High-silica Zeolites Research Articles

Maximizing the hydrothermal stability of Cu-LTA for NH3-SCR by control of Cu content and location

Hydrothermal stability is crucial for the application of NOx emission control catalysts on diesel vehicles. In this study, the influences of Cu content and Cu species on the catalytic performance and hydrothermal stability of copper-exchanged LTA (Cu-LTA) zeolite catalysts for the selective catalytic reduction of NOx with NH3 (NH3-SCR) have been systematically investigated. The Cu-LTA(n)−0.41 catalyst with the maximum amount and ratio of Cu2+-2Z species in Cu-LTA, with Cu/Al = 0.41 and Si/Al = 16, was produced by full occupation of Al pairs in the single 6-rings (s6rs) with Cu2+ ions. This catalyst showed higher deNOx activity after hydrothermal aging under extremely harsh conditions of 920 °C for 24 h than any of the other catalysts. The results suggest that maximizing the amount of Cu2+-2Z species and diminishing the formation of comparatively vulnerable [Cu(OH)]+-Z species in the high-silica LTA zeolite is an effective strategy for developing hydrothermally stable NH3-SCR catalysts.

Dual organic-structure directing agents in the synthesis of high-silica MOR Zeolite: Cooperation or competition?

Constructing a high-silica framework of a given zeolite topology via a conventional hydrothermal synthetic strategy is still an open question. In this work, In this contribution, we report the high-silica MOR zeolite (Si/Al ratio: ∼30) was fabricated by a dual OSDA combination strategy at a fixed alkalinity (OH−/SiO2) ratio of around 0.4. Additionally, we found that bulky TEA+ cation and smaller neutral heterocycle molecules severed as the pore-filling agent and cooperatively squeeze the occupation of sodium cation resulting in the low sodium content of high-silica MOR zeolite. However, such cooperative behavior can be easily spoiled by the high alkalinity ratio above 0.5. With the increment of the alkalinity ratio, the sodium cations can retake the occupied sites and facilitate the MOR framework with a relatively low Si/Al ratio. As unveiled by the powder X-ray diffraction crystallography and combined spectroscopy strategies, the nature of different dual OSDA combinations occluded in the high-silica MOR zeolite with deduced aluminum enriched T sites provide the reference for the forthcoming target design of zeotype catalysts.

High-silica FAU zeolite through controllable framework modulation for VOCs adsorption under high humidity

Although FAU zeolites have been widely used as absorbents for volatile organic compounds (VOCs) removal, preparation of high-silica FAU zeolite to expand its application under high humidity is still a big challenge. Here, two high-silica FAU zeolites were successfully prepared through the controllable framework modulation strategy. By carefully matching the degrees of dealumination and Si-insertion, the zeolite (FAU-H) with an ultra-high SiO2/Al2O3 ratio of 957 was obtained. FAU-H exhibited similar high adsorption capacities of VOCs (e.g. m-xylene and toluene) under dry and humid conditions, more importantly, its adsorption capacity of m-xylene was 25% higher than the industrial high-silica FAU zeolite (HSZ-390HUA) from Tosoh Corporation. When the dealumination degree was mismatched with that of Si-insertion, imperfect FAU-M (SiO2/Al2O3 = 108) with the existence of some defect sites was obtained that becomes a good candidate for the adsorption removal of small VOCs molecules (e.g. isopropanol). The moderate interaction between VOCs and zeolite framework is proved a key factor for small molecules adsorption on basis of the DFT calculations. Moreover, the fitted kinetic models, desorption tests, and adsorption−desorption cycling tests proved the good regenerability of FAU-H and FAU-M. Therefore, the proposed high-silica FAU zeolites are promising adsorbents for VOCs removal.

Synthesis and Structure of RUB-58: An Almost Ordered, Highly Crystalline Member of the ZSM-48 Family of Zeolites

