Macroporous network

In a particle having a bidispersed pore structure comprising spherical adsorptive subparticles of radius forming a macroporous aggregate, separate flux equations can be written for the macroporous network in terms of Eq. (16-64) and for the subparticles themselves in terms of Eq. (16-70) if solid diffusion occurs. 

Bidispersed Particles For particles of radius Cp comprising adsorptive subparticles of radius r, that define a macropore network, conservation equations are needed to describe transport both within the macropores and within the subparticles and are given in Table 16-11, item D. Detailed equations and solutions for a hnear isotherm are given in Ruthven (gen. refs., p. 183) and Ruckenstein et al. [Chem. Eng. Sci., 26, 1306 (1971)]. The solution for a linear isotherm with no external resistance and an infinite fluid volume is ... 

This paper describes a methodology that can be used to determine the macroscopic (> -30 pm) spatial arrangement of characteristic pore sizes within the macropore network of a bidisperse porous medium. 

Many reactions taking place within catalyst or absorbent pellets in industrial plants are diffusion-limited. Under the typical operating conditions for many absorbents, diffusion of gases into the porous solid occurs in the Knudsen regime. In such circumstances the rate of gas pick-up of these materials is strongly dependent on the pore structure. The pore structure for absorbent pellets that will deliver the most efficient operation of an absorbent bed requires a pervasive system of macropores which provide rapid transport of the gas flux into the centre of the pellet. A network of ramified mesopores branching off the macropores then provides extensive surface area for absorption of gas molecules. Therefore, when manufacturing an absorbent it is necessary to be able to determine the spatial distribution of the macropore network in a product to ensure that the pore structure is the most appropriate for the peirticular duty for which it is intended. 

The performance of the sorbent could be substantially improved if the active phase in the extrudate is formed in a proper degree of dispersion or a suitable macropores network. Provided that macropores network could be generated, during sorbent calcination at high temperature and many sulfidation-regeneration cycles, by using convenient pore-forming additives [15]. 

It must be stressed that this model of macroporous networks is a rather simplistic one and far more details are available in [30]. 

In addition, the solvent generates pores and, in conjunction with other parameters such as temperature, influences the morphology of the material (size, shape and size distribution of the cavities). For chromatographic applications, macroporous networks are preferable. As the pores are more accessible, recognition is enhanced and retention times reduced. For instance, the use of acetonitrile as solvent in acrylate networks leads to a more macro-porous structure than chloroform [130]. 

The rate of polymerization in the water/ethanol system is lower as compared to the other heterogeneous system, thus, polymers can phase-separate and cross-linking occurs in the separated phase mostly. Since the rate of transfer reaction should be almost the same as for the pure water system, no additional cross-links are formed. Thus, the swelling of water/ethanol gels are mostly governed by the expansion of the macroporous network. 

In the context of transport, the presence of macropores plays a particular role. Macropores are large pores, which form at the macroscopic level an obviously distinguished pore system from the soil matrix pore system. Macropores constitute sometimes a separate and/or continuous network in which particle velocities may deviate systematically from those in the soil matrix. As a result, the solutes released in the macropore network will be subjected to a preferential flow as compared to flow in the matrix system and will not completely mix with the total pore water volume at short time intervals. Preferential flow through macropores is considered here as a macroscopic process since concentrations in the macropores cannot easily be determined separately from the concentrations in the micropore system. 

Zeolites are widely used as acid catalysts, especially in the petrochemical industry. Zeolites have several attractive properties such as high surface area, adjustable pore size, hydrophilicity, acidity, and high thermal and chemical stability. In order to fully benefit from the unique sorption and shape-selectivity effects in zeolite micropores in absence of diffusion limitation, the diffusion path length inside the zeolite particle should be very short, such as, e.g., in zeolite nanocrystals. An advantageous pore architecture for catalytic conversion consists of short micropores connected by meso- or macropore network [1]. Reported mesoporous materials obtained from zeolite precursor units as building blocks present a better thermal and hydrothermal stability but also a higher acidity when compared with amorphous mesoporous analogues [2-6]. Alternative approaches to introduce microporosity in walls of mesoporous materials are zeolitization of the walls under hydrothermal conditions and zeolite synthesis in the presence of carbon nanoparticles as templates to create mesopores inside the zeolite bodies [7,8]. 

The second stage in the preparation of carbon gels is drying of the wet gel to remove the solvent trapped in its gel strnctnre. The volnme occnpied by the solvent will essentially constitute the meso- and/or macropore network of the... 

As a rule, ACF not only presents a higher adsorption capacity than conventional GAC, but the pore network is also different due to the fibril structure, which ensures a much higher adsorption kinetics. The reason is that in GAC, the adsorbate must diffuse throughout the macro and mesopores before reaching the micropore or adsorption sites, whereas the micropores are directly accessible ftom the external surface in the ACF (Fig. 23). Consequently, there is no resistance to the diffusion of adsorbates through to the adsorption pores because there is no meso/macropore network. 

In this respect, colloidal particles are ideal candidates serving as templates for the formation of amorphous and highly ordered porous networks, particularly for macroporous networks, owing also to the maturing synthesis routes of various colloidal particles with controlled size, geometry, and composition [14-18]. Due to their small sizes and the forces exerted upon... 

Fig.1 Scheme of preparation routes to macroporous networks by colloidal crystal templating... 

Instead of infiltration with neat metal nanoparticles, the interstitial voids of the template opal can also be filled wifh a mefal precursor. The impregnation of the preformed colloidal crystals with the metal precursor, followed by transformation of the precursor to the neat metal and removal of the template, results in metallic inverse opals. For example, nickel oxalate was precipitated in a PS opal and converted into a NiO macroporous network by calcination of the metal salt and combustion of the polymer. In a subsequent step, the nickel oxide was reduced to neat Ni in a hydrogen atmosphere to yield a macroporous metal network [82]. It was further suggested by the authors that by the same technique other metal networks (e.g.. Mg, Mn, Fe, Zn from their oxides and Ca, Sr, Ba etc. from their carbonates) should be accessible. 

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