Confined Growth of Silicalite-1 Nanocrystals by

Copyright © 2013 American Scientific Publishers All rights reserved Printed in the United States of America

Advanced Porous Materials Vol. 1, 294–303, 2013

Confined Growth of Silicalite-1 Nanocrystals by Ethylenediamine-Induced Immobilization of Loaded Silica in Thermo-Shrinkable Carbonaceous Templates Chunfeng Xue1 ∗ , Jianwei Fan2 , and Xiaogang Hao1 ∗ 1

RESEARCH ARTICLE

Department of Chemical Engineering, College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan, 030024, P. R. China 2 State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai, 200092, P. R. China Zeolite with mesopore systems can facilitate large molecule diffusion and enhance the accessibility of their active centers in catalytic reactions. Many carbonaceous materials or composite were prepared as hard templates for introducing inter- or intracrystalline mesopores into zeolite crystals using different strategies. Interestingly, an elegant impregnation method for precursors is as crucial as excellent mesostructures of carbonaceous templates for preparing hierarchically porous zeolite. In the paper, a base-induced immobilization method of impregnated silica precursor was demonstrated for preparing zeolite crystals with mesopore system in a thermo-shrinkable carbon matrix. Volatile organic base ethylenediamine was applied to immobilize the loaded tetraethyl orthosilicate in mesopores of carbon template FDU-15, which was directly prepared from evaporation-induced co-assembly of phenolic resin by andPublishing amphiphilicTechnology surfactant. The encapsulated Delivered to: Guest User silica was “in-situ” IP: 162.218.208.135 Thu, 09 Oct 2014template 01:47:37 transformed into silicalite-1 nanocrystals in On: the thermo-shrinkable under the assistance of Copyright: American Publishers subtle vapor. High-resolution transmission electronScientific microscopy observations reveal that the obtained silicalite-1 nanocrystals are about 6 nm in size, which is consistent with pore size of the template. The results confirmed that silicalite-1 nanocrystals was formed in a well pre-defined space. The final products possess intercrystalline mesopores with diameters of 4.0 nm, which built with stacked zeolite crystals. The silicalite-1 zeolite with multi-point BET surface area of 269 m2 g−1 and pore volume of 0.76 cm3 g−1 was revealed by nitrogen isotherm sorption measurements. They also exhibit good hydrothermal stabilities when being exposed in 100% steam at high temperature of 700 C. The proposed method should be capable of synthesizing other zeolite with mesopore system by using given structure-directing agent.

Keywords: Zeolite, Impregnation, Encapsulation, Nanocrystal, Preparation.

1. INTRODUCTION Zeolites are a family of porous crystals and have become successful as catalysts for oil refining, petrochemistry, and organic synthesis of specialty chemicals.1–4 Their uniform micropores provide size and shape selectivity for guest molecules in reactions. However, the sole presence of micropores imposes diffusion limitations on reactants, intermediates and products in zeolites.4 Thus, only about 10% of active centers of zeolites was actually used in catalytic reactions.4 A reasonable approach to overcome such drawbacks would be incorporating mesopores in zeolites. Moreover, many reports have confirmed that the mesopore systems of given zeolites basically increased the ∗

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accessibility of large molecules to the external opening of micropores.1 2 4 In the last two decades, various strategies have been proposed for establishing intracrystalline and/or intercrystalline mesopore system in zeolites.1 5–23 Dealumination, desilication, and detitanation post-treatments were developed for creating intracrystalline mesostructures in conventional zeolites.1 4 24 Zeolite crystals containing mesopore systems within each individual crystal were prepared by inserting carbon particles into growing crystals.11 25 Carbon nanotubes or nanofibers were used for forming mesochannels throughout entire zeolite crystals.13 23 Using mesostructured carbon-silica composites as digestible templates, researchers obtained zeolites with intracrystalline mesoporosity.26–34 Mesopore systems 2327-3941/2013/1/294/010

doi:10.1166/apm.2013.1031

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Confined Growth of Silicalite-1 Nanocrystals by Ethylenediamine-Induced Immobilization of Loaded Silica

