Platinum on silica gel

A platinum on silica gel catalyst was prepared by impregnation of silica gel (BDH, for chromatographic adsorption) by a solution containing 0.5% (wt.) of sodium hydroxide and 0.5% (wt.) of chloroplatinic acid (both of analytical grade). The dried catalyst contained 1% (wt.) of platinum and a corresponding amount of the alkaline component. The BET surface area of the catalyst was 40 m2/g, the mean pore radius 150 A. The catalyst was always reduced directly in the reactor in a stream of hydrogen at 200°C for 2 hr. 

A procedure similar to that used in the investigation of the hydro-demethylation of xylenes was also employed in a study of the consecutive hydrogenation of phenol via cyclohexanone to cyclohexanol in gaseous phase on a platinum on silica gel catalyst (p. 27) at 150°C [scheme (VI)] at this temperature both reactions were irreversible under the excess hydrogen used. 

The rate of dehydrocyclization is probably often inhibited by (bicyclic) product desorption. Apparent first-order rate constants in the dehydrocyclization of -butylbenzene over platinum-on-silica-gel decreased with increasing levels of bicyclic aromatic product (Fig. 4). Sinfelt and co-workers also found that product desorption is rate-controlling in methylcyclohexane aromatization (20). 

Fig. 4. Dehydrocyclization of n-butylbenzene over 2% platinum on silica gel catalyst. (—) Cyclization to methylindan and methylindenes and (---) cyclization to naphthalene. According to Csicsery (13).

The methyl group in the y-position of the side-chain interferes with cyclization. The rate of C5-cyclization of n-butylbenzene at 371°C over platinum-on-silica gel is 3.5 times higher than that of 2-phenylpentane (Table IV) (14). The difference in C6-cyclization is even larger. Now, the side-chain carbon atoms involved in the cyclizations of n-butylbenzene and 2-phenylpentane have identical natures (i.e., secondary in five-membered ring closure and primary in six-membered ring closure). The difference between the two molecules (the extra methyl group) is far removed from the two carbon atoms involved in the formation of the new bond ... 

Cyclization selectivities are very different over platinum on silica-alumina than over platinum on silica gel (Table IV). In the case of n-butylbenzene, for example, methylindan/naphthalene ratios differ by about an order of... 

Platinum on alumina has properties between those of platinum on silica gel and platinum on silica-alumina. Only about one-fifth of the methylindan is produced by the acid-catalyzed route (13). Also, the isomerization of... 

Cyclization of phenylbutenes over the platinum-on-silica gel catalyst almost exactly parallels that of -butylbenzene. Since rapid hydrogenation of the phenylbutenes results in about the same n-butylbenzene/phenylbutene ratio as observed with the n-butylbenzene feed, this is hardly unexpected. 

Isomerization over the neutral platinum-on-silica gel catalyst proceeds by two different mechanisms. 

Figure 11 shows conversion to iso-heptanes to be negligible for (0.5 wt. %) platinum supported on activated carbon (Pt/C) as the only catalyst, and also for (0.4 wt. %) platinum on silica-gel (Pt/Si02). No detectable conversion was obtained with silica-alumina. A mechanical mixture of either of the Pt-bearing particles with silica-alumina of about 150 m.Vg-surface area, both in millimeter diameter particle size (1000m), immediately resulted in appreciable isomerization ( SiAl with Pt/C SiAl with Pt/Si02). Isomerization increases rapidly for smaller component particle sizes, of 70/i and S i diameters. It approaches the performance of a silica-alumina that has been directly impregnated with platinum, and which has... 

Of interest is the tendency of platinum on silica-gel to hydrogenolyze the five-membered ring. The cyclane-alkane part of the catalysate obtained in contact with this catalyst contained 85.2% alkanes, of which 55.2% were the isoalkanes of the composition Cv-Ce. 

A platinum-iron on silica gel catalyst was prepared by impregnating silica gel (BDH, for chromatographic adsorption) with an aqueous solution of chloroplatinic acid (analytical grade) and sodium hydroxide (analytical grade). The dry product was then impregnated by a ferrous sulfate solution (C.P. grade) and the water was removed in a rotating evaporator. The prepared catalyst contained 1% Pt, 0.7% Fe, and 2% NaOH (by... 

