PST-29: A Missing Member of the RHO family of Embedded

Sep 10, 2018 - Inorganic and Structural Chemistry and Berzelii Center EXSELENT on Porous Materials, Department of Materials and Environmental Chemistr...
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PST-29: A Missing Member of the RHO family of Embedded Isoreticular Zeolites Hwajun Lee, Jiho Shin, Wanuk Choi, Hyun June Choi, Taimin Yang, Xiaodong Zou, and Suk Bong Hong Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03311 • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 12, 2018

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Chemistry of Materials

PST-29: A Missing Member of the RHO family of Embedded Isoreticular Zeolites Hwajun Lee,†,‡ Jiho Shin,†,‡ Wanuk Choi,† Hyun June Choi,† Taimin Yang,§ Xiaodong Zou,§ and Suk Bong Hong*,† †

Center for Ordered Nanoporous Materials Synthesis, Division of Environmental Science and Engineering, POSTECH, Pohang 37673, Korea §

Inorganic and Structural Chemistry and Berzelii Center EXSELENT on Porous Materials, Department of Materials and Environmental Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden

ABSTRACT: Here, we report the synthesis and structure of PST-29, the second generation of the RHO family of embedded isoreticular zeolites, the structure of which was proposed about 50 years ago but has remained undiscovered until now. We were able to synthesize this missing zeolite using N,N´-dimethyl-diazabi-cyclo[2.2.2]octane dications as an organic structure-directing agent (SDA) in the presence of both Na+ and K+ ions. When adding a small amount of seed crystals to the synthesis mixture, we were also able to synthesize PST-29 with a higher Si/Al ratio (4.5 vs 3.5), and thus with a higher stability, in the same mixed-SDA system. The Na+ form of this PST-29 has a comparable CO2 uptake to the corresponding cation form of zeolite rho, but is characterized by much faster adsorption kinetics, suggesting its high potential as a CO2 adsorbent.

Zeolites are crystalline microporous aluminosilicates with wellcharacterized and uniform pore structures that are finding a wide variety of commercial applications including catalysis, ion exchange, gas adsorption, and separations.1 We have recently discovered a novel zeolite family with expanding complexity and embedded isoreticular structures, denoted the RHO family (Figure 1).2-4 This zeolite family starts from zeolite rho (framework type RHO) consisting of 10-hedral ([4882]) double 8-ring (d8r) and 26-hedral ([4126886]) lta cages. Its scaffolds are extended by inserting an extra pair of d8r and 18-hedral ([41286]) pau cages between the lta cages along each unit-cell edge, which increases the isoreticular dimension by approximately 10 Å per generation. The space between the scaffolds is filled by embedded cages to form fully tetrahedrallyconnected frameworks. The natural zeolite paulingite (PAU) and the synthetic zeolite ZSM-25 (MWF) are the third (RHO-G3) and fourth (RHO-G4) generations of the RHO family, respectively. By extending this approach, we have predicted several more complex members (i.e., PST-20 (RHO-G5), PST-25 (RHO-G6), PST-26 (RHO-G7), and PST-28 (RHO-G8)) and synthesized them via a rational approach.2,3 However, although the second generation (RHO-G2) with one pau and two d8r cages per unit-cell edge was first proposed by Gordon et al. in 1966,5 it has remained a missing member of the RHO family so far. It should be noted that RHO-G2 contains only two different types of embedded cages (i.e., 14-hedral ([466286]) t-plg and 8-hedral ([4583]) t-oto cages), whereas all the higher generations include four different types of embedded cages, i.e., 10-hedral ([4684]) t-gsm and 12-hedral ([4785]) t-phi cages in addition to the two mentioned above. In the present communication, we report the synthesis of this missing generation of the RHO family, denoted PST-29, using N,N´-dimethyl-1,4-diazabicyclo[2.2.2]octane (Me2-DABCO) dihydroxide as an oragnic structure-directing agent (SDA) in the presence of both Na+ and K+. Me2-DABCO has been previously used as an organic SDA in synthesizing various zeolites, e.g., ZK5 (KFI), ZSM-10 (MOZ), SSZ-98 (ERI), SSZ-102 (ESV), and

