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Rediscovery of the Importance of Inorganic Synthesis Parameters in the Search for New Zeolites Jiho Shin,† Donghui Jo,† and Suk Bong Hong*

Acc. Chem. Res. Downloaded from pubs.acs.org by AUBURN UNIV on 04/23/19. For personal use only.

Center for Ordered Nanoporous Materials Synthesis, Division of Environmental Science and Engineering, POSTECH, Pohang 37673, Korea CONSPECTUS: Zeolites and related crystalline microporous materials with cavities and channels of molecular dimensions are of major importance for applications ranging from ion-exchange to adsorption and to catalysis. Because their unique shape-selective properties are closely related to the size, shape, and dimensionality of the intracrystalline channels and cavities, much interest has been devoted to the discovery of novel zeolitic materials over the last several decades. As a result, a dramatic expansion in the structural domain of crystalline microporous materials, as well as in their compositional range, has been achieved. This is largely due to the development of innovative synthetic strategies, for example, organic structure-directing agent (OSDA) design, introduction of heteroatoms like Ge in OSDA-mediated zeolite synthesis, topotactic transformation of two-dimensional layered zeolite precursors, assembly−disassembly−organization−reassembly method, etc. However, although many of these methodologies are quite successful in finding unprecedented zeolite structures, the resulting materials tend to be (hydro)thermally unstable and are often commercially impractical from a manufacturing perspective because of the high cost of the OSDA and/or heteroatom employed. Therefore, we focused on inorganic synthesis parameters as the key phase selectivity factor that has received relatively little attention in the search for new industrially relevant zeolites. This Account describes our recent efforts to find previously undiscovered aluminosilicate zeolites by boosting the roles of inorganic structure-directing agents (ISDAs) in the presence of OSDAs. They include the multiple inorganic cation and excess fluoride approaches, which aim to promote a synergistic cooperation between ISDAs and/or OSDAs and thus to hold a rational design concept, although the latter is not friendly to the practical zeolite manufacturing process due to the toxicity of fluoride. Using these two approaches, we were able to synthesize not only the second generation (PST-29) and four higher generations (PST-20 (RHO-G5), PST-25 (RHO-G6), PST-26 (RHO-G7), and PST-28 (RHO-G8)) of the RHO family of embedded isoreticular zeolites but also three other novel zeolite structures (EU-12, PST-21, and PST-22). We also explored the synthesis of a number of heteroatom-containing aluminophosphate (AlPO4) molecular sieves with different framework structures and unusually high framework charge density through the cooperative structure direction of alkali metal and small OSDA cations or under wholly inorganic conditions. Although we need to clarify the nature and extent of interactions between the inorganic cations and framework components in synthesis mixtures, we believe that our synthetic concepts, shedding new light on the importance of inorganic synthesis parameters, will open a door for achieving many other novel zeolite structures and compositions.

1. INTRODUCTION Because of their unique shape- and/or surface-selective properties, zeolites and molecular sieves, a class of crystalline microporous solids with extremely uniform cavities and channels, are one of the key elements of modern chemical industry.1 This has been largely fueled by the discovery of new zeolite structures and compositions constantly achieved over more than a half century.2−4 It was in the early 1960s when Barrer and Denny first added methylammonium cations to the zeolite synthesis gels.5 Since then, the use of a wide variety of organic structure-directing agents (OSDAs) in zeolite synthesis has been a central theme in zeolite research, because the pore architecture of the crystallized zeolite product can be dependent on the size, shape, conformational rigidity, and charge density of the OSDA.2,3 However, the phase selectivity © XXXX American Chemical Society

of the crystallization is also strongly affected by the type and concentration of inorganic structure-directing agents (ISDAs), such as alkali and/or alkaline earth metal cations, hydroxide or fluoride anions, and heteroatoms other than Si, in zeolite synthesis mixtures. Apart from the OSDA design approach, therefore, considerable efforts have been directed toward developing unprecedented synthetic strategies, with the aim of directing the formation of previously unseen zeolite structures by fostering cooperation between OSDAs and ISDAs. The most successful strategy among those reported so far is the combined use of Ge and OSDA in concentrated fluoride Received: February 12, 2019

