The Catalyst Matrix

More effective binders are now used to give sufficient strength to the new high-zeohte catalysts required for the processing of feedstocks containing high levels of recycled heavy ends. 

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An active matrix provides the primary cracking sites. The acid sites located in the catalyst matrix are not as selective as the zeolite sites, but are able to crack larger molecules that are hindered from entering the small zeolite pores. The active matrix precracks heavy feed molecules for further cracking at the internal zeolite sites. The result is a synergistic interaction between matrix and zeolite, in which the activity attained by their combined effects can be greater than the sum of their individual effects [2J. 

Matrix Activity. Increasing the catalyst matrix activity increases the octane. 

The selectivity and activity of the catalyst matrix will continue to improve. The emphasis on bottoms cracking and steady reduction in the reaction residence time demands an increase in the quantity of active matrix. 

On non-zeolitic particles in the absence of a vanadium passivator, vanadium (when present at the 0.4 wt% level) makes a greater contribution to contaminant coke and hydrogen yields than nickel at constant surface area and metals loading. Incorporation of a vanadium passivator into the catalyst matrix can greatly alter the selectivity effects of vanadium, and can essentially negate its effect on non-zeolitic particles as in the case of magnesium. 

However, the thermal conductivity of the catalyst matrix is usually larger than that of the gas. This means that the external gas-catalyst heat transport resistance exceeds the thermal conduction resistance in the catalyst particles. The temperature and concentration profiles established in a spherical catalyst are illustrated for a partial oxidation reaction in Figure 4. 

In some cases, researchers have recorded the reference spectra at the same temperature and in the same gas atmosphere as were applied for the respective measurements of the sample. Particularly in the NIR range, a temperature correction can become necessary. For example, Sels et al. (2001) used a spectrum of the catalyst matrix (layered double hydroxide) suspended in methanol and aqueous H2O2. Brik et al. (2001) recorded their BaSC>4 reference spectra at various temperatures. Lu et al. (2005) recorded the spectra of their catalyst support, CaC03, as references under the conditions of the catalytic reaction, that is, in a mixture of ethene or propene in O2 and He at 473 K. Rao et al. (2004) heated their reference material to reaction temperature to generate the baseline. 

The rate of vanadium mobility in FCCU s is dependent on many operating factors. These might be whether the unit is operated in lull or partial combustion, which will effect the average oxidation state of the vanadium. Other factors will be freshness of the vanadium, presumably older vanadium has had more of an opportunity to react with the catalyst matrix and become immobile. Steam concentration in the regenerator, catalyst make-up rate, temperature, and two-stage regeneration will all effect the mobility of vanadium. 

Pore size. The pore size distribution of the catalyst matrix plays a key role in the catalytic performance of the catalyst. An optimum pore size distribution usually helps in a balanced distribution of smaller and larger pores, and depends on feedstock type and cracking conditions. The pore size distribution of the matrix changes when another component is added e.g. by adding 35-40% kaolin to a silica-alumina gel, a pore structure with a significant amount of micropores can be obtained. Figure 27.9 Pore volume. Pore volume is an indication of the quantity of voids in the catalyst particles and can be a clue in detecting the type of catalyst deactivation that takes place in a commercial unit. Hydrothermal deactivation has very little effect on pore volume, whereas thermal deactivation decreases pore volume. 

The results shown in Fig. 2 suggest that the fast deposition of additive coke occurred mainly in the top zone as ARO passed through the catalyst bed, resulting in a much higher amount of coke on catalyst than other zones. Since the additive coke has much less influence on catalyst aaivity than catalytic coke, we conclude that the major pan of additive coke formed from large molecular weight R+AT deposited on the catalyst matrix, not affecting the active acid sites of the zeolite as much as catalytic coke. 

The pore size distribution of the catalyst matrix is important for the catalytic performance. The optimal matrix pore size distribution will depend on a balance of mesopores and macropores depending on feedstock quality and reactor conditions (e.g. conventional vs. short contact time riser operation). SAM-technology catalysts (SPECTRA, RESIDCAT, ULTIMA) exhibit different pore size distributions that are matched to various types of feedstock and unit conditions. Figure 5 exhibits typical pore size distribution of SPECTRA-944, SPECTRA-444 and ULTIMA-444 catalysts. Since the only differentiating characteristic of these three catalysts is the matrix formulation, the pore size distribution variation is characteristic of the different matrix design ... 

All industrially-produced methanol is made by the catalytic conversion of synthesis gas containing carbon monoxide, carbon dioxide, and hydrogen as the main components. Methanol productivity can be enhanced by synthesis gas enrichment with additional carbon dioxide to a certain limit [14]. However, a C02—rich environment increases catalyst deactivation and shortens its lifetime, and produces water which adversely affects the catalyst matrix stability resulting in crystallite growth via hydrothermal synthesis phenomena [14]. Thus, a special catalyst has been designed to operate under high C02 conditions. This catalyst s crystallites are located on energetically stable sites that... 

The catalytic behavior of enzymes in immobilized form may dramatically differ from that of soluble homogeneous enzymes. In particular, mass transport effects (the transport of a substrate to the catalyst and diffusion of reaction products away from the catalyst matrix) may result in the reduction of the overall activity. Mass transport effects are usually divided into two categories - external and internal. External effects stem from the fact that substrates must be transported from the bulk solution to the surface of an immobilized enzyme. Internal diffusional limitations occur when a substrate penetrates inside the immobilized enzyme particle, such as porous carriers, polymeric microspheres, membranes, etc. The classical treatment of mass transfer in heterogeneous catalysis has been successfully applied to immobilized enzymes I27l There are several simple experimental criteria or tests that allow one to determine whether a reaction is limited by external diffusion. For example, if a reaction is completely limited by external diffusion, the rate of the process should not depend on pH or enzyme concentration. At the same time the rate of reaction will depend on the stirring in the batch reactor or on the flow rate of a substrate in the column reactor. 

A key issue in the activity of these catalysts concerns the specific role of the Cu and Cr active sites which are expected to be located on the surface of the catalyst particles. Since both Cu(II) and Cr(III) are paramagnetic, EPR spectroscopy can be used to identify the nature of the heterometallic species within the catalyst matrix after different thermal and chemical treatments. However, a detailed picture of the local environment of tlie transition metal center camiot be obtained by conventional continuous wave EPR spectroscopy alone. These can, nevertlieless, be obtained by pulsed EPR methods, namely the electron spin echo envelope modulation (ESEEM) tecluiique. 

Continued research led to improvements so that the first commercial high alumina catalyst was produced by American Cyanamid Co. in 1954, this time in cooperation with Shell Development Co. (53). These catalysts contained about 25% alumina and had significantly higher pore volumes than earlier formulations. These new high-alumina catalysts had higher activity with essentially the same product distribution, exhibited a slower deterioration of activity and had better attrition properties. In later years, with zeolitic catalysts, high alumina contents in the catalyst matrix were employed for stability enhancement, and then for a host of other improvements in overall performance. 

The hydrocarbon product spectrum produced by a Fischer-Tropsch (FT) catalyst is highly dependent upon the catalyst temperature and the rate of diffusion of reactants into the catalyst matrix. The reaction is highly exothermic and, when it is carried out in a fixed bed reactor, if rates of heat removal from the catalyst are not high, hotspots will form, resulting in variation in the product spectrum and also catalyst deactivation. Thin catalyst coatings coated to heat transfer surface areas in a CPR have been found to greatly enhance the yield of the desirable products per unit volume as compared to conventional fixed bed reactors. The reduction in reactor volume and reduction in associated equipment and low-pressure drop make the FT-CPR an attractive technology. 

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