(651d) Selective Oxidative Dehydrogenation of Propane to Propylene Using Boron Nitride Catalysts

The oxidative dehydrogenation of
propane (ODHP) as a method of “on-purpose” propylene production has the
potential to be a game-changing technology in the chemical industry.[1]
However, even after decades of research establishing supported vanadium oxide (V/SiO₂)
as the state-of-the-art catalyst for ODHP,[2,3] selectivity to the olefin
product remains too low to be commercially attractive because of overoxidation
of propylene into CO and CO₂ (CO_x).

Here, we report that hexagonal
boron nitride (h-BN) and boron nitride nanotubes (BNNTs) are highly selective
catalysts for the ODHP reaction.[4,5] Despite structurally similar materials
(graphene and carbon nanotubes) showing catalytic activity for partial
oxidations in recent years, BN materials have yet to be explored for their own
catalytic activity. Both h-BN and BNNT are among the most-selective catalysts
reported for ODHP (Figure 1A). Aside from only propylene, it is equally
interesting to compare the complete product distribution between BN and V/SiO₂ catalysts
(Figure 1B). While the main byproducts of V/SiO₂ are
overoxidized products CO_x, the main byproduct of h-BN and BNNT is
ethylene, another important small olefin. For reasons that are still under
investigation, BN materials are able to largely avoid the overoxidation
reaction pathway.

We have kinetic evidence for an
Eley-Rideal reaction mechanism when using h-BN, showing the rate of propane
consumption dependent on the adsorption of oxygen and second order with respect
to P_C3H8. Our spectroscopic investigations tell us the
surface of h-BN and BNNT is functionalized by oxygen only when exposed to ODHP
reaction. Considering these insights, as well as previous DFT studies
investigating stable O-terminated BN armchair edges, we calculated reaction
intermediates suggesting the reaction active site may be a peroxo-like
>B-O-O-N< armchair edge.

Figure 1. (A)
Selectivity to propylene plotted against propane conversion for ODHP, comparing
h-BN (green), BNNT (blue) and V/SiO₂ (black) to previously
reported catalysts (open shapes); (B) Comparisons of product selectivity
between V/SiO₂ (X_C3H8=5.8%), h-BN (X_C3H8)=5.4%)
and BNNTs (X_C3H8=6.5%). Product selectivities are represented
by colored bars.

References

1.    T. Degnan, Focus
on Catalysts, 2 (2016) 1

2.    C.
Carrero, R. Schlogl, I. Wachs, R. Schomaecker, ACS Catal., 4 (2014)
3357

3.    J. Grant,
C. Carrero, A. Love, R. Verel, I. Hermans, ACS Catal., 5 (2015)
5787

4.    J. Grant,
C. Carrero, F. Goeltl, J. Venegas, P. Mueller, S. Burt, S. Specht, W.
McDermott, A. Chieregato, I. Hermans, Science, 354 (2016) 1570

5.    J. Grant,
C. Carrero, J. Venegas, A. Chieregato, I. Hermans, US Patent Application
15/260,649; Filed September 9, 2016.