SAPO-34 Zeolite Membranes Research Articles

Highly efficient and stable hydrogen permeable membrane reactor for propane dehydrogenation

The synthesis of SAPO-34 hollow fiber membranes employs the ball-milling assistant method to enhance membrane yield and quality. These membranes are then combined with CrOx/Al2O3 catalyst to create a packed bed membrane reactor (PBMR) for propane dehydrogenation (PDH). The study investigates the impact of operating temperatures, weight hourly space velocities (WHSV), and sweep gas flow rates on the SAPO-34 zeolite membrane reactor's performance, comparing it to a traditional packed bed reactor (PBR). The removal of hydrogen from the reaction side significantly improves propane conversion, surpassing thermodynamic equilibrium. In PBMR, propylene selectivity slightly exceeds that in PBR, influenced by side reactions like hydrogenolysis and cracking after hydrogen removal. Furthermore, the SAPO-34 zeolite membrane reactor undergoes a prolonged stability test with frequent regeneration using pure propane as feedstock to simulate industrial conditions. It exhibits consistent performance over 120 regeneration cycles within 240 h, maintaining similar H2 permeability and H2/C3H8 selectivity as the fresh membrane post long-term testing.

Synthesis of SAPO-34 Zeolite Membrane: Influence of Sources of Silica

The research described the production and characterization of various materials, particularly alpha-alumina ceramic supports, zeolite SAPO-34, and zeolite membranes. Ceramic supports were manufactured through dry uniaxial compaction. Sintering of the supports was carried out at 1300°C for 2 h. SAPO-34 zeolites and zeolite membranes were synthesized through a hydrothermal process involving two steps: a first step at 38°C for 24 h and a second step at 200°C for 24 h. The research aimed to determine how different silica sources, namely Aerosil 380, colloidal silica, and TEOS, influenced the outcome of the synthesis. The study identified that Aerosil 380 silica was the most suitable source for synthesizing SAPO-34 zeolites and membranes. Zeolite membranes (SAPO-34/alpha-alumina) displayed a uniform and homogeneous distribution of SAPO-34 phase zeolitic crystals. The absence of defects or cracks in these membranes confirmed the successful formation of the SAPO-34 zeolite membrane structure. This research has significant implications, particularly in materials science and applications utilizing zeolites and membranes. The choice of silica source plays a crucial role in determining the quality and properties of the synthesized materials, and the detailed characterization provides valuable insights into their performance in practical applications. Overall, the research contributes to the understanding and optimization of zeolite synthesis processes.

Mild ultraviolet detemplation of SAPO-34 zeolite membranes toward pore structure control and highly selective gas separation

SAPO-34 zeolite membranes were activated by ultraviolet (UV) radiation at low temperatures to reduce grain boundary defects. The successful detemplation of SAPO-34 zeolite powders using UV radiation was confirmed by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), and nitrogen adsorption-desorption. The irradiance exerted a significant impact on the quality of UV-detemplated SAPO-34 membranes. Under optimal UV detemplation conditions, SAPO-34 membranes showed a moderate H2 permeance of ∼10−7 mol Pa−1 m−2 s−1, with a H2/i-C4H10 permselectivity over 75 under room temperature. The membranes showed an exceptionally low CO2 permeance in comparison with N2 and CH4 despite the smaller molecular size of CO2, resulting in an unprecedentedly high H2/CO2 permselectivity over 50, which was ascribed mainly to the intense interaction between the CO2 molecules and confined organic structure directing agent (OSDA) residue in membrane channels. The temperature dependence of singe-gas permeance through SAPO-34 zeolite membranes showed that gases, including He, H2, CO2, CH4, C2H6, C3H8, and i-C4H10, markedly increased as the temperature increased, indicating an activated diffusion mechanism. The activation energies for H2 permeation through the SAPO-34 membranes were 17–23 kJ mol−1, which were reasonable based on the correlation between the activation energy for H2 permeation and the average pore size for reported membranes by assuming minimal grain boundary defects in the UV-detemplated SAPO-34 membranes.

