Superior performance of Ga-MFI zeolite with a hierarchical structure and moderate acidity in the catalytic cracking of low-density polyethylene

Abstract

Addressing the global challenge of waste plastic management, catalytic cracking by utilizing zeolite catalysts offers a sustainable approach for converting plastics into valuable products. The acidity and diffusion of zeolites play a critical role in determining the efficiency of this process. However, conventional aluminium-containing zeolites exhibit excessive acidity, promoting excessive cracking and secondary bimolecular reactions, leading to coke formation. Additionally, microporous zeolites with limited diffusion of large molecules significantly impair their activity and overall utilization efficiency. In this study, we developed gallium-modified MFI zeolite catalysts (Ga-MFI) that possess a well-balanced micro- and mesoporosity along with controlled acidity. These Ga-MFI catalysts exhibited enhanced diffusion properties and moderate acidity, which contributed to an improved gasoline yield (64.84–77.69%), alongside a significant reduction in liquefied gas yield (from 23.21% to 13.07%) during the catalytic cracking of low-density polyethylene (LDPE) when compared to the ZSM-5 catalyst. Furthermore, the Ga-MFI catalysts demonstrated a significant enhancement in the selectivity for cycloalkenes within the liquid products, increasing from 35.62% to 51.53%. This improvement can be attributed to the promotion of bimolecular cracking reactions occurring on moderate acidic sites, which facilitate olefin formation, as well as the high concentration of Lewis acid sites that aid in the dehydrogenation of cycloalkanes to cyclo-olefins. These findings underscore the potential of Ga-MFI zeolite as a promising catalyst for the sustainable conversion of plastic wastes.

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1. Introduction

Plastics are extensively utilized in daily life;¹ however, their waste poses significant challenges for natural degradation, and conventional disposal methods, such as incineration, contribute to considerable environmental issues.^2,3 The process of cracking plastics into valuable hydrocarbons, such as gasoline, facilitates the resource recovery of waste plastics while simultaneously mitigating environmental pollution.⁴ The application of catalysts can effectively reduce the cracking temperature of plastics (50–100 °C lower compared with non-catalytic cracking) and selectively yield desirable products.^5–8 Among various catalytic materials, zeolites are prominently employed in the catalytic cracking of plastics due to their abundant acidic sites, robust stability, and well-defined pore structures.^9–12 Plastics predominantly consist of long-chain hydrocarbon polymers. In the catalytic cracking process, the acidic sites present on the surface of the zeolite initially protonate the polymer chains, resulting in the formation of positively charged carbenium ions.^5,13,14 The protonation facilitates the cleavage of the polymer chains into smaller olefines and alkanes. Subsequently, these smaller molecules diffuse into the zeolite pores, where they undergo secondary transformations including isomerization, aromatization, and hydride transfer reactions.^5,12 Consequently, the diffusion ability and acidic characteristics of zeolite catalysts are regarded as critical factors influencing their catalytic performance in the cracking of plastics.^13,15–17

Extensive research has been conducted to investigate the impact of zeolite acidity on the catalytic cracking of plastics, particularly by manipulating the Si/Al ratio of zeolites to modulate their acidic properties.^13,18,19 It is well established that reducing the Si/Al ratio increases the density of acid sites, which in turn enhances the yield of aromatic hydrocarbons.¹³ Conversely, the low density of acid sites on zeolites with a high silicon content suppresses bimolecular reactions, which reduces the yield of aromatic hydrocarbons and increases the yield of light olefins.²⁰ Furthermore, the strength of acid sites is a critical determinant of the cracking activity of zeolites.²¹ Since variations in the Si/Al ratio simultaneously affect both the density and strength of acid sites,^22,23 it poses a challenge to independently regulate these two factors. BAS generated by tetrahedral Al are often excessively acidic, resulting in the production of significant amounts of coke, which leads to zeolite deactivation.²⁴ The introduction of heteroatoms, such as some other trivalent metals like gallium (Ga) and iron (Fe), when introduced into the zeolite framework can produce BAS with reduced acid strengths.^25,26 Notably, Ga exhibits a weaker G–O bond compared to the Al–O bond. As a result, Ga-containing zeolites demonstrate a milder BAS strength.²⁷ Due to their reduced acid strength, Ga-containing zeolites exhibit high selectivity and excellent resistance to coking in applications such as propane aromatization,²⁸ dodecane cracking,²⁵ and ethane aromatization.²⁹ In addition to the mild BAS, Ga-containing zeolites, following the partial substitution of cationic aluminum sites with Ga species, exhibit abundant Lewis acid sites (LAS).^30–32 During the catalytic cracking process, LAS possess strong dehydrogenation capabilities, enabling the extraction of a hydride ion to form a carbonium ion^33,34 and promoting dehydrogenation reactions,³⁵ thereby enhancing the catalytic activity and olefin selectivity of zeolites. Overall, heteroatom-substituted zeolites hold significant potential in the catalytic cracking of plastics. However, there remains a paucity of studies focused on the application of heteroatom-substituted zeolites in this aspect.^36,37

On the other hand, the large molecules generated during the initial cracking of plastics exhibit slow diffusion in pure zeolites that contain only micropores, which hinders access to all active centers and consequently results in low utilization efficiency of the zeolites. It is well established that the incorporation of mesopores can significantly enhance the diffusion capabilities of zeolites.^38,39 Following the introduction of mesopores, zeolites demonstrate improved cracking activity for plastics. Additionally, the rapid elution of products from the mesopores inhibits excessive reactions, thereby enhancing the selectivity of liquid products.^40–42 However, traditional methods for introducing mesopores are often associated with high costs and environmental pollution, which restricts their large-scale application.⁴³ As an alternative, the synthesis of zeolites containing heteroatoms has been reported to alter both the morphology and acidity of zeolites.^44–47 For instance, the incorporation of Sn results in a marked reduction in the growth of the pyramid faces (h0l) of BEA zeolites along the c direction, while the growth of the axial faces (00l) is promoted, leading to a plate-like morphology of the synthesized zeolites.⁴⁷ By varying the amount of Fe added, the morphology of the zeolites can vary from flake-like to rectangular microcrystalline aggregates.⁴⁶ Furthermore, the introduction of Ga results in a noticeable roughening of the surface of the MFI zeolite.⁴⁵

Inspired by the effect of heteroatoms on the acidity and morphology of zeolites, it is envisaged that zeolites exhibiting both a mesoporous structure and moderate acidic sites may be synthesized through the incorporation of heteroatoms, which may exhibit excellent performance in the catalytic cracking of plastics. In this work, we propose to tune the morphology and acidity of MFI zeolite catalysts by the introduction of Ga. The results show that Ga species significantly varied the morphology of the zeolites. In the absence of organic mesoporous templates, the incorporation of Ga species induced the formation of mesoporous structures within MFI zeolites. When compared to the conventional ZSM-5 zeolite, the Ga-MFI zeolite demonstrated milder surface acidity and improved diffusion capabilities, which resulted in a reduced yield of liquefied gas, dry gas, and heavy oil components during the cracking of low-density polyethylene (LDPE) and led to a substantial increase in the yield of gasoline fraction. Besides gasoline, a high content of cyclo-olefins was also observed in the liquid products of LDPE cracking, which are important industrial intermediates utilized in the production of high-value cyclo-olefin copolymers (COCs). Furthermore, owing to the presence of rich LAS, the Ga-MFI zeolite exhibited enhanced dehydrogenation activity, which contributed to a higher degree of unsaturation in the liquid products and a marked increase in the yield of cyclo-olefins.

2. Experimental

2.1. Chemicals

Low-density polyethylene (LDPE, average molecular weight: 65 000) was purchased from Lanzhou Petrochemical Company. Gallium nitrate (Ga(NO₃)₃·xH₂O) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Tetrapropylammonium hydroxide (TPAOH, 25 wt% aqueous solution), tetraethyl orthosilicate (TEOS), and aluminium nitrate (Al(NO₃)₃·9H₂O) were purchased from Sinopharm Chemical Reagent Co. Ltd. All materials were used as received without further purification.

2.2. Catalyst preparation

In the preparation of the Ga-MFI zeolite catalyst, 28.8 g of TPAOH (25 wt% aqueous solution) was initially combined with 23.1 g of TEOS. This mixture was stirred for 12 h to ensure complete hydrolysis of the TEOS. Subsequently, 69 g of H₂O and varying amounts of Ga(NO₃)₃·xH₂O (0.28, 0.56, and 1.12 g) were incorporated to achieve the desired Si/Ga ratios (100, 50, and 25), respectively. The resulting mixture was stirred vigorously for 30 h. The precursor gels obtained were then transferred to an autoclave and subjected to crystallization at 150 °C for 4 days. Following crystallization, the samples were purified via high-speed centrifugation (16 000 rpm, 30 min) and subsequently dried at 100 °C for 12 h. The dried samples were calcined at 550 °C for 4 h to eliminate the organic template. The zeolite samples were named Ga-MFI-x, where x denotes the corresponding Si/Ga ratio. The synthesis process is illustrated in Scheme S1. The preparation process of ZSM-5 was identical to that of Ga-MFI-x, with the sole difference that Ga(NO₃)₃·xH₂O was substituted with an equivalent molar amount of Al(NO₃)₃·9H₂O. Sample S-1, which is an all-silica MFI, was prepared using the same methodology as Ga-MFI, with the absence of Ga(NO₃)₃·xH₂O. Ga-loaded S-1 (Ga/S-1) was prepared through incipient wetness impregnation, with Ga₂O₃ accounting for approximately 5 wt%.

