Trash to Treasure: Microwave-Assisted Conversion of Polyethylene to

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Trash to treasure: Microwave-assisted conversion of polyethylene to functional chemicals Eva Bäckström, Karin Odelius, and Minna Hakkarainen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04091 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 9, 2017

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Industrial & Engineering Chemistry Research

Trash to treasure: Microwave-assisted conversion of polyethylene to functional chemicals

Eva Bäckström, Karin Odelius and Minna Hakkarainen*

Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, Stockholm 100 44, Sweden * Correspondence should be addressed to M.H. ([email protected])

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT Effective microwave-assisted process for recycling low-density polyethylene (LDPE) waste into value-added chemicals was developed. To achieve fast and effective oxidative degradation aiming at production of dicarboxylic acids, nitric acid was utilized as an oxidizing agent. Different conditions were evaluated; where recycling time and concentration of oxidizing agent were varied and the end-products were characterized by FTIR, NMR and HPLC. After just one hour of microwave irradiation at 180 °C in relatively dilute nitric acid solution (0.1 g/ml), LDPE powder was totally degraded. This transformation led to few welldefined water-soluble products, mainly succinic, glutaric and adipic acid as well as smaller amounts of longer dicarboxylic acids, acetic and propionic acid. The length of the obtained dicarboxylic acids could to some extent be tuned by adjusting the reaction time, temperature and amount of oxidizing agent. Finally the developed process was verified by recycling LDPE freezer bags as model LDPE waste. The freezer bags were converted mainly into dicarboxylic acids with a yield of 71 % and a carbon efficiency of the process was 37 %. The developed method can, thus, contribute to a circular economy and offers new possibilities to increase the value of plastic waste. Keywords: Polyethylene, Low-density Polyethylene, Recycling, Valorization, Microwave, Plastic waste, Dicarboxylic acids.


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INTRODUCTION The large volume of plastics production and the problems of pollution that come with them calls for new, sustainable and effective ways to recycle plastics and to take care of their material value.1,2 Worldwide, more than 300 million tons of plastic materials are manufactured annually.3 Large part of this production is used for short term applications, such as packaging, resulting in huge amounts of plastic waste. This “waste” could be a valuable resource for production of new materials and it is also important to recover and recycle it due to the persistence of traditional plastics in natural environments.4 Today conventional plastics like polyethylene (PE) and polypropylene (PP) are often placed in landfills, or as a better alternative energy recycled or mechanically recycled into low-value products.3,5 However, ideally, these waste products are an important resource for production of new materials.6,7 New efficient upcycling methods would have a high economic and environmental value and be in-line with the principals of a sustainable closed-loop society.8,9 During feedstock recycling the polymer is degraded into new molecules that in turn can be used as platform chemicals.8,10,11 Today, this method is not used to any large extent to recycle polymers and addition polymers in particular due to the generally high cost, low effectivity and low yields.12,13 Efficient processes for feedstock recycling would, however, be a good complement to mechanical recycling, which eventually decreases the mechanical properties and/or is prevented by contamination in the plastic stream.13,14 Feedstock recycling could then retain or even increase the material value while producing clean feedstock streams. PE is the most abundant plastic in the plastic waste stream. It has been shown that subjecting PE to mild oxidative conditions produces oxygen-containing low molecular-mass degradation products on the surface of the substrate, mainly mono- and dicarboxylic acids along with gaseous products depending on the conditions during the degradation. Oxidation can take place in air or in water and is facilitated by oxidizing chemicals such as nitric acid and 3 ACS Paragon Plus Environment


