Effect on catalyst performance

Composition. The results of elemental analyses are almost always included among the specifications for a commercial catalyst. Depending on the accuracy desired and whether or not the catalyst can be rendered soluble without great difficulty, elemental analysis may be performed by x-ray methods, by one of the procedures based on atomic absorption, or by traditional wet-chemical methods. Erequentiy it is important to determine and report trace element components that may have an effect on catalyst performance. 

Although no explanation for the underlying mechanistic details can be given, the support and pretreatment conditions clearly have a strong effect on catalyst performance. Highest mean rates are obtained with reduced catalysts based on silica whereas the precursor seems to have only a negligible effect. 

Natural products and common industrial chemicals in massive form are seldom useful as catalysts because they have low specific surface areas, may contain various amounts of impurities that have deleterious effects on catalyst performance, do not usually have the exact chemical composition desired, or are too expensive to use in bulk form. The preparation of an industrial catalyst generally involves a series of operations designed to overcome such problems. Many catalysts can be produced by several routes. The actual choice of technique for the manufacture of a given catalyst is based on ease of preparation, homogeneity of the final catalyst, stability of the catalyst, reproducibility... 

In general, the choice of counteranion has a minor effect on catalyst performance, with typical examples being selected from BF4, OTD, PFg, or BARF-. In one example, however, it was noted that [(R.R)-Et-DuPhos Rh CODJOTf gave superior selectivity for the reduction of / -/ disubstituted a-dehydroamino acid derivatives than the corresponding BARF complex when performed in a range of solvents, including supercritical carbon dioxide [39]. 

Methylcyclopentadienyl manganese tricarbonyl (MMT) is an additive in many commercial unleaded gasolines, and thus its potential effect on catalyst performance is an important consideration. It was reported by Faggan et al. 8) that MMT at a recommended level of 0.125 g Mn/gal shows no adverse effect on emissions, when compared to unleaded gasoline, in tests on cars operated on the 50,000-mile EPA certification schedule. These findings have been confirmed by a number of unpublished test reports from several industrial laboratories. In fact, it is indicated that some of the manganese deposits can aid catalytically in the removal of CO, and possibly to some extent also of hydrocarbons. The consequence of such a catalytic effect of Mn has still to be explored. 

Nafion-H has also been found to be a suitable catalyst to carry out Koch-type carbonylation of a variety primary, secondary, and tertiary alcohols.402 Under optimal conditions, tertiary carboxylic acids are formed in moderate-to-good yields (Table 5.29). Increased CO pressure was shown to increase acid yields by enhancing the carbonylation of the intermediate tertiary carbocation prior to dimerization and oligomerization. Solvents had a small effect on catalyst performance, which is in contrast to the findings of Lange.403 Over Nafion NR50, acid yields of 39% and 42%... 

The analysis of petroleum feedstocks for the percentages of carbon, hydrogen, nitrogen, oxygen, and sulfur is perhaps the first method used to examine the general nature, and perform an evaluation, of a feedstock. The atomic ratios of the various elements to carbon (i.e., H/C, N/C, O/C, and S/C) are frequently used for indications of the overall character of the feedstock. It is also of value to determine the amounts of trace elements, such as vanadium and nickel, in a feedstock since these materials can have serious deleterious effects on catalyst performance during refining by catalytic processes. 

We conclude this section on gas-phase oxidation catalysts with an illustrative catalytic study where it can be demonstrated that minor changes in preparation have a major effect on catalyst performance, and a very high degree of sophistication is demanded of the accuracy of analysis to detect those changes. 

Addition of inorganic acids (HC1, H2S04) has a large detrimental effect on catalyst performance. Apparently, halides block the catalytic cycle by coordinating to the Pd(II) center, thereby reducing its electrophilicity (entry 8). Furthermore, the presence of halide anions can favor protonolysis of Pd-alkyl bonds over /3-H elim-... 

