Fe-containing Zeolites Research Articles

Roles of Acidic Proton for Fe-Containing Zeolite in Direct Oxidation of Methane

Roles of Acidic Proton for Fe-Containing Zeolite in Direct Oxidation of Methane

Revealing Active Sites and Reaction Pathways in Direct Oxidation of Methane over Fe-Containing CHA Zeolites Affected by the Al Arrangement.

Fe-containing zeolites are effective catalysts in converting the greenhouse gases CH4 and N2O into valuable chemicals. However, the activities of Fe-containing zeolites in methane conversion and N2O decomposition are frequently conflated, and the activities of different Fe species are still controversial. Herein, Fe-containing aluminosilicate CHA zeolites with Fe species at different spatial distances affected by the arrangement of framework Al atoms were synthesized in a one-pot manner in the presence or absence of Na. The arrangement of framework Al atoms was identified by 27Al and 29Si MAS NMR spectra and thermogravimetry-differential thermal analysis (TG-DTA) curves. Ultraviolet (UV)-vis, X-ray absorption spectroscopy (XAS), and NO adsorption fourier transform infrared spectroscopy (FTIR) spectra were adopted to analyze Fe speciation. The higher proportion of Fe species in the 6 MR of Fe-CHA zeolites in the presence of Na was confirmed by the NO adsorption FTIR spectrum. The activities of proximal and distant Fe sites in reactions including direct oxidation of methane to methanol, methanol to hydrocarbon, and N2O decomposition were compared at different temperatures to provide the corresponding active sites and reaction pathways. The distant, isolated Fe and isolated proton were more active in the direct oxidation of methane to methanol and tandem conversion of methanol to hydrocarbon reactions than the proximal, isolated Fe and paired protons, respectively. Additionally, proximal, isolated Fe sites afforded higher activity in N2O decomposition. These findings guide the design of highly active catalysts in methane oxidation, methanol to hydrocarbon, and N2O decomposition reactions, addressing energy and environmental concerns.

Sonochemical Preparation of Fe-Y Zeolite Catalyst for the Conversion of Benzene to Phenol

Enzyme-mimetic materials such as Fe-containing zeolites are of research interest due to the high selectivity for the one-step selective oxidation of benzene to phenol over the Fe-catalytic active center at room temperature and ambient pressure. This study investigates a Fe-modified Y zeolite catalysts prepared via a sonochemical method for the oxidation of benzene-to-phenol using water as solvent. These catalysts were subjected to a screening and optimization experiment by a response surface methodology (RSM), evaluating their potential for the synthesis of phenol. Achieving the preparation of a catalyst with an increased phenol selectivity from 3% to ~91% and almost 100% when evaluated at 30 ºC. Various Fe species, including the α-Fe(II) active site, were detected using UV-Vis spectroscopy, FTIR spectroscopy, and magnetic thermal gravimetric analysis (MTGA). Highlighting the potential of sonochemistry for preparing at mild temperatures (<100 ºC) iron-modified zeolite Y catalysts with room temperature catalytic active in water-based systems.

Direct Oxidation of Methane to Methanol over Transition-Metal-Free Ferrierite Zeolite Catalysts.

Direct oxidation of methane to methanol was reported to be highly dependent on the transition- or noble-metal-loading catalysts in the past decades. Here, we show that the transition-metal-free aluminosilicate ferrierite (FER) zeolite effectively catalyzed methane and N2O to methanol for the first time. The distorted tetracoordinated Al in the framework and pentacoordinated Al on the extra framework formed during calcination, activation, and reaction processes were confirmed as the potential active centers. The possible reaction pathway similar to the Fe-containing zeolites was advocated based on the reaction results using different oxidants, N2O adsorption FTIR spectra, and 27Al MAS NMR spectra. The stable and efficient methanol production capacity of FER zeolite was ascribed to the two-dimensional straight channels and its distinctive Al distribution of FER zeolite (CP914C) from Zeolyst. The transition-metal-free FER zeolite performed better than the record in the literature and our recent results using transition-metal-containing catalysts in terms of selectivity and formation rate of methanol and stability. This work has great significance and prospects for utilizing CH4 and N2O as resources and will open new avenues for methane oxidation.

Direct Hydroxylation of Benzene with Hydrogen Peroxide Using Fe Complexes Encapsulated into Mesoporous Y-Type Zeolite.

