Interzeolite Transformation\nthrough Cross-Nucleation:\nA Molecular Mechanism for Seed-Assisted Synthesis

Polymorph selection and efficient crystallization are\ncentral goals\nin zeolite synthesis. Crystalline seeds are used for both purposes.\nWhile it has been proposed that zeolite seeds induce interzeolite\ntransformation by dissolving into structural units that promote nucleation\nof the daughter crystal, the seed’s structural elements do\nnot always match those of the target zeolite. This discrepancy raises\nthe question of how the seed promotes the daughter phase. Here, we\npresent the first molecularly resolved investigation of seed-assisted\nzeolite synthesis. Using molecular simulations, we reproduce the experimental\nfinding that a parent zeolite can promote the nucleation of a daughter\nzeolite even when it lacks common composite building units (CBUs)\nor crystal planes. Modeling the seed-assisted synthesis of an AFI-type\nzeolite using zeolite CHA, our simulations indicate that stand-alone\nCBUs from the parent seed do not facilitate daughter crystal formation.\nHowever, introducing the intact seed significantly reduces the synthesis\ntime, supporting that seed integrity is key to increased efficiency.\nThis reduction arises from the cross-nucleation of the AFI-type zeolite\non the CHA (001) face. We find that parent and daughter zeolites are\nconnected by an interfacial transition layer with an order distinct\nfrom that of both zeolites. Simulations reveal that cross-nucleation\noccurs over a broad range of synthesis conditions. We argue that cross-nucleation\nwould be most favorable for zeolite pairs that share crystalline planes\nsuch as those forming intergrowths. Our findings suggest that the\nprevalence of intergrowths with a common lattice plane in zeolite\nsynthesis is likely a kinetic effect of accelerated cross-nucleation.