3.1 HBr solution catalytic degradation
The HBr solution with a concentration of 48% HBr was initially employed for the degradation of PLA to produce 2BA and LA over a temperature range of 80 oC to 110 oC. Although PLA (white particles) is insoluble in the HBr solution at room temperature, complete dissolution of PLA occurred when heated for 4h at 90 oC or 1h at 110 oC, resulting light yellow solution or dark yellow solution, respectively (Fig. 1). The color of solution gradually intensified with reaction time and eventually turned black after prolonged reaction times, i.e. 24 h at 90 oC f or 6 h at 110 oC (Fig. 1). The dissolution and color changes observed indicates the feasibility of PLA degradation in HBr solution is practicable, even though the formation of black substances at higher temperatures and longer reaction times suggested the occurrence of undesired side reactions.
As expected, the desired products, LA and 2BA, were generated, and their formation was confirmed through the detection of subsequent methylesterification of the PLA degradation products, i.e. ML and M2B (Fig. 2). The production trends of LA and 2BA were evaluated and shown in Fig. 3A-3D. Generally, the total molar yield of LA and 2BA initially increased and then decreased with the increase of reaction time. The inflection point, representing the highest molar yield, depends on the reaction temperature. For instance, at 80 oC, a high total molar yield close to 100% was initially achieved after 8 h reaction, and this high yield was maintained even with an extended reaction time of 24 h (Fig. 3A). The sustained high yield observed over a longer period suggests the stability of LA and 2BA, regardless of their mutual transformation at relatively low temperatures. In contrast, at 110 oC, an inflection point was observed after a short reaction time of 1 h, and further extension of reaction time led to a decrease in the total molar yield due to the occurrence of undesired by-reactions. It should be noted that the formation trends of LA and 2BA are different under the tested conditions. The molar yield of LA initially increased and then decreased, reaching a maximum value of ≥ 99 mol% at all measured temperatures (80–110 oC) (Fig. 3A-3D).
Conversely, the molar yield of 2BA increased under more severe reaction conditions (i.e. longer reaction time and higher temperature), with the highest molar yield observed at 110 oC for 6 h, reaching only 12.8 mol%. The more severe reaction conditions also favored a higher molar ratio of 2BA to LA in the HBr solution, although the ratio remained below 0.16 under the tested conditions (Fig. 4A). The observed trends indicates that LA serves as an intermediate for the formation of 2BA, which was confirmed through a controlled experiment where 2BA was formed from LA in HBr solution (Fig. 3E) and the molar yield of 2BA increased with the conversion of LA. However, the conversion rate of LA to 2BA was relatively slow, with an LA conversion ratio of ~ 6.1% and a 2BA yield of 80.7 mol% at 100 oC for 6 h. Although more severe conditions promoted the 2BA formation, LA remained the dominant product under the test conditions.
In summary, based on the observed results, it can be concluded that HBr solution is an efficient solvent for depredating PLA to LA, providing a high molar yield of ≥ 99% through a one-step simple process. However, HBr solution is not recommended for the producing 2BA due to its low formation rate and molar yield. The poor efficiency in 2BA formation from LA may be attributed to the high water concentration (the initial water concentration is 52%) in the HBr solution, which can inhibit bromination reaction. To overcome this potential water-inhibition issue and achieve a higher yield of 2BA, it is suggested to use a water-free solvent that is miscible with HBr. Therefore, the HBr-HAc solution was adopted for PLA degradation and discussed below.
3.2 HBr-HAc solution catalytic degradation
HBr-HAc solution was employed for the degradation of PLA at 80–110 oC to enhance the production of 2BA. Compared to the reaction in HBr solution, the PLA degradation in HBr-HAc solution resulted in a more effective dissolution of PLA, as observed in real-time (Fig. 1 and Fig. 5). Even before the heating process, PLA was partially dissolved in the HBr-HAc solution, and a relatively mild condition (e.g., within 1h at 90 oC) converted the heterogeneous solid-liquid system into an almost homogeneous solution. The presence of ML and M2B confirmed that LA and 2BA are still the primary products of PLA degradation.
Similar to the reaction in HBr solution, the yield of LA in the HBr-HAc solution initially increased and then decreased with the extended reaction time (see Fig. 6A-6D). The inflection point (highest point) of LA yield was strongly affected by the reaction temperature, with higher temperatures resulting in a shorter inflection point. For example, at 80 oC, the inflection point occurred at 6 h with a LA yield of 77.7 mol%, while the inflection point occurred at 1 h with a LA yield of 86.9 mol% at 100 oC. However, the maximum LA yield achieved in the HBr-HAc solution was much lower than in the HBr solution (> 99 mol%), primarily due to the rapid conversion of LA to 2BA and other products.
