Construction Of Highly Efficient Friedel-Crafts Acylation Catalysts Based On Core-shell Carbon-coated Iron Nanoparticles

Aromatic ketones are kinds of important fine chemical intermediates that have been broadly used in polyphenylene sulfite,pharmaceutical,dye,spices,agricultural and other fields.Friedel-Crafts acylation reaction is a classical chemical reaction for producing fundamental intermediates.Conventionally,the homogeneous Lewis acid catalysts such as AlCl₃ and FeCl₃ have widely been used in the process of industrial production.However,the use of these homogeneous catalysts often suffers from difficulty in product separation,high cost of wastewater treatment,high environmental pressure and so on.For overcoming these drawbacks,considerable efforts have been focused on developing efficient heterogeneous catalysts such as zeolites,metal oxides,heteropoly acids and metal-organic frameworks（MOFs）.However,these solid acid catalysts commonly suffer from either easy leaching of the active species or rapid deactivation during reaction term,thus bringing serious limitation for practical application.Hence,the search for designing eco-friendly and easily recyclable and efficient solid acid catalysts for various types of Friedel-Crafts acylation reactions is still of great scientific significance and value for practical application.In recent years,with the continuous development and progress of material science and catalyst preparation chemistry,a variety of new-type materials of catalyst and technologies for the preparation of highly efficient catalysts have been emerging.Among them,high-temperature pyrolysis of MOFs precursors has received much attention since it may generate catalysts of nanostructure materials composed of a carbon matrix and metal nanoparticles.The highly ordered metal ions in MOFs are separated well by organic ligands,which is beneficial to solving the problem of particles agglomeration caused by metal sintering during the pyrolysis process,thus leading to the formation of carbon supported metal（oxide）catalysts with high dispersion and stability.Based on the above results,the research for the preparation and catalytic performance of the catalysts of a series of carbon-based iron nanoparticles were studied in this work,which is focusing on Friedel-Crafts acylation reactions of aromatics with acyl chloride.By choosing different types of Fe-based MOFs as precursors,several kinds of catalysts with the structure of carbon wrapped iron-based nanoparticles were obtained after high-temperature pyrolysis of the MOFs precursors in inert gas,and the resultant catalysts showed excellent catalytic performance as well as easily recyclability for Friedel-Crafts acylation.By combining with various characterization results,the relationship between the composition,structure of the catalysts and their catalytic performance was systematically studied,and the nature of the active sites in the catalysts and the catalytic mechanism of the acylation reaction were also discussed.The main research contents and achievements of this thesis are as follows:1.Fe-containing MOF of Fe-DABCO-TPA was first synthesized through a solvothermal method,in which DABCO and TPA represented 1,4-diazabicyclo[2.2.2]octane,and terephthalic acid,respectively,and then was used as precursor for preparing a series of carbon based Fe nanoparticles catalysts Fe@NC-T（T represents the pyrolysis temperature）.Various characterization results showed that a large number of Fe nanoparticles coated by a few graphitic layers（forming core-shell structure）are uniformed distributed in the architecture of carbon in the catalysts,while the presence of N species in the ligand of DABCO is beneficial for achieving the high dispersion of Fe-based nanoparticles.The catalytic performance of the Fe@NC-T catalysts was investigated for the Friedel-Crafts acylation reaction of m-xylene with benzoyl chloride.The results indicated that the pyrolysis temperature of the catalysts could efficiently influence the catalytic activity and stability of the catalysts.Among them,Fe@NC-800,prepared by high temperature pyrolysis at 800 ^oC,showed high catalytic activity and stability,and the catalyst can be easily recycled for several times without any high temperature treatment.A small amount of oxidized iron(Fe³⁺)species existed on the surface of the Fe nanoparticles,which are encapsulated within the carbon shell,bring a certain amount of Lewis acid centers for the acylation reaction.In addition,the HCl generated during the reaction may interact with the Fe species inside the carbon shell,which can produce more Lewis acid centers of Fe³⁺-Cl_x,thus further enhancing the catalytic activity of the recycled catalyst.The coated structure of carbon matrix in Fe@NC-800 provided effective protection for the Fe-based nanoparticles,thus avoiding the leaching of Fe species from the catalyst and leading to the formation of highly stable carbon encapsulated Fe nanoparticle catalysts for acylation reaiton.2.A series of carbon-based Fe nanoparticles catalysts Fe_xC/NC-n were prepared from the bimetallic organic framework of BMOFs-ZnFe_n through high temperature pyrolysis（n represents the Fe/Zn molar ratio）.The results indicated that the iron carbide（Fe_xC）nanoparticles coated by graphitic carbon shell are more uniformly dispersed in the carbon material;During the pyrolysis process for preparing the catalysts,the majority of Zn species in the BMOF precursors could be reduced to metallic Zn,and then sublimated during the high temperature treating process,which can inhibit the aggregation of Fe_xC nanoparticles to a certain extent,and promote the formation of mesopores and the increase of the specific surface area of the catalysts.The Fe_xC particle size,distribution and the thickness of the carbon-coated graphite layers could be adjusted within a certain range by changing the ratio of Fe/Zn.It was found the catalyst of Fe_xC/NC-0.05,prepared under optimized conditions,showed very high catalytic activity and stability for the Friedel-Crafts acylation reaction of aromatics with acyl chloride.Compared with other catalysts,Fe_xC/NC-0.5 catalyst have a smaller particle size（～25 nm）of Fe_xC nanoparticles,and the thickness of the carbon shell on the external surface of the Fe-based nanoparticles is also moderate（～4 nm）,in which can improve the structural stability of Fe_xC nanoparticles,meanwhile,can also effectively adjust the ability of the catalysts to adsorb and diffuse reactant/product molecules,thus ensuring the effective transmission of electrons/charges to a certain extent for achieving high catalytic performance in acylation reaction.3.A series of carbon wrapped Fe Ni bimetallic nanoparticle catalysts of Fe_xNi_1-x@NC were prepared by high-temperature pyrolysis of the precursors of bimetallic organic frameworks of BMOFs-Fe_xNi_1-x.Compared with Fe@NC catalyst,Fe_0.8Ni_0.2@NC with optimized composition ratio showed higher catalytic activity in Friedel-Crafts acylation reaction of aromatics with acyl chloride.The introduction of an appropriate amount of Ni species into the precursor of BMOFs-Fe_xNi_1-x could inhibit the aggregation of Fe species to a certain extent,and produce carbon coated Fe Ni alloy nanoparticles with smaller particle size and uniform distribution.According to the theoretical calculation results,it was known that a small amount of metal ions present inside the carbon shell could cause weak electron transfer from the outer surface of the graphitic carbon layers to the inner surface,thus bringing some relatively weak positive charge on the outer surface of the carbon shell,which could act as the initial catalytically active sites for the Friedel-Crafts acylation reaction.With the proceeding of the acylation the reaction,the generated Cl^-can diffuse into the inside of the carbon shell through the ion channels generated by the carbon defect structure,and leading to the formation of Fe³⁺-Cl_xspecies.This change could significantly increase the positive charge of the outer surface of the carbon shell,which in turn improve the catalytic activity of the carbon shell encapsulated Fe-based nanoparticle catalysts.In addition,the resulting aromatic ketone products cannot pass through the ion channel in the carbon shell due to its large size,thus effectively avoiding the catalyst deactivation caused by the direct contact between the aromatic ketone and the metal ion（forming stable complexes）.As a result,the core-shell carbon wrapped Fe-based nanoparticle catalysts have excellent stability and recyclability for the Friedel-Crafts acylation reaction.