Hierarchically porous MFI zeolite synthesized by zeolite seeding and alkaline steaming-mediated crystallization

Advanced Powder Technology xxx (2016) xxx–xxx

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Original Research Paper

Hierarchically porous MFI zeolite synthesized by zeolite seeding and alkaline steaming-mediated crystallization Jia-jia Xiao a, Hua Li a,⇑, Guo-bin Zhu b a Department of Inorganic Materials, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 199 Renai Road, Suzhou 215123, Jiangsu Province, PR China b College of Physics, Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, Jiangsu 215006, PR China

a r t i c l e

i n f o

Article history: Received 17 February 2016 Received in revised form 27 April 2016 Accepted 28 April 2016 Available online xxxx Keywords: Hierarchical structures Zeolites Zeolite seed Alkaline steaming Mesoporous materials

a b s t r a c t A new strategy, zeolite seed-mediated alkaline steaming treatment, has developed for synthesizing hierarchically porous zeolite. This approach involves successive transformation from an initially amorphous mesophase to a crystallized mesophase with zeolite framework under alkaline steaming. This is the first demonstration that single-crystallized hierarchically porous MFI zeolite particles could be simply synthesized under alkali steaming assisted by a small amount of zeolite seeds. The mesoporosity and particle morphology of the obtained hierarchically porous zeolites can be well tuned by changing the composition of alkaline steaming. Steaming in alkaline vapor containing TBAOH and EDA, the synthesized hierarchically porous single crystalline MFI zeolite particles show an interlamellar sheet-packed oriented structure and highly developed mesoporosity with hierarchy factor (HF) of as high as 0.25 in comparison with most previous reports where HF values are lower than 0.20. This hierarchically porous zeolite shows capacious adsorption for heavy metal ions, such as Cu (II). Ó 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

1. Introduction Zeolites are crystalline, microporous materials with strong acidity and high hydrothermal stability that have found numerous applications in catalysis, separations, ion exchange, etc. Their small micropores, however, make diffusion of bulky molecules difficult [1,2]. Fabrication of hierarchical zeolite, such as integrating mesoporosity into zeolite crystals, is a proven solution to circumvent the diffusion limitation [3–5]. Similar to most of other hierarchical porous material with well defined pore dimensions and topologies [6–8], hierarchical porous zeolites minimize diffusive resistance to mass transport by mesopores and offer high surface area for active site dispersion over micropores. Considerable efforts have been devoted to the synthesis of hierarchical zeolites, including acid/ base leaching [9,10], confined synthesis by hard templating [3], meso/micro-soft templates-directed co-assembly (one-pot route) [11–14] and using dual-function templates [15,16]. In meso/micro-template-directed co-assembly, since mesoporous structure can be formed through self-assembly in as short as minutes while crystallization of zeolites usually takes much longer time [17], hierarchically porous structure could be obtained only under the delicate balance between thermodynamic and ⇑ Corresponding author. Fax: +86 512 65880089. E-mail addresses: [email protected] (H. Li), [email protected] (G.-b. Zhu).

kinetic processes. Therefore, the synthesis becomes difficult to control and sometimes, phase separation would take place, which results in the formation of single microporous zeolitic phase, or single amorphous mesoporous silica, or a mixture of them. Until now, synthesis of ordered and hierarchically porous zeolites by self-assembly is one of major challenges in chemical sciences [5]. In a previous work [18], we found alkaline condition is indispensable to crystallize zeolite MFI, which easily resulted in phase separation from the beginning of mixing mesoporogen with microporogen. As a result, it is important to carefully control pH value within a narrow range to obtain mesoporous zeolites, which made it difficult to synthesize the material in large quantity. On the other hand, from the viewpoints of kinetics, zeolite crystals grew too rapid to control once it reached critical high temperature, which often results in the blockage in mesopore by those grown-up zeolite crystallites and unavailability of mesostructure in the final product. In order to keep mesoporous structure, low temperature treatment with nanocrystalline zeolites as precursor became one of main approaches [2,3]. Such hierarchically porous frameworks were composed of the aggregated and poorly crystallized zeolite nanocrystals which resulted in weak acidity unfavorable for catalytic applications [19]. Considering the importance of minimizing the interaction between mesoporogen and microporogen, it would be simple and controllable to obtain hierarchical porosity if it is possible to

http://dx.doi.org/10.1016/j.apt.2016.04.034 0921-8831/Ó 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

