Direct synthesis of self-assembled ZSM-5 microsphere with controllable mesoporosity and its enhanced LDPE cracking properties

Abstract

Self-assembled mesoporous ZSM-5 microspheres (∼5 μm), which are composed of inter-growthed primary nano-strips, are directly synthesized using n-hexylamine as the single template. The mesopore distribution can be facilely controlled by adjusting the size of primary particles via modulating the alkalinity of the synthesis gel. The samples obtained by varying Na₂O/SiO₂ molar ratios from 0.07 to 0.15 are denoted as ZM-X (X = 1–5). The ZM-X samples are characterized by techniques including XRD, TEM, SEM, N₂ adsorption–desorption and FTIR. It is found that ZM-3 exhibits a uniform mesopore distribution around 10 nm and possesses the largest amount of external acid sites, which lead to the remarkably enhanced catalytic performance in polyolefin cracking. A possible growth mechanism of self-assembled ZSM-5 microspheres directed by n-hexylamine is proposed.

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1 Introduction

Zeolites are widely used in separation and various catalytic processes due to their unique properties such as tunable silicon to aluminum ratios (SAR), uniform pores of molecular dimensions, intrinsic acidity and the ability to confine active metal species.^1–4 Among them, ZSM-5 with unique zig-zag channel as one of the most commonly used zeolites has been applied widely in many industrial acid-catalyzed processes including alkylation,⁵ disproportionation,⁶ isomerization⁷ and aromatization⁸ due to its superior shape selectivity. However, the relatively small pore size of conventional microporous ZSM-5 zeolite limits its further applications because of diffusion resistance of large molecules and mass transfer constraints which would lead to diffusion limitation, poor accessibility of acidic sites⁹ and fast deactivation during specific catalysis processes.^10–12 To overcome the drawbacks of these limitations, hierarchical ZSM-5 which possesses at least 2 levels of porosity (typically microporosity and mesoporosity) has been intensively developed. Various strategies such as dual-template methods with hard or soft additives^13,14 and post-treatments^15,16 have been described in the literature during the past few decades. However, the dual-template methods are limited by the template-removal problem and impurity introduction. The post-treatments usually destroy the zeolitic framework and leads to the material loss. Therefore, self-assembled zeolite aggregates have attracted more and more attention. Nano-sized zeolitic aggregates particles reserve the advantages of reduced diffusion limitation, increased external surface area, large mesopore volume, etc. The micron-sized morphology can solve the difficulty in separation due to its larger secondary particle size.^17,18

Nano-sized zeolitic aggregates can be prepared with single templates using various nitrogen-containing organics, including traditional quaternary ammonium salts,^19,20 amines^21–23 and other new develop organic molecules.²⁴ The hierarchical ZSM-5 aggregates are effective in avoiding the steric hindrance of bulky molecules,²⁵ enlarging coke tolerance to prolong the catalytic lifetime²⁶ and enhancing desirable products selectivity.²⁷ Singh et al.²⁸ synthesized ZSM-5 using a specially designed SDA with three diquaternary ammonium-terminated alkyl chain branches. The obtained ZSM-5 with highly specific surface area exhibited excellent catalytic performance and unique selectivity in the cracking of 1-octene, isomeric conversion of o-xylene and acetalization of cyclohexanone with pentaerythritol. Choi et al.²⁹ applied an appropriately designed template with long-chain alkyl group and two quaternary ammonium groups spaced by a C₆ alkyl linkage in MFI zeolite directing. And the obtained ZSM-5 with reduced crystal thickness facilitated diffusion and thereby dramatically suppressed catalyst deactivation in methanol-to-gasoline conversion. Although significant advances have been made in this subject, these procedures have some drawbacks because they are very complicated or those SDAs were very expensive. There is still an opportunity to develop simple methods or low-cost SDAs for the synthesis of meso-structured ZSM-5 zeolite.

