Enhanced one-step purification of C2H4 from C2H2/C2H4/C2H6 mixtures by fluorinated Zr-MOF

The extraction of C₂H₄ from C₂H₆/C₂H₄/C₂H₂ mixtures is of great significance in the chemical industry for C₂H₄ production but the process remains challenging due to the similarity of these C₂ hydrocarbon species in their molecular size and physical properties. Here, we report the fluorination of a stable Zr-MOF, UiO-66, to fine-tune the pore dimensions and pore functionality. In particular, UiO-66-CF₃ shows notably preferential adsorption of C₂H₆ and C₂H₂ over C₂H₄, with C₂H₂/C₂H₄ and C₂H₆/C₂H₄ selectivities of 1.4 and 1.9, respectively. Theoretical calculations provide insight into the binding sites of UiO-66-CF₃ for C₂ hydrocarbon adsorption. Breakthrough experiments further confirmed the capability of the material for purification of C₂H₄ from C₂H₂/C₂H₄/C₂H₆ ternary mixtures, evidenced by the high purity C₂H₄ (99.9%+) obtained directly from outlet gas.     

Mechanochemically synthesized ZIF-8 nanoparticles blended into 6FDA-TrMPD membranes for C3H6/C3H8 separation

The mixed matrix membranes (MMMs) consisting zeolitic-imidazolate framework-8 (ZIF-8) nanoparticles in a polymer have been of considerable interest in separation applications. The fillers used are mostly synthesized using the solvothermal method. In this study, the ZIF-8 nanoparticles were synthesized using a solvent-less and salt-free mechanochemical method and were added to 6FDA-TrMPD polyimide to prepare MMMs. The single gas permeation of C₃H₆ and C₃H₈ through the MMMs was investigated. The C₃H₆ permeability and C₃H₆/C₃H₈ ideal selectivity of a 20 wt% mechano-synthesized ZIF-8/6FDA-TrMPD MMM were 70% and 32% higher than those of the neat polymer membrane at 0.1 MPa and 308 K, respectively. The C₃H₆/C₃H₈ separation performance of the mechano-synthesized ZIF-8 MMM was similar to that of the conventional solvothermal-synthesized ZIF-8 MMM. This separation performance was in good agreement with the Maxwell model. Temperature and pressure dependence analyses confirmed that the mechano-synthesized ZIF-8 nanoparticles acted as molecular sieves in the MMMs for the C₃H₆ and C₃H₈ permeation.     

Permeation of H2, N2, CH4, C2H6, and C3H8 through asymmetric polyetherimide hollow‐fiber membranes

Permeation properties of pure H₂, N₂, CH₄, C₂H₆, and C₃H₈ through asymmetric polyetherimide (PEI) hollow‐fiber membranes were studied as a function of pressure and temperature. The PEI asymmetric hollow‐fiber membrane was spun from a N‐methyl‐2‐pyrrolidone/ethanol solvent system via a dry‐wet phase‐inversion method, with water as the external coagulant and 50 wt % ethanol in water as the internal coagulant. The prepared asymmetric membrane exhibited sufficiently high selectivity (H₂/N₂ selectivity >50 at 25°C). H₂ permeation through the PEI hollow fiber was dominated by the solution‐diffusion mechanism in the nonporous part. For CH₄ and N₂, the transport mechanism for gas permeation was a combination of Knudsen flow and viscous flow in the porous part and solution diffusion in the nonporous part. In our analysis, operating pressure had little effect on the permeation of H₂, CH₄, and N₂. For C₂H₆ and C₃H₈, however, capillary condensation may have occurred at higher pressures, resulting in an increase in gas permeability. As far as the effect of operating temperature was concerned, H₂ permeability increased greatly with increasing temperature. Meanwhile, a slight permeability increment with increasing temperature was noted for N₂ and CH₄, whereas the permeability of C₂H₆ and C₃H₈ decreased with increasing temperature. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 698–702, 2002     

Simulation of partial oxidation of natural gas with detailed chemistry: Influence of addition of H2, C2H6 and C3H8

