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Ordered Highly-Zeolitized Mesoporous Aluminosilicates Produced by a Gradient Acidic Assembly Growth Strategy in a Mixed Template System Arepati Azhati, Songhai Xie, Weiwen Wang, Ahmed A. Elzatahry, Yueer Yan, Jian Zhou, Daifallah Al-Dhayan, Yahong Zhang, Yi Tang, and Dongyuan Zhao Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02219 • Publication Date (Web): 16 Jun 2016 Downloaded from http://pubs.acs.org on June 16, 2016

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Chemistry of Materials

Ordered Highly-Zeolitized Mesoporous Aluminosilicates Produced by a Gradient Acidic Assembly Growth Strategy in a Mixed Template System Arepati Azhati,‡,† Songhai Xie,‡,† Weiwen Wang,‡,† Ahmed A. Elzatahry,§ Yueer Yan,† Jian Zhou, Daifallah ||

Al-Dhayan,¶ Yahong Zhang,*,† Yi Tang,† Dongyuan Zhao† †

Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Laboratory of Advanced Materials, Collaborative Innovation Centre of Chemistry for Energy Materials (iChEM), Fudan University, 220 Handan Road, Shanghai 200433, P. R. China §

Materials Science and Technology Program, College of Arts and Sciences, Qatar University, P. O. Box 2713, Doha, Qatar

||

Shanghai Research Institute of Petrochemical Technology, SINOPEC, Shanghai 201208, P. R. China

¶

Department of Chemistry-College of Science, King Saud University, Riyadh 11451, Saudi Arabia

ABSTRACT: Tremendous efforts have been made in recent years to synthesize ordered mesoporous zeolite materials due to the accelerating demands of industrial bulky molecule conversion. Here, we develop a novel gradient acidic assembly growth strategy to prepare ordered highly-zeolitized mesoporous aluminosilicate (SBA-16) materials in a mixed template system. This gradient acidic assembly growth strategy can achieve the high zeolitization of mesoporous aluminosilicate walls without any ordering loss of the mesostructure. The resultant highly-zeolitized mesoporous materials, composed of the intergrown zeolite sub-crystal particles (2-3 nm), exhibit high surface area 2 -1 3 -1 (∼834 m g ) and pore volume (∼0.64 cm g ), typical channel of MFI framework (0.52 nm) and uniform mesopore (∼5.75 nm), respectively. Moreover, these highly-ordered crystallized mesostructures endow them with high exposed active sites and excellent hydrothermal stability, which consequently make their catalytic activities in bulky molecule transformations at least 10 times higher than conventional zeolites or amorphous mesoporous materials. Without the use of any special surfactants, this general synthetic process provides a brand new view for the synthesis and application of highly-crystalline ordered mesoporous materials.

1.

INTRODUCTION

The discovery of ordered mesoporous materials has opened great 1-5 opportunities for heterogeneous catalysis, thanks to their larger pore size (2-50 nm) than those of microporous zeolite materials. However, due to their poorer hydrothermal stability and lower acid strength compared to zeolites, the mesoporous materials have not achieved wide applications as zeolites in chemical and petro6 leum industries so far. The need to overcome these limitations has stimulated researchers to develop innovative strategies for the synthesis of large pore zeolitized materials. The ideal one is to prepare an ordered mesoporous material with crystalline mi7 croporous walls. The initial solution proposed by researchers is to use a mixed template of surfactants and small organic ions, or introduce the alternatively pre-made zeolite precursors and surfac8-13 tants during the synthesis of mesoporous materials. Sometimes, 14 the secondary hydrothermal conversion has been followed. Unfortunately, these attempts have generally resulted in incomplete crystallization or phase separated composites with microporous 7, 8, 12, 15-17 and mesoporous domains. It has even been speculated that the formation of the zeolitic framework and assembly of the ordered mesostructure are incompatible with each other in such 18 synthesis system. Recently, Ryoo′s group has reported an alter-

