# Co-pyrolysis of beech wood and polyamide-6: Effect of HZSM-5 catalyst on the properties of pyrolysis oils

William de Rezende Locatel, Chetna Mohabeer, Dorothée Laurenti, Y. Schuurman, Nolven Guilhaume

► To cite this version:

William de Rezende Locatel, Chetna Mohabeer, Dorothée Laurenti, Y. Schuurman, Nolven Guilhaume. Co-pyrolysis of beech wood and polyamide-6: Effect of HZSM-5 catalyst on the properties of pyrolysis oils. Journal of Analytical and Applied Pyrolysis, 2022, 167, pp.105696. 10.1016/j.jaap.2022.105696. hal-04287953

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Submitted on 15 Nov 2023

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<figure><img src="image_1.png" /></figure>
1 Co-pyrolysis of beech wood and polyamide-6: Effect of HZSM-5 catalyst on the properties
2 of pyrolysis oils

3 William de Rezende Locatel, Chetna Mohabeer, Dorothée Laurenti, Yves Schuurman, Nolven
4 Guilhaume*

5 Univ. Lyon, Université Claude Bernard Lyon 1, CNRS, IRCELYON – UMR 5256, 2 Avenue
6 Albert Einstein, F-69626 Villeurbanne Cedex, France

7 * Corresponding author, nolven.guilhaume@ircelyon.univ-lyon1.fr

8

9 Abstract

10 Biomass residues and waste can be converted into bioliquids by pyrolysis, which can contribute to
11 both the production of renewable fuels and the recycling of waste. In this paper, the pyrolysis of
12 pure beech wood (BW) and of a mixture of BW + polyamide-6 (PA6), a nitrogen-containing
13 plastic, was investigated to assess the effect of PA6 on the properties of bio-oils, obtained either
14 by thermal pyrolysis or by pyrolysis with ex-situ catalytic treatment of the vapors. The presence
15 of 20 wt.% PA6 in the blend affected the pyrolysis products yields and distribution (gases, liquids,
16 chars), but also the composition of the bioliquids. In absence of a catalyst, the production of
17 phenolic compounds (guaïacol, syringol...), sugars, furans decreased significantly with the
18 BW+PA6 blend compared to pure BW, and N-containing compounds derived from PA6 were
19 found in the bioliquids. When HZSM-5 catalyst was applied for the treatment of vapors, a
20 beneficial effect of the catalyst on the reduction of the sugars and furans content in the bioliquids
21 and on the production of deoxygenated compounds was observed with pure BW. This effect was
22 mitigated with BW+PA6 blends and the formation of fully deoxygenated compounds was almost
23 totally suppressed, suggesting that the strong catalyst acid sites were poisoned. Caprolactam was
24 by far the main product of PA6 degradation found in the bioliquids, but the N-containing by-
25 products were different depending on the absence or presence of HZSM-5 catalyst. In particular,
26 the formation of 5-cyano-1-pentene in a fairly large proportion was only observed in the presence
27 of the catalyst.

28 **Keywords:** Biomass pyrolysis; polyamide pyrolysis; co-pyrolysis; Bio-oils; Catalytic upgrading.

29

30
## 1. Introduction

Plastics have assumed a crucial role in modern societies. Since the 1950s, the production and consumption of plastics has grown exponentially due to their multiple properties, such as versatility, durability, mechanical and chemical inertness, lightness, low cost etc. In 2019, the global plastic production reached 370 million tons[1]. Plastic materials can be employed in many sectors such as packaging, building and construction, automotive, electrical and electronics. Thus, it is expected that a significant part of generated plastic will end in municipal solid waste (MSW) to be disposed of with other waste materials or incinerated.

According to a report on solid waste management published in 2018[2], the world waste production is expected to increase to 3.4 billion tons by 2050, while the plastic fraction present in MSW was estimated between 6 and 13 wt.% in 2016, depending on the countries considered. Indeed, Areeprasert et al.[3] assembled data about the plastics present in MSW from different countries and the values fluctuated between 3 and 27%. Similarly, Patrício Silva and co-workers[4] reported plastic concentration between 2 and 20% for different countries. Additionally, these authors related the impact of the COVID-19 pandemic on the generation of waste plastics, due to the massive use of personal protective equipment (PPE), achieving a contribution of 3.5% of PPE in the plastic share of MSW that was sent to various landfill sites around the world in 2020.

The European Union fixed the targets of recycling at least 55% of municipal waste by 2025 and 65% by 2035[5]. However, recycling the plastics present in this waste is difficult due to sample contamination and to the presence of blends of different polymers. In addition, recycling plastic materials is economically unprofitable due to their low production and processing costs[6]. Therefore, thermal conversion processes such as gasification, incineration and pyrolysis, are promising alternatives to deal with this complex mixture of solid wastes.

