Synthesis of core–shell ZSM-5@meso-SAPO-34 composite and its application in methanol to aromatics

Abstract

A core/shell-structured ZSM-5@meso-SAPO-34 composite catalyst was hydrothermally synthesized through overgrowing SAPO-34 molecular sieve on the external surface of ZSM-5. The catalyst was thoroughly characterized with regards to its crystallinity, morphology, elemental composition, surface area, pore volume, and acidity. The catalytic performance of the as-obtained ZSM-5@meso-SAPO-34 was tested by the conversion of methanol to aromatics reaction. ZSM-5@meso-SAPO-34 exhibited higher aromatics selectivity compared to that of pristine ZSM-5 and its physical mixture with SAPO-34. The unique catalytic synergistic behavior of ZSM-5@meso-SAPO-34 was ascribed to its effective enhancement of aromatization as a result of the acid sites on the external surface covered by the SAPO-34 shell.

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1. Introduction

Currently, processes like the production of aromatics, including benzene, toluene, and xylene (BTX), are of great significance in the petrochemical industry. Classically, the production of aromatics mainly relies on the catalytic reforming of naphtha. Other alternate processes, such as methanol-to-aromatics (MTA), as replacement for the fossil-based processes have grown to be a new focus for the production of aromatics, owing to the rapid diminishing of fossil resources.¹ Recent studies have demonstrated that methanol can be selectively converted to aromatics using zeolite-based materials.^2–4

Aluminosilicate ZSM-5 zeolite, as a shape selective catalyst,^5,6 has been extensively applied in the fields of petrochemical industry and fine chemical industry involving the MTA process.⁷ Furthermore, it possesses MFI-type topology connected by two types of 10-membered ring channels, desirable catalytic properties, e.g. high surface area, thermal stability and acidity, as well as adjustable pore size comparable to the molecular dimension of BTX.⁸ Accordingly, ZSM-5 zeolite exhibits both higher catalytic activity and remarkable selectivity to aromatics in the process of aromatization.^9–11 However, the catalytic activity and selectivity for aromatics formation are usually alleviated significantly by deactivation and formation of coking associated with carbon deposition, which may lead to the covering of the active sites on the external surface.^12,13 In addition, the acid sites on the external surface of ZSM-5 can effectively enhance the BTX-selectivity as well as side reactions, which inevitably influence the BTX-selectivity of the catalyst.¹⁴ Therefore, it is highly desirable to develop suitable catalysts for the production of aromatics. To obtain higher catalytic properties, various strategies have been attempted to reduce contributions from external surface acid sites via the introduction of mesopores.^15–17 Nevertheless, it still resulted in partial destruction and species deposition in the pore structure, consequently influencing zeolite molecular transport properties.

Recent progress has demonstrated that for the fabrication of core–shell composite materials particular attention has been paid to the fields of catalysis due to the acidic properties of the external surface of zeolites, and that diffusion efficiency can be altered in a desired way.^18–21 Besides, it is noteworthy that core–shell composite materials show their synergistic effects and multifunctional properties, e.g., nanoarchitectures, alterable compositions and sizes in catalytic reactions. Generally, the synthesis of uniform core–shell materials involves various synthetic approaches such as polytypism and epitaxy or overgrowth.^19,21–24 In particular, overgrowth on different materials can combine their different intrinsic properties based on structure and acidity. For instance, ZSM-5/SAPO-11 composites have been synthesized by in situ overgrowth, which were composed of an acidic H-ZSM-5 zeolite core covered with a SAPO-11 shell.²⁵ It has been previously reported that the core–shell type composites have desirable acidity, which is favourable to enhance the synergism between Brönsted and Lewis acids. In addition, binary structure ZSM-5/SAPO-5 composite zeolites synthesized by overgrowing SAPO-5 over the crystal surface of ZSM-5 have shown a higher propylene yield and conversion of heavy oil, and a striking decrease in the number of surface active sites.²⁶ More recently, ZSM-5/SAPO-34 (ref. 27) composites with binary structures have been successfully fabricated employing pre-heated ZSM-5 suspension followed by the secondary growth of the SAPO-34 layer, which represented excellent catalytic performance and synergistic effect between ZSM-5 and SAPO-34 in propane dehydrogenation reaction.²⁸ Despite the above described references, reports on the synthesis and application of ZSM-5/SAPO-34 core–shell structure composites are restricted. To the best of our knowledge, the fabrication of this type of composite zeolites, applying overgrowth strategy, and its application in the MTA have never been reported before. Silicoaluminophosphate SAPO-34 exhibits higher performance in methanol-to-olefin (MTO) reactions owing to its characteristic CHA topology and small 8-ring pore (3.8 Å × 3.8 Å) opening, as well as mild acidity.^29,30 However, the diffusion of bulky molecules such as aromatics in the catalytic reaction is severely hindered by the microporous channels of SAPO-34 zeolites, which is probably a result of the presence of small pores.^31,32 This limitation could be overcome if the channels of the mesopore SAPO-34 system are introduced in the configuration of the core–shell ZSM-5@SAPO-34.

