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Project

Composite membranes for gas separation

This work is focused on the investigation of different aspects of composite membranes development: i) synthesis of nanoparticles and fillers, with surface modified by silane coupling agents, used for preparation of mixed matrix membranes (MMMs), ii) flat dense MMMs preparation using commercial and home-synthesized polyimides and different zeolitic fillers, and iii) Thin Film Composite (TFC) membranes study.

In the first part of the thesis, different nanoparticles were synthesized. MFI titanosilicates, particularly TS‑1 in the form of nanoparticles (200-400 nm), was synthesized and modified using six different silane coupling agents. Silanization was carried out in order to improve the compatibility of the filler with the polymeric matrix. These particles were characterized by nitrogen, carbon dioxide and methane adsorption, and FTIR. Using the modified filler and lab-synthesized copolyimide based on 6FDA dianhydride the MMMs were prepared. A new crosslinking approach by means of the silane coupling agent previously used for surface modification was applied to improve the CO2 induced plasticization resistance. Membranes were characterized by SEM in order to check the compatibility of the modified filler with the polymeric matrix, by solvent uptake analysis for the evaluation of the membrane crosslinking and swelling, and gas permeation at high pressure (up to 40 bar) and room temperature using 50/50 vol./vol CO2/CH4 mixed gases in order to check the CO2 induced plasticization resistance of the membrane. It was shown that a silane coupling agent may have a double effect not only to improve the interaction between the continuous and the dispersed phase but also to crosslink the polymeric phase avoiding the CO2-induced plasticization.

Dense self-supporting MMMs were produced using different titanosilicates (TS-1 with different Si/Ti ratio and ETS-10) and the commercial polyimide Matrimid® 5218 in order to study the effect of the titanosilicates on the separation performances of the membranes. The fillers were characterized using SEM, XRD, XPS, and AES. The MMMs with different loadings (10, 20 and 30 wt.%) were produced and characterized by SEM, TGA, and DSC. Gas separation performance was evaluated using 50/50 vol./vol CO2/CH4 mixed gases experiment at 35 °C and 8 bar transmembrane pressure. Furthermore, different mathematical models were applied to predict the final performance of MMMs and compared with obtained experimental data. The results indicate that the content of titanium in TS-1 particles leads to a different gas separation behavior mainly due to the presence of TiO2 nanoparticles on the surface of the zeolite. The ETS-10 increments the separation factor of the polymer.

Another type of composite membranes was also studied. TFC membranes were manufactured using crosslinked Matrimid® 5218 as a support layer and 6FDA‑DAM:DABA copolyimides as the top layer. Four different polyimides were used: three copolyimides with different ratios of DAM:DABA diamines: 2:1, 3:1, 9:1 and the homopolyimide 6FDA-DAM. Polyimides were characterized by GPC, FTIR, DSC, and TGA. Thermal annealing of the TFC was carried out in order to promote the crosslinking of the top layer by means of decarboxylation of the DABA moiety. TFC membranes were characterized by SEM and FTIR-ATR. In order to study the aging behavior of these membranes at conditions closer to the real working conditions, the gas performance was evaluated with: 50/50 vol./vol. CO2/CH4 mixed gases experiments carried out at 35 °C and 8 bar of transmembrane pressure for 180 h. Aging behavior of non-treated and thermally annealed membranes was compared. The thermally annealed TFC membranes showed a very slight decrease of the permeability during the performance test evaluated time. The aging behavior of non-treated TFC membranes is influenced by the content of carboxylic acid present in the copolyimide. Even though, the aging experienced by the TFC membranes under CO2/CH4 conditions is much lower than at ambient conditions reported in the literature. The reason is the CO2 adsorbed on the polymer hinders the mobility of the polymeric chains avoiding its rearrangement and the aging of the polymer.

Finally, a new type of MMMs was developed using the as-synthesized copolyimide 6FDA-DAM:DABA (3:1) as a continuous phase and the zeolite SSZ-16 as filler. The particles were characterized by XRD, SEM, and nitrogen, carbon dioxide and methane adsorption. MMMs were manufactured with low inorganic loadings (5, 10, and 15 wt.%) and they were characterized by SEM, DSC, and TGA. Afterwards, the MMMs were tested at room temperature for different transmembrane pressures (2, 4, 6, and 8 bar) and different feed compositions (25/75, 50/50, and 75/25 vol./vol. CO2/CH4) in order to evaluate the effect of pressure and feed composition, respectively. The incorporation of SSZ‑16 zeolite doubles the permeability of the unfilled membrane. Furthermore, the filler keeps the separation factor constant in the whole pressure range tested, increasing the gas separation stability of the membrane at higher pressures and higher CO2 molar fraction feed. On the contrary, unfilled membranes show a decreasing of the separation factor at the highest pressure tested.

Date:21 Oct 2014 →  6 May 2019
Keywords:membrane, gas separation
Disciplines:Analytical chemistry, Macromolecular and materials chemistry
Project type:PhD project