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hydrogen peroxide, in combination with an elevated temperature or UV-light.15–23 Due to its energetically favorable transition state, succinic acid is often the most abundant product formed during decomposition.15 To completely degrade PE to low molecular weight products, such as dicarboxylic acids, harsh conditions and relatively long degradation times are generally required. For example, when PE was subjected to a gas mixture of nitrogen oxide and dioxygen at 170 °C for 16 hours, the PE was completely degraded and dicarboxylic acids, mainly succinic acid as well as gaseous products were formed. When the lower temperature of 140 °C was used, a clear trend was seen where less succinic acid and more of the longer dicarboxylic acids, pimelic and adipic acid, were formed. Oxidation at 170 °C for six hours and then at 350 °C for additional twelve hours, resulted in formation of succinic acid as the dominant product followed by glutaric and adipic acid.24,25 These acids have a high economic value and are important monomers for production of polyesters and polyamides.26,27 Microwave-assisted processes could provide a new efficient route to feedstock recycling of polymers.28–30 Compared to conventional heating, microwave irradiation accelerates the chemical reactions thus shortening the reaction times and temperatures, while achieving high chemo-selectivity and high yields. Using water as a reaction medium is a great choice since it absorbs the energy well and can be super-heated. Microwave-irradiation has recently been shown to depolymerize common biopolymers such as starch, PHB or cellulose under mild conditions in a quick, effective, and environmentally friendly way.31–35 These method also contribute to a circular economy as the obtained chemicals could be used as property enhancers in polymers or as starting materials for polymerization.5,32 Here we aimed to utilize the effectiveness of microwave-assisted processes to achieve fast and efficient conversion of LDPE plastic waste to platform chemicals, dicarboxylic acids. The


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possibility to tune the product mixtures by changing the reaction time, temperature and concentration of oxidation catalyst was also evaluated.

EXPERIMENTAL Materials. Low Density Polyethylene (LDPE) powder (Mw ≈ 35 000 g/mol), (thickness 70 µm, length 200-300 µm) disodium succinate (98 %) and nitric acid (70 %) were purchased from Sigma Aldrich. Aliphatic dicarboxylic acids were obtained from PolyScience Corp, sodium hydroxide-pellets (99 %) were purchased from Merck Millipore, the LDPE waste was modelled by LDPE plastic freezer bags (film thickness 22.4 µm) from Hemköp, Sweden and deuterium oxide (99.9 %) for NMR analysis was obtained from Cambridge Isotope Laboratories. All chemicals and materials were used as received. Microwave-Assisted Recycling of LDPE. Two different catalysts, nitric acid (0- 0.50 g/ml water solution) and sulfuric acid (0.01- 0.03 g/ml), were first evaluated for their ability to oxidatively degrade the LDPE-powder, aiming to develop a method for feedstock recycling of LDPE to dicarboxylic acid end-products. The preliminary testing was performed in a Milestone UltraWAVE single reaction chamber microwave (Shelton, CT) for 2 hours with 20 min RAMP-time at 160 °C or 180 °C, the pressure was kept constant at 40 bars for the duration of the reaction. Based on the preliminary tests, nitric acid was chosen as a catalyst for further development of the microwave-assisted feedstock recycling process. For process optimization a FlexiWave microwave system from Milestone, Sorisole Italy, with a maximum power of 1900 W was utilized. 0.5 g of LDPE powder was placed in a FlexiWave Teflon vial together with 20 ml aqueous nitric acid solution with the nitric acid concentration varying from 0 g/ml to 0.50 g/ml. The run times were set to be from 0.5 to 2 hours. The temperature and pressure were



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followed by a probe inserted to a reference vessel. To reach the desired temperature, a 20 min RAMP-time was needed, thereafter the temperature was held constant at 180 °C and the effect was 1200 W for the duration of the reaction. After the reaction, the solution was neutralized with a 3 M sodium hydroxide solution and freeze dried in a Heto LyoLab 3000. The obtained products, denoted the end-products, were further identified and quantified by FTIR, NMR and HPLC. In the few cases, when some solid polymer remained, it was separated from the solution before the neutralization step. To further verify the microwave process for real LDPE products, a LDPE plastic bag (0.5 g), as a model of typical LDPE waste, was subjected to microwave irradiation in the FlexiWave unit. A recycling time of three hours at a nitric acid concentration of 0.1 g/ml was chosen based on the results of the degradation of the LDPE-powder and some initial testing of process conditions to degrade the model LDPE. Somewhat longer processing time was required to degrade the model LDPE waste, because of the higher molecular weight. After the reaction, the soluble content was collected. Product Analysis Fourier Transform Infrared Spectroscopy (FTIR). The FTIR spectra of the end-products, neat LDPE-powder, model LDPE waste and disodium succinate standard as a model for the expected end-products were recorded by a PerkinElmer Spectrum 2000 FTIR spectrometer (Nowwalk, CT) equipped with a single reflection attenuated total reflectance (ATR) accessory (golden gate) from Graseby Specac (Kent, U.K.). The images were taken at a resolution of 8 cm−1 between 4000 and 600 cm−1 with 16 individual scans. Nuclear Magnetic Resonance (NMR). The end-products and standard dicarboxylic acids, modelling the expected end-products, were analyzed by a Bruker Avance 400 Fourier transform Nuclear Magnetic Resonance Spectrometer (FT NMR) operating at 400 MHz (1H


