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Synthesis of MOF having hydroxyl functional side groups and optimization of activation process for the maximization of its BET surface area
Journal of Solid State Chemistry 197 (2013) 261–265
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Synthesis of MOF having hydroxyl functional side groups and optimization of activation process for the maximization of its BET surface area Jongsik Kim 1, Dong Ok Kim n,1, Dong Wook Kim, Kil Sagong Hanwha Chemical Research & Development Center, 6, Shinseong-dong, Yuseong-gu, Daejeon 305-804, Republic of Korea
a r t i c l e i n f o
abstract
Article history: Received 17 May 2012 Received in revised form 16 August 2012 Accepted 20 August 2012 Available online 4 September 2012
To accomplish the postsynthetic modification of MOF with organic-metal precursors (OMPs) described in our previous researches more efficiently, synthesis of MOF (HCC-2) possessing relatively larger pore size as well as higher number of hydroxyl functional side groups per its base unit than those of HCC-1 has been successfully conducted via adopting 1,4-di-(4-carboxy-2,6-dihydroxyphenyl)benzene as an organic ligand and Zn(NO3)2 6H2O as a metal source, respectively. Also, optimization about the Activation process of HCC-2 was performed to maximize its BET (Brunauer–Emmett–Teller) surface area which was proved to be proportional to the number of exposed active sites on which its postsynthetic modification occurred. However, Activation process having been validated to be so effective with the acquirement of highly-purified HCC-1 (CO2 supercritical drying step followed by vacuum drying step) was less satisfactory with the case of HCC-2. This might be attributed to relatively higher hydrophilicity and bulkier molecular structure of organic ligand of HCC-2. However, it was readily settled by simple modification of above Activation process. Moreover, indispensable residues composed of both DMF and its thermally degraded derivatives which were chemically attached via coordination bond with hydroxyl functionalities even after Activation process III might enable their H2 adsorption properties to be seriously debased compared to that of IRMOF-16 having no hydroxyl functionalities. & 2012 Elsevier Inc. All rights reserved.
Keywords: MOF Postsynthetic modification Organic-metal precursor Hydroxyl functional side group Activation process CO2 supercritical drying
1. Introduction Metal-organic frameworks (MOFs), a kind of coordination polymers, have been widely synthesized and investigated by many research groups [1–11] throughout the world since the introduction of MOF-5 by Yaghi’s group in the late 1990s [12,13]. Generally, MOFs are structured by the simple combination of two different kinds of building blocks, linker (organic ligand) and node (metal cluster), so that innumerable kinds of MOFs having different structures and properties from one another can be facilely obtained through diverse variations of these two blocks. This trait of MOFs helps furnish themselves with great advantages in designing their pore size and unique structure and designates themselves as more competitive nano-materials for the application to chemical reaction catalyst, special purpose membrane, selective gas adsorption material, etc. However, while deepening our knowledge of MOFs, it has become widely recognizable that their postsynthetic modification, the modification with specific chemical reagents (such as OMPs)
n
Corresponding author. Fax: þ82 42 865 6570. E-mail address: [email protected] (D.O. Kim). 1 Contributed equally to this work.
0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2012.08.046
following the formation of their crystal lattice, would be far more efficient way to endow them with a variety of functionalities and novel physical properties than time- and effort-consuming synthesis of new MOFs. Specifically, Cohen et al. [14] researched that IRMOF-3 in which one amine functional side group was included on the organic ligand could be postsynthetically modified via a series of its covalent bonds with alkyl anhydrides for the application to varied fields. Moreover, they [15] also suggested that one of the essential problems MOFs generally possessed, the significant instability under moisture atmosphere, could be remarkably resolved by introducing hydrophobic alkyl chain via postsynthetic modification. Meanwhile, Britt et al. [16] reported the successful incorporation of sulfonate and ethylenediamine groups on IRMOF-3. Still, the functional side group of the MOF structure has been commonly reported to be fairly rare owing to its propensity to interact with the metal source (metal ion) which was applied as one of the essential reactants during its synthesis. Being distinguished from aforementioned researchers to study the postsynthetic modifications of MOFs with various organic chemical reagents, Doonan et al. [17] reported successful synthesis of metal-complexed MOF via two-step postsynthetic modification. Using similar technique, Wu et al. [18] tried to obtain catalytically active chiral porous MOF. In the concrete, they
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synthesized MOF to be characterized by triclinic P1 space group as well as orthogonal chiral 2,20 -dihydroxy secondary functionality, and then successfully modified it with inorganic chemical reagent, Ti(O iPr)4. In terms of the conclusive remarks relevant to the postsynthetic modification of MOFs mentioned above, since the fact that the structure of IRMOF-3 contains amino-terephthalic acid as its organic ligand enables it to possess relatively small pore size, IRMOF-s has a tendency to retard the rate and to lower the yield of its postsynthetic modification. Furthermore, the number of applicable chemical reagents which can be chemically bound to the amine functional side group of organic ligand for IRMOF-3 is also restricted in that its organic ligand has only one active functional side group. On the contrary, now that MOFs investigated by Doonan et.al. [17] and Wu et al. [18] have relatively larger pore sizes, these research groups could eradicate the mass transfer problem of applied chemical reagents toward MOFs’ pores to some extent during their postsynthetic modification. Nevertheless, a crucial problem germane to aforementioned MOFs’ organic ligand in which limited number of functional side group existed still remained. On top of that, the fact that the molecular structure of MOFs’ organic ligands has no room for its additional chemical modification (e.g., the addition of bonds containing more than one of hydroxyl, amine, and thiol functional side groups to the organic ligand) makes further systematic research about their postsynthetic modification be almost out of the question. In an attempt to overcome abovementioned problems, our group has been trying to synthesize new MOFs having both larger pore size and higher number of functional side groups per their base unit at the same time. In order to attain this purpose, a homologous series of p-terphenyl-4,40 -dicarboxylic acid having hydroxyl functional side groups, ranging from 0 to 4, have been prepared. As a first result of this study, we [19] have recently reported successful synthesis of modified IRMOF-16 (HCC-1) using one of above linkers, 1,4-di(4-carboxy-2-hydroxyphenyl)benzene in which two hydroxyl functional side groups were contained, as an organic ligand and Zn(NO3)2 6H2O as a metal source, respectively. Besides, our studies [20,21] relevant to its postsynthetic modification with various kinds of OMPs have been performed. Although HCC-1 did provide us with a wonderful platform for postsynthetic modification, it still needed further structural improvement because its organic ligand has just two hydroxyl functional side groups. Thus, we recently prepared another organic ligand, 1,4-di-(4-carboxy-2,6-dihydroxyphenyl)benzene having four hydroxyl functional side groups and then finally succeeded in synthesizing HCC-2 via solvothermal method with Zn(NO3)2 6H2O as a metal source. Subsequently, the modified Activation process including CO2 supercritical drying step has been adapted in order to minimize its pore structure collapse and to facilitate the elimination of remnant chemicals (such as unreacted organic ligand, its undesirably formed oligomeric compounds, and reaction solvent) included in HCC-2’s pores.
