Nitrogen- and Halogen-Free Multifunctional Polymer-Directed Fabrication of Aluminum-Rich Hierarchical MFI Zeolites

Aluminum-rich hierarchical MFI-type zeolites with high acidic-site density exhibit excellent activity and selectivity in bulky molecule-involved reactions. However, it is challenging to develop a facile and environmentally benign method for fabricating them. Herein, we employ a polymer that does not contain nitrogen and halogen elements to successfully synthesize aluminum-rich hierarchical ZSM-5 zeolite with a Si/Al ratio of 8 and a significant number of mesopores comprised of oriented-assembled nanocrystals. It is demonstrated that the nitrogen- and halogen-free polymer is instrumental in the formation of the ZSM-5 zeolite by serving as a template for constructing the hierarchical micro/mesoporous structure. Moreover, this polymer also acts as a crystal growth modifier to form a single-crystalline zeolite. Notably, the resultant zeolite shows a better catalytic performance in converting waste plastic into hydrocarbons than a commercial one. Our work enables the synthesis of high-quality hierarchical zeolites without requiring quaternary ammonium templates.


Introduction
ZSM-5 zeolite with an MFI framework has been widely applied as a promising catalyst in a variety of fields such as petroleum refining and petrochemical industries because of its tunable acidity, hydrothermal stability, and uniform micropores of molecular dimensions [1][2][3]. Its catalytic performance mainly depends on the following two key factors: acidity and accessibility of the active sites. The acid sites of ZSM-5 are derived from the presence of protons balancing the negatively charged [AlO 4 ] − tetrahedra [4], which are often directly related to the Al content in the zeolite framework, i.e., the lower Si/Al ratio, the more acid sites. Therefore, aluminum-rich ZSM-5 zeolites should provide a high acid-site density increasing the catalytic activity. However, most of the acid sites in ZSM-5 zeolite are imbedded in the interior of microporous channels with sizes of 0.52-0.56 nm resulting in poor accessibility during catalytic reactions. Therefore, fabrication of ZSM-5 zeolites with high acidic-site density and improved accessibility to acid sites is of great importance.
ZSM-5 zeolite is a highly siliceous zeolite that can typically be synthesized in good quality with Si/Al ratios scoping from about 12 to an indefinitely high number. The fabrication of Al-rich ZSM-5 zeolites with Si/Al ratios lower than 12 and an enhanced concentration of Brønsted acid sites is of considerable interest but remains a great challenge [5]. Desilication in basic media results in only a marginal increase in the aluminum content. Thus, the framework aluminum content is usually increased during the synthesis process. Beck et al. [6] synthesized ZSM-5 with a Si/Al ratio of 7.5, but the sample was impure. When the aluminum content of the synthesis gel was increased, analcime and mordenite were formed as impurities [7]. Romannikov et al. [8] had synthesized ZSM-5 zeolite with a Si/Al ratio of 11; however, a part of an octahedrally coordinated Al species was observed in 1,4-Cyclohexanedimethanol, 1,5-pentanediol, 2,2-dimethoxypropane, and p-toluene sulfonic acid (PTSA) were purchased from the Aladdin Reagent Company (Shanghai, China). Cyclohexane, ethyl acetate, triethylamine, and n-hexane were bought from Sinopharm Chemical Reagent Company (Shanghai, China) and distilled with calcium hydride. NaAlO 2 (57.1% Al 2 O 3 and 37.2% Na 2 O) and NaOH (96 wt%) were gained from Sinopharm Chemical Reagent Company (Shanghai, China) and fumed silica was obtained from the Shanghai Tengmin Industrial Company (Shanghai, China). Commercial H type ZSM-5 zeolite with a Si/Al molar ratio of 12, denoted as C-Z5, was acquired from Nankai University Catalyst Co., Ltd. (Tianjin, China). Low-density polyethylene (LDPE) powder used as a model feedstock in the cracking reaction was provided by Alfa Aesar (Stock #42607) (Tianjin, China).