Please cite this article in press as: J.-j. Xiao et al., Hierarchically porous MFI zeolite synthesized by zeolite seeding and alkaline steaming-mediated crystallization, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.04.034

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separate alkaline condition (zeolitization) from the formation of mesostucture. Based on this hypothesis, we propose a new route to synthesize hierarchically porous single-crystalline zeolite: Crystalline-seed-mediated synthesis under alkaline steaming treatment (Fig. 1). Instead of alkaline used in the solution of mesoporogen (SDA) with microporogen, alkaline steaming was employed after the formation of amorphous aluminium-silicate mesophase with small amount of zeolite seeds. Under co-operation between alkaline steaming and crystalline seeding, highly crystallized mesoporous zeolite has been successfully obtained. 2. Experimental 2.1. Preparation of zeolite seeds NaAlO2 (0.11 g), tetrabutylammonium hydroxide (TBAOH, 10 wt.% aqueous solution, 21.9 ml) were dissolved into deionized water (16.5 ml) at room temperature. Then, tetraethylorthosilicate (TEOS, 99%, 12.4 ml) was added to the solution under vigorous stirring. After being stirred for 2.5 h, the solution was left to statically stand overnight after the mixture became transparent. Then, the solution was statically hydrothermal-treated in a sealed 100 ml Schott-Duran bottle under 110 °C for 4 h. 2.2. Preparation of crystalline-seed-mediated amorphous mesophase aluminium-silicate Cetyltrimethylammonium bromide (CTAB, 3.64 g), NaOH (0.54 g) and NaAlO2 (0.15 g) were dissolved in deionized water (92.3 ml) to form a transparent solution. Then, TEOS (17.16 ml) was added to the solution with stirring for 30 min. Then, zeolite seeds solution (10 ml) was added to the mixture above drop by drop. The molar ratio of the resulting sol was

0.019:1:0.09:0.01:62.82:0.11, TBAOH:SiO2:Na2O:Al2O3:H2O:CTAB. After stirring for another 2 h under room temperature, the mixture was filtered directly. The resultant gel was dried at 55 °C for another 18 h. 2.3. Preparation of alkaline solutions A series steaming solution was prepared as shown in Table 1. 2.4. Preparation of mesoporous single-crystalline zeolite The dried Crystalline-seed-mediated amorphous mesophase aluminium-silicate (1 g) was transferred into a small Teflon container, which was then put into a larger autoclave, similar to the steam-assisted crystallization approach as previously reported [12]. Various alkaline solutions (1 ml) were dropped into the bottom of the autoclave, and then the autoclave was heat-treated. Afterwards, the product was washed repeatedly with distilled water, dried in air and then calcined at 550 °C for 8 h to remove the organic agents. The Si/Al molar ratio of powder obtained was around 43. The final obtained powder was labeled as C(A)T t⁄, here, ‘C’ refer to crystal zeolite seed and ‘A’ for alkaline solution while ‘T and t’ correspond to the temperature and duration of steaming treatment, respectively, and ⁄ represents the type of alkaline solution in Table 1. For example, sample CA155-48b, both crystal zeolite seed and alkaline solution (solution b) were used and steaming condition is 155 °C for 48 h. In order to comparison, crystal zeolite seeds are used in place of TEOS and NaAlO2 as reactant to prepare sample and then are aqueous steaming treated under 155 °C for 48 h, such sample was named as ‘Z155-48d’; and also, same composition of ZSM 5 (without CTAB) are synthesized under hydrothermal treatment under 155 °C for 3 days. Detailed synthesis conditions are also given in Table S1. 2.5. Heavy metal ion, Cu2+ adsorption studies The adsorption of Cu2+ was investigated in batch equilibrium experiments. Stock solutions of Cu(NO3)2 (150 mg/L) has the pH value of 6.0 at 25 °C. The pH value of solution was adjusted to the desired value with hydrochloric acid solution. 100 mg powder samples were mixed with 100 ml stock solutions of Cu(NO3)2 under continuous stirring for a given period, followed by filtration of the adsorbent. After filtration, the concentration of Cu(II) in the aqueous phase was determined by inductively coupled plasma mass spectrometry (ICP-MS). 2.6. Characterization

Fig. 1. Schematic of formation of hierarchically porous single-crystalline zeolite.

