1.2.1 Structures and properties of Cu3(BTC)2, Cu(BDC), Cu2(BDC)2(DABCO) and Cu2(BPDC)2(BPY)
Cu3(BTC)2 [46], Cu(BDC) [44], Cu2(BDC)2(DABCO) [47] and Cu2(BPDC)2(BPY) [45] constitute Cu-MOFs that contain common SBUs of two 5-coordinate copper cations bridged in a paddle wheel-type configuration (Fig. 1.5).
9 Figure 1.5. Common coordination geometry of paddle wheel building units of Cu3(BTC)2, Cu2(BDC)2(DABCO), Cu2(BPDC)2(BPY), Cu(BDC) and their framework
structures (L = Carboxylate linker, P = N-containing bidentate pillar linker and G =
Guest molecule) [44-47].
As shown in Figure 1.5, in these frameworks carboxylates act as grid-forming ligands.
In the case of Cu3(BTC)2 and Cu(BDC), each copper completes its pseudooctohedral coordination sphere with a guest ligand G (G = H2O and DMF, respectively) opposite to the Cu-Cu vector in the as-synthesized structures. The guest molecules present after synthesis can be removed through activation, to yield a desolvated structure, which has unsaturated open metal sites. In the case of Cu2(BDC)2(DABCO) and Cu2(BPDC)2(BPY), N-containing bidentate ligands P (P = DABCO and BPY, respectively) play the role of pillars. The geometry and bond distances within the
paddle-wheel fragment are comparable for all of four Cu-MOFs (Cu−Cu = 2.627–
2.628 Å, Cu−OCO = 1.952–2.001 Å, Cu–G = 2.165 Å and Cu−P = 2.101–2.103 Å).
As Cu3(BTC)2 was constructed from the tritopic ligand BTC and Cu2(BDC)2(DABCO), Cu2(BPDC)2(BPY) were constructed from two different types of ligand, the framework structures of these Cu-MOFs are three dimention. Whereas Cu(BDC) appears to have a stacking lamellar structure that forms two-dimensional
10 tunnels. According to Carson et al., the structural transformation of Cu(BDC) from a lamellar to a compact structure upon DMF desorption can be reversed by re-adsorption of molecules containing carbonyl groups (Fig. 1.6) [40].
Figure 1.6. Reversible crystalline phase transformation of Cu(BDC) from the lamellar
to the compact structure upon desorption/adsorption of DMF [40].
Long organic linkers provide larger pore and a greater number of adsorption sites within MOFs. However, the large space within the crystal framework makes it prone to form interpenetrating structures (two or more frameworks grow and mutually intertwine together). X-Ray crystal structure of Cu2(BPDC)2(BPY) shows that two equivalent networks mutually interpenetrate which may be resulted from the use of two long linkers BPDC and BPY (Figure 1.7).
11 Figure 1.7. X-Ray structure of the doubly interpenetrating pillared-grid framework
Cu2(BPDC)2(BPY) [45].
A summary of physicochemical properties of Cu3(BTC)2, Cu(BDC), Cu2(BDC)2(DABCO) and Cu2(BPDC)2(BPY) is presented in Table 1.2.
Thermogravimetric analysis showed that all the materials are thermally stable up to 300C or higher. Gas adsorption/desorption isotherm studies gave the Brunauer- Emmett-Teller (BET) surface area of the Cu-MOFs higher than 1000 m2/g except for Cu(BDC). Although Cu(BDC) has about half of the surface area of the other Cu- MOFs, it may be desired if a comparison to determine the effect of pore geometry on a particular materials property is to be made. Besides surface area values, a key parameter in porosity is the pore aperture (or pore window, pore opening). The pore apertures are the pore-opening sizes that allow access of molecules into the pore for storage, separation or conversion applications. An example of the pore apertures of Cu2(BDC)2(DABCO) is shown in Figure1.8. There are two types of cage windows in cubical cages. While the windows formed by the DABCO linkers have a much smaller diameter (3.8 4.7 Å), the window formed by the terephthalate linkers have a free diameter of approximately (7.5 7.5 Å) (Fig. 1.8) [48]. The largest pore apertures of Cu2(BDC)2(DABCO), Cu3(BTC)2 and Cu2(BPDC)2(BPY) are in the range of 7.5 – 9.0 Å which can allow average size substrates to enter the pores to reach catalytic sites.
