Rapid Synthesis Of Hierarchically Porous Metal-organic Frameworks At Room Temperature And Its Mechanism

Metal-organic frameworks?MOFs?,a novel class of functional materials,are endowed with the three-dimensional network structures through self-assembly of metal ions?or metal clusters?and organic ligands.They are typical crystalline materials with high specific surface area,large pore volume,tunable pore size and diversity of topological structures.However,the majority of MOFs reported only contain micropores?pore size<2 nm?,which greatly restricts large molecules from approaching or leaving the active sites within the internal space of MOFs.Therefore,it is highly desirable to the design and prepare hierarchically porous MOFs?H-MOFs?with micro-,meso-and macropores.Compared with conventional microporous MOFs?C-MOFs?,H-MOFs exhibit enhanced mass transfer and diffusion rates,allowing guest molecules to freely and rapidly reach the actived sites within the micropores via the mesopore or macropore channels.Consequently,application of H-MOFs is particularly attractive in various fields involving both small and large guest molecules,including adsorption,separation,drug delivery,photoelectronics,and catalysis.Templating method is one of the most useful strategies to construct order and stable H-MOFs.However,there still exsit some issues,such as high energy consumption,long synthesis time,less type of templating agent and uncertain growth mechanism of H-MOFs.In order to solve these problems,the following work has been carried out.In this work,hierarchically porous Cu-BTC?H-Cu-BTC?have been successfully synthesized under mild conditions within 60 s using an anionic surfactant in a?Cu,Zn?hydroxyl double salt?HDS?solution.The anionic surfactant served as a template for the construction of mesopores and macropores and the?Cu,Zn?HDS accelerated crystal nucleation.The as-synthesized materials displayed significantly higher Brunauer–Emmett–Teller?BET?surface area(>1156 m²·g^-1)and total volumes(>0.64 cm³·g^-1).The textural properties of the H-Cu-BTC?e.g.,BET surface area and mesopore volume?were depending on the amount of template employed.The space time yields?STYs?of the prepared H-Cu-BTC ranged from 1.94×10⁴ to 2.90×10⁴ kg·m^-3·d^-1.Importantly,the growth of H-Cu-BTC was monitored by attenuated total-reflectance Fourier-transform infrared?ATR-FTIR?spectroscopy and the mechanism of synthesis was explored using mesodynamics?MesoDyn?simulations.The as-syntehszied H-Cu-BTC exhibited enhanced the catalytic activity and adsorption capacity.H-Cu-BTC was tested in the Henry reaction—the oxidation of p-nitrobenzaldehyde and nitromethane in which the conversion rate increased by 11.5%compared with C-Cu-BTC.The adsorption capacity of toluene is improved by 67%compared with C-Cu-BTC.In this work,two hierarchical porous zeolitic imidazolate frameworks?ZIFs?were synthesized at room temperature and pressure within 60 s using N,N-diethylethanolamine as template.The resulting hierarchically porous ZIF-8?H-ZIF-8?and hierarchically porous ZIF-90?H-ZIF-90?showed hierarchically porous structure with micro-,meso-,and macropores together with excellent thermal stability.Furthermore,the porosity properties can be readily tuned by changing the amount of template.All of the H-ZIF-8 samples show higher BET surface areas(>946.8 cm²·g^-1)than conventional microporous ZIF-8?C-ZIF-8?(831 cm²·g^-1).Moreover,H-ZIF-8 samples have much higher mesopore pore volumes(>0.2cm³·g^-1)than C-ZIF-8.The space-time yield?STY?was as high as 1.59×10⁴ kg·m^-3·d^-1.The introduced template N,N-diethylethanolamine played two roles during the synthesis:one played a protonation agent role enables the ligands deprotonation,the other played a template role served as structure-directing agents of meso-and macropores.The as-syntehszied H-ZIF-8 exhibited enhanced adsorption capacity for CO₂ which is improved by>24.4%compared with C-ZIF-8.In this work,a versatile strategy was developed with anion surfactant as the template and hydroxy double salt?HDS?as the accelerant to prepare hierarchically porous ZIFs.Compared with the preparation method of the second part,this strategy expands the type of template agents and the anionic surfactants are environmentally friendly attribute to their low toxicity.The rapid nucleation of the crystals and the formation mechanism of meso-macropores were analyzed systematically.Based on the proposed strategy,the rapid preparation of the stable hierarchically porous ZIFs?hierarchically porous ZIF-8,hierarchically porous ZIF-61,hierarchically porous ZIF-90?with tunable porosity at facile conditions were achieved.Other than intrinsic micropores?d<2 nm?,H-ZIF-8 samples show broad pore size distributions from 8 to over 100 nm,encompassing the mesoporous and macroporous regions,which is absent in the C-ZIF-8.All of the prepared H-ZIF-8 samples present higher BET surface area(>1300 cm²·g^–1)than that of C-ZIF-8(831 cm²·g^–1).The micropore volume of the H-ZIF-8(>0.54 cm³·g^–1)is also higher than C-ZIF-8(0.39 cm³·g^–1).Interestingly,with the increase of amount of surfactant,the mesopore volume would at first increase and then decrease slightly.It was noticeable that the mesopore volume reaches a maximum at the molar ratio of surfactant/Zn²⁺of 1.These results indicate the optimum ratio of surfactant/Zn²⁺for synthesizing high crystalline H-ZIF-8 is 1.Moreover,the total pore volume of H-ZIF-8 was as high as 1.2 cm³·g^–1,which is higher than that of the values reported.This proposed method not only introduced mesopores and macroproes into conventional ZIFs,but also realized the low power consumption fabrication.More importantly,it could simultaneously improve the production rate with drastic reduction in the time required for crystallization of the hierarchical porous ZIFs.The space-time yields?STYs?for the as-synthesized H-ZIF-8 could be remarkably increased,with a maximal STY as high as 3.20×10⁴ kg·m^-3·d^-1,which is the highest value as ever reported.Furthermore,the mechanism underlying the rapid synthesis of H-ZIF-8 was elaborated for the first time by means of mesoscopic dynamics simulation combined with in situ experimental technique.The resulting H-ZIF-8 materials showed quite excellent sorption properties for CO₂ and CH₄.The adsorption capacity for CO₂ is improved by 60.3%and the adsorption capacity for CH₄ is improved by 35%compared with C-ZIF-8.