This paper reports on the synthesis and stability of a polymorphic system of a metal–organic framework (MOF) composed of zinc(II) and 2-methylimidazole, as well as its potential applicability in gas storage. Three polymorphs/pseudopolymorphs, ZIF-8, ZIF-L, and dia(Zn), are discussed in this work. It was found that the synthesis of dia(Zn) with a crystal morphology of hexagonal nanosheets requires a catalyst (NH₄OH, CH₃COOH, or HCOONa), and a synthesis temperature of 60 °C. In contrast, the synthesis of ZIF-8 and ZIF-L can be conducted in the absence of a catalyst and at room temperature. This suggests that the activation energy of dia(Zn) exceeds that of ZIF-8 and ZIF-L. The three crystals were subjected to hydrothermal treatment at 100 °C to evaluate their stability. ZIF-8 presented the highest hydrothermal stability, whereas ZIF-L presented the lowest. Nitrogen physisorption performed at 77 K suggests that the microporosity of ZIF-8 exceeds that of ZIF-L and dia(Zn), which were almost nonporous. Interestingly, CO₂ thermogravimetric analysis revealed that the CO₂ adsorption of ZIF-L and dia(Zn) at 323.15 K is on par with that of ZIF-8, which implies that the flexibility of the ZIF-L and dia(Zn) framework increased considerably with temperature. Our results suggest that nonporous MOFs might be useful for gas adsorption or gas separation at ambient or high temperature.