Surface organometallic chemistry on metals: II. Characterization of new bimetallic catalysts generated by reaction of Sn( n-C 4H 9) 4 with silica-supported rhodium

Abstract

Reaction of Sn( n-C 4H 9) 4 with Rh supported on silica results in a new bimetallic RhSn catalyst which is extremely active and selective in the reduction of ethyl acetate to ethanol. Whereas Rh/SiO 2 gives rise to a selectivity for ethanol of 57%, the RhSn catalyst obtained by the organo-metallic route results in a higher activity and a selectivity to ethanol as high as 98%. Above a Sn/Rh value of 0.3, the activity varies linearly with the tin content which suggests that the enhanced catalytic activity is due to a new intermetallic phase. The catalysts have been characterized at various steps of the preparation. The starting reduced catalyst Rh/SiO 2 A with CO exhibits the typical infrared absorption bands of linear and bridged CO. Reaction of oxidized A with Sn( n-C 4H 9) 4 in refluxing heptane occurs mostly between Rh 2O 3 and the organotin compound to give an unreduced Rh III7z.sbnd;SnR x bimetallic surface complex B, the existence of which has been suggested from mass balance, STEM, and IR spectroscopy. Reduction of B at 773 K under H 2 leads to bimetallic particles with an average size of 2.2 nm and which do not contain any organic fragment (catalyst C). C chemisorbs only 0.1 H/Rh t and 0.4 CO/Rh t which is in sharp contrast with the values obtained with A (1.1 H/ Rh t and 1.1 CO/ Rh t ). CO chemisorption on B gives only a single absorption band at 2000 cm −1 corresponding to linear coordination of CO. The presence of tin has apparently three effects: (i) it decreases significantly the amount of CO and H 2 adsorbed; (ii) it apparently isolates rhodium atoms from their neighbors; (iii) it increases slightly the electron density on rhodium. Redox behavior of the RhSn/SiO 2 toward O 2 and silanol groups of silica has also been observed. With a fully reduced catalyst C, Rh (0) and Sn (0) are fully oxidized by O 2 to Rh 2O 3 and SnO 2. Thermal treatment of catalyst C under flowing He results in an oxidation of tin by surface silanol (or adsorbed water) to give a partially oxidized Sn species. H 2 is evolved during this oxidation process. The origin of the high activity and high selectivity (without hydrogenolysis property) of these catalysts is ascribed to the presence of a new catalytic phase in which rhodium atoms are isolated from their neighbors without any “ensemble” able to cleave the CC and CO bonds of ethyl acetate.