Lactide Synthesis Using ZnO Aqueous Nanoparticles as Catalysts ^†

Abstract

The increasing global consumption of conventional plastics has led to significant environmental challenges due to their resistance to degradation and dependency on petroleum, a volatile resource. In this context, polylactic acid (PLA) has emerged as a promising biopolymer alternative, offering excellent physical and mechanical properties while being derived from renewable resources. The synthesis of PLA involves the production of lactide, a crucial cyclic monomer. This study focuses on lactide synthesis using zinc oxide (ZnO) nanoparticles as catalysts. The synthesis process consists of three stages: the dehydration of lactic acid, oligomerization to obtain a PLA oligomer, and depolymerization with simultaneous distillation to produce lactide. The ZnO nanoparticle catalyst proved to be highly efficient in regulating the molecular weight of the oligomer during depolymerization, which directly impacts the molecular weight of the PLA. The lactide purification using ethyl acetate resulted in an average purity of 90.0 ± 1.79%, demonstrating the effectiveness of the purification process. The quantification of lactide through high-performance liquid chromatography (HPLC) showed excellent linearities, allowing for the accurate determination of lactide content. The lactide synthesis yielded 77–80%, and the stability of the synthesized lactide was confirmed through a second purity determination after four months, with only a 1.7% loss in lactide content. Overall, this study showcases the feasibility of lactide synthesis using ZnO nanoparticles as catalysts and contributes valuable insights into producing high-quality lactides for PLA manufacturing.

1. Introduction

The global consumption of conventional plastics has witnessed a steady rise alongside improvements in quality of life. Unfortunately, these plastics pose significant environmental challenges, accumulating in landfills and oceans worldwide due to their resistance to environmental and biological degradation [1]. Furthermore, concerns over the volatile costs of petroleum, the primary precursor of conventional plastics, have added to the urgency of finding sustainable alternatives [2]. As a result, extensive research efforts have been directed towards exploring renewable and biodegradable sources of plastics [1].

Among the various alternatives, polylactic acid (PLA) has emerged as a promising biopolymer, garnering significant interest in recent years. PLA exhibits excellent physical and mechanical properties and can be processed using existing machinery with only minor adjustments [3].

Using lactide as a biopolymer offers significant environmental benefits due to its renewable and biodegradable nature. Unlike conventional plastics derived from petroleum, PLA is produced from renewable resources, which reduces dependency on fossil fuels and mitigates the environmental impact of plastic production. Additionally, PLA exhibits superior biodegradability, breaking down into harmless natural compounds over time, thus minimizing plastic waste accumulation and its adverse effects on ecosystems.

The most widely employed method for industrial PLA production is ring-opening polymerization (ROP), a process that involves the propagation of cyclic monomers initiated by various ions [4] and applied to olefins. Lactide, the intermediate cyclic monomer in the PLA production process, holds particular significance [5,6].

The synthesis of lactide typically begins with the distillation of lactic acid, followed by its oligomerization, wherein heating lactic acid at high temperatures leads to the release of condensed water. Subsequently, some of the resulting oligomers with specific molecular weights undergo depolymerization to yield lactide [7,8,9].

However, lactide synthesis via ROP is known for its high cost and low yield [9]. Therefore, researchers have focused on exploring the most suitable catalytic systems and optimal reaction conditions to enhance the overall yield and properties of synthesized PLA [4,6].

In this context, homogeneous tin-based catalysts have been widely studied for their role in increasing the molecular weight of the oligomer, thus impeding its depolymerization to lactide. Conversely, ZnO nanoparticle catalysts have shown promise in this stage of the process [10,11]. ZnO effectively maintains the depolymerization equilibrium necessary for lactide production by regulating the molecular weight of the oligomer [10]. The molecular weight of the oligomer significantly impacts the molecular weight of the resulting PLA, as reduced mobility affects the occurrence of the “back-biting” reaction [12].

