Silica Zeolite Research Articles

Cement Composites Biodeterioration by Acidithiobacillus Thiooxidans

The focus of this work was to study the biodeterioration of special cement composites by sulfur-oxidizing bacteria Acidithiobacillus thiooxidans. The laboratory prepared cement composites that contained the selected additives - silica fume (a secondary raw material) and zeolite (a natural raw material), as partial replacements for the binder. The tests were carried out in laboratory conditions. During the experiments, changes to the pH values and the Ca and Si concentrations were observed in the liquid phases. Based on the achieved results, it can be concluded that concrete composites with partial replacement of the binder by silica fume and zeolite have a higher resistance to biogenic sulfuric acid.

Synthesis and Structure Determination by 3D Electron Diffraction of the Extra‐large Pore Zeolite ITQ‐70

In this study, we present the synthesis, characterization, and structural analysis of a novel zeolite, ITQ‐70, using 3D electron diffraction tomography. This unique material was synthesized under alkaline conditions, employing tetrakis(diethylamino)phosphonium as organic structure‐directing agent, leading to the formation of a pure silica zeolite. ITQ‐70 is distinguished by its extra‐large pore apertures, which extend along all three axes and intersect one to each other. A notable feature of this zeolite is the presence of structurally ordered defects in very high concentration (38% of the silicon atoms). As a result, ITQ‐70 exhibits the lowest framework density (10.0 T/1000Å3) ever reported for any zeolite except RWY (7.6 T/1000Å3), which contains sulfur instead of oxygen connecting T‐atoms.

Synthesis and Structure Determination by 3D Electron Diffraction of the Extra-large Pore Zeolite ITQ-70.

In this study, we present the synthesis, characterization, and structural analysis of a novel zeolite, ITQ-70, using 3D electron diffraction tomography. This unique material was synthesized under alkaline conditions, employing tetrakis(diethylamino)phosphonium as organic structure-directing agent, leading to the formation of a pure silica zeolite. ITQ-70 is distinguished by its extra-large pore apertures, which extend along all three axes and intersect one to each other. A notable feature of this zeolite is the presence of structurally ordered defects in very high concentration (38% of the silicon atoms). As a result, ITQ-70 exhibits the lowest framework density (10.0 T/1000Å3) ever reported for any zeolite except RWY (7.6 T/1000Å3), which contains sulfur instead of oxygen connecting T-atoms.

Stable Silica-Based Zeolites with Three-Dimensional Systems of Extra-Large Pores.

Zeolites are microporous crystalline materials that find a very wide range of applications, which, however, are limited by the size and dimensionality of their pores. Stable silica zeolites with a three-dimensional (3D) system of extra-large pores (ELP, i.e., pores with minimum windows along the diffusion path consisting of more than 12 SiO4/2 tetrahedra, 12R) are in demand for processing larger molecules than zeolites can currently handle. However, they have challenged worldwide synthetic capabilities for more than eight decades. In this review we first present a brief history of the discovery of ELP zeolites. Next, we show that earlier successes of zeolites with 3D ELP were not actually zeolites, but rather interrupted structures with, in addition, a composition that severely detracted from their stability. Finally, we present three new fully connected stable silica-based 3D ELP zeolites ZEO-1, ZEO-3 and ZEO-5, discuss their preparation methods and stability as well as the clear advantage of their increased porosity in catalysis and adsorption processes involving large molecules. We will discuss peculiar characteristics of their preparation and present two new reaction types giving rise to zeolites (1D-to-3D topotactic condensation and interchain expansion), highlighting how new synthesis methods can provide materials that would otherwise be unfeasible.

