Effects of halloysite nanotubes on the morphology and CO2/CH4 separation performance of Pebax/polyetherimide thin-film composite membranes

Enhancing the performance of gas separation membranes is one of the major concerns of membrane researchers. Thus, in this study, poly(ether-block-amide) (Pebax)/polyetherimide (PEI) thin-film composite membranes were prepared and their CO₂/CH₄ gas separation performance was investigated by means of pure and mixed gases permeation tests. To improve the properties of these membranes, halloysite nanotubes (HNT) were added to Pebax layer at different loadings of 0.5, 1, 2, and 5 wt % to form Pebax-HNT/PEI membranes. Scanning electron microscopy, gas sorption, X-ray diffraction, Fourier-transform infrared, and differential scanning calorimetry tests were also performed to investigate the impact of HNT on structure and properties of prepared membranes. Results showed that both CO₂/CH₄ selectivity and CO₂ permeance increased by adding HNT to Pebax layer up to 2 wt %. By increasing HNT loading to 5 wt %, the CO₂/CH₄ selectivity decreased from 32 to 18, while CO₂ permeance increased from 3.25 to 4.2 GPU. Pebax/PEI and Pebax-HNT/PEI membranes containing 2 wt % of HNT were tested using CO₂/CH₄ gas mixtures at different feed CO₂ concentrations and feed pressure of 4 bar. The results showed that with increasing CO₂ concentration from 20 to 80 vol %, CO₂/CH₄ selectivity of Pebax/PEI composite membranes increased by 19%, while, in Pebax-HNT/PEI membrane, CO₂/CH₄ selectivity decreased by 40%. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48860.     

Effect of support layer on gas permeation properties of composite polymeric membranes

PES/Pebax and PEI/Pebax composite membranes were prepared by coating the porous PES and PEI substrate membranes with Pebax-1657. The morphology and performance of the prepared membranes were investigated by SEM and CO₂ and CH₄ permeation tests. The CO₂ permeances of 28 and 52 GPU were achieved for PES/Pebax and PEI/Pebax composite membranes, respectively, with CO₂/CH₄ selectivities almost equal to that of Pebax (26). The experimental data were further subjected to a theoretical analysis using the resistance model. It was found that the porosity and the thickness of the dense section of PES substrate were an order of magnitude higher than those of PEI substitute. The porosity/thickness ratio of PEI substrate was, however, higher than PES, explaining the higher permeance of PEI/Pebax composite membrane. Substrates with porosities much higher than the Henis-Tripodi gas separation membrane were used in this work, aiming to achieve the selectivity of Pebax, rather than those of the substrate membrane materials.     

Permeation Characteristics of Light Hydrocarbons Through Poly(amide-6-β-ethylene oxide) Multilayer Composite Membranes

In this paper, poly(amide-6-β-ethylene oxide) (PEBA1657) copolymer was used to prepare multilayer polyetherimide (PEI)/polydimethylsiloxane (PDMS)/PEBA1657/PDMS composite membranes by dip-coating method. Permeation behaviors of ethylene, ethane, propylene, propane, n-butane, methane and nitrogen through the multilayer composite membranes were investigated over a range of operating temperature and pressure. The permeances of light hydrocarbons through PEI/PDMS/PEBA1657/PDMS composite membranes increase with their increasing condensability, and the olefins are more permeable than their corresponding paraffins. For light hydrocarbons, the gas permeances increase significantly as temperature increasing. When the transmembrane pressure difference increases, the gas permeance increases moderately due to plasticization effect, while their apparent activation energies for permeation decrease.     

