a Department of Chemistry and Industrial Chemistry (DCCI), University of Pisa, Pisa 56124, Italy
b POLYMAT & Department of Polymers and Advanced Materials: Physics, Chemistry and Technology, Faculty of Chemistry, University of the Basque Country UPV/EHU, Donostia-San Sebastián 20018, Spain
Approximately one-third of the global nylon production is accounted for by polyamide 6,6 (PA 66), with an annual output of 2.5 million tonnes. Despite its limited biodegradability, few end-of-life recycling strategies have been developed for PA 66. In this work, PA 66 is quantitatively depolymerized into its monomers: adipic acid and hexamethylenediamine (recovered as diammonium dichloride) using a naturally abundant iron-based Lewis/Brønsted acidic deep eutectic solvents (LBDESs) at 180 °C in 5 h. After optimization of the reaction conditions and work-up procedure, the overall monomer recovery yield exceeds 85%. The process is effective not only for virgin PA 66 in pellet and fiber forms but also with real post-consumer 100% nylon hosiery. Furthermore, environmental performance metrics for this method were evaluated and compared to previously reported depolymerization processes, indicating that the present approach is competitive.
Marco Rollo, Francesca Rastelli, Marta Ximenis, Elisa Martinelli, Gianluca Ciancal eoni, Haritz Sardon.
Iron-Based Lewis/Brønsted Deep Eutectic Solvents for the Hydrolysis of Nylon-6,6.
Engineering, 2026, 58 (3) : 106-113 DOI:10.1016/j.eng.2026.02.001
Nylon 6,6 (or polyamide 6,6, PA 66) is a thermoplastic aliphatic polyamide (PA) produced through a polycondensation between adipic acid (AA) and hexamethylenediamine (HMDA). The resulting polymer chains contain amide linkages capable of forming strong intermolecular hydrogen bonds, which impart high mechanical strength, particularly under tensile stress. Owing to its robust mechanical and thermal properties, PA 66 is widely used in textiles, composite reinforcements (including those used in the automotive sector), and fishing nets. The global nylon market exceeds 8 million tonnes annually, a figure that is expected to increase by 2.2%, reaching an estimated 10.4 million tonnes by 2027, with a market value of roughly 47 billion USD. PA 66 accounts for about one-third of total nylon production, corresponding to 2.5 million tonnes per year [1]. PA 66 exhibits extremely limited biodegradation [2], and recycling strategies for this material remain underdeveloped. Consequently, post-consume PA waste is commonly directed to energy recovery processes or disposed of in landfills, contributing to long-term environmental persistence.
Almost all recycling procedures are mechanical in nature, but depolymerization is gaining increasing attention [3], as it is potentially cost-competitive with traditional methods and avoids the degradation of the mechanical properties of the recycled polymer. As the products are the two starting monomers (AA and HMDA in the case of PA 66, or ε-caprolactam in the case of PA 6), they can be recombined to produce new nylon with the same quality and properties as the virgin polymer. However, the chemical recycling of PA 66 is challenging, because strong intermolecular hydrogen-bond interactions exist within the polymer structure. As a result, PA 66 possesses a higher melting point than PA 6 on a macroscopic scale and requires high temperatures (200-380 °C) for depolymerization, which is generally detrimental from an environmental perspective.
In spite of the harsh depolymerization conditions, some depolymerization procedures have been proposed to retain the value of PA 66 after use. In particular, Meng et al. [4] used supercritical water for the hydrolysis of the polymer (temperature (T) = 380 °C, pressure (P) = 28 MPa), but with low yields. Patil and Madhamshettiwar [5] studied the reaction parameters of depolymerization of nylon waste catalyzed by HCl(aq), showing that depolymerization is highly catalyst-dependent; however, long reaction times were needed to achieve complete depolymerization. More recently, microwave heating, along with an excess of HCl as an acidic catalyst, was applied to the depolymerization of PA 66, PA 11, PA 12, or PA 10,10, including some samples containing glass fibers. For PA 66, AA and HMDA yields reached 90% and 86%, respectively, in 10 min at 200 °C. While the yields are outstanding, the use of microwave heating could limit the implementation of these approaches at an industrial scale [6].