In this study, we demonstrate a facile two-step hydrothermal synthesis strategy to prepare a series of core–shell heterostructures ZSM-5@meso-SAPO-34 formed by the growth of SAPO-34 crystalline overlayers on ZSM-5 nanocrystals, and mesoporous SAPO-34 layer was synthesized using diethylamine (DEA) and tetrapropylammonium hydroxide (TPAOH) as co-templates. The crystallization process of the ZSM-5@meso-SAPO-34 co-crystalline zeolite was systematically investigated via XRD, SEM, TEM, and nitrogen adsorption characterizations. As a hierarchically structured composite catalyst, ZSM-5@meso-SAPO-34 exhibits enhanced catalytic properties in the methanol to aromatics reaction with high selectivity for BTX, in contrast to the mechanical mixing.

2. Experimental

2.1. Material preparation

2.1.1. Synthesis of ZSM-5 and mesoporous SAPO-34.
Zeolite ZSM-5. ZSM-5 crystals with a Si/Al ratio of 25 were synthesized by the hydrothermal method. In a typical procedure, colloidal silica sol (30%) was mixed with sodium aluminate and deionized water, and the mixture was stirred for 2 h before tetrapropylammonium hydroxide (TPAOH, >25%) was added. The molar ratio of the reactants in this synthesis was set as 1.0 SiO₂ : 0.035 Al₂O₃ : 0.07 Na₂O : 0.2 TPAOH : 30 H₂O. The mixture was vigorously stirred for 4 h before it was transferred to a 100 ml Teflon-lined autoclave, which was preheated to 40 °C for 12 h. Zeolite ZSM-5 was then obtained by hydrothermal treatment at 453 K for 48 h under autogenous pressure; then, the products were washed with distilled water and dried at 383 K. The final products were obtained after calcination at 823 K for 5 h to remove any organic substances.
Mesoporous SAPO-34. Mesoporous SAPO-34 was prepared hydrothermally using DEA and TPAOH as the co-templates with the molar composition of 1.0 Al₂O₃ : 1.0 P₂O₅ : 0.6 SiO₂ : 2.0 DEA : 0.2 TEAOH : 0.03 CTAB : 60 H₂O. TPAOH and DEA were employed as structure directing agent and co-template. Typically, 2.9 g pseudoboehmite (70% Al₂O₃) mixed with the required amount of deionized water was added to 2.7 ml tetraethylorthosilicate (TEOS) under room temperature. After successive stirring for 2 h, 2.7 ml phosphoric acid (85%) was dispersed dropwise into the abovementioned solution with continuous stirring for further 2 h. Subsequently, 3.2 ml TPAOH and 4.2 ml diethylamine (DEA) was added to the mixture, and the mixture was stirred for 5 h. Next, the solution was mixed with an aqueous (80 g of water) solution of CTAB (0.21 g) and stirred for 4 h. Final mixture was transferred into an autoclave and aged at 313 K overnight, and then the resulting gel was heated at 453 K for 48 h. All the resulting solids were thoroughly washed with deionized water, filtered, and dried for 12 h at 383 K. The organic template was eliminated by calcination at 823 K for 6 h.

