PTMSP Membranes Research Articles

Hybridizing zeolite MFI nanosheets with PTMSP membranes for enhanced butane isomer separations

Hybridizing zeolite nanosheets with a high aspect ratio has the potential to improve the gas separation performance of polymeric materials due to their good mechanical strength, thermal stability, and well-defined microporous structure. However, the use of zeolite-nanosheet-based mixed matrix membranes (MMMs) has been limited, and their effects on polymeric materials have not been studied thoroughly. In this study, zeolite MFI nanosheets were directly synthesized and introduced into PTMSP polymer using the shear-coating method. This resulted in a horizontally aligned structure with preferentially b-oriented zeolite MFI fillers. The straight b-channels of zeolite MFI facilitated fast gas permeation, while the high aspect ratio increased the tortuosity for non-permeable gases. The elastic modulus of the composite increased from 306 MPa to 2069 MPa as the zeolite MFI nanosheet content was increased to 50 wt%. The membrane was thermally stable up to 285 °C due to the protection provided by the zeolite MFI filler, which prevented thermal degradation by oxygen. The gas separation performance was improved, with a butane isomer selectivity of 7.3 and a permeability of 5400 barrer, a 20% increase from the neat PTMSP membrane. The selectivity was well preserved even in the binary mixture of butane isomers gas permeation test at 1 bar, while the neat PTMSP membrane showed a dramatic decrease in selectivity under this condition. The compatibility between the nanosheets and the polymer is expected to be important in achieving high separation performance of the composite membrane, as can be inferred from the large difference between the performance predicted by the modified Cussler model and the actual performance.

High Efficiency Membranes Based on PTMSP and Hyper-Crosslinked Polystyrene for Toxic Volatile Compounds Removal from Wastewater.

For the first time, membranes based on poly(1-trimethylsilyl-1-propyne) (PTMSP) with 5–50 wt% loading of hyper-crosslinked polystyrene sorbent particles (HCPS) were obtained; the membranes were investigated for the problem of effective removal of volatile organic compounds from aqueous solutions using vacuum pervaporation. The industrial HCPS sorbent Purolite Macronet™ MN200 was chosen due to its high sorption capacity for organic solvents. It has been found that the membranes are asymmetric when HCPS content is higher than 30 wt%; scanning electron microscopy of the cross-sections the membranes demonstrate that they have a clearly defined thin layer, consisting mainly of PTMSP, and a thick porous layer, consisting mainly of HCPS. The transport and separation characteristics of PTMSP membranes with different HCPS loading were studied during the pervaporation separation of binary and multicomponent mixtures of water with benzene, toluene and xylene. It was shown that the addition of HCPS up to 30 wt% not only increases the permeate fluxes by 4–7 times, but at the same time leads to 1.5–2 fold increase in the separation factor. It was possible to obtain separation factors exceeding 1000 for all studied mixtures at high permeate fluxes (0.5–1 kg/m2∙h) in pervaporation separation of binary solutions.

Effect of Immobilization of Phenolic Antioxidant on Thermo-Oxidative Stability and Aging of Poly(1-trimethylsilyl-1-propyne) in View of Membrane Application.

The effect of phenolic antioxidant Irganox 1076 on the structure and gas permeation behavior of poly(1-trimethylsilyl-1-propyne) (PTMSP) was investigated. Isotropic films as well as thin film composite membranes (TFCM) from pure PTMSP and with added antioxidant (0.02 wt%) were prepared. PTMSP with antioxidant has a significantly higher thermal degradation stability in comparison to pure polymer. The thermal annealing of isotropic films of PTMSP with antioxidant was carried out at 140 °C. It revealed the stability of gas permeation properties for a minimum of up to 500 h of total heating time after a modest permeation values decrease in the first 48 h. X-ray diffraction data indicate a decrease in interchain distances during the heat treatment of isotropic films and indicate an increase in the packing density of macromolecules during thermally activated relaxation. Isotropic films and TFCMs from pure PTMSP and with antioxidant stabilizer were tested under conditions of constant O2 and N2 flow. The physical aging of thick and composite PTMSP membranes point out the necessity of thermal annealing for obtaining PTMSP-based membranes with predictable properties.

Hybrid Microporous Polymeric Materials with Outstanding Permeability and Increased Gas Transport Stability: PTMSP Aging Prevention by Sorption of the Polymerization Catalyst on HCPS.

