Regioselectivity of Al–O Bond Hydrolysis during Zeolites Dealumination Unified by Brønsted–Evans–Polanyi Relationship

Abstract

We determine the mechanism of the initiation of Al−O(H) bond breaking for zeolitic structures mordenite (MOR), faujasite (FAU), MFI, and chabazite (CHA) with high Si/Al ratio occurring during dealumination. Periodic density functional theory calculations demonstrate that water adsorption on the Al atom takes place in anti position to the Brønsted acid site, via a penta-or tetracoordinated Al species. A subsequent 1,2-dissociation of water on adjacent framework oxygen atoms leads to the first Al−O(H) bond breaking (with activation energies of ∼76−125 kJ/mol). A Brønsted−Evans−Polanyi relationship to estimate transition states (TS) is established and opens the door to predictions of which crystallographic sites are able to initiate dealumination.

Effect of aluminum vacancies on the H2O2 or H2O interaction with a gamma‐AlOOH surface. A solid‐state DFT study

“…The O∙∙∙Al covalent bond energy is very sensitive to the O∙∙∙Al distance and AlOAl angle. In accordance with the literature data, its value ranges from ~75 to ~156 kJ mol −1 for different slab models of the defect AlOOH surface. However, the O∙∙∙Al bonds are characterized by close energy values for hydrogen peroxide and water adsorbed at the considered slab model.…”

“…The calculated values of the energies are very sensitive to the O∙∙∙Al distance, to the “coordination state” of Al and the nature of oxygen. The energy of covalent O∙∙∙Al bond ranges from ~75 to ~156 kJ mol −1 (Table ), in accordance with the available literature data . An unexpected negative value for the v3‐H 2 O 2 model is caused by the “strained” coordination of the H 2 O 2 molecule by the aluminum atoms of the slab.…”

Developing a Descriptor-Based Approach for CO and NO Adsorption Strength to Transition Metal Sites in Zeolites

“…The underlying property of a zeolite is the distribution of Al atoms, which is determined during the synthesis of the material and changing it afterwards very often involves the partial destruction and reconstruction of the framework. 27 This substitution of…”

Steam‐Induced Coarsening of Single‐Unit‐Cell MFI Zeolite Nanosheets and Its Effect on External Surface Brønsted Acid Catalysis

“…During their use, zeolites are typically exposed to hydrothermal treatments which may alter their structure at the atomic to the nanometer scale with desirable or undesirable effects on performance . Understanding and controlling the water vapor‐induced structural rearrangements at the nanometer (single‐unit‐cell) level is of particular significance for two‐dimensional (2D) zeolites and thicker nanosheets that constitute an emerging class of catalysts, adsorbents and membranes . Herein, we demonstrate that an all‐silica single‐unit‐cell meso /microporous MFI‐type zeolite (SPP: self‐pillared pentasil) retains its crystallinity and micro‐ and mesoporosity under steaming at 350 °C, while small but detectable changes take place in the content of silanol groups and the enthalpy of transition (Δ H Transition ) relative to α‐quartz (the most stable polymorph of silica under ambient conditions).…”