Physical Chemistry Communications, Volume 3 Issue 1, April 2016 www.bacpl.org/j/pcc
Construction of Hierarchical Pores Membrane by Growing Metal Organic Frameworks on Copper Foams Wen Zhang*1, Liqiang Zhang1, Hui Wang2, Zhiguo Qu2 1 School of Science, Xiʹan Jiaotong University, Xiʹan, Shaanxi 710049, China 2 MOE Key Laboratory of Thermo‐Fluid Science and Engineering, School of Energy and Power Engineering, Xiʹan Jiaotong University, Xiʹan, Shaanxi 710049, China *1
zhangwen@mail.xjtu.edu.cn.edu
Abstract A series of hierarchical pores Cu–BTC membrane was obtained by electrochemical method when copper foams with different pore‐density were used as the electrodes. Scanning electron microscope (SEM) and X‐ray diffraction (XRD) were used to characterize the morphology and structure of the coating; while their specific surface areas were obtained by nitrogen adsorption. Key factors in the process of electrochemical synthesis such as current intensity, temperature, time and ligand concentration were investigated systematically to find out the optimal conditions based on synthesis yield and morphology of the samples. The mechanisms of Cu‐BTC formed on the surface of copper foams were discussed. The effective thermal conductivity of Cu‐BTC/Cu‐Foam film was higher than that of pure Cu‐BTC. This paper provides a simple and reproducible way to construct hierarchical pores membrane for potential industrial applications on separation and adsorption. Keywords Metal Organic Frameworks; Membrane; Electrochemical Method; Cu‐BTC
Introduction Metal organic frameworks (MOFs) have attracted a lot of attention in recent years because of their high surface areas, regular pore sizes and pore shapes, and potential for functionalization. They have many applications in different fields such as gas adsorption[1], catalysis[2] and sensors[3]. Besides, using as powder, many of the applications require the MOFs to be deposited on various surface, such as membranes for gas adsorption, thin film in luminescent sensors or microelectronic devices[4]. However, these requirements have some difficulties in that most MOFs are made as brittle crystals or insoluble powders that cannot be directly applied to general surface‐ processing techniques. In addition, the thermal conductivity of the powder is so low that the structure of the MOFs might be collapse when it is used for gas adsorption since the adsorption for gas is exothermic reaction[5]. To overcome these difficulties, membranes or thin‐films growth techniques, which are initially developed for zeolites and molecular materials such as seeded and epitaxial growth, have also been applied to MOFs recently[6]. While the central atom of the MOFs is the metal ions, the electrochemical synthesis method have enjoyed tremendous popularity in recent years. The principle of the electrochemical synthesis relies on supplying the metal ion by anodic dissolution in a synthesis mixture that contains the organic linker and an electrolyte. The metal ion is not supplied as salt but by oxidation of the electrode. The energy required to oxidize the anode can be supplied in amperometric or potentiometric mode. This experiment employed the amperometric mode, that is to say, the voltage is fixed and current can be used to measure the reaction rate expressed as the speed at which the metal ions are dissolved. A general advantage of electrochemical synthesis is that it allows synthesis under a milder condition than typical solvothermal or microwave synthesis. It is performed under atmospheric pressure and relatively low temperature. It also reduces the time required for certain synthesis: whereas solvothermal or microwave synthesis might take hours or days, electrochemical synthesis can produce the material within minutes or hours. Moreover, electrochemical synthesis
9