The influence of hyper-crosslinked polystyrene (HCPS) MacronetTM MN200 on the gas transport properties and aging of the highly permeable glassy polymer poly(1-trimethylsilyl-1-propyne) (PTMSP) was studied and analyzed in detail. The gas transport characteristics of dense PTMSP membranes containing 0–10.0 wt % HCPS were studied. It was shown that the introduction of a small amount of HCPS into the PTMSP matrix led to a 50–60% increase of the permeability coefficients of the material for light gases (N2, O2, CO2) and slowed down the deterioration of polymer transport properties over time. The lowest reduction in gas permeability coefficients (50–57%) was found for PTMSP containing HCPS 5.0 wt % after annealing at 100 °C for 300 h. It was found that HCPS sorbed residues of tantalum-based polymerization catalyst from PTMSP. In order to investigate the influence of catalysts on transport and physical properties of PTMSP, we purified the latter from the polymerization catalyst by addition of 5 wt % HCPS into polymer/chloroform solution. It was shown that sorption on HCPS allowed for almost complete removal of tantalum compounds from PTMSP. The membrane made of PTMSP purified by HCPS demonstrated more stable transport characteristics compared to the membrane made of the initial polymer. HCPS has a complex effect on the aging process of PTMSP. The introduction of HCPS into the polymer matrix not only slowed down the physical aging of PTMSP, but also reduced chemical aging due to removal of active reagents.

Construction of ultrathin PTMSP/Porous nanoadditives membranes for highly efficient organic solvent nanofiltration (OSN)

Organic solvent nanofiltration (OSN), also known as solvent resistant nanofiltration (SRNF), has enormous potential for industrial applications. Among the emerging high-performance membrane materials, some have proven challenging to fabricate ultrathin selective layers and at a scale large enough to be practical for industrial consideration. This work demonstrates a facile and scalable solution casting method to prepare ultrathin composite films from advanced polymeric materials and porous additives. Using this method, poly(trimethylsilylpropyne) (PTMSP) membranes could be controlled to just 250 nm thickness resulting in 2 to 30-fold higher solvent permeance than other PTMSP membranes reported to date. Two highly porous additives, poly-dichloroxylene (p-DCX) and Porous Aromatic Framework (PAF-1) were employed to fabricate ultrathin PTMSP composite membranes. Compared to the native PTMSP membrane (methanol permeance of 11.5 L m−2 h−1 bar−1), these additives enhanced membrane permeance by 75% (20.1 L m−2 h−1 bar−1) and 50% (17.2 L m−2 h−1 bar−1), respectively. Inclusion of the porous additives also enhanced membrane stability, demonstrated by 300 h of continuous OSN operation. These membranes showed improved molecular weight cut-off (MWCO) and significant potential in the separation of active pharmaceutical ingredients (APIs), including the almost complete rejection of Vitamin B12 in methanol. This work demonstrates that the facile production of ultrathin membranes using intrinsically microporous polymers is possible, enabling these exciting membrane materials to be considered for industrial applications.

How is mixed-gas permeation through poly(1-trimethylsilyl-1-propyne) membranes influenced by elevated temperatures?

Poly(1-trimethylsilyl-1-propyne) (PTMSP), one of the most permeable membrane materials, has been extensively studied for gas permeation in the past. Even though it is in a glassy state, PTMSP permeates larger molecules selectively over small inert gases. Its mixed-gas selectivity in gas mixtures containing condensable gases is preferential towards the condensable components. This peculiar behavior is due to its extremely high non-equilibrium frozen-in excess free volume. This behavior also makes PTMSP and its transport performance highly relevant to study the effect of the high free volume - often called intrinsic microporosity - for applications in natural gas treatment, petroleum processing, or membrane reactors. While condensability strongly depends on the temperature, a detailed analysis of the temperature behavior in terms of the enhanced mixed-gas selectivity has not yet been performed for polymers with intrinsic porosity.Within this work, pure- and mixed-gas experiments with 1-butene (C4H8) and nitrogen (N2) were conducted at different C4H8 partial pressures and for temperatures ranging from 20∘C up to 120∘C. At 20∘C, a minimal C4H8 partial pressure of 0.5 mbar already depressed the N2 flux through PTMSP membranes in C4H8/N2 mixtures. With increasing temperatures, the extent of the C4H8 influence on N2 permeation vanishes. Elevated temperatures caused the mixed-gas selectivity αC4H8/N2 to decrease, but even at 120∘C it remained above the pure-gas selectivity.PTMSP is a membrane material with high-excess non-equilibrium free volume, which makes it prone to aging. Therefore, the physical aging of PTMSP membranes was recorded throughout the experiments. For temperatures exceeding 50∘C, pronounced aging occurred. Although the physical aging was reversed to a small degree when contacted by C4H8, the resulting permeabilities remained below the initial values. For temperatures ≤50∘C however, an in-situ rejuvination of membrane permeabilities was possible with small concentrations of C4H8 in the gas mixture. This in-situ rejuvination makes the long-term use of PTMSP membranes possible at low temperatures where condensable gases are present at sufficiently high concentration in the polymer.

