Effects of oxidation–reduction treatments of Pt/Al2O3 on catalytic activity and selectivity for hexane reforming

Abstract

Consecutive oxidation-reduction cycles involving successive oxidation treatments of 0.3 % Pt/AI,O, at various temperatures, each followed by a reduction treatment in hydrogen to produce active catalyst, lead to changes in catalytic activity for the reactions of n-hexane with hydrogen. Decreased activity is paralleled by changes in selectivity brought about by the increased tendency of large platinum particles towards coking. Changes in selectivity are not observed where an increase in activity occurs. Results show that the reforming reactions of n-hexane can be used to gain information about the state of metal particles following sintering/redispersion of Pt on alumina.Catalytic reforming of hydrocarbons in industry currently involves a bimetallic or multimetallic catalyst containing from 0.2 to 0.75 YO platinum by weight, supported on chlorided alumina.' In order to counteract losses in catalytic activity and to maintain a desirable product distribution in the reformate, an increase in operating temperature with time is required. Regeneration of deactivated catalysts involves the oxidation of carbon-rich deposits, and redispersion of the metallic elements of the catalyst. The redispersion mechanism by oxychlorination is not fully understood, although oxygen and chlorine are essential requirements for the process. 2, A previous paper4 described the effect of the temperature of oxygen treatment during oxidation-reduction cycles on the dispersion of platinum supported on chlorided y-Al,03. Infrared spectroscopy was used to monitor CO adsorption, which gave an indication of changes in the dispersion of platinum during the cycle^.^ This paper reports the effect of corresponding oxidation-reduction cycles on the activity and selectivities5 for the reaction of n-hexane with hydrogen over chlorided Pt/A120,.

Metal Redispersion Strategies for Recycling of Supported Metal Catalysts: A Perspective

“…The results of the study with CCl4 reinforced the previously reported view 163,164,166 that the creation of oxychloroplatinum or chloroplatinum species was the important step in the redispersion process. Two sintered Pt/Al2O3 catalysts (0.6 and 1 wt.% Pt) underwent successful Pt re-dispersion at 500 °C in an oxychlorination treatment with 2% O2 in a HCl/H2O/N2 feed (H2O to HCl molar ratio of 56) for up to 8 h. 167 The treated samples were analysed using TPR, hydrogen chemisorption, In another study of a Pt/Al2O3 catalyst (which was also prechlorided with HCl during impregnation) which used HCl in the oxychlorination treatment, the effect of oxygen partial pressure (from 0 to 100%) and HCl concentration (from 0.0005 to 0.003 atm) on re-dispersion were assessed.…”

“…161 The presence of this complex was also confirmed with UV-Vis in the re-dispersed catalysts prior to sintering. 162 The Pt redispersion process on pre chlorided Pt/Al2O3 catalysts has also been investigated in IR studies 163,164 as well as XRD and TEM. 84 It was noted that Pt re-dispersion was accompanied by an increase in Pt oxidation state; the oxidation of crystalline Pt and Pt re-dispersion were found to be conjoined processes.…”

n-Alkane hydroconversion over carbided molybdena supported on sulfated zirconia

“…Note that under the reaction conditions employed, no cyclised or dehydrocyclised products were detected. This contrast with the behavior of Pt-based catalysts under equivalent conditions (Anderson et al 1989). With n-octane and n-nonane, the selectivity to hydrocracking products reached 100% at all the temperatures.…”

Sintering of Supported Metal Catalysts

“…Consecutive oxidation-reduction cycles involving successive oxidation treatments of 0.3%Pt/Al 2 O 3 c a t a l y s t w e r e m a d e a t v a r i o u s temperatures, each followed by a reduction treatment in H 2 to produce an active catalyst, lead to changes in catalytic activity for the reactions of n-hexane with H 2 . Decreased activity is paralleled by changes in selectivity brought about by the increased tendency of large Pt particles towards coking [Anderson,et al,1989]. Results show that the reforming reactions of n-hexane can be used to gain information about the state of metal particles following sintering/redispersion of Pt on alumina.…”