12 Figure 1.8. Pore apertures of Cu2(BDC)2(DABCO) [48].
Table 1.2: Physicochemical properties of Cu3(BTC)2, Cu(BDC), Cu2(BDC)2(DABCO) and Cu2(BPDC)2(BPY)
MOFs Decomposition
temperature (°C)
BET surface area (m2/g)
Pore aperture
(Å2)
Ref.
Cu3(BTC)2 300 1000-1450 8.0 9.0 [49, 50]
Cu(BDC) 325 545-625 _ [51]
Cu2(BDC)2(DABCO) 300 1461 7.5 7.5 [48]
4.7 3.8
Cu2(BPDC)2(BPY) 320 1210 12.3 7.8 [35, 45]
8.8 8.0
We have been also described the structures and properties of Cu3(BTC)2, Cu2(BDC)2(DABCO), Cu2(BPDC)2(BPY) and Cu(BDC). In reality, their structures and properties are much dependent on synthetic methods and experimental conditions.
In the next section, the synthesis methods for those Cu-MOFs are reviewed.
13
1.2.2 Synthesis of Cu3(BTC)2, Cu(BDC), Cu2(BDC)2(DABCO) and Cu2(BPDC)2(BPY)
There are various approaches for MOFs preparation as illustrated in Figure 1.2. The most commonly used method for MOFs synthesis is solvothermal condition (Fig. 1.9).
Figure 1.9. Solvothermal synthesis of MOFs [10].
Cu3(BTC)2, Cu2(BDC)2(DABCO), Cu2(BPDC)2(BPY) and Cu(BDC) were synthesized by solvothermal methods as described in Scheme 1.1.
Scheme 1.1. Solvothermal synthesis of Cu3(BTC)2, Cu2(BDC)2(DABCO),
Cu2(BPDC)2(BPY) and Cu(BDC) [44-47].
14 In 1997, Mori et al. initially synthesized Cu(BDC) from copper (II) formate and terephthalic acid (H2BDC) in methanol solution at room temperature after several weeks [44]. In 2009, Cantwell et al. reported new synthetic conditions for Cu(BDC) [52]. Herein, a mixture of Cu(NO3)2.3H2O and H2BDC was dissolved in DMF and then heated at 110oC in an isothermal oven for 36 hours to form a blue powder. The BET surface areas of 625 m2/g and Langmuir surface areas of m2/g 752 were achieved.
Other groups also followed this method to synthesize Cu(BDC) for catalytic applications [37, 40, 41].
In 1999, Cu3(BTC)2 was initially discovered by Chui et al. and named as HKUST-1. In that synthesis, Cu(NO3)2.3H2O was heated with 1,3,5-benzenetricarboxylic acid (H3BTC) in a mixed solvent system H2O:EtOH at high temperature and pressure [46].
Since the publication of Chui et al., there have been a large number of synthesis studies conducted under different synthesis conditions (solvent, synthesis temperature, time,…) in an effort to improve the physicochemical properties of Cu3(BTC)2. A summary of Cu3(BTC)2 synthesis studies was provided by Jun Kim et al. [50].
Cu3(BTC)2 synthesized in a mixed solvent system H2O:EtOH may have Cu2O impurity with low porosity [53]. Reflux synthesis in EtOH [54], DMF [55], or DMF/H2O [46]
afforded Cu2O-free Cu3(BTC)2 with excellent specific surface areas (BET surface areas of 1624, 1239, and 1333 m2/g, respectively).
