To examine the catalytic effects of GO, several factors need to be explored. Owing to the harsh oxidation and sonication processes during its synthesis, GO sheets are usually coated by debris made of highly oxidized, and acidic molecular fragments 26 , Without a careful washing step to remove these debris, it is not clear whether the catalysis effects originate from this oxidative debris or from the intrinsic oxygen functional groups in GO.
Second, contributions from metallic contaminations cannot be excluded 28 unless the GO is well purified. Therefore, as-synthesized GO sheets were subjected to the sequential base and acid treatments to eliminate acidic debris and metallic contamination Fig. The base reduction NaOH, 0. To check whether metallic impurities were removed after the purification steps, modified GO abbreviated as ba-GO and its precursor graphite were analysed by inductively coupled plasma mass spectrometry Supplementary Table S1.
As shown, the concentrations of all metal impurities in ba-GO are much lower than the graphite precursor and as-prepared GO.
Aerobic oxidative coupling of primary amines, which often requires the use of precious metal catalyst, 29 , 30 , 31 , 32 was chosen as a model reaction, where GO and ba-GO were used as the substitute for the metal catalysts. Interestingly, ba-GO shows dramatically improved conversion. Control experiments were also carried out using alternative carbon catalysts similar to oxidized nanodiamond and carbon black for comparison, which all showed low activity Supplementary Table S2 , entries 12— These results prove that observed catalytic activity is unique to ba-GO, and the oxidative debris and metal contaminants do not have any significant role.
In small-scale reactions, ba-GO has demonstrated its effectiveness in the oxidative coupling of various primary amines Supplementary Table S3. As GO-based materials can be readily scaled up in large quantities 37 , it is important to determine whether the scale of the catalytic process can be increased. Therefore, a 1-g scale reaction for oxidative coupling of benzylamine 9. We have also verified that the catalysis can be carried out in open air and similar conversion and yield can be achieved Supplementary Table S2 , entry The dramatically enhanced activity of ba-GO in large-scale reaction suggests that higher concentration of substrate significantly contributes to better performance.
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With these optimized conditions, the effectiveness of ba-GO for oxidative coupling of various primary amines in a large scale was assessed Table 1. The electronic effects of the substituents on the phenyl ring are found to be well tolerated under mild reaction conditions Table 1 , entries 3—7. Moreover, para-, meta- and ortho-substituted benzylamine prove to be suitable substrates, revealing that the steric hindrance effect of the aryl ring has little impact on the efficiency of the coupling process Table 1 , entries 5—7. More impressively, the ba-GO catalyst shows good recyclability and can be reused by simple filtration and rinsing in acetonitrile.
There is little difference in both relative activity and selectivity Fig. Above all, the performance of ba-GO is comparable or even superior to that of metal catalysts 29 , 30 , The low catalyst loading is unprecedented compared with previous catalysis reaction using GO 16 , 17 , The combination of solvent-free and metal-free catalysis using low catalyst loading is a good model for practical application.
It is instructive to ask this question. Why does sequential base and acid treatment ba-GO improve the catalytic properties of GO so significantly? To correlate the catalytic activity of GO with variation in its chemical structure following sequential base and acid treatments, thermal gravimetry analysis TGA and solid state 13 C NMR studies were performed.
This result indicates that the oxygen functional groups of ba-GO are largely removed after the base—acid treatment. The broad resonance peak at X-ray photoelectron spectroscopy analysis Fig. S3a also confirms that the populations of hydroxyl and epoxide groups become significantly reduced after stepwise base and acid treatments. According to the recent studies on the mechanism of base reduction of GO, 26 , 27 the removal of oxygen functionalities is attributed to the effective removal of oxidative fragments and to a dehydration reaction driven by the base reflux conditions Components in the spectra due to epoxide, carboxyl and hydroxyl are highlighted in green, yellow and aqua, respectively.
Our purification process as well as its subsequent characterizations has excluded the role of impurities arising from debris or metallic impurities, thus the intrinsic oxygen functional groups in ba-GO should have an important role in the carbocatalysis.
Fourier transform infrared FTIR absorption spectroscopy was performed in-situ in a vacuum environment, see the Supplementary Methods for details at room temperature RT to analyse the functional groups in base-treated GO b-GO and ba-GO sequential base and acid treatments. A representative infrared absorbance spectrum of GO collected in the transmission mode is shown in Fig. As previously reported, 39 infrared spectra of as-synthesized GO indicate contributions from various oxygen functional groups. To find out whether the functional groups that are affected by the base treatment can be regenerated, b-GO was immersed in a HCl solution 0.
This observation also confirms the presence of such carboxyl groups after the base treatment.
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S4 further revealed the fact that the base treatment produced similar effect as thermal treatment in chemically reducing the hydroxyl and epoxy species. After sequential base and acid treatments, in-situ FTIR analysis reveals that the residual oxygen functional groups in ba-GO are mainly ketones, epoxides significantly reduced and carboxylic acids. The hydroxyl groups have been largely removed. To test how the catalysis is affected by the further reduction of residual oxygen functional groups, ba-GO were treated with NaBH 4 Fig.
However, reduction in the catalytic efficiency can either originate from the reduction of the oxygen-functionalities such as quinones, epoxides or from the deprotonation of the carboxylic acid groups. To investigate whether the base effect of NaBH 4 has affected the catalytic reaction, a neutralization step with acid Fig. This finding therefore provides strong evidence that the carboxylic acid groups have a vital role in the catalysis reactions as it is unlikely that the neutralization treatment can recover the reduced oxygen functionalities or introduce additional oxygen functionalities.
