Isosteric Enthalpy Of Adsorption Research Articles

Influence of temperature on water vapour sorption isotherms and kinetics of hardened cement paste and concrete

Water vapour sorption isotherms (WVSIs) have contributed significantly towards current knowledge of the porous nanostructure of cementitious materials and are an important component in modern durability calculations. In this paper, two newly developed multi-chamber testing apparatus for determining WVSIs of hardened cementitious materials are described. The adsorption, desorption and scanning behaviour of hardened cement paste and concrete are studied in detail over temperatures of 23°C to 80°C. Experiments provide confirmation of a monotonic reduction in sorption hysteresis with increasing temperature and show a disappearance of low humidity hysteresis by 60°C. Time and history dependence of apparent isosteric enthalpy of adsorption is demonstrated. Typically overlooked anomalous (non-Fickian) sorption kinetics are observed to significantly influence the equilibrium process. The magnitude and effect of temperature and humidity on kinetics is examined. Evidence supporting the role of microstructural change in observed size-independent kinetics and WVSI changes with temperature are discussed.

Organic adsorbates have higher affinities to fluorographene than to graphene

The large surfaces of two-dimensional carbon-based materials, such as graphene and fluorographene, are exposed to analytes, impurities and other guest molecules, so an understanding of the strength and nature of the molecule–surface interaction is essential for their practical applications. Using inverse gas chromatography, we determined the isosteric adsorption enthalpies and entropies of six volatile organic molecules (benzene, toluene, cyclohexane, n-hexane, 1,4-dioxane, and nitromethane) to graphene and graphite fluoride powders. The adsorption entropies of the molecules ranged from −17 to −34cal/molK and the maximum adsorption-induced entropy loss occurred for nitromethane at the high-energy sites. The enthalpies of bulkier adsorbates were almost coverage-independent on both surfaces and ranged from −11 to −14kcal/mol. Despite the fact that fluorographene has lower surface energy than graphene and graphene represents an ideal surface for the π–π stacking, the adsorbates had lower adsorption enthalpies to fluorographene than to graphene by ∼9%. These findings imply that bulkier airbone organic contaminants readily adsorb to the investigated surfaces and can modify the measured surface properties.

Adsorption of Cefocelis Hydrochloride on Macroporous Resin: Kinetics, Equilibrium, and Thermodynamic Studies

The adsorptions of Cefocelis hydrochloride in aqueous solution on macroporous resin (HP-20) were studied. On the basis of static experiments with HP-20 resin, the adsorption kinetics, isotherms, and thermodynamics of Cefocelis hydrochloride in aqueous solution on macroporous resin (HP-20) were investigated. Adsorption equilibrium data were correlated with the Langmuir and Freundlich equations. Adsorption data of kinetic were modeled using the Crank equation and fitting for the diffusion coefficient (De), and the value of the liquid film mass transfer coefficient (kf) was estimated from Carberry equation. Dynamic adsorption was performed on HP-20 resin packed in a glass column to obtain optimal parameters. The initial concentration, bed height, and residence time were considered to determine the dynamic test for the breakthrough curve. The values of thermodynamic parameters including the isosteric adsorption enthalpy (ΔH), free energy (ΔG), entropy (ΔS), and adsorption activation energy (Ea) demonstrated t...

NH2-MIL-125 as a promising material for adsorptive heat transformation and storage

With increasing demand for heating and cooling, the effective utilization of renewable and waste thermal energy becomes a significant challenge. Due to its high energy saving potential AHT (Adsorption Heat Transformation) has gained more and more interest during the last three decades. Large advances in the enhancement of AHT performance can be achieved with the development of new adsorbents. In this paper, the results of a comprehensive study on water adsorption on NH2-MIL-125 and evaluation of the feasibility of the “NH2-MIL-125 – water” working pair for AHT are presented. It is shown that NH2-MIL-125 exchanges 0.39 g H2O g−1 under typical adsorptive chilling cycle conditions, which exceeds the uptake variation for common and innovative adsorbents. NH2-MIL-125 can be regenerated at 348 K which allows use of low-temperature heat sources. The isosteric enthalpy of water adsorption varies from −49.7 ± 1.0 kJ mol−1 to −54.8 ± 2.0 kJ mol−1 at an uptake of 0.03–0.40 g g−1. The adsorption capacity and porosity of NH2-MIL-125 change slightly during the first adsorption cycle and then remain constant. Based on the data obtained, the efficiency and specific power of the chilling cycle utilizing the “NH2-MIL-125 – water” pair were assessed as 0.77–0.80 and 3.2 kW kg−1, respectively, which is of high practical interest.