TRANSPORT PROPERTIES OF LIQUID PHASE PERCOLATION CLUSTER IN HIGHLY PERMEABLE GLASSY POLYMERS

Porosity, permeability, and conductivity of PTMSP membranes in ethanol-water mixtures were determined by analyzing the impedance characteristics. The critical volume fraction of porosity at which through conductivity channels are formed was calculated. When the alcohol content in the solution increases, it was 0.29; when it decreases, it was 0.17. The obtained dependencies indicate that the transfer of matter in the membrane occurs the liquid phase elements, forming a percolation cluster as the amount of sorbed liquid increases. The hysteresis effect was observed in the measured values of porosity, permeability, conductivity, and capacity. The porous structure of a dry PTMSP membrane consists of free volume elements that are unconnected to each other. At ⁓30% alcohol concentration, through channels, begin to form, connecting both sides of the membrane. In this case, both permeability and electrical conductivity increase as a power function, in contrast to the porosity that increases linearly. Lichtenecker and Rother’s formula helped to calculate the volume fractions of the conducting and non-conducting phases in the system. The obtained values of the amount of liquid in the membrane at various spatial arrangements of the channels made it possible to establish the structure of the liquid-filled channels. The best agreement with the experimental values was obtained for the chaotic structure. This is consistent with the existing ideas about the structure of the studied membranes and confirms the assumption that the percolation cluster is formed from free volume elements filled with the liquid phase. The observed low values of the zeta potential and, consequently, the low values of the electrostatic component of the wedging pressure indicate that the increase in the size or the number of transport channels is associated with non-electrostatic forces.

High-Performance Reinforced PTMSP Membranes for Thermopervaporation Removal of Alcohols from Aqueous Media

PTMSP membranes reinforced with a metal wire mesh have been fabricated and experimentally studied to increase the permeability and mechanical properties of poly(1-trimethylsilyl-1-propyne) (PTMSP) membranes in the course of thermopervaporation (TPV) removal of butanol from fermentation broths. The effect of the material wire of the mesh (stainless steel and bronze) and mesh size (30–40 μm) on the flux and separation factor in the course of thermopervaporation separation of aqueous solutions of butanol has been experimentally studied. It is shown in this work that the use of a metal wire mesh as a support instead of traditional commercial porous supports does not reduce the separation properties of a PTMSP membrane in the course of TPV removal of butanol from model fermentation mixtures. Separation modes that make it possible to obtain permeate fluxes above 1 kg m−2 h−1 have been found. It is shown in this work that the permeability coefficient of water through PTMSP membranes is 2.5 × 10−5 mol m−2 h−1 kPa−1 and does not depend on the temperature of the feed and thickness of the membranes.

Poly(trimethylsilylpropyne) Membranes for Removal of Alcohol Fermentation Products by Thermopervaporation with a Porous Condenser

Dense membranes made of poly[1-(trimethylsilyl)-1-propyne] (PTMSP) have been studied in thermopervaporation (TPV) separation of a butanol–water binary mixture and a real acetone–butanol–ethanol (ABE) broth. When comparing PTMSP with commercially used membranes based on polydimethylsiloxane (PDMS) in the TPV process, a significant advantage of PTMSP with respect to the butanol/water selectivity is shown. The value of the separation factor for the PTMSP membrane (114) with a thickness of 61 μm is more than ninefold higher than that of the most selective Pervap 4060 commercial membrane (Sulzer Chemtech, Switzerland). The permselectivity of PTMSP membranes with respect to the butanol/water components exceeds that of commercial membranes more than fivefold. The removal of acetone, butanol, and ethanol from real fermentation broths using PTMSP membranes in the TPV process with a porous condensation surface has been studied for the first time. It has been shown that the transport and separation characteristics of PTMSP membranes in the process of separation of real fermentation broths decline with time, while demonstrating high values of the butanol/water permselectivity (4.7).

