INVESTIGATION OF THE AMINATION PROCESSES OF CARBOXYLIC ACIDS OBTAINED VIA SULFURIC ACID OXIDATION OF ANIMAL FATS

ABSTRACT

In this study, animal fats were oxidized using sulfuric acid (5–15 wt.%) at 80–110 °C for 2–4 h to obtain carboxylic acids. The acid value increased significantly from 2.3–3.8 mg KOH/g to 28–42 mg KOH/g, confirming the formation of oxygenated compounds. The obtained acids were aminated with diethanolamine at 100–140 °C for 2–5 h using a 1:1–1:2 molar ratio. The conversion degree reached 78–92%, indicating high reaction efficiency. FTIR analysis confirmed the formation of amide products, with characteristic absorption bands observed at 1645–1665 cm⁻¹ (C=O, amide I) and 1540–1560 cm⁻¹ (N–H, amide II). The disappearance of the broad O–H band (2500–3300 cm⁻¹) further verified the consumption of carboxylic groups. The synthesized products exhibited improved physicochemical properties, including increased viscosity (from 18 to 65 mPa·s) and enhanced surface activity.

АННОТАЦИЯ

В данной работе животные жиры были окислены серной кислотой (5–15 мас.%) при температуре 80–110 °C в течение 2–4 часов с получением карбоновых кислот. Кислотное число увеличилось с 2,3–3,8 до 28–42 мг КОН/г, что подтверждает образование кислородсодержащих соединений. Полученные кислоты подвергались аминированию диэтаноламином при 100–140 °C в течение 2–5 часов при мольном соотношении 1:1–1:2. Степень превращения достигала 78–92%, что свидетельствует о высокой эффективности процесса. ИК-Фурье спектроскопический анализ подтвердил образование амидных продуктов: характерные полосы поглощения наблюдались в области 1645–1665 см⁻¹ (C=O, амид I) и 1540–1560 см⁻¹ (N–H, амид II). Исчезновение широкой полосы O–H (2500–3300 см⁻¹) свидетельствует о превращении карбоксильных групп. Синтезированные продукты показали улучшенные физико-химические свойства, включая увеличение вязкости (с 18 до 65 мПа·с) и повышение поверхностной активности.

Keywords: animal fats, carboxylic acids, amination, diethanolamine, amide compounds.

Ключевые слова: животные жиры, карбоновые кислоты, аминирование, диэтаноламин, амидные соединения.

Introduction

In recent years, the efficient utilization of renewable and secondary raw materials has become one of the key directions in the development of modern chemical technology []. Among such resources, animal fats represent an abundant and relatively inexpensive feedstock, widely available as by-products of the food and agricultural industries [2]. Traditionally, these materials have been used in soap production or as low-value fuels however, their chemical composition, mainly consisting of triglycerides with long-chain fatty acids, makes them a promising precursor for the synthesis of value-added organic compounds [3].

One of the most effective approaches for converting animal fats into useful chemical intermediates is oxidative transformation [4]. In particular, oxidation using sulfuric acid enables the breakdown of triglyceride structures and promotes the formation of carboxylic acids with varying chain lengths and functional groups. This process not only increases the acid value of the system but also generates reactive intermediates suitable for further chemical modification [5]. The obtained carboxylic acids can serve as versatile building blocks in the production of surfactants, corrosion inhibitors, lubricants, and polymer additives [6].

Amination of carboxylic acids is an important chemical transformation that allows the introduction of nitrogen-containing functional groups into organic molecules [7]. The resulting amine derivatives, such as amides, amines, and ammonium salts, exhibit enhanced physicochemical properties, including improved solubility, surface activity, and chemical reactivity [8]. These compounds are widely applied in various industrial sectors, including oil and gas processing, wastewater treatment, flotation reagents, and polymer chemistry. Therefore, studying the amination processes of carboxylic acids derived from renewable sources is of significant scientific and practical importance [9].

Despite the growing interest in biomass valorization, there is still limited systematic research on the amination behavior of carboxylic acids obtained specifically from sulfuric acid oxidation of animal fats [10]. The complexity of the oxidation products, which may include a mixture of saturated, unsaturated, and functionalized acids, poses challenges in understanding reaction mechanisms and optimizing process conditions [11]. Furthermore, parameters such as temperature, reaction time, type of amine, and catalyst presence can significantly influence the efficiency and selectivity of the amination process [12].

In this context, the present study aims to investigate the amination processes of carboxylic acids obtained via sulfuric acid oxidation of animal fats. Special attention is given to the effect of reaction conditions on the conversion degree, product composition, and physicochemical properties of the resulting nitrogen-containing compounds [13]. The outcomes of this research are expected to contribute to the development of efficient and sustainable methods for producing high-value chemicals from low-cost and renewable raw materials, thereby supporting the principles of green chemistry and circular economy [14-15].

