Phase Formation Study of Ca-Terephthalate MOF-Type Materials

Ca­(BDC)­(DMF)­(H₂O) with square lattice topology (sql) and rhombic-shaped channels crystallizes under solvothermal conditions (H₂O/DMF mixture) in orthorhombic (Ca-BDC-orth) and triclinic (Ca-BDC-tric) conformations. The latter structure is solved and described here for the first time. The main difference between the two phases lies in the orientation of the coordinated DMF molecule located within the channels of the structure, influencing their porosity properties. In spite of the small deviances in crystal structure, the formation of individual phases can be controlled by the amount of base agent (TEA) and the temperature of crystallization. Triclinic phase is formed at lower synthesis temperature, and it is transformed into orthorhombic phase at increased temperatures. Systematic phase formation study varying TEA/Ca²⁺ molar ratios and the synthesis temperature showed that higher temperature of the synthesis is required for the Ca-BDC-orth phase to occur from Ca-BDC-tric using the TEA/Ca²⁺ ratio between 0.63–1.30 compared to the syntheses with TEA/Ca²⁺ values out of this range. Within the range of 0.63–1.30 the dissociation of the base is in the buffer region indicating that stability of triclinic phase is related to the thermodynamics at equilibrium conditions.