Catalytic Role of Metal Nanoparticles in Selectivity Control over Photodehydrogenative Coupling of Primary Amines to Imines and Secondary Amines

Potenital pathways for the deactivation of hindered amine light stabilisers (HALS) have been investigated by observing reactions of model compounds--based on 4-substituted derivatives of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)--with hydroxyl radicals. In these reactions, dilute aqueous suspensions of photocatalytic nanoparticulate titanium dioxide were irradiated with UV light in the presence of water-soluble TEMPO derivatives. Electron spin resonance (ESR) and electrospray ionisation mass-spectrometry (ESI-MS) data were acquired to provide complementary structural elucidation of the odd- and even-electron products of these reactions and both techniques show evidence for the formation of 4-oxo-TEMPO (TEMPONE). TEMPONE formation from the 4-substituted TEMPO compounds is proposed to be initiated by hydrogen abstraction at the 4-position by hydroxyl radical. High-level ab initio calculations reveal a thermodynamic preference for abstraction of this hydrogen but computed activation barriers indicate that, although viable, it is less favoured than hydrogen abstraction from elsewhere on the TEMPO scaffold. If a radical is formed at the 4-position however, calculations elucidate two reaction pathways leading to TEMPONE following combination with either a second hydroxyl radical or dioxygen. An alternate mechanism for conversion of TEMPOL to TEMPONE via an alkoxyl radical intermediate is also considered and found to be competitive with the other pathways. ESI-MS analysis also shows an increased abundance of analogous 4-substituted piperidines during the course of irradiation, suggesting competitive modification at the 1-position to produce a secondary amine. This modification is confirmed by characteristic fragmentation patterns of the ionised piperidines obtained by tandem mass spectrometry. The conclusions describe how reaction at the 4-position could be responsible for the gradual depletion of HALS in pigmented surface coatings and secondly, that modification at nitrogen to form the corresponding secondary amine species may play a greater role in the stabilisation mechanisms of HALS than previously considered.

Oxidation of 4-substituted TEMPO derivatives reveals modifications at the 1- and 4-positions

Paired electroreduction and electrooxidation of organics with water as a feedstock to produce value-added chemicals is meaningful. A comprehensive understanding of reaction mechanism is critical for the catalyst design and relative area development. Here, we have systematically studied the mechanism of the paired electroreduction and electrooxidation of organics on Fe-Mo-based phosphide heterojunctions. It is shown that active H* species for organic electroreduction originate from water. As for organic electrooxidation, among various oxygen species (OH*, OOH*, and O*), OH* free radicals derived from the first step of water dissociation are identified as active species. Furthermore, explicit reaction pathways and their paired advantages are proposed based on theoretical calculations. The paired electrolyzer powered by a solar cell shows a low voltage of 1.594 V at 100 mA cm-2, faradaic efficiency of ≥99%, and remarkable cycle stability. This work provides a guide for sustainable synthesis of various value-added chemicals via paired electrocatalysis. 

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Unraveling the mechanism for paired electrocatalysis of organics with water as a feedstock

Light-driven thermocatalysis has emerged as an attractive approach for catalytic oxidation of volatile organic compounds (VOCs). Herein, we have synthesized Pt-mTiO2/USY nanocomposites for the catalytic degradation of toluene under full solar spectrum. The mTiO2/USY with increasing Pt content show increased activity, among which 0.9Pt-mTiO2 exhibits toluene conversion of 86.6% and CO2 yield of 74.5% at 243 °C with a light intensity of 490 mW/cm2 under 5 vol.% water vapor. The high catalytic performance of Pt-mTiO2/USY can be attributed to the large surface area, strong light absorption, highly efficient conversion of light-to-heat, abundant oxygen vacancies, uniform Pt distribution, and Ti3+ species on the catalyst surface. The introduction of Pt nanoparticles (NPs) in mTiO2/USY enhances light absorption and improves the mobility of surface lattice oxygen. The combination of EPR, O2-TPD, and in situ DRIFTS analysis reveals that light irradiation further stimulates more active lattice oxygen to participate in the toluene oxidation. 

