Table of Contents
Introduction and significance of adsorption isotherms
Principle of gravimetric sorption analysis
The gravimetric method for determining adsorption isotherms is characterized by its accuracy, sensitivity and wide measuring range. The method is based on the exact measurement of the change in mass of a zeolite while it is brought into contact with an adsorptive. This method enables the direct recording of sorption kinetics and equilibrium data.
The zeolite is activated before the measurement, i.e. thermally dried, in order to remove adsorbed water or residual substances and make “fresh” sorption sites available (3). This is essential as it is the only way to ensure a reproducible starting point for the isotherm determination. The sample is then placed in the measurement setup on a highly sensitive microbalance.
At the heart of gravimetric sorption analysis is the continuous measurement of the change in mass of the sample during exposure to defined partial pressures of the adsorptive. The sample is placed in a high-precision microbalance in a measuring chamber. After setting a specific temperature, the partial pressure of the sorbent is gradually increased or decreased (4).
The change in the mass of the sample – caused by adsorption or desorption processes – is recorded in real time. Once equilibrium is reached at each pressure level, the load is determined. The combination of a precision balance with typical microgram resolution and controlled atmosphere adjustment ensures very accurate isotherms even at low loadings or low pressures. For temperature variation, the sample is held in an oven or thermostat so that isothermal measurements can be performed at different temperatures.
Measurement procedure and data evaluation
Temperature preparation:
- Setting and stabilization of the desired measuring temperature.
- Temperature and humidity are critical for zeolites – even small fluctuations can influence measurement results.
Measuring atmosphere:
- Gradual increase of the partial pressure or the concentration of the adsorptive in the measuring cell.
- Each step is held until equilibrium is reached (constant sample mass).
Mass measurement:
- Continuous recording of the change in mass with a microbalance.
- Mass increase corresponds to adsorption – the adsorbed quantity is recorded per step.
Evaluation of the adsorption:
- An adsorption isotherm is created from the individual values (loading vs. pressure at constant temperature).
- Typical evaluation models: Freundlich, Langmuir and Dubinin-Astakhov equation.
- Particularly suitable for zeolites: Dubinin-Astakhov equation (captures micropore properties and energetic heterogeneity).
Data analysis:
- Model-based evaluation of the raw data.
- Determination of characteristic parameters:
- Maximum sorption capacity
- Heterogeneity parameters
- Affinity of the adsorbent to the adsorptive
Factors influencing accuracy:
- Stability of the scales
- Homogeneity of the sample
- Precise control of temperature and pressure
Temperature influence on adsorption
Temperature has a decisive influence on the measurement and course of adsorption isotherms in zeolites. As the temperature increases, the equilibrium loading of the zeolite typically decreases at a constant partial pressure. The reason is that adsorption is an exothermic process: higher temperatures promote desorption as more thermal energy is available to overcome the adsorption forces (5).
The maximum load of a zeolite is directly and significantly influenced by the temperature: as the temperature increases, the maximum amount of adsorptive that can be absorbed by the zeolite generally decreases. At lower temperatures, more adsorptive is bound, while at higher temperatures adsorption becomes more difficult and desorption increases. Experiments show, for example, that the nitrogen loading of zeolite 13X is around 30 % higher at 0 °C than at 30 °C (5).
At low temperatures, isotherms often show a steeper curve and a higher saturation load; at higher temperatures, they are usually flatter and reach lower maximum values. At sufficiently high temperatures, the isotherms can become almost linear and the typical saturation characteristic weakens.
By comparing isotherms of the same material and adsorptive at different temperatures, the isosteric adsorption enthalpies can be calculated, relevant key figures for technical and thermodynamic design. Measurements should always be carried out at a controlled and precisely documented temperature, as even moderate temperature fluctuations can lead to considerable deviations in the adsorption capacities determined.
Practical application and case studies
A typical application is the investigation of the water vapor uptake of a zeolite at 25°C and increasing partial pressures. The isotherms show a steep increase in loading at relatively low partial pressures, which is due to the high affinity of zeolites for polar molecules. The regenerability is tested by re-drying the sample in vacuum or at elevated temperature – a key aspect for cyclic heat storage applications (3). For CO2, the gravimetric method can be used analogously, whereby zeolites allow high loadings even at moderate pressures.
Typical evaluation parameters include the maximum loading and accessibility in the working pressure range, affinity and interaction parameters such as the sorption enthalpy, as well as the selectivity towards other gases or components. Additional series of measurements are required to determine the kinetics.
The scientific literature confirms the central role of gravimetric analysis in modern sorption material characterization. The gravimetric determination of adsorption isotherms is a methodological backbone for the targeted development and evaluation of zeolites in the field of energy storage. Process engineering innovation, coupled with high-quality measuring systems, offers laboratory, research and development teams maximum data quality and application reliability – crucial for progress in sustainable heat management and thermal energy storage.
Conclusion
Gravimetric sorption analysis has established itself as an indispensable method for the characterization of zeolites in heat storage. Its high accuracy and reproducibility make it possible to determine precise adsorption isotherms, which serve as a basis for material selection and process optimization. Precise temperature control in particular is proving to be a critical factor, as even small temperature fluctuations have a significant impact on the storage capacity.
The method not only provides quantitative data on sorption capacity, but also valuable insights into the thermodynamic properties of the materials. This makes it an essential tool for the development of efficient energy storage systems and contributes significantly to progress in sustainable energy technology. Modern measuring systems enable automated, standardized measurements that guarantee the highest quality and comparability of results.
List of sources
(1) https://mediatum.ub.tum.de/doc/820976/820976.pdf – Materials science studies on zeolitic adsorbents for heat storage
(2) https://webdoc.sub.gwdg.de/ebook/diss/2003/tu-berlin/diss/2002/hauer_andreas.pdf – Evaluation of solid adsorbents in open sorption systems for heat storage (Hauer, Diss. 2002)
(3) https://www.eso.org/sci/facilities/develop/detectors/optdet/docs/diploma_hose.pdf – Investigation of activated carbon and zeolites – adsorption isotherms gravimetric (Hose, 2000)
(4) https://opendata.uni-halle.de/bitstream/1981185920/34866/1/ArifianYosefBenediktAwan_Untersuchung_zurSorption_von_Kohlendioxid_in_neuartigen_por%C3%B6sen_Materialien.pdf – Bachelor thesis: Investigation of the sorption of carbon dioxide in novel porous materials – measuring principle gravimetry
(5) https://duepublico2.uni-due.de/servlets/MCRFileNodeServlet/duepublico_derivate_00074130/Diss_Schmittmann.pdf – Influence of temperature on the dynamics of adsorption of short-chain alkanes on zeolites (Schmittmann, Diss. 2021)