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NMR) or 100 MHz (13C NMR). The end-products (10-20 mg) was dissolved in 0.5 mL of D2O using 150 µl of methanol as reference, standard compounds were made by dissolving corresponding dicarboxylic acids (10-20 mg) in 0.5 ml of 0.0025 M sodium hydroxide solution (D2O) also using 150 µl of methanol as reference. High-performance liquid chromatography (HPLC). The end-products from recycled LDPE powder and standard dicarboxylic acids were dissolved in MilliQ-water and analyzed using a Dionex–Thermofisher (CA, USA) coupled to a UV detector (210 nm) using a Rezex ROA-Organic acid column (300 × 7.8 mm; phenomenex, Torrance, CA, USA). The mobile phase used was 2.5 mM sulfuric acid at a flow rate of 0.5 ml/min and temperature was set to 50 °C. The water fraction after recycling of model LDPE waste was analyzed by the HPLC without any further treatment. The amount of each end-product was calculated against calibration curves made by standard carboxylic acids where the concentration was given. The yield was calculated against the original amount of LDPE waste, whereas the carbon efficiency was calculated against the amount of carbon in the original amount of LDPE waste.

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RESULTS AND DISCUSSION Microwave-assisted oxidation was evaluated as a means to feedstock recycle LDPE to dicarboxylic acids as value-added end-products. The effect of recycling time and nitric acid concentration was monitored by FTIR, HPLC and

13

C-NMR to find the best conditions for



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turning model LDPE waste to dicarboxylic acids. The recycling process is schematically presented in Figure 1. Method development for recycling of LDPE. For initial method development LDPE in powder form was utilized. The microwave-assisted feedstock recycling of LDPE-powder lead in most cases in rapid conversion to clear solutions without solid residues. This demonstrates the high effectiveness of the microwave process leading to total degradation of LDPE into low molecular weight products under relatively mild conditions and short recycling times. In some cases, the recycling time was not long enough and/or the acid concentration was not high enough for complete degradation and there was small amount of residual LDPE left. This was only observed after 0.5 hours of recycling when the concentration of nitric acid was 0.10 or 0.25 g/ml and after 2 hours of recycling in combination with no or very low nitric acid concentration (0.00, 0.01 or 0.05 g/ml).

Figure 1. Schematic illustration of the LDPE conversion process to dicarboxylic acids at 180 °C. The recycling time varied from 0.5 to 2 hours and the concentration of nitric acid varied from 0.00 to 0.5 g/ml. Identification of the end-products. To follow the recycling process and to preliminary identify the end-products from the recycling process, FTIR was utilized. Previous studies have shown that when LDPE is subjected to oxidation by nitric acid, dicarboxylic acids are the main products formed.16,36 It has also been proven that by amplifying the degree of oxidation, the relative amount of dicarboxylic acid and especially succinic acid increases.15 8 ACS Paragon Plus Environment