Stable Silica‐Based Zeolites with Three‐Dimensional Systems of Extra‐Large Pores

AbstractZeolites are microporous crystalline materials that find a very wide range of applications, which, however, are limited by the size and dimensionality of their pores. Stable silica zeolites with a three‐dimensional (3D) system of extra‐large pores (ELP, i.e., pores with minimum windows along the diffusion path consisting of more than 12 SiO4/2 tetrahedra, 12R) are in demand for processing larger molecules than zeolites can currently handle. However, they have challenged worldwide synthetic capabilities for more than eight decades. In this review we first present a brief history of the discovery of ELP zeolites. Next, we show that earlier successes of zeolites with 3D ELP were not actually zeolites, but rather interrupted structures with, in addition, a composition that severely detracted from their stability. Finally, we present three new fully connected stable silica‐based 3D ELP zeolites ZEO‐1, ZEO‐3 and ZEO‐5, discuss their preparation methods and stability as well as the clear advantage of their increased porosity in catalysis and adsorption processes involving large molecules. We will discuss peculiar characteristics of their preparation and present two new reaction types giving rise to zeolites (1D‐to‐3D topotactic condensation and interchain expansion), highlighting how new synthesis methods can provide materials that would otherwise be unfeasible.

Strength of London Dispersion Forces in Organic Structure Directing Agent-Zeolite Assemblies.

Herein, we study the London dispersion forces between organic structure directing agents (OSDAs)-here tetraalkyl-ammonium or -phosphonium molecules-and silica zeolite frameworks (FWs). We demonstrate that the interaction energy for these dispersion forces is correlated to the number of H atoms in OSDAs, irrespective of the structures of OSDAs or FWs, and of variations in charges and thermal motions. All calculations considered-DFT-D3 and BOMD undertaken by us, and molecular mechanics from an accessible database-led to the same trend. The mean energy of these dispersion forces is ca. -2 kcal.mol-1 per H for efficient H-O contacts.

Effects of silicate stabilizers on cadmium reduction and the quality of rice grains in acidic paddy soil

Silicate has been proven to be highly-effective at immobilizing soil heavy metals, but the effects of silicate stabilizers on rice grain cadmium (Cd) reduction and rice quality under field conditions are not clear. In this study, a field experiment was conducted over three consecutive years was conducted to examine the Cd reduction in rice grains and to reveal the potential effects of silicate stabilizers on rice grain nutrients, by setting different amounts of bentonite (B), silica‒calcium fertilizer (SC) and zeolite powder (ZP). The results revealed that the application of the B, SC and ZP significantly decreased the soil CaCl2‒Cd concentration (> 39%) and significantly reduced the grain Cd concentration in both early rice (> 70%) and late rice (> 18%) under field conditions; the silicate stabilizers reduced the soil available iron (Fe) but did not limit rice grain Fe nutrition. Additionally, the three silicates promoted rice yield and improved the rice grain Ca and Mg contents; and the application of B increased the amylose concentration of the late rice grains. In conclusion, high amounts of silicate stabilizers did not adversely influence the soil conventional nutrient indices, rice minerals or rice taste, but changes in rice selenium content need attention. Overall, in comparison with lime, silicate stabilizers can improve not only the safety of rice but also the nutritional and taste qualities of rice and are more eco-friendly for long-term use in soil.

Unveiling the water sorption kinetics, thermodynamics, and air dehumidification performance of sustainably synthesized metal–organic frameworks

This study focuses on the feasibility of sustainably synthesized MIL-100(Fe) as an alternative to conventional desiccants, such as silica gel and zeolites, which have low adsorption capacity and poor thermal characteristics for air dehumidification. MIL-100(Fe), a metal–organic framework, shows promise due to its excellent adsorption and customizable thermal properties. However, its synthesis typically involves toxic materials and high temperature/pressure conditions, which restrict its bulk production for large-scale applications. This study aims to explore sustainable synthesis methods for MIL-100(Fe) using non-toxic materials and milder reaction conditions and investigate its material characteristics and dehumidification performance. Experimental analyses are carried out to measure adsorption isotherms, kinetics, sorption heat, activation energy, and cyclic stability, and numerical simulations are conducted in COMSOL Multiphysics platform to estimate dehumidification performance. The test results demonstrate that MIL-100(Fe) has a higher water adsorption capacity compared to silica gel, with a unique stepwise adsorption into its two types of mesoporous cages. Increased temperatures cause the adsorption and desorption isotherms to shift towards higher relative humidity levels. The heat of water adsorption and desorption ranges between 2630–2906 kJ/kg and 2763–2848 kJ/kg, respectively. MIL-100(Fe) demonstrates high stability and no significant loss in water uptake capacity. The simulated performance indicate that MIL–100(Fe) has superior reaction rates and is 82 % more effective than the silica gel rotary wheel dehumidifiers.

