Design And Preparation Of Porous Liquids And Their Applications In CO₂ Sorption

Porous materials are widely used in catalysis,petrochemical industry,optoelectronic materials,gas adsorption/desorption,energy storage,biomedicine and many other fields due to their developed pore structure and high specific surface area.Traditional porous materials generally exist in solid state,with relatively stable cavity structure,strong durability and easy storage,but it is difficult to be applied in industrial production because of their solid state.The traditional liquid solvent has fluidity,and the liquid circulation system is easier to install into the existing industrial equipment,which is convenient to realize the recycling of the system,but it lacks permanent pore structure.Therefore,how to design and prepare new type of materials with both porosity and fluidity,that is porous liquids（PLs）,is of great significance and far-reaching influence on technological progress and the improvement of industrial production efficiency.At present,several successful preparation cases of PLs have been reported by researchers,but the problems such as fewer types,complex synthesis routines and structural functionalization processes,effumability of organic solvents,high viscosity,precipitation after setting a long time and so on restrict further development and application of PLs.At the same time,the study of PLs is limited to gas adsorption capacity test,but the study of PLs regeneration performance,gas penetration experiment and gas adsorption kinetics is still lacking.Therefore,this paper attempts to design and develop new PLs with excellent performance and study the dispersion stability and fluidity mechanism of PLs from the microscopic level,what’s more,systematically study the carbon capture performance of PLs.The main research contents and conclusions of this paper are as follows:（1）A new type of silicon dioxide（SiO₂）based PLs was prepared by using the electrostatic interaction between SiO₂ nanoparticles and polymerized imidazole cations to make the surface of hollow SiO₂ particles with positive charge,and the outer surface was covered with sulfonated polyethylene glycol to obtain fluidity.Experimental results showed that the hollow SiO₂ nanoparticles of the PLs have uniform particle size,unoccupied micropores,the space steric hindrance of the outer crown makes the nanoparticles disperse uniformly and form homogeneous liquid.Compared with the existing SiO₂ based liquid materials,the PLs has small viscosity and good fluidity,which can be flowed and transported in the pipeline,providing a feasible scheme for the preparation of subsequent PLs.（2）A series of large-size porous/hollow SiO₂ based materials were designed and prepared based on the principle of covalent bonding between cores and crowns.The effects of core and canopy structures on the phase state,stablility and fluidity,and gas adsorption properties of porous materials were explored.Experimental results showed that when the pore size of the core was large,the canopies and solvent molecules were easy to enter the core,resulting in cavity blockage and damaging the dispersion stability of PLs.The higher the solid content was,the worse the fluidity of PLs.The larger the specific surface area of the core would provide more the carbon dioxide（CO₂）adsorption sites,therefore,the CO₂ adsorption amount was higher.At room temperature,the PLs can be shown as solid,gel and liquid in sequence with the increase of the polymerization degree of canopies.The thermal stability of PLs is improved compared with pure canopies.The lower canopy molecular weight,and higher branch will decrease the viscosity and increase the fluidity of PLs.（3）Taking the interface of large-size SiO₂ based PLs molecules as the research object,the effects of different grafting densities,pore size and canpopy structures on the stability and fluidity of PLs were studied by molecular dyamics simulation（MD simulation）.Simulation results showed that the canopies occupied the free space between SiO₂ nanoparticles.If the nanoparticles were too close,the space will be compressed,and canopies would produce steric hindrance effect to separate nanoparticles from each other,so as to maintain the dispersion stability of PLs;The higher the grafting density was,the better the dispersion property of PLs was.The steric hindrance effect and entropy effect made canopies arrange spontaneously and orderly,the degree of entanglement between molecules was reduced,and the fluidity of PLs should be improved.Compared with nanofluids,the canopy of PLs is difficult to enter the core cavity due to the entropy effect caused by sequence arrangement,which maintains the unoccupancy of PLs cavities.The core with small pore sizes will make canopies difficulty in entering the pores of cores maintain the porosity of PLs.Increasing the pore size of the core can make canopy atoms enter the core pores,and the atoms are closely attached to the inner cavity due to the effective interaction with the inner wall of the core cavity,which is closely attached to the cavity,resulting in poor fluidity of the PLs.Increasing the degree of canopies helps to reduce the melting point of PLs to below room temperature,while the branching structure has a negative impact on reducing the melting point of PLs.The canopies with higher degree of polymerization occupy more free space between interfaces of cores,which make the steric hindrance effect more obvious,and the the dispersion of PLs is better;Compared with linear chain,the distance between the core of PLs with branched chain is smaller,resulting in poor dispersion of PLs;Short and branched polymer chains are more likely to enter the core cavities in the cavity occupation stage,resulting in the core cavity occupied,or even blocked,affecting the dispersion stability of PLs.