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H2 and Olefin/Paraffin Separation with Porous Materials: State of the Art and New Developments
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There are different membrane materials that can be considered for H2 separation and purification technologies. One membrane type is metallic which are typically dense sheets which H2 permeates through as its component protons and electrons. It requires the conduction of free electrons and the presence of specific catalytic surfaces to dissociate H2 on the raw feed stream side and reassociate the protons and electrons on the product side, which is the fundamental mechanism in these dense metallic membranes. However, metal membranes have some limitations, thus, a need for alternative membranes like silica. They are easy to fabricate and is low cost. Such membranes are made of three layers: a membrane layer, intermediate layer and a support. Silica membranes also accommodate the separations of small molecules because they are inorganic membranes. Inorganic membranes are also being proved for use in synthesis such as gas separation, pervaporation and reverse osmosis or in the development of chemical sensors and catalytic membranes. One good membrane is zeolite, which has inherent mechanical, thermal and chemical stability. Another new approach is hydrogen rejection and contaminate permeation and can be used for H2 purification by the use of carbon-based membranes. The have distinct economic advantage and minimizes the need for H2 recompression steps. Another is polymer membranes in which proton exchange membrane is the primary technology. This converts the chemical energy of H2 directly and efficiently to electrical energy and reduce greenhouse gases.
Alkenes (olefins) production by the catalytic dehydrogenation of light alkanes (paraffins) is an alternative to conventional heavy hydrocarbons cracking. Alkenes are important intermediate materials for a variety of applications and the catalytic alkanes dehydrogenation allows for their production from low-cost feedstocks, such as natural gas. The dehydrogenation is endothermic in nature and limited by the thermodynamic equilibrium, therefore, it is performed at elevated temperatures. However, operation at high temperatures results in side reactions (e.g., thermal cracking) and catalyst deactivation due to the carbon deposition (coking). Consequently, the catalytic membrane reactor concept is a logical choice for improvement and the dehydrogenation process performance. Hydrogen can be continuously separated by a hydrogen perm-selective membrane, increasing the conversion due to the shift of the equilibrium toward the alkene production. This in turn will allow operation at lower temperatures, preventing thermal cracking reactions, and coking. In addition, hydrogen, which is a valuable by-product, is generated. Design and optimization of dehydrogenation processes requires a choice of membrane, catalysts, thermal regimes, flow regime, and other issues that are discussed below. Pd–Ag-supported membranes are very selective and can be purchased off-the-shelf, their cost may still be prohibitive. The catalysts employed are those used in regular DH processes; there is a need for catalysts that show high activity and stability at low hydrogen pressures. New ideas are required in order to develop a reliable thermally independent process.
The mixed-matrix membrane (MMM) is a new membrane material for gas separation and plays a vital role for the advancement of current membrane-based separation technology. Blending between inorganic fillers like carbon molecular sieves, zeolite, metal oxides, silica and silica nanoparticles, carbon nanotubes, zeolitic imidazolate framework, metal organic framework, and glassy and rubbery polymers etc. is possible. Due to mechanical, thermal, and chemical stability, these membranes achieve high permeability and selectivity as compared to pure polymeric materials. Despite of these advantages, the MMM performances are still below industrial expectations because of membrane defects and related processing problems as well as the nonuniform dispersion of fillers in MMMs. Material selection for organic and inorganic phases, preparation techniques, material advancements, and performance of MMMs are discussed. Issues and challenges faced during MMM synthesis as well as problem solutions are highlighted.
Membranes playa central role in our daily life, or as indicated by one of my foreign colleagues, Richard Bowen, 'If you are tired of membranes, you are tired of life' . Biological membranes are hardly used in industrial applications, but separations with synthetic membranes have become increasingly important. Today, membrane processes are used in a wide range of applications and their numbers will certainly increase. Therefore, there is a need for well educated and qualified engineers, chemists, scientists and technicians who have been taught the basic principles of membrane technology. However, despite the growing importance of membrane processes, there are only a few universities that include membrane technology in their regular curricula. One of the reasons for this may be the lack of a comprehensive textbook. For me, this was one of the driving forces for writing a textbook on the basic principles of membrane technology which provides a broad view on the various aspects of membrane technology. I realise that membrane technology covers a broad field but nevertheless I have tried to describe the basic principles of the various disciplines. Although the book was written with the student in mind it can also serve as a first introduction for engineers, chemists, and technicians in all kind of industries who wish to learn the basics of membrane technology.
Synthetic organic chemicals are produced by the transformation of carbonaceous feedstocks into functionalized molecules through one or more chemical reactions. Such transformations are accomplished at vast industrial scales and the resulting products permeate every aspect of modern society. The molecules produced find use largely as monomers for polymer synthesis of ubiquitous plastics, or as task-specific ingredients for a myriad of applications as divergent as paint leveling agents to food preservatives. Advances in technology, significant increases in energy efficiency, as well as the utilization of fossil-fuel derived starting materials has resulted in unprecedented economy of scale and relatively stable product costs in spite of large relative increases in the price of oil and natural gas. The section entitled “Chemical Raw Materials and Feedstocks” covers the most important carbonaceous feedstocks currently utilized in the chemical processing industries; all derived from fossil-fuel based raw materials.
