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Crystallization Mechanism of a Family of Embedded Isoreticular Zeolites Jung Gi Min, Hyun June Choi, Jiho Shin, and Suk Bong Hong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04499 • Publication Date (Web): 14 Jul 2017 Downloaded from http://pubs.acs.org on July 18, 2017

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The Journal of Physical Chemistry

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Crystallization Mechanism of a Family of Embedded Isoreticular Zeolites

Jung Gi Min, Hyun June Choi, Jiho Shin, and Suk Bong Hong* Center for Ordered Nanoporous Materials Synthesis, Division of Environmental Science and Engineering, POSTECH, Pohang 37673, Korea E-mail: [email protected]

ACS Paragon Plus Environment


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ABSTRACT We have recently been successful in predicting and synthesizing a series of body-centered cubic zeolites with expanding structural complexity and embedded isoreticular structures, termed the RHO family. Here we propose a plausible formation pathway for ECR-18, ZSM25, and PST-20, the three members of this zeolite family, in the presence of tetraethylammonium ions as an organic structure-directing agent, based on the

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C MAS

NMR, IR, and CO2 adsorption results obtained from the solid products recovered as a function of time during their crystallization processes, together with the quantum-chemical calculation results. The nucleation of these three zeolites, all of which consist of seven different structural units, begins with the almost simultaneous construction of 26-hedral lta and 14-hedral t-plg cages and their subsequent connection via shared 8-rings in the diagonal direction of the cubic unit cell. As a logical next step, 8-hedral t-oto and 12-hedral t-phi cages are built around the preorganized t-plg cages. The remaining embedded spaces are readily filled up with 10-hedral t-gsm cages, as well as with t-oto and t-phi cages. Finally, 10-hedral d8r and 14-hedral pau cages are alternately constructed along the cubic edges. Over the outer surface of the resulting nuclei of the RHO family zeolites, the crystal growth may occur in a similar manner as described above.


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INTRODUCTION Zeolites and related crystalline materials are important in established fields such as catalysis and molecular separations and in emerging applications in energy and environment, and the outstanding performance of these microporous solids comes primarily from their unique pore architectures and chemical compositions.1-4 Hence, the synthesis of zeolites with desired framework structures and/or compositions for specific applications is one of the utmost concerns in the modern chemical industry. This, however, remains unsettled, mainly due to the poor understanding of the molecular-level mechanisms for zeolite crystallization.5-10 Recently, we have discovered a novel family of zeolites characterized by embedded isoreticular structures, as well as by expanding structural complexity.11-14 The existence of this zeolite family came to light when the structure of ZSM-25 (framework type MWF), first described by Mobil researchers in 1981,15 was determined by the combined use of electron diffraction data and phase information from the already known structures of zeolites Rho (RHO) and paulingite (PAU). While ZSM-25 was found to have the largest unit cell volume of all zeolites known by that time, we were also able to predict several more complex members of the so-called RHO family with a body-centered cubic structure using new structural principles and to synthesize them (i.e., PST-20, PST-25, PST-26, and PST-28) via a rational approach.11,12 Upon the consecutive insertion of a pair of 10-hedral ([4882]) d8r and 18-hedral ([41286]) pau cages between the two 26-hedral ([4126886]) lta cages of each scaffold or unit-cell edge of the ZSM-25 structure, the unit cell dimension of the RHO family increases by ca. 10 Å per one generation. Because the space between the scaffolds is completely filled up with four other types of cages (i.e., 14-hedral ([466286]) t-plg, 8-hedral ([4583]) t-oto, 10-hedral ([4684]) t-gsm, and 12-hedral ([4785]) t-phi cages), in addition, all the family members are both isoreticular and embedded in nature.



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The structures of Rho and paulingite, the first (RHO-G1) and third (RHO-G3) generations in the RHO family, can be made by removing three pairs and one pair of d8r and pau cages from all scaffolds of the ZSM-25 structure (RHO-G4), respectively. Also, those of PST-20 (RHO-G5), PST-25 (RHO-G6), and PST-26 (RHO-G7), and PST-28 (RHO-G8) can be obtained by inserting one, two, three, and four pair(s) of d8r and pau cages into the ZSM-25 unit cell edges, respectively. Because the formation of all these members is energetically favorable, the possible number of its members is essentially infinite.11,12 Hence, understanding how such a zeolite family nucleates and grows is of fundamental importance for synthesizing zeolites with designed pore structures and properties in a fully rational manner. In this paper we present the results obtained not only from

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C MAS NMR, IR, and CO2

adsorption measurements of the solid products isolated at different times during the synthesis of ECR-18 (the synthetic analog of paulingite),16 ZSM-25, and PST-20, but also from theoretical calculation results, which have enabled us to propose their reliable formation pathway. We selected these three materials as model systems for elucidating the crystallization mechanisms of the RHO family of zeolites, because all of the reported syntheses include the use of the identical organic structure-directing agent (SDA), i.e., tetraethylammonium (TEA+) ions,11,16-18 unlike the case of Rho that can crystallize even under whole inorganic conditions.19 It is well established that the 13C chemical shift of organic SDA molecules is highly sensitive to the size of the cavity within which they become encapsulated during zeolite synthesis.20 Thus,

