Hydrocarbon-Rich Bio-Oil Production from Ex Situ Catalytic Microwave Co-Pyrolysis of Peanut Shells and Low-Density Polyethylene over Zn-Modified Hierarchical Zeolite

Abstract

Peanut shells, a major economic and oil crop in China, boast an abundant availability and remarkably high lignin content compared to other agricultural residues. Previous work indicated that the modified hierarchical zeolite (Zn-ZSM-5/MCM41) was effective in promoting the conversion of intermediate macromolecules during the lignin pyrolysis reaction and enhancing the yield and selectivity of liquid products. Thereby, this study aims to improve the quality of liquid products in the ex situ catalytic microwave co-pyrolysis of peanut shells and LDPE by utilizing Zn-ZSM-5/MCM41. Employing a compound center experimental design, we optimized reaction conditions through response surface analysis. The impact of microwave pyrolysis temperature and the catalyst-to-feedstock ratio on yield distribution and liquid product selectivity was explored. Results indicated a marginal increase in liquid product yield with rising pyrolysis temperatures. Moreover, an initial increase followed by a subsequent decrease in liquid product yield was observed with an increase in the catalyst-to-feedstock ratio. Optimal conditions of 450 °C and a catalyst-to-peanut hull ratio of 2.34% yielded the highest bio-oil yield at 34.25%. GC/MS analysis of the bio-oil revealed a peak in hydrocarbon content at 68.36% under conditions of 450 °C and a catalyst-to-feedstock ratio of 13.66%. Additionally, the quadratic model effectively predicted bio-oil yield and the selectivity for major chemical components. This study underscores the potential of Zn-ZSM-5/MCM41 in optimizing liquid product quality during catalytic co-pyrolysis, offering insights into bio-oil production and its chemical composition.

1. Introduction

Energy stands as an indispensable factor for human survival and progress throughout history. However, the swift pace of human development in recent years has thrust us into an imminent energy crisis. Presently, fossil fuels serve as the primary energy source across numerous countries worldwide, yet their utilization significantly pollutes the environment. This pollution predominantly arises from the array of gases, solid waste, and heat generated during fossil fuel combustion [1,2,3].The alarming depletion rate of fossil fuels, considering current global consumption levels, signals an inevitable shortage in the foreseeable future. Consequently, there exists an urgent need to explore alternative energy sources capable of supplanting fossil fuels. This pursuit becomes crucial in mitigating the escalating energy crisis and curbing environmental degradation [4,5,6].

Biomass resources rank among the most vital renewable resources available on our planet. Acting as a renewable substitute for fossil resources, biomass exhibits low sulfur content and emits zero CO₂ [7,8,9]. Chemical analysis reveals biomass as a feedstock abundant in oxygen but deficient in hydrogen [10,11]. In tandem with societal progress, the accumulation of waste plastics has surged annually, presenting a substantial pool for recycling [12]. Notably, conventional disposal methods involving direct incineration or landfilling of most waste plastics, including plastic bags and various plastic products, have resulted in severe environmental pollution and wasted energy sources [13,14,15]. Given the hydrogen-rich nature of waste plastics, their depolymerization not only yields hydrocarbon compounds but also generates significant amounts of hydrogen and hydrogen ions. Consequently, the co-feeding of biomass and waste plastics for biofuel production via pyrolysis has garnered considerable global attention, owing to their mutually advantageous traits [16,17,18,19].

The chemical nature of biomass underscores the formidable challenge of converting lignin into aromatic compounds due to its inherent resilient 3D network structure [11,20]. Additionally, the selectivity for desired products often faces disruption from repolymerization reactions during lignin depolymerization [21,22,23,24]. Addressing these hurdles in lignin valorization requires innovative solutions. Microwave-assisted heating, known for its manifold advantages over conventional electrical heating—such as rapid and selective heating, energy efficiency, and the facilitation of chemical reactions—stands out as a promising avenue. Consequently, this technique has found widespread application in processing lignocellulosic biomass feedstock, ranging from commercial residual lignin to bamboo, Kraft lignin, rice straw, biomass model compounds, and even cow dung [25,26,27,28,29,30,31,32].