Propane dehydrogenation was studied in hydrogen permselective packed-bed membrane reactors and conventional packed-bed reactors. Two different types of developmental membranes were investigated: microporous silica-based membranes and a palladium thin film supported by a porous ceramic substrate. The membrane reactors were operated at liquid hourly space velocities (LHSVs) similar to those used in commercial reactors for propane dehydrogenation. Although the initial hydrogen permselectivity of the palladium membrane was substantially higher than that of the silica-based membranes, this membrane deactivated and eventually failed after several hours of exposure to reaction conditions. Moderate improvements in propylene yield were obtained with the silica-based membrane reactors. A propylene yield of 39.6% was obtained at an LHSV of 3 and a temperature of 823 K compared to a yield of 29.6% in a conventional packed-bed reactor operated with the same flow rate. The reaction selectivity for propylene was generally above 97 mol % in both the conventional and membrane reactors, however, catalyst deactivation rates were generally higher in the membrane reactors.
Polymers formed between three ligands and selected metal ions were prepared and their thermal stability in air was investigated. 1,5-Dihydroxynaphthalene-2,6-dicarboxylic acid (1,5-N-2,6) formed coordination polymers with Zn, Ni, Al, and Fe+3. Zn-1,5-N-2,6 was more stable than the corresponding Fe and Ni polymers. Pyromellitic acid (PMA) formed coordination polymers of high molecular weight with Th and U+4. The Th-PMA was stable up to 405°C., while the corresponding uranium compound decomposed below 400°C. The coordination polymer of 2,3,6,7-naphthalenetetracarboxylic acid (NT) with Th was less stable (decomposition temperature: 360°C.) than the corresponding pyromellitic polymer because the coordination of Th in Th-NT was partially accomplished by water molecules rather than carboxylic oxygens as in the Th-PMA polymer. The thermal stability of these polymers was a function of the metal involved in coordination. Higher thermal stability resulted when Zn or Th were used than if metals capable of changing their ionic valency were involved in polymer formation.
Zeolite membranes are the focus of much active research due to their extensive potential applications, particularly their superior catalytic and separation properties. Synthesis methods and properties are discussed, and compared with zeolite crystalline powders.
Room to breathe: An efficient method for the synthesis of zeolitic imidazolate framework-7 (ZIF-7) in ethanol/water at room temperature is reported. The ZIF-7 undergoes a reverse-phase transition from large pore (lp)-phase to narrow pore (np)-phase under heat treatment and from np-phase back to lp-phase in ethanol solution.
High-performance zeolitic imidazolate frameworks (ZIFs)/polybenzimidazole (PBI) nanocomposites are molecularly designed for hydrogen separation at high temperatures, and demonstrate it in a useful configuration as dual-layer hollow fibers for the first time. By incorporating as-synthesized nanoporous ZIF-8 nanoparticles into the high thermal stability but extremely low permeability polybenzimidazole (PBI), the resultant mixed matrix membranes show an impressive enhancement in H2 permeability as high as a hundred times without any significant deduction in H2/CO2 selectivity. The 30/70 ZIF-8/PBI dense membrane has a H2 permeability of 105.4 Barrer and a H2/CO2 selectivity of 12.3. This performance is far superior to ZIF-7/PBI membranes and is the best ever reported data for H2-selective polymeric materials in the literature. Meanwhile, defect-free ZIF-8-PBI/Matrimid dual-layer hollow fibers are successfully fabricated, without post-annealing and coating, by optimizing ZIF-8 nanoparticle loadings, spinning conditions, and solvent-exchange procedures. Two types of hollow fibers targeted at either high H2/CO2 selectivity or high H2 permeance are developed: i) PZM10-I B fibers with a medium H2 permeance of 64.5 GPU (2.16 ×10−8 mol m−2 s−1 Pa−1) at 180°C and a high H2/CO2 selectivity of 12.3, and, ii) PZM33-I B fibers with a high H2 permeance of 202 GPU (6.77 ×10−8 mol m−2 s−1 Pa−1) at 180°C and a medium H2/CO2 selectivity of 7.7. This work not only molecularly designs novel nanocomposite materials for harsh industrial applications, such as syngas and hydrogen production, but also, for the first time, synergistically combines the strengths of both ZIF-8 and PBI for energy-related applications.
Hydrogen-based energy is a promising renewable and clean resource. Thus, hydrogen selective microporous membranes with high performance and high stability are demanded. Novel NH2-MIL-53(Al) membranes are evaluated for hydrogen separation for this goal. Continuous NH2-MIL-53(Al) membranes have been prepared successfully on macroporous glass frit discs assisted with colloidal seeds. The gas sorption ability of NH2-MIL-53(Al) materials is studied by gas adsorption measurement. The isosteric heats of adsorption in a sequence of CO2 > N2 > CH4 ≈ H2 indicates different interactions between NH2-MIL-53(Al) framework and these gases. As-prepared membranes are measured by single and binary gas permeation at different temperatures. The results of singe gas permeation show a decreasing permeance in an order of H2 > CH4 > N2 > CO2, suggesting that the diffusion and adsorption properties make significant contributions in the gas permeation through the membrane. In binary gas permeation, the NH2-MIL-53(Al) membrane shows high selectivity for H2 with separation factors of 20.7, 23.9 and 30.9 at room temperature (288 K) for H2 over CH4, N2 and CO2, respectively. In comparison to single gas permeation, a slightly higher separation factor is obtained due to the competitive adsorption effect between the gases in the porous MOF membrane. Additionally, the NH2-MIL-53(Al) membrane exhibits very high permeance for H2 in the mixtures separation (above 1.5 × 10−6 mol m−2 s−1 Pa−1) due to its large cavity, resulting in a very high separation power. The details of the temperature effect on the permeances of H2 over other gases are investigated from 288 to 353 K. The supported NH2-MIL-53(Al) membranes with high hydrogen separation power possess high stability, resistance to cracking, temperature cycling and show high reproducibility, necessary for the potential application to hydrogen recycling.
