Beijing National Laboratory for Molecular Sciences, New Cornerstone Science Laboratory, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
The random disposal and incineration of plastic materials have caused a significant waste of resources and environmental pollution, which contradicts the recent emphasis on energy conservation and emission reduction. Carbon and hydrogen sources stored in plastic wastes have immense potential for the development of a carbon-neutral future. In this study, we use a two-step process for upcycling polyethylene terephthalate (PET), the most common polyester plastic, with methanol into high-value products, that is, lactic acid (LA) and 1,4-cyclohexanedicarboxylic acid (CHDA), using a commercial Ru/C catalyst. After the depolymerization of PET in a NaOH-methanol solution, the produced ethylene glycol can further react with methanol to obtain LA and hydrogen, and the hydrogen is then employed in the hydrogenation step to obtain CHDA in high yield. Notably, our method does not require an external supply of hydrogen gas (H2). This study reveals a new pathway for upcycling the two monomers from PET.
The abuse of plastic has resulted in approximately 400 million tonnes of waste annually [1], creating environmental challenges. In terms of energy conservation and emission reduction, significant attention is being given to the recycling and upcycling of waste plastics. Compared with discarding or burning plastic, which not only generates environmental pollution but also considerable resource waste, integrating plastic waste as a valuable resource of carbon, hydrogen, and oxygen by recycling or upcycling into valuable chemicals or materials could increase its value [2-5]. Currently, physical and chemical recycling processes can yield reusable plastics and monomers designed for the reuse or refabrication of original plastics [6-11]. In addition, the upcycling of the polymer or the recycled monomer could produce new materials or value-added chemicals, exploiting the potential of each polymer component to increase the benefits of plastic waste and ultimately yield a collection of value-added products [12-15].
In contrast to the conversion of polyolefins, the selective transformation of polyesters into specific value-added compounds is relatively more feasible because of the abundance of functional groups that can be effectively retained and transformed [16-25]. For example, polyethylene terephthalate (PET), the most commonly used polyester with an annual production exceeding 70 million tonnes, can be subjected to various upcycling processes. These include thermocatalytic, photocatalytic, or electrocatalytic methods. Through these processes and with the correct catalysts and reaction routes, valuable products such as 1,4-cyclohexanedicarboxylic acid (CHDA) (path 2 in Fig. 1) [26], dimethyl cyclohexanedicarboxylates [18], 1,4-cyclohexanedimethanol [27], methyl p-methyl benzoate [28], 1,4-phenylenedimethanol [29,30], BTX (short for benzene, toluene, and xylene) [31, 32,33], and liquid organic hydrogen carriers [34] can be obtained under catalytic hydrogenation conditions. Additionally, the ethylene glycol (EG) block of PET can be converted to products such as hydrogen [35,36], formate [37,38], glycolic acid [39], or 1,2-dichloroethane [17] through sophisticated reaction pathways. However, for most of these reported processes, typically, only one block of the polymer (terephthalic acid (TPA) or EG in PET) is considered and upcycled, whereas another block is either maintained or degraded.
Lactic acid (LA) is a widely used chemical for the production of various value-added products, such as alcohols (1,2-propanediol), organic acids (pyruvate and acrylic acid), aldehydes (2,3-pentanedione), esters (lactate esters), and polymers (polylactic acid) [40,41]. The global market size of LA is estimated at 3.1 billion USD by 2022. CHDA is also a crucial chemical intermediate and raw material for the production of drugs, environmentally friendly polyester resins, and coatings. In addition, LA (sale price of approximately 1250 USD·t-1) and CHDA (sale price of approximately 7000 USD·t-1) have higher market prices than EG (sale price of approximately 480 USD·t-1) and TPA (sale price of approximately 860 USD·t-1).
We propose that to achieve a sustainable upcycling process, the main goal in upcycling plastics is to treat waste as a valuable resource, with the goal of increasing the use of the constituent atoms—carbon, hydrogen, and oxygen. Therefore, the efficient use of plastics should involve not only the carbon cycle but also the hydrogen cycle. This approach, which is known as the "carbon-hydrogen cycle," is crucial for optimizing the use of these resources in plastic waste. Consequently, designing reaction routes that yield value-added products with high atom economy is of important [5].
