Facilitated propylene transport in mixed matrix membranes containing ZIF-8@Agmim core-shell hybrid material

The ZIF-8@Agmim core-shell hybrid material was synthesized via a favorable post-modification method of ion exchange (PMIE). This infrequent ZIF-8@Agmim core-shell structure maintains a well-integrated pore size that is almost the same as ZIF-8. The similar equilibrium isotherms with ZIF-8 and better kinetic separation toward propylene/propane than ZIF-8 render ZIF-8@Agmim to be an interesting candidate for propylene/propane separation. The core-shell hybrid nanomaterial was further used as nanofillers in the polymer of intrinsic microporosity matrix (PIM-1) for propylene/propane separation. The resultant mixed-matrix membranes (MMMs) exhibited a simultaneous increase in C₃H₆ permeability and C₃H₆/C₃H₈ ideal selectivity compared to pure polymer membrane owing to a synergistic effect of molecular sieving from ZIF-8 and π-complexation of Ag⁺ with propylene. The separation performance of the prepared MMM surpasses the upper bound line of polymer membranes. Furthermore, the hybrid materials possess superb photochemical stability and the corresponding MMMs exhibit excellent anti-aging property and long-term stability.     

Enhanced compatibility and selectivity in mixed matrix membranes for propylene/propane separation

Mixed matrix membranes (MMMs) based on metal–organic framework (MOF) have great promising application in separation of gas mixtures. However, achieving a good interfacial compatibility between polymer and MOF is not straightforward. In this work, focusing on one of the most challenging olefin/paraffin separations: propylene/propane (C₃H₆/C₃H₈), we demonstrate that modification of the MOF filler via dopamine polymerization using a double solvent approach strongly improves interfacial compatibility. The resulting membranes show an outstanding separation performance and long-term stability with propylene permeability nearly 90 Barrer and propylene/propane selectivity close to 75. We anticipate that similar MOF modification strategies may help solve the problem of interface defects in the manufacture of MMMs and be extended to other porous fillers.     

Boehmite-phenolic resin carbon molecular sieve membranes—Permeation and adsorption studies

Composite carbon molecular sieve membranes (c-CMSM) were prepared in a single dipping–drying–carbonization step from phenolic resin solutions (12.5–15 wt.%) loaded with boehmite nanoparticles (0.5–1.2 wt.%). A carbon matrix with well-dispersed Al₂O₃ nanowires was formed from the decomposition of the resin and dehydroxylation of boehmite. The effect of the carbon/Al₂O₃ ratio on the porous structure of the c-CMSM was accessed based on the pore size distribution and gas permeation toward N₂, O₂, CO₂, He, H₂, C₃H₆ and C₃H₈. c-CMSM with higher carbon/Al₂O₃ ratios had a more open porous structure, exhibiting higher permeabilities and lower permselectivities. c-CMSM performance was above the upper bound curves for polymeric membranes for several gas pairs, particularly for C₃H₆/C₃H₈ (permeability toward C₃H₆ of 420 barrer and permselectivity of 18.1 for a c-CMSM with carbon/Al₂O₃ ratio of 4.4).     

Facilitated transport separation of propylene–propane: Experimental and modeling study

Supported liquid membrane, as one type of facilitated transport membranes, was used for the separation of propylene–propane mixtures. The effect of trans-membrane pressure and carrier concentration on membrane separation performance were evaluated in terms of mixed-gas selectivity, propylene and propane permeances and propylene and propane permeation fluxes. A general dimensionless model for the transport of components across the membrane was proposed and solved numerically by orthogonal collocation method. Experimental results showed that for a 70:30 (vol.%) propylene–propane mixture, at pressure 120 kPa and carrier concentration 20 wt.%, a propylene permeation flux of 1.46 × 10⁻⁴ mol/m² s was obtained. Mathematical results are in well agreement with experimental results. The average deviation between experimental and modeling results was found to be 5.3% for propylene permeation flux and 0.03% for propane permeation flux.     

Carbon–Al2O3–Ag composite molecular sieve membranes for gas separation

Phenolic resins loaded with two different inorganic fillers (boehmite (γ-AlO(OH)) and silver (Ag)) were used to prepare composite carbon membranes. Polymer solutions containing γ-AlO(OH) and AgNO₃ were prepared and the effect of Ag on the transport properties of the composite membrane was evaluated. The polymer solutions were coated on α-Al₂O₃ tubes and carbonized in a single dipping-drying-pyrolysis step. After pyrolysis at 550 °C, γ-AlO(OH) yielded γ-Al₂O₃, and Ag agglomerated forming spherical nanoparticles of 30 nm in diameter. Ag loading enhanced the carbon membrane performance for several gas pairs of interest, especially for C₃H₆/C₃H₈ separation, where the C₃H₆/C₃H₈ permselectivity increased from a maximum of 15 to 38.     

