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J. Phys. Chem. B 2002, 106, 1380-1385
Syntheses and Characterization of Microporous Coordination Polymers with Open Frameworks K. Seki*,† and W. Mori‡ Department of Research and DeVelopment, Osaka Gas Co., Ltd., 6-19-9 Torishima, konohana-ku, Osaka 554-0051, Japan, and Department of Chemistry, Faculty of Science, Kanagawa UniVersity, Hiratsuka, Kanagawa 259-1293. ReceiVed: August 7, 2001; In Final Form: October 19, 2001
The reactions of porous two-dimensional copper dicarboxylates (copper fumarate, copper terephthalate, copper styrene dicarboxylate, and copper 4,4′-biphenyl dicarboxylate) with triethylenediamine as a pillar ligand yielded porous three-dimensional coordination polymers. The characterization by gas adsorption indicated that these coordination polymers have uniform micropores, high porosities, and gas adsorption capacities. These properties depend on the kind of dicarboxylate, and by changing it, the porosity and the pore size of the polymer can be controlled. The measurements of methane adsorption isotherms revealed that all coordination polymers have methane adsorption capacities, and especially, polymers synthesized from copper styrene dicarboxylate and copper 4,4′-biphenyl dicarboxylate, which have ideal pore sizes and distributions for methane adsorption, have higher methane adsorption capacities than that of the theoretical maximum for activated carbon.
1. Introduction A great deal of attention has been directed toward the use of coordination polymers in the designs and syntheses of new porous materials.1-28 These coordination polymers with open frameworks are widely regarded as attractive materials for applications in catalysis,20,22,27,28 separation,23 gas adsorption,1-3,5,7-16 and molecular recognition. Compared with conventional porous materials such as zeolites or activated carbons, these coordination polymers have a higher potential because of designable framework, high microporosity, and flexible framework based on a variety of coordination geometries of metal centers and multifunctionality of bridging organic parts. However, most of these porous materials are not sufficiently characterized as adsorbents, except for the characterizations for a few porous compounds with open frameworks reported recently.29,30 Recently, we have briefly reported on the synthesis and characterization of a coordination polymer having a threedimensional network structure bridging a two-dimensional layer of porous copper(II) terephthalate1 with triethylenediamine (TED) as a pillar ligand.3 In this paper, we wish to expand on this study and to describe the syntheses and characterization of a series of coordination polymers having a three-dimensional network structure bridging a two-dimensional layer of porous copper(II) dicarboxylate1,2 with TED as a pillar ligand. From the results, we demonstrate the relations between the structures and adsorption properties. In addition, methane adsorption property, which is one of applications for porous materials, is evaluated at high pressure at 298 K. 2. Experimental Section Synthesis. Cu(trans-OOCsCHdCHsCOO)‚1/2 TED (1). A methanol solution (27 cm3) of copper(II) sulfate pentahydrate * To whom correspondence should be addressed. E-mail: ken@ osakgas.co.jp. Fax: +81-6-6462-3433. † Osaka Gas Co., Ltd. ‡ Kanagawa University.
(1.25 g) was added to a DMF solution (17 cm3) of fumaric acid (0.58 g) and formic acid (1.0 cm3). After the mixture was allowed to stand for several days at 313 K, a toluene solution (12.5 cm3) of TED (0.19 g) was added to the mixture, which was then allowed to react at 433 K in an autoclave for several hours. A light blue precipitate was collected, washed with mehanol, and dried at 373 K under a vacuum. Anal. Found: C, 35.42; H, 3.31; N, 5.50%. Calcd for C7H8CuNO4: C, 35.98; H, 3.44; N, 5.99%. Cu(p-OOCsPhsCOO)‚1/2 TED (2). A methanol solution (10 cm3) of copper(II) sulfate pentahydrate (0.31 g) was added to a methanol solution (200 cm3) of terephthalic acid (0.21 g) and formic acid (2.0 cm3). After the mixture was allowed to stand for several days at 313 K, a toluene solution (12.5 cm3) of TED (0.07 g) was added to the mixture, which was then allowed to react at 433 K in an autoclave for several hours. A light blue precipitate was collected, washed with mehanol, and dried at 373 K under a vacuum. Anal. Found: C, 46.12; H, 3.44; N, 4.97%. Calcd for C11H10CuNO4: C, 46.57; H, 3.54; N, 4.94%. Cu(OOCsPhsCHdCHsCOO)‚1/2 TED (3). An ethanol solution (15 cm3) of