The effect of Nb₂O₅ and ZrO₂ additions on the behaviour of Li/MgO and Li/Na/MgO catalysts for the oxidative coupling of methane

Abstract

Incorporation of Nb,O, or ZrO, into both Li/MgO and Li/Na/MgO systems produced ternary and quaternary catalysts, respectively, capable of attaining optimal C, yields and selectivities at lower temperatures relative to the unprornoted materials. The degree of enhancement effected by these metal oxide additives was compared to that produced by L,i/MgO and Li/Na/MgO catalysts promoted with SnO, or Co30,. At reaction temperatures < 70O0C, the LilCoiMgO ternary system showed marked differences in behaviour compared to the other ternary catalysts tested. This was particularly evident in the variation in C, selectivity with time on stream during ageing studies of (i) untreated materials, (ii) CO, et (iii) des materiaux dosCs periodiquement avec du CHCI,.Keywords: oxidative coupling, methane, LiiMgO, Li/Na/MgO, metal oxide additives he oxidative coupling of methane to higher hydro-T carbons, principally ethylene, represents one of the alternate possibilities to that of well-established technology (Van der Wicle, 1986) whereby methane is converted to ethylene via syngas and methanol. The initial step in the latter process, involving steam reforming of methane to CO + HZ, requires high energy input at high temperature and hence proves to be considerably cost intensive. The production of higher hydrocarbons directly from methane is therefore highly desirable. Furthermore, with the advent of large worldwide reserves of natural gas which are often located in remote areas, direct on-site conversion of methane (the principal component) to light transportable fuels (derived from C2H4) is necessary to effectively offset current high transportation costs associated with shipping natural gas as a liquid or via pipelines.These observations have fuelled the current intensive research campaign aimed at finding catalysts capable of oxidatively coupling methane to C2 hydrocarbons with associated high yield and selectivity. Since the pioneering research (Keller and Bhasin, 1982) was carried out, a wide variety of promising catalytic systems have been developed including: Na-promoted PbO/y-A1203 (Bytyn and Baerns, 1986). rare earth oxides such as Sm203 and Dy203 (Otsuka et al., 1985), NaMn04/Mg0 (Sofranko et al., 1987), and Li/ZnO (Matsuura et al., 1986). Since its emergence as one of the most active catalysts screened in this area (Ito et al., 1985), Li/MgO has remained at the forefront of the continuing research in this field. Optimisation of the catalytic potential of the material for the coupling process has centred around improving its activity (at lower reaction temperatures) and in particular its stability on stream. At the relatively high reaction temperature of 780°C required for optimal performance of the catalyst, deactivation occurs due to a progressive reduction in the number of active sites corresponding to a gradual loss of Li as volatile LiOH. A previous study from this laboratory (Korf et al., 1989) showed that incorporation of oxides of Tb, Dy, Ti, V, Co, Ni, or Sn into Li/MgO resulted not only in a lowering of re...

Solid state aspects of oxidation catalysis

“…Korf et al [151] and McNamara et al [152,153] published a series of papers concerning the influence of dopants on the behaviour of Li/MgO catalysts, but did not try to interpret the results in terms of the defect chemistry and the influence of the dopants thereon.…”

Li/MgO with spin sensors as catalyst for the oxidative coupling of methane

“…To improve the catalytic performance of Li/MgO, it was doped with numerous additional metal oxides [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. In many cases an improved stability and/or higher selectivity was found, however, deactivation of these catalysts was often retarded, but still observed.…”