Synthesis Strategies for Preparing Useful Small ... - ACS Publications

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Synthesis Strategies for Preparing Useful Small Pore Zeolites and Zeotypes for Gas Separations and Catalysis Manuel Moliner, Cristina Martínez, and Avelino Corma* Instituto de Tecnología Química (UPV-CSIC), Universidad Politécnica de Valencia, Consejo Superior de Investigaciones Científicas, Valencia, 46022, Spain ABSTRACT: In the last years, the preparation of small pore zeolites, especially those presenting large cavities in their structure, have received much attention because they have shown excellent application in catalysis (such as methanol-toolefins or selective catalytic reduction of NOx) and gas separations. In the present review, we will focus on the diverse synthetic routes followed to direct the crystallization of small pore zeolites and, on the other hand, the most outstanding applications of those small pore microporous materials. KEYWORDS: small pore zeolites, aluminophosphates, gas separation, catalysis, SCR of NOx dimensional”, “two-dimensional”, or “three-dimensional” zeolites if the molecular sieve shows one, two, or three channels in different directions. Despite the large number of reported zeolites, the mechanisms governing the nucleation and crystallization of molecular sieves still remain unclear due to the interconnection of several variables influencing those processes.1 However, the knowledge acquired since the very first synthetic zeolite description5 allows rationalizing the preparation of specific zeolites by direct6 or postsynthetic methodologies.7 From all the potential synthetic strategies described in the literature, we would like to highlight in this manuscript those used for the preparation of zeolites containing exclusively small pores. The reason is the remarkable industrially relevant applications related to those materials which have appeared in the last years.8 For example, small pore zeolites containing large cavities in their structure have been introduced as catalysts for methanol-to-olefins (MTO)9 and for selective catalytic reduction (SCR) of NOx.10 In addition to those, other significant applications have been reported for small pore zeolites, especially in the field of gas separation.11 In the first part of the present manuscript, we will focus on the diverse synthetic routes described to direct the zeolite synthesis toward materials with small pores, and later we will review the most outstanding applications in catalysis and separations reported for those microporous materials presenting small pores in their structure.

1. INTRODUCTION Zeolites are a class of crystalline microporous oxides with welldefined pores and cavities of molecular dimensions that find application in catalysis, separation, and ion exchange.1 Although their chemical composition was first limited to aluminosilicate polymorphs, many more heteroatoms such as B, P, Sn, Ti, Fe, Ge, Ta, and V, among others, can now be introduced into zeolitic frameworks in addition to Si and Al.2 This large chemical versatility has allowed controlling the physicochemical functionalities of zeolites (such as acidity, redox properties, or hydrophobic−hydrophilic nature), and, consequently, the number of applications has been broadened.3 Zeolites are usually prepared at temperatures ranging from 100 to 200 °C under autogenous pressure from aqueous gels containing the different heteroatom sources, the inorganic and/ or organic cations performing as structure directing agents (SDA), and the mobilizing agents (hydroxyl or fluoride anions).1 In general, the modification of the synthesis conditions, including crystallization temperature, type of heteroatoms, molar ratios of the gel components, shape and size of the organic structure directing agents (OSDA), and type of mineralizing agent, among others, influences the crystallization of those microporous materials, directing toward different crystalline framework topologies.1,2 Today, the International Zeolite Association (IZA) recognizes 206 different molecular sieves with different pore architectures.4 Those microporous materials can be classified attending to the size of their pores or channels, and zeolites with pore openings limited by 8 atoms in tetrahedral coordination are defined as “small pore zeolites” (8-rings), by 10 atoms are “medium pore zeolites” (10-rings), by 12 atoms are “large pore zeolites” (12rings), and finally, those presenting more than 12 atoms are defined as “extra-large pore zeolites”. On the other hand, zeolites can also be classified attending to the number of pores running in different directions within the zeolitic structure, describing “one© 2013 American Chemical Society

Special Issue: Celebrating Twenty-Five Years of Chemistry of Materials Received: May 7, 2013 Revised: June 14, 2013 Published: June 17, 2013 246

dx.doi.org/10.1021/cm4015095 | Chem. Mater. 2014, 26, 246−258

Chemistry of Materials

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2. SYNTHESIS OF ZEOLITES WITH SMALL PORES It is important to note that the first synthetic small pore zeolites were prepared using exclusively inorganic cations in the synthesis gels, such as, for example, LTA,12 KFI,13 RHO,14 or CHA,15 among others. However, those former materials present very low Si/Al ratios, and their potential applications were restricted by their low hydrothermal stability and acidity. When the 206 zeolites recognized by the IZA are carefully analyzed, it can be observed that 61 different structures show exclusively small pores in their framework. However, most of those materials have not received significant attention in the literature because their preparation within a very narrow chemical composition limits their hydrothermal stability or physicochemical properties. The most significant molecular sieves presenting exclusively small pores are summarized in Table 1. This selection has been

