Microwave Assisted Hydrothermal Carbonization and Solid State

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Microwave assisted hydrothermal carbonization and solid state post modification of carbonized polypropylene Karin H Adolfsson, Chia-feng Lin, and Minna Hakkarainen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02580 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 17, 2018

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ACS Sustainable Chemistry & Engineering

Microwave assisted hydrothermal carbonization and solid state post modification of carbonized polypropylene

Karin H. Adolfsson†, Chia-feng Lin†, Minna Hakkarainen†,* a

Department of Fibre and Polymer Technology, KTH Royal Institute of Technology,

Teknikringen 58, SE-100 44 Stockholm, Sweden

*Corresponding author: Minna Hakkarainen. E-mail address: [email protected]

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ACS Sustainable Chemistry & Engineering 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

ABSTRACT

Functional carbon materials produced through an hydrothermal treatment of waste products have gained interest. Particularly the method is considered more facile and green compared to conventional decomposition methods. Here, we demonstrated an upcycling of polypropylene (PP) waste to carbon materials by a microwave assisted hydrothermal treatment. The solid product obtained from the hydrothermal treatment was analysed by multiple techniques to reveal the structure and the influence of processing conditions on PP degradation and hydrothermal carbonization. Chemical analyses showed the presence of carbonaceous material independent on acid amount (20 and 30 ml), temperature (210 and 250 °C) and time (20-80 min). A complete transformation of PP content to amorphous carbon required 60 min at 250 °C. The mass yield of the solid product decreased as a function of harsher processing conditions. At the same time, thermogravimetric analysis illustrated products with increasing thermal stability and larger amount of remaining residue at 600 °C. The solid products consisted of irregular fragments and sheet like structures. A solid state microwave process in air atmosphere was performed on a product with incomplete carbonization. The modification resulted in a decreased C/O ratio and TGA analysis in nitrogen showed high thermal stability and degree of carbonization as indicated by the remaining residue of 86.4 % at 600 °C. The new insights provided on the hydrothermal carbonization, and post modification in air atmosphere, can catalyse effective handling of plastic waste by enabling transformation of low quality waste into functional carbon materials.

Keywords: Plastic waste, Polypropylene, Microwave, Hydrothermal, Hydrochar, Carbonization


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INTRODUCTION The global production of plastics is expected to double from the current 322 million tonnes in the next 20 years. In Europe alone, 25.8 million tonnes of plastic waste are produced each year. Full utilization of this waste as raw material for production of new materials requires innovation and new processes. Over the last decade, the plastic incineration rate in Europe increased to 39 % and landfilling decreased to 31 %. However, only 30% is collected for material recycling.1 Mechanical recycling is suitable for some material groups, but it is restricted by the need of separation, limited knowledge of material compositions and decreasing material properties as a function of recycling cycles.2 As an alternative recycling method, suitable also for low quality or to some extent contaminated waste, upcycling of plastics to carbon materials has drawn interest.

Polypropylene (PP) is one of the most commonly used plastics. It can be found in packaging, electrical machines, automobiles, and other industrial applications.3 There have been several studies on the production of different carbon materials with PP as feedstock applying thermal and catalytic decompositions.4–6 Recently, the hydrothermal treatment of waste has attracted great attention for its potential in producing carbon spheres or hydrochar that are of high interest for multiple applications. The method is considered both facile and green. The treatment takes place in aqueous solution, commonly at relatively low temperatures compared to decomposition processes, and under self-generated pressure resulting in a subcritical system.7,8 Depending on the processing conditions and raw materials,9 the properties and suitable applications differ from adsorption10

to

catalysis,11

and

agricultural,12

electrochemical,13

environmental

(CO2

sequestering)14 and biomedical applications.15 Multifuntional bionanocomposites were previously prepared in our group,16 by utilizing hydrothermal carbonization products produced from



cellulose as property enhancing fillers. From a waste handling perspective, the hydrothermal treatment reduces the energy needed for dewatering and drying required before other common thermal treatments.17

