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Ind. Eng. Chem. Res. 1997, 36, 3594-3601
Oxidative Coupling of Methane over a Sr-Promoted La2O3 Catalyst Supported on a Low Surface Area Porous Catalyst Carrier Vasant R. Choudhary,* Balu S. Uphade, and Shafeek A. R. Mulla Chemical Engineering Division, National Chemical Laboratory, Pune 411 008, India
Oxidative coupling of methane (OCM) to higher hydrocarbons over Sr-promoted La2O3 supported on commercial low surface area porous catalyst carriers (containing mainly alumina and silica) at 800 and 850 °C and a space velocity of 102 000 cm3‚g-1‚h-1 has been thoroughly investigated. Effects of support, catalyst particle size, linear gas velocity (at the same space velocity), Sr/La ratio, CH4/O2 ratio in the feed, and catalyst dilution by inert solid particles on the conversion, yield, or selectivity and product ratios (C2H4/C2H6 and CO/CO2) in the OCM process have been studied. The catalysts have been characterized for their basicity, acidity, and oxygen chemisorption by the TPD of CO2, ammonia, and oxygen, respectively, from 50 to 950 °C and also characterized for their surface area. The supported catalysts showed better performance than the unsupported one. The best OCM results (obtained over Sr-La2O3/SA-5205 with a Sr/La ratio of 0.3 at a space velocity of 102 000 cm3‚g-1‚h-1) are 30.1% CH4 conversion with 65.6% selectivity for C2+ (or 19.7% C2+-yield) at 800 °C (CH4/O2 ) 4.0) and 12.8% CH4 conversion with 85.1% selectivity for C2+ (or 10.9% C2+-yield) at 850 °C (CH4/O2 ) 16.0). The basicity is strongly influenced by the Sr/La ratio; the supported catalysts showed the best performance for their Sr/La ratio of about 0.3. The methane/O2 ratio also showed a strong influence on the OCM process. However, the influence of linear gas velocity and particle size is found to be small; it results mainly from the temperature gradient in the catalyst. The catalyst dilution has little or no effect on the conversion and selectivity. However, it has beneficial effects for achieving a higher C2H4/C2H6 ratio and also for reducing the hazardous nature of the OCM process because of the coupling of the exothermic oxidative conversion reactions and the endothermic thermal cracking reactions and also due to the increased heat transfer area. Introduction In order to convert methane into higher hydrocarbons for its effective utilization, worldwide efforts have been made during the last 13-15 years for the oxidative coupling of methane (OCM) to ethane/ethylene over a number of basic solid catalysts (Anderson, 1989; Lee and Oyama, 1988; Hutchings et al., 1989; Lunsford, 1990, 1995). The OCM process occurs at high temperatures (750-900 °C). Hence, for avoiding catalyst deactivation due to the loss of volatile active components, efforts are concentrated on developing the OCM catalyst containing nonvolatile promoters or active components, such as Lapromoted MgO (Choudhary et al., 1989a; Chaudhari, 1992), La-promoted CaO (Choudhary et al., 1989b; Becker and Baerns, 1991), alkaline earth-promoted La2O3, and other rare earth oxides (DeBoy and Hicks, 1988a,b; Yamagata et al., 1990; Zhang et al., 1990; Yamashita et al., 1991; Conway et al., 1992; Rane, 1992; Choudhary et al., 1996a). These catalysts show good activity/selectivity and high thermal stability and hence long life in the OCM process. However, for converting these laboratory catalysts into commercial ones, it is preferable to support them on a porous matrix (i.e. catalyst carriers or supports) for providing high mechanical strength and resistance to abrasion to avoid a high-pressure drop across the catalyst bed and also for increasing the dispersion of the catalyst. Our earlier studies showed that when Li-promoted MgO (Choudhary et al., 1996b) and La-promoted MgO (Choudhary et al., 1996c) OCM catalysts [which showed high activity/selectivity, high productivity, and long life * To whom all correspondence should be addressed. Telephone: (+91)212-333941. Fax: (+91)212-333941/330233. E-mail:
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(Choudhary et al., 1991a and 1989a)] were supported on commonly used catalyst carriers, their activity and selectivity were reduced drastically. The reduction in the catalytic activity and selectivity was attributed to the consumption of the active catalyst components (i.e. promoter) by their chemical interactions with the active components (viz. silica and alumina) of the catalyst carriers during the high-temperature (900 °C) calcination of the supported catalysts. However, in the case of La2O3 catalyst (Choudhary et al. 1996c), its activity and selectivity are influenced only to a small extent of supporting it on different catalyst carriers. Lanthana, after promotion with strontium, shows lower activity but higher selectivity in the OCM process (Conway et al., 1992). It is, therefore, interesting to study the influence of support on the catalytic activity/selectivity of La2O3 promoted by strontium for developing a supported OCM catalyst having high activity/selectivity, high productivity, and also high thermal stability or long life. The present work was undertaken with the above objective. The OCM process over Sr-promoted La2O3 catalyst supported on low surface area porous silicaalumina catalyst carriers (SA-5205 and SA-5218, obtained from M/S Norton Co., Akron) with different Sr/ La ratios, support particle sizes, and catalyst dilutions has been thoroughly investigated. Experimental Section Supported Sr-promoted La2O3 catalysts (Sr-La2O3/SA5205 and Sr-La2O3/SA-5218 with Sr-La2O3 loadings of 16 ( 1.5 and 13 ( 1.0 wt %, respectively) were prepared by impregnating 1 mm size particles of commercial low surface area porous inert catalyst carriers (viz. SA-5205 and SA-5218, obtained from Norton Co.) by the active catalyst mass. The impregnation of mixed nitrates of © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3595 Table 1. Chemical Composition and Surface Properties of Commercial SA-5205 and SA-5218 Supports Chemical composition (wt %) SiO2 Al2O3 Fe2O3 alkali and alkaline earth oxides surface area (m2‚g-1) pore volume (cm3‚g-1) porosity (%) average pore diameter (µm)
SA-5205
SA-5218
11.8 86.1 0.2 1.8
