Hierarchical Hexagonal Zeolites: Intrinsic Transformation Directed by

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Materials and Interfaces

Hierarchical Hexagonal Zeolites: Intrinsic Transformation Directed by Precise Control of Synthetic Conditions He Ding, Zixing Xiao, Jingshuang Zhang, Tianyi Fu, Peng Bai, and Xianghai Guo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00389 • Publication Date (Web): 27 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018

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Industrial & Engineering Chemistry Research

Hierarchical Hexagonal Zeolites: Intrinsic Transformation Directed by Precise Control of Synthetic Conditions He Ding, Zixing Xiao, Jingshuang Zhang, Tianyi Fu, Peng Bai and Xianghai Guo * Department of Pharmaceutical Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, P.R. China Key Laboratory of Systems Bioengineering (Ministry of Education), Tianjin University, Tianjin, 300072, P.R. China *Corresponding author, Email: [email protected] Key words: multi-ammonium surfactant, hierarchical zeolite, temperature-gradient method, Claisen-Schmidt condensation

Abstract

Hierarchical hexagonal Zeolites with narrow pore size distribution were obtained by a simple temperature-gradient method using a multi-ammonium surfactant as micro-mesopore generating agent. The pore textures of the as-synthesized samples were characterized by electron tomography and nitrogen adsorption-desorption. The samples possessed a disordered and interconnected micro- and meso-pore

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structure, in which the mesoporous structure was introduced at lower temperature and then partially converted to microporous zeolite at higher temperature. The acid sites were characterized by FT-IR pyridine adsorption experiments under different temperatures. A relative higher amount of Brønsted and Lewis acid sites (0.074 and 0.240 mmol/g, respectively) were found in HAM(9) sample, which also had a bigger external surface. Catalytic performance of the samples was evaluated by the Claisen-Schmidt condensation of benzaldehyde and 2’-hydroxyacetophenone. An outstanding conversion as high as 86% by sample HAM(9) demonstrated the synergistic effect of micropores with acid sites of high activities and mesopores for effectively transporting bulky reactants. The stability of catalyst was also illustrated, where less than 5% change in conversion was found after four cycles.

1. Introduction Zeolites are crystalline aluminosilicates with TO4 tetrahedra linked together, which have been widely applied as shape-selected acid catalysts and adsorbents for gas separation, due to the regular pore diameter (< 2 nm), high specific surface area and good performance of hydrothermal stability1-3. However, the sole micropores in zeolites cause serious diffusion limitation. Only small molecules can diffuse through the apertures, whereas the bulkier reactants cannot reach catalytic active sites (e.g, Al sites) sitting inside the micropores4. Noticeably, microporous molecular sieves, such as zeolite Y, ZSM-5, and β, own abundant uniform microporous structures and relative high hydrothermal stability, which can be attributed to their crystallographic structures. So, it’s conspicuous if “crystallinity” could be introduced into the amorphous pore wall of the mesoporous materials, a ‘zeolite-type’ structure that simultaneously maintains mesoporous or superpore structures could be developed. Based on the possibilities, numerous efforts have been made to synthesize zeolites possessing mesopores and micropores stimultaneously (for abbreviation, denoted as ‘mesoporous zeolite’ hereafter). For preparing mesoporous zeolite, one major synthetic strategy is to induce a secondary larger pore system into zeolite crystal by different Structure Directing Agents (SDAs). Thereby, dual-templating approaches were extensively explored. By using a dual-template method with multi-steps, micro- and


2

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mesoporous composite materials could be synthesized with improved hydrothermal stability, such as MFI/MCM-415, 6, β/MCM-417, 8 and Y/MCM-419. In addition to methods of templates, direct assembly of nano-sized zeolite particles could also create interparticle mesoporous voids. Thus, seed crystal method is provided and has made significant advances in improving the hydrothermal stability of mesoporous structures10-14. Through adding proto-zeolite nanoclusters or “zeolite seeds” as secondary building blocks, more ordered and partial crystalline micro/mesoporous bimodal composites were obtained. Recently,

