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. 2026 Aug 17. Online ahead of print. doi: 10.1039/d6ra04762a

Catalytic pyrolysis of polypropylene over acid-modified coal gangue: product distribution and catalyst performance

Ying He a,b,c, Xuemei Zheng b,c, Deshun Kong b,c, Xulong Yang b,c, Aiyuan Ma b,c,✉
PMCID: PMC13480375  PMID: 42610085

Abstract

To address the dual challenges of coal gangue solid waste and waste plastic pollution, this work develops a series of coal gangue-based catalysts via hydrochloric acid modification of coal gangue samples with different iron contents, and applies them to the catalytic pyrolysis of polypropylene (PP). The results showed that, compared with unmodified coal gangue, the acid-modified catalyst increased the PP pyrolysis gas yield by up to 20.06 wt%, and the gas product exhibited potential as an alternative to natural gas. The initial decomposition temperature of PP was reduced by up to 78 °C. The quality of the liquid oil was significantly improved, with hydrocarbon content increasing by up to 33.26 area %, while the proportion of oxygenated compounds decreased by up to 42.97 area %. Characterization by XRF, XRD, and BET indicated that acid treatment enlarged the specific surface area and pore volume of coal gangue, promoted the formation and enrichment of active iron-based oxides, and generated acidic sites, thereby enhancing catalytic activity. This work offers a reference for the co-processing and high-value utilization of coal gangue and waste plastics.

Synergistic catalysis by iron and acidic sites on acid-modified coal gangue promotes PP dehydrocracking, boosting gas yield and liquid quality.graphic file with name d6ra04762a-ga.jpg

1. Introduction

The widespread use of plastics has led to a growing crisis of “white pollution,” with global plastic production projected to reach 1100 Mt by 2050.1 Polypropylene (PP), one of the most common general-purpose plastics, accounts for the largest share of global plastic production and is widely applied in numerous fields, including food packaging, the construction industry, medical devices, and textile fibers.2 Due to its high production volume, short lifespan, and difficulty in terms of recycling, PP plastic generates a large amount of waste, posing a threat to the environment.3 Therefore, selecting appropriate and efficient recycling technologies is crucial for mitigating plastic pollution and resource waste. The use of chemical methods to process waste plastics and recover high-value materials has garnered significant attention, with many studies focusing on converting plastics into high-value-added fuels or chemicals, such as hydrogen-rich gases, pyrolysis oil, and olefin monomers.4 Pyrolysis is a chemical method capable of converting waste plastics into high-value products, and it occupies a central position among numerous recycling methods.5 Lu et al.6 coupled a multifunctional pyrolyzer with a pyrolysis-gas chromatography-mass spectrometry (PY-GC/MS) system to investigate the composition of PP rapid pyrolysis and catalytic pyrolysis liquid oils. They found that in the presence of a fluid catalytic cracking (FCC) catalyst, the content of C9–C21 small-molecule mono-olefins in the PP pyrolysis liquid oil increased by 13.6%. Compared to conventional pyrolysis, the use of catalysts can enhance pyrolysis efficiency and product selectivity.7 Catalyst selection influences the product distribution, reaction pathways, and reaction conditions of plastic pyrolysis.8 Zeolite molecular sieves, metals, metal oxides, and carbon-based catalysts are commonly used catalytic materials in plastic pyrolysis.9 The introduction of highly efficient catalysts facilitates the catalytic pyrolysis of PP plastics to produce high-value materials such as hydrogen-rich gas,10 carbon nanotubes (CNTs),11 and light hydrocarbons.12 However, some catalysts suffer from limitations such as high energy consumption and high production costs during synthesis. Selecting low-cost materials to develop highly efficient catalysts is crucial for the cost-effective and efficient catalytic pyrolysis of plastics.13

Low-cost materials, such as natural zeolites, fly ash, and red mud, have been modified and used as catalysts in plastic pyrolysis, with some notable research progress. Kim et al.14 employed natural zeolite (NZ), bentonite, olivine, and Al-MCM-41 as catalysts, and through combined thermogravimetric and kinetic analyses found that both bentonite and NZ could effectively reduce the pyrolysis temperature of PP and lower the activation energy (Ea) of the degradation process. Eldahshory et al.15 investigated the catalytic pyrolysis of PP using kaolin, hematite, and white sand as catalysts and found that kaolin effectively improved both the yield and quality of the pyrolysis oil. Xi et al.16 used ZSM-5 and red mud as catalysts for PP pyrolysis, obtaining liquid oil yields of 30.88–67.83 wt% with an aromatic selectivity of 73.8–94.4 wt%. Nalluri et al.17 used fly ash as a catalyst for the pyrolysis of high-density polyethylene (HDPE) and low-density polyethylene (LDPE); a 5 wt% catalyst loading afforded the highest oil yield, with the HDPE pyrolysis oil composition being similar to diesel and the LDPE oil composition similar to gasoline. These studies indicate that the macroporous structure, high acidity, high Si/Al framework ratio, and the combination of iron-based oxides with a silica–alumina matrix, particularly the high content of active metal oxides, are key to the catalytic activity.