A

DOI: 10.1021/acs.accounts.9b00073 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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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. Five new structures (shown in blue) have been synthesized via the multiple inorganic cation approach. Adapted with permission from ref 15. Copyright 2016 Wiley-VCH.

media, which makes double 4-ring (d4r or [46]) units abundant in synthesis mixtures and thus leads to numerous novel zeolite structures with low framework densities (the number of tetrahedral atoms (T atoms) per 1000 Å3).2,3 Unfortunately, the hydrothermal stability of the resulting germanosilicate molecular sieves is generally low, limiting their use as catalysts and adsorbents. Among a total of 124 zeolite structures discovered during the last two decades and given framework type codes by the Structure Commission of the International Zeolite Association, in addition, there are only 18 high-silica (Si/Al > 5) zeolites with high hydrothermal stability.4 One major challenge facing the zeolite community is therefore the development of such types of materials with industrial relevance. In the present Account we highlight our understanding of how to directly synthesize unprecedented aluminosilicate zeolites by adequately capturing the interdependent nature of OSDAs and ISDAs. We summarized our strategies, which are quite simple but are intended to enhance the role of ISDAs under the synthesis conditions largely unexplored so far. We also show that ISDAs can play an integral role in the synthesis of phosphate-based molecular sieves, that is, metalloaluminophosphate (MeAPO) and silicoaluminophosphate (SAPO) materials.

(RHO) and ZK-5 (KFI) were reported to crystallize from the Na+-Cs+ and K+-Sr2+ mixed cation systems, respectively, where 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) was employed as an OSDA.9,10 Even more impressive are UZM-22 (MEI) and UZM-35 (MSE) synthesized from the Li+-Sr2+-(2-hydroxyethyl)trimethylammonium (choline) and Na+-K+-dimethyldipropylammonium ISDA-OSDA systems, respectively.11,12 Considering that all these zeolites have not been synthesized yet in the presence of only one type of ISDA cation, the use of multiple inorganic cations in zeolite synthesis when properly combined with the ability of a certain OSDA could be a viable alternative to new materials. Nevertheless, little systematic attention has been paid to this synergistic effect. Therefore, we have begun to focus on the cooperative structure direction among multiple inorganic cations in hydroxide media in the presence of ordinary OSDAs, enabling us to find a series of novel zeolite structures described below. 2.1. The RHO Family of Embedded Isoreticular Zeolites

In 2015 our group, together with the Wright and Zou groups, reported a family of embedded isoreticular zeolites (EIZs) with expanding structural complexity, denoted the RHO family.13 The existence of this new zeolite family came into the open when the structural model of ZSM-25 (MWF), an over 30year-old zeolite,14 was derived by combining electron diffraction and a structural coding concept. Its structure expansion, which starts from rho, operates at two levels: (i) the cubic scaffold is expanded by inserting of a pair of double 8ring (d8r or [4882]) and 18-hedral ([41286]) pau cages between the 26-hedral ([4126886]) lta cages along each unit cell edge, leading to an increase per generation in the unit cell dimension of the RHO family by ∼10 Å, and (ii) the space between the scaffolds is completely filled by other four types of embedded cages, that is, 14-hedral ([466286]) t-plg, 8-hedral ([4583]) t-oto, 10-hedral ([4684]) t-gsm, and 12-hedral ([4785]) t-phi cages, to create a rigid and fully tetrahedrally coordinated framework. Consequently, each family member is always embedded and isoreticular and has the same maximum ring size (8-ring) but a different framework topology (Figure 1). Paulingite (PAU), a natural zeolite, and ZSM-25 are the third (RHO-G3) and fourth (RHO-G4) generations of this family, respectively. We attempted to synthesize a series of higher RHO-family members predicted based on the above expansion rules.13,15 Since both ECR-18, a synthetic counterpart of paulingite, and ZSM-25 were synthesized using a combination of Na+ and tetraethylammonium (TEA+) ions, together with K+ for ECR18,14,16 we reasoned that the simultaneous use of the same ISDA (Na+) and the same OSDA (TEA+) could induce the formation of the building units necessary for higher