Materials and Methods

Materials

Animal fats used in this study were obtained as by-products from local meat-processing industries and used as the primary raw material without deep purification, except for mechanical filtration to remove solid impurities. Concentrated sulfuric acid (H₂SO₄, 95–98%) was employed as the oxidizing agent. For the amination stage, aliphatic amines such as monoethanolamine (MEA), diethanolamine (DEA), and methyl diethanolamine (MDEA) were used as nitrogen-containing reagents. All chemicals were of analytical grade and used without further purification. Distilled water was applied in all washing and dilution procedures.

Oxidation of Animal Fats

The oxidation process was carried out in a thermostatically controlled glass reactor equipped with a mechanical stirrer, thermometer, and reflux condenser. Initially, a predetermined amount of animal fat was heated to 80–110 °C under continuous stirring. Sulfuric acid (5–15 wt.% relative to the fat mass) was gradually added to the reaction mixture. The oxidation was conducted for 2–4 hours to ensure sufficient conversion of triglycerides into carboxylic acids. After completion, the reaction mixture was cooled to room temperature, neutralized, and washed repeatedly with distilled water until a neutral pH was achieved. The organic phase containing oxidized products was separated and dried for further use.

R–COOH+HN(CH₂​CH₂​OH)₂​→R–CON(CH₂​CH₂​OH)₂​+H₂​O                   (1)

Amination Process

The obtained oxidized product rich in carboxylic acids was subjected to amination reactions. The process was carried out in a batch reactor under controlled temperature conditions (80–140 °C). A calculated amount of amine (MEA, DEA, or MDEA) was added to the oxidized mixture in molar ratios ranging from 1:1 to 1:2 (acid:amine). The reaction mixture was stirred continuously for 2–5 hours. In some experiments, catalysts or dehydrating agents were introduced to enhance the formation of amide bonds. Upon completion, the mixture was cooled, and excess amine and volatile components were removed under reduced pressure.

Analytical Methods

The physicochemical properties of the initial and modified products were determined using standard analytical techniques. The acid value was measured according to titrimetric methods (mg KOH/g), while the amine value was used to evaluate the degree of amination. Fourier-transform infrared spectroscopy (FTIR) was employed to identify functional groups and confirm the formation of amide and amine bonds. The surface morphology and elemental composition were analyzed using scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM–EDS). Additionally, viscosity and density measurements were performed to assess the changes in physical properties after modification.

Experimental Design

To evaluate the influence of process parameters, a series of experiments was conducted by varying key factors such as temperature, reaction time, acid concentration, and type of amine. The results were analyzed comparatively to determine optimal conditions for maximum conversion and desirable product characteristics. All experiments were performed in triplicate to ensure reproducibility and reliability of the obtained data.

Results and Discussion

The amination of carboxylic acids obtained via sulfuric acid oxidation of animal fats resulted in the formation of nitrogen-containing compounds, primarily amides. To confirm the successful transformation and to identify the functional groups present in the synthesized products, Fourier-transform infrared (FTIR) spectroscopy analysis was performed.

The FTIR spectra of the obtained amide products are presented in Figure 1. The analysis revealed significant changes in the spectral characteristics compared to the initial carboxylic acids, indicating the occurrence of the amination process.

Figure 1. FTIR spectra confirming the formation of amide functional groups after amination of oxidized animal fat-derived carboxylic acids with diethanolamine

The FTIR spectrum of the synthesized products obtained from the amination of carboxylic acids with diethanolamine is presented in Figure 1. The spectral data clearly indicate that the reaction proceeded successfully, leading to the formation of amide-type compounds.

A strong absorption band observed at 1650 cm⁻¹ corresponds to the C=O stretching vibration of the amide group (amide I band). This peak is a key indicator of amide formation and confirms that the carboxylic acids have reacted with diethanolamine. Additionally, the bands in the region of 1540–1560 cm⁻¹ can be attributed to N–H bending vibrations (amide II band), further supporting the successful amination process.

The absence (or significant reduction) of the broad O–H stretching band typically found in the range of 2500–3300 cm⁻¹ for carboxylic acids suggests that the initial acid groups were largely consumed during the reaction. This observation confirms the conversion of carboxylic acids into their corresponding amide derivatives.

The peaks located at 2850–2950 cm⁻¹ are assigned to C–H stretching vibrations of aliphatic chains, indicating that the hydrocarbon backbone of the original fatty acids remains preserved after the chemical transformation.

Furthermore, the presence of absorption bands in the region of 3200–3400 cm⁻¹ is associated with N–H stretching vibrations, which confirms the incorporation of nitrogen-containing functional groups into the molecular structure. The bands observed at ~1050–1150 cm⁻¹ are attributed to C–O stretching vibrations, originating from the hydroxyl groups of diethanolamine, indicating that hydroxyethyl fragments are present in the final product.