ZSM-48 describes a family of disordered high-silica zeolites with a one-dimensional pore system of straight 10-ring channels. Although many different synthesis procedures and various organic structure-directing agents have been tried, all ZSM-48-type materials produced to date have proven to be highly disordered. A few structural models were proposed by Schlenker et al. ( Zeolites 1985, 5, 355−358) and by Lobo and van Koningsveld ( J. Am. Chem. Soc. 2002, 124, 13222−12230) to describe the framework topology and the disorder of the real structure, but it has not been possible to refine the structure of a ZSM-48-type material using experimental diffraction data. RUB-58 was synthesized under hydrothermal conditions at 160 to 200 °C in the system SiO2/H2O/ethylenediamine/pyridine. The product consists of spherelike aggregates of small, needlelike, colorless crystals and has an X-ray powder diffraction (XPD) pattern similar to those published for ZSM-48-type materials. The pattern, however, contains a much larger number of sharp reflections indicative of a more ordered, crystalline material. The reflections could be indexed with the unit cell parameters a = 14.2095(3) Å, b = 20.0874(5) Å, c = 8.3752(2) Å in the space group Pna21, a sub-subgroup of Imma, which is the highest possible symmetry of the ordered ZSM-48 polymorph A (see the Database of Disordered Zeolite Structures). The structure analysis was performed in successive steps: (I) analysis of the average structure, which showed that the framework structure of RUB-58 can be described in terms of polymorph A and that the pyridine molecules occupy defined positions in the elliptical 10-ring channels; (II) refinement of a faulted structure using two separate, but identical polymorph A-type framework structures, with one shifted by exactly 0.5 of the unit cell along the channel direction (volume ratio: 92/8); (III) analysis of the structure of guest-free RUB-58 to examine the geometry of the channels in the empty framework (elliptical cross section nearly identical to that of the as-made material); and (IV) development of convincing disordered models to describe the real structure of RUB-58.

Synthesis of an IMF zeolite membrane for the separation of xylene isomer

The synthesis of a continuous IMF zeolite membrane was fabricated on tubular substrates by seeded growth for the first time. The straight channels of IMF zeolite with diameters of 0.53–0.59 nm are distinguishable for p-xylene from o-xylene molecules. Pure IMF-phase high-silica IM-5 zeolite seeds with uniform and fine crystal size were fabricated by a new sonication-assisted aging process. The seeds were coated on the support by dipcoating and induced the formation of continuous membrane. Separation performance in p-/o-xylene mixture was investigated at various temperature and pressure. The typical IM-5 zeolite membrane had p-/o-xylene separation factor of 3.7. Our results suggest that IM-5 zeolite is a potentially good membrane material for the separation of xylene mixtures.

On the Use of Water and Methanol with Zeolites for Heat Transfer.

Reducing carbon dioxide emissions has become a must in society, making it crucial to find alternatives to supply the energy demand. Adsorption-based cooling and heating technologies are receiving attention for thermal energy storage applications. In this paper, we study the adsorption of polar working fluids in hydrophobic and hydrophilic zeolites by means of experimental quasi-equilibrated temperature-programmed desorption and adsorption combined with Monte Carlo simulations. We measured and computed water and methanol adsorption isobars in high-silica HS-FAU, NaY, and NaX zeolites. We use the experimental adsorption isobars to develop a set of parameters to model the interaction between methanol and the zeolite and cations. Once we have the adsorption of these polar molecules, we use a mathematical model based on the adsorption potential theory of Dubinin-Polanyi to assess the performance of the adsorbate-working fluids for heat storage applications. We found that molecular simulations are an excellent tool for investigating energy storage applications since we can reproduce, complement, and extend experimental observations. Our results highlight the importance of controlling the hydrophilic/hydrophobic nature of the zeolites by changing the Al content to maximize the working conditions of the heat storage device.

ECNU-13: A High-Silica Zeolite with Three-Dimensional and High-Connectivity Multi-Pore Structures for Selective Alkene Cracking.

A high-silica zeolite ECNU-13 (Si/Al=23) with a new three-dimensional (3D) pore system and a nanosized morphology has been developed, consisting of multitudes of 10-membered ring (10-R) medium pores and one set of 8-R small pores. A phase-discrimination strategy was proposed to synthesize ECNU-13 by regulating the gel compositions and nucleation processes that were used for preparing 12-R large-pore germanosilicate IM-20 with the known UWY topology. The crystallization was directed towards forming one set of single four-ring (s4r) composite building units together with one set of double four-ring (d4r) rather than two different types of d4r units in IM-20. The electron crystallographic investigations elucidated that the ECNU-13 structure was composed of two kinds of polymorphs as a result of distinct atomic positionings in s4r units. In catalytic cracking of 1-butene, ECNU-13 exhibited high propene selectivity (55.6 %) and propene to ethylene molar ratio (>4.7) superior to well-studied conventional ZSM-5 zeolite catalyst.