To date, hydrothermal treatment has mainly been applied on different biobased products including both low molecular weight compounds12,18 and biopolymers or more complex raw materials.12,13,19-23 Only a few plastics have so far been processed through hydrothermal treatment, either as single synthetic polymers, mixtures of plastics or municipal solid waste (MSW) consisting of biomasses, plastics and inorganics. Considering the treatment of a solely synthetic polymer, a reaction mechanism producing hydrochar from polyvinylchloride (PVC) was suggested.24 The reaction pathways proceeded through polyol and polyene formation to aromatic compounds and low molecular weight products controlled by reaction temperature (400500 °C) and water density (0-930 kg m-3).

Formerly, citric acid as catalyst for PVC degradation was evaluated.25 Complete removal of chlorine was nearly reached at 240 °C after 15 h under self-generated pressure at a temperature range of 180-260 °C where citric acid showed negligible influence on the degradation. However, the pH dropped to 2.4 due to the release of hydrochloric acid (HCl) during degradation at the highest temperature applied. In another study of PVC degradation,26 95-98 % of the chlorine content was removed at 240 °C within 2 h utilizing an initial pressure of 3 MPa where it was shown that alkali had limited effect on the removal of chlorine.


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MSW containing shredded plastic bottles was hydrothermally treated at 250 °C for 20 h.27 Both the H/C and O/C ratio was lowered for the hydrochar, though, it was also concluded that polyethylene terephthalate was still present. Another example of hydrothermally treated waste mixtures included plastics, wood and other organics that were processed at 190 °C and 1.5 bar for 30 min resulting in an homogenous solid powder.28 A mixture of merely plastics, containing PP, polyethylene and PVC was processed at 280 °C for 30 min and other feedstocks from MSW were studied separately as well.29 Hydrothermal treatment of waste wood, paper, food and rubber tires produced hydrochars, whereas the plastics formed a melt under the applied conditions.

Previously, we have demonstrated a process for hydrothermal carbonization of cellulose,21 starch,20 waste paper22 and coffee grounds23 by utilizing a microwave assisted reaction in dilute sulphuric acid solution. Here, we hypothesized that microwave assisted hydrothermal treatment could provide a route for upcycling of PP to carbon materials under acidic conditions. This route was already shown promising for PVC degradation and should have great potential even for other plastics. A modification of the hydrothermally treated PP was also investigated through a solid state microwave process in air to further promote the carbonization of PP derived materials. Multiple hydrothermal reaction conditions were evaluated followed by careful material characterization to provide new insights into the hydrothermal carbonization of PP inclusive the post modification step. The developed methods are promising options for production of carbon materials from waste and suitable even for low quality or contaminated plastic waste.

EXPERIMENTAL SECTION



Materials and chemicals. PP pellets were obtained from Domolen (MFI 16.9 g/10 min). HCl (37 %, solution in water) was purchased from ACROS Organics and HNO3 (70 %, ACS reagent) from Honeywell-Fluka. Silicon carbide (SiC, 400 mesh particle size, ≥97.5 %) was bought from Sigma Aldrich. Dialysis bags (1 kD MWCO, 45 mm flat width) were purchased from Spectrum Labs. Sodium hydroxide (NaOH) pellets were purchased from Eka Nobel. Filter paper (0.22 µm hydrophilic mixed cellulose esters) from Millipore was used for vacuum filtration and acetone from VWR. All materials and chemicals were used with no further modifications.

