Structural Modulation Of Mesoporous Alumina And Its Derived Nickel-Based Catalysts For CO Methanation

As an important route of the clean coal technologies,the production of the s ynthetic natural gas(SNG)via syngas has attracted much attention.Among different un its,the highly exothermic CO methanation reaction(CO+3H₂?CH₄+H₂O,?H₂98K=-206.1kJ mol-1)is the key sector.Indeed,the fixed-bed technology with seriesadiaba tic reactors has been successfully industrialized for the production of SNG.Howeve r,the series arrangement of the reactors requires the catalyst to be sufficiently active at a lower reaction temperature and highly resistant to the sintering at a higher tempera ture.As a result of these contradictory requirements,the development of a high-perf ormance catalyst for the CO methanation is still challengeable.Based on the comprehensive analysis of the literature reports on the CO methanati on technologies and Ni-based catalysts,this dissertation targets the design and synth esis of both active and highly sintering-resistant Ni-based catalyst for the CO methanati on with the assembly of ordered mesoporous alumina(OMA).Thus,the method for the controlled synthesis of OMA ? Ni-OMA was explored,and highly long-range O MA ? Ni-OMA was prepared with the developed method.Moreover,the relationship b etween the structural properties of Ni/(-)OMA and their catalytic performance fo r the CO methanation was rigorously revealed.With the developed strategies for synthesizing OMA,the methods for improving the Ni dispersion and the introduction of second metallic Co catalyst were systematically studied.The experimental and main conclusions are summarized as follows:(1)Highly long-range ordered OMA was prepared via alpha-amino acid assisted solvent evaporati on induced self-assembly(EISA)method.The mesoporous order and textural propertie s of OMA were significantly influenced by the type,molecular size,and amount of alpha-amino acid.The addition of L-lysine,L-arginine and L-histidine with higher pKa value was not conducive to the synthesis of highly ordered OMA.For the remaining alpha-ami no acid,the highly long-range ordered OMA could be obtained at the alpha-amino acid to aluminum precursor molar ratios between 0.075 and 0.15.With increasing the size of alpha-amino acid calculated from the optimized geometry by using Gaussian 09 sof tware,the average pore diameter of OMAs was deceased while the pore wall thick ness of OMAs was increased.Based on the above understanding,Ni-OMA was prepared via one-pot EISA method by using hydrochloric a cid,hydrochloric acid and alpha-amino acid,and nitric acid as the pH adjuster,res pectively.The results of the CO methanation showed that Ni-OMA prepared in the nitr ic acid system exhibited higher catalytic performance.By correlating the results of chara cterization and the CO methanation performance over Ni-OMA catalysts,the cataly tic activity and stability of Ni-OMA depend on the long-range order of OMA,the size an d distribution of Ni particles,the dispersion of Ni and the interaction between Ni and a lumina.(2)The OMA precursors prepared by one-pot EISA method were pretreated at differen t temperatures(T=60?,90?,120?,150?,180?)for 12 h,and then ground together wit h nickel precursors for 30 min,a series of Ni/OMA-T catalysts with NiO loading of 15 wt% were prepared,which T was the pretreatment temperature.The pretreatment tem peratures significantly influenced the porous structure of the OMA precursor,the co ntent of P123 and the amount of surface hydroxyl groups.With the increase of pretre atment temperature,the mesoporous order of OMA precursor was increased,and both BET surface area and pore volume were increased.However,the the contents of template P123 and surface hydroxyl were decreased.The low-temperature activity an d high-temperature stability of Ni/OMA-T for the CO methanation could be signif icantly regulated by simply changing the pretreatment temperatures of OMA precurs ors.The optimal catalyst of 15Ni/OMA-120 showed a high activity with CH₄ forma tion rate of 89.2 mol kgcat-1 h-1 at a temperature of as low as 300 ? and a long-term sta bility for a time-on-stream of 150 h without an observable deactivation under the conditions of 600? and an extremely high GHSV of 120000 mL g-1 h-1,which was mainly due to the highly ordered mesoporous of OMA,high dispersion and reductio n degree of Ni,and strong interaction between Ni and OMA.(3)The Co-Ni bimetallic catalysts were synthesized via post-impregnating the cobalt precursor within the mesopo rous channel of Ni-embedded ordered mesoporous alumina(Ni-OMA).The low-temperatu re activity and high-temperature stability of Co/Ni-OMA for the CO methanation we re significantly regulated by easily tuning the ratio of free Co and confined Ni sp ecies.The optimal catalyst of 8Co/15Ni-OMA showed a high activity with the CH₄ formation rate of 126 mol-1 kgcat-1 h-1 at a temperature of as low as 300? and a long-term durability for a time-on-stream of 200 h without an observable deactivation under the conditions of 600? and an extremely hi gh GHSV of 180000 mL-1 g-1 h-1.Kinetics results reveal that the apparent activation ene rgy of the CO methanation over 8Co/15Ni-OMA(100.2 kJ-1 mol-1)was clearly low er than that over 15Ni-OMA(124.0 kJ-1 mol'1)or 15Co-OMA(131.8 kJ-1 mol-1).In the absence of mass transport and heat transfer limitations,three microkinetic models were developed following the H-assisted CO dissociation and Langmuir-Hinshelwood mecha nism,which the H-assisted CO dissociation,the hydrogenation of surface carbon species(C^*)or surface CH₃^* species are proposed as the rate-determining step,respect ively.The kinetics behaviors over 15Ni-OMA and 8Co/15Ni-OMA are matched well wit h all of the kinetic models,indicating the same elementary sequence and rate-dete rmining step.In the case of 15Co/OMA,the CH₄ formation rate was predicted very well b y the kinetics models derived from the stepwise hydrogenation of surface carbon spec ies as the rate-determining step,and the kinetics model based on the H-assisted CO dissociation as the rate-determining step could be ruled out,indicating that the rate for the H-assisted CO dissociation rate is faster than that of the following stepwise hydr ogenation.Based on the discrimination of different kinetics models,Ni species confine d within OMA matrix were proposed as the dominant active sites for catalyzing the CO methanation,while the post-impregnated Co was acted as a promoter for the H-assisted CO dissociation.As a result,an enhanced low-temperature activity was achie ved over the optimal 8Co/15Ni-OMA catalyst.(4)The OMA with confined Ni in the pore wall(Ni-OMA)was prepared via the one-pot EISA metho d.By using the incipient impregnation method,different amounts of free Ni were load ed over Ni-OMA(Ni/Ni-OMA)at a fixed total NiO content of 15wt%.Characterization results confirmed the formation of well-structured Ni-OMA,and the ordered struc ture was still well preserved even after impregnating NiO at a content of as high as 12 wt%.The catalysts were evaluated for the CO methanation as a model reaction under varied conditions.Importantly,the activity and stability of Ni/Ni-OMA for the titled reaction were significantly regulated by simply changing the ratio of the confined to free Ni.Over the optimum catalyst of NiO(2 wt%)/NiO(13wt%)-OMA,the high activit y at a temperature of as low as 300? was achieved with with the CH₄ formation r ate of 94.5 mol-1 kgcat-1 h-1 while a long-term stability for a time-on-stream of 400 h was confirmed without observable deactivation under the conditions of 600 ? and an extremely high GHSV of 120000 mL g-1 h-1.The results were well explained as the integrated merits of the free Ni for a high dispersion and the confined Ni in OMA for the anti-sintering property.