Silica Supported Catalyst

The selectivity for the linear aldehyde of sol-gel immobilised 2 was found to be as high as 93%, which is similar to that of the homogeneous catalysed reaction (Table 3.2, entries 1 and 6). Sol-gel immobilised 3 gave a much lower selectivity (70%) for the linear aldehyde, and in the absence of ligand the selectivity to the linear aldehyde is only 26%. This proves that catalysts based on these ligands retain the selectivity upon immobilisation, which means that the selectivity is dominated by the size of the P-Rh-P bite-angle of ligand [18], 

TABLE 3.2 Hydroformylation of 1-octene using silica immobilised [Rh(2)CO]+ a 

Entry Catalyst Time 00 Conv. (% TOFb Or1) l b Ratio 1-aldehyde (% b-aldehyde %) 1-alcohol %) 2-octene, octane %) 

The method of catalyst immobilisation appeared to affect its performance in catalysis. Catalyst obtained by method II showed a low selectivity in the hydroformylation of 1-octene (l b aldehyde ratio was even lower than 2) at a very high rate and high yields of isomerised alkenes (Table 3.2, entry 2), whereas procedure IV resulted in a catalyst that was highly selective for the linear aldehyde (with a l b ratio of 37) (entry 5). In accordance with examples from literature it is likely that procedure II gave rise to the ionic bonding of ligand-free rhodium cations on the slightly acidic silica surface [29], 

It is interesting to note that using the sol-gel procedure (I) the pre-formation of the rhodium diphosphine complex suppressed the formation of ligand free rhodium-cations on the silica surface. This approach gave rise to a well-defined, very selective hydroformylation catalyst. All immobilised catalysts were 10 to 40 times slower than the homogeneous catalyst under the same conditions, the sol-gel procedure yielding the fastest catalyst of this series. 

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Cu/ Zn0/Si02 catalyst obtained with different doses of 5 keV Ne" ions (see insert, spectra are shifted vertically for clarity). Catalyst reduction temperature 700 K. Solid lines fitted Gauss peaks [3.147]. (b) The relative coverage of Cu and ZnO on the silica-supported catalyst, reduced at 700 K, as a function of the ion dose [3.147]. 

As in the case of graphite-supported catalysts, some metal particles were also encapsulated by the deposited carbon (Fig. 4). However, the amount of encapsulated metal was much less. Differences in the nature of encapsulation were observed. Almost all encapsulated metal particles on silica-supported catalysts were found inside the tubules (Fig. 4(a)). The probable mechanism of this encapsulation was precisely described elsewhere[21 ]. We supposed that they were catalytic particles that became inactive after introduction into the tubules during the growth process. On the other hand, the formation of graphite layers around the metal in the case of graphite-supported catalysts can be explained on the basis of... 

Fig. 3. Carbon species obtained after acetylene decomposition for 5 hours at 973 K on the surface of silica-supported catalysts made by pore impregnation (a) Co-SiOj-l (b) Co-Si02-2.

Top silica-supported catalysts bottom alumina-supported catalysts left 25,000 space velocity and right 95,000 space velocity... 

A silica-supported catalyst was prepared by anaerobic impregnation of Mo2Rh(/a-CO)(CO)4((7 -C,H,s),i (Fig. 70) from CHiCF solution, followed by evacuation at room temperature. Decomposition processes were observed at the... 

The commercial process for the production of vinyl acetate monomer (VAM) has evolved over the years. In the 1930s, Wacker developed a process based upon the gas-phase conversion of acetylene and acetic acid over a zinc acetate carbon-supported catalyst. This chemistry and process eventually gave way in the late 1960s to a more economically favorable gas-phase conversion of ethylene and acetic acid over a palladium-based silica-supported catalyst. Today, most of the world s vinyl acetate is derived from the ethylene-based process. The end uses of vinyl acetate are diverse and range from die protective laminate film used in automotive safety glass to polymer-based paints and adhesives. 

X-ray dififtaction (XRD) analysis of the freshly calcined catalysts as well as samples used for several hours in the isomerization reaction, only presented the peaks corresponding to the tetragonal form of zirconia. At the same time, for the silica series, XRD confirmed the presence of NiO on the unsulfated catalysts and NiS04 on the sul ted ones. However, XRD did not show any evidence of any of these species for the zirconia series, probably due to their high state of dispersion. Similarly, the XPS data clearly showed the presence of NiO and NiS04 on the unsulfated and sulfated silica-supported catalysts, respectively, but they were not conclusive in the case of zirconia series since both sulfate and oxide species were observed. 

