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Synthesis of MOF having functional side group
Inorganica Chimica Acta 370 (2011) 76–81
Contents lists available at ScienceDirect
Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Synthesis of MOF having functional side group Dong Ok Kim, Jeasung Park ⇑, Gui Ryong Ahn, Hyo Jin Jeon, Jong Sik Kim, Dong Wook Kim, Mi Sun Jung, Sang Wook Lee, Seong Hwan Shin Hanwha Chemical Research & Development Center, 6, Shinseong-dong, Yuseong-gu, Daejeon 304-804, Republic of Korea
a r t i c l e
i n f o
Article history: Received 9 September 2010 Received in revised form 4 January 2011 Accepted 11 January 2011 Available online 21 January 2011 Keywords: MOF Organic ligand Postsynthetic modification Supercritical drying Activation
a b s t r a c t For the preparation of MOFs which can be modified via postsynthetic modification, a homologous series of p-terphenyl-4,40 -dicarboxylic acid having hydroxyl side groups, ranging from 0 to 4, as functional groups have been synthesized. Then the modified IRMOF-16, named as HCC-1, has been successfully synthesized via solvothermal and microwave method using 1,4-di(4-carboxy-2-hydroxyphenyl)benzene as organic ligand and Zn(NO3)26H2O as metal source, respectively. A series of structural analyses have identified that it has the same crystal structure and dimensions as those of IRMOF-16. Despite the successful synthesis, conventional method of activation did not proved to be adequate for HCC-1 because of structural collapse. However, this problem could be minimized with the help of continuous CO2 supercritical drying (SCD) process. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Metal–organic framework (MOF), also known as porous coordination polymer (PCP), has been intensively studied by many research groups throughout the world since it was introduced in the late 1990s by Yaghi’s group [1,2]. Because it usually consists of two parts, linker (organic ligand) and node (metal cluster), numerous kinds of MOFs having different pore size and crystal structures can be easily generated by the simple combination change of these two parts. With the help of above characteristics, synthesis of MOF, having specific pore size and structure based on the molecular level design, was possible and it characterized MOF as very novel material compared to other porous materials such as silica, zeolites or hyper cross-linked polymer particles. Thus it has been widely investigated for the purpose of application to chemical reaction catalyst, special purpose membrane, selective gas absorption material, etc. Among the numerous MOF structures, cubic structure, known with the introduction of MOF-5 (IRMOF-1) which was followed by IRMOF-3, IRMOF-6, IRMOF-8, IRMOF-16, etc., is considered relatively more stable and easy for analysis. Due to these two advantages, the properties of cubic structured MOFs, gas adsorption, gas separation, catalytic capability, etc., are being far more intensively investigated compared with other structured MOFs [3–8]. However, while deepening our understanding of MOFs, it was turned out that modification of MOF using small chemical reagents
⇑ Corresponding author. Tel.: +82 42 865 6564; fax: +82 42 865 6570. E-mail address: [email protected] (J. Park). 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.01.030
after the lattice is formed, so-called postsynthetic modification, was much more efficient way to get MOFs with a variety of functionalities and novel physical properties compared to timeand effort-consuming new MOF design and synthesis. Since then, numerous researches for postsynthetic modification of MOFs have been widely performed throughout the world [9–11]. For example, Wu et al. [11] reported successful postsynthetic modification of MOF, having (R)-6,60 -dichloro-2,20 -dihydroxy1,10 -binaphthyl-4,40 -bipyridine as linker, by using Ti(OiPr)4 as chemical reagent. Tanabe et al. [9,10] also claimed that postsynthetic modification of IRMOF-3 with a series of alkyl aldehydes could be successfully achieved. However, IRMOF-3 studied by Tanabe et al. [9,10] had relatively small pore size that it kept its rate and conversion low especially when the length of alkyl anhydride was long. In case of MOF investigated by Wu et al. [11], even if it had relatively larger pore size than that of IRMOF-3, it still experienced the similar problem and its further systematic study via chemical modification of linker was almost impossible because of its inflexible chemical structure. On the basis of above observations, we fully understood the importance of ‘modification technology of MOFs via postsynthetic modification’ and started systematical research. As a first step, a homologous series of p-terphenyl-4,40 -dicarboxylic acid having hydroxyl side groups, ranging from 0 to 4, as functional groups has been synthesized for linkers. Then the first modified IRMOF16 (HCC-1) has been successfully obtained via solvothermal and microwave method using one of above linkers, 1,4-di(4-carboxy2-hydroxyphenyl)benzene, as organic ligand and Zn(NO3)26H2O as metal source, respectively. Thus HCC-1 will allow us to study the systematic investigation of postsynthetic modification of
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MOFs. In this paper, we present the highlights of our findings during HCC-1 synthesis. 2. Experimental 2.1. Synthesis of organic ligand 1,4-Di(4-carboxy-2-hydroxyphenyl)benzene for the synthesis of HCC-1 was prepared using the following procedures. As a first step, 1,4-di(4-methoxycarbonyl-2-methoxyphenyl)benzene was synthesized by slowly adding 1.7 g of benzene-1,4-diboronic acid, purchased from Sigma–Aldrich, 5 g of methyl 4-bromo-3methoxybenzoate, purchased from Capotchem, and 0.69 g of Pd(PPh3)4, purchased from TCI, to mixed solution of 80 ml of THF and 15 ml of 2 M aqueous K2CO3 solution under stirring at 80 °C. The reaction mixture was filtered and washed well with methanol and then dried. As a second step, 5 g of 1,4-di(4-methoxycarbonyl2-methoxyphenyl)benzene was added to the mixed solution of 200 ml of THF and 200 ml of 1 M NaOH solution under stirring at 80 °C. The suspension was stirred and refluxed for 5 h, then diluted with additional water until the salt of the product dissolved completely. After that, a solution of hydrochloric acid was added to the above suspension until it became distinctly acidic. Product recovery was done by the same procedure mentioned above. As a third step, 1,4-di(4-carboxy-2-hydroxyphenyl)benzene, was obtained by adding 2.7 g of 1,4-di(4-carboxy-2-methoxyphenyl)benzene to 30 ml of 1 M BBr3 solution, purchased from Aldrich, under stirring at 25 °C. The product mixture was filtered and washed well with deionized water and then dried. The product obtained was collected and recrystallized three times from DMF/water mixture. The sample was dried under vacuum for 3 days, and kept in a well-sealed glass bottle. Product of each step was cautiously confirmed by 1H NMR and DSC.
