Zeolite Crystals Research Articles

A Study on the Performance of Self-Leveling Mortar Utilizing Tungsten Tailings as the Aggregate

A significant quantity of tailings is produced during the development of different metal mines in China. In particular, fine-grained tailings pose challenges to the sustainable development of the mining industry. This study examines the utilization of finely ground tungsten tailings as a replacement for natural aggregates in self-leveling mortar (SLM). The study examined the impact of the aggregate-cement ratio, cement mix ratio, and varying substitution levels of different grain sizes of tungsten tailings on the flow properties, mechanical properties, and dimensional change rate of SLM. Additionally, the role of tungsten tailings in SLM was analyzed using XRD, FTIR, and SEM methods. The findings demonstrated that the utilization of sulphoaluminate cement (SAC) had a notable impact on improving the initial strength of the SLM. Additionally, a high aggregate-cement ratio negatively affected the fluidity of the SLM. The doping of tungsten tailings improved the grading relationship of the SLM. Substituting tungsten tailings of 38–75 μm grain size for natural aggregates in the preparation of SLM did not have a negative impact on its performance. In fact, substituting 60% tungsten tailings had a positive effect on the 28-day mechanical properties of the SLM. The compressive and flexural strengths of the SLM after 28 days were 26.53 MPa and 9.06 MPa, respectively, which were enhanced by 18.81% and 26% compared to the control group (C0). According to the environmental leaching test, SLM can effectively fix the heavy metal ions in tungsten tailings, and the leaching concentration of heavy metals is significantly reduced after long-term curing. The doping of finely fragmented tungsten tailings accelerated the process of hydration, resulting in the creation of hydrocalcium zeolite crystals in the latter phases of hydration. Furthermore, an increase in tailings substitution resulted in the production of a greater amount of hydration products, specifically C-S-H gels.

Morphology and Microstructural Optimization of Zeolite Crystals Utilizing Polymer Growth Modifiers for Enhanced Catalytic Application

Rationally controlling the morphology and microstructure of the zeolite crystals could play a significant role in optimizing their physicochemical properties and catalytic performances for application in various zeolite-based heterogeneous catalysis processes. Among different controlling strategies, the utilization of zeolite growth modifiers (ZGMs), which are molecules capable of altering the anisotropic rates of crystal growth, is becoming a promising approach to modulate the morphology and microstructural characteristics of zeolite crystals. In this mini-review, we attempt to provide an organized overview of the recent progress in the usage of several easily available polymer-based growth modifiers in the synthesis of some commonly used microporous zeolites and to reveal their roles in controlling the morphology and various physicochemical properties of zeolite crystals during hydrothermal synthesis processes. This review is expected to provide some guidance for deeply understanding the modulation mechanisms of polymer-based zeolite growth modifiers and for appropriately utilizing such a modulation strategy to achieve precise control of the morphology and microstructure of zeolite crystals that display optimal performance in the target catalytic reactions.

Biochar-mediated synthesis of ZSM-5 zeolite for the methanol-to-olefins reaction

The present study reports the successful synthesis of ZSM-5 zeolites with high external specific surface area by using biochar as a hard template. The physicochemical properties of the biochars (originated from grass, tea and lignin) were studied using XRD, SEM EDX, FTIR and other characterization techniques. The results showed that biochars obtained from different plants, due to the differences in morphology, particle size, composition and surface functional groups, led to significant effects on the morphology and acidity of the zeolite crystals. These ZSM-5 zeolites exhibited a significantly improved performance in the Methano-To-Olefins (MTO) reaction. Indeed, the catalyst lifetime of parent ZSM-5 started to decline after 6 h, whilst ZSM-5 modified by Grass-C only suffered a loss in conversion after 10 h on stream (at 400 °C, WHSV=8 h−1).

Acceleration of Zeolite Crystallization: Current Status, Mechanisms, and Perspectives.

