In collaboration with Mr Kevin Byrne (of the Schmitt group, School of Chemistry, Trinity College Dublin) some gas adsorption studies of 64Cd and 65Cd were carried out. Following solvent exchange with MeCN, a sample of 64Cd was activated under dynamic vacuum at 100 °C for 24 hours and the internal pore volume was characterised using CO2 and N2 as probe
molecules. Relatively minor uptake of N2 was observed at 77 K (ca. 6 cc(STP) per g) from
microporous features in the pressure range P/Po 0-0.01 followed by a shallow monotonic
increase to P = 0.99Po up to a maximum loading of 14 cc(STP) per g, Figure 3.14. This low-
capacity type I behaviour (with calculated BET surface area of ca. 5 m2 g -1, see Appendix) indicates poor affinity of the host for the N2 probe molecule under these conditions. A type 1
isotherm is observed when adsorption is limited to, at most, a few molecular layers.153 In physical adsorption, type 1 isotherms are encountered with microporous powders who have a pore size that does not exceed a few adsorbate molecule diameters. A gas molecule, when inside the pores, encounters the overlapping potential from the pore walls, enhancing the quantity of gas adsorbed at low relative pressures. At higher pressures, the pores are filled by adsorbed or condensed adsorbate leading to the plateau, thus indicating that little or no additional adsorption occurs once the pores are filled. Adsorption that results in a type 1 isotherm indicates that the pores are microporous and that the exposed surface resides almost exclusively within the pores and once filled with adsorbate, little or no external surface is available for additional adsorption.153 BET analysis, obtained by applying the theory of Brunauer, Emmett and Teller,
Chapter 3. Coordination chemistry of flexible benzene-1,3,5-tricarboxamides
is the standard method for determining surface areas from nitrogen adsorption isotherms.154
MOFs have been shown to display very large surface areas. The adsorption of CO2 at 278 K
displayed a much greater uptake, with a total adsorption capacity of 103 cc(STP) per g (17 wt%) at 1 bar, as seen in Figure 3.15. The Dubinin-Radushkevich (DR) method was used to calculate the surface area from these data, yielding a surface area at 278 K of 207 m2 g-1. This figure is more consistent with the microporous features apparent from the single crystal structure. The DR equation is widely used for describing the overall adsorption process of CO2 in microporous
materials, especially those of a carbonaceous origin. It has semi-empirical origins and is based on assumptions of a change in the potential energy between the gas and adsorbed phases and a characteristic energy of a given solid.155 This selectivity for CO2 is likely due to the relatively
narrow and polar micropores present in 64Cd, expected to favour the larger quadrupole of CO2
compared to N2. The smooth adsorption and desorption curves, with minor hysteresis, show no
obvious indication of framework flexibility or reorganisation during the adsorption process.
Chapter 3. Coordination chemistry of flexible benzene-1,3,5-tricarboxamides
The adsorption cycle was repeated at 293 K and resulted in a similar curve with a maximum capacity of 83 cc(STP) per g at 1 bar. The datapoints from the two adsorption cycles were fitted using a virial-type model using the Clausius-Clapyeron relation and the enthalpy of adsorption across the loading range was estimated. Figure 3.16. shows the zero-loading enthalpy of adsorption for CO2 is estimated at approximately -28 kJ mol-1, reaching a plateau
value of approximately -23 kJ mol-1 at higher loadings. These values are consistent with those Figure 3.16 Estimatedisosteric heat of adsorption for CO2 in 64Cd as a function of adsorbed quantity.
Figure 3.17 Offset X-ray powder diffraction of 64Cd at room temperature (blue) and MeCN soaked sample of
64Cd after adsorption experiment (green) compared to the simulated pattern of 64Cd from the single crystal data collected at 100 K (orange) and (bottom).
Chapter 3. Coordination chemistry of flexible benzene-1,3,5-tricarboxamides
typically observed for physisorption onto organic pore walls with modest polarity, suggesting little direct interaction from the metal sites or potential hydrogen bond donors in the adsorption process. Although the preferential adsorption of CO2 at 278 K over N2 at 77 K is not surprising
for a microporous material, the CO2 capacity displayed by 64Cd under mild conditions is
unusually high for a 2-dimensional framework showing only intralayer porosity; the capacity at 293 K/1 bar is comparable to that shown by well-known 3-dimensional materials including HKUST-1, MIL-53(Al) and UiO-66.156
Complex 65Cd was also subject to gas uptake investigations, with evacuation of a freshly isolated sample carried out at 100 °C under dynamic vacuum overnight. In comparison to 64Cd, both CO2 (293 K) and N2 (77 K) adsorption experiments showed negligible
adsorption. The uptake of N2 fell below the instrumental detection limit (see appendix), while
CO2 adsorption reached a maximum loading of ca. 17 cc(STP) per g at 1 bar with no obvious
Chapter 3. Coordination chemistry of flexible benzene-1,3,5-tricarboxamides
inflection point in the absorption branch, both consistent with adsorption on the external particle surfaces only, Figure 3.18.
For both complexes 64Cd and 65Cd, X-ray powder diffraction following gas adsorption showed crystalline materials closely related in structure to the freshly synthesised compounds were recovered following exposure to air, as seen in Figure 3.17 and 3.19.
To conclude this section, 64Cd was found to be permanently porous, with an unusually high capacity for CO2 for a two-dimensional material with intralayer porosity and retained its
structure following the adsorption experiments. Many structurally similar compounds were subject to collapse upon evacuation,69,144 thus indicating the structural resilience of the interconnected cadmium-carboxylate rod building units and the hydrogen bonding interactions present within the structure. 64Cd also benefits from the extra π-π interactions on the BTA arms. On the other hand, 65Cd did not display this permanent porosity, presumably due to a combination of factors, namely restricted pore size and greater ligand flexibility arising from the discrete cadmium-carboxylate cluster nodes, in comparison to the more inter-connected cadmium-carboxylate columns in 64Cd.