Argon Isotherms Research Articles

Universal Langmuir and Fractal Analysis of High-Resolution Adsorption Isotherms of Argon and Nitrogen on Macroporous Silica.

High-resolution isotherms of argon and nitrogen adsorption on macroporous silica have been simulated with universal Langmuir and fractal models. A four-parameter, fractal universal Langmuir equation is a good fit to the data at low pressures. Standard Gibbs energy changes calculated from equilibrium adsorption coefficients show a series of broad peaks that indicate adsorbate structural transformations as a function of pressure and coverage. The Freundlich equation or mean fractal model is also a good fit to isotherms at low pressures. Pressure-varying fractals are accurate fits to the data. Fractal exponents provide information on adsorbate coverage and surface access. Broad peaks in pressure-varying exponents are indicators of adsorbate structure. From adsorptive gas amounts, mean and pressure-varying fractal exponents provide details of adsorbate fractal dimensions and surface roughness. Both Ar and N2 adsorption cause increases in mean surface roughness when compared with pure silica. Surface roughness fluctuations from pressure-dependent adsorptive gas fractal dimensions are associated with adsorbate structure. At one trough, the surface is smooth and is linked to close-packed Ar or N2. For Ar adsorption at 87 K, this structure is a complete monolayer (1.00(4)), while for Ar (77 K), 1.15(4) layers and for N2 (87 K), 2.02(10) layers. The universal Langmuir specific area of the silica is 10.1(4) m2 g-1. Pressure- and coverage-dependent adsorbate structures range from filling defects and holes on the surface to cluster formation to adsorbed Ar or N2 evenly distributed or packed across the surface. The Ar (87 K) isotherm is most sensitive to adsorbate structural transformations.

Brunauer-Emmett-Teller Areas from Nitrogen and Argon Isotherms Are Similar.

Despite recommendations from the 2015 International Union of Pure and Applied Chemistry (IUPAC) technical report, surface areas of porous materials continue to be characterized by an N2 adsorption isotherm using the Brunauer-Emmett-Teller (BET) method. In this study, we provide the basis for such a practice by carrying out systematic large-scale molecular simulations on homogeneous and heterogeneous model surfaces. Specifically, we investigated the purported "orientational effect" of the N2 molecule on these surfaces. Grand canonical Monte Carlo (GCMC) simulation results from 257 diverse metal-organic frameworks show that the BET areas from Ar and N2 are similar in the range of 250-7500 m2/g with a mean deviation of 4%. Detailed analyses based on the consistency criteria for BET equations reveal that the large deviation (>10%) between the BET areas from Ar and N2 are materials specific and more prone to materials that are not able to satisfy the 3 and 4 consistency criteria. For materials that satisfy all four consistency criteria, the BET areas predicted from Ar and N2 isotherms are comparable.

Influence of layer slipping on adsorption of light gases in covalent organic frameworks: A combined experimental and computational study

Sorption of gases in micro- and mesoporous materials is typically interpreted on the basis of idealized structural models where real structure effects such as defects and disorder are absent. For covalent organic frameworks (COFs) significant discrepancies between measured and simulated adsorption isotherms are often reported but rarely traced back to their origins. This is because little is known about the real structure of COFs and its effect on the sorption properties of these materials. In the present work molecular simulations are used to obtain adsorption isotherms of argon, nitrogen, and carbon dioxide in the COF-LZU1 at various temperatures. The (perfect) model COF has a BET surface that is higher than the experimental BET surface by a factor of approximately 1.33, suggesting defects or inclusions are present in the real structure. We find that the saturation adsorption loading of small gaseous species in COF-LZU1, as determined from grand canonical Monte Carlo simulations, is also higher by approximately the same factor compared to the experimental saturation loading. The influence of interlayer slipping on the shape of the adsorption isotherm and the adsorption capacity is studied. Comparison between simulation and experiment at lower loadings suggests the layers to be shifted instead of perfectly eclipsed. The sensitivity of the adsorption isotherms in this regime towards the underlying framework topology shows that real structure effects have significant influence on the gas uptake. Accounting for layer slipping is important to applications such as catalysis, gas storage and separation.

