B-oriented MFI Zeolite Research Articles

Highly selective thin B-oriented MFI zeolite membranes on scalable modified stainless steel supports

Enhancing the separation performance of molecular sieve membranes relies significantly on precise control over their membrane morphology and microstructure. This involves creating thin, closely packed, oriented, and uniform coatings of seed crystals, enabling subsequent intergrowth to establish continuous membranes. To address industrial needs, it is crucial to generate a thin and flawless membrane efficiently producible on a large-scale support. Porous stainless supports offer scalability and easy integration into membrane modules. In this study, we present a novel and straightforward process for modifying stainless-steel supports to seed zeolite membrane layers. High-performance, b-oriented MFI zeolite membranes can be synthesized on these modified stainless-steel supports through scalable filtration seeding of MFI zeolite nanosheets followed by secondary growth. By employing a carefully designed pre-treatment procedure, we achieved the growth of well-intergrown, highly b-oriented MFI zeolite membranes on the modified stainless-steel support, establishing a robust interaction with the substrate. The membranes, fabricated through gel-free secondary growth of nanosheets deposited on a scalable stainless-steel support using this technique, demonstrate ultra-selective capabilities in separating para-xylene from ortho-xylene, with a high separation factor of 388.

Fabrication of highly b-oriented MFI zeolite film over broad temperature window

Despite that oriented MFI zeolite films exhibit significant prospects for a wide range of applications due to microstructural superiority, their precise orientation control entails rigorous demands on reaction conditions (e.g., reaction temperature). In this work, we achieved facile synthesis of continuous b-oriented MFI zeolite films with few twins over a wide crystallization temperature range of 30–210 °C through adopting NH4F-mediated gel as nutrient during epitaxial growth. Both ammonium ions and fluorine ions were found crucial for enhancing the gel reactivity, endowing epitaxial growth with higher crystallization rates; moreover, the above morphology could be maintained even under fluctuating temperature conditions during film processing, thereby showing great potential for their bath production in a facile and reproducible manner.

Fabrication of b-oriented MFI zeolite membranes by a novel printing-transfer seed layer procedure.

Highly b-oriented MFI zeolite (abbreviated as BOMZ) membranes are attractive due to less grain boundary defects and straight channels normal to the substrate, enhancing selectivity and flux in membrane separation. Herein, we demonstrate a novel, effective and easily-amplified printing-transfer oriented-seed-layer technique to manufacture uniform BOMZ seed monolayer on porous supports. Furthermore, a facile and effective approach for the synthesis of highly BOMZ membranes by introducing poly(hexamethylene biguanide) hydrochloride as a twin crystal inhibitor during seeded growth is demonstrated. Well-intergrown BOMZ membranes (~650 nm thick) obtained on porous Al2O3 supports show a flux of 2.8 kg m-2 h-1 with a separation factor as high as 71 for pervaporation in the 60 oC feed of EtOH/H2O (5 wt%), which is much higher than those of random membranes. The developed seed assembly technique on porous supports underlines great potential for facile preparation of oriented seed layers on porous supports.

B-oriented MFI zeolite membranes for xylene isomer separation - Effect of xylene activity on separation performance

MFI zeolite membranes, especially b-oriented ones, show superb vapor permeation/separation performance for xylene isomer separation at low vapor pressures. In industrial applications these membranes should be operated at high xylene vapor pressures in the vapor separation mode or with liquid feed in the pervaporation mode. This paper reports the influence of xylene activity on xylene vapor and liquid permeation/separation performance of b-oriented MFI zeolite membranes. High quality b-oriented MFI zeolite membranes were synthesized with an organic template on modified silica substrates. For vapor permeation with binary p/o-xylene feed, both permeance (for p-xylene and o-xylene) and p-/o-xylene separation factor increase, with separation factor reaching maximum of 515 at 150oC, then decreases with increasing temperature due to combined effects of temperature dependence of diffusivity and adsorption. The vapor permeance for both xylenes and separation factor decrease monotonically with increasing xylene activity (or vapor pressure). Lowest separation factor is observed when operated in the pervaporation mode giving p-/o-xylene separation factor of about 2. Compared to randomly oriented MFI zeolite membranes synthesized with an organic template, the b-oriented MFI zeolite membranes show better vapor permeation/separation performance and less negative effect of xylene activity on separation factor. However, both b- and randomly oriented MFI zeolite membranes exhibit a similarly poor separation factor when operated in the pervaporation mode.

