Catalysts desirable characteristics

Many agents have been proposed and patented including copper sulfate (34), zinc chloride (35), ferric chloride (36), aluminum chloride (36), and phosphoms pentoxide (37) ferric chloride, zinc chloride, and phosphoms pentoxide have been most widely used. The addition of these agents may vary from 0.1 to 3%, depending upon the feedstock and the desired characteristics of the product (Table 5) and all asphalt feedstocks do not respond to catalysts in the same way. Differences in feedstock composition are important qualifiers in determining the properties of the asphalt product. The important softening point-penetration relationship, which describes the temperature susceptibiUty of an asphalt, also varies with the source of the feedstock. Straight-reduced, air-blown, and air-blown catalytic asphalts from the same cmde feedstock also vary considerably. 

Considerable work has been conducted on a water-soluble catalyst using sulfonated phosphine-modified rhodium. Details of this chemistry will be described in Chapter 5. The general concept (Figure 2.3) is to make the catalyst water soluble, then after product formation, decant the product. In order for the water-soluble catalyst to be effective, the alkene must dissolve in the aqueous layer. This has been demonstrated on a commercial basis using propene. The low solubility of higher alkenes in the aqueous catalyst layer has proven problematic. The desirable characteristic of the ligand, water solubility, is needed in the separation step but is a disadvantage in the reaction step. 

Zeolite-Based Alkylation. Zeolites have the advantage of being noncot-rosive and environmentally benign. The Mobil-Badger vapor-phase ethylbenzene process was ihe lirsl zeolite-based process to achieve commercial success. It is based on a synthetic zeolite catalyst. ZSM-5. and has the desirable characteristics of high activity, low oligomerization, and low coke formation. See also Molecular Sieves. 

The role of a catalyst is to facilitate a desired transformation, yet not be consumed as part of the reaction nor be incorporated in the final product. Catalysts afford characteristics associated with green chemistry as indicated below ... 

Many desirable characteristics of honeycomb monolithic catalyst beds were illustrated by these two examples (1) catalyst loading lower than on pellets was adequate to maintain activity (2) the axial heat transfer rate resulted in rapid heat dissipation from the reaction zone (3) a vanation of void-to-caialysi-surface ratio with new turbulent interfaces at each interval introduced a convenient control over the extent of reaction. Two other variables that were investigated are loading of catalyst over bed length and type of catalyst used however, the comparative analyses have not been as systematically demonstrated for the particular operation described in these tests. 

This brief review of catalysis needs and relevant zeolite chemistry indicates that zeolites have many desirable characteristics for fiiture advanced catalysts. These attributes should provide the impetus to maintain progress in zeolite catalysis both through evolution and through step change. 

For the present purpose, a high Rh surface area even after thermal deactivation is desirable. Therefore, one of the improved catalysts, in which a relatively high amount of Rh is loaded, GEC-01 (Pt = 3 g/1, Rh = 0.6 g/1), was selected as a long life catalyst which would have high poison resistance. Figure 6 shows the activity of model Pb-poisoned catalysts. The characteristics of the activity of Pb-poisoned catalyst are clearly observed on the conventional catalyst, but hardly observed on GEC-01. 

Industrial scale processes involving immobilized enzymes are normally carried out in fixed-bed reactors. Hence, the desired characteristics of the catalyst support are closely akin to those for the heterogenous catalysts commonly employed in the chemical industry, namely ... 

The results presented here demonstrate that well-defined model catalysts with particle sizes in the 10-100-nm regime can be efficiently processed with lithographic methods. These model catalysts possess most of the desirable characteristics that are required in fundamental heterogeneous catalysis research. There are considerable advantages, compared to model catalysts prepared by, e.g., incipient wetness impregnation methods, which are enumerated ... 

The operating conditions for the gas turbine determine many of the desirable characteristics of the catalyst used in the catalytic combustor. The catalyst must be sufficiently active to operate at the lowest possible inlet temperature, preferably below 400 °C, in order to minimize the use of the prebumer. Unless the catalyst temperature is limited by some means, the catalyst must also be able to withstand the combustor outlet temperature of 1300°C. The catalyst must also operate efficiently at the very high gas velocities in the combustor. 

In 1979 Chem Systems initiated a program to develop a liquid-entrained catalyst reactor which would provide improved contacting of syngas with the catalyst in a three phase system (ref. 38). This reactor system uses much finer catalyst particles than the fluidized bed reactor, and the catalyst-liquid slurry circulates through the reactor. The syngas can be contacted with the catalyst-liquid slurry either counter currently or co-currently. It appears that this process is more efficient than the original fluidized bed process. However, a major problem with this type of three phase system will no doubt be the development of a suitable catalyst since it is unlikely that conventional co-precipitated Cu-ZnO-A Oj catalysts will have the desired characteristics, particularly mechanical strength. 

In addition, catalysts must have high thermal stability to resist sintering, particularly where there is a need for periodic regeneration by combustion of coke deposited on the catalyst. A further desirable characteristic is the ability to resist deactivating influence of poisons, notaby sulfur compounds, which are often present in reactant streams. 

Ketones represented by 83 are a new generation of catalysts for mediating asymmetric epoxidation of alkenes by Oxone . Ketone 84 exhibits desirable characteristics in the epoxidation of c/s-alkenes. For example, it does not cause isomerization of the double bond. 

These results can be helpful to minimize the influence of lot-to-lot variations of diffent activated carbon lots in the manufacture of precious metal containing catalysts and therefore to better tailor the final catalyst to the desired characteristics. 

Foraminate catalysts appear to have a number of desirable characteristics which make for economic and efficient operations in converting alcohols to amines. The advantages gained by using them are (1) high conversions and yields are obtainable, (2) relatively high space velocities can be employed, (3) the ratio of product amines can be varied over a wide range, and (4) the cost of the catalyst is comparatively low. 

As has been indicated, the catalyst particle sites control the structural arrangement of the polymer. Unfortunately, not all of the sites on a ZN catalyst behave the same way. There are always some sites that produce a low molecular weight, highly branched material. In HDPE, this waxy material must be removed to produce a polymer with the desired characteristics. 

The most important challenge is to confer desired characteristics to the solid, aiming to design specific catalysts. In most cases, there are conventional direct procedures... 

The molecular structure and morphology of individual polypropylene resins can be readily modified at the reactor stage via new catalyst systems. Postblends of available resins with various additives promote impact resistance, controlled rheology, thermal stability, and other desirable characteristics of the polymer matrix. The incorporation of chemical coupling agents and mineral-filler or glass-... 

Synthesis of phenol-formaldehyde oligomers uses as the main raw materials phenol and formic aldehyde. Both are used in the form of aqueous solutions of 90% and 40-45% concentration, respectively [1], Since formaldehyde contains small amounts of formic acid, the condensation process can take place in the absence of catalysts, so that phenol-formaldehyde oligomers can be obtained. The reaction rate is, however, quite low when the process takes place at atmospheric pressure and a temperature around 100°C. The use of acid or basic catalysts is compulsory in industry to produce phenol-formaldehyde oligomers of desired characteristics. Novolac type oligomers are obtained in acid catalysis in basic catalysis, resol type oligomers are produced. 

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