Zeolite Molecular Sieves Research Articles

Study On the Adsorption Performance of Zeolite Molecular Sieve for CO2

In this paper, CO2 was used as the target adsorbate to study the adsorption capacity of three kinds of zeolite molecular sieves for CO2 and the selective adsorption of CO2/N2. The results show that the adsorption capacity of NaX zeolite molecular sieve for CO2 is significantly higher than that of NaA and CaA zeolite molecular sieves.

Solid‐phase synthesis of silicalite‐1 molecular sieve based on fly ash and its CO2 adsorption performance

AbstractIn this work, an alkali melting‐pickling assisted solid phase synthesis method of S‐1 zeolite molecular sieve with excellent adsorption properties for CO2 was successfully developed by using solid waste fly ash. SiO2 with a purity of up to 97.84% was successfully extracted by using alkaline fusion activation, high temperature calcination and pickling. The CO2 adsorption capacity of the prepared SiO2 was 0.51 mmol/g at 298 K and 1 bar. Silicalite‐1 molecular sieve was prepared by solid phase synthesis method using SiO2 extracted from fly ash as silicon source. The results showed that the prepared Silicalite‐1 had good morphology and relatively high crystallinity. The specific surface area is 623.30 m2/g, and the total pore volume is 0.31 cm3/g. In addition, the adsorption capacity of CO2 was 2.05 mmol/g at 298 K and 1 bar. Compared with the prepared SiO2, the adsorption capacity of CO2 by Silicalite‐1 molecular sieve increased by four times. Moreover, under the test condition of 298 K, it has a high selectivity coefficient for CO2/N2 mixed gas, and after 10 times of adsorption‐desorption cycle tests, the adsorption capacity of Silicalite‐1 molecular sieve for CO2 does not change significantly, and its adsorption rate can still be as high as 89.31%. The results indicate that Silicalite‐1 molecular sieve prepared by solid phase synthesis method has good adsorption selectivity and adsorption–desorption cycle regeneration stability, and can be used in the field of CO2 adsorption, separation and purification. © 2024 Society of Chemical Industry and John Wiley & Sons, Ltd.

Boron Removal in Aqueous Solutions Using Adsorption with Sugarcane Bagasse Biochar and Ammonia Nanobubbles.

Boron is a naturally occurring trace chemical element. High concentrations of boron in nature can adversely affect biological systems and cause severe pollution to the ecological environment. We examined a method to effectively remove boron ions from water systems using sugarcane bagasse biochar from agricultural waste with NH3 nanobubbles (10% NH3 and 90% N2). We studied the effects of the boron solution concentration, pH, and adsorption time on the adsorption of boron by the modified biochar. At the same time, the possibility of using magnesium chloride and NH3 nanobubbles to enhance the adsorption capacity of the biochar was explored. The carbonization temperature of sugarcane bagasse was investigated using thermogravimetric analysis. It was characterized using XRD, SEM, and BET analysis. The boron adsorption results showed that, under alkaline conditions above pH 9, the adsorption capacity of the positively charged modified biochar was improved under the double-layer effect of magnesium ions and NH3 nanobubbles, because the boron existed in the form of negatively charged borate B(OH)4- anion groups. Moreover, cations on the NH3 nanobubble could adsorb the boron. When the NH3 nanobubbles with boron and the modified biochar with boron could coagulate each other, the boron was removed to a significant extent. Extended DLVO theory was adopted to model the interaction between the NH3 nanobubble and modified biochar. The boron adsorption capacity was 36 mg/g at room temperature according to a Langmuir adsorption isotherm. The adsorbed boron was investigated using FT-IR and XPS analysis. The ammonia could be removed using zeolite molecular sieves and heating. Boron in an aqueous solution can be removed via adsorption with modified biochar with NH3 nanobubbles and MgCl2 addition.

