Design Of The "Pseudo-homogeneous" Heterogeneous Catalysts And Study On Their Catalytic Performance

Although many homogeneous catalysts demonstrate high activity and selectivity,they are difficult to be recovered for reuse.Heterogeneous catalysts are easily separated form reaction systems,but often exhibit lower catalytic efficiency due to the limited mass transform.To address the issues,we attempted to develop novel kinds of “pseudo-homogeneous” heterogeneous catalysts which allow heterogeneous reactions to be carried out under pseudo-homogeneous reaction conditions.The strategy makes efficient catalysis couple with facile separation of catalysts come true.A series of novel chiral dimeric salen Mn?III?complexes functionalized by the task specific imidazolium-based ionic liquids?ILs?at the C-5 position of the salicylaldehyde moieties were prepared.The obtained complexes containing ILs were well characterized by FT-IR,UV-vis,ESI-MS spectra,and elemental analysis.The imidazolium-based ILs unit bonded as a bridge between the two monomeric salen Mn?III?complexes could combine the nature of the complexes with the ILs properties at molecular level.Therefore,the resultant IL-functionalized chiral dimeric salen Mn?III?complexes could act as a solvent-regulated phase transfer catalysts in the oxidative kinetic resolution of secondary alcohols using diacetoxyiodobenzene?PhI?OAc?2?as an oxidant.The positive effect of the imidazolium cation in the IL-functionalized chiral dimeric salen Mn?III?complex on the catalytic activity and enantioselectivity was observed.Under the optimum reaction conditions,up to 99% of enantiomeric excess?ee?of R-configuration with 67% conversion of ?-methyl benzyl alcohol was achieved.Furthermore,the complexes with unique solubility could be conveniently separated from the reaction system by simple addition of hexane and could be reused without significant loss in catalytic activity and enantioselecitvity.Pristine L-proline was non-covalently loaded on the graphene oxide?GO?sheet in a simple route by mixing them in aqueous solution.Technologies of characterization well suggested that L-proline was efficiently loaded on the two sides and edge of the GO sheet through hydrogen-bonding or/and ionic interaction,giving the excellent L-proline/GO hybrid catalyst for the direct asymmetric aldol reaction.The unique multilayered stucture of the GO carrier with sufficient interlayer space favored reagents' diffusion toward L-proline chiral moiety,and therefore resulted in the high catalytic efficiency of the heterogeneous L-proline.Excellent yield?96%?with high enantiomeric excess?79% ee?was obtained in the direct aldol reaction of 2-nitrobenzaldehyde with acetone catalyzed by L-proline/GO hybrid,which was comparable to that observed in the reactions promoted by L-proline itself.Furthermore,the L-proline/GO hybrid used as a heterogeneous catalyst could be easily recovered and recycled for seven times without significant loss of the reactivity.Keggin-type phosphotungstic acid(H3PW₁₂O₄₀)was supported on the amino-functionalized GO sheets through protonating amino groups on GO surface.The obtained heterogeneous catalyst?PW-NH-Si-GO?was efficient and universal in selective oxidation of alcohols with H2O2.High conversion?78%?of benzyl alcohol with excellent chemoselectivity?87%?to benzaldehyde was observed over 3mol% of the PW-NH-Si-GO within 6 h.The efficiency was comparable to that over the homogeneous counterpart.After completion of the reaction,the catalyst could be facilely recovered by centrifugal separation and exhibited well reusability.Benefiting from the layered structure and the ability to intercalate a diverse range of interlayer anions,layered double hydroxides?LDHs?were used as supports of H3PW₁₂O₄₀ to prepare pillared catalysts [PW₁₂O₄₀]3--LDHs by anionic exchange method using Mg/Al–CO3 LDH as the precursor.Technologies of characterization of FT-IR and XRD well suggested the incorporation of [PW₁₂O₄₀]3-anion in LDHs interlayer through ionic interaction.As expected,the catalytic efficiency of [PW₁₂O₄₀]3--LDHs was similar to that of homogeneous counterpart in epoxidation of cyclohexene.High conversion?88.9%?with excellent selectivity?97%?was obtained over the [PW₁₂O₄₀]3--LDHs.Furthermore,the catalyst could be facilely recovered from reaction system for efficient reuse.