Upgrading Mixed Agricultural Plastic and Lignocellulosic Waste to Liquid Fuels by Catalytic Pyrolysis

Abstract

Agriculture generates non-recyclable mixed waste streams, such as plastic (netting, twine, and film) and lignocellulosic residues (bluegrass straw/chaff), which are currently disposed of by burning or landfilling. Thermochemical conversion technologies of agricultural mixed waste (AMW) are an option to upcycle this waste into transportation fuel. In this work, AMW was homogenized by compounding in a twin-screw extruder and the material was characterized by chemical and thermal analyses. The homogenized AMW was thermally and catalytically pyrolyzed (500–600 °C) in a tube batch reactor, and the products, including gas, liquid, and char, were characterized using a combination of FTIR, GC-MS, and ESI-MS. Thermal pyrolysis wax products were mainly a mixture of straight-chain hydrocarbons C₇ to C₄₄ and oxygenated compounds. Catalytic pyrolysis using zeolite Y afforded liquid products comprised of short-chain hydrocarbons and aromatics C₆ to C₂₃. The results showed a high degree of similarity between the chemical profiles of catalytic pyrolysis products and gasoline.

1. Introduction

Plastics have long been used in agriculture to increase the yield and quality of the crop [1]. At the end of the harvest season, agricultural lignocellulosic and plastic wastes are often disposed by open burning or landfilling. Improper disposal of agricultural mixed waste (AMW) can have significant adverse environmental consequences; for example, open burning causes significant air pollution [2], and landfilling leads to the generation of greenhouse gases [3] and microplastics [4]. Alternative waste management technologies, such as thermochemical conversion, can help mitigate the negative environmental impact of AMW. Particularly, because agricultural plastic waste is petroleum-based, it is a good candidate to be exploited for product and energy recovery [5]. The use of thermochemical conversion technologies, such as pyrolysis, for lignocellulosic agricultural residue results in low quality energy products. The liquid pyrolysis products (bio-oil) of agricultural residue contain numerous oxygenated compounds, such as sugars, aldehydes, ketones, acids, and phenols. The presence of these compounds leads to low heating values, thermal instability, and corrosiveness of the energy product [6]. In contrast to the lignocellulosic residue, synthetic polymers deliver liquid products (oils and waxes) of higher quality due to their high carbon content. Co-pyrolysis of lignocellulosic biomass with synthetic polymers has shown promise in enhancing the properties of the obtained oil.

Studies have shown that co-pyrolysis of lignocellulosic biomass and synthetic polymers generates liquid products of high quality. Paradela et al. have reported that an increase in the plastics content in the blend of pine, plastics, and tires not only increases liquid yield (from 33% to 92% w/w) and favors the formation of lighter compounds (less distillation residue) but also promotes the conversion of aromatic compounds into alkanes and alkenes [7]. Rutkowski studied the bio-oil characteristics of beverage carton packaging waste and reported that non-catalytic pyrolysis of their feedstock leads to the formation of levoglucosan as a major liquid product of cardboard decomposition [8] and long-chain hydrocarbons as the product of thermal decomposition of polyethylene (PE) layers [9]. Chen et al. also reported that the co-pyrolysis of wastepaper and PE significantly enhances the oil yield and the fuel properties of the obtained oil [10]. Waste plastics exhibit synergistic effects during co-pyrolysis and provide the proper hydrogen/carbon ratio. Waste plastic is a good hydrogen source, and its use as a co-reactant in the catalytic pyrolysis of biomass increases aromatic carbon efficiency and decreases coke production. Up to 40% of gross waste plastics are made up of PE, making it a great candidate for catalytic co-pyrolysis [11].

Other studies have shown that pyrolysis of plastic waste can result in the generation of a mixture of hydrocarbons under various pyrolysis conditions [12,13,14,15,16,17]. However, there are concerns about the economic and energy consumption feasibility of the process. Moreover, quite often, the pyrolysis process produces liquid oil that contains long carbon chain compounds [18]. The oil has lower quality because of its low octane number, presence of solid residues [19], and impurities such as sulfur, nitrogen, chlorine, and phosphorous [20]. To overcome these issues, catalytic pyrolysis of plastics has become a topic of interest in the past decade [21]. Various catalysts have been utilized, such as Red Mud and ZSM-5 [22], fluid catalytic cracking (FCC) [23], HZSM-5 [24], zeolite Y [25], ferric oxide (Fe₂O₃) [26], and natural zeolite [27], to enhance the quality of the catalytic pyrolysis oil.

The first step in catalytic pyrolysis is the thermal cracking on the external surface of the catalyst. The porous structure inside the catalyst works as a channel for selective movement and breakdown of large hydrocarbon chains into smaller fragments [23]. Generally, the degradation of heavier alkenes occurs on the outer surface of the catalyst, and further degradation and product selectivity take place in the internal pores of the catalyst [28]. It was possible to combine the catalyst with the feedstock in the reactor or with the organic vapors produced in a separate catalyst chamber. Lopez et al. [22] and Chen et al. [15] reported that a liquid phase or direct contact with feedstock improves the cracking process by reducing the reaction temperature and retention time. However, in the case of direct contact, it is difficult to recover the catalyst due to the sticky nature of plastic feedstock [27]. Several catalysts, such as zeolite Y and ZSM-5, have shown to be effective under vapor phase contact with feedstock [22].

Catalyst characteristics such as surface area, pore size, pore volume, and acidity are the main factors influencing its activity in the pyrolysis process [28]. Syamsiro et al. have reported that using a catalyst with a high surface area allows for more contact between reactants and the catalyst surface, which results in an increased rate of cracking reaction to produce more gases than liquid oil [27].

The three main types of catalysts used in catalytic pyrolysis include FCC, silica-alumina catalysts, and zeolites [27]. FCC catalysts are mainly used in petroleum refineries for cracking heavy oil into gasoline and liquid oil petroleum. These catalysts have been used in the pyrolysis process successfully [27]. Silica-alumina catalysts are amorphous catalysts that have Lewis acid sites as electron acceptors and Brønsted acid sites with ionizable hydrogen atoms [27]. It has been shown that the acidity of these catalysts affects the production of liquid oil from plastic waste, and lower acidity results in higher yield [29]. Zeolite catalysts are crystalline alumino-silicates molecular sieves that have a 3D framework consisting of cavities and channels. The main characteristic of these catalysts is their ion-exchange capabilities and open pores. Different ratios of SiO₂/Al₂O₃ in zeolites determine their reactivity and affect the final products of the pyrolysis process. Zeolite catalysts generally increase volatile hydrocarbon production and have a low rate of deactivation.

Among these catalysts, FCC catalysts increase liquid oil yields and the use of spent catalysts (i.e., previously used in refineries) instead of virgin catalysts makes them more economical [30]. It is noteworthy that the introduction of zeolite Y in FCC catalyst formulations in the 70s and 80s resulted in a drastic increase in the gasoline yield [31].

