Direct synthesis of high-silica nano ZSM-5 aggregates with controllable mesoporosity and enhanced catalytic properties

Abstract

High-silica nano-sized ZSM-5 aggregates have been rapidly synthesized from a low-templated dry gel by solid-like state and a steam-assisted conversion; the crystallization of ZSM-5 aggregates is well induced within 48 h at a TPAOH/SiO₂ of 0.05. The influence of synthesis conditions such as NaOH/SiO₂ ratio on the final products was investigated. The mesopore distribution can be facilely controlled by adjusting the size of primary particles via modulating the alkalinity of the synthesis gel. The ZSM-5 aggregates obtained by varying Na₂O/SiO₂ molar ratios from 0.05 to 0.13 are denoted as NA-X (X = 1–4). The NA-X samples are characterized by techniques including XRD, SEM, TEM, N₂ adsorption–desorption, NH₃-TPD and Py-IR. SEM revealed that the obtained NA-X were ellipsoidal spheres that consisted of nano-sized crystallites with 20–70 nm. It is found that NA-2 exhibits a mesopore distribution around 12 nm and possesses the largest amount of external acid sites, which lead to the remarkably improved catalytic performance in methanol to propylene (MTP) reaction. A possible growth mechanism of self-assembled NA-X zeolites directed by tetrapropylammonium hydroxide (TPAOH) is proposed. The crystallization process indicated that the size of raw materials gradually decreased, formed nano-sized particles and the stacking of the resulting small zeolite crystals will form NA-X zeolites with prolonged time. Compared with the conventional hydrothermal route, the solid-like state and steam-assisted conversion not only significantly shortened the crystallization time, but also greatly reduced the emissions of waste liquids. In addition, for the MTP reaction, the NA-X zeolites obtained by this method showed much better catalytic performance, propylene selectivity and longer lifetime to endure coke deposition than the sample obtained by the hydrothermal route.

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1. Introduction

Zeolites with Mobil Fifth (MFI) structure have been extensively applied in the petrochemical industry, adsorption and separation fields as shape-selective catalysts because of their uniform and ordered micropores, large specific surface area, and high hydrothermal stability.^1,2 Especially, ZSM-5 zeolite as an important shape-selective catalyst is widely used in various catalytic processes, such as naphtha catalytic cracking, alkylation, isomerization, and methanol to hydrocarbons reaction, etc.³ With the increasing demand for propylene and rapid depletion of oil resources in a post-oil society, it is crucial to find a better way to maximize propylene production using a dedicated process. MTP reaction developed by Lurgi, as one of the important routes to manufacture propylene, besides conventional methods such as fluid catalytic cracking and steaming cracking, has drawn significant attention.⁴ Currently, zeolite ZSM-5 with a high SiO₂/Al₂O₃ ratio and thus less acid sites has been proved to be a remarkably effective material for the MTP reaction and it plays a critical role in the high yield of propylene and long catalytic lifetime of the MTP process.⁵

Recent studies have demonstrated that the conversion of methanol to dimethyl ether (DME) takes place mainly on the weak acid sites whereas the conversion of DME (and methanol) to light olefins occurs mainly on the strong acid sites.⁶ The dehydration of methanol produces DME, which further transformed to ethylene by dehydration during the initial reaction period. The ethylene can be transformed into propylene by its sequential interaction with Brønsted acid sites (i.e., H⁺ sites) of a protonic zeolite and methanol, followed by a dehydration process. Propylene can also transform into butylenes through a similar series of reaction stages.⁷ The primary products are ethylene and propylene, and these molecules will undergo further reactions (such as polymerization, isomerization, aromatization, cracking, and hydrogenation) that lead to the formation of higher hydrocarbons.⁸

As typical crystalline microporous sieves, zeolite ZSM-5 possesses intricate channels, adjustable acidity and high thermal/hydrothermal stability.⁹ However, due to its narrow channels, the size of molecules that can undergo reactions using the ZSM-5 zeolite as a catalyst is limited and the regular pores in molecular dimensions always seriously affect the stability of zeolite catalysts.¹⁰ It is well known that the deactivation of the MTP catalyst is dominated by coke deposition due to methanol conversion over ZSM-5 zeolite is a typical solid-acid catalytic reaction.¹¹ Hence, zeolite ZSM-5 still suffers from rapid/severe coking especially on the outer surface of the catalyst, leading to the covering of the active sites or the blocking of pore openings of the zeolite.¹² This slows down reactions, increases the possibility of secondary reactions and coke formation, and result in a degradation in material catalytic performance.¹³

In order to remedy this problem of rapid/severe deactivation and develop a highly stable catalyst for the MTP reaction, a variety of options have been established to reduce the diffusion resistance, and downsizing the crystal size to nanometers and generating hierarchically structured zeolites (HSZs) are the main solutions.^14,15 Nanosized zeolites with high external surface areas can also shorten the diffusion path length and thus reduce diffusion limitations.¹⁶ However, the low yield and difficulty of nanozeolite separation limits their practical application. HSZs that combine crystalline microporous frameworks and auxiliary mesoporous structures have attracted attention recently,¹⁷ as they can enhance the mass transfer rate and they retain the intrinsic properties of traditional zeolites with respect to surface acidity and hydrothermal stability.