Simultaneously, the strategic design and integration of catalysts play a pivotal role in augmenting both the yield and selectivity in catalytic lignin conversion processes. Among the commonly employed catalysts for lignin pyrolysis, zeolites and metal oxides take precedence, including HZSM-5 [33,34], Zn-HZSM-5 [35], HZSM-5@Al-KIT-6 [36], WO_X-TiO₂-Al₂O₃ [37], MgO [34], commercial Al₂O₃ [38], and LaFe1-xCu_xO₃ perovskites [39]. Notably, zeolites like HZSM-5 and MCM-41 catalysts have gained traction owing to their uniform molecular-sized pores and active acid catalytic sites, proving beneficial for lignin conversion [40,41,42]. However, the intricacies of the reaction mechanism reveal a challenge: intermediate molecules in lignin pyrolysis face substantial steric barriers, hindering their entry into the microporous structures of zeolite catalysts like HZSM-5. Meanwhile, although mesoporous zeolite materials like MCM-41 satisfy the need for larger pores to accommodate the sizable intermediate molecules in lignin pyrolysis, their acid strength for catalytic reactions is often inadequate. Moreover, their hydrothermal stability at high temperatures is weak, impacting their catalytic activity.

Consequently, the burgeoning focus on hierarchical zeolite catalysts stems from their amalgamation of mesoporous material pore advantages with the robust acidity and high hydrothermal stability of microporous zeolite materials. This paradigmatic shift toward hierarchical zeolite catalysts has garnered significant attention, notably due to their demonstrated ability to enhance catalytic performance and prevent coke formation due to the promotion of diffusion and avoidance of carbon deposition [43,44]. Furthermore, in order to adjust the external acidic sites and mitigate the catalyst deactivation led by coking during catalytic lignin pyrolysis, zeolites or hierarchical zeolites are usually modified by a transition metal. Literature reports demonstrated that zinc-modified zeolites were very effective in improving the selectivity for aromatics from lignin catalytic pyrolysis [35,45,46,47]. Meanwhile, our prior research corroborates this trend, revealing the remarkable catalytic efficacy of zinc-modified hierarchical zeolite (Zn-ZSM-5/MCM41) in the co-pyrolysis of lignin and LDPE, paving the way for the production of hydrocarbon-rich biofuels.

Peanuts stand as a cornerstone in both economic and oil crop sectors, with China holding the esteemed title of being the world’s largest producer. Consequently, each year, substantial by-products—such as peanut shells, accounting for 30% of peanut production—are generated. Presently, these peanut shells predominantly find use as fertilizers and animal feed, contributing to a low utilization rate and limited added value [48]. However, the chemical composition of peanut shells unveils a promising potential for further processing, given their rich lignin components [49]. This abundance of lignin positions peanut shells as viable raw materials for generating high-value products, particularly platform chemicals and liquid fuels, via leveraging emerging biorefinery technologies. Remarkably, to date, there remains a gap: the utilization of the high lignin content in biomass and the vast reserves of waste plastics. Notably, there is a conspicuous absence of reports on the production of hydrocarbon-rich bio-oil through the microwave co-pyrolysis of peanut shells and waste plastics, catalyzed by modified hierarchical zeolite. This unexplored avenue presents a unique opportunity to harness these resources efficiently, unveiling a potential solution yet to be explored. Furthermore, admittedly, the effect that reaction conditions (including reaction temperature, reaction time, and catalyst dosage) of co-pyrolysis have on product yield and selectivity is pivotal and complicated. Subsequently, the application of quadratic polynomial algorithms has made a significant contribution to simplifying experiments and cost savings during the co-pyrolysis study. Morgan et al. used a central composite design (CCD) to optimize the product yield and component composition during the catalytic co-pyrolysis of lignin and LDPE. Here, the quadratic equation for syn-gas yield, hydrocarbons, and guaicol components in the derived bio-oils were obtained [50]. Bu et al. applied response surface methodology to explore the implications of reaction conditions on the microwave co-pyrolysis of biomass and LDPE. It was pointed out that the co-pyrolysis temperature and catalyst dosage were key factors affecting either bio-oil yield or selectivity for hydrocarbons, ketones, furan, and phenolics, due to their statistical significance [51].

