Organically templated inorganic membranes for gas separation

Abstract

This work is an attempt to develop inorganic gas separation membranes for the purposes of separating high temperature binary gaseous mixtures. Carbon dioxide and nitrogen mixtures are the focus of this work but other mixtures could be used. The membrane synthesis route is derived from the sol-gel technique. It relies upon micropores being produced within the membrane and this is accomplished by the thermal removal of organic ligands (the "templates"). The thermal stability and structural evolution with temperature of these materials has been characterised with TGA, DTA, FTIR, 13C CP MAS NMR 11B MAS NMR and 29Si MAS NMR investigations. The research was demarcated into the comparisons between two systems: non-borosilicate and borosilicate. The borosilicate systems were thought to merit special investigation due to the known property of the boron atom in borosiloxane bonds to act as a network enhancer. Three different organic ligands; methyl, ethyl and phenyl have been investigated. The higher thermal stability (~770K) and the known CO2 affinity of the phenyl ligand, led to the production of materials containing both the methyl ligand (to generate porosity) and the phenyl ligand (to hopefully provide CO2 affinity). Other structures with methyl as a backbone but containing boron were found to have superior performance in terms of separation factors, robustness and durability. The permeability of CO2, N2 Ar and He was measured through all the membranes systems, as a function of pressure, temperature and time. In both the borosilicate and non-borosilicate systems, CO2 was found to permeate preferentially over He in the best specimens. This was despite its much larger molecular diameter and for both classes of system, permeance was observed to decrease with elevated temperatures. The general conclusion that for both classes of system the mechanism of preferential CO2 transport is activated surface diffusion. Evidence of gradual adsorption of CO2 by the non-borosilicate systems was indicated by their steady decrease in performance with time when exposed to this species. (Such degradation in permeance performance was not observed for those non-borosilicate systems that had not been exposed to CO2 but just N2, He or Ar. The borosilicate systems however, were far more robust. Any decrease in permeance with time, after exposure to CO2 under pressure, was orders of magnitude slower than with the non-borosilicate systems. For the non-borosilicate systems the decrease in permeability is deemed to be due to CO2 chemisorption and must be related to the surface diffusion. For the non-borosilicate systems however, chemisorption appears to play a far less important role. Structural studies (NMR and FTIR) of all the systems indicated that the pyrolysis of the organic templates produces both siloxane and in the case of the borosilicate systems, borosiloxane linkages as well. These are assumed to be the generators of the sites through which surface diffusion occurs. For the non-borosilicate systems, surface diffusion seems to be improved by the incorporation of phenyl ligands within the siloxane network. However, this is associated with accelerated adsorption and decrease in overall performance. For the borosilicate systems, the most successful system had a methyl backbone and decreased in performance very gradually and after that remained constant except for long-term modulations which were mirrored by the inert species as well. Thermally rejuvenating the degraded non-borosilicate membranes did not meet with success. However, the borosilicate systems did partially respond to this treatment and regained a significant fraction of their original performance. The conclusion is that in the non-borosilicate system chemisorption dominates over physisorption as a CO2 selectivity mechanism, whilst for the borosilicate systems the reverse appears to be true