The effect of ZIF-90 particle in Pebax/Psf composite membrane on the transport properties of CO2, CH4 and N2 gases by Molecular Dynamics Simulation method

Nowadays, mixed matrix membranes (MMMs) have considered by many researchers to overcome the problems of polymeric membranes. In addition, molecular dynamics (MD) and Monte Carlo (MC) simulation Methods are suitable tools for studying transport properties and morphology in MMMs. For this purpose, in this study using material studio 2017 (MS) software, the transport properties of CO₂, CH₄ and N₂ in Pebax, Psf neat Pebax/Psf composite and Pebax/Psf composite filled with ZIF-90 particles have been investigated. By adding Psf to Pebax matrix, the selectivity of CO₂/CH₄ and CO₂/N₂ gases has significantly increased. In addition, adding ZIF-90 particles to the Pebax/Psf composite increased the permeability of CO₂, CH₄ and N₂ compared to neat and composite membranes. The morphological properties of the membranes, such as the fractional free volume (FFV), radial distribution function (RDF), glass transition temperature (T_G), X-ray diffraction (XRD) and equilibrium density have calculated and acceptable results have obtained.     

界面聚合制备含丙烯氧基团的复合膜用于CO2分离

含有醚氧基团的膜与CO₂分子具有较强的极性作用,可以实现对CO₂/N₂物系的高效分离.其中,含丙烯氧(PO)基团的聚合物链段自由体积较大且不易结晶,是一类具有发展潜力的溶解选择性膜材料.以均苯三甲酰氯为油相单体,含PO基团的多胺为水相单体,通过界面聚合,成功制备了含PO基团的复合膜.分别采用聚醚胺D400、D230及T403为多胺水相单体,考察了膜内PO基团数量和交联度对复合膜分离性能的影响.结果表明,采用D400所制的复合膜由于具有最高的PO基团含量及较低的交联度,因此具有最高的CO₂渗透速率和CO₂/N₂分离因子.之后,考察了单体浓度、酸吸收剂种类以及水相溶液pH对复合膜分离性能的影响.通过优化这些制膜条件,制备出了CO₂/N₂分离性能较好的复合膜.     

Ceramic Supported PDMS and PEGDA Composite Membranes for CO2 Separation

Composite membranes have attracted increasing attentions owing to their potential applications for CO2 separation. In this work, ceramic supported polydimethylsiloxane （PDMS） and poly （ethylene glycol） diacrylate （PEGDA） composite membranes were prepared. The microstructure and physicochemical properties of the compos- ite membranes were characterized. Preparation conditions were systematically optimized. The gas separation performance of the as-prepared membranes was studied by pure gas and binary gas permeation measurement of CO〉 N2 and H〉 Experiments showed that PDMS, as silicone rubber, exhibited larger permeance and lower separation factors. Conversely, PEGDA composite membrane presented smaller gas permeance but higher ideal selectivity for CO2/N2. Compared to the performance of those membranes using polymeric supports or freestanding membranes, the two kinds of ceramic supported composite membranes exhibited higher gas permeance and acceptable selectivity. Therefore, the ceramic supported composite membrane can be expected as a candidate for CO2 separation from light gases.     

CeO2复合改性ZSM-5分子筛用于催化甲醇制丙烯研究

在甲醇制丙烯(MTP)反应中,ZSM-5分子筛较强的酸性易使甲醇与ZSM-5接触发生氢转移、芳构化等二次反应,堵塞孔道,使得其微孔结构更加不利于分子的扩散,加速催化剂积碳失活,导致丙烯选择性和丙烯/乙烯(P/E)比值下降。因此,本文利用催化活性较高的CeO₂对ZSM-5分子筛进行复合改性以达到有效降低其酸性并增大介孔的目的来提高丙烯选择性和P/E比。通过XRD、NH₃-TPD和N₂吸脱附技术表征,研究了不同ZSM-5硅铝比(摩尔比)、两相质量比(m(CeO₂)/m(ZSM-5))对CeO₂/ZSM-5复合催化剂物化性质的影响。在反应温度480 ℃、重时空速2.6 h^-1、N₂流量100 mL·min^-1、常压纯甲醇进料的条件下,考察了所制备的复合催化剂催化MTP的性能。结果表明,硅铝比为250、m(CeO₂)/m(ZSM-5)为1∶4的复合催化剂比以往研究结果具有更优异的MTP催化性能,甲醇转化率为99.9%,丙烯选择性为42.78%,P/E比为6.3。     