Powder XRD patterns were recorded by using a Rigaku D/Max 2200PC diffractometer with CuKa radiation (40 kV and 40 mA) with a scanning rate of 0.6 min 1 for small-angle testing and 10 min 1 for large-angle testing. The N2 sorption isotherms were measured using Micromeritics 3H-2000PM2 porosimeters at 77 K. The mesoporous specific surface area, pore-size distribution, and pore volume were calculated using the Brunauner–Emmett– Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively. The micropore specific surface area and volume were calculated by the t-plot method and DFT method, respectively. FE-SEM (field-emission-scanning) electron microscopy analysis was performed on a Hitachi SU8010 electron microscope. TEM (transmission electron microscopy) images were obtained on a JEOL-2010F electron microscope operated at 200 kV. Sample for TEM used embedded thin section technology. Temperatureprogrammed desorption of ammonia (NH3-TPD) was performed by using PCA-1200 (chemical adsorption recorder) loaded with 100 mg of sample. The sample was pretreated at 323–923 K with

Please cite this article in press as: J.-j. Xiao et al., Hierarchically porous MFI zeolite synthesized by zeolite seeding and alkaline steaming-mediated crystallization, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.04.034

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J.-j. Xiao et al. / Advanced Powder Technology xxx (2016) xxx–xxx Table 1 Steaming solution compositions. EDAa

Solution Solution Solution Solution a b

a b c d

Et3Nb

TBAOH

Volume (ml)

Mass concentration (wt.%)

Volume (ml)

Mass concentration (wt.%)

Volume (ml)

Mass concentration (wt.%)

Deionized water (ml)

12 12 9 N/A

41.8 43.0 62.4 N/A

N/A N/A 4.2 N/A

N/A N/A 22.6 N/A

N/A 8 N/A N/A

N/A 1.12  10 N/A N/A

15 7 1.8 15

2

Ethylenediamine, EDA. Triethylamine, Et3N.

Fig. 2. Wide-angle XRD patterns for samples in Table 1.

a heating rate of 10 K min 1 and the amount of desorbed ammonia was detected using gas chromatography at ambient temperature. Chemical analysis of Cu(II) in Cu2+ adsorption studies was carried out using a inductively coupled plasma mass spectrometry (ICPMS, iCAPTM Qc). 3. Results and discussion 3.1. Syntheses conditions and structures of hierarchically porous zeolites MFI Syntheses of hierarchical porous MFI zeolites were carried out under various steaming conditions. Fig. 2 shows XRD patterns of all prepared samples. And the corresponding phase information from X-ray diffraction (XRD) is given in Table 2. For comparison, sample synthesized with zeolite

seeding as precursor steaming in pure aqueous vapor-solution d (Z155-48d) or samples synthesized with amorphous aluminosilicate steaming in solution b or pure aqueous vapor-solution d (A155-48b and A155-48d) are also analyzed. Samples using crystalline zeolite seeds show high crystallinities of MFI structure under steaming condition while samples without zeolite seeding show mostly amorphous structure with very weakly crystallized SiO2 regardless of the steaming solutions: Coesite SiO2 (pdf#14-0654) for sample A155-48b and SiO2 (pdf#501431) for sample A155-48d (Fig. 2, Table 2). This indicates that crystalline zeolite seeding is necessary to induce the crystallization of the framework into zeolites under the present experimental conditions. The seeds provide nucleation site and therefore further promote zeolitization from amorphous precursors. On contrast, without zeolite seeds, those precursors mostly remain amorphous with small amount of preferentially crystallized pure SiO2 phase. All other samples containing zeolite seeds show well zeolitization with characteristic peaks at 8.8–9.2° and 23.0–25.0° indicating their MFI structure. Apart from it, a weak broad peaks around 23.0–25.0 was observed in sample C155-48d, indicating the existence of amorphous phase or its weak crystallization. Therefore, it could conclude that alkaline steaming would promote the zeolititation. The coexistence of mesopores and micropores is manifested in all samples of C(A)155 series except CA155-48c: there are combined features of type I and type IV with two adsorption steps in P/P0 < 0.01 and 0.4 < P/P0 < 0.8 regions according to the N2 sorption isotherms, corresponding to the filling in the microporous volumes and capillary condensation in mesopores, respectively (Fig. 3a). The corresponding pore structural parameters of all samples are given in Table 3. The mesoporous size distributions (Fig. 3b) for all these samples except CA155-48c clearly indicate the presence of mesoporous structure of about 3.5 nm in pore size. Typically, CA155-48b shows well-defined mesoporosity of 3.8 nm in diameter, and its BET surface area and total pore volume have been calculated to be

