Design,Preparation And Catalysis Performance Of Janus Mesoporous Silica Nanosheet

Janus nanoarchitectures,an emerging class of nanostructures named after the Roman god having two faces,have received increasing concerns in recent years.Two internal materials with distinct physical and chemical properties can be integrated in one nanoparticle via designing this Janus structure.Such an asymmetric structure endows this new nanomaterial great potential application in various areas,such as optical imaging,emulsion stabilizers,catalysis,drug delivery,etc.Particularly,in catalysis,two and even different active sites can be integrated in a nanoparticle towards a multifunctional heterogeneous catalyst.Obviously,this provides an efficient platform for designing various cascade reactions,understading cooperative catalysis effect,constructing micro/nanomotors,improving the catalytic efficiency of biphasic reactions and achieving in-suit separation and recycling of special nanocatalysts.In the second chapter,we report a new Janus mesoporous silica nanosheet with perpendicular mesochannels for enhanced biphasic catalysis.PS microspheres are used as the hard template for coating mesoporous silica.After selective surface modification and selective removal of the PS microspheres,we obtain the Janus mesoporous silica hollow microspheres.A simple grinding process is then conducted to crush the microspheres to numerous of tiny Janus mesoporous silica nanosheets.When one side of this nanosheet shows many hydrophilic silanol groups,another side is grafted with high-density of hydrophobic octyl groups.This leads to the formation of the asymmetric structure,endowing the nanosheet excellent interface activity that is favourable for forming thermodynamically stable Pickering emulsions.After immobilizing metal Pd nanoparticles inside the mesochannels of Janus nanosheets via a simple impregnation-reduction process,we obtain the Janus nanocatalyst Pd/m SiO₂.This unique nanocatalyst shows a significantly enhanced activity in aqueous nitroarene hydrogenation reactions,13-fold and 4.6-fold higher than those of the hydrophilic counterpart and conventional silica-based interfacial catalyst,respectively.It is found that the Janus nanosheets parallelly assembled at oil-water interfaces during reactions can provide an extremely short diffusion distance for oil and water soluble reactants,resulting in the superior catalytic performances.In addition,the permanent interfacial activity of Janus nanosheets and the anchoring effect of –NH₂ make the Janus catalyst very stable.After being resued for five times,it can still give a good catalytic activity.In the third chapter,on the basis of the previous chapter,Pd Ag alloy nanoparticles are loaded into the mesopores of Janus nanosheets via a simple impregnation-reduction process,presenting a unique Janus catalyst Pd Ag/m SiO₂.This interface active nanocatalyst is then used to construct a highly efficient Pickering interface catalysis system for dehydrogenation-hydrogenation reactions.Owing to the superior interface activity and the perpendicular mesochannels,offering a high oil-water interface area and a high mass transport for both oil and water soluble reactants,the obtained catalyst gives a good performance in the biphasic nitroarene reduction reactions（formic acid as the reductant）.Moreover,we study the influence of the ratio of formic acid and nitroarene and the thickness of the Janus nanosheet on the catalytic performance.It is found that the catalytic active is the highest when the ratio is 4:1 and the thickness is 40 nm（compared with the thickness of 66 nm and 95 nm）.Additionally,the catalyst can still present a good catalytic activity after being resued for five times.In the fourth chapter,the interface activity of the Janus mesoporous silica nanosheets is precisely adjusted via varying the density of the surface octyl groups.When the water contact angle is about 90°,solvent-switchable reversible inversion of Pickering emulsions,using the corresponding Janus nanosheet as the solid emulsifier,can be achieved.Such an inversion is a consecutive and general process,which is adaptive in various oil-water systems.More importantly,this inversion can be applied as a general strategy for efficiently in-suit separating and recycling biocatalysts and metal nanocatalysts.In the CALB-catalyzed ester hydrolysis reaction,we can obtain a conversion of 49% with a product ee value of 99.9% in 3 hours（the theoretical conversion rate is 50%）.The conversion rate can still reach 42% after seven recycling runs,while the conversion rate of a typical enzyme-immobilized catalyst is reduced from pristine 49% to 23% after being resued for only three times.In the Pd-catalyzed nitrobenzene hydrogenation reaction,the conversion can be still remained 95% after ten consecutive runs.