In 2001, Seki initially synthesized Cu2(BDC)2(DABCO) by heterogeneous reactions between porous copper dicarboxylates and DABCO as a pillar ligand in methanol solution [56]. In that synthesis, a methanol solution of copper (II) sulfate pentahydrate, terephthalic acid and formic acid was stand for several days at 313 K. A toluene solution of DABCO was then added to the mixture, which was allowed to react at 433 K for several hours. The BET surface areas of 3265 m2/g were achieved for this material but no structural confirmation was reported. With the same synthetic condition, the structure of Cu2(BDC)2(DABCO) was confirmed by SC-XRD after one year [47]. In 2007, Lee et al. reported a new shorter procedure for Cu2(BDC)2(DABCO) synthesis. Herein, Cu(NO3)2.3H2O was mixed with H2BDC and triethylenediamine (DABCO) in DMF and heated at 120C 36 h. Other groups also
15 followed this method to prepare Cu2(BDC)2(DABCO) for liquid phase separation [48, 57], gas adsorption [58], loading ferrocene [59] and catalytic applications [33, 60].
In 2007, Cu2(BPDC)2(BPY) was initially discovered by James and co-workers [45]. In that synthesis, a mixture of Cu(NO3)2.3H2O, 4,4’-biphenyldicarboxylic acid (H2BPDC) and 4,4’-Bipyridine (BPY) was dissolved in DMSO. The mixture was then heated at 125 C for 24h. The small green crystals were obtained as Cu2(BPDC)2(BPY). Although the structure of Cu2(BPDC)2(BPY) was confirmed by SC-XRD, its surface area was not reported. In 2013, Phan et al. reported new synthetic conditions for Cu2(BPDC)2(BPY) for catalytic application [35, 61]. Herein, the solid
mixture of Cu(NO3)2.3H2O, H2BPDC and BPY was dissolved in a mixture of DMF:EtOH:Pyridine. The resulting solution was then heated at 120 C for 24h to yield Cu2(BPDC)2(BPY) with a Langmuir surface areas of 1547 m2/g.
1.2.3 Characterization of Cu3(BTC)2, Cu(BDC), Cu2(BDC)2(DABCO) and Cu2(BPDC)2(BPY)
Cu3(BTC)2, Cu(BDC), Cu2(BDC)2(DABCO) and Cu2(BPDC)2(BPY) can be characterized by various techniques, such as single crystal X-ray diffraction (SC- XRD), powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), inductively coupled plasma mass spectrometry (ICP-MS), and gas physisorption measurement, etc.
Single crystal X-ray diffraction (SC-XRD) is the best technique for structure determination of a new crystalline MOF material. The data obtained from this analysis provide information about unit cell dimensions, space group, atomic coordinates, bond lengths and bond angles. The most characteristic bond lengths from SC-XRD data of Cu3(BTC)2, Cu(BDC), Cu2(BDC)2(DABCO) and Cu2(BPDC)2(BPY) are summarized in Table 1.3.
16 Table 1.3: The most characteristic bond lengths from SC-XRD data of Cu3(BTC)2,
Cu(BDC), Cu2(BDC)2(DABCO) and Cu2(BPDC)2(BPY)
MOFs Cu−Cu
(Å)
Cu−OCO (Å)
Cu−G or Cu−P (Å)
Ref.
Cu3(BTC)2 2.628 1.952 2.165 [46]
Cu(BDC) 2.63 1.992 2.163 [52]
Cu2(BDC)2(DABCO) 2.627 1.996 2.101 [47]
Cu2(BPDC)2(BPY) 2.628 1.990-2.001 2.103 [45]
Although SC-XRD is the best technique for structure solution of a new MOF, its use is limited by the quality of crystal obtained. SC-XRD requires a truly single and clean crystal that can generate high quality diffraction data. When single crystal cannot be obtained, powder X-ray diffraction (PXRD) is the method of choice for structure determination. In that case the PXRD pattern of the unknown MOF is compared with a simulated pattern of the optimized plausible structure. Moreover, PXRD is the most powerful technique for identification of crystalline phases and estimation of phase fractions after MOFs synthesis. Various features of a PXRD pattern reveal different information such as the unit cell dimensions, the degree of crystallinity, the crystallite size, the type of atoms and their positions in the crystal. In addition, the simulated PXRD pattern from the SC-XRD data can also be regarded as a fingerprint of a crystalline phase. A comparison between an experimental PXRD pattern and a simulated PXRD pattern from the SC-XRD data of already known compounds will help to identify the crystalline phase of the sample and its purity. PXRD patterns simulated from the SC-XRD data of Cu3(BTC)2, Cu(BDC), Cu2(BDC)2(DABCO) and Cu2(BPDC)2(BPY) are shown in Figure 1.10.