Yuan et al. co-pyrolyzed rice husks and PVC with MgO/MgCO₃, which resulted in a significant decrease in acid content and increased hydrocarbon content [32]. Miandad et al. have reported that natural and synthetic zeolite catalysts can be used for the catalytic pyrolysis of four major types of plastic wastes such as PE, PS, PP, and PET. Their resultant liquid oils have high higher heating values (HHV, 40.2–45 MJ·kg⁻¹), which is similar to conventional diesel [33]. Syamsiro et al. also used zeolite Y and natural zeolite catalysts for sequential pyrolysis and catalytic reforming of municipal plastic wastes and produced high-quality liquid and solid products with higher HHV than those of biomass and low-rank coal [27]. Generally, the primary function of the catalysts is to increase the proportion of lighter hydrocarbons in the oil through cracking reactions [34] and improve the overall process energy efficiency [22] (i.e., achieving higher quality products at lower temperatures). Zeolite Y has been used as the main cracking catalyst in FCC. It contains an internal porous structure that can convert longer-chain hydrocarbons to smaller molecules through the formation of carbenium ions via proton transfers in the hydrocarbon’s Brønsted and Lewis acid sites [35]. Since the pores of zeolite Y are small (7.3 Å), the larger molecules in the wax oil will need to be thermally cracked first before passing through the pores. Lee successfully used multiple zeolite catalysts, including zeolite Y, for the upgrading of pyrolysis wax oil, obtained from municipal plastic waste, in a continuous plug flow reactor at 450 °C [25]. The catalyst pore dimensions were shown to play a vital role in the conversion of wax into light hydrocarbon and catalysts with more than one dimension, such as zeolite Y, show a high conversion rate.

Thus, this work aims to investigate the thermal and catalytic (using Zeolite Y) decomposition of agricultural plastic waste mixed with lignocellulosic biomass via pyrolysis on a batch reactor and characterize the gaseous, liquid, and solid products by a combination of electrospray ionization-mass spectrometry (ESI-MS), gas chromatography-mass spectrometry (GC-MS), GC, proximate/ultimate analysis, surface area measurements, and Fourier transform infrared (FTIR) spectroscopy. The biomass and plastic waste feed stocks were also characterized by compositional analysis, thermogravimetric analysis (TGA), and FTIR spectroscopy.

2. Results and Discussion

2.1. Agricultural Mixed Waste (AMW) Characterization

Two batches of AMW were collected from rural Idaho farms and used as feedstock. The first batch contained wheat chaff and mixed plastic (CMP), and the second batch contained bluegrass straw and mixed plastics (BMP). The lignocellulosic portion of each batch was separately analyzed. The plastic portion of the waste consisted of 69% net wrap (NW), 22% twine 1 (T1), and 9% twine 2 (T2), all of which were separately analyzed and then extruded. Homogenized mixed plastic (MP) was also analyzed after the extrusion of NW, T1, and T2.

Density was used to identify the ratio of lignocellulosic content (L) to plastic (P) in each feedstock. L:P was 16.8:83.2 for CMP and 36.2:63.8 for BMP. FTIR spectroscopy was used to identify the plastic-type and lignocellulosic material as well as the blended AMW. NW was identified as PE, while T1 and T2 were polypropylene (PP) (detailed results are presented in Figure S1 in Supplementary Materials).

The results of chemical composition, proximate, ultimate, and calorific value analyses of bluegrass straw (BG), chaff, MP (extruded mixture of NW, T1, T2), BMP, and CMP are presented in

Table 1. Proximate and ultimate analyses, density, calorific values, and chemical content of bluegrass (BG), chaff, mixed plastic (MP), BMP, and CMP.

Analysis	BG	Chaff	MP	BMP	CMP
C (%)	37.0 ± 1.8	32.2 ± 1.6	74.2 ± 2.9	69 ± 3.1	72.7 ± 3.5
N (%)	1.2 ± 0.04	1.2 ± 0.06	0.9 ± 0.03	0.3 ± 0.02	0.4 ± 0.01
Ash (%)	17.1 ± 0.8	5.46 ± 0.26	1.02 ± 0.04	3.73 ± 0.17	5.9 ± 0.27
Fixed Carbon (%)	21 ± 0.9	15.5 ± 0.7	0.0	8.1 ± 0.4	5.3 ± 0.25
Volatile Matter (%)	61.9 ± 1.3	71 ± 0.9	98.98 ± 0.3	88.17 ± 0.5	89 ± 0.8
Density (g cm−3)	1.45 ± 0.07	1.34 ± 0.03	0.95 ± 0.02	1.09 ± 0.01	1.04 ± 0.04
Calorific Value (MJ·kg−1)	19.7 ± 0.4	17.5 ± 0.2	46.2 ± 0.9	35.4 ± 0.3	39 ± 0.2
Extractives (%)	2.1 ± 0.3	3.2 ± 0.5	-	-	-
Carbohydrate (%)	55.0 ± 1.8	58.0 ± 0.6	-	-	-
Lignin Content (%)	22.98 ± 0.5	23.6 ± 0.23	-	-	-

. The low N contents of BG and chaff were expected as both feedstocks have very little protein. This finding was in line with the literature [36]. Fixed carbon (FC) values of 21.5% and 15.5% for BG and chaff, respectively, were in the range reported for straw [37,38,39]. The calorific values for BG and chaff were 19.7 and 17.5 MJ·kg⁻¹, respectively. The calorific value for MP was 46.21 MJ·kg⁻¹ and was in the range of reported values [40]. BMP and CMP calorific values were 35.4 and 39 MJ·kg⁻¹, respectively. Considering the L:P ratio found via density analysis, these values were reasonable.

Lignin contents of BG and chaff were 23.0% and 23.6%, respectively. Acid-soluble lignin and Klason lignin contents were 4.34% and 18.64% for BG and 3.86% and 19.69% for chaff, respectively. Lignin values were consistent with previous findings [41]. The carbohydrate contents of BG and chaff were, respectively, 55% and 58%, and comparable to wheat straw [41].

The CH₂Cl₂ extractives yields were 3.2% and 2.1% for BG and chaff, respectively. Fatty acid methyl esters (FAME) analysis of the extract identified palmitic acid (C16:0), linoleic acid (C18:2), and oleic acid (C18:1) as the most abundant fatty acids (

Table 2. Fatty acid methyl ester analysis of bluegrass (BG) and chaff CH₂Cl₂ extracts.

Fatty Acid	Retention Time (min)	M+ (m/z)	(mg/g Dry BG)	(mg/g Dry Chaff)
Lauric Acid	24.70	200	0.04 ± 0.004	0.06 ± 0.004
Myristic Acid	28.70	228	0.08 ± 0.01	0.51 ± 0.04
Palmitic Acid	32.80	270	5.25 ± 0.4	12.47 ± 0.04
Linoleic Acid	34.72	294	0.57 ± 0.05	1.87 ± 0.09
Oleic Acid	36.08	296	0.67 ± 0.05	0.82 ± 0.03
Stearic Acid	36.56	298	0.12 ± 0.01	0.26 ± 0.02
Arachidic Acid	40.01	304	0.10 ± 0.01	0.16 ± 0.01

). Trace amounts of lauric (C12:0), myristic (C14:0), stearic (C18:0), and arachidic (C20:0) acids were also detected. FAME results were consistent with prior findings for lignocellulosic agricultural residues [42,43].

2.2. TGA

TGA was used to study the thermal kinetic decomposition characteristics of the AMW samples with and without a 50% catalyst (Figure 1). Differential thermogravimetric (DTG) curves are also shown in Figure 1. The thermal decomposition onsets, major peak, and final decomposition temperatures are given in

Table 3. Thermal degradation behavior determined by TGA of bluegrass (BG) straw, chaff, net wrap (NW), twine 1 (T1), twine 2 (T2), mixed plastic (MP), bluegrass mixed plastic (BMP), chaff mixed plastic (CMP), BMP-catalyst (1:1), and CMP-catalyst (1:1) at β 20 °C/min heating rate.