Synthesis of Ketal Polymer
PK3 was synthesized according to the reference procedure [30]. First, 35 mmol of 1,4-cyclohexanedimethanol and 9 mmol of 1,5-pentanediol were dissolved in 30 mL of cyclohexane and heated at 100 • C for 0.5 h under magnetic stirring. Subsequently, 0.02 mmol of PTSA dissolved in 4 mL of ethyl acetate was added. After the ethyl acetate was evaporated, 44 mmol of DMP was added to start the reaction. After reaction for 6 h, to make up for the DMP and cyclohexane that had been boiled off an additional 3 mL of DMP and 5 mL of cyclohexane were subsequently added into the reaction mixture (this was repeated every 6 h for a total of five times). At last, 2 mL of triethylamine was added to stop the reaction. After quenching and purification, a solid product with the formula of OH-[CH 2 -C 6 H 10 -CH 2 22% -* and molecular weight of 3.9 KDa was obtained and designated as PK3. The molecular weight of the synthesized product could be tuned by the number of additions of DMP. The product obtained with a single addition of DMP was denoted as PK3-1 with a molecular weight of 0. 36

Synthesis of Z5-PK3
0.24 g of NaAlO 2 and 0.32 g of NaOH were added to 27 mL of deionized water and stirred until the solution was clarified. Then 1.7 g of PK3 was added and remained under magnetic stirring for 0.5 h. Afterward, 1.9 g of fumed silica was added slowly to obtain a gel with a composition of 20 SiO 2 /1 Al 2 O 3 /3.42 Na 2 O/961.6 H 2 O/0.28 PK3. The resultant gel was agitated at 25 • C for 2 h and then transferred into a Teflon-lined stainless-steel autoclave that was placed in a homogeneous reactor at a rotational speed of 60 rpm and crystallized at 160 • C for 96 h. After crystallization, the obtained powder was filtered, washed with deionized water, dried at 100 • C for 12 h, and calcinated at 550 • C for 6 h to yield a Na-type ZSM-5 denoted as Na-Z5-PK3. Na-Z5-PK3 was further ion-exchanged with 1 M NH 4 NO 3 solution at 80 • C for 7 h (this process was repeated two times), and then calcinated at 450 • C for 7 h to yield a H-type ZSM-5 denoted as H-Z5-PK3. PK3-1 was also applied in the synthesis of ZSM-5 zeolite by replacing PK3 with the similar synthesis composition used for synthesizing Z5-PK3, and the obtained product was marked as H-Z5-PK3-1. For comparison, another synthesis without the addition of PK3 was also carried out under the same conditions as those used for synthesizing Z5-PK3, and the obtained sample was labeled as Z5-PK3-free. 29 Si, 13 C, and 27 Al magic-angle spinning (MAS) and 1 H NMR were performed using a Bruker Avance III/WB-600 spectrometer (Bruker Inc., Zurich, Switzerland). Solid-state two-dimensional (2D) 23 Na { 1 H} heteronuclear correlation (HETCOR) NMR spectra were recorded with a ramped contact pulse of 3 ms. The morphologies of zeolites were detected by double-beam field emission scanning electron microscopy (SEM) using a Helios G4 CX/Helios G4 CX electron microscope (Thermo Scientific Inc., Waltham, MA, USA) equipped with an EDAX Inc. Octane Elect Super X-ray energy dispersive spectrometer (EDS) system. Transmission electron microscopy (TEM) images were obtained using a Tecnai F20 and Talos F200X (FEI Inc., Hillsboro, OR, USA) electron microscope operated at 200 kV. The relative crystallinities and phase purities of the as-prepared samples were determined by powder X-ray diffraction (XRD) using a Rigaku D-Max 2550 diffractometer (Bruker Inc., Karlsruhe, Germany) with Cu Kα radiation (λ = 1.5418 Å). Ar adsorptiondesorption measurements were performed on a Micromeritics 2460 analyzer (Micromeritics Inc., Atlanta, GA, USA) at -186 • C after the samples were degassed at 300 • C under vacuum. Thermogravimetric analysis (TGA) was performed using a PerkinElmer STA 6000 instrument (PerkinElmer Inc., Waltham, MA, USA). Inductively coupled plasma (ICP) was carried out on a Perkin-Elmer Optima 3300 DV ICP instrument (PerkinElmer Inc., Waltham, MA, USA). The pyridinated zeolites were characterized by Fourier transform infrared spectroscopy (FTIR) performed on a Bruker Vertex 80 spectrometer (Bruker Inc., Karlsruhe, Germany) with a Harrick praying mantis diffuse reflection (DRIFTS) attachment and analyzed using OPUS 7.0 software.

