solid (5) from the
reaction of Cu(NO3)2 and compound 4 (top).
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underscoring the potential utility of MCPs with similar pore
size and shape for adsorptive desulfurization.4b
One attractive strategy to enhance the adsorption capacity
of UMCM-150 for DBT and DMDBT, while maintaining
pore size and shape, is the reversible charge-transfer com-
plexation of these compounds by a p-acceptor.10 In this
context linkers 2–4 should enhance the electronic interaction
between the electron-rich organosulfur compounds and the
MCP framework with electron-deficient linkers; such effects
might be expected to be more dramatic than differences
between interactions with light gases.
Fig. 3 displays the adsorption isotherms to 2000 ppmw S
for DBT and to 500 ppmw S for DMDBT (due to solubility
limitations) in isooctane for UMCM-150 and its analogs. In
contrast to the nearly identical H2 and CO2 adsorption isotherms
for these three isostructural MCPs, UMCM-150(N)2 adsorbs
more DBT than both UMCM-150 and UMCM-150(N)1 over
the concentration range investigated. While the latter two MCPs
exhibit similar adsorption behaviors at low concentrations, the
N-heteroaryl decorated MCPs demonstrate enhanced capacities at
high concentrations when compared to UMCM-150. For
Fig. 2 (a) Low pressure H2 and CO2 adsorption isotherms. (b) High
pressure H2 adsorption isotherms at 77 K of UMCM-150 and analogs.
clusters are responsible for the majority of the adsorption
capacities of H2 and CO2 at low pressure. Isolated linker effect
upon adsorption can be estimated at high pressure where most
adsorptive metal sites are already occupied. However, high
pressure H2 adsorption isotherms (Fig. 2b) for this UMCM-150
series do not show a significant difference up to 50 bar,
shedding some doubt on the strategy of altering linker electronic
structure alone as a means of tuning gas adsorption properties.
Instead, linker design needs to focus upon the formation of
optimally sized and shaped pores near the metal cluster to
enhance gaseous H2 and CO2 uptake.2g Indeed, the C–HꢀꢀꢀN
interactions in the planar biaryl linkers of UMCM-150(N)2 and
UMCM-150(N)1 might negate the predicted lateral interactions
between N and gas molecules6 but it is noted that this is in itself
an electronic effect of the type we seek to probe. Hydrogen or
other guests could displace this interaction if alternative modes of
interaction were favored, although this appears not to happen.
Although the substitution of N for C–H does not signifi-
cantly affect the adsorption capacity for the gases H2 and CO2,
there is still potential for enhanced uptake in the liquid phase,
as the factors governing gas phase adsorption capacities are
not the same as those governing liquid phase adsorption
capacities.3,4 The removal of organosulfur compounds from
hydrocarbon fuels has recently attracted attention because of
stringent government regulations on the sulfur content of
gasoline and diesel.4 One desulfurization strategy to obtain
ultralow sulfur fuels is to adsorb refractory organosulfur
compounds such as dibenzothiophene (DBT) and 4,6-dimethyl-
dibenzothiophene (DMDBT), challenging conventional hydro-
desulfurization approaches.9 Cychosz et al. have shown that
MCPs are suitable for this application.4 Notably, among the
MCPs tested, UMCM-150 demonstrated excellent capacity
and selectivity for removing DBT and DMDBT from diesel,
Fig. 3 (a) Adsorption isotherms for DBT (top) and DMDBT
(bottom) in isooctane for UMCM-150 and analogs. The curves
represent a fit to the Langmuir equation and are intended as guides
to the eye. (b) Proposed packing scheme of DMDBT molecules in
UMCM-150 unit cell (left) and in UMCM-150(N)2 unit cell (right); the
loading of DMDBT molecules in the unit cell is estimated from the
interpolated values at 300 ppmw S DMDBT.
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DMDBT, the adsorption isotherm for UMCM-150 rises quickly
up to 100 ppmw S and then increases moderately. On the other
hand, the DMDBT adsorption isotherms for UMCM-150(N)2
and UMCM-150(N)1 increase gradually with concentration,
exceeding the uptake for UMCM-150 at high concentrations.