The partial oxidation (POX) of natural gas to produce acetylene was studied using detailed chemistry simulation to determine the influences of adding H₂, C₂H₆ and C₃H₈ to the CH₄ feed. Four detailed chemistry mechanisms, codenamed GRI 3.0, GRI 486, Petersen, and Curran, for describing the POX of natural gas under fuel-rich conditions were evaluated by comparing calculated results with ignition delay time and homogeneous oxidation data. The Curran mechanism gave the best performance, and was further used to examine the influences brought about by changes in the natural gas composition due to the addition of H₂, C₂H₆ and C₃H₈. The addition of H₂, C₂H₆ and C₃H₈ reduced the consumption of natural gas per ton of acetylene produced, and would enhance the economy of the POX process and help use up excess H₂, C₂H₆ and C₃H₈ from other processes in chemical plants. For natural gases that contain small amounts of higher hydrocarbons, the adding of H₂, C₂H₆ and C₃H₈ significantly decreased the ignition delay time, and the residence time of the feed in the mixing zone has to be reduced when adding these species to avoid uncontrolled combustion.     

Boosting C2H6/C2H4 separation in scalable metal-organic frameworks through pore engineering

The development of ethane (C₂H₆)-selective adsorbents for ethylene (C₂H₄) purification, although challenging, is of prime industrial importance. Pillared-layer metal-organic frameworks (MOFs) possess facilely tunable pore structure and functionality, which means they have excellent potential for high-performance C₂H₆/C₂H₄ separation applications. Herein, we report a family of isostructural pillared-layer MOFs with various metal centers M and co-ligands L, M₂(D-cam)₄L₂ (denoted M-cam-L; M = Cu, Co, Ni; L = pyz, apyz, dabco), with a variety of pore surface properties. All of the M-cam-L materials exhibit preferential adsorption for C₂H₆ over C₂H₄. In particular, Ni-cam-pyz exhibits the highest C₂H₆ capture capacity (68.75 cm³ g⁻¹ at 1 bar and 298 K), Cu-cam-dabco possesses the greatest C₂H₆/C₂H₄ adsorption selectivity (2.3), and the lowest isosteric heat of adsorption is demonstrated for Cu-cam-pyz (20.1 kJ mol⁻¹). Dynamic column breakthrough experiments also confirmed the excellent separation performance of M-cam-pyz and M-cam-dabco materials. The synthesis route of the M-cam-L materials is easily scaled-up under laboratory conditions, and hence this class of MOFs is promising for practical C₂H₄ purification.     

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.     

Enhancement of C2H6 Oxidation by O2 in the Presence of N2O over Fe Ion-Exchanged BEA Zeolite Catalyst

Selective catalytic reduction (SCR) of N₂O with C₂H₆ took place effectively over Fe ion-exchanged BEA zeolite catalyst (Fe-BEA) even in the presence of excess oxygen. The mechanism in the SCR of N₂O with C₂H₆ over Fe-BEA catalyst was studied by a transient response experiment and an in situ DRIFT spectroscopy. No oxidation of C₂H₆ by O₂ took place below 350 °C (in C₂H₆/O₂). In the N₂O/C₂H₆/O₂ system, however, it was found that the reaction of C₂H₆ with O₂ was drastically enhanced by the presence of N₂O even at low temperatures (200-300 °C). Therefore, it was concluded that N₂O played an important role in the oxidation of C₂H₆ (i.e., activation of C₂H₆ at an initial step). On the basis of these findings, the mechanism in the SCR of N₂O with C₂H₆ is discussed.     

Influence of the type of reducing agent (H2, CO,C3H6 and C3H8) on the reduction of stored NO X in a Pt/BaO/Al2O3 model catalyst

In this investigation, a comparative study for a NO _X storage catalytic system was performed focusing on the parameters that affect the reduction by using different reductants (H₂, CO, C₃H₆ and C₃H₈) and different temperatures (350, 250 and 150 °C), for a Pt/BaO/Al₂O₃ catalyst. Transient experiments show that H₂ and CO are highly efficient reductants compared to C₃H₆ which is somewhat less efficient. H₂ shows a significant reduction effect at relatively low temperature (150 °C) but with a low storage capacity. We find that C₃H₈ does not show any NO _X reduction ability for NO _X stored in Pt/BaO/Al₂O₃ at any of the temperatures. The formation of ammonia and nitrous oxide is also discussed.

