Silicalite-1 Zeolite Membrane Research Articles

Organic template residues confined in silicalite-1 zeolite membranes for tuning pore structures and boosting H2/CO2 separation

Silicalite-1 zeolite membranes were successfully activated at 300–350 °C in a hydrogen atmosphere. FTIR, TGA, and N2 adsorption-desorption characterizations confirmed that the amount of the organic structure directing agent (OSDA) residue in silicalite-1 zeolite channels was tunable. The H2-calcined membrane showed a lower gas permeance than that of the air-calcined membrane due to the partial pore blockage by the OSDA residue. The permeance of H2 and CO2 increased with increasing temperature, showing an activated diffusion-like mechanism. The activation energies for the permeation of H2 and CO2 were in the range of 1.63–4.15 and 0.76–5.90 kJ mol−1, respectively, which confirmed the reduced average membrane pore size and different pore surface chemistry due to the presence of the OSDA residue in the membrane. Significant interactions between CO2 molecules and OSDA residues confined in the membrane resulted in a notably lower CO2 permeance compared to N2 and CH4, despite CO2 having a smaller kinetic diameter. Enhanced H2/CO2 selectivity in the silicalite-1 zeolite membrane, featuring confined OSDA residue, was attributed to the reduction in membrane pore size and the fine-tuning of pore surface chemistry.

Long alkyl chain-containing organosilica/silicalite-1 composite membranes for alcohol recovery

A long alkyl chain-containing organosilica layer obtained from 1, 8-bis(triethoxysilyl)octane (BTESO) was used to modify silicalite-1 membranes via a sol−gel method to obtain bi-layered BTESO/silicalite-1 composite membranes on macroporous α-Al2O3 supports for the recovery of diluted ethanol by pervaporation. Silicalite-1 zeolite membranes were hydrothermally synthesized using two seeding methods: those prepared using wet−rubbing seeding method exhibited larger separation factors than those obtained using the dip−coating method in the pervaporation of ethanol/water due to the thinner seed layer in the former. After modified with BTESO, the surface became smoother and denser with higher hydrophobicity, and the thickness of BTESO layer was approximately 300 nm on both types of membranes. In pervaporation, two types of silicalite-1 membranes unexpectedly showed similar performances after BTESO modification. Ethanol flux increased or was maintained while water flux decreased greatly for silicalite-1 membranes after BTESO coating with increasing sol concentrations, and a higher separation factor was attained without sacrificing ethanol flux. The long alkyl chains in BTESO and silanols reacted with hydroxyl groups in silicalite-1 provided flexibility and hydrophobicity for ethanol transport with minor resistance. The optimized 2.0% BTESO/silicalite-1 membranes synthesized using wet−rubbing method showed a maximum flux of 6.84 kg m−2 h−1 and separation factor of 24.6 with overall pervaporation separation index of 161 in the pervaporation of 5.0 wt% ethanol/water mixture at 60 °C.

Characterization of defect-containing zeolite membranes by single gas permeation experiments before and after calcination

The synthesis of defect-free zeolite gas separation membranes is challenging. In addition to the desired transport by surface and Knudsen diffusion through the zeolite pores, the undesired transport by Knudsen and viscous flows through the defects are typically present to a different extent. This paper proposes a method to assess the extent of the undesirable defect-transport in zeolite membranes, which is essential for fine-tuning membrane synthesis protocols. Also, knowing the undesirable defect transport contribution allows determining the corrected diffusivity of gases in the zeolite crystals incorporated in the membrane. The latter is the intrinsic property of the zeolite. The method relies on gas permeation tests with not-adsorbed and adsorbed gases before and after the membrane's calcination. The adsorption isotherms of the gases with the zeolite crystals incorporated into the membrane are also required.We have demonstrated the proposed characterization method using silicalite-1 zeolite membranes prepared by a pore-plugging method inside porous TiO2 supports. The validity of the parameters, which characterize the crystal and defect transport, were verified by comparing the experimental and predicted permeances of He, N2, CO2 and CH4 at different pressures. Also, the calculated corrected diffusivity values of CO2 and CH4 in the silicalite crystals of 1.69⋅10−8m2⋅s−1 and 6.51⋅10−8m2⋅s−1, respectively, are comparable to the values reported in the literature.