Hydrothermal treatment. A microwave oven of model flexiWAVE from Milestone Inc. was utilized for carrying out the hydrothermal treatments and the modification of a partly hydrothermally carbonized product in solid state. For the hydrothermal treatment, 1 g of the PP powder and 20-30 ml of 5M HCl/5M HNO3 (1:1, v:v) were placed in 100 ml sealed Teflon vessels. A grinder (Retsch) was used to grind the PP pellets into a fine powder. It was anticipated that PP in powder form would provide better contact with the acidic solution as compared to the pellets with lower surface area. Furthermore, the heat would reach the inner PP parts faster. First, the PP pellets were cooled by liquid N2 for approximately 1 h to avoid melting upon grinding, then the pellets where grinded through a 0.5 mm steeliness sieve. The reaction temperature for the hydrothermal treatments was set to 210 or 250 °C for a reaction time varying from 20 to 80 min with a constant ramp time of 20 min. Actual input of irradiation power was varied automatically, to reach and be stationary at the specified temperature.

After the hydrothermal treatment, the Teflon vessels were cooled by electric fan to room temperature (RT) and the formed gas was thereafter released. The solid product was collected on


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a 0.22 µm filter paper and rinsed with 50 ml deionized (DI) water, and placed in a vacuum oven operating under RT. The liquid product was purified using dialysis. The dialysis bag was placed in DI water over a period of five days with continuous replacement of DI water. To avoid damaging the dialysis bag, the pH of the liquid product was adjusted with NaOH before purification. After the dialysis purification, the product was freeze-dried and kept in a vacuum oven at RT. The products were denoted with C for carbonized product (solid phase) and L for product in liquid phase. Naming was then sequentially followed by acid aqueous amount (ml), processing temperature (°C) and time (min). As an example, the products after hydrothermal treatment with 20 ml acid at a temperature of 210 °C for 20 min (excluding ramp time) were given the abbreviation C20-210-20 (Acid amount-Temperature-Time). The liquid phase was similarly named with results provided in SI.

Modification in solid state. The solid product (C20-210-20) obtained from the hydrothermal treatment of PP powder was modified by a solid state microwave process using a self-built reactor (Figure S1). To perform the modification, the sample was placed in a glass vial (volume of 2 ml) and positioned in a larger beaker filled with SiC powder. The ramp time was fixed to 20 min, and heating was applied for 60 min at a temperature of 180 °C under air. Actual input of irradiation power was varied automatically to meet the requirements of specified temperature. After full processing time the sample was left to be cooled down to RT. The modified product was named mC20-210-20. The mass yields (%) of the solid phase products and the product after the solid state modification were calculated according to (obtained product/PP powder)∗100 (%) after drying in vacuum oven at RT.



Melt flow index (MFI). MFI of PP pellets was measured through an orifice of 2 mm in diameter. The measurements were performed according to the regulation ISO-1333. 230 °C/2.16 kg was the standard for all the measurements. An average MFI was calculated from six measurements, see Table S1.

Rheology. DHR-2 (TA instrument) with disposable and smooth aluminium plates of 25 mm in diameter was used to analyse the rheology of PP pellets and powder. The geometry gap was set to 1000 µm and the operating temperature was 190 °C. Both amplitude and frequency sweeps were performed, where the former was to determine the oscillation strain within the linear region of the sweep to assure viscoelasticity of the materials. Based on the amplitude sweep, an oscillation strain of 1.0 % was therefore selected and applied for the frequency sweep. All analyses were done with the software TA instrument, version 4. The measurements were performed two times for each sample.

Differential scanning calorimetry (DSC). Mettler-Toledo 820 was used to measure the melting temperature (Tm) and specific heat capacity (J g-1) of PP pellets, powder and some of the solid products after hydrothermal treatment. 2-4 mg of each sample was placed in a 100 µl aluminium cup with a pinhole on the lid. PP powder was also analysed using an amount of 0.5 mg as a measure of the sensitivity of the instrument as to compare with the analyses of hydrochars. The applied heating rate was 10 °C min-1 from 30 to 200 °C in a nitrogen atmosphere (rate 50 ml min1

). Each sample was analysed 2-3 times.


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Fourier transform infrared spectroscopy (FTIR). FTIR was performed on a Perkin Elmer Spectrum 100 with 16 scans from 4000 to 600 cm-1 through a resolution of 4 cm-1. The golden gate was from Graseby Specac (Kent, UK). The products after the hydrothermal treatment and the solid state modified product were dried before they were analyzed using the same settings.

