Supports Cabosil

The catalysts employed in this study consisted of V205/Si02 and Mo03/Si02 prepared by incipient wetness impregnation of the support (Cabosil L90) wiA aqueous solutions of vanadyl oxalate (Aldrich, 99.99%) and ammonium molybdate (Aldrich, 99%), respectively. The catalysts were dried at 623 K for 6 h and calcined at 773 K for 6 h. 

This tethered ferrocenyl-based Pd complex on MCM-41 (17) was then used for the catalytic amination reaction between cinnamyl acetate and benzylamine (40 °C, THF) [59]. In this case, confinement of the catalyst results in profound changes in regio- and enantioselectivity. When the homogeneous equivalent is used to catalyze the reaction, the straight chained derivative is the sole product. Similar results (only 2% of the branched product) were obtained when the catalyst was tethered to the surface of the non-porous silica Cabosil. When tethered inside the pores of MCM-41 a major change occurred in that now the branched product accounts for about 50% and a change in e.e. from 49% e.e. when anchored to the Cabosil support to +99% when anchored inside the MCM-41 pore could be observed. If the catalyst s chirality was reversed in the MCM-41 immobihzed case, so was the chirality of the product (measured at 93% e.e.) [60]. 

When two V/Si02 catalysts of 1 and 10 wt% V205 were tested for butane oxidation, the data in Table IX were obtained. The data show that the lower loading sample was much more selective than the higher loading sample. This difference was not due to the effect of impurities in the support because the data were obtained on an acid-washed Davison 62 silica which contained less than 10 ppm of Na and 200 ppm of Ca. The same effect was observed on catalysts prepared with Cabosil silica, which contained no detectable impurities. 

The details of the sample preparation and studies of the nature of the supported-metal samples have been described in a paper dealing with the effect of surface coverage on the spectra of carbon monoxide chemisorbed on platinum, nickel, and palladium (1). The samples consist of small particles of metal dispersed on a nonporous silica which is produced commercially under the names Cabosil or Aerosil.f This type of silica is suitable as a support because it is relatively inert and has a small particle size (150-200 A.). The small particle size is important because it reduces the amount of radiation which is lost by scattering. A nonporous small particle form of gamma-alumina, known as Alon-C, is also available. This material is not so inert as the silica and will react with gases such as CO and CO2 at elevated temperatures. 

A specific example of how the nature of the carrier can influence the structure of gases adsorbed on dispersed metals is shown in Fig. 10. In this figure, A is the spectrum of CO chemisorbed on Cabosil-supported platinum and B is the spectrum of CO chemisorbed on 7-alumina-(Alon-C) supported platinum. 

In the former structure, two of the bonding electrons would be provided by the platinum, while in the latter the CO would be coordinated to the platinum. The force constant for the metal-carbon bond for CO on Cabosil-supported platinum was found to be 4 X 106 dynes/cm. On the simplest basis this force constant would indicate a single platinum-carbon bond. There is, however, considerable uncertainty in going from force constants to bond order so that a contribution, perhaps as high as 50 %, of the doublebond species could not be ruled out. The simple bridged structure... 

Differences in the chemical behavior of the Cabosil-supported and 7-alumina-supported platinum samples are evident. The initial reduction of the chloroplatinic acid will start at 35° C. when Cabosil is used, but it requires a temperature of 200° C. before reduction is started on the alumina-supported samples. The CO chemisorbed on 7-alumina-supported platinum is much more difficult to oxidize, especially the bridged form, than is CO on Cabosil-supported platinum. 

D2 at room temperature. If, however, the Cabosil is impregnated with platinum, the exchange goes rapidly (14). The results of such an exchange are shown in Fig. 18. Spectrum A is due to partially dried Cabosil which is used as a support for 9.2 wt. % platinum. The bands at 2.67 and 2.80 n are due to surface OH groups and adsorbed water. The bands in the 3.4-/ region are due to hydrocarbon impurities on spectrometer windows and the bands near 4.27-ju are due to atmospheric C02. Spectrum B was obtained 10 min. after the sample had been exposed to gaseous D2. The... 

In these experiments, nickel hydroxide was mixed in a proportion of 12.4% with finely divided silica (Cabosil), pressed in a die and dehydrated at 200°, under vacuum, in the infrared analysis cell. Composition of the sample was therefore different from the composition of the samples used in the gravimetric or calorimetric work [NiO(200°)] and possible effects of the support cannot be, a priori, completely excluded. Calorimetric experiments with the supported samples have shown, however, that their reactivity toward CO is very similar to the reactivity ofNiO(200°). 

Alumina and silica supported MnC>2 catafysts were prepared by inc ient wetness impregnation of supports (Degussa, Atuminoxide C and Cabosil L-90) using manganese acetate (Aldrich) as the precursor and were calcined in air at 773 K for 3 h prior to use. The crystal phases of MnC>2 in the catalysts were identified using X-ray diffraction (XRD) and their surface areas were determined by Nz physisorption u g the BET method. 

This result suggests that, for this family of structures at least, the value of 1.6 mL g 1 provides sufficient weakness to allow total breakdown and full access to all of the catalyst surface. This inference is also supported by a comparison of results obtained with the best commercial silica gels and with a pyrogenic or "fumed silica (Cabosil) formed by flame hydrolysis of SiCl4. The latter has no pore structure, and no such structural limitations. That the two exhibit similar activities indicates that the silica gel had disintegrated to the level at which nearly all of the surface contributed to the polymerization. Furthermore, once friability of the solid is obtained because the pore volume is sufficiently high, activity can still be influenced by the surface area. However, these are only general trends, and some small exceptions are evident in the data in the table as well. It is the structure itself, rather than any porosity measurement, that determines friability. 

This work has been undertaken as part of a program to add to our knowledge of the oxidation of CO over Pt via models based on elementary steps (refs.1,2,3,4). Here we measure the reaction of C0(g) with adsorbed oxygen, and 0 (g) with adsorbed CO. These processes, although not elementary steps, are simpler to analyze than the full reaction. In the present study we are concerned only with the amounts of adsorbed CO and oxygen. As such, the measurements are equivalent to the titration of one adsorbed species by the other gaseous species. We are particlarly interested in the effect of temperature. The fraction exposed (FE) of the Pt particles supported on Si0 (Cabosil) may have an effect on the titrations also. 

Catalyst A - a Davidson Grade 952 silica supported catalyst Catalyst B - a modified, silica supported catalyst Cabosil 5-17 Values in brackets are % deviation from NAA Source Battiste and co-workers. Analytical Chemistry [68] ... 

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