Catalysts Prepared on Silica

Amorphous silica may be obtained commercially over a broad range of product specifications from a number of chemical companies across the globe. A typical silica suitable for ethylene polymerization catalysts has a particle size of between about 5-200 microns, a pore volume of about 1.4-3.0 cc/g and a surface area of 100-500 meterVg. 

As discussed previously, U.S. Patents 3,989,881 and 4,125,532, contain a large number of magnesium/titanium/chlorine/electron donor complexes that are suitable candidates for the preparation of a wide variety of high-activity catalysts for ethylene polymerization catalysts supported on silica, using the physical impregnation method. In this method, the Mg/Ti/Cl/ electron donor complexes are prepared in the particular electron donor solvent and then this solution is added to silica, after which the electron donor solvent is evaporated to cause the solid complex to be deposited into the silica pore structure. The silica only acts as a carrier of the Mg/Ti/elec-tron donor complex, which becomes the active species once the complex is reacted with an alumimun alkyl. 

An example of the physical impregnation method is provided by U.S. Patent 4,302,565, which was discussed above. The dissolution of a magnesium compound and a titanium compound in a suitable solvent, usually an electron donor solvent such as tetrahydrofuran (THF), can lead to the formation of a new species (catalyst precursor) that provides a highly-active Ziegler system when activated. Removing the solvent at elevated temperatures in the presence of a support material such as silica will precipitate the precursor into the pores of the silica. In this method it is necessary that 

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The catalyst prepared on silica exhibited very high reactivity with 1 -hexene as indicated by a r value of 26, whereas the catalyst prepared without silica exhibited a r value of 125. Hence, the silica-supported catalyst reacts approximately five times better with 1-hexene than the catalyst prepared without the silica. 

The catalyst based on Bis(triphenylsilyl)chromate, [(CgH5)3Si0]2Cr02, was reported by Carrick and coworkers at Union Carbide [17] and is prepared on a dehydrated (110-800 C) support material of silica/alumina or only silica. Catalysts prepared on silica require the addition of an aluminum alkyl to the catalyst preparation step to achieve high activity, while catalysts prepared on silica/alumina [19] do not require an aluminum alkyl in the preparation procedure. 

Examination of Table 3.5 shows that Bis(triphenylsilyl)chromate supported on silica and then treated with various aluminum alkyls is about 100 times more active than similar catalysts prepared on silica/alumina supports without an aluminum alkyl. On a chromium basis, the activity of the silica-supported catalyst is approximately 170,000 g PE/g Cr under the slurry polymerization conditions reported in Table 3.5. Under commercial polymerization conditions in which the effect of impurities is relatively less, the catalyst activity is approximately 1 x 10 g PE/g Cr, assuming a catalyst productivity of approximately 3 Kg PE/g silica-supported catalyst with the catalyst containing 0.3 wt% Cr. 

In this catalyst, Cr[CH(SiMe3)2]3 was supported on silica (surface area of 300 m /g and a pore volume of 1.6 cc/g) that was dehydrated at either 200 or 600°C using hexane as the slurry solvent. The catalyst prepared on silica dehydrated at 600°C also produces 1 -hexene in situ, which results in polymer containing a relatively high number of short-chain branches that reduce the density of the polyethylene. 

Oxazoborolidinone 8 is an example of catalyst supported on silica gel. It is prepared by immobilizing the N-tosyl-0-allyl-(S)-tyrosine with mercaptopropyl silica and treatment with BF3 and has been used to catalyze the Diels-Alder reaction of methacrolein with cyclopentadiene [17] (Equation 4.2). The cycloaddition occurs with good diastereoselectivity but with low enantioselec-tivity. 

Co-Rh catalysts were prepared on silica gel by impregnation with a pentane solution of Rl Cc (CO), After reduction at 823 1C for 20 hours, there were diffraction peaks due to the Co-Rh alloy, but also peaks from the metals themselves. 

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). 

The alkylation of phthalimide has been carried out under phase-transfer conditions by Landini and Rolla,272 using soluble phosphonium salts and later by Tundo,273 using heterogenous phase transfer catalysts immobilized on silica gel. Later the preparation of N-alkylphthalimide has been carried out directly from phthalimide by Santaniello and Ponti.274... 

For comparison with the carbon-based catalysts, several MgO catalysts were prepared on silica and alumina supports, and a very high-sur-face MgO aerogel was also prepared. Testing these under the same conditions gave very poor yields of the desired alcohols. It is clear that the carbon structures promote the activity, very likely by catalyzing the dehydrogenation and hydrogenation steps. 

Bianchini, C., Burnaby, D. G., Evans, J., Frediani, P., Meli, A., Oberhauser, W., Psaro, R., Sordelli, L. and Vizza, F. Preparation, characterization, and performance of tripodal polyphosphine rhodium catalysts immobilized on silica via hydrogen bonding, J. Am. Chem. Soc., 1999, 121, 5961-5971. 

Alternatively, polar liquids other than water can be used to generate supported liquid-phase catalysts with sulfonated ligands. For example, supported liquid-phase Pd-TPPTS catalysts have been prepared on silica with ethylene glycol as the liquid phase. " ... 

Yet another method for the preparation of hydroxycitronellal (35) has been developed it depends on the fact that the immonium salt (36) is hydrated by aqueous sulphuric acid, hydrolysis of the imine group taking place with sodium hydroxide. The effect of catalysts supported on silica gel on the well-known thermal conversion of citronellal to isopulegol has been studied. The abstract... 