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were also built by incorporating silane-functionalized or Mesoporous carbons FDU-15 with uniform pores below cationic polymers in the zeolite crystals.16 35 Ryoo group 10 nm in diameter can be directly prepared through synthesized zeolite nanosheets by introducing amphiphilic evaporation induced co-assembly proces38–40 rather than 6 7 12 Pinnavaia organosilanes as supramolecular templates. replicas from silica templates.41 The mesopores can be group developed multi-step approaches including assemused as ideal nanoreactors for preparing uniform nanobling zeolite “seeds” into mesostructured materials and crystals, which are easy to form intercrystalline mesoembedding zeolite nanoparticles in pre-made mesoporous pores. An obvious framework shrinkage (more than silicates.9 10 However, some carbons were observed in 20%) for FDU-15 polymer occurred during its carbonizathe resultant materials, which are not entirely stable tion process.40 The thermo-shrinkable mesostructure may frameworks. encapsulate and immobilize guest species. Herein an Another popular strategy is to fabricate intercrystalline ethylenediamine-induced immobilization of impregnated mesopore systems in zeolites by packing nanosized cryssilica in a thermo-shrinkable carbon FDU-15 was develtals, which can obviously shorten diffusion paths of reacoped for zeolite nanocrystals featuring intercrystalline tants. Colloidal zeolites in the size range of 50∼100 nm mesoporosity. Interestingly, the dimensions of final zeolite can be prepared by a clear solution synthesis route.36 Hownanocrystals are coincident with those of mesopores of the ever, the difficulty in post-synthesis processing of colthermo-shrinkable template. loidal zeolites are the main stubborn issues that should be resolved in an economic style, which hinder their practical 2. EXPERIMENTAL DETAILS application.36 Only when zeolite crystals are below 50 nm 2.1. Chemicals in size, can significant mesoporosity be observed in the Tri-block poly(ethylene oxide)-b-poly(propylene Tri-block final products. In view of this point, several approaches involving solid templates were developed for preparing poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethynanosized zeolite crystals (< 50 nm). Anna group prepared lene oxide) copolymer Pluronic F127 (EO106 PO70 zeolite nanocrystals (∼ 20 nm) within voids of an inert EO106 was purchased from Aldrich. Other chemicarbon matrix.11 17 25 Colloidal imprinted carbon was also cals including phenol, formaldehyde, sodium hydroxide applied as a confined space for forming nanosized ZSM(NaOH), hydrochloric acid (HCl), tetraethyl orthosilicate 5.22 Tao et al. prepared nanosized zeolite MFI and LTA (TEOS), ethylenediamine (EDA), ammonia water (NH3 · Delivered by Publishing Guest User crystals by using disordered resorcinol-formaldehyde aero- Technology H2 O) andto: tetrapropylammonium hydroxide (TPAOH) were IP: 162.218.208.135 On: 21 Thu, 09 Oct 2014 01:47:37 gels and their carbonized analogues as templates.1 20 purchased from the Shanghai Chemical Co. Ltd. All chemCopyright: American Scientific Publishers Differing from colloidal zeolites, the products can be easicals were used as received without further purification. ily recovered by removing the carbon templates through a simple calcination. 2.2. Preparation of Carbonaceous Templates For a confined growth, an elegant impregnation method 2.2.1. Preparation of Carbon Precursor of precursors is as crucial as uniform mesopores of temResol with low molecular weight was polymerized from plates for preparing zeolite nanocrystals below 10 nm. phenol and formaldehyde and used as carbon precursor. Generally, precursors can be dispersed and/or confined Typically, phenol (30.5 g) was melted at 40 C, then NaOH in carbon matrices in the case of a low content. Howsolution (6.5 g, 20 wt%) was added with stirring in 10 min. ever, with a high precursor loading level for replicatAfter formaldehyde (52.6 g, 37 wt%) was added into the ing mesostructures of templates, the zeolite precursors mixture, polymerization was proceeded at 70 C for 1 h. are inclined to aggregate severely into bulk particles. After cooling the mixture to room temperature, its pH It is fairly challenging to immobilize highly concenvalue was adjusted to 7.0 by HCl solution. To separate trated zeolite precursors into pre-defined mesopores of carout NaCl, water in the mixture was removed under vacbons without aggregating outside pores during “in-situ” uum below 50 C and the resol was redissolved in ethanol transformation into zeolite. Therefore, most reports show to achieve a concentration of 20 wt%. Finally, the NaCl a mixture of aggregated large zeolite particles. Such drawpowder was removed by a centrifugation process. backs are largely a result of the lack of efficient control to avoid substantial diffusion/aggregation of the precur2.2.2. Preparation of Carbon FDU-15 Template sors during their impregnation process. Compared with Mesoporous carbon FDU-15 was prepared by evaporaversatile carbon templates, however, few impregnation tion induced co-assembly of phenolic resin and tri-block method was specially developed except incipient wetcopolymer Pluronic F127 according to the literature.40 In ness impregnation of raw materials11 17 25 or synthesis 16 20 22 27 29 30 35 37 a typical procedure, F127 (10 g) was dissolved in ethanol The key for obtaining ideal gel mixture. (160 g) and stirred for 1 h at 35 C to form a clear solunanocrystals is to effectively confine the precursors in tion. The resol (50 g, 20 wt%) containing phenol (6.1 g) pre-defined space. Therefore, it is necessary to design a and formaldehyde (3.9 g) was slowly added under stirring practicable impregnation technique for nanosized zeolite for 2 h. The mass ratio of F127/phenol/formaldehyde/EtOH crystals.