Researchers at Chevron have described the synthesis of several high silica small pore zeolites with large cavities in their structure by using different cyclic or polycyclic ammonium cations as organic structure directing agents (OSDAs) (see Figure 1). In this sense, the synthesis of the high silica SSZ-13 zeolite was first reported by Zones in 1985.19 SSZ-13 is a three-dimensional small pore zeolite presenting large cavities in its structure with the chabazite (CHA) topology (see Figure 1). Chabazite was first described in the literature with very low Si/Al ratios (lower than 3), synthesized using alkaline cations as inorganic directing cations.20 The introduction of the bulky polycyclic N,N,Ntrimethyl-1-adamantammonium (TMAda, see Figure 1) as OSDA directs the synthesis to high-silica CHA with Si/Al ratios higher than 10.19 In an insightful publication, Gies et al. described correlations between the size and shape of the OSDA and those of the crystallized molecular sieves.21 In a similar way, Zones et al. were able to control the size and shape of the cavities of different small pore zeolites by increasing the size of the OSDA (see Figure 1, top). A smaller organic cation than TMAda, such as N-methyl quinuclidine, directs the crystallization of SSZ-17. This is a twodimensional small pore zeolite containing medium-sized cavities with Levyne structure.22 When a larger OSDA is used, such as the diquaternary ammonium compound formed by two quinuclidine units connected with a polymethylene bridge, the synthesis was directed toward SSZ-16, which is a three-dimensional small-pore zeolite containing very large cavities (see Figure 1, top).23 In fact, the cage dimensions of those three small-pore materials are 8.05 × 8.05 × 6.95 Å, 8.35 × 8.35 × 8.23 Å, and 8.35 × 8.34 × 13.03 Å, for SSZ-17, SSZ-13, and SSZ-16, respectively. As it will be shown later, the control of the zeolite cage size will have an important impact on the catalytic properties of that family of materials.24 SSZ-39 is another attractive high-silica three-dimensional small pore zeolite with large cavities described by Zones et al.25 This molecular sieve is very similar to CHA since both are built up of the same double-6-rings (D6R) units but arranged differently. Both materials present cavities of comparable size (see Figure 1) and the same framework density (15.1 T atoms per 1000 Å3). Interestingly, SSZ-39 zeolite can be prepared by using several simple medium-sized alkyl substituted cyclic ammonium cations, as described in its original patent.26 Other bulky bicyclic OSDAs designed by Chevron researchers have allowed the preparation of high-silica small-pore zeolites with large cavities in their frameworks, such as SSZ-5027 and SSZ-73,28 with RTH and SAS topologies, respectively. Particularly interesting is the dramatic directing effect of two geometric polycyclic isomers (see Isomer A and Isomer B in Figure 1), with whom the synthesis can be preferentially directed to SSZ-50 or SSZ-73 depending on the enrichment in isomer.29 Pure-Silica Small Pore Zeolites. Researchers at ITQ first reported the synthesis of zeolites using concentrated gels in fluoride media.30 In these systems, fluoride anions stabilize small zeolitic cages when occluded inside of them and, in this way, can balance the positive charge of the OSDA cation.31 Thus, concentrated gels in fluoride media have allowed the preparation of several new pure silica zeolites which, in general, show large crystal sizes with very few structural defects.32 The combination of concentrated gels in fluoride media with cyclic or polycyclic OSDAs has permitted the synthesis of some small-pore pure silica zeolites with large cavities. Indeed, pure silica CHA structure can only be achieved up to now using the bulky TMAda as OSDA in fluoride media and concentrated gels.33

Table 1. Significant Zeolites Presenting Exclusively Small Pores IZA code AEI AFX ANA CHA DDR ERI IHW ITE ITW KFI LEV LTA NSI RHO RTE RTH RWR SAS SAV UFI

zeolite names 25

98

pore architecture 99

SSZ-39, AlPO-18, SIZ-8 SAPO-56,100 SSZ-1623 Analcime,101 AlPO-24102 SSZ-13,19 SAPO-34103 ZSM-58104 UZM-12,105 AlPO-17106 ITQ-3235 ITQ-3107 ITQ-1234 ZK-5108 Levyne,109 SAPO-35103 Linde Type A,110 ITQ-2911a Nu-6(2)64 RHO14 RUB-3111 RUB-13,112 SSZ-5027 RUB-2466 STA-6,44a SSZ-73113 STA-744b UZM-539a

8×8×8 8×8×8 8×8×8 8×8×8 8×8 8×8×8 8×8 8×8 8×8 8×8×8 8×8 8×8×8 8×8 8×8×8 8 8×8 8×8 8 8×8×8 8×8

performed attending to the excellent physicochemical properties of those microporous materials, which have been achieved thanks to rational synthesis procedures. The preparation of most of those materials is revised in the next section. 2.1. Cyclic/Polycyclic Ammonium Cations and Amines as Templates for Small Pore Zeolite Synthesis. High-Silica Small Pore Zeolites. Organic molecules acting as OSDAs or templates in zeolite synthesis are frequently used since the earlier description by Barrer and Denny.16 As a general description, the introduction of OSDAs in zeolite synthesis allowed the preparation of new zeolite polymorphs with higher Si/Al ratios.17 Those organic cations are much bulkier than alkaline inorganic cations, and consequently, the total positive charges introduced into the hybrid inorganic−organic matrix are lower, requiring less aluminum atoms in the zeolitic framework (note that a trivalent aluminum atom in tetrahedral coordination creates a negative charge). The preparation of high silica zeolites allows increasing the hydrothermal stability and the acidity strength and tuning the hydrophobic/hydrophilic character of the zeolites.18 247



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Figure 1. Small pore zeolites synthesized by Zones et al. with the OSDAs used and the large cavities present in those molecular sieves.

Figure 2. Small pore zeolites synthesized at ITQ under concentrated fluoride media.

perpendicular 8-ring pores, connected by relatively short 12rings channels.35 Decadodecasil 3R (DD3R) is another reported pure silica zeolite presenting exclusively small pores in its structure, which is synthesized using the polycyclic adamantyl-amine as OSDA.36 Its preparation as pure silica form showed some reproducibility problems.37 Recently, pure silica DD3R synthesis has been improved by introducing fluoride anions to the synthesis mixture.37 Moreover, the crystallization rate of this zeolite can be notably accelerated by seeding with small amount of DD3R crystals.36a Indeed, the seeding methodology is a well-known technique in zeolite synthesis, which allows reducing the nucleation and crystallization times and increasing the selectivity toward the required crystalline phase.1 As it will be discussed, all-silica small pore zeolites offer exceptional properties for their application in gas separation,

In addition to pure silica chabazite, other all-silica small pore materials have been reported using cyclic OSDAs in fluoride media (see Figure 2). ITQ-12, a new pure silica polymorph presenting a bidimensional small pore system, can be prepared using imidazole-derived rings as OSDA in the presence of fluoride anions.34 ITQ-3 is also a bidimensional pure silica smallpore zeolite, which has been prepared using a bulky bicyclic OSDA (see Figure 2) in fluoride media. ITQ-3 zeolite shows a large cavity very similar in volume to that found in RTH (see SSZ-50 in Figure 1) but presenting slight differences in shape. Although the OSDAs used for the preparation of SSZ-50 and ITQ-3 are also very similar (see Figures 1 and 2), their different shapes have allowed the crystallization of two related small pore zeolites but with dissimilar cavities. Finally, a third twodimensional small pore zeolite, ITQ-32, has been prepared in its all-silica form using also a cyclic OSDA in fluoride media (see Figure 2).35 The pore topology of ITQ-32 zeolite exhibits two 248



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Figure 3. Supramolecular self-assembling OSDA methodology for the synthesis of pure silica LTA. Reproduced with permission from ref 11a. Copyright 2004 Nature Publishing Group.

