Catalysts modifier effect

The same type catalyst modified with boron (41), magnesium (42), or phosphoms (43) to reduce the pore size can be used to alkylate toluene with ethylene to produce predominantly -ethyltoluene. Since -ethyltoluene [622-96-8] has the smallest effective diameter of the ethyltoluene isomers, the selectivity to this isomer is favored because it can most easily escape the ZSM-5 pore stmcture. For the same reason, the alkylation of toluene [108-88-3] to xylene [106 2-3] also is favored over the usual acid catalyzed equiHbrium mixture of isomers when it is carried out over magnesium- or phosphoms-modified ZSM-5 (44). 

The Photocatalytic Effects of Ti02 based catalysts modified by transition metals for removal of pollutants in liquid phase... 

Figure 7. Effect of HMTA auxiliary. Selectivity to 2-KLG as a function of L-sorbose conversion over 5 wt% Pt/C. Unmodified catalyst ( ) catalyst modified with HMTA (O).

Raney-nickel catalysts - The effect of NH3 and base modifier on the activity and selectivity of RNi-C catalyst is shown in Table 1. The addition of NH3 significantly decreased the pseudo first-order rate constants, the conversion of RCN and the selectivity to R2NH. Upon increasing the reaction time (t) on... 

Under relatively mild conditions the Ru/C catalyst poisoned with Sn (lines 1 and 2), the Ir/C catalyst (lines 14 and 15), and the Raney-cobalt catalyst modified with CoCl2 (line 19) seem likely systems to try when initiating a search for an effective method for selectively hydrogenating the C=0 bond in an a, 3-unsaturated aldehyde. 

We named this catalyst modified catalyst instead of poisoned catalyst, because the effect of treatment with the chelate reagents on the activity of the catalyst depended on the nature of the substrate. 

Since RNi contains a large amount of aluminum and 2-hydroxy acid is a strong chelating reagent, one difference between RNi and RNiA could be ascribed to their difference in aluminum contents. Table XII (49) shows the correlation between the aluminum content and the EDA of those catalysts modified with tartaric acid. The aluminum content of RNi was decreased by pretreatment with hydroxy acid. Moreover, reduced nickel prepared from NiO (HNi-1) gives an effective modified catalyst and its pretreatment with hydroxyacid does not affect its EDA. 

In our discussion of micellar catalysts in Chapter 8, we noted that effective catalysts have two features the ability to accelerate the rate of a reaction and the ability to do so selectively. Chemistry students are familiar with the general notion that catalysts modify the mechanistic path of a reaction in such a way as to lower the activation energy and make the conversion of reactants to products more probable. One of the easiest places to see this is in reactions of diatomic gas molecules. In the gas phase the mechanism for the reaction of hydrogen and oxygen to form water involves the following steps, among others ... 

Even higher linearities are characteristic of modified rhodium catalysts. The effect is most pronounced when hydroformylation is carried out in molten triphe-nylphosphine,41 resulting in linearities as high as 9. The active catalytic species with two phosphine ligands (6) ensures the increased selectivity through steric effects. Platinum complexes with bidentate ligands exhibit the highest linearity to form linear aldehydes with 99% selectivity.27... 

The effect of electrons in the unsaturation in the ultimate and penultimate monomer groups on the ionicity of the catalysts has been shown by Wilke (133). His work shows that, when the catalyst contains triphenylphosphine, the added increasing nucleophilic effect of the double bonds converts the catalyst to an ethylene incorporating specie. After incorporation of the ethylene, cyclization and reduction of the metal occurs to produce cyclodecadiene and cydohexene. This effect is analogous to that shown earlier (88) of a diene system complexing with the catalyst modifying the ionicity to favor ethylene incorporation. 

A Raney Ni catalyst modified by tartaric acid and NaBr is fairly effective for enantioselective hydrogenation of a series of (3-keto esters (Scheme 1.41) [203a,214,215]. The enantio-discrimi-nation ability of the catalyst is highly dependent on the preparation conditions such as pH (3—4), temperature (100°C), and concentration of the modifier (1%). Addition of NaBr as a second modifier is also crucial. Ultrasonic irradiation of the catalyst leads to even better activity and enantioselectivity up to 98% ee [214d-f. The Ni catalyst is considered to consist of a stable, selective and weak, nonselective surface area, while the latter is selectively removed by ultrasonication. 

Nature of the catalyst and its concentration. In the case of hydrolysis/polycondensation of 40 and 73, the effect of the nature and concentration of the catalyst was investigated by using one of the following catalysts TBAF, HC1, NaOH, DMAP or NMI. Changing the catalyst modifies both the specific surface area and the relative percentages of micro-and mesopores (Table 6)149,152,155... 

Heterogeneous catalysts modified by the addition of chiral substances have been used to hydrogenate olefins asymmetrically, but only a few effective chiral heterogeneous catalyst systems have been found. Palladium deposited on silk fibroin was used to hydrogenate 4-benzylidene-2-methyl-5-oxazolone asymmetrically to give, after hydrolysis, optically active phenylalanine (Fig. lc). The optical purity1 of the product was found to be dependent on the origin of the fibroin and its chemical pretreatment (4-6). 

It was found by Nis XPS studies of pyridine-adsorbed samples that after deactivation the surface acidic function changes in a different manner with the bulk acidity measured by infrared characteristic absorption bands of pyridine adsorbed samples [7], which would suggest different distributions of the acidic properties in the sample catalysts. The effects of additive elements on the overall acidic features of modified zeolite catalysts are dependent on sample pretreatment and/or reaction condition, which will contribute differently to the induced acidity on the surface and in bulk bifunctional properties, as examined by the reaction of n-heptane shown in Figure 1. 

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