Mesoporous Ceramic Membranes Research Articles

Development of mesoporous ceramic membranes of diatomite/TiO2/Nb2O5 nanocomposites for the treatment of contaminated water

There is an urgent need for low cost and efficient methods for the treatment of contaminated water; in this regard the combination of natural minerals for the preparation of sieves is of special interest. The aim of this work is to develop mesoporous composite ceramic membranes using low-cost minerals: diatomite, niobium pentoxide (Nb2O5) and titanium dioxide (TiO2) powders. The multilayer samples were prepared by the tape casting technique. Initially, four tapes with different relative amounts of the minerals are prepared (diatomite %/ TiO2 %/ Nb2O5 %); the samples are A (1/0/0), B (0.9/0.1/0), C (0.9/0.05/0.05) and D (0.9/0/0.1). These tapes were cut, arranged in a multilayer, pressed, and calcined at 500 °C in presence of air. Then, these multilayers were sintered at 1200 °C in presence of argon to form the ceramic membranes (CM): CM-A, CM-B, CM-C and CM-D. A fifth ceramic membrane CM-E was prepared by combining tapes B and D. The X-ray diffraction characterization identified the phases cristobalite, quartz, anatase, rutile, and H-Nb2O5. The three-point bending test demonstrated a 225 % increase in strength after the incorporation of Nb2O5 and TiO2 particles to the diatomite matrix. For instance, the CM-A and CM-C membranes showed 4.28 and 13.90 MPa, respectively. The monolithic ceramic membranes showed high surface area, the BET analysis indicated a narrowing of pore sizes when are added Nb2O5 and TiO2 particles to diatomite; and showed surface areas of up to 6.161 m2 g-1. The composites presented band gap energies in the range of 3.19–3.70 eV. The ability of the ceramic membranes in the photodegradation of methylene blue in aqueous solutions was tested, the results showed a degradation of up to 87.3 % of the dye.

Mesoporous Polymer-Derived Ceramic Membranes for Water Purification via a Self-Sacrificed Template.

Membrane separation has been widely used in water purification, and mesoporous ceramic membranes show a high potential in the future because of their high stability and resistance to harsh environments. In the current study, a novel polymer-derived ceramic silicon oxycarbide (SiOC) membrane was developed via a preceramic reactive self-sacrificed method and was further applied in a homemade dead-end system for water purification. A cyclosiloxane hybrid polymer was selected as the precursor and polydimethylsiloxane (PDMS) was used as the sacrificial template. Membrane pores were formed because of template removal during the sintering process, creating channels for water transportation. The pore size and porosity could be readily adjusted by changing the amounts and types of PDMS used in the fabrication process. The as-prepared SiOC membrane showed a high water permeability (140 LMH@2.5 bar) and high removal rate of rhodamine B (RhB), demonstrating its potential applications in water treatment. This work would provide an easy and scalable method to prepare ceramic membranes with a controlled pore size, which could be used for different water treatment applications.

High Temperature and Pressure Alkaline Electrochemical Reactor for Conversion of Power to Chemicals