Towards continuous deoxygenation of acetic acid catalyzed by recyclable mono/bi/trimetallic zeolite catalysts

A series of mono (Ni, Cu, Co), bi (NiCu, NiCo, CuCo), and trimetallic (NiCuCo) mordenite supported catalysts were characterized and evaluated for the continuous, near-atmospheric pressure, thermal deoxygenation of acetic acid. Acetic acid was selected as a model reactant due to its abundance in both biomass pyrolysis upgrading streams and its analogous structure to carboxylic acids found in vegetable oil upgrading streams. Surface area and metal loading were nearly constant for all studied catalysts, allowing an in-depth direct comparison of the metallic activity for reaction pathways including gasification, decarboxylation, decarbonylation, hydrodeoxygenation, and ketonization. In the absence of Ni or in the absence of H2, the observed catalytic activity was low. Results from the bimetallic samples at 400 °C highlighted the importance of Ni for H2 activation and Co for the simultaneous promotion of hydrodeoxygenation and gasification. A nearly linear correlation between the Ni content and the conversion of acetic acid was observed at 400 °C. Cryogenic argon isotherms revealed the location of metal sites within the mordenite pore, further emphasizing the impact of confining environments on the deoxygenation of bio-derived compounds. TGA, TPR, and TPD studies provided information on catalyst stability, reducibility, and acidity, respectively. HRTEM indicated that agglomerates formed under the studied reaction conditions. The fresh Ni/MOR catalyst, under differential conversion conditions, was stable for ~ 24 h at 460 °C, for an additional ~ 20 h upon regeneration at 460 °C, and for an additional 30 h upon regeneration at 500 °C. Long-exposure activity was observed in the Ni/MOR catalyst, favoring hydrodeoxygenation and decarbonylation for at least 70 h under integral conversion conditions. In the bimetallic NiCo/MOR catalyst, Co stabilized Ni active sites in a co-located configuration which promoted hydrodeoxygenation, but also promoted gasification, and eventually lead to coking/lower conversion.

General Cluster Sorption Isotherm

Abstract Adsorption isotherms are an essential tool in chemical physics of surfaces. However, several approaches based on a different theoretical basis exist and for some isotherms existing approaches can fail. Here, the sorbate-sorbate interaction is not limited to an orientation vertical to the adsorbent surface. Instead, a formal orientation-independent clustering, also comprising the possibility of multilayers as a special case, is underlain. The flexible clusters are described by classical thermodynamics including a free energy relationship depending on the cluster size. In this paper a rigorous unified treatment of the adsorbate-adsorbate-adsorbent interaction is shown to result in a general isotherm equation which is applied to literature data both concerning type IV isotherms of argon and nitrogen in ordered mesoporous silica, and type II isotherms of disordered macroporous silica. The new isotherm covers the full range of partial pressure (10−6 - 0.7). The determination of surface areas is not possible by this isotherm because the cross-sectional area of a cluster is unknown. Based on the full description of type IV isotherms, most known isotherms including BET are accessible by respective simplifications. The presented model is an extension of the classical derivation of the ζ-isotherm which was shown to describe type IV isotherms restricted to adsorbates with very strong tendency of clustering such as water on hydrophobic surface (Phys. Chem. Chem. Phys. 21 (2019) 5614).

Wedge pore modelling of gas adsorption in activated carbon: Consistent pore size distributions

The pore size distribution (PSD) of a porous solid is usually obtained by matching the experimental adsorption isotherm for argon or for nitrogen at their boiling points against a theoretical isotherm constructed with models of non-connected pores. The seminal work of Rosalind Franklin in 1951, showed that the pore spaces have wedge geometry rather than the parallel side slits commonly used in carbon pore modelling. Based on this, we proposed a wedge geometry for pores as a basis for modelling adsorption in activated carbon; thereby introducing a paradigm shift from the customary slit geometry. Not only does the wedge geometry better reflect the physical pore space, it accounts for the linear connectivity which is absent in the current PSD modelling. We have used extensive simulations to obtain a theoretical isotherm and use it to match experimental isotherms for argon and nitrogen to check the invariance of the PSD with respect to the adsorbate molecule. In this new model, the artefact of zero pore volume at 1 nm, which can occur in the slit model, is absent. Apart from the self-consistency of the derived PSD, the new wedge model also gives a good account for the isosteric heat versus loading.

