Relative Pressure Range Research Articles

Entropy in multiple equilibria. Argon and nitrogen adsorption isotherms of nonporous, microporous, and mesoporous materials

Analysis of multiple equilibria of compounds with different coordination sites is extended to the description of adsorption isotherms with focus on the low relative pressure range. The entropy evolution is described using the particle distribution theory which also holds for adsorbents consisting of materials bearing more than one type of sites and applies for the condition that the adsorptive-adsorbent binding strength is larger than the adsorptive-adsorbate binding strength, so that monolayer coverage is favored. This allows to accurately determine the adsorption enthalpy. No assumption concerning the growth mechanism and specifics regarding the structure of the surface is needed. We find on a rigorous basis that this leads to Langmuir's equation for each site independently, that the total fractional amount of bound adsorptive can be described as a linear combination of individual Langmuir isotherms, and that such a linear combination has never the shape of the original Langmuir isotherms. The results are successfully applied to argon and nitrogen adsorption isotherms of nonporous, microporous, and mesoporous adsorbents which allows comparing systems for which the properties of the active surface span a large range. We observe that all experimental data can accurately be described by means of a linear combination of two Langmuir isotherms in the low relative pressure range up to a coverage of 60%–95%. This means that the shape of all adsorption isotherms is essentially determined by the entropy decrease with increasing coverage. The two site interactions involved exhibit substantially different adsorption enthalpies. Interestingly the Ar enthalpy of adsorption Δ a d s H 1 ∅ of the sites 1 for the Stöber-type silica and of the three investigated MCM-41 adsorbents (with pore size of 2.7 nm, 4.1 nm, and 4.4 nm) are similar, namely −11 kJ/mol. The situation is analogous for the enthalpy of adsorption Δ a d s H 2 ∅ for the sites 2, which amounts to −8 kJ/mol. A significantly larger enthalpy of adsorption Δ a d s H 1 ∅ for the sites 1, namely −14.3 kJ/mol, and Δ a d s H 2 ∅ = − 11.7 kJ / mol for the sites 2 is measured for potassium zeolite L thus reflecting the more polar nature of this adsorbent. The measured specific surface area for these samples ranges from 14 m 2 /g for the Stöber-type silica up to 1100 m 2 /g for the MCM-41(4.1 nm) adsorbent. The information provided by the lc2-L (linear combination of 2 Langmuir functions) analysis allows calculating the evolution of the coverage of site 1 and of site 2 as a function of increasing pressure. The inflection points of the isotherms, which mark the point where the curvature changes sign, were determined by numerically evaluating the second derivatives which vanish at this point and are compared with values obtained using BET analysis.

The influence of the ammonia concentration and the water content on the water sorption behavior of ambient pressure dried silica xerogels

Porous silica xerogels were synthesized within 10 h by a two-step sol–gel process under atmospheric conditions. In the first step, tetraethylorthosiloxane (TEOS) was hydrolyzed with water using sulfuric acid as a catalyst. In the second step, water and ammonia were added to the prehydrolyzed silica sol prior to the drying of the samples at 150 °C. The influence of the ammonia concentration and the water content on the physicochemical properties and the water sorption behavior of silica xerogels produced in the aforementioned way was investigated. The resulting silica xerogels were characterized by helium pycnometry, scanning electron microscopy, fourier-transform infrared spectroscopy, nitrogen sorption, and water sorption. It was shown that a higher ammonia concentration leads to an increased pore size of the silica xerogel which starts to adsorb water at a higher relative pressure. With an increased water content during gelation, the silica xerogel has a higher specific surface area and a higher water sorption capacity. Furthermore, silica xerogels can be tailored by a controlled addition of ammonia and water to have a higher water sorption capacity than the reference silica gel over the whole range of relative pressure.