Preparation and characterization of carbon membranes made from poly(phthalazinone ether sulfone ketone)

Carbon membranes were prepared from a novel polymeric precursor of poly(phthalazinone ether sulfone ketone) (PPESK), of which the changes of microstructure and chemical compositions during pyrolysis from 500 °C to 950 °C were monitored by thermal gravimetric analysis, X-ray diffraction, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. It has been found that the weight loss of the PPESK precursor up to 800 °C is about 43.0 wt%. After the heat treatment, the typical chemical structure of the PPESK precursor disappears, at the same time a graphite-like structure with more aromatic rings is formed. The interlayer spacing (i.e., d value) decreases from 0.471 nm to 0.365 nm as the pyrolysis temperature increases. The gas permeation performance of carbon membranes has been tested using pure single gases including H₂, CO₂, O₂ and N₂. For the carbon membrane obtained by carbonizing the PPESK precursor at 800 °C, the maximum ideal permselectivities for H₂/N₂, CO₂/N₂ and O₂/N₂ gas pairs could reach 278.5, 213.8 and 27.5, respectively.     

Transmission electron microscopy characterization of nanostructured carbon derived from Cr3C2 and Cr(C5H7O2)3

The preparation, characterization and comparison of nanostructured carbons derived by direct chlorination of Cr₃C₂ and Cr(C₅H₇O₂)₃ are reported in this work. Cr₃C₂ precursor was treated at 400 and 900 °C with a reaction time of 1 h. The nanostructure of the products has been characterized in some detail by means of transmission electron microscopy and associated techniques, such as electron energy-loss and X-ray energy dispersive spectroscopies and high-angle annular dark field imaging. Remains of Cr₃C₂ encapsulated in an amorphous carbon shell were observed at 400 °C, whereas carbon with higher ordering degree was produced at 900 °C. In the latter case, the sample can be described as a continuous variation from poorly-stacked graphene-like carbon to graphitic agglomerates. Remains of the reaction by-product, CrCl₃, are detected in the carbon particles, forming monolayers intercalated inside the graphitic agglomerates and amorphous nanoparticles. As a comparison, carbon samples derived from Cr(C₅H₇O₂)₃ were prepared at 300 and 900 °C. They mainly consist of highly disordered carbon, with local graphite-like stacking in the sample prepared at 900 °C.     

Sequential-refining of crude glycerol derived from waste used-oil methyl ester plant via a combined process of chemical and adsorption

A sequential stage physico-chemical refining of crude glycerol, derived from a waste used-oil utilizing biodiesel (methyl ester) production plant, was performed by acidification, polar solvent extraction and activated carbon adsorption at a laboratory scale and ambient temperature. The effect of varying the acid type (H₃PO₄, H₂SO₄ and CH₃COOH) and pH (1-6), the type of polar solvent (CH₃OH, C₂H₅OH and C₃H₇OH) and their ratio to glycerol (3:1-1:3 v/v), and adsorption with activated carbon at different ratios of activated carbon to glycerol (40-200 g/l) on the purity of crude glycerol was explored. The highest glycerol purity (95.74 wt.%) was obtained with the sequential acidification to pH 2.5 with H₃PO₄ and phase separation, followed by extraction with C₃H₇OH at a solvent:crude glycerol ratio of 2:1 (v/v). Finally, adsorption with commercial activated carbon at 200 g/l also achieved a 99.7% color reduction.     

Preparation and Characterization of C60‐Filled Ethyl Cellulose Mixed‐Matrix Membranes for Gas Separation of Propylene/Propane

Mixed‐matrix membranes (MMMs) consisting of ethyl cellulose as continuous matrix and inorganic particle C₆₀ as dispersed phase were prepared for propylene/propane separation. The impact of the C₆₀ content on the separation properties of MMMs without and with ultraviolet cross‐linking was investigated. The increment of decomposition temperature and single glass temperature of ethyl cellulose/C₆₀ MMMs indicates a strong interfacial interaction between polymer and fullerenes. After UV irradiation, the gas permeability coefficient of propylene and ideal separation factor of propylene/propane decreased, and new features appeared in scanning electron microscopy and atomic force microscopy images, testifying the photopolymerization reaction of C₆₀ at a depth near to the surface. C₆₀ could be acted as a possible replaced carrier for the separation of olefin/paraffin using membrane separation technology.     

