Regulation of the Si/Al ratios and Al distributions of zeolites and their impact on properties

Thiourea-Driven Mild Synthesis of Photochromic Zeolite without H2 for Transient Information Storage

The synthesis of hackmanite has been a topic of research for nearly 100 years, but up to now, the reported synthesis methods inevitably use reducing H2 to introduce the F-center through high-temperature calcination to make it change color, which extremely hinders its energy efficient large-scale synthesis before commercial applications. Herein, we present a facile route to synthesize hackmanite by using organic sulfur source thiourea as a precursor in a mild and hydrogen-free process. The synthesized hackmanite exhibits UVC-specific response, fast response, high contrast coloring, and fast fading at room temperature. In view of its photochromic features, a proof-of-concept is presented for transient information storage and burning-after-reading protection; the written information can be automatically erased within 8 min at room temperature.

Advances and challenges in designing active site environments in zeolites for Brønsted acid catalysis

Zeolites contain proton active sites in diverse void environments that stabilize the reactive intermediates and transition states formed in converting hydrocarbons and oxygenates to chemicals and energy carriers. The catalytic diversity that exists among active sites in voids of varying sizes and shapes, even within a given zeolite topology, has motivated research efforts to position and quantify active sites within distinct voids (synthesis-structure) and to link active site environment to catalytic behavior (structure-reactivity). This Feature Article describes advances and challenges in controlling the position of framework Al centers and associated protons within distinct voids during zeolite synthesis or post-synthetic modification, in identifying and quantifying distinct active site environments using characterization techniques, and in determining the influence of active site environments on catalysis. During zeolite synthesis, organic structure directing agents (SDAs) influence Al substitution at distinct lattice positions via intermolecular interactions (e.g., electrostatics, hydrogen bonding) that depend on the size, structure, and charge distribution of organic SDAs and their mobility when confined within zeolitic voids. Complementary post-synthetic strategies to alter intrapore active site distributions include the selective removal of protons by differently-sized titrants or unreactive organic residues and the selective exchange of framework heteroatoms of different reactivities, but remain limited to certain zeolite frameworks. The ability to identify and quantify active sites within distinct intrapore environments depends on the resolution with which a given characterization technique can distinguish Al T-site positions or proton environments in a given zeolite framework. For proton sites in external unconfined environments, various (post-)synthetic strategies exist to control their amounts, with quantitative methods to distinguish them from internal sites that largely depend on using stoichiometric or catalytic probes that only interact with external sites. Protons in different environments influence reactivity by preferentially stabilizing larger transition states over smaller precursor states and influence selectivity by preferentially stabilizing or destabilizing competing transition states of varying sizes that share a common precursor state. We highlight opportunities to address challenges encountered in the design of active site environments in zeolites by closely integrating precise (post-)synthetic methods, validated characterization techniques, well-defined kinetic probes, and properly calibrated theoretical models. Further advances in understanding the molecular details that underlie synthesis-structure-reactivity relationships for active site environments in zeolite catalysis can accelerate the predictive design of tailored zeolites for desired catalytic transformations.

Zeolite Crystallization Inside Chemically Recyclable Ordered Nanoporous Co3O4 Scaffold: Precise Replication and Accelerated Crystallization

The synthesis of mesoporous zeolites has garnered attention with regard to improving their catalytic and adsorption performances. While the hard-templating method provides opportunities to design precisely controlled hierarchical micro- and mesoporous structures, synthesizing mesoporous zeolites without external precipitation requires significant work. This is mainly due to the absence of usable templates other than carbon with hydrophobic surfaces. Herein, it is demonstrated that the Co3O4 template is valuable in preparing mesoporous silicalite-1 and ZSM-5 with a precisely controlled porous structure through hydrothermal synthesis. Unlike conventional carbon templates, the Co3O4 template is relatively hydrophilic, effective in suppressing external precipitation, and is reusable by dissolving in an acidic solution. The crystallization process also differs from that of the carbon template, as the silicate precipitates on a 3D ordered nanoporous Co3O4 scaffold, followed by crystallization and crystal growth. Furthermore, it is unexpectedly observed that the zeolite crystallization is accelerated in the Co3O4 template. The synthetic approach utilizing nanoporous metal oxides opens new doors for the control of the hierarchical structure of zeolites, as well as for the design of metal oxide-zeolite nanocomposite catalysts, due to the potential extensibility of the combination of metal oxides and zeolites.

