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Boron nitride (BN) as an emerging and revolutionary metal-free catalyst has demonstrated great potential in various oxidative dehydrogenation reactions. However, traditional BN sheets always presen...

Metal-free boron- and carbon-based catalysts have shown both great fundamental and practical value in oxidative dehydrogenation (ODH) of light alkanes. In particular, boron-based catalysts show a superior selectivity toward olefins, excellent stability and atom-economy to valuable carbon-based products by minimizing CO2 emission, which are highly promising in future industrialization. The carbonaceous catalysts also exhibited impressive behavior in the ODH of light alkanes helped along by surface oxygen-containing functional groups. This review surveyed and compared the preparation methods of the boron- and carbon-based catalysts and their characterization, their performance in the ODH of light alkanes, and the mechanistic issues of the ODH including the identification of the possible active sites and the exploration of the underlying mechanisms. We discussed different boron-based materials and established versatile methodologies for the investigation of active sites and reaction mechanisms. We also elaborated on the similarities and differences in catalytic and kinetic behaviors, and reaction mechanisms between boron- and carbon-based metal-free materials. A perspective of the potential issues of metal-free ODH catalytic systems in terms of their rational design and their synergy with reactor engineering was sketched.

Zero or negative emissions of carbon dioxide (CO2) is the need of the times, as inexorable rising and alarming levels of CO2 in the atmosphere lead to global warming and severe climate change. The electrochemical CO2 reduction (eCO2R) to value‐added fuels and chemicals by using renewable electricity provides a cleaner and more sustainable route with economic benefits, in which the key is to develop clean and economical electrocatalysts. Carbon‐based catalyst materials possess desirable properties such as high offset potential for H2 evolution and chemical stability at the negative applied potential. Although it is still challenging to achieve highly efficient carbon‐based catalysts, considerable efforts have been devoted to overcoming the low selectivity, activity, and stability. Here, we summarize and discuss the recent progress in carbon‐based metal‐free catalysts including carbon nanotubes, carbon nanofibers, carbon nanoribbons, graphene, carbon nitride, and diamonds with an emphasis on their activity, product selectivity, and stability. In addition, the key challenges and future potential approaches for efficient eCO2R to low carbon‐based fuels are highlighted. For a good understanding of the whole history of the development of eCO2R, the CO2 reduction reactions, principles, and techniques including the role of electrolytes, electrochemical cell design and evaluation, product selectivity, and structural composition are also discussed. The metal/metal oxides decorated with carbon‐based electrocatalysts are also summarized. We aim to provide insights for further development of carbon‐based metal‐free electrocatalysts for CO2 reduction from the perspective of both fundamental understanding and technological applications in the future.

Novel polymerizations based on alkyne monomers are becoming a powerful tool to construct polymers with unique structures and advanced functions in the areas of polymer and material sciences, and scientists have been attracted to develop a variety of novel polymerizations in recent decades. Therein, catalytic systems play an indispensable role in the influence of polymerization efficiencies and the performances of the resultant polymers. Concerning the shortcomings of metallic catalysts, much of the recent research focus has been on metal-free polymerization systems. In this paper, metal-free catalysts are classified and the corresponding polymerizations are reviewed, including organobase-catalyzed polymerizations, Lewis-acid-catalyzed polymerizations, as well as catalyst-free polymerizations. Moreover, the challenges and perspectives in this area are also briefly discussed.

Phosphorus-doped carbon materials are promising alternatives to noble metal-based catalysts for the highly selective oxidation of benzyl alcohol to benzaldehyde, but it is challenging to achieve high loadings of high-activity P dopants in metal-free catalysts. Here, the preparation of high-loading and well-dispersed P atoms confined to the surfaces of cellulose-derived carbon via a dissolving-doping strategy is reported. In this method, cellulose is dissolved in phosphoric acid to generate a cellulose-phosphoric supramolecular collosol, which is then directly carbonized. The as-prepared carbon possesses a high specific surface area of 1491 cm

PGM-free catalysts for oxygen reduction represent a long-term, high-risk research and development approach with high potential impact on the single greatest cost contributor to automotive fuel cell stacks.

