User:Minihaa/Carboxylate–based Metal–organic framework

1.4.3.4 MIL-100 & MIL-101
MIL-100 and MIL-101 comprise the same trimeric building unit as the MIL-88(above).68-70 The rigid carboxylate linkers connect trimers such that they form a super-tetrahedra which is the main building unit of the material. The difference between the two structures is that the MIL-10070 is prepared with trimesic acid wherethe super-tetrahedral unit has a trimesate on the face of each tetrahedra whereas, theMIL-10168 is prepared with terephthalic acid and consists of tetrahedra with aterephthalate on each edge with the trimeric units on the vertices.The two structures share the same overall network connectivity of tetrahedral units, inzeolite nomenclature the MTN topology, a framework type reported for the zeolite ZSM-3971 where the AlO4 and SiO4 tetrahedra are replaced by ‘super-tetrahedral’building units. The difference in size between the metal tetrahedra of ZSM-39 and thesuper-tetrahedral unit of the MIL-100 and MIL-101 means that the pore size is alsosignificantly larger. The structures contain two types of cage with porosity into themesoporous region. The internal diameter of the cages in MIL-101(Cr) is 29 Å for thesmaller cage and 34 Å for the larger. The smaller cage is accessible throughmicroporous pentagonal windows with a cross-sectional diameter of 1.2 nm, and thelarger cage through both the pentagonal windows and larger hexagonal windows ofdiameter 1.6 nm. The MIL-101 structure is best represented by comparison to the zeolitic MTN network, a framework type reported for the zeolite ZSM-39. Where the zeolitecontains AlO4 and SiO4 tetrahedra, in MIL-101(Sc) these are replaced my much larger‘super-tetrahedral’ units.

The relative size of the tetrahedral units in MIL-101(Cr) creates a highly accessibleporous network of cages accessible through pentagonal and hexagonal windows between cages. The same trimeric units as in MIL-88 are present in thestructure, with three terminal coordination sites. One terminal site must be occupied by a negatively charged anion (either hydroxide or fluoride - hydrofluoric acid is usedin the synthesis) to balance the charge on the framework. The remaining two sites are occupied by H2O molecule, as evidenced by IR studies of the material.72 Removal of these terminal H2O molecules results in the formation of Lewis acidic free metal sites.The MIL-101 structure requires multi-stage activation to render the full volume accessible. Terephthalic acid molecules from the original synthesis can become trapped in the cages and must be removed. There are various activation methods reported for the material but the most effective involves first treating the material in hot ethanol for 8 hours followed by dispersion in aqueous ammonium fluoride and washing in hot water.4 Following this activation, the BET surface area of the MIL-101(Cr) is reported at 4230 m2g-1. Recent work on MIL-101(Cr) has explored a range of applications including the incorporation of catalytically active species such as polyoxometalates73 within the cages and also the use of MIL-101 in thechromatographic separations of substituted aromatics.74

1.4.2 High surface area materials
Metal-organic frameworks currently boast the highest reported surface areas of any known materials.55 MOFs with exceptionally high surface areas are of interest in applications such as hydrogen storage. The weak intermolecular interactions between the hydrogen molecules mean that the dominating interaction is the between the hydrogen molecules and the surface of the material therefore the higher the surface area the greater the adsorption capacity for hydrogen. Physisorption is more energetically efficient than other storage methods such as metal hydrides which, although they have a density of stored hydrogen that is greater, require a higher temperature and therefore greater energy cost to release the hydrogen. Some of the highest surface area MOFs reported to date are based on the inorganic units of the prototypical MOF materials MOF-5 and HKUST-1 (described above) but with more complex extended organic linkers, examples of which are shown in Figure 1.15.55,56