Адсорбционные характеристики материалов ограждающих конструкций

Знание численных значений адсорбционных харак­теристик материалов ограждающих конструкций зданий необходимо для проведения расчётов теплотехнических показателей этих конструкций. Адсорбционными характери­стиками материала являются: константа с уравнения Брунау- эра-Эммета-Тейлора (БЭТ) сорбции паров воды, равновесная сорбционная влажность wm материала, соответствующая за­полнению первого монослоя адсорбированных молекул воды (ёмкость монослоя), относительной влажности воздуха фт , соответствующей заполнению первого монослоя, и удельная поверхности А материала. Существует два метода обработки изотерм сорбции паров воды исследуемым материалом с це­лью определения этих характеристик. Первый метод основан на использовании уравнения БЭТ. Второй метод, названный Ж-метод, представляет собой модификацию первого.
 Уравнение БЭТ применимо для анализа изотермы сорб­ции только в диапазоне относительных давлений адсорбата от 0,05 до 0,35. N-метод применим для анализа изотермы сорбции в диапазоне относительных влажности воздуха от 0,05 до 0,5. Таким образом, основное преимущество N-метода по сравнению с методом, основанным на уравнении БЭТ, заключается в том, что N-метод позволяет получить более точную информацию об адсорбционных характеристиках исследуемого материала, так как эту информацию получают путём обработки большего числа экспериментально полу­ченных значений равновесной сорбционной влажности этого материала.
 С целью сопоставления двух вышеупомянутых методов по этим методам были обработаны полученные на вакуумной сорбционной установке при температуре +20 °С изотермы сорбции восьми строительных материалов: цементного камня, тяжёлого бетона, керамзитобетона, азеритобетона, красного кирпича, цементно-песчаного раствора и пенобетона. По ре­зультатам обработки были определены адсорбционные харак­теристики и получены уравнения начальных участков изотерм сорбции исследованных материалов при температуре +20 °С.

Adsorption cooling system employing novel MIL-101(Cr)/CaCl2 composites: Numerical study

Due to the significant increase in the global temperature, the demand for cooling is dramatically rising. Most of this demand is met by conventional systems driven by electricity generated using fossil fuels contributing in the global warming phenomenon. The adsorption system is a sustainable system being driven by waste or low-grade heat sources such as solar energy. MIL-101(Cr) is a metal–organic framework (MOF) material with exceptional properties and high-water uptake. Nevertheless, it is not suitable for adsorption cooling application as the high capacity is taking place only at high relative pressure range (>0.5). The water adsorption characteristics of MIL-101(Cr) were significantly enhanced through incorporating the material with calcium chloride. Results showed that at a desorption temperature of 90 °C and a chilled water inlet temperature of 10 °C, the SCP increased from 168 for the neat MIL-101(Cr) to 248 and 388 W kg‒1 for Comp_1:5 and Comp_1:8 CaCl2 composites, respectively outperforming the long dominating silica gel.

Ultrasound-assisted Synthesis and Characterization of Silicon Centered Porous Aromatic Frameworks

Silicon centered porous aromatic frameworks( Si-PAF) was synthesized by ultrasound-assisted Yamamoto coupling reaction. The ultrasound amplitude and reaction time were adjusted to optimize the reaction parameters. The obtained Si-PAF exhibits high chemical stability. It is insoluble in common organic solvents such as CHCl3,tetrahydrofuran( THF),ethanol,acetone and dimethylformamide( DMF). The polymer was characterized by Fourier transform infrared spectroscopy( FTIR),scanning electron microscopy( SEM),highresolution transmission electron microscopy( TEM),powder X-ray diffraction( PXRD),and N2 gas absorption.From the disappearance of the C—Br bands( at 529 cm-1) in the FTIR spectra,the formation of the product can be confirmed. SEM image shows that Si-PAF affords a spherical morphology. Simultaneously,TEM reveals that it possesses a worm-like texture. PXRD result indicates the amorphous texture which might be due to distortion and interpenetration of the phenyl rings. The N2 sorption isotherms were measured at 77 K to characterize the porosity of the Si-PAF networks. All networks give rise to typeⅠ nitrogen gas adsorption isotherms with a distinct hysteresis for the whole range of relative pressure. Si-PAF exhibits high surface areas( up to 601 m2/g) and good adsorption ability of N2.

High-Pressure in Situ 129Xe NMR Spectroscopy: Insights into Switching Mechanisms of Flexible Metal-Organic Frameworks Isoreticular to DUT-49.

