Extra-framework Species Research Articles

Revealing the Bro̷nsted Acidic Nature of Penta-Coordinated Aluminum Species in Dealuminated Zeolite Y with Solid-State NMR Spectroscopy.

The inevitable dealumination process of zeolite Y is closely related to ultrastabilization, enhanced Bro̷nsted acidity, and deactivation throughout its life cycle, producing complex aluminum and acidic hydroxyl species. Most investigations on dehydrated zeolites have focused on the Bro̷nsted acidity of tetra-coordinated Al (Al(IV)) and Lewis acidity associated with tricoordinated Al (Al(III)) sites, which has left the penta-coordinated Al (Al(V)) in dealuminated zeolites scarcely discussed. This is largely due to the oversimplified view of detectable Al(V) as an exclusively extra-framework species with Lewis acidity. Here we report the formation of Bro̷nsted acidic penta-coordinated Al species (Al(V)-BAS) in the dealumination process. Two-dimensional (2D) through-bond and multiquantum 1H-{27Al} correlation solid-state NMR experiments demonstrate the presence of a bridging Si-OH-Al(V) structure in dealuminated Y zeolites. Different from the conventional belief that water attack leads to the breaking of zeolite framework Al-O bonds in the initial stage of zeolite dealumination, the observed Al(V) as a dealumination intermediate is directly correlated with a BAS pair because of the direct dissociation of water on the framework tetrahedral aluminum (Al(IV)), thus bypassing the cleavage of Al-O bonds. 1H double-quantum solid-state NMR experiments and theoretical calculations provide further evidence for this mechanism, whereas pyridine adsorption experiments confirm stronger acidity of Al(V)-BASs than the traditional bridging hydroxyl groups associated with Al(IV). We were also able to detect the Al(V)-BAS site from dealuminated SSZ-13 zeolite with CHA topology, suggesting that its creation is not specific to the framework structure of zeolites.

Single-crystal X-ray diffraction study of a largely Cs-exchanged natural Ca-chabazite: Crystal-chemical factors for its excellent Cs-exchange ability

A hydrated natural Ca-chabazite, (Ca1.86Na0.13K0.09)(Al3.98Si8.03)O24·12.38H2O, and its Cs-exchanged form, (Cs2.66Ca0.45Na0.04K0.10)(Al4.04Si8.04)O24·8.52H2O, have been studied by the structure analyses based on single-crystal X-ray diffraction data. In the hydrated natural Ca-chabazite, all of extraframework species were found at the essentially identical locations to those in previously reported room-temperature structure. On the other hand, we revealed that the Cs-exchanged form has the essentially ten occupied-sites in extraframework: four water sites (OW2′, OW3, OW4, OW5), essentially two Cs sites (Cs1/Cs1′, Cs2) and four Ca sites (Ca1, Ca2, Ca3, Ca4). The Cs+ ions more preferentially occupy the Cs1/Cs1′ site, located at/around the centers of the 8-membered ring windows of [4126286]-cavities, than the Cs2 site. The Cs1/Cs1’ and Cs2 sites are essentially identical to the OW2 and OW3 sites observed in the hydrated natural Ca-chabazite, respectively; thus, these Cs sites are produced by replacing water molecules in both OW sites with Cs+ ions. In terms of interatomic distances, the coordination environments of the extraframework species in the chabazite crystals before and after the Cs-exchange treatment are discussed. In particular, both samples have a common feature that possible hydrogen bonds are relatively weak between water molecules and framework O atoms, whereas those are relatively strong between water molecules. On the basis of the present findings, we discuss the crystal-chemical key factors for an excellent Cs-exchange ability of chabazite as a highly efficient radioactive-element remover.

