Zeolite Crystals Research Articles

Conversion of lignite fly ash into synthetic zeolite by hydrothermal, fusion‐hydrothermal, and hydrothermal‐sonochemical processes

AbstractOBJECTIVEThe purpose of this study was the conversion of calcareous lignite fly ash (LFA) into synthetic zeolite by hydrothermal, fusion‐hydrothermal, and hydrothermal‐sonochemical processes.METHODExperiments were carried out using raw and washed (5 N HCl) fly ash. Initially, the fractional factorial experimental design was used to plan the hydrothermal experiments and assess the process parameters, such as solid to liquid ratio (S/L), silica to aluminum oxide ratio (SiO2/Al2O3), calcium oxide content (% CaO), time, and temperature of the hydrothermal process. The optimized conditions were then applied to a two‐stage fusion‐hydrothermal process, while the influence of sonication on the conversion efficiency was tested before and after the hydrothermal process. Mineralogical analysis was performed by x‐ray diffractometer (XRD) to detect zeolite formation. Further characterization was conducted by x‐ray fluorescence (XRF), specific surface area, and laser particle analysis to assess process efficiency.RESULTS AND DISCUSIONThe results indicated the formation of four types of zeolite crystals. The washed LFA (wLFA) yielded the maximum zeolite yield when it was hydrothermally treated for 6 h at 150 °C, S/L = 1:2, and SiO2/Al2O3 = 6.3. In such conditions, phillipsite was synthesized at 27%. The yield of phillipsite zeolite was improved to 32% when the hydrothermal process was followed by a 30 min sonication process. At the same processing conditions, the use of raw LFA produced 72% crystalline tobermorite. The hydrothermal‐late sonication process can effectively convert waste fly ash into valuable products. © 2023 The Authors. Journal of Chemical Technology and Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry (SCI).

The influence of deuterium on sodium mobility and viscosity of colloidal precursor suspensions yielding template-free nanosized zeolites

The isotopic substitution in chemistry is characterized by two main effects, i.e., the kinetic isotope effect (KIE), associated with differences in the reaction rates, and the geometric isotope effect (GIE) related to crystallographic features. In this work, considering the difference in the weight of deuterium (D) compared to the weight of hydrogen (H), we use the isotopic substitution to investigate the crystallisation mechanism of template-free nanosized faujasite (FAU) type zeolite in colloidal precursors. Тhe basic hydrothermal synthesis solvent - water (H2O), is substituted with heavy water (D2O), aiming to understand the isotopic influence and the effect of H-bond networks on the zeolite crystallisation. The crystallinity, viscosity, conductivity, chemical composition, morphology and local order of the FAU zeolites synthesized in both hydrated (H) and deuterated (D) medium are evaluated with X-ray diffraction, TEM microscopy, EDX and ICP analysis, FT-IR and solid-state NMR spectroscopy. The structural directing agent - Na+, exhibits a lower mobility in the deuterated medium, because of a characteristic higher viscosity compared to hydrated medium. This leads to a slower nucleation and crystallisation in the initial stages of hydrothermal treatment in the colloidal suspensions but also to a significant difference in the Al distribution.

Highly productive framework bounded Ni2+ on hierarchical zeolite for ethylene oligomerization

The production of higher linear olefins via ethylene oligomerization is an applied industrial process using homogeneous catalysts in the liquid phase. Heterogeneous catalysts based on Ni supported on zeolites are attractive materials for gas phase oligomerization but typically offer a low selectivity or low conversion. Here, we investigate a tailored method to introduce the Ni2+ species within the hierarchical zeolite crystallization step (in situ) and compare it with the standard impregnation procedure (ex situ). The in situ engineered catalyst has a very high concentration of Ni2+ species, seamlessly inserted and well dispersed into the zeolite framework, with increased accessibility through meso- and micropores. This catalyst has a unique 1-butene cumulative productivity (32.7 g of 1-butene per g of catalyst) and stability for at least 48 h. This framework bounded Ni2+ promotes oligomerization over isomerization, cracking, and hydride transfer, while the hierarchical zeolite structure enables the discharge of coke precursors. These results pave the way for a more efficient and effective ethylene oligomerization process.

