MFI Zeolite Crystals Research Articles

Efficient Catalytic Production of Hydrogen Peroxide Using Tin‐containing Zeolite Fixed Palladium Nanoparticles with Oxidation Resistance

AbstractThe metal surfaces tend to be oxidized in air through dissociation of the O−O bond of oxygen to reduce the performances in various fields. Although several ligand modification routes have alleviated the oxidation of bulky metal surfaces, it is still a challenge for the oxidation resistance of small‐size metal nanoparticles. Herein, we fixed the small‐size Pd nanoparticles in tin‐contained MFI zeolite crystals, where the tin acts as an electron donor to efficiently hinder the oxidation of Pd by weakening the adsorption of molecular oxygen and suppressing the O−O cleavage. This oxidation‐resistant Pd catalyst exhibited superior performance in directly synthesizing hydrogen peroxide from hydrogen and oxygen, with the productivity of hydrogen peroxide at ≈10,170 mmol gPd−1 h−1, steadily outperforming the catalysts tested previously. This work leads to the hypothesis that tin is an electron donor to realize oxidation‐resistant Pd within zeolite crystals for efficient catalysis to overcome the limitation of generally supported Pd catalysts and further motivates the use of oxidation‐resistant metal nanoparticles in various fields.

Efficient Catalytic Production of Hydrogen Peroxide Using Tin-containing Zeolite Fixed Palladium Nanoparticles with Oxidation Resistance.

The metal surfaces tend to be oxidized in air through dissociation the O-O bond of oxygen to reduce the performances in various fields. Although several ligand modification routes have alleviated the oxidation of bulky metal surfaces, it is still a challenge for the oxidation resistance of small-size metal nanoparticles. Herein, we fixed the small-size Pd nanoparticles in tin-contained MFI zeolite crystals, where the tin acts as an electron donor to efficiently hinder the oxidation of Pd by weakening the adsorption of molecular oxygen and suppressing the O-O cleavage. This oxidation-resistant Pd catalyst exhibited superior performance in directly synthesizing hydrogen peroxide from hydrogen and oxygen, with the productivity of hydrogen peroxide at ~10,170 mmol gPd-1 h-1, steadily outperforming the catalysts tested previously. This work leads to the hypothesis that tin is an electron donor to realize oxidation-resistant Pd within zeolite crystals for efficient catalysis to overcome the limitation of generally supported Pd catalysts, and further motivates the use of oxidation-resistant metal nanoparticles in various fields.

Fischer-Tropsch synthesis to olefins boosted by MFI zeolite nanosheets.

Catalytic reactions are severely restricted by the strong adsorption of product molecules on the catalyst surface, where promoting desorption of the product and hindering its re-adsorption benefit the formation of free sites on the catalyst surface for continuous substrate conversion1,2. A solution to this issue is constructing a robust nanochannel for the rapid escape of products. We demonstrate here that MFI zeolite crystals with a short b-axis of 90-110 nm and a finely controllable microporous environment can effectively boost the Fischer-Tropsch synthesis to olefins by shipping the olefin molecules. The ferric carbide catalyst (Na-FeCx) physically mixed with a zeolite promoter exhibited a CO conversion of 82.5% with an olefin selectivity of 72.0% at the low temperature of 260 °C. By contrast, Na-FeCx alone without the zeolite promoter is poorly active under equivalent conditions, and shows the significantly improved olefin productivity achieved through the zeolite promoter. These results show that the well-designed zeolite, as a promising promoter, significantly boosts Fischer-Tropsch synthesis to olefins by accelerating escape of the product from the catalyst surface.

