Metal-free Hydrogenation Catalysts Research Articles

Metal-free catalysts for hydrogenation.

Metal-free catalysts for hydrogenation.

N, B dual-doped carbons as metal-free catalysts for hydrogenation of quinoline with formic acid

Metal-free catalyst with good biocompatibility and sustainability is a fascinating alternative for metal-based catalyst, especially in the production of fine chemicals sensitive to metal impurities. Catalytic hydrogenation with formic acid (FA) is regarded as a sustainable and green strategy for organic hydrogenation, in which the development of metal-free catalyst is an arduous but significant challenge. In this work, a dual-doping strategy of N and B was chosen to build the desired base and acid sites in carbon materials for simultaneous activation of hydrogen donor FA and acceptor quinoline. In the additive-free system, N, B dual-doped metal-free catalyst BNC-900 showed outstanding catalytic performance for the hydrogenation of quinoline with FA, 91.64% conversion of quinoline and 93.62% selectivity for 1,2,3,4-tetrahydroquinoline were achieved. Moreover, it exhibited satisfying stability and tolerance for diverse substrates. According to the experimental data and DFT calculation results, pyridinic N and BC2O were revealed to be the primary active sites for FA and quinoline respectively and act synergistically. Besides, it was found that the formation of B-N bond derived from the neutralization of the electron donor and acceptor may hinder the construction of highly active sites.

In situ oxygen-induced zigzag graphene as metal-free catalyst for hydrogen activation in nitroarenes hydrogenation

Metal-free catalysts for hydrogenation reactions using molecular hydrogen (H2) are a major challenge in the field of catalysis. The Frustrated Lewis Pairs (FLPs) systems and heteroatom-doped carbon materials as metal-free catalysts are currently the mainstream research in hydrogenation reactions. Here, a series of oxygen-induced zigzag graphene (OZG) was constructed over thermal treatment of reduced graphene oxide (rGO) and applied as metal-free catalysts in nitrobenzene hydrogenation. The OZG-800 (thermal treatment at 800 °C) shows promising performance activity with 99.7% nitrobenzene conversion and 94.4% selectivity to aniline. The experimental results indicate that the zigzag edge and medium-strong acid sites formed by oxygen species may be the active centers in nitrobenzene hydrogenation. The density functional theory (DFT) calculations are in accord with experimental results that the zigzag edge and oxygen species play a critical role in H2 activation and dissociation. This work provides a promising route for the rational design of metal-free catalysts for hydrogenation.

Nitrogen-doped carbon nanotubes as a highly active metal-free catalyst for nitrobenzene hydrogenation

A series of metal-free N-doped carbon nanotubes (CNTs) are fabricated using an in-situ synthesis and post nitridation treatment. N-doped carbon nanotubes with various defects, nitrogen contents, and nitrogen species are employed as the catalysts for nitrobenzene hydrogenation. The potential active sites of N-doped carbon nanotubes are explored in terms of catalytic performance in nitrobenzene hydrogenation as well as DFT calculations of hydrogen adsorption on carbon nanotubes and nitrogen species. Active and selective hydrogenation of nitrobenzene is achieved over N-doped carbon nanotubes prepared by post nitridation. Pyrrolic-N in the N-doped carbon nanotubes is found to play an active site for nitrobenzene hydrogenation, which is greatly in favor of the chemisorption and dissociation of hydrogen molecules. N-doped carbon nanotubes with pyrrolic-N show an encouraging catalytic hydrogenation activity in nitrobenzene reduction. These findings provide a promising route for the rational design of the metal-free hydrogenation catalysts and the development of carbon nanotube-based materials.

