SOLVENT EFFECT ON THE STYRENE OXIDATION OVER Co-ZSM-5.

SILVANA C. CAGLIERI, LILIANA B. PIERELLA AND CLARA SAUX.

Córdoba - Argentina.

Conferencia; 9ª Conferencia Latinoamericana de Físico-Química Orgánica. 9th Latin American Conference on Physical Organic Chemistry.; 2007

The styrene oxidation by hydrogen peroxide (30 wt% in water) over Co-HZSM-5 (0.6 wt% of Co by AA and > 98 % of crystallinity by XRD) has been studied. The zeolite was obtained by hydrothermal method and the Co incorporation was performed by ion-exchange. The partial oxidation of styrene was carried out in a glass batch reactor with a magnetic stirring immersed in a thermostatized bath, equipped with a reflux condenser, and a thermometer, at 60°C, styrene/H2O2 molar ratio: 1.7 and 0.2 g of the catalyst for 7 h. The mains products of reaction were benzaldehyde and styrene oxide and were analyzed by gas chromatography and mass spectrometry.The nature of solvent has an important effect on the reaction. In order to investigate the solvent effect on the styrene conversion, we have used both aprotic solvents such as: acetonitrile, acetone, ethyl methyl ketone and protic solvents such as: isopropyl alcohol and secbutyl alcohol. The results reported in figure show an increase of styrene conversion with the reaction time. The increase of the activity is more pronounced with the aprotic solvents. This behaviour may be explained by the increase of substrate concentration in the hydrophilic pores of Co-HZSM-5. Solvents Dielectric Constant Styrene conversion (mol %) at 7 h. acetonitrile 37.5 26.2 acetone 20.7 14.23 Ethyl methyl ketone 18.5 11.97 isopropyl alcohol 19.9 7.2 secbutyl alcohol 16.5 9.87                                               Hydrophilic character of Co-HZSM-5, would act forming stable complexes between alcohols (as solvents) with active sites in detriment of organic substrate adsorption over the surface of the zeolite, decreasing the activity for the reaction. New Brönsted acid sites could be generated by the protic molecules adsorption over Lewis acid sites of the Co-HZSM-5 samples, decreasing the activity for the reaction. Acetonitrile (aprotic solvent with the highest dielectric constant showed in the table) reveals to be the best solvent in term of activity for the reaction. This behaviour could be associated with the poisoning of Brönsted acid sites by the basic acetonitrile molecules. High benzaldehyde selectivity (> 80 mol%) and low styrene oxide selectivity (< 10 mol%)  could be obtained in all the experiments.