Coal gangue, a solid waste produced during coal mining and washing, is mainly composed of SiO2 (30–60%) and Al2O3 (15–40%), and contains a certain amount of active metal oxides such as Fe2O3 and CaO.18 With the structural characteristics of a natural aluminosilicate framework and active metal oxides, it has the potential to be developed into a highly active catalytic material.19 Zhang et al.20 used coal gangue as a support to prepare Ni/CG catalysts via the impregnation method for the pyrolysis of polyethylene (PE). At a Ni loading of 10 wt% and a pyrolysis temperature of 750 °C, the H2 yield reached 31.48 mmol g−1 plastic, with a concentration of 66.48 vol%. Hu et al.21 prepared Ni-based catalysts supported on coal gangue for the catalytic cracking of PE. The study found that the thermal decomposition of coal gangue generates a macroporous structure, which further enhances the interaction between the coal gangue and the supported metal, enabling the simultaneous generation of H2 and CNTs. Zhang et al.22 used coal gangue as a support to prepare coal gangue-based catalysts loaded with 10 wt% Ni for the pyrolysis of PP and PE, achieving high yields of H2 and CNTs. It was found that coal gangue not only serves as a support for Ni loading, but the Fe2O3 it contains itself also acts as active sites in the catalytic reaction, promoting the generation of CNTs and H2. Previous studies have confirmed the feasibility of using coal gangue as a support for active metals in the catalytic pyrolysis of plastics. However, the loading of active metals inevitably increases the catalyst preparation cost and process complexity.23 Notably, coal gangue itself contains active metal oxides (e.g., Fe2O3 and CaO) and naturally occurring aluminosilicate frameworks, endowing it with the potential to directly catalyze plastic pyrolysis without additional active components. Through a simple acid modification, inert constituents can be selectively leached out, the specific surface area can be increased, and more metal active sites can be exposed. This approach is expected to enhance the dehydrogenation and partial oxidation ability during plastic cracking, thereby improving the selectivity toward hydrogen-rich gas while maintaining a low preparation cost.

At present, coal gangue is mostly used as a carrier to load active metals for plastic pyrolysis. There are no literature reports on the direct use of coal gangue as a catalyst for the pyrolysis of polypropylene (PP), and the role of active components such as Fe2O3 and CaO in coal gangue itself in the catalytic pyrolysis of PP has not been well interpreted24–26 in this study, different coal gangue based catalysts were prepared by hydrochloric acid modification, and PP catalytic pyrolysis experiments were carried out to investigate the effect of acid-modified coal gangue on the yield distribution characteristics of PP pyrolysis products. Combined with XRF, XRD, BET, XPS characterization analysis, the effect of acid-modified coal gangue on the composition of PP pyrolysis gas and liquid oil and the thermal decomposition process was clarified.

2. Materials and methods

2.1. Experimental materials

The four coal gangue (CG) samples, designated CG1, CG2, CG3, and CG4 in ascending order of iron content, were all collected from Liupanshui, Guizhou, China. The results of their X-ray fluorescence (XRF) analysis are listed in Table 1. Polypropylene powder (100 mesh) was purchased from Dongguan Huachuangxin Plastic Co., Ltd. Its ultimate and proximate analyses yielded the following results: C, 86.99%; H, 13.97%; volatile matter, 99.96%; and moisture, 0.04%. Nitrogen gas (N2, industrial grade) was supplied by Guizhou Qiancheng Wangda Gas Co., Ltd. Hydrochloric acid (analytical reagent, purity ≥ 99.7%, mass fraction) was purchased from Chongqing Chuandong Chemical Group Co., Ltd.

Table 1. Chemical composition of raw coal gangue (wt%).

Sample SiO2 Al2O3 Fe2O3 TiO2 CaO K2O ZnO MgO SO3 P2O5 MnO2 Other
CG1 47.7 24.6 11.2 4.84 3.02 2.27 0.353 0.806 2.73 1.39 0.135 0.965
CG2 46.4 22.8 14.6 5.20 4.88 2.37 0.042 0.786 0.883 1.54 0.207 0.292
CG3 41.0 17.3 23.4 4.06 7.72 2.29 0.052 1.35 0.417 1.64 0.406 0.365
CG4 32.2 16.3 31.2 2.48 11.2 0.705 0.025 0.793 3.21 1.25 0.511 0.126
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2.2. Preparation of catalysts

The coal gangue was crushed, ground, and sieved through a 100-mesh screen, then dried at 105 °C for 12 h and stored in sealed containers for subsequent use. The raw coal gangue was placed in a muffle furnace and calcined at 500 °C for 3 h under an air atmosphere to remove residual combustible matter and moisture. To minimize the leaching of active iron species from the sample,27 the calcined material was mixed with 1 mol L−1 HCl solution at a liquid-to-solid ratio of 20 : 1 (mL g−1) and stirred in a water bath at room temperature for 4 h. The mixture was then filtered and washed with deionized water until the filtrate was neutral. The resulting solid residue was dried at 105 °C for 12 h, ground for 5 min, and calcined at 500 °C for 2 h in a muffle furnace. After cooling to room temperature in the furnace, the material was removed and ground for 5 min, yielding acid-modified coal gangue-based catalysts (ACG), designated ACG1, ACG2, ACG3, and ACG4, respectively. See Fig. 1 for the preparation procedure.

Fig. 1. Schematic illustration of the preparation of acid-modified coal gangue and its catalytic pyrolysis of polypropylene.

Fig. 1

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2.3. Characterization

The elemental composition of coal gangue (reported as oxides) was determined by X-ray fluorescence spectrometry (XRF, Supermini200, Rigaku, Japan). The phase structures of coal gangue before and after acid modification were analyzed by X-ray diffraction (XRD, LabX XRD-6100, Shimadzu, Japan). Porous characteristics were measured by N2 physisorption (ASAP 2020 Plus, Micromeritics, USA). The specific surface areas were calculated using the Brunauer–Emmett–Teller (BET) method, while pore size distributions and average pore diameters were determined from the desorption isotherms by the Barrett–Joyner–Halenda (BJH) method. The surface elemental composition and chemical states were characterized by X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Scientific, USA).

