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Porous Nanosized Particles: Preparation, Properties, and Applications Valentin Valtchev*,† and Lubomira Tosheva*,‡ †Laboratoire Catalyse & Spectrochimie, ENSICAEN, Universite ́ de Caen, CNRS, 6 Boulevard du Marećhal Juin, 14050 Caen, France ‡Division of Chemistry and Environmental Science, Manchester Metropolitan University, Chester Street, Manchester M1 5GD, United Kingdom CONTENTS 1. Introduction A 2. Zeolites and Related Microporous Crystals C 2.1. Mechanism of Zeolite Formation C 2.2. Conventional Hydrogel Synthesis of Zeolite Nanocrystals E 2.3. Confined-Space Synthesis of Nanozeolites I 2.4. Microreactor Synthesis J 2.5. Top-Down Approaches J 2.6. Nanometer-Thick Zeolite Sheets K 3. Ordered Mesoporous Silica Nanoparticles L 3.1. Stöber-Modified Syntheses L 3.2. Synthesis in an Acidic Medium N 3.3. Co-Condensation Methods N 3.4. Other Approaches O 3.5. Template Removal and Colloidal Stability O 4. Metal−Organic Framework Nanomaterials O 4.1. Synthesis of Nanosized Metal−Organic Frameworks P 4.2. Formation Mechanism Q 5. Applications of Porous Nanomaterials R 5.1. Preparation of Porous Carbons R 5.2. Thin Films and CoatingsSynthesis and Applications S 5.3. Optical and Sensing Aplications T 5.4. Catalytic Applications T 5.5. Biomedical Applications U 5.6. Other Potential Applications U 6. Summary and Outlook U Author Information V Corresponding Author V Notes V Biographies V Acknowledgments W References W 1. INTRODUCTION The different local environment of atoms exposed at solid surfaces compared to atoms in the bulk is the driving force for many physical and chemical processes. There are two approaches to increase the number of surface atoms in solids, namely, to decrease the size of dense particles or to create an open pore network within the bulk of the solid. Both approaches result in an increase in the specific surface area of materials. An elegant way to synergize the two approaches and to maximize the fraction of exposed atoms to the surface is to prepare nanosized particles containing accessible and uniform nanopores. These three strategies toward increasing the surface area of solids and hence the material’s reactivity are schematically illustrated in Figure 1. The steady interest in nanosized porous solids is due to the potential of these materials to offer sustainable solutions to global issues such as increasing energy demands and at the same time more rigorous environmental standards for industrial pollutants, depletion of resources, and health improvement. Considering the accumulated number of publications dedicated to porous nanoparticles and their somewhat limited outreach in cross-disciplinary fields, the aim of this review is to provide an overview of recent developments in the area of synthesis and applications of the different groups of porous nanomaterials. The porous materials considered in this work have ordered pore structures and pore sizes of up to 50 nm. According to the International Union of Pure and Applied Chemistry (IUPAC), materials with pore widths of less than 2 nm are classified as microporous, mesoporous materials have pore sizes between 2 and 50 nm, and solids with pore sizes exceeding 50 nm are macroporous.1 Only the microporous and mesoporous pore Received: November 1, 2012 Figure 1. Schematic illustration of the ways to increase the surface area of dense solids. Review pubs.acs.org/CR © XXXX American Chemical Society A dx.doi.org/10.1021/cr300439k | Chem. Rev. XXXX, XXX, XXX−XXX ranges are relevant to nanosized particles. These two classes of materials are further subdivided into families of porous materials depending on their structural and compositional characteristics. Generally, particles with sizes of 100 nm in at least one dimension will be covered, although submicrometer- sized particles are also included where appropriate. Examples of materials with ordered porous structures can be found in nature, for instance, microporous zeolite minerals. The classical definition of a zeolite is a crystalline aluminosilicate built of oxygen-linked tetrahedral silicon and aluminum atoms that form a three-dimensional microporous structure compris- ing channels and voids occupied by alkali or alkali-earth cations and water molecules. This definition reflects the chemical composition of natural zeolites, whereas synthetic zeolite-type materials exhibit much broader variations in the Si/Al ratio and include other framework elements, such as P, Ti, Ge, Ga, B, and Fe. Despite the abundance of some natural zeolites, their application in industrial processes is limited because of the variation of the zeolite chemical composition within the deposit and the presence of impurities. In the early 1960s, synthetic zeolites (FAU-type) successfully replaced amorphous alumi- nosilicate catalysts in fluid catalytic cracking (FCC) process. After realizing the potential of zeolites in the petroleum industry, there were continuous efforts to optimize