Table 2 Morphological information from XRD and SEM. Sample name a

CA155-48a CA155-48ba CA155-48ca C155-48da CA170-24ba A155-48bb A155-48db Z155-48dc a b c

XRD results and morphologies Cubic MFI, numerous small crystallites aggregated into particles Multilamellar MFI, watermelon-like single crystal Smooth surface, long-rod shape MFI Irregular morphology, severe phase separation with MFI and amorphous products Multilamellar MFI, spindle-like single crystal Coesite SiO2 (pdf#14-0654), only SiO2 (pdf#50-1431), only Highly crystallized with spindle-like

Using zeolite seeds mixed with amorphous aluminum-silicate as precursor. Only using amorphous aluminum-silicate as precursor. Only using zeolite seeds as precursor.

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Fig. 3. Nitrogen adsorption and desorption isotherms of C(A)155 series (a) and the corresponding pore size (b) distributions calculated by the BJH method using the desorption branch of the isotherms. The isotherms in the left are shifted by 0, 100, 200, and 300 cm3 g 1, respectively, along the y axis to avoid overlapping. The isotherms in the right are shifted by 0, 100, 170, 220 cm3 g 1 nm 1, respectively, along the y axis.

404 m2 g 1 and 0.36 cm3 g 1, while its mesopore surface area and micropore volume are 215 m2 g 1 and 0.17 cm3 g 1, respectively. Sample CA155-48a, steamed in vapor of solution a (EDA of 41.8 wt.%, Table 1) is demonstrated similar porous structure to CA155-48b (steamed in vapor of solution b – EDA of 43.0 wt.%): the ratio of microporous volume and the ratio of mesoporous surface area for CA155-48a are 53% and 49%, respectively while the

ratio of microporous volume and mesoporous surface area for CA155-48b accounts for 47% and 53%, respectively. In comparison, sample CA155-48c shows a low peak and a broad pore size distribution at 2–4 nm as well as similar ratio of microporous volume (53%) and much lower ratio of mesoporous surface area (37%). Based on literatures [20], the crystallization process belongs to the solid-phase transformation process, during which, inducing by zeolite seeds, the amorphous phase would nucleate around those seeds and then grow up. Nucleation is a primary step to zeolititation while the process of growth is the key to realize the co-existence of mesopores and micropores. Such process is realized through the assumption and dissolution of amorphous phase: one extreme end is that the growth process is too fast as to destroy mesoporous structure and the other end is the unavailability of crystallization or only part crystallization of amorphous phase when it progresses too slowly. Therefore it is important to cooperate the process of growth and nucleation. Aiming at it, the concentration of alkaline steaming solution plays important role. High concentration of alkaline vapor (in solution c, EDA (62.4 wt.%) and Et3N (22.6 wt.%) greatly speed up growth, as a result of which highly crystallized zeolite with high ratio of microporous volume but low ratio of mesoporous surface area is obtained. On contrast, steamed in proper alkaline condition, such as solution b (EDA, 43.0 wt.%; TBAOH, 1.12e( 2) wt.%) and solution a (EDA, 41.8 wt. %), the growth would cooperate with nucleation and accordingly highly developed hierarchically porous materials can be obtained. In order to explore the coexistence of mesopores and micropores, i.e., to understand whether there were serious phase separations or not in the samples, hierarchy factor (HF) [9] was used to characterize these samples: both conventional zeolites and pure amorphous mesoporous materials have low HF (conventional zeolites, HF 6 0.1; amorphous mesoporous materials, HF 6 0.05), while nano-zeolite has much higher HF (>0.15). Therefore, HF for phase-separated sample would be in between the values of pure zeolites and amorphous mesoporous materials. The HFs of most hierarchy porous zeolites in literatures [2,9,21], were lower than 0.2. According to this rule, samples CA155-48a, CA155-48b and CA155-48c possess well-developed mesoporous structure within the zeolite crystals, which have high HFs (>0.2). On contrast, C155-48d, CA170-24b and Z155-48d have relative low HFs (<0.15) which imply the phase separation, or single mesoporous or microporous structure of the samples. As far as we know, it is the first report that HF reached as high as 0.26. The hydrothermal stability of CA155-48b was tested by subjecting the calcined sample (0.2 g) in water (50 mL) to 100 °C for 50 h under vigorously stirring. This sample is labeled as CA155-48b-HT. The sample showed a BET surface area loss around 10% (Table 3),

