Article pubs.acs.org/cm
Heteroatom-Tolerant Delamination of Layered Zeolite Precursor Materials Isao Ogino,†,¶ Einar A. Eilertsen,†,⊥ Son-Jong Hwang,§ Thomas Rea,‡ Dan Xie,‡ Xiaoying Ouyang,† Stacey I. Zones,*,†,‡ and Alexander Katz*,† †
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Department of Chemical and Biomolecular Engineering, University of California at Berkeley, Berkeley, California 94720, United States ‡ Chevron Energy Technology Company, Richmond, California 94804, United States § Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States S Supporting Information *
ABSTRACT: The synthesis of the first delaminated borosilicate layered zeolite precursor is described, along with its aluminosilicate analogue, which consists of Alcontaining UCB-3 and B-containing UCB-4 from as-made SSZ-70. In addition, the delamination of PREFER (which is the precursor to ferrierite zeolite) under similar conditions yields delaminated layered zeolite precursors consisting of Al-containing UCB-5 and Ti-containing UCB-6. Multinuclear solid-state NMR spectroscopy (11B and 27Al), diffuse-reflectance UV-vis spectroscopy, and heteroatom/Si ratios measured via elemental analysis are consistent with a lack of heteroatom leaching from the framework following delamination. Such mild delamination conditions are achieved by swelling the zeolite precursor in a fluoride/chloride surfactant mixture in DMF solvent, followed by sonication. Powder X-ray diffraction, argon gas physisorption, and chemisorption of bulky base probes strongly suggest delamination, and demonstrate a 1.5-fold increase in the number density of external acid sites and surface area of calcined UCB-3, relative to calcined Al-SSZ-70. The synthesis of microporous pockets in materials UCB-3−UCB-5 suggests the possibility of interlayer porosity in SSZ-70, which is a layered zeolite precursor material whose structure remains currently unknown. The mildness of the delamination method presented here, as well as the lack of need for acidification in the synthesis procedure, enables the delamination of heteroatom-containing zeolites while preserving the framework integrity of labile heteroatoms, which could otherwise be leached under harsher conditions. KEYWORDS: delamination, exfoliation, layered zeolite precursor, SSZ-70, ferrierite zeolite, MCM-22(P)
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INTRODUCTION Delamination of layered zeolite precursors allows bulky reactants access to the catalytically active sites of the zeolite surface,1−6 which are usually restricted to small molecules, because of their location within microporous frameworks. Such sites are known to be difficult to replicate within nonzeolitic oxide networks with more open porosity. The synthesis of delaminated materials therefore expands the range of molecules usable for zeolite-catalyzed reactions and may also enable shape-selective catalysis involving large molecules. Conventional methods of layered zeolite precursor delamination require high pH conditions (typically pH 13.5)7,8 for swelling the zeolite precursor layers, followed by sonication and acidification for exfoliating layers, and such harsh pH conditions inevitably lead to partial destruction of zeolites.6−8 Such conditions are especially problematic from the perspective of undesired leaching of labile framework heteroatoms,5,9 such as boron (B) atoms.5,9 Under controlled conditions, these B atoms have been previously shown to be post-synthetically exchangeable with other metal atoms, such as aluminum (Al), to synthesize active catalysts. 10 Thus, delamination of borosilicate layered zeolite precursors with the boron remaining © 2013 American Chemical Society
intact within the framework could enable the synthesis of a wide variety of catalysts consisting of heteroatoms on external surfaces within well-defined coordination environments; this is something that, currently, is difficult to accomplish. Here, we demonstrate exfoliation of layered zeolite precursors under sufficiently mild conditions that preserve the integrity of heteroatoms within the framework, including the first example of doing so with boron. The manuscript first demonstrates the attempt at delamination of the layered, zeolite precursor SSZ-70, in our case containing either B or Al as substituting heteroatoms in the lattice.11−13 SSZ-70 has a unique topology and is separate from the MCM-22(P) material; thus, we thought it would be good to characterize its delamination using our milder method, which we described for MCM-22(P) in making UCB-1 in a previous report.14 SSZ-70 was first described in the patent literature in 2006 and was prepared from a system using boron and fluoride Received: October 10, 2012 Revised: February 22, 2013 Published: February 27, 2013 1502
