Preparation of Interlayer-Expanded Zeolite from Lamellar Precursor

Nov 5, 2013 - The SEM images were taken on a Hitachi S-4800 microscope. The amounts of Si and Al were ...... Design of Microporous Material HUS-10 wit...
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Preparation of Interlayer-Expanded Zeolite from Lamellar Precursor Nu-6(1) by Silylation Jin-gang Jiang,† Lili Jia,† Boting Yang, Hao Xu, and Peng Wu* Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, North Zhongshan Road 3663, Shanghai 200062, People’s Republic of China S Supporting Information *

ABSTRACT: Interlayer-expanded zeolite, IEZ-NSI, has been post-synthesized from a two-dimensional (2D) layered precursor Nu-6(1) (Si/Al ratio = 30 − ∞) by intercalating with diethoxydimethylsilane in hydrochloric acid solution. Ethanol was demonstrated to be a suitable solvent for inducing an effective interlayer pillaring with monomeric silane. IEZNSI was revealed to possess a 3D crystalline structure composed of 10-membered ring (MR) channels by using Rietveld crystal structure refinement of the PXRD pattern of IEZ-Nu-6(1)-H (Si/Al = ∞). The 10-MR pores in IEZ-NSI were constructed through the linkages between the pillaring Si atoms and the NSI layers. IEZ-Nu-6(1)-H(Si/Al = ∞) crystallizes in space group P121/a1 (No. 14) with a = 22.9372(4) Å, b = 5.0205(5) Å, c = 13.8209(1) Å, and β = 103.294(22)°. IEZ-NSI with more accessible channels gave a higher catalytic activity in the esterification of acetic acid with ethanol in comparison to Nu-6(2). KEYWORDS: silylation, lamellar zeolite, Nu-6(1), interlayer pore expansion and UCB-221 have been prepared successfully from MCM22(P) and PREFER precursors, respectively, although preswelling at pH 9 in a solution containing surfactant, F− ions, and Cl− anions. The zeolite ZSM-35 can be obtained from the FER layer reassembly.22 Corma et al. developed a novel hybrid zeolitic material (MWW-BTEB) by pillaring the swollen MCM22 material with 1,4-bis(triethoxysilyl)benzene (BTEB).23 Interlayer silylation of the precursors with monomeric silane proves to be a versatile technique for post-synthesizing new zeolites with larger porosities. Interlayer-expanded zeolites (IEZ) have been prepared from various precursors such as MWW(P), PREFER, PLS-1, MCM-47,18 RUB-36,24 and RUB39.25 In some cases, the acid treatment in the absence of silane may also convert the lamellar precursors to interlayer expanded structures, in which soluble and/or removable silicon species from the crystals are assumed to serve as pillars supporting the zeolite layers or sheets. The typical examples include Ti-YUN117 and APZ materials.26 These post-synthesized zeolites have expanded pore windows, high crystallinity, and outstanding hydrothermal stability, and then they serve as promising heterogeneous catalysts for processing bulky molecules as well as selective adsorbents for adsorption and separation usages. Lamellar precursor Nu-6(1), synthetized using 4′4-bipyridine as SDA, was converted by calcination into Nu-6(2), a 3D zeolite with the NSI topology.27,28 The Nu-6(1) precursor is formed by [SiO4] and [SiO3OH] tetrahedral units, with two

1. INTRODUCTION There are currently more than 200 types of zeolites with welldefined crystalline structures,1 some of which originate from zeolitic lamellar precursors, and these are forming an important family in microporous materials. The precursors with 2dimensional (2D) layered structures, e.g., MCM-22,2 PREFER,3 PLS-1,4 Nu-6(1),5 EU-19,6 RUB-15,7 RUB-18,8 RUB39,9 PreAFO,10 etc., are converted to three-dimensional (3D) crystalline zeolites through topotactic transformation by calcination. Recently, new lamellar materials have also been developed. The germanosilicate IM-12 with the UTL topology can be converted into a lamellar material as the Ge-rich double 4-MR units turn to be hydrolyzed easily under hydrothermal conditions.11 On the other hand, multilayered MFI zeolites are hydrothermally synthesized readily by using specially designed bifunctional quaternary ammoniums as structure-directing agents (SDAs).12 The lamellar precursors with malleable features could be reconstructed to new crystalline structures by postsynthesis treatments. A series of methods have been developed, e.g., full or partial phase delamination,13−15 interlayer pillaring with amorphous silica,16 and inserting a monomeric Si species originating from the framework or adscititious silanes into the interlayer spaces.17,18 Full phase delamination and silica pillaring are based on the surfactant-assisted preswelling of the lamellar precursors in a strong basic media at high temperatures.14,16 The swelling of MCM-22(P) precursor can also be realized at room temperature, but the swollen material is only useful for pillaring with silica, but not suitable for delamination.19 Recently, two delaminated materials UCB-120 © 2013 American Chemical Society

Received: July 6, 2013 Revised: November 4, 2013 Published: November 5, 2013 4710

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on an Elementar VarioEL III CHN elemental analyzer. 29Si and 13C solid-state MAS NMR spectra were recorded on a Varian Model VNMRS-400WB spectrometer under one pulse condition and crosspolarization, respectively. 29Si NMR spectra were acquired with a 7.5 mm T3HX probe at 79.43 MHz and a spinning rate of 3 kHz. The chemical shift was referred to 2,2-dimethyl-2-silapentane-5-sulfonic acid sodium salt ((CH3)3Si(CH2)3SO3Na). 13C NMR spectra were recorded with a 7.5-mm T3HX probe at 100.54 MHz and a spinning rate of 5 kHz. 2.4. Synchrotron Radiation XRD Experiment and Structure Refinement. Synchrotron radiation XRD analysis was performed on the IEZ-Nu-6(1)-H. High-resolution powder diffraction data were collected at room temperature with Synchrotron Beamline 14B at Shanghai Synchrotron Radiation Facility (SSRF). The wavelength of the incident monochromatic X-ray is 1.2438 Å. To improve preferred orientation of the sample, the transmission geometry were used with 0.5 mm glass capillary sample holders. The effective X-ray energy is 10 keV. Further details of the diffraction experiment are summarized in Table 1.

