Crystallization Mechanism of Zeolite UZM-5 - Chemistry of Materials

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Crystallization Mechanism of Zeolite UZM-5 Min Bum Park, Nak Ho Ahn, Robert W. Broach, Christopher Nicholas, Gregory J Lewis, and Suk Bong Hong Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm504079m • Publication Date (Web): 29 Jan 2015 Downloaded from http://pubs.acs.org on February 2, 2015

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Chemistry of Materials 1

Crystallization Mechanism of Zeolite UZM-5 Min Bum Park,†,§ Nak Ho Ahn,† Robert W. Broach,‡ Christopher P. Nicholas,‡ Gregory J. Lewis,‡ and Suk Bong Hong*,† †

Center for Ordered Nanoporous Materials Synthesis, Department of Chemical Engineering and School of Environmental Science and Engineering, POSTECH, Pohang 790-784, Korea

‡

New Materials Research, UOP LLC, A Honeywell Company, Des Plaines, Illinois 60017, United States

ACS Paragon Plus Environment

Chemistry of Materials

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Abstract

A reliable formation pathway for UZM-5 zeolite crystals in the presence of tetraethylammonium, tetramethylammonium, and Na+ ions at 150 °C has been proposed based on the

13

C MAS NMR and IR spectra of a series of solid products recovered as a function of time

during the crystallization process, as well as on the crystal structure of as-made UZM-5 determined using synchrotron powder X-ray diffraction and Rietveld analyses. The nucleation of this cage-based small-pore zeolite begins with the construction of the largest 26-hedral lta-cages among its four different structural units. The next step is the attachment of 14-hedral wbc-cages to the preorganized lta-cage at shared 6-rings in an appropriate orientation that will allow the growth of two wbc-cage layers linked by 8-hedral rth-cage formation along both a- and b-axes. The resulting interlayer space is readily converted to a layer of lta-cages by interconnecting two opposing wbc-cages, with the concomitant formation of interlayer d4r-cages and 8-rings. Over the outer surface of the resulting UZM-5 nuclei, which resembles one half of an lta-cage, the crystal growth may take place in a self-assembled manner as described above.


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Introduction

The charge density mismatch (CDM) approach to zeolite synthesis, which was developed by Lewis and co-workers in the early 2000s, is a rational synthetic strategy employing the structuredirecting agent (SDA) cooperation that can be used for finding previously unobserved zeolite framework structures and/or compositions.1-10 This approach begins with creating a clear aluminosilicate synthesis solution that cannot crystallize by itself due to the large mismatch between the low charge density of the CDM SDA, the hydroxide content, and the higher charge density of the anticipated final aluminosilicate framework. To overcome this CDM barrier to any solid formation (not just zeolite crystal formation), it is necessary to perturb the starting synthesis solution with a small but sufficient amount of crystallization SDAs with high charge densities and enough heat so that eventually zeolite crystallization is achieved. UZM-9 (framework type LTA) is a high-silica version of zeolite A synthesized via the CDM approach and is known to crystallize from a CDM aluminosilicate solution using tetraethylammonium (TEA+) as a CDM SDA and tetramethylammonium (TMA+) and Na+ as crystallization SDAs.1,2,5,6 We have recently demonstrated that while the nucleation stage of this cage-based small-pore zeolite is dominated by the incorporation of Na+ and TMA+ into a very small amount of the solid phase with a Si/Al ratio with ca. 2.5, much lower than that (8.0) of its initial synthesis mixture, the larger TEA+ ion plays a more important role during the crystal growth process.9 More interestingly, its CDM synthesis was found to involve the initial formation of lta-cages as a nucleation center rather than of much smaller sod- or d4r-cages, which is contrary to the proposed mechanism for the crystallization of traditional LTA zeolites.11 Among the zeolites synthesized via the CDM approach, on the other hand, UZM-5 (UFI) was the first to be identified as a novel framework topology.1,5,6,12-16 Of particular interest is that it can be synthesized using the same synthesis mixture as that used for UZM-9 formation but at a higher temperature (150 vs 100 °C). As shown in Figure 1, the UFI structure can be distinguished by four different building units: 26-hedral ([4126886]) lta-, 14-hedral ([45546481]) wbc-, 8-hedral ([4454]) rth-, and 6-hedral ([46]) d4r-cages.17 Within the ab-plane of UZM-5, the lta-cages are connected through shared 8-rings and linking d4r-cages just as they are in UZM-9. Unlike in UZM-9, however, they are stacked along the c-axis so that an intervening layer of alternating up and down oriented wbc-cages resides between the lta-cage layers, connected to the lta-cages of