Flexible metal–organic frameworks (MOFs) are capable of changing their crystal structure as a function of external stimuli such as pressure, temperature, and type of adsorbed guest species. DUT-49 is the first MOF exhibiting structural transitions accompanied by the counterintuitive phenomenon of negative gas adsorption. Here, we present high-pressure in situ 129Xe NMR spectroscopic studies of a novel isoreticular MOF family based on DUT-49. These porous materials differ only in the length of their organic linkers causing changes in pore size and elasticity. The series encompasses both, purely microporous materials as well as materials with both micropores and small mesopores. The chemical shift of the adsorbed xenon depends on xenon–wall interactions and thus on the pore size of the material. The xenon adsorption behavior of different MOFs can be observed over the whole range of relative pressure. Chemical shift adsorption/desorption isotherms closely resembling the conventional, uptake-measurement-based isotherms were obtained at 237 K where all materials are rigid. The comparable chemical environment of the adsorbed xenon in these isoreticular MOFs allows to establish a correlation between the chemical shift at a relative pressure of p/p0 = 1.0 and the mean pore diameter. Furthermore, the xenon adsorption behavior of MOFs is studied also at 200 K. Here, structural flexibility is found for DUT-50, a material with an even longer linker than that of the previously known DUT-49. Its structural transitions are monitored by 129Xe NMR spectroscopy. This compound is the second known MOF showing the phenomenon of negative gas adsorption. Further increase in the linker length results in DUT-151, a material with an interpenetrated network topology. In situ 129Xe NMR spectroscopy proves that this material exhibits another type of flexibility compared to DUT-49 and DUT-50. Further surprising observations are made for DUT-46. Volumetric xenon adsorption measurements show that this nonflexible microporous material does not exhibit any hysteresis. In contrast, the in situ 129Xe NMR spectroscopically detected xenon chemical shift isotherms exhibit a hysteresis even after longer equilibration times than in the volumetric experiments. This indicates kinetically hindered redistribution processes and long-lived metastable states of adsorbed xenon within the MOF persisting at the time scale of hours or longer.

Carbon–GO Composites with Preferential Water versus Ethanol Uptake

The elimination of small amounts of water from alcohols is by no means a trivial issue in many practical applications like, for instance, the dehumidification of biocombustibles. The use of carbonaceous materials as sorbents has been far less explored than that of other materials because their hydrophobic character has typically limited their water uptake. Herein, we designed a synthetic process based on the use of eutectic mixtures that allowed the homogeneous dispersion of graphene oxide (GO) in the liquid containing the carbon precursor, e.g., furfuryl alcohol. Thus, after polymerization and a subsequent carbonization process, we were able to obtain porous carbon-GO composites where the combination of pore diameter and surface hydrophilicity provided a remarkable capacity for water uptake but extremely low methanol and ethanol uptake along the entire range of relative pressures evaluated in this work. Both the neat water uptake and the uptake difference between water and either methanol or ethanol of our carbon-GO composites were similar or eventually better than the uptake previously reported for other materials, also exhibiting preferential water-to-alcohol adsorption, e.g., porous coordination polymers, metal-organic frameworks, polyoxometalates, and covalent two-dimensional nanosheets embedded in a polymer matrix. Moreover, water versus alcohol uptake was particularly remarkable at low partial pressures in our carbon-GO composites.

MIL-101(Cr)/calcium chloride composites for enhanced adsorption cooling and water desalination

The adsorption capacity of adsorbent materials at low relative pressure range is a crucial parameter that guarantee the high overall performance and efficiency of adsorption cooling and water desalination systems. MIL-101(Cr) is a metal-organic framework (MOF) material with a high-water vapour uptake of 1.36–1.47 gH2O gads−1. However, MIL-101(Cr) exhibits an IV isotherm (S- shaped isotherm) which means that the high-water uptake only takes place at high relative pressure (≥0.5). This drawback makes MIL-101(Cr) impractical for adsorption applications working at lower relative pressure such as adsorption cooling and desalination with cooling effect which operate at relative pressures ≤0.4. In this work, MIL-101(Cr)/CaCl2 composites were synthesized, the unsupported and CaCl2 supported materials were fully characterized in terms of their structure (XRD), thermal stability, vibrational spectroscopy structure, morphology (SEM), water adsorption capacity and BET surface area. As a result, incorporating CaCl2 into the MIL-101(Cr) structure significantly enhanced water adsorption characteristics in the desired relative pressure range as the water vapour uptake for Comp_1:8 CaCl2 composite increased from only 0.1 gH2O gads−1 for as-synthesized MIL-101(Cr) to 0.65 gH2O gads−1 at a relative pressure of 0.3.

