Unsupported catalysts

In the case of methyloxirane, however, on Pt and Pd catalysts the extent of the rupture of the sterically hindered bond is indicative of the electrophilic character of the catalyst. Unsupported or silica-supported ion-exchanged catalysts cleave the sterically less hindered bond, whereas on the impregnated catalysts, the rupture of the more hindered C-O bond is dominant.290 It is likely that Pt or Pd surface metal ions are responsible for the rupture of the sterically more hindered bond and residual chlorine from the catalyst preparation can stabilize these ions in the hydrogen atmosphere. 

Noble metal catalysts are known to be easily poisoned by sulfur. Tungsten carbide appeared to show noble metal characteristics, yet was found to be sulfur resistant. The n-heptane isomerization reaction described above was repeated with the two catalysts unsupported W2C and 0.3% Pt/Al203. 

Fig. 5.5. (a) Left Rate of methanol formation as a function of the Cu+ concentration (detected by XPS). Catalyst unsupported Cu, promoted by various alkalis [61]. (b) Right Rate of methanol formation as a function of the Pdn+ concentration (detected by chemical extraction and a.a.s.). Catalyst Pd/Si02 promoted by MgO/MgCk (various Mg/Pd ratios lead to various Pn + contents)... 

Hydrodesulphurization Catalysts Based on Mo or W. - In this Section structural studies of Mo- and W-containing catalysts (unsupported and supported, without and with promoters) and relations between catalyst composition and structure and catalytic properties are reported. Work on sulphides of other metals and on the kinetics and mechanism of hds is reported in the following two Sections. 

The presence of the cyclopentadienyl ligand at the chromium center provides a catalyst with a unique high response to hydrogen as a chain transfer agent (97). A number of ir- and O-bonded transition metal compounds in solution or supported (34) have been described as polymerization catalysts. Unsupported and supported transition metal-ally 1 compounds have been proposed to initiate polymerization by reaction with monomer in a manner illustrated by Reaction 22 for (allyl)3ZrBr/Si02 and ethylene. 

Key words PEMFC/DMFC/Carbon Supported Catalyst/Unsupported Catalyst... 

The presence of an oxide support, and therefore of a gold-support interface, is of definihve importance to get achve catalysts. Unsupported gold particles are poorly... 

It is essential that a solubilized form of an alkaline anion-exchange polymer be developed to improve the interface between the electrodes and the AEM electrolyte. Success in this effort will decrease MEA resistances. A water-based soluble form which can be rendered water insoluble when cast would be preferred, as there are safety concerns (primarily with industrial scale production) about using organic solvents near finely dispersed (pyrophoric) metal catalysts (unsupported or supported on carbon). 

These catalysts were developed from the generation. At low temperatures (below 100 °C) the active violet y or 5 form of the brown P-TiCls is formed. Through the smaller size of the primary crystallites, the surface area and activity of the catalyst was increased. The and generation catalysts (unsupported catalysts) were used in suspension processes with hexane as a solvent, in mass polymerisation processes (Rexene, Phillips), in the BASF gas phase process (vertical agitation) and in the solution process (Eastman). 

Table 1.4. Oxidation of Cl—C4 alkanes over Pd, Pt and Rh catalysts (unsupported or supported on alumina). From Ref. 9.

The anhydride of 1,8-naphthalenedicarboxyHc acid is obtained in ca 95—116 wt % yield by the vapor-phase air-oxidation of acenaphthene at ca 330—450°C, using unsupported or supported vanadium oxide catalysts, with or without modifiers (96). 

Most catalysts for solution processes are either completely soluble or pseudo-homogeneous all their catalyst components are introduced into the reactor as Hquids but produce soHd catalysts when combined. The early Du Pont process employed a three-component catalyst consisting of titanium tetrachloride, vanadium oxytrichloride, and triisobutjlalurninum (80,81), whereas Dow used a mixture of titanium tetrachloride and triisobutylalurninum modified with ammonia (86,87). Because processes are intrinsically suitable for the use of soluble catalysts, they were the first to accommodate highly active metallocene catalysts. Other suitable catalyst systems include heterogeneous catalysts (such as chromium-based catalysts) as well as supported and unsupported Ziegler catalysts (88—90). 

Eluidized-bed reactors are highly versatile and can accommodate many types of polymerization catalysts. Most of the catalysts used for LLDPE production are heterogeneous Ziegler catalysts, in both supported and unsupported forms. The gas-phase process can also accommodate supported metallocene catalysts that produce compositionaHy uniform LLDPE resins (49—51). 

LLDPE by itself does not present any health-related hazard on account of its chemical inertness and low toxicity. Consequently, film, containers, and container Hds made from LLDPE are used on a large scale in food and dmg packaging. Some LLDPE grades produced with unsupported metallocene catalysts have an especially high purity due to high catalyst productivity and a low contamination level of resins with catalyst residue. FDA approved the use of film manufactured from these resins for food contact and for various medical appHcations (80). However, if LLDPE articles contain fillers, processing aids, or colorants, thek health factors must then be judged separately. 

