J. Phys. Chem. C 2010, 114, 1827–1834
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Cu2(pyrazine-2,3-dicarboxylate)2(4,4′-bipyridine) Porous Coordination Sorbents: Activation Temperature, Textural Properties, and CO2 Adsorption at Low Pressure Range Omar J. Garcı´a-Ricard and Arturo J. Herna´ndez-Maldonado* Department of Chemical Engineering, UniVersity of Puerto Rico-Mayagu¨ez Campus Mayagu¨ez, Puerto Rico 00681-9000 ReceiVed: October 28, 2009; ReVised Manuscript ReceiVed: December 16, 2009
The effect of activation temperature on the textural properties and low-pressure adsorption performance of the porous coordination polymer Cu2(pzdc)2(bpy) [pzdc ) pyrazine-2,3-dicarboxylate, bpy ) 4,4′-bipyridine], better known as CPL-2, was considered to elucidate the material potential for separations. The effective activation temperature range was estimated via coupled thermal gravimetric and Fourier transforms infrared spectroscopy analysis. A textural property analysis via the Rs-plot, Dubinin-Radushkevich and Horvath-Kawazoe methods show that a significant reduction in effective surface area and micropore volume occurs when the activation temperature is increased from 373 to 423 K. Cooling of the sample in a moisture-free environment revealed that such reduction is nonreversible, as evidenced by single-component CO2 equilibrium adsorption tests. Although CO2 equilibrium adsorption isotherms exhibit a linear behavior in the ambient pressure range, an increase in activation temperature eventually decreases the pore size of the structure resulting in a considerable decrease in loading amounts. This was also corroborated by means of in situ high-temperature X-ray diffraction, which was used to monitor the lattice semiquantitative changes of CPL-2 during the thermal activation sequence. In addition, adsorption uptake data was gathered to estimate a diffusion time constant and elucidate preliminary information about the kinetics involved during the transport of CO2 through the micropores of CPL-2. After inspection of the adsorbent particle morphology via scanning electron microscopy, it became ostensible that the transport phenomenological model suitable to fit the uptake data was that of a slab-shape particle. For the sample pretreated at 373 K the analysis yields an average diffusion time constant of ca. 0.5 s-1 at 298 K. 1. Introduction Porous coordination polymers (PCPs), also called metalorganic frameworks (MOFs), are microporous materials with regular pore size and shape which, like zeolites and zeolitelike materials, have gotten the attention of many scientists and engineers due to their potential application in molecular storage, heterogeneous catalysis, and gas separation via adsorption.1-13 These compounds are classified as polymers because of the infinite structure of coordination bonds, formed by the metal ions acting as nodes (or connectors) and bridging ligands acting as linkers. Depending on the valence of the metal element and its electronic configuration, different coordination geometries can be obtained, e.g., linear, T-shape, tetrahedral, octahedral. Recently, many porous coordination polymers have been synthesized using copper,14-20 zinc,21-23 and other transition metals24-27 as their mono- or dinuclear14,22 nodes. For the case of the organic ligand linkers, multidentate building units with two or more donor atoms are used to create the infinite structure, mostly N, O, and/or S-donor atoms.5,28-32 The size and shape of the linkers are also taken into consideration when designing a given structure in order to produce certain functionality and such is the case of the CPL-n PCPs series. The CPL-n series, originally developed by Kitagawa’s group15,20,33-36 and also studied by Kaneko et al.,4,19 consists of neutral 2D layers formed by the Cu2+ and the anionic pyrazine2,3-dicarboxylate (pzdc2-) and separated by a neutral and lineal heterocyclic N-donor pillar ligand. This series possesses three * To whom correspondence should be addressed: Phone: 787-832-4040 x3748. Fax: 787-834-3655. E-mail:
[email protected].
