Platinum catalysts hydrocarbon conversion

P. Biloeu, J.N. Helle, H. Verbeek, F.M. Dautzenberg and W.M.H. Sachtler, The role of rhenium and sulfur in platinum-based hydrocarbon-conversion catalysts, J. Catal. 63 (1980) 112-118. 

Complete reforming kinetics have been developed for several commercial catalysts, including those used in Mobil reformers. Since KINPTR affects Mobil s business strategy, the complete reforming kinetics are proprietary. However, as an example, KINPTR C6 kinetics will be presented for UOP s R16H platinum-rhenium-alumina catalyst. Both the hydrocarbon conversion and the deactivation equations [Eqs. (36), (40)] can be directly applied to the C6 system. For the C6 hydrocarbon conversion, Eq. (40) becomes... 

Davis, S.M., Zaera, F. and Somorjai, G.A. (1982) The reactivity and composition of strongly adsorbed carbonaceous deposits on platinum. Model of the working hydrocarbon conversion catalyst. J. Catal., 77, 439. 

In the field of hydrocarbon conversions, N. D. Zelinskii and his numerous co-workers have published much important information since 1911. Zelinskii s method for the selective dehydrogenation of cyclohexanes over platinum and palladium was first applied to analytical work (155,351,438,439), but in recent years attempts have been made to use it industrially for the manufacture of aromatics from the cyclohexanes contained in petroleum. In addition, nickel on alumina was used for this purpose by V. I. Komarewsky in 1924 (444) and subsequently by N. I. Shuikin (454,455,456). Hydrogen disproportionation of cyclohexenes over platinum or palladium discovered by N. D. Zelinskii (331,387) is a related field of research. Studies of hydrogen disproportionation are being continued, and their application is being extended to compounds such as alkenyl cyclohexanes. The dehydrocyclization of paraffins was reported by this institute (Kazanskil and Plate) simultaneously with B. L. Moldavskil and co-workers and with Karzhev (1937). The catalysts employed by this school have also been tested for the desulfurization of petroleum and shale oil fractions by hydrogenation under atmospheric pressure. Substantial sulfur removal was achieved by the use of platinum and nickel on alumina (392). 

The reforming of petroleum fractions boiling between about 90 and 200°C. to high octane gasolines constitutes one of the largest scale industrial catalytic operations of our times. The quantity processed over platinum catalysts exceeds 2 X 10 liters/day. A majority of the reactions involved are polystep hydrocarbon conversions (see refs. 10,11, and the extensive review of the art by Ciapetta et al., 31). 

The comparison (Fig. 1 and 6) between NO oxidation by oxygen in the absence of hydrocarbon and NO reduction in the presence of C10H22 and O2 shows that the oxidation and the reduction of NO are comparable and occur in the same temperature range with platinum or ruthenium catalysts, the conversion rates being very low with ruthenium. With rhodium or iridium and more particularly with palladium or copper NO oxidation is very limited at the temperature where NO reduction occurs. 

The choice of hydrocarbon nature determines N2 and N2O selectivities. But the temperature range of NO conversion and the yield in nitrogen are function of the chain length (Table 2). On platinum catalysts, the best nitrogen yield are obtained with long chain alkanes at low reduction temperature and with unsaturated hydrocarbons like toluene and ethylene at higher reduction temperature. 

The selective reduction of NO by hydrocarbon in an excess of oxygen was studied using a platinum catalyst doped or not with zinc. Successive impregnetion or co-impregnation of Pt and Zn on alumina were made. In some cases, in the presence of Zn, the NO conversion is increased in parallel with N2 formation. A better conversion of hydrocarbons was also observed. EXAFS experiments and N2O decomposition experiments have been carried out to explain these observations. 

Figure 7.1. Block diagram of hydrocarbon conversion over platinum catalysts showing the approximate range of reaction rates and temperature ranges that are most commonly studied.

Structure Sensitivity of Hydrocarbon Conversion Reactions on Platinum Surfaces How does the reaction rate depend on the atomic structure of the platinum catalyst surface To answer this question, reaction rate studies using flat, stepped, and kinked single-crystal surfaces with variable surface structure were very useful indeed. For the important aromatization reactions of n-hexane to benzene and Ai-heptane to toluene, it was discovered that the hexagonal platinum surface where each surface atom is surrounded by six nearest neighbors is three to seven times more active than the platinum surface with the square unit cell [155, 156]. Aromatization reaction rates increase further on stepped and kinked platinum surfaces. Maximum aromatization activity is achieved on stepped surfaces with terraces about five atoms wide with hexagonal orientation, as indicated by reaction rate studies over more than 10 different crystal surfaces with varied terrace orientation and step and kink concentrations (Figure 7.38). 

Carbonaceous Overlayers What is the composition of the working platinum catalyst surface When the surface is examined after carrying out any one of the hydrocarbon conversion reactions, it is always covered by a near-monolayer amount of carbonaceous deposit. 

S.M. Davis, F. Zaera, and G.A. Somorjai. The Reactivity and Composition of Strongly Adsorbed Carbonaceous Deposits on Platinum. Model of the Working Hydrocarbon Conversion Catalyst. J. Catal. ll A >9 (1982). 

For these tests both Catalyst "C" and Catalyst "D" had platinum loading levels of ca. 2 g/ft. The test results for gas phase hydrocarbon conversion and SOF removal and conversion are shown in Figure la and lb, respectively. 

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