The pyrolysis behavior of mixtures of PP and coal gangue (before and after acid modification) was investigated by thermogravimetric analysis (TGA, STA449F3, Netzsch, Germany). The release characteristics of gaseous products during catalytic pyrolysis of PP over acid-modified coal gangue were examined by thermogravimetric analysis coupled with infrared spectroscopy (TG-IR, TGA55, TA Instruments, USA, coupled with Nicolet iS50, Thermo Fisher Scientific, USA). Quantitative analysis of the pyrolysis gas was performed using a portable infrared gas analyzer (TY-6330P, Wuhan Tianyu Intelligent Control Technology Co., Ltd., China). The liquid oil derived from pyrolysis was analyzed by gas chromatography-mass spectrometry (GC-MS, 7890A GC coupled with 5975C MS, Agilent, USA). Chromatographic separation was performed on an HP-5MS column with the following temperature program: held at 40 °C for 5 min, ramped to 300 °C at 5 °C min−1, and held for 5 min. High-purity helium was used as the carrier gas at a flow rate of 1 mL min−1, and the injection volume was 1.5 µL. Mass spectra were acquired in electron ionization (EI) mode (70 eV) with an ion source temperature of 230 °C and a quadrupole temperature of 150 °C; full-scan mode was used over the mass range m/z 33–533 with a solvent delay of 5 min. Given that the pyrolysis oil contains approximately 200 individual compounds, performing separate calibration for each constituent is impractical. Accordingly, the relative content of each component was estimated from its percentage of the total ion chromatogram (TIC) peak area.

2.4. Catalytic pyrolysis experimental apparatus and product yield calculation

Catalytic pyrolysis of polypropylene (PP) was conducted in a single-stage reactor, with a schematic diagram of the experimental apparatus presented in Fig. S1. In each experiment, PP (4 g) and a coal gangue-based catalyst (2 g) were accurately weighed and mixed uniformly, after which the mixture was loaded into a ceramic boat. This ratio was selected based on the optimization results for natural clay catalysts,15 while also taking into account the inherently low specific surface area and limited active phase content of coal gangue, so as to ensure sufficient contact between the active sites and PP. However, the high catalyst-to-plastic ratio employed in this study represents a limitation. Future work will investigate the catalytic performance at lower catalyst loadings to better assess the practical potential of this system. The boat was then placed in the middle of the quartz tube within the tubular furnace. Prior to pyrolysis, N2 was introduced into the reactor at a flow rate of 200 mL min−1 for 15 min to purge air and ensure an inert atmosphere. The sample was subsequently heated to 500 °C at a heating rate of 5 °C min−1 and held at this temperature for 1 h. Upon completion of pyrolysis, N2 at a controlled flow rate of 100 mL min−1 was introduced into the reactor for 30 min to sweep the pyrolysis gas out of the reactor. The gas was collected throughout the process using gas collection bags. After the experiment, the masses of the ceramic boat, the quartz tube, and the condenser were recorded, from which the yields of liquid, solid, and gas products were calculated.

The yields28 of the gas-liquid-solid three-phase products, the calorific value of the gas LHV,29 and the yield30 of hydrogen are calculated respectively according to eqn (1)–(5).

2.4. 1

In the formula, M1 (g) is the mass of the quartz tube and condenser before the reaction, M2 (g) is the mass of the quartz tube and condenser after the reaction, and Mplastic (g) is the mass of PP.

2.4. 2

In the formula, Mtot (g) is the total mass of the catalyst and solid carbon after the reaction, Mcat (g) is the mass of the catalyst, and Mplastic (g) is the mass of PP.

Gas yield (wt%) = 100 wt% − liquid yield − solid yield 3
LHV (MJ N m−3) = 0.126 × CO + 0.108 × H2 + 0.358 × CH4 + 0.665 × CnHm 4

In the formula, CO, H2, CH4, and CnHm are volume fractions of the gas produced.

2.4. 5

In the formula, mole of H2 produced (mmol) represents the number of H2 moles produced; mass of plastic (g) represents the mass of the plastic.

The volume fraction of each gas (H2, CH4, CO2, CO, CnHm) is calculated.31

2.4. 6
2.4. 7
2.4. 8
2.4. 9
2.4. 10

3. Results and discussion

3.1. Characterization of coal gangue before and after acid modification

The XRF analysis results of the acid-modified coal gangue (ACG) are presented in Table 2. Compared with the XRF composition of the raw coal gangue (CG) shown in Table 1, the amount of iron leached during the acid modification process was low. Among the ACG samples, the iron content was enriched in ACG1 and ACG4. As reported in the literature,27 when the HCl concentration is 1 mol L−1, the leaching rate of iron remains low, whereas the CaO and MgO contents are significantly reduced. The results in Table 2 demonstrate that by appropriately controlling the acid concentration, readily soluble alkaline metal oxides (CaO, MgO, and MnO2) and sulfates can be effectively removed from the coal gangue with minimal iron dissolution, thereby enriching the active iron species in certain ACG samples.

Table 2. Chemical composition of acid-modified coal gangue (wt%).

Sample SiO2 Al2O3 Fe2O3 TiO2 CaO K2O ZnO MgO SO3 P2O5 MnO2 Other
ACG1 52.6 25.8 12.1 4.75 0.308 2.10 0.164 0.307 0.141 1.16 0.047 0.523
ACG2 53 24.1 12.9 4.98 0.625 2.05 0.693 0.024 0.01 1.12 0.106 0.392
ACG3 48.7 17.4 21.7 5.08 1.38 2.13 0.032 1.66 0.158 1.21 0.261 0.289
ACG4 38.9 18.1 36.5 2.81 0.41 0.747 — 0.542 0.210 1.16 0.376 0.245
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Fig. 2a presents the XRD patterns of the different CG and ACG samples. The main mineral phases identified in the four CG samples are siderite (FeCO3), quartz (SiO2), kaolinite (Al2Si2O5(OH)4), calcite (CaCO3) and anatase (TiO2). Siderite is the predominant iron-bearing phase, and all four CG samples display FeCO3 diffraction peaks at 2θ values of 24.8°, 32.0°, 38.1°, and 42.2° (PDF#97-010-0678). After acid modification, CaCO3 and Al2Si2O5(OH)4 were dissolved and removed. The iron in the CG transformed from the FeCO3 phase into α-Fe2O3 and MgFe2O4 phases, with characteristic α-Fe2O3 diffraction peaks appearing at 2θ = 24.1°, 33.1°, 35.6°, and 40.9° (PDF#97-006-6756). ACG3 exhibits MgFe2O4 diffraction peaks at 2θ = 30.2° and 35.6° (PDF#97-004-3744). In ACG1, ACG2, and ACG4, FeCO3 is mainly converted to α-Fe2O3, and ACG4 shows stronger α-Fe2O3 peak intensities and higher crystallinity. In ACG3, FeCO3 primarily transforms into MgFe2O4, owing to the enrichment of Mg after acid treatment; under Mg-rich conditions, FeCO3 is more readily converted to MgFe2O4.32 The formation of α-Fe2O3 and MgFe2O4 demonstrates that the iron in the CG underwent a phase transformation from carbonate species to iron-based oxide active phases via acid treatment. Iron-based oxides typically exhibit favorable catalytic activity when used as catalysts for plastic pyrolysis.33,34

