Progress in Continuous-flow Oxidation of Aromatic Hydrocarbons

• Abstract
• Introduction
• Current Research on Synthetic Process
• Aromatic Branched-chain Oxidation to Alcohols
• Oxidation of Aromatic Branched Chains to Aldehydes or Ketones
• Aromatic Branched Chain Oxidation to Acid
• The Aromatic Branch Chain Oxidizes to Other Compounds
• Conclusion
• References

Abstract

Aromatic alcohols, aromatic aldehydes, and aromatic acids are usually synthesized by oxidation of the corresponding aromatic compounds and can be used as intermediates for the synthesis of pharmaceuticals, dyes, plasticizers, and fragrances. However, implementing these oxidations in conventional batch reactors, especially on larger scales, is often associated with severe safety risks and process challenges. The potential of explosion propagation within the reactor can be reduced using continuous-flow technology, which can eliminate or alleviate these safety hazards by reducing the size of the reactor and the size of the channel. In this paper, the research progress on aromatic hydrocarbon catalytic oxidation reactions using continuous-flow technology, as well as various catalytic systems and the corresponding selectivity, is reviewed. The translation of aerobic oxidation from batch oxidation to continuous-flow processes, including process intensification and equipment study, has also been discussed.

Introduction

The catalytic oxidation of hydrocarbons using air, or O₂ in the liquid phase, is one of the most important processes for the production of a wide range of high-value-added chemical industrial products including alcohols, aldehydes, ketones, acids, esters, and phenols.[1] [2] [3] For example, the oxidation of o-xylene using liquid-phase air or O₂ produces o-phthalaldehyde or phthalic acid.[4] [5] However, the process may occur with cascade reaction characteristics, or expose to a safety risk of explosion.[6] [7] For such oxidation reactions, the selection of oxidation reaction devices and reactor types, the choosing of catalyst systems, and the optimization of reaction conditions to improve the conversion of reaction substrates, the selectivity of target products, and the safety issue during the reaction process, have been a hot research topic.[8] [9] Some traditional oxidation systems are highly reactive and have high-risk factors, whereas continuous-flow heat and mass transfer have received considerable attention for their high efficiency, precise control, and low risk.[4] [5] [6] By comparing several types of synthetic methods, a photocatalytic oxidation system may be more economical and environment-friendly, providing a novel technological pathway for the preparation of aromatic acids.[7] [8] [9]

Current Research on Synthetic Process

Currently, hundreds of techniques are available for catalytic oxidation of alky arenes, and they can be divided into three categories: (1) chemical oxidation methods, including oxygen oxidation,[10] [11] hydrogen peroxide oxidation,[12] [13] and nitric acid oxidation[14]; (2) electrocatalytic oxidation[15]; and (3) photocatalytic oxidation.[16] The first two are conventional oxidation processes for alkyl arenes while the last one is a newly developed green oxidation technique. Among other things, [Fig. 1] illustrates the conventional oxygenation synthesis of alkylated arenes. Since 1980, the oxidation of aromatic hydrocarbons using air or oxygen in the liquid phase has received more and more attention, and it can be classified into three categories: solvent-free, and acid or alkali liquid-phase oxidation methods. The alkali and solvent-free method is very promising because of its high selectivity and lack of the drawbacks of the acid method. The acid method has the advantages of high yields, a simple post-treatment process, low production costs, and the product precipitated by crystals, while a serious drawback is the use of organic acid as a solvent affecting the corrosion of the equipment. Although solvent-free and alkaline methods avoid the problems of the acid procedures, the selection of suitable catalytic systems with a high selectivity is very challenging.

Light is the ideal energy source for green synthetic chemicals due to its accessibility and safety. As a gentle method, it has been widely used to construct organic molecules. In 1967,[17] Akira Fujishima of the University of Tokyo (known as the “father of photocatalysis” and the discoverer of “photocatalyst”) and his mentor Kenichi Hondo jointly discovered that water molecules on the surface of the titanium dioxide (TiO₂) electrode photolyzed—that is, broke down into hydrogen and oxygen—when exposed to ultraviolet light.[18] [19] This discovery opened a new chapter in photocatalysis research, followed by significant advances in photoinduced catalytic synthesis of organic compounds over the past few decades.

[Table 1] summarizes the benefits and drawbacks of the main oxidation modes. Additionally, coupling with flow chemistry is a popular subject, and its use in oxidation reactions can effectively address the risk issue and provide a safer guarantee for oxidation reactions.