Developing a novel calcium silver zeolite for caries management

OBJECTIVE

To develop a novel calcium silver zeolite (Ca-Ag-Zeo) and assess its biocompatibility, physiochemical properties and antimicrobial effects.

METHODS

Ca-Ag-Zeo was synthesized using ion-exchange method with calcium chloride, silver nitrate and Zeolite X (Zeo). Silver zeolite X (Ag-Zeo) and Zeo were set as control. The chemical structure, morphology, crystal structure and elemental composition of Ca-Ag-Zeo was characterized by X-ray diffraction spectrum, scanning electron microscopy, transmission electron microscopy and energy dispersive spectroscopy, respectively. Its biocompatibility on the human gingival fibroblasts was assessed by cell counting kit-8 assay. Its physiochemical properties were determined by the released calcium and silver ion using Inductive Coupled Plasma Emission Spectrometry for up to 12 weeks. The antimicrobial properties on Streptococcus mutans, Lactobacillus acidophilus, Lactobacillus casei, and Candida albicans were assessed by minimum bactericidal concentration (MBC) or minimum fungicidal concentration (MFC) assay.

RESULTS

Ca-Ag-Zeo with a hexagonal cage structure was synthesized. As for biocompatibility, the half-maximal inhibitory concentration (± SD in mg/mL) of Ca-Ag-Zeo, Ag-Zeo and Zeo in human gingival fibroblasts were 0.52 ± 0.05, 0.15 ± 0.01 and 3.35 ± 0.58, respectively (Zeo > Ca-Ag-Zeo > Ag-Zeo; p < 0.05). As for physiochemical properties, the accumulated ion release (± SD in mg) of Ca-Ag-Zeo, Ag-Zeo and Zeo were 0.011 ± 0.003, 0 and 0 for calcium ion, respectively (Ca-Ag-Zeo > Ag-Zeo, Zeo; p < 0.001), and 0.213 ± 0.032, 0.209 ± 0.019 and 0 for silver ion, respectively (Ca-Ag-Zeo, Ag-Zeo > Zeo; p < 0.001). As for anti-microbial ability, the MBC/MFC (mg/mL) of Ca-Ag-Zeo, Ag-Zeo and Zeo were 32, 16 and > 256 against Streptococcus mutans; 32, 16, > 256 against Lactobacillus acidophilus; 16, 16, and 256 against Lactobacillus casei; 0.25, 0.125; and 2, 1, > 256 against Candida albicans, respectively.

CONCLUSION

A novel Ca-Ag-Zeo was developed. It presented better biocompatibility compared to Ag-Zeo. It released calcium and silver ions sustainably, and it could inhibit the growth of common cariogenic microorganisms.

Zeolite-based nanoparticles drug delivery systems in modern pharmaceutical research and environmental remediation

This review explores the potential of zeolite-based nanoparticles in modern pharmaceutical research, focusing on their role in advanced drug delivery systems. Zeolites, integrated into polymeric materials, offer precise drug delivery capabilities due to their unique structural features, biocompatibility, and controllable properties. Additionally, zeolites demonstrate environmental remediation potential through ion exchange processes. Synthetic zeolites, with modified release mechanisms, possess distinctive optical and electronic properties, expanding their applications in various fields. The study details zeolites' significance across industrial and scientific domains, outlining synthesis methods and size control techniques. The review emphasizes successful encapsulation and functionalization strategies for drug delivery, highlighting their role in enhancing drug stability and enabling targeted delivery. Advanced characterization techniques contribute to a comprehensive understanding of zeolite-based drug delivery systems. Addressing potential carcinogenicity, the review discusses environmental impact and risk assessment, stressing the importance of safety considerations in nanoparticle research. In biomedical applications, zeolites play vital roles in antidiarrheal, antitumor, antibacterial, and MRI contrast agents. Clinical trials featuring zeolite-based interventions underscore zeolite's potential in addressing diverse medical challenges. In conclusion, zeolite-based nanoparticles emerge as promising tools for targeted drug delivery, showcasing diverse applications and therapeutic potentials. Despite challenges, their unique advantages position zeolites at the forefront of innovative drug delivery systems.

