**1. Introduction**

Environmental protection efforts in the industry is focused on reducing wastes by recycling some of these materials [1] and replacing the raw materials of petroleum-based products used in different industries with organic fluids [2,3]. At the same time, modern technologies should be friendly with the environment [4–6], obviously in terms of energy and economic efficiency. Industrial soybean oil is often used, by further processing, as an ingredient for paints, plastics, fibers, detergents, cosmetics and lubricants that show similar viscosity to commercial lubricants [7]. Natural solvent and for example, clear liquid soy derived from soybean oil as methyl esters could serve as a green alternative to synthetic solvents. In addition, due to their low evaporation rate and longer contact time, maintained with a target material, they are recommended as natural adjuvant and surfactant in order to increase crop yields while lowering input costs by improving contact of spray droplets on plant surfaces and more effectively penetrating waxy surfaces. Soy esters present some benefits as safe to handle and store, low toxicity compared to other common substances. Vegetable oils, containing non-toxic and ecofriendly fatty acids, are successfully used by esterification and transesterification syntheses in biodiesel production [8–10], which is one of the main important biodegradable products [2,11–15]. Color properties of certain substances is often used in describing their properties, either organic or inorganic ones, as well as in food field [16–18]. As known, rheological behavior is important because it provides information regarding flow and storage of relevant materials under operation conditions [19] especially when the products are used in paint industry. Rheology is used

**Citation:** Popa, S.; Tamas, A.; Simulescu, V.; Jurcau, D.; Boran, S.; Mosoarca, G. A Novel Approach of Bioesters Synthesis through Different Technologies by Highlighting the Lowest Energetic Consumption One. *Polymers* **2021**, *13*, 4190. https:// doi.org/10.3390/polym13234190

Academic Editor: Edina Rusen

Received: 9 November 2021 Accepted: 28 November 2021 Published: 30 November 2021

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in many researches, as for the analysis of engine oil lubricants [16–24], hydrogels [25,26], different polymer—plasticizer systems [27,28], heterocycles [29–31], nanofluids [32,33] collagen solutions [34], cyclodextrin nanospongies [35] and other. Energy efficiency represents another important issue in today's technology, in order to find and implement low cost processes [36,37]. At present, the esterification processes in classical batch technology require much energy consumption, because of the necessity of high temperatures (up to 220–250 ◦C), long reaction time, and vigorously stirring. That is why researches are made to find new energy-saving technological methods [12,13,37].

At present, the microwave heating is gaining more and more influence in technological processes due to its energy economy and environmental advantages, being used in different fields [38–44].

To overcome these shortcomings, the present paper refers to the synthesis of a series of fatty acid esters via two modern technologies that do not deal with solvent extraction of the azeotrope namely a bubble column reactor [11,45] and a microwave field reactor. In this reactor the process time is short, microwave heating being a widely accepted tool for synthetic chemists [26,46]. For the sake of comparison, the synthesis was performed in a classic batch reactor as well. To the best of our knowledge, there is no reported work on a comparison between energy consumption in the various esterification reactors, or a comparative rheological study of the obtained products. Therefore, this paper investigated these aspects, revealing that the bubble column reactor and the microwave field reactor are energy-saving technological methods for the bioester synthesis. All products properties are similar, regardless the synthesis method.

#### **2. Materials and Methods**

The soybean oil fatty acids were received from Baichim SRL Bucharest. The organic alcohols n-propanol, n-butanol and n-pentanol, and the catalyst p-toluene-sulfonic acid were purchased from Fluka Honeywell (Charlotte, NC, USA). The fatty acids (R-COOH) from hydrolyzed soybean oil have a typical composition containing 11% palmitic acid, 4% stearic acid, 25% oleic acid, 50% linoleic acid and 9% linolenic acid. The physicochemical properties of fatty acids from used soybean oil are: viscous liquid without mechanical impurities; yellow color; 0.89 g/cm3 density at 20 ◦C; 14–16 ◦C melting point; 193.4 mg KOH/g acid number; −1.458 refraction index at 20 ◦C.

