Enhancement of the Sorption Ability of Aluminum Oxide Desiccants by Alkaline Modification

The influence of impregnation of aluminum oxide desiccants prepared by centrifugal thermal activation of hydrargillite with alkali (KОН and NaOH) and carbonate (Na₂CO₃ and K₂СО₃) solutions on the physicochemical properties of the products was studied. Impregnation with alkali solutions increases the dynamic capacity of the desiccants by a factor of 2 and more, whereas impregnation with carbonate solutions decreases the sorption characteristics of the desiccants at similar texture characteristics. Introduction of alkaline modifiers leads to a considerable decrease in the concentration of Lewis acid sites on the surface and to an increase in the concentration of strong base sites. Linear correlation was revealed between the concentration of strong base sites on the surface of the desiccants and their dynamic capacity in drying of humid air. The desiccants modified by impregnation exhibit not only high static and dynamic capacity, allowing improvement of the drying efficiency, but also considerably enhanced mechanical strength.     

Evaluation of acid-base properties of ammonia plasma-treated polypropylene by means of XPS

Acid-base properties of ammonia plasma-treated polypropylene (APTPP) were characterized by X-ray photoelectron spectroscopy (XPS) in conjunction with the molecular probe technique and using chloroform (TCM) as a reference Lewis acid. It is shown that TCM is retained by the basic surfaces of APTPP but not by the untreated PP. The retention of TCM is shown to be entirely due to the formation of TCM:APTPP acid-base complexes. This is supported by the C12p^3/2 binding energy (BE) and the shape of the Cl2p peak from the adsorbed TCM. ΔH^AB, the heat of TCM:APTPP acid-base interaction was found to be in the range of 3.1–4.3 kcal/mol using a published Cl2p^3/2 BE−ΔH^AB correlation. This ΔH^AB compares remarkably well with the values of 4.3–4.9 kcal/mol determined for TCM:amine complexes, and shows indeed that TCM is complexed by nitrogen containing basic groups grafted at the surface. However, the TCM/N ratio has an optimal value up to 1 second of plasma treatment and then decreases sharply, showing that less specific interaction sites are accessible at the surface for longer treatment times. This parallels previous findings about the metallization of APTPP by aluminium which was found to be optimal for treatment times lower than 1 second in our experimental conditions. This work shows that XPS can now indeed be used to quantitatively assess the acid-base properties of modified polymer surfaces.     

Effect of modifying alumina desiccants with sulfuric acid on their physicochemical properties

In the production of alumina desiccants by extrusion, the introduction of sulfuric acid at the stage of preparing a mouldable paste based on hydroxides containing bayerite or pseudoboehmite increases the sorption capacity of the product. This effect is most pronounced for the pseudoboehmite-based materials. The dynamic capacity of these desiccants increases to the level characteristic of the bayerite-containing hydroxide (>5 g H₂O/100 cm³) for a dew point of ?40°C and a contact time of 1.5 s), and their static capacity exceeds this value (increasing from 21.13 to 23.1 g H₂O/100 cm³). This procedure changes the phase composition and textural characteristics of the pseudoboehmite-based desiccants and increases Brønsted acidity and generates strong Lewis acid sites on the surface of all oxides. The dynamic capacity of desiccants with similar textural characteristics depends on the acid-base properties of their surface.     

Selective water sorbents for multiple applications, 5. LiBr confined in mesopores of silica gel: Sorption properties

This paper presents sorption properties of a selective water sorbent based on mesoporous KSKG silica gel as a host matrix and lithium bromide as a hygroscopic salt. Sorption isobars, isochores and isotherms measured at T=40–120°C and partial vapor pressures of 7.5–81.0 mbar indicated two types of water sorption: 1) formation of a solid crystalline LiBr monohydrate at low amounts of sorbed water, and 2) vapor absorption by the salt solution at higher sorptions. Sorption properties of the LiBr monohydrate are found to change significantly due to salt impregnation into the mesoporous silica gel, whereas the solution confinement to the mesopores did not change its water sorption properties as compared to the bulk solution. Desorption curves follow a first order kinetics in the temperature range of 60–130°C at different vapor pressures.     