Characterization of acid sites on Hf-Beta zeolite by CD3CN IR-TPD

To identify Hf species and quantitatively evaluate their acid properties, combined measurement of infrared (IR) and temperature-programmed desorption (TPD) with deuterated acetonitrile (CD3CN) as probe molecule was performed on Hf-Beta and HfO2/Si-Beta. IR spectra of CD3CN adsorbed on Hf-Beta showed five types of Hf species including framework open and closed Hf sites, extra-framework HfO2, and two types of Hf species bearing hydroxy groups. IR measurement showed structural change in Hf-Beta upon exposing to water vapor with generation of the two types of Hf species bearing hydroxy groups. Integrated molar extinction coefficients for C N stretching vibration of CD3CN adsorbed on these Hf species were estimated and the number of each Hf acid site was calculated. Acid strength of these Hf species was estimated from CD3CN-TPD measurement in combination with IR; the acid strength of the open and closed Hf sites, extra-framework HfO2 species, and the other Hf species was found to be strong, moderate, and weak, respectively.

Pairing Ga/Al-Zeolites with tailored acidity as tandem catalysts for the conversion of alcohols to olefins

Zeolites are versatile catalysts owing to their tunable acidity where the active site(s) can be placed within the crystal framework or as extra-framework species in confined pores. A common approach to tailor Brønsted acidity is heteroatom exchange involving the replacement of framework aluminum with alternative elements. Introduction of heteroatoms often results in less acidic catalysts that can be used in tandem reactions for targeted reactive intermediates. In this study, we prepare a series of three different zeolites with MWW, CHA, and MFI frameworks as aluminosilicates and gallosilicates to demonstrate their performance in alcohol dehydration reactions. The two reactions examined in this study are methanol conversion to dimethyl ether and ethanol conversion to ethylene. Our findings reveal that Ga-zeolites exhibit superior performance with Ga-MCM-22 (MWW) achieving nearly 100 % alcohol conversion and selectivity to desired products at contact times that are significantly less than most dehydration catalysts reported in literature. The unique properties of Ga-zeolites are attributed to their reduced acid site strength via a direct (one-pot) synthesis that avoids conventional time-intensive, multi-step post-synthesis modifications of Al-zeolites. Here we demonstrate the use of Ga-zeolites as tandem catalysts when paired with a downstream Al-zeolite in a dual-bed reactor configuration to convert cheaper reagents (alcohols) upstream to more desired intermediates as feeds for downstream catalysts to produce light olefins. We demonstrate that dual beds using Ga-zeolite – Al-ZSM-5 (upstream–downstream) pairings outperform single bed configurations with Al-ZSM-5 or a physical mixture (Ga-/Al-zeolite) for methanol to hydrocarbons reactions, with notable improvements in catalyst lifetime and increased light olefins selectivity. Similarly, dual beds using Ga-zeolite – Al-SSZ-13 (upstream–downstream) pairings result in improved performance for ethanol to propylene reactions with significant increases in propylene selectivity (>20 %) compared to a conventional single bed configuration of Al-SSZ-13. Overall, this study offers a new perspective on the use of heteroatom-exchanged zeolite catalysts in tandem reactions as a means of capitalizing on their reduced acid site strength and suggests differences in the intrinsic acidity of zeolite crystal structures.

Transformation of Light Alkanes into High-Value Aromatics

This research work is focused on the transformation of light alkane (propane) into high-value aromatics using gallo-alumino-silicate catalysts. Two sets of gallo-alumino-silicates were synthesized for this study. In the first set, the ratio of Ga/(Al+Ga) was modified, while the Si/(Al+Ga) ratio was held constant. In the subsequent set, the Si/(Al+Ga) ratio was adjusted, while maintaining a consistent Ga/(Al+Ga) ratio. This approach aimed to directly assess the impact of each ratio on catalyst performance. The comprehensive characterization of all catalysts was conducted using various instrumental techniques, i.e., BET surface area, XRD, NH3-TPD, 27Al, 71Ga and 29Si MAS NMR, and XPS. A gradual reduction in the percentage of crystallinity and rise in meso-surface area was noticed with a rise in Ga/(Al+Ga) ratio. The total acidity (NH3-TPD) demonstrated a decline as the Si/(Al+Ga) ratio increased, attributed to an overall decline in Al3+ or Ga3+ species. The XPS intensity of the Ga 2p3/2 peak rose in correlation with an elevated ratio of Ga/(Al+Ga), suggesting the formation of extra-framework Ga species. The propane conversion, aromatic yield, and aromatization/cracking ratio exhibited an increase with an increasing Ga/(Al+Ga) ratio, reaching an optimum value of 0.46 before declining. Conversely, an appreciable drop in the conversion of propane and yield of aromatics was detected with the rise in Si/(Al+Ga) ratio, attributing to the decline in acidity. The catalyst having a Ga/(Al+Ga) ration of 0.46 exhibited the highest propane conversion and aromatic yield of 83.0% and 55.0% respectively.