Uniformity control and ultra-micropore development of tubular carbon membrane for light gas separation

Tubular carbon molecular sieve (CMS) membranes have been recognized as a potential module for commercial application due to its high mechanical strength and large surface area. However, the carbon layer uniformity was restricted by substrate texture and dope fluidity when the dip-coating method was used. This study evaluated the influence of various parameters of dip-coating with an integrated vacuum-assisted system, including solvent vaporization rates, vertical immersion/withdrawal velocity, vacuum degree, dope composition, coating cycles on the microstructure, and gas separation performance of CMS membranes. Using vacuum assistance and a low-vaporization solvent minimized the influence of viscosity and gravity on dope fluidity as a result of fast phase inversion. The as-prepared tubular CMS membranes showed enhanced perm-selectivity according to a H₂/N₂ gas selectivity of 8.8, a CO₂/N₂ gas selectivity of 6.7, a H₂ permeability of 464 barrer, and a CO₂ permeability of 356 barrer.     

Growth of carbon nanofibers from the iron-copper catalyzed decomposition of CO/C2H4/H2 mixtures

The growth of carbon nanofibers from Fe-Cu catalyzed decomposition of CO/C₂H₄/H₂ mixtures at temperatures over the range 500-650 °C has been investigated. Based on analysis of the gas phase and solid products it is apparent that co-adsorption of CO and C₂H₄ induces major perturbations in the surfaces of the bimetallic catalyst particles. These features are reflected in an increase in the yield of solid carbon and subtle changes in the structural characteristics of the carbon nanofibers. Optimum performance with respect to the yield of carbon nanofibers is found for iron-rich particles treated in CO/C₂H₄/H₂ (1:3:1) at 600 °C. Deactivation of the catalyst is observed to occur with high Cu concentrations and at reaction temperatures in excess of 600 °C. It is suggested that under these conditions the surface of the particles in contact with the reactant gas mixture become enriched in Cu, which does not possess the ability to dissociatively chemisorb either CO or C₂H₄.     

Structural study of carbon nanomaterials prepared by chlorination of tungsten carbide and bis(cyclopentadienyl)tungsten dichloride

Hollow carbon nanobags have been obtained by the chlorination of bis(cyclopentadienyl)tungsten dichloride (W(C₅H₅)₂Cl₂) at 400 and 900 °C. Transmission electron microscopy images indicate an incipient graphitisation at higher reaction temperatures and an increase in the average dimensions of the particles. When using tungsten carbide (WC) as precursor, carbide-derived carbon has been observed at 900 °C, whereas at lower temperatures core-shell-like structures have been found as intermediate reaction steps. In both type of materials, electron energy-loss spectroscopy shows a very similar sp² carbon bonding content (∼94%). Textural studies show Type 1 adsorption isotherms with surface areas of 1250 and 1320 m²/g for WC and W(C₅H₅)₂Cl₂ respectively at the higher temperature treatment.     

Hydrothermal gasification of olive mill wastewater as a biomass source in supercritical water

In this study, the hydrothermal gasification of biomass in supercritical water is investigated. The work is of peculiar value since a real biomass, olive mill wastewater (OMW), is used instead of model biomass compounds. OMW is a by-product obtained during olive oil production, which has a complex nature characterized by a high content of organic compounds and polyphenols. The high content of organics makes OMW a desirable biomass candidate as an energy source. The hydrothermal gasification experiments for OMW were conducted with five different reaction temperatures (400, 450, 500, 550 and 600 °C) and five different reaction times (30, 60, 90, 120 and 150 s), under a pressure of 25 MPa. The gaseous products are mainly composed of hydrogen, carbon dioxide, carbon monoxide and C₁-C₄ hydrocarbons, such as methane, ethane, propane and propylene. Maximum amount of the gas product obtained is 7.71 mL per mL OMW at a reaction temperature of 550 °C, with a reaction time of 30 s. The gas product composition is 9.23% for hydrogen, 34.84% for methane, 4.04% for ethane, 0.84% for propane, 0.83% for propylene, 49.34% for carbon dioxide, and 0.88% for minor components such as n-butane, i-butane, 1-butene, i-butene, t-2-butene, 1,3-butadiene and nitrogen at this reaction conditions.     

