Permporometry Measurements Research Articles

Functionalization of silica membranes for CO2 separation

Five organic CO2-philic functional groups were incorporated in silica matrixes for preparation of functionalized silica membranes to explore the CO2 separation performance. Chemical groups including acetate, trifluoromethyl, methacrylate, urea and vinyl groups were anchored in the silica network using the co-condensation method.The information from 29Si solid-state NMR and FTIR analyses indicates the successful formation of a covalent bond between functional groups and the silica network. The thickness of the functionalized silica layers was measured by SEM and the thermal stability of the organic groups was determined by thermogravimetric analysis (TGA).The gas permeance and mixed gas selectivity of CO2/N2 was measured in the temperature range of 253–373 K with a feed pressure of 9 bar. A maximum selectivity of as high as 10 was observed for a trifluoromethyl functionalized silica membrane with a CO2 permeance of 5.5 × 10−7 mol s−1 m−2 Pa−1. Permporometry measurements indicated that the contribution of flow through micropores to the total flow for all the functionalized silica membranes varied between 62 and 82%. All membranes were CO2 selective.

Rejection modeling of ceramic membranes in organic solvent nanofiltration

In this work, a transport model based on equations for aqueous nanofiltration has been adapted to describe the rejection of ceramic membranes for nanofiltration in organic solvents. Required membrane parameter were taken from permporometry measurements of the respective membrane. The calculated results were validated by rejection measurements with different membranes of MWCOs between 350 and 1200g/mol. By selecting an appropriate standard deviation of the pore size, rejection curves measured with polystyrenes in THF were successfully described using the model without any fitting parameters. Differences between the calculated and the experimental rejections were smaller than 0.7% for the membrane with the smallest pore size. The use of the model for a real life solute system has been checked with different specialty chemicals and indicates a good transferability. Variations of process parameters as temperature, pressure and feed velocity were in excellent accordance with the experimental observations. However, to reflect the incomplete rejection of membranes with a high amount of defect pores (detectable by the permporometry measurement), the model has to be extended as well as for the applicability to different solvents.

Thermoresponsive ultrafiltration membranes for the switchable permeation and fractionation of nanoparticles

Poly(ethylene terephthalate) track-etched ultrafiltration membranes (110nm pore diameter) have been functionalized with the thermo-responsive polymer poly(N-isopropylacrylamide) (PNIPAAm) via surface-initiated Atom Transfer Radical Polymerization (SI-ATRP). The PNIPAAm chain lengths, i.e. degree of graft functionalization, inside the membrane pores could be controlled very well with polymerization time. Importantly, gas flow/pore dewetting permporometry measurements demonstrated that the pore diameter in the dry state could be reduced and that the narrow pore size distribution of the membranes was not changed during the grafting process. Both hydrodynamic pore diameters of the membranes and grafted hydrodynamic layer thickness on the pore walls as well as their response to temperature could be estimated by measuring water permeability and applying Hagen–Poiseuille law. Defined temperature-induced swelling/deswelling ratios of ~2 had been observed. These data indicate that PNIPAAm chains in the brush state had been achieved. The ultrafiltration membrane pores could be switched between more open and more closed states. For example, the hydrodynamic pore diameter could be switched from 21nm at 23°C to 69nm at 45°C. For the same type of membrane the rejection of monomodal 21nm silica nanoparticles could be switched from 99% at 23°C to only 35% at 45°C. The rejection for larger monomodal 35nm silica nanoparticles was above 90% for every functionalized membrane irrespective of the temperature. For an exemplary functionalized membrane evidence for a switchable size-selective NP fractionation has been found. A mixture of the 21 and the 35nm silica nanoparticles was ultrafiltered through the membrane and at 23°C only the smaller ones could be found in the permeate whereas at 45°C also the larger nanoparticles were able to pass the membrane.

Stimuli–responsive track-etched membranes via surface-initiated controlled radical polymerization: Influence of grafting density and pore size

The surface-functionalization of poly(ethylene terephthalate) track-etched membranes of different nominal pore sizes (400, 1000 and 3000 nm) with stimuli–responsive poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) via surface-initiated (SI) atom transfer radical polymerization (ATRP) was performed. Variations of grafting density and grafted chain length were achieved by variation of synthesis conditions. It could be clearly demonstrated that mixtures of reaction solutions containing different ratios of acyl bromides, only one bearing the initiator group necessary for the SI ATRP, led to different initiator group densities on the resulting track-etched membrane surface which had been verified by X-ray photoelectron spectroscopy. Moreover the mass increase as function of reaction time strongly correlated with the amount of initiator bound to the membrane surface indicating that the ATRP reaction was not limited by monomer diffusion into the pores. Scanning electron microscopy images and permporometry measurements indicated an even functionalization on the entire membrane surface which was the basis for further investigations. The stimuli–responsive properties of PDMAEMA grafted track-etched membranes were studied by permeability measurements with citrate and glycine buffers as function of pH (2 and 10) and temperature (25 and 60 °C). By that the barrier properties of the membranes could be effectively changed in two steps. The results agree with the expectation that a change in grafting density and chain length has an effect on the stimuli–responsive properties of the membrane. Results for membranes having similar degrees of grafting clearly showed that the reversible swelling of grafted polymeric layers was more pronounced for lower grafting density.

