Aging: After visible formation of gel, processing proceeds to the aging step where the structure and the properties of the formed network continue to change up to the point that yields the target density. It represents the time between the formation of the gel  and the removal of solvent. As long as the pore liquid remains in the matrix, a gel is not static and can undergo many transformations.  This step includes four processes: polycondensation, syneresis, coarsening, and phase transformation.

Polycondensation between surface functional groups continue s to occur after the gel point. This process is actually desirable as it leads to a more cross-linked network that is mechanically stronger and consequently easier to handle. However, extensive condensation can lead to shrinking of the gel to such an extent that the solvent is actually expelled in a phenomenon called syneresis. Parameters that affect this process include temperature, time, and pH of the pore liquid. However, studies of these effects are still very qualitative.

A typical preparation of zirconia aerogel is shown in Fig. 6. Zirconium propoxide is used as precursor for zirconium. The sol prepared from precursor solution is aged for 2h and then supercritically dried. In this example calcination is done in two steps.

Fig. 6. Solgel parameters in preparation of zirconia aerogels

Book References :

•  K.P. de Jong. , Synthesis of solid catalysts , Wiley –VCH, 2009

•  J.T. Richardson, Principle of catalysts development, Plenum Press, 1989

•  G. Ertl, H. Knozinger & J. Weitkamp, Handbook of Heterogeneous Catalysis Vol 1, Wiley – VCH, 1997

•  R. J. Farrauto & C. H. Bartholomew, Fundamentals of Industrial Catalytic Processes, Blackie Academic & Professional, 1997