Review on the Catalytic Conversion of Naphtha to Aromatics: Advances and Outlook

Naphtha is the main feedstock used in the production of gasoline and benzene, toluene, and xylene (BTX) aromatics, which are widely applied as chemical intermediates in the fields of packaging, synthetic fibers, and solvents. This article reviews the recent literature regarding the catalytic conversion of naphtha to higher-value BTX in terms of catalysts used, catalyst properties, reaction mechanisms, kinetic modeling, operating conditions, and product selectivity. Most studies focused on ZSM-5 and ZSM-5 modified using Ga, Zn, Ni, Co, Cu, Fe, Mo, W, Sn, etc. The promoting effects of metals and catalyst properties on naphtha conversion and the selectivity toward aromatics were critically analyzed. Pt-promoted zeolite L exhibited an excellent catalytic performance in naphtha aromatization for the selective production of benzene using n-hexane. Other studies investigated different types of zeolites, such as ZSM-11, BEA, X, Y, MOR, ZRP, and TsVM high-silica zeolites, with distinct properties. Nonzeolitic catalysts included Pt, Re, and Sn supported on Al2O3; activated carbon; and mesoporous silicates (MCM-41 and SBA-15). Finally, the challenges and perspectives of naphtha aromatization related to emerging naphtha types, cofeeding, and improved catalysts are highlighted.

Constructing PtCe cluster catalysts by regulating metal-support interaction via Al in zeolite for propane dehydrogenation

Pt-based catalysts as one of the most used industrial catalysts in heterogeneous catalysis, it is essential for the on-purpose catalytic dehydrogenation of propane to propylene (PDH). Here, the highly dispersed PtCe clusters are constructed by high-silica Beta zeolite as support to improve the catalytic stability and propylene selectivity in the PDH reaction. The defect sites derived from silanol nests are responsible for dispersing PtCe clusters inside the channels of the high-silica Beta-557 zeolite, while the trace Al sites play an important role in further stabilizing the active site structure by regulating the metal-support interaction. The closely associated Pt and Ce species are critical to embody the outstanding catalytic activity (propane conversion of ∼42.2%, propylene selectivity of ∼91.9%) with excellent stability (deactivation constant: 0.013 h−1) at 550 °C. This work proposed an efficient strategy for designing stable PtCe bimetallic clusters by utilizing silanol nests and Al sites to regulate the metal-support interaction synergistically.

Direct observation of Cu in high-silica chabazite zeolite by electron ptychography using Wigner distribution deconvolution

Direct observation of Cu in Cu-chabazite (CHA) zeolite has been achieved by electron ptychography using the Wigner distribution deconvolution. The imaging properties of ptychographically reconstructed images were evaluated by comparing the intensities of six-membered-ring columns of the zeolite with and without Cu using simulated ptychography images. It was concluded that although false contrast may appear at Cu-free columns for some acquisition conditions, ptychography can discriminate columns with and without Cu. Experimental observation of CHA with and without Cu was performed. Images obtained from the Cu-containing sample showed contrast at the six-membered-rings, while no contrast was observed for the Cu-free sample. The results show that ptychography is a promising technique for visualizing the atomic structures of beam-sensitive materials.

Spatial Proximities between Brønsted Acid Sites, AlOH Groups, and Residual NH4+ Cations in Zeolites Mordenite and Ferrierite

The interaction between Brønsted acid sites (BAS), AlOH groups, and residual NH4+ cations in zeolites mordenite and ferrierite was investigated by solid-state 1H magic-angle spinning and 1H–27Al rotational-echo adiabatic-passage double-resonance (REAPDOR) nuclear magnetic resonance (NMR) techniques in combination with density functional theory (DFT) cluster calculations. Higher temperatures are needed for the full elimination of NH4+ (853 K instead of 723 K compared to other high-silica zeolites). NMR data reveal that residual NH4+ cations are primarily in the vicinity of BAS and other NH4+ in the case of mordenite and AlOH groups in ferrierite materials. DFT calculations for a variety of cluster models with BAS and NH4+ ions embedded in their zeolite environment rationalize the experimental data. It is suggested that NH4+ ions are preferably stabilized within 8-ring pores, explaining the higher temperature required to decompose them.

