Macroporous ceramics with pore sizes from 400 nm to 4 mm and porosity within the range 20%–97% have been produced for a number of well-established and emerging applications, such as molten metal filtration, catalysis, refractory insulation, and hot gas filtration. These applications take advantage of the unique properties achieved through the incorporation of macropores into solid ceramics. In this article, we review the main processing routes that can be used for the fabrication of macroporous ceramics with tailored microstructure and chemical composition. Emphasis is given to versatile and simple approaches that allow one to control the microstructural features that ultimately determine the properties of the macroporous material. Replica, sacrificial template, and direct foaming techniques are described and compared in terms of microstructures and mechanical properties that can be achieved. Finally, directions to future investigations on the processing of macroporous ceramics are proposed.

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Processing Routes to Macroporous Ceramics: A Review

2.3. Amorphous Minerals 4354 3. Biological Routes to Controlling Morphology 4354 3.1. General Mechanisms 4356 3.1.1. Soluble and Insoluble Organic Molecules 4356 3.1.2. Control over Crystal Polymorph 4357 3.1.3. Control over Crystal Orientation 4359 3.2. Single-Crystal Biominerals 4359 3.2.1. Organic and Inorganic Soluble Additives 4360 3.2.2. Templating of Single-Crystal Morphologies 4361 3.3. Polycrystalline Biominerals 4366 3.3.1. Nacre Formation in Mollusks 4366 3.3.2. ForaminiferasA Biogenic Mesocrystal 4368 3.4. Amorphous Biominerals 4370 3.4.1. Silicification in Diatoms 4370 3.4.2. In Vitro Studies of Silicification in Diatoms 4371 4. Bioinspired Routes to Controlling Crystal Morphologies 4371

Controlling Mineral Morphologies and Structures in Biological and Synthetic Systems

Owing to their immense potential in energy conversion and storage, catalysis, photocatalysis, adsorption, separation and life science applications, significant interest has been devoted to the design and synthesis of hierarchically porous materials. The hierarchy of materials on porosity, structural, morphological, and component levels is key for high performance in all kinds of applications. Synthesis and applications of hierarchically structured porous materials have become a rapidly evolving field of current interest. A large series of synthesis methods have been developed. This review addresses recent advances made in studies of this topic. After identifying the advantages and problems of natural hierarchically porous materials, synthetic hierarchically porous materials are presented. The synthesis strategies used to prepare hierarchically porous materials are first introduced and the features of synthesis and the resulting structures are presented using a series of examples. These involve templating methods (surfactant templating, nanocasting, macroporous polymer templating, colloidal crystal templating and bioinspired process, i.e. biotemplating), conventional techniques (supercritical fluids, emulsion, freeze-drying, breath figures, selective leaching, phase separation, zeolitization process, and replication) and basic methods (sol–gel controlling and post-treatment), as well as self-formation phenomenon of porous hierarchy. A series of detailed examples are given to show methods for the synthesis of hierarchically porous structures with various chemical compositions (dual porosities: micro–micropores, micro–mesopores, micro–macropores, meso–mesopores, meso–macropores, multiple porosities: micro–meso–macropores and meso–meso–macropores). We hope that this review will be helpful for those entering the field and also for those in the field who want quick access to helpful reference information about the synthesis of new hierarchically porous materials and methods to control their structure and morphology.

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Hierarchically porous materials: synthesis strategies and structure design

In this review, we discuss the phenomenon of complementary macropore incorporation into mesoporous and/or microporous solids in order to enhance their catalytic performance in fuels and chemicals synthesis.

Hierarchical porous materials: catalytic applications.