Graphitic N-dominated nitrogen-doped carbon nanotubes as efficient metal-free catalysts for hydrogenation of nitroarenes

A series of metal-free N-doped carbon nanotubes (NCNTs) are synthesized using melanin or protoporphyrin IX as the dopant via a pyrolysis process. Multiwalled carbon nanotubes with various diameters are employed as the support, and their performance is compared with those of graphene and carbon black. The content of doped graphitic N is effectively controlled by varying the heating rate during pyrolysis. The obtained metal-free catalysts are employed to catalyze the hydrogenation of various nitroarenes under mild reaction conditions (2 MPa of H2, 120 °C). Efficient and selective hydrogenation of 14 nitroarenes is achieved over NCNTs with outer diameters of ≤20 nm synthesized via pyrolysis of melanin at 800 °C. Graphitic N in the as-prepared NCNTs is found to play a key role in efficient hydrogenation. Kinetic data reveal that a zero-order reaction occurs on the NCNTs, and the apparent activation energy of 4-nitrophenol hydrogenation is as low as 35.69 kJ mol−1. Recycling experiments indicate that the NCNTs have good stability and recyclability for hydrogenation of 4-nitrophenol.

A Heterogeneous Metal-Free Catalyst for Hydrogenation: Lewis Acid-Base Pairs Integrated into a Carbon Lattice.

Designing heterogeneous metal-free catalysts for hydrogenation is a long-standing challenge in catalysis. Nanodiamond-based carbon materials were prepared that are surface-doped with electron-rich nitrogen and electron-deficient boron. The two heteroatoms are directly bonded to each other to form unquenched Lewis pairs with infinite π-electron donation from the surrounding graphitic structure. Remarkably, these Lewis pairs can split H2 to form H+ /H- pairs, which subsequently serve as the active species for hydrogenation of different substrates. This unprecedented finding sheds light on the uptake of H2 across carbon-based materials and suggests that dual Lewis acidity-basicity on the carbon surface may be used to heterogeneously activate a variety of small molecules.

A strategy for developing metal-free hydrogenation catalysts: a DFT proof-of-principle study.

We computationally formulated a metal-free strategy to construct bifunctional active sites for reversible H2 activation, using a N-centered group as a Lewis basic site and a PV-centered group as a Lewis acidic site. By embedding the active sites into five-membered and 6-membered rings, we designed a series of molecules, some of which have feasible kinetic barriers to perform catalytic hydrogenation, calling for experimental realizations.

ChemInform Abstract: Frustrated Lewis Pairs

AbstractReview: 203 refs.

Frustrated Lewis Pairs.

The articulation of the notion of "frustrated Lewis pairs" (FLPs), which emerged from the discovery that H2 can be reversibly activated by combinations of sterically encumbered Lewis acids and bases, has prompted a great deal of recent activity. Perhaps the most remarkable consequence has been the development of FLP catalysts for the hydrogenation of a range of organic substrates. In the past 9 years, the substrate scope has evolved from bulky polar species to include a wide range of unsaturated organic molecules. In addition, effective stereoselective metal-free hydrogenation catalysts have begun to emerge. The mechanism of this activation of H2 has been explored, and the nature and range of Lewis acid/base combinations capable of effecting such activation have also expanded to include a variety of non-metal species. The reactivity of FLPs with a variety of other small molecules, including olefins, alkynes, and a range of element oxides, has also been developed. Although much of this latter chemistry has uncovered unique stoichiometric transformations, metal-free catalytic hydroamination, CO2 reduction chemistry, and applications in polymerization have also been achieved. The concept is also beginning to find applications in bioinorganic and materials chemistry as well as heterogeneous catalysis. This Perspective highlights many of these developments and discusses the relationship between FLPs and established chemistry. Some of the directions and developments that are likely to emerge from FLP chemistry in the future are also presented.

Are intramolecular frustrated Lewis pairs also intramolecular catalysts? A theoretical study on H2 activation.

