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Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 2, 1979
The Influence of Chloriding on Isomerization and Hydrogenolysis of n-Pentane on Pt-AI,03 Reforming Catalysts P. Govlnd Menon, Rudy P. De Pauw,' and Gilbert F. Froment" Laboratorium voor Petrochemische Techniek, Rijksuniversiteit Gent, Krijgslaan 27 1, 9000 Gent, Belgium
The influence of chloriding with CCI, in H, at 300-436 "C upon the isomerization and hydrogenolysis of n-pentane on Pt-AI2O3 reforming catalysts was investigated in a pulse micro-reactor and in a bench-scale integral reactor. The chemisorption of H, and its temperature programmed desorption (TPD) were used to characterize the F't surface resulting from the chloriding and other pretreatments. I f H, is desorbed profusely from the catalyst at 50-250 "C, it is an indication of the strong hydrogenolysis activity of the catalyst. Chlorided catalysts exhibit H, desorption mostly in the 250-500 "C range. The coke deposited on the catalyst a s a result of chloriding with CCll affects the hydrogenolysis more than the isomerization. Chlorine alone on the catalyst, without coke, has only a minor effect on hydrogenolysis; this was shown by comparable CI content (5-7 wt % ) on the catalyst brought about by using dry HCI gas instead of CCI4 in H,. The rate of coking in hydrogenolysis on a Li-doped nonacidic catalyst is found to be more than double that during bifunctional catalysis on the chlorided catalyst.
Introduction The isomerization of n-paraffins is one of the most desired reactions in the catalytic reforming of naphtha to produce high-octane-number gasoline. It is also carried out as a separate process at relatively lower temperatures than in reforming. The influence of chloriding on isomerization activity of Pt-A1203 bifunctional catalyst has been studied by several workers. Goble and Lawrence (1964) found that with CC14 in N2 or air the basic chlorination reaction involved an exchange of two C1 atoms in C C 4for each surface oxygen in alumina to give an active lowtemperature isomerization catalyst, but chlorination in H2 or with HC1 yielded an inactive catalyst. Giannetti and Sebulsky (19691, who activated their catalysts with chlorides of sulfur, found that a pretreatment with HCl enhanced the effect of subsequent sulfo-chlorination. Myers (1971) used anhydrous HC1 a t 480-650 "C to activate catalysts for butane isomerization at 60 "C; he found that a pretreatment of the catalyst with H2 before its chlorination enhanced the catalytic activity. From the identical weight increase observed on chlorinating A1203 and Pt-A1203 with HC1, Massoth (1972) concluded that the chlorination reaction per se was independent of the presence of Pt. On the other hand, results obtained from infrared spectra by Primet et al. (1975) and from electron spin resonance by Sivasankar et al. (1977) suggest the existence of Pt-C1 species in the Pt-A1203 catalyst as it is normally used, even before any additional chloriding. These somewhat contradictory results raise a few basic questions: What is the role of the chloride on the catalyst in isomerization and hydrogenolysis reactions? During chloriding, does chlorine add on to A1203only, or to Pt also? How does the added chlorine affect the activity stability of the catalyst a t conditions a t or near those of catalytic reforming? To what extent is this added chlorine stripped off from the catalyst by moisture in the feed or during regeneration (coke burn-off) of the catalyst? Answers to the above questions have been sought in the present work by studies on commercial Pt-A1203 reforming catalysts using the following techniques: (a) H2 chemisorption on the exposed Pt metal surface in the catalyst, (b) temperature programmed desorption of H2 from the catalyst, and (c) pulse micro-reactor studies on the reTexaco Belgium, Brussels. 0019-7890/79/1218-0110$01 .OO/O
actions of n-pentane. The inferences from the results of these studies were then tested for n-pentane isomerization on a bench-scale continuous-flow integral reactor. The composite reaction mechanism proposed was finally subjected to a kinetic simulation, based on the earlier kinetic work on the n-pentane-Pt-A1203 system in this laboratory by Hosten and Froment (1971) and De Pauw and Froment (1975). Experimental Section A pulse micro-reactor, provided with a gas sampling valve and a temperature programmed tubular heater (Figure 1) was used for the major part of the work in the present studies. This setup could be used (a) as a micro-catalytic pulse reactor for catalytic activity studies, (b) a t room temperature for H2-02 titration of the Pt metal surface, and (c) for temperature-programmed desorption (TPD) of H2from the catalyst in the range 2 0 0 "C. For (b) and (c), the gas chromatographic column was shortcircuited and only the thermal conductivity detector (TCD) of the gas chromatograph (GC) was needed, but a capillary tube or a packed GC column was necessary at the exit of the gas stream (after the TCD) to give a line pressure of about 1.3 bar in the system. For (c), the U-tube between the reactor and the TCD was cooled in liquid nitrogen to freeze out any moisture or HC1 evolved from the catalyst during the heating to 600 "C. Also, 1-2 g of catalyst (0.1-0.4 mm size) was taken for (b) and (c), but only 150-500 mg (