Study of methods of chemisorption purification of hydrogen sulfide in natural gases with natural sorbent

We know from the literature that the cost-effectiveness of using zeolites to purify natural gas is much higher than other natural resources. In the laboratory, CaCl2∙ZnCl2∙MnCl2/HQZ were prepared by treating bentonite obtained from local raw materials with calcium, zinc, and magnesium salts. Local raw materials are activated by mechanical-thermal and chemical processing and are delivered in the form of dark-colored granules (tablets) up to 5 mm in diameter. The sorption potential of high-silica zeolite obtained from local raw materials by metal salts and heat treatment was studied, and a technological scheme was developed. A catalyst containing CaCl2∙ZnCl2∙MnCl2/HQZ was developed for drying the obtained high-silica zeolite oil satellite and natural gas from water vapor and purification from sulfur compounds, and its texture and sorption properties were studied. The total amount of SiO2, Ca2+, Zn2+, and Mg2+ ion phases was studied in the radiological analysis of the surface of the catalyst CaCl2∙ZnCl2∙MnCl2/HQZ. According to IR spectroscopy data, surface adsorption of hydrogen sulfide at 25°C occurs in the presence of hydroxyl groups. UZBEKGAZ JSC -Mubarek Gas Processing Plant conducted experimental tests to study the sorption properties of this catalyst, as well as a comparative study of the possibility of sorption of sulfides with the participation of catalysts CaCl2∙ZnCl2∙MnCl2/HQZ and CaA (5A). The tests showed that another experimental test of the selected CaA (5A) compound catalyst was also performed. The absorption of this adsorbent hydrogen sulfide showed that the catalyst CaCl2∙ZnCl2∙MnCl2/HQZ has a total capacity of 8.3% by mass and 14.3% by mass.

Hydrogen bonding to oxygen in siloxane bonds drives liquid phase adsorption of primary alcohols in high-silica zeolites.

Upon liquid phase adsorption of C1-C5 primary alcohols on high silica MFI zeolites (Si/Al = 11.5-140), the concentration of adsorbed molecules largely exceeds the concentration of traditional adsorption sites: Brønsted acid and defect sites. Combining quantitative in situ1H MAS NMR, qualitative multinuclear NMR and IR spectroscopy, hydrogen bonding of the alcohol function to oxygen atoms of the zeolite siloxane bridges (Si-O-Si) was shown to drive the additional adsorption. This mechanism co-exists with chemi- and physi-sorption on Brønsted acid and defect sites and does not exclude cooperative effects from dispersive interactions.

Fluoride-free and seed-free microwave-assisted hydrothermal synthesis of nanosized high-silica Beta zeolites for effective VOCs adsorption.

High-silica Beta zeolites, typically synthesized by hydrothermal synthesis with the assistance of F- or seeds, are highly important in volatile organic compounds (VOCs) adsorption. Fluoride-free or seed-free synthesis of high-silica Beta zeolites attracts great attention. Herein, highly dispersed Beta zeolites with a size of 25-180 nm and Si/Al ratios of 9-∞ were successfully synthesized by a microwave-assisted hydrothermal strategy. We have for the first time revealed that microwave irradiation can induce the formation of hydroxyl free radicals (˙OH), promoting the formation of the Si-O-Si bond. Thanks to the high total surface area, pore volume, and excellent hydrophobicity, the as-prepared pure-silica Beta zeolite presents a higher toluene adsorption capacity in VOCs adsorption compared to other pure-silica Beta zeolites prepared by traditional methods. This work provides a facile avenue for fluoride-free and seed-free synthesis of nanosized high-silica zeolites, promising their important applications in VOCs adsorption.

Coking Mechanism and the Distribution of Agglomeration Products in High-Silica MFI-Type Zeolites

Coking Mechanism and the Distribution of Agglomeration Products in High-Silica MFI-Type Zeolites

Coking Mechanism and the Distribution of Agglomeration Products in High-Silica MFI-Type Zeolites

Coking Mechanism and the Distribution of Agglomeration Products in High-Silica MFI-Type Zeolites

EVOLUTION AND ROLE OF THE DEWAXING PROCESS IN OIL REFINING

Currently, the dewaxing process is used throughout the oil industry. There is a lot of research and interest in this process as necessary in the production of fuels for cold regions, as well as lubricating oils. For dewaxing, mesoporous high-silica zeolites with different structures have been used for a long time, offering sufficient activity in the process. Dewaxing can be divided into several types and is actively used not only in the oil industry, but also in biology and medicine. In this article, an overview of the dewaxing process development stages and the most popular research on this topic was made, various indicators and physicochemical parameters were considered.

