ARTICLE pubs.acs.org/crystal
Sonochemical Synthesis and Characterization of Submicrometer Crystals of the MetalOrganic Framework Cu[(hfipbb)(H2hfipbb)0.5] Cantwell G. Carson,† Andrew J. Brown,‡ David S. Sholl,†,* and Sankar Nair†,* †
School of Chemical & Biomolecular Engineering and ‡School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States
bS Supporting Information ABSTRACT: The metalorganic framework (MOF) Cu(4,40 -hexafluoroisopropylidenebis-benzoate)1.5 (Cuhfipbb) has been projected as an important new nanoporous material for fabricating membranes with applications in gas separation and CO2 capture, among others. Synthesis of submicrometer crystals of Cuhfipbb, however, is impeded by several factors, including the extreme hydrophobicity of the hfipbb ligand. We report a fast synthesis of submicrometer particles of Cuhfipbb via a sonochemical technique, at temperatures as low as 0 °C, and with the addition of 2-propanol to control the particle morphology. The particles were characterized by powder X-ray diffraction, thermogravimetry, light scattering, and electron microscopy to ascertain the effects of synthesis parameters on the size distribution, structure, and morphology. The presence of a small amount of 2-propanol substantially alters the particle morphology from needles to a more isotropic shape. The Cuhfipbb particles produced by this approach are suitable for use in applications involving fabrication of membranes and thin films.
’ INTRODUCTION Metalorganic frameworks (MOFs) denote a class of crystalline solids in which metal cations are connected by multifunctional organic ligands to form extended macromolecular structures in one, two, or three dimensions.1 Variations in the metal cation and organic ligand have resulted in the creation of a very large number of MOFs, many of which are expected to have useful properties in practical applications. In this paper, we focus on the MOF Cu(4,40 -hexafluoroisopropylidene-bis-benzoate)1.5. Cu[(hfipbb)(H2hfipbb)0.5],2 abbreviated below as Cuhfipbb, is promising as a nanoporous material for a variety of chemical separations applications. For example, this MOF was experimentally shown to preferentially adsorb n-butane over pentane3 and computationally predicted to be highly effective in gas separations such as CO2/CH4.4 Recently, Watanabe et al. used quantum chemistry and molecular modeling to predict the adsorption and transport properties of CO2 and CH4 in Cuhfipbb.5 While both molecules can adsorb in the MOF, the diffusivity of CO2 through the MOF’s one-dimensional pores was predicted to be several orders of magnitude faster than the diffusivity of CH4, leading to an unusually high overall selectivity (>1000) for CO2 over CH4.68 The above investigations show that Cuhfipbb has a unique combination of structural properties that leads to attractive CO2 and hydrocarbon adsorption and transport properties. The technological implications5,9 of these properties make Cuhfipbb an excellent candidate material for fabrication of membrane-based separation devices. These are also the reasons that inorganic nanoporous (zeolite) materials and membranes have attracted considerable attention for gas separation applications r 2011 American Chemical Society
(e.g., CO2/CH4).1013 However, a number of challenges exist in the practical fabrication of membranes that incorporate MOFs. One possible method is to grow a dense MOF film from an initial seed layer of submicrometer MOF particles on a porous support using the same concept as in the large literature on zeolite membranes.14 MOF membranes of this kind have been reported using Cu3BTC2 and MOF-5, but they do not show high selectivities for gas separations.1517 These low selectivities are consistent with theoretical models for these materials.18 Recently, a Cuhfipbb membrane was synthesized by modifying the procedures used in initial syntheses of Cuhfipbb powder samples.19 The performance of this membrane for CO2/N2 separations was examined, but both the gas flux and the selectivity were low. It appears likely that this outcome was related to morphological control of the MOF crystals and the intergrowth of these crystals during membrane synthesis, rather than intrinsic