On the effect of hafnium loading and location in mesoporous silica on the activity of catalysts in oxidation of dibenzothiophene

Oxidation of thiophenes to sulfones (easily extracted from fuel) is of crucial importance in petrochemical and automotive industry. The rational design and synthesis of stable catalysts of this process still remains one of the biggest challenges. In this work, a series of HfSBA-15 mesoporous materials with different hafnium content (3, 5 or 10 wt.%) has been prepared and studied in the liquid phase oxidation of dibenzothiophene (DBT) with H2O2. The activity of HfSBA-15 samples prepared by one-pot method was compared to that of Hf/SBA-15 synthesized by impregnation and that of HfMCF type of silica. It has been shown that hafnium loading and its location in the silica structure (framework and/or extra-framework positions) affect the efficiency of DBT oxidation. Moreover, the reaction conditions (i.e. oxidation temperature, reaction time, oxidant/sulfur ratio and catalyst amount) were optimized to achieve maximum conversion of dibenzothiophene (99 %). The location of hafnium species in SBA-15 and MCF materials was influenced by the loading of the metal and the type of silica structure (determined by the preparation method). Hafnium located in the framework of SBA-15 promoted generation of a higher number of active Lewis acid sites (LAS) and higher activity in oxidation of dibenzothiophene compared to extra-framework HfO2 species anchored in HfMCF material. It indicates that the location (framework vs extra-framework) and formation of isolated Hf4+ species are decisive for high activity of hafnium-doped mesoporous molecular sieves. This finding opens up a pathway for the development of highly efficient, stable and relatively cheap solid acid catalyst for oxidation of thiophenes to sulfones.

Acid-modulated synthesis of ultra-thin Ti-MWW zeolite nanosheets for the efficient epoxidation of cycloalkenes with tert-butyl hydroperoxide

The Ti-MWW zeolites with a layered morphology, synthesized by a conventional method, usually show low catalytic activities in the epoxidation of bulky alkenes due to the serious steric constraint imposed by the zeolite micropores and the adverse effects from some extra-framework octahedral Ti species. In the present work, the Ti-MWW zeolite was prepared by the reaction of an ultra-thin layered borosilicate MWW-type zeolite (D-ERB-1) with tetrabutyl orthotitanate (TBOT) in a HNO3 aqueous solution, wherein D-ERB-1 was hydrothermally synthesized by introducing an organosilane into the synthetic gel. In combination with the results from extensive characterizations for the synthesized zeolites, it is found that the currently synthesized Ti-MWW zeolite without extra-framework octahedral Ti species has thinner nanosheets that are very helpful to the epoxidation of cycloalkenes with tert-butyl hydroperoxide (TBHP), and is more active than the conventional Ti-MWW and other titanosilicate zeolites. Moreover, the developed catalyst is recyclable after the regeneration of the deactivated zeolite by washing with acetonitrile (MeCN). Therefore, the present work provides some guidance in the development of an efficient titanosilicate catalyst for the epoxidation of bulky alkenes.

The nature of extraframework aluminum species and Brønsted acid site interactions under catalytic operating conditions

A systematic investigation of hydrated extraframework aluminum (EFAl) species interacting with Brønsted acid sites (BAS) in H-ZSM-5 is presented to understand the cooperative active site structure under catalytic operating conditions. Static models of EFAl species confined in the H-ZSM-5 unit cell show that isolated BAS protonate neutral EFAl species to form cations. Ab-initio molecular dynamics (AIMD) simulations and enhanced sampling performed at the temperature for methanol-to-hydrocarbon conversion reveal two regimes of stable EFAl species, namely the [Al(OH)2]+ ion existing with two bonds to the zeolite scaffold or as a pore-guest in the form of [Al(OH)2(H2O)2]+. Our results indicate that hydrogen-bonding plays a significant role in BAS-EFAl active site structure, especially at higher BAS density and that EFAl species can function as both Bronsted and Lewis acidic components to alter proton transfer kinetics as well as shape selectivity within these microporous solids.