Figure 4. Cyclic polyethers, cyclic polyamines, and metal−polyamine complexes used for the synthesis of small-pore zeolites.

charge density mismatch (CDM) approach,38 a methodology that studies the cooperative roles of different OSDAs in the synthesis gels in order to achieve the best match with the negative framework charges of crystallized zeolites. Following this strategy, different new zeolites or known microporous materials

such as propylene−propane and linear and branched olefins, or in the upgrading of natural gas. 2.2. Small Pore Zeolites Prepared by the Charge Density Mismatch Approach. Researchers at UOP have nicely rationalized the synthesis of zeolites by using the so-called 249



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previously described.41 Both materials present a three-dimensional small pore system with large cavities that, up to that moment, were synthesized under organic-free media using alkaline cations. Those molecular sieves showed very low Si/Al ratios (values lower than 3).42 However, the presence of the crown ether in the synthesis gels enhances crystallinity and phase selectivity and increases the Si/Al ratios (values up to 5) of both materials, demonstrating the beneficial effect of employing this polycyclic ether on the synthesis of small pore zeolites with large cavities. In a similar way, Caro et al. used a bulkier crown ether, such as Kryptofix 222 (see Figure 4), for the preparation of different polymorphs of the LTA zeolite.43 In their work, the authors describe the synthesis of the silicogermanate43a and the aluminophosphate43b forms of the LTA zeolite. Interestingly, Kryptofix 222 molecule shows the adequate size and shape to fit the large cages of LTA structure. On the other hand, Wright et al. first introduced some macrocyclic polyamines [cyclam, tmtac (1,4,8,11-tetramethyl1,4,8,11-tetraazacyclotetradecane), and hmhaco (1,4,7,10,13,16hexamethyl-1,4,7,10,13,16-hexaazacyclooctadecane), see Figure 4] as novel OSDAs for directing the preparation of new small pore metalloaluminophosphates (MAPOs).44 The use of tmtac as OSDA was particularly productive since two novel small pore MAPOs, such as STA-644a and STA-7,44b were prepared containing different divalent metals in framework tetrahedral sites (M = Mg, Mn, Fe, Co, or Zn). In fact, the ability for introducing diverse type of metals in those small pore zeotypes by isomorphic substitution offers them unique acid and redox properties. The framework topologies of STA-6 and STA-7 are very different, presenting monodimensional and three-dimensional small pores, respectively. However, both materials show similar large cavities with comparable shape and size.44b The authors reveal by molecular modeling that STA-6 and STA-7 frameworks can be considered to be built up of cages or cavities, and tmtac macrocycles proceed as extraordinary templates for those cages.44b 2.5. Cyclic Polyamine−Metal Complexes. Extra-framework cationic metals or metal clusters are usually introduced in microporous materials by postsynthetic cationic exchange or impregnation procedures of metal precursors.45 However, metalcontaining molecular sieves prepared by those postsynthetic methodologies not only require numerous steps in their preparation but also suffer from a lack of uniformity in the metal distribution within the crystals when metal-containing small pore zeolites are prepared.46 Indeed, the efficient metal diffusion through small pores can be severely restricted depending on its ionic radius. In this context, the ability to prepare metal-containing small pore zeolites by direct methodologies would allow reducing the overall steps required and, furthermore, control the metal dispersion within zeolitic crystals.6 The most rational way to disperse, while protecting the metal during the hydrothermal synthesis, is the “in-situ” formation of organometallic complexes, which must be stable under the severe conditions required in zeolite preparation (high pH and temperature). Following these premises, Wright et al. described a complex of nickel with the azamacrocycle 1,4,8,11-tetramethyl-1,4,8,11tetraazacyclotetradecane (Ni-tmtac, see Figure 4) for the direct crystallization of silicoaluminophosphates STA-6 and STA-7.47 Different characterization techniques demonstrated that nickelcomplex molecules remained intact after crystallization processes inside the STA-6 and STA-7 cavities. The as-prepared samples

with improved Si/Al ratios have been prepared, especially small pore zeolites.39 In this sense, Lewis et al. reported the synthesis of UZM-5 using the cooperation of two of the most common OSDAs used in zeolite synthesis, i.e., tetramethylammonium (TMA) and tetraethylammonium (TEA).39a This zeolite shows a new pore topology (UFI), presenting a two-dimensional 8-ring pore system.39a Thanks to the rational combination of different sized OSDAs, this novel molecular sieve was prepared with Si/Al ratios from 5 to 12. UZM-12 is the aluminosilicate form of the ERI topology, which has been reported for the first time by Miller et al.39b This material was prepared following the CDM approach, requiring the combination of two different OSDAs, i.e., TEA and hexamethylbutanediammonium molecules, and potassium cations. Depending on the synthesis conditions, the crystal size (from nano- to micrometer-sized) and the morphology (spheres, plates, or rods) of UZM-12 material was tuned.39b A very remarkable example of cooperative OSDAs following the CDM approach is the preparation of the UZM-9 material, which has the LTA topology.38 LTA is a three-dimensional small pore zeolite with spherical cavities of 1.14 nm, resulting in a molecular sieve with very high void volume. The first description of LTA using an organic structure directing agent, tetramethylammonium, was the ZK-4 zeolite.40 This former synthesis performed by Kerr allowed increasing the Si/Al ratio of the LTA zeolite from one to almost two. Lewis et al. followed the CDM methodology and used a very complex mixture of organic and inorganic cations, including diethyldimethylammonium (DEDMA), TEA, TMA, and Na.38 The unusual combination of those cations allowed decreasing the content of aluminum atoms in the LTA framework up to a Si/Al ratio of 6. In general, higher Si/Al ratios improve the hydrothermal stability and the acidity of zeolites, opening new opportunities for zeolites. 2.3. “Supramolecular Self-Assembling” Methodology. As described above, the synthesis of LTA zeolite with a Si/Al ratio of up to 6 by UOP researchers using the charge density mismatch approach was a clear success. However, the chemical composition window where LTA could be synthesized was still limited. Moreover, as we will see later, some important separations required LTA samples with much lower or non -Al content. Thus, in order to increase the Si/Al ratio in the LTA material, a large and rigid OSDA should be designed to achieve a good fit within the large LTA cavity, while reducing the total amount of positive charges introduced in the as-prepared solid. However, the design of an effective bulky OSDA is not always simple, because the organic molecule must present an adequate hydrophobicity to achieve good organic−inorganic interactions during the nucleation and crystallization processes.17 Then, Corma et al. proposed the “supramolecular selfassembling” approach as a new concept in zeolite synthesis.11a The authors demonstrated the “in situ” formation of a very large OSDA by the supramolecular self-assembling of two aromatic quinolinium-derived molecules through π−π interactions (see Figure 3). This type of paired-OSDA fits perfectly the spherical LTA cavity, and most importantly, the size to charge ratio is maximized. Following this self-assembly (see Figure 3), a highsilica and pure silica LTA zeolite, named ITQ-29, was formed.11a 2.4. Crown Ethers and Azamacrocycles. Patarin et al. have used crown ethers, such as 18-crown-6 ether (see Figure 4), as OSDAs for the synthesis of small pore zeolites with large cavities in their structure.41 They reported the preparation of RHO-type and KFI-type zeolites with higher Si/Al ratios than 250