properties of Cuhfipbb. Similar issues are well known for zeolite membranes, and extensive efforts to grow membranes via “secondary growth” (i.e., beginning from submicrometer “seed crystal” layers deposited on supports) have allowed these challenges to be overcome in several cases.19,20 An alternative route is the fabrication of composite (“mixed matrix”) membranes by incorporation of submicrometer MOF particles into polymeric membranes. Relative to pure MOF films, mixed matrix membranes have the important advantage that they can be envisioned as an extension of the commercial technology that Received: June 9, 2011 Revised: August 1, 2011 Published: August 16, 2011 4505
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Crystal Growth & Design is already widely applied to produce polymeric membranes.21,22 Mixed matrix membranes have been investigated for CuBDC,23 MOF-5,24 Cu3BTC2, Mn(HCOO)2,25 and {[Cu2(PF)6(NO3)(4,4-bpy)4] 3 2PF6 3 2H2O}n26 using crystals of these materials incorporated in polymeric films. These initial studies suggest that achieving good interfacial contact between MOF particles and polymer matrices is relatively straightforward in comparison to polymer/inorganic nanocomposites, where this issue has been problematic for the development of mixed matrix membranes using zeolites.27 In order to fabricate either MOF films or MOF-containing mixed matrix membranes, it is vital to produce submicrometer crystals of the MOF of interest. In the former application, it is desirable to use seed particles that are smaller than the final thickness (ideally ∼15 μm) of the dense film grown by secondary growth. For mixed matrix membranes, the MOF particle size must be smaller than the thickness of the polymeric films used as the membrane (typically less than 1 μm in commercial hollow fiber polymeric membranes). In both applications, it is also desirable to control the particle morphology. In mixed matrix membrane applications, for example, anisotropic particles would lead to alignment of particles due to the flow fields associated with producing hollow fiber membranes. This situation can be avoided if more isotropic particles are used. In comparison to the literature on the synthesis of large MOF crystals, much less attention has been paid to controlled synthesis of submicrometer MOF crystals. The synthesis of Cuhfipbb has several unusual features in comparison to most other MOFs. First, the control of its previously reported hydrothermal synthesis is impeded by the marked hydrophobicity of one of the main reactants, H2hfipbb. Only above 90 °C does one observe a solubility of H2hfipbb high enough to form a clear aqueous solution. Second, the MOF formation requires a protonated H2hfipbb ligand. Most MOF syntheses rely on the use of a base to deprotonate a carboxylic acid, which then becomes incorporated into the MOF and creates a basic environment inside the MOF. As a result, attempts to solvothermally synthesize Cuhfipbb in an aprotic solvent (such as N,N-dimethylformamide) are unsuccessful. These two factors precluded the use of conventional methods to synthesize submicrometer Cuhfipbb.28 In this paper, we report a fast sonochemically aided method and use of a morphology control additive to synthesize submicrometer Cuhfipbb particles. Sonochemical synthesis of MOFs is potentially attractive because of its capability to supply controlled amounts of energy to the solution and influence reactivity via microscopic pressure and temperature fluctuations.29 It has been used recently to synthesize submicrometer-sized particles of Cu3BTC2,30 MOF-2,31 and other MOF structures.3234 In the present case, evidence of Cuhfipbb formation (by X-ray diffraction) could be found after only 30 s of room-temperature sonication, at which conditions H2hfipbb is still extremely hydrophobic. Previous studies have documented the effect of sonication time on MOF particle size and yield. However, we could find no studies investigating the effect of other important experimental parameters on MOF yield and particle morphology. Accordingly, we also studied the effect of temperature, sonication power, and solvent composition on the resulting size, structure, yield, and CO2 uptake of the product.