Decoding gas-solid interaction effects on adsorption isotherm shape: II. Polar adsorptives

A unique set of 6 polar adsorptives of relatively large dipole moment and of increasing kinetic diameter were used to probe pore volumes available and their mechanism of adsorption on a well-characterised microporous carbon. Multiple adsorption isotherm measurements were made and repeatable results with relatively small standard deviations in amount adsorbed at low relative pressures were obtained. Inconsistencies were observed between calculated Gurvitsch volumes. Sources of these were analysed and identified as contributions from one or more of: (a) molecular sieve effects; (b) molecular packing effects, and; (c) 2D molecular structure formation due to hydrogen bonding. These inconsistencies were further studied by comparison with pore volumes derived via the Dubinin-Radushkevich (DR) equation. Qualitative analyses of the micropore filling processes were proposed, and substantiated by complementary DR analyses. Although most of the isotherms showed Type I character, recasting the relative pressure axis in logarithmic format highlighted clear differences as contributions from fluid-fluid and fluid-solid interactions during pore filling. Overall, the adsorptives were classified into three groups: (a) polar adsorptives with primarily specific interactions adsorbing as a condensation process over a relatively narrow relative pressure range in a medium and late pressure range (iso-PrOH, MeOH, 2-methyl, 2-butanol, H2O); (b) polar adsorptives with potential for non-specific interactions adsorbing as a condensation process over a relatively narrow pressure range in a medium pressure range (pyridine, iso-PrOH, 2-methyl, 2-butanol); and, (c) halogenated adsorptives adsorbing with an S-shaped uptake extending over a broad relative pressure (dichloromethane).

Fractal Characterization of Nanopore Structure in Shale, Tight Sandstone and Mudstone from the Ordos Basin of China Using Nitrogen Adsorption

The characteristics of the nanopore structure in shale, tight sandstone and mudstone from the Ordos Basin of China were investigated by X-ray diffraction (XRD) analysis, porosity and permeability tests and low-pressure nitrogen adsorption experiments. Fractal dimensions D1 and D2 were determined from the low relative pressure range (0 < P/P0 < 0.4) and the high relative pressure range (0.4 < P/P0 < 1) of nitrogen adsorption data, respectively, using the Frenkel–Halsey–Hill (FHH) model. Relationships between pore structure parameters, mineral compositions and fractal dimensions were investigated. According to the International Union of Pure and Applied Chemistry (IUPAC) isotherm classification standard, the morphologies of the nitrogen adsorption curves of these 14 samples belong to the H2 and H3 types. Relationships among average pore diameter, Brunner-Emmet-Teller (BET) specific surface area, pore volume, porosity and permeability have been discussed. The heterogeneities of shale nanopore structures were verified, and nanopore size mainly concentrates under 30 nm. The average fractal dimension D1 of all the samples is 2.1187, varying from 1.1755 to 2.6122, and the average fractal dimension D2 is 2.4645, with the range from 2.2144 to 2.7362. Compared with D1, D2 has stronger relationships with pore structure parameters, and can be used for analyzing pore structure characteristics.

Ultrahigh benzene adsorption capacity of graphene-MOF composite fabricated via MOF crystallization in 3D mesoporous graphene

Highly porous composites were synthesized via facile crystallization of a metal-organic framework (MOF) in mesopores of a three-dimensional (3D) graphene (MG). First, MG was prepared via facile thermal exfoliation of graphite oxide in air, which enabled sufficient separation of graphene sheets in the form of an interconnected 3D structure. Next, aluminum-based MOF [MOF520: Al8(OH)8(BTB)4(HCOO)4, BTB = 4,4′,4”-benzene-1,3,5-tryil-tribenzoate] was predominantly crystallized in mesopores of 3D structure having a large pore volume. The resulting MG-MOF composites possessed specific surface areas and pore volumes between those of MG (640 m2/g and 3.0 cm3/g) and MOF (3160 m2/g and 1.3 cm3/g). The composite consisting of MG and MOF with a weight ratio of 1:2 showed almost twice-larger benzene adsorption capacity, i.e., 24.5 mmol/g at 20 °C and relative pressure close to unity, in comparison to that of pure MOF. Although at the same conditions the pure MG captured an ultrahigh amount of benzene, i.e., 33.6 mmol/g, adsorption of this adsorbate at low and moderate relative pressures was much lower than that on pure MOF. Thus, the MG-MOF composites featured much better benzene adsorption performance in a wide range of relative pressures as compared to pure MG and MOF, respectively. Additionally, the composites featured better thermal stability in comparison to the pristine MOF. The use of MG in this synthesis method afforded the well-dispersed, small crystals of MOF, which is essential for the design of MOF-based composites.

