Hexene, isomerization reaction

Figure 6. Variation of equilibrium conversion and yield of 1-hexene isomerization reactions with temperature. (Reprinted with permission from ref. 8. Copyright 1988 Pergamon.)...

The reaction takes place on the catalyst housed in three stationary beds in the reactor. The catalyst used for the l-hexene isomerization reaction is a commercial E-302 reforming catalyst, supplied by Engelhard corporation. The bifunctional catalyst is composed of 0.6 wt% Platinum supported on 1/16" right cylindrical gamma-alumina extrudates. To minimize external mass-transfer resistances and to achieve CSTR behavior, the fluid phase containing the reactants is kept mixed by an impeller powered by a 0.75 hp MagneDrive assembly that can provide stirring speeds up to 3,000 rpm. Unconverted reactant, product and the SCF medium exit via a port located at the top of the reactor. 

Bis(imino)pyridine iron complex 5 acts as a catalyst not only for hydrogenation (see 2.1) but also for hydrosilylation of multiple bonds [27]. The results are summarized in Table 10. The reaction rate for hydrosilylations is slower than that for the corresponding hydrogenation however, the trend of reaction rates is similar in each reaction. In case of tra s-2-hexene, the terminal addition product hexyl (phenyl)silane was obtained predominantly. This result clearly shows that an isomerization reaction takes place and the subsequent hydrosilylation reaction dehvers the corresponding product. Reaction of 1-hexene with H2SiPh2 also produced the hydrosilylated product in this system (eq. 1 in Scheme 18). However, the reaction rate for H2SiPh2 was slower than that for H3SiPh. In addition, reaction of diphenylacetylene as an atkyne with phenylsilane afforded the monoaddition product due to steric repulsion (eq. 2 in Scheme 18). 

Of course, certain features of overall kinetics are inaccessible via a cluster model method, such as the influence of pore structure on reactivity. The cluster model method cannot integrate reaction rates with concepts such as shape selectivity, and an alternative method of probing overall kinetics is needed. This has recently been illustrated by a study of the kinetics of the hydroisomerization of hexane catalyzed by Pt-loaded acidic mordenite and ZSM-5 (211). The intrinsic acidities of the two catalysts were the same, and differences in catalyst performance were shown to be completely understood on the basis of differences in the heat of adsorption of hexene, an intermediate in the isomerization reaction. Heats of adsorption are strongly dependent on the zeolite pore diameter, as shown earlier in this review (Fig. 11). 

Employing 1-hexene isomerization on a Pt/y-ALOj reforming catalyst as a model reaction system, we showed that isomerization rates are maximized and deactivation rates are minimized when operating with near-critical reaction mixtures [2]. The isomerization was carried out at 281°C, which is about 1.1 times the critical temperature of 1-hexene. Since hexene isomers are the main reaction products, the critical temperature and pressure of the reaction mixture remain virtually unaffected by conversion. Thus, an optimum combination of gas-like transport properties and liquid-like densities can be achieved with relatively small changes in reactor pressure around the critical pressure (31.7 bars). Such an optimum combination of fluid properties was found to be better than either gas-phase or dense supercritical (i.e., liquid-like) reaction media for the in situ extraction of coke-forming compounds. 

In the late 1960s Drozd and coworkers reported that 5-hexenyllithium 4, which was prepared from l-bromo-5-hexene by reaction with lithium metal, undergoes an isomerization to cyclopentylmethyllithium 5 at room temperature, but few details were given in these short communications (Scheme l)6. 

This homogeneous hydroformylation reaction was conducted in a batch reactor, and because of the nature of the catalyst, isomerization reactions of 1-hexene to 2- and 3-hexenes and hydrogenation reactions of hexenes to hexanes and aldehydes to alcohols were minimized. The following data were obtained at 323 K with an initial concentration of 1-hexene at 1 mol/L in toluene and Pco... 

The maximum allylic ester isomerization possible can be measured when hexene is used in the reaction. Both hexene-l-yl acetates and hexen-2-yl acetates will give hexen-3-yl acetates upon allylic ester isomerization (Reactions 8a, 8b, and 8c). Therefore, the hexanol-3-acetate found after hydrogenating the hexenyl acetate product will be representative of the sum of both allylic attack on hexene during vinylation and allylic ester isomerization after vinylation and olefinic isomerization. If hexanol-3-acetate is found in only low levels in the reaction product, then both allylic attack during vinylation and allylic ester isomerization can be discounted in considering the major reaction pathways. 