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Therefore, succinic acid was selected as the first reference compound to be analyzed by FTIR for comparison against the recycling end-product spectra. Succinic acid was analyzed as disodium succinate because this is the expected form for the recycling end-products after the solution is neutralized by sodium hydroxide and freeze-dried, original LDPE, standard disodium succinate and selected end-product mixtures after different degradation times and nitric acid concentrations were all examined by FTIR (Figure 2). The FTIR spectra of LDPE illustrated the typical absorption bands at 1470 cm-1, indicative of the CH2-scissoring, and at 2900 cm-1, indicating the CH-stretching vibration.37 These peaks were not present in the spectra of the end-products, demonstrating a total transformation from LDPE-powder to low molecular weight products. New absorbance bands appeared in the spectra of the endproducts. Two large peaks at 1560 cm-1 and 1410 cm-1 were identified to originate from the deprotonated carboxyl group asymmetric stretching (Figure 2a and b).38 The spectra of the end-products after different recycling processes and the spectra of standard disodium succinate were all very similar, indicating that dicarboxylic acids were the main end-products formed during microwave-assisted oxidative recycling of LDPE. For the samples processed for 2 h with nitric acid concentration 0.25 g/ml or 0.50 g/ml the peak at 1410 cm-1 is masked by the very large peak at 1340 cm-1, which originates from the salt added during neutralization (see supporting information, Figure S1). Although the peak at 1410 cm-1 is masked, it is clear that the intensity of the carboxyl band is very low for the 0.25 g/ml sample and it basically disappears when the nitric acid concentration is further increased to 0.50 g/ml (Figure 2b). This indicates too harsh conditions and further degradation of initially formed dicarboxylic acids and/or direct degradation to gaseous products. When the microwave recycling time was only 30 min at all nitric acid concentrations, no peaks were detected by the FTIR, indicating that longer recycling times and/or higher concentration of nitric acids is needed for the transition from LDPE to water-soluble dicarboxylic acids.



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Figure 2. FTIR spectra of the obtained end-products after microwave-assisted oxidative recycling. FTIR spectra of end-product mixtures after a) different recycling time ranging from 1 to 2 h at constant nitric acid concentration of 0.10 g/ml, b) different nitric acid concentrations from 0.10 g/ml to 0.50 g/ml at constant recycling time of 2 h. LDPE and disodium succinate spectra are included for comparison. The initial identification by FTIR indicated that the end-products of LDPE recycling, after neutralization and freeze-drying, were mainly carboxylic acids in salt form. This was further supported by 13C-NMR (Figure 3), as the characteristic peaks of the CH2 group around 15-40 ppm and the carbonyl carbon peak around 180 ppm were identified (See supporting information, Figure S3 and Figure S4). The characteristic peaks of disodium succinate (34.22 ppm), disodium glutarate (22.47 and 36.72 ppm) and disodium adipate (37.43 ppm and 25.87 ppm) were identified in the spectra of the end-products and confirmed by comparison with the 13

C-NMR of corresponding standard compounds. In the spectra of the end-products (Figure 3a

and b) the broad peak at approximately 37 ppm is believed to represents both disodium glutarate and disodium adipate. Since the shift of the

13

C depends on the local environment

the peaks can be slightly more or less shifted depending on the amount of added salt. In accordance with the FTIR results (Figure 2b), at high concentration of nitric acid (0.50 g/ml), no carboxylic acids could be detected, indicating further degradation of the initially formed products. Moreover, when the recycling time was only 30 min, no peaks connected to the


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formation of dicarboxylic acids could be detected indicating too short degradation time at the used nitric acid concentrations.

Figure 3. 13C-NMR spectra of the end-products from microwave-assisted recycling a) after 2 h with nitric acid concentration ranging from 0.10 g/ml to 0.50 g/ml and b) after different recycling times ranging from 1 to 2 h and a constant nitric acid concentration of 0.10 g/ml. The spectra of the end-product mixtures are compared to spectra of standard dicarboxylic acid salts. As third method to further confirm the identity and to attain the relative amount of the endproducts formed during recycling of LDPE-powder, HPLC was employed, see Figure 4. The chromatograms showing the end-products after neutralization and freeze-drying were compared to the chromatograms of standard dicarboxylic acids. The HPLC analysis in correlation with the FTIR and

13

C-NMR analysis show that dicarboxylic acids are the main

products formed during thermo-oxidative recycling of LDPE-powder in the microwave. Dicarboxylic acids as well as some acetic and propionic acid were identified in varying ratios depending on the experimental conditions.