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adjacent layers via shared 8-rings. This layer effectively blocks off diffusion along the c-axis, restricting diffusion in UZM-5 to two dimensions. In the wbc-cage layer, adjacent wbc-cages are fused together via shared 4-rings and linked together through rth-cages via shared 5-rings. Very recently, we have developed a charge density model of aluminosilicate zeolite synthesis and applied it to the CDM syntheses of UZM-9 and UZM-5.10 The initial synthesis solution has been assigned a “global charge density” equivalent to the initial OH-/(Si + Al) ratio, and the CDM syntheses have been described in terms of a temperature-driven charge density disproportionation of the reaction mixture to zeolitic solids with lower charge densities and solution-based products with higher charge density. Confronting the CDM barrier, solid formation is motivated by the Coulombic stabilization enabled by the crystallization SDAs (TMA+ and Na+), with low synthesis temperatures driving the disproportionation to a lesser extent, resulting in relatively high charge density solids with Si/Al ratios significantly lower than the ratio (8.0) of the initial synthesis solution. Higher synthesis temperatures drove the disproportionation to a greater extent, allowing the formation of lower charge density, less Coulombically stabilized, higher Si/Al ratio products with the concomitant incorporation of CDM SDA (TEA+) with a low charge density. Because of the tunable nature of the charge density of the solid and incorporation of TEA+ with temperature in this system, it was possible to attain the amorphous mixed cation compositions that are eventually converted to crystalline UZM-9 and UZM-5. The purpose of this work is to elucidate the formation pathway for UZM-5 zeolite crystals in the TEA+-TMA+-Na+ mixed SDA system. Here we use 13C MAS NMR and IR spectroscopies in order to show that like the case of UZM-9, the nucleation of UZM-5 begins with the formation of lta-cages that are largest among its four different structural building units. We have also determined the structure of as-made UZM-5 crystallized in the presence of organic SDAs only (i.e., TEA+ and TMA+) using synchrotron powder X-ray diffraction and Rietveld analyses, which provide clear evidence for the construction of the second largest wbc-cages around the preorganized lta-cages as the next logical step. Finally, rth- and d4r-cages have been proposed to be in turn built up by the coupling of wbc-cages along the a- and b-axes and along the c-axis, respectively, leading to embryonic UZM-5 zeolite crystals. We also discuss alternatives to this scenario, including assemblage from secondary building units (SBUs) and nanoparticles.


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Figure 1. UFI structure and its four different building units.

Experimental Section

Synthesis.

A

clear

synthesis

solution

with

the

composition

8.0TEAOH·(1-

x)TMACl·xNaCl·0.5Al2O3·8.0SiO2·240H2O, where x is 0 or 0.5, was prepared by combining TMACl (97%, Aldrich), TEAOH (35% aqueous solution, Sachem), NaCl (≥99.5%, Aldrich), aluminum tri-sec-butoxide (Al[O(s-Bu)]3, 97%, Aldrich), tetraethylorthosilicate (TEOS, 98%, Aldrich), and deionized water. Further details on its preparation can be found in our recent papers.9,10 The final synthesis solution was stirred at room temperature for 1 day, charged into Teflon-lined 23-mL autoclaves, and then heated as a function of time under rotation (60 rpm) at 150 °C. The solid products and mother liquors were separated by centrifugation (15000 rpm, 10 min). The recovered solids were redispersed in deionized water using an ultrasonic bath (100 W, 42 kHz) for 60 min and followed by centrifugation, which was repeated three times. Finally, the resulting solids were dried overnight at room temperature. Some solid products obtained in this study were repeatedly exchanged in 1.0 M NaNO3 solutions (1 g solid/100 ml solution) at 80 °C for 4 h. For convenience sake, we will refer to the UZM-5 samples prepared in the presence and absence of Na+ ions as Na-UZM-5 and UZM-5, respectively. Analytical Methods. Powder X-ray diffraction (XRD) patterns were recorded on a PANalytical X΄Pert diffractometer (Cu Kα radiation) with an X΄Celerator detector. The relative crystallinities of a series of solid products recovered after heating at 150 °C for different times were determined by comparing the areas of a rather intense X-ray peak around 2θ = 10.2°,



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corresponding to the (110) reflection of the UFI structure,17 with that of a fully crystallized sample. Crystal morphology and average size were determined by a JEOL JSM-6510 scanning electron microscope (SEM). Solution 27Al, 29Si, and

13

C NMR spectra were recorded in 5-mm

quartz tubes using a Bruker DRX-500 spectrometer, and

27

Al,

29

Si, and

13

C MAS NMR spectra

were measured on a Varian Inova 300 spectrometer at a spinning rate of 6.0 kHz. Further details of solution and solid-state NMR measurements can be found elsewhere.9 The 27Al chemical shifts are reported relative to Al(H2O)63+, and the 29Si and 13C chemical shifts are referenced relative to TMS. Deconvolution and simulation of the 29Si and 13C MAS NMR spectra obtained in this work were carried out using the PeakFit curve-fitting program. The IR spectra in the structural region were recorded on a Nicolet 6700 FT-IR spectrometer using the KBr pellet technique. The concentration of solid sample in KBr pellets was kept constant at 0.02 g of sample per g of KBr, and 256 scans were accumulated for each IR spectrum. The high-resolution powder XRD pattern for the as-made form of the UZM-5 sample obtained using only TEA+ and TMA+ as SDAs was collected on the 9B beamline at Pohang Acceleration Laboratory (Pohang, Korea) using monochromated X-rays (λ = 1.46390 Å). The detector arm of the vertical scan diffractometer consists of seven sets of Soller slits, flat Ge(111) crystal analyzers, anti-scatter baffles, and scintillation detectors, with each set separated by 20°. Data were obtained on the sample at room temperature in flat plate mode, with a step size of 0.01° and overlaps of 0.5° to the next detector bank over the 2θ range 3 – 123.5°. Structure refinements were performed by the Rietveld method18 with the program TOPAS.19 The profile was matched in the range 8.5° ≤ 2θ ≤ 80°. High angle data was not used because of the poor signal to noise ratio. Low angle data was not used because the fits at low angle are poor due to the very small crystallite size in the [110] direction which leads to unusual peak shapes as discussed in details previously.12 Corrections for crystallite size and strain anisotropies were applied. Throughout the course of refinements, T-O distance and O-T-O angles were given soft constraints of 1.63 Å and 109.5°, respectively. The final refinement used 71 variables and gave Rwp and Rp values of 6.7 and 5.4%, respectively.


