Organic template-free synthesis of zeolite mordenite nanocrystals through exotic seed-assisted conversion

Abstract

Seed-assisted synthesis has been demonstrated as a green and facile route to produce useful zeolites with several frameworks, and meanwhile shows the potential to selectively template the formation of specific structured crystals. In view of the difficulty of directly synthesizing zeolite mordenite (MOR) with short 1D, 12-MR channels, we proposed a plausible synthetic route of exotic seed-assisted conversion (ESAC), starting from aluminosilicate gel containing zeolite beta (BEA) seeds to MOR nanocrystals. Through judiciously adjusting the synthetic conditions, including NaF/Si, OH⁻/Si, hydrothermal temperature and added zeolite BEA seeds, nano-crystallite assembled zeolite MOR nanoparticles with rich inter-crystallite mesoporosity, high crystallinity, perfect framework, and high catalytic activity for low-density polyethylene cracking, were fast transformed under a low seeded, organic template-free system. The combined investigation of synthetic conditions and the crystallization process deepened the understanding on this special crystallization behaviour.

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1. Introduction

Zeolite mordenite (MOR) is one of industrially important zeolites that is widely used in petroleum refining and the petrochemistry, for instance in isomerization, alkylation, dewaxing, cracking, NO_X control, and selective hydrogenation.^1–3 The MOR-type zeolite possesses a one-dimension (1D) channel system composed of a parallel 12-membered ring (12-MR) micropores (0.67 × 0.70 nm) along the c [001] direction.⁴ The catalytic reactions mostly take place in these 1D, 12-MR pores, so that the shape and the size of MOR crystals are of critical importance for the transport of reactants and products. However, these 1D channelled zeolites always tend to form anisotropic crystals with the channels oriented along the longest crystal axis and the pore opened at low surface-energy faces, such as LTL, TON, MTT, MTW and MOR zeolites.^5–8 As such, the most common morphology of MOR zeolite is characterized by needles with c-axis elongation, which would limit molecules access to the pores at crystal surfaces and increase the internal path for molecular diffusion.

In order to subtly and controllably synthesize MOR zeolites, the influences of various synthesis parameters have been widely investigated, including molar composition (silica, alumina, and hydroxide ions), water content, the selection of extraframework cations, temperature, aging and heating time, etc.^8–12 Amongst these researches, de Jong and coworkers⁹ prepared large MOR crystals (>10 μm) and altered the thickness of MOR platelets along c-axis by changing SiO₂/H₂O, SiO₂/Al₂O₃ and NaOH/SiO₂ ratios. Recently, Rimer et al.¹¹ further attempted to reduce the aspect ratio, even using zeolite growth modifiers, to generate thin platelets with shorter 1D channels, and concluded that preparing MOR crystals with sizes less than 1 μm is a challenging task. Suib and coworkers¹⁰ successfully synthesized MOR with crystal diameter smaller than 100 nm by using mordenite seeds and modifying synthetic parameters, but the crystal length in the direction of 1D, 12-MR pores was still larger than 300 nm. In addition, Valtchev et al.¹² also proposed that the synthesis of nanosized MOR-type crystals is particularly difficult from a clear solution method, since the presence of Na⁺ is indispensable, which leads to aggregation of the initial particles. Therefore, a facile, effective and inexpensive synthesis of zeolite MOR with dramatically shortened microporous diffusion path is still challenging and highly desirable.

Seed-assisted synthesis has been adopted for the synthesis of zeolites with several frameworks to avoid the use of organic templates, and it is also accompanied with lots of advantages, such as decreasing the crystallization time, increasing the crystallinity, and facilely regulating the particle size.^13,14 As generally reported, a target zeolite can be well guided by the aid of zeolite seeds with the same framework in the synthesis gel.^15–18 Furthermore, it is found in recent researches that these seed crystals can function as not only the “templates” to induce the formation of microporous framework but also that for fine-tuning the morphology/meso- or macroporosity.^19–23 In contrast, the use of seeds could also yield other zeolites through modulating the gel composition or hydrothermal condition.^24–26 Okubo and coworkers²⁶ recently proposed a hypothesis for organic structure directing agent (OSDA)-free seed-induced synthesis that the framework of final product could be demonstrated by the building units in seed crystals and zeolite growth solutions of desired products; as such, they successfully grew zeolites ZSM-11 (MEL), ZSM-5 (MFI), beta (BEA) and MTW-type crystals. Additionally, the crystallization transformation sometimes occurs in zeolite synthesis, usually with a gradual process and follows the sequence: (1) amorphous phase, then (2) less stable zeolite, afterward (3) most stable zeolite.^27–29 On this basis, transformations of one zeolite structure into another one (interzeolite transformations) have been explored with or without the aid of organic templates and/or seed species.^30–34 For example, Iglesia and coworkers³⁴ recently synthesized high-silica MFI (ZSM-5), CHA (chabazite), STF (SSZ-35), and MTW (ZSM-12) zeolites from FAU (faujasite) or BEA (beta) parent materials via interzeolite transformations.

