Heptanes, isomerization

Figure 2. The effect of adamantane addition on n-heptane isomerization turnover rates [473 K, 650 kPa H2, 100 kPa n-heptane].

Figure 4 illustrates the surface area-reaction temperature relationship for maximum isomer yield in the n-heptane isomerization experiments. The curve shows the reaction... 

Heptane Isomerization—It is not expected that isomerization of a heptane fraction per se will be commercially feasible since straight run heptanes are a choice stock for catalytic reforming. The estimated equilibrium octane number (RON clear) for C7 paraffins at 98°F is only about 82 using Rossini s equilibrium data. [These data have been checked experimentally at 98.2°F by G. M. Kramer and A. Schriesheim (44). Good agreement was obtained except for the 2,2-DMP and 3,3-DMP which underwent side reactions.]... 

Figure 20.11 Mechanistic scheme for n-heptane isomerization according to a bond-shift mechanism with a metallocyclobutane intermediate.

Figure 20.7 Activation period for rc-heptane isomerization over Mo03-carbon-modified at...

Noble metal catalysts are known to be easily poisoned by sulfur. Tungsten carbide appeared to show noble metal characteristics, yet was found to be sulfur resistant. The n-heptane isomerization reaction described above was repeated with the two catalysts unsupported W2C and 0.3% Pt/Al203. 

The acidity of perfluorinated sulfonic acids can be increased further by complexa-tion with Lewis acid fluorides, such as SbF5, TaF5, and NbF5.183 They have been found to be effective catalysts for n-hexane, n-heptane isomerization, alkylation of benzene, and transalkylation of alkylbenzenes (see Chapter 5). 

The applicability of the intimacy criterion has been demonstrated 19) in a series of tests on n-heptane isomerization under conventional operating conditions, i.e., at elevated hydrogen partial pressure where catalyst deactivation is minimized. The reaction was examined over mechanically distinct but mixed particles of X (Pt-bearing particles) and Y (silica-alumina) of varying particle size R (equal for both types of particles) in 50-50 volume proportion as well as over single type catalyst in the reaction zone. The reaction conditions provided a partial pressure of n-heptane of 2.5 atm., of hydrogen of 20 atm., and a residence time of 17 sec. 

Fig. 13. n-Heptane isomerization and hydrogenative cracking over mixed catalysts of varying proportion of platinum and acidic component. 

A transition to nontrivial polystep conditions can easily be made by increasing pressure and/or lowering temperature. At 380°C., and 12 atm. pressure, equilibrium partial pressure of cyclo-olefin is of the order of 10 atm. and the equilibrium conversion for the first step is then no greater than 0.1%. The required particle intimacy (component particle size) now approaches that in the case of n-heptane isomerization above. Table VII below shows an experimental demonstration of the mixed component catalyst operation under these conditions. 

SiC-Supported M0O3-Carbon-Modified Catalyst for the //-Heptane Isomerization... 

M. Ledoux, C. Pham-Huu, P. Del Gallo and E. Peschiera, n-Hexane and n-heptane isomerization at atmospheric and medium pressure on MoO3-carbon-modified supported on SiC and AI2O3, Appl. Catal., A, 132, 77-96 (1995). 

TABLE 7.36. Kinetic Parameters for n-Heptane Isomerization Over Metal Catalysts... 

It has been proposed that the reaction mechanism for n-hexane and n-heptane isomerization over platinum promoted WZ (PtAVZ) at temperatures about 200 °C is a bifunctional non-classic one [9,10]. Active sites are formed by the interaction of hydrogen dissociated on platinum, migrating by spillover, with WOx surface species. Room temperature... 

Catalysts prepared from the two different Pt precursors present an induction period for catalytic activity, as shown in Fig. 1 for Pt(h,500)AVZ. The same behavior was found for Pt(n,500)AVZ (not shown for this reason). A similar induction period has been found for Fe-Mn and Ni promoted sulfated zirconia [19,20]. We consider that the induction period can be ascribed to the generation of active sites under reaction conditions, as found also for n-heptane isomerization [10]. After that induction period, conversion for catalysts calcined at 500 °C and lower temperatures, remains constant during the 240 min-on-stream slightly decreasing for those calcined at 600 °C (not shown). 

Weisz [100] utilized typical values of parameters and observed reaction rates for hydrocarbon isomerizations to show that grain sizes of less than I fan for catalyst one or two can have extremely low intermediate (R) partial pressures of 10 atm, which would be unobservable even though the overall reaction had a finite rate. Also, for n-heptane isomerization, the thermodynamic equilibrium concentration under typical conditions, can be used in Eq. h to compute the minimum catalyst grain siro, or intimate, required for appreciable reaction to occur, not influenced by diffiisional limitations. At a 40 percent conversion to isoheptanes at 470°C, the result was... 

Related alkanes such as pentanes, hexanes and heptanes isomerize by similar pathways with increasing tendency towards cracking (i.e. C—C bond cleavage). 

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