Light-driven Selective Oxidation Research Articles

Tailoring β-ketoenamine covalent organic framework with azo for blue light-driven selective oxidation of amines with oxygen

Covalent organic frameworks (COFs) present bright prospects in visible light photocatalysis with abundant active sites and exceptional stability. Tailoring an established COF with photoactive group is a prudent strategy to extend visible light absorption toward broad photocatalysis. Here, a β-ketoenamine COF, TpBD-COF, constructed with 1,3,5-triformylphloroglucinol (Tp) and 4,4′-biphenyldiamine (BD), is tailored with azo to validate this strategy. The insertion of azo into BD affords 4,4′-azodianiline (Azo); TpAzo-COF is successfully constructed with Tp and Azo. Intriguingly, the insertion of azo enhances π-conjugation, thereby facilitating visible light absorption and intramolecular electron transfer. Moreover, TpAzo-COF, with an appropriate electronic structure and impressive specific surface area of 1855 m2 g−1, offers substantial active sites conducive to the reduction of oxygen (O2) to superoxide. Compared with TpBD-COF, TpAzo-COF exhibits superior performance for blue light-driven oxidation of amines with O2. Superoxide controls the selective formation of product imines. This work foreshadows the remarkable capacity of tailoring COFs with photoactive group toward broad visible light photocatalysis.

Blue light-driven selective aerobic oxidation of amines by benzothiadiazole metal–organic framework photocatalysis

A zirconium-based benzothiadiazole metal–organic framework, UiO-68-BT, performs blue light-driven selective oxidation of benzyl amines with O2 by photocatalysis.

Visible light-driven selective oxidation of amines by cooperative photocatalysis of niobium oxide nanorods with an electron–proton transfer mediator

Cooperative photocatalysis has gained increasing interest to better an existing redox reaction with a mediator. Herein, an ingenious system is constructed for the visible light-driven selective oxidation of a variety of amines by merging niobium oxide (Nb2O5) photocatalysis with an electron–proton transfer mediator, 2,2,6,6-tetramethylpiperidine-1-oxoammonium hexafluorophosphate (TEMPO+PF6−). The visible light-driven dissociative adsorption of amine over Nb2O5 nanorods gives rise to an amine–Nb2O5 complex with a weak response to visible light (λ < 460 nm). With adding 4 mol% of TEMPO+PF6−, the conversion of benzylamine over Nb2O5 is approximately 2.5 times that without TEMPO+PF6−. The proton and electron transfer between the amine–Nb2O5 complex and TEMPO+PF6− is the key to assuring cooperative photocatalysis. Besides, the oxidant dioxygen is predominantly reduced to superoxide anion in affording the imine products. This work highlights it is feasible to ameliorate selective organic conversions over metal oxide photocatalysts with a suited mediator.

2D Ti-based metal–organic framework photocatalysis for red light-driven selective aerobic oxidation of sulfides

Ti-based metal-organic frameworks (MOFs) consisting of organic linkers and Ti-oxo clusters have tremendous proneness in visible light photocatalysis. Here, two-dimensional (2D) Ti-based porphyrin MOF (PMOF (Ti)) and bulk PMOF (Ti) were synthesized and fully characterized. Consequently, the porphyrin linker imparts broad absorption of visible light and the appropriate reduction potential of Ti‐oxo clusters can activate O2 to superoxide (O2•–). The 2D PMOF (Ti) exhibits more satisfying results for the red light-driven selective oxidation of sulfides with O2 than bulk PMOF (Ti). In addition, 2D PMOF (Ti) is highly robust which could be recycled 8 times without any loss of photocatalytic activity. Mechanistic investigations reveal that O2•– is the crucial reactive oxygen species (ROS) in forming sulfoxides with high selectivities. This work highlights that 2D MOFs could have a great potential in selective photocatalytic transformations of organic molecules.

A two-dimensional Bi-based porphyrin metal–organic framework photocatalyst for white light-driven selective oxidation of sulfides

A single crystal of a Bi-based porphyrin metal–organic framework was synthesized by a hydrothermal method. It exhibited significant photocatalytic activity for the selective oxidation of sulfides, maintaining high activity after 10 catalytic cycles.

Hydrazone-linked 2D porphyrinic covalent organic framework photocatalysis for visible light-driven aerobic oxidation of amines to imines

Covalent organic frameworks (COFs) have recently gained rising consideration for visible light photocatalysis. Their property could be accurately established with specific reactions in which the most investigated one turns out to be the aerobic oxidation of amines. In this contribution, a hydrazone-linked 2D (two-dimensional) porphyrinic COF, Por-DETH-COF, was assembled from 5,10,15,20-tetrakis(4-benzaldehyde)porphyrin (p-Por-CHO) and 2,5-diethoxyterephthalohydrazide (DETH) and its photocatalytic activity was duly appraised with the aerobic oxidation of amines. Thereby, the red light-driven selective oxidation of benzyl amines to imines was obtained in very high conversions and selectivities with ambient air as the oxidant. Importantly, the photocatalytic system exhibited remarkable compatibility of functional groups and extensive scope of benzyl amines. Notably, the Por-DETH-COF photocatalyst displayed outstanding recyclability after five successive cycles. This work suggests that 2D COFs could contribute a unique juncture for selective organic transformations by photocatalysis.