native route to synthesize hierarchically ordered micromesoporous materials and mesostructured zeolites, that is, a new class of dual-function surfactants featuring multi-ammonium groups was used as sole template to direct both micropore and 19-21 mesopore structures during the crystallization of zeolite. Their work demonstrates the feasibility of directing ordered mesoporous zeolite via a soft template strategy. Nonetheless, as indicated by 22 Xiao et al., its long-range ordering of mesoporous and zeolitic structures simultaneously is still very limited. There is an inherent compromise between the zeolitic framework and the mesoporous structure in the degree of their structure ordering. These researches trigger us to review the initial mixed template system, and rethink on the reason that it fails to obtain ordered high-zeolitized mesoporous materials. Obviously, a basic treatment condition causes the dissolution of mesopore wall before zeolite crystallization starts, and the well-crystallized zeolite wall cannot also fit with the large curvatures of mesostructured. To avoid this, a contrarian strategy, i.e. an acidic medium, has to be adopted to inhibit the dissolution of silica species. We can use the pre-crystallized zeolite sub-crystal (SC) particles to construct the mesostructure for the purpose of easy and uniform zeolitization of mesopore walls in the acidic condition. Moreover, we can also

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achieve the high zeolitization of ordered mesopore walls by intergrowth of SCs rather than crystallization towards single crystals. This intergrowth will not only assure the high crystallinity (but not long-range ordering) of mesopore walls but also bypass the strain field caused by the ordering requirement of both microporous and mesoporous scales. Herein, we demonstrate that ordered highly-zeolitized mesoporous materials can be obtained via a new gradient acidic assembly growth strategy in a typical mixed template synthesis system. The synthetic system of mesoporous aluminosilicate SBA-16 with body-center cubic structure is used to deduce this process (Figure 1). It has been found that the pH gradient control of the synthetic system is the crux to avoid the phase separation and assemble highly-ordered mesoporous materials with well intergrown zeolite walls. Moreover, the ordered highly-zeolitized mesoporous materials possess high hydrothermal stability and accessible strong acid sites, owing to their highly ordered mesostructures and crystalline walls. This feature endues them with high activity and stability in bulky molecule transformations, e.g. the cracking of 1,3,5-triisopropyl-benzene (TIPB) and Friedel-Crafts alkylation of anisole with benzyl alcohol.

Figure 1. Schematic gradient acidic assembly growth process of ordered highly-zeolitized mesoporous aluminosilicate SC-SBA-16. Firstly, pre-crystallized SC particles of ZSM-5 zeolite with a size of 2-3 nm are prepared hydrothermally. Secondly, the SC colloidal solution without any pre-treatment is added in acidic solution containing surfactants. Then, pH of the system is adjusted to 0.5 by HCl solution to assemble the ordered mesostructure and then is hydrothermally o treated at 100 C for 3 h to grow interpenetrating zeolitized networks. Finally, pH value of the above assembly system is further changed to o 1.9 by triethylamine, and then is treated at 120 C for 1 h. All the processes are done in turn without any additional pretreatment except the change of pH values. All the samples before characterization are o calcined at 550 C.

2.

EXPERIMENTAL SECTION

Materials. Tetraethylorthosilicate (TEOS, 99%, Shanghai Lingfeng Chemical Reagent Co. Ltd.), Tetrapropylammonium hydroxide (TPAOH, 25 wt.%, Yixing Dahua Chemical Co. Ltd.), Aluminum isopropoxide (C9H21AlO3, Al2O3 ≥ 24.7%, Sinopharm Chemical Reagent Co. Ltd.), F127 (EO106PO70EO106, Maver. = 12600, Sigma), Hexadecyl trimethyl ammonium bromide (CTAB, ≥ 99%,

