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Page 4 of 17 Li et al. Chem Synth 2023;3:30 https://dx.doi.org/10.20517/cs.2023.16

EXPERIMENTAL
Preparation of MIL-53 (Al)
[41]
MIL-53 (Al) was successfully synthesized through a hydrothermal reaction . In detail, aluminum nitrate
nonahydrate [Al(NO ) .9H O, VWR, 3.9 g], terephthalic acid (TPA, VWR, 0.864 g), and deionized water
2
3 3
(15.3 mL) were transferred into 50 mL Teflon-lined stainless steel autoclaves. The mixtures were then
stirred for 30 mins and kept at 220 °C in an oven for three days for reaction. Afterward, the mixture was
filtered, washed three times with deionized water and ethanol, and dried at 80 °C. The resulting white
product was then purified in a muffle furnace (280 °C, three days). This high temperature can make sure
complete removal of unreacted TPA species and the occluded TPA molecules within the structure. Finally,
the light yellow powdery MIL-53(Al) was obtained.

Preparation of MIL-68 (Al)
The synthesis of MIL-68 (Al) was carried out following the reported work [42,43] . Typically, TPA (VWR, 5 g)
and aluminum chloride hexahydrate (AlCl .6H O, VWR, 4.88 g) were dissolved in 300 mL of N, N-
3
2
dimethylformamide (DMF, VWR). The mixture was then stirred for 2 h and transferred into autoclaves
with Teflon insets, where it was kept at 130 °C for 18 h. Afterward, the mixture was cooled down to room
temperature and collected by filtration. To remove any free acid that may still remain in the pores, the as-
synthesized product was dispersed in 50 mL of DMF under stirring three times at room temperature. To
further remove the DMF from the pores, the same procedure was repeated four times using 50 mL of
methanol instead of DMF.

Preparation of MIL-100 (Al)
The synthesis of MIL-100 (Al) follows the reported protocol [44,45] . In detail, 0.75 g Al(NO ) .9H O and 0.37 g
3 3
2
trimesic acid (H BTC, VWR) were mixed in a deionized water (10 mL) solution (pH = 1.9-2.0). More
3
importantly, 0.192 g of DMF was added dropwise into this solution, and the quantity of DMF was 2.63
mmol based on their double CH groups (pH = 1.9). After heating the mixture solution to 200 °C for about
3
4 h, a light yellow sample was obtained after filtration. Then, washing the powder three times with DMF and
methanol, respectively, the purified MIL-100 (Al) was obtained after drying at 60 °C for 6-10 h.

Preparation of selenium@porous carbon composites
Each of the as-synthesized MIL-53 (Al), MIL-68 (Al), and MIL-100 (Al) was calcinated at 700, 800, and 900
-1
°C, respectively, with a heating rate of 5 °C min for 4 h in an argon (Ar) atmosphere. To remove the
aluminum species, the resulting black powder was immersed in a 4 M HCl aqueous solution at room
temperature for 12 h. After washing with deionized water to remove any remaining acid, the products were
dried in a vacuum oven overnight. The porous carbon materials derived from Al-MIL were labeled as MIL-
53-700, MIL-53-800, MIL-53-900, MIL-68-700, MIL-68-800, MIL-68-900, and MIL-100-700, MIL-100-800,
MIL-100-900, respectively. The Se-porous carbon composites were synthesized by a two-step melt-diffusion
procedure. As all the samples calcined at 800 °C gave the highest surface area and highest pore volume, only
the samples calcined at 800 °C were selected for the study on Se confinement. For example, with a weight
ratio of 2:1, bulk Se (Sigma-Aldrich) and MIL-53-800 were thoroughly mixed by ball milling (labeled as Se-
MIL-53-800). The mixture was then heated to 260 °C and maintained for 16 h, followed by heating to 300
°C for 4 h in a tube furnace filled with flowing Ar to obtain the composites. The final products were labeled
as Se@MIL-53-800. The same procedures were carried out with MIL-68-800 and MIL-100-800, leading to
the fabrication of Se@MIL-68-800 and Se@MIL-100-800, respectively.

Materials characterization
X-ray diffraction (XRD) patterns were obtained on a Panalytical with Cu Kα radiation (λ = 0.15406 nm)
with 45 kV and 30 mA. The scanning electron microscopy (SEM) observation was carried out using a JEOL