Samples	1st Onset (°C)	2nd Onset (°C)	1st Peak (°C)	Major Peak (°C)	Final Decomposition (°C)
BG	243	-	-	352	385
Chaff	291	-	-	349	401
NW	472	-	-	519	530
T1	479	-	-	511	521
T2	478	-	-	514	524
MP	482	-	-	512	526
BMP	316	488	365	520	534
CMP	310	492	332	527	539
BMP + Cat	316	423	373	447	485
CMP + Cat	330	426	346	453	484

.

According to Raveendran et al., the process of biomass pyrolysis typically involves three steps: (i) dehydration; (ii) devolatilization, which produces biochar; and (iii) the gradual transformation of the biochar that has already been produced [44]. The evaporation of water and light volatiles was responsible for the initial decomposition stage for chaff and BG, which occurred at <300 °C and was associated with very little mass loss [45]. The majority of the weight loss occurred between 300 and 410 °C, where two distinct DTG peaks were present and corresponded to the steep weight loss [46]. The weight loss between 325 and 400 °C was attributed to cellulose degradation [45,47,48]. Over 90% of the material eventually deteriorated. Levoglucosan and other oligomers were created from the breakdown of cellulose through trans-glucosidation [49]. The peak around 470 °C was likely related to the breakdown of lignin [50,51]. A modest weight loss was observed above 500 °C, which was attributed to the biochar’s sluggish transformation [52].

All plastic samples exhibited one-step breakdown [53,54]. The highest degradation rate for T1, T2, NW, and MP (extruded mixture of NW, T1, and T2) occurred between 520 and 530 °C [55]. The major DTG peaks reduced in size in the presence of catalysts because low molecular weight alkanes degraded more quickly at lower temperatures. According to other studies, the presence of zeolite catalysts with a high acidity level speeds up the breakdown of polymers into shorter fragments and gaseous products [56,57]. In thermal conversion procedures, the catalyst’s crystalline structure encourages hydrogen transfer reactions that result in high conversions [58,59].TGA was a helpful tool to establish a suitable operating temperature frame for pyrolysis. The results suggested that 500 to 600 °C was an appropriate temperature range for pyrolysis.

Based on a linear regression model, the apparent E_a was estimated using the isoconversional technique, as described below, and data given in

Table 4. Activation energy values (E_a) at various conversion factors (α) for chaff, bluegrass straw (BS), net wrap (NW), twine 1 (T1), twine 2 (T2), mixed plastic (MP), chaff mixed plastic (CMP), and bluegrass mixed plastic (BMP) determined by TGA.

Conversion	Chaff		BS		NW		T1		T2		MP		CMP		BMP
α	Ea (J/mol)	R2	Ea (J/mol)	R2	Ea (J/mol)	R2	Ea (J/mol)	R2	Ea (J/mol)	R2	Ea (J/mol)	R2	Ea (J/mol)	R2	Ea (J/mol)	R2
10%	117	0.99	152	0.986	203	0.957	246	0.996	169	0.995	236	0.987	195	0.988	176	0.999
20%	146	0.99	173	0.985	200	0.92	236	0.995	176	0.994	231	0.985	229	0.988	186	0.999
30%	155	0.99	198	0.991	201	0.949	228	0.996	179	0.993	227	0.984	214	0.994	194	0.883
40%	158	0.99	206	0.992	201	0.963	223	0.996	178	0.992	224	0.985	212	0.996	232	0.989
50%	160	0.99	212	0.993	201	0.973	219	0.996	178	0.993	222	0.984	212	0.997	228	0.994
60%	156	0.99	221	0.993	200	0.977	214	0.996	176	0.993	218	0.983	210	0.998	229	0.996
70%	170	0.98	246	0.998	200	0.981	211	0.996	175	0.993	217	0.982	209	0.998	229	0.996
80%	303	0.76	494	0.607	200	0.984	209	0.995	175	0.994	215	0.981	210	0.999	233	0.996
90%	336	0.68	312	0.480	200	0.988	207	0.995	175	0.995	214	0.98	213	0.999	235	0.995
Average	189		246		201		222		176		223		211		216

. Figure 2 shows a linear regression of the FWO method in the conversion (α) range of 10% to 90%. For BG and chaff, Figure 2a,b display two sets of parallel iso-conversional lines, one set representing 10% ≤ α ≤ 70% and the other representing α ≥ 80%. For the T1, T2, NW, and MP plastic samples (Figure 2c–f), the slope was similar for all conversion rates, suggesting a similar kinetic behavior. In the BMP and CMP thermographs (Figure 2g–h), there were two sets of parallel iso-conversional lines due to the mixed nature of the feedstock. The behavior was likely due to different response mechanisms between the plastic and lignocellulosic portions [46,60,61].

2.3. DSC and DTA

DSC was used to determine the melting behavior of T1, T2, and NW and the extruded samples, MP, BMP, and CMP. DSC and DTA thermograms of the analyzed samples are shown in Figure 3. Average DSC melting peaks and percent crystallization of NW, T1, T2, MP, BMP, and CMP samples are given in

Table 5. Average DSC melting peaks and percent crystallinity of NW, T1, T2, MP, BMP, and CMP samples.

Samples	First Peak Tm (°C)	$\mathbf{\Delta }{\mathit{H}}_{\mathit{m}} (\mathbf{J}/\mathbf{g})$	Xc (%)	Second Peak Tm (°C)	$\mathbf{\Delta }{\mathit{H}}_{\mathit{m}} (\mathbf{J}/\mathbf{g})$	Xc (%)
Net wrap (NW)	130.9 ± 0.4	123 ± 1	41.9 ± 0.4	-	-
Twine 1 (T1)	155.2 ± 0.3	29.8 ± 3	14.4 ± 0.6	165.6 ± 1	104 ± 2	50.1 ± 1
Twine 2 (T2)	154.0 ± 0.2	30.7 ± 3	14.8 ± 0.6	163.8 ± 1	97.1 ± 3	46.9 ± 1
Mixed Plastic (MP)	133.3 ± 0.5	106 ± 2	40.1 ± 0.5	160.2 ± 0.8	12.1 ± 2	57.7 ± 1
BMP	131.6 ± 0.4	60.8 ± 1	23.3 ± 0.7	158.9 ± 0.8	7.53 ± 2	32.8 ± 0.3
CMP	132.7 ± 0.3	75.0 ± 1	28.9 ± 0.8	159.7 ± 0.7	9.51 ± 2	40.3 ± 0.3

. All thermograms show a major and a minor endothermic melting peak, with the distinct peaks for MP, BMP, and CMP. The NW thermogram show a single peak at 130 °C t that corresponds to PE. T1 and T2 display two endothermic peaks around 150 and 160 °C associated with PP. MP, BMP, and CMP show two peaks at 132 and 160 °C consistent with the mix of PE and PP. The peak at 132 °C was sharper and more distinct than the peak at 160 °C because of the larger proportion of net wrap compared to twine in the mixture.

