Non-zeolitic Pores Research Articles

Characterizing Nonzeolitic Pores in MFI Membranes

Methods that use n-hexane (n-hexane permporosimetry and n-hexane/2,2-dimethylybutane (DMB) separation) are shown to not be effective for characterizing MFI zeolite membranes because n-hexane adsorption swells MFI crystals and shrinks the size of nonzeolitic pores. Measurements on a membrane in which 30% of its helium flux at 300 K was through nonzeolitic pores demonstrate that benzene permporosimetry and isooctane vapor permeation as a function of feed activity provide better characterizations. Isooctane condensed in nonzeolitic pores at high activities, and this was used to estimate the sizes of those pores. The average nonzeolitic pore size in this membrane decreased from approximately 3.0 to 1.5 nm as the temperature increased from 300 to 348 K, apparently due to thermal expansion of MFI crystals. Benzene permporosimetry yielded dramatically different results from n-hexane permporosimetry because benzene does not swell the MFI crystals significantly. Single-component pervaporation fluxes as a function ...

Influence of desorption conditions before gas separation studies in nanocomposite MFI–alumina membranes

The gas permeation and separation performance of polycrystalline MFI-type zeolite membranes is strongly dependent on the number and type of intercrystalline pores in its structure. Herein we show that the role of such domains is affected by how a membrane is pre-treated before use to remove adsorbed species (e.g. moisture and organics). This ‘pre-treatment’ step appears to be crucial not only to obtain reliable permeation data, but also to improve the membrane separation performance in practical applications. We illustrate this idea by using a collection of tubular nanocomposite MFI–alumina membranes showing different quality for the separation of n-butane/H 2 mixtures and submitted to different pre-treatment protocols. The influence of each protocol on the final separation performance of the membranes depends on their quality, namely on the density of intercrystalline defects or non-zeolite pores in their structure. Moreover the quality of the support affects the final membrane performance.

Effects of exposure to water and ethanol on silicalite-1 membranes

The effects of long exposures to ethanol, water and 0.1 M aqueous solutions of ammonia, sodium hydroxide, tetrapropylammonium hydroxide (TPAOH) and hydrochloric acid on thin TPA-silicalite-1 membranes were studied. Single gas permeation experiments, porosimetry and scanning electron microscopy were used to characterize the membranes. It was found that a short exposure (24 h) will only dissolve synthesis residues and will not affect membrane quality negatively. The only medium that had an effect after 24 h was sodium hydroxide, which almost dissolved the film completely. After exposing TPA-silicalite-1 membranes for 30 days in the various liquids, the membrane quality decreased in the order ethanol < 0.1 M hydrochloric acid < 0.1 M TPAOH < water < 0.1 M ammonia < 0.1 M sodium hydroxide due to dissolution of the silicalite-1 crystals. This study has shown that prolonged exposure to aqueous solutions will lead to dissolution of silicalite-1 crystals causing an increase in micro- and mesopores in the film. The amount and size of the pores will depend on the pH of the aqueous medium. Higher pH gives a higher dissolution and hence more non-zeolitic pores in the silicalite-1 film. Ethanol has no effect on the dissolution of the zeolite film even after 30 days. This finding has an effect in membrane preparation and in several membrane applications such as pervaporation and separation of hydrocarbons isomer mixtures.

Separation Behavior of Steam from Hydrogen and Methanol through Mordenite Membrane

A mordenite membrane was prepared on the outer surface of a porous α-alumina tubular support by a secondary-growth method. Permeation tests for water/methanol/hydrogen mixtures were performed at 423 and 473 K. A mordenite membrane free from non-zeolitic pores allowed us selective permeation of steam and blocked the permeation of hydrogen and methanol. In order to elucidate the mechanism of such selective permeation behavior, adsorption, permeation, and structural properties of the mordenite membrane were discussed. As a result, it was considered that selective permeation of steam against hydrogen was based on its preferential adsorption of steam, while boundaries between mordenite crystals in the membrane may play a key role for inhibiting the permeation of methanol.