We investigate computationally a series of intramolecular frustrated Lewis pairs (FLPs), with the general formula Mes2PCHRCH2B(C6F5)2, that are known from the literature to either activate molecular hydrogen (FLPs with R = H (1) or Me (4)), or remain inert (FLPs with R = Ph (2) or SiMe3 (3)). The prototypical system Mes2PCH2CH2B(C6F5)2 (1) has been described in the literature (Grimme et al., Angew. Chem., Int. Ed., 2010; Rokob et al., J. Am. Chem. Soc., 2013) as an intramolecular reactant that triggers the reaction with H2 in a bimolecular concerted fashion. In the current study, we show that the concept of intramolecular H2 activation by linked FLPs is not able to explain the inertness of the derivative compounds 2 and 3 towards H2. To cope with this, we propose an alternative intermolecular mechanism for the investigated reaction, assuming stacking of two open-chain FLP conformers, and formation of a dimeric reactant with two Lewis acid–base domains, that can split up to two hydrogen molecules. Using quantum-chemical methods, we compute the reaction profiles describing these alternative mechanisms, and compare the derived predictions with earlier reported experimental results. We show that only the concept of intermolecular H2 activation could explain both the activity of the FLPs having small substituents in the bridging molecular region, and the inertness of the FLPs with a bulkier substitution, in a consistent way. Importantly, the intermolecular H2 activation driven by intramolecular FLPs indicates the key role of steric factors and noncovalent interactions for the design of metal-free systems that can efficiently split H2, and possibly serve as metal-free hydrogenation catalysts.

On the Mechanism of Hydrogen Activation by Frustrated Lewis Pairs

We report herein a comprehensive theoretical study of the thermodynamics and kinetics of molecular hydrogen activation by frustrated Lewis pairs (FLPs). A series of intermolecularly combined boranes (Lewis acids) and phosphines (Lewis bases), with experimentally established different reactivities towards H2, have been subjected to DFT and (SCS-)MP2 calculations, and analyzed in terms of their structural properties, the energetics of association of the FLPs, and the kinetics of their interactions with H2 and hydrogenation to the ion-pair products. The analysis included the following steps: 1) assessment of the ability/inability of the Lewis species to preorganize into FLPs with an optimum arrangement of the acid and base sites for preconditioning the reaction with H2 , 2) comprehension of the different thermodynamics of hydrogenation of the selected FLPs by comparing the Gibbs energies of the overall reactions, and 3) estimation of the mechanism of the activation of H2 by identifying the reaction steps and the associated kinetic barriers. The results of our studies correlate well with experimental findings and have clarified the reasons for the observed different reactivities of the investigated systems, ranging from reversible or nonreversible activation to no reaction with H2. The derived predictions could assist the future design of Lewis acid-base systems with desired properties and applicability as metal-free hydrogenation catalysts.

Improving the Industrial Feasibility of Metal-Free Hydrogenation Catalysts Using Chemical Scavengers

A modified process using inexpensive poison scavengers has been developed that allows for more economical and practical scale-up of metal-free catalytic hydrogenation. The scavengers remove impurities such as water and aldehydes that can hinder catalysis allowing for the use of commercial-grade solvents, substrates and gases. In addition, the scavengers have the unique ability to regenerate poisoned catalysts, allowing for increased turnover numbers and longer catalyst lifetimes. Hydrogenations of unpurified imine substrates proceed with high yield using a variety of metal-free hydrogenation catalysts, demonstrating the general compatibility of this process.

Electronic effects of triarylphosphines in metal-free hydrogen activation: a kinetic and computational study

The frustrated Lewis pair-mediated reversible hydrogen activation is studied as a function of the electron-donor quality of a series of phosphines. The increasing acidity of the generated phosphonium species leads to a stepwise lowering of the temperature for the highly reversible H2-activation and permits concrete classification for the first time. The influence of the acid strength on the metal-free hydrogenation of selected olefins is investigated by kinetic experiments and quantum chemical calculations. Detailed information for the rate-determining steps fully support our mechanistic model of a protonation step prior to hydride transfer. The rate of hydrogenation is strongly dependent on the electronic nature of the phosphine and of the acidity of the corresponding phosphonium cation. A careful balance of these two factors provides highly efficient metal-free hydrogenation catalysts. The provided findings are used to revise the reactivity of Lewis bases in the hydrogenation of imines, one of the most recognized applications of FLPs.

