Selective Oxidation Of Toluene Research Articles

S-scheme TiO2/Bi2WO6 heterojunction for enhanced photocatalytic selective oxidation of toluene

Selective activation of saturated C-H bonds in hydrocarbons, such as the activation of the C-H bond in toluene, is an effective method for the production of high value-added chemicals. However, improving the C-H bond activation of toluene remains a challenge. In this study, S-scheme TiO2/Bi2WO6 composites were synthesized by loading TiO2 nanoparticles on Bi2WO6 flakes using a simple hydrothermal method. The physical and chemical properties of the prepared materials were characterized using X-Ray Diffraction (XRD), Fourier Transform Infrared Spectoscopy (FT-IR), Scanning Electronic Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS) and other characterization methods. Selective oxidation of toluene to benzaldehyde under visible light irradiation using air as an oxidizing agent. When the molar ratio of TiO2 and Bi2WO6 was 0.20, the composites exhibited excellent photocatalytic performance with a benzaldehyde production rate of 1725.41 μmol·g-1·h-1, which was 2.31 and 1.91 times higher than that of TiO2 and Bi2WO6, respectively. This excellent performance is attributed to the formation of heterojunction structure by the loaded TiO2 nanoparticles, which improves the separation efficiency of photogenerated carriers and thus enhances its photocatalytic performance. In addition, the active substances involved in the photocatalytic reaction were identified by free radical trapping experiments as ·O2- and h+. Finally, the photocatalytic mechanism of S-scheme TiO2/Bi2WO6 composites was proposed by energy band structure analysis and radical trapping experiments. This study provides a simple design pathway for the construction of visible-light responsive bismuth tungstate-based heterojunction photocatalysts and some ideas for the development of new and efficient photocatalytic toluene selective oxidation catalysts.

An Investigation of N-Hydroxyphthalimide Catalyzed Aerobic Oxidation of Toluene without Metal Ions in Liquid Phase: Effect of Solvents and Phase Transfer Catalysts.

The selective oxidation of toluene to yield value-added oxygenates, such as benzyl alcohol, benzaldehyde, and benzoic acid, via dioxygen presents a chlorine-free approach under benign conditions. Metal-free catalytic processes are preferred to avoid metal ion contamination. In this study, we employed N-hydroxyphthalimide (NHPI) as a catalyst for the aerobic oxidation of toluene to its oxygenated derivatives. The choice of solvent exerted a significant impact on the catalytic activity and selectivity of the catalyst NHPI at reaction temperatures exceeding 70 °C. Notably, hexafluoroisopropanol substantially enhanced the selective production of benzaldehyde. Furthermore, we identified didecyl dimethyl ammonium bromide, featuring two symmetrical long hydrophobic chains, as a potent enhancer of NHPI for the solvent-free aerobic oxidation of toluene. This effect is ascribed to its unique symmetrical structure, extraction capabilities, and resistance to thermal and acid/base conditions. Based on the product distribution and control experiments, we proposed a plausible reaction mechanism. These findings may inform the industrial synthesis of oxygenated derivatives from toluene.

Single-crystal oxygen-rich bismuth oxybromide nanosheets with highly exposed defective {10–1} facets for the selective oxidation of toluene under blue LED irradiation

Reactive radicals are crucial for activating inert and low-polarity C(sp3)-H bonds for the fabrication of high value-added products. Herein, novel single-crystal oxygen-rich bismuth oxybromide nanosheets (Bi4O5Br2 SCNs) with more than 85 % {10–1} facets exposure and oxygen defects were synthesized via a facile solvothermal route. The Bi4O5Br2 SCNs demonstrated excellent photocatalytic performance in the selective oxidation of toluene under blue light. The yield of benzaldehyde was 1876.66 μmol g−1 h−1, with a selectivity of approximately 90 %. Compared to that of polycrystalline Bi4O5Br2 nanosheets (Bi4O5Br2 PCNs), the activity of Bi4O5Br2 SCNs exhibit a 21-fold increase. Experimental studies and density functional theory (DFT) calculations have demonstrated that the defect Bi4O5Br2 (10–1) facets exhibits exceptional adsorption properties for O2 molecules. In addition, the single-crystal structure in the presence of surface defects significantly increases the separation and transport of photogenerated carriers, resulting in the effective activation of adsorbed O2 into superoxide radicals (•O2–). Subsequently, the positively charged phenylmethyl H is readily linked to the negatively charged superoxide radical anion, thereby activating the CH bond. This study offers a fresh perspective and valuable insights into the development of efficient molecular oxygen-activated photocatalysts and their application in the selective catalytic conversion of aromatic C(sp3)-H bonds.