Zeolites are important classes of crystalline materials and possess well-defined channels and cages with molecular dimensions. They have been extensively employed as heterogeneous catalysts and gas adsorbents due to their relatively large specific surface areas, high pore volumes, compositional flexibility, definite acidity, and hydrothermal stability. The zeolite synthesis normally undergoes high-temperature hydrothermal treatments with a relatively long crystallization time, which exhibits low synthesis efficiency and high energy consumption. Various strategies, e.g., modulation of the synthesis gel compositions, employment of special silica/aluminum sources, addition of seeds, fluoride, hydroxyl (·OH) free radical initiators, and organic additives, regulation of the crystallization conditions, development of new approaches, etc., have been developed to overcome these obstacles. And, these achievements make prominent contributions to the topic of acceleration of the zeolite crystallization and promote the fundamental understanding of the zeolite formation mechanism. However, there is a lack of the comprehensive summary and analysis on them. Herein, we provide an overview of the recent achievements, highlight the significant progress in the past decades on the developments of novel and remarkable strategies to accelerate the crystallization of zeolites, and basically divide them into three main types, i.e., chemical methods, physical methods, and the derived new approaches. The principles/acceleration mechanisms, effectiveness, versatility, and degree of reality for the corresponding approaches are thoroughly discussed and summarized. Finally, the rational design of the prospective strategies for the fast synthesis of zeolites is commented on and envisioned. The information gathered here is expected to provide solid guidance for developing a more effective route to improve the zeolite crystallization and obtain the functional zeolite-based materials with more shortened durations and lowered cost and further promote their applications.

Tuning of the acid sites in the zeolite-alumina composite Ni catalysts and their impact on the palm oil hydrodeoxygenation reaction

The hydrodeoxygenation (HDO) of bio-oil is one of the potential approaches to produce green diesel. However, HDO catalyst requires the development of bifunctionality which translates to the simultaneous presence of acidic and metal sites for desired catalytic activity and selectivity. Zeolites and their composites are attractive candidates for the conversion of biomass to fuels. In the present work, a series of Ni-incorporated Al2O3-zeolite beta bifunctional composite catalysts with distinct Al2O3 (25–75 wt%) and Ni contents (5–15 wt%) were synthesized via a facile one-pot method directly from nickel acetate, nano-boehmite (γ-AlO(OH)) and the NH4+ form of beta zeolite (NH4+-BZ). The nano-boehmite particles, due to the positive charges on their surface, electrostatically attract negatively charged beta zeolite crystals, which leads to the assembly of a hierarchical pore structure upon calcination. Interestingly, the composite catalysts synthesized were quite homogeneous with uniform dispersion of Ni particles. All composite catalysts were thoroughly characterized using XRD, SEM-EDX, SEM-mapping, HR-TEM, H2-TPR, H2-TPD, NH3-TPD, XPS, 31P MAS NMR, 27Al MAS NMR and N2 sorption analysis. The synthesized composite catalysts with distinct Al2O3 contents showed diverse textural properties, distinct nature of acid sites and improved performance in hydrodeoxygenation of palm oil. Particularly, the Ni/BZ-Al50 (with BZ:Al2O3 = 50:50) composite catalyst significantly enhanced the conversion of palm oil (up to 90%) and yield of n-C15-C18 hydrocarbons (up to 69%) at moderate temperature of 375 °C as compared to 10Ni/BZ catalyst (Conv. = 75%, yield of n-C15-C18 = 52%). The higher catalytic performance realized with composite catalysts can be ascribed to its hierarchical pore structure, moderate acidity (chemisorption studies), tuned acid sites nature (solid state NMR) and homogeneous distribution of active Ni sites (H2 chemisorption) which helps to improve product selectivity by minimizing side reactions. The time-on-stream (TOS) experiments were carried out up to 20 h which clearly showed that composite catalysts are more stable suggesting the lower amount of coke deposition (TPO studies) and suppression of metal sintering.

Coupling Different Periodic Building Units for Intergrowth Zeolites.

Multiple-phase disordered zeolites, i.e., intergrowth zeolites, are important industrial catalysts, like single-phase ordered zeolites, but little is known about their rational synthesis and phase competition, mainly due to current poor understanding of the zeolite crystallization mechanism. Here, we theoretically demonstrated that sodalite and cancrinite cage layers, the periodic building units (PerBUs) of FAU/EMT and SBT/SBS structures, respectively, could be nondefectively connected to each other across double rings of 6 tetrahedral atoms when inverted and mirrored. We then synthesized an unprecedented family of FAU/SBT/SBS intergrowths with controllable FAU portions (named as the PST-34 family of intergrowth zeolites) using a multiple inorganic cation approach, providing clear experimental evidence for the layer-by-layer crystal growth mechanism of zeolites. This study shows that control of interactive cooperation extent between different inorganic structure-directing agents in the presence of an unselective organic structure-directing agent may enable repeated stacking of different but structurally related PerBUs in intergrowth zeolite synthesis.

Remarkably facilitated crystallization of MSE-type zeolite by using an efficient organic structure-directing agent

Abstract A very powerful organic structure-directing agent (OSDA) for the synthesis of MSE-type zeolite was designed and successfully prepared. When using conventional OSDA, more than 10 d of crystallization time was necessary, whereas the use of the new type of OSDA gave a highly crystalline MSE-type zeolite using only 32 h of crystallization time.

