Study On The Preparation Of Homogeneous Or Heterogeneous Co-salen Catalysts And Its Catalytic Performance In The Oxidative Carbonylation Of Amines

Co-salen complexes are widely used as catalysts in the fields of oxidation and asymmetric synthesis,which have the advantadges of powerful oxygen complexation and simple synthesis.There are few reports about their use in oxidative carbonylation of aniline, and even if most are used as homogeneous catalysts in the reaction.We successfully synthesized a series of Co-salen complexes with different substituents and applied them as homogeneous catalysts in oxidative carbonylation of aniline.In the further studies,we prepared supported Co-salen catalysts by different methods in order to solve the problems of separation catalyst from products,to maintain their activity and increase their stability and reusability.The activities of heterogeneous Co-salen catalysts are tested by oxidative carbonylation of aniline to methyl N-phenyl carbamate(MPC).Carbamates are mainly used in the fields of polymers as intermediate of isocyantes,pesticides and synthesis of drugs.Their traditional manufacture is "phosgene process" in which toxic and pollutive phosgene used as raw material,and recently,carbonylation using CO offers a new green process for the manufacturation of MPC.CO is a reagent of low toxicity,low cost and accessibility,despite acting as reductant in reactions.In recent years,application of CO as a carbonylating reagent to synthesize many kinds of carbonyl compounds in the field of organic synthesis has been attracted more and more attention.All kinds of CO-induced carbonylation need catalyst,and a great amount of research work has been focused on the development of catalytic system of high catalytic activity,recovery and environmentally benignity.In the green process for carbamates via carbonylation,the widely reported catalysts include groupâ…§metal salts,complexes and some S,Se,Te compounds,for the latter,the inherit toxicity of catalysts,inseparable from product,limit its further application in industry.Firstly in this paper,we studied the oxidative carbonylation of aniline to N, N'-diphenyl urea without presence of alcohols,and proposed novel non- corrosive and effective Co(â…¡)salen in combined with pyridine base ligands catalytic system.Using salicylaldehyde derivatives with different substituents and various diamine as starting materials,we synthesized six salen ligands and corresponding Co(â…¡)salen complexes,the catalytic activity of various Co(â…¡)salen/pyridine catalytic system was investigated,the effect of substituent of salen,promoter,mole ratio of Co(â…¡)salen to promoter,catalyst loading,temperature,pressure and partial pressure ratio of feed gas CO to O₂ on the catalytic activity of catalyst were also studied.The best results emerged from Co(â…¡)salophen(OH)₂/4-methyl pyridine composite catalytic system,the conversion of aniline is 85.3%and the separate yield of diphenyl urea is 79.2.On the basis of the experimental results,a plausible mechanism was proposed based on the formation of carbamyl compound.Although homogeneous Co(â…¡)salen catalysts have comparable activity to noble base catalysts and cheaper than them,the disadvantages companied with the homogeneous catalysts such as separation,requirement of large amount of organic solvent and recovery are also serious.The efficient route to solve the problems is the heterogenization of homogeneous catalyst.Firstly we use a "ship in bottle" method to heterogenize six Co(â…¡)salen complexes.They are well characterized by FT-IR,UV-Vis,BET,XRD,XPS and thermal analysis(TA),the content and leaching of Co are determined by AAS.The immobilized catalysts and their homogeneous analogues are evaluated by oxidative carbonylation of aniline to methyl N-phenyl carbamate(MPC).The immobilized catalysts have superior activity to their homogeneous analogues.For example,when the reaction was performed over homogeneous Co(â…¡)- salophen -(OH)₂,the conversion of aniline is 51.6%and the yield of MPC is 38.8%,when it was encapsulated in Y zeolite,the conversion of aniline is promoted to 63.2%and the yield of MPC