In this article, we proposed a plausible route of exotic seed-assisted conversion (ESAC) synthesis to prepared MOR nanocrystals with dramatically short 1D, 12-MR channels. Herein, nanosized BEA zeolites were adopted as seeds to transform and then recrystallize to target MOR zeolite, in which this conversion is favoured thermodynamically and kinetically in some extent. Through judiciously adjusting the synthetic conditions (mainly including salt concentration/kind, alkalinity, hydrothermal temperature and the added exotic seeds), the relationship between product phase/morphology and detailed synthetic condition is discussed, meanwhile the suitable condition is revealed to obtain pure MOR nanocrystals. In addition, the whole crystallization process from BEA to MOR is preliminarily studied, focusing on a longstanding induced period and a quick conversion/recrystallization process. Finally, typical samples are further tested to represent the advantages of as-synthesized nanosized products by comparing their detailed properties, involving morphology, framework aluminium site, acidity and catalytic performance in low-density polyethylene (LDPE) cracking reaction.

2. Experimental

2.1 Sample synthesis

In practice, a mixture of n_SiO2/n_Al2O3/n_Na2O/n_NaF/n_H2O was first prepared from 40% colloidal silica, Al₂(SO₄)₃·18H₂O, NaOH and sodium fluoride, meanwhile the pre-prepared zeolite BEA seed was added; the resulting gel solution was aged at room temperature for a certain period (normally 3 h), and then heated up to higher temperature (T/°C) for another period (t/min) to prepare the zeolite product denoted as n_SiO2/n_Al2O3/n_Na2O/n_NaF/n_H2O-T(t). After hydrothermal treatment, the solid products were recovered by filtration and washed with deionized water. The BEA zeolite with a size of ca. 200 nm and a Si/Al ratio of 20 was used as exotic seeds. The quantity of pre-added seeds typically equaled to 7.0 wt% of total SiO₂ weight in the starting gel. The BEA seeds were synthesized by a clear solution method,³⁵ and the obtained suspension was directly used as seed without further treatment.

For investigating the detailed crystallization process, a series of samples at different crystallization periods were synthesized under a typical composion of n_SiO2/n_Al2O3/n_Na2O/n_NaF/n_H2O-T(t) = 10/0.25/3.25/4.5/270-160(t), where t equals 0, 4, 12, 24, 28 and 32 h, respectively. Meanwhile, a series of as-synthesized products with different morphologies were also prepared under different NaF/Si ratios, OH⁻/Si ratios, reaction temperature and BEA seeds. The detailed synthetic conditions involved in this work are listed in Table 1.

Table 1 Initial synthesis compositions or conditions, and corresponding product phase in ESAC synthesis

No.	Pre-added seeds^a	NaF/Si^b	OH^−/Si^b	T^b/°C	Product phase^d
a The BEA zeolite seeds with a size of ca. 200 nm and a Si/Al ratio of 20 was used as seeds in general. Smaller sized nano-BEA zeolite (nano-BEA*) and large sized BEA zeolite (bulk-BEA) were also adopted for comparison.b Other synthetic conditions are maintained at H2O/SiO2 = 27, Al2O3/SiO2 = 0.025 and t = 48 h.c KF was replaced for NaF in synthesis of this sample.d Am. = amorphous phase, Lay. = layered material, and Qu. = quartz.
1	Nano-BEA	0	0.65	160	BEA + Am.
2	Nano-BEA	0.15	0.65	160	BEA + Am.
3	Nano-BEA	0.30	0.65	160	MOR + minor BEA
4	Nano-BEA	0.45	0.65	160	MOR
5	Nano-BEA	0.60	0.65	160	MOR
6	Nano-BEA	0.45 (KF)^c	0.65	160	BEA
7	Nano-BEA	0.45	0.45	160	Lay
8	Nano-BEA	0.45	0.55	160	MOR + minor MFI
9	Nano-BEA	0.45	0.75	160	MOR + BEA
10	Nano-BEA	0.45	0.65	140	MOR + minor BEA
11	Nano-BEA	0.45	0.65	180	MOR/MFI + Qu.
12	None	0.45	0.65	180	Am.
13	Nano-BEA*	0.45	0.65	160	MOR + minor BEA
14	Bulk-BEA	0.45	0.65	160	MOR