copper(II) acetate monohydrate (0.10 g) was added to a DMF solution (30 cm3) of styrene dicarboxylic acid (0.10 g) and formic acid (0.3 cm3). After the mixture was allowed to stand for several days at 313 K, a toluene solution (12.5 cm3) of TED (0.03 g) was added to the mixture, which was then allowed to react at 433 K in an autoclave for several hours. A light blue precipitate was collected, washed with mehanol, and dried at 373 K under a vacuum. Anal. Found: C, 49.34; H, 2.70; N, 4.42%. Calcd for C13H12CuNO4: C, 50.90; H, 2.96; N, 4.42%. Cu(4,4′-OOCsPhsPhsCOO)‚1/2 TED (4). A methanol solution (25 cm3) of copper(II) formate (0.50 g) was added to a DMF solution (70 cm3) of 4,4′-biphenyl dicarboxylic acid (0.25 g) and formic acid (0.3 cm3). After the mixture was allowed to stand for several days at 313 K, a toluene solution (12.5 cm3) of TED (0.06 g) was added to the mixture, which
10.1021/jp0130416 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/17/2002
Microporous Coordination Polymers was then allowed to react at 433 K in an autoclave for several hours. A light blue precipitate was collected, washed with mehanol, and dried at 373 K under a vacuum. Anal. Found: C, 56.52; H, 3.55; N, 3.52%. Calcd for C17H14CuNO4: C, 57.22; H, 3.11; N, 3.93%. Physical Measurements. X-ray powder diffraction data were collected on a Rigaku RINT 2400 by using Cu KR radiation. Temperature control was achieved with a temperature control unit. The simulated powder patterns were obtained by using Cerius2 DIFFRACTION module. Thermal gravimetric (TG) analyses were carried out with a Seiko Instruments SSC5200 in a air atmosphere (heating rate: 5 K/min). Ar Adsorption Measurement. Adsorption isotherms were measured using ASAP 2000M volumetric adsorption equipment from Micromeritics. Before adsorption measurements, the samples were degassed under vacuum at 373 K. Adsorption isotherms were measured in relative pressure range from 10-6 to 1. Mercury Penetration Measurement. The mercury penetrations were measured using PoreMaster 60-GT (Quantachrome Corp.) or AutoPore 9500 (Micromeritics Instrument Corp.) where maximum applied pressure was 420 MPa. The values of surface tension and contact angel were taken as 4.85 N/m and 130°, respectively. Methane Adsorption Measurement. The apparatus was equipped with the Cahn-2000 microbalance contained within a SUS steel pressure chamber which was connected with two separate lines for evacuation and adsorbate gas pressurization. After the each sample was set in the apparatus, the adsorbed molecules in the pore and on the surface were removed heating under reduced pressure. After becoming the constant weight, methane was dosed into the adsorption chamber. After the equilibrium of adsorption, the change of the weight of sample was measured. After the buoyancy had been corrected to the obtained amount of the weight change, the absorbed amount was calculated. 3. Results and Discussion Syntheses and Structures of Porous Copper Dicarboxylates. Three-dimensional coordination polymers of Cu(OOCR-COO)‚1/2C6H12N2 were synthesized by heterogeneous reactions between porous copper dicarboxylates and TED as a pillar ligand. The temperature dependence of the magnetic susceptibilities31 for the obtained coordination polymers indicate that they have the same dinuclear structures32-34 as those of porous copper dicarboxylates1 which have two-dimensional structures of dicarboxylic acids bridging the center copper ions. On the basis of these results and elemental analyses, these structures are supposed to be three-dimensional ones in which two-dimensional layers bridging the copper (II) ions with the dicarboxylate ions are linked with TED as a pillar ligand as shown in Figure 1. The structures and the stabilities of three-dimensional coordination polymers were studied by X-ray powder diffraction (XRPD) and thermal gravimetric (TG) analysis. Figure 2 showed that the observed XRPD patterns are in good accordance with the simulated patterns of the optimized plausible structures by using Cerius2, indicating the structures of the obtained compounds are confirmed to be the plausible structures. The TG curve of 3 illustrates the release of the adsorbed molecules up to about 343 K, followed by the thermal decomposition of the structure at 473 K. No chemical decomposition was observed between 343 and 473 K. The structure of this stable phase was studied by measuring the XRPD pattern
J. Phys. Chem. B, Vol. 106, No. 6, 2002 1381
Figure 1. Plausible three-dimensional structures of coordination polymers.
