Non-zeolitic Pores Research Articles

Regeneration of zeolite membranes deactivated by condensable molecules

A zeolite membrane is a microporous film on a porous substrate that can discriminate molecules in the gas and liquid phases through its pores. Although zeolite pores play a critical role in achieving a high separation performance, they tend to be clogged by condensable molecules during the separation process, resulting in a significant drop in the separation performance. In this study, the thermal regeneration of zeolite membranes deactivated by condensable molecules was investigated to achieve their reusability without significant changes in the separation performance. Silicoaluminophosphate (SAPO-34) and pentasil (MFI) membranes were prepared, and condensable vapors, including water, n-butanol, n-methyl-2-pyrrolidone, and thermally degenerated organic vapors were chosen as the foulants to block the pores of the zeolite membranes. The optimized regeneration temperature differed according to the type of foulant and zeolite framework. Overall, the optimized thermal desorption temperature of every foulant was lower for the MFI membrane because of its larger pores and more hydrophobic surfaces compared to those of the SAPO-34 membrane. For the membranes deactivated by thermally degenerated organic vapors, the pores of the zeolite membranes were opened based on organic solvent treatment followed by thermal desorption. In addition, the MFI membrane was more sensitive to heat than the SAPO-34 membrane; therefore, slight changes in the membrane selectivity were observed depending on the thermal regeneration protocols. This study demonstrated that deactivated zeolite membranes can be regenerated, and excess heat exposure during thermal regeneration can decrease gas selectivity through the creation of nonzeolitic pores.

Optimal synthesis of nanosize seeds for secondary growth of high performance hydroxy-sodalite (H-SOD) zeolite membranes for small gas and water separations

Nanosize H-SOD particles were hydrothermally synthesized as seeds for the secondary growth of hydroxy-sodalite (H-SOD) zeolite membrane. The size and phase purity of H-SOD zeolite particles were carefully controlled by changing several hydrothermal conditions. Especially, NaOH concentration in the hydrothermal solution made a significant effect on the phase purity and particle size. The high phase purity particles with a mean diameter of 80 nm were successfully synthesized at a 3 M NaOH concentration and applied for secondary growth. Overall, uniform H-SOD zeolite membranes with various thicknesses (3, 7, and 9.5 μm) were prepared by varying secondary growth time. The 9.5 μm thick H-SOD zeolite membrane exhibited high H 2 /N 2 permselectivity of 95 at 473 K and a transmembrane pressure of 0.3 MPa. And, the pervaporation performance was evaluated for 90 wt% methanol-water mixture at 423 K, an elevated temperature. It showed a high water/methanol separation performance; its water/methanol separation factor was 2726 and the total flux, 1.02 kg/m 2 h. The high small gases and water separation performances were because interparticular voids between the nano size seeds were small and the surface of seed layer was flat. Therefore, the secondary growth remained a continuous zeolite layer with few non-zeolitic pores. In the present work, it was well proven experimentally that application of nanosize seed is an effective option to prepare high performance zeolite membrane in H-SOD zeolite system. • High purity H-SOD zeolite particles with a mean diameter of 80 nm were carefully prepared for the secondary growth of H-SOD zeolite membranes. • Synthesis temperature, time and NaOH concentrations played major roles on the particle size. • The H-SOD zeolite membranes prepared by using the nanosize seeds showed excellent performances for H 2 /CO 2 , H 2 /N 2 and water/methanol separations. • The good performance was due to smaller interparticular voids between nano size seeds and the increased surface flatness of seed coating layer.

SSZ-13 zeolite membranes on four-channel α-Al2O3 hollow fibers for CO2 separation

High-silica CHA (SSZ-13) zeolite membranes exhibit great potential for CO2 separation. However, the practical application is significantly hindered by the high investment cost. Here, we synthesized SSZ-13 zeolite membranes on four-channel hollow fibers with pore size of 150 nm, 370 nm and 600 nm, respectively. The CO2/CH4 selectivity of 127 ± 11 as well as CO2 permeance of (3.1 ± 0.6) × 10-7 mol m−2 s−1 Pa−1 was achieved on the membranes synthesized on the hollow fibers with 150 nm pore size at 433 K for 96 h. The single gas permeation was quantitively assigned to evaluate surface diffusion, Knudsen diffusion and viscous flow through the as-synthesized membranes. The permeation of small molecules (e.g., CO2 and N2) is mainly contributed by the zeolitic pores, however, the permeation of relatively big molecule (e.g., CH4) is more sensitive to the non-zeolitic pores. For the membranes with αCO2/CH4 of 130 and 48, the non-zeolitic pores contributed 24.7% and 69% of CH4 permeance, respectively.

