Distribution of Aluminum over the Tetrahedral Sites in ZSM-5 Zeolites and Their Evolution after Steam Treatment

Abstract

Zeolite ZSM-5 is one of the most widely used zeolites, in particular in heterogeneous catalysis. This work investigates the incorporation of Al in the silica framework of monoclinic ZSM-5 and the Al speciation during steam treatment for four ZSM-5 samples with different Si/Al ratios, using ultrahigh field 27 Al NMR (22.3 T), 29 Si NMR, X-ray diffraction, and IR spectroscopy. 27 Al MQMAS NMR at 22.3 T allows identification and quantification of 10 distinct tetrahedral framework resonances which are assigned to the crystallographic sites by their average T−O−T angles, determined from powder XRD patterns for the same samples. A clear Al site preference is observed and found to be dependent on the Si/Al ratio of the parent zeolite. The framework sites facing the intersections in ZSM-5 are found to be most prone to dealumination, whereas Al in the straight and sinusoidal-shaped channels are more stable toward steam treatment at high temperatures. Extra-framework Al species with 5-and 6-fold coordination and two distorted tetrahedral AlO 4 sites have also been identified. The spectroscopic data are supported by catalytic activities from α-test cracking experiments which are in accordance with a reinsertion of Al in the framework at mild steaming and the onset of dealumination at higher temperatures.

Zeolite (In)Stability under Aqueous or Steaming Conditions

“…This Al(O h ) is quantitatively converted to the tetrahedral Al(T d ) form at temperature above 395 K. [51] Although moved from their initial framework positions, not all Al atoms are completely framework-independent, as a fraction of them remain partially bonded to the framework via hydroxyls or water molecules. [52] Removal of framework aluminum can result in i) a decrease in the Brønsted acid site concentration, due to the direct destruction of acid centers and interaction of expelled aluminum ions with remaining bridging hydroxyl groups at ion-exchanged positions [53] ii) a change in the distribution of acid site strength (not only due to a general decrease in the framework aluminum density, [54] but also due to the selective removal of a particular fraction of Al atoms from specific framework positions [55] ) iii) an increase in the apparent acid strength of a fraction of Brønsted acid sites [56] iv) formation of additional mesopores and, upon severe dealumination, formation of amorphous deposits that can be removed by post-treatment acid washing (however, after harsh steaming, micropores, and mesopores can still be filled with expelled species, resulting in a decreased apparent concentration of active sites). [57] Under mild steaming, when a noticeable fraction of mesopores is not yet formed, the accessibility of Brønsted acid sites can be improved due to structural damage of zeolite crystal entrances.…”

A Practical Review of NMR Lineshapes for Spin-1/2 and Quadrupolar Nuclei in Disordered Materials

“…DFT (Density Functional Theory) calculations have been used to directly relate NMR parameters to structural differences and variations, modeled either as many distinct crystalline sites or continuous distributions from disorder. [11] Such an approach has prompted increased interest in recent research in heterogeneous catalysis, such as revealing aluminum/proton site distributions in zeolite [32,35,36] and amorphous silica-alumina catalysts [37].…”

Location of the Active Sites for Ethylcyclohexane Hydroisomerization by Ring Contraction and Expansion in the EUO Zeolitic Framework

“…8 This is first of all due to the limited number of zeolites with more or less known location of aluminum atoms. 5,[9][10][11][12][13][14][15][16][17] Then, even once the location is known, it is generally not unique so that identifying the precise location of the most stable intermediates and transition states is not straightforward. This is however a condition for the design of better catalysts.…”