Adsorption of components from air on silica aerogels

Abstract Aerogels are porous media that have large internal surface areas causing them to have significant gas adsorption. The adsorption isotherms of carbon dioxide, nitrogen, oxygen, and argon with a tetramethoxysilane (TMOS) silica aerogel, a [trimethylsilyl]oxy (TMS)-modified silica aerogel, and a silica gel were measured in the pressure range of 0–6.5 atm at 10 °C, 30 °C, 50 °C, 70 °C, and 90 °C. These isotherms were fitted to Langmuir, Freundlich, Sips, and Toth isotherms, as well as the temperature dependent Toth model and their cumulative adsorption of the components of air were discussed. Isosteric heats of adsorption were quantified. With this data, we can see the impact of the surface groups on adsorption as well as the pore size, with a significant increase in the quantity and strength of adsorption of CO2 reported between silanol and TMS groups. The relationship between adsorption capacity and the thermal properties of silica aerogels were also discussed in this study.

Surface Area Determination of Porous Materials Using the Brunauer–Emmett–Teller (BET) Method: Limitations and Improvements

Surface areas of metal–organic frameworks (MOFs) have been commonly characterized using the Brunauer–Emmett–Teller (BET) method based on adsorption isotherms of nonreactive nitrogen or argon. Recen...

Physical-Constant Equations-of-State for Argon Isotherms

Using argon as the exemplary fluid, we report a representation of equations-of-state in fluid regions bounded by percolation loci with mainly physical constants that define state bounds and characterize the fluid phase diagram. Both gas- and liquid-state pressures can be represented by 3- or 4-term virial expansions. Gaseous states require only known virial coefficients and physical constants belonging to the fluid, i.e., Boyle temperature (TB), Tc, pc and coexisting densities of gas (ρcG) and liquid (ρcL) at Tc. A notable finding is that for isotherms below TB, the contribution of the fourth virial term is negligibly small within experimental uncertainty. In the supercritical mesophase, both gaseous and liquid states percolate the configurational phase volume, whereupon state functions of the density obey a linear combination equation-of-state in this region. We also find evidence for the percolation line that bounds the low-temperature existence of the pure liquid argon state giving rise to an equilibrium prefreezing mesophase. In this region, which probably exists for all liquids, there is a colloidal dispersion of the metastable crystalline structure with an energy density closest to that of the liquid, wherein coexisting amorphous and crystalline states both percolate the phase volume. We report equations-of-state for isotherms of argon, over the whole equilibrium fluid range; i.e., for four regions, gas, supercritical mesophase, liquid, and a liquid + bcc prefreezing and metastable fluid mesophase up to a glass transition pressure. We compare with experiment using the Tegeler–Span–Wagner equation via the NIST fluid thermophysical property database.

Quantification of rhenium oxide dispersion on zeolite: Effect of zeolite acidity and mesoporosity

The anchoring nature and spatial distribution of rhenium oxide (ReOx) supported on zeolites (ReOx/zeolite) were determined as a function of the support acidity and mesoporosity. The ReOx/zeolite samples were characterized collectively using Argon isotherm, XRD, Raman, XAS, DRIFTS, and organic chemical titrations. The results show that the ReOx species formed the isolated distorted tetrahedral ReO4− structure. On the zeolite support, ReOx species anchored predominantly on Brønsted acid sites (Si-OH+-Al) enclosed in micropores (internal acid sites), by replacing the protons in Si-OH+-Al sites at a H+/Re ratio of ∼1.1 in zeolites with high acidity. The decrease in zeolite acidity led to the anchoring of ReOx species onto both Si-OH+-Al and silanol group (Si-OH). The increase in zeolite mesoporosity migrated the ReOx species onto both internal acid sites and those on the external surface and mesopores (external acid sites), as well as Si-OH groups. The implication of distribution of ReOx in zeolite was explored by direct non-oxidative methane conversion reaction.