As we all known, ZSM-5 could be efficiently synthesized by using various primary amine molecules as templates such as ethylamine, n-propylamine, iso-propylamine, n-butylamine and n-pentylamine.³⁰ Nonetheless, the obtained ZSM-5 always displays the traditional coffin-like or lath-shape morphology without mesoporosity.³¹ In our previous work, we have shown that 1,6-diaminohexane can be used as SDA for the synthesis of hierarchical ZSM-5 materials.²¹ However, the secondary porosity mainly belongs to the macroporosity instead of mesoporosity with proper pore size. It is known to all that the mesopore diameter of the hierarchical zeolite usually has a significant influence on the catalytic performance, especially on the distributions of the products.^32–34 And the commonly controllable adjusting of mesopore diameter is achieved by varying the carbon chain length of the surfactant,³⁵ using swelling agent³⁶ and post-synthesis.³⁷ Herein we choose the simple n-hexylamine as the single template in a facile method for direct synthesis of ZSM-5 with tunable mesopore. Differing from the above traditionally expensive and tedious methods of mesopore diameter tuning, we modulate the mesopore diameter of the self-assembled microspheres by varying the alkalinity of the synthesis gel to control the sizes of the primary nanoparticles. Catalytic cracking of low density polyethylene (LDPE) is used to correlate the catalytic performance with the external acid density of the ZSM-5 materials. A mechanism for the growth of the hierarchical microspheres is also proposed in the present report.

2 Experimental

2.1 Zeolite synthesis

2.1.1 Synthesis of hierarchical ZSM-5 microspheres. The silica and alumina sources for the hierarchical ZSM-5 microspheres synthesis were Na₂SiO₃·9H₂O and NaAlO₂ (Al₂O₃ 53.18 wt%, Na₂O 40.18 wt%), respectively. N-Hexylamine was used as template. A typical preparation occurred as follows: the HCl solution was added drop-by-drop to the aqueous solution of Na₂SiO₃·9H₂O under stirring. After being continuously stirred for 2 h, the freshly precipitated silica was quickly recovered by filtration and a following wash was carried out by hot deionized water to neutral. Then the n-hexylamine was dropped into the precipitated silica to ensure the interaction between the hydrophilic groups and amino groups. Subsequently the obtained solid was added into the pre-determined aqueous solution containing NaAlO₂ and NaOH to obtain synthetic mixtures with molar composition of 1SiO₂ : 0.033Al₂O₃ : xNa₂O : 0.2n-hexylamine : 19H₂O, where x = 0.07, 0.085, 0.10, 0.12 and 0.15. After being stirred for 4 h, the mixture was transferred into Teflon-lined stainless steel autoclaves, and then heated under autogenous pressure for 5 days at 175 °C. The obtained products were filtered, washed and then dried overnight at 100 °C. The organic component was removed by calcinations in air at 550 °C for 6 h. Then the zeolites were ion-exchanged with NH₄Cl twice and calcined at 550 °C for 6 h. The obtained H-form samples were denoted as ZM-X (X = 1–5), with respect to the variable value of Na₂O/SiO₂ (x = 0.07, 0.085, 0.10, 0.12 and 0.15) respectively.

2.1.2 Synthesis of comparable ZSM-5 (Con.Z5). For comparison, conventional ZSM-5 was prepared as follows according to the traditional method.³⁸ NaAlO₂ (Al₂O₃ 53.6 wt%, Na₂O 42.6 wt%) was dissolved in TPAOH solution (tetrapropylammonium hydroxide, 25 wt% in water). Then tetraethyl orthosilicate (TEOS) as the silica resource was added dropwise to the mixture and then added the water to achieve the mixture with a molar ratio of 1SiO₂ : 0.033Al₂O₃ : 0.2TPAOH : 60H₂O. After being stirred for 2 h, the mixture was then transferred into Teflon-lined stainless steel autoclave, and was heated under autogenous pressure for 3 days at 180 °C. The resultant solid product was treated in the same subsequent procedures as the ZSM-X samples were done to convert to H-form zeolite. The obtained H-form conventional sample was denoted as Con.Z5.

2.2 Characterization

Powder X-ray diffraction patterns (XRD) were collected on a Rigaku Ultima IV powder diffractometer using Cu-K_α radiation (λ = 0.154184 nm). The relative crystallinity of the products was determined from the peak area between 2θ = 22.5–25° using a commercial ZSM-5 sample (Nankai) as the reference. The SiO₂/Al₂O₃ ratios were quantified by inductively coupled plasma atomic emission spectroscopy (ICP-AES) on a Thermo IRIS Intrepid II XSP atomic emission spectrometer. N₂ adsorption–desorption isotherms were recorded with a Quantachrome Autosorb-3B system. Scanning electron microscopy (SEM) was performed on a scanning electron microscopy (type HITACHI S-4800) with an accelerating voltage of 3 kV. Transmission electron microscopy (TEM) images were collected on a JEM-2010 operating at 200 kV. NH₃-temperature programmed desorption (NH₃-TPD) profiles were obtained on a Tianjin XQ TP5080 auto adsorption apparatus equipped with a TCD detector. The nature of acid sites was further investigated using collidine and pyridine as the probe molecules. Self-supported wafers of the samples with the similar amount were activated at 500 °C for 1 h. Collidine/pyridine vapor was admitted in dose until the catalyst surface was saturated at 80 °C and then was desorbed until a pressure of 10⁻⁶ mbar. IR spectra were recorded using a Nicolet IS-50 FTIR spectrometer.