High activity and sinter-resistance of Ni@silicalite-1 catalyst for dry reforming of methane

This paper reports a highly active and stable Ni@silicalite-1 catalyst for dry reforming of methane. In the reaction conducted at 800[Formula: see text]C, CH4/CO2=1/1 and GHSV=100,000 mL⋅g[Formula: see text]⋅h[Formula: see text] for 100 h, the Ni@silicalite-1 delivered CH4 conversions of 93% without deactivation. This superior property of the catalyst comes from the special structure that large amounts of Ni (13.7 wt.%) are embedded in the silicalite-1 zeolite crystals as small particles ([Formula: see text]4.7 nm), with the zeolite channels transferring molecules while with the zeolite framework resisting the Ni particle migration.

Hollow STW‐Type Zeolite Single Crystals with Aluminum Gradient for Highly Selective Production of p‐Xylene from Methanol‐Toluene Alkylation

AbstractZeolites with uniform micropores are important shape‐selective catalysts. However, the external acid sites of zeolites have a negative impact on shape‐selective catalysis, and the microporosity may lead to serious diffusion limitation. Herein, we report on the direct synthesis of hierarchical hollow STW‐type zeolite single crystals with a siliceous exterior. In an alkalinous fluoride medium, the nucleation of highly siliceous STW zeolites takes place first, and the nanocrystals are preferentially aligned on the outer surface of the gel agglomerates to grow into single crystalline shells upon crystallization. The lagged crystallization of the internal Al‐rich amorphous gels onto the inner surface of nanocrystalline zeolite shells leads to the formation of hollow cavities in the core of the zeolite crystals. The hollow zeolite single crystals possess a low‐to‐high aluminum gradient from the surface to the core, resulting in an intrinsic inert external surface, and exhibit superior catalytic performance in toluene methylation reactions.

Hollow STW-Type Zeolite Single Crystals with Aluminum Gradient for Highly Selective Production of p-Xylene from Methanol-Toluene Alkylation.

Zeolites with uniform micropores are important shape-selective catalysts. However, the external acid sites of zeolites have a negative impact on shape-selective catalysis, and the microporosity may lead to serious diffusion limitation. Herein, we report on the direct synthesis of hierarchical hollow STW-type zeolite single crystals with a siliceous exterior. In an alkalinous fluoride medium, the nucleation of highly siliceous STW zeolites takes place first, and the nanocrystals are preferentially aligned on the outer surface of the gel agglomerates to grow into single crystalline shells upon crystallization. The lagged crystallization of the internal Al-rich amorphous gels onto the inner surface of nanocrystalline zeolite shells leads to the formation of hollow cavities in the core of the zeolite crystals. The hollow zeolite single crystals possess a low-to-high aluminum gradient from the surface to the core, resulting in an intrinsic inert external surface, and exhibit superior catalytic performance in toluene methylation reactions.

Adsorption and Desorption Behaviors of Ammonia on Zeolites at 473 K by the Pressure-Swing Method.

Adsorption and desorption behaviors of ammonia on the zeolites were investigated using the pressure-swing method to utilize the ammonia adsorbent at 473 K. For various (Y, A, L, and mordenite) types of zeolites, the effects of their pore sizes, countercation species, and number of adsorption sites on the ammonia adsorption behavior were evaluated. The adsorption capacity increased with increasing the pore size. The differences in the countercation species and the number of adsorption sites contributed to the ammonia adsorption process on the zeolites. Sodium cations strongly adsorb ammonia molecules. The adsorption/desorption of ammonia expanded and shrunk the zeolite crystal, suggesting that the pores in the zeolite were enlarged by the adsorption of ammonia molecules. Despite repetitive lattice expansion and shrinkage, the morphologies of the zeolite particles were not degraded. These results confirm that the zeolite showed high durability for the adsorption and desorption of ammonia at 473 K.

