Hydrophobicity Boosts Catalytic Activity: The Tailoring of Aluminosilicates with Trimethylsilyl Groups**

Abstract

Introducing organic groups into metal silicate catalysts and thus supposedly changing the surface hydrophobicity has been shown to enhance the catalyst performance in various reactions. However, the organic groups introduction does not unambiguously guarantee hydrophobicity control. Therefore, a thorough characterization is necessary to provide a complete view of the interaction between the catalyst surface, reactants, and products. Herein, an aluminosilicate catalyst with welldispersed Al atoms was prepared via the non-hydrolytic sol-gel method. This material was post-synthetically modified with trimethylsilyl groups; their number on the catalyst surface was controlled via a temperature-vacuum pretreatment. In such a way, aluminosilicate materials with similar porosity, structure, and acid site strength and quality were obtained. Notably, the water sorption measurements showed that trimethylsilylated aluminosilicates adsorb 2.5-3 times less water than the parent material (p/p 0 = 0.3). The turn-over-frequency in epoxide ring opening and ethanol dehydration scaled up with the number of trimethylsilyl groups grafted on the catalyst surface. Particularly, the heavily trimethylsilylated sample achieved three to five times higher turnover-frequency in styrene oxide aminolysis than the parent aluminosilicate material. To the best of the authors' knowledge, it exhibited the most active Al sites for epoxide aminolysis in the present literature.

Lewis Acidic Aluminosilicates: Synthesis, 27Al MQ/MAS NMR, and DFT-Calculated 27Al NMR Parameters

“…In the case of py–Al­(OCUBE) 3 , the observed C Q min = 11.8 MHz maps very well to the previously reported value of 9.4 ± 0.5 MHz for the analogous sites observed after the adsorption of pyridine onto dehydrated zeolites . Similarly, the observed C Q min = 10.1 MHz for TEPO–Al­(OCUBE) 3 and 4.8 MHz for [AlO 4 ] − compare well to the reported values of 11.3 and 4.6 MHz, assigned to highly distorted sites of the corresponding type in synthetic amorphous aluminosilicate materials after the adsorption of TEPO . The larger positive deviations of C Q min compared to | C Q DFT | for L = Et 3 N (2.3 MHz) and TEPO (2.1 MHz) could be explained as an additional disorder due to the conformational freedom of the pendant ethyl substituents of the two ligands.…”

“… 8 Similarly, the observed C Q min = 10.1 MHz for TEPO–Al(OCUBE) 3 and 4.8 MHz for [AlO 4 ] − compare well to the reported values of 11.3 and 4.6 MHz, assigned to highly distorted sites of the corresponding type in synthetic amorphous aluminosilicate materials after the adsorption of TEPO. 55 The larger positive deviations of C Q min compared to | C Q DFT | for L = Et 3 N (2.3 MHz) and TEPO (2.1 MHz) could be explained as an additional disorder due to the conformational freedom of the pendant ethyl substituents of the two ligands. As illustrated above, the experimental fits of the asymmetry parameter η avg for the L–Al(OCUBE) 3 sites are highly unreliable with values generally larger or comparable to the corresponding predicted η DFT .…”

“…8 Similarly, the observed C Q min = 10.1 MHz for TEPO− Al(OCUBE) 3 and 4.8 MHz for [AlO 4 ] − compare well to the reported values of 11.3 and 4.6 MHz, assigned to highly distorted sites of the corresponding type in synthetic amorphous aluminosilicate materials after the adsorption of TEPO. 55 The larger positive deviations of…”