Catalysis Of Polyoxometalate Composites In The Oxidation Of Alcohol

Oxidation of alcohol to carbonyl compounds is an important chemical reaction,which is widely used in medicine,agriculture and fine chemical industry.The traditional oxidants involved in alcohol oxidation reactions are generally compounds containing Cr(VI)or Mn(VII),high valence iodine compounds,sulfoxide,etc.,which are toxic oxidants and can cause pollution to the environment.With the increasing environmental problems,chemists are increasingly pursuing "green chemistry".Therefore,it is very meaningful to replace traditional toxic oxidants with non-toxic oxidants such as H2O2 or molecular oxygen(O2)However,most organic substrates and hydrogen peroxide or oxygen will form two-phase system without cosolvent such as oil-water or oil-gas,and the development of multiphase catalysts in the two-phase system is an important subject in green chemistry Polyoxometalates(POMs)are used in the oxidation of various alcohols by H2O2 because of their unique acidity and redox properties.POMs can be used as a homogeneous catalyst with high catalytic activity due to its high water solubility.But this also makes it difficult to recover from the system and is not suitable for industrial productionIn order to recycle POMs,we used hydrotalcite-like compound(LDH)as the supporter,and distributed the POM anion clusters to the interlayer of LDH through ion exchange,and made intercalated Mg3Al-POM LDH and Mg2AlNi-POM LDH.These materials were used as catalysts for the oxidation of cyclohexanol by H2O2.The results showed that the catalysts can not only effectively catalyze the oxidation of cyclohexanol,but also can be recovered from the system and reused.Moreover,the activity of the catalyst enhanced with the increase of Ni content and the decrease of Mg contentAlthough the above POM-LDH successfully heterogenize POM,when they are used in two-phase system,the two-phase system will undoubtedly be transformed into a complex three-phase system(i.e.,oil-water-solid or oil-gas-solid),which limits the effective contact between the reactants and the catalyst.In order to achieve effective contact between catalyst and reactant in the multiphase system,this paper adopts polyoxometelates(POMs)with catalytic activity as the inorganic hydrophilic part,attaches to the hydrophobic organic chain segment through the covalent bond,and finally forms the POMs hybrid molecule with both interface activity and catalytic activity.Firstly,we used the hybrid molecules as a catalyst to introduce a mixing system of cyclohexanol/H2O2 aqueous solution.It is proved that the designed catalyst has interface activity,which can emulsify the two-phase system of alcohol/H2O2 aqueous solution,and the resulting emulsion has high stability.Under the condition of ambient temperature,we systematically studied the catalytic performance and catalytic mechanism of the catalyst for the oxidation of cyclohexanol.The experimental results showed that the emulsion system can significantly increase the phase interface of the alcohol/H202 polyphase system,which is the main reason why the catalytic activity of the catalyst is significantly higher than that of the homogeneous POM.In addition,raising the temperature will agglomerate the organic hybrid POMs,thus achieving emulsion demulsification and catalyst separation.This system provides a reference for the application of emulsion catalyst in industrial productionMeanwhile,we introduced the above mentioned hybrid molecules into the alcohol/O2 system,and found that the hybrid molecules can not only change the interfacial tension of water-oil,but also change the surface tension of organic compounds,making it possible to transform into an unstable foam system.Through the study of the catalytic performance and mechanism of the catalyst in the foam system,it is found that the foam system can realize direct contact between reactants and catalysts at the interface,and also create more reaction interfaces for the system,increasing the overall concentration of catalyst and oxygen,resulting in higher catalytic activity of hybrid molecules than traditional heterogeneous catalysts.Foam system provides a new idea for organic synthesis reaction involving gas.