crystallographically independent 4′4-bipyridine molecules located between layers. After transformation to Nu-6(2), through interlayer condensation by calcination, two different interlayer 8-MR pores are formed, which have relatively narrow entrances (2.4 Å × 4.8 Å and 3.2 Å × 4.3 Å).28 The catalytic application of Nu-6(2) is limited because of its small porosity. A delaminated derivative ITQ-18 was thus post-synthesized by expanding and exfoliating the lamellar precursor Nu-6(1). Possessing a high surface area (588 m2 g−1), ITQ-18 offered a higher activity in synthesis of diamino diphenyl methane (DADPM) for producing polyurethanes.29−31 On the other hand, delaminated Nu-6(1) with a lower surface area (151 m2 g−1) has been applied to the reaction of dehydration of xylose into furfural.32 In addition, extracting the SDA species in Nu6(1) gives rise to a derivative MCM-39, which can be further converted to intercalated, swollen, and pillared materials.33 The preparation of interlayer-expanded zeolite from Nu-6(1) by silylation has not yet been reported. After removing the intercalated SDA species in Nu-6(1), the interlayer pores become much narrower, which poses difficulties to incorporating guest silane molecules. In this study, we found a suitable treatment media for interlayer silylation of Nu-6(1) and succeeded in preparing IEZ-Nu-6 materials.

Table 1. Experimental Parameters for Structure Analysis of IEZ-Nu-6(1)-H (Si/Al = ∞) parameter sample unite cell composition conditions of data collection diffractometer sample holder wavelength 2θ range step size number of points total number of reflections fwhm [deg] peak profile number of profile parameters number of structural parameters lattice parameters a b c β number of atoms space group Rwp Rwp (w/o bck) Rp

2. EXPERIMENTAL SECTION 2.1. Synthesis of Nu-6(1) Lamellar Precursors. The lamellar precursors of Nu-6(1) were synthesized following the procedures reported previously in the literature.28 4′4-Bipyridine (1.82 g) was dissolved in 10.08 g ethanol to form solution A. Then, 18.45 g of sodium silicate (27.099% SiO2, 8.693% Na2O) was diluted with 14.92 g of distilled water to give solution B. Aluminum sulfate (0, 0.31, 0.62, and 0.92 g corresponding to gel Si/Al ratios of infinite, 90, 45, and 30, respectively) and 1.52 g of sulfuric acid were dissolved in 22.78 g of distilled water to form solution C. Then, solution B and C were added dropwise into solution A under mechanical stirring. The final gels were crystallized in a Teflon-lined stainless steel autoclave under rotation (100 rpm) at 408 K for 3 days. The products were collected by filtration, washed with water, and dried overnight; the result was Nu6(1) precursors with different Al contents. Two-dimensional (2D) lamellar Nu-6(1) precursors were calcined at 823 K for 6 h, resulting in three-dimensional (3D) Nu-6(2) zeolite with the NSI topology. 2.2. Interlayer Silylation of Nu-6(1) Precursors. The Nu-6(1) precursors were silylated with Me2Si(OEt)2 (DEDMS) using 2 M HCl either in water or EtOH. The solid-to-liquid weight ratio was fixed at 1:50, whereas the amount of DEDMS was varied from 0 to 0.16 g per gram of precursor. The mixture was retreated in a Teflon-lined stainless steel under static conditions at 403−473 K for 1 day. To investigate the solvent effects on the formation of an interlayer expanded structure, the silylation procedures were traced in water or ethanol as a function of treatment time (from 15 min to 24 h). The treated samples were filtered, washed with water, and then dried at 363 K overnight. The interlayer expanded zeolite products were named as IEZ-Nu-6(1). They were further calcined in air at 423−823 K, giving rise to IEZ-Nu-6(2). 2.3. Characterization Methods. Powder X-ray diffraction (XRD) patterns were collected on a Rigaku Ultima IV X-ray diffractometer using Cu Kα1 radiation (λ = 1.5405 Ǻ ). Nitrogen adsorption isotherms were recorded at 77 K on a BELSORP-MAX instrument after activating the calcined samples at 573 K or the dried samples at 393 K under vacuum for at least 10 h. The SEM images were taken on a Hitachi S-4800 microscope. The amounts of Si and Al were quantified by inductively coupled plasma (ICP) spectroscopy on a Thermo IRIS Intrepid II XSP atomic emission spectrometer. The thermogravimetric and differential thermal analyses (TG-DTA) were performed on a Mettler−Toledo Model TGA/SDTA851e apparatus from room temperature to 1073 K at a heating rate of 10 K min−1 in air. The elemental analyses for carbon and nitrogen contents were performed

value/remark IEZ-Nu-6(1)-H (Si/Al = ∞) Si26O56H8 room-temperature data collection SSRF BL14B1 capillary 1.2438 Å 2°−48° 0.0135° 2θ 3407 457 0.168° (2θ = 16.02°) pseudo-Voigt 8 123 22.9372(4) Å 5.0205(5) Å 13.8209(1) Å 103.294(22)° 24 P121/a1 (No. 14) 6.04% 18.97% 4.53%

Based on the indexed powder pattern, a structure model for IEZNu-6(1)-H was derived, adapted from the layered precursor Nu-6(1), and energy-minimized using high-level density functional theory (DFT) calculations (see the Supporting Information). The calculations were performed with the quantum mechanical code Dmol3 to optimize the structure. The exchange-correlation functional is expressed by the generalized gradient corrected (GGA) functional with the Perdew−Burke−Ernzerhof parametrization.34 Subsequently, the coordinates of the optimized tetrahedral network were used as starting parameters for the Rietveld refinement of the XRD data, using the Reflex powder refinement module in the Materials Studio suite of programs.35 Further details of the diffraction experiment and the fractional coordinates obtained from the Rietveld analysis are summarized in Tables 1−5. 2.5. Catalytic Reaction. The esterification of acetic acid and ethanol was carried out in a fixed bed reactor under atmospheric pressure conditions. The zeolite powder (0.2 g) was placed in a tubular quartz reactor that had been pretreated in N2 flow (30 mL h−1) at 533 4711