Table 3 Textural properties of samples under various steaming conditions. Solution

Sa

Vb

Smicro m2 g CA155-48a CA155-48b CA155-48c C155-48d CA170-24b Z155-48d ZSM 5 CA155-48b-HT a b c

a b c d b d N/A N/A

231 189 237 272 275 193 329 176

Sexternal 1

Vmicro cm3 g

218 215 138 108 153 259 108 186

Sext/SBET

Vmicro/Vtotal

HFc

0.49 0.53 0.37 0.28 0.36 0.43 0.22 0.51

0.53 0.47 0.53 0.46 0.43 0.31 0.45 0.48

0.26 0.25 0.20 0.13 0.15 0.14 0.10 0.25

Vtotal 1

0.19 0.17 0.16 0.16 0.18 0.19 0.18 0.18

0.36 0.36 0.30 0.35 0.42 0.62 0.40 0.37

Smicro (micropore surface areas) and Sexternal (external surface areas) by t-pot method. Vmicro (micropore volumes) and Vtotal (total pore volumes) was calculated by DFT method. HF = ((Sext/SBET) ⁄ (Vmicro/Vtotal), calculated according to literature [9].

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Fig. 4. SEM images of samples obtained under various steaming conditions. (a) CA155-48a; (b) CA155-48b; (c) CA155-48c; (d) C155-48d; and (e) CA170-24b; scale bar: 500 nm.

which is remarkably lower than the reported one of higher than 82% for conventional MCM-41 after hydrothermal treatment for 24 h [22], proves the as prepared hierarchical zeolites to be enough stable for applications in catalytic reactions. Furthermore, SEM images and XRD patterns (Fig. S1, supporting information) also demonstrate that this material possesses high hydrothermal and steam stability. Field-emission-scanning electron microscope (FE-SEM) images (Fig. 4) give much more information of these samples. Samples steamed in various alkaline vapors show regular morphology: sample steamed in vapor of solution a (EDA aqueous solution) shows cubic-shaped particle of around 600–1000 nm in size composed of numerous small crystallites (Fig. 4a, sample CA155-48a), while sample CA155-48b (Fig. 4b) steamed by solution b (EDA + TBAOH, mixed aqueous solution) shows watermelon shape with clearly oriented multilamellar structure of around 900–2000 nm in particle size. These layers with thickness of around 20–50 nm assembled into an ordered arrangement, and there are around 10 nm inter-layer spaces in between. Combining dynamic light scattering (DLS) analysis of the seed crystals (Fig. S2), whose sizes peak at around mainly 20 nm and 10 nm, one can see the thickness of nanosheets in CA155-48b coincides with the size of several seeds. Under present exploration, EDA vapor facilitates the controllable crystallization, while TBAOH (boiling temperature, 100 °C), evaporating at 100 °C, cooperatively induce those crystallized zeolite particle to assemble and further to grow up into nanosheetspacked watermelon shape particles. The controlled crystallization