Research on Modification of Oxygen-Producing Adsorbents for High-Altitude and Low-Pressure Environments

In oxygen production on plateaus, pressure swing adsorption (PSA) oxygen production is currently the most commonly used oxygen production method. In plateau regions, low pressure leads to a decrease in adsorbent nitrogen–oxygen separation performance, which affects the performance of PSA oxygen production, so it is particularly important to enhance adsorbent nitrogen–oxygen separation performance. In this paper, Li-LSX (lithium low-silicon aluminum X zeolite molecular sieve) adsorbents were modified using the liquid phase ion exchange method, and five kinds of modified adsorbents were obtained, namely AgLi-LSX, CaLi-LSX, ZnLi-LSX, CuLi-LSX, and FeLi-LSX, respectively. The influences of different metal ions and modification time lengths on the adsorbent nitrogen adsorption and nitrogen–oxygen separation coefficients were analyzed. Through theoretical calculations, the nitrogen and oxygen adsorption and separation performances of the modified adsorbents at different altitudes and low adsorption pressures were investigated. It is shown that the nitrogen adsorption capacity of the AgLi-LSX-1 adsorbent obtained from the modification experiment reaches 27.92 mL/g, which is 3.24 mL/g higher than that of Li-LSX; the nitrogen–oxygen separation coefficients of S1 and S2 are 19.24 and 7.54 higher, respectively; and the nitrogen–oxygen separation coefficients of S4 are 20.85 and 7.54 higher than those of Li-LSX, respectively. With the increase in altitude from 50 m to 5000 m, the nitrogen–oxygen separation coefficient of the AgLi-LSX-1 adsorbent increased rapidly from 20.85 to 57, and its nitrogen–oxygen separation coefficient S4 exceeded that of the Li-LSX adsorbent to reach 47.61 at an altitude of 4000 m. Therefore, the modified adsorbent AgLi-LSX-1 in this paper can enhance the performance of the PSA oxygen process for oxygen production in plateau applications.

Molecular Insight into the Electric Field Regulation of N2 and CH4 Adsorption in the Trapdoor ZSM-25 Zeolites.

Controlling gas admission by regulating pore accessibility in porous materials has been a topic of extensive research. Recently, the electric field (E-field) has emerged as an external stimulus to alter the adsorption behavior of some microporous adsorbents. However, the mechanism behind this phenomenon is not yet fully understood. Here, we demonstrate the crucial role of the trapdoor cations of zeolite molecular sieves in E-field-regulated gas adsorption. The E-field activation caused framework expansion and cation deviation, significantly reducing the energy barrier for gas molecules passing through the pore aperture gated by the trapdoor cation. This led to an increase in the N2 adsorption capacity of ZSM-25 and a 60% improvement in N2/CH4 selectivity in the quest for nitrogen rejection for natural gas processing. By combining experimental and computational approaches, we elucidated the influence of E-field activation as a concurrent effect of the reduced heat of adsorption caused by framework expansion and the decrease in the energy barrier resulting from promoted cation oscillation. These findings pave the way for the material design of E-field-regulated adsorption and its application in molecular separation.

Efficient removal of ammonia–nitrogen in wastewater by zeolite molecular sieves prepared from coal fly ash

Zeolite molecular sieves are potential adsorbents for wastewater treatment, characterized by high efficiency, simple process, easy regeneration, and low treatment cost. In this study, zeolite A molecular sieves were prepared using coal fly ash (CFA), which is an effective method for the utilization of CFA. The results showed that the CFA-based zeolite molecular sieves synthesized under optimized conditions exhibited excellent adsorption and removal rates (> 40%) for ammonia–nitrogen in wastewater of different concentrations and properties. The analysis of adsorption kinetics revealed that the adsorption process followed pseudo-second-order kinetics model, indicating that the adsorption of ammonia–nitrogen on zeolite is primarily controlled by chemisorption rather than physisorption. The adsorption process can be divided into two stages, with a higher adsorption rate and a smaller diffusion boundary layer thickness in the first stage, and a lower adsorption rate and an increased diffusion boundary layer thickness in the second stage. This indicates that as the adsorption proceeds, the internal diffusion resistance within the particles gradually increases, leading to a decrease in the adsorption rate until reaching equilibrium, where both the diffusion and adsorption become stable. The adsorption isotherms of ammonia–nitrogen on zeolite A conformed to the assumptions of the Langmuir model, suggesting that the adsorption mechanism primarily involves uniform monolayer adsorption on the surface without intermolecular interactions.

