Selective Hydrogenation Reaction At Pickering Droplet Interfaces

As an important reaction in chemical processes,hydrogenation,especially the selective hydrogenation of multiple unsaturated bonds containing compounds,has been widely applied in the production of fine chemicals like drugs,chemical intermediates,dyes and spices et al.So far,the efficiency of selective hydrogenation was mainly improved by regulating the electronic or steric effects of metal surface.However,some obstacles always exist in these pathways,which significantly hindered their practical applications,for example,the separation of catalysts and products,catalyst recycling,low catalytic efficiency and uncontrollable selectivity.Pickering emulsion,which is stabilized by amphiphilic solid particles,is regarded as one of the most important green chemical platform to solve these problems,since its superior specialities like large water-oil interface,tunable interface properties and high stability.Especially,the unique polar gradient and microenvironment at the interface local can influence the catalytic species and change the hydrogenation selectivity.As such,Pickering emulsion might show great potential in engineering the catalytic efficiency of selective hydrogenation,but seldom studied in previous reports.In this thesis,we focused on improving catalytic efficiency of hydrogenation reactions via regulating the Pickering emulsion micro-interface,like the textural parameters of droplets or the distribution of catalytic species.The main results are summarized as follows.Ti O₂nanoparticles appropriately modified with hydrophobic organosilane could be utilized as efficient emulsifier to stabilize different types of Pickering emulsion,which could also be supported with noble metal NPs to act as active species in water phase.The emulsion type?o/w or w/o?,droplet size or the distance between droplets could be finely regulated by varying the amount of grafted organosilane,the shearing force or oil/water ratio.Such a tuning is curial for the mass transport-sensitive,multistep catalysis reactions such as selective hydrogenation of benzene.As the case study shows,the Pickering emulsion system gave much higher cyclohexene selectivity?51.2%?and yield?43.3%?than the conventional biphasic system even under lower-speeded stirring.The catalyst together with the solid emulsifier could be recycled effectively.After six reaction cycles,the selectivity and yield of cyclohexene were still as high as 54.8%and 38.4%,respectively.Impressively,this strategy and the case study presented here will help to understand the behavior of biphasic catalysis and push biphasic reactions toward a more efficient and controllable level.The titanate nanotubes modified with hydrophobic organosilane could also be utilized as emulsifier to stabilize Pickering emulsions.The assembly of nanotubes at different loci within w/o Pickering emulsions?for example the inner interfacial layer,within the interior of droplets,the conventionally-called of interfaces or the outer interfacial layer of droplets?can be regulated by the interaction between ligands and nanotubes.The interaction between nanotubes was revealed by means of fluorescence confocal spectroscopy,FT-IR spectra,cryo-TEM,dynamic interfacial tension curve and other characterization methods.The spatially controlled assembly of tubular particles at liquid-liquid interfaces of Pickering emulsion droplets provide a platform for further revealing the essence of interfacial catalytic process.We used?,?-unsaturated aldehydes hydrogenation as model reaction to investigate the influence of the Pickering emulsion systems with the spatially controlled assembly of tubular catalyst particles at the different locations of droplets.It is found that for all investigated?,?-unsaturated aldehydes the selectivity and catalysis efficiency for the inner interfacial layer reaction are much higher than those obtained for all of the composite interface reaction within Pickering emulsions,the outer interfacial layer reaction within Pickering emulsions,the reaction within droplet interiors and reactions in single aqueous or organic phases.92.0-98.0%selectivity to the thermodynamically and kinetically unfavorable C=O hydrogenation over C=C hydrogenation was achieved unexpectedly.The effects of mass transfer,basicity,droplet size,thickness of interfacial layer and ligand for the hydrogenation at inner interfacial layer of droplets were systematically investigated,and the liquid-liquid interface effect was revealed.This regulation method for hydrogenation selectivity breaks through the limitation of traditional method.