Pillared Metal-Organic Frameworks: Properties and Applications

By Lida Hashemi and Ali Morsali

In the last two decades, metal-organic frameworks (MOFs) have provoked considerable interest due to their potential applications in different fields such as catalysis, gas storage and sensing. The most important advantages of MOFs over other porous materials is the ability of tailoring their pore size, functionality and even the topology of the framework by rational selection of the molecular building blocks. Therefore, many chemists have tried to engineer the structure of MOFs to achieve specific functions.

Pillared metal organic frameworks are a class of MOFs composed of inorganic secondary building units (SBUs) and two sets of organic linkers, generally oxygen- and nitrogen-donor ligands. Typically, in the structure of pillared MOFs, the oxygen-donor struts link the metal clusters into a two-dimensional (2D) sheet and the N-donor struts pillar the sheets to generate a three-dimensional (3D) framework. Thus, the construction of MOFs by utilizing two sets of organic linkers could provide an extra possibility for further tuning of MOF’s pore walls. A variety of functional groups including imine, amide and heterocycles were successfully incorporated into bidentate pillar ligand skeleton. Interestingly, by using pillaring linkers with different length, a wide diversity of metal-organic frameworks with tunable pore dimensions and topologies can be obtained. In this book, we introduce pillared metal organic frameworks with their properties and applications.

• Language: English
• Publisher: Wiley
• Release date: Apr 8, 2019
• ISBN: 9781119460374

Book preview

Chapter 1

Introduction to Metal-Organic Frameworks

1.1 What are the Metal-Organic Frameworks?

Metal-organic frameworks (MOFs) are made by linking inorganic and organic units by strong bonds (reticular synthesis). The flexibility with which the constituents’ geometry, size, and functionality can be varied has led to more than 20,000 different MOFs being reported and studied within the past decade. The organic units are ditopic or polytopic organic carboxylates (and other similar negatively charged molecules), which, when linked to metal-containing units, yield architecturally robust crystalline MOF structures with a typical porosity of greater than 50% of the MOF crystal volume. The surface area values of such MOFs typically range from 1000 to 10,000 m²/g, thus exceeding those of traditional porous materials such as zeolites and carbons. To date, MOFs with permanent porosity are more extensive in their variety and multiplicity than any other class of porous materials.

Crystalline metal-organic frameworks (MOFs) are formed by reticular synthesis, which creates strong bonds between inorganic and organic units. Careful selection of MOF constituents can yield crystals of ultrahigh porosity and high thermal and chemical stability. These characteristics allow the interior of MOFs to be chemically altered for use in gas separation, gas storage, and catalysis, among other applications. The precision commonly exercised in their chemical modification and the ability to expand their metrics without changing the underlying topology has not been achieved with other solids. MOFs whose chemical composition and shape of building units can be multiply varied within a particular structure already exist and may lead to materials that offer a synergistic combination of properties [1].

The building blocks of a MOF (SBU) are carefully chosen such that their properties are retained and exhibited by the product material. Whereas the nature and concentration of the monomers in an organic polymer determine its processability, physical and optical characteristics, it is the network connectivity of the building units that largely determines the properties of a MOF. These may include magnetic exchange, acentricity for non-linear optical (NLO) applications, or the definition of large channels available for the passage of molecules. The inclusions of chiral centers or reactive sites within an open framework are also active goals for generating functional materials. Consequently, MOF synthesis not only requires the selection and/or preparation of desired modules, but also some foresight as to how they will be assembled in the final solid. In order to aid the process of structure prediction, the concept of secondary building units (SBUs) as structural entities was adopted from zeolite structure analysis [2]. These are simple geometric figures representing the inorganic clusters or coordination spheres that are linked together by the (typically linear) organic components to form the product framework. Examples of some SBUs that are commonly encountered in MOFs are illustrated in Figure 1.1.

Figure shows inorganic secondary building units and organic linkers commonly encountered in Metal-organic frameworks.

Figure 1.1 Some inorganic secondary building units (A) and organic linkers (B) [1].

Although many of these units have been observed in molecular species [3,4], they are generally not introduced directly, but are formed in situ under specific synthetic conditions. Conversely, branched organic links with greater than two coordinating functionalities constitute preformed SBU s. The success of an SBU in the design of open frameworks relies both on its rigidity and directionality of bonding, which must be reliably maintained during the assembly process.