Experimental evaluation of a hydrogel composite desiccant wheel for dehumidification with high water uptake and low regeneration temperature

Rotary dehumidification, a key method for continuous solid adsorption dehumidification, is extensively utilised in various industrial and practical applications. The performance of the adsorbent within the desiccant wheel is a major determinant of its energy efficiency in practical applications. While studies on Metal Organic Frameworks (MOFs) and polymer desiccant wheels exist, the majority of rotary dehumidification systems continue to employ traditional adsorbents like silica gel and zeolite, which are plagued by low dehumidification capacity and high energy consumption issues. Super hygroscopic hydrogel, enhanced with a hygroscopic agent, has gained prominence in air treatment because of its outstanding hygroscopicity and low regeneration temperature. With the goal of integrating advanced materials into practical engineering applications and mitigating the heat and mass transfer loss and adsorption/desorption performance degradation due to volume expansion and viscous agglomeration post-water uptake in large-scale super hygroscopic hydrogel applications, this study successfully developed a hydrogel wheel, measuring 350 mm in diameter and 200 mm in thickness, using a honeycomb-shaped glass fibre paper substrate. Experimental findings demonstrate that the hydrogel desiccant wheel outperforms both the MOF and commercial low-temperature silica gel wheels under all tested environmental conditions. This advantage is particularly noticeable at 15 °C and 30 % Relative Humidity (RH), where the moisture content differential (Δd) in the hydrogel system is 2.96 g/kg, 1.89 times greater than the MOF system’s 1.56 g/kg and 5.19 times higher than the silica gel’s 0.57 g/kg. Furthermore, the controlled variable method was used to assess the impact of each operational parameter on the hydrogel wheel’s performance in both low and high moisture conditions, offering design insights for engineering applications.

Study on the catalytic performance of Fe in-situ modified small crystallite Silicalite-1 zeolite in Chichibabin condensation reaction

Pyridine and its derivatives, collectively referred to as pyridine bases, are widely used in industries such as pesticides and pharmaceuticals, serving as crucial intermediates in the chemical industry. In recent years, with the development of the pesticide and pharmaceutical industries, the demand for pyridine bases has rapidly increased. The Chichibabin condensation reaction is the most commonly route for industrial production of pyridine bases. Currently, the most used ZSM-5 zeolite catalyst is limited by the instability of its silicon-aluminum framework structure, resulting in a short active reaction cycle (5 h). To address this limitation, this study selected the thermally stable and hydrothermally stable Silicalite-1 zeolite. Polyvinylpyrrolidone (PVP) was employed as a colloidal dispersant and Fe was introduced into the MFI framework through in-situ modification during the hydrothermal synthesis of zeolite. The influence of PVP dosage, template agent dosage, and other crystallization conditions on the crystallinity, pore structure, and acidity of Silicalite-1 zeolite products was investigated using XRD, SEM, TG, and N2 adsorption-desorption measurement. The acidity of Fe-modified Silicalite-1 zeolites was characterized using NH3-TPD, Py-FTIR, FT-IR, and XPS. These results indicated that the introduction of seed crystals effectively reduced the particle size of the zeolite to about 200 nm. Fe-modified Silicalite-1 displayed a disk-like morphology with excellent crystal dispersion. The highest relative crystallinity of the zeolite reached 103% with 15% seed crystal input and 3.75% PVP addition. The Fe-modified Silicalite-1 possessed a significantly enhanced abundance of both Lewis (L) and Brønsted (B) acid sites, resulting in an increase in the initial activity from 66% to 85% for the pyridine bases synthesis through the Chichibabin condensation. Compared to ZSM-5, Fe-modified Silicalite-1 exhibited superior catalytic stability, maintaining the total carbon conversion and pyridine bases yield above 66% and 40%, respectively, over a 15 h reaction period. Furthermore, the strategy proposed in this study, employing polyvinylpyrrolidone as a colloidal stabilizer to modify Silicalite-1 zeolite, could significantly broadened the application prospects of weakly acidic pure silica zeolites in the field of acid catalysis. This approach has demonstrated significant scientific value and industrial potential.

