Conversion of polyethylene to gasoline: Influence of porosity and acidity of zeolites

Abstract

Plastic waste is causing serious environmental problems. Developing efficient, cheap and stable catalytic routes to convert plastic waste into valuable products is of great importance for sustainable development, but remains to be a challenging task. Zeolites are cheap and stable, but they are usually not efficient for plastic conversion at a low temperature. Herein a series of microporous and mesoporous zeolites were used to study the influence of porosity and acidity of zeolite on catalytic activity for plastics conversion. It was observed that H-Beta zeolite was an efficient catalyst for cracking high-density polyethylene to gasoline at 240 °C, and the products were almost C₄–C₁₂ alkanes. The effect of porosity and acidity on catalytic performance of zeolites was evaluated, which clearly visualized the good performance of H-Beta due to high surface area, large channel system, large amount accessible acidic sites. This study provides very useful information for designing zeolites for efficient conversion of plastics.

Graphical abstract

Keywords

plastics conversion / polyethylene / zeolites / acidity / porosity

1 Introduction

As a class of essential material in modern life, plastics is an important polymer that is used in many areas of daily life. Plastics product has substantially increased during the past 50 years. From 1950 to 2015, approximately 8.3 billion tonnes of plastics have been produced and 5.8 billon tonnes of plastic waste have been generated [1]. Plastic waste is accumulated in landfills or the natural environment which causes waste of land resources and pollution of environment [2,3].

Recycling waste plastics by traditional mechanical methods is limited by significant technical and economic challenges [4–6]. Chemical recycling provides an opportunity to degrade plastics to valuable chemicals [7,8]. Polyethylene (PE) is one of the most extensively used plastics, but the waste PE is hardly degraded due to the inert carbon–carbon bond [9–12]. It is urgent to develop catalytic systems that can efficiently degrade PE. By using a “pincer”-ligated iridium complex and external hexane, 82% of C₃–C₁₂ products were generated through cross metathesis of the PE chain and hexane at 150 °C [13]. Diesel-range (C₁₀–C₂₂) and longer-chain alkanes were obtained by hydrogenolysis of C(sp3)–C(sp3) bonds in PE over noble metals with external H₂ [9,14–17]. A yield of 31% of narrowly-ranged (C₉–C₁₅) alkanes was obtained over a mesoporous SiO₂/Pt/SiO₂ catalyst at 250 °C and 1.38 MPa of H₂ for 5.5 days [18]. Hydrocracking of PE over a dual-catalyst system (Pt/WO₃/ZrO₂ + HY zeolite) afforded a high yield of gasoline of 73% at 250 °C and 3 MPa of H₂ [19]. In the absence of external H₂, long-chain alkylaromatics and alkylnaphthenes (an average of approximately C₃₀) were obtained over Pt/Al₂O₃ by tandem hydrogenolysis and aromatization [20]. However, these catalytic systems have the disadvantages of requiring noble metals or addition of H₂/alkane/alkene, which significantly increase the cost.

Zeolites are cheap and stable catalysts which are widely used for cracking and hydrocracking processes in petroleum refineries. The key step of PE degradation is breaking carbon-carbon bond, which is similar to the process in petroleum refineries. Due to the advantages of zeolites, i.e., cheap, stable, and efficient for breaking carbon–carbon bond [21], it is promising to develop zeolite catalysts for polyolefins degradation for practical application. Vollmer et al. [22] used an industrial fluid catalytic cracking catalyst (the main component being zeolite Y) to catalyze polyolefin cracking to produce gasoline, and around 80% yield of gasoline was obtained at 450 °C. In addition, other microporous and mesoporous zeolites such as modified-ZSM-5 [23,24], H-Beta [25,26], and Al-SBA-15 [27] showed a good catalytic activity for catalytic cracking of high density polyethylene (HDPE), but often needs high reaction temperatures (around 400 °C). At present, zeolite as a catalyst for the catalytic degradation of polyolefin generally requires high temperature conditions, and the catalytic performance is poor under mild conditions (< 250 °C). To develop zeolite for efficient conversion of PE under mild conditions, it is necessary to systematically study the key factor of zeolite on polyolefin degradation.

