Neutron powder diffraction (NPD) has been used extensively to determine the location of guest molecules in porous framework materials, and this work extends to CO2 [31-36]. Two MOFs that have been explored intensively for their selective sorption properties are M2(dobdc) (M = Mg, Mn, Co, Ni, Zn; dobdc = 2, 5- dioxido-1, 4-benzenedicarboxylate), also known as MOF-74 or CPO-27, and M3(btc)2 (M = Cu, Cr, Mo; btc =1,3, 5-benzenetricarboxylate) with Cu3(btc)2 also known as HKUST-1. Both materials contain exposed M2+ sites, with the M2(dobdc) material possessing exceptionally large densities of such sites. The location of CO2 in the two MOF materials Mg2(dobdc) and Cu3(btc)2, along with the host-CO2 structure, was determined using NPD. The nature of the host-CO2 interaction in both materials was identified to be binding at metal sites via an oxygen with the remainder of the molecule remaining relatively free (see Fig. 3.1), where the adsorbed CO2 is clearly located above the open Mg ions in Mg2(dobdc) [32]. Importantly, the presence of coordinatively-unsaturated metal sites in MOFs such as M2(dobdc) and Cu3(btc)2 leads to enhanced interactions between adsorbates such

as CO2 and the host framework, but also guest molecules such as CH4 and H2. Indeed, we will show that the application of NPD to examine competitive binding between CO2 and these other gases represents an area of significant current interest.

Density-functional theory (DFT) calculations performed using the NPD-deter — mined structures allowed evaluation of the representative CO2 motions in Mg2(dobdc) and Cu3(btc)2. These calculations show that the O bound to the open metal can be approximated as the rotational centre. In both materials the open metal and the associated carboxyls from the ligands form a nearly square-planar surface at the CO2 binding-site, such that the metal-CO2 interaction closely represents a surface normal. The CO2 rotations are shown by arrows in Fig. 3.1(b-c), occurring about the surface normal (red arrows) and away from the surface-normal (blue arrows). The mode energies for the motions denoted by the red and blue arrows are

4.3 and 8.5 meV for Mg2(dobdc), respectively, and 0.2 and 3.4 meV for Cu3(btc)2, respectively. To gain more direct information about the CO2-host interaction in Cu3(btc)2 the energy at the open-metal sites (assuming a rigid host framework) was calculated for these two CO2 motions as a function of CO2 rotational angle, and is shown in Fig. 3.1d. As expected, the energy curves are shallow, particularly in the ±10° region, allowing for significant CO2 orientational disorder in the MOF at this site with little effect on the total energy of the MOF-CO2 system. These findings are in excellent agreement with the relatively-large atomic displacement parameters of CO2 adsorbed at open metal sites obtained from the NPD measure­ments, in particular for Cu3(btc)2. These results also point to the presence of dis­order (either static or dynamic) in the orientation of the CO2 molecule, resulting in a relatively large apparent O-C-O bond bend obtained from the NPD data, which is a structural average and strongly biased by the relatively large disorder of the adsorbed CO2. Since CO2 is reversibly physisorbed on these open metal sites, a large degree of CO2 bond activation and bending is unlikely.

Vacancy-containing Prussian blue analogues of the formula M(1)II3[M

(2) III(CN)6]2 (where M(1) and M(2) are transition metals) are excellent candidate gas adsorbents as 1/3 of their octahedral MIII(CN)3 units are vacant for charge neutrality, generating both non-vacancy and vacancy pores. Each vacancy pore will possess some of the six bare-metal sites per formula unit (eight per unit cell). The M (1)3[Co(CN)6]2 system (M(1) = Mn, Co, Ni, Cu, Zn, Fig. 3.2) displays good selectivity for CO2 over CH4 and N2 [38], with a NPD study revealing two sites for CO2 binding in the Fe3[Co(CN)6]2 material, which has a CO2 uptake of

2.20 mmol g-1 at 35 °C and 1 bar [39]. At one of these sites CO2 was found to bridge between two open-metal sites, with the quadrupolar CO2 molecule inter­acting strongly with the positively-charged Fe sites. The saturation of this site by CO2 at relatively-low CO2 concentrations indicated the favourable nature of the interaction, explaining the selectivity of the material.

CO2 hydrates, consisting of an H2O-cage encapsulating a CO2, are another porous material that have great potential for application as CO2 adsorbents, and these too have been studied using NPD to determine the locations of CO2 within the cage [40]. This study used a cage in which D was substituted for H, allowing structural details of the cage atoms and their interaction with the CO2 to be determined. The study also included the temperature-dependence of this CO2-cage interaction. Data indicate that the CO2 molecule in the tetrakaidecahedral cage rotates rapidly even at low temperatures and that the interaction between the CO2 molecule and the D atoms of the cage is strong enough to provide the site dependence of the atomic displacement parameters of the D atoms. Further work on CO2 hydrates [41] using NPD found CO2 to have different motions in the small and large cages of this system. In both cages the CO2 resides at the cage centre, however, in the small cage the O atoms revolved freely around the C atom, in contrast to the large cage where the O atoms revolved around the C atom along the plane parallel to the hexagonal facets of the cage. The analysis of CO2 hydrates using NPD has also been extended to studies of their formation, including kinetics, using in situ NPD [42]. This work also derived the occupancy of CO2 in the small and large cage during the formation of the hydrate.