Besides hydrolysis, other approaches such as ammonolysis of PA 66 have been reported to be catalyzed by Nb2O5 [7] and hard Lewis acids [8], giving HMDA and the diamide of AA as products. Another viable method is the hydrogenolysis of PA 66, through a ruthenium catalyst, potassium tert-butoxide as co-catalyst and harsh reaction conditions (T = 150-200 °C, hydrogen pressure = 70-100 bar (1 bar = 105 bar), reaction time = 48-50 h) [9,10]. Although several approaches have been studied, a process that could be potentially scaled up does not yet exist.
We recently prepared a series of bifunctional deep eutectic solvents possessing both Lewis and Brønsted acidic sites (LBDESs) based on iron [11,12] and zinc [13]. In particular, iron-based LBDESs have been found to be effective in the depolymerization of polyethylene terephthalate (PET), with almost quantitative yields of highly pure terephthalic acid, under flexible reaction conditions: If the reactor is irradiated with microwaves, the reaction is completed after 10 min (T = 180 °C, autogenous pressure) [12], while decreasing the temperature down to 100 °C leads to an increase of the reaction time to 1-3 h. These LBDESs not only act as catalyst but also favor the interaction with PET, facilitating its depolymerization in milder conditions.
Interestingly, like PET, PAs are condensation polymers and possess a carbonyl moiety capable of interacting with Lewis acids. Inspired by this similarity, in this paper, we demonstrate that iron-based LBDESs can be utilized as industrially relevant alternatives for the depolymerization of PA 66. Several conditions have been screened to optimize both the depolymerization procedure and the work-up method to obtain the relative monomers, that is, AA and HMDA (here obtained in the form of ammonium salt) at high yield and purity. Among the many LBDESs previously reported [11], we found that FeCl3∙6H2O/methanesulfonic acid (MSA) (1:1 molar ratio, A1_1) and FeCl3∙6H2O/acetic acid (AcOH) (1:1 molar ratio, B1_1) were the most suitable for the depolymerization process (Fig. 1). The former was found to be the best-performer in the depolymerization of PET [11], while the latter is more suitable for a potential scale-up of the process, being cheaper, less corrosive, and more sustainable.
2. Experimental details
Iron(III) chloride hexahydrate (FeCl3∙6H2O, 97%) and glacial acetic acid (AcOH, 98%) were purchased from Alfa Aesar. MSA, AA (> 98%), hydrochloric acid (HCl, 37%), formic acid (96%), sodium hydroxide (NaOH, 98%), ethyl acetate (EtOAc, 99%), ethanol (99%), diethyl ether (99.8%), acetone (99.5%), deuterium oxide (D2O, 99.8%), and deuterated dimethyl sulfoxide (DMSO-d6, 99.8%) were purchased from Sigma Aldrich. All the chemicals in this study were used without further purification. The water content of acetic acid was determined via Karl Fischer titration (HI 904 Karl Fischer Coulometric Titration, Hanna Instruments), and it was 1600 ppm.
2.1. LBDES preparation
Different mixtures were prepared by mixing FeCl3·6H2O with MSA (A1_1) or acetic acid (B1_1) in the appropriate molar ratio under mild heating (approximately 80 °C) until homogeneous red dark liquid appeared. All the systems studied here have been already characterized (Refs. [11,12]).
2.2. General instruments
1H and 13C nuclear magnetic resonance (NMR) measurements were carried out on a Fourier transform (FT) NMR Jeol JNM-ECZR 500 MHz with HFX probe at room temperature. The Fourier transform infrared (FTIR)-attenuated total reflectance (FTIR-ATR) spectra of terephthalic acid were recorded using Agilent Cary 630 FTIR or Thermo Scientific Nicolet iS50 spectrometer. The conditions for FTIR-ATR measurements were a scan range of 400-4000 cm-1 with 32 scans, and a resolution of 4 cm-1. Elemental analyses were carried out using an Elementar Vario Micro Cube (carbon, hydrogen, nitrogen, and sulfur, CHNS).
