Size-specific catalyst

Physical properties of catalysts also may need to be checked periodically, includiug pellet size, specific surface, porosity, pore size and size distribution, and effective diffusivity. The effectiveness of a porous catalyst is found by measuring conversions with successively smaller pellets until no further change occurs. These topics are touched on by Satterfield (Heterogeneous Cataly.sls in Jndustiial Practice, McGraw-Hill, 1991). 

The bias of a given diene for RCM depends on the ring size formed, on conformational constraints of the substrate, on the presence of functional groups, and on interactions with the specific catalyst used. 

A series of experiments were performed using various sizes of catalyst spheres. The reaction was first order irreversible. The first two columns of the table record the diameter dp in cm and the rate in mol/(h)(cc). The surface concentration was Cs = 0.0002 mol/cc. Find the true specific rate and the effective diffusivity. 

Besides the 29Si and 27 A1 NMR studies of zeolites mentioned above, other nuclei such as H, 13C, nO, 23Na, 31P, and 51V have been used to study physical chemistry properties such as solid acidity and defect sites in specific catalysts [123,124], 129Xe NMR has also been applied for the characterization of pore sizes, pore shapes, and cation distributions in zeolites [125,126], Finally, less common but also possible is the study of adsorbates with NMR. For instance, the interactions between solid acid surfaces and probe molecules such as pyridine, ammonia, and P(CH3)3 have been investigated by 13C, 15N, and 31P NMR [124], In situ 13C MAS NMR has also been adopted to follow the chemistry of reactants, intermediates, and products on solid catalysts [127,128],... 

Nonhazardous spent catalysts can be also reused in the production of bricks. Specifically, catalysts are crushed and decreased in size to form alumina/silica sand that can replace the sand used in the manufacture of bricks. Moreover, spent fluidized-bed catalysts can be reused as cement components. Specifically, the catalyst is used to replace clinker in the final grinding (Cardenosa el al., 1992). For the disposal of catalysts, the techniques presented in Section 4.3 can be largely applied. 

The industrial rates obtained earlier from the pseudohomogeneous model actually include diffusional limits and are suitable for the specific reactor with the specific catalyst particle size for which the data was extracted. Such pseudohomogeneous models do not account explicitly for the catalyst packing of the reactor. On the other hand, heterogeneous models account for the catalyst explicitly by considering the diffusion of reactants and of products through the pores of the catalyst pellet. 

Solvent Tempcraliiic Catalyst Reagent size Specificity... 

We now consider an adiabatic reactor of fixed size or catalyst weight and investigate what happens as the feed temperature is varied. The reaction is reversible and exothermic. At one temperature extreme, using a very high feed temperature, the specific reaction rate will be large and the reaction will proceed rapidly, but the equilibrium conversion will be close to zero. Consequently, very little product will be formed. A plot of the equilibrium conversion and the conversion calculated from the adiabatic energy balance,... 

Studies of catalysts deactivation by coke are abundant in the literature most of them are usually conducted at high temperatures (around 500°C) using metal catalysts supported on oxides with low surface area such as silica, aluminas or silica-alumina [2 and references therein]. The deactivation by coke of zeolite catalysts has also been studied and such studies have mostly been done for high temperature reactions such as the conversion of n-hexane or the isomerization of xylenes [2,4]. However, low temperature coke formation (20-25°C) combining the effect of high acidity and size specificity for a high coking component such as nickel, has not yet been considered from the point of view of the presence of compounded effects of crystalline structure and location of metal particles. 

For every commercial catalyst an optimal combination of unit operation sequence exists for the manufacture of that specific catalyst and there will for each unit operation exist preferential process equipment, i.e. fluid bed calciner for calcination. The sequence of unit operations with the special selection of process equipment and all process parameters forms the know-how for manufacturing a catalyst product of large commercial value. But know-how does not mean that you always know why the desired properties are obtained due to the insufficient scientific characterisation of the catalyst material as described above under 2.1. Even small adjustments of the process can change strength, pore size distribution, bulk density, crystallite size etc. of the product and, thus, harm the performance in the industrial reactor. It has normally been costly and time-consuming to reach the final recipe and, therefore, all catalyst companies want to keep it secret. If a single unit operation is changed it will often influence the optimisation of most of the other unit operations, and much of the development will have to be redone. 

A controversial issue related to cobalt catalysts in Fisher-Tropsch synthesis is the structure-sensitive character of this reaction. Iglesia and co-workers [126,127] reported a large increase in activity when the cobalt particle size was decreased from 200 nm to 9 nm, whereas the specific activity [turnover frequency (TOF)] was not influenced by the cobalt particle size. However, other authors have reported that the TOF suddenly decreased for catalysts with cobalt particle sizes smaller than 10 nm [122,128]. Bezemer et al. [125] were the first to investigate the influence of cobalt particle size in the range 2.6 to 27 nm on performance in Fischer-Tropsch synthesis on well-defined catalysts supported on carbon nanofibers. It was found that the TOF for CO hydrogenation was independent of cobalt particle size for catalysts with particles larger then 6 nm (at atmospheric pressure) or 8 nm (at 35 bar). But both the TOF and the C5+ selectivity decreased for catalysts with smaller particles. It was proposed that the cobalt particle size effects could be attributed to a strucmre-sensitivity characteristic of the reaction, together with a CO-indnced reconstmction of the cobalt surface. 

The right choice of supporting material as well as the choice of suitable properties (pore size, specific surface, chemical surface composition) are important factors influencing the immobilization of the metallocene catalyst and the fragmentation of the support during polymerization. Commercially applied porous silica gels are prepared by neutralization of aqueous alkali metal silicate with acid. The pore structure and pore size distribution can be controlled by the type of chemical reaction and experimental conditions. ... 

We have strived to include as many examples of catalysts and their scope as possible within the confines of a book of this size, while maintaining readability of this text and its use as a teaching aid. Further coverage of specific catalysts can be obtained from the many comprehensive reviews referenced. 

Preparation of a titanium-aluminium (Ti-Al) catalytic system without modifying diene additives (piperylene) results in the formation of relatively large catalyst particles (with an average radius of about 3-4 micrometres). The turbulent mode hydrodynamic impact on a two-component Ti-Al catalytic system does not influence the size of catalyst particles (and therefore its specific surface) compared with traditional process technology (Figure 3.12, curves 1 and 2). Preparation of Ti-Al with piperylene additives (three-component catalytic system) results in a decrease of the catalyst particles size to r 1.5 micrometres... 

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