Synthesis of 4A Zeolite Molecular Sieves by Modifying Fly Ash with Water Treatment Residue to Remove Ammonia Nitrogen from Water

The widespread presence of ammonia nitrogen (NH4+–N) pollutants poses a serious threat to water environment health. In this study, a novel zeolite (WTR–CFA zeolite) with excellent adsorption performance is synthesized using CFA as the raw material and water treatment residue (WTR) as the aluminum source through an ultrasonic–assisted alkali melt hydrothermal method. Compared with traditional CFA–zeolite, WTR–CFA zeolite only generates 4A zeolite with a single crystal phase, and the peak shape is sharp, which results in better crystallization. WTR–CFA zeolite perfectly solves the technical problems of the low utilization rate and poor controllability of the crystal form in traditional artificially synthesized zeolites. The maximum NH4+–N adsorption capacity of WTR–CFA zeolite is 29.80 mg/g, which is higher than that of most adsorbents reported in previous studies. After five cycles of adsorption regeneration, the regeneration efficiency of WTR–CFA zeolite only decreased from 98.84% to 97.12%, which demonstrates excellent environmental value. The adsorption isotherms and kinetics of NH4+–N conform to the Langmuir model and quasi–second order kinetic model, respectively, which indicates that ion–exchange–dominant chemical adsorption plays a major role in the adsorption mechanism. In summary, this study combines the use of CFA and WTR resources with the treatment of aquatic pollution to reduce material synthesis costs, control the crystal structure of WTR–CFA zeolite, and increase adsorption capacity. This approach achieves the goals of “waste treatment and turning waste into treasure”.

Effect analysis on the hydrocarbon adsorption performance enhancement of the different zeolite molecular sieves in the gasoline engine under the cold start process

In this work, the adsorption performance of ZSM-5, Beta-A, and NON zeolites on hydrocarbons emitted from the engine is investigated for selecting the best suited zeolite molecular sieves for hydrocarbon traps during the cold-start phase of the engine. Based on the giant canonical Monte Carlo (MC) method for adsorption simulation, the single-component, multi-component maximum adsorption, adsorption isotherms, adsorption sites of different hydrocarbon molecules on ZSM-5, Beta-A, and NON molecular sieves were investigated by using C2H2, C2H4, C3H6, and C7H8 molecules as probe molecules. The single-component adsorption results showed that for the macromolecular hydrocarbon toluene (C7H8) Beta-A zeolite had the best adsorption effect, followed by ZSM-5 zeolite, and NON had the worst adsorption effect. The adsorption of propylene (C3H6) on ZSM-5 and Beta-A is close to each other, and the adsorption of ethylene (C2H4) and acetylene (C2H2) are much different relative to C3H6 and C7H8. This is due to the fact that the adsorption properties of zeolites are determined by the molecular diameter of the hydrocarbons. The results of multicomponent adsorption showed that the adsorption amounts of multicomponent competitive adsorption were reduced relative to single-component adsorption, but were affected by other components in different ways, from small to large C7H8<C3H6<C2H4<C2H2.

A "Liquid-In-Solid" Electrolyte for High-Voltage Anode-Free Rechargeable Sodium Batteries.

Developing anode-free batteries is the ultimate goal in pursuit of high energy density and safety. It is more urgent for sodium (Na)-based batteries due to its inherently low energy density and safety hazards induced by highly reactive Na metal anodes. However, there is no electrolyte that can meet the demanding Na plating-stripping Coulomb efficiency (CE) while resisting oxidative decomposition at high voltages for building stable anode-free Na batteries. Here, a "liquid-in-solid" electrolyte design strategy is proposed to integrate target performances of liquid and solid-state electrolytes. Breaking through the Na+ transport channel of Na-containing zeolite molecular sieve by ion-exchange and confining aggregated liquid ether electrolytes in the nanopore and void of zeolites, it achieves excellent high-voltage stability enabled by solid-state zeolite electrolytes, while inheriting the ultra-high CE (99.84%) from liquid ether electrolytes. When applied in a 4.25V-class anode-free Na battery, an ultra-high energy density of 412W h kg-1 (based on the active material of both cathodes and anodes) can be reached, which is comparable to the state-of-the-art graphite||LiNi0.8Co0.1Mn0.1O2 lithium-ion batteries. Furthermore, the assembled anode-free pouch cell exhibits excellent cycling stability, and a high capacity retention of 89.2% can be preserved after 370 cycles.

