Assemblies of hybrid core–shell ZSM-5 zeolite materials

Abstract

Here we report a one-step dual template strategy to achieve the facile synthesis of two different types of hybrid core–shell ZSM-5 zeolite materials, consisting of a crystalline bulk ZSM-5 zeolite core and a vesicular or lamellar ZSM-5 zeolite shell. The structures and morphologies of these two hybrid core–shell zeolite materials were elucidated by mutually complementary techniques.

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Hierarchical porous materials possessing pores of different scales can lead to superior application properties according to size and shape.¹ Zeolites, the best-known group of molecular sieves, are a family of crystalline aluminosilicate materials which have been widely used as shape-selective catalysts, adsorbents, ion exchangers, microreactors and separators in the fields of oil refining and petrochemical process due to their unique properties such as tunable acidity, high specific surface area, well-defined porosity, high thermal and hydrothermal stability.² However, serious diffusional restriction imposed by the sole presence of crystalline microporous network limited their practical applications when large molecular reactions were involved. Hence, the targeted design of the pore hierarchy desired in transport-based applications will result in improved performance in such a single zeolite material. Great efforts have been made to solve this problem over the past years and led to new ways of controlling molecular diffusion limitations in zeolite materials, including dealumination,³ desilication,⁴ assembly of zeolite nanocrystals^5,6 and templating strategies.^7–14 To date, mesoporous zeolites, possessing the structure characteristics and chemical properties of microporosity as well as the fast diffusion paths and mass transport consequences of mesoporosity in such materials, have been studied for applications as broad as possible.¹⁵

Recently, the synthesis of hybrid zeolite materials with hierarchical pore structures and various morphologies has also been developed to expand the use of zeolites, which gathered considerable research interest for a number of applications.^16,17 The hybrid zeolite materials with interconnected meso-/micro-porous components and different shapes are of not only great scientific but technological significance as well because the pore sizes on different length scales and morphologies strongly affected their physicochemical properties to varying degrees.¹⁸ In particular, epitaxial growth of a 2-D layered zeolite on the surface of a bulk (3-D) zeolite to construct 2D–3D hybrid materials with tunable microporous and mesoporous domains has been reported. For instance, Nair et al. employed a two-step synthesis method, which involved the pre-synthesis of the bulk zeolite core with different particle sizes, followed by the epitaxial growth of the 2D layered zeolite shell on it, thereby giving rise to a pure-silica hybrid material containing a unique morphology of hierarchical pore structures.¹⁶ As the hybrid lamellar-bulk material combines the merits of bulk zeolite (acid sites in micropore network) and lamellar zeolite (large surface roughness and mesoporosity) in a single particle, it may find applications as advanced materials in various fields such as catalysis, separation and sensor technologies. Therefore, it would be interesting and useful if new and facile strategies could be developed to direct the formation of this hybrid zeolite materials with tunable mesoporosity and strong acidity. It should be noted that, so far, the facile synthesis of hierarchical hybrid zeolite materials remains an unsolved challenge.

Previous studies have focused attention on the preparation of hybrid siliceous zeolite materials.^16,17 However, incorporation of aluminum into the framework of siliceous zeolite is significant as it improves the acidity in micropore network and extends their potentional use as much as possible. Thus it is more important to convert siliceous zeolites into ZSM-5 zeolites with a crystalline aluminosilicate framework.¹⁹ Herein, we present a new and facile route to synthesize two different types of hybrid core–shell ZSM-5 zeolite materials by a mixed template system in one-pot. Tetrapropylammonium Bromide (TPABr) and [3-(trimethoxysilyl)propyl]octadecyldimethylammonium chloride ([(CH₃O)₃SiC₃H₆N(CH₃)₂C₁₈H₃₇]Cl, TPOAC) were employed as dual templates to direct the formation of one hybrid vesicular-bulk ZSM-5 zeolite (denoted as HVBZ) consisting of crystalline bulk ZSM-5 zeolite core and vesicular ZSM-5 zeolite shell. Furthermore, the addition of small amount of adipic acid disodium salt (NaOOC(CH₂)₄COONa) into the initial synthesis composition gave another hybrid lamellar-bulk ZSM-5 zeolite with a new morphology (denoted as HLBZ) coupling crystalline bulk ZSM-5 zeolite core and lamellar ZSM-5 zeolite shell.

