Activity and selectivity

The relatively high activities of these catalysts can in most cases be attributed to the high dispersions of the active species. These are normally incorporated as cations via an ion-exchange process and thus remain bound onto the extensive inner surface of the zeolites by electrostatic forces. The selectivities observed, for example, in oligomerization reactions where, in general, dimers are formed in preference to higher oligomers, may be a direct consequence of the spatial limitations imposed on transition-state complexes within the small zeolite cavities. 

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In 1968 a new methanol carbonylation process using rhodium promoted with iodide as catalyst was introduced by a modest letter (35). This catalyst possessed remarkable activity and selectivity for conversion to acetic acid. Nearly quantitative yields based on methanol were obtained at atmospheric pressure and a plant was built and operated in 1970 at Texas City, Tex. The effect on the world market has been exceptional (36). 

Propylene Oxidation. The propylene oxidation process is attractive because of the availabihty of highly active and selective catalysts and the relatively low cost of propylene. The process proceeds in two stages giving first acrolein and then acryUc acid (39) (see Acrolein and derivatives). 

The bulk stmcture of the catalyticaHy active phase is not completely known and is under debate in the Hterature (125,131—133). The central point of controversy is whether (Valone or in combination with other phases is the most catalyticaHy active for the conversion of butane to maleic anhydride. The heart of this issue concerns the role of stmctural disorder in the bulk and how it arises in the catalyst (125,134,135). Most researchers agree that the catalysts with the highest activity and selectivity ate composed mainly of (Vthat exhibits a clustered or distorted platelet morphology (125). It is also generaHy acknowledged that during operation of the catalyst, the bulk oxidation state of the vanadium in the catalyst remains very close to the +4 valence state (125). 

The H2SO4 concentration is controlled above 90% to provide the optimum activity and selectivity. Purity is maintained by the withdrawal of system acid and replacement with fresh 98% acid. The spent acid is returned to an acid manufacturiag plant for reprocessiag. 

Most soHd catalysts used on a large scale are porous inorganic materials. A number of these and the reactions they catalyze are summarized ia Table 1 (10). Catalysis takes place as one or more of the reactants is chemisorbed, chemically adsorbed, on the surface and reacts there. The activity and selectivity of the catalyst depend strongly on the surface composition and stmcture. 

Catalyst testing and evaluation have been revolutionized by computers, automated test reactors, and analytical methods. With modem equipment, researchers can systematically prepare and screen many catalysts in a short time and efftciendy deterrnine, not only the initial catalytic activity and selectivity, but also the stabiUty and the appearance of trace products that may indicate some new catalytic properties worthy of further development. 

Triethyl aluminum, complexed with another electron donor, typically ethyl -anisate [94-30-4J, was used as cocatalyst with the FT-1 catalyst and acted to reduce and stabilize the active titanium-containing catalytic site. The early versions of the FT-1 catalyst required extremely high molar ratios (>400 1) of aluminum to titanium to obtain satisfactory activity and selectivity to isotactic polymer. This resulted in excessively high aluminum residues in the polymer. Later versions of the FT-1 catalyst attained much higher activity. 

Volume 67 Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis. 

Aside from the above reforming reactions, a small amount of feed components are converted to polymeric hydrogen deficient products which deposit on the catalyst as "coke." A coke buildup results in activity and selectivity loss which ultimately requires catalyst regeneration. In semi-regenerative operation, the coking rate is maintained at a low level to provide cycles of at least three to six months. In cyclic units, coking conditions are inherently much more severe so that frequent regenerations are required. 

Control of chemical activity and selectivity by catalytic antibodies in reactions of heterocycles 95MI11. 

The different furanones 104 were tested for their potency as inhibitors of PGE2 production both in transfected Chinese hamster ovarian (CHO) cells expressing human COX-2 and in human whole blood. Compound 104r proved to be an orally active and selective COX-2 inhibitor that is devoid of the ulcerogenic effect at >100 times the dose for antiinflammatory, analgesic, and antipyretic effects (99BMC3187). 

A few catalysts display activity and selectivity levels suflicteady bigli for application in organic syntliesis. Hieir utilization in tlie syntliesis of a number of diiral budding blocks and target molecules is emerging as summarized in tlie second pan of tills diapter. 

