Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
George S. Hammond wikipedia , lookup
Asymmetric induction wikipedia , lookup
Fischer–Tropsch process wikipedia , lookup
Ene reaction wikipedia , lookup
Ring-closing metathesis wikipedia , lookup
Wolff–Kishner reduction wikipedia , lookup
Strychnine total synthesis wikipedia , lookup
Petasis reaction wikipedia , lookup
Hofmann–Löffler reaction wikipedia , lookup
Correlation Between Acidity, Basicity and Catalytic Performance of Binary SolGel MgO-Al2O3 Catalysts L. F. Chen1, J. A. Wang1,*, T. López 2 and R. Gómez2 1 Laboratory of Catalysis and Materials, ESIQIE, National Polytechnic Institute, 07738 Mexico City, Mexico. 2Departamento de Química, Universidad Autónoma Metropolitana-I, A. P. 55-534, 09340 México City, Mexico Introduction Recently, by using reaction of alcohols like isopropanol on metal oxide surface as probe reaction, correlation between acidic-basic properties of the oxide catalysts and reaction selectivity has been reported (1). The dehydration selectivity is postulated to proceed at acidic sites, while, dehydrogenation selectivity is presumed to be catalysed by basic sites or by both acidic and basic sites (2). However, care must be taken when one uses such probe reaction as simple diagnostics of surface acid-base properties since 2-propanol decomposition is recently found to be a function of not only the surface properties but also the reaction condition employed (3,4). It is the purpose of the present work to study the influences of experimental parameters on dehydration and dehydrogenation selectivity of isopropanol decomposed over different sol-gel MgO-Al2O3 catalysts. Reaction mechanism involving the added water function is assumed. Results and discussion The sol-gel magnesia-alumina oxides show a surface area ranging from 237 to 323 m2/g with bimodel pore size distribution. Acid and basic sites coexist on the surface of the samples, depending on the molar ratio of MgO/Al2O3. When isopropanol was decomposed over the magnesia-alumina oxides, propene, acetone and isopropylether were identified in the outline stream, showing that dehydrogenation, dehydration and bimolecular polymerisation reaction occur. The total conversion of isopropanol linearly increased with increasing of the reaction temperature. Below 150°C, the main product was acetone; however, above 200°C, propene was the dominant. The acetone selectivity was correlated to basicity of the solids as well as the reaction parameters such as the reaction time and temperature. However, the selectivity to propene is independent of acidity of the catalysts, differing from the conclusion reported by Kckenzie (2). When water was present in the reaction mixture, both the activity and selectivity of the catalysts are remarkably altered (Table 1): (i) isopropanol conversion is increased or decreased, depending on the reaction temperature; (ii) acetone concentration and selectivity are profoundly enhanced in the whole reaction temperature range. Below 250 °C, the total conversion of isopropanol significantly increased, however, above 300 up to 400 °C, the conversion decreased when water stream is present. These results show that water may directly involve in the reaction, promoting the activity below 250 °C but inhibiting the activity at higher temperature. For explanation the promotion/inhibition originated from the added water, it is assumed that below 250 °C, water molecule is adsorbed on the catalyst surface to form new active species those are involved in the reactions, promoting isopropanol decomposition. However, above 250 °C, a competitive adsorption between the reactant and water molecules may occur, the strong adsorption of water molecules poisons some active sites, decreasing isopropanol adsorption and therefore suppressing the total activity. Table 1. Differences of isopropanol conversion and acetone concentration and selectivity between the cases of water presence and absence in the stream. ____________________________________________________________________ ∆Ca ∆Ca/Ca Sd Sd/Sa Temp. (°C) ∆Ct ___________________________________________________________________ 400 -2.63 0.96 0.93 1.01 0.98 350 -3.58 2.73 3.42 2.85 3.56 300 -2.23 5.81 7.57 7.91 7.91 250 33.80 13.07 107.03 26.80 28.82 200 17.99 12.24 200.66 56.95 6.40 150 4.85 3.96 69.47 18.02 0.28 100 1.10 0.91 9.29 11.05 0.14 ___________________________________________________________________ *∆Ct=Ctw–Ct, (%), the difference of isopropanol conversion between the cases of water presence (Ctw) and absence (Ct) in the stream; ∆Ca=Caw–Ca (%), the difference of acetone concentration between the cases of water presence (Caw) and absence (Ca) in the stream; Sd =Saw-Sa (%), the acetone selectivity difference between the cases of water presence (Saw) and absence (Sa). Since the added water enhanced acetone concentration and selectivity even when the total conversion decreased at higher temperature, it is a clear indication that the diminishing of the total conversion is resulted from inhibition of dehydration pathway. This indicates that acetone formation is through more than one pathway: a normal dehydrogenation route and a hydroxyl-assisted mechanism. In the latter case, it is assumed that the H atom within OH linked to isopropanol reacts with hydroxyls on the catalyst surface to dehydrate molecular water and an alkoxide carbonion species bounded on the cationic lattice ((CH3)2HCO-); and then the active α-H in the alkoxide carbonion species reacts with the H atom linking on the oxygen lattice to yield hydrogen and acetone molecules. Therefore the hydroxyl-assisted mechanism is a modified dehydrogenation route accompanying dehydration. References 1. 2. 3. 4. C. F. Fishel, R. J. Davis, Langmuir 10 (1994) 5. A. L. McKenzie, C. T. Fishel, R. J. Davis, J. Catal. 138 (1992) 547. J. Rekoske, M.A. Barteau, J. Catal. 165 (1997) 57. J. A. Wang, X. Bokhimi, O. Novaro, T. Lopez, R. Gomez, J. Mol. Catal. A, 145(1999) 291.