Cyclopropane adsorption

Figure 6. The optimized geometry of the cyclopropane adsorption complex.

The same type of porphyrin-Ru complex was immobilized by coordina-tive adsorption on aminopropylsilicas (Fig. 26) as either amorphous or crystalline supports [79]. Mesoporous crystalline MCM-48 was the best support, as shown by the improved results obtained in the epoxidation of styrene with 2,6-dichloropyridine N-oxide (TON > 13 000 and 74% ee). The versatility of this catalyst was demonstrated in the intramolecular cyclopropanation of frans-cinnamyl diazoacetate. TON was ten times higher than that obtained in solution and 85% ee was observed. The solid was recycled and reused, although partial loss of selectivity occurred. 

Hj Dj exchange on, 26 39-43 heteropolyanion-supported, 41 230-231 high MiUer index, 26 12-15,35,36 -H-USY zeoUte, 39 186-187 hydrocarbons adsorption, 38 229-230 reactions of cyclopropane, cyclohexane, and n-heptane, 26 51-53 structural effects, 30 25-26 hydrogen adsorption on, 23 15 hydrogenation, 30 281-282 olefins, in ethanol, 30 352-353 in hydrogenation reaction, 33 101 -iron alloys, 26 75 isomerization, 30 2-3 isotope, NMR properties, 33 213,274 kinetic oscillations, 37 220-228 ball models of densely packed surfaces, 37 221-222... 

Muller and Gault proposed (57) an adsorbed cyclopropane-like intermediate (Scheme VIIIB). The McKervey-Rooney-Samman mechanism involves a n complex like adsorption of two carbon atoms (88) (Scheme Vine). 

The adsorption of cyclopropanes at room temperature has been characterized by infrared spectroscopy for a number of silica-supported catalysts, viz., Ni (86), Pt (86), Pd (266), and Rh (91). The spectra are identical with those obtained from the adsorption of propene on the same metals. They give absorptions from CH3 groups showing that the C3 ring has been opened, and the nature of the spectra has already been discussed (140, and Part I, Section Vl.C.l.b). Typical spectra of species formed from cyclopropane on Ni/Si02 and Pt/Si02, obtained by Ward at room temperature, are shown in Figs. 9C and 9D. 

Avery (43) and Ward (71) have shown that at room temperature closely similar spectra are obtained by the adsorption of cyclopropane or propene on Pd/Si02 and Ni/Si02, respectively. Isomerization of cyclopropane to propene most probably occurs as an intermediate situation. 

Ni-exchanged zeolite A Dehydration and adsorption of N20, ethene, propene, cyclopropane State of nickel, aquo complexes DR/IS Klier and Ralek, (1968)... 

An example of the role of strongly bonded intermediates is afforded by the work of Ponec et al. (ifast reaction takes place only on a small fraction of the surface, whereas on the remainder the dissociated and dehydrogenated species are removed only slowly adsorption of both components is competitive. 

Cerny and co-workers measured calorimetrically heats of hydrocarbon adsorption on Pt (243) and Dy (244) films at room temperature. The heats of adsorption on Pt were significantly higher than those found in the previous study and increased in the order methane ethylene < propylene < acetylene methylacetylene < allene < cyclopropane ethane < propane. 

It appears that this facile bond cleavage is the result of an assisted adsorption of the cyclopropane ring promoted by the adsorption of the unsaturated species on the catalyst surface adjacent to the hydrogenolysis active site as depicted in Fig. 20.3. Unsaturated substituents are not the only species that can assist in the adsorption of the cyclopropane ring. Cyclopropyl amines are hydrogenolyzed over palladium or Raney nickel at 80°C and 50 atmospheres by breaking one of the bonds adjacent to the amine group.26,27... 

As far as the adsorption and skeletal isomerization of cyclopropane and the product propene are concerned, results mainly obtained by infrared spectroscopy, volumetric adsorption experiments and kinetic studies [1-4], revealed that (i) both cyclopropane and propene are adsorbed in front of the exchangeable cations of the zeolite (ii) adsorption of propene proved to be reversible accompanied by cation-dependent red shift of the C=C stretching frequency (iii) a "face-on" sorption complex between the cyclopropane and the cation is formed (iv) the rate of cyclopropane isomerization is affected by the cation type (v) a reactant shape selectivity is observed for the cyclopropane/NaA system (vi) a peculiar catalytic behaviour is found for LiA (vii) only Co ions located in the large cavity act as active sites in cyclopropane isomerization. On the other hand, only few theoretical investigations dealing with the quantitative description of adsorption process have been carried out. 