Fixed- and fluidised-bed synthesis of coiled carbon fibres on an in situ generated H2S-modified Ni/Al2O3 catalyst from a NiSO4/Al2O3 precursor

NiSO₄/Al₂O₃ and NiO/Al₂O₃ catalyst precursors were formed by calcination of NiSO₄·6H₂O/Al₂O₃ at 500 and 800 °C, respectively. The catalyst precursor was reduced under H₂ and N₂ and then reacted under C₂H₂, H₂ and N₂ at 650 °C. Coiled carbon fibres were formed in fixed- and fluidised-bed reactors using the NiSO₄/Al₂O₃ catalyst precursor. Thermodynamic modelling using an infinite equilibrium stage construction predicted complete reduction of NiSO₄ to Ni and simultaneous H₂S formation occurs in both fixed- and fluidised-bed systems. XRD measurements confirmed that Ni was the only catalytically active crystalline species present at concentrations >0.5 wt.% (XRD detection limit) post-reduction, however XRF and XPS measurements additionally detected the presence of small quantities (<0.9 wt.% S) of S species. S is adsorbed onto the Ni surfaces during reduction when H₂S is released and dissociates on the Ni surface. Non-coiled carbon fibres produced on the Ni/Al₂O₃ catalyst formed from the NiO/Al₂O₃ precursor demonstrated that modification of Ni/Al₂O₃ with S is required for coiled carbon fibre synthesis.     

High H2 permeable SAPO-34 hollow fiber membrane for high temperature propane dehydrogenation application

SAPO-34 hollow fiber zeolite membranes are successfully synthesized on α-Al₂O₃ hollow fiber ceramic substrates by secondary growth method, and used to separate H₂ from a binary mixture (H₂, C₃H₈) or ternary mixture (H₂, C₃H₈, and C₃H₆) under a wide temperature range (25–600°C) with the aim of using them for propane dehydrogenation (PDH) reactions at high temperature. The results show excellent performance for H₂/C₃H₈ and H₂/ C₃H₈ & C₃H₆ separation, with high H₂ permeance of 3.1 × 10⁻⁷ mol/m²/s/Pa and H₂/C₃H₈ selectivity of 41 at 600°C. Additionally, the membrane shows stable performance for 140 hr of H₂/C₃H₈ separation test at 600°C. The high performance of this membrane is mainly attributed to the thin (∼2 μm) zeolite layer and asymmetric-wall of the hollow fiber support. So far, this membrane offers the highest hydrogen permeation and selectivity for H₂/C₃H₈ separation at high temperature (600°C) compared to those reported in literature.     

Membranes of poly(phenylene oxide) and its copolymers: Synthesis,characterization, and application in recovery of propylene from a refinery off‐gas mixture

Copolymers of 2,6‐dimethyl‐phenol (DMP) and 2,6‐diphenyl‐phenol (DPP) were synthesized in the initial molar ratio of 100 : 0 (S(PPO)), 90 : 10 (Co‐A), 75 : 25 (Co‐B), 65 : 35 (Co‐C), and 0 : 100 (PDPPO). Dense membranes of 30 μm thickness were tested for single gas permeation and binary mixture separation of 55:45 (in mol %) propylene‐propane at 30°C ± 2°C. Their performance was ultimately examined in the enrichment of propylene from a refinery off‐gas mixture (ROG or also called as absorber tail gas, ATG) having the same composition as the ATG of a fluid catalytic cracking (FCC) unit of HPCL refinery, Visakhapatnam. The mixture contains C₁–C₅ hydrocarbons and nonhydrocarbons such as CO, CO₂, H₂, and N₂. A detailed permeation study of the hydrocarbon part of ATG revealed that using S(PPO) and Co‐A, propylene could be upgraded from ～ 29 mol % (on nonhydrocarbon free basis) to 62.2 and 74.4 mol % with propylene/propane selectivity ratio of 5.99 and 8.45, respectively. The structure of polymers was characterized by Fourier transform infrared (FTIR), proton nuclear magnetic resonance (Proton NMR), viscosity measurements. Scanning electron microscope (SEM), wide angle X‐ray diffraction (WAXD), density and fractional free volume measurements were used for studying membrane morphology. Dynamic mechanical thermal analyzer (DMTA) and tensile testing were carried to find glass transition temperature (T_g) and mechanical properties. The relative differences observed in gas permeation of these polymers were correlated with the physical properties measured. S(PPO) and Co‐A were identified as potential materials for the upgradation of propylene from refinery off‐gas streams. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

A new yttrium-based metal–organic framework for molecular sieving of propane from propylene with high propylene capacity

Developing energy-efficient alternatives for propylene (C₃H₆) and propane (C₃H₈) separation is of great significance and challenge in the petrochemical industry. Herein, we report the rational design of a new yttrium-based ultramicroporous metal–organic framework (MOF) comprised of 12-connected hexanuclear [Y₆(OH)₈(COO)₁₂]²⁻ cluster and 5-(3,5-dicarboxybenzoylamino)isophthalic acid (H₄dbai) with ftw topology. It possesses a suitable pore window size and a relatively large pore volume for molecular sieving separation of C₃H₈ from C₃H₆ with a high C₃H₆ capacity. At 298 K and 100 kPa, the adsorption capacity of C₃H₆ was 2.57 mmol/g, which is the highest among the reported C₃H₆/C₃H₈ molecular sieving MOF adsorbents. The molecular simulation revealed that the steric hindrance effect together with the electrostatic interaction of the oxygen sites in the window resulted in the molecular sieving separation of C₃H₆/C₃H₈. The breakthrough experiments confirmed its excellent separation performance under dynamic conditions to produce high purity (97.1%) of C₃H₆ with a working adsorption capacity of 1.75 mmol/g.     