Molecular Modeling
To study the adsorption of PK3 on the (010), (100), and (101) crystal planes of the synthesized MFI zeolites, umbrella sampling molecular dynamics (USMD) simulations were conducted using GROMACS-2020. 6. The details of computation were according to the reference [31].

Catalytic Tests
The LDPE cracking reaction was conducted on TGA. The catalysts were mixed with LDPE powder at a catalyst/LDPE weight ratio of 1/3, and the mixture was then placed into the corundum crucible of the TGA instrument, and N 2 at a flow rate of 50 mL min −1 was used to remove the air in the oven. The reactions were carried out in the TGA instrument by elevating the temperature to 600 • C at a ramp rate of 10 • C min −1 in N 2 . The conversion of LDPE was calculated from the weight loss during the reaction. To regenerate the zeolite catalyst and quantify the amount of coke after LDPE cracking, the spent sample was heated from 25 • C to 700 • C at a ramp rate of 10 • C min −1 in O 2 (90 mL min −1 ). The regenerated sample was collected and weighed, then mixed with LDPE in a catalyst/LDPE weight ratio of 1/3 to continue the pyrolysis-regeneration cycles.  These nanocrystals are assembled in the same orientation, displaying oriented growth, and the entire particle has a single-crystal structure. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image ( Figure 3) has good structural contrast and shows a somewhat three-dimensional distribution of the hierarchical structure, showing the coexistence of both micropores and intracrystal mesopores formed by the oriented assembly of nano-sized crystals. The coexistence of micropores and mesopores of H-Z5-PK3 can be further confirmed by the Ar adsorption-desorption isotherms. The steep rise uptake at a low relative pressure (P/P 0 < 0.1) reveals the presence of micropores in H-Z5-PK3 (Figure 4a). At higher relative pressure, the combined features of type I and IV isotherms with a H3-type hysteresis loop (P/P 0 that ranges from 0.80 to 0.99, Figure 4a) are observed. These are ascribed to the adsorption of Ar molecules onto the mesopores, which results from the intercrystalline voids. The Barrett-Joyner-Halenda (BJH) pore size distribution curve ( Figure 4b) exhibits a large pore-size distribution in the range of 10-100 nm. Based on the data in Table 1, when comparing with a reference H-form ZSM-5 zeolite (C-Z5), H-Z5-PK3 shows an apparently larger BET surface area (421 vs. 345 m 2 /g) and a higher mesopore volume (0.16 vs. 0.03 cm 3 /g) owing to the high crystallinity and number of mesopores. Notes: 1 Measured by ICP; 2 total specific surface area calculated by applying the Brunauer-Emmett-Teller (BET) equation using the linear part (0.05 < P/P 0 < 0.30) of the adsorption isotherm; 3 micropore and mesopore specific surface areas, and micropore and mesopore volumes calculated using the t-plot method; 4 determined by IR spectra of adsorbed pyridine.

