Synthesis and characterization of nanocrystallite MgAl2O4 spinels as catalysts support

Abstract

Because of the high melting point, good chemical stability and mechanical strength, spinel-type solid, MgAl2O4, is widely applied as ceramic materials. Recently, there has been growing interest in the utilization of magnesium-aluminate spinel as catalysts or catalysts support in the fields of environmental catalysis, petroleum processing and fine chemicals production [1–3]. For example, MgAl2O4 is an important base catalyst which efficiently catalyzes the coupling reactions of short chain linear alcohols [4] and mercaptan oxidation [5]. Due to its strong basicity and the high capacity of SO2 picking-up, magnesium-aluminate is also used as a co-catalyst, mixed with FCC catalysts for controlling SOx emission in the fluid catalytic cracking units [6–8]. It is reported that when Pt-Sn metals are supported on MgAl2O4, instead of alumina, in the case of propane dehydrogenation, the Pt-Sn/MgAl2O4 catalysts show a high resistance-sintering ability [9]. Catalytic tests also confirm that this kind of catalysts have satisfactory activity and selectivity in propane dehydrogenation reaction in steam as reaction media [10]. Magnesium-aluminate spinel is, therefore, of considerable interest in catalysis from a technical point of view. Differing from ceramic materials, magnesiumaluminate spinel used as catalysts support or catalysts by itself, show a relatively large surface area, small crystalline size and special active sites, which can be controlled by its preparation method. The ceramic magnesium-alumina spinels can be prepared by wet mixing of magnesium oxide and aluminum hydrogel, coprecipitation and sol-gel techniques with the calcination temperatures between 1200 and 2000 ◦C, resulting in a bulk-sintered body with very small surface area [11–14]. In the catalysis field, however, magnesiumaluminate spinel is generally prepared at a moderate temperature range from 600 to 900 ◦C to obtain a suitable pore distribution and large surface area. In such case, coprecipitation and sol-gel methods are often used [3, 6, 8, 10]. In the present work, we focus on the preparations of magnesium-aluminate spinels which will be used as Pt-Sn catalysts support for propane dehydrogenation, by using traditional coprecipitation techniques with low cost precursors. Samples A, B and C were prepared by using Mg(NO3)2·5H2O and Al(NO3)3·9H2O as precursors and concentrated ammonia solution as precipitating agent. Two solutions were respectively prepared by dissolving a given amount of magnesium and aluminum nitrates which was dependent on the designed composition of the samples. At the same rate, these two solutions were added into a 2000 ml flask containing 500 ml of deionized water. During the addition, the formed slurry was stirred and the pH value was respectively controlled at about 7, 10 or 12 by adding ammonia solution. Afterwards, the slurry was continuously stirred for an hour and then aged at room temperature overnight. The aged slurry was filtered and washed with deionized water. The filtered cake was dried at 120 ◦C for 16 h and then calcined at 800 ◦C for 8 h in air in a furnace. Samples D and E were also prepared by coprecipitation of magnesium nitrate and aluminum nitrate solutions but with (NH4)2CO3 as precipitating agent. The preparation procedures were the same as shown above. Samples F, G and I which respectively correspond to a nominal atomic ratio of Al to Mg of 1.8, 2.0 and 2.3, were prepared by wet mixing method. Alumina hydrogel (boehmite) and magnesium oxide were mixed in 1 liter of deionized water with continuous stirring to obtain a suspended solution. The filtered materials were dried at 120 ◦C for 16 h and then calcined at 800 ◦C for 8 h. The real chemical compositions of the samples were determined by using atomic absorption spectroscopy (AAS). Measurements were carried out on atomic absorption apparatus (Perkin Elmer Company model 5000). The specific surface area, pore volume and pore size distribution of the various samples were obtained from nitrogen adsorption–desorption isotherms measured on a model Digisorb 2600 analyzer.