DTA was used to observe the exothermic or endothermic behavior of the MP, BMP, and CMP samples at higher temperatures with and without a catalyst. The DTA thermogram of the MP showed major endothermic and exothermic peaks at 148 and 462 °C, respectively. The peak at 148 °C was associated with the plastic melting, and the peak at 462 °C was associated with the decomposition of the plastic. Consistent with these results, Çanlı et al. reported 110 °C as the starting melting point and 475 °C as the degradation point for LDPE in DTA analysis [62]. The first endothermic peak appeared at approximately 147 °C in MP (−109.4 J/g), BMP (−85.8 J/g), and CMP (−93.1 J/g), and major exothermic peaks appeared at approximately 467 °C in MP (1133 J/g), BMP (560 J/g), and CMP (664 J/g). The exothermic peaks were associated with the degradation of plastic and biomass [63]. MP, BMP, and CMP mixed with a Zeolite Y catalyst (1:1) showed two distinct endothermic peaks at approximately 146 and 430 °C. The first peak was associated with the melting of the plastic in MP (−78.7 J/g), BMP (−41.56 J/g), and CMP (−80.2 J/g) samples, and the second peak was associated with catalytic degradation of the plastic and biomass in MP (−380.1 J/g), BMP (−307.3 J/g), and CMP (−309 J/g) samples.

2.4. Analytical Py-GCMS of AMW

A wide range of volatile organic compounds were released during the thermochemical breakdown of lignocellulosic biomass and plastic. These biomass-derived substances are typically produced by the thermal cracking of lignin, hemicellulose, and cellulose fractions. Py-GCMS provided a reliable way to identify potential chemical components that may have resulted from thermal degradation processes [64]. To identify the potential pyrolysis products of each individual component of BMP and CMP, analytical pyrolysis GC-MS was carried out at 500 °C (Supplementary Material Figures S1 and S2). The detected chemicals along with their concentrations (%) are presented in the Supplementary Materials, Table S1. Acetic acid and cyclopropyl-carbinol and 1-hydroxyl-2-propanone were found to be the most abundant chemicals in chaff and BG samples. The majority of these came from the breakdown of hemicellulose [64]. There were also aliphatic oxygenated organic molecules, phenol, furan derivatives, and aromatic compounds. Py-GC-MS results for NW, T1, and T2 samples indicated the presence of PE and PP degradation products such as alkanes and alkenes from C₃ to C₃₅, suggesting a high potential for conversion to fuel [65,66].

2.5. Pyrolysis Process and Product Yield

To produce lower molar mass products, BMP and CMP samples were subjected to thermal and catalytic pyrolysis at 500, 550, and 600 °C. Most of the condensable products were collected in the U-tube condenser and impinger. The non-condensable (gaseous) products were collected in a gas sampling bag. Pyrolysis product yields are shown in Figure 4. The thermal pyrolysis at 500 °C had the highest wax yield at 70% for BMP and 73% for CMP. The highest total liquid product yield was 50% in BMP and 46% in CMP for catalytic pyrolysis at 600 °C. Compared to the thermal pyrolysis process, catalytic pyrolysis produced substantially more gaseous products, with the maximum at 500 °C at 50% for CMP. The higher percentage of gaseous products implied that significantly larger number of lighter hydrocarbons were produced during catalytic pyrolysis. These findings corroborated with previous findings for the products of catalytic pyrolysis of plastic waste [33]. Syamsiro et al. reported a 52% liquid product yield for catalytic pyrolysis of municipal solid plastic waste using zeolite Y and reported a maximum liquid product yield of 54% for catalytic pyrolysis of polystyrene using natural zeolites [27]. The slightly higher yield figures in this work may have been the result of adding the impinger collection stage. All samples were catalytically pyrolyzed initially with the new catalyst and three further times with the recovered catalyst. The catalyst remained effective, and the product yields from experiments using recovered catalyst remained comparable to those from the initial experiment using new catalyst. The product yields were within the reported range given in the literature [66].

2.6. Products Characterization

2.6.1. Analysis of Wax and Liquid Products by ESI-MS

Positive ion ESI-MS of wax and liquid products of thermal and catalytic pyrolysis of BMP and CMP was carried out to measure the molar mass distribution of volatile and non-volatile compounds in the wax and liquid products (Figure 5 and Supplementary Material Figure S3). The results of the computed M_w and M_n are provided in

Table 6. Weight (M_w) and number (M_n) average molar mass of thermal pyrolysis and catalytic pyrolysis of BMP and CMP at 500, 550, and 600 °C were determined from positive ion ESI-MS data.

Sample Name	Pyrolysis	Temperature (°C)	Mn	Mw	Monomer/Oligomer
BMP	Thermal	500	695	952	0.29
		550	662	873	0.33
		600	645	907	0.35
	Catalytic	500	594	827	0.31
		550	562	787	0.37
		600	506	753	0.41
CMP	Thermal	500	702	969	0.27
		550	690	979	0.30
		600	648	879	0.34
	Catalytic	500	647	895	0.29
		550	622	859	0.36
		600	592	837	0.39

.

Significant [M+H]⁺ peaks were found in the thermal pyrolysis products of BMP and CMP at m/z 125, 139, 151, 163, 165, 179, 193, and 323, which were tentatively attributed to guaiacol, hydroxymethylfurfuryl, ethyl guaiacol, eugenol/isoeugenol, coniferyl aldehyde, and cellobiosan. These compounds were derived from the lignocellulosic bluegrass or chaff portion of the BMP and CMP [8,67,68]. A series of ions (m/z 14 apart) corresponding to hydrocarbon dienes from C₁₄ (m/z = 196) to C₂₇ (m/z = 252), alkenes from C₇ (m/z = 113) to C₃₃ (m/z = 282), and alkanes from C₇ (m/z = 102) to C₃₈ (m/z = 298) were also identified in the ESI-MS. There were several intense peaks in the m/z 300–350 range, possibly associated with lignin [69,70,71]. The ESI-MS results were further confirmed by the GC-MS results presented in the next section.

Products of catalytic pyrolysis did not contain any peaks for oxygenated compounds or dienes, in contrast to those of thermal pyrolysis. Instead, the results showed substantial [M+H]⁺ peaks for aromatics from C₈ (m/z = 109) to C₁₃ (m/z = 179), alkanes from C₇ (m/z = 105) to C₁₅ (m/z = 215), and alkenes from C₈ (m/z = 115) to C₁₀ (m/z = 143). Benzene and toluene were not observed because they had a [M+H]⁺ below m/z 100. The ESI-MS findings demonstrated that, in comparison to thermal pyrolysis, catalytic pyrolysis led to greater breakdown of aliphatic molecules into short-chain hydrocarbons and aromatics.

The M_w and M_n for the products of thermal and catalytic pyrolysis and their monomer to oligomer ratio are listed in Table 6. During thermal and catalytic pyrolysis, as temperature increases, the M_n decreases by an average of 7% (thermal) and 15% (catalytic) in BMP and 8% (thermal) and 9% (catalytic) in CMP. Thermal degradation and sample reactions with the catalyst led to the drop in average molar mass [66]. Ion intensities were used to estimate the composition of monomer (m/z 100–300) to oligomer (m/z 301–2000) compounds (Table 6). The ranges were selected to represent low and high molar masses [8]. In thermal pyrolysis, the monomer to oligomer ratio increased by 21% in BMP and 26% in CMP. In catalytic pyrolysis, the ratio increased by 32% in BMP and 41% in CMP. The larger increase in the ratio during catalytic pyrolysis was the result of the formation of substantially more short-chain alkanes and aromatic compounds.