With respect to PET transformation, we aim to transform waste into designated high-value products in a meticulously designed upcycling process. One possible method is to use H2, which is generated from the EG block transformation, in the hydrogenation of the TPA block. This method enables the production of a high-value product, CHDA. While reforming the EG with water is an option to produce hydrogen (path 4 of Fig. 1), it concurrently generates a large amount of CO2 [35,36,42]. In this study, we propose another strategy (path 3 of Fig. 1): the dehydrogenative coupling of EG with methanol added [43-45], which replaces conventional EG reforming or direct dehydrogenation. This method generates not only sufficient hydrogen for the transformation of the TPA block but also LA, a high-value product and monomer for biodegradable polymers, from the C-C coupling reaction. The produced hydrogen can be used for the hydrogenation of TPA to create CHDA. Successfully implementing this strategy would enable the concurrent upcycling of both components in PET. Notably, the possible methods for synthesizing LA and CHDA from PET require complex, multistep reactions (up, Fig. 1).
To achieve this goal without relying on external H2 gas, it is crucial to efficiently harness the H2 produced from the cross-coupling of EG with methanol for TPA hydrogenation when stoichiometric amounts of EG and TPA are derived from the saponification reaction of PET (Eq. (1)). However, only 2 mol of H2 is produced with the consumption of 1 mol of EG (Eq. (2)), whereas 3 mol of H2 is needed to convert 1 mol of TPA to CHDA (Eq. (3)). Drawing on insights from our previous work [46], the dehydrogenation of methanol (Eq. (4)) normally occurs concurrently with the cross-coupling reaction. This process produces additional H2, which can combine the hydrogen produced from the coupling reaction for the subsequent hydrogenation of TPA. Thus, the overall reaction could be the co-conversion of PET and methanol to LA and CHDA (path 5 in Fig. 1).
$ \left(\mathrm{C}_{10} \mathrm{H}_{8} \mathrm{O}_{4}\right)_{\mathrm{n}}+2 n \mathrm{NaOH} \rightarrow n \mathrm{C}_{8} \mathrm{H}_{4} \mathrm{O}_{4} \mathrm{Na}_{2}+n\left(\mathrm{CH}_{2} \mathrm{OH}\right)_{2} $
In this study, we accomplished a two-step reaction to upcycle PET plastics with methanol into LA and CHDA without the need for external hydrogen (path 5 in Fig. 1). During depolymerization, EG underwent dehydrogenization and combined with methanol to produce LA and H2, which was catalyzed by a commercial Ru/C catalyst. The in situ generated H2 was subsequently efficiently reused for TPA hydrogenation when the same Ru/C catalyst was used. The isolated yields of LA and CHDA were 55% and 84%, respectively. Importantly, the entire process was conducted at a mild reaction temperature (160 °C) without the need for catalyst replacement or external H2.
2. Discussions and results
First, we conducted a comprehensive evaluation of the reactions involving methanol (Eq. (4)), EG (Eq. (2), and PET in a methanol solution of NaOH (Fig. 2(a)) with/without a commercial 5 weight percent (wt%) Ru/C catalyst. The PET was recovered from plastic bottles, and the molecular weight was analyzed via gel permeation chromatography (GPC) analysis (number-average molecular weight (Mn) = 18 832 g·mol-1; Fig. S1 in Appendix A). Ru was chosen because it was reported to be an efficient catalyst for the designed reactions of EG and TPA [46, 47]. The dehydrogenation of methanol occurred in the alkaline solution (R.1 in Fig. 2(a)), which resulted in the production of approximately 110 mmol of HCOONa and approximately 130 mmol of H2 at 160 °C after 10 h (Fig. 2(a) and Table S1 in Appendix A). After EG (15 mmol) was added, methanol and EG began to undergo dehydrogenative coupling, resulting in the formation of 8 mmol of sodium lactate (LA-Na; yield = 55%) at 160 °C after 10 h (R.2 in Fig. 2(a)). Simultaneously, traces of sodium glycolate (GA-Na) were produced, which is consistent with our previous results [46]. When PET was added to replace EG without the addition of a Ru catalyst, the PET was simply completely depolymerized into monomers, disodium terephthalate (TPA-Na), and EG (R.3 in Fig. 2(a)), without methanol dehydrogenation or LA formation. After Ru/C was added, the EG derived from PET was successfully converted to LA-Na with a yield of 62%, and TPA-Na was obtained with a yield of nearly 100% yield (R.4 in Fig. 2(a)). These results suggest that the proposed reaction route for LA synthesis and H2 production is feasible, although TPA hydrogenation was not observed.
