same or different, are equal to 2 or 3, excluding the case where b=3 and where the polyazacycloalkane is of formula (XIV) in which (i) n and p are equal to 2 and m and q are equal to 3 or (ii) n and p are equal to 3 and m and q are equal to 2, so as to obtain the grafted polyazacycloalkane of formula: ##STR33## in which S is a silica gel and P and b have the meanings given hereinbefore.