Water-wax behaviour in porous silica at low temperature Fischer-Tropsch conditions

Water is a major product of Fischer-Tropsch synthesis, and hence the behaviour of water within Fischer-Tropsch synthesis catalysts and its potential influence on catalyst rate and selectivity are questions of long-standing interest. The present work applies three different magnetic resonance techniques to study how water interacts with a model wax, n-octacosane, within the pore space of a porous silica of mean pore size ∼18 nm. 1H magnetic resonance spectroscopy, spin-lattice relaxation time and pulsed-field gradient measurements were performed at 195 °C, and for water pressure in the range 3–13.6 bar, conditions relevant to low temperature Fischer-Tropsch synthesis. The uptake of water within this system is shown to be very similar to that observed for capillary condensation of water within the empty pore space of the same porous silica under the same experimental conditions; suggesting that capillary condensation of water within the wax-saturated pores is occurring. The behaviour of water is characterised by two regimes. At low water relative pressures of ∼0.3 ≤ P/P0 ≤ ∼0.8 water moves into the pore space, displacing wax from the pore surface and existing as a water-rich layer between the pore surface and an oil-rich phase in the centre of the pore; the strong interaction with the pore surface is evidenced by the short nuclear spin relaxation time values of water at the lowest pressures which then increase slightly as multi-layer adsorption at the pore surface occurs with increase in pressure. In the water relative pressure range ∼0.8 ≤ P/P0 ≤ ∼0.97, condensation of water within the pores is observed, characterised by increases in both spin-lattice relaxation time and molecular diffusivity. Analysis of the data suggests that as much as ∼40% of the pore surface is occupied by condensed water after condensation has occurred. It is suggested that these two regimes of water behaviour inside initially wax-filled pores might explain previously reported aspects of the behaviour of Fischer-Tropsch catalyst performance as a function of pore size and amount of co-fed water.

Explaining water adsorption in one-dimensional channels in AlPO4-5 on molecular scale

Abstract The adsorption of water in one-dimensional channels in porous aluminophosphate material AlPO4-5 has been studied by a combination of gravimetric analysis techniques, neutron and X-ray diffraction and neutron spectroscopy. Molecular structure of AlPO4-5 consists of 12-membered rings of alternating, corner-sharing AlO4 and PO4 tetrahedra connected by oxygen bridges into sheets in the (a, b) plane. The sheets are connected by oxygen bridges along the c crystal axis and form one-dimensional channels of which the largest, formed by the 12-membered rings, have a van der Waals diameter of about 8.3 A. Gradually increasing the amount of adsorbed water we could follow the evolution of the confined water mobility in a systematic way and identify the molecular mechanism of water adsorption. We focused particularly on the range of low and medium relative pressures up to p/p0 = 0.32, where a change from a hydrophobic behavior to a steep, capillary condensation like water intake has been observed. At the initial adsorption stages water occupies positions close to the pore walls causing the contraction of channels in the (a, b) plane and the prolongation of the channels along c axis in AlPO4-5. With the progressing intake water molecules form chains along the main channels. The cooperative interactions between water molecules lead to the onset of phonon-like cooperative modes and, surprisingly, to the increase of diffusive-like motion, which slow down only in the final adsorption stages when AlPO4-5 channels are completely filled.

