Carbocatalysis in Liquid-Phase Reactions

Cooperative Surface-Particle Catalysis: The Role of the "Active Doughnut" in Catalytic Oxidation

We consider the factors that govern the activity of bifunctional catalysts comprised of active particles supported on active surfaces. Such catalysts are interesting because the adsorption and diffusion steps, which are often discounted in "conventional" catalytic scenarios, play a key role here. We present an intuitive model, the so-called "active doughnut" concept, defining an active catalytic region around the supported particles. This simple model explains the role of adsorption and diffusion steps in cascade catalytic cycles for active particles supported on active surfaces. The concept has two important practical implications. First, the reaction rate is no longer proportional to the number of active sites, but rather to the number of "communicative" active sites-those available to the reaction intermediates during their respective lifetimes. Second, it generates an important testable prediction concerning the dependence of the total reaction rate on the particle size. With these tools at hand, we examine six experimental examples of catalytic oxidation from the literature, and show that the active doughnut concept gives valuable insight even when detailed mechanistic information is hard to come by.

Metal-Free Carbocatalysis in Advanced Oxidation Reactions

Catalytic processes have remarkably boosted the rapid industrializations in chemical production, energy conversion, and environmental remediation. As one of the emerging applications of carbocatalysis, metal-free nanocarbons have demonstrated promise as catalysts for green remediation technologies to overcome the poor stability and undesirable metal leaching in metal-based advanced oxidation processes (AOPs). Since our reports of heterogeneous activation of persulfates with low-dimensional nanocarbons, the novel oxidative system has raised tremendous interest for degradation of organic contaminants in wastewater without secondary contamination. In this Account, we showcase our recent contributions to metal-free catalysis in advanced oxidation, including design of nanocarbon catalysts, exploration of intrinsic active sites, and identification of reactive species and reaction pathways, and we offer perspectives on carbocatalysis for future environmental applications. The journey starts with the discovery of peroxymonosulfate (PMS) and peroxydisulfate (PDS) activation by graphene-based materials. With the systematic investigations on most carbon allotropes, for the first time the carbocatalysis for PMS or PDS activation was correlated with the pristine carbon configuration, oxygen functionality (ketonic groups), defect degree (exposed edge sites and vacancies), and dimensional structure. Moreover, an intrinsic difference in catalytic oxidation does exist between PMS and PDS activation. For example, the PMS/carbon reaction is dominated by free radicals, while PDS/carbon catalysis was unveiled as a singlet oxygen- or nonradical-based process in which the surface-activated PDS complex directly degrades the organic pollutants without relying on the generation of free radicals. Nitrogen doping significantly enhances the carbocatalysis because of the positively charged carbon domains, which strongly bind with persulfates to form reactive intermediates toward organic reactions. More importantly, N doping substantially alters the catalytic oxidation from a radical process to a nonradical pathway in PMS activation. Codoping of sulfur or boron with nitrogen at a rational level will synergistically promote the catalysis as a result of the formation of more catalytic centers by improved charge/spin redistribution of the carbon framework. Furthermore, a structure-performance relationship was established for annealed nanodiamonds with a characteristic sp³/sp² (core/shell) hybridization, where the catalytic pathways were intimately dependent on the thickness of the graphitic shells. Interestingly, the introduction of structural defects and N dopants into the well-defined graphitic carbon framework and alteration of graphene/diamond hybrids can transform the persulfate/carbon system from a radical oxidation pathway to a nonradical pathway. Encapsulation of metal nanoparticles within carbon layers further modulates the electronic states of the interacting carbon via charge transport to increase the electron density. Overall, this Account contributes to unveiling the mist of carbocatalysis in AOPs and to summarizing the achievements of metal-free remediation. We also present future research directions on underpinning the knowledge base to facilitate the applications of nanocarbons in sustainable catalysis and environmental chemistry.

Highly dispersed Co and Ni nanoparticles encapsulated in N-doped carbon nanotubes as efficient catalysts for the reduction of unsaturated oxygen compounds in aqueous phase

N-Doped carbon nanotube-encapsulated Co and Ni nanoparticles exhibit excellent catalytic performance for the selective reduction of unsaturated oxygen compounds in aqueous phase.

Hollow carbon anchored highly dispersed Pd species for selective hydrogenation of 3-nitrostyrene: metal-carbon interaction

Hollow nanocarbon supported Pd species are highly active (TOF of 21 845 h⁻¹), selective (97%), and stable (4 cycles) for selective hydrogenation of 3-nitrostyrene to 3-ethylnitrobenze.