Fischer-Tropsch reaction mixture permeation through a silicalite-1 membrane reactor and its effect on the produced hydrocarbons distribution

Abstract The quantification of the permeation of the Fischer-Tropsch reaction mixture through a silicalite-1 zeolite membrane in which is integrated in to fixed bed reactor was theoretically investigated. The approach is based on the prediction of the permeation parameters by using two different mechanisms including surface diffusion and gaseous diffusion. It was found that under our investigated conditions, the total permeation could be governed by surface diffusion model since the contribution of this mechanism is dominant versus the gaseous diffusion. Noteworthy, our results show that except for the selective gas permeation of carbon dioxide, the measuring factors of different permeates were proportional to the operating pressure. Hydrocarbons with low molecular weight diffuse greater than long-chain hydrocarbons. Furthermore, the high adsorbed molecules are more likely to be affected by the high processing temperature. It can be also highlighted that the permeate amounts has no important effect on the product distribution which is characterized by the olefins to paraffins ratios. So the assumption that considers the separation of CO 2 without assuming other components permeation is well supported.

Surface-gel-conversion synthesis of submicron-thick MFI zeolite membranes to expedite shape-selective separation of hexane isomers

Ultrathin zeolite membranes are of paramount importance in accelerating gas transport during membrane separation, and lowering down their membrane thicknesses to submicron scale is deemed to be very challenging. Herein, we develop an advanced approach of surface gel conversion for synthesis of submicron-thick pure silica MFI (silicalite-1) zeolite membranes. Viscous gel is prepared by finely adjusting the precursor composition, enabling its reduced wettability. The unfavorable wetting of the support surface can effectively prevent gel penetration into alumina support voids. Aided by the seeds, the surface gel is directly and fully crystallized into an MFI zeolite membrane with minimal water steam. A membrane with a thickness of 500 nm is successfully acquired and it is free of visible cracks. Additionally, the as-synthesized membranes exhibit rapid and selective separation of hexane isomers by virtue of unprecedentedly high n-hexane permeance of 24.5×10−7 mol m−2 s−1 Pa−1 and impressive separation factors of 13.3-22.6 for n-hexane over its isomers. This developed approach is of practical interest for sustainable synthesis of high-quality zeolite membranes.

Fast synthesis of thin Silicalite-1 zeolite membranes at low temperature

The preparation of zeolite membranes usually involves long hydrothermal synthesis at elevated temperature, which leads to issues like lengthy synthesis, high energy consumption, safety concern etc. It is desirable to prepare zeolite membranes at low temperature with short synthesis time. In this study, thin Silicalite-1 zeolite membranes were prepared in 4–6 h by conventional hydrothermal synthesis at ~373K. The use of ammonium hexafluorosilicate ((NH4)2SiF6), a highly reactive silica source, is the key to the ultrafast crystallization kinetics even at low temperature, which greatly increased productivity of membrane synthesis. The crystal growth rates of Silicalite-1 crystals and membranes were at least 10 times faster than using regular silica source. Hexane permporometry indicated the presence of low defect density in the membrane. High n-butane permeance of 4.3 × 10−7 mol m-2 s-1·Pa-1 and decent n-butane/i-butane selectivity of 17 were obtained on the Silicalite-1 membranes, which is comparable to the leading Silicalite-1 membranes. The fast synthesis of high performance Silicalite-1 membranes at benign condition greatly simplified membrane preparation and lowered membrane cost. This approach might be used for the synthesis of other types of all-silica zeolite membranes.

Silicalite-1 zeolite membrane: Synthesis by seed method and application in organics removal

Silicalite-1 (S-1) zeolite membrane synthesized by seed method with superior features attracts intensive attentions, while the influences of key parameters during synthesis process and its applications for organics removal require further investigation. This study revealed the morphology and the structure of the prepared membranes under different crystallization temperatures and seed concentrations by using a suite of characterization methods. The as-prepared membrane under optimal condition (crystallization temperature of 175 °C and seed concentration of 1.0 wt. %) possessed high membrane integrity, with ideal separation factor of 4.0. It also exhibited outstanding performance for organics removal, with dyes retention of 99.9% and 99.2% for 500 mg L−1 neutral red and 500 mg L−1 methyl blue, respectively. Excellent antifouling property of the synthesized membrane was also proved. Results of this work can guide the characteristic improvement of the S-1 zeolite membrane by adjusting key parameters and broaden its applications in dye wastewater treatment.