In this paper, the influence of porosity and acidity of zeolites on the catalytic performance for PE conversion were investigated. A variety of zeolites, including H-Beta, ZSM-5, mordenite, ZSM-35, MCM-22, HY, MCM-41, and SBA-15 were used for conversion of PE at 240 °C. The Brunauer-Emmet-Teller (BET) surface area, external surface area, channel property, cage property, acid strength, and accessible acid were examined. In addition, the relationships between these properties and the catalytic performance were studied, which provides a guideline for designing efficient zeolite catalysts for polyolefins conversion.

2 Experimental

2.1 Materials

Beta zeolite (SiO₂/Al₂O₃ ratio of 25), ZSM-5 zeolite (SiO₂/Al₂O₃ ratio of 25), MCM-22 zeolite (SiO₂/Al₂O₃ ratio of 30), mordenite zeolite (SiO₂/Al₂O₃ ratio of 25), ZSM-35 zeolite (SiO₂/Al₂O₃ ratio of 25), and MCM-41 zeolite (SiO₂/Al₂O₃ ratio of 25) were purchased from Nankai University Catalyst Co., Ltd. (Tianjin, China). All chemicals needed to prepare SBA-15 and HY catalysts were as follows: tetraethylorthosilicate (Si(OC₂H₅)₄, TEOS, 98%, Alfa Aesar), pluronic P123 (PEO₂₀PPO₇₀PEO₂₀, molecular weight = 5800, Sigma-aldrich), distilled water, hydrochloric acid (HCl, 37%, Sinopharm Chemical Reagent Co., Ltd.), aluminum sulfate octadecahydrate (Al₂(SO₄)₃·18H₂O, ≥ 99%, Sinopharm Chemical Reagent Co., Ltd., China), ammonia solution (NH₃·H₂O, 28% in water, Sinopharm Chemical Reagent Co., Ltd., China). Hydrofluoric acid (HF, ≥ 40%), nitric acid (HNO₃, 66.5 wt.% (wt, mass fraction)), methylene chloride (CH₂Cl₂, ≥ 99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. HDPE (melting index: 0.66–0.9 g/min, particle size: approximately 200 mesh) was purchased from Macklin. 2,6 di-tert-buthylpyridine (DTBPy, Innochem, 98 wt.%). Pyridine (≥ 99 wt.%).

2.2 Preparation of SBA-15 and HY

The zeolites were prepared using the method outlined in Refs. [28,29]. By adding 4 g of Pluronic P123 and 125 mL of 2 mol/L HCl aqueous solution to a 250 mL double-necked round-bottomed flask, a mixture was obtained, which was stirred at 40 °C until the surfactant was completely dissolved. Then, 8.5 g of TEOS was added to the above solution by drops and the combination was stirred magnetically for 4 h. Following the addition of 0.9913 g of Al₂(SO₄)₃·18H₂O (SiO₂/Al₂O₃ = 25), magnetic stirring was performed for 20 h. The reaction mixture was put into a stainless-steel autoclave lined with Teflon and subjected to hydrothermal treatment at 100 °C for 48 h. It was then transferred to a polypropylene beaker after cooling to ambient temperature. The pH of the mother liquor was raised to 7.5 by swirling a 4 mol/L ammonia solution into it. With the use of special indicator paper (pH 6.4–8.0), the pH was checked. A second hydrothermal treatment of the mixture was conducted in a stainless-steel autoclave lined with Teflon at 100 °C for 72 h. Filtration was used to separate the final solids which were washed multiple times with distilled water. Then, the solids were calcined at 550 °C in the air for 12 h after being dried at 60 °C overnight.

By adding 1 g of commercial HY (SiO₂/Al₂O₃ ratio of 6) and 30 mL of 0.225 mol/L HNO₃ aqueous solution to a 50 mL round-bottomed flask, a mixture was obtained which was refluxed at 80 °C for 5 h. Filtration was used to separate the final solids which were washed multiple times with distilled water. Then, the solids were calcined at 550 °C in the air for 12 h after being dried at 80 °C overnight. The Si/Al₂ molar ratio of prepared HY sample was determined by ICP-OES to be 24 (Table S1).