Tuning RHO Zeolite Crystallization Time and Precursors for Stable and Improved Methanol Conversion to Dimethyl Ether Compared to Conventional Catalysts

Tuning RHO Zeolite Crystallization Time and Precursors for Stable and Improved Methanol Conversion to Dimethyl Ether Compared to Conventional Catalysts

Current progress in the synthesis of zeolite crystals at low temperatures and their catalytic applications

Current progress in the synthesis of zeolite crystals at low temperatures and their catalytic applications

Can the combination of in situ differential impedance spectroscopy and 27Al NMR detect incongruent zeolite crystallization?

Crystallizing zeolites with isotropic properties is critical to the chemical industry but can be extremely challenging as small deviations in the synthesis conditions can have extreme effects on the final products. Easily implemented in-situ monitoring systems could make a real difference, but very few experimental methodologies cater to the specific needs of applications relying on harsh, hyper-alkaline conditions involving multiphasic systems such as Hydrated Silicate Ionic Liquids. Differential impedance spectroscopy (DIS) promises to enable such studies. It remains highly accurate despite possible electrode degradation or scaling. This study showcases how in-situ differential impedance measurements not only enable reliable detection of crystallization of even minimal amounts of zeolite product but also illustrates how the unique combination of in situ DIS and in situ, 27Al NMR provides insight into complex, incongruent zeolite crystallization processes.

Hydrothermal zeolitization: Towards a paradigm shift for producing stronger and more sustainable construction materials

The construction sector provides 14.8 million jobs in the European Union. However, it also accounts for 36% of greenhouse gas emissions and 35% of total waste. High-temperature ceramic manufacturing consumes substantial energy, yielding basic structural products. The alternatives presented, such as geopolymers, still do not replace traditional materials and are not as sustainable as believed. This innovative study combines hydrothermal zeolitization with kaolin-based ceramics fired at both conventional (900°C) and much lower temperatures (600°C), exploring varied conditions, including the use of microplastic wastes as pore-forming agents. Significant zeolite crystallization (15–74%) and mechanical strength increase (2–37-fold compared to untreated materials) is demonstrated in granular specimens, especially when adding microplastics. Geopolymerization appears as a secondary process during treatment. This innovative method offers energy-efficient lower temperatures, transforming fired specimens into zeolite-enriched, robust materials, adaptable to current technology. This study paves the way for further research, opening up a new field of study on hydrothermally manufactured Zeolitic Construction Materials.

Synthesis of zeolite X from fly ash in an impinging stream reactor and its mechanisms

The conventional hydrothermal method with magnetic stirring for the synthesis of zeolite X from fly ash has some problems, such as low micromixing efficiency, low mass transfer rate, and long synthesis time. In this study, impinging stream reactor is used to improve the mass transfer and micromixing and the mechanism how the nucleation and crystallization of zeolite is accelerated is explored. The results show that premixing in the impinging stream reactor can reduce the stirring and aging time from 4 h to 20 min because the hydroxyl free radicals resulting from the cavitation effect can accelerate the crystallization of zeolite. After collision, the viscosity of the system is increased from 0.22 Pa·s to 3 Pa·s, which can increase the residence time of cavitation bubbles in the gel solution and effectively inhibit the aggregation of crystal nuclei. As a result, the crystal nuclei in the system are well dispersed. The impinging stream reactor can also accelerate the mass transfer between liquid and solid phases and improve the micromixing of raw materials, so that the polymerization between Si and Al is increased and the nucleation of zeolite X is accelerated. The degree of order of zeolite X samples obtained by premixing in the impinging stream reactor is much higher than that obtained by magnetic stirring. The relative crystallinity after crystallization for 12 h (100 %) is higher than that obtained by magnetic stirring (94.93 %); and the n(Si/Al) is lower than that obtained by magnetic stirring. It is concluded that premixing in the impinging stream reactor can improve the polymerization of aluminosilicate and accelerate the nucleation and crystallization of zeolite X. Under the same conditions, zeolite X synthesized by premixing in the impinging stream reactor has better CO2 adsorption capacity than those synthesized by magnetic stirring and industrial synthesis.

Interzeolite Transformation from FAU-to-EDI Type of Zeolite.