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was finally fixed at 10:6.1:3.9:200. The obtained mixture was poured into Petri dishes for forming membranes. It took 5–8 h to evaporate the solvent at room temperature and 20 h to thermopolymerize at 100 C. Finally, the sample was heated at 350 or 600 C for 3 h under N2 flow (60 cm3 s−1 to obtain FDU-15-350 or FDU-15600, respectively. The heating rate was 1 C min−1 below 600 C.

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vapor at 700 C for 6 h in a tubular oven. The resultant sample was denoted as NS-570-S700. To compare with NS, conventional silicalite-1 (CS) was also hydrothermally synthesized with molar composition of 18TPAOH:60SiO2 :1800H2 O. Briefly, TPAOH solution (11.3 g, 50 wt%) was dissolved into deionized water (44.4 g). After the solution was stirred for 0.5 h, TEOS (19.3 g) was added under vigorous stirring for 4 h. Finally, the obtained mixture was sealed into a PTFE-lined stainless-steel autoclave. After aged at room temperature overnight, it was heated at 130 C for 72 h. The sample was washed with deionized water, and dried at 100 C. Finally, it was calcined at 570 C for 6 h to remove the TPAOH template. The as-synthesized sample and calcined one were named as CS and CS-570, respectively.

2.3. Synthesis of Nanosized Silicalite-1 An EDA-catalyzed hydrolysis approach for immobilizing silica on FDU-15 template was developed for preparing nanosized silicalite-1 (NS). Here, FDU-15-350 (2.00 g) was activated at 150 C for 3 h in a vacuum drying oven and used as thermo-shrinkable template. After cooled to room temperature, it was immediately impregnated with 2.4. Measurements TEOS (1.22 g) according to its pore volume and heated at Small-angle X-ray scattering (SAXS) patterns were col150 C again for 0.3 h to evaporate the detained TEOS outlected on a Nanostar U small-angle X-ray scattering side mesopores. Then the mixture was cooled and mixed system using Cu K radiation at 40 kV and 35 mA. Highwith 2 ml of EDA solution, which was composed of angle X-ray diffraction (XRD) patterns were recorded on EDA, H2 O and EtOH at volume ratio of 17:2:1. Cata Brucker D8 powder X-ray diffractometer using Cu K alytic hydrolysis of the loaded TEOS by EDA was perradiation at 40 kV and 40 mA. The d-spacing values formed at room temperature for 0.2 h. After that, EDA was and unit-cell parameters (a0 of samples were removed from the mixture by heated at 100 C for 2 h. √ calculated by the formula d = 2/q and a = 2d / 3, respec0 100 Subsequently, the mixture was impregnated with TEOS tively. Transmission electron microscopy (TEM) images and EDA for three times according to the above procedure. were collected on JEOL 2011 and JEM-2100F electron After the multi-step impregnation was performed for given Technology to: Guest User Delivered by Publishing The ground samples for the measurements composite was On: car- Thu,microscopes. times, resultant SiO2 @FDU-15-350 IP: 162.218.208.135 09 Oct 2014 01:47:37 were suspended in Copyright: American framework shrink- Scientific Publishers ethanol and supported onto carbonbonized at 600 C in N2 flow. With the coated copper grids. Scanning electron microscope (SEM) age occurred, SiO2 was encapsulated by the mesopores images were collected on Philips XL30 electron microof carbon FDU-15-600 and a composite of SiO2 @FDUscope operated at 20 kV. The samples were coated with 15-600 was obtained. Its SiO2 content was measured as a thin gold film before the measurements. N2 adsorption about 42 wt% by using thermogravimitric analysis in air. isotherms of samples were measured with a Micromeritics Finally, TPAOH solution (1.71 g, 50 wt%) was added ASAP2020 M + C analyzer at −196 C. Before the meainto the SiO2 @FDU-15-600 composite at room tempersurements, all samples were degassed at 300 C in vacuum ature. The molar composition of the obtained mixture overnight. The pore volumes and pore size distributions was fixed at 18TPAOH:60SiO2 :203H2 O. The final mixwere derived from adsorption branches of isotherms by ture was transferred into an open polytetrafluoroethylene using non-local density functional theory (NLDFT) model. (PTFE) cup and sealed in a 50 ml PTFE-lined autoclave The Brunauer–Emment–Teller (BET) method was utilized containing 0.07 ml water to produce vapor. After aged to calculate specific surface area. Thermogravimitric (TG) at room temperature overnight, it was heated at 130 C analysis of sample was performed from 25 to 800 C in air for 72 h. After the autoclave was cooled to room temon a Mettler Toledo TG-SDTA851 analyzer with a heating perature, the resultant nanosized silicalite-1 (NS) sample rate of 5 C min−1 . (NS@FDU-15-600) was washed with deionized water, filtered, and dried at 100 C. The carbonaceous templates 3. RESULTS AND DISCUSSION were removed by calcination in a muffle furnace at 570 C for 6 h. The calcined NS@FDU-15-600 was denoted as 3.1. Mesoporous Carbon Template NS-570. Here, the temperature was elevated from room Ordered mesoporous carbon FDU-15 was prepared through temperature to 570 C at a ramp rate of 1 C min−1 . evaporation induced co-assembly of pre-polymerized pheFor comparisons, TPAOH solution (50 wt%) and ammonia nolic resin and Pluronic F127. The resin and F127 temwater (28 wt%) were also used to hydrolyze the loaded plate were dissolved in ethanol and the resultant solution TEOS. The corresponding SiO2 @FDU-15-600 composites was poured into dishes. After evaporation of ethanol and were also transformed into silicalite-1 molecular sieve as thermopolymerization, as-made FDU-15 membrane with mentioned above. Hydrothermal stability of NS-570 zeoirregular shape was collected from the dishes. After it was pyrolyzed at 350 C in N2 to remove the F127 template, lite was evaluated by exposing the sample in 100% water 296


