CO2 Adsorption of Metal Organic Framework Material

CO2 Adsorption of Metal Organic Framework Material Cu-BTC via Different Preparation RoutesMei Mei Peng, Dae Kyung Kim, Abidov Aziz, Kyung Ran Back, Ung Jin Jeon, and Hyun Tae Jang*Chemical Engineering Department, Hanseo University, Seosan 356-706, South Korea htjang@hanseo.ac.krAbstract. In this work, Cu3(BTC)2 (Cu-BTC) metal organic framework (MOF), also known as HKUST-1, was prepared by 3 different synthetic route. The synthesized Cu-BTC materials were characterized by powder X-ray diffraction (XRD) for phase structure, scanning electron microscopy (SEM) for crystal structure, thermogravimetric analysis (TGA) for thermal stability, and nitrogen adsorption-desorption for pore textural structure. The results showed that the sample synthesized by microwave method has the high surface area and pore volume of 1721 m2g-1 and 0.7424 cm3g-1, respectively. Among the three synthetic methods, the sample synthesized by microwave method exhibited a CO2 uptake of 23.3% at 25 °C and ambient pressure. Keywords: MOFs, Cu-BTC, CO2 adsorption, porous material.1IntroductionMetal-organic frameworks (MOFs) have been of considerable interest in recent years because of their novel three dimensional porous architectures. As a new class of nanoporous materials MOF have many potential advantages over the traditional adsorbents, such as well-defined structures, controlled pore size, high thermal and chemical stability, high surface area, low density, desired chemical functional groups [1-5]. Applications of MOFs in gas storage, gas/vapor separation, size-, shape-, and enantio-selective catalysis, luminescent and fluorescent materials, and drug storage and delivery have been explored [6]. The attractive properties of MOFs make them promising materials for gas separation and storage [7-12]. MOF-210 synthesized by Yaghi et al. currently holds the record breaking CO2 storage of 2400 mg/g of MOF at room temperature and at ca. 50 bar [13], the surface area and pore volume of this MOF is 6240 m2g-1 and 3.6 cm3g-1, this is the highest surface area reported for any crystalline material. Like this MOF-177 also have highest CO2 adsorption capacity of about 33.5 mmol g-1 at 32 bar [14]. Cu3(BTC)2, (Cu-BTC) also known as HKUST-1 is a widely studied MOF, which was first reported by Chui et al. [15]. Cu-BTC is a typical coordination framework*Corresponding author.T.-h. Kim et al. (Eds.): MAS/ASNT 2012, CCIS 341, pp. 244–251, 2012. © Springer-Verlag Berlin Heidelberg 2012CO2 Adsorption of Metal Organic Framework Material Cu-BTC245with open metal sites, this unsaturated metal centers exhibits a high CO2 adsorption capacity, this open metal sites can participate in the binding of CO2 directly. Bordiga et al. investigated the CO2 adsorption in Cu-BTC by IR spectroscopy [16]. Their results showed that the coordinatively unsaturated Cu2+ centers in this MOF act as specific interaction sites and play an important role in CO2 adsorption. The mechanism of adsorption can be described by CO2 coordination to the metal center in an end-on fashion, i.e., O=C=O···Cu2+. CO2 adsorption of Cu-BTC has been reported by several groups [9, 14, 17], among which the highest CO2 uptake capacity has been reported as 12.7 mmol g-1 at 15 bar and ambient temperature [17]. Based on the previous reports, in the present study, Cu-BTC was synthesized in 3 different methods and studied for its CO2 adsorption performance at ambient temperature and pressure.22.1ExperimentalChemicalsCopper (II) nitrate trihydrate [Cu(NO3)2·3H2O, Daejung chemicals & metals, 99%], benzene-1,3,5-tricarboxylic acid (BTC, Sigma Aldrich, C9H6O6, 95%). The solvents, ethyl alcohol anhydrous (C2H5OH, Daejung chemicals & metals 99.9%), N,Ndimethylformamide [DMF, HCON(CH3)2, Daejung chemicals & metals 99.5%], chloroform [CHCl3, Daejung chemicals & metals 99.5%], deionized (DI) water were used in the synthesis. 