Separation of propylene/propane mixture with new 8-ring zeolite, pure silica chabazite (SiCHA), has been studied in this work. Since the diffusion of propane molecules in SiCHA is extremely slow, thus equilibrium information for propane has been indirectly estimated using available uptake data at 80 °C and 600 Torr. Moreover, molecular simulation has been used to obtain equilibrium information of propylene and propane and verify our estimation. The ideal kinetic selectivity of propylene/propane mixture is 28 at 80 °C, which increases with decreasing temperature. A four-step, kinetically controlled pressure swing adsorption process has been suggested for this separation and studied in detail using a nonisothermal micropore diffusion model, developed and verified in an earlier study. In this model, Langmuir isotherm represents adsorption equilibrium and micropore diffusivity depends on adsorbate concentration in the micropores, according to chemical potential gradient as the driving force for diffusion.
Isobutene is an important intermediate compound in the petrochemical industry for the production of polymers (butyl rubber, polybutene, and isoprene) and methyl tert-butyl ether. In this work, the n-butane dehydroisomerization reaction in a membrane reactor (MR) was investigated by thermodynamic analysis in a wide range of temperatures, reaction pressures, and equilibrium hydrogen partial pressures, by means of a simplified reaction scheme. The shift of the equilibrium conversion in an MR was evaluated by taking into account the chemical reaction equilibrium and the permeative equilibrium through a 100% hydrogen-selective membrane. The evaluated limits imposed by thermodynamics on an MR are much wider than those of a traditional reactor so that a conversion of about 7 times higher could be obtained over that of the traditional process under a set of operating conditions. This gives a powerful indication on how the use of an MR can extend the thermodynamic limits of this reaction, in terms of conversion, even at thermodynamically unfavorable operating conditions.
The technical and economic feasibility of a hybrid separation process in which gas separation membranes are combined with conventional distillation are assessed for the separation of ethylene from ethane and of butadiene from a C4-mixture. The potentials for increased energy efficiency and debottlenecking were determined in relation to the required membrane performances. The energy saving potential for the separation of ethylene from ethane is rather low owing to the required very high membrane selectivity. Energy savings can be expected when the membrane selectivity for ethylene is >60. However, the possibility to increase the column capacity in an existing plant by using a membrane is very high. This can become economically attractive if the membrane has a selectivity for ethylene of ≥10. In the case of butadiene separation, the energy savings can be as high as 30% depending on membrane selectivity and process configuration. This high value can be reached when the membrane selectivity for butadiene relative to saturated hydrocarbons equals 15. Again, an increase in the production capacity of butadiene can be achieved in an economic viable fashion.
Modified MFI-type zeolite membranes were investigated as high-temperature water-gas shift (WGS) membrane reactors (MRs) in combination with a nanocrystalline Fe/Ce WGS catalyst. The effects of the MR operating conditions and the membrane separation performance on the CO conversion (χCO) were studied experimentally and by calculations using a simple one-dimensional plug-flow reactor (PFR) model. The experimental results showed that, at high temperatures (e.g., >500 °C), the zeolite MR with moderate H2 selectivity (e.g., αH2/CO2 31, and αH2/CO 25) and permeance (Pm,H2 0.9 × 10–7 mol s–1 m–2 Pa–1) was capable of overcoming the limit of equilibrium CO conversion and χCO of the MR could be further enhanced by increasing the reaction pressure while keeping the permeate pressure unchanged. At high temperatures and high reaction pressures, CO is rapidly consumed by a fast reaction that minimizes the membrane permeation of unreacted CO; meanwhile, the efficiency of H2 removal is improved as a result of the increased H2 partial pressure difference across the membrane. The model calculations have indicated that the current membrane has the potential to achieve high CO conversion of χCO > 99% under practically meaningful operating conditions.
In this article, the integrated process technology is introduced for production of hydrogen, which incorporates pyrolysis of biomass and catalytic steam reforming. The process adopts a fluidized bed reactor for pyrolysis and a fixed bed reactor for steam reforming, in which technological parameters, such as pyrolysis temperature, ratio of steam over biomass, reforming temperature, gas hourly space velocity, catalyst size, and the life of the catalyst, were investigated in order to conclude their effects on the hydrogen production. Results from experiments indicate the concentration of hydrogen in the gaseous product and thus the hydrogen yield increases as both operation temperatures of pyrolysis and reforming increase while catalyst size reduces. The increase in steam over biomass, however, results in hydrogen yield varying from increase to decrease.