Flexible ZIFs: probing guest‐induced flexibility with CO2, N2 and Ar adsorption

AbstractBACKGROUNDThe effect of framework topology on the guest‐induced flexibility of several crystalline zeolitic imidazolate frameworks (ZIF‐7, ZIF‐9, ZIF‐11 and ZIF‐12) was investigated via analysis of experimental nitrogen (N2), carbon dioxide (CO2) and argon (Ar) isotherms at 77 K (N2 and Ar) and 273 K (CO2) for gas pressures up to 0.13 MPa.RESULTSThe experimental isotherms of these frameworks were analysed in order to investigate their structural flexibility using gases with kinetic diameters equal to or larger than the diameters of their static pore apertures. The results of gas sorption measurements indicate guest‐induced phase changes for ZIF‐7 and ZIF‐9 (SOD topologies). ZIF‐12 (RHO topology) also shows uptake for gases, despite its pore‐limiting diameter being smaller than the kinetic diameters of the adsorbed molecules.CONCLUSIONSThis work highlights the ability of ZIFs with different framework topologies to change their structure and increase their pore aperture upon interaction with certain gases. These findings are key in the development of more selective ZIF‐based materials for important industrial applications including low‐energy gas separations, catalytic nanoreactors and sensor technology. © 2019 The Authors. Journal of Chemical Technology & Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Molecular simulations of COFs, IRMOFs and ZIFs for adsorption-based separation of carbon tetrachloride from air

Covalent organic frameworks (COFs), metal organic frameworks (MOFs) and zeolitic imidazolate frameworks (ZIFs) have been widely studied in gas separation applications due to their large surface areas, high pore volumes, tunable pore sizes and chemical stabilities. In this study, separation performances of 153 COFs, 14 IRMOFs and 8 ZIFs were assessed for efficient removal of carbon tetrachloride (CCl4) from CCl4/Ar, CCl4/N2, CCl4/O2 mixtures at 298 K and infinite dilution. The top performing three materials in each group, namely, borazine-linked polymer (BLP-2H-AA), IRMOF-11 and ZIF-6 were identified. Single-component, binary mixture and quaternary mixture adsorption isotherms of argon (Ar), CCl4, nitrogen (N2) and oxygen (O2) in these materials were computed at 298 K and various total pressures from 10−3 to 1.5 × 104 kPa. Mixture adsorption selectivities and separation potentials were then calculated and the effect of relative humidity on the performance of adsorption-based CCl4 separation was examined. Single-component and quaternary mixture diffusion coefficients of Ar, CCl4, N2 and O2 were finally computed. Our results showed that ZIF-6 exhibits the highest adsorption selectivity and the highest separation potential for CCl4/Ar, CCl4/N2 and CCl4/O2 mixtures, followed by IRMOF-11 and BLP-2H-AA. Results of this computational study will be highly useful to identify the promising materials for removal of CCl4 from air.

Polylayer Adsorption on Rough Surfaces of Nanoaerosols Obtained via the Rapid Cooling of Droplets

An approach is developed for studying polymolecular adsorption on the modeled rough surface of a small aerosol obtained from a liquid droplet on its rapid cooling. A way of estimating the specific surface of adsorbent droplets with rough surfaces is proposed, and the temperature and size dependences of the specific surface are established. Isotherms of N2 and Ar polymolecular adsorption on a heterogeneous surface of small spherical particles of SiO2 are derived. The possibility of using this approach to describe an experiment is demonstrated. Comparison to the experimental isotherms reveals agreement with isotherms of argon and nitrogen on silica surfaces, with an error of up to 4.5%.

Estimation of the average heat of gas adsorption from the adsorption isotherm measured at supercritical temperatures and pressures

Using a high-precision volumetric—gravimetric experimental unit, the adsorption iso-therms of methane, argon, and nitrogen were measured at pressures of 0.1—40 MPa and at temperatures of 303—373 K on four carbon adsorbents with different porosities. To calculate the characteristic energy and average heat of adsorption of gases in the supercritical region, two parameters, which characterize an adsorptive were introduced: critical temperature T c of the gas and the analog of the saturated vapor pressure “P s”, which is defined assuming that densities of an adsorbate and an adsorptive are equal. The average heats of gas adsorption well agree with the average isosteric heats for the adsorption systems over the entire pressure and temperature intervals under study.