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Table 2. Comparison of Unit-Cell Parameters among IEZ-Nu-6(1)-H, lamellar Precursor Nu-6(1), and Zeolite Nu-6(2) sample

a (Å)

b (Å)

c (Å)

α (deg)

β (deg)

γ (deg)

IEZ-Nu-6(1)-H Nu-6(1) Nu-6(2)

22.9372(4) 27.7287(6) 17.257(2)

5.0205(5) 4.9731(1) 4.9881(4)

13.8209(1) 13.9350(2) 13.848(1)

90 90 90

103.294(22) 103.73(1) 106.09(1)

90 90 90

free volume between adjacent layers. The calcined sample then possessed a much narrower pore entrance between the layers than the original lamellar precursor. In fact, the 8-MR pores thus formed are of extremely distorted shape in the NSI structure. The structural reconstruction by calcination also altered the XRD diffractions in the wide angle region. Nevertheless, the diffractions without correlation to the a-axis remained almost intact. For example, the [002] reflection was observed at the same position for both Nu-6(1) and Nu-6(2), irrespective of a large shift of the [200] one. Thus, it is presumed that the structure change essentially occurred along the layer stacking direction. When the precursor Nu-6(1) was silylated with DEDMS, a new structure was obtained as evidenced by XRD investigation (Figures 1c−f). Compared with the 3D Nu-6(2) zeolite (Figure 1b), the silylated samples showed the layer-related diffractions at lower angles. However, the [200] diffraction was relatively broad and low in intensity when the silylation was carried out in an aqueous solution (Figure 1c), although the same condition always led to well-ordered interlayer expanded structures when applied to the silylation of other precursors such as MCM22(P), PREFER, RUB-39, RUB-36, PLS-1.16,24,25 Thus, the silylation of Nu-6(1) in aqueous solution only constructed a relatively disordered interlayer expanded structure. The interlayer expansion and pillaring by silylation are considered to take place only when the extraction rate of host molecules well matches that of the incorporation of guest silane molecules. As mentioned above, the interlayer spacing was narrowed greatly when Nu-6(1) was calcined to remove the bulky SDA species of 4′4-bipyridine, implying the pore entrance was easily closed to a great extent. Thus, lowering the extraction rate for the SDA species would be helpful to introduce the silane molecules into the layer spaces and to obtain more-ordered structures. This strategy was realized here by adjusting the rate of SDA extraction by increasing the portion of ethanol in silylation media. With increasing volume of ethanol added, the layer-related diffractions became sharper and more ordered (see Figures 1d and 1e). In particular, in ethanol solution (with water only from hydrochloric acid), a well-ordered expanded structure was formed, showing an intensive and sharp [200] diffraction (Figure 1f). Different to the other zeolite precursors, the silyation media are thus critical to induce the interlayer silyaltion for Nu-6(1). Chemical element analyses indicated that the precursor Nu6(1) contained 14.68 wt % carbon and 3.08 wt % nitrogen species, corresponding to a C/N ration of 4.8, which was very close to the C/N ratio in 4′4-bipyridine. It is deduced that the original molecular structure of SDA was preserved inside zeolite pores after crystallization. The mass percentage of the nitrogen specie decreased with prolonging the silylation time (Figure 2), simply due to a gradual removal of organic species located in interlayer spaces by acid extraction. The removal of the organic species occurred extremely rapidly at initial stage. Within first 75 min, the SDA species were extracted by 98% when the silylation was conducted in water (Figure 2a). After that, the nitrogen content decreased gently. In contrast, the removal of

Table 3. Final Fractional Coordinates for IEZ-Nu-6(1)-H after DFT Calculationa atom

x/a

y/b

z/c

Si1 Si2 Si3 Si4 Si5 Si6 Si7

0.31868 0.63250 0.29005 0.31574 0.37016 0.71005 0.49926

0.20354 0.81090 0.53065 0.70753 0.68139 0.96698 0.91337

0.30213 0.27750 0.00733 0.80229 0.22282 0.49367 0.21447

O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12 O13 O14 O15

0.34142 0.36746 0.65934 0.72911 0.32131 0.44048 0.36488 0.22954 0.32862 0.32131 0.32664 0.25156 0.56095 0.49689 0.50079

0.56088 0.44718 0.93025 0.2831 0.61566 0.75826 0.95088 0.71536 0.94583 0.11395 2.45395 0.32516 0.72652 0.9254 0.19743

0.11108 0.30597 0.38907 0.50682 0.91708 0.23572 0.80496 0.00911 0.23337 0.41624 0.73237 0.24874 0.25924 0.09502 0.27219

H1 H2

0.52892 0.46893

0.34139 0.04526

0.26748 0.05169

a

The calculations start from the proposed crystal structure. Unit-cell parameters: a = 22.82 Å, b = 5.01 Å, c = 13.82 Å, β = 103.78°, space group P121/a1 (No. 14).

K for 2 h. The reactor temperature was then decreased to 473 K; at this point, the mixture of ethanol (99.7%) and acetic acid (>99.9%) was fed into the reactor at a molar ratio of 3:1 and a feeding rate of 1.2 mL h−1. The reaction products were collected by a cooling trapper and analyzed on a gas chromatograph (Shimadzu Model 2014, FID detector) equipped with a 30 m DB-WAX capillary column.

