Facile Room-temperature Synthesis Of Hierarchical Porous MOFs And Two Dimensional MOFs Nanosheet

Metal-organic frameworks（MOFs）,which are highly adjustable and possess high specific surface area and porosity,have been applied in gas storage,adsorption separation and heterogeneous catalysis.However,the most of the conventional MOFs are bulk crystals and only possess micropores.The bulk structure and narrow pore size are unfavorable for the molecules rapidly and freely approaching or leaving the internal active sites,which limits the application of MOFs in many aspects.Therefore,the synthesis and application of hierarchically porous MOFs（H-MOFs）and two-dimensional MOFs（2D-MOFs）have attracted attention.Hierarchically porous MOFs possess micropores,mesopores or macropores at the same time,thus the guest molecules can rapidly diffuse through the meso/macropores,reducing their mass transfer resistance and greatly improving the material properties.Moreover,for the two-dimensional MOFs materials,their active sites are directly exposed on the surface of the 2D plane,thus the molecule can diffuse directly to the active site,greatly reducing the mass transfer resistance.These properties make two-dimensional MOFs have great application prospects in catalysis.However,there are still some challenges in the synthesis of hierarchically porous MOFs and two-dimensional MOFs.First,the species and the synthesis routes of the H-MOFs and 2D-MOFs are still limited.Moreover,the synthesis conditions are harsh leading to the the low space time yield(STY<300 kg·m^-3·d^-1),which usually require long synthesis time（>12h）,high energy consumption（synthesis temperature>100℃）.Thus,the mild,green,environmentally friendly and rapid synthesis routes for the H-MOFs and 2D-MOFs are in great urgence.In order to improve the performance of MOFs and simplify the synthesis route,the organic amines were employed as a bifunctional template to synthesize the nanoscale hierarchically porous ZIF-67 first,which possess micropores,mesopores and macropores simultaneously and the crystal size<200nm.The synthesis procedure of the nanoscale hierarchically porous ZIF-67 can be shortened to 5 minutes,which improved material performance greatly and reduced the cost of synthesis.The crystal size and porosity of the synthesized nanoscale hierarchically porous ZIF-67 could be regulated by the reaction time and the type of organic amine.The crystal size could be regulated to 50nm at least while the mesoporous pore volume V_meso could be as high as 0.86cm³?g^-1.The as-synthesized nanoscale hierarchically porous ZIF-67 material was applied in the adsorption of the malachite green in aqueous solution.The adsorption rate of the nanoscale hierarchically porous ZIF-67 are much higher than that of conventional ZIF-67 material,and MG adsorption rate and capacity increased with the temperature（Enthalpy of adsorptionΔH>0）.At 40℃,the saturated MG adsorption capacity of nanoscale hierarchically porous ZIF-67 could reach to 114.1 mg/g.The meso-dynamics（MesoDyn）simulation was used to investigate the phase behavior of organic amine modulator thus a possible synthesis mechanism is obtained as follow:On the one hand,the organic amine modulator self-assembled to form the columnar aggregations,serving as structure directing agents.Thus the precursor of the conventional microporous MOFs could crystallize over the modulator aggregations;On the other hand,the introduced organic amine enhanced the deprotonation degree of ligand and accelerated the nucleation of MOF crystal.Thus the crystal size of product would be limited in nanoscale during the short nucleation period.The nanoscale hierarchically porous structure formed after the removal of the modulator.However,most of the template could not be directly used to rapidly synthesize hierarchically porous MOFs at room temperature.In order to broaden the synthetic routes of the hierarchically porous MOFs and the types of template,and improve the yield of the fabrication of hierarchically porous MOFs,a simple,universal and high-yield method was developed to rapidly synthesize the hierarchically porous Cu-BTC at room temperature by adding ZnO and template to the reactant solution simultaneously.The synthesis time is only 30minutes while the STY is as high as 1322 kg·m^-3·d^-1,and the synthesis route is suitable for a lot of templates including 1-bromodecane,1-bromotetradecane,1-bromohexane,tetradecylnitrile and hexylamine.The pore structure of the the hierarchically porous Cu-BTC could be tailored by tuning the amount and type of the template,where the mesoporous pore volume V_meso could be as high as 0.16 m³·g^-1.Furthurmore,the rapid synthesis route has good universality,which has been applied in the synthesis of hierarchically porous ZIF-8.The synthesized hierarchically porous Cu-BTC exhibited higher catalytic activity than the conventional Cu-BTC material.After reacting for 24 hours,the yield of the Henry reaction of4-nitrobenzaldehyde catalyzed by hierarchically porous Cu-BTC reached 26.3%while that of reaction catalyzed by conventional Cu-BTC is 16.5%.In addition,the mesoscopic kinetic simulation and comparative experiments were used to investigate the formation mechanism of H-MOFs.A possible mechanism is as follow:Firs,the added ZnO reacts with the metal nitrate aqueous solution to form the（Zn,Cu）hydroxy double salt and the BTC^3-ligand rapidly react with the（Zn,Cu）-HDS to form MOF precursor;the MOFs precursor self-assemble under the direction of the template and crystalline on the surface of surfactant micelles under the synergy of HDS and template micelles;the meso-/macropores were formed after the removal of the template,which were constructed by the microporous framework.Furthermore,a two-dimensional CuBDC material uniformly loaded with ZnO was rapidly synthesized at room temperature by adding ZnO and 1-bromopentane to a conventional Cu-BDC reactants DMF solution,and the synthesis time was only 30 minutes.The as-synthesized two-dimensional ZnO@CuBDC is a petal-like dispersed sheet structure with high specific surface area and hierarchically porous structures.The pore structure of the two-dimensional ZnO@CuBDC could be regulated by tuning the addition order and the amount of 1-bromolane.When 1-bromopentane was added to the solution containing metal ions and ZnO and the ratio of 1-bromopentane/Cu was n_{1-bromopentane}:n_Cu=2:1,the as-synthesized ZnO@CuBDC possessed the maxmium of specific surface area and pore volume as S_BET=427.5 m²?g^-1 and V_t=0.24cm³?g^-1.When 1-bromopentane was added to the ligand solution and the ratio of 1-bromopentane/Cu was n_{1-bromopentane}:n_Cu=1:1,the as-synthesized ZnO@CuBDC possessed the maxmium of V_meso/V_micro recached to 6.23.The synthesized ZnO@CuBDC material was used for the removal of mercaptan in gasoline.The removal rate of mercaptan reached 100%,and the sulfur capacity reached 23.9 mg-S/g,which is much higher than the traditional material Cu-13X（10.0 mg-S/g）.