The powder X-ray diffraction (XRD) patterns of the two hybrid core–shell ZSM-5 zeolites are shown in Fig. 1. The reflections in the low-angle XRD patterns (2θ ∼ 1–7°) of the two samples before calcination are very much alike. Two resolved Bragg reflections (2θ ∼ 2.5°, 5.0° for HVBZ and 2θ ∼ 2.3°, 4.6° for HLBZ), corresponding to a mesoscale lattice with 2D lamellar symmetry, could be observed, demonstrating that the materials have a highly long-range ordered lamellar structure.²⁰ However, surfactant removal leads to the partial condensation of the zeolite layers. The diffraction peaks of calcined materials in the low-angle XRD patterns disappear, which is consistent with the reported results.^21,22 The wide-angle XRD patterns shows well-resolved peaks of a typical MFI-type structure in the 2θ = 7–40° range with no evidence of other crystalline phase.²³

Fig. 1 Low-angle (A) and wide-angle (B) XRD patterns of as-synthesized HVBZ and HLBZ.

Fig. 2 shows the morphology of HVBZ visualized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations. The HVBZ is in spherical shapes with rough surface (Fig. 2a). Most of them are actually intergrown. This is a typical growth pattern for ZSM-5. The core–shell structure could be better seen in Fig. 2b after some grinding. The shell is the composite of zeolites and zeolite vesicles (marked by red arrows) or consist of only zeolite vesicles, as indicated by the TEM images in Fig. 2c and d, respectively. However, the representative SEM images in Fig. 3 reveal that the two hybrid samples have distinct morphologies. The HLBZ exhibits platy structures aggregated with each other, similar to coffinlike morphology typically observed for silicalite-1, involving twinning of the layered zeolite crystal (Fig. 3a and b). Elemental analysis (EA) gives a Si/Al molar ratio of 18 for both HVBZ and HLBZ, which is in the proper range for ZSM-5 zeolite. The ²⁷Al MAS NMR spectra of HVBZ and HLBZ (see ESI†) showed a single peak at δ = 53.94 ppm and δ = 53.20 ppm, respectively, owing to tetrahedrally coordinated aluminium species in crystalline zeolite. The absence of a peak at δ = 0–10 ppm (corresponding to octahedral aluminium species) demonstrated that Al atoms were entirely incorporated into the framework. Lamellar ZSM-5 is seen to grow at the smooth surface of conventional ZSM-5, and the particle surface is highly roughened after the growth of lamellar zeolite as shown in Fig. 3c and d, where a part of shell was detached after some grinding. TEM image in Fig. 3e displays the morphology and core–shell structure observed under SEM. TEM image at high magnification (Fig. 3f) confirms the lamellar structure of the shell (marked by red arrows). The shell of HLBZ has been further studied by TEM after some grinding. As expected, the shell is crystalline structure (as indicated by the corresponding Fast Fourier Transform) with lattice fringes attributed to zeolite as shown in ESI.† These results indicate that the addition of adipic acid disodium salt induces a dramatic change in the morphology.

Fig. 2 (a and b) SEM images and (c and d) TEM images of HVBZ.

Fig. 3 (a–d) SEM images and (e and f) TEM images of HLBZ. The concentration of NaOOC(CH₂)₄COONa was 2.3 × 10⁻² mol L⁻¹. Squares in one image show the area enlarged in the next image.

The dual template strategy has been found to be successful in the synthesis of hierarchical zeolites. The hydrolyzable methoxysilyl moieties of TPOAC could strongly interact with growing crystal domains because of their formation of covalent bonds with the SiO₂ and Al₂O₃ sources and thus phase-separation phenomenon was avoided, resulting in zeolite materials with short-range correlation between mesopores.^9,24 Note that the formation of hybrid core–shell MFI zeolites induced by the use of TPA⁺ and TPOA⁺ or other zeolite types induced by the use of TPOAC has not been observed. To our knowledge, no work about the synthesis of zeolite vesicle is presented in the literature. The different micellar assemblies behavior in this case might be caused by the interplay between TPA⁺ and the TPOA⁺ molecules in the given synthesis composition, which collaboratively, directs the formation of bulk zeolite core and zeolite vesicle shell in a single zeolite particle. Yet the organic salt NaOOC(CH₂)₄COONa plays an important role in the morphology control process. A decrease of concentration of NaOOC(CH₂)₄COONa from 2.3 × 10⁻² to 1 × 10⁻² mol L⁻¹ as well as the use of sodium hexanoate (CH₃(CH₂)₄COONa) only generated hybrid vesicular-bulk zeolite with spherical morphology, similar to HVBZ. However, when the concentration of NaOOC(CH₂)₄COONa was increased to 4 × 10⁻² mol L⁻¹, very few particles like HLBZ were observed. The coffinlike particles interacted with each other and were fabricated into larger crystals like zeolite membrane as clearly shown in Fig. 4. Instead of NaOOC(CH₂)₄COONa, the suberic acid disodium salt (NaOOC(CH₂)₆COONa) can also be applied to synthesize hybrid lamellar-bulk zeolite with coffinlike morphology (not shown). These observations demonstrated that the alkyl dicarboxylates could direct the formation of hybrid zeolites with different morphologies in the presence of different salt concentration. This phenomenon might be related to the electrostatic interaction between the double decomposition of negatively charged organonic salt (–OOC(CH₂)₄COO–) and positively charged dual templates (TPA⁺ and TPOA⁺), which generates bulk zeolite core and lamellar zeolite shell as single zeolite particles with a twinned crystal growth. Such principle has also been reported for the synthesis of mesoporous silica.²⁵