Competitive Adsorption of 4-Hydroxybenzaldehyde and Toluene onto High Silica Zeolites

To evaluate the ability of zeolites to remove natural low molecular organic substances, p-hydroxybenzaldehyde (p-HBA), a phenolic compound derived from lignin, was chosen as a representative of naturally occurring dissolved organic substances. Two distinct high-silica zeolite materials, namely ZSM-5 and Y, were utilised for the study, and the adsorption process was investigated under a wide range of conditions. It has been observed that p-HBA is adsorbed by both zeolites, and the pH significantly impacts the adsorption of p-HBA, particularly within the low concentration range, while exerting minimal influence on the saturation capacity. For both zeolites, various isotherm models were assessed to accurately describe the adsorption data obtained from aqueous solutions of p-HBA. In addition, to comprehend the selectivity of the adsorbents towards natural organic substances and xenobiotics, the competitive adsorption of mixtures of p-HBA and toluene (TOL) was investigated. The zeolite’s saturation capacity for p-HBA diminishes with increasing contaminant concentration. Conversely, the adsorption of toluene remains minimally affected by p-HBA, and it has been demonstrated that toluene can displace adsorbed p-HBA from the zeolites’ sites. This finding has been confirmed by diffractometric study that indicates that TOL and p-HBA occupy “the same” adsorption sites. Furthermore, Rietveld refinements reveal the formation of p-HBA complexes interacting with the framework and stabilising the guest structures within the zeolite porosity. The results obtained are important for the selection of proper adsorbent for the removal of hydrocarbons in environmental application (natural waters).

Breaking the stability limit of zeolite-based alkane dehydrogenation catalysts

Metal catalysts confined inside zeolite micropores or crystals are widely applied in the chemical industry because of their distinctive catalytic functions, especially with regard to the conversion of small and stable alkane molecules [1,2]. Silicalite-1 (S-1), a pure silica zeolite with an MFI topology, has a robust framework capable of withstanding harsh thermal and chemical conditions. Recently, S-1-based catalysts have demonstrated significant advantages in the construction and stabilization of various active metal centers and have attracted considerable attention for their potential in alkane dehydrogenation reactions. The absence of acidic sites prevents various acid-catalyzed side reactions, rendering S-1 a promising candidate for high-temperature reactions such as non-oxidative propane dehydrogenation (PDH). Sun et al. revealed that encapsulating subnanometer bimetallic PtZn clusters in S-1 resulted in a high propylene selectivity of 99%, with no catalyst deactivation even after 200h in the PDH reaction [3]. However, sustaining high stability at high conversion rates is still challenging for PDH reactions, and commercial processes are still based on Pt- and Cr-based catalysts, which need frequent regeneration.

Structural stability and adsorption behaviour of CO2-loaded pure silica CHA and ITW zeolites upon compression

The present study investigates the structural stability and adsorption behavior of ultrahigh CO2-loaded pure-silica zeolites chabazite (CHA) and ITQ-12 (ITW) under high pressure conditions. To analyze these properties, we have utilized in situ synchrotron-based X-ray powder diffraction techniques. Lattice indexation provides information of the filling process and, through Rietveld refinements and Fourier recycling methods, we have been able to (i) determine the location and amount of guest carbon dioxide molecules within the cavities of pure-SiO2 CHA zeolite and (ii) tentatively determine that within the channels of the porous pure-SiO2 ITW framework. The filling of the zeolite pores with CO2 molecules was found to have a positive impact on the structural stability of both CHA and ITW under compression, which do not undergo pressure-induced amorphization up to 12.2 GPa and 15.9 GPa, respectively. Interestingly, low compressibility takes place in CHA zeolite below 4 GPa during CO2 loading and a second-order phase transition occurs in CO2-filled ITW zeolite at 2.1 GPa. These results highlight the influence of CO2 adsorption on the compressibility behavior of these zeolites. Overall, our study provides detailed insights into the structural behavior of CO2-loaded CHA and ITW under high pressure and allows comparison with other pure silica zeolites described in the literature.