at RT, 423 and 473 K. These results demonstrated that the porous network structure was retained up to 473 K in the absence of the included guest molecules. The similar analyses revealed that other porous network structures are retained up to 473, 523, and 513 K for 1, 2, and 4, respectively. Characterization of Coordination Polymers by Ar Adsorption. To characterize coordination polymers, high-resolution adsorption isotherms of Ar at 87.3 K were measured in a relative pressure range from 10-6 to 1. The adsorption isotherms are shown in Figure 3. All of the adsorption isotherms show typical isotherms of Langmuir type, confirming the presence of micropores without mesopores. The sharp rise of argon adsorption at low relative pressures indicates that the micropores are extremely uniform. The rise of argon adsorption at high relative pressures arises from the condensation of adsorbates into interparticle voids between the primary particles which sizes are about 10-100 nm. From these data, the BET surface area, micropore volume, pore diameter, and pore distribution {using Dubinin-Radushkevitch (DR) methods35 and Horvath-Kawazoe (HK) methods36} were derived to characterize the obtained coordination polymers. The results are summarized in Table 1. The pore size distributions are shown in Figure 4. For 1 and 2, one sharp peak is observed at 7.0 and 7.4 Å, indicating that the obtained coordination polymers have uniform micropores. To compare experimental values of pore sizes with calculated values, the plausible structures were optimized by molecular mechanics (MM) and molecular dynamics (MD) and the pore sizes were calculated from these structures geometrically. The optimized plausible structures were shown in Figure 5. The effective pore sizes calculated from these optimized structures of 1 and 2 were about 6.8 and 7.4 Å. These calculated values are in good accordance with experimental values of the HK method. On the other hand, for 3 and 4 (especially 4), the pore size distributions exhibit two peaks. This result does not indicate the existence of two kinds of pores having different pore sizes but indicates the existence of two adsorption sites having the different potential values in one pore. These sites are the corner and the edge of the square pore. Despite uniform micropores for 3 and 4, the pore size distributions exhibit two peaks because the HK method cannot be used when calculating the pore size distributions for pores of more than 8 Å in width.37 By the simulations mentioned above, geometric pore sizes for 3 and 4 were calculated to be 9.4 and 10.5 Å, respectively. These calculated values are in good accordance with experimental values of median pore sizes in the range to less than 20 Å in the HK method. The pore size is larger as the size of carboxylic acid is longer. The surface area and micropore volume are
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Figure 2. X-ray diffraction patterns of 2, 3, and 4. (a) Experimental X-ray powder diffractions after removal of the included guest molecules. (b) Simulated X-ray powder diffractions of the optimized structures by using Cerius2.
Figure 3. Ar adsorption isotherms at 87.3 K for coordination polymers.
TABLE 1: Microporosities of Coordination Polymers Obtained by Ar Adsorption
compound
S.A./ m2 g-1
1 2 3 4
606 1891 3129 3265
micropore volume/ cm3g-1 a HK DR 0.23 0.71 1.07 1.18
pore size/ Å
porosity/ %
7.0 7.4 9.5 10.8
33 58 65 68
0.24 0.70 1.11 1.26
Figure 4. Pore size distributions for coordination polymers.
TABLE 2: Characteristic Parameters of Argon Adsorption Isotherms at 87.3 K micropore volume/ βE0/ qst,)1/e/ W0/ cm3 g-1 kJ mol-1 kJ mole-1 compound cm-3 (STP)g-1 a
a Estimated from the obtained value at P/P ) 0.11 which is 0 associated with the value of 20 Å pore size.
greater as the pore size is larger, indicating that the pore size can be controlled freely by varying the kind of carboxylic acid. DR analysis35 was used to determine the saturated amount of adsorption and the adsorption energy in a series of coordination polymers. The DR equation is expressed as follows:
ln W ) ln W0 - (A/βE0)
2
where W and W0 are the amount of adsorption at P/P0 and the saturated amount of adsorption, E0 is a characteristic adsorption energy, and the parameter of A is Polanyi’s adsorption potencial defined as A ) RT ln(P0/P). The parameter of β is affinity coefficient related to the adsorbate-adsorbent interaction. All DR plots were almost liner in the higher P/P0 region, giving the saturated amount of adsorption W0 and adsorption energy βE0 (Table 2).