2.1.2. Synthesis of core–shell ZSM-5@meso-SAPO-34. Core–shell ZSM-5@meso-SAPO-34 materials were synthesized by adding a certain amount of pre-made ZSM-5 precursor into the gel mentioned in SAPO-34 in the absence of TPAOH and DEA. Subsequently, TPAOH and DEA were added dropwise into the resulting solution at room temperature, and then the mixture was dispersed in an aqueous solution of CTAB with continuous stirring until a uniformly dispersed suspension was obtained. After aging at 313 K overnight, the gel was crystallized at 453 K for 1–3 days. Finally, the product was filtered, washed thoroughly with deionized water, and then dried overnight at 383 K. The organic template was eliminated by calcination at 823 K for 4 h. The final samples were designated as CZS(n), where n represents the weight ratio of ZSM-5 precursor and SAPO-34 suspension gel.

2.1.3. Synthesis of mechanical mixture of HZSM-5 and SAPO-34. For the control experiment, a mechanical mixture of ZSM-5 and SAPO-34 was prepared by mixing as-synthesized ZSM-5 and a certain amount of as-synthesized mesoporous SAPO-34 with a weight ratio of 2 : 1. The composite zeolites were calcined at 873 K for 4 h and crushed into 20–40 meshes; these were denoted as HZS.

2.2. Catalyst characterizations

X-ray diffraction (XRD) patterns were performed to determine the phase structures of the samples using a D/max-RB diffractometer with Cu-Kα radiation (40 kV, 50 mA). The morphology and size of the crystal materials were measured by scanning electron microscopy (SEM) on a HITACHI SU-8010 microscope equipped with an energy-dispersive X-ray spectrometer (EDS) operated at 10–15 kV. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) of the samples were performed on a JEOL JEM-2100F with an acceleration voltage of 200 kV.

Infrared absorption spectra were recorded on a WQF-510 Fourier transform infrared (FT-IR) spectroscope with the resolution of 4 cm⁻¹. The sample powder and KBr were extruded into the self-supported wafer with the mass ratio of 1 : 300, and the spectra were recorded in the range of 4000–400 cm⁻¹. N₂ adsorption–desorption isotherms were obtained on a Micromeritics ASAP 2405 analyzer at −196 °C. Prior to the measurement, each sample (100 mg) was degassed at 300 °C for 10 h. The specific surface areas were calculated according to the BET method. The surface area and pore volume of the samples was calculated by BET, t-plot and BJH methods.

The acidic properties of the samples were determined by NH₃-TPD with a Micromeritics AutoChem II 2920 in the range of 150–600 °C at a ramp rate of 20 °C min⁻¹, and the desorbed ammonia was monitored by a gas chromatograph with a TCD detector.

2.3. Catalytic activity measurement

MTA reaction was carried out in a continuous-flow fixed-bed reactor with an inner diameter of 10 mm. The catalyst (5 ml) was loaded into the reactor. Methanol was pumped into the reactor after the catalyst was pretreated in nitrogen flow at 748 K for 3 h. The reaction was conducted under a pressure of 0.5 MPa and at a weight hourly space velocity of 1.2 h⁻¹. Liquid products were analyzed on a gas chromatograph (Agilent 7890A) equipped with a flame ionization detector and a capillary column.

3. Results and discussion

3.1. Preparation and characterization of core–shell ZSM-5@meso-SAPO-34

3.1.1. XRD. ZSM-5@meso-SAPO-34 is prepared using a hydrothermal method by adding a certain amount of pre-made ZSM-5 precursor into the gel of SAPO-34. Displayed in Fig. 1 are the X-ray diffraction patterns of a composite comprising ZSM-5 and SAPO-34 treated for 48 h with the weight ratio of ZSM-5 precursor and SAPO-34 suspension gel varying from 1 to 2. Both the samples show well-resolved diffraction peaks at 2θ = 7.9°, 8.8°, 22°–25°, and 9.5°, which are assigned to the MFI and CHA phases.³³ The XRD results indicate the presence of both ZSM-5 and SAPO-34 in the intergrowth zeolites. In addition, it appears that the crystallinity and intensity of CZS-2 are higher than those of CZS-1. In contrast, the reflections of the ZSM-5@meso-SAPO-34 core–shell are comparable to those of a mechanical mixture except for weaker relative intensity and crystallinity of the diffraction peak. When compared with those of the pristine zeolites (ZSM-5 and SAPO-34), the characteristic peaks of core–shell ZSM-5@meso-SAPO-34 shift slightly to higher angles due to the distortion of the crystal lattice, which is in agreement with previously reported XRD analysis.³⁴ In the case of the core–shell zeolite, the shrinkage that results from intergrowth is less noticeable because the ionic radius of silicon is similar to those of Al and P ions when it forms composite zeolites. The observed phenomena evidenced that SAPO-34 has successfully grown simultaneously as an outer layer of ZSM-5 in CZS.