2. MULTIPLE INORGANIC CATION APPROACH When aluminosilicate zeolites are synthesized in hydroxide media, inorganic cations (alkali metal ions and sometimes alkaline earth ones) are generally used in the presence or absence of OSDAs. In many cases, they are added as their hydroxide form, where the OH− ions contribute to the solubilization of (alumino)silicate species and serve as the counterions balancing the framework negative charges. Both alkali and alkaline earth metal ions are much less flexible for the rational design of synthesis conditions compared to OSDAs because of their limited sizes and shapes in aqueous solution. However, these ISDAs also affect the phase selectivity of the crystallization, since the ordering of water molecules and the conformations of OSDAs through the formation of metal− organic complexes can differ according to their type.6−8 Therefore, much interest has been devoted to the harmonization of the structure-directing ability of ISDAs with that of OSDAs during zeolite synthesis but which has been performed mainly in a trial-and-error manner. It is also remarkable that the use of more than one type of inorganic cations, together with a particular OSDA, in zeolite synthesis has long been applied. For example, although synthesizable under OSDA-free conditions, zeolite rho B

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PST-20 are located mainly within the 8-rings of t-oto, t-gsm, and t-phi cages of the as-made form of this zeolite.13 Given that the use of a second type of inorganic cations has promoted the formation of several particular types of structural building units and thus increased the overall structural complexity of the zeolite product, the multiple inorganic cation approach appears to be useful for finding other novel families of EIZs.17

generations. More importantly, we noticed that the number of embedded cages increases much more rapidly than that of scaffold cages with increasing generation number, whereas all the natural analogues of GIS- and PHI-type zeolites, except amicite, which consist of t-gsm cages only and t-oto and t-phi cages as building units, respectively, contain a considerable amount of alkaline earth metal cations such as Ca2+ and even Ba2+ as extraframework cations.4 To encourage the formation of embedded cages, we intentionally added some amount of Ca2+, Sr2+, or both to the ZSM-25 synthesis gel containing Na+ and TEA+ ions. This strategy with a degree of rational design, termed the “multiple inorganic cation” approach, allowed us to synthesize four targeted members with more complex framework structures in the RHO family, namely, PST-20 (RHO-G5), PST-25 (RHOG6), PST-26 (RHO-G7), and PST-28 (RHO-G8) (Table 1).13,15 Note that the presence of alkaline earth metal ions in synthesis gels is essential for directing their crystallization, because, for example, the Sr2+ ions introduced in the gel of

2.2. PST-29 and EU-12

The structure of RHO-G2 consisting of one pau and two d8r cages at the edges of the unit cell, first proposed by Gordon and co-workers ∼50 years ago,18 has not been discovered until a very recent date. We were able to crystallize this missing second generation, denoted PST-29, using both Na+ and K+ in the presence of N,N′-dimethyl-1,4-diazabicyclo[2.2.2]octane (Me2-DABCO) as an OSDA.19 We found that the Na+/K+ ratio in the synthesis mixture is a critical factor governing the crystallization of PST-29, and the sole use of either of the two alkali metal cations with Me2-DABCO did not give this phase at all (Figure 2a). Again, the cooperative structure-directing effects of multiple inorganic cations in the presence of a particular OSDA was observed for aluminosilicate zeolites. However, the synthesis and structures of all the known higher generations than PST-29 in the RHO family always include the use of TEA+ as an OSDA and seven different types of cages, respectively.13−16 Thus, the fact that PST-29 with no t-gsm and t-phi cages can be synthesized using Me2-DABCO indicates that the OSDAs for the lower generation members of a given family of EIZs, particularly when not all cage or composite building unit types available to the corresponding zeolite family have emerged, do not need to be the same as one another. It is interesting to note here that, unlike Na-rho, NaPST-29 exhibits fast adsorption kinetics, as well as a high CO2 uptake, revealing its great potential as a CO2 adsorbent. This can be rationalized by considering Na+ locations in the single 8-ring windows, thinner than d8r cages, between the t-oto cages and the pau or t-plg cages. Through the interactions with CO2, therefore, many cations in Na-PST-29 can be more easily relocated compared to Na-rho, permitting other CO 2 molecules to pass.19 The cooperative effects of multiple inorganic cations on the formation of novel zeolite structures can also be observed in our recent work on the synthesis of EU-12 using choline as an OSDA, with both Na+ and Rb+ ions present,20 the framework structure of which has remained unsolved for the past 30 years.21 We were able to crystallize this zeolite through the simultaneous use of Na+ and Rb+ ions within a narrow range of concentrations: when either only Na+ or Rb+ was used, no signs of EU-12 formation were observed (Figure 2b). EU-12 is a small-pore zeolite containing a two-dimensional channel system consisting of two parallel straight 8-ring channels along the a axis, where only one of them is linked with the sinusoidal channel by sharing 8-rings in the ac plane. H-EU-12 shows a considerably higher ethene selectivity in the low-temperature (200 °C) dehydration of ethanol than H-mordenite (MOR), known as the best catalyst for this reaction,22 which is a result of product shape selectivity.20

