MFI-type Zeolite Research Articles

Facile preparation of hierarchically porous diatomite/MFI-type zeolite composites and their performance of benzene adsorption: The effects of NaOH etching pretreatment

Hierarchically porous diatomite/MFI-type zeolite (Dt/Z) composites with excellent benzene adsorption performance were prepared. The hierarchical porosity was generated from the microporous zeolite coated at the surface of diatom frustules and from the macroporous diatomite support. A facile NaOH etching method was employed for the first time to treat the frustule support, followed by hydrothermal growth of MFI-type zeolite at the surface of frustules previously seeded with nanocrystalline silicalite-1 (Sil-1). NaOH etching enlarged the pores on diatom frustules and further increased the coated zeolite contents (Wz). The central macropore size of the diatom frustules increased from approximately 200–500nm to 400–1000nm after NaOH etching. The Wz could reach 61.2%, while the macroporosity of the composites was largely preserved due to more voids for zeolite coating being formed by NaOH etching. The Dt/Z composites exhibited higher benzene adsorption capacity per unit mass of zeolite and less mass transfer resistance than Sil-1, evaluated via a method of breakthrough curves. These results demonstrate that etching of a diatomite support is a facile but crucial process for the preparation of Dt/Z composites, enabling the resulting composites to become promising candidates for uses in volatile organic compounds emission control.

Deactivation mechanism of MFI-type zeolites by coke formation during n-hexane cracking

Diffusivity and adsorption properties of n-hexane within coked MFI-type zeolites, the micropore volume, and number of acid sites were measured to investigate the deactivation mechanism of MFI-type zeolite during n-hexane cracking. The coked MFI-type zeolite was prepared by n-hexane cracking over the zeolite at 923K. The diffusivity and adsorption isotherm of n-hexane within the coked zeolite was obtained using the constant volume method. Micropore volume and acid sites number of the coked zeolite decreased with increasing coke loading, resulting in a decrease in n-hexane adsorption on the coked zeolite. In contrast, intracrystalline diffusivity of n-hexane within the coked zeolite increased with coke loading, and effective diffusivity remained constant regardless of coke loading. Coke formation on the acid site inside the zeolite pores caused a decrease in n-hexane adsorption onto the acid site, resulting in geometrical limitations that promoted the major mass transfer resistance. Effect of diffusivity on MFI-type zeolite deactivation during n-hexane cracking was small. The main cause of the deactivation was considered to be a decrease in reaction rate due to the decrease in the number of acid sites for n-hexane cracking by coke formation.

Application of robust MFI-type zeolite membrane for desalination of saline wastewater

Zeolites are potentially a more robust desalination alternative as they are chemically stable and possess the essential properties needed to reject ions. This work proposes to use zeolite membranes for desalination of saline recycled wastewater for the possibility of avoiding the costly pre-treatment needed for polymeric reverse osmosis membranes. The MFI-type zeolite membrane was developed on a tubular α-alumina substrate by a combined rubbing and secondary hydrothermal growth method. The prepared membrane was characterised by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and single gas (He or N2) permeation, and underwent desalination tests with saline wastewater under different conditions. When saline wastewater was fed at 7MPa to the zeolite membrane it showed a salt rejection of 80% based on electrical conductivity (EC) and a flux of 4Lm−2h−1. A254 (organics absorption at the wavelength of 254nm measured by a HACH DR5000 spectrophotometer) removal exceeded 90%. Slightly lower salt removal and A254 results were observed when operating at a lower pressure of 3MPa. During batch concentration runs on the saline wastewater, the EC of the feed water increased from the initial value of 1770µScm−1 to 3100µScm−1 over the 48h test time which indicates that a 43% water recovery was achieved. EC reduction remained >70% and flux was maintained at around 2Lm−2h−1 throughout the test period, indicating that the membrane resisted organic fouling. Chlorine stability studies showed that a long-term (7 days) strong hypochlorite clean did not significantly alter the flux or rejection confirming the chemical stability of zeolite membranes. Overall, the zeolite membrane showed excellent chemical resilience and produced a desalinated product suitable for reuse applications (e.g. irrigation, residential or industrial). Fluxes, however, need to be improved to be competitive with current polymeric membranes as do rejections for higher purity water applications.