Mixed matrix membranes (MMMs) composed of metal organic framework (MOF) fillers embedded in a polymeric matrix represent a promising alternative for CO2 removal from natural gas and biogas. Here, MMMs based on NH2‐MIL‐53(Al) MOF and polyimide are successfully synthesized with MOF loadings up to 25 wt% and different thicknesses. At 308 K and ΔP = 3 bar, the incorporation of the MOF filler enhances CO2 permeability with respect to membranes based on the neat polymer, while preserving the relatively high separation factor. The rate of solvent evaporation after membrane casting proves key for the final configuration and dispersion of the MOF in the membrane. Fast solvent removal favours the contraction of the MOF structure to its narrow pore framework configuration, resulting in enhanced separation factor and, particularly, CO2 permeability. The study reveals an excellent filler‐polymer contact, with ca. 0.11% void volume fraction, for membranes based on the amino‐functionalized MOF, even at high filler loadings (25 wt%). By providing precise and quantitative insight into key structural features at the nanoscale range, the approach provides feedback to the membrane casting process and therefore it represents an important advancement towards the rational design of mixed matrix membranes with enhanced structural features and separation performance.
This study investigated the effect of annealing time and temperature on gas separation performance of mixed matrix membranes (MMMs) prepared from polyethersulfone (PES), silicoaluminophosphate (SAPO-34), and 2-hydroxy 5-methyl aniline (HMA). A postannealing period at 120°C for a week extensively increased the reproducibility and stability of MMMs, but for pure PES membranes no post-annealing was necessary for stable and reproducible performance. The effect of operation temperature was also investigated. The permeabilities of H2, CO2, and CH4 increased with increasing permeation temperature from 35°C to 120°C, yet CO2/CH4 and H2/CH4 selectivities decreased. PES/SAPO-34/HMA ternary and PES/SAPO-34 binary MMMs exhibited the highest ideal selectivity and permeability values at all temperatures, respectively. For H2/CO2 pair, when temperature increased from 35°C to 120°C, selectivity increased from 3.2 to 4.6 and H2 permeability increased from 8 to 26.5 Barrer for ternary MMM, demonstrating the advantage of using this membrane at high temperatures. The activation energies were in the order of CH4 > H2 > CO2 for all membranes. PES/SAPO-34/HMA membrane had activation energies higher than that of PES/SAPO-34 membrane, suggesting that HMA acts as a compatibilizer between the two phases. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40679.
Ethylene/ethane separation via cryogenic distillation is extremely energy-intensive, and membrane separation may provide an attractive alternative. In this paper, ethylene/ethane separation performance using polymeric membranes is summarized, and an experimental ethylene/ethane polymeric upper bound based on literature data is presented. A theoretical prediction of the ethylene/ethane upper bound is also presented, and shows good agreement with the experimental upper bound. Further, two ways to overcome the ethylene/ethane upper bound, based on increasing the sorption or diffusion selectivity, is also discussed, and a review on advanced membrane types such as facilitated transport membranes, zeolite and metal organic framework based membranes, and carbon molecular sieve membranes is presented. Of these, carbon membranes have shown the potential to surpass the polymeric ethylene/ethane upper bound performance. Furthermore, a convenient, potentially scalable method for tailoring the performance of carbon membranes for ethylene/ethane separation based on tuning the pyrolysis conditions has also been demonstrated. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3475–3489, 2013
The substitution of fossil fuels by renewable energy sources is needed to decrease greenhouse gas emissions, especially CO 2 . Wind and solar power are today considered as attractive alternatives for electric power generation, but are not suitable for providing base load. Thus, efficient storage of electrical energy is inevitable. Liquid hydrocarbons (HCs) exhibit an excellent volumetric energy density and offer various opportunities for storing electric energy. They can be produced by CO 2 and renewable H 2 (generated by water electrolysis) in a two step process. The first step is generation of syngas by reverse water-gas shift (RWGS) at elevated temperatures; the second step comprises the production of liquid hydrocarbons by Fischer-Tropsch (FT) synthesis. The experiments on RWGS with a commercial Ni-catalyst show that a CO 2 conversion of around 80 % can be reached at 800 °C within a very short residence time of less than < 0.1 s. The experiments on FTS with Fe as catalyst and syngas containing different amounts of CO 2 indicate that the influence of CO 2 on CO conversion and product selectivities (including net CO 2 production by water-gas shift) is insignificant if the inlet partial pressures of H 2 and CO are kept constant. If CO is substituted by CO 2 , less HCs are formed, the water-gas shift is repressed, and methane selectivity increases.
A combined experimental and theoretical investigation of propane and propylene adsorption in the metal–organic framework CuBTC is presented. The dependence of adsorption enthalpies on the adsorbed amount was determined by microcalorimetry up to 8 mmol g–1 coverage (roughly C3Hn/Cu2+ ratio of 1.5). Trends observed experimentally were interpreted on the basis of accurate calculations carried out at the hybrid DFT–ab initio level. Three types of adsorption sites were identified; however, qualitatively different results were obtained for propane and propylene. Propane preferentially adsorbs at the cage center sites (−ΔH° = 43 kJ mol–1), followed by adsorption at the cage window sites (31 kJ mol–1), while the interaction with the coordinatively unsaturated sites (CUS) is relatively weak (24 kJ mol–1). On the contrary, propylene preferentially interacts with the CUS (56 kJ mol–1), while the adsorption at the cage center and cage window sites was found to be only 45 and 34 kJ mol–1, respectively. Due to the topology of CuBTC, lateral interactions are significantly more important among the adsorbates located at the cage center and cage window sites (populated in the case of propane) than among adsorbates at the CUS and cage center sites (populated in the case of propylene). Therefore, adsorption energies obtained for coverages above 6 mmol g–1 of adsorbed amount were larger for propane than for propylene. Consequently, the presence of small cages makes the CuBTC MOF less suitable for propane/propylene separation than MOFs having the Cu2+ CUS but without small cages (e.g., CPO-27).