monochromated Cu Ka radiation (k = 1.5418 Å) and with a scan speed of 2 s per step and a step size of 0.02°. SEM observation for the morphology of MOF was preformed using a Hitachi S-4800, and elemental mapping was conducted using EDS (energy dispersive X-ray spectrometer). The specific surface areas of the MOFs were calculated by the BET (Brunauer, Emmett and Teller) method, using N2 adsorption on ASAP2420 from Micromeritics. Samples were evacuated under 105 Torr dynamic vacuum at 100 °C for 3 h and then measured over the range of 0.01 < P/P0 < 0.1. 3. Results and discussions The synthesis of HCC-1 has been performed via solvothermal method using one of the previously prepared linkers, 1,4-di(4-carboxy-2-hydroxyphenyl)benzene, as organic ligand and Zn(NO3)2 6H2O as metal source, respectively. Adopted synthetic conditions which had been refined through several sets of experiments were shown separately in Table 1. As for solvent, known as one of the most important components, DEF and DMF were selected based on the previous researches [12] – basic chemicals generated by thermal decomposition of above solvents may play an important role in MOF synthesis by inducing deprotonation of organic ligand. However, to the best of our knowledge more detailed synthetic mechanism of MOF and each chemical’s role are not available so far.
2.2. Synthesis of MOF (solvothermal method) A 0.438 g of 1,4-di-(4-carboxy-2-hydroxyphenyl)benzene and 2.23 g of Zn(NO3)26H2O, purchased from Aldrich, were dissolved in 200 ml of N,N-dimethylformamide (DMF, 99.0%, Samchun). The bottles containing above mixture were sealed and heated at 85 °C for 86 h. And then the bottles were cooled down to room temperature, and the reaction mixtures were decanted. After washing the small amber block crystal with fresh DMF, the recovered HCC-1 was treated by SCD process.
Fig. 1. SEM images of synthesized HCC-1 with the change of solvent (a) DMF (b) DEF.
IRMOF-16
2.4. Characterization of MOF
3
10
(500)
(400)
(210)
(200) (210)
(100)
HCC-1
(110)
After heated at 85 °C for 20 h, the dissolved mixture of 0.175 g of 1,4-di-(4-carboxy-2-hydroxyphenyl)benzene and 0.892 g of Zn(NO3)26H2O in 80 ml of N,N-dimethylformamide was transferred into several containers for microwave. Then three consecutive microwaving (800 W) were applied. After given time, the containers were cooled down to room temperature and the reaction mixtures were decanted. After washing the small solid with fresh DMF, the recovered HCC-1 was treated by SCD process.
Intensity (a.u.)
2.3. Synthesis of MOF (microwave method)
20
30
2θ (deg.)
Powder X-ray diffraction (PXRD) was conducted using a Bruker D8 Advanced diffractometer operated at 40 kV and 40 mA with
Fig. 2. Powder X-ray diffraction patterns of IRMOF-16 and HCC-1.
Table 1 Synthetic conditions of HCC-1 (solvothermal method). Organic ligands
Zn(NO3)26H2O
Solvent (ml)
Temp. (°C)
Time (h)
0.438 g (1.25 mmol)
2.23 g (7.5 mmol)
DMF 200
85
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It was observed that DMF was more efficient than DEF for the synthesis of HCC-1 when other conditions were remained fixed. As can be seen in Fig. 1, HCC-1 using DMF has relatively bigger size and more regular shape compared to HCC-1 using DEF. Powder X-ray diffraction pattern of HCC-1 was investigated to determine its crystal structure. Since HCC-1 is modified from IRMOF-16, powder X-ray diffraction pattern of IRMOF-16 was also displayed with HCC-1 for the structure comparison. In Fig. 2, HCC-1 presented very similar diffraction pattern to that of IRMOF-16, including
(a) Conventional method
Intensity (a.u.)
78
(b) CO2 SCD
Simulated IRMOF-15 (no guest solvent)
3
10
20
30
2θ (deg.) Fig. 6. The change of powder X-ray diffraction patterns with the change of activation method.
Simulated IRMOF-16 (no guest solvent)
120
100
HCC-1 (evacutated by SCD)
3
30
20
10
80
BET (%)
2θ (deg.) Fig. 3. Comparison of powder X-ray diffraction patterns of MOFs.
60
40
20 BET : 2600m2/g BET : 4724m2/g
0
0
100
200
300
Temperature (oC) Fig. 4. MOF structures of (a) IRMOF-16 and (b) HCC-1.
8 9 P
T
5
3
10
T
7
H.
H.
Intensity (a.u.)
P
Fig. 7. Comparison of thermal stabilities of HCC-1 having different BET surface area values.
4
1 6 2 1. CO2 cylinder 2. Cooler
5. Reactor 6. Bath circulator
3. Syringe pump 4. Oven
7. Ventilation 8. Pressure transducer
9. Thermometer 10. Back pressure regulator
Fig. 5. Custom-built continuous CO2 supercritical drying equipment.
3
10
20
2θ (deg.) Fig. 8. Effect of crushing on X-ray diffraction pattern of HCC-1.
