Selective Oxidation Of Styrene To Prepare Benzaldehyde Over Metal Oxide-Quaternary Ammonium Chloride Catalyst

Benzaldehyde is an important organic intermediate, which is widely used in the productions of medicine, pesticides, spices, and other fine chemicals. The current production methods include the hydrolysis of benzyl dichloride and the oxidation of toluene, which is produced as a byproduct of benzoic acid production. However, the drawbacks of these processes are the environmental pollution problem and the limited production of benzaldehyde. The present investigation explored a new method for the preparation of benzaldehyde by making use of phase transfer catalytic oxidation of styrene. The selective oxidation of styrene was conducted over metal oxide-quaternary ammonium chloride catalyst by using hydrogen peroxide as oxidant. This method has the following advantages:simple catalyst that do not require preparation,, easy catalyst recovery process, high catalytic efficiency, and greener reaction pathway.In the present investigation, V2O5-quaternary ammonium chloride is used as the catalyst. Reaction conditions, such as quaternary ammonium chloride species, reaction temperatures, catalyst mass, styrene/oxidant molar ratio, and H2O2 flow, were systematically optimized. A maximum benzaldehyde selectivity of 92% with styrene conversion of 72% was reached.The phase transfer capabilities of several quaternary ammonium chlorides were studied. The amount of vanadium transferred from the aqueous phase to styrene phase by different quaternary ammonium chloride was measured by color metrical analysis on a UV-visible spectrophotometry. By comparing the amount of vanadium that was transferred into the styrene phase, the phase transfer capability of quaternary ammonium chloride was evaluated.Over the V2O5-quaternary ammonium chloride catalyst, the reaction mechanism is suggested. Based the amount of V2O5 used in the reactions, the V2O5 is fully dissolved in H2O to form vanadate acid, which reacts with H2O2 to form peroxyvanadate (V) anions, and then the peroxyvanadate (V) anions react with quaternary ammonium in aqueous phase to exchange cations to form quaternary ammornium peroxyvanadate (V)(QAPV). The QAPV diffuses from aqueous phase into styrene. The styrene reacts with QAPV to form styrene oxide, phenylacetaldehyde, and acetophenone as the primary oxidation products. The styrene oxide might be hydrolyzed to form 1-Phenyl-1,2-ethanediol. Both styrene oxide and 1-Phenyl-1,2-ethanediol could be oxidized to benzaldehyde at the reaction conditions. The succeeding oxidation of benzaldehyde forms benzoic acid. After reaction in styrene, the QAPV becomes simple vanadate and diffuses back to the aqueous phase, and then reacts with H2O2 and quaternary ammonium chloride to regenerate QAPV for the next cycle of reaction.The other catalyst investigated is the MoO3-quaternary ammonium chloride. Over the catalyst, the influences of the quaternary ammonium chloride, the reaction temperature, the quitity of catalyst, and the mol ratio of styrene/oxidant on reaction were investigated. At the optimal reaction conditions, a styrene conversion of 28% with a benzaldehyde selectivity of 69% was obtained.