The main esterification reactions were performed in a bubble column reactor (B), in a microwave reactor (M) and in a classic batch reactor (C), using soybean oil fatty acids as the acid component and three hydroxyl-compounds: n-propanol (1), n-butanol (2), and n-pentanol (3), with the 1:2 mole fraction between the fatty acids from soybean oil and the organic alcohols respectively. The catalyst p-toluene-sulfonic acid was used in proportion of 0.4%. The obtained bioesters with n-propanol are B1, M1 and C1, with n-butanol are B2, M2, C2, and n-pentanol are B3, M3, C3 respectively.

A cylinder glass column with an internal diameter of 0.03 m and a height of 0.3 m provided with a heating mantle was used as bubble column reactor. Agitation was achieved by bubbling nitrogen through a nozzle at the base of the column. The energetic efficiency calculation of the bubble column reactor was developed upon esterification reaction among benzoic acid and propylene glycol, using different reaction conditions [47]. Considering the best conditions achieved in terms of energy consumption (the nozzle of 0.6 mm, argon pressure of 123.6 Pa and with no filling material) esterifications in the bubble column reactor were carried out. Esters of soybean oil fatty acids with the organic alcohols were synthesized in two steps. The first step took place in a flask fitted with a thermometer and a water reflux cooler, where the preheating of the reaction mixture was carried out for one hour under continuous stirring, at 60 ◦C on an electric stove of 5 kW. Then the product was transferred directly into the column reactor, where the synthesis was carried out at the reflux temperature, to give the products B1, B2 and B3. The reaction conditions and times are presented in Table 1.


**Table 1.** The reaction conditions for soybean esters production in different technologies.

Fatty acid esters, namely M1, M2 and M3 were obtained in a chemical reactor with a microwave heating oven (model DB-001, China Doble Best, China) provided with a reflux cooler, in a single step in the presence of the catalyst—p-toluene sulfonic acid at the reflux temperature (Table 1). The characteristics of the chemical reactor with microwave (M) heating are: microwave power 0~800 W; microwave frequency 50 MHz 2450+; Shaking magnetic stirrer.

In the classic technology, the reactor (Model ELN9.1, Carl Roth GmbH + Co. KG, Karlsruhe, Germany) was heated with an electric stove of 5 kW. The synthesis was performed in a solution esterification process, using p-toluen sulfonic acid as catalyst. The water azeotrope was extracted with toluene. The esterification was carried out in a single step according to the reaction conditions presented in Table 1, with the formation of products C1, C2 and C3.

In all cases, the esterification was monitored by periodic determination of the acid number over the whole synthesis, and the process was considered to be completed when acid index (IA) was below 1 mg KOH/g. The synthesized esters were purified by neutralization with 10% aqueous sodium carbonate solutions, washed with demineralized water to neutral pH, then vacuum distillation and decolourisation with activated charcoal and filtration. The purified compounds were then subjected to specific analysis.

The physicochemical properties of the bioesters were determined by using standardized techniques: density—SR EN ISO 12185-03, refractive index—SR 7573-95, acidity index—SR EN ISO 660:2009, iodine value—SR EN ISO 3961:2013, color—visual, rheological study was performed using a Brookfield CAP2000+L viscometer (AMETEK GmbH/B.U. Brookfield, Dresden, Germany), according to ASTM D445, temperature range 5–70 ◦C.

The calculated data are the mean of three independent replicates. Before running the one-way ANOVA analysis, Equal Variances tests (Multiple comparisons and Levene's methods) were performed to verify that the samples have equal variances. The 95% confidence level was adopted and the Tukey pairwise comparisons was applied to establish the significant differences between samples. Minitab 19 software (Minitab, LLC, USA) was utilized to perform the required calculations.

