Hierarchical porous silica materials

Taubert A, Li Z (2007) Inorganic materials from ionic liquids. Dalton Trans p 723 Kuang DB, Brezesinski T, Smarsly B (2004) Hierarchical porous silica materials with a trimodal pore system using surfactant templates. J Am Chem Soc 126 10534 Wu XF, Tian YJ, Cui YB, Wei LQ, Wang Q, Chen YF (2007) Raspberry-like silica hollow spheres hierarchical structures by dual latex-surfactant templating route. J Phys Chem C 111 9704... 

Sen T, Tiddy GJT, Casci JL, Anderson MW (2004) Synthesis and characterization of hierarchically ordered porous silica materials. Chem Mater 16 2044 Deng Y, Liu C, Yu T, Liu F, Zhang F, Wan Y, Zhang L, Wang C, Tu B, Webley PA, Wang H, Zhao D (2007) Facile synthesis of hierarchically porous carbons from dual colloidal crystal/block copolymer template approach. Chem Mater 19 3271 Luo Q, Li L, Yang B, Zhao D (2000) Three-dimensional ordered macroporous structures with mesoporous silica walls. Chem Lett 29 378... 

Based on the control of sol-gel deposition, hierarchically porous silica-based materials with a bimodal pore system (mesopores/large meso/macropores) and a diversity of dopant elements (Al, Ti, V, and Zr) could be prepared by using a one-pot surfactant-assisted procedure [78]. Another example includes the preparation of nonionically templated [Si]-MSU-X mesoporous silicas with bimodal pore systems by adding dilute electrolytes. 

Windows or external surfaces that are self-cleaning are possible with superhydrophobic surfaces. Based on a monolayer polystyrene (PS) colloidal crystal, large-scale two-dimensional (2D) hierarchical porous silica (orderly arranged macropores and disordered mesopores in its skeleton) with a high specific surface area have been fabricated by the sol-gel technique. Such material has demonstrated reversible superhydrophobicity (16). 

In principle the bicontinuous 3-dimensional network structure of MCM-48 would act as a good catalytic support.[7] However, its lower hydrothermal and thermal stability has led to much less application of MCM-48 in catalysis. Recently, a family of mesoporous molecular sieves (denoted as MSU-G) with vesicle-like hierarchical structure, worm-like mesoporous structure and bicontinuous nano-porous silica had been synthesized.[8-10] It was proposed that highly accessible mesoporous materials could be obtained through different synthetic procedure and composition. 

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. 

HMC is a very interesting porous carbon material with unique hierarchical macro/mesoporous spherical morphology. HMCs with various core sizes and/or shell thicknesses can be fabricated through the independent control of the core sizes and/or shell thicknesses of the SCMS silica templates [28,62,63], while the micro- and mesoporosity of the HMCs can be controlled to some extent by the source type and amount of carbon precursor incorporated into the SCMS silica template. HMC with different core shapes (non-spherical) have been synthesised through nanocasting techniques [61,64], A key factor for the s)mthesis of HMCs with diverse shapes and sizes lies in the fabrication of SCMS silica replica templates. 

Because of the attractive physicochemical properties and potential applications in catalysis, biotechnology, adsorption, and separation, fabrication of hierarchically porous (macro/mesoporous) materials, especially for the three-dimensional ordered macro/ mesoporous (3DOM) materials, has been a focus in the research on materials science and engineering in recent years [99,199,200], By using close-packed arrays of monodisperse spheres, such as polystyrene (PS), poly(methyl methacrylate) (PMMA), and silica as template, metals [201,202], metal oxides [203-208], metal chalcogenides [209], silica [204,210,211], carbon [212,213], polymers [214,215], and hydroxyapatite [216] with 3DOM structures have been generated. 

In view of catalytic potential applications, there is a need for a convenient means of characterization of the porosity of new catalyst materials in order to quickly target the potential industrial catalytic applications of the studied catalysts. The use of model test reactions is a characterization tool of first choice, since this method has been very successful with zeolites where it precisely reflects shape-selectivity effects imposed by the porous structure of tested materials. Adsorption of probe molecules is another attractive approach. Both types of approaches will be presented in this work. The methodology developed in this work on zeolites Beta, USY and silica-alumina may be appropriate for determination of accessible mesoporosity in other types of dealuminated zeolites as well as in hierarchical materials presenting combinations of various types of pores. 

Several approaches towards the synthesis of hierarchical meso- and macro-porous materials have been described. For instance, a mixture that comprised a block co-polymer and polymer latex spheres was utilized to obtain large pore silicas with a bimodal pore size distribution [84]. Rather than pre-organizing latex spheres into an ordered structure they were instead mixed with block-copolymer precursor sols and the resulting structures were disordered. A similar approach that utilized a latex colloidal crystal template was used to assemble a macroporous crystal with amesoporous silica framework [67]. 

Mesoporous materials have been extensively studied since the discovery of mesoporous silica. The great progresses made in sol-gel chemistry open the way to the synthesis of hierarchically nanostructured porous materials. 

The materials which have been mentioned here so far are predominantly shaped in planar films of hierarchical order. However, the synthesis of hierarchically structured particles is also highly desirable, as they might be further processed and used for the preparation of composite porous materials. Wu et al. showed the synthesis of raspberry-like hollow silica spheres with a hierarchically structured, porous shell, using individual PS particles as sacrificial template [134]. In another intriguing approach by Li et al. [135], mesoporous cubes and near-spherical particles (Fig. 10) were formed by controlled disassembly of a hierarchically structured colloidal crystal, which itself was fabricated via PMMA latex and nonionic surfactant templating. The two different particle types concurrently generated by this method derive from the shape of the octahedral and tetrahedral voids, which are present in the template crystal with fee lattice symmetry. 

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