2.3 Catalyst characterization

The catalysts were characterized by different methods. The structure of the catalysts was analyzed by X-ray powder diffractometer (XRD) on a Rigaku Ultima IV X-ray diffractometer (35 kV, 25 mA), with Cu-Kα radiation (λ = 1.5405 Å). In the range of 0.5°–35°, the step size was 0.04°, the scan rate was 10 °/min, and the diffractograms were collected. The crystal size and morphology of the samples were determined using scanning electron micrograph (SEM) micrographs on Germany ZEISS GeminiSEM 300 instrument. Total pore volumes, surface areas, and pore size distribution were characterized on a BEL-SORP-MAX physical adsorption instrument. All products were degassed at 300 °C for 3 h under dynamic vacuum and then the adsorption measurements were conducted. Nitrogen adsorption isotherms were determined at ‒196 °C in the pressure range of 0–0.1 MPa, and then the textual properties of all samples were obtained from the adsorption isotherms. The total pore volume (V_t) was calculated by Gurvich-rule at P/P₀ = 0.99, and the specific surface area and pore size distribution (PSD) of the samples were calculated by the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively. On an Optima 8300 equipment, inductively coupled plasma emission spectrometer (ICP-OES) analysis was conducted to determine the chemical composition (SiO₂/Al₂O₃ mole ratios) of the calcined catalysts. Before analysis, 10 mg of the sample was completely dissolved in HF under microwave. The solution was then diluted to 50 mL with deionized water. Solid-state 27Al NMR spectra were conducted using a 400-MHz solid-state NMR spectrometer (400 MHz Avance II+, Bruker Biospin). The acidic properties of the catalysts were characterized by ammonia temperature programmed desorption (NH₃-TPD) by the tp-5080 unit using He as carrier gas. The acid type and concentration of the samples were conducted by pyridine adsorption Fourier-transform infrared (Py-IR spectroscopy) analysis on a tensor 27 equipped with a BX-5 in situ transmission FT-IR spectroscopy device. 20 mg of zeolites were finely ground in an agate mortar and pressed into thin wafers. The vacuum was evacuated at 500 °C for 2 h. Then, the temperature was decreased to 30 °C and the background spectrum was collected. Finally, adsorption of pyridine for 30 min was conducted. After the evacuation to remove physically adsorbed pyridine, it was measured by IR spectroscopy at 150 °C. The IR spectra of 2,6 di-tert-butylpyridine adsorbed on samples were obtained as described above, by using 2,6 di-tert-butylpyridine instead of pyridine.

2.4 Catalytic tests

The cracking of HDPE was conducted in a 10 mL Tefonlined stainless-steel autoclave. In a typical experiment, 0.5 g of HDPE and 0.1 g of zeolite were ground together evenly in an agate mortar and loaded into the autoclave. The autoclave was purged with N₂ three times and the pressure was then regulated to ambient pressure. After sealing the reactor, the reactions were conducted at 240 °C for several hours (Fig. S1). Finally, after cooling to ambient temperature, the gaseous products were collected with a gas bag and analyzed by a gas chromatography (GC, Agilent 8890) equipped with a thermal conductivity detector (TCD), an flame ionization detector (FID), a Al₂O₃ column, a 5 A molecular sieve column, and two hayesep Q columns. The liquid products were dissolved in CH₂Cl₂ and were analyzed by gas chromatography (GC, Agilent 8890) and gas chromatography-mass spectrometry (GC-MS, Agilent 7890A) equipped with a HP-5ms column. The GC column temperature program for the gas products analysis: held at 60 °C for 10 min, at 20 °C/min to 130 °C with a holding time of 12 min, at 25 °C/min to 150 °C with a holding time of 20 min. The GC column temperature program for the liquid products analysis: held at 40 °C for 3 min, 20 °C/min to 280 °C with a holding time of 1 min. The conversion of polyolefin and selectivity (S) and yield of products were calculated by