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2.2 Sample characterization

Powder XRD patterns were obtained on a Bruker D8-Advanced diffractometer with Cu-K_α radiation at 40 kV and 40 mA. The morphology information was obtained by SEM (Philips XKL30 D6716), field emission-SEM (Hitachi S-4800) and TEM (JEOL JEM-2011). The chemical microanalysis of the products was performed by EDX equipped on the TEM equipment. The particle size distribution curves of the zeolite samples were collected by DLS method on zetasizer (Nano-ZS90, Malvern). The N₂-sorption isotherms were measured by a Micromeritics ASAP-2010 instrument at liquid nitrogen temperature. Before analysis, all of samples were degassed in vacuum at 300 °C for 9 h. Solid-state ²⁷Al MAS NMR experiments were performed on a Bruker DSX 300 spectrometer at a rotation rate of 12 kHz. The acid amount and strength were measured by NH₃-TPD using a Micromeritics AutoChem 2920 analyzer. The catalyst sample (0.1 g) was heated at 550 °C in a He flow for 3 h and then cooled to 80 °C. NH₃ adsorption was performed under a flow of 10 vol% NH₃/He (30 ml min⁻¹) for 0.5 h. The NH₃-TPD was promptly started at a heating rate of 10 °C min⁻¹ from 80 to 600 °C.

3. Results and discussion

Zeolites are kinetically stable, and the relative empirical phenomenon shows that primarily-formed metastable structures could transform into more thermodynamically stable structures with time at suitable situation.^36,37 Chemical reactions in the transformation of metastable materials are governed by an intertwined combination of thermodynamic driving forces and kinetic barriers. For the synthesis of MOR from BEA, we referred to the database of International Zeolite Association.³⁸ MOR-type zeolites possess denser framework structure (framework density (FD) 17.0; defined as T atom per nm³, where T stands for Si or Al atoms in the zeolite framework) than BEA-type ones (FD 15.3), which illustrates that the transformation is thermodynamically favoured. In addition, the presence of common MOR composite building unit (CBU) between parent (BEA) and target (MOR) structures, as shown in Scheme 1, helps to overcome the kinetic hurdles during transformations, especially for assisting the nucleation.^26,32,34 Thus, we surmise their conversion can proceed, and attempt to use nanosized BEA zeolite as seed crystals to tailor the crystallization process and control the morphology of target MOR zeolite.

Scheme 1 Correlation of common composite building unit between BEA, MOR and MFI.

3.1 Investigation on the optimal ESAC conditions for synthesis of MOR nanocrystals

For a successful conversion, chemical composition of the gel and hydrothermal conditions are adjusted to favour the formation of desired MOR framework. Referring to the typical synthetic conditions of BEA and MOR,³⁹ we mainly investigate the influence of NaF/Si ratio (Na⁺ concentration alteration along with F⁻), OH⁻/Si ratio (alkalinity), hydrothermal temperature, and the properties of parent zeolite seeds. Here, the utilization of NaF salt can not only regulate Na⁺ ion concentration, but also introduce a certain amount of F⁻ ion which is an important mineralising agent as well as plays the roles to etch the zeolite crystal to somewhat modify the crystallization process.^19,40,41 The detailed hydrothermal synthesis conditions and product phases involved in this work are listed in Table 1.

At first, we investigated the effect of NaF addition on the final products. Sample 1 is the one prepared without adding any NaF salt, which exhibits a mixture of zeolite BEA and amorphous phase (Fig. 1). After adding a small amount of NaF (NaF/Si = 0.15), the crystal phases in product (Sample 2) is similar to that in Sample 1 but with increased crystallinity. Further adding NaF leads to the formation of high-crystalline MOR zeolite (Sample 3) but co-existing with a little amount of BEA phase. After the addition amount increases to NaF/Si = 0.45, a product of pure MOR phase is obtained (Sample 4). Then no obvious alteration is observed in XRD pattern when NaF/Si increases to 0.60 (Sample 5). These results indicate that the addition of NaF both elevates the crystallization rates of zeolites and promotes the conversion from BEA to MOR.

Fig. 1 XRD patterns of products of Sample 1–5 obtained at various salt concentrations of NaF/Si = 0, 0.15, 0.30, 0.45 and 0.60; and Sample 6 obtained with different kind salt of KF/Si = 0.45.

