Research On The Preparation Of Bimetal MOFs By Steam-assisted Method And Their Gas Adsorption Performance

As a new member of porous materials,metal-organic framework materials（MOFs）have received extensive attention from researchers in recent years,especially in gas separation and adsorption.However,in the course of the research,it has been found that there are already many MOFs that are currently available.Simply preparing new MOFs without increasing or widening the performance of MOFs cannot solve it practical application problems.Based on this,it has become a significant research topic to improve or increase the properties of a classical MOF through modified methods.Theself-assembly synthesis mode of MOFs makes it easy to be functionalized.Usually,two methods often be used to functionalize MOFs:grafting functional groups on organic ligands or metal centers and completely or partiallyexchanging organic ligands or metal centers of the original MOFs with other organic ligands or metal centers.In this article,PCN-6’and PCN-6 were selected as the parent MOFs,due to PCN-6’has the same topological structure as Cu-BTC but has a larger specific surface area,and the structure of PCN-6 can be reproducedby two identical interpenetrated nets of PCN-6’.The method of metal exchanging was selected.This article,bimetallic MOFs were firstly prepared by a steam-assisted manner,in subsequent experiments examined in detail the synthesis temperature and material ratio on the samples synthesized to get the optimum reaction condition.The samples was confirmed to be a bimetallic material by EDS elemental analysis,and the structure and properties were determined by basic characterization methods such as PXRD and BET.The main contents and conclusions include the following:（1）PCN-6’has the same diffraction peaks as PCN-6,and the preparation conditions are also very close.PCN-6’is a non-interpenetrated MOF isostructural,in the preparation process,a little of hydrochloric acid needs to be added to increase the space steric hindrance.While,PCN-6 can be reproduced by two identical interpenetrated nets of PCN-6’,itdo not need add hydrochloric acid.The optimal exchange conditions for each of the four different metal PCN-6’（M）series materials and the five different metal PCN-6（M）series materials were not the same.（2）The phase of the PCN-6’（M）and PCN-6（M）was determined.The XRD diffraction pattern and morphology of PCN-6’（M）were confirmed to be consistent with PCN-6’by XRD and SEM,PCN-6 and PCN-6（M）had the same situation.The specific surface area of PCN-6’（M）and PCN-6（M）was smaller than that of their respective parent materials by BET characterization,and the BET specific surface area was between 860m²·g¹ and 1230 m²·g¹.The metal exchange amount of PCN-6’（M）is between 10%and 20%,and PCN-6（M）is between 10%and 25%by the characterization of EDS.（3）CO₂ and H₂ adsorption were tested on bimetallic materials PCN-6’（M）and PCN-6（M）.Although both PCN-6’（M）and PCN-6（M）have lower CO₂ and H₂ adsorption capacities than their respective parent materials at 0.1 MPa,in the low pressure range,it can be seen that most of the bimetallic materials have higher CO₂ and H₂ adsorption capacities than their parent materials.This may be due to the increased binding energy of CO₂ and H₂ after metal exchange.However,in the process of metal exchange,the force of steam is strong,therefore,it is effect on the structure of the material and the specific surface area of the obtained bimetallic MOFs is significantly lower than their parent materials.As a result,the CO₂ and H₂ adsorption capacity is lower than their parent materials at the final test pressure.（4）Four PCN-6’（M）have higher CH₄ adsorption capacity than its parent material PCN-6’at atmospheric pressure,and PCN-6（Zn）、PCN-6（Co）and PCN-6（Ni）higher than its parent material PCN-6,this is a manifestation of the superior performance of bimetallic materials over parent materials.In addition,the CO₂/CH₄ adsorption selectivity of the PCN-6’series materials and the PCN-6 series materials are consistent,and they follow the order of Fe²⁺>Cu²⁺>(Ni²⁺)>Co²⁺>Zn²⁺>Mn²⁺.（5）Compared with their respective parent materials,the performance improvement of PCN-6’（M）is more pronounced than that of PCN-6（M）,this may be due to the fact that the PCN-6 with the interpenetrated structure that is more stable and is not easily to be sufficient metal exchanged,thus the its bimetal materials with general properties.