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small-angle X-ray scattering (SAXS) patterns of obtained FDU-15-350 polymer show one strong and three wellresolved peaks, which could be indexed as 100, 110, 200 and 310 reflections of a 2-D hexagonal mesostructure with space group p6mm (Fig. 1(a)).40 41 The intense peak (100) reflects a d-spacing of ∼ 12.5 nm, corresponding to a unit-cell parameter a0 of 14.4 nm. With further carbonization at 600 C, the resultant FDU-15-600 show Technology to: Guest User Delivered bycarbon Publishing 3.2. of Nanosized Silicalite-1 three well-resolved peaks at high values in SAXS patIP:q162.218.208.135 On: Thu, 09Structures Oct 2014 01:47:37 After the SiO @FDU-15-600 composite was mixed with Copyright: American Scientific Publishers tern (Fig. 1(b)), suggesting a stable but shrunk mesostruc2 TPAOH solution, the encapsulated SiO2 was “in-situ” ture. The calculated value of its parameter a0 is 11.5 nm, transformed into nanosized silicalite-1 (NS) in mesopores indicating that there was about 20% framework shrinkof FDU-15-600 template under water vapor. The nanoage due to further crosslinking during the carbonization. sized silicalite-1@FDU-15-600 (NS@FDU-15-600) comHere, FDU-15-350 polymer was activated at 150 C in posite exhibits two peaks in its SAXS patterns, which were air and used as a thermo-shrinkable template. Tetraethyl similar to those of the SiO @FDU-15-600 composite, sugorthosilicate (TEOS) was selected as a silica precursor 2 gesting a stable mesostructure. After the templates includand impregnated into mesopores of the template. After the ing TPAOH and FDU-15-600 were removed by calcination TEOS detained outside mesopores was removed by heatat 570 C, however, no obvious peak was observed for NSing at 150 C, the TEOS loaded in mesopores was “in570 zeolite from SAXS pattern (Fig. 1(d)), implying a dissitu” hydrolyzed and converted into silica species under ordered mesostructure. It is reasonable that NS-570 zeolite the catalysis of ethylenediamine (EDA) solution, which did not replicate mesostructure of FDU-15-600 template was composed of EDA, H2 O and EtOH at the volume ratio owing to the poor interconnectivity between cylindrical of 17:2:1. Resultant composite is denoted as SiO2 @FDUmesopores of the template. 15-350. After it was carbonized at 600 C in N2 , resulX-ray diffraction (XRD) patterns of the NS@FDUtant SiO2 @FDU-15-600 composite shows a less-resolved 15-600 composite prior to the removal of templates are SAXS pattern with two peaks compared with FDU-15-600 illustrated (Fig. 2(a)). The composite contained silicalite-1, template (Fig. 1(c)), implying an inferior mesostructure. Table I. Properties of thermo-shrinkable carbonaceous templates and calcined silicalite-1 samples. Sample FDU-15-350 FDU-15-600 NS-570 CS-570

a0 [nm]

Smeso [m2 g−1 ]

SBET [m2 g−1 ]

Vmicro [cm3 g−1 ]

Vtotal [cm3 g−1 ]

D [nm]

Crystallinity

14.4 11.5 / /

408 392 171 68

650 743 269 181

0.081 0.18 0.069 0.076

0.65 0.53 0.76 0.10

7.2 5.0 0.48, 4.0 0.48

/ / 31% 100%

Notes: a0 is unit cell parameter; SBET is BET specific surface area; Vmicro is pore volume derived from micropores; D is pore diameter and Vtotal is total pore volume.


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Fig. 1. SAXS patterns of samples: FDU-15 template pyrolyzed at 350 (a), and 600 C (b), SiO2 @FDU-15-600 composite (c) and NS-570 zeolite (d).

