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Zeolites from a Materials Chemistry Perspective Mark E. Davis Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States ABSTRACT: Zeolites and zeolite-like materials are continually finding new applications. Because of the uniformity of these solids, the expression of macroscale materials properties that are controlled by the materials chemistry at the atomic/molecular scale are achievable. In this Perspective, I discuss the following areas of current interest in zeolites and zeolite-like materials that rely on manipulation of the materials chemistry for their preparation and provide new opportunities for application: (i) exploitation of organic structure-directing agents (SDAs) for new materials, (ii) the synthesis of zeolites without SDAs, (iii) the synthesis of very hydrophobic materials, (iv) conversions of two-dimensional (2D) to 3D materials and vice versa, (v) hierarchically organized materials, (vi) chiral materials, and (vii) direction of tetrahedral atoms to specific framework positions. KEYWORDS: zeolites, synthesis, applications, future

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INTRODUCTION It is my pleasure to provide this Perspective on the materials chemistry of zeolites for the 25 Year Celebratory Special Issue of Chemistry of Materials. As a previous editor of this journal, I have closely watched its evolution. I am excited by the many developments in materials chemistry over the past few decades and feel that Chemistry of Materials has been a showcase for the field. In 1992, Raul Lobo and I published an extensive review on “Zeolite and Molecular Sieve Synthesis” in Chemistry of Materials.1 That review has now been cited over 750 times (Web of Knowledge, June 2013) and continues to be cited today. In that review, Raul and I summarized zeolite and molecular sieve synthesis in order to identify significant trends and suggest future areas of research. Also, we discussed the plausibility of new types of materials that may be achievable through designed, synthetic efforts. Here, I provide a perspective on zeolites and zeolite-like materials and emphasize how the materials chemistry drives new properties and new applications. A few of these issues that have emerged over the past decades are ones that Raul and I discussed and got right, while others have been totally unanticipated. Zeolites and molecular sieves continue to find more and new uses in commercial applications.2 The traditional areas of zeolite and molecular sieve use, such as ion exchangers, adsorbents, and catalysts (for example, with catalysis, see ref 3), all continue to grow. In addition to these more traditional applications (that have been used by traditional industries like the petrochemicals industries), there are now many other uses outside those areas and industries. For example, low Si/Al zeolites are very hydrophilic and well-known to adsorb water. Most of us have used them at some point to dehydrate organic solvents. The ability of zeolites to adsorb water (along with other features) has led to their use as a blood-clotting agent. © XXXX American Chemical Society

QuikClot contains a dehydrated zeolite that was approved for use by the FDA in 2002.4,5 It is currently being used by the military and emergency personnel and can even be purchased for consumer use. In addition to preparing zeolites with specific pore sizes for shape-selective adsorption/catalysis, zeolites can be modified in order to tune their surface properties. An interesting application of the tuned adsorption properties of a zeolite is in the wine industry. The contamination of wine with 2,4,6-trichloroanisole (TCA), known as “cork taint,” is an increasing problem for wine producers. A zeolite with particular adsorption properties is now being used to remove TCA from wine without altering its taste.6 An area that has emerged since our review is that of hydrophobic materials. The chemistry of the zeolite materials can now be controlled to provide very hydrophobic solids that can perform a number of new applications. I will discuss this topic further and illustrate a new and unanticipated application in the context of new catalysis below. However, the use of hydrophobic materials as adsorbents to remove organic compounds such as methyltertbutylether (MTBE) and chlorinated hydrocarbons like 1,2-dichloroethane and chlorobenzenes has already been commercialized.7,8 In our 1992 review, Raul and I presented numerous issues with zeolites in the context of their materials synthesis and/or properties. For example, at that time, extra-large pore materials (crystalline molecular sieves with pores composed of greater Special Issue: Celebrating Twenty-Five Years of Chemistry of Materials Received: June 12, 2013 Revised: August 29, 2013