Construction of Pt-mTiO2/USY multifunctional catalyst enriched with oxygen vacancies for the enhanced light-driven photothermocatalytic degradation of toluene

Simultaneous photocatalytic H2 production and preparation of high-value chemicals by the selective oxidation of organic substances is of great significance for solar energy utilization and conversion. Herein, a Pt-modified 2D/2D CdS/Fe2O3 Z-scheme heterojunction was dexterously designed for photocatalytic dehydrogenation coupling of benzylamine with high performance and product selectivity. The Z-scheme charge transfer in CdS/Fe2O3 greatly boosts charge separation and maintains the strong redox capacity of photogenerated electrons and holes. Impressively, the loading of the Pt cocatalyst not only provides rich H2 production active sites to further accelerate charge separation and H2 production kinetics but also facilitates the condensation of N-benzylideneamine with benzylamine for N-benzylidenebenzylamine production and NH3 releasing. The selectivity for C═N coupling products is significantly enhanced and the C–C coupling side reaction is greatly suppressed. With sufficient benzylamine substrate and visible light irradiation, the optimal Pt/CdS/Fe2O3 composite displays a high H2 production rate of 39.4 mmol h–1 g–1 with a quantum efficiency of 32.81% at 420 nm, and the selectivity of benzylamine oxidation to N-benzylidenebenzylamine is above 90%. 

Optimizing the Reaction Pathway by Active Site Regulation in the CdS/Fe2O3 Z-Scheme Heterojunction System for Highly Selective Photocatalytic Benzylamine Oxidation Integrated with H2 Production

Photocatalytic removal of single volatile organic compounds (VOCs) has been widely investigated; however, photodecomposition of VOC mixtures has been rarely addressed, which may bring safety doubts in indoor air purification due to possible formation of harmful compounds. Here we show that in photocatalytic oxidation of formaldehyde–toluene and formaldehyde–xylene mixtures, the introduction of Pd cocatalyst on TiO2 photocatalyst successfully inhibits the formation of toxic methylphenols, thus promoting the complete mineralization of VOC mixtures into CO2 via the harmless benzaldehyde intermediates. Mechanistic analysis reveals that the loading of Pd cocatalyst effectively removes the inherent surface −OH groups of TiO2, which significantly promotes the activation of O2 into radical dotOH radicals. The Pd cocatalyst also directs the radical dotOH radicals to attack the methyl group instead of the aromatic ring for the formation of benzaldehyde and its further oxidation to CO2, thus yielding a better overall photocatalytic performance.

Inhibit the formation of toxic methylphenolic by-products in photo-decomposition of formaldehyde-toluene/xylene mixtures by Pd cocatalyst on TiO2

The construction of C–C bonds by coupling reactions is an important process in synthetic chemistry though expensive catalysts are required. Heterogeneous photocatalysis offers a platform for C–C co...

Rationally Designed Metal Cocatalyst for Selective Photosynthesis of Bibenzyls via Dehalogenative C–C Homocoupling

Sulfur vacancy (SV) engineering is an evolving approach to improve the performance of metal sulfides for hydrogen evolution reactions (HER); however, a simple and universal method for creating SV i...

Surface Sulfur Vacancy Engineering of Metal Sulfides Promoted Desorption of Hydrogen Atoms for Enhanced Electrocatalytic Hydrogen Evolution

Paired Electrochemical N–N Coupling Employing a Surface-Hydroxylated Ni3Fe-MOF-OH Bifunctional Electrocatalyst with Enhanced Adsorption of Nitroarenes and Anilines

Wei Qiao

Papers

Inhibit the formation of toxic methylphenolic by-products in photo-decomposition of formaldehyde-toluene/xylene mixtures by Pd cocatalyst on TiO2

Catalytic Role of Metal Nanoparticles in Selectivity Control over Photodehydrogenative Coupling of Primary Amines to Imines and Secondary Amines

Rationally Designed Metal Cocatalyst for Selective Photosynthesis of Bibenzyls via Dehalogenative C–C Homocoupling

Surface Sulfur Vacancy Engineering of Metal Sulfides Promoted Desorption of Hydrogen Atoms for Enhanced Electrocatalytic Hydrogen Evolution

Paired Electrochemical N–N Coupling Employing a Surface-Hydroxylated Ni3Fe-MOF-OH Bifunctional Electrocatalyst with Enhanced Adsorption of Nitroarenes and Anilines