A significant volume of literature relates to our work. Concerning choice of support, Montassier et al. have examined silica-supported catalysts with Pt, Co, Rh Ru and Ir catalysts.However, these systems are not stable to hydrothermal conditions. Carbon offers a stable support option. However, the prior art with respect to carbon-supported catalysts has generally focused on Ru and Pt as metals.Additionally, unsupported catalysts have also been reported effective including Raney metals (metal sponges).Although the bulk of the literature is based on mono-metallic systems, Maris et al. recently reported on bimetallic carbon-supported catalysts with Pt/Ru and Au/Ru. In contrast, our work focuses primarily on the development of a class of rhenium-based carbon supported catalysts that have demonstrated performance equal to or better than much of the prior art. A proposed reaction mechartism is shown in Figure 34.2 °l... 

In addition to this work on charcoal- and silica-supported catalysts and on evaporated platinum films, a number of studies have been made on alumina-supported platinum catalysts (e.g., 111-114, 81,115) in which the aim has been the study of reactions at the platinum alone. In these cases, one cannot automatically dismiss the possibility of participation of the alumina support (i.e., of dual function behavior of the catalyst) because it is known that alumina may have acidic properties, particularly when retained halogen is present. In general terms, there is no immediate answer to this problem because the nature of this sort of catalyst wall be much dependent on the details of catalyst history, preparation, and use. However, there can be little doubt that in many experimental studies using plati-num/alumina, and in which the assumption has been made that the alumina support is inert, this assumption is essentially valid. For instance, one may note the inert alumina used by Davis and Venuto (111) and the justification provided by Gault et al. (116) for the inertness of the alumina used in a substantial body of previous work irrespective of whether the catalyst was... 

Platinum is an important example of a metal where, even on an uncontaminated surface such as is offered by an evaporated film, there is a strong tendency for only one C—C bond to be ruptured in any particular reacting molecule. On this basis, one may express the distribution of reaction products in terms of relative C—C bond rupture probabilities. Some data of this sort are contained in Table XI for thick and ultrathin film catalysts, and for comparison there are included some data for reactions on a silica-supported catalyst containing 0.8% platinum. These data all refer to reactions carried out in the presence of a large excess of hydrogen, although the results of Kikuchi et al. (128) indicate that on platinum catalysts the position of C—C bond rupture (in n-pentane) is very little dependent on hydrogen pressure. The data in Table XI show that, on the whole, the 0.8% platinum/silica catalyst used by Matsumoto et al. (110) was inter-... 

The work of Kikuchi et al. (123) with silica-supported catalysts also shows the high tendency of iron (370°-400°C), cobalt (330o-360°C) and nickel (330°-370°C) to catalyze fragmentation (of n-pentane) to methane. This work also showed that with cobalt and nickel, the extent of methane formation tended to decrease with increasing hydrogen partial pressure. Some data are listed in Table XII. 

These silica-supported catalysts (Figure 13) are, according to their authors, more advantageous because the functionalization is performed from the easily available linker, 3-isocyanato-propyl-... 

Moreno described the cycloaddition of 2,5-dimethylfuran (42) catalyzed by silica-supported Lewis acids under solvent-free conditions in closed Teflon vessels using a commercial microwave oven (Scheme 9.11) [28, 52]. Under these conditions coordination of the silica-supported catalyst with the oxygen bridge favors ring opening, thus leading to the aromatic compounds in one step. The use of Si (71) gave the best results for aromatic compounds. 

Ethanol can be derived from biomass by means of acidic/enzymatic hydrolysis or also by thermochemical conversion and subsequent enzymatic ethanol formation. Likewise for methanol, hydrogen can be produced from ethanol with the ease of storage/transportation and an additional advantage of its nontoxicity. Apart from thermodynamic studies on hydrogen from ethanol steam reforming,117-119 catalytic reaction studies were also performed on this reaction using Ni-Cu-Cr catalysts,120 Ni-Cu-K alumina-supported catalysts,121 Cu-Zn alumina-supported catalysts,122,123 Ca-Zn alumina-supported catalysts,122 and Ni-Cu silica-supported catalysts.123... 

For several silica-supported catalysts in condensed phase, including the SAPC system, the rates are disappointing. This can be assigned to slow mass transfer, and perhaps to incomplete rhodium hydride formation as we have discussed and observed. The sol-gel catalyst is relatively fast and is sometimes only a few times slower than the homogeneous one. Since only limited ways of preparation were tested, there is probably more scope for sol-gel catalysts. Space-time yields are promising at the present state of affairs. 

Figure 3.7 Comparison of the monochromatic Mo 3d XPS spectra of M0O3 in an insulating silica-supported catalyst and in a conducting, thin silica film-supported model catalyst, showing the effect of inhomogeneous charge broadening (courtesy of H. Korpik, Eindhoven).

