MFI纳米片沉积层气相转化制备超薄b轴取向沸石膜

利用乙二胺-水蒸汽进行气相转化(VPT)制备超薄、取向MFI沸石膜,通过将MFI纳米片沉积层转化为致密的沸石膜,实现了膜厚度的有效控制。扫描电子显微镜和X射线衍射表明,制备的沸石膜膜厚度约为280 nm,具有高度b轴取向的致密结构。丁烷异构体双组分分离测试结果表明,在333 K下,等物质的量的正丁烷/异丁烷混合物的正丁烷渗透速率和分离因子分别为1.5×10^-7 mol·m^-2·s^-1·Pa^-1和14.8。Na₂SiO₃作为低聚硅源在MFI沸石纳米片二次生长过程中能够提供硅源和碱度,通过在胺类蒸汽中实现MFI沸石纳米片间的融合生长,进一步提高了膜的取向度和致密性。     

MFI纳米片沉积层气相转化制备超薄b轴取向沸石膜

利用乙二胺-水蒸汽进行气相转化（VPT）制备超薄、取向MFI沸石膜,通过将MFI纳米片沉积层转化为致密的沸石膜,实现了膜厚度的有效控制。扫描电子显微镜和X射线衍射表明,制备的沸石膜膜厚度约为280 nm,具有高度b轴取向的致密结构。丁烷异构体双组分分离测试结果表明,在333 K下,等物质的量的正丁烷/异丁烷混合物的正丁烷渗透速率和分离因子分别为1.5×10^-7 mol·m^-2·s^-1·Pa^-1和14.8。Na₂SiO₃作为低聚硅源在MFI沸石纳米片二次生长过程中能够提供硅源和碱度,通过在胺类蒸汽中实现MFI沸石纳米片间的融合生长,进一步提高了膜的取向度和致密性。     

Open‐Pore Two‐Dimensional MFI Zeolite Nanosheets for the Fabrication of Hydrocarbon‐Isomer‐Selective Membranes on Porous Polymer Supports

Two‐dimensional zeolite nanosheets that do not contain any organic structure‐directing agents were prepared from a multilamellar MFI (ML‐MFI) zeolite. ML‐MFI was first exfoliated by melt compounding and then detemplated by treatment with a mixture of H₂SO₄ and H₂O₂ (piranha solution). The obtained OSDA‐free MFI nanosheets disperse well in water and can be used for coating applications. Deposits made on porous polybenzimidazole (PBI) supports by simple filtration of these suspensions exhibit an n‐butane/isobutane selectivity of 5.4, with an n‐butane permeance of 3.5×10^?7 mol m^?2 s^?1 Pa^?1 (ca. 1000 GPU).     

薄片状MFI晶体无凝胶蒸汽辅助晶化制备b轴取向沸石膜

采用新型薄片状MFI沸石晶体作为晶种,通过无凝胶蒸汽辅助晶化法(gel-less steam-assisted crystallization,GLSAC)在烧结氧化硅多孔载体上制备了致密平整、b轴取向的MFI沸石膜。考察了GLSAC中模板剂浓度、釜底水量、生长温度及时间对MFI沸石膜的影响。扫描电子显微镜和X射线衍射结果显示,适量的模板剂和釡底水量能抑制晶种的面外生长。成功制备了厚度750 nm、致密平整的MFI沸石膜。制备的对丁烷异构体气体分离测试表明,25℃时,等物质的量的丁烷异构体的分离因子(SF_A/B)可达36,对正丁烷的渗透速率为1.5×10^-7mol·m^-2·s^-1·Pa^-1。     

以开孔的二维MFI纳米片为构筑单元制备沸石片膜

以食人鱼溶液(体积比为3∶1的95%～98%(w/w)H_2SO_4和30%(w/w)H_2O_2混合液)处理多层MFI(ML-MFI)除去有机结构导向剂(OSDA),经超声剥离和沉降纯化后得到了开孔的MFI沸石纳米片。采用X射线衍射(XRD)、扫描电镜(SEM)、透射电镜(TEM)、N_2吸附-脱附、傅立叶变换红外光谱(FT-IR)和热重分析(TGA)等手段对得到的MFI沸石纳米片进行表征,发现食人鱼溶液处理可移除ML-MFI中的OSDA,再经超声剥离得到可分散、开孔的MFI沸石纳米片。将MFI沸石纳米片用简单抽滤的方式沉积到自制Al_2O_3载体上,不经二次生长得到了连续的沸石纳米片膜。单组分气体渗透性能测试结果表明,制备的MFI沸石纳米片膜对正/异丁烷的理想选择性为4.1～5.8,正丁烷的渗透速率为2.2×10~(-7)～4.1×10~(-7) mol·m~(-2)·s~(-1)·Pa~(-1)。     