Enrichment of spent SF6 gas by zeolite membranes for direct reuse in gas-insulated switchgear units

Zeolite membranes have attracted considerable attention in the separation industry owing to their high separation selectivity and permeance. However, their industrial applications have been hindered owing to a lack of case studies focusing on their commercialization. Herein, MFI and SAPO-34 zeolite membranes were employed to purify spent SF6 gas taken from a gas-insulated switchgear (GIS). This study demonstrated that separation by a single zeolite membrane produced ultrapure SF6 gas (>99.7 %) satisfying the reuse guidelines for GIS units. This level of purity is difficult to achieve using polymeric membranes due to the plasticization effect. The synthesized MFI membrane exhibited high N2 permeance with ∼ 2,000 gas permeation units (GPU) at 2 bar and low N2/SF6 selectivity (<50), whereas the SAPO-34 membranes exhibited the opposite trends, i.e., high N2/SF6 selectivity (>500) and relatively low permeance (<600 GPU at 2 bar). Interestingly, the MFI membrane system effectively enriched the SF6 gas, while lower feed pressures achieved by decreasing the feed flows were required for the SAPO-34 membranes to achieve the same purity. Changes in the feed pressure led to corresponding changes in the membrane separation performance, and the concentration polarization increased at high feed pressures. A MFI membrane area of ∼ 0.088 m2 was sufficient to purify the entire spent SF6 gas produced by GIS units at 145 kV in 6 h; this indicates an opportunity to realize compact SF6 enrichment systems. Thus, considering the high separation performance and the small size of the separation system, MFI membranes represent a promising option for the industrial purification of spent SF6 gas from GIS units.

Recent Progress of SAPO-34 Zeolite Membranes for CO2 Separation: A Review.

In the zeolite family, the silicoaluminophosphate (SAPO)-34 zeolite has a unique chemical structure, distinctive pore size, adsorption characteristics, as well as chemical and thermal stability, and recently, has attracted much research attention. Increasing global carbon dioxide (CO2) emissions pose a serious environmental threat to humans, animals, plants, and the entire environment. This mini-review summarizes the role of SAPO-34 zeolite membranes, including mixed matrix membranes (MMMs) and pure SAPO-34 membranes in CO2 separation. Specifically, this paper summarizes significant developments in SAPO-34 membranes for CO2 removal from air and natural gas. Consideration is given to a variety of successes in SAPO-34 membranes, and future ideas are described in detail to foresee how SAPO-34 could be employed to mitigate greenhouse gas emissions. We hope that this study will serve as a detailed guide to the use of SAPO-34 membranes in industrial CO2 separation.

H2 permeation and its influence on gases through a SAPO-34 zeolite membrane

This work analysed the permeation of binary and ternary H2-containing mixtures through a SAPO-34 membrane, aiming at investigating how hydrogen influences and its permeation is influenced by the presence of the other gaseous species, such as CO2 and CH4. We considered the behaviour of various gas mixtures in terms of permeability and selectivity at various temperatures (25–300 °C), feed pressures (400–1000 kPa) and compositions by means of an already validated mass transport model, which is based on surface and gas translation diffusion. We found that the presence of CO2 and CH4 in the H2-containing mixtures influences in a similar way the H2 permeation, reducing its permeability of about 80% compared to the single-gas value because of their stronger adsorption. On the other hand, H2 promotes the permeation of CO2 and CH4, causing an increment of their permeability with respect to those as single gases. These combined effects reflected in interesting selectivity values in binary mixture (e.g., CO2/H2 about 11 at 25 °C, H2/CH4 about 9 at 180 °C), which showed the potential of SAPO-34 membranes in treating of H2-containing mixtures.

Thin SAPO-34 zeolite membranes prepared by ball-milled seeds

• Small seeds led to thinner SAPO-34 membranes due to enhanced secondary nucleation. • Ball-milling generated nano seed particles easily. • Ultrafine seed particles produced by ball-milling resulted into thinner membrane. • Outstanding CO 2 -CH 4 separation performance realized on 1.6 μm thick SAPO-34 membrane. Zeolite membranes demonstrated great separation performance for many important separation processes, however, high membrane cost is a major obstacle for its commercial application. Reducing membrane thickness is an effective way of lowering its unit cost, which requires a delicate control of nucleation and crystallization processes. Herein, the effect of seed size on the separation performance of SAPO-34 membranes was studied. Smaller seeds facilitated secondary nucleation which led to thinner membrane with higher CO 2 permeance. Ball-milling of regular SAPO-34 crystals can generate ultra-small seed particles easily, which is even better than tailor-made nano SAPO-34 seeds on reducing membrane thickness. Thin SAPO-34 membrane with thickness ~1.4 μm can be prepared with high reproducibility and high CO 2 permeance ~10.2 × 10 −7 mol·m −2 ·s −1 ·Pa −1 and CO 2 -CH 4 selectivity of 59 were obtained for the separation of equalmolar CO 2 -CH 4 mixture at 4.0 MPa and room temperature.

A mini-review on recent developments in SAPO-34 zeolite membranes and membrane reactors

Schematic diagram of a SAPO-34 membrane for various gas separation.

Synthesis of thin SAPO-34 zeolite membranes in concentrated gel

Synthesis of thin zeolite membranes in the sub-micron range is desirable but a great challenge. Despite the successful synthesis of ultrathin Silicalite-1 and CHA membranes, the pursuit of SAPO-34 membranes thinner than 1 μm remains difficult. Herein, thin SAPO-34 membranes were prepared in concentrated gel by conventional hydrothermal synthesis. Concentrated membrane gel (TEAOH/Al2O3 = 4 and H2O/Al2O3 = 10) enhanced nucleation and facilitated the formation of small SAPO-34 crystals and therefore thin membranes. High TEAOH/Al2O3 ratio and special mixing sequence resulted into a homogeneous membrane gel with low viscosity which is beneficial for membrane synthesis with high reproducibility. Al(OH)3 as Al source led to formation of cubic SAPO-34 crystals which facilitated crystal intergrowth and reduced defects. These membranes (∼1.2 μm thick) exhibited high CO2 permeance of 3.9 × 10−6 mol/(m2 s Pa) and CO2–CH4 selectivity of 152 for equimolar CO2–CH4 mixture at room temperature and 0.14 MPa pressure drop. Further optimization of aging treatment or limiting the excessive membrane growth by a two-step hydrothermal synthesis + dry gel conversion method, the membrane thickness was successfully reduced to ∼0.9 μm, which increased the CO2 permeance to ∼5.0 × 10−6 mol/(m2 s Pa) without sacrificing CO2–CH4 selectivity.

A new seeding method, dry rolling applied to synthesize SAPO-34 zeolite membrane for nitrogen/methane separation

A new seeding method for the secondary growth of zeolite membrane, dry rolling, was applied to synthesize SAPO-34 zeolite membrane. In dry rolling, seeds are introduced on the outer surface of tubular support by mechanically rolling support in a cylindrical container with dry seed particles. During the process, plate-like SAPO-34 seeds fractured into smaller particulates which were uniformly distributed on the support and partially infiltrated into the pores of the support. The low speed and long seeding provided a more homogeneous seed layer due to uniform distribution of fractured tiny seeds and no mechanical damage of support. The homogeneous seed coating reliably developed SAPO-34 separation layer with better performance during the secondary growth process compared with conventional manual rubbing. The membrane synthesized at an optimum condition showed N2/CH4 permselectivity of 4.38 and N2 and CH4 permeance of 399 and 91 GPU, respectively. The performance was comparable or better than generally reported single gas performance, but the performance reliability was much better. Therefore, it could be concluded that dry rolling can be one of the reliable seeding processes for the preparation of zeolite membranes.

