Cobalt Entrapped in N,S-Codoped Porous Carbon: Catalysts for Transfer Hydrogenation with Formic Acid

Recent progress of heterogeneous catalysts for transfer hydrogenation under the background of carbon neutrality

Under the background of carbon neutrality, the direct conversion of greenhouse CO2 to high value added fuels and chemicals is becoming an important and promising technology. Among them, the generation of liquid C1 products (formic acid and methanol) has made great progress; nevertheless, it encounters the problem of how to use it efficiently to solve the overcapacity issue. In this review, we suggest that the catalytic transfer hydrogenation using formic acid and methanol as the hydrogen sources is a critical and potential route for the substitution for the fossil fuel-derived H2 to generate essential bulk and fine chemicals. We mainly focus on summarizing the recent progress of heterogeneous catalysts in such reactions, including thermal- and photo-catalytic processes. Finally, we also propose some challenges and opportunities for this development.

One-Pot Conversion of Furfural to γ-Valerolactone over Co- and Pt-Doped ZSM-5 Catalysts

γ-Valerolactone (GVL) is one of the useful biomass compounds produced via different reaction pathways from hemicellulose. In this study, Co- and Pt-doped/ZSM-5 catalysts with different Co loadings (0–10 wt.%) and Pt loadings (0.5–2 wt.%) were prepared by impregnation method and employed in a one-pot conversion of furfural to GVL. The yield of GVL increased with increasing reaction temperature from 100 to 140 °C. At the reaction temperature of 120 °C, higher amounts of secondary products such as AL and IPL can be converted to GVL, especially on the Co- and Pt-modified ZSM-5 catalysts. Compared to the non-modified H-ZSM-5 (GVL yield 35.4%), Co- and Pt-doped ZSM-5 catalysts exhibited much higher yield of GVL with the 1%Pt/ZSM-5 catalyst showing the highest yield of GVL at 85.4% at 120 °C and 1 bar N2 without the use of liquid acid or external H2 supply. The catalyst performances were correlated to the physicochemical properties of the catalysts such as the amount and type of acid sites. The NH3-TPD and in situ FTIR spectra of pyridine adsorption results revealed that Co- and Pt-loaded on ZSM-5 enhanced Lewis and weak acid sites, which are beneficial for the reaction. The results present a simple strategy to obtain high GVL yield under relatively mild conditions.

Polyaniline coated Pt/CNT as highly stable and active catalyst for catalytic hydrogenation reduction of Cr(VI)

Liquid phase catalytic hydrogenation reduction is a feasible method to eliminate Cr(VI) in water, while supported noble metal catalysts are liable to deactivation. In this study, carbon nanotube supported Pt catalyst (Pt/CNT) coated by polyaniline (Pt/CNT@PANI) was prepared and applied in the liquid phase catalytic hydrogenation of Cr(VI). Characterization results disclose that after coating Pt/CNT is completely wrapped by PANI layers and active Pt particles are no longer accessible. Despite complete embedment of Pt particles by PANI layers, Pt/CNT@PANI remains highly active for Cr(VI) reduction in liquid phase catalytic hydrogenation. The catalytic Cr(VI) reduction on Pt/CNT@PANI can be described by a PANI oxidation-reduction mechanism, by which PANI is first oxidized by Cr(VI) to form Cr(III), and oxidized PANI is reduced by catalytic hydrogenation. The Cr(VI) reduction on Pt/CNT@PANI complies with the Langmuir-Hinshelwood model, reflecting the pivotal role of Cr(VI) adsorption. Furthermore, the catalytic activity of Pt/CNT@PANI differs with PANI layer thickness and Cr(VI) reduction is positively correlated with reaction temperature. Catalyst recycling results show that after 4 cycles Pt/CNT loses 92.4% of catalytic activity, while the initial activity of Pt/CNT@PANI slightly decreases by 11.6%, demonstrating its high catalyst stability.