Ryoo

and

coworkers

investigated

a

multiammonium

surfactant

molecule

{[C22H45-N+(CH3)2-C6H12-N+(CH3)2-C6H13] Br2-, in short, ‘C22-6-6N2’} as SDA to generate mesoporous MFI zeolites with unit-cell (2 nm) thickness15, 16. It was clear that the quaternary ammonium head groups spaced by C6 alkyl linkage served as MFI structure directing agent, while the long-chain C22 alkyl group guided lamellar assemblies. Since then, multiammonium surfactant directed synthesis is quite attractive as an approach for generating micro- and mesoporous zeolites, specially, for the purpose of controllable porosity. However, the broad distribution of mesopore sizes (2-15 nm) of their synthesized samples, caused by randomly stacked nanocrystals and collapse of the multilayer structure after calcination, suggested an intriguing direction of improvement for these ultrathin zeolites. Due to the important effect of the micelle precursors in the induction process on the nucleation, many efforts have been exerted on controlling their morphology. Based on the polymerization degree and structure regularity, the micelle precursors could be approximately labeled into oligomers, nanoparticles and nanocrystals. Through aging the zeolite precursor of MFI at 90 oC, Kirschlock and coworker17 found the structure of nanoslabs and tablets in precursors and then generated ordered mesoporous materials by sequence reaction control. By dynamic controlling of the precursors forming zeolites, Shi and coworkers18 fabricated mesoporous ZSM-5 zeolites using hexadecyl trimethyl ammonium bromide (CTAB) by seeding method. Fortunately, the morphology of micelle precursors can be regulated by temperature control. It provides a continuously direct synthesis system which could be attractively demanded to achieve uniform pore size.


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Herein we demonstrated the multiammonium surfactant (C22-6-6N2) was effective to direct zeolite crystallization inside a uniform mesopores, exhibiting scattered MFI structure among the uniform mesoporous structures. A one-pot temperature-gradient strategy was applied to obtain hierarchical MFI/MCM-41 composites with different ratios of Si/Al, in which a competition between the assembling of micro- and meso-directing agents with aluminosilicate precursors during hydrothermal synthesis was significantly avoided. Lower temperature (100 oC) was the crucial condition for facilitating assembly of C22-6-6N2 to form the uniform mesopores and higher temperature provided possibilities for intrinsic transformation to crystalline zeolites. Meanwhile, the catalytic performance of as-synthesized materials in a typical condensation reaction between bulky organic reactants was evaluated. The results showed that these hierarchical aluminosilicate catalysts displayed significantly enhanced catalytic activity, by combining the advantages of mesoporous and microporous zeolites.

2. Materials and Methods 2.1. Reagents and Materials Tetraethyl orthosilicate (TEOS, TCI, 96%), sodium hydroxide (2 M in water, TCI), 1-bromodocosane

(C22H45Br,

TCI,

>98%),

1-bromohexane

(C6H13Br,

TCI,

>98%),

N,N,N’,N’-tetramethyl-1,6-diamino -hexane (C10H24N2, TCI, >98%), Aluminum sulfate octadecahydrate [Al2(SO4)3.18H2O,

Aladdin],

2-hydroxyacetophenone

(C8H8O2,

Aladdin,

98%),

Benzaldehyde

(C6H5CHO, Aladdin, GCS), Ammonium acetate and (CH3COONH4, Macklin, 1.0 M) and DI water. One kind of zeolite Organic Structure-Directing Agents (OSDA) was prepared according to the previous report15. The OSDA has the molecular formulas of [C22H45-N+(CH3)2-C6H12-N+(CH3)2-C6H13] Br2-. For brevity, the OSDA is denoted by C22-6-6N2.

2.2. Preparation of zeolites The hierarchical zeolite was hydrothermally synthesized using C22-6-6N2 surfactant as the OSDA. For a typical Huo’s procedure19, 20, a synthetic mixture with final molar composition 1 OSDA : 8 SiO2 : 3


4

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NaOH: 1054 H2O : 32 EtOH : x Al2(SO4)3 was prepared. TEOS was added dropwise to an aqueous solution of NaOH containing C22-6-6N2 surfactant. Aluminum sulfate octadecahydrate was dissolved in the deionized water (DI water) and then the clear solution was dropwise added to the solution with continuous stirring. The resultant gel mixture was aged at room temperature for 1 h. The final gel mixture was transferred to a HF-washed Teflon-lined stainless autoclave and statically heated at 100 oC for 3 d and following 160 oC for 3, 6, 9 d, respectively under autogenous pressure. The solid product was centrifuged, washed with DI water, dried in an oven at 80 oC for 12 h and calcined at 550 oC for 6 h under air atmosphere with a heating rate of 0.75 oC/min.