Here, the reduced intensity was ascribed to the small contrast between carbon framework and pore void.42 N2 sorption isotherms of the FDU-15-350 templates obviously exhibit type IV curves with capillary condensation step at relative pressure P /P0 = 05–0.7 and an H1 -type hysteresis loop, which is typical of cylindrical mesopores.40 41 A narrow pore-size distribution with a mean value of 7.2 nm was calculated from the adsorption branch. Multi-point BET surface area and pore volume of FDU-15-350 template was 650 m2 g−1 and 0.65 cm3 g−1 , respectively (Table I). Among them, 408 m2 g−1 and 0.57 cm3 g−1 derived from ordered mesostructure. After carbonizations at 600 C, a capillary condensation step at P /P0 = 04–0.65 and an H1 -type hysteresis loop were observed for resultant FDU-15-600 template, implying shrunk mesopores. A narrow pore size distribution centered at 5.0 nm was obtained for the FDU-15-600 template. Based on FDU-15-350, about 30% pore shrinkage occurred during the carbonization. Multi-point BET surface area and pore volume of FDU-15-600 was 743 m2 g−1 and 0.53 cm3 g−1 , respectively. Among them, about 47% and 34% contributed from microstructure. Additionally, no N2 sorption isotherm curve was observed for the SiO2 @FDU-15-600 composite, implying that its pores were unavailable for N2 and might be blocked with the SiO2 precursors.

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Fig. 2. XRD patterns of NS@FDU-15-600 composite (a), NS-570 zeolite (b), NS-570-S700 zeolite steamed at 700 C vapor (c), and CS-570 zeolite (d).

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size of the template and indicating a confinement effect of the template. Morphologies of NS-570 crystals were different from the boat-shape of conventional silicalite-1 crystals.11 The results further confirm that the particles were formed in the cylindrical mesochannels. The highresolution TEM images of the NS-570 zeolite (Figs. 3(e) and (f)) show lattices of different nanoparticles, which confirm that the nanoparticles were crystallized to some extent. By combining the results with XRD analysis, some obvious lattices were indexed as crystal planes of (214), (253), (313), and (400) for silicalite-1. The adjacent NS-570 nanocrsytals were packed together and formed intercrystalline mesopores during calcination. Intercrystalline mesopore networks are exhibited in the domains, as labeled with capped letters “T ” (Fig. 3(e)). Triangular mesopores may be imprints of pore walls of the template. The pore size was about 4 nm, smaller than pore wall thickness (∼ 6.5 nm) of the template, which may result from slight aggregation of zeolite nanocrystals during the template removal. Therefore, it can be concluded that NS-570 nanocrystals (< 10 nm) featuring intercrystalline mesopore system were crystallized from the immobilized silica in thermo-shrinkable carbon matrix, and the EDA-catalyzed hydrolysis of the loaded TEOS was an effective procedure for depositing silica species in mesopores and preparing zeolite nanocrystals.

as can be judged from locations of diffraction peaks,7 11 suggesting that the encapsulated silica was converted into silicalite-1 zeolite. After templates were removed, main XRD peaks for NS-570 zeolite (Fig. 2(b)) were well coincident with those of conventional silicalite-1. Weak intensities and broadened peaks were observed for NS-570 zeolite, suggesting the presence of nanosized crystals. The Delivered byatPublishing crystallinity of NS-570 zeolite was estimated about 31% Technology to: Guest User Isotherms IP:reference 162.218.208.135 On: Thu,3.3. 09 N Oct 2014 01:47:37 2 Adsorption according to its XRD pattern by to that of conCopyright: American Scientific Publishers adsorption isotherms of NS-570 zeolite exhibited type I N 2 ventional silicalte-1 (CS-570) zeolite, as listed in Table I. overlapped type IV curves with several capillary condensaCalculated from Scherrer formula, the mean grain size of tion steps at different relative pressure ranges (Fig. 4A(a)), NS-570 zeolite was about 7 nm, basically matching with suggesting a hierarchical pore system. The uniform pore the mesopore size of FDU-15-600 template. The results size of ∼ 0.48 nm was calculated by using the NLDFT imply that the zeolite nanocrystals grew in the mesopores method (Fig. 4B(a)), which derived from silicalite-1 topolof the template. After being steamed at 700 C for 6 h ogy. The H1 hysteresis loop occurred at a relative presin 100% water vapor, resultant NS-570-S700 zeolite still sure range of P /P0 = 015–0.25, implying the presence of show broadened peaks in its XRD patterns similar to that intercrystalline mesopores. A pore size distribution with a of the sample NS-570 (Fig. 2(c)), indicating a good stabilmean value of 2.2 nm was calculated for NS-570 zeolites ity. Calcined conventional silicalte-1 (CS-570) shows typ(Fig. 4B(a)). Another hysteresis loop was observed at P /P0 ical peaks of MFI topology in XRD patterns (Fig. 2(d)), of above 0.45, corresponding to packed mesopores with coinciding with that reported previously.27 28 Strong intenlarge diameter. A pore size distribution with a mean value sities and sharp peaks were observed for the CS-570 zeoof 4.0 nm was calculated from the adsorption branch. The lite, suggesting the presence of large crystals. results were coincident with those concluded from TEM Transmission electron microscopy (TEM) images of the observations. Some macropores were also detected at high FDU-15-600 template show large domains of hexagonal pressure range, which were attributed to stacked NS-570 mesostructure, as viewed along [001] direction (Fig. 3(a)). crystals. Multi-point BET surface area and total pore volAn obvious stripe-like pattern was also observed along ume of the NS-570 zeolite were 269 m2 g−1 and about [110] direction (Fig. 3(b)), confirming an ordered hexago0.76 cm3 g−1 , respectively (Table I). Among them, about nal mesostructure. The results agree with those concluded surface area of 171 m2 g−1 and pore volume of 0.59 cm3 g−1 from SAXS analysis. TEM images of the NS-570 zeolite derived from intercrystalline mesopore system. display lots of nanosized particles across the whole domain On the other hand, CS-570 zeolite mainly showed type I (Figs. 3(c) and (d)). The results reveal that nanoparticurve with capillary condensation at low relative pressure cles were spatially located in the mesopores of the car(Fig. 4A(b)). A uniform pore size of about 0.48 nm was bon template without formation of any nanowires. The calculated from the adsorption branch (Fig. 4B(b)), which nanoparticles were spherical and their diameters were centered at about 6 nm, coinciding with the mesopore agreed with micropore feature of silicalite-1. A small 298