2. Experimental The minute delineation with regard to the synthesis and characterization of HCC-2 was provided in the supplementary material. The contents contained in this supplementary material can be briefly listed as follows: Synthesis of organic ligand, Synthesis of HCC-2 via solvothermal synthesis, Activation process of HCC-2, and Characterization of HCCs. 3. Results and discussion In this study, HCC-2 has been synthesized via solvothermal method which is widely accepted as one of the most common and
powerful methods for MOF synthesis. Specifically, this was originated to the following presumption that solvents, such as DMF and DEF, produce basic chemicals through thermal decomposition with the help of Zn2 þ introduced as metal source and that they activate MOF synthesis by inducing deprotonation of organic ligands [22]. However, to the best of our knowledge, more detailed information about its synthetic mechanism as well as each chemical’s role has not been affirmative so far. With a view to ultimately fulfilling successful postsynthetic modification of a series of HCCs, complete removal of unwanted residues (e.g., unreacted organic ligand, its oligomeric compounds, and reaction solvent) presented in MOFs’ pores, so-called Activation step, should be done first. Even though a conventional thermal evacuation has been usually proved to be fairly effective for this purpose via both computer simulation and experiment [22,23], it does not always show satisfactory results, especially in case that it is applied to MOFs having organic ligands with relatively longer chains. It is renowned that strong capillary force could be generated during the removal of reaction solvent from MOF’s pores and that this force might give rise to dramatically collapse its pore structure. Thus, solvent exchange (from either DMF or DEF to chloroform) followed by either thermal evacuation at moderate temperature or CO2 supercritical drying would have been widely exploited nowadays in an effort to overcome this problem [24–26]. Especially, CO2 supercritical drying was proved to be undeniably efficient for the elimination of abovementioned residues in MOF’s pores [19–21,24–26]. However, unlike the result studied by Hupp et al. (in case of IRMOF-16, BET surface area increased up to 306% from 470 m2/g to 1910 m2/g) [25,26,37], two step comprising of solvent exchange with absolute ethanol followed by supercritical CO2 drying step alone could not remove aforementioned unwanted residues inside HCCs’ pores thoroughly (not shown). This might be attributed to hydroxyl functional side groups of HCCs which IRMOF-16 did not possess. Moreover, as compared to DMF (reaction solvent), absolute ethanol which highly tends to bind with functional groups of HCCs via hydrogen bonding mechanism might prevent these residues from being extinguished completely inside HCCs’ pores [19–21]. In this study, three kinds of Activation processes which was based on the combination of previous studies [19–26] and characterized by a series of consecutive steps were proposed (Fig. 1). Among them, CO2 supercritical drying step was proved to be a key process and was performed via contacting HCC-2 with continuous flow of supercritical state CO2 with the speed of 10 ml/min at 50 1C and 200 bar for 6 h using custombuilt equipment designed by our research group (Fig. S5). After executing Activation process I, cubic shaped HCC-2 crystal was finally observed through SEM (scanning electron microscopy) as displayed in Fig. 2(a) and (c). The value of its BET surface area reached up to 580 m2/g, while it is far smaller than those of HCC-1, 2000, 2600, 3000 and 4724 m2/g, reported in our previous studies [19,20]. Although not being shown here, TGA (thermogravimetric analysis) was also conducted in order to scrutinize the critical factor resulting in such a low BET value for HCC-2. Unlike the TGA result obtained in HCC-1, the TGA result in HCC-2 showed continuous mass reduction above room temperature. As a first action to cope with this problem, temperature of vacuum drying process increased up to 200 1C in order to promote the efficiency of the Activation process [Activation process II, Fig. 1(b)]. However, as compared to Activation process I, no considerable change in BET surface area (Table 1), TGA, and SEM image [Fig. 2(d)] was still observed. This was presumably attributed to the fact that significant amount of undesirable impurities including reaction solvent was not perfectly eliminated from HCC-2’s pores and still existed inside them. This presumption
J. Kim et al. / Journal of Solid State Chemistry 197 (2013) 261–265
was additionally validated by the results via XRF, EA and pore size distribution (PSD) displayed in Table S2 and Fig. S10. Specifically, as indicated in CASE B (percent error) of Table S2, considerably large amount of DMF was not yet fully removed from HCC-2’s pores. Moreover, noticeable capillary force arising from the extant DMF presented in these pores might result in the serious damage upon these pore structures as shown on CASE B of Fig. S2. Thus, the majority of its pores were distributed within the wide range from 39 nm to 755 nm. As another measure to solve this problem, solvent exchanging step being soaked in the chloroform for about 30 h (6 h 5 times) was inserted between ‘solvent decanting step’ and ‘CO2 supercritical drying step’ [Activation process III, Fig. 1(c)]. It was based on the assumption that solvent exchange of DMF with chloroform might be helpful to dramatically increase the efficiency of CO2 supercritical drying due to three decisive factors. Specifically, as one of these factors, it is well-known that DMF has a high