Effect analysis on hydrocarbon adsorption performance of transition metal modified zeolites in the gasoline engine during cold start process using Monte Carlo method

Zeolite molecular sieves are commonly used for adsorption of hydrocarbons(HCs) during cold start phase of gasoline engines. In this work, the adsorption properties of three zeolites modified with three transition metal ions(Fe2+, Cu2+ and Zn2+) were investigated for the HCs adsorption using the Monte Carlo method. Adsorption simulations were carried out with ethylene and propylene as probe molecules or with a mixture of five hydrocarbon and water molecules. The introduction of transition metal cations enhances the local positive electric field within the molecular sieve, enabling it to engage in stronger adsorption interactions with adsorbate molecules, which significantly improves the adsorption performance of the molecular sieve. For single-component adsorption, Zn2+-MFI with a Si/Al ratio of 7 showed the best adsorption of ethylene, with a high adsorption capacity of 149.19 molecules per cell at 298 K, which was 1.47 times higher than that of pure silicon. The enhanced hydrophilicity of the modified zeolite resulted in a substantial increase in the adsorption of water molecules during multicomponent adsorption. The limited adsorption sites and intermolecular competition for adsorption resulted in the occupation of adsorption sites by large hydrocarbons and a decrease in the adsorption of the remaining small molecules.

A Study of ZSM-5 Molecular Sieve Shaping for an Innovative Cyclohexene Hydration Process

The development of the zeolite molecular sieves (ZSM-5) molding technique is essential for the cyclohexene hydration reaction distillation process in both industrial and research settings. The reactive distillation process can solve many of the drawbacks associated with the existing technique of manufacturing cyclohexanol via cyclohexene hydration, such as high catalyst digestibility, low conversion rate, and operational challenges. This study developed a series of molded ZSM-5 catalysts with various binder types and contents were constructed. The suggested pseudo-boehmite and hydroxypropyl methylcellulose were chosen as binders for extrusion molding of ZSM-5. The effects of binders on the strength reliability were investigated by strength tests and statistical analysis of the Weibull function. The effects of binders on the physical structure, acidity, and catalytic performance of ZSM-5 were investigated by X-ray diffraction, scanning electron microscopy, physical adsorption of N2, and desorption of NH3. The findings demonstrate that the addition of binder has no effect on ZSM-5's crystal structure. The experiment's results showed that the molded catalyst could be used for the hydration process with over 95% selectivity and a yield of 10.45% cyclohexanol. Copyright © 2024 by Authors, Published by BCREC Publishing Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).

Synthesis of ZSM‐5 zeolite from fly ash by one‐pot hydrothermal method for methylene blue removal

AbstractUsing fly ash to synthesize zeolite molecular sieves can improve the reuse value of coal fly ash resources, reduce the synthesis cost of the zeolites, and protect the ecology. A novel approach was proposed to prepare zeolite socony mobil‐5 (ZSM‐5) zeolite via one‐pot hydrothermal synthesis. The mesoporous ZSM‐5 synthesized by adjusting the molar ratio of chemical reagents has higher crystallinity, larger specific surface area, and more adsorption sites. An analysis of methylene blue (MB) adsorption on mesoporous ZSM‐5 was conducted at many different adsorption parameters, including solution concentration, temperature, and duration of adsorption. The Langmuir model can objectively describe the adsorption behavior of MB on ZSM‐5, and the adsorption characteristics of MB on ZSM‐5 are better described by the pseudo‐second‐order kinetic model. The thermodynamic model provides the adsorption behavior as an endothermic reaction.

Solvation‐Tailored PVDF‐Based Solid‐State Electrolyte for High‐Voltage Lithium Metal Batteries

AbstractPoly(vinylidene fluoride) (PVDF)‐based polymer electro‐lytes are attracting increasing attention for high‐voltage solid‐state lithium metal batteries because of their high room temperature ionic conductivity, adequate mechanical strength and good thermal stability. However, the presence of highly reactive residual solvents, such as N, N‐dimethylformamide (DMF), severely jeopardizes the long‐term cycling stability. Herein, we propose a solvation‐tailoring strategy to confine residual solvent molecules by introducing low‐cost 3 Å zeolite molecular sieves as fillers. The strong interaction between DMF and the molecular sieve weakens the ability of DMF to participate in the solvation of Li+, leading to more anions being involved in solvation. Benefiting from the tailored anion‐rich coordination environment, the interfacial side reactions with the lithium anode and high‐voltage NCM811 cathode are effectively suppressed. As a result, the solid‐state Li||Li symmetrical cells demonstrates ultra‐stable cycling over 5100 h at 0.1 mA cm−2, and the Li||NCM811 full cells achieve excellent cycling stability for more than 1130 and 250 cycles under the charging cut‐off voltages of 4.3 V and 4.5 V, respectively. Our work is an innovative exploration to address the negative effects of residual DMF in PVDF‐based solid‐state electrolytes and highlights the importance of modulating the solvation structures in solid‐state polymer electrolytes.