An Extrinsic-Pore-Containing Molecular Sieve Film: A Robust, High-Throughput Membrane Filter.

MFI type zeolites with 10-membered-ring pores (ca. 0.55 nm) have the ability to separate p-xylene (ca. 0.58 nm) from its bulkier isomers. Here, we introduced non-zeolitic micropores (ca. 0.6-1.5 nm) and mesopores (ca. 2-7 nm) to a conventional microporous MFI type zeolite membrane, yielding an unprecedented hierarchical membrane structure. The uniform, embedded non-zeolitic pores decreased defect formation considerably and facilitated molecular transport, resulting in high p-xylene perm-selectivity and molar flux. Specifically, compared to a conventional, crack network-containing MFI membranes of similar thickness (ca. 1 μm), the mesoporous MFI membranes showed almost double p-xylene permeance (ca. 1.6±0.4×10-7 mol m-2 s-1 Pa-1 ) and a high p-/o-xylene separation factor (ca. 53.8±7.3 vs. 3.5±0.5 in the conventional MFI membrane) at 225 °C. The embedded non-zeolitic pores allowed for decreasing the separation performance degradation, which was apparently related to coke formation.

Synthesis of all-silica ZSM-58 zeolite membranes for separation of CO2/CH4 and CO2/N2 gas mixtures

Abstract We prepared all-silica ZSM-58 zeolite membranes on α-alumina tubular supports and optimized their synthesis conditions. High-purity ZSM-58 zeolite membranes were prepared using a concentrated precursor solution (methyltropinium iodide/SiO2 and NaOH/SiO2 molar ratios > 0.05) with a H2O/SiO2 molar ratio of 52 at 413 K for 24 h. A ZSM-58/DOH mixed layer or a pure DOH layer was generated with ZSM-58 when the synthesis temperature exceeded 413 K and the synthesis time was longer than 24 h. The membranes prepared with a H2O/SiO2 molar ratio higher than 52 were not covered ZSM-58 zeolite and/or generated DOH zeolite. Further, we investigated the separation performance of the ZSM-58 zeolite membrane for CO2/CH4 and CO2/N2 gas mixtures as a function of feed pressure. At a pressure drop of 0.2 MPa, the membrane exhibited a CO2 permeance and CO2/CH4 mixture selectivity of 17 × 10−8 mol s−1 m−2 Pa−1 and 290, respectively, for a CO2/CH4 system, and a CO2 permeance and CO2/N2 selectivity of 29 × 10−8 mol m−2 s−1 Pa−1 and 44, respectively, for a CO2/N2 system. The mixture selectivity was lower than the single gas selectivity in CO2/CH4 separation due to competitive adsorption since CH4 mainly passes through non-zeolite pores. In contrast, the mixture selectivity was higher than the single gas selectivity in CO2/N2 separation because N2 permeation was inhibited by CO2 adsorption in the zeolite pores since N2 mainly passes through the zeolite pores. Thus, high-purity ZSM-58 zeolite membranes with high CO2 separation performances were obtained by the optimized synthesis conditions.

Pervaporation of Water–Alcohol Mixtures on Cation-Exchanged LTA Zeolite Membranes

Na-LTA membranes have been synthesized on porous nickel supports by in situ crystallization from a true solution. K-LTA and Ca-LTA membranes have been prepared from Na-LTA membranes by ion exchange in 1 M KCl and CaCl2 solutions, respectively. The formation of the LTA structure, its preservation during ion exchange, and achievement of nearly 100% replacement in the near-surface layer of the zeolite membranes have been proven by means of XPD and SEM-EDX. All the zeolite membranes synthesized have been tested in pervaporation of isopropanol–water, ethanol–water, and methanol–water mixtures containing 10 wt % water. It has been found that the mass flux of alcohol does not depend on the nature of alcohol or zeolite cation if the kinetic diameter of alcohol molecule is greater the effective diameter of zeolite pores. A new method for evaluating of the mass flux through nonzeolite pores based on pervaporation data has been proposed. The estimated mass flux through nonzeolite pores for all the zeolite membranes tested is 15 ± 3 g m−2 h−1 which is 1.6 to 7.6% of the total transmembrane mass flux depending on the alcohol chosen.