Diverse synthetic methods have been established to generate HSZs, for instance, post-synthetic treatments (dealumination and desilication)^18,19 can generate HSZs through the selective removal of aluminum (dealumination) or silicon (desilication) from the framework. However, the major disadvantage of dealumination is a partial reduction in crystalline nature of the zeolite, which reduces the catalytic activity.²⁰ Desilication is considered more effective to introduce intracrystalline mesopores in the zeolite crystals than dealumination, but a decrease in crystallinity and thus a lower hydrothermal stability result.²¹ In these dissolution (dealumination and desilication from the zeolites) methods, it is difficult to control the zeolite mesoporosity, which leads to a change in chemical composition² (the obtained mesoporous zeolites possess a different chemical composition from the original zeolites, because the SiO₂/Al₂O₃ ratio in the zeolites has been changed). On the other hand, it is common to employ nanoscale materials, including carbon materials,^22,23 polymers^24–26 and surfactants,^27,28 as sacrificial mesoscopic templates to generate HSZs, which work cooperatively with a molecular zeolite structure-directing agent (SDA) and can be facilely removed by combustion.

Aggregates of nanosized zeolite particles display advantages of nanosized zeolites and HSZs, for example, reduced diffusion limitation, an increased external surface area, and a large mesopore volume, while being able to solve difficulties in separation because of their larger secondary particle size.¹⁶ Various attempts to synthesize nano-sized ZSM-5 aggregates have been developed. In a typical approach, nanosized aggregates of Zeolite Socony Mobile-Five (ZSM-5) were synthesized by careful control of gel composition and crystallizing conditions in a hydrothermal environment. Pluronic triblock copolymer F127 has been used for the in situ assembly of zeolite silicalite-1 (S-1) nanocrystals into microspheres, but the synthesis of Al-containing zeolite ZSM-5 microspheres has not been reported yet.²⁹ Another strategy is to restrict the nanocrystal growth, termed the confined space^30,31 or silanization method.^32,33 Recently, researchers in Ryoo's group reported the synthesis of ZSM-5 zeolite aggregates with sheet-like morphology using an appropriately designed bifunctional template.³⁴ However, the high cost of some of the various types of templates or troublesome preparation is still inevitable in most cases.³⁵ ZSM-5 aggregates could also be fabricated using a primary template of tetrapropylammonium hydroxide (TPAOH),³⁶ but a long crystallization time and large amounts of templates are usually needed, which makes the process less economical. From another point of view, the crystallization time was as long as several days, and the yield of nano-sized ZSM-5 aggregates was no more than 50%.³⁷ Besides, the generated gas from combustion for these organic species removal is environmentally unfriendly.³⁸ To reduce the usage of porous templates and/or simplify synthesis, a new methodology is desired for the nano-sized ZSM-5 aggregates preparation.

Steam-assisted conversion (SAC), as one of the typical dry gel conversion methods for zeolite synthesis, has recently attracted an increasing interest due to its high zeolite yield, few amounts of wastes and low template consumption.³⁹

Herein, a fast high-silica (SiO₂/Al₂O₃ ≥ 100) nano ZSM-5 aggregates (denoted as NA-X) synthesis method is explored in a low-templated dry gel system via SAC method. The resulting material showed ellipsoidal spheres consisted of nano-sized crystallites with 20–70 nm, when used in the MTP reaction, the NA-X catalysts exhibited a high propylene selectivity and a longer catalytic lifetime. Influencing factors such as NaOH/SiO₂ ratio on the final products in the crystallization process are systematically discussed for fine tuning the zeolite crystal size and microstructure. In addition, the primary synthesis mechanism of the ZSM-5 crystal is proposed. Moreover, benefiting from the synergistic effects of TPAOH and SAC method, TPAOH/SiO₂ molar ratio is greatly reduced to 0.05, and an interesting smaller particles size was achieved compared to what is achieved in literature.¹⁶ This synthesis process was rapid, economical and shows potential for large-scale production. This catalyst design concept provides an efficient way to developing highly efficient heterogeneous metal catalysts, which will have potential for their wide-range application in selective catalytic reactions with larger molecules in industry. For comparison purposes, a conventional ZSM-5 sample (denoted as C-ZSM-5) was crystallized by conventional hydrothermal methods from the gel with the same composition at 120 °C for 48 h.

2. Experimental section

2.1 Synthesis of high silica C-ZSM-5 zeolite

The high-silica C-ZSM-5 zeolite has been hydrothermally synthesized with colloidal silica (40% of SiO₂) was used as silica source and NaAlO₂ was used as aluminium source, which was designated as 100% crystallinity. In a typical procedure, 0.090 g of sodium aluminate (NaAlO₂, 99.9%), 3.69 g of TPAOH, (Acros, 25%), and 0.47 g of NaOH were completely dissolved in 30.00 g of H₂O. 13.65 g of colloidal silica (40% SiO₂) was added dropwise under stirring to the resultant mixture. The final molar composition of the mixture was 100SiO₂ : 0.6Al₂O₃ : 7Na₂O : 2500H₂O : 5TPAOH. After stirring for 3 h, the gel mixture was transferred into a stainless-steel autoclave lined with polytetrafluoroethylene to crystallize at 120 °C for 48 h under autogeneous pressure. After centrifugation at 10 000 rpm for 5 min at room temperature and washed with distilled water three times, dried at 120 °C for 5 h, and calcined at 550 °C for 6 h to remove the template, the crystalline product of C-ZSM-5 was achieved. The calcined products were ion exchanged three times in aqueous 1.0 M NH₄NO₃ solutions at 80 °C for 2 h and then calcined at 550 °C for 6 h to obtain the acidic H-form.