2.2 Synthesis Procedures for Cu-BTC with Different Methods2.2.1 Solvothermal Method (Cu-BTC-ST1) Cu(NO3)2·3H2O (1.75 g, 7.2 mmol) was dissolved into 24 ml DI water, and BTC (0.84 g, 4 mmol) was dissolved into 24 ml ethanol under stirring at room temperature. Then the copper solution was added to the BTC solution and stirred for 1 h. The mixture was transferred to a Teflon lined stainless steel autoclave and kept at 120 °C for 12 h. The reaction vessel was cooled to ambient temperature, centrifuged and the solid product was vacuum dried at room temperature to yield sample, CuBTC-ST1 [18]. 2.2.2 Solvothermal Method (Cu-BTC-ST2) Cu-BTC-ST2 was synthesized following the procedure reported by Xiang et al. [19]. BTC (2.5 g, 12 mmol) and Cu(NO3)2·3H2O (5 g, 20.7 mmol) were added in a 250 ml bottle in 125 ml solvent consisting of DMF, ethanol, and DI water with a ratio of 1:1:1, after sonication for 15 min, the bottle was placed in an oven at 85 °C for 20 h. The product was isolated by filtration and rinsed with 3x10 ml DMF, and then immersed in CHCl3 for 3 days, during which the activation solvent was decanted and replaced freshly thrice. The product was dried at 200 °C in vacuum to yield Cu-BTCST2.246M.M. Peng et al.2.2.3 Microwave Method (Cu-BTC-MW) Cu-BTC-MW was synthesized following the procedure reported by Xiang et al. [19]. BTC (2.5 g, 12 mmol) and Cu(NO3)2·3H2O (5 g, 20.7 mmol) were added in a 500 ml beaker in 125 ml solvent containing DMF, ethanol, and DI water with a ratio of 1:1:1. After sonication for 15 min, the beaker was then placed inside household microwave oven (Goldstar, MH-700H) and heated at 700 W for 4 min, during which the beaker was put out from microwave oven for a little while with slight stir every 30 s to avoid explosion. A blue suspension was obtained. The product was isolated and then rinsed with 3x10 ml DMF. After exchanging with CHCl3 for three days, the product was dried at 200 °C in vacuum to yield Cu-BTC-MW. 2.3 CharacterizationPowder X-ray diffraction (XRD) patterns were recorded using a Rigaku D/Max 2200+Ultima diffractometer with Cu-Kα radiation (λ=0.154 nm). The diffraction data were recorded in the 2θ range 5-50° with a step of 0.02 °/s. Thermogravimetric analysis (TGA) was performed by Scinco TGA N-1000 thermo gravimetric (TG) analyzer, the sample was heated from room temperature to 800 °C under N2 at a temperature ramp of 10 °C/min. The nitrogen adsorption-desorption isotherms were measured at 77 K on a Belsorp mini II volumetric adsorption analyzer. Prior to each adsorption measurement the samples were evacuated at 200 °C under vacuum (p<10-5 mbar) for 6 h in the degassing port. The surface area (SBET) was determined from the linear part of the Brunauer-Emmett-Teller (BET) equation, and the pore volume was calculated using a BET plot based on the amount of nitrogen gas adsorbed at the last adsorption point (p/p0 = 0.99) and the pore size distribution using the Barrett-JoynerHalenda (BJH) method. SEM images were captured on JEOL JSM 5600 scanning electron microscope. 2.4 CO2 AdsorptionCO2 adsorption-desorption measurements for Cu-BTC MOFs were performed using Scinco TGA N-1000 thermo gravimetric (TG) analyzer. A sample weight of approximately 10 mg was loaded into an alumina sample pan in a TG unit and tested for CO2 adsorption-desorption performance. The initial activation of the samples was carried out at 150 °C for 1 h in a nitrogen atmosphere. Then adsorption run was conducted using CO2 gas (99.99%), and the desorption run was conducted in N2 flow. The adsorption runs were conducted at 25, 50, 75 °C under atmospheric pressure, and desorption was determined at 150 °C.33.1Results and DiscussionsCharacterizationFig.1 A. shows the XRD patterns of Cu-BTC sample prepared by solvothermal and microwave methods. The XRD patterns of the Cu-BTC obtained by the three methodsCO2 Ads sorption of Metal Organic Framework Material Cu-BTC247were almost similar with ea other, and coincided closely with the Cu-BTC repor ach rted in literature [18]. It indicate that the three samples synthesized by different meth ed hods have the same crystal stru ucture. The SEM images of the synthesized samples are presented in Fig. 1 B. Th showed octahedral morphology [20]. The crystals of hey s samples synthesized by th solvothermal method were almost similar while the he sample synthesized by m microwave method was smaller in size which was in e accordance with the previou report [19]. Smaller dimensions of the latter were due to us homogeneous effect. More seeds were formed in a short time, leading to a li e ittle chance for the seeds to grow up while using microwave. wFig. 1. A. XRD patterns of Cu u-BTC prepared with different methods.