Characterization of MFI Zeolites
Nanomaterials 2022, 12, x FOR PEER REVIEW 6 of 13 Notes: 1 Measured by ICP; 2 total specific surface area calculated by applying the Brunauer-Emmett-Teller (BET) equation using the linear part (0.05 < P/P0 < 0.30) of the adsorption isotherm; 3 micropore and mesopore specific surface areas, and micropore and mesopore volumes calculated using the tplot method; 4 determined by IR spectra of adsorbed pyridine.    Notes: 1 Measured by ICP; 2 total specific surface area calculated by applying the Brunauer-Emmett-Teller (BET) equation using the linear part (0.05 < P/P0 < 0.30) of the adsorption isotherm; 3 micropore and mesopore specific surface areas, and micropore and mesopore volumes calculated using the tplot method; 4 determined by IR spectra of adsorbed pyridine.    The chemical states of aluminum and silicon species in the Na-and H-type Z5-PK3 were further studied by 27 Al and 29 Si MAS NMR techniques, respectively. The 27 Al MAS NMR spectra of Na-Z5-Pk3 shows a single sharp signal at ca. 55 ppm, while the resonance at 0 ppm is absent, indicating it consists of a tetrahedral framework Al-species (Al IV ) (top in Figure 5a). After ion exchange and calcination, the spectrum of H-Z5-PK3 (bottom in Figure 5a) shows a small quantity of aluminum at 0 ppm that relates to the extraframework Al species (Al VI ). The 29 Si MAS NMR spectra primarily include five resolved signals [32,33]: the peak at −98 ppm corresponds to a defective Q 2 (2Al) unit; the peak at −102 ppm corresponds to Q 3 (0Al) at the external surface of the zeolites; and the peaks between −106 and −109 ppm are attributed to Q 4 (1Al) Si species, that is, sites with one neighboring aluminum atom in the second coordination sphere. Those at −112 and −115 ppm belong to Q 4 (0Al) framework Si species. The relative distribution of these Si species as obtained by the deconvolution of the 29 Si NMR spectra is summarized in Table 2   The chemical states of aluminum and silicon species in the Na-and H-type Z5-PK3 were further studied by 27 Al and 29 Si MAS NMR techniques, respectively. The 27 Al MAS NMR spectra of Na-Z5-Pk3 shows a single sharp signal at ca. 55 ppm, while the resonance at 0 ppm is absent, indicating it consists of a tetrahedral framework Al-species (Al IV ) (top in Figure 5a). After ion exchange and calcination, the spectrum of H-Z5-PK3 (bottom in Figure 5a) shows a small quantity of aluminum at 0 ppm that relates to the extraframework Al species (Al VI ). The 29 Si MAS NMR spectra primarily include five resolved signals [32,33]: the peak at −98 ppm corresponds to a defective Q 2 (2Al) unit; the peak at −102 ppm corresponds to Q 3 (0Al) at the external surface of the zeolites; and the peaks between −106 and −109 ppm are attributed to Q 4 (1Al) Si species, that is, sites with one neighboring aluminum atom in the second coordination sphere. Those at −112 and −115 ppm belong to Q 4 (0Al) framework Si species. The relative distribution of these Si species as obtained by the deconvolution of the 29 Si NMR spectra is summarized in Table 2  The chemical states of aluminum and silicon species in the Na-and H-type Z5-PK3 were further studied by 27 Al and 29 Si MAS NMR techniques, respectively. The 27 Al MAS NMR spectra of Na-Z5-Pk3 shows a single sharp signal at ca. 55 ppm, while the resonance at 0 ppm is absent, indicating it consists of a tetrahedral framework Al-species (Al IV ) (top in Figure 5a). After ion exchange and calcination, the spectrum of H-Z5-PK3 (bottom in Figure 5a) shows a small quantity of aluminum at 0 ppm that relates to the extra-framework Al species (Al VI ). The 29 Si MAS NMR spectra primarily include five resolved signals [32,33]: the peak at −98 ppm corresponds to a defective Q 2 (2Al) unit; the peak at −102 ppm corresponds to Q 3 (0Al) at the external surface of the zeolites; and the peaks between −106 and −109 ppm are attributed to Q 4 (1Al) Si species, that is, sites with one neighboring aluminum atom in the second coordination sphere. Those at −112 and −115 ppm belong to Q 4 (0Al) framework Si species. The relative distribution of these Si species as obtained by the deconvolution of the 29 Si NMR spectra is summarized in  29 Si MAS NMR spectrum (Figure 5b and Table 2), it can be obtained that the framework Si/Al ratio of the Na-Z5-PK3 is 7 and the H-Z5-PK3 is 8. From TEM-EDX mapping (Figure 3c,d), it shows the uniform distribution of both Al and Si, and there is no Al-enrichment zone in H-Z5-PK3. These results show that this nitrogen-and halogen-free polymer can successfully induce the synthesis of aluminum-rich single-crystalline hierarchical ZSM-5 zeolites with high purity. crystalline hierarchical ZSM-5 zeolites with high purity. Notes: 1 Chemical shift after deconvolution; 2 calculated from the peak areas of the different δSi from the 29 Si MAS NMR profiles in Figure 5. Chemical shift (ppm) Figure 5. Solid-state (a) 27 Al and (b) 29 Si MAS NMR spectra of Na-Z5-PK3 (top) and H-Z5-PK3 (bottom).

