Reaction of Sn(n-C4H9)4 with Rh supported on silica results in a new bimetallic RhSn catalyst which is extremely active and selective in the reduction of ethyl acetate to ethanol. Whereas Rh/SiO2 gives rise to a selectivity for ethanol of 57%, the RhSn 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/SiO2 A with CO exhibits the typical infrared absorption bands of linear and bridged CO. Reaction of oxidized A with Sn(n-C4H9)4 in refluxing heptane occurs mostly between Rh2O3 and the organotin compound to give an unreduced RhIII7z.sbnd;SnRx 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 H2 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/Rht and 0.4 CO/Rht which is in sharp contrast with the values obtained with A (1.1 H/Rht and 1.1 CO/Rht). 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 H2 adsorbed; (ii) it apparently isolates rhodium atoms from their neighbors; (iii) it increases slightly the electron density on rhodium. Redox behavior of the RhSn/SiO2 toward O2 and silanol groups of silica has also been observed. With a fully reduced catalyst C, Rh(0) and Sn(0) are fully oxidized by O2 to Rh2O3 and SnO2. 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. H2 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 CC and CO bonds of ethyl acetate.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry