| In recent years,with the continuous increase of industrial oxygen use,the demand for air separation adsorbent（Li X）is increasing.In the face of the shortage of lithium resources,the search for new N2/O2 adsorbent has gradually become a research hotspot.In addition,with the large-scale use of natural gas,low quality natural gas purification（N2/CH4 separation）also put forward higher requirements for nitrogen removal technology.Compared with cryogenic distillation,the balanced adsorption separation process can achieve the separation of the gas mixture without phase transition.In view of the advantages of low energy consumption and simple equipment,the application of adsorption separation technology in air separation and natural gas purification can not only reduce the energy consumption in the process of nitrogen removal,but also alleviate the large demand of oxygen and natural gas in the international society.However,due to the scarcity of N2 selective adsorbent,the further application of this process has been faced with great challenges.It was found that MOFs with unsaturated metal sites of d3 electronic structure showed a strong affinity for N2.This kind of material can achieve the selective separation of N2/O2 and N2/CH4 throughπbackbonding between metal sites and N2,showing a good application prospect in air separation and natural gas purification.The continuous emergence of excellent chemical materials has created an opportunity to solve the problem of chemical industry.MOFs material with strong N2 affinity provides an excellent adsorbent for nitrogen removal technology.However,due to the late start of research on N2 selective adsorbents,the available adsorbents are limited and the research on their mechanism is still unclear.Therefore,it is of great significance to develop new N2 selective MOF adsorbents and study their structure-activity relationship.At present,MOFs adsorbents with Cr（Ⅲ）unsaturated sites have been proved to have strong adsorption force on N2 molecules.In 2017,Chang et al.obtained the open Cr（Ⅲ）sites through high-temperature activation of MIL-100Cr for the first time.The results show that the open Cr（Ⅲ）site could adsorbed N2 molecules by Cr（Ⅲ）···N feedbackπbond,exhibiting excellent N2/CH4 and N2/O2 separation performance.In addition,Cr（Ⅲ）-MOF with Cr（Ⅲ）unsaturated sites has good hydrothermal stability and relatively simple synthesis,so it has a promising prospect as an industrial adsorbent.In this paper,the synthesis/modification of Cr（Ⅲ）-MOFs has been studied,and the relationship between the adsorption capacity of N2,the selectivity of N2/O2 and N2/CH4 and the structure of the skeleton was analyzed.The main research work and conclusions are as follows:（1）In this chapter,a novel metal-organic framework TYUT-96Cr with high density chromium sites was constructed.Its structure and composition were analyzed in detail by PXRD,77 K nitrogen adsorption and inductively coupled plasma emission spectroscopy.The results showed that the structure of TYUT-96Cr contained two different forms of chromium vacancy,and the content of chromium vacancy reached 3.54 mmol/cm3,which was much higher than that of MIL-100Cr（2.42 mmol/cm3）.TYUT-96Cr exhibited the highest N2 adsorption capacity（37.46 cm3/cm3）,and the selectivity of N2/O2（5:95,v/v）and N2/CH4（20:80,v/v）reached 26.95and 4.60.The results of CO adsorption infrared and DFT calculation indicate that the Cr（Ⅲ）site in TYUT-96Cr and N2 molecule interact with the terminal Cr-N,thus achieving the purpose of N2 capture.Under dynamic conditions,TYUT-96Cr（5825 mmol/L）showed higher high-purity oxygen yield than MIL-100Cr（1085 mmol/L）and commercial air separation adsorbents（13X and Li-LSX）.For N2/CH4 separation,the penetration experiment shows that TYUT-96Cr is more suitable for the removal of trace nitrogen in the gas mixture.When the gas mixture ratio is 5/95 N2/CH4,the high-purity methane yield of TYUT-96Cr（720 mmol/L）was much higher than that of MIL-100Cr（440 mmol/L）and MIL-101Cr（10 mmol/L）.Benefiting from the kinetic inertness of Cr（Ⅲ）and the tight spatial arrangement of Cr3O,TYUT-96Cr exhibits excellent thermal and chemical stability.（2）It was found that there was a multi-site synergistic N2 adsorption nano-trap in MIL-102Cr,which increased the binding energy of N2 molecules.The adsorption heat of MIL-102Cr for N2 reached 45 k J/mol,which was much higher than that of TYUT-96Cr（37 k J/mol）and MIL-100Cr（40 k J/mol）.According to the results of adsorption isotherm,the N2 adsorption capacity of MIL-102Cr at 0.01 bar reaches 6.42 cm3/g,which is higher than that of TYUT-96Cr and MIL-100Cr.The calculated IAST selectivity of 79/21 N2/O2 and 20/80 N2/CH4 for this material are 13.11 and 1.80.The structure information of MIL-102Cr was analyzed by means of XRD and DFT calculation.The results show that F atoms exist in the channel in contraption mode,where F/Cr ratio is 1/3.Due to the special spatial position of Cr and F in the structure,N2 molecule is adsorbed by a side-on end-on mode.Under dynamic conditions,the penetration selectivity of MIL-102Cr（79:21 N2/O2）reaches 3.90,which is higher than that of TYUT-96Cr（2.10）.In addition,for different proportions of N2/CH4 mixture,high purity CH4 product gas can be obtained through a single separation process.（3）It was found in the previous study that Lewis acidity at Cr（Ⅲ）site had a great effect on N2 adsorption.In this chapter,an anion replacement method is proposed,which successfully realizes the regulation of Lewis acidity,thus the N2 adsorption capacity of the material can be regulated.From the results,due to the difference in electronegativity of the replaced anions,the density of metal central electron cloud will be affected,which will lead to the change of its Lewis acidity.When the anions are F-and Cl-,the Lewis acidity of Cr（Ⅲ）site will be improved.Compared with the parent MIL-101-NO3（18 k J/mol）,the modified material showed a higher N2 adsorption heat,which reached 24 k J/mol for MIL-101-Cl.The adsorption data showed that the adsorption capacity of N2 and the separation capacity of N2/CH4 increased with the increase of Lewis acidity of the unsaturated site in MIL-101-X.The adsorption capacity of MIL-101-Cl reached 36.00 cm3/g with a N2/CH4 IAST selectivity of 2.70 at 273 K and 1bar.（4）Since the Lewis acidity of unsaturated Cr（Ⅲ）sites has a great influence on N2adsorption,this chapter further uses the substituent induction strategy to regulate the Lewis acidity of metal sites,so as to improve the adsorption and separation performance of nitrogen.It can be seen from the results that the presence of the electron-drawing group-NO2 is conducive to improving the Lewis acidity of the unsaturated metal site in the material,while the electron-donating group-CH3 has the opposite effect.The adsorption heat of MIL-101Cr-NO2 for N2 reached 30 k J/mol,which was much higher than that of MIL-101Cr（24k J/mol）and MIL-101Cr-CH3（14 k J/mol）.The adsorption data showed that the selectivity of MIL-101Cr-NO2（10.80）for N2/O2 was higher than that of MIL-101Cr（4.70）and MIL-101Cr-CH3（3.20）.Interestingly,MIL-101Cr-NO2 and MIL-101Cr-CH3 exhibit opposite N2,CH4 selectivity due to the difference Lewis acid strength of Cr（Ⅲ）site.The results of adsorption heat and DFT calculations showed that the binding energy of N2 molecule on Cr（Ⅲ）site was positive correlated with the Lewis acid strength of Cr（Ⅲ）site.The dynamic penetration experiments show that MIL-101Cr-NO2 can effectively separate the mixture of N2/O2 and N2/CH4. |
PDF Full Text Request