Adsorption structure of dimethyl ether on silicalite-1 zeolite determined using single-crystal X-ray diffraction.

The adsorption structures of dimethyl ether (DME) on silicalite-1 zeolite (MFI-type) are determined using single-crystal X-ray diffraction. The structure of low-loaded DME-silicalite-1 indicates that all DME molecules are located in the sinusoidal channel, which is the most stable sorption site based on the van der Waals interaction between DME and the framework. The configuration of guest molecules (linear or bent) plays an important role in determining where the stable sorption site is in the pore system of MFI-type zeolites. Bent molecules favor the sinusoidal channel, while linear molecules favor the straight channel. The contribution of DME-DME interactions is considerable in the high-loaded DME-silicalite-1 structure.

Remarkable acceleration of the fructose dehydration over the adjacent Brønsted acid sites contained in an MFI-type zeolite channel

5-Hydroxymethylfurfural (HMF) is an important platform chemical that can be obtained from biomass by acid catalysis. In the current investigation, an acceleration of the dehydration of fructose to HMF with an increased reaction rate and decreased activation energy is identified over the adjacent acid sites in MFI-type zeolites. The spatial proximities of these sites become prominent with the decrease of the Si/Al ratio in HZSM-5 and increase the charge density on the adsorbed reactant molecule according to 13C MAS NMR results. The cooperative catalysis by the adjacent acid sites is deduced and it is very important to develop new effective acid catalysts.

Microstructural optimization of MFI-type zeolite membranes for ethanol–water separation

High quality pure-silica MFI-type zeolite membranes are successfully prepared by simply controlling the amount of structure directing agent (SDA), i.e., tetrapropylammonium (TPA+) in the synthesis solution for seeded growth. The effects of several synthesis parameters such as alkalinity (OH−/Si), TPA+ concentration (TPA+/Si), and crystallization time on the membrane pervaporation performance are investigated in detail. The synthesized MFI-type zeolite membranes are thoroughly characterized by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy, the water contact angle test, X-ray diffraction (XRD), the pervaporation test and gas permeation. The membrane microstructure is very sensitive to the TPA+/Si ratio of the synthesis solution. At a high TPA+/Si ratio (typically 0.17), parasitic twin crystals are intergrown in the zeolite layer, thus resulting in the formation of membrane defects during SDA removal treatment (calcination at 500 °C). When the membrane is prepared with a low TPA+/Si ratio (≤0.05), the appearance of twin crystals can be significantly suppressed and the synthesized dense MFI-type zeolite membranes exhibit pervaporation separation factors higher than 85 for 5 wt% ethanol–water mixtures at 60 °C. We demonstrate for the first time that controlling the SDA concentration in the precursor solution is beneficial to the elimination of membrane microstructural defects, showing a pathway to high quality MFI-type zeolite membranes.

An Important Factor in CH4 Activation by Zn Ion in Comparison with Mg Ion in MFI: The Superior Electron-Accepting Nature of Zn2+

We present clear IR and density functional theory (DFT) evidence demonstrating that the electron-accepting nature of Zn2+ ion exchanged in MFI-type zeolite (ZnMFI) plays a dominant role in CH4 activation. The IR study revealed that the heterolytic dissociation of CH4 takes place on the Zn2+ ion exchanged in MFI under a CH4 atmosphere even near room temperature, whereas a similar reaction scarcely occurred on Mg2+ ion exchanged in MFI, although the ionic radius and charge of Mg2+ are almost the same as those of Zn2+. These data indicate that the dissociation reaction of CH4 on Zn2+ in MFI is facilitated not only by the electrostatic interaction but also by the electron-transfer interaction. This interpretation was clearly evidenced by the observed v1 mode of the C–H symmetric stretching vibration, i.e., a larger band shift toward lower wavenumbers, for the molecular CH4 adsorbed on ZnMFI, compared with those for a gaseous CH4 molecule. Additional experiments were also performed by the IR method utilizing C...

