No 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.

Synthesis of 1,3-butadiene (BD) from ethanol has experienced a true renaissance in recent years due to ecological and economic reasons. The open porosity and number of Lewis acid sites in metal silicates (M = Zr, Ta) have been reported in numerous studies as key factors enabling reaching high BD productivity. However, some microporous zeolites recently displayed very high BD productivity. To gain a deeper insight, we have applied non-hydrolytic sol-gel (NHSG) – a method well-known to produce highly porous and homogeneous metal silicates – in the preparation of zirconosilicates with varying micropore volume. The porosity (N2 adsorption-desorption experiments), structure (IR, XPS, NMR, and DRUV-Vis spectroscopy, XRD, MAS NMR), and acidity (IR spectroscopy combined with pyridine adsorption) of these materials have been described in detail and compared to a benchmark sample prepared by dry impregnation. Above mentioned characterization methods proved that NHSG preparation provided highly homogeneous Zr dispersion in silica leading to almost doubled Lewis acid site numbers and higher activity in ethanol-to-butadiene (ETB) transformation, Meerwein-Ponndorf-Verley (MPV) redox reaction, and aldol condensation, in comparison to the catalyst prepared by dry impregnation. The fraction of micropore volume in micro-mesoporous samples (ranging from 27 % to 69 %) did not play a significant role: The activity in all three catalytic reactions followed the acid site numbers. The selectivity and long-term stability in ETB process were similar for catalysts prepared by NHSG and dry impregnation.

Hybrid materials based on metallosilicates are extensively studied for their enhanced catalytic performance. Introducing organic groups into metal silicate catalysts and thus supposedly change the surface hydrophilicity and hydrophobicity has been shown to have an appreciable effect on the catalyst performance in various reactions, including epoxide ring opening reactions and alcohol dehydration. However, the sole organic groups introduction does not unambiguously guarantee hydrophilicity control and might lead to porosity drop, acidity modulation, structural changes, etc. Therefore, a thorough and complex characterization is necessary to provide a complete view of the interaction between the catalyst surface, reactants, and products. Herein, we have prepared aluminosilicate catalyst with well-dispersed Al atoms in silica-based matrices via the non-hydrolytic sol-gel (NHSG) 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, we have obtained aluminosilicate materials with similar porosity, structure, and acid site strength and quality. Notably, the water sorption measurements showed that trimethylsilylated aluminosilicates adsorb 2.5−3 times less water (p/p0 = 0.3) than the parent aluminosilicate catalyst. 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 with aniline than the parent aluminosilicate material. To the best of our knowledge, it exhibited the most active Al sites for epoxide aminolysis in the present literature.

Borane cluster based porous covalent networks, named activated borane (ActB), were prepared by co-thermolysis of decaborane(14) (nido-B10H14) and selected hydrocarbons (toluene – ActB-Tol, cyclohexane – ActB-cyHx, and n-hexane – ActB-nHx) under anaerobic conditions. These amorphous solid powders exhibit different textural and Lewis acid (LA) properties that vary depending on the nature of the constituent organic linker. For ActB-Tol, its LA strength even approaches that of the commonly-used molecular LA, B(C6F5)3. Most notably, ActBs can act as heterogeneous LA catalysts in hydrosilylation/deoxygenation reactions with various carbonyl substrates, as well as in the gas-phase dehydration of ethanol. These studies reveal the excellent potential of ActBs in catalytic applications, showing a) the possibility for tuning catalytic reaction outcomes (selectivity) in hydrosilylation/deoxygenation reactions by changing the material’s composition, and b) the very high activity toward ethanol dehydration that exceeds commonly used γ-Al2O3 by achieving a stable conversion of ~93 % with a selectivity for ethylene production of ~78 % during a 17 h continuous period on stream at 240°C.