Preparation and Performance of Three-dimensional Ordered Porous Zinc Oxide Sorbents for Mid-temperature Gas Stream Deep Desulfurization

Author:Li Zuo

Supervisor:sun tong hua


Degree Year:2018





Hydrogen sulfide(H2S)is a normal air pollutant occurs naturally which is highly toxic,corrosive,and flammable in nature.Even though,most H2S comes from various hydrocarbon industrial sources such as coal syngas,crude petroleum,natural gas,and biogas.Ultra low amount of H2S is harmful to human health,environment and industry.Because of the ever increasing efficiency standards required for environmental protection and industrial acceptalble level,sulfur-containing gas must be treated prior to its emission into the atmosphere.Non-reactive sorption of H2S was reported on various metal oxides sorbents in recent decades,especially ZnO,which is the favorable thermodynamics of the sulfurization reaction.However,such processes suffer from low sulfur uptake capacity,sintering,hard to regeneration.To conquer the issues,the ZnO and stabilization agent,promoter were combined to structure the 3D ordered mesoporous and macroporous ZnO based sorbents,with characterization and activity tests,the content of the thesis contains the following parts:1.A series of 3 dimensional(3D)structural mesoporous silica materials,SBA-16,MCM-48 and KIT-6,was synthesized and supported different ZnO loading amount by the incipient wetness method to evaluate the performances on H2S removal.The breakthrough tests showed all the ZnO-loaded sorbents exhibited the H2S removal capacity of bellow 0.1ppm.With the best ZnO loading percentage of 30 wt%on MCM-48 and KIT-6,20 wt%on SBA-16,further increasing ZnO loading caused the decrease of the sorption capacity due to the agglomeration of ZnO.With the largest pores in these 3D arrangement materials,KIT-6 showed the best performance of supported material for ZnO,due to its retained superior physical properties as well as large pore diameter to allow faster gas-solid interaction and huge pore volume to disperse ZnO on the surface of it.Moreover,the apparent activation energy(Ea=21.18 kJ/mol)as well as the predicted results were studied with deactivation model.The results of regression fitting reveal the accurate prediction breakthrough behaviors for the sa.2.Rare earth elements(RE=Ce,La,Sm and Gd)doped ZnO supported on KIT-6 sorbents(RE-ZnO/KIT-6)were synthesized by sol-gel method and their H2S removal performances at medium temperature were tested.The RE dopants(except Ce)significantly enhanced the desulfurization capacity of ZnO/KIT-6 sorbentIt was found that RE could hinder the ZnO crystal ripening during calcination resulted in smaller ZnO particles,enhance the interaction of ZnO and silica matrix to improve the dispersion of active phase on KIT-6.Furthermore,by increasing the outlayer electron density of Zn atom and oxygen transfer ability,the synergistic effect considered to be favorable for RE-ZnO/KIT-6 sulfidation,which resulted in the best desulfurization performance of La-ZnO/KIT-6.H2,O2 and steam in the gas stream could impact the performance of sorbents.H2 and steam inhibited the capture of H2S by ZnO in the sorbents,in the case of La-ZnO/KIT-6,the steam content should control as lower as5 vol%to ensure the desulfurization efficiency and precision.However 1vol%O2 content could possibly enhanced the desulfurization efficiency due to the oxidation catalysis synerginetic effect,while increase the O2concentration to 3 vol%would deacrease the efficiency significantly.3.Three dimensional ordered microporous(3DOM)ZnO-SiO2sorbents with controlled pore sizes and different Zn/Si ratios are successfully created via a polystyrene(PS)colloidal crystal template method,and their performances on H2S removal are systematically investigated.The resulting sorbents possess well-defined 3DOM structures with adjustable pore sizes from 152 to 290 nm when Zn/Si ratio was lower,which showed SiO2 was important to support the 3DOM structure as a stabilization agent.Under 300°C,the desulfurization efficiency was increased with the zinc content at beginning,meant the amount of active phase was crucial when structural parameters were stable.Nevertheless,the performance was declined if the zinc oxides content continueing increasing due to the disordered pore arrangement and decreased surface area.Investigation on different pore diameter 3DOM sorbents indicated the desulfurization efficiency was decreased while increasing the pore size.At last,the pore size of 152 nm prepared with 230 nm PS sphere revealed the best performance.The enhanced impact of La promoter was weaker on macroporous range compared with that of mesoporous range.4.3DOM zinc ferrite and silica composites were prepared by polymethyl methacrylate(PMMA)colloidal templates and applied on the removal of H2S at 550°C.The morphology analysis showed the as-prepared ZnFe2O4-SiO2 materials had a typical 3DOM structure,which was constructed with about 186 nm-sized macropores and 20 nm-sized walls.The 3DOM structure sorbents exhibited remarkable desulfurization performance due to the unique structure features with high surface area,nano zinc ferrite particles,well arranged macropores with interconnected mesopores,which significantly enhances gaseous reactant diffusion during sulfidation compared with those of bulk zinc ferrite sorbent but still slightly lower than that of ZnFe2O4/KIT-6.However,the regeneration of sulfided 3DOM ZnFe2O4-SiO2 was efficient than that of mesoprous or bulk zinc ferrites sorbent due to it accomplished the lower regeneration temperature.Sulfates could be formed during both sulfidation and regeneration process,while the residues were different,with ferrous sulfate in sulfided sample and zinc sulfate in regenerated sample.Due to the residual sulfate was negligible after regeneration,3DOM structure and silica participation enhanced the breakthrough capacity of 50 wt%ZnFe2O4-SiO2 sorbent and maintained higher desulfurization capacities upon multiple sulfidation/regeneration cycles more ascribed to its special structural feature.A lower H2 content,5 vol%,could significantly promote the desulfurization efficiency of 3D-50ZFS due to the reaction between ZnFe2O4 and H2S needs the participation of H2.As the same,the performance would decend if the H2 content was too high due to the ferrious oxides could be reduced easily under high reduction atmosphere.