1 2 3 4 a
185 556 845 992
0.24 0.71 1.08 1.27
10.57 15.32 8.16 5.87
17.09 21.84 14.68 12.39
The inherent micropore volume determined by the Langmuir plot.
Here by using the results of these values W0, we gain insight into the schematic of the filling states of the micropore with argon. The filling states are determined from W0 under the assumption of square pores and spherical argon molecule having a 3.4 Å pore diameter. Figure 6 shows the schematic of the argon filling states of micropores for coordination polymers. These results indicate that the spherical argon molecules can fill the square micropores regularly in high packing density. This is the reason the amount of argon adsorbed is much larger than other coordination polymers synthesized previously and zeolite 5A.
Microporous Coordination Polymers
J. Phys. Chem. B, Vol. 106, No. 6, 2002 1383
Figure 5. Optimized structures of three-dimensional coordination polymers by using simulation.
Figure 6. Schematic of the argon filling states of micropores for coordination polymers.
Figure 7. High-pressure methane adsorption isotherms at 298 K.
Methane Adsorption Properties at High Pressure. The adsorption isotherms of methane were measured gravimetrically up to 3.5 MPa. Figure 7 shows the methane adsorption isotherms of coordination polymers at 298 K. All of the adsorption isotherms are the Langmuir type. The amount of methane adsorbed per weight at 3.5 MPa is larger as the surface area,
micropore volume, and pore size are larger. The smaller the pore size is, the sharper the rise of the amount at low pressure is, indicating adsorbents having smaller pore sizes have the stronger micropore field. These values are much higher than that of zeolite 5A {about 83 cm3 (STP) g-1 at 3.5 MPa} and higher than that of Cu(4,4′-bipy)2(SiF6)10 which has the highest methane adsorption capacity among other coordination polymers. The values for 3 and 4 are nearly the same as that of the high surface area activated carbon (AX-21) which is a methane adsorbent with the highest capacity among conventional materials.38 The amount of methane adsorbed at 3.5 MPa of 3 and 4 is the same despite the fact that the porosity of 4 is larger than that of 3, indicating that the interaction energy of 4 for methane is weaker than that of 3 because of larger pore size. From this result, the more expansion of pore size is not effective for improvement of methane adsorption capacity up to 3.5MPa. In three-dimensional coordination polymers, the suitable pore size for methane adsorption at 3.5 MPa is supposed to be close to that of 3. Furthermore, this value of 2 was compared with those of activated carbon fibers (ACF) which were the best carbon for methane adsorption because of their uniform pore dimen-
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TABLE 3: Characteristic Parameters of Methane Adsorption Isotherms at 298 K compound
WL/ cm3 (STP)g-1 a
βE0/ kJ mol-1
qst,φ)1/e/ kJ mol-1
P0q/ MPa
1 2 3 4
121 302 513 564
8.87 8.08 8.31 8.50
17.04 16.25 16.48 16.67
13.9 32.9 82.4 109.7
a
The inherent micropore volume determined by the Langmuir plot.
Figure 8. Mercury penetration curve which is a differential intrusion versus a pore diameter.