Mechanism of Zeolite Growth
Na + can be used as an inorganic template to direct the formation of MFI structure; however, in this work, the only use of Na + results in a mordenite (MOR) phase with larger particle sizes (~180 nm) (Figures 1 and 6). The solid-state 13 C MAS NMR spectra ( Figure  7a) of Na-Z5-PK3 exhibits the peaks associated with the PK3 species. This means that PK3 remains intact during the crystallization process and is incorporated into Na-Z5-PK3 because its flexible polymer chain allows it to fill the micropores. Site-specific interactions of the framework 23 Na species with PK3 molecules were established by solid-state 2D HETCOR NMR spectra of Na-Z5-PK3 (Figure 7b). With the use of 2D NMR correlation spectra, mutual proximities or covalent connectivities of corresponding 1 H, 23 Na species can be manifested by correlated signal intensities [34]. 2D 23 Na { 1 H} HETCOR NMR spectra of Z5-PK3 show two strong correlated 2D signal intensities, the 23 Na signal at −5  29 Si MAS NMR spectra of Na-Z5-PK3 (top) and H-Z5-PK3 (bottom). Notes: 1 Chemical shift after deconvolution; 2 calculated from the peak areas of the different δ Si from the 29 Si MAS NMR profiles in Figure 5.

Mechanism of Zeolite Growth
Na + can be used as an inorganic template to direct the formation of MFI structure; however, in this work, the only use of Na + results in a mordenite (MOR) phase with larger particle sizes (~180 nm) (Figures 1 and 6). The solid-state 13 C MAS NMR spectra (Figure 7a) of Na-Z5-PK3 exhibits the peaks associated with the PK3 species. This means that PK3 remains intact during the crystallization process and is incorporated into Na-Z5-PK3 because its flexible polymer chain allows it to fill the micropores. Site-specific interactions of the framework 23 Na species with PK3 molecules were established by solid-state 2D HETCOR NMR spectra of Na-Z5-PK3 (Figure 7b). With the use of 2D NMR correlation spectra, mutual proximities or covalent connectivities of corresponding 1 H, 23 Na species can be manifested by correlated signal intensities [34]. 2D 23 Na { 1 H} HETCOR NMR spectra of Z5-PK3 show two strong correlated 2D signal intensities, the 23  CH 2 ) and 2.21-1.45 (m, 8.2H, CH 2 )), and are especially correlated to the 1 H signal at 4 ppm originating from the PK3 coordinated to Na + cations [35]. It can be concluded that Na + collaborates with PK3 to direct the formation of ZSM-5 (the obtained product in the absence of Na + is amorphous).  [35]. It can be concluded that Na + collaborates with PK3 to direct the formation of ZSM-5 (the obtained product in the absence of Na + is amorphous).  Macromolecules decorated with hydroxyl groups tend to effectively control the growth of crystals by tailoring the morphology and size of zeolites [36,37]. It is likely that PK3 with alcohol groups can form H-bonds with surface silanols, adsorb on crystal and/or precursor surfaces, and inhibit the growth by particle attachment through steric stabilization [38]. To verify the hypothesis that preferential interactions between PK3 and zeolite surfaces lead to the [010] orientation of zeolite nanocrystals, we probed the interaction of PK3 with different MFI surfaces by means of USMD simulations (Figure 8) to compute the adsorption free-energy profile F(z). As shown in Figure 8, the function F(z) is nearly constant for z beyond 0.5 nm for each surface, manifesting that the interactions between PK3 and ZSM-5 are short ranged. Obviously, the adsorption free energies of PK3 for the (010) and (101) surfaces of MFI are comparable and more favorable than those for the (100) surface (Figure 8). Previous studies demonstrated that the (010) surface was the most energetically favorable orientation among the three MFI orientations, (001), (010), and (101); thus, this surface is favorable for adsorption of organic additives [39]. Thus, PK3 preferentially binds to the MFI (010) surface through the hydrogen binding between alcohols on PK3 and the exposed ≡SiOH or ≡AlOH groups presented on the (010) surface. Crystal growth in the (010) direction is inhibited and in an identical orientation is obtained, while the surfaces of the crystals are bridged and filled with absorbed PK3, inducing the formation of (010)-oriented-assembly single crystals.  