Kinetic Analysis of C8 Dehydrocyclization Reactions on Mono and Bifunctional Platinum -Alumina Catalyst
The dehydrocyclization of n-octane and iso-octane to ethyl benzene, o-, m-, and p- xylenes was investigated on a mono - and bifunctional Pt/A12O3 catalyst, using pulse technique with hydrogen as carrier gas at a pressure of 1.8 atm and between 563 and 7630K. Similarly, the isomerization of the C8 - aromatics was also studied in the temperature range 513 -6730K and 1.8atm pressure. The total conversions of n-octane and iso-octane on bifunctional catalyst were found to decrease with increasing temperatures for all pulse sizes investigated. On monofunctional Pt/A12O3, however, the total conversion of n-octane increased with temperature, passing through a maximum at 6130K. The total conversion of each of the xylene isomers and of ethyl benzene also went through a maximum as the temperature increased. There was a large initial production of iso-octane when n-octane reacted on bifunctional Pt/A12O3, catalyst, the iso-octane production went through minimum as the temperature increased: For iso-octane reaction on hifunctional Pt/A12O3 and n-octane reaction on monofunctional Pt/A12O3, however, there was a large initial production of ethylbenzene which also went through a minimum as the temperature increased. Each of the yields of ethyl benzene and o-xylene went through a maximum as temperature increased for n-octane dehydrocylization on acidic Pt/A12O3. As regards iso-octane dehydrocylization on Pt/A12O3, O, p- xylenes yield went through their respective maxima. The same behaviour was observed when n-octane reacted on monofunctional Pt/A12O3. The selectivity of ethylbenzene to O-xylene increased with temperature for all pulse sizes investigated, while that of ethylbenzene to iso -octane went through a maximum when noctane reacted on bifunctional catalyst. On mono-functional catalyst, however, the selectivity of ethyl benzene to o-xylene decreased with temperature and passed through a minimum. With respect to the reactions of the C8-aromatics on bifunctional and mono functional Pt/A12O3, the onset of reaction occurs at a lower temperature and the initial rates were higher on the bifunctional Pt/A12O3. Furthermore the bifunctional catalyst was found to be very selective in the production of p-xylene and p- and o-xylene at temperature less than 563 and 5930K, respectively. The Wei-Prater method was used for the calculation of absolute rate constants for ethylbenzene isomerization on both catalysts. Generally, the rate constants were higher on bifunctional Pt/A12O3. Estimated activation energies appears to account for the high yield of ortho from meta xylene during ethylbenzene isomerization on bifunctional Pt/A12O3.