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inclination to generate bonding with the hydrogen atom of hydroxyl functional side groups presented in HCC-2’s pores via hydrogen bonding mechanism. Thus, with a view to minimizing the chance to form this bonding, solvent exchange step mentioned above might be indispensably conducted. Moreover, as another of these factors, the chloroform, relatively possessing higher solubility in the supercritical state CO2 (mimic to the liquid pentane) as well as higher capability to solve both unreacted organic ligand and its oligomeric compounds than those of DMF, might enable these impurities to be removed more prominently. Furthermore, as the other of these factors, executing Activation process III [with solvent exchange, Fig. 1(c)] might be of great help in eliminating trace amount of chloroform (boiling point: 61.2 1C) within HCC-2’s pores more predominantly than removing small amount of DMF (boiling point: 153.1 1C) by conducting Activation process II [without solvent exchange, Fig. 1(b)]. Interestingly enough, in case of following Activation process III, the value of BET surface area was remarkably increased by up to 534% (602 m2/g-3820 m2/g) as summarized in Table 1. The effect resulting from the insertion of solvent exchange step in the Activation process III (CASE C) was reaffirmed by its PSD result shown in Fig. S10. Being compared to the PSD result caused by Activation process II (CASE B), CASE C was characterized by the Table 1 Effect of the change in the Activation conditions on the BET surface areas (SBET, m2/g) for prepared HCCs. Sample
HCC-2 w/o solvent exchange
w/solvent exchange
HCC-1 w/o solvent exchange
w/solvent exchange Fig. 1. Suggested activation processes applied to HCCs for the maximization of their BET surface areas and the acquirement of their highly-purified forms: (a) activation process I, (b) activation process II, and (c) activation process III.
Vacuum drying temperature (1C)
BET surface area (SBET, m2/g)
150 180 200 150 180 200
580 (CASE A) 596 602 (CASE B) 3520 3715 3820 (CASE C)
150 180 200 150 180 200
3815 4280 3600 (CASE D) 5095 5166 5634 (CASE E)
Fig. 2. SEM images of HCC-2 with low magnification ( 100) resulting from (a) activation process I/CASE A and (b) activation process III/CASE C as well as ones of HCC-2 with high magnification ( 2000) resulting from (c) activation process I/CASE A, (d) activation process II/CASE B, and (e) activation process III/CASE C.
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fact that majority of the pores was distributed within the smaller size ranging from 5 nm to 22 nm. This indicated that relatively smaller capillary force originating from the slight amount of chloroform presented in HCC-2’s pores might be hardly exerted on the collapse of these pores. Moreover, as compared to the percent errors of CASE B in Table S2, those of CASE C decreased significant. This meant that there might be significantly trace amount of chloroform in HCC-2’s pores. Above effect was finally confirmed by observing totally different shape of TGA result (not shown here) and SEM images [Fig. 2(b) and (e)]. TGA result showed no mass reduction till thermal degradation temperature of HCC-2 (about 300 1C) while substantial difference in HCC-2’s morphology was observed after Activation process III as shown in Fig. 2(e). This indicated that the removal of impurities to block HCC-2’s pores was proceeded visually and significantly during Activation process III. Activation process III was also applied to HCC-1 to deeply elaborate its effect on BET surface area of HCC-1 having lower number of hydroxyl functional side groups per its base unit than that of HCC-2. Although the effect of Activation process III on HCC-1 was not as prominent as one observed from HCC-2, the value of BET surface area has also increased by up to 56.5% (3600 m2/g-5634 m2/g). Based on the observations made above, it is speculated that the Activation process II, namely, ‘CO2 supercritical drying step followed by vacuum drying step’ was not efficient enough for a series of HCCs to be properly activated, especially in case of HCC-2 possessing relatively higher number of hydroxyl functional side groups per its base unit as well as relatively bulkier molecular structure of its organic ligand than those of HCC-1. To be specific, higher number of hydroxyl functional side groups within HCC-2’s pores might force effective elimination of DMF more arduous due to the larger amount of hydrogen bonding between hydroxyl functional side group and DMF. This assumption could be proved in the comparison of PSD between HCC-2 (CASE B) and HCC-1 (CASE D) displayed in Fig. S10. Considerably larger capillary force of remnant DMF exerted on HCC-2’s pores might be in charge of the severe degradation of its structure which might ideally ˚ In other words, DMF might possess uniform pore size (21.5 A). be responsible for the significant increase in HCC-2’s pore size mostly ranging within the realm of both mesopore and macropore (from 39 nm to 755 nm). Furthermore, bulkier molecular structure of HCC-2’s organic ligands might enable effective elimination of either unreacted organic ligands or its oligomeric compounds more difficult owing to the higher possibility to the generation of steric hindrance among them. The significant difference about the percent errors (%) between CASE B and CASE D (Table S2) might additionally elucidate the validity of aforementioned assumption. With the preparation of activated HCC-2, its structural analysis was also conducted via PXRD (Powder X-ray diffraction). In that the structure of HCC-2 was modified from that of either IRMOF-16 or HCC-1, three XRD patterns were displayed together for the comparison of their structures. In Fig. 3, HCC-2 showed an exactly same diffraction pattern as those of IRMOF-16 and HCC-1, and this drove us to conclude that these three MOFs have the same crystalline structure, cubic structure having no interpenetration ˚ In other words, no change in with each axis of about 21.5 A. crystalline structure has been made from the introduction of various number of hydroxyl functional side groups to the backbone of organic ligand molecules of IRMOF-16. For reference, further intensive discussion about X-ray crystallography can be available from our previous studies [19–21]. FT-IR analysis to verify the structural characteristics of HCCs (the presence of hydroxyl functionality) definitely was also conducted as shown in Fig. 4. Hydroxyl functional side groups were detected
Fig. 3. Powder X-ray diffraction (PXRD) patterns of IRMOF-16, HCC-1, and HCC-2.