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Figure 2. Powder XRD patterns for a series of solid products obtained after Na-UZM-5 synthesis under rotation (60 rpm) at 150 °C. Results and Discussion

Figure 2 shows the powder XRD patterns of a series of solid products obtained after NaUZM-5 synthesis as a function of time under rotation (60 rpm) at 150 °C, and Supporting Information Table S1 lists their chemical composition data. Two X-ray peaks around 2θ = 10.2 and 22.9° due to the (110) and (310) reflections of the UFI structure, respectively,17 began to be detectable after 24 h, and crystallization was almost complete after 72 h. The lack of (00l) reflections at the early stage of Na-UZM-5 synthesis, when the crystalline order was detectable by powder XRD, suggests that the growth rate of Na-UZM-5 crystals is considerably higher along the a- and b-axes than along the c-axis. To follow the formation pathway of Na-UZM-5 crystals, solution and solid-state

29

Si,

27

Al,

and 13C NMR spectroscopies were used to characterize the mother liquors and solids isolated in the time series described above, respectively. The

29

Si solution NMR spectra in Figure 3 reveal

that the concentration of silicate prismatic hexamers (d3r-cages) in the mother liquor is



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Figure 3. (a) Solution 29Si NMR spectra of a series of mother liquors and (b) 29Si MAS NMR spectra of the corresponding solid products separated after Na-UZM-5 crystallization under rotation (60 rpm) at 150 °C for different times. considerably higher than that of any of the other (alumino)silicate species, including d4r clusters, even after 36 h of heating, when two X-ray peaks around 2θ = 10.2 and 22.9° due to the (110) and (310) reflections of the UFI structure, respectively, are clearly visible (Figure 2). This suggests that like Na-UZM-9 crystallization,9 Na-UZM-5 crystallization does not require the presence of a particular type of its structural building units (e.g., d4r-cages) in the liquid phase. However, there is a notable decrease in intensity of all 29Si lines, including the most intense line around -90 ppm due to the silicate double 3-ring unit in the spectrum of a starting aluminosilicate synthesis solution,20 with increasing crystallization time. After 120 h, 29Si lines for the various Si species are hardly detectable. We also found that the 29Si MAS NMR spectra of X-ray amorphous solids are characterized by one very broad line around -101 ppm. When crystallization time is longer than 24 h, this 29Si line disappears and four much narrower lines around -97, -102, -108, and -114 ppm begin to be resolved. This is in good agreement with the concomitant dissolution of the initially formed amorphous solids and the growth of Na-UZM-5 crystals (Figure 2). While deconvolution of the

27

Al NMR spectrum of an unheated synthesis solution indicates

the presence of four components around -75, -70, -65, and -60 ppm assignable to qn(nSi) species with n = 1 – 4, respectively,21 the spectrum becomes featureless after heating at 150 °C for only 18 h, giving one broad

27

Al line around 70 ppm (Supporting Information Figure S1). This

indicates that Al species in the solution phase during Na-UZM-5 synthesis are mainly nonmonomeric. We have previously observed similar

27

Al NMR line shape changes from the


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solution 27Al NMR spectra of the mother liquors recovered from Na-UZM-9 synthesis at 100 °C for different times.9 The

27

Al MAS NMR spectra show only one broad line around 51 ppm,

typical of tetrahedral Al,22 for both crystalline and X-ray amorphous solids. On the other hand, the solution

13

C NMR results show that two new lines appearing around 11 and 46 ppm, which

should be assigned to the CH3 and CH2 carbons of triethylamine (NEt3), respectively,23 are clearly observed after 18 h at 150 °C and become stronger at longer synthesis times (Supporting Information Figure S2). Since they are hardly detectable from the mother liquors separated after Na-UZM-9 crystallization at 100 °C for 48 h or so,9 it is clear that the decomposition of TEA+ is more severe at 150 °C, i.e., during the synthesis of Na-UZM-5. However, the absence of NEt3 lines in the

13

C MAS NMR spectra of the solid products isolated simultaneously indicates that

this small amine takes no part in Na-UZM-5 crystallization (see below). Figure 4 shows the 13C MAS NMR spectra of a series of solid products recovered after NaUZM-5 synthesis at 150 °C for different times, and Table 1 lists the chemical shifts, line widths, and relative intensities of 13C MAS NMR lines of TMA+ and TEA+ ions in this series of solids. It has long been recognized that the

13

C chemical shift of organic SDAs, especially of TMA+, is

highly sensitive to the size of the cavity within which they become encapsulated during zeolite synthesis, but it is almost independent of the framework Al content of the zeolite host.22 For example, the TMA+ ions within the small sod-cages and the larger lta-cages in LTA zeolites have been repeatedly shown to exhibit the