A comprehensive comparative study on morphology and pervaporative performance of porous-supported mesoporous zeolitic membranes

Abstract Mordenite (MOR) and faujasite (FAU) were formed as a membrane layer on tubular mullite and disk-shaped α-alumina supports for pervaporation (PV) dehydration of ethanol (EtOH). Structural analyses were carried out using XRD, SEM and EDAX. EtOH/water PV process was utilized to assess the membranes performances. The experiments show the fairly high separation factor representing the defect-free membrane layer and legible nonzeolitic pores on the top-layer. The effects of synthesis parameters including ceramic support, repetition of coating, seeding method, crystallization time (14 and 18 h) and temperature (160, 170 and 180 °C), and Si/Al ratio of the precursor gel formulation (12 and 16) on the membranes structures and their PV performances were investigated. Furthermore, the effective operational conditions (feed concentration (50, 70 and 90 wt% EtOH) and temperature (25 and 45 °C), and vacuum on permeate side (80 and 130 mmHg)) on PV performance of the membranes were studied. According to a combined performance parameter, pervaporation separation index (PSI), the MOR layer on α-alumina (MOR_D-II) can be reported as the best membrane for EtOH/water PV process. Permeation flux (43.43%), separation factor (30.39%) and PSI (62.35%) decrease with increasing EtOH concentration in feed (50–90 wt%). However, increasing feed temperature increases permeation flux and decreases separation factor, whereas applying higher vacuum in the permeate side leads to the opposite trend.

Preparation of a non-hydrothermal NaA zeolite membrane and defect elimination by vacuum-inhalation repair method

This work reports the preparation of a non-hydrothermal NaA zeolite and its stainless steel-supported membrane using geopolymer-gel-thermal-conversion (GGTC) combined with a dip-coating method, which does not require hydrothermal processing. The XRD and high-resolution transmission electron microscopy (HRTEM) analyses indicate that the 1.1Na2O–Al2O3–2SiO2–8H2O geopolymer gels cured at 25°C nucleate in the geopolymerization stage. After heat-treatment above 60°C, the crystal nuclei in the geopolymer gels continue to grow and transform into NaA zeolite crystals with good crystallinity. To improve the separation efficiency of the stainless steel-supported NaA zeolite membrane in the pervaporation (PV) process of an ethanol/water mixture, this paper reports a simple vacuum-inhalation repair method for non-zeolitic pores or other defects in the NaA zeolite membrane. After vacuum-inhalation repair of the non-hydrothermal NaA zeolite membrane with sodium alginate solutions and CaCl2 solutions, in that order, the repaired NaA zeolite membrane exhibited a separation factor that was enhanced approximately 7- to 14-fold over that of the unrepaired NaA zeolite membrane.

Synthesis of thin CrAPSO-34 membranes by microwave-assisted secondary growth

CrAPSO-34 crystals were synthesized by conventional hydrothermal method with Cr isomorphously substituted into their structures. CrAPSO-34 membranes were prepared by microwave-assisted heating on porous α-Al2O3 substrates seeded with CrAPSO-34 crystals. These membranes were characterized by SEM, EDX, XRD, FT-IR, and UV–Vis techniques. The dense CrAPSO-34 membrane with a thickness of about 2.5 μm was formed after secondary growth. FT-IR and UV–Vis provided direct evidence for Cr substitution into the CHA zeolite membrane structure. The defect size distribution of CrAPSO-34 membranes indicated that non-zeolitic pores were negligible, which was further conformed by the result of single-gas permeances for CrAPSO-34 membrane versus kinetic diameter of the permeating molecule. The separation performance of CrAPSO-34 membranes for equimolar CO2/CH4 gas mixtures was presented. These membranes displayed CO2 permeances as high as 7.9 × 10−7 mol/m2 s Pa with CO2/CH4 separation selectivities in the 152–156 range at 302 K and a pressure drop of 138 kPa.

