Oxygen‐Functionalized Few‐Layer Graphene Sheets as Active Catalysts for Oxidative Dehydrogenation Reactions

Abstract

Nanostructured graphitic forms of carbons have shown intersting potential for catalysis research and are ideal candidates to substitute the conventional metal-oxide catalysts because they can be easily disposed, which enables a greener, more sustainable catalytic process. Few-layer graphene and its functionalized form offer the opportunity to investigate the nature of graphitic active sites for oxidation reactions in well-defined carbon-based catalysts. In this paper, we report the utilization of oxygen-functionalized few-layer graphene sheets containing variable amounts of oxygen in the heterogeneous catalytic oxidative dehydrogenation (ODH) reaction of isobutane at 400ºC. Interestingly, there is poor correlation between oxygen content and catalytic performance. Carbonyl groups were found to be highly stable, and graphene that had higher sp(2) character, the lowest oxygen content, and fewer edge sites presented the lowest specific rate of isobutane reaction, although the isobutene selectivity remained high. The reoxidation of the graphene surface occurred at the same rate as the ODH reaction suggesting a Mars-van Krevelen type of mechanism, similar to that which takes place on oxide surfaces. These results appear to suggest that a higher fraction of exposed edges where oxygen active sites can be formed and exchanged should lead to more active catalysts for ODH reactions.

Highly dispersed buckybowls as model carbocatalysts for C–H bond activation

“…Since the nominal carbon loading was kept constant, this behavior is a strong indicator of, not only the catalytic activity function of the buckybowls, but also the welldispersed nature of the buckybowls fragments resulting in proportionality between the number of active sites and the fullerene amount deposited. The steady-state activity level obtained with the supported buckybowl fragments is well-superior to the previously studied graphitic mesoporous carbon (~1.5x10 -7 μmol s -1 g -1 at 400 °C) 12,35 and represents a 1.5 fold increase when compared to the previously studied few-layer graphene material (~2x10 -7 μmol s -1 g -1 at 360 °C). This comparison was done by calculating the reaction rate for the few-layer graphene sheets based on our previously reported isobutane ODH activity and activation energy carried out at temperatures between 380-400 o C 12,35 .…”

“…Therefore, it is imperative to have model compounds with well-defined crystal structures if one aims to fully unravel the catalytic function of graphitic materials. Previously, we have used few-layer graphene (FLG) containing varying amounts of oxygen as model catalysts for oxidative dehydrogenation (ODH) reactions 12 . The goal was to investigate the catalytic properties of graphitic carbon by exposing both, basal plane and functionalized edges, in a high surface area material.…”

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

“…4). This opposite behavior clearly demonstrates that surface area plays no direct role in the catalytic performance [26]. Textural characterization after reaction also reveals that these catalysts preserve 45-80% of their initial surface area despite coke deposition after 5 h. Similar reductions were found in a microporous commercial AC for the same reaction [6].…”

“…Considering just the active sites is essential to obtain a correct correlation. When all oxygen groups are considered, no coherent correlation is observed [26,27]. A positive intercept indicates that active sites are introduced onto the catalyst surface by oxygen present on the reaction mixture.…”

Doped graphenes in catalysis

“…However, a major difference between CNTs and GO is the high O content of the latter, with a large population of epoxide groups and this could determine that the active sites of GO for this process could be different than those of CNTs, even for the same process. In fact, it has been proposed that the oxidative dehydrogenation of light alkanes catalyzed are promoted by epoxides in GO rather than quinones as in the case of CNTs [44][45][46]. Thus, it is the methodology rather than the conclusions what has to be applied when transferring the knowledge from other allotropic forms to catalysis by Gs.…”