13

C MAS NMR line around 59 and 57 ppm,

respectively.6,9,24-26 Compared with the 13C line of the corresponding cation in D2O, these values are shifted to lower field by ca. 3 and 1 ppm, respectively. The spectrum of the X-ray amorphous solid recovered after 12 h, which has a considerable organic content of 13.6 wt % (Supporting Information Table S1), gives the TMA+ CH3 13C NMR line at 56.6 ppm, as well as the TEA+ CH2 and CH3 13C NMR lines at 52.8 and 6.5 ppm, respectively. When this sample was exchanged in 1.0 M NaNO3 solutions at 80 °C for 4 h, its organic content was decreased to ca. 9 wt % (Supporting Information Figure S3). However, we found that while no further significant decrease in organic content was caused by repeated ion exchange, the TEA+ 13C NMR lines are hardly observable (Supporting Information Figure S4). Therefore, it is most likely that most TMA+ ions in this amorphous solid are placed within the non-exchangeable sites, probably within lta-cages and/or analogous cages which can be readily transformed into lta-cages. Figure 4 also shows that although some cages in the initially formed solids could be slightly larger/smaller than



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Figure 4. 13C MAS NMR spectra of the solid products recovered after Na-UZM-5 synthesis under rotation (60 rpm) at 150 °C for different times. To more clearly display changes in the 13C NMR line intensity, the relative intensities are referenced relative to the height of the CH3 13C line due to TMA+ around 57 ppm. The solution 13C NMR spectra of TMACl, and TEABr in D2O (bottom traces) are also given for comparison. an lta-cage or even incomplete, this may have little influence on the

13

C chemical shift of the

organic SDAs occluded, like the case of Na-UZM-9.9 Another interesting observation obtained from Figure 4 is that the CH3 13C NMR line not only of TMA+, but also of TEA+, shifts gradually to lower field with increasing synthesis time. As evidenced by the curve deconvolution results in Table 1 and Supporting Information Figure S5, in fact, two new 13C lines at 57.6 and 7.4 ppm, which should also come from TMA+ and TEA+, respectively, become stronger until Na-UZM-5 fully crystallizes. This trend is essentially identical with that observed for the

13

C MAS NMR spectra of the solid products separated as a


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Table 1. Chemical Shifts, Line Widths, and Relative Intensities of 13C MAS NMR Resonances of TMA+ and TEA+ Ions within the Solid Products Recovered as a Function of Time during Na-UZM-5 Synthesis under Rotation (60 rpm) at 150 °C 13 C NMR δ,b ppm from TMS TMA+ CH3 TEA+ CH2 TEA+ CH3 wbc-cage lta-cage or wbc-cage interlayer space lta-cage wbc-cage interlayer space samplea TMACl 55.3 TEABr 53.8 10.4 solid(12 h) 56.6 (170) [1.00] 52.8 (290) [0.74] 6.5 (280) [0.86] solid(18 h) 56.6 (160) [1.00] 52.6 (230) [0.72] 7.4 (160) [0.18] 6.5 (160) [0.59] 6.5 (250) [0.53] solid(24 h) 57.6 (90) [0.11] 56.8 (180) [0.89] 52.6 (240) [0.76] 7.4 (240) [0.37] solid(36 h) 57.6 (120) [0.41] 56.8 (130) [0.59] 52.4 (190) [0.60] 7.5 (150) [0.59] 6.5 (120) [0.29] 6.4 (270) [0.37] solid(48 h) 57.6 (110) [0.64] 56.5 (160) [0.36] 52.5 (220) [0.91] 7.3 (240) [0.71] solid(72 h) 57.5 (110) [0.72] 56.4 (130) [0.28] 52.8 (220) [0.88] 7.4 (210) [0.59] 6.4 (230) [0.46] solid(120 h) 57.6 (110) [0.69] 56.3 (140) [0.31] 52.7 (200) [0.94] 7.5 (230) [0.57] 6.4 (240) [0.44] a b Values in parentheses are times heated to obtain the corresponding solid products. Values in parentheses and square brackets are full widths at half-maximum in Hz and relative intensities referenced to the intensity of the deconvoluted component(s) from TMA+ in wbc- or lta-cages, respectively.



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function of time during Na-UZM-9 synthesis at 100 oC.9 Assuming an ellipsoidal shape for both cages, the volume (140 Å3) of the wider but truncated wbc-cages calculated using Marler’s equation27 is slightly smaller than that (160 Å3) of sod-cages. Thus, if TMA+ is accommodated within the former cavity, the CH3

