Hydrodenitrogenation Catalysis

AuNP/MIL-88B-NH2 Nanocomposite for the Valorization of Nitroarene by Green Catalytic Hydrogenation

The efficiency of a catalytic process is assessed based on conversion, yield, and time effectiveness. However, these parameters are insufficient for evaluating environmentally sustainable research. As the world is urged to shift towards green catalysis, additional factors such as reaction media, raw material availability, sustainability, waste minimization and catalyst biosafety, need to be considered to accurately determine the efficacy and sustainability of the process. By combining the high porosity and versatility of metal organic frameworks (MOFs) and the activity of gold nanoparticles (AuNPs), efficient, cyclable and biosafe composite catalysts can be achieved. Thus, a composite based on AuNPs and the nanometric flexible porous iron(III) aminoterephthalate MIL-88B-NH2 was successfully synthesized and fully characterized. This nanocomposite was tested as catalyst in the reduction of nitroarenes, which were identified as anthropogenic water pollutants, reaching cyclable high conversion rates at short times for different nitroarenes. Both synthesis and catalytic reactions were performed using green conditions, and even further tested in a time-optimizing one-pot synthesis and catalysis experiment. The sustainability and environmental impact of the catalytic conditions were assessed by green metrics. Thus, this study provides an easily implementable synthesis, and efficient catalysis, while minimizing the environmental and health impact of the process.

Elucidation of the Pyridine Ring-Opening Mechanism of 2,2'-Bipyridine or 1,10-Phenanthroline Ligands at Re(I) Carbonyl Complexes

The cleavage of the C-N bonds of aromatic heterocycles, such as pyridines or quinolines, is a crucial step in the hydrodenitrogenation (HDN) industrial processes of fuels in order to minimize the emission of nitrogen oxides into the atmosphere. Due to the harsh conditions under which these reactions take place (high temperature and H2 pressure), the mechanism by which they occur is only partially understood, and any study at the molecular level that reveals new mechanistic possibilities in this area is of great interest. Herein, we unravel the pyridine ring-opening mechanism of 2,2'-bipyridine (bipy) and 1,10-phenanthroline (phen) ligands coordinated to the cis-{Re(CO)2(N-RIm)(PMe3)} (N-RIm= N-alkylimidazole) fragment under mild conditions. Computational calculations show that deprotonation of the pyridine ring, once dearomatized, is crucial to induce ring contraction, triggering extrusion of the nitrogen atom from the ring and cleavage of the C-N bond. It is noteworthy that different products (regioisomers) are obtained depending on whether the ligand used is bipy or phen due to the additional rigidity and stability conferred by the central ring of the phen ligand, an issue also addressed and clarified computationally. Strong support for the proposed mechanism is provided by the characterization and isolation, including three single-crystal X-ray diffraction structures, of several of the proposed reaction intermediates.

Low temperature catalytic hydrodeoxygenation of lignin-derived phenols to cyclohexanols over the Ru/SBA-15 catalyst

Cyclohexanol and its derivatives are widely used as chemical intermediates and fuel additives. Herein, Ru/SBA-15 catalysts were prepared via impregnation, and used for the production of cyclohexanols from lignin-derived phenols. The catalyst samples were characterized by XRD, XPS, TEM, etc., where the Ru⁰ species was speculated as the active phase. 5 wt% Ru/SBA-15 with small Ru particle size (4.99 nm) and high Ru dispersion (27.05%) exhibited an excellent hydrogenation activity. A high cyclohexanol yield of >99.9% was achieved at 20 °C for 5 h in an aqueous phase, and the catalyst indicated stable activity and selectivity after five runs. Crucially, Ru/SBA-15 exhibited a zero-order reaction rate with an apparent activation energy (Ea) as low as 10.88 kJ mol⁻¹ and a TON of 172.84 at 80 °C. Simultaneously, demethoxylation activity was also observed in the hydrodeoxygenation (HDO) of G- and S-type monophenols, and a high yield of 37.4% of cyclohexanol was obtained at 80 °C and 4 h when using eugenol as substrate.