Whereas the effect of water on deactivation and on the overall activity of the FTS varies with the support, similar effects of water on the selectivity is reported for all catalysts, to a certain degree independent of the support, promoter and conditions. The effect can be summarized as an increase in C5 + selectivity, a decrease in methane selectivity, and in some instances a weak enhancement of the C02 selectivity is observed. Fig. 4 illustrates the effect on the C5 + and methane selectivity of adding water to cobalt catalysts supported on alumina, silica and titania, and both unpromoted and Re-promoted catalysts are shown. At the outset these selectivities are strong functions of the conversion, the C5 + selectivity increasing and the methane decreasing with increasing conversion, as illustrated by the trendlines in the figures. The points for methane are below, and C5 + -selectivity is above the line when water is added. Similar results were reported by many authors for alumina-supported catalysts,16-19 23 30 silica-supported catalysts,30 37 46-48 and titania-supported catalysts.19 30... 

The impact of the new activation procedure on the WGS rate, using either Na2C03 or NaOH, on silica-supported complexes at 150 °C, is reported in Table 51 156,163 Coitions 1-1.4 g dried silica-supported catalyst with 1.6 wt% loading of Ru. 

Silica-supported catalysts, 23 54-55 Silica-supported metallocene catalysts, 76 88... 

Mayoral and colleagues210 studied the same reaction catalyzed by a menthoxyaluminum catalyst supported on silica gel and alumina. The catalyst was prepared by treatment of the solid support with diethylaluminum chloride and (—)-menthol. The silica-supported catalyst proved more active than the alumina-supported catalyst. The reaction rates and enantioselectivities depended strongly on the amount of (—)-menthol used. The highest ee obtained was 31% at 81% conversion (endo/exo = 10/90). 

Interestingly, the dimeric Cr(salen) catalyst 64 supported on silica showed enhanced activity for ARO of 1,2-epoxyhexane and cyclohexene oxide in the presence of ionic liquids particularly with [BMIM][PF6] (64-IL) [86] (Table 6). A significant increase in the product selectivity was also observed with silica supported ionic liquid (64-SILP) for ARO of 1,2-epoxyhexane and cyclohexene oxide (ee, of 87% and 75% respectively) as eompared to silica supported catalyst minus the ionie liquid (Table 6, entries 5,6). However, after repeated recycling, the silica support material deteriorates due to the abrasive forees in the stirred reactor. As a result, silica material was non-recoverable, but the expensive dimeric Cr(salen) catalyst 64 and the ionic liquid was recovered quantitatively by Soxhlet extraction with acetone. SILP-catalyst system was also used in a eontinuous-flow reactor. 

C6Hii)7Si70i2MgTiCl3] catalyzes ethylene polymerizahon in the presence of triethylaluminium co-catalyst. Its achvity (of about 111 kg-PE gj h" ) and the characterishcs of the polymer (M = 140000, M /Mn = 5.5, MI = 1.02) are comparable to those of a typical commercial Ti/Mg/Si02 silica-supported catalyst [88]. 

The catalyst-loading was in the range of 42 mg (Nucleosil 300-7) to 63 mg catalyst/g (Lichrospher). RCM reactions carried out with these two silica-supported catalyst versions allowed TONs of up to 90 for a series of simple Q ,cu-dienes [56]. 

The nature of the support can have a very profound influence on the catalyst activity. Thus, phosphinated polyvinyl chloride supports are fairly inactive (75), and phosphinated polystyrene catalysts are considerably more active (57), but rather less active particularly when cyclic olefins are the substrates than phosphinated silica supports (76). Silica-supported catalysts may be more active because the rhodium(I) complexes are bound to the outside of the silica surface and are, therefore, more readily available to the reactants than in the polystyrene-based catalysts where the rhodium(I) complex may be deep inside the polymer beads. If this is so, the polystyrene-based catalysts should be more valuable when it is desired to hydrogenate selectively one olefin in a mixture of olefins, whereas the silica-based catalysts should be more valuable when a rapid hydrogenation of a pure substrate is required. 

The adsorption of cyclopropanes at room temperature has been characterized by infrared spectroscopy for a number of silica-supported catalysts, viz., Ni (86), Pt (86), Pd (266), and Rh (91). The spectra are identical with those obtained from the adsorption of propene on the same metals. They give absorptions from CH3 groups showing that the C3 ring has been opened, and the nature of the spectra has already been discussed (140, and Part I, Section Vl.C.l.b). Typical spectra of species formed from cyclopropane on Ni/Si02 and Pt/Si02, obtained by Ward at room temperature, are shown in Figs. 9C and 9D. 

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