Abstract
How aluminum distributes during synthesis and rearranges after processing within the zeolite framework is a central question in heterogeneous catalysis, as it determines the structure and location of the catalytically active sites of the one of the most important classes of industrial catalysts. Here, exploiting the dipolar interaction between paramagnetic metal ions, we derive the spatial distribution of single aluminum sites within the ZSM-5 zeolite framework in the nanometer range, in polycrystalline samples lacking long-range order. We use a Monte Carlo approach to validate the findings on a pristine ZSM-5 sample and demonstrate that the method is sensitive enough to monitor aluminum redistribution induced in the framework by chemical stress.
Highlights
How aluminum distributes during synthesis and rearranges after processing within the zeolite framework is a central question in heterogeneous catalysis, as it determines the structure and location of the catalytically active sites of the one of the most important classes of industrial catalysts
The MFI topology comprises either a monoclinic (P21/n symmetry) or orthorhombic structure (Pnma symmetry) with 24 and 12 crystallographically distinct T sites, respectively, the 24 T sites of the monoclinic form corresponding to the 12 T sites of the orthorhombic form.[8]
The high Si/Al ratios and the very similar scattering factors for Al and Si make the determination of the Al atoms at the different T sites of ZSM-5 by X-ray diffraction (XRD) very difficult
Summary
The form factor is computed by multiplying the relevant dipolar trace by the number of counts determined through the Monte Carlo approach and summing up the individual dipolar traces This last approximation holds as the low occupancy and randomness in ZnI distribution allow one to consider the experimental time trace as the sum of pairwise dipolar oscillations, not as the product of all possible dipolar traces. Spectroscopic properties of ZnI, dependence of the dipolar frequency on the interspin distance, details on the RIDME pulse sequence, RIDME primary data and repeats, RIDME data on independent sample preparations, Monte Carlo computations on all T sites, the ESEEM effect, DEER data, independent sites approximation, effect of ZnI occupancy (PDF). The data are available from the corresponding author upon reasonable request
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