Functionalized metal oxide silica membranes for gas separation

Abstract

The functionalization of silica derived membranes has been generally investigated by embedding materials into silica matrices such as templates, metal oxides or through the use of hybrid organosilicas. The careful selection of an appropriate material can confer silica films with molecular sieving properties, or create preferential adsorption sites, or enhance chemical and hydrothermal stability. In pursuing novel approaches, this thesis explores the synergistic effects of integrating surfactants and metal oxides into to the functionalization of silica matrices. To this end, cobalt oxide silica materials were produced using a one step sol-gel method, initially incorporating a small cationic surfactant (hexyl triethyl ammonium bromide) but still bearing in mind the importance of the microporous character of the final material. The influence of surfactant load, surfactant alkyl tail length and calcination temperature on material properties and membrane separation performance was systematically evaluated.This work shows for the first time that surfactant incorporation during sol-gel synthesis alters the oxidation state of the final cobalt embedded into the silica matrix after calcination. This major finding was evidenced by the shift of Co3O4 formation within calcined xerogels, from 45% to almost 80%, as the surfactant concentration increased from the reference cobalt silica (Surfactant/Co=0) to the equimolar mixture (Surfactant/Co=1), respectively. This finding is counter intuitive, as the concentration of cobalt precursor was kept constant in the sol-gel process. Further addition of surfactant above the equimolar ratio induced a drop in the Co3O4 phase. This synergistic effect between the surfactant and metal oxide is postulated to arise from the promotion of tetrahedral coordination between cobalt and Br (surfactant counter ion) within the as-synthetised materials, which alters the oxidation pathway during calcination. Based on these results, a model was proposed to explain the mechanisms leading to the changes in cobalt oxidation state. In the first instance when considering low surfactant concentrations, the tetrahedral cobalt bromide complex was more easily oxidised than its octahedral counterpart. In the second instance, when the Surfactant/Co ratio exceeds 1, there were additional strong interactions between the cobalt bromide complex and the surfactant head groups which in turn hinder the oxidation process. Additionally, these complex interactions also delayed the surfactant removal from the silica framework, thus affecting the final morphology. Furthermore, by changing the alkyl length of the surfactant, it was observed a direct dependency between its aggregation ability and the presence of these metal-surfactant interactions.Further proof of the proposed mechanism is supported by Synchrotron work, as the initial FTIR, UV-Vis and XPS data was corroborated with X-ray absorption analysis. In particular, XAS analysis allowed for the individual cobalt species to be identified and quantified. A significant alteration in the cobalt phase distribution was observed as surfactant concentration increased. The CoO phase was dominant within the pure cobalt silica without surfactant, whilst a secondary Co3O4 phase was measured at ~44%. The Co3O4 phase increased towards the equimolar ratio, thereby becoming the dominant phase. Above this ratio, the CoO like phase was promoted at the expense of Co3O4. Additionally, the EXAFS data shows the presence of Si within the nearest neighbourhood of a fraction of the cobalt atoms, thus suggesting that some of the cobalt exists as a Co2+ ion directly connected to the silica matrix. The second major finding relates to the cobalt phase influencing the thermodynamic properties associated with the sorption of gases. The isosteric heat of adsorption (Qst) for H2 increased but decreased for CO2 as a function of increasing Co3O4 content within the silica matrix. This trend was contrary with increasing Co2+ (or CoO) content. This additional functionality was accompanied by further structural changes in the silica itself. For instance, microporosity was observed for matrices with surfactant/Co ratios ≤1, though an increasing degree of mesoporosity was measured for ratios that exceeded 1.A final test of the resultant materials was carried out by investigating the performance of these materials as functionalised cobalt oxide silica membranes for single gas permeation of He, H2, N2 and CO2. Temperature dependent transport related to activated transport was observed for surfactant/Co ratios <1, thus suggesting membrane films with a pore size distribution within the ultra-micropore range (2-5 A). Interestingly, the membrane performance was consistent with the functionalization trends found during materials characterisation and the transport behaviour was identified to be an integral function of the pore size and cobalt oxidation state. Essentially, the separation of gases with small kinetic diameters (He and H2) from larger gases (N2 and CO2) was higher near the equimolar surfactant/Co ratio silica membranes, and then decreased at higher surfactant ratios. Coupled with activated transport and H2 and CO2 adsorption, these results strongly suggest that the dominant Co3O4 phase in the silica matrix offers smaller interstitial cavities at the metal-silica interface when compared with a those films dominated by a CoO or Co2+ phase.