www.bacpl.org/j/pcc Physical Chemistry Communications, Volume 3 Issue 1, April 2016
facilitates metal salt‐free and continuous production, which is a major advantage for industrial application. In this work, Cu‐BTC membranes on different specifications of the copper foams were prepared quickly by using electrochemical method, and then, optimized electrochemical condition for crystal formation was obtained. Based on the principle of crystal engineering, the relationship between structure and properties such as adsorption and thermal conductivity were discussed for potential applications. Experiment Procedure Material Preparation The electrodes were copper foams (25×25×4 mm) of the different PPI (the number of pore per specific surface area) with purities of 99.5%, and the porosity is 0.9. Importantly, the copper foams must be cleaned thoroughly before equipped in electrochemical cell. Cu‐BTC/Cu Foams were synthesized in a typical two‐electrode system[7]. The solution with 9.5 mmol (2.0g) of 1,3,5‐benzenetricarboxylic acid (H3BTC) and 127 mmol (4.0g) Tributylmethylammonium methyl sulfate (MTBS) in 150mL ethanol was used. The mixture was heated up to 55℃ and then kept at this temperature during the whole synthesis course by constant magnetic stirring. 50 mA fixed current was passed through the electrochemical cell with two copper electrodes spaced 20 mm for 90 min. Material Characterization The structure and morphology of the samples was characterized by scanning electron microscopy (SEM, JEOL S4800, Japan). Scanning was performed on a sample previously sputter coated with a thin layer of gold to avoid charging. The crystal structures of the material were investigated by X‐ray powder diffraction (XRD) using a LabX XRD‐6000 with Cu K radiation (1.54 Å), operating at 40 kV and 35 mA. The effective thermal conductivity of samples was measured by 3‐ method based on transient hot wire method (TC3000E, Xi’an Xiaxi) which was mainly used to test the film. The accuracy of this instrument is 2%[8]. Two same samples were put together around the test probe to test the effective thermal conductivity of through plane at room temperature and ambient pressure. The gases adsorption isotherms were measured using an automatic gas sorption analyzer (Quantachrome Autosorb IQ, USA). In the analysis, the samples were pretreated at 120 °C under vacuum for 12 h to remove any physically adsorbed gaseous from their surface. Results and Discussion The Structure Characterization of Cu‐BTC /Cu Foam Membrane During the reaction, bubbles were rising from the cathode quickly and the solution was turning blue. Meanwhile, copper foam as the anode was covered by blue crystals gradually. Fig.1 shows the typical SEM picture of Cu‐BTC membrane grown on Cu Foam (80PPI). The octahedral‐shaped crystals were observed. The film became more obvious with increased pore density.
FIG.1 TYPICAL SEM FOR CU‐BTC /CU FOAM MEMBRANE SYNTHESIZED BY ELECTROCHEMICAL METHOD.
Fig.2 shows the X‐ray diffraction patterns of the Cu‐BTC crystalline powder and the copper‐supported MOF
10
Physical Chemistry Communications, Volume 3 Issue 1, April 2016 www.bacpl.org/j/pcc
membrane, respectively. The XRD pattern of the membrane is similar to the Cu‐BTC crystalline phase, thus indicating the phase purity and homogeneity of the constructed crystalline membrane. 7000
Intensity(a.u.)
6000
Cu-BTC powder Cu-BTC membrane on 80PPI Cu foam
5000 4000 3000 2000 1000 0 10
20
30
40
2 ( )
50
FIG. 2 XRD PATTERNS OF CU‐BTC POWDER AND CU‐BTC/CU FOAM MEMBRANE
The Adsorption Abilities of Cu‐BTC /Cu Foam Membrane Fig.3 proves the N2 adsorption ability of Cu‐BTC powder and Cu‐BTC films on Cu Foam at ‐196°C. The increase of N2 adsorption lies in Cu‐BTC films covered on the skeleton of Cu foam, but this layer is rather thin and light, the enhancement is no much. The BET of Cu‐BTC films on Cu Foam(80 PPI) calculated based on N2 adsorption isothermal is about 70 cm3/g. XRD and BET prove the successfully formation of Cu‐BTC film on the Cu foam in several minutes by electrochemical method. 500