Aromatic Branched-chain Oxidation to Alcohols

In 2022, Long and collogues reported the synthesis of phenols by electrochemical C–H hydroxylation of arenes in a continuous flow ([Fig. 2]).[20] The method has a broad scope (compatible with arenes spanning diverse electronic properties) and is performed under mild conditions without any catalysts or chemical oxidants. Among them, a graphite anode (exposed electrode area = 10 cm²) and a Pt cathode plate are separated by 0.15 mm, at a constant current of 61 mA, pumping 1-ethyl-4-methoxybenzene at 0.4 mL/min. 1-Methoxy-4-(trifluoromethyl)benzene was chosen as a model substrate for reaction optimization. The desired phenol was obtained in 83% yield in less than 22 seconds. They revealed that the substrates with relatively rich electronic properties afforded good yields. Furthermore, continuous production of phenol at 1 mol quantity was achieved, further suggesting the synthetic utility of the procedure.

The next year, the team performed highly site-selective monooxygenation of benzylic C(sp³)-H bonds using continuous-flow reactors ([Fig. 3]). The method, which does not require catalysts or chemical oxidants, separated graphite anode sheet (exposed electrode area = 10 cm²) from Pt cathode sheet by 0.15 mm, at a constant current of 6.1 mA, pumping 1-ethyl-4-methoxybenzene at 0.4 mL/min. As a result, 1-(4-methoxyphenyl)ethanol was obtained at a residence time of 22 seconds, with a yield of 86%.[21]

To prepare the corresponding alcohol compounds, Zhu et al created a photocatalyst based on g-C₃N₄ doped Co ions that can activate molecular oxygen and oxidize the benzylic C(sp³)-H bond of alkyl aromatic hydrocarbons at ambient temperature.[22] The catalyst favors high selective conversion under both UV and visible light conditions and is suitable for a wide range of light sources ([Fig. 4]). The product can be prepared in the batch reaction system without the need for an extra solvent and free radical initiator. No oxygen atmosphere was needed in continuous-flow and fixed-bed devices. Using cumene (CM) as a substrate, the selectivity of 2-phenyl-2-propanol (BP) was improved from 89.3 to 96.8% and 97.7%, respectively, and the reaction time was reduced from 9 hours to 100 and 165 minutes. This study also illustrates the benefits of highly active carbon nitride–based photocatalysts for environment-friendly and sustainable chemistry.

Patrick's group reported the creation of a novel mesofluidic flow oxygenation method for benzylic C–H oxidation ([Fig. 5]).[23] Activation of the photocatalyst using a UV lamp generates a radical on the substrate's benzylic position. Singlet oxygen (¹O₂) was incorporated into the radical intermediate to create ketone or alcohol. This technique works well and has a variety of substrates. Notably, when oxidation occurs selectively on benzylic sites without over-oxidizing the heterocyclic atoms, the necessary carbonyl or hydroxyl derivatives are produced. A flow process makes possible a more productive and sustainable protocol. By reducing the time and increasing substrate concentration attained in the flow, it is possible to achieve scalability to reach chemical levels that could be challenging to produce in the batch. It has also been successfully applied to the synthesis and late-stage modification of bioactive compounds, as demonstrated by the production of a single hydroxyl regioisomer (a nonsteroidal anti-inflammatory analgesic medication) from ibuprofen in a good yield (55%) of oxidation.

The tube-in-tube reactor was also used for biocatalytic production of 3-phenylcatechol from 2-hydroxybiphenyl, which was catalyzed by 2-hydroxybiphenyl 3-monooxygenase ([Fig. 6]). Formate dehydrogenase was added for cofactor recycling to convert sodium formate to carbon dioxide.[24] However, high substrate loadings can be achieved by using an organic liquid feed containing substrate and an aqueous stream consisting of enzymes, cofactor, and sodium formate. Under optimized conditions, a productivity of approximately 18 g • L⁻¹ h⁻¹ of the desired catechol was achieved, which is 38 times higher than that of the conventional batch reactions.

Oxidation of Aromatic Branched Chains to Aldehydes or Ketones

Recently, Zhang and coworkers systematically studied continuous aerobic oxidation of ethylbenzene to acetophenone over homogeneous and heterogenous NHPI in a micro-packed bed reactor.[25] The microreactor platform provided an enhanced gas–liquid mass transfer, enabling multiphase oxidation under kinetic control ([Fig. 7]). Compared with the conventional batch reactor, the space-time yield (STY) of oxidation is increased by 2 orders of magnitude. When the liquid flow rate is 0.1 mL/min, the gas flow rate is 10 mL/min, the selectivity is 90.1%, the conversion rate is 93%, and the STY value reaches 2.33 × 10⁵ mol /(L • h • kg).