2.3. Catalytic Reaction The Claisen-Schmidt condensation of benzaldehyde and 2’-hydroxyacetophenone were performed in a 10-mL heat-resistant glass bottle under N2 atmosphere. For ion exchange, the zeolites were stirred in ammonium acetate (1 M) aqueous solution at 90 oC for 6 h for three times. Afterwards, the samples were filtered, washed and dried at 80 oC for 12 h. The zeolites were converted to H+ form after calcination at 550 oC in air. Before the catalytic reactions, the zeolites described above were pre-activated at 150 oC for 3 h in a vacuum oven. Typically, 1.0 mmol of 2-hydroxyacetophenone (0.136 g) and 1.5 mmol benzaldehyde (0.159 g) were mixed with 0.5 mL DMF solvent under stirring. Then, 15.0 mg of catalyst was added at once to the reaction mixture under N2 atmosphere, and the mixture was heated at 150 oC for 24 h. The products were 2’-hydroxychalcone and flavanone. The liquid samples were filtered off and analyzed by LC3000I High Performance Liquid Chromatography (HPLC) with a UV detector and a Kromasil 100-5C18 (250 × 4.6 mm i.d.) to determine the selectivity and conversion. The conversion and selectivity of products are defined as [(initial moles of reactant - final moles of reactant)/ initial moles of reactant] × 100% and (moles of product i/moles of total products) × 100%, respectively.

2.4. Characterizations


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Power X-ray diffraction (XRD) patterns were obtained at 2θ angles between 1o and 30o using Bruke AXS D8-A-Ddvance equipped with Cu-Ka radiation (λ = 0.1548 nm) at 15 mA and 40 kV, operating with a step size of 0.012o and dwell time of 0.4 s at room temperature. High-resolution scanning electron microscopy (SEM) images were taken with SU8010 field-emission scanning electron microscope operating at 3 kV. Transmission electron microscopy (TEM) images were taken by a JEM-2100F (JEOL) transmission electron microscopy operated at an accelerating voltage of 200 kV. Before the measurements, the powder samples were suspended in ethanol by ultra-sonication. A drop of the dispersion was supported on a copper grid with holey carbon and then dried under ambient conditions. N2 adsorption-desorption isotherms were measured at -196 oC on 3H-2000PM2 volumetric adsorption apparatus. Prior to each measurement, about 0.2 g of all products were outgassed at 260 oC to vacuum (-0.1 MPa). The specific surface are was calculated from the adsorption branch in the P/P0 range of 0.04-0.32 using Brunauer-Emmett-Teller (BET) equations. The micropore diameter distribution was derived from the adsorption branch at P/P0 < 0.1 via nonlocal density functional theory (NLDFT) analysis. The solid

29

Si and

27

Al magic angle spinning (MAS) NMR spectra were recorded using

Infinityplus 300 of Varian at room temperature. The elemental compositions of Si and Al were analysed by inductively coupled plasma optical emission spectroscopy (ICP-OES) using a Horiba Ultima 2 instrument equipped with photomultiplier tube detection. 0.01 g of samples was dissolved in HF (40%, 3 mL) at room temperature for 30 min, diluted with saturated boric acid solution (45 mL). The Fourier transform infrared (FT-IR) spectra were collected using Nicolet 6700 by Thermo Scientific. The experiments were done on the powdered samples with KBr addition. Pyridine desorption measurements were performed using a Nicolet Nexus 470 FI-IR spectrometer with a transmission MCT/A detector. The baseline spectra were recorded at 35, 100, 200 and 350 oC. After adsorption, pyridine was desorbed under dynamic vacuum at different temperature for 1 h. Concentration of Lewis (cL) and Brønsted (cB) acid sites were calculated by the intergral intensities of bands at 1452 cm-1 and 1545 cm-1, respectively. The purity of the zeolite SDAs was confirmed by 1H NMR spectroscopy (Bruker, 400 MHz).


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2.5. Hydrothermal Stability Test The hydrothermal stability of the hierarchical samples was tested by boiling the samples at 100 oC for 24 h21. 0.2 g of samples was loaded into a 50 mL autoclave with 20 mL DI water and kept at 100 oC for 24 h. After treatment, the samples were recovered by centrifugation, washed with DI water, and dried at 80 oC in vacuum overnight.