Mutual influence in permeation of CO2-containing mixtures through a SAPO-34 membrane

The presence of adsorbed CO2 significantly affects the permeation of weakly adsorbed species in zeolite membranes. At the same time, CO2 is itself influenced by the presence of another component, deviating its permeation with respect to the single-gas condition. In this work, we investigated the mutual interaction between CO2 and the other permeating gases through SAPO-34 zeolite membranes for several binary mixtures with H2, N2 or CH4, varying temperature (25–100 °C), feed pressure (200–1000 kPa) and CO2 composition (0.05–0.95). We used a validated model (Zito et al., J. Membr. Sci. 564, 2018, 166–173) considering the competition between surface and gas translation diffusion. CO2 permeance is found higher in mixture with H2 in the whole range of operating conditions investigated, whereas CH4 slows down CO2 permeation. SAPO-34 shows a selectivity in mixture quite higher from the one evaluated from the permeance of single gas. The choice of appropriate operating conditions allows high permeance and selectivity, making SAPO-34 particularly suitable for CO2 separation.

Hydrophobic modification of SAPO-34 membranes for improvement of stability under wet condition

Abstract SAPO-34 zeolite membranes show high efficiency for CO2/CH4 separation but suffer from the reduction of separation performance when exposed to humid atmosphere. In this work, n-dodecyltrimethoxysilane (DTMS) was used to modify the hollow fibers supported SAPO-34 membranes to increase the external surface hydrophobicity and thus sustain their performance under moisture environment. The modified membranes were fully characterized. Their separation performance was extensively investigated in both dry and wet gaseous systems and compared with the un-modified ones. The un-modified SAPO-34 membrane exhibited a high separation selectivity of 160 and CO2 permeance of 1.18 × 10−6 mol·m−2·s−1·Pa−1 for separation of dry CO2/CH4 at 298 K. However, its separation selectivity declined to 0.9 and the CO2 permeance was only about 1.7 × 10−8 mol·m−2·s−1·Pa−1 for wet CO2/CH4 at same temperature. High temperature (e.g. 353 K) could reduce the effect of moisture to improve SAPO-34 separation selectivity, but further increasing temperature (e.g. 373 K) led to decrease in CO2/CH4 separation selectivity. A significant decrease of selectivity was observed at higher pressure drop. The modified SAPO-34 membrane showed decreased CO2 permeance but increased separation selectivity for dry CO2/CH4 gas mixture, and super performance for wet CO2/CH4 gas mixture due to the improved hydrophobicity of membrane surface. A separation selectivity of 65 and CO2 permeance of 4.73 × 10−8 mol·m−2·s−1·Pa−1 for wet CO2/CH4 mixture can be observed at 353 K with a pressure drop of 0.4 MPa. Furthermore, the modified membrane exhibited stable separation performance during the 120-hour test for wet CO2/CH4 mixture at 353 K. The hydrophobic modification paves a way for SAPO-34 membranes in real applications.

Vapor separation of methanol-dimethyl carbonate mixture on SAPO-34 zeolite membrane

The separation of methanol-dimethyl carbonate (DMC) is an important but challenging process. Polymer membranes suffered from poor separation performance and stability. High performance SAPO-34 zeolite membrane was prepared on porous α-alumina supports with oil-bath heating method. Vapor separation of methanol-DMC mixtures on SAPO-34 membranes was systematically studied, including pressure drop, feed composition, temperature, flow rate, membrane degradation etc. SAPO-34 membranes demonstrated outstanding vapor separation performance, with methanol flux ~ 15 kg/(m2 h) and methanol/DMC separation factor ~ 1000 at 403 K and 0.6 MPa pressure drop for 90methanol:10DMCwt% feed. SAPO-34 membranes exhibited great stability in ~ 20 d continuous test. Methanol flux decreased substantially with time but it can be recovered to 80% of original value by a simple methanol rinse. Methanol/DMC separation factor exhibited significant increase with time, which can be attributed to the defect blocking by the chemical bonding of DMC in the defects, as verified by XPS and FT-IR analysis. Too much DMC adsorption on crystal surface hindered the free permeation of methanol. The methanol adsorption induced expansion of SAPO-34 crystals also helped shrinking defects effectively, which contributed to high separation factor. The addition of 1% water in the feed had little impact on separation performance. Compared with polymer membranes and other inorganic membranes, SAPO-34 membranes exhibited significantly improved flux and separation factor for the vapor separation of methanol-DMC mixture.