30
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D.O. Kim et al. / Inorganica Chimica Acta 370 (2011) 76–81 Table 2 Synthetic conditions of HCC-1 (microwave method). Organic ligands
Zn(NO3)26H2O
Power (W)
1.25 mmol (DMF 200 ml)
7.5 mmol
Preheating (85 °C/20 h/200 ml) 800 85/105/130
unique sharp peak representing (1 0 0) plane. This observation drove us to conclude that HCC-1 is cubic structure with the each axis of 21.45 Å. It is well documented [1,2] that interpenetrated structured MOF (e.g., IRMOF-15) is more likely to be formed when relatively longer molecules are adopted as linkers. With the formation of these structures, the pore size of MOF becomes smaller and this gives rise to an adverse effect on the efficiency of postsynthetic modification – decreased pose size makes mass diffusion of chemical reagent in MOF more difficult. For this reason, determining whether HCC-1 has interpenetrated structure or not is very important. Thus a supplementary experiment, generation of X-ray diffraction patterns of IRMOF-15 and IRMOF-16 via computer simulation as shown in Fig. 3, was performed to observe the differences between them. As can be seen in Fig. 3, since the interpenetrated structured never has diffraction plane whose MOF, having space group im3m, value of (h + k + l) is odd number, HCC-1 should not have distinctive (1 0 0) plane if it has interpenetrated structure. However HCC-1 showed very distinctive (1 0 0) plane, like IRMOF-16, and this led us to conclude that HCC-1 has no interpenetrated structure. This conclusion was confirmed by the study of Bae et al. [8]. They generated several X-ray diffraction patterns of IRMOF-16 derivatives such as IRMOF-16, IRMOF-16-2fold interpenetrated, IRMOF-16-3fold interpenetrated, etc. via computer simulation. Among them, nothing showed similar pattern to that of HCC-1 except IRMOF-16. On top of X-ray studies, measurements of pore size and distribution provided us with another experimental evidence to support above conclusion. We observed very narrow distribution of pore size, with average value of 22.4 Å, via NLDFT (non-local density functional theory) method. On the basis of above observations, the structure of HCC-1 was generated and presented in Fig. 4. For the quantification of porous material’s degree of porosity, measurement of BET surface area is widely used. In case of MOF, solvent used as synthetic medium (guest solvent) should be removed, so-called activation, in advance of BET surface area measurement. For that purpose, guest solvent having high boiling point (e.g., DEF or DMF) is exchanged with solvent having low boiling point (e.g., chloroform) and then dried under vacuum. However, for HCC-1, this conventional activation method not only
Preheating
Temp. (°C)
Time (min)
Solution for microwave (ml)
120/60/60
80
was so time-consuming job but also had adverse effect on its structural stability. Structural collapse was confirmed by the dramatic
Microwaving
Temp.
(c)
(b) 20 hr
(a)
60 min
60 min
60 min 120 min 180 min
Time Fig. 9. Protocol of microwave method for HCC-1 synthesis.
Fig. 10. SEM images of synthesized HCC-1 with the increase of microwaving time of first step. (a) 60 min, (b) 12 min, and (c) 180 min.
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decreases of X-ray diffraction peak intensity and BET surface area. However this kind of phenomenon is not so common with MOFs having relatively shorter length linkers (e.g., IRMOF-1 or IRMOF3). Thus above observation suggests that the structural stability of HCC-1 is more or less weaker because of its longer length of linker. Nelson et al. [13] and Wang et al. [14] have also reported the similar phenomenon, severe collapse of IRMOF-16, during the conventional activation, and insisted that the generation of capillary force of solvent during thermal evacuation was one of the main reasons for the collapse of MOF structure. In an effort to solve this problem, we introduced continuous CO2 supercritical drying (SCD) method for HCC-1. For that, a certain amount of slurry state HCC-1, right after synthesis, was placed in the cell of 50 ml and then extracted by continuous flow of supercritical state CO2 (50 °C and 200 bar) for 6 h. Fig. 5 shows our custombuilt continuous CO2 SCD equipment. Fig. 6 describes (a) X-ray diffraction pattern of HCC-1 activated by conventional activation method – guest solvent DMF was exchanged with chloroform and then dried under vacuum at 60 °C and (b) X-ray diffraction pattern of HCC-1 activated by CO2 SCD. From the comparison of two patterns, it was observed that the structure of HCC-1 was severely damaged by conventional activation but the structure of HCC-1 remained almost undamaged by CO2 SCD. As a supplementary experiment, observation of the variation of BET surface area was also made to confirm above observation. Fig. 6a showed as low as 250 m2/g, whereas Fig. 6b showed 4724 m2/g. And its measured amount of residual solvent, using TGA, was below 1 wt%. All these observations clearly indicate that CO2 SCD is very effective method for the activation of HCC-1 without severe structural damage. Since HCC-1 was synthesized to be used for the postsynthetic modification, its thermal stability was also important. If it has higher thermal stability, its possible temperature range of postsynthetic modification can be widened. For this reason, we prepared two kinds of HCC-1, having BET surface areas of 2600 and 4724 m2/g, respectively, and placed them in the temperature con-
trolled autoclave (10 bar of H2) for 6 h, and investigated their thermal stabilities by monitoring their variations of BET surface areas. As can be seen in Fig. 7, while the value of BET surface area of HCC1 with 4724 m2/g remained more or less constant till 250 °C, the value of BET surface area of HCC-1 with 2600 m2/g decreased during the entire range of experimental temperature. This observation gave us an idea that there is intimate relationship between thermal stability and BET surface area. We speculate that HCC-1 having lower BET surface area has more unsaturated nodes, chemically unstable part, than HCC-1 having higher BET surface area and this causes lower thermal stability. However, there are some more important factors to be considered for the efficient postsynthetic modification. One of them is the size of MOF. Unlike many other conventional chemical reactions, no stirring is applied to prevent structural collapse of MOF. To compensate that, very long reaction time is usually given for making sure of diffusion of chemical reagents deep into the MOF. Thus synthesizing MOFs having smaller size could be an effective way to make postsynthetic modification more competitive because it can help to reduce reaction time [15]. Although obtaining smaller sized MOF via simple mechanical crushing is the easiest way for above purpose, its structural damage is inevitable. Fig. 8 shows the adverse effect of crushing on MOF structure. We can see that the peak intensity of X-ray diffraction pattern of crushed HCC-1 decreased remarkably, and it is clear evidence of structural collapse. Moreover, the decrease of BET surface area, from 2845 to 939 m2/g, provides us with additional experimental evidence showing structural collapse. Thus we introduced microwave method to obtain smaller sized HCC-1 having no structural collapse problem mentioned above. It is widely documented [16–20] that microwave makes faster and more even heating of reaction system possible, and it promotes to increase the number of generated MOF nuclei. For this reason, smaller sized MOF could be finally obtained. However, as far as we know, more detailed synthetic mechanism is not available yet.