（4）Taking the SiO₂ based PLs systems with small particle sizes and reference system as the research object,the differences in the stability of flow between PLs and pure nanoparticles,nanofluid（NF）and porous organic-inorganic hybrid materials（NOHMs）were studied by MD simulation.Results show that the nanoparticles first move irregularly in water,then approach and adhere to each other,and finally reach a relative balance.The addition of water molecules can delay the aggregation of nanoparticles,but not enough to form stable nanofluids.The interaction energy between nanoparticles in the nanofluid is-632 k J/mol,showing mutual attraction.Once the aggregation of the pure nanoparticles or the nanoparticles in nanofluid occurs,they cannot disperse spontaneously.The diffusion coefficient of the water molecules near the surface of nanoparticles in nanofluids is close to zero.The further the distance from the surface of nanoparticles,the greater the diffusion coefficient of the water molecules,up to 2.43×10^-9 m²/s.It means that the approaching speed of nanoparticles is faster at the beginning of motion,and the approaching speed becomes slower with the decrease of the distance between nanoparticles.The compositions of PL,NF and NOHMs are similar,but the stability and fluidity of the resulting materials vary greatly due to the slightly different preparation process and solid content.The results of site distribution function（SDF）and radial distribution function（RDF）show that the coronas and canopies of PL and NOHM can be closely integrated around the core,which hinders the proximity of nanoparticles to form stable fluid materials.However,the coronas and canopies of NF are randomly distributed in the system.Because the non-bond interaction between nanoparticles is stronger than the non-bond interaction between nanoparticles and coronas,the nanoparticles near the nanoparticles will preferentially aggregate with each other during the movement.At this time,the NF cannot exist stably.RDF of all materials was characterized by short-range order and long-range disorder.Therefore,the materials are liquid at room temperature.The glass transition temperature of the material increases with the increase of solid content.The solid content of PL is higher than that of pure ionic liquid（IL）and lower than that of NOHM,and the glass transition temperature is higher than that of IL and lower than that of NOHM.The viscosity of IL,PL,NF and NOHM were 2.32m Pa·s,4.06m Pa·s,3.34m Pa·s and5.66m Pa·s,respectively.The canopy that provides mobility in NF is less bound by the nano-cores,and its viscosity is smaller than PL and NOHM.However,canopies of PL and NOHM are greatly bound by the nano-cores,and the free movement is constrained.Therefore,the two materials have larger viscosity.The viscosity of PL and NOHM is determined by the solid content.Therefore,PL has small viscosity and good fluidity.（5）Design and build a gas adsorption experimental system for SiO₂ based PLs.Through the measurement of gas pressure in the adsorption chamber,the CO₂ gas absorption variation of PLs with time was calculated and numerical fitted.The effects of core and canopy structures,ambient temperature and pressure on CO₂ gas absorption performance and recycling performance were studied.The experimental results showed that the CO₂ adsorption process of PLs not only existed physical adsorption,but also existed chemical adsorption.Higher temperature decresed the CO₂ solubility in the PLs and the lowered the saturated adsorption capacity;Higher pressure improved the physical dissolved saturated CO₂ adsorption capacity.The core occupancy of canipies has a great influence on CO₂ adsorption performance.Reducing the branching structure of canopies,increasing the polymerization degree of canopies and reducing the pore size of PLs can prevent the canopies from entering the core pores,prevent occupying the CO₂ adsorption site,and effectively improve the saturated CO₂ adsorption capacity.After 10 adsorption and desorption cycles,all the prepared PLs still maintain more than 90%of the original CO₂ adsorption capacity,which displayed a good cycle performance.In this paper,different types of PLs were designed and prepared and the effects of different factors on their stablility,fluidity and gas adsorption performance were also studied through the combination with experiments and simulations,aiming to provide theoretical guidance for the designing and preparation of PLs,transportation,storage and gas adsorption.