Adsorption performance was further determined under iodine vapor conditions. As shown in Fig.4, the maximum saturated adsorption capacities of
Th-BPYDC and
Th-UiO-67 were 2.23 and 2.10 g∙g
–1, respectively after 24 h. Then, in order to explore the recycling performance of the materials, the cycling experiments of iodine in cyclohexane solution were performed on
Th-BPYDC. As shown in Fig.4(d), the adsorption percentage of iodine still remained at 70% after four cycles, indicating that
Th-BPYDC had high stability with good reusability (Fig. S5, cf. ESM). With the increase in the number of cycles, the decreased removal rate was derived from the mass loss of the material during the elution process. The
Th-BPYDC and
Th-UiO-67 before and after iodine adsorption were characterized and the results were shown in Fig.5. The PXRD of
Th-BPYDC and
Th-UiO-67 after adsorption in iodine cyclohexane solution and iodine vapor matched the PXRD of the two materials before adsorption, implying that the structures of the two materials did not change before and after the adsorption, which further indicated that the two materials were very stable (Fig.5(a) and Fig.5(b)). The iodine attached to the crystal surface and entered the pore, resulting in the disappearance of the diffraction peaks at 5° to 7° of the
Th-UiO-67 (Fig. S6, cf. ESM). TGA analysis further revealed that iodine occupied the cavities of the
Th-UiO-67 and
Th-BPYDC (Fig. S7, cf. ESM). To further demonstrate the stability of the two materials, Fourier transform infrared spectroscopy (FTIR) was applied to characterize the
Th-BPYDC and
Th-UiO-67 before and after absorbing iodine vapor or iodine cyclohexane, and we found that the functional groups of
Th-BPYDC and
Th-UiO-67 did not change after adsorption (Fig.5(c) and Fig.5(d)). The above complementary analysis confirms that the structures of
Th-BPYDC and
Th-UiO-67 are very stable. In order to evaluate the adsorption mechanism of iodine cyclohexane and iodine vapor by
Th-BPYDC and
Th-UiO-67, the analysis was carried out by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The XPS spectra clearly showed the existence of iodine signals after adsorption, including I
3– and I
2, indicating that charge transfer was formed between the MOFs framework and iodine, and part of the guest molecular iodine entered the pores of the MOFs (Fig.5(e), Fig.5(f), S8, S9, cf. ESM) [
28]. The adsorption of I
3– and I
2 on the two MOFs was further confirmed by Raman spectroscopy by the appearance of peaks at 110 cm
–1 (I
2 stretching vibration) and 160 cm
–1 (I
3– stretching vibration) (Figs. S10 and S11, ESM) [
29]. The above characterizations demonstrated that the bipyridine units enhanced the binding affinity towards I
2 and I
3− anions, thus significantly boosting the adsorption uptake capacity and kinetics. The high uptake capacity of
Th-BPYDC is comparable to other reported MOFs [
28,
29,
36,
45] (Tab.1). One can see that the
Th-BPYDC shows higher adsorption capacity than other materials.