Carboxylate-based metal–organic frameworks

More recent work on divalent carboxylates with longer and more complex organic components is pushing the limits of gas adsorption and storage properties with the highest surface areas and lowest densities of all known crystalline materials.

The two copper ions in the paddlewheel coordinate to the oxygen of two water molecules to create a double square pyramidal geometry for the two metal sites in the hydrated form of the structure.

The rigid, porous structure of the HKUST-1 framework combined with the accessibility of the activated metal sites upon dehydration has led to a lot of interest in adsorption, separation and catalysis applications.

[5][6][7] While surface area and pore volume is important to the adsorption properties of MOF materials, another consideration is the availability of coordinatively unsaturated metals sites.

This unfavourable coordination environment means that the activated metal site has a high enthalpy of adsorption and is readily filled by adsorbed guest species.

The channels in as-prepared vanadium MIL-47 contain some residual guest terephthalic acid and is reported as having the formula VIII(OH)(CO2-C6H4-CO2) and a hydroxide μ2-OH ion forming the infinite chains.

When activated MIL-53(Cr or Al) is exposed to moisture, X-ray diffraction shows the material adopts a ‘closed’ structure, due to the strong hydrogen bonding interaction between the hydroxyl groups of the inorganic chains and the adsorbed water molecules.

Such large structural changes, in response to adsorption of gas or solvent molecules, is commonly referred to as ‘breathing’.

al,[17] disordered over two positions, with hydrogen bonding between the oxygen of the DMF and the hydroxyl group of the inorganic chain.

The activated samples were stored under inert atmosphere to prevent rehydration which would lead to hydrolysis and ultimately decomposition of the structure.

A number of activation procedures were attempted and NMR analysis used to verify complete removal of guest molecules to obtain the surface area results.

Notably, a recent computational study of the theoretical surface area, gave a value of 3333 m2g−1 for MIL-68(V) suggesting that there may still be activation issues with all the MIL-68 derivatives, rendering some of the porosity in accessible.

Further study on the metal carboxylate systems of trivalent iron and chromium yielded a series of materials referred to as MIL-88(A-D).

Solvent exchange experiments on the terephthalate form, MIL-88B, show that large organic molecules such as lutidine and butanol are able to enter the framework and induce an increase in cell volume over the dried material.

Individual Cu 2 (CO 2 ) 4 (H 2 O) 2 ‘paddlewheel’ cluster coordinated by trimesic acid (left) and the porous cubic framework (right) of HKUST-1. Blue polyhedra represent the square pyramidal CuO5. Black spheres represent the carbon atoms of the organic linker.
Single chain (left) and view along the hexagonal channels (right) of the nickel dihydroxyterephthalate, CPO-27(Ni). [ 8 ] Nickel octahedra are shown in green, black spheres represent the carbon atoms of the organic linker and red spheres the oxygen of adsorbed water molecules.
Corner sharing vanadium chains (left) and large pore framework of MIL-68(V) (right). 64 Vanadium octahedra are shown in red, black spheres represent the carbon atoms of the organic linker.