| Directly emptying underground recovered low concentration coalbed methane(5%-35%,volume fraction)due to permeation of air is not only a waste of resources,but also a cause of serious air pollution.To take full advantage of CH4 and O2 contained in the low concentration coalbed methane,the efficient transformation of low concentration coalbed methane to syngas process was proposed.The reaction system is partial oxidation of methane(POM)essentially.To solve sintering of active component and carbon deposition problems for load type catalysts,M@Si02 catalyst with core-shell structure was developed,properties were characterized by means of XRD,TPR,BET,TEM and catalytic performance evaluation,the prepared conditions and reaction conditions of partial oxidation of methane were optimized.1.Nano-sized metal particles were prepared by coprecipitation method,thesize of nanoparticles can be adjusted by controlling the calcination temperatures.The higher the preparation temperatures were,the larger sizes of nanoparticles were.Metal oxide particles with sizes of 8-17 nm were prepared at the temperatures of 350?-600? respectively.Ni@Si02 catalysts coated with Si02 shell structure were prepared using ethyl orthosilicate hydrolysis condensation in alkaline environment.The diameters of three kinds of catalysts reduced(Ni-350@SiO2?Ni-500@SiO2?Ni-600@SiO2)were 44.67 nm?44.82 nm?46.52 nm respectively.The thickness of the SiO2 layer is about 20 nm,diameter of pore in SiO2 shell is about 5 nm.Nanoparticles tend to aggregation during catalyst preparation process,monodispersity of particles should be solved first.The larger Ni nuclear diameter was,the higher the reduction temperature was.The surface pretreatment of the nanoparticles effected interation forces between Ni nuclear and SiO2 shell,which affected the reduction temperatures of the catalysts sequentially.2.The conversion of CH4 and the selectivity of CO and H2 for Ni@SiO2 catalyst were 95%?93%?80%respectively at reaction temperature of 750? and space velocity of 5×104ml·g-1·h-1,while those were 85%?90%?68%respectively for traditional load type catalyst Ni/SiO2,it was obvious that the performance of core-shell type catalyst was better than load type catalyst,because the reaction occured on the entire surface of the active component for core-shell type catalyst.It is also noticed that the catalytic performance of traditional load type catalyst had a significant decline as the reaction time prolonged.Whilecore-shell type catalyst showed excellent heat stability due to hindering effect of SiO2.3.The smaller sizes of the particles were,the higher catalytic activitives were.Ni@SiO2 catalysts calcinated at 350? show higher CH4 conversion ratio and H2/CO ratio,but carbon deposition was more serious on the catalyst surface area.Catalysts whose sizes of Ni particle is less than 10nm is unfavorable for POM reaction.Carbon deposition was associated with NiO which active component was oxidized to,formation of NiO is one of the important factors that lead to catalyst deactivation.It is supposed that there was dynamic balance between Ni and NiO on the surface of the active component,the proportion of two kinds of Ni species states influenced CH4 conversion and product selectivity.Particle size of Ni nuclear and the the hole structure in SiO2 layer affected the proportion of the two kinds of Ni species states,which then influenced the performance of catalysts further.The catalytic activity can be improved by added additives(ZrO2?CeO2?CoO),ZrO2?CeO2 can inhibit growth up of particles and provide lattice oxygen which prevented the oxidation of the metal Ni,which inhibited the carbon deposition.Ni/ZrO2@SiO2 with nickel and zirconium ratio of 1:1 showed best catalytic activity.4.When the reaction temperatures were higher and space velocities were smaller,the catalysts activitives were better.For Ni/ZrO2@SiO2 catalyst(Ni/Zr=1:1),CH4 convered completely closely at 750?,CO selectivity and H2 selectivity were about 90%,85%,H2/CO ratio was 1.8 at space velocity of 5×104 ml·g-1·n-1.CH4 conversion had a downward trend with the increase of space velocity,when reaction temperature was 750?,CH4 convered completely closely at space velocity of 5×104 ml·g-1·h-1,CO selectivity and H2 selectivity were about 92%,80%.Moreover,space velocity had more influence on H2 selectivity than CO selectivity. |
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