A comparative study of several microporous materials to store methane by adsorption

Methane adsorption was studied in microporous materials with differences in their pore geometry and surface chemistry in order to evaluate the characteristics that enhance methane storage in porous materials. The studied materials included metal-organic frameworks (Cu-BTC, MIL-53 and ZIF-8), zeolites (5A and 13X) and an activated carbon (Maxsorb). Textural properties such as pore volume and specific surface area were studied by nitrogen adsorption–desorption isotherms. Methane adsorption was analyzed by means of measurements at low and high pressures (0–5000 kPa) at various temperatures (278–323 K). The effect of the adsorbate–adsorbent interaction was evaluated by the determination of the Henry constant using the adsorption data of methane at low pressures and 298 K. In addition, the isosteric enthalpy of adsorption using data of adsorption at several temperatures was obtained. A linear trend was observed between the Henry constant, the enthalpy of adsorption and the pore size, indicating that the latter is the most influential factor in the interaction of a microporous adsorbent and methane at room temperature. Also, it was found that the textural properties (i.e. specific surface and micropore volume) are directly related with the methane storage capacity at 298 K and 3.5 MPa. However, we obtained a deviation of this behavior for ZIF-8 that is related to its geometry and pore size.

Crystal engineering of a family of hybrid ultramicroporous materials based upon interpenetration and dichromate linkers.

A new family of 2-fold interpenetrated primitive cubic (pcu) networks of formula [M(L)2(Cr2O7)] n (M = Co2+, Ni2+, Cu2+ and Zn2+; L = 4,4'-azopyridine), DICRO-3-M-i, has been synthesised and their structures, permanent porosity and gas sorption properties were comprehensively characterised. Molecular simulations indicate that CO2 molecules occupy both of the two distinct ultramicropores that run through this isostructural series. The orientation of the Cr2O72- pillars is thought to contribute to high isosteric enthalpy of adsorption (Qst) towards CO2 and temperature programmed desorption experiments reveal that DICRO-3-Ni-i selectively adsorbs CO2 from gas mixtures that simulate flue gas. Performance in this context is among the highest for physisorbents measured to date and these materials are readily regenerated at 50 °C.

English

A new approach using reed pulping black liquor by hydrothermal reaction followed by fast carbonization at 350 degrees C in fluidized bed reactor for 5 minutes was proposed in this paper. The BET specific surface area generated at optimum conditions reached 390 m(2) g(-1). The CO2 selectivity over CH4 and N-2 comparable to commercial activated carbons was obtained. The isosteric enthalpies of adsorption for CO2, CH4 and N-2 on the resultant carbon sample prepared at optimum conditions show surface heterogeneity. The kinetic results indicate its potential for separation of carbon dioxide from carbon dioxide/methane mixture.

Interplay between Ethanol Adsorption to High-Energy Sites and Clustering on Graphene and Graphite Alters the Measured Isosteric Adsorption Enthalpies

We present a combined experimental and theoretical study aimed at understanding the behavior of polar probe ethanol on graphene and graphite hydrophobic surfaces. We measured isosteric adsorption enthalpies and entropies by inverse gas chromatography for coverages ranging from 0.1 to 20%. The adsorption enthalpies were found to vary with surface coverage and differed considerably between the materials at low coverage. However, they approached the same adsorption enthalpy value of −12.0 ± 0.4 kcal/mol for T centered at 303–393 K and coverages above 5%. We explained the observed behavior using molecular dynamics simulations by employing empirical force-field and density functional theory calculations on two graphene models: circumcoronene and infinite graphene. The simulations showed that various hydrogen-bonded ethanol clusters formed spontaneously from isolated ethanol molecules on graphene and provided a distribution of cluster sizes. Nonlocal density functional theory was used to calculate adsorption en...

Adsorption equilibrium of carbon dioxide on ammonia-modified activated carbon

The equilibrium adsorption isotherms of carbon dioxide on a commercial granular activated carbon (GAC) and an ammonia-modified GAC (OXA-GAC) were measured using a static volumetric method. CO2 adsorption measurements were performed at three different temperatures (303, 318, and 333K), and pressures up to 1atm. The obtained equilibrium data were fitted to the Freundlich, Sips, and Toth isotherms using a semi-empirical approach to differentiate the contributions of physical and chemical adsorption to the total CO2 uptake. The isotherm parameters were determined independently for each mechanism by non-linear regression. To evaluate the adequacy of the fit of the isotherm models, two different error functions (i.e., the average relative error and the nonlinear regression coefficient) were calculated. The Toth semi-empirical equilibrium model provided the best fit to the experimental data, with average relative errors of less than 3% observed at all temperatures. The isosteric heats of CO2 adsorption onto the ammonia-modified adsorbent and onto the untreated adsorbent were determined using the Clausius–Clapeyron equation. The loading dependence of the isosteric enthalpy of CO2 adsorption over the OXA-GAC reflected an energetic heterogeneity of the adsorbent surface. The initial isosteric heats of adsorption of 70.5kJmol−1 and 25.5kJmol−1 correspond to the adsorption of CO2 on the modified and untreated adsorbents, respectively, and these values were in excellent agreement with the zero-coverage heats of adsorption obtained using the temperature-dependent parameters of the proposed model.