The reactions and product distributions thus far reported have been exclusively concerned with hexene. It was of interest to see whether the high specificity of positional substitution could be maintained with the other hexene isomers. By positional substitution specificity is meant ester attachment on ether of the carbons involved in the original carbon-carbon double bond. Table VII shows the results of these studies. The internal olefins reacted more slowly than the a-olefin, and with both palladium chloride-cupric chloride and 7r-hexenylpalladium chloride-cupric chloride systems high substitutional specificity (> 95% ) was also maintained with 2-hexene (Table VII). However, with 3-hexene the specificity is considerably lower (80%). Whether this is caused by 3-hexene isomerization prior to vinylation or by allylic ester isomerization is not known. A surprisingly high ratio of 2-substitution to 3-substitution is found ( 7 1) in the products from 2-hexene. An effect this large... 

The objective of this paper is to demonstrate the importance of phase and reaction equilibria considerations in the rational development of SCF reaction schemes. Theoretical analysis of phase and reaction equilibria are presented for two relatively simple reactions, viz., the isomerizations of n-hexane and 1-hexene. Our simulated conversion and yield plots compare well with experimental results reported in the literature for n-hexane isomerization (4) and obtained by us for 1-hexene isomerization. Based on our analysis, the choice of an appropriate SCF reaction medium for each of these reactions is discussed. Properties such as viscosity, surface tension and polarity can affect transport and kinetic behavior and hence should also be considered for complete evaluation of SCF solvents. These rate effects are not considered in our equilibrium study. 

For 1-hexene Isomerization, lower temperatures slightly favor equilibrium conversion. Therefore, decreasing reaction temperature through the addition of a low T solvent such as CO2 is not thermodynamically unfavorable. However, the reaction may become klnetlcally limited. On the other hand, as seen from Figure 6, because temperature does not significantly affect equilibrium conversion. 

A possible solution to such a problem would be to dilute the 1-hexene feed with a suitable amount of CO2 so that the critical temperature of the reaction mixture is reduced to such levels as to provide reasonable isomerization reaction rates yet low coking rates at dense supercritical conditions of temperature and pressure. 

Table I compares the results for the hydrogenation of 1-hexene in methanol with the intercalated and homogeneous catalyst systems. Under the reaction conditions employed, the hydrogen uptake rate is lower for the intercalated catalyst than for the homogeneous catalyst. However, the intercalated catalyst greatly reduces the extent of 1-hexene to 2-hexene isomerization, relative to homogeneous solution. The ability of the intercalated catalyst to inhibit substrate isomerization has been attributed to the existence of a surface equilibrium between a monohydride...

Fig. 17. Reaction energy diagram of the isomerization reaction of hexene (energy values in kJ/mol) [136].

The possibility of inducing cataljrtic activity by displacement of atoms was demonstrated (10) using the hexene-1 isomerization reaction before and after exposure of silica to a fast neutron flux. The realization of a catalytic effect caused by displaced electrons was achieved by choosing a reaction, hydrogen-deuterium exchange on alumina, which could be studied at lower temperature where such defects are stable (11). 

These kinetic parameters take into account the formation of a five-membered ring intermediate, the H-abstraction reaction of two H-atoms of allyl type and the formation of the resonantly stabilized l-hexen-3-yl radical. These facts explain why the reverse isomerization reaction requires greater activation energy. As clearly shown in Fig. 6, there is a new class of important reactions, i.e. ring decomposition (e.g., cyclo-hexyl to form hexenyl radical) and the reverse cyclo-addition reaction. The activation energies of ring decomposition to form primary radicals are 31,500 and 28,000 kcal/kmol respectively for the... 

Medium pore aluminophosphate based molecular sieves with the -11, -31 and -41 crystal structures are active and selective catalysts for 1-hexene isomerization, hexane dehydrocyclization and Cg aromatic reactions. With olefin feeds, they promote isomerization with little loss to competing hydride transfer and cracking reactions. With Cg aromatics, they effectively catalyze xylene isomerization and ethylbenzene disproportionation at very low xylene loss. As acid components in bifunctional catalysts, they are selective for paraffin and cycloparaffin isomerization with low cracking activity. In these reactions the medium pore aluminophosphate based sieves are generally less active but significantly more selective than the medium pore zeolites. Similarity with medium pore zeolites is displayed by an outstanding resistance to coke induced deactivation and by a variety of shape selective actions in catalysis. The excellent selectivities observed with medium pore aluminophosphate based sieves is attributed to a unique combination of mild acidity and shape selectivity. Selectivity is also enhanced by the presence of transition metal framework constituents such as cobalt and manganese which may exert a chemical influence on reaction intermediates. 