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Table 1. Summary of Synthesis Conditions for the 4 Samples of Cu[(hfipbb)(H2hfipbb)0.5] A
B
C
D
synthesis time (h)
6
6
6
1
temperature (°C) percent 2-propanol
0 1.2
0 0
0 1.2
90 1.2
sonicating power (W)
91
91
26
91
’ EXPERIMENTAL SECTION Reagents were used as purchased. 4,40 -(Hexafluoroisopropylidene)bis(benzoic acid) (98%), copper(II) nitrate hemi(pentahydrate) (98%), and 2-propanol (99.5%) were purchased from Sigma-Aldrich. To synthesize Cu[(hfipbb)(H2hfipbb)0.5], 200 mg of H2hfipbb was dispersed in 75 mL of deionized water by shaking and mild sonication. In a separate container, 79 mg of copper(II) nitrate hemi(pentahydrate) was dissolved in 5 mL of water. The molar ratio of copper nitrate to H2hfipbb to water was 1:1.5:12 900. The copper nitrate solution was then added to the H2hfipbb suspension and sonicated as follows. All syntheses were sonicated at intervals of 1 s on and 1 s off. Sonication was carried out with a Sonics Vibracell VCX 130 source equipped with a model CV 18 horn operating at 20 kHz. Our detailed parametric studies are illustrated by four representative experimental conditions (labeled AD and summarized in Table 1). For reaction conditions A, C, and D, 1 mL of 2-propanol was added. For reactions A, B, and D, the sonicating power was set at 77 W, and for reaction C it was set at 22 W. For syntheses A, B, and C, the reaction vessel was maintained inside a 3 L block of ice to keep the reaction temperature close to 0 °C for the 6.6 h duration of the experiment. For synthesis D, the reaction container was placed on a hot plate and heated to 90 °C prior to and during the synthesis for 1 h. After synthesis, all samples were centrifuged and washed with 2-propanol to remove excess H2hfipbb and then in water. For purposes of comparison, Cuhfipbb was also synthesized by the previously published hydrothermal method.2 The synthesis conditions shown in X-ray diffraction data were collected on a well-aligned PANalytical X’Pert Pro MPD with Cu Kα radiation (45 kV, 40 mA) and an X’Celerator detector. Pawley fits of the unit cell were carried out using the Accelrys package in Materials Studio. Scanning electron microscopy (SEM) images were collected on a LEO-1530 operating at 10 kV. Light-scattering measurements were made on a Brookhaven Instruments BI-APD detector equipped with a 632.8 nm/75 mW laser using samples that were dispersed in hexane and with oleic acid added as a surfactant. All samples were filtered through a 5 μm filter prior to DLS measurements. Thermogravimetric measurements were made on a Netzch STA 449 F1 Jupiter with a Aeolos QMS 403 C mass spectrometer with a heating rate of 10 °C/min. CO2 adsorption was carried out on a Hiden-Isochema Intelligent Gravimetric Analyzer at 25 °C after activation for 4 h under vacuum at 200 °C.
’ RESULTS AND DISCUSSION Scanning electron micrographs of samples from experimental conditions AD are shown in Figure 1. Sample A is the representative result that clearly shows formation of submicrometer particles of Cuhfipbb. The powder X-ray diffraction patterns from all four conditions AD (Figure 2) show that Cuhfipbb is produced. Only sample A, however, consists of pure Cuhfipbb; samples BD also contain a small amount of impurity phase associated with XRD peaks at 6.6°, 8.7°, and 11.5° 2θ. Although the assignment of these impurity peaks is currently unclear, it is worth noting that the impurity phase can also be synthesized by using the reaction conditions for sample A with stirring and no 4506
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Figure 1. SEM images of samples AD of Cu[(hfipbb)(H2hfipbb)0.5] crystals. Synthesis conditions are summarized in Table 1. The scale bar in B differs from those in A, C, and D.
sonochemical energy input. The yield of Cuhfipbb in sample A was approximately 11% (Table 2). The yield for all syntheses was lower in comparison to other reports of sonochemically synthesized MOFs31 and of conventionally synthesized Cuhfipbb.2 A number of factors may contribute to this reduced yield. A stoichiometric ratio of metal to ligand (1:1.5) was used, as opposed to other syntheses that used up to a 4-fold excess of H2hfipbb. In addition, the lack of solubility of H2hfipbb in water meant that the amount of reactant in solution was low in comparison to sonochemical syntheses of other MOFs in which the ligand is dissolved in a suitable solvent. Sample D, synthesized at an elevated temperature, produced a yield of 40%, comparable to that of hydrothermal syntheses. For the other samples, use of low temperature was designed to reduce the growth rate of the crystallites after crystal nucleation. Initially, 2-propanol was added to our syntheses to increase the solubility of H2hfipbb in water, as this might result in a higher synthesis yield, but subsequent microscopy of the particles revealed changes in the morphology. The presence of 2-propanol in the synthesis mixture had the effect of converting long needles into more isotropic particles (Figure 1). In the absence of 2-propanol, the needle-shaped particles in sample B appear to have their long axis parallel to the substrate surface. In order to
Table 2. Summary of Results from Dynamic Light-Scattering Measurements A
B
C
D
percent yield (%)
11
7
7
average size (nm)
660
692
794
40 571; 1713a
standard deviation (nm) scattering % < 1 μm
84 88
89 80
140 80
172; 589a 49b
a
The average size and standard deviation of both distributions of sample D are shown, as they were comparable in the sample fraction. b Much of the sample appeared to have been trapped in the 5 μm filter, and the true yield of submicrometer particles may be lower.