Controlled growth for synthesizing a compact mordenite membrane

Controlling the growth of zeolite crystals on a porous alumina support is essential for preparing a compact zeolite membarne. First, mordenite seed crystals applied on a nonporous α-alumina disk were grown and morphological change of mordenite crystals were observed in the course of growth. Then, mordenite membranes were synthesized on a porous α-alumina tube under the same conditions employed in the study using the alumina disks. We found that seed crystal growth was widely controllable by changing water content in reaction solution, which resulted in better control of the morphology of mordenite crystals for synthesizing a thin compact mordenite membrane. Separation properties for mordenite membranes were studied in water–hydrogen binary system at 473 K with 10 kPa of water partial pressure, where no capillary condensation was expected in non-zeolitic pores. Separation factor for a mordenite membrane with a few defects was poor; however, a defect-free mordenite membrane prepared under a suitable condition highly separated steam from hydrogen.

Characterizing non-zeolitic pore volume in zeolite membranes by temperature-programmed desorption

Temperature-programmed desorption is shown to be an effective method to measure the volume of non-zeolitic pores in a MFI membrane. A molecule that was too large to adsorb into the MFI pores at room temperature and the experimental time scale, 2,2-dimethylbutane, was adsorbed in the non-zeolitic pores, and its liquid density was used to estimate their pore volume. When n-hexane or n-octane adsorbed in the MFI pores, the non-zeolitic pore volume decreased by 55%, whereas benzene adsorbed in the MFI pores had no effect. The non-zeolitic pore volume decreased because n-hexane and n-octane swell the MFI crystals of the membrane and decrease the non-zeolitic pore size (the space between the MFI crystals), but benzene does not.

Synthesis and characterization of ( h 0 h)-oriented silicalite-1 films on α-Al 2O 3 substrates

A simple and well-designed synthesis procedure is proposed to fabricate silicalite-1 films on porous α-Al 2O 3 substrates on purpose of preventing the aluminum leaching. The continuous and 2 μm thick seed layer of silicalite-1 crystals is fabricated by using a spin coater. The first-time seeded growth is performed to synthesize a thin layer of intergrown ZSM-5 crystals on the silicalite-1 seed layer, where the use of low alkalinity and short synthesis time is to reduce the aluminum leaching. The intergrown layer of ZSM-5 crystals serves as a barrier to block the aluminum leaching from porous α-Al 2O 3 substrates in the second-time seeded growth, leading to the formation of ca. 11 μm thick intergrown and oriented silicalite-1 films with an extremely high Si/Al ratio. According to SEM images and XRD measurements, the as-synthesized silicalite-1 film is dense, continuous, and (1 0 1)-oriented. The electron probe microanalysis (EPMA) of the resulting film demonstrates that there is no aluminum leaching in the second-time seeded growth. The leaking tests confirm that non-zeolitic pores in the silicalite-1 film are negligible.

2,2-Dimethylbutane adsorption and diffusion in MFI zeolite

Adsorption rates of 2,2-dimethylbutane (DMB) from the liquid phase into MFI zeolite crystals were measured by a volumetric/gravimetric method. Desorption rates in the presence of liquid n-hexane were measured by a non-adsorbing solvent (isooctane) method. Dimethylbutane desorbed, and was replaced by n-hexane, approximately three orders of magnitude faster than it adsorbed. Adsorption was slow and reached only 40% of saturation loading in 6-μm crystals after 45 days at 295 K, but this DMB loading desorbed into n-hexane in less than 1 h. Desorption may be faster due to slight expansion of the MFI unit cell by n-hexane adsorption. Although DMB diffused slower than n-hexane in MFI crystals, its transient time through a silicalite-1 membrane was an order of magnitude shorter than the n-hexane time at 313 K because DMB diffused through defects, whereas n-hexane diffused mainly through zeolite pores. The n-hexane transient time was longer because n-hexane: (1) expanded the MFI crystal size slightly and shrank non-zeolitic pores, and (2) adsorbed in the zeolite as it moved through the remaining non-zeolitic pores. A silicalite-1 membrane saturated with DMB at 423 K was sealed so effectively, and DMB desorbed slowly enough, that the helium permeation flux at 313 K was below the detection limit.