Pyrolysis is an interesting process because it can process different types of feedstocks and allows converting their organic fractions into bio-oils, which can then be integrated into conventional refinery processes (fluid catalytic cracking or hydroprocessing) to produce fuels, or be used as a source of high-value products[7,8]. Several studies[9–13] and the recent review by Ansari et al.[8] reported the beneficial effects of processing plastics and biomass together. However, the co-pyrolysis of plastic and biomass can be affected by different parameters such as
60 the biomass and plastic types, the biomass to plastic ratio, the process parameters (e.g. temperature,
61 residence time, heating rate...) and the reactor configuration[14].

62 Table 1 brings together some recent studies devoted to co-pyrolysis of biomass and plastics.

63
<table><caption>Table 1 - Summary of recent publications on co-pyrolysis of biomass and plastics</caption><thead><tr><td>Reference</td><td>Catalyst</td><td>Biomass</td><td>Catalyst/biomass ratio</td><td>Plastic</td><td>Plastic/biomass ratio</td><td>Conditions</td></tr></thead><tbody><tr><td>Yu, 2021[15]</td><td>HZSM-5</td><td>wallnut shell</td><td>1:8; 1:4; 1:2; 1:1</td><td>LDPE</td><td>1:1</td><td>450 – 850°C</td></tr><tr><td>He et al., 2021[12]</td><td>HZSM-5</td><td>corn stalk</td><td>1:1</td><td>HDPE</td><td>0:1; 4:1; 3:1; 2:1; 1:1; 2:1; 3:1; 4:1; 1:0</td><td>550 – 800°C</td></tr><tr><td>Qian et al., 2021[16]</td><td>Hierarchical ZSM-5</td><td>Douglas fir</td><td>0.75:1</td><td>LDPE</td><td>0; 0.15; 0.5; 0.85; 1</td><td>429 – 571°C</td></tr><tr><td>Dyer, Nahil, and Williams, 2021[17]</td><td>ZSM-5</td><td>waste wood pellets</td><td>2:1</td><td>HDPE, LDPE, PP, PS and PET</td><td>1:1 and 1:9; 1:4 (only for HDPE and PS)</td><td>500°C catalytic bed and temperature ramp of 10°C/min to 500°C</td></tr><tr><td>Likun, Zhang, and Fan, 2021[18]</td><td>USY and ZSM-5</td><td>torrefied bagasse</td><td>1:1</td><td>HDPE, PS, EVA and PP</td><td>1:9; 3:7; 1:1; 7:3</td><td>600°C</td></tr><tr><td>Praveen Kumar and Srinivas, 2020[9]</td><td>Spent FCC catalyst</td><td>groundnut shell</td><td>1:10</td><td>PP and PS</td><td>1:2; 1:1; 2:1</td><td>510°C</td></tr><tr><td>Lin et al., 2020[19]</td><td>HZSM-5</td><td>corn stover</td><td>1:1</td><td>HDPE</td><td>1:1</td><td>550°C</td></tr><tr><td>Razzaq et al., 2019[20]</td><td>(Fe, Ni, Co, Zn) ZSM-5</td><td>wet straw</td><td>2:1</td><td>PS</td><td>1:1</td><td>500 – 650°C</td></tr><tr><td>Y.-M. Kim et al., 2017[21]</td><td>HZSM-5 and Al-MCM-41</td><td>torrefied yellow poplar</td><td>10:1</td><td>HDPE</td><td>1:1</td><td>600°C</td></tr><tr><td>B.-S. Kim et al., 2016[22]</td><td>ZSM-5 and HY</td><td>cellulose</td><td>10:1</td><td>PP and LLDPE</td><td>1:1</td><td>500 and 600°C</td></tr></tbody></table>
The plastics most frequently studied recently were polyolefins, such as PE and PP, with plastic concentrations around 50 wt.% and the catalysts employed were essentially zeolites, particularly ZSM-5. Thus, co-pyrolysis with these main plastics has been exhaustively studied and its impacts on the pyrolysis process has already been well investigated. As general trends, an increase in the production of liquid aromatics and a decrease in the formation of light aromatics and chars were identified.

Studies involving plastics that contain heteroatoms, such as N atoms in polyamides, are scarce. Polyamide-6 (PA6) is a whitish semi-crystalline thermoplastic polymer; its structure is depicted in Figure 1. It has a wide range of applications due to its properties of high strength, rigidity, chemical resistance and simple processing. Thus, PA6 is used in the automotive and construction industries, in electrical and electronic materials, in fibers applied for clothing and, more recently, in the confection of personal protective equipment (PPE), such as disposable surgical masks[23]. As a consequence of the COVID-19 pandemic, the concentration of this plastic in MSW has significantly increased.

