Light Olefin Isomerization

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). 

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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). 

Olefin isomerization has also been mediated by the photolysis of Fe(C0)s.144 Recently, a detailed study of alkene isomerization by photolysis of Fe(CO)5 has shown that the reaction is truly photocatalytic.14S The very high quantum yields ( 1.0), Table 24, and the fact that the pentenes are ultimately equilibrated to the thermodynamic ratio support the notion that the role of the light is to generate a thermally active catalyst. A mechanism similar to that in reactions (53)-(57) involving Fe(CO)3 as the repeating unit can be used to account for the results. 

A favorable factor tending to limit isomerization is that in any practical olefin process, only the lower molecular weight olefins, Cio and below, will be recycled for alkylation. As long as a reasonable recovery of the triethylaluminum can be achieved, there will be more TEA available than required for stoichiometric addition of all the recycled light olefins. This, of course means that it will not be necessary to push for complete alkylation of the recycled TEA. Since it can be shown that isomerization increases rapidly as alkylation is pushed past aluminum-dialkylethyl toward complete alkylation, the advantage of a partial alkylation is obvious (Figure 4). 

Distillate dewaxing Lube dewaxing Gas to aromatics Gas to Aromatics Light olefins to gasoline and distillate Methanol to gasoline Methanol to light olefins Xylene isomerization Toluene disproportionation Ethylbenzene synthesis 58-59) p-Xylene synthesis p-ethyltoluene synthesis... 

High naphtha and light olefin yields are favored by operation at high temperatures. The naphtha becomes excellent feed for naphtha crackers to make ethylene, the diesel is a green diesel with a cetane number of about 70, and obviously zero sulfur and aromatics, therefore it is an excellent blending component for poorer quality counterparts. The high molecular weight wax can either be cracked to fuels products or isomerized to reduce the pour point and produce lube base stocks. 

MFI ZSM-.5 Silicalite 5.3 X 5.6 5.1 X 5.5 7Z-P, MBP. Xylenes 2,2,4-TMCs, TMB FCC, dewaxing, aromatization. xylene isomerization, toluene disproportionation, ethylene-benzene alkylation Methanol—fuels or light olefins... 

The feed can be obtained from cat naphtha, coker naphtha, and steam cracker s C4 s and pyrolysis gasoline. The largest source of olefinic feedstock molecules is cat naphtha, which contain 20-60% olefins. Most of linear cat naphtha olefins are converted to light olefins, and at the same time, an increased octane and reduced olefin content naphtha is produced by the concentration of higher octane aromatics, plus isomerization and some additional aromatics formation. 

Trombetta, M., Busca, G., Rossini, S., Piccoli, V., andComaro, U. FT-IR studies on light olefin skeletal isomerization catalysis. Part I the interaction of C4 olefins with pure y-alumina. J. Catal 1997,168, 334-348. 

A key portion of the SHOP process is the isomerization—disproportionation (I/D) process in which excess light (C —C q) and heavy olefins (Cjg ) are converted to detergent range odd and even linear internal olefins. Eor each pass through this system, only 10—15% of the olefins fed are... 

The higher degree of stereospecificity in the singlet reaction relative to that of the triplet is not unexpected in light of the greater lifetime of the triplet bira cal allowing bond rotation or bond breaking to yield acetone and an isomerized olefin. 

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