DDR Zeolite Membrane Research Articles

Hydrogen-permeable DDR zeolite membrane packed with Zn/HZSM-5 catalyst for methane co-aromatization with ethylene

Co-aromatization of methane (CH4) and ethylene (C2H4) over a membrane reactor (MR) using hydrogen-permeable DDR zeolite membrane packed with Zn/HZSM-5 catalyst was studied for the first time. The adopted DDR zeolite membrane showed high H2 selectivity for separations of binary H2/CH4 and trinary H2/CH4/C2H4 mixtures. Typically, the selectivities for H2/CH4 and H2/C2H4 were 40 and 29 for trinary mixture separation at room temperature, respectively. The effects of reaction temperature, methane-ethylene feed ratio, space velocity and H2 removal on the membrane reaction performance were investigated systematically. The membrane reactor can achieve 8.43% methane conversion with BTX selectivity of 80.68% within 1 h under the optimal reaction conditions (reaction temperature 450 °C, methane-ethylene feed ratio 3:2, weight hourly space velocity (WHSV) 800 mL/(gcat h), sweep helium flow rate 15 mL/min). Compared to a fixed-bed reactor (FBR) with similar reaction condition, the achieved methane conversion and BTX yield were improved by ∼42% and ∼10%, respectively, due to the timely removal of H2 from the reaction system. The membrane also showed good stability, and the methane conversion maintained at about 7% for about 30 h. After burning the coke formed on the catalyst, the initial reaction performance can be restored, showing a good regeneration ability.

Synthesis of Nickel Loaded Ddr Zeolite Membrane for Separation of Hydrogen from Blast Furnace Flue Gas

Synthesis of Nickel Loaded Ddr Zeolite Membrane for Separation of Hydrogen from Blast Furnace Flue Gas

Preparation of Al-Containing ZSM-58 Zeolite Membranes Using Rapid Thermal Processing for CO2/CH4 Mixture Separation.

The synthesis of DDR-type zeolite membranes faces the problem of cracks that occur on the zeolite membrane due to differences in the thermal expansion coefficient between zeolite and the porous substrate during the detemplating process. In this study, Al-containing ZSM-58 zeolite membranes with DDR topology were prepared by rapid thermal processing (RTP), with the aim of developing a reproducible method for preparing DDR zeolite membrane without cracks. Moreover, we verified the influence of RTP before performing conventional thermal calcination (CTC) on ZSM-58 membranes with various silica-to-aluminum (Si/Al) molar ratios. Using the developed method, an Al-containing ZSM-58 membrane without cracks was obtained, along with complete template removal by RTP, and it had higher CO2/CH4 selectivity. An all-silica ZSM-58 membrane without cracks was obtained by only using the ozone detemplating method. ZSM-58 crystals and membranes with various Si/Al molar ratios were analyzed by using Fourier-transform infrared (FTIR) spectroscopy to confirm the effects of RTP treatment. Al-containing ZSM-58 zeolites had higher silanol concentrations than all-silica zeolites, confirming many silanol condensations by RTP. The condensation of silanol forms results in the formation of siloxane bonds and stronger resistance to thermal stress; therefore, RTP caused crack suppression in Al-containing ZSM-58 membranes. The results demonstrate that Al-containing ZSM-58 zeolite membranes with high CO2 permeance and CO2/CH4 selectivity and minimal cracking can be produced by using RTP.

Facile synthesis of seed crystals and gelless growth of pure silica DDR zeolite membrane on low cost silica support for high performance in CO2 separation

Microporous membrane has shown great potential for the separation of carbon dioxide from the gas mixture. Beside the membrane performance, the membrane preparation method is a factor to be concerned in this technique. In this work, pure silica DDR zeolite membranes on silica porous substrate were synthesized from an economical and environmentally efficient process. Porous silica substrate was fabricated using a low cost material. The membrane growth reaction used a minimum amount of chemical, produced no waste generation. Part of the reactant is supplied from porous substrate. The membrane quality is strongly dependent on the exposed faces of seed crystals and the pH value of the template solution. The obtained membranes show very high permeance and selectivity for CO2 from a CO2/CH4 gas mixture in both dry and humid conditions and hot gas flow. The membrane maintains stability in high performance with the separation factor of 440–500 for a test period of one week.