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Sinopharm Chemical Reagent Co. Ltd.), Hydrochloric acid (36 ~ 38 wt.%, analytical grade, Zhitang Co. Ltd.), Triethylamine (C6H15N ≥ 99%, Sinopharm Chemical Reagent Co. Ltd.), nbutylamine (C4H11N ≥ 99%, Sinopharm Chemical Reagent Co. Ltd.), tert-butylamine (C4H11N ≥ 99%, Sinopharm Chemical Reagent Co. Ltd.), acetonitrile (C2H3N ≥ 99.5%, Sinopharm Chemical Reagent Co. Ltd.) Synthesis. The ZSM-5 zeolite sub-crystal (SC) particles were synthesized under microwave irradiation with a gel composition of SiO2:0.013Al2O3:0.31TPAOH:17.72H2O. The synthesis solution o was stirred at room temperature for 24 h and pretreated at 80 C for 90 min under microwave irradiation (CEM Discover SP). o Subsequently, the final solution was treated at 120 C for desired time under microwave irradiation. The SC particles were used as the silica/aluminum sources to assembly the zeolitized mesopo23 rous SC-SBA-16 in the presence of Pluronic F127 and CTAB. Typically, 112.0 mg of F127 and 12.0 mg of CTAB were added into a certain amount of HCl solution under magnetic string, respectively. Subsequently, 970.0 mg of the SC solution without any pretreatment was added to above solution and stirred at room temperature for 10 min. Keeping the total mass at 15.0 g, pH values of the system were adjusted to the desired by HCl, respeco tively. Then the mixture was hydrothermally treated at 100 C for 3 h under microwave irradiation. The final solid products were o o washed, dried overnight at 60 C and calcined at 550 C for 6 h. The pH values of the SC-SBA-16-0.5 solution obtained via above process at pH = 0.5 were further adjusted to the desired pH values by adding the different amounts of triethylamine, respectively. o The final solution was hydrothermally treated at 120 C for 1 h. o The resultant solid product was washed, dried overnight at 60 C o and calcined at 550 C for 6 h. In addition, the ZSM-5 zeolite SC particles was further hydroo thermally treated at 180 C for 60 min to obtain the ZSM-5 zeolite as the referenced sample in the catalytic reaction. The other referenced Al-SBA-16 sample was prepared by using TEOS and aluminum isopropoxide as silica/aluminum sources via the above o o same procedures, i.e. pH = 0.4, at 100 C for 3 h and at 120 C for 1 h. The resultant solid product was washed, dried overnight at 60 o o C and calcined at 550 C for 6 h. Characterization. Characterization. Dynamic light scattering (DLS) measurements were used to monitor the particle size in the zeolite subcrystal solution. Malvern Zetasizer Nano-ZS90 instrument with a o detector collecting scattered light at 90 was used to estimate the variation of average particle size with respect to time. All the light o scattering measurements were carried at 25 C, using diluted mother liquors. Dilutions were continued until the reproducible size results were obtained. Each measurement was repeated at least six times. Fourier transform infrared (FT-IR) spectra of SC particles were performed on PerkinElmer Spectrum two. Smallangle X-ray scattering measurements were taken on a Nanostar U small-angle X-ray scattering system (Bruker, Germany) using Cu Kα radiation (40 kV, 35 mA). The wide-angle X-ray diffraction were taken on Bruker AXS D8 diffractometer with Cu Kα radia29 tion at 40 kV and 40 mA. The solid-state Si magic angle spin27 ning nuclear magnetic resonance (MAS NMR) spectrum and Al


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MAS NMR were recorded on Bruker AVAVCE DMX 500 instrument. The morphology information was obtained by Field Emission Scanning Electron Microscope (Hitachi S-4800) and Field Emission Transmission Electron Microscope (Tecnai G2 F20 S-Twin). The ultrathin specimen for high-resolution transmission electron microscopy (HRTEM) observations were prepared on a UC-7 (Leica, Germany) ultra-microtome. The ultrathin crosssections with a thickness of 30-50 nm were obtained by using a -1 DIATOME 35° diamond knife at cutting speed of 0.8 mm s . The sections were collected by holy grids. N2 and Ar sorption isotherms were measured by Quantachrome Autosorb instrument at 77 and 87 K, respectively. The pore size distribution was calculated by adsorption branch. The micropore size distribution was obtained by nonlocal density function theory (NLDFT) model. The mesopore pore size was obtained from Barrett-JoynerHalenda (BJH) method. The total surface area was obtained by application of Brunauer-Emmett-Teller (BET) equation. The mesopore area and the micropore volume were calculated by t-plot method. The amount and strength of accessible strong acid sites on the external surface or total surface were determined by a nonaqueous titration on a potentiometric titration meter (ZDJ-5, Shanghai Leici Instrument Factory) using tert-butylamine or nbutylamine. The sample (~100 mg) was suspended in 25 ml of acetonitrile and agitated for 3 h. Then, the suspension was titrated -1 with 0.01 mol L n-butylamine or tert-butylamine in acetonitrile -1 at a rate of 0.1 mL min . The electrode potential variation was measured with a continuous titration model using a double junction electrode. The addition continued until no further change of mV was recorded. Pyridine-adsorbed FT-IR spectra of different samples were measured with a Nicolet FTIR 360 spectrometer. The selfsupported wafer of samples were degassed in a vacuum cell for 2 o h at 400 C. Then, the background spectra were recorded at 200, o 300 and 400 C, respectively. Next, consecutive doses of pyridine were added until saturation. Finally, the samples were heated at o 200, 300 and 400 C under vacuum environment for 2 h, respectively, and the spectra were then recorded. The relative concentration of the Brönsted and Lewis acid sites were calculated from the + -1 intensity of the PyH and PyL bands (1544 and around 1454 cm , respectively). The hydrothermal stability measurement was carried out with o 100% steam at 600 C under a weight hourly space velocity -1 (WHSV) of 6.0 h . The sample was placed in a stainless steel, o and heated at 600 C for 0.5 h in flowing nitrogen. Then the water -1 with WHSV of 6 h was pumped into the preheated zone of the o reactor (200 C). The samples were hydrothermally treated at 600 o C for 2 h. No carrier gas was used during the hydrothermal treatment. Catalytic Test. Friedel-Crafts alkylation of anisole with benzyl alcohol: Alkylation reaction was performed in a glass reactor under microwave irradiation (NOVA-2S). Typically, 2.7 g of anisole (25 mmol), 0.182 g of benzyl alcohol (1.68 mmol) and 100 mg of catalyst were placed into the Pyrex glass reactor and

