High pressure adsorption of hydrogen, nitrogen, carbon dioxide and methane on the metal–organic framework HKUST-1
Graphical abstract
Research highlights
► High Pressure Adsorption of Supercritical fluids on MOF material up to 50 MPa. ► Calculation of Surface Excess data from experimental data. ► Applying to models for the calculation of the absolute amount adsorbed.
Introduction
A new group of porous materials was developed in the mid 90ties by Yaghi and co-workers [1], [2], [3], the so called metal–organic frameworks or coordination polymers. Such materials consist of metal atoms or metal/oxygen-cluster, which are connected by organic linkers. Several linkers and also motifs have been described to form hundreds of different porous materials [4], [5], [6], [7], [8], [9]. One of the most important specimens of this class of materials is HKUST-1 (also named Cu3(BTC)2 or, as a commercially available product, BasoliteTM C300) [10], [11], [12].
As already mentioned in the Refs. [10], [11], HKUST-1 consists of Cu–Cu-dimers which are connected by 1,3,5-tricarboxylate linkers to form a 3-dimensional network with micropores in a range of 0.7–0.9 nm. Spectroscopic analysis of the commercially available HKUST-1 prepared by electrochemical synthesis is given in Ref. [11].
During the last decade, the well known material HKUST-1 has been widely studied in adsorption processes and adsorptive separations by several groups. Among those, adsorption applications like purification and separation processes [11], [13], [14], [15], [16], [17], gas storage, especially hydrogen storage under ambient or cryogenic temperatures [18], [19], [20], [21], [22] or gas capture in the presence of environmental applications or for vehicle industry [23], [24] are of the main interest.
To the best of our knowledge, only one recent publication by Senkovska and Kaskel [25] has dealt with high pressure adsorption data of gases (>15 MPa at T = 298 K for methane) on HKUST-1 at room temperature up to 20 MPa. These authors have reported storage capacities for methane in synthetic HKUST-1-samples of about 15.7 wt.% at 303 K. Thus, we report for the first time high pressure (>20 MPa) adsorption data for the MOF-type material HKUST-1. In this study, we measured the sorption capacities of HKUST-1 (BasoliteTM C300) for the supercritical gases hydrogen, nitrogen, methane and carbon dioxide in a temperature range of 303–343 K and pressures up to 50 MPa.
However, the adsorption of a supercritical gas on a microporous solid up to high pressures has been widely studied [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41]. It is therefore known, that the adsorption isotherm of a supercritical fluid on a microporous material consists of a maximum in the surface excess amount at a certain pressure. The experimental surface excess amount should be the starting point for the calculation of the absolute amount adsorbed for an experimentally working scientist, because it is not possible to determine the absolute adsorbed quantities by experimental techniques. This is of interests, when experimental data have to be compared with results from simulation experiments, where the absolute amount adsorbed can be obtained. Therefore, models must be used to calculate the absolute amount adsorbed from the experimentally available surface excess. The major aim of this work is a comparison between two different models for the calculation of the absolute amount adsorbed from the surface excess amount. Here, we use two different models. On the one hand, we use the total pore volume of the adsorbent to calculate the absolute amount adsorbed [42], [43], [44], [45]. On the other hand, a second model gives us the volume and the density of adsorbed phase [38], [46], [47], [48], [49]. The results with both models were further compared to each other. Especially, the second model can be used to characterize specific properties of the adsorbed phase.
Furthermore, the isosteric heat of adsorption for each gas (hydrogen, nitrogen, methane and carbon dioxide) was calculated using the Clausius–Clapeyron equation up to the saturation regime of the isotherms.
Section snippets
Materials
The adsorbent HKUST-1 was used in the form of the commercially available material, BasoliteTM C300, as purchased from Sigma Aldrich (US). For a more detailed description of the material see also Ref. [11].
All used gases were obtained from Air Products (US) with variable purity (CO2 99.995%, N2 99.995%, CH4 99.5%, H2 99.995% and He 99.9992%). Finally, methanol was purchased from Fluka (Germany) with a purity of 99.9%.
Nitrogen physisorption at 77 K
Nitrogen adsorption isotherms at 77 K on HKUST-1 were obtained by using the
Determination of the specific pore volume
Generally, the textural properties of nanoporous solids such as MOFs, can be investigated by physisorption experiments, e.g., by nitrogen (77 K) or argon (87 K) sorption isotherms [42], [43], [44], [45]. Not only the specific surface area and the pore size distribution are of interest, but also the specific pore volume is an important parameter, especially for the determination of the absolute amount adsorbed from the surface excess amount [45]. A detailed analysis of such procedures can be found
Conclusion
Pure gas adsorption data of nitrogen, hydrogen, carbon dioxide and methane on the commercially available metal–organic framework HKUST-1 (BasoliteTM C300) were measured at temperatures between 303 and 343 K and pressures up to 50 MPa by using the gravimetric method. Textural informations were obtained from adsorption isotherms of several adsorptives at different temperatures and pressures, e.g., nitrogen adsorption at 77 K and carbon dioxide adsorption at 273 K.
A detailed description of the effect
Acknowledgement
The authors thank the “Deutsche Forschungsgemeinschaft” DFG-project SPP 1362 MOF (STA428/17-1) for financial support.
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