Additionally, SEM observations in Fig. 2 reveal that the obtained samples exhibit distinct morphologies and particle sizes although the pre-added seeds are the same nano-sized BEA crystals. Among them, Sample 1 shows a bulk aggregated morphology with the particle size of ca. 1 μm (Fig. 2a), and Sample 2 is similar to Sample 1 in morphology but with a little smaller size (Fig. 2b). For Samples 3 and 4 (Fig. 2c and d), the particle sizes are dramatically decreased, especially for Sample 4 which possesses the size even less than 150 nm. The alteration of particle size is also evidenced by the number distribution data detected on DLS measurement (Fig. 3). However, further adding NaF (Sample 5) seems not to work well and leads to some bulk aggregations (Fig. 2e).

Fig. 2 SEM images of products of Sample 1–5 obtained at different salt concentrations of NaF/Si = (a) 0, (b) 0.15, (c) 0.30, (d) 0.45, (e) 0.60; and (f) Sample 6 of different kind salt at KF/Si = 0.45. All scale bars indicate 1 μm.

Fig. 3 Particle size distribution of different products detected by DLS.

Besides, we also attempted to replace NaF by equal moles of NH₄F or KF to investigate the effect of alkaline cations. Although the addition of NH₄F cannot lead to any crystalline product, the high-crystalline nanosized BEA-type zeolite is obtained by adding KF under hydrothermal treatment for 48 h (Sample 6 in Fig. 1f and 2f). Interestingly, Samples 4 and 6 are similar in morphology and particle size but with completely different frameworks. Combining the results in previous literatures,^42,43 it indicates that the selection of extraframework cations also plays a critical role in the type of framework in OSDA-free media.

Next, we investigated the influences of the alkalinity and hydrothermal temperature for the interconversion from BEA to MOR. Under a low alkalinity of OH⁻/Si = 0.45, a layered aluminosilicate material of magadiite is obtained with a bulk layered or aggregated structure in micrometer level (Sample 7 in Fig. 4). Further increasing OH⁻/Si ratio to 0.55, MOR zeolite is obtained but co-existing with a little amount of MFI phase (Sample 8 in Fig. 4). Interestingly, this sample shows a special lamellate structure. When the alkalinity increases to OH⁻/Si = 0.75, we observes nano-sized zeolite crystals, in which MOR phase co-existed together with some residual BEA (Sample 9 in Fig. 4). Here, we should note that the framework structures and CBU of BEA, MOR and MFI all include a common MOR structural motif (Scheme 1).³⁸ It seems plausible that the common MOR units within the BEA-derived gel assist to reduce kinetic hurdles for nucleation and formation of these zeolites or their intergrown crystals, while the obtained product frameworks also depend on their favour synthesis conditions (low alkalinity for MFI, median one for MOR, and high one for BEA).^26,32,34

Fig. 4 XRD patterns and SEM images of products of Samples 7–11 obtained at different alkalinity and hydrothermal temperature. All scale bars indicate 1 μm.

These results were also evidenced by further investigating the influence of hydrothermal temperature to this interconversion. At low hydrothermal temperature of 140°, MOR phase is observed but without fully transforming BEA resource (Sample 10 in Fig. 4). At T = 180°, although the transformation of BEA is fulfilled, a small amount of impurity phases of MFI and alpha quartz, appear in the obtained product (Sample 11 in Fig. 4). It indicates that the selected synthesis conditions (e.g., OH⁻/Si and T) should conducive to the target structure, instead of alternate structures, during interzeolite transformation, even if the addition of NaF greatly enhances the trend for the formation of MOR framework.

Finally, the synthesis of MOR from BEA was carried out by using BEA seeds of different sizes in view of their probable differences on induced effects and product morphologies. We firstly explored the condition with no addition of BEA seeds in the preparation. Only amorphous aluminosilicate material is obtained in 48 h, as shown in XRD pattern of Sample 12 in Fig. 5. When the seed crystals (ca. 200 nm) are replaced by the smaller ones (ca. 120 nm), the formation of nano-sized zeolite crystals were observed (Sample 13 in Fig. 5). However, the starting BEA phase still remains a majority. In addition, a bulk commercial BEA-type zeolite (around 500–600 nm with aggregation) was also adopted to induce the zeolite crystallization, and the conversion products are harvested with pure MOR single-crystal particles of the size larger than 10 μm (Sample 14 in Fig. 5). By comparing the results of Samples 4 and 12–14, we can conclude that: (1) the crystallization rates of zeolites are notably elevated with the addition of BEA seeds comparing to that from amorphous aluminosilicate gels; (2) the smaller size of BEA seeds used, the stronger induced effect to preserve the parent phase, so that the conversion needs more harsh condition to favour the formation of MOR; and (3) product morphologies are closely related with the size of parent seeds.

Fig. 5 XRD patterns and SEM images (inset) of products of Samples 12–14 obtained with the addition of different BEA seeds.