o

irradiated under stirring for 4 h at 160 C. After being cooled to room temperature, the solid catalysts were filtered and the remaining liquid was analyzed on a gas chromatograph (GC-122) equipped with a flame ionization detector and a capillary column (SE-30: a 30 m-long, 0.25 mm-i.d., and 0.3 μm-thick). The conversion was calculated based on benzyl alcohol. The selectivities of products were calculated as the ratios of yield of product to conversion of benzyl alcohol.

Cracking of 1,3,5-triisopropyl-benzene (TIPB) and isopropylbenzene (IPB): The catalytic activities of the samples toward TIPB/IPB cracking were tested in a pulse microreactor. The samo ple (30 mg) was preheated at 500 C for 1 h before reaction. N2 -1 with a flow rate of 40 mL min was used as the carrier gas. 0.4 µL of TIPB/IPB was injected for each test. The reaction was performed at different temperatures to detect the activities. The deactivation tests were conducted in the same way as above whereas o the temperature was fixed at 500 C. The products were analyzed by an on-line gas chromatograph (Agilent 5820) equipped with a 24 flame ionization detector. 3.

RESULTS AND DISCUSSION

Ordered Highly Highlyighly-zeolitized Mesoporous SCSC-SBASBA-16. 16. The ordered highly-zeolitized mesoporous aluminosilicate SC-SBA-16 was synthesized via a gradient acidic assembly growth strategy in a typical mesoporous synthetic system using pre-crystallized zeolite SC particles as Si/Al sources (Figure 1). Scanning electron microscopy (SEM) image of the zeolitized mesoporous material SC-SBA-16, obtained by the gradient acidic assembly growth process, displays uniform adherent spherical morphology with a size of 500-800 nm, and no other separated particles or zeolite crystals are detected throughout the entire sample (Figure 2a). Transmission electron microscopy (TEM) images of SC-SBA-16 taken along [100] (Figure 2b), [110] and [111] (Figure S1, a and b) zone axes display well-ordered cubic mesostructures with the cell parameter a0 = 14.28 nm, which match well with those ordered 25, 26 structures (body-centre cubic) of SBA-16 reported previously, demonstrating that the pre-crystallized SC particles can be well assembled into highly ordered mesoporous materials. In order to observe the mesopore wall structure clearly, the specimen for HRTEM observation was made into ultrathin cross-sections with a thickness of 30-50 nm. The corresponding HRTEM images (Figure 2, c and d, and Figure S1, e-h) clearly show ordered arrays of mesopores and their crystalline microporous frameworks. The crystal lattice fringes with spacings of 0.65, 0.32 and 0.29 nm along [100] zone axis of the cubic mesopore walls can be clearly observed, which are attributed to the (220) (-214) and (034) lattice planes of MFI zeolitic frameworks, respectively. Moreover, the selected area electron diffraction (SAED) pattern (Figure S1d) taken perpendicularly to Figure S1c and their Fourier-transform pattern (Figure 1d and Figure S1h, inset) focused on [100] zone axis of mesopore walls further verify these lattice fringes in the HRTEM images. These results demonstrate that the mesopore walls possess a typical MFI framework homogeneously, and they are consisted of the intergrown MFI sub-crystals rather than single crystals with throughout lattice fringes.