made it possible to cooperative formation of mesoporous structure and microporous structure. However, the seeds sizes are still much bigger than the micelle size formed by CTAB (2–4 nm). Therefore, it would be difficult to create the well-ordered mesoporous structure. Similar phenomenon was observed on sample CA170-24b (Fig. 4e): steamed in the same alkaline solution but different heat treatments, sample CA170-24b also shows multilamellar structure with much thicker layer (50 nm) than sample CA155-48b. This observation indicates TBAOH specifically boosted the formation of lamellar zeolite. These layers further aggregated and grew up and packed into thicker lamellar structure at 170 °C, resulting in the partial disappearing of mesoporosity. And this result also indicates that the growth of zeolite at 170 °C is extremely fast even though the steaming duration is much shorter than CA155-48b. Too strong alkaline steaming condition greatly promoted the growth of zeolite by destroying mesoporous structure, as shown in sample CA155-48c (Fig. 4c), where the resultant particles are completely crystallized, and have smooth surface, long-rod-like shape. Steamed under pure water, the sample C155-48d shows irregular shape. Combined with low magnification image (Fig. S3d, supporting information), high amount of amorphous phase can be found to mix with spherical particles, which indicates its insufficient crystallization (Fig. 4d) and the co-existence of crystalline zeolite and amorphous aluminium-silicate. Although there is also TBAOH trapped within precursor of C155-48d, the strong interaction between TBAOH and aluminosilicate make it difficult

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Fig. 5. TEM image of a CA155-48b crystal (a); (b) HR-TEM image taken from a square area in (a); and (c) the corresponding SAED of the particle.

to evaporate and further to facilitates crystallization and orientation of the amorphous precursor. As a result, only low HF (0.13, Table 3) was obtained. Although Z155-48d, using crystal zeolite seeds as precursors shows highly crystallization according to XRD pattern, SEM (Fig. S4, supporting information) demonstrates lot of spongy-like amorphous matters mixes with spindle-like particles, indicating imbalance between nucleation and growth. This observation is also consistent with low HF (0.14, Table 3). In addition, homogeneous spindle-like particles with size around 500 nm for ZSM 5 combined with its strong characteristic peaks in XRD pattern indicates its highly crystallization (Fig. S5, supporting information). These particles are smaller than all of the other samples whose sizes are mostly bigger than 700 nm. The small particles aggregate to form inner macropores with size around several hundred nanometers, resulting high total porous volume (0.4 cm3 g 1). Although those macropores exist among particles, relative low ratio of external surface area still lowers down the HF of 0.1, indicating its unavailability of hierarchically porous structure. Fig. 5 displays the transmission electron microscopic (TEM) images of sample CA155-48b. Fig. 5a shows the representative sheet morphology with a dimension around 1.0  0.6 lm. In the left-up corner of the image, there are several parallel sheet array (white arrows as shown in the Fig. 5a), which indicates that there is a lamellar structure with lamellar thickness of around 20–50 nm, as also be observed in SEM image (Fig. 4b). A wormhole-like mesoporous structure with pore size of around 2–5 nm can be found in the image. In the HR-TEM image (Fig. 5b) taken from the square at the rim of the particle in Fig. 5a, the highly ordered structure of CA155-48b demonstrates the crystallization of the mesoporous framework of the sample. The single-crystalline selected area electron diffraction pattern (SAED, Fig. 5c) indicates that the small crystallites in a

particle of CA155-48b are highly lattice-coherent with each other, i.e., these crystallites are preferentially oriented in the same direction. In another word, the frame work of the whole particle is a single-crystalline with a penetrating mesopore network wherein. Because of the throughout single-crystallization in particles with defined mesoporous structure is retained in the sample CA155-48b, it can be concluded that the framework of the mesoporous structure has successfully crystallized into single-crystal in the whole particle. CA170-24b was also observed by TEM for further clarifying the development of porous texture. Instead of developed mesoporous texture, CA170-24b is found isolated macropores within particles

Fig. 6. NH3-TPD profiles of samples ZSM5 and CA155-48c.

