Isomerization n-heptane

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... 

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

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. 

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... 

Bimetallic Pt-Mo catalysts were prepared by successive chemical vapor deposition of Mo(CO)g onto nanometer-size platinum particles dispersed in EMT zeolite [264]. Bimetallic particles of imiform composition in a highly dispersed form were formed. Subsequent decomposition of the Mo precursor at 600 K in hydrogen formed a supported molybdenum carbide phase that was characterized by XPS, TEM and EX AES. Nitridation at 973 K in flowing NH3 led to a nanometer-sized Pt-core covered by a Mo-nitride layer. The coverage of the Pt-clusters by ca. 0.3 nm of carbide or nitride modified greatly the selectivities in n-heptane isomerization and hydrogenolysis. 

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