Table 1. Syntheses from Gel Composition 5.2TEABr· 1.9Na2O·0.5MII(NO3)2·xAl2O3·7.2SiO2·yH2Oa gel composition

run 1 2 3c 4 5 6e

MII

SiO2/ Al2O3

H2O/ SiO2

crystallization conditions (T/t, °C/d)

productb

7.2 7.2 7.2 7.2 7.2 7.2

54.2 54.2 54.2 41.7 34.7 34.7

140/7 145/4 145/2 145/2 145/2 145/2

ZSM-25 PST-20 + (ZSM-25) PST-20 PST-20 + (analcime) P1 + Ud PST-25 + PST-20

7.2

54.2

145/2

PST-20 + ZSM-25 + P1 PST-25 + PST-20 + P1 PST-26 + PST-25 + P1 P1 + (PST-26) + (PST-25) P1 P1 PST-25 + PST-26 + PST-20 PST-26 + PST-28 + PST-25 PST-26 + PST-28 + PST-25 + mordenite mordenite + PST-25 + PST-20 mordenite + (PST-26) + (PST-25)

7

Sr Sr Sr Sr Sr, Ca Ca

8

Ca

7.2

41.7

145/2

9

Ca

7.2

34.7

145/2

10

Ca

7.2

27.8

145/2

11 12 13

Ca Ca Ca

4.8 4.8 9.6

34.7 27.8 34.7

145/2 145/2 145/2

14

Ca

9.6

27.8

145/2

15

Ca

9.6

20.8

145/2

16

Ca

14.4

34.7

145/2

17

Ca

14.4

27.8

145/2

x and y are varied between 0.5 ≤ x ≤ 1.5 and 150 ≤ y ≤ 390, respectively. The final synthesis mixture was stirred at room temperature or 80 °C for 24 h, and crystallization was performed under rotation (60 rpm) at 140−145 °C for 2−7 d. bThe phase appearing first is the major phase, and the product obtained in a trace amount is given in parentheses. cRun performed after adding a small amount (2 wt % of anhydrous raw materials) of the material, which was obtained from run 2 as seeds to the synthesis mixture. d Unknown, probably dense material. eSr/Ca = 1.0. Adapted from with permission refs 13 and 15. Copyright 2015 Nature Publishing Group. Copyright 2016 Wiley-VCH. a

3. EXCESS FLUORIDE APPROACH In their seminal 1978 patent Flanigen and Patton first used fluoride anions as the mineralizing agent in zeolite synthesis, which enabled the production of pure-silica ZSM-5 (MFI) with an exceptional degree of hydrophobicity at nearly neutral C

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Figure 2. Zeolite syntheses at 120 and 150 °C from gel compositions (a) 1.0Me2-DABCO(OH)2·xNa2O·(4.0 − x)K2O·1.0Al2O3·15SiO2·400H2O and (b) 2.0ChCl·yNa2O·(1.0 − y) Rb2O·0.25Al2O3·5.0SiO2·100H2O, where x and y are varied between 0 ≤ x ≤ 4.0 and 0 ≤ y ≤ 1.0, respectively. OSDA abbreviations: Me2-DABCO, N,N′-dimethyl-1,4-diazabicyclo[2.2.2] octane; Ch, choline. The framework structures of PST-29 (top) and EU-12 (bottom) are also given. The EU-12 structure consists of two types of straight 8-ring channels, labeled A and B, respectively. While the Type A channels intersect with the sinusoidal channels, the Type B channels link to the sinusoidal ones via their 8-rings. Adapted with permission from refs 19 and 20. Copyright 2018 American Chemical Society. Copyright 2016 Wiley-VCH.

