Organosilica Membranes Research Articles

Mixed-Matrix Organo-Silica-Hydrotalcite Membrane for CO2 Separation Part 2: Permeation and Selectivity Study.

This study introduces an innovative approach to designing membranes capable of separating CO2 from industrial gas streams at higher temperatures. The novel membrane design seeks to leverage a well-researched, high-temperature CO2 adsorbent, hydrotalcite, by transforming it into a membrane. This was achieved by combining it with an amorphous organo-silica-based matrix, extending the polymer-based mixed-matrix membrane concept to inorganic compounds. Following the membrane material preparation and investigation of the individual membrane in Part 1 of this study, we examine its permeation and selectivity here. The pure 200 nm thick hydrotalcite membrane exhibits Knudsen behavior due to large intercrystalline pores. In contrast, the organo-silica membrane demonstrates an ideal selectivity of 13.5 and permeance for CO2 of 1.3 × 10-7 mol m-2 s-1 Pa-1 at 25 °C, and at 150 °C, the selectivity is reduced to 4.3. Combining both components results in a hybrid microstructure, featuring selective surface diffusion in the microporous regions and unselective Knudsen diffusion in the mesoporous regions. Further attempts to bridge both components to form a purely microporous microstructure are outlined.

Ti-substituted organosilica membranes for H2 sieving: Sol-gel and DFT insights

The fabrication of gas-sieving amorphous silica membranes exhibiting superior thermal/hydrothermal/chemical stability by the facile, scalable and controllable synthesis strategies is highly desired to bring the silica membranes a step closer to large scale deployment. Sol-gel modification of bis(triethoxylsilyl)ethane (BTESE) membrane pore network with Ti substitution resulted with a shift in the molecular level cut-off to smaller molecules establishing ideal selectivity improvement from 13 ± 2 to 20 for H2/N2 and 17 ± 4 to 28 for H2/CH4 at 200 °C. BTESE and analogous Ti-substituted BTESE membranes (BTESE-Ti) exhibited H2 permeances of (0.9–1.2)x10−6 mol/(m2sPa) and 5.8 × 10−7 mol/(m2sPa), respectively, reflecting a difference in their thickness. Decrease in the sol concentration by 2-fold led to an increase in the H2 permeance value to 8.9 × 10−7mol/(m2sPa) at 200 °C for BTESE-Ti membranes. We further theoretically designed amorphous BTESE and BTESE-Ti structures, and simulated their interactions with the gas molecules along with the pore diffusion of H2 as a model system to elucidate the membrane performance through the density functional theory (DFT) calculations. Theoretical findings suggest that the only transport mechanism is the direct transport from the gas phase to the pore entrance by excluding the surface diffusion possibility.

Rigid-flexible coupled organosilica membranes toward high-efficiency molecules separation

Membrane-based technology has garnered significant interest in the separation and purification of molecules due to its inherent advantages, such as high efficiency, low energy consumption, and ease of operation. In this study, rigid-flexible coupled organosilica membranes were fabricated through co-polymerization using 1,2-bis(triethoxysilyl)acetylene (BTESA) containing rigid acetylene bridges and 1,2-bis(triethoxysilyl)propane (BTESP) containing flexible propane bridges. The incorporation of flexible propane bridges into BTESA networks allowed for precise adjustment of the network structures, resulting in composite BTESA-P membranes with improved molecular sieving properties. BTESA-P membrane demonstrated significant promise for applications in both CO2 capture and pervaporation (PV) dehydration due to its high potential. BTESA-P30 membrane, fabricated using the rigid-flexible coupled strategy, demonstrated a CO2 permeance of 3807 GPU and a CO2/N2 selectivity of approximately 40. These results surpassed those of numerous previously reported membranes, indicating significant potential for CO2 capture applications. BTESA-P30 membrane exhibited significant competitiveness in the decarbonization process of natural gas. More significantly, analogous occurrences are evident in gas permeation and PV separation process, where the permeation mechanism of gas and PV separation is primarily governed by molecular sieving. The utilization of diverse organosilica precursors in the rigid-flexible coupled strategy proposed in this study offers increased potential for membrane structure design and energy-efficient separation of molecules.

