Catalysts pore-size distribution function

The final element in the analysis is the pore size distribution function of the experimental catalyst COMOX 1661. 

Important physical properties of catalysts include the particle size and shape, surface area, pore volume, pore size distribution, and strength to resist cmshing and abrasion. Measurements of catalyst physical properties (43) are routine and often automated. Pores with diameters <2.0 nm are called micropores those with diameters between 2.0 and 5.0 nm are called mesopores and those with diameters >5.0 nm are called macropores. Pore volumes and pore size distributions are measured by mercury penetration and by N2 adsorption. Mercury is forced into the pores under pressure entry into a pore is opposed by surface tension. For example, a pressure of about 71 MPa (700 atm) is required to fill a pore with a diameter of 10 nm. The amount of uptake as a function of pressure determines the pore size distribution of the larger pores (44). In complementary experiments, the sizes of the smallest pores (those 1 to 20 nm in diameter) are deterrnined by measurements characterizing desorption of N2 from the catalyst. The basis for the measurement is the capillary condensation that occurs in small pores at pressures less than the vapor pressure of the adsorbed nitrogen. The smaller the diameter of the pore, the greater the lowering of the vapor pressure of the Hquid in it. 

The surface area of the catalyst as well as the pore size distribution can easily be measured, and the zeolite and matrix surface areas of the catalyst can be determined by the t-plot method. The different FCC yields can then be plotted as a function of the ZSA/MSA ratio, zeolite surface area or matrix surface area, and valuable information can be obtained [9], The original recommendation was that a residue catalyst should have a large active matrix surface area and a moderate zeolite surface area [10,11]. This recommendation should be compared with the corresponding recommendation for a VGO catalyst a VGO catalyst should have a low-matrix surface area in order to improve the coke selectivity and allow efficient stripping of the carbons from the catalyst [12], Besides precracking the large molecules in the feed, the matrix also must maintain the metal resistance of the catalyst. 

Occluded coke is a function of the operating conditions of the FCCU stripper, of catalyst pore volume and pore size distribution. This coke that has the higher content of hydrogen is the lowest refractory between the different kinds of coke produced in the FCC process. Althongh in this study it was not directly evalnated, it may be associated with the peak A at the lowest temperature ( 500°C-550°C). So once the... 

PCH materials offer new opportunities for the rational design of heterogeneous catalyst systems, because the pore size distributions are in the supermicropore to small mesopore range (14-25A) and chemical functionality (e.g., acidity) can be introduced by adjusting the composition of the layered silicate host. The approach to designing PCH materials is based on the use of intercalated quaternary ammonium cations and neutral amines as co-surfactants to direct the interlamellar hydrolysis and condensation polymerization of neutral inorganic precursor (for example, tetraethylorthosilicate, TEOS) within the galleries of an ionic lamellar solid. 

In the sol-gel preparation of supported metals, a metal precursor is usually added directly to the solution prior to gelling. Regardless of whether the metal precursor participates in hydrolysis and/or condensation, it will become part of the network as the gel forms. Thus, any parameters that are important in solution chemistry (Table 1) could affect the properties of the metal upon activation. An example is the work of Zou and Gonzalez [39] cited in Section 2.I.4.3.A. When these authors used water content as a variable to change the pore size distribution of a series of Pt/Si02 catalysts, they found that the particle size distribution of reduced Pt (in the form of crystallites) is also dependent on the hydrolysis ratio. The average Pt particle size nearly doubles (from about 1.7 to 3nm) as the hydrolysis ratio increases from 10 to 60. As noted earlier, the stability of these catalysts, in terms of the resistance of Pt particles towards sintering, is a function of how well the pore diameter and particle size match. 

The macro-porosity emacro and the correlation function corresponding to the macro-pore size distribution of the washcoat were evaluated from the SEM images of a typical three-way catalytic monolith, cf. Fig. 25. The reconstructed medium is represented by a 3D matrix and exhibits the same porosity and correlation function (distribution of macro-pores) as the original porous catalyst. It contains the information about the phase at each discretization point— either gas (macro-pore) or solid (meso-porous Pt/y-Al203 particle). In the first approximation, no difference is made between y-Al203 and Ce02 support, and the catalytic sites of only one type (Pt) are considered with uniform distribution. 

The nature and arrangement of the pores determine transport within the interior porous structure of the catalyst pellet. To evaluate pore size and pore size distributions providing the maximum activity per unit volume, simple reactions are considered for which the concept of the effectiveness factor is applicable. This means that reaction rates can be presented as a function of the key component. A only, hence RA(CA). Various systems belonging to this category have been discussed in Chapters 6 and 7. The focus is on gaseous systems, assuming the resistance for mass transfer from fluid to outer catalyst surface can be neglected and the effectiveness factor does not exceed unity. The mean reaction rate per unit particle volume can be rewritten as... 