Oxygen and oil barrier properties of microfibrillated cellulose films and coatings

The preparation of carboxymethylated microfibrillated cellulose (MFC) films by dispersion-casting from aqueous dispersions and by surface coating on base papers is described. The oxygen permeability of MFC films were studied at different relative humidity (RH). At low RH (0%), the MFC films showed very low oxygen permeability as compared with films prepared from plasticized starch, whey protein and arabinoxylan and values in the same range as that of conventional synthetic films, e.g., ethylene vinyl alcohol. At higher RH’s, the oxygen permeability increased exponentially, presumably due to the plasticizing and swelling of the carboxymethylated nanofibers by water molecules. The effect of moisture on the barrier and mechanical properties of the films was further studied using water vapor sorption isotherms and by humidity scans in dynamic mechanical analysis. The influences of the degree of nanofibrillation/dispersion on the microstructure and optical properties of the films were evaluated by field-emission scanning electron microscopy (FE-SEM) and light transmittance measurements, respectively. FE-SEM micrographs showed that the MFC films consisted of randomly assembled nanofibers with a thickness of 5–10 nm, although some larger aggregates were also formed. The use of MFC as surface coating on various base papers considerably reduced the air permeability. Environmental scanning electron microscopy (E-SEM) micrographs indicated that the MFC layer reduced sheet porosity, i.e., the dense structure formed by the nanofibers resulted in superior oil barrier properties.     

Water sorption and permeation in chitosan films: Relation between gas permeability and relative humidity

The water sorption of chitosan has been studied at 20 °C. Water transport is governed by a Fickian process for relative humidities lower than 0.4, and in that range of partial pressures, the diffusion coefficient is concentration‐dependent. At a higher activity, anomalous diffusion is observed. The sorption isotherm is well described by the Guggenheim‐Anderson‐de Boer (GAB) model, and the clustering phenomenon observed at high relative pressures can be studied with the parameters of this model. The water permeability coefficient greatly increases with the relative pressure, and the water plasticization effect leads to a loss of the gas barrier properties under wet conditions. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 3114–3127, 2001     

A sorption balance-based method to study the initial drying of dispersion droplets

The initial drying of dispersion samples with varying geometries and surface areas was continuously recorded in a sorption balance at different temperatures and relative humidities. The samples were applied as small (5–20 mg) droplets on glass. We were able to show a linear dependence of the evaporation rate on the exposed surface area of the samples if the evaporative cooling was taken into account. The method outlined in the present paper facilitates scaling and comparison of results obtained from drying experiments with different-sized dispersion samples.     

Increasing the sorption activity of carbon adsorbents by electron-beam processing and fullerene microadditives

The sorption capacity of activated carbon with respect to Cu²⁺ cations was found to be enhanced considerably upon its chemical modification with fullerenes and during its electron-beam processing. It was discovered that introducing fullerenes (20 μg/g) into activated carbon leads to a change in the chemical composition of its surface, due to changes in the system of conjugated bonds in activated carbon leading to an increase in the content of Br?nsted acid (pK _a 0–5) and weakly base (pK _a 8–11) sites capable of cation exchange on the material surface. We conclude that electron-beam processing (optimal dose, 25–50 kGy) facilitates the rearrangement of bonds on the surface of activated carbon, thereby increasing the number of Lewis base and Br?nsted acid sites capable of adsorbing metal ions in accordance with the donor-acceptor and cation-exchange mechanism, respectively.     

Sol–gel derived TiO<Subscript>2</Subscript> wood composites

The sol–gel process was applied to enhance properties of pine sapwood. For this purpose wood prisms were soaked in nanoscaled precursor solutions prepared from titanium(IV) n-butoxide and titanium(IV) iso-propoxide, respectively, using vacuum impregnation technique. The wet composites were cured by special program with final heat treatment at 103 °C. Weight percent gains (WPG) of the wood specimen in the range of 19–25% were obtained due to these procedures. SEM investigations show that precursor solutions penetrate into the whole wood body and the titania formed after heat treatment in the composites is deposited in the pores (lumen) and partly in the cell walls of the wooden matrix. The moisture sorption was investigated in long term tests for a period of some months by storage at 20–23 °C in humid air (relative humidity of 99%) and ambient atmosphere (relative humidity 40–60%), respectively. For untreated reference samples the moisture sorption results in increasing of mass and volume according to saturation values of 24 and 13%, respectively, after about 15 days. The incorporation of titania reduces the saturation values of the moisture sorption by up to 12% in mass and by up to 5% in volume at a relative humidity of 99%. Thus, an enhancement of the dimensional stability of about 60% is obtained at best. The results demonstrate that modification of wood with sol–gel derived precursors can enhance its dimensional stability, which prevents the formation of cracks. Because of that reduced moisture sorption biological attacks should be delimited. Additionally, thermal analyses show a retarded combustion of the wood matrix due to titania infiltration.     