Mesoporous Y-type zeolite (MYZ) was prepared by an acid and base treatment of commercial Y-type zeolite (YZ). The mesopore volume of MYZ was six times higher than that of YZ. [Fe(terpy)2]2+ complexes encapsulated into MYZ and YZ with different Fe contents (Fe(X)L-MYZ and Fe(X)L-YZ; X is the amount of Fe) were prepared and characterized. The oxidation of benzene with H2O2 using Fe(X)L-MYZ and Fe(X)L-YZ catalysts was carried out; phenol was selectively produced with all Fe-containing zeolite catalysts. As a result, the oxidation activity of benzene increased with increasing iron complex content in the Fe(X)L-MYZ and Fe(X)L-YZ catalysts. The oxidation activity of benzene using Fe(X)L-MYZ catalyst was higher than that using Fe(X)L-YZ. Furthermore, adding mesopores increased the catalytic activity of the iron complex as the iron complex content increased.

Fenton-like oxidation of phenol with in-situ generated hydrogen peroxide and Pd/Fe-zeolite catalysts

Two types of iron-containing zeolites with different surface hydrophobicity, Fe-ZSM5 (SiO2/Al2O3 = 26) and Fe-ZSM5 (SiO2/Al2O3 = 236) were studied as adsorbents and catalysts for oxidation of phenol by means of a Fenton-like process at ambient conditions and nearly neutral pH, with and without in-situ generation of H2O2. Adsorption of phenol is more favorable on high silica Fe-ZSM5 (236) zeolite due to its higher surface hydrophobicity. Palladium (Pd) immobilization on Fe-ZSM5 zeolites has a positive impact on phenol degradation with a synergistic role of Pd and Fe (II)/(III) for activation of H2O2. The best result for phenol conversion and mineralization was observed over both hydrophilic and hydrophobic 0.1 wt.% Pd/Fe-ZSM5 with commercial H2O2 achieving ≥ 90% conversion of phenol (100 mg/L) in 4 h and 60–63% mineralization in 6 h with 5 g/L catalyst and 5 g/L H2O2. In addition, Pd/Fe-ZSM5 can be used for in-situ formation of H2O2 using formic acid as H-source and externally supplied oxygen. The combination of these processes provided by this adsorbent/catalyst material is exploitable for on-site oxidative regeneration of zeolite adsorbents. This study shows that Fe-containing zeolites are promising catalysts for a combined approach of adsorption/oxidative degradation of phenol by commercial or in-situ produced H2O2.

Assessment of the capability of Fe and Al modified BEA zeolites to promote advanced oxidation processes in aqueous phase

Iron-containing zeolites were prepared using two-step postsynthesis and wet impregnation methods and characterized using conventional techniques. The materials were tested for production of hydroxyl radicals using N,N-dimethyl-p-nitrosoaniline (pNDA) as radical scavenger and discoloration of Congo Red (CR) using advanced oxidation processes (AOPs). The characterization showed that the materials were morphologically homogeneous and had different iron amounts incorporated into the framework and extra-framework of the BEA structure. The pNDA bleaching curves suggested that FexHAlBEA is more effective for producing hydroxyl radicals because of the presence of higher amounts of octahedral Fe(III) in the extra-framework positions. A similar trend was found for CR discoloration. The use of hydrogen peroxide increased both hydroxyl radical generation and CR discoloration, which suggests that the Fe in the zeolites decomposed the oxidant and increased the production of free radicals. Fe-containing zeolites were found as interesting catalysts for AOPs to remove organic contaminants in water.

Quasi-solid-phase synthesis of Fe-MFI zeolites by using Fe-containing zeolite seed sol for hydroxylation of benzene with H2O2

Abstract Fe-MFI zeolites were fabricated by the direction of Fe-containing zeolite seed sol via a dry-gel conversion method. The textural properties and crystallinity of these zeolites were characterized by using N2 adsorption-desorption isotherms, SEM (Scanning Electron Microscope) and XRD (X-ray diffraction), respectively. The iron coordination of these samples was characterized by UV–vis (ultraviolet–visible) and ESR (electron spin resonance) techniques. Fe-containing zeolite seed sol enhanced the distribution of the Fe species that exists mainly in the zeolites’ framework and in a small amount in oligomeric FexOy clusters. Fe-MFI zeolite catalysts were applied in the direct hydroxylation of benzene to phenol with H2O2. Under optimized conditions, Fe-MFI zeolite synthesized from Fe-containing seed sol showed 12.1% conversion with 97% phenol selectivity, which was greater than the Fe-MFI zeolite from pure silica seed sol with 5.6% conversion with 95% phenol selectivity.