H 2/CO 2 permeation through a silicalite-1 composite membrane

Single and binary H 2/CO 2 gas permeation was studied through a silicalite-1 composite membrane consisting of a thin zeolite film (<1 μm) supported on α-alumina. The temperature range for permeation measurements was 25–300 °C. To determine the quality of the membrane, i.e. the quantity and size of defects, n-hexane/helium permporometry measurements were performed. In general, single component fluxes decreased with increasing temperature whereas binary component fluxes showed a maximum value followed by a continuous decrease. A mass transport model that takes into account the surface diffusion and gas translational diffusion in the zeolite pores, Knudsen diffusion in defects, as well as viscous flow and Knudsen diffusion in the support material was developed to simulate the single and binary gas permeation measurements. Simulation results show that the surface diffusion was the dominant mass transport mechanism in the membrane. In addition, the transport resistance of the support material was not negligible and it was found to influence the permeation selectivity. The model adequately described the experimental results for both single and binary permeation. The model predictions indicated that a CO 2/H 2 separation factor exceeding 9.8 and a CO 2 flux exceeding 4 mol/(m 2 s) could be obtained at 0 °C and a feed pressure of 10 bar and atmospheric permeate pressure.

Performance and stability of multi-channel MFI zeolite membranes detemplated by calcination and ozonication in ethanol/water pervaporation

Two multi-channel defect-free high-silica MFI membranes have been tested for the removal of ethanol from a 7 mol% and 3 mol% aqueous solution under pervaporation conditions. One membrane is detemplated by calcination at 773 K, and one is ozonicated at 473 K. The flux and ethanol separation factor were determined at 348 K, 360 K, and 373 K. The ethanol separation factors of both membranes were similar and ranged from ∼ 20 at 373 K to ∼ 40 at 348 K. The ethanol and water fluxes through both membranes in this temperature range are proportional to the component fugacity difference over the membranes. The calcined membrane showed an 80% higher flux than the ozonicated membrane, showing the ozonication could not completely remove the template. After the ozonicated membrane was calcined at 723 K, the flux increased to a similar value as the initially calcined membrane, while the separation factor did not change. Both membranes showed an increase in flux by a factor of two after a subsequent calcination at 823 K, also without affecting the separation factor. The high calcination temperature required for complete detemplation indicates the presence of aluminum in the zeolite framework of the membrane. The membrane stability was monitored for 40 h and 177 h of operation for the initially calcined and ozonicated membrane, respectively. At 348 K the membranes showed stable performance, but the stability of the membranes at elevated temperatures is limited, showing a decrease in both flux and selectivity during operation at 373 K. The effect of the exposure to a water/ethanol mixture on the zeolite membrane is investigated by permporometry measurements of the membranes and by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, magic angle spinning nuclear magnetic resonance (MAS NMR), thermogravimetric analysis (TGA), and nitrogen adsorption experiments using several MFI-type zeolite powders. The most plausible mechanism causing the instability of the membrane is ethoxy formation of the ethanol with the silanol groups and Brønsted acid sites in the zeolite framework combined with the formation of hydrophilic extra-framework pathways by hydrolysis.

In situ evaluation of defect size distribution for supported zeolite membranes

The permporometry measurements are performed with respect to a series of zeolite membranes with different defect sizes, which can be further applied for in situ measurement of the defect size distribution in zeolite membranes. Gas permeation experiments are conducted for CO 2/N 2 gas mixture to test the separation performance of the studied zeolite membranes. By taking into account the “ t-layer” on defect walls, a mathematical model and the corresponding procedure are developed so that the defect size distribution in zeolite membranes can be calculated by using the results of permporometry measurements. The defect size distribution and the maximal defect size show a good correlation with the separation performance of CO 2/N 2 gas mixture for zeolite membranes. It is demonstrated that the separation performance of zeolite membranes is mainly determined by large defects. It has been shown that the permporometry-based methodology proposed in this contribution is an effective way for the quality evaluation of zeolite membranes.