Thermo-gravimetric (TG/DTG) analyses were performed with NETZSCH STA 449F1 STA449F1A-0220-M (NETZSCH-Gerätebau GmbH, Germany)—approximately 3–7 mg of sample was heated in an Al2O3 crucible, with 5 ◦C/min, in nitrogen atmosphere, within the range 20–600 ◦C. An ion trap mass spectrometer ITQ 1100 coupled with Gas Chromatograph Trace 1310 (Thermo Fisher Scientific, Waltham, MA, USA) was used for qualitative analysis of soybean fatty acids bioesters. MS parameters were set as following: transfer line temperature at 310 ◦C, source temperature at 170 ◦C and scan range between 30 and 350 amu. The reaction product structure was established based on the *m*/*z* ratio. Fourier Transform Infrared (FT-IR) spectra of the samples were obtained in attenuated total reflectance (ATR) mode on a Bruker Vertex 70 (Bruker Daltonik GmbH, Bremen, Germany) spectrometer equipped with a Platinum ATR, Bruker Diamond Type A225/Q. Spectra were collected in the range 4000–400 cm−<sup>1</sup> with a resolution of 4 cm−<sup>1</sup> and with 40 scans/min. A MINOLTA CM 3220d spectrophotometer (Konica Minolta Sensing Europe B.V., Nieuwegein, The Netherlands) was used for the colorimetric analysis of the applied pigment in films in the following conditions: the CIE D65 illuminant (natural day light) and the standard 10◦ observer function.

### **3. Results and Discussion**

In the present study, three esterification methods were used for obtaining of bioesters from soybean fatty acids with different alcohols. Raw materials are well mixed and esterification reactions can be completed within 3 to 10 h depending on esterification method. The aim was to determine which synthesis method is more energy efficient. As it was expected, microwave and bubble column methods are promising routes with lower energetic consumption.

#### *3.1. Energetic Efficiency Comparison*

Taking into account the properties of the heating devices used in the three technologies of this research, the calculated energy consumption of the process developed in the bubble column reactor was approximately 18,000 kJ for one synthesis of 300 g bioester. In the reactor with a microwave field the energy consumption for the same esterification process was approximately 8700 kJ, and in the classical reactor 144,000 kJ. As can be seen, for the same bioester production, the use of the classical reactor requires a much more energy consumption than in the case of the other two technologies, which are also environmental protective ones.

#### *3.2. Bioesters Analysis*

In order to verify the properties of the esters of soybean oil fatty acids with various alcohols synthesized via the technologies mentioned above, the purified products were characterized by physicochemical analyzes specific to this class of substances (Table 2 and Figure 1).



All products showed viscous, opalescent aspect, with a light brown-orange color. The unsaturated degree of oils being appreciated by the iodine index, the iodine values of the synthesized bioesters are in the range of 120–140 g I2/100 g, according to the one of the soybean oil. As known, acidity is an indication of the presence of free fatty acids that are to be limited due to the formation of soaps, which may lead to the emulsions formation. The values of the acid index indicate a low content of free fatty acids. The values of the refractive index and density at 20 ◦C, presents a slightly variation. The dynamic viscosity increases with the number of carbon atoms brought by the alcoholic rest. Similar results were reported in the literature [8].

The results from Figure 1 show that there was no statistically significant difference of this parameter between esters, obtained with n-propanol (B1, M1, C1), synthetized in bubble column reactor, microwave reactor and classic batch reactor. The same conclusion can be drawn by observing the results concerning the esters obtained with n-butanol (B2, M2, C2) and n-pentanol (B3, M3, C3) respectively.

**Figure 1.** Dynamic viscosity at 25 ◦C, for the synthetized bioesters. Values are expressed as means of three independent replicates and error bars represent the standard deviation. Columns denoted by different letters indicated significant (*p* < 0.05) differences among different synthesis conditions.

All that suggested the formation of similar esters despite the different reactors used for their synthesis.