Thermogravimetric analysis (TGA). Mettler-Toledo TGA/SDTA 851e was utilized for TGA of the solid products after the hydrothermal treatments and the solid state modified sample. 2-4 mg of each sample was placed into a 70 µl alumina cup. The samples were heated at a rate of 10 °C min-1 from 30 to 600 °C under 80 ml min-1 nitrogen flow. Each sample was analysed 2-3 times.

Wide angle X ray diffraction (WAXD). PANalytical X’Pert PRO diffractometer with a silica mono-crystal sample holder was used for the WAXD measurements of all the solid products after the hydrothermal treatment and the solid state modified sample. Cu-κ alpha (λ= 1.541874 nm) was operating at a voltage of 45 kV and a current of 45 mA. The intensity was measured at 2Theta (2θ) angular range between 5-60° with a step of 0.017° for all the analyses.

Confocal raman spectroscopy. Raman measurements were applied for the solid products after hydrothermal treatment and the modified product, which were analysed on a HR800 UV Jobin Yvon Raman (Horiba, Kyoto, Japan) combined with a solid state laser utilizing a lambda excitation (λex) of 514 nm. A 100x objective and a 600 groove mm-1 density grating was used for all measurements. Spectra were acquired at a resolution of 1 cm-1 utilizing 5 accumulations with an acquisition time of 30 s.



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Scanning electron microscopy (SEM). SEM was used to determine the morphology of all materials. The characterization was done by using an Ultra high resolution FE-SEM (Hitachi S4800). The solid part after hydrothermal treatment and post modification was analysed in powder form and drop casted from a dispersion in acetone of 1 mg ml-1 (bath sonicated for 30 min). The liquid products obtained from the hydrothermal treatment were drop casted onto the grid at a concentration of 0.1 mg ml-1 in DI water. All samples were coated with platinum/palladium at a thickness of 5.0 nm.

Nuclear magnetic resonance (NMR). NMR spectrum was acquired for a liquid product gained from the hydrothermal treatment (L20-250-40). 400 MHz 1H-NMR and 100 MHz

13

C-NMR

spectra were recorded on the Bruker Advance 400 spectrometer at 298 K. Approximately 10 mg of the vacuum-dried product at RT was dispersed in 1 ml deuterium oxide (D2O).

Ultraviolet-visible spectroscopy (UV-Vis). SHIMADZU UV-2550 was used to measure the UV-Vis absorption of L20-250-40. The concentration of the sample was 0.01 mg ml-1 in DI water.

Fluorescence spectroscopy. Fluorescence properties was examined for the liquid product L20250-40 at a concentration of 0.1 mg ml-1 in DI water on a Varian Cary Eclipse spectrophotometer. The excitation wavelength was altered from 330 to 490 nm with a 20 nm increment.


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Zeta potential (ζ ζ−potential). Zetasizer Nano ZS from Malvern Instruments was applied to measure the ζ-potential of the products in the liquid phase after the hydrothermal treatments. A concentration of 0.3 mg ml-1 in DI water was used at a temperature of 20 °C.

pH measurements. The pH value of the liquid products from the hydrothermal treatments were measured by utilizing a VWR symphony meter SB70P at RT in a concentration of 0.3 mg ml-1. Calibration was performed with buffer solutions at pH 4 and 7.

X ray photoelectron spectroscopy (XPS). XPS analysis was performed on C20-210-20 and mC20-210-20, which are the samples before and after solid state modification. The area analysed was 300x700 µm. Spectra were collected by a Kratos Axis Ultra DLD electron spectrometer using monochromated Al Kα source operating at 150 W. For wide survey spectra, a pass energy of 160 eV was applied and 20 eV was used for individual photoelectron lines. To stabilize the surface potential a spectrometer charge neutralization system was employed. The binding energy scale was referenced to C 1s at 284.8 eV for both 20-210-20 and mC20-210-20. Kratos software was used for data processing.