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C MAS NMR spectroscopy is useful for understanding

zeolite crystallization at the molecular level. The structures of ECR-18, ZSM-25, and PST-20, together with their seven different cage types are shown in Figure 1, and the numbers of the seven cage types in each of the units can be found in Supporting Information Table S1. EXPERIMENTAL SECTION


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Synthesis and Na+ ion exchange. Three TEA+–containing aluminosilicate gels with different oxide compositions, which proved to give pure ECR-18, ZSM-25, and PST-20, were prepared according to the procedures reported in the literature,11,17,18 and their oxide compositions are given in Table S2. In the case of PST-20 synthesis, seed crystals (2 wt% of anhydrous raw materials) were added prior to stirring at room temperature for 24 h to reduce the level of ZSM-25 impurity to lower than 5%. The seed crystals used here were PST-20 zeolite which was previously prepared at 418 K for 48 h,11 without using seed crystals. The final synthesis mixture was charged into 23 mL Teflon-lined autoclaves and heated under rotation (60 rpm) at a given temperature for different times. The solid products and mother liquors were separated by centrifugation (15000 rpm, 10 min). In the case of the solid products obtained during the synthesis of PST-20, which includes the use of both Na+ and Sr2+ as inorganic SDAs, further washing with copious hot water to remove the excess Sr2+, probably existing as Sr(OH)2 on the external surface of zeolite crystallites, was required. The recovered solids were redispersed in deionized water using an ultrasonic bath (100 W, 42 kHz) for 60 min and reisolated by centrifugation. This process was repeated three times. The resulting solids were dried overnight at room temperature. When necessary, they were stirred several times in 1.0 M NaNO3 solutions (1 g solid/100 mL solution) for 6 or 4 h at 353 K in order to obtain the CO2 sorption isotherms of their TEA+-Na+ form or to check whether the occluded TEA+ ions were exchangeable. Analytical methods. Powder X-ray diffraction (XRD) patterns were collected on a PANalytical X’Pert diffractometer (Cu Kα radiation) with an X’Celerator detector. The relative crystallinities of a series of solid products recovered at different time intervals during ECR-18, ZSM-25, and PST-20 syntheses were determined by comparing the areas of the intense X-ray peaks around 2θ = 10.6, 11.0, and 11.4°, corresponding to the (330), (440), and



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(550) reflections of these three zeolites, respectively,11,21 with those of the corresponding zeolites in the fully crystallized form, respectively. Thermogravimetric analysis (TGA) was conducted on a SII EXSTAR 6000 thermal analyzer, and the weight losses of occluded organic SDAs were further confirmed by differential analyses (DTA) using the same analyzer. Elemental analysis for Si, Al, Na, K, and Sr were carried out using a Jarrell-Ash Polyscan 61E inductively coupled plasma spectrometer in combination with a PerkinElmer 5000 atomic absorption spectrophotometer. The C, H, and N contents of the solid product samples were analyzed by a Vario EL III elemental organic analyzer. The

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C MAS NMR spectra were recorded on an Agilent Unity Inova 600 spectrometer

(KBSI Western Seoul Center) at a 13C frequency of 150.90 MHz with a π/2 rad pulse length of 3.0 µs, a recycle delay of 1.0 s, and an acquisition of ca. 5000 pulse transients. The

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C

chemical shifts are referenced relative to TMS. Deconvolution and simulation of the 13C MAS NMR spectra obtained in this work were preformed using the PeakFit curve-fitting program. The IR spectra in the structural region were collected on a Nicolet 6700 FT-IR spectrometer using the KBr pellet technique. The CO2 adsorption isotherms were measured at 273 K and at pressures up to 1.0 bar using a Mirae SI nanoPorosity-XG analyzer. Prior to the experiments, 0.1 g of sample was evacuated at 523 K for 6 h. The equilibrium conditions for each point on the isotherms were fixed at 98% of the calculated uptake or a maximum equilibration time of 2 h. Quantum-chemical calculations. Seven different cage systems of the MWF structure were used to calculate the stabilization energy for various combinations of Na+, [Na(H2O)6]+, and/or TEA+ ions. The unit cell parameters and atomic coordinates for the MWF structure with space group Im3m were taken from the International Zeolite Association tabulation,21 and all calculations were carried out using the Gaussian 09 program package.22 An M06-