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Synthesis of PREFER. PREFER aluminosilicate was synthesized from a gel mixture of SiO2:0.10 Al2O3:1.0 SDA:1.5 NH4F:1.0 HF:10 H2O, according to previously published literature.16 Typically, a mixture of 1.47 g of ammonium fluoride (Sigma−Aldrich, >98%), 0.50 g of hydrofluoric acid solution (49%), 1.60 g of fumed silica (Sigma− Aldrich, particle size = 0.007 μm), and 0.38 g of alumina (Sasol, Catapal B) was mixed in a Teflon container, and the highly viscous gel was stirred using a spatula until homogeneous. To this mixture, a solution of 4.16 g of 4-amino-2,2,6,6-tetramethylpiperidine (Sigma− Aldrich, 98%) in 4.43 g of distilled water was added, and the resulting gel was stirred until homogeneous. The gel was subsequently transferred to a 23-mL Teflon-lined Parr stainless steel autoclave, and heated while tumbling the reactor at 60 rpm for a period of 5 days at 448 K. The titanium version of PREFER was synthesized similarly except using titanium ethoxide (Sigma−Aldrich). Synthesis of Delaminated Materials. A mixture of 0.50 g of asmade SSZ-70 or PREFER, 0.55 g of cetyltrimethylammonium bromide (CTAB) (Sigma−Aldrich, ≥99.0%), 0.85 g of tetrabutylammonium fluoride trihydrate (TBAF) (Sigma−Aldrich, ≥97%), and 0.85 g of tetrabutylammonium chloride (TBACl) (Sigma−Aldrich, ≥97%) in 20 mL of DMF (Fisher Scientific) was placed in a screw-capped 50-mL PFA tube, and stirred using a magnetic stir bar at 373 K for 16 h in an oil bath. After cooling, the slurry was sonicated for 1 h in an ice bath, using a Branson digital sonifier 450 (Branson, USA) operating under pulse mode (1.0 s on and 0.1 s off). The sonicated slurry was filtered, to separate a solid from a brown-colored filtrate. The solid was washed with DMF and then with ethanol, thoroughly, and dried at 323 K overnight, yielding a white solid. The delaminated materials synthesized from Al-SSZ-70, B-SSZ-70, Al-PREFER, and Ti-PREFER were denoted as UCB-3, UCB-4, UCB-5, and UCB-6, respectively. Typical silicate yields are in excess of 90%. Ion Exchange of Al-SSZ-70 or UCB-3. Calcined material (calcined according to a previously published procedure14) was contacted with 0.2 M aqueous NH4NO3 (NH4+/Al ratio >10) at 353 K for 1 h. This procedure was repeated four times, and the slurry was filtered. The resulting solid was dried at 353 K in a convection oven overnight. Characterization. Powder X-ray diffraction (XRD) patterns were collected on either a Siemens D5000 diffractometer or a Bruker D8 Advance, both using a Cu Kα radiation. Transmission electron microscopy (TEM) images were recorded on a JEOL Model JEM2010 (200 kV) at the University of California, Davis. Argon gas physisorption isotherms were measured on a Micromeritics ASAP2020 instrument at 86 K. Prior to measurement, samples were evacuated at 623 K for 4 h. 29Si solid-state CP MAS NMR (CP contact time of 2 ms at 8 kHz sample spinning) spectra were measured using a Bruker Avance 500 MHz spectrometer with a wide-bore 11.7 T magnet and employing a 4-mm MAS probe (Bruker). The spectral frequencies were 500.23 MHz for the 1H nucleus, and 99.4 MHz for the 29Si nucleus. 29Si MAS NMR spectra were acquired after a 4 μs−90° pulse with application of a strong 1H decoupling pulse. The spinning rate was 12 kHz, and the recycle delay time was 300 s. Amine chemisorption experiments were performed using a TG Instruments Model 2920 system. Approximately 30 mg of sample in the proton form was heated to 823 K in 100 mL/min of a dry nitrogen flow, followed by a soak at this temperature for 3 h. After cooling to 423 K, ∼50 μL of pyridine (Sigma−Aldrich, spectroscopic grade) or collidine (Sigma−Aldrich, 99%) are injected to the inlet line via syringe, and the system remains at 423 K for 3000 min. Then, the temperature was raised to 473 K, followed by a soak at this temperature for 2 h. After this step, the temperature of the sample was further increased to 523 K, followed by a soak at this temperature for 2 h. Diffuse reflectance infrared Fourier transform (DRIFT) UV-vis spectroscopy of solid samples was performed on a Varian Cary 400 Bio UV-vis spectrophotometer equipped with a Harrick Praying Mantis accessory at room temperature. Elemental analysis was performed at Galbraith Laboratories.