The conceptual approach by which a metal–organic framework is designed and assembled is termed reticular synthesis and is based upon identification of how building blocks come together to form a net, or reticulate. It is hypothesized, and indeed observed for a large number of compounds, that the various network topologies adopted by MOFs are represented by only a small number of simple, high symmetry structures [5]. These have been likened to the nets underlying simple inorganic compounds, such as diamond, graphite, SrSi2, and PtS. Foreknowledge as to which topology will be adopted by a given set of building blocks is particularly relevant to the development of porous materials, as it is precisely the expansion of these simple nets by the organic links that defines voids within the solid. Knowledge may also be gleaned about the likelihood of catenation, where two or more identical frameworks are intergrown at the expense of pore volume. This may take the form of interpenetration [6], where the networks are maximally displaced from each other, or inter weaving, where they are minimally displaced and exhibit close contacts that may result in mutual reinforcement [7]. The former is commonly cited as one of the major obstacles that must be overcome in the development of a porous MOF. The possibility of either of these events is directly dependent on the network topology [8] or distortions and the nature of metal, ligand or counter-ions [9,10].

To prepare MOFs with even higher surface area (ultrahigh porosity) requires an increase in storage space per weight of the material. Longer organic linkers provide larger storage space and a greater number of adsorption sites within a given material. However, the large space within the crystal framework makes it prone to form interpenetrating structures (two or more frameworks grow and mutually intertwine together). The most effective way to prevent interpenetration is by making MOFs whose topology inhibits interpenetration because it would require the second framework to have a different topology. Additionally, it is important to keep the pore diameter in the micropore range (below 2 nm) by judicious selection of organic linkers in order to maximize the BET surface area of the framework, because it is known that BET surface areas obtained from isotherms are similar to the geometric surface areas derived from the crystal structure.

1.2 Synthesis of Metal-Organic Frameworks

MOFs are typically synthesized by combining organic ligands and metal salts in solvothermal reactions at relatively low temperatures (below 300 °C). The characteristics of the ligand (bond angles, ligand length, bulkiness, chirality, etc.) play a crucial role in dictating what the resultant framework will be. Additionally, the tendency of metal ions to adopt certain geometries also influences the structure of the MOF. The reactants are mixed in high boiling, polar solvents such as water, dialkyl formamides, dimethyl sulfoxide or acetonitrile. The most important parameters of solvothermal MOF synthesis are temperature, the concentrations of metal salt and ligand (which can be varied across a large range), the extent of solubility of the reactants in the solvent, and the pH of the solution. Although experience often dictates the best conditions for growing these crystalline frameworks, experimentation and trial-and-error methods are still often necessary [11] (Scheme 1.1).

Figure shows different synthesis process and applications of Metal-organic frameworks of which most promising applications of metal–organic frameworks is gas storage and selective gas adsorption.

Scheme 1.1 Schematics showing the synthesis and applications of MOFs.

In addition to this standard method, several other synthetic methodologies are described in the literature including the mixture of non-miscible solvents, an electrochemical route, and a high-throughput approach. One of the most promising alternatives is microwave irradiation which allows access to a wide range of temperatures and can be used to shorten crystallization times while controlling face morphology and particle size distribution. A serious limitation of this approach is the general lack of formation of crystals large enough to obtain good structural data.

1.3 Structural Highlights of Metal-Organic Frameworks

When considering the structure of MOFs, it is helpful to recognize the secondary building units (SBUs), which dictate the final topology of a framework. While the organic linkers are also important SBUs, their structure seldom changes during MOF assembly.

Recently, the geometry of the SBU has been proven to be dependent on not only the structure of the ligand and type of metal utilized, but also the metal to ligand ratio, the solvent, and the source of anions to balance the charge of the metal ion. Several publications discussed the topic of SBU formation and structure in depth. Pores are the void spaces formed within MOFs (or any porous materials) upon the removal of guest molecules. In general, large pores are advantageous for conducting host–guest chemistry such as catalysis, therefore mesoporous (openings between 20 and 500Å) or even macroporous (openings greater than 500Å) materials are attractive. Microporous materials have pores less than 20Å which result in strong interactions between gas molecules and the pore walls making them good candidates for gas storage and gas separation applications. In all cases, measurements of these openings are done from atom to atom while subtracting the van der Waals radii to give the space available for access by guest molecules.

The pores of MOFs are usually occupied by solvent molecules that must be removed for most applications. Structural collapse can occur and, in general, the larger the pore, the more likely the collapse. Permanent porosity results when the framework remains intact and is more difficult to achieve in mesoporous MOFs than in microporous analogues. Although MOFs can be constructed with ligands designed to generate large pores, frameworks will often interpenetrate one another to maximize packing efficiency. In such cases, the pores sizes are greatly reduced, but this may be beneficial for some applications. Indeed interpenetrated frameworks have been intentionally formed and found to lead to improved performance, for example, in H2 storage.

