Synthesis and characterization of Co, Ni, Zr and Cu MOFs based on 1,4-naphthalenedicarboxylic acid linker for hydrogen generation
Mostafa Farrag
Abstract
Cobalt-, nickel-, copper-, and zirconium-cluster-based metal-organic frameworks (MOFs) were synthesized via the solvothermal method using 1,4-naphthalenedicarboxylic acid (1,4-NDC) as the organic linker. The synthesized MOFs were characterized by PXRD, XPS, TEM, and the BET model. Powder X-ray diffraction (PXRD) confirmed that the MOFs possessed highly crystalline structures. These MOFs were utilized as innovative catalysts to produce hydrogen through the hydrolysis of sodium borohydride (NaBH₄), a promising hydrogen source for energy applications. Among them, Co(1,4-NDC) and Ni(1,4-NDC) catalysts exhibited the highest catalytic activity, producing the maximum hydrogen volume within 5.1 and 6.0 min of stirring at room temperature, respectively. The high catalytic activity is attributed to the small particle sizes of the cobalt and nickel clusters in the MOFs, as confirmed by TEM analysis, and to their large surface areas, with Co(1,4-NDC) and Ni(1,4-NDC) exhibiting specific surface areas of 820 and 770 m²/g, respectively. The effects of catalyst weight and reaction temperature (30-60 °C) were investigated. Co(1,4-NDC) and Ni(1,4-NDC) showed high hydrogen generation rates (HGR) of 1777 and 1333 mL H₂ g⁻¹ min⁻¹ at 60 °C, respectively. The apparent activation energy of NaBH₄ hydrolysis over Ni(1,4-NDC) was 33.94 kJ/mol, which is lower than that of many MOFs with different linkers reported in the literature. The presence of a base (NaOH) increased the hydrogen generation rate and reduced the activation energy in the case of Co(1,4-NDC), used here as a model catalyst. The HGR reached 1785.7 mL H₂ g⁻¹ min⁻¹ at room temperature and 6250 mL H₂ g⁻¹ min⁻¹ at 60 °C in the presence of 15 mg NaOH. The activation energy for NaBH₄ hydrolysis over Co(1,4-NDC) decreased from 40.53 to 34.14 kJ/mol in the presence of the base. Moreover, Co(1,4-NDC) demonstrated high recyclability for at least seven cycles, both with and without the base. These synthesized catalysts are considered highly efficient, noble-metal-free catalysts for energy-related applications.