A facile strategy based on the metal-free design of carbon to deliver an insight into the active sites for liquid phase carbocatalysis.

An effective method to study the active sites for carbocatalysis is proposed based on designing a carbon catalyst in the absence of metal as the growth catalyst. The results suggest that the oxygenated groups on the aromatic carbons are mainly responsible for the catalytic reduction of nitrobenzene and some other reactions.

High Yield Controlled Synthesis of Nano-Graphene Oxide by Water Electrolytic Oxidation of Glassy Carbon for Metal-Free Catalysis.

The strong quantum confinement effect as well as abundant edges and oxygen functional groups enable nano-graphene oxide (NGO) a variety of intriguing applications such as catalysis, bioimaging, drug delivery and photovoltaic devices. However, the development of NGO is severely hindered because of the difficulty in controlled mass production. Here, we report the efficient synthesis of NGO with a high yield of ∼40 wt % by water electrolytic oxidation of glassy carbon (GC). The NGO shows a high oxidation degree (C/O atomic ratio, ∼1.4) and excellent dispersion stability. Moreover, its size can be easily tuned by the graphitization degree of GC, which enables the controlled synthesis of NGO with average size of 4, 8, and 13 nm and different oxygen functional groups. As metal-free catalysts, the 13 nm sized NGO is found to be beneficial for the oxidative coupling reaction of benzylamine, while the 4 nm sized NGO shows a conversion rate of 88 times higher than 13 nm sized NGO for the oxidation reaction of benzene. In addition, the water electrolytic oxidation mechanism of graphitic materials is systematically studied. It is found that sulfuric acid has a protective effect on the graphite electrode during the water electrolytic oxidation process, and 50 wt % sulfuric acid solution well balances the protection and oxidation processes, leading to the highest oxidation efficiency and production rate.

A comparative study of nitrobenzene reduction using model catalysts.

A zigzag-type quinone performs better than an armchair-type quinone in the reduction of nitrobenzene. When different kinds of functionalities co-exist, the reaction is dominated by the most active sites, but the most negative sites should also be taken into consideration if the acitive sites have zigzag structures.

Biomass-Derived Graphene-like Carbon: Efficient Metal-Free Carbocatalysts for Epoxidation.

N-doped graphene-like layered carbon (NG) could be synthesized via a metal-free pyrolysis route from glucose, fructose, and 5-hydroxymethylfurfural (5-HMF), which are cheap and widely available biomass or biomass derivatives. A well-developed thin-layer structure with large lateral dimensions could be obtained when 5-HMF was used as the precursor. More importantly, the 5-HMF-derived NG gave superior performance in epoxidation reactions compared with the conventional carbon catalysts and the performance of 5-HMF derived NG was even similar to that of a cobalt catalyst. Characterizations by TEM and XPS accompanied by EPR analysis revealed that the enhanced catalytic properties for NG arise from its high activation ability for both alkenes and O2 , which are attributed to the graphitic layered structure and graphitic N species, respectively.

Carbocatalysis in Liquid-Phase Reactions.

There is broad interest in metal-free catalysis for sustainable chemistry. Carbocatalysis is a "green" option for catalytic transformations in the gas phase as well as in the liquid phase. This is evident by the numerous reports on gas-phase dehydrogenation and selective oxidation where carbon can be used as a successful alternative to metal oxide systems. Carbocatalysis for liquid-phase reactions, especially for organic synthesis, is an emerging research discipline and has undergone rapid development in recent years. This Review provides a critical analysis on the state-of-the-art of carbocatalysts for liquid-phase reactions, with a focus on the underlying mechanisms as well as the advantages and limitations of metal-free carbocatalysts.

Hydrothermal Carbon Enriched with Oxygenated Groups from Biomass Glucose as an Efficient Carbocatalyst.

Metal-free carbocatalysts enriched with specific oxygenated groups with different morphology and size were synthesized from glucose by hydrothermal carbonization, in which cheap and widely available biomass could be converted into functionalized carbon using an environmentally benign process. The hydroxy- and carbonyl-enriched hydrothermal carbon (HTC) could be used in nitrobenzene reduction, and higher conversion was obtained on the sphere morphology with smaller size. In the Beckmann rearrangement of cyclohexanone oxime, carboxyl-enriched HTC exhibited superior performance compared with conventional solid acid (such as HY and HZSM-5), on which the strong acid sites and weak Lewis acid sites lead to high selectivity for the side product. Although the intrinsic acidity of carbon is weak, the carboxyl-enriched carbon was used in weak Brønsted acid-catalyzed reactions, such as the Beckmann rearrangement.

Active sites and mechanisms for direct oxidation of benzene to phenol over carbon catalysts.

The direct oxidation of benzene to phenol with H2 O2 as the oxidizer, which is regarded as an environmentally friendly process, can be efficiently catalyzed by carbon catalysts. However, the detailed roles of carbon catalysts, especially what is the active site, are still a topic of debate controversy. Herein, we present a fundamental consideration of possible mechanisms for this oxidation reaction by using small molecular model catalysts, Raman spectra, static secondary ion mass spectroscopy (SIMS), DFT calculations, quasi in situ ATR-IR and UV spectra. Our study indicates that the defects, being favorable for the formation of active oxygen species, are the active sites for this oxidation reaction. Furthermore, one type of active defect, namely the armchair configuration defect was successfully identified.

Carbon nanotubes oxidized by a green method as efficient metal-free catalysts for nitroarene reduction.

Hydrogen peroxide (H2O2) functionalized carbon nanotubes exhibited better catalytic performance than their nitric acid oxidized counterparts in the reduction of nitrobenzene. One important reason may be attributed to the notably less negative oxygenated groups on the surface of the former one.

Mixed template effect adjusted by amine concentration in ionothermal synthesis of molecular sieves.

Cationic templating of imidazolium adjusted by the amine concentration in an ionothermal system results in products with larger channels or cage-like structures by reassembling the inorganic hosts around the changed organic guests.