Triazine Frameworks for the Photocatalytic Selective Oxidation of Toluene.

Investigations into the selective oxidation of inert sp3 C-H bonds using polymer photocatalysts under mild conditions have been limited. Additionally, the structure-activity relationship of photocatalysts often remains insufficiently explored. Here, a series of thiophene-based covalent triazine frameworks (CTFs) are used for the efficient and selective oxidation of hydrocarbons to aldehydes or ketones under ambient aerobic conditions. Spectroscopic methods conducted in situ and density functional theory (DFT) calculations revealed that the sulfur atoms within the thiophene units play a pivotal role as oxidation sites due to the generation of photogenerated holes. The effect of photogenerated holes on photocatalytic toluene oxidation was investigated by varying the length of the spacer in a CTF donor-acceptor based photocatalyst. Furthermore, the manipulation of reactive oxygen species was employed to enhance selectivity by weakening the peroxidative capacity. As an illustrative example, this study successfully demonstrated the synthesis of a precursor of the neurological drug AMG-579 using a photocatalytic protocol.

Enhanced Charge Transfer Process and Photocatalytic Activity over a Phosphonate-based MOF via Amorphization Strategy.

Recently, amorphous materials have gained great attention as an emerging kind of functional material, and their characteristics such as isotropy, absence of grain boundaries, and abundant defects are very likely to outrun the disadvantages of crystalline counterparts, such as low conductivity, and ultimately lead to improved charge transfer efficiency. Herein, we investigated the effect of amorphization on the charge transfer process and photocatalytic performance with a phosphonate-based metal-organic framework (FePPA) as the research object. Comprehensive experimental results suggest that compared to crystalline FePPA, amorphous FePPA has more distorted metal nodes, which affects the electron distribution and consequently improves the photogenerated charge separation efficiency. Meanwhile, the distorted metal nodes in amorphous FePPA also greatly promote the adsorption and activation of O2. Hence, amorphous FePPA exhibits a better performance of photocatalytic C(sp3)-H bond activation for selective oxidation of toluene to benzaldehyde. This work illustrates the advantages of amorphous MOFs in the charge transfer process, which is conducive to the further development of high performance MOFs-based photocatalysts.

Enhanced Charge Transfer Process and Photocatalytic Activity over a Phosphonate‐based MOF via Amorphization Strategy

AbstractRecently, amorphous materials have gained great attention as an emerging kind of functional material, and their characteristics such as isotropy, absence of grain boundaries, and abundant defects are very likely to outrun the disadvantages of crystalline counterparts, such as low conductivity, and ultimately lead to improved charge transfer efficiency. Herein, we investigated the effect of amorphization on the charge transfer process and photocatalytic performance with a phosphonate‐based metal–organic framework (FePPA) as the research object. Comprehensive experimental results suggest that compared to crystalline FePPA, amorphous FePPA has more distorted metal nodes, which affects the electron distribution and consequently improves the photogenerated charge separation efficiency. Meanwhile, the distorted metal nodes in amorphous FePPA also greatly promote the adsorption and activation of O2. Hence, amorphous FePPA exhibits a better performance of photocatalytic C(sp3)−H bond activation for selective oxidation of toluene to benzaldehyde. This work illustrates the advantages of amorphous MOFs in the charge transfer process, which is conducive to the further development of high performance MOFs‐based photocatalysts.

Photocatalytic selective oxidation of toluene under encapsulated air conditions.

Benzaldehydes are indispensable building blocks in chemistry. However, the selective oxidation of toluene to benzaldehyde remains an ongoing challenge due to the low oxidation potential of benzaldehyde compared to toluene. We report herein a mild protocol that combines hydrogen atom transfer (HAT) with encapsulated air conditions and suitable catalyst loading for selective oxidation of toluene with high selectivity as well as good functional-group tolerance and a broad substrate scope for the synthesis of various high-value aromatic aldehydes. Moreover, the compatibility of this reaction with toluene derivatives of bioactive molecules further demonstrated the practicality of this approach. Mechanism studies have demonstrated that the collaboration between the oxygen quantity and the HAT catalytic system has a major impact on the high selectivity of the reaction. This study not only showcases the effectiveness of HAT strategies toward selective oxidation of toluene to benzaldehyde, but also provides an approach to controlling the selectivity of HAT reactions.

Highly efficient selective oxidation of toluene to benzaldehyde over Cu–V oxides supported on amorphous SiO2

Selective oxidation of toluene into high-value-added benzaldehyde is meaningful and challenging.