3. Results and Discussion 3.1 Characterization of hierarchical MCM-41/MFI composites The hierarchical MCM-41/MFI composites with different ratio of Si/Al were successfully synthesized by a simple one-pot temperature-gradient strategy with C22-6-6N2 as sole surfactant, and we denoted these new hierarchical aluminosilicate materials as HAM (abbreviation of Hierarchical Aluminosilicate Materials). All samples with different ratio of Si/Al were firstly treated at 100 oC for 3 d, following by different days at higher temperature. The Si/Al molar ratios measured by ICP-OES were shown in Table 1.

Table 1. Physical properties of HAM. Sam ples[a]

Si/ Al

HA M (0)

0

HA M (9)

9

HA M (18)

18

HA M (29)

29

D(nm)[

a[c]

dp[d]

dw[e]

5.74

6.62

4.41

1.10

b]

Synthesis conditions[f] 1. 100 oC 3 d 2. 160 oC 2 d

5.82

6.72

4.25

1.24

1. 100 oC 3 d 2. 160 oC 2 d

5.78

6.67

4.25

0.90

1. 100 oC 3 d 2. 160 oC 2 d

5.78

6.67

4.89

0.89

1. 100 oC 3 d 2. 160 oC 2 d

[a] Numbers in brackets were the ratio of Si/Al except for HAM(0). HAM(0) is a pure silica material. [b] D(nm) was calculated based on the maximum intensity peak (100) in the low-angle XRD patterns


7


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[c] “a” was the cell parameter, calculating and estimated by = ( )*D(100), which was equal to the distance √

between centres. [d] “dp” was the primary mesopore diameter, calculated by Nonlocal Density Functional Theory (NLDFT) using BJH pore size distribution curve. [e] “dw” was the thickness of pore wall and calculated by substracing “dp” from “a”, dw = a - dp. [f] The number of 1 and 2 denoted as a sequence of synthesis conditions.

XRD patterns of as-synthesized HAM are shown in Figure 1. One peak was observed in low angle (2θ < 5°) in HAM(0), with d-spacing of 5.74 nm, which could be indexed to the (100) reflection of 2D hexagonal symmetry (P6mm) lattice with a unit cell parameter a = 6.62 (Table 1), similar to MCM-41. From high angle (5o-30o), two peaks (2θ = 8.08° and 9.04°) occurred in HAM(0), which were attributed to (101) and (200) crystal faces of MFI zeolite structure15. Figure S1 shows the XRD patterns of HAM(0) reacting for different days. After reacting for 3 d at 100 oC, one special MCM-41 peak was observed without crystalline MFI peaks in high angle. When reacting 3 d at 100 oC and 2 d at 150 oC, the order degrees of MCM-41 were reduced, while four MFI peaks occur, indicating MCM-41 zeolites are partially crystallized after increasing temperature. For a longer time (100 oC 3 d and 150 oC 4 d), HAM(0) formed the MFI zeolite structure, where at least four special peaks in 2θ = ~ 8°, ~ 9°, ~ 23° and ~ 24°) and the MCM-41 peaks disappear. The characterization of HAM(0) suggests that 100 oC is important for the generating MCM-41 structure and increasing the synthesis temperature would lead to further crystallization of the hexagonal mesoporous walls, resulting in enhanced thickness of the crystalline walls. The hierarchical aluminosilicate materials shown in Figure 1 possessed characteristic peaks of MCM-41, suggesting HAM with different ratios of Si/Al could form the mesoporous structure, and four peaks are observed in the range of 2θ=5-30o, which are attributed to (101), (200), (501), and (303) crystal faces, respectively, indicating they also have MFI microporous structure15. However, the peaks intensities and their sharpness reduce with the decreasing of the ratio of Si/Al, HAM(9) < HAM(18) < HAM(29), and the result is consistent with the decrease of Al content in the materials. According to previous researches, the higher Al content in materials, the longer hydrothermal treatment time for crystallization of zeolites22. The decrease of the crystalline domains results in the reduction of special peak intensity, which is confirmed by means of the Scherrer equation relating with the main diffraction peaks23, 24.


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Therefore, we continually extend the hydrothermal treatment time to generate the crystalline MFI zeolites. As shown in Figure S2, the intensity of HAM(9) peaks increased with retained original mesoporous structure, where (100) still could be found in low-angle XRD patterns. However, the longer time under hydrothermal treatment finally results in the total conversion of MCM-41 phase to MFI structure, suggesting the importance of controlling synthesis time.