Fig. 4 SEM images of hybrid zeolite material synthesized in the presence of NaOOC(CH₂)₄COONa. The concentration of NaOOC(CH₂)₄COONa was 4 × 10⁻² mol L⁻¹. Squares in one image show the area enlarged in the next image.

The textural properties of the HVBZ and HLBZ samples were further confirmed by nitrogen (N₂) adsorption–desorption measurement. As depicted in Fig. 5, they all exhibit a type IV adsorption–desorption isotherm with a pronounced hysteresis loop at a relative pressure of 0.4 < P/P₀ < 0.95, corresponding to the capillary condensation of N₂ in the mesoporous structures. Accordingly, the Barrett–Joyner–Halenda (BJH) pore size analysis based on the adsorption branches of the isotherms shows a broad distribution of mesopore diameters owing to the irregular distortion of zeolite layers. The textural properties of both samples determined from the corresponding N₂ isotherm are summarized in Table 1.

Fig. 5 (A) Nitrogen adsorption–desorption isotherms and (B) BJH pore size distribution of HVBZ and HLBZ. The isotherm and distribution of mesoporous diameter for HVBZ was offset vertically by 50 and 0.2 cm³ g⁻¹, respectively.

Table 1 Textural properties of two hybrid zeolites

Sample	SBET [m^2 g^−1]	Sext^a [m^2 g^−1]	Vt [cm^3 g^−1]	Vmicro^a [cm^3 g^−1]	Vmeso [cm^3 g^−1]
a Determined from t-plot method.
HVBZ	322	129	0.23	0.1	0.13
HLBZ	436	171	0.29	0.09	0.20

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It can be seen that the micropore volume (V_micro) of HLBZ is slightly lower than that of HVBZ whereas its Brunauer–Emmet–Teller (BET) surface area (S_BET), external surface area (S_ext), total pore volume (V_t) and mesopore volume (V_meso) are much higher. It is conclusively shown that the salt NaOOC(CH₂)₄COONa has dramatic influence on both morphology and textural properties, indicating the feasibility of systematically tuning zeolite properties by simply adding alkyl dicarboxylates in the dual template synthesis.

Conclusions

In summary, the structure and morphology of two hybrid core–shell zeolite materials (denoted as HVBZ and HLBZ) synthesized by a facile one-step dual template (TPABr and TPOAC) strategy were elucidated by mutually complementary techniques. The HVBZ is in spherical shapes consisting bulk ZSM-5 core and vesicular ZSM-5 zeolite shell. However, the HLBZ shows coffinlike morphology coupling crystalline bulk ZSM-5 zeolite core and lamellar ZSM-5 zeolite shell. The capability of growing a lamellar zeolite or zeolite vesicle on a conventional bulk zeolite creates the possibility of a new class of hybrid zeolite materials whose morphologies and textural properties can be rationally controlled by the addition of alkyl dicarboxylates. The synthesis approach demonstrated here is potentially applicable to form other types of hybrid zeolite materials when proper templates are selected. Moreover, the availability of a large variety of organosilane surfactant and alkyl dicarboxylates, varying in size and chemical nature, provides new opportunities to facilitate the emergence of new hierarchical porous materials specifically tailored to applications as diverse as catalysts, sorbents, sensors and optoelectronic devices.

Acknowledgements

This work was supported by National Natural Science Foundation of China (Grant 20906012), Shanghai Municipal Natural Science Foundation (no. 14ZR1400800) and Fundamental Research Funds for the Central Universities of China (Grant 2232013D3-05).

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Footnote

† Electronic supplementary information (ESI) available: Experimental and characterization details. See DOI: 10.1039/c4ra15263h

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