Molecular Characterization of Hybrid Organic-Inorganic Membranes for Gas Separation under Harsh Conditions – MOLHYB

Molecular dynamics simulations (MD) are used at LEPMI-USMB to screen a novel set of candidate polyPOSS-imides for improved thermomechanical resistance, i.e. up to at least 400°C, without compromising their gas separation function over the 0-60 bar pressure range. Associated molecular modelling techniques are used to either build the models or characterize the new structures. These include Density-Functional Theory (DFT) for the development of the force-field, cross-linking algorithms for creating the networks, Test-Particle Insertion (TPI) via Excluded-Volume Mapping Sampling (EVMS) for the gas solubility, Grand Canonical Monte Carlo (GCMC) for the gas sorption, Trajectory-Extending Monte Carlo (TEKMC) for the gas diffusion and various analyses programs.

The most-promising polyPOSS-imide structures are synthesized via interfacial polymerization on ceramic supports at the University of Twente. The ultrathin membranes are characterized by techniques such as Infrared Spectroscopy (FTIR-ATR) for the nature of the chemical groups, X-ray Photoelectron Spectroscopy (XPS) for the imide:POSS ratios, in-situ Thermo-Ellipsometric Analysis (TEA) for the thickness and refractive indices, goniometry for the contact angles, pycnometry for the density, Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) for the morphology, Thermal Gravimetric Analysis (TGA) for the degradation, Differential Scanning Calorimetry (DSC) for the heat capacity, Permeability Tester for the gas permeance, and both Spectroscopic Ellipsometry or Magnetic Suspension Balance for the gas sorption.

A phenyl derivative of the inorganic precursor, OAPS (octa(aminophenyl)silsesquioxane), has been identified in order to increase the thermoresistance of polyPOSS-imide networks. However, there are three possible positions for the NH2 group on the phenyl ring. In the literature, the relative proportions of the meta:ortho:para isomers are found to depend on the sources and on the two synthesis routes, which are either the nitration of octa(phenylsilsesquioxane) followed by reduction, or the condensation of aminophenyltrialkoxysilanes. In order to characterize the proportions of the isomers, 29Si-NMR, 13C-NMR and 1H-NMR spectra were measured on OAPS originating from both routes. We are currently trying to assign the peaks to the different isomers by comparing the experimental spectra with model NMR spectra calculated by DFT. Densities were measured by pycnometry, and infrared spectra of both OAPS were also obtained.

In parallel with the characterization of the inorganic precursor, more than twenty molecular models of crosslinked OAPS-imide structures have been built from the pure isomers of OAPS and the PMDA, 6FDA, ODPA dianhydrides with different degrees of connectivity. Extensive structural characterizations, isotropic dilation and unaxial stress elongations were performed on the model structures at 22°C, 300°C and 400 ° C in order to assess the effect of OAPS on thermomechanical resistance. The most promising candidates included polyOAPS-6FDA.

The synthesis of the first polyOAPS-6FDA membranes has already been carried out and the characterization tests on these new hybrid membranes are promising. They seem to retain their mechanical integrity and their permeation properties up to at least 400 °C, which is significantly higher than for the initial polyPOSS-imides.

The prediction of the NMR spectra for the isomers of the OAPS precursor must be refined and the conditions for the interfacial polycondensation must be optimized, especially for the choice of appropriate solvents.

The selection will be further refined by modelling gas transport for CO2/CH4, N2/CH4, CO2/N2, H2S/CH4 and CO2/H2S separations over a wide range of pressures and temperatures, which is difficult to carry out in the laboratory. The novel polyPOSS-imide membranes showing both improved thermoresistance and optimized gas separation properties will then be assembled at Twente atop inorganic porous hollow fibres in order to obtain supported materials that can be used for upscaling.

Communications:
«Hybrid organic/inorganic polyPOSS-imide model networks for gas separation applications« S. Neyertz, D. Brown, S. Salimi, M.J.T. Raaijmakers et N.E. Benes, STEPI 11 - Polyimides and High Performance Polymers, Juin 2019, Montpellier, France

«Molecular design of hybrid organic-inorganic poly(POSS-imide) networks for gas separations under harsh conditions«, S. Neyertz, D. Brown, S. Salimi, F. Radmanesh et N.E. Benes, ICOM 2020 (International Congress on Membranes and Membrane Processes), Dec. 2020, London, UK

Gas separation by dense polymer membranes is a very promising alternative to the cryogenic distillation or adsorption separation processes due to its much lower energy costs. This is critical for chemical industries, where the separation of mixtures accounts for over 50% of the energy costs. More energy-efficient methods should improve economical viability and lower greenhouse gas emissions. In addition, membrane devices are compact and fairly easy to operate. Unfortunately, polymers do not perform well under harsh conditions since they tend to lose their structural integrity at high temperatures and pressures.

The University of Twente, The Netherlands, has recently developed new hybrid ultrathin membranes based on inorganic POSS cross-linked with organic imides in order to improve the thermomechanical resistance while maintaining the gas-sieving properties. The synthesis is suited to large-scale production and these hybrid polyPOSS-imide networks are indeed able to perform under tougher conditions than conventional polymers. However, the aliphatic arms of the POSS precursors used so far are too flexible and prone to thermal degradation, which prevents their use above ~300°C. In addition, the gas sieving abilities are strongly dependent on the precursors, the cross-linking densities, the temperatures and the pressures. Furthermore, experiments under harsh conditions are difficult to carry out and it has not been possible to characterize them over a large range of high temperatures and pressures.

The aim of MOLHYB is to exploit the possibilities opened up by this innovative class of materials in order to develop new hybrid membranes capable of performing at very high temperatures and pressures, based on a combined molecular modelling and experimental approach. During an initial collaboration with Twente, the LEPMI at the University Savoie Mont Blanc, France (LEPMI-USMB), has developed realistic molecular models of two polyPOSS-imides at one cross-linking density. We intend to design more robust materials within MOLHYB. Molecular dynamics simulations will be used at LEPMI-USMB to pre-screen a novel set of candidate polyPOSS-imides for improved thermomechanical resistance, i.e. up to at least 400°C, without compromising their gas separation function. Their physical and mechanical properties will be characterized at the molecular-level as a function of the precursors, the cross-linking densities and the temperature. Only the most promising structures will then be synthesized and characterized experimentally by Twente. In parallel, single-gas sorption and transport in the selected model polyPOSS-imides will be studied at LEPMI-USMB for penetrants with different plasticizing capabilities, i.e. N2, CH4, CO2 and H2S, under a full set of both normal and harsh conditions. The latter are industrially-relevant conditions that are difficult to attain safely in the laboratory. Twente will carry out single-gas permeation experiments under a limited set of conditions to validate the model results. This will provide ideal selectivities for CO2/CH4, N2/CH4, CO2/N2, H2S/CH4 and CO2/H2S separations under a large range of conditions. To assess the influence of mixed-gas reservoirs, LEPMI-USMB will also consider CO2+CH4+H2S mixtures. The novel polyPOSS-imide membranes showing both improved thermoresistance and optimized gas separation properties will then be assembled at Twente atop inorganic porous hollow fibres in order to obtain supported materials that can be used for upscaling.

Based on this combined approach, MOLHYB should lead to better materials for selective separations under harsh conditions, i.e. with mixed-gas reservoirs at high temperatures and pressures. Since The Netherlands are not part of the countries selected for PRCI, Twente will entirely provide its own funding. The ANR demand only concerns the French partner LEPMI-USMB and is mainly aimed at funding a Ph. D. student.