Pervaporation and vapour permeation of methanol – dimethyl carbonate mixtures through PIM-1 membranes

The separation of dimethyl carbonate (DMC) from its mixtures with methanol was studied using pervaporation (PV) and vapour permeation (VP) through thick (∼0.5 mm) PIM-1 membranes; PV characteristics of PDMS and PTMSP membranes are provided for comparison. DMC is a “green” chemical with numerous applications in chemistry, but its production is energy- and cost-intensive. As their azeotrope contains 82 mol.% of methanol at 40 °C, DMC-selective rather than common methanol-selective membranes can allow for energy efficient separation and thus production of this “green chemical”. PV of the azeotropic mixture through the PIM-1 membrane showed a separation factor of 2.3, which is comparable to that observed for PV through the PDMS membrane; the PTMSP membrane showed practically no separation. The total PV fluxes followed the order: PTMSP ≫ PIM-1 > PDMS. When the PIM-1 membrane was operated in the VP mode, a separation factor of up to 5.1 was reached for the vapours having the azeotropic composition, while total fluxes dropped ca 50-times compared to PV. The highest observed separation factor of 6.5 was found for VP of DMC-rich vapour mixtures highly diluted with inert gas. To our knowledge, VP through PIM-1 membranes enables to date the most selective membrane-based removal of DMC from its azeotrope with methanol.

Evaluation of liquid transport properties of hydrophobic polymers of intrinsic microporosity by electrical resistance measurement

In this work, the electrical resistance method was used to get an insight of the liquid transport through the polymers with excess of fractional free volume. The dense films based on the first generation of polymers of intrinsic microporosity, disubstituted polyacetylenes PMP and PTMSP, were used; water-ethanol binary mixtures allowed to adjust the affinity of the liquid phase to selected polymers continuously. In this work, the liquid-membrane interaction was considered as a stepwise occupation of accessible free volume elements within the polymer, while the appearance of continuous (pseudo)liquid channels in the polymeric matrix, so-called percolation clusters, was required for formation of hydrodynamic liquid transport. Since the appearance of such percolation clusters might dramatically change the conductivity of hydrophobic materials, it was proposed to use the electrical resistance method to investigate the evaluation of selected polymers from being a barrier (water) to being permeable (ethanol) with respect to the liquid composition. To highlight the electrical resistance of the swollen films, all water/ethanol solutions contained NaCl (0.5 g/L). The steady-state values of liquid permeation, sorption/swelling, and electrical conductivity data were considered together and discussed. The electric resistance was significantly varied for 30 µm dense films of PMP (1–1300 kΩ) and PTMSP (0.15–910 kΩ) by the adjustment of ethanol content in water. The phase shift (25– 100,000 Hz) illustrated that the behavior of PTMSP and PMP membranes changed from mainly capacitive at low ethanol concentrations to the ion conductive as higher ethanol content. The very good quantitative agreement was found between the relative change of the membrane electrical resistance and the permeability as a function of the ethanol content in the liquid phase. Consequently, electrical resistance measurements could be used as an express method to determine the membrane permeability of low permeable materials, since the time required to get steady-state results was found to be much shorter for electrical resistance measurements than for permeability measurements.

Stabilization of gas transport properties of PTMSP with porous aromatic framework: Effect of annealing

This work was focused on the study of physical aging of PTSMP and preventing this effect via incorporation of porous aromatic frameworks (PAF-11). The dense PTMSP membranes with PAF-11 content of 0, 1, 5 and 10wt% were stepwise annealed in air at 100°C for up to 510h with constant monitoring of O2, N2 and CO2 permeability (measured at 30°C). As-cast and aged PTMSP samples were characterized by means of helium pycnometry, dynamic mechanical analysis and IR spectroscopy. Gas transport characteristics of the PTMSP sample containing 10wt% of PAF-11 became stable upon annealing at 100°С within the short time interval (100–200h). However, for all other samples containing 5wt% of PAF-11 or less, gas permeability gradually decreased with time. According to the IR analysis, upon high-temperature treatment, PAF-11 nanoparticles tended to migrate from the sub-layer region into the membrane matrix bulk. According to the helium pycnometry, the introduction of PAF-11 to the PTMSP matrix provided a loosened packaging of polymer chains, and the structure became more stabilized since the porous filler served as a “physical” cross-linker.