Network engineering of organosilica membranes for efficient pervaporation dehydration

In the petroleum industry, the mixing of ethanol (EtOH), isopropanol (IPA), and n-butanol (n-BuOH) with water is a prevalent occurrence in both production and recovery phases. Obtaining the required degree of purity via dehydration poses a significant challenge that needs to be addressed. Pervaporation has emerged as a promising method for separating azeotropic mixtures due to its energy efficiency and independence from vapor-liquid equilibrium constraints. In this study, organosilica precursors, namely 1,2-bis(triethoxysilyl)ethane (BTESE), 1,2-bis(triethoxysilyl)ethylene (BTESEthy), and 1,2-bis(triethoxysilyl)acetylene (BTESA), were utilized for the fabrication of organosilica membranes using the sol–gel method. Subsequently, these membranes were employed for pervaporation dehydration of EtOH, IPA, and n-BuOH. This study investigates the effect of the degree of unsaturation of the bridged group, feed composition, and C atom count of alcohol on membrane dehydration performance. The membranes demonstrated superior separation performance for the n-butanol/water mixtures compared to other alcohol/water systems investigated. In this particular context, BTESE membrane exhibited a significant separation factor, although accompanied by a reduced permeation flux. Conversely, BTESA membrane demonstrated a decreased separation factor but an increased permeation flux. The comparison between gas permeation and pervaporation highlighted that the separation mechanism was primarily governed by molecular sieving through the organosilica membranes.

A broad spectrum of electrospun organosilica membrane properties by tuning the chemical nature of the precursor building block

Tetraethyl orthosilicate (TEOS) directly electrospun nanofibrous membranes are well-established for high-demanding applications. However, further expansion and exploitation of these materials require broadening the portfolio of Si-based precursors. In the present work, a part of the Si–O–Si linkages is therefore replaced by terminal groups (Si–R) or bridged organic linkages (Si-R-Si), playing with the inorganic-organic precursor balance considering triethoxymethylsilane (TEOMS), bis(triethoxysilyl)methane (BTESM) or bis(triethoxysilyl)ethane (BTESE). Both a rheological and kinetic analysis for the sol-gel synthesis of these three precursors is first done in comparison with the TEOS-based reference system, followed by an in-depth study of the electrospinning potential and the resulting organosilica nanofibrous materials properties. The production of the nanofibrous materials is achieved by electrospinning of a partly cross-linked sol-gel system with an optimal initial precursor/H2O/EtOH ratio selecting a sufficiently low temperature. It is demonstrated that the precursor/H2O ratio mainly controls the reactivity of the polycondensation by hydrolysis enhancement, and the precursor/EtOH ratio allows control over the dynamic viscosity due to dilution of the system. Depending on the chemical structure, a different range of workable viscosity conditions is obtained, highlighting the relevance of the construction of structure-property relationships. The rheological and kinetic analysis underpinned the relation of chemistry building blocks and organosilica network synthesis for material applications in general for the first time. Overall, a wide portfolio of membrane properties is reported, including the control over water take-up, acidity and basicity strength, and thermal stability starting from molecular thus chemical variations.

Pervaporation properties of silylated ionic liquid-derived organosilica membrane

We previously reported gas/vapor permselectivities, microstructures, and permeation mechanisms of silylated ionic liquid (IL)-derived organosilica membranes (IL organosilica membranes), which are a class of chemically stabilized IL membranes. In this study, an IL organosilica membrane was used for organic liquid separation using pervaporation (PV) for the first time. Unary PV tests were conducted using alcohols, ketones, aromatic hydrocarbons, and alkanes. We investigated the relationship between the permeances of the organic compounds and differences in the Hansen solubility parameters between the membrane and organic compounds to evaluate the selective permeability of the organic compounds through the membrane. Second, binary PV tests were conducted to separate (1) alcohols/alkanes and (2) aromatics/alkanes. In the toluene/n-hexane (toluene 48 mol%) separation tests, a toluene/n-hexane permeance ratio of 11 was obtained with a toluene permeance of 2.6 × 10−6 mol m−2s−1Pa−1. Further, the membrane exhibited high selectivity for 1-hexanol in a binary 1-hexanol/n-hexane mixture. The IL organosilica membrane exhibited excellent potential for affinity-based organic liquid separation.