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2X/6-31+G-(d):MNDO ONIOM method23-25 was applied to optimize the geometries of various structures described above. During the geometric optimization, the 5T cluster active center [Al(SiO)4] and located cations were treated at the high M06-2X level, whereas the rest of the tetrahedral atoms (T-atoms) fixed at their crystallographic locations were treated at the low MNDO level. The terminal Si atoms at each cluster edge were capped with H atoms at a Si-H bond distance of 1.47 Å oriented along the direction of the corresponding Si-O bond. The stabilization energy was calculated as the difference between the total energy of the absorption complex and the sum of the energies of the separated guest cations and the cluster model. RESULTS AND DISCUSSION Figure 2 shows the powder XRD patterns and relative crystallinities for the solid products obtained after ECR-18, ZSM-25, and PST-20 syntheses as a function of time under rotation (60 rpm) at 373, 408, and 418 K, respectively, and Table S3 lists the chemical composition data of some selected products from each synthesis. As expected from their crystallization temperatures, the crystallization kinetics was found to be faster in the order ECR-18 < ZSM25 < PST-20. Since the phase purity of PST-20 is more sensitive to the crystallization temperature than those of ECR-18 or ZSM-25,11 it was difficult to isolate the impurity-free crystallization intermediates. Our recent computer simulations and structural analysis of as-made ZSM-25 have shown that the TEA+ cations are located only within the pau and t-plg cages.11 Therefore, the fact that the largest lta cage among a total of seven different types of cages is occupied by inorganic SDA only (i.e., hydrated Na+) is unexpected. This should also be the case of asmade ECR-18 and PST-20, because the structural units present and the organic SDA used are the same for these three zeolites. The

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C MAS NMR spectra of a series of solids isolated



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after ECR-18, ZSM-25, and PST-20 syntheses at 373, 408, and 418 K for different times, respectively, are shown in Figure 3, and the results from their curve deconvolution are given in Table 1. It is well established that the 13C chemical shift of organic SDA molecules can be sensitive to the size of the cavity within which they become encapsulated during zeolite synthesis.20 Thus,

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C MAS NMR spectroscopy is quite useful for understanding zeolite

crystallization at the molecular level. It is clear from Figure 3 that the TEA+ cation remains intact and has not decomposed under the synthesis conditions employed here. We also note that the 13C MAS NMR spectra of the solid products recovered after heating the ECR-18, ZSM-25, and PST-20 synthesis mixtures for 54, 40, and 20 h, respectively, when they are still X-ray amorphous (Figure 2), show two resonances around 53 and 7 ppm due to the CH2 and CH3 carbons of TEA+. Although these materials, which have organic contents of 3.0, 3.0, and 2.6 wt%, respectively (Table S3), were repeatedly refluxed in 1.0 M NaNO3 solution for 4 h, no significant decrease in organic content was observed (Figure S1). This suggests that their TEA+ cations are mainly located at nonexchangeable sites. Another interesting observation from Figure 3 is that when the ECR-18, ZSM-25, and PST20 synthesis gels are heated for prolonged times, their solid products give a new peak at 52.5 ppm or so. It should be noted here that the volume (440 Å3) of 18-hedral ([41286]) pau cage is considerably larger than that (280 Å3) of 14-hedral ([466286]) t-plg cages (Figure S2). Since, if the TEA+ ions are accommodated within pau cages, the CH2 13C peak should be observed at a higher field than the peak of the cation within t-plg cages, we can assign the peak around 52.5 ppm in Figure 3 to the CH2 carbons of TEA+ within pau cages. In fact, both the aluminosilicate zeolite merlinoite (MER) and the silicoaluminophosphate molecular sieve STA-14 (KFI), which were synthesized using TEA+ as an organic SDA and confirmed to have


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this cation within pau cages only, are reported to give the TEA+ CH2

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C NMR peak at an

even higher field (51.5 ppm).26,27 Therefore, it is obvious that during the synthesis of the RHO family of embedded isoreticular zeolites, the formation of t-plg cages and/or such similar cages takes precedence over that of pau cages in solid phase and/or at the solid-liquid phase. However, the interpretation of the 13C NMR resonances at 6.8 – 8.0 ppm in Figure 3, which should be due to the CH3 carbons of TEA+, is more complicated and difficult. Apparently, the chemical shift of the outer CH3 carbons of TEA+ can be more strongly affected than that of the inner CH2 carbons, when this organic SDA is encapsulated within a particular type of zeolite cages. We were able to detect three TEA+ CH2 13C NMR peak at 6.7-6.9, 7.4-7.5, and 7.9-8.1 ppm from the 13C MAS NMR spectra of the solids products separated from the ECR18 and ZSM-25 synthesis series, whereas there is only one broad and asymmetric resonance around 8 ppm in a series of solid products of PST-20. Due to the reason mentioned earlier, at any rate, a CH3 13C peak at 6.8 ppm in the spectra of the solids obtained after 54 and 40 h of heating of ECR-18 and ZSM-25 synthesis mixtures, respectively, must be assigned to the TEA+ ions occluded within the t-plg cage. The CH3 13C peak appearing around 8 ppm in the 13

C MAS NMR spectrum of the solid isolated after 20 h of heating during PST-20 synthesis

was considerably broader than that observed for any of the other solids prepared here (Table 1) so that its deconvolution has not been tried. Figure 3 also shows that a new 13C resonance appears at 8.0 ppm in the spectra of the solids isolated after 108 and 81 h of heating in ECR-18 and ZSM-25 syntheses, respectively. However, there is still no noticeable peak at about 52.5 ppm, corresponding to the CH2 carbons of TEA+ located within pau cages. This strongly suggests that the two

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C peaks at

6.8 and 8.0 ppm are closely related to the formation of t-plg cages. Considering that the

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C

chemical shift of the outer CH3 carbons of TEA+ occluded within t-plg cages can be more



