Juana Herrero

INFRARED STUDIES OF SURFACE ACIDITY AND REVERSIBLE FOLDING IN PALYGORSKITE

The infrared absorption spectra of a palygorskite sample from Cáceres, Spain, showed two previously unreported bands in the OH-stretching region at 3420–3440 and 3220–3230 cm−1 after evacuation at 90°–230°C. These bands, which reached maximum intensity after the sample was heated at 150°C, were assigned to OH in the

Kinetic studies on the selective hydrogenation of phenylacetylene catalyzed by [Rh(NBD)(PPh3)2]BF4 (NBD=2,5-norbornadiene)

\begin{array}{*{20}{c}} H \\: \\ {Si - O - Si\,and\,} \\ \end{array}\begin{array}{*{20}{c}} H \\: \\ {Si - O - A1} \\ \end{array}

Surface acidity of palygorskite-supported rhodium catalysts

groups, respectively. To characterize the nature of these OH groups, pyridine was adsorbed on the sample. The resultant spectra suggest that at 150°C the palygorskite folded and OH groups protonated, resulting in the formation of a deformed pyridinium ion between 150° and 290°C. A high concentration of thermally stable Lewis-acid sites on the surface of the palygorskite was also noted.

CO adsorption on rhodium(i) and on metallic rhodium supported on titanium dioxide

Abstract A kinetic investigation of the selective hydrogenation of 2,5-norbornadiene to norbornene catalyzed by [Rh(NBD)(PPh 3 ) 2 ]BF 4 ( 1 ) has been carried out in dichloromethane at room temperature. The reaction is independent of the substrate concentration, while it is first order in catalyst and hydrogen pressure. Furthermore, the addition of triphenylphosphine inhibits the reduction. Moreover, it has been observed that under hydrogen atmosphere and at low temperature, complex 1 is in equilibrium with the dihydrido cis – trans- [RhH 2 (NBD)(PPh 3 ) 2 ]BF 4 , and that the addition of one equivalent of [Rh(NBD){P( p -Tol) 3 } 2 ]BF 4 to a chloroform- d 1 solution of 1 affords [Rh(NBD)(PPh 3 ){P( p -Tol) 3 }]BF 4 in 54% yield. On the basis of these observations and other spectroscopic results, we propose that the hydrogenation of 2,5-norbornadiene to norbornene catalyzed by 1 proceeds by the five-coordinate dihydrido [RhH 2 (NBD)(PPh 3 )] + , which is formed by oxidative addition of molecular hydrogen to both 1 and a tricoordinate species [Rh(NBD)(PPh 3 )] + , depending on the concentration of free phosphine in the catalytic solution.

Decomposition and reducibility of rhodium clay catalyst precursors

Abstract The complex [Rh(NBD)(PPh 3 ) 2 ]BF 4 catalyzes the selective hydrogenation of phenylacetylene to styrene. In dichloromethane solution at 25°C, selectivities close to 80% are achieved. The reaction is first order in catalyst and substrate, and second order in hydrogen pressure, while the addition of triphenylphosphine inhibits the reduction. The mechanism deduced for this hydrogenation, on the basis of the above mentioned kinetic results, suggests that the reduction of the diolefin of the catalyst does not occur during the hydrogenation of the alkyne.

EXAFS Characterization of Rhodium-Clay Catalysts and Their Precursors

Abstract Rhodium catalysts, prepared by reduction of a cationic organometallic rhodium compound anchored to various treated clays, silica and titania, were tested in the hydrogenation of a sunflower oil in acetone solution. The reaction was carried out at atmospheric pressure and room temperature. Preliminary results on the effect of different supports on the activity, selectivity and trans-isomer content are presented. Results show that rhodium supported on palygorskite, pretreated at 423 K in vacuum, gave rise to the highest activity of all the catalysts at an iodine value of more than 55, and the lowest trans-isomer formation among the catalysts supported on palygorskite for an iodine value of more than 65. The rate of hydrogenation is of the same order or less than that found for 1-hexene hydrogenation.

Exclusive formation of cis-PhCH:CH(SiEt3) by addition of triethylsilane to phenylacetylene catalyzed by ruthenium complex [(Me2CH)3P]2RuHCl(CO)

Infrared spectra of adsorbed pyridine have been used to obtain qualitative information on the nature of the interaction of pyridine with different rhodium catalysts supported on palygorskite and silica. Based on these data, qualitative definitions of the adsorption sites of these catalysts have been deduced. The catalysts were prepared with natural palygorskite, and palygorskite dehydrated in vacuo at 150°C and 400°C. In this way, catalysts were obtained that had different water contents and, therefore, different acidities. Lewis-bound pyridine was detected by infrared spectroscopy from room temperature to 500°C. The greatest acidity was found in a catalyst prepared with the palygorskite support dehydrated at 150°C before preparation of the catalyst. With this catalyst, strongly hydrogen-bound pyridine was observed when evacuation was carried out at temperatures between 150°C and 300°C. Catalysts prepared with the palygorskite support pretreated at 400°C did not exhibit strongly hydrogen-bound pyridine, and Lewis acidity decreased significantly. As expected, hydrogen-bound pyridine was also detected for rhodium supported on silica. However, it was desorbed at temperatures below 150°C. The results of the acidity studies follow the same pattern as those for 1 -hexene double-bond migration under hydrogénation reaction conditions.

Selective hydration of nitriles to amides promoted by an Os-NHC catalyst: Formation and X-ray characterization of κ2-amidate intermediates

Abstract The interaction between a cationic organometallic Rh(I) compound and titanium dioxide in acetone leads to Rh(I) supported on titanium dioxide. The reduction of this preparation with molecular hydrogen at atmospheric pressure and ambient temperature yields a supported metallic rhodium catalyst which is active in 1-hexene hydrogenation. Both precursor and catalyst have been characterized by XPS and CO adsorption infrared spectroscopy. The dicarbonyl species is the predominant one in both samples. Dissociative CO adsorption is discussed.