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metal clusters during the zeolite hydrothermal crystallization.58 They stabilized the metal precursors in the preparative gel with the bifunctional (3-mercaptopropyl)trimethoxysilane ligands (see Figure 5). On one hand, mercapto groups interact with

were calcined in air to remove the organic species and later treated with hydrogen, directing the reduction of the original extra-framework divalent nickel into Ni(I) species, which are very active in different reactions, such as oligomerization of ethylene and propylene and cyclodimerization of acetylene.48 Wright et al. have also reported the direct preparation of Nicontaining CHA silicoalumiphosphate, but using in this case the dimethylenetriamine (deta) complex of nickel (see Figure 4) as OSDA.49 The use of this type of organometallic complex is very attractive because it is less expensive than the above-described azamacrocycle-based complex. Very recently, Xiao et al. have performed the direct synthesis of the copper containing small pore aluminosilicate CHA structure, Cu-SSZ-13 material, using a similar low-cost amine complex, as Cu2+-tetraethylenepentamine (Cu-TEPA, see Figure 4).50 The authors described a structure directing effect of the Cu-TEPA complex toward the CHA cavities determined by theoretical calculations.50 The integrity of the complex molecule in the asprepared solid was confirmed by different characterization techniques. The calcination in air of the as-prepared sample resulted in the Cu-SSZ-13 material with extra-framework Cu2+ species, which have been described as the active sites for the selective catalytic reduction (SCR) of NOx.51 The synthesis methodology described by Xiao et al. shows an efficient and inexpensive “one-pot” approach to prepare Cu-SSZ13, since it not only prevents the numerous synthesis steps of traditional ion-exchange procedures but also avoids the use of the expensive N,N,N,trimethyl-1-adamantammonium cation. However, this methodology shows important limitations in the control of the copper content and the Si/Al ratio in the final solids, achieving always very high copper loadings, with values close to 10 wt % and Si/Al molar ratios lower than 8.50 Both parameters introduce low hydrothermal stability to those metalcontaining zeolites when severe conditions are required, as it is the case of the SCR of NOx (high temperatures in the presence of steam).52 Martı ́nez-Franco et al. have presented a new methodology for the direct synthesis of the silicoaluminophosphate form of the CHA structure, SAPO-34, containing extra-framework Cu2+ species by using two cooperative templates.53 In this rationalized synthetic procedure, Cu-TEPA (see Figure 4) was combined with an inexpensive small organic molecule (diethylamine), which was previously described as an efficient OSDA in the SAPO-34 synthesis.54 This cooperative OSDA method allows the control of the Cu loading in a catalytically interesting range (3 to 8 wt %).53 A similar OSDA cooperative synthesis procedure has been recently described for the direct synthesis of the CuSSZ-13 zeolite.55 Wright et al. have reported an analogous OSDA cooperative approach for the direct synthesis of the Cu-STA-7 molecular sieve.56 For this purpose, the authors combined a polyamine macrocyclic complex, such as Cu-cyclam, with tetraethylammonium cations. As seen, those polyamine macrocycles, such as cyclam or tmtac, show extraordinary directing effects toward STA-7 cavities.44b Then, this rational methodology also allowed the control of the amount of Cu2+ species within STA-7 cavities. 2.6. Mercaptosilane-Assisted Metal Encapsulation. A few years ago, it was presented that mercaptoalkyl trimethoxysilane ligands can interact with metal nanoparticles (gold) through the mercapto groups and with a source of silica (TEOS) through the alkoxysilane groups, encapsulating the metal clusters within silica and avoiding sintering during calcination.57 In a step forward, Choi et al. have described a novel strategy to encapsulate

Figure 5. Scheme of the mercaptosilane-assisted metal encapsulation. Reproduced with permission from ref 58. Copyright 2010 American Chemical Society.

metals, and on the other hand, alkoxysilane groups react with Si and Al atoms to form the zeolitic framework (see Figure 5). Using these bifunctional ligands, successful encapsulation of Pt, Pd, Ir, Rh, and Ag clusters within the NaA zeolite has been accomplished.58 2.7. OSDA-Free Synthesis by Seeding Methodologies. The preparation of zeolites without using organic structure directing agents is a matter of great interest for industry, since those organic molecules are frequently the most expensive component in zeolite synthesis. However, OSDA-free zeolites is not a new concept, since the first synthetic zeolites were prepared in the lab in the absence of organic molecules.5 Those traditional OSDA-free descriptions required the presence of a large amount of alkaline cations, and consequently, the Si/Al ratios in the final solids were often lower than 2.18 Unfortunately, the potential industrial applications of those materials were limited in many cases, due to their poor hydrothermal stability and acidity. Nevertheless, in recent years, the OSDA-free synthesis of microporous materials has renewed its industrial interest thanks to the direct preparation of OSDA-free zeolites with higher Si/Al ratios by seeding procedures.6 This method consists in the introduction of a large amount of preformed crystals (10−25 wt %) of the desired zeolitic structure into the synthesis gels without the presence of organic molecules, and those crystals will act as “seeds” directing the zeolite nucleation.59 The described crystallization mechanisms comprise either the core−shell growth32 or the partially dissolution of seeds and posterior growth.60 251



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Figure 6. Topotactic transformations of (A) Nu-6(1) into small pore zeolite Nu-6(2) and (B) RUB-18 into small pore zeolite RUB-24 [B].