Preparation and characterization of non-ionic block co-polymer templated mesoporous silica membranes

We report on the preparation and characterization of mesoporous silica membranes via micellar templating using the non-ionic ethylene oxide–propylene oxide–ethylene oxide tri-block copolymer surfactant, Pluronic P123, as a template under various template/silica volume percentages ( V TS%). The silica membranes were prepared by dip coating silica sols onto porous α-alumina supports followed by calcination to remove the template. The membranes were characterized by single gas permeation and permporometry measurements. Using a V TS% of 33 we found that silica membranes comprised of three dip/fire cycles resulted in highly reproducible continuous membranes with minimal defects. Helium and nitrogen permeance values were of the order of 10 −6 mol m −2 s −1 Pa −1. The pore size distribution, determined from permporometry measurements, indicated a dominant peak at 5 nm diameter and approximately 10% microporosity. In addition, there was 8–10% permeance through pores with diameters between 15 and 40 nm. At higher V TS% (47.5 and 65) the silica membranes had considerable bypass flow. The pore size distribution (PSD) of the membrane via permporometry indicated a shift to smaller values for the membrane when compared to the PSD of the powder from the calcined dipping sol, determined by nitrogen adsorption/desorption.

Zirconium-containing mesoporous silica Zr-MCM-48 for alkali resistant filtration membranes

Zirconium-containing mesoporous silica (Zr-MCM-48) membranes were synthesized on a porous alumina support from an aqueous solution containing tetraethylorthosilicate (TEOS), zirconium propoxide (ZrPr) and cetyltrimethylammonium bromide (C 16TAB). The Zr-MCM-48 membranes show high resistance in alkaline solutions at pH 9–12 although the structure of the pure silica MCM-48 membranes collapsed even at pH 10. Zirconium species is considered to present near the pore surface of MCM-48 because the reactivity of ZrPr with surfactant (C 16TAB) molecules is much higher than TEOS in the starting solution. Zr-MCM-48 shows more stable in alkaline solutions than Ti-containing MCM-48 (Ti-MCM-48). The gas permeation and permporometry measurements suggest that there are no large pinholes and cracks in the Zr-MCM-48 membranes. From permporometry the average pore size of the Zr-MCM-48 membrane is estimated to be 2.5 nm. The filtration of polyethylene glycol (PEG) in water shows that the membranes has molecular weight cutoff (MWCO) of about 3800 u. The corresponding molecular size is 2.3 nm. The sharp increase in the rejection of PEG with increase in molecular weight shows narrow pore size distribution of the membrane. These results show that Zr-MCM-48 membranes have great potential for nanofiltration and ultrafiltration with high performance.

Pore size distributions of silylated mesoporous silica MCM-48 membranes

Abstract Mesoporous silica MCM-48 membranes were synthesized on a porous alumina support using surfactant molecules which act as template for the polymerization of silica. Higher synthetic temperatures resulted in thicker silica wall. However, the average pore size of MCM-48 was independent of the synthetic temperature. The permeation of N2 through the calcined MCM-48 membrane was governed by the Knudsen diffusion. There was no contribution of viscous flow, which can be observed in large pinholes. The pore size distributions of silylated mesoporous silica MCM-48 membranes were evaluated by permporometry measurements in which N2 flux was monitored in the presence of methanol vapor. A sharp decrease in the N2 flux at a relative vapor pressure of methanol equal to 0.5 was observed, indicating a narrow pore size distribution of the MCM-48 membrane. The corresponding Kelvin diameter of the membranes was about 2.0 nm, which is consistent with the value for the silylated MCM-48 powder calculated with Barrett–Joyner–Halenda (BJH) method using N2 adsorption isotherm.

Modification of Porous Alumina Membranes Using Al2O3 Atomic Layer Controlled Deposition

Al2O3 films were deposited with atomic layer control inside the pores of Anodisc alumina membranes. To achieve this controlled deposition on a high aspect ratio structure, a binary reaction for Al2O3 chemical vapor deposition (2Al(CH3)3 + 3H2O → Al2O3 + 6CH4) was separated into two half-reactions: (A) AlOH* + Al(CH3)3 → Al-O-Al(CH3)2* + CH4 and (B) AlCH3* + H2O → AlOH* + CH4, where the asterisks designate the surface species. The trimethylaluminum [Al(CH3)3] (TMA) and H2O reactants were employed alternately in an ABAB... binary reaction sequence to deposit the Al2O3 film. Because each half-reaction is self-limiting, atomic layer controlled Al2O3 deposition was achieved on the surface of the high aspect ratio pores. To determine the necessary reaction conditions, surface species during each half-reaction were periodically monitored using in situ transmission FTIR spectroscopy. Ex situ gas flux and permporometry measurements were also performed to determine the effect of the binary reaction sequence on the...