Chromium-based catalysts are applied in industry for the production of lower alkenes such as ethene, propene and isobutene through the dehydrogenation of the corresponding alkanes. A significant amount of coke is deposited on chromium catalysts during the dehydrogenation process. There are many detailed publications devoted to chromium supported on silica and silicas. Advantage of such catalysts is their relative resistance to coke deposition. Nevertheless, the coke is formed on these catalysts and it takes the catalyst regeneration in practice. The introduction of oxidants in the reaction mixture results not only in the shift of equilibrium for dehydrogenation process but in an oxidation of formed coke and of the chromium on catalyst surface. Dehydrogenation of light paraffins in the presence of oxidants on CrOx/silica prepared by impregnation is widely presented in literature. But these data are distinguished by activity, selectivity and tolerance to irreversible deactivation. In this connection we tried to study the influence some factors in preparation of impregnated catalysts on their activity and physical-chemical properties. One can separate the following main stages at preparation of impregnated catalysts: a) the preparation of support, b) the preparation of chromium salt solution, c) the impregnation, d) the precipitation, e) an activation of the air-dry samples. The conditions of each stage can influence on properties of prepared catalysts. Catalysts were prepared by impregnation of commercial silica (KSKG – 374 m2/g) from water solutions of Cr(NO3)3 .9H2O. Chromium amount was changed in the range (%, wt) 1.0-7.0. Catalytic studies were carried out in a flow reactor at 600oC, 200 h-1 and at atmospheric pressure. Initial reaction mixtures were of the following compositions (%, vol.): (C3H8) (15), CO2 (30), N2 (55) and (C3H8) (15), CO2 (30), N2 (47.5), O2 (7.5). Catalysts were investigated by DR-UV-Vis spectroscopic method and temperature-programmed reduction (TPR-H2). Methods of TPR-H2 and DRS allow to evaluate the uniformity of active phase and relative catalytic activity. The influence of time for the previous calcination of silica, conditions of impregnation and activation studied. To check the influence of acidity of chromium nitrate solution at Cr precipitation, catalysts were prepared at the following pH: 5.0, 3.0, 1.7; 1.0 and 0.25. It was shown that the preliminary calcination of silica and an increase of precipitation time have a positive effect on activity, stability, tolerance to irreversible deactivation of chromium-oxide catalysts and on the homogeneity of impregnated active phase. The increase of pH of chromium nitrate solution at Cr precipitation has a negative effect on the activity of chromium catalysts and, especially, on the deactivation rate. Data of TPR-H2 and DRS investigations showed that the active surface is considerably changed during the process of propane oxidative dehydrogenation, because of the agglomeration of chromium-oxide particles.
Specifity of silica-supported chromium-oxide catalysts preparation for propane oxidative dehydrogenation
BOTAVINA, Maria;
2009-01-01
Abstract
Chromium-based catalysts are applied in industry for the production of lower alkenes such as ethene, propene and isobutene through the dehydrogenation of the corresponding alkanes. A significant amount of coke is deposited on chromium catalysts during the dehydrogenation process. There are many detailed publications devoted to chromium supported on silica and silicas. Advantage of such catalysts is their relative resistance to coke deposition. Nevertheless, the coke is formed on these catalysts and it takes the catalyst regeneration in practice. The introduction of oxidants in the reaction mixture results not only in the shift of equilibrium for dehydrogenation process but in an oxidation of formed coke and of the chromium on catalyst surface. Dehydrogenation of light paraffins in the presence of oxidants on CrOx/silica prepared by impregnation is widely presented in literature. But these data are distinguished by activity, selectivity and tolerance to irreversible deactivation. In this connection we tried to study the influence some factors in preparation of impregnated catalysts on their activity and physical-chemical properties. One can separate the following main stages at preparation of impregnated catalysts: a) the preparation of support, b) the preparation of chromium salt solution, c) the impregnation, d) the precipitation, e) an activation of the air-dry samples. The conditions of each stage can influence on properties of prepared catalysts. Catalysts were prepared by impregnation of commercial silica (KSKG – 374 m2/g) from water solutions of Cr(NO3)3 .9H2O. Chromium amount was changed in the range (%, wt) 1.0-7.0. Catalytic studies were carried out in a flow reactor at 600oC, 200 h-1 and at atmospheric pressure. Initial reaction mixtures were of the following compositions (%, vol.): (C3H8) (15), CO2 (30), N2 (55) and (C3H8) (15), CO2 (30), N2 (47.5), O2 (7.5). Catalysts were investigated by DR-UV-Vis spectroscopic method and temperature-programmed reduction (TPR-H2). Methods of TPR-H2 and DRS allow to evaluate the uniformity of active phase and relative catalytic activity. The influence of time for the previous calcination of silica, conditions of impregnation and activation studied. To check the influence of acidity of chromium nitrate solution at Cr precipitation, catalysts were prepared at the following pH: 5.0, 3.0, 1.7; 1.0 and 0.25. It was shown that the preliminary calcination of silica and an increase of precipitation time have a positive effect on activity, stability, tolerance to irreversible deactivation of chromium-oxide catalysts and on the homogeneity of impregnated active phase. The increase of pH of chromium nitrate solution at Cr precipitation has a negative effect on the activity of chromium catalysts and, especially, on the deactivation rate. Data of TPR-H2 and DRS investigations showed that the active surface is considerably changed during the process of propane oxidative dehydrogenation, because of the agglomeration of chromium-oxide particles.
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