Hierarchical Porous Carbon Materials: Synthesis and Introduction

Hierarchical porous materials, possessing multi-level pore structures—macropores (diameter > 50 nm), mesopores (2-50 nm), and micropores (<2 nm)—exhibit high specific surface areas, high pore volume ratios, enhanced permeability, low mass transfer characteristics, and substantial storage capacities. These attributes have led to their widespread adoption across various fields, including catalysis, adsorption, separation, energy, and life sciences, showcasing superior performance over simpler porous materials.

Drawing Inspiration from Nature

Many designs of hierarchical porous materials are inspired by natural structures. These materials can enhance mass transfer, enable selective permeation, create significant hydrophilic-hydrophobic environments, and modulate the optical properties of materials.

Strategies for Synthesizing Hierarchical Porous Materials

1. Surfactant Templating Method

How can we utilize surfactants to form hierarchical mesoporous materials? Employing two surfactants of different molecular sizes as templates is a straightforward strategy. Surfactant self-assembled molecular aggregates or supramolecular assemblies have been used as structure-directing agents for constructing porous structures. By carefully controlling phase separation, hierarchical pore structures can be synthesized using dual surfactant templating.

In diluted surfactant aqueous solutions, the reduction of hydrocarbon chain contact with water decreases the system's free energy. The hydrophilicity of surfactant terminal groups determines the type, size, and other characteristics of the aggregates formed by many surfactant molecules. The CMC of surfactant aqueous solutions is related to the chemical structure of the surfactant, temperature, and/or cosolvents used in the system.

Bimodal mesoporous silica gels are prepared using solutions containing block copolymers (KLE, SE, or F127) and smaller surfactants (IL, CTAB, or P123).

2. Replication Method

What is the classical approach to synthesizing porous carbon materials? The general templating replication procedure for porous carbon involves preparing a carbon precursor/inorganic template composite, carbonization, and subsequent removal of the inorganic template. This method can be divided into two categories. The first category involves embedding inorganic templates within the carbon precursor, such as silica nanoparticles. After carbonization and template removal, the resulting porous carbon materials have isolated pores initially occupied by the template species. The second method introduces the carbon precursor into the template pores. The porous carbon materials generated after carbonization and template removal possess interconnected pore structures.

3. Sol-Gel Method

How is the sol-gel method used to synthesize hierarchical porous materials? It begins with the formation of a colloidal particle suspension (sol), followed by the formation of a gel composed of aggregated sol particles. Thermal treatment of the gel yields the desired material and morphology, such as powders, fibers, films, and monoliths. Precursors are typically metal organic compounds, such as alkoxides, chelated alkoxides, or metal salts like metal chlorides, sulfates, and nitrates. Initial hydrolysis of alkoxides or deprotonation of coordinated water molecules leads to the formation of reactive hydroxyl groups, which then undergo condensation processes to form branched oligomers, polymers, nuclei with a metal oxide skeleton, and reactive residual hydroxyl and alkoxide groups.

4. Post-Treatment Method

What post-treatment methods are used to prepare hierarchical porous materials by introducing secondary pores? These methods generally fall into three categories. The first category involves grafting additional porous materials onto the original porous material. The second involves chemical etching or leaching of the original porous material to obtain additional pores. The third involves assembling or arranging precursors of porous materials (usually nanoparticles) using chemical or physical methods (such as multilayer deposition and inkjet printing) to create new pores. The significant advantages of post-treatment are: (i) the ability to design various functionalities to meet different requirements; (ii) the ability to obtain a variety of structures to design organized patterns and morphologies; (iii) the ability to combine various types of pores to expand the desired applications.

5. Emulsion Templating Method

How can adjusting the oil phase or water phase in an emulsion form hierarchical structures with pore sizes ranging from nanometers to micrometers? Precursors solidify around droplets, and then solvents are removed through evaporation, resulting in porous materials. In most cases, water is one of the solvents. Emulsions can be formed by dispersing water droplets in the oil phase, known as "water-in-oil (W/O) emulsions," or by dispersing oil droplets in water, known as "oil-in-water (O/W) emulsions."

To manufacture porous polymers with hydrophilic surfaces, W/O emulsions are widely used to adjust their hydrophobic porous structures. To enhance hydrophilicity, functionalizable copolymers (such as vinyl benzyl chloride) are added to non-functionalizable monomers (such as styrene) in the emulsion. By adjusting droplet sizes, hierarchical porous materials with interconnected porosities and continuous pore diameters can be obtained.

6. Zeolite Synthesis Method

How can zeolite synthesis strategies, combined with other synthesis strategies, generate hierarchical porous materials? Overgrowth strategies based on phase separation control during zeolite synthesis can be used to obtain bi-microporous zeolites with hierarchical core/shell structures, which can be divided into three types. The first type involves overgrowth through isomorphous cores (such as ZSM-5/silicalite-1), where core crystals act as structure-directing agents. The second type is epitaxial growth, such as zeolite LTA/FAU types, involving the same building units with different spatial arrangements. In this method, due to selective overgrowth of zeolite layers, coating can only be performed on certain specific crystal faces. The third type is overgrowth on different zeolites, such as FAU/MAZ, BEA/MFI, and MFI/AFI types. These zeolites are composed entirely of different zeolite structures, thus possessing distinct chemical and structural characteristics.

7. Colloidal Crystal Templating Method

How does the colloidal crystal templating method, compared to other methods, manufacture materials with ordered, periodic pore structures over a larger size range? The porosity generated using this method is a direct replica of the periodic array of uniform colloidal particles used as hard templates, making it easier to construct hierarchical size levels compared to other templating methods. Using colloidal crystal templates can yield additional porosity beyond the assembled colloidal voids.

The basic steps of colloidal crystal templating are illustrated, including the formation of colloidal crystal templates, precursor infiltration, and template removal. Generally, both surface and volume template structures can be generated. Three-dimensional ordered macroporous (3DOM) structures generated through surface templating feature interconnected "balloon" and strut-like networks.

8. Bio-templating Method

How are hierarchical porous materials manufactured through biomimetic strategies that directly replicate natural materials or spontaneous assembly processes? Both methods can be defined as bio-inspired processes.

A wide variety of natural materials with hierarchical porous structures can be used directly as bio-templates due to their low cost and environmental friendliness. Among these materials, bacterial threads, diatom frustules, eggshell membranes, insect wings, pollen grains, plant leaves, wood cellulose, protein aggregates, spider silk, diatoms, and other organisms have been reported.

9. Polymer Templating Method

How can polymer structures with macropores be used as templates for manufacturing hierarchical porous materials? Macroporous polymers can act as scaffolds, with chemical reactions or infiltration of nanoparticles occurring around or within them, guiding the material's morphology. After the polymer is removed, the material retains the structural characteristics of the original template.

10. Supercritical Fluid Method

How can materials with well-defined porous structures be synthesized using only water and carbon dioxide, without the need for volatile organic solvents, thus offering broad application prospects? The removal of the droplet phase is straightforward because carbon dioxide reverts to a gaseous state upon depressurization. Supercritical fluids, which are neither gases nor liquids, can be gradually compressed from low to high densities. Therefore, supercritical fluids are crucial as tunable solvents and reaction media in chemical processes. Supercritical fluid technology is an important method for synthesizing and processing hierarchical porous materials.

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