2096
Ind. Eng. Chem. Res. 1997, 36, 2096-2100
Oxidative Coupling of Methane over Supported La2O3 and La-Promoted MgO Catalysts: Influence of Catalyst-Support Interactions Vasant R. Choudhary,* Shafeek A. R. Mulla, and Balu S. Uphade Chemical Engineering Division, National Chemical Laboratory, Pune 411 008, India
Methane-to-C2-hydrocarbon conversion activity and selectivity (or yield) of MgO and La-promoted MgO catalysts in the oxidative coupling of methane and strong basicity of the catalysts are decreased appreciably when these catalysts are deposited on commonly used commercial low surface area porous catalyst carriers containing Al2O3, SiO2, SiC, or ZrO2 + HfO2 as the main components. The decrease in the strong basicity and catalytic activity/selectivity or yield is mostly due to strong chemical interactions between the active catalyst component (viz. MgO and La2O3) and the reactive components of the catalyst support (viz. Al2O3 and SiO2), resulting in the formation of catalytically inactive binary metal oxides on the support surface. However, the influence of support on the activity/selectivity of La2O3 is relatively very small, and also the chemical interactions of La2O3 with the supports (except that containing a high concentration of SiO2) are almost absent. The catalyst-support interactions are thus found to be strongly dependent upon the nature (chemical composition) of both catalyst and support. For developing better supported catalysts for the oxidative coupling of methane, supported La2O3 with some promoters shows high promise. Introduction La-promoted MgO (Choudhary et al., 1989a,b) shows high activity and selectivity, very high productivity (or space time yield), and high stability or long life in the oxidative coupling of methane (OCM) to C2-hydrocarbons, which is a process of great practical importance. The individual components of this catalyst, La2O3 (Otsuka et al., 1985; Lin et al., 1986; Taylor and Schrader, 1991; Choudhary and Rane, 1991, 1994), and MgO (Choudhary et al., 1994), also show good activity and selectivity in the OCM process. In industrial practice, active catalysts are generally deposited on catalyst carriers (or porous supports), which provide to the catalyst a porous matrix having a high mechanical strength (i.e., high crushing strength and also high resistance to attrition loss), a high thermal/hydrothermal stability, and also a low-pressure drop across the reactor bed. These are essential features apart from the activity, selectivity, and productivity of a commercial catalyst. Deposition of catalyst on support may also lead to a better dispersion of the catalyst, increasing its surface area. It is, therefore, interesting to study the influence of commonly used catalyst carriers on the performance of the MgO, La2O3, and La-promoted MgO catalysts in the OCM process. Since these OCM catalysts are prepared (or thermally treated) and operated at high tempertures (800-900 °C), it is also interesting to know chemical interactions between the active catalyst component(s) and the reactive components of different supports. Although strong metal-support interactions (commonly known as SMSI effect) have been extensively studied for supported metal catalysts (Tauster, 1987; Haller and Resasco, 1989), the information on metal oxide-support interactions is scarce (Stone, 1990). The present investigation was undertaken for the above purpose. In this study, influence of the deposition of MgO, La2O3, and La-promoted MgO over commonly * To whom all correspondence should be addressed. S0888-5885(96)00318-1 CCC: $14.00
used supports (low surface area porous catalyst carriers such as silica, silica-alumina, silicon carbide, and zirconia, used for supporting oxidation catalysts) on their bulk and surface properties (viz. crystalline phases, surface area, surface basicity, and oxygen adsorption) and catalytic activity/selectivity in the OCM process has been thoroughly investigated. Experimental Section Unsupported MgO(I) and MgO(II) catalysts were prepared using magnesium acetate and magnesium nitrate, respectively, as precursors. Unsupported La2O3 and La-promoted MgO (La/Mg mole ratio ) 0.1) were prepared using lanthanum nitrate and mixed lanthanum nitrate and magnesium nirate, respectively, as precursors. The precursors were ground with distilled water sufficient to form a thick paste and dried at 110 °C for 12 h. The dried catalyst mass was decomposed at 600 °C for 6 h in static air, pressed binder-free, crushed to 22-30 mesh size particles, and calcined at 900 °C for 5 h in static air. Supported MgO, La2O3 and La-promoted MgO (Tables 1-3) catalysts were prepared by impregnating commonly used low surface area porous supports (SA5552, SC5532, SS5231, and SZ5564, obtained from Norton Co., Worcester, MA), crushed to 22-30 mesh size particles, with an aqueous solution containing the same precursors as used in the preparation of the unsupported catalysts, by the incipient wetness impregnation technique, drying at 110 °C for 12 h, and calcining in static air at 900 °C for 5 h. The loading of MgO, La2O3, and La-MgO in the supported catalysts was 5 ( 0.2, 12 ( 0.5, and 17 ( 0.5 wt %, respectively. The physiochemical properties of the SA5552, SC5532, SS5231, and SZ5564 supports are as follows: surface area (m2‚g-1), 0.33, 0.10, 0.22, and 0.10, respectively; porosity (%), 59, 45, 35, and 45, respectively; chemical composition, 93.1% Al2O3, 5.6% SiO2, and
