[0005] Patent application FR 2842438 A1 discloses a process for preparing beads containing a cross-linked mineral matrix. The process is suitable for the preparation of alumina- or silica-based millimeter-scale beads by a sol-gel process. The production of these beads comprises the step of preparing gelled beads by pouring a suspension comprising a precursor of the inorganic matrix and an alginate dropwise into a solution of a polyvalent cation salt, at a pH of less than 3. The combined actions of the polyvalent cation and of the acidity variations of the medium contribute to the gelling of this alginate and to a congealing of the drops as "soft" beads. Thus, the mineral matrix is homogeneously distributed throughout the bead. However, dissolution of silica occurs over time in these prepared hybrid alginate-silica beads as observed by Dandoy et al (201 1 ).