Moving away from fossil fuels requires harvesting more and more intermittent renewable energy resources and establishing a sustainable system for the production of chemicals. This brings forward the need for efficient large scale energy storage technologies 1-3 and technologies for the conversion of renewable electricity to chemicals. Electrochemical reactors can play a crucial role in this endeavor, since they can efficiently and reversibly transform electricity to high-value chemicals, and thus serve as energy storage and recovery devices for balancing the grid, while offering a means for the sustainable production of chemicals 4-6. A novel type of alkaline electrochemical cell that can operate at elevated temperature and pressure has been developed that relies on corrosion resistant high temperature diaphragms, based on mesoporous ceramic membranes where aqueous KOH is immobilized by capillary forces. Raising the operating temperature offers a means to boost performance, as both ionic transport and reaction kinetics are exponentially activated with temperature. Indeed, we have demonstrated alkaline electrolysis cells operating at 200-250 °C and 20-50 bar at very high efficiencies and power densities. This work will provide an overview of our efforts to develop components of such high temperature alkaline electrochemical reactors for different applications. Low-cost large-scale production methods have been successfully employed for the production of ceramic diaphragms and full cells. The influence of composition and microstructure on the long-term chemical stability and mechanical durability of the mesoporous ceramic membranes has been explored. Instrumentation for electrochemical testing at elevated pressures (up to 99 bar) and temperatures (up to 300 °C) with in-line chemical analysis has been established enabling experiments with gaseous or liquids reactants/products at cell sizes of up to 25 cm2. Efforts are currently directed towards the investigation of the intrinsic activity of mixed oxides for the oxygen evolution reaction at elevated temperatures and pressures, and of the intrinsic activity, selectivity and stability of supported metal catalysts towards the electrocatalytic conversion of biomass derivatives to high-value chemicals. Finally, the use of selected electrocatalysts for the production of high performance electrodes will be reported.

High Temperature Alkaline Electrolysis - Status and Potential

Alkaline electrolysis is a proven technology with several large scale facilities for hydrogen production realized and operated reliably for decades. Nevertheless, its broader deployment is hindered by the relatively high cost for hydrogen production. To overcome this obstacle, it is necessary to improve cell efficiency, increase the production rate, and decrease capital cost. Since conventional alkaline electrolysis technology has reached maturation, only small incremental improvements can be expected. To achieve a drastic step forward, we have developed a new generation of alkaline electrolysis cells that can operate at elevated temperature and pressure, producing pressurized hydrogen at high rate (m3 H2/hžm2 cell area) and high electrical efficiency. The concept relies on the development of corrosion resistant high temperature diaphragms, based on mesoporous ceramic membranes where aqueous KOH is immobilized by capillary forces, in combination with gas diffusion electrodes that overcome mass transport limitations at large production rates. Raising the operating temperature offers a means to drastically improve performance, as both ionic transport and reaction kinetics are exponentially activated with temperature. Indeed, we have demonstrated alkaline electrolysis cells operating at 200-250 °C and 20-50 bar at very high efficiencies and power densities. This enables high production rates near the thermoneutral voltage, thereby overcoming the need for cooling. This work will provide an overview of the exploratory technical studies undertaken so far. Two electrochemical test stations have been established to carry our experiments at elevated pressures (up to 99 bar) and temperatures (up to 300 °C). The conductivity of aqueous KOH was investigated at elevated temperatures to establish the optimum concentration at 200-250 °C. An optimum value of 0.84 S cm-1 was established at 200 °C for 45 wt% aqueous KOH immobilized in mesoporous SrTiO3. Gas diffusion electrodes were developed using metal foams loaded with different non-precious metal electrocatalysts in order to reduce the overpotentials for oxygen and hydrogen evolution. Small cells have been fabricated and operated at current densities of up to 1.75 A cm-2 and 3.75 A cm-2 at cell voltages of 1.5 V and 1.75 V at 200 °C at 20 bar, corresponding to electrical efficiencies of almost 99 % and 85 %, respectively. Long-term operation at 200 °C was successfully demonstrated for 400 h, suggesting relatively stable cell performance. Finally, low-cost production methods have been utilized for a first scale-up of the cell size from 1 cm2 to 25 cm2. Efforts are currently directed towards the investigation of the intrinsic activity of mixed oxides for the oxygen evolution reaction at elevated temperatures and pressures, and towards establishing the required infrastructure for testing of 25 cm2 cells.

