(713c) Structural and Catalytic Stability of Aluminosilicate Hierarchically-Structured Two-Dimensional MFI Zeolite Nanosheets Under Steaming
Among the factors that determine the efficacy of these materials is their hydrothermal stability. Their thin dimensionality and the high surface-to-volume ratio may increase their reactivity to steam and render their functionality.
We studied the effect of mild steam treatment on the structural, textural, morphological and acidic properties of aluminosilicate (Si/Al =135) hierarchically-structured two-dimensional MFI zeolite nanosheets orthogonally-intergrown in a house-of-cards architecture (Self-Pillared Pentasil zeolite), motivated by the importance of the MFI framework in catalyzing several solid-acid-catalyzed-reactions in the chemical industry.
Under the applied steaming conditions, the material preserved its crystallinity, microporosity and the mesoporosity created within the interlamellar space of the intergrown sheets as determined by X-ray diffraction and Ar physisorption. The steamed material showed no change in its morphology and hierarchical construction upon a bright-field high-resolution transmission electron microscopy examination.
Solid-state 29Si magic angle spinning nuclear magnetic resonance spectra of the starting and the steamed SPP are identical; indicating no detectable change in the local environments of the Si framework sites upon the applied steam-treatment. Solid-state 27Al magic angle spinning nuclear magnetic resonance spectrum of the steamed material, on the other hand, shows a clear decrease in its overall intensity relative to that of the starting material. This denotes the creation of 27Al nuclear magnetic resonance invisible Al sites. Such sites are believed to be in hydrophobic regions of the zeolitic system making their detection not possible at the applied 27Al nuclear magnetic resonance hydration pre-treatment. The used parameters of the 27Al nuclear magnetic resonance analysis did not allow to resolve the different extraframework Al species, if any, other than the octahedral sites present in the starting and the steamed material. Comparison of the 27Al nuclear magnetic resonance results with those of 29Si nuclear magnetic resonance suggests that Al is more susceptible to steaming-induced local rearrangement than Si under the tested steaming.
Total number of Brønsted acid sites as quantified by Hoffman elimination of t-butylamine, decreases upon steaming. This is indicative of a dealumination behavior of the steamed two-dimensional sheets. The determination of the fraction of external Brønsted acid sites by the bulky 2,6-di-tert-butylpyridine base (can only access the sites on the external surface and at pore mouth region) titration during bimolecular ethanol dehydration, suggests a higher steam-stability of external Brønsted acid sites over internal ones in two-dimensional Self-Pillared Pentasil zeolite. The fraction is higher in the steamed material.
The effect of the reported acidity changes on the catalytic properties of the material was assessed using liquid-phase benzyl alcohol self-etherification (catalyzed by internal and external Brønsted acid sites) and benzyl alcohol alkylation with mesitylene (catalyzed by the external Brønsted acid sites only). We found that the selectivity towards the formation of the ether product over the alkylate, is enhanced. This finding contradicts the expectation of a decrease in such selectivity upon the determined increase in the fraction of external Brønsted acid sites. We explain this improvement in shape-selectivity by changes in the acidity of the zeolitic system. Most probably, the loss of the dealuminated external Brønsted sites comes mainly from the most-active sites that were responsible for catalyzing the alkylation reaction. The remaining internal Brønsted sites seem to be active enough to still catalyze the etherification reaction. The captured phenomenon may be used for surface-passivation applications of hierarchically-structured two-dimensional zeolites that are known for their poor shape-selectivity in diffusion-limited solid-acid-catalyzed reactions.
The conducted investigation sheds light on the importance of not only successfully preparing two-dimensional zeolites, but also inspecting their structural and catalytic stability under industrially-applicable operating conditions.