高性能AlPO4-14分子筛膜的制备及气体分离性能

制备了高性能的AlPO₄-14分子筛膜. 首先通过控制反应溶胶中水和模板剂的含量制备了形貌均一的AlPO₄-14分子筛, 分子筛的尺寸为15~18 mm; 然后采用晶种法即在反应凝胶中加入分子筛作为晶种进一步调控分子筛的大小, 使得AlPO₄-14分子筛的尺寸从15~18 mm减小到2~3 mm, 得到形貌均一的纯相片状晶体, 同时有效缩短了制备时间; 最后以多孔管状莫来石为支撑体, 采用二次生长法制备AlPO₄-14分子筛膜. 考察了2种不同大小的晶种对膜形貌和性能的影响, 发现以大尺寸的分子筛(15~18 mm)作为晶种制备的分子筛膜的分离层存在较多缺陷, 而采用小尺寸的晶种(2~3 mm)制备的膜层较均一致密. AlPO₄-14分子筛膜经高温脱除模板剂后仍然保持着纯相的AlPO₄-14晶型, 表明二次生长法促进了AlPO₄-14晶体在膜层中的生长且使其具有更高的结晶度和热稳定性. 在25 ℃, 100 kPa下, AlPO₄-14分子筛膜对H₂/CH₄, CO₂/CH₄和H₂/CF₄的理想分离因子分别为28, 40和1047, 且H₂和CO₂的渗透速率分别为6.3×10 ^-7和9×10 ^-7 mol·(m ²·s·Pa) ^-1; 对等摩尔CO₂/CH₄混合气体的分离因子为81.5, 且CO₂的渗透速率为8.8×10 ^-7 mol·(m ²·s·Pa) ^-1.     

基底表面性质与前驱液化学环境对原位定向构筑钛硅分子筛膜的影响

b取向MFI型分子筛膜能够显著促进分子的传输效率, 被广泛应用于混合物分离及催化领域. 虽然传统的原位水热晶化法已较为成熟, 然而仍难以调控膜层的b轴取向生长. 本文以304不锈钢片为基底, 采用经典的原位水热晶化法研究了基底表面物化性质、 前驱液配比及晶化条件对钛硅分子筛膜b轴取向生长的影响. 结果表明, TiO₂中间层表面的羟基能够定向诱导分子筛晶粒的吸附, 进而提高钛硅分子筛膜的b轴取向程度. 同时, 前驱液中模板剂和水含量对晶粒的大小及膜层的定向生长具有显著影响, 即仅在合适的碱度下才能形成高b轴取向的钛硅分子筛膜.     

Synthesis of submicron polycrystalline MFI zeolite films on porous ceramic supports

This paper reports preparation and properties of ultra-thin MFI-type zeolite (silicalite) films on sol–gel derived mesoporous 8 mol% yttria doped zirconia (YZ) supports by the in situ synthesis method. Only non-continuous zeolite films were obtained on the as-synthesized YZ supports (calcined at 450°C). Continuous polycrystalline pure silicalite films, with a thickness of 0.5–0.7 μm, could be grown on the YZ supports pretreated at 700°C. The difference in the oxygen vacancy defect concentrations in these two types of YZ supports is suggested to explain the different results of growing silicalite films. Moreover, defect free 2–3 μm thick silicalite films were also prepared on the α-alumina supports under the same synthesis conditions. The silicalite layer on the YZ support offers a significantly lower mass transfer resistance as compared to that on the alumina support. Single gas permeation data of hydrogen, methane and ethane through these membranes were measured and compared with those of silicalite membranes prepared by other groups. The alumina supported silicalite membranes prepared in this work show better separation factors than the literature values. The zeolite film on the YZ support is less resistant to the stress induced in the step of template removal as compared to the zeolite film on the alumina support.     

Regulating Nonclassical Pathways of Silicalite‐1 Crystallization through Controlled Evolution of Amorphous Precursors

Differentiating mechanisms of zeolite crystallization is challenging owing to the vast number of species in growth solutions. The presence of amorphous colloidal particles is ubiquitous in many zeolite syntheses, and has led to extensive efforts to understand the driving force(s) for their self‐assembly and putative roles in processes of nucleation and growth. In this study, we use a combination of in situ scanning probe microscopy, particle dissolution measurements, and colloidal stability assays to elucidate the degree to which silica nanoparticles evolve in their structure during the early stages of silicalite‐1 synthesis. We show how changes in precursor structure are mediated by the presence of organics, and demonstrate how these changes lead to significant differences in precursor–crystal interactions that alter preferred modes of crystal growth. Our findings provide guidelines for selectively controlling silicalite‐1 growth by particle attachment or monomer addition, thus allowing for the manipulation of anisotropic rates of crystallization. In doing so, we also address a longstanding question regarding what factors are at our disposal to switch from a nonclassical to classical mechanism.     