Remarkable Catalytic Activities of Hydrothermally Synthesized Bilayered HZSM-5 Coatings with Additional Nonzeolite Pores

Bilayered HZSM-5 zeolite coatings with additional nonzeolite pores were synthesized on the inner surface of stainless steel tubes by a secondary growth method with the addition of ethanol into the synthesis batch. The as-synthesized coatings were characterized by scanning electron microscopy and gas permeation, indicating that additional nonzeolitic pores were formed in the presence of ethanol. A remarkable greater than 1000-fold improvement in N2 permeation was observed. Catalytic cracking of supercritical n-dodecane (500 °C, 4 MPa) over a series of bilayered HZSM-5 coatings showed that the HZSM-5 coatings with additional nonzeolite pores also exhibit a remarkable catalytic activity improvement by ca. 150% and that the amount of nonzeolite porosity may play a determining role in the enhancement of the catalytic performance. According to cracking products and the permeation test, the performance enhancement may be attributed to the effective access of reactant to the acid sites in the zeolite pores due to...

Preparation and optimization of mordenite nanocrystal-layered membrane for dehydration by pervaporation

Mordenite nanocrystal-layered membranes consisting of a mordenite nanocrystal layer and protection layer were successfully prepared. The obtained mordenite membranes were applied to the separation of water from water/organic solutions (organic solvents: ethanol, acetone, iso-propanol, or acetic acid) using a pervaporation method at temperatures from 60 to 100 °C. Water permeance through the mordenite membrane in each water/organic solution system was almost identical regardless of organic solvent types in the feed solution. In contrast, the permeance of the organic molecules depended on their polarities. The pre-aging temperature of the mother liquid and the heating rate for formation of the protection layer affected the secondary growth process that formed the protection layer, leading to different morphologies and sizes of the crystals in the protection layer. Water flux increased with decreasing crystal size in the protection layer because the number of non-zeolitic pores among the mordenite crystal increased as the crystal size decreased.

Mercaptoundecanoic acid capped palladium nanoparticles in a SAPO 34 membrane: a solution for enhancement of H₂/CO₂ separation efficiency.

In this work, the high quality Pd/SAPO 34 membranes were grown on the support using a secondary (seeded) growth hydrothermal technique followed by insertion of 11-mercaptoundecanoic acid capped palladium (MUA-Pd) nanoparticles (NPs) to the membrane surface. For this, first, the indigenous low cost clay-alumina support was treated with poly diallyldimethylammonium chloride (PolyDADMAC) polymer, and subsequently, a seed layer of SAPO 34 crystals was deposited homogeneously in a regular orientation. Since PolyDADMAC is a high charge density cationic polymer, it assisted in reversing the charge of the support surface and produced an attractive electrostatic interaction between the support and zeolite crystals. This may facilitate the zeolite grain orientation in the synthesized membrane layer. Here, the Pd NPs were deposited in the membrane matrix by a simple dip-coating method. After thermal treatment of the Pd/SAPO 34 membrane, the defects were formed because of the removal of the structure-directing agent (SDA) from the zeolite pores but the presence of Pd NPs, which were entrapped inside the nonzeolitic pores and clogged the defects of the membrane. Field emission scanning electron microscopy (FESEM) and elemental mapping of the membrane cross-section confirmed that most of the Pd NPs were deposited at the interface of the membrane and the support layer which may increase the membrane efficiency, i.e., separation factor, as well as permeability of H2 through the membrane. As the membrane structure was associated with the oriented crystal, the pores were more aligned and permeation adequacy of H2 through the membrane enhanced. These membranes have a relative hydrogen permeance of 14.8 × 10(-7) mol·m(-2)·s(-1)·Pa(-1). The selectivity of H2/CO2 based on single gas permeation was 10.6, but for the mixture gas (H2/CO2 55:45), the H2/CO2 mixture separation factor increased up to 20.8 at room temperature. It is anticipated that this technique may be useful for making a defect free membrane and also a hydrogen selective Pd loaded membrane with lower cost (as the quantity of Pd is low) which can be utilized for a "clean energy" related application.