17 Figure 1.10. PXRD patterns of Cu3(BTC)2, Cu(BDC), Cu2(BDC)2(DABCO) and
Cu2(BPDC)2(BPY) [45-47, 52]
The stability of a crystalline MOF material towards various conditions can be studied by in situ PXRD. Changes in the crystalline phase caused by the modification of the
surrounding conditions such as temperature and pressure can be detected. A common use of in situ PXRD is to study the thermal stability of a MOF material. In 2009,
Carson et al. have carried out in situ PXRD experiments to examine the effect of
temperature and solvent on the Cu(BDC) crystal structure (Fig. 1.11, [45-47, 52]). The
in situ PXRD patterns of Cu(BDC) showed that it undergoes a phase change between
160 and 220C and remains unchanged up to 300C.
18 Figure 1.11. In situ PXRD patterns of Cu(BDC) [10].
The crystal morphology (size and shape) of a MOF material can be observed by Scanning electron microscopy (SEM). In addition, the shape of a crystal may provide information about the crystal system of its structure. SEM can be used to observe crystals of 10 to 100 nm in size and is useful to control the sizes and shapes of the crystals from different synthesis conditions. For example, Figure 1. 12 shows that Cu3(BTC)2 crystals exhibit a cubic octahedral morphology and Cu(BDC), Cu2(BDC)2(DABCO), Cu2(BPDC)2(BPY) presented as well-shaped cubic crystals [10, 33, 35, 37]. Moreover, the porosity of a MOF material can be observed by Transmission electron microscopy (TEM).
19 Figure 1.12. SEM images of Cu3(BTC)2, Cu(BDC), Cu2(BDC)2(DABCO) and
Cu2(BPDC)2(BPY) [10, 33, 35, 37].
The thermal stability and decomposition of compounds are studied using thermogravimetric analysis (TGA). A common process of a MOF material is first the release of uncoordinated water and solvent from the pores/channels followed by crystal water (coordinated water molecules) and finally the loss of the linkers resulting in decomposition of the MOF. As shown in Figure 1.13, the stability of Cu3(BTC)2, Cu(BDC), Cu2(BDC)2(DABCO) and Cu2(BPDC)2(BPY) are over 300 °C [33, 35-37].
20 Figure 1.13. TGA of Cu3(BTC)2, Cu(BDC), Cu2(BDC)2(DABCO) and
Cu2(BPDC)2(BPY) [33, 35-37].
Fourier-Transform Infra-Red (FT-IR) spectroscopy is a fast, non-destructive method for identifying functional groups and coordination modes in MOFs materials. The presence of water and solvent can also be observed. The coordination mode between metal and carboxylate can be determined. The FT-IR spectra of Cu3(BTC)2, Cu(BDC), Cu2(BDC)2(DABCO) and Cu2(BPDC)2(BPY) [33, 35-37] are shown in Figure 1.14.
21 Figure 1.14. FT-IR spectra of Cu3(BTC)2, Cu(BDC), Cu2(BDC)2(DABCO) and
Cu2(BPDC)2(BPY) [33, 35-37].
Moreover, the permanent porosity of MOFs materials needs to be proven by adsorption and desorption isotherms (nitrogen physisorption measurement). An isotherm can give information about the kind of pores that exist micropores or mesopores, surface area, pore volume and pore size distribution. The content of metal can be measured by inductively coupled plasma mass spectrometry (ICP-MS).
After synthesis and characterization, the applications of Cu3(BTC)2, Cu(BDC), Cu2(BDC)2(DABCO) and Cu2(BPDC)2(BPY) should be investigated. In the next section, catalytic applications of Cu3(BTC)2, Cu(BDC), Cu2(BDC)2(DABCO) and Cu2(BPDC)2(BPY) will be reviewed.