Catalysis by hybrid sp2/sp3nanodiamonds and their role in the design of advanced nanocarbon materials

Hybrid sp²/sp³nanocarbons, in particular sp³-hybridized ultra-dispersed nanodiamonds and derivative materials, such as the sp³/sp²-hybridized bucky nanodiamonds and sp²-hybridized onion-like carbons, represent a rather interesting class of catalysts still under consideration.

Selective hydrogenation of quinolines into 1,2,3,4-tetrahydroquinolines over a nitrogen-doped carbon-supported Pd catalyst

In this study, we have prepared a nitrogen-doped carbon supported Pd catalyst with abundant porous structures (abbreviated as Pd/CN) for the hydrogenation of quinolines.

Sulfonation of carbonized xylan-type hemicellulose: a renewable and effective biomass-based biocatalyst for the synthesis of O- and N-heterocycles

The application of biomass-based carbonaceous solid acids in catalysis is attracting increasing attention in the field of chemistry.

Phosphorus oxide clusters stabilized by carbon nanotubes for selective isomerization and dehydrogenation of β-isopentene

Phosphorus oxide clusters (POCs) exhibit more selective isomerization and dehydrogenation of β-isopentene than metal oxide clusters (MOCs).

Nanocarbons for Catalytic Desulfurization

Nanocarbon catalysts are green and sustainable alternatives to metal-based catalysts for numerous catalytic transformations. The application of nanocarbons for environmental catalysis is an emerging research discipline and has undergone rapid development in recent years. In this focus review, we provide a critical analysis of state-of-the-art nanocarbon catalysts for three different catalytic desulfurization processes. In particular, we focus on the advantages and limitations as well as the reaction mechanisms of the nanocarbon catalysts at the molecular level.

Metal-Free, Initiator-Free Graphene Oxide-Catalyzed Trifluoromethylation of Arenes

The direct C-H trifluoromethylation of arenes catalyzed by graphene oxide (GO) under safe conditions is described. This strategy is metal-free, initiator-free, safe, and scalable. It employs a readily available CF₃ source and the reaction can be easily controlled to obtain a mono-trifluorinated product. This method opens a new avenue for GO-catalyzed chemistry.

Selective cobalt nanoparticles for catalytic transfer hydrogenation of N-heteroarenes

Nitrogen modified cobalt catalysts supported on carbon were prepared by pyrolysis of the mixture generated from cobalt(ii) acetate in aqueous solution of melamine or waste melamine resins, which are widely used as industrial polymers. The obtained nanostructured materials catalyze the transfer hydrogenation of N-heteroarenes with formic acid in the absence of base. The optimal Co/Melamine-2@C-700 catalyst exhibits high activity and selectivity for the dehydrogenation of formic acid into molecular hydrogen and carbon dioxide and allows for the reduction of diverse N-heteroarenes including substrates featuring sensitive functional groups.

Metal-Free Oxidation of Glycerol over Nitrogen-Containing Carbon Nanotubes

Nitrogen rich carbon nanotubes have been used as a metal free catalyst for the conversion of glycerol into dihydroxyacetone using tert-butyl hydroperoxide as an oxidant. Pyridine nitrogen groups embedded in a carbon matrix are identified as active sites for the reaction. Computational studies have demonstrated that oxidation of pyridine groups to pyridine oxime followed by hydrogen abstraction from secondary alcohol is likely responsible for the oxidation process.

Graphenes as Metal-Free Catalysts with Engineered Active Sites

This Perspective article highlights how recent discoveries on the activity of defective graphene to promote different organic reactions as metal-free catalysts has led to propose certain substructures present on these defective graphenes as active sites. The sustainability of using as catalysts graphenes obtained from biomass and the possibility to generate active sites by introducing defects on the sheet are the two main characteristics triggering research in this area. Emphasis is made in the need to gain understanding on the nature of the active sites and how this understanding requires the combination of conventional kinetic experiments as well as advanced characterization tools. The relationship between catalysis by graphene and that by organocatalysis has also been remarked.

Synthesis of nitrogen and sulfur co-doped hierarchical porous carbons and metal-free oxidative coupling of silanes with alcohols

Hierarchically porous N and S co-doped carbon was used as an efficient and robust metal-free catalyst for oxidative coupling of silanes with alcohols.

Unravelling the radical transition during the carbon-catalyzed oxidation of cyclohexane by in situ electron paramagnetic resonance in the liquid phase

The selective oxidation of hydrocarbons is of great importance in the chemical industry.