The OH−/T-atom ratio and the Al-source are identified as critical parameters for the successful crystallization of AEI and AFX type zeolites when sufficient organic structure directing agent (OSDA) molecules are present. Especially the use of a zeolite as the Al-source is essential. When a complete zeolite-to-zeolite transformation of FAU is explored it is found to proceed without any solid crystalline intermediates. The optimal OH−/T-atom ratio can also be decreased when the Al-content in the reactant zeolite is increased to resemble the product composition better. This makes higher yields and better utilization of the OSDA possible compared to gels with less Al. During successful zeolite transformations the lattice parameter of FAU, which is proportional to the Al-content, seems to converge at a certain range before the onset of product crystallization. This indicates that successful nucleation and/or formation of the target zeolite is dependent on this type of intermediate and dependent on the dissolution of the starting zeolite. Based on the findings of optimal OH−/T-atom ratios and synchronization of Si/Al ratio in the reactant zeolite with the product zeolite we also show that AEI and AFX can be obtained from CHA, which has similar structural features, but a higher framework density (FD) than e.g. FAU. This indicates that zeolite-to-zeolite transformations does not have to proceed from zeolites with low FDs (i.e. high stabilization energies) to higher FDs (i.e. lower stabilization energies), but is mainly driven by favorable OSDA-zeolite interactions. Overall, results are rationalized in a scheme where the dissolution rate of a starting zeolite with key structural features must be lower than the crystallization of the zeolite product in order to obtain a successful zeolite-to-zeolite transformation.
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