1.2.4 Catalytic activities of Cu3(BTC)2, Cu(BDC), Cu2(BDC)2(DABCO) and Cu2(BPDC)2(BPY)
Among several popular MOFs, copper-based frameworks previously exhibited high activity in various organic reactions due to their unsaturated open copper metal sites
22 [40, 41, 62-65]. Herein, the catalytic activities of Cu3(BTC)2, Cu(BDC), Cu2(BDC)2(DABCO) and Cu2(BPDC)2(BPY) are shown in Table 1.4.
Table 1.4: List of conversion reactions and catalytic reactions used Cu3(BTC)2, Cu(BDC), Cu2(BDC)2(DABCO) and Cu2(BPDC)2(BPY) as heterogeneous catalysts
MOFs Substrates Catalytic
reaction
Conversion (isolated yield) (%)
Ref
Cu3(BTC)2 Benzaldehyde Acetalization 88 [64]
Methanol Benzylic compounds Oxidation 80 [66]
Phenylacetylene Cycloaddition >99 [41]
Benzyl Azide
Benzylamine Aza-Micheal 100 [34]
Ethylacrylate
Iodobenzene Arylketone 74 [36]
Acetylacetone
Phenol Ullmann 89 [38]
Benzaldehyde
Cu(BDC) 2-
aminobenzylalcohol Friedlọnder 100 [37]
Acetophenone
Alkyne cyclization 97 [40]
Amine Aldehyde Cu2(BDC)2(DABCO) Nitrobenzaldehyde Ullmann 100 [33]
Phenol
Imidazole Arylation 97 [60]
4-Iodoacetophenone
Cu2(BPDC)2(BPY) 2-
hydrobenzaldehyde Oxidation 100 [61]
1,4-dioxane
Phenol Oxidation 97 [35]
Formamide
23
Benzoxazole Arylation 95 [32]
Iodobenzene
Cu3(BTC)2 which is one of Cu-MOFs materials has been used intensively in catalysis as its structure is stable, high thermal stability, moisture tolerance [50]. In addition, the free coordination Cu (II) open metal sites orientated towards the center of one of the largest pores. These copper ions could play a role as Lewis acids in many reactions.
For example, Garcia and co-workers reported that Cu3(BTC)2 could be used as a reusable solid catalyst for acetalization of various aldehydes with methanol in good yields (Scheme 1.2). The reaction occurred at room temperature without the need of water removal [64]. Especially, Cu3(BTC)2 showed better catalytic activity than conventional homogeneous (ZnCl2) and other heterogeneous (zeolites, clays) catalysts [50, 64].
Scheme 1.2. The reaction of various aldehydes with methanol using the
Cu3(BTC)2 as catalyst [64].
Garcia et al. also revealed that Cu3(BTC)2 with large pores is an efficient and reusable solid catalyst for the oxidation of benzylic compounds with t-butylhydroperoxide as oxidant in acetonitrile offering moderate to good yields [66] (Scheme 1.3). The solid catalyst was stable under the reaction condition and could be reused several times.
Therefore, the availability of MOFs and the simple product isolation made this system quite attractive for academic as well as industry communities [66].
Scheme 1.3. The oxidation of the benzylic compounds with t-butylhydroperoxide
using the Cu3(BTC)2 as catalyst [66].
24 It was previously indicated that the activity of Cu can change depended on the organic linkers in the MOFs. In specific, Corma and co-workers demonstrated that various Cu-
MOFs (i.e. Cu(2-pymo)2, Cu(im)2, Cu3(BTC)2, Cu(BDC)) are all efficient catalysts for
‘‘click” reactions (1,3-dipolar cycloaddition reactions) with activities and selectivities being as high as the case of using homogeneous catalysts [41] (Scheme 1.4). However, it should be noted that MOFs containing CuN4 are more active than those with CuO4 centers. It means that the organic component of the MOFs plays an important role on the overall activity.
Scheme 1.4. The 1,3-dipolar cycloaddition reaction catalyzed by various Cu-
MOFs catalysts [41].