In this work, we present the synthesis of lactide through ROP catalyzed with ZnO nanoparticles, starting from a solution of commercial lactic acid. We assess the efficiency of this process by determining the production and conversion yield while also measuring the purity of the lactide using HPLC and its thermal properties through DSC-TGA analysis. Herein, we provide a comprehensive account of our observations and discuss the obtained results.

2. Materials and Methods

2.1. Materials

L-lactic acid was procured from Mater Food, Paraguay, with a monomer concentration of 88.2% in water. ZnO nanoparticles were obtained from US Research Nano-materials Inc., Houston, TX, USA, as an aqueous dispersion of Nano-ZnO, 30–40 nm, with a concentration of 20 wt% in water. Ethyl acetate (analytical-grade organic solvent) was used for lactide purification. Chromatographic-grade acetonitrile and water (Merck) were utilized for lactide quantification. All chemicals were used directly without further purification.

2.2. Lactide Synthesis

Lactide synthesis was carried out using ZnO nanoparticle dispersion (30–40 nm, 20 wt%) as a highly efficient catalyst at a load ratio of 0.6 wt%, following the method reported by Hu et al. [10].

The process consists of three stages, each with specific conditions: the dehydration of lactic acid, oligomerization to obtain a PLA oligomer, and depolymerization with simultaneous lactide distillation. The process was performed in triplicate.

The lactide synthesis was conducted using a laboratory-scale system, as depicted in Figure 1.

Initially, an aqueous solution of the commercial lactic acid (250 g) and Nano-ZnO catalyst (20 wt%) at a load ratio of 0.6 wt% was introduced into a round-bottom flask. The temperature was raised to 80 °C using a thermal oil bath, and a vacuum pump decreased the pressure down to 60 kPa, as optimized by Hu et al. [10]. Dehydration proceeded until no more water was expelled from the solution.

For the oligomerization of lactic acid, it was important to avoid excessively high molecular weight, as this would hinder the subsequent formation of lactide. Therefore, the temperature was gradually increased to 150 °C, while the pressure was reduced to 10 kPa. During this step, water was produced by the formation of ester bonds between the hydroxyl and carboxyl groups and had to be removed [7]. To achieve this, the conditions were maintained for three hours or until no further water condensed in the double-neck flask. The product obtained at this stage was the PLA oligomer, characterized by a lower molecular weight due to the fewer repeating units in the chain [13].

After completing the oligomerization, the subsequent stage was undertaken, using the same setup, but without the condenser. To depolymerize the oligomer and obtain lactide, the pressure was gradually reduced to 3 kPa, and the temperature was raised to 220 ºC. This allowed the removal of lactide from the reaction system through distillation [10]. Lactide was collected in a round-bottom flask submerged in an ice bath for rapid solidification. The process continued until no more product was obtained from the heated flask.

2.3. Lactide Purification

To ensure the suitability of lactide for PLA production, it is essential to remove any residual acid and water. Recrystallization in an appropriate solvent is an effective purification method, with ethyl acetate demonstrating excellent capability for this purpose [10,14].

The recrystallization process followed the conditions reported by Hu et al. [10] and Sanglard et al. [14]. The crude lactide was dissolved in ethyl acetate (1:1.5 $w/v$) and stirred at 80–90 °C for 10 min. The hot solution was filtered to remove any undissolved impurities and the filtrate was then cooled to room temperature and subsequently to 4 °C for lactide recrystallization. The resulting crystals were collected using vacuum suction and dried in a vacuum oven at 40 °C to complete the purification process.