Figure 1. XRD patterns of the HAM materials with different ration of Si/Al, synthesized by temperature-gradient strategy for 7 d.

The obvious evidence of zeolite framework can also be identified by FT-IR spectroscopy. Figure 2 shows the FT-IR patterns of HAM samples. As shown in Figure 2, the characteristic peaks of amorphous silica are marked in black. The peak around 550 cm-1 marked in blue are related to crystal zeolite6, 25, 26, which are used to determine the crystalline degree of the HAM samples. The optical ratio of peak I (~550 cm-1)/peak II (~450 cm-1) is related to the degree of crystallization, due to the amorphous silica lacking of absorbance near 550 cm-1. Thus, it could be inferred that the microcrystalline MFI structure existed in all the HAM samples. Peak II in HAM(0), HAM(9), HAM(18) and HAM(29), appear ratios of the two peaks


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as 0.29, 0.50, 0.57, and 0.69, respectively. The results could be supported by wide-angle XRD patterns. A strong band sited in near 1094 cm-1 with a pronounced shoulder at 1231 cm-1 is corresponding to T-O-T (T represents Si or Al) asymmetric stretching mode. While the weak peak around 800 cm-1 is assigned to the T-O-T symmetric stretching mode27.

Figure 2. FT-IR patterns of the HAM materials with different ratios of Si/Al.

Figure 3(A) is the 29Si MAS NMR of HAM. The HAM samples present similar spectra around 100 ppm and 110 ppm, which correspond to Q3 sites deriving from the silanol groups on the surface of zeolites, (SiO)3Si-OH, and crystallographically nonequivalent Q4 sites, (SiO)4Si, respectively28. The polymerization degree of silica frameworks can be represented by the ratio of Q3 to Q4 29, 30. The Q3/Q4 29

values of MCM-41 and ZSM-5 are 0.62 and 0.05 . For HAM(0), the values are 0.60, 0.40 and 0.19 for 3 d, 5 d and 7 d respectively, suggesting again the degrees of crystallization increased successively with time (Figure S3). As for the value of Q3/Q4 of HAM(9), HAM(18) and HAM(29), they were 0.63, 0.41, and 0.22. The results are corresponded with XRD patterns, proving again the reduction of crystallization degree with Si/Al ratio. Compared with MCM-41, the HAM zeolites exhibit high ratio of (SiO)4Si sites in


10

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silica-based zeolites, where the hydrothermal stability could be enhanced by condensation of pore walls ( Q4 sites)31. On the other hand, cross-linking degree in the framework was increased by SiO4 unit providing further structural stability32. For testing, the HAM zeolites are boiling in water for 100 h, and the as-synthesized samples still exhibit the diffraction peaks of meso- and microporous structure, while they disappear in MCM-41 sample (Figure S4A). And Figure S4B demonstrate the existence of Al active sites in boiling-treated samples.

Figure 3. (A) 29Si MAS NMR of HAM and (B) 27Al MAS NMR of HAM

Figure 3(B) is the 27Al MAS NMR of HAM. Al provides the acid catalytic sites in MFI zeolites, and the coordination number of Al atoms in framework decides the catalytic performance of solid-catalysts. Before measurement, HAM zeolites are moistened with distilled water. The spectrum of HAM present an intense peak near 54 ppm, which is signal of the AlO4 unit with tetrahedral coordination in the zeolite framework33. The signal shown in 0 ppm reflects the octahedral form of extra-framework Al. The intensity of peak on 0 ppm in HAM is much lower than that in MCM-41, indicating mostly Al atoms are incorporated into the framework. After raising temperature, extra-framework Al atoms fill in the defects of silanol nests. Firstly, the hydrolysis of siloxane (Si-O-Si) bonds accompany with the degradation of pure silica MCM-41 structure, and then the incorporation of Al in framework creates Si-O-Al bonds. Meanwhile, the negative charge on framework, due to the tetrahedral coordinated Al, is available to repel the OH- for promoting the hydrolysis of siloxane bands31. Due to the more uniform Al coordination in