2.2 Synthesis of high silica NA-X zeolites

In the synthesis of NA-X zeolites, colloidal silica (40% of SiO₂) was used as silica source and NaAlO₂ was used as aluminium source. NaOH was added to adjust the Na₂O/SiO₂ ratio. After dissolving NaOH and NaAlO₂ in a certain amount of H₂O, TPAOH was dropped into the clear solution in sequence. Finally, colloidal silica were added dropwise under stirring to the resultant mixture. After stirring for 3 h, mixture with a molar composition of 100SiO₂ : 0.6Al₂O₃ : XNa₂O : 2500H₂O : 5TPAOH was heated to uncomplete dryness at 80 °C for about 4 h and the templated dry gel was obtained. 8.00 g of dry gels were transferred into a 8 mL Teflon cup, which itself was placed into a 50 mL stainless steel autoclave with 2.00 mL water in the bottom, and then the autoclave was sealed and transferred into a preheated oven at the desired reaction temperature of 120 °C and heated for 48 h. After washed with distilled water three times, dried at 120 °C for 5 h, and calcined at 550 °C for 6 h to remove the template, the crystalline product of NA-X was achieved. The calcined products were ion exchanged three times in aqueous 1.0 M NH₄NO₃ solutions at 80 °C for 2 h and then calcined at 550 °C for 6 h to obtain the acidic H-form.

2.3 Characterization

X-ray powder diffraction (XRD) analysis was conducted on a RIGAKU Smart Lab diffractometer, using Cu Kα radiation. Nitrogen adsorption–desorption isotherms were obtained using a Micromeritics ASAP2020 apparatus at the temperature of liquid nitrogen (−196 °C). The samples were outgassed at 300 °C 4 h prior to the sorption measurements. The sample specific surface area (S_BET) was calculated by the Brunauer–Emmett–Teller (BET) method and the pore size distribution by the Barrett–Joyner–Halenda (BJH) method using the adsorption branch. The external surface area (S_ext) and micropore volume (V_micro) were obtained by the t-plot method. The total pore volumes (V_pore) were evaluated at P/P₀ = 0.99. The mesopore volume (V_meso) was obtained by the calculated total data minus the corresponding micropore data. Scanning electron microscopy (SEM) images were obtained using a field-emission scanning electron microscope (SUPRA55) operating at an acceleration voltage of 3 kV. Transmission electron microscopy (TEM) images were obtained using a Tecnai G2 F20 instrument operating at 200 kV. Surface acidities were measured by NH₃-temperature program desorption (NH₃-TPD) on an apparatus TP-5080 (Tianjin Xianquan Technology Development Co., Ltd).

Pyridine-adsorbed Fourier transform infrared spectroscopy (Py-FTIR) was used to determine the amount of Brønsted and Lewis acid sites using a Bruker Tensor 27 equipment. Approximately 32 mg of catalyst was pressed into a regular wafer (R = 1.3 cm⁻¹) and placed inside an infrared cell. The determination of the infrared spectrum was performed after sample treatment at room temperature for 2 h under vacuum, and this spectrum was used as background for the adsorbed pyridine experiments. Next, pyridine vapour was introduced into the cell at room temperature for 1 h; pyridine was then adsorbed at a 5.0 × 10⁻² Pa equilibrium pressure at room temperature for 2 h, the spectra were then recorded after evacuation at 200 °C for 1 h.

The coke amount was determined using a TG/DTA6300 (Japan) thermogravimetric analyzer from 30 to 800 °C under air flow with a heating rate of 10 °C min⁻¹. The amounts of Si and Al in zeolites were quantified by inductively coupled plasma (ICP) on a Thermo IRIS Intrepid II XSP atomic emission spectrometer after dissolving the samples in HF solution.

2.4 Catalytic performance test

The MTP reaction was carried out at 470 °C in an auto-sampled fixed-bed microreactor under atmospheric pressure. The catalyst was 0.5 g and the WHSV for methanol was 2 h⁻¹ with a feed of methanol solution (n(CH₃OH) : n(H₂O) = 1 : 1). The total products were analyzed by an on-line thermo trace gas chromatograph equipped with a Flame Ionization Detector (FID) and a 50 m CP-ParaPLOTQ capillary column. Both methanol and DME are regarded as reactants for calculation.

3. Results and discussion

3.1 Physiochemical properties

The NaOH concentration in the precursor solution played a predominant role in the formation of both microporous and mesoporous structures in the ZSM-5 products.⁴⁰ The NaOH concentration could influence the crystallization process parameters, such as solubility, polymerization and depolymerization rate of the silicate species, rate of zeolite nucleation and crystal growth. Therefore, the purity and the relative crystallinity of the zeolites obtained depend on the synthesis conditions.⁴¹ Furthermore, the sodium ions in the hydrated form could also act as an inorganic template for ZSM-5 formation.

