Molecular Sieve Membranes Research Articles

Techno-economics and sensitivity analysis of hybrid process combining carbon molecular sieve membrane and distillation column for propylene/propane separation

The conceptual designs of a carbon molecular sieve (CMS) membrane and a distillation process with heat pump integration were used to evaluate propylene/propane separation system economics. A CMS membrane prepared on a porous alumina hollow fiber was employed to establish the conceptual design of the integrated system. The technologies were evaluated in terms of capital and operating costs and compared to a conventional C3-splitter to elicit the one that minimizes total costs. Different configurations are suggested to satisfy the design specifications of 99.6 wt% product purity and 99% propylene recovery rate. When the heat pump with membrane system was applied in condition of stage cut 0.5, pressure ratio 3, propylene/propane selectivity 28, and propylene permeance 20 GPU, the capital and operating costs decreased by 2.3% and 68.7% compared to distillation only process, respectively. The cost saving variation of the hybrid system was confirmed with membrane specifications such as selectivity and permeance. The simulation results indicated that the heat pump with membrane process is advantageous in terms of both energy and economic savings. A correlation contour between membrane permeance/selectivity and cost saving was developed for the hybrid systems. The findings provide a helpful guide to more economically separate gas mixtures.

Bottom up approach to study the gas separation properties of PIM-PIs and its derived CMSMs by isomer monomers

Resolving the relationships between the molecular structure and gas separation properties of microporous polymers and their derived carbon molecular sieve membranes (CMSMs) are extremely important in obtaining highly efficient gas separation polymer membranes and CMSMs. To elucidate the correlations, we designed two microporous polyimides (CTPI and MTPI) derived from two triptycene-based diamine isomer monomers, that is, 2,6-triptycene diamine (C2 symmetry, CTA) and 2,7-triptycene diamine (Cs symmetry, MTA). The MTA shows a higher dipole moment than CTA (2.47 vs 1.83 Debye). As a result, the MTPI derived from MTA exhibits a slightly higher density (1.25 vs 1.23 cm3 g−1), smaller fractional free volume (0.250 vs 0.262), lower surface area (148 vs 193 m2 g−1) and smaller micropore size (7.5 vs 9.0 Å), higher glass transition temperature (345.8 vs 329.3 °C) and lower gas permeability but a slightly higher gas pair selectivity than CTPI. The MTPI also demonstrates a higher maximum derivative of weight loss temperature (547.6 vs 546.4 °C) than CTPI, and the 550 °C pyrolyzed MTPI (MTPI-550) shows a higher surface area (772 vs 703 m2 g−1) and larger micropore (6.4 vs 5.4 Å) than the corresponding CTPI-550. Consequently, the MTPI-550 shows higher gas permeability than CTPI-550. The CO2 permeability of MTPI-550 is 9878 Barrer combined with CO2/CH4 ideal selectivity of 44.5, which is one of the best performances among reported CMSMs. The barometric sorption results indicate that the higher permeability of CMSMs than the PIM-PI precursors is due to the larger diffusion and solubility coefficients because of huge enhanced microporosity, and the higher selectivity is attributed to their diffusion selectivity by the smaller pore size of CMSMs. The above findings clearly show that the larger dipole moment isomer leads to a compacter polymer chain packing of the corresponding PIM-PI, and a more open structure in the resulting CMSM due to the larger dipole-dipole interaction delayed its decomposition speed during the CMSM formation.

In situ construction of an immobilized b-oriented titanium silicalite spherical molecular sieve membrane

Oriented molecular sieve membranes prepared on the surface of a specifically shaped substrate have been applied in the separation and catalysis process, but there is no report on the directional growth of molecular sieve membranes on the surface of spherical substrates. Here, a simple in situ crystallization method was employed to successfully prepare a single layer of b-oriented continuous titanium silicalite-1 (TS-1) molecular sieve membrane on the surface of 2-mm alumina beads. First, a titania intermediate layer was loaded on the surface of beads by the dipping method, and the surface was then modified with the TBOT modification solution. The ratio of the precursor solution was n(TEOS):n(TBOT):n(TPAOH):n(H2O) = 1:0.010:0.15:40 and was crystallized at 180 °C for 72 h to prepare the membrane. The content of the templating agent and water affected the size of TS-1 crystal grains and membrane crosslinking—both in turn influenced the continuity and orientation of the molecular sieve membrane. Spherical membrane catalyst was applied to catalyze the epoxidation of chloropropene in a fixed bed reactor, and the conversion of hydrogen peroxide and selectivity of epichlorohydrin attained 87.4% and 93.2%, respectively. This work provides a foundation for the design of novel TS-1 immobilized catalyst.

