Alkanes oligomerization

On the other side, the addition of a second metal like Re, Ir or Sn, which increases the stability of Pt/Al203 catalyst, has been studied extensively in the literature. In summary, the effect of Sn is ascribed to i) a reduction of the number of Lewis sites [17] ii) an increase of selective hydrogenation of coke precursors (dienes) [18] Hi) a suppression of hydrogenolysis reactions [19]. Moreover, Sn can promote alkane oligomerization or even Diels-Alder type reactions [20]. 

In this work we have studied the mechanism of alkane oligomerization, establishing a network for the primary and secondary reactions occurring on Ga ZSM-5 catalysts. Normal C3 and normal and branched C4 alkanes and alkenes and a LSR have... 

There are few noteworthy features of these reactions which were not mentioned in context with the previous section and Figure 3. First, the product distributions depend in some cases, on the presence or absence of a hydrogen evolution catalyst such as Pt(0). They change in predictable ways [39, 58, 75, 79, 80, 82, 87, 88, 91]. Second, the subsequent reactions of initial alkane derived products are sometimes significant, informative, and provocative. Not surprisingly, both alkane oligomerization... 

Pellet el al. reported on the reaction of propene over SAPO-n materials. Though the main products over ZSM-5 type aluminosilicate are aromatics and lower alkanes, oligomerization proceeds selectively over SAPO-11 and SAPO-31, indicating that these materials do not possess strong acid sites which are capable of promoting hydride transfer reactions. 

Table 5 gives typical results of the wax cracking process to surfactant olefins. Compared with the pure a-olefins produced by the oligomerization reactions of ethylene the crack olefins are decreased in quality, especially due to the conjugated diene part (2-4%). Moreover, there are some problems in guaranteeing the wanted amounts of C20-C30 n-alkanes. Therefore in industrially de-... 

FIGURE 6.30 a,cu-Bis(tocopheryl)alkanes (36) and other a, >-bis(hydroxyphenyl)-alkanes (37-40) as starting materials for the spiro oligomerization/spiro polymerization reaction. 

The preparation of similar precursors suitable for the deposition of metal nitrides is analogous to the preparations of phosphorus and arsenic compounds. The initial reaction of metal trialkyls MR3 (M = A1, Ga, In) with amines (NHR 2) results in the formation of oligomeric amido compounds [R2MNR 2] (n = 2 or 3) which eliminate alkanes on thermolysis. The incorporation of a proton as a substituent on the pnictide bridging ligand has been examined, and many compounds of the type [R2MNHR ]2 have been synthesized. The presence of this proton may facilitate /3-elimination, allowing lower deposition temperatures to be used. 

Metallacyclobutanes have been proposed as intermediates in a number of catalytic reactions, and model studies with isolated transition metallacyclobutanes have played a large part in demonstrating the plausibility of the proposed mechanisms. Since the mechanisms of heterogeneously catalysed reactions are especially difficult to determine by direct study, model studies are particularly valuable. This article describes results which may be relevant to the mechanisms of isomerization of alkanes over metallic platinum by the bond shift process and of the oligomerization or polymerization of alkenes. 

The reversal of the insertion reaction [Eq. (10)] is not normally observed [in contrast to nickel hydride addition to olefins, Eq. (9)]. An exception is the skeletal isomerization of 1,4-dienes (88, 89). A side reaction—the allylhydrogen transfer reaction [Eq. (5)]—which results in the formation of allylnickel species such as 19 as well as alkanes should also be mentioned. This reaction accounts for the formation of small amounts of alkanes and dienes during the olefin oligomerization reactions (51). 

Table I gives the compositions of alkylates produced with various acidic catalysts. The product distribution is similar for a variety of acidic catalysts, both solid and liquid, and over a wide range of process conditions. Typically, alkylate is a mixture of methyl-branched alkanes with a high content of isooctanes. Almost all the compounds have tertiary carbon atoms only very few have quaternary carbon atoms or are non-branched. Alkylate contains not only the primary products, trimethylpentanes, but also dimethylhexanes, sometimes methylheptanes, and a considerable amount of isopentane, isohexanes, isoheptanes and hydrocarbons with nine or more carbon atoms. The complexity of the product illustrates that no simple and straightforward single-step mechanism is operative rather, the reaction involves a set of parallel and consecutive reaction steps, with the importance of the individual steps differing markedly from one catalyst to another. To arrive at this complex product distribution from two simple molecules such as isobutane and butene, reaction steps such as isomerization, oligomerization, (3-scission, and hydride transfer have to be involved.

Ozone also reacts with ethane in the gas phase at room temperature. Rather than a direct molecular reaction, however, evidence points to the initiation of radical-chain reactions by the very small O-atom concentrations present in ozone at room temperature. Added oxygen scavenges the radicals and slows the build-up, leading to induction periods which may be in excess of 3 h. Recent advances in mechanistic investigations of gas-phase ozonolysis of alkanes have been reviewed. Oligomeric peroxides dominate the products of oxidation of nitrotoluenes with ozone in acetic acid. °... 

Formation of C8 alkanes in the alkylation of isobutane even when it reacts with propene or pentenes is explained by the ready formation of isobutylene in the systems (by olefin oligomerization-cleavage reaction) (Scheme 5.2). Hydrogen transfer converting an alkane to an alkene is also a side reaction of acid-catalyzed alkylations. Isobutylene thus formed may participate in alkylation Cg alkanes, therefore, are formed via the isobutylene-isobutane alkylation. 

Fast side reactions under the conditions of acid-catalyzed alkylation are oligomerization (polymerization) and conjunct polymerization. The latter involves polymerization, isomerization, cyclization, and hydrogen transfer to yield cyclic polyalkapolyenes. To suppress these side reactions, relatively high (10 1) alkane alkene ratios are usually applied in commercial alkylations. 

Condensation of Q—C4 alkanes to produce highly branched oligomeric and polymeric hydrocarbons41 was also achieved by their condensation in FSO3H—SbF5. Block methylene units in the polymeric chain were observed even when methane was brought into reaction with alkenes under similar conditions.42... 

The major problem of the application of zeolites in alkane-alkene alkylation is their rapid deactivation by carbonaceous deposits. These either strongly adsorb on acidic sites or block the pores preventing the access of the reactants to the active sites. A further problem is that in addition to activity loss, the selectivity of the zeolite-catalyzed alkylation also decreases severely. Specifically, alkene formation through oligomerization becomes the dominant reaction. This is explained by decreasing ability of the aging catalyst to promote intermolecular hydride transfer. These are the main reasons why the developments of several commercial processes reached only the pilot plant stage.356 New observations with Y zeolites reconfirm the problems found in earlier studies.358,359... 

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