**2. Experimental Section**

#### *2.1. Chemicals and Materials*

NH4Al(SO4)2·12H2O and NH3·H2O were purchased from Tianjin Zhiyuan Chemical Reagent Co. LTD. (Tianjin, China)

#### *2.2. Sample Preparation*

In the hydrothermal (HT) synthesis process, 40 mL of 2.5% NH3·H2O solution and 5 g of solid NH4Al(SO4)2·12H2O were added to a Teflon-lined stainless steel autoclave and kept at different temperatures (150 ◦C and 200 ◦C) for a certain time. The samples were collected by centrifugation and washed with distilled water several times. The samples were named as "HT- temperature-time" (e.g., HT-150-24 h means that the sample was kept at 150 ◦C for 24 h).

#### *2.3. Sample Characterization*

X-ray diffraction (XRD) patterns were performed using a Rigaku SmartLab SE diffractometer equipped with a Cu Kα X-ray source and a 2θ range of 10◦–80◦ with 5◦/min. The morphology of the sample was monitored using a scanning electron microscope (SEM, TESCAN MIRA LMS). The surface areas and pore volumes of the samples were evaluated by nitrogen physisorption using a surface area and pore size analyzer (Gold APP Instruments V-Sorb 2008P, Beijing, China). The surface areas were estimated using the BET

(Brunauer−Emmett−Teller) method and the pore volumes were determined by the BJH (Barrett−Joyner−Halenda) method.
