Article pubs.acs.org/Macromolecules
Nanoporous Porphyrin Polymers for Gas Storage and Separation Zhuo Wang,† Shengwen Yuan,‡ Alex Mason,‡ Briana Reprogle,‡ Di-Jia Liu,*,‡ and Luping Yu*,† †
Department of Chemistry and The James Franck Institute, The University of Chicago, 929 East 57th Street, Chicago, Illinois 60637, United States ‡ Chemical Sciences & Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States S Supporting Information *
ABSTRACT: This article describes the synthesis of four porous polymers containing Ni−porphyrin units with Brunauer−Emmet−Teller (BET) specific surface areas up to 1711 m2/g achieved. The isotherm gas adsorptions of hydrogen, methane and carbon dioxide over these polymers were measured. The adsorption selectivity for methane and carbon dioxide over nitrogen were also investigated. While the initial isosteric heat of adsorption (ΔHads) was around 8−9 kJ/ mol for hydrogen, it reached 23 kJ/mol for methane and 29 kJ/mol for carbon dioxide. CO2/N2 selectivity as high as 19 (calculated from single gas adsorption isotherms) was also achieved with one of these four polymers.
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INTRODUCTION Advanced gas adsorbent materials could play critical roles in the next generation clean energy technologies such as hydrogen storage for fuel cell in transportation, natural gas adsorbent for light-duty vehicle, and CO2 separation and sequestration from the flue gas, etc. Adsorbents based on physisorption principle are generally the preferred materials, because such physisorption process does not require exchange of chemical bonds therefore less undesired kinetic barrier and reaction heat generation. Similarly, the desorption process is more robust and consumes less parasitic energy. Physisorption of the gas molecules is achieved through van der Waals (vdW) interaction with the adsorbent surface. In this regard, the microporous materials are particularly advantageous due to enhanced vdW interaction in confined pore space. There are several wellknown classes of microporous material including zeolites, activated carbons, silica, metal organic frameworks (MOFs) and porous organic polymers (POPs).1−19 Among them, POPs are attractive due to their lightweight, high thermal stability, high tolerance to humidity, stable against gas contaminants and great flexibility in surface structure and chemical environment modification. In the case of hydrogen storage, for example, one of the major challenges is the weak interaction between the absorbent and the nonpolar H2 molecules if the adsorption is only dominated by the London−dispersion force, which is typically in the range of 3−6 kJ/mol. For near ambient temperature hydrogen storage, it has been suggested that a value of ΔHads > 15 kJ/mol is needed.20 Theoretical calculations as well as experimental results showed that an improvement in adsorption energy could be achieved by incorporating unsaturated metal ions into a porous network such as MOF through d-s electronic orbital interaction.21 Such improvement in adsorption between CH4 or CO2 with © 2012 American Chemical Society
unsaturated metal ions through charge-induced dipole interaction is also very interesting with only limited studies available at present.21c,d Metalloporphyrins belong to a class of metal complexes related to naturally abundant porphyins. They have facilitated some of nature’s most important chemical processes including photosynthesis, oxygen transport and catalytic oxidations. With the square-planar coordination site, a large number of metal ions could be readily incorporated into the porphyrin center, which could serve as the building block of polymers with the coordinationally unsaturated metal center. Porous network polymers based on porphyrin and phthalocyanine have been previously synthesized via dioxane formation reactions with BET surface areas below 1000 m2/g.22 We recently reported two polyporphyrins with BET surface area as high as 1522 m2/g using thienyl oxidative coupling polymerization.23a Herein, we report our new effort in preparing several highly porous porphyrinic polymers using Sonagashira−Hagihara coupling, alkyne−alkyne homocoupling and alkyne trimerization reactions.18,23b,c These polymers exhibited narrow pore size distributions and high BET surface areas up to 1711 m2/g. We conducted isotherm measurements of H2, CH4, and CO2 adsorption over these polyporphyrins at several temperatures. We found that they demonstrated excellent hydrogen uptakes with capacity larger than 3.5 mass % at 77 K and 50 bar achieved. These nickel-chelated porphyrin polymers also produced promising potentials for methane storage and carbon dioxide capture with relatively high adsorption enthalpies and storage capacities. For example, up to 12.3 wt % of adsorbed Received: July 10, 2012 Revised: August 29, 2012 Published: September 17, 2012 7413
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Scheme 1. Synthesis of Polyporphyrin Polymers
Figure 1. FTIR spectra for polymers Ni-Por-1 (red), Ni-Por-3 (green) and Ni-Por-4 (blue), in comparison with that of monomer 2 (black).
