Construction Of Pickering Droplet-based Fixed-Bed Reaction System For Continuous-flow Catalysis

Biphasic catalytic systems,such as organic/water and ganic/ionic liquids are of an important class of green chemistry synthesis platforms that are extensively used in laboratory synthesis and in industrial fabrication.However,due to the"immiscibility"of two phases,the overall tendency of the two liquids is to reduce their interface,resulting in inefficient contact of the catalyst with the reactants.In order to improve the reaction efficiency of the biphasic reaction system,a phase transfer catalyst has to be added to provide a"carrier"for the transfer of substances,or a surfactant is added to form micelles or emulsions to increase the reaction interface.Unfortunately,the introduction of these additional additives presents some more difficulties for product separation and purification.In industry,biphasic reactions typically require external mechanical agitation to obtain high reaction efficiency.However,the separation of the two phases is also need to recover the product when the reaction is completed,which has to be implemented in batch reactors and seriously restricts their large-scale application.Nonetheless,the breakthroughs up to date in this context are only achieved in continuous stirred tank reacors,supported liquid technonlogy,and microfluidic technology.Dispite significant advances,these methods still rely on external or special equipment or meterials in achieving a continuous flow manner.In this regard,they are still not suitable for large-scale industrial applications.To overcome the problems,an interface-active SiO₂ nanoparticle was used to stabilize droplets and then filled them into a packed-bed reactor to construct a droplet-based continuous flow packed-bed system.Key fundamental principles underpinning this method such as the oil phase flow behavior,the stability of compartmentalized droplets and the confinement capability of these droplets toward catalysts are experimentally and theoretically investigated.Due to the high stability,such droplets can be packed in a column reactor like particulate catalysts in conventional packed-bed reactors.The catalysts such as enzyme,acid and base are thus“immobilized”in the column reactor,while the interstices among the droplets allow the oil phase and substrates dissolved in it to flow down.During passage through the column reactor the reactants contact the catalyst at droplet interfaces,where catalytic reactions occur.Case studies including an enzymatic chiral reactions,a sulfuric acid-catalyzed addition reaction and a heteropolyacidcatalyzed ring opening reaction demonstrate the generality and versatility of this method.Impressively,the droplet-based reaction system exhibit not only excellent durability even over a span as long as 2000 h,but also exhibit up to10-fold reaction efficiency in comparison to their batch counterparts.On the basis,the relationship between catalytic efficiency and properties of the droplets interface were also revealed.In above system,catalysis reactions occur only at the interfaces of water droplets in oil rather than within the droplets,and the catalysts inside droplets are not accessible to reactant molecules because organic reactants are mostly insoluble in water.Herein,to continuously process enzymatic or homogeneous catalysis reactions more efficiently,a new droplet-based continuous flow packed bed reaction system was eatablished by using organic/ionic liquid?IL?biphasic system.The reasion is that the ionic liquid can dissolve organic compounds and the reactant molecules can effectively diffuse into the droplet and contact with the catalyst,which allows reactions to take place within the droplets.Similarly,this system adoptes ionic liquid?IL?droplets in an immiscible oil as building blocks to fill into a column reactor.Case studies including enzymatic chiral reactions,CuI-catalyzed cycloaddition demonstrate its generality and versatility.Impressively,the IL droplet-based reaction system exhibit not only excellent durability even over a span as long as 4000 h,but also exhibit up to 20-fold reaction efficiency in comparison to their batch counterparts.Furthermore,it was found that 77%of the initial specific activity of enzyme was still maintained.Morover,the theoretical investigation reveales that there exists a concentration gradient within the droplet for both reactant and product,which resulted the diffusion of the reactants into the droplets and the constant removal of products from droplets,thus mitigating the product inhibition effect,and the catalysis efficiency is also significantly boosted.The theotetical model not only allows to predict the the relationship between reaction efficiency and droplet size,but also suppoted by the experimental results.To further verify the superiority of the continuous flow packed-bed reaction system,a mathematical model based on catalytic efficiency of the batch reaction and the droplet-based packed-bed reaction system was also established.Furthermore,the calculations reivels that the droplet-based packed bed system exhibit several order of magnitude enhancement in catalysis efficiency than their batch counterparts.For a given model reaction,for example,the CuI-catalyzed cycloaddition reaction is still researched here,not only suooprting the theoretical results,but also proving that the droplet-bed packed bed reaction system can significtly improve the catalytic efficiency.