<figure><img src="image_2.png" /><figcaption>Figure 1 – Structure of polyamide-6</figcaption></figure>

In this work, the co-pyrolysis beech wood (BW) chips, as lignocellulosic biomass, and PA6 was studied and coupled with catalytic upgrading of the pyrolysis vapors using a HZSM-5 catalyst. For this purpose, the pyrolysis products (gases, liquids and solids) were characterized in depth using various analytical techniques, specifically optimized for this type of samples. This study aims at assessing the role of the catalyst on the products composition after catalytic treatment of the pyrolysis vapors and the effect of PA6 on the catalytic activity, by comparing experiments carried out in the absence or in the presence of the catalyst, using pure BW or a blend of BW + 20 wt.% PA6.
2. Experimental section

2.1. Feedstocks

The lignocellulosic biomass used consists of beech wood chips supplied by J. Rettenmaier & Söhne; they were sieved to obtain particle sizes between 0.75 and 1.7 mm. Polyamide-6 (BASF - Ultramid® B) was supplied as pellets, which were crushed and sieved before use to obtain particle sizes in the same range as the wood chips. BW chips were mechanically mixed with 5, 10 and 20 wt.% of PA6 (on dry basis). All feedstock samples were dried in an oven at 110°C for 1h before each pyrolysis experiment and stored in airtight flasks.

2.2. Catalyst

Zeolite HZSM-5 with a molar Si/Al ratio of 40 was supplied by Céramiques Techniques Industrielles (CTI) Salindres, France. The catalyst characterization has been described previously[24] and its main properties are summarized in Table 2.

<table><caption>Table 2 - Properties of HZSM-5 catalyst</caption><thead><tr><td>Property</td><td>Value</td></tr></thead><tbody><tr><td>Particle size (μm)</td><td>100-300</td></tr><tr><td>Water content<sup>a</sup> (% wt.)</td><td>3.9</td></tr><tr><td>Si/Al atomic ratio<sup>b</sup> (SAR)</td><td>40.5</td></tr><tr><td>Total SSA<sup>c</sup> (m<sup>2</sup>g<sup>-1</sup>)</td><td>423</td></tr><tr><td>Microporous surface (m<sup>2</sup>g<sup>-1</sup>)</td><td>363</td></tr><tr><td>External surface (m<sup>2</sup>g<sup>-1</sup>)</td><td>60</td></tr><tr><td>Average pore diameter<sup>d</sup> (nm)</td><td>0.55</td></tr><tr><td>Total pore volume (cm<sup>3</sup>g<sup>-1</sup>)</td><td>0.18</td></tr><tr><td>Brønsted acid sites<sup>e</sup> (μmol/g)</td><td>127</td></tr><tr><td>Lewis acid sites<sup>e</sup> (μmol/g)</td><td>45</td></tr></tbody></table>

Pyrolysis set-up

<footnote>a measured by TGA; b measured by ICP; c BET method; d from International Zeolite Association database; e measured by FT-IR of pyridine adsorbed at 150°C.</footnote>
Co-pyrolysis of beech wood and PA6 was carried-out in a lab-scale pyrolytic bench already described [25], with a few modifications to adapt the system to the mixed wood/plastic feedstocks (Figure 2). The jacketed reactor has an internal zone where the thermal pyrolysis is carried out and a bed of catalyst can be placed (or not) just below to treat the vapors which come out of the pyrolysis zone. The BW chips or BW+PA6 blends were previously dried at 110°C for 1.5 h before being placed in the gas-tight hopper and kept under N₂ atmosphere.

Unlike the previous description [25], the new inner reactor (pyrolysis zone) was a straight quartz tube, in which the solid chips were fed semi-continuously from the top for 1 h, by small increments of ≈0.045 g every 10 s, and fell down by gravity onto a quartz porous frit at the bottom of this section, where the chars and solid residues were retained. The total mass of solid chips fed in the reactor was ≈16 g in 1 h of experiment. The temperature in the pyrolysis zone was 450-500°C along the tube and 480°C on the porous frit where the wood chips were stopped. The pyrolysis vapors (formed by thermal pyrolysis) were entrained downstream by a nitrogen flow of 260 smL/min and passed through the catalyst bed (in catalytic experiments) placed just below the pyrolysis zone on a second quartz frit. The temperature of the catalyst bed was 453 ± 2°C. In this external part of the reactor, an additional gas flow composed of 240 smL/min N₂ and 20 smL/min He (internal standard) was added to prevent retro-diffusion of the vapors and to adjust the total gas flow rate. The residence time of the vapors in the pyrolysis zone was estimated at 41 s, while the contact time with the catalyst (when present) was around 2 s. The pressure in the reactor was continuously monitored by a pressure sensor connected to a safety device which cuts off the experiment automatically if the pressure exceeds 1.3 bars. The pressure measured during the experiments was 1.1-1.15 bar.
<figure><img src="image_3.png" /><figcaption>Figure 2 - Pyrolysis setup scheme</figcaption></figure>

The vapors were condensed downstream the reactor and collected in a series of traps including a condenser (1) and a round bottom flask (2) both cooled at -5°C, an electrostatic precipitator (3), a trap filled with glass beads at -5°C (4) and a trap at ambient temperature filled with silica gel beads (5), that prevented any condensable compound from being introduced in the micro-GC for online gas phase analysis. The total mass of bio-oils formed in the experiments was assessed by mass difference, by weighing the different glassware parts before and after the experiments.