Continuous-flow reactors can be used in a wide variety of hazardous processes. Chemists have begun to design flow systems that even allow new biocatalytic reactions to occur. Chapman et al decomposed H₂O₂ in the presence of galactose oxidase to produce a quantitative amount of oxygen for the oxidation of benzyl alcohol with a yield of 92% at a residence time of 8 minutes ([Fig. 8]).[26] The advantage of this scheme is that, by using the same flow-through system, incompatible enzymes can work together, which provides a new research line for catalytic oxidation. Gutmann et al investigated the oxidation of ethylbenzene with hydrogen peroxide and molecular oxygen catalyzed by cobalt and bromide ions in acetic acid ([Fig. 9]).[27] When hydrogen peroxide was used for oxidation, a mixture of products, including ethylbenzene hydroperoxide, acetophenone, 1-phenylethanol, and 1-phenylethyl acetate, was produced. Ethylbenzene and reaction intermediates were not fully converted to acetophenone. However, when atmospheric oxygen was used for oxidation, no catalyst deactivation was observed. Ethylbenzene was oxidized to acetophenone at 80°C with 74% selectivity at 150 minutes. Reaction conditions are converted with a tubular gas–liquid reactor to a continuous-flow process, which typically provides superior mass transfer characteristics and prevents oxygen depletion during the initial stages of rapid oxidation. A continuous-flow scheme allows the reaction temperature to be increased to 110 to 120°C, thereby reducing the reaction time to only 6 to 7 minutes without affecting the reaction selectivity.

A novel method for the catalytic oxidation of o-chlorotoluene to o-chlorobenzaldehyde has also been investigated. In 2020, Yang et al reported the liquid phase selective oxidation of o-chlorotoluene (OCT) to o-chlorobenzaldehyde (OCBD) using oxygen as an oxidant and cobalt acetate/manganese acetate/potassium bromide system as catalysis, and the reaction medium was acetic acid doped with a small amount of water ([Fig. 10]).[28] The reaction parameters can be easily controlled by performing the reaction in a microchannel reactor. Under the conditions of 0.88 mol% catalyzing substrate concentration of 2.39 mol/L, Co/Mn/Br molar ratio of 0.3/0.3/1, oxygen/substrate molar ratio of 5.5, reaction pressure of 0.5 MPa, and reaction temperature 150°C, OCT conversion was maintained at 10.3% and the selectivity to OCBD was as high as 71.8%.

Yun et al performed selective aerobic oxidation of benzylic sp³ C-H bonds to generate the corresponding ketones under continuous-flow conditions.[29] The reaction was driven by N-hydroxyphthalimide (NHPI) and tert-butyl nitrite (TBN) as catalysts under aerobic conditions ([Fig. 11]). The residence time of 54 seconds was 466-fold higher than the batch parallel reaction (7 hours), giving benzophenone in 87.9% yield.

Notably, recovery of catalysts and solvents (92.6 and 94.5%) and scale-up experiments (0.87 g/h run for 28 hours) of the oxidation of 1,2,3,4-tetrahydronaphthalene to α-tetralone proved the versatility and applicability of the scheme for large-scale production with a certain degree of cost control.

Quite recently, Bannon et al proposed a continuous-flow method for the aerobic photo-oxidation of benzyl substrates to ketones and aldehydes.[30] The process exploits UV-A LEDs (375 nm) in combination with a Corning AFR reactor that ensures effective gas–liquid mixing ([Fig. 12]). With an airflow rate of 24.8 mL/min, solvent flow rate of 1 mL/min, temperature of 50°C, and pressure of 14.4 bar, the conversion rate of benzophenone was 98% in 1 minute. Under the optimized conditions, the substrates were screened, obtaining a wide range of ketones. Overall, this continuous-flow approach offers several improvements over alternative oxidation methods due to the combined use of air as an oxidant and sodium anthraquinone-2 sulfonate (SAS) as a water-soluble photocatalyst. The use of greener and safer conditions as well as process-strengthening principles make this flow method attractive for further industrial applications.

Gary et al used a specially designed continuous-flow photochemical reactor to selectively photooxidize alkyl benzenes.[31] The reactor was fitted with a low-power UV light source and a fine bubble generator as an oxidizing agent in conjunction with sodium anthraquinone sulfonate, a water-soluble catalyst ([Fig. 13]). The creation of small bubbles accelerates the reaction and significantly increases the two phases' interaction efficiency. The efficiency of the air-based slug-flow system with fine bubbles was 1.4 times at a lower feed-flow rate of 2 mL/min, and 1.8 times at a higher feed-flow rate of 5 mL/min. Ethylbenzene was selectively oxidized to acetophenone in continuous flow at room temperature during a brief residence period of 5 minutes, with a conversion of ethylbenzene of 90% and 92% selectivity to acetophenone. Compressed air can be used as an oxidant instead of pure O₂ because of improved mass transfer and increased efficiency, thus alleviating the potential safety concerns and making the process amenable for scale-up.