Unveiling Ru(bpy)3 2+-Encapsulated Zeolite Y as Photocatalyst: Harnessing Photocatalytic Singlet Oxygen Generation for Mustard Gas Simulant Detoxification

This study explores the encapsulation of Ru(bpy)3 2+ within Zeolite Y (ZY) to improve photocatalytic singlet oxygen generation for the degradation of a mustard gas simulant, namely 2-chloroethyl ethyl sulfide (CEES). Mustard gas simulants are known to disrupt several biological processes; thus, their effective degradation is essential. Zeolite Y, with its hierarchical structure and adjustable Si/Al ratios, is an ideal host for Ru(bpy)3 2+, significantly improving its photocatalytic efficiency and stability. It is demonstrated through XRD and spectroscopic analyses that encapsulated Ru(bpy)3 2+ maintains its structural and photophysical properties, which are essential for generating singlet oxygen. Ru(bpy)3(1.0) loaded ZY(15) (where 1.0 and 15 represent the encapsulated amount of Ru(bpy)3 2+ and Si/Al ratio, respectively) outperforms other investigated photocatalytic systems in the oxidation of CEES, demonstrating high conversion rates and selectivity toward nontoxic sulfoxide products. Immobilization of Ru(bpy)3 2+-encapsulated zeolite Y onto cotton fabric results in effective degradation of CEES. The experimental results, validated by theoretical calculations, indicate an improved oxygen affinity and accessibility in zeolites with higher Si/Al ratios. This study advances the design of photocatalytic materials for environmental and defense applications, providing sustainable solutions for hazardous chemical degradation.

Ecofriendly synthesized Zeolite 4A for the treatment of a multi-cationic contaminant-based effluent: Central composite design (CCD) statistical approach

One of the key aspects of futureproofing the sustainability of life on earth lies in the protection of the hydrosphere, particularly from soluble heavy metal ion pollutants. In the current study, the central composite design and optimization of the ion-exchange process have been carried out for the simultaneous removal of selected cations; Cd2+, Cu2+, and Zn2+ cations using synthesized zeolite 4A. X-ray diffraction analysis confirmed the formation of zeolite 4A. The Brunauer-Emmett-Teller (BET) surface area of the synthesized zeolite was 32 m2/g. Results mainly indicate that there is a strong relationship between the experimental data and central composite design-based models of ion removal efficiency with R2 > 0.9 and the lack of fit less than 0.1 %. All the selected ion exchange parameters (time, dosage, pH, and temperature) were found to be statistically significant, with a p-value less than 0.05. For the complete simultaneous removal of selected cations, the optimal zeolite dosage, pH, and contact time are 1.2 g/100 cm3, 6, and 3 h. The optimal temperature ranges from 25 to 27 °C. The initial concentration of each selected cation is 450 mg/L. The ion exchange is in good agreement with the Freundlich and Langmuir isotherm models. Based on the Langmuir isotherm model, the maximum Cd2+, Cu2+, and Zn2+ uptake capacity values of zeolite are 103, 99.89, and 82.08 mg/g, respectively. In this study, it has been mainly inferred that CCD can be considered a useful tool for the modeling and optimization of zeolite ion exchange applications.

Nanomaterials-modified reverse osmosis membranes: a comprehensive review

Because of its great efficiency and widespread application, reverse osmosis (RO) is a popular tool for water desalination and purification. However, traditional RO membranes have a short lifespan due to membrane fouling, deterioration, decreased salt rejection rate, and the low water flux with aging. As a result, membrane modification has received a lot of attention recently, with nanomaterials being extensively researched to improve membrane efficacy and lifespan. Herein, we present an in-depth analysis of recent advances of RO membranes modification utilizing nanomaterials. An overview of the various nanomaterials used for membrane modification, including metal oxides, zeolites, and carbon nanomaterials, is provided. The synthesis techniques and methods of integrating these nanomaterials into RO membranes are also discussed. The impacts of nanomaterial change on the performance of RO membranes are addressed. The underlying mechanisms responsible for RO membrane enhancements by nanomaterials, such as improved surface hydrophilicity, reduced membrane fouling via surface repulsion and anti-adhesion properties, and enhanced structural stability, are discussed. Furthermore, the review provides a critical analysis of the challenges and limitations associated with the use of nanomaterials to modify RO membranes. Overall, this review provides valuable insights into the modification of RO membranes with nanomaterials, providing a full grasp of the benefits, challenges, and future prospects of this challenging topic.