Hydrothermal conversion of FAU and ∗BEA-type zeolites into MAZ-type zeolites in the presence of non-calcined seed crystals

To gain further insight into the interzeolite conversion, we investigated the seed-assisted synthesis of MAZ-type zeolite from various starting zeolites with different framework structures, such as FAU, ∗BEA, and MFI-type zeolites, without the use of an organic structure-directing agent (OSDA). The OSDA-free synthesis of MAZ-type zeolite from FAU and ∗BEA-zeolites was successfully achieved in the presence of non-calcined seed crystals. However, a pure MAZ-type zeolite could not be obtained from MFI-type zeolite. The crystallinity of the obtained MAZ-type zeolite strongly depended on the kind of framework structure of the initial zeolite. The crystallinity of MAZ from FAU was higher than that of MAZ from ∗BEA. It was confirmed that the structural similarity between the starting and the finally crystallized zeolites is a crucial factor for the interzeolite conversion process. The presence of composite building units composed of a common structural entity, namely a four-membered ring, is a key factor for the OSDA-free synthesis of MAZ-type zeolite.

High-pressure-induced structural changes, amorphization and molecule penetration in MFI microporous materials: a review.

This is a comparative study on the high-pressure behavior of microporous materials with an MFI framework type (i.e. natural mutinaite, ZSM-5 and the all-silica phase silicalite-1), based on in-situ experiments in which penetrating and non-penetrating pressure-transmitting media were used. Different pressure-induced phenomena and deformation mechanisms (e.g. pressure-induced over-hydration, pressure-induced amorphization) are discussed. The influence of framework and extra-framework composition and of the presence of silanol defects on the response to the high pressure of MFI-type zeolites is discussed.

Enhanced selectivity to benzaldehyde in the liquid phase oxidation of benzyl alcohol using nanocrystalline ZSM-5 zeolite catalyst

In the present paper, nanocrystalline hierarchical ZSM-5 zeolites were successfully synthesized by the hydrothermal method in the presence of tetrapropylammonium hydroxide as a single template with the gel composition of 58SiO2:Al2O3:20TPAOH:1,500H2O. The prepared zeolite catalysts were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Nitrogen adsorption–desorption (BET), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HR-TEM) techniques. The formation of pure and highly crystalline ZSM-5 zeolite phase is confirmed by XRD. The IR vibration band at 550 cm−1 is assigned to the double 5-rings of MFI-type zeolites. N2 adsorption–desorption isotherms showed that the synthesized product had high BET surface area and possessed composite pore structures with both micro and mesopores. The catalytic performance of hierarchical ZSM-5 zeolite was investigated in the selective oxidation of benzyl alcohol (BzOH) with hydrogen peroxide (H2O2) under mild conditions. The results showed that the conversion of BzOH and the selectivity to benzaldehyde were about 94 and about 99 % respectively, when using 0.08 g ZSM-5 catalyst with acetonitrile as the solvent and H2O2 as the oxidant at 90 °C. This catalyst can be retrieved and reprocessed for five times without a significant loss in its activity and selectivity.

Fabrication of a highly b-oriented MFI-type zeolite film by the Langmuir-Blodgett method.

sec-Butanol-modified rounded-coffin-shaped silicalite-1 (SL) microcrystals were assembled into a compact and highly b-oriented monolayer extending over the centimeter scale via the Langmuir-Blodgett (LB) technique. For comparison, methanol- or ethanol-modified SL microcrystals could not float and were compressed into a dense film in an LB trough. Subsequently, highly b-oriented MFI films with a thickness of ∼1.5 μm were successfully obtained on the solid substrates by secondary growth of the LB monolayer using tetrapropylammonium hydroxide (TPAOH) as the structure-directing agent. The electrochemical experiments confirmed that the prepared films were defect-free. In general, the LB method is a highly controllable and reproducible method of organizing anisotropic zeolite crystals with a preferred orientation over a relatively large surface area. The LB technique could be further applied as an effective platform for the oriented assembly of different types of zeolite particles and the growth of variously oriented zeolite films.