and an imidazolium template. The chemistry was greatly expanded as to usable SDA, host compositions, and the use of hydroxide (OH) as well as fluoride (F) as mineralizers.12,13 In the work by Archer and Davis, it was clear from 29Si NMR that there could well be a structural relationship to MCM-22 materials, because of many similar chemical shifts. In fact, it may be the case that there is construction of a material, in SSZ70, which contains subunits also used in MCM-22 materials and, as such, made the prospect of delamination, leading to exposed “cups” on external surfaces such as is the case for MWW derivatives, that much more appealing for us to study. Here, we also get a chance to apply this delamination approach to a borosilicate material as well as an aluminosilicate since SSZ-70 can be prepared with both as heteroatom substituents for Si. While there is a large body of research for the delaminated or exfoliated materials and their applications in catalysis when Al is present in the framework, there is very little for the borosilicate materials. They can be catalysts of interest in their own right, as previously demonstrated by Chen et al.15 Our delamination procedure relies on the treatment of as-made SSZ-70 with a surfactant mixture containing fluoride and chloride anions in N,N-dimethylformamide (DMF), 14,16 followed by sonication. This treatment obviates the need for basic/acidic conditions, which were used previously1−9,16 and would have the undesirable consequence of leading to heteroatom leaching from the framework. In the second part of the present work, a similar approach described was applied to PREFER (precursor to ferrierite zeolite)17 containing either aluminum or titanium, in order to further demonstrate the generality of the delamination method.
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MATERIALS AND EXPERIMENTAL METHODS
Materials. All reagents used in zeolite synthesis, delamination, and chemisorption were of reagent-grade quality and were used asreceived. Preparation of 1,3-Diisobutylimidazolium Hydroxide (SDA) Solution. Synthesis of 1,3-diisobutylimidazolium bromide (SDA+Br−) was performed according to previously described literature.12 SDA+Br− was ion-exchanged to the hydroxide form by contacting with 2 equiv of an ion-exchange resin (Bio-Rad, AG1-X8) in water at room temperature for 24 h. Synthesis of Al-SSZ-70. Synthesis of Al-SSZ-70 was performed according to previously described literature.11,13 Typically, 0.171 g of aluminum hydroxide (Reheis F-2000, 50−53 wt % Al2O3) was dissolved in a mixture of 8.89 g of 1 N sodium hydroxide aqueous solution (Sigma−Aldrich), 6.88 g of distilled water, and 35.6 g of 0.5 mmol/g of SDA+OH− solution. To this mixture, 5.5 g of fumed silica (Sigma−Aldrich, particle size = 0.007 μm) and seed crystals (as-made Al-SSZ-70, 1% SiO2) were added. The gel composition was SiO2:0.010 Al2O3:0.050 Na2O:0.20 SDA:30 H2O. After the gel was stirred at room temperature overnight, it was divided into four portions, and each gel was transferred to a 23-mL Parr reactor equipped with a Teflon liner. Each reactor was sealed and heated while tumbling at 60 rpm at 423 K for a period of 1−2 weeks. After cooling, the solid product was collected by centrifugation, washed thoroughly with distilled water, and dried at 353 K in a convection oven overnight. Calcination of AlSSZ-70 was performed in an O2/Ar flow at 823 K for 12 h. Syntheses of B-SSZ-70. Synthesis of B-SSZ-70 was performed similarly to Al-SSZ-70 synthesis, except that boric acid (J.T. Baker) was used instead of aluminum hydroxide. Gel compositions were SiO2:0.033 B2O3:0.050 Na2O:0.20 SDA:30 H2O. The gel was sealed in a 23-mL Parr reactor and heated while tumbling the reactor at 60 rpm at 423 K for a period of 1−2 weeks. After cooling, the solids were collected by centrifugation, washed thoroughly with distilled water, and dried at 353 K in a convection oven overnight. Calcination of the as-made materials was performed in an O2/Ar flow at 823 K for 12 h. 1503
