Hierarchical porous materials

Hierarchical porous materials are sohds that are ordered at different length scales. Materials with multiple porosities are of high interest for apphcations in catalysis and separation, because these apphcations can take advantages of different pore structures. For example, microporous mesoporous composites have shown superior catalytic activities by the combination of strong acidity from zeohtes with high reactant or product mobility due to large uniform mesopores. Several approaches have been reported on the design and synthesis of hierarchical porous materials, as discussed below. 

Colloidal zeolites have been used as building blocks to fabricate hierarchical porous materials. Infiltrating ethanol sol of zeolite nanoparticles into an ordered array of polystyrene spheres resulted in macroporous zeolites, which involves a self-assembly process. After ethanol evaporation, zeolite nanoparticles were aggregated by capillary forces. High concentration of external silanol groups favored the formation of hydrogen bonds between particles and eventually Si-O-Si bonds after calcination. The method has been further developed to produce transparent and self-standing zeolite membranes with controlled mesoporosity. Concurrently, the preformed zeolite-coated polystyrene spheres have been... 

Shin, Y.S. Liu, J. Wang, L.Q. Nie, Z.M. Samuels, W.D. Fryxell, G.E. Exarhos, G.J. Ordered hierarchical porous materials towards tunable size- and shape-selective microcavities in nanoporous channels. Angew. Chem. Int. Ed. Engl. 2000, 39, 2702-2707. Katz, A. Davis, M.E. Molecular imprinting of bulk, microporous silica. Nature 2000, 403, 286-289. 

Bnmo MM, Coiti HR, Balach J, Cotella NG, Barbero CA (2009) Hierarchical porous materials capillaries in nanoporous carbon. Frmct Mat Lett 2 135-138... 

Nakanishi K, Tanaka N (2007) Sol-gel with phase separation. Hierarchically porous materials optimized for high-performance liquid chromatography separations. Acc Chem Res 40 863... 

Hierarchical porous materials are predominately based on zeolitic systems where mainly mesopores have been introduced in the microporous framework of a zeolite crystal in a similar way as motorways would intersect a narrow road system of a downtown area [110-113]. The hierarchy in such materials will result in an optimized performance in transport-limited applications. Thus, hierarchical zeolite-containing materials combine characteristics of pore size regimes of at least two different length scales [113, 114]. It has already been proved that such micro/mesoporous bimodal systems reduce the diffusion limitations for molecules within zeolite catalysts [115-118]. Several methods for the implementation of additional transport pores have been developed during the past few years, however. 

It is important to note that the commonly used term average pore size is insufficient to describe a complex pore structure, and it is even misleading for many porous materials because it does not reflect (i) the magnitude, and (ii) the modality of the pore size dispersion that is, how narrow the pore size distribution centers around one or several maxima. Only carbons with a very narrow pore size distribution, such as CNTs, some carbide-derived carbons (CDCs), and many template-produced carbons, exhibit a meaningful pore size average, whereas most activated carbons or hierarchic porous materials exhibit a much broader distribution of pore sizes. 

Use of Dual and Multiple Templates in Hierarchically Porous Material Synthesis... 

Besides the biomaterials mentioned above, the cuttlebone [49] and chito-san [50] with unique structure have also been used as templates for the formation of hierarchically porous materials, which maintained the biological structure. Starch gel and dextran were also used to produce hierarchically sponge-like micro-, meso/macroporous monoliths of silicalite and meso/macro-porous metal oxides [51,52]. 

Hierarchically porous materials have been prepared using air bubbles as additional templates combined with micellar tempiating by a bubbling process that can produce air-liquid foams [64-67]. This synthesis process may allow concisely control over the cell sizes and shapes of the bubbles and a more easily... 

Posttreatments, including chemical etching or postsynthesis, have also been successfully exploited in the preparation of hierarchically porous materials. 

Hierarchically porous metal oxide networks can be formed via a spontaneous self-formation phenomenon from metal alkoxides in aqueous solution [113]. Two chemical processes, hydrolysis and condensation, are involved in this spontaneous self-formation procedure to target hierarchically porous structures [114,115]. In fact, the hydrolysis and condensation rates are generally comparable for metal alkoxides [116]. The condensation rate is directly proportional to the rapid hydrolysis rate of reactive metal alkoxides [117,118]. It is well known that the rapid reaction rate of metal alkoxides plays the key role in the formation of hierarchically porous metal oxides [119,120]. The self-formation procedure to form hierarchically porous materials can be achieved by dropping liquid metal alkoxide precursors into an aqueous solution. In this section, the features of self-formation procediu-e and the resulting hierarchically porous materials are summarized. 

Very simple method An important benefit of this spontaneous self-formation procediu-e is its simplicity. Hierarchically porous materials can be obtained by just dropping the liquid metal alkoxide precursors into an aqueous solution, without any posttreatment such as calcination or solvent extraction to remove the exotemplate. Fiuthermore, the spontaneous self-formation pro-cediu-e generally takes place at room temperature in an open system. This feature allows the scaling up of this method, which is quite desirable in industry. 

Wide range of mild synthesis conditions The spontaneous self-formation pro-cediu-e can take place under a wide range of synthesis conditions. Hierarchically porous materials can be synthesized by this simple method in pure water or aqueous acidic or alkaline solution, in the presence or absence of a siu actant or a cosolvent, and with or without hydrothermal treatment... 

The key features of the resulting hierarchically porous materials are as follows ... 

Unique porous hierarchy The hierarchically porous materials obtained via this spontaneous self-formation procediu-e possess well-ordered tubular or funnel-like macrochannels with micro/mesoporous walls. The spontaneous self-formation procedure can be optimized by carefully adjusting the synthesis conditions relative to various parameters, such as the central metal atom of the alkoxide, the alkyl group in the alkoxide, hydrolysis ratio, pH, and temperature of the solution. 

Tunable chemical composition Hierarchically porous materials with single or multiple chemical compositions can be prepared via this spontaneous selfformation procedure by using single or multiple alkoxide precursors. Pure oxide materials can be obtained by starting with just the alkoxide or alkylme-tal precursors in aqueous solutions, as opposed to other synthesis strategies where the resulting products are often contaminated by residual species. Furthermore, metal phosphates with hierarchically porous structures can also be prepared by this self-formation process. 

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