Greene et al developed a continuous-flow process to establish a homogeneous Cu(I)/TEMPO catalyst system for the aerobic oxidation of primary alcohols to aldehydes ([Fig. 14]).[32] When 4-nitrobenzylic alcohol was used as a substrate, 4-nitrobenzaldehyde was achieved in 99% yield in the presentence of 5 mol% of Cu(OTf)/bpy, 10 mol% of N-methylimidazole (NMI), and 5 mol% of TEMPO in a 5-minute residence period. Additionally, a 100-g scale scaling experiment was conducted using benzyl alcohol as the model, resulting in 99% quantitative formation of benzaldehyde after 5 minutes of residence time, which was successfully allowed to remain for 24 hours. The findings offer a crucial basis for the application of flow-based, large-scale aerobic oxidation processes in pharmaceutical process chemistry.

Naik et al reports a straightforward and safe continuous-flow oxidation procedure that selectively converts primary and secondary alcohols into the corresponding ketone and aldehyde compounds using catalytic quantities of TEMPO in a biphasic solvent system in the presence of sodium hypochlorite and sodium bromide ([Fig. 15]).[33] The entire experimental apparatus was supplied by two peristaltic pumps, and the reaction was performed through a coil with a static mixer at 0.32 equiv. NaOCl, 20°C, and 7.5 minutes of retention time. For online quenching of the reaction 10% Na₂S₂O₃ was used. In this case, a trifluoromethylated oxazole building block and a precursor to the anti-HIV drug maraviroc were scaled up, and the desired phenylpropyl aldehyde was synthesized in 93% isolated yield.

Using supported photocatalysts on silica, Blanchard et al combined a continuous-flow reactor with a high-performance LED array to achieve a uniform distribution and the most efficient use of light energy ([Fig. 16]).[34] Compared with the reactor reaction mode, the reaction does not require expensive high-pressure equipment and reduces light energy loss and carbon emission for the photooxidation reaction of 1,5-dihydroxynaphthalene. The continuous-flow mode improves the reaction efficiency by 24 times with an STY value of 5.35 g/(L • h).

Aromatic Branched Chain Oxidation to Acid

A metal-free-catalyzed oxidation process of 4-(methylsulfonyl)-2-nitrotoluene for the production of 4-(methylsulfonyl)-2-nitrobenzoic acid has been developed by Su's group in 2022 ([Fig. 17]).[35] The process uses molecular oxygen as an oxidant, nitric acid as a promoter, and N,N′,N″-trihydroxyisocyanuric acid (THICA) as a catalyst. The residence time, reaction temperature, and oxygen/substrate molar flow ratio were optimized to give a reaction conversion of 90%, a selectivity of 97%, and a residence time of 50 minutes. The use of continuous-flow technology gives a decent yield and selectivity and is safer and more environment friendly than the batch oxidation method.

In the following year, the team established a continuous-flow process from OCT to o-chlorobenzoic acid (OCBA), using pure oxygen as the oxidant, acetic acid as a cosolvent, and CoBr₂/MnBr₂ as a catalyst; by creating a slug flow, the reaction was rapidly stimulated ([Fig. 18]).[36] The reaction parameters can be easily controlled using the continuous-flow reactor, and 90% conversion of OCBA was achieved at a residence time of 15 minutes. The isolated yield of OCBA was up to 94%. Shorter residence times, higher product yields, and operational safety can be achieved using a simple continuous-flow system, as compared with conventional batch processes.

Prieschl et al presented a continuous-flow reactor for the oxidation of aldehydes to carboxylic acids using an in situ–generated performic acid ([Fig. 19]).[37] This low-molecular-weight, high-performance acid is an environment-friendly and inexpensive oxidizer that can be easily produced in situ from formic acid and hydrogen peroxide. This eliminates the safety hazards associated with handling this potentially explosive reagent, and the product is easily separated after the reaction by simple decompression distillation. The reactor has an effective volume of 12 mL, a backup pressure of 5 bar, a residence time of 20 minutes, and a temperature of 100°C. Both the yield and conversion rate are up to 99%.