A series of NA-X samples were synthesized with varied Na₂O/SiO₂ ratio and corresponding product yields of 59.4–76.8% (Table S1, ESI†). In this work, the molar ratio of Na₂O/SiO₂ ranging from 0.05 to 0.13 was studied. Dry gels were subjected to steam treatment to respectively prepare NA-X (X = 1–4) zeolites, which are in comparison with C-ZSM-5 zeolite synthesized via common hydrothermal synthesis. Fig. 1 shows the powder XRD results for NA-X and C-ZSM-5 zeolites. All the samples synthesized exhibited typical and well-resolved peaks of zeolite with MFI structure, thus confirming that the product was indeed ZSM-5. No impurities or amorphous phase were detected by XRD, which suggests the almost complete conversion of the amorphous raw materials into crystalline zeolites. Compared with C-ZSM-5, the NA-X samples synthesized have a slightly lower intensity of XRD signals, indicating the smaller size of crystalline domains. On varying the Na₂O/SiO₂ ratio from 0.05, 0.07 and 0.09 to 0.13, the crystallinity of the samples is 97%, 93%, 95% and 96%, respectively. With an increase in the Na₂O/SiO₂ ratio, the relative crystallinity decreased first, then increased. There is a slight difference in the characteristic peaks among NA-X samples obtained by varying the slurry alkalinity from Na₂O/SiO₂ = 0.05 to 0.13, implying that the synthetic system possesses broad operating flexibility. Moreover, full width at half maximum (FWHM) of the XRD peaks corresponding to the NA-X samples are listed in the Table 1, showing the broader characteristic diffraction peaks than that of C-ZSM-5. The broadened peaks of NA-X samples could be ascribed to the smaller size of the primary particles of the NA-X samples.⁴² However, the increase of Na₂O/SiO₂ ratio could lead to reduction in the yields (from 76.78% to 59.37%, see Table S1, ESI†) and the SiO₂/Al₂O₃ ratio in the synthesized NA-X samples. A SiO₂/Al₂O₃ ratio of 163 can be found in the product when the Na₂O/SiO₂ ratio in the batch was 0.05, while it decreased to 147 when the Na₂O/SiO₂ ratio increased to 0.13, indicating that the higher alkalinity leads to the higher concentration of free Si species in slurry.

Fig. 1 The XRD patterns of NA-X samples and C-ZSM-5.

Table 1 The zeolitic properties of the C-ZSM-5, C₁-ZSM-5 and the NA-X samples

Sample	Na2O/SiO2	Crys.^a (%)	SBET^b (cm^2 g^−1)	Sext^c (cm^2 g^−1)	Vmicro^c (cm^3 g^−1)	Vpore^d (cm^3 g^−1)	Vmeso^d (cm^3 g^−1)	FWHM^e
a R.C.: relative crystallinity.b Calculated by the Brunauer–Emmett–Teller (BET) method applied to the adsorption branch of the isotherms.c Calculated by the t-plot method.d Vmeso = Vads, P/P0 = 0.99 − Vmicro.e Calculated by the MFI-characteristic peaks at 2θ = 23°.
NA-1	0.05	97	337	185	0.07	0.22	0.15	0.188
NA-2	0.07	93	388	193	0.07	0.25	0.18	0.345
NA-3	0.09	95	361	184	0.07	0.24	0.17	0.320
NA-4	0.13	96	343	175	0.07	0.22	0.15	0.290
C-ZSM-5	0.10	100	322	112	0.1	0.18	0.08	0.182
C1-ZSM-5	0.10	99	330	147	0.07	0.20	0.13	0.085

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The obtained C-ZSM-5 zeolite has the traditional morphology of the bulk particles and the particles size distributes around 3 μm (see Fig. S1, ESI†), while the size of primary particles was 200 nm.

Significantly different from C-ZSM-5 zeolite, the SEM image in Fig. 2A–D reveals that the obtained NA-X samples were ellipsoidal aggregates in similar size (700–800 nm) compared to ZSM-5 aggregates in literature 7 (1.5–2.0 μm), the high-resolution SEM image (inset of Fig. 2A–D) shows that the structure of NA-X samples are relatively loose with numerous holes in the framework. As mentioned above, an interesting smaller particles size was achieved compared to what is achieved in literature.¹⁵ The primary particle sizes of NA-X samples from ∼70 nm to smaller than 20 nm while that of ZSM-5 aggregates in literature 7 from ∼100 nm to smaller than 50 nm. On varying the Na₂O/SiO₂ ratio from 0.05, 0.07 and 0.09 to 0.13, the size of primary particles was first decrease and then increase. At the relatively low Na₂O/SiO₂ ratio of 0.05 and 0.07, the primary particle sizes dramatically decreased from 70 nm to smaller than 20 nm. As the Na₂O/SiO₂ ratio increased to 0.09, the primary particles increased from 20 nm to much larger than 40 nm. Further increasing the Na₂O/SiO₂ ratio from 0.09 to 0.13 saw a progressive increase in the primary particle sizes (much larger than 70 nm), indicating some fusion of nanocrystals appeared as the Na₂O/SiO₂ ratio increased. It could also be attributed to the crystallization promotion by the increase of system basicity, which is consistent with SEM images and FWHM in XRD patterns.⁴³

Fig. 2 The SEM images of (A) NA-1, (B) NA-2, (C) NA-3, (D) NA-4.

The TEM images of NA-2 sample in Fig. 3A agree with the SEM observations, showing nanometer-sized particles with an ellipsoidal morphology (Fig. 2B).

Fig. 3 The TEM images of NA-2 (A–C), the corresponding SAED patterns of NA-2 (D).

Because of the density difference of the crystallized frameworks and the mesopore areas, in the TEM image of Fig. 3B, the clear bright-to-dark contrast reflects the development of mesoporosity in the synthesized NA-2 sample. Primary particles with clear lattice fringes are visible from the high-resolution TEM image (Fig. 3C) taken at the edge of the NA-2 sample, and confirm that the NA-2 is formed from zeolite nanocrystals rather than from amorphous silica. The high-resolution TEM image (Fig. 3C) and corresponding selected area electron diffraction (SAED) in Fig. 3D display parallel lattice fringes throughout the entire particle and periodic diffraction spots, which demonstrates the single crystalline property of NA-2, and that the introduction of mesopores maintains the single crystal integrity.