Effective C4 Separation by Zeolite Metal-Organic Framework Composite Membranes

Inorganic zeolites have excellent molecular sieving properties, but they are difficult to process into macroscopic structures. In this work, we use metal-organic framework (MOF) glass as substrates to engineer the interface with inorganic zeolites, and then assemble the discrete crystalline zeolite powders into monolithic structures. The zeolites are well dispersed and stabilized within the MOF glass matrix, and the monolith has satisfactory mechanical stabilities for membrane applications. We demonstrate the effective separation performance of the membrane for 1,3-butadiene (C4H6) from other C4 hydrocarbons, which is a crucial and challenging separation in the chemical industry. The membrane achieves a high permeance of C4H6 (693.00±21.83 GPU) and a high selectivity over n-butene, n-butane, isobutene, and isobutane (9.72, 9.94, 10.31, and 11.94, respectively). This strategy opens up new possibilities for developing advanced membrane materials for difficult hydrocarbon separations.

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.

Cu-OFF/ERI Zeolite: Intergrowth Structure Synergistically Boosting Selective Catalytic Reduction of NOx with NH3

Cu-SSZ-13 has been commercialized for selective catalytic reduction with ammonia (NH3-SCR) to remove NOx from diesel exhaust. As its synthesis usually requires toxic and costly organic templates, the discovery of alternative Cu-based zeolite catalysts with organotemplate-free synthesis and comparable or even superior NH3-SCR activity to that of Cu-SSZ-13 is of great academic and industrial significance. Herein, we demonstrated that Cu-T with an intergrowth structure of offretite (OFF) and erionite (ERI) synthesized by an organotemplate-free method showed better catalytic performance than Cu-ERI and Cu-OFF as well as Cu-SSZ-13. Structure characterizations and density functional theory calculations indicated that the intergrowth structure promoted more isolated Cu2+ located at the 6MR of the intergrowth interface, resulting in a better hydrothermal stability of Cu-T than Cu-ERI and Cu-OFF. Strikingly, the low-temperature activity of Cu-T significantly increased after hydrothermal aging, while that of Cu-ERI and Cu-OFF substantially decreased. Based on in situ diffuse reflectance infrared Fourier transform spectra analysis and density functional theory calculations, the reason can be attributed to the fact that NH4NO3 formed on the CuxOy species within ERI polymorph of Cu-T underwent a fast SCR reaction pathway with the assistance of Brønsted acid sites at the intergrowth interfaces under standard SCR reaction conditions. Significantly, Cu-T exhibited a wider temperature window at a catalytic activity of over 90% than Cu-SSZ-13 (175-550 vs 175-500 °C for fresh and 225-500 vs 250-400 °C for hydrothermal treatment). This work provides a new direction for the design of high-performance NH3-SCR catalysts in terms of the interplay of the intergrowth structure of zeolites.

Site-directed cation ordering in chabazite-type AlxGa1-xPO4-34 frameworks revealed by NMR crystallography

We report the first synthesis of the mixed-metal chabazite-type AlxGa1-xPO4-34(mim) solid solution, containing 1-methylimidazolium, mim, as structure directing agent (SDA), from the parent mixed-metal oxide solid solution, γ-(AlxGa1-x)2O3. This hitherto unreported family of materials exhibits complex disorder, arising from the possible distributions of cations over available sites, the orientation of the SDA and the presence of variable amounts of water, which provides a prototype for understanding structural subtleties in nanoporous materials. In the as-made forms of the phosphate frameworks, there are three crystallographically distinct metal sites: two tetrahedral MO4 and one octahedral MO4F2 (M = Al, Ga). A combination of solid-state NMR spectroscopy and periodic DFT calculations reveals that the octahedral site is preferentially occupied by Al and the tetrahedral sites by Ga, leading to a non-random distribution of cations within the framework. Upon calcination to the AlxGa1-xPO4-34 framework, all metal sites are tetrahedral and crystallographically equivalent in the average R3̄ symmetry. The cation distribution was explored by 31P solid-state NMR spectroscopy, and it is shown that the non-random distribution demonstrated to exist in the as-made materials would be expected to give remarkably similar patterns of peak intensities to a random distribution owing to the change in average symmetry in the calcined materials.

Formation of paired Ga sites in CHA-type zeolite frameworks via a transcription-induced method

Paired Ga sites represented by the Ga-O-Si-O-Ga sequence were firstly formed intentionally in CHA-type zeolite frameworks via the transcription of pre-formed paired Ga species in a Ga-rich amorphous silica-gallia under seed-assisted hydrothermal conditions. Such paired Ga sites behaved as ion-exchange sites for capturing divalent cation, Co2+.