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than 12 oxygen atoms that can give pore sizes above 1 nm) were just emerging. These materials were prepared as aluminophosphate compositions. We suggested at that time that large organic structure-directing agents (SDAs) could likely lead to silica-based, extra-large pore materials with significantly different properties from the aluminophosphates. This has indeed occurred.9,10 However, we did not anticipate the discovery of the ordered mesoporous materials in that review. Thus, the development of many types of porous materials with pores larger than 1 nm in diameter has occurred and will not be discussed further here. Interestingly, not only has there been a movement to larger pore materials, but also much work has occurred with small pore materials (pores composed of 8 oxygen atoms). This is because of the discovery of materials like SAPO-34 that now commercially convert methanol to primarily ethylene and propylene in China, and Cu-SSZ-13 that is now used commercially as a deNOx catalyst (these topics will be covered elsewhere in this issue of Chemistry of Materials). Here, I am going to discuss the following topics as they are emerging areas of current interest that all rely on manipulation of the materials chemistry for their preparation and provide new opportunities for application: (i) exploitation of SDAs for new materials, (ii) the synthesis of zeolites without SDAs, (iii) the synthesis of very hydrophobic materials, (iv) conversions of 2D to 3D materials and vice versa, (v) hierarchically organized materials, (vi) chiral materials, and (vii) direction of tetrahedral atoms to specific framework positions. The latter two topics have not yet been realized but are of high current and future interest.

Work continues on understanding how the properties of the SDAs lead to the production of crystalline structures.14−17 Since synthetic organic chemistry is quite advanced, the ability to understand what properties of the organic SDAs provide for good structure direction remains crucial to future successes. The diversity of organic molecules that are being utilized as SDAs is ever increasing (a recent example involves organic molecules that are proton sponges18). Of importance is the fact that the inorganic chemistry cannot be ignored when exploiting the concept of structure direction.17,19 It is the correct combination of SDAs and inorganic chemistry that are married together in the assembly processes to create the final structures. In 1991, Raul and I asked the question, “can zeolite synthesis be designed?” At that time, the answer was no. Today, the answer remains no, but the field is moving closer to being able to produce structures with given properties by design, e.g., pore sizes and presence or absence of cages. A particularly interesting methodology that illustrates the importance of having both the “right” organic molecule and the “right” inorganic chemistry is now called the charge density mismatch (CDM) approach.20 The concept of CDM involves creating a synthesis mixture that contains an organic agent like a tetraethylammonium cation. The mixtures are heated, but they do not produce a crystalline material due to a number of reasons, including a large mismatch in charge that would be required if a structure were to form via structure direction of the organic cation. When additional organic molecules, e.g., tetramethylammonium cations, are added, these “precursor” mixtures then crystallize into a solid. This approach has provided a number of new structures that have higher aluminum contents than normally achieved with the use of traditional syntheses employing SDAs.20,21 When an organic SDA is used to crystallize a molecular sieve of interest, the cost of the organic is often thought to be prohibitive of commercialization. Zones has recently discussed this issue.2 Two synthetic methods have been developed to address the use of costly organic SDAs. Lee et al. showed that recyclable SDAs can be prepared.22 For example, the synthesis of ZSM-5 has been illustrated with this methodology using a SDA that contains a cyclic ketal group that is removed from the SDA while in the pores of the formed ZSM-5 (Figure 1). The remaining organic is then small enough to be removed via ion exchange. The collected organic fragments are then reassembled and used again. This method has not yet been commercialized. A second approach exploits the use of two organic molecules in the synthesis. The expensive SDA is used

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SYNTHESIS OF ZEOLITES AND MOLECULAR SIEVES Structure Direction via the Use of Organic Species. The synthesis of zeolites and molecular sieves can involve the use of organic molecules that influence the crystal assembly processes. The term “templating” has been used in this context in the past. Raul and I suggested that true templating may have occurred in only one case (ZSM-18) at the time of our review. We emphasized that the organic molecules that were described in the literature as templates should be thought of as structuredirecting agents (SDAs) rather than as true templates. To date, I believe that there is no example of true templating, as the one example we mentioned previously1 has now been shown to not be templating but rather what we would classify as structure direction. The concept of structure direction via organic molecules has provided a wealth of new structures with new properties. Previously, we suggested that the synthesis of extra-large pore materials via appropriately sized organic SDAs could occur. This has indeed happened, and there are several examples of extra-large pore materials, e.g., UTD-1, CIT-5, and SSZ-53,10 that have been synthesized via the use of large organic SDAs. In addition to creating extra-large pore materials, the use of SDAs also enabled the synthesis of molecular sieves that have intersecting 12 and 10 member-ring pores. This pore architecture is particularly desirable for certain catalytic reactions, and we speculated that these structures could be synthesized because the natural zeolite Boggsite has a pore system of this type.11 Through the use of SDAs, materials with 12−10 pore systems have been prepared. SSZ-33 and SSZ-26 were the first synthetic structures of this type,12 and there are now many others. In fact, even the synthetic analog of Boggsite has now been prepared using an organic SDA.13