RESULTS AND DISCUSSION

Microwave assisted hydrothermal treatment was applied to transform PP into a carbonized material (Figure 1). The influence on the degradation and carbonization of PP of various process parameters such as acid amount, temperature and time were studied. The hydrothermal treatment



can result in products that will be present in liquid or gas phase as well as solid products.29 Here, both the solid products and the products from the liquid phase (see SI) were thoroughly examined. To further explore the carbonization of PP derived materials, one of the hydrothermally treated samples with incomplete carbonization, C20-210-20, was modified by heating the sample by a solid state microwave process in air.

Figure 1. Schematic description of the microwave assisted hydrothermal treatment of PP and post modification in air atmosphere by a solid state microwave process.

Chemical functionalities and morphology. FTIR analysis of the original PP powder and the solid products from the microwave process showed that the hydrothermal treatment in acidic solution led to changes in the chemical structure of PP, Figure 2a. PP was represented by its typical absorptions around 2900 cm-1, 1465 cm-1 and 1376 cm-1. These wavenumbers represent the vibration stretches of sp3 CHx, and the bending of CH3 and CH2.31 All solid products showed


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in addition stretches in a broad range over 3000 cm-1 attributed to COOH/C-OH, 1704 and 1596 cm-1 representing C=O and C=C, and a peak at 1217 cm-1 for C-O.19

The mass yields of the products, C20-210-20, C20-250-20 and C20-250-40, were 68.5, 61.6 and 56.6 % respectively. Thus, demonstrating a decrease with increasing processing temperature and time, Figure 2b, which could indicate both increased degree of carbonization and higher amount of liquid and gaseous products.17 It should be noted that these mass yields most probably include carbonized material, some remaining PP and salt. Hydrochar derived from MSW showed likewise a trend of decreasing mass yield from 76 to 57 % with higher processing temperature and longer time.32 The amount of liquid and gaseous products increased, though, the mass of the gaseous products was much lower compared to the solid and liquid phase.

When utilizing a reaction time of 60 min at 250 °C (C20-250-60) the sp3 CHx, CH3 and CH2 stretches diminished. The same was seen for the samples, C20-250-80 and C30-250-60, gained under harsher conditions. As for feedstocks containing biomass, the C/O and C/H ratio carbon generally decreases as an effect of hydrothermal carbonization.19,27,32,33 The amount of solid product continued to decrease as a function of processing time from 44.6 to 33.3 % for the samples subjected to 60 and 80 min of hydrothermal treatment. An abrupt decrease in mass yield was observed when the acid amount was increased from 20 to 30 ml to 1.40 %. Similar effect was observed during microwave assisted oxidation of LDPE to low molecular weight dicarboxylic acids, where increasing the acid concentration led to formation of larger amount of gaseous products instead of targeted dicarboxylic acids.34



Figure 2. FTIR of PP powder and solid products after the hydrothermal treatment (a), images of PP powder and C20-250-60 (b), and mass yield (%) (c) solid products after the hydrothermal treatment.


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After the hydrothermal treatment, the physical appearance of the solid parts was shown to possess various morphologies (Figure 3). After dispersion of the samples in acetone, the dominant part of the samples contained both smaller and larger irregular fragments. The solid parts also consisted of sheet structures with folds as observed for graphene materials.35 The morphologies of the samples were shown to be independent on different processing conditions. Though, clear physical differences were shown between the original PP and the solid products after the hydrothermal treatment (Figure S3). The PP powder had a smooth and continuous surface, whereas the solid products had rough exteriors mixed with fragments of various structures. The matrix of the solid products possessed a foam like structure, similar to the foam structure that was previously seen after thermal decomposition of starch.36 It was suggested that the structure was initially formed due to melting of the granules. Hydrothermal treatment of biobased raw materials commonly results in the formation of spherical carbonized particles,19,33,36 while also densely aggregated forms23,33 and nanoporous structures have been reported.12



Figure 3. SEM images of the solid products after the hydrothermal treatment after dispersion in acetone.