A simple strategy for synthesis of b-axis-oriented MFI zeolite macro-nanosheets

Zeolite nanosheets with shortened diffusion paths and high external surface areas have drawn significant attention. This unique morphology of zeolites can endow them with reduced diffusion resistance and high accessibility of active sites, and thereby good catalytic performance. Exploring simple synthesis strategies for zeolite nanosheets is mostly desired in zeolite chemistry. Herein, MFI zeolite nanosheets with a thickness of 50-100 nm along the b -axis and a length of 1 μm along the c -axis, namely zeolite macro-nanosheets, have been successfully synthesized via the strategy of combining pre-aging and pH regulation without the use of complex templates/additives and fluorine media. Pre-aging at a relatively high temperature of 90 °C and low alkalinity for crystallization (pH = 7.0~8.5) are found to play key roles in controlling macro-nanosheet synthesis, which can promote the formation of MFI zeolite precursors and then dissociate them into smaller ones for the following oriented aggregation during hydrothermal treatment. Crystallization kinetic studies suggest that the two-dimensional crystal growth is the intrinsic behavior of zeolite crystallization under employed conditions. This synthesis strategy can be easily extended to the synthesis of heteroatom-containing MFI macro-nanosheets, such as ZSM-5 and TS-1.

A strategy of constructing the Ni@silicalite-1 catalyst structure with high activity and resistance to sintering and carbon deposition for dry reforming of methane

Dry reforming of methane (DRM) as a promising reaction has both benefits reducing greenhouse gasses’ emissions and producing syngas for Fischer-Tropsch synthesis, whereas the Ni based DRM catalysts significantly suffer from deactivation due to the problems of metal sintering and carbon deposition. This paper reports a strategy for preparing the Ni@silicalite-1 catalyst, with which the nickel metal was encapsulated into the silicalite-1 zeolite crystals as Ni nano-particles (Ni NPs) with much high dispersion. Being benefited from this special structure, the catalyst displayed much high activity and durability under ultra-high space velocity (GHSV = 900,000 mL⋅gcat−1⋅h−1) reaction conditions. The high durability of the catalyst is originated from the strong interaction of Ni with the support and the encapsulation of the composite in the zeolite framework, which made the catalyst highly resistant to sintering and coke deposition. The much high activity of the catalyst should be the result of that Ni was highly dispersed in the catalyst and channels of the zeolite provide pathway for the reactant molecules contacting the Ni NPs and for the product molecules releasing there.

Sulfide-occluded zeolites: Cooperative adsorption-precipitation system for near perfect decontamination of aqueous Hg species

Sulfide-occluded zeolites could allow both adsorption reaction of zeolites and precipitation reaction of sulfides to operate within one system, which leads to synergic effects on removal of toxic cationic heavy metals from aqueous systems. Here we proved that near complete decontamination of aqueous Hg cations bellow WHO guideline level of 1 ppb could be accomplished with sulfide-occluded zeolites. Interestingly, adsorption of cationic Hg species resulted in the formation of discrete HgS crusts on the surfaces of the zeolite crystals. TEM and Hg removal isotherms strongly indicated that the HgS crust played a crucial role not only in preventing the back-release of adsorbed Hg species but also in scavenging residual aqueous Hg species. Therefore, in-situ cooperation of adsorption and precipitation reactions performed by sulfide-occluded zeolites could be applied to reliable and safe Hg decontamination of aqueous systems.

Illuminating the Black Box: A Perspective on Zeolite Crystallization in Inorganic Media.