Operando HERFD-XANES/XES studies reveal differences in the activity of Fe-species in MFI and CHA structures for the standard selective catalytic reduction of NO with NH3

Fe-containing zeolites were studied as catalysts for the standard NH3-SCR reaction with the primary aim of gaining insight into the structure-function relationship of these materials. Catalysts with different Fe nuclearity (i.e. isolated species, clusters, large particles) were synthesised by incipient wetness impregnation, using H-ZSM-5, H-SSZ-13 and Silicalite-1 as supports, and characterised by in situ and operando X-ray emission spectroscopy (XES) and high energy resolution fluorescence detected X-ray absorption near-edge spectroscopy (HERFD-XANES) under NH3-SCR conditions. The combination of these techniques allowed us to obtain a detailed understanding of the changes in Fe coordination, oxidation state and geometry occurring during reaction. The results obtained suggested that isolated octahedral Fe3+ species on H-ZSM-5 are highly active under the conditions studied, undergoing reduction when exposed to NH3 or under SCR conditions. In contrast, isolated tetrahedral Fe3+ sites present in Silicalite-1 exhibited lower redox properties, leading to a reduced NO conversion. Clusters and FexOy particles on H-SSZ-13 exhibited low SCR activity.

Experimental Evidence of the Mechanism of Selective Catalytic Reduction of NO with NH3 over Fe-Containing BEA Zeolites.

Various temperature-programmed techniques were used as tools in mechanistic studies of selective catalytic reduction (SCR) of NO with ammonia in the presence of Fe-containing BEA zeolites. Moreover, FTIR studies of adsorbed NH3 and NO were conducted to determine the interactions of reactants with the catalyst surface. Iron was introduced into BEA zeolite by three different methods: i) two-step post-synthesis; ii) conventional wet impregnation; iii) ion exchange. The catalytic activity was dependent on the method used for iron introduction. The reactivities of NH3 and NO adsorbed on iron-modified zeolites obtained by impregnation and ion-exchange methods were higher than those measured for the catalyst obtained by a two-step post-synthesis method. The activity of Fe-containing zeolites in SCR was related to the form of deposited iron species, as well as to the nature, strength, and concentration of acid sites. Possible reaction pathways of NO reduction over the FeBEA zeolite catalysts were presented and discussed.

Identification of α-Fe in High-Silica Zeolites on the Basis of ab Initio Electronic Structure Calculations.

α-Fe is the precursor of the reactive FeIV═O core responsible for methane oxidation in Fe-containing zeolites. To get more insight into the nature and stability of α-Fe in different zeolites, the binding of Fe(II) at six-membered-ring cation exchange sites (6MR) in ZSM-5, zeolite beta, and ferrierite was investigated using DFT and multireference ab initio methods (CASSCF/CASPT2). CASPT2 ligand field (LF) excitation energies of all sites were compared with the experimental DR-UV-vis spectra reported by Snyder et al. From this comparison it is concluded that the 16000 cm-1 band of α-Fe, observed in all three zeolites, can uniquely be assigned to a high-spin square-planar (SP) Fe(II) located at a 6MR with an Al-Si-Si-Al sequence, where the Al atoms are positioned opposite in the ring and as close to each other as possible. The stability of such conformations is also confirmed by the binding energies obtained from DFT. The bands at 10000 cm-1 in the experimental spectra, assigned to spectator Fe(II), are attributed to six-coordinated trigonal-prismatic Fe(II) species, as calculated for the γ-site in ZSM-5. The entatic effect of the zeolite lattice on the stability of the SP sites was investigated by making use of the unconstrained Fe(II) model complex FeL2 (with L = [Al(OH)4]-). The SP conformer is approximately 2 kcal/mol more stable than the tetrahedral form, indicating that the SP coordination environment of α-Fe is not imposed by the zeolite lattice but rather electronically preferred by Fe(II) in the environment of four O ligands. A significant contribution to the stability of the SP conformer is provided by mixing of the doubly occupied 3dz2 orbital with the higher lying 4s.