Entrapped NbOx clusters in MFI zeolite for sustainable acid catalysis

Abstract Entrapping the small NbOx clusters into MFI zeolite crystals was directly prepared via a facile hydrothermal method, assisting with the sodium citrate and EDTA-2Na to dissolve and chelate the niobium precursors. The existing states and configurations of Nb species in the zeolites were carefully characterized by a series of techniques including XRD, FTIR, SEM, TEM, UV–Vis, XPS and Raman. These results clearly indicated that the entrapped oligomeric NbOx clusters in MFI zeolite crystals contained Nb-OH, Nb=O and Nb-O-Si bonds. These characters were the origin of both Lewis and Bronsted acid sites for these Nb-MFI samples, which was vividly confirmed by FTIR spectroscopy with pyridine absorption and solid-state NMR spectroscopy with ammonia and TMPO as probing molecules techniques. The as-prepared Nb-MFI zeolites were employed as sustainable Lewis acids in several catalytic reactions. Especially, Nb-MFI zeolites exhibited remarkable catalytic performance in the cross-aldol condensation between furfural and acetone for biomass upgrading, surpassing traditional Lewis acidic zeolites Sn-MFI and Sn-BEA in terms of activity and selectivity, respectively. The 2.6%Nb-MFI also showed very good stability during reaction and it could be recycled for six times without loss in activity. Finally, 2.6%Nb-MFI was verified as a general catalyst for the cross-aldol condensation reaction between small-molecule aldehydes and ketones. All these results demonstrate the great potential of Nb-MFI as a robust catalyst for the shape-selective Lewis acid catalysis. The synthesis strategy developed herein could be readily extended to the construction of zeolite entrapped high-valence transition metal clusters for catalytic applications.

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.

Concerted Bimetallic Nanocluster Synthesis and Encapsulation via Induced Zeolite Framework Demetallation for Shape and Substrate Selective Heterogeneous Catalysis.

Bimetallic nanoparticle encapsulation in microporous zeolite crystals is a promising route for producing catalysts with unprecedented reaction selectivities. Herein, a novel synthetic approach was developed to produce PtZnx nanoclusters encapsulated inside zeolite micropores by introducing Pt2+ cations into a zincosilicate framework via ion exchange, and subsequent controlled demetallation and alloying with framework Zn. The resulting zeolites featured nanoclusters with sizes of approximately 1 nm, having an interatomic structure corresponding to a PtZnx alloy as confirmed by pair distribution function (PDF) analysis. These materials featured simultaneous shape and substrate specificity demonstrated by the selective production of p-chloroaniline from the competitive hydrogenation of p-chloronitrobenzene and 1,3-dimethyl-5-nitrobenzene.

Importance of hydrogen for low-temperature detemplation of high-silica MFI zeolite crystals

Many attempts have been made for template removal from inorganic porous materials at mild temperature (below 350 °C), because high-temperature calcination is attributed as the trigger of the framework damages. In this study, a new two-step method, coupling by low-temperature hydrocracking and oxidation, is proposed for efficient removal of the organic templates in high-silica MFI zeolite crystals of different size at a mild temperature (300 °C). With this approach, unlike in conventional calcination above 400 °C, the pristine zeolite framework is preserved and no coke-like residues are formed, consistently verified by NMR, TGA, and Raman spectra. The reason for this phenomenon is proved as a special propyl-radical- mechanism in H2 atmosphere, indicated by GC measurement and analysis online. This provides an alternative option for the low-temperature detemplation of high-silica microporous materials.

Synthesis of Wool-Ball-Like ZSM-5 with Enlarged External Surfaces and Improved Diffusion: A Potential Highly-Efficient FCC Catalyst Component for Elevating Pre-cracking of Large Molecules and Catalytic Longevity

A hierarchical ZSM-5 zeolite composed of nano-sized MFI zeolite crystals was synthesized without using secondary template by a traditional hydrothermal procedure. The as-synthesized hierarchical ZSM-5 and a reference catalyst were both characterized by XRD, SEM, FT-IR, in situ infrared (IR) spectrometry of pyridine, NH3-TPD, N2 adsorption–desorption, intelligent gravimetric analyzer, and by thermogravimetry analyses. After examining and comparing the results, it is discovered that the hierarchical ZSM-5 catalyst displays excellent catalytic performance with an improved conversion of isopropylbenzene and a longer catalytic life because of its dramatically enlarged external surfaces. The results also display that the shortened diffusion path length contributes to enhancing the stability of the hierarchical catalyst by depressing the coking deposit during the catalytic cracking of n-octane. A wool-ball-like zeolite composed of nano-sized MFI crystals was synthesized without a secondary template by the traditional hydrothermal procedure. Nanocrystallization of crystal particles in the hierarchical ZSM-5 zeolite gives the catalyst an improved diffusion and a dramatically enhanced external surface which contributes to elevating pre-cracking of large molecules and catalytic longevity, and to offering a potential and high-efficiency FCC catalyst component.

