A hierarchical titanium(iv)-based metal–organic framework with oxygen vacancies for enhanced methylene blue adsorption in a wide pH range

Abstract

Hierarchical pores and oxygen vacancies play a vital role in enhancing the adsorption behavior of metal–organic frameworks (MOFs). Herein, a 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 analyses. The incorporation of hierarchical pores and oxygen vacancies facilitates diffusion of guest molecules and endows the surface of the material with 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 isotherm model, and the maximum adsorption capacity was 525.7 mg g⁻¹. Kinetic studies indicate that the adsorption process follows a pseudo-second-order kinetic model, and 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 bonding, n–π* interactions, and pore filling. Reusability studies and continuous operation on a fixed bed both exhibited its potential for practical applications. This research provides valuable insights into the synthesis of novel adsorbents with an ultra-large adsorption capacity and ultra-high adsorption rate in the full pH range, and broadens their potential application in the continuous removal of cationic dyes from wastewater.