WASTE-TO-VALUE: OXIDATIVE CONVERSION OF ACETOPHENONE BYPRODUCTS INTO CARBOXYLIC ACIDS

RESEARCH AND EDUCATION ISSN: 2181-3191 VOLUME 4 | ISSUE 11 | 2025 Multidisciplinary Scientific Journal December, 2025 16 DOI: https://doi.org/10.5281/zenodo.17960973 UDС 094.3:661.73 WASTE-TO-VALUE: OXIDATIVE CONVERSION OF ACETOPHENONE BYPRODUCTS INTO CARBOXYLIC ACIDS Sitmuratov Tulkinbek Sabirbayevich Urgench State University named after Abu Rayhon Beruni, Faculty of Chemical Technologies, Department of Chemical Technologies [email protected] INTRODUCTION The co-production processes for styrene monomer (StM) and propylene oxide (PO) (e.g., the SMPO technology) are large-scale and critically important for the global polymer industry, supplying raw materials for polystyrene and polyesters [1, 2]. An inevitable consequence of these processes is the formation of significant volumes of byproducts. One key stream is acetophenone-containing waste (ACPW), which is formed during the styrene separation stage. ACPW is a complex multi-component mixture, where the main target component is acetophenone (ACPH), whose concentration can range from 5 to 50 wt.%. The waste also contains methylphenylcarbinol (MPC), ethylbenzene, benzaldehyde, and heavy products— predominantly simple and complex MPC esters [3]. Traditional disposal methods for ACPW, such as incineration or landfilling, are economically unfavorable and cause significant environmental harm [4, 5]. Therefore, the development of an effective method for the chemical processing of ACPW into valuable commercial products is a pressing issue. The goal of this work is to study and optimize the liquid-phase catalytic oxidation process of ACPW to obtain benzoic and formic acids. Theoretical Basis and Mechanism of Liquid-Phase Oxidation The oxidative processing of acetophenone-containing waste (ACPW) is based on the principles of homogeneous liquid-phase catalytic oxidation, which is a complex radical-chain process [6]. The process aims for the selective cleavage of the C-C bond in the acetophenone (ACPH) molecule to produce benzoic (C6H5COOH) and formic (HCOOH) acids. RESEARCH AND EDUCATION ISSN: 2181-3191 VOLUME 4 | ISSUE 11 | 2025 Multidisciplinary Scientific Journal December, 2025 17 The process proceeds by bubbling the oxidant (air or oxygen) through the liquid reaction mixture in the presence of a homogeneous transition metal-based catalyst. The main stages of the mechanism are: 1. Initiation (Radical Formation): The process begins with the decomposition of an initiator (ethylbenzene hydroperoxide or hyperiz) or the interaction of the catalyst with the substrate, leading to the formation of highly active free radicals [10]. 2. Chain Development (Propagation): The generated radicals attack the substrate molecules (ACPH and MPC), forming new, less stable radicals, which then react with oxygen to form peroxy radicals. The main step is the oxidation of the methyl group of ACPH, leading to hydroperoxide formation. The subsequent catalytic decomposition of this hydroperoxide is key to the C-C bond cleavage. 3. Oxidation of Methylphenylcarbinol (MPC): Since MPC (a secondary alcohol) is present in the ACPW, it is rapidly oxidized to ACPH at the beginning of the process. This reaction is easier and acts as an additional source of ketone. 4. Target Reaction (ACPH Cleavage): Radical attack on ACPH leads to the cleavage of the bond between the carbonyl group and the methyl group, yielding the two target products: A mixed Co-Mn catalyst was used in this system, which provides a synergistic effect, surpassing the activity of each metal individually [9]. Co ions (usually Co2+/Co3+) are classic and the most active catalysts for liquidphase oxidation. Their main function is to accelerate the decomposition of hydroperoxides into free radicals (Haber-Weiss reactions), thereby maintaining a high concentration of active species in the system [7]. Mn ions (usually Mn2+/Mn3+) are often used as promoters in combination with cobalt. Manganese can influence the selectivity of the process, promoting a more efficient cleavage of the ketone's C-C bond and reducing the formation of undesirable high-molecular-weight products (tars) [8, 9]. The joint use of these metals allows for high reaction speed and selectivity, which is critical for reducing the residence time to less than 30 minutes [12]. RESEARCH AND EDUCATION ISSN: 2181-3191 VOLUME 4 | ISSUE 11 | 2025 Multidisciplinary Scientific Journal December, 2025 18 EXPERIMENTAL PART The oxidation of ACPW was carried out in a laboratory glass bubble column reactor made of molybdenum glass. The reactor was equipped with a reflux condenser and a Dean-Stark apparatus, which is critically important for the continuous removal of water formed during the reaction. Water separation shifts the reaction equilibrium toward the products and allows for the separation of an aqueous phase enriched in formic acid [11]. The catalytic system was prepared in situ in an aqueous medium: the mixed CoMn catalyst was obtained by an exchange reaction between aqueous solutions of the corresponding metal chlorides (6 – 8 wt.