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strongly affected by the surrounding embedded cages such as t-oto and t-phi cages around the preorganized t-plg cages than that of its inner CH2 carbons, we have divided t-plg cages into two groups, i.e., t-plg(i) and t-plg(ii) cages: the former type of t-plg cages is connected only with itself and lta cages only before the formation of the other types of embedded cages around it, whereas the latter one is the t-plg cages around which t-oto and t-phi cages are constructed. If such is the case, the two 13C peaks at 6.8 and 8.0 ppm could then be attributed to the TEA+ ions within the relatively less restricted t-plg(i) and more restricted t-plg(ii) cages, respectively. A quite similar interpretation has already been made in the discussion of the 13C MAS NMR spectra of the solid products recovered during the synthesis of cage-based, smallpore zeolites UZM-5 (UFI) and UZM-9 (LTA) in the presence of tetramethylammonium, as well as TEA+.8,9 Another 13C resonance can be observed at 7.5 ppm when the ECR-18, ZSM-25, and PST-20 synthesis gels are heated for 110, 86, and 38.3 h or longer, respectively. While these crystallization times led the main X-ray reflections of both zeolite structures to clearly appear, the peak at 7.5 ppm remains in the

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C MAS NMR spectra of fully crystallized ECR-18,

ZSM-25, and PST-20 zeolites. As shown in Figure 3, in contrast, the

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C NMR peak at 6.9

ppm assignable to the t-plg(i) cages is now missing in their spectra. Therefore, it can be concluded that the peak at 7.5 ppm is due to the CH3 carbons of TEA+ within pau cages. Figure 4 shows the IR spectra in the structural region for a series of solid products recovered as a function of time during the synthesis of the three members of the RHO family. These results reveal that a weak IR band at 620 cm-1 due to the d8r cage begins to be observable in the spectra of the solid products separated after heating the ECR-18, ZSM-25, and PST-20 synthesis mixtures for 110, 86, and 38.3 h, or longer, respectively.28 It is worth noting that the crystallization times employed to obtain the first two solids are the same as


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those required for the formation of pau cages characterized by 13C MAS NMR spectroscopy (Figure 3). Because differences in the detection limit for the atomic ordering of zeolites between IR and 13C MAS NMR are negligible, the d8r and pau cages in the RHO family of zeolites appear to be formed simultaneously. This can be reasoned by the fact that the cubic scaffolds of the RHO family are constructed with chains of alternating d8r and pau cages between two lta cages. Figure 5 shows the CO2 uptakes at 273 K up to 1.0 bar on the solid products isolated after crystallization of ECR-18, ZSM-25, and PST-20 for different times. When these three members of the RHO family are fully crystallized, all of their seven different types of cages are interconnected via 8-ring windows, the diameters of which are larger than the LennardJones size (3.30 Å) of CO2.29 However, this gas molecule cannot diffuse through the t-plg and pau cages because of the occupation by TEA+ ions. Then, we could exclude the possibility of CO2 adsorption on the lta and d8r cages which are attached to t-plg and d8r cages only and to pau and/or lta cages only, respectively. One may speculate that the lta cages in amorphous or incompletely crystallized RHO family zeolites can adsorb CO2 molecules, if these large cages are formed in a discrete manner during the nucleation step, that is, without connecting to t-plg or d8r cages. However, even if it were so, since there are only two lta cages per unit cell in ECR-18, ZSM-25, and PST-20 (Table S1), the CO2 uptakes on such lta cages in fully crystallized zeolites cannot be significant. Assuming that four CO2 molecules are adsorbed per lta-cage as reported in the literature for zeolite Rho,30 in fact, the lta cages in ECR-18, ZSM-25, and PST-20 were calculated to have CO2 uptakes of only 0.17, 0.08, and 0.04 mmol g-1, respectively. This clearly shows that the high CO2 uptakes (~ 3.5 mmol g-1) at 273 K and 1.0 bar observed for the Na+-TEA+ form of fully crystallized RHO family zeolites (Figure 5) may be mainly due to



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the adsorption on their t-oto, t-gsm, and t-phi cages.13 Figure 5 also shows that the CO2 uptakes (< 0.9 mmol g-1) of amorphous solid products obtained at the early stage of crystallization of ECR-18, ZSM-25, and PST-20 are always considerably smaller compared to the corresponding, fully crystallized materials, although their 13C MAS NMR spectra give a clear indication of the formation of t-plg cages (Figure 3). Of particular interest is a slight but non-negligible increase in CO2 uptake when the crystallization time at 373, 408, and 418 K is 81, 60, and 30 h, or longer, respectively, when each of these solid products is still X-ray amorphous (Figure 2). This cannot be understood without considering that the t-oto, t-gsm, and t-phi cages, which are available for CO2 adsorption, are formed after the times stated above. On the basis of the experimental results presented thus far, therefore, it is clear that these three embedded cages are built up after the formation of t-plg cages, but before that of d8r and pau cages at the nucleation stage of the RHO family zeolites. However, we still have no clear evidence when the largest lta cages are constructed. Despite this, we can reason that from a structural point of view, the lta and t-plg cages would be built around the same time or so in the nucleation process, since the t-plg cages cannot be fully connected by themselves without having lta cages in the body-centered cubic unit cell. To further address this issue, we carried out quantum-chemical calculations of the stabilization energies for various combinations of Na+, TEA+, and [Na(H2O)6]+ in seven different cage systems: discrete lta, t-plg, and pau cages, and cage clusters containing 1 lta cage + 8 t-plg cages, 1 lta cage + 6 d8r cages, 1 t-plg cage + 4 t-oto cages, and 3 t-plg cages interconnected (Figure S4), each with one or two Al atoms in tetrahedral positions. The latter four cluster systems are the possible initial structures built by assuming that the nucleation of ECR-18, ZSM-25, and PST-20 all starts with the simultaneous formation of lta and t-plg