is raised to 45.4%.The XPS and AAS results demonstrated that the encapsulated catalysts have perfect stability and reusability,after 5 cycles,their activities keeps unchanged.Using muti-step procedure heterogenizing homogeneous complex catalysts on silica matrix is a widely applied strategy and well reported in a great amount of papers.This strategy involves the standard four steps,that is,functionalization of ligand with reactive group,reaction with silane coupling agent,complexation with metal ion and hydration and co-condensation with organic silica source.To the best of our knowledge,the application of immobilized Co(â…¡)salen catalyst on silica via covalent bond in the oxidative carbonylation of aniline to MPC has not been reported.We choose two salen ligands with different diamine bridge,functionalizing them with chloromethyl group,then reacting with 3-aminopropyl triethoxysilane to form silylating salen ligand.The silylated ligand coordinated with Co²⁺ ion,followed by hydration and co-condensation with TEOS in some ratio to prepare two hybrid catalysts.The two hybrid materials are characterized by FT-IR, UV-Vis,TA and XPS,and the possible structural formula is determined.The two supported catalysts also show good activity in the oxidative carbonylation of aniline to MPC.Under the experimental conditions,the conversion of aniline and selectivity of MPC are 62.6% and 87.2%for Co(â…¡)salen-NH-SiO₂,63.8%and 88.6%for Co(â…¡)salophen-NH-SiO₂.The supported catalysts are recycled for five times without the loss of activity,and no Co⁺⁺ leaching is detected by AAS.In the further research work,considering the mixed TiO₂-SiO₂ oxide with low content of Ti has selective oxidation activity for some reactions,we conclude that the introduction of low content TiO₂ in silica matrix is favored by partial oxidation of reactive substrates. We prepared supported Co(â…¡)salen on SiO₂ or TiO₂-SiO₂ modified with low content of Ti, their activity were tested and compared by oxidative carbonylation of aniline to MPC.The hybrid materials,Co(salen)-C=N-TS(x),the letter x in bracket represents the Ti/Si ratio in preparation,were characterized by FT-IR,UV-Vis,XRD,XPS,TEM and TG,the IR spectra demonstrated the existence of Ti-O-Si hetero- linkages.Using pure TiO₂-SiO₂ as catalyst to catalyze oxidative carbonylation of aniline,we found that the acidic TiO₂-SiO₂ support showed some activity with 31.2%of the conversion of aniline and 10.9%of the yield of MPC.The activity of Co(â…¡)salen supported on TiO₂-SiO₂ is higher than that supported on SiO₂.Co(â…¡)salophen-C=N-TS(0.1) showed the best activity with 60.7%of the conversion of aniline,53.5%of the yield of MPC and 88.1%of the selectivity of MPC.The AAS results also confirm that the Co content of recovered catalyst is same as the fresh catalyst, and no Co⁺⁺ leaching in the reacted mixture is detected.The investigation into the mechanism of oxidative carbonylation catalyzed by Co(â…¡)salen/KI catalytic system is also conducted,and a plausible mechanism based on carbamyt intermediate is proposed.To study the effect of supports,organic crosslinked polystyrene,hexagonal mesoporous molecular sieve MCM-41,and amorphous SiO₂ were chosen to anchor the same Co(â…¡)salen complex via covalent bond.The catalytic activities of these heterogeneous catalysts were compared by oxidative carbonylation of aniline.According to the catalytic results,the Co(â…¡)salen supported on MCM-41 shows the lowest activity,and the activity of the Co(â…¡)salen complex supported on polystyrene shows a sharp decrease when the catalyst is reused for the second time,the AAS results demonstrates the serious leaching of Co⁺⁺ into solution.Comparing the two support,the Co(â…¡)salen complex supported on amorphous SiO₂ via sol-gel process shows good activity,and the activity can be perfectly maitained when they are reused for several times.These facts indicate that activity of the complex has some relationship to the regularity of the support,and the mesoporous molecular sieve with uniform pores distribution and partial crystallinity is not a suitable support for Co(â…¡)salen in oxidative carbonylation of aniline.