Prior to starting the reaction, the reactor (empty in thermal experiments, or loaded with 2 g of catalyst in *ex-situ* catalytic experiments) was heated to 500 °C under a N₂ flow for 2 h to ensure fully stabilized temperatures in the whole system (reactor and cold traps) and to desorb the water adsorbed on the catalyst.

## 2.3. Products recovery
Non-condensable products (hereinafter called “gas phase”) were analyzed and quantified online using a micro-GC.

The fractions called “aqueous phase” and “organic phase” were recovered directly in the round bottom flask and simply separated by centrifugation; thus, they constituted the most representative share of the bio-oil produced in the process. Other fractions of bio-oils were also deposited in the downstream traps and remained stuck on the glassware parts. In order to obtain a detailed mass balance, these fractions were recovered by washing the glassware with methanol, followed by evaporation of the methanol at 50°C under a pressure of 400 mbar, yielding the “recovered organic phase”. However, the evaporation step was found to modify the composition of this bio-oil fraction due to simultaneous evaporation of volatile components. In addition, ¹³C-NMR analysis showed the presence of residual methanol in this fraction. Therefore, it was decided not to mix the “recovered organic phase” with the native “organic phase”. Finally, a part of the bio-oil could not be recovered because it was adsorbed on the silicagel beads or lost during the different steps of recovering the other fractions. This fraction was called “organic not recovered” and was quantified by weighing the glassware parts before and after experiment.

The chars were recovered from the first quartz frit, weighed and are hereinafter called “solid phase”.

## 2.4. Product characterization

Gaseous products were analyzed online using a micro gas chromatograph Inficon 3000 (SRA instruments) equipped with three analytical modules and TCD detectors, calibrated with appropriate compound mixtures. A MolSieve column equipped with a Plot U pre-column was used to analyze He, H₂ and CO; a Plot U column was used to analyze CO₂, C₂H₄ and C₂H₆; an alumina column was used to analyze the remaining hydrocarbons (C₃-C₅). The duration of each analysis was 6 minutes and the analyses followed each other for 90 minutes. Helium was used as internal standard to account for the variations of total gas flow rates at the reactor inlet and outlet.

Elemental compositions (CHNS) were obtained with a Flash2000 (Thermo Scientific) equipment. For each analysis, a known mass of sample (1-3 mg) was placed in a tin crucible and the experiment was repeated three times to determine the measurement uncertainty.
The HHV was calculated from the elemental analysis using the equation proposed by Channiwala and Parikh [26].

$$HHV = 0.3491[C] + 1.1783[H] - 0.1034[O] - 0.0151[N]$$

In this equation the HHV is expressed in MJ/kg and [C], [H], [N] and [O] in mass percentages on dry basis. This equation is valid for the range of values: C: 0.00 – 92.25 %; H: 0.43 – 25.15%; O: 0.00 – 50.00%; N: 0.00 – 5.60%.

The water content of bio-oils was determined by Karl-Fischer (KF) titration using a Metrohm Titrando 852 analyzer. The samples were diluted in THF at a concentration of 10 wt.% and the volumetric method was employed. To avoid side reactions of aldehydes and ketones with traditional KF reagents, which produce water, specific reagents for samples containing these organic compounds were chosen: Karl Fischer ROTI hydroquant C5K (5 mg of water/mL of solution) and Karl Fischer ROTI hydroquant working medium K. The calibration was carried out using a solution of 10 wt.% of H₂O in THF.

Molar mass distributions were obtained by gel permeation chromatography (GPC) using an Agilent equipment (1200 series) equipped with two PL gel columns with different pore sizes (500 and 50 Å) using THF as the mobile phase (1 mL/min). Two detectors were used, a refractive index detector (RID) and diode-array detector (DAD), collecting data each 0.4 s in the range of 200-450 nm with a resolution of 2 nm. The samples were prepared with concentration of 1 wt.% in THF and filtered before analysis (PTFE – 0.45µm). The measurements were carried out at 35°C and the calibration was performed using hydrocarbons standards (molecular weight between 86 and 1000 g/mol).

¹³C nuclear magnetic resonance (¹³C-NMR) spectra were recorded using a Bruker AVANCE 400 MHz spectrometer and the spectra were analyzed with the TopSpin 3.0 software. The analyses were carried out at room temperature with an accumulation of 4500 scans. Trioxane was used as internal standard, diluted in DMSO-D6 in a coaxial tube configuration that prevented the contact of the solvent with the bio-oil samples.