Characterization of sorption mechanisms of solid-phase microextraction with volatile organic compounds in air samples using a linear solvation energy relationship approach

The linear solvation energy relationship (LSER) model was used to characterize interactions responsible for sorption of volatile organic compounds (VOCs) in air samples on six different solid-phase microextraction (SPME) fibers at 296K and zero relative humidity. The polydimethylsiloxane and polyacrylate fibers sorption data were also modeled at different relative humidities in the range of 10-90% and influence of water vapors on the extraction process is discussed. The LSER equations were obtained by a multiple regression of the distribution coefficients of 14 probe solutes on an appropriate SPME fiber against the solvation parameters of the solutes. The derived LSER equations successfully predicted the VOC distribution coefficients and the selectivity of individual SPME fibers for the various volatile solutes. The LSER approach coupled with SPME is a relatively simple and reliable tool to rapidly characterize the sorption mechanism of VOCs with various stationary phases and may potentially be applied to design and test new chromatographic materials for sampling or separation of VOCs.     

Arsenic sorption by modified clinoptilolite–heulandite rich tuffs

Recent works show that modified natural zeolites improve the remotion of anionic or non-polar organic pollutants from water. In this work the arsenic sorption from aqueous solutions onto clinoptilolite–heulandite rich tuffs modified with lanthanum, hexadecyltrimethylammonium or iron was investigated considering the arsenic chemical species and the pH of the arsenic solutions. Clinoptilolite–heulandite rich tuffs were characterized by scanning electron microscopy and X-ray diffraction analysis. The elemental composition of the zeolitic samples was also determined. According to the Langmuir isotherm model the arsenic (V) sorption capacity of the zeolites was 75.4 μg As/g at pH 3, 3.9 μg As/g at pH 5 and 53.6 μg As/g at pH 6, for the lanthanum, HDTMA and iron modified clinoptilolite–heulandite rich tuff from Chihuahua (México), respectively. In general, the results suggested that the arsenic retention depends on the precedence of zeolitic material, the nature of arsenic chemical species, pH as well as the characteristics of modified natural zeolites. In this work the arsenic adsorption mechanisms are also discussed.     

Efficient Solar-Driven Water Harvesting from Arid Air with Metal–Organic Frameworks Modified by Hygroscopic Salt

Freshwater scarcity is a global challenge threatening human survival, especially for people living in arid regions. Sorption-based atmospheric water harvesting (AWH) is an appealing way to solve this problem. However, the state-of-the-art AWH technologies have poor water harvesting performance in arid climates owing to the low water sorption capacity of common sorbents under low humidity conditions. We report a high-performance composite sorbent for efficient water harvesting from arid air by confining hygroscopic salt in a metal–organic framework matrix (LiCl@MIL-101(Cr)). The composite sorbent shows 0.77 g g⁻¹ water sorption capacity at 1.2 kPa vapor pressure (30 % relative humidity at 30 °C) by integrating the multi-step sorption processes of salt chemisorption, deliquescence, and solution absorption. A highly efficient AWH prototype is demonstrated with LiCl@MIL-101(Cr) that can enable the harvesting of 0.45–0.7 kg water per kilogram of material under laboratory and outdoor ambient conditions powered by natural sunlight without optical concentration and additional energy input.     