B-Oriented Mfi Zeolite Membranes for Xylene Isomer Separation - Effect of Xylene Activity on Separation Performance

B-Oriented Mfi Zeolite Membranes for Xylene Isomer Separation - Effect of Xylene Activity on Separation Performance

Insights into 1,2-dihydroxybenzene addition on twin suppression during the secondary hydrothermal synthesis of b-oriented MFI zeolite films

Elimination of parasitic twin crystals on the surface of highly b-oriented MFI zeolite films remains an interesting but challenging issue. 1,2-Dihydroxybenzene (C6H6O2), as complex agent to restrict the activity of siliceous species, was introduced into the secondary hydrothermal synthesis solution. The effect of the synthesis parameters such as crystallization temperature and reaction duration on the morphology of the resulting oriented MFI zeolite films was systematically investigated. The addition of C6H6O2 suppressed the growth rate of MFI crystals in the films along both the c-axis and a-axis directions. The suppression was even more severe for the a-axis directions because of only very limited siliceous species available for the seed crystal growth as well as the relatively fast depletion of them for c-axis growth. The twin elimination effect of the C6H6O2 addition was further validated by the successful achievement of highly b-oriented MFI films on various film supports under the optimum synthesis condition.

Solvent-Free Secondary Growth of Highly b-Oriented MFI Zeolite Films from Anhydrous Synthetic Powder.

Up to date, zeolite films have been mainly fabricated by in situ crystallization, secondary growth in a solution/hydrogel, or occasionally by vapor phase transformation of dry gel. Here we demonstrate for the first time a solvent-free secondary growth method for b-oriented silica MFI zeolite films using the synthetic powder from ground anhydrous raw solids in the presence of NH4F. Typically, precisely b-oriented MFI zeolite films are synthesized from seed layers of highly b-oriented MFI zeolite crystals in the synthetic powder of 1SiO2:0.035TPABr:0.05NH4F at 175 °C for 6 h. If needed, b-oriented MFI zeolite multilayer films can be acquired by changing the synthesis time or the amount of NH4F in the synthetic powder. Compared with the traditional hydrothermal synthesis, the approach developed here may provide a new avenue for fabricating high quality zeolite films/membranes.

Enhanced compactness of oriented MFI zeolite films by heat treatment of seed layer

The achievement of compact and defect-free film structure is crucial for the application of b-oriented MFI zeolite film. In this work, a novel heat treatment technique was used to treat zeolite seeded substrates prior to secondary growth. The influence of the heat treatment parameters such as treat temperature and time on the final film morphology were systematically investigated. The relationship between film compactness and the parameters was established. Under the optimized treat temperature of 120 °C and treat time of 1 h, compact and uniform b-oriented MFI zeolite film was achieved. The applicability of the optimum heat treatment condition was validated by employing various film substrates.

A facile fabrication of highly b-oriented MFI zeolite films in the TEOS-TPAOH-H2O system without additives

A facile secondary growth method for fabrication of highly b-oriented MFI zeolite films is demonstrated. This method enables the growth of b-oriented MFI seed monolayers into highly b-oriented dense films in the TEOS-TPAOH-H2O system without any additive or extra treatment. Suppression of twin growth is achieved via the increase of water content in the synthesis solution. A clear correlation between the film orientation and the synthesis composition for secondary growth is presented. Compared with previously reported strategies in controlling the orientation of MFI zeolite films, this method offers greater operating window of the synthesis composition (H2O/Si = 1500–3000, TPA+/Si = 0.1–0.5, OH−/Si = 0.1–0.2) and synthesis condition (150–180 °C, 4–8 h). The thickness of b-oriented MFI films (370 nm-610 nm) can be tuned by adjusting the water amount or the synthesis condition. The new synthesis method uses lower amount of organic template and silica source making it potentially more economical and environmentally friendly and thus attractive for industrial implementation.

Fabrication of b-oriented MFI Film via Langmuir-Blodgett Technique

A compact and highly b-oriented MFI-type zeolite monolayer was fabricated by assembling anisotropic zeolite micro-crystals extending over the centimeter scale by Langmuir–Blodgett (LB) method. The LB monolayer was subsequently employed as a seed layer for the secondary growth of highly b-oriented and defect-free MFI zeolite films. Firstly, LB technique was used to assemble a highly b-oriented MFI type zeolite monolayer on stainless steel substrates and platinum electrodes at an optimum target pressure of 20 mN/m constantly. Secondly, the precursor pretreatment method for preparing highly b-oriented MFI films was proved to be achievable. The crystal intergrowth and orientation of the MFI films were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The experimental results confirmed that the precursor pretreatment method was an effective route to suppress the undesired a-oriented twin growth during the secondary growth step. Finally, the highly b-oriented MFI-type zeolite film modified platinum electrodes were further tested as the working electrodes in an aqueous solution including [Fe(CN)6] with a diameter of 0.72 nm. Cyclic voltammetry experimental results demonstrated that the b-oriented MFI films fabricated by the Langmuir-Blodgett method were defect-free.

