Butane isomerization selectivity

These reactions can also be performed over a strong acid catalyst at reaction temperatures that are lower than over zeolites. Because of this, isomerization of M-butane over Zr02-supported sulfate catalysts was initially proposed by Hino and Arata. They proposed these catalysts as being effective in butane isomerization at room temperature, a reaction that does not take place, even in 100% sulphuric acid. For this reason, these catalysts were considered as solid superacids, since they are active and selective in the isomerization of n-butane to isobutane at... 

Hydrocarbons higher than C3 can also undergo isomerization on Pt, running in parallel to hydrogenolysis. With butane or isobutane the selectivity for isomerization is rather low, also on Pt, but the higher hydrocarbons show more of isomerization reactions. With higher hydrocarbons some other metals (Ir, Pd) also show some isomerization selectivity. 

The product distributions obtained over the catalysts are summarized in Table II for the reaction of butane (29, 31, 32). The activities varied with the kinds of metal oxides that were treated with SbF,. SbF,/ Si02-Ti02 showed the highest activity, and SbF,/Ti02 was highly selective for the skeletal isomerization of butane, the selectivity being 72%. On... 

The coexistence of metal and HPA reveals a unique catalytic bifunctionality. For Pt-Cs2.5Ho.sPWi204o, the activity of the -butane isomerization is remarkably enhanced in the presence of H2. The rate of isobutane formation at 573 K is comparable with or higher than those of Pt-H-ZSM5 and Pt-SC>4 /ZrC)2 while the selectivity of HPA is much higher [100, 101]. 

Solid superacids of the sulfated zirconia type were found active for n-butane isomerization at low reaction temperatures (50). These catalysts, however, were rapidly deactivated with time on stream. The isomerization selectivity and the stability of sulfated zirconia catalysts can be incerased by the introduction of Pt and by carrying out the reaction in the presence of H2. Higher catalytic activities were obtained when Pt was impregnated after the impregnation of zirconia gel with 0.5 M H2SO4 (51). Sulfated zirconia promoted with Fe or Mn showed an even higher activity than unpromoted SZ for the low temperature isomerization of n-butane (52). 

Davis et al. (23) also studied the isomerization and hydrogenolysis of isobutane, n-butane, and neopentane over flat, stepped, and kinked Pt single-crystal surfaces. The rates and selectivities of butane isomerization and consecutive rearrangements were maximized on (100) portions of the surfaces. Competing hydrogenolysis reactions were most rapid on surfaces containing the greatest step- and kink-site densities. 

Anderson considers hydrogenolysis reactions in detail, along with isomerization reactions, on supported metals. The model is illustrated using butane in Figure 1. The parameters used are X, the fractional conversion of reactant the isomerization selectivity, the selectivity, S, to a j-carbon-number product (C ) the ratio, T., of the desorption rate of adsorbed C. (C. ) to the rate... 

The reactivity pattern displayed by platinum crystal surfaces for alkane isomerization reactions is completely different from that for aromatization. Studies revealed that maximum rates and selectivity (rate of desired reaction /total rate) for butane isomerization reactions are obtained on the flat crystal face with the square unit cell. Isomerization rates for this surface are four to seven times higher than those for the hexagonal surface. Isomerization rates are increased to only a small extent by surface irregularities (steps and kinks) on the platinum surfaces (Figure 7.39). 

Since SSITKA can decouple the apparent rate of reaction into the contribution from the intrinsic activity ( the reciprocal of surface residence time of intermediates) and the nrnnber of active sites ( surface concentration of intermediates), the cause of deactivation of a catalyst during reaction can often be revealed. SSITKA has been used in a number of studies for this purpose. Catalyst deactivation during n-butane isomerization and selective CO oxidation are good examples. Deactivation studies are conducted by collecting isotopic transient data at particular times-on-stream as deactivation occurs. 

As mentioned above, n-alkanes cannot be alkylated by conventional acid catalysts except in some cases when they undergo isomerization to form branched isomers. Superacids, however, are capable of inducing the reaction of any alkanes with alkenes (63). Alkylations of small alkanes (even that of methane and ethane) with ethylene and propylene were reported. When high excess of methane or ethane (required to prevent oligomerization of the alkene) was reacted with ethylene (10 1 HF-TaFs, 40°C), propane (58% selectivity) and n-butane (78% selectivity), respectively, were produced. 

Yaluris, G., Larson, R.B., Kobe, J.M., Gonzales, M.R., Fogash, K.B., and Dumesic, J.A. Selective poisoning and deactivation of acid sites on sulfated zirconia catalysts for n-butane isomerization. J. Catal 1996,158, 336-342. 

The temperature dependence of the selectivity for isomerization versus hydrogenolysis depends on the type of catalyst. Thus, over thick platinum film catalysts this selectivity was temperature independent for the reaction of the butanes and neopentane (24). However, in Boudart and Ptak s (122) reaction of neopentane over platinum/carbon the selectivity to isomerization decreased slightly with increasing temperature while Kikuchi et al. (128) found an increased trend for isomerization in the reaction of n-pentane over platinum/silica and platinum/carbon catalysts. 

The combination of Pt or Pd with CS25H0.5PW12O40 (Cs2.5) is also very effective for the isomerization of n-butane to isobutane (381). The reaction rate and selectivity for conversion to isobutane are summarized in Table XXXIV (381, 382). The activity in the presence of H2 changed little with time. Pt- and Pd-Cs2.5 show very high selectivities (94-96%) relative to those of Pt/SO -/ Zr02 (47%) and Pt/HZSM-5 (34%), whereas the activities of Pt- and Pd-Cs2.5 for the formation of isobutane are comparable to those of Pt/HZSM-5 and Pt/S04 /Zr02. Pt-Cs2.5 catalyzes the reaction even at 473 K and 0.05 atm of H2. 

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