Recent advancements in molecular separation of gases using microporous membrane systems: A comprehensive review on the applied liquid absorbents

Industrial-based molecular separation of detrimental gaseous molecules (especially CO2) from various mixtures using different types of chemical absorbents has been recently of paramount attention all around the world to mitigate their severe impacts on ecosystem such as acid rain and global warming. This paper aims to present a state-of-the-art review about the recent advancements in the molecular separation of CO2 applying hollow fiber membrane contactors (HFMCs). Additionally, all employed membrane materials for separating CO2 contaminant including polymeric membranes (PMs), microporous organic polymers (MOPs), fixed-site-carrier membranes (FSCMs), inorganic membranes (IMs), mixed matrix membranes (MMMs), and carbon molecular sieve membranes (CMSMs) are comprehensively discussed. Discussion about the most important operating challenges towards the separation of CO2 inside the HFMCs (i.e., mass transfer resistance and membrane pores wettability) is considered as another objective of this paper. Another aim of this paper is to classify the prevalent types of liquid absorbents and critically discuss the advantages and disadvantages of each group in the field of membrane-based gas separation process.

A MEMS µ-Preconcentrator Employing a Carbon Molecular Sieve Membrane for Highly Volatile Organic Compound Sampling

This paper presents the synthesis and evaluation of a carbon molecular sieve membrane (CMSM) grown inside a MEMS-fabricated μ-preconcentrator for sampling highly volatile organic compounds. An array of µ-pillars measuring 100 µm in diameter and 250 µm in height were fabricated inside a microfluidic channel to increase the attaching surface for the CMSM. The surface area of the CMSM was measured as high as 899 m2/g. A GC peak amplification factor >2 × 104 was demonstrated with gaseous ethyl acetate. Up to 1.4 L of gaseous ethanol at the 100 ppb level could be concentrated without exceeding the capacity of this microchip device. Sharp desorption chromatographic peaks (<3.5 s) were obtained while using this device directly as a GC injector. Less volatile compounds such as gaseous toluene, m-xylene, and mesitylene appeared to be adsorbed strongly on CMSM, showing a memory effect. Sampling parameters such as sample volatilities, sampling capacities, and compound residual issues were empirically determined and discussed.

Nano-Confinement Effects on Structural Development and Organic Solvent-Induced Swelling of Ultrathin Carbon Molecular Sieve Films.

Successful implementation of carbon molecular sieve (CMS) membranes in large scale chemical processes inevitably relies on fabrication of high performance integrally skinned asymmetric or thin-film composite membranes. In principle, to maximize separation efficiency the selective CMS layer should be as thin as possible which requires its lateral confinement to a supporting structure. In this work, we studied pyrolysis-induced structural development as well as ethanol vapor-induced swelling of ultrathin CMS films made from a highly aromatic polyimide of an intrinsic microporosity (PIM–PI) precursor. Utilization of a light polarization-sensitive technique, spectroscopic ellipsometry, allowed for the identification of an internal orientation within the turbostratic amorphous CMS structure driven by the laterally constraining support. Our results indicated a significant thickness dependence both in the extent of pyrolytic collapse and response to organic vapor penetrant. Thinner, substrate-confined films (∼30 nm) collapsed more extensively leading to a reduction of microporosity in comparison to their thicker (∼300 nm) as well as self-supported (∼70 μm) counterparts. The reduced microporosity in the thinner films induced changes in the balance between penetrant-induced dilation (swelling) and filling of micropores. In comparison to thicker films, the initial lower microporosity of the thinner films was accompanied by slightly enhanced organic vapor-induced swelling. The presented results are anticipated to generate the fundamental knowledge necessary to design optimized ultrathin CMS membranes. In particular, our results reinforce previous findings that excessive reduction of the selective layer thickness in amorphous microporous materials (such as PIMs or CMS) beyond several hundred nanometers may not be optimal for maximizing their fluid transport performance.