Fabrication of a highly b-oriented MFI-type zeolite film-modified electrode with molecular sieving properties by Langmuir–Blodgett method

A novel highly compact and b-oriented MFI-type zeolite monolayer was obtained by assembling sec-butanol-modified rounded-coffin-shaped silicalite-1 microcrystals by Langmuir–Blodgett (LB) method. The LB monolayer was subsequently tested as a seed layer for the secondary growth of uniformly b-oriented and defect-free MFI zeolite films on stainless steel substrates and platinum electrodes. Three different methods were evaluated for identifying a suitable synthesis route to suppress the undesired a-oriented twin growth during the synthesis of MFI films. The crystal intergrowth and orientation of the MFI films were characterized by scanning electron microscopy and X-ray diffraction (XRD). The crystallographic preferred orientation indices and preferred orientation coverage values calculated using the XRD data were used to quantitatively determine the preferred orientation and surface coverage of the prepared zeolite films. The experimental results confirm that the low tetrapropylammonium hydroxide (TPAOH) method is the most effective route to suppress the undesired a-oriented twin growth, when the highly b-oriented LB monolayer was used as the seed layer for the secondary growth of the b-oriented MFI film. The highly b-oriented MFI-type zeolite film-modified electrodes were further tested as the working electrodes in electrochemical experiments. Cyclic voltammetry experimental results confirmed that the b-oriented MFI films fabricated at low TPAOH concentration were defect free and possessed a distinct molecular sieving property after calcination. The LB method, a highly reproducible and controllable method for the organization of zeolite crystals, can be further applied as an effective platform for the promotion and development of zeolite-modified electrodes as selective sensors.

Fabrication of b-Oriented MFI Zeolite Films under Neutral Conditions without the Use of Hydrogen Fluoride.

The fabrication of MFI zeolite films with particular b-axis orientation is especially fascinating. Unlike the conventional alkaline or hydrofluoric acid (HF) assisted neutral synthesis route, here we develop a novel neutral synthesis solution system of TPABr/fumed silica/H2 O without the use of HF and successfully synthesize highly b-oriented MFI zeolite films on glass-plate substrates by secondary growth. The localized weak alkaline environment created by the dissolved Na2 O species from the substrate is identified as the key factor for the depolymerization of fumed silica and subsequently the in-plane growth of zeolite seed layers. Continuous b-oriented MFI films can also be synthesized on other substrates in the presence of a glass plate or a trace amount of NaOH, which making our neutral synthesis route promising for the direct synthesis of MFI zeolite films and membranes on various substrates.

Microwave-assisted seeded growth of the submicrometer-thick and pure b-oriented MFI zeolite films using an ultra-dilute synthesis solution

A chitosan layer is utilized for controlled fabrication of MFI zeolite films in dilute synthesis solution under microwave irradiation. The dense, ca. 600 nm-thick and solely b-oriented silicalite-1 films can be formed onto the chitosan supported silicalite-1 monolayer under microwave irradiation for 1 h.

Dynamic Hydrothermal Synthesis of a b-Oriented MFI Zeolite Film

Continuous b -oriented MFI zeolite films were prepared on stainless steel substrates by dynamic hydrothermal synthesis using a rotating convection oven. The influences of rotation speed, crystallization temperature and crystallization time were studied. X-ray diffraction was used to identify the orientation of the zeolite films. Compared with the static method, a shorter synthesis time, a more uniform particle size distribution and better film orientation were observed for the dynamic method, and this is attributed to the low concentration and thermal gradients that exist in the bulk solution and to the washing action of the bulk solution toward the substrate surface. ： 在旋转烘箱中采用动态水热法, 在不锈钢片支撑体上合成了连续的 b 轴取向 MFI 型分子筛膜, 并运用扫描电镜和 X 射线衍射考察了合成釜转速、晶化温度和晶化时间对所得分子筛膜性能的影响. 与静态水热法相比, 动态水热合成过程中合成液的温差和浓度差大大降低, 且合成液持续冲刷支撑体表面, 使得动态法具有合成时间短、取向性好和分子筛颗粒粒径分布均一的优点. Continuous b -oriented MFI zeolite films were prepared using a rotating convection oven. The advantages of this dynamic method include a shorter synthesis time, a more uniform particle size distribution, and better film orientation.

Continuity Control of b-Oriented MFI Zeolite Films by Microwave Synthesis

Influences of aging and crystallization conditions on the continuity of b-oriented MFI zeolite films on stainless steel substrates are systematically investigated. A long aging time (e.g., 20 h) at room temperature is needed to obtain a continuous b-oriented MFI zeolite film by the microwave heating at 165 °C. However, the aging time can be shortened from 20 h to 30 min by elevating the aging temperature to 80 °C. Compared with the conventional heating (2 h), continuous b-oriented MFI zeolite films are prepared on stainless steel substrates with a much shorter crystallization time by the microwave heating (35 min). Continuous b-oriented MFI zeolite films can be synthesized by controlling nuclei concentration and crystal size, which are mainly determined by the conditions of the aging and crystallization respectively. On the basis of the results from these experiments, a film continuity−synthesis condition map is obtained and a film formation mechanism is discussed.

Preparation of b-oriented MFI zeolite membranes on porous α-alumina substrates precoated with mesoporous silica sublayer

Preparation of b-oriented MFI zeolite membranes on porous α-alumina substrates precoated with mesoporous silica sublayer