ConspectusSince the discovery of synthetic zeolites in the 1940s and their implementation in major industrial processes involving adsorption, catalytic conversion, and ion exchange, material scientists have targeted the rational design of zeolites: controlling synthesis to crystallize zeolites with predetermined properties. Decades later, the fundamentals of zeolite synthesis remain largely obscured in a black box, rendering rational design elusive. A major prerequisite to rational zeolite design is to fully understand, and control, the elementary processes governing zeolite nucleation, growth, and stability. The molecular-level investigation of these processes has been severely hindered by the complex multiphasic media in which aluminosilicate zeolites are typically synthesized. This Account documents our recent progress in crystallizing zeolites from synthesis media based on hydrated silicate ionic liquids (HSIL), a synthesis approach facilitating the evaluation of the individual impacts of synthesis parameters such as cation type, water content, and alkalinity on zeolite nucleation, growth, and phase selection. HSIL-based synthesis allows straightforward elucidation of the relationship between the characteristics of the synthesis medium and the properties and structure of the crystalline product. This assists in deriving new insights in zeolite crystallization in an inorganic aluminosilicate system, thus improving the conceptual understanding of nucleation and growth in the context of inorganic zeolite synthesis in general. This Account describes in detail what hydrated silicate ionic liquids are, how they form, and how they assist in improving our understanding of zeolite genesis on a molecular level. It describes the development of ternary phase diagrams for inorganic aluminosilicate zeolites via a systematic screening of synthesis compositions. By evaluating obtained crystal structure properties such as framework composition, topology, and extraframework cation distributions, critical questions are dealt with: Which synthesis variables govern the aluminum content of crystallizing zeolites? How does the aluminum content in the framework determine the expression of different topologies? The crucial role of the alkali cation, taking center stage in all aspects of crystallization, phase selection, and, by extension, transformation is also discussed. New criteria and models for phase selection are proposed, assisting in overcoming the need for excessive trial and error in the development of future synthesis protocols.Recent progress in the development of a toolbox enabling liquid-state characterization of these precursor media has been outlined, setting the stage for the routine monitoring of zeolite crystallization in real time. Current endeavors on and future needs for the in situ investigation of zeolite crystallization are highlighted. Finally, experimentally accessible parameters providing opportunities for modeling zeolite nucleation and growth are identified. Overall, this work provides a perspective toward future developments, identifying research areas ripe for investigation and discovery.

Tailoring the pore structure、morphology and acidity of MFI zeolites by the regulation of Si/Al ratio in the synthetic gel

In this work, we report a facile and low-cost approach for controllable preparation of hierarchical mesoporous MFI zeolites via simply manipulating the Si/Al ratio in the initial synthetic precursor. The results demonstrate that Si/Al ratio significantly influences not only the acidity but also the pore structure as well as morphology of MFI zeolites. With the increase of Si/Al ratio, the mesoporous structures are weakened due to the increasingly severe crystal fusion leading to the formation of bulk zeolite crystal and the acid amounts decrease as well. The optimal sample Z-55 with spherical morphology is composed of numerous superfine nanocrystals. These primary results are quite beneficial to guiding reasonable design of the zeolites with other topologies. In LDPE cracking, the sample Z-55 with tri-modal mesopores distributions performs enhanced catalytic activity compared with convention MFI zeolite and the T50 is reduced to 391 °C, mainly ascribed to the luxuriant mesoporous structures.

In Situ Ultrasonic Measurements as a Tool to Characterize the Stages of the Zeolite A Crystallization Process

AbstractZeolite A (LTA) was synthesized at 80 °C under hydrothermal conditions from a gel with the molar composition of 2.5 Na2O : 2 SiO2 : Al2O3 : 80 H2O. In the present study, we report the use of home‐made ultrasound devices as a real‐time, in situ diagnostic tool for monitoring the progress of zeolite formation. In addition, the specific aim of this study was to show that the measured ultrasound parameters – ultrasonic (US) velocity and attenuation – simultaneously provide independent information about the changes in the liquid and solid phase, respectively, during the crystallization process. Both views are necessary for extracting kinetic data about the zeolite crystallization processes and allow elucidating the different stages of zeolite LTA formation process. The obtained in situ US data were compared with conventional ex situ powder X‐ray diffraction measurements following the formation of zeolite LTA from dense gels. The results demonstrate that a good correlation exists between the crystallization progress of zeolite LTA in terms of crystallinity changes and the ultrasonic attenuation and velocity.