Ultrasound-assisted synthesis of CoFe2O4/Ce-UiO-66 nanocomposite for photocatalytic aerobic oxidation of aliphatic alcohols

During the past years, light-driven selective oxidation of various alcohols has attracted increasing attention as a green and eco-friendly manner to convert visible light energy into valuable compounds. In this work, magnetic CoFe2O4/Ce-UiO-66 embedded structure composites are elaborately designed for the photocatalytic oxidation of aliphatic alcohols under visible-light irradiation and aerobic condition at room temperature. The CoFe2O4/Ce-UiO-66 structure was prepared using a simple and fast ultrasound-assisted technique in 60 min at room temperature. As compared with the unmodified CoFe2O4, the embedded composite exhibited better visible-light sensitization performance. The catalyst showed high chemical stability in the reaction conditions and can be recovered quickly and reused for at least five reaction runs in the aerobic oxidation reaction condition.

Boosting Photocatalytic Activity and Stability of Lead-Free Cs3Bi2Br9 Perovskite Nanocrystals via In Situ Growth on Monolayer 2D Ti3C2Tx MXene for C-H Bond Oxidation.

Light-driven selective oxidation of saturated C-H bonds with molecular oxygen, as an alternative to conventional thermochemical catalysis, allows a sustainable and eco-friendly manner to convert solar energy into highly value-added oxygenates. However, the photocatalytic oxidation of hydrocarbons still remains a great challenge owing to the low efficiency in the separation and transfer of photogenerated charge of the currently available photocatalytic materials. Herein, we report a novel perovskite-based heterostructure photocatalyst, in which ligand- and lead-free all-inorganic perovskite Cs3Bi2Br9 nanocrystals (NCs) with uniform crystal size and high crystallinity were homogeneously distributed on the surface of ultrathin two-dimensional (2D) monolayer Ti3C2Tx MXene nanosheets in an in situ growth manner. The resultant heterostructure featured with intimate interface between Cs3Bi2Br9 NCs and Ti3C2Tx MXene and strong visible-light adsorption not only exhibits significant enhancement in the performance of photocatalytic oxidation of challenging aromatic and aliphatic alkanes under visible-light irradiation but also greatly improves the stability of Cs3Bi2Br9 NCs under a reaction environment. Comprehensive characterizations reveal that the formation of an intimate interface between Cs3Bi2Br9 NCs and highly conductive ultrathin 2D Ti3C2Tx MXene nanosheets via strong interaction markedly accelerates the separation and transfer efficiency of photogenerated electron-hole pairs and simultaneously suppresses their recombination, resulting in improved utilization of the excited charges, which account for the highly enhanced photocatalytic performance.

Designing fluorene-based conjugated microporous polymers for blue light-driven photocatalytic selective oxidation of amines with oxygen

Conjugated microporous polymers (CMPs) have been showcased with a brilliant prospect in organic semiconductor photocatalysis, attributing to accessible molecular design, chemical stability and environmental friendliness. Here, two novel fluorene-based CMPs were designed and conveniently synthesized with carbazole as an electron donor. Importantly, subtle variation of substituent at the methylene bridge (9-position) of fluorene precursor results in different performances in which dimethyl substituent was proven to be more efficient than difluoro substituent for blue light photocatalysis. MFC [9,9'-(9,9-dimethyl-9H-fluorene-2,7-diyl)bis(9H-carbazole)]-CMP has a much larger specific surface area, a more favourable redox position, and resultantly a superior photocatalytic performance during blue light-driven selective oxidation of amines into imines with oxygen (O2) in an environmentally benign solvent ethanol (C2H5OH). This work suggests that subtle tweaking in electron acceptors could give rise to superior photocatalytic activity for CMPs in selective chemical conversions.

Cooperative TiO2 photocatalysis with TEMPO and N-hydroxysuccinimide for blue light-driven selective aerobic oxidation of amines

TiO2 has been the focus of attention in semiconductor photocatalysis for several decades because it can potentially settle the grand energy and environmental issues with earth-abundant elements of Ti and O. However, because of its wide band gap, TiO2 can only collect UV light, hindering its practical applications under the illumination of sunlight. In view of this, an interesting phenomenon of light-driven adsorption of amines onto TiO2 to form a visible light-absorbing complex was adapted to assemble smart photocatalysis. The endurance of this complex was eminently refurbished by blue light-driven continuous adsorption of amines. This in turn promoted a vital selective chemical transformation, blue light-driven selective oxidation of amines into imines with atmospheric dioxygen (O2). More importantly, the inclusion of TEMPO and N-hydroxysuccinimide (NHS) into the smart photocatalytic system could cooperatively expedite the blue light-driven selective aerobic oxidation of amines into imines through dual independent reaction channels, resembling that of enzymatic catalysis. This work underscores the importance of manoeuvring multiple reaction channels by cooperative photocatalysis during selective chemical transformations.