This review focuses on important aspects of applying physisorption for the pore structural characterization of hierarchical materials such as mesoporous zeolites. During the last decades major advances in understanding the adsorption and phase behavior of fluids confined in ordered nanoporous materials have been made, which led to major progress in the physisorption characterization methodology (summarized in the 2015 IUPAC report on physisorption characterization). Here we discuss progress and challenges for the physisorption characterization of nanoporous solids exhibiting various levels of porosity from micro- to macropores. While physisorption allows one to assess micro- and mesopores, a widely employed method for textural analysis of macroporous materials is mercury porosimetry and we also review important insights associated with the underlying mechanisms governing mercury intrusion/extrusion experiments. Hence, although the main focus of this review is on physical adsorption, we strongly emphasize the importance of combining advanced physical adsorption with other complementary experimental techniques for obtaining a reliable and comprehensive understanding of the texture of hierarchically structured materials.

Recent advances in the textural characterization of hierarchically structured nanoporous materials

‘Hierarchy’ is a property which can be attributed to a manifold of different immaterial systems, such as ideas, items and organisations or material ones like biological systems within living organisms or artificial, man-made constructions. The property ‘hierarchy’ is mainly characterised by a certain ordering of individual elements relative to each other, often in combination with a certain degree of branching. Especially mass-flow related systems in the natural environment feature special hierarchically branched patterns. This review is a survey into the world of hierarchical systems with special focus on hierarchically porous zeolite materials. A classification of hierarchical porosity is proposed based on the flow distribution pattern within the respective pore systems. In addition, this review might serve as a toolbox providing several synthetic and post-synthetic strategies to prepare zeolitic or zeolite containing material with tailored hierarchical porosity. Very often, such strategies with their underlying principles were developed for improving the performance of the final materials in different technical applications like adsorptive or catalytic processes. In the present review, besides on the hierarchically porous all-zeolite material, special focus is laid on the preparation of zeolitic composite materials with hierarchical porosity capable to face the demands of industrial application.

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Hierarchy concepts: classification and preparation strategies for zeolite containing materials with hierarchical porosity

During the past several years, different kinds of hierarchical structured zeolitic materials have been synthesized due to their highly attractive properties, such as superior mass/heat transfer characteristics, lower restriction of the diffusion of reactants in the mesopores, and low pressure drop. Our contribution provides general information regarding types and preparation methods of hierarchical zeolitic materials and their relative advantages and disadvantages. Thereafter, recent advances in the preparation and characterization of hierarchical zeolitic structures within the crystallites by post-synthetic treatment methods, such as dealumination or desilication; and structured devices by in situ and ex situ zeolite coatings on open-cellular ceramic foams as (non-reactive as well as reactive) supports are highlighted. Specific advantages of using hierarchical zeolitic catalysts/structures in selected catalytic reactions, such as benzene to phenol (BTOP) and methanol to olefins (MTO) are presented.

Zeolitic materials with hierarchical porous structures.

A topical review of the use of non-porous and porous inorganic materials as supports for the immobilization of ILs and their catalytic applications is given. The basic principles and the supports that are being used in various concepts emerging from supported ionic liquids (SILs) including supported ionic liquid films (SILFs), supported ionic liquid catalysis/catalysts (SILC/SILCAs), solid catalysts with ILs (SCILs), solid catalysts with IL layer (SCILL), supported IL nanoparticles (SILnPs) and supported ionic liquid phase (SILP) catalysts (SILPC) are first discussed. In addition, we provide a comprehensive review of the SILs concept dealing with inorganic supports, such as layered silicates, porous glass, zeolites, mesoporous (MCM) and PMO (periodic mesoporous organosilica) materials; and their applications.

https://www.tpbin.com/Uploads/Subjects/6bac8b60-e73d-4b66-aa2d-b8345e2004aa.pdf

Thangaraj Selvam

Papers

Hierarchy concepts: classification and preparation strategies for zeolite containing materials with hierarchical porosity

Zeolitic materials with hierarchical porous structures.

Supported ionic liquids on non-porous and porous inorganic materials—A topical review

Zeolite coatings on microcellular ceramic foams: A novel route to microreactor and microseparator devices

Biotemplating of Luffa cylindrica sponges to self-supporting hierarchical zeolite macrostructures for bio-inspired structured catalytic reactors