PTMSP-ZIF 복합막의 기체투과 특성

PTMSP-ZIF composite membranes were prepared by the addition of zeolitic imidazolate framework (ZIF-8) into poly (1-trimethylsilyl-1-propyne) (PTMSP) having high gas permeability to improve trade-off relationship of the polymer membrane. PTMSP-ZIF composite membranes were prepared with different amounts of ZIF-8; 0, 5, 10, 20, 30 and 40 wt%. Gas permeation properties for H2, N2, CO2, and CH4 were investigated by increasing the amount of ZIF-8 in the PTMSP. The gas permeability of PTMSP-ZIF composite membranes within 5∼30 wt% of ZIF-8 contents increased as ZIF-8 contents went up and decreased thereafter. The gas permeability for CO2 showed the maximum value of 76080 barrer at 30 wt% of ZIF-8 content and PTMSP-ZIF composite membrane containing 20 wt% of ZIF-8 content had the highest selectivity (CO2/N2) with the value of 8.2. The selectivity (H2/N2) and selectivity (CO2/CH4) were almost the same as PTMSP in the range 10∼40 wt% of the ZIF-8. Overall, PTMSP-ZIF composite membranes resulted in maintained selectivity and increased permeability compared to those of PTMSP membranes.

Predictive membrane transport models for Organic Solvent Nanofiltration: How complex do we need to be?

From feasibility tests at laboratory scale to large industrial scale processes, modelling can be applied to Organic Solvent Nanofiltration (OSN) for two purposes: (i) parameter estimation, when experimental data for standard solute + solvent systems are available, and it is desired to estimate relevant parameters for as yet uncharacterised systems, and (ii) prediction, when the model parameters for the solvent–solute are available, and modelling can be applied to describe the performance of a different solute + solvent system or operations at a different process scale. Both estimation and prediction require the choice of a transport model, to carry out regression and simulation, respectively. This paper reports a systematic comparison of a range of different transport models (irreversible thermodynamics, solution–diffusion, pore-flow and transient transport models, such as the solution–diffusion with imperfections model) using selected experimental data for various solutes and solvents through polymeric OSN membranes with different physico-chemical properties. Commercial integrally skinned asymmetric polyimide (PI, glassy) and thin film composite silicone-coated PI (rubbery) membranes from Evonik® MET were systematically tested in this study, with styrene oligomers and Safranin-O dye in different organic solvents under uniform operating conditions (temperature, pressure, cross-flow velocity and solute concentration). In addition, experimental data from the literature were taken for non-commercial thin film composite silicone-coated PI, glassy poly[1-(trimethylsilyl)-1-propyne] (PTMSP) and poly[4methyl2pentyne] (PMP) membranes, to include a broad spectrum of positive and negative rejection values. The different transport models were used to perform regression of experimental data and prediction at different pressure values, based on the regressed model parameters. The models were then compared in terms of regression performance and experimental/numerical effort required to use them for parameter estimation. Negative rejection data was used to further discriminate among the models. Solution–diffusion-based models gave a better description of permeation through flexible-chain glassy membranes than pore-flow models. On the other hand, pore-flow-based models gave a better description of permeation through glassy PTMSP and PMP membranes. For integrally skinned asymmetric PI membranes, the prediction of a concentration process using the best performing regression models (i.e. the Maxwell–Stefan and the classical solution–diffusion model) was also performed. Interestingly, no significant difference was observed between the two models for both a membrane with complete solute rejection (Duramem® 200) and a membrane with partial solute rejection (Duramem® 500). Process modelling by accounting for a classical solution–diffusion-type transport mechanism is therefore sufficient to obtain a reliable transport description for this membrane family.

Novel PTMSP-based membranes containing elastomeric fillers: Enhanced 1-butanol/water pervaporation selectivity and permeability