NaX分子筛填充PDMS膜对丙烯／丙烷分离性能的研究

采用了对丙烯／丙烷有吸附选择性的NaX型分子筛填充PDMS,通过真空涂敷法涂敷至PPESK／PEI中空纤维基膜上,制备了NaX／PDMS中空纤维复合膜。研究了固化温度、固化时间、涂敷时间（抽真空时间）、NaX填充含量对丙烯／丙烷分离性能的影响。实验结果表明,室温下,在固化温度80℃,固化时间80min,涂敷时间6min,NaX填充含量30％时,复合膜对丙烯,丙烷的分离性能最佳,分离系数达到2．68。为丙烯／丙烷的分离提供了一种新的可能性,开辟了一条研制高性能丙烯／丙烷分离有机膜的新途径。     

Hydrophobic modification of SAPO-34 membranes for improvement of stability under wet condition

SAPO-34 zeolite membranes show high efficiency for CO₂/CH₄ separation but suffer from the reduction of separation performance when exposed to humid atmosphere. In this work, n-dodecyltrimethoxysilane (DTMS) was used to modify the hollow fibers supported SAPO-34 membranes to increase the external surface hydrophobicity and thus sustain their performance under moisture environment. The modified membranes were fully characterized. Their separation performance was extensively investigated in both dry and wet gaseous systems and compared with the un-modified ones. The un-modified SAPO-34 membrane exhibited a high separation selectivity of 160 and CO₂ permeance of 1.18×10^-6 mol·m^-2·s^-1·Pa^-1 for separation of dry CO₂/CH₄ at 298 K. However, its separation selectivity declined to 0.9 and the CO₂ permeance was only about 1.7×10^-8 mol·m^-2·s^-1·Pa^-1 for wet CO₂/CH₄ at same temperature. High temperature (e.g. 353 K) could reduce the effect of moisture to improve SAPO-34 separation selectivity, but further increasing temperature (e.g. 373 K) led to decrease in CO₂/CH₄ separation selectivity. A significant decrease of selectivity was observed at higher pressure drop. The modified SAPO-34 membrane showed decreased CO₂ permeance but increased separation selectivity for dry CO₂/CH₄ gas mixture, and super performance for wet CO₂/CH₄ gas mixture due to the improved hydrophobicity of membrane surface. A separation selectivity of 65 and CO₂ permeance of 4.73×10^-8 mol·m^-2·s^-1·Pa^-1 for wet CO₂/CH₄ mixture can be observed at 353 K with a pressure drop of 0.4 MPa. Furthermore, the modified membrane exhibited stable separation performance during the 120-hour test for wet CO₂/CH₄ mixture at 353 K. The hydrophobic modification paves a way for SAPO-34 membranes in real applications.     

PPESK中空纤维复合膜对丙烯／丙烷分离性能的研究

以含二氮杂萘联苯结构的聚芳醚砜酮PPESK（Tg263—305℃）为膜材料,以干-湿相转化法制备中空纤维基膜,采用内涂法涂覆聚乙烯基吡咯烷酮（PVP）,并配合Ag＋,制备了促进输送中空纤维复合膜。研究了复合膜聚电解质涂敷时间、2次涂层、不同银盐以及不同过渡金属离子等对复合膜丙烯／丙烷分离性能的影响。实验结果表明．室温下,n（Ag＋）：n（C=O）=1：1,涂敷时间为10min时,复合膜对丙烯／丙烷的分离性最好,PVP／AgNO3体系分离系数为3．98,PVP／AgBF4体系的分离系数为5．69。经过2次涂敷后,PVP／AgNO3体系的分离系数升高至4．20,PVP／Ag-BF4体系的分离系数升高至10．78。由此可见,相同条件下,AgNO3的促进传递效果明显低于AgBF4。     

