Sulfur poisoning of rhodium-alumina catalysts in toluene and ethylbenzene steam reforming.
Résumé
Sulfur poisoning in steam reforming of PhMe is studied on well-characterized Rh-Al2O3 catalysts by using 2 poisoning procedures, i.e., addition of 0-250 ppm S (as thiophene) in the PhMe and presulfiding of the catalyst. These procedures result in very different behaviors of the catalysts with respect to poisoning. When S is added to the feed, the catalyst undergoes a quasi-frontal deactivation. Coke is, by far, the main poisoning agent at the beginning of the reaction (0-1 h). This period is followed by a linear deactivation with time-on-stream depending on the S content in the feed. The proper effect of S on catalyst deactivation is assessed by assuming that both effects (coke and S) are additive. The S coverage at 0 conversion slightly decreases as particle size increases (0.9 to 0.6 for particle sizes 10 and 55 .ANG., resp., indicating an increased S resistance for well-dispersed catalysts. S added to the feed has no effect on selectivity. When the catalysts are presulfided, the residual catalytic activity is proportional to the H chemisorption capacity of the sample. In this case, S exhibits a strong effect on the selectivities: the ring opening reaction is practically suppressed and the catalyst becomes very selective to C6H6. These results are explained by a possible electronic effect of S on the Rh particles. In steam reforming of PhEt, presulfiding has practically no inhibiting influence on dehydrogenation to styrene; on the other hand, ring opening is virtually suppressed, the presulfided catalyst being also more selective to dehydrogenation and, to a lesser extent, more selective to dealkylation.