Design And Preparation Of Heterogeneous Silver Catalysts And Their Performance For Cyclization And Carboxylation With Carbon Dioxide

Carbon dioxide?CO₂?is one of the main greenhouse gases which can be utilized as an important C1 source in organic synthesis and fine chemical manufacturing due to its non-toxic,harmless and renewable characteristics.However,the thermodynamic stability and kinetic inertia of CO₂ limit its efficient conversion.Based on this,researchers have carried out various explorations which include homogeneous and heterogeneous catalytic systems on these issues.However,it is undeniable that the existing methods generally require longer reaction time,higher gas pressure,and have higher requirements on operation and reactor,which seriously limit their application value.Therefore,it is of great value to develop efficient and stable catalysts for CO₂ conversion and utilization to bring about green manufacturing of fine chemicals.In this regard,a series of novel supported Ag-based nanocatalysts was designed and prepared in this paper,which were then applied in the carboxylation of carbon dioxide to achieve green conversion under mild conditions.Firstly,porous organic polymer?POP?modified graphite carbonitride?g-C₃N₄?was prepared and utilized as a support.Ag nanoparticles were supported on this support by chemical reduction method and revealed high dispersion.A new type of Ag/POP@g-C₃N₄catalyst was then prepared and applied in the reaction of CO₂ with propynyl alcohol to synthesize alkylidene cyclic carbonates.It was found that grafting POP to g-C₃N₄ can significantly increase the specific surface area and nitrogen sites?N-H and C=N?,while providing rich anchoring sites.The anchoring sites could promote the anchoring and disperation of Ag nanoparticles which resulted in excellent catalytic activity and stability.CO₂ physical adsorption and ¹H NMR characterizations were used to explore the reaction mechanism,and a synergistic catalytic mechanism of the dual effects of Ag/POP@g-C₃N₄ on CO₂ adsorption and activation was proposed.Subsequently,SBA-15 molecular sieve with a large specific surface area and adjustable pore size was used as a carrier.To enrich its adsorption and anchor points,the surface and channel was modified by amino groups and constructed an encapsulated Ag nanoparticle catalyst,which can efficiently catalyze the reaction of CO₂ and propynol to synthesize alkylidene cyclic carbonates at room temperature and pressure.Studies have shown that rich pore channels and amino structures can effectively limit Ag nanoparticles into an extremely small size of 1.50 nm and promote their adsorption of CO₂,thereby improving their catalytic activity.Finally,a covalent triazine framework material?CTF?was prepared by Friedel-Crafts reaction using melamine and thiophene as raw materials.Nitrogen-sulfur doped carbon-nitrogen?NS-CN?was then prepared by high-temperature calcination the CTF.Ag nanoparticles were further anchored and confined into NS-CN by an in situ reduction method and then applied in the carboxylation of CO₂ with terminal alkyne.It was found that the calcination temperature has a great influence on the activity of the catalyst,and 600 ^oC is the best calcination temperature.In addition,the new CTF-derived NS-CN material not only retains the original structure,but also derives rich pore structure and surface groups,which can effectively improve the dispersibility and stability of Ag nanoparticles with the average particle size of only 2.56 nm.These advantages account for its excellent catalytic performance in the carboxylation of CO₂ and terminal alkynes.