Fig. 11. (a) SEM images of synthesized HCC-1 from Fig. 10(c) and (b) elemental mappings (C, O and Zn) using EDS.
D.O. Kim et al. / Inorganica Chimica Acta 370 (2011) 76–81
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4. Conclusions
Fig. 12. Powder X-ray diffraction pattern of synthesized HCC-1 via microwave method.
We tried to find out our own optimized reaction conditions through several independent sets of experiments and summarized them in Table 2 and Fig. 9, respectively. One of our noteworthy findings was that the time length of first microwaving step, shown in Fig. 9, was more important than any other conditions for obtaining better shaped HCC-1. As shown in Fig. 10, the outer shape of HCC-1 improved with the increase of microwaving time from 60 to 180 min. Besides, EDS scanning for the mapping of atoms (C, O and Zn) was also performed to understand the origin of ball shaped small particles observed on the surface of MOF shown in Fig. 11. On the basis of mapping results, it was speculated that they were mainly residual organic materials (linker) since distinctive ball shaped mapping patterns were obtained only from C and O. Thus we tried to remove these organic by-products as much as possible via solvent cleaning and decanting, and finally obtained HCC-1 with the BET surface area of 2755 m2/g. Fig. 12 represents the X-ray diffraction pattern of synthesized MOF via microwave method. It is not difficult to derive a conclusion that it has the same crystal structure as that of HCC-1 synthesized via solvothermal method shown in Fig. 2. On the other hand, because its observed average size was about 20 lm, much smaller than HCC-1 shown in Fig. 1, microwave method proved to be very powerful and effective method to obtain smaller sized MOF. However, the effect of MOF size on the efficiency of postsynthetic modification is beyond the scope of this paper, and it will be discussed separately.
The modified IRMOF-16, HCC-1, having functional groups for postsynthetic modification, has been successfully synthesized via solvothermal and microwave method using 1,4-di(4-carboxy-2hydroxyphenyl)benzene as organic ligand and Zn(NO3)26H2O as metal source, respectively. From the solvothermal method, MOF having average size of 500 lm and BET surface area of 4724 m2/g was synthesized. On the other hand, MOF having average size of 20 lm and BET surface area of 2755 m2/g was obtained from microwave method. However, it was found that both MOFs have identical crystal structure–cubic structure. Thermal stability was also investigated. MOF having higher value of BET surface area showed higher thermal stability than that of MOF having lower value of BET surface area, and this indicated that obtaining MOFs with higher value of BET surface area is essential for the wider temperature range of postsynthetic modification. Despite the successful synthesis, conventional method of activation was not proved to be effective for HCC-1 because structural degradation was observed. However, this problem could be minimized with the help of CO2 supercritical drying (SCD) process. References [1] M. Eddaoudi, J. Kim, N.L. Rosi, D.T. Vodak, J. Wachter, M. O’Keeffe, O.M. Yaghi, Science 295 (2002) 469. [2] H. Li, M. Eddaoudi, M. O’Keeffe, O.M. Yaghi, Nature 402 (1999) 276. [3] N.L. Rosi, J. Eckert, M. Eddaoudi, D.T. Vodak, J. Kim, M. O’Keeffe, O.M. Yaghi, Science 300 (2003) 1127. [4] J. Gascon, U. Aktay, M.D. Hernandez-Alonso, G.P.M. van Klink, F. Kapteijn, J. Catal. 261 (2009) 75. [5] H. Frost, T. Duren, R.Q. Snurr, J. Phys. Chem. B 110 (2006) 9565. [6] M.A. Miller, C.Y. Wang, G.N. Merrill, J. Phys. Chem. C 113 (2009) 3222. [7] A.R. Millward, O.M. Yaghi, J. Am. Chem. Soc. 127 (2005) 17998. [8] Y.S. Bae, D. Dubbeldam, A. Nelson, K.S. Walton, J.T. Hupp, R.Q. Snurr, Chem. Mater. 21 (2009) 4768. [9] K.K. Tanabe, Z. Wang, S.M. Cohen, J. Am. Chem. Soc. 130 (2008) 8508. [10] Z. Wang, K.K. Tanabe, S.M. Cohen, Inorg. Chem. 48 (2009) 296. [11] C. Wu, A. Hu, L. Zhang, W. Lin, J. Am. Chem. Soc. 127 (2005) 8940. [12] J.L.C. Rowsell, O.M. Yaghi, Microporous Mesoporous Mater. 73 (2004) 3. [13] A.P. Nelson, O.K. Farha, K.L. Mulfort, J.T. Hupp, J. Am. Chem. Soc. 131 (2009) 458. [14] K. Wang, N. Petkov, M.A. Morris, J.D. Holmes, Stud. Surf. Sci. Catal. 170 (2007) 1796. [15] L. Huang, H. Wang, J. Chen, Z. Wang, J. Sun, D. Zhao, Y. Yan, Microporous Mesoporous Mater. 58 (2003) 105. [16] Z. Xiang, D. Cao, X. Shao, W. Wang, J. Zhang, W. Wu, Chem. Eng. Sci. 65 (2010) 3140. [17] G.A. Tompsett, W.C. Conner, K.S. Yngvesson, Chemphyschem 7 (2006) 296. [18] S.H. Jhung, T. Jin, Y.K. Hwang, J.S. Chang, Chem. Eur. J. 13 (2007) 4410. [19] J.S. Choi, W.J. Son, J.H. Kim, W.S. Ahn, Microporous Mesoporous Mater. 116 (2008) 727. [20] C.-M. Lu, J. Liu, K. Xiao, A.T. Harris, Chem. Eng. J. 156 (2010) 465.