To clarify the pore structure development of NA-X samples, N₂ adsorption–desorption isotherm characterization was conducted for NA-X and C-ZSM-5 samples. As shown in Fig. 4A, C-ZSM-5 exhibits a representative type I isotherm reflecting its microporous structure, while NA-X samples exhibited similar combination of type I + IV isotherms, indicating that they have a similar pore structure. The synthesized NA-X samples show a clear hysteresis loop at a relative pressure p/p₀ of 0.45–0.95, which is caused by capillary condensation in mesoporous channels and contributions from intercrystalline voids between the nanophase crystalline particles. The pore-size distribution curves of the NA-X and C-ZSM-5 samples are shown in Fig. 4B. According to the pore-size analysis by the Barrett–Joyner–Halenda (BJH) method using the adsorption isotherm, no pore size distribution of the C-ZSM-5 could be observed in the range of mesopores, indicating its microporosity dominated textural properties, while the NA-X samples have mesopores with widths between 4 and 30 nm with a maximum at 12 nm, which belongs to the stacking intercrystalline mesopores of nanosized ZSM-5 zeolite. Table 1 summarizes the textural properties of the synthesized NA-X and C-ZSM-5 zeolites.

Fig. 4 (A) N₂ adsorption–desorption isotherms and (B) pore size diameter of the C-ZSM-5 and NA-X samples.

As shown in Table 1, NA-X samples have a similar micropore volume of about 0.07 cm³ g⁻¹. The NA-X samples possess a higher Brunauer–Emmett–Teller (BET) surface area (337–388 m² g⁻¹, 322 m² g⁻¹ and 330 m² g⁻¹, respectively), higher external surface area (175–193 m² g⁻¹, 112 m² g⁻¹ and 147 m² g⁻¹, respectively) and a larger mesopore volume (0.15–0.18 cm³ g⁻¹, 0.08 cm³ g⁻¹ and 0.13 cm³ g⁻¹, respectively) compared with the C-ZSM-5 and ZSM-5 aggregates in literature 7.

On varying the Na₂O/SiO₂ ratio from 0.05, 0.07 and 0.09 to 0.13, the BET surface area, external surface area, mesopore volume of NA-X samples were increase first and then decrease. The NA-2 has the largest BET surface area, the largest external surface area, the largest mesopore volume in all NA-X (X = 1, 2, 3, 4) samples, which is because it has a minimum primary particles. Besides, the mesopore volume as well as the external surface area of the NA-X samples correlates well with the FWHM of the XRD peaks. In comparison, C-ZSM-5 shows no distinct mesopore distribution because of the absence of sharp uptake at high p/p₀.

3.2 Acidic properties

Temperature-programmed desorption of ammonia (NH₃-TPD) was carried out to investigate the zeolite sample acidity and the NH₃-TPD profiles are shown in Fig. 5A. The profiles of NA-X and C-ZSM-5 are characterized by two desorption peaks with maxima at 184 °C and 396 °C, respectively. The low-temperature peak is usually ascribed to the chemisorption of ammonia molecules on weak acid sites, which are catalytically inactive in the methanol conversion reaction, whereas the high-temperature peak is attributed to ammonia desorption from strong Brønsted and strong Lewis acid sites. The peak areas allow for an estimation of the number of acid sites, assuming one NH₃ molecule per acid site. The peak area is proportional to the number of acid sites on the sample. Values of the sample acidity are given in Table 2. The ammonium desorption profiles of the NA-X and C-ZSM-5 zeolites are similar. The number of acid sites at 184 °C and 400 °C for the NA-X samples is slightly lower than that of C-ZSM-5 (Table 2). The weak acid strength of the NA-X zeolites exist because of the small amount of aluminum incorporated in the zeolite framework. Moreover, as shown in Fig. 5A, with the increase in Na₂O/SiO₂ ratio, both the peak areas of weak and strong acid sites were increased, indicating the increase in the total amount of acid sites. Hence, the NH₃-TPD data agree with the ICP analysis results (see Table S1 and results 3.1, ESI†).

Fig. 5 (A) NH₃-TPD profiles of as-synthesized zeolite: NA-X and C-ZSM-5 samples; (B) Py-IR spectra of NA-X and C-ZSM-5 samples.

Table 2 The acidic properties data of NH₃-TPD and pyridine –FTIR

Samples	Peak area of weak acid	Peak area of strong acid	Concentration of B acid sites [μmol pyd g^−1]	Concentration of L acid sites [μmol pyd g^−1]	B/L
NA-1	227	467	74	200	0.37
NA-2	259	605	122	178	0.69
NA-3	322	614	119	196	0.61
NA-4	354	673	117	271	0.43
C-ZSM-5	433	674	146	273	0.53
C1-ZSM-5	218	435	103	285	0.36

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Pyridine (Py) adsorption was followed by infrared (IR) spectroscopy to identify the number and nature of acid sies in the zeolites. Fig. 5B shows the Py-IR spectra of the NA-X and C-ZSM-5 samples. Fourier-transform IR spectra of the catalysts after Py adsorption at ambient temperature and subsequent evacuation at 200 °C in order to release physisorbed pyridine exhibit two characteristic bands at ∼1539 and 1440 cm⁻¹, which are ascribed to pyridinium ions chemisorbed on Brønsted acid sites and coordinatively bound Py on Lewis acid sites, respectively. Brønsted and Lewis acidities have been quantified according to the integrated areas of the peaks at 1539 and at 1440 cm⁻¹, respectively, and the data are summarized in Table 2. The number of Brønsted acid sites present over NA-X and C-ZSM-5 catalysts decreases as C-ZSM-5 > NA-2 > NA-3 > NA-4 > NA-1, whereas the number of Lewis acid sites of the NA-X and C-ZSM-5 catalysts decreases as C-ZSM-5 > NA-4 > NA-1 > NA-3 > NA-2. As shown in Fig. 5A and B and Table 2, it is clearly observed that both the amount of Brønsted acid sites and the ratio of B/L increases first and then decreases with the increasing of Na₂O/SiO₂ ratio. For the Lewis acid sites, but it is the opposite trend. NA-2 possesses largest amount of Brønsted acid sites and the highest ratio of B/L in all NA-X samples. Accordingly, it is expected that reactions, such as the MTP reaction, that are catalyzed by solid-acid will occur predominantly.⁴⁴