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HAM, it manifests its potential advantages in wide range of acid catalytic applications. Additionally, the higher Al content, the better hydrothermal stability. HAM zeolites own the percentage of tetrahedral coordinated Al species higher than 97%, compared with 81.4% of MCM-41, suggesting an enhancing catalytic activity34, 35. For elucidating the role of lower temperature (100 oC) in temperature-gradient strategy and the role of C22-6-6N2 directing meso- and microporous structure, the control samples (H-S1 and H-S2) are prepared. H-S1 is directly synthesized under 160 oC without lower temperature treatment, and H-S2 is synthesized without surfactant in the same way as HAM. As shown in Figure S5, HS-1 samples exhibit characteristic peaks of MFI, whereas no special peaks of MCM-41, which indicates that reaction at 100 o

C is key for introducing mesoporous structure. Lower temperature is the crucial condition for facilitating

assembly of C22-6-6N2 to form the uniform mesopores. And the H-S2 synthesized gel does not show any obvious solid precipitate after centrifugation. The results show that C22-6-6N2 serves as structure-directing agents, which induce the formation of the zeolite framework during the hydrothermal treatment.

Figure 4. SEM images of HAM


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SEM images of HAM zeolites are shown in Figure 4 with different morphologies. HAM(0) synthesized for 3 d at 100 oC possess granular morphology with a granularity of 0.15 µm, which has been depicted as a “wormy structure36. For longer synthesis time, the granular morphology transfers into reticulated pieces and finally exhibits a lamellar morphology. After increasing temperature, the surface of HAM become rough, which could be caused by the incorporation of microporous phase. And HAM zeolites of higher Si/Al ratio show the smaller particle size.

Figure 5. TEM images of HAM(9). (A) TEM images viewed in large scale. (B) The d-spacing calculated from ED pattern is 1.91 nm, which could be indexed to the (200) crystal faces of MFI framework. (C) The microporous MFI crystalline domains are generated from the surface of mesoporous structure. The insert image is the fast Fourier transformation (FFT) of selected area in red rectangle. (D) Enlarged image of the region marked by a rectangle confirms the coordination of pentasil chain of MFI structure in the crystalline walls.


13


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More details about morphologies are shown in TEM images (Figure 5 and Figure S6). Figure 5A-D confirms that the microporous MFI crystalline domains are generated from the surface of the mesoporous wormhole structure. The crystalline MFI structure is confirmed by ED pattern (inserted in Figure 5B). Also, the calculated pore diameter of 5.50 nm form FFT patterns inserted in Figure 5C was corresponding with the (100) d-spacing of MCM-41 and the (101) crystal face was attributed to silicalite-1. The spot marked (200) corresponding to a d-spacing = 1.91 nm which is corresponding with the alternating MFI layer in Figure 5D. TEM cross-section of the HAM(9) (Figure 5D, the insert picture is enlarged from selected domain marked by red rectangle) reveals that the stacking of MFI layers (1.91 nm) are composed of three pentasil sheets corresponding to the thickness of a single unit cell dimension along b-axis of b=1.9738 nm15. XRD patterns only present h0l reflections confirming that the framework thickness along the b-axis is actually small. The more information of HAM(0) is provided in Figure S6. HAM(0) shows the partially crystalline MFI domains distributed in mesoporous materials, which also demonstrated by the insert FFT pattern in Figure S6B. The framework of MFI zeolites contains 10-MR elliptical channels with pore-sized of 5.5×5.1 Å sized running along the (100) direction, which are interconnected in a sinusoidal manner by 5.6×5.3 Å sized elliptical channel running straight along (010) direction. The MFI framework structure of mesoporous HAM is confirmed by FFT pattern. The perfect-matched verification of MFI zeolite framework in HAM demonstrate that through controlling synthesis conditions and their intimate mixing, we could lead parallel formation of micro-/mesoporous composites.


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Figure 6. Nitrogen adsorption isotherms and pore size distribution of HAM.

Nitrogen adsorption-desorption isotherms are depicted in Figure 6. HAM materials show a typical IV-type isotherm (according to the IUPAC) with various hysteresis loops37. The pore size distributions are derived from adsorption branch in BJH method. In the case of the HAM materials, the N2 adsorption isotherms of the zeolites samples shown in Figure 6 present a steep increase in the adsorption branch in the low-relative pressure region of P/P0 < 0.01, which is consistent with the adsorbate filling in the micropores. A moderately steep increase in the region of 0.4 < P/P0 < 0.8 indicates capillary condensation in the mesopores. Another gradually continuous increases of the adsorption in the range of 0.8 < P/P0 < 0.98 are attributed to the N2 in the large pores, which are probably the interparticle mesopores. Evidently, the N2 adsorption isotherms with two distinct features of micropores and mesopores are characteristic of the manners in the hierarchical zeolites. For HAM(0), the N2 adsorption isotherms and pore size distribution of different syntheses time are shown in Figure S7. At first temperature section, a narrow dumbbell-shaped hysteresis loops appears with one head of the “dumbbell” in the range of P/P0 = 0.45-0.75 and the other head over 0.85, which indicate the existence of mesopores within the materials and the interparticle mesopores, respectively. The results demonstrate again the introduction of MCM-41


