Study On The Preparation And CO₂ Adsorption Performance Of Hydrotalcite-like Composites

The continuous and rapid rise of carbon dioxide concentration in the atmosphere has caused numerous environmental problems,so advanced CO₂ capture technology is required to solve this problem.Among them,the adsorption method has been regarded as one of the most promising carbon capture technologies due to its low energy consumption and cost,easy regeneration and high cycle stability,and the core of this method is the adsorbent with excellent performance.Hydrotalcite-like material（LDH）,as a CO₂ adsorbent,has attracted extensive attention because of its good adsorption capacity,rapid adsorption/desorption kinetics and good regeneration performance.In this thesis,a new type of high-performance CO₂ adsorption composite material was obtained by modifying the hydrotalcite-like material.At the same time,the adsorption mechanism of the composite adsorbents was explored through a series of characterizations.The specific research contents are as follows:（1）The LDO-Cx samples（x=2～18）of monocarboxylate anion intercalated hydrotalcite-like oxides（LDO）with different carbon chain lengths（C2～C18）were prepared,and their CO₂ adsorption properties were investigated.A series of characterization and adsorption performance studies show that the adsorption performance of intercalated long carbon chain carboxylate anions（x>10）are better than that of intercalated short carbon chain carboxylate anions（x≤10）hydrotalcite-like oxides,which may be due to the more stable quasi-amorphous structure,lower crystallinity and smaller particle size of the intercalated long carbon chain carboxylate anion samples.Among the LDO-Cx samples,LDO-C18 exhibited the best adsorption capacity（0.55 mmol/g）at 200°C.It was found that the optimal calcination temperature of the LDO-C18 sample was 400°C.Too low calcination temperature results in incomplete decomposition of the layered structure,while too high calcination temperature leads to the transformation of amorphous mixed oxides into spinel oxides.In addition,the loading of alkali metals has a significant effect on promoting the adsorption performance of the LDO-C18,and the 8K/LDO-C18 sample still maintains a good regeneration performance after 10 cycles of testing.（2）In order to further improve the medium and high temperature adsorption performance of the above LDO-Cx materials,a novel nanocomposite nanocomposite（x GO-LDO-SA）of graphene oxide（GO）-doped C18 stearate（SA）-intercalated LDO（LDO-SA）.The results show that with the increase of GO doping amount,the CO₂adsorption capacity of x GO-LDO-SA samples increases first and then decreases.Among them,6GO-LDO-SA nanocomposite show the highest CO₂ adsorption capacity（0.83 mmol/g）,which is better than that of common hydrotalcite-like composites.Further studies show that the doping of GO can improve the dispersion and stability of the composites,and the high adsorption performance of the6GO-LDO-SA sample was associated with its increase in the specific surface area and the number of alkaline sites.The 6GO-LDO-SA adsorbent exhibited a fast kinetics（0.863 mmol/（g·min））under the medium-temperature conditions.The adsorption process can be divided into three stages:extra-membrane diffusion,intra-particle diffusion and equilibrium reaction process,and the adsorption of the sample is in good agreement with the Freundlich isotherm model.The adsorption process of CO₂ on composite is mainly chemical adsorption,and the adsorbed CO₂mainly exists in the form of bicarbonate,monodentate carbonate or bidentate carbonate.（3）The M/GO-LDO-SA were prepared by loading different alkali metals M（M=Li,Na,K and Cs）on the above optimal 6GO-LDO-SA sample by impregnation method.Among all the M/GO-LDO-SA samples,the K/GO-LDO-SA and Cs/GO-LDO-SA samples exhibited good CO₂ adsorption capacity at medium temperature conditions.Compared with the Cs/GO-LDO-SA sample（83%）,the K/GO-LDO-SA sample（87%）has better regeneration efficiency.It was found that the CO₂ adsorption process of the modified adsorbent is mainly chemical adsorption,so the strength of the alkaline site is the main factor affecting its CO₂ adsorption performance.Among them,the basicity follows the basic trend of alkali metal oxides,and as the radius of the alkali metal ion increases,the partial charge of the relevant oxygen anion becomes more negative,which induces the formation of alkaline sites with higher electron-donor capacity.Subsequently,the adsorption properties of x K/GO-LDO-SA samples with different K contents were studied,and the results showed that with the increase of K loading,the adsorption capacity first increased and then decreased.The 11K/GO-LDO-SA sample not only has a good adsorption capacity（1.14 mmol/g）,but also exhibits good regeneration performance and fast adsorption kinetics.（4）The CO₂ adsorption performance of hydrotalcite-like material at low temperature is also an important indicator in its practical application.Therefore,the SBA-15-supportedpolyethyleneimine（PEI）-modifiedhydrotalcite（x LDH-y PEI/SBA-15）were prepared by the in situ coprecipitation method,and their low-temperature CO₂ adsorption performance and involved mechanisms were studied.After loading of PEI and LDH on the surface of SBA-15,the composite materials still maintained an ordered two-dimensional hexagonal porous structure and a high specific surface area（441～585 m²/g）.The optimal CO₂ adsorption capacity（2.01mmol/g）of 40LDH-2PEI/SBA-15 at 50°C was 1.88 times larger than that（1.07mmol/g）of SBA-15.The SBA-15 sample fits the pseudo-first-order kinetic model,while the 40LDH-2PEI/SBA-15 sample fits the pseudo-second-order kinetic model.The isosteric heat of adsorption(Q_st)of the 40LDH-2PEI/SBA-15 sample is in the range of 58.5～66.5 k J/mol,and it has a good affinity for CO₂ molecules.Such an excellent CO₂ adsorption capacity of 40LDH-2PEI/SBA-15 was associated with its high BET surface area,abundant basic sites,and fast adsorption kinetics.In addition,the 40LDH-2PEI/SBA-15 sample has high CO₂/N₂ selectivity,and it still maintains a high regeneration efficiency of more than 94%after 10 cycles of regeneration experiments.（5）A novel kind of CO₂ solid adsorbents（x LDH/MA）was prepared by loading of LDH on the mesoporous alumina（MA）support.The LDHs in x LDH/MA were evenly distributed on the surface of MA.x LDH/MA possessed a large BET surface area（278～378 m²/g）,narrow pore-size distribution（4.37～6.23 nm）,and a highly crystalline framework.Among all of the x LDH/MA samples,30LDH/MA showed an excellent CO₂ adsorption capacity at 50°C（1.56 mmol/g）,which was 1.55 times larger than that of unmodified MA.Due to the synergistic effect between MA and LDH,30LDH/MA exhibited a higher surface area and a more amount of adsorption sites,which were the main factors improving its CO₂ adsorption performance.The adsorption curve of 30LDH/MA sample fits well with the pseudo-second-order kinetic model,which confirms that the main adsorption mechanism of the composite is chemical adsorption,and the adsorbed CO₂ mainly exists in the form of carbonate.In addition,the 30LDH/MA composite material showed fast adsorption kinetics and good regeneration efficiency.