Zeolite decorated highly porous acicular calcium silicate ceramics

Macrocellular calcium silicate (wollastonite) ceramic foams possessing ~86vol% total porosity were produced from commercial calcium silicate hydrate (xonotlite) nano-size acicular crystals and poly-methyl-methacrylate microbeads (PMMA) (used as sacrificial pore formers). Open cell wollastonite foams had a bi-modal pore size distribution with major modes located ~100nm due to interparticle porosity and ~100µm due to the sacrificial pore former. These macrocellular ceramics were then used as a scaffold for MFI type zeolite (silicalite-1) synthesis. Monolayer coffin shaped zeolite crystals (~3µm size, measured from the c-axis and ~300nm thickness) were observed with almost full coverage on the inner macro-cell walls. The specific surface area of the components increased from 9.6m2/g to 108.2m2/g via zeolite functionalization, leading to components possessing multiscale porosity.

Adsorption Process of CO2 on Silicalite-1 Zeolite Using Single-Crystal X-ray Method

The process of CO2 adsorption on silicalite-1 zeolite (MFI-type) is revealed using a single-crystal X-ray method. The structure of CO2-silicalite-1 with a small amount of CO2 in the pore is determined, wherein most of CO2 molecules are located in the straight channel. It indicates the straight channel is the most stable sorption site based on the van der Waals interactions between the CO2 and the framework, and the CO2 molecules initially adsorb in the straight channel in the adsorption process. This is the first report to describe the structure of MFI-type zeolites with the adsorbate molecules occupying only the straight channel.

Understanding the Role of the Cosolvent in the Zeolite Template Function of Imidazolium-Based Ionic Liquid

In this work, a study for understanding the role played by [ClBmim], [BF4Bmim], [PF6Bmim], and [CH3SO3Bmim] ionic liquids (ILs) in the synthesis of zeolites is presented. The use of [ClBmim] and [CH3SO3Bmim] ILs, as reported earlier [ Chem. Eur. J. 2013 , 19 , 2122 ] led to the formation of MFI or BEA type zeolites. Contrary, [BF4Bmim] and [PF6Bmim] ILs did not succeed in organizing the Si-Al network into a zeolite structure. To try to explain these results, a series of quantum mechanical calculations considering monomers ([XBmim]) and dimers ([XBmim]2) by themselves and plus cosolvent (water or ethanol) were carried out, where X ≡ Cl(-), BF4(-), PF6(-), or CH3SO3(-). Our attention was focused on the similarities and differences among the two types of cosolvents and the relation between the structure and the multiple factors defining the interactions among the ILs and the cosolvent. Although a specific pattern based on local structures explaining the different behavior of these ILs as a zeolite structuring template was not found, the calculated interaction energies involving the Cl(-) and CH3SO3(-) anions were very close and larger than those for BF4(-) and PF6(-) species. These differences in energy can be used as an argument to describe their different behavior as structure directing agents. Moreover, the topology of the cosolvent is also an ingredient to take into account for a proper understanding of the results.

Synthesis of multilamellar MFI-type zeolites under static conditions: The role of gel composition on their properties

The formation of multilamellar MFI-type zeolite has been studied systematically by investigating the crystallization kinetics and the influence of different synthesis parameters under static conditions. Depending on the template amount and alkalinity, multilamellar MFI-type zeolites with different degrees of purity were obtained. In accordance with X-ray diffraction, SAXS and SEM results, highly crystalline multilamellar MFI-type zeolites were obtained within 4days of crystallization time. Depending on the Si/Al ratio of the synthesis gel, different crystallization rates, textural properties and product morphologies were observed.