Fast synthesis of thin all-silica DDR zeolite membranes with inorganic base as mineralizing agent for CO2-CH4 separation

Compared with other small pore zeolite membranes, all-silica DDR membrane has unique advantages like smaller pore size and outstanding stability, which makes it perfect for the separation of light gases, like CO2-CH4 and O2-N2 separations. However, the development of DDR membrane has fallen behind due to the following issues like long synthesis time, poor control on membrane thickness, use of toxic solvent etc. In this study, fast synthesis of high-performance all-silica DDR membrane was realized by conventional hydrothermal synthesis in a mother liquor with inorganic base as mineralizing agent and reduced template concentration. Thin DDR membranes with thickness ~1 μm can be prepared in 3–6 h with high reproducibility. High CO2 permeance of 8.6 × 10-7mol/(m2·s·Pa) and CO2-CH4 selectivity of 150 were obtained on 1.3 μm thick DDR membrane (0.14 MPa pressure drop and room temperature, permeability = 3339 Barrers). The elimination of organic solvent (ethylene diamine), reduced consumption of organic template and use of cheap inorganic bases make membrane synthesis cheaper and more environment-friendly. Compared with other all-silica DDR membranes, the current method is superior on membrane performance (permeance and membrane thickness), material cost, synthesis time and waste disposal.

Fast synthesis of thin all-silica DDR zeolite membranes by co-template strategy

The separation performances of all-silica DDR membranes are inferior to other small pore zeolite membranes, which is the result of poor understanding of the corresponding zeolite powder and membrane syntheses. Issues like lengthy synthesis, tough detemplation, poor reproducibility, use of toxic solvent, make it difficult to prepare high performance DDR membranes. In this study, fast synthesis of thin all-silica DDR membranes (~1.6 μm) was achieved in only 3 h by conventional hydrothermal synthesis in a ethylenediamine-free gel. The addition of tetraethylammonium hydroxide (TEAOH), functioned as mineralizing agent and phase controller, is the key. The presence of TEAOH made high temperature synthesis possible by suppressing the formation of other phases, which led to significantly increased crystallization kinetics and shortened synthesis time. As a mineralizing agent, TEAOH may increase the solubility of silica source, favor nucleation and result into the formation of small crystals and thin membrane. The removal of ethylenediamine make the synthesis environment-friendly. Mild template removal under wet ozone environment oxidized template molecules effectively and minimized the formation of defects. Outstanding CO2–CH4 separation performance was obtained, with CO2 permeance of 4.1 × 10−7mol/(m2·s·Pa) and CO2–CH4 selectivity of 160 at 0.14 MPa pressure drop.

CO2 Separation via a DDR Membrane: Mutual Influence of Mixed Gas Permeation

The permeation properties of DDR zeolite membranes in the separation of mixed gases are strongly affected by the gas species present in the mixture, their concentrations, and the operating conditio...

Memory effect in DDR zeolite powder and membrane synthesis

In this work, syntheses of crystalline DDR powder and DDR membranes were studied with regard to memory effects happening to the repetitively used synthesis vessel. It was found that promoted nucleation and growth towards DDR framework structures occur during a certain number of synthesis sequences. X-ray diffraction (XRD), scanning electronic microscopy (SEM), thermogravimetry (TG) and differential scanning calorimetry (DSC) and single-gas permeation tests were partially used to characterize obtained DDR crystals, as-synthesized DDR membranes as well as detemplated DDR membranes. The results implied that intermediate residues remain as traces on or within the re-used synthesis liner wall and act as seeds in the subsequent synthesis approaches. The obtained crystals derived from the different synthesis sequences were used as seed materials for DDR membrane growth. It could be shown that thin films of pure DDR phase start to form onto alumina supports within 18 h secondary growth and a defect-free DDR membrane can be achieved after 24 h of synthesis.

Ultrafast synthesis of thin all-silica DDR zeolite membranes by microwave heating

All-silica DDR zeolite membrane has great potential for CO2-CH4 separation because of its suitable pore size, great stability and insensitivity to moisture. However, the long synthesis time and tough template removal created great challenges for the synthesis of high quality DDR zeolite membrane. All-silica DDR zeolite membrane was prepared on porous ceramic tubes with microwave-aided hydrothermal synthesis. Template removal of DDR membranes was studied with rapid thermal processing and low temperature calcination under ozone environment. Compared with traditional hydrothermal synthesis, microwave heating significantly accelerated the crystallization kinetics of DDR membranes by reducing the synthesis time from 2 to 6 d to 1 h. Membrane thickness was also reduced from ~5 µm to less than 1.5 µm. These improvements can be attributed to the fast and homogeneous heating from microwave heating, which resulted into fast nucleation and crystallization as well as smaller crystals and thinner membrane. Template removal by rapid thermal processing was not successful for DDR membranes which could be the result of high thermal stress at high calcination temperature. Low temperature calcination under ozone environment can remove template molecules effectively without generating defects, which can be ascribed to small thermal stress at low calcination temperature. Outstanding CO2-CH4 separation performance was obtained, with CO2 permeance of 4.7 × 10-7 mol/(m2 s Pa) and CO2-CH4 selectivity of 190 at 0.14 MPa pressure drop. The high separation permeance is the synergy of optimized seeding, microwave-aided synthesis and mild template removal under ozone environment.