3.2 Investigation on the ESAC process of MOR nanocrystals

It is found above that the product of uniform MOR nanocrystals can be successfully gained from a BEA seed-containing gel with the optimal composition around NaF/Si = 0.45 and OH⁻/Si = 0.65 at 160 °C for 48 h. We attempted to further insight the detailed crystallization process, especially for the formation of MOR nanocrystals. Fig. 6 displays the representative XRD data of the samples collected at different stages referring to the synthetic condition of Sample 4. Before heating (t = 0 h), the tiny diffraction peaks at 2θ = 6.8–8.5° and 21.0–23.5° result from a small amount of pre-added parent seeds of BEA framework. After hydrothermally treated for 4 h, the mixed gel shows an increased BEA crystallinity; and the crystallinity goes up slowly during subsequent crystallization from 4 to 12 h. Then, it is interesting that the crystallinity of BEA slowly fades in some extent from 12 to 24 h. The broad diffraction around 2θ = 15–30° is observed in the samples during 0–24 h, indicating that amorphous phase exists in all intermediates during the first long period. Surprisingly, within a very short time from 24 to 28 h, the interzeolite transformation from BEA to MOR takes place accompanied with a rapid crystallization process to produce high-crystalline MOR samples. Further prolonging heating time (32 h) has a little role for enhance crystallinity. We also altered the added amount of BEA seeds to 5 wt% to explore the alteration of solids phase and crystallinity during crystallization. The similar tendency is observed for the case of 5 wt% seeds, but the whole alteration rate is slowed down (Fig. S1†).

Fig. 6 XRD patterns of products collected at different hydrothermal treatment time.

SEM images clearly represent the morphology evolutions in Fig. 7. At t = 0 h (Fig. 7a), many aggregations composed of very small amorphous nanoparticles (ca. 10–20 nm) are observed, and the parent BEA seeds seem to be embedded in them. After reaction of 4 h (Fig. 7b), these intermediate gel nanoparticles get more aggregating and form interconnected worm-like morphological particles. At 12 h, the well-dispersed nano-crystallites aggregated particles appear (Fig. 7c). These crystal particles further dissolve and fuse to new agglomerates during the period from 12 to 24 h (Fig. 7d) referring to its XRD pattern (Fig. 6). However, with further prolonging the reaction time to 28 h (Fig. 7e), the embryonic crystal particles of 200 nm with a nano-crystallites assembled structure almost form through gradually consuming the formerly formed bulk BEA-derived gel intermediates. After 32 h, the high-crystalline MOR product is displayed in Fig. 7f without any amorphous species.

Fig. 7 SEM images of products obtained at different hydrothermal treatment time of (a) 0 h, (b) 4 h, (c) 12 h, (d) 24 h, (e) 28 h and (f) 32 h.

It is worth to note that the whole ESAC process could divided into two important stages: (1) a longstanding induced period of first 24 h and (2) a quick process for zeolite conversion and recrystallization from 24 to 28 h. In Stage I, the partial crystallization of the parent zeolite takes place in the beginning 0–12 h, but this BEA-containing intermediates can't further crystallize to fully crystalline form in the following time under the synthetic conditions conducive to MOR zeolite formation. Meanwhile, the decomposition/dissolution of the starting zeolite product seems to occur to generate locally ordered aluminosilicate species.^32,34 In Stage II, it is interesting that the transformation from BEA to MOR occurs fast and spontaneously without requiring the presence of either seeds or OSDA. Referring to some literatures,^26,32,34 we regards that the structural similarity of subunits between the parent zeolite (BEA) and the final crystallized zeolite (MOR) is an essential factor for this interzeolite conversion, while the fast conversion and re-crystallization process in 24–28 h is related to the generation and restructuration of partially ordered aluminosilicate species during the longstanding Stage I.

3.3 Investigation on the properties of as-synthesized MOR nanocrystals

The advantages of synthesized typical MOR nanocrystals (Samples 4) are further investigated by comparing with the bulk MOR of Sample 14 prepared via the similar ESAC method, including the detailed morphology, aluminium state, acidity and catalytic performance. According to the FE-SEM images in Fig. 8a and b, Sample 4 exhibits a ca. 200 nm sized uniform ellipsoid-like morphology with very rough external surface. TEM images of Fig. 8c and d show that the individual MOR particles are composed of abundant small nano-crystallites with the average diameter of ca. 20 nm. The in situ assembly of these nano-sized crystallites gives rise to abundant intercrystallite mesopores within these zeolite particles. Moreover, the high-resolution TEM image in Fig. 8d presents that each of the nano-crystallites is high crystalline with very clear crystal lattice fringes, and remarkably, these lattice fringes seem to be somewhat along the same orientation. In contract, Sample 14 displays a bulk single-crystal structure (inset in Fig. 5).