(a) Cu-BTC-ST1, (b) CuBTC-ST2 and (c) Cu-BTC-MW B. SEM images of Cu-BTC prepared with different meth W. hods. (a) Cu-BTC-ST1, (b) Cu-BT TC-ST2, (c) Cu-BTC-MW and (d) Cu-BTC-MW at 10, ,000 magnification.The nitrogen adsorption n-desorption isotherms of Cu-BTC samples prepared by different methods are show in Fig. 2. All of them displayed Type I isotherm a wn m, signature characteristic of m microporous materials. The surface area and pore volu ume Cu-BTC-MW was found to be 1721 m2g-1 and 0.7424 cm3g-1 respectively, which w o was higher than Cu-BTC-ST1 a Cu-BTC-ST2 (Table 1). The results are in line with the and previous reports [19]. The s surface area and pore volume of Cu-BTC-MW was alm most nearer to value reported by Millward and Yaghi [14]. The thermograms of the synthesized samples are presented in Fig. 3. Prior to e r ater analysis, the samples were preheated in a vacuum oven remove the physisorbed wa and gases. Initial weight loss below 150 °C is attributed to desorption of the coordinated water. A weigh loss of about 5.2 and 3.4 % was observed for Cu-BT ht TCST 1 and 2 while a loss of about 7.6% was noted for Cu-BTC-MW, hence the la f atter retains more amount of water. The second weight loss around above 300 °C was due to the decomposition of th organic network. Cu-BTC-MW was found to be sta he able upto 350 °C.248M.M. Peng et al.Fig. 2. N2 adsorption-desorption isotherms of Cu-BTC prepared with different methods Table 1. Porosity properties of Cu-BTC prepared with different methodsSample Cu-BTC-ST1 Cu-BTC-ST2 Cu-BTC-MW 3.2 CO2 AdsorptionSBET (m2g-1)1465.0 1656.8 1721.0Total pore volume (cm3g-1)0.6079 0.7130 0.7424Average pore diameter (nm)1.6600 1.6571 1.7924The CO2 adsorption and desorption profiles of Cu-BTC samples are presented in Fig. 4, the CO2 adsorption capacity of the three samples decreased with increase in temperature, which reveals that the CO2 adsorption in Cu-BTC was mainly by physical adsorption. High CO2 uptake was exhibited by Cu-BTC-MW of about 23.3 wt% while for Cu-BTC-ST 1 and 2 exhibited 16.7 and 22.3 wt% respectively, at 25 °C. Hence, Cu-BTC-MW is better than other as it showed high adsorption at room temperature (25 °C) and atmospheric pressure The repeatability of Cu-BTC-MW was verified by carrying four consecutive CO2 adsorption-desorption runs at 25 °C (adsorption) and 150 °C (desorption) with pure CO2 (99.99%) feed gas. No apparent change in CO2 uptake was observed (Fig. 5). This result suggests that the Cu-BTC-MW is reusable.CO2 Ads sorption of Metal Organic Framework Material Cu-BTC249Fig. 3. TGA cu urves of Cu-BTC prepared with different methodsFig. 4. CO2 adsorption-deso orption profiles of Cu-BTC prepared with different methods. (a) Cu-BTC-ST1, (b) Cu-BTC-ST2 and (c) Cu-BTC-MW.250M.M. Peng et al.Fig. 5. Recycla ability of CO2 adsorption at 25 °C of Cu-BTC-MW4ConclusionCu-BTC crystals were sy ynthesized under solvothermal and microwave assis sted solvothermal conditions. Compared to the 2 solvothermal methods, Cu-B BTC ave ume synthesized under microwa condition possessed high surface area and pore volu of 1721 m2g-1 and 0.7424 c 3g-1. The CO2 uptake of Cu-BTC-MW was found to be cm o about 23.3 wt% at 25 °C a ambient pressure. So from this study, it is concluded and that, for the synthesis of C Cu-BTC, microwave method is a better than others a it as requires short crystallizati ion time and also exhibit high CO2 uptake at ro oom temperature and pressure. Acknowledgements. We a acknowledge the financial support by grants from Ko orea CCS R&D center, founded by the Ministry of Education, Science and Technology of y the Korean Government.References1. Banerjee, R., Furukawa, H Britt, D., Knobler, C., O’Keeffe, Y.O.M.: Control of Pore S H., Size and Functionality in Isore eticular Zeolitic Imidazol M. ate Frameworks and Their Car rbon Dioxide Selective Capture Properties. J. Am.Chem. Soc. 131, 3875–3877 (2009) e 2. 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