Heating-driven assembly of covalent organic framework nanosheets for gas separation

Covalent organic frameworks (COFs) are a family of promising membrane materials because of their intrinsic pores with uniform size, tunable functionalities, and high stability. Especially, two-dimensional (2D) COFs can be exfoliated into single- or few-layered nanosheets and restacked into molecular sieving membranes through controlling restacking modes and thickness. However, traditional methods mainly rely on increasing the thickness (vertical restacking control) to achieve high separation selectivity but with compromised permeance. Herein, the fabrication of ultrathin COF (CTF-BTD) membranes through the restacking of the nanosheets with the aid of introducing thermal perturbation is reported, i.e., a heating vacuum filtration method to optimize the relative displacement of nanosheets without sacrificing thickness control. As a result, a membrane exhibits high H2 permeance (655.6 GPU) and H2/CO2 selectivity (43.1) for CO2 pre-combustion capture, which are superior than those of the membranes prepared at room temperature. This work realizes an enhanced separation performance through the control of restacking modes, which will inspire the optimization of other membrane architectures.

Ultra-thin and high hydrogen permeable carbon molecular sieve membrane prepared by using polydopamine as carbon precursor

Carbon molecular sieve membranes (CMSMs) have attracted much attention because of its high stability. Attributing to its non-toxic, excellent adhesion and high carbon yield, polydopamine is a promising carbon precursor for the fabrication of CMSMs. In this study, ultra-thin (80 nm) and high hydrogen permeable CMSMs are prepared by using polydopamine as the carbon precursor. At 25 °C and 1 bar, the CMSM shows a high H2 permeance of 2.7 × 10−6 mol m−2 s−1 Pa−1, which is much higher than that of the previously reported CMSMs, and ideal separation factors of H2/CO2, H2/CH4 and H2/C3H8 are 18.3, 31.4 and 49.1, respectively.

Carbon Molecular Sieve Membranes Comprising Graphene Oxides and Porous Carbon for CO2/N2 Separation.

To improve the CO2/N2 separation performance, mixed-matrix carbon molecular sieve membranes (mixed-matrix CMSMs) were fabricated and tested. Two carbon-based fillers, graphene oxide (GO) and activated carbon (YP-50F), were separately incorporated into two polymer precursors (Matrimid® 5218 and ODPA-TMPDA), and the resulting CMSMs demonstrated improved CO2 permeability. The improvement afforded by YP-50F was more substantial due to its higher accessible surface area. Based on the gas permeation data and the Robeson plot for CO2/N2 separation, the performances of the CMSMs containing 15 wt % YP-50F and 15 wt % GO in the mixed polymer matrix surpassed the 2008 Robeson upper bound of polymeric membranes. Hence, this study demonstrates the feasibility of such membranes in improving the CO2/N2 separation performance through the appropriate choice of carbon-based filler materials in polymer matrices.

Realizing the impact of the intermediate layer structure on the CO2/CH4 separation performance of carbon molecular sieving membranes: Insights from experimental synthesis and molecular simulation

This work is a continuation of our previous study investigating the role of the intermediate layer in carbon molecular sieve membrane (CMSM) formation and its impact on the gas separation performance. Herein, the effect of key preparation variables of CMS/TiO2 membrane prepared from the polyetherimide (PEI) precursor on the membrane’s structure and CO2/CH4 separation quality was studied. A set of CMS/TiO2/Al2O3 membranes produced under different conditions (pH of the sol-gel for the intermediate TiO2 layer preparation, the TiO2 interlayer thickness and the carbon separation layer thickness) was prepared, characterized by the AFM, FE-SEM and EDXS methods and tested for CO2/CH4 separation efficiency. In addition, a molecular simulation was applied to investigate the effect of intermediate TiO2 layer.All prepared membranes had good separation quality (the separation factor between 37 and 65). The best results for membranes with the thickest TiO2 interlayer prepared at pH = 5 were identified; notably, membrane samples with the thin defect free carbon separation layer achieved the best balance of permeance vs. selectivity and exceeded the Robeson upper bound. The molecular simulation results support the finding of the intermediate layer plays a vital role for the synthesis of gas separation membranes.