Figure 1. Schematic representation a recyclable SDA. From ref 22. B

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calcination that involves environmental concerns over the exhaust gases that can contain toxic oxides of carbon and nitrogen). The zeolite ECR-1, normally prepared via the use of organic SDAs, was synthesized without an organic SDA in 2006.23 Interestingly, the Si/Al of the organic-free synthesis was the same as that from the preparation using organic SDAs (Si/Al = 14). The organic-free synthesis was obtained by elucidating a specific inorganic mixture. Thus, this methodology would be difficult to generalize to other structures. A more generalized approach to the synthesis of zeolites that were originally prepared with organic SDAs, but now prepared in their absence, exploits seeding the reaction mixture. Xie et al. first prepared zeolite beta without the use of an organic SDA in 2008.24 While the natural mineral tschernichite exists and has the zeolite beta structure,25 the preparation of organic-free zeolite beta is a significant breakthrough in zeolite synthesis. The Si/Al of tschernichite is ca. 3. Thus, the zeolite beta structure is able to exist with this low Si/Al. The organic-free syntheses of zeolite beta give Si/Al as low as ca. 5,26 whereas the SDA mediated syntheses all give values larger than 10. The seed-assisted synthesis of organic-free zeolites has now been reported for a number of zeolites that had previously only been prepared via the use of an organic SDA, e.g., ZSM-11,27 ZSM12,28 and RUB-13.29 Thus, it appears that the use of seedassisted synthesis can be applied to prepare a variety of zeolite structures. Lower Si/Al products may not be as useful as their higher Si/Al analogs. This is likely the case for their use as catalysts. However, aluminum can be removed from the lower Si/Al materials (this requires further chemical processing) to provide useful catalytic materials.30 The use of organic SDA-free syntheses of zeolites provides a number of advantages for scale up and commercialization. However, there are also downsides to this type of synthesis. High yields are typically not achieved with these types of preparations even when high amounts of seeds (ca. 10 wt %) are used, and control of the product Si/Al and crystal sizes are difficult. Thus, one needs to consider the trade-offs of preparing zeolites with methods using organic SDAs like those described by Zones2 to the lower yields of the organic SDA-free syntheses. Higher yields may be achievable using synthetic methods that do not involve bulk solvent,31 but it is not clear

to nucleate the synthesis, and a second organic molecule that is much less costly is utilized in the crystal growth process to fill the internal void space (Figure 2). Zones provides a nice

Figure 2. Schematic representation of the use of a SDA and another organic molecule that can fill pore space during the synthesis of a zeolite.2

example of this method showing how a much less expensive organic cation can be used in combination with a series of more expensive organic cations to produce a series of structures (SSZ-13, SSZ-35, SSZ-33, SSZ-42).2 This methodology has been used to commercialize the synthesis of molecular sieves.2 Thus, there continues to be strong efforts in creating new structures and/or existing structures with new properties via the use of structure direction. Bias against this method of synthesis because of perceived cost barriers is declining for the reasons outlined by Zones.2 Synthesis in the Absence of Organic SDAs. The first synthetic zeolites were aluminum-rich and prepared using alkali metal cations.1 Higher Si/Al zeolites are normally obtained when using organic SDAs, as these cations are larger than alkali cations (less framework charge needed to balance their positive charge) and they are more hydrophobic (the larger the Si/Al, the more hydrophobic is the pore space). Recently, there are renewed efforts to synthesize zeolites and molecular sieves without the use of organic SDAs. The advantages of doing so are the elimination of the organic SDA cost and the need to remove the organic species prior to use (normally through air

Figure 3. Stereoselective reactions accomplished in water with hydrophobic zeolites. C

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Figure 4. Illustration of methods to form porous materials from layered precursors. From ref 47.