Table 2. Syntheses of Aluminosilicate Zeolites Using Imidazolium-Based OSDAs at Different HF Concentrationsa

productb

HF/R 0.5 1.0 2.0 3.0 4.0

ferrierite A + (U) ferrierite + A D D

SSZ-50 + ITQ-12 SSZ-50 + ITQ-12 PST-21 ITQ-12 D

ferrierite ferrierite + (U) PST-22 ZSM-22 + A D

SSZ-50 SSZ-50 + ITQ-12 PST-22 PST-22 + A D

SSZ-50 SSZ-50 SSZ-50 SSZ-50 + A D

The synthesis mixture composition is 0.5ROH·xHF·1.0SiO2·0.05Al2O3·5.0H2O, where R is OSDA, and x is varied as 0.25 ≤ x ≤ 2.0. All syntheses were performed under rotation (60 rpm) at 175 °C for 14 d. bThe product appearing first is the major phase, and the product obtained in a trace amount is given in parentheses. A, U, and D indicate amorphous, unknown, and dense phases, respectively. Adapted with permission from ref 36. Copyright 2018 Wiley-VCH. a

pH.23 This chemistry was revisited by Guth and co-workers in the late 1980s. They extended it not only to silica-based zeolites containing various trivalent (Al, B, Fe, Ga) or tetravalent (Ge, Ti) framework elements but also to phosphate-based molecular sieves such as aluminophosphate (AlPO4), SAPO, and gallophosphate (GaPO4) materials.24,25 The fluoride route is substantially different from the hydroxide route, for example, in that the F− ions easily end up entrapped inside as-made zeolites unlike the OH− ions. As a result, the OSDA cations in high- and pure-silica zeolites can be counterbalanced by F− ions in place of framework Al atoms, notably reducing the concentration of Si−O− framework defects. More interestingly, these anions prefer to be encapsulated within small cages, especially within d4r units, making the resulting cages more stabilized, and thus play a structure-directing role rather than a mineralizing one. A breakthrough in the fluoride route came in the late 1990s when researchers at ITQ combined it with the highly concentrated conditions (i.e., low H2O/SiO2 ratios, typically 15 at all. This implies that the concentration of both Al and F− in the synthesis mixture is crucial for directing the formation of both zeolites. PST-21 and PST-22 are composed of 1,3-stellated cubic units (6-hedral ([4254]) bre units) as a secondary building unit in a nonjoint manner (Figure 3a−c). The former material is

3.2. High-Silica ERS-7

Despite its simplicity, choline, when used together with various alkali and/or alkaline earth metal cations, can direct the synthesis of more than 10 different zeolite structures, many of which are different from those crystallized in the presence of TMA+ or TEA+.37 We recently synthesized a high-silica (Si/Al = 14) version of zeolite ERS-7 (ESV) using this OSDA in excess fluoride conditions (HF/OSDA = 2.0).38 Given that choline gave other phases like ferrierite when HF/OSDA ≤ 1.0, the excess fluoride approach may be a valuable tool to obtain zeolites with known framework structures but new compositions. We also note that, like as-made PST-21 and PST-22, the F− content in our high-silica ERS-7, although synthesized in the presence of a large concentration of this anion, is not so high.