Reverse microemulsion assisted synthesis of copper nano-catalyst for highly selective toluene oxidation

Reverse microemulsion assisted synthesis of copper nano-catalyst for highly selective toluene oxidation

Triggering photocatalytic performance of La2CoxMn2-xO6 via heat activation.

The La-based perovskite (LaBO3) exhibits excellent optical properties. However, its valence band (VB) potential is not sufficiently positive to reach the oxidation potential required for the cleavage of chemical bonds (such as benzylic C-H), limiting its application in photocatalysis. Herein, we report the unconventional effects of heat activation on the reduction of the dissociation energy of benzylic C-H and aqueous H-O, thereby triggering the photocatalytic activity of La2CoxMn2-xO6 perovskites. Additionally, we demonstrate that photocatalysis is the main contributor to substrate conversion in the selective oxidation of toluene and reduction of CO2. Particularly, La2Co1.5Mn0.5O6 shows excellent performance with a product yield of 550.00 mmol gcat-1 and a toluene conversion of 22,866.67 μmol gcat-1 h-1. To the best of our knowledge, this is the highest reported product yield for the selective oxidation of benzylic C-H bond of toluene. Our findings provide insight into the specific role of heat activation in photocatalysis, which is crucial for breaking and overcoming the VB barrier to realize challenging reactions.

Multi-dimensional lead-free hybrid double perovskite toward efficient and stable photocatalytic selective oxidation of toluene

With unique photoelectronic properties, perovskites have drawn attention in photocatalysis. However, their activity and stability especially in moist environments is not appealing, despite it was demonstrated that the construction of three-dimensional (3D) and low-dimensional counterparts could result in boosting of activity and stability, respectively. Here, a lead-free hybrid double perovskite ((DETA)BiBr6/Cs2AgBiBr6, DCABB) was fabricated by uniformly dispersing 0D DETABiBr6 (DETA3+ = NH3+(CH2)2NH2+(CH2)2NH3+) in the matrix of 3D Cs2AgBiBr6. Benefited from the passivation of Br defects and resistance of water by DETA3+, the stability of DCABB improved in comparison to pristine 3D Cs2AgBiBr6. Moreover, the formation of DCABB increased the adsorption of O2 and the accumulation of photogenerated electrons on the surface of catalyst, consequently boosting the generation of active O2− and OH species. The DCABB displayed higher photocatalytic performance than its components in toluene selective oxidation under visible light illumination, achieving a toluene conversion of up to 100 %.

Tuning the oxygen vacancy and acidity of V-Ag-Ce/TiO2 catalyst for selective gas-phase oxidation of toluene to benzaldehyde

Toluene gas-phase oxidation is a green, chlorine-free production process for benzaldehyde but its conversion efficiency is still unsatisfying due to the lack of highly efficient catalyst. Here the selective oxidation of toluene to benzaldehyde over V-Ag-Ce/TiO2 under an air atmosphere was investigated. The catalyst was prepared by impregnation and characterized by scanning electron microscope (SEM), N2 adsorption-desorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Electron paramagnetic resonance (EPR), H2 temperature programmed reduction (H2-TPR), O2 temperature programmed desorption (O2-TPD), and NH3 temperature programmed desorption (NH3-TPD). The amount of oxygen vacancy and acid of the catalyst can be tuned through adjusting the V:Ag:Ce ratio. Over the optimum catalyst (V:Ag:Ce = 4:2:1, molar ratio), 92% benzaldehyde selectivity with a 27% toluene conversion can be obtained at 550 °C. Medium amounts of oxygen vacancy (0.43) and acidity (0.10 mmol/g) are found to be conducive to the conversion of toluene to benzaldehyde. This study has important theoretical guiding significance for improving conversion efficiency from toluene to benzaldehyde by gas-phase aerobic oxidation.

Achieving High Selectivity in Photocatalytic Oxidation of Toluene on Amorphous BiOCl Nanosheets Coupled with TiO2.

The inert C(sp3)-H bond and easy overoxidation of toluene make the selective oxidation of toluene to benzaldehyde a great challenge. Herein, we present that a photocatalyst, constructed with a small amount (1 mol %) of amorphous BiOCl nanosheets assembled on TiO2 (denoted as 0.01BOC/TiO2), shows excellent performance in toluene oxidation to benzaldehyde, with 85% selectivity at 10% conversion, and the benzaldehyde formation rate is up to 1.7 mmol g-1 h-1, which is 5.6 and 3.7 times that of bare TiO2 and BOC, respectively. In addition to the charge separation function of the BOC/TiO2 heterojunction, we found that the amorphous structure of BOC endows its abundant surface oxygen vacancies (Ov), which can further promote the charge separation. Most importantly, the surface Ov of amorphous BOC can efficiently adsorb and activate O2, and amorphous BOC makes the product, benzaldehyde, easily desorb from the catalyst surface, which alleviates the further oxidation of benzaldehyde, and results in the high selectivity. This work highlights the importance of the microstructure based on heterojunctions, which is conducive to the rational design of photocatalysts with high performance in organic synthesis.