To optimize the production of LA and H2, we initially varied the dosage of PET, because the ratio of EG to methanol significantly affects LA formation and methanol dehydrogenation [42,46]. The results clearly indicate the following trend: TPA and EG production increased after more PET was added, whereas LA initially increased but then decreased. Notably, methanol dehydrogenation was inhibited to some degree when PET was present, and the amount of HCOONa and H2 generated in the system decreased with increasing PET feeding amount (Fig. 2(b)). This pattern suggests that the methanol dehydrogenation reaction was inhibited when the PET input was increased. A lower PET input causes an overly rapid methanol dehydrogenation reaction, generating more H2 than is required for TPA hydrogenation, thus unnecessarily consuming more methanol. Conversely, more PET input excessively inhibits methanol dehydrogenation, resulting in an insufficient amount of H2 generated within the system to meet the hydrogenation demand of TPA. Because TPA hydrogenation requires a H2/TPA molar ratio greater than 3, we set it to approximately 5 by adding 3 g of PET (Fig. 2(b)). Moreover, the optimal LA yield (62%) was obtained (Table S2 in Appendix A).
The dosage of NaOH was subsequently tuned because it plays a crucial role in the dehydrogenation and coupling reactions [46]. Without NaOH (Fig. 2(c)), PET was depolymerized to EG and dimethyl terephthalate (DMT) in methanol at 160 °C after 10 h. The dehydrogenation of methanol or the formation of LA barely occurred, highlighting the importance of adding NaOH. As the NaOH dosage increased, the methanol dehydrogenation reaction accelerated, accompanied by a gradual increase in LA formation. The highest LA yield and a suitable H2/TPA molar ratio were achieved with 8 g of NaOH (Fig. 2(c) and Table S3 in Appendix A). Given the excess NaOH employed in the reaction process, either recovering NaOH via differences in solubility between the components in the reaction products or reducing its consumption through catalyst optimization and reaction condition adjustments would significantly increase both the economic viability and the sustainability of the process. Additionally, the reaction temperature and time were also important for LA formation because higher temperatures only resulted in higher methanol dehydrogenation rates, and longer reaction times reduced the LA yield because of increased side reactions [44,46]. Similar phenomena were observed in the reactions of PET (Figs. 2(d) and (e)). We determined that lower temperatures (< 160 °C) were not conducive to the dehydrogenative coupling of EG and that hydrogen production was insufficient. High temperatures (> 160 °C) cause side reactions and excessive methanol dehydrogenation (Fig. 2(d) and Table S4 in Appendix A). When the reaction time was extended to 160 °C from 10 to 25 h, TPA hydrogenation was still not realized, whereas methanol dehydrogenation and LA side reactions continued (Fig. 2(e) and Table S5 in Appendix A). Although the hydrogenation of TPA was not successful at this stage, the optimized reaction conditions for PET depolymerization and LA synthesis with H2 production were determined to be 3 g of PET in a 5 mol·L-1 NaOH methanol solution at 160 °C for 10 h. This setup ensured complete depolymerization of PET, a 62% yield of LA, and the preparation of H2 for the subsequent hydrogenation of TPA (H2/TPA molar ratio of approximately 5).
