| Though coalbed methane reserves of our country are abundant,the utilization rate of coalbed methane accounts only about a third of extraction volume,most are direct vented.A large number of low concentration coal-bed methane emissions are not only a waste of resources,also can increase the greenhouse effect.Making full use of CH4 and O2 in low concentration coalbed methane and transform them into syngas is one of the effective utilization way.Its essence is the partial oxidation of methane(POM).Because of ordered mesoporous alumina with high specific surface area,ordered-pore structure.and good thermal stability,load of transitional metal mesoporous alumina catalyst has became the research hotspot and applied in the systerm of POM reaction.In this paper,in order to resolve active component sintering and carbon deposition of the supported catalysts,four kinds of Ni-based alumina catalysts prepared by different method on the basis of previous work.The monolithic mesoporous NiO-Al2O3 catalyst prepared by sol-gel method was selected out.A series of mesoporous Al2O3,NiO-Al2O3 catalysts were synthesized for POM reaction to syngas,with isopropanol aluminum as aluminum source,F127 as template agent,nitric acid as acidity regulator,and nickel nitrate as the active component precursor.Physical and chemical properties of catalysts were characterized by means of XRD,BET,TPR,TEM.The impacts of catalysts preparation conditions and reaction process conditions on catalytic performance were studied.The main conclusions were as follows:1.The effect of different preparation methods on the catalyst performanceFour different catalysts were synthesized by impregnation method and sol-gel method respectively,and carried on the BET,XRD,TEM characterization and activity evaluation under the same reaction conditions.It was found that obvious catalytic activity differences appeared on four different kinds of catalysts,and the catalyst prepared by sol-gel method with template agent showed better catalytic activity,higher nickel dispersion and larger specific surface area,more ordered structure and more uniform pore size distribution.2.The influence of catalyst preparation conditions on the catalytic performanceThe specific surface area,pore volume and pore diameter of the monolithic catalyst NiO-Al2O3 decreased with the increase of Ni content.When nickel load reached 30%,may lead to the NiO particles aggregation and uneven distribution in Al2O3.When reaction temperature was less than 600 ?,the CH4 conversion,H2 selectivity and CO selectivity slowed down with the increase of Ni load,and 5%NiO-Al2O3 catalyst showed the best catalytic activity.This phenomenon showed that Ni load was not the more the better.5%NiO-Al2O3 catalyst was calcined under different temperature(400?-800?)in order to study the influence of catalyst calcination temperature on catalytic properties.The results showed that catalyst specific surface area and pore volume gradually decreased,while the interaction between active component and carrier increased with the calcination temperature increasing.But when the calcination temperature was over 600?,catalyst activity decreased.The experimental results showed that the catalyst calcined at 600? presented the best catalytic activity.3.The effect of reaction conditions on the catalytic activityReaction temperature had a great influence on the catalytic activity and product distribution.With the increase of reaction temperature,CH4 conversion,H2 and CO selectivity were also rising.When the temperature was 750?,CH4 conversion rate reached 97%,closed to completely convert.In the temperature range of 400 ? to 600 ?,CH4 conversion rate was increased by a big margin.While in 600 ? to 750 ? temperature range,the increase amplitude of the CH4 conversion rate gradually decreased.The temperature had a greater influence on CO selectivity,a relatively smaller effect on H2 selectivity.The catalytic activity of 5%NiO-Al2O3 calcined at 600? was evaluated catalyst after reduction at different temperature(600?,700?,800?).The results showed that reduction temperature had a great influence on catalytic performance.When the reduction temperature was 600 ?,the catalyst could be completely reduced and showed the best catalytic activity compared with other catalysts.Meanwhile,CH4 conversion rate could reach almost 90%,the selectivity of CO was close to 92%.However,when the reduction temperature was over 600?,catalyst activity decreased insead.4.Catalyst stability testThe catalyst stability of 5%NiO-Al2O3 calcined at 600? was evaluated after reduction at 600?.The results showed that catalyst exhibited efficient catalytic activity and stability after 96 h reaction.CH4 conversion and CO selectivity could reach 89%and 89%,respectively,far exceeding the thermodynamic equilibrium values(600 ?,55%,59%).TEM results showed that after long period of reaction,the particles of Ni were still dispersed uniformly.There was still no obvious aggregation phenomenon,and catalyst pore structure remained orderly rule.All phenomena showed that the catalyst had good resistance to sintering capability and stability. |
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