Fig. 2. Physicochemical characterization of coal gangue before and after acid modification (CG and ACG series): (a) X-ray diffraction (XRD) patterns, (b) N2 adsorption–desorption isotherms and (c) pore size distribution curves.

Fig. 2

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The specific surface area and pore structure of the coal gangue before and after acid modification were analyzed using a nitrogen physisorption analyzer. The specific surface areas and total pore volumes of the various samples are summarized in Table 3, and the corresponding N2 adsorption–desorption isotherms and pore size distributions are presented in Fig. 2. As shown in Table 3, acid modification increased the specific surface area and total pore volume of all four coal gangue samples. In particular, CG3 and CG4 exhibited pronounced increases in specific surface area, by 48% and 131.83%, respectively. Fig. 2b displays the N2 adsorption–desorption isotherms. According to the IUPAC classification, all samples before and after acid modification exhibit type IV isotherms, characteristic of mesoporous materials. After acid modification, the adsorption capacity in the high relative pressure region increased more steeply, with ACG3 showing a notably sharp rise. This further suggests that acid treatment dissolves carbonates, metal oxides, and other impurities in the coal gangue, thereby generating new mesopores.35Fig. 2c shows the pore size distributions. The pore sizes of all samples are predominantly distributed in the range of 20–100 nm, with a distinct main peak in the 25–40 nm range, indicating a typical mesoporous structure. Acid modification did not alter the dominant pore size range; instead, it cleared the pore channels, enlarged the pores, and made the pore structure more uniform. Due to differences in mineral composition, the effects of acid modification varied among the samples. After acid treatment, CG3 and CG4 displayed significantly enhanced specific surface areas and pore volumes, more uniform pore size distributions, and a higher proportion of mesopores.

Table 3. Specific surface area and pore size distribution of coal gangue before and after acid modification.

Sample S BET (m2 g−1) V pore (cm3 g−1) Average pore size (nm)
CG1 16.90 0.05 10.01
ACG1 24.29 0.09 12.61
CG2 26.07 0.06 8.07
ACG2 26.90 0.08 9.45
CG3 27.79 0.07 8.5
ACG3 41.13 0.16 13.17
CG4 11.06 0.04 11.08
ACG4 25.64 0.13 16.00
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Fig. 3a and b presents the Fe 2p XPS spectra of CG1, ACG1, CG4, and ACG4. In Fig. 3a, CG1 exhibits Fe2+ peaks at approximately 710.01 eV and 723.10 eV36 and Fe3+ peaks at approximately 712.26 eV and 726.70 eV.37 The fitted peak area of Fe2+ (69.32%) is larger than that of Fe3+ (30.68%), indicating that iron in CG1 exists predominantly in the Fe2+ valence state. For ACG1, the Fe2+ peaks appear at approximately 710.08 eV and 723.35 eV, while the Fe3+ peaks appear at approximately 712.36 eV and 726.40 eV. After modification, the relative peak area of Fe3+ increased by 54.51%, suggesting that acid modification converted most of the Fe2+ to Fe3+. This observation is consistent with the XRD results, which confirm that iron in coal gangue exists as trivalent iron-based oxides after acid modification. As shown in Fig. 3b, CG4 displays Fe2+ peaks at approximately 709.90 eV and 723.14 eV and Fe3+ peaks at 712.60 eV and 726.30 eV. The peak area of Fe2+ (59.41%) is higher than that of Fe3+ (40.59%), further confirming that iron in CG4 is also predominantly present as Fe2+. This indicates that, irrespective of the iron content, iron in unmodified coal gangue primarily exists in the divalent state. In ACG4, Fe2+ peaks are observed at approximately 709.80 eV and 723.50 eV, while Fe3+ peaks are located at 711.40 eV and 726.20 eV. After modification, the relative peak area of Fe3+ reached 70.68%, representing an increase of 30.09% compared with that before modification. These results further demonstrate that acid modification can alter the valence state of iron in coal gangue, effectively converting iron species into trivalent iron-based oxides and exposing more active sites.

Fig. 3. High-resolution XPS spectra and peak fitting results of coal gangue before and after acid modification: (a) Fe 2p spectra of CG1 and ACG1, (b) Fe 2p spectra of CG4 and ACG4, (c) O 1s spectra of CG1 and ACG1 and (d) O 1s spectra of CG4 and ACG4.