the characteristics of existing zeolite materials and to prepare new zeolitic framework types. As a result, zeolite framework compositions were stretched far beyond the limits imposed by nature. The main avenues to achieve these new compositions/structures are based on the use of organic structure-directing agent(s) (SDA) (i) to increase the Si/Al ratio for the preparation of high-silica or pure-silica zeolites and direct the synthesis of new aluminosilicate zeolites and (ii) to aid the successful synthesis of new zeolitic framework types from synthesis gels containing framework atoms other than Si and Al.2−4 New framework types are approved every year by the Crystallographic Commission of the International Zeolite Association, and the current number of zeolite framework types is 206.5 The nonclassical (nonaluminosilicate) zeolitic frame- works comprise, for example, all silica, alumino-, galo-, and transition metal phosphates and zinc, gallium, and germanium silicates.4,6−8 The templating strategy has been extended beyond the use of simple organic molecules to macromolecules and supramolecular species that resulted in silicates with pores in the mesoporous range.9−12 These materials are termed as ordered mesoporous silica materials and are characterized with uniform pores surrounded by amorphous walls. Zeolitic and ordered mesoporous silica materials have been converted into carbon replicas benefiting from the “parent” pore structures and at the same time possessing the characteristics of carbon.13−17 In the most recently discovered family of microporous solids, metal−organic framework (MOF) materials, the solvent employed in the synthesis fills the pores of the materials.18,19 The application-driven design of solid surfaces to conform to the operating conditions of industrial processes has involved not only the discovery of new types of porous materials but also the direct or postsynthesis modification of existing structures. For instance, the replacement of OH− with F− as a mineralizing agent in the zeolite synthesis allowed the preparation of hydrophobic defect-free zeolite crystals.20−23 The partial substitution of framework atoms by heteroatoms such as B, Fe, Cr, and Ti substantially changes the nature of the catalytic sites.24 The ion-exchange of the charge-balancing cations in the zeolite channels by other cations is used to introduce/modify the active sites in zeolites and in addition to tune the effective pore diameter.25 The accessible pore volume can also be modified by grafting molecular entities at the pore mouths or on the channel walls.26−30 The introduction of secondary mesoporosity in microporous materials is currently attracting considerable attention due to minimized adverse effects, such as pore blocking and coke formation.31−33 Control of the crystal morphology to maximize the number of pore openings per unit external surface and/or to advantage the accessibility to a particular channel system has also been actively investi- gated.34−37 The reduction of the size of porous particles to nano- dimensions offers an additional potential to optimize the performance of porous solids in traditional catalytic and sorption applications. For example, nanosized porous particles have shown improved catalytic activity in diffusion-limited reactions. Manipulation of nanocrystalline suspensions using colloidal chemistry approaches has resulted in the preparation of 2D and 3D structures of microporous materials with controlled characteristics for separation, purification, catalytic, sensing, optical, and semiconducting applications. Stable dispersions of mesoporous nanoparticles have shown potential for drug delivery and imaging in biomedicine. The intense developments in the area of nanosized porous materials resulted in numerous reviews published in the past 10 years or so.38−45 However, these reviews are dedicated to a particular class of materials, zeolites, ordered mesoprous materials, or MOFs. One of the aims of the present review is to summarize recent developments for all these classes of nanosized porous materials. Zeolites in the form of colloidal zeolite suspensions from SDA-containing clear solutions were first reported in 1993,46 almost 30 years after the introduction of organic templates into the zeolite synthesis.47 There had been intense research in the area of nanozeolites in the following 10−15 years to develop procedures for the synthesis of zeolitic suspensions of different zeolite-type frameworks and to fabricate tailored structured materials using bottom-up approaches. The nature of the synthesis of colloidal zeolites, namely, clear solutions and low temperatures, also allowed expansion in the range of instrumentation used, resulting in considerable advancement of the understanding of the zeolite formation mechanism. Despite the fine-tuning of the structural characteristics of films and hierarchical structures achieved, the number of zeolite-type structures prepared in the form of stable