Optimized rapid thermal processing for the template removal of SAPO-34 zeolite membranes

The synthesis of defect-free zeolite membrane remains a big challenge due to the fact that thermal expansion mismatch between the zeolite film and the porous support can generate thermal stress and cracks during template removal process. Rapid thermal processing (RTP) for the template removal of SAPO-34 zeolite membrane was systematically studied and an optimized procedure was provided. A short RTP at 973 K led to substantially increased selectivity and permeance. The higher selectivity is the result of bonding between zeolite crystals generated during the rapid heating, which reduced cracking during the flowing slow calcinations. Moreover, RTP also facilitated the decomposition of template molecules which resulted into higher permeance. Long exposure at high temperature and thermal cycling may lead to the formation of defects in SAPO-34 membranes. The integration of RTP and conventional calcination, using a 1 min RTP at 973 K, fast cooling to 673 K, then 4 h calcination and slow cooling to room temperature, led to significantly improved selectivity and permeance. This optimized template removal procedure inherited the advantage of RTP treatment and also reduced the exposure time at high temperature, thus led to membranes with both high permeance and selectivity. Permporometry analysis confirmed the reduced defect size and area caused by RTP treatment. The best membrane exhibited CO2 permeance of 6.1 × 10−7 mol/(m2 s Pa) and CO2/CH4 selectivity of 88 at 4.0 MPa pressure drop, which were improved by 49% and 80% respectively, compared with membranes calcined by conventional template removal procedure.

Ultrafast synthesis of thin SAPO-34 zeolite membrane by oil-bath heating

The lengthy synthesis time is a major contributor to zeolite membrane's overall cost, which limits its commercial applications. Ultrafast synthesis of SAPO-34 zeolite membrane was realized by oil-bath heating. The synthesis time was significantly shortened from 5∼8 h to 1 h and membrane thickness was reduced from 3∼5 μm to 0.8 μm. This is by far the fastest synthesis method and the thinnest SAPO-34 membrane available. Oil-bath heating eliminated the thermal-lag associated with regular oven heating, which led to enhanced nucleation and crystallization. The obtained SAPO-34 membranes exhibited superior high pressure CO2CH4 separation performance. The oil-bath heating method might serve as a cheap alternative for the expensive microwave system, which inherits all benefits of microwave heating without adding extra cost. This method might be used for the synthesis of other types of zeolite membranes for different separation applications.

Thin Hydrogen-Selective SAPO-34 Zeolite Membranes for Enhanced Conversion and Selectivity in Propane Dehydrogenation Membrane Reactors

We demonstrate significantly enhanced conversion and selectivity in high-temperature propane dehydrogenation (PDH) membrane reactors via the synthesis and use of small-pore SAPO-34 zeolite membranes. The reduction of SAPO-34 membrane thickness to ∼1 μm levels while maintaining good H2 selectivity is challenging. Initial reductions in membrane thickness were achieved using a single structure-directing agent (SDA) and concentrated precursor solution, leading to a large permeance increase and good H2/hydrocarbon selectivity. We further show that thinner (∼1 μm) SAPO-34 membranes can be fabricated on a large area (∼18 cm2) tubular ceramic supports by the use of both base and salt forms of the SDA to independently control the SDA concentration and pH. EDX analysis and elemental mapping confirmed the formation of micrometer-thin (∼1 μm) SAPO-34 zeolite membrane layers on α-alumina tubular supports. Micrometer-thin SAPO-34 membranes reach propylene selectivities greater than 80% as well as high propane conversio...

