Modulation of coordinative unsaturation degree and valence state for cerium-based adsorbent to boost phosphate adsorption

The electronic structure and associated chemical characteristics of metal-based adsorbent are directly relevant to the selectivity and efficiency of phosphate uptake. However, few studies focus on the nature of metal centers’ electronic orbit (i.e., the coordination number and valence state). Herein, we report a coordinatively unsaturated Ce(III)-based materials, which exhibits excellent potential in effective phosphate removal. Via controlled partial thermolysis and the following reduction process, the valence state and coordination number of original Ce(III)-MOF (denoted as CM) can be tuned and optimized. The manufacture of more coordination vacancy was fulfilled through total release of solvent molecules under annealing at 300 °C in air. Meanwhile, the reduction procedure could precisely tuned the ratio of Ce(III)/Ce(IV). The result shows that samples only annealed induce a sharp decrease of the phosphate capacity due to the high amount of Ce(IV) state. After the reduction process, the XPS spectra reveal the growth of oxygen vacancy content calculated as 11.6% and the increase of Ce(III)/Ce(IV) values from 0.79:1 to 1.36:1. Based on those great improvement of the unsaturated coordination numbers and the recovery of Ce(III) content for metal centers, the maximum capacity of CM-300(R) to adsorb phosphate is up to 273 mg/g, 2.6 times larger than that of pristine CM. Those new insights provide a novel strategy for synthesizing a highly active adsorbent by controlling the electronic structure of metal centers for efficient phosphate removal.