The shale gas revolution has shifted propylene production from naphtha cracking to on-purpose production with propane dehydrogenation (PDH) as the dominant technology^1-9. Because PDH is endothermic and requires high temperatures that favour sintering and coking, the challenge is to develop active and stable catalysts^1-3 that are sufficiently stable^10,11. Zeolite-supported Pt-Sn catalysts have been developed to balance activity, selectivity and stability^12,13, and more recent work documented a PDH catalyst based on zeolite-anchored single rhodium atoms with exceptional performance and stability¹⁴. Here we show for silicalite-1 (S-1) that migration of encapsulated Pt-Sn₂ clusters and hence agglomeration and anchoring within the zeolite versus agglomeration on the external surface can be controlled by adjusting the length of the S-1 crystals’ b-axis. We find that when this axis is longer than 2.00 μm, migration of Pt-Sn₂ monomers during PDH results in intra-crystalline formation of (Pt-Sn₂)₂ dimers that are securely locked in the channels of S-1 and capable of converting pure propane feed to propylene at 550 °C for more than 6 months with 98.3% selectivity at 91% equilibrium conversion. This performance exceeds that of other Pt-based PDH catalysts and approaches that of the Rh-based catalyst. While synthesis requirements and cost are currently prohibitive for industrial use, we anticipate that our approach to controlling the migration and lockup of metals in zeolites may enable to development of other noble metal catalysts that offer extended service lifetimes in industrial applications^15-17.