Catalysts. Nearly aU. of the industrially significant aromatic alkylation processes of the past have been carried out in the Hquid phase with unsupported acid catalysts. For example, AlCl HF have been used commercially for at least one of the benzene alkylation processes to produce ethylbenzene (104), cumene (105), and detergent alkylates (80). Exceptions to this historical trend have been the use of a supported boron trifluoride for the production of ethylbenzene and of a soHd phosphoric acid (SPA) catalyst for the production of cumene (59,106). 

Heterogeneous hydrogenation catalysts can be used in either a supported or an unsupported form. The most common supports are based on alurnina, carbon, and siUca. Supports are usually used with the more expensive metals and serve several purposes. Most importandy, they increase the efficiency of the catalyst based on the weight of metal used and they aid in the recovery of the catalyst, both of which help to keep costs low. When supported catalysts are employed, they can be used as a fixed bed or as a slurry (Uquid phase) or a fluidized bed (vapor phase). In a fixed-bed process, the amine or amine solution flows over the immobile catalyst. This eliminates the need for an elaborate catalyst recovery system and minimizes catalyst loss. When a slurry or fluidized bed is used, the catalyst must be separated from the amine by gravity (settling), filtration, or other means. 

An even more ambitious goal is to characterize an unsupported catalyst, because the surface is extremely rough and the target rapidly deteriorates under bombardment. Energy deposition leads to enormous erosion, because the substrate cannot get rid of the energy deposited, owing to the low heat conductivity. As a consequence static LEIS conditions have to be used to obtain information on the surface alone. In Fig. 3.60a we show a series of LEIS spectra obtained with 5 keV Ne" ions on a... 

Another example is the hydrogenation of the homoallylic eompound 4-methyl-3-cyclohexenyl ethyl ether to a mixture of 4-methylcyclohexyl ethyl ether and methylcyclohexane. The extent of hydrogenolysis depends on both the isomerizing and the hydrogenolyzing tendencies of the catalysts. With unsupported metals in ethanol, the percent hydrogenolysis decreased in the order palladium (62.6%), rhodium (23 6%), platinum (7.1%), iridium (3.9%), ruthenium (3.0%) (S3). 

Reduction of unsaturated aldehydes seems more influenced by the catalyst than is that of unsaturated ketones, probably because of the less hindered nature of the aldehydic function. A variety of special catalysts, such as unsupported (96), or supported (SJ) platinum-iron-zinc, plalinum-nickel-iron (47), platinum-cobalt (90), nickel-cobalt-iron (42-44), osmium (<55), rhenium heptoxide (74), or iridium-on-carbon (49), have been developed for selective hydrogenation of the carbonyl group in unsaturated aldehydes. None of these catalysts appears to reduce an a,/3-unsaturated ketonic carbonyl selectively. 

Palladium, platinum, or nickel, supported or unsupported, are the metals usually used in nitro-group reductions. The choice of catalyst often depends on what other functions are present and on the products desired,... 

Anilines have been reduced successfully over a variety of supported and unsupported metals, including palladium, platinum, rhodium, ruthenium, iridium, (54), cobalt, and nickel. Base metals require high temperatures and pressures (7d), whereas noble metals can be used under much milder conditions. Currently, preferred catalysts in both laboratory or industrial practice are rhodium at lower pressures and ruthenium at higher pressures, for both display high activity and relatively little tendency toward either coupling or hydrogenolysis,... 

Fig. 4 shows the XRD patterns of PS bead, unsupported PM012, and PM012-PS samples. PS bead showed no characteristic XRD patterns due to its amorphous nature. Unsupported PM012 catalyst showed the characteristic XRD patterns of the HPA catalyst. On the other hand, however, PM012-PS catalyst showed no characteristic XRD patterns of PM012, as observed for... 

Fig. 5. (a) 2-Propanol conversions and (b) product yields over unsupported PM012 and supported PMon-PS catalysts. 

The final probe of molecular clusters is that of selected chemical reactions. The use of probe reactions to study supported cluster catalysts is well established, and we are attempting the development of similar probes of unsupported clusters. The first steps in this direction are the design of a pulsed chemical reactor to go with the pulsed cluster source and the development of criteria for reactions. It is important to recall that at present... 

The performance of a supported metal or metal sulfide catalyst depends on the details of its preparation and pretreatraent. For petroleum refining applications, these catalysts are activated by reduction and/or sulfidation of an oxide precursor. The amount of the catalytic component converted to the active ase cind the dispersion of the active component are important factors in determining the catalytic performance of these materials. This investigation examines the process of reduction and sulfidation on unsupported 00 04 and silica-supported CO3O4 catalysts with different C03O4 dispersions. The C03O4 particle sizes were determined with electron microscopy. X-ray diffraction (XRD), emd... 

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