2.3. Optimized procedure for PA 66 hydrolysis
For the catalytic experiments, a 25 mL Carius tube equipped with screw cap and a magnetic stirrer was charged with an appropriate amount of PA 66 and 8.24 equivalents of LBDES (relative to the repeating unit of PA 66), using either A1_1 or B1_1. The hydrolysis reactions were carried out at temperatures ranging from 100 to 180 °C, for 1 to 5 h, with 1-3 equivalents of PA 66. After the depolymerization was complete, the reaction vessel was cooled down to room temperature and the reaction crude was treated with aqueous NaOH (pH 13-14) to precipitate iron, which was subsequently removed by vacuum filtration. The resulting alkaline solution was then acidified with 37% HCl (pH 1-2) to restore the charges of the species typically present during and at the end of the depolymerization process. The solution was dried first by using a rotary evaporator, followed by overnight drying in an oven at 60 °C. The dried reaction mixture was subjected to organic extraction with approximately 30 mL of ethyl acetate, stirred at room temperature for 1 h. The mixture was then filtered, separating the organic phase, which contained the extracted AA, from the solid phase, consisting of HMDA·2HCl and sodium chloride. HMDA·2HCl was subsequently extracted with 50-60 mL of ethanol and separated from NaCl by filtration. Both organic phases, containing the monomers (AA + ethyl acetate and HMDA·2HCl + ethanol) were dried by using a rotary evaporator, followed by overnight drying in an oven at 60 °C.
The PA 66 conversion (CPA) was calculated through the ratio between the mass of the PA 66 recovered at the end of the reaction (WPA-final) and the mass of the PA 66 loaded into the reaction (WPA-initial) according to Eq. (1):
$C_{\mathrm{PA}}=\frac{W_{\mathrm{PA}-\text { final }}}{W_{\mathrm{PA}-\text { initial }}} \times 100 \%$
AA and HMDA·2HCl yields (YAA and YHMDA∙2HCl) were calculated according to Eqs. (2), (3), respectively:
where WAA is the weight of isolated AA, MAA is the molecular weight of AA, WHMDA∙2HCl is the weight of isolated HMDA·2HCl, MHMDA∙2HCl is the molecular weight of HMDA·2HCl, W1 is the initial weight of PA 66, and MPA is the molecular weight of PA 66 monomeric unit. For all the analyses, AA obtained from depolymerization was compared with a commercially available AA (Sigma Aldrich, purity > 98%), taken as a reference sample.
2.4. Determination of PA 66 degradation degree
To evaluate the degree of depolymerization of PA 66, the dried reaction mixtures, obtained after the removal of iron, were homogenized and analyzed by 1H NMR at room temperature in DMSO-d6.
PA depolymerization degree (DDPA) to AA (the same is also valid for the other monomer HMDA·2HCl) was evaluated according to Eq. (4), by taking into account the signal integrals of the methylene group next to the carbonyl of the amide bond (-CH2CONH-) typical of the residual oligomers and the methylene group next to the carboxyl (-CH2COOH) present either in the AA monomer or in the residual oligomers as terminal functional group.
where I-CH2COOH and I-CH2CONH- are the signal integrals of methylene group next to the carbonyl of the amide bond and that next to the carboxyl, respectively.
2.5. Determination of the viscosimetric molecular weight
The molecular weight of pristine PA 66 and PA 66 residues (after depolymerization reaction) was determined by an Ubbelohde capillary viscometer at 25 °C, employing five solutions at different concentrations of PA 66 or the residues, in 96% formic acid. The five solutions were prepared by adding 1 mL of stock solution (0.66 g per 100 mL) of PA 66 (or residue) to pure formic acid. The setup used for these analyses is shown in Fig. 2.
The measurements were carried out by allowing the polymer solutions to flow through tube N and recording the efflux time for the fluid level to pass from line E to line F. The viscometer was immersed in a thermostatic water bath maintained at 25 °C, as viscosity is highly temperature dependent. The viscosimetric molecular weight (Mη) was determined using the Mark-Houwink equation (Eq. (5)):
$M_{\eta}=\sqrt[\alpha]{\frac{[\eta]}{k}}$
where [η] is the intrinsic viscosity, α and k are the Mark-Houwink parameters which depend on the particular polymer-solvent system as well as temperature.
For the pair of 96% formic acid/PA 66 (or residue), the values of k and α considered were 0.000353 dL∙g-1 and 0.786, respectively [14].
2.6. Environmental factors
The environmental factor, E, is defined as follows:
where mwaste and mprod are the mass of waste and product, respectively; msolv is the mass of the solvent; mPA is the mass of PA 66; Yprod is the yield of one of the two products (AA or HMDA·2HCl); and Mprod is the molecular weight of the chosen product. In our case, we did not consider solvent recycling, which would decrease E values, and nor the work-up procedure, which would increase E values. These approximations make the comparison with other literature procedures easier.