Hence, this study employed microwave-assisted heating to co-pyrolyze peanut shells alongside waste plastics, aiming to examine the influence of reaction parameters—such as pyrolysis temperature and the quantity of Zn-ZSM-5/MCM41 catalyst added—on both product yield and the selectivity of bio-oil chemical components. Utilizing Design Expert software (version 13), predictive models for product yield were developed through fitting. Subsequently, GC/MS analysis was employed to scrutinize the chemical composition of the bio-oil. The impact of reaction conditions on the bio-oil’s chemical composition was explored using response surface analysis. Furthermore, employing the model, a correlation between reaction conditions and the primary bio-oil components was established, facilitating the determination of optimal reaction parameters. This comprehensive investigation not only sheds light on the influence of reaction conditions but also delves into the potential of microwave co-pyrolysis for peanut shells and waste plastics. By mitigating environmental pollution and enabling the judicious utilization of resources to produce high-value liquid fuels via highly efficient catalysts, this approach holds promise for fostering a low-carbon economy and advancing circular agriculture.

2. Results and Discussion

2.1. The Effect Reaction Conditions Have on Product Yield Distribution

Table 1. Effects of reaction conditions on product yield distribution during catalytic microwave co-pyrolysis experiments.

Sample	Pyrolytic Temperature (°C)	Ratio of Catalyst to Biomass (%)	Bio-Oil (%)	Syn-Gas (%)	Volatiles (%)
K-1	308.56	8	25.02	34.7	59.72
K-2	350	4	25.55	36.36	61.91
K-3	350	12	29	38	67
K-4	450	2.34	34.25	33.12	67.37
K-5	450	8	28.4	37.47	65.87
K-6	450	8	28.15	39.89	68.04
K-7	450	8	27.75	37.65	65.4
K-8	450	13.66	24.45	38.16	62.61
K-9	550	4	25.75	38.34	64.09
K-10	550	12	30.25	34.2	64.45
K-11	591	8	32.2	28.35	60.55
K-12	450	-	22.81	39	61.81

illustrates the impact of reaction conditions on the yield distribution observed during the microwave co-pyrolysis of peanut shells and LDPE utilizing Zn-ZSM-5/MCM41 as a catalyst. The bio-oil yield ranged from 24.45% to 34.25%, reaching its maximum at 34.25% under the conditions of 450 °C and a catalyst-to-peanut shell ratio of 2.34%. Obviously, a significant portion of the bio-oil yield fell between 25% and 30%. In comparison to the control (K-12), addition of the Zn-ZSM-5/MCM41 catalyst led to a remarkable 25% increase in bio-oil yield. Furthermore, the syn-gas yield varied from 28.35% to 39.89%, predominantly within the range of 36% to 39%. The obtained volatiles accounted for approximately 60% to 68% of yield. This signified a change in residual solid product (bio-char) yield from 32% to 40% of the feedstock during catalytic microwave co-pyrolysis of peanut and LDPE facilitated by the Zn-ZSM-5/MCM41 catalyst.

