Review pubs.acs.org/CR
Porous Nanosized Particles: Preparation, Properties, and Applications Valentin Valtchev*,† and Lubomira Tosheva*,‡ †
Laboratoire Catalyse & Spectrochimie, ENSICAEN, Université de Caen, CNRS, 6 Boulevard du Maréchal Juin, 14050 Caen, France Division of Chemistry and Environmental Science, Manchester Metropolitan University, Chester Street, Manchester M1 5GD, United Kingdom 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 CONTENTS nanopores. These three strategies toward increasing the surface 1. Introduction A area of solids and hence the material’s reactivity are 2. Zeolites and Related Microporous Crystals C schematically illustrated in Figure 1. 2.1. Mechanism of Zeolite Formation C The steady interest in nanosized porous solids is due to the 2.2. Conventional Hydrogel Synthesis of Zeolite potential of these materials to offer sustainable solutions to Nanocrystals E global issues such as increasing energy demands and at the 2.3. Confined-Space Synthesis of Nanozeolites I same time more rigorous environmental standards for industrial 2.4. Microreactor Synthesis J pollutants, depletion of resources, and health improvement. 2.5. Top-Down Approaches J Considering the accumulated number of publications dedicated 2.6. Nanometer-Thick Zeolite Sheets K to porous nanoparticles and their somewhat limited outreach in 3. Ordered Mesoporous Silica Nanoparticles L cross-disciplinary fields, the aim of this review is to provide an 3.1. Stö ber-Modified Syntheses L overview of recent developments in the area of synthesis and 3.2. Synthesis in an Acidic Medium N applications of the different groups of porous nanomaterials. 3.3. Co-Condensation Methods N The porous materials considered in this work have ordered 3.4. Other Approaches O pore structures and pore sizes of up to 50 nm. According to the 3.5. Template Removal and Colloidal Stability O International Union of Pure and Applied Chemistry (IUPAC), 4. Metal−Organic Framework Nanomaterials O materials with pore widths of less than 2 nm are classified as 4.1. Synthesis of Nanosized Metal−Organic microporous, mesoporous materials have pore sizes between 2 Frameworks P and 50 nm, and solids with pore sizes exceeding 50 nm are 4.2. Formation Mechanism Q macroporous.1 Only the microporous and mesoporous pore ‡
5. Applications of Porous Nanomaterials 5.1. Preparation of Porous Carbons 5.2. Thin Films and CoatingsSynthesis and Applications 5.3. Optical and Sensing Aplications 5.4. Catalytic Applications 5.5. Biomedical Applications 5.6. Other Potential Applications 6. Summary and Outlook Author Information Corresponding Author Notes Biographies Acknowledgments References
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S T T U U U V V V V W W Figure 1. Schematic illustration of the ways to increase the surface area of dense solids.
1. INTRODUCTION The different local environment of atoms exposed at solid surfaces compared to atoms in the bulk is the driving force for © XXXX American Chemical Society
Received: November 1, 2012
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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 investigated.34−37 The reduction of the size of porous particles to nanodimensions 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 mesoporous 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
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 submicrometersized 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 comprising 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 aluminosilicate 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 frameworks 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 B
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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 nanocrystalyielding 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. Consequently, 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 (