Separation of CO2/CH4 through carbon molecular sieve membranes derived from P84 polyimide

The separation of CO₂/CH₄ separation is industrially important especially for natural gas processing. In the past decades, polymeric membranes separation technology has been widely adopted for CO₂/CH₄ separation. However, polymeric membranes are suffering from plasticization by condensable CO₂ molecules. Thus, carbon molecular sieve membranes (CMSMs) with excellent separation performance and stability appear to be a promising candidate for CO₂/CH₄ separation. A commercially available polyimide, P84 has been chosen as a precursor in preparing carbon membranes for this study. P84 displays a very high selectivity among the polyimides. The carbonization process was carried out at 550–800 °C under vacuum environment. WAXD and density measurements were performed to characterize the morphology of carbon membranes. The permeation properties of single and equimolar binary gas mixture through carbon membranes were measured and analyzed. The highest selectivity was attained by carbon membranes pyrolyzed at 800 °C, where the pyrolysis temperatures significantly affected the permeation properties of carbon membranes. A comparison of permeation properties among carbon membranes derived from four commercially available polyimides showed that the P84 carbon membranes exhibited the highest separation efficiency for CO₂/CH₄ separation. The pure gas measurement underestimated the separation efficiency of carbon membranes, due to the restricted diffusion of non-adsorbable gas by adsorbable component in binary mixture.     

Separation of propylene and propane with a microporous metal–organic framework via equilibrium-kinetic synergetic effect

Adsorption separation of olefin and paraffin can greatly lower the energy consumption associated with the currently utilized distillation technique but remains a great challenge. Herein, we report the efficient separation of propylene (C₃H₆) and propane (C₃H₈) in a phosphate anion-functionalized metal–organic framework (MOF) ZnAtzPO₄ by synergetic effect of equilibrium and kinetics. The material features periodically expanded and contracted apertures decorated with electronegative groups, offering eligible pore shape and pore chemistry to effectively trap C₃H₆ under moderate isosteric heat of adsorption (27.5 kJ mol⁻¹) while obstruct the diffusion of C₃H₈. It simultaneously combines excellent thermodynamic selectivity (uptake ratio of 1.71) and kinetic selectivity (~31) for C₃H₆/C₃H₈ separation, meanwhile can be easily regenerated. Breakthrough experiment for C₃H₆/C₃H₈ gas mixture was conducted and confirmed the outstanding separation capability of ZnAtzPO₄. The equilibrium and kinetics cooperative C₃H₆/C₃H₈ adsorption separation was for the first time found in anion-functionalized MOFs, and further confirmed by computational studies.     

Hydrogen separation by dense cermet membranes

Novel cermet (i.e. ceramic-metal composite) membranes have been developed to separate hydrogen from mixed gases, particularly product streams generated during coal gasification and/or methane reforming. Hydrogen separation with these membranes is non-galvanic, i.e. it does not use electrodes or an external power supply to drive the separation, and hydrogen selectivity is nearly 100% because the membranes contain no interconnected porosity. The hydrogen permeation rate has been measured as a function of temperature (500-900 °C), membrane thickness (≈22-210 μm), and partial pressure of hydrogen (0.04-1.0 atm) in the feed gas. The hydrogen flux varied linearly with the inverse of membrane thickness, and reached ≈20 cm³(STP)/min cm² for a membrane with a thickness of ≈22 μm at 900 °C with 100% H₂ (at ambient pressure) as the feed gas. The results indicate that the hydrogen flux is limited by bulk diffusion and might be higher for a thinner (<22 μm) membrane. Some of the membranes were tested in a simulated syngas mixture containing H₂, CO, CO₂, and CH₄, and showed no degradation in performance. Hydrogen flux measurements made in H₂S-containing atmospheres for times approaching ≈270 h showed that a 200-μm-thick cermet membrane was stable in gases containing up to ≈400 ppm H₂S. While longer-term studies are needed, these results suggest that the cermet membranes may be suitable for practical hydrogen separation applications.