2.3 Catalytic cracking of LDPE

2.3.1 Reaction in TG analyzer. The LDPE catalytic cracking in TG analyzer was carried out in a temperature-programmed condition. Before the test LDPE was mixed with zeolite in weight ratio of 3 : 1 by grinding. After thorough mixing, about 10 mg mixture was placed in the micro-crucible for testing. Thermogravimetric (TG) analysis of the LDPE cracking was performed with NETZSCH STA449F3. The measurements were executed in flowing N₂ with a heating rate of 10 °C min⁻¹ from 30 °C to 700 °C. The data was collected and processed with NETZSCH-Proteus-6. The resulting curves of LDPE cracking had been normalized.

2.3.2 Reaction in batch reactor. The catalytic cracking of LDPE were conducted in a batch reactor with continuous stirring. Zeolite was previously mixed with LDPE at weight ratio of 1 : 50 by thorough grinding. The mixture was placed in the reactor and a stream of N₂ flow swept to remove air. Afterward, the reactor was heated to 340 °C and kept at 340 °C for 2 h. The conversion was measured by the weight loss of solid residual compared with the initial weight of reactant. The cracking products with the boiling points lower than 230 °C was evaporated in to the analysis system and measured by an online gas chromatography (GC7900, HP-5 capillary column) and an offline gas chromatography (GC-14B, PLOT-Q capillary column) both equipped with flame ionization detectors. The product selectivity was calculated by the area percent of GC spectra after normalizing, the yields were determined by the conversion and product selectivity.

3 Results and discussion

3.1 Physiochemical properties

XRD patterns of the ZM-X samples obtained after crystallization at 175 °C for 120 h are given in Fig. 1. The XRD peaks of all ZM-X samples show a series of characteristic diffraction peaks at 2θ of 7.9°, 8.9°, 23.0°, 23.9°, 24.4°, correlating well with that of Con.Z5. No impurities can be observed in the XRD patterns. This demonstrates that all ZM-X samples are highly crystalline MFI-structured ZSM-5. There is no distinctions in the characteristic peaks among ZM-X samples obtained by varying the slurry alkalinity from Na₂O/SiO₂ = 0.07 to 0.15, implying that the synthetic system possesses broad operating flexibility. Full width at half maximum (FWHM) of the XRD peaks corresponding to the ZM-X samples are listed in the Table 1, showing the broader characteristic diffraction peaks than that of Con.Z5. The broadened peaks of ZM-X samples could be ascribed to the smaller size of the primary particles of the microspheres.^29,39,40 SiO₂/Al₂O₃ as well as the solid yields (listed in Table S1†) slightly decreases with the slurry alkalinity increasing, indicating that the higher alkalinity leads to the higher concentration of free Si species in slurry.

Fig. 1 The XRD patterns of ZM-X samples and Con.Z5.

Table 1 The zeolitic properties of the Con. Z5 and the ZM-X samples

Samples	SAR^a	R.C.^b (%)	SBET^c, m^2 g^−1	Sext, m^2 g^−1	Vmic^d, cm^3 g^−1	Vmeso, cm^3 g^−1	Vtotal, cm^3 g^−1	FWHM^e
a SAR: SiO2/Al2O3, detected by the ICP-AES.b R.C.: relative crystallinity.c Calculated by the Brunauer–Emmett–Teller (BET) method applied to the adsorption branch of the isotherms.d Calculated by the t-plot method.e Calculated by the MFI-characteristic peaks at 2θ = 23°.
Con.Z5	28	100	356	30	0.14	0.06	0.20	0.231
ZM-1	30	91	355	96	0.11	0.10	0.21	0.392
ZM-2	28	95	368	115	0.12	0.11	0.23	0.447
ZM-3	26	95	377	119	0.12	0.12	0.24	0.404
ZM-4	27	97	375	96	0.13	0.10	0.23	0.345
ZM-5	25	96	380	90	0.14	0.09	0.23	0.306