Owing to the excessive amount of methanol used in the reaction system, performing a precise quantitative analysis of its conversion is challenging. However, we can estimate methanol conversion by monitoring the production of HCOONa and LA during the reaction process, because methanol primarily participates in the dehydrogenative coupling reaction with EG (Eq. (2)) and the dehydrogenation reaction of methanol itself (Eq. (4)). In accordance with the calculation method described above, we determined the methanol conversion under various reaction conditions (Fig. S2 in Appendix A), and the methanol conversion under optimal conditions (62% yield of LA) was 4.8%. We calculated the turnover frequency (TOF), turnover number (TON), and activity of the catalysts at different reaction times. First, the size of the metal Ru particles on Ru/C was analyzed by transmission electron microscopy, as shown in Fig. S3 in Appendix A, and the image reveals that the Ru nanoparticles were uniformly anchored on the carbon support and had a narrow size distribution centered at 2.2 nm. From this, we calculated the dispersion of Ru on the carbon surface to be 50.6%. We subsequently evaluated the performance at shorter reaction times to obtain lower EG conversion for the calculation of the TOF. When the reaction time is less than 0.5 h, the EG conversion does not exceed 20%: When the reaction times are 0.2 and 0.5 h, the EG conversions are 3.3% and 9.3%, respectively. With these results, we calculated that the TOF was 111.4 h-1 at 0.5 h of reaction. When the EG was almost completely converted and LA was obtained with a yield of 62%, the TON and activity were calculated to be 187.9 mol of LA (molLA) per mole of metal (molmetal) and 18.8 molLA·molmetal-1·h-1, respectively.
Three steps are required to transform PET into LA and CHDA, including the saponification of PET into TPA-Na and EG (step 1), the dehydrogenative cross-coupling of EG and CH3OH into LA with H2 released (step 2), and the hydrogenation of TPA into CHDA (step 3). Because saponification in the presence of alkali (step 1) is apt to occur and the selective hydrogenation of aromatic rings on Ru species (step 3) has been extensively investigated [47-49], more discussions about dehydrogenative coupling under the catalysis of Ru/C and alkali were performed.
The dehydrogenative cross-coupling reaction of EG and CH3OH proceeds via the processes of dehydrogenation, aldol condensation, dehydration, and the Cannizzaro reaction, as reported in previous research on homogeneous catalysts [43-45]. Recently, through controlled experimental studies, we reported the first example of heterogeneous catalytic dehydrogenative coupling of EG with primary alcohols and reported that the aldol condensation, dehydration, and Cannizzaro reactions can be catalyzed only by alkali, whereas metal catalysis is essential for dehydrogenation [46]. To provide a better understanding of the important and initial processes of metal catalysts, we performed isotopic labeling experiments using deuterated methanol (CD3OD) or deuterated EG (EG-d4, HOCD2CD2OH) as reactants and performed liquid chromatography to better quantify the products.
First, the formation rates of the target product LA were compared when CH3OH and EG (denoted as rLA), CD3OD and EG (denoted as rLA*), or CH3OH and EG-d4 (denoted as rLA#) reactants were used at low conversions. As shown in Fig. S4 in Appendix A, the kinetic isotope effect (KIE) values of LA production were 2.4 and 1.5 with respect to the reactions of EG-d4 with CH3OH (rLA/rLA#) and EG with CD3OD (rLA/rLA*), respectively. The results indicate that in terms of the dehydrogenative coupling of EG and CH3OH into LA on Ru/C catalysts, EG dehydrogenation had a stronger effect on the reaction rates than CH3OH dehydrogenation did. In addition, the gas products were analyzed with an online mass spectrometer (Fig. S5 in Appendix A). When EG-d4 replaces EG to react with CH3OH, a distinct HD signal can be detected in the system. This finding indicates that the D in EG-d4 is liberated during the dehydrogenative cross-coupling reaction between EG-d4 and CH3OH. These findings further indicate that a portion of the hydrogen generated in the system originates from EG.
Second, the formation rates of the byproduct formic acid (FA) were compared when it was generated from CH3OH (denoted as RFA in terms of the reactions in the absence of EG and rFA in terms of the reactions in the presence of EG) or CD3OD (denoted as RFA* in terms of the reactions in the absence of EG and rFA* in terms of the reactions in the presence of EG) at low conversions. As shown in Figs. S4 and S6 in Appendix A, the KIE values of FA production were 2.1 and 8.9 in terms of the reactions in the absence of EG (RFA/RFA*) and in the presence of EG (rFA/rFA*), respectively. The greatly increased KIE value of FA formation when EG was added as a reactant suggests that competitive adsorption or dehydrogenation occurred between CH3OH and EG on the surface of the catalyst and that the FA formation reaction was significantly inhibited when EG was present.