Amorphous carbon and thermal stability. As shown by the FTIR results, the chemical structure of PP changed considerably due to the hydrothermal treatment, and hence, changes were also expected in the semicrystalline character of PP. As seen in Figure 4a, the PP powder showed clear diffraction peaks at 14.0, 16.9, 18.5, 21.2, 21.7° corresponding to the monoclinic α-crystal planes of (110), (040), (130), (131) and (111).37 Thermal analysis of PP displayed a single


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melting peak at 165 °C further supporting the presence of α-crystals and semicrystalline PP (Figure S2).3

Significant changes were induced after the hydrothermal treatment. Already for the product subjected to the mildest hydrothermal treatment, C20-210-20, the PP diffraction peaks became diffused (Figure 4a). No remaining PP crystals could be detected in C20-210-10 by DSC, indicating absence or very low amounts of remaining PP (Figure S5). The WAXD pattern of C20-250-20 and C20-250-40 remained very similar to C20-210-20. However, after a reaction time of 60 min, the product C20-250-60, shown in the image in Figure 4a, retained no semicrystalline PP diffraction peaks. There was also a broad diffraction peak around 26° allocated to the graphitic plane of (002) and typically assigned to amorphous carbon.11,38 The remaining samples gained under harsher conditions showed similar pattern.

To further evaluate the changes on a molecular structural level, Raman analyses of the solid products were performed. All solid products showed D and G bands at 1357 and 1590 cm-1, which is in agreement to what is observed for carbonaceous materials (Figure S4).39 The G band stands for the bond stretching of sp2 atoms in rings and chains, and the D band represents the breathing modes of sp2 atoms in rings.40 The D band was broader than the G band as typically observed for amorphous carbon. No obvious difference in the WAXD pattern was shown when 30 ml acid was used e.g. C30-250-60 as compared to samples carbonized in 20 ml of acid, except for the elimination of crystalline NaCl. The absence of NaCl could be related to the smaller sized fragments observed by SEM, which might have led to less trapped NaCl in the carbon material and was thus easier rinsed off.



Thermal stability of the original PP powder and the solid products were analysed under nitrogen atmosphere as seen in Figure 4b. The degradation onset temperature of PP was 431 °C while the solid products showed different thermal behaviour. They started to degrade slowly already before 100 °C. Oxygen functionalities and low molecular weight products could be the cause of the initial thermal instability and weight loss at the relatively low temperature as was previously observed for carbon nanodots41 and hydrochar.42 The thermal degradation onset temperature for the hydrochars was chosen at the point where the rate of the weight loss was the greatest, i.e., which was between 377 and 386 °C depending on the sample (Table S3).

Between 30 and 200 °C, the sample subjected to the harshest acidic conditions with 30 ml acid was the least thermally stable product. A trend of increasing thermal instability in this temperature range was also seen for the samples treated with higher processing temperature and longer times. An exception was C20-250-40 in comparison to C20-250-20. All solid products had remaining masses ranging from 43.7 to 59.0 % at 600 °C, Table S2. The hydrothermally treated products most likely carbonized further while heating and therefore became thermally stable at high temperatures.43 PP on the other hand showed only a remaining mass of 3.7 % at the same temperature, showing no tendency to carbonize. Samples formed under longer processing times showed increased residues up to 60 min, and the most thermally stable product was the one carbonized with higher acid amount.


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Figure 4. WAXD of the original PP powder and the solid products after the hydrothermal treatment (a), and TGA curves under nitrogen for the PP powder and the solid products after the hydrothermal treatment (b).