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Figure 4. Examples of HPLC chromatograms of the end- products after microwave-assisted oxidative recycling at 180 °C with the process times varying from 1 to 2 h and constant nitric acid concentration of 0.10 g/ml. The salt is formed during neutralization. Figure 5 summarizes the relative amounts of the end-products after recycling under different conditions as determined by the relative peak area in the HPLC chromatograms after microwave-assisted oxidative recycling of LDPE-powder. Succinic acid and glutaric acid were the dominating end-products in most cases irrespective of the recycling time and nitric acid concentration (Figure 5a and b). This is in agreement with the results reported earlier after thermo-oxidative feedstock recycling of LDPE, MDPE and HDPE under dioxygen and nitrogen oxide gas.24,25 However, the reaction time required for full degradation of LDPE by this conventional oxidative method was long, 16 hours at 170 °C. In the present study, LDPE was completely degraded in the microwave after only one hour at moderate conditions, i.e. a nitric acid concentration of 0.1 g/ml and at 180 °C. Furthermore, the product distribution 12 ACS Paragon Plus Environment

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could be tuned to some degree by controlling the recycling time and the concentration of nitric acid. Harsher conditions, i.e., 1 and 1.5 hours of microwave irradiation at a nitric acid concentration of 0.50 g/ml or 2 hours at a nitric acid concentration of 0.30-0.40 g/ml at 180 °C produced simpler product mixtures with only succinic and glutaric acid. Lower concentration of nitric acid produced a wider range of dicarboxylic acids. Similar results were found during the conventional thermo-oxidative recycling of LDPE, where decreasing the reaction temperature from 170°C to 140°C led to lower amount of succinic acid in relation to pimelic and adipic acid.21 The opposite was observed when the reaction temperature was increased to 350°C after initial 6 hours at 170 °C. This resulted in the formation of succinic acid, pimelic acid and adipic acid in the molar ratio 6:3:1. The oxidation of PE by nitric acid has been described before,

39

and our results are in agreement with their findings. The

increasing amount of dicarboxylic acids, especially succinic acid, has also been observed when the conventional thermo-oxidation or photo-oxidation time was increased.11 It was earlier suggested that most of the dicarboxylic acids are initially formed by a zip depolymerization reaction.

11

However, on pro-longed reaction the dicarboxylic acids can

further loose carbon dioxide from a carboxyl radical followed by further oxidation of the resulting alkyl radical.20 This leads to changes in the product patterns increasing the relative amount of shorter dicarboxylic acid, like succinic acid with time, which is in full agreement with the results reported here. The radicals formed during the process can either abstract hydrogen leading to the formation of monocarboxylic acids, or the radical reaction enables a cyclic transition state through a backbiting mechanism. The cyclic transition state explains the dominant formation of succinic acid under harsher conditions during the recycling of both LDPE-powder and model LDPE waste. As expected a certain recycling time or concentration of nitric acid was needed to reach complete degradation of the original LDPE (Figure 5a). However, when the reaction time was 13 ACS Paragon Plus Environment


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2 hours under high nitric acid concentration (Figure 5b), no dicarboxylic acids could be detected by HPLC which confirms the results from FTIR and

13

C-NMR analysis (Figure 2a

and 4).24,25 The most likely explanation is formation of low molecular weight gaseous products, e.g. through splitting of carbon dioxide from the originally formed dicarboxylic acids as described above. Dicarboxylic acids were also the main product formed in a previous study,16 where different types of nitric acid solutions (concentrated, fuming or red fuming) were utilized to degrade HDPE for different times (16-96 h) and at temperatures adapted for the boiling temperature of the different types of nitric acid. Compared to our study, the concentration of nitric acid was much higher (71 %-95 %) and the reaction produced dicarboxylic acids with significantly higher molecular weights, from 1300 to 3000, further illustrating the effectiveness of the microwave-assisted feedstock recycling presented in this work. Another study was performed previously to oxidize PE by microwave irradiation in conventional household microwave oven.40 After a maximum of 90 seconds irradiation in the conventional microwave oven with potassium permanganate as oxidizing agent, vinylic and hydroxyl groups were formed on the surface of the substrate. Here, however, we show that the PE can be completely degraded through microwave irradiation using relatively mild conditions and short reaction time.