The effect of Si/Al ratio on local and nanoscale water diffusion in H-ZSM-5: A quasielastic neutron scattering and molecular dynamics simulation study

The dynamics of water confined in H-ZSM-5 (protonated form of the Zeolite Socony Mobil – 5) has been studied using quasielastic neutron scattering (QENS) and classical molecular dynamics simulations (MD). QENS measurements probed water confined in ZSM-5 samples with Si/Al ratios of 15, 40 and 140 at 2.8 wt% loadings. In the lower silica samples, fitting of the elastic incoherent structure factor (EISF) showed that water diffusion was confined to a sphere (with radii ranging from 3.4 to 4.3 Å), suggesting the mobile water was located within the MFI (framework type of H-ZSM-5) channel intersections, giving localised diffusion coefficients in the range of ∼0.9–1.8 × 10−9 m2s−1. In the high silica zeolite, the diffusion was observed to be far less confined and more long range in nature, with diffusion coefficients significantly higher than in the lower silica systems (∼1.8–4.8 × 10−9 m2s−1). MD simulations further investigated the effect of the Si/Al ratio on water diffusivity at 2.8 wt% loading (9 molecules/unit cell (UC)) in H-ZSM-5 with Si/Al ratio = 15, 47, 95 and fully siliceous. The Si/Al ratio had a significant effect on the MD calculated nanoscale diffusivity of water, reducing the self-diffusion coefficient by a factor of 2 from a fully siliceous system to that with Si/Al = 15, due to the strong coordination and increased residence time of water molecules at the Brønsted acid sites which range from ∼5 ps to ∼2 ps in the Si/Al = 15 and Si/Al = 95 systems respectively. QENS observables, both the EISF and quasielastic line broadenings, were reproduced from the MD trajectories upon sampling the experimental timescale giving both qualitative and quantitative agreement with the QENS experiments. Fitting of the MD calculated EISF showed that the experimentally observed diffusion confined to a sphere of radii ranging from 3.5 to 6.8 Å was also present in our simulations, with diffusion coefficients calculated to within a factor of 0.5 of experiment.

Efficient synthesis of high-silica SSZ-13 zeolite and its catalytic performance in MTO reaction

Highly efficient synthesis of zeolites is an important goal given the many catalytic applications of zeolites in industrial chemical processes. The conventional zeolite syntheses, i.e., hydrothermal synthesis in a batch setup are usually challenged by certain limiting factors deriving from the long synthesis period or the low yield. Herein, we report that a combined strategy of organic promoter and zeolite seeding can remarkably improve the synthetic efficiency of high silica SSZ-13 zeolite. The low-cost promoter diethylamine (DEA) and zeolite seeds could both remarkably accelerate the crystallization rate of SSZ-13 zeolite, and DEA has a much higher beneficial effect on the solid yield as compared to the zeolite seeds. Consequently, the crystallization time could be unprecedentedly shorted by a factor of 30, instead of multiple days normally required. Besides, this novel protocol also exhibits high atom efficiency (93% of yield). The resultant high-silica nano-sized SSZ-13 zeolite shows good (hydro)thermal stability along with excellent catalytic performance for the MTO reaction.

Biased Al distribution of high-silica FAU-type zeolite synthesized by fast manner at high temperature

The flexible control of composition and atomic arrangement is the ultimate goal in materials science. The control of the Si/Al ratio and Al distribution in the zeolite framework is a hot research topic in the field of zeolites, particularly for catalytic applications. To date, the direct synthesis of high-silica faujasite zeolites (FAUs) with a Si/Al ratio higher than 5 has been limited so far, which is desired to achieve high catalytic performance in various processes such as fluid catalytic cracking and hydrocracking catalysts. In addition, the Al distribution in high-silica FAU has been rarely discussed. In this study, we rapidly synthesized a high-silica FAU with a Si/Al ratio of approximately 6 using tetrapropylammonium cations and seed crystals at a high temperature of 180 °C for 4 h, by approximately one-tenth of conventional FAU syntheses using an OSDA. In silico analysis, based on the results of solid-state 29Si magic-angle spinning nuclear magnetic resonance spectra, revealed that the obtained FAU had a biased Al distribution compared with previously reported FAUs. We also succeeded in the first synthesis of FAU with biased Al distribution using N,N- dimethyl- 3, 5- dimethylpiperidinium cations that acted as organic structure-directing agents just under high-temperature conditions.