Although the gaseous product from the first reaction step was exclusively high-purity H2 (Fig. S7 in Appendix A), a potential poisoning effect from the gas phase was investigated initially by venting and recharging the system with 3 MPa of pure and fresh H2 at 120 °C for 6 h. In this test, the entire liquid solution from the PET reaction showed reaction with external H2, with only traces of hydrogenation products (yield of CHDA = 0.2%, Fig. 3(a) and Table S6 in Appendix A). However, when pure TPA in CH3OH solution was tested under the same conditions as the Ru/C catalyst, a significantly higher yield of CHDA was achieved (54%), which indicates that methanol is not a barrier to TPA hydrogenation. The hydrogenation was subsequently markedly inhibited when NaOH was added, confirming that the presence of CH3OH and NaOH hinders the hydrogenation of TPA. Therefore, combining LA formation and TPA hydrogenation in one step is not feasible. The TPA must be separated from one or both CH3OH and NaOH. Afterward, pure TPA or TPA separated from the PET reaction was tested in water as the solvent, resulting in decent yields of CHDA (Fig. 3(a), right panel). The addition of NaOH in H2O further facilitated the reaction (yield of CHDA = 91%). Therefore, H2O is an appropriate green solvent for TPA hydrogenation. However, the addition of EG to the aqueous NaOH solution again prevented TPA hydrogenation. This finding indicates that alcohol and NaOH cannot coexist for TPA hydrogenation, and the detailed reaction conditions need to be further optimized.
Given the requirement of product separation, the possibility of replacing the Ru/C catalyst with a more efficient catalyst for TPA hydrogenation was explored. Various catalysts (Pt, Pd, and Ru) that have been reported for TPA hydrogenation were screened [47, 50], revealing that Pt, Pd, and Ru performed similarly in water (Fig. 3(b)). However, in NaOH solutions, Ru outperformed Pt and Pd in water, whereas all three catalysts failed in methanol (Table S7 in Appendix A). Consequently, the Ru/C catalyst was chosen for the second step to avoid the inconvenience of catalyst replacement.
While the coexistence of alcohols and NaOH hinders TPA hydrogenation, the detailed reaction conditions remain ambiguous. To elucidate the counterbalance between NaOH and CH3OH amounts for TPA hydrogenation catalyzed by Ru/C, various conditions were explored. An increase in the amount of either NaOH or methanol clearly inhibits TPA hydrogenation when they coexist (Fig. S8 and Table S8 in Appendix A). Thus, TPA should be completely separated from alcohols and bases. Moreover, we evaluated the product distribution of TPA hydrogenation under different conditions, including different pressures, temperatures, and reaction times (Figs. S9-S11 and Tables S9-S11 in Appendix A). The results indicate that TPA can be fully hydrogenated with increasing H2 pressure, temperature, and reaction time, achieving high CHDA yields of > 93%. However, more stringent conditions result in the formation of more decarboxylation byproducts (benzoic acid (BA) and cyclohexane carboxylic acid (CCA)). Therefore, TPA hydrogenation was conducted under the optimized conditions of 3 MPa H2 at 160 °C for 10 h.
