Preparation Of Melamine-based Organic Porous Materials And Their Catalytic SO₂ Copolymerization

The sulfur-containing polymer as a highly valuable product has been used in variety of fields including photoresists,urea fertilizer,proton exchange membrane fuel cells,and thermosetting adhesives,it is an advanced strategy for the copolymerization of SO₂ with epoxide into the sulfur-containing polymer.At present,a variety of catalysts involving organometallic complexes and Lewis acid/base catalysts have been developed due to catalysts play an indispensable role,however,these catalyst systems have the problems of low selectivity and high cost of available catalysts.Therefore,the exploration of green nonmetallic catalysts with high activity and selectivity and recyclability is the key to solve the harsh conditions of copolymerization of SO₂ and epoxide.The abundant pore structure of porous organic materials can increase the number of active sites in the material structure,the pores of the material make it easy for gas in contact with active site,and the epoxide is activated by the hydrogen bond donor at the same time.Notably,Schiff-base network（SNW）materials of porous organic materials have various synthesis methods,and contain a large number of triazine rings and nitrogen-rich structural units.The active sites can be highly dispersed in the frame structure,which promotes substrates to combine with the catalytic active sites,the mass transfer of substrates,and theπ-πstack between the benzene ring and the triazine ring is more favorable for charge transfer.SNW materials can well make up for the shortcomings of traditional catalysts such as difficult recovery and harmful to the environment.Therefore,based on the concept of resource utilization of SO₂,two new and efficient SNW catalysts were synthesized and designed in this paper to study their catalytic performance in the copolymerization of SO₂,and the possible catalytic mechanism was proposed.The main research contents are as follows:（1）A series of SNW porous organic materials（SNW-1,SNW-1-30,SNW-1-45）were prepared by using melamine and terephthalaldehyde based on the Schiff-base reaction.The copolymerization of SO₂ with cyclohexene oxide was firstly catalyzed by SNW catalyst,and it was found that SNW-1 possessed the best catalytic performance.The performance of SNW-1 under different conditions was investigated.The results exhibit that under the optimal conditions,the unit content of sulfite and the selectivity of poly（cyclohexene sulfite）（PCS）reach 93%and 88%,respectively,and conversion of cyclohexene oxide（CHO）is 98%.Moreover,the SNW-1 can be reused more than 6times without obvious decrease of the catalytic activity,and it presents a wide applicability to various epoxides.（2）In order to improve the hydrogen bond donor and selectivity of the catalyst,a new organic porous material SNW-KUST1 was synthesized by Schiff-base reaction with terephthalaldehyde which modified by hydroxyl group and melamine.Under the optimal conditions of copolymerization,the catalytic performance of SNW-1 and SNW-KUST1 was compared.The results show that the SNW-KUST1 exhibits the excellent catalytic performance for the copolymerization of SO₂ and CHO with a record PCS selectivity of 93%,the unit content of sulfite of 95%,and conversion of CHO of98%.Moreover,the SNW-KUST1 can be reused more than 6 times without obvious decrease of the catalytic activity,and presents a wide applicability to various epoxides.（3）Flynn-Wall-Ozawa,Kissinger and Coats-Redfern methods were used to calculate the thermal degradation kinetics of the PCS.The most probable mechanism of thermal degradation kinetics of PCS prepared by SNW-1 catalyzed copolymerization of SO₂ is consistent with the F₃ mechanism of random nucleation with three nuclei on a single particle,the integration mode is 1/（1-α）²,activation energy is 101.02 k J mol^-1.The most probable mechanism of thermal degradation kinetics of PCS prepared by SNW-KUST1 catalyzed copolymerization of SO₂ is consistent with the D₃ three-dimensional diffusion mechanism,the integration mode is[1-（1-α）^1/3]²,activation energy is 83.72 k J mol^-1.Based on the research results and thermal degradation kinetics analysis of PCS,a possible mechanism for the SO₂ copolymerization over SNW catalysts is proposed.