13

C line should not be observed at higher field than the

corresponding line of the cation within the latter one. As shown in Figure 4, however, the new CH3 13C NMR line is observed at higher field (57.6 ppm) than the line around 59 ppm of TMA+ within the sod-cages in Na-UZM-9.9 While the CH3 13C line appearing around 57 ppm (Table 1) could be assigned to the TMA+ ions occluded within lta-cages and/or the analogous cages that are readily transformable to lta-cages, therefore, it is very difficult to ascertain whether the line at 57.6 ppm may be due to the TMA+ ions positioned within wbc-cages of Na-UZM-5. However, we note that the chemical shift of this line is exactly the same as that (57.6 ppm) of the 13C NMR line observed for zeolite Nu-1 (RUT), in which TMA+ is encapsulated within a 16-hedral [44566581] cage with 28 tetrahedral atoms (T-atoms) that is slightly larger than the 14-hedral [45546481]) wbc-cage with 24 T-atoms, but is also truncated with an 8-ring.28 Anyhow, if the line at 57.6 ppm in Figure 4 is not due to the TMA+ within wbc-cages, it could then correspond to the identical cations within lta-cages, but some CH3 groups of which should be located near wbc-cages. A similar assignment can also be applied to the new TEA+ CH3 13C NMR line at 7.4 ppm after 18 h of heating at 150 °C. A combination of elemental and thermal analyses indicates that the fully crystallized NaUZM-5 sample obtained after 72 h at 150 °C contains approximately four TEA+ and four TMA+ ions per unit cell, as well as about one Na+ ion (Supporting Information Table S1). This suggests that there are, on average, two TEA+ ions per lta-cage and one TMA+ ion per wbc-cage. Because accurate information on the TMA+ location within this small-pore zeolite is necessary to elucidate its crystallization mechanism, we attempted to solve the structure of as-made UZM-5 by synchrotron powder XRD and Rietveld analyses. Here we selected the sample prepared in the Na+-free system rather than that synthesized in the Na+-containing system in order to avoid any possible influence of the presence of Na+ on the location of TMA+ in UZM-5. For the Rietveld refinement, initial framework atom positions were taken from the published structure data for a calcined UZM-5 sample containing no organic SDAs. 12 The approximate positions of intrazeolitic TEA+ and TMA+ ions were located at the difference Fourier map, and all cations


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Figure 5. Rietveld plot for as-made UZM-5: observed (blue), calculated (red), and difference (lower gray trace). The tick marks indicate the positions of allowed reflections.

Figure 6. (a) The structure of TMA+ occluded within the wbc-cage of as-made UZM-5 and (b) the lowest-energy conformation of the free TMA+ cation (blue for the T-atom, where T is Si or Al, red for O, black for C, pale blue for N, and pink for H). were added in general positions as rigid bodies using the Z-matrix formalism. The rigid bodies were allowed to translate and rotate as a whole, and all torsional angles about the C-N and C-C bonds were allowed to vary. The single TEA+ and TMA+ ions were refined to disordered positions within the lta- and wbc-cages, respectively, with an occupancy of near 0.125 in both cases. The final unit cell parameters obtained were a = 12.3415(10) Å, c = 28.342(7) Å (tetragonal I4/mmm). The final Rietveld plot for as-made UZM-5 is displayed in Figure 5, and the final atomic positions can be found in Supporting Information Table S2. The average T-O bond distance (1.62 Å) and average O-T-O and T-O-T angles (109.4 and 152.2°, respectively) were found to be in good agreement with those expected for zeolites. To check the possibility that the



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Table 2. Selected Distances between the CH3 Carbons of TMA+ and the Oxygen Atoms of wbc-Cages in As-Made UZM-5a C-O bond distance (Å) C-O bond distance (Å) C1-O4 2.84 C3-O1 3.73 C1-O5 3.49 C3-O3 3.27 C1-O6 2.53 C3-O5 3.48 C1-O7 2.87 C4-O6 3.20 C2-O3 3.32 C4-O7 3.50 C2-O4 3.76 C2-O5 3.35 a Selected only when the distance between the CH3 carbon and framework oxygen is shorter than 4.0 Å. TEA+ and TMA+ ions were located within the wrong cavities, refinements with the two ions switched were tried. However, these refinements gave significantly poorer fits. Figure 6 shows the structure of TMA+ within the wbc-cage. Therefore, it is clear that the CH3 13

C line appearing at 57.6 ppm in Figure 4 must be due to the TMA+ ions located within the wbc-

cages in Na-UZM-5 (Table 1). This indicates that Na-UZM-5 nucleation begins with the formation of large lta-cages and/or analogous cages in the solid phase and/or at the solid-liquid interface and their formation takes precedence over that of smaller wbc-cages, like the formation pathway for Na-UZM-9 crystals in the identical mixed-SDA system.9 The shortest distance among the distances between the CH3 hydrogen atom of TMA+ and the framework oxygen atom of wbc-cages was determined to be 2.19 Å, which falls on the borderline that characterizes moderate and weak hydrogen bonds.29 The distance (2.53 Å) between the CH3 carbon and the framework oxygen, where this shortest hydrogen bonding is involved, is fairly shorter than the average distance (3.06 Å) between the TMA+ carbon atom and the framework oxygen atom of 14-hedral ([4668]) sod-cages in TMA-sodalite (SOD) prepared in the absence of inorganic cation. However, the average distance (3.28 Å) between the CH3 carbon and the framework oxygen in as-made UZM-5 was calculated to be longer than that (3.06 Å) of TMA-sodalite, probably due to the wider girth of and the presence of 8-rings truncating the wbc-cages.30 We should note here that the all C-H···O angles of C-O bonds listed in Table 2 are always greater than 100°, suggesting their hydrogen bonding character.31 Therefore, it is not difficult to rationalize why the CH3 13C NMR line of TMA+ within the wbc-cages in Na-UZM-5 is observed at higher field (57.6 ppm) than the line around 59 ppm of TMA+ within the sod-cages in Na-UZM-9.9 When comparing the 13C MAS NMR spectra (Supporting Information Figure S6) of the solid products recovered as a function of time during the synthesis (Supporting Information Figure S7) of UZM-


