Sequential Topotactic Transformations Traverse Metastable Phases in Complex Aluminophosphate Frameworks with Tunable Pore Architectures.

Crystalline porous frameworks are irreplaceable platforms for catalysis, separation, new energy, sensing, and biomedicine, yet the majority exist as inaccessible metastable structures across an extensive structural landscape. Here we introduce a sequential topotactic transformation (STT) strategy, in which a simple steaming treatment induces a cascade of structurally correlated topotactic transformations, systematically uncovering a family of hidden metastable frameworks. Using this STT approach, the small-pore aluminophosphate molecular sieve (AlPO MS) DNL-17 undergoes sequential crystalline-state transitions to yield three previously unknown MSs (DNL-18, DNL-19, and DNL-20). Structural characterization and DFT calculations reveal that these transformations preserve partial ABC-6 stacking motif through selective bond cleavage and framework-fragment sliding. This strategy not only accesses diverse metastable states but also enables pore-size modulation from small-pore to ultramicroporous regimes, leading to efficient propane/propylene and water/carbon dioxide separations. It broadens the structural diversity of MSs and provides a strategy for tailoring pore architectures toward application-specific performance.