shown in Figure 1. Two peaks at 3290 and 2108 cm−1 in the spectrum of metal porphyrin were assigned to C−H stretching and CC triple bond vibration, respectively. These peaks disappeared after forming polyporphyrin polymers. It is known that the band at 3033 cm−1 is associated with the C− H stretch in phenyls. The band at 1600 cm−1 is associated with the CC stretch in phenyls, and 1500 cm−1 with CC stretch in porphyrin. The thermal properties of these polymers were characterized by thermal gravimetric analysis (TGA). It was found that, after an initial weight loss of 5%-15%, which may be due to loss of the trapped solvents and moisture, most of these polymers were stable up to 260−300 °C Surface Properties of Polyporphyrins. Surface area and pore size distribution were characterized with a Micromeritics ASAP 2020 accelerated surface area and porosity analyzer using nitrogen as probing gas at 77 K. All polymers exhibited the type I adsorption isotherm (Figure S1 in the Supporting Information), which is the characteristics of a microporous POP material. The calculation using the nonlocal density
CH4 at 35 bar and 12.8 wt % of adsorbed CO2 at 1.08 bar were achieved, respectively.
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RESULTS AND DISCUSSION
Synthesis of Monomers and Porous Polymers. The polyporphyrin polymers were synthesized according to Scheme 1. The 5,10,15,20-tetrakis(4-(ethynylphenyl)porphyrin (1) was synthesized according to the literature procedure.24 The monomer [5,10,15,20-tetrakis(4-(ethynylphenyl)porphyrin]nickel(II) (2) was synthesized by heating compound 1 with metal acetate in DMF. The resulting porphyrin monomer was polymerized through three types of polymerization reactions, namely Sonagashira−Hagihara coupling with tetraiodophenylmethane, alkyne trimerization, and alkyne−alkyne homocoupling reactions. Structural Characterizations. Since the resulting polymers are insoluble, the structural information was obtained using Fourier transform infrared (FTIR) spectrometer. The FTIR spectra of these polymers and monomer porphyrin 2 are 7414
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3.13 2.66 2.55 2.26 0.98 0.85 0.80 0.68 8.0 8.2 8.8 7.8 0.69/0.90 0.54/0.96 0.38/0.59 0.38/0.62 0.0025 0.0025 0.0020 0.0022 0.0038 0.0032 0.0022 0.0023 0.023 0.023 0.017 0.016 0.035 0.028 0.019 0.017 1711 1393 894 778
sample
Ni-Por-1 Ni-Por-2 Ni-Por-3 Ni-Por-4
2286 1921 1323 1041
CH4 uptake (IB, 1.08 CO2 Uptake (IB, 1.08 bar) (mmol/g bar) (mmol/g adsorbent) adsorbent) hydrogen ΔHads (kJ/mol) H2 max vol. uptake (77K) (kgH2/L adsorbent)
H2 max gr. uptake (RT, 70 bar) (kgH2/kg adsorbent+H2ads)
vol. uptake (RT, 70 bar) (kgH2/L adsorbent)
μ-pore/total pore (cm3 g‑1/ cm3 g‑1) 7415
H2 max gr. uptake (77K) (kgH2/kg adsorbent+H2ads)
where SV is the skeleton volume per gram of polymer, MV is noncompressible microrpore volume per gram of polymer. The heats of adsorption as the function of hydrogen loading for NiPor-1 is shown in Figure 2b, where the initial ΔHads could be extrapolated to be about 8 kJ/mol, indicating an incremental improvement over the commonly reported heat of adsorption for pure carbon−hydrogen porous polymers.25 Figure S3, in the Supporting Information, includes the heats of adsorption versus the hydrogen uptake for all four polymers, with Ni-Por-3 having the highest initial heat of adsorption value at ∼8.8 kJ/mol. Methane and Carbon Dioxide Adsorption and Separation. Porous organic polymers were previously reported with excellent capacities for methane and carbon dioxide adsorption.26 Thus, we have also investigated the sorption capability of these nickel-doped porphyrin polymers for methane and carbon dioxide using the Micromeritics ASAP 2020 system. Adsorption isotherms of methane and carbon dioxide were measured at both ice/water bath and 25 °C water bath at a gas pressure up to 1.08 bar. As shown in Figure 3, CH4 uptake at ice/water bath reached 15.7 mgCH4/g_ads, while CO2 uptake at the same temperature reached 138 mgCO2/g_ads. The heats of adsorption for both CH4 and CO2 generally followed the same trench: ΔHNi‑Por‑1 < ΔHNi‑Por‑2 < ΔHNi‑Por‑3 ≤ ΔHNi‑Por‑4, as shown in Figure 4. The initial heat of adsorption was estimated to be up to 23 kJ/mol for CH4 and 29 kJ/mol for CO2, respectively. Gas adsorption selectivity of CO2 over N2 was calculated using the similar approach by Rosi et al27a based on single gas sorption isotherms, the selectivity follows the sequence of Ni-Por-1 < Ni-Por-2 < Ni-Por-4 < Ni-Por-3, with values of 14.4, 15.4, 17.4, and 19.1, respectively; while the selectivity of CO2 over CH4 is 3.6, 3.7, 4.15, and 4.22 respectively for Ni-Por-1, Ni-Por-2, Ni-Por-3, and Ni-Por-4.27