Effect of Si/Al ratio and amount of deposited MFI-type seed crystals on the separation performance of silicalite-1 membranes for ethanol/water mixtures in the presence of succinic acid

Silicalite-1 zeolite membranes were prepared on porous tubular α-Al2O3 supports by a secondary growth method. The influences of the Si/Al ratio and the amount of deposited MFI (silicalite-1 and ZSM-5) seed crystals on membrane pervaporation (PV) performance for the separation of ethanol/water (binary) and ethanol/water/succinic acid (ternary) mixtures were investigated. The prepared membranes were characterized by X-ray diffraction and scanning electron microscopy, and the Si and Al distributions in the membrane layers were determined by energy-dispersive X-ray line scanning. Each silicalite-1 membrane was composed of two layers: an MFI-seed-crystal layer that grew on the support surface, and another gel-derived silicalite-1 top layer that formed on the seed crystal layer. The texture of the MFI seed layer (i.e., the deposited amount and the Si/Al ratio) was found to play an important role in the separation process. When silicalite-1 seed crystals were used, the ethanol separation factor was high in the binary ethanol/water system, but lower in the presence of succinic acid. However, when ZSM-5 seeds were used, the ethanol separation factor in the ternary system improved. These results suggest that silicalite-1 membranes prepared using ZSM-5 seed crystals are effective for the recovery of ethanol in the presence of organic acids.

High-performance silicalite-1 membranes on porous tubular silica supports for separation of ethanol/water mixtures

Abstract Production of bioethanol, a renewable energy source, is limited by the costly, energy-consuming concentration processes involved. Membrane separation techniques have emerged as promising alternatives to overcome these limitations. In this study, silicalite-1 zeolite membranes with high pervaporation (PV) performance were prepared on porous tubular silica supports by a secondary growth method. The effects of the deposited seed amount on the support and synthesis parameters, namely, the tetra- n -propylammonium concentration (TPA + /Si), alkalinity (OH − /Si), and crystallization time, were investigated. A dense zeolite layer was formed between the typical columnar crystal layer of silicalite-1 membranes and the silica support. The PV performance was mainly dependent on the thickness of the dense layer: as the thickness increased, the separation factor increased. The silicalite-1 membrane prepared using a synthesis solution with a molar composition of 1SiO 2 :0.05TPABr:0.08NaOH:75H 2 O at 433 K for 8 h exhibited the highest separation performance with a separation factor of 92 and a flux of 3.00 kg m −2 h −1 for 10 wt% ethanol/water mixtures at 323 K, which is the best result reported to date for tubular supports. Thus, silica supports proved to be suitable for the preparation of high-performance silicalite-1 membranes for ethanol/water mixture separation.

Preparation, Characterization and Catalytic Performance of Supported Titanium Silicalite-1 Zeolite Membrane Catalyst

Preparation, Characterization and Catalytic Performance of Supported Titanium Silicalite-1 Zeolite Membrane Catalyst

Development of dual-membrane coated Fe/SiO2 catalyst for efficient synthesis of isoparaffins directly from syngas

A novel dual-membrane coated catalyst (Fe/SiO2-S-Z) with Fe/SiO2 core and Silicalite-1 and H-ZSM-5 zeolite membranes was developed and employed for isoparaffins direct synthesis from syngas via Fischer–Tropsch synthesis (FTS) reaction. The Silicalite-1 zeolite membrane, as a structural membrane, was prepared under a close-to-neutral synthesis condition, which avoided the structure destruction of the core catalyst Fe/SiO2 and favored the in-situ growth of H-ZSM-5 zeolite membrane. In H-ZSM-5 zeolite membrane, as a functional membrane, the micropores and strong acidic sites were preserved in the dual-membrane catalyst, which played important roles in the isoparaffins direct synthesis. Activity tests indicated that the dual-membrane catalyst exhibited an excellent performance for isoparaffins direct synthesis. The isoparaffins selectivity of the dual-membrane catalyst was up to 29.8%, much higher than those of individual core catalyst (12.9%) and physical mixture catalyst of H-ZSM-5 zeolite and Fe/SiO2 catalyst (16.6%). It was found for the first time that the hydrogenation and isomerization of olefins over the dual-membrane catalyst were the main reasons for the high isoparaffins selectivity. In addition, due to the spatial confinement effect of the microporous zeolite membranes, the hydrocracking and isomerization of heavy hydrocarbons (C12+) also contributed to the high selectivity of isoparaffins.