4. SYNTHETIC STRATEGIES FOR PHOSPHATE-BASED MOLECULAR SIEVES Since the monumental discovery of AlPO4 molecular sieves by Flanigen and co-workers in 1982,39 the compositional regime of zeolitic materials, as well as their structural realm, has greatly E

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Figure 4. (a) Ternary diagrams illustrating the chemical compositions of MeAPO (top, PST-16 (■), PST-17 (●), PST-19 (▲), ZnAPO-88 (▼)) and SAPO (bottom, SAPO-69 (□), SAPO-79 (○), MER (△), EDI (▽), GIS (◇), ANA (◁)) molecular sieves synthesized using alkali metal cations as ISDAs in both presence and absence of OSDAs. The elemental analysis data were taken from refs 48 and 49. (b) Powder XRD patterns with SEM images (scale bar (from bottom to top): 20, 4, 2, and 2 μm) of the hydrated SAPO molecular sieves with (from bottom to top) MER, EDI, GIS, and ANA topologies synthesized under wholly inorganic conditions. Adapted with permission from ref 49. Copyright 2018 Wiley-VCH.

eventually crystallize a zeolite product cooperatively with the CDM SDA. Recently, Lewis and our groups have applied the CDM concept to the MeAPO and SAPO systems.48 Using various combinations of small OSDA cations and alkali metal ions corresponding to CDM and crystallization SDAs, respectively, we were able to synthesize four zincoaluminophosphate molecular sieves (PST-16 (CGS), PST-17 (BPH), PST-19 (SBS), and ZnAPO-88 (MER)) and two SAPO materials (SAPO-69 (OFF) and SAPO-79 (ERI)), the FWCDs of which range from 0.17 to 0.33 per T atom (Figure 4a). We note here that the balance between the charge density of synthesis mixtures, defined as OH−/H3PO4, and the relative content of OSDA and alkali cations should be delicately adjusted for achieving their cooperative structure direction.

expanded: various heteroatom-substituted AlPO4 materials with more than 50 different framework topologies, such as SAPO and MeAPO molecular sieves, have been reported.4,40−43 Of particular interest is that, from the viewpoint of framework charge density (FWCD), silica- and AlPO4-based molecular sieves have evolved in opposite directions. Because the use of OSDAs in aluminosilicate zeolite synthesis yielded less negatively charged (high-silica) frameworks, that is, escaping from low-silica ones, the neutrally charged AlPO4 frameworks synthesized using OSDAs solely has been simply considered as the genesis of heteroatom-substituted AlPO4 molecular sieves. This may be the reason ISDAs have been hardly employed in the synthesis of the latter materials until now. 4.1. Synthesis of MeAPO and SAPO Molecular Sieves using Both ISDAs and OSDAs

4.2. OSDA-Free Synthesis of SAPO Molecular Sieves

In the early 2000s, Lewis and co-workers developed a charge density mismatch (CDM) approach to aluminosilicate systems to encourage the cooperation between multiple SDAs toward a single zeolite structure.44 This two-step synthetic strategy, which can influence not only the heteroatom distribution in zeolite products but also their phase selectivity,45,46 begins with preparing a homogeneous mixture that has a low Si/Al ratio (ca. 1−10; a potential high FWCD aluminosilicate network) and large, low charge density SDAs (designated the CDM SDAs) like TEA+ and thus cannot crystallize by itself due to charge density mismatch.47 High charge density SDAs (designated crystallization SDAs) that are added later, such as TMA+ and inorganic cations, settle such a situation and

There are currently few reports on the synthesis of SAPO molecular sieves under wholly inorganic conditions, and none of them provided any clear evidence for the framework substitution of P atoms.49 However, we recently were able to synthesize SAPO materials with MER, EDI, GIS, and ANA topologies (Figure 4b), in which the presence of framework Si, Al, and P atoms is demonstrated by a combination of multinuclear magic-angle spinning (MAS) NMR and singlecrystal X-ray diffraction (XRD) analyses.49 More recently, we also were successful in synthesizing SAPO-34 (CHA), a commercial methanol-to-olefins catalyst, using Na+ or K+ ions as an ISDA both in the presence and absence of a small amount of previously prepared SAPO-34 as seed crystals.50 F

