Hierarchically structured porous materials applications

In this chapter, we present an overview of the synthesis strategies based on the sol-gel process for the preparation of multimodal porous networks of hierarchically structured functional materials and a brief introduction to their applications is given at the end of the chapter. 

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. 

Hierarchical porous materials are solids that are ordered at different length scales. Materials with multiple porosities are of high interest for applications in catalysis and separation, because these applications can take advantages of different pore structures. For example, microporous-mesoporous composites have shown superior catalytic activities by the combination of strong acidity from zeolites 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. 

Wide application Applications of these hierarchically porous materials are emerging due to their multiscale porous structures, high accessibility, and high storage capacity. These materials have been applied to photocatalysis, catalysis, solar cells, separation and purification processes, catalytic supports, and energy conversion and storage. 

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. 

From an application point of view, the volumetric capacities are even more important than that on a gravimetric basis, due to the limited volume of the gas storage tank. Under this consideration, Qian et al. [131] optimized the stractural features of hierarchical porous carbon monolith by incorporating the advantages of MOFs (Cu3(BTC)2) to maximize the volumetric based CO2 capture capability (CO2 capacity in cm per cm adsorbent). The mesoscopic structure of the HCM-Cu3(BTC)2 composites and the parent materials (HCM and Cu3(BTC)2) were characterized by SEM. The SEM micrograph (Fig. 2.24) clearly displays that Cu3(BTC)2 crystallites are bom within the macropores of the HCM matrix. The sponge-like skeleton of HCM before and after the MOF growth remains... 

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