Water Adsorption Properties Research Articles

Improvements in the synthesis of zeolites with low Si/Al ratio from Venezuelan sodium silicate for an environmentally friendly process

<p>LTA and FAU zeolites were successfully synthesized from a Venezuelan sodium silicate solution, by hydrothermal crystallization under autogenous pressure at 100°C, with 2–24h crystallization times. The synthesized materials were characterized by XRD, BET specific surface area and SEM. A series of synthesis tests were performed to study the influence of the molar composition of the starting mixture over zeolites crystallization. The effect of crystallization time for a particular synthesis mixture composition was studied for both zeolites types. The reuse as alkaline medium of the mother liquor separated during filtration, and the effect of the aging before crystallization were additionally studied. The experimental result are in agreement with the crystallization mechanism proposed for zeolites synthesis in liquid phase. The use of a 2SiO2:Al2O3:6.Na2O:240H2O synthesis mixture composition allows obtaining LTA zeolite within 2h of crystallization. For FAU zeolite, no aging period was needed when starting with a 4SiO2:Al2O3:6.6Na2O:264H2O composition. It was possible to synthesize both zeolites with high purity and crystallinity and with adequate water adsorption properties.</p>

Water Adsorption Properties of Free and Dehydrated β-Cyclodextrin Studied by near Infrared Spectroscopy and Gravimetry

β-cyclodextrin, like other carbohydrates has a tendency to adsorb water molecules and the properties are attributed to the hydroxyl groups in the molecules. β-cyclodextrin, the cyclic oligomer of glucose has a hydrophobic interior and hydrophilic exterior. The cyclic structure favours the formation of hydrogen bonds between the OH groups on the adjacent glucose units and affects the formation of hydrogen bonds with water molecules. The hydoxyl groups engaged in hydrogen bondings can be eliminated at high temperatures and the adsorption properties of the dehydrated β-cyclodextrin will depend on the new functional groups formed. The aim of the report is to discuss the issue of the water adsorption properties of free and dehydrated β-cyclodextrin. Dry β-cyclodextrin and dehydrated β-cyclodextrin at temperatures 250, 300 and 350 °C were allowed to adsorb water from a humidity controlled air environmennt and the evolving near infrared spectra were measured using a near infrared spectrometer equipped with a transflectance accessory. The near infrared spectra in the region 10,000-4000 cm-1 and their second and fourth derivative profiles were used in studying the variation in the adsorption characteristics of dehydrated β-cyclodextrin. The results of the analyses show that the adsorption of water by β-cyclodextrin decreses at 300 °C compared to 200 and 250 °C. Dehydration forms more of the ethereal type-O-bonds in the molecule and explains the decrease in the water molecular adsorption at higher dehydration temperatures.

Adsorption of water and ethanol on noble and transition-metal substrates: a density functional investigation within van der Waals corrections.

We report the results of extensive computational investigation of the adsorption properties of water and ethanol on several Cu-, Pt-, and Au-based substrates, including the close-packed unreconstructed Cu(111), Pt(111), and Au(111) surfaces, defected metal substrates with on-surface low-coordinated sites generated by the intermixing of Pt-Cu and Pt-Au in the topmost surface layers and strained on-surface and sub-surface Pt-layers at Cu(111) and Au(111) substrates. The calculations are based on the density functional theory (DFT) within the van der Waals (vdW) correction. For all the substrates, we found that water and ethanol bind via the anionic O atom to the cationic one-fold coordinated on-top metal sites, which enhances the adsorbate-substrate Coulomb interactions. For water, both DFT and DFT + vdW calculations predict a flat geometry. For ethanol, the DFT and DFT + vdW results are in contrast, namely, DFT yields a perpendicular orientation of the C-C bond with respect to the surface, while we obtained a parallel orientation of the C-C bond using DFT + vdW, which maximizes the adsorption energies. Despite expected deviations due to the nature of the weak adsorbate-substrate interactions, we found that the adsorption energy of water and ethanol shows a linear dependence as a function of the position of the center of gravity of the occupied d-band, and hence, the magnitude of the adsorption energy increases as the d-band center position shifts towards the Fermi energy. Thus, it indicates hybridization between the O p- and metal d-states, which determines the magnitude of the adsorption energy of water and ethanol on clean, low-coordinated, and strained noble and transition-metal substrates.