Highly Selective Oxidation of Toluene to Benzaldehyde in Alkaline Systems

For the selective oxidation of toluene to benzaldehyde, the biggest challenge is to prevent the further oxidation of benzaldehyde as toluene conversion increases. Using benzyl benzoate as the solvent and adding benzoate to adjust the alkalinity of the system, the formation of the benzoyl radical can be well inhibited, but the oxidation of toluene is weakened. The decomposition of benzyl benzoate to benzaldehyde can activate the reaction at the initial stage, and the negative influence of alkalinity is avoided to some extent. Therefore, by using atmospheric air as an oxidant, a high selectivity and yield of benzaldehyde can be obtained in a benzyl benzoate catalytic system with alkali as an additive. The mechanism of alkaline regulation and selective oxidation of toluene was investigated by in situ infrared (IR) spectroscopy experiments. The benzaldehyde capacity is defined to characterize the tolerance of benzaldehyde in the system. Catalysts and alkaline regulators can be easily reused with high-boiling solvents, making it a convenient process for product separation and catalyst–alkaline solvent recycling.

One-Pot Synthesis of Bismuth Basic Nitrate-Bi2MoO6 Photocatalyst for Selective Oxidation of Toluene

The key issues to improve the performance of photocatalysts for selective oxidation of toluene are promoting the migration and separation of charge carrier, and enhancing the generation of active oxygen species. It is known that the construction of compact heterojunction is an efficient protocol to inhibit photogenerated electron-hole recombination. In this work, a 2D-2D [Bi6O6(OH)3](NO3)3·1.5H2O-Bi2MoO6 (denoted as BBN-BMO) composite heterojunction has been prepared by one-step hydrothermal method for the first time. The tetragonal phase BBN in the composite plays the role of transferring electrons from the visible light activated orthorhombic phase BMO and promoting the generation of active •O2−, the holes left in BMO are used to activate toluene and produce benzyl radical, thus greatly improving the photocatalytic performance for selective oxidation of toluene. The toluene conversion rate of BBN-BMO is 3466 µmol·g−1·h−1, which is three times that of pure BMO. The selectivity to benzaldehyde is 94.2%. In addition, reasonable mechanism has been speculated based on a series of control experiments.

Ferroelectric polarization enhancing photocatalytic performance of 2D carbon and oxygen co-doping g-C3N4/α-In2Se3 heterostructure: A conversion of traditional type-II to S-scheme

Ferroelectric materials with internal spontaneous polarization are conducive to enhancing photocatalytic performance by promoting photogenerated carriers separation. However, the traditional perovskite-type ferroelectric photocatalysts possess a typical 3D structure that is constrained by few exposed catalytic active sites and low specific surface area when compared to a 2D structure. In our study, the electronic properties of the 2D ferroelectric heterostructure for carbon and oxygen co-doping g-C3N4 (COCN)/In2Se3 with different out-of-plane ferroelectric polarization directions are investigated by first-principle calculations, namely, COCN/DOWN and COCN/UP heterostructures. The results show that when the ferroelectric polarization of the 2D In2Se3 layer in heterostructures is reversed, the heterostructure switches from traditional type-II (COCN/DOWN heterostructure) with an indirect bandgap of 1.58 eV to S-scheme (COCN/UP heterostructure) with a direct bandgap of 1.43 eV, in which the band edge positions of the S-scheme COCN/UP heterostructure satisfy the redox potential of the efficient photocatalytic selective oxidation of toluene to benzaldehyde. Further investigations revealed that the application of an electric field 0 ∼ +0.3 V/Å can reduce the bandgap and enhance the out-of-plane polarization of the COCN/UP heterostructure, which improve the photocatalytic activity of the S-scheme COCN/UP heterostructure. This work highlights the significance of ferroelectric polarization for charge transfer in heterostructures and provides theoretical guidance for the design of high-performance S-scheme photocatalysts.

