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Literature Cited Huang, C. P.; Richardsen, J. T. J . Catal. 1978, 51, 1. Morbidelli, M.; Servida, A.; Varma, A. Ind. Eng. Chem. Fundam. 1982, 21, 278. Schneider, P.; Gelbin, D. Chem. Eng. Sci. 1985, 40, 1093. Wedel, S.; Luss, D. Ind. Eng. Chem. Fundam. 1984, 23, 280.
Wu, H.; Yuan, Q.; Zhu, B. Huagong Xuebao 1984,4, 283. Wu, H. Ph.D. Dissertation, Dalian Institute of Chemical Physics, Chinese Acadamy of Sciences, 1986. WU, H.; Yuan, Q.;Zhu, B. Huagong Xuebao 1988, in press. Received for review September 4, 1987 Accepted February 2, 1988
Hydroisomerization and Hydrocracking of n -Alkanes. 3. n -Heptane Transformation on PtH Offretite Catalysts Giuseppe E. Giannetto Universidad Central de Venezuela, Facultad de Zngenieria, Dipartimento de Quimica Aplicada,
Los Chaguaramos, Caracas, Venezuela
Fernanda B. Alvarez and Michel R. Guisnet* U A CNRS 350, Catalyse e n Chimie Organique, Uniuersit6 de Poitiers, UFR Sciences, 40 avenue du Recteur Pineau, 86022 Poitiers, Cedex, France
The transformation of n-heptane was studied a t 250 "C, 1 atm, and pH /pnSheptsne = 9 on a series of PtH offretite catalysts (PtHOFF) containing from 0.06 to 1.25 w t % platinum with a dispersion equal to or higher than 90%. Adsorption measurements show that platinum does not modify the crystallinity of offretite but causes a blockage of some of the large channels. This explains why platinum has only a slight positive effect on the rate of n-heptane transformation. The reaction products can be divided in two families: (i) linear cracking products formed in the gmelinite cages and in the large channels inaccessible to branched hydrocarbons because of the platinum (their distributions, 55-60 mol 90CB,30-35% n-C4, and 5 1 0 % n-C6,can be explained by cracking and alkylation reactions whose linear products alone can diffuse in the gas phase); (ii) isomers and isobutane plus an equimolar amount of propane formed through the classical bifunctional mechanism in the unobstructed large channels. We have shown (Giannetto et al., 1986) that the activity, the stability, and the selectivity of PtHY catalysts for n-heptane transformation depend essentially on the ratio of the number of their active acid sites to the number of their hydrogenating sites. The activity per acid site increases at first with this ratio and then remains constant, as can be expected with the classical bifunctional mechanism. For low values of this ratio, the monobranched isomers M, the bibranched isomers B, and the cracking products C (isobutane + propane) are apparently primary products, whereas at high values the step by step process n-C7 e M F! B C can be observed. The pore structure of the zeolite used as a platinum carrier also plays a significant role (Martens et al., 1984; Guisnet et al., 1987). Thus, when the diffusion of the molecules is monodimensional like on PtH mordenite, platinum and/or carbonaceous compounds deposited during the first moments of the reaction can easily block access to the active sites, which is not the case with tridimensional diffusion (like on PtHY). The size of the pores as well as the size and the shape of the space available near the acid centers can also affect the selectivity of n-heptane transformation on bifunctional zeolitic catalysts. Thus, the differences in selectivity between PtHY and PtHZSM-5 catalysts are due to the slow migration of certain olefinic intermediates in the ZSM-5 channels and to steric constraints (or to a good matching of acid sitesintermediates) at channel intersections (Guisnet et al., 1987). The study of the effect of the pore structure on n-heptane transformation is extended here to the case of PtH offretite catalysts (PtHOFF). Offretite is a zeolite which has remarkable selectivity as a dewaxing catalyst (Chen et al., 1984). It has two types of channels: rectilinear
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*To whom correspondence should be addressed.
cylindrical channels of about 6.5 A in diameter connected by cavities known as gmelinite cages whose elliptical apertures measure 3.6 X 5.2 A (Whyte et al., 1971). Numerous organic compounds (linear or branched alkanes or olefins, methylbenzenes, pyridine, etc.) can diffuse in the large channels, whereas only linear hydrocarbons can enter the gmelinite cages. By pyridine poisoning it was shown that both the large channels and the gmelinite cages of HOFF have very strong acid sites (Bourdillon et al., 1986). The same technique was used here in order to distinguish between n-heptane transformation in the gmelinite cages and in the large channels of PtHOFF catalysts.
