Design And Catalytic Mechanism Of Metal-free Catalyst For The Cycloaddition Of CO₂ With Epoxide

In view of the environmental issues caused by CO₂,the capture and chemical utilization of CO₂ are indeed the effective ways to pursue a sustainable development and meet growing energy demands.The cycloaddition of CO₂ with epoxide to produce cyclic carbonate is 100%atom-economic.Due to the high polarity and bioactivities,the cyclic carbonate has been widely employed in electrolytes for lithium batteries,fuel additives,polymer synthesis and intermediates for pharmaceuticals/fine chemicals.However,due to the inherent thermodynamic stability and kinetic inertia of CO₂,effective catalysts have to be introduced to reduce energy barriers for industrial production of cyclic carbonates.Although metal based catalysts have achieved favorable catalytic effects,metal-free catalysts have been most intensively researched in this field for the consideration of green chemical production.In this paper,a series of metal-free catalysts for the preparation of cyclic carbonates were designed based on the viewpoint that functional groups such as hydrogen bond donors（acidic sites）and unsaturated amine species（basic sites）synergistically catalyze CO₂ cycloaddition.The influence rule of these catalysts on the product yield was systematically explored,and some metal-free catalysts without halides and solvents and with excellent activity under mild reaction conditions were screened out.The main research contents are as follows:1.Three metal-free Schiff base homogeneous catalysts containing hydrogen bond donor（-OH）and nitrogen base sites were synthesized,and their activities for the cycloaddition of CO₂ with epoxides were tested under the mild conditions of halogen-and solvent-free.The Schiff base-enol（Acen-H）homogeneous catalyst synthesized with 2,4-pentanedione as a raw material has remarkable activity.The effects of the compositions and chemical structures of three Schiff base homogeneous catalysts on their catalytic ability were systematically studied by characterization methods,and the excellent universality of Acen-H was further investigated under the optimized reaction parameters（yield～98.5%）.The proposed mechanism of Acen-H catalyzed cycloaddition was strongly supported by kinetics and density functional theory（DFT）.It was confirmed that the high activity of Acen-H from the active hydrogen bond donors（-O-H,=N...H）in its unstable enols and basic imines（-N=）had a synergistic effect on the cycloaddition.2.In order to solve the difficult separation of homogeneous Acen-H and improve its adsorption and activation capacity for CO₂,tow novel triazinyl-enol polymer（TEPNs）heterogeneous catalysts were prepared by selecting 2,4-pentanedione and N-rich melamine and used in the cycloaddition of CO₂ and epoxides.The physicochemical properties of TEPNs were analyzed in detail using various characterization methods to explore the structure of TEPNs and the key factors determining the catalytic activity.As a metal-free heterogeneous catalyst,TEPNs-M1P3 exhibited a positive cycloaddition effect in the absence of halides and solvents,with favorable recyclability.In particular,the experimental design（response surface methodology）was used to optimize the reaction conditions and build a prediction model for propylene carbonate yield and the practical effectiveness of the model in predicting the response was verified.Based on the experimental results,the mechanism of synergistic enhancement and cycle decay between the functional active sites of TEPNs-M1P3 was revealed.TEPNs-M1P3overcomed the difficult separation of homogeneous catalysts,but its activity was much weaker than that of Acen-H.3.In order to improve the activity of TEPNs-M1P3,four environmentally-friendly bio-based triazine polyamide（BTP-MF）heterogeneous catalysts were prepared by"one-pot"method using 2,5-furandicarboxylic acid with stronger hydrogen bond donor capacity（Br?nsted acidity）and N-rich melamine.BTP-MF-Me OH,a novel non-metallic catalyst synthesized at room temperature,exhibited excellent and broad cycloaddition ability（yield～98.8%）under mild conditions without halides and solvents.The chemical structures and properties of the catalysts prepared by different solvents were comprehensively evaluated by standard analytical techniques.The kinetics and cycle stability of BTP-MF-Me OH were investigated,and the proposed cycloaddition mechanism was verified based on experiments and density functional theory（DFT）calculations.The results showed that the excellent catalytic performance of BTP-MF-Me OH was attributed to the synergistic interaction among hydrogen bond donors（-N-H,C=O…H or=N…H）,imino groups（-N=）of triazine-ring,and hydrazine group（N-H）with edge defects.4.In order to verify the assumption that increasing hydrogen bond donor types and unsaturated N species can synergistically improve the catalytic activity,three novel triazinyl polyamides（PMP-TDNs）were synthesized by"one-pot"method using 4,5-imidazoledicarboxylic acid and 2,5-thiophenedicarboxylic acid with N-rich triazine derivatives,respectively.In addition,PMP-TDNs,as metal-free heterogeneous catalysts were used for the cycloaddition of CO₂ with epoxides.The composition and physicochemical properties of PMP-TDNs were evaluated via different analytical methods to explore the crucial factors affecting catalytic activity.Especially,triazinyl-imidazole polyamide（PMP-TDNs-MI）with multiple hydrogen bond donors and high-density N basic sites showed remarkable activity and excellent universality（yield～99.9%）,without halides and solvents.The kinetics,thermodynamics and reusability of PMP-TDN_S-MI were investigated,and the synergistic catalytic mechanism between the multifunctional active sites of PMP-TDNs-MI and the main reasons for its remarkable catalytic activity were revealed based on the experimental results.Generally speaking,the activity of PMP-TDNs-MI was better than that of other heterogeneous catalysts developed in this paper,and was equivalent to that of homogeneous Acen-H.The key to the high activity of the catalyst was the synergistic enhancement effect between multifunctional active groups such as multiple types of strong hydrogen bond donors and unsaturated N species.