pubs.acs.org/Langmuir © 2010 American Chemical Society
Synthesis, Characterization, and Application of Metal Organic Framework Nanostructures Carlos A. Fernandez,† Satish K. Nune,† Radha Kishan Motkuri,† Praveen K. Thallapally,*,† Chongmin Wang,‡ Jun Liu,§ Gregory J. Exarhos,§ and B. Peter McGrail† §
† Energy and Environment Directorate, ‡WR Wiley Environmental Molecular Sciences Laboratory, and Fundamental and Computational Science Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, United States
Received September 7, 2010. Revised Manuscript Received October 6, 2010 The considerable number of important physical properties, including optical, electronic, and magnetic properties, of Prussian blue (PB) analogues have attracted fundamental and industrial interest. Nevertheless, the gas sorption properties of PB coordination compounds were only investigated very recently. In this work, we report the synthesis and gas sorption properties of PB nanocomposites with different size and shape obtained by using poly(vinylpyrrolidone) (PVP), chitosan, and dioctyl sodium sulfosuccinate (AOT) as stabilizers and structure directing agents. All three porous nanocrystals show high and selective CO2 adsorption over CH4 or N2. No distinct relationship was found between the size (or shape) of the nanosorbents and their gas uptake capacities. To our knowledge, this is the first report on the use of PB nanocomposites for CO2 capture applications.
Porous coordination polymers (PCPs) or metal-organic frameworks (MOFs) represent an exciting class of materials with high thermal stability, adjustable chemical functionality, and remarkably high porosity.1-7 The Prussian blue (PB) family exemplifies the earliest known coordination compounds. Their diverse magnetic and electronic properties have stimulated extensive fundamental research activity.8 However, their gas sorption properties were only investigated very recently by us and others.9-16 The Prussian blues of chemical formula M3[M(CN)6]2 3 nH2O are best described as a cubic a-Po type network topology where M2þ and M3þ ions are connected through cyanide bridges that are filled with water molecules. The water molecules can be removed at high temperatures under vacuum to result in a porous framework. In *To whom correspondence should be addressed. E-mail: Praveen.Thallapally@ pnl.gov. Fax: 509- 3717249. Telephone: 509-3717183. (1) Yaghi, O. M.; O’Keeffe, M.; Ockwig, N. W.; Chae, H. K.; Eddaoudi, M.; Kim, J. Nature 2003, 423, 705. (2) Kitaura, R.; Seki, K.; Akiyama, G.; Kitagawa, S. Angew. Chem. Int. Ed. 2003, 42, 428. (3) Dalgarno, S. J.; Thallapally, P. K.; Barbour, L. J.; Atwood, J. L. Chem. Soc. Rev. 2007, 36, 236. (4) Ferey, G.; Mellot-Draznieks, C.; Serre, C.; Millange, F. Acc. Chem. Res. 2005, 38, 217. (5) Zhao, X. B.; Xiao, B.; Fletcher, A. J.; Thomas, K. M.; Bradshaw, D.; Rosseinsky, M. J. Science 2004, 306, 1012. (6) Li, J. R.; Kuppler, R. J.; Zhou, H. C. Chem. Soc. Rev. 2009, 38, 1477. (7) Perry, J. J. t.; Perman, J. A.; Zaworotko, M. J. Chem. Soc. Rev. 2009, 38, 1400. (8) Verdaguer, M.; Bleuzen, A.; Marvaud, V.; Vaissermann, J.; Seuleiman, M.; Desplanches, C.; Scuiller, A.; Train, C.; Garde, R.; Gelly, G.; Lomenech, C.; Rosenman, I.; Veillet, P.; Cartier, C.; Villain, F. Coord. Chem. Rev. 1999, 190-192, 1023. (9) Windisch, C. F., Jr.; Thallapally, P. K.; McGrail, B. P. Spectrochim. Acta, Part A 2010, 77, 287. (10) Beauvais, L. G.; Long, J. R. Inorg. Chem. 2006, 45, 236. (11) Kaye, S. S.; Long, J. R. J. Am. Chem. Soc. 2005, 127, 6506. (12) Culp, J. T.; Matranga, C.; Smith, M.; Bittner, E. W.; Bockrath, B. J. Phys. Chem. B 2006, 110, 8325. (13) Natesakhawat, S.; Culp, J. T.; Matranga, C.; Bockrath, B. J. Phys. Chem. C 2007, 111, 1055. (14) (15) Windisch, C. F., Jr.; Thallapally, P. K.; McGrail, B. P. Spectrochim. Acta, Part A 2009, 74, 629. (16) Thallapally, P. K.; Motkuri, R. K.; Fernandez, C. A.; McGrail, B. P.; Behrooz, G. S. Inorg. Chem. 2010, 49, 4909.