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Con.Z5 has the traditional morphology of the bulk particles, and the particles size distributes around 1 μm (Fig. 2). ZM-X samples are aggregates of nano-sized zeolite crystals in size of ∼5 μm. The primary nano-particles are well faceted and crossly intergrowth, favoring the abundant intercrystalline mesopores. Furthermore, the microspheres plump and the length and thickness of primary zeolite nano-particles increase with the alkalinity, indicating the promoting growth of the primary particles under higher alkalinity.⁴¹ The TEM images of ZM-2, ZM-3 and ZM-4 are also shown in the Fig. 2. The thickness of the primary nanostructure enlarges from ∼30 nm in ZM-2 to ∼90 nm in ZM-4. It could also be attributed to the crystallization promotion by the increase of system basicity,⁴¹ which is consistent with SEM images and FWHM in XRD patterns. The expansion of primary particles on the assembled microspheres increases the intercrystalline mesopore diameter.^19,42

Fig. 2 The SEM images of (A) Con.Z5, (B) ZM-1, (C) ZM-2, (D) ZM-3, (E) ZM-4, (F) ZM-5 and the TEM images of (G) ZM-2, (H) ZM-3 and (I) ZM-4.

The N₂ adsorption–desorption isotherms and the pore size distribution of ZM-X samples are shown in Fig. 3 and the physical properties of all samples are listed in Table 1. The Con.Z5 displays the type I isotherm corresponding to its microporous properties. The isotherms of all ZM-X samples are type IV with hysteresis loops in the relative high pressure, indicating the presence of inter-crystalline secondary pores formed by the random aggregation of the primary nano-sized particles. No pore size distribution of the Con.Z5 could be observed in the range of mesopores, indicating its microporosity dominated textural properties. ZM-X samples exhibit different distributions based on the alkalinity of the synthesis slurry. The obvious pore diameters centering around 10 nm of ZM-2 and ZM-3 show the relative abundant mesoporosity. Comparably, the porosity in ZM-4 and ZM-5 mainly appears as the microporosity and macroporosity. All the assembled samples have the similar micropore volumes, consistent with their high relative crystallinity. The mesopore volume varies from 0.10 to 0.13 cm³ g⁻¹ in a slight fluctuation, depending on their porosity domination. Besides, the mesopore volume as well as the external surface area of the ZM-X samples correlates well with the FWHM of the XRD peaks. The mesoporosity of ZM-X samples is originated from the intercrystalline porosity of the primary particles. The nice correlation indicates the close relation between the size of primary particles and the mesoporosity on ZM-X samples. With similar specific surface area of 350–380 m² g⁻¹, the different proportion of mesoporosity and microporosity between Con.Z5 and ZM-X samples exists for the morphology and growth disparity distinctions.

Fig. 3 (A) N₂ adsorption–desorption isotherms and (B) pore size diameter of the Con.Z5 and ZM-X samples.

3.2 Acidic properties

Similar NH₃-TPD curves and pyridine-FTIR spectra (see Fig. S1 and Table S2†) can be observed for all the ZM-X samples, implying that the ZM-X samples possess similar bulky acidity with the Con.Z5. The acid distribution of the obtained samples was further investigated using collidine as the probe molecule to explorer the external properties and the FTIR spectra of collidine adsorbed on the samples are shown in Fig. 4A. The band at 1619 cm⁻¹ represents collidine adsorbed on the external Si–OH sites. Collidine adsorbed on the external Brønsted acid sites gives the band at 1638 cm⁻¹ and 1650 cm⁻¹, and the band at 1633 cm⁻¹ is ascribed to the collidine adsorbed on the external Lewis acid sites.^43,44 No obvious signal at 1650 cm⁻¹, 1638 cm⁻¹ and 1633 cm⁻¹ can be observed for the Con.Z5, but for the ZM-X samples, pronounced resonances can be found, demonstrating that the acid sites of the zeolitic microspheres are easily accessed by the large collidine molecule. From the peak splitting of the resonance in the range of 1580 cm⁻¹ to 1660 cm⁻¹ and the calculated concentration of external Brønsted acid sites (Fig. 4B and Table S2†), ZM-2 and ZM-3 possess much higher concentration of external Brønsted acid sites than the other ZM-X samples. Furthermore, the ratio of the external to the bulky Brønsted acid concentration, which is defined as the accessibility index of collidine molecules,^45,46 correlates well with the external textural properties. The same trend of Lewis acid concentration also could be inferred from the existing information. ZM-3 gets the highest concentration of external Brønsted acid and the largest proportion of external Brønsted acid concentration in bulky Brønsted acid concentration, indicating its nature of easier accessibility for steric impedimental molecules than other samples. This might have a great effect on catalytic performance in the large molecule involved reactions.