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Figure 2. 2 SEM (a), TEM (b) and HRTEM images (c, d), small-angle XRD patterns (e) and Ar sorption isotherm and pore size distributions (f) of ordered highly-zeolitized mesoporous aluminosilicate SC-SBA-16 prepared by gradient acidic assembly growth strategy. The insets in b and d are corresponding Fourier-transform patterns. The used ultrathin specimen for HRTEM observations (c, d) is ultrathin cross-section with a thickness of 30-50 nm. The micropore size distribution is obtained by NLDFT model, and the mesopore one is calculated by BJH method.

The small-angle XRD pattern of SC-SBA-16 products in Figure 2e shows two main Bragg reflections and four minor reflection. The first three small-angle peaks can be indexed to the 110, 200 and 211 reflection (d = 10.1, 7.57, 6.10 nm), and the cell parameter (a0) is calculated to be 14.19 nm, in good agreement with that calculated from TEM measurements. Furthermore, because of the thinner ordered mesoporous walls (thickness wt = 11.88 nm) less than several single-unit-cell of MFI zeolites and their intergrown nature displayed in HRTEM images (Figure 2, c and d, and Figure S1, e-h), no any visible diffraction peak of MFI framework appears in the wide-angle XRD region (Figure S2a). The Ar sorp-

tion measurement of SC-SBA-16 shows type IV isotherms with a H2 hysteresis loop (Figure 2f), similar to that of Al-SBA-16 with the cage-like mesostructure obtained from TEOS and aluminum 2 -1 isopropoxide (Figure S2b). It has a high surface area of 834 m g 3 -1 and a large pore volume of 0.64 cm g (Table 1). Obviously, the SC-SBA-16 products display an increasing micropore area from 2 -1 3 -1 76 to 310 m g and micropore volume from 0.03 to 0.1 cm g , without loss of the mesoporosity compared to Al-SBA-16 (Table 1). Moreover, it shows two narrow pore size distributions at the mean value of 0.52 and 5.75 nm (Figure 2f, inset and Figure S2, c and d), respectively.

Table 1. Textural properties of the ordered highlyhighly-zeoliti zeolitized tized mesoporous SCSC-SBASBA-16, 16, referenced AlAl-SBASBA-16 and ZSMZSM-5 as well as their their total and external surface acid amounts calculated by their potentiometric titration curves with n-butylamine and tert-butylamine. a

Smicro

b

Smeso

(m g )

-1

(m g )

-1

(m g )

(cm g )

(cm g )

SC-SBA-16

834

310

524

0.64

Al-SBA-16

505

76

429

ZSM-5

637

564

73

Sample

SBET 2

2

2

-1

Vtotal

c

3

-1

Vmicro

b

3

Total strong acid d amount

External surface strong e acid amount

-1

-1

-1

(mmol g )

(mmol g )

0.10

0.063 (0.022)

0.060

0.49

0.03

0.028 (0.0)

0.024

0.66

0.21

0.056

0.018

a

b

BET surface area calculated from the Ar adsorption data obtained at P/P0 between 0.05 and 0.2, using the BET equation. Micropore area and c d Micropore volume estimated using t-plot method. Single point total pore volume at P/P0 = 0.995. Total strong acid amount calculated from no butylamine titration amount at E ≥ 100 mV. The data in parenthesis represented the acid amount after treatment with 100% steam at 600 C for 2 h -1 e under a WHSV of 6.0 h . External surface strong acid amount calculated from of tert-butylamine titration amount at E ≥ 100 mV.