“Frustrated Lewis pair” hydrogenations

This perspective article discusses developments of metal-free hydrogenation catalysts derived from "frustrated Lewis pair" (FLP) systems. The range of catalysts uncovered and the applications to reductions of imines, aziridines, enamines, silyl enol ethers, diimines, metallocene derivatives and nitrogen-based heterocycles are described. In addition, FLP aromatic reduction of aniline derivatives to the cyclohexylamine analogs is discussed. The potential applications of these metal-free reductions are considered.

Metal-free catalysts for hydrogenation of both small and large imines: a computational experiment

This study extends our previous work of using π-FLP strategy to develop metal-free hydrogenation catalysts. Using small MeN=CMe(2) imine (im1) as a model, we previously designed cat1 and cat2 catalysts. But it is unclear whether they are capable of catalyzing the hydrogenations of bulky imines. Using tBuN=C(H)Ph (im2) as a representative of large imines, we assessed the energetics of the cat1- and cat2-catalyzed im2 hydrogenations. The predicted energetics indicates that they can still catalyze large imine hydrogenations with experimentally accessible kinetic barriers, although the energetics becomes less favorable. To improve the catalysis, we proposed new catalysts (cat3 and cat4) by tailoring cat1 and cat2. The study indicates that cat3 and cat4 could have better performance for the hydrogenation of the bulky im2 than cat1 and cat2. Remarkably, cat3 and cat4 are also found suitable for small imine (im1) hydrogenation. Examining the hydrogen transfer substeps in the eight hydrogenations involved in this study, we observed that the mechanism for the hydrogen transfer step in the catalytic cycles depends on the steric effect between catalyst and substrate. The mechanism can be switched from stepwise one in the case of large steric effect (e.g.im2/cat2) to the concerted one in the case of small steric effect (e.g.im1/cat3). The new catalysts could be better targets for experimental realization because of their simpler constructions.

Cover Picture: Expanding the Scope of Metal-Free Catalytic Hydrogenation through Frustrated Lewis Pair Design (Angew. Chem. Int. Ed. 37/2010)

The evolution of deep and shallow flowers is a response to a “race” with pollinating organisms that determines the length and form of the bill of hummingbirds. Likewise, the selection of the Lewis basic component for a frustrated Lewis pair is dictated by the steric congestion around the boron center of the Lewis acid. In their Communication on page 6559 ff., T. Soós and co-workers show that this “selection phenomenon” can be exploited as a design concept for metal-free hydrogenation catalysts.

Fullerenes as Metal-Free Hydrogenation Catalysts

A highly efficient C60-fullerene-catalyzed hydrogenation of nitrobenzene is reported. The reaction pathway could be either driven by light, utilizing UV irradiation and atmospheric hydrogen, or by pressure, affording the aniline at elevated temperatures and 5 MPa hydrogen. In the latter case a mixture of C60 and its corresponding monoanion C60 - proved to be a more efficient catalyst. During the pressure-driven reaction process azo- and azoxybenzene could be observed as side products; however, the separated catalytic hydrogenations of those species were not successful compared to the original transformation, proposing a direct ­reaction route without intermediates rather than a sequential reduction mechanism.

Computational design of metal-free catalysts for catalytic hydrogenation of imines

Using MeN=CMe(2) as an imine model, computational chemistry has been applied to design metal-free hydrogenation catalysts. The implementation includes designing proper electronic structures to split H(2) and building appropriate chemical scaffolds to prevent possible side reactions which may deactivate the catalysts. Interestingly, the designed catalysts bear resemblances to the well-known metal-ligand bifunctional hydrogenation catalysts in terms of both the activation principle and the hydrogenation mechanisms. The hydrogenations catalyzed by the designed catalysts proceed via two major steps, hydrogen activation and hydrogen transfer. The predicted energetics for completing the catalytic cycles indicate that these reactions have feasible kinetics and thermodynamics for experimental realizations under ambient conditions. We also showed how to improve the catalysis by using the "cooperative effect" and the non-bonding interactions. The reported catalysts can be the targets for experimental synthesis. The strategy can be borrowed to design similar catalysts.

Metal‐Free Catalytic Hydrogenation

Metal‐Free Catalytic Hydrogenation