Surface-Modified Carbon Nanotubes Catalyze Oxidative Dehydrogenation of n -Butane

Abstract

Butenes and butadiene, which are useful intermediates for the synthesis of polymers and other compounds, are synthesized traditionally by oxidative dehydrogenation (ODH) of n-butane over complex metal oxides. Such catalysts require high O 2 /butane ratios to maintain the activity, which leads to unwanted product oxidation. We show that carbon nanotubes with modified surface functionality efficiently catalyze the oxidative dehydrogenation of n-butane to butenes, especially butadiene. For low O 2 /butane ratios, a high selectivity to alkenes was achieved for periods as long as 100 hours. This process is mildly catalyzed by ketonic C=O groups and occurs via a combination of parallel and sequential oxidation steps. A small amount of phosphorus greatly improved the selectivity by suppressing the combustion of hydrocarbons.Transition metal oxides have been widely used as catalysts for the conversion of butane to C 4 alkenes, important industrial precursors for producing synthetic rubbers, plastics, and a number of industrially important chemicals. Despite a great deal of research, alkene selectivity in the current butane-to-butadiene process is severely limited (1). One important reason is that the unsaturated products are much more readily oxidized to CO 2 than is the starting alkane. The chemical complexity of polyvalent metal oxides, although found to be necessary for catalytic activity, impedes satisfactory selectivity through isolation of active sites (2-6). For this reason, the origin of the catalytic activity is debated, and there is as yet no generally accepted picture of the reaction mechanism (7, 8).Carbon materials have been reported to catalyze the oxidative dehydrogenation (ODH) of an aromatic molecule, ethylbenzene. However, conventional carbons, in particular activated carbon, underwent unavoidable deactivations due to coking or combustion (9-12). Recently, it was shown that only wellnanostructured carbons are stable and coke-free catalysts for styrene synthesis (12, 13). Activation of C-H bonds in the ethyl group is considered to be coordinated by the ketonic carbonyl (C=O) group. Ethylbenzene has an aromatic moiety that enables relatively facile activation. Here, we report on surfacemodified carbon nanotubes (CNTs) as a high-performance catalyst for the ODH of the much less active butane. Relative to metal-based catalysts, CNTs displayed an enhanced selectivity to C 4 alkenes, especially butadiene.We conducted the reaction at 400° or 450°C with an O 2 /butane ratio of 2.0. The product mixture contained only 1-butene, 2-butene, butadiene, CO 2 , CO, and residual reactants; the resulting carbon balance was 100 ± 3% ( fig. S1A) (14). In a blank experiment without catalyst, the alkene yield was as low as 0.9%. Over pristine CNTs, 88.9% of the converted butane was burnt, yielding 1.6% alkenes (Fig. 1A). Considering the intensive stability of CNTs in O 2 ( fig. S1B) (14), we conclude that the CO 2 during the reaction mainly originated from the oxidation of the hydrocarbon feedstock and not from burning of t...

Catalytic performance of carbon nanotubes in H2O2 decomposition: Experimental and quantum chemical study

“…Besides serving as a support, CNTs alone can be used as catalysts for some specific reactions, including methane and NO x decomposition [3,4], oxidative dehydrogenization of aromatic hydrocarbons and alkanes [5,6,7,8,9], oxidation of aniline [10], p-toluidine [11], and benzyl alcohol [12], catalytic wet air oxidation of phenol [13,14] and pcoumaric acid [15,16], aerobic oxidation of cyclohexane [17,18], ozonation of oxalic acid [19,20], selective oxidation of H 2 S [21], and heterogeneous hydroxylation of organics [22]. Nitrogen-doped CNTs were used as basic catalyst in Knoevenagel condensation [23] and CO oxidation [24] reactions.…”

Oxidative Dehydrogenation of Ethane over Multiwalled Carbon Nanotubes

“…The latter is contradictory to the TPD results, but possibly resulted from the relatively slow thermal decomposition of the P 2 O 5 precursor salt and its spreading on the CNTs surface masking the generation of new ketonic functionalities on the surface of P-oCNTs. With regard to the overall reaction rate in comparison with other hydrocarbons, the ODH of C 2 H 6 is the slowest of all the C 2 -C 4 alkanes, [1,2] as can be seen from the low conversions (Figure 2 a). This result was expected, as the reactivity of the alkanes is known to increase with the chain length.…”

“…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.…”

“…[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.…”

Controlled growth of metal-free vertically aligned CNT arrays on SiC surfaces

“…Due to their extraordinary physico-chemical properties, carbon nanotubes (CNTs) may find extensive applications in heterogeneous catalysis and fuel cell technology, either as metal catalyst supports [1,2] or directly as metal-free catalysts [3,4]. For such purpose it is often necessary to immobilize CNTs arrays on solid supports such that the immobilized CNT materials can be directly used in practical catalytic processes [4,5].…”