Synthesis, characterization, and water adsorption properties of a novel multi-walled carbon nanotube/MIL-100(Fe) composite

Conventional small-scale adsorption chillers generally employ silica-gel/water or zeolite/water working pairs given the relatively high level of mesoporosity and water affinity in these adsorbent materials. However, the coefficient of performance (COP) and specific cooling power (SCP) evaluated for the adsorption chiller using these adsorbent/adsorbate pairs cannot be still considered practically feasible in the context of a commercial system. Metal organic frameworks (MOFs) are not only characterized by much higher water adsorption capacities than these materials, but also can be mass-produced using much simpler methods than the template-assisted synthesis routes of most zeolites. However, the low intrinsic thermal conductivity of these materials limits their use as adsorbents in commercial-scale adsorption chillers. In this study, a novel composite composed of multi-walled carbon nanotubes (MWCNTs) incorporated in a MIL-100(Fe) framework has been synthesized using a molecular-level mixing process. The resulting composite, with varying volume fraction of MWCNTs, has been characterized for microstructure, degree of crystallinity, thermal stability, water sorption kinetics and hydrothermal cyclic stability for potential use as an adsorbent in commercial adsorption chillers.

Water-adsorption properties of Xiazijie montmorillonite intercalated with polyamine

AbstractPolyamine is a novel inhibitor applied to water-based drilling fluids in order to reduce hydration of clays, especiallymontmorillonite (MT). In the present study, thewater-adsorption properties of montmorillonite intercalated with polyamine were characterized by X-ray diffraction (XRD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) and adsorption isotherm tests using water, benzene and heptane. According to the XRD, DRIFT and water-adsorption test results, polyamine (PA)- intercalated montmorillonite (PA-MT) adsorbs less water than either K+- or NH+4-intercalated montmorillonite (K-MT, NH4-MT). The surface energy of MT was analysed using the van Oss- Chaudhury-Good (VCG) approach based on thewater, benzene and heptane adsorption results. The surface energy between PA-MTand water was less than that of NH4-MT and K-MT. Compared with K-MT and NH4-MT, the surface energy of the Lewis acid portion of PA-MT underwent a more significant decrease, suggesting that the hydration of unexchanged Na+ did not make a discernible contribution. The experimental water-adsorption data were fitted using Langmuir, Brunauer-Emmett-Teller (BET), and Freundlich adsorption models. The Langmuir adsorption models can be used to fit the data for water adsorption on PA-MTacross the entire p/p0 range. After plotting the experimental data in the linear format of the Freundlichmodel, only one linear region for PA-MTwas observed, indicating a singlewater-adsorption mechanism for PA-MT. When compared with PA in the absence of clay, DRIFT spectroscopy showed that the NH2 bending mode for PA-MT shifted from 1571 cm–1 to 1619.7 cm–1. This suggested an increase in hydrogen bonding (H bonding) between the ammonium group and the interlayer water. Across the entire p/p0 range, water is suggested to have adsorbed onto PA-MT through H-bonding interactions between the ammonium group and the interlayer water.