Zeolites as coating materials for Fiber Bragg Grating chemical sensors for extreme conditions

Important progress has been made towards the expanding of the limits of applicability of Fiber Bragg Grating (FBG) optical sensors. The development of new inorganic coatings consisting of MFI zeolite crystals (silicalite and ZSM-5) has opened up the possibility towards the use of this technology at high temperatures and pressures. Using two different, purposely-built experimental setups, it was shown that zeolite-coated FBG sensors can be used for chemical gas sensing, both at ambient as well as at extreme conditions, i.e. at temperatures of up to 260°C and pressures of up to 200bar. The sensors are selective for linear alkanes, show no significant cross-sensitivity towards H2O and CO2, are thermally and chemically robust and have response times of 5min or less. Additionally, molecular simulations are used to bring insight into the underlying nature of the sensing mechanism.

Platinum Nanoparticles Encapsulated in MFI Zeolite Crystals by a Two-Step Dry Gel Conversion Method as a Highly Selective Hydrogenation Catalyst

A unique and well-controllable synthesis route to encapsulate metallic nanoparticles in the interior of MFI zeolite crystals has been developed. In the first step, hierarchical micro-mesoporous ZSM-5 zeolite was obtained by alkali treatment, and the platinum was deposited mainly in the pores. Then the precursor was covered with a gel similar in composition to silicalite-1 zeolite, which was structurally converted as whole to the Pt-encapsulated MFI zeolite employing the dry gel conversion method. With this method, the metal species, content, size, and encapsulation in the zeolite are easily controllable. The highly thermally stable Pt nanoparticles encapsulated in MFI zeolites kept their original size after a high-temperature catalytic test for CO oxidation. Because of the size selectivity of the MFI zeolite, the current Pt@MFI catalyst was highly active for hydrogenation of nitrobenzene but inert for hydrogenation of 2,3-dimethylnitrobenzene. Also, the Pt@MFI catalyst is highly selective for the hydrogen...

Synthesis of Zeolite Nanomolecular Sieves of Different Si/Al Ratios

Nanosized zeolite molecular sieves of different Si/Al ratios have been prepared using microwave hydrothermal reactor (MHR) for their greater application in separation and catalytic science. The as‐synthesized molecular sieves belong to four different type zeolite families: MFI (infinite and high silica), FAU (moderate silica), LTA (low silica and high alumina), and AFI (alumina rich and silica‐free). The phase purity of molecular sieves has been assessed by X‐ray diffraction (XRD) analysis and morphological evaluation done by electron microscopy. Broad XRD peaks reveal that each zeolite molecular sieve sample is composed of nanocrystallites. Scanning electron microscopic images feature the notion that the incorporation of aluminum to MFI zeolite synthesis results in morphological change. The crystals of pure silica MFI zeolite (silicalite‐1) have hexagon lump/disk‐like shape, whereas MFI zeolite particles with Si/Al molar ratios 250 and 100 have distorted hexagonal lump/disk and pseudo spherical shapes, respectively. Furthermore, phase pure zeolite nanocrystals of octahedron (FAU), cubic (LTA), and rod (AFI) shape have been synthesized. The average sizes of MFI, FAU, LTA, and AFI zeolite crystals are 250, 150, 50, and 3000 nm, respectively. Although the length of AFI zeolite rods is in micron scale, the thickness and width are of a few nanometers.

Methods of increasing the stability of the catalytic effect of MFI type zeolites and the total service life of catalysts based on them

Increasing the operational stability of MFI-type zeolite catalysts is topical for the creation of highly efficient catalysts for the conversion of methanol or C2-C4 hydrocarbon gases into aromatic hydrocarbons, i.e., important industrial processes. This study proposes three new approaches to increasing the stability of zeolite catalysts: selective dealumination on the outer surface of MFI zeolite crystals, structurally selective ion exchange on the outer surface of crystals, and the application of an isothermal (multitube) reactor. The effect of selective dealumination and structurally selective ion exchange conditions on the SiO2/Al2O3 molar ratio in the zeolite on the interregeneration period duration in the conversion of methanol into hydrocarbons has been studied. It has been established that the selective dealumination of the outer surface increased the interregeneration period duration by 3–5 times. Structurally selective ion exchange on the outer surface of zeolite crystals allows us to increase the interregeneration period duration by 2–4 times and to decrease the regeneration temperature and duration due to the change of the properties of deposited coke. The application of a multitube reactor facilitates the procedure of regeneration in comparison with an adiabatic reactor due to the formation of less condensed coke depositions.

