Zeolite Framework Structure Research Articles

Efficient conversion of benzene and syngas to toluene and xylene over ZnO-ZrO2&H-ZSM-5 bifunctional catalysts

A series of ZnO-ZrO 2 solid solutions with different Zn contents were synthesized by the urea co-precipitation method, which were coupled with H-ZSM-5 zeolite to form bifunctional catalysts. As a new benzene alkylation reagent, syngas was used instead of methanol to realize the efficient conversion of syngas and benzene into toluene and xylene. A suitable ratio of ZnO-ZrO 2 led to the significant improvement in the catalytic performance, and a suitable amount of acid helped to increase the selectivity of toluene/xylene and reduce the selectivity of the by-products ethylbenzene and C 9 + aromatics. The highest benzene conversion of 89.2% and toluene/xylene selectivity of 88.7% were achieved over 10% ZnO-ZrO 2 &H-ZSM-5 (Si/Al = 23) at a pressure of 3 MPa and a temperature of 450 °C. In addition, the effect of the zeolite framework structure on product distribution was examined. Similar to the molecular dynamics of aromatic hydrocarbons, H-ZSM-5 zeolites comprise 10-membered-ring pores, which are beneficial to the activation of benzene; hence, the conversion of benzene is higher. H-ZSM-35 and H-MOR zeolites exhibited small eight-membered-ring channels, which were not conducive to the passage of benzene; hence, the by-product ethylbenzene exhibits a higher selectivity. The distance between the active centers of the bifunctional catalysts was the main factor affecting the catalytic performance, and the powder mixing method was more conducive to the conversion of syngas and benzene.

In situ DRIFT studies on N2O formation over Cu-functionalized zeolites during ammonia-SCR

The influence of the zeolite framework structure on the formation of N2O during ammonia-SCR of NOx was studied for three different copper-functionalized zeolite samples, namely Cu-SSZ-13 (CHA), Cu-ZSM-5 (MFI), and Cu-BEA (BEA).

Study on preparation of CuCl/REY adsorbent with high CO adsorption and selectivity

In this paper, rare earth elements are used to modify Y zeolite, and CuCl-loaded rare earth Y zeolite adsorbents (CuCl/REY) were prepared by solid monolayer dispersion method. The adsorption performance of CuCl/REY on CO was studied, and the adsorbents were characterized via nitrogen adsorption/desorption at 77 K, X-ray fluorescence (XRF), X-ray diffraction (XRD) and scanning electron microscopy (SEM). We also characterize the state of Cu through temperature-programmed reduction using hydrogen (H2-TPR). The results show that rare earth can slightly increase the pore size, while maintaining the stability of the zeolite framework structure. Also, it can inhibit the production of copper oxide (CuO) to increase the CO capacity, and reduce the adsorption of N2 to increase the CO/N2 selectivity. Under the condition of 298 K and 1 bar, CuCl/REY showed a high CO adsorption capacity of 2.64 mmol/g, and the selectivity of CO/N2 reached 53, which was nearly double the performance of the adsorbent prepared with NaY zeolite.

Pinpointing and Quantifying the Aluminum Distribution in Zeolite Catalysts Using Anomalous Scattering at the Al Absorption Edge.

The location of aluminum in a zeolite framework structure defines the accessibility and geometry of the catalytically active sites, but determining this location crystallographically is fraught with difficulties. Typical zeolite catalysts contain only a small amount of aluminum, and the X-ray scattering factors for silicon and aluminum are very similar. To address this problem, we have exploited the properties of resonant X-ray powder diffraction across the Al K edge, where the aluminum scattering factor changes dramatically. By combining conventional synchrotron powder diffraction data with those collected at energies near the X-ray absorption edge, aluminum is highlighted. In this way, the different distributions of aluminum in two FER-type zeolites with identical chemical compositions but different catalytic properties could be determined unambiguously. The results are consistent with previous studies, but quantitative. This approach constitutes a major advance in our fundamental understanding of the relationship between zeolite structure and catalytic activity.

Development of Methanol Permselective FAU-Type Zeolite Membranes and Their Permeation and Separation Performances.