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The former is corresponding to the channel dimension of MFI zeolitic frameworks (Figure S2c), and the latter further manifests the highly ordered mesopore arrays similar to Al-SBA-16 although it is larger than that (4.95 nm) of Al-SBA-16 (Figure S2d). Besides, the total and external surface strong acid amounts and strengths of the ordered highly-zeolitized SC-SBA-16 were measured quantitatively by titrating it with n-butylamine (Figure S2e) 27, 28 Herein, n-butylamine can and tert-butylamine (Figure S2f). enter the micropore channels of MFI zeolites whereas tertbutylamine can only bind exclusively to external acid sites in the 27-29 mesopores. According to the earlier investigation, the initial electrode potential (Ei) indicates the maximum strength of the acid sites, and the amount of n-butylamine titration at E ≥ 100 mV is corresponding to the total number of strong acid sites. The SCSBA-16 sample titrated with both n-butylamine and tertbutylamine displays strong acidity (Ei > 500 mV), which is much higher than those of Al-SBA-16 (Ei = 200 mV). Moreover, its -1 -1 total (0.063 mmol g ) and external surface (0.060 mmol g ) strong acid amounts are almost identical, implying its maximum exposed active sites. The acidic strength (Ei > 500 mV) and the -1 total strong acid amount (0.056 mmol g ) of ZSM-5 zeolites are comparable with SC-SBA-16. However, its external surface acid -1 amount (0.019 mmol g ) is much smaller than that of SC-SBA-16. And it is worthy to mention that when the ordered highlyo zeolitized SC-SBA-16 is treated with 100% steam at 600 C for 2 -1 h under a WHSV of 6.0 h , the ordering of its mesopore structure can be retained well, and its total strong acid amount is still 0.022 -1 mmol g (Table 1 and Figure S3). However, the acidity of AlSBA-16, after the above 100% steam treatment, completely loses though its mesostructure is retained (Table 1 and Figure S3), indicating the well hydrothermal stability of the high-zeolitized SCSBA-16 sample. PrePre-crystallized ZSMZSM-5 Zeolite SC Particles Particles. articles. In the preparation of the highly-zeolitized ordered mesoporous aluminosilicates SCSBA-16, the SC particles with appropriate size and crystallization degree are firstly selected to assure the ordered assembly and high zeolitization of mesopores. According to the dynamic light scattering (DLS) measurements of the synthetic process of ZSM-5 zeolites (Figure 3A), the particles around inflection point region (red region, Figure 3A) are selected as Si/Al source to assemble

the SC-SBA-16. The most probable particle size of SCs in this region is 6.38 nm (Figure 3A, inset), which can well match the wall thickness of SBA-16. Moreover, FT-IR spectrum (Figure 3Ba) of the SC particles reveals their MFI-type framework struc-1 tures, although the band at 580 cm is displaced from the one -1 usually observed at 550 cm which has been specifically assigned 30 to five-ring structures in Pentasil zeolites. The frequency shift can be ascribed to the small size of the SC particles containing the 31 five rings as well as the limited connectivity along the five rings. 29 Solid state Si MAS NMR spectrum (Figure 3Bb) of the SC particles displays two peaks at -102 and -110 ppm, which are corre4 3 sponding to Si(OSi)4 (Q ) and Si(OSi)3(OH/OAl) (Q ) groups, 27 respectively. Its Al MAS NMR spectrum (Figure 3Bc) shows 4coordinate tetrahedral framework aluminum around 53 ppm and 6-coordinate octahedral extra-framework aluminum around 0 ppm. Furthermore, the HRTEM image (Figure 3C) clearly reveals the crystalline nature and the size distribution of 2-3 nm of these SC particles, which is smaller than the result of DLS measurement (Figure 3A, inset). This could be caused by the hydration of the SC particles in the DLS measurements. Investigation of Gradient Acidic Assembly Growth Process. The above as-prepared SC colloidal solution containing abundant + TPA ions was added into the typical synthetic system of the mesoporous silica SBA-16 materials. Such system containing two different templates was used to assemble SC-SBA-16 under pH o 0.5 at 100 C for 3 h. The obtained sample was named as SCSBA-16-0.5. As shown in Figure S4, the small-angle diffraction peak of SC-SBA-16-0.5 shifts toward left compared to that of the parent SBA-16 material (P-SBA-16), indicating its increasing dspacing (Table 2). Besides, no Bragg diffraction is observed in the wide-angle region, demonstrating that no large zeolite crystals are formed during the assembly. However, the micropore surface area 2 -1 2 -1 (400 m g ) and the total surface area (713 m g ) of the SC-SBA2 -1 16-0.5 are much larger than those (212 and 523 m g ) of P-SBA16 (Table 2). The thick mesopore wall (14.5 nm) guarantees the accommodation and intergrowth of the SC particles within the frameworks, which is thought to be the prerequisite to formation 32 of ordered highly-zeolitized mesoporous materials. Furthermore, taking alkaline synthesis principle of zeolite materials into con33 sideration, we also

Figure 3. 3 (A) Variation of particle diameter as a function of crystallization time of ZSM-5 zeolite. The inset is particle size distribution curve of 29 27 as-prepared sub-crystal particles in the red region. (B) FT-IR spectrum (a), Si MAS NMR spectrum (b) and Al MAS NMR spectrum (c) of asprepared SC particles in the red region of Figure 3A. (C) HRTEM image of as-prepared SC particles in the red region of Figure 3A.