Guo et al reported a continuous flow process for the preparation of 2,4-dichloro-5-fluorobenzoic acid (BA).[38] They chose 2,4-dichloro-5-fluoroacetophenone (AP) as the starting material and acetic acid as cosolvent, and achieved excellent results in the continuous-flow oxidization system ([Fig. 20]). BA nitric acid oxidation is a violent exothermic reaction. However, a continuous-flow system with advantages such as good mass and heat transfer ensures the safety of the reaction. With the optimal reaction conditions, 100% yield was obtained at 70°C for 9 minutes. Compared with conventional batch reactions, a continuous-flow system achieves lower nitric acid consumption, higher product yield, shorter reaction time, environment-friendliness, and process continuity, thus ensuring higher operational safety.

Vanoye et al reported a safe, straightforward, and atom-economic approach for the oxidation of aliphatic aldehydes to the corresponding carboxylic acids within a continuous-flow reactor, using pure oxygen as the reaction's oxygen source.[39] The reaction is typically performed at room temperature using 5 bar oxygen in PFA tubing without additional catalysts or radical initiators. A steady Taylor flow is produced by varying the rate ratio of the two materials. The reaction was examined in real time using online GC analysis ([Fig. 21]). The residence time was 17.4 minutes with 95% conversion and 98% selectivity. A catalytic quantity of an Mn(II) catalyst is occasionally added. The benefits given by continuous-flow technology make the procedure an effective substitute for conventionally catalyzed aerobic processes.

Photosensitizer 2-tert-butylanthraquinone (2-t-Bu-AQN) was used to oxidize aromatics in a glass continuous-flow microreactor built by Itoh's group ([Fig. 22]).[40] By evenly applying a particular wavelength of light on the market's surface—a glass chip microreactor—a particular chemical can be oxidized. There is something special about the glass continuous-flow microreactor. Internal circulation within each slug accelerates the mixing of the liquid and gas phases, increasing the response pace in slug flow. Benzoic acid was synthesized with a yield of 48% in 2 hours and p-tert-butylbenzoic acid with a yield of 83% yield in 2 hours under the conditions of 5 μL/min, 0.1 MPa of oxygen flow, and a 375-nm LED. The photosensitizer used can be synthesized and recycled, resulting in a certain cost savings.

The Aromatic Branch Chain Oxidizes to Other Compounds

Kabeshov et al described an atom-economy continuous-flow electrooxidation approach to benzamide without further reagents.[41] The substrate and electrolyte were added to the solvent, and they were moved at a rate of 0.5 mL/min to the coil's electrolytic cell. The reaction was monitored by in-line UV. After adding methanesulfonic acid, N-(4-(tert-butyl)benzyl)acetamide was obtained from p-tert-butyltoluene with a yield of 72% ([Fig. 23]). Investigations on the stability of the process revealed that the process yielded 9.12 g of amide per day.

According to an overview of the role of flow technology in preparing indole derivatives provided by Luisi's group, the derivatization of indole rings in the field of photosynthesis is achieved by connecting two reaction units in series.[42] There is a 34-W LED light in the center of the reaction unit ([Fig. 24]), which is encircled by fluorinated ethylene propylene (FEP). When exposed to light with methylene blue as a photosensitizer and oxygen as an oxidant, indoles were obtained in 60 to 99%. This process transforms various indole derivatives in 1 to 3 hours as opposed to the 24 hours required by the batch technique.

Conclusion

The continuous-flow process for various oxidation reactions described above, in addition to improving the reaction rate while maintaining intrinsic safety, addresses issues of spray material, local hot spots in the kettle, harsh reaction conditions, and poor reaction selectivity that exist in the conventional kettle oxidation process. There are numerous instances of photooxidation described previously, which is unique due to its low energy use, great selectivity, and mildness. In recent years, photocatalysis has garnered widespread attention due to its benign reaction conditions and environmental friendliness; however, the system's low catalytic efficiency and inefficient use of visible light still plague it. Notably, photoredox has also been used to methylate C(sp³)-H bonds in alkanes,[43] further demonstrating the great potential of photoredox and its wide range of applications. With the development of photocatalytic technology, the oxidation of aromatic hydrocarbons will be a successful, environment-friendly, and promising technique with a variety of applications.

Conflict of Interest

None declared.

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Publication History

Received: 10 October 2024

Accepted: 04 March 2025

Article published online:
22 April 2025

© 2025. The Author(s). This is an open access article published by Thieme under the terms of the Creative Commons Attribution License, permitting unrestricted use, distribution, and reproduction so long as the original work is properly cited. (https://creativecommons.org/licenses/by/4.0/)

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