Solid state modification. The product C20-210-20 was chosen to be further modified by a solid state microwave process by heating in air atmosphere. The purpose was to increase the carbonization as was seen for thermally oxidized linear low density polyethylene (LLDPE).43 Solid state microwave processes were earlier applied to carbonaceous materials44 and in several



studies with graphene oxide, graphite oxide, graphene and char as microwave absorbers and susceptors.45-47 It was therefore anticipated that the hydrothermally treated sample would absorb microwaves to some extent. However, since the analyses indicated presence of some remaining PP in the C20-210-20, SiC powder was used as a susceptor for additional heating of the sample.48

A morphological study of the products as seen in Figure 5 showed that the modification gave rise to changes in the material, where additional small fragments were formed and covered the matrix in the modified sample (Figure 5a). The solid state modification resulted in a mass yield of 40 % and images of the powder implied that the modified product was more homogenous in size as compared to the unmodified product. However, the modified product kept large fragments in intact in higher degree after dispersion in acetone (Figure 5b), which could be due to crosslinks in the material.

Figure 5. SEM images and pictures of products before and after modification (a) and SEM images after dispersion in acetone (b).


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XPS and FTIR analyses on the unmodified and modified products were performed to probe the chemical differences. XPS confirmed the introduction of oxygen due to thermal oxidation as the C/O ratio decreased from 7.4 to 5.6 through the modification (Figure 6). Wide survey spectra showed that the unmodified product also contained small amounts of N and Cl, which originated from the HNO3/HCl solutions used for the hydrothermal treatment (Figure 6a and d). NaCl was previously observed with WAXD in the unmodified product and XPS showed likewise trace amounts of Na at a binding energy of 1070 eV.49,50

Focus was put on attributing the main peaks in each spectrum. High resolution spectra of C 1s and O 1s of the unmodified product are given in Figure 6b and c. The C 1s showed the presence of C-C,H/C=C, C-O, C=O and COOH with binding energies of 284.4, 286.1, 287.2 and 288.9 eV.36,42 C-N and C-Cl have reported peak binding energies of around 286.0 eV,51,52 which are close to the value of C-O. In the O 1s spectra, two peaks at 531.9 and 533.7 were assigned to C=O/COOH and C-O.53 The modified product had the same oxygen functionalities as the unmodified product (Figure 6e and f). There was an additional broad peak at 290.6 eV for the modified product that could be due to π−π∗ shake up, which indicates aromaticity.53,54

The N1s spectrum displayed the presence of small amounts of organic nitrogen (Figure S6). These peaks were allocated at 400.0 and 410.2 eV.55 In the modified product, interestingly, an additional nitrogen group was given by 399.0 eV, possibly attributed to the pyridinic nitrogen.56 In Cl 2p both C-Cl and inorganic chlorine were noted for the unmodified product, where the inorganic chlorine was attributed to the binding energies below the approximate value of 200



eV.52 The modified product consisted of the same elements but without Cl. The ratio of C-O,N,Cl and C=O/COOH was 0.66 for the unmodified and 1.0 for the modified product. Since the fraction of C-O,N,Cl to (organic N+C-Cl) also increased from 2.35 to 8.35 after modification, it was confirmed that the amount of C-O bonds increased in relation to C=O/COOH.

Figure 6. XPS wide survey spectra of C20-210-20 and mC20-210-20 (a, d) with the corresponding high resolution spectra of C 1s (c, d) and O1s (e, f) obtained from the deconvolution of the fitted curves.


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The modified product mC20-210-20 showed the same FTIR vibration stretches as the unmodified product: C=O, C=C, CH3, CH2 and C-O at the wavenumbers 1704, 1596, 1465, 1376 and 1217 cm-1, respectively (Figure 7a). Though, both the relative difference in the absorption between C=O and C=C and the broad contour of C-OH/COOH around 3000 cm-1 decreased and shifted to 3300 cm-1 for the modified product, confirming the XPS results. Another change noticed by FTIR was the vanishing of the sp3 CHx stretches for the modified product as seen at 2900 cm-1 before modification. The organic N and Cl were not noticed in FTIR, which could be explained by twofold causes. First, XPS is even more of a surface analytical method and secondly there were low amounts of N and Cl in comparison to the other elements.

