2.2 | Preparation of MOF-74 and py-MOF-74
Synthesis of MOF-74. Co-MOF-74 crystals were achieved by reaction of Co(NO3)2·6H2O (1.03 mmol) and 2,5-dihydroxyterephthalic acid (dhtp, 0.330 mmol) in a mixed solvent (DMF/ethanol/water = 1: 1: 1 in volume, 24 mL in total) under a solvothermal condition (at 100oC for 24 h). Fresh-synthesized Co-MOF-74 were activated at 250℃ under a vacuum system for 12 h to generate open metal sites.
Synthesis of py-MOF-74. The activated MOF-74 was immediately transferred into a stainless-steel autoclave containing a certain amount of pyrazine in the glovebox filled with N2. The autoclave was heated at 110℃ for 1 d, in which MOF-74 was soaked in the high-density of pyrazine steam, producing pyrazine-interior-embodied MOF-74 (py-MOF-74). The pyrazine amount in the autoclave was changed in the post vapor modification process, resulting in the reactive molar ratio of Co2+ nodes in MOF-74 to pyrazine of 1: 0.5 for py-MOF-74a, 1: 1 for py-MOF-74b and 1: 10 for py-MOF-74c. After 24 h, py-MOF-74 was washed with methanol to remove the weakly binding pyrazine molecules, and treated under vacuum condition for sample activation.