Fig. 1 XRD patterns of the as-synthesized mesoporous SAPO-34 (a), CZS-1 (b), CZS-2 (c), HZS (d), and ZSM-5 (e).

Fig. 2 shows the XRD patterns of ZSM-5@meso-SAPO-34 (CZS-2) synthesized at different crystallization times, all of which exhibit well-resolved peaks in the range of 5°–50° and match well with those of Fig. 1c. With continuous increase in the crystallization time, the crystallinity and intensity of CHA in the ZSM-5@meso-SAPO-34 was relatively weaker than that of pure SAPO-34 crystal; moreover, the intensity of MFI gradually increased. In Fig. 2e, it can be observed that the intensity of MFI is higher than that of CHA. This can be explained as follows: on the one hand, the growth rate of the ZSM-5 phase is fast enough to surpass that of the SAPO-34 phase when crystallization time is extended: the former will protrude out of the latter, as shown in Fig. 3e. Therefore, the intensity of MFI is observed to be higher than that of CHA. On the other hand, since the weight ratio of ZSM-5 precursor and SAPO-34 suspension gel is 2, the diffraction peaks of ZSM-5 are higher than those of the SAPO-34 phase during the secondary crystallization process.

Fig. 2 XRD patterns of ZSM-5@meso-SAPO-34 (CZS-2) synthesized at different crystallization times: 24 h (a); 36 h (b); 48 h (c); 60 h (d); and 72 h (e).

Fig. 3 SEM images of the as-synthesized ZSM-5 (a), hierarchically mesoporous SAPO-34 (b), core–shell ZSM-5@meso-SAPO-34 (CZS-2) synthesized at different crystallization time: 48 h, high-magnification (c) and corresponding low-magnification (d), 60 h (e), 72 h (f).

3.1.2. SEM and TEM. To further demonstrate the targeted core–shell structure in ZSM-5@meso-SAPO-34, scanning electron microscopy (SEM) was used to characterize the morphology of the as-synthesized. As displayed in Fig. 3a, the sample of the as-synthesized ZSM-5 possesses very uniform spherical-like morphology with particle sizes ranging from 1 to 2 μm and consists of hexagonal columnar monocrystals. Similar morphologies are observed in ZSM-5@meso-SAPO-34 core–shell (Fig. 3c–f), and the size of the crystal particle is in agreement with the abovementioned observations. This is attributed to the aggregation of ZSM-5 crystals under the action of Gibbs free energy. In addition, the morphology did not suffer any influence from the gel system of secondary crystallization.

Fig. 3 mainly displays the morphology of ZSM-5@mesoporous SAPO-34 synthesized with different crystallization times. Initially, only uniform cubic-like morphology SAPO-34 phase with smooth crystal-face, and particle sizes of ca. 10 μm in diameter was observed in the composite structure after 24 h crystallization (Fig. 3b), which is the typical morphology of the crystalline CHA zeolite. Furthermore, no trace of ZSM-5 crystal was detected on the surface of ZSM-5@meso-SAPO-34 other than the SAPO-34 phase. It was assumed that a densely layered shell of the SAPO-34 formation begins to appear on the surface of ZSM-5, and ZSM-5 further grew into core phase until it was entirely covered by the SAPO-34 shell. However, this did not ensure that the core–shell structure only relied on the results. When crystallization time was prolonged to 48 h (Fig. 3c and d), spherical ZSM-5 particles began to appear on the surface of ZSM-5@meso-SAPO-34, and part of the particle was embedded into the composite structure, which changed the surface of the skeletons from smooth to coarse. With the crystallization time increased to 72 h (Fig. 3e and f), the amount of spherical ZSM-5 particles increased continuously and the particles were scattered on the surface of the SAPO-34 shell. This suggests that ZSM-5 precursors are gradually formed in the original silicoaluminophosphate gel of SAPO-34 and core–shell composite structures were subsequently fabricated with increase in crystallization time. Thus, appropriate synthesis time is a critical factor in the synthetic procedure for the formation of self-assembled core–shell structure.