Gas chromatography coupled with a mass spectrometer was performed with a GCMS-QP2010 SE apparatus (Shimadzu) equipped with a Zebron ZB Wax-Plus column (length 60 m, internal diameter 0.25 mm, film thickness 0.25 µm). The samples were diluted in methanol (20 wt.% for
201 organic phase and 30 wt.% for aqueous phase) and filtered before the analysis (Cellulose acetate
202 – 0.45µm). The oven temperature program started at 70 °C (held for 10 min), then was heated at a
203 rate of 5°C/min up to 150 °C. At this point the heating rate changed to 3°C/min up to 250 °C (held
204 for 15 min).

205 Two-dimensional gas chromatography coupled with mass spectrometry (GCxGC-MS) was
206 carried out with an Agilent GC 6890N apparatus. The first separation was performed by a polar
207 column VF-1701 MS (I.D 0.25 mm; film thickness 0.25 µm; length 30 m) followed by a second
208 apolar column DB1 (I.D 0.25 mm; film thickness 0.25µm; length 1.5m) for the second dimension
209 of separation. The modulation was performed by a cryogenic modulator (Zoex Corporation USA)
210 each 12s. The compounds were identified by the coupled MS detector (5975B). The use of a first
211 polar column followed by a less polar column favored the separation of oxygen and nitrogen-
212 containing compounds, widely present in bio-oils[27]. The temperature program was set as
213 follows: first column from at 50°C to 300 °C with a ramp of 1.75 °C/min, then to 320°C with a
214 ramp of 1.8 °C/min. The second column went from 50 °C to 320°C at 1.8 °C/min. The samples
215 concentration was the same as described for GC-MS analyses.

216 The spent catalyst (recovered after reaction) was analyzed by thermogravimetric analysis
217 (TGA) under air (40 mL/min) using a TGA/DSC-1- Star<sup>c</sup> system (Mettler Toledo) equipped with
218 a gas controller GC200. The temperature was initially held at 25°C for 5 minutes, then increased
219 up to 900°C with a heating rate of 10°C/min.

220 3. Results and discussion

221 3.1. Characterization of pyrolysis and co-pyrolysis products

222 To assess the effect of ex-situ catalytic treatment of pyrolysis vapors, the results obtained with
223 and without catalyst (HZSM-5) have been compared for experiments with pure beech wood (BW)
224 or a blend of BW + 20 wt. % PA6. The mass balance was close to 100 % in all experiments (98-
225 103 %) and liquid products were the main fraction obtained, representing 57-67 wt.% of the total
226 mass of products. Figure 3 displays the selectivity for solid, liquid and gas products, calculated as
227 the mass of a given fraction over the total mass of products. In the absence of a catalyst, the mass
228 of solid products corresponded to those collected on the first frit glass at the outlet of the pyrolysis
229 zone, while in the presence of the catalyst it also included the mass of coke deposited on the spent
230 catalyst, which was also quantified separately.
<figure><img src="image_4.png" /><figcaption>Figure 3 - Product selectivity in experiments with pure BW and BW+PA6 (20%) blends</figcaption></figure>

For both feedstocks, the total mass of liquid products (aqueous + organic phases) decreased in the presence of HZSM-5, while the mass of gaseous products increased slightly. The selectivity towards the organic phase was higher with BW + PA6 than with pure BW, and with BW + PA6 the selectivity for the organic phase was higher in thermal than in catalytic pyrolysis.

Regarding the gas products, the increase in the formation of non-condensable gases in the presence of HZSM-5 is due to its strong acid sites that promote the cracking of pyrolysis vapors into small compounds [28,29]. This effect, however, was marginal when BW + PA6 was used. Figure 4 shows the total mass of gases produced, normalized by the mass of feedstock. The experiment with catalyst and 20% of PA6 (▲) was repeated 3 times and the error bar is displayed on the figure.
<figure><img src="image_5.png" /><figcaption>Figure 4 - Total mass flow of gaseous products normalized by the initial mass of feedstock</figcaption></figure>

The increase in gas production in the presence of HZSM-5 was significant in experiments with BW. However, this effect was marginal when PA6 was added to BW: although in the first 20 min there were slightly more gases produced in the presence of HZSM-5 (▲), afterwards no more difference was noticeable compared to experiments without a catalyst (●) and the trend ended up being reversed for the last data point. This suggests a catalyst deactivation in the presence of PA6, which seems likely related to the adsorption of N-containing compounds on acid sites than to coke deposition, given that the two data sets were close at the reaction onset. The decomposition of PA6 resulted mainly in caprolactam, as will be discussed later, but it can also generate compounds such as pyrroles that may remain adsorbed even at high temperatures.