Reanalysis of CO2-silicalite-1 structure as monoclinic twinning

Abstract Structural analysis of CO2-silicalite-1 crystals using the single-crystal X-ray diffraction method reveals that the crystal system is twinned monoclinic. The structure of silicalite-1 (MFI-type zeolites) loaded with CO2 has previously been solved in orthorhombic system [1], but the R value is still large. Since CO2 molecules are too small to maintain the orthorhombic structure, it is considered that the CO2-silicalite-1 crystals should be in the relaxed monoclinic twin phase. The diffraction data in the previous report is recalculated in P21 /n as the monoclinic twinning.

Zeolite films as building blocks for antireflective coatings and vapor responsive Bragg stacks.

Zeolite films (LTL, BEA and MFI) are prepared with a thickness in the range 50-170 nm through a multistep spin-on deposition method. The optical properties of the zeolite films including refractive index, extinction coefficient and thickness are determined from the reflectance spectra using a nonlinear curve fitting method. The total free pore volume of the films using the Bruggeman effective medium theory is calculated. The potential of the zeolite films for broadband antireflection (AR) application is demonstrated. Five times reduction of the reflectance of a silicon substrate covered with the double AR films comprising of MFI type zeolite (120 nm) deposited on Nb2O5 (60 nm) is achieved. Additionally, the MFI zeolite film is used as a building block of vapor responsive Bragg stacks with a strong response towards acetone. The reversible response of the Bragg stacks towards acetone without additional annealing opens up the possibility of preparing sensors with optical read-out by incorporation of sensitive and transducer elements into a single device.

A continuous process for glyoxal valorisation using tailored Lewis-acid zeolite catalysts

The aqueous-phase heterogeneously catalysed isomerisation of bio-oil derived glyoxal is herein introduced as a novel route for the sustainable production of glycolic acid. While commercial ultra-stable Y zeolites displayed only moderate performance, their evaluation enabled us to highlight the crucial role of Lewis acidity in the reaction. Gallium incorporation into these zeolites boosted the glycolic acid yield, although the best catalytic results were obtained over tin-containing MFI-type zeolites, reaching 91% yield of the desired product at full conversion. These materials comprised hydrothermally-synthesised Sn-MFI as well as a novel catalyst obtained by the introduction of tin into silicalite-1 by means of a simpler and more scalable method, i.e. alkaline-assisted metallation. In-depth spectroscopic characterisation of these systems uncovered a substantial similarity of the tin centres obtained by the top-down and bottom-up synthetic approaches. NMR spectroscopic studies gave evidence that the reaction follows a 1,2-hydride shift mechanism solely catalysed by Lewis-acid sites. The Sn-MFI analogue could be reused in 5 cycles without the need for intermediate calcination, did not evidence any tin leaching, and demonstrated suitability for utilisation under continuous-flow operation. The tin-based zeolites exhibited remarkable performance also in alcoholic solvents, leading to the one-pot production of relevant alkyl glycolates.

MFI-type zeolite nanosheets for gas-phase aromatics chlorination: a strategy to overcome mass transfer limitations

Mass transfer limitations and catalytic activity were studied for various ZSM-5 zeolite crystal sizes in the chlorination of deactivated arenes. An estimation of the quantity of mild acidic external silanol groups of zeolite nanosheets was made.

Acid property of MFI-type zeolites probed by trimethylphosphine oxide studied by solid-state NMR

Acid properties of ZSM-5 type zeolite are studied by solid-state NMR using trimethylphosphine oxide (TMPO) as a probe molecule. TMPO is introduced from the gas phase at 373K. The amounts of the loaded TMPO are quantified by the signal intensities of 1H MAS NMR spectra, which are compared with the numbers of Brønsted acid sites estimated by the Si/A ratio of the framework derived from 29Si MAS NMR spectra and the content of fourfold-coordinated Al from 27Al MAS NMR spectra. The use of 373K for loading the probe molecules increases the loading level of TMPO, and thus almost all the acid sites are probed. The 31P chemical shifts in the range between 90 and 60ppm are attributed to TMPO adsorbed on Brønsted acid sites, whereas the signals between 60 and 40ppm are ascribed to TMPO adsorbed on other sites than Brønsted acid sites. The 13C chemical shift of methyl groups in the adsorbed TMPO depends mainly on the confined space as well as on the acid strength.