Heterogeneous Metal-Free Hydrogenation over Defect-Laden Hexagonal Boron Nitride

Abstract

Catalytic hydrogenation is an important process used for the production of everything from foods to fuels. Current heterogeneous implementations of this process utilize metals as the active species. Until recently, catalytic heterogeneous hydrogenation over a metal-free solid was unknown; implementation of such a system would eliminate the health, environmental, and economic concerns associated with metal-based catalysts. Here, we report good hydrogenation rates and yields for a metal-free heterogeneous hydrogenation catalyst as well as its unique hydrogenation mechanism. Catalytic hydrogenation of olefins was achieved over defect-laden h-BN (dh-BN) in a reactor designed to maximize the defects in h-BN sheets. Good yields (>90%) and turnover frequencies (6 × 10^-5-4 × 10^-3) were obtained for the hydrogenation of propene, cyclohexene, 1,1-diphenylethene, (E)- and (Z)-1,2-diphenylethene, octadecene, and benzylideneacetophenone. Temperature-programmed desorption of ethene over processed h-BN indicates the formation of a highly defective structure. Solid-state NMR (SSNMR) measurements of dh-BN with high and low propene surface coverages show four different binding modes. The introduction of defects into h-BN creates regions of electronic deficiency and excess. Density functional theory calculations show that both the alkene and hydrogen-bond order are reduced over four specific defects: boron substitution for nitrogen (B_N), vacancies (V_B and V_N), and Stone-Wales defects. SSNMR and binding-energy calculations show that V_N are most likely the catalytically active sites. This work shows that catalytic sites can be introduced into a material previously thought to be catalytically inactive through the production of defects.

Figure 1

Hydrogenation of (E)-1,2-diphenylethene [6] under excess hydrogen at 170 °C with different catalyst loadings. Five grams (27.7 mmol) of olefin was used. Both 1 g of dh-BN activated for 48 h under hydrogen (a) and nonactivated h-BN-coated milling media (b) effectively hydrogenated the olefinic bond.

Figure 2

Hydrogenation of propene [1] with an excess of hydrogen at 220 °C. Two grams of catalyst and 37.9 mmol of propene [1] were used. The reaction using dh-BN activated in a large batch had slower kinetics (black trace) than that of the reaction using dh-BN activated in a smaller batch (gray trace).

Figure 3

¹³C SSNMR of dh-BN exposed to high and low partial pressures of propene. The peaks around 25 ppm are due to the methyl carbon on propene and show four distinct binding modes. The peaks at 62 and 83 ppm are likely due to partial oxidation of the bound propene molecule. The spectra were obtained using a cross-polarization technique.

Figure 4

TPD of ethene on dh-BN. The desorption profile is similar to that from defect-laden surfaces.^,

Figure 5

Electronic density cross sections plotted along the vertical plane, passing through the center of the two carbon atoms of gas-phase (A) ethene (C₂H₄), (B) ethane (C₂H₆), and (C) C₂H₄/dh-BN for the defects B_N, SW, V_N, and V_B. Contours are drawn in a linear scale (nine contours from 0 to 0.27 e/bohr³). It can be seen that the electronic density of ethene on dh-BN exhibits a similar structure to that of C₃H₆, indicating a reduction of the bond order of the C–C bond in the adsorbed ethene.

Figure 6

The barrier (in electronvolts) for each elementary reaction step is calculated using the climbing image nudged elastic band method and is shown by the number (eV) between the states. The largest barrier in the minimum energy pathway of propene hydrogenation over V_Nis 1.53 eV (148 kJ/mol). The zero potential energy corresponds to propene and hydrogen in the gas phase without any interaction with h-BN. The thick horizontal bars represent intermediate states. *Adsorbed species. The insets are structures of (a) co-adsorbed propene and hydrogen (C₃H₆* + 2H*) and (b) intermediate state C₃H₇*.