Figure 5 shows the detailed product distribution in the gas phase.
<figure><img src="image_6.png" /><figcaption>Figure 5 - Cumulative mass of NCG normalized by the initial mass of feedstock</figcaption></figure>

Whatever the feedstock used, CO and CO₂ were the main products in all experiments, representing almost 90% of the total mass of gaseous products. In the presence of HZSM-5, the CO production increased slightly, particularly when using BW, whereas the CO₂ production did not change significantly. Indeed, in literature[30–33], the strong Brønsted acid sites present in HZSM-5 have been reported as being more efficient to remove oxygen from biomass through decarbonylation of carbohydrate-derived compounds rather than through decarboxylation reactions. Methane was also the main light hydrocarbon produced whatever the conditions used,
while the yields of light olefins (ethylene and propylene) increased strongly in the presence of HZSM-5 when pure BW was used, but less significantly with BW + PA6. Finally, the hydrogen production, which represented less than 0.1 wt.%, was similar in all experiments (the differences observed were within the margin of error).

The water content, the ultimate analysis and the HHV of the organic and aqueous phases are shown in Table 3.

<table><caption>Table 3 - Water content, ultimate analysis and higher heating value of organic and aqueous phases</caption><thead><tr><th rowspan="2"></th><th rowspan="2">%H<sub>2</sub>O</th><th colspan="4">Elemental analysis (wt.% on dry basis)</th><th rowspan="2">HHV (MJ/kg)</th></tr><tr><th>C</th><th>H</th><th>N</th><th>O*</th></tr></thead><tbody><tr><td colspan="7" style="text-align: center; font-weight: bold;">Organic phase</td></tr><tr><td colspan="7" style="text-align: center; font-weight: bold;">Beech wood</td></tr><tr><td>no catalyst</td><td>18.7±1.0</td><td>65.4±1.0</td><td>6.0±0.4</td><td>0.6±0.1</td><td>28.0±1.5</td><td>27.0±1.0</td></tr><tr><td>HZSM-5</td><td>11.9±1.8</td><td>70.6±1.2</td><td>6.4±0.6</td><td>0.5±0.6</td><td>22.4±0.6</td><td>30.0±0.2</td></tr><tr><td colspan="7" style="text-align: center; font-weight: bold;">BW + 20% PA6</td></tr><tr><td>no catalyst</td><td>21.2±0.3</td><td>66.1±1.0</td><td>7.2±0.2</td><td>5.1±0.4</td><td>21.6±0.7</td><td>29.3±0.6</td></tr><tr><td>HZSM-5</td><td>13.8±1.4</td><td>71.6±2.4</td><td>7.5±0.5</td><td>6.1±0.4</td><td>14.8±2.2</td><td>32.2±0.7</td></tr><tr><td colspan="7" style="text-align: center; font-weight: bold;">Aqueous phase</td></tr><tr><td colspan="7" style="text-align: center; font-weight: bold;">Beech wood</td></tr><tr><td>no catalyst</td><td>48.3±1.2</td><td>52.1±0.4</td><td>6.2±3.0</td><td>1.2±0.6</td><td>40.4±2.6</td><td>21.3±3.8</td></tr><tr><td>HZSM-5</td><td>78.9±1.3</td><td>61.3±1.6</td><td>3.9±3.2</td><td>2.1±1.3</td><td>32.7±2.2</td><td>23.5±2.6</td></tr><tr><td colspan="7" style="text-align: center; font-weight: bold;">BW + 20% PA6</td></tr><tr><td>no catalyst</td><td>56.2±0.1</td><td>60.7±2.5</td><td>6.4±2.8</td><td>6.2±0.8</td><td>26.7±1.6</td><td>25.9±2.7</td></tr><tr><td>HZSM-5</td><td>65.8±2.3</td><td>54.5±1.8</td><td>6.2±7.2</td><td>8.7±1.8</td><td>30.6±5.3</td><td>23.1±8.9</td></tr></tbody></table>

*by difference

The effect of the catalyst was obvious considering the water content of the organic and aqueous phases: the organic phase obtained after *ex situ* catalytic treatment contained about half as much water, whereas the aqueous phase contained more water. Besides the decarbonylation and decarboxylation reactions mentioned previously, the oxygen from biomass-derived oligomers can be removed through dehydration of OH groups, which results in water formation [34–36]. Using BW, the total yield of water (sum of water content in organic and aqueous phases) changed from 19 without a catalyst to 21.5 % in the presence of HZSM-5. Changing the concentration of water in the organic and aqueous phases also modifies their polarity and consequently the products
distribution between the two phases, depending on their more or less polar nature. Polar compounds such as acids and sugars will tend to remain in the aqueous phase during the phase separation process. Although the effect of the catalyst was still very clear with BW + PA6, it was less important, confirming a partial catalyst deactivation related to PA6.

The nitrogen content in both organic and aqueous phases was much higher when BW + PA6 was used, and also higher in the presence of the zeolite, which suggests that the catalyst also promoted the depolymerization of PA6, leading to small N-compounds that are condensed in the liquid fractions.