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Figure 5. Relative amount of the dicarboxylic acids after microwave-assisted recycling of LDPE-powder at 180 °C a) the effect of recycling time, b) the effect of nitric acid concentration. Microwave-assisted recycling of LDPE plastic bag modelling LDPE waste. To verify that the process developed for the relatively low molecular weight LDPE powder also works for real LDPE waste, a LDPE-plastic freezer bag, was subjected to microwave-assisted oxidation. After initial testing to find required conditions for complete degradation, the recycling time was increased to three hours to ensure complete degradation. This longer reaction time was required due to the higher molecular weight and lower surface area of the model LDPE waste compared to the LDPE-powder. A relatively low nitric acid concentration of 0.1 g/ml was chosen since it was shown to be a high enough concentration to rapidly and completely degrade LDPE-powder during the method development step. The model LDPE waste was completely degraded into soluble low molecular weight products during the three hours and the reaction resulted in the formation of carboxylic acids as identified by HPLC, see Figure 6. The type and amount of carboxylic acids found correlated well with those identified after recycling of LDPE-powder under similar conditions.



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Figure 6. HPLC -chromatogram of the final degradation products formed after 3 hours of oxidation of LDPE- plastic bag at a nitric acid concentration of 0.10 g/ml at 180 °C. The relative and absolute amounts of the produced carboxylic acids after the thermo-oxidative recycling of model LDPE waste are shown in Figure 7. The identified carboxylic acids weighed 0.36 g. This amount was calculated by the method described above, using the concentration of end-products determined from the calibration curves prepared with standard carboxylic acids. The weight of the original plastic bag was 0.5 g. The yield of the reaction products, thus, amounted 71 % which is comparable to previous studies (65 %) ,24 where 16 h at 170 °C was needed to transform LDPE to carboxylic acids by a gas mixture. This clearly illustrates that the method developed here is effective yet mild. The carbon efficiency of the process was 37 %. Mainly dicarboxylic acids were formed with succinic acid as the most abundant degradation product followed by glutaric acid. Much effort was previously focused on turning LDPE waste into fuels by cracking or pyrolysis.5, 13, 41 These methods require high temperatures and they usually involve use of harmful solvents such as toluene. Here, valuable degradation products were formed in high yields under relatively mild conditions with water as a green


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solvent. It is envisioned that the obtained dicarboxylic acids could e.g. be used as monomers for polymer or plasticizer production.26, 27

Malonic acid Succinic acid Glutaric acid Adipic acid Pimelic acid Acetic acid Propionic acid

Relative amount

0,5

19.82% 25.35%

0,3

0,2

6.49%

Starting material

0,4

grams

7.91% 1.58% 2.63% 36.22%

0,1

ac Pr id op io ni c ac id

ac i el ic

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ac id

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in ic Su cc

M

al on ic

ac id

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LD PE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. Dicarboxylic acids produced during thermo-oxidative degradation for three hours at a nitric acid concentration of 0.10 g/ml and at 180 °C of model LDPE waste. The products were identified and their yields were calculated from the HPLC results. The shaded area represents the amount of carbon from the initial weight of the carbon in the model LDPE waste. CONCLUSIONS The developed microwave-assisted oxidative process effectively recycled LDPE waste to functional chemicals. Moderate concentration of nitric acid as oxidizing agent combined with microwave irradiation promoted complete degradation of LDPE. To follow the recycling process and to characterize the end-products; FTIR, 13C-NMR and HPLC were utilized. After only three hours of microwave-assisted recycling in 0.1 g/ml nitric acid solution, the model LDPE waste was transformed to mainly water-soluble dicarboxylic acids- the carbon efficiency was 37 % and the total yield 71 % Due to its favorable transition state, succinic



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acid was the main degradation product. However, by adjusting the recycling time and nitric acid concentration, the length of the dicarboxylic acids could be tuned to some extent. Microwave-assisted recycling of polyethylene waste is a promising technique for retaining the intrinsic material value of the polymer via chemical recycling to functional chemicals.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: . FTIR spectra of the model LDPE waste, and plain nitric acid solution neutralized by sodium hydroxide solution and thereafter freeze-dried; 13C-NMR spectra of standard dicarboxylic acids and chosen end-products after microwave-assisted recycling.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel.: +46-8-7908271 ORCID Minna Hakkarainen 0000-0002-7790-8987 Author contribution All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS Authors gratefully acknowledge the financial support from J. Gust. Richert stiftelse (Grant 2015-00153) and The Swedish Research Council for Environment, Agricultural Sciences and 18 ACS Paragon Plus Environment

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Spatial Planning, Formas (Grant 2015-386). Basile Guillaume is sincerely thanked for his assistance during this project.

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