1-Hexene, isomerization

The plot of data for the carbonyl, Rh6(CO)i6, (Figure 1) shows that the rate of 1-hexene isomerization exceeds that of hydroformylation. At olefin conversion in excess of 50%, little 1 isomer remains. An increase in branched aldehyde relative to linear aldehyde accompanies the change in isomer distribution. The absence of aldehyde hydrogenation is complete even at very high conversion levels using the conditions cited. 

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. 

The complex [Rh(nbd)(amphos)2], which contains the water-soluble ligand amphos (33), has been prepared. In a two-phase system this complex acted as a hydroformylation catalyst for 1-hexene. Isomerization of the alkene occurred to a limited extent. In the presence of H2 and CO the dicarbonyl [Rh(CO)2(amphos)2] was thought to be formed. 

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. 

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

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. 

Both batch and continuous runs were performed. Batch runs were performed mainly to check the validity of the predicted equilibrium conversions. A batch conversion of 90% was obtained after 11 hours of operation when 1-hexene isomerization was performed at a supercritical temperature of 265 0 (1.07 T ) and subcritical pressures varying between 250 and 150 psig (0.54-0.33 P ). The liquid product collected in the phase separator bore no color and contained virtually no oligomers. Combined conversions to 2-hexenes and 3-hexenes, are about 69% and 21% respectively. These values compare well with the simulated equilibrium values of 72.4% and 24.3% respectively (see Figure 6). 

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. 

MC Clark, B Subramaniam. 1-Hexene isomerization on a Vtly-MiOi catalyst The dramatic effects of feed peroxides on catalyst activity. Chem Eng Sci 51 2369-2377,... 

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... 

Fig. 6. Arrhenius plots for 1-hexene isomerization on a Pt/y-Al203 catalyst (54).

These conclusions confirm the results with Ir(Xantphos) complexes by Eisenberg s group from 2006 [11]. For some of these H2lr complexes, a trans coordination of the diphosphine was found. The hydrido complexes HIr(CO)2(Xantphos) and H3lr(CO)(Xantphos) exhibited only modest hydroformylation activity for the transformation of 1-hexene and styrene (Hj/CO = 2 1, 3 atm 75 °C). The aldehydes were produced in a yield of 10%. More than 50% 1-hexene isomerization was observed. Complete inhibition of the reaction took place in the presence of a twofold excess of the bidentate ligand. It was speculated that in some cases dissociation of Xantphos could be a precondition for catalysis to occur. 

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