2.3. Zeolite characterization

The samples were analyzed using a Bruker's D8 Advance X-ray diffractometer, with Cu Kα (λ = 0.1542 nm) as the light source with the excitation voltage and current of 40 kV and 40 mA, respectively. The Rietveld refinement was applied to refine the structure and size of crystallites using Topas software from Bruker. After refinement, the goodness of fitting (GOF) value is less than 2, and the R-weighted pattern (R_wp) value is less than 10. The morphology of the sample was characterized using the JEOL JSM-7900F field emission scanning electron microscope equipped with an upper electron detector (UED) probe. The operating mode is gentle beam super high resolution (GBSH). During the SEM observation process, an ultra-low electron excitation voltage (less than 1 kV) was applied to obtain high-resolution surface details of the sample.^48,49 Energy Dispersive Spectroscopy (EDS) was performed with the Oxford Instruments X-MaxN mounted on the JSM-7900F, with an electron voltage of 15 kV and a current of 8 μA. Nitrogen adsorption and desorption was used to analyze the pore structure of the sample with a Quantachrome QUADRASORB evo™ automated surface area and pore size analyzer. Prior to analysis, the zeolite samples were treated under vacuum at 300 °C for 6 h. The surface area was calculated using the Brunauer–Emmett–Teller (BET) method, and the selected points for calculation were determined with Quantachrome's BET assistant. The total pore volume was calculated based on the nitrogen adsorption at P/P₀ = 0.98. The micropore surface area and micropore volume of the sample were calculated using the t-plot method. The pore size distribution (PSD) of zeolite samples was obtained using adsorption branch data and calculated using the Barret–Joyner–Halenda (BJH) method. The pyridine adsorption infrared spectrum (Py-FTIR) of the zeolite was collected on a Bruker VERTEX 70V spectrometer. Firstly, about 20 mg of powder sample was pressed into a thin circular plate and fixed in the infrared cell. The sample was heated under vacuum to 500 °C to remove the adsorbed water, and then cooled to room temperature; this IR spectrum was used as a background. At room temperature, pyridine was adsorbed until saturation, and the remaining pyridine was removed by opening the vacuum valve. Then, the temperature was increased and the infrared spectra were recorded after desorption for 10 minutes at 150 °C, 250 °C, 350 °C, and 450 °C. Ammonia temperature-programmed desorption (NH₃-TPD) was performed on a Micromeritics AutoChem II 2920 chemical adsorption instrument. The sample was treated at 500 °C for 1 h in a helium flow to remove the adsorbed water, and then cooled to 100 °C with ammonia adsorption for 40 minutes. The gas was switched to helium, removing excess ammonia. After the baseline became stable, the temperature was ramped to 500 °C at a rate of 10 °C min⁻¹ while synchronously recording the NH₃-TPD desorption curve. The UV–Vis spectra of catalysts were obtained from the spectrometer (UV-2700, Shimadzu, Japan) furnished with an integrating sphere device. X-ray photoelectron spectroscopy (XPS) analyses were carried out on a Nexsa G2 spectrometer from Thermo Fisher Scientific using a monochromatic Al Kα source and the instrument base pressure was lower than 10 × 10⁻⁷ Pa. The analysis area was 500 × 500 μm and the pass energy was 30 eV, and the data were acquired with 0.1 eV steps.

The kinetic study of LDPE catalytic cracking was performed on a thermogravimetric-mass spectrometry instrument (TG-MS, Netzsch STA449 F5). Firstly, the zeolite was mixed with LDPE in a mass ratio of 1 : 4 and ground to achieve a uniform mixture. Under a nitrogen atmosphere, the temperature was ramped at a rate of 10 °C min⁻¹, and the LDPE conversion was calculated based on the mass change.

where α(t) is the conversion rate at time t, w₀ is the initial weight of the sample, w(t_∞) is the final mass of the sample, and w(t) is the mass of the sample at time t.

The kinetic simulation was conducted with the Coats–Redfern method:

T is the reaction temperature (K), A is the pre-exponential factor (min⁻¹), β is the heating rate (K min⁻¹), E is the activation energy (J mol⁻¹), R is the gas constant (8.314 J mol⁻¹ K⁻¹), and g(x) is a term related to the conversion rate α, where g(x) = −ln(1 − α).

2.4. Catalytic cracking of LDPE using zeolite catalysts

The catalytic cracking of LDPE was carried out in a batch reactor. Firstly, 2 g of zeolite was mixed with 40 g of LDPE and heated to 250 °C under a N₂ atmosphere. Then, the system was switched to a steam flow of 3 mL min⁻¹ and heated up to 500 °C and maintained for 1 h. After the reaction, the steam was closed. The system was switched to a N₂ gas flow and heated up to 600 °C, then switched to an air flow and calcined for 1.5 h. An online analyzer was applied to detect the CO/CO₂ content and calculate the coke yield.

The liquid and gas phase products were separated through a −6 °C trap, and gas and liquid products were collected using gas bags and glass bottles, and their weights were recorded. A gas chromatograph (Agilent7890B, USA) equipped with a flame ionization detector (FID) and a thermal conductivity detector (TCD) was applied to analyze the components in gas products. The liquid product composition was analyzed with a simulated distillation chromatograph (Varian SCION400-GC, USA) equipped with a FID detector. A high precision M4 chromatograph, consisting of an Agilent 8890 gas chromatograph, an electronic pressure control system and a FID detector, an Agilent 7693A automatic sampler, and an AC independent column temperature control system, was used to analyze the fine composition of the liquid product. For specific instrument and analysis methods, please refer to our previous work.⁵⁰

3. Results and discussion

3.1. Characterization of the Ga-MFI zeolite catalyst

Fig. 1a presents the XRD patterns of the zeolite samples. All samples display characteristic diffraction peaks associated with the MFI zeolite at 8.1°, 8.9°, 23.1°, 24.0°, and 24.5°, with no evidence of impurity phases.⁵¹ In comparison to samples S-1 and ZSM-5, Ga-MFI shows an increase in the lattice parameters (Fig. 1b), which is further enlarged with the increase of Ga content, which probably is attributed to the larger atomic size of Ga in comparison to Si and Al.⁵² The increase of Ga content in the zeolite samples results in a notable broadening of the diffraction peaks for Ga-MFI-50 and Ga-MFI-25 (Fig. 1c), suggesting that the incorporation of Ga reduces the crystallite size of the zeolite.⁵³ In contrast, no broadening is observed in samples S-1 and ZSM-5, indicating that their crystallite sizes remain constant. The refinement results for the zeolite catalysts further indicate that at elevated Ga content, the crystallite size of Ga-MFI-25 is significantly reduced (Table S1). All samples exhibit an infrared absorption band at approximately 560 cm⁻¹ (Fig. S1), which is attributed to the lattice vibrations of the five-membered ring structure,^54,55 thereby confirming the presence of the MFI crystal structure. Additionally, the absorption bands observed at approximately 1115 cm⁻¹ and 800 cm⁻¹ are attributed to the asymmetric stretching of Si–O–T (T = Al or Si) and the symmetric stretching of Si–O–Si, respectively.^56,57 UV-Vis was applied to investigate the chemical state of Ga (Fig. S2). The peak observed at 200–210 nm is attributed to Ga species incorporated within the framework of the zeolite, while the broad peak at 270 nm corresponds to Ga species located outside the framework.^45,58 Both internal and external Ga species are present in Ga-MFI-x, and their concentrations increase with increased Ga content. In contrast, for Ga₂O₃/S-1, Ga species are predominantly found to be located outside the framework. The chemical state of Ga was further examined by XPS (Fig. 1d). The binding energies of Ga-MFI-x for Ga 2p_3/2 and 2p_1/2 are 1118.8 and 1145.7 eV, respectively, assignable to highly dispersed tetracoordinated Ga(III) species within the zeolite framework, or as cationic species. For Ga₂O₃/S-1, the binding energies shift to 1118.4 and 1145.3 eV for Ga 2p_3/2 and 2p_1/2, suggesting a relatively weak interaction with the zeolite surface for the Ga species.^59–63

Fig. 1 XRD patterns and Rietveld refinement fitting curves (a), unit cell parameter (b), local magnification of XRD patterns (c), and Ga 2p XPS spectra (d) of zeolite samples.

The adsorption isotherms and PSD curves of the zeolite samples are illustrated in Fig. 2, with the corresponding textural properties summarized in Table S2. All samples have high adsorption capacity at low relative pressures and a high micropore surface area typical of MFI type zeolites. In contrast, sample Ga-MFI-25 shows a marked increase in N₂ adsorption at P/P₀ > 0.1, accompanied by the appearance of a distinct hysteresis loop, indicating the presence of mesopores (Fig. 2a). The PSD curve further reveals the existence of a hierarchical pore structure (Fig. 2b). Additionally, Ga-MFI-25 exhibits a significantly high external surface area and a large external pore volume (Table S2). As observed with SEM images (Fig. 3), ZSM-5 displays a rough surface morphology, primarily attributed to the presence of protruding small particles; however, the secondary pore channel structure is not discernible. The all-silica zeolite S-1 presents a smooth particle surface, while the introduction of a small amount of Ga in sample Ga-MFI-100 results in a slightly rough surface due to minor crystalline plane displacement. With an increase in Ga content, the surface roughness becomes increasingly pronounced, leading to the formation of blocks composed of nanoscale particle aggregates. When the Si/Ga is 25, the particle size of Ga-MFI-25 further decreases, which correlates with the decrease in crystallite size calculated via Rietveld refinement (Fig. 1 and Table S1), and the particle aggregation for sample Ga-MFI-25 becomes loose. Discrete particles form agglomerates, thereby constructing abundant interparticle macroporosity and mesoporosity, as verified by N₂ adsorption, and this phenomenon is nearly absent in samples Ga-MFI-100 and Ga-MFI-50. With a further increase in Ga content, excessive Ga adversely affects zeolite growth, resulting in the formation of an amorphous structure. As is reported, the introduction of Al requires additional OH⁻ and charge compensating cations (TPA⁺) to induce the formation of a tetrahedral environment of Al.⁶⁴ This process promotes the dissolution of the nucleus by OH⁻ and generates charged and separated subunits that can subsequently assemble into small zeolite particles.⁶⁵ Consequently, during the dissolution–recrystallization process of zeolites, the dissolution of particles and secondary nucleation on the external surface of gel particles become predominant in Al-doped systems in contrast to pure silicon systems. This phenomenon elucidates the formation of nanocrystals attached to the surface of ZSM-5. In comparison to Al, Ga exhibits weaker Ga–O bonds, which accelerates the dissolution process and contributes to the formation of nanocrystalline aggregates (Fig. 3).

