Porous Catalysts

Pt/Ru Catalyst Polymer Pt Catalyst Porous Gas Layer Electrolyte Layer Diffusion Membrane Electrode... 

Fig. 50. Contribution of energy for crack propagation to the total fracture energy for sol-vent-modified epoxies prepared via CIPS with 1 wt % catalyst porous, and macroporous epoxies prepared via kinetically controlled CIPS with 1 wt % catalyst...

The changing catalyst porous texture is modelled using a Bethe network originating from percolation concepts. Preliminary results indicate that reliable metal deposition profiles and catalyst life-time predictions can be made by the proposed catalyst deactivation model. 

Prerequisite for hydrodemetallisation is the diffusion of the large porphyrins into the catalyst porous texture prior to the sequential reaction mechanism. Diffusion of these large molecules can be limited by geometric exclusion and hydrodynamic drag. When the solute molecular size is significant as compared to the pore size, a restrictive factor can be introduced to account for the reduction in difftisivity. As a consequence, clarification of detailed HDM reaction kinetics may be obscured by diffusion limitations. 

The phenomena relevant to HDM catalyst deactivation are intrinsic reaction kinetics, restrictive intraparticle diffusion and (changing) catalyst porous texture. 

We are not going to deal with all these examples of application of percolation theory to catalysis in this paper. Although the physics of these problems are different the basic numerical and mathematical techniques are very similar. For the deactivation problem discussed here, for example, one starts with a three-dimensional network representation of the catalyst porous structure. Systematic procedures of how to map any disordered porous medium onto an equivalent random network of pore bodies and throats have been developed and detailed accounts can be found in a number of publications ( 8). For the purposes of this discussion it suffices to say that the success of the mapping techniques strongly depends on the availability of quality structural data, such as mercury porosimetry, BET and direct microscopic observations. Of equal importance, however, is the correct interpretation of this data. It serves no purpose to perform careful mercury porosimetry and BET experiments and then use the wrong model (like the bundle of pores) for data analysis and interpretation. 

Metal deposition in hydrotreating of heavy oils is one of the most important phenomenon causing catalyst deactivation. Present work focuses on the modeling of hydrodemetallisation catalyst deactivation by model compound vanadyl-tetraphenylporphyrin. Intrinsic reaction kinetics, restrictive diffusion and the changing catalyst porous texture are the relevant phenomena to describe this deactivation process. The changing catalyst porous texture during metal depositon can be described successfully by percolation concepts. Comparison of simulated and experimental metal deposition profiles in catalyst pellets show qualitative agreement. 

The general approach for modelling catalyst deactivation is schematically organised in Figure 2. The central part are the mass balances of reactants, intermediates, and metal deposits. In these mass balances, coefficients are present to describe reaction kinetics (reaction rate constant), mass transfer (diffusion coefficient), and catalyst porous texture (accessible porosity and effective transport properties). The mass balances together with the initial and boundary conditions define the catalyst deactivation model. The boundary conditions are determined by the axial position in the reactor. Simulations result in metal deposition profiles in catalyst pellets and catalyst life-time predictions. 

Catalyst Porous Texture. The initial porous texture of a catalyst pellet... 

Figure 4. Catalyst porous texture as described by the cubic tessellation model and the Bethe network a- by cubic tessellation model, b- Bethe network.

Percolation concepts can be successfully applied to describe the changing catalyst porous texture during HDM. 

The results show that catalyst porous structure has a strong impact both on the structure of Co species and on the catalytic behavior of the catalysts. The size of supported cobalt particles is related to the size of silica pores supports with small pores stabilize smaller... 

Besides the compact membrane catalysts described in Section II, there are two types of composite membrane catalyst porous and nonporous. Composite catalyst consists of at least two layers. The first bilayered catalyst was prepared by N. Zelinsky [112], who covered zinc granules with a porous layer of palladium sponge. The sponge became saturated with the hydrogen evolved during hydrochloric acid reaction with zinc and at room temperature actively converted hydrocarbon iodates into corresponding hydrocarbons. 

A previously proposed state-of-the-art catalyst deactivation model, based on percolation concepts (5, 6), is proposed to tackle the problem of the changing catalyst porous texture (see Figure 2). 

Ni/Si02 catalysts (18 wt. % Ni), prepared by incipient wetness impregnation [9] and deposition-precipitation [10-12], were reduced by TPR up to 700°C (INi) and 900°C (DPNi) with a 5 % H2/Ar mixture, i.e., up to the minimum temperature required to obtain full nickel reduction [9,10]. For both catalysts, porous silica Spherosil XOA400 (Rh6ne Poulenc, A 400 m. g i) was used as a support. 

Supported liquid-phase catalysts, porous solids with liquids held in the narrow pores, function even at high temperatures. 

The methods of soft chemistry include sol-gel, electrochemical, hydrothermal, intercalation and ion-exchange processes. Many of these methods are employed routinely for the synthesis of ceramic materials. - There have been recent reviews of the electrochemical methods, intercalation reactions, and the sol-gel technique. The sol-gel method has been particularly effective with wide-ranging applications in ceramics, catalysts, porous solids and composites and has given rise to fine precursor chemistry. Hydrothennal synthesis has been employed for the synthesis of oxidic materials under mild conditions and most of the porous solids and open-framework structures using organic templates are prepared hydro-thermally. The advent of supramolecular chemistry has started to make an impact on synthesis, mesoporous solids being well known examples. ... 

Pt/Ru Catalyst pt Catalyst Porous Gas Layer Polymer Layer Diffusion Electrol)rte Electrode... 

Clay is a commonly occurring porous material that is frequently used in engineering endeavors, but there are also other important porous materials, such as those used in some types of electrodes and as catalysts. Porous media should be examined giving due regard to both the physics and chemistry of the constituents. The purpose of this book is to present an approach that examines porous media from both these aspects, outlining a procedure that combines microscale (or even nanoscale) characteristics with macroscale behavior. 

The first one is a general methodology developed by Abu-Reziq et al [26,27] for the conversion of fully hydrophobic catalytic reactions - the catalyst, the substrate, and the product, are all hydrophobic - into a catalytic reaction that is carried out in water, eliminating the need for organic solvents. The method is based on a three-phase system composed of an emulsion (oil in water of the substrate and product molecule) and a solid (the catalysts), and was termed the EST (emulsion/solid transport) process. The idea (Figure 31.11) relies on the transport of hydrophobic substrates to an entrapped catalyst, and the transport of the resulting product from the catalyst porous solid back into the bulk. Specifically, the catalyst is entrapped inside a hydrophobically modified porous sol-gel matrix the hydrophobic substrate for that catalyst is emulsified in water in the presence of a suitable surfactant and the powdered catalytic sol-gel material is dispersed in that emulsion. Upon contact of the surfactant with the hydrophobic interface of the sol-gel matrix, it reorients and spills the substrate into the pores... 

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