Nitrogen‐Doped sp²‐Hybridized Carbon as a Superior Catalyst for Selective Oxidation

Abstract

Current approaches for efficient C À H bond activation are usually mediated by heterogeneous [1] or homogeneous [2] catalysts. The basis is the employment of transition metals or organometallic centers, which is pivotal for the successful attack on the targeted C À H bonds. [3] However, we have reported that it is feasible to use carbon-based nanomaterials to activate short-chain alkanes in catalytic dehydrogenation reactions [4] although relatively high reaction temperatures are required. It is of particular interest to know whether it is possible to activate CÀH bonds to get high value-added products at a moderate reaction temperatures by using cheap metal-free catalysts. To this end, an elegant approach using metal-or boron-doped carbon nitrides as catalysts [5] has been developed for the selective oxidation of allylic and benzylic hydrocarbons in organic solvents with moderate conversion. Attempts to achieve higher activity also include the application of N-alkoxysulfonyloxaziridines for the activation of C(sp 3 ) À H bonds, [6] although a complicated catalytic system for efficient reaction circulation was required.Layered carbon, that is, highly exfoliated graphitic structures containing one or a few graphene layers, [7] has an unconventional electronic structure, [8] which was speculated to have a high chemical reactivity. [9] Indeed, researchers observed that layered carbon can catalyze hydrogenation, [10] ring-opening polymerization, [11] and CÀH oxidation reaction, [12] and that it could serve as a support for metal oxide catalysts. [13] Herein we describe nitrogen-doped graphene materials that can activate the benzylic C À H bond with exceptionally high activity. The nitrogen atoms introduced are preferentially bound at graphitic sites in the carbon framework. This induces high charge and spin density at the adjacent ortho carbon, which promotes the formation of reactive oxygen species and the materials display exceptional catalytic activity even at room temperature.Firstly, we examined the oxidation of ethylbenzene in aqueous phase with tert-butyl hydroperoxide (TBHP) as the oxidant and without using catalyst. However, no obvious activity was observed by GC after a reaction time of 24 h (Table 1, entry 1). Then we used a graphene sample prepared by the arc-discharge method (referred to as Arc-C) [14] as the catalyst for this reaction. Surprisingly, Arc-C activated ethylbenzene at 353 K to generate acetophenone in 20.7 % yield (Table 1, entry 2). As Arc-C had been prepared by a directcurrent arc-discharge method with a pure graphite rod as the electrode in an NH 3 /He atmosphere, besides trace nitrogen (0.7 %), no element other than carbon was detected by elemental analysis (EA) (oxygen cannot be detected by this method). The full X-ray photoelectron spectrum showed a C content of 97.9 % and low amounts of nitrogen and oxygen of 0.9 % and 1.1 %, respectively. This promising observation suggests that it is layered carbon material itself that catalyzed the oxyfunctionalization of the hydrocarbon. As Arc-C...

Heterogeneous PdAg alloy catalyst for selective methylation of aromatic amines with formic acid under an additive-free and solvothermal one-pot condition

“…The catalytic performance was evaluated using the model reaction between aniline and formic acid under an additive-free and solvothermal one-pot condition (140°C, 10 h), in comparison to mono-and tri-metallic catalysts (Table 1 and see ESI † Supplementary Table S4). Both monometallic Pd/N-rGO and commercial Pd/C catalysts produced a mixture of cyclohexanone and dicyclohexylamine via active hydrogenation, as expected (entries 1 and 2, Table 1), which was consistent with the reported results, [21][22][23][24][25] while a metal-free carbocatalyst (GO and N-rGO) 28,29 and mono-metals (Ag, Co, Au, Pt, Ni and Sn) loaded on N-rGO showed almost no catalytic effect (see ESI † entries 1-10, Supplementary Table S4). Positively, the Pd-based bimetallic nano-alloys with transition metals (Co, Au, Pt, Sn and Ni) produced N-methyl-N-phenylformamide (5) as a methylation product in a yield range of 18-70% (entries 3-7, Table 1).…”

“…Three N1s peaks at~399, 400 and 402 eV were assigned to the pyridinic, pyrolic and graphitic or quaternary N (see ESI † Supplementary Figure S4c). 28,29,31 The large amount of nitrogen (~4 at %) and oxygen atoms (~18 at %) in Supplementary Table S5 coordinated metal NPs on the graphene in a well dispersed and stable manner, preventing the re-oxidation of noble metal M 0 (M = Ag, Pd), as reported for N-doped carbon. 15,32 Therefore, a structural model of Pd 47 Ag 53 /Fe 3 O 4 /N-rGO could be hypothetically proposed (in ESI † Supplementary Scheme S1).…”

Direct Observation of Quantum‐Confined Graphene‐Like States and Novel Hybrid States in Graphene Oxide by Transient Spectroscopy

“…This could be helpful for highly active photocatalytic processes. [ 43,56 ] It also renews the understanding of role of oxygen-containing functional groups in GO and its part-reduced derivatives. Moreover, a greater understanding of the fundamental physics of carbon nanomaterials will eventually be a benefi t for design and fabrication of novel carbon-based nano optical-electronic devices in the not-too-distant future.…”

Synthesis and electrochemical applications of nitrogen-doped carbon nanomaterials

“…Graphene is considered as an interesting, exciting, and eye-capturing two-dimensional (2-D) structure formed by the sp 2 hybridized carbon [145][146][147][148][149]. The nitrogenenriched graphene (NG) materials have been prepared by the various synthesis techniques and routes ( Figure 7) [151,152] in order to meet the desired requirements for the various electrochemical and biochemical applications [139,[153][154][155][156].…”