The role of defects in high-silica zeolite hydrolysis and framework healing

The stability of high silica zeolites under standard laboratory and mild steaming conditions is understood to be due to the lack of hydrophilic Al-O-Si moieties, which can be targeted by water via hydrolysis reactions with relatively low barriers. However, in hydrophobic high-silica and siliceous frameworks, the specific interactions between water and the internal framework sites are incompletely understood. In particular, the behaviour of common internal defects, including partial hydrolysis species and silanol nests are not established, despite their expected role in accelerating the decomposition of the framework. In this work, we utilise machine learning potentials combined with density functional calculations to rigorously sample the hydrolysis processes in siliceous zeolites with topologies CHA and MFI under low water conditions and quantify the effect of defects. Internal silanol defect sites are found to accelerate zeolite decomposition primarily by bypassing the initial, high-barrier hydrolysis step. Subsequent steps proceed with lower reaction barriers. However, all reaction steps are found to be highly activated, and unlikely to occur rapidly at room temperature under conditions of low internal water concentration. Exchange-healing routes, which reverse hydrolysis while incorporating oxygen from water molecules are found to be competitive along the entire hydrolysis pathway in CHA, providing an additional source of stabilization against hydrolytic decomposition.

Heat and mass transfer analysis of desiccant-coated heat exchanger with desiccants and metal-organic framework for bus air conditioning applications

Desiccant-coated heat exchangers (DCHEs) in air conditioning offer advantages like independent temperature and humidity control, reduced energy consumption, and high performance. However, the availability of desiccants with high water adsorption and low thermal energy requirements is limited. The present study proposes that aluminum fumarate (Al-Fum) MOF-coated heat exchangers will outperform conventional desiccants, especially at low regeneration temperatures (around 50°C). In order to evaluate this hypothesis, the performance of DCHEs was evaluated with a validated analytical model for different solid desiccant coatings: silica gel, zeolite, and Al-Fum MOF. Water vapor sorption characteristics, heat and mass transfer, and adsorption and desorption capacities were determined. The results showed that the Al-Fum MOF-coated heat exchanger outperformed silica gel and zeolite at low regeneration temperatures under adiabatic conditions. The MOF exhibited superior water uptake capacity (0.39 g/g) between 20% and 80% RH compared to the other studied conventional desiccants. Moreover, the MOF achieved the lowest outlet air temperature. At 19.5 g/kgda, the MOF demonstrated higher adsorption rates (4 % and 0.3 %) than zeolite and silica gel. These findings highlight the potential of Al-Fum MOFs as energy-efficient solutions for controlling indoor air humidity, particularly for latent-sensible heat separation systems with conventional vapor compression bus air-conditioning systems.

Polymer-grade propylene production through temperature swing adsorption: Molecular simulation and techno-economic analysis

Producing polymer-grade propylene from the propylene-rich mixtures can be quite challenging since the most commonly employed technology, distillation, requires high-energy input and is expensive. Therefore, identifying less-costly, efficient, and environmentally friendly methods, such as the use of the adsorptive process, is highly desirable. In this regard, six pure silica zeolites (AFV, AST, AVL, IFR, LEV, and SWY) as propane selective adsorbents are investigated for the separation of 10 mol.% propane and 90 mol.% propylene mixture. Molecular simulation Monte Carlo (MC) is used to calculate framework structure parameters such as pore volume, surface area, and framework density of pure silica zeolites. Also, adsorption isotherms, propane selectivity, and working capacity are presented using MC calculations. The Langmuir-Freundlich isotherm model is utilized to calculate isotherm parameters. A system model is implemented in Aspen Adsorption software (version 11) and breakthrough curves are achieved. The Temperature Swing Adsorption (TSA) cycle is simulated and key performance parameters such as propylene purity, recovery, and productivity for all six silica zeolites are calculated. Finally, economic analysis of the TSA process is performed using the Aspen Capital Cost Estimator (version 11) and Aspen Process Economic Analyzer (version 11) and compared with the cost of the conventional distillation process. Results showed that the capital cost and operating cost of temperature swing adsorption are 42.3 % and 91.37 % less than those of distillation, respectively.