\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}=\frac{[\mathrm{P}\mathrm{o}\mathrm{l}\mathrm{y}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{f}\mathrm{i}\mathrm{n}{]}_0-[\mathrm{P}\mathrm{o}\mathrm{l}\mathrm{y}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{f}\mathrm{i}\mathrm{n}{]}_t}{[\mathrm{P}\mathrm{o}\mathrm{l}\mathrm{y}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{f}\mathrm{i}\mathrm{n}{]}_0}\times 100\mathrm{\%},

S_i=m_i/\sum m_i\times 100\mathrm{\%},

\mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}=\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\times S_i\times 100\mathrm{\%},

where [Polyolefin]₀ and [Polyolefin]_t (t referring to reaction time) denote the mass of polyolefin before and after reaction, respectively, and m_i denotes the mass of product i. Among solid residues (referring to solid hydrocarbons, the mass of used catalyst having been subtracted) after reaction, there might be small parts of PE that occurred to some reactions (for example it may be converted to PE with shorter chains), but it is hard to determine each solid product. Thus, all solid residues (mass of used catalyst having been subtracted) were treated as [Polyolefin]_t. This leads to a slightly overestimated selectivity and underestimated conversion, but the yield is accurate.

3 Results and discussion

3.1 Catalytic results

A series of zeolites with various porosity and acidity were tested for catalytic conversion of HDPE at 240 °C (Tab.1). It shows that the main products are gasoline over all the studied zeolites, and the i-alkanes were the main components of produced gasoline with only small amounts of alkenes and aromatics (Tab.1). Zeolites catalyzed the conversion of PE to i-alkanes via isomerization, β-scission, and hydrogen transfer [30–35]. i-alkanes can increase the octane number of gasoline, while alkenes decrease the stability of gasoline and aromatics cause environment pollution. Thus, the zeolites are suitable catalysts for producing high quality gasoline. However, the conversion of HDPE was significantly different over a variety of zeolites. Controlled test without catalyst shows negligible conversion of PE (Tab.1 entry 1). H-Beta and ZSM-5 zeolites show a higher catalytic performance than other microporous zeolites (MCM-22, mordenite, ZSM-35, and HY) (Tab.1, entries 2‒7). In addition, the conversion of PE over both the mesoporous materials (MCM-41 and SBA-15) was low (Tab.1, entries 8 and 9). HDPE is bulky molecule and mesoporous material should be favorable for HDPE conversion, but the conversion of HDPE over MCM-41 and SBA-15 is notably lower than that over microporous material H-Beta. To understand these phenomena, the relationship between the activity, the porosity, and the acidity of zeolites was further investigated.

3.2 Relationship between catalytic performance and porous properties

The N₂ adsorption-desorption isotherms of the zeolites showed that H-Beta, ZSM-5, mordenite, ZSM-35, MCM-22, and HY displayed type I isotherms, typical for microporous materials [36], while MCM-41 and SBA-15 showed type IV isotherms, typical for mesoporous materials [37] (Fig. S2). H-Beta, MCM-22, MCM-41, and SBA-15 showed a larger specific surface area and external surface area. ZSM-35 had a relatively small specific surface area and external specific surface area (Tab.2). The external surface area of catalyst is a factor for bulky molecule conversion. However, the link between external surface area and catalytic performance was not straightforward. Therefore, the porous structure of zeolites was further investigated. The X-ray diffraction patterns confirmed the certain crystal structure of each zeolite [29,38] (Fig. S3). Solid-state ²⁹Si NMR and ²⁷Al MAS NMR spectra of zeolites verified the chemical environment of Si and Al in zeolite structure (Fig. S4 and Table S2). As shown in Fig. S4(a), the eight zeolites contain a small amount of extra-framework Al atoms (0 ppm) in octahedral coordination and the tetrahedrally coordinated Al species in the zeolite framework (54 ppm) was primarily found. As shown in Fig. S4(b), ²⁹Si NMR spectra of microporous zeolite showed that Si atoms had rich coordination states. Besides the common forms of Si(OSi)₄, Si(OSi)₃OH, and Si(OSi)₃OAl, etc., they also contained a considerable amount of Si(OSi)₂(OAl)₂ and an extremely small amount of Si(OSi)₂(OH)₂ signals. ²⁹Si NMR spectra of mesoporous zeolite showed that there mainly existed Si(OSi)₃OAl and Si(OSi)₃OH configuration (−108 to −101 ppm) in MCM-41 and SBA-15 [39,40]. The corresponding porosity [41] are shown in Tab.3 and Fig.1. All the microporous zeolites studied here had a maximum microporous size of 6.3‒11.3 Å. The diffusion channels of H-Beta, ZSM-5, and HY zeolite were large in all three-dimensional directions of a, b, and c, whereas mordenite, ZSM-35, and MCM-22 had narrow diffusion channels in either one or two directions. Interestingly, H-Beta and ZSM-5 (Tab.1, entries 2 and 3) demonstrated a significantly higher catalytic performance than mordenite, ZSM-35, and MCM-22 (Tab.1, entries 4‒6), and the products were almost C₄‒C₁₂ alkanes. Obviously, the narrow diffusion channels of mordenite, ZSM-35, and MCM-22 limited the diffusion of reactant and/or intermediates to active sites in pores, and thus their catalytic performance was poor, compared to other literature where similar results were found [42–47]. Based on these phenomena, the catalytic performance of zeolites for PE conversion strongly link to the diffusion channels.