Contents lists available at ScienceDirect
Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Synthesis of MOF having functional side group Dong Ok Kim, Jeasung Park ⇑, Gui Ryong Ahn, Hyo Jin Jeon, Jong Sik Kim, Dong Wook Kim, Mi Sun Jung, Sang Wook Lee, Seong Hwan Shin Hanwha Chemical Research & Development Center, 6, Shinseong-dong, Yuseong-gu, Daejeon 304-804, Republic of Korea
a r t i c l e
i n f o
Article history: Received 9 September 2010 Received in revised form 4 January 2011 Accepted 11 January 2011 Available online 21 January 2011 Keywords: MOF Organic ligand Postsynthetic modification Supercritical drying Activation
a b s t r a c t For the preparation of MOFs which can be modified via postsynthetic modification, a homologous series of p-terphenyl-4,40 -dicarboxylic acid having hydroxyl side groups, ranging from 0 to 4, as functional groups have been synthesized. Then the modified IRMOF-16, named as HCC-1, has been successfully synthesized via solvothermal and microwave method using 1,4-di(4-carboxy-2-hydroxyphenyl)benzene as organic ligand and Zn(NO3)26H2O as metal source, respectively. A series of structural analyses have identified that it has the same crystal structure and dimensions as those of IRMOF-16. Despite the successful synthesis, conventional method of activation did not proved to be adequate for HCC-1 because of structural collapse. However, this problem could be minimized with the help of continuous CO2 supercritical drying (SCD) process. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Metal–organic framework (MOF), also known as porous coordination polymer (PCP), has been intensively studied by many research groups throughout the world since it was introduced in the late 1990s by Yaghi’s group [1,2]. Because it usually consists of two parts, linker (organic ligand) and node (metal cluster), numerous kinds of MOFs having different pore size and crystal structures can be easily generated by the simple combination change of these two parts. With the help of above characteristics, synthesis of MOF, having specific pore size and structure based on the molecular level design, was possible and it characterized MOF as very novel material compared to other porous materials such as silica, zeolites or hyper cross-linked polymer particles. Thus it has been widely investigated for the purpose of application to chemical reaction catalyst, special purpose membrane, selective gas absorption material, etc. Among the numerous MOF structures, cubic structure, known with the introduction of MOF-5 (IRMOF-1) which was followed by IRMOF-3, IRMOF-6, IRMOF-8, IRMOF-16, etc., is considered relatively more stable and easy for analysis. Due to these two advantages, the properties of cubic structured MOFs, gas adsorption, gas separation, catalytic capability, etc., are being far more intensively investigated compared with other structured MOFs [3–8]. However, while deepening our understanding of MOFs, it was turned out that modification of MOF using small chemical reagents
⇑ Corresponding author. Tel.: +82 42 865 6564; fax: +82 42 865 6570. E-mail address: [email protected] (J. Park). 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.01.030
after the lattice is formed, so-called postsynthetic modification, was much more efficient way to get MOFs with a variety of functionalities and novel physical properties compared to timeand effort-consuming new MOF design and synthesis. Since then, numerous researches for postsynthetic modification of MOFs have been widely performed throughout the world [9–11]. For example, Wu et al. [11] reported successful postsynthetic modification of MOF, having (R)-6,60 -dichloro-2,20 -dihydroxy1,10 -binaphthyl-4,40 -bipyridine as linker, by using Ti(OiPr)4 as chemical reagent. Tanabe et al. [9,10] also claimed that postsynthetic modification of IRMOF-3 with a series of alkyl aldehydes could be successfully achieved. However, IRMOF-3 studied by Tanabe et al. [9,10] had relatively small pore size that it kept its rate and conversion low especially when the length of alkyl anhydride was long. In case of MOF investigated by Wu et al. [11], even if it had relatively larger pore size than that of IRMOF-3, it still experienced the similar problem and its further systematic study via chemical modification of linker was almost impossible because of its inflexible chemical structure. On the basis of above observations, we fully understood the importance of ‘modification technology of MOFs via postsynthetic modification’ and started systematical research. As a first step, a homologous series of p-terphenyl-4,40 -dicarboxylic acid having hydroxyl side groups, ranging from 0 to 4, as functional groups has been synthesized for linkers. Then the first modified IRMOF16 (HCC-1) has been successfully obtained via solvothermal and microwave method using one of above linkers, 1,4-di(4-carboxy2-hydroxyphenyl)benzene, as organic ligand and Zn(NO3)26H2O as metal source, respectively. Thus HCC-1 will allow us to study the systematic investigation of postsynthetic modification of
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MOFs. In this paper, we present the highlights of our findings during HCC-1 synthesis. 2. Experimental 2.1. Synthesis of organic ligand 1,4-Di(4-carboxy-2-hydroxyphenyl)benzene for the synthesis of HCC-1 was prepared using the following procedures. As a first step, 1,4-di(4-methoxycarbonyl-2-methoxyphenyl)benzene was synthesized by slowly adding 1.7 g of benzene-1,4-diboronic acid, purchased from Sigma–Aldrich, 5 g of methyl 4-bromo-3methoxybenzoate, purchased from Capotchem, and 0.69 g of Pd(PPh3)4, purchased from TCI, to mixed solution of 80 ml of THF and 15 ml of 2 M aqueous K2CO3 solution under stirring at 80 °C. The reaction mixture was filtered and washed well with methanol and then dried. As a second step, 5 g of 1,4-di(4-methoxycarbonyl2-methoxyphenyl)benzene was added to the mixed solution of 200 ml of THF and 200 ml of 1 M NaOH solution under stirring at 80 °C. The suspension was stirred and refluxed for 5 h, then diluted with additional water until the salt of the product dissolved completely. After that, a solution of hydrochloric acid was added to the above suspension until it became distinctly acidic. Product recovery was done by the same procedure mentioned above. As a third step, 1,4-di(4-carboxy-2-hydroxyphenyl)benzene, was obtained by adding 2.7 g of 1,4-di(4-carboxy-2-methoxyphenyl)benzene to 30 ml of 1 M BBr3 solution, purchased from Aldrich, under stirring at 25 °C. The product mixture was filtered and washed well with deionized water and then dried. The product obtained was collected and recrystallized three times from DMF/water mixture. The sample was dried under vacuum for 3 days, and kept in a well-sealed glass bottle. Product of each step was cautiously confirmed by 1H NMR and DSC.