Feng, Kostrov and Stewart (1974) reported multicomponent diffusion data for gaseous mixtures of helium (He), nitrogen (N2) and methane (CH4) through an extruded platinum-alumina catalyst as functions of pressure (1 to 70 atm), temperature (300 to 390 K), and terminal compositions. The experiments were designed to test several models of diffusion in porous media over the range between Knudsen and continuum diffusion in a commercial catalyst (Sinclair-Engelhard RD-150) with a wide pore-size distribution. 

The effect of crystal structure may be investigated by preparing catalysts, as described above, at various temperatures which assures a set of catalysts having variable surface areas, pore size distributions, and crystallinity. Measuring the catalytic activity as a function of these physical properties will help to define the role of crystal structure for the particular transition metal sulfide. In general, the HDS is poorly correlated to N2 BET surface area. This non-correlation can be most easily seen by preparing a... 

Catalyst characterization tests include measurement of surface areas, chemisorption, pore-size distributions, crystal structure as determined by X-ray crystallography, reaction mechanisms as revealed by kinetics, and isotopic tracers and diagnostic catalytic reactions to test functional capabilities. These have been interpreted in terms of variation of catalyst preparation-structure-performance relationships. 

Internal diffusion limitations of reactant molecules can affect the activity properties and this depends on the pore size distribution as well as on the particle size of the catalyst. Figure 3 shows the variation of the carbon productivity as a function of the particle size of the catalyst. 

The density functional theory was employed by Neimark et al. [44,86,87] to study the adsorption of N2 and Ar on MCM-41 mesoporous silica and V catalysts supported on MCM-41. In another group of papers, Neimark et al. employed the density functional theory to study N2 and Ar adsorption MCM-48 mesoporous silica [28,29]. Their main goal was to characterize the pore size distribution of those mesoporous solids. According to the density functional approach it possible to calculate the adsorption isotherm on an individual pore. If a cylindrical symmetry is assumed for the pore, the adsorption... 

Figure 6.7 Catalyst engineering involves an optimal combination of interdependent structural elements that yields the catalytic, mechanical, and physicochemical (specific surface area and pore size distribution, density, surface functionality, and acidity) properties desired for successful industrial applications. Solid arrows indicate primary contributions of catalyst components to the desired properties. Dashed arrows indicate secondary influences of these components via the interdependent nature of some properties.

In Refs. [18,19], the macrohomogeneous theory was extended to include concepts of percolation theory. The resulting structure-based model correlates the performance of the CCL with the volumetric amounts of Pt, C, ionomer, and pores. A detailed review of macroscopic catalyst layer theory can be found in Ref. [17]. A further extension of this theory in Ref. [25] explores the key role of the CCL for the fuel cell water balance. This function is closely linked to the pore size distribution. Major principles of these models will be reproduced here. The details can be found in the literature cited. 

Figure 2.7. Catalyst utilization at macroscopic scale, i.e. the catalyst utilization factor /(Xptc, Xei) (>Sr)/Xptc in the exchange current density (top panel, cf. Eqs. (2.65) and (2.66)), and oxygen diffusion coefficient (bottom panel, cf. Eq. (2.63)) as functions of the liquid water saturation, plotted for the three different pore size distributions in Figure 2.2 [35]. The plots reveal the effect of porous structure on the basic competition between activity (top) and mass transport (bottom) in the CCL. The structure with a large fraction of primary pores is beneficial for catalyst utilization and detrimental for gas diffusion, and vice versa.

FIGURE 23.23 (a) CapiUary condensation radius for typical hydrotreater light oil feed mixed with 50 mole % hydrogen, plotted as a function of relative pressure referred to the dew point. The dew point is in the region of 32 har. (b) Pore size distribution for an alumina hydrotreatment catalyst support. 

The influence of capillary condensation upon catalyst effectiveness factor has been assessed both by approximate calculations and by pore network simulations. It was found that catalyst effectiveness could be affected by the presence of capillary condensation, depending on the ratio of reaction rates in the gas and liquid phases. The effectiveness factor under conditions of capillary condensation is sensitive to operating conditions of the reactor, such as pressure, and to properties of the catalyst pore structure like pore-size distribution and connectivity. Once the catalyst pellet contains some pores filled with liquid, the kinetics of the process become dependent upon the phase equilibria of the system. This can lead to multiple steady states in the reaction rate as a function of temperature or pressure, because the current state of the catalyst pellet depends on the history of temperature and pressure profiles to which it has been subjected. 

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