Fig. 8 SEM (a, b) and TEM images (c, d) of Sample 4.

These differences in morphology can also be evidenced by N₂ sorption isotherm (Fig. 9). Sample 4 shows a type IV isotherm with a H3 hysteresis loop, in which the steep increase at low relative pressure (P/P₀ < 0.2) illustrates the perfect microporous structure, while the enhanced adsorption and hysteresis loop at intermediate and high pressures (0.4 < P/P₀ < 1) are associated with the adsorption on inter-crystallite mesopores. In contrast, Sample 14 with large-sized and perfect single-crystal features performs almost no hysteresis loop (Fig. 9c), and presents only a steep at low relative pressure (P/P₀ < 0.2), illustrating the only existence of microporous structure. The detailed textural properties of these samples are summarized in Table 2 which are calculated according to the isotherms in Fig. 9. Although both samples have large specific surface area (S_L, >530 m² g⁻¹) and micropore volume (V_micro, >0.14 cm³ g⁻¹), Sample 4 possesses much larger S_ext and V_meso. This result further proves that Sample 4 possesses both the intact microporous structure and the abundant mesopores and external surface.

Fig. 9 Nitrogen sorption isotherms at 77 K of (a) Sample 4 and (b) Sample 14.

Table 2 Physicochemical parameters for C-MOR, Sample 4 and Sample 14

Sample name	Si/Al^a	SL, m^2 g^−1	Sext^b, m^2 g^−1	Vmicro^b, cm^3 g^−1	Vmeso^c, cm^3 g^−1
a Determined by XRF.b By t-plot method.c Using BJH method by desorption data. SL: Langmuir surface area; Sext: external surface area; Vmicro: micropore volume; Vmeso: mesopore volume.
C-MOR	8.0	550	75	0.16	0.13
Sample 4	9.7	598	140	0.14	0.26
Sample 14	10.2	531	26	0.17	0.05

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In addition to the morphology and porosity of MOR nanocrystals, we also investigated its properties on aluminium state and acidity. Fig. 10 presents ²⁷Al MAS NMR spectra of these two samples. All samples exhibit a main resonance (>99%) located at 56 ppm corresponding to the tetrahedral coordination framework aluminum atoms with Brönsted acid site nature. And the Brönsted acid sites of zeolites are expected to be the key active sites for various acid catalytic reactions.^20–23 In addition, the similar Si/Al ratios (ca. 10 in Table 2) indicate their similar amount of acid sites. NH₃-TPD measurement further proves this result. As shown in Fig. 11, almost the same total peak areas of Samples 4 and 14 indicate their similar amount of acid sites. But, Sample 14 possesses more strong acid sites, which should result from its large single-crystal structure with relative higher XRD peak intensity (Fig. S2, ESI†) and larger micropore volume (Table 2).

Fig. 10 ²⁷Al NMR spectra of (a) Sample 4 and (b) Sample 14.

Fig. 11 NH₃-TPD profiles of (a) Sample 4 and (b) Sample 14.

To understand the catalytic activity of the synthesized MOR nanocrystals by using ESAC method, a temperature-programmed catalytic test of LDPE cracking was carried out in a TG analyzer (50–600 °C at a ramp of 10 °C min⁻¹).²¹ Sample 4 was used as catalyst to show the positive effect of its specific structure on the catalytic performance. For comparison, (a) blank tests without the catalyst, (b) that with the commercial zeolite mordenite (C-MOR, Si/Al = 8, provided by the Catalyst Plant of Nankai University, China), and (c) that with Sample 14 were also conducted under the same conditions. The important features of C-MOR are shown in Table 2 and ESI (Fig. S2–5†). The T₅₀ (degradation temperature at 50% of LDPE conversion) is considered as a measure to evaluate the relative activity of all catalysts.²¹ As shown in Fig. 12, pure LDPE itself decomposes at the highest temperature of T₅₀ = 475 °C, whereas, the addition of zeolite MOR catalysts can greatly reduce the decomposing temperatures. The cracking temperature is progressively shifted to lower values in the order of C-MOR (T₅₀ = 451 °C) > Sample 14 (T₅₀ = 446 °C) > Sample 4 (T₅₀ = 383 °C). The T₅₀ over the catalyst Sample 4 is shifted by 68 °C with respect to C-MOR, presenting better catalytic activity of MOR nanocrystals from the ESAC procedure. This is probably due to more accessible acid sites from the rich mesoporosity on the external surface in Sample 4 (Fig. 8 and 9, and Table 2). Interestingly, the cracking activity for Sample 14 is slightly higher than that of C-MOR, we think that it should result from the high crystallinity, perfect microporous framework and strong acidity of Sample 14.