Novel hollow fiber membrane reactor for high purity H2 generation from thermal catalytic NH3 decomposition

Although storage of H2 in various liquid chemicals makes it very attractive for practical applications, production and purification of H2 from decomposition of these chemicals under relatively mild conditions is still challenging. In this work, we reported production of high-purity H2 from NH3 decomposition using a combined packed bed/membrane reactor loaded with Ru-based catalyst. Three types of membranes (modified MFI zeolite membrane, carbon molecular sieve membrane, and Pd/Ag membrane) were employed and compared for H2 production and purification from NH3 decomposition. All these membrane reactors exhibited high NH3 conversion of >99% with H2 recovery of >90% under pressurized NH3 feed of 7 bar. Nevertheless, H2 purity varied because of the different separation performance of these membranes. High H2 purity of >99.999% with <10 ppb NH3 concentration in permeate was achieved using high-quality Pd/Ag membrane reactor. These results suggest a feasible and highly energy efficient option for producing high-purity H2 from NH3 decomposition.

Tailoring pore structure and surface chemistry of microporous Alumina-Carbon Molecular Sieve Membranes (Al-CMSMs) by altering carbonization temperature for optimal gas separation performance: An investigation using low-field NMR relaxation measurements

In this work, we applied low-field, NMR spin–lattice measurements to evaluate for the first time the effect of carbonization temperature (range 600–1000 ℃) on the preparation of Alumina-Carbon Molecular Sieve Membranes (Al-CMSMs), providing new insights into intra-pore fluid interactions. The results show that the average Al-CMSM pore size generally increases with carbonization temperature whilst the hydrophilicity of the pore surface, and the amount of strongly adsorbed H2O, decreases with an increasing carbonization temperature. As such, lower carbonization temperatures produce more hydrophilic membranes, with further evidence provided by FTIR measurements demonstrating the presence of polar functional groups on the surface, with water interacting more strongly with the membrane surface, as evidenced by NMR.It was found that the Al-CMSM carbonization temperature significantly affected permeance and H2O/CH4 permselectivity by altering the membrane pore size distribution and pore hydrophilicity. H2O permeance values are seen to be up to 100 times larger than respective CH4 permeance values. The greater permeance of H2O is attributed to the larger kinetic diameter of CH4 relative to H2O and the adsorption of water in the hydrophilic pores enhancing the adsorption-diffusion transport mechanism. Optimal water permeation temperatures are thus higher for the more hydrophilic membranes, obtained at lower carbonization temperatures, as more energy is required to remove strongly adsorbed water blocking the pores. At higher carbonization temperatures, the Knudsen diffusion mechanism of permeance dominates over the adsorption-diffusion mechanism thereby reducing permeance as diffusion slows due to collisions between gas molecules and the pore walls. CH4 permeation always occurs via Knudsen diffusion with CH4 permeance increasing with permeation temperature due to the increased rate of CH4 diffusion.

Measuring the permeabilities of binary gas mixtures with a novel time‐lag technique

AbstractMeasuring permeation properties of gas mixtures is an important but challenging issue. This work reports on a novel time‐lag technique by selectively depositing (ie, desublimation) one of the gas components in a binary mixture at the downstream side using a liquid nitrogen cold trap, thereby allowing for determination of the permeation properties of both permeating components from the pressure response in the permeate chamber. The measurement technique was demonstrated for the permeation of several binary gas mixtures (H2/CO2, and He/CO2 with various compositions) through a carbon molecular sieve membrane. It was found that the true permeation properties of a binary mixture can be significantly different from the ideal values based on the permeation of pure gas components. This method is time and cost efficient and works well for binary gas mixtures where the difference in the boiling points of the gas components is sufficiently large so that one of the components in the permeate can be selectively desublimated at the permeate side.

