Selectivity of catalyst

Either product can be favored over the other by proper selection of catalyst and reaction conditions. However, the principal source of DIPE is as a by-product from isopropyl alcohol production. Typically, excess DIPE is recycled over acidic catalysts ia the alcohol process where it is hydrated to isopropyl alcohol. DIPE is used to a minor extent ia iadustrial extraction and as a solvent. 

By proper selection of catalyst and reaction conditions, hydrocarbons and oxygenates ranging from methane and methanol through high (> 10,000) molecular weight paraffin waxes can be synthesized as iadicated ia Figure 11 (44). 

The technique of catalytic hydrogenation can be applied almost universally to unsaturated systems, and therein lies its chief advantage (7). By appropriate selection of catalyst, pressure, and temperature, a remarkable variety of substrates can be made to undergo hydrogenation, many of them under hydrogen pressure not exceeding 50 psi (see Appendix 3 for description and use of low-pressure hydrogenation apparatus). 

Performance of an FCC unit is often maximized when the unit is operated against multiple constraints simultaneously. It is essential that the specified constraints allow for minimum comfort zones. An operator-friendly advanced control program, coupled with proper selection of catalyst formulation, would allow optimizing the performance of the unit on a daily basis. 

It is important to observe that the electrochemically promoted Pt surface (Uwr O V) gives SN2 selectivity values above 70% vs 35% on the unpromoted surface (Uwr>0 V). The Pt surface is thus made as selective as a Rh surface would be under similar conditions. The ability of electrochemical promotion to alter the product selectivity of catalyst surfaces is one of its most attractive features for practical applications. 

Changing catalyst support from carbon to calcium carbonate leads to dramatic improvement of the cis/tran ratio from 6 1 to 18 1, that is the cis selectivity increases from 85.7% to 94.7%. The reason for better selectivity on CaC03 supported catalyst is attributed to its lower surface area leading to lower hydrogenation activity, but more selective to the desired product. The successful commercialization of the new route for sertraline synthesis demonstrates that for a stereoselective hydrogenation reaction, improve product selectivity can be achieved by proper selection of catalyst support. 

Early workers viewed carriers or catalyst supports as inert substances that provided a means of spreading out an expensive material like platinum or else improved the mechanical strength of an inherently weak material. The primary factors in the early selection of catalyst supports were their physical properties and their cheapness hence pumice, ground brick, charcoal, coke, and similar substances were used. No attention was paid to the possible influence of the support on catalyst behavior differences in behavior were attributed to variations in the distribution of the catalyst itself. 

In the direct ammoxidation of propane over Fe-zeolite catalysts the product mixture consisted of propene, acrylonitrile (AN), acetonitrile (AcN), and carbon oxides. Traces of methane, ethane, ethene and HCN were also detected with selectivity not exceeding 3%. The catalytic performances of the investigated catalysts are summarized in the Table 1. It must be noted that catalytic activity of MTW and silicalite matrix without iron (Fe concentration is lower than 50 ppm) was negligible. The propane conversion was below 1.5 % and no nitriles were detected. It is clearly seen from the Table 1 that the activity and selectivity of catalysts are influenced not only by the content of iron, but also by the zeolite framework structure. Typically, the Fe-MTW zeolites exhibit higher selectivity to propene (even at higher propane conversion than in the case of Fe-silicalite) and substantially lower selectivity to nitriles (both acrylonitrile and acetonitrile). The Fe-silicalite catalyst exhibits acrylonitrile selectivity 31.5 %, whereas the Fe-MTW catalysts with Fe concentration 1400 and 18900 ppm exhibit, at similar propane conversion, the AN selectivity 19.2 and 15.2 %, respectively. On the other hand, Fe-MTW zeolites exhibit higher AN/AcN ratio in comparison with Fe-silicalite catalyst (see Table 1). Fe-MTW-11500 catalyst reveals rather rare behavior. The concentration of Fe ions in the sample is comparable to Fe-sil-12900 catalyst, as well as... 

Microwave irradiation of catalysts before their use in chemical reactions has been found to be a new promising tool for catalyst activation. Microwave irradiation has been found to modify not only the size and distribution of metal particles but probably also their shape and, consequently, the nature of their active sites. These phenomena might have a significant effect on the activity and selectivity of catalysts, as found in the isomerization of 2-methylpentene on a Pt catalyst [2],... 

In this and subsequent studies [41 14], Takeuchi and coworkers described the scope and selectivity of catalysts derived from [fr(COD)Cl]2 and achiral phosphorus ligands. Many of the trends that they uncovered have also been observed with more recently developed catalyst systems derived from chiral phosphoramidite ligands (Table 1). 

Firstly and primarily, it seeks to disclose the elementary (microscopic) mechanism of the catalytic act. Every heterogeneous catalytic process, like any chemical process, is based in the final reckoning on an electronic mechanism. It is the aim of the theory to elucidate this mechanism. This is necessary if the theory of catalysis is to rise above vulgar empiricism and to show how to control the activity and selectivity of catalysts, i.e., how to vary them to the required degree and in the required direction. 

The investigation of activity and selectivity of catalysts was carried out in a flow quartz reactor at atmospheric pressure and following i eaotion conditions. 

Table 1. Composition, Ammonia and Oxygen Uptakes, Surface Coverage, Active Site Density, Methanol Conversion and Product Selectivities of Catalysts...

One of the simplest optimization tasks is aimed to select the proper catalyst combination and the corresponding process parameters. In this case the main task is to create a proper experimental space with appropriate variable levels as shown in Table 1. This experimental space has 6250 potential experimental points (N) (N = 2 x 5 = 6250). This approach has been used for the selection of catalysts for ring hydrogenation of bi-substituted benzene derivatives. The decrease of the number of variable levels from 5 to 4 would result in significant decrease in the value of N (N= 2 X 4 = 2048). 

Electrochemical Reaction/Transport. Electrochemical reactions occur at the electrode/electrolyte interface when gas is brought to the electrode surface using a small pump. Gas diffuses through the electrode structure to the electrode/electrolyte interface, where it is electrochemically reacted. Some parasitic chemical reactions can also occur on the electrocatalytic surface between the reactant gas and air. To achieve maximum response and reproducibility, the chemical combination must be minimized and controlled by proper selection of catalyst sensor potential and cell configuration. For CO, water is required to complete the anodic reaction at the sensing electrode according to the following reaction ... 

The catalysts applied in hydroprocessing operations are typically sulfided CoO—Mo03—A1203 or NiO—M0O3—A1203. The results of relevant studies281 and the application in refinery processes of these and other transition-metal sulfide catalysts were reviewed.282 Selection of catalysts and reactors for particular feeds and products is also an important issue.257,280,283,284... 

Characteristics of Selection of Catalysts for Conversion of Sulfur-containing Heterocycles A. V. Moshkina, Katalitich. Sintez i Prevrashcheniya Geterotsilcl. Soedin. Geterogen. Kataliz., 1976, 31-52. 

The selectivity of catalysts in COj is always lower than 3 % in all the temperature range investigated (410 - 490°C). An excess of chromia deposited on the support reduces the activity of the catalysts which become more oxidant and less selective in nitriles. Sample containing 50 % of chromium tends to behave as pure Cr O which favours deep oxidation and yields 71.5 % of CO2 at 440°C (see Table 2). 

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