Ru/SBA-15 showed excellent activity for phenol to cyclohexanol at 20 °C and exhibited a zero-order character with low Ea of 10.88 kJ mol⁻¹. A high yield of 37.4% of cyclohexanol was obtained at 80 °C and 4 h when using eugenol as substrate.

Hydrogenation of Pyridine and Hydrogenolysis of Piperidine overγ-Mo2N Catalyst: A DFT study

Increasing demands on producing environmentally friendly products are becoming a driving force for designing high active catalysts. Thus, surfaces that efficiently catalyse the nitrogen reduction reactions are vastly sought in moderating air-pollutant emissions. This contribution aims to computationally investigate the hydrodenitrogenation (HDN) networks of pyridine over γ-Mo2N(111) surface via density functional theory (DFT) approach. Various adsorption configurations have been considered for the molecularly adsorbed pyridine. Findings indicate that pyridine can be adsorbed via side-on and end-on modes in six geometries in which one adsorption site is revealed to have the lowest adsorption energy of (-45.3 kcal/mol(. Over nitrogen hollow site adsorption site, initial HDN steps proceed by the stepwise hydrogenation of pyridine into piperidine followed the Langmuir−Hinshelwood mechanism. The obtained findings are the first to theoretically model the hydrogenation pathways of pyridine to form piperidine then the hydrogenolysis of piperidine producing C5H12 and NH3 over metal nitride and paved the way for further investigations to better understanding such an important nitrogen removal reactions.

Syntheses, Structures, Reactivities, and Basicities of Quinolinyl and Isoquinolinyl Complexes of an Electron Rich Chiral Rhenium Fragment and Their Electrophilic Addition Products

Reactions of Li⁺ [(η⁵ -C₅ H₅ )Re(NO)(PPh₃ )]^- with 2- and 4-chloroquinoline or 1-chloroisoquinoline give the corresponding σ quinolinyl and isoquinolinyl complexes 3, 6, and 8. With 3 and 8 there is further protonation to yield HCl adducts, but additions of KH give the free bases. Treatment of 3 with HBF₄ ⋅OEt₂ or H(OEt₂ )₂ ⁺ BAr_f ^- gives the quinolinium salts [(η⁵ -C₅ H₅ )Re(NO)(PPh₃ )(C(NH)C(CH)₄ C(CH)(CH))]⁺ X^- (3-H⁺ X^- ; X^- =BF₄ ^- /BAr_f ^- , 94-98 %). Addition of CF₃ SO₃ CH₃ to 3, 6, or 8 affords the corresponding N-methyl quinolinium salts. In the case of [(η⁵ -C₅ H₅ )Re(NO)(PPh₃ )(C(NCH₃ )C(CH)₄ C(CH)(CH))]⁺ CF₃ SO₃ ^- (3-CH₃ ⁺ CF₃ SO₃ ^- ), addition of CH₃ Li gives the dihydroquinolinium complex (S_Re R_C ,R_Re S_C )-[(η⁵ -C₅ H₅ )Re(NO)(PPh₃ )(C(NCH₃ )C(CH)₄ C(CHCH₃ )(CH₂ ))]⁺ CF₃ SO₃ ^- ((S_Re R_C ,R_Re S_C )-5⁺ CF₃ SO₃ ^- , 76 %) in diastereomerically pure form. Crystal structures of 3-H⁺ BAr_f ^- , 3-CH₃ ⁺ CF₃ SO₃ ^- , (S_Re R_C , R_Re S_C )-5⁺ Cl^- , and 6-CH₃ ⁺ CF₃ SO₃ ^- show that the quinolinium ligands adopt Re⋅⋅⋅C conformations that maximize overlap of their acceptor orbitals with the rhenium fragment HOMO, minimize steric interactions with the bulky PPh₃ ligand, and promote various π interactions. NMR experiments establish the Brønsted basicity order 3>8>6, with K_a (BH⁺ ) values >10 orders of magnitude greater than the parent heterocycles, although they remain less active nucleophilic catalysts in the reactions tested. DFT calculations provide additional insights regarding Re⋅⋅⋅C bonding and conformations, basicities, and the stereochemistry of CH₃ Li addition.

