Alexander J. Graham

The effect of pressure on Cu-btc: Framework compression vs. guest inclusion

Here we present detailed structural data on the effect of high pressure on Cu-btc. Application of pressure causes solvent to be squeezed into the pores until a phase transition occurs, driven by the sudden compression and expansion of equatorial and axial Cu-O bonds.

Stabilization of Scandium Terephthalate MOFs against Reversible Amorphization and Structural Phase Transition by Guest Uptake at Extreme Pressure

Previous high-pressure experiments have shown that pressure-transmitting fluids composed of small molecules can be forced inside the pores of metal organic framework materials, where they can cause phase transitions and amorphization and can even induce porosity in conventionally nonporous materials. Here we report a combined high-pressure diffraction and computational study of the structural response to methanol uptake at high pressure on a scandium terephthalate MOF (Sc2BDC3, BDC = 1,4-benzenedicarboxylate) and its nitro-functionalized derivative (Sc2(NO2-BDC)3) and compare it to direct compression behavior in a nonpenetrative hydrostatic fluid, Fluorinert-77. In Fluorinert-77, Sc2BDC3 displays amorphization above 0.1 GPa, reversible upon pressure release, whereas Sc2(NO2-BDC)3 undergoes a phase transition (C2/c to Fdd2) to a denser but topologically identical polymorph. In the presence of methanol, the reversible amorphization of Sc2BDC3 and the displacive phase transition of the nitro-form are completely inhibited (at least up to 3 GPa). Upon uptake of methanol on Sc2BDC3, the methanol molecules are found by diffraction to occupy two sites, with preferential relative filling of one site compared to the other: grand canonical Monte Carlo simulations support these experimental observations, and molecular dynamics simulations reveal the likely orientations of the methanol molecules, which are controlled at least in part by H-bonding interactions between guests. As well as revealing the atomistic origin of the stabilization of these MOFs against nonpenetrative hydrostatic fluids at high pressure, this study demonstrates a novel high-pressure approach to study adsorption within a porous framework as a function of increasing guest content, and so to determine the most energetically favorable adsorption sites.

The effect of pressure on the post-synthetic modification of a nanoporous metal-organic framework.

Here we report four post-synthetic modifications, including the first ever example of a high pressure-induced post-synthetic modification, of a porous copper-based metal-organic framework. Ligand exchange with a water ligand at the axial metal site occurs with methanol, acetonitrile, methylamine and ethylamine within a single-crystal and without the need to expose a free metal site prior to modification, resulting in significant changes in the pore size, shape and functionality. Pressure experiments carried out using isopropylalcohol and acetaldehyde, however, results in no ligand exchange. By using these solvents as hydrostatic media for high-pressure single-crystal X-ray diffraction experiments, we have investigated the effect of ligand exchange on the stability and compressibility of the framework and demonstrate that post-synthetic ligand exchange is very sensitive to both the molecular size and functionality of the exchanged ligand. We also demonstrate the ability to force hydrophilic molecules into hydrophobic pores using high pressures which results in a pressure-induced chemical decomposition of the Cu-framework.

High pressure studies of metal organic framework materials

This paper gives a brief overview of our recent work in the field of high pressure structural studies of metal organic framework materials. These are nanoporous materials which have found widespread application in the fields of gas separation and storage. Pressure provides a convenient tool to modify pore size and content without the need to alter the material chemically. Our techniques (high pressure single crystal diffraction and density functional theory) have allowed us to determine how the molecular frameworks change as a function of external pressure. The results have been surprising.

A combined high-pressure diffraction and computational study on scandium MOFs

Previous high-pressure experiments have shown that pressure-transmitting fluids composed of small molecules can be forced inside the pores of metal organic framework materials, where they can cause phase transitions and amorphization and can even induce porosity in conventionally non-porous materials.1 Here we present a combined high-pressure diffraction and computational study of the structural response to methanol uptake at high pressure on a scandium terephthalate MOF (Sc2BDC3, BDC=1,4benzenedicarboxylate)2 and its nitro-functionalized derivative (Sc2(NO2-BDC)3)3 and compare it to direct compression behaviour in a non-penetrative hydrostatic fluid, Fluorinert-77. In Fluorinert-77, Sc2BDC3 displays amorphization above 0.1 GPa, reversible upon pressure release, whereas Sc2(NO2-BDC)3 undergoes a phase transition (C2/c to Fdd2) to a denser but topologically-identical polymorph. In the presence of methanol, the reversible amorphization of Sc2BDC3 and the displacive phase transition of the nitroform are completely inhibited (at least up to 3 GPa). Upon uptake of methanol on Sc2BDC3, the methanol molecules are found by diffraction to occupy two sites, with preferential relative filling of one site compared to the other: grand canonical Monte Carlo simulations support these experimental observations and molecular dynamics simulations reveal the likely orientations of the methanol molecules, which are controlled at least in part by H-bonding interactions between guests. As well as revealing the atomistic origin of the stabilization of these MOFs against non-penetrative hydrostatic fluids at high pressure this study demonstrates a novel high pressure approach to study adsorption within a porous framework as a function of increasing guest content, and so to determine the most energetically favourable adsorption sites.

The effect of solvent and pressure on the post-synthetic modification of a metal-organic framework

Porous metal-organic frameworks (MOFs) have an array of potential applications including gas storage, separation processes and catalysis. As such, hundreds of MOF-themed research papers are now published annually[1], with many reporting synthetic approaches to making more sophisticated, novel frameworks. Recently, this has led to strong interest in the concept of post-synthetic modification (PSM).[2] This has proven to be a very elegant technique in which to modify MOFs after they have been synthesised, since it offers the potential to tune the pore size, topology and functionality while conserving the integrity of the structure, and is an attractive route for covalent modification that is unachievable by established synthetic routes. To date, two main approaches have been used for PSM of MOFs. In the first method, the organic linker is covalently modified by introducing new functional groups. The second method involves first exposing a free site on the metal, usually by removing a ligand. Here we have investigated the PSM behaviour of the porous MOF, STAM¬1 (St Andrews MOF1).[3] We report four new phases of the material, including the first ever example of a pressureinduced PSM. STAM-1 is comprised of copper ‘paddle-wheels’ linked by monomethyl-esterified benzene-1,3,5-tricarboxylic acid ligands, with water molecules axially coordinated on the CuII ions. The ester and water groups yield a framework containing both hydrophobic and hydrophilic channels, respectively. Here we show how singlestep PSM is possible via ligand exchange at the axial metal coordination site with a number of organic solvents, resulting in significant changes in the pore size and functionality. Specifically, hydrophilic channels in native STAM-1 can be converted into discrete hydrophobic pores. A range of organic solvents have also been used as hydrostatic media for high-pressure single-crystal X-ray diffraction experiments. This has allowed us to induce ligand exchange using pressure and investigate the effect of ligand exchange on the stability and compressibility of the framework, demonstrating that post-synthetic ligand exchange is very sensitive to both the molecular size and functionality of the solvent. This work is, to the best of our knowledge, the most extensive study conducted on the importance of hydrostatic media selection for the investigation of porous materials under pressure. We also demonstrate the ability to force hydrophilic molecules into hydrophobic pores using high pressures, and present the first example of a pressure-induced chemical decomposition of a porous material.