2.6.2. Analysis of Wax and Liquid Products by GC-MS

The wax and liquid products of thermal and catalytic pyrolysis were analyzed by GC-MS. The wax product of thermal pyrolysis of BMP and CMP was brown in color. The wax samples contained a variety of long-chain alkanes, alkenes, and dienes as well as oxygenated chemicals. Alkanes and olefins with different chain lengths (C₇–C₄₄) and a negligible number of aromatic hydrocarbons (0.26 to 0.36% in BPM and 0.41 to 1% in CMP) were produced during thermal pyrolysis (Supplementary Material Figure S4 and Table S2). These results corroborated previous waste plastic thermal pyrolysis findings [72]. A significant number of oxygenated compounds were observed in the wax product (9 to 16% in BPM and 16 to 22% in CMP). In addition, long-chain alkanes and alkenes (C₁₇–C₃₈) were the most abundant substances in the condenser, and short-chain alkenes were the most abundant substances in the impinger (C₇–C₁₀) (Supplementary Material Figure S5 and Table S3). The results suggested that the conversion was incomplete during thermal pyrolysis. The compounds from BMP and CMP pyrolysis were only partially cracked and their average chain lengths decreased with temperature [67]. The presence of oxygenated compounds and the lack of aromatics were discouraging factors as they significantly lowered the quality of the liquid product.

The catalytic pyrolysis product was a yellow, transparent liquid. The catalytic pyrolysis products displayed a chemical composition profile (Figure 6 and Figure 7) that resembled that of gasoline (Supplementary Material Table S4) [73,74]. The samples contained a mixture of aromatics (C₆–C₂₃) and short-chain alkanes (straight-chain and isomerized) commonly found in gasoline, such as toluene p-xylene, o-xylene, mesitylene, benzene, ethylbenzene, naphthalene, 1-methyl-, 1,2,4-trimethyl benzene, and 1-ethyl-2-methyl-benzene. The alkane, olefin, and aromatic compositions were higher than those observed in gasoline. The chemical profiles of the catalytic pyrolysis liquid products produced at 550 °C for BMP and CMP were closest to that of gasoline (

Table 7. Yields (g/100g) of branched alkanes, straight alkanes, alkanes, diene, aromatics, and oxygenated compounds in the pyrolysis (thermal and catalytic) products at 500, 550 and 600 °C from BMP and CMP. The proportions are also shown for gasoline as a basis of comparison.

Pyrolysis	Sample Name	Alkanes (g/100 g)		Olefins (g/100 g)		Aromatics (g/100 g)	Oxygenated (g/100 g)
		Branched	Straight	Alkene	Diene
Thermal	BMP500	3.02	27.56	50.32	8.89	0.26	9.96
	BMP 550	3.32	19.89	55.49	9.66	0.46	11.19
	BMP 600	2.81	18.15	51.37	10.76	0.36	16.54
Catalytic	BMP 500	3.79	4.21	1.6	1.58	88.82	0
	BMP 550	12.84	6.16	1.28	1.17	78.54	0
	BMP 600	12.63	37.99	14.86	1.01	33.51	0
Gasoline		46.9	0.5	4.00		48.6
Thermal	CMP500	4.06	20.73	49.11	9.07	1.02	16.01
	CMP 550	5.26	21.26	49.66	10.11	0.41	13.29
	CMP 600	4.17	15.83	43.65	13.54	0.41	22.40
Catalytic	CMP 500	11.98	3.19	8.02	3.08	73.73	0
	CMP 550	7.68	17.01	19.28	2.34	53.70	0
	CMP 600	5.89	40.59	8.04	0.58	44.89	0

). As anticipated, the products collected at the impinger were shorter chain compounds (Figure 7 and Supplementary Material Table S5) than those collected from the condenser (Figure 6 and Supplementary Material Table S6). Toluene, 2-methyl- pentane, and p-xylene were the most abundant compounds [60]. The existence of aromatic compounds and the absence of oxygenated compounds represented a major improvement over thermal pyrolysis products [66]. The aromatic chemical profile of the catalytic pyrolysis products in this study closely matched the one reported by Bagri et al. for catalytic pyrolysis of polyethylene [75]. In comparison with thermal pyrolysis, catalytic pyrolysis resulted in significant changes to the carbon number range in products. However, the range of aromatic carbon numbers in the products of catalytic pyrolysis was considerably wider (Supplementary Material Table S7). This result suggested that aromatic carbon mass increased and alkane and olefin carbon mass decreased (Supplementary Material Tables S5 and S6). There were several common steps in aromatization reaction pathways. In mixed feedstock such as BMP and CMP, cellulose must first be cracked and deoxygenated to create small olefins such as C₂–C₅. The small olefins were oligomerized to create C₆–C₁₀ olefins, which proceeded through hydrogen transfer processes to create dienes, and then cyclization and aromatization reactions to create aromatics [11].

Previous studies showed that the amount of aromatic chemicals increased when zeolite Y was added to the process. Furthermore, the 3D structure of zeolite Y was essential for the transformation of wax into light hydrocarbons [25]. Zheng et al. reported that the highest yield of aromatic hydrocarbons in catalytic pyrolysis was for LDPE and PP, both of which were present in BMP and CMP [11]. Long-chain hydrocarbons produced by thermal pyrolysis can be transformed into lighter-branched (isomerized) hydrocarbons and aromatics, like those found in gasoline and this study, when using zeolite Y in the catalytic pyrolysis process [76,77]. A list of all identified substances and their abundance concentration is given in the Supplementary Material Tables S2–S6.

2.6.3. Gas Analysis by GC-MS

Catalytic pyrolysis led to more efficient breakdowns than thermal pyrolysis of AMW, which subsequently resulted in more gaseous compounds. Zeolite Y facilitated the secondary breakdown of the condensable vapors [54]. The gaseous products from thermal and catalytic pyrolysis were collected and directly analyzed. Thermal pyrolysis products consisted of alkanes, and alkenes (C₁ to C₆) such as methane, ethyl-cyclopropane, 1-hexene, 1-pentene, and 2-butene in both BMP and CMP. Oxygenated compounds including dimethyl-cyclopropane, acetaldehyde, CO₂, and CO were also identified in both samples. In general, CMP showed a greater number of identified gaseous products likely because of the lower L:P ratio (Supplementary Material Table S8). Products of catalytic pyrolysis included alkanes, alkenes, and aromatics (C₁ to C₇) [78]. CO₂ and CO were the only oxygenated compounds identified in catalytic pyrolysis gaseous products (

Table 8. Identified gas products of catalytic pyrolysis of BMP and CMP at 500,550 and 600 °C. The units are percent compound of total collected gas.

Compound Name	Formula	M+	RT	BMP 500	BMP 550	BMP 600	CMP 500	CMP 550	CMP 600
			Min	%	%	%	%	%	%
Carbon monoxide	CO	28	1.13	72.9	76.1	32.9	89.5	56.3	3.98
Methane	CH4	16	1.2	-	12.7	8.36	-	13.2	0.75
Carbon dioxide	CO2	44	1.48	-	-	15.72	-	1.65	3.99
Ethylene	C2H4	28	2.02	6.48	1.33	15.29	4.41	3.92	5.99
Ethane	C2H6	30	2.55	2.20	1.06	6.42	0.64	3.05	5.80
Propene	C3H6	42	8.77	1.00	2.20	8.52	0.53	2.69	15.7
Propane	C3H8	44	9.57	2.40	1.72	2.03	0.65	3.18	15.1
Isobutane	C4H10	58	16.24	3.11	1.99	1.39	0.84	5.67	17.8
2-methyl-1-propene	C4H8	56	17.15	0.12	0.44	1.03	-	-	4.90
2-Butene	C4H8	17.59	56	0.14	0.27	1.14	-	2.23	0.29
Butane	C4H10	58	18	0.97	0.36	1.24	-	1.42	4.89
1-Butene	C4H8	56	18.27	0.11	0.36	0.59	0.37	0.99	2.05
(Z)-2-Butene	C4H8	56	18.55	0.26	0.35	0.29	-	-	1.13
3-methyl-1-Butene	C5H10	70	22.89	-	-	0.32	-	-	0.11
2-methyl-Butane	C5H12	72	23.61	-	-	-	0.38	3.77	7.28
ethyl-Cyclopropane	C5H10	70	24.16	-	-	0.63	-	-	1.16
Isoprene	C5H8	68	24.57	-	-	0.45	-	-	0.07
1-Pentene	C5H10	70	24.76	-		0.23	0.37	1.99	3.97
1,4-Pentadiene	C5H8	68	25.77	0.96	-	0.26	-	1.02	-
2-methyl-Pentane	C6H14	86	29.61	2.56	-	-	0.41	-	0.88
2-methyl-2-Butene,	C5H10	70	29.74	5.25	0.74	0.63	-	-	0.06
2,3-dimethyl-pentane	C7H16	100	30.02	-	-	-	-	-	0.56
1-Hexane	C6H14	86	31.07	0.36	-	-	-	-	0.25
2-ethyl-1,3-Butadiene	C6H10	82	31.22	0.11	-	-	0.22	-	-
Benzene	C6H6	78	31.25	0.25	0.46	1.32	0.55	-	2.28
Toluene	C7H8	92	32.23	0.73	-	1.15	1.11	-	-

). The most abundant compound was CO in all pyrolysis experiments which has been previously observed and reported by Paradela et al. [7].

2.6.4. FTIR

The AMW components and extruded materials before pyrolysis and the thermal and catalytic pyrolysis products of BMP and CMP samples were analyzed using FTIR spectroscopy to obtain their chemical functional groups (Figure 8 and Figure 9). Using the literature as a guide,

Table 9. FTIR spectra band assignments for products of thermal and catalytic pyrolysis in BMP 500 °C, 550 °C, 600 °C, and CMP 500 °C, 550 °C, 600 °C.

Bond/ Functional Group	Therma Pyrolysis BMP			Catalytic Pyrolysis BMP			Thermal Pyrolysis CMP			Catalytic Pyrolysis CMP
	Wax			Liquid			Wax			Liquid
	500 °C	550 °C	600 °C	500 °C	550 °C	600 °C	500 °C	550 °C	600 °C	500 °C	550 °C	600 °C
	Wavenumber (cm−1)
Out-of-plane Bend O–H	-	-	-	-	618	620	-	-	-	-	-	-
Out-of-plane Ring C=C bending	-	-	-	692	692	691	-	-	-	691	691	692
=C–H Bend (Alkene)	719	719	719	-	-	-	719	719	719	-	-	-
Aliphatic CH2 Rocking	730	730	730	728	729	730	730	730	730	728	728	728
=C–H Bend (Alkene)	-	-	-	742	742	742	-	-	-	742	742	742
C–H “oop” aromatics	-	-	-	770	770	770	-	-	-	768	768	768
Aromatic C–H out of plane bending	-	-	-	784	784	784	-	-	-	784	784	784
=C–H Bending Alkene	-	-	-	795	-	-	-	-	-	795	795	795
Aromatic C–H out of plane bending	-	-	-	809	811	811	-	-	-	807	807	807
Aromatic C–H out of plane bending	-	-	-	835	835	835	-	-	-	835	835	835
Aromatic C–H out of plane bending	-	-	-	-	848	847	-	-	-	847	847	847
C–H “oop” aromatics	886	887	887	875	875	874	887	887	887	874	877	877
O–H bend carboxylic acids	908	908	908	-	-	-	908	908	908	-	-	-
CH2 wagging or twisting	970	974	974	-	960	-	974	974	974	-	-	966
=C–H bend alkenes	993	993	993	-	-	-	993	993	993	-	-	-
aromatic C-H in-plane deformation	1047	1047	1047	1034	1037	-	1047	1047	1047	1037	1038	1038
C–C–H bending	-	-	-	1120	1124	-	-	-	-	1120	1120	1120
C–H wag (–CH2X) alkyl halides	-	1166	1166	-	1157	-	1166	1166	1166	1157	1157	1157
C–H in-plane bending	-	-	-	-	1267	-	-	-	-	1271	1271	1271
CH3 Symmetric bending	1376	1376	1376	1378	1378	1378	1376	1376	1376	1377	1377	1377
C–H in-plane wagging	1461	1462	1462	1455	1455	1455	1462	1462	1462	1456	1456	1456
Aromatic skeletal vibration (C=C)	-	1472	1472	-	1495	1495	1472	1472	1472	1495	1495	1495
C–C stretch (in-ring) aromatics/Phenyl compounds	1516	-	-	1508	1508	1506	-	-	-	1506	1506	1506
C–C (aromatic) ring stretch, sp2 CH2 (olefinic) Conjugation	-	-	-	1607	1605	1606	-	-	-	1607	1607	1607
–C=C– stretch alkene	1641	1641	1641	-	-	-	1641	1641	1641	-	-	-
H–C=O: C–H stretch aldehydes	-	-	-	2730	2730	2730	-	-	-	2730	2730	2730
C–H symmetrical stretching from CH2 from aliphatic chain	2848	2847	2847	-	-	-	2847	2847	2847	-	-	-
C–H Stretch Methyl (–CH3)	-	-	-	2860	2860	2859	-	-	-	2859	2859	2859
C–H asymmetrical stretching from CH2 from aliphatic chain	2915	2915	2915	2921	2921	2921	2915	2915	2915	2923	2923	2923
C–H asymmetrical stretching from CH3 from aliphatic chain	2957	2960	2960	2961	2964	2962	2960	2960	2960	2957	2957	2957
sp2 CH2 olefinic (nonterminal = CHC)	-	-	-	3017	3018	3016	-	-	-	3016	3016	3019
O-H stretching Cellulose, Hemicellulose, Lignin	3077	3076	3076	-	3050	3049	3080	3076	3076	-	-	-
O–H stretching vibration, H–bonded, (alcohols, phenols)	3407	-	-	-	3411	3407	-	-	-	-	-	-

(wax, liquid), and Supplementary Material Table S9 (char) provide band assignments for all samples.

2.6.5. FTIR of AMW and Extruded Materials

NW and twine (T1 and T2) spectra resembled those of PE and PP (Supplementary Material Table S10 and Figure S6). At 2949–2850 cm⁻¹, a high, sharp absorption band was observed that corresponds to C–H stretching vibrations and distinguishes saturated hydrocarbon bonds. The presence of alkenes was indicated by the significant absorption peaks at 1650 cm^–1, and 1000–717 cm^–1 as those regions represent =C–H stretching vibration, –C=C– stretching vibration, and –C=C– bending vibration, respectively. Due to the scissoring vibration of CH₂, the absorption bands at 1460 cm⁻¹ indicated the presence of the methylene group [79]. Band assignments at 1370, 1730, and 2914 cm⁻¹ represented all three lignocellulosic components including cellulose, hemicellulose, and lignin [80].