Investigation of the Mechanism of Colloidal Silicalite‐1 Crystallization by Using DLS,SAXS, and 29Si NMR Spectroscopy

Colloidal silicalite‐1 zeolite was crystallized from a concentrated clear sol prepared from tetraethylorthosilicate (TEOS) and aqueous tetrapropylammonium hydroxide (TPAOH) solution at 95 °C. The silicate speciation was monitored by using dynamic light scattering (DLS), synchrotron small‐angle X‐ray scattering (SAXS), and quantitative liquid‐state ²⁹Si NMR spectroscopy. The silicon atoms were present in dissolved oligomers, two discrete nanoparticle populations approximately 2 and 6 nm in size, and crystals. On the basis of new insight into the evolution of the different nanoparticle populations and of the silicate connectivity in the nanoparticles, a refined crystallization mechanism was derived. Upon combining the reagents, different types of nanoparticles (ca. 2 nm) are formed. A fraction of these nanoparticles with the least condensed silicate structure does not participate in the crystallization process. After completion of the crystallization, they represent the residual silicon atoms. Nanoparticles with a more condensed silicate network grow until approximately 6 nm and evolve into building blocks for nucleation and growth of the silicalite‐1 crystals. The silicate network connectivity of nanoparticles suitable for nucleation and growth increasingly resembles that of the final zeolite. This new insight into the two classes of nanoparticles will be useful to tune the syntheses of silicalite‐1 for maximum yield.     

Silicalite crystal growth investigated by atomic force microscopy

Atomic force microscopy has been used to image the various facets of two morphologically distinct samples of silicalite. The smaller (20 microm) sample A crystals show 1 nm high radial growth terraces. The larger (240 microm) sample B crystals show growth terraces 1 to 2 orders of magnitude higher than the terraces on sample A with growth edges parallel to the crystallographic axes. Moreover, the terraces on the (010) face are significantly higher than the terraces on the (100) face - inconsistent with the previously proposed 90 degrees intergrowth structure. Sample A highlights that under certain synthetic conditions, silicalite grows in a manner akin to zeolites Y and A, via the deposition of layers comprising, in the case of silicalite, pentasil chains. It is probable that the rate of terrace advance is identical on the (010) and (100) faces, and it is the rate of terrace nucleation that dictates the overall growth rate of each facet and hence the relative size expressed in the final crystal morphology. Analysis of the growth terraces of sample B and detailed consideration of the structures of both MFI, and a closely related material MEL, lead to the proposal of a generalized growth mechanism for silicalite including the incorporation of defects within the structure. These defects are thought to be responsible for both the relative and the absolute terrace heights observed and may also explain the hourglass phenomenon observed by optical microscopy. The implications of this growth mechanism, supported by results of infrared microscopy, generate a new dimension to the continuing debate on the existence of intergrowths within one of the most important structures relevant to zeolite catalysis.     

Molecular simulation of adsorption and diffusion of chlorinated alkenes in ZSM-5 zeolites

In this work, adsorption and diffusion of trichloroethylene (TCE) and tetrachloroethylene (PCE) in ZSM-5-type zeolites were studied using molecular simulation methods. Grand canonical Monte Carlo technique was to calculate adsorption isotherms and heats of vaporization of TCE and PCE in zeolite. The results demonstrated that the Pnma-P2(1)2(1)2(1) symmetry transition of the zeolite framework has no significant effect on the TCE adsorption capacity of the silicalite, but it causes an increase of the PCE adsorption capacity. Simulations using a silicalite framework with Pnma symmetry showed that the adsorption capacity of the silicalite was limited to five molecules per unit cell. However, when a framework with P2(1)2(1)2(1) symmetry was used in the simulations, the capacity reached to eight molecules per unit cell, which is the actual adsorption capacity. To calculate intracrystalline diffusion coefficients of these compounds, molecular dynamics simulations were performed at different temperatures and loadings. The results show that the zeolite symmetry has a significant impact on diffusion coefficients of the sorbate molecules.     

