Thermal stability of alkali metals deposited on oxide supports and their influence on the surface area of the support

Abstract

Different alkali metal-promoted supports were prepared by impregnating SiO 2, Al 20 3, Cr 2O 3 and MgO with solutions of lithium, sodium or potassium nitrate or carbonate. They were calcined under an air flow at 773,1023 and 1173 K in order to measure the thermal stability of the alkali metal and the modifications of the surface area of the support. It was found that the deposited alkali metals are fairly stable on Cr 2O 3 or Al 2O 3 and slightly less so on SiO 2- With MgO, an important loss of alkali metal was observed with calcination temperatures > 773 K. This effect, increasing from lithium to potassium, was attributed to a reaction with the walls of the reactor. The alkali metal impregnation decreases the specific surface area of the support, whatever the temperature. Moreover, for the lithium-magnesia system, it was observed that there was a total collapse of the texture after treatment at 1023 K (0.4 m 2 g -1 instead of 54 m 2 g -1 for pure MgO). The results are interpreted in terms of superficial solid-state reactions with formation of oxy salts. Such formation does not occur with MgO, which explains the departure of the alkali metal from the support.