Fig. 3

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Fig. 3c and d display the O 1s XPS spectra of CG1, ACG1, CG4, and ACG4. The spectra were deconvoluted into three components: the Oα peak at approximately 529.85–530.3 eV is assigned to lattice oxygen associated with metal oxides (e.g., Fe–O, Ca–O); the dominant Oβ peak at approximately 531.4–532.1 eV is attributed to bridging oxygen within the aluminosilicate framework (Si–O–Si and Al–O–Si), with its high proportion (82.46–91.56%) consistent with the elevated SiO2 and Al2O3 contents determined by XRF analysis; and the Oγ peak at approximately 532.9–533.2 eV corresponds to surface silanol groups (Si–OH) and adsorbed water. Following acid modification, the Oα peak-area proportion increased by 10.84 percentage points for ACG1 relative to CG1 and by 7.70 percentage points for ACG4 relative to CG4. The enhanced Oα contribution indicates that acid treatment facilitates the surface exposure of metal oxide species in coal gangue, thereby increasing the density of accessible active sites.38

3.2. Distribution of PP product yields from catalytic pyrolysis of coal gangue before and after acid modification

Fig. 4 illustrates the influence of different coal gangue samples, before and after acid modification, on the product distribution obtained from the catalytic pyrolysis of PP. The three-phase product distribution (Fig. 4a) reveals that, irrespective of whether raw gangue (CG) or acid-modified gangue (ACG) is used, the pyrolysis products consist predominantly of gas and liquid oil, while the yield of solid products remains persistently low. Compared with CG, ACG catalysts significantly increase the overall gas yield and correspondingly decrease the liquid oil yield. In particular, the most pronounced increase in gas yield was observed for ACG1, derived from CG1 by acid modification, which exhibited a 20.06 percentage point rise relative to that obtained with the unmodified CG1. This enhancement may be attributed to the optimization of the pore structure of coal gangue through acid modification, along with the effective liberation and exposure of originally encapsulated iron-bearing minerals, which facilitates their directed transformation into active iron-based oxide phases such as α-Fe2O3 and MgFe2O4. These iron-based active sites can efficiently promote C–C bond cleavage, intensify the cracking and dehydrogenation of long-chain PP macromolecules, and thereby drive the secondary catalytic cracking of intermediate macromolecular liquid components into non-condensable gaseous products.39

Fig. 4. Effect of coal gangue before and after acid modification on PP pyrolysis: (a) yields of three-phase products, (b) composition of the pyrolysis gas, (c) XRD patterns after reaction, and (d) calorific value of the pyrolysis gas.

Fig. 4

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The compositional characteristics of PP pyrolysis gas under different coal gangue are shown in Fig. 4b, and the gas data from the pyrolysis of coal gangue alone are listed in Table S3. The gas generated under unmodified CG catalysis contains small-molecule hydrocarbons (CnHm), H2, CH4, CO2, and a small amount of CO, whereas in the products obtained with acid-modified ACG, CO2 and CO are absent, leaving only CnHm, H2, and CH4. Self-pyrolysis experiments of coal gangue alone (Table S3) demonstrate that when CG is pyrolyzed independently, CO2, CH4, and trace amounts of H2 are released, originating from the decomposition of carbonate minerals and the self-pyrolysis of minor associated low-rank coal.40 These results indicate that the CO2 and a portion of the CH4 detected under CG catalysis derive primarily from the coal gangue itself. The gaseous products from the separate thermal decomposition of ACG under identical conditions were also examined. None of the four ACG samples produced detectable gases, confirming that acid treatment effectively removed carbonate minerals and other gas-releasing components. In contrast, CG alone released measurable quantities of CO2 and CH4 during thermal decomposition, attributable to carbonate decomposition and residual organic matter pyrolysis. Therefore, the gas composition observed under CG catalytic conditions (Fig. 4b) reflects dual contributions from both PP cracking and the thermal decomposition of coal gangue itself. After acid modification, however, the interfering sources such as carbonates have been eliminated, and the gaseous products obtained over ACG can be regarded as the intrinsic products of PP catalytic cracking, excluding non-catalytic contributions from the catalyst matrix. This interpretation is consistent with the weakening or disappearance of carbonate diffraction peaks in the XRD patterns after modification (Fig. 2a). The above results indicate that acid modification eliminates non-catalytic interferences such as carbonate decomposition in coal gangue, so that the pyrolysis gas consists mainly of H2, CH4, and small-molecule hydrocarbons (CnHm), forming a combustible gas mixture.

A comparison of the gas compositions obtained over CG and ACG reveals a significant increase in H2 concentration upon acid modification. Specifically, the H2 concentration over CG4 was 21 vol%, whereas that over ACG4 reached 35 vol%, indicating that the α-Fe2O3 and MgFe2O4 species exposed by acid modification not only promoted the deep cleavage of C–C bonds but also likely facilitated dehydrogenation, thereby enhancing the conversion of hydrocarbon intermediates into H2.41 Except for ACG3 the XRD pattern of the spent catalyst (Fig. 4c) shows that the diffraction peaks of α-Fe2O3 weakened or disappeared, while a pronounced Fe3O4 peak emerged. This indicates that Fe3+ in the active phase was partially reduced to a mixed Fe2+/Fe3+ valence state, accompanied by a phase transformation from α-Fe2O3 to Fe3O4. The Fe3+ redox behavior suggests that lattice oxygen in the active phase participated in the dehydrogenation and partial oxidation of hydrocarbons. This is further confirmed by the O 1 s spectrum of the spent ACG4 (Fig. S3): the area percentage of the lattice oxygen Oα peak at 530.33 eV decreased from 12.48% to 6.40%, relative to the fresh ACG4. No CO or CO2 was detected in the gaseous products. This is likely because the lattice oxygen in the coal gangue preferentially oxidizes hydrocarbon fragments into oxygen-containing organic compounds rather than promoting deep oxidation to CO and CO2. It can therefore be inferred that α-Fe2O3 in the coal gangue function as the primary metal active sites for PP pyrolysis. Moreover, the H2 yield exhibited a clear upward trend with increasing iron content in the acid-modified coal gangue, further corroborating the essential role of iron-based active sites in H2 generation; higher iron contents consistently led to greater H2 production. This observation is consistent with the literature: Wang et al.42 loaded 0%–25% Fe onto activated carbon for PP pyrolysis and found that both the total gas yield and H2 yield increased continuously with Fe loading. At a Fe loading of 15%, the combined yield of gas and H2 reached 172.09 mmol g−1 plastics.

Fig. 4d compares the lower heating values (LHV) of the gases produced from the catalytic pyrolysis of PP over CG and ACG catalysts. Acid modification markedly improves the fuel quality of the gaseous products, with the LHV values obtained from the ACG series being consistently higher than those from CG. Specifically, the LHV of the gases generated over the CG catalysts ranges from 33.78 to 53.61 MJ N m−3, whereas that over the ACG catalysts ranges from 42.67 to 55.24 MJ N m−3. According to the relevant Chinese standard for city gas,43 the LHV values of the gases produced by both CG and ACG catalytic systems far exceed the minimum requirement of 7 MJ N m−3, highlighting their considerable potential for resource utilization. The gaseous products from ACG-catalyzed pyrolysis of PP exhibit promising utilization potential. H2 can be purified via mature technologies to serve as a clean energy carrier, while the remaining light hydrocarbon-rich gas, after purification and conditioning, shows potential for use as a fuel gas.

To further distinguish the contributions of calcination and acid leaching to the catalytic activity of coal gangue, control samples (denoted as CCG1, CCG2, CCG3, and CCG4) were prepared by calcining CG1, CG2, CG3, and CG4 at 500 °C for 2 h without acid leaching, and subsequently evaluated for PP catalytic pyrolysis under identical conditions. The three-phase product yields and gas composition are presented in Fig. S4 and Table S2. Taking H2 content as an indicator, the gas obtained from CCG1 contained only 6.4 vol% H2, slightly higher than that of CG1 (4 vol%) but substantially lower than that of ACG1 (14.7 vol%). For CCG4, the H2 content was 18 vol%, marginally lower than that of CG4 (21 vol%) and far below that of ACG4 (35 vol%). These results demonstrate that calcination alone provides limited enhancement of the catalytic activity of coal gangue. The acid leaching step plays a critical role in removing carbonates, enriching active iron oxides on the surface, and generating acidic sites. Therefore, the significant improvement in the catalytic activity of acid-modified coal gangue is primarily attributable to the acid leaching step.

3.3. Thermogravimetric analysis

To investigate the effect of different coal gangue samples, before and after acid modification, on the PP pyrolysis process, the thermal decomposition behavior of coal gangue–PP mixtures was analyzed using a thermogravimetric analyzer (TGA). The results are presented in Fig. 5. At a heating rate of 5 °C min−1, pure PP exhibited only a single stage of weight loss. After the addition of CG or ACG, PP still decomposed in a single step. As shown in Fig. 5a and c, the thermal decomposition of PP in the presence of CG occurred over the temperature range of 350–500 °C, which is similar to that of pure PP. When ACG was used as the catalyst, the decomposition temperature range shifted downward to 300–450 °C, indicating that ACG lowers the onset decomposition temperature of PP. Compared with pure PP, the onset temperature was reduced by 41.15–78.12 °C. This observation can be attributed to the acid modification optimizing the pore structure of the coal gangue, which facilitates the diffusion of PP macromolecules, while simultaneously increasing the number of iron-based active sites. The presence of transition metal active sites in the catalyst reduces the apparent activation energy of the PP pyrolysis reaction,44 thereby promoting PP decomposition. As shown in Fig. 5b and d, the temperature corresponding to the maximum weight loss rate of pure PP is 446.86 °C. The addition of ACG significantly decreased this temperature compared with CG; in particular, ACG4 lowered the temperature of the maximum weight loss rate by 59.68 °C, indicating that the catalytic activity of acid-modified coal gangue is significantly enhanced, which helps to reduce the energy consumption of the PP pyrolysis process.45

Fig. 5. TG and DTG curves of coal gangue/PP composites before and after acid modification. (a) and (b) before modification and (c) and (d) after modification.

Fig. 5

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3.4. Analysis of liquid-phase products

To investigate the effect of acid modification on the composition of the liquid oil derived from coal gangue-catalyzed pyrolysis of PP, the liquid products obtained using CG1, ACG1, CG4, and ACG4 were analyzed by gas chromatography–mass spectrometry (GC-MS). The composition of the obtained liquid oils is presented in Fig. 6, and the specific peak-area proportions of individual components are listed in Table S4.

Fig. 6. Distribution characteristics of pyrolysis oil components over different catalysts. (a) Chemical composition of pyrolysis oil (area %) and (b) carbon number distribution (C8–C16) of pyrolysis oil (area %).

Fig. 6

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As shown in Fig. 6a, the four liquid oils are mainly composed of alkanes, olefins, aromatic hydrocarbons, and oxygen-containing compounds. The peak area proportion of oxygen-containing compounds in the liquid oil obtained from CG catalytic pyrolysis is relatively high, while the content of hydrocarbon substances (alkanes, olefins, and aromatic hydrocarbons) is relatively low. The high abundance of oxygen-containing compounds may be related to the metal oxides on the surface of coal gangue and the oxygen-containing functional groups present in the coal gangue itself.46

After catalytic pyrolysis with ACG, the peak area proportion of hydrocarbon substances in the liquid oil increased significantly. The peak area proportions of hydrocarbon substances from PP pyrolysis using CG1 and CG4 were 27.86 area% and 45.53 area%, respectively, while those for ACG1 and ACG4 increased to 61.12 area% and 60.55 area%, respectively. This increase in hydrocarbon content may be attributed to the acid treatment promoting the exposure and dispersion of active iron oxides (α-Fe2O3) on the surface of coal gangue, thereby providing more metal active sites for the cracking of PP macromolecules and facilitating the cleavage of C–C bonds. The peak area proportion of oxygen-containing compounds in the liquid oil decreased significantly, from 67.79 area% and 51.73 area% for CG1 and CG4 to 24.82 area% and 31.86 area% for ACG1 and ACG4, respectively. This decrease is mainly due to the removal of carbonate and other oxygen-containing mineral components from the coal gangue by acid treatment.