Preparation of hydrophilic silicalite-1 nanocrystal-layered membrane for separation of water from water–acetone solution by pervaporation

Hydrophilic silicalite-1 nanocrystal-layered membrane was successfully prepared on an alumina ceramic filter. The silicalite-1 nanocrystals were layered on the outer surface of cylindrical alumina ceramic filters, followed by a hydrothermal synthesis to form a silicalite-1 layer (protection layer) on the nanocrystal layer. In order to achieve high hydrophilic properties of the membrane with the remaining silanol group, the liquid phase oxidation technique was applied to remove the template. From the SEM observation, the nanocrystal layer and the protection layer were clearly observed, and the silicalite-1 protection layer was uniformly formed on the nanocrystal-layer. Pervaporation experiments to separate water from water–acetone solution (acetone: 90 wt%) were conducted using the silicalite-1 nanocrystal-layered membrane. The membrane exhibited the selective permeation as well as the high flux of water. In the nanocrystal-layered membrane, water–silanol networks were formed on the non-zeolitic pores surface among the nanocrystals, which were the main channels for water permeation. The increase in the number of pores among the silicalite-1 crystals leads to a high flux of water through the membrane.

A high reproducible fabrication method for industrial production of high flux NaA zeolite membrane

A high reproducible fabrication method for high-flux NaA zeolite membrane has been developed for industrial mass production. The synthesized NaA membranes exhibited higher water flux (5.6 kg m −2 h −1) by pervaporation (PV) experiments in a mixture of water (10 wt.%)/ethanol (90 wt.%) at 348 K compared with those of previous reported for the comparable water/alcohol separation factors, α (10 3–10 4). Vapor permeation (VP) experiments on saturated vapor with the above mixture up to 418 K and 930 kPa showed so high performance of flux of 31 kg m −2 h −1 with α > 10 4 that can be applied practically to such a hybrid system of distillation-vapor permeation. High reproducibility in the developed fabrication method for membranes was confirmed by permeation tests. Pore size distribution estimated by permporosimetry showed that an average pore size was 0.7 nm in Kelvin diameter and half of the gas flow permeate through nonzeolitic pores. The thickness of effective layer in the membrane was determined by a focused ion beam (FIB) preparation method to be ca. 4 μm that was thinner than those of previous studies. It could be suggested that the higher flux is attributed mainly to lower transporting resistance by the thin layer and existence of nonzeolitic pores in the membranes.

Flexible nanostructure of MFI zeolite membranes

Pervaporation and vapor permeation of hydrocarbons were used to characterize three MFI zeolite membranes that had significant fluxes through non-zeolitic pores. The single-component pervaporation fluxes of n-hexane were only 5–55% of the fluxes of larger molecules (2,2-dimethylbutane (DMB), isooctane, and 1,3,5-trimethylbenzene (TMB)) at ∼300 K, all of which did not adsorb in the MFI pores under the conditions used. In spite of this behavior, these membranes were selective for C 6 isomer separation, and the n-hexane/DMB vapor-phase separation factor was 3200 for one membrane at low hydrocarbon partial pressures. The pervaporation behavior was consistent with the explanation that MFI membrane structures are flexible; MFI crystals increase in size when n-hexane adsorbs, and this expansion dramatically decreases the non-zeolitic pore sizes. Thus, less than 0.8 mol% n-hexane reduced DMB and TMB pervaporation fluxes to ∼5% of their single-component fluxes for one membrane. At high vapor concentrations, molecules condensed in the non-zeolitic pores, which had almost no selectivity, and this capillary condensation decreased the n-hexane/DMB separation factor. Non-zeolitic pores were shown to increase in size after a membrane was calcined at 1000 K.

Influence of operation parameters on the separation of mixtures by pervaporation and vapor permeation with inorganic membranes. Part 1: Dehydration of solvents