Synthesis of (h0l)-oriented zeolite Fe-beta membrane

The continuous and intergrown zeolite Fe-beta membrane was synthesized on the porous α-Al2O3 substrate by fluoride route in combination with a secondary seeded growth method. XRD and SEM results show that the synthesized zeolite Fe-beta membrane and all-silica zeolite beta membrane are (h0l)-oriented. The thickness of zeolite Fe-beta membrane is just 5.6μm, which is thinner than that of the all-silica zeolite beta membrane (8.5μm). Leaking tests confirm that non-zeolitic pores in the synthesized membranes are negligible. XRD patterns of the powdery samples which were collected from the bottom of the autoclave after secondary growth indicated that most of iron is incorporated in the zeolite beta framework.

TS-1 zeolite as an effective diffusion barrier for highly stable Pd membrane supported on macroporous α-Al2O3 tube

Thin and defect-free supported palladium membranes were prepared on macroporous α-alumina ceramic tubes using TS-1 zeolite as the intermediate and diffusion barrier layer. It is necessary to modify the pore size of the substrate and form an effective diffusion barrier between the palladium membrane and the support in order to obtain a continuous and thin Pd layer. The detailed microstructure of the composite membranes were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) spectroscopy. Pd membranes supported on a TS-1 layer with a thickness of 2–5 μm showed high H2 flux and ideal selectivity of H2/N2 in the working temperature range. The result reveals that the palladium penetrated into the TS-1 films through the nonzeolitic pores existing in the zeolite films. The resultant Pd-TS-1 membrane displays very high stability due to the excellent anchorage of Pd layer inside the zeolite barrier grown on the support. This membrane could be able to tolerate rapid and repeated cycling of gases, temperature and pressure without sacrificing its good permeance and selectivity, even below the Pd embrittlement temperature (<573 K). These results suggest that TS-1 zeolite is an effective intermediate substrate modifier to bridge the large pore size of the support for the synthesis of a thin Pd layer.

Coke deposition in the SAPO-34 membranes for examining the effects of zeolitic and non-zeolitic pathways on the permeation and separation properties in gas and vapor permeations

SAPO-34 membranes were hydrothermally synthesized using 75 nm-sized SAPO-34 crystals as seed crystals. The SAPO-34 membrane showed a high selectivity to water for water/2-propanol (H2O/IPA) vapor permeation (separation factor>3200). Selective coke deposition within the pores of SAPO-34 via methanol-to-olefins reaction was applied to examine the effects of zeolitic and non-zeolitic pathways on the separation properties using various SAPO-34 membranes with different qualities. The results imply that H2O molecules are selectively adsorbed and the permeation of IPA is blocked even through the non-zeolitic pores (less than 1 nm) of SAPO-34 membranes. The size and hydrophilicity of non-zeolitic pores are important factors for inhibiting the IPA permeation.

A simple method for healing nonzeolitic pores of MFI membranes by hydrolysis of silanes

In this paper, we proposed a simple method with hydrolysis of silanes to heal nonzeolitic pores of pure silica MFI (silicalite) zeolite membranes. Three silane precursors, tetraethoxysilane (TEOS), tetramethoxysilane (TMOS) and dimethoxydimethylsilane (DMDS) were used for membrane modification at room temperature. The modified membranes were evaluated by the separation of p-/o-xylene mixture. It was found that the hydrolysis of silanes in nonzeolitic pores could obviously improve separation performance of the MFI zeolite membranes. The modification results of nonzeolitic pores were related to the silane precursors with different physical–chemical properties and molecular kinetic diameter. Compared with TEOS and TMOS, DMDS was more suitable as a precursor for healing nonzeolitic pores. For a zeolite membrane after DMDS modification, p-/o-xylene separation factor could be increased from 2.34 to 10.84 while p-xylene flux decreased only about 12.6%.

Synthesis, ethanol dehydration and thermal stability of NaA zeolite/alumina composite membranes with narrow non-zeolitic pores and thin intermediate layer