determine the crystallographic direction corresponding to the long axis, we carried out a Rietveld fitting of an X-ray diffraction pattern from a sample of the needle-like particles aligned with the long axis parallel to a substrate (Supporting Information). The results of the Rietveld fit, including the MarchDollase preferred orientation parameters, show that the long axis of the needles corresponds to the [010] direction, which is also the direction of the onedimensional pores in the MOF (Figure S1, Supporting Information). Rietveld refinement of the X-ray diffraction patterns 4507
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Figure 3. Crystal structure of Cuhfipbb, showing the nanopores in the [010] direction: white atoms, hydrogen; gray, carbon; red, oxygen; green, fluorine; brown, copper.
Figure 2. X-ray powder diffraction patterns for samples AD, and predicted powder pattern of Cuhfipbb based upon the crystal structure. Samples BD also have peaks at 6.6°, 7.8°, and 11.5° 2θ, denoted by an asterisk (*), indicating the presence of an additional phase not present in Cuhfipbb.
of samples A, C, and D was unable to resolve the direction of the channels using the preferred orientation effect, as was successfully carried out for the needle-like sample B. This is expected in the case of sample A wherein the crystals have an isotropic appearance. However, we were unable to prepare samples of materials C and D with a large degree of orientational anisotropy. This observation is consistent with the particle morphology predicted by BFDH crystal morphology theory and with a previous pole figure analysis.19,35 Of the three principal directions of the Cuhfipbb structure (Figure 3), the (010) planes have the smallest dhkl spacing (7.271 Å), and are therefore predicted by BFDH theory to grow the fastest. On the other hand, addition of 2-propanol leads to formation of more isotropic particles which do not show any clear preferred orientation effects. This change in morphology could be caused by competition for the apical site on Cu(II) between the OH group of 2-propanol and the protonated carboxylic acid group on H2hfipbb. This may be similar to the effect of methanol on the conventional hydrothermal synthesis, as demonstrated by Ranjan et al.19 Such a method of control of over particle morphology will be useful in a variety of applications of this MOF and most immediately for membrane fabrication. For example, isotropic particles with a significant fraction of the surface area containing nanopore openings are more suitable for use as minority components in mixed matrix membranes than needle-like particles. Similarly, more isotropic particles are likely to be well suited for use as seed crystals for fabrication of dense MOF films by secondary growth. Figure 1 also illustrates the effect of synthesis temperature on the particle morphology. Sample D, synthesized at 90 °C under
reflux conditions, yields large crystals. This is notable because, unlike the other samples, reaction D was only sonicated for 1 h. By this time, the product was depositing on the sides of the container and was no longer suspended in solution. In previously reported sonochemical syntheses, the sample is allowed to heat up under the power of the sonicating horn.31 This approach can be expected to result in a wide distribution of particle sizes, which is often undesirable. To measure the particle size distribution from our syntheses, dynamic light-scattering (DLS) measurements were made. DLS autocorrelation spectra (Figure 4) were collected from samples that had been filtered through a 5 μm filter. These spectra were fitted with a maximum entropy fit using log-normal distributions for the particle size distributions. The final fit parameters are shown in Table 2, and the fitted DLS autocorrelation functions are also shown in Figure 4. Sample A shows a high percentage of submicrometer crystals, with an average size of 660 nm and a narrow standard deviation of 84 nm. This average particle size and quite narrow size distribution makes the present Cuhfipbb particles well suited for use in membrane fabrication applications. Figure 5 shows a comparison of the thermogravimetric measurements between sonochemically synthesized sample A and the hydrothermal synthesis of Pan et al.2 As expected, the hydrothermal synthesis showed no mass loss before 330 °C, but sample A has a 5% mass loss around 190 °C. Mass spectrometry traces of m/z ratios of 43 and 45 during the TGA measurement for sample A showed that the mass loss could be attributed to residual 2-propanol. There was no change in the m/z ratio of 18, indicating that the mass loss did not come from water in the framework. Furthermore, the mass loss due to solvent evacuation can be reduced by washing the crystals in DMF and then in DI water. The hydrophobicity of the pore indicates that this would be a good candidate for use in humid environments. The effect of the thermally activated desorption of 2-propanol on the Cu hfipbb framework was also monitored via in situ high-temperature powder X-ray diffraction (Figure S2, Supporting Informatio). Heating sample A and the hydrothermally synthesized sample from room temperature to 250 °C led to a unit cell volume increase of only 0.4% and 1.3%, respectively. Beyond this, no phase change or structural degradation is evident below 250 °C. 4508
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Figure 4. Dynamic light-scattering autocorrelation functions for dispersions of the four samples AD in hexane. The predicted autocorrelation functions after the maximum entropy fits described in the text are also shown as solid lines. Also shown are the predicted particle size distributions based on the fits to the autocorrelation functions. The final parameters are reported in Table 2.