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cages. Here we did not introduce TEA+ into the lta cage in cage systems 1 lta cage + 8 t-plg cages and 1 lta cage + 6 d8r cages, as well as into the discrete lta cage, due to the absence of any organic SDA in this largest cage (Figure 3). The calculation results in Table 2 reveal that the cluster model 1 lta cage + 8 t-plg cages is thermodynamically more stable by at least ca. 16 kcal mol-1 of SDA than the formation of discrete t-plg and pau cages and the other three cage clusters, regardless of the type(s) of guest species present. This indicates that a combination of lta and t-plg cages is considerably more practical than any other cage systems studied here. The 13C MAS NMR results in Figure 3 clearly shows that the formation of t-plg cages takes precedence over that of pau cages. As shown in Table 2, however, the formation of a discrete t-plg cage in the presence of one TEA+ cation was calculated to be energetically less favorable by ca. 36 kcal mol-1 than that of a discrete pau cage. These arguments taken in total led us to believe that the formation of both lta and t-plg cages during the synthesis of RHO family zeolites may be kinetically, rather than energetically, controlled. Given the structure of the cluster system 1 lta cage + 8 t-plg cages, it is most likely that the formation of lta cages cannot be behind that of t-plg cages. Figure 6 shows the proposed formation pathway for PST-20, which we selected as a representative zeolite of the RHO family, based on the overall experimental and theoretical results of this work. As the first stage of nucleation, small (alumino)silicate species are organized around the hydrated Na+ and/or TEA+ cations to almost simultaneously construct the largest lta and the fairly smaller t-plg cages in the solid phase and/or at the solid-liquid interface, respectively (step a). Here, it does not matter whether or not they are structurally incomplete. The lta and t-plg cages formed are then connected by sharing 8-ring windows in the diagonal direction of the cubic unit cell to form a more thermodynamically favorable structure



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(step b), which provides a way to build the PST-20 structure in a more energetically favorable manner. It is worth noting that the phase selectivity in the synthesis of the ‘body-centered cubic’ RHO family of zeolites can be determined by the number of diagonally interconnected t-plg cages between the corner-located and body-centered lta cages. This, at least in part, explains why all the members of the RHO family, except its first generation (Rho) with no tplg cages, have been synthesized using the same organic SDA, i.e., TEA+.11,12,15-18 Because the second generation of this family, which is still hypothetical, has eight t-plg cages per unit cell, we believe that its crystallization may also be possible in the presence of TEA+, when the inorganic synthesis parameters such as the type and concentration of inorganic SDAs are properly controlled. According to the definition of embedded isoreticular zeolites, their structures are composed of embedded cages, some of which should appear with increasing the generation number in order to maintain the structural integrity, as well as of scaffold cages that are expanded in an isoreticular manner.11 From a structural point of view, the t-plg cage is thus not a scaffold cage. From a crystallization point of view, however, its role in the nucleation process is much more important than expected. As shown in Figure 6, four diagonal cage rods, which consist of lta and t-plg cages only, cross one another by sharing an lta cage at the body-centered cubic point in the PST-20 unit cell. We believe that such cage rods may be robust enough to allow the construction of the other embedded type cages around them. This hypothesis looks sensible because the framework structures of all the members in the RHO family are thermodynamically favorable in principle.12 On the other hand, the inorganic cations residing outside the preorganized t-plg cages can promote the formation of real embedded cages, i.e., t-oto, t-gsm, and t-phi cages, in a consecutive order (Figure 6). Considering their spatial shapes, t-oto and t-phi cages are first


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constructed around the preorganized cages (steps c and d). Then, the remaining embedded spaces are subsequently filled up with t-gsm cages, as well as with t-oto and t-phi cages, while sharing 8-rings with t-oto and t-phi cages (steps e and f). It is interesting to note here that the amorphous solid obtained after heating the PST-20 synthesis mixture at 418 K for 20 h has the highest Na+/Al ratio among all solid products in the synthesis series (Figure 7). Therefore, the role of Na+ as an inorganic SDA during PST-20 nucleation appears to become less important after this crystallization time. This can be further supported by the synchrotron powder XRD and Rietveld analyses that the Sr2+ cations in PST-20 are mainly located within these three cages,8 which is also the case of K+ ions in ECR-18.31 Finally, as supported by the 13C MAS NMR and IR results in Figures 3 and 4, lta cages are connected with pairs of d8r and pau cages along the unit cell edges, leading to PST-20 nuclei. A similar self-assembly process may allow the formed nuclei to further grow in the presence of TEA+, Na+, and Sr2+ ions. CONCLUSIONS The overall characterization results have enabled us to propose a general crystallization mechanism of the RHO family of body-centered cubic zeolites with embedded isoreticular structures containing seven different types of cages. The formation pathway for this zeolite family was found to start with the construction of the largest lta cages and considerably smaller t-plg cages. In the next step, the diagonally crossed cage rods, consisting of these two cages only, are constructed. From a crystallization point of view, these cage rods may in our view play a role as a skeleton in the nucleation of the body-centered cubic crystals of the RHO family. Then, both t-oto and t-phi cages are built around the preorganized t-plg cages. The rest of embedded spaces are filled up with t-oto or t-phi cages again, as well as with tgsm cages. The nucleation of the RHO family zeolites is further facilitated by alternately building d8r and pau cages along the cubic unit cell edges. The fact that the t-plg cage is one