2.6.6. FTIR of Wax and Liquid Products

FTIR spectra of thermal pyrolysis products (Figure 8) resembled that of paraffin [66]. The O-H stretching vibrations (3600–3200 cm⁻¹) appeared to have been eliminated by thermal pyrolysis in all samples [81]. FTIR spectra bands around 2915 cm⁻¹ and 2847 cm⁻¹ showed that aliphatic functional groups exhibited both symmetric and asymmetric C–H stretching.

FTIR spectra of the liquid products from catalytic pyrolysis of BMP and CMP showed a high degree of similarity with that of gasoline (Figure 9). The liquid products had bands at 2960 cm⁻¹ and 2925 cm⁻¹ associated with the asymmetric stretching of methyl (CH₃) and methylene (CH₂), respectively. The bands between 720 and 840 cm⁻¹ that could be attributed to the aromatic C–H out-of-plane bending [82]. For all bands between 1375 and 1610 cm⁻¹, the liquid products showed lower intensities. C–H deformation bands were observed at 1465 cm⁻¹. The bands at about 1608 cm⁻¹ were associated with aromatic C–C stretching vibrations, and the band around 1375 cm⁻¹ was attributed to CH₃ symmetrical deformations. The coupled vibrations of the methyl and methylene groups’ symmetric stretching were responsible for the vibrational band between 2875 and 2850 cm⁻¹. In comparison to the one found in gasoline, these bands were present in all liquid products, but they were stronger and had a smaller shoulder (for methylene stretches) [82,83,84].

2.6.7. Gas Analysis by FTIR

The gaseous products from thermal and catalytic pyrolysis of BMP and CMP were analyzed by FTIR spectroscopy (Supplementary Material Figures S7 and S8). Natural gas (consisting of methane, ethane, propane, butane, pentane, and hexane), CO, CO₂, N₂, and H₂ were analyzed separately as reference standards. Gaseous products from thermal and catalytic pyrolysis for BMP and CMP showed the presence of natural gas (methane), CO, and CO₂ fingerprints which aligned with the GC-MS results.

2.6.8. FTIR of Solid Products

The solid product (char) from the pyrolysis of AMW was analyzed by FTIR spectroscopy (Supplementary Material Figure S9). FTIR band and functional groups are given in Supplementary Material Table S9. The FTIR spectra of thermal and catalytic pyrolysis char samples only showed bands associated with lignocellulosic materials, suggesting that the plastic position in both BMP and CMP had fully degraded. In lignocellulosic biomass, cellulose, hemicellulose, and lignin were the three primary components [80]. Absorptions due to C–H stretching occurred at 2914 cm⁻¹ and were likely associated with cellulose, hemicellulose, and lignin. Additionally, the band at 1417cm⁻¹ was found to be attributed to symmetric CH₂ bending vibration in cellulose, carboxyl vibration in glucuronic acid with xylan, and C–H in-plane deformation with aromatic ring stretching in lignin. The aromatic ring stretching and vibration (C=C–C) in lignin was associated with the absorption band at 1584 cm⁻¹ [85]. The C–O–C stretching at the β-(1→4)-glycosidic links in cellulose and hemicellulose causes the bands at 873 cm⁻¹ [80]. The bands between 645 and 798 cm⁻¹ were aromatic C–H stretching vibrations and =C–H bending (alkene) indicating that the biochar included hydrocarbons. Biochar can be added to agricultural soil to enhance soil quality through adding aggregates and solids, expanding microbiome populations, minimizing fungal populations, and reducing the requirement for fertilizer. Crop yields and soil quality can be improved by biochar; however, certain drawbacks need to be considered [86].

3. Materials and Methods

3.1. Materials

Agricultural mixed waste (AMW) was collected from farms in northern Idaho. The AMW contains NW, twine (composed of low-density polyethylene, and PP), mixed with lignocellulosic residues such as wheat chaff and bluegrass straw (Figure 10). Zeolite Y (Alfa Aesar (45870), hydrogen, powder form, surface area: 780 m²/g, SiO₂:Al₂O₃: 30:1) was used as received. Regular gasoline (Conoco gas station in Moscow, ID, USA) was used as standard.

3.2. Density

A Quantachrome Ultra-Pycnometer 1000 was used to measure the density of all AMW components. Using each component density in Equations (1) and (2), the L:P ratio was determined.

$$Y=\frac{dF-d1}{d2-d1 }$$

(1)

$$X=1-Y$$

(2)

where, dF was the density of mixture (CMP or BMP), d1 was the density of mixed plastic, d2 was the density of lignocellulosic portion (chaff or bluegrass straw), Y was the percent of the lignocellulosic material in AMW batch, and X was the percentage of the plastic mixture in AMW batch.

3.3. Compounding

The AMW was manually fed and compounded into an extruded rod (9 mm OD) using a co-rotating twin-screw extruder (Leistritz 18 mm dia, L/D ratio of 40, 200 rpm, barrel temperature 160 °C, 4.7 kW motor, and base torque of 18%). The extruded rod was milled using a plastic granulator with a screen size of 6 mm (Sterling BP608, New Berlin, WI, USA), and the granules were re-extruded again using a K-Tron weight loss feeder operating at 0.5 kg·h⁻¹ to create a homogeneous material for subsequent pyrolysis experiments.

3.4. Calorific Value, Fixed Carbon, Volatile Matter (VM) on Raw and Extruded AMW

Calorific value was determined following the ASTM D5865-04 using a Parr oxygen bomb calorimeter (model number 1261) on 1 g of extruded sample (in duplicate). Ash content, fixed carbon (FC), and volatile matter (VM) were all measured in duplicate using proximate analysis following ASTM E870-82. Samples for FC and VM were burned at 950 °C for 7 min in a muffle furnace. Ash content was determined at 600 °C for at least 16 h. The elements C, N, and H were measured using an elemental analyzer (Costech, the ESC 4010).

3.5. Moisture Content Measurement and Wax Extraction

A Mettler Toledo moisture analyzer was used to obtain the moisture content of the separated chaff and BG samples. The lignocellulosic samples (4.0 g) were Soxhlet extracted in duplicate using CH₂Cl₂ (150 mL) for 16 h, and extractives were determined gravimetrically, according to ASTM D1108-96.

3.6. Carbohydrate, Lignin, and FAME Analysis on Lignocellulosic Residue

Fatty acid methyl ester (FAME) derivatives were used to determine lipids content on chaff and BG CH₂Cl₂ extracts. The samples (2 mg) were heated for 90 min at 90 °C in a sealed 5 mL reacti-vialTM containing CH₃OH, H₂SO₄, and CHCl₃ (1.7:0.3:2.0 v/v/v, 2 mL) to create their FAME derivatives. 1-Naphthaleneacetic acid was added as an internal standard (200 g·mL⁻¹ in CHCl₃). Gas chromatography-mass spectrometry (GC-MS) was used to analyze the FAME derivatives using an ISQ-Trace1300 (ThermoScientific) system with a ZB-5 (30 m × 0.25 mm, Phenomenex) capillary column under a temperature gradient of 40 °C (1 min) to 320 °C at 5 °C·min⁻¹. Genuine C₁₂ to C₂₀ fatty acid standards and spectrum matching with the NIST mass spectral library from 2017 were used to identify the eluted chemicals.

The extractive-free chaff and BG samples were analyzed for carbohydrate and lignin contents. Klason and acid-soluble lignin content were measured by digesting extractive-free biomass (200 mg) in sulphuric acid (2 mL, 72%) for 60 min at 30 °C, followed by secondary hydrolysis (4% H₂SO₄, 30 min, 121 °C) in an autoclave in accordance with ASTM D 1106-96. The Klason lignin concentration was determined gravimetrically, and the acid-soluble lignin was determined by measuring the absorbance at 205 nm of the filtered hydrolysate with an absorption coefficient of 110 L·g⁻¹·cm⁻¹ (Genesys 50, Thermoelectron).