Several zeolites with very different pore architectures can be prepared by this seed-direct method,59b,61 but we will describe those presenting small pores and large cavities. On one hand, Xiao et al. have described the LEV synthesis under organotemplate-free conditions following the seeding methodology.62 LEV is a two-dimensional small pore zeolite presenting large cavities in its framework. This material was previously reported using N-methylquinuclidinium cations as OSDA (see SSZ-17 in Figure 1). The OSDA-free LEV prepared by Xiao et al. avoids the use of the expensive organic molecule, and furthermore, shows excellent crystallinity, high surface area, uniform crystals, and tetrahedral aluminum species. On the other hand, Tatsumi et al. have reported the OSDA-free synthesis of the RTH zeolite.63 RTH is a two-dimensional small pore structure with large cavities in its framework, and as occurs with LEV, it was previously prepared using different expensive OSDAs (see SSZ-50 in Figure 1).27 The OSDA-free procedure proposed by Tatsumi et al. allows the direct preparation of this zeolite, named as TTZ-1, with a Si/Al ratio of 41.63 These are only two examples, but surely this seeding methodology can be applied to the synthesis of other small pore zeolites that, at present, require the presence of expensive OSDAs. 2.8. Topotactic Transformations. A completely different methodology to direct the formation of small pore zeolites is the postsynthetic topotactic transformation of preformed crystalline materials, such as, for example, layered zeolitic precursors (see Figure 6). On one hand, those layered precursors can be pillared or delaminated to produce high-surface-area materials with large accessibility to bulky reactants, as reported for the ITQ-18

material (see Figure 6A). But on the other hand, these silicate layered materials can also be condensed to form interesting 3D small pore framework structures upon calcination processes.64 A classical example is the topotactic transformation of the layered silicate Nu-6(1) into the small pore Nu-6(2) zeolite.64 Whittam described the first report of the Nu-6(1) and Nu-6(2) materials in the 1980s, where Nu-6(1) was prepared using 4,4′-bipyridyne as OSDA and Nu-6(2) obtained by treating Nu-6(1) at high temperatures.65 Afterward, Zanardi et al. solved the structure of both materials by using synchrotron X-ray diffraction.64 A more recent description involving topotactic condensation toward small pore zeolites is the transformation of the layered silicate RUB-18 into small pore pure silica RUB-24 molecular sieve (RWR topology, see Figure 6B).66 Marler et al. prepared this new zeolite by heating at 500 °C the as-prepared layered RUB-18 precursor.66

3. APPLICATIONS OF SMALL PORE ZEOLITES In this part of the manuscript, some relevant applications of small pore zeolites will be highlighted. 3.1. Gas Separations. The development of new efficient gas separation techniques in order to improve current expensive and high-energy-demanding industrial separation processes is, even more today, a matter of interest.67 The total market in 2010 for gas phase separations, including H2 and mixtures of air and natural gas, was $350 million, and it is estimated that it will reach $760 million in 2020.68 The combination of regular pore architecture, large adsorption capacity, and high structural stability of zeolites offers unique properties for their application as efficient materials for gas 252



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and cis-but-2-ene molecules are excluded by differences in their kinetic diameters.37 On the other hand, ITQ-32 zeolite separates trans-2-butene and 1-butene from the linear C4 hydrocarbon fraction coming from the steam reformer.67a ITQ-32 can also achieve excellent propene−propane separations (see Figure 7A).

separations. These separations using zeolites can be achieved for kinetic or thermodynamic reasons. In this sense, preferential adsorption or diffusion rates of the adsorbates within the zeolite occurs in kinetic separations, whereas size exclusion of one gas from the gas mixture by the zeolitic pores occurs in steric separations.67b Very interestingly, small pore zeolites have become attractive selective solids for industrially relevant gas separations in the last years. Light Hydrocarbon Separations (Olefins−Paraffins). Probably, the separation of propene from propane is one of the most challenging industrial separations. This mixture mainly comes from the catalytic cracking unit in the refinery, and its current separation is performed by cryogenic distillation, requiring high energy consumption.69 Their separation is of great importance because propene can be used for polypropylene production, while propane can be applied for household heating. Small pore zeolites could be adequate materials for this separation process, because their pore sizes are very similar to the size of those gases. In addition, zeolites with no acidity (pure silica zeolites) would be highly desired for the olefin−paraffin separations in order to avoid olefin oligomerizations.67a All-silica hydrophobic deca-dodecasil 3R (DD3R) is a very effective material for the selective adsorption of propene/propane, showing high adsorption propene selectivity.36a,70 This selective adsorption behavior can be explained by the slight difference in the critical-molecular diameter. In this sense, propene shows a critical diameter (0.431 nm) smaller than the 8-ring windows (0.45 nm) present in the DD3R zeolite, whereas the critical diameter of propane is very similar to the 8-ring windows (0.45 nm).70a Pure silica ITQ-12 is also able to adsorb preferentially propene more than 100 times faster than propane at 30 °C.34 Interestingly, this material shows an extreme adsorption temperature behavior, revealing a complete absence of propane adsorption at 80 °C.71 The authors hypothesized that higher temperatures induce slight deformations in the ITQ-12 cages, avoiding the adsorption of the slightly larger propane molecule. Other pure silica small pore zeolites have been studied for propane−propene separations, such as CHA and ITQ-3.72 Those materials show very different sorption capacities, as seen in Table 2. All silica CHA presents the highest sorption capacity

Figure 7. (A) Propene and propane sorption on ITQ-ITQ-32 at 60 °C. Reproduced with permission from ref 67a. Copyright 2007 Royal Society of Chemistry. (B) CO2 and CH4 adsorption isotherms of RHO zeolite at 30 °C. Reproduced with permission from ref 75. Copyright 2010 American Chemical Society.