Solvation-Tailored PVDF-Based Solid-State Electrolyte for High-Voltage Lithium Metal Batteries.

Poly(vinylidene fluoride) (PVDF)-based polymer electro-lytes are attracting increasing attention for high-voltage solid-state lithium metal batteries because of their high room temperature ionic conductivity, adequate mechanical strength and good thermal stability. However, the presence of highly reactive residual solvents, such as N, N-dimethylformamide (DMF), severely jeopardizes the long-term cycling stability. Herein, we propose a solvation-tailoring strategy to confine residual solvent molecules by introducing low-cost 3 Å zeolite molecular sieves as fillers. The strong interaction between DMF and the molecular sieve weakens the ability of DMF to participate in the solvation of Li+, leading to more anions being involved in solvation. Benefiting from the tailored anion-rich coordination environment, the interfacial side reactions with the lithium anode and high-voltage NCM811 cathode are effectively suppressed. As a result, the solid-state Li||Li symmetrical cells demonstrates ultra-stable cycling over 5100 h at 0.1 mA cm-2, and the Li||NCM811 full cells achieve excellent cycling stability for more than 1130 and 250 cycles under the charging cut-off voltages of 4.3 V and 4.5 V, respectively. Our work is an innovative exploration to address the negative effects of residual DMF in PVDF-based solid-state electrolytes and highlights the importance of modulating the solvation structures in solid-state polymer electrolytes.

Experimental and simulation investigation on the different iron content beta zeolite for controlling the cold-start hydrocarbon emission from a gasoline vehicle

In this work, cold start experiments results showed that the HC catcher coated with 1% iron content is more effective than that with 3% iron content, the peak HC emission is reduced from 0.0029 g to 0.0018 g. To further reveal the reasons for the effects of BEA zeolite molecular sieves with different iron contents on HC adsorption, the measured HC emission components (butene, propene, ethene, acetaldehyde, and acetylene) were used as HC simulation molecules for molecular simulation experiments, the simulation results showed that, for single-component adsorption, the adsorption effect of beta zeolite molecular sieves with high iron content was better; for multi-component adsorption, acetaldehyde and butene maintained higher adsorption amounts, and the adsorption of acetaldehyde increased with the increase of the iron content in the beta zeolite molecular sieves; in the presence of water and CO2, the effect of water is greater than that of carbon dioxide, and its effect increases with the increase of iron content in beta zeolite molecular sieves. The maximum adsorption capacity of water can reach 4.5mmol/zeolite, which seriously affected the adsorption of HC molecules. Properly reducing the iron content of BEA zeolite molecular sieve is useful for enhancing its adsorption effect on the main HC simulated molecules.

Analysis of the integral zeolite molecular sieve process for helium CPS application

Zeolite Molecular Sieves (ZMSs) are commonly adopted in tritium handling facilities for impurity removal from gaseous streams, particularly for tritiated water trapping. Several activities have been conducted to characterize the adsorption and desorption behaviour of the sieving materials. Instead, less attention has been put on the analysis of the integral process comprising the adsorption of the tritiated humidity, the regeneration of the molecular sieve and the recovery of the desorbed water. Focusing on the application for the EU-DEMO helium Coolant Purification System (CPS), this work presents a process simulator relying on a MATLAB dynamic model of the ZMS bed and on a RELAP5/MOD3.3 model for the regeneration loop. The dynamic model can simulate both the adsorption and regeneration modes of the ZMS by providing the temperature and the adsorbate concentration profile along the column, while the thermal-hydraulic model is used to assess the conditions of the regeneration loop. By coupling the outcomes of the two models it is possible to establish the effective regeneration efficiency of the process and the amount of the tritiated water recovered. For the case of helium CPS, the analysis demonstrates the feasibility of the proposed operative scheme and the capabilities of the regeneration procedure to ensure a 74.4 % regeneration efficiency of the ZMS bed.