To account for any potential lactide loss during recrystallization, three batches of purified lactide were re-purified together in ethyl acetate. This approach ensured a unified lactide purity level and facilitated a comparison of yields for each recrystallization process. The recrystallization yield was calculated using Equation (1).

$${\eta }_{\mathrm{recry}}=\frac{\mathrm{Crude}\mathrm{lactide}\left[\mathrm{g}\right]}{\mathrm{Purified}\mathrm{lactide}\left[\mathrm{g}\right]}\times 100$$

(1)

2.4. Lactide Quantification

The quantity of lactide produced was determined through dry-weight measurement, accounting for the conversion yield of lactic acid into lactide (Equation (2)) and the production yield (Equation (3)). The theoretical production of lactide was calculated based on the actual lactic acid content in the solution.

$${\eta }_{\mathrm{conv}}=\frac{\mathrm{Lactide}\left[\mathrm{g}\right]}{\mathrm{Lactic}\mathrm{acid}\left[\mathrm{g}\right]}\times 100$$

(2)

$${\eta }_{\mathrm{prod}}=\frac{\mathrm{Actual}\mathrm{lactide}\mathrm{production}\left[\mathrm{g}\right]}{\mathrm{Theoretical}\mathrm{lactide}\mathrm{production}\left[\mathrm{g}\right]}\times 100$$

(3)

2.5. Lactide Characterization

For the determination of purity, a quantitative HPLC method was employed, analyzing the lactide content using a standard calibration curve prepared in the range of 25–125 μg/mL.

The quantification was performed using Shimadzu LC-20A HPLC equipment, with a low-pressure quaternary pump, an autosampler in which 25 μL of the sample is injected, and an Octadecylsilane (C18) column measuring 25 cm × 4 mm internal diameter × 5 microns of padding.

The mobile phase consisted of a 2:8 ($v/v$) mixture of acetonitrile and water, with a flow rate of 0.8 mL/min. The detector wavelength was set at 250 nm, and the oven temperature was maintained at 30 °C. Lactide samples (0.1 g) were dissolved in anhydrous alcohol in 25 mL flasks, followed by shaking and filtration. The resulting filtrate was used for HPLC analysis.

The quantification method was based on the work of Feng et al. [15], who developed an innovative method for lactide quantification using hydrolytic kinetics. To assess the stability of the synthesized lactide, a second determination was performed.

Thermal analysis of the synthesized lactide was conducted using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). A sample of the synthesized lactide (5 mg) was analyzed using a NETZSCH STA 449 F3 Jupiter simultaneous thermal analyzer (Selb, Germany), with a heating rate of 15 °C/min from 25 °C to 410 °C under a nitrogen atmosphere with a flow rate of 50 mL/min.

3. Results

3.1. Lactide Purification

Lactide synthesis was performed in triplicate, and Figure 2 shows the appearance of the crude synthesized lactide, while Figure 3 displays the recrystallized lactide after purification.

From the figure, we can see that the purification of lactide produces the formation of white and homogeneous crystals.

3.2. Lactide Quantification

The results of the lactide synthesis and subsequent purification yields are summarized in

Table 1. Yields in the synthesis and purification of lactide.

Run Number	${\mathit{\eta }}_{\mathbf{conv}}$	${\mathit{\eta }}_{\mathbf{prod}}$	${\mathit{\eta }}_{\mathbf{recry}}^1$	${\mathit{\eta }}_{\mathbf{recry}}^2$
1	68.71	77.91	54.51
2	67.75	76.70	47.63	36.59
3	71.04	79.55	57.40

.

The maximum conversion of concentrated lactic acid to lactide was 71.04%, which is in agreement with the value reported by Hu et al., where 3 more grams of lactide was obtained for every 100 g of lactic acid [10].

Regarding crystallization, a maximum yield of 57.4% was reached.

3.3. Lactide Characterization

The quantification of lactide using HPLC exhibited excellent linearity, with an R² value of 0.9998 in the equation of the line area = 5851.3[X] − 33,532, as shown in Figure 4, obtained from the injection of concentrated lactic acid standards between 25 and 125 μg mL${}^{-1}$. The purity of the synthesized lactide, determined through triplicate analysis of the sample dilutions, was found to be an average of 90.0% ± 1.79 g.

The synthesized lactide was stored in a vacuum-sealed desiccator to maintain its stability. Determination and subsequent applications were performed promptly to minimize potential degradation. The stability of the synthesized lactide was evaluated by repeating the purity determination 120 days later, resulting in an 88.3% purity.

