Synthesis of Lanthanide and Lanthanide-Silicate Aerogeis

In this paper we report on the preparation of mixed lanthanide-silicate and pure lanthanide aerogels from the chlorides of erbium, praseodymium, and neodymium. A two-step sol-gel method is described for preparing the mixed aerogels, using a sub-stoichiometric amount of water in the first step to prepare a partially condensed silica-lanthanide precursor. The pure lanthanide aerogels are prepared directly from the chlorides using propylene oxide as a scavenger for reaction generated hydrochloric acid. The aerogel microstructures vary from colloidal for the pure lanthanide and high weight percent lanthanide-silicate aerogels to polymeric for the low weight percent lanthanide-silicate aerogels. This change in microstructure is also indicated by the BET analyses, which show that the surface areas decrease with increasing lanthanide concentrations. In general, we measured reductions of lanthanide contents during the supercritical drying step due to insufficient linking and subsequent "washing out" of the lanthanides from the gels. Also, the retention efficiency for the lanthanide improves with higher silica concentrations, making quantitative doping by this method practical only for the lower lanthanide concentrations.