In addition to E, the energy economy coefficient, ε, is defined as
$\varepsilon=\frac{Y_{\mathrm{prod}}}{T \cdot t}$
where T is the reaction temperature, and t is the reaction time.
Finally, the energy impact factor, ξ, is the ratio between E and ε.
3. Results and discussion
The preparation and characterization of the iron-based LBDESs A1_1 and B1_1 has been previously reported [11]. As for the other deep eutectic solvent (DES) systems, in these cases as well, mixing the components in the suitable stoichiometry and gently heating the mixture for some minutes (generally 80 °C for 5-15 min) easily led to homogeneous, deep red liquids.
The performance of A1_1 and B1_1 in the hydrolysis of PA 66 to its monomer components, AA and HMDA, was initially studied by using virgin PA 66 pellets with a viscosimetric molecular weight of (124 ± 4) kDa (Fig. 1). Initially, the depolymerization reaction was conducted by using a PA 66/LBDES molar ratio of 1:8.24, at a temperature of 180 °C for 3 h. As the LBDESs contain volatile components, such as water or acetic acid, the reaction was carried out in a thick-wall Carius tube equipped with a screw cap. At the end of the reaction, the mixture was cooled down, and a 24-hour crystallization process from cold water (5 °C) was performed to recover AA. This approach is consistent with conventional industrial methods for the recovery of AA [15,16]. A nearly white powder was obtained through vacuum filtration, which was confirmed by FTIR and NMR (Fig. 3) to be predominantly AA.
In particular, from the analysis of the FTIR spectra, the powder exhibited minor bands at 3296, 1684, and 1540 cm-1, which were assigned to N-H stretching, C=O stretching, and a combination of N-H bending and C-N stretching vibrations, respectively, indicative of amide functional groups [17]. These spectral features strongly suggest the presence of residual short-chain oligomers in the product. This observation aligns with previous studies [5], where the co-precipitation of oligomers with AA during cooling has been reported. The presence of these oligomers was indicative of incomplete depolymerization, which prompted us to optimize the reaction conditions. At this stage, the reaction progress was monitored only by FTIR spectroscopy. This approach allowed us to confirm the success of depolymerization by ensuring that no amide-related peaks remained in the spectrum, thereby confirming the absence of oligomers.
The efficiency of A1_1 was investigated by varying key reaction parameters, such as temperature, reaction time, and PA 66/solvent ratio. Initially, the reaction time was studied, maintaining a temperature of 180 °C and a PA 66/A1_1 molar ratio of 1:8.24 (Fig. 4 and Fig. S1 in Appendix A). A rough estimation of the polymer conversion, calculated by the disappearance of the FTIR N-H stretching (3296 cm-1, see Table S1 in Appendix A), indicated that almost complete depolymerization was achieved at 5 h. The experimental data can be fitted with an exponential growth (coefficient of determination R2 = 0.965). To confirm this data, the degradation degree to AA (the same also applies to the other product, HMDA·2HCl) was determined from 1H NMR spectroscopy according to Eq. (4) (Fig. S2 in Appendix A). In order to do this, a solution of NaOH was added to the reaction mixture to cause the precipitation of iron hydroxide, then the acidic pH was restored, and the solvent was removed. The resulting powder was dissolved in DMSO-d6 and analyzed, providing a depolymerization degree of 86.6% and 100% after 1 and 5 h of reaction, respectively. The difference between the conversion obtained from FTIR spectra and the degradation degree measured by NMR spectra is that the former includes long, insoluble oligomers, whereas the latter does not.
Any reaction temperature lower than 180 °C led to uncomplete depolymerization (Fig. S3 in Appendix A) and the FTIR spectra resembled the spectrum of PA 66. The average molecular weight of the residues obtained at the end of the depolymerization reactions was determined by viscosity measurements as described in Section 2.5 and according to previously reported methods in literature [18]. Mη values are listed in Table S2 in Appendix A.
Experimental Mη values indicate that depolymerization already occurs at 100 °C, even if still incomplete after 5 h of reaction, resulting in the formation of shorter chains (30 kDa, Fig. 4(b)). A complete depolymerization requires higher reaction temperatures, consistently with the literature [19,20].