Copper salts of α,ω-dibasic acids, HOOC(CH2)n−2COOH, in which n = 2–10, show (cupric malonate excepted) abnormal magnetic moments, which are lower than the theoretical spin value (1.73 B.M.) for one free electron. (Therefore structures may be expected containing two copper atoms, as there are in cupric acetate monohydrate crystals) in cristalline cupric salts of α-ω-dicarboxylates which show subnormal magnetic moments. A nonplanar structure of oxalic radicals is confirmed experimentally as well by radiostructural analysis as by electron diffraction. These data indicate that cupric oxalate crystals have a dimeric structural network or polymeric molecules containing coordination links and do not behave as isolated molecules with one copper atom or as dimeric molecules, as do the crystals of cupric acetate monohydrate. Analogous network structures are possible for cupric salts of α-ω-dicarboxylic acids having an even number of carbon atoms, although the carbon chains may be bent more or less from their normal configurations; the Cu-Cu links could also be simultaneously changed. Indeed, the magnetic moment of the copper salts of the even series increases with increasing length of carbon chains. For the malonate, the rotation of chains within the central carboxylic groups forbids the formation of such network structures. For the other acids of the odd series the flexibility of the carboxylic groups must take part in bringing two copper atoms together. Indeed, cupric malonate shows a normal magnetic moment as observed for most cupric derivatives, and the effective moment per copper atom in the cupric salts of the odd series decreases with increasing length of carbon chains of the acids until the curve of the magnetic moments of the dibasic acids of the even series practically coalesces with that of the odd series (above pimelic acid). These conclusions, which are based on the magnetic moment data of the polymeric structures of cupric salt crystals of α-ω-dicarboxylic acids, are also confirmed by infrared spectral data, especially by the variations of the CO stretching and the CH2 rotation bands. Analogous structures are also proposed for the abnormal magnetic moments observed for the cupric salts of rubeanic acid, thioxamide, and similar derivates.
1.
The possibility of using the isomerization of 1-butene to 2-butenes to facilitate the isolation of highpurity isobutylene from the C4 fraction of pyrogas has been shown.
Two types of metal-organic frameworks (MOFs) have been synthesized and evaluated in the separation of C2 and C3 olefins and paraffins. Whereas Co2(dhtp) (=Co-CPO-27 = Co-MOF-74) and Mg2(dhtp) show an adsorption selectivity for the olefins ethene and propene over the paraffins ethane and propane, the zeolitic imidazolate framework ZIF-8 behaves in the opposite way and preferentially adsorbs the alkane. Consequently, in breakthrough experiments, the olefins or paraffins, respectively, can be separated.
Pure and binary adsorption equilibrium data of ethylene and ethane on zeolite 5A were collected with a volumetric method for the temperature range 283 K to 323 K and pressure up to 950 kPa. The applicability of the binary adsorption prediction by the vacancy solution theory (VST) was investigated. Further individual adsorption and selectivity were obtained by VST prediction. According to the experimental results, zeolite 5A has a high adsorption capacity and selectivity for ethylene in the ethylene/ethane system. VST predicts that ethylene selectivity increases with pressure; it also shows that the amount of ethylene separated by zeolite 5A increases as the temperature decreases at a specified pressure.
Gate-opening of ZIF frameworks is an important microscopic phenomenon to explain the adsorption, diffusion, and separation processes for large guest molecules. We present a force field, with the input from density functional theory (DFT) calculations, for the molecular dynamics simulation on the gate-opening in ZIF-8. The computed self-diffusivities for sorbed C1 to C3 hydrocarbons were in good agreement with the experimental values. The observed sharp diffusion separation from C2H6 to C3H8 was elucidated by investigating the conformations of the guest molecules integrated with the flexibility of the host framework.
The alkylation of benzene with ethane has been studied over the Pt/H-MFI catalyst in the presence of intermetallic compounds and titanium as hydrogen acceptors. The addition of hydrogen scavengers shifts the thermodynamic equilibrium of the reaction and allows for an improvement in the catalytic activity. In the presence of Ti, Zr2Fe, and ZrV2, the initial conversion of ethane increases by 5–7 times with respect to the neat catalyst. However, the positive effect of hydrogen acceptors decreased with time on stream, owing to saturation of their capacity.
A new force field and a hybrid Monte Carlo/molecular dynamics simulation method are developed to investigate the structural transition of zeolitic imidazolate framework-8 (ZIF-8)induced by N sorption. At a high loading (approximately 50 Nmolecules per unit cell), ZIF-8 shifts from low-loading (LL) to high-loading (HL) structure. A stepped sorption isotherm is predicted with three distinct regions, which agrees well with experimental data. The orientation of imidazolate rings and the motion of framework atoms exhibit sharp changes upon structural transition. Furthermore, pronounced changes are observed in various contributions to potential energies (including stretching, bending, torsional, van der Waals, and Coulombic). The analysis of radial distribution functions between N and framework atoms suggests N interacts strongly with the imidazolate rings in ZIF-8. The simulation reveals that the structural transition of ZIF-8 is largely related to the reorientation of imidazolate rings, as attributed to the enhanced van der Waals interaction between N and imidazolate rings as well as the reduced torsional interaction of framework in the HL structure. This is the first molecular simulation study to describe the continuous structural transition of ZIF-8 and, it provides microscopic insight into the underlying mechanism.