The Van Krevelen diagram obtained for the organic phases is shown in

<figure><img src="image_7.png" /></figure>

Figure 6. With both feedstocks, the effect of HZSM-5 on the deoxygenation of bio-oils was evidenced by the shift to the left of O/C ratios compared to un-catalyzed experiments. The H/C ratio remained close to that of the initial biomass. A similar figure with N/C ratios as a function of H/C ratios can be found in the supplementary information (Fig. S1).
<figure><img src="image_8.png" /><figcaption>Figure 6 - Van Krevelen diagram of organic phases (on dry basis).</figcaption></figure>

The molar mass distribution of organic fractions is presented in Figure 7. The refractive index detector (RID) signal (dw_logM) was normalized by the mass of dry bio-oil used in each analysis. For experiments with pure BW, the catalyst caused the peak at ≈270 g/mol (in eq. HC) to decrease and the peak at ≈100 g/mol (in eq. HC) to increase, while the average molar mass decreased from 210 to 191 g/mol (in HC equivalent). This behavior can be associated to the promotion of biomass depolymerization in the presence of the catalyst. Conversely, when plastic was added to the feedstock, this difference was less pronounced and the average molar mass did not change in the presence of the catalyst (219 g/mol in HC eq.).
<figure><img src="image_9.png" /><figcaption>Figure 7 – GPC molar mass distribution of organic phases</figcaption></figure>

¹³C-NMR (Figure 8) was used to identify the different functional groups that can be found in the bio-oils. It allows a global analysis of all bio-oils components, independently of their solubility in solvents and without any limitation of molar mass range or volatility/thermal stability. The functional groups and their respective chemical shifts were reported in previous publications[24,25].
<figure><img src="image_10.png" /><figcaption>Figure 8 - ¹³C-NMR functional group distribution in the organic phases</figcaption></figure>

The presence of the catalyst resulted in an increase in the production of aromatics (from 46 to 64% using BW only); when using BW + PA6, however, this increase was less pronounced. Similarly, the decrease in sugars, alcohols and ethers production was less significant when PA6 was present in the feedstock. This corroborates the hypothesis of a partial deactivation of acid sites, which are responsible for the reactions of cracking, aromatization and deoxygenation [16,37–39]. In addition, a significant increase in aliphatic C-C units was observed in co-pyrolysis experiments, which could be attributed to the production of compounds originating from the degradation of PA6.

GC-MS analyses provided a more detailed insight on the composition of bio-oils, in terms of compounds identification. Due to the large number of compounds present in the bio-oils, the calibration of each compound was not possible and the analysis was hence not quantitative, but the peak areas of similar compounds can be compared. The compounds that can be analyzed using GC-MS are also limited to molecules that can be vaporized in the injector without decomposition (which probably excludes some sugars) and elute at 250°C (maximum temperature that can be applied to the column). The main groups of compounds produced by pyrolysis of beech wood identified by GC-MS have been published previously[24,25]. The detailed list of compounds
identified individually using their mass fragmentation spectra can be found in the supporting information. The characterization of the organic fractions obtained in the present study are displayed in Figure 9.

<figure><img src="image_11.png" /><figcaption>Figure 9 - Relative areas of compounds families identified by GC-MS for the organic phases</figcaption></figure>

As observed with ¹³C-NMR, GC-MS analyses also showed that the presence of the catalyst resulted in an increase in aromatic compounds families such as phenols and aromatic ethers, but this increase was less significant for experiments making use of BW + PA6. Furthermore, the effect of the catalyst in reducing the amounts of furans and sugars was clearly diminished in the presence of PA6, in agreement with the reduced production of CO, CO₂, confirming a partial catalyst deactivation[34,40]. Finally, the formation of aromatic and polyaromatic deoxygenated compounds (PAH), which was observed only in the presence of HZSM-5 catalyst, was drastically reduced with BW + PA6, suggesting that the catalytic sites responsible for their formation were almost totally suppressed.
345

Figure 10 shows the GC-MS analyses of the aqueous phase.

<figure><img src="image_12.png" /></figure>

346

347

Figure 10 - Relative areas of compounds families identified by GC-MS for aqueous phase

348

349

350

351

352

353

354

355

Using pure BW, the HZSM-5 catalyst led to a strong increase in the concentration of acid/ester and phenol families in the aqueous phase. This trend was strongly mitigated using BW + PA6, which can also be associated to the catalyst deactivation. Organic acids, especially acetic acid, can be produced by depolymerization of the three biopolymers constitutive of lignocellulosic biomass (cellulose, hemicellulose and lignin), but hemicellulose mainly contributes to the formation of organic acids by cleavage of the acetyl groups linked to the polysaccharide structure [41–44]. These reactions are promoted by acid sites and the poisoning of these sites might impair the production of such compounds.

