Adsorption and separation of harmful organic vapors by flexible hydrophobic bis-pyrazolate based MOFs

The last years have witnessed a blooming of research interest on the preparation, characterization and optimization of porous functional materials (MOFs, metalorganic frameworks) based on transition metal ions linked by long organic spacers in three dimensional networks [1]. The pioneering work of Yaghi’s, Kitagawa’s and Férey’s groups in the synthesis of MOFs was initially based on porous metal polycarboxylates. In this context, several chemically stable and thermally inert porous systems have been isolated, their main drawback being, however, the easily hydrolysable nature of the M-carboxylate bond. Polyazolate systems, like pyrazolate and tetrazolate [2], offer a closely related stereochemistry to carboxylate and, at the same time, a higher basicity, possibly guaranteeing more robust coordinative bonds. Thus, planning to isolate new porous MOFs with long polytopic spacers possessing nitrogen donor sites, we turned our attention to the 1,4-(4-bispyrazolyl)benzene ligand (C12H10N4, H2bpb) [3]. Reacting the deprotonated form of the H2bpb moiety with nickel, copper and zinc salts, we isolated the highly porous homoleptic Ni(bpb) and Zn(bpb) species, and a dense Cu2(bpb) material. Ab-initio powder diffraction methods and thermodiffractometry allowed the determination of their crystal structures and flexibility. The different stereochemical requirements induce, in the 3-D Ni(II) and Zn(II) species, rhombic and square channels respectively, capable of hosting guest molecules and accounting for the 57 and 65% of the total volume. Indeed, both species possess permanent nanoporosity and adsorb large quantities of small gaseous molecules (N2, Ar at 77 K and CO2, CH4 at 273 K), as a consequence of their unusually large surface areas (peaking well above 2000 m2 g-1). The hydrophobic nature of the channels is also adequate for the incorporation of organic vapors (benzene, cyclohexane, 2,2-dimethylbutane) at 298 K, as witnessed by selectivity studies.