A Uniformly Oriented MFI Membrane for Improved CO2 Separation

Membrane separation of CO₂ from natural gas, biogas, synthesis gas, and flu gas is a simple and energy‐efficient alternative to other separation techniques. But results for CO₂‐selective permeance have always been achieved by randomly oriented and thick zeolite membranes. Thin, oriented membranes have great potential to realize high‐flux and high‐selectivity separation of mixtures at low energy cost. We now report a facile method for preparing silica MFI membranes in fluoride media on a graded alumina support. In the resulting membrane straight channels are uniformly vertically aligned and the membrane has a thickness of 0.5 μm. The membrane showed a separation selectivity of 109 for CO₂/H₂ mixtures and a CO₂ permeance of 51×10^?7 mol m^?2 s^?1 Pa^?1 at ?35 °C, making it promising for practical CO₂ separation from mixtures.     

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/H₂O 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 Na₂O 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.     

A Two‐Dimensional Lamellar Membrane: MXene Nanosheet Stacks

Two‐dimensional (2D) materials are promising candidates for advanced water purification membranes. A new kind of lamellar membrane is based on a stack of 2D MXene nanosheets. Starting from compact Ti₃AlC₂, delaminated nanosheets of the composition Ti₃C₂T_x with the functional groups T (O, OH, and/or F) can be produced by etching and ultrasonication and stapled on a porous support by vacuum filtration. The MXene membrane supported on anodic aluminum oxide (AAO) substrate shows excellent water permeance (more than 1000 L m⁻² h⁻¹ bar⁻¹) and favorable rejection rate (over 90 %) for molecules with sizes larger than 2.5 nm. The water permeance through the MXene membrane is much higher than that of the most membranes with similar rejections. Long‐time operation also reveals the outstanding stability of the MXene membrane for water purification.     

Direct Synthesis of Nanosheet-Stacked Hierarchical “Honey Stick-like” MFI Zeolites by an Aromatic Heterocyclic Dual-Functional Organic Structure-Directing Agent

Soft template designing is the most promising strategy for the synthesis of zeolite nanosheets. MFI nanosheets directed by soft templates (containing long-chain alkyl groups or aromatic groups as hydrophobic component) can be found frequently; however, so far, MFI nanosheets synthesized by soft templates with aromatic heterocycle groups (e. g., s-triazine groups) are rare. Herein, a nanosheet-stacked hierarchical MFI zeolite (NSHM) has been synthesized by using a triply branched s-triazine-based surfactant as a bifunctional organic structure-directing agent. On the basis of a geometrical match relationship, a formation model has been proposed. Synthesized NSHM had abundant mesopores stacked by nanosheets and exhibited a high surface area (430 m² ⋅ g⁻¹). The 1 wt% Pd/NSHM attained a significant increase in yield of cyclohexanol/cyclohexanone mixture (from 66 to 85 %) in the oxidation of cyclohexane compared with Silicalite-1 and SBA-15 as supports.     

Crack formation in α-alumina supported MFI zeolite membranes studied by in situ high temperature synchrotron powder diffraction

Cracks are frequently formed in α-alumina supported MFI membranes during calcination. To better understand crack formation, in situ powder diffraction data were collected during calcination of a type of MFI membrane (ca. 1800 nm thick) which is known to crack reproducibly. In addition, data for MFI powder and a blank support were also collected. Both a synchrotron radiation facility and an in-house instrument were used. The unit cell parameters were determined with the Rietveld method, and the strain in the direction perpendicular to the film surface was calculated for the film as well as for the support. The microstrain in the support was also estimated. Based on the results obtained here, a model for crack formation in this type of MFI membrane was proposed. The lack of cracks in other types of MFI membranes (ca. 500 nm) prepared in our laboratory is also explained by the model. In thicker MFI films, the crystals are well intergrown. During heating, the MFI crystals contract and the α-alumina support expands. Consequently, a thermal stress develops in the composite which eventually leads to formation of cracks in the film and structural defects in the support. In thinner films, the crystals are less well intergrown and the thermal expansion mismatch leads to opening of grain boundaries rather than cracks.     