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Fig. 3.

TEM images of FDU-15-600 template (a), (b) and NS-570 zeolite (c)–(f).

adsorption at above P /P0 = 045 was also observed for CS-570 zeolite, indicating small amount of mesopores and macropores. The macropores may be contributed from aggregated zeolite particles. About surface area of 68 m2 g−1 (38%) derived from the aggregated structure. BET surface area of 181 m2 g−1 was calculated for CS-570 zeolite (Table I). Total pore volume of CS-570 zeolite was Adv. Porous Mater. 1, 294–303, 2013

0.10 cm3 g−1 , obviously smaller than that of NS-570 zeolite. By a simple calculation, only about 24% pore volume can be ascribed to the mesostructure. 3.4. Thermogravimetric (TG) Analysis TG curves of SiO2 @FDU-15-600 composite in air show that about 58 wt% of weight loss occurred at around 299

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Fig. 5. TG curves recorded in air for SiO2 @FDU-15-600 composite (a), NS-570 zeolite (the curve was vertically offset by −5 wt%) (b), and NS-570-S700 zeolite steamed at 700 C vapor for 6 h (c).

For example, HCl is often used to catalyze the hydrolysis of TEOS in preparing mesoporous silica materials such as SBA-15.41 NH3 · H2 O is selected to prepare silica microsphere in water-alcohol system.43 Generally, the hydrolysis of TEOS catalyzed by alkali is faster than that by acid. In present study, to effectively restrict silica species in the mesopores, the hydrolysis of loaded silica precursors was expected to: to be accomplished as soon as possible in a conDelivered by Publishing Technology Guest User addition to EDA solution, TPAOH soluIP: 162.218.208.135 On: Thu,trollable 09 Octstyle. 2014In01:47:37 Copyright: American Scientific tion and Publishers NH3 · H2 O were also used to hydrolyze the loaded TEOS. The corresponding SiO2 @FDU-15-600 composFig. 4. N2 adsorption–desorption isotherms (A) and pore size distribuites were transformed into silicalite-1 zeolite under the tions (B) of calcined zeolites: (a) NS-570 and (b) CS-570. same conditions. Sharp peaks with strong intensities were observed for obtained silicalite-1 zeolite in XRD patterns 500 C (Fig. 5(a)), mainly derived from the combustion (Fig. 6), indicating large crystals with good crystallinity. of FDU-15-600 carbon template. This result indicated that SEM images of resultant samples revealed many regular the template could be easily removed by calcination at crystals with about 10∼20 m in size (Fig. 7), which were above 500 C. The resultant SiO2 content of SiO2 @FDU15-600 composite was calculated as about 42 wt%. Calcined NS-570 zeolite showed weight loss of 8 wt% below 500 C, corresponding to the dehydration of silica species (Fig. 5(b)). Weight loss of about 2 wt% was observed above 500 C, implying the complete removal of FDU15-600 template by calcination. After NS-570 zeolite was steamed at 700 C for 6 h, resultant NS-570-S700 sample still showed weight loss of 10 wt% in TG curve, which was very similar to NS-570 zeolite (Fig. 5(c)). The results implied that the NS-570 zeolite was free of carbon residue after the calcination at 570 C. The TG and XRD analyses suggest that NS-570 zeolite possessed good hydrothermal stability in 100% water vapor at 700 C. 3.5. Selecting Catalyst for Hydrolyzing Silica Precursor It has been known that pre-hydrolysis of silica precursor such as TEOS can be catalyzed by acid or alkali. 300

Fig. 6. XRD patterns of silicalite-1 prepared from composites catalyzed by TPAOH solution (a) and ammonia water (b).