Fig. 2 N₂ adsorption isotherms (a) and PSD curves (b) of zeolite catalysts.

Fig. 3 SEM images with schematic illustration representing the morphology of zeolite samples: ZSM-5 (a, b), S-1 (c, d), Ga-MFI-100 (e, f), Ga-MFI-50 (g, h), and Ga-MFI-25 (i, j).

The acidity of zeolite catalysts was investigated using Py-FTIR (Fig. 4). The absorption bands observed at 1540 cm⁻¹ and 1450 cm⁻¹ for all samples are attributed to the BAS generated by the Si–OH–Ga bridging hydroxyl groups in the zeolite and the LAS generated by the unsaturated Ga orbitals, respectively.^66,67 The area of the adsorption bands associated with pyridine was integrated to quantify the amounts of BAS and LAS, as summarized in Table S3.⁶⁸ It can be observed that the Si/Ga ratio significantly influences the acidity of zeolites. Among all samples, Ga-MFI-100 exhibits the lowest concentration of acid sites, with pyridine nearly completely desorbing at 350 °C. In contrast, the Ga-MFI-50 and Ga-MFI-25 samples retain some adsorbed pyridine even at 450 °C, indicating the stronger acidity and the presence of more acid sites. The ZSM-5 sample also exhibits BAS and LAS (Fig. 4d) due to the Si–OH–Al bridging hydroxyl groups and the empty orbitals of Al, but it exhibits less LAS compared to samples Ga-MFI-x. This is likely due to the partial substitution of cationic sites by Ga species, which eliminates the original BAS sites and generates new LAS sites.³² Compared to Ga-MFI samples, the ZSM-5 shows a slower decrease in BAS content with increasing temperature and retains more BAS at 450 °C, which can be ascribed to its stronger BAS. Conversely, the LAS in ZSM-5 are weak, resulting in nearly complete disappearance of the LAS band at 450 °C. The distribution of acid strength among the zeolite samples was further analyzed by NH₃-TPD. The acid sites are classified as weak, medium, and strong based on the NH₃ desorption temperature (Fig. 5 and Table S4). With the increase of Ga content, the number of acid sites significantly increases, and the desorption temperature of NH₃ from strong acid sites also increases (from 287 to 322 °C), indicating enhanced acid strength. Compared to the Ga-MFI-x, ZSM-5 has a desorption temperature of 372 °C for strong acid sites, indicating higher acid strength, which is attributed to the stronger Al–O bond, reducing the charge density of the bridging hydroxyl groups.²⁷ Correspondingly, the medium acid strength of ZSM-5 is also significantly stronger than that of Ga-MFI-x. In cracking and other reactions, excessively strong acid sites often lead to a significant increase in gaseous products and coke yield.⁶⁹ The moderate acid sites on Ga-MFI-x may be favorable for the production of liquid products like gasoline.

Fig. 4 The Py-FTIR spectra on zeolite samples Ga-MFI-100 (a), Ga-MFI-50 (b), Ga-MFI-25 (c), and ZSM-5 (d) measured at 150, 250, 350 and 450 °C.

Fig. 5 NH₃-TPD profiles of zeolite samples.