The separation performance of two different commercially available tubular inorganic membranes was studied for solvent dehydration. The separation layers consisted of A-type zeolite and microporous silica. The membrane characteristics were determined as function of operating conditions such as feed composition, temperature, and permeate pressure in pervaporation and vapor permeation. Among different membranes of the same batch, flux and selectivity were reproducible within 10%. The partial flux of water as the preferentially permeating component increases linearly with the water vapor pressure difference between feed and permeate and depends only marginally (viscosity influence) upon the properties of the organic component. The flux of the organic (retained) component is low and can best be described by assuming a substance and membrane specific permeance (flux over partial pressure difference) that is independent of composition. At very low water concentration in the feed one would expect a strong increase in permeability of the retained component through non-zeolite pores and larger silica pores as predicted by pure component measurements. However, this effect was not observed in mixtures within the concentration range studied here. A temperature rise improves flux rates exponentially while selectivity remains high. Thus, higher module cost in comparison to polymeric membranes can be compensated by reduced membrane area if a higher operating temperature can be chosen. Flux and selectivity decline as a function of permeate pressure with decreasing driving force. In vapor permeation with inorganic membranes superheating of the vaporous feed improves their performance while for polymeric materials a steep flux decline is observed. High flux and selectivity are obtained in the separation of water from alcohols. The normalized flux values of the A-type zeolite membrane are roughly 10 kg/m 2 h bar with a mixture selectivity of 2000 for methanol, 4000 for ethanol and 8000 for n -butanol. The average permeance of the amorphous silica membrane lies above 12 kg/m 2 h bar with mixture selectivity of 50 for methanol, 500 for ethanol and 2000 for n -butanol. The separation mechanism is mainly based on adsorption and diffusion enhanced by shape selectivity and size exclusion in some cases. The transport characteristics could be described with a simple transport model based on normalized permeate fluxes. With regard to the operation stability of the membranes, no deterioration of the performance was observed for the A-type zeolite in solvent dehydration or in separation of water from reaction mixtures. The silica membrane showed an initial conditioning effect involving a rearrangement of Si–OH groups with an increase in selectivity and decrease in flux of about 30%. After a few hours the performance stabilized and remained constant during further operation.

Modification of Zeolite Membranes for H2Separation by Catalytic Cracking of Methyldiethoxysilane

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 unc...

SAPO-34 membranes for CO 2/CH 4 separation

Continuous SAPO-34 membranes with low concentrations of non-zeolite pores were synthesized by in situ crystallization on porous stainless steel and α-Al 2O 3 supports. These membranes separated CO 2 from CH 4 with selectivity as high as 67 at 297 K for a CO 2 permeance of 1.6 × 10 −7 mol/m 2 s Pa. The membranes are selective for CO 2 because CO 2 preferentially adsorbs, and it also diffuses faster than CH 4. The CO 2 and CH 4 heats of adsorption were measured on SAPO-34 crystals as 24 ± 0.8 and 16 ± 0.8 kJ/mol, respectively. Selectivity depended on temperature, feed pressure, and pressure drop. A maximum in CO 2 permeance was observed near 275 K for both the single gas and a CO 2/CH 4 mixture. The CO 2/CH 4 selectivity decreased as temperature increased from 253 to 473 K. As trans-membrane pressure drop increased, the CO 2 flux increased, but the CO 2 permeate concentration showed a maximum of 94.7 mol% for a 50/50 feed. Both permeance and selectivity were stable for at least 1 month.

Transport of C 6 isomers through ZSM-5 zeolite membranes

The separation of binary mixtures of hexane isomers using ZSM-5 zeolite membranes on porous stainless steel and alumina tubes was modeled for both vapor permeation and pervaporation. The fluxes and selectivities for single-component and mixture permeation of n-hexane, 3-methylpentane (3-MP), and 2,2-dimethylbutane (DMB) were described by combining two diffusion mechanisms. The flux through zeolite pores was modeled by Maxwell–Stefan diffusion without molecular interaction, whereas the flux through the grain boundaries between the crystals was modeled by activated Knudsen diffusion. Adsorption isotherms and diffusion coefficients for the zeolite pores were obtained from the literature. Two fitting parameters for the size and the amount of non-zeolite pores were included in the model to determine membrane quality. The influence of membrane support, membrane preparation, operation mode, and process parameters on the separation characteristics is illustrated with the transport model.