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of the cages first formed in the presence of TEA+ during the nucleation process of this family may explain why all of its already known members, except Rho, have thus far been synthesized using the same organic SDA, i.e., TEA+. We hope that this work will provide a basis for the synthesis of new families of zeolites with embedded isoreticular structures. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxxx/jacs.xxxxxx. Numbers of each of cage types in RHO family zeolites; chemical composition data of

synthesis

plg

cage

mixtures

dimensions;

and

various

solid cage

products;

systems

pau

employed

and in

t-

quantum-

chemical calculations.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Jiho Shin: 0000-0002-0279-4006 Suk Bong Hong: 0000-0002-2855-1600 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Creative Research Initiative Program 2012R1A3A-2048833) through the National Research Foundation of Korea.


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REFERENCES (1) Davis, M. E. Ordered porous materials for emerging applications. Nature 2002, 417, 813-821. (2) Camblor, M. A.; Hong, S. B. In Porous Materials; Bruce, D. W., Walton, R. L., O’Hare, D., Eds.; Wiley: Chichester, U.K., 2011; pp 265-325. (3) Moliner, M.; Rey, F.; Corma, A. Towards the rational design of efficient organic structure-directing agents for zeolite synthesis. Angew. Chem. Int. Ed. 2013, 52, 1388013889. (4)

Moliner, M.; Martínez, C.; Corma, A. Multipore zeolites: synthesis and catalytic applications. Angew. Chem. Int. Ed. 2015, 55, 3560-3579.

(5) Aerts, A.; Kirschhock, C. E. A.; Martens, J. A. Methods for in situ spectroscopic probing of the synthesis of a zeolite. Chem. Soc. Rev. 2010, 39, 4626-4642. (6) Férey, G.; Haouas, M.; Loiseau, T.; Taulelle, F. Nanoporous solids: how do they form? An in situ approach. Chem. Mater. 2014, 26, 299-309. (7) Grand, G.; Awala, H.; Mintova, S. Mechanism of zeolites crystal growth: new findings and open questions. CrysEngComm. 2016, 18, 650-664. (8) Park, M. B.; Lee, Y.; Zheng, A.; Xiao, F.-S.; Nicholas, C. P.; Lewis, G. J.; Hong, S. B. Formation pathway for LTA zeolite crystals synthesized via a charge density mismatch approach. J. Am. Chem. Soc. 2013, 135, 2248-2255. (9) Park, M. B.; Ahn, N. H.; Broach, R. W.; Nicholas, C. P.; Lewis, G. J.; Hong, S. B. Crystallization mechanism of zeolite UZM-5. Chem. Mater. 2015, 27, 1574-1582. (10) Ahn, S. H.; Lee, H.; Hong, S. B. Crystallization mechanism of cage-based, small-pore molecular sieves: a case study of CHA and LEV structures. Chem. Mater. DOI: 10.1021/acs.chemmater.7b00980.



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(11) Guo, P.; Shin, J.; Greenaway, A. G.; Min, J. G.; Su, J.; Choi, H. J.; Liu, L.; Cox, P. A.; Hong, S. B.; Wright, P. A.; Zou, X. A zeolite family with expanding structural complexity and embedded isoreticular structures. Nature 2015, 524, 74-78. (12) Shin, J.; Xu, H.; Seo, S.; Guo, P.; Min, J. G.; Cho, J.; Wright. P. A.; Zou, X.; Hong, S. B. Targeted synthesis of two super-complex zeolites with embedded isoreticular structures. Angew. Chem. Int. Ed. 2016, 55, 4928-4932. (13) Mayoral, A.; Min, J. G.; Hong, S. B. Aberration-corrected STEM analysis of the RHO family of zeolites with embedded isoreticular structures. Microporous Mesoporous Mater. 2006, 236, 129-133. (14) Min, J. G.; Kemp, C. K.; Hong, S. B. Zeolites ZSM-25 and PST-20: selective carbon dioxide adsorbents at high pressures. J. Phys. Chem. C 2017, 121, 3404-3409. (15) Doherty, H. G.; Plank, C. J.; Rosinski, E. J. U.S. Patent 4,247416, 1981. (16) Vaughan, D. E. W.; Strohmaier, K. G. Synthesis of ECR-18 – a synthetic analog of paulingite. Microporous Mesoporous Mater. 1999, 28, 233-238. (17) Kim, D. J.; Shin, C.-H.; Hong, S. B. Synthesis and characterization of a gallosilicate analog of zeolite paulingite. Microporous Mesoporous Mater. 2005, 83, 319-325. (18) Hong, S. B.; Paik, W. C.; Lee, W. M.; Kwon, S. P.; Shin, C.-H.; Nam, I.-S.; Ha, B.-H. 02-P-10-Synthesis and characterization of zeolite ZSM-25. Stud. Surf. Sci. Catal. 2001, 135, 186. (19) Robson, H. E.; Shoemaker, D. P.; Ogilvie, R. A.; Manor, P. C. Synthesis and crystal structure of zeolite Rho - a new zeolite related to Linde type A. Adv. Chem. Ser. 1973, 121, 106-115. (20) Engelhardt, G.; Michel, D. High-resolution Solid-State NMR of Silicates and Zeolites; Wiley: Chichester, U.K., 1987.