The quadratic model equations for predicting the yield of bio-oil, syn-gas, and volatiles are displayed as follows:

Y_Bio-oil = 28.1 + 2.31H₁ + 0.37H₂ + 1.97H₁H₂ + 0.19H₁₂ − 2.22H₂₂

(1)

Y_Syn-gas = 38.34 − 2.57H₁ + 1.80H₂ + 1.00H₁H₂ − 3.23H₁₂ − 1.17H₂₂

(2)

Y_Volatiles = 66.44 + 0.1H₁ + 1.28H₂ − 1.18H₁H₂ − 3.21H₁₂ + 1.25H₂₂

(3)

The quadratic model presented in Equation (1) effectively predicts the bio-oil yield, demonstrating an R² value of 0.9837. This high R² value indicates a strong correlation between pyrolysis temperature, catalyst dosage, and bio-oil yield, affirming the model’s efficacy. Moreover, the p-value associated with pyrolysis temperature (0.0019) highlights its significant influence on bio-oil yield distribution in this process. Similarly, the model depicted in Equation (2) achieves an R² of 0.9454, effectively predicting syn-gas yield. The p-values for pyrolysis temperature and the ratio of catalyst to peanut shell (0.0057 and 0.0195, respectively) indicate their noteworthy impact on syn-gas yield distribution during the microwave catalytic co-pyrolysis reaction. Furthermore, Equation (3) yields an R² of 0.9409, indicating its suitability for predicting volatile yield in the catalytic microwave co-pyrolysis of peanut shell and LDPE. This underscores the predictive capability of the model for volatile yield in this process.

Figure 1 depicts the response surface analysis detailing product yield distribution during the catalytic microwave co-pyrolysis of peanut shell and LDPE, under varied reaction conditions. In Figure 1a, the overall bio-oil yield ranged between 24% and 32%. At lower pyrolysis temperatures, a gradual increase in bio-oil yield was noted, followed by a decline within the 4% to 12% range of catalyst dosage. Conversely, at temperatures above 500 °C, a clear augmentation in bio-oil yield occurred with increased catalyst dosage. Additionally, it was evident that bio-oil yield rose proportionally with reaction temperature when the catalyst dosage remained constant, attributed to heightened pyrolytic volatiles from secondary thermal decomposition reactions [11,20]. Moving to Figure 1b, a marginal rise, and subsequent decline, in syn-gas yield was observed with increasing catalyst quantity. Although an initial slight increase in syn-gas yield was noted with rising pyrolysis temperature, a significant decline occurred beyond 450 °C, attributed to competitive reactions between gas and liquid phases during biomass pyrolysis at higher temperatures. Figure 1c demonstrates the trend in volatile matter yield, indicating an initial rise and subsequent decrease with increasing pyrolysis temperature. Meanwhile, a trend toward increasing volatile yield was observed with greater catalyst dosage. The peak volatile yield (68.04%) occurred at 450 °C and with a catalyst dosage of 8%.

2.2. GC/MS Analysis of the Bio-Oils Obtained from Microwave Catalytic Co-Pyrolysis Reactions

Figure 2 displays the chemical compositions analyzed via GC/MS of bio-oils obtained from the microwave catalytic co-pyrolysis of peanut shells and LDPE. The results showcased a predominant presence of hydrocarbons, phenols, alcohols, and ethers, collectively accounting for over 90% of the detected bio-oils. Notably, hydrocarbons and phenols emerged as the two most substantial compounds, collectively contributing to over 90% of the detected bio-oils. Compared to the control (K-12), the hydrocarbon content in the bio-oils varied significantly under different experimental conditions, ranging from 50.99% to 68.36%. The maximum hydrocarbon content (68.36%) was achieved at 450 °C with a catalyst-to-peanut shell ratio of 13.66%. Aromatic hydrocarbon content, ranging from approximately 21.67% to 33.45% in the obtained bio-oils (as shown in Figure 3), proved to be a significant component influencing hydrocarbon selectivity. Interestingly, these were not observed in the control (K-12). The highest aromatic hydrocarbon content coincided with the conditions that yielded the highest hydrocarbon content (K-8, 450 °C, and a catalyst dosage of 13.66%). Additionally, phenols, another major compound, ranged from 17.81% to 30.92%, which is significantly lower compared to the control K-12 (62.89%). The lowest phenol content of 17.81% corresponded with the bio-oil exhibiting the highest hydrocarbon content (K-8). These findings reveal that the addition of the Zn-ZSM-5/MCM41 catalyst substantially augmented hydrocarbon content, especially aromatic hydrocarbons. This enhancement could be attributed to the advantageous acid and pore channel characteristics of micro-mesoporous zeolites modified by transition metals. These characteristics likely played a pivotal role in promoting the hydrodeoxygenation of phenols, facilitating aromatic hydrocarbon production during the microwave co-pyrolysis of peanut shells and LDPE [43,52].