Bis(triethoxysilyl)ethane (BTESE)-Organosilica Membranes for H2O/DMF Separation in Reverse Osmosis (RO): Evaluation and Correlation of Subnanopores via Nanopermporometry (NPP), Modified Gas Translation (mGT) and RO Performance.

A 40 cm length Bis(triethoxysilyl)ethane (BTESE) membrane having different pore sizes was successfully prepared by changing the number of coating times for gas permeation (GP) and organic solvent reverse osmosis (OSRO) separation study. It was found that BTESE-6 membranes prepared through six-time coating consisted of small-sized pores in the range 0.56 to 0.64 nm estimated using modified Gas Translation (mGT) method and 0.59 to 0.67 nm estimated by nanopermporometry (NPP) method, respectively. These membranes demonstrated a high DMF rejection, RDMF > 95% with total flux, Jv total > 5 kg m-2 h-1 at operating condition feed pressure, Pf: 8 MPa; feed temperature, Tf : 50 °C; and feed flowrate, Qf : 30 mL/min; and they exhibited a high degree selectivity of He/SF6 in the range of ~ 260-3400 at a permeation temperature 200 °C. On the other hand, the larger pore sizes of the BTESE-4 membranes (pore size estimates > 0.76 nm to 1.02 nm) exhibited low DMF rejection and a low degree selectivity of He/SF6 around ~30% and 25, respectively, at the same operating condition as BTESE-6. Both GT and NPP methods can be considered as an indicator of the measurement membrane pore size. From this study, it was found that He and SF6 gases can be some of the potential predictors for water and DMF permeance. Furthermore, by comparing our OSRO membrane with other PV membranes for DMF/H2O separation, our BTESE-6 membranes still exhibited high flux in the range of 3-6 kg m-2 h-1 with a separation factor H2O/DMF in the range of 80-120.

Steam recovery from flue gas by organosilica membranes for simultaneous harvesting of water and energy

Steam recovery from the spent gases from flues could be a key step in addressing the water shortage issue while additionally benefiting energy saving. Herein, we propose a system that uses organosilica membranes consisting of a developed layered structure to recover steam and latent heat from waste. Proof-of-concept testing is conducted in a running incinerator plant. The proposed system eliminates the need for a water supply while simultaneously recovering latent heat from the waste stream. First, the long-term stability of an organosilica membrane is confirmed over the course of six months on a laboratory-scale under a simulated waste stream. Second, steam recovery is demonstrated in a running waste incinerator plant (bench-scale), which confirms the steady operation of this steam recovery system with a steam recovery rate comparable to that recorded in the laboratory-scale test. Third, process simulation reveals that this system enables water-self-reliance with energy recovery that approximates 70% of waste combustion energy.

Gas permeation properties of bridged-type organosilica membranes at extremely low temperatures and the application to oxygen separation

A bridged-type organosilica, bis(triethoxysilyl)methane (BTESM) membrane was evaluated in detail for single-gas permeation at temperatures ranging from −130 to −200 °C, and for binary (O2/He, O2/N2) separation at 30 and -115 °C, respectively. The permeance of He, Ar, and N2 demonstrated a trend of activated diffusion as permeance increased with an increase in the temperature; O2 permeance, however, reached its maximum value at about −80 °C. O2 permeance increased with a decrease in the temperature (30 to −80 °C), and permeance then gradually decreased (−80 to −130 °C), which demonstrates surface diffusion. In the case of O2/He binary separation, both O2 and He permeated according to molecular sieving at 30 °C, but at −115 °C, O2/He selectivity was enhanced approximately 10-fold due to permeation blockage by adsorbed O2. On the other hand, in the case of O2/N2 binary separation at −115 °C, the effect of permeation blocking by adsorbed O2 permeance was lessened due to the effect of competitive adsorption, which translated to a level of O2/N2 selectivity that was only 1.5-fold higher than that at 30 °C.

