Identifying Active Functionalities on Few‐Layered Graphene Catalysts for Oxidative Dehydrogenation of Isobutane

Abstract

The general consensus in the studies of nanostructured carbon catalysts for oxidative dehydrogenation (ODH) of alkanes to olefins is that the oxygen functionalities generated during synthesis and reaction are responsible for the catalytic activity of these nanostructured carbons. Identification of the highly active oxygen functionalities would enable engineering of nanocarbons for ODH of alkanes. Few-layered graphenes were used as model catalysts in experiments to synthesize reduced graphene oxide samples with varying oxygen concentrations, to characterize oxygen functionalities, and to measure the activation energies for ODH of isobutane. Periodic density functional theory calculations were performed on graphene nanoribbon models with a variety of oxygen functionalities at the edges to calculate their thermal stability and to model reaction mechanisms for ODH of isobutane. Comparing measured and calculated thermal stability and activation energies leads to the conclusion that dicarbonyls at the zigzag edges and quinones at armchair edges are appropriately balanced for high activity, relative to other model functionalities considered herein. In the ODH of isobutane, both dehydrogenation and regeneration of catalytic sites are relevant at the dicarbonyls, whereas regeneration is facile compared with dehydrogenation at quinones. The catalytic mechanism involves weakly adsorbed isobutane reducing functional oxygen and leaving as isobutene, and O2 in the feed, weakly adsorbed on the hydrogenated functionality, reacting with that hydrogen and regenerating the catalytic sites.

Carbon Nanofibers Modified with Heteroatoms as Metal‐Free Catalysts for the Oxidative Dehydrogenation of Propane

“…In addition, the CO/CO 2 ratio is larger for B-and P-doped CNFs, which suggests the pivotal role of groups that desorb CO in the catalysis of the selective dehydrogenation to propene in agreement with the literature. [5,8] Operation temperatures below 673 K not only ensure the long-term stability of the catalyst but also enable the determination of apparent activation energies ( Figure S11), which were masked if the reactions were performed at higher temperatures because of diffusional limitations ( Figure S12). The apparent activation energies (E a ; Table 5) were calculated for the conversions both of O 2 and C 3 H 8 lower than 10 %.…”

“…The selective sites are reported to be quinone or dicarbonyl groups on the carbon surface, while carboxylic acids are unselective to propene and favour the complete oxidation of its double bond. [5,8,9] CNFs have been used as a catalyst for the ODH of ethylbenzene, in which the graphitic platelet orientation has a significant effect on the catalyst activity and selectivity. [10] In that case, the conversion was promoted by enhanced p-p interactions of the ethylbenzene phenyl ring with the carbon basal planes.…”

“…This can be accomplished by a precise deactivation of unselective sites with dopant elements or by tailoring the oxygenated groups, which can be accomplished, for example, by exploiting the different thermal stability of the oxygen functionalities. [8] In this regard, the triphenylphosphine precursor was more effective to deactivate unselective sites while keeping selective sites active compared to ammonium phosphate or boric acid precursors. Further research should be performed to gain more insight into the interaction between the dopant precursors and unselective oxygen sites.…”

Oxidative dehydrogenation of isobutane catalyzed by an activated carbon fiber cloth exposed to supercritical fluids

“…The oxidative dehydrogenation of isobutane is usually catalyzed by metal oxides [2][3][4][5]. However, different carbon materials, such as activated carbons (ACs) [6,7], carbon xerogels [8] or graphitic carbons [9][10][11], have emerged as alternative metal-free catalysts capable of producing isobutene.…”

Direct dehydrogenation of isobutane to isobutene over carbon catalysts

“…To date, carbon has been used primarily as a support because it offers high surface areas and tunable adsorption properties. Recent advances, however, suggest that various forms of carbon, such as activated carbon [20,21], carbon nanotubes [22][23][24], graphitic carbon [17,25], graphene [22,26], and nanodiamonds [23,27,28], can themselves act as catalysts generating high selectivity and stability during the oxidative dehydrogenation (ODH) reactions of alkanes. Some researchers have even suggested that, in the case of metal-based catalytic systems, the "active coke" formed during the initial stage of the reaction plays a primary role in the ODH process [29][30][31].…”