Hierarchically catalytic 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. 

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. 

Hierarchical porous materials have attracted much interest in recent years due to their intensive use in different fields ranging from catalysis to ceramics [1,2]. In the present work an original environmentally friendly tecluiique combining simple synthesis method with use of waste biomass was utilized for the production of Ce-Zr oxide, which can be used both as a support and as a catalyst in various processes. Thanks to its oxygen storage capacity (OSC) Ce-Zr oxide is the main component in three-way catalysts and catalytic filters for oxidation of soot emitted by diesel engines [3], In the latter case the hierarchical porous stracture of the systems is of great importance in order to capture the particulates of soot... 

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. 

Friedel-Crafts acylation is widely used for the production of aromatic ketones applied as intermediates in both fine chemicals and pharmaceutical industries. The reaction is carried out by using conventional homogenous catalysts, which represents significant technical and environmental problems. The present work reports the results obtained in the Friedel-Crafts acylation of aromatic substrates (anisole and 2-methoxynaphthalene) catalyzed by Beta zeolite obtained by crystallization of silanized seeds. This material exhibits hierarchical porosity and enhanced textural properties. For the anisole acylation, the catalytic activity over the conventional Beta zeolite is slightly higher than with the modified Beta material, probably due to the relatively small size of this substrate and the weaker acidity of the last sample. However, the opposite occurred in the acylation of a bulky substrate (2-methoxynaphthalene), with the modified Beta showing a higher conversion. This result is interpreted due to the presence of a hierarchical porosity in this material, which favors the accessibility to the active sites. 

The catalytic activity of hierarchical and conventional Beta zeolites for acylation of 2-MN is displayed in Figure 2(a) The Beta (PHAPTMS) sample shows a superior catalytic activity than the conventional one, due to its enhanced textural properties. In this case, the bulky nature of both substrate and products may cause the existence of diffusional problems inside the zeolitic channels, which are attenuated in the modified Beta sample due to the presence of the hierarchical porosity. Regarding the product distribution (Figure 2(b)), two main products are observed and a third isomer, 8-A,2-MN isomer is produced just in minor amounts. Interestingly, the selectivity towards the desired isomer increases in the material obtained from silanized seeds, reaching values around 75%. Probably, the active sites located on the surface of the secondary porosity are able to catalyze also the formation of 6-A,2-MN by transacylation. However, this reaction is expected to be strongly hindered in the conventional Beta zeolite since it requires the participation of two bulky molecules as reactants. 

The Beta material prepared by seed silanization show interesting catalytic properties in aromatic acylation reaction, especially when using a bulky substrate, such as 2-methoxynaphthalene. The superior activity and selectivity exhibited by this sample has been related to the presence of a hierarchical porosity, which decreases the steric and diffusional hindrances, favoring the accessibility to the active sites and allowing the occurrence of the transacylation reaction. 

While modeling the structure and properties of porous materials one usually is interested in structural properties of a desirable hierarchical level. For example, for chemical properties the molecular structure is major, and the specific adsorption and catalytic properties are guided by the structure and composition of particle surface. Diffusion permeability is determined by the supramolecular... 

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. 

The inherent limitations of the use of zeolites as catalysts, i.e. their small pore sizes and long diffusion paths, have been addressed extensively. Corma reviewed the area of mesopore-containing microporous oxides,[67] with emphasis on extra-large pore zeolites and pillared-layered clay-type structures. Here we present a brief overview of different approaches to overcoming the limitations regarding the accessibility of catalytic sites in microporous oxide catalysts. In the first part, structures with hierarchical pore architectures, i.e. containing both microporous and mesoporous domains, are discussed. This is followed by a section on the modification of mesoporous host materials with nanometre-sized catalytically active metal oxide particles. 

At the root of these behaviors is the phenomenon of synchronization of a macroscopic oscillating system. This topic has been discussed occasionally in the literature but has rarely been explicitly treated. In principle there are several stages or hierarchical levels on which oscillations can occur (1) the single-crystal plane, (2) the catalytically active metal crystallite, (3) the catalyst pellet, (4) arrangements of several pellets in one layer of a flow reactor or a CSTR, and finally (5) the catalytic packed-bed reactor (327). On each of these levels, different types of oscillations may exist, but to become observable on the next level oscillations on the respective sublevels must be synchronized. For example, if oscillations of the CO/O2 reaction on a Pt(lOO) face of a Pt crystallite supported on a pelletized support material in a packed-bed reactor occur, the reaction on the (100) facet as a whole must oscillate in synchrony, other (100) facets of the crystallite have to synchronize, other crystallites in the pellet must couple to the first crystallite, and, finally, all pellets in one layer of the bed must display oscillations in synchrony. If the synchronization on one of these levels fails, different oscillators will superimpose and their effects will cancel. One would then only observe a possible increased level of noise in the measured conversion. On the other hand, if synchronization occurs independently over several regions of a system, then it might exhibit apparently chaotic behavior caused by incomplete coupling. 

The synthesis of a hierarchical pore structure, combining the macroporous diatomaceous earth with microporous zeolites, is reported. Diatomaceous earth is an abundant and varied source of macroporous silica which has been zeolitisatised to produce a bifunctional, hierarchical composite. A range of different zeolites have been synthesised to generate different pore architectures, hydrophobic/hydrophilic materials and ion-exchange/catalytic properties. 

The remarkable and singular properties shown by hierarchical zeolites have brought about potential catalytic applications for these materials in numerous reactions, especially those wherein steric or diffusion limitations are encountered. Table 8.3 summarizes the literature dealing with the application of hierarchical zeolites in a variety of reactions (oil refining and petrochemical reactions, and fine chemicals reactions). 

Nb-based catalysts are among the investigated systems in total removal of -butanol due to the capacity of niobium to adopt variable oxidation states. Thus, Nb-doped hierarchically micro(meso) macroporous Ti02 (anatase phase) (Figure 17.10) synthesized via a self-formation procedure showed close performances with catalysts in which the same materials served as supports for noble metals [46]. The efficiency of these catalytic systems in the total oxidation of butan-l-ol was enhanced by the improved diffusion through the intrinsic macro-porous network. An effect of the niobium content was evidenced as welL... 

In summary, the hierarchically porous monolithic HSQ materials are highly useful for their in-built reactive Si—H moiety, for the synthesis of supported noble metal and alloy metal nanoparticles for catalytic applications. Furthermore, they can be used for reversible grafting of organic alcohols, which is a major advantage because they overcome the difficulties of traditional hydrolyzable precursors. 

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