Nanoporous spongy graphene: Potential applications for hydrogen adsorption and selective gas separation

In the present work, a nanoporous (pore width~0.7nm) graphene-based sponge-like material with large surface area (~350m2/g) was synthesized by wet chemical reduction of graphene oxide in combination with freeze-drying. Surface morphology and elemental composition were studied by scanning and transmission electron microscopy combined with energy dispersive X-ray spectroscopy. Surface chemistry was qualitatively examined by Fourier-transform infrared spectroscopy, while the respective structure was investigated by X-ray diffraction analysis. Textural properties, including Brunauer–Emmet–Teller (BET) surface area, micropore volume and surface area as well as pore size distribution, were deduced from nitrogen gas adsorption/desorption data obtained at 77K and up to 1bar. Potential use of the spongy graphene for gas storage and separation applications was preliminarily assessed by low-pressure (0–1bar) H2, CO2 and CH4 sorption measurements at different temperatures (77, 273 and 298K). The adsorption capacities for each gas were evaluated up to ~1bar, the isosteric enthalpies of adsorption for CO2 (28–33kJ/mol) and CH4 (30–38kJ/mol) were calculated using the Clausius–Clapeyron equation, while the CO2/CH4 gas selectivity (up to 95:1) was estimated using the Ideal Adsorbed Solution Theory (IAST).

Krypton Adsorption on Zeolite-Templated Carbon and Anomalous Surface Thermodynamics.

Krypton adsorption was measured at eight temperatures between 253 and 433 K on a zeolite-templated carbon and two commercial carbons. The data were fitted using a generalized Langmuir isotherm model and thermodynamic properties were extracted. Differing from that on commercial carbons, krypton adsorption on the zeolite-templated carbon is accompanied by an increasing isosteric enthalpy of adsorption, rising by up to 1.4 kJ mol(-1) as a function of coverage. This increase is a result of enhanced adsorbate-adsorbate interactions promoted by the ordered, nanostructured surface of the adsorbent. An assessment of the strength and nature of these adsorbate-adsorbate interactions is made by comparing the measured isosteric enthalpies of adsorption (and other thermodynamic quantities) to fundamental metrics of intermolecular interactions of krypton and other common gases.

Isotherms, kinetics and thermodynamics for adsorption of pyridine vapors on modified activated carbons

The adsorption isotherms of pyridine have been determined gravimetrically on different samples of granulated, fibrous and spherical activated carbons. Several chemical treatments such as oxidation, degassing and impregnation have been carried out on as-received activated carbon cloth (ACC) so as to modify the surface characteristics of the activated carbon. The porous and chemical structure of modified and unmodified activated carbons has been characterised by XRD, TGA–DTG, SEM and pore size distribution techniques. The adsorption increases on oxidation while decreases on degassing which can be correlated to increase and decrease in the concentration of surface oxygen groups. Loading with Ag+ and Cu2+ increases the adsorption of pyridine compared with the unloaded sample while loading with Cr6+ decreases the adsorption of these vapors. The adsorption–desorption branches of pyridine vapors on metal impregnated do not meet even at zero relative vapor pressure indicating the chemisorption or other specific interactions. The adsorption kinetics data has been examined using the linear driving force and empirical diffusion models. The initial decrease in isosteric enthalpy of adsorption for modified and unmodified ACC indicates the effect of surface heterogeneity and later increase indicates the influence of lateral interactions between adsorbate molecules. The results suggest that the adsorption of pyridine involves the physisorption as well as specific interactions with carbon oxygen functional groups and metal complexes present on the carbon surface.