This study reports for the first time the transformation of the pre-made FAU type of zeolite to the EDI type of zeolite. The concentration of the KOH solution controls this interzeolite transformation, which unusually occurs at both room temperature and under hydrothermal conditions. The transformation involves the amorphization and partial dissolution of the parent FAU phase, followed by the crystallization of EDI zeolite. At room temperature, the transformation (11-35 days) provides access to well-shaped nano-sized crystals and hollow hierarchical particles while the hydrothermal synthesis results in faster crystallization (6-27 h). These findings reveal an example of an interzeolite transformation to a potassium zeolite that lacks common composite building units with the parent zeolite phase. Finally, this work also demonstrates the first room-temperature synthesis of EDI zeolite from a gel precursor.

Mechanical Properties of Aeolian Sand Concrete Made from Alkali-Treated Aeolian Sand and Zeolite Powder.

The aim of this study is to promote the application of the excited zeolite powder (ZP)with aeolian sand powder (ASP) in the field of aeolian-sand concrete (ASC) production. This study utilises NaOH to treat composite cementitious materials containing aeolian sand and zeolite powders, which were used to replace 50% of the cement in aeolian-sand concrete (ASC). Production of alkali-inspired cement-based windswept concrete(AAZC).The mechanical properties of treated ASC considerably improved, especially when the NaOH dosage was 4% by mass. After curing this sample (denoted as AAZC-4) for 28 d, its compressive strength improved by 17.2%, and its split tensile increased by 16.3%. Potassium feldspar and montmorillonite in zeolite powder and SiO2 in the sand were decomposed by OH- and combined with other elements to generate various silicate gels and A-type potassium zeolite crystals inside the concrete. Microscopic examination showed that the gels and crystals intertwined to fill the pores, decreasing (increasing) the percentage of large (small) pores, thus optimising the pore structure. This substantially improved the mechanical properties of ASC. Freeze-thaw salt-intrusion tests showed that the extent of mass loss, degree of damage and loss of compressive strength of AAZC-4 were similar to those of ordinary concrete but were reduced by 36.8%, 19% and 52.1%, respectively, compared with those of ASC. Therefore, AAZC-4 has a sustainable working performance in chloride-ion permeable environments in cold and arid areas.

SSZ-39 zeolite-based Ru catalysts for selective hydrogenation of levulinic acid to γ-valerolactone: Influence of synthesis method and zeolite acidity

Novel SSZ-39 zeolite-based Ru catalysts were synthesized by impregnation on as-synthesized metallic and protonated SSZ-39 and a one-pot synthesis strategy. The study focused on the influence of the SSZ-39 zeolite characteristics and catalyst preparation method type toward levulinic acid (LA) selective conversion to γ-valerolactone (GVL) under relatively mild reaction conditions (150 °C, 4 h and 2 MPa of H2 with H2O and THF as solvents). Catalysts exhibited excellent catalytic efficiency (100 % LA conversion and 98.3 % GVL selectivity) and remarkable hydrothermal stability. The results revealed a correlation between excellent catalytic activity and the structural features of the synthesized catalysts, predominantly dependent on their appropriate surface acidity (strong Lewis acid sites) and enhanced electronic interaction between Ru species and SSZ-39. The one-pot method promoted the formation of mesopores besides the zeolitic micropores present in SSZ-39; therefore, the catalyst prepared by the one-pot method (Ru/SSZ-39) showed purer SSZ-39 crystallites and a slightly larger catalytic surface area of 439 m2/g than the original zeolite surface area of 424 m2/g. In addition, the original zeolite crystals retained their morphology as spherical aggregates and remained unaffected by Ru impregnation on the metallic or protonated support during the synthesis of Ru/Na-SSZ-39 and Ru/H-SSZ-39 catalysts, respectively. Notably, it has been demonstrated that SSZ-39 zeolite effectively acts as a support for the preparation of Ru catalyst in biomass conversion reactions. Moreover, the reused catalyst exhibited consistent activity over six cycles, maintaining its structural framework without significant loss in activity.

Synchrotron radiation SAXS study of the species structure evolution in the synthesis process of MCM-56 zeolite

In the process of synthesizing zeolite, the evolution behavior of aluminosilicate species is crucial for investigating the crystallization mechanism and controlling the structure of desired zeolite. Herein, the synthesis process of MCM-56 zeolite was investigated by using the synchrotron radiation Small Angle X-ray Scattering (SAXS) technique. Based on the distance distribution function curves and Porod analysis, the particle distribution of the sol in the synthesis system transformed from a monodisperse spherical particle to a polydisperse lamellar structure. The results indicated that the particle size distribution, fractal dimension, and the evolution behavior of particles in sol vary with the variation of crystallization time. Specifically, the initial step involves the depolymerization of particles forming small particles approximately 1 nm. Subsequently, these small particles polymerize to form structural units ranging in size from 1 to 5 nm, and finally grow to form MCM-56 zeolite crystal by consuming aluminosilicate species. The SAXS results in conjunction with the XRD, SEM, FT-IR, and N2 adsorption-desorption analysis elucidate the evolution of aluminosilicate species and the crystallization process of MCM-56 zeolite at three stages, including the induction/nucleation period (0–84 h), the crystal growth period (84–108 h) and the crystal stability period (108–168 h). By virtue of the understanding for the structural evolution of the sol species, this work is expected to highlight the controllable synthesis of MCM-56 zeolite.