Nitrogen and Phosphorus Dual-Coordinated Single-Atom Mn: MnN2P Active Sites for Catalytic Transfer Hydrogenation of Nitroarenes

The coordination environment of atomically metal sites can modulate the electronic states and geometric structure of single-atom catalysts, which determine their catalytic performance. In this work, the porous carbon-supported N, P dual-coordinated Mn single-atom catalyst was successfully prepared via the phosphatization of zeolitic imidazolate frameworks and followed by pyrolysis at 900 °C. The optimal Mn₁-N/P-C catalyst with atomic MnN₂P structure has displayed better catalytic activity than the related catalyst with Mn-N_x structure in catalytic transfer hydrogenation of nitroarenes using formic acid as the hydrogen donor. We find that the doping of P source plays a crucial role in improving the catalytic performance, which affects the morphology and electronic properties of catalyst. This is the first Mn heterogeneous catalyst example for the reduction of nitroarenes, and it also revealed that the MnN₂P configuration is a more promising alternative in heterogeneous catalysis.

Superhydrophobic Ru Catalyst for Highly Efficient Hydrogenation of Phenol under Mild Aqueous Conditions

Selective hydrogenations of lignin-derived phenolic compounds represent essential processes in the chemical industry, especially for production of a multitude of fine chemicals. However, selective hydrogenation of phenolic compounds in water phase suffers from low conversion. Here we report a catalyst of well-dispersed Ru clusters fixed in N-doped mesoporous hollow carbon spheres (Ru@N-CS) for enhanced cyclohexanol productivity in phenol hydrogenation at mild aqueous condition. This superhydrophobicity carbon spheres appear to selectively allow diffusion of phenol and hydrogen molecules to the electron-rich coordination unsaturated Ru active sites, while confining the reactants there to enhance its reaction probability. The Ru@N-CS catalyst can selectively hydrogenate phenol at 80 °C and 0.5 MPa of H2 in 30 min in aqueous medium with phenol conversions of 100% and ~100% cyclohexanol selectivity, corresponding to cyclohexanol productivity up to 471 per g of Ru per minute. The TOF value is up to 9980 h−1, which 14 times more than Ru nanoparticles supported on N-doped carbon hollow spheres (Ru/N-CS). This work provides an important catalytic system for upgrading of bio-oil into value-added chemicals under mild aqueous-phase.

Water-soluble and reusable Ru-NHC catalyst for aqueous-phase transfer hydrogenation of quinolines with formic acid

Water-soluble Ru-NHC complexes were synthesized and their catalytic activity was tested in the transfer hydrogenation of quinoline-type N-heteroarenes using a formic acid/sodium formate buffer solution. The unique multifunctional features of the designed ligand within the catalyst backbone endowed it with excellent durability, reusability and compatibility with a simple aqueous-phase operation. Thus, it was possible to reuse as little as 0.25 mol% of the catalyst for three consecutive catalytic runs to provide an overall turnover number of around 900. A mechanistic investigation suggested that hydride generation was the rate-limiting step, whereas hydride transfer was relatively facile. Furthermore, computational studies supported that the reaction pathway was dominated by 1,4-hydride insertion at the N-heteroarene substrates.

Recent Advances on Heteroatom-Doped Porous Carbon/Metal Materials: Fascinating Heterogeneous Catalysts for Organic Transformations

Design and preparation of low-cost, effective, and novel catalysts are important topics in the field of heterogeneous catalysis from academic and industrial perspectives. Recently, heteroatom-doped porous carbon/metal materials have received significant attention as promising catalysts in divergent organic reactions. Incorporation of heteroatom into the carbon framework can tailor the properties of carbon, providing suitable interaction between support and metal, resulting in superior catalytic performance compared with those of traditional pure carbon/metal catalytic systems. In this review, we try to underscore the recent advances in the design, preparation, and application of heteroatom-doped porous carbon/metal catalysts towards various organic transformations.

Recent developments of nanocatalyzed liquid-phase hydrogen generation

Hydrogen is the most effective and sustainable carrier of clean energy, and liquid-phase hydrogen storage materials with high hydrogen content, reversibility and good dehydrogenation kinetics are promising in view of "hydrogen economy". Efficient, low-cost, safe and selective hydrogen generation from chemical storage materials remains challenging, however. In this Review article, an overview of the recent achievements is provided, addressing the topic of nanocatalysis of hydrogen production from liquid-phase hydrogen storage materials including metal-boron hydrides, borane-nitrogen compounds, and liquid organic hydrides. The state-of-the-art catalysts range from high-performance nanocatalysts based on noble and non-noble metal nanoparticles (NPs) to emerging single-atom catalysts. Key aspects that are discussed include insights into the dehydrogenation mechanisms, regenerations from the spent liquid chemical hydrides, and tandem reactions using the in situ generated hydrogen. Finally, challenges, perspectives, and research directions for this area are envisaged.