As shown in Table S4, the peak area proportions of olefins and aromatic hydrocarbons in the liquid oil obtained from ACG-catalyzed pyrolysis of PP are significantly higher than those obtained from CG-catalyzed pyrolysis. The enhanced formation of these unsaturated hydrocarbons can be attributed to the acidic sites generated on the coal gangue during acid treatment. Specifically, calcination induces dehydroxylation of kaolinite in coal gangue, and the subsequent acid leaching promotes re-hydroxylation of the aluminosilicate surface, generating silanol (Si–OH) and aluminol (Al–OH) groups that serve as Brønsted acidic sites.47 Additionally, acid leaching may partially remove framework Al, creating coordinatively unsaturated Al3+ ions that act as Lewis acidic sites. Figures S2a and S2b present the NH3-TPD profiles of CG4 and ACG4. Before modification, CG4 exhibited two desorption peaks at approximately 500 °C and 730 °C. Comparison with the thermal analysis curve of CG4 (Fig. S2c) reveals that these peaks correspond to the release of gaseous species during the thermal decomposition of coal gangue rather than NH3 desorption from acidic sites, indicating that the surface acidity of unmodified CG4 was negligible. After acid modification, the NH3-TPD profile of ACG4 displayed a distinct NH3 desorption peak centered at approximately 400 °C, indicating that acid treatment generated acidic sites in the coal gangue. It can therefore be inferred that the metal sites provided by α-Fe2O3 and these acid-generated Brønsted and Lewis acidic sites48 may act cooperatively to promote chain scission, dehydrogenation, cyclization, and aromatization reactions, thereby enhancing the formation of olefins and aromatic hydrocarbons.

From the perspective of carbon number distribution, aviation fuel fractions typically correspond to hydrocarbon mixtures in the C8–C16 range.49Fig. 6b shows the distribution of hydrocarbon components in the liquid oil within this carbon number range. Compared with CG, the peak area proportion of C8–C16 hydrocarbons obtained from ACG-catalyzed pyrolysis of PP increased significantly, from 58.34 area% and 65.65 area% for CG1 and CG4 to 90.03 area% and 86.87 area% for ACG1 and ACG4, respectively. This distribution characteristic indicates that the liquid oil obtained from ACG-catalyzed pyrolysis of PP exhibits a certain similarity to aviation fuel in terms of carbon number range, and holds potential for use as a blending component after further hydroprocessing. It is worth noting that a carbon number distribution falling within the aviation kerosene range does not automatically qualify the oil for direct use as aviation fuel. The actual certification of aviation fuel must satisfy a number of strict physical property specifications, such as flash point, freezing point, and aromatic hydrocarbon content limits. Although the pyrolysis oil produced by ACG exhibits increased hydrocarbon content and decreased oxygenated compound content, the remaining oxygenated compounds (ACG1: 24.82 area%, ACG4: 31.86 area%) and olefins (ACG1: 32.69 area%, ACG4: 41.00 area%) still render it unsuitable for direct use as a fuel. Nevertheless, its carbon number distribution characteristics and high hydrocarbon content indicate its potential as a fuel precursor or chemical feedstock. Through chemical upgrading processes such as hydrotreating, it is expected to be converted into specification-compliant clean fuels or value-added chemical products.

3.5. Analysis of TG-IR in the pyrolysis process

To investigate the release behavior of gaseous products during the catalytic pyrolysis of PP over different acid-modified coal gangue samples, thermogravimetric–Fourier transform infrared (TG-FTIR) analysis was performed on ACG1/PP and ACG4/PP mixtures under a nitrogen (N2) atmosphere. The results are presented in Fig. 7. As shown in Fig. 7a and b, the infrared absorption intensity of the evolved gases reached a maximum in the temperature range of 380–400 °C, which coincides with the temperature of the maximum weight loss rate in the DTG curves. In the case of ACG1, the absorption peaks were weak at 340 °C and gradually intensified with increasing temperature. Based on the wavenumber positions and literature assignments, the C–H vibrational peaks of alkanes, olefins, and aromatics are observed at 1377, 976, and 890 cm−1, respectively, indicating the evolution of the corresponding gaseous products;50 the absorption peaks at 2882 and 1459 cm−1 are assigned to CH2 vibrations in aromatics;51 the peaks at 2968 and 2936 cm−1 are attributed to aliphatic C–H stretching vibrations, and the peak at 2363 cm−1 is assigned to aliphatic C Created by potrace 1.16, written by Peter Selinger 2001-2019 O stretching vibrations.50,52 Unlike ACG1, the FTIR spectrum of the products obtained from ACG4-catalyzed pyrolysis of PP exhibits no distinct characteristic peaks for alkenes and aromatics at 976 and 890 cm−1. Comprehensive analysis of the FTIR spectra from the pyrolysis of PP with the two acid-modified coal gangues reveals that the gaseous products mainly consist of alkanes, olefins, aromatics, and H2. A comparison of the peak intensities indicates that ACG1 yields more alkanes, olefins, and aromatics than ACG4, which is consistent with the results shown in Fig. 4b. These differences can be attributed to the differing iron contents of the two coal gangue samples. ACG4 possesses a higher iron content; its abundant iron-based oxide active sites promote the deep cracking of small-molecule hydrocarbons, causing intermediate products such as olefins and aromatics to undergo further scission into H2 and resulting in the weakening or disappearance of the characteristic peaks at 976 and 890 cm−1. In contrast, ACG1, with its lower iron content, induces relatively mild cracking, thereby retaining more alkanes, olefins, and aromatics. This systematic variation in product distribution with iron content further suggests that iron-based oxides play an important role in the coal gangue-catalyzed pyrolysis of PP.

Fig. 7. TG-FTIR spectra of evolved gases from the pyrolysis of (a) ACG1/PP and (b) ACG4/PP blends at different temperatures.