Predicting the adsorption of n-perfluorohexane in BAM-P109 standard activated carbon by molecular simulation using SAFT-γ Mie coarse-grained force fields

This work is framed within the Eighth Industrial Fluid Properties Simulation Challenge, with the aim of assessing the capability of molecular simulation methods and force fields to accurately predict adsorption in porous media for systems of relevant practical interest. The current challenge focuses on predicting adsorption isotherms of n-perfluorohexane in the certified reference material BAM-P109 standard activated carbon. A temperature of [Formula: see text] K and pressures of [Formula: see text], 0.3, and 0.6 relative to the bulk saturation pressure p0 (as predicted by the model) are the conditions selected in this challenge. In our methodology we use coarse-grained intermolecular models and a top-down technique where an accurate equation of state is used to link the experimental macroscopic properties of a fluid to the force-field parameters. The state-of-the-art version of the statistical associating fluid theory (SAFT) for potentials of variable range as reformulated in the Mie group contribution incarnation (SAFT- γ Mie) is employed here. The parameters of the SAFT- γ Mie force field are estimated directly from the vapour pressure and saturated liquid density data of the pure fluids using the equation of state, and further validated by molecular dynamic simulations. The coarse-grained intermolecular potential models are then used to obtain the adsorption isotherm kernels for argon, carbon dioxide, and n-perfluorohexane in graphite slit pores of various widths using Grand Canonical Monte Carlo simulations. A unique and fluid-independent pore size distribution curve with total micropore volume of 0.5802 cm3/g is proposed for the BAM-P109. The pore size distribution is obtained by applying a non-linear regression procedure over the adsorption integral equation to minimise the quadratic error between the available experimental adsorption isotherms for argon and carbon dioxide and purpose-built Grand Canonical Monte Carlo kernels. The predicted adsorption levels of n-perfluorohexane at 273 K in BAM-P109 are 72.75 ± 0.01, 73.82 ± 0.01, and 75.44 ± 0.05 cm3/g at Standard Temperature and Pressure (STP) conditions for [Formula: see text], 0.3, and 0.6, respectively.

Applicability of using CO2 adsorption isotherms to determine BET surface areas of microporous materials

While nitrogen and argon isotherms at their respective boiling points are commonly used for the BET analysis of porous materials, CO2 isotherms at 273 K have been suggested as an alternative option for materials containing very small pores in which nitrogen and argon molecules cannot access at cryogenic temperatures. However, it has been known that the BET surface areas obtained from CO2 isotherms are not really meaningful in an absolute sense due to the strong CO2–CO2 interaction. In this study, CO2 isotherms in metal–organic frameworks (MOFs) and zeolites were predicted by grand canonical Monte Carlo (GCMC) simulations and used to evaluate the BET analysis for these materials. For all the microporous materials with varied pore sizes, the BET surface areas calculated from the simulated CO2 isotherms agreed roughly with their geometric surface areas from the crystal structures. Moreover, a clear selection of a proper BET linear region was possible for the material possessing only ultra-micropores. These results indicate that the BET surface areas determined from CO2 isotherms at 273 K may be considered reasonable for adsorbents containing only ultra-micropores where the BET surface areas based on N2 or Ar isotherms at cryogenic temperatures are not available.

Towards a better understanding of water adsorption hysteresis in activated carbons by scanning isotherms

The occurrence of water sorption hysteresis associated with the filling of micro and narrow mesopores (particularly for pores of widths smaller than ca. 3 nm, where nitrogen and argon isotherms at their boiling temperatures, i.e. 77 K and 87 K, respectively, are known to be reversible) provides additional opportunities for textural characterization. In this work systematic water scanning desorption isotherms within the hysteresis loop were carried out on well-characterized activated carbons with varied textural features and surface chemistry. Accurate micro-mesopore analysis was obtained by means of nitrogen, argon and carbon dioxide adsorption experiments coupled with advanced density functional theory methods (i.e., NLDFT, QSDFT). The obtained results indicated that water adsorption/desorption phenomena for pores of different sizes take place independently from each other. This investigation constitutes a starting point for the interpretation of water adsorption hysteresis by means of scanning desorption measurements.

Existence of ultrafine crevices and functional groups along the edge surfaces of graphitized thermal carbon black.