Furthermore, GC/MS analysis revealed notable impacts of pyrolysis temperature and catalyst dosage on ether content. At a temperature of 450 °C and a catalyst dosage of 2.34% relative to the peanut shell, the obtained bio-oil exhibited an ether content of up to 12.28%. However, as either the pyrolysis temperature or catalyst dosage increased, the yield of ether compounds initially decreased before rising again. Specifically, identified ethers included compounds like 1-methoxy-2-(methoxymethyl)-benzene, 4-ethyl-1,2-dimethoxy-benzene, and 3-ethylbenzene. Interestingly, the lowest content of ethers and derivatives (~1.4%) was observed at 550 °C with a catalyst dosage of 12%. This outcome suggested that an elevated catalyst amount led to the degradation and conversion of ethers and their derivatives into other substances. This transformation might be associated with alterations in phenol and its derivatives, aromatic hydrocarbons, furans, and their derivatives.

2.3. Response Surface Analysis of the Major Chemical Components of Bio-Oil

In this study, we employed the central composite experimental design (CCD) method to assess the influence of reaction temperature and catalyst dosage on the distribution of hydrocarbons, aromatic hydrocarbons, phenols, and ethers within the resulting bio-oils. The independent variables selected were the reaction temperature (H₁, °C) and the catalyst quantity (H₂, %). Subsequently, we derived the quadratic model equations for hydrocarbons, aromatic hydrocarbons, phenols, and ethers, as follows:

Y_Hydrocarbon = 62.03 + 1.1H₁ + 1.74H₂ + 5.61H₁H₂ − 4.96H₁₂ + 1.65H₂₂

(4)

Y_{Aromatichydrocarbon} = 23.58 + 2.37H₁ + 0.972H₂ + 3.46H₁H₂ + 1.40H₁₂ + 3.64H₂₂

(5)

Y_Phenol = 20.79 − 2.37H₁ − 1.09H₂ − 1.25H₁H₂ + 4.78H₁₂ − 0.25H₂₂

(6)

The coefficients’ p-values for hydrocarbons, aromatic hydrocarbons, phenols, and ethers were 0.0003, 0.0466, 0.0496, and 0.045, respectively—each below 0.05. These outcomes signify the reliability of the models in predicting the major chemical compositions of bio-oils derived from the microwave catalytic pyrolysis of peanut shells and LDPE. The R² value of Equation (4) stood at 0.9915, indicating a strong ability of the model to illustrate the relationship between hydrocarbon content and the given variables. Moreover, the p-values associated with pyrolysis temperature and catalyst dosage for hydrocarbons were 0.0289 and 0.006, respectively, suggesting their significant influences on hydrocarbon content in this process. Similarly, Equation (5) exhibited an R² of 0.941, highlighting its applicability in predicting the content distribution of aromatic hydrocarbons during microwave catalytic co-pyrolysis. The pyrolysis temperature’s p-value concerning aromatic hydrocarbon content was 0.0252, unveiling its significant influence on the distribution of aromatic hydrocarbons. Furthermore, Equation (6) showed an R² of 0.8353, affirming the quadratic model’s effectiveness in predicting phenol content.

Figure 4 depicts the response surface analysis results showcasing the main chemical components (hydrocarbons, aromatic hydrocarbons, phenols, and ethers) within the obtained bio-oils. As shown in Figure 4a, the hydrocarbon content initially increased and then decreased with rising pyrolysis temperature. At 450 °C, the optimal hydrocarbon content of 68.36% was achieved at a catalyst/peanut shell ratio of 13.66%. Moving to Figure 4b, the aromatic hydrocarbon content exhibited an initial decline followed by an increase with increasing pyrolysis temperature and catalyst dosage. The highest aromatic content of 30.28% was observed at a pyrolysis temperature of 591 °C and a catalyst dosage of 8%. Figure 4c illustrates the phenol content, which decreased initially and then rose as the pyrolysis temperature increased. Notably, at a reaction temperature of 450 °C and a catalyst amount of 13.66%, the phenol content reached its minimum at 17.81%.