Experimental Section HOFF (K0,ZH3~gA14~1Si13.9036) was supplied by Grace Davison. X-ray diffraction spectrum shows that this zeolite is perfectly crystallized but probably contains some stacking faults (this is indicated by the existence of small typical peaks (Chen et al., 1984)). Its adsorption capacity was measured for nitrogen (0.25 cm3 g-l), for n-hexane (0.217 cm3g-l), and for m-xylene (0.13 cm3 g-l). The values obtained were consistent with the offretite structure (Breck and Grose, 1973). Five PtHOFF catalysts with 0.06,0.15, 0.25, 0.40, and 1.25 wt % platinum were prepared by competitive ion exchange with a 1/200 (PtNH3)4C12/ NH4NO3solution. Calcination and reduction of the samples were carried out at 573 K and at 773 K, respectively, under the conditions already reported for PtHZSM-5 catalysts (Giannetto et al., 1985). The average size of the platinum crystallites was estimated by transmission electron microscopy. The n-heptane transformation was carried out at atmospheric pressure in a flow reactor at 523 K with a hydrogen/hydrocarbon molar ratio equal to 9. The activities ((molar flow rate x conversion)/weight of catalyst) were measured below a 10% conversion. Different conversions
0888-58851 8812627-1174$01.50/0 0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 7,1988 1175
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Figure 1. Electron microscopy transmission view of 0.4 wt % PtHOFF.
were obtained by modifying the contact time (200-500 mg of catalyst, 0.1-12 cm3 h-l n-heptane flow rate). Two procedures were followed to determine the effect of pyridine poisoning on the activity and on the selectivity of the samples for n-heptane transformation. On HOFF, whose deactivation by coke is very fast, the behavior of a previously poisoned sample and the behavior of a nonpoisoned sample were compared, whereas on 0.06 and 0.4 wt % PtHOFF catalysts the effect of pyridine was determined by using a fresh sample and then the same sample poisoned. In both cases, poisoning was carried out at 423 K by injection of a large excess of pyridine (about 4 cm3g-l of catalyst) in several successive pulses, the catalyst being under a dry hydrogen flow. The reaction temperature was then increased up to 523 K and the n-heptane transformation studied after about 30 min at this temperature under hydrogen flow.
Results 1. Characterization of PtHOFF Catalysts. 1.1. Crystallinity and Porosity. The X-ray diffraction spectra of 1.25 w t % PtHOFF and of HOFF were identical, which shows that the introduction of platinum does not modify the crystallinity. Moreover, this is confirmed by nitrogen adsorption at 77 K the porous volumes accessible to nitrogen are practically the same in HOFF (0.25 cm3g-l) and in 1.25 wt % PtHOFF (0.245 cm3 g-l). However, blockage of the porosity was shown by adsorption experiments with n-hexane at 273 K and particularly with m-xylene at 303 K: the volume accessible to n-hexane was slightly smaller in 1.25 wt % PtHOFF than in HOFF (0.19 against 0.217 cm3 g-l), whereas the volume accessible to m-xylene was 10 times smaller in 1.25 wt % PtHOFF than in HOFF (0.013 cm3 g-' against 0.13 cm3 g-l). 1.2. Platinum Crystallite Size. By electron microscopy (see for example 0.4 wt % PtHOFF in Figure l), it was shown that on all the catalysts the size of the platinum crystallites is less than 10 A (dispersion