Thermogravimetric and differential scanning calorimetry analyses (TGA/DSC) (Figure 5 and Figure 6) show the melting point ($T_m$), decomposition temperature ($T_d$), and enthalpy of the fusion ($\Delta H_m$) of the lactide.

The mass decomposition curve and the respective derivative are presented in Figure 5, revealing $T_d=227.8$ °C. Decomposition initiated at 108.32 °C with a weight loss of 0.54%, and it was nearly complete at 237.11 °C, with a weight loss exceeding 99.9%.

The melting properties of the synthesized lactide are depicted in Figure 6, with $T_m=97.2$ °C and $\Delta H_m=98.05$ J/g.

4. Discussion

4.1. Conversion and Production Yields of Lactide

The production yield of lactide varies significantly among different works, depending on the chosen method, catalyst, and reaction conditions [16].

In this study, we adopted the conditions determined by Hu et al. [10], who achieved a remarkably higher production yield of 91%, coupled with a high lactide purity of 95%. This notable result might be attributed to the low pressure (1 kPa) used in their process, which not only prompted the reaction to progress forward but also facilitated the removal of water, a crucial factor for lactide molecule formation [9,17].

In our experimental setup, the vacuum pump could only reach a working pressure of 3 kPa, whereas other investigations employing the distillation of lactide operated at much lower pressures, typically between 1300 and 400 Pa [18,19,20]. Such conditions allowed for better yields and faster distillation of lactide.

Regarding the recrystallization yield during the purification process, limited information is available in the literature. For instance, Sanglard et al. [14] researched the recrystallization of lactide using different solvents, reaching a recrystallization yield using ethyl acetate of approximately 70% [14], slightly higher than the results obtained in this investigation.

4.2. Characterization of Lactide

The storage conditions of lactide influence its purity. Prolonged exposure time to oxygen and moisture can lead to the conversion of lactide back to lactic acid [21]. Therefore, in this study, all synthesized lactide was stored in a vacuum-sealed desiccator to minimize such degradation. On the other hand, although a second recrystallization may improve purity, it inevitably results in a loss of 20–30% of lactide [4].

The purity of lactide (90.0 ± 1.79%) is in agreement with that in other investigations. Sanglard et al. [14] reported lactide purities ranging from 84% to 96%, while Feng et al. [15] achieved an average purity of 90.73% with a relative standard deviation of 1.5%. However, the research by Hu et al. [10], on which our methodology was based, yielded lactide with a purity of 95%.

Remarkably, the lactide remained stable with a purity of 88.3% after four months of storage under the described conditions. This is noteworthy compared to studies where lactide stored for only 14 days experienced a 5% loss in purity [21].

Regarding the thermal properties of lactide, the melting and decomposition temperatures ($T_m=97.2$ °C and $T_d=227.8$ °C) are in agreement with those in other investigations. For example, Peñaranda et al reported $T_m=95.65$ °C and $T_d=195.12$ °C [22], while Nyiavuevang et al. reached $T_m=94$ °C and $T_d=240$ °C [23].

5. Conclusions

This study demonstrates the feasibility of synthesizing lactide, a crucial intermediate in the production of poly(lactic acid) (PLA), using zinc oxide aqueous nanoparticles as efficient catalysts. The use of these catalysts resulted in lactide yields of 77–80%, indicating the effectiveness of the catalytic system.

The purification process using ethyl acetate showed a yield of approximately 57%, consistent with prior research studies. The average purity of the obtained lactide was 90.0 ± 1.79%, affirming the effectiveness of the purification method.

Overall, this study contributes to the understanding of lactide synthesis using zinc oxide aqueous nanoparticles as catalysts and provides valuable insights into the production of high-quality lactide for PLA manufacturing. Further research can focus on process optimization and scale-up studies to enhance the efficiency and yield of lactide production, further advancing the development of sustainable and environmentally friendly biopolymers as alternatives to conventional plastics.