The separation of non-aqueous mixtures is important for chemical production, and zeolite membranes have great potential for energy-efficient separation. In this study, the influence of the framework structure and composition of zeolites on the permeation and separation performance of methanol through zeolite membranes were investigated to develop a methanol permselective zeolite membrane. As a result, the FAU-type zeolite membrane prepared using a solution with a composition of 10 SiO2:1 Al2O3:17 Na2O:1000 H2O showed the highest permeation flux of 86,600 μmol m−2 s−1 and a separation factor of 6020 for a 10 wt% methanol/methyl hexanoate mixture at 353 K. The membrane showed a molecular sieving effect, reducing the single permeation flux of alcohol with molecular size for single-component alcohols. Moreover, the permeation flux of methanol and the separation factor increased with an increase in the carbon number of the alcohols and methyl esters containing 10 wt% methanol. In this study, the permeation behavior of FAU-type zeolite membranes was also discussed based on permeation data. These results suggest that the FAU-type zeolite membrane has the potential to separate organic solvent mixtures, such as solvent recycling and membrane reactors.

Highly selective zeolite T membranes with different ERI stacking faults for pervaporative dehydration of ethanol

The effect of stacking arrangements of ERI into OFF structure on the separation performance of zeolite T membrane, which was obtained from a modified gel containing a small number of fluoride ions for ethanol dehydration, was investigated for the first time. The addition of fluoride ions and water content largely influenced the zeolite framework structure, the content of ERI stacking, and separation performance.The membrane M3 with MF(NaF: KF = 3:1)/SiO2 and H2O/SiO2 ratio of 0.15 and 35 respectively showed high crystallinity of zeolite T and ordered intergrowth of ERI with the content of about 50–60% estimated by comparing the simulated and observed ratios of (201) and (211) ERI odd lines. The M7 with an H2O/SiO2 ratio of 25 exhibited disordered stacking of ERI in the T membrane. Further increasing MF/SiO2 ratio to 0.20 and 0.25 (M4-M5) or H2O/SiO2 ratio to 45 and 55 (M8-M9) led to the formation of zeolite T membrane with impurity of PHI or CHA structure. The membrane M3 and M7 showed the highest separation index among the reported T membranes for ethanol dehydration with water flux of 2.68 and 1.94 kg·m-2·h-1 and separation factors of 9163 and 10000, respectively. The optimized membrane also exhibited good hydrothermal stability and reproducibility.

Impact of the Zeolite Cage Structure on Product Selectivity in CO-assisted Direct Partial Oxidation of Methane over Rh Supported AEI-, CHA-, and AFX-type Zeolites

In this manuscript, we have demonstrated CO-assisted methane conversion into small oxygenates using a Rh catalyst supported on zeolites with AEI-, CHA-, and AFX-type structures (specifically, SSZ-39, SSZ-13, and SSZ-16 zeolites, respectively). Methanol, formic acid, and acetic acid were obtained as the oxygenate products and their selectivities were unique to each of the zeolite framework structures. Smaller cage structures were preferable to produce smaller oxygenates and vice versa. Among them, the AEI-type structure was highly selective (>80%) in the formation of methanol.

Melting Point Depression and Phase Identification of Sugar Alcohols Encapsulated in ZIF Nanopores

Author(s): Dames, C; Urban, JJ; Kang, H | Abstract: Sugar alcohols (SAs) have attractive characteristics as phase-change materials, but their relatively high melting temperature limits their application in the real world. Nanoconfinement can be a useful parameter to reduce the melting temperature to pragmatic ranges. Using molecular dynamics simulations, we investigate the phases and behaviors of encapsulated SA in ZIF-8 and ZIF-11, which cannot be experimentally observed. Based on reliable partial charges for the zeolitic imidazolate framework (ZIF) structures calculated by a density functional theory, structural analysis shows that the SA's attractive interaction with the ZIF structure frustrates the SA crystallization and also elucidates the second-order phase transition between amorphous phases. A methodology is suggested to determine the phase transition temperature of confined materials and used to quantify the melting temperature depression of the ZIF-confined SAs. We also explored the thermal conductivity of SA-in-ZIF composites. Phonon frequency analysis verifies that the presence of SA molecules enhances the heat transfer by adding heat pathways between the nanoporous structure of ZIFs.