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Since our SAPO molecular sieve syntheses include the use of a considerable amount of alkali metal hydroxides (MOH/ Al2O3 ≥ 2.5), the synthesis gels are highly basic (typically pH ≥ 10) compared to the typical OSDA-containing SAPO gels, reminiscent of low-silica aluminosilicate gels, giving the resulting materials with unusually high FWCDs (0.25−0.46; Figure 4a). We also found that, unlike that of the other SAPO phases, the framework Si fraction of the MER-type SAPO molecular sieve is controllable by changing Si content in the K+ ion-containing synthesis gel. In consequence, a K-SAPO-MER material with the highest Si fraction (Si/T = 0.53) selectively adsorbed CO2 over CH4 and N2, because the presence of silica islands in its framework leads to a decrease in the number of extraframework cations per unit cell (and thus an increase in microporosity).49

†

J. S. and D. J. contributed equally to this work.

Notes

The authors declare no competing financial interest. Biographies Jiho Shin got his Ph.D. in Environmental Science and Engineering from POSTECH in 2013 under the supervision of Prof. Suk Bong Hong. He worked as a postdoctoral researcher with Prof. Xiaodong Zou (2015−2016) at Stockholm University. Currently, he is a research professor at the Center for Ordered Nanoporous Materials Synthesis in POSTECH. His research focuses on the synthesis and structure determination of zeolites and their applications. Donghui Jo received his Ph.D. in Environmental Science and Engineering in 2018 from POSTECH under the supervision of Prof. Suk Bong Hong. He is currently a postdoctoral fellow in the same group.

5. SUMMARY AND OUTLOOK In this Account we have outlined our recent attempts to adjust inorganic synthesis parameters (i.e., alkali and alkaline earth metal cations and fluoride anions) to find zeolites and zeolitelike materials with novel framework structures or compositions. The multiple inorganic cation and excess fluoride approaches, when appropriately combining the structuredirecting ability of already known OSDAs with various choices of inorganic synthesis conditions in the aluminosilicate system, have enabled us to crystallize a series of unprecedented zeolite structures. We have also demonstrated that MeAPO and SAPO materials with different framework topologies and unusually high FWCDs can be synthesized using alkali metal ions, by applying the CDM concept, originally developed for aluminosilicate zeolite chemistry, or without the aid of any OSDAs. The greatest benefit of our synthetic strategies appears to lie in their versatility. Both the multiple inorganic cation and excess fluoride approaches are applicable to a broad spectrum of relatively simple OSDAs so that they may contribute to create novel zeolite structures of technological relevance. While they could also be utilized in the synthesis of AlPO4-based molecular sieves, it is necessary to understand the underlying mechanisms of cooperative structure direction among multiple ISDAs and a given OSDA in crystallization conditions. For example, elucidating the exact role of each type of inorganic cations during the formation of zeolite nuclei, especially those consisting of more than one type of structural building units, may be the key to attaining the rational synthesis of zeolites with designed pore structures. The Al concentration effects on the phase selectivity of the crystallization in excess fluoride conditions is another issue to be addressed. Because of similar reasons, the nature and extent of interactions between the alkali or alkaline earth metal ions and framework components in MeAPO and SAPO synthesis gels are directly related to the successful synthesis of such AlPO4-based molecular sieves with new framework structures and compositions in the presence or absence of OSDAs.

■

Suk Bong Hong is a professor of Environmental Science and Engineering and the director of Center for Ordered Nanoporous Materials Synthesis at POSTECH. He received his Ph.D. in Chemical Engineering from Virginia Tech in 1992 under Prof. Mark Davis. His current research is on the synthesis, characterization, and applications of zeolites and molecular sieves.

■

ACKNOWLEDGMENTS We acknowledge financial support from the National Creative Research Initiative Program (2012R1A3A2048833) through the National Research Foundation of Korea and from the National Research Council of Science & Technology (CRC14-1-KRICT) grant by the Korea Goverment (MSIP). We also thank our many co-workers whose names appear on specific references for their invaluable contributions.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Suk Bong Hong: 0000-0002-2855-1600 G

DOI: 10.1021/acs.accounts.9b00073 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.accounts.9b00073 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research (50) Hong, S. B.; Park, S. H. Silicoaluminophosphate Molecular Sieves with CHA Topology and Their Manufacturing Method Using Inorganic Structure-Directing Agent. K.R. Patent Application 102018-0173283, 2018.

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DOI: 10.1021/acs.accounts.9b00073 Acc. Chem. Res. XXXX, XXX, XXX−XXX