colloidal suspensions remained limited, syntheses were not industrially friendly, and as a result the researchers in this area somewhat diversified their efforts in recent years to include other materials/applications. The synthesis of ordered meso- porous materials initially followed the historical trends of zeolites in terms of synthesis, characterization, and applications. This resulted in the accelerated development of nanosized mesoporous materials, less than 10 years after the first report on the synthesis of ordered mesoporous materials.48,49 The potential of ordered mesoporous materials as drug delivery carriers was first demonstrated in 2001.50 The exploration of ordered mesoporous nanoparticles for biomedical applications is currently flourishing. In the case of MOFs, developments have been so rapid that the synthesis of MOF nanocrystrals did not have the same impact as in the case of the former two classes of materials, and their potential is yet to be evaluated. MOFs also followed the general zeolite development route, namely, synthesis of new structures, tuning of crystal size and morphology, and fabrication of nanocrystals, films, and 3D Chemical Reviews Review dx.doi.org/10.1021/cr300439k | Chem. Rev. XXXX, XXX, XXX−XXXB structures. Often, transferable synthesis approaches, character- ization methods, and applications have been applied within the area of porous nanoparticles. At the same time, the number of researchers entering the field is steadily increasing. These “newcomers” would highly benefit from a review, where recent developments in all three classes of ordered porous nanoma- terials, namely, zeolites, mesoporous silicas, and MOFs, are presented together. The avalanche of papers published in recent years, particularly in the area of MOFs, may result in unnecessary systematic exploration of approaches already applied for another class of porous materials because of unawareness of past research. The present review aims at giving a thorough picture of developments in the area of nanosized zeolites and related materials reported mainly after 2005, when a comprehensive review on nanozeolites was published.38 Recent developments in the area of nanosized ordered mesoporous silicas and MOFs will also be presented but in smaller depth. The reason is that many excellent review articles by leading researchers are regularly appearing in the literature following the very intense research effort in these two areas. Carbon replicas of nanosized porous materials with regular pore structures will also be discussed. This review on nanosized porous materials bringing the recent developments in the different classes of ordered porous nanomaterials together is expected to promote the transfer of knowledge from past research into interdisciplinary areas and to generate new ideas for future developments. 2. ZEOLITES AND RELATED MICROPOROUS CRYSTALS 2.1. Mechanism of Zeolite Formation Zeolite syntheses are performed in closed systems, where a reaction between the components of the initial gel leads to nucleation and further growth of the kinetically most favorable phase. Under such conditions, control of the nucleation allows one to direct the ultimate crystal size. In other words, the nutrient pool is limited and after exhaustion of a building component, the crystal growth stops. The relationship between nucleation and crystal size is presented in Figure 2. As can be seen, an increase in the number of nuclei leads to a decrease of the ultimate crystallite size. Hence, the formation of small zeolite crystals requires conditions that favor nucleation over crystal growth. In contrast, large zeolite crystals can be formed only if nucleation is repressed and the conversion of nutrients into zeolite-type materials is completed.51 Thus, directing of the zeolite crystal size requires close control of the zeolite nucleation process and simultaneous growth of crystals. The latter conditions are particularly important when the goal is the synthesis of nanosized crystals with narrow particle size distribution. Prior to reviewing the literature devoted to the synthesis of nanosized microporous crystals, a brief overview of today’s understanding of the mechanism of zeolite formation will be provided. The discussion on the mechanism of zeolite formation will also allow the introduction of the main type of zeolite systems used in the preparation of nanosized crystals, since most of the recent studies devoted to the zeolite nucleation−growth mechanism are performed on nanocrystal- yielding precursors. The advantages of these systems in fundamental studies are due to their homogeneity and thus temporal and spatial uniformity of nucleation events, which facilitate the interpretation of the experimental results. It is generally accepted that zeolite nucleation deviates from the classical crystallization mechanism describing crystal growth from supersaturated solutions.52−56 This is probably one of the most complex cases of hydrothermal