Synthesis and Characterization of Pure and Ag-TiO<sub>2</sub>-Modified Diatomaceous Aluminosilicate Ceramic Membranes for Water Remediation

Mesoporous ceramic membranes were prepared from raw and modified diatomaceous earth alumi-nosilicate mineral precursors. The main modification component of the ceramic membranes was Ag-loaded TiO2 nanoparticles (STOX). Chemical and microstructural characterizations of the raw materials and the modified precursors were carried out using Fourier Transform Infrared (FTIR) Spectroscopy, Particle Induced X-ray Emission (PIXE-IBA), Energy Dispersive X-ray Spectroscopy (EDX) and Scanning Electron Microscopy (SEM). The precursors and membranes were prepared and subsequently subjected to a high temperature sintering treatment for physico-chemical modification and stability. Remediation functionalities of the ceramic membranes on water samples were studied using Atomic Absorption Spectrophotometry (AAS), Total Bacterial Count Enumeration; Total Dissolved Solids (TDS), pH, and Electroconductivity (EC). Remediation experiments showed reductions in the concentration of certain cations such as Mg2+, Mn2+, Cd2+, Ni2+ and K+ by the modified ceramic membrane samples, while increased concentrations were observed for Ca2+, Na+ and Mg2+. The antimicrobial microfiltration process showed 100% bacterial removal and 70% fungi removal in most of the samples. Membranes exhibited good flux output from 5.607 L/hr&#183m2 (STOX-Z) to 39.245 L/hr&#183m2 (ZEO-T) under a pressure of 0.0196 MPa.

Influence of coal power plant exhaust gas on the structure and performance of ceramic nanostructured gas separation membranes

In this work, we investigate the effect of coal power plant exhaust gas on amino-modified mesoporous ceramic membranes. The testing of ceramic membranes in the flue gas of coal-fired power plants represents a new approach, as testing under simulated flue gas conditions has already been undertaken, but not yet during direct exposure to exhaust gas. Flue gas exposure trials were carried out at a lignite-fueled power plant and a hard-coal-fueled power plant. Most experiments were conducted using a test rig designed to bring planar membrane samples in direct contact with unconditioned flue gas in the exhaust gas channel. Another test rig was designed to test membrane modules with pre-treated flue gas. The tested membranes had an asymmetrical structure consisting of a macroporous α-Al2O3 support coated with a mesoporous γ-Al2O3 or 8YSZ interlayer. The microporous functional top layer was made of amino-functionalized silica. The tests revealed different degradation mechanisms such as gypsum/fly ash deposition on the membrane surface, pore blocking by water condensation, chemical reactions and phase transformation. A detailed analysis was carried out to evaluate their impact on the membrane in order to assess membrane stability under real conditions. The suitability of these membranes for this application is critically discussed and an improved mode of membrane operation is proposed.

Removal of hazardous volatile organic compounds from water by vacuum pervaporation with hydrophobic ceramic membranes

Hydrophobic alumina and titania micro and mesoporous ceramic membranes were prepared by grafting of C6F13C2H4Si(OEt)3 (C6) molecules and subsequently applied in a pervaporation (PV) process to the removal of hazardous organic solvents (MTBE, EtAc and BuOH) from binary aqueous solutions. The transport and separation properties of investigated membranes were discussed. Additionally it was found that membrane material and grafting time have an important impact of the pervaporative properties. All tested membranes were selective toward organics. Titania membrane was the most efficient and was characterized by the highest value of PSI and the highest value of permeate flux of organic compounds in water–EtAc (JEtAc=1.1kgh−1m−2; PSIEtAc=140kgh−1m−2) and water–MTBE (JMTBE=1.0kgh−1m−2; PSIMTBE=194kgh−1m−2) systems. This behavior of titania membrane was also discussed based on the Hansen solubility parameters. In presented work the evaluation of MTBE removal degree with titania modified membrane from water binary mixture in the pervaporation process was discussed.