Synthesis and Characterization of Hybrid Metal Zeolitic Imidazolate Framework Membrane for Efficient H2/CO2 Gas Separation

In this paper, we propose mixed metal ions in the node of the zeolitic imidazolate framework (ZIF) structure. The hybrid metal ZIF is formed for the gas separation of hydrogen and carbon dioxide. In the first stage, the nanoparticles were prepared as a coating on a substrate, and acting as secondary growing nuclei. The hybrid metal ZIF structures were characterized by X-ray diffractometry (XRD) and Fourier transform infrared spectroscopy (FTIR). N2 adsorption–desorption isotherms determined surface area, and scanning electron microscopy (SEM) was used to observe the microstructure and surface morphology. The hybrid metal ZIF-8-67 powder had the largest surface area (1260.40 m2 g−1), and the nanoparticles (100 nm) could be fully dense-coated on the substrate to benefit the subsequent membrane growth. In the second stage, we prepared the hybrid metal ZIF-8-67 membrane on the pre-seeding substrate with mixed metal nanoparticles of cobalt and zinc, by the microwave hydrothermal method. Cobalt ions were identified in the tetrahedral coordination through UV–Vis, and the membrane structure and morphology were determined by XRD and SEM. Finally, a gas permeation analyzer (GPA) was used to determine the gas separation performance of the hybrid metal ZIF-8-67 membrane. We successfully introduced zinc ions and cobalt ions into the ZIF structure, where cobalt had a strong interaction with CO2. Therefore, GPA analysis showed an excellent H2/CO2 separation factor due to lower CO2 permeability. The CO2 permeance was ~0.65 × 10−8 mol m−2 s−1 Pa−1, and the separation factors for H2/CO2 and H2/N2 were 9.2 and 2.9, respectively. Our results demonstrate that the hybrid metal ZIF-8-67 membrane has a superior H2/CO2 separation factor, which can be attributed to its very high specific surface area and structure. Based on the above, hybrid metal ZIF-8-67 membranes are expected to be applied in hydrogen or carbon dioxide gas separation and purification.

Tutorial: structural characterization of isolated metal atoms and subnanometric metal clusters in zeolites

The encapsulation of subnanometric metal entities (isolated metal atoms and metal clusters with a few atoms) in porous materials such as zeolites can be an effective strategy for the stabilization of those metal species and therefore can be further used for a variety of catalytic reactions. However, owing to the complexity of zeolite structures and their low stability under the electron beam, it is challenging to obtain atomic-level structural information of the subnanometric metal species encapsulated in zeolite crystallites. In this protocol, we show the application of a scanning transmission electron microscopy (STEM) technique that records simultaneously the high-angle annular dark-field (HAADF) images and integrated differential phase-contrast (iDPC) images for structural characterization of subnanometric Pt and Sn species within MFI zeolite. The approach relies on the use of a computational model to simulate results obtained under different conditions where the metals are present in different positions within the zeolite. This imaging technique allows to obtain simultaneously the spatial information of heavy elements (Pt and Sn in this work) and the zeolite framework structure, enabling direct determination of the location of the subnanometric metal species. Moreover, we also present the combination of other spectroscopy techniques as complementary tools for the STEM-iDPC imaging technique to obtain global understanding and insights on the spatial distributions of subnanometric metal species in zeolite structure. These structural insights can provide guidelines for the rational design of uniform metal-zeolite materials for catalytic applications.