CO2-Methane and CO2−N2 Separations. Natural gas is a hydrocarbon gas mixture formed in nature, presenting primarily methane, carbon dioxide, and nitrogen. The removal of CO2 for upgrading natural gas is a very attractive gas separation process, since CO2 is the major contaminant of natural gas feedstocks.74 The current CO2/CH4 separation technology is based on aqueous amine scrubbing, which requires very complex and high energy-demanding plants.74 In contrast, the use of zeolites for the selective adsorption of one component of the natural gas stream is very simple, avoiding the use of chemicals in the separation process, and requiring very low energy.75 Corma et al. have shown that two small pore zeolites, LTA75 and RHO,11b are very selective molecular sieves for CH4 purification from natural gas. First, the control of the Al content in the LTA framework and, consequently, of the polarity of the zeolite permits the optimization of the adsorption properties in the CO2/CH4 separation.75 And second, RHO zeolite shows the highest selectivity ever described for the CO2/CH4 separation (see Figure 7B).11b The authors proposed that this unique separation approach is due to the combination of the effective pore aperture of RHO zeolite (3.6 Å), which lies between kinetic diameters of CO2 and methane and the structural modifications occurring during the CO2 adsorption in RHO zeolite.11b

Table 2. Propane−Propene Separation over Different Small Pore Zeolitesb adsorption capacity (mg/g) zeolite

pore system

ring size (Å)

80 °C

30 °C

ratio D’sa

Si-CHA ITQ-3 DD3R

3-d 2-d 2-d

3.8 × 3.8 3.8 × 4.3 3.7 × 4.4

90 46 34

120 63 45

46000 690 12000

a D(propene)/D(propane), 30 °C. bAdapted with permission from ref 72. Copyright 2004 Elsevier.

and the highest ratio of propene to propane diffusion constants (>40 000).72 This excellent separation behavior of CHA can be explained by its smaller pore dimension, which mostly impedes the propane diffusion (see Table 2).72 In addition, pure silica DD3R and ITQ-32 zeolites have been reported as excellent zeolites for the separation of linear C4 olefins.37,67a,73 DD3R zeolite shows high applicability for the separation of but-2-ene isomers.37 The DD3R cages are only accessible to trans-but-2-ene and buta-1,3-diene, while but-1-ene 253



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Two very recent reports have also described unique CO2/CH4 separations using RHO76 and CHA zeolites.77 On one hand, Lozinska et al. describe an adsorption mechanism where cations occupying 8-ring sites on cation-exchanged RHO zeolite show high mobility to allow the uptake of CO2.76 CO2 strongly interacts with cations placed in those window sites, allowing CO2 to pass through pore openings thanks to the temporary mobility of these cations, while CH4 does not interact strongly enough to permit cation motion from the 8-ring sites. On the other hand, Shang et al. describe similar CO2/CH4 separations using cationexchanged (Na, K, Cs) CHA zeolites, which also occur through the different ability of gas molecules to induce cation deviation from their 8-ring window sites.77 This selective “molecular trapdoor” allows the preferential CO2 adsorption over CH4. DD3R hydrophobic zeolitic membranes have also been prepared and tested in the CO2/CH4 separation.78 Interestingly, those membranes present very high selectivies for CO 2 adsorption, which is 200 times higher than that of CH4. On the other hand, CO2 capture to reduce the atmospheric CO2 concentration mainly provoked by combustion of fossil fuels is a current important issue. For this purpose, the adequate adsorbents must show high selectivity to CO2 over N2, high regenerability, and low pressure drop.79 Lobo et al. have demonstrated that high silica small pore ZK-5 zeolite, synthesized using the 18-crown-6 ether as OSDA, has an excellent high CO2 working capacity and good CO2 selectivity over N2 at ambient temperature and pressure.79 Similarly, Cheung et al. have also studied different small pore silicoaluminophosphates (SAPO-17, SAPO-35, SAPO-56, and SAPO-RHO) for the CO2/N2 separation, observing that SAPORHO presents the highest CO2 uptake with lowest N2 uptake at 273 K.80 3.2. Catalysis. Methanol-to-Olefins (MTO). Most of the light olefins, such as ethylene, propylene, and butylene, are mainly produced from steam cracking of naphtha.81 However, the increase of the crude oil price and the large emission of CO2 situate MTO as a feasible alternative process to produce light olefins.81a From the different catalysts described in the literature, small pore silicoaluminophosphate SAPO-34 molecular sieve with the CHA structure is the most efficient catalyst to produce ethylene and propylene.82 However, small pore SAPO-34 deactivates fast, requiring frequent regenerations.9 On the basis of SAPO-34, UOP and Norsk Hydro developed the commercial UOP/Hydro MTO technology for the production of light olefins (see the product selectivities reported by UOP for SAPO-34 compared to ZSM-5 in Figure 8A).82 This process is based on a low-pressure fluidized-bed reactor, which allows the continuous catalyst regeneration.83 Researchers at Dalian Institute of Chemical Physics developed a similar fluidized-bed process based on the SAPO-34 catalyst.84 A commercial plant using this technology started in China in 2010. The good catalytic behavior of SAPO-34 in the MTO process can be explained by the combination of large cavities and small pores. On one hand, it has been described that MTO reaction mechanism proceeds through the formation of a “hydrocarbon pool” of substituted aromatic molecules.85 In the case of SAPO34, this “hydrocarbon pool” is selectively formed within the large cavities and is formed mainly by hexamethyl benzenium ions from which the light olefins are produced.86 On the other hand, the small pores avoid the diffusion of aromatic or branched molecules, favoring the high selectivity to linear light olefins. Other interesting silicoaluminophosphates presenting small pores and large cavities, such as SAPO-17 (ERI), SAPO-18

Figure 8. (A) Comparative product selectivities of SAPO-34 and ZSM-5 in the MTO process reported by UOP. Reproduced with permission from ref 82. Copyright 2005 Elsevier. (B) Methanol conversion versus reaction temperature in SAPO-34 and SSZ-13. Reproduced with permission from ref 89. Copyright 2009 Springer.