     

Study of the preparation of bulk tungsten carbide catalysts with C2H6/H2 and C2H4/H2 carburizing mixtures

The influence of the preparation procedure of tungsten carbide on the mechanism of carburization is discussed. This work is focused on the reduction and the carburization of tungsten trioxide by a mixture of hydrocarbon and H₂ to form WC. Temperature-programmed reaction spectra obtained with CH₄, C₂H₆ and C₂H₄ have been measured. In presence of the CH₄-H₂ mixture, H₂ is the reducing agent and the hydrocarbon is consumed for the carburization whereas C₂H₆ or C₂H₄ participates in the reduction of the tungsten oxide. The temperatures of reduction and carburization are lower by about 150 K using C₂H₆ or C₂H₄ instead of CH₄. Such a decrease of the temperature of reduction of tungsten oxide is needed to avoid the formation of poorly reducible compounds that can occur during the preparation of supported tungsten carbide. Furthermore, the surface area of the resulting carbide is 25 m²/g with C₂H₆ and C₂H₄ and 10 m²/g with CH₄. During the carburization, the deposit of excess carbon on the WC surface is larger with the C₂ hydrocarbons than with CH₄, but it protects the carbide and can be removed by hydrogen treatment. This revised version was published online in July 2006 with corrections to the Cover Date.     

Design and screening of ionic liquids for C2H2/C2H4 separation by COSMO‐RS and experiments

Ionic liquids (ILs) have been proposed as promising solvents for separating C₂H₂ and C₂H₄, but screening an industrially attractive IL with high capacity from numerous available ILs remains challenging. In this work, a rapid screening method based on COSMO‐RS was developed. We also present an efficient strategy to improve the C₂H₂ capacity in ILs together with adequate C₂H₂/C₂H₄ selectivity with the aid of COSMO‐RS. The essence of this strategy is to increase molecular free volume of ILs and simultaneously enhance hydrogen‐bond basicity of anions by introducing flexible and highly asymmetric structures, which is validated by a new class of tetraalkylphosphonium ILs featuring long‐chain carboxylate anions. At 298.1 K and 1 bar, the solubility of C₂H₂ in ILs reaches 0.476 mol/mol IL, very high for a physical absorption, with a selectivity of up to 21.4. The separation performance of tetraalkylphosphonium ILs to the mixture of C₂H₂/C₂H₄ was also evaluated. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2016–2027, 2015     

Dinuclear Rhodium Complexes Containing the Diyne 1,4-C6H4(C≡CH)2 and the Isomeric Bisvinylidene 1,4-C6H4(CH=C:)2 as Bridging Units