Fig. 4. FT-IR analysis (Nujol mulls between two NaCl plates, 500–4000 cm 1) of (a) IRMOF-16, (b) HCC-1, and (c) HCC-2.
as a broad peak at 3572 cm 1 for HCC-1 and 3566 cm 1 for HCC-2, respectively while no distinguishable peak near these regions was observed for IRMOF-16 [27,28]. As an effort to verify the effect of hydroxyl functionalities of both HCCs, H2 adsorption isotherms at different temperatures (77 K and 87 K) and isosteric heats of adsorption (Qst) for HCC-1, HCC-2, and IRMOF-16 were obtained as shown Fig. S11–S13 and Table S3. Concretely, three MOFs possessing the similar range of BET surface areas (SBET, 3000 m2/g) and being equally adapted to Activation process III were yielded in order to minimize the effect of impurities inside their pores bound coordinately to imperfectly formed nodes (such as coordinatively unsatured sites and exposed metal sites) of HCCs as well as the effect of pore properties (pore size, pore volume, etc.) [29–36]. The experimental results from these MOFs seemed that hydroxyl functionalities, if any, could seriously degrade the H2 adsorption ability of MOFs. As shown Fig. S11 and S13, IRMOF-16 (without hydroxyl functionality) could adsorb relatively larger amount of H2 gas than two HCCs (with functionality) and its Qst was the highest among them. Under the assumption that the
J. Kim et al. / Journal of Solid State Chemistry 197 (2013) 261–265
unexpectedly negative effect of hydroxyl functionalities of HCCs in terms of H2 adsorption ability might be ascribed to the presence of DMF and its derivatives which could be still resided inside their pores even after Activation process III, three MOFs were thermally treated via custom-built continuous thermal treatment equipment (Fig. S14). The gas collected during the thermal treatment under high-purified Argon atmosphere (99.9999%) in the temperature range from 25 to 600 1C (50 1C/min) was qualitatively analyzed via GC–MS. As a result, DMF and its derivatives which might originate from the thermal decomposition of DMF during the course of either the formation of MOF or its Activation process would be detected (not shown here). Moreover, from the previous observation (no weight loss until 300 1C in TGA analysis), these impurities proved not to be detached from the HCCs’ pores even at the 300 1C. This might result from the fact that hydroxyl functional side groups of HCCs were the preferential adsorption sites for H2 gas [37–41] and that DMF and its derivatives which preoccupied these sites predominantly via significantly rigid bonds with abovementioned side groups incurred the noticeable decrease in H2 adsorption ability for HCCs. 4. Conclusions To sum up, HCC-2 possessing both relatively larger pore size and higher number of hydroxyl functional side groups per its base unit at the same time than those of HCC-1 has been prepared via solvothermal method. In order to execute its postsynthetic modification with various organic-metal precursors more efficiently, the research about the optimization relevant to the maximization of its BET surface area has been simultaneously conducted. The inclusion of solvent exchange step (from DMF to the chloroform) between ‘solvent decanting step’ and ‘CO2 supercritical drying step’ would be of great importance in maximizing the BET surface area of HCC-2. This might be ascribed to the fatal role of chloroform which could compensate for two drawbacks of HCC-2’s organic ligand which has relatively higher number of hydroxyl functional side group and bulkier molecular structure than those of HCC-1’s organic ligand. The fact that XRD pattern of HCC-2 mimic those of IRMOF-16 and HCC-1 suggested that HCC-2 have a cubic-shaped structure with each axis of about 21.5 A˚ without structural interpretation. Also, the structural characteristics of HCCs having hydroxyl functionality were directly proved via FT-IR analysis. However, DMF and its thermally decomposed derivatives which coordinately preoccupied hydroxyl functional side groups of HCCs rendered their hydrogen adsorption abilities to be seriously debased.