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Fig. 7.


SEM photographs of silicalite-1 prepared from composites catalyzed by TPAOH solution (a), (b) and ammonia water (c), (d).

obviously larger than the pore size of carbon template. NH3 · H2 O is volatile, it is a weak alkali with low conDelivered User media, some loaded TEOS was centration.to:InGuest a water-rich The results further confirmed that some of by thePublishing loaded sil- Technology IP: 162.218.208.135 On: Thu, 09 Oct 2014 01:47:37 easily transferred out of the mesopores of the template durica was crystallized outside the mesopores of FDU-15-600 Copyright: American Scientific Publishers ing the low rate hydrolysis catalyzed by NH3 · H2 O. In a template. word, to effectively immobilize the silica species in preAccording to the results above, EDA was a good catformed mesopores, a volatile, high concentration, strong alyst for “in-situ” hydrolysis of loaded silica precursors alkali was a good candidate of hydrolysis catalyst for in the carbon template, which might be ascribed to its multistep impregnation of silica precursors. Here, EDAproperties. EDA is an organic alkali and able to speed catalyzed hydrolysis of loaded TEOS effectively restricted up the hydrolysis and deposition of the loaded TEOS at silica species in the carbon mesopores. The silica immobiroom temperature, which would greatly squeeze times and lized in pre-defined mesopores was convenient to be furopportunities for the possible transfer and migration of ther crystallized into zeolite nanocrystals. loaded silica precursors. Secondly, EDA is accessible and easy to give solutions with desired pH value. In this study, an EDA solution with high concentration was necessary 3.6. EDA-Catalyzed Hydrolysis for for rapid hydrolysis at room temperature. Generally, NH3 · Immobilizing Silica H2 O and TPAOH are available in an aqueous style with According to the results above, EDA-catalyzed hydrollots of water (> 50 wt%), which is inconvenient to conysis of impregnated silica precursors in mesoporous trol the possible transfer of the loaded precursor. Multicarbon templates was an effective approach for constep impregnation was needed for loading high content of fined growth of zeolite nanocrystals. The method for silica species. Therefore, the catalysts for “in-situ” hydrolhydrolyzing the impregnated silica towards zeolite nanoysis were needed to be completely removed by a simcrystals mainly underwent following steps: impregnation, ple method before next impregnation of silica precursor. hydrolysis, encapsulation, crystallization, and calcination It was noted that EDA is volatile and easy to be evaporated (Scheme 1). Firstly, mesostructured carbon template by heating. Thus, TEOS could be smoothly impregnated was prepared through co-assembling triblock copolymer once again after the removal of the remained EDA. On Pluronic F127 and pre-polymerized resol. Compared with the other hand, TPAOH with large molecule weight was CMK-5, mesoporous carbon FDU-15 was easier to be difficult to evaporate completely. The remained TPAOH prepared as a template.44 45 After activated FDU-15 temwould detain silica precursor and keep it outside mesoplate was impregnated with silica precursor TEOS (boiling pores. Thus, large zeolite crystal was produced outside the point, 165.5 C), the detained TEOS outside the mesopores was removed by heating at around 150 C. Subsequently, carbon template during crystallization process. Although Adv. Porous Mater. 1, 294–303, 2013

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Scheme 1. EDA-induced immobilization of impregnated silica precursor aiming for confined growth of zeolite nanocrystals featuring intercrystalline mesopore systems.

In summary, an ethylenediamine-induced hydrolysis procedure for immobilizing silica precursors in thermoshrinkable carbon scaffolds was demonstrated for preparing zeolite nanocrystals. The volatile, strong organic alkali acted as a good candidate of hydrolysis catalyst for “in-situ” hydrolyzing silica sources and restricting them in the predefined mesopores. Silicalite-1 nanocrystals were formed in the confined space after the loaded silica was “in-situ” transformed under subtle vapor. The nanocrystals with size of about 6 nm had surface area of 269 m2 g−1 , pore volume of 0.76 cm3 g−1 and dual sets of pores centered at 0.48 and 4.0 nm. They also showed a good stability in 100% water vapor at 700 C.

