Understanding microstructural evolution of mixed metal oxide silsesquioxane glasses through wet-chemical synthesis

Abstract

Membrane separation offers energy efficient alternatives for distillation in separation of small molecules. Among a large variety of materials that can be used, microporous hybrid silica membranes are (1) suitable for the separation of the smallest molecules eg: H2/N2, (2) have a superior (hydro)thermal and chemical stability and (3) unlike polymers do not suffer from chain-rearrangement-induced permeability loss at elevated temperatures. This research explores the possibilities of manipulating the membrane properties by the incorporation of metal ions / metal oxide clusters within the hybrid silica network, with the aims to (1) further improve the (hydro)thermal and chemical stability and (2) manipulate the affinity and therefore the selectivity for the separation of particular molecules. Nevertheless, the homogeneous incorporation of metal ions within is not straightforward from a synthesis point of view, since metal alkoxides rapidly condense nanosized clusters that phase separate from the glassy matrix. Agglomeration studies by means of in-situ small angle X-ray Scattering (SAXS) which involves acid catalysed hydrolysis/condensation of niobium pentaethoxide (NPE)/1,2-bis-(triethoxysilyl)-ethane (BTESE) mixtures reveal a rapid nucleation of nanosized metal oxide clusters within 5s of measurement. However, subsequent growth was clearly affected yielding a reversible agglomeration kernel. Growth was suppressed by the surrounding of silsesquioxane groups. While drying the sol the presence of nanosized phase separated clusters became apparent in the SAXS-pattern. Hydrolysis of BTESE prior to the addition of NPE had only a small effect on the size of the niobia clusters. Alternatively, the use of the chelating agent acetylacetone was more effective in suppressing the fast nucleation of niobia clusters as implied by the increased number of Si-O-Nb couplings being found by 17O-NMR. To promote the dispersion of metal ions within the hybrid silica matrix, the sol-gel precursor N,N,N',N'-tetrakis-(3-(triethoxysilyl)-propyl)-malonamide (TTPMA) was synthesized. The malonamide ligands clearly coordinated the Ce4+ and Ni2+ metal centers and enhanced their dispersion however did not completely circumvent redistribution upon calcination. Nevertheless, these Ce-TTPMA and Ni-TTPMA membranes showed higher H2/N2perselectivities as compared to previously reported hybrid silica membranes.