Properties and separation performance of Ge-ZSM-5 membranes

Germanium-substituted ZSM-5 zeolite membranes with various Si/Ge ratios were synthesized by in situ crystallization on porous tubular stainless steel and α-Al 2O 3 supports. For a Si/Ge ratio in the synthesis gel of 25, the Si/Ge ratio in the membrane was 41–82, depending on the support. When the Si/Ge ratio was 5, extra framework compounds that contained Ge formed. The fraction of permeation through non-zeolite pores increased as the Si/Ge ratio decreased. The Ge-ZSM-5 membranes were effective for separating acetone from water by pervaporation. The highest acetone/water selectivity of 330 (at 323 K) and the highest total flux of 0.95 kg/m 2 h (at 333 K) were obtained for a membrane with a Si/Ge ratio of 41. This membrane selectively removed alcohols, ketones, carboxylic acids, esters, and propionaldehyde from 5 wt.% aqueous solutions because the organics preferentially adsorbed. The C 3 organics showed higher organic/water selectivities than C 2 or C 4 organics with the same functional group. The organic/water separation selectivity increased as the liquid-phase fugacity of the organic in the feed increased.

Synthesis of silicalite tubular membranes by in situ crystallization

AbstractSilicalite membrane preparation conditions related to membrane separation properties were studied focusing on in situ crystallization to prepare highly selective silicalite membranes on porous tubular supports by a single hydrothermal treatment. The membranes morphologies prepared under typical synthesis conditions were characterized by scanning electron microscopy. The silicalite membrane, well‐oriented with c axis normal to the support surface, could be prepared by in situ crystallization and a seeding method. The membrane separation properties were controlled by synthesis conditions such as seeding, temperatures, supports, and silica sources. Under certain conditions, such factors as tube pore size, played an important role. Silicalite membranes would be prepared better by in situ crystallization than by a seeding method under the same conditions. For example, the highest ethanol/water separation factor of 106 with a flux of 0.9 kg/m2·h for a feed concentration of 5 wt. % ethanol at 60°C was obtained for the silicalite membrane prepared on an unseeded mullite tube at 175°C for 16 h with colloidal silica, whereas the membranes prepared on the silicalite‐seeded tubes had separation factors up to 70. The higher separation selectivity for ethanol over water would be attributed to reduction in nonzeolitic pores, supported by gas permeations of n‐butane and i‐butane.

Synthesis and Separation Performance of SSZ-13 Zeolite Membranes on Tubular Supports

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...

Separation of 1,3-propanediol from glycerol and glucose using a ZSM-5 zeolite membrane

A Na-ZSM-5 zeolite membrane was effective for separating 1,3-propanediol from glycerol and glucose in water by pervaporation. Binary, ternary, and quaternary mixtures were studied as a function of temperature. At 308 K, the total flux for the quaternary feed was 0.21 kg/m 2 h. The 1,3-propanediol/glycerol selectivity decreased from 54 to 21 over the temperature range 308–328 K, whereas for the same range 1,3-propanediol/glucose selectivity increased from 330 to 2100. The 1,3-propanediol/glycerol selectivity was controlled by both preferential adsorption and differences in diffusion rates. The 1,3-propanediol/glucose selectivity was mainly controlled by differences in diffusion rates, with the larger glucose diffusing through non-zeolite pores. Glycerol inhibited glucose permeation and thus increased the 1,3-propanediol/glucose selectivity. The presence of glucose decreased the total flux but increased the 1,3-propanediol/glycerol selectivity.

GAS PERMEATION PROPERTIES OF ION-EXCHANGED LTA-TYPE ZEOLITE MEMBRANES

NaA-type zeolite membranes were prepared on a 1.7-mm i.d. porous α-alumina tube by repeating a hydrothermal reaction. The membranes, which were prepared by four repetitions, were ion-exchanged with Ca2+ and K+, and the permeation for single-component and mixed gases were investigated at 35°C. The NaA-type membrane showed a hydrogen permeance of (0.8–2.6) × 10−8 mol · m−2 s−1 Pa−1 and a hydrogen/n-butane ideal separation factor of 12–15. The permeability properties of the membrane for binary mixtures were dependent on the size of the permeating molecules and their adsorptivity to the zeolite. The KA-type zeolite membrane showed a higher ideal separation factor for a hydrogen/nitrogen system than did the NaA- and CaA-type zeolite membranes. The pores of the CaA-type zeolite membrane were larger than those in the NaA- and KA-type zeolite membranes, and showed higher permeances to n-butane and i-butane. All the zeolite membranes contained non-zeolitic pores, which decreased the selectivity of permeation.