Fig. 4 (A) The collidine-FTIR spectra and (B) the peak fit for the collidine-FTIR spectra of assembled ZM-X samples collected at 100 °C.

3.3 Catalytic performance

The introduced interparticle porosity among the primary ZSM-5 nano-crystals may have a positive influence on the catalytic performance due to the improved accessibility of active sites. LDPE cracking, a suitable probe reaction for diffusion limited reactions, is used here to evaluate the activity of these zeolites due to the nature of the branched polyethylene chain (diameter of 0.494 nm).⁴⁷ TG curves of the mixture of LDPE over corresponding catalysts and the decomposition temperature at conversion of 50% are illustrated in Fig. 5. We can easily find that the LDPE decomposes at very high temperature in uncatalyzed thermal-degradation, but the addition of ZSM-5 as catalysts greatly reduces the decomposing temperatures. This can be observed more obviously in Fig. 5B. T₅₀ for the uncatalyzed degradation is 486 °C and it reduces to 397 °C when Con.Z5 is used as catalyst, which shifts by ∼90 °C to lower temperature. When hierarchical ZM-X samples are used, T₅₀ further decreases to 358–381 °C, with a decrease of T₅₀ by 105–140 °C. Although possessing similar bulky acidity and cracking capacities of small alkyl benzene molecules (Table S3†), the ZM-X samples exhibit better catalytic performance than bulk Con.Z5 ​in LDPE cracking. This could be ascribed to the increased external surface area and amount of accessible acid sites, as a consequence of the introduction of secondary interparticle pores. The strengthened accessibility of the external acidity favors the primary crack of the polymers, leading to the remarkable drops of the LDPE cracking temperatures. It should be pointed out that ZM-3 gives more pronounced performance than the others.

Fig. 5 (A) The TG curves of LDPE catalytic degradation and (B) the decomposition temperatures at conversion of 50%.

The product distributions of the catalyzed LDPE cracking were further investigated in a batch reactor and shown in the Fig. 6 and Table S4.† Thermal cracking of LDPE without catalysts exhibits a poor conversion of 5.1% at 340 °C. Ascribing to the lower cracking degree of LDPE without catalysts,⁴⁸ higher selectivity of C₆⁽⁺⁾ (heavier than C₅), especially C₁₆⁺ (heavier than C₁₆) products can be observed. Remarkably increased conversions were obtained when Con.Z5 and ZM-X samples were used as catalysts. Consistent with the catalytic performance in TG analyzer of LDPE cracking, the ZM-X samples lead to higher conversions (66.3–82.9%) than that of Con.Z5 (37.9%). Owing to the enhanced accessibility of LDPE to the acidic sites, ZM-3 gets the highest conversion among the ZM-X samples. Product distributions are different in the catalytic cracking of LDPE. Higher gaseous product selectivity and lower liquid product selectivity can be observed over Con.Z5 (57.4% and 42.6%, respectively), whereas predominant products of C₆–C₁₆ components which could be used as liquid fuels are obtained over ZM-3 samples (74.1%). Generally gaseous products are usually recognised as the terminal products of β-scission process of hydrocarbons.²⁵ The high branching chains and large molecular size make it difficult for LDPE to transfer in the microporous channels. Thus, the formed secondary chain-products on the limited external surface of Con.Z5 tend to further crack in the micropores, resulting in higher selectivity of gaseous products. While on the ZM-X samples, the presence of mesoporosity reduces the spatial constraint of bimolecular reactions and enhances the secondary reactions such as oligomerization and aromatization,⁴⁹ leading to higher selectivity of liquid products. As shown in the Fig. 6, the yields of liquid products increase with the external surface area. This trend is closely related to the external surface area and the accessibility of acid sites. Owing to the brilliant textural and external acidic properties, the highest conversion and selectivity for C₆–C₁₆ products are obtained over ZM-3 in the catalytic cracking of LDPE.

Fig. 6 The product yields of LDPE catalytic cracking over Con.Z5 and ZM-X samples.