The reaction of [RhCl(PiPr₃)₂] ( 1 ) with 1,4-C₆H₄(C≡CH)₂ at 0°C leads almost quantitatively to the formation of the bis(alkyne) complex [(PiPr₃)₂ClRh-(HC≡C₆H₄-C≡CH)RhCl(PiPr₃)₂] (2). At elevated temperatures (THF, 60°C) it rearranges to give the isomeric bis(vinylidene) complex [(PiPr₃)₂ClRh-(=C=CH-C₆H₄-CH=C=)RhCl(PiPr₃)₂] (3). A one-pot synthesis of 3 is also described. Treatment of either 2 or 3 with pyridine affords the bis(alkynyl)dihydrido compound [(PiPr₃)₂(py)Cl(H)Rh(-C≡C-C₆H₄-C≡C-)Rh(H)Cl(py)(PiPr₃)₂] ( 4 ) in which both metal centers are octahedrally coordinated. Whereas the reaction of 2 with NaC₅H₅ produces the complex C_sH₅(PiPr₃)Rh(HC≡C-C₆H₄-C≡CH)Rh(PiPr₃)C₅H₅ ( 7 ), the bis(vinyl-idene) isomer C₅H₅(PiPr₃)Rh(=C=CH-C₆H₄-CH=C=)Rh(PiPr₃)C₅H_s ( 8 ) is obtained from 4 and NaC₅H₅. Electrophiles preferably attack the Rh=C bonds of 8 and thus on protonation with CF₃CO₂H the bis(vinyl) complex C₅H₅(PiPr₃)(CF₃CO₂)-Rh(Z,Z-CH=CH-C₆H₄-CH=CH)Rh(O₂CCF₃)(PiPr₃)C₅H_s ( Z-9 ) is formed. In acetone solution, it rearranges to give the E isomer. Reaction of 8 with sulfur affords the bis(thioketene) complex C₅H₅(PiPr₃)Rh(≡²-C,S; η²-C,S-S=C=CH-C₆H₄-CH=C=S)-Rh(PiPr₃)C₅H₅ ( 12 ), for which only one diastereomer is observed. All attempts to prepare mononuclear rhodium compounds containing the diyne HC≡C-C₆H₄-G≡CH or the isomeric vinylidene: C=CH-C₆H₄-G≡CH as ligand failed.     

Application of a C6‐OH of chitosan immobilized cyclodextrin derivates on an electrochemical H2O2 biosensor

Biosensor detecting techniques have attracted much attention in the content determination of H₂O₂, which has been used illegally as a food additive. An electrochemical biosensing membrane for the detection of H₂O₂ was developed with C6‐OH of chitosan immobilized cyclodextrin derivates (6‐CD–CTS), which possessed a high cyclodextrin loading capacity (2.12 × 10^?4 mol/g), as the carrier. The biosensor was prepared through the inclusion of ferrocene as the electron mediator in a hydrophobic cavity of cyclodextrin and crosslinking catalase (CAT) to 2‐NH₂ of 6‐CD–CTS. The ferrocene‐included complex was evaluated by ultraviolet–visible spectrophotometry and thermogravimetric analysis. Its electrochemical behavior was also studied. The impact of the reaction conditions on the CAT immobilization capacity was evaluated. When previous membrane was used to detect the concentration of H₂O₂ (C_H2O2), we found that the catalysis of CAT and the signal amplification of ferrocene had a major impact on the cyclic voltammograms. The optimal working pH of the modified electrode was 7.0. The peak current (I) had a linear relationship with the H₂O₂ concentration (CH₂O₂) in the range 1.0 × 10^?4 to 1.0 × 10^?3 mol/L. The linear regression equation was I = 0.00475C_H2O2 ? 0.03025. The detection limit was 10^?6 mol/L. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41499.     

Thermodynamic adsorption data of CH4, C2H6, C2H4 as the OCM process hydrocarbons on SAPO-34 molecular sieve

Adsorption of CH₄, C₂H₆ and C₂H₄, the feed and main products of oxidative coupling process of methane (OCM) has been studied on silicoalumina-phosphate molecular sieve (SAPO-34) in mild conditions. The experiments were conducted in a batch system based on volumetric adsorption measurement technique for determination equilibrium adsorption capacity in the absolute pressure range of 100–1000 kPa and at the isothermal temperatures of 303, 313 and 323 K. Various isotherm equations were fitted on the adsorption equilibrium data and the model parameters were predicted as a function of temperature. Isosteric heats of adsorption were determined using Clausius–Clapeyron equation at different surface coverage. Maximum capacity of SAPO-34 was observed at 303 K and 880–900 kPa equilibrium pressure with 1.25, 2.02 and 4.67 mmol/g adsorbed amount for methane, ethane and ethylene adsorption, respectively. The adsorption selectivity of ethane and ethylene against methane were determined and the appropriate potential of SAPO-34 was observed for separation of OCM products from methane. The isotherm models and enthalpy of adsorption can be efficiently used for the simulation of the adsorption process constructed at the downstream of the OCM process for separation of ethane and ethylene from methane.     