Book Review Advanced Materials for Wastewater Treatment and Desalination: Fundamentals to Applications

Book Review Advanced Materials for Wastewater Treatment and Desalination: Fundamentals to Applications

Atmospheric-pressure plasma modification to control porous structure and affinity of organosilica membranes for pervaporation dehydration: Effect of plasma modification time

The effects of the plasma treatment time on the gas permeation and pervaporation dehydration properties of organosilica membranes modified by atmospheric-pressure plasma were investigated. The He/N2 permeance ratio increased from 10 to over 100 as the plasma modification time increased, indicating that the pore size decreased by plasma modification. As measures of the pervaporation properties, the H2O permeance gradually increased, and the EtOH permeance gradually decreased with increasing plasma modification time, resulting in a higher H2O/EtOH permeance ratio from about 20 to over 1000. The enhanced separation performance is attributed to the densification of the pore structure with increasing plasma modification time. A comparison of the temperature dependence of the permeance for gas permeation and pervaporation dehydration indicated that plasma modification increased the affinity of the membrane matrix for water molecules. This enhanced permeation property can be attributed to the increased adsorption of water on the membrane surface with increasing plasma modification time. It was clarified that the permeation properties could be controlled through the tuning of the pore structure and water adsorption properties of the membranes by varying the plasma treatment time.

Planetary cross-linked structure design of hybrid organosilica membrane by amine-driven polymerization for CO2 separation

A hybrid organosilica membrane, with a unique 3-dimensional planetary crosslinked network was fabricated for investigating the effect of in-situ crosslinked network synthesis on the membrane structure. Herein, a dual reaction approach was introduced for the copolymerization. 3-Aminopropyltriethoxysilane (APTES) consisting of amide acts as a shell covering the core cage polyhedral oligomeric silsesquioxane (POSS). Epoxy POSS was selected to construct a tough and 3D free volume as the host for organosilica, to promote gas transport and facilitate a mild reaction with the amide functional group, through the ring-open polymerization without a catalyst. As a branched-chain modified unit, APTES can increase the crosslink density to form the final structure such as a planetary orbit. As a result, it was confirmed that the as-prepared M-0.1A1P(a) formed a planetary structure, from the Si–C main structure surrounded by N using FE-TEM. Moreover, M-0.1A1P(a) showed a maximum selectivity of 77.71 towards CO2/N2, 93.79 towards CO2/CH4, 121.32 towards H2/N2, and 144.86 towards H2/CH4, and an adequate hydrogen permeance of 1.41 × 10−8 mol/(m2sPa) at around 25 °C, confirming that this type of cross-linked network has full potential for the application of small molecular gas separation, especially for CO2.

Sub-nanometer scale tailoring of the microstructures of composite organosilica membranes for efficient pervaporation of toluene/n-heptane mixtures

Amorphous organosilica-based membranes are of enormous potential for pervaporation separation of organic mixtures, however, engineering organosilica membranes with the on-demand pore size and functionality for precise molecular sieving still remains a challenge. Herein, as a proof-of-concept, we proposed a facile and novel co-condensation strategy by pairing the biphenyl-bridged organosilica precursor (BTESBP) with another organosilica precursor (PhTES) equipped with a phenyl pendant group for efficient pervaporation of aromatic/aliphatic mixtures. It was demonstrated that the strong aromatic π-π interactions between BTESBP and PhTES as well as the steric hindrance effect of PhTES allowed the tailoring of membrane pore size at the sub-nano scale, and enhanced the accessibility of biphenyl groups to aromatic hydrocarbons. At the optimal BTESBP/PhTES mass ratio of 98/2, the resulting BTESBP/PhTES composite organosilica membrane achieved superior toluene/n-heptane pervaporation separation performance with a total flux surpassing 1 kg m−2 h−1 and a separation factor of 8–10 for a 50 wt% toluene/n-heptane feed mixture. Hopefully, this study can stimulate the rethinking of the design principles for composite organosilica membranes and the subsequent development of advanced membranes for energy-efficient pervaporation of organic mixtures.