As seen in Figure 7b, WAXD spectrum of the modified product showed a broad diffraction peak around 24°, which is indicative of amorphous carbon. As previously mentioned, the same was observed for some of the solid products after the hydrothermal treatment. C20-250-60, C20-25080 and C30-250-60 possessed similar WAXD patterns with a broad diffraction peak around 26°. After modification, three small diffraction peaks with 2θ values above 33° also emerged. Figure 7c displays the Raman spectra where the I(D)/I(G) ratio increased after the modification from 0.66 to 0.76. The G band was nearly constant, whereas the intensity and width of the D band increased. In amorphous carbon, the intensity correlates to sp2 clustering and the width to the disorder e.g. distribution of orders and dimensions of clusters.40 The modification was most likely an oxidation as earlier observed for LLDPE.43

The modification resulted in carbonization of mC20-210-20 in nitrogen atmosphere. mC20-21020 was a more thermally stable product with very low weight loss up to 600 °C (Figure 7d)



compared to before modification. Heat treatment of polyacrylonitrile give rise to N containing ladder type structure, which can withstand high temperatures and enable for carbonization and graphitization,57 whereas thermally oxidized LLDPE showed similar behahivor due to crosslinking.43 The modification e.g. thermal oxidation could have induced crosslinks as indicated by C=C bonds in chemcial and molecular structural analysis. For the unmodified product, the thermal degradation proceeded slowly as a function of temperature up to an onset temperature of 377 °C, whereas the modified product was still thermally stable at the same temperature. Though, in the temperature range 30-150 °C, the modified product mC20-210-20 gave rise to earlier degradation onset in comparison to the unmodified product, which most likely was arising from the supplementary oxygen groups. Altogether, for the modified product, a low weight loss with remaining weight of 86.2 % was realised at 600 °C as compared to 46.7 % for the unmodified product.


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Figure 7. FTIR (a), WAXD (b), Raman (c) and TGA at 600 °C with residues (%) (d) of C20210-20 and mC20-210-20, respectively.



CONCLUSIONS A microwave assisted hydrothermal process for carbonization of PP and post modification of the obtained product in air to further promote the carbonization were developed. The influence of the process parameters; acid amount, temperature and time on the structure of the solid products was established. All obtained solid products contained carbonized materials but a processing time of 60 min at 250 °C was demanded to completely transform PP to amorphous carbon. The mass yield of the solid products decreased as a function of harsher processing conditions, showing that the processing parameters had an impact on the structure of the formed solids, and also resulted in products with higher thermal stability and increased amount of residue at 600 °C. During the hydrothermal treatment, the solid products consisted of irregular fragments of smaller and larger sized, and sheet structures were observed in all samples. After the post modification by a microwave solid state process of the solid product, C20-210-20, the C/O ratio decreased due to thermal oxidation. Most interesting was the pronounced thermal stability of the modified product with the large remaining residue of 86.2 % at temperatures up to 600 °C, which indicates good potential for the developed process for further carbonization of PP. The microwave assisted hydrothermal treatment and post modification in air provides new insights in to future possibilities for the upcycling of PP to interesting carbon materials.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: The supporting Information includes description of the microwave set up for solid state process, MFI and rheology analysis of original PP, SEM, Raman, TGA and DSC analysis of the solid


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fraction after microwave hydrothermal treatment, and XPS analysis before and after solid state modification and analysis of the liquid phase after the microwave treatment.

AUTHOR INFORMATION Corresponding Author *M.H., Email: [email protected]. Notes The authors declare no competing financial interest.

Acknowledgments The authors gratefully acknowledge financial support from Stiftelsen Olle Engkvist Byggmästare (grant number 2015/441). Astrid Ahlinder is greatly appreciated for help with the rheology measurements.

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For Table of Contents Use Only

Synopsis: Sustainable and circular economy requires new processes that can give plastic waste a second life.


Microwave Assisted Hydrothermal Carbonization and Solid State

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