Catalytic Hydrogenation of γ-Butyrolactone to Butanediol over a High-Performance Cu-SiO2 Catalyst

High-performance Cu catalysts were developed for the selective hydrogenation of γ-butyrolactone (GBL) to 1,4-butanediol (BDO). Among the various catalysts prepared by ammonia evaporation (AE) and impregnation (IM) methods with silica or MFI zeolite supports, the 5% Cu-SiO2-AE catalyst was the best one. It exhibited 95% selectivity for BDO and 71% conversion of GBL after 2–8 h reaction at 200 °C and 4 MPa H2, with high stability in five-cycle runs. Comprehensive characterizations showed that the AE method favored generating nano Cu particles with an average size of 2.9 nm on the 5% Cu-SiO2-AE catalyst. The silica support derived from a sol demonstrated an advantage over the MFI zeolite in the preparation of a highly dispersed and stable Cu catalyst, in view of its anti-sintering and robust composition of Cu0, Cu+, and Cu2+ in the cycling operation. The reaction pathways for GBL to BDO over the Cu catalysts were found to commonly involve reversible reactions of hydrogenation and dehydrogenation, along with subsequent dehydration to form THF. The high performance of the Cu catalysts in the conversion of GBL to BDO was attributed to the high dispersion of Cu, the presence of stable active sites, and fewer strong acid sites in the catalyst.

Machine Learning and IAST-Aided High-Throughput Screening of Cationic and Silica Zeolites for Alkane Capture, Storage, and Separations

Machine Learning and IAST-Aided High-Throughput Screening of Cationic and Silica Zeolites for Alkane Capture, Storage, and Separations

Asymmetric double-layer design of LiCl@CC/PAM for highly efficient water uptake from atmosphere

Water scarcity has become a global problem and caused a lot of conflicts. Atmospheric water harvesting (AWH) is a promising way to address water shortage issue. However, traditional adsorbent materials such as silica gels and zeolites suffer from low water productivity and high regeneration temperature. Hygroscopic salts like LiCl are regarded as a perspective substitution. Salt-modified metal organic frameworks (MOFs) and hydrogels are employed to alleviate the salt aggregation but they are limited to expensive fabrication or sluggish adsorption kinetics. Herein, we demonstrate an asymmetric double-layer design of LiCl@CC (carbon cloth)/PAM with separated function of water adsorption/desorption and water storage in different layer. This double-layer design not only enhances the mechanical properties but also shows an improved water adsorption kinetics compared to common salt-composite hydrogels. In experimental test, the LiCl@CC/PAM exhibits excellent water uptake of 1.5 g g−1 under 60 % RH in 4 h and fast water desorption under sunlight in 1.5 h. In outdoor test, LiCl@CC/PAM based device presents a daily water uptake of 5.12 g g−1 day−1. Meanwhile, all the materials used and fabrication process are feasible to scale up, which provides a possible solution to the global crisis of water scarcity.

Effects of silica fume and zeolite on the durability of engineered cementitious composite containing limestone powder and blast furnace slag in a magnesium sulphate medium

ABSTRACT Engineered cementitious composite (ECC) is a highly ductile high-performance fibre-reinforced cementitious composite whose design, mixing ratios and other important parameters can be optimised using the theory of micromechanics. This study investigated the effects of different quantities of silica fume and zeolite on the mechanical properties and durability of ECC specimens containing lime stone powder and blast furnace slag when treated in the 5% magnesium sulphate solution at 28, 150, 180 and 210 days of age. The results obtained showed that the compression strengths in lab environment were enhanced up to 25% at the age of 150 days by increasing the silica fume content of the ECC, whereas the modulus of rupture (MOR) and mid-span deflection (MSD) were decreased in all the specimens tested. Meanwhile, a 17% increment in blast furnace slag content was observed to enhance both MSD and MOR at all ages, e.g. it causes 23% increase in MSD and 9% increase in MOR at the age of 28 days. In addition, a 46% increase in lime stone powder content was found to increase compressive strength but to reduce MOR by 7% and 8% at the age of 28 days, respectively.