Molecular simulation of the adsorption of MTBE in silicalite, mordenite, and zeolite beta

The use of methyl tertiary butyl ether (MTBE) as a gasoline additive has resulted in serious environmental problems following spills and leaks, primarily due to MTBE's high solubility in water. Remediation technologies have involved air stripping, advanced oxidation, and sorption on granular activated carbons (GAC). Hydrophobic zeolites, such as silicalite, dealuminated Y, mordenite, and beta, have been of interest in recent studies for the removal of MTBE from water. Some of these materials have shown a better performance than GAC particularly in the microg/L range. We made Monte Carlo and molecular dynamics simulations of the adsorption of pure MTBE in silicalite, mordenite, and zeolite beta with different Na+ loadings at room temperature to reveal the factors affecting the adsorption process. The results show that although the three zeolites studied here have similar pore volumes, the pore structure of zeolite beta causes a significant difference on the predicted amount of MTBE adsorbed. It was found that the position of the Na+ cations has an important effect at lower pressures. Within the range of [Na+] studied, the amount of Na+ was not found to be critical on the adsorption capacity of any of the zeolites studied, except at very low pressures in silicalite and zeolite beta.     

A novel urea conductometric biosensor based on zeolite immobilized urease

A new approach was developed for urea determination where a thin film of silicalite and zeolite Beta deposited onto gold electrodes of a conductometric biosensor was used to immobilize the enzyme. Biosensor responses, operational and storage stabilities were compared with results obtained from the standard membrane methods for the same measurements. For this purpose, different surface modification techniques, which are simply named as Zeolite Membrane Transducers (ZMTs) and Zeolite Coated Transducers (ZCTs) were compared with Standard Membrane Transducers (SMTs). Silicalite and zeolite Beta with Si/Al ratios 40, 50 and 60 were used to modify the conductometric electrodes and to study the biosensor responses as a function of changing zeolitic parameters. During the measurements using ZCT electrodes, there was no need for any cross-linker to immobilize urease, which allowed the direct evaluation of the effect of changing Si/Al ratio for the same type of zeolite on the biosensor responses for the first time. It was seen that silicalite and zeolite Beta added electrodes in all cases lead to increased responses with respect to SMTs. The responses obtained from ZCTs were always higher than ZMTs as well. The responses obtained from zeolite Beta modified ZMTs and ZCTs increased as a function of increasing Si/Al ratio, which might be due to the increased hydrophobicity and/or the acid strength of the medium.     

Zeolite synthesis from tetraalkylammonium silicate gels

Siliceous zeolite synthesis gels containing tetraalkylammonium (TAA⁺) and sodium cations were studied using X-ray diffraction, elemental analysis, ion exchange, ²⁹Si magic-angle spinning nuclear magnetic resonance spectroscopy, and scanning electron microscopy. The TAA⁺ cations are encapsulated in silicate cages, and silicalite is formed via the rearrangement of these cages by the breaking and reformation of siloxane bonds. Tetrabutylammonium (TBA⁺) cations promote silicalite growth, but not as effectively as tetrapropylammonium (TPA⁺) because the larger TBA⁺ cations do not conform as well to the silicalite lattice, thus forming an intergrowth of the silicalite-1 and silicalite-2 structures. The time to nucleate silicalite is not affected by the TBA⁺ content of the gel, but the rate of silicalite crystal growth increases with increasing TBA⁺ in the gel. The TBA⁺ occupies all the channel intersections of the silicalite formed. Tetraethylammonium (TEA⁺) cations are encapsulated in silicate cages, but not to the same extent as TPA⁺ and TBA⁺, because TEA⁺ is not as hydrophobic. No silicalite forms in the TEA⁺ silicate gel. The addition of tripropylamine (TriPA) to a TPA⁺ silicate gel has no effect on the kinetics of silicalite formation. TriPA does not incorporate into the gel because it is neutral and, therefore, does not experience a coulombic attraction to the negatively charged surface of the gel.     

Rapid synthesis of MFI zeolite nanocrystals

A rapid synthesis procedure for nonagglomerated silicalite nanocrystals has been developed. This was achieved by concentrating the precursor sol before 10-12 h of aging at 80 degrees C, followed by hydrothermal synthesis at 175 degrees C for 90 min. The high silica concentration in the concentrated sol accelerated the aggregation of primary units that were present early in the system. Thus, little silica nutrients were left for growth when the secondary particles were converted to zeolite during hydrothermal reaction. As a result, fully dispersible nanocrystals were obtained within a day instead of weeks as reported previously. The aggregation of primary units during the 80 degrees C aging process as well as the conversion of these aggregates into zeolite has been followed by DLS, XRD, and FTIR. In light of the new results, the nucleation and growth mechanisms of MFI zeolite that have been under debate in the literature were reexamined.     