Structural study of Ni- or Mg-based complexes incorporated within UiO-66-NH2 framework and their impact on hydrogen sorption properties

Nickel and magnesium acetylacetonate molecular complexes were post-synthetically incorporated into microporous zirconium-based MOF (UiO-66-NH2) in order to introduce active open-metal sites for hydrogen sorption. Elemental analysis, nitrogen physisorption and DFT calculations revealed that 5 molecules of Ni(acac)2 or 2 molecules of Mg(acac)2 were incorporated into one unit cell of UiO-66-NH2. 1H–13C CPMAS and 1H MAS NMR spectroscopy showed that, although embedded within the pores, both Ni- and Mg-complexes interacted with the UiO-66-NH2 framework only through weak van der Waals bonds. Inclusion of metal complexes led to the decrease of hydrogen sorption capacities in Ni-modified as well as in Mg-modified samples in comparison with the parent UiO-66-NH2. The isosteric hydrogen adsorption enthalpy slightly increased in the case of Ni-modified material, but not in the case of Mg-modified analogue.

Hydrogen cryo-adsorption by hexagonal prism monoliths of MIL-101

Hexagonal prism shaped monoliths of envelope density 0.40–0.467 g/cm3 and remarkable mechanical stability were obtained from MIL-101 powder. The hydrogen adsorption isotherms within an extended pressure range show that the excess adsorption decreases with the increasing density of the pellets. At 77 K and 150 bar, the total volumetric capacity is 46.5 g/L; the discharge to 159 K and 5 bar leads to 45 g/L (38.8 g/L referring to the outer tank volume) supporting MIL-101 as a promising candidate for applications in the 77–160 K range of interest for cryo-adsorption hydrogen storage method. The isosteric adsorption enthalpy evaluated from the experimental data with the van't Hoff equation, using fugacity, is in agreement with the calorimetric heat of adsorption reported in literature. Monoliths of this shape allow the best possible packing density of any sorbent in a container and the primary data reported here on MIL-101 could serve as material engineering properties required for modeling hydrogen storage tanks.

Gas Storage and Diffusion through Nanocages and Windows in Porous Metal–Organic Framework Cu2(2,3,5,6-tetramethylbenzene-1,4-diisophthalate)(H2O)2

A novel nanoporous metal–organic framework NPC-4 with excellent thermal stability was assembled from 2,3,5,6-tetramethylbenzene-1,4-diisophthalate (TMBDI) and the paddle-wheel secondary building unit (Cu2(COO)4). The porous structure comprises a single type of nanoscale cage (16 A diameter) interconnected by windows (5.2 × 6.3 A), which give a high pore volume. CH4 (195–290 K), CO2 (198–303 K), N2 (77 K), and H2 (77 K) adsorption isotherms were studied for pressures up to 20 bar. NPC-4 exhibits excellent methane and carbon dioxide storage capacities on a volume basis with very high adsorbate densities, under ambient conditions. Isobars were investigated to establish the relationship for adsorption capacities over a range of storage temperatures. The isosteric enthalpies of adsorption for both CH4 and CO2 adsorption did not vary significantly with amount adsorbed and were ∼15 and ∼25 kJ mol–1, respectively. The adsorption/desorption kinetics for CH4 and CO2 were investigated and activation energies, enthal...

Methane preconcentration by adsorption: a methodology for materials and conditions selection

Methane (CH $$_4$$ ) adsorption has been widely studied, mainly in the context of natural gas purification. A much less prominent, but highly relevant application is the preconcentration of CH $$_4$$ from ambient air. In this study, we compare six different commercial adsorbent materials with respect to their effectiveness for methane preconcentration: a macroporous polymeric resin (HayeSep D), multi-walled carbon nanotubes, two microporous metal-organic frameworks (HKUST-1 and ZIF-8), and two zeolites (5A and 13X). The most relevant properties, such as isosteric enthalpy of adsorption, specific surface area and the selectivity for CH $$_4$$ adsorption over N $$_2$$ were characterized by analyzing adsorption/desorption isotherms. Using these parameters, we discuss the tested adsorbents with respect to the most important properties and identify the most promising candidates. Furthermore we identify the experimental conditions that are expected to give the best results with respect to practical applications.

The surface reactivity of chalk (biogenic calcite) with hydrophilic and hydrophobic functional groups