high-silica levyne (LEV).6-10 The structure of PST-29 has been solved by synchrotron single-crystal X-ray diffraction (XRD), as well as by synchrotron powder XRD and Rietveld analyses. We also show that PST-29 exhibits a large CO2 adsorption capacity, fast adsorption kinetics, and long-term stability, rendering it potentially useful as a CO2 adsorbent. Table 1. Representative Synthesis Conditions and Resultsa gel composition run (NaOH + KOH)/SiO2 Na/K H2O/SiO2 productb 1 0.27 ∞ 26.7 amorphous 2 0.27 7.0 26.7 PST-29 c 3 0.27 7.0 26.7 chabazite + erionite 4 0.27 3.0 26.7 PST-29 + (philipsite) 5 0.27 1.0 26.7 PST-29 + erionite + phillipsite 6 0.27 0.3 26.7 erionite 7 0.27 0 26.7 ZSM-10 + erionite 8 0.27 7.0 20.0 ECR-18 + PST-29 + phillipsite 9 0.27 7.0 13.3 PST-29 + ECR-18 + phillipsite 10 0.27 7.0 33.3 amorphous 11 0.22 5.5 26.7 amorphous 12d 0.22 5.5 26.7 PST-29 13c,d 0.22 5.5 26.7 erionite + ECR-18 + (PST-29) 14d 0.20 5.0 26.7 amorphous 15e 0.22 5.5 26.7 amorphous f 16 0.22 5.5 26.7 ECR-18 g 17 0.22 5.5 26.7 ECR-18 18h 0.22 5.5 26.7 ECR-18 a The composition of the synthesis mixture is 1.0R(OH) ∙xNa O∙yK O∙ 2 2 2 1.0Al2O3∙15SiO2∙zH2O, where R is Me2-DABCO, and x, y, and z are varied between 0 ≤ x ≤ 4.0, 0 ≤ y ≤ 4.0, and 200 ≤ z ≤ 500, respectively. All syntheses were carried out under static conditions at 120 oC for 14 days, unless otherwise stated. b The product appearing first is the major phase, and the product obtained in a trace amount is given in parentheses. c Performed using TEA+ instead of Me2-DABCO. Some synthesis runs were carried out at 120 oC for 7 days after adding a small amount (2 wt % of the silica in the gel) of previously prepared PST-29,d rho,e ECR-18,f ZSM-25,g and PST-20h as seeds to the gel, respectively.2,11-14