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RESULTS AND DISCUSSION Syntheses of Al-SSZ-70 and B-SSZ-70. PXRD data for the as-made Al- and B-SSZ-70 materials are shown in Figures 1a and 1b. The peak at 3.3° (d-spacing, 27 Å) and 6.6° (d-
suggests that the as-made Al-SSZ-70 was at least partially delaminated by this treatment. For B-containing UCB-4 in powder pattern b′ in Figure 1, there is a broad secondary peak at 4° (d-spacing = 2.2 Å), which may arise from some swollen layers remaining with interdigitized surfactants, because of incomplete delamination. Since PXRD alone is insufficient proof of delamination, because of the general loss in the longrange order of information accompanying delamination in the PXRD pattern, additional evidence for delamination is provided using multiple techniques below (see Table 1 for details). Table 1. Characterization Techniques to Examine Delamination of Layers and Preservation of Structural Integrity of Zeolite Framework and Retention of Heteroatoms in the Silicate Framework characterization technique
assessment
powder X-ray diffraction, PXRD transmission electron microscopy, TEM argon physisorption
Figure 1. Powder XRD patterns characterizing the following as-made materials: Al-SSZ-70 (pattern a), UCB-3 (pattern a′), B-SSZ-70 (pattern b), and UCB-4 (pattern b′). The dashed lines indicate the positions of the peaks at 3.3° and 6.6° that represent the lamellar structure of as-made SSZ-70.
loss of long-range order, possibly in more than one dimension of the layers direct observation of delaminated layers
elemental analysis solid-state NMR spectroscopy (29Si and heteronuclear) base titration of acid sites using differently sized probe molecules
spacing, 13 Å) represents the lamellar structure of SSZ-70, and the sharp peak at 26° is consistent with previously reported SSZ-70.13 Elemental analysis data in Table 2 show that the asmade Al-SSZ-70 and B-SSZ-70 materials have Si/heteroatom ratios of 43.4 and 28.7, respectively. The corresponding data for the swollen materials (see Figure S1 in the Supporting Information) shows the appearance of a new, low-angle peak in the range of 2.1°−2.2°. This indicates that there has been an expansion along the c-axis to ∼40 Å. This is a clear sign that the material has been swollen. In addition, the elemental analysis of the swollen material, after washing, shows an increase in organic content close to 40 wt %, up from the as-made SSZ-70 material that typically contains ∼18 wt % SDA.12,13 Again, this demonstrates uptake of the surfactant in the swelling step. The ratio of C/N+ also now has increased, as the CTAB surfactant has a value of 19, which is more than twice the value observed for the SDA alone. The loss of the peak intensity at 3.3° and 6.6° also indicates changes in the ordering along a c-axis. The remaining peaks in the PXRD for powder pattern a′ in Figure 1 also are almost all broad now. This means that the long-range order, or periodicity along c, is not present at this stage. Furthermore, the intralayer and interlayer mixed peaks (hkl where l is not 0 and h and k are not both 0) also become weaker and broader, which could indicate a less-parallel alignment and stacking of a potential MWW-type (recall that our NMR data in ref 13 have some similarities to MWW) set of layers in an a−b plane. These two changes in the PXRD data also support the success of the partial delamination of the as-made SSZ-70 starting material. Powder pattern a in Figure 1 bears some resemblance to a pattern “E” described in ref 18. There, the authors describe the loss of an 002 peak and the merging of 101 and 102 into a single broad peak. This type of pattern is consistent with a material experiencing some extent of delamination. This
decrease in micropore volume representing disappearance of pore channels between layers; increase in external surface area (quantitative) Si/heteroatom atomic ratio (quantitative) structural integrity of silicate framework; coordination of heteroatoms accessibility of total and surface acid sites (quantitative)