Sulfur-vacancy-rich ZnS/CdIn2S4 heterojunction for efficient photocatalytic selective oxidation of toluene to benzaldehyde

The conversion of the volatile organic pollutant to industrially valuable chemicals using photocatalysis is a promising endeavor. Herein, ZnS/CdIn2S4 heterojunction with rich sulfur vacancy was constructed via a simple one-pot method for the selective photocatalytic oxidation of toluene to benzaldehyde. ZnS/CdIn2S4 has a suitable band gap and valence band position, thus possessing a mild oxidation ability to generate benzaldehyde while avoiding excessive oxidation. In addition, the presence of tightly bound contact interfaces and sulfur vacancy in ZnS/CdIn2S4 heterojunction facilitates the rapid separation of photogenerated electron-hole. These characteristics of the heterojunction synergistically promoted the selective photocatalytic oxidation of toluene, resulting in ZnS/CdIn2S4 exhibiting the highest benzaldehyde production rate (1025 μmol g−1 h−1) and the highest conversion rate (13.7%), which were 93.1 and 56.9 times higher than those of ZnS and CdIn2S4, respectively. Furthermore, a possible response mechanism has been suggested based on a series of experiments. This study provides new insights into the combination of volatile organic pollutant degradation and high-value chemicals production.

Efficient Photo-Assisted Thermal Selective Oxidation of Toluene Using N-Doped TiO2.

Selective oxidation of toluene is a key reaction to produce high value-added products but remains a big challenge. In this study, we introduce a nitrogen-doped TiO2 (N-TiO2) catalyst to create more Ti3+ and oxygen vacancy (OV), which act as active sites for selective oxidation of toluene via activating O2 to superoxide radical (•O2-). Interestingly, the resulting N-TiO2-2 exhibited an outstanding photo-assisted thermal performance with a product yield of 209.6 mmol·gcat-1 and a toluene conversion of 10960.0 μmol·gcat-1·h-1, which are 1.6 and 1.8 times greater than those obtained under thermal catalysis. We showed that the enhanced performance under photo-assisted thermal catalysis was attributed to more active species generation by making full use of photogenerated carriers. Our work suggests a viewpoint to apply a noble-metal-free TiO2 system in the selective oxidation of toluene under solvent-free conditions.

Effect of dissolved organic matter on selective oxidation of toluene by ozone micro-nano bubble water

The oxidation capacity of ozone micro-nano bubble water (OMBW) was always higher than ozonated water due to enhanced contact by bubble interface, while the effect of coexisted dissolved organic matter (DOM) on the oxidation efficiency was still unclear. In this paper, batch experiments were carried out to investigate the selective oxidation of toluene by both OMBW and ozonated water (OW) with coexisted DOM in water. Five types of background solutions were applied in this study, including humic acid solution, fulvic acid solution and three types of diluted landfill leachates at the same content of total organic carbon. Results showed that coexisted DOM had a greater inhibition effect on toluene oxidation rate by OMBW, and the oxidation rate of toluene by OMBW and OW became close. It was mainly caused by the decreased reaction rate between toluene and hydroxyl radical (kT-OH·) in OMBW after the introduction of DOM, which competed for the adsorption sites on micro-nano bubble interface. The fraction of ozone to oxidize toluene as well as kT-OH· was in positive correlations with SUVA254 and the content of humic acid-like substances, but negatively correlated with E2/E3, content of tryptophan-like proteins and content of fulvic acid-like substances. In addition, increasing the ozone dose was not effective in increasing the utilization rate of ozone in OMBW due to limited adsorption sites on micro-nano bubble interface. The paper was conductive to the application of ozone micro-nano bubble water in groundwater remediation with complex water matrices.

Selective liquid-phase oxidation of toluene over heterogeneous Mn@ZIF-8 derived catalyst with molecular oxygen in the solvent-free conditions

In this work, liquid-phase catalytic oxidation of toluene was carried out under solvent-free conditions, and highly selective synthesis of benzaldehyde (BAL) and benzyl alcohol (BOL) and benzoic acid (BAC) in the presence of Mn@ZIF-8 calcined material as catalyst with oxygen molecules. As a heterogeneous catalyst, the zeolitic imidazolate framework Mn@ZIF-8 derived material exhibited reasonable substrate-product selectivity (70.3% of selectivity to BAL and BOL, 95.1 % of selectivity to BAL, BOL and BAC) and conversion (6.5%) under optimum reaction conditions. The catalysts were characterized by BET-specific surface area determination, XRD, XPS, FT-IR, TG-DTG and SEM-EDS-Mapping. The results demonstrated that the catalytic capacity of the catalysts was enhanced by the good dispersion of amorphous Mn species in ZIF-8 derivatives and high specific surface area. The possible reaction pathway for the catalytic oxidation of toluene was also suggested. Maybe this method employing Mn@ZIF-8 as efficient catalyst affords a new and environmentally friendly route for the synthesis of BOL and BAL from the selective oxidation of toluene.