Oxidative Dehydrogenation of Ethane over Multiwalled Carbon Nanotubes

Abstract

The suitability of nanocarbons as catalysts for oxidative dehydrogenation (ODH) reactions has been investigated for numerous hydrocarbon substrates, such as ethylbenzene, 1-butene, isobutene, n-butane, and propane. [1][2][3][4][5][6][7][8][9] Carbon catalysts are regarded as a clean and sustainable alternative to metal oxide catalysts, [10] as their disposal after a certain lifetime by combustion is safe and environmentally benign. In addition, carbon is free of lattice or structural oxygen and can be used as a metal-free catalyst, allowing for an in-depth theoretical treatment. [1] Industrially relevant ODH product yields, however, have been achieved only for ethylbenzene due to the kinetic stabilization of the product styrene by conjugated p electrons. Hodnett described how the product yield of ODH reactions is limited by the difference in bond strengths of the weakest CÀ H bonds in the reactant and product molecules, respectively. [11] Higher differences lead to lower product yields due to consecutive combustion. On the other hand, the barrier for chemical activation of alkanes increases with decreasing chain length. [12] Thus, the oxidative activation of ethane and methane over carbon catalysts is highly challenging due to the limitation of the reaction conditions by the carbon catalyst combustion. The oxidative coupling of methane (OCM) is apparently out of reach for this catalytic system, whereas successful ethane ODH was reported over mixed metal oxides at temperatures as low as 673 K. [13] For carbon-mediated ODH catalysis, the surface oxygen groups, especially the quinoidal groups, are regarded as the active sites. [1-3, 14, 15] Oxidative stress under ODH reaction conditions, however, inevitably leads to combustion of the carbon catalyst with acidic oxygen groups as intermediates. In contrast to the ketonic groups, these groups are electrophilic and thus ultimately cause the combustion of both the alkanes and the alkenes by attack of the carbon chain. For ODH of propane, it was shown that both the combustion of the catalyst and the formation of electrophilic oxygen could be reduced by modification of carbon nanotubes with boron and phosphorus oxide. [2] Besides an empirical test of activation capability for ethane, with a high CÀH bond strength of 420.5 kJ mol À1 , this study also contains a kinetic approach to further investigate the nature of this protection effect and a thorough study of stable and unstable surface oxygen groups that might be involved in ODH catalysis.Regarding the characterization of the catalysts, temperatureprogrammed desorption (TPD) experiments of the freshly impregnated samples clearly established that the precursor salts used for B and P modification both almost completely decompose under release of H 2 O and NH 3 at below the ODH reaction temperature of 673 K (Figure 1 a). Thus, during ODH reactivity tests, the presence of the heteroatoms in the form of partially hydrated oxides can be assumed. The pronounced hygroscopic character of P 2 O 5 was nicely reflected in the lar...

Carbon-catalysed reductive hydrogen atom transfer reactions

“…A prolonged reaction time of 36 h was required to produce a high yield of the desired product when p-isopropylbenzyl alcohol was used as the substrate (entry 9). The C-1 catalyst also exhibits promise for direct coupling of aniline with simple electron-poor heteroaromatic alcohols under similar conditions in moderate yields (entries [10][11][13][14]. p-Methylthiobenzyl alcohol was converted to the respective secondary amine in high yield.…”

“…The primary byproducts observed by gas chromatography-mass spectrometry and GC-FID were azobenzene and azoxybenzene. (7,(10)(11)13,16). wIsolated yield.…”

“…Carbon materials, including amorphous carbon, ordered mesoporous carbon, graphite/graphene (oxide) and carbon nanotubes, are widely applied in many catalytic transformations in modern organic chemistry [9][10][11] , and good performance has been obtained in the oxidation of an alkane [12][13][14][15] , alcohol [16][17][18] , amine 19 , thiol and sulphide 20,21 . These reactions encompass the oxidative hydrogen atom transfer reactions.…”

Promoted NiO Catalysts for the Oxidative Dehydrogenation of Ethane

“…For many years the typical catalysts for the ODHE reported in the literature consisted of vanadium oxide supported on different metal oxides [6,7] and recently unconventional materials such as nanocatalysts [8,9], catalysts derived from heteropolyacids [10] and carbon nanotubes [11] have demonstrated to be efficient in this reaction but their catalytic performance is still far from the best catalytic systems, which are NiO-based catalysts [12][13][14][15][16][17][18][19][20][21][22] and MoVNb-mixed metal oxide materials [23][24][25][26].…”

Activation processes of highly ordered carbon nanofibers in the oxidative dehydrogenation of ethylbenzene

“…In the present work highly ordered CNFs were used as model catalyst to determine and interpret the evolution the catalytic performance and get some additional information regarding the active sites and the catalyst activation during the reaction. Using elemental carbon as a catalyst for oxidation reactions, a pronounced initial (de)activation profile is generally observed, which is related to coke deposition in micropores [13], surface restructuring [9], and equilibration of surface oxygen groups [14][15][16][17]. The present study focuses on different methods of surface treatment, which are expected to result in different states of activated surfaces with different catalytic performances.…”