A Mechanistic Study on the Aerial Alcohols Oxidation Catalyzed by TPAP-Doped Sol-Gel Materials

1. Living A Mechanistic Study on the Aerial Alcohols Oxidation Catalyzed by TPAP-Doped Sol-Gel Materials S. Campestrini1, M. Carraro1, R. Ciriminna2, M. Pagliaro2, U. Tonellato1 1 Dipartimento di Scienze Chimiche, Università degli Studi di Padova, via Marzolo 1, 35131, Padua, Italy 2 CNR-Istituto per lo Studio dei Materiali Nanostrutturati, via Ugo La Malfa 153, 90146 Palermo, Italy Tetra-n-propylammonium perruthenate (TPAP) is an effective catalyst for high-selective aerobic oxidation of a wide variety of alcohols to the corresponding carbonyl compounds. The catalyst is still active even when it is entrapped in a solid matrix like organic-modified silicates (ORMOSILS). RR’-CHOH + ½ O2ORMOSILS-TPAP RR’-C=O + H2O toluene or scCO2 , 75°C Catalytic activity for the aerial oxidation of benzyl alcohol to benzaldehyde in toluene over TPAP entrapped in fresh and “aged” 75%-methyl-modified silica matrices (Me3A). The external surface Ru/Si atomic molar ratio of fresh and “aged” Me3A . (by electron-probe microanalysis) 18 months later (A) (B) What happens to the catalyst? In order to understand the activity variation it is important to know that the Organic Modified SILicates, wherein the TPAP is dispersed, were prepared by the sol-gel route. The xerogel Me3A by hydrolisys and condensation of a mixture of methyltrimethoxysilane (75%) and tetraethylortosilicate (25%) under neutral condition. And since the growth of the material is under kinetic control, the thermodynamically unstable texture of xerogel continues to evolve with time by condensation. What is the factor, then, responsible for the ten-fold improvement in catalityc activity? See on the right… (C) (D) Kinetic costant for aerobic oxidation of benzyl alcohol to benzaldehyde in supercritical CO2 over TPAP entrapped in fresh and “aged” 75%-methyl-modified silica matrices. The electron-probe microanalysis shows that the Ru/Si atomic ratio at the surface decreases with ongoing time. At the early stage of the sol-gel process (A), pseudomicellar aggregates form in solution in which the alkyl tails tend to orientate towards the inner core of the aggregate (B). In the resulting hydrophobic environment, lipophilic TPAP ion pairs (in red) concentrate. This agglomerate of perruthenate eventually end up in the inner, clogged porosity where it is not completely available for catalysis (C). With time the material continues to evolve and the superficial hydroxyls further condense creating a new open microporosity wherethese TPAP molecules migrate and disperse (D). The oxidation of various alcohols with oxygen catalysed by TPAP-doped ORMOSILS in sc-CO2 at 75°C and 22.0 MPa has been studied in detail in order to elucidate the reaction mechanism. The system preserves TPAP from self-aggregation (leading to oxidation-inactive ruthenium derivatives) and allows free diffusion of the reactants dissolved in the supercritical phase. Kinetic order in substrate relative to benzyl alcohol oxidation with O2 (1 bar at 25°C) catalysed by Me3A-TPAP (content of TPAP: 5 x 10-3 mmol per 100 mg ormosil), in scCO2 at 75°C, and 22 MPa Kinetic order in catalyst relative to benzyl alcohol oxidation with O2 (1 bar at 25°C) catalysed by Me3A-TPAP (content of TPAP: 5 x 10-3 mmol per 100 mg ormosil), in scCO2 at 75°C, and 22 MPa Kinetic order in oxygen relative to benzyl alcohol oxidation with O2 catalysed by Me3A-TPAP (content of TPAP: 5 x 10-3 mmol per 100 mg ormosil), in scCO2 at 75°C, and 22 MPa FIRST ORDER SUSTRATE The negative order in oxygen is compatible with a “ternary complex” in wich the oxygen can saturate the perruthenate. FIRST ORDER CATALYST NEGATIVE ORDER OXYGEN The relative reactivities of 3-Chlorobenzyl alcohol (f), 4-Methylbenzyl alcohol (g)and Benzyl alcohol (b)determine an Hammet correlation with a r>0, thus suggesting that a partial negative charge in the transition state. This also indicates that the higher reactivity of primary alcohols respect with secondary is due to electronic rather than to steric effect. 1-D-Benzyl (a)alcohol and Benzyl alcohol (b)show a KIE>1; this means that the rate-determining-step is the hydride abstraction. kcat 1-Phenylethanol (c) is less reactive than Benzyl alcohol (b) such as 2-octanol (e) versus 1-octanol (d); Me3A-TPAP show a preference in oxidation of primary versus secondary alcohols as found for the stoichiometric oxidation with TPAP [K. Yamaguchi, N. Mizuno, Angew. Chem. Int. Ed.2002,41, 4538]. The substrate Benzyl methyl ether (h)does not react, thus suggesting the formation of an ester-type rather than an organo-metallic intermediate. Kinetic costant for benzyl alcohol oxidation with O2 (1 bar at 25°C) catalysed by Me3A-TPAP (content of TPAP: 5 x 10-3 mmol per 100 mg ormosil), in scCO2 at 75°C, and 22 MPa