Nonuniformly distributed catalysts

Catalysts, prepared by impregnation of the porous support, in many cases exhibit intrapellet activity gradients, which are traditionally thought to be detrimental to catalyst performance. The effect of a deliberate nonuniform distribution of the catalytic material in the support, on the performance of a catalyst pellet received attention as early as the late 1960s [18,19]. These, as well as later studies, both experimental and theoretical, demonstrated that nonuniformly distributed catalysts can offer superior conversion, selectivity, durability, and thermal sensibility characteristics over those wherein the activity is uniform. 

Some studies of potential commercial significance have been made. For instance, deposition of catalyst some distance away from the pore mouth extends the catalyst s hfe when pore mouth deactivation occui s. Oxidation of CO in automobile exhausts is sensitive to the catalyst profile. For oxidation of propane the activity is eggshell > uniform > egg white. Nonuniform distributions have been found superior for hydrodemetaUation of petroleum and hydrodesulfuriza-tion with molybdenum and cobalt sulfides. Whether any commercial processes with programmed pore distribution of catalysts are actually in use is not mentioned in the recent extensive review of GavriUidis et al. (in Becker and Pereira, eds., Computer-Aided Design of Catalysts, Dekker, 1993, pp. 137-198), with the exception of monohthic automobile exhaust cleanup where the catalyst may be deposited some distance from the mouth of the pore and where perhaps a 25-percent longer life thereby may be attained. 

With nickel/alumina catalysts (cf. 4 ) preparation by coprecipitation or by the decomposition of a high dispersion of nickel hydroxide on fresh alumina hydrogel, yields nickel aluminate exclusively. On the other hand, when, as in impregnation, larger particles of nickel compound are deposited, the calcination product is a mixture of nickel oxide and nickel aluminate. The proportion of nickel oxide increases when occlusion of the impregnation solution leads to a very nonuniform distribution (49). 

Active centers, nature of, 10 96 Active site, 27 210-221 in catalysts, 17 103-104, 34 1 for olefin chemisorption, 17 108-113 dual-site concept, 27 210 electrical conductivity, 27 216, 217 ESCA, 27 218, 219 ESR, 27 214-216 infrared spectroscopy, 27 213, 214 model, 27 219-221 molybdena catalyst, 27 304-306 Mdssbauer spectroscopy, 27 217, 218 nonuniform distribution, transport-limited pellets, 39 288-291... 

An important aspect concerning catalytically active membrane reactors, is the distribution of the active phase within the membrane system. Modem modification techniques (van Praag et al. 1989, Lin, de Vries and Burggraaf 1989) allow control over the catalyst distribution and preferential deposition of the active phase at different places in the membrane (top layer/support) system. Studies on conventionally used plate-shaped and cylindrically-sha-ped catalytically active pellets (Vayenas and Pavlou 1987a, b, Dougherty and Verykios 1987) have shown that nonuniformly activated catalysts (catalysts with nonuniform distribution of active sites according to a certain profile)... 

One of the drawbacks of this CAVERN device is the occurrence of a nonuniform distribution of reactant on catalysts because adsorption occurs on a deep bed of catalyst packed in a MAS rotor. To overcome this problem, we developed several shallow-bed CAVERN devices (95), and Fig. 10 shows a version of one such design. A thin layer of catalyst is supported on a glass trapdoor, and the device is evacuated. A furnace is clamped in place so that the catalyst can be activated if necessary. The catalyst is cooled with a cryogen bath, and a controlled amount of adsorbate is introduced from the vacuum line. The trapdoor is raised, the loaded catalyst falls into the MAS rotor, and the seal is driven into place. Finally the cold, sealed rotor is manually transferred into the cold MAS probe. The added advantages of the shallow-bed CAVERN is that all manipulations can be carried out without using a glovebox in any step. 

The performance of adsorptive (and indeed almost all multifunctional) reactors benefits from an expedient nonuniform distribution and integration of the functionalities at various levels. Simply combining given proportions of catalyst and adsorbent in a fixed-bed reactor seldom realizes the full potential available [52]. The objective may be to maximize utilization of adsorbent capacity or to optimize catalyst productivity. Although these aims need not be mutually exclusive, they often give rise to different strategies. 

F(c/c,) denotes the dimensionless form of an arbitrary rate expression./(x) is a nonuniform, normalized catalyst activity distribution inside the pellet. A(x) is an auxiliary function, subject to the following linear differential equation ... 

We conclude that the dispersion of the elements in the oxide-form catalysts depends not only on the phosphorus content but also on the preparation procedures. In any case, large amounts of phosphorus favor the agglomeration of metal oxides. The discrepancies in the literature suggest that both the preparation conditions and the activation conditions (such as the nature of the presulfiding mixture and the sulfidation temperature) may also affect the textural properties and morphology of the sulfided catalysts. The nonuniform distribution of molybdenum in the catalyst particles at low phosphorus loadings may affect the XPS results discussed in terms of phase dispersion. 

Binkerd CR, Ma YH, Moser WR, and Dixon AG. An experimental study of the oxidative coupling of methane in porous ceramic radial-flow catalytic membrane reactors. Proceedings of ICIM4 (Inorganic Membranes), Gatlinburg, TN D.E. Fain (ed.), 1996 441-450. Yeung AKL, Sebastian JM, and Varma A. Mesoporous alumina membranes synthesis, characterization, thermal stability and nonuniform distribution of catalyst. J. Membr. Sci. 1997 131 9-28. 

If a metal particle resides in a pore of a similar size, a considerable part of its surface is in intimate contact with the pore walls and, therefore, inaccessible to adsorbates [6]. For Pd/C catalysts dispersed uniformly over the surface of a microporous support, the true dg) and apparent d) sizes of Pd particles may indeed be very different (Table 5, nos. 1-6 Table 1). The influence of micropores is much lower when metal particles are stabilized in wider pores or in the form of rather coarse particles (Table 5, nos. 9, 10, 12, 14, 15). The same happens if the metal is nonuniformly distributed through the grain of microporous support (nos. 12-15), which is achieved, for example, by adsorption of colloidal catalyst precursors. 

Sequential precipitation is used when a nonuniform distribution of catalyst components throughout a catalyst particle is desirable. In this method, the pH of the precipitation is controlled at specific levels at different... 

In excess solution adsorption, the support material is submerged in excess amount of impregnation solution (the volume of impregnation solution is much higher than the pore volume of the support). The excess solution is filtered after adsorption equilibrium is reached. In many cases, competitive adsorption between solvent and solutes and/or between different solutes leads to a nonuniform distribution of active components throughout the support particles. This phenomenon can be utilized to enhance performance (normally selectivity) of certain types of catalysts. The distribution of the active components can also be tailored by the manipulation of the pore structure of the support, pH and viscosity of the solution. ... 

In supercritical solutions, in a microscopic sense, molecules are nonuniformly distributed. There is aggregation of the solvent molecules around the solute and clusters are formed. The local clustering of solutes or solvents under supercritical condition increases the local concentration of the substrate or the catalyst in the solution and may result in enhanced reaction rates. 

The precious metal composition is typically uniform in the radial and axial directions of the monolith structure, although different designs have been described in the patent literature and have even been used in some selected applications. However, much more common is a nonuniform distribution of the precious metals within the washcoat layer. One - macroscopic - example of nonuniform distribution is that the amount of one precious metal component decreases from the part of the washcoat that is in contact with the gas phase towards the part of the washcoat that is in contact with the monolith wall and eventually vice-versa for the second precious metal component. Another - microscopic - example of nonuniform distribution within the washcoat is that each precious metal component is selectively deposited on different washcoat components. These nonuniformities are intentional and are desirable for kinetic reasons or because of specific beneficial interactions between the precious metals and the washcoat oxides. The type of nonuniformity that can be achieved depends strongly on the production procedure of the catalyst. 

In accord with numerical simulations, a nonuniform distribution of the exhaust gas flow might have a positive influence on the catalyst light-off, as an increased fraction of the energy contained in the exhaust gas is concentrated on a smaller... 

The solid phase could be a reactant, product, or catalyst. In general the decision on the choice of the particle size rests on an analysis of the extra-and intra-particle transport processes and chemical reaction. For solid-catalyzed reactions, an important consideration in the choice of the particle size is the desire to utilize the catalyst particle most effectively. This would require choosing a particle size such that the generalized Thiele modulus < gen, representing the ratio of characteristic intraparticle diffusion and reaction times, has a value smaller than 0.4 see Fig. 13. Such an effectiveness factor-Thiele modulus analysis may suggest particle sizes too small for use in packed bed operation. The choice is then either to consider fluidized bed operation, or to used shaped catalysts (e.g., spoked wheels, grooved cylinders, star-shaped extrudates, four-leafed clover, etc.). Another commonly used procedure for overcoming the problem of diffu-sional limitations is to have nonuniform distribution of active components (e.g., precious metals) within the catalyst particle. 

One potential drawback of devices in which adsorption occurs on a deep bed of catalyst packed in an MAS rotor is the occurrence of a nonuniform distribution of reactant. This problem is illustrated by the Cs MAS spectra of zeolite CsZSM-5 in Fig. 5 [47]. Without adsorbates, the Cs chemical shift was —157 ppm at 298 K. An amount of methanol- C equivalent to 1 methanol molecule for every Cs in the zeolite was then introduced using the apparatus in Fig. 4, and the spectrum in Fig. 5b was obtained. The spectrum shows the consequences of a deep-bed adsorption of a slowly diffusing adsorbate. Rather than a single Cs resonance reflecting the adsorption of one equivalent of the methanol, there are two signals one at — 157 ppm originating from the bottom of the catalyst bed and a second at — 82 ppm from the top of the catalyst bed. The... 

DistriWtion of Catalyst in Pores Because of the practical requirements of manufacturing, commercial impregnated catalysts usually have a higher concentration of active ingredient near the outside than near the tip of the pores. This may not be harmful, because it seems that effectiveness sometimes is better with some kind of nonuniform distribution of a given mass of catalyst. Such effects may be present in cases where the rate exhibits a maximum as a function of... 

TABLE 23-3 Normalized Catalyst Activity Profiles Nonuniform Distribution of Catalyst on the Inner Walls of Straight Channels with Rectangular Cross Section"... 

Yeung AKL, Sebastian JM, and Varma A. Mesoporous alumina membranes Synthesis, characterization, thermal stability and nonuniform distribution of catalyst. J. Membr. Sci. 1997 131 9-28. 

---