We conducted cycling stability experiments for LA formation and TPA hydrogenation. Moreover, to further validate the universality of this process for various real-life PET plastics, we tested distinct postconsumer PET substrates in each cycle, including water bottles, food containers, pillow fibers, stained beverage bottles, and cotton-containing textiles (the content of PET is 45%, as measured by nuclear magnetic resonance (NMR), Fig. S12 in Appendix A). As shown in Fig. S13 in Appendix A, as the number of LA formation reaction cycles increases, the catalytic performance of Ru/C gradually decreases. This deactivation becomes significantly more obvious during subsequent TPA hydrogenation cycles. We subsequently characterized the catalysts before and after the cyclic reaction. The X-ray diffraction (XRD) patterns (Fig. S14(a) in Appendix A) reveal that the crystalline phase structure of the catalyst remained largely unchanged before and after the reaction. However, the diffraction peaks attributed to Ru became more distinct, suggesting that the particle size of Ru increased after multiple reaction cycles. To validate this hypothesis, transmission electron microscopy (TEM) analysis of the postreaction catalyst was conducted (Fig. S14(b) in Appendix A). The particle size statistics indicated that the Ru particles were predominantly distributed at approximately 3.2 nm after the reaction, which was slightly larger than the 2.2 nm observed in the fresh catalyst (Fig. S3(a)). Inductively coupled plasma optical emission spectrometry (ICP-OES) analysis further indicated a decrease in Ru loading from 3.72 wt% in the fresh catalyst to 2.15 wt% after the reaction, indicating partial loss of metallic Ru active centers under acidic/alkaline conditions. The compromised reaction performance could be attributed to both Ru particle agglomeration and partial leaching during the reaction process. Compared with the LA formation reaction, TPA hydrogenation is more sensitive to alterations in the properties of the active sites. Subsequent efforts will focus on developing acid/base-resistant catalysts with improved stability.
Last, a concise two-step reaction was designed to convert real-life PET bottles in methanol without the need for external hydrogen (Fig. 4 and Figs. S15 and S16 in Appendix A). After a 10 h reaction at 160 °C for 3 g of plastic bottles in a methanol solution of NaOH (5 mol·L-1), the system pressure increased from 0 to 1.2 MPa, indicating the formation of H2 (∼103 mmol). After it was cooled to room temperature, all the gas produced in the autoclave was collected in a vacuum tank by pumping water into the autoclave. At this stage, the yields of LA-Na and TPA-Na were determined to be 63% and 100%, respectively, by NMR analysis (Fig. S17 in Appendix A). The liquid phase products were then collected and separated via acidification. The dissolved products were further separated and purified to obtain LA, and the precipitated TPA and Ru/C catalyst were transferred back to the autoclave with 180 mL of H2O. The remaining air in the autoclave was vacuumed before the H2 stored in the tank was reintroduced back into the autoclave. TPA was completely hydrogenated within 30 h at 160 °C, as indicated by the decrease in the hydrogen pressure from 2.4 to 0.3 MPa, and TPA was barely detected in the products (Fig. 4 and Table S12 in Appendix A). Eventually, 0.74 g of LA (purity > 88%, Fig. S18 in Appendix A) and 2.16 g of CHDA (purity > 99%, Fig. S19 in Appendix A) were obtained, with isolated yields of 55% and 84%, respectively. The NMR spectrum of the obtained CHDA from PET-derived-TPA hydrogenation in the second step shows that the product is a mixture of cis- and trans-CHDA (chemical shift (δ) = 2.00-2.14, 1.78-1.93, and 1.21-1.36 ppm, respectively) (Fig. S20 in Appendix A). The cis/trans ratio is 3.9, which is close to that in the literature (cis/trans ration = 4.5) [47].
These results confirm the feasibility of upcycling of PET waste plastics and methanol into high-value chemicals (LA and CHDA) without the need for external hydrogen, achieved through a two-step atom-economic process. Notably, an excess amount of H2 was produced from methanol dehydrogenation, because we designed to meet the needs of the hydrogenation step. Thus, even though the C atoms from PET were transformed into high-value products, the efficiency of the H atom was imperfect under these conditions. The development of better catalytic systems that can fully hydrogenate TPA at low H2 pressures should be able to improve it significantly.
3. Conclusions
We designed an innovative two-step process to upcycle PET waste plastics and methanol into value-added LA and CHDA. Notably, both building blocks, EG and TPA, in the plastic were upcycled using only one commercial Ru/C catalyst without the need for external hydrogen. By efficiently collecting and reusing the H2 produced from LA formation and methanol dehydrogenation, TPA was fully hydrogenated into CHDA. Successfully coupling these two reactions rendered the entire process atom economical. Furthermore, we obtained reasonable isolated yields with high purities for LA (yield = 55%, purity > 88%) and CHDA (yield = 84%, purity > 99%) using real-life PET bottles as feedstocks. This approach not only reflects the atomistic economy in converting waste resources but also increases the possibility of PET upcycling by integrating plastic upgrading with C1 chemistry.
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