In this study the preparation of Li-promoted MgO catalysts is described using, respectively, (i) wet impregnation and (ii) sol-gel method. In the case of Li-promoted MgO catalysts, defects sites, due to the surface substitution of Mg2+ ions by a Li+ ion in the MgO matrix, are reported to play a significant role in processes involving oxidation reactions. More specifically, the impregnation of MgO supports with aqueous solutions of Li salts (i.e., LiNO3), as route to prepare Li-promoted MgO catalysts, allows a homogeneous distribution of lithium on the catalyst surface. However, in this specific case, high temperature treatments are required. In fact, incorporation of lithium ions in MgO (forming a substitutional solid solution) takes place only at ≥700°C causing drastic sintering effect that result in materials with low surface area and thus low catalytic activity. Sol-gel preparation is here presented as an alternative and promising route for the preparation of Li-promoted MgO catalysts. In this study, Li-promoted MgO catalysts were prepared via co-gelling Mg(OCH3)2 and LiNO3. Our observations during gel studies suggested that the presence of lithium ions in the sol-gel system drastically influenced the extent of hydrolysis and condensation. In particular lithium ions can be incorporated already in magnesia at the Li-Mg-gel stage facilitating formation of a substitutional solid solution. Furthermore, the results showed that high temperature treatments are not required and very active materials are formed after calcination at temperatures below 600°C. In addition, the enhanced lithium incorporation minimizes the amount of free lithium phases present. Our observations suggest that in the case of Li-promoted MgO catalysts both those effects are responsible for the high thermal stability and high surface area obtained after calcination. In this work, IR spectroscopic characterization of Lewis acid sites (Mg2+ LC) using carbon monoxide is also extensively reported and presented as a tool to investigate the effect of the incorporation of lithium ions in MgO. Our results suggest that sol-gel catalysts possess a higher amount of incorporated lithium ions in MgO. These findings are in agreement with the results obtained from active site titration using CO2 sorption experiments and oxidation reduction cycles with H2. Interestingly, the results showed that incorporated lithium ions efficiently provide the stabilization of active oxygen species [O-] in MgO forming [Li+O-] sites. Therefore, the activity and selectivity improvements during the oxidative conversion of propane can be explained by the promoting effect of lithium to enhance the creation of active oxygen species [O-] in MgO. Accordingly, during the oxidative dehydrogenation/cracking of propane over Li-promoted MgO catalysts prepared using sol-gel route, a higher number of active [Li+O-] sites per cm3 of reactor volume was achieved compared to conventionally prepared materials and superior yields were recorded (same amounts of catalyst in the reactor).
|Award date||29 May 2008|
|Place of Publication||Enschede|
|Publication status||Published - 29 May 2008|