Isomerization selective hydrogenation

FCC after removal of C5 olefins via selective hydrogenation step passes to the isomerization unit. It has been proposed that after the formation of a hutyl carhocation, a cyclopropyl carhocation is formed which gives a primary carhenium ion that produces isobutene ... 

Figure 1. Isomerization selectivity on Pt/WOx-Zr02 (0.3% wt. Pt, 12.7% wt. W, H-atoms formed from H2 or adamantane on Pt, 073 calcination) and Pt/SOx-ZrOj sites are involved in carbocation hydrogen (0.4% wt. Pt, 4.5% wt. S, 723 K...

Citronellol is formed through selective hydrogenation of the C=C bond activated by the presence of the OH group, whereas menthol 3 is the product of a three-functional process involving isomerization of the allylic alcohol 1 to citronellal 4, ene reaction to isopulegol 5 and final hydrogenation (Scheme 2). 

Metal complexes, especially involving transition metals, are known for their role as catalysts in a broad variety of chemical processes including isomerization, oxidization, hydrogenation, and polymerization. Such catalytic reactions play an important role not only in many industrial processes, such as petroleum and polymer industries, but also in many biological systems, e.g., a variety of selective oxidation catalysts with heme (1) and nonheme (2) iron centers. The transition metals in these systems usually constitute a fundamental part of the catalyst, due to their... 

Cyclooctadiene isomers (i.e., 1,5-cod or 1,3-cod) are selectively hydrogenated by [Ru(/74-cod)(/76-C8H1o)] (51) to produce exclusively cyclooctene in THF, under ambient temperature (20 °C) and 1 bar H2 pressure [64]. Again, cyclooctane is only detected when the diene substrate is completely transformed to the monoene. The rate of hydrogenation is higher in case of the conjugated 1,3-cycloocta-diene substrate, whereas isomerization of the non-conjugated 1,5-cyclooctadiene... 

Similarly, 1,2-cyclononadiene in methanol with 10% palladium on carbon catalyst gave cis-cyclononene122. The cis isomer is not necessarily the primary product of allene hydrogenation, since the initially formed trans isomer is rapidly isomerized under the reaction conditions. Bond and Sheridan showed that allene resembles acetylene in its ease of hydrogenation123. They suggested that it is selectively adsorbed and held more strongly by the catalyst than 1-propene. Allene was selectively hydrogenated with Pd, Pt and Ni in the presence of 1-propene without its further reduction. 

In the case of PCI/AI2O3 catalysts the morphology of the metal particles is also important because it determines the hydrogenation and isomerization selectivity. On flat metal surfaces isomerization is preferred whereas rough surfaces are more active in hydrogenation129. 

W. Jakubetz and B. L. Lan. A simulation of ultrafast state-selective IR-laser-controlled isomerization of hydrogen cyanide based on global 3D ab initio potential and dipole surfaces. Chem. Phys., 217 375-388(1997). 

Detailed studies were conducted with ZnO, which is a hydrogenation catalyst and effects isomerization without hydrogen.129,130 Spectroscopic observations130-132 led to the postulation of surface ally lie species and, consequently, a 1,3 hydrogen shift was suggested to account for isomerization. The tt-allylic species is formed and stabilized on adjacent acid-base (Zn2+-02 ) site pairs. The characteristic cis selectivity also supports a mechanism similar to that suggested to be effective in homogeneous systems. 

Fig. 22. Selective hydrogen mixing between C2H4 and C2D4 during induction time Tor the isomerization of m-2-butene-d0, and superimposed hydrogen scrambling between C2D4 and cis-2-butene-dQ after the induction time on cut single crystal catalyst of MoS2 at 100°C (8,14).

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