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. 2023 Oct 30;13(22):14614-14626.
doi: 10.1021/acscatal.3c04011. eCollection 2023 Nov 17.

"Activated Borane": A Porous Borane Cluster Polymer as an Efficient Lewis Acid-Based Catalyst

Martin Lamač  1 , Béla Urbán  1 , Michal Horáček  1 , Daniel Bůžek  2 , Lucie Leonová  3 , Aleš Stýskalík  3 , Anna Vykydalová  2 , Karel Škoch  2 , Matouš Kloda  2 , Andrii Mahun  4 , Libor Kobera  4 , Kamil Lang  2 , Michael G S Londesborough  2 , Jan Demel  2
Affiliations

"Activated Borane": A Porous Borane Cluster Polymer as an Efficient Lewis Acid-Based Catalyst

Martin Lamač et al. ACS Catal. .

Abstract

Borane cluster-based porous covalent networks, named activated borane (ActB), were prepared by cothermolysis 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 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 the 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.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Scheme 1
Scheme 1. Synthesis of ActB Materials
Figure 1
Figure 1
SEM images of ActB-Tol (left), ActB-cyHx (middle), and ActB-nHx (right). For additional images, see the SI.
Figure 2
Figure 2
Adsorption isotherms of Ar at 87 K (top) and CO2 at 195 K (bottom) for ActB-Tol, ActB-cyHx, and ActB-nHx.
Figure 3
Figure 3
FTIR (ATR-Si) spectra of ActB-Tol (top), ActB-cyHx (middle), and ActB-nHx (bottom); red lines correspond to samples exposed to air for 1 min.
Figure 4
Figure 4
1H MAS NMR (left column), 13C CP/MAS NMR (middle column), and 11B MAS NMR (right column) spectra of ActB-Tol, ActB-cyHx, and ActB-nHx samples.
Figure 5
Figure 5
31P MAS ssNMR spectra from top: BCF+TEPO adduct, TEPO, ActB-Tol+TEPO, ActB-cyHx+TEPO, and ActB-nHx+TEPO.
Figure 6
Figure 6
11B 3Q/MAS NMR spectra of ActB-Tol+TEPO (a), ActB-cyHx+TEPO (b), and ActB-nHx+TEPO (c). The newly detected signal is highlighted by gray boxes.
Figure 7
Figure 7
TPD curves for ActB-Tol (left), ActB-cyHx (middle), and ActB-nHx (right).
Scheme 2
Scheme 2. Deoxygenation of Benzophenone (1) Performed Using Different ActB Catalysts, Yields Determined by GC (Isolated Yield in Parentheses)
1 mol % B(C6F5)3 (BCF) used as a catalyst (reaction time 2 h). kinetic profiles (bottom) of the deoxygenation of 1 catalyzed by ActBs at 60 or 100 °C. Conversion of 1 determined by GC.
Scheme 3
Scheme 3. Hydrosilylation/deoxygenation of Acetophenone (4): (a) Screening of Different ActB Materials Using 1.5 or 3 equiv of Silane and Toluene as the Solvent; (b) Reaction Temperature Optimization Using ActB-Tol in Toluene; and (c) Variation of Reaction Conditions in THF Solvent. Yields Determined by GC
Scheme 4
Scheme 4. Reactions Involved in the Mechanism of 4-Hydrosilylation/Deoxygenation by ActB-Tol: (a) Deoxygenation of Silyl Ether 5 in the Presence of Silane, (b) without Silane, (c) Attempted Reaction of Styrene (6), (d) Reaction of Alcohol 9 in the Presence of Silane, and (e) without Silane. Conditions: 60 °C, 22 h, Toluene Solvent (1 mmol Substrate, 20 mg of ActB). GC Yields Given in Parentheses
Scheme 5
Scheme 5. Hydrosilylation/Deoxygenation of (a) Benzaldehyde (10), (b) Trans-chalcone (13), (c) Benzil (16), (d) Cyclohexanone (22), and (e) 2-Heptanone (27), using ActB-Tol or ActB-cyHx as the Catalysts at Various Conditions (Silane Stoichiometry, Temperature, Time) as Indicated. All Reactions Performed in Toluene. Yields Determined by GC
Figure 8
Figure 8
Time-dependence of the catalytic activity of ActB-Tol in benzophenone (1) deoxygenation and ActB-cyHx in trans-chalcone (13) 1,4-hydrosilylation under flow conditions (X-Cube, 100 °C, flow rate: 0.1 mL min–1—solid lines—or 0.2 mL min–1—segmented line, concentration of substrates 0.5 mM mL–1).
Figure 9
Figure 9
Ethanol conversion (top) and ethylene yield (bottom) during ethanol dehydration at 170, 190, 210, and 240 °C. Weight hour space velocity (WHSV) was kept for all measurements at 2.2 g g–1 h–1.
Figure 10
Figure 10
Ethanol conversion (top) and ethylene yield (bottom) during ethanol dehydration at 240 °C overnight (stability test). WHSV was kept at 4.4 g g–1 h–1 except for HZSM-5 due to its high activity. WHSV was set to 17.6 g g–1 h–1; for details, see the SI.
See this image and copyright information in PMC

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