Characterization of natural resin materials using water adsorption and various advanced techniques

A physicochemical characterization of natural raw resin material was evaluated and reported. The studied material is a natural resin, a natural product from pinus halepensis trees which is collected from the forests of Chalkidiki region of North Greece. The plurality of this product combined with its special property of removing water from commercial liquid fuels commands the detailed physicochemical characterization of this material. In particular, various techniques, such as water adsorption at 22 °C, thermogravimetric analysis, differential scanning calorimetry, Fourier transform infrared spectroscopy and X-ray diffraction, were used in order to evaluate the structural and surface properties of the material. The water adsorption isotherm was also measured and fitted using the Guggenheim, Anderson and De Boer model in order to correlate the water activity characteristics. In addition, the kinetics of the adsorption was also fitted with good accuracy using the exponential Chapman model. Furthermore, as the results show, the natural resin presents good thermal characteristics. Finally, the studied material presents efficient water adsorption properties, up to 246.8 mmol/g, and it can be proposed as a promising dehydration material.

Performance of an open thermal adsorption storage system with Linde type A zeolites: Beads versus honeycombs

The process of the discharging performance of an open thermal adsorption storage was investigated by tests in a 1.5-L-lab-storage accompanied by computational fluid dynamics (CFD) simulations. The storage materials were Linde-type 4A-zeolite beads and honeycomb structures both with and without binder. Additional information about the water adsorption properties und secondary pore structure was obtained by thermogravimetry, isotherm measurements and mercury intrusion. In accordance to the experimental observations and different to the CFD-results the binderless zeolite honeycombs showed the best adsorption dynamics but not the binder containing honeycombs. The accelerated adsorption kinetics in the walls of the binderless honeycombs may be caused by the optimal secondary pore size distribution and leads to an improved adsorption/discharge performance.

Water adsorption in SAPO-34: elucidating the role of local heterogeneities and defects using dispersion-corrected DFT calculations.

The chabazite-type silicoaluminophosphate SAPO-34 is a promising adsorbent for applications in thermal energy storage using water adsorption-desorption cycles. In order to develop a microscopic understanding of the impact of local heterogeneities and defects on the water adsorption properties, the interaction of different models of SAPO-34 with water was studied using dispersion-corrected density-functional theory (DFT-D) calculations. In addition to SAPO-34 with isolated silicon atoms, the calculations considered models incorporating two types of heterogeneities (silicon islands, aluminosilicate domains), and two defect-containing (partially and fully desilicated) systems. DFT-D optimisations were performed for systems with small amounts of adsorbed water, in which all H2O molecules can interact with framework protons, and systems with large amounts of adsorbed water (30 H2O molecules per unit cell). At low loadings, the host-guest interaction energy calculated for SAPO-34 with isolated Si atoms amounts to approximately -90 kJ mol(-1). While the presence of local heterogeneities leads to the creation of some adsorption sites that are energetically slightly more favourable, the interaction strength is drastically reduced in systems with defects. At high water loadings, energies in the range of -70 kJ mol(-1) are obtained for all models. The DFT-D interaction energies are in good agreement with experimentally measured heats of water adsorption. A detailed analysis of the equilibrium structures was used to gain insights into the binding modes at low coverages, and to assess the extent of framework deprotonation and changes in the coordination environment of aluminium atoms at high water loadings.

Insights into the microscopic behaviour of nanoconfined water: host structure and thermal effects

The hydration of nanoporous materials is relevant to many fundamental and industrial applications. In this context, zeolites are usually used, but metal–organic frameworks constitute an emerging class of useful materials. The singular properties of water ascribed to the molecular association lead to a variety of behaviors in the confining systems that are not well understood. There is little of both experimental and computational information available, and most of which are limited to room temperature. This work addresses the adsorption and structural properties of water in a series of porous materials for a fixed topology, in particular LTA, and at various temperatures in the range of 298–573 K. The targeted structures were the all-silica zeolite ITQ-29, the aluminosilicate form with charge-balancing sodium and calcium cations LTA-5A, and the Zn-based zeolitic imidazolate framework. The adsorption process was computed using Monte Carlo simulations in the grand canonical ensemble and comprehensively rationalized at the molecular level on the basis of energetic factors and radial distribution functions. The structure of confined water for the different hydration levels was characterized in detail by using a specific criterion of hydrogen bond formation. The influence of both host characteristics and temperature on the microscopic behaviour of adsorbed water was assessed. Overall, this work proves that water–water hydrogen bonding is enhanced by the hydrophobic character of the pore walls and the large confining spaces. The increase in temperature induces progressive destruction of hydrogen bonds, but the majority of water molecules remain associated even when saturation is not reached. Concerning the thermodynamics of water adsorption, it is mainly affected by the peculiarities of the pore structure.