Graphene Oxide Facilitates Solvent‐Free Synthesis of Well‐Dispersed,Faceted Zeolite Crystals

Zeolites with molecular dimension pores are widely used in petrochemical and fine‐chemical industries. While traditional solvothermal syntheses suffer from environmental, safety, and efficiency issues, the newly developed solvent‐free synthesis is limited by zeolite crystal aggregation. Herein, we report well‐dispersed and faceted silicalite ZSM‐5 zeolite crystals obtained using a solvent‐free synthesis facilitated by graphene oxide (GO). The selective interactions between the GO sheets and different facets, which are confirmed by molecular dynamics simulations, result in oriented growth of the ZSM‐5 crystals along the c‐axis. More importantly, the incorporation of GO sheets into the ZSM‐5 crystals leads to the formation of mesopores. Consequently, the faceted ZSM‐5 crystals exhibit hierarchical pore structures. This synthetic method is superior to conventional approaches because of the features of the ZSM‐5 zeolite.     

Graphene Oxide Facilitates Solvent‐Free Synthesis of Well‐Dispersed,Faceted Zeolite Crystals

Zeolites with molecular dimension pores are widely used in petrochemical and fine‐chemical industries. While traditional solvothermal syntheses suffer from environmental, safety, and efficiency issues, the newly developed solvent‐free synthesis is limited by zeolite crystal aggregation. Herein, we report well‐dispersed and faceted silicalite ZSM‐5 zeolite crystals obtained using a solvent‐free synthesis facilitated by graphene oxide (GO). The selective interactions between the GO sheets and different facets, which are confirmed by molecular dynamics simulations, result in oriented growth of the ZSM‐5 crystals along the c‐axis. More importantly, the incorporation of GO sheets into the ZSM‐5 crystals leads to the formation of mesopores. Consequently, the faceted ZSM‐5 crystals exhibit hierarchical pore structures. This synthetic method is superior to conventional approaches because of the features of the ZSM‐5 zeolite.     

Investigations of the adsorption of n-pentane in several representative zeolites

We have examined the adsorption of n-pentane in several representative zeolites such as silicalite (MFI), ferrierite (FER), zeolite L (LTL), and faujasite zeolites with FAU structure including siliceous Y (Si-Y) and Na-Y by using FT-Raman spectroscopy in combination with thermogravimetric analysis (TGA) with particular attention being paid to the conformational and dynamic behavior of the guest molecule. The results indicate that the framework topology mainly dictates the conformation of n-pentane in a zeolite. For the zeolites with channel systems such as silicalite, ferrierite, and zeolite L, the population of the all-trans conformer increases upon loading, given that the geometry of the isomer fits better in the channel. When n-pentane is adsorbed in zeolites with a large cavity, such as Si-Y and Na-Y, the distribution of the all-trans (TT) and trans-gauche (TG) conformers is similar to that of pure liquid, suggesting that the large supercage in the framework imposes minimal effect on the conformational equilibrium. The dynamics of the guest molecule is, however, influenced significantly by the existence of cations. Adsorption of n-pentane in a siliceous framework such as silicalite and Si-Y results in extensive molecular motion at room temperature, the degree of which decreases with decreasing temperature. In zeolites ferrierite, L, and Na-Y, the presence of cations in the framework markedly hinders the overall molecular motion. The cations clearly play a role in the observed static disorder of the guest molecule in zeolite L. Important information regarding the location of the n-pentane molecules within silicalite and ferrierite is also obtained.     

Molecular sieving MFI-type zeolite membranes for pervaporation separation of xylene isomers

Molecular sieving MFI-type zeolite membranes were prepared by a secondary growth method without using an organic template. Silicalite membranes with intercrystalline pores minimized or eliminated were obtained by this synthesis method which avoids the template removal step. The silicalite membrane exhibits molecular sieving characteristics with pervaporation separation factor for p-xylene to o-xylene or m-xylene of as high as about 70, the highest ever reported for a pervaporation membrane.     