Effect of gas adsorption on acoustic wave propagation in MFI zeolite membrane materials: experiment and molecular simulation.

The present study reports on the development of a characterization method of porous membrane materials which consists of considering their acoustic properties upon gas adsorption. Using acoustic microscopy experiments and atomistic molecular simulations for helium adsorbed in a silicalite-1 zeolite membrane layer, we showed that acoustic wave propagation could be used, in principle, for controlling the membranes operando. Molecular simulations, which were found to fit experimental data, showed that the compressional modulus of the composite system consisting of silicalite-1 with adsorbed He increases linearly with the He adsorbed amount while its shear modulus remains constant in a large range of applied pressures. These results suggest that the longitudinal and Rayleigh wave velocities (VL and VR) depend on the He adsorbed amount whereas the transverse wave velocity VT remains constant.

Separation of CO2 and N2 with a lithium-modified silicalite-1 zeolite membrane

Carbon dioxide capture and storage is gaining attention as a strategy to abate greenhouse gas emissions. However, currently available commercial processes to remove CO2 from flue gas streams, mainly from N2 are costly and energy intensive. A silicalite-1 membrane was prepared on a porous alumina support using hydrothermal synthesis. The resulting silicalite-1 membranes were modified using Li solutions in order to achieve better CO2 separation. SEM micrographs exhibited a thin, uniform and compact silicalite-1 film well adhered to the alumina support. Crystallinity of the membrane was confirmed by XRD. CO2 and N2 permeation experiments were carried out in a stainless steel module. CO2 single gas permeation was measured through a silicalite-1 membrane from room temperature to 400°C, having a 25psi pressure on the feed side. The resulting CO2 permeance measurements indicated molecular sieving as the principal transport mechanism for the unmodified silicalite-1 membrane, whereas surface diffusion is the principal transport mechanism for lithium-modified silicalite-1 membranes. The separation between CO2 and N2 takes place due to preferential CO2 adsorption and diffusion, which in turn hinders N2 permeation through the silicalite-1 pore network. A CO2/N2 separation factor up to 6 at 25psi and 400°C was obtained using the silicalite-1 modified membrane, whereas this was 1.46 using the unmodified silicalite-1 membrane.

Predicting gas flux in silicalite-1 zeolite membrane using artificial neural networks

In this paper, artificial neural network (ANN) as a powerful tool for solving complicated problems is used to predict gas flux through silicalite-1 zeolite membrane. Network training was fulfilled using a collected database of the practiced operation including gas flux under various operating conditions (e.g. feed pressure and operating temperature) with different kinetic diameter of the permeating species (e.g. CO2, O2, N2 and CH4). Trying various types of the networks, a network with one hidden layer including 5 neurons was found to be optimum. Performance of the ANN model was compared with statistical analysis using datasets that were kept apart from the original database. The results showed that there is an excellent agreement between the experimental data and the predicted values, with high correlation (R2 = 0.9952) and less error (RMSE = 8.9E−4). In addition, sensitivity analysis revealed that the input feed pressure is the most sensitive parameter on the output gas flux.

Hydroformylation of 1-Hexene on Silicalite-1 Zeolite Membrane Coated Pd–Co/A.C. Catalyst

A core–shell-like encapsulated catalyst was designed where Pd–Co/activated carbon (A.C.) catalyst was enwrapped by silicalite-1 zeolite membrane, which greatly improved normal to iso ratio of aldehyde products during hydroformylation of 1-hexene with syngas (CO + H2), due to its spatial confined structure and shape selective property derived from zeolite membrane.

Silicalite-1 zeolite membranes on unmodified and modified surfaces of ceramic supports: A comparative study

Silicalite-1 zeolite membranes were prepared hydrothermally on the porous ceramic supports, both unmodified and modified with 3-aminopropyl triethoxysilane (APTES) as a coupling agent following ex situ (secondary) crystal growth process. The microstructure of the membranes was examined by scanning electron microscopy (SEM). The permeation study with a single gas, nitrogen (N2) was performed through the membranes. For the surface modified support, a more surface coverage of the seed crystals on the porous support was observed resulting in a relatively higher dense packing of the crystals during secondary crystal growth process compared to that obtained from the unmodified support. The membrane developed on surface modified support rendered lower permeance value i.e. 9 × 10−7 mol m−2 s−1 Pa−1 of N2 compared to that formed on the unmodified support which gave permeance value of 20 × 10−7 mol m−2 s−1 Pa−1 of N2.