monochromated Cu Ka radiation (k = 1.5418 Å) and with a scan speed of 2 s per step and a step size of 0.02°. SEM observation for the morphology of MOF was preformed using a Hitachi S-4800, and elemental mapping was conducted using EDS (energy dispersive X-ray spectrometer). The specific surface areas of the MOFs were calculated by the BET (Brunauer, Emmett and Teller) method, using N2 adsorption on ASAP2420 from Micromeritics. Samples were evacuated under 105 Torr dynamic vacuum at 100 °C for 3 h and then measured over the range of 0.01 < P/P0 < 0.1. 3. Results and discussions The synthesis of HCC-1 has been performed via solvothermal method using one of the previously prepared linkers, 1,4-di(4-carboxy-2-hydroxyphenyl)benzene, as organic ligand and Zn(NO3)2 6H2O as metal source, respectively. Adopted synthetic conditions which had been refined through several sets of experiments were shown separately in Table 1. As for solvent, known as one of the most important components, DEF and DMF were selected based on the previous researches [12] – basic chemicals generated by thermal decomposition of above solvents may play an important role in MOF synthesis by inducing deprotonation of organic ligand. However, to the best of our knowledge more detailed synthetic mechanism of MOF and each chemical’s role are not available so far.
2.2. Synthesis of MOF (solvothermal method) A 0.438 g of 1,4-di-(4-carboxy-2-hydroxyphenyl)benzene and 2.23 g of Zn(NO3)26H2O, purchased from Aldrich, were dissolved in 200 ml of N,N-dimethylformamide (DMF, 99.0%, Samchun). The bottles containing above mixture were sealed and heated at 85 °C for 86 h. And then the bottles were cooled down to room temperature, and the reaction mixtures were decanted. After washing the small amber block crystal with fresh DMF, the recovered HCC-1 was treated by SCD process.
Fig. 1. SEM images of synthesized HCC-1 with the change of solvent (a) DMF (b) DEF.
IRMOF-16
2.4. Characterization of MOF
3
10
(500)
(400)
(210)
(200) (210)
(100)
HCC-1
(110)
After heated at 85 °C for 20 h, the dissolved mixture of 0.175 g of 1,4-di-(4-carboxy-2-hydroxyphenyl)benzene and 0.892 g of Zn(NO3)26H2O in 80 ml of N,N-dimethylformamide was transferred into several containers for microwave. Then three consecutive microwaving (800 W) were applied. After given time, the containers were cooled down to room temperature and the reaction mixtures were decanted. After washing the small solid with fresh DMF, the recovered HCC-1 was treated by SCD process.
Intensity (a.u.)
2.3. Synthesis of MOF (microwave method)
20
30
2θ (deg.)
Powder X-ray diffraction (PXRD) was conducted using a Bruker D8 Advanced diffractometer operated at 40 kV and 40 mA with
Fig. 2. Powder X-ray diffraction patterns of IRMOF-16 and HCC-1.
Table 1 Synthetic conditions of HCC-1 (solvothermal method). Organic ligands
Zn(NO3)26H2O
Solvent (ml)
Temp. (°C)
Time (h)
0.438 g (1.25 mmol)
2.23 g (7.5 mmol)
DMF 200
85
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It was observed that DMF was more efficient than DEF for the synthesis of HCC-1 when other conditions were remained fixed. As can be seen in Fig. 1, HCC-1 using DMF has relatively bigger size and more regular shape compared to HCC-1 using DEF. Powder X-ray diffraction pattern of HCC-1 was investigated to determine its crystal structure. Since HCC-1 is modified from IRMOF-16, powder X-ray diffraction pattern of IRMOF-16 was also displayed with HCC-1 for the structure comparison. In Fig. 2, HCC-1 presented very similar diffraction pattern to that of IRMOF-16, including
(a) Conventional method
Intensity (a.u.)
78
(b) CO2 SCD
Simulated IRMOF-15 (no guest solvent)
3
10
20
30
2θ (deg.) Fig. 6. The change of powder X-ray diffraction patterns with the change of activation method.
Simulated IRMOF-16 (no guest solvent)
120
100
HCC-1 (evacutated by SCD)
3
30
20
10
80
BET (%)
2θ (deg.) Fig. 3. Comparison of powder X-ray diffraction patterns of MOFs.
60
40
20 BET : 2600m2/g BET : 4724m2/g
0
0
100
200
300
Temperature (oC) Fig. 4. MOF structures of (a) IRMOF-16 and (b) HCC-1.
8 9 P
T
5
3
10
T
7
H.
H.
Intensity (a.u.)
P
Fig. 7. Comparison of thermal stabilities of HCC-1 having different BET surface area values.
4
1 6 2 1. CO2 cylinder 2. Cooler
5. Reactor 6. Bath circulator
3. Syringe pump 4. Oven
7. Ventilation 8. Pressure transducer
9. Thermometer 10. Back pressure regulator
Fig. 5. Custom-built continuous CO2 supercritical drying equipment.
3
10
20
2θ (deg.) Fig. 8. Effect of crushing on X-ray diffraction pattern of HCC-1.