Silicon Carbide Membranes Modified by Chemical Vapor Deposition Using Species of Low Sticking Coefficients in a Silane/Acetylene Reaction System

Asymmetric SiC membranes are modified by a SiH₄/C₂H₂/Ar low-pressure chemical vapor deposition (LPCVD) system at a temperature of 800°C. The pore size of the membrane is reduced to increase its selectivity at the expense of its permeance. The chemical vapor deposition (CVD) modification, using a chamber to minimize the gas-phase reaction, is superior to that without a chamber, because the film-forming species of low sticking coefficients can improve the pore size at a lower cost of permeance reduction. With the knowledge of pore structure and CVD kinetics, the alteration of membrane properties can be properly predicted.     

Effect of coordination number on the formation of silver nanoparticles in polymer/silver salt complex membranes

Polymer/silver salt complex membranes consisting of AgBF₄ dissolved in poly(2-ethyl-2-oxazoline) (POZ) exhibited higher separation performance for propylene/propane mixtures but gradual decrease of separation properties, primarily due to the reduction of silver ions to silver nanoparticles. In this study, the effect of salt concentration on the formation of silver nanoparticles in POZ/AgBF₄ complex membranes was investigated. Separation test showed that 1/0.5 POZ/AgBF₄ complex membrane exhibited improved long-term stability on the membrane separation performance for propylene/propane mixtures compared to 1/1 POZ/AgBF₄ complex membrane. The stability improvement would be attributed to the suppression of the reduction of silver ions to silver metal nanoparticles, resulting from the higher coordination number of silver ions to carbonyl oxygens of POZ in 1/0.5 POZ/AgBF₄ membrane than 1/1 membrane. Transmission electron microscopy (TEM) and UV–visible spectra results confirmed that the rate of silver metal nanoparticle formation in the 1/0.5 POZ/AgBF₄ complex membrane was significantly retarded compared to that of 1/1 POZ/AgBF₄ complex membrane. It is therefore concluded that the coordination number of silver ions for carbonyl oxygens is of pivotal importance in controlling the reduction reaction of silver ions to silver metals and consequently improving the membrane separation performance for propylene/propane mixtures.     

PtSn负载量对PtSn/Al2O3催化剂上丙烷脱氢性能的影响

由丙烷直接催化脱氢制取丙烯已经成为增产丙烯的重要手段之一。以水热法制备Al_2O_3载体,采用等体积浸渍法制备不同PtSn负载量的PtSn/Al_2O_3催化剂。通过XRD、N2-吸附、拉曼光谱和H2-TPR等对其进行表征,并考察不同PtSn负载量对催化剂催化丙烷脱氢性能的影响。结果表明,在制备的催化剂中,Pt1.5Sn3/Al_2O_3具有最高的催化丙烷脱氢活性和稳定性,丙烷初始转化率高达55.6%,丙烯选择性98.1%。反应330 min后,丙烷转化率仅降约10%,选择性保持不变。     

Gradient nanoporous phenolics as substrates for high-flux nanofiltration membranes by layer-by-layer assembly of polyelectrolytes

Thin film composite(TFC) membranes represent a highly promising platform for efficient nanofiltration(NF)processes. However, the improvement in permeance is impeded by the substrates with low permeances. Herein,highly permeable gradient phenolic membranes with tight selectivity are used as substrates to prepare TFC membranes with high permeances by the layer-by-layer assembly method. The negatively charged phenolic substrates are alternately assembled with polycation polyethylenimine(PEI) and polyanion poly(acrylic acid)(PAA)as a result of electrostatic interactions, forming thin and compact PEI/PAA layers tightly attached to the substrate surface. Benefiting from the high permeances and tight surface pores of the gradient nanoporous structures of the substrates, the produced PEI/PAA membranes exhibit a permeance up to 506 L? m~(-2)?h~(-1)?MPa~(-1), which is ～2–10 times higher than that of other membranes with similar rejections. The PEI/PAA membranes are capable of retaining N 96.1% of negatively charged dyes following the mechanism of electrostatic repulsion. We demonstrate that the membranes can also separate positively and neutrally charged dyes from water via other mechanisms.This work opens a new avenue for the design and preparation of high-flux NF membranes, which is also applicable to enhance the permeance of other TFC membranes.     