Hydrogen reduction treatment of boron carbon nitrides for photocatalytic selective oxidation of alcohols

Light-driven selective oxidation of aromatic alcohols allows a sustainable and eco-friendly manner to convert solar energy into highly valuable aromatic aldehydes. However, the low separation and transfer efficiency of photogenerated charge carriers and the sluggish reaction kinetics seriously restrict the efficiency of the organic photosynthesis of desired compounds. Herein, a facile strategy is adopted to regulate the boron carbon nitrides (BCN) semiconductors by hydrogen reduction to precisely tune the structural and surface properties. The reduced BCN materials can effectively enhance charge separation and migration as well as promote O2 activation and mass transfers. As a result, this BCN catalyst therefore displays a remarkable enhancement in photosynthesis of aromatic aldehydes from the alcohols with high conversion and selectivity compared to pristine BCN.

Phenol–TiO2 complex photocatalysis: visible light-driven selective oxidation of amines into imines in air

A new photocatalytic protocol utilizing surface complexation of phenol with TiO2 for the selective oxidation of amines into imines in air was presented.

Salicylic acid complexed with TiO2 for visible light-driven selective oxidation of amines into imines with air

The interplay of salicylic acid (SA) and reactive oxygen species (ROS) can modulate biotic and abiotic stress of plants, essential biological aerobic oxidation processes. Meanwhile, ROS plays a critical role in TiO2 photocatalytic system for the degradation of organic species. Herein, we developed a system consisted of SA and TiO2 aiming at the selective oxidation of organic molecules by ROS. Interestingly, SA complexed with TiO2 leads to ligand-to-metal charge transfer (LMCT) under visible light irradiation. The charge transfer from ligand (chemically adsorbed SA) to metal (conduction band of TiO2) activates O2 to ROS, superoxide (O2−). The positive charge located at ligand SA is connected with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) catalysis for direct two-electron oxidation of amines and later regenerated by O2−. SA and its derivatives were screened as ligands of TiO2 for the selective aerobic oxidation of amines into imines under blue light-emitting diode (LED) irradiation in which 5-CH3O-SA (5-methoxysalicylic acid, 0.8 mol%) complexed with anatase TiO2 and coupled with TEMPO (5 mol%) confers significantly better results than the others. By this visible light LMCT route, both primary amines and secondary amines can be selectively oxidized into corresponding imines with atmosphere O2 as the terminal oxidant. Importantly, the desired product of N-benzylidenebenzylamine can be isolated in 92% yield.

Influence of tetraalkylammonium cations on quality of decatungstate and its photocatalytic property in visible light-triggered selective oxidation of organic compounds by dioxygens

Three decatungstates (DTs) were conveniently synthesized respectively using quaternary ammonium cations of tetramethyl, tetrapropyl and tetrabuthyl as counterions (marked as TMADT, TPADT and TBADT) and their quality, photoluminescence (PL) property and redox capacity were measured by UV–vis, XPS and PL spectra and cyclic voltammograms (CVs), respectively. The photo-catalysis activities of three DTs were checked using visible light-driven selective oxidation of cyclohexane, toluene, ethylbenzene and benzyl alcohol by dioxygens (O2) in acetonitrile (MeCN). The results showed that the photo-catalysis activity of DT anion for these oxidations gradually increases with the shortening of its cationic alkyl chain, which is attributed to the continuous improvement of its quality from TBADT to TMADT. Furthermore, the additives water, 12 M hydrogen chloride (HCl) solution and especially the mixture of both can markedly promote the DTs-photocatalyzed oxidations and such promotion presents an enhancing trend from TBADT to TMADT. which is likely due to the beneficial effects of these additives as follows: i) Water can significantly stabilize the structure of DT anion under light illumination and improve its redox cycling in MeCN; ii) 12 M HCl may obviously improve the oxidative capacity and redox cycling of DT anion and stabilize its photo-excited states. iii) The respective promotion effects of such two additives on photo-catalysis can be well combined and such combined effect is enhanced drastically upon TMADT owing to its hydrophilic feature, thus significantly improving the photo-catalytic efficiency of TMADT under the joint action of water and 12 M HCl.

Engineering a polyoxometalate-based metal organic framework with more exposed active edge sites of Ag for visible light-driven selective oxidation of cis-cyclooctene

The polyoxometalate-based MOF with exposed active edge sites of Ag was designed for visible light-driven selective oxidation of cis-cyclooctene.