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Table 2. Textural properties of P-SBAgrowth.. SBA-16 and SCSC-SBASBA-16 obtained during the gradient acidic assembly growth

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a

Smicro

(m g )

-1

P-SBA-16

wt f

(nm)

(nm)

(nm)

3.41

9.01

12.74

12.19

0.46

4.89

11.20

15.84

14.51

0.64

5.64

11.08

15.67

13.55

(m g )

-1

(cm g )

(cm g )

523

212

0.08

0.33

SC-SBA-16-0.5

713

400

0.17

SC-SBA-16-0.5-1.9

933

474

0.20

2

2

3

-1

BJH average adsorpd tion pore diameter

a0e

Vmicro

SBET

b

d110

b

Sample

Vtotal

c

3

-1

(nm)

a

b

BET surface area calculated from the N2 adsorption data obtained at P/P0 between 0.05 and 0.2, using the BET equation. Micropore area and c d Micropore volume estimated using t-plot method. Single point total pore volume at P/P0 = 0.995. Average pore diameter (nm) calculated from e f the adsorption isotherm by the BJH method. Unit-cell parameter (a0) estimated from the position of the [110] diffraction aline, . Pore 0 = d110 2 3 wt = a0 2 − d p wall thickness (wt) estimated from equation, .

try to assemble SC-SBA-16 under higher pH value. However, it is found that the surface of the obtained samples begins to become rough and ordering of mesostructured decreases with the increasing pH values (Figures. S5 and S6). When the pH value is higher than 0.6, the phase separation occurs, i.e. the mesopore walls of SC-SBA-16 begin to dissolve partly and the separated particles appear (Figure S6). This dissolvent of the pore walls at relatively high pH value forms rich cavities, so as to lead to the increases of their surface areas (Table S1). When the system become alkaline, only ZSM-5 nanozeolites can be obtained in the synthetic system and the mesostructural ordering disappears completely (Figures. S5 and S6). Obviously, an acidic media is the premise to obtain highly-ordered SC-SBA-16 during its construction. Moreover, when the pH value of the SC-SBA-16-0.5 system was increased to 1.9, a secondary hydrothermal treatment process, 34 a wide-used approach in the zeolite crystallization, was employed to grow the interpenetrating zeolitic networks and further improve the zeolitization degree of the mesopore walls. The obtained sample was termed as SC-SBA-16-0.5-1.9. As shown in Figure S4, the (110) reflection peak of SC-SBA-16-0.5-1.9 has a little change compared to that of SC-SBA-16-0.5. However, its 2 -1 2 -1 micropore area (474 m g ) and total surface area (933 m g ) 2 -1 greatly increase compared to those (400 and 713 m g ) of SCSBA-16-0.5 (Table 2), although no any diffraction appear in its wide-angle XRD pattern. These results indicate the further zeolitization of mesopore walls and the intact retainment of mesopore ordering. However, if the pH value of the system further increases, the ordering of mesostructure decreases and the phase separation occurs (Figures. S7 and S8, and Table S2). Instead, if the pH value of the SC-SBA-16-0.5 system is still fixed at 0.5 during the 2 -1 secondary hydrothermal treatment, the micropore area (368 m g ) 2 -1 and total surface area (763 m g ) of the resultant SC-SBA-160.5-0.5 just have few change compared with SC-SBA-16-0.5 although its mesostructure is retained (Figure S9), implying the indispensability of this gradient acidity control strategy. It is a remarkable fact that SC particles can interactively grow + in an appropriate acidity in the presence of TPA ions, whereas a high pH value or a basic condition can result in the phase separation. The intergrowth among SC units under an acidic condition can be further confirmed by the growth of SC particles under the