Photoinduced Electron Transfer in Host-Guest Complexes of 2-Naphthyl-O(CH2) n -adamantanamines with Mono-6-O-p-nitrobenzoyl-β-cyclodextrin and Mono-6-O-m-nitrobenzoyl-β-cyclodextrin

A number of naphthalene derivatives containing adamantanamine binding moiety and an (CH₂) _n (n=2, 3, 4, 5, 6) spacer were prepared as the electron donor. A supramolecular assembly was fabricated by the inclusion between the donor substrates and the host molecules, i.e., mono-6-O-p-nitrobenzoyl-β-cyclodextrin (pNBCD) and mono-6-O-m-nitrobenzoyl-β-cyclodextrin (mNBCD), in water. The fluorescence quenching in these systems was studied in detail. It revealed efficient photoinduced electron transfers (PET) between the naphthalene donors and the cyclodextrin acceptors. This PET process was partitioned into a dynamic quenching component caused by bimolecule collision reactions and a static quenching component due to hydrophobic binding between the donor and acceptor molecules. Detailed Stern–Volmer constants were measured and they were partitioned into dynamic Stern–Volmer quenching constants (dynamic quenching) and static binding constants (static quenching). In these two pathways, the static quenching was found to be highly efficient and dominant in the presence of NBCD.     

Sorption-fluorimetric determination of zinc

The sorption of ions of zinc on carriers modified by 8-oxychinoline and its derivative is investigated. The optimum parameters of sorption are revealed, based on which silochrome S-60 with immobilized 8-oxychinoline (pH 7.1, time of contact 30 min, weight of sorbent 0.3 g, capacity of sorbent on the modifier 60 μmol/g) is chosen. A yellow-green fluorescence sorbate arises on irradiation by ultra-violet light in a phase sorbent. The spectrum of fluorescence represents a wide unstructured strip with a maximum at 505 nm. In the construction of a calibrated graph, Cu(II), Cd(II), Pb (II), Bi(III), and Fe(III) do not prevent the determination of 5–50 μg of zinc; however, equal amounts of Al(III) (50–100 μg) do prevent it. The technique is applied for the sorption-fluorimetric determination of zinc in river and waste water. The relative standard deviation does not exceed 0.05 (n = 5).     

Effect of the Preparation Methodof Al–Mg–O Catalysts on the Selective Decomposition of Ethanol

Summary.  Selective decomposition of ethanol was used as a test reaction at 350°C to evaluate the catalytic activity of two Al–Mg–O mixed oxides prepared by two different methods (wet impregnation and coprecipitation). The catalyst precursors were examined by TG and DTA and were calcined between 500–900°C for 5 h in air. The surface area of all catalysts was measured by N₂ sorption using the BET method. The total acidity and basicity were determined by TPD using pyridine and formic acid. The catalysts were characterized by XRD analysis. It was found that the preparation method of Al–Mg–O catalyst has a great effect on the selective decomposition of ethanol. Al–Mg–O (I) catalysts, prepared by wet impregnation, were more selective towards ethene formation during dehydration of ethanol. This is ascribed to their high total surface acidity. On the other hand, Al–Mg–O (II) catalysts, prepared by coprecipitation, were highly selective in the oxidative dehydrogenation of ethanol to yield acetaldehyde. This could be attributed to their high concentration of basic sites. In addition, the production of traces of diethyl ether was also observed (three times more for Al–Mg–O (II) than for Al–Mg–O (I)). Corresponding author. E-mail: shalawy99@yahoo.com Received October 12, 2001. Accepted (revised) January 7, 2002     

Influence of water on the dielectric behaviour of chitosan films

 The dielectric properties of chitosan films with a degree of deacetylation of 86% have been investigated in the frequency range of 10^3–10⁶ Hz covering a broad range of temperatures from −150 to 150 °C. The variation of the dielectric pro-perties with temperature has been associated with two dielectric processes: (a) a local relaxation attributed to the presence of hydrogen-bonded water appearing at low temperatures (b) a conduction process related to water molecules which becomes desorbed upon heating at T>80 °C. Isothermal dielectric experiments have been performed in order to follow, in real time, the occurrence of both, the water sorption and desorption processes. Received: 20 June 1996 Accepted: 19 November 1996     

Sorption of radiocesium on soils in the presence of electrolytes

The influence of background electrolytes (KCl, NH₄Cl, CTABr) in different concentrations on the sorption ability of radiocesium by measuring the distribution coefficient has been studied. Sorption isotherms of cesium for characterization of soil sorption ability were used. Sorption of cesium depends on its concentration and at least three different sorption sites are active in the sorption process. In the case of low cesium concentration, two very selective sites with high distribution coefficients are responsible for the sorption. With increasing cesium concentration in the aqueous phase, distribution coefficient is decreasing. Frayed edge sites of illite in soil and exchangeable potassium are probably responsible for nonlinear isotherms at low cesium concentrations. From sorption isotherms and determination of potassium by activation analysis, it was found that the capacity of very selective sites for different concentrations of background electrolyte was up to 7 mmol·kg^–1.     