Although adsorption capacity is closely correlated with MCP pore
size and shape,4 the uptake enhancement for UMCM-150(N)2
among the isostructural series implies that the electron deficient
linker of UMCM-150(N)2 augments the interaction with the
electron rich organosulfur compound by means of donor–acceptor
p–p interaction in the framework (Fig. 3b), leading to unparalleled
refractory organosulfur compound adsorption capacities.
Laboratory (DEFG26-04NT42121) and the 21st Century Jobs
Trust Fund received through the SEIC Board from the State
of Michigan (Grant 454). K.A.C. acknowledges financial
support from a Graham Environmental Sustainability Institute
fellowship.
Notes and references
z Crystal data: (a) UMCM-150(N)2ꢀ3H2O(C26H16N4O15Cu3, CCDC
775769): 95 K, hexagonal, space group P63/mmc (no. 194), a =
18.4456(3), c = 39.5369(7) A, V = 11 649.8(5) A3, Z = 6, 36 091
reflections measured, 3827 unique (Rint = 0.062). R1 = 0.0786
(I > 2s(I)), Rw = 0.2868 (I > 2s(I)) after SQUEEZE routine applied
in PLATON;11 (b) UMCM-150(N)1ꢀ2H2O (C28H16N2O14Cu3, CCDC
775770): 95 K, hexagonal, space group P63/mmc (no. 194),
a = 18.2056(3), c = 40.6655(7) A, V = 11 672.6(4) A3, Z = 6,
47 715 reflections measured, 3824 unique (Rint = 0.049), R1 = 0.0615
(I > 2s(I)), Rw = 0.2020 (I > 2s(I)) after SQUEEZE routine applied
in PLATON.11
UMCM-150 displays a distinctive DMDBT adsorption
isotherm and the local structural features in these MCPs lead
to different adsorption behaviors for the large DMDBT
molecule. In contrast to linkers 2 and 3, the phenyl C–H steric
interactions in linker 1 give rise to twisting between benzene
rings7a (Fig. 3b). It is postulated that this structural characteristic
leads to increased interaction between DMDBT and the
UMCM-150 framework at low concentrations: the methyl
group of DMDBT interacts favorably with one of the tri-
nuclear carboxylates which is sterically less hindered due to the
twist of the phenyl–carboxylate bond. This methyl–carboxylate
contact,3a however, leads to pore blockage, resulting in the
modest capacity increase beyond B100 ppmw S. Fig. 3b
depicts the proposed DMDBT molecule packing in the
UMCM-150 unit cell at 300 ppmw S, where the pores are
filled with B5 DMDBT molecules (37 g S per kg MCP). In
contrast, the electron-deficient linker of UMCM-150(N)2
facilitates p–p interactions with the electron-rich organosulfur
compound, leading to close packing in the framework. At
300 ppmw S, where there are B8 DMDBT molecules per
UMCM-150(N)2 unit cell (54 g S per kg MCP), the efficient
p–p stacking of the framework and the adsorbate leaves space
for further adsorption of DMDBT molecules, in agreement
with the continuous rise of the adsorption isotherm.
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In summary we have successfully synthesized three isostructural
MCPs possessing nearly uniform surface areas from homo-
logous biaryl tricarboxylate linkers containing phenyl,
pyrimidine, and pyridine units, building the ideal system to
probe linker influence upon guest adsorption. The almost
identical isotherms of H2 and CO2 in these MCPs imply that
the N-heteroaryl linkers in the UMCM-150 analogs do not
considerably affect the gas phase adsorption behavior. On the
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UMCM-150(N)1 have demonstrated better adsorption perfor-
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enhance interactions between the host MCP framework
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We acknowledge support of this work through US
Department of Energy through the National Energy Technology
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1454 Chem. Commun., 2011, 47, 1452–1454
This journal is The Royal Society of Chemistry 2011