Carbon molecular sieve membranes for hydrogen purification from a steam methane reforming process

Asymmetric carbon molecular sieve (CMS) membranes prepared from cellulose hollow fiber precursors were investigated for H2/CO2 separation in this work. The prepared carbon membrane shows excellent separation performance with H2 permeance of 111 GPU and an H2/CO2 selectivity of 36.9 at 10 bar and 110 °C dry mixed gas. This membrane demonstrates high stability under a humidified gas condition at 90 °C and the pressure of up to 14 bar. A two-stage carbon membrane system was evaluated to be techno-economically feasible to produce high-purity H2 (>99.5 vol%) by HYSYS simulation, and the minimum specific H2 purification cost of 0.012 $/Nm3 H2 produced was achieved under the optimal operating condition. Sensitivity analysis on the H2 loss and H2 purity indicates that such membrane is still less cost-effective to achieve ultrapure hydrogen (e.g., >99.8 vol%) unless the higher operating temperatures for carbon membrane systems are applied.

Ionothermal synthesis of AlPO-34 membranes on macroporous α-Al2O3 supports without using organic template

Continuous and compact AlPO-34 molecular sieve membranes were successfully synthesized on the outer surface of macroprous tube α-Al2O3 supports under secondary ionothermal growth with the proposed wetting seed layer. The recyclable ionic liquids (ILs) can both act as solvent media and the structure-directing agent (SDA) of the aiming topological structure of CHA, largely reducing the wastewater. Wetting the seed layer could accelerate the diffusion of ILs to the surface of the support and further increase the binding force between the membrane and the support. Compared with the open system, the closed system led to the achievement of the high-quality AlPO-34 membrane. The H2 permeance of the resulting membrane was 9.74 × 10−8 mol m−2 s−1 Pa−1 and the ideal selectivities of H2/CO2, H2/N2, H2/CH4, H2/C3H8 and H2/i-C4H10 were 7.48, 8.54, 14.67, 39.25 and 65.83, respectively, which were higher than Knudsen diffusion ratios. The obtained membrane was firstly explored for the PV dehydration of isopropyl alcohol (90 wt%)/water mixtures, exhibiting good performance with the separation factor of 147 and high water flux of 1.09 kg m−2 h−1.

Insights into the Role of Polymer Conformation on the Cutoff Size of Carbon Molecular Sieving Membranes for Hydrogen Separation and Its Novel Pore Size Detection Technology.

In this study, three polymer precursor conformations, dilute, semi-dilute, and concentrated, were used to fabricate carbon molecular sieving (CMS) membranes via a fixed carbonization protocol. The effects of the precursor conformation on the microstructure of the resultant CMS membranes were characterized by Raman analysis. Their ability to separate light gases, such as H2/CH4 and H2/N2, was assessed with a single-gas system. Additionally, a novel method was proposed to detect the cutoff size of the CMS membranes created in this study. The method combined high-resolution transmission electron microscopy (HR-TEM) and a focused ion beam (FIB) system. Finally, due to the semi-dilute solution's denser polymer chains and lack of severe polymer entanglement, highly graphited CMS membranes with excellent gas separation performance were successfully synthesized using a semi-dilute polyetherimide dope solution. Interlayer distances in the carbon matrix were visualized and measured using our novel probing tool (HR-TEM and FIB) and software. The CMS membrane fabricated with a semi-dilute dope exhibited the best gas separation performance of the tested membranes. It had the most ordered carbon sheet orientation and exhibited a superior selectivity of H2/CH4 = 293 with a hydrogen permeability of 1138.7 Barrer, far surpassing the reported permselectivity of other membranes. We believe that the high H2/CH4 selectivity presented here is unprecedented for CMS membranes reported in the literature.

An efficient vapor-phase processing method derived mesoporous N-C@SnO2-Co3O4 hollow nanoboxes with abundant surface oxygen vacancy for highly improved gas sensing application