3. RESULTS AND DISCUSSION 3.1. Interlayer Silylation of Nu-6(1). Figure 1 shows the XRD patterns of Nu-6(1) precursor (pure silicalite) before and after calcination, as well as those of after the silylation with DEDMS in water, ethanol, and their mixture. The precursor Nu-6(1) exhibited the characteristic [200] diffraction at 2θ = 6.6°, because of the layered structure in the low-angle region (Figure 1a), which was in agreement with the literature.28 After a direct calcination in air, the precursor was converted to Nu6(2) with the 3D NSI topology. The layer-related [200] diffraction was shifted to a higher 2θ region 2θ = 10.5° (Figure 1b). The structural change implied that the interlayer silanol groups were then reduced to form cross-linkages between after burning off the organic SDA species pillaring the NSI sheets. The direct calcination made the d spacing of the [200] plane decrease from 13.54 Å to 8.47 Å, indicating a large shrinkage of 4712

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Table 4. Final Fractional Coordinates with Site Occupancy Factors (SOF) for IEZ-Nu-6(1)-H after Rietveld Analysis

a

atom

x/aa

y/ba

z/ca

Uisob

SOF

Si1 Si2 Si3 Si4 Si5 Si6 Si7

0.31675(56) 0.63153(56) 0.28876(69) 0.31427(64) 0.36898(54) 0.70920(50) 0.49845(57)

0.21072(194) 0.81078(170) 0.54214(329) 0.70859(163) 0.68829(196) 0.96231(281) 0.91724(153)

0.30079(61) 0.27796(93) 0.00673(53) 0.80194(68) 0.22262(52) 0.49454(105) 0.21497(66)

0.058 0.018 0.023 0.013 0.023 0.230 0.031

1 1 1 1 1 1 0.5

O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12 O13 O14 O15

0.34012(62) 0.36589(53) 0.65849(52) 0.72726(53) 0.31964(72) 0.43942(55) 0.36377(67) 0.22850(65) 0.32781(53) 0.31919(54) 0.32484(56) 0.24982(55) 0.55985(56) 0.49616(65) 0.49890(62)

0.56985(253) 0.45312(171) 0.92810(213) 0.27783(297) 0.61518(203) 0.76330(163) 0.95072(188) 0.72829(367) 0.95359(212) 0.11966(158) 0.45562(150) 0.33461(250) 0.72836(149) 0.93080(182) 0.20058(146)

0.11060(51) 0.30524(53) 0.38981(96) 0.50622(122) 0.91651(69) 0.23579(56) 0.80521(67) 0.00882(64) 0.23368(61) 0.41468(65) 0.73149(62) 0.24758(65) 0.25942(77) 0.09557(69) 0.27112(77)

0.036 0.041 0.154 0.068 0.061 0.052 0.015 0.067 0.011 0.194 0.010 0.047 0.104 0.07 0.055

1 1 1 1 1 1 1 1 1 1 1 1 1 0.5 0.5

H1 H2

0.52725(60) 0.46694(82)

0.34391(146) 0.05190(209)

0.26676(77) 0.05029(84)

0.063 0.052

1 1

EFA1c EFA2c EFA3c

0.40100 0.25135 0.57979

0.41470 0.15255 0.24929

0.53031 0.24535 0.93243

The number between brackets gives the esd. bUiso represents the isotropic thermal parameters. cEFA extra-framework electron density maxima.

Table 5. Lattice Constants Bond Distances and Angles of the Refined and Simulated IEZ-Nu-6(1)-H Lattice Parameters a (Å) b (Å) c (Å) β (deg) Bond Distance (Å) minimum Si−Si maximum Si−Si minimum Si−O maximum Si−O Bond Angle (deg) minimum Si−O−Si maximum Si−O−Si minimum O−Si−O maximum O−Si−O

DFT

Rietveld Refinement

22.82 5.01 13.82 103.78

22.9372(4) 5.0205(5) 13.8209(1) 103.294(22)

2.9877 3.1946 1.6183 1.6811

2.9910 3.2097 1.5948 1.7289

128.733 157.982 102.410 116.930

129.024 157.987 102.708 116.159

Figure 1. XRD patterns of Nu-6(1) precursor (spectrum a), directly calcined sample Nu-6(2) (spectrum b), and as-silylated Nu-6(1) with DEDMS and 2 M HCl in water (spectrum c), ethanol/H2O (1:1) (spectrum d), ethanol/H2O (2:1) (spectrum e), and ethanol (spectrum f). Other conditions: Nu-6(1), 0.5 g; DEDMS, 0.08 g; temperature, 473 K; and time, 24 h.

SDA was obviously slow in ethanol, showing a relatively tempered downtrend (Figure 2b). Thermogravimetric investigation was identical to the CHN analysis results (see Figure S1 in the Supporting Information). Thus, it is assumed that, before the silane molecules are introduced into the layers to pillar the structure, removing the organic species too fast would close or narrow the interlayer entrance, and then result in an insufficient silylation. In other words, the rate matching

between SDA removal and silane incorporation should be carefully controlled to expand the interlayer pore structure for Nu-6(1). Employing ethanol as a suitable solvent for the interlayer silylation, we have investigated the effects of various treatment 4713

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Figure 2. Dependence of remaining nitrogen content in Nu-6(1) on the silylation time in water (curve a) and in ethanol (curve b). The Nu-6(1) precursor contains 3.08 wt % nitrogen from SDA. Others silylation conditions: Nu-6(1), 0.5 g; DEDMS, 0.08 g; and temperature, 473 K.

Figure 4. XRD patterns of Nu-6(1) precursor (spectrum a), assilylated with DEDMS in ethanol using 0.5 M HCl (spectrum b), 1 M HCl (spectrum c), 2 M HCl (spectrum d), and 3 M HCl (spectrum e). Other conditions: Nu-6(1), 0.5 g; DEDMS, 0.08 g; temperature, 473 K; and time, 24 h.

parameters on the degree of silylation and the order of expanded structure. The diffractions increased in intensity with a rising temperature from 403 K to 473 K (Figure 3), indicating

Figure 5. XRD patterns of Nu-6(1) precursor (spectrum a), assilylated with 0 g (spectrum b), 0.08 (spectrum c), 0.13 g (spectrum d), and 0.18 g of DEDMS in ethanol. Other conditions: Nu-6(1), 0.5 g; HCl, 2 M; temp., 473 K; and time, 24 h.