Toward three-dimensionally ordered nanoporous graphene materials: template synthesis, structure, and applications

Precise template synthesis will realize three-dimensionally ordered nanoporous graphenes (NPGs) with a spatially controlled seamless graphene structure and fewer edges. These structural features result in superelastic nature, high electrochemical stability, high electrical conductivity, and fast diffusion of gases and ions at the same time. Such innovative 3D graphene materials are conducive to solving energy-related issues for a better future. To further improve the attractive properties of NPGs, we review the template synthesis and its mechanism by chemical vapor deposition of hydrocarbons, analysis of the nanoporous graphene structure, and applications in electrochemical and mechanical devices.

Catalytic upcycling of post-consumer multilayered plastic packaging wastes for the selective production of monoaromatic hydrocarbons

In order to obtain extended storage life of food-grade materials and better barrier properties against environmental factors, a multilayer plastic packaging (MLP) is often used. The multilayer packaging plastics are labelled as "other" (SPI#7) category, and are manufactured with a combination of barrier plastics, rigid plastics and printing surface. Owing to their complex composition and difficulty in separating the layers of MLP, its mechanical recycling is challenging. In this study, MLP wastes (MLPWs) were collected from zero-waste garbage collection center of IIT Madras, India, and thoroughly characterized to determine their composition and plastic types. MLPWs were characterized using various physico-chemical methods such as thermogravimetric/differential scanning calorimetric analysis, Fourier transform infrared spectroscopy, bomb calorimetry, and proximate and ultimate analyses. The MLPWs were mainly made up of polyethylene (PE) and polyethylene terephthalate (PET). Further, the non-catalytic and zeolite-catalyzed fast pyrolysis of these MLPWs were studied using analytical pyrolysis coupled with gas chromatograph/mass spectrometer (Py-GC/MS). The non-catalytic fast pyrolysis of MLPWs primarily produced a mixture of aliphatic and alicyclic hydrocarbons, while zeolite catalyzed fast pyrolysis resulted in the formation of mono-aromatic hydrocarbons (MAHs). The activity of HZSM-5, zeolite Y (HY) and zeolite beta (Hβ) catalysts were evaluated, and the salient products were quantified. The yields of MAHs like benzene, toluene, ethylbenzene and xylene using the zeolites followed the trend: HZSM-5 (14.9 wt%) > HY (8.1 wt%) > Hβ (7.8 wt%), at 650 °C. The use of HZSM-5 resulted in highest yield of MAHs, viz. 16.1 wt%, at the optimum temperature of 550 °C and MLPW-to-catalyst ratio of 1:15 (w/w). The superior activity of HZSM-5 is due to its nominal acidity and larger pore size of 4.24 nm, as compared to HY and Hβ. The MAHs yield from three other types of MLPWs varied in the range of 9-16 wt%. The present study demonstrates a promising pathway for the catalytic upcycling of highly heterogeneous MLPWs in the context of circular economy.

Synthetic Control of the Defect Structure and Hierarchical Extra-Large-/Small-Pore Microporosity in Aluminosilicate Zeolite SWY

The SWY-type aluminosilicate zeolite, STA-30, has been synthesized via different routes to understand its defect chemistry and solid acidity. The synthetic parameters varied were the gel aging, the Al source, and the organic structure directing agent. All syntheses give crystalline materials with similar Si/Al ratios (6-7) that are stable in the activated K,H-form and closely similar by powder X-ray diffraction. However, they exhibit major differences in the crystal morphology and in their intracrystalline porosity and silanol concentrations. The diDABCO-C82+ (1,1'-(octane-1,8-diyl)bis(1,4-diazabicyclo[2.2.2]octan)-1-ium)-templated STA-30 samples (but not those templated by bisquinuclidinium octane, diQuin-C82+) possess hierarchical microporosity, consisting of noncrystallographic extra-large micropores (13 Å) that connect with the characteristic swy and gme cages of the SWY structure. This results in pore volumes up to 30% greater than those measured in activated diQuin-C8_STA-30 as well as higher concentrations of silanols and fewer Brønsted acid sites (BASs). The hierarchical porosity is demonstrated by isopentane adsorption and the FTIR of adsorbed pyridine, which shows that up to 77% of the BASs are accessible (remarkable for a zeolite that has a small-pore crystal structure). A structural model of single can/d6r column vacancies is proposed for the extra-large micropores, which is revealed unambiguously by high-resolution scanning transmission electron microscopy. STA-30 can therefore be prepared as a hierarchically porous zeolite via direct synthesis. The additional noncrystallographic porosity and, subsequently, the amount of SiOHs in the zeolites can be enhanced or strongly reduced by the choice of crystallization conditions.