CH2)), and are especially correlated to the 1 H signal at 4 ppm originating from the PK3 coordinated to Na + cations [35]. It can be concluded that Na + collaborates with PK3 to direct the formation of ZSM-5 (the obtained product in the absence of Na + is amorphous).  Macromolecules decorated with hydroxyl groups tend to effectively control the growth of crystals by tailoring the morphology and size of zeolites [36,37]. It is likely that PK3 with alcohol groups can form H-bonds with surface silanols, adsorb on crystal and/or precursor surfaces, and inhibit the growth by particle attachment through steric stabilization [38]. To verify the hypothesis that preferential interactions between PK3 and zeolite surfaces lead to the [010] orientation of zeolite nanocrystals, we probed the interaction of PK3 with different MFI surfaces by means of USMD simulations (Figure 8) to compute the adsorption free-energy profile F(z). As shown in Figure 8, the function F(z) is nearly constant for z beyond 0.5 nm for each surface, manifesting that the interactions between PK3 and ZSM-5 are short ranged. Obviously, the adsorption free energies of PK3 for the (010) and (101) surfaces of MFI are comparable and more favorable than those for the (100) surface (Figure 8). Previous studies demonstrated that the (010) surface was the most energetically favorable orientation among the three MFI orientations, (001), (010), and (101); thus, this surface is favorable for adsorption of organic additives [39]. Thus, PK3 preferentially binds to the MFI (010) surface through the hydrogen binding between alcohols on PK3 and the exposed ≡SiOH or ≡AlOH groups presented on the (010) surface. Crystal growth in the (010) direction is inhibited and in an identical orientation is obtained, while the surfaces of the crystals are bridged and filled with absorbed PK3, inducing the formation of (010)-oriented-assembly single crystals. Macromolecules decorated with hydroxyl groups tend to effectively control the growth of crystals by tailoring the morphology and size of zeolites [36,37]. It is likely that PK3 with alcohol groups can form H-bonds with surface silanols, adsorb on crystal and/or precursor surfaces, and inhibit the growth by particle attachment through steric stabilization [38]. To verify the hypothesis that preferential interactions between PK3 and zeolite surfaces lead to the [010] orientation of zeolite nanocrystals, we probed the interaction of PK3 with different MFI surfaces by means of USMD simulations (Figure 8) to compute the adsorption freeenergy profile F(z). As shown in Figure 8, the function F(z) is nearly constant for z beyond 0.5 nm for each surface, manifesting that the interactions between PK3 and ZSM-5 are short ranged. Obviously, the adsorption free energies of PK3 for the (010) and (101) surfaces of MFI are comparable and more favorable than those for the (100) surface (Figure 8). Previous studies demonstrated that the (010) surface was the most energetically favorable orientation among the three MFI orientations, (001), (010), and (101); thus, this surface is favorable for adsorption of organic additives [39]. Thus, PK3 preferentially binds to the MFI (010) surface through the hydrogen binding between alcohols on PK3 and the exposed ≡SiOH or ≡AlOH groups presented on the (010) surface. Crystal growth in the (010) direction is inhibited and in an identical orientation is obtained, while the surfaces of the crystals are bridged and filled with absorbed PK3, inducing the formation of (010)-oriented-assembly single crystals. Z5-PK3-1 synthesized by using PK3-1 with molecular weight of 0.36 KDa as a template is comprised of spheroid-shaped ZSM-5 nanocrystals and does not exhibit noticeable anisotropy (Figures 1 and 9). This phenomenon demonstrates that macromolecules tend to be more effective modifiers than small organic molecules because their proximal binding moieties can simultaneously bind to the crystal surfaces [38]. PK3 also functions as a "porogen" on the mesoscale, and the Ar adsorption-desorption isotherms and pore-size distribution curves ( Figure 4 and Table 1) indicate that the mesopore volume and size of the MFI zeolite increase with the molecular weight of PK3. These mesopores are formed after the removal of the PK3 that occupies the interstitial spaces between the nanocrystals.