Drastic Photoemission Color Alternation from a Single Molecule as a Starting Material Introduced in Acid-Treated Zeolites: From Pure Blue to White.

Since high-purity blue- and white-light emitters are an indispensable group of materials for the creation of next-generation optical devices, a number of light-emitting materials have been developed from both inorganic and organic synthetic chemistry. However, these synthetic chemical methods are far from the perspective of green chemistry due to the multistep synthetic process and the use of toxic reagents and elements. Herein, we demonstrate that the introduction of simple unsubstituted anthracenes into zeolite-like pores can create a wide variety of luminescent materials, from ultrapure blue luminescent materials (emission peak at 465 nm with a full width of half-maximum of 8.57 nm) to efficient white luminescent materials [CIE coordination at (0.31, 0.33) with a quantum efficiency of 11.0% under 350 nm excitation light]. The method for rational design of the luminescent materials consists of the following two key strategies: one is molecular orbital confinement of the anthracene molecules in the zeolite nanocavity for regulating the molecular coordination associated with photoexcitation and emission and the other is the interaction of unsubstituted anthracenes with extra-framework aluminum species to stabilize the 2-dehydride anthracene cation in the zeolite cavity.

Generation of mesoporosity in crystals of zeolite omega by post-synthesis treatments with surfactants

The large-pore zeolite omega (SiAl = 3.1) has been treated with solutions of the surfactant hexadecyltrimethylammonium (CTA) in aqueous ammonia in order to generate intracrystalline mesoporosity. For this purpose, the as-made sample was calcined, transformed in the acid form by ion exchange with ammonium chloride solution followed by calcination, and the resulting acid form subsequently treated with steam at 620 ºC to partially dealuminate the framework. The samples were characterized by XRD, N2 adsorption, TGA, SEM, 29Si and 27Al MAS NMR, pyridine adsorption for acidity determination, and tested in the isomerization/disproportionation of m-xylene. It has been found that partial dealumination of the framework (Si/Al ∼ 7.3) followed by mild acid leaching (HCl = 0.3 M) to remove only the extraframework aluminium species is required for CTA to generate intracrystalline mesopores with a narrow pore diameter of 4 nm. The CTA treatment is ineffective when applied to the acid form (Si/Al ∼ 5.7) or the steamed sample before acid leaching. The concentration of strong acid sites and the average acid strength of the CTA samples are lower than those of the steamed/acid leached sample, which suggests the presence of weak acid sites located at the mesopores. The mesopores greatly enhanced the catalytic activity in the m-xylene conversion by decreasing diffusional constraints. However, the selectivity to the bulky trimethylbenzenes is not improved by the CTA treatment, which suggests that the reaction mainly takes place at the strong acid sites located at the micropores.

Solar-Driven Reforming of Methane and Nitrogen to Methanol and Ammonium on Iron-Modified Zeolite under Ambient Conditions in Water.

Artificial photosynthesis from selective methane oxidation or nitrogen reduction to value-added chemicals provides a promising pathway for the sustainable chemical industry, while still remaining a great challenge due to the extreme difficulty in C-H and N≡N bond cleavage under ambient conditions. Catalysts that can cocatalyze these two reactions simultaneously are rarely reported. Here, Fe-ZSM-5 with highly dispersed extra-framework Fe-oxo species enables efficient and selective photocatalytic conversion of methane and nitrogen to coproduce methanol and ammonia using H2O as the redox reagent under ambient conditions. The optimized Fe-ZSM-5 photocatalyst achieves up to 0.88 mol/molFe·h of methanol products with 97% selectivity. Meanwhile, the productivity of ammonia is 0.61 mol/molFe·h. In situ EPR and DRIFT studies disclose that water serves as a redox reagent to provide hydroxyl radicals for methane oxidation and protons for nitrogen hydrogenation. Quantum chemical calculations revealed that Fe-oxo species play a significant role in the coactivation of methane and nitrogen molecules, which lowers the energy barriers of rate-determining steps for methanol and ammonia generation.

