Macro- and mesoporous

Figure 2.20. Micro-, macro-, and mesopores in a porous sorbent. (Reprinted with permission from Ref. 56. Copyright 1996 Barnebey Sutcliffe Corporation.)...

In macro- and mesoporous membrane layers the nature of the flow is determined by the relative magnitude of the mean free path X of the molecules and the pore size dp. When the mean free path of the gas molecules is much larger than the pore size, i.e. X dp, collisions of molecules with the pore walls are predominant and the mass transport takes place by the well-known selective Knudsen diffusion process. If the pore radius is much larger than the mean free path of the molecules and a pressure difference over the membrane exists the mass transport takes place by non-selective viscous flow. 

The measurements of external and internal specific surface area have already been discussed in Chapter 1, Section 1.1.3. The principles and the isotherm equation of the BET method to measure external specific surface area, including macro- and mesopores, have been presented in Chapter 1, Section 1.3.4.1.5. The external specific surface area is usually determined by nitrogen gas adsorption at the temperature of liquid nitrogen. Both static (one-point) and dynamic (five-point) methods are applied. The calculations are made by Equation 1.75 (Chapter 1), using one or five different pressure values. The external specific surface area is calculated from the maximum number of surface sites, that is, monolayer and the cross-sectional area of nitrogen molecules. 

Figure 6 Comparison of bimodal macro- and mesopore arrangements of agglomerates in dependence of the synthesis temperature of primary MCMoidal particles.

The growth in the use of molecular sieves as catalysts as compared with macro- and mesoporous oxides was stimulated by several factors (i) The high concentration of active sites (in comparison with oxides) results in very active catalysts, (ii) The defined pore structure allows to exclude reactants from being converted and/or products to be formed or transported out of the pores due to a too large size, (iii) The active site and the environment of that site can be designed on an atomic level for exEimple by ion exchange [9] or chemical functionalization of the framework [10]. (iv) It is possible to tailor the chemical properties of molecular sieves better than those of conventional macro and mesoporous oxides. 

Microfiltration and ultrafiltration are the two main filtration techniques for which ceramic membranes have been widely used to date. As described in Section 6.2.1.2, MF and UF ceramic membranes exhibit macro- and mesoporous structure, respectively, which result from packing and sintering of ceramic particles. Liquid flow in such porous media is convective in nature and the simplest description of permeation flux, J, is given by the Darcy s equation [20] ... 

An electrochemical Pt deposition from the diamine nitrite solution onto monoctystalline, macro- and mesoporous silicon is presented. Pt grain size versus deposition time was determined from the SEM data. A catalytic reactivity of the Pt coated electrodes was estimated by the calculation of the effective surface area with a voltammetry technique. 

Monocrystalline, macro- and mesoporous silicon were used for the electrochemical deposition of Pt. A 10 pm thick macroporous silicon layer was formed by anodizing of p-type Si wafers of 12 Ohm-cm resistivity in an aqueous solution of HF acid and DMSO (10 46 by volume parts) at the current density of 8 mA-cm [1]. Pore channels distributed with the surface density of 6T0 cm look like long straight holes with inlet diameters of 1.5 pm. An uniform 1 pm thick mesoporous silicon layer was fabricated by anodizing of n" -type Si wafers of 0.01 Ohm-cm resistivity in a solution of HF acid, water and isopropanol (1 3 1 by volume parts) at the current density of 60 mA-cm . The mesoporous silicon sample formed looks like Si layer perpendicularly pierced through by pore channels with diameter of about 20 nm. The number of pores per square centimetre is up to 2-10 [2]. 

These curves were obtained experimentally for Pt electrodeposited onto the monocrystalline, macro- and mesoporous silicon surfaces. There are two peaks corresponding to the oxidation of adsorbed hydrogen, a potential area of a double layer, and an area of the oxygen adsorption. The current increase at the potential of 1.45 V and higher is associated with the release of molecular oxygen. In the cathodic polarization, peaks concerned with the reduction of adsorbed oxygen, a feebly marked potential area of the double layer, and a maximum of the hydrogen adsorption are observed. 

