Hierarchical titanium(IV)-based metal organic framework with oxygen vacancies for enhanced methylene blue adsorption in a wide pH range

The hierarchical pores and oxygen vacancies play a vital role for enhancing adsorption behaviors of metal organic framework (MOFs). Herein, the hierarchical Ti-MOF with oxygen vacancies was synthesized via a temperature-regulated linker defect engineering strategy for the first time. The materials were characterized by XRD, FTIR, TGA, XPS, SEM, TEM, EPR, and BET analysis. The incorporation of hierarchical pores and oxygen vacancies facilitate diffusion of guest molecules and endow the surface of the material with negative electronegativity, resulting in efficient adsorption performance for methylene blue (MB) dye over a wide pH range. Isotherm studies indicate that the adsorption process follows a Langmuir isotherms model, the maximum adsorption capacity was 525.7 mg g-1. Kinetic studies indicate that the adsorption process follows a pseudo-second-order kinetic model, the adsorption equilibrium time was only 10 min. The ultra-large adsorption capacity and ultra-low adsorption equilibrium time originated from the hierarchical pores and oxygen vacancies, exceeding the performance of most reported MOFs. Thermodynamic studies reveal that adsorption processes are spontaneous and endothermic in nature. The adsorption mechanism can be attributed to electrostatic attraction, π-π stacking, hydrogen band, n-π* interactions and pore-filling. Reusability studies and continuous operation on a fixed bed both exhibited usage potential for practical applications. This research provides valuable insights for the synthesis of novelty adsorbents with an ultra-large adsorption capacity and ultra-high adsorption rate in full pH range, and broadened their potential applications in the continuous removal of cationic dye wastewater.