Tailoring the water adsorption properties of MIL-101 metal–organic frameworks by partial functionalization

Partial functionalization of the metal–organic framework, MIL-101, with –COOH groups is proven effective for enhancing the water uptake capacity at low pressure.

Thermodynamic properties, kinetics and adsorption mechanisms of Swiss cheese bioaroma powder

Water adsorption kinetics of Swiss cheese bioaroma microparticles were determined by a static method at 15°C, 25°C, 35°C and 45°C. This work used the Guggenheim–Anderson–de Boer (GAB) model to calculate thermodynamic properties of water adsorption. Additionally, this work predicted conditions for the stable storage of microparticles obtained through a spray drying encapsulation process with maltodextrin and modified corn starch (Capsul®) at a 1:1 (w/w) ratio. We evaluated the mechanisms that direct water vapor adsorption by studying enthalpy–entropy compensation. The product reached equilibrium phases after 120h of storage. The observed minimum values for the net integral entropy were indicative of maximized stability conditions of the product. At these conditions, water activity values ranged from 0.391 to 0.537, and the temperature ranged from 15°C to 35°C. An isokinetic compensation was confirmed for the adsorption phenomenon. This approach resulted in two zones, low and intermediate-high water activities. Under low moisture, the process was controlled by entropy; under intermediate-high moisture, it was driven by enthalpy.

Characterization and water adsorption properties of Ag- and Zn-exchanged A zeolites

Ag- and Zn-exchanged A zeolites are prepared by the liquid ion exchange method following the procedure reported in our previous studies. Samples with Ag and Zn metal loadings are obtained and characterized by physicochemical and thermal analyses. The obtained materials with improved physicochemical properties are promising candidates for use as haemostatic agents. Both powder X-ray diffraction and nitrogen adsorption measurements show good conservation of the zeolitic structure and texture after ionic exchange. Porosity measurements indicate that the variation of the porous volumes and the specific surface areas depends on the number and nature of exchanged cations. Thermal behavior of water adsorption studied by using TGA microbalance and differential scanning calorimetry highlights clearly the complexity of the adsorption and diffusion mechanisms of water molecules within the micropores of exchanged zeolites. The differences between sorption rates and hydration heats appear to be due mainly to the location and migration of Ag, Ca and Zn species in the cationic position of the A zeolite framework. The exothermicity of adsorption is attenuated by Ag and Zn exchange.

Dynamics of Water in Hierarchical Mesoporous H-ZSM-5 by Fast Field-Cycling NMR Relaxometry

The dynamics of water molecules adsorbed in hierarchical mesoporous H-ZSM-5 was investigated by fast field-cycling NMR relaxometry. Three distinct sites in H-ZSM-5 that can host water molecules, designated a, b, and c, have been identified; no significant exchange of water molecules among these sites was observed. Investigations into the relative populations of molecules confined at sites c (Pc) and b (Pb) revealed that the population ratio (Pc/Pb) does not change significantly over the temperature and water-content ranges investigated. Furthermore, the motion of water at site b is very slow and shows almost no dependence on temperature. While H-ZSM-5 has pores of two different sizes (mesopores and micropores), the low population and observed slow motion of water at site b indicates a strong interaction between water molecules and the zeolite materials, which is possible only when water is adsorbed on the surface. The temperature of the phase transition of water provides insight into the properties of water adsorption. The transition temperature of water confined at site a varies with water content in the sample, indicating that it is located in a larger space; thus, site a represents mesopores. The transition temperature of water and the population of water molecules at site c are independent of water content, indicating that this site has been saturated; therefore, site c is the micropore, which is saturated because of capillary condensation.