Fig. 12 Temperature-programmed mass loss curves of LDPE over (a) blank, (b) C-MOR, (c) Sample 14 and (d) Sample 4 determined by TG.

4. Conclusions

In summary, we have demonstrated that by adopting a simple exotic seed-assisted conversion (ESAC) method, the low-seeded (nano-BEA seeds) and organic template-free aluminosilicate gel can be successfully converted to MOR nanocrystals. This nanosized product synthesized under optimized conditions represents a nano-crystallite assembled structure, and owns abundant inter-crystallite mesopores, higher crystallinity and perfect framework. The obtained MOR nanocrystals show remarkable activity in the low-density polyethylene cracking reaction, in which both the large external surface and the preserved distinct zeolite acidity play the important roles. Additionally, we also systematically investigated the effects of various synthetic conditions for ESAC process. It is found that although the addition of NaF can greatly direct the conversion from BEA to MOR, the other synthesis conditions (e.g., OH⁻/Si, T) should also favour the formation of target framework. The added BEA seeds also take an important role in this process, which not only affect the conversion but determine the crystal size and morphology. Furthermore, through tracking the crystallization process, we evidence two important stages: (1) a longstanding induced period, and (2) a quick conversion and recrystallization process. These findings indicate that the ESAC route might be a facile and green alternative strategy for the selective synthesis of specific structured and nanosized zeolites, and could provide several special crystallization cases to helpfully understand the mechanistic details.

Acknowledgements

This work was supported by National Key Basic Research Program of China (2013CB934101), China Postdoctoral Science Foundation (2015M580289), NSFC (21433002, 21573046, 21473037 and U1463206), Sinopec (X514005) and National Plan for Science and Technology of Saudi Arabia (14-PET827-02).