Adsorption and IR spectroscopic measurements proved that significant structural changes occured in the adsorbed cyclopropane and propene molecules upon interaction with the cations. Similarly, the results of X-ray diffraction measurements revealed that significant changes took place in the geometry of the adsorption sites, first of all in the positions of the... 

An infrared study of CO adsorption on Ru-Au supported on magnesia suggested that this bimetallic behaves differently from Ru-Cu, with no evidence of Au segregation at the cluster surface, (nor separate Au clusters although ruthenium and gold are practically immiscible in the bulk). At temperatures below 383 K where the reaction between cyclopropane and hydrogen adopted routes to propane or methane + ethane, no interaction between Au and Ru containing up to 36% Au was evident from the kinetic parameters.However, a more complete examination (unpublished) of these catalysts by XPS, EXAFS, SAXS, and other techniques has been made and it is believed that the surface contained Ru atoms only. 

From these results, it is concluded that, after exchange with Ca2+, zejolite LTA can selectively adsorb n-butanol and higher normal alcohols, n-butene and higher normal alkenes, propane and C4 Ci4 normal alkanes, and cyclopropane, etc., which cannot be adsorbed by zeolite 4A. On the other hand, the presence of divalent Ca2+ promotes the adsorption selectivity of 5A zeolite for polar molecules and other unsaturated compounds. 

In bi- and oligocyclic systems the cleavage reaction can also take a different course depending on the preferred mode of adsorption of the molecule on the catalyst surface. For example, in exo-homotriquinacene, 14 and 15 and in hexacyclo[4.4.0.0 .0 . 0 ]decane (diademane), both types of cyclopropane C — C bonds were cleaved by hydrogenation in the presence of palladium on charcoal. ... 

The presence of electrolyte, its possible adsorption on the electrocatalyst, and the electrode-electrolyte potential can alter the strength of reactant adsorption, the surface coverage, and the reaction rate (5,7,8). Thus, electro-generative hydrogenation of ethylene on platinum and palladium electrodes in acidic electrolytes proceeds more slowly than the corresponding gas phase catalytic reactions (33). However, electrocatalytic reduction of cyclopropane is faster than the catalytic one, probably due to a decrease in hydrogen and reactant competitive chemisorption. Some electrolyte ions and impurities can also poison the electrocatalysts (34). 

Similarly, the difficulty for electrocatalytic, electrogenerative hydrogenation of alkenes on platinum parallels the strength of gas phase adsorption of the substrate (55) acetylene > ethylene > propylene > cyclopropane. Palladium is a more active electrocatalyst for ethylene reduction than platinum (55), in agreement with adsorption strength on each metal. Selectivity and reduction rate of substituted alkenes also depends on adsorption... 

Reduction of acetylene as well as ethylene, propylene, and cyclopropane, at positive potential was first reported by Langer and co-workers (25, 26, 33). Mass spectrometric and coulometric analyses showed quantitative hydrogenation of the unsaturates. Reductions were fastest in acidic electrolytes in agreement with the potentiometric results discussed earlier. The relative rates of the reactants on Pt black appeared to follow the strength of adsorption, with acetylene the most difficult to hydrogenate and with cyclopropane reducing readily. 

Bassett and Habgood (24) deliberately applied this chromatographic method, in addition to the microcatalytic chromatographic method of Hall and Emmett 25), to the isomerization of cyclopropane on a molecular sieve. They were able to assess the heat of adsorption of the reactant, the activation energy and the order of the reaction. 

Stereospecific Adsorption of Nitrous Oxide, Cyclopropane, Water, and Ammonia on the Co(II)A Synthetic Zeolite... 

The interaction of many hydrocarbons (both aliphatic and aromatic) with zeolites has been investigated. HY zeolite catalyses the conversion of cyclopropane at room temperature to isobutane. The proposed mechanism involves a non-classical protonated cyclopropane ion intermediate. At 200 cyclopropane isomerizes to propene and also forms aromatic species. Adsorption and transformation of but-ene has been widely studied. It is useful to draw a distinction between hydroxylated and dehydroxylated samples. On hydroxylated samples but-ene isomerizes and also oligo-The -OH groups vibrating at 3640 cm" were found to be... 

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