Synthetic zeolite pellets incorporated to air lime–metakaolin mortars: Mechanical properties

In this study synthetic fine and coarse zeolite pellets were chosen in the development of air lime–metakaolin mortars for repairing ancient masonry to be used in conservation and restoration of cultural heritage. Synthetic zeolite was used due to their particular water adsorption properties and act as an artificial pozzolan promoting the development of hydraulic phases.Physical, chemical, mineralogical and microstructural tests were accomplished to characterize the materials used in mortars’ preparation.Flexural, compressive strength and dynamic modulus of elasticity tests were performed in mortars at 28, 90 and 180days of curing. Incorporation of both fine and coarse zeolite pellets caused improvement of mechanical strength of mortars. The highest flexural strength value (∼0.5MPa) was achieved in both mortars with fine zeolites pellets at 90days and 20 and 30wt.% metakaolin, and coarse zeolite pellets at 180days and 30wt.% metakaolin as lime replacement. 1.0MPa was the highest compressive strength value obtained at 180days for mortars with both fine zeolite pellets and coarse zeolite pellets, with 20 and 30wt.% of metakaolin, respectively.Elasticity modulus ranged from 2.3GPa to 3.9GPa confirming the high deformation capability of these mortars.Zeolite pellets type A is a promise synthetic material that could be successfully used in air lime–metakaolin render mortars for applications in the conservation and restoration of cultural heritage.

Synthesis, structure and water sorption in Zr metal-organic frameworks

Metal-organic frameworks (MOFs) based on zirconium secondary building units (SBUs) have proven to have great thermal and chemical stability,[1,2] which make them ideal for their use in different applications. We have prepared a series of six new MOFs made from the Zr6O4(OH)4(-CO2)nsecondary building units (n = 6, 8, 10, or 12) and variously shaped carboxyl organic linkers to make extended porous frameworks, with the aim of studying their performance as water adsorbents. Thus, we have evaluated the water adsorption properties of these new MOFs and other reported porous materials to identify the compounds with the most promising materials for use in applications such as thermal batteries or delivery of drinking water in remote areas. An X-ray single-crystal and a powder neutron diffraction study reveal the position of the water adsorption sites in one of the best performing materials, and highlight the importance of the intermolecular interactions between adsorbed water molecules within the pores.

Structural Diversity in Multinuclear PdII Assemblies that Show Low‐Humidity Proton Conduction

Systematic investigation on synergetic effects of geometry, length, denticity, and asymmetry of donors was performed through the formation of a series of uncommon Pd(II) aggregates by employing the donor in a multicomponent self-assembly of a cis-blocked 90° Pd(II) acceptor and a tetratopic donor. Some of these assemblies represent the first examples of these types of structures, and their formation is not anticipated by only taking the geometry of the donor and the acceptor building units into account. Analysis of the crystal packing of the X-ray structure revealed several H bonds between the counteranions (NO3 (-) ) and water molecules (OH⋅⋅⋅ON). Moreover, H-bonded 3D-networks of water are present in the molecular pockets, which show water-adsorption properties with some variation in water affinity. Interestingly, these complexes exhibit proton conductivity (1.87×10(-5) -6.52×10(-4) Scm(-1) ) at 296 K and low relative humidity (ca. 46 %) with activation energies of 0.29-0.46 eV. Moreover, the conductivities further increase with the enhancement of humidity. The ability of these assemblies to exhibit proton-conducting properties under low-humidity conditions makes these materials highly appealing as electrolytes in batteries and in fuel-cell applications.