Catalytic Performance for LDPE Cracking
The catalytic performance of the synthesized hierarchical zeolites (H-Z5-PK3-1 and H-Z5-PK3) and the reference zeolite (C-Z5) for LDPE pyrolysis was evaluated. These zeolites have different pore structures and acid strengths (Table 1). It is obvious that H-Z5-PK3 with the highest acid amount efficiently converts LDPE to hydrocarbons at the lowest temperature (Table 3 and Figure 10a-c). Four consecutive pyrolysis-regeneration cycles were also carried out. During these tests, the spent catalysts were regenerated by calcination at 700 °C, then reused in subsequent reaction cycles [40,41]. Two different temperatures, T20 and T50, at which 20 wt% and 50 wt% of the LDPE is pyrolyzed to hydrocarbons, are compared to evaluate the performances of the various ZSM-5 zeolites. At T20, cracking occurred mainly at acid sites on the external surface. At T50, the degraded Z5-PK3-1 synthesized by using PK3-1 with molecular weight of 0.36 KDa as a template is comprised of spheroid-shaped ZSM-5 nanocrystals and does not exhibit noticeable anisotropy (Figures 1 and 9). This phenomenon demonstrates that macromolecules tend to be more effective modifiers than small organic molecules because their proximal binding moieties can simultaneously bind to the crystal surfaces [38]. PK3 also functions as a "porogen" on the mesoscale, and the Ar adsorption-desorption isotherms and pore-size distribution curves ( Figure 4 and Table 1) indicate that the mesopore volume and size of the MFI zeolite increase with the molecular weight of PK3. These mesopores are formed after the removal of the PK3 that occupies the interstitial spaces between the nanocrystals. Z5-PK3-1 synthesized by using PK3-1 with molecular weight of 0.36 KDa as a template is comprised of spheroid-shaped ZSM-5 nanocrystals and does not exhibit noticeable anisotropy (Figures 1 and 9). This phenomenon demonstrates that macromolecules tend to be more effective modifiers than small organic molecules because their proximal binding moieties can simultaneously bind to the crystal surfaces [38]. PK3 also functions as a "porogen" on the mesoscale, and the Ar adsorption-desorption isotherms and pore-size distribution curves ( Figure 4 and Table 1) indicate that the mesopore volume and size of the MFI zeolite increase with the molecular weight of PK3. These mesopores are formed after the removal of the PK3 that occupies the interstitial spaces between the nanocrystals.