Page 19 of 30

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(21) Baerlocher, Ch.; McCusker, L. B. Database of Zeolite Structures, http://www.izastructure.org/databases/ (accessed January 24, 2017). (22) Frishch, M. J.; et al. Gaussian 09, revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. (23) Boronat, M.; Matínez-Sánchez, C.; Law, D.; Corma A. Enzyme-like specificity in zeolites: a unique site position in mordenite for selective carbonylation of methanol and dimethyl ether with CO. J. Am. Chem. Soc. 2008, 130, 16316-16323. (24) Zhao, Y.; Truhlar, D. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor. Chem. Acc. 2008, 120, 215-241. (25) Hohenstein, E. G.; Chill, S. T.; Sherrill, C. D. Assessment of the performance of the M05-2X and M06-2X exchanged-correlation functionals for noncovalent interactions in biomolecules. J. Chem. Theory Comput. 2008, 4, 1996-2000. (26) Barrett, P. A.; Valencia, S.; Camblor, M. A. Synthesis of a merlinoite-type zeolite with an enhanced Si/Al ratio via pore filling with tetraethylammonium cations. J. Mater. Chem. 1998, 8, 2263-2268. (27) Castro, M.; Garcia, R.; Warrender, S. J.; Slawin, M. Z.; Wright, P. A.; Cox, P. A.; Fecant, A.; Mellot-Draznieks, M.; Bats, N. Co-templating and modelling in the rational synthesis of zeolitic solids. Chem. Commun. 2007, 3470-3472. (28) Lercher, A. J.; Jentys, A. Infrared and raman spectroscopy for characterizing zeolites. Stud. Sur. Sci. Catal. 2007, 168, 435-476. (29) Li, J.; Kuppler, R. J.; Zhou, H. Selective gas adsorption and separation in metal-organic frameworks. Chem. Soc. Rev. 2009, 38, 1477−1504. (30) Lozinska, M. M.; Mangano, E.; Mowat, J. P. S.; Shepherd, A. M.; Howe, R. F.;



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Thompson, S. P.; Parker, J. E.; Brandani, S.; Wright, P. A. Understanding carbon dioxide adsorption on univalent cation forms of the flexible zeolite Rho at conditions relevant to carbon capture from flue gases. J. Am. Chem. Soc. 2012, 134, 17628-17642. (31) Richens, D. T. The Chemistry of Aqua Ions; Wiley: Chichester, U.K., 1997. (32) Lapshin, A.E.; Magdysyuk, O.V.; Goluveba, O. Yu.; Nikolaeva, E. A. Distribution of extraframework cations and water molecules in the structure of synthetic paulingite. Glass Phys. Chem. 2011, 37, 72-77.


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Table 1. Chemical Shifts, Peak Widths, and Relative Intensities of 13C MAS NMR Resonances of TEA+ Ions within a Series of Solid Products Separated after Heating at 373, 408, and 418 K for Various Times during Synthesis of ECR-18, ZSM-25, and PST-20, Respectively 13

C NMR δ,b ppm from TMS

CH2 a

sample TEABr

t-plg cage

pau cage

t-plg(i) cage

53.9

CH3 pau cage 10.5

t-plg(ii) cage

ECR-18 synthesis solid (54 h) solid (108 h) solid (110 h) solid (116 h) solid (122 h)

53.3 (130) [1.00] 53.3 (110) [1.00] 53.2 (90) [1.00] 53.2 (90) [1.00] 53.2 (90) [1.00]

52.3 (90) [0.25] 52.4 (60) [0.30] 52.4 (90) [0.46]