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Figure 1. Framework representations of cross-sections (ca. 12 Å thick) of RHO-G1 to RHO-G8 in the RHO family of embedded isoreticular zeolites. Adopted from ref 3. Note that PST-29 has the RHO-G2 structure that has not been discovered yet. Table 1 lists the results from syntheses carried out using Me2DABCO-containg aluminosilicate gels with the same Si/Al ratio (7.5) but a wide range (0-∞) of Na/K ratios at 120 oC for 7 or 14 days. We note that the gel composition leading to pure PST-29 is very narrow. When the (NaOH + KOH)/SiO2 ratio in the gel was fixed at 0.27, for example, the Na/K ratio yielding PST-29 was 7.0 only (run 2 in Table 1). Here we refer to this zeolite as PST-29(u), where u represents ‘unseeded’. It is worth noting that the use of TEA+ instead of Me2-DABCO in the PST-29(u) synthesis conditions gave a mixture of chabazite (CHA) and erionite (ERI). Also, when Na+ was used as a sole inorganic SDA, the synthesis mixture remains amorphous. A continuous Na/K ratio decrease to 0 in the gel resulted in the formation of phillipsite (PHI), erionite, and/or ZSM-10. Interestingly, when the (NaOH + KOH)/SiO2 and Na/K ratios were fixed to 0.27 and 7.0, respectively, a decrease in H2O/SiO2 ratio from 26.7 to 20.0 yielded ECR-18, a synthetic version of paulingite, although not phase-pure. Like the case of PST29, its appearance is unexpected, because TEA+ is the only known organic SDA yielding ECR-18.12 Powder XRD measurements show that PST-29(u) loses its structural integrity when calcined at 550 oC (Figure 2a), probably due to the high framework Al content (Si/Al = 3.5; Table S1). To obtain PST-29 with a higher Si/Al ratio, we decreased OH- concentration in the synthesis mixture, while keeping the H2O/SiO2 ratio (26.7) constant. A decrease in (NaOH + KOH)/SiO2 ratio from 0.27 to 0.22 directed no crystallization of zeolite products. However, when adding a small amount (2 wt% of the silica in the gel) of previously prepared PST-29(u) crystals as seeds to the above synthesis mixture, we were able to obtain pure PST-29, denoted PST-29(s), where s indicates ‘seeded’. Unlike that of PST-29(u), the structure of PST-29(s) was found to maintain its overall structural integrity not only after calcination to remove the occluded organic SDA molecules (Figure S1) but also after conversion into its proton form, as evidenced by the XRD, 27Al and 29Si MAS NMR, and N2 adsorption data (Figures 2a and S2-S4). Elemental analysis indicates that this material has a Si/Al ratio of 4.5. A further slight decrease in the (NaOH + KOH)/SiO2 ratio of the gel to 0.20 gave no crystalline solids even after seeded synthesis at 120 oC for 28 days. This again shows that the structure-directing ability of Me2-DABCO itself is not strong enough to govern the crystallization of PST-29. We also performed zeolite syntheses under the conditions where the crystallization of PST-29(s) proved to be highly reproducible, after replacing PST-29(u) seed crystals by the equivalent amount of as-made rho, ECR-18, ZSM-25, or PST-20 crystals. As shown in Table 1, the use of rho seeds yielded an amorphous phase. When using ECR-18, ZSM-25, or PST-20 seeds, in addition, we always obtained pure ECR-18, regardless of the structure type of seed crystals added (Figures S6-S8). To our knowledge, on the other hand, the TEA+-mediated synthesis of rho consisting of only two d8r and lta cages, the first RHO family member, has not been reported yet. However, TEA+ directs the synthesis of all known

higher members than PST-29, where all seven types of cages are always present.2,3,12-14 The successful Me2-DABCO-mediated synthesis of PST-29, in which two (i.e., t-gsm and t-phi cages) out of the seven cage types in the RHO family are still lacking, suggests that the organic SDAs for the lower generations of a particular family of EIZs, especially when not all types of cages or structural building units available to the corresponding zeolite family have been manifested, may not be necessarily identical with one another. We also note that the Na/K ratios (< 0.8) in as-made PST-29(u), PST-29(s), and ECR-18 synthesized here are much smaller than those (Na/K ≥ 5.5) in their synthesis mixtures (Tables 1 and S1). It thus appears that K+ plays a more important role as an inorganic SDA in the Me2-DABCO-mediated synthesis of PST-29 and ECR18 than Na+, although both inorganic cations are necessary for their crystallization. Although as-made PST-29(u) were found to appear as spherelike crystals of ca. 10 μm length with highly stepped surfaces (Figure 2b), we were able to collect the single-crystal XRD data of its Na+-exchanged form i.e., Me2-DABCONa-PST-29(u), at the 2D beamline of the Pohang Acceleration Laboratory (PAL; Pohang, Korea) using synchrotron X-rays (λ = 0.62000 Å) and to solve the structure by the direct methods: cubic, Im-3m, a = 25.039(3) Å , V = 15699(5) Å 3 (Table S2). As expected from powder XRD data, the structure obtained was consistent with the proposed hypothetical RHO-G2 structure.2,5,12,15,16 The final atomic positions and selected bond lengths and angles for Me2-DABCONa-PST-29(u) can be found in Tables S3-S5. The cubic scaffold of PST-29 is arranged in the sequence lta-d8r-pau-d8r-lta along unit-cell edges (Figure 3b), which can be obtained by adding or removing a pair of pau and d8r