The effect of protic ionic liquids incorporation on CO2 separation performance of Pebax-based membranes

The separation of carbon dioxide (CO₂) is of great importance for environment protection and gas resource purification. The ionic liquids (ILs)-based gas separation membrane provides a new chance for efficient CO₂ separation, while high permeability and selectivity of membranes is a great challenge. In this study, the influence of two protic ILs with different anion ([TMGH][Im] and [TMGH][PhO]) on the CO₂ separation performance of the prepared ILs/Pebax blended membranes were systematically investigated at different temperature. The results showed the CO₂ permeability exhibits the rising trend for ILs/Pebax blended membranes with the increment of IL content. Especially, the [TMGH][Im] with low viscosity and high CO₂ absorption capacity leads to the blended membranes showing better CO₂ permeability and ideal CO₂ selectivity than that of membranes with [TMGH][PhO] at high IL content. Besides, with operating temperature increasing, the gas permeability of 20% (mass) [TMGH][Im]/Pebax blended membrane increases due to the decreasing viscosity of IL and the rising chain mobility of polymer. Inversely, the gas selectivity shows decreasing trend because CO₂ absorption capacity obviously decreased at higher temperature.     

Propylene-induced plasticization in silver polymer electrolyte membranes

Silver polymer electrolyte membranes consisting of AgBF₄ or AgCF₃SO₃ dissolved in poly(2-ethyl-2-oxazoline) (POZ) or poly(N-vinyl pyrrolidone) (PVP) have previously shown outstanding separation performance for propylene/propane mixtures. The ideal separation factor (i.e. pure gas selectivity) of propylene over propane reached 15,000, but the actual selectivity (i.e. mixed gas selectivity) was approximately 50. In this study, the origin of large difference between pure and mixed gas selectivities has been elucidated in terms of membrane plasticization by propylene molecules. The membrane plasticization was confirmed by the decrease of selectivity with increasing propylene concentration in the feed, resulting from the increase of both propylene and propane permeances. The decrease of glass transition temperature (T_g) in silver polymer electrolytes upon the sorption of propylene also supported the membrane plasticization, determined by in situ high-pressure differential scanning calorimetry (HPDSC). Further analysis of configuration entropy model revealed that the interaction of propylene/silver ion in POZ/AgBF₄ was approximately 2-fold stronger than that in POZ/AgCF₃SO₃, which is in good agreement with the results of propylene solubility and FT-IR spectra.     

Covalent organic frameworks-incorporated thin film composite membranes prepared by interfacial polymerization for efficient CO2 separation

Thin film composite (TFC) membranes with nanofillers additives for CO₂ separation show promising applications in energy and environment-related fields. However, the poor compatibility between nanofillers and polymers in TFC membranes is the main problem. In this work, covalent organic frameworks (COFs, TpPa-1) with rich —NH— groups were incorporated into polyamide (PA) segment via in situ interfacial polymerization to prepare defect-free TFC membranes for CO₂/N₂ separation. The formed covalent bonds between TpPa-1 and PA strengthen the interaction between nanofillers and polymers, thereby enhancing compatibility. Besides, the incorporated COFs disturb the rigid structure of the PA layer, and provide fast CO₂ transfer channels. The incorporated COFs also increase the content of effective carriers, which enhances the CO₂ facilitated transport. Consequently, in CO₂/N₂ mixed gas separation test, the optimal TFC (TpPa_0.025-PIP-TMC/mPSf) membrane exhibits high CO₂ permeance of 854 GPU and high CO₂/N₂ selectivity of 148 at 0.15 MPa, CO₂ permeance of 456 GPU (gas permeation unit) and CO₂/N₂ selectivity of 92 at 0.5 MPa. In addition, the TpPa_0.025-PIP-TMC/mPSf membrane also achieves high permselectivty in CO₂/CH₄ mixed gas separation test. Finally, the optimal TFC membrane showes good stability in the simulated flue gas test, revealing the application potential for CO₂ capture from flue gas.     