H2O/SiO2 that exceed 10 and often can reach into the 100s (for some clear solution syntheses). What is surprising about these low water content syntheses is that they tend to produce zeolites and molecular sieves with high porosity. The fundamental reasons behind this trend remain unknown. However, Zones and co-workers have shown that the lower water amounts in the synthesis media do lead to frameworks of lower framework density and that these frameworks contain a greater number of rings with four tetrahedral atoms.39 Additionally, this type of synthesis has led to the crystallization of new and interesting structures. For example, a pure silica structure with a helical pore system has recently been reported.40 I will discuss this structure further in the section on chirality.

how this type of preparation could be accomplished at large scale. Also, zeolites are not used in their as-synthesized form in applications, so subsequent steps like ion change, etc. will require processes involving solvents. Thus, it is not clear how the lack of a bulk solvent in the synthesis step would influence the overall process of preparing a zeolite material for application. Synthesis Using F− as the Mineralizing Agent Rather than OH−. In our 1991 review of zeolite synthesis, we discussed the emerging use of F− as a mineralizing agent (HF, NaF, NH4F) for zeolite and molecular sieve synthesis.1 While initially discovered by Flanigen and Patton in 1978, this synthetic method was developed further by Guth and Kessler in the late 1980s and early 1990s.1 Two issues of importance have emerged since that time. First, the high silica structures that are prepared using organic SDAs in F− media have low defect density (internal silanol groups) because the F− balances the positive charge of the SDA rather than creating a defect in the structure to perform the balancing.32 Because of the low internal defect density, the structures can be very hydrophobic.33,34 The ability to create hydrophobic environments within the pore system of the materials has opened new applications in adsorption and catalysis. For example, glucose can be isomerized to fructose in bulk aqueous solvent using a zeolite beta that is hydrophobic and contains Lewis acid sites created by either framework Ti4+ or Sn4+ substitutions for Si4+.35 The catalysis proceeds by a mechanism that is analogous to that which occurs with metalloenzymes.36 Some of the key features of both the zeolite and the enzyme catalysis are a Lewis acid mediated intramolecular hydride shift that is accomplished in a hydrophobic environment of the catalyst.35,36 Thus, the ability to create hydrophobic zeolites by the use of F− mediated syntheses is opening the area of Lewis acid catalysis in water.37 The types of stereoselective reactions that have been reported over these hydrophobic catalysts are shown in Figure 3. Second, Camblor and co-workers have shown that, with F−based syntheses, the amount of water that is necessary for crystallization can be very small.38 For example, a variety of different materials can be synthesized in F− media with water contents below a H2O/SiO2 of 10 (values can even be as low as 1).38 Typical OH-mediated zeolite syntheses have values of

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ZEOLITE AND MOLECULAR SIEVE ARCHITECTURES AND ARRANGEMENTS 2D to 3D Structures and Vice Versa. Numerous zeolite structures can be visualized as layers that are connected to form the three-dimensional framework. Early attempts to create framework silicates by topotactic condensation of layered silicates such as magadiite were described by Lagaly et al.41 The structures of the materials formed could not be solved, so it remains unknown whether true frameworks were prepared at that time. Since then, there have been several successes in synthesizing framework materials by topotactic condensation of layered precursor materials. Marler and Gies provide an excellent review of this area.42 The first successful zeolite material prepared from a 2D material was MCM-22.43 The precursor to MCM-22 (MCM-22(P)) contains one unit cell thick layers separated by organic molecules that were used as SDAs in the synthesis. Upon calcining MCM-22(P), the layers condense to form the MCM-22 framework (3D-MWW) (Figure 4). This discovery opened numerous synthetic pathways to produce porous materials. The pillaring of clays and other layered materials has been explored for many decades. We pillared layered silicates with silanes in attempts to create organosilicate precursors that upon calcination might form framework structures.44 As with the materials prepared by Lagaly et al., we were not able to solve the structures of these new microporous materials. Thus, we were not able to prove that framework silicates could be formed via this synthetic D