crystallization, where typically several hundred atoms are involved in the formation of one unit cell. The development of the zeolite framework includes weak and strong interactions between building components, resulting in the formation of a covalently bonded framework stabilized by extraframework species. The formation and dissociation of the nuclei take much more complex routes than the formation of salt compounds in solutions. Con- sequently, the zeolite formation proceeds via a number of steps that are still not well understood. Besides the complexity of the process, a serious obstacle in its detailed understanding is the large variety of initial systems employed in the zeolite synthesis. The zeolite-yielding systems can be subdivided into three main groups: (i) ultradense gels, where no bulk liquid is present; (ii) hydrogel systems comprising bulk solid and liquid phases; and (iii) optically clear sols that contain only discrete gel particles. The latter two systems were found to be particularly convenient in the investigation of zeolite nucleation−growth processes, and most of the investigations of crystallization mechanism of zeolite formation in the past decade have been performed on such model systems. It is worth noting that systems (i) and (iii) have no practical uses and their utilization is limited to laboratory investigations. The current discussion on zeolite formation is not aimed at the generally accepted scheme of zeolite framework formation around charged templating species, that is, alkali-metal cation− water complexes or organic molecules.57 It seems that lately there is also general agreement on the heterogeneous character of zeolite nucleation. The uncertainties about zeolite nucleation concern the species involved in the self-assembly processes leading to the formation of entities with structural order and their spatial and temporal location. Most of the recent works on the mechanism of zeolite formation were performed on optically clear sols comprising very small (<6 nm) particles. Schoeman first used such a system to study the nucleation of silicalite-1.58 According to the model proposed by Schoeman, a cluster−cluster aggregation mechanism is dominating until a certain size is reached (ca. 1 nm), and after that, the growth mechanism is via deposition of low molecular weight species onto growing crystals. On the basis of the extended Derjaguin− Landau−Verwey−Overbeek (DLVO) theory, he suggested that it is not likely that silicalite-1 crystals grow via a particle− particle aggregation mechanism.59 Today, there is no doubt that small (a few nanometer) particles play an important role in zeolite formation from clear sols. The ongoing discussion is how these protoentities are transformed into zeolite crystals. Figure 2. Relationship between the number of viable nuclei and the ultimate crystal size in a zeolite-yielding system. Chemical Reviews Review dx.doi.org/10.1021/cr300439k | Chem. Rev. XXXX, XXX, XXX−XXXC Opposing opinions can be summarized into two major groups: (i) aggregation of fully amorphous particles that undergo chemical and structural reorganization until reaching zeolite organization60−63 and (ii) aggregation of entities (nanoslabs) that possess the actual zeolite structure from early on in the process.64,65 An intermediate mechanism suggests that aggregated particles have already possessed some intermediate order.55 Clear sols are the preferred systems in the synthesis of nanozeolites with narrow particle size distribution, since the uniformity of the nucleation events and homogeneous distribution of viable nuclei in the system ensure the formation of crystals with narrow particle size distribution. The above studies were performed on model systems, namely, diluted clear solutions comprising discrete gel particles. Mass zeolite production, however, is based on hydrogel systems with a high level of heterogeneity. Zeolite nucleation and growth in such systems is more difficult to control, and the ultimate crystalline material often contains crystallites and aggregates of fairly different size. Tracking the mechanism of zeolite formation in such systems is also a serious challenge, due to the high heterogeneity and complexity of events in the solid and liquid phases of the system. Nevertheless, in recent years substantial progress has been achieved and the details of key events leading to zeolite nucleation have been agreed upon to a great extent.66−68 For instance, it is now clear that alkaline hydroxide determines the size of the precursor gel particles in the system yielding industrially important zeolites like FAU- and LTA-type. Namely, during the initial polymerization of the aluminosilicate precursor, the concentration of sodium hydroxide in solution increases, which restricts the extent of the polymerization process and leads to the formation of small randomly aggregated gel particles with open structure. Throughout the induction period, a partial depolymerization tak