Volume @ STP(cc/g)
400 300
ads. Cu-BTCpowder des. Cu-BTCpowder ads. Cu-BTC/Cu Foam 80PPI ads. Cu-BTC/Cu Foam80 PPI
200 100 0 0.0
0.2
0.4
0.6
0.8
1.0
P/P0
FIG. 3 NITROGEN ADSORPTION ISOTHERMAL OF CU‐BTC POWDER AND CU‐BTC/CU FOAM MEMBRANE
Table‐1 illustrates the specific surface area of different Cu‐BTC membrane/ Cu foams. Fig.4 shows the adsorption‐ desorption isotherms of nitrogen at ‐196°C on different Cu‐BTC samples. It shows clearly that the specific surface area drops a lot compared with that of powder Cu‐BTC because the mass ratio of Cu‐BTC and Cu foam which it grew on is rather low. With the increase of pore density, specific surface area increased until 60 PPI than decreased. It is likely because the diffusion of the metal ions and ligands is restricted by the remaining of large pores during the membrane growth, which might inhibit the nucleation and growth of Cu‐BTC crystals. TABLE‐1 THE SPECIFIC SURFACE AREA AND MASS RATIO OF DIFFERENT CU‐BTC POWER AND CU‐BTC MEMBRANE SAMPLES
Samples
Cu‐BTC powder
20 PPI
40 PPI
60 PPI
80 PPI
Specific surface area (cm3/g)
1504.9
37.1
67.7
95.9
69.3
mass ratio of Cu‐BTC on Cu foam（wt%）
100 %
8.7 %
11.2 %
17.6 %
15.3 %
11
www.bacpl.org/j/pcc Physical Chemistry Communications, Volume 3 Issue 1, April 2016
Volume @ STP(cc/g)
100 ads. ads. ads. ads.
des. des. des. des.
20PPI 40PPI 60PPI 80PPI
50
0 0.0
0.2
0.4
0.6
0.8
1.0
P/P0
FIG. 4 THE ADSORPTION ISOTHERM OF CU‐BTC MEMBRANE ON DIFFERENT CU FOAMS (20 PPI, 40PPI, 60 PPI, 80 PPI)
The influence of synthesis time on Cu‐BTC membrane mainly embodies in two aspects. One is that the formation of Cu‐BTC membrane is impossible when the synthesis time is too short because the formation of Cu‐BTC membrane must involve some procedures including the oxidation of copper electrode, the deprotonation of the BTC acid and the activation and combination of Cu2+ and ligands, otherwise, the amount of crystal is too few to form the Cu‐BTC membrane. Another is electrochemical deposition, the longer the synthesis time is, the thicker membrane will be. However, it is not better when the synthesis time is too long in that the film will be detached and the mechanical strength will be strongly dropped. The Optical Conditions of Cu‐BTC /Cu Foam Membrane As we observed, the rate of formation of Cu‐BTC is very high, and it quickly takes place in the double layer around the electrode, which led to the Cu‐BTC crystals forming directly on the surface of electrode. Usually, large crystals are formed on the surface of the electrode, while small crystals are directly formed in solution. As the synthesis continues, more crystals grow on the uncovered metal surface. When the surface of electrode is fully covered, the charge transfer and the transport of both Cu2+ and the linker become more difficult[9]. If it continues to apply current at this stage, further dissolution of copper would cause the Cu‐BTC membrane crack and some large crystals detach and the new electrode surface would be available for further reaction. Therefore, the optimum conditions obtained based on synthesis yield and morphology of the samples, are 60 min at 55℃ with current of 55 mA. The Thermal Conductivity of Cu‐BTC /Cu Foam Membrane
Thermal conducitivity(W/mk)
2.4 2.0
Metal foam+Cu-BTC Cu-BTC
1.6 1.2 0.8 0.4 0.0 -10
0
10
20
30
40
(PPI)
50
60
70
80
90
FIG. 5 THE EFFECTIVE THERMAL CONDUCTIVITY AT DIFFERENT PORE DENSITY
Fig.5 shows the thermal conductivity results of Cu‐BTC powder and Cu‐BTC loaded on different copper foams. Usually, thermal conductivity of Cu‐BTC powder is as low as that of air since it possesses extremely high porosity. Poorly effective thermal transport properties impede their actual application such as gas storage, gas separation,