(AEI), SAPO-35 (LEV), STA-7 (SAV), and STA-14 (KFI), have been studied for the MTO reaction.87 Their mild acidity and their pore topologies offer good activities and selectivities to lower olefins in the MTO process, but always lower than with SAPO-34. In addition to those SAPOs, some small pore aluminosilicates have also been tested in the MTO process.24,88 Although aluminosilicates present higher hydrothermal stability than their SAPO counterparts, they have not been intensively studied in the literature, probably because they show a faster catalyst decay.88a Indeed, a more rapid deactivation when acid strength and acid site density are increased has been reported.81a A comparative activity study between two isostructural CHA materials, such as silicoaluminophosphate SAPO-34 and aluminosilicate SSZ-13, has recently been described.89 In this study, both materials were prepared with similar acid site density and crystal size but with different acid strengths, and they were tested at different reaction temperatures (from 300 to 425 °C).89 As expected, the most acidic SSZ-13 material showed higher activity and faster deactivation rates than SAPO-34. However, the authors observed that the more acidic SSZ-13 material showed good catalytic behavior when the MTO reaction was performed at lower temperatures, suggesting its potential use as commercial catalyst in the MTO process (see Figure 8B). The other important issue is the effect of the cage size of small pore aluminosilicates on the product selectivity in the MTO process. A recent report dealing with this has been presented by Bhawe et al.24 The authors observed that the ethylene selectivity decreases as the cage size increases, while propylene selectivity is highest with intermediate cage sizes. 254



Perspective

exchanged small pore zeolites containing D6R units in their structure have been proposed as good catalysts for the SCR of NOx, such as LEV, SSZ-16,92 and SSZ-39.52b Cu-exchanged SSZ-39 not only performs extremely well for the SCR of NOx (see Figure 10A) but also shows better hydrothermal stability than Cu-CHA when treated under very harsh conditions.52b As described in the synthesis section, different new procedures using organometallic Cu complexes have allowed the “one-pot” preparation of different Cu-containing small pore zeolites with large cavities, such as Cu-STA-7,56 Cu-SSZ-13,50 and Cu-SAPO34.53 Those direct procedures are very attractive since the metal ion exchange step is avoided. Cu-STA-7 shows very good activity for the SCR of NOx, even when working in the presence of steam.56 However, the use of the expensive Cu−macrocyclic (Cu-cyclam) complex as OSDA prevents its potential industrial application. The direct synthesis of Cu-SSZ-13 reported by Xiao et al. only requires the inexpensive Cu-TEPA complex as OSDA (see Figure 4) showing good activities for the SCR of NOx when tested in the absence of water.50 Recently, Martı ́nez-Franco et al. have also described the one-pot synthesis of Cu-SSZ-13 using a rational combination of templates (Cu-TEPA and TMAda), allowing the achievement of very active and stable catalysts for the SCR of NOx when working under severe reaction conditions (high temperature, presence of steam, and very high gas hourly space velocity).55 Finally, the same authors have described the direct synthesis of Cu-SAPO-34.53 They followed a low-cost preparation of Cu-containing zeolites by using the combination of inexpensive OSDAs (Cu-TEPA and diethylamine) and allowing the Cu-content control in the catalysts. As a result, the authors show that intermediate Cu contents in Cu-SAPO-34 materials present exceptional catalytic activities and hydrothermal stabilities for the SCR of NOx (see Figure 10B).53 Other Catalytic Applications. Small pore zeolites have also been applied as acid catalysts for the synthesis of small methylamines, which are important intermediates in solvent or pesticide preparations.93 The synthesis of methylamines is performed by reacting methanol with ammonia through a vapor-phase reaction.94 The thermodynamic equilibrium favors the formation of the undesired trimethylamine instead of desired mono- or dimethylamines. Then, several small pore shapeselective catalysts have been used to minimize the formation of

Finally, a very remarkable recent description on using aluminosilicates for MTO reaction has been reported by Tatsumi et al.63 In this work, the authors describe not only the OSDA-free synthesis of the RTH zeolite but also its excellent activity in the MTO process with extraordinary propylene selectivity.63 Selective Catalytic Reduction (SCR) of NOx. Nitrogen oxide (NOx) is a large atmospheric contaminant produced from fossil fuel combustion. Several processes have been described for the NOx emission control in mobile vehicles, but the selective catalytic reduction (SCR) of NOx by ammonia or urea has emerged as the most applied technology.90 Cu-exchanged zeolites, as Cu-ZSM-5 or Cu-Beta, were described as efficient catalysts for SCR of NOx.91 However, those medium and large pore metal-exchanged zeolites do not meet the most demanding hydrothermal stability targets when treated in the presence of steam at high temperatures. Very recently, BASF researchers have reported that Cu-containing small pore aluminosilicate CHA shows good activity for the SCR of NOx and, most importantly, a higher hydrothermal stability than medium and large pore Cu-exchanged zeolites.10 Lobo et al. have elucidated that the better hydrothermal stability of Cu-CHA can be attributed to the preferential coordination of cationic copper species to the double 6-ring (D6R) cages located in the CHA cavities (see Figure 9).51 In addition to CHA, other Cu-

Figure 9. Preferential location of Cu2+ species coordinated to D6R units of CHA. Reproduced with permission from ref 51. Copyright 2010 American Chemical Society.