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this regard, the formation of micro- and mesoporous frameworks and deposition of various metallic nanoparticles has been very well documented in the literature.17-20 However, reducing the size of these materials to nanometer scale improves the mass transfer properties compared to the bulk.21,22 In this regard, nanometersized materials including PBs often exhibit remarkable size- and shape-dependent physical and chemical properties, which cannot be observed in their bulk analogues.23-30 For example, PB nanocomposites have been shown to exhibit improved optical switching performance with excellent contrast and switching speeds over bulk inorganic PB films.31 In addition, we have recently demonstrated that inorganic PB analogues can be potentially used for highly specific and efficient CO2 capture and separation applications because the physically adsorbed gas can be removed from the matrix with a small amount of externally applied heat energy. (17) Klein, N.; Senkovska, I.; Gedrich, K.; Stoeck, U.; Henschel, A.; Mueller, U.; Kaskel, S. Angew. Chem., Int. Ed. 2009, 48, 9954. (18) Kishan, M. R.; Tian, J.; Thallapally, P. K.; Fernandez, C. A.; Dalgarno, S. J.; Warren, J. E.; McGrail, B. P.; Atwood, J. L. Chem. Commun. (Cambridge, U.K.) 2010, 46, 538. (19) Muller, M.; Zhang, X.; Wang, Y.; Fischer, R. A. Chem. Commun. (Cambridge, U.K.) 2009, 119. (20) Schroder, F.; Esken, D.; Cokoja, M.; van den Berg, M. W.; Lebedev, O. I.; Van Tendeloo, G.; Walaszek, B.; Buntkowsky, G.; Limbach, H. H.; Chaudret, B.; Fischer, R. A. J. Am. Chem. Soc. 2008, 130, 6119. (21) Nune, S. K.; Thallapally, P. K.; Dohnalkova, A.; Wang, C. M.; Liu, J.; Exarhos, G. J. Chem. Commun. 2010, 46, 4878. (22) Tanaka, D.; Henke, A.; Albrecht, K.; Moeller, M.; Nakagawa, K.; Kitagawa, S.; Groll, J. Nat. Chem. 2010, 2(5), 410. (23) Zhai, J. F.; Zhai, Y. M.; Wang, L.; Dong, S. J. Inorg. Chem. 2008, 47, 7071. (24) Johansson, A.; Widenkvist, E.; Lu, J.; Boman, M.; Jansson, U. Nano Lett. 2005, 5, 1603. (25) Uemura, T.; Kitagawa, S. J. Am. Chem. Soc. 2003, 125, 7814. (26) Uemura, T.; Ohba, M.; Kitagawa, S. Inorg. Chem. 2004, 43, 7339. (27) Qiu, J. D.; Peng, H. Z.; Liang, R. P.; Li, J.; Xia, X. H. Langmuir 2007, 23, 2133. (28) Fiorito, P. A.; Goncales, V. R.; Ponzio, E. A.; de Torresi, S. I. C. Chem. Commun. 2005, 366. (29) Pajerowski, D. M.; Frye, F. A.; Talham, D. R.; Meisel, M. W. New J. Phys. 2007, 9. (30) Gotoh, A.; Uchida, H.; Ishizaki, M.; Satoh, T.; Kaga, S.; Okamoto, S.; Ohta, M.; Sakamoto, M.; Kawamoto, T.; Tanaka, H.; Tokumoto, M.; Hara, S.; Shiozaki, H.; Yamada, M.; Miyake, M.; Kurihara, M. Nanotechnology 2007, 18. (31) DeLongchamp, D. M.; Hammond, P. T. Adv. Funct. Mater. 2004, 14, 224.
