Hydrogenolysis selectivity

Karpinski and Clarke found that small incorporations of tin in platinum films led to a decrease of hydrogenolysis selectivity of about one order of magnitude (106a). Incorporation of copper in rhodium (106b) and of gold in palladium (106c) gave a similar behavior. 

In so far as it is possible to summarize the vast body of information available, it appears in the main EUROPT-1 behaves as expected for a Pt catalyst having particles of about 2 nm in size. Compared to other catalysts, however, its hydrogenolysis selectivity is low, as is its tendency to form carbonacous residues [7] these characteristics may be connected. An attempt has been made [17] to show how the various reactive intermediates might be accommodated on the faces of a 55-atom cubooctahedron (Fig. 5). 

Benzylidene acetals, very fragile in an acidic medium, are hydrolysed by aqueous acetic acid or cleaved by hydrogenolysis. Selectivity can be observed in the hydrolysis of isopropylidene acetals. With compound 5.10, only the ring spanning positions 5 and 6 is hydrolysed by aqueous acetic acid. Hydrolysis of the acetal function which involves the anomeric oxygen requires heating in mineral acid (0.1 M) for a few hours, comparable conditions to those of hydrolysis of a glycoside. 

Although hydrogenation of aromatic systems requires more energetic conditions than do those of olefinic double bonds, carbocyclic aromatics are readily hydrogenated to the fully saturated product. Controlling hydrogenolysis, selectivity and stereochemistry are major problems connected with arene hydrogenations. The substituents play an important role, and their effects are not the same for all catalysts. ... 

Catalyst Surface area Nitrogen content (m g" ) (mmol g ) N/Mo ratio HDS rate C-S (pmol g h ) hydrogenolysis /Selectivity ... 

Electron-rich metals Cu, Ag, Au, Hg, Sn, Pb, Ge (a) Improved selectivity in the hydrogenation of multiple bonds (b) Suppression of hydrogenolysis in alkane transformations (c) Suppression of carbon deposition (d) Alteration of hydrogenolysis selectivity ... 

The structure-sensitivity of alkane hydrogenolysis catalysed by platinum was first established by Oles Poltorak many years ago since then, comparatively little work has been done on this system. The TOP for cyclopentane decreased about five-fold on Pt/A Os catalysts of 7 to 65% dispersion at 573 K, this dependence being similar to that of the stepwise exchange of methane, but less than that for multiple exchange. The trends of isomerisation and hydrogenolysis selectivities with particle size observed with n-butane (Section 13.3) have... 

Relation Between the Hydrogenolysis Selectivity and Catalyst Structure of Rh-ReO, ... 

Fig. 22 Hydrogenolysis selectivity trend on various substrates over Rh-ReO, Rh-MoO, and Ir-ReO,...

---