3. Materials and Methods

3.1. Materials

Peanut shells were sourced from the rural region of Zhenjiang City, Jiangsu Province, while low-density polyethylene (LDPE) was procured from China Petroleum & Chemical Corporation, Beijing, China. Prior to experiments, the peanut shells underwent a 48 h drying process in an oven set at 105 °C. Chemicals including Zn(NO₃)₂·6H₂O, NaOH, and Cetyltrimethylammonium bromide (CTAB, 99%) were purchased from Aladdin, Shanghai, China, and Sigma-Aldrich, St. Louis, USA. The ZSM-5 zeolite (Si/Al ratio 46) utilized in the preparation of Zn-ZSM-5/MCM41 was obtained from Nankai University Catalyst Company, Tianjing, China.

3.2. Catalyst Preparation

The process commenced by weighing the ZSM-5 precursor powder, characterized by a silicon-to-aluminum ratio of 46, which was then placed into a beaker. Subsequently, 100 mL of 2 mol/L NaOH solution was added and stirred vigorously with a glass rod until dissolved. This step yielded a 3 mol/L aluminum silicate solution after an hour of magnetic stirring. Following this, a CTAB solution of 125 mL, with a 10 wt% mass fraction, was introduced and stirred for an additional hour. The resulting solution was then transferred into a hydrothermal reactor for crystallization, facilitated by incubation in an oven set at 110 °C for 24 h. After cooling, the solution’s pH was adjusted to 8.5 before subjecting it to another 24 h crystallization cycle under identical oven conditions. The resulting crystallized material underwent washing and filtration using distilled water. Subsequently, the filtered solid material was dried in a quartz boat in an oven at 110 °C for 12 h. This dried sample was then placed into a high-temperature tube furnace set at 550 °C for 6 h in an atmospheric environment. An NH₄Cl solution, with a concentration of 1.0 mol/L, was added and stirred for ion exchange. Post washing, filtration, and drying, the samples were subjected to calcination in a high-temperature tube furnace, culminating in the production of ZSM-5/MCM41 composite molecular sieves. The modification of ZSM-5/MCM41 with zinc was achieved through the impregnation method.

3.3. Microwave Co-Pyrolysis Experiment

The experimental setup for ex situ catalytic microwave pyrolysis is illustrated in Figure 5. The volatiles from the microwave co-pyrolysis of peanut shells and LDPE were catalytically upgraded by an ex situ catalytic fixed-bed reactor. The microwave pyrolysis equipment (1 KW, 2.45 GHz) was procured from Nanjing Xianou Technology Co., Ltd. (Nanjing, China). A 500 mL quartz flask served as the reactor, positioned within the microwave pyrolytic chamber. Temperature measurements were facilitated using an infrared thermometer (Guangzhou Huahong Automation Equipment Co., Ltd., Guangzhou, China) with a range of 0 to 900 °C. The microwave pyrolysis/co-pyrolysis reaction temperature was correlated/controlled by adjustment of the input microwave power. In general, when the reaction temperature reached or surpassed the targeted value, the automatic temperature/power control of the microwave reactor applied a minimum input power (0–100 W) to retain the designed reaction temperature. However, the automatic temperature/power control of the microwave reactor would switch to maximum power or a set value when the detected temperature was lower than the actual target parameter. To this end, it is known that the input power setting of microwave-assisted heating may have a critical influence on the heating rate of the pyrolysis reaction; moreover, the heating rate plays a key role in the products’ yield distribution during biomass/solid organic waste pyrolysis. In concordance with our previous study on liquid bio-fuel production from biomass pyrolysis/co-pyrolysis [30,50], this work set an input power of 750 W for the co-pyrolysis reaction. The catalytic co-pyrolysis had a retention time of 15 min to ensure complete thermal cracking of the reactants. Preceding the pyrolysis reaction, the entire system was purged with nitrogen at a rate of 10 mL/min for 15 min, creating an oxygen-free environment conducive to pyrolysis. The experimental materials comprised LDPE and peanut shells, totaling 50 g in mass, with LDPE constituting 25% of the peanut shell mass, and activated carbon (5 wt% of biomass) was utilized as a microwave absorber during co-pyrolysis reaction. For the Ex Situ catalytic reforming reaction, Zn-ZSM-5/MCM41 composite zeolite was employed as a catalyst, and the catalytic fixed-bed reactor operated at 650 °C.