The surface properties of calcium carbonate minerals play an important role in a number of industrial and biological processes. Properties such as wettability and adsorption control liquid–solid interface behaviour and thus have a strong influence on processes such as biomineralisation, remediation of aquifers and oil recovery. We investigated how two model molecules of different polarity, namely water and ethanol, interact with reservoir and outcrop chalk samples and we compared their behaviour with that of pure, inorganically precipitated calcite. Thermodynamic quantities, such as the work of wetting, surface energy and isosteric adsorption enthalpy, were determined from vapour adsorption isotherms. The chalks were studied fresh and after extraction of organic residues that were originally present in these samples. The work of wetting correlates with the amount of organic matter present in the chalk samples but we observed a fundamental difference between the adsorption properties of chalk and pure, inorganically precipitated calcite toward the less polar, ethanol molecule. Further analysis of the chemical composition of the organic matter extracted from the chalk samples was made by gas chromatography (GC–MS). Monitoring surface composition by X-ray photoelectron spectroscopy (XPS) before and after extraction of the organic material, and with atomic force microscopy (AFM), showed that nanometer sized clay crystals observed on the chalk particle surfaces could be an important part of the reason for the differences. Removal of the extractable portion of the hydrocarbons liberates adsorption sites that have different wetting properties than the rest of the chalk and these have an energy distribution that is similar to clays. Thus, the results exemplify the complexity of biogenic calcite adsorption behaviour and demonstrate that chalk wetting in drinking water aquifers as well as oil reservoirs is controlled partly by the nanoparticles of clay that have grown on the chalk surfaces and partly by adsorbed organic material.

Tuning the Host–Guest Interactions in a Phosphine Coordination Polymer through Different Types of post-Synthetic Modification

The porous Phosphine Coordination Material, PCM-10 contains abundant free P(III) donor sites that can be subjected to a variety of post-synthetic modifications. The diverse P(III)/P(V) organic reactivity and coordination chemistry available to aryl phosphines have been exploited to decorate the pores of PCM-10, allowing for an extensive structure-function study. Polar P═O moieties, charged P(+)-CH3 phosphonium species with exchangeable coanions (I(-), F(-), BF4(-), and PF6(-)) and P-AuCl groups have been successfully post-synthetically incorporated. These modifications directly affect the strength of the resulting host-guest interactions, as demonstrated by comparative sorption studies of CO2, H2, and other gases in the solid-state. Broad tunability of the enthalpy of CO2 adsorption is observed: incorporation of BF4(-) ions inside the pores of PCM-10 results in 24% enhancement of the isosteric adsorption enthalpy of CO2 compared to the parent material, while F(-) anions induce a 36% reduction. Meanwhile, AuCl-decorated PCM-10 shows a high H2 sorption capacity of 4.72 wt % at 77 K and 1.0 bar, versus only 0.63 wt % in the unmodified material.

Isosteric enthalpies for hydrogen adsorbed on nanoporous materials at high pressures

A sound understanding of any sorption system requires an accurate determination of the enthalpy of adsorption. This is a fundamental thermodynamic quantity that can be determined from experimental sorption data and its correct calculation is extremely important for heat management in adsorptive gas storage applications. It is especially relevant for hydrogen storage, where porous adsorptive storage is regarded as a competing alternative to more mature storage methods such as liquid hydrogen and compressed gas. Among the most common methods to calculate isosteric enthalpies in the literature are the virial equation and the Clausius–Clapeyron equation. Both methods have drawbacks, for example, the arbitrary number of terms in the virial equation and the assumption of ideal gas behaviour in the Clausius–Clapeyron equation. Although some researchers have calculated isosteric enthalpies of adsorption using excess amounts adsorbed, it is arguably more relevant to applications and may also be more thermodynamically consistent to use absolute amounts adsorbed, since the Gibbs excess is a partition, not a thermodynamic phase. In this paper the isosteric enthalpies of adsorption are calculated using the virial, Clausius–Clapeyron and Clapeyron equations from hydrogen sorption data for two materials—activated carbon AX-21 and metal-organic framework MIL-101. It is shown for these two example materials that the Clausius–Clapeyron equation can only be used at low coverage, since hydrogen’s behaviour deviates from ideal at high pressures. The use of the virial equation for isosteric enthalpies is shown to require care, since it is highly dependent on selecting an appropriate number of parameters. A systematic study on the use of different parameters for the virial was performed and it was shown that, for the AX-21 case, the Clausius–Clapeyron seems to give better approximations to the exact isosteric enthalpies calculated using the Clapeyron equation than the virial equation with 10 variable parameters.

Preparation of carbon sphere from lactose by hydrothermal reaction and its performance in gas separation

Carbon spheres were produced from mild hydrothermal reaction of biomass lactose, followed by carbonization with low‐activation temperatures (400°C). The Brunauer‐Emmett‐Teller (BET)‐specific surface area of carbon at optimum condition was 403 m2/g. The resultant carbon shows good performance in preferential adsorption of CO2 to CH4 below 50 kPa at 293 K. The results of the isosteric enthalpies of adsorption for different gases on prepared carbon indicate mildly heterogeneous surfaces. The relatively good performance of the resultant carbon in kinetics indicates its potential for low‐pressure kinetic separation of carbon dioxide from carbon dioxide and methane mixture at room temperature.. © 2013 American Institute of Chemical Engineers Environ Prog, 33: 581–587, 2014