Effect of ZIF-7 doping content on H2/CO2 separation performance of 1,2-bis(triethoxysilyl)ethane-derived organosilica membranes

Organosilica membranes with good hydrothermal stability are promising for pre-combustion capture, but the trade-off effect still constrains their gas separation performance. Herein, we prepared hybrid membranes by incorporating ZIF-7 nanoparticles into 1, 2-bis(triethoxysilyl)ethane (BTESE) networks to improve the H2/CO2 permselectivity by the intrinsic properties of nanoparticles. The effect of inorganic fillers was investigated by adjusting the doping content of ZIF-7 nanoparticles. We found that the H2 permeance of hybrid membranes remained at ∼10−6 mol·m−2·s−1·Pa−1, while the H2/CO2 permselectivity decreased with increasing nanoparticle doping content. The E-ZIF-7-0.4 hybrid membrane showed H2 permeance of 1.2 × 10−6 mol·m−2·s−1·Pa−1 and H2/CO2 permselectivity of 21.7. Our results may offer valuable insights into preparing hybrid membranes with optimal nanoparticle doping content.

High hydrogen selectivity Pd-Ni alloy film hydrogen sensor with hybrid organosilica membranes

Hydrogen sensors are very important for real-time detection of hydrogen leakage. In this study, a Pd-Ni film hydrogen sensor with high hydrogen response and selectivity was fabricated. A 1.3 µm-thick organosilica membrane was coated on the surface of Pd-Ni alloy film by sol-gel process and hot-coating method to fabricate a 1,2-bis(triethoxysilyl)methane (BTESM)-coated Pd-Ni film hydrogen sensor, which endows the sensor with superior H2 selectivity due to the molecular sieve property. And the gas sensing performance was tested at optimum operating temperature. The results show higher response is obtained in both N2 and air atmosphere by hybrid organosilica membrane modification on the Pd-Ni film sensor. Very importantly, the response of the sensor to hydrogen is greatly improved and well maintained in the presence of oxygen, although the response/recovery time increased. The organosilica membrane has high permeability to hydrogen with the kinetic dimater of 0.289 nm while effectively inhibits the permeation of relatively larger gas molecule such as oxygen (0.34 nm) and methanol (0.38 nm), resulting in excellent selectivity to H2. The mechanism of response and selectivity enhancement is explored.

Pervaporation performance of BTESE/TEOS-derived organosilica membrane and its stability in isopropanol aqueous solutions

Pervaporation performance of BTESE/TEOS-derived organosilica membrane and its stability in isopropanol aqueous solutions

The Effect of C/Si Ratio and Fluorine Doping on the Gas Permeation Properties of Pendant-Type and Bridged-Type Organosilica Membranes.

A series of pendant–type alkoxysilane structures with various carbon numbers (C1–C8) were used to fabricate sol–gel derived organosilica membranes to evaluate the effects of the C/Si ratio and fluorine doping. Initially, this investigation was focused on the effect that carbon-linking (pendant–type) units exert on a microporous structure and how this affects the gas-permeation properties of pendant–type organosilica membranes. Gas permeation results were compared with those of bridged–type organosilica membranes (C1–C8). Network pore size evaluation was conducted based on the selectivity of H2/N2 and the activation energy (Ep) of H2 permeation. Consequently, Ep (H2) was increased as the C/Si ratio increased from C1 to C8, which could have been due to the aggregation of pendant side chains that occupied the available micropore channel space and resulted in the reduced pore size. By comparison, these permeation results indicate that pendant–type organosilica membranes showed a somewhat loose network structure in comparison with bridged–type organosilica membranes by following the lower values of activation energies (Ep). Subsequently, we also evaluated the effect that fluorine doping (NH4F) exerts on pendant−type [methytriethoxysilane (MTES), propyltrimethoxysilane (PTMS)] and bridged-type [1,2–bis(triethoxysilyl)methane (BTESM) bis(triethoxysilyl)propane (BTESP)] organosilica structures with similar carbon numbers (C1 and C3). The gas-permeation properties of F–doped pendant network structures revealed values for pore size, H2/N2 selectivity, and Ep (H2) that were comparable to those of pristine organosilica membranes. This could be ascribed to the pendant side chains, which might have hindered the effectiveness of fluorine in pendant–type organosilica structures. The F–doped bridged–type organosilica (BTESM and BTESP) membranes, on the other hand, exhibited a looser network formation as the fluorine concentration increased.