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(Fig. S6a, supporting information). Similar to CA155-48b, CA17024b is highly crystallized into single crystalline structure but absence of mesoporous structures, which results in its defined morphology but low HF of 0.15. This observation indicates that zeolitization under alkaline steaming is extremely fast and uncontrollable at 170 °C. Its zeolitization was firstly initiated from outer surface by alkaline steaming. Further zeolitization by mass transport from inner core to outside resulted in the formation of those macropores within CA170-24b. 3.2. NH3-TPD The amount of total acid sites in CA155-48b and its acid strength have been investigated by a temperature-programmed desorption of ammonia (NH3-TPD). The results indicate that CA155-48b contains both strong (432 °C) and weak acid sites (around 207 °C, Fig. 6). CA155-48b has a higher amount of weak acidic sites (around 200 °C) and all acidic sites than ZSM 5. Apart from it, both CA155-48 and ZSM 5 shows a similar amount of medium to strong acidic sites from 300 to 550 °C, in accordance with the microporous structural data: a close microporous volume percentage as well as microporous volume are observed in CA155-48 (47% from microporous volume/total volume, 0.17 cm3 g 1) and ZSM 5 (45%, 0.18 cm3 g 1). Such results indicate CA155-48 is highly crystallized as ZSM 5. 3.3. Cu2+ adsorption The presence of three dimensional mesoporous structure enables the free accessibility of guest molecules to the zeolitic micropore system, which favors the processes such as adsorption and catalysts involving bulky molecues. The purification of water with absorbents and ion-exchangers for heavy metal ion removal is an important topic for many scientific disciplines [23]. Among various adsorption applications, the mesoporous zeolite for heavy metal adsorption is one of the most promising methods for environmental cleanup since it can be directly used as adsorbents without any further surface modification [24]. The investigation of heavy metal ions adsorption conducted on CA155-48b and ZSM 5 will be discussed with an aim to gain insight into the effect of textual properties. The results of the Cu(II) removal performances on different zeolites are shown in Fig. 7. The reaction was run for 5 h with an initial Cu(II) concentration of 150 mg/L. As expected, the metal ions exchanging activity of CA155-48b was much higher than that of

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ZSM 5. In the first 3 h of ion exchanging process, CA155-48b shows significantly different characteristic from ZSM 5: Cu(II) exchanging of ZSM 5 is nearly 0 while that of CA155-48b increased linearly. The highly developed mesoporous structure in CA155-48b (53% for the ratio of external surface area/total surface area and 215 m2 g 1) endows a faster diffusive speed in contrast with ZSM 5 (22%, and 108 m2 g 1). On contrast, the network with small sized pore (micropores) makes ZSM 5 highly inefficient for processes involving diffusion of liquid, i.e., the species diffusion in the micropore network is the controlling step. Once the liquid solution thoroughly penetrated throughout all of the pores, ion exchanging process would accelerate. As a result, after 4 h, one can see ion exchanging speed of ZSM 5 became fast with sharp slope in the curve, indicating the liquid have penetrated throughout all pores and ion exchanging process became controlling step. In comparison, CA155-48b, still kept a nearly constant ion exchanging speed, which indicating the highly developed mesopores efficiently benefits diffusion of liquid process and ion exchanging was the controlling step from the beginning. Therefore, it could be anticipated that such developed mesoporous zeolites as sample CA155-48b prove their application prospects in those diffusion-control reactions. 4. Conclusions We have developed a new strategy for synthesizing mesoporous zeolites: zeolite seed-assisted alkaline steaming. Crystalline zeolite seeding alone results in limited zeolitization with low HF values (<0.15) while simply amorphous and/or silica phase can be obtained under only alkaline steaming. Under cooperative effect of alkaline steaming and small amount of crystal zeolite seeding, hierarchical zeolites with high HF values (>0.20) can be obtained. Steamed in vapor containing EDA, cubic-shaped mesoporous zeolite particles can be obtained while single-crystallized hierarchically porous zeolite with oriented multilamellar structure is synthesized under the steaming containing both EDA and TBAOH. Because zeolitization occurs firstly from surfaces of particles under alkaline steaming, sample became hollow structure with isolated macropores within particles as steaming temperature raised at 170 °C. Inducing by alkaline steaming with TBAOH, nanosheet zeolites primarily formed and assembled into array with layer space around 10 nm, and at the same time large amount of mesopores around 3.5 nm were dispersed among each layer. The obtained hierarchically porous zeolite demonstrates highly efficient adsorption of Cu(II) due to its well-developed mesostructure, which indicates its promising prospects for applications in separation or catalysis. Acknowledgement The financial support of National Natural Science Foundation of China (Grant No. 21301123), is gratefully acknowledged. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apt.2016.04.034. References

Fig. 7. The removal of Cu (II) as a function of time on CA155-48b and ZSM5. Reaction conditions: 150 mg/L Cu (II); solid/liquid = 100 mg/100 ML; pH = 6.0.

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Please cite this article in press as: J.-j. Xiao et al., Hierarchically porous MFI zeolite synthesized by zeolite seeding and alkaline steaming-mediated crystallization, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.04.034