Catalyst‐controlled functionalization of unmodified carbonyl compounds is a relevant operation in organic synthesis, especially when high levels of site‐ and stereoselectivity can be attained. This objective is now within reach for some subsets of enolizable substrates using various types of activation mechanisms. Recent contributions to this area include enantioselective transformations that proceed via transiently generated noncovalent di(tri)enolate‐catalyst coordination species. While relatively easier to form than simple enolate congeners, di(tri)enolates are ambifunctional in nature and so control of the reaction regioselectivity becomes an issue. This Minireview discusses in some detail this and other problems, and how noncovalent activation approaches based on metallic and metal free catalysts have been developed to advance the field., Extend and divert. Extended enolates may be generated by deprotonation of β,γ‐ or α,β‐unsaturated carbonyl compounds smoothly, to react with a variety of electrophiles in situ. Easier to form than the parent simple enolates, di‐ and trienolates present, however, two nucleophilic carbons (typically Cα and Cγ) potentially leading to regioisomeric products. In recent years, methods based on both metallic and organic chiral catalysts have been developed able to address some of these site‐ and stereoselectivity issues, enabling direct asymmetric C−H functionalization of carbonyl compounds.

The targeted choice of specific photocatalysts has been shown to play a critical role for the successful realization of challenging photoredox catalytic transformations. Herein, we demonstrate the successful implementation of a rational design strategy for a series of deliberate structural manipulations of cyanoarene-based, purely organic donor-acceptor photocatalysts, using 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) as a starting point. Systematic modifications of both the donor substituents as well as the acceptors' molecular core allowed us to identify strongly oxidizing as well as strongly reducing catalysts (e.g., for an unprecedented detriflation of unactivated naphthol triflate), which additionally offer remarkably balanced redox potentials with predictable trends. Especially halogen arene core substitutions are instrumental for our targeted alterations of the catalysts' redox properties. Based on their preeminent electrochemical and photophysical characteristics, all novel, purely organic photoredox catalysts were evaluated in three challenging, mechanistically distinct classes of benchmark reactions (either requiring balanced, highly oxidizing or strongly reducing properties) to demonstrate their enormous potential as customizable photocatalysts, that outperform and complement prevailing typical best photocatalysts.

Current metal-free catalysts for hydrogen evolution reaction (HER) are mainly carbon-based. In this work, HER catalytic activity on two-dimensional (2D) boron sheets (α and β12) are explored using periodic density functional theory. Using the binding free energy of H (ΔGH*) as the descriptor, we found that both α and β12 sheets present superior activity, with ΔGH* being close to zero. It is expected that 2D boron sheets would be a promising metal-free catalyst in the electrolysis of water and may offer novel thoughts for the design of new catalysts for other reactions.

Efficient ionic liquid-coated carbon nanotubes (IL@CNTs) for the synthesis of chromene derivatives were developed. The characteristic features of IL@CNTs are that they are metal free and have positively charged surfaces. The catalytic activity of IL@CNTs was evaluated for the synthesis of chromene derivatives in water under sonothermal conditions, affording high yields of the desired products. The present process is an eco-friendly method for the synthesis of chromene derivatives, authenticated with a high EcoScale score, low E-factor, and metal-free conditions.

Hybrid sp2/sp3 nanocarbons, 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. Their characteristics, properties and catalytic reactivity are presented, with an analysis of the state-of-the-art of their use in gas- and liquid-phase reactions, including photo- and electro-catalysis. It is remarked that intrinsic differences exist between these and other nanostructured carbon catalysts. The analysis shows how different features make nanocarbons unique with respect to other types of catalysts and are the bases for an advanced design of nanocarbon-type catalysts. The aspects discussed regard the presence of hybrid sp2/sp3 configurations, nano-engineering related to the role of defects and vacancies in their catalytic behaviour, the creation of active sites by modification in the charge density at carbon atoms or C–C bonds, the generation of strained C–C bonds by curvature and other mechanisms, and the formation of semiconducting areas and defect sites at the interface with supported nanoparticles. The advanced strategies for identifying and quantifying active sites of carbon catalysts are highlighted.