For widely used semiconductor-based chemoresistive sensors, low sensitivity and poor anti-interference in the complex environment (humidity resistance, etc.) are two main constraints of application. To enhance the sensitivity, many effective strategies have been developed such as constructing heterostructures with synergistic effects, introducing abundant oxygen vacancies as active sites, designing hollow/cavity structure to increase surface area to facilitate target gas adsorption, and so on. Besides, the interference caused by water vapor can be blocked.via MOF or molecular sieve membranes but is accompanied with the shielding of some target molecules. The above methods are faced with the problems of complex process, large workload of material screening and failure to maintain the device stability. Hence, an effective vapor-phase method derived N-C@SnO2-Co3O4 complex that combined the hydrophobicity, acetone selectivity, p-n heterostructure with mesoporous hollow characteristics was proposed, named mesoporous N-C@SnO2-Co3O4 hollow nanoboxes (HNBs). Particularly, the chemoresistive sensor based on N-C@SnO2-Co3O4 HNBs (2.0%) showed satisfactory selectivity to acetone vapor at relatively low working temperature (160 °C), and the response remained stable even under high humidity (with the R.H. of 90%). Impressively, a three-fold enhancement in response signal was observed for the sensor based on N-C@SnO2-Co3O4 HNBs when compared with its counterpart of SnO2-Co3O4, which can be ascribed to the consequent unpaired electrons from oxygen vacancies and hollow mesoporous structures. Additionally, a sensing prototype based on the N-C@SnO2-Co3O4 HNBs was practically fabricated. The satisfactory sensing response and stability further verified the potential applications in industrial acetone detection. This facile vapor-phase approach sheds light on designing sensing materials with enhanced sensitivity and humidity resistance, as well as device stability simultaneously.

Carbon hollow fiber membranes for a molecular sieve with precise-cutoff ultramicropores for superior hydrogen separation

Carbon molecular sieve (CMS) membranes with rigid and uniform pore structures are ideal candidates for high temperature- and pressure-demanded separations, such as hydrogen purification from the steam methane reforming process. Here, we report a facile and scalable method for the fabrication of cellulose-based asymmetric carbon hollow fiber membranes (CHFMs) with ultramicropores of 3–4 Å for superior H2 separation. The membrane fabrication process does not require complex pretreatments to avoid pore collapse before the carbonization of cellulose precursors. A H2/CO2 selectivity of 83.9 at 130 °C (H2/N2 selectivity of >800, H2/CH4 selectivity of >5700) demonstrates that the membrane provides a precise cutoff to discriminate between small gas molecules (H2) and larger gas molecules. In addition, the membrane exhibits superior mixed gas separation performances combined with water vapor- and high pressure-resistant stability. The present approach for the fabrication of high-performance CMS membranes derived from cellulose precursors opens a new avenue for H2-related separations.

MOF-in-COF molecular sieving membrane for selective hydrogen separation

Covalent organic frameworks (COFs) are promising materials for advanced molecular-separation membranes, but their wide nanometer-sized pores prevent selective gas separation through molecular sieving. Herein, we propose a MOF-in-COF concept for the confined growth of metal-organic framework (MOFs) inside a supported COF layer to prepare MOF-in-COF membranes. These membranes feature a unique MOF-in-COF micro/nanopore network, presumably due to the formation of MOFs as a pearl string-like chain of unit cells in the 1D channel of 2D COFs. The MOF-in-COF membranes exhibit an excellent hydrogen permeance (>3000 GPU) together with a significant enhancement of separation selectivity of hydrogen over other gases. The superior separation performance for H2/CO2 and H2/CH4 surpasses the Robeson upper bounds, benefiting from the synergy combining precise size sieving and fast molecular transport through the MOF-in-COF channels. The synthesis of different combinations of MOFs and COFs in robust MOF-in-COF membranes demonstrates the versatility of our design strategy.

High-performance porous carbon-zeolite mixed-matrix membranes for CO2/N2 separation

Nano-sized PS-MFI, ETS-10 and SAPO-34 zeolites were utilized as CO2-selective fillers to fabricate high-performance mixed-matrix carbon molecular sieve membranes (CMSMs) for application in the CO2/N2 separation process. In this work, an in-house polyimide, ODPA-TMPDA, was used as a polymer precursor for CMSMs, due to its higher intrinsic gas permeability compared with commercial polyimides. Zeolite-filled CMSMs were then successfully fabricated without any appreciable defects at the filler/matrix interfaces. Gas permeation testing revealed that both CO2 permeability and CO2/N2 selectivity can be significantly improved upon incorporation of such zeolite fillers, enabling selective adsorption and transport of CO2. In particular, SAPO-34 yielded the best results among the zeolite fillers tested, resulting in an excellent performance far beyond the Robeson upper bound limit for CO2/N2 separation.