Butene Skeletal Isomerization

K. P. (2001) Deactivation of solid add catalysts for butene skeletal isomerization on the benefldal and harmful effects of carbonaceous deposits. Appl. Catal A, 212, 97-115. 

Figure 2. Dimerization/cracking mechanism in n-butene skeletal isomerization over ferrierite catalysts.

Deactivation of Ferrierite during the 1-Butene Skeletal Isomerization... 

In this study, coke formation on ferrierite during the 1-butene skeletal isomerization was investigated. Tlie influence of temperature, 1-butene partial pressure, WHSV, and pretreatment conditions on the activity, selectivity and stability was analyzed in order to understand the process of coke formation. The coke was characterized by TPO and DRIFTS. 

Butene Isomerization. - The coke formed on ferrierite during 1-butene skeletal isomerization was observed with TEM °. The images show that for a ferrierite containing 9.1 wt% coke, the platelets are bordered by a layer of amorphous material which is probably coke. 

The grounds of this process based on the use of medium-pore zeolites, such as PER, were laid down by the research works at Shell [25] and Texaco [26]. Ferrierite exhibits a better isobutene selectivity in 1-butene skeletal isomerization, and produces less products and coke. The zeolites for this process have been commerciahzed by Mobil and British Petroleum (ISOFIN process) and Lyondell. 

In the case of n-butene isomerization it was demonstrated (Figure 2) that the ideal micro-pore topology led to retardation of the C8 dimer intermediate and that the catalyst based on the ferrierite structure was close to optimal in this respect [1). For selective isodewaxing a one-dimensional pore structure which constrained the skeletal isomerization transition state and thereby minimized multiple branching such as the SAPO-11 structure was found to meet these criteria. Clearly, these are ideal systems in which to apply computational chemistry where the reactant and product molecules are relatively simple and the micro-porous structures are ordered and known in detail. 

ISOFIN [Iso olefins] A cataylytic process for making iso-olefins from normal olefins by skeletal isomerization. The principle example converts //-butenes to isobutylene, needed as a feedstock for making methyl /-butyl ether. Developed by BP Oil Company, Mobil Corporation, and MW Kellogg from 1992. 

SKIP [Skeletal isomerization process] A process for converting linear butenes into isobutene. Developed by Texas Olefins in the 1990s and operated by that company in Houston, TX. 

Melhyl-l-propanol Isobutyl Alcohol) and 2-Phenyl-1-propanol Herling and Pines (SO) studied the dehydration of 2-methyl-l-propanol and 2-phenyl-1-propanol. The two alcohols were passed over alumina under nonacidic conditions at temperatures of 350° and 270°, respectively (Tables III and IV). The 2-methyl-l-propanol underwent, in part, skeletal isomerization forming butenes, whereby the ratio of cisjtrans 2-butene produced was four to six times greater than the equilibrium ratio. The extent of skeletal isomerization depended to some extent on the method of preparation of the alumina. 

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). 

Lee, S.-H., Shin, C.-H., and Hong, S.B. (2004) Investigations into the origin of the remarkable catalytic performance of aged H-ferrierite for the skeletal isomerization of 1 -butene. J. Catal.,... 

Byggningsbacka, R., Lindfors, L.-E., and Kumar, N. (1997) Catalytic activity of ZSM-22 zeolites in the skeletal isomerization reaction of 1-butene. 

Beta scission of a carbenium ion is an elementary step that is inihated by the weakening of the bond beta to the positive charge, leading to a smaller carbenium ion and an alkene. This elementary step is further discussed in Sections 13.8.1, 13.8.3.1 and 13.8.4 within the context of alkene skeletal isomerization, isobutane-2-butene alkylation and alkane cracking, respectively. 

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... 

The skeletal isomerization of C4 and C5 n-olefins is an acid-catalyzed reaction requiring relatively strong acid sites that proceeds via carbenium ion intermediates formed upon protonation of the double bond (17). Double bond cis-trans isomerization usually occurs on the acid sites before skeletal isomerization. The general reaction mechanism for branching isomerization is depicted in Fig. 2 2. Protonation of the double bond leads to a secondary carbenium ion, which then rearranges into a protonated cyclopropane (PCP) structure. In the case of n-butenes,... 

In contrast, only methane, propylene, butadiene (all three in approximately equal quantities), and hydrogen comprise the major products obtained from decomposition of either cis- or trans-2-butenes under a variety of conditions (5). While isomerization occurred in all cases, equilibrium isomer distribution was never achieved. Ethylene was observed among the products at high temperatures and high conversion levels. Skeletal isomerization has not been observed however, at low temperatures (6,7) substantial conversion to 1-butene has been reported. 

Over the range of conditions, 1-butene decomposes more rapidly than either of the 2-butene isomers. Double-bond shift and geometrical isomerization accompany the decomposition of the n-butenes however, skeletal isomerization does not occur, as isobutene is not found among the products of the pyrolysis. Isomerization reactions apparently are kinetically controlled, as equilibrium distributions are not generally observed. Trans cis ratios in the products do not correspond to equilibrium at either the maximum or the average reactor temperatures, and in some cases the ratio falls below equilibrium values based on American Petroleum Institute (API) data (14). However, none of these data exceed the equilibrium values based on more recent thermodynamic data (15). 

Skeletal Isomerization of n-Butenes Cataiyzed by Medium-Pore Zeolites and Aiuminophosphates... 

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