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Figure 7. IR spectra in the structural region of the solid products recovered after Na-UZM-5 synthesis under rotation (60 rpm) at 150 °C for different times. 5 in the presence of TEA+ and TMA+ ions only, on the other hand, we were able to confirm that the formation of wbc-cages occurs after that of larger lta-cages. This implies that the formation pathway for UZM-5 zeolite crystals is hardly altered by the presence of inorganic SDAs like Na+ ions in the synthesis mixture. Figure 7 shows the IR spectra in the structural region of a series of solid products, which were subject to 13C MAS NMR analysis. While a weak band around 545 cm-1 assignable to the single 5-ring (s5r) unit, a part of both wbc- and rth-cages, begins to be detected in the IR spectra of the solid products isolated after 30 h or longer of heating at 150 °C, a band around 570 cm-1 due to the d4r-cage unit9,12,32 is clearly observable only after 48 h. Given the connecting mode of building units in the UFI structure (Figure 1),16 we can reason that d4r-cages are formed at least after wbc-cages. The evolution of the compositions of solid products during the crystallization of Na-UZM-9 and Na-UZM-5 has been detailed in our recent paper.10 The early solid products observed for



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both Na-UZM-9 and Na-UZM-5 syntheses are zeolitic compositions, judged by the cation/Al ratios and the entrapment of organic SDAs within nonexchangeable sites. This state corresponds to the “secondary amorphous phase” described by Cundy and Cox that exhibits some features of the zeolites being synthesized and immediately precedes nucleation.33 The initially observed solids, formed in cation-assisted syntheses by the action of crystallization SDAs (TMA+ and Na+), begin a migration from lower to higher Si/Al ratio (higher to lower charge density) during the course of zeolite crystallization, with an accompanying increase in incorporation of CDM SDA (TEA+) with a low charge density as the reactions proceed. At any synthesis condition, Coulombic stabilization favors higher charge density materials, but with increasing temperature, it is possible to stabilize lower charge density, less Coulombically stabilized, higher Si/Al ratio frameworks. Hence, the early kinetic products with high charge densities dominated by incorporation of crystallization SDAs are slowly transformed to the products with lower charge densities which can be supported by the crystallization temperature and the existing CDM barrier to TEA+ incorporation. During this charge density migration, one can imagine that at the local level at any time there can be a range of charge densities (Si/Al ratios) that can be stabilized by various combinations of TEA+, TMA+, and Na+. During the induction time (24 h) for Na-UZM-5, the Si/Al ratio increased from 5.45 (12 h) to 5.59 when the appropriate nuclei appeared that could lead to the Na-UZM-5 product with Si/Al ratio of 7.13 after 72 h.10 After 24 h, in addition, the fractional yield of Na+ is a maximum of 0.39, while that of TMA+ 0.48 (Table 2, ref. 10). After 72 h, when Na-UZM-5 is fully crystallized, on the other hand, the fractional yields of Na+ and TMA+ are 0.20 and 1.04, respectively. This indicates that Na+ has been released by dissolution of the initially formed solids, while TMA+ is a limiting reagent in the crystallization. As Na+ is released from the solid product, in the same period between 24 and 72 h, the TEA+/Al ratio increases from 0.33 to 0.48 to accommodate Na-UZM-5 with a lower charge density. Meanwhile, in the Na+-free synthesis of UZM-5, the TMA+-TEA+ combination with a lower charge density promotes the formation of a solid with Si/Al = 6.12 only after 12 h, which is closer to the Si/Al ratio (7.1) of fully crystallized Na-UZM-5.10 It is worth noting that UZM-5 is among the zeolites with low Si/Al ratios containing 5-rings in their framework structures. Therefore, it appears that the migration of the initially formed amorphous solids to crystalline products with higher Si/Al ratios appears to be requisite before 5-rings are stabilized.


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Figure 8. Schematic illustration of a possible formation pathway for UZM-5 crystals in the TEA+-TMA+ mixed-organic SDA system, both with and without a small amount of inorganic SDAs present. Full wbc-cage layers, a product of step c, contain two different orientations U (up) and D (down) with respect to the orientation of their 8-ring along the c-axis. Figure 8 shows a possible formation pathway for UZM-5 crystals in the TEA+-TMA+ mixedorganic SDA system, which has been derived based on the characterization results presented thus far. First of all, aluminosilicate species with relatively low molecular weight are organized around not only TMA+ but also TEA+ to construct large lta-cages as a nucleation center in the solid phase and/or at the solid-liquid interface (step a). This early TEA+ incorporation is possible at 150 °C because the additional energy allows networks with low charge densities (high Si/Al ratios) to be stabilized. Unlike the synthesis of Na-UZM-9, however, the presence of Na+ is not crucial for that of UZM-5, because UZM-5 can crystallize without the aid of any inorganic SDAs. The initially formed lta-cages should also contain some Na+ or TMA+ ions, as well as TEA+, because crystallization SDAs with higher charge densities are required for the condensation of solids from the CDM aluminosilicate solution. This mutual incorporation of both CDM and



