Molecular Dynamics Simulation Study On The Association Of FLPs In ILs In The CO₂+H₂ Reaction

A large number of greenhouse gases in the atmosphere,especially carbon dioxide emitted from the burning of fossil fuels,have caused serious global warming problems.Therefore,reducing carbon dioxide in the atmosphere is crucial.The catalytic conversion of carbon dioxide into methanol can not only alleviate the environmental crisis caused by climate waring,but also solve the energy crisis caused by the shortage of fossil energy.Frustrated Lewis pairs（FLPs）have the ability to activate small molecules（H₂）,so it has become a research hotspot in non-metal catalyzed CO₂hydrogenation reactions.Ionic liquids（ILs）solvents can improve the effective configuration of FLPs,thereby enhancing there catalytic activity.At the same time,ILs have important applications in carbon capture.Therefore,it is very important to study the mechanism of the interaction between ILs and FLPs.However,there are few studies on the interaction mechanism between FLPs and ILs in the FLPs-ILs system,and the precise mechanism of action is still ambiguous.Therefore,this paper selects a typical FLPs model（tBu）₃P/B（C₆F₅）₃and 5 ILs solvents for research.Molecular dynamics simulations of this series of FLPs-ILs systems are carried out to study the interaction mechanism of these 5 ILs solvents and traditional toluene solvents with（tBu）₃P/B（C₆F₅）₃,and reveal the influences of ILs solvents on the structure of FLPs.The specific content is as follows:（1）Molecular dynamics simulation of the mixed system of[C_nmim][NTf₂]（n=6,10 and14）ILs with different cations and（tBu）₃P/B（C₆F₅）₃was carried out to study the mechanism of interaction between ILs and（tBu）₃P/B（C₆F₅）₃and the effects of ILs with different cations on the association state of（tBu）₃P/B（C₆F₅）₃.ILs with three different cations all tended to enhance the formation of（tBu）₃P/B（C₆F₅）₃.Compared with traditional toluene solvents,due to the larger molecular structure of anions and cations in ILs,voids are formed between the ionic liquid molecules.The voids in ILs are then occupied by Lewis acid（tBu）₃P molecules,resulting in the association of Lewis acid（tBu）₃P and Lewis base B（C₆F₅）₃in（tBu）₃P/B（C₆F₅）₃.Associated（tBu）₃P/B（C₆F₅）₃shows the typical characteristics of FLPs,that is,the center B-P of Lewis acid and base face each other and the distance is less than 5?.This typical FLPs configuration is conducive to the activation of CO₂and H₂.In addition,due to the large cation-anion interaction value in ILs,ILs solvents tend to associate（tBu）₃P and B（C₆F₅）₃molecules into a compact pair.Finally,the longer the alkyl chain length of the[C_nmim]⁺（n=6,10 and 14）cations,the more effective（tBu）₃P/B（C₆F₅）₃ratio.（2）Molecular dynamics simulation of the mixed system of[C₆mim][xxx]（xxx=NTf₂,PF₆and CTf₃）ILs with different anions and（tBu）₃P/B（C₆F₅）₃was carried out to study the mechanism of interaction between ILs and（tBu）₃P/B（C₆F₅）₃and the effects of ILs with different cations on the association state of（tBu）₃P/B（C₆F₅）₃.[C₆mim][NTf₂]and[C₆mim][PF₆]tend to strengthen the formation of（tBu）₃P/B（C₆F₅）₃,while[C₆mim][CTf₃]will weaken the formation of（tBu）₃P/B（C₆F₅）₃.For[NTf₂]^-and[PF₆]^-anionic ionic liquids,the content of effective Lewis acid-base pairs in ILs is higher than that of traditional toluene solvents.However,in[C₆mim][CTf₃],due to the large number of active O atoms in[CTf₃]^-anion and the loose accumulation of other particles around the B（C₆F₅）₃molecule,[CTf₃]^-anion will competes with P atom of（tBu）₃P/B（C₆F₅）₃and associates with the B atom of（tBu）₃P/B（C₆F₅）₃,which weakens the association between the P atom and the B atom,resulting in the weakening of the formation of（tBu）₃P/B（C₆F₅）₃.[CTf₃]^-anions have a very large negative impact on the（tBu）₃P/B（C₆F₅）₃association,and even make the effective structure ratio lower than traditional solvents（1.08%<2.32%）.