A Hierarchical MFI Zeolite with a Two‐Dimensional Square Mesostructure

A conceptual design and synthesis of ordered mesoporous zeolites is a challenging research subject in material science. Several seminal articles report that one‐dimensional (1D) mesostructured lamellar zeolites are possibly directed by sheet‐assembly of surfactants, which collapse after removal of intercalated surfactants. However, except for one example of two‐dimensional (2D) hexagonal mesoporous zeolite, no other zeolites with ordered 2D or three‐dimensional (3D) mesostructures have been reported. An ordered 2D mesoporous zeolite can be templated by a cylindrical assembly unit with specific interactions in the hydrophobic part. A template molecule with azobenzene in the hydrophobic tail and diquaternary ammonium in the hydrophilic head group directs hierarchical MFI zeolite with a 2D square mesostructure. The material has an elongated octahedral morphology, and quaternary, ordered, straight, square channels framed by MFI thin sheets expanded along the a–c planes and joined with 90° rotations. The structural matching between the cylindrical assembly unit and zeolite framework is crucial for mesostructure construction.     

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 SF₆. 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 H₂/SF₆ideal selectivity as high as 260 because of SF₆-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 SF₆ swell MFI crystals when they adsorb, but benzene and CO₂ 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.     

堇青石栽体表面ilicalite-1沸石膜的合成与表征

本文首次采用堇青石为载体,用水热合成方法,制备了致密的高硅MFI沸石膜,并对其结构、成分以及气体渗透性质进行表征。     

High-resolution physical adsorption on supported borosilicate MFI zeolite membranes: comparison with powdered samples

We have performed physical adsorption measurements to analyze the structure of intact MFI membranes with a nominal Si:B ratio of 12.5:1. Argon adsorption isotherms of membranes at 77 K are nearly identical to isotherms of corresponding zeolite powders throughout most of the domain of relative pressures, the exception being the micropore-filling region (between 10⁻⁵ and 10⁻⁴ relative pressure). The isotherm in the micropore-filling region is steeper by a factor of 4–5 for the borosilicate membranes than it is for a powder made under the same conditions, while the saturation loadings are found to be similar. This implies a difference in how the zeolite pores fill with argon – either thermodynamic or kinetic in origin – and that fluxes for argon permeation through such powder and membrane samples may differ. Our membrane adsorption measurements reveal no hysteresis and hence no mesoporsity in the zeolite membranes. This finding, from adsorption measurements, stands in contrast to recent alkane permeation measurements on these membranes, which suggest the presence of mesopores. We reaffirm the conclusion, found in previous work, that zeolite membranes can be approximated as a zeolite powder plus a support for the purposes of adsorption characterization.     

Single-Crystalline MFI Zeolite with Sheet-Like Mesopores Layered along the a Axis

Designing a templating strategy for directing mesopore growth along different crystallographic directions is essential for fabricating two- or three-dimensional single-crystalline mesoporous zeolites. However, so far, mesopores formed in MFI zeolites by soft templates have mostly been generated by disrupting growth along the b axis; generating mesopores by disrupting growth along the a axis is rare. Herein, a single-crystalline mesoporous MFI zeolite (SCMMZ) with sheet-like mesopores layered along the a and b axes was synthesized using a triply branched surfactant with diquaternary ammonium groups connected to 1,3,5-triphenylbenzene by a six- and eight-carbon alkyl chain (TPB-6 and 8). The sheet-like mesopores were embedded in the MFI framework and were retained even after calcination. Molecular mechanics calculations provided evidence of low binding energy configurations of the surfactant that directed the growth of straight and zigzag channels along the b and a axes, respectively. The formation of nanosheets was attributed to the geometric matching of the arrangement of the aromatic groups to the zeolite framework.     

para‐Xylene Ultra‐selective Zeolite MFI Membranes Fabricated from Nanosheet Monolayers at the Air–Water Interface

The control of membrane morphology and microstructure is crucial to improve the separation performance of molecular‐sieve membranes. This can be enabled by making thin, dense, and uniform seed‐crystal coatings, which are then intergrown into continuous membranes. Herein, we show a novel and simple floating particle coating method can give closely packed monolayers of zeolite nanosheets on nonporous or porous supports. The zeolite nanosheet monolayer is formed at the air–water interface in a conical Teflon trough. As the water in the trough is drained, the monolayer is deposited on a support placed below. Membranes prepared by gel‐free secondary growth of the nanosheets deposited by this method show unprecedented ultra‐selective performance for separation of para‐ from ortho‐xylene (separation factor >10 000).     