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crystallization SDAs into the lta-cages and/or the analogous cages is consistent with the 13C MAS NMR results in Figure 4, which reveals a more spacious environment for TEA+ at this early stage of Na-UZM-5 crystallization vs what is observed at the final stages. The next step is the construction of wbc-cages around the preorganized lta-cage by sharing 6-rings (step b). To balance the negative framework charges created by Al incorporation, TMA+ should be placed in the proximity of the outer surface of the lta-cage, as corroborated by the fact that each wbc-cage is occupied by this small organic SDA (Figure 6). On the other hand, attachment of wbc-cages to the preorganized lta-cage at shared 6-rings can occur in random orientations that will not allow further nucleation. In fact, two adjacent wbccages attached to the lta-cage via shared adjacent 6-ring must be oriented in proper registry so that an rth-cage can be formed by interconnecting a pairs of wbc-cages along the a- or b-axis. Considering only the attachment of wbc-cages to lta-cages via shared 6-rings, probability of attaching a single wbc-cage to an lta-cage in a manner suitable for the subsequent growth of wbccage layers, which is characterized by orienting the 8-rings of both lta- and wbc-cages along the same axial direction, is 3/6, i.e., three of the six orientations related by a 60° rotation are suitable. The orientation for the wbc-cage layers is established after the attachment of the first cage, so to build further upon this unit, only one of the six orientations for the wbc-cages is now appropriate. If such is the case, the probability of attaching a total of eight wbc-cages to the lta-cage (step b) in a manner similar to that described above could then be calculated to be 3/6 × (1/6)7, i.e., 1.8 × 10-6. This suggests that while the formation of particular, viable zeolite nuclei may be difficult, a prominent dissolution/recrystallization process should exist to rectify the errors. The early formation of similar wbc-lta cage aggregates, but unviable for growth, may account for the early observation of 5-rings before the d4r-cages that come with crystallization (Figure 7). Once such a composite cage system consisting of one lta-cage surrounded by eight wbc-cages in a particular orientation are formed, each of its wbc-cages connects to another wbc-cage through an rth-cage along both a- and b-axes (step c), which provides two different types of sites to propagate the growth of wbc-cage layers. One type of sites is located at the 8-rings on the top and bottom (along the c-axis) of the lta-cage, where a next step would be the formation of a wbc-cage by sharing an 8-ring with the lta-cage and 4-rings and 5-rings with the adjacent four wbc-cages and rth-cages, respectively. These wbc-cages have the opposite orientation along the c-axis than the four wbc-cages initially attached to the initial lta-cage. The other type of sites is a concave


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surface formed by two 4-rings on two adjacent wbc-cages and the 5-ring of the rth-cage that connects them. A wbc-cage could be built around a TMA+ cation placed at these sites, again leading to a wbc-cage with the opposite orientation along the c-axis than the two wbc-cages attached to each other. Continuous growth at the two different types of sites results in two 2-dimensional layers of up and down oriented wbc-cages. These wbc-cage layers grow comparatively rapidly not only because of their high bond density, but also because of the strong structure-directing ability of TMA+ in wbc-cage formation. As shown in Figure 8, consequently, an open wbc-cage interlayer space, which is actually flooded with TEA+, the most abundant cation in the synthesis mixture, should be shortly converted to a layer of lta-cages along the a- and b-axes (step d). This annealing step includes the interconnection of the 4-rings of two opposing wbc-cages layers to form interlayer d4r-cages and 8-rings. Recall that d4r-cage formation cannot be in advance of wbc- and rth-cage formations (Figure 7). Because step d is not cation-dependent, in addition, TMA+ and Na+, as well as TEA+, can be encapsulated within the newly formed interlayer ltacages. The fact that the cation/Al ratio of all solid products isolated during Na-UZM-5 synthesis is always larger than unity (Supporting Information Table S1) suggests the occlusion of even their OH- form in lta-cages. It is interesting to note here that when the solid products separated after Na-UZM-5 synthesis at 150 °C for different times are exchanged in 1.0 M NaNO3 solutions at 80 °C for 4 h, the

13

C line at 6.5 ppm due to the CH3 carbons of TEA+ becomes hardly

detectable, unlike the line at 7.4 ppm (Supporting Information Figure S4). This suggests that the TEA+ cations responsible for the appearance of the former

13

C line are located in exchangeable

sites, most likely in wbc-cage interlayers. It is also remarkable that UZM-5 typically appears as thin plate-like crystallites.6,12 This implies that the crystal growth along the c-axis is energetically less favorable than the extension of wbc-cage layers along the a- and b-axes. The SEM images of a series of solid products isolated as a function of time during Na- UZM-5 synthesis at 150 °C (Supporting Information Figure S8) reveal that such crystallites begin to be observable after 36 h. We also note that while there is a lack of (00l) reflections (e.g., the (002) around 2θ = 6.2°) of the UFI structure in the powder XRD pattern of the solid recovered after 24 h, the (110) and (310) reflections around 2θ = 10.2 and 22.9°, respectively,17 are already detectable (Figure 2). One possible option for the growth along the c-axis is the addition of a d4r-cage at the site of a 4-ring of the outer wbc-cage surface.