On the Mixed Gas Behavior of Organosilica Membranes Fabricated by Plasma-Enhanced Chemical Vapor Deposition (PECVD).

Selective, nanometer-thin organosilica layers created by plasma-enhanced chemical vapor deposition (PECVD) exhibit selective gas permeation behavior. Despite their promising pure gas performance, published data with regard to mixed gas behavior are still severely lacking. This study endeavors to close this gap by investigating the pure and mixed gas behavior depending on temperatures from 0 °C to 60 °C for four gases (helium, methane, carbon dioxide, and nitrogen) and water vapor. For the two permanent gases, helium and methane, the studied organosilica membrane shows a substantial increase in selectivity from αHe/CH4 = 9 at 0 °C to αHe/CH4 = 40 at 60 °C for pure as well as mixed gases with helium permeance of up to 300 GPU. In contrast, a condensable gas such as CO2 leads to a decrease in selectivity and an increase in permeance compared to its pure gas performance. When water vapor is present in the feed gas, the organosilica membrane shows even stronger deviations from pure gas behavior with a permeance loss of about 60 % accompanied by an increase in ideal selectivity αHe/CO2 from 8 to 13. All in all, the studied organosilica membrane shows very promising results for mixed gases. Especially for elevated temperatures, there is a high potential for separation by size exclusion.

Water permeation in gas and liquid phases through organosilica membranes: A unified theory of reverse osmosis, pervaporation, and vapor permeation

Reverse osmosis (RO), pervaporation (PV), vapor permeation (VP), and gas separation (GS) are the possible permeation modes for molecular separation via membranes. In the present study, the relationship among them was formulated based on thermodynamics and verified via experiments. Water permeation via RO, PV, and VP was performed at a constant temperature of 150 °C and at a transmembrane pressure of 100 kPa using a recently developed hydrothermally stable organosilica membrane. This allowed an accurate evaluation of the permeance obtained from RO, PV, and VP, which confirmed the formulated model. In addition, the effect of concentration in the membrane pores was further included in the model, which successfully predicted the permeance of not only water but also methanol and hydrogen under different levels of pressure. The present study provides a general understanding of permeation and enables estimation of the performances of RO, PV, VP, and GS.

Transesterification membrane reactor with organosilica membrane in batch and continuous flow modes

A membrane reactor (MR) using a 1,2-bis(triethoxysilyl)acetylene (BTESA)-derived organosilica membrane was applied for the transesterification reaction by extracting methanol (MeOH) in batch and flow modes. The BTESA membrane exhibited a MeOH flux exceeding 10 kg/(m2 h) for MeOH/butyl acetate (BA) binary pervaporation with 10 wt% MeOH at 100 °C and a MeOH separation factor exceeding 103. The effect of temperature on the liquid-phase batch MR (Batch-MR) performance was investigated at 60–100 °C. The BA yield for methyl acetate (MA)/n-butanol (BuOH) = 1 at 100 °C reached 84% after 10 h, which was significantly higher than that achieved at equilibrium (48%). Moreover, the performances of the Batch-MR, continuous stirred tank MR (CST-MR), and plug flow MR (PF-MR) were compared under the same reaction conditions. The PF-MR exhibited a low MeOH permeance owing to concentration polarization, whereas the Batch-MR and CST-MR exhibited a higher permeance because of vigorous agitation, leading to higher BA yields. Furthermore, the gas-phase PF-MR by vapor permeation (PF-VPMR) was used to prevent concentration polarization. The MeOH permeance of the PF-VPMR was three times higher than that of the liquid-phase PF-MR (PF-PVMR); however, the BA yield at 100° C of the PF-VPMR was similar to that of the PF-PVMR due to the formation of ethers as by-products. Nevertheless, the BA yield at 80 °C of the PF-VPMR was enhanced by more than 10% in relation to that of the PF-PVMR. Thus, the transesterification MR with the BTESA membrane can be used for efficient ester production.