356

357

358

GC-MS analyses also provided a specific insight on N-compounds. Bio-oils prepared from pure beech wood contained small amounts of nitrogen compounds, as shown in Table 3, from the presence in the wood of proteins involved in biological functions and plants metabolism. The bark
of beech trees also contains chlorophyll, of which the four pyrrole rings can decompose into N-compounds. Three compounds were identified: 1-(2-propenyl)-piperidine (1.92 % of the total peak area), 5,6-dihydro-5-methyl-uracil (1.46 %) and 3-methylpyridazine (0.53 %).

In order to better understand the impact of the catalyst on the degradation PA6, Figure 11 presents the main N-containing products analyzed in the organic and aqueous phases. Over 100 compounds were identified, the complete list of which can be found in the supplementary information. However most of them are present only in very small amounts.

<figure><img src="image_13.png" /><figcaption>Figure 11 - N-containing products distribution in organic and aqueous phases obtained by pyrolysis of BW+PA6 without or with HZSM-5 catalyst</figcaption></figure>

In all experiments, caprolactam was by far the main product, in agreement with the literature [45–49], and it was found in both organic and aqueous phases. This compound can be formed by two reaction pathways: (i) cleavage of two C-N bonds of the polymeric chain and cis-elimination, leading to formation of hexene-5-amide, followed by cyclization into caprolactam (Figure 12A), or (ii) hydrolysis of two amide linkages of PA6, leading to the formation of aminocaproic acid,
also followed by cyclization [46,47] (Figure 12B). This pathway could be preferred in the presence of BW, the pyrolysis of which produces a lot of water. Bockhorn et al. found that the thermal pyrolysis of pure PA6 between 380 and 460°C led to caprolactam being formed with yields >92% [47]. This suggests that in co-pyrolysis of PW+PA6 blends, the presence of wood-derived products decreased the selectivity of PA6 degradation. The secondary products differed depending on the presence or not of the catalyst. Without a catalyst, the secondary products are also formed through cleavage of C-N bonds of PA6, but without cyclization, leading to linear compounds such as acexamic acid found in the organic phase, and N-butyldiene-1-butanamine (BDBA) found in the aqueous phase. When *ex situ* catalytic treatment of vapors was applied, 5-cyano-1-pentene was the second most abundant product identified in both phases. This linear mono-nitrile can be formed by dehydration of caprolactam, catalyzed by Brønsted acid sites [49,50], which clearly exemplifies the effect of HZSM-5 catalyst in modifying the composition of bio-oils.

<figure><img src="image_14.png" /><figcaption>Figure 12: Possible reaction pathways for the formation of caprolactam from PA-6.</figcaption></figure>

## 3.2. Coke deposition on the spent catalyst

The catalyst was heavily coked after pyrolysis: the coke content was 13.4 wt.% after pyrolysis of BW and 11.8 wt.% after co-pyrolysis of BW+PA6, which suggests that PA6 contributed to reducing the catalyst coking. Ultimate analysis of this coke, however, revealed that the N-content of this coke was 0.5 wt.% after pyrolysis of BW and 6.1 wt. % after pyrolysis of BW+PA6, a twelve-fold increase much greater than could be expected from the initial PA6 concentration in the feedstock (20 wt.%). This confirms that N-containing compounds derived from PA6 adsorb on
398 the catalyst more strongly than compounds derived from BW and are probably responsible for the
catalyst deactivation.
399

400
401 4. Conclusion

402
403 In this study, the co-pyrolysis of a N-containing plastic, polyamide-6 (PA6) and beech wood (BW) was investigated in the absence or presence of a HZSM-5 catalyst for *ex-situ* treatment of
404
405 the pyrolysis vapors. The catalyst modified the composition of pyrolysis oils and products distribution in both liquid phases (aqueous and organic). Compared to the pyrolysis of pure BW,
406
407 the effect of the catalyst on the composition of pyrolysis oils was less pronounced in the presence of the plastic, due to a partial catalyst deactivation, which essentially affected its deoxygenation
408
409 properties. This deactivation can be assigned to the adsorption of N-containing compounds, produced by degradation of PA6, on strong acid sites of the zeolite that are responsible for the
410
411 formation of deoxygenated compounds. Despite this deactivation, the catalyst remained capable of promoting the depolymerization of BW and PA6, which led to an increase in the concentration
412
413 of N-containing products in the liquid phases, with caprolactam as the main product. The catalyst
414
415 also modified the N-containing by-products found in bio-oils, notably the formation of 5-cyano-1-pentene was observed only in the presence of HZSM-5 catalyst.

416
417 ACKNOWLEDGEMENTS
418

This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement N° 818120 (Waste2Road). The authors also thank Véronique Bounor-Légaré (CNRS and University of Lyon) for providing different samples of plastics.
419
420

421
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