Hybrid Hydrogen-Bonded Organic Frameworks For Gas Adsorptive Separation

Separation is a cost-and energy-intensive chemical process.Exploring precise separation technology for mixtures with similar structures and properties is of revolutionary significance for resource deep utilization and industrial green development,but it also faces great technical challenges.As a non-heat-driven separation process,adsorptive separation is an efficient method to solve this separation challenge.The core of adsorptive separation lies in the development of highly efficient adsorbents and separation processes.Hydrogen-bonded organic frameworks(HOFs)are a new class of crystalline organic solids that with features of mild synthesis conditions as well as easy purification and regeneration.However,the construction difficulties of HOFs with good stability,permanent porosity,and rich functional sites restrict HOFs’ applications in the fields of adsorptive separation and so on.The purine organic ligands with π-conjugated rigid structure as well as abundant hydrogen bond binding sites and metal coordination sites are perfect modules for constructing stable,highly porous,and functionalized HOFs.In this paper,a series of stable metalnucleobase hybrid HOFs(HOF-ZJU-201,HOF-ZJU-202,and HOF-ZJU-203)with microporous structures and charge differentially distributed pore surfaces were constructed.The adsorptive separation performance and industrial application potential of hybrid HOFs were investigated for the typical gaseous similar mixtures including rare gases(Xe,Kr),light alkanes,CO2/CH4,and CH4/N2.The adsorptive separation mechanism and the role the confined micro-electric field enhanced adsorptive separation strategy played in different systems were deeply explored.Furthermore,the shaping of materials,adsorption thermodynamics and diffusion kinetics in the fixed bed,and process simulation results systematically confirmed the industrial application feasibility of hybrid HOFs.The main research contents are as follows:Based on the polarizability difference of rare gases Xe and Kr,the metalnucleobase hybrid HOFs with unique pore surface micro-electric fields were used to selectively capture Xe with greater polarizability and achieve highly separation of Xe/Kr.Hybrid HOFs exhibited the Xe capacity of 1.32-2.09 mmol·g-1 under 0.1 bar,and the Xe/Kr selectivity of 15.1-21.0,achieving a balance between adsorption capacity and separation selectivity.By employing GCMC simulation,DFT-D theoretical calculation,and differential charge density analysis,it was clarified that the confined micro-electric field in the hybrid HOFs formed by the staggered electron-deficient and electron-rich regions could serve as a strong adsorption site to produce a more significant polarization effect on Xe with larger polarizability.The increasing difference of charge distribution on the pore surface can further enhance the polarization effect.The fixed bed breakthrough experiments showed that the hybrid HOFs have excellent Xe/Kr dynamic adsorptive separation performances.The dynamic capture capacity of Xe at low pressure of HOF-ZJU-201 reaches 25.8 mmol·kg-1,which shows great application potential for the direct capture of trace rare gases in the waste gas of used nuclear fuel.Inspired by the above research,based on the different polarizability of CH4,C2H6,and C3H8,the metal-nucleobase hybrid HOFs were further applied to the adsorptive separation of light alkanes.Under low pressure of 0.1 bar and 0.05 bar,the materials have high adsorption capacity of C2H6(1.39-2.52 mmol·g-1)and C3H8(1.06-2.23 mmol·g-1)respectively.The C2H6 adsorption capacity of HOF-ZJU-201(1.26 mmol·g1)at 0.01 bar ranks front those reported MOFs and HOFs.The GCMC simulation and DFT-D theoretical calculation results showed that the hydrogen-bonded frameworks achieve selective adsorption of light alkanes through multiple C-H…N and C-H…F hydrogen bonds as well as C-H…π interactions.The fixed bed breakthrough experiments illustrated their outstanding separation potentials for light alkane mixtures.Especially,products with high-purity of CH4(>99.99%)can be directly obtained from three-component gaseous mixtures,which shows great application value for the deep removal of alkane impurities in conventional natural gas.The good stability of materials reveals their industrial application prospect.Given the fact that there exist unique highly polarized channels in the metalnucleobase hybrid HOFs and abundant amino groups and hexafluoro anions in the frameworks,the possibility of adsorptive separation of CO2,CH4,and N2 was further explored.The hybrid HOFs exhibited high adsorption capacity of CO2(2.31-3.35 mmol·g-1)and CH4(0.99-1.73 mmol·g-1)at ambient conditions.The lower adsorption heat is beneficial to their rapid regeneration in practical industrial applications.The DFT-D calculations and Hirshfeld Surface analysis revealed the main existing Cδ+… Fδdipole-dipole,N-Hδ+…Oδ-hydrogen bonds and van der Waals interactions between CO2 and hydrogen-bonded frameworks.The fixed bed breakthrough experiments verified their efficient dynamic separation performance of CO2/CH4 and CH4/N2.The single-column desorption experiment of HOF-ZJU-201 can obtain products with CH4 content higher than 90%,indicating that this kind of material can be utilized for the efficient upgrading of CH4 in unconventional natural gas.The enlarged synthesis and shaping of materials provided data support for the further industrial applications of hybrid HOFs.To further verify the feasibility of hybrid HOFs’industrial application,the process simulation and parameter optimization of the HOF-based PSA process for CH4/N2 separation were carried out by Aspen Adsorption V11 software with HOF-ZJU-201 shaping pellet as adsorbent and low methane concentration gas upgrading as the target.The single-bed breakthrough performance of the adsorbent was investigated,and the adsorption wave and concentration wave of the adsorbate in the fixed bed were clarified.A four-bed and six-step PSA process including adsorption,equilibrium depressurization,replacement,vacuum desorption,equilibrium repressurization,and final pressurization was designed.The effects of tower length/diameter ratio,desorption pressure,as well as the flow rate,pressure,and composition of replacement feed on product purity,recovery,and yield were investigated.The parameter ranges that obtain optimum purity and optimum index balance were determined by operating parameters optimization.The PSA process simulations provided theoretical support for the subsequent development of hybrid HOFs as well as a useful reference for the efficient upgrading of low-concentration coalbed methane.