Figure 3. XRD patterns of Nu-6(1) precursor (spectrum a), assilylated with DEDMS in ethanol at 403 K (spectrum b), 443 K (spectrum c), and 473 K (spectrum d). Other conditions: Nu-6(1), 0.5 g; DEDMS, 0.08 g; HCl, 2 M; and time, 24 h.

expanded structure was constructed even in the absence of DEDMS silane (Figure 5b). Similar phenomena have already been reported on the direct acid treatment of post-synthesized Ti-MWW precursor,17 and PLS-n (n = 1−4) precursors.36 Soluble silicon species from the zeolite crystals probably serve as pillars to cross-link the layers. However, the diffractions increased in intensity obviously with increasing amount of silane added (see Figures 5c and 5d). Thus, the amount of silicon species to be removed was not enough to realize a sufficient silylation unless with the coexistence of the external silicon source supplied by DEDMS silane. As a consequence, a successful interlayer pillaring is carried out, preferrably in ethanol using 2 M HCl at 473 K and with the addition of a suitable amount of DEDMS. The optimized conditions were then applied to the silylation of the Nu-6(1) precursors with various Al contents. The interlayer-expanded aluminosilicates with well-ordered structures and high crystallinity were postsynthesized successfully (see Figure 6). Table 6 summarizes the physicochemical properties of Nu6(1) lamellar precursor, Nu-6(2), as-made IEZ-Nu-6(1) and

that higher temperatures were favorable to constructing wellordered structures with interlayer expanded pores. The acid concentration also altered the interlayer expansion greatly as shown in Figure 4. The acid may have two functionalities, that is, extracting the SDA species and assisting the bond formation between the silane groups and the silanols on zeolite layers. At a lower HCl concentration (0.5 M), the resultant material turned to be a mixture of Nu-6(1) precursor and IEZ-Nu-6(1), showing two separate diffractions at 2θ = 5°−10° (Figure 4b). This is probably because that the SDA species was extracted insufficiently from the interlayer spaces in 0.5 M HCl. The interlayer expansion and pillaring then occurred to a limited extent. When the acid concentration was over 1 M, the [200] diffraction due to the Nu-6(1) precursor disappeared completely, and that which was attributed to the expanded zeolite structure was developed instead (see Figures 4c and 4d). According to the diffraction intensity, 2 M HCl was sufficient to achieve a good silylation. Figure 5 shows the effect of silane amount on the structure order of IEZ-Nu-6(1). The interlayer 4714

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Figure 7. SEM images of (a) Nu-6(1) and (b) IEZ-Nu-6(1).

K (see Figure S2 in the Supporting Information). Although the XRD patterns clearly evidenced a structural expansion along the direction of layer stacking after silylation (Figure 1), the SEM images did not provide observable change in crystal thickness. Therefore, the formation of new crystalline structure by silylation took place not by destroying and reconstructing the macro crystals but at an atomic or molecular level. Infrared (IR) spectra indicated that the silylation developed two new bands at 850 and 2965 cm−1, and they were intensified with increasing amount of added DEDMS (Figure 8). These

Figure 6. XRD patterns of IEZ-Nu-6(1)-Si prepared from the precursor with a Si/Al ratio of 30 (spectrum a), 45 (spectrum b), 90 (spectrum c), and infinity (∞) (spectrum d).

Table 6. Physicochemical Properties of Lamellar Precursors Nu-6(1), Directly Calcined Sample Nu-6(2), and IEZ-Nu-6 Zeolites

a

No.

sample

d200-spacing (Å)

SSAa (m2 g−1)

1 2 3 4

Nu-6(1) Nu-6(2) IEZ- Nu-6(1) IEZ-Nu-6(2)-Si-550

13.54 8.47 10.98 9.72

85 50 192 83

Langmuir specific surface area (SSA) determined by N2 adsorption.

calcined IEZ-Nu-6(2) samples. The interlayer spacing was expanded by silylation by 2.5 Å for IEZ-Nu-6(1) when compared to Nu-6(2). The expansion value was at the same level reported for the silayltion of MCM-22(P) and PREFER with the same silane.18 The [200] reflection shifted gradually to high angles with rising calcination temperature (see Figure S2 in the Supporting Information). A distinct shift was observed after the calcination at >523 K, together with a partial loss of cystallinity. Presumably, this is closely related to distorted and twisted interlayer linkage natures in NSI zeolite. The incorporated silane molecules existed in relaxed states before when without calcination, but with the methyl moieties attached to the silane were burned off at high temperatures, Si−O−Si bonds formed between the pillaring Si atoms and the zeolite sheets may change greatly in bond angle to release the energy. After calcination at 823 K, and the d-spacing of the silylated material was reduced from 10.98 Å to 9.72 Å. However, the value was still larger than that of the directly calcined sample Nu-6(2) (8.47 Å). N2 adsorption at 77 K indicated that Nu-6(2) had a specific surface area of only 50 m2 g−1 (see Table 6, No. 2), because the NSI zeolite possesses two sets of narrow 8-MR pores in a distorted manner (4.5 Å × 2.6 Å, 4.8 Å × 2.4 Å).37 The silylation increased the surface area to 192 m2 g−1 for as-made IEZ-Nu-6(1) and 83 m2 g−1 after calcination at 823 K. 3.2. Investigation into Interlayer Silylation with IR and NMR Spectroscopies. Figure 7 shows the SEM images of Nu6(1) precursor and IEZ-Nu-6(1). The materials were composed of petaloid crystal aggregates. The primary crystals were plate-shaped, exhibiting a morphology typical of lamellar zeolites. There was no obvious difference in crystal morphology before and after silylation, even after further calcination at 823

Figure 8. IR spectra of Nu-6(1) precursor (spectrum a), as-silylated with 0 g (spectrum b), 0.08 g (spectrum c), 0.13 g (spectrum d), and 0.18 g of DEDMS in ethanol (spectrum e). Other conditions: Nu6(1), 0.5 g; HCl, 2 M; temperature, 473 K; and time, 24 h.

