Metal-organic frameworks (MOFs) are constructed from metal ions/clusters coordinated by organic linkers (or bridging-ligands). The hallmark of MOFs is their permanent porosity, which is frequently found in MOFs constructed from metal-clusters. These clusters are often formed in situ, whereas the linkers are generally pre-formed. The geometry and connectivity of a linker dictate the structure of the resulting MOF. Adjustments of linker geometry, length, ratio, and functional-group can tune the size, shape, and internal surface property of a MOF for a targeted application. In this critical review, we highlight advances in MOF synthesis focusing on linker design. Examples of building MOFs to reach unique properties, such as unprecedented surface area, pore aperture, molecular recognition, stability, and catalysis, through linker design are described. Further search for application-oriented MOFs through judicious selection of metal clusters and organic linkers is desirable. In this review, linkers are categorized as ditopic (Section 1), tritopic (Section 2), tetratopic (Section 3), hexatopic (Section 4), octatopic (Section 5), mixed (Section 6), desymmetrized (Section 7), metallo (Section 8), and N-heterocyclic linkers (Section 9).

The need for alternative fuels is greater now than ever before. With considerable sources available and low pollution factor, methane is a natural choice as petroleum replacement in cars and other mobile applications. However, efficient storage methods are still lacking to implement the application of methane in the automotive industry. Advanced porous materials, metal-organic frameworks and porous organic polymers, have received considerable attention in sorptive storage applications owing to their exceptionally high surface areas and chemically-tunable structures. In this critical review we provide an overview of the current status of the application of these two types of advanced porous materials in the storage of methane. Examples of materials exhibiting high methane storage capacities are analyzed and methods for increasing the applicability of these advanced porous materials in methane storage technologies described.

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A porous polymer network (PPN) grafted with sulfonic acid (PPN-6-SO(3)H) and its lithium salt (PPN-6-SO(3)Li) exhibit significant increases in isosteric heats of CO(2) adsorption and CO(2)-uptake capacities. IAST calculations using single-component-isotherm data and a 15/85 CO(2)/N(2) ratio at 295 K and 1 bar revealed that the sulfonate-grafted PPN-6 networks show exceptionally high adsorption selectivity for CO(2) over N(2) (155 and 414 for PPN-6-SO(3)H and PPN-6-SO(3)Li, respectively). Since these PPNs also possess ultrahigh physicochemical stability, practical applications in postcombustion capture of CO(2) lie well within the realm of possibility.

One of the most pressing environmental concerns of our age is the escalating level of atmospheric CO 2 , which is largely correlated to the combustion of fossil fuels. For the foreseeable future, however, it seems that the ever-growing energy demand will most likely necessitate the consumption of these indispensable sources of energy. Carbon capture and sequestration (CCS), a process to separate CO 2 from the flue gas of coal-fired power plants and then store it underground, has been proposed to reduce the anthropogenic CO 2 emissions. Current CO 2 capture processes employed in power plants worldwide are post-combustion "wet scrubbing" methods involving the chemical adsorption of CO 2 by amine solutions such as monoethanolamine (MEA). The formation of carbamate from two MEA molecules and one CO 2 molecule endows the scrubber with a high capacity and selectivity for CO 2 . However, this process suffers from a series of inherent problems, such as high regeneration costs that arise from heating the solution (ca. 30 % of the power produced by the plant), fouling of the equipment, and solvent boil-off. [1] To sidestep the huge energy demand, corrosion problem, and other limitations of traditional wet scrubbers, intensive efforts have been made to investigate the use of solid adsorbents as an alternative approach. [2] Compared to wet scrubbing, in which a large amount of water (70 % w/w) must be heated and cooled during the regeneration of the dissolved amines, the solid adsorbent approach has the tremendous advantage of improving the energy efficiency of the regeneration process by eliminating the need to heat water.Porous materials, such as MOF-210, [3] NU-100, [4] and PPN-4, [5] have been deemed to be viable storage alternatives because of their high porosity and, therefore, significantly increased accessible contact area with gas molecules. This could be advantageous because separation and regeneration could be performed under relatively mild conditions compared to amine wet scrubbing systems. Unfortunately, the record high storage capacities do not translate to high selectivities and only moderate CO 2 -uptake capacities were observed under carbon capture conditions.The polarizability and large quadrupole moment of CO 2 can be taken advantage of by introducing CO 2 -philic moieties that create strong interactions between the material surface and the CO 2 . This will improve the loading capacities and selectivity of CO 2 over other gases. Indeed, this approach has already been proven to be very successful in enhancing the enthalpy of CO 2 adsorption, [6] which can be calculated from CO 2 sorption isotherms at different temperatures and used to quantify the interaction between the material and CO 2 . It is worth pointing out that the porosity of the material will be compromised by the introduction of functional groups. CO 2 loading capacities at ambient conditions are dependent on the adsorption enthalpy and porosity (both surface area and pore volume), which must be balanced to achieve high loading.Besides the load...

Three porous polymer networks (PPNs) have been synthesized by the homocoupling of tetrahedral monomers. Like other hyper-cross-linked polymer networks, these materials are insoluble in conventional solvents and exhibit high thermal and chemical stability. Their porosity was confirmed by N2 sorption isotherms at 77 K. One of these materials, PPN-3, has a Langmuir surface area of 5323 m2 g−1. Their clean energy applications, especially in H2, CH4, and CO2 storage, as well as CO2/CH4 separation, have been carefully investigated. Although PPN-1 has the highest gas affinity because of its smaller pore size, the maximal gas uptake capacity is directly proportional to their surface area. PPN-3 has the highest H2 uptake capacity among these three (4.28 wt %, 77 K). Although possessing the lowest surface area, PPN-1 shows the best CO2/CH4 selectivity among them.

A unique strategy, sequential linker installation (SLI), has been developed to construct multivariate MOFs with functional groups precisely positioned. PCN-700, a Zr-MOF with eight-connected Zr6O4(OH)8(H2O)4 clusters, has been judiciously designed; the Zr6 clusters in this MOF are arranged in such a fashion that, by replacement of terminal OH(-)/H2O ligands, subsequent insertion of linear dicarboxylate linkers is achieved. We demonstrate that linkers with distinct lengths and functionalities can be sequentially installed into PCN-700. Single-crystal to single-crystal transformation is realized so that the positions of the subsequently installed linkers are pinpointed via single-crystal X-ray diffraction analyses. This methodology provides a powerful tool to construct multivariate MOFs with precisely positioned functionalities in the desired proximity, which would otherwise be difficult to achieve.

Despite tremendous efforts, precise control in the synthesis of porous materials with pre-designed pore properties for desired applications remains challenging. Newly emerged porous metal-organic materials, such as metal-organic polyhedra and metal-organic frameworks, are amenable to design and property tuning, enabling precise control of functionality by accurate design of structures at the molecular level. Here we propose and validate, both experimentally and computationally, a precisely designed cavity, termed a 'single-molecule trap', with the desired size and properties suitable for trapping target CO 2 molecules. Such a single-molecule trap can strengthen CO 2 -host interactions without evoking chemical bonding, thus showing potential for CO 2 capture. Molecular single-molecule traps in the form of metal-organic polyhedra are designed, synthesised and tested for selective adsorption of CO 2 over N 2 and CH 4 , demonstrating the trapping effect. Building these pre-designed singlemolecule traps into extended frameworks yields metal-organic frameworks with efficient mass transfer, whereas the CO 2 selective adsorption nature of single-molecule traps is preserved.