1.4.4 Framework flexibility in MOFs
Flexiblility in MOF systems typically refers to reversible crystal-to-crystal transitionswhere the framework connectivity of the bulk material is retained but a changeoccurs in the geometry of the framework components.12 This could be as simple as arotation within an organic molecule (changing the internal surface with minimalvolume change) or as complex as a 3-dimensional hinging motion increasing theoverall unit cell volume. This type of flexibility is not unique to MOF systems. Somezeolite frameworks are known to have a degree of flexibility, typically arising fromexchanging out the cations which stabilize certain framework components such asring systems between cages. A good example of this flexibility is observed in thezeolite-rho structure.75 Flexibility in MOFs can occur in a number of ways, some common examples arerepresented in Figure 1.28 (below). Example A is observed for the MIL-53 series in2D29,63 (Figure 1.19) and as a 3D example in MIL-88 (Figure 1.23). Example B is a casewhere a layered structure can be reversibly separated by intercalation of a gas orsolvent between the layers.26 Example C is typical of interpenetrated frameworkswhere the void space and 3D connectivity is such that two fully connectedframeworks can exist within each other. In materials such as MOF-50876 and somemembers of the IRMOF series discussed earlier (Figure 1.3), there can be translationof one framework with respect to the other, again driven by the adsorption of gas orsolvent molecules. The final example shown (D) is representative of a case where theflexibility arises from rotational flexibility of the organic ligand pillaring the layers.26 Inthis case rigid layers are separated by the flexible linker which can expand andcontract in response to the adsorption and desorption of guest molecules.

1.4.5.1 General scandium chemistry
The existence of scandium as an element was first proposed by Dimitri Mendeleevwhen he created the periodic table and predicted the presence of an element that heproposed would resemble boron in it properties and he called it ekaboron. Scandiumoxide was isolated by Swedish mineralogist Lars Frederick Nilson in 1876 from euxeniteand gadolinite minerals from scandanivia, the name scandium derives from the latinScandia meaning Scandinavian. Scandium is not a rare element on Earth. The abundance in the Earth’s crust is stated as being similar to that of more readily available elements such as cobalt and lead but the deposits are more dispersed and minerals containing scandium as the main constituent are rare. One of the main sources of scandium is uranium or rare earth mining where the scandium is abyproduct, sold as scandium oxide. The main application of scandium to date is inscandium-aluminium alloys used in some aircraft bodies. The presence of a smallamount of scandium <1 % limits the grain growth that occurs when welding aluminiumand results in a stronger alloy without the need for rivets although it is still moreexpensive than titanium alloys currently in use.The chemical behaviour of scandium is similar to that of aluminium and yttrium.Naturally occurring scandium is exclusively the 45Sc isotope a quadrupolar nuclei whichhas a nuclear spin of 7/2. A major advantage of this is that it permits the use of solid-state NMR analysis of scandium materials, providing information on the local structureof the solids.In organic chemistry, scandium triflate is commonly used as a Lewis acid catalyst,prepared by reaction of scandium oxide with trifluoromethanesulfonic acid (referredto as triflic acid).77,78 This is used as a recyclable, homogenous, Lewis acid catalyst,which unlike most common Lewis acidic compounds, is not deactivated in the presenceof water.77 In terms of ionic radius, scandium sits between indium and the first rowtransition metals.79Extensive literature studies have been conducted on the other trivalent metals in the exploratory synthesis of novel MOFs, yielding a wide range of framework topologies. Scandium, being towards the larger and lighter end of the scale, presents as an excellent choice of trivalent ion for MOF synthesis (Figure 1.29).

1.4.5.2 Scandium in porous materials
Scandium bearing open framework materials were first reported as recently as 2002.Bull et. al. synthesised a scandium sulphate phosphate,80 prepared hydrothermally andcharacterized by single-crystal X-ray diffraction, electron diffraction and solid-stateNMR (Figure 1.30). The framework was templated using the cyclen macrocycle(1,4,7,10-tetraazacyclododecane) resulting in a cubic arrangement of interconnectedcages. At a similar time, the work of Riou et. al. in Versailles yielded anethylenediammonium templated scandium phosphate (Figure 1.30).81 This material,solved using single-crystal X-ray diffraction and solid-state NMR, is composed of cornershared ScO6 octahedra and HPO4 tetrahedra formed around the ethylenediammoniunorganic template.