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structure. After increasing temperature, the ordered degrees reduce, and the MFI phase are generated while remain the original mesoporous phase. The decreasing of ordered degree could attribute to the mesopores wall transferring into zeolitic framework. For a longer synthesis time, the mesopores are totally destroyed and from MFI-type zeolites.

Table 2. Textural property measured by N2 adsorption-desorption isotherm. SBET

Smicro

Sext

Vtotal

Vmicro

Vmeso

Vmicro/Vtotal

Vmeso/Vtota

[m2g−1]

[m2g−1]

[m2 g−1]

[cm3 g−1]

[cm3 g−1]

[cm3 g−1]

HAM(0)

1046

197

849

0.89

0.23

0.66

0.26

0.74

HAM(9)

540

61

479

0.81

0.02

0.79

0.03

0.97

HAM(18)

512

171

341

0.95

0.08

0.87

0.08

0.92

HAM(29)

459

240

219

0.84

0.09

0.75

0.11

0.89

l

Table 2 lists the textural properties of HAM materials derived from N2 adsorption-desorption isotherms. BET surface area (SBET) is determined by multi-point BET method in relative pressure range P/P0 = 0.05-0.20. Micropore volume (Vmicro) and surface are (Smicro), external surface are (Sext) are determined by t-plot method. Total pore volume (Vtotal) is determined at P/P0 = 0.97. Mesopore volume (Vmeso) is calculated through abstracting Vmicro from Vtotal. In general, HAM(0) synthesized for 3 d has a large specific surface area of 1268 m2/g (Figure S7). The high value of SBET could be explained by the formation of the composite of MFI and MCM-41 structure. After increasing temperature, the SBET of HAM(0) reduces to 1046 m2/g, suggesting the transformation from mesopore to micropore. For a longer time, HAM(0) has the smaller specific surface area of 288 m2/g. The mesoporous structure is totally broken and forms a layered structure with high aspect ratio. The results showed precise control over reaction time is important to induce the hierarchical porosity in these zeolite structures.


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Figure 7. FI-IR spectra after pyridine adsorption and evacuation at different temperatures (35 oC, 100 oC, 200 oC and 350 oC) on HAM(9). The absorption band characteristics for specifically adsorbed pyridine are described by H for hydrogen-bonded pyridine, B for on Brønsted acid sites, and L for Lewis acid sites.

The pyridine adsorption experiment is an efficient way to characterize the acid sites on catalyst. The FI-IR results of pyridine adsorbed on HAM(9) at different temperatures are presented in Figure 7. The bands sitting in 1452, 1489 and 1620 cm-1 could be attributed to Lewis acid sites. In addition, the bands (1489, 1545 and faint peak 1635 cm-1) are corresponding to pyridine adsorbed on Brønsted sites. The decreasing intensity of 1489 and 1545 cm-1 peaks with increasing temperatures suggests a weak Brønsted sites. The HAM(9) sample possesses relative higher amount of Brønsted and Lewis acid sites (0.074 and 0.240 mmol/g, respectively) than HAM(18) (0.052 and 0.218 mmol/g, respectively) and HAM(29) (0.048 and 0.196 mmol/g, respectively) samples, which is in good agreement with the Al content of HAM samples confirmed by ICP-OES. In addition, HAM(9) samples have a relatively large amount of Brønsted sites at the external surface (0.041 mmol/g), which are calculated by adsorbing 2,6-di-tert-butypyridine (DTBP) on the external surface of HAM(9)38. The result could be attributed to the quite high external


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surface area of the as-synthesized samples as shown in Table 2. Compared with conventional MFI zeolite (the amount of external Brønsted sites is 0.005 mmol/g, corresponding 0.8% of overall acid sites34), the amount of external Brønsted sites of HAM(9) is 55%. Noticeably, the fraction of Lewis acid centers in HAM (70%) is much higher than conventional MFI, which may influence the catalytic performance.

