Preparation And Methanation Performance Of Mo-based Sulfur-resistant Catalyst

Coal to natural gas utilizes the abundant coal resources in our country to produceclean fuel, which has important significance in mitigating the energy crises.Methanation reaction is the key step in the coal to natural gas process. Sulfur-resistantmethanation reaction can be carried out without the need of deep desulfidationprocess, which makes the process simplified and then the cost decreased. Thedevelopment of highly active sulfur resistant methanation catalyst is very importantfor methanation process. Therefore, in this paper we studied the effects of preparationof Mo-based catalyst and reaction conditions on methanation performance.Preliminary discussions on the active sites for methanation reaction and thestructure-activity relationship were done, which will lay the basis and guidance forcatalyst design and process optimization.The effects of precursor and MoO3content on sulfur-resistant methanationperformance of the Mo-based catalysts were investigated. Mo sulfides and oxides areall have methanation activity. Also, the amorphous MoS2phase is more active formethanation than the highly crystalline MoS2phase. The methanation activity ofMoO3/Al2O3catalyst reaches a maximum CO conversion at25%MoO3loading,corresponding to the monolayer saturated coverage of4.04Mo/nm2on the Al2O3support with the MoO3in the form of amorphous state. It was found that thecrystalline MoO3and Al2(MoO4)3are formed at MoO3loading larger than30%.As a promoter, Co, Ni or La improved the methanation activity of15%MoO3/Al2O3catalyst to different extent. When the concentration of H2S inreaction gas is0.2%, all three additives improve CO conversion on the occasion ofMoO3loading less than the monolayer coverage with Co and Ni perform better thanLa. The addition of La did not affect the selectivity of the products, but addition of Coor Ni made the selectivity of CH4decreased slightly. But CO conversion decreasedslightly with adding Co or Ni on the catalyst with MoO3loading larger than thesaturated monolayer coverage. When the concentration of H2S in reaction gas is1.2%,additive Co and Ni are conducive to improve the methanation activity no matter whatthe MoO3loading. Similar improvement effect was also found by addition of Co, Nior La to15%MoO3/25%CeO2-Al2O3catalyst. Except the catalyst with Ni promoter, the catalysts with other promoters achieved good stability. The activity deteriorationwith time on NiMoCeAl catalyst can be attributed to the more serious carbondeposition.The catalyst supported on CeO2-Al2O3, MgO-Al2O3, TiO2-Al2O3or ZrO2-Al2O3composite oxides, respectively, showed different methanation performances. Thecatalyst on CeO2-Al2O3shows the highest methanation activity among the testedcatalysts. Also, the catalyst using ceria-alumina composite support prepared bydeposition-precipitation method achieves the best sulfur-resistant methanation activitydue to its smaller ceria particle size, better ceria dispersion and weak interactionbetween ceria-alumina, but its stability is not so good. The catalyst usingco-precipitation CeO2-Al2O3as support has better stability.On catalysts of25%MoO3/Al2O3and5%CoO-15%MoO3/25%CeO2-Al2O3,the effects of reaction parameters on the methanation activity were studied. With theincrease of space velocity, CO conversion decreased. The increased reactiontemperature and H2/CO ratio are conducive to improve the methanation activity.Adding CH4has little effect on CO conversion. Addition of H2O promoted the watergas shift reaction but inhibited the methanation reactions. The presence of waterresults in the loss of the active sites of methanation reaction, leading to partiallyirreversible deactivation of the catalyst. The presence of CO2inhibited the formationof methane. Increase of the H2S concentration increases the methanation activity ofcatalyst CoMoCeAl, but it had little effect on Mo/Al catalyst. In high H2Sconcentration (1.2%), additive Co protected the active Mo species from beingpoisoned by H2O. The deactivation caused by adding10%H2O or10%CO2isreversible on Co-Mo/CeAl catalyst. However, the protective effect of additive Co isnot significant at low H2S concentration as0.2%.A continuous300h stability test on the optimized catalyst was carried out andthe catalyst deactivation at reaction temperature lower than600°C is negligible,indicating a good stability of the catalyst. But the catalyst activity and stabilitydeteriorate with time when the reaction temperature was raised as high as650°C.