Following the synthesis, MOFs, like other coordination polymers, may participate in further chemical reactions to decorate the frameworks with molecules or functional groups in what is known as post-synthetic modification (PSM). Sometimes the presence of a certain functional group on a ligand prevents the formation of the targeted MOF. In this situation, it is necessary to first form a MOF with the desired topology, and then add the functional group to the framework. This may be applied to MOFs that are designed for catalysis and gas storage, as these applications require functional groups to modify the surface property and pore geometry. It is important to keep in mind that the two most important factors in PSM are making sure that the reagent used to enhance the functionality is small enough to fit inside the cavity of the MOF and that the reaction conditions will not destroy the framework. If the reagent is too small to enter the cavity or the framework is destroyed by the reaction, the modification will be useless.

The assembly of metal ions and organic ligands from solution into a solid-state phase can be accomplished in various ways, and may give rise to different products. The factors determining the assembly pathway can be relatively subtle. To date, little explicit connection between preparative conditions and the exact structure of the resultant product has been revealed. A wide variety of structures may arise from small differences in synthesis conditions. In general, factors such as identity of solvent, solvent concentration, nature of counterion, metal–ligand ratios, metal coordination geometries, pH values, temperature, and nature of guest molecules are thought to play important roles in formation of thermodynamically favoured products.

For example, combination of zinc nitrate and terephthalic acid (H2BDC) produces MOF-5 from N,N-diethylformamide (DEF), but this combination can also yield another phase, MOF-2, by simply changing the solvent to N,N-dimethylformanide (DMF). Although the two MOFs are synthesized from similar starting materials, their molecular building blocks are quite different: an octahedral basic Zn4O (O2C–)6 cluster in MOF-5 and a square planar paddlewheel Zn2(O2C–)4 in MOF-2 (Figure 1.2).

Figure shows structures of MOF-5 and MOF-2 through combination of zinc nitrate and terephthalic acid from N,N-diethylformamide and N,N-dimethylformanide, respectively.

Figure 1.2 Crystal structures of MOF-5 and MOF-2 with different MBB.

In general, the structural differences between isomeric MOFs having identical metal and ligand components lie in their distinct SBBs, even when the same starting materials are employed to synthesize these MOFs. However, the same SBBs and organic linkers can also assemble into different phases by changing synthetic conditions. For example, the recently reported two-fold interpenetrated Zn/BTB-ant and Zn/BTB-tsx species show the same Zn4O(O2C–)6 SBBs and organic linkers as those in MOF-177 but with different topological structures. The topological differences between these three phases result from differences in their preparative methods:

MOF-177 being synthesized in DEF, whereas Zn/ BTB-ant and Zn/BTB-tsx are synthesized in DMF and DMF–H2O, respectively (Figure 1.3).

Figure shows structures of Zn4O(BTB)2 where MOF-177 is synthesized in DEF, Zn/BTB-ant and Zn/BTB-tsx are synthesized in DMF and DMF–H2O, respectively.

Figure 1.3 Crystal structures of Zn4O(BTB)2 prepared in different solvothermal reactions.

1.4 Expansion of Metal-Organic Frameworks Structures

An x-ray diffraction study performed on a single crystal of MOF-5 dosed with nitrogen or argon gas identified the adsorption sites within the pores. The zinc oxide SBU, the faces, and, surprisingly, the edges of the BDC²– linker serve as adsorption sites. This study uncovered the origin of the high porosity and has enabled the design of MOFs with even higher porosities. Moreover, it has been reported that expanded tritopic linkers based on alkyne rather than phenylene units should increase the number of adsorption sites and increase the surface area.

For many practical purposes, such as storing gases, calculating the surface area per volume is more relevant. By this standard, the value for MOF-5, 2200 m²/cm³, is among the very best reported for MOFs. Note that the external surface area of a nanocube with edges measuring 3nm would be 2000m²/cm³. However, nanocrystallites on this scale with clean surfaces would immediately aggregate, ultimately leaving their potential high surface area inaccessible.

A family of 16 cubic MOFs-IRMOF-1 [also known as MOF-5, which is the parent MOF of the isoreticular (IR) series] to IRMOF-16 with the same underlying topology (isoreticular) was made with expanded and variously functionalized organic linkers. This development heralded the potential for expanding and functionalizing MOFs for applications in gas storage and separations. The same work demonstrated that a large number of topologically identical but functionally distinctive structures can be made. Note that the topology of these isoreticular MOFs is typically represented with a three-letter code, pcu, which refers to its primitive cubic net. One of the smallest isoreticular structures of MOF-5 is Zn4O(fumarate)3; one of the largest is IRMOF-16 [Zn4O(TPDC)3; TPDC²– = terphenyl-4,4"-dicarboxylate]. In this expansion, the unit cell edge is doubled and its volume is increased by a factor of 8. The degree of interpenetration, and thus the porosity and density of thesematerials, can be controlled by changing the concentration of reactants, temperature, or other experimental conditions (5).