The formation of the synthesized core–shell ZSM-5@meso-SAPO-34 was also confirmed by SEM-EDX analysis, and the results are presented in Fig. 4. Fig. 4 reveals the results of SEM-EDS scanning along the edge of the external surface of the ZSM-5@meso-SAPO-34 core–shell (CZS-2). According to the EDS spectra in Fig. 4, obviously, the number of P element is larger than Si element. Moreover, the P element only exist in SAPO-34 phase, but not exist in ZSM-5. Thus, the EDS investigation further verified that ZSM-5 phase in the composite may be covered by the SAPO-34 layer generated in the secondary synthesis.

Fig. 4 SEM-EDS line scanning of ZSM-5@meso-SAPO-34 core–shell (CZS-2).

Fig. 5 shows the TEM micrographs of the as-synthesized core–shell ZSM-5@meso-SAPO-34, which clearly exhibit that the ZSM-5 was covered tightly by an SAPO-34 shell that was about 20–30 nm-thick with a core–shell structure, and no other crystalline phase was detected except for the characteristic cubic SAPO-34 phase and a few spherical ZSM-5 particles, which is in good agreement with SEM results (Fig. 3d). Moreover, the lattice fringes between core and shell can be better observed in the enlarged image (Fig. 5B). The results indicate that SAPO-34 was successfully grown on the surface of ZSM-5 crystallites and a core–shell structure composite was fabricated.

Fig. 5 TEM images of core–shell ZSM-5@meso-SAPO-34 (CZS-2) synthesized with a crystallization time at 48 h: low-magnification (A), high-magnification (B), and 72 h low-magnification (C), and high-magnification (D).

3.1.3. FT-IR. FT-IR spectroscopy assists in identifying surface species of shell formed during crystallization. Fig. 6 displays the FT-IR spectra of the as-synthesized samples of ZSM-5, SAPO-34, ZSM-5@meso-SAPO-34 core–shell (CZS-2) and mechanical mixture (HZS). Framework vibrations at 455, 793 and 1094 cm⁻¹ are observed in CZS-2, which correspond to the Si–O bending mode vibration of tetrahedron inner links of T–O–T that are similar to those of ZSM-5 zeolites. In addition, two bands at 546 cm⁻¹ and 640 cm⁻¹ for CZS-2 and HZS are assigned to the asymmetric vibration of the five-membered rings of T–O–T (T = Si or Al) in the MFI frameworks and the bend of the double 6-ring, respectively. These results indicate that the core–shell composite sample contains the primary units of CHA zeolites in addition to MFI-type zeolites, which further proved the formation of a shell phase around the core phase during the second crystallization.

Fig. 6 IR spectra of samples: (a) ZSM-5, (b) CZS-2, (c) HZS, and (d) mesoporous SAPO-34.

It is worth noting that distinct absorptions appear at about 1221 cm⁻¹. The peak for CZS-2 slightly shifts to 1218 cm⁻¹, and the intensity is lower than those in HZS and pure ZSM-5. This phenomenon can be ascribed to a particular conjunction form of tetrahedrons and a special skeleton structure at the interface between core and shell zeolites. Because the IR band at 1221 cm⁻¹ is sensitive to change in the zeolite structure, the decrease in intensity proves the presence of CHA structure besides the MFI phase. In case of CZS-2 and HZS, all the bands exhibit a lower intensity, corresponding to pure ZSM-5 and SAPO-34. It is inferred that the introduction of SAPO-34 (shell) decreases the crystallinity of the as-synthesized core–shell zeolite, which is in accordance with the result of XRD.

