Selective Oxidations on Recoverable Catalysts Assembled in Emulsions

Abstract

Catalysts assembled in emulsions are found to be potentially recoverable and efficient for a number of catalytic reactions. The catalysts composed of polyoxometalate anions and quaternary ammonium cations have been designed and synthesized according to the catalytic reactions and by optimizing the structures of cations and anions. The catalysts act essentially as surfactants, which are uniformly distributed in the interface of the emulsion droplets, and accordingly behave like homogeneous catalysts. The catalysts show remarkable selectivity and activity in the oxidation of sulfur-containing molecules to sulfones in diesel and the selective oxidation of alcohols to ketones, using H 2 O 2 as oxidant. For an example, the catalyst demonstrated over 96% efficiency of H 2 O 2 and~100 % selectivity to sulfones for the selective oxidation of sulfur-containing molecules in real diesel. Moreover, the catalysts can be separated and recycled by a simple demulsification and re-emulsification.

Construction of Amphiphilic Polyoxometalate Mesostructures as a Highly Efficient Desulfurization Catalyst

“…[ 18,19 ] To overcome this obstacle, a number of amphiphilic POM catalytic systems, both homogeneous and heterogeneous, have been reported. Common systems include phase transformation, [ 20 ] encapsulation by dendritic cations, [ 21 ] microemulsion formation, [22][23][24] and immobilization into a polymer or silica matrix. [ 19 , 25,26 ] However, generally both homogeneous and heterogeneous systems suffer from limitations.…”

Intriguing Role of a Quaternary Ammonium Cation in the Dissociation Chemistry of Keggin Polyoxometalate Anions

“…This kind of study relates to a special class of compounds: POM-based supramolecular assembles. This study allows us to better understanding the important role of cations on the formation of the POM anionic frameworks as structure directing agents [23][24][25] and on the control of catalytic activity of POM clusters [26][27][28].…”

Hybrid Organic-Inorganic Materials Based on Polyoxometalates and Ionic Liquids and Their Application in Catalysis

“…The microemulsion is achieved when a proper balance of attractive and repulsive interactions on the hydrophobic and hydrophilic side of the interface is attained. The water oil biphasic emulsion system with a surfactant type of catalyst designed and synthesized by using a suitable combination of hydrophilic POMs and lipophilic quaternary ammonium cations are reported by Li et al [49][50][51] the catalysts depends on the proper quaternary ammonium cation and its ability to form metastable emulsions in diesel with the H 2 O 2 . The higher the ability to form a metastable emulsion, higher the oxidation activity and the better the separation after reaction either by demulsification or by decantation.…”