The infrared spectra in the hydroxyls region of zeolite samples are depicted in Fig. 6a. The band at approximately 3726 cm⁻¹ is attributed to the stretching vibration of isolated external and internal hydroxyl groups.⁷⁰ The band at 3700–3680 cm⁻¹ is attributed to the hydroxyl groups carried by the Ga outside the zeolite framework, while the band at approximately 3617 cm⁻¹ is attributed to the bridging hydroxyl groups of the Si–OH–M structure.⁷¹ With an increase in the amount of Ga in the zeolite samples, the band at 3617 cm⁻¹ increases, which confirms the increase in acid sites and the presence of Ga in the zeolite framework (see the sample Ga-MFI-25 sample). The Si–OH–M bridging hydroxyl groups in ZSM-5 undergo a ‘red shift’ compared to Ga-MFI-x, indicating a higher acid strength in the former.⁷² Ga-MFI-25 exhibits a strong absorption band at 3726 cm⁻¹, which is likely due to its large external surface area, while the band at 3700–3680 cm⁻¹ is relatively weak, indicating the presence of few Ga species outside the framework as extra framework cations. The broader band in the range of 3600–3200 cm⁻¹ is attributed to the zeolite nest type of defects.⁷³ Compared with Ga-MFI-x and ZSM-5 samples, S-1 has more defect sites, which may be due to the fact that pure silicon generates less negative charge relative to Si–O–T groups during zeolite growth, resulting in a weaker interaction with the positive template agent.⁷⁴ Ga-MFI-25 has the least defect sites, while in general, nanosized zeolites tend to have more defect sites,⁷⁵ indicating that Ga repairs the hydroxyl nests in the defect sites. To investigate the acid site accessibility of the Ga-MFI zeolites, a probe molecule with appropriate size needs to be selected. As 2,4,6-collidine with a kinetic diameter of about 0.74 nm can only interact with the acid sites located in the micropore mouths of the zeolite,⁷⁶ it is employed to evaluate the external acid sites of zeolite samples. Three bands in the range of 1550–1675 cm⁻¹ appear after the adsorption of 2,4,6-collidine on zeolite samples (Fig. 6b, right). The bands at 1618 cm⁻¹ and 1572 cm⁻¹ are attributed to the 2,4,6-collidine bonded to Si–OH groups, and the band at 1638 cm⁻¹ is attributed to protonated 2,4,6-collidine formed upon interaction with external acid sites of the MFI zeolite.⁷⁷ Correspondingly, after the adsorption of 2,4,6-collidine, part of external and bridging hydroxyl groups in the zeolite sample disappear (Fig. 6b, left), indicating that they have been covered by 2,4,6-collidine. Compared to other zeolite samples, Ga-MFI-25 exhibits significantly stronger absorption peaks for 2,4,6-collidine and a more pronounced reduction in hydroxyl groups (Fig. 6b). This phenomenon is likely due to its relatively abundant external surface, which facilitates the adsorption of 2,4,6-collidine onto the acidic sites and silicon hydroxyl groups present on the external surface.

Fig. 6 FTIR spectra highlighting the OH stretching region (a), and FTIR difference spectra of zeolite samples upon 2,4,6-collidine outgassing at 125 °C for 30 min (b).

3.2. Evaluation of zeolites in LDPE catalytic cracking

The evaluation of zeolite catalysts in LDPE cracking was initially performed using a thermogravimetric analyzer under a N₂ atmosphere (Fig. 7). The cracking of LDPE mainly occurs at 430–450 °C and completely decomposed without detectable carbon deposit. The addition of zeolite catalysts significantly promotes the cracking of LDPE and substantially reduces the cracking temperature. The temperature of LDPE catalytic cracking varies greatly among different samples. The LDPE cracking occurs at about 440 °C when utilizing the Ga-MFI-100 zeolite catalyst which possesses a limited number of acidic sites (Fig. 4 and 5) without secondary porosity (Fig. 2). As the Ga content increases, the cracking temperature of LDPE decreases from 380 °C for Ga-MFI-50 to 325 °C for Ga-MFI-25, which is attributed to the gradual increase of acid sites and enhanced diffusion facilitated by the presence of mesopores. Although ZSM-5 possesses stronger acidity, the cracking temperature was still slightly higher than that of the Ga-MFI-25 zeolite catalyst, possibly due to the rapid diffusion through the secondary mesopores of Ga-MFI-25 (Fig. 2). For Ga₂O₃/S-1, the absence of tetracoordinated Ga species (Brønsted acid sites) results in extremely weak catalytic activity, with LDPE cracking occurring at 451 °C. The Coats–Redfern method is a commonly employed integral approach for analyzing LDPE cracking.^78–80 At g(x) = −ln(1 − α), the linear fitting was good (Fig. 7b and Table S5), thus indicating that the cracking process is described with the first-order reaction mechanism.⁷⁹ According to the results from the Coats–Redfern calculation, the introduction of zeolites significantly reduced the activation energy of LDPE cracking, implying that the acidic sites of zeolites would generate carbon cations from LDPE, which facilitates the cracking reactions.⁵ With the increase of Ga amount, the activation energy gradually decreases, and the Ga-MFI-25 zeolite catalyst exhibits a significantly lower activation energy than pure ZSM-5, which is consistent with the lower initial cracking temperature of Ga-MFI-25. Mass spectroscopy was used to analyze the gas products of LDPE cracking without and with ZSM-5 and Ga-MFI-25 zeolite catalysts (Fig. S3). It can be seen that in the absence of zeolite catalysts, only a minimal quantity of gaseous products was produced through radical-induced thermal cracking. Conversely, the use of the ZSM-5 catalyst resulted in a higher yield of CH₄ and C₂H₆ in the gas phase compared to the process involving the Ga-MFI-25 catalyst. This is explained with the strong BAS sites that directly attacked C–H bonds and C–C bonds, resulting in more single-molecule cracking.⁸¹ Throughout all zeolite-catalyzed LDPE cracking processes, the yield of olefins was significantly higher than that of alkanes, which is due to the propensity of long-chain LDPE to produce more olefins following multiple carbon cation cracking compared with radical-induced thermal cracking.

Fig. 7 Thermal gravimetric analysis of LDPE catalytic cracking in the presence of zeolite catalysts (a), and the fitting curves of the Coats–Redfern model (b).