Notes and references

1. C. Martinez and A. Corma, Coord. Chem. Rev., 2011, 255, 1558–1580.
2. M. Moliner, C. Martinez and A. Corma, Angew. Chem., Int. Ed., 2015, 54, 3560–3579.
3. J. Shi, Y. D. Wang, W. M. Yang, Y. Tang and Z. K. Xie, Chem. Soc. Rev., 2015, 44, 8877–8903.
4. W. M. Meier, Z. Kristallogr., 1961, 115, 439.
5. A. I. Lupulescu, M. Kumar and J. D. Rimer, J. Am. Chem. Soc., 2013, 135, 6608–6617.
6. O. Muraza, Ind. Eng. Chem. Res., 2015, 54, 781–789.
7. A. W. Burton, K. Ong, T. Rea and I. Y. Chan, Microporous Mesoporous Mater., 2009, 117, 75–90.
8. L. Zhang, S. J. Xie, W. J. Xin, X. J. Li, S. L. Liu and L. Y. Xu, Mater. Res. Bull., 2011, 46, 894–900.
9. L. Zhang, A. N. C. van Laak, P. E. de Jongh and K. P. de Jong, Microporous Mesoporous Mater., 2009, 126, 115–124.
10. B. O. Hincapie, L. J. Garces, Q. H. Zhang, A. Sacco and S. L. Suib, Microporous Mesoporous Mater., 2004, 67, 19–26.
11. J. D. Rimer, M. Kumar, R. Li, A. I. Lupulescu and M. D. Oleksiak, Catal. Sci. Technol., 2014, 4, 3762–3771.
12. V. Valtchev and L. Tosheva, Chem. Rev., 2013, 113, 6734–6760.
13. K. Iyoki, K. Itabashi and T. Okubo, Microporous Mesoporous Mater., 2014, 189, 22–30.
14. X. J. Meng and F. S. Xiao, Chem. Rev., 2014, 114, 1521–1543.
15. Y. Kamimura, S. Tanahashi, K. Itabashi, A. Sugawara, T. Wakihara, A. Shimojima and T. Okubo, J. Phys. Chem. C, 2011, 115, 744–750.
16. G. Majano, A. Darwiche, S. Mintova and V. Valtchev, Ind. Eng. Chem. Res., 2009, 48, 7084–7091.
17. N. Ren, Z. J. Yang, X. C. Lv, J. Shi, Y. H. Zhang and Y. Tang, Microporous Mesoporous Mater., 2010, 131, 103–114.
18. B. Xie, H. Y. Zhang, C. G. Yang, S. Y. Liu, L. M. Ren, L. Zhang, X. J. Meng, B. Yilmaz, U. Muller and F. S. Xiao, Chem. Commun., 2011, 47, 3945–3947.
19. H. B. Zhang, Y. Zhao, H. X. Zhang, P. C. Wang, Z. P. Shi, J. J. Mao, Y. H. Zhang and Y. Tang, Chem.–Eur. J., 2016 DOI:10.1002/chem.201600028.
20. H. B. Zhang, Z. J. Hu, L. Huang, H. X. Zhang, K. S. Song, L. Wang, Z. P. Shi, J. X. Ma, Y. Zhuang, W. Shen, Y. H. Zhang, H. L. Xu and Y. Tang, ACS Catal., 2015, 5, 2548–2558.
21. H. B. Zhang, Y. C. Ma, K. S. Song, Y. H. Zhang and Y. Tang, J. Catal., 2013, 302, 115–125.
22. H. B. Zhang, K. S. Song, L. Wang, H. X. Zhang, Y. H. Zhang and Y. Tang, ChemCatChem, 2013, 5, 2874–2878.
23. Z. J. Hu, H. B. Zhang, L. Wang, H. X. Zhang, Y. H. Zhang, H. L. Xu, W. Shen and Y. Tang, Catal. Sci. Technol., 2014, 4, 2891–2895.
24. H. Y. Zhang, Q. Guo, L. M. Ren, C. G. Yang, L. F. Zhu, X. J. Meng, C. Li and F. S. Xiao, J. Mater. Chem., 2011, 21, 9494–9497.
25. Y. Kamimura, K. Iyoki, S. P. Elangovan, K. Itabashi, A. Shimojima and T. Okubo, Microporous Mesoporous Mater., 2012, 163, 282–290.
26. K. Itabashi, Y. Kamimura, K. Iyoki, A. Shimojima and T. Okubo, J. Am. Chem. Soc., 2012, 134, 11542–11549.
27. H. Greer, P. S. Wheatley, S. E. Ashbrook, R. E. Morris and W. Z. Zhou, J. Am. Chem. Soc., 2009, 131, 17986–17992.
28. T. Moteki and R. F. Lobo, Chem. Mater., 2016, 28, 638–649.
29. Y. Wang, X. G. Li, Z. Y. Xue, L. S. Dai, S. H. Xie and Q. Z. Li, J. Phys. Chem. B, 2010, 114, 5747–5754.
30. K. Honda, A. Yashiki, M. Sadakane and T. Sano, Microporous Mesoporous Mater., 2014, 196, 254–260.
31. H. Jon, N. Ikawa, Y. Oumi and T. Sano, Chem. Mater., 2008, 20, 4135–4141.
32. H. Jon, K. Nakahata, B. W. Lu, Y. Oumi and T. Sano, Microporous Mesoporous Mater., 2006, 96, 72–78.
33. T. Sano, M. Itakura and M. Sadakane, J. Jpn. Pet. Inst., 2013, 56, 183–197.
34. S. Goel, S. I. Zones and E. Iglesia, Chem. Mater., 2015, 27, 2056–2066.
35. M. A. Camblor, A. Corma, A. Mifsud, J. PerezPariente and S. Valencia, Stud. Surf. Sci. Catal., 1997, 105, 341–348.
36. C. S. Cundy and P. A. Cox, Microporous Mesoporous Mater., 2005, 82, 1–78.
37. A. Navrotsky, O. Trofymluk and A. A. Levchenko, Chem. Rev., 2009, 109, 3885–3902.
38. http://www.iza-structure.org/databases.
39. H. Robson and K. P. Lillerud, Verified Syntheses of Zeolitic Materials, Elsevier, 2nd edn, 2001.
40. L. L. Yu, S. J. Huang, S. Miao, F. C. Chen, S. Zhang, Z. N. Liu, S. J. Xie and L. Y. Xu, Chem.–Eur. J., 2015, 21, 1048–1054.
41. Z. Qin, L. Lakiss, J. P. Gilson, K. Thomas, J. M. Goupil, C. Fernandez and V. Valtchev, Chem. Mater., 2013, 25, 2759–2766.
42. L. Van Tendeloo, E. Gobechiya, E. Breynaert, J. A. Martens and C. E. A. Kirschhock, Chem. Commun., 2013, 49, 11737–11739.
43. M. D. Oleksiak and J. D. Rimer, Rev. Chem. Eng., 2014, 30, 1–49.

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Footnote

† Electronic supplementary information (ESI) available: XRD, SEM, nitrogen sorption and NH₃-TPD characterization results for C-MOR. See DOI: 10.1039/c6ra08211d

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