two bands are assigned the asymmetric stretching vibration of methyl groups and the rocking of methyl groups attached to silicon, respectively.38,39 Thus, the silylation incorporated the (CH3)2Si groups into the zeolite probably pillaring the layers. The incorporation of (CH3)2Si groups into the precursor by silylation has been further investigated by 13C and 29Si MAS NMR techniques. First, the Nu-6(1) precursor showed, in 13C NMR spectra, three resonances at 149.6, 141.5, and 121.0 ppm (Figure 9a), which are attributed to the carbon atoms with unequivalent chemical positions in 4′4-bipyridine molecules. The silylation removed the SDA-related resonances almost completely, while developed for the as-made IEZ-Nu-6(1) a new resonance at −2.0 ppm (Figure 9b), attributed to the configuration of (CH3)2Si groups.38 Two weak resonances at 57.0 ppm and 17.6 ppm were due to the CH2 and CH3 groups in residual ethanol, respectively, since the silylation was conducted in ethanol solvent. Figure 10 shows the 29Si MAS NMR spectra of the Nu-6(1) precursor before and after silylation. For the precursor, the two resonances at −112 and −103 ppm are assigned to the Si(OSi)4 groups (Q4) and the silicon-bearing OH groups, (OH)Si(SiO)3 (Q3), respectively (Figure 10a). After silylation, a new resonance at −11.8 ppm appears (Figure 10b and c), which is attributed to the Si(CH3)2(OSi)2 groups (D2).40 Note that the silylation decreased the intensity of Q3 resonance by 65%, 4715

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Scheme 1

Figure 9. 13C MAS NMR spectra of Nu-6(1) precursor (a) and assilylated with DEDMS (b). The asterisks indicate spin side bands.

on the IEZ-Nu-6(1) sample (Si/Al = ∞), which was prepared by direct acid treatment in the absence of silane as the sample (Figure 5b). It is then not necessary to consider the complex nature of the problem concerning the methyl groups in silane molecules and the framework aluminum then could be omitted. The pillars supporting the layers are assumed to be the  Si(OH)2 groups. The structure model was thus built for IEZNu-6(1) based on the collection of the NSI sheets expanded by the tetrahedral Si(OH)2 units. Based on indexed powder pattern using the program x-cell,41 a unit cell with space group P121/a1 with parameters a = 22.82 Å, b = 5.01 Å, c = 13.82 Å, and a monoclinic angle of 103.78° can be obtained. The lattice parameters b and c of IEZ-Nu-6(1) very closely resembled the corresponding parameters of the Nu-6(2) structure (a = 17.257 Å, b = 4.988 Å, c = 13.848 Å, β = 106.09°), but the a parameter was found to be significantly longer. The bc-planes in Nu-6(1) are parallel to the layers, so it was a straightforward conclusion that these layers were preserved in the IEZ-Nu-6(1). The structure model was thus built for IEZ-Nu-6(1), based on the collection of the NSI layers expanded by the tetrahedral Si(OH)2 units. Through coordinate transformation of the atomic positions of the Nu6(2) framework, the layers were translated into the new unit cell. The geometry of new unit cell was optimized by DFT based on the lattice parameters obtained from the indexed powder XRD diagram. The fractional coordinates obtained from the geometrical refinement were used as starting parameters for the Rietveld refinement of the crystal structures of IEZ-Nu-6(1) . The final results of the Rietveld analyses are represented by Rwp = 6.04%, Rp = 4.53%, Rwp(w/o bck) = 18.97%, indicating that the simulated powder pattern fitted well the experimental one. Technical details and results of the structure analysis are summarized in Table 1, and representative distances and angles are compiled in Table 5. From the analysis of the geometrical data, all angles and distances are within expected values for silicate frameworks. Figure 11 shows that the simulated diffraction pattern matched closely the experimental data, indicating that the interlayer-expanded structure was highly crystalline. Table 2 compares the unit cell constants among uncalcined IEZ-Nu6(1), Nu-6(1) precursor, and Nu-6(2). The lattice parameter along the layer stacking direction (a-axis) was increased from 17.257 Å to 22.9372 Å after acid treatment, indicating an

Figure 10. 29Si MAS NMR spectra of Nu-6(1) precursor (spectrum a) and as-silylated with DEDMS (spectrum b).

demonstrating that the silylation occurred through the reaction of silane groups with the silanols on the layer surface to form the pillaring silicon species. Upon calcination, the peak at −11.8 ppm disappeared, and a new signal at −93 ppm with the same intensity showed up, indicating that the methyl groups were replaced by (OH)2Si(SiO)2 groups (Q2) (see Figure S4b in the Supporting Information). Thus, it is deduced that the partial condensation of Si−OH barely took place during the calcination process. Nevertheless, the intensity decrease of Q3 resonance was not as much as the silylation of the MCM-22(P), PREFER, and PLS-1 precursors reported previously.18,24,25 This implied that a part of pillaring sites remained unoccupied by silane molecules after silylation. The same things happened on the material IEZNu-6(1)-H. According to the 29Si MAS NMR spectrum (see Figure S4c in the Supporting Information), only 31.7% of the bridging sites were occupied by the silanol Q2. Based on the above-mentioned investigations, the interlayer silylation of Nu6(1) with alkoxysilane can be described graphically in Scheme 1. The inserted Si(CH3)2 groups pillar the up-and-down NSI sheets, forming expanded interlayer pores, in comparison to directly calcined material (Nu-6(2)). 3.4. Crystal Structure Analysis. In order to simplify the resolution process, we have carried out the structural simulation 4716

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it is assumed that only a part of sites were occupied. When the material was subjected to calcination, the shrinkage of unit cells occurred due to the flexibility of the Q2 linker groups. These two reasons may correspond to the degradation of the structure. 3.5. Esterification of Acetic Acid and Ethanol over IEZNu-6(2). Table 7 shows the catalytic results of over Table 7. Comparison of Acetic Acid Conversion among Nu6(2), IEZ-Nu-6(2)-H, and IEZ-Nu-6(2)-Sia No.

sample

Si/Al ratiob

acetic acid conversion (%)

1 2 3

Nu-6(2) IEZ-Nu-6(2)-H-550 IEZ-Nu-6(2)-Si-550

35 64 68

42.0 50.2 72.3

a

Reaction conditions: catalyst, 0.2 g; ethanol/acetic acid molar ratio, 3; feed rate, 2 mL h−1; N2, 30 mL min−1; temperature, 473 K; and time, 2 h. bDetermined by ICP analysis.