M2B was measured from LA in the HBr-HAc solution followed by methylesterification (Fig. 2A). In a quantitative experiment in HBr-HAc solution at 100 oC for 12 h, a high LA conversion rate of 89.0% and a 2BA yield of 48.0 mol% were obtained (Fig. 6E). Compared to the HBr solution under the same condition (100 oC for 12 h), the HBr-HAc solution demonstrates a 14.6 fold increase in LA conversion and 8.7 fold increase in 2BA yield, although the overall yield was lower. This demonstrates that the HBr-HAc solution promoted the formation of 2BA along with other byproducts, at the expense of LA consumption. The conversion of LA to 2BA was enhanced under severe conditions, with the molar ratio of 2BA to LA reaching approximately 54 at 110 oC for 7 h (Fig. 4B). At the mild temperatures of 80–100 oC, the 2BA yield increases with the reaction time (see Fig. 6A-6C), reaching a maximum yield of 53.6 mol% at 100 oC for 16 h. However, further increase in reaction temperature to 110 oC had an adverse effect on 2BA formation due to the occurrence of by-reactions, leading to a decrease in the highest 2BA yield decreased to 45.2 mol% (at reaction time of 9 h, Fig. 6D).
To exam the impact of the material ratio (PLA : HBr-HAc solution = 1 g : 2.5–7.5 ml), reactions were conducted at 100 oC for various duration (1–13 h). Generally, a higher dosage of HBr-HAc solution and longer reaction time favored the yield of 2BA but had adverse effect on the yield of LA (Fig. 7). This trend was also evident in the molar ratio of 2BA to LA (Fig. 4C). Furthermore, a higher dosage of HBr-HAc solution allowed for shorter reaction time to achive a high 2BA yield. When the liquid (HBr-HAc solution) to solid (PLA) ratio was 7.5 (ml/g), the highest 2BA yield of 55.2 mol% was obtained at a reaction time of 11 h. However, it should be noted that the stability of 2BA in the reaction system with high dosage of HBr-HAc solution was compromised. Extending the reaction time to 13 h resulted in a lower 2BA yield of 54.2 mol% (Fig. 7).
3.3 Esterification
The methylesterification of PLA-derived LA and 2BA with methanol is a well-established process facilitated by acid catalysis. The conversion of LA and 2BA to ML and M2B, respectively, were achieved by heating at 50 oC for 3 h. The efficiency and effectiveness of the methylesterification process mirrored the trends observed for the formation of LA and 2BA. Interestingly, other derivatives such as ethyl lactate and ethyl 2-bromopropionate, isopropyl lactate and isopropyl 2-bromopropionate, n-butyl lactate and n-butyl 2-bromopropionate + tert-butyl 2-bromopropionate were synthesized via esterification of PLA degradation products with ethanol, isopropanol, n-butanol and tert-butyl alcohol, respectively (Fig. 2B).
Figure 8 showed the conversion pathway of PLA to 2-bromopropionates and lactates via a two-step process. In the first step of catalytic degradation, PLA is converted into LA, 2BA and other byproducts (steps (1)-(5)), while LA also serves as an intermediate for the production of 2BA via bromination (steps (6)). Although lactide and acrylic acid are typically intermediates in PLA degradation (steps (7) and (8)) or in LA dehydration (lactide from intermolecular dehydration, while acrylic acid from intramolecular dehydration) [20–24], they were not detected in the reaction solutions. Lactide is efficiently converted to LA and 2BA (steps (9) and (10)) (Fig. 2A), while acrylic acid maybe be rapidly transformed into 2BA or other byproducts under acid catalysis (steps (11) and (12)). Experimental results confirmed that acrylic acid serves as an intermediate in 2BA formation through double bond addition of HBr (Fig. 2A). In the second step of esterification (step (13)), the generated LA and 2BA were converted into various lactates and 2-bromopropionates by adding corresponding alcohols under the acid catalysis.
3.4 Impacts pf reaction factors
The Software JMP13 is used to quantitatively analyze the impacts of reaction time, temperature, and solution system (variables). The cross impact of different variable on the molar yield of LA and 2BA (strains) are displayed in Fig. 9. The results confirm the previously discussed general trends regarding the impact of these factors on the molar yield and products. Table 1 listed the significance of the correlation between variables and strains. It indicates that most factors do not have significant impacts on LA yield, which can be attributed to the complexity of LA concentration changes as an intermediate during the reaction.
Table 1
The Prob > [t] values of the different variables and strains.
Prob > [t] | 2BA (wt%) | LA (wt%) |
Temperature | < 0.0001 | 0.0663 |
HBr solution | < 0.0001 | < 0.0001 |
HBr-HAc solution | NA | NA |
Reaction time | < 0.0001 | 0.5436 |
Figures 10 and 11 provide detailed analysis of the impact of each factor. While the molar yield of 2BA shows a relatively clear relationship with the impacting factors (R2 = 0.78), the relationship between LA (mol%) and the studied factors is complex and nonlinear (R2 = 0.47). Comparing these figures, it can be concluded that high temperature favors the generating 2BA and reduces LA. This observation suggests that the intermediate M3H is not stable at high temperatures, causing the reaction equilibrium to shift towards the final product of 2BA. Both solution systems support the formation of 2BA and LA. The figures also illustrate that longer reaction times tend to produce more 2BA rather than LA. This aligns with the conclusion that 2BA can be considered as a more thermodynamically stable final product.