Pickering Emulsion Microreactor Hosting Ionic Liquid For Synthesis Of Cyanohydrin

Cyanohydrins are highly versatile synthetic intermediates,which can easily be converted into a wide variety of β-amino alcohols,α-hydroxycarboxylic acids,α-aminonitriles and α-hydroxy esters.They are also components of commercially important compounds.This transformation typically requires the use of Lewis acids,Lewis bases as the catalyst.Unfortunately,there are many problems in the above process suffer from the disadvantages of high dosage,low product selectivity,large use of organic solvents or harsh reaction conditions,which do not meet the requirements of green chemistry.Ionic liquids(IL)can be used as both green solvents and catalysts under mild reaction conditions,and they are widely used in catalytic reaction systems.However,it is directly used in batch reaction system,there are some cumbersome problems such as large amount of IL catalyst,difficult to separate and recover from the reaction system.In order to solve the above problems,homogeneous IL catalysts are usually immobilized on supports to form solid heterogeneous catalysts and then filled into fixed beds to realize continuous-flow catalytic reactions.However,this method requires multi-step modification of IL structure units,which will destroy the internal structure of ionic liquid.At the same time,the active site of catalyst is embedded in the carrier,and it is difficult to fully contact with the reactants.Moreover,mechanical agitation is not particularly efficient because of the high viscosity of IL,which is especially true for large-scale production.In order to solve these problems,herein,we have developed a novel method,which extends the concept of continuous-flow system with emulsion droplets in fixed bed to the IL/O Pickering emulsion system.“Immobilize” IL catalysts for continuous-flow reaction based on the confinement of an IL catalyst within other IL-based Pickering emulsion droplets.A new IL/OPickering emulsion continuous-flow fixed-bed reaction system was constructed.This system will not affect the structure of IL catalyst unit and facilitate the regulation of catalyst dosage.Futhermore,each [bmim]BF4droplet microreactor not only provides reaction medium but also serves as an ionic “sea” for hosting the other IL catalysts against loss through strong electrostatic interactions.So the efficient utilization of catalysts can be realized.The IL emulsion microreactor based on [bmim]Cl catalyst was filled into a fixed-bed to achieve efficient catalytic synthesis of cyanohydrin in the continuous-flow system.The catalysis efficiency was significantly enhanced compared to the batch reaction,the catalytic efficiency was improved by 2-3times and the conversion remained at 95% without obvious decay after 100 h of continuous reaction.The diffusion-reaction process of reactant molecules in droplets was predicted and the relationship between droplet size and catalytic efficiency was studied by theoretical calculation.The experiment further confirmed that the catalytic efficiency was the highest within the optimum size range of the droplets.Because of the single IL catalyst is difficult to obtain highly selective chiral cyanohydrins in the above reaction system,a one-pot cascade system in which different kinds of catalysts in concert was constructed based on the IL/O Pickering emulsion system.Cyanohydrin was synthesized by the interaction of bimetallic titanium complex catalyst and organic alkali.Then,(S)-cyanohydrin enantiomer was separated by CALB enzyme kinetics,and the chiral products were obtained by minor enantiomer cycling.Since various homogeneous catalysts inhibit activity with each other,in order to further improve chiral selectivity,multicompartalized reactors was constructed inspired by biological cells.The three-dimensional dendritic mesoporous Si O2 was used as a catalyst carrier(nanoreactor)assembled in the interior of the IL-based Pickering emulsion(micron reactor)to realize a cell-like system in which multiple catalysts were efficiently synergistic effect in a micro-nanomulti-space.Compared with the batch reaction system,the catalytic efficiency was increased by 12 times.