The water vapour sorption behaviour of three celluloses: analysis using parallel exponential kinetics and interpretation using the Kelvin-Voigt viscoelastic model

The objective of this study was to examine the water vapour sorption behaviour of three celluloses, which were originally derived from cotton fibers, using a dynamic vapour sorption apparatus, including analyses of the sorption rate and hysteresis occurring in the isotherm run. Cotton linter, α-cellulose, and microcrystalline cellulose (MCC), respectively attained equilibrium moisture contents of 14.2, 20.6, and 16.9% at a relative humidity (RH) of 95%. All three cellulosic materials exhibited sorption hysteresis to varying degrees throughout the full RH range; the MCC and α-cellulose displayed the lowest and highest total hysteresis, respectively. The sorption kinetics were analysed in terms of the parallel exponential kinetics (PEK) model, with excellent fits to the data being obtained. The PEK data is further interpreted on the basis of two Kelvin-Voigt elements operating in series. Clear differences in behaviour were seen between the α–cellulose and the other two celluloses in this study. The relative importance of capillary condensation and matrix viscoelasticity with respect to sorption hysteresis is discussed.     

Study of heat of micellization and phase separation for Pluronic aqueous solutions by using a high sensitivity differential scanning calorimetry

Heat of micellization and phase separation temperature (known as cloud point) for the poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (abbreviated by PEO–PPO–PEO) triblock copolymers, the Pluronics F108, F98, F88, F68, F38, P65, and L62, in water are carefully determined by using a high sensitivity differential scanning calorimeter. It is interesting to find out that there exists a maximum heat of micellization for all these Pluronics. In this study, the heat of micellization of all of the Pluronics decreases as the temperature increases, as expected, at high temperature region (low Pluronic concentration region). However, the enthalpy change has a surprisingly positive relationship with temperature at low temperature region (high Pluronic concentration region). The critical micelle temperature consistently decreases as the Pluronic concentration increases. This unexpected behavior of the positive heat capacity changes of Pluronic aqueous solutions at higher concentration region is somewhat related to the variation of water accessible polar (PEO groups) and non-polar (PPO groups) surface areas in the micellization process. Especially, the removal of polar surface area from water may dominate the contribution to the positive heat capacity change upon micellization. In addition, the cloud points of Pluronic solutions are also discussed. The enthalpy–entropy compensation phenomenon for the micellization of Pluronics is discussed, and the enthalpy–entropy compensation temperature is calculated.     

Efficient Solar‐Driven Water Harvesting from Arid Air with Metal–Organic Frameworks Modified by Hygroscopic Salt

Freshwater scarcity is a global challenge threatening human survival, especially for people living in arid regions. Sorption‐based atmospheric water harvesting (AWH) is an appealing way to solve this problem. However, the state‐of‐the‐art AWH technologies have poor water harvesting performance in arid climates owing to the low water sorption capacity of common sorbents under low humidity conditions. We report a high‐performance composite sorbent for efficient water harvesting from arid air by confining hygroscopic salt in a metal–organic framework matrix (LiCl@MIL‐101(Cr)). The composite sorbent shows 0.77 g g^?1 water sorption capacity at 1.2 kPa vapor pressure (30 % relative humidity at 30 °C) by integrating the multi‐step sorption processes of salt chemisorption, deliquescence, and solution absorption. A highly efficient AWH prototype is demonstrated with LiCl@MIL‐101(Cr) that can enable the harvesting of 0.45–0.7 kg water per kilogram of material under laboratory and outdoor ambient conditions powered by natural sunlight without optical concentration and additional energy input.