Catalyst utilization

Novel palladium catalysts show marked improvements in both yields and selectivities, compared to nickel carbonyl catalysts utilized in eadier commercial carbonylation processes (83,84). The palladium catalysts are also expected to be less hazardous. 

Organochromium Catalysts. Several commercially important catalysts utilize organ ochromium compounds. Some of them are prepared by supporting bis(triphenylsilyl)chromate on siUca or siUca-alumina in a hydrocarbon slurry followed by a treatment with alkyl aluminum compounds (41). Other catalysts are based on bis(cyclopentadienyl)chromium deposited on siUca (42). The reactions between the hydroxyl groups in siUca and the chromium compounds leave various chromium species chemically linked to the siUca surface. The productivity of supported organochromium catalysts is also high, around 8—10 kg PE/g catalyst (800—1000 kg PE/g Cr). 

This arrangement ensures more efficient overall catalyst utilization and a significant increase in the yield of octenes. As an example, dimer selectivity in the 90-92 % range with butene conversion in the 80-85 % range can be obtained with a C4 feed containing 60 % butenes. Thanks to the biphasic technique, the dimerization... 

Here R is a methyl or ethyl group, R is C4 including branched all l, R is the olefin derived from dehydration of R OH, and R ° is an oligomer of R ". It is demonstrated here that these pathways can be controlled by the properties of the acid catalyst utilized,... 

The size of the cataly.st particle influences the observed rate of reaction the smaller the particle, the less time required for the reactants to move to the active catalyst sites and for the products to diffuse out of the particle. Furthermore, with relatively fast reactions in large particles the reactants may never reach the interior of the particle, thus decreasing the catalyst utilization. Catalyst utilization is expressed as the internal effectiveness factor //,. This factor is defined as follows ... 

In order for diffusional limitations to be negligible, the effectiveness factor must be close to 1, i.e. nearly complete catalyst utilization, which requires that the Thiele modulus is suffieiently small (< ca. 0.5), see Figure 3.32. Therefore, the surface-over-volume ratio must be as large as possible (particle size as small as possible) from a diffusion (and heat-transfer) point of view. There are many different catalyst shapes that have different SA/V ratios for a given size. 

Microlevel. The starting point in multiphase reactor selection is the determination of the best particle size (catalyst particles, bubbles, and droplets). The size of catalyst particles should be such that utilization of the catalyst is as high as possible. A measure of catalyst utilization is the effectiveness factor q (see Sections 3.4.1 and 5.4.3) that is inversely related to the Thiele modulus (Eqn. 5.4-78). Generally, the effectiveness factor for Thiele moduli less than 0.5 are sufficiently high, exceeding 0.9. For the reaction under consideration, the particles size should be so small that these limits are met. 

In a slurry reactor (Fig 5.4.74), the catalyst is present as finely divided particles, typically in the range 1-200 pm. A mechanical stirrer, or the gas flow itself, provides the agitation power required to keep the catalytic particles in suspension. One advantage is the high catalyst utilization not only is the diffusion distance short, it is al.so possible to obtain high mass-transfer rates by proper mixing. 

Optimization of the catalyst layer composition and thickness in PEFCs for maximum catalyst utilization in operation on air and on impure hydrogen feed streams [Wilson, 1993 Springer et al., 1993]. 

Catalyst layer architecture As a consequence of the diminishing remrns from ever higher dispersion, the effort to increase the active catalyst surface area per unit mass of Pt has centered in recent years primarily on optimization of catalyst layer properties, aiming to maximize catalyst utilization in fuel cell electrodes based on Pt catalyst particle sizes of 2-5 nm. High catalyst utilization is conditioned on access to the largest possible percentage of the total catalyst surface area embedded in a catalyst... 

The most recent improvements in Pt catalyst utilization U by optimization of catalyst layer composition and stmcture have led to catalyst utilizations as high as 80%, or more, determined as the ratio between measured ORR current per geometric square centimeter of electrode area and the current expected from the total measured Pt surface area per geometric square centimeter of the electrode, i.e.,... 

Paulus UA, Veziridis V, Schnyder B, Kuhnke M, Scherer GG, Wokaun A. 2003. Fundamental investigation of catalyst utilization at the electrode/solid polymer electrolyte interface. Part I. Development of a model system. J Electroanal Chem 541 77-91. 

When processing flue gas containing dust, the reactors are typically vertical, with down flow of flue gas. The catalyst is typically arranged in a series of two to four beds, or layers (Figure 1.2). For better catalyst utilization, it is common to use three or four layers, with provisions for an additional layer which is not initially installed. 

Effect of fragmentation on catalyst utilization when intraparticle diffusion is rate controlling (shaded areas represent regions of the catalyst with insignificant concentrations of reactants). 

Slurry Reactors. Slurry reactors are commonly used in situations where it is necessary to contact a liquid reactant or a solution containing the reactant with a solid catalyst. To facilitate mass transfer and effective catalyst utilization, the catalyst is usually suspended in powdered or in granular form. This type of reactor has been used where one of the reactants is normally a gas at the reaction conditions and the second reactant is a liquid, e.g., in the hydrogenation of various oils. The reactant gas is bubbled through the liquid, dissolves, and then diffuses to the catalyst surface. Obviously mass transfer limitations can be quite significant in those instances where three phases (the solid catalyst, and the liquid and gaseous reactants) are present and necessary to proceed rapidly from reactants to products. 

The first use of room temperature ionic liquids as potential novel soluble phases for combinatorial synthesis has recently been described. As model reaction the Knoevenagel condensation of salicyl aldehyde grafted on to an imidazolium-derived ionic liquid was studied under the action of microwave irradiation (Scheme 12.19) [66]. Reactions were performed without additional solvent in the presence of a basic catalyst, utilizing microwave irradiation in a designated monomode microwave reac-... 

For facilities where many different materials are stored, processed or mixed, it may be desirable to use a matrix approach to investigating all possible combinations of materials, including contaminants, materials of construction, additives, catalysts, utilities and common substances such as air and water. The development of a compatibility matrix is discussed in Section 4.3. 

The first catalysts utilized in phase transfer processes were quaternary onium salts. In particular, benzyltriethylammonium chloride was favored by Makosza (7 ) whereas Starks utilized the more thermally stable phosphonium salts (6,8). In either case, the catalytic process worked in the same way the ammonium or phosphonium cation exchanged for the cation associated with the nucleophilic reagent salt. The new reagent, Q+Nu , dissolved in the organic phase and effected substitution. 

Antoine et al. [28] inveshgated the gradient across the CL and found that the Pt utilization was dependent on the CL porosity. In a nonporous CL, catalyst utilization was increased through the preferential locahon of Pt close to the gas diffusion layer in a porous CL, catalyst utilization efficiency was increased through the preferential location of Pt close to the polymer electrolyte membrane. In PEM fuel cells, fhe CL has a porous structure, and better performance is expected if higher Pf loading is used af preferential locahons close to the membrane/catalyst layer interface. 

During spraying, the carbon substrate is moved by means of an X-Y axes coordinated system controlled by computer software. The CLs formed from the electrospray technique show both morphological and structural improvements, which could contribute to better catalyst utilization than is achieved... 

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