The HY zeolite had wide diffusion channels in all three directions. However, it exhibited a poor catalytic performance. By examining the reaction residue, it was found that the reaction residue of HY zeolite was black (Fig.2(f)), while those obtained over other catalysts were yellow, brown, or green. This indicates that the HY zeolite promoted the formation of coke. Then, the HY solid residue was performed by an SEM examination (Fig. S5). It was discovered that there was a clear phenomenon of carbon deposition by comparing the solid residue after the reaction with pure HDPE. HY is an FAU type zeolite, whose structure is shown in Fig.2(i). It has a super-cage which is larger (11.2 Å) than the opening (7.3 Å). Thus, the coke formation in super-cage hardly diffused and blocked the active sites, resulting in the low catalytic performance of the HY zeolite. As a result, the cage structure of zeolite was harmful to the conversion of HDPE.

In addition, for the zeolites with similar structures, the catalytic performance is related to the specific surface area and external specific surface area [23,48–50]. For example, H-Beta that have a higher specific surface area and external specific surface area show a higher catalytic performance than ZSM-5 (Tab.1, entries 2 and 3). ZSM-35 had a lower specific surface area (Tab.2), and showed a lower catalytic performance than mordenite and MCM-22 (Tab.1, entries 4‒6).

In summary, the catalytic performance of zeolite for PE catalytic conversion is related to diffusion channels, cage structure, specific surface area, and external specific surface area of zeolites. However, mesoporous zeolite such as MCM-41 and SBA-15 with a significantly large pore structure (20‒500 Å) have a very poor catalytic performance for PE conversion. Zhang et al. [27] used SBA-15 to crack low-density polyethylene (LDPE) at 200‒550 °C, and the conversion of PE was also very low (< 5%) at 240 °C. Not only porosity but also other factors such as acidity may affect the activity. Thus, the influence of acidity on PE conversion was further investigated.

3.3 Relationship between catalytic performance and acidity

The amounts and strengths of acid sites in zeolites were characterized by NH₃-TPD (Fig.3(a)). All the microporous zeolites had similar amount of acid sites (Tab.4), as they have a similar SiO₂/Al₂O₃ ratio (Table S1). Typically, the NH₃-TPD curves of microporous zeolites had two peaks at around 150‒200 °C and > 300 °C which corresponded to NH₃ eluted from the weak and strong acid sites, respectively [51]. As exhibited in Fig.3(a) and Tab.4, all the microporous zeolites had both weak and strong acid sites, while the mesoporous zeolite (MCM-41 and SBA-15) had no strong acid sites due to amorphous framework [29,52–54], and thus they exhibited low catalytic activities for PE conversion. These results indicates that strong acid sites (peak at > 300 °C) were essential for PE conversion. A sufficient number of strong acid sites can efficiently transform PE. According to the Haag–Dessau mechanism, alkanes are protonated by acid sites to form carbonium ions which crack to give carbenium ions followed by β-scission to produce smaller alkane/alkene. The transient state, i.e., carbonium ion is unstable, thus it requires strong acid sites to lower the activation energy (E_a) [55]. The relationship between acid strength and catalytic performance can be explained by the decrease of E_a as zeolite acid strength increases [56]. Therefore, enough amount of strong acid sites is a key factor for efficient conversion of PE.