3.2 Catalytic test The catalytic performances of HAM are investigated using bulky organic molecules, where diffusion of the reactant molecules constrains the reaction. 2-hydroxychalcone and flavanone, members of chalconoid and flavonoid family, are synthesized by the condensation of benzaldehyde with 2’-hydroxyacetophenone and subsequent isomerization39, 40. In our experiments, HAM zeolites with different ratio of Si/Al are used for catalyzing the Clasien-Schmidt condensation of benzaldehyde and 2’-hydroxyacetophnone.

Figure 8. Conversion of 2-hydroxychalcone (left) and selectivity of flavanone and chalcone (right) over HAM(9) as a function of reaction time.


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The results in Fig. 8 showed that the 2’-hydroxyacetophenone conversion by HAM(9) catalyst gradually increased up to 80% within 24 h of the initial reaction period. The HAM catalysts shows a relative high selectivity (92%-97%) to the targeted products, flavanone and chalcone. As expected, the catalytic activities (per weight of catalyst) of the HAM zeolites significantly surpass those of conventional MCM-41 and MFI zeolites (see Table 3). The conversion of 2’-hydeoxyacetophenone under HAM(9) could reach 86%, which is better than the result with unilamellar MFI nanosheets reported by Ryoo et al15. These enhanced catalytic activities can be attributed to a large number of acid sites incorporated into the mesoporous structure with the combination of uniform mesopores and framework acidity. For testing the catalytic stability, we recycled the HAM zeolites for three times, and the fluctuation of conversion was under 5%. Thus, the HAM zeolites possessed high stability.

Table 3. Catalytic conversion of bulky molecules over different catalysts. Reaction

Samples[a]

Conversion [b]

Multilamellar/Unilamellar MFI nanosheets (48/53)15

48(62/28/10)/ 76 (64/32/5)

Flavanone

Chalcone

MCM-41 (50)18

9 (58/42/0)

Conventional MFI (41)15

16 (50/50/0)

HAM(9)

86 (58/30/8) *

HAM(18)

63 (68/29/3) *

HAM(29)

77 (57/38/5) *

Catalytic activities are compared on the basis of the same weight of catalyst. [a] Numbers in brackets were the ratio of Si/Al. [b] Numbers in parentheses were selectivity: (flavanone/chaconne/others). * Numbers indicate the percentage reactant conversion, reproducible within 3% over three runs.

4. Conclusions


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In summary, we demonstrated a general and simple temperature-gradient strategy for the construction of uniform mesoporous zeolite using long-chain multi-ammonium surfactant as single structure-directing agent. Our synthetic method allows the generation of mesoporous zeolites with MFI zeolitic framework walls, balancing the intrinsic compromise in the growth of the zeolitic and mesoporous structure. It is possible to get hierarchical zeolites with as well as crystalline MFI domains using only one template and in one pot. The as-synthesized hierarchical zeolites exhibit improved catalytic performance and good hydrothermal stability, which could be developed as potential catalysts for acid-catalyzed conversion of bulky molecules.

ASSOCIATED CONTENT Supporting Information. Experimental procedure and characterization data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *[email protected] Acknowledgements We gratefully acknowledge the funding for this work provided by the National Natural Science Foundation of China (No. 21202116), Independent Innovation Foundation of Tianjin University of China (No. 2017XZY-0052), and Natural Science Foundation of Tianjin of China (No. 16JCYBJC20300).

Reference


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Table of Contents graphic Title: Hierarchical Hexagonal Zeolites: Intrinsic Transformation Directed by Precise Control of Synthetic Conditions Industrial & Engineering Chemistry Research Year, Volume, Page – Page He Ding, Zixing Xiao, Zixing Xiao, Jingshuang Zhang, Tianyi Fu,Peng Bai and Xianghai Guo *

Hierarchical hexagonal zeolites with narrow pore size distribution were obtained by a simple temperature-gradient method using a multi-ammonium surfactant as micro-mesopore generating agent. Catalytic performance of these materials was evaluated by the Claisen-Schmidt condensation of benzaldehyde and 2’-hydroxyacetophenone. An outstanding conversion as high as 86% by sample HAM(9) demonstrated synergic effect of micropores with highly active acid sites and mesopores for effectively transporting bulky reactants.


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Hierarchical Hexagonal Zeolites: Intrinsic Transformation Directed by

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