Skeletal Isomerization Butenes, Pentenes, Hexenes

Olefins, unlike paraffins, do not show significant gains in octane number with skeletal isomerization (see Table 14.2). As a result, olefin isomerization is not a useful octane boosting strategy. However, tertiary olefins (olefins with three alkyl substituents on the double bond), do react fairly readily with olefins to form ethers, which do have good octane numbers-for example, methyl tert-butyl ether (MTBE). 

Several review articles [37-40] have been written discussing the use of molecular sieves for the isomerization of light olefins, especially butene. The major driving force was the requirement in the 1990 amendments to the Clean Air Act in the United States that required the addition of oxygenates to gasoline in amounts up to 2.7wt% oxygen in the final gasoUne product [37]. The primary additive chosen to meet these requirements was MTBE, with lesser amounts of the ethyl ether (ETBE) or tert-amyl methyl ether (TAME) as supplements. One major possible route to meet these requirements was the isomerization of linear butenes (1-butene, cis-2-butene and trans-2-butene) into isobutene (2-methyl-l-propene). 

Early reports of reaction over borosiUcate [42], siUcaUte [43], Theta-1 [44], ZSM-22 [45] and ZSM-23 [46] gave relatively low selectivities for isobutene. With the use of the aluminophosphates SAPO-11 [47] and MgAPSO-31 [48] improved selectivities were reported, reaching a final plateau with ferrierite [49]. 

AH these processes faced competition from the on-demand production of isobutene through a combined process of isomerization of n-butane to isobutane 

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