As a major determining factor on the performance of fuel cells and metal–air batteries, the cathodic oxygen reduction reaction (ORR) plays a vital role in electricity production. The extremely complicated reaction process during ORR catalysis makes theoretical simulations highly important to clarify its catalytic mechanism as well as to develop high-performance catalysts. To substitute high-cost and scarce noble-metal catalysts (like Pt-based catalysts), novel metal-free ORR catalysts have been widely proposed by experimental and theoretical approaches. In this review, we summarize recent theoretical progress on the microscopic reaction process and catalytic performance of metal-free ORR catalysts, in both fuel cells and metal–air batteries (mainly Li–O2 batteries). The metal-free ORR catalysts are sorted by the origin of catalytic activity, such as defects and doping, intrinsic hybridization, substrate interaction, as well as alloying. Based on the presented research results, we summarize the theoretical simulations on revealing the ORR process catalyzed by metal-free catalysts, which is helpful for designing high-performance metal-free catalysts to substitute the traditional noble-metal catalysts.

A straightforward method to construct oxazolidinones through a three-component reaction involving CO2, epoxides, and amines promoted by a combination of Bu4NI and 1,8-diazabicyclo[5.4.0]undec-7-ene was developed. A wide range of aromatic and aliphatic amines and monosubstituted epoxides were converted into 3,5-disubstituted-2-oxazolidines in up to 95 % yield. This metal-free and easily available catalytic system was applicable to a broad range of substrates, including conventionally challenging ones such as aliphatic epoxides, at atmospheric pressure. Preliminary mechanistic studies suggested a reaction pathway involving β-amino alcohols.

Metal-free catalytic materials have recently received broad attention as promising alternatives to metal-involved catalysts. This is owing to their inherent capability to overcome the inevitable limitations of metal-involved catalysts, such as high sensitivity to poisoning, the limited reserves, high cost and scarcity of metals (especially noble metals), etc. However, the lack of shape-controlled metal-free catalysts with well-defined facets is a formidable bottleneck limiting our understandings on the underlying structure-activity relationship at atomic/molecular level, which thereby restrains their rational design. Here, we report that catalytically active crystals of a porphyrin, 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin, could be shaped into well-defined cubes and sheet-like tetradecahedrons (TDHD), which are exclusively and predominantly enclosed by {101} and {001} facets, respectively. Fascinatingly, compared to the cubes, the TDHDs display substantially enhanced catalytic activity toward water decontamination under visible-light irradiation, although both the architectures have identical crystalline structure. We disclose that such interesting shape-sensitive catalytic activity is ascribed to the distinct spatial separation efficiency of photogenerated electrons and holes induced by single-channel and multichannel charge transport pathways along noncovalent supramolecular chains, which are arranged as parallel-aligned and 2D network patterns, respectively. Our findings provide an ideal scientific platform to guide the rational design of next-generation metal-free catalysts of desired catalytic performances.

Hydrogen peroxide (H2O2), as an environmentally friendly and highly efficient oxidation reagent, is now widely and increasingly used in many areas. The traditional anthraquinone synthesis process, however, is energy and capital intensive with low efficiency. Electrochemically reducing oxygen (O2) to H2O2 with metal‐free catalysts is considered a promising alternative. This work describes the two‐electron oxygen reduction process from both thermodynamic and kinetic aspects and summarizes the designing rules for effective H2O2 formation catalysts. This includes tuning the binding energy of the intermediate species, the concentration of dissociation active sites, and adjusting the dielectric constant of solvents. These principles are successfully applied to the design of various metal‐free catalysts by proper structure engineering and heteroatom doping, which shows both high activity and efficiency. Furthermore, pH and temperature effects are presented with a view toward reaction optimization. An outlook of future challenges is also discussed.

Graphenes and related materials have attracted growing interest as metal-free catalysts. The present review is focused on describing the active sites that have been proposed to be responsible for the catalytic activity observed for such systems. It will be shown that diverse defects and chemical functionalities on the graphene layers can catalyze reactions, including oxygenated functional groups, carbon vacancies and holes, edge effects, and the presence of dopant elements. Besides discrete active sites, the catalytic activity arising from the collective properties of graphenes as materials by adsorbing substrates and reagents and activating them by charge transfer is also commented. The review has an introductory general section summarizing the general methodologies that have been used to support the proposed structure of the active sites, including theoretical calculations, comparison of the catalytic activity of graphene samples with different compositions, the use of organic molecules as models of the active centers, and selective masking of functional groups. The review is concluded with our view on future developments in the field.