Acknowledgments This research has been bolstered by the Research Consortium having been leaded by Professor Jisoon Ihm affiliated to the Department of Physics and Astronomy in Seoul National University, Republic of Korea. This Research Consortium has been mainly sponsored by Hanwha Chemical Corporation for the development of state-of-the-art hydrogen materials to adsorb hydrogen molecules via Kubas-type adsorption mechanism.
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Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jssc.2012.08.046.
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Rapid Communication
Synthesis of MOF having hydroxyl functional side groups and optimization of activation process for the maximization of its BET surface area Jongsik Kim 1, Dong Ok Kim n,1, Dong Wook Kim, Kil Sagong Hanwha Chemical Research & Development Center, 6, Shinseong-dong, Yuseong-gu, Daejeon 305-804, Republic of Korea
a r t i c l e i n f o
abstract
Article history: Received 17 May 2012 Received in revised form 16 August 2012 Accepted 20 August 2012 Available online 4 September 2012
To accomplish the postsynthetic modification of MOF with organic-metal precursors (OMPs) described in our previous researches more efficiently, synthesis of MOF (HCC-2) possessing relatively larger pore size as well as higher number of hydroxyl functional side groups per its base unit than those of HCC-1 has been successfully conducted via adopting 1,4-di-(4-carboxy-2,6-dihydroxyphenyl)benzene as an organic ligand and Zn(NO3)2 6H2O as a metal source, respectively. Also, optimization about the Activation process of HCC-2 was performed to maximize its BET (Brunauer–Emmett–Teller) surface area which was proved to be proportional to the number of exposed active sites on which its postsynthetic modification occurred. However, Activation process having been validated to be so effective with the acquirement of highly-purified HCC-1 (CO2 supercritical drying step followed by vacuum drying step) was less satisfactory with the case of HCC-2. This might be attributed to relatively higher hydrophilicity and bulkier molecular structure of organic ligand of HCC-2. However, it was readily settled by simple modification of above Activation process. Moreover, indispensable residues composed of both DMF and its thermally degraded derivatives which were chemically attached via coordination bond with hydroxyl functionalities even after Activation process III might enable their H2 adsorption properties to be seriously debased compared to that of IRMOF-16 having no hydroxyl functionalities. & 2012 Elsevier Inc. All rights reserved.
Keywords: MOF Postsynthetic modification Organic-metal precursor Hydroxyl functional side group Activation process CO2 supercritical drying
1. Introduction Metal-organic frameworks (MOFs), a kind of coordination polymers, have been widely synthesized and investigated by many research groups [1–11] throughout the world since the introduction of MOF-5 by Yaghi’s group in the late 1990s [12,13]. Generally, MOFs are structured by the simple combination of two different kinds of building blocks, linker (organic ligand) and node (metal cluster), so that innumerable kinds of MOFs having different structures and properties from one another can be facilely obtained through diverse variations of these two blocks. This trait of MOFs helps furnish themselves with great advantages in designing their pore size and unique structure and designates themselves as more competitive nano-materials for the application to chemical reaction catalyst, special purpose membrane, selective gas adsorption material, etc. However, while deepening our knowledge of MOFs, it has become widely recognizable that their postsynthetic modification, the modification with specific chemical reagents (such as OMPs)
n
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following the formation of their crystal lattice, would be far more efficient way to endow them with a variety of functionalities and novel physical properties than time- and effort-consuming synthesis of new MOFs. Specifically, Cohen et al. [14] researched that IRMOF-3 in which one amine functional side group was included on the organic ligand could be postsynthetically modified via a series of its covalent bonds with alkyl anhydrides for the application to varied fields. Moreover, they [15] also suggested that one of the essential problems MOFs generally possessed, the significant instability under moisture atmosphere, could be remarkably resolved by introducing hydrophobic alkyl chain via postsynthetic modification. Meanwhile, Britt et al. [16] reported the successful incorporation of sulfonate and ethylenediamine groups on IRMOF-3. Still, the functional side group of the MOF structure has been commonly reported to be fairly rare owing to its propensity to interact with the metal source (metal ion) which was applied as one of the essential reactants during its synthesis. Being distinguished from aforementioned researchers to study the postsynthetic modifications of MOFs with various organic chemical reagents, Doonan et al. [17] reported successful synthesis of metal-complexed MOF via two-step postsynthetic modification. Using similar technique, Wu et al. [18] tried to obtain catalytically active chiral porous MOF. In the concrete, they
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synthesized MOF to be characterized by triclinic P1 space group as well as orthogonal chiral 2,20 -dihydroxy secondary functionality, and then successfully modified it with inorganic chemical reagent, Ti(O iPr)4. In terms of the conclusive remarks relevant to the postsynthetic modification of MOFs mentioned above, since the fact that the structure of IRMOF-3 contains amino-terephthalic acid as its organic ligand enables it to possess relatively small pore size, IRMOF-s has a tendency to retard the rate and to lower the yield of its postsynthetic modification. Furthermore, the number of applicable chemical reagents which can be chemically bound to the amine functional side group of organic ligand for IRMOF-3 is also restricted in that its organic ligand has only one active functional side group. On the contrary, now that MOFs investigated by Doonan et.al. [17] and Wu et al. [18] have relatively larger pore sizes, these research groups could eradicate the mass transfer problem of applied chemical reagents toward MOFs’ pores to some extent during their postsynthetic modification. Nevertheless, a crucial problem germane to aforementioned MOFs’ organic ligand in which limited number of functional side group existed still remained. On top of that, the fact that the molecular structure of MOFs’ organic ligands has no room for its additional chemical modification (e.g., the addition of bonds containing more than one of hydroxyl, amine, and thiol functional side groups to the organic ligand) makes further systematic research about their postsynthetic modification be almost out of the question. In an attempt to overcome abovementioned problems, our group has been trying to synthesize new MOFs having both larger pore size and higher number of functional side groups per their base unit at the same time. In order to attain this purpose, a homologous series of p-terphenyl-4,40 -dicarboxylic acid having hydroxyl functional side groups, ranging from 0 to 4, have been prepared. As a first result of this study, we [19] have recently reported successful synthesis of modified IRMOF-16 (HCC-1) using one of above linkers, 1,4-di(4-carboxy-2-hydroxyphenyl)benzene in which two hydroxyl functional side groups were contained, as an organic ligand and Zn(NO3)2 6H2O as a metal source, respectively. Besides, our studies [20,21] relevant to its postsynthetic modification with various kinds of OMPs have been performed. Although HCC-1 did provide us with a wonderful platform for postsynthetic modification, it still needed further structural improvement because its organic ligand has just two hydroxyl functional side groups. Thus, we recently prepared another organic ligand, 1,4-di-(4-carboxy-2,6-dihydroxyphenyl)benzene having four hydroxyl functional side groups and then finally succeeded in synthesizing HCC-2 via solvothermal method with Zn(NO3)2 6H2O as a metal source. Subsequently, the modified Activation process including CO2 supercritical drying step has been adapted in order to minimize its pore structure collapse and to facilitate the elimination of remnant chemicals (such as unreacted organic ligand, its undesirably formed oligomeric compounds, and reaction solvent) included in HCC-2’s pores.