3.4 Crystallization mechanism

Among all samples, ZM-3 exhibits the superior textural and acidic properties. In order to track the crystallization process, crystallized ZM-3 with different time is investigated using XRD, FTIR and SEM. No obvious XRD peaks related to the MFI phase could be observed after crystallizing for 41 h (Fig. 7A). Correspondingly, no absorption band at 550 cm⁻¹, which is attributed to the characteristic of pentasil zeolites, could be observed (Fig. 7B). These suggest the amorphous nature of the obtained solids and no crystalline phase could be detected. Characteristic peaks of MFI phase together with the absorption band at 550 cm⁻¹ in the IR spectra appears after crystallizing for 53 h. With the prolonging of crystallization, the IR optical density ratio of 550 cm⁻¹ and 450 cm⁻¹ enhance, corresponding to the increase of the relative crystallinity. Meanwhile, the C–H and N–H vibration of the template further confirm the growth of ZM-3. The δ(N–H) bending modes at ∼1500 cm⁻¹ and 1610 cm⁻¹ as well as the ν(C–H) and δ(C–H) vibrational modes of the alkyl groups in 2800–3000 cm^−l and 1380–1490 cm^−l regions could be observed in the samples with MFI phase detected from the XRD.⁵⁰ This demonstrates that the as-synthesized zeolites process high relative crystallinity, and the template molecules are filled in the zeolite channels.

Fig. 7 (A) The XRD patterns and (B) FTIR spectra of ZM-3 at different crystallization time.

The particles morphology at different crystallization period is captured and showed in Fig. 8. At the beginning of crystallization, only irregularly-shaped gels could be observed. After 17 h of crystallization, fusiform aggregates composed of nano-particles appear in small size. Then the fusiform aggregates grow up gradually, and the oriented cluster on the surface develop in different directions in accordance with the “defect intergrowth mechanism”.^24,51 To reach the balance state of minimum energy,⁵² the fusiform aggregates have experienced 3D expansion to form microspheres with cauliflower-like morphology in the subsequent crystallization. The TEM images (Fig. S2†) gives the information that the core of the crystal particle at crystallization time of 28 h seems to be compact, indicating that the quick growth of the particles resulted in primary particles fusion growth and the decrease of inner intercrystalline porosity. The grain boundaries in the TEM images of ZM-3 inner section (Fig. S3†) show the intergrowth in particle interior, confirming the cross-growth of primary fusiform aggregates in different directions.

Fig. 8 The SEM images of ZM-3 at different crystallization time.

According to the morphology transition in the crystallization process, we propose a possible route of the hierarchical zeolitic microspheres formation as shown in Fig. 9A. Firstly, fusiform aggregates of nano-crystals occur. At the same time, lattice defects may form on the particle surface. The enriched hydroxyl groups on the defects could interact with the organic templates and the silica-alumina gel before a subsequent oriented growth occurring to form the oriented ordered framework. With the amorphous silica-alumina gel gradually crystallizes into zeolitic structure, the length of the clusters expand to a macroscopical spheres within the Principle of Minimum Energy and the primary particles bundle on surface get further development. Finally a cauliflower-like microsphere surfacely staked with nano-sized primary particles form in size of ∼5 μm. As mentioned before, with the enlargement of primary particles on ZM-X samples, the mesopore diameter increases gradually. From the TEM images we know the mesoporosity is mainly provided by the interstice of surface primary particles. The larger primary particles would lead to the larger intercrystalline mesopore diameter, thus the size of mesoporosity is closely dependent on the size of primary particles, as shown in Fig. 9B.

Fig. 9 (A) The proposed route for the formation of hierarchical zeolite and (B) the schematic of mesopore expanding with the enlargement of primary particles.

4 Conclusions

Hierarchical ZSM-5 microspheres aggregated with primary particles are prepared using n-hexylamine as the single template. The hierarchical ZSM-5 aggregates exhibit mesopore volume and external surface area of as large as ~0.12 cm³ g⁻¹ and ~120 m² g⁻¹, respectively. The mesopore diameter of the obtained microspheres could be regulated from ∼7 nm to 50 nm by adjusting the alkalinity of the synthesis gel. During the hydrothermal crystallization, small aggregates form firstly and then spontaneously assemble into uniform cauliflower-like microspheres. The crystallization process is in accordance with the “defect intergrowth mechanism” and the Principle of Minimum Energy. The obtained hierarchical microspheres exhibit more pronounced performance with respect to the bulky Con.Z5 in the LDPE catalytic cracking due to improved accessibility of the polymer molecules to the active sites.

Acknowledgements

We gratefully acknowledge the financial support by NSFC (21403070, 21373088, 21503081), National Key Technology R&D Program (2012BAE05B02) and Shanghai Leading Academic Discipline Project (No. B409).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra03047e

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