Molecular shape-selectivity of MFI zeolite nanosheets in n-decane isomerization and hydrocracking

MFI zeolite nanosheets with thickness of 2 and 8nm were synthesized, transformed into bifunctional catalysts by loading with platinum and tested in n-decane isomerization and hydrocracking. Detailed analysis of skeletal isomers and hydrocracked products revealed that the MFI nanosheets display transition-state shape-selectivity similar to bulk MFI zeolite crystals. The suppressed formation of bulky skeletal isomers and C5 cracking products are observed both in the nanosheets and the bulk crystals grown in three dimensions. This is typical for restricted transition-state shape-selectivity, characteristic for the MFI type pores. It is a first clear example of transition-state shape-selectivity inside a zeolitic nanosheet. Owing to the short diffusion path across the sheets, expression of diffusion-based discrimination of reaction products in the MFI nanosheets was limited. The 2-methylnonane formation among monobranched C10 isomers and 2,7-dimethyloctane among dibranched C10 isomers, which in MFI zeolite are favored by product diffusion, was much less favored on the nanosheets compared to the reference bulk ZSM-5 material.

Preparation and Gas Adsorption Characteristics of Zeolite MFI Crystals with Organic-Functionalized Interiors

We show that the selective gas adsorption properties of pure-silica MFI zeolite crystals are significantly altered by modifying their internal pore structure with molecules such as aliphatic alcohols, aromatic alcohols, amino-alcohols, and amines. NMR and TGA/DSC characterization indicates that the functional groups are chemisorbed to the internal pores, and the organic molecule loading is strongly dependent on the concentration of internal silanol sites. Adsorption isotherms of CO2, CH4, and N2 are measured for the functionalized samples up to 400 kPa. In the low pressure region (1–10 kPa), the organic-modified materials show gas adsorption behavior significantly different from that of the bare zeolite MFI, ranging from an enhancement in CO2/CH4 sorption selectivity in the amine-functionalized materials to an enhancement in CH4 adsorption in the aromatic-functionalized material. The CO2/N2 sorption selectivity decreases for all the modified materials compared to bare zeolite MFI. This investigation of organic-modified MFI zeolites points toward the tuning of adsorption and molecular sieving behavior of zeolites through internal modification of the pores.

Preparation of nano-crystalline MFI zeolite via hydrothermal synthesis in water/surfactant/organic solvent using fumed silica as the Si source

The preparation of nano-crystalline MFI zeolites (Silicalite-1 and ZSM-5) was carried out by hydrothermal synthesis in a water/surfactant/organic solvent using fumed silica and aluminum sulfate as the Si and Al source, respectively. It was confirmed that the surfactant in the solution affected the nucleation rate of the MFI zeolite. Moreover, the crystal size of the MFI zeolite decreased with increasing surfactant concentration, and nanometer-sized MFI zeolites was obtained at a surfactant concentration of 0.25–0.5 mol/L. The successful preparation of MFI zeolite nanocrystals was ascribed to the stabilization of the MFI zeolite precursors and/or crystals by the adsorbed surfactant on their surface. This method was applied to the preparation of nano-crystalline ZSM-5 zeolite. ZSM-5 zeolite with a crystal size of approximately 50 nm was obtained. The nano-crystalline ZSM-5 zeolite was well-crystallized without octahedral Al atoms in the external framework, and exhibited almost the same acidity as a reference ZSM-5 zeolite.

Solvothermal deposition and characterization of magnesium hydroxide nanostructures on zeolite crystals

A methodology to engineer roughened inorganic nanostructures on zeolites by deposition and growth of metal hydroxides is reported. Pure-silica (PS) MFI and (aluminosilicate) LTA zeolite crystals are treated by solvothermal methods to deposit inorganic hydroxide (e.g., Mg(OH) 2) coatings with nanoscale roughened features on the crystal surfaces in a controlled manner. Detailed characterization of the surface-modified MFI crystals by nitrogen physisorption allows the quantification of the surface roughness of the Mg(OH) 2 nanostructures. Pore volume characterization shows that the nanostructure deposition has only a marginal effect on the porosity of the MFI. The surface-roughened zeolites are used to fabricate zeolite/polymer mixed matrix films with considerably improved interfacial adhesion properties. Solvothermal deposition of inorganic nanostructures is also demonstrated on zeolite LTA crystals, which present aluminosilicate surfaces. In this case, the intracrystalline sodium ions in LTA are partially substituted with magnesium ions from the reagent solution during treatment. The solvothermal treatment is thus modified to deposit smaller Mg(OH) 2 nanostructures, resulting in more roughened zeolite LTA surfaces. Our detailed characterization reveals that the surface-treated LTA crystals may be promising materials for applications in mixed matrix membranes.