Synthesis of high performance SAPO-34 zeolite membrane by a novel two-step hydrothermal synthesis + dry gel conversion method

High performance SAPO-34 zeolite membranes were prepared by a novel two-step method, hydrothermal synthesis + dry gel conversion method. A short hydrothermal synthesis was used to coat a thin gel layer on the porous ceramic support and the gel layer was converted into zeolite membrane in the following dry gel conversion treatment. XRD and SEM were used to monitor this two-step synthesis process. The final membrane thickness was determined by the hydrothermal synthesis time (or gel layer thickness), and no further growth occurred in the dry gel conversion treatment, since no mother liquor provided nutrients for membrane growth. Membrane thickness was reduced from 7 μm (hydrothermal synthesis) to 2 μm (two-step method). The obtained SAPO-34 zeolite membranes have CO2 permeances 13–16 × 10−7 mol/(m2 s Pa) and CO2–CH4 selectivity 40–60 at 4.0 MPa pressure drop.

Synthesis of SAPO-34 zeolite membranes with the aid of crystal growth inhibitors for CO2–CH4separation

Small SAPO-34 zeolites and thin membranes were synthesized by using triethylamine as the template and β-cyclodextrin as the crystal growth inhibitor (CGI). The thus obtained SAPO-34 membranes exhibited CO2–CH4 separation selectivities as high as 186. In addition, they displayed CO2 permeance values greater than 2.32 × 10−6 mol m−2 s−1 Pa−1.

Synthesis of CuO–ZnO–Al2O3 @ SAPO-34 core@shell structured catalyst by intermediate layer method

AbstractThe present study focuses on synthesis of SAPO-34 zeolite membrane on the surface of CuO–ZnO–Al2O3 (CZA) catalyst particles to form CZA@SAPO-34 core@shell structured catalyst. In contrast to the traditional support of porous alumina, CZA catalyst particles have a relatively brittle surface, which leads to a big challenge to coat SAPO-34 zeolite membrane on their surface. Moreover, the hydrothermal synthesis of SAPO-34 zeolite membrane is carried out under weakly alkaline condition at 200 °C for hours, which causes part of the surface of CZA to be fragmented. To overcome these shortcomings, the intermediate layer of alumina is introduced to the surface of the CZA particles and acts as a barrier to the high-temperature hydrothermal and alkaline condition. It also takes as a transition to enhance SAPO-34 zeolite seeds adherence to the surface of CZA particles. With the help of an alumina layer, a continuous and dense zeolite membrane has been obtained on the surface of CZA particles. The prepared core@shell structured catalyst has better selectivity in CO hydrogenation for producing light hydrocarbons because of the synergetic effects between the membrane and core catalyst.

Seeded-gel synthesis of SAPO-34 single channel and monolith membranes, for CO2/CH4 separations

▸ SAPO-34 zeolite membranes were synthesized on 7-channel alumina monolith supports. ▸ The membranes had high CO 2 /CH 4 selectivities even at high pressures. ▸ SAPO-34 seeds were added to the synthesis gel instead of seeding the support surface. ▸ Seeding the gel simplified synthesis and improved reproducibility. SAPO-34 zeolite membrane synthesis was scaled up by preparing membranes on seven-channel monolith alumina supports. These monoliths have CO 2 permeances and CO 2 /CH 4 separations selectivities at 4.6 MPa feed pressure that were similar to SAPO-34 membranes on single channel supports. They also exhibited the same pressure dependence. SAPO-34 membrane preparation was improved by adding SAPO-34 seed crystals to the synthesis gel instead of placing seeds on the support surface. Membranes prepared by seeded-gel synthesis had higher CO 2 permeances, higher CO 2 /CH 4 separation selectivities, and smaller standard deviations for these values. Using seeded-gel synthesis also decreased the number of synthesis steps, but increased the amount of seeds needed by two orders of magnitude.