The majority of the published literature on improved adsorbents for H2 purification by PSA deals with equilibrium adsorption properties (adsorption capacities of the impurities and their selectivities over H2) of the materials. The adsorbents are generally chosen in such a way that the kinetics of adsorption of the impurities into the adsorbents are relatively fast, primarily being controlled by macro- and mesopore diffusion within the adsorbent particles. The kinetics of adsorption may, however, become an issue for the removal of the trace amounts (ppm) of a relatively weakly adsorbed impurity (N2 or CH4) at the product end of an H2 PSA due to the existence of a very low driving force for the adsorption process. It was suggested that a layer... 

In Eq. 6.25 the transport in the pore fluid is modeled as free diffusion in the macro-and mesopores, but the diffusion coefficient Dpolej is usually lower than in the liquid mobile phase because of the random orientation and variations in the diameter of the pores (tortuosity) (Section 6.5.8). 

In the case of macro- and mesoporous supports their flow resistance should be as small as possible. If the transport resistance is not negligible corrections must be applied in the study of the separation properties of the separating layers. It is shown that even small pressure gradients across the support can cause a considerable decrease of the permeation and of the separation factor of the top layer, especially in the case of adsorbing gases. The absolute value of the permeate pressure is important in addition to the pressure ratio of feed and permeate streams. Increasing support resistance causes an increase of the permeate pressure on the interface between support and separation (top) layer in the case of supported membranes. 

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 general two fimctionahzation approaches are available either (1) direct functionalization of the macro- and mesopores during the hierarchical network formation or (2) postfimctionahzation of the porous material via the... 

Note that in this equation the axial dispersion = 0) and also the micropore diffusion are neglected. The model takes into account only the diffusion in the macro- and mesopores and the resistance in a concentration boundary layer around a pellet. 

Both active carbons differ in the development of porous structure within particles, as well as in the contribution of various kinds of pores into their total porosity. AG carbon is a typical polydisperse adsorbent, of a comparable contribution of all kinds of pores. On the contrary, RN active carbon is a microporous adsorbent with dominant microporous structure and lower content of macro- and mesopores. 

The total volume of mercury VHg(p) penetrating the pores of the material at pressure p leads via equation (1.2) to the integral volume Vp(r) of all pores with radii (p) larger than r < p < oo, i. e. Vp(r) = VHg(p). By differentiation to the pore radius r this yields the differential pore size distribution of the material. This method is valuable to investigate macro- and mesopores (lUPAC, cp. Sect. 3), but not for micropores, i. e. it is limited to pore radii r > 1 nm. 

Mercury intrusion data also may be misleading for porous materials having many inkbottle type pores, cp. middle portion of Fig. 1.1. In such situations high pressures are needed to overcome resistance of mercury to pass the narrow neck of the pore, i. e. the wider portion of the inkbottle pores will not be adequately reflected in the experimentally taken Vhj = Vng (p) curve. However, despite these disadvantages, mercury intrusion experiments often gives valuable information concerning the macro- and mesopores of a sorbent and hence very well may be used for comparative measurements and quality tests of sorbent samples. 

In Fig. 6.22 the spectrum of the initial state is indicated by Vac. . It can be seen that within the first hour of the adsorption process the capacitance of the system increases with a shift of its maximum value to somewhat higher frequencies. This effect is possibly due to a primary adsorption process of the CO-molecules at locations in macro- and mesopores of the AC where, due to their permanent dipole moment (pco = 0.1 D), they add to both the capacitance and the resonance frequency of the loaded AC. Now microbalance measurements have shown that approximately 80 % of the total mass adsorbed during the whole process is adsorbed within the first hour. In view of this, the changes of the capacitance spectrum which were observed afterwards for nearly 40 h are doubtless due to an internal diffusion or dissoziation process of the CO molecules taking place without uptake of more molecules from the gas phase. 

An industrial DMTO fluidized bed catalyst pellet is basically composed of SAPO-34 zeofite particles and catalyst support (or matrix). The pores of zeolite particles and matrix are interconnected as a complex network. The pores inside zeofite particles are typically micropores (less than 2 nm) and the matrix normally has either mesopores (2-50 nm) or macropores (>50 nm), or both (Krishna and Wesselingh, 1997). The bulk diffusion coefficients in the meso- and macropores might be several orders of magnitude larger than surface diffusion coefficients in the micropores. Kortunov et al. (2005) found that the diffusion in macro- and mesopores also plays a crucial part in the transport in catalyst pellets. Therefore, other than a model for SAPO-34 zeofite particles, a modeling approach for diffusion and reaction in MTO catalyst pellets, which are composed of SAPO-34 zeofite particles and catalyst support, is needed. 

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