Preparation of defect-free DDR zeolite membranes by eliminating template with ozone at low temperature

A dilute synthesis precursor was developed to prepare high-quality DDR zeolite membranes on α-Al2O3 four-channel hollow fibers by secondary growth method. However, the as-membranes were susceptible to crack during high temperature activation (>873K) for the template removal. A molecular simulation demonstrated that a large contraction of ~ 5% occurred for the DDR zeolite framework after template removal. Ozone environment was employed for membranes activation at low temperature (473K) to prevent the formation of cracks. The occluded template in zeolite cages was successfully removed by this way and high quality DDR zeolite membranes were achieved reproducibly without any cracks. The membranes were further evaluated for single gas permeation and separation of CO2/CH4 mixture. The ideal selectivities for CO2/CH4, CO2/N2, CO2/C3H8, CO2/H2, N2/CH4 and O2/N2 were 305, 32, 1500, 5, 10 and 1.7, respectively. For the separation of equimolar CO2/CH4 mixture, a high selectivity of 500 and CO2 permeance of 3.5×10−8molm−2s−1Pa−1 could be achieved. The membranes also exhibited high stability in wet environment. A stable permeance of 3.5×10−8molm−2s−1Pa−1 and selectivity of 140 could be achieved during 40h test for the separation of CO2/CH4 mixture containing 1.5% vapor at 353K.

DDR-type zeolite membrane synthesis, modification and gas permeation studies

DDR-type zeolite membrane was synthesized on porous α-alumina substrate by hydrothermal treatment of a ball-milled Sigmal-1 crystal seed layer in an aluminum-free precursor solution containing 1-Adamantylamine as the structure directing agent (SDA). The as-synthesized DDR zeolite membranes were defect-free but the supported zeolite layers were susceptible to crack development during the subsequent high-temperature SDA removal process. The cracks were effectively eliminated by the liquid phase chemical deposition method using tetramethoxysilane as the precursor for silica deposits. The modified membrane was extensively studied for H2, He, O2, N2, CO2, CH4, and i-C4H10 pure gas permeation and CO2/CH4 mixture separation. At 297K and 2-bar feed gas pressure, the membrane achieved a CO2/CH4 separation factor of ~92 for a feed containing 90% CO2, which decreased to 62 for a feed containing 10% CO2 with the CO2 permeance virtually unchanged at ~1.8×10−7mol/m2sPa regardless of the feed composition. It also exhibited an O2/N2 permselectivity of 1.8 at 297K. The gas permeation behaviors of the current aluminum-containing DDR type zeolite membrane are generally in good agreement with the findings in both experimental and theoretical studies on the pure-silica DDR membranes in recent literature.

Pore modification of deca-dodecasil-rhombohedral zeolite membrane by carbon loading from in situ decomposition of 1-adamantanamine for improved gas separation

Carbon loading by decomposition of 1-adamantanamine in the pores of a DDR zeolite membrane increases H2–CO2 permeance and separation efficiency.

Sonication mediated hydrothermal process – an efficient method for the rapid synthesis of DDR zeolite membranes

The formation and growth of a DDR zeolite membrane was developed on the low cost indigenous clay–alumina substrate for separation of H2 from H2–CO2 mixture by selective deposition of oriented seed crystals, followed by secondary growth method with sonication mediated hydrothermal technique. The formation of free radicals by ultrasonic irradiation in the sonochemical method enhances the rate of nucleation which ultimately reduces the DDR zeolite crystallization time. Surface seeding not only accelerates the zeolite crystallization on the support surface but also enhances the formation of an homogenous zeolite membrane layer. The DDR seeds were synthesized by a sonication mediated hydrothermal technique within a short crystallization time i.e. 2 days and used to provide nucleation for the membrane growth. Accordingly DDR zeolite membranes were synthesized on seeded substrate within 5 days. The membrane thickness was found to be ∼26 μm. The synthesized membranes along with seed crystals were characterized by XRD, FTIR, FESEM and EDAX analysis. The performance of the membrane formed was evaluated by single gas as well as mixture gas permeation measurement for H2 and CO2. The H2–CO2 separation selectivity of the membrane increased up to 3.7 at room temperature which is more than the reported values. To the best of our knowledge, there is no report on the synthesis of a DDR zeolite membrane within 7 (2 days for seed crystal and 5 days for membrane synthesis) days by a secondary growth technique.