important characteristics that should be highlighted: (1) compounds are obtained rapidly through a simple synthesis procedure under normal temperature and pressure, (2) has proven to be flexible under the inclusion of a guest-molecule,20,32,37,38 and (3) different compounds within the series can be obtained just by changing the pillar ligands (e.g., pyrazine for CPL-1, 4,4′bipyridine for CPL-2, trans-1,2-bis(4-pyridyl)ethylene for CPL5) to increase or decrease the spacing between the neutral 2D layer, therefore tailoring the pore size and sieving functionality for a given application. In this work, we present the degassing or activation temperature as a critical parameter in the activation or regeneration of Cu2(pzdc)2(bpy) [pzdc ) pyrazine-2,3-dicarboxylate, bpy ) 4,4′bipyridine], better known as CPL-2. The textural properties and adsorption performance are presented for three different activation temperatures all below the decomposition limit. For practical purposes, the effect of the activation temperature on the adsorption performance of CPL-2 was studied at room temperature from pure component adsorption isotherms of CO2, O2, and N2. For CO2, the selected approach is a simple way of estimating the materials storage capacity and the potential ability to remove this gas at bulk quantities from air at room temperature. This work also presents a brief discussion on the morphology and size of the CPL-2 particles and a qualitative description of some relevant lattice changes observed during in situ hightemperature X-ray powder diffraction (XRD). Additional information regarding CO2 adsorption in CPL-2 is given, including (1) the diffusion time constant at room temperature as a function
10.1021/jp9103068 2010 American Chemical Society Published on Web 01/07/2010
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J. Phys. Chem. C, Vol. 114, No. 4, 2010
of loading and (2) the isosteric heat of adsorption for the different activation temperatures. 2. Experimental Section 2.1. Materials. Reagents used were 2,3-pyrazinedicarboxylic acid (H2pzdc, 97% purity), 4,4-bipyridine (bpy, 98% purity), and copper(II) perchlorate hexahydrate (Cu(ClO4)2 · 6H2O, 98% purity). All three were obtained from Sigma-Aldrich and used without further purification. Adsorbate gases used were O2 (ultra high purity grade, Linde), N2 (high purity grade, Linde), and CO2 (ultra high purity grade, Praxair). He (high purity grade, Praxair) was used as a carrier gas in thermogravimetric and Fourier transform infrared (TGA-FTIR) spectroscopy analyses and in situ high temperature XRD. Denatured ethanol used during synthesis and the methanol used for sample washing purposes were both obtain from Sigma-Aldrich with 95 and 99% purity, respectively. 2.2. Synthesis of CPL-2. CPL-2 was synthesized slightly varying previously reported methods;15,20 instead of sodium 2,3pyrazinedicarboxylate (Na2pzdc), 2,3-pyrazinedicarboxylic acid (H2pzdc) was used. A mixture solution (100 mL) containing 1 mmol of H2pzdc (0.1681 g) and 0.5 mmol of bpy (0.078 g) dissolved in NaOH (0.04M)/EtOH (1:1) was added to the solution (100 mL) containing 1 mmol of Cu(ClO4)2 · 6H2O (0.37 g) at room temperature. The mixture was stirred for one day and the precipitate was then filtered and washed with copious amounts of water and methanol, followed by drying for several hours at 363K. 2.3. TGA, FT-IR Spectroscopy, and Porosimetry. TGA were performed in a TA-Q500 microbalance. Samples were heated from ambient temperature to 1173 K at a heating rate of 10 K/min using a constant helium (high purity grade, Praxair) flow of 60 mL/min. The gas was pretreated with presorbers (i.e., 3A Zeolites and hydrocarbon traps) to remove traces of water or other contaminants that could have been present. The gas exhaust of the TGA instrument was analyzed in an attempt to identify the species evolving from the CPL-2 decomposition by means of FT-IR spectroscopy. The IR spectrometer setup consisted of a Nicolet 6700 Optical Spectrometer Mainframe equipped with a Nicolet X700 TGA/ IR external interface module. This module houses a highefficiency condensing and collection optics, a DLa TGS detector, a nickel-coated stainless steel gas cell, and a heated transfer line. Both the transfer line and the gas cell are thermally insulated to prevent cold spots and condensation of high boiling point materials onto the system. Although the IR interface is operable at temperatures up to 600 K, the TGA can be operated up to ca. 1273 K. During the experiments, about 12 mg of sample were placed in the TGA instrument and heated from room temperature to 1173 K at a rate of 10 K/min in a flow of dry, high-purity helium. The transfer line between the TGA exhaust and IR bench and the IR gas cell were heated to 510 and 520 K, respectively. During the tests, the spectral resolution (data spacing) was kept at 3.857 cm-1 and signal averaging was performed for 50 scans/sample every 30 s. These analyses were conducted to determine the thermal decomposition temperature of CPL-2 and specify the degassing (pretreatment) temperature for the adsorption analyses. Decomposition species were identified using IR reference data available elsewhere. A Micromeritics ASAP 2020 static low-pressure volumetric adsorption unit equipped with turbo molecular drag pumps was used for the porosimetry tests and the gathering of gas adsorption isotherm data. Degassing of the samples was achieved using the instrument’s sample activation module in two stages: (1)
Garcı´a-Ricard and Herna´ndez-Maldonado evacuation at a rate of 50 mmHg/s and unrestricted evacuation below 5 mmHg to reach the degassing pressure (