Mesoporous α-Fe2O3 membranes as proton conductors: Synthesis by microwave-assisted sol–gel route and effect of their textural characteristics on water uptake and proton conductivity

This paper reports novel inorganic proton exchange electrolytes based on mesoporous α-Fe2O3 ceramic membranes. The unsupported mesoporous hematite proton ceramic membranes with high specific surface areas, high pore volumes, and narrow pore size distributions have been synthesized from a hydrolytic ferric oxide polymer prepared by a microwave-assisted sol–gel route. The effect of their textural characteristics on water uptake and proton conductivity has been studied.Microwave heating allows us to obtain homogeneous hydrosols at short times (2s) meanwhile conventional heating gives rise to inhomogeneities in the hydrosol independently of heating times. Electron diffraction TEM observations show that the xerogels calcined at 200°C are polycrystalline in nature and correspond to α-Fe2O3 in accordance with the two characteristic vibrations in hematite observed by FTIR spectroscopy. According to EMF measurements, proton transport is observed in these ceramic membranes and shows an Arrhenius dependence on temperature for all relative humidities studied. A sigmoidal dependence of the water uptake and the proton conductivity with the RH at a constant temperature was observed with the greatest increase detected between 58% and 81% RH. The pore volume and the average pore size of the α-Fe2O3 mesoporous ceramic membranes seem to be the main factors which influence the water uptake and consequently the proton conductivity in the studied ranges. The membranes with the largest pore volume, and the largest pore size have the highest water uptake and in turn the highest values of proton conductivity in the whole range of relative humidities (RHs). According to the activation energy values, proton migration in this kind of materials could be dominated by the Grotthuss mechanism in the whole range of RH.

Sol–gel synthesis and characterization of mesoporous yttria-stabilized zirconia membranes with graded pore structure

Mesoporous yttria-stabilized zirconia (YSZ) membranes can be used for liquid phase applications in harsh environments and as supports for ultra-thin dense ceramic, carbonate, or metallic membranes. This article reports on the synthesis and characterization of three-layer mesoporous ceramic membranes consisting of a mesoporous YSZ layer, a macroporous YSZ intermediate layer, and macroporous α-alumina support. The macroporous YSZ intermediate layer was coated on the alumina support using a suspension of submicron-sized YSZ powders, and the mesoporous YSZ layer was obtained by dip-coating with diluted zirconia sol doped with yttrium nitrate. The mesoporous YSZ layer has desired cubic phase structure. Crack-free mesoporous YSZ membranes could be obtained by multiple dip-coating, drying, and calcination using a dilute YSZ sol at a concentration of 0.014 M with the help of using a drying control chemical additive. The 5 times dip-coated mesoporous YSZ membranes were about 1 μm in thickness with an average pore diameter of 3 nm. The mesoporous YSZ membranes exhibited Knudsen separation factor. The characteristics of the dip-coating process for the mesoporous YSZ membranes on the macroporous YSZ support are similar to those on the macroporous alumina support.

Hydrogenation of nitrates in water using mesoporous membranes operated in a flow-through catalytic contactor

The hydrogenation of nitrates in aqueous solution has been studied using Pd–Cu catalysts deposited in mesoporous ceramic membranes operated in a flow-through catalytic membrane reactor. The effect of the top-layer pore size (5, 10 or 25 nm) on the catalytic activity was explored. The activity increased with trans-membrane flow rate for the three membranes, although moderately with the 25 nm pore membrane. At a similar flow rate, the activity increased when the pore size decreased. Concentration polarization of nitrates was evidenced by ultrafiltration experiments with bare membranes, and was enhanced when the pore size decreased. Concentration polarization appeared effective in improving the catalytic activity, due to a local increase in nitrate concentration in the catalytic top-layer of the membranes.

Solvent-dependent permeability in asymmetric ceramic membranes with tortuous or non-tortuous mesopores