Synthesis and property of silicalite-1 membranes by restricting growth method with dilute solution

A specific synthesis mode, based on precursor diffusion through the support channels, the restricting growth method (RGM) with ultra-dilute precursor has been designed to prepare silicalite-1 zeolite membranes on seeded macroporous α-Al 2O 3 tubular supports. Several parameters were systematically investigated to evaluate their influence on performance of the membranes, including the H 2O/SiO 2 ratio, template type and crystallization temperature. X-ray diffraction (XRD), scanning electron microscopy (SEM) and single-gas permeation tests were used to characterize the as-prepared membranes. Results showed the membrane prepared under optimum parameters (H 2O/SiO 2 ratio of 800, TPAOH as template and crystallization at 443 K) was continuous, dense and with a controllable section thickness almost the same as that of the seed layer. It displayed preferable hydrogen permeance (1.2 × 10 −6 mol m −2 s −1 Pa −1) and permselectivity (α H2/SF6 = 134) at 298 K under 0.1 MPa pressure drop. Moreover, due to the utilization of ultra-dilute precursor, this technique has the advantage of reduced chemical consumption. On the contrary, the uncompacted membranes with poor permeation behavior were obtained by the conventional secondary growth method under the same synthesis parameters.

Pervaporation characteristics of trichlorinated organic compounds through Silicalite-1 zeolite membrane

Silicalite-1 membrane synthesized on the inside of a porous stainless steel tube was used for pervaporation of trichloromethane, 1,1,2-trichloroethane and trichloroethylene from their aqueous solutions. Fluxes of trichloromethane, 1,1,2-trichloroethane and trichloroethylene were observed to be 3.0∼39.8 g/m 2/h, 0.3∼4.6 g/m 2/h and 0.1∼2.0 g/m 2/h while their separation factors were 16.4∼29.6, 4.2∼10.3 and 5.7∼18.1, respectively depending on the mole fraction of a trichlorinated organic compound in the feed solution and the operating temperature. The silicalite-1 membrane showed the best separation ability for trichloromethane, since trichloromethane has the smallest kinetic diameter and the highest fugacity among the trichlorinated organic compounds.

Preparation of alkali-resistant, Sil-1 encapsulated nickel catalysts for direct internal reforming-molten carbonate fuel cell

Abstract A thin layer of silicalite-1 zeolite membrane was grown on the surface of Ni/SiO 2 and Ni/Al 2 O 3 catalyst beads after seeding and secondary regrowth to create core–shell catalysts that are resistant to alkali poisoning from direct internal reforming-molten carbonate fuel cell (DIR-MCFC). The zeolite shell thickness was optimized to prevent poisoning and minimize diffusion resistance. An out-of-cell test was designed to simulate the fuel cell operating conditions, which showed that the new core–shell catalysts maintained a high activity similar to the original fresh catalyst in spite of the exposure to alkali vapor at high temperature. The conventional Ni/SiO 2 and Ni/Al 2 O 3 catalysts suffered higher than 80% decrease in activity for steam reforming of methane reaction (SRM).

Silicalite-1 Zeolite Membrane Reactor Packed with HZSM-5 Catalyst for meta-Xylene Isomerization

A silicalite-1/α-Al2O3 zeolite membrane with high para-xylene (PX) selectivity was tested for isomerization of meta-xylene (MX) to PX. Two HZSM-5 catalyst packing methods were compared in the membrane reactor: (1) catalyst in contact with the Al2O3 substrate and (2) catalyst in contact with the membrane layer. The second packing method exhibited higher efficiency for MX isomerization. Compared with a conventional reactor, the membrane reactor with the second catalyst packing method had increased rates of 26.0% and 31.2% for the PX yield and selectivity, respectively, at a temperature of 270 °C. PX selectivity and yield increased as the flow rate of the sweep gas increased because of the enhanced PX flux through the zeolite membrane. Increasing the gas hourly space velocity (GHSV) could reduce the contribution of membrane permeation to PX productivity. It was also revealed that MFI zeolite membranes exhibit a high stability for xylene isomerization.