Acknowledgments: This work is supported by Tal“in-situ” hydrolysis of the loaded TEOS was performed ents Foundation of Taiyuan University of Technology rapidly in the mesochannels of FDU-15 template after (No. TYUT-RC201113A), and China Postdoctoral SciEDA solution was added. As a result, silica species were ence Foundation (No. 20100480532). We thank Profesrestricted in the mesopores and a pseudo-core–shell strucsor Dongyuan Zhao in Fudan University and Professor tured composite SiO2 @FDU-15-350 was formed. By a Xuguang Liu in Taiyuan University of Technology for their simple calculation, the “in-situ” hydrolysis of the loaded discussion. TEOS to silica was accompanied by significant volume shrinkage, with ∼ 88% of the pore space pre-occupied by References and Notes the precursor being released after the conversion. Thus, the 1. Y. Tao, H. Kanoh, L. Abrams, and K. Kaneko, Chem. Rev. 106, 896 SiO2 @FDU-15-350 composite still possessed high meso(2006). porosity for the impregnation of TEOS and EDA once 2. A. Corma, Chem. Rev. 97, 2373 (1997). @FDU-15- Technology again. After the hydrolysis finished, the SiO Delivered by 2Publishing to: Guest 3. M. E. Davis, NatureUser 417, 813 (2002). Egeblad, C. 01:47:37 H. Christensen, M. Kustova, and C. H. Christensen, 350 composite was activated and remained EDA On: was Thu,4.09K.Oct IP: the 162.218.208.135 2014 Chem.Publishers Mater. 20, 946 (2008). Copyright: C. The Scientific removed for next impregnation by heating at 150American 5. A. Karlsson, M. Stocker, and R. Schmidt, Micropor. Mesopor. Mater. impregnation of TEOS and corresponding hydrolysis were 27, 181 (1999). repeated for four times. After the final SiO2 @FDU-15-350 6. K. Na, M. Choi, W. Park, Y. Sakamoto, O. Terasaki, and R. Ryoo, composite was carbonized at 600 C, the loaded SiO2 was J. Am. Chem. Soc. 132, 4169 (2010). 7. M. Choi, K. Na, J. Kim, Y. Sakamoto, O. Terasaki, and R. Ryoo, tightly encapsulated by mesopore walls because polymeric Nature 461, 246 (2009). frameworks of FDU-15-350 template is further shrank 8. L. Wang, Z. Zhang, C. Yin, Z. Shan, and F. S. Xiao, Micropor. during the carbonization. Framework shrinkage ratio of Mesopor. Mater. 131, 58 (2010). FDU-15-600 templates was calculated as about 20% from 9. Y. Liu and T. J. Pinnavaia, Chem. Mater. 14, 3 (2002). 10. T. O. Do, A. Nossov, Springuel-Huet, M. A. C. Schneider, J. L. their unit-cell parameters (Table I), which agreed with preBretherton, C. A. Fyfe, and S. Kaliaguine, J. Am. Chem. Soc. 126, viously reported results.40 The obtained SiO2 @FDU-1514324 (2004). 600 composite was further mixed with structure-directing 11. Claus J. H. Jacobsen, C. Madsen, J. Houzvicka, I. Schmidt, and agent TPAOH solution at room temperature and crysA. Carlsson, J. Am. Chem. Soc. 122, 7116 (2000). tallized in a sealed autoclave containing subtle vapor. 12. M. Choi, H. S. Cho, R. Srivastava, C. Venkatesan, D. H. Choi, and R. Ryoo, Nat. Mater. 5, 718 (2006). It was expected that an equilibrium can be established 13. I. Schmidt, A. Boisen, E. Gustavsson, K. Ståhl, S. Pehrson, S. Dahl, between the vapor and water in the composite during heatA. Carlsson, and C. J. H. Jacobsen, Chem. Mater. 13, 4416 (2001). ing process. The vapor–liquid equilibrium could ensure 14. W. Fan, M. A. Snyder, S. Kumar, P. S. Lee, W. C. Yoo, A. V. the conversion of SiO2 to zeolite in the confined space. McCormick, R. L. Penn, A. Stein, and M. Tsapatsis, Nat. Mater. 7, 984 (2008). Meanwhile, the carbon framework could restrict the migra15. R. Srivastava, M. Choi, and R. Ryoo, Chem. Commun. 4489 (2006). tion and aggregation of nanosized zeolite during the 16. H. Wang and T. J. Pinnavaia, Angew. Chem. 118, 7765 (2006). crystallization. Finally, zeolite nanocrystals featuring inter17. I. Schmidt, C. Madsen, and C. J. H. Jacobsen, Inorg. Chem. 39, 2279 crystalline mesopores were obtained after all the carbona(2000). ceous templates were removed by calcination at 570 C. 18. X. T. Wei and P. G. Smirniotis, Micropor. Mesopor. Mater. 89, 170 (2006). The nanocrystal size of about 6 nm coincided with the 19. W. C. Li, A. H. Lu, R. Palkovits, W. Schmidt, B. Spliethoff, and pore size of FDU-15-600 template. The pathway described F. Schüth, J. Am. Chem. Soc. 127, 12595 (2005). here should be capable of synthesizing other zeolite nano20. Y. S. Tao, Y. Hattori, A. Matumoto, H. Kanoh, and K. Kaneko, crystals such as FAU by replacing the structure-directing J. Phys. Chem. B 109, 194 (2005). agent. 21. Y. Tao, H. Kanoh, and K. Kaneko, Langmuir 21, 504 (2005). 302


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Received: 19 September 2013. Accepted: 18 November 2013.

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