Preparation And CO₂Adsorption Performance Of Metal-organic Frameworks/Aminated Graphite Oxide Composites

CO2capture and sequestration is considered to be the most efficient way to control greenhouse effect. Adsorption is one of the effective postcombustion methods of CO2sequestration owing to its low energy requirements and a relatively simple technology. As physical adsorbent, metal-organic frameworks (MOFs) is considered as one of the promising separation media for CO2capture. To further enhance CO2adsorption, the composites of MOFs with aminated graphite oxide were synthesized. These materials and the parent composite components were characterized using XRD, FT-IR, potentiometric titration, sorption of nitrogen, thermal analysis, elemental analysis, Raman spectroscopy, and SEM/EDX techniques. CO2adsorption on these composites was evaluated at different temperatures under wide pressure range. The main research contents and obtained results are summarized as follows:First, aminated graphite oxide was prepared for CO2capture. On the basis of the interaction between amines and oxygen containing groups, basic nitrogen-functionalities were successfully introduced into the graphite oxide (GO). Three types of aminated GO samples modified by excess EDA, DETA or TETA were synthesized for CO2adsorption. Their CO2capture performance was investigated by dynamic adsorption experiments with N2-CO2mixed gases at30℃. It was found that the aminated GO was an efficient adsorbent for CO2capture. Among them, the sample aminated by EDA exhibited the highest adsorption capacity with the longest breakthrough time of CO2. Before saturation, its adsorption capacity was up to53.62mg CO2/g sample. In addition, among the adsorbents of graphite oxide modified by10wt%,50wt%and100wt%EDA, graphite oxide with50wt%EDA had the highest adsorption capacity.Based on the above results, a typical metal-organic framework MOF-5was chosen and its composites with aminated graphite oxide (MOF-5/AGO) were synthesized to expect the synergy on structure and chemistry with the intended application for CO2capture. The incorporation of GO or AGO brings higher pore volume in the MOF composites, which is attributed to the new pores generated at the interface between the GO layers and the MOF "blocks". MOF-5, MOF-5/GO and MOF-5/AGO samples with suitable specific surface area and pore diameter were investigated on their CO2adsorption performance and stability under humidity conditions. The results indicate that high surface area and pore volume, pore similar in size to the size of gas adsorbate, and extra reactive sites modified in the composites contribute to the high CO2capacity. Besides, the composites involved by GO or AGO show better anti-moisture performance than the parent MOF.Due to the poor stability of MOF-5, new composites of a copper based MOF Cu-BTC and graphite oxide modified by urea (GO-U) were synthesized because of the good stability and easy to large-scale synthesis of Cu-BTC and the low cost of urea. The introduction of GO-U didn’t prevent the formation of MOF crystal structure, but resulted in the modified chemical environment of copper sites, led to an increase in the porosity, and caused defects in MOF crystals and thus an exposure of more unsaturated copper sites than those in the parent MOF. All of these features are beneficial for CO2adsorption. CO2uptake at dynamic conditions and CO2adsorption isotherms in the pressure range up to1.5MPa at three different temperatures close to ambient were measured. The results indicate that CO2adsorption on the parent MOF and its composites with GO or aminated GO follows a typical physical absorption mechanism. The primary adsorption sites are open copper centers and cage window sites. Among all samples analyzed the composite of Cu-BTC and modified graphite oxide with the highest N content (MOF/GO-U3) is the best sample providing a significant advance in CO2capture. The CO2capacity on this material reaches4.65and7.27mmol/g at298K and277K at0.1MPa, and13.41and15.00mmol/g at298K and277K at1.5MPa. This CO2capacity is competitive with many best performing adsorbents addressed in the literature. Besides, MOF/GO-U3also exhibits higher CO2/CH4selectivity than the parent MOF and similar CO2/N2selectivity. A relatively low heat of CO2adsorption on MOF/GO-U3caused that this composite can be fully regenerated and used in multicycle adsorption with the minimum energy demand.To recycle the graphite oxide with pre-adsorbed NH3, it was mixed with Cu-BTC precursor to synthesize the composites of MOF and ammoniated graphite oxide with the intended application for CO2capture. They showed higher amount of CO2adsorbed than that of the parent MOF. The incorporation of ammoniated graphite oxide affected two main adsorption sites of CO2on Cu-BTC including open copper centers and cage window sites, which both belong to physical forces, and contributed to the higher CO2uptake. In one word, the composites not only recycled the waste GO with NH3, but also increased CO2adsorption capacity of the parent MOF.