3.3 Catalytic performance

The prepared samples NA-X, C-ZSM-5 and a commercial ZSM-5 (denoted as C₁-ZSM-5: Purchased from Catalyst Plant of Nankai University, SiO₂/Al₂O₃ = 167) zeolites were tested by MTP reaction in a fixed bed reactor under identical conditions (T = 470 °C, P = 1 atm, WHSV = 2.0 h⁻¹). The methanol conversion over NA-X, C-ZSM-5 and C₁-ZSM-5 are plotted as a function of reaction time in Fig. 6A. Here, the catalyst deactivation is set as methanol conversion falling to around 95%. In view of methanol conversion, their catalytic lifetime follows 192 h, 144 h, 136 h, 132 h, 114 and 104 h with order of NA-2 > NA-3 > NA-1 > NA-4 > C₁-ZSM-5 > C-ZSM-5. As can be seen, the NA-X catalysts showed obvious higher catalytic stability than the C-ZSM-5 and C₁-ZSM-5 catalysts. The NA-X showed the longest lifetime among these three types of catalysts. In addition, the catalytic lifetime of the NA-X samples increases first and then decreases with the increase of Na₂O/SiO₂ ratio from 0.05 to 0.13. As is clearly indicated in Fig. 6A, the C₁-ZSM-5 exhibited shorter lifetime than that of the NA-X catalysts. In particular, C-ZSM-5 deactivated very quickly and presented the even worse lifetime in methanol conversion among all catalysts.

Fig. 6 (A) Methanol conversion vs. time: T = 470 °C, p = 1 bar, n(methanol) : n(water) = 1 : 1, WHSV = 2 h⁻¹; (B) TG profiles of C₁-ZSM-5, C-ZSM-5 and NA-2 in the MTP reaction test at 470 °C for 10 h.

The distribution of products over the NA-X, C-ZSM-5, and C₁-ZSM-5 zeolites in steady-state conditions are listed in Table 3. The products obtained are classified into light hydrocarbons (C₁–C₄), light olefins (C₂⁼–C₄⁼) and C₅⁺ hydrocarbons.

Table 3 Product selectivity at steady state in MTP conversion over NA-X, C-ZSM-5 and C₁-ZSM-5 catalysts

Catalyst	Selectivity (C-mol%)					C2^=-C4^=	P/E
	C1–4^a	C2H4	C3H6	C4H8	C5^+^b
a C1–C4 saturated hydrocarbons.b C5 and higher hydrocarbons.
NA-1	4.7	9.2	40.3	19.6	26.2	69.1	4.4
NA-2	5.2	8.5	40.9	20.0	25.4	69.4	4.8
NA-3	6.1	9.4	39.2	19.6	25.7	68.2	4.2
NA-4	6.9	11.0	38.4	18.4	25.3	67.8	3.5
C-ZSM-5	10.2	13.2	33.0	14.8	28.8	61.0	2.5
C1-ZSM-5	7.6	14.8	38.3	11.2	28.1	65.3	2.4
References	10.2	15.5	37.6	13.2	21.5	66.3	2.4

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It can be clearly seen that compared with the C-ZSM-5, and C₁-ZSM-5 samples, the NA-X zeolites show higher selectivities to propylene and butylenes, while their selectivities toward ethylene, C₁–C₄ alkanes, C₅⁺ hydrocarbons are relatively lower. In addition, the NA-X catalysts exhibited a considerably higher P/E ratio (3.5–4.4) than the C-ZSM-5 (2.5), and C₁-ZSM-5 (2.4) samples.

The C₁-ZSM-5 zeolite delivers a lower propylene selectivity of 38.3% with a lower (C₂⁼–C₄⁼) olefins selectivity of 65.3%, which are associated with the formation of a lot of C₁–C₄ alkanes (7.6%) and C₅⁺ products (28.1%) compared with the NA-X zeolites. In addition, the C-ZSM-5 zeolites delivers a lowest propylene selectivity of 33.0% with a (C₂⁼–C₄⁼) lowest olefins selectivity of 61.0%, which are associated with the formation of a lot of C₁–C₄ alkanes (10.2%) and C₅⁺ products (28.8%) in all catalysts. The comparison between the yields of propylene as well as light olefins obtained from the present study and that reported in the literature⁴⁵ for MTP reaction are presented in Table 3 (references). It can be seen that, with our efficient NA-X catalyst, the yields of the propylene and light olefins are both reasonably improved.

Clearly, at the initiation stages of the reaction, due to the availability of the majority of acid sites and methanol's small size, all catalysts show almost 100% methanol conversion, indicating the high initial activity of all samples. However, as the reaction progresses with increased time, all the tested samples are reducing their catalytic activity with different deactivation rates. This is generally attributed to the coverage of acid sites and blockage of the pore mouth by carbon deposit, based on the previous literature.^46,47

Moreover, for this acid-catalyzed reaction (MTP), microporous C-ZSM-5 catalyst still tends to undergo rapid coking formation on the acid sites, leading to blocking the diffusion path of reactants and products and thus restrict them transport to/from the active sites in microporous channels.⁴⁷ As a result, only the molecules whose size is smaller than that of ZSM-5 channels can diffuse out as reaction products. The larger products would be more likely to become trapped and thereby form a coke precursor.⁴⁸ So, the solely microporous C-ZSM-5 zeolite is gradually deactivated as the micropores are filled with coke deposits, or their pore openings become blocked. Consequently, C-ZSM-5 deactivated very quickly and presented the even worse lifetime in methanol conversion among all catalysts.