Adsorption and diffusion of alkylbenzene in microspherical ZSM-5 zeolite assembled with nanocrystals

Adsorption isotherms and zero length column (ZLC) desorption curves of alkylbenzene (toluene and isopropylbenzene) in a series of microspherical ZSM-5 zeolites with inter- and intracrystalline mesopores were measured for revealing structural and surface characteristics of the hierarchical zeolites. The isotherms showed that adsorption capacities of toluene and isopropylbenzene increase with increase of mesoporosities in the range of higher relative pressure. Henry’s constants (KH) and initial heats of adsorption (Qst) turned out that introduction of mesoporous structure into the zeolites weakens interaction between alkylbenzene molecules and zeolite surface. Moreover, isopropylbenzene and toluene have different adsorption and diffusion behavior in the zeolites. The adsorption capacity, KH and Qst for isopropylbenzene to microporous and mesoporous ZSM-5 zeolites are lower than these for toluene. The effective diffusion time constants (Deff/R2) of alkylbenzene from the ZLC desorption curves increase with increase of mesoporosities in the ZSM-5 zeolites, but diffusion activation energy (Ea) expresses a contrary tendency. Due to low interaction between isopropylbenzene and zeolite surface, Deff/R2 values of isopropylbenzene with lower activation energy in all ZSM-5 samples are higher than these of toluene. The molecular sizes of two alkylbenzenes are not the key factor for their adsorption and diffusion in the mesozeolites.

Modifying the BET model for accurately determining specific surface area and surface energy components of aggregates

When adopting the vapor adsorption method to measure the surface energy components of aggregates, a critical step is to determine the specific surface area (SSA) using the traditionally-used BET (T-BET) model. However, this model is demonstrated to be valid only for a limited relative pressure range of 0.05–0.40. Furthermore, the determined total SSA from this model is arbitrarily utilized to calculate the spreading pressure of probe vapors on aggregates, which does not comply with the multilayer adsorption theory. All these would definitely lead to the erroneous values of SSA and surface energy components of aggregates.To overcome these deficiencies, this study develops a modified BET (M−BET) model in the light of the multilayer adsorption theory, based on which five steps are designed to determine the SSA and the surface energy components for aggregates. The vapor adsorption tests of three probe vapors are performed on four types of aggregates. The SSA, spreading pressure and surface energy components are then separately calculated using the two BET models. It is observed that the linear fit of M-BET model applies satisfactorily to the entire adsorption isotherm with the R2 value larger than 0.98. The adsorption heat of the probe vapor in the second to the ith layer is demonstrated to be less than the heat of liquefaction, which accounts for the derivations of the data points of the T-BET model from linearity over the limited pressure range. The accompanying calculation results of the M-BET model reveal significant differences from those of the T-BET model, while the separation of SSA of the non-zero adsorption site from the total SSA also produces a non-negligible effect on the calculated surface energy components, identifying the significance of accurate measuring the SSA. Thus, in the vapor adsorption method it is imperative to employ the M-BET model to determine the SSA of the non-zero adsorption site so as to obtain the accurate surface energy components for aggregates.

Pore structure of kapok fiber

Kapok is a natural fiber with an extremely high degree of hollowness. Numerous pores in its cell wall form the pathways for gas molecules. The delamination of the cell wall in scanning electron microscopy images and the hysteresis loop in nitrogen adsorption–desorption isotherms are interpreted to indicate that the kapok fiber has slit-shaped pores. Nitrogen adsorption method based on Brunauer–Emmett–Teller theory was used to analyze the mesopore. The specific surface area of kapok fiber is evaluated as 2.99 m2/g in the relative pressure range (0.05–0.35). The pore size of the fiber is distributed in 4–40 nm and mainly concentrated on 3–4 nm. The volume of the pores with diameters of 2–40 nm accounted for about 80% of the total pore volume, and those with diameters over 40 nm accounted for about 20%. By the Va ~ αs micropore evaluating method, it is found that the micropore volume of the fiber is about 7.0399×10−11 ml/g.

Metal-Organic Frameworks for Water Harvesting from Air.

Water harvesting from air in passive, adsorption-based devices holds great potential for delivering drinking water to arid regions of the world. This technology requires adsorbents that can be tailored for a maximum working capacity, temperature response, and the relative pressure range in which reversible adsorption occurs. In this respect, metal-organic frameworks (MOFs) are promising, owing to their structural diversity and the precision of their functionalization for adjusting both pore size and hydrophilicity, thereby facilitating the rational design of their water-sorption characteristics. Here, chemical and structural factors crucial for the design of hydrolytically stable MOFs for water adsorption are discussed. Prevalent water adsorption mechanisms in micro- and mesoporous MOFs alongside strategies for fine-tuning of their adsorption behavior by means of reticular chemistry are presented. Finally, an approach for the selection of promising MOFs with respect to water harvesting from air is proposed and design concepts for next-generation MOFs for application in passive adsorption-based water-harvesting devices are outlined.