Evolution of the pore and framework structure of NaY zeolite during alkali treatment and its effect on methanol oxidative carbonylation over a CuY catalyst

Alkali treatment is widely used on aluminosilicate zeolites with high Si/Al ratios in order to fabricate mesopores in the framework. However, for zeolites with low Si/Al ratios, the effect of alkali treatment on the pore and framework structure needed further study. In this work, Y zeolite is treated with NaOH solutions of different concentrations and is used as the support for Cu-based catalysts for oxidative carbonylation of methanol to dimethyl carbonate. The physicochemical properties of the supports and corresponding catalysts are characterized by N2 adsorption–desorption, X-ray diffraction, X-ray fluorescence, transmission electron microscopy, inductively coupled plasma mass spectrometry, X-ray photoelectron spectroscopy, and H2-temperature-programmed reduction analyses. The results show that no obvious mesopores are formed under alkali treatment, even at high NaOH concentration. However, amorphous species present in the micropores of Y zeolite are removed, which increases the micropore surface area as well as the crystallinity. Simultaneously, the cage structure is partially destroyed, which also leads to a slight increase of the pore volume and surface area. The altered micropore structure eventually increases the content and accessibility of the exchanged Cu species, which is beneficial to the catalytic activity. When the concentration of NaOH is 0.6 M, the space time yield of dimethyl carbonate for the corresponding catalyst was 151.4 mg g−1 h−1 which is 3.3-fold higher than that of the untreated-Y-zeolite-supported Cu catalyst. However, further increasing the alkali treatment strength can seriously destroy the basic aluminosilicate structure of the Y zeolite and decrease its intrinsic ion-exchange capacity. This results in the formation of agglomerated CuO on the catalyst surface, which was not conducive to catalytic activity.

Synthesis of core-shell flower-like WO3@ZIF-71 with enhanced response and selectivity to H2S gas

Core-shell heterostructures of metal oxide semiconductor (MOS) and metal organic framework (MOF) improve gas-sensing properties due to the MOF's molecular sieving effects. However, the selection of MOF is limited by the requirement of identical metal element in MOF to that in MOS, which is crucial for valid interface adhesion. In this paper, WO3@ZIF-71 (zeolitic imidazolate framework) structure with different metal elemental components (W and Zn) was fabricated by a step-by-step (SBS) approach. As a result, ZIF-71 was uniformly coated on the surface of WO3. In order to explore the influence of ZIF-71 (pore size: 4.8 Å), the gas sensitivity of WO3 and WO3@ZIF-71 were tested. Different kinetic diameters gases: H2S (3.62 Å), CH3COCH3 (4.6 Å), CH3CH2OH (4.53 Å), NO2 (5.8 Å) were selected as testing gas. Gases with small molecules (H2S, CH3COCH3 and CH3CH2OH) can pass through the membrane of ZIF-71 and contact with WO3, while larger ones (NO2) were blocked outside the membrane. Excitingly, the response of WO3@ZIF-71 toward H2S gas was significantly improved, which soars from 2.24 of 20 ppm for pure WO3 to 19.12 for H2S at 250 °C, increased about 9 times. These results indicate that the MOS@MOF core-shell structure can be synthesized by the SBS method without the restriction of the same metal elements in MOS and MOF, which is also an effective approach to regulate the selectivity and sensitivity of the MOS@MOF gas sensor.

Adsorption of hydrogen and carbon dioxide in zeolitic imidazolate framework structure with SOD topology: experimental and modelling studies

The aim of this work is to develop insights into adsorption of hydrogen and carbon dioxide in a zeolitic imidazolate framework, ZIF-8, using high-pressure adsorption studies, adsorption isotherm model fitting, and DFT investigation of preferential adsorption sites and binding energies. The robustness of ZIF series metal–organic frameworks has drawn interest towards its utility in large scale applications in gas storage and separation. We use room temperature synthesis of ZIF-8 using DMF as a solvent, and benchmarked it against typical solvothermal synthesis. The resulting material is characterized using XRD, SEM, TG–DSC and N2 adsorption isotherm. High-pressure volumetric adsorption of the activated materials is conducted to analyze the hydrogen and carbon dioxide storage capacities up to 50 and 40 bar, respectively. ZIF-8 shows maximum H2 storage capacity of 3.13 wt% at 50 bar and 77 K, and CO2 storage capacity of 46 wt% at 40 bar and 300 K. The parameters of Unilan adsorption isotherm are estimated from the equilibrium adsorption data and isosteric heats of adsorption for H2 and CO2 on ZIF-8 are computed. DFT calculations are used to obtain preferential adsorption sites of H2 and CO2. Adsorption enthalpy values were calculated from DFT as − 7.08 and − 25.98 kJ/mol, respectively for H2 and CO2 at the most preferred sites. We found a close agreement between isosteric heat of adsorption of hydrogen (− 4.68 kJ/mol) and the enthalpy of hydrogen adsorption from DFT (− 6.04 kJ/mol) at 77 K.

