Gold Nanoparticles Stabilized With Functionalized Ionic Liquids: Catalysis In Liquid Phase Oxidation

Gold nanoparticles(GNPs)have shown promising application in the fields of catalysis,optics,electronics and biology sensor,etc.,due to the good stability,the quantum size effect,the surface and optics effect,and the special affinity for biology molecules.GNPs,which have attracted much attention in the last decades,are often used as a catalyst for the oxidation reaction in gas phase,for example,CO oxidation at low temperature.Relatively fewer researches were focused on the liquid phase oxidation,such as selective oxidation of alcohols and epoxidation of styrene.Although the styrene epoxidation has been successfully achieved with GNPs as a catalyst,the catalytic activity and water resistance of the GNPs is not satisfactory.Organic peracids,anhydrous t-butyl hydroperoxide(TBHP)and anhydrous H₂O₂ are always used as oxidants in the liquid phase oxidation of styrene.These oxidants are expensive,hazardous to be handled and very difficult to be applied for industrial practical merit.Therefore,how to improve the catalytic activity and use the environment-friendly oxidant(e.g.,H₂O₂ aqueous solution)in the GNPs catalytic liquid phase oxidation reaction of styrene are still challenging.To solve these problems,one of the key is how to prepare small size and uniform distribution GNPs.In this dissertation,three colloidal GNPs were prepared which were stabilized with ionic liquids functionalized by thiol,thioether or disulfide. And other three supported GNPs were synthesized which were captured and stabilized with the disulfide-functionalized ionic liquids,thioether or tetra-sulfide groups immobilized in hybrid inorganic-organic mesoporous silicates.The obtained colloidal and supported GNPs were characterized by UV-vis,FT-IR,XRD,TEM,N₂ adsorption-desorption,²⁹Si MAS-NMR and TG-DSC,etc.The catalysis in the liquid-phase oxidation of styrene over the GNPs was investigated systemically.The main results and conclusions are included as follows:Three functionalized ionic liquids were synthesized:(a)1-(2',3'-dimercaptoacetoxypropyl) -3-methylimidazolium,3″-mercapto-1″-propanesulfonic acid(the thiol-functionalized ionic liquids,RSH-IL);(b) 1-(2′,3′-epithiopropane)-3-methylimidazolium chloride(the thioether-functionalized ionic liquids,RSR-IL);(c)bis{6-[3-(methyl imidazolyl-hexyl-disulfide-hexyl)-imidazolyl]-hexyl}-disulfide chloride (the oligomeric disulfide-functionalized ionic liquids,RSSR-IL).The three synthesized ionic liquids were used as stabilization reagents to prepare GNPs,which were referred to GNPs-RSH-IL,GNPs-RSR-IL and GNPs-RSSR-IL,respectively.The particle size of these GNPs was 2.7±0.3 nm,2.8±0.8 nm and 3.4nm±1.0 nm,respectively.The size differences of GNPs indicated that the ability of the sulfur groups in the synthesized ionic liquids to stabilize GNPs was ordered as:RSH＞RSR＞RSSR.FT-IR spectra showed that the structure of the functionalized ionic liquids had not been destroyed during the preparation process of GNPs,and the ionic liquids had been integrated to the prepared GNPs.The catalytic performances of the GNPs stabilized by the three ionic liquids functionalized with thiol,thioether and disulfide were investigated in the epoxidation of styrene with m-CPBA as the oxidant.It was found that the catalytic activity of the GNPs was related to the sulfur groups in the functionalized ionic liquids.The order of the catalytic activity was GNPs-RSSR-IL＞GNPs-RSR-IL＞GNPs-RSH-IL.It was attributed to the different ability of the functionalized ionic liquids to stabilize and protect GNPs.The strongest ability to protect the formed GNPs of RSH-IL leaded to the lowest catalytic activity among the three functionalized ionic liquids.Therefore,choosing the thioether or disulfide functionalized ionic liquids as a stabilization reagent should be beneficial to reduce slightly the stability of GNPs and improve the catalytic activity.GNPs supported by hybrid inorganic-organic mesoporous silicates were prepared using disulfide-functionalized ionic liquids units, tetra-sulfide and thiol groups in the mesoporous silicates as a stabilization reagent,respectively.These three hybrid inorganic-organic mesoporous silicates of SBA-15 were synthesized by one pot hydrolysis and condensation reactions of organosilica precursors containing sulfur group and tetraethoxysilane(TEOS)in the presence of P123:(a)bridged organosilica precursor containing disulfide-functionalized ionic liquids units were incorporated into the network structure of the silica matrixe of periodic mesoporous organosilica(PMO),which was referred to 2S-IL-PMO-SBA-15.(b)bridged organosilica precursor containing tetra-sulfide units were incorporated into the network structure of the silica matrixe of PMO,referred to 4S-PMO-SBA-15.(c)thiol group anchored on the channel surface of SBA-15,referred to SH-SBA-15.The obtained supports were characterized by XRD,TEM,N₂ adsorption-desorption.The results indicated that they had the typical characteristics of SBA-15.The ²⁹Si MAS-NMR spectra and TG-DSC analysis revealed that the organic moieties had been covalently bonded to the silicate framework.Aqueous chloroaurate ions were in-situ reduced by the silanol groups on the surface of the prepared hybrid inorganic-organic mesoporous silicates to form GNPs that were subsequently captured by RSH,RSR and RSSR to form supported GNPs which were denoted as Au-2S-IL-PMO-SBA-15,Au-4S-PMO-SBA-15 and Au-SH-SBA-1 5 respectively.The results of TEM observation showed the average size of GNPs entrapped into the channels of the mesoporous silicates was ca.1.8 nm,3.4nm and＞5nm,respectively,which revealed that both the functionalized ionic liquids group and unique structure of PMO contributed to the formation of the small sized and high dispersed gold particles on the support of silica analog.This finding solved the problem that small size and high dispersion of GNPs are difficult to be obtained onto the inert supports of pure silica.The supported GNPs were used as catalysts in the epoxidation of styrene,using anhydrous TBHP,70%TBHP and 30%H₂O₂ as oxidants. The results showed that the Au-2S-IL-PMO-SBA-15 exhibited excellent catalytic activity and water resistance.With 30%H₂O₂ as the oxidant,the catalytic activity followed the order as:Au-2S-IL-PMO-SBA-15＞Au-4S-PMO-SBA-15＞Au-SH-SBA-15.99.6%of styrene conversion and 93.3%of selectivity to styrene oxide were obtained over the novel Au-2S-IL-PMO-SBA-15 catalyst.Furthermore,the catalyst could be conveniently recovered for recycled use without significant loss of catalytic activity and selectivity.The results further indicated that the functionalized ionic liquid group and PMO structure both contributed to the small size and high dispersion of gold particles,which endowed Au-2S-IL-PMO-SBA-15 high catalytic activity.The catalysis of the Au-2S-IL-PMO-SBA-15 in the selective oxidation of benzyl alcohol by aqueous TBHP was investigated.It was found that 29.1%of benzyl alcohol conversion with 100%of selectivity to benzaldehyde could be obtained.Furthermore,the catalyst could be conveniently recovered for recycle without significant loss of catalytic activity and selectivity.