According to ASTM E 1758-01, neutral carbohydrate analysis was carried out on the secondary hydrolysis filtrate (5 mL). The monosaccharides were measured using differential refractive index detection (Waters model 2414) with HPLC (two Rezex RPM columns, 7.8 mm × 300 mm, Phenomenex) at 85 °C on elution with water (0.5 mL·min⁻¹) [22].

3.7. Thermal Analysis

3.7.1. TGA

The thermal stability and impact of catalyst addition on the thermal breakdown of AMW samples TGA was performed using a Perkin–Elmer TGA-7 instrument (Shelton, CT, USA). The AMW and mixture of AMW and zeolite Y catalyst (1:1) were used in the experiments. 8 mg of the material was heated under nitrogen (30 mL·min⁻¹) from 30 °C to 900 °C at 10 °C·min⁻¹.

3.7.2. Differential Scanning Calorimetry (DSC) and Differential Thermal Analysis (DTA)

Using Perkin–Elmer DSC-7 instruments, triplicate analyses of the AMW samples (6–8 mg) were performed from 40 to 200 °C at 10 °C·min⁻¹ under N₂ (20 mL·min⁻¹). The Pyris v.13.3.1 software was used to evaluate the DSC data. The following equation (Equation (3)) is used to determine the percentage of crystallinity in the plastics:

$$X_c=\frac{\Delta H_m}{\Delta H_0 }\times 100\%$$

(3)

$X_c$ is the percent crystallinity of the wax, $\Delta H_m$ is the melting enthalpy or enthalpy of fusion calculated from the area under the peak, and $\Delta H_0$ is the theoretical enthalpy of fusion for low-density polyethylene (293 J·g⁻¹) and polypropylene (207 J·g⁻¹).

The exothermic and endothermic reactions of the MP, BMP, and CMP samples with temperature were analyzed using a Perkin Elmer DTA-7 instrument from 40 °C to 1000 °C at 20 °C·min⁻¹ under N₂ (30 mL·min⁻¹) [51].

3.8. Analytical Py-GCMS

Using a Projector II unit (SGE Analytical Science) connected to a GC-MS (Trace 1300-ISQ, Thermo Scientific, Waltham, MA, USA), analytical pyrolysis was carried out, in duplicate, at 500 °C, and the compounds were separated on a ZB-5 capillary column (30 m × 0.25 mm Ø, 0.25 µm coating, Phenomenex) from 50 °C (1 min) to 250 °C (10 min) at 5 °C·min⁻¹. By comparing mass spectra, utilizing NIST 2017 library matching, and comparing the compounds to standards, the compounds were identified. Each compound’s relative abundance was determined relative to the CO₂ peak.

3.9. Thermal and Catalytic Pyrolysis

Samples were thermally pyrolyzed at three temperatures (500, 550, and 600 °C) in a quartz tube reactor (20 mm ID × 300 mm) under N₂ (100 mL·min⁻¹) using a mass flow controller (Dakota instruments). For catalytic pyrolysis experiments, a 60 mL·min⁻¹ N₂ flowrate was used. The reactor setup is shown in Figure 11. All reactor components were weighed before and after each experiment to be able to determine a mass balance and yield. Samples (1.0 g) were secured between glass wool plugs inside a carrier glass tube (16 mm dia × 125 mm) and placed in the furnace heated zone. For catalytic pyrolysis, the sample (0.5 g) was placed in a carrier tube and held in place with glass wool and sandwiched between a mixture of Zeolite Y and sand (1:1, 1.0 g on each side), and secured with class wool plugs. The pyrolysis products were condensed using a U-tube condenser immersed in liquid nitrogen followed by an impinger (10 mL of CH₂Cl₂ containing 1,2,4-trichlorobenzene as internal standard) to capture lighter-weight hydrocarbons. In separate experiments, the gaseous products were collected in a Tedlar gas sampling bag (500 mL, SASSCO) after the condenser.

3.10. GC-MS

The products from thermal pyrolysis and catalytic pyrolysis were analyzed in duplicates by GC-MS (Trace 1300-ISQ, Thermo Scientific, Waltham, MA, USA). The wax oil and liquid samples (1.0 mg) were dissolved in CH₂Cl₂ (1 mL) containing 1,2,4-trichlorobenzene as an internal standard. Separation was achieved using a temperature program of 40 °C (1 min) to 320 °C at 5°C·min⁻¹ on a ZB-5 capillary column (30 m × 0.25 mm Ø, 0.25 µm coating, Phenomenex). The gas products were injected directly into the GC-MS using a 100 µL syringe and separation was achieved using a TG-BondQ packed column (30 m × 0.32 mm Ø, 10 µm thickness, Thermo Scientific, Waltham, MA, USA) with a temperature program of 35 °C (5 min) to 150 °C (10 min) at 5°C·min⁻¹.

3.11. ESI-MS

The molar mass of wax and liquid products from the thermal and catalytic pyrolysis experiments was determined by negative ion ESI-MS (m/z 100–2000) on a Finnigan LCQ-Deca instrument (Thermo-Quest). Samples (2 mg mL⁻¹) were dissolved in CH₂Cl₂/methanol/acetic acid (50:49:1) and injected into the ESI-MS at 10 µL min⁻¹. The capillary voltage and ion source are 50 V at 275 °C and 4.5 kV, respectively. The number-average molar mass (M_n) and weight-average molar mass (M_w) were determined using Equations (4) and (5), respectively, where M_i was mass after accounting for the charge and N_i was the ion intensity.

$$M_n=\sum N_i{M}_i/\sum N_i$$

(4)

$$M_w=\sum N_i{M}_i{}^2/\sum N_i{M}_i$$

(5)

3.12. Fourier Transform Infrared Spectroscopy (FTIR)

FTIR spectra of solid samples were obtained using a Thermo-Nicolet iS5 spectrometer with a ZnSe attenuated total reflection (iD5-iTR) accessory. For the dark char samples, a Ge iD5-iTR accessory was employed. Gas was collected and analyzed on a Thermo-Nicolet iS10 FTIR spectrometer equipped with a gas cell (KBr 32 mm windows and 100 mm path length). Omnic v9 software is used to correct the baseline and average the FTIR spectra (Thermo-Nicolet).

4. Conclusions

Agricultural mixed waste (AMW) comprising of plastic (netting and twine) and lignocellulosic fiber (wheat chaff and bluegrass) was successfully characterized to determine its composition. The AMW was compounded by extrusion to produce a homogeneous feedstock and then pyrolyzed into wax (paraffins and olefins), liquid oil (oxygenated compounds), gas and char products. The highest wax/oil yields were achieved at 600 °C. The wax products contained oxygenated compounds but could potentially be used as bunker fuel in marine vessels. The gaseous products could be used as a petrochemical feedstock. Catalytic pyrolysis of the AMW using Zeolite Y (catalyst) was successfully applied to produce aromatic compounds in about a 45% yield at 600 °C. The profile of the aromatic components was similar to gasoline, making the liquid product suitable for use as a “drop-in“ fuel. The recovered catalyst proved to be as effective as the new catalyst, which could help with the economics of the industrial implementation of the process.