Various Pd based catalysts supported on silica and containing manganese have been prepared and characterized. Two Mn species have been detected, i) reduced Mn in direct interaction with Pd on the metallic particles ii) oxidised Mn layed on Si02 and showing reactive oxygens An increase in activity for reduction of NO is observed on the catalysts containing Mn with an optimum for a Pd/Mn atomic ratio of about one. This enhancement of activity is due to either the presence of Pd-Mn dual sites or to a bifiinctional mechanism between reduced Pd and oxidised Mn at the vicinity of Pd. 

In this context, the present work has thus been undertaken to study the influence of manganese on the Pd catalytic properties. Series of Pd-Mn catalysts supported on silica with various Pd/Mn ratios have therefore been prepared and characterized. Their activity has been measured in presence of different CO-NO-62-hydrocarbons mbrtures. 

The phase transformations occurring during sintering of the catalysts are indicated in Fig. 1. Table III shows the relationship between crystalline phase transformations in the standard catalyst, the corresponding pore structure collapse, and the loss in activity upon sintering. Data on the catalysts prepared from silica sols are given in Table IV. Electron micrographs of the sols used for the preparation of catalysts 1, 2, and 3 are shown in Fig. 2. 

The Reppe process is a method that was developed in the 1940s and typical manufacturers include BASF, Ashland, and Invista. Cu-Bi catalyst supported on silica is used to prepare the 1,4-butynediol by reacting formaldehyde and acetylene at 0.5 MPa and 90-110 C (Eq. (10.2)). The copper used in the reaction is converted to copper(I) acetylide, and the copper complex reacts with the additional acetylene to form the active catalyst. The role of bismuth is to inhibit the formation of water-soluble acetylene polymers (i.e., cuprenes) from the oligomeric acetylene complexes on the catalyst [5a]. The hydrogenation of 1,4-butynediol is accom-pUshed through the use of Raney Ni catalyst to produce 1,4-butanediol (Eq. (10.3)). The total yield of 1,4-butanediol production is 91% from acetylene [5b]. Since acetylene is a highly explosive compound, careful process control is necessary. 

As listed in Section S.2.2.2.2, cogelation has been used to prepare Cu-Ru, Pd-Ni, Cu-Ni, Pd-Ag, Pd-Cu, and Pt-Cu catalysts supported on silica. Heinrichs et al. [33, 158] and Lambert et al. [152, 177] examined the size and composition of metal particles in Pd-Ag/SiOj and Pd-Cu/SiOj cogelled xerogel catalysts by X-ray diffraction (XRD) and TEM. XRD results are illustrated in Figure 8.12 in the case of three Pd-Cu/SiOj catalysts. 

In this paper, we present a study in which combustion catalysts based on silica-coated metal monoliths were prepared. The aim of this study was to prepare washcoated metal monoliths with controlled properties. The properties varied are specific surface area of the washcoat and washcoat thickness or washcoat loading. Furthermore, we discuss how the preparation procedure affects the resulting catalyst properties and related performance. We deposited washcoats based on colloidal silica sols. Colloidal silica sols give porous materials with rather narrow pore size distributions when dried and calcined. This gives us excellent control over the pore size distribution of the washcoat, as will be discussed. The technique presented here, allows deposition of washcoats with controllable thickness in one step, unlike techniques based on pure silica sols, reported elsewhere [7,8]. Washcoats were impregnated with paUadium salts to make active catalysts that were tested in methane combustion. The effects of the preparation procedure of the silica and of the impregnation procedure were studied using particulate catalysts. 

The presence of thallium(0) led to an increase in activity and selectivity of metallic palladium catalysts supported on silica in aldose oxidation reactions. However, silica-supported thallium(0) had no activity by itself (entry 3). ° Similarly, the bimetallic catalyst platinum-thallium/ZSM-5, prepared by impregnation of thallium sulfate and chloroplatinic acid on Zeolite Socony Mobil-5 (ZSM-5), showed greater selectivity in propane aromatisation and almost the same catalytic activity as monometallic thallium/ZSM-5 (entry 4). Similar comparison of vanadium-caesium-copper and vanadium-caesium-copper-thallium catalysts supported on TiOa.SiC demonstrated that addition of thallium improved the catalytic activity in partial oxidation of p-tert-butyltoluene to p-tert-butyl-benzaldehyde (entry The application of solid-supported thallium-based catalysts in different processes includes (a) iron-thallium catalysts in carbon monoxide hydrogenations to form hydrocarbons and alcohols, and catalytic reforming of... 

In a manner similar to the four methods mentioned above, rhodium complex catalysts immobilized on silica modified by 2-(MeO)3Si(CH2)2C5H4N and (Me0)3Si(CH2)30C0CMe=CH2 using [Rh(CO)2Cl]2 as the precursor are prepared These immobilized pyridine-Rh complexes are shown to be active catalysts in the... 

Fig. 13 Molar mass of HDPE/UHMWPE reactor blends prepared on silica-cosupported Cr-1/ Fe-2/Cr-3 multi-site catalysts with variable amount of Cr-3 (Reprinted with the permission of [190] Copyright 2010 Wiley VCH)...

Carvone-derived 2,3-epoxy alcohol derivatives (317) have been found to rearrange with stereoselective formation of ring-contracted ketones (318) in a process where the stereochemical result seems to be independent of epoxide configuration. On the basis of this study, the authors concluded that the rearrangement of tetrasubstituted cyclic epoxy alcohol derivatives remains a transformation for which both regiose-lectivity and stereochemical outcome are difficult to predict. New solid acid catalysts based on silica-supported zinc triflate have been prepared for use in the rearrangement of a-pinene oxide to campholenic aldehyde. ... 

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