12
Physical Chemistry Communications, Volume 3 Issue 1, April 2016 www.bacpl.org/j/pcc
catalysis because highly exothermic heat of adsorption can lower the adsorption or even destroy the framework of the materials. However, Cu‐BTC loaded on the copper foams makes its thermal conductivity improved a lot, amounting to ten times as high as that of powder Cu‐BTC (60PPI) because copper has high thermal conductivity. Besides, the thermal conductivity of Cu‐BTC loaded on the copper foams increases with the pore density increased. Pore density controls the amount of larger pore in the two stage pore structure, and the size of large pore gets smaller with the pore density increasing, resulting in the air remaining in the copper foams which was replaced by Cu‐ BTC, so the thermal conductivity greatly improved. Conclusions Cu‐BTC/Cu foam hierarchical pores membranes are obtained by electrochemical method when using different pore‐density copper foams as electrodes. The optimum conditions for electrochemical synthesis based on synthesis yield and morphology of the samples are 60 min at 55℃ with current of 55 mA. The thermal conductivity of the Cu‐BTC/Cu foam improved a lot comparing with Cu‐BTC powder. It proves a rapid approach to prepare hierarchical pores membranes for potential applications on separation or adsorption. ACKNOWLEDGMENT
This work is financially sponsored by the National Natural Science Foundation of China (No. 51176149) , the National Key Projects of Fundamental R/D of China (973 Project: 2011CB610306) and the Fundamental Research Funds for the Central Universities (xjj2012102). REFERENCES
[1]
Kaye S S, Dailly A, Yaghi O M, et al. “Impact of preparation and handling on the hydrogen storage properities of Zn4O(1,4‐ bennzenedicarboxylate)3 (MOF‐5).” J. Am. Chem. Soc.2007, 129(46): 14176 ‐14169.
[2]
Zhu X, Zheng H Y, Wei X F, Lin Z Y, et al. “Metal‐organic framework (MOF): a novel sensing platform for biomolecules.” Chem. Commun., 2013, 49(13): 1276‐1278.
[3]
Chen B L, Wang L B, Xiao Y Q, et al. “A luminescent metal‐organic framework with Lewis basic pyridyl sites for the sensing of metal ions.” Angew. Chem. Int. Ed. 2009, 48(3):500‐503.
[4]
Liu J, Sun F X, Zhang F, et al. “In situ growth of continuous thin metal organic framework film for capacitive humidity sensing.” J. Mater. Chem., 2011, 21(11):3775‐3778.
[5]
Farha O K, Yazaydin A O, Eryazici I, et al. “De novo synthesis of a metal organic framework material featuring ultrahigh surface area and gas storage capacities.” Nature Chemistry, 2010, 2(11):944‐948.
[6]
Liu J, Sun F X, Zhang F, et al. “In situ growth of continuous thin metal organic framework film for capacitive humidity sensing.” J. Mater. Chem., 2011, 21(11):3775‐3778.
[7]
Doménech A, Garcia H, Doménech‐Carbó M T, et al. “Electrochemistry of metal–organic frameworks: a description from the voltammetry of microparticles approach.” J Phys Chem C, 2007, 111(37): 13701‐13711.
[8]
Finefrock S W, Wang Y, Ferguson J B, et al. “Measurement of thermal conductivity of PbTe nanocrystal coated glass fibers by the 3ω method.” Nano Lett. 2013, 13(11):5006‐5012.
[9]
Millange F, Osta R El, Medina M E, et al., “A time‐resolved diffraction study of a window of stability in the synthesis of a copper carboxylate metal–organic framework.” Cryst Eng Comm, 2011, 13:103‐108.
W. Zhang (China, 1974‐)is an associated professor in school of science, Xi’an Jiaotong University, Xi’an Shaanxi, China. She earned her doctor degree on Material Science and Engineering at Xi’an Jiaotong University in 2005. Her research interest lies in the design and application of composites about metal organic frameworks and metal oxides for gas separation and sensing applications.
13