SAPO-RHO zeolite was synthesized hydrothermally at 473 K for 48 hours, using DEA as the OSDA. The as-synthesized SAPO-RHO exhibits well-defined crystal morphology and high crystallinity, as shown in fig. . To prepare ion-exchanged samples, the protonated DEA molecules were partially replaced with alkali metal cations. This process involved synthesizing Liexchanged SAPO-RHOs (denoted as Li-x-SAPO-RHOs, where x represents the number of exchange cycles) and other metal (M)-exchanged SAPO-RHOs (M = Na, K, and Cs, each subjected to three exchange cycles). Thermogravimetric (TG) analysis confirmed the successful ion exchange (figs. S3 to S5).