Adsorption of different gases on graphitized thermal carbon black (GTCB) has been studied with a new molecular model to examine the consequences of micropore crevices and functional groups at the junctions between adjacent basal planes. Adsorption was simulated in the Grand Canonical Monte Carlo ensemble and the theoretical Henry constants were calculated by Monte Carlo volume integration over the Boltzmann factor of the solid-fluid potential. The simulation results are in good agreement with high-resolution experimental isotherms for argon on mineralogical graphite measured by Lopez-Gonzalez et al.1 From detailed inspection of the argon isotherms at extremely low coverages, we find two distinct Henry law regions, separated by a plateau (suggesting saturation of the stronger sites) that spans over a few decades of pressure. The first Henry law region is attributed to adsorption in the ultrafine crevices at the junctions between two adjacent basal planes, and the second region corresponds to adsorption on the basal plane, as confirmed by the theoretical Henry constant. The simulated isosteric heat and snapshots of molecular configurations show that argon adsorbs preferentially in the ultrafine crevices where there is a deep potential well due to overlap from the opposite pore walls. Similar behavior was found for other nonassociating fluids (Ar, N2, and CO2); however, for associating fluids (NH3 and H2O), the strong sites for adsorption and nucleation come from the combined effects of functional groups and ultrafine crevices, since the latter cannot alone account for the observed adsorption.

Impact of Chemical Heterogeneity on the Accuracy of Pore Size Distributions in Disordered Solids

Disordered porous solids, such as heterogeneous catalysts, possess internal void spaces with high degrees of both chemical and geometric heterogeneity. For pore size distributions (PSDs) to be useful void space descriptors for aiding understanding of phenomena such as catalyst activity, their level of accuracy must be known. Due to differences in specific interactions and wetting behavior, the choice of adsorbate can severely impact the accuracy of PSDs derived for disordered materials using gas sorption. Wetting effects in the capillary condensation region have recently been studied extensively by simulation, but there have been no complementary, definitive experimental studies of sorption mechanisms. However, in this work, the novel integrated gas sorption and mercury porosimetry technique has been used to prepare a particular subset of pores, located within the disordered network of a given amorphous material (eg silica or alumina), that had dead-ends with different chemical properties to the walls, and then definitively compare the individual sorption behavior of two different, common adsorbates (nitrogen and argon) within just these pores. The presence of a heavy metal surface was found to lead to a shift to higher pressure in the positions of adsorption and desorption branches and a widening of the hysteresis for argon isotherms for silica pores, but not for alumina pores. In contrast, no such effect was observed for nitrogen. Hence, using argon adsorption rather than nitrogen adsorption was found likely to lead to pore size inaccuracies of at least ∼200% in characterizing silica-supported platinum catalysts, for example. The experimental findings on effects of wetting and pore length were found to be consistent with findings from recent Monte Carlo and MFDFT simulations. Further, analysis of hysteresis widths also suggested that the standard approach using NLDFT would be unsuitable for the type of samples studied here. The results highlight particular issues for the accuracy of PSDs for catalysts consisting of metal nanoparticles supported on oxide materials.

Adsorption of pentane isomers on metal-organic frameworks Cu-BTC and Fe-BTC

Adsorption isotherms of n-pentane, isopentane and cyclopentane on Cu-BTC and Fe-BTC metal-organic frameworks, measured in a temperature range from 273 to 333K, have been examined. For correlations, adsorption of nitrogen at 77K and argon at 87K has been studied as well. The analysis of adsorption isotherms was based on the calculation of differential distributions of the amount adsorbed with respect to the adsorption potential. It appears that the filling of Cu-BTC micropores with nitrogen occurs in two steps due to specific interaction of nitrogen molecules with open metal sites. In contrast to nitrogen, the filling of Cu-BTC micropores with argon takes place in one step. The micropores of Fe-BTC are filled with nitrogen and argon gradually in two featureless stages indicating the unordered porous structure. Adsorption isotherms of n-pentane, isopentane and cyclopentane display especially at 273 and 293K a rectangular course with a long vertical part. This shape of C5 hydrocarbon isotherms on Cu-BTC is consistent with the shape of argon isotherms since the interactions with saturated hydrocarbons are nonspecific and thus the pores of Cu-BTC are filled in one step. The isotherms of C5 hydrocarbons on Fe-BTC show one saturation step being less sharp than in the case of Cu-BTC. Calculated isosteric adsorption heats suggest that the energy of lateral interactions between adsorbed molecules dominates the gas–solid energy heterogeneity. This domination is especially pronounced for cyclopentane adsorbed on Cu-BTC. In the case of Fe-BTC, almost constant isosteric adsorption heats show that the variations of the gas–solid interaction energy are compensated by the energy of lateral interactions.