Published on Web 10/19/2010
DOI: 10.1021/la103590t
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Figure 1. Schematic illustration of the stabilizers used for the synthesis of PB nanocomposites.
In this work, we focused our attention on the development of PB nanocomposites as effective CO2 sorbent materials for industrial gas streams. We rationalized that the PB nanocomposites should exhibit a relatively high CO2 storage capacity due to the increase in surface area while maintaining the structural characteristics of the original lattice. Herein, we report the synthesis and gas sorption features of PB nanocomposites with different size and shape using polymers such as poly(vinylpyrrolidone) (PVP), chitosan, and dioctyl sodium sulfosuccinate (AOT) (Figure 1) as stabilizers and structure directing agents.32 To our knowledge this is the first report on the use of PB nanocomposites for CO2 capture applications. Cobalt based nano-Prussian blue analogues of chemical formula Co3[Co(CN)6]2 3 H2O were successfully synthesized using PVP (here onward nCoCo1), chitosan (nCoCo2), and AOT (nCoCo3) as stabilizers based on reported procedures for the synthesis of Fe3[Fe(CN)6]2 nanocrystals.33,34 Spherical shaped particles of nCoCo1 (M3þ, Co3þ = 10 mM, PVP/M2þ = 50, with M2þ = Co2þ) were synthesized by dropwise addition of aqueous K3[Co(CN)6] (332 mg, 1 mmol) solution (20 mL) to an aqueous solution (80 mL) of CoNO3 3 6H2O (291 mg, 1 mmol) and PVP (K-30; average Mw = 40 000, 5.50 g, 50 mmol) at room temperature upon vigorous stirring. The cube shaped nCoCo2 material was synthesized by mixing 10 mL of 0.1% chitosan solution containing cobalt nitrate hexahydrate (50.20 mg, 0.17 M) and 10 mL of 0.1% chitosan solution containing K3[Co(CN)6] (32.9 mg, 0.1 M). The resulting colloidal suspension was continuously agitated overnight to obtain nCoCo2. Similarly, nCoCo3 with two different sizes was synthesized as follows. First, two waterin-hexane microemulsions containing the precursors were prepared with a water-to-surfactant molar ratio (W value) of 5. One microemulsion was obtained by mixing 200 mL of hexane with 8.8 g of AOT and 1.8 mL of aqueous 0.3 M K3[Co(CN)6]. The second microemulsion was prepared by the combination of 16 mL of hexane with 0.73 g of Co(AOT)2 3 7H2O (see the Supporting Information) and 128 μL of water. Once the micelle solutions became translucent, dropwise addition of the first microemulsion was added to the second microemulsion while being vigorously agitated. For the smaller size AOT-CoCo nanoparticles, the same procedure was employed but a decreased concentration of K3[Co(CN)6] and Co(AOT)2 3 7H2O precursors to 1/3 was used while adjusting the amount of water and hexane in the second microemulsion to maintain the W value of 5. In all cases, the reaction mixtures turned pink after the first few seconds, indicating the formation of PB nanocomposites. The isolated PB nanocomposites were further purified by either precipitation with absolute ethanol (32) Wang, Y. T.; Zhu, J. Z.; Zhu, R. J.; Zhu, Z. Q.; Lai, Z. S.; Chen, Z. Y. Meas. Sci. Technol. 2003, 14, 831. (33) Ding, Y.; Hu, Y.-L.; Gu, G.; Xia, X.-H. J. Phys. Chem. C 2009, 113, 14838. (34) Vaucher, S.; Li, M.; Mann, S. Angew. Chem., Int. Ed. 2000, 39(10), 1793.
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Figure 2. TEM micrographs of nCoCo1 (left) and nCoCo2 (right); the inset shows a higher magnification image.
