甲烷在MnOx/SiO_2催化剂上氧化偶联反应的研究

STUDIES OF METHANE OXIDATIVE COUPLING OVER MnO_x/SiO_2 CATALYSTS

本文研究了催化剂MnOx/SiO_2在改进为连续进料反应条件下的催化活性及其催化反应机理.实验结果表明;锰负载量升至10(Wt)%时,甲烷转化率和C_2选择性均最高;进一步增大锰负载量时,催化剂中Mn_3O_4晶相形成,此时催化活性开始下降,说明催化剂中形成的硅酸锰盐与甲烷氧化偶联反应直接关联.反应初期,催化反应活性较高,但C_2选择性较低,反应几小时后,活性有所下降,但C_2选择性提高,最后.活性和选择性趋近稳定.进一步研究表明:催化剂中Mn^(2+)配位环境从反应前的八面体中介场转变为四面体强场,而且催化剂中Mn^(2+)浓度增大约100倍,说明Mn^(2+)浓度提高有利C_2选择性提高.反应稳定后的催化剂表面观察到Carbide(282.9 eV)和CHx(a)(X=0-3)(284.5 eV)碳物种存在,说明甲烷在催化剂表面吸附并分解:CH_4(g)→CH_3(a)→一CH_2(a)→→→Carbide,并讨论了甲烷通过CHx(a)在催化剂表面上迁移,直接偶联生成乙烯(X=2)和乙烷(X=3)的可能性.

Methane is the main component of natural gas. The direct oxidative conversion of methane into higher hydrocarbons was a challenge and the attention of many researchers. In this paper , the catalytic activities and the reaction mechanism for methane oxidative coupling over MnOx/ SiO2 catalysts under continuous flow conditions were studied by GC, ESR, IR, and XRD. The experimental conditions were chosen as follow: weight of catalyst = 500mg, particle of catalyst= 40-60 mesh, reaction temperature= 1073 K, pressure of reaction = 0.1 MPa. He : CH4 : O2 = 383.8 : 282. 0 : 94.2, the total flow to the reactor= 50.0 mL/ min. The catalysts used were prepared by impregnating SiO2 with Mn (CH3COO)2 solution, drying the solution at 393 K and finally calcinating in air at 1073 K for 10 h. The experimental results showed that the conversion of methane and C2 selectivity reached maximum when the amount of manganese oxide loaded on SiO2 reached 10 wt% , Further increase of manganese oxide would result in the formation of crystalline Mn3O4 and the decrease of methane conversion and C2 selectivity. The results indicated the catalytic activity was correlated to the manganese silicate formed at low loading (< 10 wt%). During the beginning several hours of reaction over the 10 wt% MnOx/SiO2 catalyst, the conversion of methane decreased and C2selectivity increased slightly before becoming stable. The intensity of Mn2+ESR signal in the 10 wt% MnOx/ SiO2catalyst after reaction (Sample B) was about 100 time bigger compared with that before reaction (Sample A), and the shape of Mn2+ ESR signal changed from six resolved peaks of mediate octahedral ligand field structure to one broad peak of strong tetrahedral ligand field structure. XPS studies of Mn (2p) peaks showed the typical shake-up peaks for Mn2+ at about 5 eV higher binding energy of Mn (2p3 / 2) and Mn2pl / 2 on the surface of Sample B. These results indicated the increase of Mn2+ on the surface of catalyst would improve the C2 selectivity. After Sample B was treated in oxygen atmosphere at 1073 K for two hours (Sample C), there was no change in shape and intensity of the Mn2+ ESR signal observed,implying the Mn2+ centers in'Sample B were very stable and could not be easily reoxidized. Two possible mechanisms for catalytic recycling were proposed: (1) Mn2+ centers experience no valence change during the catalytic reaction, and the function of the catalyst was just to provide surface orbitals with suitable energy and symmetry for the reaction; (2) Mars-Van Krevelen type mechansim proposed by Sofranko et al., although the redox reaction of Mn2+ ions was not observed by ESR under our experimental conditions. Two carbon species, carbide at 282.9 eV and CHx(X = 0-3) at 284.5 eV were observed on the surface of Sample B by XPS studies of C(ls) peaks, indicating the existence of the following process: CH4 (g)→CH3 (a)→CH2(a)→→→carbide The possible mechanism of coupling two CHx(a) groups to form ethylene (X = 2) and ethane (X = 3) on the surface was also discussed.