Insights into the Synthesis of Spiral Beta Zeolite with Enhanced Catalytic Performance in VOC Abatement.

The rational synthesis of zeolites with designed morphology is a highly challenging task. In this study, we propose 1,5-bis(methylpiperidine)pentylammonium hydroxide (BMPPAOH) as an organic structure-directing agent (OSDA) based on theoretical calculations. The morphology of zeolite samples is characterized by XRD, SEM, TEM, N2 sorption isotherms, and UV Raman spectroscopy. This simple bis-quaternary ammonium salt favored the formation of spiral morphology in Beta zeolite spheres (S-Beta). The crystallization of zeolite in the presence of BMMPAOH is a two-stage process, where nanoparticles agglomerate into spheres in the early stages followed by the emergence of S-Beta crystals with spiral morphology. The synthesized Pt-S-Beta catalysts show higher catalytic activity in VOC abatement compared with other Pt-Beta samples.

The Current Understanding of Mechanistic Pathways in Zeolite Crystallization.

Zeolite catalysts and adsorbents have been an integral part of many commercial processes and are projected to play a significant role in emerging technologies to address the changing energy and environmental landscapes. The ability to rationally design zeolites with tailored properties relies on a fundamental understanding of crystallization pathways to strategically manipulate processes of nucleation and growth. The complexity of zeolite growth media engenders a diversity of crystallization mechanisms that can manifest at different synthesis stages. In this review, we discuss the current understanding of classical and nonclassical pathways associated with the formation of (alumino)silicate zeolites. We begin with a brief overview of zeolite history and seminal advancements, followed by a comprehensive discussion of different classes of zeolite precursors with respect to their methods of assembly and physicochemical properties. The following two sections provide detailed discussions of nucleation and growth pathways wherein we emphasize general trends and highlight specific observations for select zeolite framework types. We then close with conclusions and future outlook to summarize key hypotheses, current knowledge gaps, and potential opportunities to guide zeolite synthesis toward a more exact science.

Magnetic properties of bauxites and pre-fused hydrothermal product (zeolite)

Bauxites are deposits rich in aluminium and iron oxides/hydroxides, formed by the weathering of aluminosilicates rocks under warm and humid conditions. Due to their mineralogical composition, bauxites represent the primary source for aluminum and, subordinately, iron production. However, these sediments rich in Si and Al can be also used as raw material for the synthesis of zeolite with magnetic properties thanks to the high availability of Fe. In this work, three bauxite samples collected from Upper Cretaceous deposits of southern Italy were characterized for their magnetic parameters and used for the synthesis of zeolite with LTA topology (zeolite A). The results indicate differences in antiferromagnetic behaviour among the three raw materials. All the synthetic products, instead, display the same antiferromagnetic behaviour thus suggesting that the method used for the zeolite crystallization neutralizes the parameters responsible for raw bauxite magnetic differences.

MOR nanosheets with tunable c-axis thickness and their catalytic performance in DME carbonylation

Controlling the anisotropic growth of crystals to achieve desired morphology is a challenge in zeolite crystallization, which often requires the use of bulky organic surfactants or additional crystal growth modifiers. Here, MOR crystals with tunable c-axis thickness (12-MR channel direction) were readily synthesized with commercial tetraethylammonium hydroxide (TEAOH) as an organic-structure directing agent (OSDA). It is revealed that the relative growth rates of different crystal faces of MOR zeolite can be tuned by the crystallization temperature and TEAOH amount in the developed gel system. Higher temperature and lower TEAOH concentration can effectively retard the growth of MOR crystals along the c-axis, yielding MOR nanosheets with short c-axis thickness (ca. 40 nm). Investigation on the crystallization process demonstrates that the crystal growth follows a particle attachment mechanism. Theoretical calculation was further employed to elucidate the c-axis thickness variation of MOR zeolite under different conditions. A kilogram-scale synthesis of MOR nanosheets has been realized with a 10 L autoclave, achieving high solid yield of 92 %. The MOR nanosheets, offering short 12-MR diffusion path, exhibit substantially enhanced catalytic activity and stability in the dimethyl ether (DME) carbonylation to methyl acetate reaction.