3.1.4. N₂ adsorption–desorption characterization. N₂ adsorption–desorption isotherms and pore size distributions were used to estimate the porosity features of samples, as shown in Fig. 7. The initial part of all the samples exhibit the type I isotherm curves with a sharp uptake at relative pressure P/P₀ below 0.1, indicating a characteristic micropore framework. The ZSM-5@meso-SAPO-34 sample displays the type IV isotherm with a hysteresis loop at a relative pressure P/P₀ of 0.45–0.95 as a result of the presence of mesopores, which is associated with capillary condensation.³⁵ Comparatively, the isotherm curves of ZSM-5@meso-SAPO-34 core–shell and the mechanical mixture are more or less similar to those of pristine ZSM-5 zeolite combined micropores and mesopores, whereas typical hysteresis loops of them obviously show different behaviour. Compared with the latter, the isotherm curve of the core–shell composite sample appears as a relatively flat curve with a type H4 hysteresis loop. The phenomenon is associated with narrow slit-like pores in accordance with the IUPAC,³⁶ which is attributed to the presence of a large number of mesopores in the structured composite. In contrast, sharp uptake in the adsorption–desorption isotherms of the HZS sample can be observed at higher relative pressures, P/P₀ > 0.9. Perhaps, it arises from the formation of an agglomerate of particles from ZSM-5 and SAPO-34.

Fig. 7 (a) N₂ adsorption–desorption isotherms and (b) pore size distributions of HZS, SAPO-34, CZS-2 and ZSM-5.

The textural properties of samples are listed in Table 1, in which the specific BET surface area and total pore volume of the pristine and as-synthesized ZSM-5 are 398 m² g⁻¹ and 0.21 cm³ g⁻¹, respectively, with lower contribution to mesopores from intrinsic ZSM-5; only 81 m² g⁻¹and 0.13 cm³ g⁻¹, respectively. In contrast, the ZSM-5@meso-SAPO-34 composites possess lower micropore volumes but more mesopores. The micropore volumes and total pore volumes of ZSM-5@meso-SAPO-34 core–shell are higher than the mechanical mixture, with about 0.16 cm³ g⁻¹ and 0.23 cm³ g⁻¹, respectively. This is in accordance with the result of Fig. 7.

Table 1 Textural properties of the zeolites samples

Sample	Smeso^a (m^2 g^−1)	Smicro^b (m^2 g^−1)	SBET^c (m^2 g^−1)	Vmeso^d (cm^3 g^−1)	Vmicro^e (cm^3 g^−1)	Vtotal^f (cm^3 g^−1)
a Surface area covered by mesopores calculated from t-plot.b Surface area covered by micropores calculated from t-plot.c Total surface area calculated using BET equation in a range of relative pressure from 0.05 to 0.3.d Pore volume covered by mesopores calculated from t-plot.e Pore volume covered by micropores calculated from t-plot.f Single point pore volume at P/P0 = 0.99.
ZSM-5	81	317	398	0.08	0.13	0.21
CZS-2	68	309	377	0.16	0.07	0.23
HZS	57	299	356	0.14	0.08	0.22
SAPO-34	52	270	322	0.09	0.16	0.25

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3.2. The acidity of ZSM-5@meso-SAPO-34 core–shell catalysts

The surface acidities of the as-synthesized samples were determined by the temperature programmed desorption of ammonia (NH₃-TPD). As illustrated in Fig. 8, the spectra of the ZSM-5@meso-SAPO-34 display two distinct desorption peaks; a high-temperature peak at ca. 440 °C and a low-temperature peak centered at ca. 183 °C, which mainly arise from the strong and weak acid sites, respectively. It can be seen that the area of the high-temperature peak shows an obvious decrease and simultaneously slightly shifts towards lower temperatures compared to that of pristine ZSM-5. It can be deduced that a portion of the strong acid sites of ZSM-5 or the surface of ZSM-5 core may be covered and replaced by the weaker acidity of the SAPO-34 shell after the incorporation of SAPO-34, which is in agreement with our previous results. In addition, the mechanical mixture shows lower temperature and smaller acidity relative to that of core–shell ZSM-5@meso-SAPO-34.

Fig. 8 NH₃-TPD patterns of different samples: ZSM-5, CZS-2, HZS and SAPO-34.