Figure 2. (a) Powder XRD patterns of as-made and calcined (bottom two traces) PST-29(u) and as-made and calcined (top two traces) PST-29(s). (b) FE-SEM images of as-made PST-29(u) (bottom) and PST-29(s) (top).

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Chemistry of Materials

Figure 3. (a) The five different building units found in PST-29. The (b) three-dimensional framework structure of Me2-DABCONa-PST29(u) and its (c) refined Me2-DABCO position within the t-plg cage and (d) Na+ positions: (clockwise, from top left) Na1 at 8-ring windows between the lta and d8r cages (12e, occupancy 0.36), Na2 at 8-ring windows between the pau and d8r cages (12e, occupancy 0.27), Na3 at 8-ring windows between the t-plg and t-oto cages (48k, occupancy 0.24), and Na4 at 8-ring windows between the pau and t-oto cages (48k, occupancy 0.26). The locations of Me2-DABCO ions within the pau cages are not given here, because of their disordered nature. Si, Al, or O, blue; C, purple; N, orange; Na, pink. cages from the scaffolds of rho and paulingite, respectively. The lta and t-plg cages are connected in the sequence lta-plg-lta-plg-lta along the diagonal direction of the cubic unit cell. The remaining space is filled with t-oto cages, leading to a fully four-connected framework. The PST-29 structure has three topologically distinct tetrahedral sites (T-sites) with a multiplicity ratio of 1:2:2, eight symmetry-independent O atoms, and a framework density (defined as the number of T-atoms per 1000 Å 3) of 15.4, falling within the category of small-pore materials. Figure 3 also shows the refined positions of extraframework Me2-DABCO and Na+ ions in Me2-DABCONa-PST-29(u). A combination of 13C and 1H MAS NMR and Raman spectroscopies reveals that the Me2-DABCO ions in this zeolite remain intact, which is also the case of ECR-18 synthesized using the same organic SDA (Figures S9-S11). From the single-crystal XRD analysis, we realized that the organic cations are located only within the pau and tplg cages among the five different cages in PST-29, which is also supported by the energy minimization calculations (Figure S12 and Table S6). However, because the Me2-DABCO ions are located in the high-symmetry (4/mmm) pau cages, they can adapt different orientations and are disordered within the pau cages (Figure S13). The Na+ ions are located not only at both sides (8-ring windows) of d8r cages, but also at 8-ring windows between the t-oto cages and the pau or t-plg cages. To obtain the structure of organic-free NaPST-29(s) in its hydrated form, in addition, we obtained powder XRD data at the 9B beamline of the PAL using monochromated Xrays (λ = 1.5167 Å ) and carried out Rietveld refinements based on the initial structure model of Me2-DABCONa-PST-29(u). An acceptable final Rwp value of 8.6% was obtained. The final refined unit cell parameters obtained were a = 24.97157(3) Å (Table S7). The final atomic positions and selected bond lengths and angles are listed in Tables S8-S10 with the final Rietveld plot displayed in Figure S14. Figure 4a shows the adsorption isotherms of CO2, N2, and CH4 on Na-PST-29(s) at 25 oC. The CO2 uptake of this cage-based small-pore zeolite was 4.3 mmol g-1 at 1.0 bar which is lower than the value (5.8 mmol g-1) of Na-X with Si/Al = 1.3 (FAU), a largepore zeolite and commercial CO2 adsorbent. However, it is higher than the uptakes (3.6 and 3.7 mmol g-1, respectively) of Na-A with Si/Al = 1.0 (LTA) and K-chabazite with Si/Al = 3.0 (Figure S15), two small-pore zeolites widely studied for CO2 adsorption, and is comparable to the value (4.5 mmol g-1) of Na-rho, the highest among the known RHO family zeolites.2,17-19 The CO2 uptake of Na-PST-29(s) was found to remain unchanged over 50 adsorptiondesorption cycles at 70 oC (Figure 4a inset). This zeolite also shows CO2 selectivities (30 and 30, respectively) over CH4 and N2 that are