2. Experimental The minute delineation with regard to the synthesis and characterization of HCC-2 was provided in the supplementary material. The contents contained in this supplementary material can be briefly listed as follows: Synthesis of organic ligand, Synthesis of HCC-2 via solvothermal synthesis, Activation process of HCC-2, and Characterization of HCCs. 3. Results and discussion In this study, HCC-2 has been synthesized via solvothermal method which is widely accepted as one of the most common and
powerful methods for MOF synthesis. Specifically, this was originated to the following presumption that solvents, such as DMF and DEF, produce basic chemicals through thermal decomposition with the help of Zn2 þ introduced as metal source and that they activate MOF synthesis by inducing deprotonation of organic ligands [22]. However, to the best of our knowledge, more detailed information about its synthetic mechanism as well as each chemical’s role has not been affirmative so far. With a view to ultimately fulfilling successful postsynthetic modification of a series of HCCs, complete removal of unwanted residues (e.g., unreacted organic ligand, its oligomeric compounds, and reaction solvent) presented in MOFs’ pores, so-called Activation step, should be done first. Even though a conventional thermal evacuation has been usually proved to be fairly effective for this purpose via both computer simulation and experiment [22,23], it does not always show satisfactory results, especially in case that it is applied to MOFs having organic ligands with relatively longer chains. It is renowned that strong capillary force could be generated during the removal of reaction solvent from MOF’s pores and that this force might give rise to dramatically collapse its pore structure. Thus, solvent exchange (from either DMF or DEF to chloroform) followed by either thermal evacuation at moderate temperature or CO2 supercritical drying would have been widely exploited nowadays in an effort to overcome this problem [24–26]. Especially, CO2 supercritical drying was proved to be undeniably efficient for the elimination of abovementioned residues in MOF’s pores [19–21,24–26]. However, unlike the result studied by Hupp et al. (in case of IRMOF-16, BET surface area increased up to 306% from 470 m2/g to 1910 m2/g) [25,26,37], two step comprising of solvent exchange with absolute ethanol followed by supercritical CO2 drying step alone could not remove aforementioned unwanted residues inside HCCs’ pores thoroughly (not shown). This might be attributed to hydroxyl functional side groups of HCCs which IRMOF-16 did not possess. Moreover, as compared to DMF (reaction solvent), absolute ethanol which highly tends to bind with functional groups of HCCs via hydrogen bonding mechanism might prevent these residues from being extinguished completely inside HCCs’ pores [19–21]. In this study, three kinds of Activation processes which was based on the combination of previous studies [19–26] and characterized by a series of consecutive steps were proposed (Fig. 1). Among them, CO2 supercritical drying step was proved to be a key process and was performed via contacting HCC-2 with continuous flow of supercritical state CO2 with the speed of 10 ml/min at 50 1C and 200 bar for 6 h using custombuilt equipment designed by our research group (Fig. S5). After executing Activation process I, cubic shaped HCC-2 crystal was finally observed through SEM (scanning electron microscopy) as displayed in Fig. 2(a) and (c). The value of its BET surface area reached up to 580 m2/g, while it is far smaller than those of HCC-1, 2000, 2600, 3000 and 4724 m2/g, reported in our previous studies [19,20]. Although not being shown here, TGA (thermogravimetric analysis) was also conducted in order to scrutinize the critical factor resulting in such a low BET value for HCC-2. Unlike the TGA result obtained in HCC-1, the TGA result in HCC-2 showed continuous mass reduction above room temperature. As a first action to cope with this problem, temperature of vacuum drying process increased up to 200 1C in order to promote the efficiency of the Activation process [Activation process II, Fig. 1(b)]. However, as compared to Activation process I, no considerable change in BET surface area (Table 1), TGA, and SEM image [Fig. 2(d)] was still observed. This was presumably attributed to the fact that significant amount of undesirable impurities including reaction solvent was not perfectly eliminated from HCC-2’s pores and still existed inside them. This presumption
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was additionally validated by the results via XRF, EA and pore size distribution (PSD) displayed in Table S2 and Fig. S10. Specifically, as indicated in CASE B (percent error) of Table S2, considerably large amount of DMF was not yet fully removed from HCC-2’s pores. Moreover, noticeable capillary force arising from the extant DMF presented in these pores might result in the serious damage upon these pore structures as shown on CASE B of Fig. S2. Thus, the majority of its pores were distributed within the wide range from 39 nm to 755 nm. As another measure to solve this problem, solvent exchanging step being soaked in the chloroform for about 30 h (6 h 5 times) was inserted between ‘solvent decanting step’ and ‘CO2 supercritical drying step’ [Activation process III, Fig. 1(c)]. It was based on the assumption that solvent exchange of DMF with chloroform might be helpful to dramatically increase the efficiency of CO2 supercritical drying due to three decisive factors. Specifically, as one of these factors, it is well-known that DMF has a high
