Catalytic reactions, activity patterns

Structural changes of alkaline earth and alkali ions promoted alkaline earth oxides as well as other catalytic systems after eatalytic reaction have been studied by XRD, IR, DTA and other analytical techniques (Table 1 ). The formation of hydroxides and basic carbonates has been identified in XRD patterns for the catalyst samples consisting of either CaO or MgO. IR spectroscopic studies also support the formation of hydroxides layer after catalytic reaction. Activities and selectivities of different samples were shown in Table- 2. 

The catalytic pyrolysis of R22 over metal fluoride catalysts was studied at 923K. The catalytic activities over the prepared catalysts were compared with those of a non-catalytic reaction and the changes of product distribution with time-on-stream (TOS) were investigated. The physical mixture catalysts showed the highest selectivity and yield for TFE. It was found that the specific patterns of selectivity with TOS are probably due to the modification of catalyst surface. Product profiles suggest that the secondary reaction of intermediate CF2 with HF leads to the formation of R23. 

The activity of the transition metals, especially for the chemisorption of molecular hydrogen and in hydrogenation reactions has been correlated, in the past, with the existence of partially filled d bands. Many alloy studies were prompted by the expectation that catalytic activity would change abruptly once these vacancies were filled by alloying with a group IB metal. Examples of such behavior have been collected together for the Pd-Au system (1). It is to be expected also that various complications might superimpose on the simple activity patterns observed for primitive... 

The model shown in Scheme 2 indicates that a change in the formal oxidation state of the metal is not necessarily required during the catalytic reaction. This raises a fundamental question. Does the metal ion have to possess specific redox properties in order to be an efficient catalyst A definite answer to this question cannot be given. Nevertheless, catalytic autoxidation reactions have been reported almost exclusively with metal ions which are susceptible to redox reactions under ambient conditions. This is a strong indication that intramolecular electron transfer occurs within the MS"+ and/or MS-O2 precursor complexes. Partial oxidation or reduction of the metal center obviously alters the electronic structure of the substrate and/or dioxygen. In a few cases, direct spectroscopic or other evidence was reported to prove such an internal charge transfer process. This electronic distortion is most likely necessary to activate the substrate and/or dioxygen before the actual electron transfer takes place. For a few systems where deviations from this pattern were found, the presence of trace amounts of catalytically active impurities are suspected to be the cause. In other words, the catalytic effect is due to the impurity and not to the bulk metal ion in these cases. 

In a number of cases, a common pattern for catalytic activity seems to be developing, where the zinc centre, already four-coordinate with three protein ligands and an aqua group, binds the substrate to give a five-coordinate complex. The zinc-bound aqua group is ionized to give hydroxide, which participates in the catalytic reaction. Changes in coordination number of the zinc from four to five and back to four appear to be a common feature of zinc metalloenzymes. 

The system ESKA (Expert System for Selection and Optimization of Catalysts [20]) was designed at BASF specifically for hydrogenation reactions. The main component of a catalyst is proposed on the basis of activity patterns which describe the applicability of catalysts for different types of hydrogenations. The system also is able to propose secondary catalyst components and, if necessary, a support material which is stable under reaction conditions and does not have any undesired catalytic properties. Based on heuristics for required as well as undesired side reactions and for different catalytically active components, the system also proposes reaction conditions as temperature, pressure, the solvent or the pH. 

A particularly difficult problem appeared to be the systems of two active metals [27,28]. While, in several cases [27], the product patterns of the catalytic reaction show the presence of both active metals (Pt-Re, Pt-Co, Pt-Ir, Pd-Ni) in the surface, the chemisorption data, such as e.g. IR spectra of adsorbed CO, are less definite on this point. Recently Joyner and Shipiro [28] even speculated that — at least with Pt alloys — it is only Pt which forms the surface. Important information on the last mentioned problem has been supplied by single-crystal experiments, in which one metal (B) is covered by one, two or more monolayers of the second metal (A). It appeared [29] that, to see the bulk properties of a metal A, with regard to XPS and/or CO chemisorption, at least two or three layers of A should be laid down on metal B. This means that an ensemble of three or four contiguous surface A atoms must also have the A atoms underneath (atoms in the next layer, filling the holes of the first layer), to behave like corresponding ensembles of A in bulk metal A. This could be one of the reasons why the size of the necessary ensemble formally derived from the overall kinetic and the topmost layer composition is sometimes unreasonably large. 

Reactions of diethylzinc with benzaldehyde and heptanal were carried out in the presence of a catalytic amount (5 mol%) of 8 or 9, in order to examine the effect of the substitution pattern (R, R ) of catalysts 8 or 9 on the enantioselectivity. Results are summarized in Table 3-2. The secondary amine 8a was catalytically less active and less enantioselective than tertiary amine catalysts (entry 1). Tertiary amines 9a, 9b and 9c afforded (S)-l-phenylpropanol by reaction with benzaldehyde... 

Figures 6 and 7 show for catalyst samples coked in laboratory the catalytic relative activity tor the same reactions shown in Figs. 4 and 5, respectively. It can be seen that the catalytic function deactivation follows the same pattern as in the case of commercially coked samples. The main difference between the coked samples is the larger deactivation of the metallic function in laboratory samples, as more coke was deposited on the metallic function of these samples, as shown tn Fig. 1.

There aie indications fmm the relative small line width in both XRD patterns and Messbauer spectra that the imn(ir)sulfatc easily sinters, even at the relative low temperatures of the catalytic reaction. This is also indicated by the catalytic performance test of the catalyst B. Catalyst B containing iron oxide not interacting with the support shows a large deactivation. The time required for stabilization is much longer, and the remaining activity and selectivity is low. The effect of the dispersion, and the interaction with the carrier stresses the great importance of the preparation procedure [4]. 

No matter how active a catalyst particle is, it can be effective only if the reactants can reach the catalytic surface. The transfer of reactant from the bulk fluid to the outer surface of the catalyst particle requires a driving force, the concentration difference. Whether this difference in concentration between bulk fluid and particle surface is significant or negligible depends on the velocity pattern in the fluid near the surface, on the physical properties of the fluid, and on the intrinsic rate of the chemical reaction at the catalyst that is, it depends on the mass-transfer coefficient between fluid and surface and the rate constant for the catalytic reaction In every case the concentration of reactant is less at the surface than in the bulk fluid. Hence the observed rate, the global rate, is less than that corresponding to the concentration of reactants in the bulk fluid. 

Activity patterns follow acid strength, as shown in Fig. 4.20. Parallel trends arc obeyed for gasoline production and cumene dealkylation, and the latter is commonly used as a modeP reaction for catalytic cracking. 

These considerations are strikingly demonstrated by the volcano-shaped pattern of variation of catalytic activity as shown schematically in Figure 7.3. While the heat of adsorption is steadily decreasing from left to right, the catalytic reaction rates peak at the group VIII metals in the periodic table. Figure 7.3 shows the pattern of variation of catalytic reaction rates across the series of transition metals Re, Os, Ir, Pt, and Au for the hydrogenolysis of the C—C bond in ethane, the C —N bond in methylamine, and the C —Cl bond in methyl chloride. 

---