3.4. Experimental Design

Utilizing Design Expert software (version 13), we conducted an analysis to optimize the reaction conditions for the microwave catalytic co-pyrolysis of peanut shells and LDPE. The study focused on dependent variables encompassing bio-oil, syngas, and volatiles yields, with the independent variables being the microwave pyrolytic temperature and the quantity of catalyst added. To determine the optimal response conditions, the data underwent central composite design (CCD).

Table 2. Experimental factor level and coding.

Factor	Coding	Scope and Level
		−q (−1.41)	−1	0	1	+q (1.41)
Temperature (°C)	H1	309	350	450	550	591
Catalyst ratio (%)	H2	2.34	4	8	12	13.65

details the experimental factor levels and coding used in the analysis.

The calculation equation is as follows [52]:

M = 2^m + 2m + mD

(7)

m indicates that there are several factors, and m_D indicates the center point of the repeated experiments that need to be performed.

The response of the dependent variables can be estimated using a quadratic polynomial:

Z_j = e₀ + e₁H₁ + e_2×2 + e₁₁H₁₂ + e₂₁H₂H₁ + e₂₂H₂₂

(8)

where Z_j represents the predicted response value; H₁ and H₂ represent the independent variables; and e₀, e₁, e₂, e₁₁, e₂₂, and e₂₁ are the regression coefficients.

3.5. GC/MS Analysis of Bio-Oils

In this experiment, the chemical composition of the liquid product (bio-oil) was analyzed using a gas chromatography-mass spectrometer (Model: Agilent7890A/5975C (Agilent Technologies Inc., Santa Clara, CA, USA). The GC conditions included a DB-5 capillary column measuring 30 m × 0.25 mm × 0.25 μm. The column temperature followed a programmed procedure: starting at 40 °C, it increased at a rate of 5 °C per minute until reaching 300 °C, which was then maintained for 5 min. Helium served as the carrier gas at a flow rate of 0.6 mL per minute, with a split ratio set at 50:1. The MS conditions encompassed an EI source with an electron energy of 70 eV and a scanning range of 30–550 amu.

4. Conclusions

In this study, we employed modified hierarchical zeolite (Zn-ZSM-5/MCM41) and LDPE co-feeding to bolster bio-oil yield and selectivity for biofuel production during the microwave pyrolysis of peanut shells. Our aim was to determine optimal conditions for generating hydrocarbon-rich bio-oils. The outcomes indicated a noteworthy increase in overall bio-oil yield, up by approximately 12–30% compared to the control, with the highest yield of 34.25% achieved at 450 °C with a catalyst dosage of 2.34%. GC/MS analysis unveiled hydrocarbon contents ranging from 51% to 68.4%, with aromatic hydrocarbons being the predominant constituents in the obtained bio-oils. The peak hydrocarbon content (68.36%) emerged at 450 °C with a catalyst dosage of 13.66%. Concurrently, as hydrocarbon composition increased, a significant reduction in phenol content was observed. Response surface analysis underscored the substantial influence of pyrolysis temperature and catalyst dosage on hydrocarbon content in this process. These findings hold promise for advancing biofuel production and promoting circular economy practices by efficiently converting organic waste via catalytic thermochemical conversion.