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inclination to generate bonding with the hydrogen atom of hydroxyl functional side groups presented in HCC-2’s pores via hydrogen bonding mechanism. Thus, with a view to minimizing the chance to form this bonding, solvent exchange step mentioned above might be indispensably conducted. Moreover, as another of these factors, the chloroform, relatively possessing higher solubility in the supercritical state CO2 (mimic to the liquid pentane) as well as higher capability to solve both unreacted organic ligand and its oligomeric compounds than those of DMF, might enable these impurities to be removed more prominently. Furthermore, as the other of these factors, executing Activation process III [with solvent exchange, Fig. 1(c)] might be of great help in eliminating trace amount of chloroform (boiling point: 61.2 1C) within HCC-2’s pores more predominantly than removing small amount of DMF (boiling point: 153.1 1C) by conducting Activation process II [without solvent exchange, Fig. 1(b)]. Interestingly enough, in case of following Activation process III, the value of BET surface area was remarkably increased by up to 534% (602 m2/g-3820 m2/g) as summarized in Table 1. The effect resulting from the insertion of solvent exchange step in the Activation process III (CASE C) was reaffirmed by its PSD result shown in Fig. S10. Being compared to the PSD result caused by Activation process II (CASE B), CASE C was characterized by the Table 1 Effect of the change in the Activation conditions on the BET surface areas (SBET, m2/g) for prepared HCCs. Sample
HCC-2 w/o solvent exchange
w/solvent exchange
HCC-1 w/o solvent exchange
w/solvent exchange Fig. 1. Suggested activation processes applied to HCCs for the maximization of their BET surface areas and the acquirement of their highly-purified forms: (a) activation process I, (b) activation process II, and (c) activation process III.
Vacuum drying temperature (1C)
BET surface area (SBET, m2/g)
150 180 200 150 180 200
580 (CASE A) 596 602 (CASE B) 3520 3715 3820 (CASE C)
150 180 200 150 180 200
3815 4280 3600 (CASE D) 5095 5166 5634 (CASE E)
Fig. 2. SEM images of HCC-2 with low magnification ( 100) resulting from (a) activation process I/CASE A and (b) activation process III/CASE C as well as ones of HCC-2 with high magnification ( 2000) resulting from (c) activation process I/CASE A, (d) activation process II/CASE B, and (e) activation process III/CASE C.
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fact that majority of the pores was distributed within the smaller size ranging from 5 nm to 22 nm. This indicated that relatively smaller capillary force originating from the slight amount of chloroform presented in HCC-2’s pores might be hardly exerted on the collapse of these pores. Moreover, as compared to the percent errors of CASE B in Table S2, those of CASE C decreased significant. This meant that there might be significantly trace amount of chloroform in HCC-2’s pores. Above effect was finally confirmed by observing totally different shape of TGA result (not shown here) and SEM images [Fig. 2(b) and (e)]. TGA result showed no mass reduction till thermal degradation temperature of HCC-2 (about 300 1C) while substantial difference in HCC-2’s morphology was observed after Activation process III as shown in Fig. 2(e). This indicated that the removal of impurities to block HCC-2’s pores was proceeded visually and significantly during Activation process III. Activation process III was also applied to HCC-1 to deeply elaborate its effect on BET surface area of HCC-1 having lower number of hydroxyl functional side groups per its base unit than that of HCC-2. Although the effect of Activation process III on HCC-1 was not as prominent as one observed from HCC-2, the value of BET surface area has also increased by up to 56.5% (3600 m2/g-5634 m2/g). Based on the observations made above, it is speculated that the Activation process II, namely, ‘CO2 supercritical drying step followed by vacuum drying step’ was not efficient enough for a series of HCCs to be properly activated, especially in case of HCC-2 possessing relatively higher number of hydroxyl functional side groups per its base unit as well as relatively bulkier molecular structure of its organic ligand than those of HCC-1. To be specific, higher number of hydroxyl functional side groups within HCC-2’s pores might force effective elimination of DMF more arduous due to the larger amount of hydrogen bonding between hydroxyl functional side group and DMF. This assumption could be proved in the comparison of PSD between HCC-2 (CASE B) and HCC-1 (CASE D) displayed in Fig. S10. Considerably larger capillary force of remnant DMF exerted on HCC-2’s pores might be in charge of the severe degradation of its structure which might ideally ˚ In other words, DMF might possess uniform pore size (21.5 A). be responsible for the significant increase in HCC-2’s pore size mostly ranging within the realm of both mesopore and macropore (from 39 nm to 755 nm). Furthermore, bulkier molecular structure of HCC-2’s organic ligands might enable effective elimination of either unreacted organic ligands or its oligomeric compounds more difficult owing to the higher possibility to the generation of steric hindrance among them. The significant difference about the percent errors (%) between CASE B and CASE D (Table S2) might additionally elucidate the validity of aforementioned assumption. With the preparation of activated HCC-2, its structural analysis was also conducted via PXRD (Powder X-ray diffraction). In that the structure of HCC-2 was modified from that of either IRMOF-16 or HCC-1, three XRD patterns were displayed together for the comparison of their structures. In Fig. 3, HCC-2 showed an exactly same diffraction pattern as those of IRMOF-16 and HCC-1, and this drove us to conclude that these three MOFs have the same crystalline structure, cubic structure having no interpenetration ˚ In other words, no change in with each axis of about 21.5 A. crystalline structure has been made from the introduction of various number of hydroxyl functional side groups to the backbone of organic ligand molecules of IRMOF-16. For reference, further intensive discussion about X-ray crystallography can be available from our previous studies [19–21]. FT-IR analysis to verify the structural characteristics of HCCs (the presence of hydroxyl functionality) definitely was also conducted as shown in Fig. 4. Hydroxyl functional side groups were detected
Fig. 3. Powder X-ray diffraction (PXRD) patterns of IRMOF-16, HCC-1, and HCC-2.