6.7 (80) [1.62] 7.9 (110) [0.99] 6.8 (70) [1.10] 8.0 (70) [1.33] 7.4 (80) [0.25] 6.8 (70) [0.46] 8.0 (60) [1.34] 7.5 (100) [0.32] 6.9 (70) [0.40] 8.0 (70) [1.53] 7.5 (80) [0.46] ZSM-25 synthesis solid (40 h) 53.3 (110) [1.00] 6.8 (110) [1.31] solid (81 h) 53.4 (200) [1.00] 8.1 (80) [1.06] 6.8 (70) [0.91] solid (86 h) 53.2 (130) [1.00] 52.5 (70) [0.27] 8.0 (80) [1.82] 7.5 (40) [0.26] 6.8 (80) [0.58] solid (96 h) 53.2 (110) [1.00] 52.5 (50) [0.32] 8.1 (60) [1.78] 7.4 (80) [0.27] 6.8 (50) [0.24] solid (106 h) 53.2 (110) [1.00] 52.5 (90) [0.37] 8.1 (90) [1.87] 7.5 (80) [0.37] PST-20 synthesis solid (20 h) 53.4 (390) [1.00] 8.0 (210) [3.91] solid (38.3 h) 53.1 (150) [1.00] 52.4 (90) [0.69] 8.0 (120) [2.95] 7.5 (110) [1.56] solid (38.7 h) 53.1 (130) [1.00] 52.5 (110) [0.49] 8.0 (90) [2.93] 7.5 (95) [0.98] solid (72 h) 53.2 (120) [1.00] 52.5 (110) [0.40] 8.0 (50) [2.38] 7.4 (60) [0.79] a The first sample is the pure reagents; the other samples are the solids products synthesized at the indicated times. bValues in parentheses and square brackets are full widths at half-maximum in Hz and relative intensities referenced to the intensity of the peak(s) appearing around 53.2 ppm due to CH2 carbons of TEA+, respectively.



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Table 2. Stabilization Energies Calculated for Various Combinations of SDAs in Seven Different Cage Systems of the RHO Family of Embedded Isoreticular Zeolites stabilization energy (kcal mol-1 of SDA)c Al/systema guest cation(s)b,c lta t-plg pau 1 lta + 8 t-plg 1 lta + 6 d8r 1 t-plg + 4 t-oto 3 t-plg 1 1 Na+ -154.2 -139.9 -151.5 -167.9 -145.1 -143.4 -145.6 + d d 1 TEA -2.8 -38.9 -70.1 -40.3 -43.0 + -117.9 -98.3 -94.6 -136.1 -108.4 -104.8 -81.4 1 [Na(H2O)6] + 2 2 Na -330.7 -307.2 -310.1 -353.6 -310.6 -312.5 -323.2 + + d d d d 1 Na + 1 TEA -266.2 -214.7 -207.5 2 TEA+ -d -d -d -168.1 -d -d -71.0 + d d d 2 [Na(H2O)6] -312.1 -308.4 -260.3 -240.5 a The number of tetrahedral Al atoms in the framework of the given cage system. For their distribution that directly affects the location of guest cations in each cage system, see Figure S3. bThe guest cations include both organic (TEA+) and inorganic (Na+) SDAs. In general, the Na+ ion in aqueous solution forms the octahedral aqua complex [Na(H2O)6]+. The lowest-energy structure for the octahedral [Na(H2O)6]+ complex in which the bond distance between an O atom of a water molecule and the Na+ ion was fixed at 2.37 Å31 was derived using the ONIOM method.25 cThe TEA+ ion assumed to be located within the t-plg or pau cages but not within the considerably larger lta-cages or smaller t-oto cages (for details, see the discussion related to the 13C MAS NMR results in Figure 3). dNot calculated due to the absence of TEA+ in lta cages or to one of either TEA+ or [Na(H2O)6]+ cannot be accommodated without severe steric hindrance.


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Figure 1. The structures of the RHO family of embedded isoreticular zeolites ranging from zeolite Rho (RHO-G1) to PST-20 (RHO-G5) and their seven different building units. Adopted from ref. 11.



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Figure 2. Powder XRD patterns (bottoms) and relative crystallinities (top) for a series of solid products obtained after crystallization of ECR-18 (left), ZSM-25 (middle), and PST-20 (right) under rotation (60 rpm) at 373, 408, and 418 K for different times, respectively.


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Figure 3.

13

C MAS NMR spectra of the solid products isolated after heating under rotation

(60 rpm) at 373, 408, and 418 K for different times during the synthesis of (a) ECR-18, (b) ZSM-25, and (c) PST-20, respectively. The solution

13

C NMR spectrum of TEABr in D2O

(bottom trace) is also given for comparison.



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Figure 4. IR spectra in the structural region of the solid products recovered after crystallization of (a) ECR-18, (b) ZSM-25, and (c) PST-20 as a function of time under rotation (60 rpm) at 373, 408, and 418 K, respectively.


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Figure 5. CO2 uptakes at 273 K and 1.0 bar for a series of solid products obtained after heating under rotation (60 rpm) at 373, 408, and 418 K for different times during the synthesis of (a) ECR-18, (b) ZSM-25, and (c) PST-20, respectively.



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Figure 6. Schematic illustration of a possible formation pathway for PST-20 in the TEA+Na+-Sr2+ mixed SDA system.


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Figure 7. Si/Al (○), TEA+/Al (▲), Na+/Al (■), Sr2+/Al (●), and (TEA+ + Na+ + Sr2+)/Al (▼) ratios in the solid products isolated after crystallization of PST-20 as a function of time at 418 K.



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TOC Graphic of Min et al., “Crystallization Mechanism of a Family of Embedded isoreticular Zeolites”


Crystallization Mechanism of a Family of ... - ACS Publications

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