Current Situation of Fly Ash Based Zeolite Molecular Sieve and Its Application in Water Treatment

Current Situation of Fly Ash Based Zeolite Molecular Sieve and Its Application in Water Treatment

Self-synthesis and performance analysis of a Cu-Fe composite ZSM-5 zeolitic catalyst for NOx reduction and particulate matter removal using NH3 SCR

The NH3-SCR has gained widespread usage due to its exceptional denitrification efficiency. It achieves high nitrogen oxides (NOx) reduction rates within the temperature range of 300⁰C to 400℃. Nonetheless, most of the catalyst have a limited temperature range for effective NOx removal applications. For sustainable utilization of common combustion units, it has become essential to develop a catalyst which can provide a broader spectrum of temperature range for NOx reduction. This study is thus aimed to enhance the structure of ZSM-5 zeolite molecular sieves using tetrapropylammonium bromide. Copper and iron ions (total 3% weight) were incorporated to boost the reaction at temperatures of 180⁰C, 250⁰C and 320⁰C. The overall efficiency of NOx removal was above 81%, highest being 97.6%. The carrier significantly impacts the NOx conversion rate with increased copper ion content in the samples at medium and low temperatures. The Particulate Matter removal efficiency ranged from 90.1% to 93.1%.

Synthesis of zeolite molecular sieve 13X from coal-fired slag for efficient room temperature CO2 adsorption

Coal-fired slag (CFS) was used to successfully synthesize zeolite molecular sieve (ZMS) 13X by the alkali fusion hydrothermal method for efficient room-temperature CO2 adsorption. The optimum synthesis conditions of ZMS 13X were confirmed using the single-factor and Box-Behnken methods. With a specific surface area of 585.67 m2·g−1, the resultant ZMS 13X is a mesoporous material with good CO2 adsorption selectivity and cyclic performance. At 25 °C and 1 bar, the initial CO2 adsorption amount presented 3.69 mmol·g−1 and still maintained 3.37 mmol·g−1 of CO2 adsorption amount after 10 cycles. The Langmuir isotherm model can well describe the CO2 adsorption process of ZMS 13X as an entropy-decreasing physical adsorption process with the intermolecular interaction of van der Waals forces. Therefore, this work proposes a new strategy for the low-cost synthesis of ZMS 13 from the CFS solid waste for efficient room-temperature CO2 capture.

Preparation of NaA zeolite molecular sieve based on solid waste fly ash by high-speed dispersion homogenization-assisted alkali fusion-hydrothermal method and its performance of ammonia-nitrogen adsorption

A high-speed dispersion homogenization-assisted alkali fusion-hydrothermal method was successfully developed for the preparation of NaA zeolite molecular sieve with excellent adsorption performance for ammonia nitrogen using solid waste fly ash. The zeolite molecular sieve, prepared at a calcination temperature of 700 °C and a crystallization temperature of 80 °C, was identified as NaA zeolite molecular sieve through XRD and SEM tests, exhibiting a uniform and ordered cubic crystal structure. The result of BET analysis indicates that the NaA zeolite molecular sieve has a specific surface area of 288.219 m2/g and a plentiful pore structure. The effect of pH, adsorption time, and initial concentration of ammonia nitrogen on the adsorption capacity of NaA zeolite molecular sieve was also investigated. The fitting analysis of experimental adsorption data using adsorption kinetics and isotherm models indicates that the kinetic adsorption of ammonia nitrogen solution by zeolite molecular sieve follows the second-order kinetic model more closely. The Langmuir model shows a better correlation than the Freundlich model, suggesting that the adsorption of ammonia nitrogen solution by the zeolite molecular sieve is mainly based on the surface-uniform monolayer physical adsorption process. Under the conditions of pH = 7 and an initial ammonia nitrogen concentration of 100 mg/L, the adsorption capacity of the zeolite molecular sieve for ammonia nitrogen within 60 min can reach 27.5 mg/g, showing excellent adsorption performance. This study establishes a vital research foundation for the utilization of fly ash and the treatment of ammonia nitrogen wastewater, with great ecological and social benefits.