(nCoCo1, nCoCo3) or by repeated centrifugation (nCoCo2) to remove excess polymer, Co(III) ions, and any KNO3 that was present from the composite. Transmission electron microscopy (TEM) micrographs of the as-synthesized nCoCo1and nCoCo2 are shown in Figure 2. Images of nCoCo1 reveal the formation of irregular sphericalshaped particles with an average size of 36 ( 5 nm, whereas the chitosan stabilized nCoCo2 resulted in the formation of irregular cubic-shaped nanocrystals with an average size of 30 ( 7 nm. A closer look at higher magnification in nCoCo1 shows what might be a homogeneous distribution of nanopores on the PB nanoparticle. The nanoparticle size distributions of nCoCo1 and nCoCo2 reveal the formation of uniform nanoparticles (Figure 3). The possible mechanism for the particle formation on nCoCo1 includes coordination of the amide moiety of PVP to Co ions during particle nucleation and growth. In this fashion, PVP provides steric stabilization, preventing particle agglomeration. It has also been reported the size-controlled synthesis of Fe3[Fe(CN)6]2 nanocrystals by modifying the PVP/Fe2þ feed ratio.33 The driving force of nanoparticle growth on nCoCo2 comes from the system interior, in other words from the reducing agent provided by the hydrolysis product of chitosan in an acidic condition. Therefore, nCoCo2 nanoparticles with different shapes and sizes can be prepared by controlling the hydrolysis rate Langmuir 2010, 26(24), 18591–18594
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Figure 3. Nanoparticle size distribution of nCoCo1 (A) and nCoCo2 (B) obtained from analysis of at least 300 nanoparticles on multiple TEM micrographs.
Figure 4. TEM micrographs of 5.2 ( 0.6 nm (left) and 1.3 ( 0.3 nm (right) nCoCo3 nanocrystals; the insets show a higher magnification image clearly showing the lattice spacing that corresponds to 0.41 nm and the corresponding transmission electron diffraction (TED) pattern.
of chitosan.34 Figures 4 and 5 show representative TEM images and the corresponding histograms for nCoCo3 with two different sizes, 5.2 ( 0.6 nm and 1.3 ( 0.3 nm, that were obtained by simply varying the concentration of both cobalt precursors, K3[Co(CN)6] (inside the water droplet in one of the microemulsions) and surfactant Co(AOT)2 (in the second microemulsion). The TEM micrographs in Figure 4 of the as-synthesized nanocrystals show 5.2 nm cubic and rhombohedral shapes when higher precursor concentrations were used as compared to the small spherical nanoparticles obtained at lower concentrations. The mechanism of nanoparticle growth on nCoCo3 seems to begin by precursor exchange inside the aqueous nanodroplet increasing the intramicellar supersaturation which generates nucleation and growth of AOTencapsulated nanocrystals.22 Therefore, the reverse micelle acts as a template and is responsible for the final nanoparticle size (and shape) which can be adjusted by changing the size of the nanodroplet (through modification of the W value) or by changing the concentration of precursors inside the droplet as was done here.22 Once all the nano-Prussian blue analogues were synthesized, thermal analysis was performed on nCoCo1-3, which shows sharp weight losses of 28, 23, and 19% (Supporting Information Figures S1-S3) while the weight loss in bulk Co3[Co(CN)6]2 (CoCo) is about 30% between room temperature and 200 °C. The weight loss in PB nanocomposites (nCoCo1-2) corresponds to the removal of water molecules trapped in the crystal lattice during the synthesis of these nanomaterials. In order to know the structural similarity and thermal stability of the synthesized nanomaterials toward bulk, Powder X-ray diffraction (PXRD) measurements were also performed. Identical PXRD patterns of the bulk and nanoform (Supporting Information Figure S4)11 demonstrate the successful synthesis of polymer stabilized nanosized Prussian blue analogues. In addition, variable temperature PXRD under vacuum shows no change in PXRD, indicating the Langmuir 2010, 26(24), 18591–18594
Figure 5. Nanoparticle size distribution of 5.2 nm (left) and 1.3 nm (right) nCoCo3 obtained from analysis of at least 300 nanoparticles on multiple TEM micrographs.