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not controllable, and often, the composition of the remaining regions that have microporosity may not be at the framework composition that is required. To overcome these limitations, there have been a variety of methods developed to create hierarchically organized microporous materials with greater control on the arrangement of the microporous and mesoporous areas of the material. Numerous techniques have been used to create hierarchically organized microporous materials, and several recent reviews on this topic are available.57−59 Some of these methods use sophisticated structure directing agents. For example, Ryoo and co-workers first introduced the use of an organosilane that contains three structural features: (i) an alkoxysilane terminus that gets incorporated into the final inorganic structure, (ii) an adjacent quaternary ammonium group that is used to structure direct a unit cell of a zeolite (e.g., MFI), and (iii) an alkyl tail that then is used to create the mesopore configuration.57−59 These types of SDAs can produce hierarchically organized microporous materials of controllable thickness and with fairly uniform mesopores that can be tuned by the size of the alkyl tail. Additionally, the use of a SDA allowed for the growth of hierarchically organized MFI zeolites with plate-like morphology and 90 degree rotational intergrowths.60,61 The mesoporosity created via the two synthetic methods used to create these MFI-based materials is significantly different. Additionally, Tsapatsis and co-workers have recently demonstrated that tetrabutylammonium cations can be used as the SDAs for their preparation.62 While these hierarchically organized materials are scientifically interesting and quite elegant, the question remains as to whether there are any technical advantages to the use of these materials over those that have randomly oriented mesopores. Because these materials will be much more costly to prepare (although the recent work by Tsapatsis et al.62 could change this), their application will most likely have to be for a use that takes advantage of the uniformity of the overall structure. In many ways, this argument is similar to that for the application of the ordered mesoporous materials. That is, one must find an application that utilizes the uniformity in the pore system in order to justify the added cost of production over solids that have nonuniform porosity.

methodology. However, the use of precursor materials like MCM-22(P) enabled the creation of delaminated structures (ITQ-2) that in principle could be only one layer45 or pillared materials that gave well-defined 3D solids (MCM-36).46 These synthetic methods (illustrated in Figure 4 with MCM-22(P)) have been used to form a variety of materials starting from layered precursors other than MCM-22(P); e.g., the FER precursor (PREFER) can be delaminated to give ITQ-6 or pillared to prepare ITQ-36.47 Reviews on these methods of preparation and the structures and properties of the 3D materials formed are available.47−49 This area of materials synthesis is quite active, and precursor 2D materials like MCM22(P) continue to provide for new materials.49 Synthetic issues such as the control of the pillaring and exfoliation processes remain important topics of research.50 Recently, there have been studies involved in deconstructing 3D frameworks into 2D solids. Previous attempts to perform the 3D to 2D transformations using silicate and zeolites have not led to well-defined solids. However, the use of framework germanosilicates has enabled the 3D to 2D transformation.51 Chlubna et al. used this concept to convert the framework structure UTL to a layered solid (Ge removed from fourmembered rings).52 The layered material was then pillared and calcined to yield high surface area solids with both micropores and mesopores. Verheyen et al. prepared the framework material IM-12 that has four-membered rings that connect layers in the structure.53 By synthesizing the material such that the four-membered rings contain Ge and the layers Si, the Ge could be acid leached from the framework material to produce a layered solid (Si−O−Ge and Ge−O−Ge linkages are more readily hydrolyzed than Si−O−Si). Calcination annealed the layered structure to create a new 3D framework.53 Likewise, Roth et al. disassembled the framework UTL and reassembled the formed layers into two different framework materials.54 This is an exciting new area of synthesis that should open pathways to structures that may not be obtainable via standard hydrothermal synthesis. Hierarchically Organized Microporous Materials. The exploitation of the microporosity in zeolites and zeolite-like materials has led to a number of commercial applications like shape selective adsorption (e.g., adsorb only linear molecules in the presence of branched molecules) and catalysis (e.g., production of para-xylene). However, the creation of hierarchically organized microporous materials, that is, materials with a secondary porosity (most typically mesoporosity) that is connected to the microporosity, can produce materials that outperform purely microporous materials in certain applications. For example, the observed catalytic rates from zeolites with mesopores can be larger than those with only micropores due to the increased transport rates in the mesopores that connect to the micropores where the shape selective reactions can occur. In the past, hierarchically organized microporous materials were prepared via dealumination (by steaming and/or acid treatments; see, for example, the work by Janssen et al. on three-dimensional transmission electron microscopy of dealuminated zeolite Y samples55). Highly dealuminated zeolites contain both mesopores and micropores56 and have been commercialized as catalysts, e.g., the 3DDM (3-dimensional dealuminated mordenite) catalysts that Dow has commercialized (Dow/Kellogg Process) for the synthesis of cumene. These methods of creating hierarchically organized microporous materials are economically viable for large scale preparation. The size, shape, and number of mesopores are