30
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D.O. Kim et al. / Inorganica Chimica Acta 370 (2011) 76–81 Table 2 Synthetic conditions of HCC-1 (microwave method). Organic ligands
Zn(NO3)26H2O
Power (W)
1.25 mmol (DMF 200 ml)
7.5 mmol
Preheating (85 °C/20 h/200 ml) 800 85/105/130
unique sharp peak representing (1 0 0) plane. This observation drove us to conclude that HCC-1 is cubic structure with the each axis of 21.45 Å. It is well documented [1,2] that interpenetrated structured MOF (e.g., IRMOF-15) is more likely to be formed when relatively longer molecules are adopted as linkers. With the formation of these structures, the pore size of MOF becomes smaller and this gives rise to an adverse effect on the efficiency of postsynthetic modification – decreased pose size makes mass diffusion of chemical reagent in MOF more difficult. For this reason, determining whether HCC-1 has interpenetrated structure or not is very important. Thus a supplementary experiment, generation of X-ray diffraction patterns of IRMOF-15 and IRMOF-16 via computer simulation as shown in Fig. 3, was performed to observe the differences between them. As can be seen in Fig. 3, since the interpenetrated structured never has diffraction plane whose MOF, having space group im3m, value of (h + k + l) is odd number, HCC-1 should not have distinctive (1 0 0) plane if it has interpenetrated structure. However HCC-1 showed very distinctive (1 0 0) plane, like IRMOF-16, and this led us to conclude that HCC-1 has no interpenetrated structure. This conclusion was confirmed by the study of Bae et al. [8]. They generated several X-ray diffraction patterns of IRMOF-16 derivatives such as IRMOF-16, IRMOF-16-2fold interpenetrated, IRMOF-16-3fold interpenetrated, etc. via computer simulation. Among them, nothing showed similar pattern to that of HCC-1 except IRMOF-16. On top of X-ray studies, measurements of pore size and distribution provided us with another experimental evidence to support above conclusion. We observed very narrow distribution of pore size, with average value of 22.4 Å, via NLDFT (non-local density functional theory) method. On the basis of above observations, the structure of HCC-1 was generated and presented in Fig. 4. For the quantification of porous material’s degree of porosity, measurement of BET surface area is widely used. In case of MOF, solvent used as synthetic medium (guest solvent) should be removed, so-called activation, in advance of BET surface area measurement. For that purpose, guest solvent having high boiling point (e.g., DEF or DMF) is exchanged with solvent having low boiling point (e.g., chloroform) and then dried under vacuum. However, for HCC-1, this conventional activation method not only
Preheating
Temp. (°C)
Time (min)
Solution for microwave (ml)
120/60/60
80
was so time-consuming job but also had adverse effect on its structural stability. Structural collapse was confirmed by the dramatic
Microwaving
Temp.
(c)
(b) 20 hr
(a)
60 min
60 min
60 min 120 min 180 min
Time Fig. 9. Protocol of microwave method for HCC-1 synthesis.
Fig. 10. SEM images of synthesized HCC-1 with the increase of microwaving time of first step. (a) 60 min, (b) 12 min, and (c) 180 min.
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decreases of X-ray diffraction peak intensity and BET surface area. However this kind of phenomenon is not so common with MOFs having relatively shorter length linkers (e.g., IRMOF-1 or IRMOF3). Thus above observation suggests that the structural stability of HCC-1 is more or less weaker because of its longer length of linker. Nelson et al. [13] and Wang et al. [14] have also reported the similar phenomenon, severe collapse of IRMOF-16, during the conventional activation, and insisted that the generation of capillary force of solvent during thermal evacuation was one of the main reasons for the collapse of MOF structure. In an effort to solve this problem, we introduced continuous CO2 supercritical drying (SCD) method for HCC-1. For that, a certain amount of slurry state HCC-1, right after synthesis, was placed in the cell of 50 ml and then extracted by continuous flow of supercritical state CO2 (50 °C and 200 bar) for 6 h. Fig. 5 shows our custombuilt continuous CO2 SCD equipment. Fig. 6 describes (a) X-ray diffraction pattern of HCC-1 activated by conventional activation method – guest solvent DMF was exchanged with chloroform and then dried under vacuum at 60 °C and (b) X-ray diffraction pattern of HCC-1 activated by CO2 SCD. From the comparison of two patterns, it was observed that the structure of HCC-1 was severely damaged by conventional activation but the structure of HCC-1 remained almost undamaged by CO2 SCD. As a supplementary experiment, observation of the variation of BET surface area was also made to confirm above observation. Fig. 6a showed as low as 250 m2/g, whereas Fig. 6b showed 4724 m2/g. And its measured amount of residual solvent, using TGA, was below 1 wt%. All these observations clearly indicate that CO2 SCD is very effective method for the activation of HCC-1 without severe structural damage. Since HCC-1 was synthesized to be used for the postsynthetic modification, its thermal stability was also important. If it has higher thermal stability, its possible temperature range of postsynthetic modification can be widened. For this reason, we prepared two kinds of HCC-1, having BET surface areas of 2600 and 4724 m2/g, respectively, and placed them in the temperature con-
trolled autoclave (10 bar of H2) for 6 h, and investigated their thermal stabilities by monitoring their variations of BET surface areas. As can be seen in Fig. 7, while the value of BET surface area of HCC1 with 4724 m2/g remained more or less constant till 250 °C, the value of BET surface area of HCC-1 with 2600 m2/g decreased during the entire range of experimental temperature. This observation gave us an idea that there is intimate relationship between thermal stability and BET surface area. We speculate that HCC-1 having lower BET surface area has more unsaturated nodes, chemically unstable part, than HCC-1 having higher BET surface area and this causes lower thermal stability. However, there are some more important factors to be considered for the efficient postsynthetic modification. One of them is the size of MOF. Unlike many other conventional chemical reactions, no stirring is applied to prevent structural collapse of MOF. To compensate that, very long reaction time is usually given for making sure of diffusion of chemical reagents deep into the MOF. Thus synthesizing MOFs having smaller size could be an effective way to make postsynthetic modification more competitive because it can help to reduce reaction time [15]. Although obtaining smaller sized MOF via simple mechanical crushing is the easiest way for above purpose, its structural damage is inevitable. Fig. 8 shows the adverse effect of crushing on MOF structure. We can see that the peak intensity of X-ray diffraction pattern of crushed HCC-1 decreased remarkably, and it is clear evidence of structural collapse. Moreover, the decrease of BET surface area, from 2845 to 939 m2/g, provides us with additional experimental evidence showing structural collapse. Thus we introduced microwave method to obtain smaller sized HCC-1 having no structural collapse problem mentioned above. It is widely documented [16–20] that microwave makes faster and more even heating of reaction system possible, and it promotes to increase the number of generated MOF nuclei. For this reason, smaller sized MOF could be finally obtained. However, as far as we know, more detailed synthetic mechanism is not available yet.
