Selective uptake of La3+ ions with polyoxometalates-functionalized mesoporous SBA-15: An EXAFS study

In this work, mesoporous materials based on inorganic/organic polyoxometalate (POMs) hybrids with the ability of further incorporating heterometallic lanthanum cations were designed. On the basis of the La3+ coordination to vacant organophosphonyle derivatives of POMs previously studied, herein POMs-grafted onto an amino-functionalized mesoporous SBA-15 ({NH2}-SBA-15) with incorporated La3+ ions were synthesized by varying the amounts of the latter. L-III-edge La EXAFS spectroscopy of the resulting materials provided local structural information around La-III ions (i.e. number, nature and distances of neighboring atoms), which are comparable to that of the La-III complexes of POMs hybrids. This technique, in addition with P-31 CP-MAS NMR spectroscopy, gave definitive clues for a selective uptake of the incorporated La3+ ions by the POMs grafted onto the {NH2}SBA-15 supports. This study is a clear demonstration that, due to their chelate effect, POMs hybrids act as efficient ligands for La3+ even in the presence of competitive adsorption sites. Consequently, a regular distribution of the La3+ ions along the channels of the SBA-15 is thus observed in these materials due to the nanostructuration of the POMs onto the support.