Precise Synthesis Of Nano-porous Carbons For Gas Separation

Mixing of gases is a thermodynamically spontaneous process.Obviously,gases seperation,as a reverse process of mixing gases,consumes energy.Therefore,an efficient separation method to minimize the energy cost of gas seperation has been the hot topic or extensively researched.Compared to cryogenic separation,chemical absorption,membrane separation and so on,adsorption technology works well in collection of trace amount of guest components based on the van der Waals interaction between adsorbents and adsorbates if selective separation materials can be developed.Adsorbents namely porous materials are the cores for efficient separation in a dynamic flow situation,which shall simultaneously integrate multiple functional criteria such as high adsorption capacity,good selectivity,fast sorption kinetics rates and long term stability.Considering the characteristics of porous carbons such as developed porosity,good stability,and excellent moisture-resistant ability,herein,we have designed and synthesized a series of porous carbon adsorbents with tailored pore architecture and special morphology through solution synthesis strategy.Furthermore,we have fundamentally investigated the adsorption/separation behavior,and their relations with the pore structure and morphology features of sorbents.Specifically,this thesis includes the following parts:(1)Inspired by the alveolar structure,we proposed a surface free energy-induced assembly approach for synthesis of multi-cavity carbon spheres(MCC).The crucial point for this assembly approach is first employment of small-sized nanoemulsions with high homogeneity as primary building block.Spontaneous aggregation and assembly of sub-structural units are processing in following hydrothermal synthesis induced by reduction of surface free energy of system.As a result,multi-cavity structure is formed,in which the size and number of cavity can be modulated by changing size of nanoemulsion and concentration of polymer.Confined nanospace pyrolysis enables to further enlarge cavity size compared to regular pyrolysis.Further K2CO3 activation of MCC can significantly improve the surface area and pore volumes.The activated MCC shows outstanding CO2 sorption and separation capacities,high selectivity.At 1 bar and 298 K,the equilibrium capacity of a representative sample is 4.5 mmol g-1;the separation factor of CO2/N2 is 57.(2)In order to enhance the adsorption kinetics as well as the utilization rate of microporosity,extra-thin carbon nanoplates(TCP)were prepared,which relies on an ingenious application of the thermoregulated phase transition of stearic acid.The stearic acid was melted and evenly dispersed into surfactant F127 aqueous solution to form a uniform emulsion.A subsequent cooling process caused the stearic acid to undergo a phase transition from liquid to solid followed by Ostwald ripening.As a result,a suspension of ultrathin stearic acid sheets was formed.Subsequently,the stearic acid sheets with an abundance of carboxylic acid groups guide the interface-assembly and polymerization of resorcinol-formaldehyde-propylamine via hydrogen bonding.Following pyrolysis,the free-standing carbon nanoplates were obtained,which still can overcome the surface tension by retaining the flat morphology.Further K2CO3 activation of such TCP can significantly improve its surface area and pore volumes,while the pore distribution is unchanged.The activated TCP shows outstanding CO2 sorption and separation capacities,high selectivity.At 1 bar and 298 K,the equilibrium capacity of the sample is 5.0 mmol g-1;the separation factor of CO2/N2 is 161.More importantly,the high equilibrium uptakes can be achieved in dynamic flow situations,highlighting its high selectivity and good adsorption kinetics.(3)To implement the same gas uptakes which are obtained at equilibrium condition in a dynamic flow situation,free-standing carbon nanoplates(FCP)with uniformly sized permeable ultramicropores(5-6 A)and precisely controlled nano-thickness(30-65 nm),were prepared from the thermoregulated phase transition of stearic acid.The unique extra-thin 2D structure facilitates the release of volatile species formed during pyrolysis and promotes the orientated growth of carbon crystallites basic units.The experimental results and coarse-grained Brownian dynamics simulation confirm a single-sized pore system,resulting from the stacking of carbon crystallites in nearly parallel orientation manner in the unique extra-thin 2D carbon due to the existence of-80%sp2-hybridized carbon.When the prepared FCP were evaluated as an adsorbent,the equilibrium uptakes of CO2,C2H4,C2H6,and C3H8 were as high as 5.2,5.6,5.3 and 5.1 mmol g-1,respectively,at 298 K and 1 bar,whilst the separation selectivity values were 7,41,71 and 386 for CO2/CH4,C2H4/CH4,C2H6/CH4,and C3H8/CH4 systems,respectively.A high adsorption capacity,excellent selectivity,and fast sorption kinetics of carbon nanoplates are complimented by the fact that the equilibrium uptakes can be achieved in dynamic flow situations.This is all due to the extremely thin free-standing carbon nanoplates providing short diffusion paths and thus facilitating the transfer of gas molecules into the inner micropores.