Toward Microscopic Design of Zeolite Crystals: Advantages of the Fluoride-Mediated Synthesis

MFI zeolite crystals were organized into three-dimensional functional microscopic assemblies such as star-like or hedgehog morphologies. These zeolite structures were prepared under acidic conditions by fluoride-mediated synthesis. The degree of crystallinity and the specific surface area have been tailored as a function of the fluoride concentration in the synthesis gel. Furthermore, the morphology of the zeolite crystals assembly has also been investigated. Indeed, the crystal growth has been modified by changing the synthesis parameters: silica source, time, pH, and fluoride anion concentration. Depending on the aging time, either a star-like morphology of the MFI crystals was observed for the first time, or a hedgehog-microscopic assembly of the zeolite crystals was observed. It is therefore possible to synthesize zeolites with properties tailored at a molecular level and having an anisotropic assembly of the crystals. These organized microstructures create peculiar diffusion conditions which allow im...

Hydrothermal synthesis of giant single crystals of MFI type zeolite: Modified bulk material dissolution method

In the present study, we aimed at the development of an easy and reproducible synthesis method of giant MFI zeolite crystals. We used a new synthesis method, modified bulk material dissolution (M-BMD) method, in which a piece of quartz glass tube was inserted as a silica source in a small PTFE sleeve together with ZSM-5 powder as seed crystals. We adopted a higher HF concentration (25 mmol/25 ml) of the synthesis solution than that usually used. By using this method, only a few crystal nuclei were formed separately from each other on the surface of the quartz glass tube in the PTFE sleeve and giant zeolite crystals were reproducibly synthesized on it. Giant single crystals of MFI as large as 5.0 mm were successfully synthesized at 453 K for 90 days by using the M-BMD method.

2,2-Dimethylbutane adsorption and diffusion in MFI zeolite

Adsorption rates of 2,2-dimethylbutane (DMB) from the liquid phase into MFI zeolite crystals were measured by a volumetric/gravimetric method. Desorption rates in the presence of liquid n-hexane were measured by a non-adsorbing solvent (isooctane) method. Dimethylbutane desorbed, and was replaced by n-hexane, approximately three orders of magnitude faster than it adsorbed. Adsorption was slow and reached only 40% of saturation loading in 6-μm crystals after 45 days at 295 K, but this DMB loading desorbed into n-hexane in less than 1 h. Desorption may be faster due to slight expansion of the MFI unit cell by n-hexane adsorption. Although DMB diffused slower than n-hexane in MFI crystals, its transient time through a silicalite-1 membrane was an order of magnitude shorter than the n-hexane time at 313 K because DMB diffused through defects, whereas n-hexane diffused mainly through zeolite pores. The n-hexane transient time was longer because n-hexane: (1) expanded the MFI crystal size slightly and shrank non-zeolitic pores, and (2) adsorbed in the zeolite as it moved through the remaining non-zeolitic pores. A silicalite-1 membrane saturated with DMB at 423 K was sealed so effectively, and DMB desorbed slowly enough, that the helium permeation flux at 313 K was below the detection limit.

Microwave Fabrication of MFI Zeolite Crystals with a Fibrous Morphology and Their Applications

Der Einbau von Metall und die mikrowellengestützte Synthese von MFI-Zeolithen führen zur Kristallstapelung in einer Richtung und somit zu einer Fasermorphologie (siehe Bilder des Ti-MFI-Zeoliths). Diese Struktur hat ungewöhnliche physikochemische Eigenschaften zur Folge, nämlich eine geringere Packungsdichte und höhere Hydrophobie als nichtfasrige Analoga, womit bessere Katalyseeigenschaften und bessere größenselektive Adsorption einhergehen.