H₂S And CO₂ Removal By Carbon Based Porous Materials With Control Structures

Due to the unique pore structures and surface chemistry, porous carbon materials have been widely used for H2S and CO2 removal. However, the relationships between structures and perforamance of the carbon based porous desulfurizers and the mechanism of H2S catalytic oxidation were not well elucidated. And for CO2 removal, carbon based capture materials only have a small capture capacity and low selectivity. Therefore, the catalytic oxidation of H2S over carbon based porous materials with control structures were detailed studied and the CO2 capture materials with a large capacity and high selectivity were developed. The main results of this thesis are summarized as follows:(1) H2S catalytic oxidation over microporous carbon desulfurizers with control structures: A highly effective desulfurizer (PASC-Ⅰ) was prepared by impregnating Na2CO3 over pitch-based microporous spherical activated carbon (PSAC). The saturation sulfur capacity of PASC-Ⅰwas 0.67 g H2S/g catalyst, about 18% higher than that of commercial desulfurizer (0.48 g H2S/g catalyst). The products of H2S oxidation over PSAC-Ⅰwere mainly elemental sulfur and a little sulfuric acid. Pore structures play an important role in the performance of the microporous carbon desulfurizers. The saturation sulfur capacities of the desulfurizers have a linear relationship with the total pore volumes and are proportional to the volumes of pores larger than 0.7 nm. So the saturation sulfur capacities are mainly determined by the total pore volumes, especially by the large pore volumes. The active sites of H2S oxidation should be edge (or defect) carbon atoms and the formed sulfur radicals. Elemental sulfur as a dominate product of H2S oxidation was mostly deposited in large pores (d>0.7 nm), while sulfuric acid was preferably produced in small micropores (d<0.7 nm).(2) H2S catalytic oxidation over mesoporous carbon desulfurizers with control structures: High performance mesoporous carbon desulfurizers were prepared by loading Na2CO3 over mesoporous carbon aerogels. The saturation sulfur capacity of this material was up to 3.31 g H2S/g catalyst,5 times higher than that of commercial desulfurizer. The activity decreased with the increase of mesopore sizes, and the performance achieved the best at a compromise between mesopore volumes and mesopore sizes. The products of H2S oxidation over this material were mainly elemental sulfur and sulfuric acid. With the increase of mesopore sizes, the content of elemental sulfur increased. The depositions of sulfur in the pores of the desulfurizers include the incubation formation, nucleation, and growth processes.(3) H2S catalytic oxidation over alkaline carbon nanotubes (CNTs):High performance alkaline carbon nanotube desulfurizers were prepared by loading Na2CO3 over CNTs. The saturation sulfur capacity of the alkaline CNTs was up to 1.86 g H2S/g catalyst, which was 3 times higher than that of commercial desulfurizers. The introduction of Na2CO3, which provided the hydrophility and alkalinity needed for H2S dissociation, significantly enhanced the catalytic performance. The catalytic performance was determined by the structures of CNTs, in which the single-walled CNT desulfurizer exhibited highest sulfur capacity. With the increase of out diameters of the CNTs, the saturation sulfur capacities decreased.(4) H2S removal by regenerable desulfurizers:Two kinds of desulfurizers were prepared by loading polyethylenimine (PEI) over mesoporous carbon aerogel and hierarchical porous silica. PEI loaded carbon aerogel has a large sulfur capacity, but the regernation performace is poor. This is mainly attributed to the oxidation of H2S to form elemental sulfur and sulfuric acid at edge carbon active sites of the mesoporous carbon.65 wt.% PEI loaded hierarchical porous silica exhibits both large sulfur capacity and excellent H2S cycle removal performance. The sorption of H2S ever PEI loaded porous silica was attributed to the reversible reactions between H2S and amine groups of PEI. The sorption of H2S was predominately determined by the thermodynamic factor rather than the kinetic diffusion of H2S in the PEI films.(5) CO2 capture by carbon based hybrid capture material:A novel high efficiency CO2 capture material was developed by loading PEI over silica-carbon hybrid aerogel with a large porosity. This material demonstrated a particularly high CO2 capture capacity up to 220 mg CO2/g sorbent, which is much higher than that of PEI loaded ordered meosporous silica (140-180 mg CO2/g sorbent). The huge pore volume of the silica-carbon hybrid aerogel will enable it to be loaded large amount of amine active sites, and the residual silica can improve the dispersion of PEI in the aerogels, which enhances the CO2 capture capacity. The sorption of CO2 was determined both by the reaction between CO2 and PEI and by the diffusion of CO2 in the PEI films. At low temperature, the diffusion of CO2 was a limited step and most of PEI was not utilized, resulting in a poor capture performance. At high temperature, the capture capacity was limited by sorption thermodynamic and decreased with increasing the temperature. The developed capture material can be regenerated easily at 110℃, and it exhibits excellent regenerability and stability.