3.3. Catalytic performance

Methanol-to-aromatics (MTA) conversion as a typical shape-selective reaction was used to evaluate the catalytic performance of the ZSM-5@mesoporous SAPO-34 catalyst. Its catalytic properties were compared with those of ZSM-5 catalyst and physical mixture of ZSM-5/SAPO-34. As shown in Table 2, SAPO-34 showed an extremely low yield of aromatics (2.5%) on account of weak acidity, as characterized by NH₃-TPD. The yields to aromatics are around 30.1 wt% on parent ZSM-5 zeolites with the Si/Al ratio = 25. However, ZSM-5 exhibits a slightly higher yield of 84.7 wt% to BTX. Compared with the purely acidic ZSM-5, ZSM-5/SAPO-34 (HZS) prepared by physical mixing improves the aromatics yield to 37.7 wt% and gives obviously higher selectivity for BTX (85.5 wt%). In contrast, ZSM-5@meso-SAPO-34 catalyst exhibited high catalytic activity. It is worth noting that the yield of aromatics is obviously enhanced with the increase in ZSM-5/SAPO-34 weight ratio. The selectivity to BTX and yields of aromatics at reaction temperature of 460 °C and WHSV = 1.2 h⁻¹ reach 81.5%, 85.1% and 40.2%, 48.6%. This result should be related to the increased acidity and acid amount in ZSM-5, which caused the enhancement in aromatics formation. Moreover, the addition of suitable amounts of SAPO-34 in the core–shell structure can be beneficial for the formation of aromatics. The direct connection between cores and shells lead to the fabrication of hierarchical porous structure and enhance the accessibility to active sites.

Table 2 Product distributions of methanol transformation over the investigated catalysts^a

Catalyst	Product distribution (wt%)							YBTX (%)	SBTX (%)
	C1–C5^b	B^c	T^d	o-X^e	m-X^f	p-X^g	C9^+^h
a Reaction conditions: 0.5 MPa, 460 °C, WHSV = 1.2 h^−1 time-on-stream = 4 h.b C1–5 alkenes.c Benzene.d Toluene.e ortho-Xylene.f meta-Xylene.g para-Xylene.h C9 and C9^+ aromatics.
ZSM-5	64.5	2.1	9.8	5.6	9.5	3.1	5.4	30.1	84.7
HZS	62.3	2.9	12.1	6.0	10.8	5.9	5.3	37.7	85.5
SAPO-34	97	0.2	0.2	0.4	1.3	0.4	0.5	2.5	87.1
CZS-1	50.1	3.4	10.5	8.1	14.9	3.3	9.7	40.2	81.5
CZS-1.5	45.8	4.1	11.2	7.9	18.1	4.1	8.8	45.4	83.8
CZS-2	42.9	4.2	13.4	7.9	18.5	4.6	8.5	48.6	85.1
CZS-2.5	43.8	4.3	13.1	7.5	18.2	4.4	8.7	47.5	84.5

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Fig. 9 shows the methanol conversion and selectivity of benzene, toluene, and xylene with time over ZSM-5@meso-SAPO-34. As presented in the figure, the selectivity of BTX increases with time and can reach the maximum of 85.1% when the methanol conversion approaches 100%, whereas selectivity levels decrease with time. The change of BTX selectivity could be related to the coking effect, which reduced the pore size of the SAPO-34 shell in ZSM-5@meso-SAPO-34.

Fig. 9 Conversion and selectivity of BTX with time on stream over ZSM-5@meso-SAPO-34. B: benzene, T: toluene, and X: xylene.

4. Conclusions

The hierarchical pore system with core–shell structured ZSM-5@meso-SAPO-34 is successfully prepared by the hydrothermal method via introducing the mesoporous SAPO-34 system. The characterization by SEM, BET, XRD, FTIR, TEM, EDS and NH₃-TPD indicated that the cetyltrimethylammonium bromide (CTAB) and TPAOH aided the preparation of ZSM-5@mesoporous SAPO-34 core–shell composites with large surface area and endowed the composites with enhanced catalytic activity for the MTA. These excellent physicochemical properties for ZSM-5@mesoporous SAPO-34 come from the synergistic effects of ZSM-5 and mesoporous SAPO-34, which shorten the diffusion path length within the composite zeolites. Under optimal conditions, the methanol to aromatics conversion on the ZSM-5@mesoporous SAPO-34 achieved a power conversion efficiency of approximately 100% and higher yield of aromatics, which is comparable with the performance of the ZSM-5 and physical mixture of ZSM-5 and SAPO-34. These results indicate that the ZSM-5@mesoporous SAPO-34 is a promising composite material and provides a novel way for the production of high-value aromatics.

Acknowledgements

This project was financially supported by the scientific and technological department of Liaoning Province (no. 201202126).We appreciate anonymous reviewers for their helpful suggestions on the quality improvement of our present paper.

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