Pt–Sn nanoalloys on Sn-Beta zeolite for efficient propane dehydrogenation

Zeolite-tailored bimetallic Pt-Sn catalysts have been shown to be effective for propane dehydrogenation (PDH). Here, we have successfully prepared and compared two Sn-Beta-supported bimetallic PtSn catalysts (Pt/Sn-Beta (DG) and Pt/Sn-Beta (DA)), in which the Sn-Beta zeolite was prepared by a dry-gel (DG) synthesis method involving in situ Sn incorporation or a post-synthesis method involving dealumination and Sn incorporation (DA), respectively. The as-prepared Pt/Sn-Beta (DG), possessing a less fraction of extra-framework Sn species comparing to Pt/Sn-Beta (DA), shows a superior performance in terms of activity (TOF, 1.8 s-1), selectivity (99.0%) and stability (7360 min) at 550 °C using 40 kPa propane and 60 kPa nitrogen gas, reflecting one of the highest PDH performances for bimetallic Pt-Sn catalysts under similar conditions. By a combination of the state-of-the-art electron microscopy-based techniques and advanced spectroscopy techniques, we found that the proper Pt-Sn nano-alloying in Pt/Sn-Beta (DG), rather than Sn coverage on Pt species in Pt/Sn-Beta (DA) owing to the migration of excess extraframework Sn species, is essential for providing an enhanced PDH performance, as the active Pt species could be well isolated, accessed and stablized in Pt/Sn-Beta (DG) by alloying.

Speciation and interconversion of atomically dispersed extra-framework Ga in ZSM-5 zeolite

Ga/ZSM-5 has been studied for a long time for its efficiency in catalytic dehydrogenation and conversion of light alkanes. Atomically dispersed extra-framework Ga species, such as [Ga]+(I), [GaH2]+(III), [GaO]+(III), [GaHOH]+(III), [Ga(OH)2]+(III) and [GaH]2+(III), etc. were proposed as reaction centers for such processes. To rationale the observed catalytic performances of Ga/ZSM-5, we investigated the electronic structures of these Ga species and pathways for their interconversion in zeolite channels by first-principles based calculations. Results from thermodynamics calculations at conditions relevant to experiments show that [Ga]+(I) is plausible at high temperature and low PH2O, and [Ga(OH)2]+(III) is plausible at low temperature and high PH2O. On pathways for interconversion of proposed Ga species, the calculated energy and free energy barriers for H2 dissociation and H2O formation are among the highest and correlate with involvement of Ga centers and their coordination with H and O-containing ligands. The coordination of framework O to Ga center are rather flexible, making dynamic formation of undercoordinated Ga species feasible at operation conditions and these species may attend subsequent chemical processes. We expect the findings would help to understand the formation and evolution of atomically dispersed reaction centers in zeolite channels under operation conditions.

Ti-MWW synthesis via acid-modulated isomorphous substitution of ERB-1 for efficient ethylene oxidative hydration to ethylene glycol

Although the oxidative hydration of ethylene with hydrogen peroxide (H2O2) to ethylene glycol (EG) over Ti-MWW zeolites take advantages of step-saving and facile operation, both H2O2 utilization efficiency and EG yield are relatively low, because some extra-framework octahedral Ti species are usually formed in the Ti-MWW zeolites synthesized by prevailing methods, which are detrimental to selective oxidation. In the present work, an acid-modulated isomorphous substitution method, i.e., layered borosilicate MWW-type zeolite (ERB-1) treated with HNO3 aqueous solution containing tetrabutyl orthotitanate (TBOT), is developed to synthesize a Ti-MWW zeolite without extra-framework octahedral Ti species under the optimized synthesis conditions. In combination with the results from extensive characterizations for the synthesized zeolites, it is found that in the zeolite prepared by the currently developed synthesis method there are more open Ti(OSi)3OH sites formed, resulting in the stronger Lewis acidity that is very helpful to EG production from the oxidative hydration of ethylene with H2O2, compared to the one synthesized by the prevailing method. Moreover, the developed catalyst is recyclable after the regeneration of the deactivated zeolite by calcination at high temperatures. Therefore, it is anticipated that the currently developed Ti-MWW might be industrially applicable in the one-pot synthesis of EG from ethylene with H2O2.