Fig. 7

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Based on the catalyst characterization and the analysis of PP pyrolysis products, the enhanced activity of acid-modified coal gangue is proposed to be associated with the combined contributions of iron species, acidic sites, and pore structure. It should be noted, however, that the currently available data are insufficient to determine which of these factors plays a dominant role, and the potential influence of uncharacterized parameters (such as surface defects or residual alkali metals) cannot be ruled out. Within these limitations, the following insights can be drawn: (1) acid treatment significantly increases the specific surface area and pore volume of the catalyst, which may shorten the diffusion paths of reactants and products and improve the accessibility of active sites, thereby contributing to the enhanced apparent catalytic activity. (2) Acid treatment promotes the surface enrichment and dispersion of active iron oxides, which may provide additional metal sites for the cleavage of C–C bonds and dehydrogenation of PP macromolecules. The increased relative content of iron oxides observed by XRD, XRF and the corresponding increase in H2 yield are consistent with this view. (3) The acidic sites may facilitate the formation and rearrangement of carbocation intermediates, thereby enhancing C–C bond scission and aromatization reactions, which agrees with the increased proportions of olefins and aromatic hydrocarbons in the pyrolysis oil.

3.6. Catalyst stability and deactivation

The stability and reusability of ACG4 were evaluated over six consecutive PP pyrolysis cycles. In the first three cycles, the spent catalyst was reused directly without regeneration, whereas in the last three cycles, it was regenerated by calcination in air at 500 °C for 2 h prior to each run (Fig. 8). During the cycles without regeneration, the H2 concentration declined progressively from 33.3 vol% in the 1st cycle to 14.7 vol% in the 2nd cycle, and further to 6.4 vol% in the 3rd cycle (Fig. 8a), indicating a continuous loss of dehydrogenation activity. In addition, the liquid products collected from the 2nd and 3rd cycles solidified at room temperature, reflecting a reduced ability of the catalyst to crack long-chain hydrocarbons. The TGA mass loss of the spent catalyst under an oxygen atmosphere decreased from 10.47 wt% in the 1st cycle to 4.53 wt% in the 3rd cycle (Fig. 8c and d). If irreversible coke had progressively accumulated, the mass loss resulting from coke combustion would be expected to increase with the cycle number; however, the observed trend was opposite. XRD analysis (Fig. 4c) revealed that α-Fe2O3 in the catalyst was reduced to Fe3O4 during pyrolysis. This phase transformation consumes lattice oxygen, significantly impairs the oxidative dehydrogenation capacity, and consequently leads to catalyst deactivation.53 Upon regeneration by air calcination, Fe3O4 was re-oxidized back to α-Fe2O3, thereby restoring the active sites. This was confirmed by the recovery of the H2 concentration to the first-cycle level in the 4th–6th cycles. These results demonstrate that the catalyst deactivation is reversible and is primarily attributed to the reduction of α-Fe2O3 to Fe3O4, rather than to irreversible coke accumulation. The catalyst can be effectively regenerated by simple air calcination, which offers significant practical advantages for plastic recycling and the application of solid-waste-derived catalysts.

Fig. 8. (a) Gas composition after six cycles of PP pyrolysis over ACG4, (b) liquid product yield after six cycles and (c and d) TG and DTG curves of the spent catalyst after the 1st and 3rd cycles, respectively.

Fig. 8

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4. Conclusion

In this work, coal gangue was used as a raw material. An acid modification strategy was adopted to prepare a catalyst for polypropylene (PP) pyrolysis. Compared with unmodified coal gangue, the acid-modified catalyst increased the gas yield and enhanced H2 selectivity. It also improved the quality of the liquid oil. The proportion of hydrocarbons in the liquid oil rose significantly, while the content of oxygenated compounds decreased markedly. These improvements are attributed to the effective regulation of the mineral composition and structural properties of coal gangue by acid modification. Inert mineral phases, such as Ca and Mg carbonates and kaolinite, were selectively leached or transformed. This process introduced acidic sites and increased the specific surface area and pore volume. Encapsulated iron species were released and converted into active iron-based oxides. These oxides provided metal active sites for the pyrolysis reaction and promoted the dehydrogenation process. This work confirms the feasibility of using acid-modified coal gangue to catalyze PP pyrolysis for improved product distribution. It provides a new approach for the design of solid-waste-based catalysts and offers an experimental basis for the synergistic resource utilization of coal gangue and waste plastics.

Author contributions

Ying He: writing – original draft, visualization, methodology, investigation. Xuemei Zheng: investigation, visualization, conceptualization. Deshun Kong: resources, supervision. Xulong Yang: investigation, methodology. Aiyuan Ma: writing – review and editing, visualization, supervision, conceptualization, funding acquisition.

Conflicts of interest

There are no conflicts to declare.

Supplementary Material

RA-OLF-D6RA04762A-s001

Acknowledgments

We are grateful for the financial support from the Guizhou Provincial Science and Technology Support Program Project (QKH Zhicheng [2026]97, QKHJC Youth [2024]015), the Major Scientific and Technological Special Project of Guizhou Province (QKH Major [2025]030), and the Scientific and Technological Innovation Platform Research Project of Guizhou Province (CXPTXM [2025]004), and the Joint Graduate Training Program of Liupanshui Normal University (LPSSYLPY202508).

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its supplementary information (SI). Supplementary information: the pyrolysis apparatus schematic, catalyst characterization data (NH3‑TPD, TG‑DTG, XPS) and product distribution plots (Fig. S1–S4). Tables S1–S4 report detailed yields, gas compositions and liquid‑product component fractions for polypropylene catalytic pyrolysis over coal‑gangue‑based catalysts. See DOI: https://doi.org/10.1039/d6ra04762a.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

RA-OLF-D6RA04762A-s001
RA-OLF-D6RA04762A-s001.pdf (558.6KB, pdf)

Data Availability Statement

The authors confirm that the data supporting the findings of this study are available within the article and its supplementary information (SI). Supplementary information: the pyrolysis apparatus schematic, catalyst characterization data (NH3‑TPD, TG‑DTG, XPS) and product distribution plots (Fig. S1–S4). Tables S1–S4 report detailed yields, gas compositions and liquid‑product component fractions for polypropylene catalytic pyrolysis over coal‑gangue‑based catalysts. See DOI: https://doi.org/10.1039/d6ra04762a.

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