Figure 10. (A) SCR of NOx activity of fresh-Cu-SSZ-39 and steamed at 750 °C. Reproduced with permission from ref 52b. Copyright 2012 Royal Society of Chemistry. (B) SCR of NOx activity of different Cu-containing SAPO-34 materials synthesized by “one-pot” methodologies. Adapted with permission from ref 53. Copyright 2012 Elsevier. 255



Perspective

On the other hand, the fluoride synthesis route seems the most effective method for the preparation of highly hydrophobic small pore pure silica crystals with potential application for light hydrocarbon separations, whereas small pore zeolites with lowmedium Si/Al ratios are valuable materials for natural gas upgrade. The design of new polycyclic or supramolecularassembled OSDAs could improve the synthesis of known small pore zeolites or direct the crystallization of new structures. However, the use of bulky OSDAs usually increases the cost of the zeolite preparation. To overcome this cost-effective issue the original expensive OSDAs can be partially replaced by a less selective but less-expensive pore filler.96 In this scenario, the original and expensive OSDA initiates the nucleation of the desired zeolite, and the secondary OSDA fills and stabilizes the structure. Zones has demonstrated this issue for the synthesis of SSZ-13 zeolite, where almost 80% of the original bulky polycyclic N,N,N-trimethyl-1-adamantammonium can be replaced by the cheaper and commercially available benzyl trimethyl ammonium.97 This significant cost reduction in OSDA could be extended to the synthesis of other high-silica small pore zeolites.

trimethylamine. From the different small pore zeolites, RHO and SSZ-16 zeolites are the most active catalysts to produce dimethylamine. 93 It has been observed that the pore dimensionality, cage volume, and acid strength have a strong influence on the product selectivity and the catalyst stability.93 Small pore zeolites have been used to entrap metal clusters for reactions involving reduction−oxidation cycles in an attempt to avoid metal agglomeration.46,58 Indeed, one of the most significant challenges in the design of catalysts containing metallic nanoparticles is that those nanoparticles remain stable and active during the catalytic test. Thus, Zhan and Iglesia have described a direct synthesis methodology to encapsulate RuO2 clusters of 1 nm diameter in the large cavities of the small pore LTA zeolite.46 These RuO2-encapsulated clusters were able to oxidize methanol over bulkier alcohols thanks to the unique shape-selectivity offered by the LTA framework and, on the other hand, were also able to hydrogenate ethene even in the presence of catalyst poisons, as organosulfur compounds. Choi et al. have also described a new general strategy for the direct encapsulation of different metal clusters (see Figure 5).58 Following their approach, controlled clusters of Pt, Pd, Ir, Rh, and Ag were introduced in the large cavities of LTA zeolite. Those metal-containing small pore zeolites were very active shape-selective catalysts for hydrogenation and oxidative dehydrogenation reactions, showing a unique resistance against sulfur poisoning and thermal sintering.58 Recently, the same authors have broadened the metal clusters encapsulation by direct synthesis to other small pore zeolites, such as SOD, GIS, and ANA.95

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Manuel Moliner was born in Valencia in 1979. He obtained his B.S. degree in Chemical Engineering at the University of Valencia in 2003 and completed his Ph.D. at the Polytechnic University of Valencia (UPV), in Chemistry, under the guidance of Prof. Avelino Corma and Dr. Maria J. Dı ́az. After a two-year postdoctoral stay at Caltech with Prof. Mark Davis, he joined the “Instituto de Tecnologı ́a Quı ́mica” (ITQ) as a “Ramón y Cajal Researcher”, where his research lies at the interface of materials design and heterogeneous catalysis.

4. PERSPECTIVES The extraordinary success obtained in recent applications of small pore zeolites in catalysis and gas separation have renewed the interest in the preparation of new small pore polymorphs, especially those presenting large cavities in their structures. As seen along this review, several strategies have been proposed for directing the preparation of small pore zeolites. However, the potential applicability of some of those microporous materials is limited because they can only be prepared under very specific conditions (i.e., low Si/Al ratios), because the structure is unstable after calcination (i.e., hydrated AlPOs or berosilicates) or due to the high cost of the required OSDA (i.e tmtac, cyclam, or Kryptofix 222). In fact, there are 61 different zeolites accepted by the International Zeolite Association (IZA) containing exclusively small pores in their structure, but only 20 of them have received some attention in the open literature (see Table 1). From those 20, as far as we know, only small pore LTA, CHA (SSZ-13 or SAPO-34), and RHO zeolites are being employed in industrial processes.96 Therefore, the rational design of small pore zeolites with the aim of preparing new or improved polymorphs for their industrial application will be a very challenging approach in the next years. On one hand, the recently described direct syntheses of small pore zeolites containing well-distributed metals within their cavities and high-silica small pore zeolites prepared by seeding OSDA-free methodologies are interesting strategies for the manufacture of small pore zeolite materials with the adequate characteristics for their implementation as industrial catalysts. Indeed, the direct preparation of Cu-SAPO-34 with controllable metal charge53 and the OSDA-free synthesis of RTH zeolite63 are two interesting discoveries, which can open alternative inexpensive synthetic routes, as well as new interesting catalytic applications.

Cristina Martı ́nez obtained her bachelor in Chemistry at the University of Valencia in 1991, and joined the group of Professor Corma at the Instituto de Tecnologı ́a Quı ́mica (ITQ, UPV-CSIC) that same year. She received her Ph.D. in 1997 at the Polytechnic University of Valencia, and at present is a Tenured Scientist at the ITQ. Her principal research topics are focused on heterogeneous catalysis, mainly zeolite based, applied to refining and petrochemistry processes and methane upgrading. Avelino Corma was born in Moncófar, Spain, in 1951. He studied Chemistry at the Universidad de Valencia (1967−1973) and received his Ph.D. at the Universidad Complutense de Madrid in 1976. He was Postdoctoral in the Department of chemical engineering at the Queen’s University (Canada, 1977−1979). He has been a research professor at the Instituto de Tecnologı ́a Quı ́mica (UPV-CSIC) at the Universidad Politécnica de Valencia since 1990. His current research field is catalysis, covering aspects of synthesis, characterization, and reactivity in acid− base and redox catalysis. Avelino Corma is co-author of more than 900 articles and 100 patents on these subjects.

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ACKNOWLEDGMENTS This work has been supported by the Spanish Government through Consolider Ingenio 2010-Multicat, the “Severo Ochoa Program”, MAT2012-37160 and MAT2012-31657 and by UPV through PAID-06-11 (n.1952). Manuel Moliner also acknowledges “Subprograma Ramon y Cajal” for the contract RYC-201108972. 256



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