The concept of the isoreticular expansion is not simply limited to cubic (pcu) structures, as illustrated by the expansion of MOF-177 to give MOF-180 [Zn4O(BTE)2] and MOF-200, which use larger triangular organic linkers. Contrary to the MOF-5 type of expanded framework, expanded structures of MOF-177 are noninterpenetrating despite the high porosity of these MOFs (89% and 90% for MOF-180 and MOF-200, respectively). These results highlight the critical role of selecting topology.

1.5 High Thermal and Chemical Stability

Because MOFs are composed entirely of strong bonds (e.g., C-C, C-H, C-O, and M-O), they show high thermal stability ranging from 250° to 500°C [19]. It has been a challenge to make chemically stable MOFs because of their susceptibility to link-displacement reactions when treated with solvents over extended periods of time (days). The first example of a MOF with exceptional chemical stability is zeolitic imidazolate framework–8 [ZIF-8, Zn(MIm)2; MIm– = 2-methylimidazolate], which was reported in 2006 [19a]. ZIF-8 is unaltered after immersion in boiling methanol, benzene, and water for up to 7 days, and in concentrated sodium hydroxide at 100°C for 24 hours. MOFs based on the Zr(IV) cuboctahedral SBU also show high chemical stability; UiO-66 [Zr6O4(OH)4(BDC)6] and its NO2- and Br-functionalized derivatives demonstrated high acid (HCl, pH = 1) and base resistance (NaOH, pH = 14). The stability also remains when tetratopic organic linkers are used; both MOF-525 [Zr6O4(OH)4(TpCPP-H2)3; TpCPP = tetra-para-carboxyphenylporphyrin] and 545 [Zr6O8(TpCPP-H2)2] are chemically stable in methanol, water, and acidic conditions for 12 hours [19d]. Furthermore, a pyrazolate-bridged MOF [Ni3(BTP)2; BTP³– = 4,4,4-(benzene-1,3,5-triyl)tris(pyrazol-1-ide)] is stable for 2 weeks in a wide range of aqueous solutions (pH = 2 to 14) at 100°C (60). The high chemical stability observed in these MOFs is expected to enhance their performance in the capture of carbon dioxide from humid flue gas and extend MOFs’ applications to water-containing processes.

1.6 Applications of Metal-Organic Frameworks

From the very beginnings of MOF research, it was recognized that not only would the framework components be alterable, but also the contents of the cavities they would define. To provide evidence of the accessibility of these void regions, ion and solvent molecule exchanges were studied. These analyses are useful as a preliminary demonstration of the integrity of an open framework when coupled with PXRD, as long as the crystallite integrity and composition of the exchange solvent are monitored to exclude the possibility of a dissolution/recrystallization mechanism. Quantitative exchange studies with a variety of small molecules have also identified MOFs with specificity towards guest shape or functionality. These observations have inspired beliefs that with proper tailoring, MOFs may be produced to act as highly selective molecular sieves, sensors, or catalysts. Sensor capabilities become realizable when the optical, electronic, or magnetic properties of the framework are altered by guest interactions. This phenomenon has been demonstrated in MOFs containing luminescent lanthanides or paramagnetic transition metals. Catalytic behavior has been reported in only a few instances and this area deserves much more attention. In other hand the most promising applications of metal–organic frameworks is gas storage and selective gas adsorption (Scheme 1.1).

1.6.1 Gas (Hydrogen and Methane) Storage in MOFs

A tank charged with a porous adsorbent enables a gas to be stored at a much lower pressure than an identical tank without an adsorbent. Thus, high pressure tanks and multi-stage compressors can be avoided providing a safer and more economical gas storage method. Many gas storage studies have been conducted on porous adsorbents such as activated carbon, carbon nanotubes, and zeolites. MOFs have received growing attention as such adsorbents due to their tunable pore geometries and flexible frameworks. The need to reduce global reliance on fossil fuels by the use of alternative technologies has pushed hydrogen and methane gases to the forefront of gas storage applications. This section will review the state-of-the-art study of hydrogen and methane storage in MOFs.

1.6.1.1 Hydrogen Storage in MOFs

Hydrogen is an ideal energy carrier. It almost triples the gravimetric heat of combustion of gasoline (120 MJ/kg vs. 44.5 MJ/kg), and the main byproduct after energy release is water. This makes hydrogen a leading candidate for on-board fuel. However, hydrogen exists in a gaseous state at ambient temperature and pressure with a density of 0.08 kg/m³. Even in its liquid state, which requires