There are a variety of porous materials have been increasingly studied such as nanotubes [1], mesoporous silicas [2], mesoporous carbons [3], microporous and mesoporous metal-organic frameworks (MOFs) [4] since the invention of aluminophosphate molecular sieves [5]. In comparison with other porous materials, MOFs possess unique structures, in which the metal ions combine with organic linkers to form secondary building units (SBUs), which dictate the final topology of a whole framework [6]. The combination of numerous kinds of linkers and metal ions can lead to the considerate diversity of this material [7]. Some examples of MOFs structures are shown in Figure 1.1.
Figure 1.1. The 3D structures of representative MOFs [7].
4 Since the exploration of MOFs, several synthetic strategies for the preparation of crystalline MOFs materials have been developed. A summary of various approaches for MOFs preparation is presented in Figure 1.2 [8].
Figure 1.2. (a) Synthesis conditions commonly used for MOFs preparation;
(b) indicative summary of the percentage of MOFs synthesized using various
preparation routes [8].
5 MOFs are usually synthesized by solvothermal method, based on the change in polarity of solvents combining with appropriate temperature. In detail, a mixture of ligands and metal salts dissolved in a solvent (or a mixture of solvents) is heated below 300 °C during 48-96h for the grow of crystals [8]. Solvent selection depended on different criteria including solubility, stability, reactivity, redox potential [8] etc.
Preferred solvents are polar solvents with high boiling points including dialkyl formamide, ethanol or water. Solvothermal method can afford MOFs with crystallinity being high enough for their structure determination by Single Crystal X-Ray Diffraction (SC-XRD). However, this method also suffers from drawbacks including long reaction time is required, large-scale synthesis is limited and many trials and errors are necessary. To overcome these disadvantages, other methods have been studied including microwave-assisted synthesis [9], biphasic solvothermal synthesis [10], electrochemical synthesis [11], high-throughput [12] or mechanochemical synthesis [13]. However, these methods cannot yield the crystals with sufficient quality for their structure determination by SC-XRD compared to solvothermal method.
Owing to their unique composition, MOFs show outstanding characteristics including diverse, predetermined and flexible structures, tunable pore size, large surface areas, high crystallinity, ultrahigh porosity, and sustainable frameworks [14]. Thanks to these exceptional properties, MOFs have been investigated for many potential applications including gas adsorption and storage [15-19], catalysis [20], drug delivery [21], chemical separation [22, 23] and chemical sensors [24]. Although investigations on catalytic applications of MOFs are relatively lagging behind other topics, the situation has improved dramatically since 2009. Figure 1.3 shows the development of MOF fields on the basis of articles appeared in the last twenty years [25]. It is clear that MOF catalysis underwent a rapid development in recent five years.
6 Figure 1.3. Development of MOF fields in comparison to the MOF catalysis in the last
ten years (SciFinder until Jan 15, 2014) [25].
In catalysis, MOFs can be used as heterogeneous catalysts to overcome drawbacks of traditional homogeneous catalytic systems such as high catalysts amount and irrecoverbility [20, 26-29]. In addition, MOFs have highly organized structures, large specific surface areas and uniform size distributions of pore and voids. The system of channels with a strict geometry in MOFs allows their use in size- and shape-selective catalysis, which is similar to zeolites [30]. Moreover, their highly open metal sites and high metal contents can lead to their highly catalytic activity [6]. In some reactions requiring harsh conditions, MOFs cannot compete with zeolites due to their low thermal stability. However, MOFs can offer striking features consisting of diverse structures and tunable pore size [20, 31]. The features and physicochemical properties of common porous materials and MOFs materials are shown in the Table 1.1.
7 Table 1.1. The comparison of structural features and physicochemical properties between some common porous materials used in industry and MOFs materials [20, 31].
Porous materials
Characteristics and pore
structures
Surface area (m2/g)
Pore size
Silica gel amorphous; shapes, pore sizes
are not uniform; surface functional groups are mainly neutral hydroxyl groups
< 1000
Average diameter:
20 – 30 Å.
Activated alumina
amorphous; shapes, pore sizes are not uniform; surface functional groups are mainly acidic or basic hydroxyl groups
Average diameter:
20 – 50 Å.
Zeolite crystalline; shapes, pore sizes
are uniform
Window opening diameter:
3 – 10 Å.
Activated carbons
amorphous; shapes, pore sizes are not uniform; different degrees of local surface polarities
>1000
Average diameter: 3 – 100 Å.
Molecular sieve carbons
amorphous; pore sizes are larger than activated carbons
Window diameter:
3 – 5 Å.
MOFs crystalline; shapes, pore sizes
and surface functional groups can be adjusted flexibly
Among a variety of transition metal MOFs, Cu-MOFs emerge as the most used materials. Many studies reported MOFs containing copper active sites as efficient heterogeneous catalysts [32-41]. According to previous reports, Cu-MOFs are usually made up from the paddle-wheel shaped SBU. These Cu-MOFs contain many open metal sites that enable the reactivity of organic compounds in organic transformations (Fig. 1.4). Among organic linkers that are often used for Cu-MOFs synthesis, 1,4- benzenedicarboxylic acid (BDC), 1,3,5-benzenetricarboxylic acid (BTC) and 4,4’-
8 biphenyldicarboxylic acid (BPDC) have advantages that they are commercial and relatively cheap. In another approach, MOFs can be constructed from mixed linkers to provide greater flexibility in terms of surface area, modifiable pore size and chemical environment [42]. Linkers BDC and BPDC could be easily combined with pillar linkers such as 1,4-diazabicyclo [2.2.2]octane (DABCO) or 4,4’-bipyridine (BPY) to form rigid Cu-MOFs [43-47]. Therefore, Cu-MOFs constructed from BDC, BTC or BPDC recently attracted great attention. In this chapter, literature review of structure, physicochemical properties, synthesis methods, characterization, and catalytic applications of four Cu-MOFs including Cu3(BTC)2, Cu(BDC), Cu2(BDC)2(DABCO) and Cu2(BPDC)2(BPY) has been discussed.
Figure 1.4. Cu-MOFs (M=Cu, L=carboxylate) contain open metal sites that enable the
reactivity of organic compounds in organic transformations.