Selective promotion effect

First of all, given the well recognised promoting effects of Lewis-acids and of aqueous solvents on Diels-Alder reactions, we wanted to know if these two effects could be combined. If this would be possible, dramatic improvements of rate and endo-exo selectivity were envisaged Studies on the Diels-Alder reaction of a dienophile, specifically designed for this purpose are described in Chapter 2. It is demonstrated that Lewis-acid catalysis in an aqueous medium is indeed feasible and, as anticipated, can result in impressive enhancements of both rate and endo-exo selectivity. However, the influences of the Lewis-acid catalyst and the aqueous medium are not fully additive. It seems as if water diminishes the catalytic potential of Lewis acids just as coordination of a Lewis acid diminishes the beneficial effects of water. Still, overall, the rate of the catalysed reaction... 

Among various methods to synthesize nanometer-sized particles [1-3], the liquid-phase reduction method as the novel synthesis method of metallic nanoparticles is one of the easiest procedures, since nanoparticles can be directly obtained from various precursor compounds soluble in a solvent [4], It has been reported that the synthesis of Ni nanoparticles with a diameter from 5 to lOnm and an amorphous-like structure by using this method and the promotion effect of Zn addition to Ni nanoparticles on the catalytic activity for 1-octene hydrogenation [4]. However, unsupported particles were found rather unstable because of its high surface activity to cause tremendous aggregation [5]. In order to solve this problem, their selective deposition onto support particles, such as metal oxides, has been investigated, and also their catalytic activities have been studied. 

Fe/4.6Si/1.44K/2.0Cu and 100Fe/4.6Si/5.0K/2.0Cu catalysts were found to be 0.92 and 0.94, respectively. Since the amount of Cu was essentially identical in both catalysts and the steady CO conversion rate of the 100Fe/4.6Si/5.0K/2.0Cu catalyst was similar to that of the 100Fe/5.1Si/5.0K catalyst, we postulate that the difference in CO conversion level between the two Cu-promoted catalysts is due primarily to a difference in the K (and perhaps Si) content. In general, selectivities in FTS are compared at a similar CO conversion level for each catalyst. However, wide variations in the CO conversion level in the current study make it difficult to evaluate the effects of different promoters on product selectivity. In spite of the differences in CO conversion rates, an effort was made to qualitatively compare the promotional effect of Cu and K on product selectivity at similar, or nearly similar, CO conversion levels. 

The readsorption and incorporation of reaction products such as 1-alkenes, alcohols, and aldehydes followed by subsequent chain growth is a remarkable property of Fischer-Tropsch (FT) synthesis. Therefore, a large number of co-feeding experiments are discussed in detail in order to contribute to the elucidation of the reaction mechanism. Great interest was focused on co-feeding CH2N2, which on the catalyst surface dissociates to CH2 and dinitrogen. Furthermore, interest was focused on the selectivity of branched hydrocarbons and on the promoter effect of alkali on product distribution. All these effects are discussed in detail on the basis... 

Based on these observations and several other experimental results with cofeeding of ethene and 1-alkene,9 the selectivity of branched hydrocarbons,11 and the different promoter effects of Li-, Na-, K-, and Cs-carbonate/oxide,1213 a novel mechanism has been proposed that is consistent with these various experimental results.14 The formulation of this mechanism follows the knowledge of analogous reactions in homogeneous catalysis and gives a detailed insight in the crucial step of C-C linkage formation. The aim of this work is to discuss in detail these experiments and their relationship to the proposed mechanism. 

Alkalization of iron catalysts causes two different effects. The selectivities of 1-alkenes are raised and both the growth probability a2 and the fraction f2 are markedly increased, as already shown in Figure 11.2. Detailed studies on the promoter effect of alkali have revealed the effect on 1-alkene selectivity to saturates at 1 mass% of K2C03, while the effect on f2 already begins at 0.2 mass% of K2C03.1213 This difference points to specific active sites in Fischer-Tropsch syn-... 

One of the most active and selective catalysts in this kind of reaction is undoubtedly Ir/support, as recently demonstrated by Jacobs and coworkers [276], Therefore, by combining the carbonyl affinity of metallic iridium with the promotion effect of the H-fl zeolite, which is a strong Bronsted acid, one can reduce a large variety of unsaturated ketones and aldehydes to allylic alcohols with high conversions, selectivities, and diasteieoselectivities. 

H. Tanaka, S.-I. Ito, S. Kameoka, K. Tomishige, and K. Kunimori, Promoting effect of potassium in selective oxidation of CO in hydrogen-rich stream on Rh catalysts, Catal. Commun. 4, 1-4... 

Another a, i-unsaturated aldehyde analyzed is cinnamaldehyde. Its liquid-phase hydrogenation has been studied in our research group [20, 51, 94], using Pt, Ni and Cu-based tin-modified hi- and organobimetaUic catalysts (in all cases with Si02 as support). The catalytic results obtained showed that in aU cases there was a marked promoting effect of Sn on the selectivity to cinnamic alcohol (UOL). The specific modification of the monometallic systems due to Sn addition from the application of SOMC/M markedly increases the selectivity to UOL, especially in the case of Ni, where it goes from zero selectivity for the monometallic to 25% for the NiSn catalyst. Pt-based systems modified by Sn yield the best Suol values. 

Table I shows the effects of Mel/DME and CO/DME ratios in the feed gas on product yields. With increasing Mel/DME ratio both methyl acetate yield and selectivity increased. The yield of methyl acetate increased with an increase in the CO/DME ratio whereas its selectivity decreased. In the case of methanol carbonylation on Ni/A.C. catalyst, the product yield and selectivity were strongly affected by CO/MeOH ratio but not by Mel/MeOH ratio (14-16). The promoting effect of methyl iodide on the methanol carbonylation reached a maximum at a very low partial pressure, that is 0.1 atm or lower. However, both CO/DME and Mel/DME ratios were important for regulating the product yield and selectivity of the dimethyl ether carbonylation. This suggests that the two steps, namely, the dissociative adsorption of methyl iodide on nickel (Equation 4) and the insertion of CO (Equation 5) are slow in the case of dimethyl ether reaction.

These species show different promoting effects on the activity and selectivity of the homologation of methyl acetate with CO + H2 (carbonylation to acetic acid, homologation to ethyl acetate and hydrogenation to methane) (5). 

Linic and co-workers provided two additional examples of modifying selectivity for the ethylene epoxidation reaction. They demonstrated the promotional effect of Cs on EO formation using DFT calculations Cs atoms increase AEa by up to 0.2 eV vs. Ag only, via an induced electric field that interacts with the different dipoles of the two TS s." More recently Christopher et al. synthesized and tested (100) facet-dominated Ag nanowire catalysts, based on the DFT results that AEa is ca. 0.1 eV larger on Ag(lOO) than on Ag(lll), because of the extra elongation of the Ag-adsorbate bonds required to form the TS to AC on Ag(lOO). The Ag nanowire catalysts were indeed more selective than conventional Ag catalysts, in which Ag particles mainly exposes the (111) facet." Incidentally,... 

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