Besides Cu3(BTC)2, the catalytic activities of Cu(BDC) was discovered. Phan et al.
synthesized and applied Cu(BDC) as effective heterogeneous catalyst for the modified Friedl-Ander reaction [37] (Scheme 1.5). In this reaction, Cu(BDC) offered significantly higher catalytic performance than that of other Cu-MOFs. Moreover, the catalyst could be recycled and reused for many times without significant loss in its catalytic activity.
Scheme 1.5. The modified Friedlọnder reaction using the Cu(BDC) as catatalyst [37].
Another report showed that Cu(BDC), accompanied Cu3(BTC)2, was found to be efficient catalysts for the hydroxylation and nitration of aryl halides [67]. The results indicated that the reaction using Cu(BDC) as catalyst gave the same excellent yield of product compared to ultilizing Cu3(BTC)2, and remarkably higher yield than cases applying Cu-MOFs such as Cu(pymo)2 and Cu(im)2 or homogeneous catalysts including CuI and Cu(OAc)2.. Therefore, the Cu(BDC) with open active sites is a greatly potential heterogeneous catalyst for several organic syntheses.
25 With the same Cu(BDC) topology, Cu2(BDC)2(DABCO), having a three-dimensional network structure bridging a two-dimension layer of porous copper (II) terephthalate with DABCO as a pillar ligand [47], has also studied in catalysis. In details, the Cu2(BDC)2(DABCO) could be used as an heterogeneous catalyst for the coupling reaction of phenols with nitroarenes to form diaryl ethers [33] (Scheme 1.6). In comparison with the conventional Ullmann reaction, the Cu2(BDC)2(DABCO) catalyst offered more advantages such as the reaction avoided the formation of halide byproducts. In addition, this catalyst could be recycled and reused more than five times with no obvious decrease in activity [33]. To the best of our knowledge, although no more reports have shown its catalytic activity so far, the Cu2(BDC)2(DABCO) is really a promising material in catalytic applications.
Scheme 1.6. The coupling of phenols with nitroarenes to form diaryl ethers using the
Cu2(BDC)2(DABCO) as catalyst [33].
Cu2(BPDC)2(BPY) has been recently attached interest due to their high catalytic activity. This MOF could be used as an efficient and reusable catalyst for the cross- dehydrogenative coupling reaction of ethers with 2-carbonyl-substituted phenols [61]
(Scheme 1.7). Interestingly, Cu-MOFs and even homogeneous copper salts exhibited significantly lower catalytic activity than Cu2(BPDC)2(BPY). The result also confirmed that the reaction could only occur in the presence of the Cu2(BPDC)2(BPY) [61].
Scheme 1.7. The cross-dehydrogenative coupling reaction of 2-hydroxybenzaldehyde
and 1,4-dioxane using Cu2(BPDC)2(BPY) as a solid catalyst [61].
26 Recently, Cu2(BPDC)2(BPY) was used as solid catalyst for the oxidative cross- dehydrogenative coupling of phenols and formamides [35] as well as the arylation of azoles with aryl halides [32]. The Cu2(BPDC)2(BPY) exhibited not only the high activity but also the good reusability in those reactions. Besides, this solid could be reused many times without a significant degradation in catalytic activity.
After using as heterogeneous catalysts for organic transformations, the recovered used Cu3(BTC)2, Cu(BDC), Cu2(BDC)2(DABCO) and Cu2(BPDC)2(BPY) were generally characterized by XRD, FT-IR, TGA to compare with the fresh ones. Although they were reused many times, the results showed that most of them have trivial charges in their structures in spite of being suffered from repeated reactions [32, 33, 35-37, 40, 41, 61-67]. Therefore, used Cu3(BTC)2, Cu(BDC), Cu2(BDC)2(DABCO) and Cu2(BPDC)2(BPY) are indeed highly potential solid catalysts for organic syntheses especially when the view of green chemistry has been emphasized increasingly. The next part of this chapter will review studies performed on CC cross coupling reactions to illustrate the high demand of recyclable heterogeneous catalysts for these important transformations.