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AREAS FOR FUTURE INVESTIGATION Chiral Structures. In our 1992 review, Raul and I concluded the presentation with a discussion on the possibility of creating chiral zeolites.1 To date, no one has synthesized a sample of a powder that is an enantiomerically pure, chiral zeolite. This goal remains elusive, and I describe some of the progress that has occurred since 1992 in this area. Chiral, crystalline frameworks and layered materials do exist. With zeolites or zeolite-like materials, we first reported a slight enantioselectivity from a zeolite beta that was synthesized using a chiral structure directing agent.1 Zeolite beta is an intergrowth of two polymorphs (A and B), and one (A) has enantiomorphs. Thus, to prepare an enantiomerically pure powder of zeolite beta, one would have to synthesize pure polymorph A and then one enantiomorph of polymorph A. Raul and I pointed out that there are frameworks that should have single crystals that are enantiomerically pure (analogous to the dense silica, quartz).1 When synthesizing these in powder form, there should be a 50−50 mixture of the two enantiomorphs. These structure types are likely to be more amenable to creating enantiomerically pure powders as one would not have to first solve the E

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can be at low pH and high salt concentrations,71 that were previously thought impossible. This area of science and technology has most certainly been driven in the past and will be in the future by the chemistry of the materials. Because of the uniformity of the materials, the ability to control the molecular-level chemistry and architectures allows one to design for macroscopic properties by tuning the molecular-level chemistry. Thus, as the chemistry of these materials (both the synthesis chemistry and the chemistry of the framework solids) is further elucidated, it will enable investigators to exploit these solids in new ways. In this brief Perspective, I have provided my opinions of what I believe are some of the most significant areas of investigation. There are other excellent reviews available that cover topics not discussed here (e.g., refs 47, 57, and 72), and I would hope that those readers interested in the chemistry of zeolite and molecular sieve materials also obtain those references.

problem of obtaining a pure polymorph. Structures of this type have been synthesized and are discussed below. In order to obtain a pure sample of a chiral zeolite, chiral organic molecules will be needed in the synthesis to direct the formation of a bulk sample of a pure enantiomorph.1 Several chiral molecules have been used in the synthesis of zeolites.63 The question arises as to why the chiral molecules used have not led to chiral framework structures. Part of the answer involves the mobility of the organic in the formed inorganic structure. Thus, the use of a rigid organic that fits into the zeolite pore structure without the ability to have any motion (even rotational motion of any portion of the molecule) will be necessary. Recently, materials have been synthesized that have enantiomorphs.64−66 First, the germanosilicate material was prepared (structure code STW) that has helical pores.64 Since germanosilicates do not have the thermal stability of silicates, it was desirable to prepare the STW structure in silica alone. Rojas and Camblor synthesized the pure-silica material they called HPM-1 that has the STW structure.65 This pure-silica material was prepared using an achiral organic molecule, and the individual crystals do contain optical activity.66 However, the bulk powder must be a racemic mixture of the two enantiomorphs. It would be quite interesting to perform the synthesis of HPM-1 with chiral organic molecules in the attempt to obtain a single enantiomorph, as separation of the individual chiral crystals into enantiomerically pure powdered samples is unlikely to be feasible. Framework Positioning of Atoms by Design. It is wellknown that the position of atoms such as aluminum can have dramatic effects on the properties of the zeolite. For example, only aluminum atoms in small pores of zeolites can be used to create acid sites that can carbonylate methanol.67 Also, different site distributions of Si and Ga in the zeolite natrolite can significantly alter the ion exchange properties.68 Therefore, it is of great scientific and technological interest to create synthetic methods that would allow for the placement of atoms into specific crystallographic sites within framework materials. At present, there are synthetic efforts aimed at altering the framework positioning between large and small cages, and the resulting materials have shown interesting catalytic behaviors.69,70 Ultimately, it would be advantageous to be able to not only alter the distribution between pore/cage size areas but create crystallographic site specific synthetic methods. One thought that we explored in this territory was the use of Zn as a substitute for Si. Since the Zn−O−Si angle is ca. 130 degrees, Zn has a tendency to order into those sites with tighter angles. While we have been able to produce ordered zincosilicates, we have thus far been unable to prepare framework materials that show good microporosity and stability and have Zn ordered into specific crystallographic positions.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest. Biography Mark E. Davis is the Warren and Katharine Schlinger Professor of Chemical Engineering at the California Institute of Technology. He has over 375 scientific publications, two textbooks, and over 50 US patents. Professor Davis was the first engineer to win the NSF Alan T. Waterman Award. He was elected in the National Academy of Engineering in 1997, the National Academy of Sciences in 2006, and the Institute of Medicine of the National Academies in 2011.