Insights into the Interfacial Effects in Heterogeneous Metal Nanocatalysts toward Selective Hydrogenation

Heterogeneous metal catalysts are distinguished by their structure inhomogeneity and complexity. The chameleonic nature of heterogeneous metal catalysts have prevented us from deeply understanding their catalytic mechanisms at the molecular level and thus developing industrial catalysts with perfect catalytic selectivity toward desired products. This Perspective aims to summarize recent research advances in deciphering complicated interfacial effects in heterogeneous hydrogenation metal nanocatalysts toward the design of practical heterogeneous catalysts with clear catalytic mechanism and thus nearly perfect selectivity. The molecular insights on how the three key components (i.e., catalytic metal, support, and ligand modifier) of a heterogeneous metal nanocatalyst induce effective interfaces determining the hydrogenation activity and selectivity are provided. The interfaces influence not only the H₂ activation pathway but also the interaction of substrates to be hydrogenated with catalytic metal surface and thus the hydrogen transfer process. As for alloy nanocatalysts, together with the electronic and geometric ensemble effects, spillover hydrogenation occurring on catalytically "inert" metal by utilizing hydrogen atom spillover from active metal is highlighted. The metal-support interface effects are then discussed with emphasis on the molecular involvement of ligands located at the metal-support interface as well as cationic species from the support in hydrogenation. The mechanisms of how organic modifiers, with the ability to induce both 3D steric and electronic effects, on metal nanocatalysts manipulate the hydrogenation pathways are demonstrated. A brief summary is finally provided together with a perspective on the development of enzyme-like heterogeneous hydrogenation metal catalysts.

Remarkable adsorptive removal of nitrogen-containing compounds from hydrotreated fuel by molecularly imprinted poly-2-(1H-imidazol-2-yl)-4-phenol nanofibers

Molecularly imprinted polymer (MIP) nanofibers were prepared by the electrospinning of poly 2-(1H-imidazol-2-yl)-4-phenol (PIMH) in the presence of various nitrogen containing compounds (N-compounds). Molecularly imprinted polymer nanofibers show selectivity for various target model nitrogen-containing compounds with adsorption capacities of 11.7 ± 0.9 mg g^-1, 11.9 ± 0.8 mg g^-1 and 11.3 ± 1.1 mg g^-1 for quinoline, pyrimidine and carbazole, respectively. Molecular modelling based upon density functional theory (DFT) indicated that hydrogen bond interactions may take place between the lone-pair nitrogen atom of model compounds (quinoline and pyrimidine) and the -OH and -NH groups of the PIMH nanofibers. The adsorption mode followed the Freundlich (multi-layered) adsorption isotherm, which indicated possible nitrogen-nitrogen compound interactions. Molecularly imprinted polymer nanofibers show potential for the removal of nitrogen-containing compounds in fuel.

Hydroprocessing of low-temperature coal tar to produce jet fuel

Jet fuel was prepared from low-temperature coal tar (LTCT) via two-stage fixed beds that were filled with two commercial catalysts.

Effects of calcination and reduction temperature on the properties of Ni-P/SiO2 and Ni-P/Al2O3 and their hydrodenitrogenation performance

A series of SiO₂-supported and γ-Al₂O₃-supported nickel phosphides were prepared by temperature-programmed reduction (TPR) with different calcination and reduction temperatures. The prepared catalysts were characterized by XRD, BET, H₂-TPR, CO titration and HRTEM. The crystal phase and CO uptake content were influenced by calcination and reduction temperature. The catalytic performance of various catalysts was tested in quinoline hydrodenitrogenation and exhibited considerable differences. The quinoline HDN activity of SiO₂-supported nickel phosphides decreases with increase of calcination and reduction temperature. In contrast to SiO₂-supported samples, the ability to remove nitrogen of γ-Al₂O₃-supported samples increases with reduction temperature.

