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Ind. Eng. Chem. Res. 1998, 37, 2142-2147
Oxidative Coupling of Methane over SrO Deposited on Different Commercial Supports Precoated with La2O3 V. R. Choudhary,* S. A. R. Mulla, and B. S. Uphade Chemical Engineering Division, National Chemical Laboratory, Pune 411 008, India
The influence of catalyst carrier or support (with different chemical compositions and surface properties), catalyst deposition method (viz., impregnation and coating), precursor for strontium oxide (SrO; Sr-nitrate, acetate, hydroxide, and carbonate), and loading of SrO and lanthanum oxide (La2O3; 0-25 wt %) on the surface properties and performance of catalyst in oxidative coupling of methane (OCM; at 850 °C, gas hourly space velocity ) 1.02 × 105 cm3‚g-1‚h-1 and CH4/O2 ) 4 or 16) was thoroughly investigated. The basicity, acidity, and O2 chemisorption of the catalysts were studied by the temperature programmed desorption (TPD) of CO2, NH3, and O2, respectively, from 50 to 950 °C. The total and strong basic sites, acidity, and OCM activity of the supported catalyst were strongly influenced by the support used and also by the La2O3 loading on the support. The catalyst with a sintered low surface area porous silica-alumina support and high (20 wt %) La2O3 and SrO loadings showed the best performance in the OCM process. The OCM activity was influenced by SrO loading, but to a smaller extent, and also by the method of SrO deposition. The OCM activity of the supported catalysts could be related to their strong basic sites (measured in terms of the CO2 desorbed between 500 and 950 °C). Introduction Oxidative coupling of methane (OCM) to C2-hydrocarbons is a futuristic process of practical importance. During the past 12-15 years, worldwide efforts have been made to develop new catalysts for the OCM process, improve its performance, and understand the process (Anderson, 1989; Lee and Oyama, 1988; Hutchings et al., 1989; Lunsford, 1990, 1995). Most of the catalysts reported for the process are unsupported ones. For commercial exploitation, it is preferable to support the active catalyst on a porous matrix (i.e. catalyst carrier or support). The support or the porous matrix provides the catalyst with the required mechanical strength and resistance to abrasion to avoid highpressure drop across the catalyst bed. Because of the matrix, the dispersion and the stability against sintering or crystal growth of the active catalyst mass may also be increased. The strontium (Sr)-promoted lanthanum oxide (La2O3) catalyst shows high activity and selectivity in the OCM process (DeBoy and Hicks, 1988a,b,c; Gulcicek et al., 1990; Feng et al.,1991a,b; Xu and Lunsford, 1991; Kalenik and Wolf; 1992). Among different alkaline earth promoted rare earth-oxide catalysts, Sr-La2O3 catalyst shows the best performance in the OCM (Mulla, 1997; Choudhary et al., 1998). Our recent studies (Choudhary et al., 1997) indicated that the Sr-promoted La2O3 catalyst shows a better performance when it is supported on a low surface area porous catalyst carrier. The improved performance is attributed to the increase in the dispersion and also to the reduction in strength of very strong basic sites of the catalyst due to its deposition on the support. In our earlier studies (Mulla, 1997), among various catalysts containing alkaline earth oxides deposited on * To whom the correspondence should be addressed. Telephone: (+91) 212-336451 (ext. 2163). Fax: (+91) 212-333941. E-mail:
[email protected].
the support precoated with different rare earth oxides, the catalyst containing SrO deposited on sintered low surface area support (SA-5205, obtained from M/S Norton, Akron, OH) showed the best performance in the OCM. This invention was undertaken for studying further the influence of support with different chemical compositions and surface properties, loading of SrO and La2O3, method of SrO deposition (viz., impregnation or coating), and precursors for SrO (viz., Sr-nitrate, hydroxide, acetate, or carbonate) used in the preparation of supported SrO/La2O3 catalyst on its surface properties (viz., surface area, basicity/base strength distribution, acidity/acid strength distribution, and O2 chemisorption) and also on its performance in the OCM process. Experimental Section The catalysts were prepared by depositing SrO on 22-30 mesh size particles of different commercial porous catalyst carriers (Table 1) precoated with La2O3. The catalyst carriers (except SA3232) are sintered low surface area refractory inert materials that have no surface acidity or bacisity. The deposition of SrO on the precoated supports and the precoating of supports with La2O3 were done by the incipient wetness impregnation technique, using strontium nitrate and lanthanum nitrate, respectively, in their aqueous solution. After each impregnation, the catalyst mass was dried at 90 °C for 16 h and then calcined in static air at 950 °C for 4 h. The catalyst carriers were obtained from M/S Norton. The surface area of the catalysts was determined by the single-point Brunauer-Emmett-Teller (BET) method, using a Monosorb Surface-Area Analyzer (Quanta Chrome Corp., Syosset, NY). Some of the catalysts were characterized by temperature programmed desorption (TPD) of CO2, NH3, and O2. High purity CO2 (>99.995%), NH3 (>99.99%), and O2 (>99.5% passed over 13X molecular sieves and
S0888-5885(97)00601-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/17/1998
Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 2143 Table 1. Properties of Supports and Supported Catalystsa surface area (m2‚g-1)
support properties support
composition (wt %)
pore volume (cm3‚g-1)
porosity (%)
support
supported catalyst SrO/La2O3/support)
active catalyst mass (SrO/La2O3)b
SA5205 SA5218 SA5552 SA3232 SC5232 SC5532 SS5231 SS5531 SZ5564
Al2O3 (86.1%), SiO2 (11.8%) Al2O3 (86.1%), SiO2 (12.0%) Al2O3 (93.1%), SiO2 (5.6%) Al2O3 (80.3%), SiO2 (17.9%) SiC (65.8%), SiO2 (28.5%) SiC (65.8%), SiO2 (28.5%) Al2O3 (4.1%), SiO2 (95.0%) Al2O3 (4.1%), SiO2 (95.0%) ZrO2 + HfO2 (94.1%), CaO (3.5%)
0.35 0.20 0.39 0.60 0.25 0.23 0.25 0.27 0.15
54 40 59 63 42 45 35 40 45