2D Zeolite Coatings: Langmuir–Schaefer Deposition of 3 nm Thick MFI Zeolite Nanosheets

Stable suspensions of zeolite nanosheets (3 nm thick MFI layers) were prepared in ethanol following acid treatment, which partially removed the associated organic structure‐directing agent. Nanosheets from these suspensions could then be dispersed at the air–water interface and transferred to silicon wafers using Langmuir–Schaefer deposition. Using layer‐by‐layer deposition, control on coating thickness was demonstrated. In‐plane X‐ray diffraction (XRD) revealed that the deposited nanosheets contract upon calcination similar to bulk MFI crystals. Different methods for secondary growth resulted in preferentially oriented thin films of MFI, which had sub‐12‐nm thickness in certain cases. Upon calcination, there was no contraction detectable by in‐plane XRD, indicating well‐intergrown MFI films that are strongly attached to the substrate.     

Zeolitic Imidazolate Framework/Graphene Oxide Hybrid Nanosheets as Seeds for the Growth of Ultrathin Molecular Sieving Membranes

A defect‐free zeolitic imidazolate framework‐8 (ZIF‐8)/graphene oxide (GO) membrane with a thickness of 100 nm was prepared using two‐dimensional (2D) ZIF‐8/GO hybrid nanosheets as seeds. Hybrid nanosheets with a suitable amount of ZIF‐8 nanocrystals were essential for producing a uniform seeding layer that facilitates fast crystal intergrowth during membrane formation. Moreover, the seeding layer acts as a barrier between two different synthesis solutions, and self‐limits crystal growth and effectively eliminates defects during the contra‐diffusion process. The resulting ultrathin membranes show excellent molecular sieving gas separation properties, such as with a high CO₂/N₂ selectivity of 7.0. This 2D nano‐hybrid seeding strategy can be readily extended to the fabrication of other defect‐free and ultrathin MOF or zeolite molecular sieving membranes for a wide range of separation applications.     

Boron Nitride Membranes with a Distinct Nanoconfinement Effect for Efficient Ethylene/Ethane Separation

A BN membrane with a distinct nanoconfinement effect toward efficient ethylene/ethane separation is presented. The horizontal and inclined self‐assembly of 2D BN nanosheets endow the BN membrane with abundant percolating nanochannels, and these nanochannels are further decorated by reactive ionic liquids (RILs) to tailor their sizes as well as to achieve nanoconfinement effect. The noncovalent interactions between RIL and BN nanosheets favor the ordered alignment of the cations and anions of RIL within BN nanochannels, which contributes to a fast and selective ethylene transport. The resultant membranes exhibit an unprecedented separation performance with superhigh C₂H₄ permeance of 138 GPU and C₂H₄/C₂H₆ selectivity of 128 as well as remarkably improved long‐term stability for 180 h, outperforming reported state‐of‐the‐art membranes.     

高硅MFI分子筛膜的合成与低温脱除膜内有机模板剂

采用旋涂法在粗糙的α-Al₂O₃载体片上制备出较完备的分子筛晶种层;以四丙基氢氧化铵(TPAOH)为有机模板剂,通过调控合成液的H₂O/Si摩尔比,实现了对分子筛晶体面内优先生长的调控;经过三次水热合成得到致密交联的h0h-轴取向高硅MFI分子筛膜,膜厚约为8 μm(包括~5 μm致密层和~3 μm过渡层)。采用先低温加氢裂解后低温空气氧化的两步法脱除工艺,有效脱除了分子筛膜内的有机模板剂。相比于传统高温煅烧法,该法可以避免分子筛膜因脱除模板剂而形成的较大晶间缺陷。因而采用低温两步法脱除模板剂的分子筛膜片在30 ℃时具有较好的CO₂分离效果,其CO₂/N₂分离因子达到5.2, CO₂渗透通量高达5.8 × 10^-7mol·m^-2·s^-11·Pa^-1。