Structure Modulations and Symmetry of Lazurite-Related Sodalite-Group Minerals

Lazurite and other lazurite-related minerals (LRMs) containing sulfur in both sulfate and sulfide forms are sodalite-type compounds with various extraframework species, of which the tendency to order leads to structural modulations with a period that is either commensurate or incommensurate with the period of the basic lattice. In this work, the structures of incommensurately modulated monoclinic LRMs are re-examined based on the superstructure of slyudyankaite, formerly known as triclinic lazurite. Similarities and differences between three one-dimensionally modulated LRMs and cubic LRM structures modulated in several directions are discussed. Assumptions are made on how the symmetry of the structure and the composition of the crystal can affect the period of structural modulation.

Synthesis of NaY zeolites with exceptionally high SiO2/Al2O3 ratios via a dissolution-recrystallization process

Commercial HY zeolites with SiO2/Al2O3 ratios between 5.1 and 80 have been treated in alkaline solutions in the presence of crown ether molecules. If the zeolites are almost totally amorphized at room temperature due to severe desilication of the framework, they completely recrystallize within less than one day when temperature is increased to 110 °C. Extra-framework species likely present in HY precursors are reincorporated in the framework, providing NaY zeolites with exclusively 4-coordinated Al species. The composition of the newly formed NaY zeolite depends not only on the SiO2/Al2O3 ratio of the HY precursor but also on the alkalinity of the desilicating solution. For each HY, the SiO2/Al2O3 ratio of the recrystallized NaY increases linearly when the Na2O/SiO2 ratio of the alkaline solution decreases, down to a limit beneath which complete recrystallization is no longer possible. FAU zeolites with framework SiO2/Al2O3 ratios up to 25 have been obtained from CBV 760, which constitutes the highest value ever reported in the literature for as-made NaY zeolites. A mechanism involving adsorption of crown ether molecules in the supercages of the partially desilicated framework followed by recrystallization has been proposed. This mechanism supports all experimental results, in particular the presence of more than 1 crown ether molecule per supercage in recrystallized solids.

Catalytic cracking of supercritical n-dodecane over macro-meso-HZSM-5 coatings by vertical secondary growth

Hierarchical HZSM-5 zeolite coatings are extensively investigated for catalytic cracking of supercritical fuels to benefit the thermal management of hypersonic aircraft. However, there is still a challenge for preparing zeolite coatings combining the high diffusion property, strong mechanical strength and a large amount of Brønsted acid sites. Herein, the macro-meso-HZSM-5 coatings with meso-HZSM-5 grains vertically welding on the compact and randomly oriented zeolite basal layer were fabricated on the inner wall of the stainless-steel tubes by ammonia-assisted vertical secondary growth, using amphiphilic organosilane as a meso-porogen. The structure benefited the breaking of the trade-off between diffusion and acidity of the coating. And by adjusting the dosage of inorganic ammonium salt and the crystallization time, the synthesis mechanism of the coatings was speculated. Inorganic ammonium salt facilitated the formation of macropores and the vertical growth of crystals with (00 h)&(10 h) orientation (pseudo-c-orientation). The chemical composition of the seed layer was adjusted using Si-MFI, Al-MFI or Ga-MFI as seeds, and the mechanical strength was enhanced by the intergrowth of zeolite phase in the seed layer. The macro-meso-HZSM-5 coating synthesized with Al-MFI as the seed (MZC-Al-2N), exhibited an excellent catalytic conversion and stability for cracking of supercritical n-dodecane (550 °C and 4 MPa), with the initial reaction rate and the deactivation rate being three times and 52 % of the conventional HZSM-5 coating, respectively, due to the combination of the improved diffusion property as well as the suitable acidity. The coating synthesized using Ga-MFI seed showed a lower activity, but a higher heat sink compared with MZC-Al-2N, probably due to the dehydrogenation of light alkanes over the synergistic effect between the Brønsted acid sites and the extra-framework Ga species.