For 1-hexene isomerization and for acid catalyzed Cg aromatic reactions all molecular sieves were evaluated in their calcined, powdered state. For the study of Cg aromatics, selected SAPO molecular sieves were aluminum exchanged or steam treated as noted in Table IV. For bifunctional catalysts used in paraffin cyclization/isomerization and ethylbenzene-xylene interconversions, the calcined molecular sieve powder was mixed with platinum-loaded chlorided gamma alumina powder. These mixtures were then bound using silica sol and extruded to form 1/16" extrudates which were dried and calcined at 500°C. The bifunctional catalysts were prepared to contain about 0.54 platinum and about 40 to 504 SAPO molecular sieve in the finished catalysts. 

Catalyst Evaluation. The powdered molecular sieves were evaluated following the treatment described above, without further activation. The 1-hexene isomerization and Cg aromatic isomerization tests were conducted in tubular, fixed bed, continuous flow microreactors. The catalyst bed contained one gram molecular sieve powder and one to three grams of similarly sized quartz chips used as diluent. The reactor was heated to the chosen reaction temperatures in a fluidized sand bath, and the reaction temperature was monitored by a thermocouple located m the catalyst bed. Typical runs lasted 3 to 5 hours during which samples were collected every 30 minutes. 

Recently it has been found that the speed of addition of diborane to olefins is remarkably increased if an ether is present (21). In spite of its great speed this hydroboration reaction in ether is fairly selective. Thus treatment of an equimolar mixture of 1- and 2-hexene with a deficiency of diborane, followed by refluxing, yielded tri-n-hexylboiane. Under the influence of heat the organoborane from 2-hexene isomerized into tri-n-hexylbornne (22). Similarly a mixture of 2-, 3-, 4-, and 5-decenes treated with diborane in ether, heated and then subsequently oxidized gave an 80% yield of 1-decanol. Diborane may thus be used to transform olefins into alcohols. 

The butene-1 is mixed with butene recycle from the autometathesis recovery section and is vaporized, preheated and fed to the autometathesis reactor (3) where butene-1 reacts with itself to form hexene-3 and ethylene over a fixed bed of proprietary metathesis catalyst. Some propylene and pentene are also formed from the reaction of butene-2 in the butene-1 feed. Reactor effluent is cooled and flows to the autometathesis recovery section (4), where two fractionation columns separate it into a hexene-3 product that flows to the hexene isomerization unit (5), an ethylene/propylene mix, and butene-1 that is recycled to the butene... 

A diazo transfer reaction takes place, however, between diazoalkanes and derivatives of cyclopropene. Thus, in the reaction of dimethyl 3,3-dimethyl-cyclo-propene-l,2-dicarboxylate (2.200) with diazomethane, the primary product 2.201, a diaza bicyclo[3.1.0]hexene, isomerizes on irradiation to the diazo compound 2.202 (2-79). Acid catalysis, however, leads to the isomerization of 2.201 to the 1,4-dihydro-pyridazine 2.203 (Franck-Neumann and Buchecker, 1969). An analogous reaction of... 

The superbasic catalysts show very high activity in alkene isomerization reactions. The isomerization of 1-pentene and 1-hexene and trans 2-pentene has been examined in the presence of MgO and MgO on to which metallic sodium has been evaporated. 

Mesoporous Metal Oxide Solid Acids Three-dimensional porous metal oxides have been recently synthesized and applied to acid-catalyzed reactions. The use of mesoporous metal oxides is an interesting approach to develop a solid acid catalyst with enhanced activity. The mesopores in the oxide allow the reactants to access additional active acid sites in the pores, resulting in improved rates of acid catalysis. Mesoporous niobium oxides and tantalum oxides treated with phosphoric acid or sulfuric acid have been examined as solid acid catalysts [57-59]. These mesoporous oxides exhibited remarkable activity in Friedel-Crafts alkylation and 1-hexene isomerization in the liquid phase. For sulfated mesoporous tantalum oxides /m-TsL O ), the effect of pore size has been investigated using... 

Table 2 shows thermodynamic data for typical reforming reactions at 500°C. The equilibrium constant of exothermic reaction decreases as the temperature increases. This is the case of the isomerization reaction. However the heat of reaction in this case is small, and the change in the equilibrium constant is not very large. The hydrogenation reactions are also exothermic with a much higher heat of reaction than the isomerization reactions. The hydrogenation of 1-hexene (the inverse reaction of dehydrogenation of n-hexane) has a heat of reaction of 31,000 cal/mol. This means that the equilibrium conversion in this reaction will be much more affected by temperature than the isomerization. 

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