The effect of the Ce content on the oxidative dehydrogenation of propane over CrOy-CeO2/γ-Al2O3 catalysts

A series of CrO_y (17.5 wt%)-CeO₂ (X wt%)/γ-Al₂O₃ catalysts (X=0, 0.5, 2, 5, 8) with various Ce contents were prepared by a wetness impregnation method and were applied to the dehydrogenation of propane to propylene at 550℃ and 0.1 MPa. The prepared catalysts were characterized by BET, H₂-TPR, O₂-TPD, XPS, XRD, SEM-EDS and Raman spectroscopy. Among the prepared catalysts, the 17.5Cr-2Ce/Al catalyst with the largest amount of lattice oxygen exhibited the best catalytic performance for the dehydrogenation of propane to propylene with lattice oxygen. The decreased presence of oxygen defects and reducibility were the factors responsible for the improved dehydrogenation activity of the catalysts. The CeO₂ layer could inhibit the evolution of lattice oxygen (O^2-) to electrophilic oxygen species (O₂^-), and the oxygen defects on the catalyst surface were reduced. The inhibited lattice oxygen evolution prevented the deep oxidation of propane or propylene, the average CO_x selectivity decreased from 24.41% (17.5Cr/Al) to 5.71% (17.5Cr-2Ce/Al), and the average propylene selectivity increased from 60.15% (17.5Cr/Al) to 85.05% (17.5Cr-2Ce/Al).     

Design of catalysts for deNOx process using synergistic phenomenon

Investigation of the mechanism of the selective reduction of NO_x by propane over the individual samples of commercial catalysts NTK, STK, and Ni–Cr-oxide catalyst and over their binary mechanical mixtures has shown that the synergistic effect observed in the latter case is caused by the oxidative activation of propane on the STK and Ni–Cr-oxide surface which results in the formation of more effective reducing agents, propylene and hydrogen correspondingly. In the case of the Ni–Cr-oxide and NTK catalytic system, hydrogen forms over the former catalyst in propane oxidation, migrates through the gas phase to the latter catalyst, where NO_x is activated with the formation of nitrate structures which interact with the said hydrogen giving the products of the overall reaction, N₂ and H₂O. When the pair of NTK and STK is concerned, the interaction of C₃H₈ and O₂ over the latter catalyst gives stable products of partial propane oxidation and/or oxidative dehydrogenation which are transported due to interphase diffusion to NTK surface. The nature of observed synergistic enhancement of catalysis in the case of binary mixtures is proposed under the terms of “remote control” mechanism described in literature and can serve a useful purpose in the design of catalysts for this reaction.     

纤维状BPO4/SiO2催化剂的制备及其丙烷氧化脱氢性能

通过静电纺丝法制备了一种高温焙烧后具有纤维状结构的磷酸硼/二氧化硅（BPO₄/SiO₂）催化剂,并考察了BPO₄负载量和焙烧温度对该催化剂的结构和催化丙烷氧化脱氢性能的影响。研究发现焙烧过程使纤维直径减小。随着BPO₄负载量的增加,丙烷氧化脱氢活性增高。当BPO₄负载量为7%（质量分数）,焙烧温度为600℃时,BPO₄/SiO₂催化剂具有最佳的催化性能;在反应温度为480℃下,丙烷转化率和丙烯产率分别达到17.0%和13.0%,且催化剂稳定性良好。当焙烧温度较低时（550℃）,催化剂中的有机物分子未被除尽,导致烯烃的选择性偏低;当焙烧温度较高时（700℃）,SiO₂结构收缩紧密,抑制了活性相的暴露。由于纤维结构可暴露更多活性位点,该催化剂较相同条件下粉末状BPO₄催化剂有着更高的催化活性。