Пероксидное каталитическое окисление метана на железосодержащих цеолитах

Пероксидное каталитическое окисление метана на железосодержащих цеолитах

Synthesis and characterization of the Fe-substituted ZSM-22 zeolite catalyst with high n-dodecane isomerization performance

A series of Fe-substituted ZSM-22 zeolites were synthesized via an organic structure-directing agent-free route with a small amount of precursor suspension. Characterizations by XRD, SEM, TEM, N2 adsorption, UV–vis, FT-IR, and XPS confirm the high crystallinity, uniform morphology, and smaller crystal size of the as-synthesized zeolites and the presence of Fe3+ in the framework of the zeolites. The acidity, especially the Brønsted acidity, decreases significantly due to the low acidity of Si–OH–Fe bridge hydroxyls compared with that of Si–OH–Al. Moreover, in the Fe-containing catalysts, Pt can easily be reduced and its dispersion slightly increases, as confirmed by H2 TPR and CO chemisorption. As a result, the Pt/[Al,Fe]-ZSM-22 catalysts exhibit superior selectivity in n-dodecane isomerization compared with the Pt/ZSM-22 catalyst. The higher monobranched isomer selectivity of the catalyst using Fe-substituted ZSM-22 zeolites may be due to more balanced bifunctionality between acid sites and metal sites and decreased crystallite size of the Fe-containing zeolites, which improves the diffusion of intermediates and prevents the products from further cracking. It is also observed that more dibranched isomers were formed with more Fe replacement even at higher temperatures due to reduced strong acidity by Fe replacement, which significantly suppressed the cracking and improved the selectivity to deeper isomerization.

A novel processing of carbon nanotubes grown on molecular sieve coated porous ceramics

The present study focuses on the growth of carbon nanotubes (CNTs) on Fe-containing zeolites coated porous ceramics by implementing three different and independent techniques, successively. Direct foaming-derived porous ceramics were subjected to hydrothermal reaction for on-site growth of NaA zeolites within it. The porous ceramics-zeolite composite was subjected to ion-exchange reaction to obtain the catalyst for CNT synthesis. Multi-walled CNTs (MWCNTs) were grown by catalytic chemical vapour deposition (CCVD) process using acetylene as carbon source. Microstructural, thermogravimetric and spectroscopic analyses showed distinctive differences in terms of hollow structural feature, yield and crystallinity of the MWCNTs with different reaction temperatures.

The catalytic activity of Fe-containing SiBEA zeolites in Fischer–Tropsch synthesis

This work is focused on catalytic activity of Red-C-FexSiBEA zeolites in Fischer–Tropsch synthesis. The Fe-containing zeolites were obtained by two-step post-synthesis method, which allowed incorporating the metal ions into zeolite framework and in this way to obtain materials with new physicochemical and catalytic properties. The TPR-H2 studies allowed to determine the reducibility of different kinds of iron species present in C-FexSiBEA zeolites. The catalytic investigations indicated that the most active catalyst in FTS is Red-C-Fe20SiBEA, which achieved the CO conversion of 70% and selectivity towards liquid products of 71%. The isoalkanes, n-alkanes, olefins and oxidative products (alcohols, ketones and aldehydes) were identified among of liquid products. The most probably reason of catalysts deactivation is coke formation, which amount, calculated on the base of DTG-DTA data, was 7% for Red-Fe4.0SiBEA, 20% for Red-C-Fe10SiBEA and 35% for Red-C-Fe20SiBEA.

The role of zeolite Fe-ZSM-5 porous structure for heterogeneous Fenton catalyst activity and stability

Four types of iron containing materials have been synthesized: conventional zeolite Fe-ZSM-5 (conv), hierarchical zeolite Fe-ZSM-5 (hier), small crystals (d=330nm) of zeolite Fe-ZSM-5 (nano) and ferric oxide species supported on the amorphous silica Fe/SiO2. Samples were prepared by hydrothermal treatment, polystyrene spheres were used as a template for Fe-ZSM-5 (hier) and Fe/SiO2. The materials were characterized by different techniques. Nature of iron-containing particles in the samples and stability of iron species in the reaction media were suggested by using thermodynamic considerations. All solid-phase Fe-containing samples as well as dissolved Fe(NO3)3 were tested in H2O2 decomposition reactions in absence or presence of iron-complexing agent Na2EDTA, which has been used to test the catalyst stability. Catalytic activity of ferric species in hydrogen peroxide decomposition for small 330-nm crystals of Fe-ZSM-5 was 1.4times higher than for large zeolite crystals, and significant decrease of the activity was observed for samples containing amorphous silica phase. Experimental results showed that ferric sites in zeolite were stable due to the limited diffusion of Na2EDTA in zeolite phase. Wet hydrogen peroxide oxidation of organic complexing agents by H2O2 using Fe-containing zeolites has a good potential for purification of nuclear waste water.

