Radical Chemistry and Reaction Mechanisms of Propane Oxidative Dehydrogenation over Hexagonal Boron Nitride Catalysts

Abstract

Although hexagonal boron nitride (h‐BN) has recently been identified as a highly efficient catalyst for the oxidative dehydrogenation of propane (ODHP) reaction, the reaction mechanisms, especially regarding radical chemistry of this system, remain elusive. Now, the first direct experimental evidence of gas‐phase methyl radicals (CH3.) in the ODHP reaction over boron‐based catalysts is achieved by using online synchrotron vacuum ultraviolet photoionization mass spectroscopy (SVUV‐PIMS), which uncovers the existence of gas‐phase radical pathways. Combined with density functional theory (DFT) calculations, the results demonstrate that propene is mainly generated on the catalyst surface from the C−H activation of propane, while C2 and C1 products can be formed via both surface‐mediated and gas‐phase pathways. These observations provide new insights towards understanding the ODHP reaction mechanisms over boron‐based catalysts.

Why Boron Nitride is such a Selective Catalyst for the Oxidative Dehydrogenation of Propane

“…According to this simple model, propane is consumed in Reactions (14)- (17), leading to the expression Equation (2a) for the propane consumption:…”

“…While the iC 3 H 7 /nC 3 H 7 ratio is not directly experimentally accessible,w ec an evaluate Equation (1a) in two limiting scenarios:1 )gas-phase activation of C 3 H 8 via Reactions (14) and (15), and 2) surface activation of C 3 H 8 via > BOC species formed from site B 6 O 3 #2 as described in Figure S4 (Reactions (16) and (17)). Ther ate coefficient ratio of Reactions (14) and (15) leads to iC 3 H 7 /nC 3 H 7 = 1.5, reflecting the slightly higher activation barriers reported for the abstraction of primary Ha toms by HOOC and the number of primary versus secondary Hatoms in propane.Similarly,weused the computed barriers for the surface Ha bstraction by BOC to predict iC 3 H 7 /nC 3 H 7 = 0.74 under scenario 2, favoring the formation of n-propyl radicals.T his result reflects the higher reactivity of BOC species relative to HOOC radicals,w hich makes the surface sites less selective for secondary CÀH bonds in propane.…”

“…This model suggests that, under ODH conditions,t here may be amix of surface and gas-phase propane Habstraction, leading to an approximately equimolar amount of i-a nd n-propyl radicals being formed. TheH 2 O 2 formed in Reactions (14) and 15is decomposed into water and oxygen, [17] or it can react barrierless with surface > BOC species to form additional HOOC radicals ( Figure S7).…”

“…successfully detected radicals during the ODH of propane over hBN catalysts via synchrotron vacuum ultraviolet photoionization mass spectroscopy (SVUV-PIMS), in support of this hypothesis. [17] Considering the possible radical pathways involved in hBN-catalyzed ODH, we aim to gain insight into the reactions that could take place both at the surface and in the gas phase. Additionally,t he role of H 2 Oh as not been extensively explored to date,d espite it being as ignificant reaction product and literature precedent suggesting its possible synergistic role in improving oxidation performance.…”

Ab Initio Multiscale Process Modeling of Ethane, Propane and Butane Dehydrogenation Reactions: A Review

“…With high h-BN at 600 • C, they report the propylene selectivity of 36% and the propane conversion of 43%. Changing the temperature to 500 • C, the conversion was only 0.3%, while the selectivity increased to 94% [43]. Cao et al investigated an oxidative dehydrogenation of propane using graphitic carbon nitride.…”