Solvent-dependent transport and the role of surface interactions were examined in commercial mesoporous ceramic membranes using permeability and thermoporometry measurements. The membranes chosen were titania (TiO 2) with tortuous interconnected pores (1, 5, and 50 kDa, corresponding to pore diameters of ca. 8.2, 18.3, and 33.2 nm, respectively) and alumina (Al 2O 3) with non-tortuous 20 nm cylindrical pores. A pre-water/solvent/post-water permeability cycle was employed to account for structural differences between membranes and to gauge the effect of residual solvent on water permeability at different temperatures. Our results suggest that in both types of membranes, restricted permeability of 1-butanol and cyclohexane was due to a combination of surface sorption and an increase in disjoining pressure due to solvation forces. Sorption and solvation forces were prevalent as their length scales were on the same order of magnitude as the pore radii. For 1-butanol, chemisorption changed the surfaces from hydrophilic to hydrophobic, and led to a significant decrease in post-water permeability. While Darcy's law could not describe 1-butanol and cyclohexane permeability, it did apply to water and 1,4-dioxane in the 20 nm alumina membranes. Thermoporometry, coupled with permeability, was further used to evaluate surface wetting within the mesopores.

Molecular simulation of permeation through alkyl-functionalized mesoporous ceramic membranes

Organic–inorganic nanocomposites have received much attention in the past decade as potential “next generation” membrane materials, especially for solubility-based gas separations. In this paper, we present a molecular simulation study of materials in which self-assembled monolayers with alkyl functionality are attached to the surfaces of a mesoporous inorganic substrate. Molecular dynamics simulation was used to gain insight on the relationship between microstructure and separation performance in a prototype solubility-based separation (propane/nitrogen). The main parameters in the study were pore size (3, 5, and 10 nm), alkyl chain length (octyl, dodecyl, and octadecyl), and surface density (2 and 4 μmol/m 2). The diffusivity, solubility, and permeability of propane and nitrogen in the model nanocomposite materials were predicted, and the permselectivity data were compared with those from experiment. Qualitative agreement with experiment was observed in several respects. We predicted improvement in selectivity of the membrane for propane, and a decrease in overall permeance, when increasing the chain length or decreasing the pore size. The best propane selectivity was achieved in cases where the alkyl chains were able to span the width of the pore; under these conditions, the surface density can be used to modify the permeability without much change in selectivity.

Role of tail chemistry on liquid and gas transport through organosilane-modified mesoporous ceramic membranes

To develop hydrophobic ceramic membranes, the permeability of liquids (water, ethanol, hexane, and CO 2(l)) and gases (CO 2 and N 2) were examined in unmodified and surface-modified 1 kDa titania and 5 nm γ-alumina membranes. Octyltrichlorosilane (Cl 3SiC 8H 17) and its fluorinated analog trichloroperfluorooctylsilane (Cl 3SiC 2H 4C 6F 13) were chosen as modifying agents. SEM/EDS analysis revealed tri-bonding of the silanes to both materials with no evidence of a thick polymerized surface layer. Characterization by CO 2 and N 2 gas permeability indicated Knudsen diffusion in all membranes with minimal CO 2 selectivity, except for 5 nm C8F (CO 2/N 2 = 2.6) where a surface-flow mechanism was apparent. In contrast, surface modification yielded significant solvent- and modifier-dependent differences in liquid permeability. For example, hexane exhibited the greatest permeability in the C8H mesoporous titania and γ-alumina membranes, while liquid CO 2 (fluorophilic) permeability was the greatest with the C8F membranes. As expected, the hydrophobic C8F membranes were impermeable to water. Low gas and liquid permeabilities in the C8F-modified membranes were consistent with large transport resistances due to the bulky fluorinated tails. These results demonstrate that silane surface modification can be used to tailor liquid transport behavior and improve apolar solvent flux in ceramic membranes relative to polar solvents. In addition, these membranes have proven amenable to liquid CO 2, a green solvent alternative.