To improve organic solvent preferential permeation of poly[1-(trimethylsilyl)-1-propyne] (PTMSP) and to provide efficient separation of dilute 1-butanol/water mixtures, the membranes were modified by the addition of minor amounts of a soft elastomeric filler, poly(dimethylsilmethylene) (PDMSM), to the host polymeric matrix. As compared with virgin PTMSP membranes, the modified membranes demonstrate better 1-butanol/water pervaporation selectivity and permeability. This unique coupled phenomenon is observed at low concentrations of the elastomeric filler (~1–3wt%) in the host matrix and for the feed aqueous mixtures containing 0.6–3.6wt% of butanol. For vacuum pervaporation of butanol solutions, introduction of only 1.2wt% of PDMSM into the PTMSP matrix makes it possible to increase the permeate flux up to 75% and the separation factor up to 67%. When studying pervaporation of the feed solution containing 2wt% of butanol, the dependences of the permeate flux and separation factor on the elastomeric filler content in the host matrix show the well-pronounced maxima at 1.2–3wt% (MW of the elastomer is 83–1000kg/mol). In this case, the attained permeate flux is 1.5–1.8 times higher than that of the virgin PTMSP, and the separation factor increases by a factor of 1.4–1.7. The corresponding values of thickness-corrected flux and separation factor for the PTMSP/PDMSM membranes are 3.5–4.0kgm/(m2h) and 112–136, respectively. More specifically, the 30µm thick PTMSP/PDMSM (98/2) membrane exhibited permeate flux 0.12kg/(m2h) and separation factor 128 upon pervaporation of 2wt% aqueous butanol solution at 25°C. The experiments on the equilibrium sorption of butanol solutions on the PTMSP membranes modified by the elastomeric filler revealed similar dependences with their well-pronounced maxima in the same region of the PDMSM content. The highest value of membrane selectivity αmem for PTMSP/PDMSM is 8.9 as compared with 5.3 for the virgin PTMSP membranes. Qualitative explanation of the enhanced pervaporation selectivity coupled with the improved permeate flux for the PTMSP/PDMSM membranes was advanced.

Application of negative retention in organic solvent nanofiltration for solutes fractionation

For the first time, the utilization of negative retention in organic solvent nanofiltration (OSN) for solute fractionation was proposed and realized. Using membranes made of poly[1-(trimethylsilyl)-1-propyne] (PTMSP) and ethanol as a solvent, this principle was successfully proved for two different pairs of dyes: (i) Orange II (anionic; 350Da) and Solvent Blue 35 (neutral; 350Da), (ii) Remazol Brilliant Blue R (anionic; 626Da) and Oil Red O (neutral; 408Da). The solutes ratio in the permeate after one step filtration with 80% of liquid recovery was achieved as follows (initial ratio in the feed – 1:1): 1:10 for Orange II to Solvent Blue 35 and 1:26 for Remazol Brilliant Blue R to Oil Red O. With compromising of liquid recovery, the solutes ratio in the permeate can be significantly improved; for example, 15% liquid recovery may provide the ratio in favor of neutral solute as 1:260 and 1:340, respectively. Pronounced difference in solutes retention was interpreted by membrane–solute interaction expressed in terms of equilibrium solute distribution coefficient K between the membrane and liquid and difference in solubility parameters δ. Low values of solute distribution coefficient K for Orange II and Remazol Brilliant Blue R (less than 1) revealed that both anionic dyes have a tendency to be concentrated in liquid phase resulting in high solute retention (more than 90%). Better rejection of Remazol Brilliant Blue R (1258Å3) in contrast to Orange II (297Å3) was explained by difference in solute size. High K values (more than 10) observed for Solvent Blue 35 and Oil Red O indicated that these two neutral solutes have a high affinity to membrane material and can be easily accommodated within interconnected free volume elements of PTMSP. Such solubility determined solute transport resulted in negative retention. Analysis of solubility parameters δ of components showed that neutral solutes have a stronger affinity to the membrane material rather than ethanol; meanwhile, PTMSP has a preferential affinity to solvent molecules over selected anionic dyes. These experimental observations allowed to conclude that the principle of one step solutes fractionation based on combination of negative and positive retention could be realized with PTMSP membranes for OSN applications.

Study on ageing/fouling phenomena and the effect of upstream nanofiltration on in-situ product recovery of n-butanol through poly[1-(trimethylsilyl)-1-propyne] pervaporation membranes

Thin film composite poly[1-(trimethylsilyl)-1-propyne] – PTMSP – pervaporation membranes have been investigated for in-situ product recovery of n-butanol from a fermentation broth using a Clostridium acetobutylicum strain. For this specific application, a strong flux decline is observed which can be attributed to ageing and fouling phenomena in the membrane. In order to have a better understanding of these complex phenomena, X-ray photoelectron spectroscopy and infra-red spectroscopy have been used to monitor the ageing of the PTMSP membranes under different steady state conditions. The fouling effect of different components in the fermentation broth has been systematically investigated through off-line pervaporation tests on model mixtures with stepwise addition of fermentation by-products, revealing the negative impact of butyric acid and long chain fatty acids on the permeate flux. Additionally, long chain fatty acids, such as stearic acid, impact negatively the butanol/water separation factor. In order to remedy or at least decrease the impact of this fouling issue, the integration of an upstream nanofiltration step has been evaluated. This pre-treatment step has led to a drastic improvement of the flux through the filled PTMSP membrane (factor 4), while the butanol–water separation factor (27.5) remained much higher than the separation factor obtained with a commercial PDMS membrane (14.7).