somewhat lower than those observed for Na-rho or TEANa-ECR18. However, we note that its CO2/CH4 selectivity is significantly higher compared to Na-X, Na-A, or K-chabazite (Table S11), mainly due to the so-called trapdoor effect (Figure S16),20,21 like the case of the other RHO-family members.2,17-19,22,23 As shown in Figure 4b, on the other hand, Na-PST-29(s) achieves equilibrium much faster (ca. 2 min vs 2 h) at 25 oC and 1.2 bar than Na-rho, rendering the former material more attractive as a CO2 adsorbent. CO2 molecules are accessible to Na-rho void space via d8r cages only, so that CO2 diffusion in Na-rho is highly

Figure 4. (a) Adsorption isotherms at 25 oC of CO2 (navy), CH4 (green), and N2 (pink) for Na-PST-29(s). Inset: CO2 adsorptiondesorption cycles at 70 oC. (b) CO2 adsorption kinetics at 25 oC and 1.2 bar of Na-rho (pink), Na-PST-29(s) (navy), TEANa-ECR-18 (green), TEANa-ZSM-25 (orange), TEANa-PST-20 (violet). Inset: zoom of the CO2 adsorption kinetics for the first 5 min.

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limited. In Na-PST-29(s), however, they can also diffuse via 8-ring windows that are located between the t-oto cages and the pau or tplg cages (Figure 3d). Therefore, it is most likely that if the Na+ ions are more weakly held in ‘thinner’ 8-ring windows than in d8r cages, the cations in the former windows would then be more readily relocated than those in the latter ones throughout the interactions with CO2 molecules and permit other CO2 molecules to pass. We suggest that the faster CO2 adsorption kinetics on the other higher generations of the RHO family (i.e., TEANa-ECR-18, TEANaZSM-25, and TEANa-PST-20) than that on Na-rho can be explained in a similar way. Structural characterization of various cation forms of PST-29(s), as well as catalytic testing of its proton form, is in progress in our laboratory, and the results will be given elsewhere. In summary, we have synthesized PST-29, the missing second generation of the RHO family of embedded isoreticular zeolites, in the Me2-DABCO-Na+-K+ mixed-SDA system and solved its structure by using both single-crystal and powder X-ray diffraction. A PST-29 zeolite, which was synthesized at a relatively low hydroxide level using a seeding method and thus has a high Si/Al ratio of 4.5, is stable not only after calcination but also after conversion into its proton form. Its Na+ form was found to show a high CO2 uptake (4.3 mmol g-1) at 25 oC and 1.0 bar, together with fast kinetics and long-term stability, making PST-29 attractive for selective CO2 adsorption.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental section, characterization data, crystallographic information, and additional results (PDF)

AUTHOR INFORMATION Corresponding Author [email protected]

Author Contributions ‡These

authors contributed equally.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the National Creative Research Initiative Program (2012R1A3A2048833) through the National Research Foundation of Korea and the Swedish Research Council (VR). We thank PAL, Pohang (2D, D. Moon; 9B, D. Ahn) for synchrotron diffraction beam time. PAL is supported by MSIP and POSTECH. We also thank J. G. Min (POSTECH) for CO2 adsorption kinetics.

REFERENCES (1) Camblor, M. A.; Hong, S. B. In Porous Materials; Bruce, D. W., Walton, R. L., O’Hare, D., Eds.; Wiley: Chichester, 2011; pp 265−325.

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