Reduction of Surface Nitrates via C3H6 Oxidation Over a Pt/Al2O3 Catalyst

Reduction of surface nitrates with C₃H₆ on a Pt/Al₂O₃ catalyst was investigated using diffuse reflectance infrared spectroscopy (DRIFTS) and a reactor system designed for monolith-supported catalysts. C₃H₆ oxidation was inhibited by the presence of NO, and vice versa, and data indicate that adsorbed NO_X reacted with gas phase C₃H₆. DRIFTS results confirm reaction between C₃H₆ and surface nitrates with linear nitrites as the reaction products. Data show that Pt is required for this reaction, which suggests the nitrates in proximity to the Pt particles are affected/relevant.     

UTSA-280 metal–organic framework incorporated 6FDA-polyimide mixed-matrix membranes for ethylene/ethane separation

Mixed-matrix membranes (MMMs), judiciously combining processability of polymer and remarkable separation performance of nanofillers, have been extensive pursuits for molecular separation process. Permeability matching between filler and polymer is one of the necessary requisites to desirable mixed-matrix effect. Considering the superior molecular sieving effect of UTSA-280 metal-organic frameworks on C₂H₄ and C₂H₆, here, we report two types of UTSA-280/6FDA-polyimide MMMs toward C₂H₄/C₂H₆ separation. The molecular sieving effect of UTSA-280 endowed 6FDA-DAM:DABA(3:2) membrane with simultaneous improvements in C₂H₄ permeability and C₂H₄/C₂H₆ selectivity. Optimally, when the filler reached 21.80 wt%, C₂H₄ permeability and C₂H₄/C₂H₆ selectivity was increased to 6.49 Barrer (by 15%) and 4.94 (by 32%), respectively. On the contrary, UTSA-280/6FDA-DAM MMMs showed undesirable mixed-matrix effect that C₂H₄ permeability decreased meanwhile C₂H₄/C₂H₆ selectivity nearly kept at polymeric pristine membrane level. It was found that permeability matching between two phases was responsible to these opposite mixed-matrix effects. More specifically, UTSA-280 had a relatively low gas permeability so that it required a less permeable polymeric matrix like 6FDA-DAM:DABA(3:2) to exert its molecular sieving effect. Furthermore, the optimal-matching 6FDA-matrix in permeability with UTSA-280 fillers was predicted by theoretical model. This work not only reports improving C₂H₄/C₂H₆ separation performance via mixed-matrix formulation, but also emphasizes the importance of permeability matching between polymer and filler to realize the mixed-matrix effect.     

Preparation and C3H8/Gas Separation Properties of a Synthesized Single Layer PDMS Membrane

In this paper, a new polydimethylsiloxane (PDMS) membrane was synthesized and its ability for separation of heavier gases from lighter ones was examined. Sorption, diffusion, and permeation of H₂, N₂, O₂, CH₄, CO_2, and C₃H₈ in the synthesized membrane were investigated as a function of pressure at 35°C. PDMS was confirmed to be more permeable to more condensable gases such as C₃H₈. This result was attributed to very high solubility of larger gas molecules in hydrocarbon?based PDMS in spite of their low diffusion coefficients relative to small molecules. The synthesized membrane showed much better gas permeation performance than others reported in the literature. Increasing upstream pressure increased solubility, permeability and diffusion coefficients of C₃H₈, while these values decreased slightly or stayed constant for other gases. Local effective diffusion coefficient of C₃H₈ and CO₂ increased with increasing penetrant concentration which indicated plasticization effect of these gases over the range of penetrant concentration studied. C₃H₈/gas solubility, diffusivity and overall selectivities also increased with increasing feed pressure. Ideal selectivity values of 4, 13, 18, 20, and 36 for C₃H₈ over CO₂, CH₄, H₂, O_2, and N₂, respectively, at upstream pressure of 7 atm, confirmed the outstanding separation performance of the synthesized mebrane.     