This explanation agrees well with our observation that the catalytic properties of base-treated GO b-GO is much poorer than those of the ba-GO and the catalytic activity is enhanced only after the acid neutralization Supplementary Table S2 , entries 8 and All these findings point strongly to the significant contribution of the carboxylic acids in the catalytic conversion here.
To confirm the role of carboxylic acids in this synthetic transformation, 1-pyrene carboxylic acid was used as a molecular analogue to mimic the graphene-based catalytic system. We also observed that pyrene itself has no activity in the aerobic oxidation of amines. According to the widely accepted Lerf—Klinowski model, 40 epoxy and hydroxyl groups reside on the GO basal plane, whereas the periphery of the GO sheets is decorated mainly by carboxyl, anhydride, lactone, phenol, lactol or pyrone groups.
As the carboxylic acids are mainly residing at the periphery of the sheet, its coverage will be expected to be relatively low compared with the other oxygenated groups and it is difficult to explain the observed catalytic activity.
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Scanning tunnelling microscope STM imaging Fig. These holes could be created during the harsh oxidation process, which were enlarged by the etching process during the base treatment. Infrared spectroscopy studies also confirmed the hole defect formation by indicating a CO 2 production during thermal annealing of base-treated GO Supplementary Fig. This fact could be further confirmed by atomic force microscopy studies Supplementary Fig.
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These findings suggested that the active catalysis sites in ba-GO are likely to be hole defects in the conjugated domain, and the edges of the hole defect are terminated by the carboxylic acids. A low temperature annealing is needed to remove adsorbates to allow clear imaging. The density of larger-sized holes is higher in b-GO relative to GO. The solid lines are fitted with Lorentzian curves. Our studies showed that the base reduction of GO generates nanovoids that are surrounded by conjugated domains.
To confirm this fact, we performed electron spin resonance ESR measurements for GO subjected to different treatments. The ESR spectra shown in Fig. Only ba-GO exhibits the broader ESR signal, with an intensity ratio of the broad peak to narrow peak of about One way to study whether the spin states are important in catalysis is to quench them.
It is well known that the coupling of diazonium to graphene occurs via a radical pathway, and therefore regions on graphene with unpaired electrons should be highly reactive towards such coupling reactions. In this case, we functionalized ba-GO with phenyl carboxylic acid using diazonium coupling reactions.
Introduction of the phenyl carboxylic groups ensures that the density of the carboxylic functionalities is preserved after the coupling reactions, although the density of unpaired spins in ba-GO will be reduced. This suggests that radical states at the edge sites are important in the catalysis besides carboxylic groups.
To shed light on the mechanism, we employed a spin-trapping electron paramagnetic resonance EPR technique using 5,5-dimethylpyrroline N -oxide DMPO as the spin trap to detect the intermediates formed during reaction. Control studies show that in the absence of ba-GO catalyst or O 2 , spin signals due to oxygen radical show very low intensity Fig. The formation of H 2 O 2 , which is one of the postulated intermediates in the aerobic oxidation reactions is also detected by using the catalysed oxidation of N , N -diethyl-1, 4-phenylenediammonium sulphate DPD by Horseradish peroxidase POD , where two absorption peaks centred at ca.
We suggest that these edge sites with unpaired electrons constitute the active catalytic sites and afford enhanced kinetics for the trapping and activation of molecular oxygen by a sequence of electron transport and reduction to superoxide radical. One plausible mechanism involving both the unpaired electron and the carboxylic groups is proposed as following: Benzylamine 1a binds to the acidic carboxyl groups via hydrogen bond, and forms an electron-donor complex B.
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At the same time, the anchored amine is oxidized by positive charge of the hole to form the cation radical complex. Meanwhile, the active site is released for another catalytic cycle, giving rise to a turnover mechanism. The imine intermediates can react with the free amines to furnish the coupling products Fig. The role of base and acid treatment in enhancing the catalytic performance of GO can now be understood. Refluxing in base helps to increase the density of the nanovoids in GO, which gives rise to a high density of edges that can have localized spins. Another important effect of the base reduction is the removal of the hydroxyl groups.
This has the effect of changing the dynamics of water solvation layer around GO, which allows greater access to the catalytic sites by the reactant. A subsequent acid neutralization step is needed to recover the active carboxylic groups, which serves as hydrogen bonding sites to the amines and also facilitates the proton transfer reactions due to its acidic properties. More excitingly, we discovered that ba-GO catalyst can be used under solvent-free, open-air condition and is recyclable, this has tremendous implication for green catalysis and the full potential of this material remains to be discovered.
In conclusion, we have identified a chemical treatment that is needed to impart high catalytic activities on GO in the aerobic oxidation of various amines and investigated the catalytic origins. As GO is an amphiphilic material with pH-sensitive functional groups, appropriate base and acid treatments are needed to activate the catalytic sites in GO.
The carboxylic groups at the edges of defects, along with the localized unpaired electrons, work synergistically to trap molecular oxygen and the amine molecules, thus facilitating intermolecular arrangements. In view of the fact that the production of GO is now being scaled up industrially, these chemical processing steps can be adopted as the standard protocol to render GO catalytically active, thus opening the possibility for the large-scale industrial applications of GO in industrial catalysis. Then, 7. The final ba-GO obtained were filtered and washed by DI water and acetone. The synthesized ba-GO was kept in vacuum desiccators.
Debris Supplementary Fig. S7a and Supplementary Fig. S8 present in the GO solution was isolated by using ammonia as the base according to the reported reference Benzylamine 1. After that, the solution was cooled to RT and 1. N -phenyl tetrahydroisoquinoline 3a Sample preparation: DPD reagent was prepared by dissolving 0.