Structure–property tuning in hydrothermally stable sol–gel-processed hybrid organosilica molecular sieving membranes

Abstract

Supported microporous organosilica membranes made from bridged silsesquioxane precursors by an acid-catalyzed sol-gel process have demonstrated a remarkable hydrothermal stability in pervaporation and gas separation processes, making them the first generation of ceramic molecular sieving membranes with sufficient performance under industrially relevant conditions. The commercial availability of various a,x-bis(trialkoxysilyl)alkane and 1,4-bis(trialkoxysilyl)benzene precursors facilitates the tailoring of membrane properties like pore size and surface chemistry via the choice of precursor(s) and process variables. Here, we describe the engineering of sols for making supported microporous thin films, discuss the thermal and hydrothermal stability of microporous organosilicas and give a short overview of the developments and applications of these membranes in liquid and gas separation processes since their first report in 2008.Graphical Abstract

Hydrothermal stability of silica, hybrid silica and Zr-doped hybrid silica membranes

“…It is possible that during the hydrothermal treatment, as used in our work some traces of oxygen were still present, which would lower the onset temperature of decomposition. Similar changes in onset temperature were reported in a review paper of Ten Elshof et al [15], he observed that the in literature reported upper temperature limits varies significantly for the bridging ethylene groups under inert atmosphere from 250 to 500°C.…”

“…In literature several features are mentioned [15] for the increased hydrothermal stability. Very often the hydrothermal stability is ascribed to an increase of the hydrophobicity or the non-polarity, introduced by the organic groups, resulting in a reduction of the diffusion of water into the membrane.…”

“…A second explanation for the increased hydrothermal stability of these Si-hybrid membranes is the increased ability to connect a monomer with the adjacent network when two silicon atoms are joined by an organic bridge [15]. Organically bridged monomers have a sixfold connectivity while pure silicon only has a four- fold connectivity.…”

“…Finally, Ten Elshof at al. [15] added another aspect by considering the improved hydrothermal stability from a more fundamental molecular point of view. Silicon dioxide consist of corner sharing tetrahedra with oxygen atoms at the corner.…”

“…The BTESE tetrahedral has also a CH 2 segment which is expect to have not the same impressive freedom as the bond angle of the SiAOASi bound resulting in a lower flexibility of the overall network as compared to the silica network. The incorporation of this organic segment shifts the balance of these interactions and as result of this the hydrothermal stability increases [15,18].…”