We present a combined computational and experimental approach to evaluate the suitability of the ITQ-12 nanoporous material (ITW) as a propane-propylene separation device. For this, we have computed adsorption and diffusion of propane and propylene in the ITQ-12 zeolite. The propane isotherm is reproduced well, but the available propylene models in the literature are unable to describe the propylene isotherm. Newly developed force field parameters for propylene were obtained by fitting to our own experimental adsorption isotherms and validated with previous data taken from the literature. To obtain self-diffusion of propane and propylene in the zeolite, we combined the configurational-bias Monte Carlo method with rare-event molecular simulation techniques. Our results support experimental observations that point out ITQ-12 as a suitable structure for propane-propylene separation. The selectivity originates mainly from a difference in adsorption, possibly enhanced by a difference in diffusion.
In this work, the concept of zeolite (zeolitic) membrane is discussed from a practical perspective. We consider the limitations of the existing synthesis methods and speculate on new opportunities of zeolites and zeolite-type materials such as metal organic frameworks for the production of membranes. This paper focuses on the barriers that need to be eliminated before the commercialization of these membranes becomes attractive. Additional opportunities for commercialization may arise in the shape either of mixed matrix membranes, taking advantage of composites with polymers, or as zeolite coatings useful for a plethora of new applications.
Metal–organic frameworks (MOFs) are hybrid organic–inorganic nanoporous materials that exhibit regular crystalline lattices with relatively well-defined pore structures. Chemical functionalization of the organic linkers in the structures of MOFs affords facile control over pore size and chemical/physical properties, making MOFs attractive for a variety of industrial applications including membrane-based gas separations. A wealth of reports exists discussing the synthesis and applications of MOFs; however, relatively few reports exist discussing MOF membranes. This disparity owes to challenges associated with fabricating films of MOF materials, including poor substrate–film interactions, moisture sensitivity, and thermal/mechanical instability. Since even nanometer-scale cracks and defects can affect the performance of a membrane for gas separation, these challenges are particularly acute for the fabrication of MOF membranes. Here, we review recent progress on MOF membranes with an emphasis on their fabrication techniques, challenges involved in membrane synthesis, reported strategies to address these challenges (issues), and gas separation performance. Finally, we conclude with our perspectives on future research directions in this area.
About 80% of the present world energy demand comes from fossil fuels. Unlike using fossil fuels, using hydrogen as an energy source produces water as the only byproduct. Use of hydrogen as an energy source could help to address issues related to energy security including global climate change and local air pollution. Moreover, hydrogen is abundantly available in the universe and possesses the highest energy content per unit of weight compared to any of the known fuels. Consequently, demand for hydrogen energy and production has been growing in the recent years. Membrane separation process is an attractive alternative compared to mature technologies such as pressure swing adsorption and cryogenic distillation. This paper reports different types of membranes used for hydrogen separation from hydrogen-rich mixtures. The study has found that much of the current research has been focused on nonpolymeric materials such as metal, molecular sieving carbon, zeolites, and ceramics. High purity of hydrogen is obtainable through dense metallic membranes and especially palladium and its alloys, which are highly selective to hydrogen. Thin membranes would not only reduce the cost of materials but also increase the hydrogen flux. Metal alloys or composite metal membranes have been used for hydrogen purification. However, metallic membranes are sensitive to some gases such as carbon monoxide and hydrogen sulfide. Therefore, ceramic membranes, inert to poisonous gases, are desirable. Inorganic microporous membranes offer many advantages over thin-film palladium membranes. More importantly, in microporous membranes, the flux is directly proportional to the pressure, whereas in palladium membranes, it is proportional to the square root of the pressure. The paper also discusses the advantages and disadvantages of different hydrogen separation membranes. Also, the paper reports performance of selected membranes in terms of hydrogen selectivity and permeability.
Boron-substituted ZSM-5 and SAPO-34 membranes were silylated by the catalytic cracking of methyldiethoxysilane (MDES) to increase their selectivity for H2 separation from light gases. The MDES reacted in the B-ZSM-5 pores and reduced their effective pore diameter, so that silylation significantly increased their H2 selectivity. The H2/CO2 separation selectivity at 473 K increased from 1.4 to 37, whereas the H2/CH4 separation selectivity increased from 1.6 to 33. However, silylation decreased the H2 permeances more than 1 order of magnitude in the B-ZSM-5 membranes. The H2 permeance and H2/CO2 and H2/CH4 separation selectivities increased with temperature. At 673 K, the H2 permeance was 1.0 × 10-7 mol·m-2·s-1·Pa-1 and the H2/CO2 separation selectivity was 47. Methyldiethoxysilane does not fit into SAPO-34 pores, but silylation apparently decreased the pore size of the nonzeolite pores in the SAPO-34 membranes. After silylation, the H2 permeances and H2/CO2 and H2/N2 separation selectivities were almost unchanged in the SAPO-34 membranes because H2, CO2, and N2 permeate mainly through SAPO-34 pores. In contrast, H2/CH4 separation selectivity increased from 35 to 59, and CO2/CH4 separation selectivity increased from 73 to 110, apparently because CH4 permeates mainly through non-SAPO-34 pores.