Separation of acetic acid-water mixtures by pervaporation through silicalite membrane

Polycrystalline silicalite membranes were prepared on two kinds of porous supports by hydrothermal synthesis. The pervaporation performance of the silicalite membrane obtained was investigated using an acetic acid-water mixture as a feed. The silicalite membrane on the sintered stainless steel support selectively permeates acetic acid in the concentration of the feed acetic acid in the region of 5 to 40 vol%. However, the membrane on the porous alumina support showed no separation for the aqueous acetic acid solution. From the fact that the top layer of the membrane on the alumina support was not composed of pure silicalite but ZSM-5 zeolite crystals, which contained Brønsted acidic sites (Si(OH)Al) in the framework, it was suggested that the acidic sites associated with the framework aluminums play an important role in the separation of the acetic acid-water mixture. A long-term test of the pervaporation was also carried out to clarify the stability of the membrane.     

Molecular dynamics simulations of mass transfer resistance in grain boundaries of twinned zeolite membranes

The net mass transfer resistance for gas molecules permeating through zeolite membranes includes contributions from intracrystalline diffusion and contributions from interfacial effects. These interfacial effects can arise either from gas-zeolite interfaces or from interfaces that exist within zeolite crystals due to grain boundaries. We present the first atomically detailed simulations that examine interfacial mass transfer resistance due to internal grain boundaries in zeolites that are relevant for membrane applications. Our calculations examine twinned silicalite crystals in crystallographic configurations that have been identified in previous experiments. We used the dual control volume grand canonical molecular dynamics method to simulate the permeance of CH(4) and CF(4) through thin twinned silicalite crystals. The magnitudes of the grain boundary resistances are quite substantial, at least for the thin crystals that are accessible in our simulations.     

Pervaporation Separation and Catalysis Activity of Novel Zirconium Silicalite-1 Zeolite Membrane

Novel zirconium silicalite‐1 zeolite membrane was hydrothermally prepared on the mullite porous support at 150–185°C for 40–72 h by an "in situ" method using tetraethyl orthosilicate (TEOS), zirconium butoxide (ZBOT) and tetrapropylammonium hydroxide (TPAOH) as silica source, zirconium source and organic structure directing agent, respectively. X‐ray diffraction (XRD) patterns, fourier transformed infrared (FT‐IR) spectra, and inductively coupled plasma‐atomic emission spectrometry (ICP) of the accompanying zeolite powder confirmed that the zirconium was isomorphously incorporated into the zeolite framework. The surface chemical compositions of the obtained membrane were measured with an energy‐dispersive X‐ray spectral analyzer (EDS), and the membrane morphologies were observed by a scanning electron microscope (SEM). The results showed that the zeolite crystals growing on the support were zirconium silicalite‐1 zeolites, and the dense membrane layer was composed of the well inter‐growing zeolite crystals. The zirconium silicalite‐1 zeolite membrane, which was derived from the synthesis solution having a molar ratio of 1.00SiO₂:0.01ZrO₂:0.17TPAOH:120H₂O, showed high ethanol permselectivity with a flux of 1.01 kg/(m²·h) accompanied with a separation factor of 73 for ethanol/water (5/95, w/w) system under a pervaporation condition at 60°C. Moreover, this membrane displayed pervaporation‐aided catalysis activity for iso‐propanol oxidation with hydrogen peroxide as oxidant, and the corresponding iso‐propanol conversion was 35%.     

Layer‐by‐Layer Approach to Superhydrophobic Zeolite Antireflective Coatings

Antireflection surfaces and coatings have attracted considerable interests because they can maximize light transmittance of the substrates. In this work, zeolite antireflective (ZAR) coatings are prepared via layer‐by‐layer (LBL) assembly of MFI ‐type zeolite silicalite‐1 and polyelectrolyte. A micro‐ and macroporous hierarchical structure was obtained which contributes to the antireflective property of the zeolite coatings. The light transmittance of the coating on quartz can achieve as high as 99.3% at 650 nm. Furthermore, a superhydrophobic ZAR coating can be obtained by chemical modification with 1H,1H,2H,2H–perfluorooctyl‐triethoxysilane. This work demonstrates that zeolites are excellent candidates as high transparent superhydrophobic coatings.     

Electrodeposition within zeolite membrane hosts

An electrochemical procedure for the incorporation of metal nanoparticles (diameter ∼1 nm) into coherent zeolite membranes is reported. The method is exemplified with the deposition of platinum in healed zeolite Y membranes but has also been applied to copper deposition within membranes of silicalite.