Langmuir surface area (m2/g)
(1)
BET SSA (m2/g)
1 SV + MV
Table 1. Surface Properties and Gas Adsorption Capacities for POP Ni-Por-1 to Ni-Por-4
TPD =
skeleton vol. (cm3 g‑1)
function theory (NLDFT) based on isotherm data showed that a majority of the pore volume of these polymers was contributed by the micropores with pore sizes distribution narrowly centered at about 0.6−1.0 nm, a desirable dimension for gas adsorption. (Figure S2, Supporting Information, is the plot of the differential pore volumes versus the pore size based on NLDFT calculation.) Table 1 summarizes the surface properties and gas adsorption capacities of the polymers. Hydrogen Storage. Hydrogen adsorption isotherms were measured with a Sievert type apparatus. Figure 2a shows the excess H2 adsorption capacities of Ni-Por-1 measured under various equilibrium pressures at three different temperatures. The maximum H2 uptake capacity for polymer Ni-Por-1 reached 3.5 mass% at 77 K and 40 bar, which followed so-called “Chahine’s rule”, similar to its rigid counterparts, such as activated carbon and MOFs.2,3,19 The isosteric heat of adsorption at low coverage region was derived from the isotherms measured at 195 and 298 K. We chose these two temperatures because the corresponding isotherms were well-separated, thus the heat of adsorption for hydrogen could be more accurately deduced, in contrast to the approach that uses isotherms at 77 and 87 K. In addition, hydrogen adsorption is more selective toward the stronger binding site at higher temperature. To calculate the volumetric hydrogen adsorption capacities, a tight-packing density (TPD) was used as the polymer density. Such tight-packing density is defined by eq 1.
0.85 0.73 0.73 0.67
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Figure 2. (a) H2 uptake isotherms for Ni-Por-1 at liquid nitrogen bath (LN, T = 77 K), dry ice/acetone bath (DIA, T = 195 K), and room temperature (RT, T = 298 K). (b) H2 heat of adsorption as a function of hydrogen uptake for Ni-Por-1.
Figure 3. (a) Methane uptake isotherms of Ni-Por-1 to Ni-Por-4 at ice/water bath. (b) Carbon dioxide uptake isotherms of Ni-Por-1 to Ni-Por-4 at ice/water bath.
Figure 4. (a) Ni-Por-1 to Ni-Por-4 heats of adsorptions as the function of methane uptake. (b) Ni-Por-1 to Ni-Por-4 heats of adsorptions as the function of carbon dioxide uptake.
Such gas selectivity sequences are consistent with the heat of adsorption data. When using room temperature isotherms for calculation, the value of gas selectivity for these two types of gases remained about the same as that obtained from ice/water bath isotherms. The pronounced selectivity for CO2 over N2 makes this type of transition metal doped POPs good candidates for CO2 capture and separation from the flue gas stream, where CO2 and N2 accounts for around 12 mol % and 64 mol % of the total gas content, respectively.
Methane adsorption at the pressure up to 45 bar was also measured with the high-pressure Sievert isotherm apparatus. At ice/water bath, the excess CH4 storage capacity of over 168 mg/g, or 153 V/V was achieved for Ni-Por-1 at 45 bar, (see Figure S4, Supporting Information). The CH4 uptake capacities at 35 bar for the other three polymers were also measured. In general, the methane storage capacities of these POPs are comparable to that of a microporous zeolite 5A28 and some MOFs.29 The isosteric heat of adsorption of Ni-Por-1 as the 7416
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data at 77 and 195 K was corrected using the temperature correction method as recommended by the DOE best practice.30 Ultra-high-purity hydrogen of 99.9995% was used for H2 update measurement, and the free volume of the sample cell at each temperature was calibrated with helium (99.9995%). Hydrogen equilibrium pressure up to 80 bar was applied during the measurement. A typical measurement error was