sions. The value for 2 is superior to that of ACF20 which has almost the same surface area. The high-pressure adsorption of supercritical methane has been studied by the followed extended DR equation:39
ln W ) ln WL - (A/βE0)2,
A ) RT ln(P0q/P)
where W is the amount of adsorption at P, βE0 is the adsorption energy, and P0q is the saturated vapor pressure of the quasivaporized supercritical methane. The inherent micropore volume, WL, is determined by the Langmuir plot. All DR plots were liner in all regions, indicating that the mechanism of methane adsorption was micropore filling and giving P0q and βE0. Furthermore, the βE0 leads to isosteric heat of adsorption qst,φ)1/e at the fractional filling of 1/e. Obtained parameter, WL, βE0, qst,φ)1/e, and P0q are summarized in Table 3. The qst,φ)1/e values of 1, 2, 3, and 4 are from 16.2 to 17.0 kJ mol-1, being almost the same as that of the activated carbon fiber (17.1 kJ mol-1) with slit-shaped micropores.39 The value of P0q depends on the pore size. The value of P0q is larger as the pore size is larger. The qst,φ)1/e value indicates that the methane adsorption of coordination polymers is physical adsorption and the mechanism of methane adsorption is explained by micropore filling which is the same mechanism as that of activated carbon. Characterization of Coordination Polymers by Microscopy and Mercury Penetration. The samples were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). SEM image of particles of 3 indicate that the particles are plate forms of ca. 0.2 µm in width, these particles aggregate to be formed in size of ca. 7 µm. The surface AMF image of 3 shows nanocrystals of ca. 100 nm in size on the surface, which are considered to be the primary particles. The similar analyses for other coordination polymers revealed that they had the same structures. Moreover, to characterize pore structures and material densities, mercury intrusion penetrations were measured from 1 to 60 000 psia. Figure 8 shows the mercury penetration curve which is a differential intrusion versus a pore diameter. These
results of mercury porosymmetries are in good accordance with those of SEM and AMF in regard to voids between particles. For example, in the case of 3, the voids between the secondary particles (layer planes) correspond to peak a of 0.1-5 µm and the voids between the primary particles correspond to peak b of 6-20 nm. The apparent density of primary particle is calculated from the result of mercury porosimetry by assuming that peaks a and b are derived from voids between particles. The apparent density was 0.983 g cc-1. The practical parameter v/v expressing a gas storage capacity (The v/v is the volume of gas at 273 K and 0.1013 MPa divided by the volume of the tank) was estimated to be 225 v/v for 3 from this density and the amount of adsorbed methane per weight of dried sample. By the same method, the v/v values of other samples were calculated to be 188 and 225 for 2 and 4, respectively. On the other hand, the adsorption simulations predict that the theoretical maximum for methane storage capacity of carbon, of which the structure is parallel planes of graphite with the optimum slit width 11.4 Å, is 198 v/v for void-free monolithic carbon.40 The amount of adsorbed methane for 3 and 4 surpasses the theoretical maximum storage capacity of carbon calculated at the same condition. 4. Summary This study has demonstrated that coordination polymers having a three-dimensional network structure bridging the twodimensional layer of copper (II) dicarboxylate by TED as a pillar ligand have uniform micropores, which pore structures can be controlled by the kind of the ligand. The construction of a threedimensional network structure is an effective method of improving porosity. These coordination polymers have a higher porosity and higher methane adsorption capacity than zeolites and other porous coordination polymers. The results of this work will lead up to the development of the porous materials which have the necessity to control the structure for applications in other gas storage, separation, catalysis, and molecular recognition. The synthesis and characterization of coordination polymers with more expanded ligands are in progress. Supporting Information Available: Figures of the temperature dependence of the magnetic susceptibilities for the obtained coordination polymers (Figure S1), of the TG curve and powder X-ray for 3 (Figure S2), of extended DR plots of the high-pressure methane adsorption for the obtained coordination polymers (Figure S3), and of SEM and AFM images of particles for 3 (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Mori, W.; Inoue, F.; Yoshida, K.; Nakayama, H.; Takamizawa, S. Chem. Lett. 1997, 1219. (2) Seki, K.; Takamizawa, S.; Mori, W. Chem. Lett. 2001, 122. (3) Seki, K.; Takamizawa, S.; Mori, W. Chem. Lett. 2001, 332. (4) Chui, S. S.-Y.; Lo, S. M-F.; Charmant, J. P. H.; Orpen, A. G.; Williams, I. D. Science 1999, 283, 1148. (5) Li, H.; Eddaoudi, M.; O’Keeffe, M.; Yaghi, O. M. Nature 1999, 402, 276. (6) Li, H.; Eddaoudi, M.; Groy, T. L.; Yaghi, O. M. J. Am. Chem. Soc. 1998, 120, 8571. (7) Kondo, M.; Shimamura, M.; Noro, S.; Minakoshi, S.; Asami, A.; Seki, K.; Kitagawa, S. Chem. Mater. 2000, 12, 1288. (8) Kondo, M.; Okubo, T.; Asami, A.; Noro, S.; Yoshitomi, T.; Kitagawa, S.; Ishii, T.; Matsuzaka, H.; Seki, K. Angew. Chem., Int. Ed. Engl. 1999, 111, 140. (9) Kondo, M.; Yoshitomi, T.; Seki, K.; Matsuzaka, H.; Kitagawa, S. Angew. Chem., Int. Ed. Engl. 1997, 36, 1725. (10) Noro, S.; Kitagawa, S.; Kondo, M.; Seki, K. Angew. Chem., Int. Ed. Engl. 2000, 39, 2081.
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