Synthesis Of Nitrogen-doped Porous Carbons Via Melt Salt Method And Their Application In H₂S Removal

Tremendous attention has been paid to the removal of hydrogen sulfide(H2S)due to its malodorous,toxic and corrosive characteristics.Dry desulfurization at room temperature has been proven to be one effective technology for dilute H2S-containing gas streams,which possesses the advantage of low energy consumption,easy operation,and high desulfurization precision.The key for H2S removal technology is the design and preparation of efficient desulfurizers.Currently the H2S removal capacities and recycle performance of the desulfurizers should be improved.What is more,the interrelationship between physicochemical properties and desulfurization performance should be disclosed.Considering the merits of N-doped porous carbons such as developed porosity,basic carbon surface and good stability,herein,we have prepared various N-doped porous carbon desulfurizers with tunable surface chemistry and pore structures via melt salt method.The relationship between physicochemical properties of N-doped porous carbons and desulfurization performance are revealed and clarified.The main contents and results are summarized as follows:(1)Aiming to address the issue of the low adsorption capacity of porous carbons,adsorption sites are introduced to porous carbon surface.In the synthesis,N-doped microporous carbons are prepared via a molten salt strategy using LiCl-KCl as molten salt,melamine as nitrogen sorce and D-glucose as carbon source,while N-doped mesoporous carbons were synthesized by a soft-template approach using triblock copolymer F127 as template,dicyandiamide as nitrogen source and phloroglucinol-formaldehyde as carbon source.The adsorption performance of H2S for the carbons is evaluated at water-free and oxidation-free condition,and the results indicate that the breakthrough sulfur capacity of N-doped microporous carbons is 3.9 times higher than that of non-doped microporous carbons,and that the breakthrough sulfur capacity of N-doped mesoporous carbons are 5.0 times higher than those of non-doped mesoporous carbons.Density functional theory calculation results suggest that both pyridinic and pyrrolic nitrogen in N-doped porous carbons contribute to the anchor sites for H2S molecules,leading to the improved H2S adsorption capacities.(2)Considering the inert surface of porous carbons,the basic nitrogen-containing functional groups were introduced to porous carbons to increase the surface basicity.A series of N-doped microporous carbons are prepared via salt melt synthesis using KCl-ZnCl2 as molten salt,melamine as nitrogen sorce and D-glucose as carbon source.These N-doped microporous carbons are used as catalysts for H2S catalytic oxidation at water and oxidation-containing condition at room temperature.The results indicate that N-doped microporous carbons can be used as highly efficient catalysts for H2S catalytic oxidation to elemental sulfur.The breakthrough capacity and saturation sulfur capacity are as high as 0.94 and 1.10 g g-1,respectively.Meanwhile,these catalysts can be regenerated easily,exhibiting excellent cycling stability.(3)In order to enhance the reactant diffusion as well as increase the storehouse for H2S oxidation products,N-doped mesoporous carbons with high nitrogen content(nitrogen content:>20 wt%)and developed porosity have been prepared via salt melt synthesis.In the preparation,KCl-ZnCl2 is used as molten salt,adenine is used as both nitrogen sorce and carbon source.The porosity of the carbons can be tuned(surface area:880?3153 m2 g-1;pore volume:0.668?4.75 cm3 g-1)by changing KCl mole percentage in KCl-ZnCl2.When used as catalysts for H2S catalytic oxidation under water and oxidation-containing condition at room temperature,high breakthrough sulfur(1.82 g g-1)and saturation sulfur capacity(2.01 g g-1)are achieved.The relationship between the pore structure and H2S catalytic performance is deduced in detail,revealing that hierarchical pores with macro-meso-micropore structure are propitious to H2S catalytic oxidation.(4)In order to enhance the mass transfer as well as improve the utilization of the inner porosity,mesopores are introduced and nanoscale thickness is shorten.N-doped mesoporous carbon nanosheets with a thickness of?20 nm are prepared by pyrolysis microporous MOFs in a molten salt medium.The evolution of N-doped mesoporous carbon nanosheets from three-dimensional MOFs in molten salt is proposed as a sequential dissolution followed by salt template confining growth mechanism.Benefiting from the mesoporosity and nanosheet structure merits,the breakthrough sulfur capacity for N-doped mesoporous carbon nanosheets is 10.6 times to that of N-doped microporous carbon obtained by direct pyrolysis of MOFs.After impregnating with 20 wt%Na2CO3,the breakthrough sulfur capacity for N-doped mesoporous carbon nanosheets is 21.1 times to that of N-doped microporous carbons.