The product distribution of catalytic cracking of LDPE in a batch reactor is shown in Fig. 8a. The products were separated into dry gas, liquefied gas, gasoline, diesel, and other components through a simulated distillation chromatograph, a gas chromatograph, and an online CO/CO₂ analyzer. The main component in the product is gasoline, which accounts for 55–78% of the total products. Among all samples, the Ga-MFI-100 catalyst exhibited the highest yields of heavy oil and diesel components. Combining with acidity analysis, this is ascribed to its low concentration of acidic sites and weak acid strength (Fig. 4), which renders it difficult to completely crack large intermediate molecules.⁶⁹ In addition, the diffusion restrictions also reduce its catalytic efficiency.⁸² With the increase of Ga content (Ga-MFI-50 catalyst), there was a notable reduction in the yields of heavy oil and diesel components, accompanied by an increase in gasoline yield, which suggests that the increased amounts of acidic sites effectively facilitate the cracking of heavy components into light products. When employing the Ga-MFI-25 catalyst, the yields of heavy oil and diesel decline, while the gasoline yield increases to 78%, and the yield of liquefied gas slightly decreases. This phenomenon can be attributed to the relatively small particle size of Ga-MFI-25, which generates a substantial amount of interparticle macropores and mesopores (Fig. 2 and 3b). These structural features improve the accessibility of acidic sites (Fig. 6) and facilitate the catalytic cracking of heavy oil and diesel. Furthermore, the reduced particle size enables rapid diffusion of reaction products out of the zeolite, thereby mitigating excessive cracking and reducing the yield of gaseous products and coke.⁴² Compared to Ga-MFI-x, the ZSM-5 catalyst yields much more liquefied gas, which can be attributed to the stronger acidic sites of ZSM-5, facilitating deep cracking.^15,83 Correspondingly, the gasoline yield decreases to 65% when using the ZSM-5 catalyst. Despite possessing stronger acidic sites, ZSM-5 produces more heavy oil and diesel components compared to Ga-MFI-25 and also exhibits a higher activation energy for LDPE cracking (Table S5). This should be due to the large external surface area (87 cm² g⁻¹) and high acid density (137.25 μmol g⁻¹) of the Ga-MFI-25 catalyst, which enhances the initial cracking of LDPE and facilitates the entry of large molecular intermediates into the micropores, and further cracking.¹⁵ Due to its relatively mild acidic sites (maximum NH₃ desorption at 322 °C), Ga-MFI, particularly Ga-MFI-25, produces significantly less dry gas (Fig. 8b) and coke compared to ZSM-5 (maximum NH₃ desorption at 372 °C).

Fig. 8 Product distribution of LDPE catalytic cracking on zeolite catalysts (a) and yields of dry gas and coke (b).

Cyclo-olefins are important industrial intermediates utilized in the production of high-value cyclo-olefin copolymers, which exhibit high transparency, high mechanical strength, and excellent corrosion resistance.^84,85 These copolymers are commonly employed in medical packaging materials, optical components, and organic light-emitting diodes (OLEDs).^86,87 Cyclo-olefins are mainly produced through the dehydrogenation of cycloalkanes and the hydrogenation of aromatic hydrocarbons. However, these processes are energy-intensive and thermodynamically limited with a low yield.^88–90 In previous studies, we employed M4 multidimensional chromatography for the fine analysis of liquid products in the catalytic cracking of LDPE. It was found that a high content of cyclo-olefins exists in liquid products obtained from LDPE cracking across all the Al-containing zeolites. The formation of cyclo-olefins was attributed to the long hydrocarbon chains in LDPE.⁵⁰ In this work, we also observed the presence of cyclo-olefins in the liquid products of LDPE cracking using Ga-MFI-x catalysts (Fig. 9), and the cyclo-olefin component was significantly increased compared to the Al-containing MFI zeolite (ZSM-5). Additionally, a significant reduction in cyclo-alkane production was observed with Ga-MFI-25, suggesting that the dehydrogenation of cycloalkanes on the LAS of Ga-MFI may primarily contribute to the enhanced production of cyclo-olefins.^91,92 Furthermore, an increase in the aromatic hydrocarbon content was observed on the Ga-MFI catalyst, which likely results from the further dehydrogenation of cyclo-olefins. The liquid products obtained with Ga-MFI-x catalysts demonstrated a high degree of unsaturation, and the oligomerization of polyenes may also be responsible for cyclo-olefin formation.⁹³ Due to the strong BAS of ZSM-5, it directly attacks C–C and C–H bonds,⁸¹ leading to the generation of small molecules such as H₂, methane, and ethane through monomolecular cracking reactions (Fig. S3), resulting in a lower olefin production. Conversely, Ga-MFI-x catalysts possess moderate BAS, large pores and abundant LAS, which favor bimolecular cracking and dehydrogenation reactions, thereby enhancing the formation of olefins⁹⁴ and improving the selectivity towards cyclo-olefins (Scheme 1). Furthermore, Ga-MFI-25, due to its elevated acid site density (137.25 μmol g⁻¹), facilitates the cyclization of olefins,⁹⁵ consequently enhancing the yield of cyclo-olefins.