TEM images for as-made SSZ-70 in Figures 2a and 2b show a rectilinear morphology, with the expected layer-to-layer distance of ∼2.6 Å.13 TEM images of delaminated SSZ-70 show an abundance of single layers, as shown in Figures 2c−f. These data are consistent with delamination but also do not provide definitive proof. To provide more quantitative data to show delamination, we performed argon gas adsorption experiments (Table 1). Argon gas adsorption−desorption isotherms characterizing calcined SSZ-70 and calcined UCB-3 and UCB-4 are shown in Figure S1 of the Supporting Information. Table 2 lists micropore and total pore volumes, as well as external surface areas, as determined by the t-plot method. Delaminated materials UCB-3 and UCB-4 show significantly lower micropore volumes and larger external surface areas, relative to the corresponding calcined SSZ-70 three-dimensional zeolite. Because UCB-4 shows a larger decrease in the micropore volume and a higher measured external surface area relative to UCB-3, the results suggest that the B-SSZ-70 layered zeolite precursor has undergone a higher degree of delamination, relative to the aluminum-containing precursor. This greater extent of delamination for the boron-containing precursor of SSZ-70 suggest a novel and more effective route for the synthesis of delaminated Al-SSZ-70, which circumvents the more difficult problem of delaminating the aluminumcontaining layered zeolite precursor. This route consists of first delaminating the easier B-SSZ-70 precursor, followed by a subsequent lattice substitution of B with Al in the framework. Both UCB-3 and UCB-4 contain micropores after delamination. Some structural similarity between these two materials might also be anticipated.13 From this perspective, the microporosity in UCB-3 and UCB-4 after delamination might 1504
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Figure 3. 29Si CP MAS NMR spectra characterizing the following asmade materials: Al-SSZ-70 (spectrum a), UCB-3 (spectrum a′), BSSZ-70 (spectrum b), and UCB-4 (spectrum b′). Contact time = 2 ms.
These are presumably due to variations in Si−O−Si (or Si−O− Al) bonds accompanying delamination. The spectrum characterizing UCB-4 in spectrum b′ in Figure 3 shows results similar to those for UCB-3: (i) an absence of a Q2 resonance at approximately −90 ppm, and (ii) slight changes in resonances between −96.5 ppm and 124.0 ppm, relative to as-made B-SSZ70 in spectrum b in Figure 3. Altogether, the PXRD, TEM, argon gas physisorption, and 29Si CP MAS NMR spectroscopic characterization data are consistent with the delamination of SSZ-70 layers via sonication of swollen SSZ-70 layers, without amorphization of the zeolitic framework, because of the mild conditions employed. Mildness of the New Delamination Method to Heteroatoms in SSZ-70. The removal of heteroatoms from the zeolite framework during delamination would be expected to cause an increase in the Si/heteroatom ratio, when comparing materials before and after delamination. The similarity of these ratios for as-made SSZ-70 and materials UCB-3 and UCB-4 in Table 2 suggests a lack of such heteroatom leaching. This is further corroborated by 27Al MAS NMR spectra of as-made Al-SSZ-70 and UCB-3 in Figures 4a and 4b, which both exclusively show tetrahedral aluminum species at 43−60 ppm.19 This is consistent with the complete retention of Al atoms within the framework after delamination. To further confirm the retention of Al atoms in the framework as well as delamination of SSZ-70 layers, the accessibility of acid sites in calcined Al-SSZ-70 and UCB-3 was measured by first converting both materials to the H-form via
Figure 2. TEM images characterizing (a, b) as-made SSZ-70 and (c−f) delaminated SSZ-70. The arrows indicate delaminated single layers.
Table 2. Results from Chemical Analysis of As-Made Materials and Argon Physisorption Measurement of Calcined Materials sample
Si/heteroatom ratioa
Vmicrob (cm3/g)
Vtotal (cm3/g)
Sextc (m2/g)
Al-SSZ-70 UCB-3 B-SSZ-70 UCB-4 Al-PREFER UCB-5 Ti-PREFER UCB-6
43.4 41.8 28.7 29.1 77.9 79.6 57.3 53.4
0.16 0.10 0.15 0.06 0.07