Fig. 4. FT-IR analysis (Nujol mulls between two NaCl plates, 500–4000 cm 1) of (a) IRMOF-16, (b) HCC-1, and (c) HCC-2.
as a broad peak at 3572 cm 1 for HCC-1 and 3566 cm 1 for HCC-2, respectively while no distinguishable peak near these regions was observed for IRMOF-16 [27,28]. As an effort to verify the effect of hydroxyl functionalities of both HCCs, H2 adsorption isotherms at different temperatures (77 K and 87 K) and isosteric heats of adsorption (Qst) for HCC-1, HCC-2, and IRMOF-16 were obtained as shown Fig. S11–S13 and Table S3. Concretely, three MOFs possessing the similar range of BET surface areas (SBET, 3000 m2/g) and being equally adapted to Activation process III were yielded in order to minimize the effect of impurities inside their pores bound coordinately to imperfectly formed nodes (such as coordinatively unsatured sites and exposed metal sites) of HCCs as well as the effect of pore properties (pore size, pore volume, etc.) [29–36]. The experimental results from these MOFs seemed that hydroxyl functionalities, if any, could seriously degrade the H2 adsorption ability of MOFs. As shown Fig. S11 and S13, IRMOF-16 (without hydroxyl functionality) could adsorb relatively larger amount of H2 gas than two HCCs (with functionality) and its Qst was the highest among them. Under the assumption that the
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unexpectedly negative effect of hydroxyl functionalities of HCCs in terms of H2 adsorption ability might be ascribed to the presence of DMF and its derivatives which could be still resided inside their pores even after Activation process III, three MOFs were thermally treated via custom-built continuous thermal treatment equipment (Fig. S14). The gas collected during the thermal treatment under high-purified Argon atmosphere (99.9999%) in the temperature range from 25 to 600 1C (50 1C/min) was qualitatively analyzed via GC–MS. As a result, DMF and its derivatives which might originate from the thermal decomposition of DMF during the course of either the formation of MOF or its Activation process would be detected (not shown here). Moreover, from the previous observation (no weight loss until 300 1C in TGA analysis), these impurities proved not to be detached from the HCCs’ pores even at the 300 1C. This might result from the fact that hydroxyl functional side groups of HCCs were the preferential adsorption sites for H2 gas [37–41] and that DMF and its derivatives which preoccupied these sites predominantly via significantly rigid bonds with abovementioned side groups incurred the noticeable decrease in H2 adsorption ability for HCCs. 4. Conclusions To sum up, HCC-2 possessing both relatively larger pore size and higher number of hydroxyl functional side groups per its base unit at the same time than those of HCC-1 has been prepared via solvothermal method. In order to execute its postsynthetic modification with various organic-metal precursors more efficiently, the research about the optimization relevant to the maximization of its BET surface area has been simultaneously conducted. The inclusion of solvent exchange step (from DMF to the chloroform) between ‘solvent decanting step’ and ‘CO2 supercritical drying step’ would be of great importance in maximizing the BET surface area of HCC-2. This might be ascribed to the fatal role of chloroform which could compensate for two drawbacks of HCC-2’s organic ligand which has relatively higher number of hydroxyl functional side group and bulkier molecular structure than those of HCC-1’s organic ligand. The fact that XRD pattern of HCC-2 mimic those of IRMOF-16 and HCC-1 suggested that HCC-2 have a cubic-shaped structure with each axis of about 21.5 A˚ without structural interpretation. Also, the structural characteristics of HCCs having hydroxyl functionality were directly proved via FT-IR analysis. However, DMF and its thermally decomposed derivatives which coordinately preoccupied hydroxyl functional side groups of HCCs rendered their hydrogen adsorption abilities to be seriously debased.
Acknowledgments This research has been bolstered by the Research Consortium having been leaded by Professor Jisoon Ihm affiliated to the Department of Physics and Astronomy in Seoul National University, Republic of Korea. This Research Consortium has been mainly sponsored by Hanwha Chemical Corporation for the development of state-of-the-art hydrogen materials to adsorb hydrogen molecules via Kubas-type adsorption mechanism.
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Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jssc.2012.08.046.
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