Hierarchical Y zeolite with accessible intracrystalline mesopores and enhanced hydrocracking performances via sequential multi-carboxyl acid and fluoroborate post-treatments

Hierarchical Y zeolite that combines the intrinsic microporosity with mesoporosity can alleviate diffusion limitation in hydrocracking catalyst. In this study, hierarchical Y zeolites with abundant mesopores that connect the interior of the zeolite crystals to the external surface were successfully synthesized by post-synthesis dealumination using citric acid (C6H8O7) and/or ammonium tetrafluoroborate (NH4BF4) as agents. The results show that C6H8O7 can effectively extraction framework aluminum species (FALs), while NH4BF4 mainly interacts with extra framework aluminum species (EFALs). Via sequentially adopting C6H8O7 and NH4BF4 (denoted as DAY-C-F), such interconnected mesopores can be effectively introduced without sacrificing the zeolite crystallinity. The results of the scaled-up hierarchical Y zeolite reveal that the above mentioned post-treatment strategy is feasible and reproducible for potential industrial manufacturing and applications. The hydrocracking test results reveal that the naphtha yield for the catalyst derived from the C6H8O7-NH4BF4 sequentially treated USY is higher than that of the counterpart made from commercial USY.

Carbonation and freeze-thaw resistance of geopolymer-zeolite cementitious composites

Geopolymer-zeolite cementitious composites (GZCC) are emerging materials which can find applications as self-supported cementitious adsorbents, self-cleaning composite panels, etc. Considering the carbonation and free-thaw attack tendency of GZCC, this study assessed the carbonation and freeze–thaw resistance of geopolymer-zeolite cementitious composites (GZCC). GZCC samples with different activator-to-binder ratios (0.8 or 1), activator composition (sodium silicate-to NaOH ratio of 0.17 or 0.5 and NaOH molarity of 5 or 8 M), and hydrothermal duration (24 or 48 h), were prepared. Samples were exposed to accelerated carbonation environment after 3 and 14 days of curing. The physical and chemical characteristics of samples before and after carbonation were assessed by means of compressive strength, x-ray diffraction (XRD), and mercury intrusion porosimetry (MIP). Free-thaw exposure was started after 14 days of initial curing. Samples were put at -20 °C during freezing and immersed in water during thawing. The test results revealed that the carbonation resistance of samples was dependent on the mix proportion. A higher activator and lower silicate content resulted in lower residual strength which can be related to the higher porosity and lower compressive strength values of reference uncarbonated samples. However, zeolitic phases were still present which directs that the functionality of GZCC might not be affected much. All the samples broke after freeze–thaw exposure between 2 to 20 cycles. The formation of zeolitic crystals enhanced the open porosity of GZCC samples which caused their poor freeze–thaw resistance owing to easy penetration of water.

Mechanical and microstructural properties of just add water geopolymer cement comprised of Thermo-Mechanicalsynthesis Sediments-Fly ash mix

This study investigated the utilization of dredged sediments for the mix design of a just add water geopolymer (one-part geopolymer). For this purpose, four aluminosilicate sources were prepared, by volume substitution of fly ash by sediment, with ratios of 15, 30 and 50%. These mixtures were thermomechanically treated to increase their reactivity. Then, commercial anhydrous sodium silicates and sodium hydroxide powder (NaOH) were used to formulate geopolymers. Several mortar formulations have been prepared with a variation of SiO2/Al2O3 molar ratio between 3; 3.5; 4; 4.5 and 5 for each aluminosilicate source. The measurements of the mechanical strengths were performed at 7 and 28, days. An optimal mixture was obtained for each mixture. Indeed, the optimal SiO2/Al2O3 ratio was equal to 3.5 for FA, FA85SD15 and FA70SD30 and 5 for FA50SD50. Subsequently, a series of tests was carried out on the selected optimal mixtures, such as microstructural characterizations (Mercury Intrusion Porosimetry, SEM) and chemical characterizations (XRD, FTIR). The results have shown that the incorporation of sediments in mixtures can improve the mechanical performance of the geopolymers. The results of XRD, FTIR and SEM have shown that sediments can constitute a source of aluminosilicate by the formation of geopolymer gels and zeolite crystals as a secondary product. In addition, the Mercury Intrusion Porosimetry results showed that the substitution of fly ash by sediments could improve durability by reducing the pore size, which also explain the gain in strength with sediments incorporation.