Hie selection of a solid catalyst for a given reaction is to a large extent still empirical and based on prior experience or analogy. However, there are now many aspects of this complex situation that are quite well understood. For example we know how the true chemical kinetics, which are an intrinsic property of the catalyst, and all the many aspects of transport of material and heat around the catalytic particles, interact. In other words, the physical characteristics around the catalyst system and their effects on catalyst performance are well known today. The chemist searching for new and better catalysts should always consider these physical factors, for they can be brought under control, and often in this way definite gains can usually be made both in activity and in selectivity. Further, this knowledge enables us to avoid... 

Loss on ignition (LOI, wt%). LOI is determined by measuring the loss of weight (mostly water) upon ignition at 815°C for an hour. The measurement is required because catalysts are paid for on a dry basis. LOI has no effect on catalyst performance. This test is performed only on the fresh catalyst. 

The addition of a Lewis base to the Al-alkyl has a remarkable effect on catalyst performance both in ethylene and a-olefin polymerization. 

In this paper we have investigated the performance of zeolites Y, EMT, beta and ZSM-5 for the reaction of isoprene (1) with methylvinylketone (2) to 4-acetyl-1-methyl-1-cyclohexene (3) and 4-acetyl-2-methyl-1-cyclohexene (4) shown in fig. 1. Furthermore, we used a silica binder in order to avoid experimental problems which arise when zeolite powder is taken as catalyst. In some cases binders have a significant influence on the catalyst activity and selectivity sometimes the activity can be enhanced, e.g., by additional active sites on the binder surface. Again, a loss of selectivity can be observed, if the spatial restrictions inside the zeolite framework, which are often required for selectivity, are not present on the binder surface. The nature of the binder and its effect on catalyst performance were also a subject of this study. 

The results demonstrate that presulfiding has an overall positive effect on catalyst performance in processing residues. The improvements are specifically in areas of significant interest in residue upgrading namely, conversion to distillates and asphaltenes reduction. The... 

Under more normal epoxidation conditions the equilibrium amount of adsorbed and absorbed oxygen is equivalent to about 9 monolayers, a similar thickness to that detected by electron diffraction after several cycles of oxidation and reduction with CO. An indirect measurement of the slow response of the subsurface oxygen concentration to changes in gas-phase oxygen pressure and its effect on catalyst performance has been carried out by Levchenko et al. A sudden reduction in oxygen partial pressure results in an immediate loss of activity followed by a much slower response with relaxation times in the range 10—100 min. The activation energy for this process was about 126 kJ mol . ... 

Impurities in the starting material stream can have positive effects (e.g., addition of halogen-containing compounds in silver-catalyzed ethylene oxide synthesis, but mostly their effects on catalyst performance are negative. In each case the catalyst must be tested with the original feed (e.g., from an integrated pilot plant). 

Determination of the Partial Pressure of Water during the Activation of the BP GTL Fischer-Tropsch Catalyst and its Effects on Catalyst performance. 

The presence of steam in the feed has a beneficial effect on catalyst performance, at least for precious metal catalysts as demonstrated by several workers (see Figure 4.27) [323-326], although it may deactivate gold catalysts rapidly (107). 

The addition of residual fractions to gas oil feed results in an increase in the impurity content of the equilibrium FCC catalyst and causes a decrease in activity. Metal impurities exist as porphyrin complexes which crack and deposit metal residues on the catalyst surface, causing catalyst deactivation. The most serious effects on catalyst performance result from nickel and vanadium compounds. Sodium can also deactivate acid sites on the catalyst, but the effect is generally reduced by desalting erode oils and by absorption of small amounts of sodium on the matrix. Sulfur compoimds in the feed contaminate products and regenerator flue gas. 

The autocatalyst operates under conditions that are constantly changing. Cold starts may be required several times a day and even on a motorway, automobiles are regularly accelerating and braking. This will influence the air/fuel ratio and engine temperature. These factors all have an effect on catalyst performance. A typical operating cycle is, therefore ... 

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