Pyridine-infrared spectroscopy (Py-IR spectroscopy) was used to characterize the various types of acid sites in zeolites (Fig.3(b)). The peaks at 1454 and 1545 cm^‒1 belonged to the coordinatively bound pyridine molecules on Lewis acid sites and pyridinium ion on Brönsted acid sites, respectively [57]. As can be seen from Fig.3(b), all the zeolites had both Brønsted and Lewis acid sites. The ratio of Brønsted acid to Lewis acid (B/L) of zeolite was in the order of MCM-22 < MCM-41 < SBA-15 = HY < H-Beta < mordenite < ZSM-5 < ZSM-35 (Tab.4). The conversion of PE over various zeolite was in the order of ZSM-35 < SBA-15 < MCM-41 < HY < mordenite < MCM-22 < ZSM-5 < H-Beta (Tab.1). Based on the studies, an apparent relationship between B/L ratio and conversion of PE was not observed [24,58].

2,6-di-tert-butylpyridine infrared spectroscopy (DTBPy-IR spectroscopy) were employed to characterize the accessibility of acid sites for bulky molecules (Fig.3(c), Table S3) [59]. After adsorption of DTBPy, H-Beta and HY showed obvious peaks at 3370, 1616, and 1530 cm⁻¹, which are characteristic signals of DTBPyH⁺ ions, indicating that they had abundant acid sites that were accessible for bulky molecules. The signals of DTBPyH⁺ ions were also observed for MCM-41 and MCM-22, as they had mesopores that were accessible for bulky molecules. In addition, mordenite had weak signals of DTBPyH⁺ ions, probably due to a wide channel in the c direction (6.45 Å, Tab.3). ZSM-5 and ZSM-35 that have a diffuse channel below 5 Å showed little adsorption of DTBPy. Thus, the DTBPy-IR spectroscopy results are consistent with the porosity of zeolite. Although the direct link between accessibility of acid sites and catalytic activities was not obvious due to existence of complicated factors, H-Beta that had super accessibility of acid sites for bulky molecules exhibited an excellent catalytic performance for PE conversion [23,60].

3.4 Establishment of structure-function relationship

Based on the above results, the catalytic activities of zeolite for PE conversion are determined by multiple factors, such as porosities (BET surface area, external surface area, channel property and cage property) and acidity (strength and accessibility). Both porosities and acidities depend on the structures of zeolite framework. Thus, it is hard to control variables to investigate the relationships between porosities or acidities and catalytic activities. Here, the effect of each factor on catalytic performance of zeolite was evaluated. The evaluation criterions are established based on the above studies.

Although the direct link between surface area or external surface area and catalytic activity was not obvious due to effects of other uncontrolled variables, it was reasonable to suppose that the larger surface area and/or external surface area were beneficial for PE conversion. Thus, the surface area and external surface area are classified into 5 levels and are assigned levels from 0 to 4 (Tab.5).

The catalytic performance of zeolite for PE conversion was related to diffusion channels and the cage structure. Wider diffusion channel favored catalytic reaction and had higher levels. The cage structure resulted in coke formation, and thus it promoted the deactivation of catalyst. The cage structure is represented by the difference between the volume of cage and diffusion channel (Table S4). The catalysts with larger difference are classified into lower levels (Tab.5).

The catalysts with large amount of strong acid sites or accessible acid sites had higher levels (Tab.5).