Zeolitic imidazolate framework decorated on 3D nanofiber network towards superior proton conduction for proton exchange membrane

Manipulating membrane nanophase-separation behavior to construct ion-nanochannels for optimizing proton transport was still a challenge. Herein, a novel strategy was proposed for constructing consecutive ion-nanochannels by combining zeolitic imidazolate framework (ZIF-8) and 3D network structure (3DNWS) of poly-m-phenyleneisophthalamide nanofibers (PMIA NFs) via a hydrothermal method. NF hybrid proton exchange membranes (PEMs) were fabricated by filling the inter-fiber voids of 3DNWS-PMIA/ZIF-8 (P/ZIF-8) NFs with Nafion matrix. The structures of P/ZIF-8 and performance of hybrid PEMs were evaluated. The construction of ZIF-8 on 3DNWS-PMIA NFs enabled channels on the basis of the acid–base pair between –SO3H (Nafion) and N–H (imidazole molecule of ZIF-8) to be longer and consecutive. Results showed that P/ZIF-8@Nafion could reach 0.258 S/cm (80 °C, 100% relative humidity) proton conductivity and 150.60 mW/cm2 power density for single-cell performance. In addition, the superiority of 3DNWS of PMIA NFs considerably promoted the thermal and dimensional stability, especially in methanol crossover for hybrid PEMs. This work provided a new approach for preparing high-performance PEMs for direct methanol fuel cells.

Halogenated Metal–Organic Framework Glasses and Liquids

The synthesis of four novel crystalline zeolitic imidazolate framework (ZIF) structures using a mixed-ligand approach is reported. The inclusion of both imidazolate and halogenated benzimidazolate-derived linkers leads to glass-forming behavior by all four structures. Melting temperatures are observed to depend on both electronic and steric effects. Solid-state NMR and terahertz (THz)/far-IR demonstrate the presence of a Zn-F bond for fluorinated ZIF glasses. In situ THz/far-IR spectroscopic techniques reveal the dynamic structural properties of crystal, glass, and liquid phases of the halogenated ZIFs, linking the melting behavior of ZIFs to the propensity of the ZnN4 tetrahedra to undergo thermally induced deformation. The inclusion of halogenated ligands within metal-organic framework (MOF) glasses improves their gas-uptake properties.

Propylene/propane separation on a ferroaluminosilicate levyne zeolite

Abstract The propylene/propane separation properties of levyne zeolites with different framework Si/Al and Fe/Al ratios have been examined and compared with those of the four well-studied zeolite adsorbents (i.e., Na-A, Ca-A, Na-X and ITQ-12) for this separation. Both zeolite framework structure and composition were found to be critical for the propylene uptake, kinetic separation and adsorbent durability. Among the zeolites studied here, the mixed CaNH4 form of a ferroaluminosilcate levyne (Si/Al = 15.5 and Fe/Al = 0.27) showed the highest propylene/propane selectivity (11). Under vacuum-swing adsorption mode at 298 K and reservoir pressure of 1.2 bar, this partially iron-substituted zeolite was characterized by a higher propylene uptake (ca. 1.0 vs 0.7 mmol g−1) than its aluminosilicate version, which can be attributed to a combination of the molecular sieve effect of its framework topology and relatively weaker acidity. The results of this study demonstrate that, in contrast to widely held belief, zeolite adsorbents selective for propylene/propane separation do not need to be a pure-silica composition.