Figure 11. Rietveld refinement of powder X-ray patterns of IEZ-Nu6(1) (Si/Al = ∞). Experimental (red) and calculated (black) XRD patterns as well as the difference profile are shown (blue). The short tick marks (green) below the patterns give the position of the Bragg reflections.

aluminosilicate catalysts of Nu-6(2) and IEZ-Nu-6(2) in the esterification of acetic acid and ethanol reaction. Two types of IEZ-Nu-6(2) zeolites were prepared by interlayer pillaring directly in acid solution without silane addition or in the presence of DEDMS. They were both further calcined at 823 K for 2 h. After the pillaring in acid solution, a dealumination occurred reasonably, leading to higher Si/Al ratios. Despite less acid sites, the two IEZ-Nu-6(2) catalysts showed higher acetic acid conversion than Nu-6(2). The 10-MR channels of IEZNu-6(2) would propose less limitation to the adsorption and diffusion of molecules, in comparison to the 8-MR channels in Nu-6(2). Thus, the activity improvement should be attributed to a higher accessibility of the acid sites in larger pores. Moreover, the IEZ-Nu-6(2) zeolite prepared using DEDMS was more active than that prepared by acid treatment only (see Nos. 2 and 3 in Table 7). On the other hand, the hydrophilicity/hydrophobicity of materials may influence their catalytic activities.42 In view of the more ordered crystalline structure (Figure 5), the IEZ-Nu-6(2)-Si-550 has more hydrophilic Si−OH groups in the micropores than IEZ-Nu6(2)-H-550, and then IEZ-Nu-6(2)-Si-550 was more hydrophilic. Because acetic acid is also hydrophilic, it would reach more easily the acid sites in the IEZ-Nu-6(2)-Si-550 catalyst.

interlayer expansion of the layered silicates. The b and c parameters confining the angle α = 90° are almost identical with the b and c parameters and angle α of Nu-6(2) (4.9881 Å, 13.848 Å, and 90°, respectively), which means that they are no structural changes happening inside the NSI layers. Figure 12 shows the crystalline structure of IEZ-Nu-6(1) as viewed along the b-direction; the expansion of the NSI-layers

Figure 12. Structure of IEZ-Nu-6(1) along the b-direction.

by one silicate tetrahedron are shown. This increases the ring size of the channels by two tetrahedra from 8-ring to 10-ring pore openings along the b-direction. The silanols exist as the bridging linker units. Structure refinement yielded surprising order of the alignment of linker groups, which is similar with the zeolite COE-3 and COE-4.24 For the channels running along the b-direction, in c-direction, the silanols of opposite linker sites point toward each other, leading to an alternation of narrow and wide 10-ring channels. From the analysis of the results of structure refinement, the small 10-ring has a maximum pore width of 4.6 Å, whereas the wide 10-ring has a pore opening of 5.8 Å × 4.8 Å, based on the refined structure and an oxygen radius of 1.35 Å. As can be seen from Table 4, there are nonframework constituents in the list of atoms. Therefore, the C-atoms were used as the scatters to similate the residual electron density inside the pore systems. The same strategy was used for the IEZ-CDO and IEZ-RRO,24,25 which improved the results of the Rietveld analysis. IEZ-Nu-6(1) was calcined at 423−823 K, resulting in a partial collapse of the structure (see Figure S2 in the Supporting Information). According to 29Si MAS NMR spectra,

4. CONCLUSIONS The interlayer structure of NSI topology is expanded successfully by silylating the lamellar precursor Nu-6(1) with DEDMS silane in acid solution. The interlayer pillaring in ethanol gives rise to the materials with a more-ordered crystalline structure than in water. The structure simulation shows that IEZ-Nu-6(1) is composed of two sets of 10-MR channels as a result of interlayer pillaring by monomeric silicon species. IEZ-Nu-6(2), which has larger pores, is more active than Nu-6(2) in the esterification of acetic acid and ethanol. The interlayer-expanded materials with opener pore windows are expected to serve as effective acid catalysts as well as shapeselective adsorbents for adsorption and separation.

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ASSOCIATED CONTENT

S Supporting Information *

This material is available free of charge via the Internet at http://pubs.acs.org. 4717

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(24) Gies, H.; Müller, U.; Yilmaz, B.; Feyen, M.; Tatsumi, T.; Imai, H.; Zhang, H. Y.; Xie, B.; Xiao, F. S.; Bao, X. H.; Zhang, W. P.; De Baerdemaeker, J.; De Vos, D . Chem. Mater. 2012, 24, 1536. (25) Gies, H.; Müller, U.; Yilmaz, B.; Feyen, M.; Tatsumi, T.; Xie, B.; Xiao, F. S.; Bao, X. H.; Zhang, W. P.; De Vos, D. Chem. Mater. 2011, 23, 2545. (26) Ikeda, T.; Kayamori, S.; Oumi, Y.; Mizukami, F. J. Phys. Chem. C 2010, 114, 3466. (27) Whittam, T. V. U.S. Patent 4,397,825, 1983. (28) Zanardi, S.; Alberti, A.; Cruciani, G.; Corma, A.; Fornes, V.; Brunelli, M. Angew. Chem., Int. Ed. 2004, 43, 4933. (29) Corma, A.; Fornés, V.; Diaz, U. Chem. Commun. 2001, 24, 2642. (30) Corma, A.; Botellaa, P.; Mitchellb, C. Chem. Commun. 2004, 17, 2008. (31) Zubowa, H.-L.; Schneider, M.; Schreier, E.; Eckelt, R.; Richter, M.; Fricke, R. Microporous Mesoporous Mater. 2008, 109, 317. (32) Lima, S.; Pillinger, M.; Valente, A. A. Catal. Commun. 2008, 9, 2144. (33) Roth, W. J.; Kresge, C. T. Microporous Mesoporous Mater. 2011, 144, 158. (34) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (35) Materials Studio Release Notes v.6.1; Accelrys Software: San Diego, CA, 2012. (36) Ikeda, T.; Kayamori, S.; Oumi, Y.; Mizukami, F. J. Phys. Chem. C 2010, 113, 3466. (37) Baerlocher, C.; McCusker, L. B.; Olson, D. H. Atlas of Zeolite Framework Types; Elsevier Press: New York, 2007. (38) Joo, J.; Hyeon, T.; Hyeon-Lee, J. Chem. Commun. 2000, 16, 1487. (39) Yamamoto, K.; Nohara, Y.; Domon, Y.; Takahashi, Y.; Sakata, Y.; Plévert, J.; Tatsumi, T. Chem. Mater. 2005, 17, 3913. (40) Engelhardt, G.; Michel, D. High-Resolution Solid-State NMR of Silicates and Zeolites; John Wiley & Sons Ltd.: New York, 1987. (41) Neumann, M. A. J. Appl. Crystallogr. 2003, 36, 356. (42) Xiao, F.; Xie, B.; Zhang, H.; Wang, L.; Meng, X.; Zhang, W.; Bao, X.; Yilmaz, B.; Müller, U.; Gies, H.; Imai, H.; Tatsumi, T.; De Vos, D. ChemCatChem. 2011, 3, 1442.

AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +86-21 62232292. E-mail: [email protected]. Author Contributions †

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support from National Natural Science Foundation of China (Nos. 20925310, U1162102, and 21373089), Ph.D. Programs Foundation of Ministry of Education (No. 2012007613000), the National Key Technology R&D Program (No. 2012BAE05B02), the Shanghai Leading Academic Discipline Project (No. B409), and Shanghai Synchrotron Radiation Facility (No. j13sr0021).

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REFERENCES

(1) McCusker, L. B.; Baerlocher, Chr. Database of Zeolite Structure, http://www.iza-structure.org/databases. (2) Leonowicz, M. E.; Lawton, J. A.; Lawton, S. L.; Rubin, M. K. Science 1994, 264, 1910. (3) Schreyeck, L.; Caullet, P.; Mougenel, J. C.; Guth, J. L.; Marler, B. Microporous Mater. 1996, 6, 259. (4) Ikeda, T.; Akiyama, Y.; Oumi, Y.; Kawai, A.; Mizukami, F. Angew. Chem., Int. Ed. 2004, 43, 4892. (5) Zanardi, S.; Alberti, A.; Cruciani, G.; Corma, A.; Fornes, Brunelli, V. M. Angew. Chem., Int. Ed. 2004, 43, 4933. (6) Andrews, S. J.; Papiz, M. Z.; McMeeking, R. A.; Blake, J.; Lowe, B. M.; Franklin, K. R.; Helliwell, J. R.; Harding, M. M. Acta Crystallogr., Sect. B: Struct. Sci. 1988, 44, 73. (7) Oberhagemann, U.; Bayat, P.; Marler, B.; Gies, H.; Rius, J. Angew. Chem., Int. Ed. Engl. 1996, 35, 2869. (8) Vortmann, S.; Rius, J.; Gies, S. H. J. Phys. Chem. B 1997, 101, 1292. (9) Wang, Y. W.; Gies, H.; Marler, B. Chem. Mater. 2005, 17, 43. (10) Wheatley, P. S.; Morris, R. E. J. Chem. Mater. 2006, 16, 1035. (11) Roth, W. J.; Shvets, O. V.; Shamzhy, M.; Chlubn, P.; Kub, M.; Nachtigall, P.; Cejka, J. J. Am. Chem. Soc. 2011, 133, 6130. (12) Choi1, M.; Na, K.; Kim, J.; Sakamoto, Y.; Terasaki, O.; Ryoo, R. Nature 2009, 461, 246. (13) Corma, A.; Fornés, V.; Pergher, S. B.; Maesen, Th. L. M.; Buglass, J. G. Nature 1998, 396, 353. (14) Wu, P.; Nuntasri, D.; Ruan, J. F.; Liu, Y. M.; He, M. Y.; Fan, W. B.; Terasaki, O.; Tatsumi, T. J. Phys. Chem. B 2004, 108, 19126. (15) Liu, G. Q.; Jiang, J. G.; Yang, B. T.; Fang, X. Q.; Xu, H.; Peng, H. G.; Xu, L.; Liu, Y. M.; Wu, P. Microporous Mesoporous Mater. 2013, 165, 210. (16) Maheshwari, S.; Jordan, E.; Kumar, S.; Bates, F. S.; Penn, R. L.; Shantz, D. F.; Tsapatsis, M. J. Am. Chem. Soc. 2008, 130, 1507. (17) Fan, W. B.; Wu, P.; Namba, S.; Tatsumi, T. Angew. Chem., Int. Ed. 2004, 43, 236. (18) Wu, P.; Ruan, J. F.; Wang, L. L.; Wu, L. L.; Wang, Y.; Liu, Y. M.; Fan, W. B.; He, M. Y.; Terasaki, O.; Tatsumi, Y. J. Am. Chem. Soc. 2008, 130, 8178. (19) Maheswari, S.; Jordan, E.; Kumar, S.; Bates, F. S.; Penn, R. L.; Shantz, D. F.; Tsapatsis, M. J. Am. Chem. Soc. 2008, 130, 1507. (20) Ogino, I.; Nigra, M. M.; Hwang, S. J.; Ha, J. M.; Rea, T.; Zones, S. I.; Katz, A. J. Am. Chem. Soc. 2011, 133, 3288. (21) Eilertsen, E. A.; Ogino, I.; Nigra, M. M.; Hwang, S. J.; Ha, J. M.; Rea, T.; Zones, S. I.; Katz. Chem. Mater. 2011, 24, 5404. (22) Zhao, Z.; Zhang, W.; Ren, P.; Han, X.; Müller, U.; Yilmaz, B.; Feyen, M.; Gies, H.; Xiao, F.; De Vos, D.; Tatsumi, T.; Bao, X. Chem. Mater. 2013, 25, 840. (23) Corma, A.; Díaz, U.; García, T.; Sastre, G.; Velty, A. J. Am. Chem. Soc. 2010, 132, 15011. 4718

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