Hydrophilic molecular sieve surface layer formation in hydrophobic poly(1-trimethylsilyl-1-propyne) membranes

The transport properties of benzene vapor, water vapor, and nitrogen gas were systematically investigated for vacuum-ultraviolet (VUV)-irradiated highly gas and vapor-permeable poly(1-trimethylsilyl-1-propyne) (PTMSP) membranes by a xenon excimer lamp at 172nm. The photon was completely absorbed by the surface membrane, and the VUV irradiation did not depend on the color center and the bulk structure of the PTMSP membrane. Moreover, the two-step surface chemical structure change of the PTMSP membrane from the highly hydrophilic C=O bond-rich surface layer to the low hydrophilic SiO2-rich surface layer was observed after VUV irradiation, in contrast to the effect on general polymer membranes. The two-step surface photooxidation and scission reaction of PTMSP membranes was likewise discovered. The surface-modified layer exhibited gas barrier properties for benzene vapor and nitrogen gas, but not for water vapor, because the benzene vapor and nitrogen permeability decreased as the VUV-irradiation time increased. This gas and vapor permeability reduction strongly depended on the molecular size, whereas the water vapor permeability did not depend on the surface hydrophilicity. This hydrophilic surface layer possessed molecular sieve properties. Thus, the PTMSP membrane in this study was changed by VUV irradiation from a benzene vapor-permselective non-molecular sieve to a water vapor-permselective molecular sieve.

Macrochain configuration, stucture of free volume and transport properties of poly(1-trimethylsilyl-1-propyne) and poly(1-trimethylgermyl-1-propyne)

The relationship between poly(1-trimethylsilyl-1-propyne) (PTMSP) and poly(1-trimethylgermyl-1-propyne) (PTMGP) microstructure, gas permeability and structure of free volume is reported. n-Butane/methane mixed-gas permeation properties of PTMSP and PTMGP membranes with different cis-/trans-composition have been investigated. The n-butane/methane selectivities for mixed gas are by an order higher than the selectivities calculated from pure gas measurements (the mixed-gas n-butane/methane selectivities are 20–40 for PTMSP and 22–35 for PTMGP). Gas permeability and n-butane/methane selectivity essentially differ in polymers with different cis-/trans-composition. Positron annihilation lifetime spectroscopy investigation of PTMSP and PTMGP with different microstructure has determined distinctions in total amount and structure of free volume, i.e. distribution of free volume elements. The correlation between total amount of free volume and gas transport parameters is established: PTMSP and PTMGP with bigger free volume exhibit higher n-butane permeability and mixed-gas n-butane/methane selectivity. Such behavior is discussed in relation to the submolecular structure of polymers with different microstructure and sorption of n-butane in polymers with different free volume.

Enhanced gas transport properties and molecular mobilities in nano-constrained poly[1-(trimethylsilyl)-1-propyne] membranes

Interfacial constraints in ultrathin poly(l-trimethylsilyl-1-propyne) (PTMSP) membranes yielded gas permeabilities and CO2/helium selectivities that exceed bulk PTMSP membrane transport properties by up to three-fold for membranes of submicrometer thickness. Maximum permeability coefficients of 110 × 103 Barrer and 27 × 103 Barrer for carbon dioxide and helium, respectively, were found to occur in membranes of ∼750 nm thickness. Indicative of a free volume increase, a molecular energetic mobility analysis (involving intrinsic friction analysis) revealed enhanced methyl side group mobility. This was evidenced by a minimum in the activation energies of ∼4 kcal/mol in thin PTMSP membranes with maximum permeation, compared to ∼5.5 kcal/mol in bulk films. Aging studies conducted over the timescales relevant to the conducted experiments signify that the free volume states in the thin film membranes are highly unstable in the presence of sorbing gases such as CO2. These results are discussed and contrasted to PTMSP bulk membrane systems, which were found to be unaffected by aging over the equivalent experimental time scale.