Study On The Mechanism Of Adsorption Of CO₂ And Typical Gas Pollutants By MOFs-modified Biochar

The continuous rise in atmospheric CO₂ concentration exacerbates global warming.Flue gas from coal combustion is not only the main source of CO₂ industrial emissions,but also contains SO₂,NO_x,volatile organic pollutants and other harmful gases,which will cause a great impact on human health and ecological environment.The multi-pollutant control technology using biochar as a green low-cost adsorbent has great potential in real application,but it is limited by the lack of adsorption performance and modification control means.Based on this,metal-organic frameworks（MOFs）were used to deeply modify biochar in this thesis.The evolution mechanism of the physicochemical properties of MOFs-modified biochar was analyzed systematically.Finally,the combined adsorption mechanism of modified biochar was analyzed.This study provides a new idea for the modification and regulation of biochar,and provides theoretical and technical support for its multi-component gas adsorption.Firstly,ZIF-8（zeolitic imidazolate framework-8）and Cu-BTC（copper benzene-1,3,5-tricarboxylate）were selected as representatives of MOFs and the feasibility of in-situ modification of MOFs to modify biochar was investigated.During grafting,MOFs interact with oxygen-containing groups on the surface of biochar,and the latter acts as a synthesis site to enable MOFs with smaller particle size to be grafted onto biochar.The composite showed inheritance to the micropore and surface groups in MOFs,and density functional theory calculation confirm that the MOFs-grafted biochar has a stronger affinity for CO₂,which is mainly reflected in the enhanced van der Waals effect.Secondly,biochar was modified with nitrogen doing by heat treatment with the help of the thermal activation and limited domain characteristics of nitrogen-rich ZIF-8.The results show that with the increase of heat treatment temperature,the introduction of ZIF-8 nitrogen acted more significantly with the aromatic rings and oxygen-containing functional groups of biochar,and many pores（especially micropores）were formed.The conversion of Zn-N-C and pyridinic N to oxidized nitrogen was facilitated by increasing the proportion of biochar.Graphitic N and C-O-C/carbonyl group promote CO₂ adsorption,and the micropores with the pore size of 0.62-0.72 nm play a major role in CO₂ adsorption.Then the“one pot”reaction of biomass and ZIF-8 precursor liquid and pyrolysis are adopted to realize the depth control of the“morphology-nitrogen groups-porosity”of biochar.The ZIF-8 modified biochar by one-pot method showed a controllable three-dimensional flower-like morphology.The surface morphology,nitrogen group type and content,pore structure parameters were significantly affected by pyrolysis temperature and doping ratio.When the pyrolysis temperature exceeds 600℃,the frame structure in the precursor completely collapses and Zn O appears.When the temperature rises to 900°C,a large number of narrow micropores can be formed by the etching effect of Zn O on carbon.The specific surface area of the modified biochar reaches 883.4 m²/g,and the CO₂ adsorption performance is outstanding（134 mg/g）.Next,the adsorption and desorption characteristics of CO₂,SO₂ and toluene in simulated flue gas of different modified biochars were studied in a fixed bed adsorption system,and the structure-activity relationship was revealed by molecular simulation and calculation.The one-pot modified biochar has the best comprehensive adsorption performance,and its adsorption capacity of each gas decreases the least with the increase of adsorption temperature.Physical adsorption was the main adsorption process,and the optimal adsorption pore size was as follows:toluene（0.66-0.76 nm）>CO₂（0.6-0.7 nm）>SO₂（0.46-0.64 nm）.Only SO₂ showed obvious chemical adsorption.Finally,the multi-component adsorption characteristics of CO₂,SO₂,NO and toluene on modified biochar at different adsorption temperatures（25-120°C）were studied,and the competitive mechanism of multi-component adsorption was revealed through continuous adsorption experiments and calculation.The adsorption capacities of CO₂,SO₂,NO and toluene of ZIF-8 modified biochar by one-pot method can reach 38.1,78.9,3.1,and 245.2mg/g,respectively.Among which CO₂,SO₂ and toluene show strong competitive effects,and their adsorption capacities are reduced by 42%,28%and 27%,respectively,compared with the single-component adsorption.The calculation results also show the three gases are ranked as follows:toluene>SO₂>CO₂.