The prominent difference in the catalytic performance of NA-X, and C₁-ZSM-5 samples can be explained by the difference in the structural properties as well as by the strength of the acid sites and accessibility of reactants to these active sites.⁴⁹

With the higher Brunauer–Emmett–Teller (BET) surface area, higher external surface area and a larger mesopore volume compared with the C₁-ZSM-5, the samples NA-X exhibit prominent catalytic performance in MTP reaction involved bulky species. Meanwhile, the high surface area of NA-X accelerates desorption of olefins.⁵⁰ While the low BET surface area, low external surface area and low mesopore volume accelerate deactivation of the C₁-ZSM-5.

As suggested by Patcas,⁵¹ the higher effectiveness of NA-X zeolites is due to the reduced internal diffusion limitations. In the case of NA-X zeolites with intercrystalline mesoporosity, coke is formed mainly on the external surface and/or mesopores, relatively high mesoporosity and shorter diffusion path length facilitate the transfer of the coke precursor towards the outside of the micropores, attenuating coke deposition in the micropore channels even at a high coking level. Consequently, the molecules of the intermediate products can easily escape from the zeolite channels, and the secondary reactions such as aromatization and hydrogen transfer are inhibited, which subsequently leads to high propylene selectivity and a low deactivation rate. Hence, the experimental results agree with the BET data.

The acidity of catalyst is viewed as a significant factor in determining the catalytic performance in MTP reaction. Recent studies⁵² have reported that the conversion of methanol to DME takes place mainly on the weak acid sites while the conversion of DME or methanol to light olefins occurs mainly on the strong acid sites. Therefore, the number and strength of acid sites influence selectivity and catalyst deactivation.

Based on the NH₃-TPD analysis results (Fig. 5A and Table 2), both weak and strong acid site densities increase with the increase of Na₂O/SiO₂ ratio. As expected, the higher amounts of strong acid site and weak acid sites for NA-X zeolites compared to the C₁-ZSM-5, which may also promote propylene production (Table 3) and improve MTP catalytic stability (Fig. 5A). Therefore, the fast conversion drop over C₁-ZSM-5 attributes to the few number of strong acid sites which dehydrate methanol and form the first C–C bond.⁵⁰ A large number of strong acid sites of NA-X may be favourable to a higher propylene selectivity and P/E ratio in MTP reaction. Although, strong acid sites are the dominant active sites for MTP reaction, they also produce the main heavy hydrocarbons and aromatics products, which are precursors of coke formation, which deactivates the catalysts. In contrast, the high weak acidity hinders various side reaction such as hydrogen-transfer and cyclization reactions on strong acid sites which result in low selectivity of saturated hydrocarbons and aromatics (C₅⁺).⁵² Thus, higher amounts of weak acid sites for NA-X catalysts compared to C₁-ZSM-5, which may also promote propylene production (Table 3) and improve MTP catalytic stability (Fig. 6A).

Finally, our main conclusion is that the NA-X zeolites represent the best catalytic performance, including highest propylene selectivity and P/E ratio as well as the longest catalyst lifetime, among all the studied catalysts. From a catalytic point of view, both the accessibility of the active centers and the number, strength of the acid sites for catalysts are essential factors.⁵³ A large number of strong acid sites of NA-X samples may be favourable to a higher propylene selectivity and P/E ratio in MTP reaction. The high strength of weak acid sites and large pore size of NA-X decrease coke formation and the catalyst deactivation. Hence, the better catalytic performance for the MTP reaction over the NA-X catalysts could be attributed to well-developed intracrystalline mesopores, which will shorten the diffusion path and improve the accessibility of reactants to reactive sites.

3.4 Thermogravimetry analysis

Coking is well known to be the main cause of H-ZSM-5 deactivation in the MTP process and therefore it is particularly desirable to study the zeolite coking behavior to obtain insight into the stability improvement by microstructure design.

The NA-2, C-ZSM-5, and C₁-ZSM-5 catalysts were analyzed by thermogravimetry after the MTP reaction test at 470 °C for 10 h. As shown in Fig. 6B, from 300 °C to 800 °C, the mass loss of C-ZSM-5 and C₁-ZSM-5 catalyst are 0.4% and 0.5%, whereas that of NA-2 is 0.6%. Compared with C-ZSM-5 and C₁-ZSM-5 zeolites, NA-2 therefore possesses a higher coke-tolerant capability. For hierarchically structured NA-2 zeolite, the auxiliary mesoporous structures increase the external surface area and consequently the amount of more accessible active sites for bulky MTP product molecules, and also helps shorten the molecular diffusion path and residence time of the large intermediates and products in the micropore channels. As a result, the possibility of coke formation near the micropore mouth, and the detrimental effect of coke formation in the mesoporous structures on catalyst performance are decreased significantly. In addition, the considerably large specific surface area and high mesoporosity of the NA-2 enable it to accommodate increased coke deposition, which also contributes to the notable improvement in catalytic lifetime. Furthermore, the high concentration of surface acid sites, especially the Brønsted acid sites and the high ratio of B/L (Fig. 5A and B and Table 2) of NA-2 would have a significant effect on the propylene distribution in the MTP reaction.^54,55

3.5 Crystallization mechanism

As suggested in previous report, here adopted SAC method for the preparation of NA-X zeolites is a kinetics-controlled nucleation/growth/aggregation process. Significantly different from the explosive crystal growth after a long-term induction period in the conventional hydrothermal synthesis, SAC method's key point lies in the moderate crystallization transformation of amorphous precursors in the discrete and localized water pools.⁵⁶

In the case of NA-X zeolites synthesis, lower Al-doping content means decreased crystallization barrier and, consequently, increased crystallization rate.¹² Thus for the relatively low Al-doping NA-X samples synthesis, the lower TPAOH/SiO₂ slowed down crystallization rate and thus be beneficial to the production of expected nano ZSM-5 aggregates.