XRD patterns of different SiO₂-supported nickel phosphides reduced at (a) 560 °C, (b) 650 °C, (c) 750 °C and different γ-Al₂O₃-supported nickel phosphides reduced at (d) 560 °C, (e) 650 °C, (f) 750 °C.

Hydrodenitrogenation of pyridines and quinolines at a multinuclear titanium hydride framework

Investigation of the hydrodenitrogenation (HDN) of aromatic N-heterocycles such as pyridines and quinolines at the molecular level is of fundamental interest and practical importance, as this transformation is essential in the industrial petroleum refining on solid catalysts. Here, we report the HDN of pyridines and quinolines by a molecular trinuclear titanium polyhydride complex. Experimental and computational studies reveal that the denitrogenation of a pyridine or quinoline ring is easier than the ring-opening reaction at the trinuclear titanium hydride framework, which is in sharp contrast with what has been reported previously. Hydrolysis of the pyridine-derived nitrogen-free hydrocarbon skeleton at the titanium framework with H2O leads to recyclization to afford cyclopentadiene with the generation of ammonia, while treatment with HCl gives the corresponding linear hydrocarbon products and ammonium chloride. This work has provides insights into the mechanistic aspects of the hydrodenitrogenation of an aromatic N-heterocycle at the molecular level.

Room-Temperature Ring-Opening of Quinoline, Isoquinoline, and Pyridine with Low-Valent Titanium

The complex (PNP)Ti═CH^tBu(CH₂^tBu) (PNP = N[2-PⁱPr₂-4-methylphenyl]₂^-) dehydrogenates cyclohexane to cyclohexene by forming a transient low-valent titanium-alkyl species, [(PNP)Ti(CH₂^tBu)], which reacts with 2 equiv of quinoline (Q) at room temperature to form H₃C^tBu and a Ti(IV) species where the less hindered C₂═N₁ bond of Q is ruptured and coupled to another equivalent of Q. The product isolated from this reaction is an imide with a tethered cycloamide group, (PNP)Ti═N[C₁₈H₁₃N] (1). Under photolytic conditions, intramolecular C-H bond activation across the imide moiety in 1 occurs to form 2, and thermolysis reverses this process. The reaction of 2 equiv of isoquinoline (Iq) with intermediate [(PNP)Ti(CH₂^tBu)] results in regioselective cleavage of the C₁═N₂ and C₁-H bonds, which eventually couple to form complex 3, a constitutional isomer of 1. Akin to 1, the transient [(PNP)Ti(CH₂^tBu)] complex can ring-open and couple two pyridine molecules, to produce a close analogue of 1, complex (PNP)Ti═N[C₁₀H₉N] (4). Multinuclear and multidimensional NMR spectra confirm structures for complexes 1-4, whereas solid-state structural analysis reveals the structures of 2, 3, and 4. DFT calculations suggest an unprecedented mechanism for ring-opening of Q where the reactive intermediate in the low-spin manifold crosses over to the high-spin surface to access a low-energy transition state but returns to the low-spin surface immediately. This double spin-crossover constitutes a rare example of a two-state reactivity, which is key for enabling the reaction at room temperature. The regioselective behavior of Iq ring-opening is found to be due to electronic effects, where the aromatic resonance of the bicycle is maintained during the key C-C coupling event.

A study on the origin of the active sites of HDN catalysts using alumina-supported MoS3 nanoparticles as a precursor

The origin of the active sites of the hydrodenitrogenation catalysts was comprehensively studied using alumina-supported MoS₃ nanoparticles (NPs) as a novel precursor.