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Because the d4r-cage is too small to encapsulate any cation used as an SDA here, however, this type of d4r-cage formation cannot be favorable from an electrostatic point of view. Therefore, it appears that the formation of new lta-cages over the outer surface of the resulting lta-wbc-rth-d4r composite cage system (step e), which resembles the bottom or top half of an lta-cage, is a more plausible option. As described above, this step can be further facilitated with the presence of TEA+ and/or TMA+ ions near the outer surface of the composite cage system. If so, the crystal growth would occur at the interface with the aluminosilicate solution with the incorporation of TEA+, TMA+, and/or Na+ into the solid phase. Another possible mechanism that might be considered include the construction of UZM-5 from SBUs34,35 or nanoslabs.36 In fact, it is possible to construct the UFI topology by connecting d4r- and rth-cages as SBUs to each other in a ratio of 1 to 2.12 However, construction from several CBUs may be problematic because of the many possibilities. For example, d4r-cages could be connected to form the LTA topology, and rth-cages could be connected to form the RTH topology. In particular, their mixtures could generate a variety of framework structures besides the UFI topology. Having the two different SBUs simultaneously available in appropriate concentrations may also be problematic. Further complications arise from the presence of multiple SDAs. Combination with lta- and rth-cages may again lead to many possible outcomes. As described above, there is no observation of 5-rings in the X-ray amorphous solid products obtained after Na-UZM-5 synthesis at 150 oC for 24 h or shorter (Figure 7), suggests the absence of rth-cages. When crystallization does start at 24 h, the aluminosilicate solution has been depleted of Si and Al, the fractional yields of Si and Al are 0.38 and 0.53, respectively, and the OH-/(Al + Si) in solution increases to 1.35 from the starting value of 0.89.10 It is counter-intuitive to expect the formation of a large 12T species like rth-cage at this stage of the synthesis when the increased OH-/T-atom level would suggest that the aluminosilicate species should be getting smaller. The flexibility required in the chemistry discussed above, such as the changing Si/Al ratios and varying SDA contents, as well as the complicated nature of the UFI topology makes it highly unlikely that a SBU assemblage mechanism could account for the formation of UZM-5 crystals. Since UZM-5 crystallization is from a clear solution, a nanoslab mechanism may also be considered. This idea has been applied mostly to the synthesis of pure-silica ZSM-5 (MFI),11,33 a very simple system which consists of one T-atom (Si) and one SDA molecule,


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tetrapropylammonium. It has also been applied to the early stages of the synthesis of zeolite beta.36 The focus of the characterization is more on the aggregation and coalescence of initially formed nanoparticles to crystals, while at the same time there does not seem to be transformational chemistry occurring with respect to the composition during this process. While we have not performed the necessary characterization to confirm or deny a nanoslab mechanism in ours work, we speculate that a nanoslab mechanism of UZM-5 formation would not have the flexibility accommodate the dynamic compositional transformation observed in a UZM-5 synthesis. When the crystallization of Na-UZM-5 begins at 24 h, ca. 60 % of the T-atoms are still in solution. Considering a coalescence of nanoparticles of “crystallization composition” formed at this stage of the synthesis, if the nanoparticle morphology resembles that seen for the NaUZM-5 crystals, namely extensive growth in the a- and b-axes, with limited growth along the caxis, it does not appear that such thin plates could aggregate on edge and coalesce to form a larger plate. To accommodate the complexities of the CDM synthesis of UZM-5, including the changing Si/Al ratios and multiple SDA contents, a flexible mechanism involving synthesis from simple species along the lines of that outlined by Cundy and Cox should prevail.33

Conclusions

The overall characterization results of our study have allowed us to propose a reliable formation pathway for UZM-5 zeolite crystals from their discrete building units: the construction of the largest lta-cages, with concomitant incorporation of TEA+ and TMA+, precedes that of the other three building units. Similar to the case of UZM-9, the second largest wbc-cages are built around the preorganized lta-cage by sharing 6-rings in an appropriate orientation for further nucleation, when TMA+ is located in the proximity of the outer surface of a single lta-cage. A logical next step is the growth of wbc-cage layers on the lta-cage surface which includes the rthcage connection along the a- and b-axes. The wbc-cage interlayer space formed is then converted to a layer of lta-cages via formation of the smallest d4r-cages. Once such UZM-5 nuclei or embryonic crystals are formed, lta-cages are built over their outer surface, which takes after the halved lta-cages, in the presence of TEA+ and/or TMA+ ions. Finally, a similar self-assembly process may take place over newly formed lta-cages, yielding UZM-5 zeolite crystals.



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Associated Content

Supporting Information Chemical composition data, powder XRD patterns, and solution and solid-state

27

Al and

13

C

NMR spectra for a series of mother liquors and solid products recovered after Na-UZM-5 and/or UZM-5 synthesis at 150 °C for different times; and final atomic coordinates for as-made UZM-5. This material is available free of charge via the Internet at http://pubs.acs.org.

Author Information

Corresponding Author E-mail: [email protected]

Present Address §

Institue for Chemical and Bioengineering, ETH Zurich, Wolfgang-Pauli-strasse 10, CH-8093

Zurich, Switzerland

Notes The authors declare no competing financial interest.

Acknowledgment This work was supported by UOP LLC and the National Creative Research Initiative (2012R1A3A2048833) and BK 21-plus programs through the National Research Foundation of Korea funded by the Korea government (MSIP). We thank PAL (Pohang, Korea) for synchrotron diffraction beam time. PAL is supported by MSIP and POSTECH.


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References

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TOC Graphic of Park et al., “Crystallization Mechanism of Zeolite UZM-5”


Crystallization Mechanism of Zeolite UZM-5 - Chemistry of Materials

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