Defect-Rich Hierarchical Porous UiO-66(Zr) for Tunable Phosphate Removal

Abstract

The introduction of defects into hierarchical porous metal-organic frameworks (HP-MOFs) is of vital significance to boost their adsorption performance. Herein, an advanced template-assisted strategy has been developed to fine-tune the phosphate adsorption performance of HP-MOFs by dictating the type and number of defects in HP-UiO-66(Zr). To achieve this, monocarboxylic acids of varying chain lengths have been employed as template molecules to fabricate an array of defect-rich HP-UiO-66(Zr) derivatives following removal of the template. The as-prepared HP-UiO-66(Zr) exhibits a higher sorption capacity and faster sorption rate compared to the pristine UiO-66(Zr). Particularly, the octanoic acid-modulated UiO-66(Zr) exhibits a high adsorption capacity of 186.6 mg P/g and an intraparticle diffusion rate of 6.19 mg/g·min^0.5, which are 4.8 times and 1.9 times higher than those of pristine UiO-66(Zr), respectively. The results reveal that defect sites play a critical role in boosting the phosphate uptake performance, which is further confirmed by various advanced characterizations. Density functional theory (DFT) calculations reveal the important role of defects in not only providing additional sorption sites but also reducing the sorption energy between HP-UiO-66(Zr) and phosphate. In addition, the hierarchical pores in HP-UiO-66(Zr) can accelerate the phosphate diffusion toward the active sorption sites. This work presents a promising route to tailor the adsorption performance of MOF-based adsorbents via defect engineering.

Keywords: defect chemistry; hierarchical pores; metal−organic frameworks; monocarboxylic acids; phosphate removal.