A novel synthesis strategy is developed for the facile synthesis of a uniform, dense, and oriented zeolite LTA molecular sieve membranes by using the cationic polymer to capture the zeolite precursor particles during hydrothermal synthesis. The obtained zeolite LTA membranes display molecular sieving performance in shape-selective mixture gas separation.
SSZ-13 zeolite membranes were synthesized on the inside surface of porous stainless steel tubes. In parallel with zeolite pores, the membranes had nonzeolite pores that were larger than the 0.38-nm zeolite pore diameter, but single-gas permeances of H2, N2, CH4, and n-C4H10 decreased with increasing kinetic diameter at 298 K. The CO2/CH4, H2/CH4 and H2/n-C4H10 ideal selectivities were 11, 9.0, and 63, respectively, at 298 K, and the separation selectivities for the mixtures of the same gas pairs were 12, 8.2, and 5.7 at 298 K. The SSZ-13 membranes selectively removed H2O from HNO3/H2O liquid mixtures by pervaporation to break the azeotrope at 69.5 wt % HNO3. The permeate concentration was 38.3% HNO3, and the total flux was 0.12 kg/m2·h at 298 K.
Absolute ethylene/ethane separation is achieved by ethane exclusion on silver-exchanged zeolite A adsorbent. This molecular sieving type separation is attributed to the pore size of the adsorbent, which falls between ethylene and ethane kinetic diameters.
We have synthesized, characterized, and computationally simulated/validated the behavior of two new metal-organic framework (MOF) materials displaying the highest experimental Brunauer-Emmett-Teller (BET) surface areas of any porous materials reported to date (∼7000 m(2)/g). Key to evacuating the initially solvent-filled materials without pore collapse, and thereby accessing the ultrahigh areas, is the use of a supercritical CO(2) activation technique. Additionally, we demonstrate computationally that by shifting from phenyl groups to "space efficient" acetylene moieties as linker expansion units, the hypothetical maximum surface area for a MOF material is substantially greater than previously envisioned (∼14600 m(2)/g (or greater) versus ∼10500 m(2)/g).
In large chemical plants multicomponent fluid mixtures are separated in fractions or in pure components. Dealing with gaseous mixtures, separations can be carried out by cryogenic distillation, adsorption followed by temperature swing adsorption (TSA) or pressure swing adsorption (PSA) or membrane processes, as gas permeation or pervaporation. When separation is carried out with membrane processes, the purification efficiency is often not sufficient if ultrapure products are the objective of the process. In the case of cryogenic distillation and adsorption product purity is not a problem. The investment cost of membrane as well as cryogenic separation processes are high in comparison to adsorption separation plants but the increase of investment cost versus capacity is smaller for cryogenic plants compared to membrane and adsorption units [1]. Therefore, the installation of a cryogenic process may be advantageous in very large plants designed for ultrapure products whereas adsorption is often the domain for plants with medium capacity. The future of membrane separation depends on the development of more efficient membranes.
DDR-type zeolite membranes were prepared by the secondary growth method on porous α-alumina disk, followed by on-stream counter diffusion chemical vapor deposition modification to eliminate the intercrystalline micropores. Single gas permeation of He, H2, CO2, and CO through this zeolite membrane before and after CVD modification was measured in 25–500°C. Intracrystalline diffusivities for these four gases in DDR-type zeolite were obtained from the permeation data above 300°C to examine the effects of the size and molecular weight of permeating gases on diffusion and permeation rate for this zeolite membrane. For the unmodified DDR-type zeolite membrane with presence of a small amount intercrystalline micropores the diffusivity (or permeance) with a low activation energy depends on both the size and molecular weight of permeating gases. For the CVD-modified DDR-type zeolite membrane with intercrystalline micropores eliminated, the activation energy for diffusion and diffusivity increases with increasing molecular size of the permeating gases. © 2008 American Institute of Chemical Engineers AIChE J, 2008.
Die Trennung von Propan/Propen-Gemischen erfolgt nach dem Stand der Technik durch Tieftemperaturrektifikation. Eine Alternative zu diesem kosten- und energieintensiven Prozess ist die Trennung der beiden Gase durch Adsorption an (mikroporösen) Feststoffen. Im vorliegenden Beitrag wird über experimentelle Untersuchungen zur Adsorption von Propan und Propen bzw. eines Gemisches aus beiden Komponenten an metallorganischen Koordinationspolymeren (sog. MOF, metal organic frameworks) berichtet.
A novel concept called Sorption Enhanced Reaction Process (SERP) for hydrogen production by steam-methane reformation (SMR) reaction uses a fixed packed column of an admixture of an SMR catalyst and a chemisorbent to remove carbon dioxide selectively from the reaction zone. The chemisorbent is periodically regenerated by using the principles of pressure swing adsorption. The SERP process steps allow direct production of high-purity hydrogen (>95 mol %) at high methane to hydrogen conversion (>80%) with dilute methane (<5 mol%) and trace carbon oxide (∼50 ppm) impurities at the reaction pressure by operating the reactor at a low temperature of 450°C. A conventional plug-flow reactor packed with catalyst alone not only needs to be operated at a much higher temperature (>650°C) to achieve the same methane to hydrogen conversion, but produces a much lower purity of hydrogen product (∼75 mol %) with a large quantity of carbon oxide (∼20 mol %) impurities. A novel chemisorbent, which reversibly sorbs carbon dioxide in the presence of excess steam at a temperature of 300–500°C, was developed for application in the SERP and the process is experimentally demonstrated in a bench-scale apparatus.