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ACKNOWLEDGMENTS I thank my long time collaborator and friend Dr. Stacey Zones for helpful discussions. I also thank Urbano Diaz for providing me with Figure 4.

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REFERENCES

(1) Davis, M. E.; Lobo, R. F. Chem. Mater. 1992, 4, 756−768. (2) Zones, S. I. Microporous Mesoporous Mater. 2011, 144, 1−8. (3) Martínez, C.; Corma, A. Coord. Chem. Rev. 2011, 255, 1558− 1580. (4) Wright, J. K.; Kalns, J.; Wolf, E. A.; Traweek, F.; Schwarz, S.; Loeffler, C. K.; Snyder, W.; Yantis, L. D.; Eggers, J. J. Trauma 2004, 57, 224−230. (5) Neuffer, M. C.; McDivitt, J.; Rose, D.; King, K.; Cloonan, C. C.; Vayer, J. S. Mil. Med. 2004, 169, 716−720. (6) Cunningham, J. Highly selective molecular confinement for the prevention and removal of taint in foods and beverages. U.S. Patent 7,629,009, December 8, 2009. (7) Vignola, R.; Bagatin, R.; D’Auris, A. D. F.; Flego, C.; Nalli, M.; Ghisletti, D.; Millini, R.; Sisto, R. Chem. Eng. J. 2011, 178, 204−209. (8) Vignola, R.; Bagatin, R.; D’Auris, A. D. F.; Massara, E. P.; Ghisletti, D.; Millini, R.; Sisto, R. Chem. Eng. J. 2011, 178, 210−216. (9) Davis, M. E. Nature 2002, 417, 813−821. (10) Jiang, J. X.; Yu, J. H.; Corma, A. Angew. Chem., Int. Ed. 2010, 49, 3120−3145. (11) Howard, D. G.; Tschernich, R. W.; Smith, J. V.; Klein, G. L. Am. Mineral. 1990, 75, 1200−1204. (12) Lobo, R. F.; Pan, M.; Chan, I.; Li, H. X.; Medrud, R. C.; Zones, S. I.; Crozier, P. A.; Davis, M. E. Science 1993, 262, 1543−1546. (13) Simancas, R.; Dari, D.; Velamazan, N.; Navarro, M. T.; Cantin, A.; Sastre, G.; Corma, A.; Rey, F. Science 2010, 330, 1219−1222. (14) Wagner, P.; Nakagawa, Y.; Lee, G. S.; Davis, M. E.; Elomari, S.; Medrud, R. C.; Zones, S. I. J. Am. Chem. Soc. 2000, 122, 263−273. (15) Zones, S. I.; Burton, A. W.; Lee, G. S.; Olmstead, M. M. J. Am. Chem. Soc. 2007, 129, 9066−9079.

FINAL THOUGHTS

Zeolites and zeolite-like materials are finding new applications as the number of structures and types of compositions of these microporous materials continues to expand. The ability to control the chemistry of these materials has greatly advanced since the time of our 1992 review,1 and it is going to be interesting to see what happens in the future. The preparation of zeolites with new properties, e.g., hydrophobic structures containing isolated Lewis acid centers, has opened new chemistries, e.g., reactions of sugars in aqueous media that F

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dx.doi.org/10.1021/cm401914u | Chem. Mater. XXXX, XXX, XXX−XXX