Hydrogen chloride removal from hydrogen gas by adsorption on hydrated ion-exchanged zeolites

Hydrogen chloride is found in important hydrogen gas streams of the chemical industry and needs to be removed for safety and environmental concerns. Here, a systematic study is presented on HCl removal from hydrogen gas by adsorption on zeolites, including cation free and high cation loaded materials, as well as ion-exchanged zeolites (with alkali, alkaline earth and transition metal ions). The HCl removal performance was studied by a fixed-bed breakthrough method under high gas velocities (>0.3 m/s), at high pressures (30 bar) and at room temperature, and with low HCl concentrations (<200 ppm). The zeolite screening indicated that the 13X zeolite outperforms the other tested materials. The ion-exchanged X zeolites were extensively characterized via SEM-EDX, XRD, ATR-FTIR, TGA and argon adsorption isotherms. The results revealed that the ion-exchange was successfully achieved with expected tendencies in XRD and ATR-FTIR spectra, and porosities. The breakthrough experiments demonstrated that the hydration of the zeolite prior HCl adsorption improves the hydrogen gas purification performance. The characterization after HCl adsorption supports the hypothesis that HCl is taken up by the material by forming salt molecules within the zeolite cavities by reaction with the cations and, which is, moreover, enhanced by the presence of pre-adsorbed water. The type of cation present in the zeolite framework structure notably affected the HCl removal adsorption capacity as well as uptake rate. Among the ion-exchanged zeolite samples, the Zn2+ form exhibited the highest adsorption capacity at saturation, attributed to the over-exchange of the zeolite cations. The investigation revealed important parameters such as cation radius, atomic mass and electronegativity, which played a noteworthy role in defining the HCl removal performance of ion-exchanged zeolites.

Citronellal cyclisation to isopulegol over micro-mesoporous zsm-5 zeolite: effects of desilication temperature on textural and catalytic properties

In this work, desilication reassembly post treatment process was applied in synthesis of mesoporous zeolite with stable phase composition and applied it in citronellal cyclisation reaction for the production of isopulegol. The desilication and temperature effects were further investigated on physical and chemical characteristics of zeolite and compared them with catalytic activity. The desilicated zeolite samples have been characterized with the help of N2-adsorption, XRD, ICP-OES, pyridine adsorption and FTIR techniques. Its performance was explored by controlling operative parameters. Experimental outcomes exhibited that desilication of zeolite would led to formation of mesopores inside the stable zeolite framework structures without substantial damage of their internal composition. These changes facilitate mass transfer and catalytic activity with an increase in surface area, mesoporosity, pore size, pore volume, acidity, and Lewis acid sites. Optimum desilication temperature (80 °C) was found as a best for an comprising extra active and selective mesoporous zeolite catalyst for citronellal cyclisation. Thus, this type of zeolite material has shown 100% conversion (e.g. complete conversion of citronellal reactants to the desired products), 95% isopulegol selectivity and highest reaction rate (0.11 min−1). This study exhibits the mesoporous zeolite (MZ-80 °C) as one of the most effective catalyst for citronellal cyclization reaction based on assessment of catalytic performance relative to other commonly available options.

Preparation of Pt/CeL reforming catalyst and its performance in the aromatization of naphtha

To solve the problems of chlorine loss and equipment corrosion caused by the conventional alumina-type reforming catalyst, zeolite L was modified with Ce3+ by ion exchange method and with Ce3+-modified L (CeL) as the support, the Pt/CeL reforming catalyst without any chlorine was prepared by impregnation method. The CeL support and Pt/CeL catalyst were characterized by XRD, N2 adsorption-desorption, NH3-TPD and Py-FTIR; the catalytic performance of Pt/CeL in aromatization was investigated in a continuous-flow fixed-bed micro-reactor, with an industrial hydrofining naphtha containing 0.50 μg/mL sulfur as the feedstock. The results indicate that the modification with Ce3+ by ion exchange has little influence on framework structure of zeolite L; however, it can raise the acid content and acid strength of the CeL support and then remarkably enhance the performance of the Pt/CeL reforming catalyst in aromatization. The activity and selectivity of the Pt/CeL catalyst are comparable to those of the commercial alumina-type reforming catalysts, suggesting that proper acidity can facilitate the aromatization reaction.

From Reticular Chemistry Design to Density Functional Theory Modeling for New Zeolitic Imidazolate Framework Topologies: Mechanical Stability, Electronic Structure, and CO2 Selectivity

Adopting the knowledge and resource of reticular chemistry, we introduce a series of new zeolitic imidazolate framework (ZIF) structures within three topological categories, pth, pts, and ast, cons...