Regarding the polymer loading, the PA 66/A1_1 molar ratio was increased from 1:8.24 to 3:8.24 (reaction temperature 180 °C, reaction time 5 h) and it was observed that the PA 66 loading could be raised up to 2.5 equivalents without altering the FTIR spectrum of the products (Fig. S4 in Appendix A). Indeed, a further increase in the PA 66 loading to 3 equivalents led to the reappearance of small amide-relative signals in the FTIR spectra of the product, suggesting incomplete depolymerization. Also the dilution of B1_1 with 1 equivalent of water led to worse performances (Fig. S5 in Appendix A). Under the optimized reaction conditions (PA 66/system A1_1 molar ratio 2.5:8.24, reaction temperature 180 °C, reaction time 5 h), AA was recovered with an average yield of 59.4% ± 1.7% through crystallization at 5 °C. Nevertheless, the use of acids that are environmentally sustainable, cost-effective, less corrosive and easy to dispose of is preferable, and acetic acid appears to be one of the most promising candidates, as it can also be produced from renewable sources [21]. Depolymerization tests were therefore conducted under optimized reaction conditions by using B1_1, resulting in a quantitative PA 66 conversion and yielding 51.2% of nearly pure AA after crystallization at 5 °C. The PA 66 degradation degree to AA was evaluated through 1H NMR analysis (Eq. (4)) and was found to be 20.4%, 80.7%, and 100% after 1, 3, and 5 h, respectively (Fig. S6 in Appendix A). Importantly, the use of nylon fibers instead of pellets (Mη of (136 ± 6) kDa), led to comparable results (AA yield reaching 50.7%).
The difference between conversion (100%) and yield (50%-60%) led us to optimize the work-up procedure. Firstly, using a simple cold crystallization, the product contained iron impurities, resulting in a brown-colored AA powder. For this reason, an alkaline treatment step was implemented. NaOH pellets were added to the crude mixture, causing the quantitative precipitation of iron as iron hydroxide. The precipitated iron was then removed by filtration, leaving the liquid phase with a final pH of 14. After this, the acidic conditions were restored through the addition of 37% HCl, allowing the removal of acetic acid by rotary evaporation. Both acetic acid and iron chloride hexahydrate can obviously be recycled to regenerate the system B1_1 [14,22]. While this may not constitute classical "solvent reuse," it still represents a closed-loop process. Nevertheless, given that acetic acid is inexpensive and biodegradable, its separation from water may prove to be more costly and environmentally burdensome than simply allowing its loss.
The resulting solid (AA, HMDA·2HCl, and NaCl) was extracted with ethyl acetate (EtOAc) to selectively dissolve AA, which was easily recovered after evaporation of the organic solvent. The residual solid mainly containing HMDA·2HCl and NaCl was extracted with EtOH to selectively dissolve and recover the ammonium salt (Fig. 5). The new residue, mainly consisting of NaCl, could be subjected to the well-known chloralkali process [23] to regenerate NaOH and HCl, closing the cycle and making the process completely circular.
The whole process was applied not only to virgin PA 66, but also to real post-consumer products, as 15- and 20-denier nylon tights, with reproducible results and high yields (85%-95%, Fig. 6).
Regarding the mechanism of action of LBDESs, a key observation is the solubility of PA 66 in the solvent system, suggesting a likely random chain scission mechanism. Recalling the case of PET [12], iron(III) may facilitate depolymerization by coordinating to the carbonyl oxygen of the amide group, while protons (H+) can protonate the amide nitrogen, resulting in dual activation of the amide bond and promoting its subsequent hydrolysis. Solvent B1_1 uses a weaker acid than A1_1 and, for this reason, its performances are slightly worse after 3 h of reaction (compare Figs. S2 and S6), but the complete polymer degradation requires 5 h in both cases.
In order to investigate the synergistic effect of the Lewis and Brønsted acids of LBDES in PA 66 depolymerization, control experiments were conducted under the same conditions. The reaction was carried out using a mixture containing only the Brønsted acid (acetic acid/water molar ratio 1:1), and a mixture containing only the Lewis acid (FeCl3⋅6H2O/ water molar ratio 1:1). While the former only led to dissolution/reprecipitation of the polymer, the Lewis acid alone achieved quantitative conversion of PA 66 in 5 h, yielding 86.8% of AA and 91.7% of HMDA∙2HCl. By reducing the reaction time to 2 h, the FeCl3⋅6H2O/water (molar ratio 1:1) liquid led to a conversion of 75% (Fig. 7), even if the FTIR spectrum of the reaction product demonstrated the formation of a small amount of AA. For comparison, in the same reaction time, B1_1 was able to lead to a quantitative conversion, but the FTIR spectrum did not show any peak relative to AA (Fig. S5).