Template method for the synthesis of a heterogeneous fenton catalyst based on the hierarchical zeolite FeZSM-5

Zeolite h-FeZSM-5 with a hierarchical micro/macropore system has been synthesized in the presence of a template based on the close-packed polystyrene (PS) spheres, and the conventional zeolite FeZSM-5 has been obtained in the absence of a PS template. The zeolites have been characterized by X-ray diffraction, scanning and high-resolution transmission electron microscopy, and N2 sorption. The macropore walls of the hierarchical zeolite consist of ZSM-5 nanocrystals and amorphous globules of silica. Compared to the conventional zeolite, the hierarchical one has a high BET and external surface areas of 245 and 472 m2/g, respectively, and a high pore volume of 0.6 cm3/g. The catalytic properties of the Fe-containing zeolites were studied in the H2O2 decomposition reaction in the absence and in the presence of EDTA ligands and in the oxidation of low- and high-molecular-weight organic compounds by hydrogen peroxide at 25°C. Hierarchical zeolite h-FeZSM-5 is highly efficient in the oxidation of large molecules.

Indications of the reactive species in a heterogeneous Fenton-like reaction using Fe-containing zeolites

Heterogeneous Fenton-like reactions have potential applications in wastewater treatment. Recently, the use of zeolites containing iron species as catalysts in Fenton-like processes has gained increasing interest. While it has been assumed in the literature that hydroxyl radicals (OH ) are the dominant reactive species formed from H 2O 2 in the presence of Fe-zeolites, no direct evidence for this hypothesis exists so far. This work aims at the elucidation of the type of reactive species involved in the heterogeneous Fenton reaction using the Fe-containing zeolites Fe-ZSM5 and Fe-Beta. Electron paramagnetic resonance (EPR) measurements using dimethyl pyrroline-N-oxide (DMPO) as spin trap were carried out; the formation of the complex DMPO-OH was observed, which is often ascribed to a reaction with OH . However, significant differences were found in the kinetic deuterium isotope effect (KDIE) for methanol oxidation between the heterogeneous Fenton-like reaction and genuine OH reactions initiated by the homogeneous Fenton reaction. The KDIE for methanol oxidation in the heterogeneous reaction increased from 1.33 to 1.41 with increasing concentration of the OH scavenger bicarbonate, whereas the KDIE in the classical Fenton reaction is 1.07 and unaffected by the presence of quenchers. The significantly different values are a clear indication for the participation of additional oxidants beside OH . In addition, Fe-zeolites catalyzed the epoxidation of cyclohexene by H 2O 2. This type of reaction suggests the participation of a high-valent oxoiron complex (ferryl) in the wet peroxide oxidation of organic compounds catalyzed by Fe-zeolites.

Hierarchical zeolites prepared by organosilane templating: A study of the synthesis mechanism and catalytic activity

The crystallization of hierarchical ZSM-5 in the presence of the organosilane octadecyl-dimethyl-(3-trimethoxysilyl-propyl)-ammonium chloride as the mesoporogen was investigated as a function of time and temperature. The synthesis by this method proceeds in two steps. The rapid formation of a predominantly amorphous disordered mesoporous aluminosilicate precursor phase is followed by the formation of globular highly mesoporous zeolite particles involving dissolution of the precursor phase. It is difficult to completely convert the initial phase into the final hierarchical zeolite. This limits the amount of aluminium built into the MFI network and the resulting Brønsted acidity. In the presence of iron, more crystalline hierarchical zeolite is obtained. These Fe-containing zeolites are excellent catalysts for the selective oxidation of benzene to phenol. Their hierarchical pore structure leads to higher reaction rates due to increased mass transfer and increased catalyst longevity despite more substantial coke formation.

Improved Automotive NO x Aftertreatment System: Metal Ammine Complexes as NH3 Source for SCR Using Fe-Containing Zeolite Catalysts

Ammonia storage is a challenge in the selective catalytic reduction of NOx in vehicles. We propose a new system, based on metal ammines as the ammonia source. In combination with iron containing zeolites as the SCR catalyst it should be possible to obtain a low temperature system for NOx removal.