A vacuum swing adsorption process using 13X zeolite pellets with five steps was designed to split an equimolar mixture of propylene/propane: pressurization with feed; high-pressure feed; high-pressure purge with product; cocurrent blowdown; and counter-current vacuum blowdown, where the enriched propylene product is withdrawn. In the process, the partial pressure of the C3-mixture is controlled with nitrogen, which is used as inert gas. With an equimolar feed of C3 diluted to 50% with nitrogen, the column is fed at 5 bar and 423 K, and the product is obtained when the total pressure is lowered to 0.1 bar. After 15–20 cycles, the cyclic steady-state condition is achieved, a propylene-enriched stream of 98% mol relative to propylene/propane mixture, with 3.2% of nitrogen, a recovery of 19% (molar basis), and a productivity of 0.785 mol/kg·h is obtained. The experimental work was complemented with numerical simulations, and the effect of different operating parameters on the performance of the VSA was considered.
Large-scale computational screening of thirty thousand zeolite structures was conducted to find optimal structures for separation of ethane/ethene mixtures. Efficient grand canonical Monte Carlo (GCMC) simulations were performed with graphics processing units (GPUs) to obtain pure component adsorption isotherms for both ethane and ethene. We have utilized the ideal adsorbed solution theory (IAST) to obtain the mixture isotherms, which were used to evaluate the performance of each zeolite structure based on its working capacity and selectivity. In our analysis, we have determined that specific arrangements of zeolite framework atoms create sites for the preferential adsorption of ethane over ethene. The majority of optimum separation materials can be identified by utilizing this knowledge and screening structures for the presence of this feature will enable the efficient selection of promising candidate materials for ethane/ethene separation prior to performing molecular simulations.
The separation of ethane/methane and propane/methane mixtures with a silicalite-1 membrane was investigated as a function of composition, total hydrocarbon pressure (up to 425 kPa), and temperature (273–373 K). The selectivity of the membrane is highly dependent on these operating conditions. The generalized Maxwell–Stefan equations were used to predict the fluxes and the associated selectivities. These model predictions, based on separately determined single-component adsorption and diffusion parameters, were in excellent agreement with the experimental data. Using the empirical Vignes relation for the prediction of the binary Maxwell–Stefan surface diffusivity, there were no fitting parameters involved in the model prediction. The importance of incorporation of adsorbate–adsorbate interactions in the model is clearly shown, both for transient and steady-state mixture permeation. Theoretical analysis showed that for mixtures of a fast, weakly adsorbing component and a slow, strongly adsorbing component the maximum selectivity obtained with microporous membrane separation is always a factor of 2 lower than the one obtained under equilibrium adsorption conditions.
Synthesis, characterization and adsorption properties of propane/propylene on mesoporous SBA-15 are reported. This material was also used as high surface area material for silver deposition to enhance propylene adsorption improving selectivity towards the olefin. Two different silver loadings were tested: Ag/SiO2 = 0.5 and Ag/SiO2 = 1.0. With the lower silver content, better selectivity and amount adsorbed was obtained. Preliminary studies were done for the use of this adsorbent in Vacuum-Pressure Swing Adsorption (VSA-PSA) units. With a four-step cycle comprising feed, pressurization, rinse and blowdown recovery of 97% propylene with chemical grade purity (91%) was obtained and also fuel grade HD-5 propane at high pressure. If higher propylene purity is required, a five-step cycle has to be used (pressurization, feed, rinse, co-current depressurization to intermediate pressure and counter-current blowdown). In this case, purity of 99% was obtained with 63% recovery.
Benzene alkylation with propane has been studied over HZSM-5 loading 3.1–15.4wt% Mo in continuous-flow microreactor under
350°C and atmospheric pressure with the highest activity obtained at 6.7wt% Mo loading. C7–9 aromatics were obtained as main products while the total amount of benzene rings kept unchanged. i-Propylbenzene and n-propylbenzene are formed primarily, while toluene, ethylbenzene, and ethyl-toluene are formed secondly from the propylbenzenes.
Catalytic performance of 6.7wt% Mo/HZSM-5(38) partially poisoned by NH3 shows that the strong acid sites play a crucial role in the alkylation. Low SiO2/Al2O3 ratio of HZSM-5 in the Mo modified catalysts gives high propane conversion. Two hydrothermal treatment methods were applied
to the 6.7wt% Mo/HZSM-5(38) catalyst, caused decrease of propane conversion but result in different product distribution.
A possible reaction mechanism concerning bifunctional active centers resulted from combination of loaded Mo species and strong
acid centers on HZSM-5 is proposed.
The general mechanisms of gas separation in microporous inorganic membranes are reviewed in this article. Emphasis has been placed on discussing the requirements of membrane pore structure and material properties for high temperature hydrogen separation from other small gases involved in processes of hydrogen production from fossil fuels. The recent research progresses in developing the crystalline zeolite membranes, and amorphous silica-based membranes for high temperature hydrogen separation are critically reviewed. The fundamental issues associated with the zeolite and silica membranes relevant to the practical applications are analyzed based on the relationships between the separation performance and membrane structural and chemical properties.