stability of the nanomaterials after solvent removal (see Supporting Information Figure S4). From these measurements, it was not clear whether polymer is still intact after heating the sample. Therefore, TEM analysis of the activated samples dispersed in solution revealed no aggregation and also showed no size difference among the macrographs of the as-synthesized nanomaterials ,indicating that the polymeric protecting shell (PVP, chitosan, or AOT) remains intact. (Supporting Information Figures S5). The apparent specific surface areas for the PB nanocomposites (nCoCo1-3) were determined by using the Brunauer-EmmettTeller (BET) method. Nitrogen adsorption measurements were performed at 77 K. The samples were degassed at 200 °C for 24 h under vacuum prior to the measurements. Type I isotherms were observed with calculated surface areas of 781, 744, and 715 m2/g for nCoCo1 through nCoCo3 (1.3 nm sized particles) (see Supporting Information). These surfaces areas are in overall agreement with the data of bulk CoCo (720 m2/g).8 As described earlier, the size and shape of nCoCo1-3 did not change upon heating the samples to 200 °C, which implies that the ligand shell is stable at these activation temperatures, thereby preventing particle aggregation. These results suggest the synthesized nanoparticles are porous and might find applications in gas storage and separation areas. Therefore, we attempted to measure and compare the gas sorption properties of nano-Prussian blue analogues to that of bulk material. DOI: 10.1021/la103590t
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Figure 6. Low and high pressure CO2 uptake in bulk as well as nCoCo1-3: (0) nCoCo1, () nCoCo2, (4) nCoCo3, and (]) bulk CoCo.
Prior to the sorption, all the nanomaterials were activated for 12 h at 200 °C, similar to the case of bulk material. Using a high pressure volumetric gas analyzer, CO2, N2, and CH4 sorption isotherms were collected at 298 K for all three dehydrated Prussian blue analogues. Figure 6 shows the adsorption and desorption isotherms for all three nCoCo1-3 where the calculated CO2 wt % at room temperature and 1 bar was 8.5, 7.8, and 7.1%, respectively. Similarly, at high pressure, the CO2 wt % at 30 bar was calculated to be 20-24 wt % for nCoCo1-3. The CO2 sorption values obtained for all three porous nanomaterials are comparable to our reported values for bulk CoCo within the experimental error (Figure 6). Also, there is no obvious trend between
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nanoparticle size of these porous Prussian blue analogues and their gas uptake capacity. Similar measurments were performed on other gases including methane and nitrogen at identical conditions. Significantly lower or no uptake was observed for CH4 and N2 in all three porous nanomaterials at both low and high pressures (Supporting Information Figures S9 and 10). From these experiments, it is clear that nCoCo1-3 can preferentially sorb more CO2 over methane at a given pressure, but we did not observe a significant amount of nitrogen at the same conditions. The pore size of nCoCo shows 5 A˚ 7.1 A˚, which is large enough to accommodate all the gases, but the gas sorption isotherms indicate no uptake of nitrogen. This could be explained by considering the quadrupole moment and polarizability of these gases. The higher uptake of CO2 is explained as being due to the higher quadrupole moment and polarizability compared to CH4 and N2, whereas the higher uptake of methane over nitrogen can be explained as being due to the higher polarizability of methane over nitrogen. The similar behavior was observed quite often in our laboratory and in the literature. In conclusion, the successful synthesis and CO2 capture properties of three Prussian blue nanoparticle analogues has been described. All three (PVP, AOT, and chitosan) stabilized PB nanocrystals show high and selective CO2 adsorption over CH4 or N2. At this time, no unambiguous relationship was found between the size (or shape) of the nanosorbents and their gas uptake capacities. Further experiments are in progress to understand the role of surfactants on the formation of spherical and square shaped nanoparticles and the effect of polymer concentration in the process of nucleation and crystal growth. In addition, appropriate choice of the stabilizing ligand may work to enhance CO2 selectivity by providing an impermeable barrier to transport of other gases. Acknowledgment. Synthesis and characterization were supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award KC020105-FWP12152. In addition, portions of the work (gas sorption measurements) were supported by the U.S. Department of Energy, Office of Fossil Energy. The Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy under Contract DE-AC05-76RL01830. Supporting Information Available: Details of thermogravimetric analysis, high temperature PXRD, TEM, and N2 adsorption isotherms. This material is available free of charge via the Internet at http://pubs.acs.org.
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