Reaction Kinetics of C3H6 Oxidation for Various Reaction Pathways Over Diesel Oxidation Catalysts

Reaction kinetics studies of C₃H₆ oxidation over Pt/Al₂O₃ and Pt/SiO₂ catalysts were characterized using temperature-programmed oxidation with different oxidants: O₂, NO₂ and surface nitrates. Activation energies and conversion performance were compared in order to determine which hydrocarbon oxidation reaction pathway(s) is relevant in diesel exhaust gas aftertreatment applications. NO _x adsorption did not occur on the SiO₂ surface so the reaction between C₃H₆ and NO₂ could be isolated, i.e. no nitrate effect would complicate the analysis and their significance could be decoupled. These results were then compared with Pt/Al₂O₃ where surface nitrates did form upon exposure to NO _x . The onset of C₃H₆ oxidation was observed at a lower temperature with O₂ than with NO₂, but the activation energy was lower with NO₂. This apparent discrepancy is related to the different oxidant concentrations used and the different adsorption pathways. The results indicate that hydrocarbons must be activated first for oxidation to begin, for either NO₂ or O₂. In analyzing the reaction between C₃H₆ and nitrates, the reaction did not occur until NO _x started to desorb from the catalyst at higher temperatures, i.e. when nitrates became unstable and decomposed, thus providing a readily available oxidant source. However, when O₂ was added to the nitrate/C₃H₆ system, the reaction began at even lower temperature than with just C₃H₆ and O₂. Nitrate consumption was also observed once oxidation began. The presence of the combination of nitrates and O₂ resulted in a lower C₃H₆ oxidation activation energy.     

Efficient adsorptive separation of propene over propane through a pillar-layer cobalt-based metal–organic framework

The purification of propene from the propene/propane binary mixture is one of the most important and challenging separation processes in the chemical industry. In this study, a cobalt-based pillar-layer metal–organic framework, Co(AIP)(BPY)_0.5, was synthesized. It exhibited excellent water and moisture stability and efficient C₃H₆/C₃H₈ separation performance. At 298 K and 100 kPa, the adsorption capacity of C₃H₆ was 1.99 mmol/g and the IAST adsorption selectivity was 21, which has exceeded most investigated MOFs adsorbents. The cyclic breakthrough experiments of C₃H₆/C₃H₈ binary mixture confirmed its efficient dynamic separation property and excellent recyclability. The molecular simulation showed that the C₃H₈ molecules with multiple binding sites (C─H) were restricted at the middle of the one-dimensional channel. Compared with C₃H₆, the large steric hindrance of C₃H₈ caused lower diffusion rate (with a C₃H₆/C₃H₈ kinetic selectivity of 29.7, at 303 K) in the narrow pore system of 1 , which has been confirmed by the kinetic adsorption experiments.     

Adsorbate-assisted NO decomposition in NO reduction by C3H6 over Pt/Al2O3 catalysts under lean-burn conditions

The rates and product selectivities of the C₃H₆-NO-O₂ and NO-H₂ reactions over a Pt/Al₂O₃ catalyst, and of the straight, NO decomposition reaction over the reduced catalyst have been compared at 240C. The rate of NO decomposition over the reduced catalyst is seven times greater than the rate of NO decomposition in the C₃H₆-NO-O₂ reaction. This is consistent with a mechanism in which NO decomposition occurs on Pt sites reduced by the hydrocarbon, provided only that at steady state in the lean NO _x reaction about 14% of the Pt sites are in the reduced form. However, the (extrapolated) rate of the NO-H₂ reaction at 240C is about 10⁴ times faster than the rate of the NO decomposition reaction thus raising the possibility that NO decomposition in the former reaction is assisted by H_ads. It is suggested that adsorbate-assisted NO decomposition in the C₃H₆-NO-O₂ reaction could be very important. This would mean that the proportion of reduced Pt sites required in the steady state would be extremely small. The NO decomposition and the NO-H₂ reactions produce no N₂O, unlike the C₃H₆-NO-O₂ reaction, suggesting that adsorbed NO is completely dissociated in the first two cases, but only partially dissociated in the latter case. It is possible that some of the associatively adsorbed NO present during the C₃H₆-NO-O₂ reaction may be adsorbed on oxidised Pt sites.