Figure 5. Thermogravimetric data for sample A (O) and for the hydrothermally synthesized sample (b) are shown on the left axis. Mass spectrometry profiles of sample A for m/z ratios of 18 ( 3 3 ), 43 ( 3 ), 44 ( 3 3 3 ), and 45(—) are also shown on the right axis.
Figure 6. CO2 adsorption isotherms at 25 °C for sample A (O) and the hydrothermally synthesized Cuhfipbb (b), and the data of Bao et al. () from their hydrothermally synthesized sample.36
The use of sonochemical synthesis does not appear to reduce the porosity of the structure, as demonstrated by the CO2 adsorption isotherms taken at 25 °C shown in Figure 6. The isotherms demonstrate that rather than sacrificing performance for a reduction in particle size the use of sonochemical methods produces a material with slightly enhanced CO2 adsorption, from 1.0 molecules of CO2 per unit cell (0.76 mmol/g) for the hydrothermally synthesized sample to 1.1 molecules per unit cell (0.87 mmol/g) for sample A at 10 bar. Figure 6 also shows that the adsorption isotherm recently reported by Bao et al.36 for CO2 in their hydrothermally synthesized Cuhfipbb is in good agreement with the adsorption isotherm of sample A in the pressure range of 01 bar. The present data clearly show that the sonochemically synthesized Cuhfipbb material has high porosity and exhibits the CO2 adsorption characteristics expected from
Cuhfipbb; however, from the limited data it is not clear whether the slight difference in CO2 uptake is due to the increased external surface area of the submicrometer particles or to a slight difference in the crystal structure of the Cuhfipbb material arising from the hydrothermal synthesis used in this work.
’ CONCLUSIONS A sonochemical synthesis method for Cu[(hfipbb)(H2hfipbb)0.5] (Cuhfipbb) has been described that results in a large fraction of submicrometer particles with high phase purity and significant yield. The effects of important synthetic parameters have been studied. The growth rate appears to be controlled by the temperature, with higher temperatures resulting in a larger distribution of particle sizes. The phase purity of the crystal structure 4509
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Crystal Growth & Design appears to be affected primarily by the sonicating power but also by the presence of elevated temperatures during sonication. The particle geometry is influenced by the presence of 2-propanol, which acts as a “crystal-shape-directing” agent. In the absence of 2-propanol, Cuhfipbb grows as long needle-like particles, but more isotropic particles are formed upon addition of 1.2% 2-propanol. The yields from the present method are comparable with those from hydrothermal synthesis on the basis of the amount of ligand used. The porosity and CO2 uptake capacity of the sample is not reduced by sonochemical synthesis. Due to their submicrometer size and more isotropic shape (with a significant fraction of surface area exposing the nanopores), the present particles of Cuhfipbb are suitable for use in membrane applications for separation of gas and hydrocarbon mixtures.
’ ASSOCIATED CONTENT
bS
Supporting Information. Preferred orientation analysis for sample B, and X-ray diffraction patterns for sample A during high-temperature treatment. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (S.N.); david.sholl@chbe. gatech.edu (D.S.S.).
’ ACKNOWLEDGMENT This work was partially supported by ConocoPhillips Co. and the National Science Foundation (#0966582). The authors thank Prof. K. S. Walton (Georgia Tech) for use of the Intelligent Gravimetric Analyzer (IGA).
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