From the study of the influencing of single reactions by products and by other added substances and from the analysis of mutual influencing of reactions in coupled systems, the following conclusions can be drawn concerning adsorption of the reaction components. (1) With the exception of crotyl alcohol on the platinum-iron-silica gel catalyst, all the substances present in the coupled system, i.e. reactants, intermediate products, and final products, always adsorbed on the same sites of the catalytic surface (competitive adsorption). This nonspecificity was established also in our other studies (see Section IV.F.2) and was stated also by, for example, Smith and Prater (32), (2) The adsorption of starting reactants and the desorption of the intermediate and final products appeared in our studies always as faster, relative to the rate of chemical transformations of adsorbed substances on the surface of the catalyst. 

The general procedure for the electrochemical preparation of (10) is as follows. A solution of (9) (3 mmol) in wet acetonitrile (40 ml, 5 vol.% of H20) containing sodium perchlorate (0.25 m) was placed in an undivided electrolysis cell equipped with a platinum plate anode and a platinum plate cathode. The system was subjected to a constant current electrolysis (300 mA current density, 20mAcnr2) at ambient temperature. After 4 faradays per mole of (9) had been consumed, the electrolysed solution was poured into water (50 ml) and extracted with dichloromethane (3 X 30 ml). The organic layer was dried with magnesium sulfate and concentrated under reduced pressure. The residue was chromatographed on silica gel to afford (10) in an excellent yield. 

In this project, we make use of platinum-type catalyst on silica gel. Although this is less selective than more modem palladium-based catalysts, kinetic data are available in the literature as an LHHW model [2], better suited for flexible reactor design. The reaction rate equations are ... 

Carbon monoxide undergoes activated adsorption on the surface of palladium oxide. The maximum for this process, at about 350 mm. pressure, is at about 100°C. The gas taken up during activated adsorption can only be recovered as C02 for the most part (57). In a CO-air stream a slight initial reduction of PdO occurs at 23°C., but in the absence of oxygen, there is no reduction below 76°. This process of reduction decreases in rate with time and does not go to completion below 156°. Carbon dioxide, when present in the gas phase, inhibits the reduction of the palladium at 100°C. because it is adsorbed strongly by the PdO (56). Catalysts have been prepared by the deposition of palladium and platinum on asbestos, on silica gel, and on charcoal. 

Cyanamide and melamine can be obtained by llCN oxidation in the gas phase on silica gel [41 j (Section 3.3) and the catalytic oxidation of HCN on platinum has been reported as the most convenient and least polluting method for HCN destruction (54). 

Yu and Halperin (162) observed a -Pt surface resonance from platinum particles initially prepared on silica gel the carrier was removed afterwards with a solution of sodium hydroxide, thus forming a self-supported pow -der sample. Its average particle diameter determined by TEM was 4 nm assuming a log-normal distribution, this corresponds to a dispersion of 0.31. The NMR samples w ere extensively w ashed with, water, dried, and left exposed to the atmosphere. A signal detected at 1.089 G/kHz w as attributed... 

Hydrogen and a Catalyst. The most common catalysts are platinum and ruthenium, but homogeneous catalysts have also been used, including copper on silica gel and a ruthenium catalyst on mesoporous silica. Before the discovery of the metal hydrides this was one of the most common ways of effecting this reduction, but it suffers from the fact that C=C, C=C, C=N, and C=N bonds are more susceptible to attack than C=0 bonds. For aromatic aldehydes and ketones, reduction to the hydrocarbon (19-61) is a side reaction, stemming from hydrogenolysis of the alcohol initially produced (19-54). 

In complete contrast with these developments, Kobosev approached the problem of supported metal catalysts from the viewpoint of atomic dispersion and from the viewpoint of the theory of active ensembles which bears his name. Over a period of years, Kobosev and co-workers investigated the behavior of catalysts containing very small amounts of supported metal. Many of their observations have been reviewed recently (2) and they inspired Poltorak to investigate very carefully dilute platinum catalysts on silica gel (3, 4). The studies from Poltorak s laboratory, originating from the desire to check the theoretical and experimental results of Kobosev, have led independently to many of the same conclusions reached by ourselves starting from the commercially available platinum catalysts for reforming (5). 

On the other hand, working with a more restricted range of highly dispersed platinum catalysts on silica gel similar to those examined by Benesi et al. (31), Poltorak and co-workers (3, 4, 37-39) succeeded in... 

Two Si-Si bonds in a molecule of bis(disilanyl)dithiane 32 oxidatively add to isonitrile-platinum(O) complex to give tetrakis(organosilyl)platinum(IV) complex 33, which is isolated by column chromatography on silica gel (Eq. 14) [23]. 

Kohli, J. C., and Kumar, S. (1991). Novel procedure for the thin-layer chromatography of terpinoids on platinum ion-silica gel layers. Natl. Acad. Sci. Lett. (India). 14 325-329. 

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