Catalytic Performance for LDPE Cracking
The catalytic performance of the synthesized hierarchical zeolites (H-Z5-PK3-1 and H-Z5-PK3) and the reference zeolite (C-Z5) for LDPE pyrolysis was evaluated. These zeolites have different pore structures and acid strengths (Table 1). It is obvious that H-Z5-PK3 with the highest acid amount efficiently converts LDPE to hydrocarbons at the lowest temperature (Table 3 and Figure 10a-c). Four consecutive pyrolysis-regeneration cycles were also carried out. During these tests, the spent catalysts were regenerated by calcination at 700 °C, then reused in subsequent reaction cycles [40,41]. Two different temperatures, T20 and T50, at which 20 wt% and 50 wt% of the LDPE is pyrolyzed to hydrocarbons, are compared to evaluate the performances of the various ZSM-5 zeolites. At T20, cracking occurred mainly at acid sites on the external surface. At T50, the degraded

Catalytic Performance for LDPE Cracking
The catalytic performance of the synthesized hierarchical zeolites (H-Z5-PK3-1 and H-Z5-PK3) and the reference zeolite (C-Z5) for LDPE pyrolysis was evaluated. These zeolites have different pore structures and acid strengths (Table 1). It is obvious that H-Z5-PK3 with the highest acid amount efficiently converts LDPE to hydrocarbons at the lowest temperature (Table 3 and Figure 10a-c). Four consecutive pyrolysis-regeneration cycles were also carried out. During these tests, the spent catalysts were regenerated by calcination at 700 • C, then reused in subsequent reaction cycles [40,41]. Two different temperatures, T20 and T50, at which 20 wt% and 50 wt% of the LDPE is pyrolyzed to hydrocarbons, are compared to evaluate the performances of the various ZSM-5 zeolites. At T20, cracking occurred mainly at acid sites on the external surface. At T50, the degraded polymeric species diffused into the pores of the zeolite and further degraded, indicating that the interior acid sites are accessible [42]. H-Z5-PK3 and H-Z5-PK3-1 are nanocrystalline zeolites, which present a larger external surface area (Table 1), shorter diffusion path, and more accessible active sites than micron-sized C-Z5. The temperatures required to achieve conversion of 20 wt% and 50 wt% over H-Z5-PK3 and H-Z5-PK3-1 display little change in the four cycles, but it gradually increased in the tests over C-Z5 (Table 3 and Figure 10a-c) because of excessive carbon formation on the surface of the micropore catalyst that blocked the active sites. The TGA analysis of the spent samples (Table 3) reveals that the weight loss of H-Z5-PK3 and H-Z5-PK3-1 is negligible during the four consecutive pyrolysisregeneration cycles. However, the weight loss of C-Z5 is noticeable. Due to the high acid strength, unique single-crystalline structure, and hierarchical porosities, H-Z5-PK3 exhibits a remarkably improved catalytic lifetime and lower rate of coke formation in the LDPE cracking reaction. polymeric species diffused into the pores of the zeolite and further degraded, indicating that the interior acid sites are accessible [42]. H-Z5-PK3 and H-Z5-PK3-1 are nanocrystalline zeolites, which present a larger external surface area (Table 1), shorter diffusion path, and more accessible active sites than micron-sized C-Z5. The temperatures required to achieve conversion of 20 wt% and 50 wt% over H-Z5-PK3 and H-Z5-PK3-1 display little change in the four cycles, but it gradually increased in the tests over C-Z5 (Table 3 and Figure 10a-c) because of excessive carbon formation on the surface of the micropore catalyst that blocked the active sites. The TGA analysis of the spent samples (Table 3) reveals that the weight loss of H-Z5-PK3 and H-Z5-PK3-1 is negligible during the four consecutive pyrolysis-regeneration cycles. However, the weight loss of C-Z5 is noticeable. Due to the high acid strength, unique single-crystalline structure, and hierarchical porosities, H-Z5-PK3 exhibits a remarkably improved catalytic lifetime and lower rate of coke formation in the LDPE cracking reaction.

Conclusions
We have developed a novel strategy to construct aluminum-rich hierarchical ZSM-5 zeolites by using a nitrogen-and halogen-free polymer as a template. The polymer PK3 serves as a dual-function template and facilitates the assembly of the zeolites into singlecrystalline form. Moreover, the obtained samples exhibit excellent catalytic performance in LDPE cracking. This improved catalytic performance is attributed to the strong acidity of the ZSM-5 zeolites, as well as the presence of mesopores. The successful application of this polymer template to the construction of aluminum-rich hierarchical ZSM-5 zeolites highlights that this strategy has potential applications in the synthesis of other uniform hierarchical materials.

Conclusions
We have developed a novel strategy to construct aluminum-rich hierarchical ZSM-5 zeolites by using a nitrogen-and halogen-free polymer as a template. The polymer PK3 serves as a dual-function template and facilitates the assembly of the zeolites into singlecrystalline form. Moreover, the obtained samples exhibit excellent catalytic performance in LDPE cracking. This improved catalytic performance is attributed to the strong acidity of the ZSM-5 zeolites, as well as the presence of mesopores. The successful application of this polymer template to the construction of aluminum-rich hierarchical ZSM-5 zeolites highlights that this strategy has potential applications in the synthesis of other uniform hierarchical materials.