Sorption and hydration effects on liquid carbon dioxide transport through mesoporous γ-alumina and titania membranes

Dehydrated and partially hydrated liquid carbon dioxide (>70 bar, 298 K) transport was examined in mesoporous ceramic membranes with γ-alumina (5 nm) and titania (1 and 5 kDa) separation layers. Liquid carbon dioxide, a green solvent alternative, is an apolar solvent with pressure-dependent physicochemical properties. Viscosity-corrected volumetric flux was used to incorporate changes in density and viscosity with increasing feed pressure. It was observed, for both dehydrated and partially hydrated liquid carbon dioxide (∼20 ppm water in feed), that solvent–membrane interactions played a significant role in transport behavior. For dehydrated feeds, consecutive permeation cycles indicated appreciable carbon dioxide sorption, reducing the effective cross-sectional pore area and permeability. When liquid carbon dioxide was partially hydrated, water preferentially adsorbed on the pore surface and led to a significant reduction in permeability relative to dehydrated conditions. Flux of hydrated liquid carbon dioxide was non-linear in all three membranes due to water stripping from the pore surface with increasing pressure (i.e. carbon dioxide density and solvent strength). The permeability of liquid carbon dioxide was compared with reported values for “conventional” solvents. Based on our findings, we discuss potential factors that will influence condensed phase carbon dioxide transport in ceramic membranes.

Recovery of Homogeneous Polyoxometallate Catalysts from Aqueous and Organic Media by a Mesoporous Ceramic Membrane without Loss of Catalytic Activity

The recovery of homogeneous polyoxometallate (POM) oxidation catalysts from aqueous and non-aqueous media by a nanofiltration process using mesoporous gamma-alumina membranes is reported. The recovery of Q(12)[WZn(3)(ZnW(9)O(34))(2)] (Q=[MeN(n-C(8)H(17))(3)](+)) from toluene-based media was quantitative within experimental error, while up to 97 % of Na(12)[WZn(3)(ZnW(9)O(34))(2)] could be recovered from water. The toluene-soluble POM catalyst was used repeatedly in the conversion of cyclooctene to cyclooctene oxide and separated from the product mixture after each reaction. The catalytic activity increased steadily with the number of times that the catalyst had been recycled, which was attributed to partial removal of the excess QCl that is known to have a negative influence on the catalytic activity. Differences in the permeability of the membrane for different liquid media can be attributed to viscosity differences and/or capillary condensation effects. The influence of membrane pore radius on permeability and recovery is discussed.

Gas permeation properties in a composite mesoporous alumina ceramic membrane

In this study, the effect of different chemical interactions on the gas permeation properties were investigated in the composite mesoporous ceramic membrane prepared with γ-alumina on the surface of a macroporous ceramic membrane. In the permeation results, the gas permeance of the strongly adsorbing gas species increased in the mesoporous ceramic membranes. It is considered that the permeation of the adsorbing gas species increased through preferential adsorption on the membrane pore surface. It was shown in this study that the modified mesoporous ceramic membrane could increase the permeation performance in the presence of the adsorbing gas species due to the surface diffusion mechanism.

An analysis of the performance of membrane reactors for the water–gas shift reaction using gas feed mixtures

The water–gas shift (WGS) reaction in membrane reactors has been widely studied by several authors. From these works, the increase of the CO conversion above the equilibrium values appears to be possible when hydrogen is removed through the membrane. However, to date, this feasibility has been verified mostly when feeding pure reagents to the reactor, although in an industrial context the feed normally contains several other compounds. The objective of this work has been to analyse the effect of the feed composition on the membrane reactor efficiency in order to determine the best conditions in terms of CO conversion. At this purpose, experimental tests with mixtures of different compositions have been carried out in three different systems of reaction: (1) traditional fixed-bed reactor; (2) membrane reactor with mesoporous ceramic membrane; (3) membrane reactor with palladium membrane. The experiments included permeation (for the membrane reactors) and reaction tests. The experimental results obtained with the various systems of reaction have been compared. A mathematical model has been also formulated for the different type of reactors used in order to verify the experimental results obtained. From the work carried out it can be concluded that by using the palladium membrane reactor it is possible to overcome the equilibrium conversion. Moreover, a complete conversion has been achieved for one of the mixtures fed to the reactor.

Importance of Electrostatic Interactions for the Selectivity of Micro and Mesoporous Ceramic Membranes

Importance of Electrostatic Interactions for the Selectivity of Micro and Mesoporous Ceramic Membranes