Porosity controls and fractal disposition of organic-rich Permian shales using low-pressure adsorption techniques

The pore structure characteristics of the Lower and Upper Permian shales belonging to the Barren Measures and Raniganj Formations, respectively, were investigated using the low-pressure N2 adsorption-desorption experiments. It was found that the kaolinite content of the Barren Measures shales strongly influenced the Brunauer-Emmett-Teller specific surface area (BET SSA). However, it was the Rock-Eval temperature maxima (Tmax) for the Raniganj Formation shales that influenced the BET SSA values. These shales are dominantly mesoporous and display a negative correlation between BET SSA and average pore radius. Nitrogen adsorption-desorption isotherms are of type IIB and type IV, displaying H2, H3, and hybrid H3-H4 hysteresis patterns. A strong positive correlation exists between average pore radius and the difference in volumes of gas adsorbed at the last-two-highest relative pressures measured. Samples with steeper isotherm slopes at the higher relative pressure range were those with the highest average pore radii. Porosity fractal dimension, D2 displayed a positive correlation with BET SSA and Tmax, and a negative correlation with average pore radius. It is thus concluded that shales with the lowest average pore sizes and highest thermal maturities are marked by larger SSA and more complex pore structures. One of the tested samples (CG 1019) with the highest D2 value is associated with the lowest D1 fractal dimension value. That counter intuitive relationship may reflect analytical constraints of the nitrogen adsorption method at lower relative pressures.

Decoding gas-solid interaction effects on adsorption isotherm shape: I. Non-polar adsorptives

A suite of non-polar adsorptives of different kinetic diameters and shape were used to determine adsorption and pore filling mechanism of a well-characterised poly(furfuryl alcohol)-based activated carbon. Triplicate measured Type I adsorption isotherms for each adsorptive were averaged to provide standard deviation in relative pressures and associated amounts adsorbed. Plateau amounts adsorbed for N2, Ar, CH4, and C6H6, provided Gurvitsch volumes averaged to 0.368 ± 0.015 cm3(liq)/g. The calculated Gurvitsch volumes were compared with those derived via the Dubinin-Radushkevich (DR) equation. Additional adsorptives were CO2, iso-butane and SF6. The results of these 7 adsorptives were used to qualitatively analyse and decode a micropore filling adsorption mechanism. The DR equation was also used for further analysis of the pore filling mechanism. Based on the adsorbate-adsorbate and adsorbate-adsorbent interactions, adsorbates were classified into three groups: (a) Non-polar with non-specific interactions (no dipole, no quadrupole, not readily polarizable: Ar, N2, CH4 and iso-butane), adsorbing as a continuous uptake over the observed relative pressure range; (b) Non-polar adsorptives with potential for specific interactions (no dipole, quadrupole moment: CO2 and C6H6), adsorbing as a condensation process over a relatively narrow relative pressure range in a medium pressure range; (c) Halogenated adsorptives (no dipole, no quadrupole, polarizable: SF6), adsorbing with an S-shaped uptake extending over a relatively broad relative pressure range.

Water adsorption capacity enhancement of ferroaluminophosphate (FAPO4-5) by impregnation of CaCl2

New composite materials were prepared by impregnating CaCl2 into FAPO4-5 to enhance the water adsorption capacity of the FAPO4-5 and limit the deliquescence of CaCl2. The structure of the FAPO4-5 was not changed after impregnation, as confirmed by the corresponding X-ray diffraction (XRD) patterns. The composite materials exhibited enhanced water adsorption capacity (up to 08 g/g-sorbent at 50.0 wt% loading) within the working relative pressure range of water vapor of adsorption heat pump (AHP) systems (0.1–0.3 of water vapor relative pressure). The composite material with CaCl2 loading up to 15.9 wt% can also minimizes deliquescence of the salt which is not preferable in AHP applications. Despite partial pore filling of CaCl2 in the matrix, the adsorption capacities of the composites were comparable to the sum of the adsorption capacities of CaCl2 and FAPO4-5 without significantly affecting that of the pristine FAPO4-5.