Benzene Oxidation over Pt Loaded on Fly Ash Zeolite X

In the present study, zeolite X (FANaX) was synthesized from coal fly ash (FA) by a two-step high-temperature method. In order to follow the effect of different contaminants in the starting coal ash, zeolite X was also synthesized from pure chemicals according to a classical recipe (NaX). Iron was loaded on this reference zeolite with the amount which was contained in the coal FA. The final catalytic samples were obtained by wet impregnation of Pt nanoparticles on both types of zeolite crystals. The most active samples in the benzene oxidation were the platinum-modified ones and, among them, the Pt-impregnated FA zeolite (Pt FANaX). The comparison of the catalytic activity of Pt FANaX with the reference PtFe NaX zeolite showed a temperature difference of 10 °C in favor of Pt FANaX at 50% benzene conversion. From these results, it can be concluded that FA zeolites are a good, cheaper and environmentally friendly alternative to traditional zeolites, synthesized from pure chemicals, which can be applied in the preparation of catalysts for the purification of gaseous mixtures from harmful organic compounds.

Synthesis and Characterization of Zeolite NaY Dispersed on Bamboo Wood.

Zeolites in powder form have the potential to agglomerate, lowering access to active sites. Furthermore, a suspension of fine zeolite powder in liquid media is difficult to separate. Such drawbacks could be improved by dispersing zeolite crystals on support materials. This work demonstrates the dispersion of zeolite NaY crystals on bamboo wood by mixing the wood with zeolite gel before hydrothermal treatment. The syntheses were performed with acid-refluxed and non-refluxed wood. The phase of zeolites, particle distribution and morphology, zeolite content in the wood, and zeolite-wood interaction were investigated using X-ray diffraction, X-ray tomography, scanning electron microscopy, thermogravimetric analysis, nitrogen sorption analysis, and X-ray photoelectron spectroscopy. Higher zeolite content and better particle dispersion were obtained in the synthesis with the acid-refluxed wood. The composite of NaY on the acid-refluxed wood was demonstrated to be an effective adsorbent for Ni(II) ions in aqueous solutions, providing a higher adsorbed amount of Ni(II) per weight of NaY.

Heat diffusion optimization in high performance perovskite solar cells integrated with zeolite

Heat accumulation inside perovskite solar cells causes the decomposition of the perovskite layer and hole transport materials (HTMs) under working conditions, yielding a decrease in long-term stability. Here, we present a zeolite-assisted heat conduction strategy by introducing economic zeolite crystals (e.g., NaX, NaY, and ZSM-5) as a cooling filter to induce heat diffusion. The fitted thermal diffusion kinetic equation from real-time infrared thermal imaging technology reveals the zeolite skeleton assisted thermal conduction mechanism of internal lattice vibration. Additionally, the nearly twofold improved conductivity of the modified HTM film is benefited from Na+ hopping on the supercages of the zeolite, therefore, the best-performed device with a rapid heat diffusion and defect inhibition obtains a remarkable power conversion efficiency of 23.42%. Both of NaX modified sprio-OMeTAD and PTAA based devices exhibit excellent operational stability after heating 1000 h at 85 °C under N2 condition. This work demonstrates the potential application of economical porous zeolite materials in improving the thermal stability of PSCs.

Encapsulating Pt clusters during the crystallization of MWW zeolite

Directly encapsulation of Pt clusters within MWW zeolite was realized by precisely regulating the synthetic parameters, especially the alkalinity and the Pt content during the hydrothermal synthesis. Simple addition of Pt(NH3)4(NO3)2 to the routine recipe for MWW synthesis resulted in a mixture of MWW to FER zeolites, while appropriately lowering the alkalinity of the initial gel composition favored the formation of pure MWW phase and simultaneously the incorporation of Pt clusters into the micropores of MWW (Pt/Na/Al2O3/H2O/HMI/SiO2 = 0.0015/0.18/0.044/15/0.34/1). At high Pt loading (Pt/SiO2>0.00225), however, the crystallinity decreased considerably and the 3D MWW structure gradually evolved into disordered layers, while Pt particles located on the surface of the disordered layers and were prone to grow into larger ones. In short, addition of Pt(NH3)4(NO3)2 in the initial gel altered the crystallization process of MWW zeolite, and Pt-MWW could be obtained by appropriately lowering the alkalinity and accurately controlling the amount of platinum.