Fig. 11. (a) SEM images of synthesized HCC-1 from Fig. 10(c) and (b) elemental mappings (C, O and Zn) using EDS.
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4. Conclusions
Fig. 12. Powder X-ray diffraction pattern of synthesized HCC-1 via microwave method.
We tried to find out our own optimized reaction conditions through several independent sets of experiments and summarized them in Table 2 and Fig. 9, respectively. One of our noteworthy findings was that the time length of first microwaving step, shown in Fig. 9, was more important than any other conditions for obtaining better shaped HCC-1. As shown in Fig. 10, the outer shape of HCC-1 improved with the increase of microwaving time from 60 to 180 min. Besides, EDS scanning for the mapping of atoms (C, O and Zn) was also performed to understand the origin of ball shaped small particles observed on the surface of MOF shown in Fig. 11. On the basis of mapping results, it was speculated that they were mainly residual organic materials (linker) since distinctive ball shaped mapping patterns were obtained only from C and O. Thus we tried to remove these organic by-products as much as possible via solvent cleaning and decanting, and finally obtained HCC-1 with the BET surface area of 2755 m2/g. Fig. 12 represents the X-ray diffraction pattern of synthesized MOF via microwave method. It is not difficult to derive a conclusion that it has the same crystal structure as that of HCC-1 synthesized via solvothermal method shown in Fig. 2. On the other hand, because its observed average size was about 20 lm, much smaller than HCC-1 shown in Fig. 1, microwave method proved to be very powerful and effective method to obtain smaller sized MOF. However, the effect of MOF size on the efficiency of postsynthetic modification is beyond the scope of this paper, and it will be discussed separately.
The modified IRMOF-16, HCC-1, having functional groups for postsynthetic modification, has been successfully synthesized via solvothermal and microwave method using 1,4-di(4-carboxy-2hydroxyphenyl)benzene as organic ligand and Zn(NO3)26H2O as metal source, respectively. From the solvothermal method, MOF having average size of 500 lm and BET surface area of 4724 m2/g was synthesized. On the other hand, MOF having average size of 20 lm and BET surface area of 2755 m2/g was obtained from microwave method. However, it was found that both MOFs have identical crystal structure–cubic structure. Thermal stability was also investigated. MOF having higher value of BET surface area showed higher thermal stability than that of MOF having lower value of BET surface area, and this indicated that obtaining MOFs with higher value of BET surface area is essential for the wider temperature range of postsynthetic modification. Despite the successful synthesis, conventional method of activation was not proved to be effective for HCC-1 because structural degradation was observed. However, this problem could be minimized with the help of CO2 supercritical drying (SCD) process. References [1] M. Eddaoudi, J. Kim, N.L. Rosi, D.T. Vodak, J. Wachter, M. O’Keeffe, O.M. Yaghi, Science 295 (2002) 469. [2] H. Li, M. Eddaoudi, M. O’Keeffe, O.M. Yaghi, Nature 402 (1999) 276. [3] N.L. Rosi, J. Eckert, M. Eddaoudi, D.T. Vodak, J. Kim, M. O’Keeffe, O.M. Yaghi, Science 300 (2003) 1127. [4] J. Gascon, U. Aktay, M.D. Hernandez-Alonso, G.P.M. van Klink, F. Kapteijn, J. Catal. 261 (2009) 75. [5] H. Frost, T. Duren, R.Q. Snurr, J. Phys. Chem. B 110 (2006) 9565. [6] M.A. Miller, C.Y. Wang, G.N. Merrill, J. Phys. Chem. C 113 (2009) 3222. [7] A.R. Millward, O.M. Yaghi, J. Am. Chem. Soc. 127 (2005) 17998. [8] Y.S. Bae, D. Dubbeldam, A. Nelson, K.S. Walton, J.T. Hupp, R.Q. Snurr, Chem. Mater. 21 (2009) 4768. [9] K.K. Tanabe, Z. Wang, S.M. Cohen, J. Am. Chem. Soc. 130 (2008) 8508. [10] Z. Wang, K.K. Tanabe, S.M. Cohen, Inorg. Chem. 48 (2009) 296. [11] C. Wu, A. Hu, L. Zhang, W. Lin, J. Am. Chem. Soc. 127 (2005) 8940. [12] J.L.C. Rowsell, O.M. Yaghi, Microporous Mesoporous Mater. 73 (2004) 3. [13] A.P. Nelson, O.K. Farha, K.L. Mulfort, J.T. Hupp, J. Am. Chem. Soc. 131 (2009) 458. [14] K. Wang, N. Petkov, M.A. Morris, J.D. Holmes, Stud. Surf. Sci. Catal. 170 (2007) 1796. [15] L. Huang, H. Wang, J. Chen, Z. Wang, J. Sun, D. Zhao, Y. Yan, Microporous Mesoporous Mater. 58 (2003) 105. [16] Z. Xiang, D. Cao, X. Shao, W. Wang, J. Zhang, W. Wu, Chem. Eng. Sci. 65 (2010) 3140. [17] G.A. Tompsett, W.C. Conner, K.S. Yngvesson, Chemphyschem 7 (2006) 296. [18] S.H. Jhung, T. Jin, Y.K. Hwang, J.S. Chang, Chem. Eur. J. 13 (2007) 4410. [19] J.S. Choi, W.J. Son, J.H. Kim, W.S. Ahn, Microporous Mesoporous Mater. 116 (2008) 727. [20] C.-M. Lu, J. Liu, K. Xiao, A.T. Harris, Chem. Eng. J. 156 (2010) 465.