NaA zeolite/alumina composite membranes were prepared by the secondary growth process using a nanometer-scale seed, preparatory to investigation of their ethanol dehydration behavior. Single-gas permeation data for hydrogen, nitrogen, n-C 4H 10 and SF 6 indicated that the prepared membranes have relatively narrow non-zeolitic pores. The water flux sharply decreased in an existence of alcohol. The phenomenon could be explained by the ethanol blockage in the α-cage with window of 4.2 Å, being activated by the hydrogen bond between adsorbed water and ethanol. In dehydrating 95 wt.% ethanol at 70 °C, the prepared membranes showed a high water/ethanol separation factor (∼10,000) and a low flux (∼0.03 kg/m 2 h). It announced that at the pervaporation condition, the water flux through the zeolitic pores is ignorable. In nine membranes among 14 total prepared, well-developed cracks were observed in the formed zeolite layers before or during a dehydration test at 70 °C. The cracks were induced by the mismatch of the thermal expansion coefficients of the NaA zeolite layer (negative expansion) and the porous alumina support (positive expansion). The cracked NaA zeolite membranes showed a low water/ethanol separation factor (30–300) and a high total flux (1–10 kg/m 2 h) in the pervaporation condition. By a simple thermal stress calculation, it was established that the zeolite membranes with a clear phase boundary between the LTA layer and α-alumina support is highly vulnerable to thermal crack formation during the heating.

High-flux ZSM-5 membranes with an additional non-zeolite pore system by alcohol addition to the synthesis batch and their evaluation in the 1-butene/i-butene separation

ZSM-5 membranes were crystallized on tubular TiO 2 supports and evaluated in the permeation of a 50%/50% 1-butene/i-butene mixture. If some of the water in the standard recipe 90 SiO 2:0.225 Al 2O 3:1 Na 2O:3.6 TPAOH:1.8 TPABr:1800 H 2O was substituted by the same molar amount of short-chain length alcohol, the fluxes and permeances increased remarkably but the 1-butene/i-butene shape selectivity decreased only slightly. This experimental finding is attributed to additional non-zeolite micropores which are formed by the presence of alcohols in the synthesis batch. Since the formation of this additional pore system is linked to a decrease of the crystal size in the membrane layer, the increased length of grain boundaries could represent the structural origin of these additional non-zeolite pores. In a membrane preparation if 25% of the molar amount of water was substituted by ethanol, the 1-butene flux increased by the factor of two, but the mixture separation factor decreased by only 30%. Therefore, relatively thick ZSM-5 layers of about 30 μm gave 1-butene permeances of the order of 1 m 3(STP) m −2 h −1 bar −1 with permselectivities of 20 and mixture separation factor of 6. The testing of the ZSM-5 membranes took place under practice-relevant conditions, i.e. with undiluted feeds and without applying sweep gases or reduced pressure on the permeate side.

Synthesis and characterization of SAPO-5 membranes on porous α-Al 2O 3 substrates

The continuous and highly intergrown SAPO-5 membrane on the α-Al 2O 3 substrate was synthesized by using in situ crystallization and microwave heating. According to the characterization using scanning electron microscopy (SEM) and X-ray diffraction (XRD), the 5 μm-thick membrane was composed of well-intergrown pure SAPO-5 crystals, which were tightly covered on the α-Al 2O 3 substrate in random orientation. The single-component pervaporation measurements revealed the resulting membrane was of high-quality with few non-zeolitic pores.

High selectivities in defective MFI membranes

Two polycrystalline MFI membranes with significant flow through defects (non-zeolitic pores that are gaps between the crystals) are shown to have high ideal and mixture selectivities. The membranes were characterized at room temperature by permporosimetry, pervaporation, separations, and single-gas permeation. These measurements indicate that one membrane (B-ZSM-5) had a relatively large number of smaller defects, whereas the second membrane (silicalite-1) had a smaller number of somewhat larger defects. The relative contributions of these defects to the overall flux changed dramatically in the presence of n-alkanes and SF 6. These molecules caused adsorption-induced expansion of the crystals, and this expansion shrank the defect sizes and thus changed the membrane permeation characteristics. The B-ZSM-5 membrane had 90% of its helium flux through defects at room temperature, but it had a H 2 /SF 6 ideal selectivity as high as 260 because of SF 6 -induced swelling that stopped 99% of the flux through the defects. In contrast, the silicalite-1 membrane had only 9% of its helium flux through defects, but the defects were large enough that crystal swelling only decreased the flux through them by 30%. Thus its selectivities were lower. These studies show that n-hexane, n-pentane, n-butane, n-propane, and SF 6 swell MFI crystals when they adsorb, but benzene and CO 2 do not. The changes in membrane microstructure due to crystal expansion not only significantly affect membrane separation ability, but also have implications on how to select appropriate characterization techniques for evaluating MFI membrane quality.