Maximizing Active Fe Species in ZSM-5 Zeolite Using Organic-Template-Free Synthesis for Efficient Selective Methane Oxidation.

The selective oxidation of CH4 in the aqueous phase to produce valuable chemicals has attracted considerable attention due to its mild reaction conditions and simple process. As the most widely studied catalyst for this reaction, Fe-ZSM-5 demonstrates high intrinsic activity and selectivity; however, Fe-ZSM-5 prepared using conventional methods has a limited number of active Fe sites, resulting in low CH4 conversion per unit mass of the catalyst. This study reports a facile organic-template-free synthesis strategy that enables the incorporation of more Fe into the zeolite framework with a higher dispersion degree compared to conventional synthesis methods. Because framework Fe incorporated in this way is more readily transformed into isolated extra-framework Fe species under thermal treatment, the overall effect is that Fe-ZSM-5 prepared using this method (Fe-HZ5-TF) has 3 times as many catalytically active sites as conventional Fe-ZSM-5. When used for the selective oxidation of CH4 with 0.5 M H2O2 at 75 °C, Fe-HZ5-TF produced a high C1 oxygenate yield of 109.4 mmol gcat-1 h-1 (a HCOOH selectivity of 91.1%), surpassing other catalysts reported to date. Spectroscopic characterization and density functional theory calculations revealed that the active sites in Fe-HZ5-TF are mononuclear Fe species in the form of [(H2O)3Fe(IV)═O]2+ bound to Al pairs in the zeolite framework. This differs from conventional Fe-ZSM-5, where binuclear Fe acts as the active site. Analysis of the catalyst and product evolution during the reaction suggests a radical-driven pathway to explain CH4 activation at the mononuclear Fe site and subsequent conversion to C1 oxygenates.

Catalytic oxidation performance and ion-exchange of Ti-MWW zeolite membrane with dual organic template agents and potassium carbonate

In the absence of boric acid, the tubular Ti-MWW zeolite membranes are prepared with dual organic template agents and potassium carbonate by secondary hydrothermal synthesis. 12MR cage and 10 MR of the MWW zeolite topology structure is stabilized by TMAdaOH and HMI organic template agents, K+ cations is a critical mineralization agent for preparing Ti-MWW zeolite membrane with bi-layer Ti-MWW zeolites layer in this work. Besides, ion-exchange could greatly improve the catalytic performance of the Ti-MWW zeolite membrane for phenol hydroxylation with H2O2, influences of ammonium salt, ion-exchange time and temperature on the micro-structure and catalytic performance are investigated. The upper network-like Ti-MWW zeolite layer could be dissolved and removed by ion-exchange with 0.5 M (NH4)2SO4 solution at 60 °C for 24 h, and the “lying” Ti-MWW zeolites layer is kept on the mullite support. On the other hand, ion-exchange could promote the transformation of hexacoordinated extra-framework titanium species to tetracoordinated state in the framework, part of K+ cations of Ti-MWW zeolites are replaced with NH4+ cations. Total flux and phenol conversion of the ion-exchanged Ti-MWW zeolite membrane are 14.89 kg·m−2·h−1 and 36.49 % for phenol hydroxylation by H2O2 at 50 °C.

Counting the Acid Sites in a Commercial ZSM-5 Zeolite Catalyst.

This work investigates the acid sites in a commercial ZSM-5 zeolite catalyst by a combination of spectroscopic and physical methods. The Brønsted acid sites in such catalysts are associated with the aluminum substituted into the zeolite lattice, which may not be identical to the total aluminum content of the zeolite. Inelastic neutron scattering spectroscopy (INS) directly quantifies the concentrations of Brønsted acid protons, silanol groups, and hydroxyl groups associated with extra-framework aluminum species. The INS measurements show that ∼50% of the total aluminum content of this particular zeolite is extra framework, a conclusion supported by solid-state NMR and ammonia temperature-programmed desorption (TPD) measurements. Evidence for the presence of extra-framework aluminum oxide species is also seen in neutron powder diffraction data from proton- and deuterium-exchanged samples. The differences between results from the different analytical methods are discussed, and the novelty of direct proton counting by INS in this typical commercial catalyst is emphasized.