Pentenes, skeletal isomerization

CDTECH Isobutylene/isoamylene n-Butenesand n-pentenes Skeletal isomerization of n-olefins to iso-olefins 3 2000... 

Sandelin, F., Salmi, T., and Murzin, D. (2006) Dynamic modelling of catalyst deactivation in fixed bed reactors skeletal isomerization of 1-pentene on ferrierite. Ind. Eng. Chem. Res., 45, 558-566. 

Cowley, M. 2006. Skeletal isomerization of Fischer-Tropsch-derived pentenes The effect of oxygenates. Energy Fuels 20 1771-76. 

The isomerization of light olefins is usually carried out to convert -butenes to isobutylene [12] with the most frequently studied zeolite for this operation being PER [30]. Lyondell s IsomPlus process uses a PER catalyst to convert -butenes to isobutylene or n-pentenes to isopentene [31]. Processes such as this were in larger demand to generate isobutene before the phaseout of MTBE as a gasoline additive. Since the phaseout, these processes often perform the reverse reaction to convert isobutene to n-butenes which are then used as a metathesis feed [32]. As doublebond isomerization is much easier than skeletal isomerization, most of the catalysts below are at equilibrium ratios of the n-olefins as the skeletal isomerization begins (Table 12.5). 

In the case of alkenes, 1-pentene reactions were studied over a catalyst with FAU framework (Si/Al2 = 5, ultrastable Y zeoHte in H-form USHY) in order to establish the relation between acid strength and selectivity [25]. Both fresh and selectively poisoned catalysts were used for the reactivity studies and later characterized by ammonia temperature programmed desorption (TPD). It was determined that for alkene reactions, cracking and hydride transfer required the strongest acidity. Skeletal isomerization required moderate acidity, whereas double-bond isomerization required weak acidity. Also an apparent correlation was established between the molecular weight of the hard coke and the strength of the acid sites that led to coking. 

Skeletal isomerization requires higher temperature and stronger acid catalysts than do double-bond migration and cis-trans isomerization. Butylenes, for example, are transformed to isobutylene over supported phosphoric acid catalysts.98 The equilibrium mixture at 300°C contains approximately equal amounts of straight-chain and branched butenes. Similar studies were carried out with pentene isomers.99 Side reactions, however, may become dominant under more severe conditions.100... 

Data obtained on the rate of skeletal isomerization of 1-pentene over a sample of catalyst containing only the acid function (no platinum) provided additional strong support for the proposed mechanism. This can be seen quite simply by referring to Fig. 5, in which the circles and the... 

Fig. 5. Isomerization rate versus pentene partial pressure (S4). Comparison of n-pentane isomerization rate over platinum-alumina catalyst with the rate of skeletal isomerization of 1-pentene over the platinum-free catalyst 372°C.

Loading of zeolites with alkali metals provides catalysts that have double bond isomerization activity and little or no accompanying skeletal isomerization. A NaX zeolite loaded with 10.4 wt % sodium showed 94% conversion of 1-pentene to 2-pentene at 25°, and 38% conversion of 1,3-cyclohexadiene at 100° to a mixture of 44% 1,4-cyclohexadiene and 56% benzene. Only a narrow range of sodium loadings proved eflective, however. 

Trans4m S A catalytic skeletal isomerization process that transforms n-butane to wobutene and -pentene to iioamylene. Developed by LyondellBasell and used in its refinery in Texas since 1995. 

Sodium on alumina. This high-surface sodium catalyst, prepared by adding sodium to dry alumina with stirring at 150° under nitrogen, is effective for isomerization of butenes and pentene-1. In contrast to acid-catalyzed isomerization, no skeletal rearrangement occurs. The results indicate some stereoselectivity. Thus butene-1 is isomerized initially to about equal amounts of cis- and trani-butene-2 eventually the thermodynamic equilibrium mixture rich in the trans-isomti is obtained. The catalyst was used to effect almost quantitative conversion of methylenecyclobutane into 1-methylcyclobutene. ... 

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