Modeling Permeation of CO2/CH4, N2/CH4, and CO2/Air Mixtures across a DD3R Zeolite Membrane

Single component (CO2, CH4, and N2) and equimolar binary mixture (CO2/CH4, N2/CH4, and CO2/Air) permeation data across a disk-shaped all-silica DDR zeolite membrane have been the subject of a thorough modeling study over a challenging broad temperature (220−373 K) and feed pressure (101−1500 kPa) range. The mass transport through the zeolite layer is evaluated for two rival, Maxwell Stefan-based, models: the Relevant Site Model (RSM) and the so-called Reed Ehrlich (RE) approach. Both models have been introduced to account for the strong loading dependency of the diffusivity in small-pore cage-like zeolites like DDR. High pressure adsorption isotherms (up to 7000 kPa) measured on DDR crystals are incorporated to describe adsorption on the zeolite. Both the RSM as the RE approach yield an excellent model fit of the single component permeation data. However, for both models the N2 and CH4 data did not allow an accurate estimation of the model fit parameters. Both models can lead to a good prediction of comparable quality of the mixture permeation data based on the single component model fit parameters. The RE approach is very sensitive toward the model input parameters and the estimated mixture loading, which both can be very hard to determine accurately in practice. The RSM does not suffer from both these issues, which is an evident advantage with respect to application of this model.

Modeling Pure Gas Permeation in Nanoporous Materials and Membranes

The low-pressure transport of simple fluids in nanopores and in disordered nanoporous networks is analyzed, using a recent oscillator model theory from the author's laboratory, considering the trajectories of molecules moving in the potential energy field of the fluid-pore wall interaction. The scaling behavior of the single-pore theory is discussed, and it is shown that the Knudsen model provides an upper bound to the diffusivity scaled with the pore radius. The single-pore theory is shown to apply well to ordered materials and successfully interprets recent literature data on the variation of permeability with diffusant molecular size for a DDR zeolite membrane. A peak in permeability is seen at a pore-size-dependent molecular size because of the opposing effects of equilibrium and transport. Application to disordered pore networks is also presented on the basis of a hybrid correlated random walk effective medium theory imbedding the oscillator model at the single-pore level, and a rigorous expression for the tortuosity is derived from the theory. A rich variety of behavior is predicted for the tortuosity, which can increase or decrease with increasing extent of pore size nonuniformity as well as with changes in temperature because the diffusing species preferentially flows through more conducting pores. Weakly adsorbing gases such as helium are seen to have a higher tortuosity than more strongly adsorbing ones. The predicted values of tortuosity are shown to be in line with those obtained from the interpretation of recent experimental mesoporous membrane transport data and are in the range of 5-10 whereas those extracted using the Knudsen model are unrealistically high, in the range of 10-20.

High temperature permeation and separation characteristics of an all-silica DDR zeolite membrane

The permeation and separation characteristics of an all-silica DDR zeolite tubular membrane have been studied in the temperature range of 303–773 K and feed pressures up to 500 kPa. The permeation experiments are complemented by single component adsorption isotherms. The permeance of He, H 2, CO 2, CO and N 2 monotonically decreases with increasing temperature. This behaviour could be described accurately for all components by a surface diffusion mechanism. Only in case of N 2 and CO small deviations are observed above 600 K. Isobutane is not able to enter the DDR pores and passes only through a very small number of defects in the membrane. The single component permeance is about equal to the permeance of this component in a binary mixture. Only below 473 K the H 2 permeance in a mixture with CO 2 or isobutane is reduced in comparison with its single component permeance. The ideal H 2/CO and CO 2/CO selectivities range from 3 to 12 and 10 to 2 between 303 and 673 K, respectively. These mixture selectivities where always below 5 and much lower than the ideal selectivities because of non-differential operation along the membrane tube. The ideal H 2/isobutane selectivity is >600 at 101 kPa feed pressure at all temperatures. The mixture selectivities at 101 kPa total feed pressure is ∼400 in an equimolar binary mixture. The high selectivities, high H 2 and CO 2 fluxes and stable membrane operation, also at high temperatures, makes this membrane a potential candidate for high temperature (reactive) separations that involve removal of H 2 and CO 2.