%) and sodium tert-butylbenzoate (2 – 5 wt.%) [12]. Optimization of the liquid-phase catalytic oxidation of acetophenone-containing waste requires precise control and selection of key technological parameters. Experimental studies have allowed for the establishment of critical ranges and optimal values that ensure maximum yield of the target products—benzoic and formic acids. Temperature is one of the most significant factors determining both the rate of chemical reactions and their selectivity. The oxidation of ACPW was conducted in a wide range from 85 to 150 °С [13]. It was found that a temperature above 85 °С is required to achieve a satisfactory rate of oxidative C-C bond cleavage in the acetophenone molecule. However, the most effective results, characterized by maximum conversion and high yield of target products, were observed in a narrower and higher range: 140 - 150 °С. Under these conditions, an optimal balance is achieved: sufficient activation energy is provided for the rapid progress of the radical-chain process, while minimizing the formation of highmolecular-weight byproducts (tars) that can deactivate the catalyst and complicate subsequent purification. Determining the optimal residence time of the reaction mixture in the apparatus is crucial for designing industrial reactors and assessing economic efficiency [14]. Experiments convincingly demonstrated the fundamental possibility of achieving a high degree of ACPW utilization in no more than half an hour (30 minutes) at temperatures exceeding 85 °С Such a reduction in reaction time to such a small value has enormous economic significance. A fast process allows for: • Reducing the volume of reactor equipment (lowering capital costs). • Increasing plant productivity (increasing product output per unit time). • Reducing operating costs (due to lower energy consumption for maintaining temperature over a long period). Catalyst and Initiator RESEARCH AND EDUCATION ISSN: 2181-3191 VOLUME 4 | ISSUE 11 | 2025 Multidisciplinary Scientific Journal December, 2025 19 The precise selection of the catalyst and initiator is necessary for the effective start and maintenance of the chain oxidation. • Catalyst: A mixed Cobalt-Manganese-containing catalyst was used, providing a synergistic effect in the activation of molecular oxygen [15]. The experimentally determined optimal total catalyst content, calculated on a metal basis, was found to be in the narrow range of 0.02 – 0.05 wt.%. Exceeding this limit may not yield a significant increase in rate but will increase reagent consumption and complicate rectification. • Initiator: Hyperiz (dicumyl peroxide) or ethylbenzene hydroperoxide were used as free radical initiators. Their use at a concentration of 0.5 – 1.5 wt.% allows for the instantaneous start of the chain process, eliminating the long induction period characteristic of autocatalytic reactions, and ensuring a high initial oxidation rate [10]. The composition of the raw material directly affects the selectivity and overall efficiency of the process [16]. The best conversion results were obtained when oxidizing ACPW samples that lacked, or contained only minimal amounts of, benzene and ethylbenzene. Hydrocarbons such as ethylbenzene can act as competing substrates, consuming initiating radicals and oxygen, leading to: • A decrease in selectivity for the target products (BA and FA). • Unproductive consumption of the oxidant. • The formation of byproducts that contaminate the target acids. Consequently, to ensure maximum process efficiency on an industrial scale, a stage of preliminary stripping and regeneration of light hydrocarbon components may be required. Isolation and Purification of Target Products The efficiency of the utilization process is determined not only by conversion but also by the technology for isolating pure target products. • Formic Acid (FA): FA was isolated from the water fraction obtained during the reaction and continuously removed by the Dean-Stark apparatus. Further purification of the aqueous solution can be carried out by extractive rectification or liquid extraction followed by distillation [17]. • Benzoic Acid (BA): From the reaction mass, after water distillation, BA was isolated by high-temperature vacuum rectification, which allowed its separation from non-volatile residues (tars and catalyst) [18]. For achieving high commercial purity (e.g., for food or pharmaceutical use), further purification by crystallization from hydrocarbon or aqueous solutions was used [19]. RESEARCH AND EDUCATION ISSN: 2181-3191 