Organically modified aluminas by grafting and sol–gel processes involving phosphonate derivatives

The surfaces of alumina particles were modified by grafting with phenylphosphonic acid and its organic-soluble ester derivatives (diethyl phenylphosphonate and bis(trimethylsilyl) phenylphosphonate). Solid-state 31P NMR spectroscopy indicated that in aqueous media the formation of bulk aluminium phosphonate phases could be avoided by using phenylphosphonic acid at pH 6. The formation of such phases was detected in organic media in the presence of phenylphosphonic acid or its silyl ester. On the other hand the use of the dialkyl ester derivative in organic media allowed controlled grafting, excluding the formation of phosphonate phases even under prolonged heating. Alternatively a two-step sol–gel process was carried out, which involved first the non-hydrolytic condensation between aluminium alkoxide and phenylphosphonic acid (or the parent bis(trimethylsilyl) ester), then the hydrolysis–condensation of the remaining Al–OR groups. 31P and 27Al NMR spectroscopy proved the homogeneity of the solids obtained, even for P/Al ratios as high as 1.