Porous M^II/pyrimidine-4,6-dicarboxylato neutral frameworks: Synthetic influence on the adsorption capacity and evaluation of CO ₂-adsorbent interactions

The understanding of the factors that affect the real pore-network structure for a given bulk material due to different synthetic procedures is essential to develop the material with the best adsorption properties. In this work, we have deeply studied the influence of the crystallinity degree over the adsorption capacity on three new isostructural MOFs with the formula {[CdM(μ₄-pmdc)₂(H₂O)₂] ×solv}_n (in which, pmdc=pyrimidine-4,6-dicarboxylate; solv=corresponding solvent; M^II=Cd (1), Mn (2), Zn (3)). Compared with other methods, the solvent-free synthesis stands as the most effective route because, apart from enabling the preparation of the heterometallic compounds 2 and 3, it also renders the adsorbents with the highest performance, which is indeed close to the expected one derived from Grand Canonical Monte Carlo (GCMC) calculations. The structural analysis of the as-synthesised and evacuated frameworks reveals the existence of a metal atom exposed to the pore. The accessibility of this site is limited due to its atomic environment, which is why it is considered as a pseudo-open-metal site. The chemical and physical characterisation confirms that this site can be modified as the metal atom is replaced in compounds 2 and 3. To assess the effect of the metal replacement on the adsorption behaviour, an exhaustive study of CO₂ experimental isotherms has been performed. The affinity of the pseudo-open metal sites towards CO₂ and the distribution of the preferred adsorption sites are discussed on the basis of DFT and GCMC calculations. MOF-CO₂ interactions: The procedure employed for the synthesis of a new family of metal-organic frameworks (MOFs) has allowed the optimisation of their crystallinity and adsorption capacity (see figure). The solvent-free method provides the opportunity to prepare heterometallic compounds, in addition to rendering the adsorbents with the highest performance. The experimental CO ₂ adsorption behaviour is discussed and supported by quantum-mechanical calculations involving the CO₂ interactions with the adsorbent surface.