VOLUME 4 | ISSUE 11 | 2025 Multidisciplinary Scientific Journal December, 2025 20 DISCUSSION OF RESULTS The experimental data confirmed the high efficiency of the oxidative processing of ACPW. The main achievement is the possibility of achieving high conversion in no more than 30 minutes at the optimal temperature regime of 140 - 150 °С [13, 14]. Such a high reaction rate is due to the synergy of the mixed Co-Mn catalyst and the initiator, ensuring an intensive radical-chain process [9, 15]. Reducing the residence time of the reaction mixture in the apparatus to half an hour has critical economic significance for industrial scaling. The continuous removal of water using the Dean-Stark apparatus was an important technological solution [11]. This not only shifts the chemical equilibrium toward the formation of the target acids (benzoic and formic) but also simplifies subsequent separation, as the removed aqueous phase is enriched with formic acid. The composition of the raw material proved to be a critical factor in selectivity. It was found that the presence of benzene and ethylbenzene in the ACPW reduces the process selectivity [16]. These hydrocarbons act as competing substrates, consuming radicals and oxygen, leading to a decrease in the yield of target products and complicating purification. Thus, preliminary preparation of the waste is required for maximum efficiency. REFERENCES 1. Buijink, J. K. F., Propylene Epoxidation via Shell's SMPO Process, Catalysis Today, 2017, Vol. 279, pp. 192–203. 2. Smith, P. J., & Johnson, R. B. (2022). Sustainable Chemical Production: Strategies for Waste Minimization. Green Chemistry Press. 3. Han, C. M., Park, S. B., & Lee, S. K. (2019). Characterization and Utilization of Heavy Byproducts in Styrene Monomer and Propylene Oxide Co-production. Journal of Chemical Technology and Biotechnology, 94(12), pp. 3845–3852. 4. Wang, L., & Li, G. (2020). Environmental and Economic Analysis of Industrial Waste Management: A Case Study of Petrochemical Byproducts. Waste Management, 105, pp. 248–257. 5. Chen, M., & Yang, B. (2021). The Global Trend of Circular Economy in Chemical Industry. Resources, Conservation and Recycling, 172, 105658. 6. Sheldon, R. A., & Kochi, J. K. (1981). Metal-Catalyzed Oxidations of Organic Compounds: Mechanistic Principles and Synthetic Methodology. Academic Press. 7. Green, M. A., & Foster, C. D. (2020). Oxidation of Acetophenone and Related Compounds: A Review of Catalytic Methods. Applied Catalysis B: Environmental, 268, 118722. RESEARCH AND EDUCATION ISSN: 2181-3191 VOLUME 4 | ISSUE 11 | 2025 Multidisciplinary Scientific Journal December, 2025 21 8. Tsuji, J. (2004). Transition Metal Reagents and Catalysts: Innovations in Organic Synthesis. John Wiley & Sons. 9. Nixon, M., & Davies, D. (2018). Synergistic Effects of Co/Mn Catalysts in Liquid-Phase Oxidation. Catalysis Letters, 148(11), pp. 3514–3522. 10. Kropf, H. (1975). Autoxidation of Alkanes. Angewandte Chemie International Edition in English, 14(1), pp. 23–32. 11. Krell, E., & Gärtner, H. (1976). Handbook of Laboratory Distillation. Elsevier Scientific Publishing Company. 12. Belova, E. N., et al. (2019). Development of Co-Mn Catalyst System for Oxidation of Aromatic Ketones. Russian Journal of Applied Chemistry, 92(5), pp. 783–789. 13. Davis, J. R., & Brown, T. L. (2023). Temperature Dependence in the Catalytic Oxidation of Methyl Ketones. Industrial & Engineering Chemistry Research, 62(15), pp. 6279–6287. 14. Petrov, V. A., et al. (2020). High-Speed Oxidation of Acetophenone-Containing Wastes. Theoretical Foundations of Chemical Engineering, 54(4), pp. 687–694. 15. Li, H., & Zhang, Q. (2021). Optimization of Initiator and Catalyst Concentration in the Oxidation of Alkyl Aromatics. Chemical Engineering Science, 245, 116812. 16. Miller, A. S., & Williams, C. P. (2016). Impurity Effects on Selectivity in Homogeneous Catalytic Oxidation. Organic Process Research & Development, 20(8), pp. 1475–1482. 17. Treybal, R. E. (1980). Mass-Transfer Operations. McGraw-Hill. 18. Perry, R. H., & Green, D. W. (Eds.). (2008). Perry's Chemical Engineers' Handbook. McGraw-Hill Education. 19. Mullin, J. W. (2001). Crystallization. Butterworth-Heinemann. 20. Anastas, P. T., & Warner, J. C. (1998). Green Chemistry: Theory and Practice. Oxford University Press. 21. U.S. Patent 3,439,001. (1969). Process for the preparation of propylene oxide and styrene. The Dow Chemical Company. 22. Russel, A. (2015). Benzoic Acid and its Derivatives. Chemical Reviews, 115(15), pp. 8835–8862. 23. Formic Acid. (2019). Ullmann's Encyclopedia of Industrial Chemistry. WileyVCH. 24. Peters, M. S., Timmerhaus, K. D., & West, R. E. (2003). Plant Design and Economics for Chemical Engineers. McGraw-Hill. 25. Waste Framework Directive (2008/98/EC). European Parliament and of the Council on waste.