Nevertheless, it is important to note that the FeCl3⋅6H2O/ water (molar ratio 1:1) system presents significant challenges. Firstly, it is not a homogeneous liquid mixture at room temperature, which poses practical challenges during experimental procedures. Increasing the water content is not an effective solution, as PA 66 is not soluble in water and the resulting acidity would be insufficient. Secondly, a substantial amount of FeCl3⋅6H2O is required, potentially increasing the cost of the process. These factors limit the overall efficiency and scalability of the process, thereby justifying the use of a DES as a more sustainable and efficient alternative. The use of a Brønsted acid, indeed, addresses these issues by maintaining high acidity, dissolving PA 66, and stabilizing iron in the liquid phase by lowering the melting point of the eutectic mixture.
The products were characterized by FTIR, NMR (Figs. S7 and S8 in Appendix A), and elemental analysis. A preliminary FTIR analysis showed that the spectrum of the isolated AA closely matched that of commercial AA (purity > 98%) used as reference. The only notable difference is the presence of a band at 1090 cm-1, possibly due to residual solvents (EtOAc). However, a simple recrystallization from cold water proved to be highly effective in further purifying AA. This straightforward method allowed for the removal of any remaining impurities, yielding a product with significantly improved purity. The elemental composition of the recrystallized monomer closely matched the theoretical value for pure AA, indicating the removal of impurities (Table S3 in Appendix A). Additionally, the 1H NMR spectrum showed no signals attributable to impurities, further validating the effectiveness of the purification process (Fig. S7).
On the other hand, from the 1H NMR spectrum of the HMDA·2HCl (Fig. S8), it is evident that the method employed in this study does not fully guarantee the complete removal of AA from this fraction. The characteristic signals of AA were clearly present in the spectra, resulting in a residual content of 3%-6%. Additionally, signals at 2.12 and 3.04 ppm, due to -NHCOCH2CH2- and -CONHCH2CH2-, indicated the presence of oligomeric species, which were undetected during the PA 66 degradation analysis, possibly due to their low concentration. Vacuum distillation (after basification) would yield a pure product, as all the impurities are non-volatile (sodium adipate, sodium chloride, and PA 66 oligomers). This is the same methodology industrially used to purify and separate the diamine.
Finally, the environmental parameters of the process were evaluated. In particular, the factor E, defined as the mass of waste for the mass of the product, was determined as well as the energy economy (ε) and the energy impact (ξ), calculated as the E/ε ratio (Table 1).
The values of E, ε, and ξ are referred only to the depolymerization reaction and not to the work-up. Consistently with previously reported literature [5,6,9], also in our case the values of E, ε, and ξ are referred only to the depolymerization reaction. Although in some cases the depolymerization conditions are markedly different from those of this work (for example in the case of the hydrogenative depolymerization [10], or in the case of microwave irradiation [6]), making strict comparisons difficult, the values obtained for the LBDES are noteworthy and suggest the potential of these solvents for the effective depolymerization of PA-based waste.
4. Conclusions
The depolymerization of PA 66 using LBDESs demonstrated that recovering AA and HMDA∙2HCl with A1_1 (FeCl3∙6H2O/MSA) and B1_1 (FeCl3∙6H2O/ acetic acid) is possible and the results are promising. Here, we evaluated the impact of key reaction parameters, including time, temperature, and PA loading, on the depolymerization efficiency, achieving complete depolymerization after 5 h at 180 °C. The process was optimized, as evidenced by the 100% conversion of PA 66, which was confirmed by FTIR and NMR spectroscopy.
Work- up procedures were refined to improve monomer recovery and purity, allowing for the isolation of AA and HMDA∙2HCl in high yields (90% and 97%, respectively). Recrystallization from water further enhanced the purity of the isolated AA, as confirmed through FTIR, NMR, and elemental analysis.
Although the study achieved significant progress in depolymerization efficiency and product recovery, some challenges remained. Residual oligomeric species were detected in HMDA∙2HCl fraction, and basification followed by vacuum distillation is required to obtain pure HMDA.
Notably, the described procedure has been applied both to virgin and real post-consumer samples, with comparable results, demonstrating that the LBDESs possess a good practical potential.
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