Water Adsorption in Porous Metal–Organic Frameworks and Related Materials

Water adsorption in porous materials is important for many applications such as dehumidification, thermal batteries, and delivery of drinking water in remote areas. In this study, we have identified three criteria for achieving high performing porous materials for water adsorption. These criteria deal with condensation pressure of water in the pores, uptake capacity, and recyclability and water stability of the material. In search of an excellently performing porous material, we have studied and compared the water adsorption properties of 23 materials, 20 of which are metal-organic frameworks (MOFs). Among the MOFs are 10 zirconium(IV) MOFs with a subset of these, MOF-801-SC (single crystal form), -802, -805, -806, -808, -812, and -841 reported for the first time. MOF-801-P (microcrystalline powder form) was reported earlier and studied here for its water adsorption properties. MOF-812 was only made and structurally characterized but not examined for water adsorption because it is a byproduct of MOF-841 synthesis. All the new zirconium MOFs are made from the Zr6O4(OH)4(-CO2)n secondary building units (n = 6, 8, 10, or 12) and variously shaped carboxyl organic linkers to make extended porous frameworks. The permanent porosity of all 23 materials was confirmed and their water adsorption measured to reveal that MOF-801-P and MOF-841 are the highest performers based on the three criteria stated above; they are water stable, do not lose capacity after five adsorption/desorption cycles, and are easily regenerated at room temperature. An X-ray single-crystal study and a powder neutron diffraction study reveal the position of the water adsorption sites in MOF-801 and highlight the importance of the intermolecular interaction between adsorbed water molecules within the pores.

The effects of ultrasonic/microwave assisted treatment on the properties of soy protein isolate/titanium dioxide films

Soy protein isolate (SPI, 5.0 g/100 mL,) films embedded with nano-TiO2 (0, 0.5, 1.0, 1.5 and 2.0 g/100 mL) were prepared by solution casting and modified by ultrasonic/microwave assisted treatment (UMAT). The effects of nano-TiO2 content and UMAT time on the films' physical properties and structure were investigated. Incorporation of nano-TiO2 significantly enhanced films' mechanical properties and barrier properties, because of the intermolecular force between nano-TiO2 and SPI. UMAT time ≤20 min obviously improved films' tensile strength values (15.77 MPa, 245% higher than the control), and reduced water vapor permeability (1.8457 × 10−11 g cm−1 s−1 Pa−1, 72.11% lower than the control) and oxygen permeability values (0.8897 × 10−5 cm3 m−2 d−1 Pa−1, 57.66% lower than the control). SEM images also revealed a more compact and dense structure of films when UMAT time ≤20 min. Films' water adsorption properties were evaluated. GAB and Henderson models exhibited the best to fit experimental data, thus it was predicted that films (1.5 g nano-TiO2/100 ml) could be stable at low moisture content (0.27 kg of water/kg dry mass).

What makes zeolitic imidazolate frameworks hydrophobic or hydrophilic? The impact of geometry and functionalization on water adsorption

We demonstrate, by means of Grand Canonical Monte Carlo simulation on different members of the ZIF family, how topology, geometry, and linker functionalization drastically affect the water adsorption properties of these materials, tweaking the ZIF materials from hydrophobic to hydrophilic. We show that adequate functionalization of the linkers allows one to tune the host–guest interactions, even featuring dual amphiphilic materials whose pore space features both hydrophobic and hydrophilic regions. Starting from an initially hydrophobic material (ZIF-8), various degrees of hydrophilicity could be obtained, with a gradual evolution from a type V adsorption isotherm in the liquid phase to a type I isotherm in the gas phase. This behavior is similar to what was described earlier in families of hydrophobic all-silica zeolites, with hydrophilic “defects” of various strength, such as silanol nests or the presence of extra-framework cations.

Steamed Zeolites for Heat Pump Applications and Solar Driven Thermal Adsorption Storage

The influence of the dealumination degree of NaY with respect to the water adsorption properties was investigated by infrared spectroscopy, thermogravimetry and isotherm measurements. The modification of the samples is a result of a steaming process at different temperatures in dependence of time. It was found that dealumination controls the hydrophilic behavior of NaY and contributes to defined low desorption temperatures. Similar to microporous SAPO's steamed Y-zeolites can be used for low temperature applications of heat transformation and thermal adsorption storage.