Formic acid enabled selectivity boosting in transfer hydrogenation of 5-hydroxymethylfurfural to 2,5-furandimethanol on highly dispersed Co–Nx sites

Catalytic transfer hydrogenation (CTH) reaction is considered as a potential route for upgrading bio-based carbonyl compounds to their corresponding alcohols.

Polyvinylpyrrolidone-Stabilized Iridium Nanoparticles Catalyzed the Transfer Hydrogenation of Nitrobenzene Using Formic Acid as the Source of Hydrogen

Catalytic nitrobenzene reduction is crucial for the synthesis of 4,4-methylene diphenyl diisocyanate, which is used to produce polyurethane foams, thermoplastic elastomers, and adhesives. The stability and activity of nanoparticle catalysts are affected by surface ligands and stabilizers. We established the complete composition of 7.0 ± 1.1 nm iridium oxide nanoparticles that were stabilized by polyvinylpyrrolidone (PVP[Ir]). PVP[Ir] and its surface stabilizers were characterized using elemental analysis (EA), high-resolution X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (PXRD), FT-IR, and UV-vis spectroscopy. Notably, PVP[Ir] contained 33.8 ± 0.4% Ir. XPS binding energy analyses suggest that 7% of the Ir is Ir(0) and 93% is IrO2. Using formic acid as the source of hydrogen, PVP[Ir] catalyzed the selective hydrogenation of nitrobenzene to give aniline as the only product in 66% yield in 1 h at 160 °C in a high-pressure metal reactor. Less than 1% of the side products (azobenzene and azoxybenzene) were detected. In contrast, using alcohol as the hydrogen source led to a low yield and a poor selectivity for aniline.

Effect of triblock copolymers on crystal growth and the photocatalytic activity of anatase TiO2 single crystals

In order to evaluate the effect of a triblock copolymer on the growth of TiO₂ crystals, anatase TiO₂ crystals with different morphologies and structures were synthesized by controlling the content and type of triblock copolymer in the solvothermal route. The resulting samples were characterized by XRD, XPS, SEM, TEM and EDX. The characterization results show that hydrofluoric acid can promote the formation of highly active (001) facets by the formation of a Ti–F bond. The triblock copolymers (P123 and F127) refine the surface structure of polycrystalline spherical TiO₂ and make the crystal surface homogeneous and smooth. Moreover, P123 causes the agglomeration effect and hinders the recrystallization process of anatase TiO₂ single crystals, and this will lead to corrosion of the crystal facets. Meanwhile, F127 destroys crystal formation and hinders crystal growth due to its special micelle structure. In addition, research on the photocatalytic activity proposed that the integrity of the (001) and (101) facets was a critical factor in the photocatalytic reaction. The resultant anatase TiO₂ single crystals could produce more hydroxyl radicals (˙OH) in the photocatalytic system, which exhibited remarkable photocatalytic performance for the degradation of three types of dye.

The effects of triblock copolymers (P123 and F127) on the growth of TiO₂ crystals were studied. Anatase TiO₂ crystals with different morphologies and structures were synthesized by controlling the content and type of triblock copolymer in a solvothermal method.

Highly dispersed nickel anchored on a N-doped carbon molecular sieve derived from metal-organic frameworks for efficient hydrodeoxygenation in the aqueous phase

ZIF-8 was employed as a template to synthesize HD-Ni/N-CMS containing highly dispersed Ni at the atomic level anchored on a N-doped carbon molecular sieve for vanillin hydrodeoxygenation. The ZIF-8 structure was inherited and Ni-N bonds were formed by the coordination of Ni with N-rich defects, therefore it exhibited a high turnover frequency (1047.1 h-1) and good stability.