Fenoldopam (76) is an antihypertensive renal vasodilator apparently operating through the dopamine system. It is conceptually similar to trepipam. Fenoldopam is superior to dopamine itself because of its oral activity and selectivity for dopamine D-1 receptors (D-2 receptors are as.sociated with emesis). It is synthesized by reduction of 3,4-dimethoxyphenylacetonitrile (70) to dimethoxyphenethylamine (71). Attack of diis last on 4-methoxystyrene oxide (72) leads to the product of attack on the epoxide on the less hindered side (73). Ring closure with strong acid leads to substituted benzazepine 74. O-Dealkylation is accomplished with boron tribromide and the catechol moiety is oxidized to the ortho-quinone 75. Treatment with 9NHC1 results in conjugate (1,6) chloride addition and the formation of fenoldopam (76) [20,21]. 

Many transition metal-catalyzed reactions have already been studied in ionic liquids. In several cases, significant differences in activity and selectivity from their counterparts in conventional organic media have been observed (see Section 5.2.4). However, almost all attempts so far to explain the special reactivity of catalysts in ionic liquids have been based on product analysis. Even if it is correct to argue that a catalyst is more active because it produces more product, this is not the type of explanation that can help in the development of a more general understanding of what happens to a transition metal complex under catalytic conditions in a certain ionic liquid. Clearly, much more spectroscopic and analytical work is needed to provide better understanding of the nature of an active catalytic species in ionic liquids and to explain some of the observed ionic liquid effects on a rational, molecular level. 

Obviously, the ionic liquid s ability to dissolve the ionic catalyst complex, in combination with low solvent nucleophilicity, opens up the possibility for biphasic processing. Furthermore it was found that the biphasic reaction mode in this specific reaction resulted in improved catalytic activity and selectivity and in enhanced catalyst lifetime. 

Further studies of Pseudomonas sp. lipase revealed a strong influence of the water content of the reaction medium (Entry 20) [48]. To be able to compare the enzyme activity and selectivity as a function of the water present in solvents of different polarities, it is necessary to use the water activity (a ) in these solvents. We used the... 

Hydrogenation of olefinic unsaturation using ruthenium (Ru) catalyst is well known. It has been widely used for NBR hydrogenation. Various complexes of Ru has been developed as a practical alternative of Rh complexes since the cost of Ru is one-thirtieth of Rh. However, they are slightly inferior in activity and selectivity when compared with Rh catalyst. 

In the preceding section, it has been shown that considerable attention has been devoted to palladium as a heterogeneous catalyst. The present section describes the homogeneous palladium catalysts developed for hydrogenation of NBR. The main drive behind the development of various catalyst systems is to find suitable substituents of the Rh catalyst. Palladium complexes are much cheaper as compared with Rh and exhibit comparable activity and selectivity to Rh and Ru complexes. 

The product distrihution is influenced hy the catalyst properties as well as the various reaction parameters. The catalyst activity and selectivity are functions of acidity, crystalline size, silica/alumina ratio, and even the synthetic procedure. Since the discovery of the MTG process. 

Currently, zeolites, especially those of ZSM-5 type, are preferred for their higher activities and selectivities. They are also more stable thermally. Modifying ZSM-5 zeolites with phosphorous, boron, or... 

The metals in the FCC feed have many deleterious effects. Nickel causes excess hydrogen production, forcing eventual loss in the conversion or thruput. Both vanadium and sodium destroy catalyst structure, causing losses in activity and selectivity. Solving the undesirable effects of metal poisoning involves several approaches ... 

Compared to amorphous silica-alumina catalysts, the zeolite catalysts are more active and more selective. The higher activity and selectivity translate to more profitable liquid product yields and additional cracking capacity. To take full advantage of the zeolite catalyst, refiners have revamped older units to crack more of the heavier, lower-value feedstocks. 

The breakthrough in FCC catalyst was the use of X and Y zeolites during the early 1960s. The addition of these zeolites substantially increased catalyst activity and selectivity. Product distribution with a zeolite-containing catalyst is different from the distribution with an amorphous silica-alumina catalyst (Table 4-3). In addition, zeolites are 1,000 times more active than the amorphous silica alumina catalysts. 

The cations that replace the sodium ions determine the catalyst s activity and selectivity. Zeolites are synthesized in an alkaline environment such as sodium hydroxide, producing a soda-Y zeolite. These soda-Y zeolites have little stability but the sodium can be easily... 

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