In the post-desilication preparation of hierarchical zeolites, Pérez-Ramírez's group⁵⁷ reported that TPA⁺ cations could act as mesopore growth moderators. The development of intracrystalline mesostructure depends on the balance between the affinity of structure-directing agent to zeolite surface and dissolution of the crystal framework.⁵⁸ Based on the characterization results, a “nucleation/growth/aggregation” mechanism is proposed here as illustrated in Fig. 7.

Fig. 7 Schematic of the growth mechanism for the NA-X samples.

During the initial hydrothermal synthesis stage, zeolite nucleation and growth are predominant and result from the highly concentrated precursor species and relevant concentration fluctuation, a large number of nano-crystals appear in small size, whereas the basic etching effect is insignificant because of protection by aluminum complexes and TPA⁺ cations that are incorporated in the micropores and dispersed in the reaction system.

During the following hydrothermal treatment, basic etching won't becomes more dominant because the high supersaturation conditions of aluminum complexes and TPA⁺ cations, while small organic quaternary ammonium salts with short alkyl chains, TPA⁺, present a strong affinity towards nanocrystals and can be occluded within the zeolite framework during crystallization. Their interaction with nanocrystals may inhibit excessive crystal growth, the nanoparticles gathered due to their high activity and grow up gradually, fusiform aggregates composed of nanoparticles appear in small size.

In the later stages of hydrothermal treatment, the fusiform aggregates grow up gradually, and the oriented cluster on the surface develop in different directions in accordance with the “defect intergrowth mechanism”.^59,60 To reach the balance state of minimum energy, the fusiform aggregates have experienced 3D expansion to form microspheres with dandelion-like morphology in the subsequent crystallization.⁶¹

Generally, the final crystal size distribution of NA-X samples strongly depends on the total number of nuclei formed during the crystallization and on the rate of their formation.^62,63 It is worth mentioning that NA-X samples cannot be obtained in the absence of NaOH by solid-like state and steam-assisted conversion. So, it could be deduced that the addition of NaOH provided alkalinity to dissolve the dry gel and Na⁺ to balance the charge of the skeleton.⁵⁸ Moreover, no ZSM-5 phase could be obtained in the final products when dry gel definitely did not contain water.³⁷ Therefore, the little water contained in the dry gel was critical for the formation of NA-X zeolites in the solid-like state system. So, it was beneficial for the nuclei to get supersaturated in the solid-like state system. Finally, the nuclei grew into nano-sized ZSM-5 aggregates.

Among all samples, NA-2 exhibits the superior textural properties and excellent catalytic performance. In order to track the crystallization process, crystallized NA-2 with different time is investigated using XRD, and SEM. No obvious XRD peaks related to the MFI phase could be observed after crystallizing for 12 h (Fig. 8). These suggest the amorphous nature of the obtained solids and no crystalline phase could be detected. With the prolonging of crystallization, the relative crystallinity of samples increase, corresponding to the transmutation of the morphology.

Fig. 8 The XRD patterns of NA-2 at different crystallization time.

The particles morphology at different crystallization period is captured and showed in Fig. 9. At the beginning of crystallization, only irregularly-shaped gels could be observed. After 18 h of crystallization, a large number of nanoparticles appear in small size. After 24 h of crystallization, the nanoparticles gathered and grow into fusiform aggregates with small size. Then the fusiform aggregates have experienced 3D expansion to form microspheres with dandelion-like morphology in the subsequent crystallization (36–48 h).

Fig. 9 The SEM images of NA-2 at different crystallization time.

4. Conclusions

In the absence of additional mesoporous template, high-silica NA-X zeolites with controllable mesoporosity were successfully synthesized by using a simple SAC and solid-like state conversion method. We have demonstrated that by using a simple SAC and solid-like state conversion method, the low-templated dry gel can be converted to high-silica NA-X zeolites with high crystallinity and abundant mesoporosity. High-silica NA-X samples with an average particle size below 70 nm are prepared using a small amount of TPAOH as the single template. Then NA-X zeolites exhibit mesopore volume and external surface area of as large as ∼0.18 cm³ g⁻¹ and ∼193 m² g⁻¹, respectively. The mesopore diameter of the obtained NA-X samples could be regulated from 4 nm to 30 nm by adjusting the alkalinity of the synthesis gel. By the synergistic effects of the TPAOH and SAC method, the TPAOH/SiO₂ ratio can be successfully reduced to be 0.05. The high-silica NA-X zeolites obtained possesses high surface area, external surface area, mesopore volume and thereby high MTP reaction activity. This synthesis process was rapid and economical and shows potential for large-scale production. Furthermore, such a SAC or solid-like state conversion method method would significantly improve the zeolite yield (59.37–76.78%) and avoid huge wastewater pollution, which are attractive in industrial applications of hierarchical zeolites. Furthermore, by using this method, different hierarchical zeolites (such as mordenite, zeolite X and Y) may be synthesised as well.

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Footnote

† Electronic supplementary information (ESI) available: Details of sample characterization and results. See DOI: 10.1039/c6ra21080e

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