The　shale　gas　revolution　has　shifted　propylene　production　from　naphtha　cracking　to　on-purpose　production　with　propane　dehydrogenation　(PDH)　as　the　dominant　technology1,　2,　3,　4,　5,　6,　7,　8-9.　Because　PDH　is　endothermic　and　requires　high　temperatures　that　favour　sintering　and　coking,　the　challenge　is　to　develop　active　and　stable　catalysts1,　2-3　that　are　sufficiently　stable10,11.　Zeolite-supported　Pt-Sn　catalysts　have　been　developed　to　balance　activity,　selectivity　and　stability12,13　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　mu　m,　migration　of　Pt-Sn2　monomers　during　PDH　results　in　intracrystalline　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　degrees　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.　Although　synthesis　requirements　and　cost　are　at　present　prohibitive　for　industrial　use,　we　anticipate　that　our　approach　to　controlling　the　migration　and　lockup　of　metals　in　zeolites　may　enable　the　development　of　other　noble-metal　catalysts　that　offer　extended　service　lifetimes　in　industrial　applications15,　16-17.