above synthetic conditions without the addition of surfactants. The FT-IR spectra of the SC particles treated under the above -1 synthetic condition clearly show that the bands in 550 - 600 cm region assigned to five-ring structure of MFI zeolitic framework -1 shift from 580 to 562 cm (Figure S10). This shift not only indicates the growth of the SC particles but also demonstrates their 30 better connectivity among the five-ring units with a framework. And the increases of the particle size can also be demonstrated by their DLS measurements (Figure S10B) and TEM observations (Figure S10, C and D) treated under the above synthetic conditions, that is, the size of the hydrated SC particles increase from 6.38 to 8.47 nm after the gradient acidic growth process. Moreover, these TEM images also reveal the nature of their intergrowth clearly (Figure S10D). Additionally, the growth of the SC particles and the high zeolitization of the mesopore walls in SC-SBA16 can be further confirmed by NMR spectra of SC-SBA-16-0.51.9 sample (Figure S11). Evaluation of Catalytic Performance. Performance. The highly-zeolitized mesoporous aluminosilicate SC-SBA-16 shows distinct advantage in Friedel-Crafts alkylation of anisole with benzyl alcohol and the cracking of TIPB compared to ZSM-5 and Al-SBA-16 (Figure 4), indicating their strong acidity and high exposed active sites (Table 1). In detail, the SC-SBA-16-0.5-1.9 and SC-SBA-16-0.5-0.5 exhibit far higher conversion of benzyl alcohol and selectivity of products than ZSM-5 and Al-SBA-16 in Friedel-Crafts alkylation of anisole (Figure 4a and Table S3). Whereas, the catalytic activity of SC-SBA-16-0.5-0.5 is much lower than that of SC-SBA-160.5-1.9, which demonstrates the necessity of the gradient acidic assembly growth strategy. For the TIPB cracking, all the mesoporous materials display higher cracking activity in all temperature points tested compared to ZSM-5 (Figure 4b), implying the advantage of mesoporous materials in the conversion of bulky molecules. What is more, the catalytic activities of the three mesoporous materials follow the order of SC-SBA-16-0.5-1.9 > SC-SBA16-0.5-0.5 > Al-SBA-16, further demonstrating the importance of the gradient acidic growth for achieving the high zeolitization of the mesopore walls. In addition, SC-SBA-16-0.5-1.9 also exhibits high anti-deactivation ability in the cracking of TIPB, only about 24% activity loss can be observed even if after 450 times of injection (Figure S12). And it is worthy to note that ZSM-5 zeolite


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displays the highest catalytic activity in the cracking isopropybenzene (IPB) with smaller size among the four catalysts (Figure S13). These results reveal the strongest microporous acidic sites of the ZSM-5 zeolite and the largest accessible external surface acidic sites of SC-SBA-16-0.5-1.9 sample, which are greatly consistent with their acid amounts conducted by FT-IR spectra after pyridine adsorption (Figure S14 and Table S4).

sites for bulky molecule substrates. These consequently bring about their substantially-improved catalytic activity and stability in gaseous/liquid phase reaction involving bulky molecules compared with conventional zeolites or amorphous mesoporous materials. This new synthesis process without the use of any special template can also be expected to achieve the adjustment of wall thickness, mesopore diameter and framework topology, by generalizing it to the synthetic system of other ordered mesoporous materials rather than design and synthesis of special templates.

ASSOCIATED CONTENT PDF document containing the details of structure, morphology, acidity and catalytic activity characterization of the materials (Figures S1S14, and Tables S1−S4). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]

Author Contributions ‡

These authors contributed equally.

The manuscript was written through contributions of all authors. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors are grateful to Dr. Z. C. Liu and Prof. Y. H. Yue for helpful discussion on catalytic reactions. Funding: This work was supported by the 973 Program (2013CB934101), NSFC (U1463206, 21473037, 21433002 and 21573046), and National Plan for Science, Technology and Innovation (MAARIFAH), King Abdulaziz City for Science and Technology, Kingdom of Saudi Arabia (14-PET827-02), Sinopec (X514005)..

REFERENCES Figure 4. 4 Catalytic activities of various catalysts in Friedel-Crafts alkylation of anisole with benzyl alcohol (a) and in the cracking of 1,3,5-triisopropyl-benzene (b). 4.

CONCLUSION

In summary, we have prepared for the first time ordered highlyzeolitized mesoporous aluminosilicate (SBA-16) materials by a novel gradient acidic assembly growth strategy, and demonstrates the feasibility to achieve the ordering of both microporous and mesoporous structures in the conventional mixed template system. The gradient acidic growth is the key to achieve the intergrowth of pre-crystallized SC particles in the mesopore walls, and to avoid the phase separation. The ordered highly-zeolitized mesoporous SC-SBA-16 materials show their strong surface acidity, excellent hydrothermal stability and maximum accessible acidic

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