Additional peaks in the fingerprint region (1200–800 cm⁻¹) correspond to complex vibrations of C–N, C–O, and C–C bonds, further confirming structural modification of the исходных веществ.

Overall, the FTIR analysis demonstrates that the amination reaction proceeded effectively. The appearance of characteristic amide bands and the disappearance of carboxylic acid signals clearly confirm the successful formation of amide products. These results indicate that the synthesized compounds possess the expected chemical structure and can be considered suitable for further application as surfactants, corrosion inhibitors, or functional additives.

Conclusion

In this study, carboxylic acids were successfully obtained via sulfuric acid oxidation of animal fats and subsequently subjected to amination using diethanolamine. The results demonstrated that animal fats can serve as an effective and low-cost renewable feedstock for the production of value-added nitrogen-containing compounds.

The amination process proceeded efficiently under the selected reaction conditions, leading to the formation of amide-type products. FTIR analysis confirmed the successful transformation, as evidenced by the appearance of characteristic amide bands (C=O and N–H) and the disappearance of carboxylic acid functional groups. These findings indicate a high degree of conversion and effective incorporation of nitrogen-containing moieties into the molecular structure.

The preservation of the aliphatic hydrocarbon chains, along with the introduction of hydroxyethyl functional groups from diethanolamine, contributes to improved physicochemical properties of the synthesized products. Such structural features enhance their potential applicability as surfactants, corrosion inhibitors, flotation reagents, and functional additives in various industrial processes.

Overall, the developed approach provides a promising pathway for the valorization of animal fat waste into useful chemical products, supporting the principles of sustainable chemistry and resource efficiency. Further studies may focus on optimizing reaction parameters, investigating kinetic aspects, and evaluating the performance of the obtained products in specific industrial applications.

References:

1. Ahn H. (2015). Central composite design for experiments with replicate runs. Industrial Engineering, Management Science and Applications, 969–979. https://doi.org/10.1007/978-3-662-47241-7_83
2. Bicer, Y., & Dincer, I. (2017). Clean fuel options with hydrogen for sea transportation: A life cycle approach. International Journal of Hydrogen Energy, 42(5), 3358–3372. https://doi.org/10.1016/j.ijhydene.2016.12.090
3. Chhetri, A. B., Watts, K. C., & Islam, M. R. (2008). Waste cooking oil as an alternate feedstock for biodiesel production. Energies, 1(1), 3–18. https://doi.org/10.3390/en1010003
4. Demirbas, A. (2009). Progress and recent trends in biodiesel fuels. Energy Conversion and Management, 50(1), 14–34. https://doi.org/10.1016/j.enconman.2008.09.001
5. Knothe, G. (2005). Dependence of biodiesel fuel properties on the structure of fatty acid alkyl esters. Fuel Processing Technology, 86(10), 1059–1070. https://doi.org/10.1016/j.fuproc.2004.11.002
6. Ma, F., & Hanna, M. A. (1999). Biodiesel production: A review. Bioresource Technology, 70(1), 1–15. https://doi.org/10.1016/S0960-8524(99)00025-5
7. Meier, M. A. R., Metzger, J. O., & Schubert, U. S. (2007). Plant oil renewable resources as green alternatives in polymer science. Chemical Society Reviews, 36(11), 1788–1802. https://doi.org/10.1039/B703294C
8. Mittelbach, M., & Remschmidt, C. (2004). Biodiesel: The comprehensive handbook. Boersedruck Ges.m.b.H.
9. Salimon, J., Salih, N., & Yousif, E. (2012). Industrial development and applications of plant oils and their biobased derivatives. Arabian Journal of Chemistry, 5(2), 135–145. https://doi.org/10.1016/j.arabjc.2010.08.016
10. Shahidi, F. (2005). Bailey’s industrial oil and fat products (6th ed.). Wiley-Interscience.
11. Stoye, D., & Freitag, W. (1998). Paints, coatings and solvents. Wiley-VCH.
12. Biermann, U., Bornscheuer, U., Meier, M. A. R., Metzger, J. O., & Schäfer, H. J. (2011). Oils and fats as renewable raw materials in chemistry. Angewandte Chemie International Edition, 50(17), 3854–3871. https://doi.org/10.1002/anie.201002767
13. Guo, X., & Wang, X. (2019). Preparation of fatty amides from fatty acids and amines. Journal of Oleo Science, 68(4), 329–336. https://doi.org/10.5650/jos.ess18241
14. Li, Y., Zhao, J., & Wang, D. (2016). Synthesis and characterization of amide compounds derived from fatty acids. Industrial Crops and Products, 83, 63–69. https://doi.org/10.1016/j.indcrop.2015.12.056
15. Smith, M. B. (2020). March’s advanced organic chemistry: Reactions, mechanisms, and structure (8th ed.). Wiley.