Fig. 9 The liquid product distribution of LDPE catalytic cracking on zeolite catalysts.

Scheme 1 Schematic illustration of the LDPE catalytic cracking process on ZSM-5 and Ga-MFI-x zeolite catalysts.

Operando FTIR was employed to investigate the catalytic cracking process of LDPE over the zeolite (Fig. 10). LDPE primarily consists of hydrocarbons with long-chain structures, with its main absorption bands attributed to –CH₂ at 2856 cm⁻¹ and 2933 cm⁻¹, as well as terminal –CH₃ absorption bands at 2965 cm⁻¹ (Fig. 10a, right). As the cracking process progresses, the external hydroxyl groups on the outer surface of the zeolite (3735 cm⁻¹) and the isolated hydroxyl groups within the pores (3700 cm⁻¹) also undergo corresponding changes (Fig. 10a, left). At the beginning of the reaction (0 min), a significant amount of LDPE (melting point of approximately 110 °C) melts as the temperature rises from room temperature to 320 °C. This melting process results in LDPE covering the external surface of the zeolite, leading to a weak absorption band of the external hydroxyl groups at 3735 cm⁻¹. Additionally, as the cracking process continues, an increased amount of LDPE melts and further obscures the zeolite surface, causing the absorption band to gradually decrease. During this phase, the absorption band of the internal hydroxyl group decreases rapidly, indicating that the intermediates formed by the initial cracking on the external surface are penetrating into the micropore of the zeolite for further reaction. Concurrently, the absorption bands of the –CH₃ and –CH₂ groups in LDPE decrease rapidly, with the –CH₃ band diminishing at a faster rate, nearly disappearing at around 10 min. This rapid decline may be attributed to the attack of branched and terminal –CH₃ groups by acidic sites.⁹⁶ After 8 min of reaction, the absorption band of external hydroxyl groups of zeolites gradually strengthens, which should be due to that the melting LPDE on the surface of the zeolite is consumed and detaches from the external surface. However, the internal hydroxyl group only shows a slow increase in intensity and does not return to its initial level, which should be due to the presence of residual hydrocarbons within the pore (Fig. 10b). The results observed through Operando FTIR analysis confirmed the reaction process of LDPE melting – initial cracking – entering micropores – further reaction, and product diffusion out of zeolite micropores (Scheme 2). This also underscores the importance of external acid sites in the LDPE cracking process.

Fig. 10 The FTIR spectra of Si–OH groups in the zeolite (a left), and –CH₃, –CH₂ in LDPE (a right) during LDPE cracking over sample Ga-MFI-25 at 320 °C. The intensity variation of Si–OH groups and organic matter during the cracking process (b).

Scheme 2 The catalytic cracking process of LDPE over zeolites.

4. Conclusion

A series of Ga-MFI zeolite catalysts with different amounts of Ga were synthesized. With the introduction of Ga, the morphology and surface features of the zeolites gradually changed from smooth particles to rough agglomerates composed of nanocrystal aggregates. The increase of Ga content resulted in the crystallization of nanocrystals and packing into agglomerates with well-developed inter-crystal mesoporosity. Compared to ZSM-5, Ga-MFI-x catalysts exhibit milder acid strength and more LAS. Due to the moderate acid sites, the yield of gaseous components in the LDPE cracking by Ga-MFI decreased from 23.21% to 13.07%, while the yield of gasoline components significantly increased from 64.84% to 77.69%. Despite the decrease in acid strength, the catalyst with the highest Ga content (sample Ga-MFI-25) shows the lowest LDPE cracking activation energy. The LDPE cracking activation energy changed from 103.26 to 88.27 kJ mol⁻¹ for ZSM-5 and Ga-MFI-25, respectively and the yield of heavy oil and diesel decreased for Ga-MFI-25 compared to ZSM-5, due to the well-developed secondary porosity of the former. Due to the increased bimolecular cracking, the unsaturation degree of the liquid products increases, resulting in an enhanced cyclo-olefin yield of 51.53% for Ga-MFI-25, compared with 35.62% for ZSM-5. Therefore, the catalytic cracking of LDPE using Ga-MFI zeolites represents a promising approach to the cost-effective production of cyclo-olefins, high value-added intermediates for COCs.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d5qi01659b.

Acknowledgements

This work was financially supported by the National Key R&D Program of China (2022YFA1503400), the National Natural Science Foundation of China (22478431, 21991091), the Key Projects of Shandong Key R&D plan (2019JZZY010506), the Taishan Scholar Foundation (tspd20210308), and the Fundamental Research Funds for the Central Universities (23CX10005A). This work was co-funded by the European Union (ERC, ZEOLIghT, 101054004). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them. This work is dedicated to Professor Zifeng Yan on the occassion of his 60th birthday.

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