Silica-based covalent organic framework composite for efficient separation and enrichment of palladium and its heterogeneous catalysis application

Considering the necessity and challenge in palladium recovery from high-level liquid waste and hydrogen energy development, the first attempt was made to adsorb Pd(II) from simulated high-level liquid waste firstly, then to achieve in situ reduction by a simple method and followed by using it directly for efficient hydrogen production through the catalytic decomposition of formic acid. In this work, a composite of covalent organic framework and SiO2 (Tp-Azo-COF/SiO2) was prepared by wet impregnation method using SiO2 as a carrier. The characteristics of the composite were examined by SEM, EDS, BET, TG, XRD, etc. The results showed that Tp-Azo-COF/SiO2 exhibited regular silicon-based spherical shapes in microscopic form and good heat resistance. In batch experiments, Tp-Azo-COF/SiO2 was demonstrated excellent adsorption and selectivity for Pd(II) at a wide range of HNO3 acidity (1-5 M), and the separation factor was as high as 2653 at 3 M HNO3. Tp-Azo-COF/SiO2 was able to reach a maximum adsorption capacity of 85.40 mg/g at 120 min at 298 K in 3 M HNO3. FT-IR and XPS analysis show that the adsorption mechanism of Pd(II) by the composite occurs mainly on the C-NH of Tp-Azo-COF, and NO3- is also involved in the coordination. In dynamic column experiments, Tp-Azo-COF/SiO2 was demonstrated a nearly complete separation and recovery of Pd(II) with good reusability, and the enrichment coefficients all maintained at about 30. Finally, an efficient and environmentally friendly process was proposed to achieve the in situ reduction of Pd(II)-loaded Tp-Azo-COF/SiO2. Pd-loaded Tp-Azo-COF/SiO2 was demonstrated better decomposition performance than commercial Pd-based catalysts hydrogen production by the catalytic decomposition formic acid.