We report on the bottom-up synthesis of a fully sp2-hybridized nitrogenated three-dimensional microporous graphitic framework (3D-MGF) starting from a highly preorganized, saddle-shaped tetraphenylene derivative under ionothermal reaction conditions. The 3D-MGF showed high stability and a surface area of 928 m2 g−1 along with a narrow pore size distribution. Our approach enabled template-free inclusion of the third dimension into the graphitic frameworks while retaining π-conjugation and conductivity, which was verified by their activity as metal-free electrocatalysts for the hydrogen evolution reaction.

A series of ZIF-derived nitrogen-doped carbon materials were prepared using melamine as the additional nitrogen sources for acetylene hydrochlorination. The results indicated that the optimal complex of Z4M1 exhibits the highest acetylene conversion of 60% under the reaction conditions of 180 °C, C2H2 gas hourly space velocity (GHSV) = 50 h−1, VHCl/VC2H2 = 1.15, suggesting a promising metal-free catalyst for acetylene hydrochlorination. It is illustrated that the additions of melamine not only make the complex of Z4M1 displays special morphological structure and proper pore structure, but also adjust the relative content ratio of three nitrogen species in ZIF-derived nitrogen-doped carbon materials.

The building blocks of the well-defined and easily modified carbon-bond structure of graphene sheet units bring unique characteristics to carbon materials. Especially when these carbon materials are nanosized, their tunable electronic states, giant conjugated structures, oxygen groups on the defects of graphene sheets, and various dopants all help make the materials active in liquid-phase catalytic reactions. In this mini-review, we mainly focus on the liquid-phase reactions catalyzed by carbon nanomaterials, and the related mechanisms are discussed on the basis of their structure fundamentals. In the final section, we also provide our perspectives on this rapidly developing carbocatalysis field.

Developing metal−free catalysts for CO oxidation has been a key scientific issue in solving the growing environmental problems caused by CO emission. In this work, the potential of the silicon carbide (SiC) monolayer as a metal−free catalyst for CO oxidation was systematically explored by means of density functional theory (DFT) computations. Our results revealed that CO oxidation reaction can easily proceed on SiC nanosheet, and a three-step mechanism was proposed: (1) the coadsorption of CO and O2 molecules, followed by (2) the formation of the first CO2 molecule, and (3) the recovery of catalyst by a second CO molecule. The last step is the rate−determining one of the whole catalytic reaction with the highest barrier of 0.65 eV. Remarkably, larger curvature is found to have a negative effect on the catalytic performance of SiC nanosheet for CO oxidation. Therefore, our results suggested that flat SiC monolayer is a promising metal-free catalyst for CO oxidation.

Summary Recent advances in research and development of carbon-based metal-free catalysts (C-MFCs) have opened up new research areas for multifunctional electrocatalysis of the oxygen reduction reaction (ORR) in fuel cells, the ORR and oxygen evolution reaction (OER) in metal-air batteries, and the OER and hydrogen evolution reaction (HER) for overall water splitting. New fundamentals governing multifunctional metal-free carbon electrocatalysis were discovered, and recent breakthroughs in this emerging field have enabled C-MFCs to be used as metal-free multifunctional electrocatalysts for various energy-related reactions even beyond the ORR, OER, and HER. This article provides an overview of C-MFCs for multifunctional catalysis, covering their structure design and synthesis and multifunctional catalysis mechanism and potential applications in clean energy conversion and storage, chemical and energy conversion, and environmental protection. Current challenges and future opportunities in this emerging but rapidly developing field are also discussed.

Platinum group metal-free electrocatalysts are an important class of materials for the sustainable energy economy and significant effort has been made toward developing platinum group metal-free catalysts to replace their costly Pt counterparts in fuel cell electric vehicles. The progress made in activity over the last decade is reviewed, along with synthesis strategies for more controlled active site formation and performance gains achieved through electrode engineering. The increased focus on durability, the growing understanding of degradation mechanisms, and the need for standard performance and accelerated stress test protocols are discussed.