Supported palladium isomerization selectivity

A large number of heterogeneous catalysts have been tested under screening conditions (reaction parameters 60 °C, linoleic acid ethyl ester at an LHSV of 30 L/h, and a fixed carbon dioxide and hydrogen flow) to identify a suitable fixed-bed catalyst. We investigated a number of catalyst parameters such as palladium and platinum as precious metal (both in the form of supported metal and as immobilized metal complex catalysts), precious-metal content, precious-metal distribution (egg shell vs. uniform distribution), catalyst particle size, and different supports (activated carbon, alumina, Deloxan , silica, and titania). We found that Deloxan-supported precious-metal catalysts are at least two times more active than traditional supported precious-metal fixed-bed catalysts at a comparable particle size and precious-metal content. Experimental results are shown in Table 14.1 for supported palladium catalysts. The Deloxan-supported catalysts also led to superior linoleate selectivity and a lower cis/trans isomerization rate was found. The explanation for the superior behavior of Deloxan-supported precious-metal catalysts can be found in their unique chemical and physical properties—for example, high pore volume and specific surface area in combination with a meso- and macro-pore-size distribution, which is especially attractive for catalytic reactions (Wieland and Panster, 1995). The majority of our work has therefore focused on Deloxan-supported precious-metal catalysts. 

With a DELOXAN supported palladium complex catalyst, DELOXAN HK I, the linoleate selectivity is further increased. In comparison to the commercial batch hydrogenation with a nickel on kieselguhr catalyst, the DELOXAN supported palladium complex catalyst in combination with sc CO2 as a solvent gives higher space-time-yields, a higher linoleate selectivity and a significantly decreased cis/trans isomerization rate. 

DELOXAN AP II supported platinum catalysts in sc CO2 are less active than DELOXAN AP II supported palladium catalysts, but they show an improved linoleate selectivity and a significantly lower cis-trans isomerization rate. The overall yield of undesirable trans fatty acids is 7.5 GC area-% in the edible oil hardening with a DELOXAN AP II supported 2 wt. % platinum catalyst. In a batch hydrogenation using the commercial powdered nickel on kieselguhr catalysts the undesirable trans fatty acid content was determinded to 40 percent. 

We shall consider first the simplest reaction so far reported (56, 94), which is the hydrogenation of 2-butyne in a flow system at room temperature and a little above, catalyzed by alumina-supported palladium (0.03%). This reaction proceeds very selectively, only a trace of butane being formed in the presence of 2-hutyne, as long as the catalyst is not completely fresh. Moreover, the reaction is highly stereoselective for the formation of cis-2-butene and only traces of mw -2-butene and 1-butene were observed. After the removal of 2-hutyne the cts-olefin both isomerized and hydrogenated, showing that a powerful thermodynamic factor is again operative (as was observed for 1-butyne, propyne and acetylene) when alkyne is present. 

The noble metal component may be either palladium or platinum the effect of the concentration of both metals on methylpentane as well as on dimethylbutane selectivity in C6 hydroisomerization on lanthanum and ammonium Y-zeolite with Si/Al of 2.5 has been studied by M.A. Lanewala et al. (5). They found an optimum of metal content for that reaction between 0.1 and 0.4 wt.-%. The noble metal has several functions (i) to increase the isomerization activity of the zeolite (ii) to support the saturation of the coke precursors and hence prevent deactivation, as was shown by H.W. Kouvenhoven et al. (6) for platinum on hydrogen mordenite (iii) to support the hydrodesulfurization activity of the catalysts in sulfur containing feedstocks. 

The literature [101, 102] cites the hydrogenation and isomerization of allylbenzene in the presence of palladium complexes on polymeric supports having oxygen-containing coordination groups like -OPh, -COOH, -OH. It was shown that the activity and selectivity of synthesized polymeric catalytic systems were affected by both the electron-donor properties of graft coordination groups and the solvent type. In the presence of the catalyst, the isomerization of allylbenzene to 1-propenylbenzene mainly yields the trans isomer. 

Lewis acidic nanoscaled metal fluorides were successfully used as cataljdically active support for platinum and palladium. These catalysts were found to be very efficient in n-pentane hydroisomerization reactions [71]. In situ preparation is an efficient approach toward very small precious metal nanoparticles (2 nm) finely distributed over the fluoride catalyst that exhibit very good activity, stability, and selectivity toward isomerization (up to nearly 1(X)%) at 350°C. The exciting requisite for this superior catalytic behavior has been shown to be the combination of very strong Lewis acidity of the HS-AIF3 with C-H and/or C-F bond activation... 

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