Cobalt-Polypyrrole/Melamine-Derived Co-N@NC Catalysts for Efficient Base-Free Formic Acid Dehydrogenation and Formylation of Quinolines through Transfer Hydrogenation

It is highly desired but remains a great challenge to develop non-noble metal heterogeneous catalysts to supersede noble metal catalysts for formic acid (FA) dehydrogenation and the corresponding transfer hydrogenation reactions. Herein, we developed a simple and feasible melamine-assisted pyrolysis strategy for the preparation of atomic cobalt-nitrogen (Co-N)-anchored mesoporous carbon with high metal loading (>6.8 wt %) and high specific surface area (750 m² g^-1). Systematic investigation reveals that both the organic carbon source polypyrrole and the nitrogen source melamine are crucial for the successful generation of such Co-N-based materials. The obtained samples (Co-N)_n@NC were demonstrated to be highly efficient and robust catalysts for FA dehydrogenation and formylation of quinolines through transfer hydrogenation, exhibiting a very high hydrogen production rate of 16 451 mL·g_Co^-1·h^-1 for FA dehydrogenation and affording excellent yields (up to 99%), selectivity (up to 98%), and stability for transfer hydrogenation. This work may provide a promising route for the fabrication of more low-cost metal-nitrogen catalysts for green fine chemical synthesis.

Biomass-derived carbon-supported Ni catalyst: an effective heterogeneous non-noble metal catalyst for the hydrogenation of nitro compounds

The biomass-derived carbon material supported Ni catalysts (Ni/C) demonstrated a high catalytic activity for the hydrogenation of nitro compounds into primary amines at room temperature.

Biomass-derived Fe-NC hybrid for hydrogenation with formic acid: control of Fe-based nanoparticle distribution

A series of Fe-NC catalysts were synthesized by pyrolyzing an Fe complex and wheat flour at 500 °C. All of them were characterized and applied in the catalytic transfer hydrogenation of nitroarenes with formic acid. It was found that the catalytic activity was significantly affected by the size and distribution of Fe-based nanoparticles (NPs), which could be easily regulated by altering the Fe source. Meanwhile, more basic nitrogen sites were preserved on the catalyst so that the reaction ran smoothly without base additives. Among all catalysts, Fe-NC-FeCl₂ exhibited the best catalytic performance due to smaller Fe₃O₄ NPs and greater N doping. Moreover, it showed excellent applicability for diverse nitroarenes. Obviously, this work demonstrates the importance of the metallic NPs' size and distribution, providing a new insight into the design of M-NC catalysts. The catalyst is economical and eco-friendly, and shows potential application value in industry.

A series of iron-based, N-doped carbon catalysts exhibits excellent activity in catalytic transfer hydrogenation of nitroarenes without base additive.

Polymer-Anchored Bifunctional Pincer Catalysts for Chemoselective Transfer Hydrogenation and Related Reactions

A series of polymer-supported cooperative PC(sp³ )P pincer catalysts was synthesized and characterized. Their catalytic activity in the acceptorless dehydrogenative coupling of alcohols and the transfer hydrogenation of aldehydes with formic acid as a hydrogen source was investigated. This comparative study, examining homogeneous and polymer-tethered species, proved that carefully designing a link between the support and the catalytic moiety, which takes into consideration the mechanism underlying the target transformation, might lead to superior heterogeneous catalysis.

Controllable Sulfur, Nitrogen Co-doped Porous Carbon for Ethylbenzene Oxidation: The Role of Nano-CaCO3

Heteroatom-doped porous carbon materials have exhibited promising applications in various fields. In this work, sulfur, nitrogen co-doped carbon materials (SNCs) with abundant pore structure were prepared by pyrolysis of sulfur, nitrogen-containing porous organic polymers (POPs) mixed with nano-CaCO₃ at high temperature. Among the resultant materials, SNC-Ca-850 possesses a relatively high level of doped heteroatoms and exhibits an excellent catalytic performance for the selective oxidation of benzylic C-H bonds. It is noteworthy that nano-CaCO₃ increases the doped sulfur content in the synthesized carbon materials to a large extent and impacts the existence modes of sulfur. In addition, it enhances the porous structure and specific surface area of the resultant SNCs significantly. This work provides a viable strategy to promote the doping of sulfur into carbon materials during the pyrolysis process.

MOF-derived Ni@NC catalyst: synthesis, characterization, and application in one-pot hydrogenation and reductive amination

MOF-derived Ni@NC is prepared and used as highly selective catalyst for one-pot hydrogenation and reductive amination.