Based on these evaluation criteria, the levels of all the studied zeolites are shown in Tab.6. The radar map in Fig.4 is derived from the data in Tab.6. The radar map clearly shows the different effects of acidities and porosities of zeolites on catalytic performance. Although HY has a large pore and specific surface area and a high acidity, it has a cage-like structure, which generates carbon deposition and leads to its deactivation [61–63]. The most prominent advantage of mesoporous materials, MCM-41 and SBA-15, is that they have large pores, but the weak acid limits their activities for PE conversion. Compared with other zeolites, ZSM-35 does not have much advantage in porosity and acidity, so its catalytic activity is poor. Although mordenite has the advantage of large diffusion pores and BET surface area, its external surface area and acidity are poor, resulting in a reduced activity. Similar to HY, MCM-22 also has a cage structure, and its acid strength and the amount of accessible acid is low, so its catalytic performance is poor. Although ZSM-5 has limited accessible acid sites for bulky molecules, the catalytic performance is not poor. This indicates that the acid sites on the external surface are only for pre-cracking of PE, and the acid sites in pore are the main active sites for converting intermediates to products. However, radar maps do not accurately reflect catalytic results. For instance, the surface area is a less efficient indicator at the ‘efficiency’ of the catalyst since as mentioned above. Some catalysts with a ‘4’ rating for surface area showed poor results and vice-versa. However, other ‘criteria’ such as the ‘cage property’ have more straightforward results where an inverse relationship can be easily established. It can be seen that each factor has a different degree of influence on the catalytic effect. Therefore, according to the experimental results, weighting is assigned to each parameter (Fig.5). The cage property, channel property, acid strength, and accessible acid have important effects on the catalytic activity of zeolite. The weighting is set higher. In contrast, the BET surface area and external specific surface area are not closely related to catalysis. The weighting is set lower. Finally, the score of each catalyst multiplied by weighting (Tab.7) is consistent with the catalytic activity, indicating that the weighting assigned is reasonable. The weightings in descending order are cage structure, diffusion channel, acid strength, accessible acid site, external specific surface area and specific surface area. Factors such as cage property, channel property, acid strength, and accessible acid (Fig.5) are relatively important for PE conversion, which provide significant guidance for subsequent catalyst design.

For H-Beta-catalyzed PE cracking, first, PE molecules are absorbed on the Brønsted acid sites. Then, they are protonated to form a carbonium ion intermediate, following a cracking reaction into an alkane and a carbenium ion. The carbenium ions undergo skeletal rearrangements and form isomerized carbenium ions. According to the structure of the carbenium ion reactant and product, they proceed through different types of β-scissions to produce shorter carbenium ions and n- or iso-alkenes. Large external surface areas, a certain strong acid sites, and large amounts of accessible acid sites of H-Beta provide the platform for activation of PE, while the suitable channel system of H-beta is beneficial for intermediates diffusion and provides environment for isomerisation and β-scissions. Thus, it has a great advantage in PE conversion. By increasing reaction temperature, the yield of gasoline range products reaches 65.8% at 280 °C (Tab.1, entries 10 and 11). However, increasing reaction temperature leads to more energy cost. The optimization reaction temperature is 260 °C, at which the energy cost is the lowest (Fig. S6).

4 Conclusions

H-Beta, ZSM-5, mordenite, ZSM-35, MCM-22, HY, MCM-41, and SBA-15 were used for HDPE conversion under mild conditions. The influences of porosity and acidity of the zeolites on HDPE conversion were studied. The catalysts with a large specific surface area, an external specific surface area, a large channel system, and free of cage-like structure are favorable for PE conversion. Moreover, strong acid sites are essential for cleavage of C─C bonds of PE. Of the catalysts studied, the H-Beta zeolite has the best catalytic activity with C₄–C₁₂ alkanes as the main products. The effect of each factor on catalytic performance of zeolite was evaluated. The H-Beta zeolite show merits in most aspects. The H-Beta zeolite with an excellent catalytic performance has a great potential for application in transformation of PE to gasoline. The H-Beta zeolite was also used to crack other polymers, such as polypropylene (PP) and LDPE (Table S5). It is found that the conversion of LDPE is similar to that of HDPE, but the conversion of PP is lower. The reason for this may be that PP has a more active branch chain, and is easy to form carbon deposits which affects the activity of catalysts in some extent. Although the conversion of polyolefins on the studied zeolites is not very high, this work reveals the effect of porosity and acidity of zeolite on the catalytic conversion of PE under mild conditions. This is very valuable for designing a cheap, stable and efficient catalytic system for the degradation of polyolefin, which pave the way for its industrial application. Thus, this paper would provide reference for the potential application of zeolites in the field of materials design, catalysis and industrial application.

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